mrs energy & sustainability: a review journal page 1 of 31 ......portable power applications...

MRS Energy & Sustainability : A Review Journal page 1 of 31 © Materials Research Society, 2015 doi:10.1557/mre.2015.4

Introduction

Globally, around 90% of the transport sector depends on fossil fuel powered internal combustion engines (ICEs). 1 While this technology has enabled an unprecedented increase in human mobility, the consumption of fossil fuels by the transport sector has also greatly contributed to the atmospheric emission of

greenhouse gases (mainly carbon dioxide) and other pollutants, such as nitrogen oxides (NO x ) and volatile organic compounds (VOCs). 2 , 3 Besides these environmental concerns, fossil fuels are considered to be unsustainable in the long-term as they represent a fi nite resource fraught with geopolitical constraints and increasing costs of extraction. 4 Because of these factors, a number of alternative, sustainable energy solutions are under active development.

Among potential long-term solutions, fuel cells based on potentially renewable fuels, such as hydrogen and methanol have received considerable attention. 5 – 7 In the portable power and transportation sector, polymer electrolyte membrane fuel cells (PEMFCs) have been the major focus, as they provide higher power density and more facile cycling performance than other fuel cell archetypes. In the transportation arena, PEMFCs can extract signifi cantly more power out of the same quantity of fuel

ABSTRACT

The direct methanol fuel cell (DMFC) enables the direct conversion of the chemical energy stored in liquid methanol fuel to electrical

energy, with water and carbon dioxide as by-products. Compared to the more well-known hydrogen fueled polymer electrolyte membrane

fuel cells (H 2 -PEMFCs), DMFCs present several intriguing advantages as well as a number of challenges.

This review examines the technological, environmental, and policy aspects of direct methanol fuel cells (DMFCs). The DMFC enables the

direct conversion of the chemical energy stored in liquid methanol fuel to electrical energy, with water and carbon dioxide as byproducts.

Compared to the more well-known hydrogen fueled PEMFCs, DMFCs present several intriguing advantages as well as a number of chal-

lenges. Factors impeding DMFC commercialization include the typically lower effi ciency and power density, as well as the higher cost of

DMFCs compared to H 2 -based fuel cells. Because of these issues, it is likely that DMFC technology will fi rst be commercialized for small

portable power applications (e.g., the displacement of batteries in consumer electronic applications), where the shorter product lifetimes

( ∼ 1–2 yrs for a battery versus 8–15 yrs for a car) and the much higher price points ( ∼ $10/W for a laptop battery vs. �∼ $0.05/W for a vehicle

engine) provide a more attractive entry point. While such applications are not likely to signifi cantly impact the global energy sustainability

picture, they provide an important initial market for fuel cell technology. As such, in this review, we provide an overview of recent research

and the challenges to the development of DMFCs for both the portable (shorter-term) and transport (longer-term) sectors.

Keywords : energy generation , ionic conductor , ceramic

REVIEW

DISCUSSION POINTS • Is methanol a renewable fuel for the future?

• Can you imagine powering your laptop computer with alcohol?

• What will be the role of fuel cells versus batteries in the future energy economy?

A review on direct methanol fuel

cells – In the perspective of

energy and sustainability

Prabhuram Joghee and Jennifer Nekuda Malik , Department of

Metallurgical & Materials Engineering , Colorado School of Mines , Golden ,

Colorado 80401 , USA

Svitlana Pylypenko , Department of Chemistry and Geochemistry ,

Colorado School of Mines , Golden , Colorado 80401 , USA

Ryan O’Hayre , Department of Metallurgical & Materials Engineering ,

Colorado School of Mines , Golden , Colorado 80401 , USA

Address all correspondence to Ryan O’Hayre at [email protected]

(Received 30 September 2014 ; accepted 14 April 2015 )

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when compared to traditional combustion engines, thereby providing 30–90% higher effi ciency (on a “well-to-wheels” basis) than regular gasoline-powered ICEs. 8 Most vehicular fuel cells are based on hydrogen-powered PEMFC technology. However, the commercialization of hydrogen-powered PEMFC vehicles suffers from challenges due to on-board storage constraints (even at 1000 bar, H 2 takes up about 3000 times more space than an equivalent amount of gasoline), and lack of a consumer H 2 distribution infrastructure. 9 , 10

The leading direct liquid fuel PEMFC technology, known as the direct methanol fuel cell (DMFC), produces electric power directly from high energy density liquid methanol fuel. Deploying DMFC in the vehicles can resolve on-board storage constraints, as liquid methanol fuel can be distributed using the existing gasoline distribution infrastructure with only minor modifications. 11 , 12 Despite this advantage, the higher materials fabrication cost and the typical lower effi ciency and power density of DMFCs pose daunting barriers for the deploy-ment of DMFC technology in the transportation sector. Weigh-ing in favor of DMFC technology, however, the transition to a methanol-powered transportation sector could be accomplished through a series of more incremental steps compared to the signifi cant infrastructure investments needed for a future H 2 -powered transportation sector. For example, methanol might fi rst be introduced into the existing ICE-based transportation sector by means of blending methanol with gasoline for f lex fuel vehicles (FFV). The FFV scenario is intriguing considering the recent abundance of inexpensive natural gas, which has rendered methanol competitive with gasoline under large-scale production scenarios. Couple that with the fact that methanol has a higher octane value and is less toxic than gasoline 13 , 14 and the case for methanol blended gasoline FFV gains strength.

Methanol is less expensive (per unit energy) and provides significantly higher volumetric and gravimetric energy den-sity compared to compressed hydrogen (at 1000 bar) or even compared to liquid hydrogen. 13 , 14 Interestingly, one liter of methanol contains more hydrogen (99 g) than a liter of liquid hydrogen (71 g). 14

Methanol can be produced by a number of methods. Methanol is mostly obtained from feedstocks such as coal and natural gas by means of the gasifi cation and steam reforming processes, respectively. 15 , 16 In recent years, interest has grown in producing methanol from renewable energy sources such as wood, agricul-tural wastes, and other biomass resources by thermochemical processes. Such processes produce around 170 gallons of meth-anol per ton of biomass. 16 While methanol production from coal and biomass is less effi cient than from natural gas (50–60% compared to 70–80% effi ciency), the renewable nature of biomass feedstock coupled with the possibility for improving production effi ciency makes methanol production from biomass an attractive long-term solution. But production of methanol from biomass raises some concern over the possibility of crop displacement, which could become a serious issue. 17

Here, it is interesting to recall the opinion of Dr. George A. Olah, winner of the 1994 Nobel Prize in chemistry, who stated that “methanol in its own right is an excellent fuel in liquid

form as it can be mixed with gasoline in any proportion, easily stored, transported and can be considered as a universal energy carrier that could eventually replace all the hydrocarbons and ethanol.” 15 Once established, a methanol transportation sector could be further enhanced by the introduction of DMFC-powered vehicles, which would yield environmental, effi ciency, and associated operating-cost advantages compared to combustion-based methanol-fueled vehicles. Thus, the attractive properties of methanol in terms of its energy density, ease of distribution, and potential for production from clean fossil-based resources (natural gas) or from renewable sources (biomass, solar) make the DMFC an intriguing, although still far-off transportation power technology. 18 – 20

Because of the daunting cost, durability, and power-density issues impeding the application of DMFC to the transportation sector, it is likely that DMFC technology will fi rst be commer-cialized for small portable power applications (e.g., the dis-placement of batteries in consumer electronic applications), where the shorter product lifetimes ( ∼ 1–2 years for a battery versus 8–15 years for a car) and the much higher price points ( ∼ $10/W for a laptop battery versus �∼ $0.05/W for a vehicle) provide a more attractive entry point. 21 , 22 In fact, a number of organizations are actively developing DMFCs for portable elec-tronics devices such as cell phones and laptop computers. 23 – 25 While such applications are not likely to signifi cantly impact the global energy sustainability picture, they provide an important initial market for fuel cell technology. As such, in this review, we provide an overview of current methanol-based FC technolo-gies and applications, recent research and development, and the challenges to their commercialization for both the portable (shorter-term) and transport (longer-term) sectors.

Concept and principles of DMFC

Figure 1 illustrates the basic operating principles of both acid- and alkaline-based DMFCs. 26 In a DMFC, the key compo-nent is the membrane electrode assembly (MEA), in which anode and cathode catalyst layers are in intimate contact on either side of the polymer electrolyte membrane (PEM) (either acid or alkaline based membranes can be used). Conventionally, carbon-supported PtRu or (unsupported) PtRu black catalysts are used in the anode and carbon-supported Pt or Pt black catalysts are used in the cathode. Gas diffusion layers (GDLs) are placed in intimate contact with the catalyst layers to aid reactant distribution, current collection, and catalyst-layer protection. GDLs are typically made by coating a carbon and polytetrafluoroethylene (Teflon) mixture on the catalyst-facing-side of a carbon paper/cloth. The presence of Teflon in the GDLs exerts hydrophobic properties necessary to trans-port oxygen molecules to the catalytic sites at the cathode or to facilitate the escape of CO 2 from the anode.

In the DMFC, a methanol/water mixture is directly fed to the anode. Methanol is directly oxidized to carbon dioxide with the possible formation of intermediate species such as carbon monoxide, formaldehyde, and/or formic acid. The formation of these intermediate species is held responsible for the sluggish





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kinetics of the methanol oxidation reaction (MOR) in the anode. 27 The protons produced during the MOR conduct from the anode to the cathode through the Nafi on membrane, while the produced electrons fl ow from the anode to cathode through an external circuit as shown in Fig. 1 . The electrons and protons react with oxygen molecules at the cathode and produce water. In the case of an alkaline DMFC, the methanol is oxidized by OH − ions that are conducted from the cathode to the anode through the alkaline membrane and water is therefore produced at the anode instead.

The overall reaction process that occurs in an acid-based DMFC is shown below:

Anode reaction:

+ −+ → + + (1)

Cathode reaction:

+ −+ + → (2)

Overall reaction:

+ → + (3)

In the case of an alkaline-based DMFC, the reaction process is shown below:

Anode reaction:

− −+ → + + (4)

Cathode reaction:

− −+ + → (5)

Overall reaction:

+ → + (6)

The free energy change, Δ G , associated with the overall reac-tion can be directly related to the reversible cell potential via:

Δ = − (7)

where n is the number of electrons involved in the chemical reaction ( n = 6 electrons per mole of methanol), F is the Faraday constant (96,487 coulombs per mole), and E is the equilibrium (reversible) cell potential.

DMFC polarization curves, effi ciency, and energy

density

In most DMFCs, the open circuit voltage (OCV) is signifi cantly lower than the reversible potential. This is mainly attributed to the factors such as the irreversible adsorption of methanol-derived intermediate species at the anode and methanol crossover from anode to cathode. These effects lead to a mixed potential at the cathode, thereby reducing the OCV to values typically lower than 0.9 V (versus a reversible potential of 1.22 V). 28 – 30 The reversible potential falls below 0.9 V in the presence of

Figure 1. (a) Principles involved in the operation of acid and (b) alkaline based DMFCs. Reproduced with permission from Springer Science, Copyright 2013. 26






even a small amount of methanol crossing through the mem-brane from the anode. As current is drawn from the cell, the cell voltage decreases further due to the combined effect of kinetic overpotentials, ohmic losses, and mass transport con-straints due to the CO 2 gas removal from anode and f looding at the cathode (see Fig. 2 ). 31

As it is possible to separately measure the anode and cath-ode potentials using a dynamic hydrogen electrode (DHE), the typical polarization responses for both electrodes ( E cathode , E anode ), as well as the overall cell polarization curve ( E cell ) are shown in Fig. 2 .

= − (8)

In contrast to PEMFCs, where most of the overall cell polarization can typically be attributed to the cathode, in DMFCs both the anode and cathode contribute significantly to the overall cell polarization.

DMFC effi ciency

For all fuel cells, including DMFCs, the voltage effi ciency is determined by the ratio between the terminal cell voltage and the reversible potential for the process during the operation of the fuel cell at the same temperature and pressure.

η = (9)

where η v is the voltage effi ciency, E cell is the operating cell volt-age, and E rev is the theoretical cell voltage. Because of the metha-nol crossover issue in most DMFCs, not all of the methanol consumed by the cell is converted into electrical current. A fuel effi ciency parameter ( η f ) can therefore be defi ned as the ratio between the measured electrical current ( I ) and that calculated from Faraday's law on the basis of methanol consumption ( I total ):

η = (10)

The overall effi ciency of the DMFC can then be expressed as

η= η × η × η (11)

Where η rev accounts for irreversible entropic losses and is gen-erally defi ned as:

η = Δ Δ (12)

where Δ G and Δ H are the Gibbs free energy and enthalpy of the overall MOR, respectively.

It has been shown that in a DMFC at 0.5 V and with 97% fuel utilization, the overall chemical to electrical conversion efficiency will be around 40%. 32 Moore et al. 33 obtained a maximum conversion efficiency around 35% for a single cell DMFC using 1M methanol in a careful study where the meth-anol crossover current density was measured along with cell current density and voltage. Most published efficiency met-rics pertain to DMFC single cells. Efficiency information for complete DMFC systems is generally not available, but should be lower than single-cell metrics due to additional power losses associated with system ancillaries.

Energy density of methanol

The energy density of a fuel can be defi ned with respect to weight (kWh/kg) or volume (kWh/L) as

( ) ( )= −Δ = −Δ ρ (13)

where W e and W s are the theoretical specifi c energy (Wh/g) and specific energy density (Wh/L), respectively; Δ H is the molar heat of combustion (kJ/mol) of the fuel; M is the molecular weight (g/mol) of the fuel, and ρ is the density (g/L) of the fuel.

Table 1 and Fig. 3 summarize the energy density of various fuels. From the fi gure and table, it is apparent that pure meth-anol has an energy density 15 times higher than the energy density of a Li-ion battery and is one order of magnitude larger than H 2 stored in a pressurized tank (at 200 bar) or in a metal hydride system (4–5%). But, the energy density of methanol is lower than that of conventional liquid fuels such as gasoline and diesel.

Although the theoretical volumetric and gravimetric energy density values of methanol are clearly much higher than the energy density of the battery systems shown in Fig. 3 , this is somewhat of an unfair comparison. The net energy density of a complete DMFC system will be much lower than the energy density of neat methanol fuel due to the mass/volume of the system components and the low (25–35%) conversion effi-ciency of methanol in DMFCs. 34 Nevertheless, recent com-parisons of actual Li-ion batteries (with conversion efficiency of above 80%) and a real DMFC system showed the DMFC to have more than twice the gravimetric energy density (130–200 Wh kg −1 for the Li-ion batteries, versus 400 Wh kg −1 for the DMFC). 35 , 36

Figure 2. DMFC single cell, anode, and cathode polarization curves

operated at 60 °C, ambient pressure, with 1M methanol and air feed at

the anode and cathode, respectively. Reproduced with permission from

Wiley-VCH, Copyright 2009. 13






State-of-the-art anode, cathode, and electrolyte

materials for DMFC

Anode catalysts

Until now, Pt-based noble metal catalysts remain the best choice for activating the MOR at the DMFC anode. When using a pure Pt catalyst, the MOR is not completely realized because of the formation and subsequent irreversible absorption of CO and CHO intermediate species, 37 – 39 which severely impede the kinetics of methanol oxidation on Pt. To mitigate the effect of these poisonous species, the Pt catalyst is usually alloyed with second metals such as Ru, Sn, Mo, Co, Ni, etc. It has been proposed that these second metals alleviate the irreversible absorption of the poisoning intermediate species by means of the bifunctional mechanism and the ligand effect. 40 , 41 The bifunctional mechanism involves the oxidation of CO/CHO

intermediates adsorbed on Pt by OH ads species formed on the second metal at a relatively lower anodic potential. 40 The ligand effect involves a decrease in the adsorption energy of CO/CHO ads species on Pt due to alloying with the second metal, thereby facilitating the oxidation of these poisonous species at relatively lower anodic potential. 41 Extensive studies of the MOR in both acid and alkaline media on various Pt-based bimetallic catalysts (PtRu, PtMo, PtW, and PtSn) have shown that PtRu catalysts exhibit the highest MOR activity 42 – 44 as well as higher DMFC power density and stability compared with other bimetallic catalysts. Ruthenium dissolution, however, poses a big issue in PtRu catalyst systems. 42 , 45 – 49

In analogy to common practice in PEMFCs, DMFCs often use supported PtRu catalysts, where nanoscale PtRu catalyst particles are supported on a high surface area carbon powder to increase mass activity and therefore bring down the cost com-pared to using an unsupported PtRu black catalyst. Lizcano-Valbuena et al. 49 compared various types of carbon-supported Pt-based alloys for the anode of the acid-based DMFC and showed that PtRu/C (75:25 by weight) provided the best perfor-mance among the various Pt-based alloys as depicted in Fig. 4 .

Further research on reducing the loading level of PtRu on the supporting material while at the same time maintaining performance and stability is needed. To improve DMFC catalyst performance and stability, various types of carbon supports have been explored. 50 – 54 PtRu supported on carbon nanofi bres, carbon nanotubes, and graphenes have exhibited particularly intriguing DMFC performance and stability. 52 , 54 , 55 Several researchers have also explored chemically modifying the carbon support itself as a means to improve catalyst performance and stability. 56 – 60 Of particular note, PtRu catalysts impregnated/deposited on nitrogen modifi ed carbon supports have been shown to exhibit signifi cantly higher performance and stability when compared to undoped counterparts. 61 – 63

Table 1. Volumetric and gravimetric energy density for various fuels for

low-temperature fuel cells. (Reproduced with permission from Wiley-VCH,

Copyright 2009. 13 )

Fuels

Volumetric energy

density (kWh L −1 )

Gravimetric energy

density (kWh kg −1 )

Hydrogen 0.18@1000psi, 25˚C 1.2

Methanol 4.82 (100 wt%) 6.1

Ethanol 6.28 (100 wt%) 8.0

Formic acid 2.10 (100 wt%) 1.7

Dimethyl ether 5.61 (in liquid of 100 wt%) 8.4

Ethylene glycol 5.87 (100 wt%) 5.3

Figure 3. Gravimetric and volumetric energy density of various fuels and

batteries. Reproduced with permission from Wiley-VCH, Copyright 2009. 13

Figure 4. Voltage versus current density plot for a single cell of the DMFC

using various types of carbon-supported Pt-based alloys. Reproduced with

permission from The Electrochemical Society, Copyright 2001. 49






Cathode catalysts

In the cathode of a DMFC, as in a PEMFC, Pt metal remains the best catalytic material even though Pt suffers a nearly 400 mV loss due to the sluggish electrochemical kinetics of the oxygen reduction reaction (ORR) and mixed potential caused by meth-anol crossover. 64 – 66 In commercial DMFC systems, carbon-supported Pt is used at the cathode to minimize the cost.

To improve the kinetics of the ORR and improve the toler-ance against methanol crossover, various carbon-supported Pt-based alloys (Pt–M, where M is Fe, Au, Pd, Cr, and W 2 C) have been developed for the cathode and these alloys show improved performance in DMFCs versus pure Pt/C catalysts. 67 – 71 It has been proposed that the formation of Pt-based alloys slightly decreases the Pt–Pt interatomic distance, facilitating cleavage of the strong O=O bond of oxygen to achieve the complete oxygen reduction that favors 4 electron transfer. 64 , 71 The sta-bility of the PtCr and PtFe alloys has been studied in PEMFC cathodes, 72 – 74 and the findings suggest that the stability of these alloys depends on the degree of alloying as well as the cat-alyst particle size. For instance, Ballard Inc. has demonstrated enhanced initial performance and better durability (over 500 h) for PtCr catalysts compared to Pt catalysts in PEMFC cathodes (see Fig. 5 ). 64

To lower cost, it is highly desirable to develop non-noble metal alternatives to Pt-based catalysts. Potential alternatives include pyrolysed Fe porphyrins and Ru-based chalcogenides, which show decent ORR performance (although still not com-petitive with Pt) and excellent methanol tolerance. 75 , 76 However, long-term stability is a potential issue with many of these cata-lysts and further breakthroughs are needed. In alkaline media, because of a wider range of stable metal alloys and inherently faster ORR kinetics, less expensive metals such as Pd, Ag, and their alloys have been successfully used as carbon-supported cathode catalysts with reasonable performance and comparable stability to Pt/C catalysts. 77 – 80 Current intensive research on Pd and Ag and their alloys for the ORR in alkaline media provides further hope that these metals could provide a lower-cost alternative to Pt and Pt-based alloys for alkaline-based PEMFC and DMFC cathodes.

Electrolyte materials

The desired traits for a PEM in a DMFC application include: (i) stable high temperature (80–90 °C) operation capability, (ii) low methanol crossover (<10 −6 mol min −1 cm −1 ) or low diffusion coefficient of the membrane (<5.6 × 10 −6 cm 2 s −1 at T = 25 °C), 81 (iii) high ionic conductivity (>80 mS cm −1 ), 81 (iv) high chemical and mechanical durability particularly at T > 80 °C, (v) low ruthenium crossover, and (vi) low cost. The most commonly used cation exchange membrane (CEM) in DMFCs is Dupont's Nafi on membrane, which does not satisfy all the above requirements due to disadvantages including high cost ($600–$1200 m −2 ), 82 high cost per unit power (300 $ kW −1 at 240 mW cm −2 ), 83 high methanol crossover, 84 and high ruthenium crossover from anode to cathode. 85 Despite these disadvantages, however, alternative membranes generally do not match the

performance and durability of Nafi on, which therefore remains the membrane of choice for most commercial DMFC applica-tions. In the search for better membranes, researchers have pur-sued the modifi cation of Nafi on-type membranes as well as the development of entirely new classes of membranes. 86 – 91 While many of these newer membranes are lower in cost than Nafi on, they have generally not yet obtained the combination of high conductivity and high stability that makes Nafi on so compel-ling. The characteristic properties of various CEM types along with their DMFC performance are provided in Table 2 .

In contrast to CEMs, anion exchange membranes (AEMs) are solid PEMs that contain positive ionic groups (typically quater-nary ammonium functional groups such as poly-N + Me 3 ) and mobile OH − ions. They are also commonly referred to as alkaline membranes. Some of the commercially available AEMs for DMFC application include MORGANE-ADP and an ammonia-based AEM membrane from Tokuyama Co. Japan. Interestingly, these AEMs exhibit low methanol crossover because of the absence of electro-osmotic drag from the anode side: the OH − ions migrate from cathode to anode in alkaline-based DMFCs, whereas in CEMs, protons migrate from the anode to the cath-ode and tend to drag methanol molecules with them. Unfortu-nately, however, the diffusion coefficient and ionic mobility of OH − are typically less than H + so AEMs tend to show lower conductivities. Most AEMs are also not as stable as their CEM counterparts at elevated operating temperatures. 94 , 95 The development of AEMs with high ionic conductivity, good chemical stability, and low methanol permeability is an area of active research. 95 – 105 The basic properties of several widely investigated AEMs are given in Table 3 .

MEA fabrication

DMFC performance and durability mainly rely on the struc-ture of the MEA, which is the key component in the DMFC. In recent years, many attempts have been made to improve the fabrication process and structural parameters of the MEAs to maximize the number of triple-phase boundaries available to facilitate the electrochemical reaction. Conventional methods to fabricate the MEAs are classified into two groups. 106 – 108 One is the catalyst-coated substrate (CCS) method and the other is the catalyst-coated membrane (CCM) method. In the CCS approach, the anode and cathode catalysts are coated on the GDL, which is made of carbon paper, felt, or cloth, and subse-quently hot pressed by sandwiching the membrane electrolyte in-between the anode and cathode catalyst-coated GDLs to form the MEA. In this method, the catalyst layer coated on the GDL is not always effectively transferred to the membrane and some of the catalytic particles get buried into the GDL and are not effectively utilized for the electrochemical reaction during fuel cell operation. Therefore, this method may not be suitable for achieving high MEA performance.

In the CCM method, the anode and cathode catalysts are directly coated on the two sides of the membrane and the GDLs are subsequently sandwiched on either side. In this approach, hot-pressing of the GDL is usually not required. 108 The catalyst






thickness can be controlled and the interfacial contact between the membrane and the catalyst layer is found to be very inti-mate due to the direct coating of the catalyst layer on the mem-brane. This helps to achieve higher cell performance. 109 The DMFC performance of the MEAs fabricated by the CCS (or

catalyst coated on diffusion backing) and CCM method is shown in Fig. 6 . 106

An alternative approach called the decal transfer method (DTM) has also been adopted to achieve higher cell performance and mass production of MEAs. 110 In the DTM approach, the

Figure 5. (a) Comparative performance of Pt and PtCr alloys in PEMFC cathodes operating at 80 °C using hydrogen/air, hydrogen/helox (helox is a mixture of

helium and oxygen), and hydrogen/oxygen at 1.5/2 and 2/10 anode/cathode stoichiometry (308/308 kPa anode/cathode pressure). (b) Durability test for Pt

and PtCr alloys under the same temperature and pressure conditions as (a) using hydrogen/air at 1.5/2 stoichiometry. Reproduced with permission from

Johnson Matthey, Copyright 2002. 64






Table 2. Properties of various types of CEMs for application in DMFC.

Membrane

Methanol permeability

(cm 2 s −1 x 10 −6 )

Ionic conductivity

(mS cm −1 )

Thermal stability

(°C) a

Current density

(mA cm −2 )

Nafi on 117 (170–180 μ m) 87 14.1–17.2(60–90 °C, 1M) 90–12(80 °C, 34–100% RH) 80 50(80 °C, 1M)

Nafi on 115 (127 μ m) 88 19.8(25 °C, 2M) 41(25 °C) 80 20(60 °C, 1M)

Nafi on + ZrP 92 ... 24–60(25 °C, 100 RH) 150 ...

Nafi on + SiO 2 89 4.17 270–390(60–90 °C) 145 240

BPSH-40 (138 μ m) 90 8.1 ... ... 150

sSPEEK 93 0.2(20 °C, 2M) 0.9(20 °C) ... 220(80 °C, 0.5M)

PBI 91 ... 10–40(130–180 °C) 160–200 500

a At higher fuel cell operating temperature (above 80 °C), Nafi on type membranes are likely to lose water and mechanical strength, which may have a detrimental

effect on the performance/durability of the fuel cell. Additives like SiO 2 and ZrP can be added to enhance the thermal stability of Nafi on as they can help to

retain water and mechanical strength even at higher fuel cell operating temperature (above 100 °C).

catalyst layers are deposited on Tefl on decal substrates and are then subsequently transferred to a dry electrolyte membrane during the hot-pressing of the MEA. MEAs fabricated by the CCM and DTM methods have an improved catalyst/membrane interface, better catalyst utilization, 108 , 110 and superior formation of the ionomer network. 107 , 110 These aspects are all generally beneficial in improving the performance and durability of a DMFC used with both acid- and alkaline-based membranes. 48 , 110 The CCM and DTM MEA fabrication processes are compared in Fig. 7 . 111

History of research on DMFC performance and

long-term stability

Basic investigations into the direct electro-oxidation of methanol were initiated in the early to mid 1980s. During these early years, research focused on the development of suitable anode and cathode electrocatalysts and characterization of their performance in half-cell configurations with aqueous acid or alkaline electrolytes. 112 – 114 DMFC research increased signifi -cantly in the 1990s, with the development of DMFC MEAs and the proliferation of single cell and DMFC stack/system studies. This section will focus on MEA and stack-level studies from the early 1990s to the current period. Table 4 summarizes some of the performance and durability data.

In 1994, a team from the Jet Propulsion Laboratory, the University of Southern California and Giner, Inc., sponsored by the Defense Advanced Research Projects Agency (DARPA), reported on the development of a liquid-fed DMFC. 115 This work showed improved performance when a solid electrolyte was used instead of an aqueous sulfuric acid electrolyte, demon-strating ∼ 300 mA cm −2 at 0.5 V and 90 °C (see Fig. 8 ). The authors discussed the effects of temperature and methanol

concentration on performance, indicating that (i) higher tem-peratures led to increased output, (ii) optimum methanol con-centrations lay between 0.5 and 2M, and (iii) methanol crossover limited the performance, particularly at methanol concentra-tions above 2M. In 1995, a team from the University of Newcastle reported a vapor-fed DMFC yielding 75 mA cm −2 at 550 mV and ∼ 100 °C, when operating with 1 vol% methanol. 30 The authors reported steady performance over 8 h without apparent degradation. In 1996, a group from Los Alamos National Laboratory demonstrated current densities as high as 370 and 670 mA cm −2 (at 110 and 130 °C, respectively) at 0.5 V by optimizing the MEA using the DTM. 28 Meanwhile, by vary-ing temperature (in the range of 60–120 °C), methanol con-centration (0.1M, 0.5M, 2.5M, and 4.0M), and the Nafion membrane thickness (comparing 117, 115, and 112), scientists from Korea Institute of Energy Research identifi ed a window of optimal DMFC operating conditions that yielded 230 mA cm −2 at 0.55 V. 116 In addition, authors also reported that performance could be decreased by changes in the catalyst particle size, crystal-linity, and composition induced by heat-treatment of the catalyst.

The method of MEA preparation has been shown to have a signifi cant impact on DMFC performance. DMFCs fabricated using CCM have generally shown better performance than those fabricated using DTM, mainly due to increase in the thickness and porosity of the cathode. In contrast, the thickness of the anode does not appear to have a signifi cant impact. 111 One study highlighted a 39% performance improvement from the CCM-based MEA fabrication, attributing the enhancement to the favorable porosity. 117 To improve anode performance, it has recently been demonstrated that anodic treatment of anode catalyst at 0.8 V versus DHE prior to DMFC operation results in a signifi cant improvement in DMFC performance, despite also






triggering some ruthenium dissolution and a decrease in elec-trochemical surface area. Based on data supporting reorganiza-tion of the Nafi on on the catalyst surface, this improvement was attributed to the formation of an improved interface between catalyst and Nafi on ionomer. 118

Long-term degradation is one of the major challenges imped-ing DMFC commercialization. Among notable studies examin-ing DMFC degradation, a 30% loss of original power density was reported for a DMFC operated at a low current density of 100 mA cm −2 for 75 h. The performance loss was primarily attributed to delamination of the catalyst layer and agglomera-tion of the catalyst, which was especially severe at the anode. 119 Not surprisingly, longer durability tests have also confi rmed degradation due to catalyst agglomeration and the consequent loss of electrochemically active surface area. 120 , 121 In a study examining 500 h of operation, performance losses of 34% were recorded and attributed to delamination at the interface, particle agglomeration and ruthenium leaching and crossover. 122 In another study, a gradual decrease in the performance of a DMFC monitored for a period of 600 h was primarily assigned to degradation of the cathode. 123 At higher temperatures, the rate of degradation is higher due to the formation of pinholes in the membrane in concert with delamination and degradation of cathode. 124 Also, at higher temperatures, ruthenium dissolution/leaching from PtRu/C is accelerated when the anode experiences

potential greater than 0.363 V versus DHE. 120 The DMFC per-formance and durability data are summarized in Table 4 .

Electrochemical impedance and physiochemical analysis of MEAs after 1000 h of continuous operation show additional degradation mechanisms beyond catalyst activity losses. One notable degradation mechanism is an increase in cathode mass transport resistance associated with changes in its morphology and decreased hydrophobicity. 132 Membrane degradation can also contribute to long-term performance losses in DMFCs. A DMFC made with poly(tetrafl uoroethylene-co-perfl uoropropyl vinyl ether)-graft-poly(styrene sulfonic acid (PFA-g-PSSA) membrane demonstrated 26.8% loss in voltage after testing for 2066 h, with losses attributed to catalyst agglomeration and reduced conductivity of the membrane. 129 Performance loss due to interfacial delamination between the electrode and membrane was shown to be greater for wholly aromatic sul-fonated poly(sulfone) membranes when compared to Nafi on membranes. 133 While methanol crossover can be reduced by using alternative membranes 134 or by microporous layers with ultra-small (˂5 μm) pores, 135 it is important to note that decreased methanol crossover does not necessarily translate into increased power. 136

DMFC performance using anion exchange PEMs instead of the more common proton-based Nafi on membrane has been evaluated by several groups, most of whom report higher ohmic

Table 3. Basic properties of various types of AEMs for DMFC application.

Membrane Ionic conductivity (S cm −1 ) Methanol permeability (cm 2 s −1 )

Tokuyama (A201) 95 2.9 × 10 —2 ...

Tokuyama (A901) 95 1.14 × 10 −2 ...

Radiation-grafted ETFE-AAEM 96 3.0 × 10 −2 (30 °C) 0.6 × 10 −6 (20 °C)

QPEK-C 97 1.6 × 10 −3 (20 °C) P < 10 −9 (30 °C)

QPES-C 98 4.1 × 10 −2 (20 °C) 5.72 × 10 −8 (30 °C)

9.2 × 10 −2 (70 °C) 1.23 × 10 −7 (70 °C)

Crosslinked QAPVA 99 2.7–7.3 × 10 −3 (30 °C) 1.0–4.1 × 10 −6 (30 °C)

Crosslinked QAPVA/HACC 100 0.3–1.3 × 10 −2 (60 °C) 0.6–4.4 × 10 −6 (30 °C)

QAPVA/SiO 2 101 3.5–6.8 × 10 −3 (30 °C) 0.9–1.2 × 10 −6 (30 °C)

PVA/TiO 2 102 0.3–4.8 × 10 −2 (30 °C) 3.7 × 10 −7 (25 °C)

CPPO/BPPO 103 2.2–3.2 × 10 −2 (25 °C) 1.4–1.5 × 10 −7 (25 °C)

Crosslinked CPPO/BPPO 104 2.2–3.2 × 10 −2 (25 °C) 1.0–1.3 × 10 −7 (25 °C)

KOH-doped PBI 105 1.8 × 10 −2 (25 °C) 2.6 × 10 −7 (25 °C)






resistance, but lower methanol crossover. 94 , 131 Matsuoka et al. report that the OCV of an AEM-DMFC is 100–200 mV higher than a comparable DMFC made with Nafi on. 130 Several other studies have demonstrated higher OCV and improved perfor-mance from AEM-DMFCs. 137 , 138 Scott et al. demonstrated an AEM-DMFC with a power density of 16 mW cm −2 , when operat-ing at 60 °C with oxygen and 6 mW cm −2 when operating at ambient temperature with air. 131 An AEM-DMFC fabricated with a Tokuyama-006 membrane using CCS method showed peak power densities up to 168 mW cm −2 . 47 Another study achieved maximum power densities of 117 mW cm −2 in oxygen and 100 mW cm −2 in air, respectively, while using Tokuyama A-201 membrane and signifi cantly lower catalyst loadings than those shown in previous reports. 48 This work also demon-strated that modifying a PtRu/C anode catalyst with nitrogen enables even higher power densities, 140 mW cm −2 in oxygen and 110 mW cm −2 in air. Ionomer content was shown to greatly

affect the performance of an AEM-DMFC fabricated using the dry spraying method. 139 While AEM-DMFCs offer higher OCV and perhaps slightly better initial performance than Nafi on-based DMFCs, the robustness of alkaline membranes typically suffers compared to Nafi on-type membranes, resulting in more rapid performance losses. 47

Moving from single-cell studies to stack development, in the late 1990s Siemens worked on the development of larger-sized DMFCs operating under air conditions, starting with 3–60 cm 2 single cells, followed by a three-cell stack. The stack produced 1.4 V at 0.1 A cm −2 under air operation, yielding a total power output of 77 W. This was the highest total power output for an air-operated multicell DMFC stack reported at that time. 11 In 2000, Los Alamos National Laboratory published on the development of DMFCs with new stack technology utilizing a 2 mm pitch per cell that was suitable for both portable power and transport applications. Among other fi ndings, the loading and nature of the catalyst and catalyst layer structure and composition were found to be essential factors in the DMFC optimization. 140 Figure 9 shows a 3-cell short stack and its performance along with a photograph of a 500 W 71-cell DMFC stack developed at Forschungszentrum Jülich. These stacks also utilized a 2 mm cell pitch. 125 Around the same time, Motorola Labs investigated ceramic and graphite-based DMFC stack designs. They eventually settled on a graphite-based stack that in comparison to a previous ceramic-based stack generated over 3 times higher peak power density (77 W L −1 versus 26.1 W L −1 ), mainly due to an increase in the active area per unit stack volume and a higher temperature inside the graphite stack under steady state operation. 126 Performance of the DMFC stack, which produced 54 W (85 mW cm −2 ) in air and 98 W (154 mW cm −2 ) in oxygen, was found to be highly sensitive to the distribution of the reactants. A 20% increase in power density was observed when implementing a counter-fl ow arrangement for the reactants. 127

In studies examining long-term DMFC operation, it has been demonstrated that the life of the fuel cell can be extended by periodic air cut-off, a strategy that was successfully imple-mented to achieve DMFC operation for over 1200 h. 126 , 128 Air interruption control was also used on a 42-cell 400 W stack fabricated with graphite bipolar plates, demonstrating that each cell could maintain stable performance for over 500 h. 141 Regulating the air on-off condition and using moderate air fl ow rates can mitigate long-term performance losses, particularly on the cathode side. 142

Another important issue for DMFC stacks is cold-start operation and operation under freezing temperatures. Investi-gation of the cold start operation of a 10-cell DMFC stack showed that self-heating operation is limited to temperatures above −10 °C. 143 A self-sustained 15-cell DMFC stack with a sur-face area of 45 cm 2 was reported to provide 40 W at ambient temperature. 144

The drive to commercialize DMFCs for portable applica-tions has led to signifi cant research in air breathing systems and to the development of confi gurations that do not require ancil-lary balance of plant components and systems that are tolerant

Figure 6. Polarization curves of MEA1 (catalyst loading anode/cathode =

1/1 mgPt cm −2 ) and MEA2 (catalyst loading anode/cathode = 1/1 mgPt cm −2 ).

Conditions: T = 80 °C, C MeOH = 0.75M, O 2 operation, cathode/anode

overpressure =0.1/0 bar. Reproduced with permission from Elsevier,

Copyright 2004. 106

Figure 7. Schematic comparison of the CCM and DTM MEA fabrication

processes. Reproduced with permission from International Journal of

Hydrogen Energy, Copyright 2011. 111






to higher methanol concentrations. Compared to DMFCs with active air supply, passive (“air breathing”) DMFC cells often require higher catalyst loadings to ensure good performance. 145 A study comparing the performance of a series of passive DMFCs made from Nafi on membranes of different thicknesses showed very similar performance at high methanol concentration (4M). 146 Guo and Faghri discussed the performance of a series of passive DMFC progressing in size from a single cell with 1 cm 2 active area to single cells with 4.5 and 9 cm 2 active area, and finally to two four-cell stacks with 18 and 36 cm 2 active area, respectively. 147 This work reported higher power densities for 2M and 3M methanol concentrations as compared to 5M

solution, with the latter resulting in higher methanol crossover and increased temperature. 147 A prototype of a 2 W passive DMFC with 8 cells using a porous carbon plate (PCP) platform was developed to accommodate 100% methanol. 148 PCP tech-nology was also used to study the performance of a 30 cm 2 active DMFC, demonstrating 42 mW cm −2 at 45 °C. 149 A passive 8-cell twin stack with total surface area of 32 cm 2 achieved 16.9 mW cm −2 when operating at ambient temperature with 4M methanol. This stack was equipped with a fuel-feed device that allowed for self-regulation of the feeding rate, based on the discharging current of the stack. 150 Li and Faghri compared two passive stacks, one using 1M methanol and the other using

Table 4. Performance and durability of single cell and stacks of DMFC.

Single cell/

Stack

Methanol

concentration

(M) Oxidant

Anode

catalyst/

loading

(mg cm −2 )

Cathode catalyst/

loading (mg cm −2 ) Membrane

Temperature

(°C)

Power

density

(mW cm −2 )

Durability

(h) a

Single cell 28 1 O 2 PtRu(2.2) Pt(2.3) Nafi on 112 130 390 ...

Single cell 30 2 Air PtRu/C(5) Pt(5) Nafi on 117 100 120 ...

Stack 116 2.5 O 2 PtRu/C(3) Pt/C(3) Nafi on 117 90 260 ...

Stack 125 1 Air PtRu(3.9) Pt/C(2.3) Nafi on 115 64 32 ...

Single cell 119 1 O 2 PtRu/C(2) Pt/C(1) Nafi on 115 75 65 75

Stack 126 1 Air PtRu(10) Pt(8) Nafi on 117 55 60 1200

Stack 127 2 Air PtRu (5) Pt(5) Nafi on 115 25 90 ...

Single cell 121 1 Air PtRu(4) Pt(4) Nafi on 115 60 70 110

Stack 128 1 Air PtRu/C(2) Pt/C(1.5) Nafi on 115 51 108.5 2000

Single cell 122 1 Air PtRu(3) Pt(2) Hydrocarbon 80 110 500

Single cell 129 1 Air PtRu/C(2) Pt/C(2) PFA-PSSA x

( x = 50) b

60 140 2000

Single cell 130 1 O 2 PtRu/C(4) Pt/C(1) AEM 50 6 ...

Single cell 131 1 Air PtRu/C(1) Pt/C(1) AEM 60 9 ...

Single cell 47 1 O 2 PtRu(8) Pt(8) AEM 90 160 ...

Single cell 48 2 O 2 PtRu/C(3) Pt/C(2) AEM 80 140 ...

a DMFC durability measurement is not standardized. Typically, either constant-current or constant-voltage durability tests are used. In Refs. 119 and

121 , a constant voltage 0.4–0.5 V (per single cell) was used for durability testing (which normally yields a current density of around 100–150 mA cm −2 ).

In the other studies, 129 – 132 durability was evaluated under a constant current density mode of 100–150 mA cm −2 (which generally yields a voltage of around

0.4–0.5 V). The voltage and current-based durability studies chosen for this table should therefore render similar levels of degradation to the cell.

b PFA-PSSA= Poly(tetrafl uoroethylene-co-perfl uoropropyl vinyl ether)-poly(styrene sulfonic acid).






pure methanol, obtaining similar performance for both stacks, although the pure methanol stack provides a clear advantage in energy storage density. 151

Status of DMFC technology for portable electronic

devices

Over the years, Li-ion batteries have increasingly been used in the portable electronic industry to power laptop computers, mobile phones, digital cameras, and so on. In recent years, con-siderable progress has been made in manufacturing Li-ion batteries with increased energy density and lifetime. 152 , 153 However, one can argue that these improvements have not kept up with the increasing power demands of portable electronic applications. Research by the Boston Consulting Group (BGP)

in 2005 illustrates the fact that mobile device energy demand growth has exceeded the growth in battery energy capacity. This phenomenon is referred as the “run time gap” ( Fig. 10 ) and while this study was published a decade ago, this issue con-tinues to constrain the design features of current portable elec-tronic devices. In addition to energy density limitations, Li-ion batteries face self-discharging and capacity fade issues that limit lifetime. For instance, Li-ion batteries can permanently lose 35% of energy capacity over the period of 12 months if exposed to 40 °C ( Table 5 ). 154

In this context, the higher energy density of methanol fuel provides an opportunity for DMFCs in the portable power sector. As discussed in the introduction, the theoretical spe-cifi c energy density of methanol is signifi cantly higher than the energy density of advanced Li-ion batteries, but a fair

Figure 9. Stacks developed at Forschungszentrum Jülich. (a) Photograph of the 3-cell short stack, (b) performance of the 3-cell short stack, and (c) photograph

of the 500 W 71-cell stack. Reproduced with permission from Elsevier, Copyright 2002. 125

Figure 8. Effect of (a) temperature and (b) fuel concentration on cell performance. Reproduced with permission from Elsevier, Copyright 1994. 115






comparison of complete DMFC systems versus Li-ion batter-ies shows a much tighter race. 35 , 36 , 155 , 156 Nevertheless, as shown in Fig. 11 , taking into account the additional system and pack-aging requirements associated with the DMFC, DMFCs offer an advantage compared to Li-ion batteries at higher energy contents and for larger system sizes.

Miniature DMFCs with methanol fuel cartridges instead of power cords and adapters allow consumers to enjoy a truly “wire-less” experience, which is particularly useful in places where the power grid is unavailable. For example, DMFC-powered laptops are projected to enable up to one month of use between refueling (assuming eight hours of usage per day for fi ve days a week). 157 The nascent portable DMFC market was estimated at approxi-mately $17.5 million in 2010. 158 Forecasts project that the DMFC market could increase to $1.1 billion by 2016 and this will account for 85% of the portable fuel cell market by the end of the forecast period. 159 As discussed in more detail later, regulatory actions could promote increased use of DMFCs. The 2007 authorization of DMFC devices and cartridges on aircraft by the International Civil Aviation Organization and the US Department of Trans-portation is an important step in this regard. 159

A number of companies and organizations are actively engaged in the development of prototype “miniature” and hybrid DMFCs for portable electronic applications. 23 , 160 – 164 Early DMFC prototypes suffered from nonoptimized cell design, high electrocatalyst loading in both the anode and the cathode, deployment of unsuitable current collectors, and improper water management that resulted in poor air diffusion in the cathode. Recognizing that these issues required further solu-tion, several signifi cant advances were made by industry over the last 15 years. In 2002, Samsung Advanced Institute of Tech-nology (SAIT, South Korea) developed a small monopolar DMFC (single cells are placed together in such a way that two positive poles or two negative poles are opposite to each other in pairs) using a new type of membrane and catalyst supported on mesoporous 162 carbon that greatly reduced catalyst load-ings. The advanced monopolar DMFC yielded excellent power densities even at ambient temperature as shown in Fig. 12 , which enabled a notebook computer to run for 10 h on a 100 cm 3 cartridge of methanol. 24

In 2006, SAIT developed a docking station, which was equipped with a DMFC. The DMFC offered a maximum power output of 20 W and could generate small amounts of power over long periods of time at low temperatures. 165 Subsequently, in 2009, SAIT developed a prototype DMFC for military applica-tions which weighed 7.7 lbs and was capable of delivering a total of 1800 Wh of energy. 166 Although the company stated that fi eld-test of the DMFC for military use would be conducted in 2010, further information on this product is not presently available.

In 2003, the Smart Fuel Cell AG Company (Germany) suc-cessfully launched its fi rst commercial DMFC (Smart Fuel Cell C25) for notebook computers. 167 This compact DMFC system, weighing just 1.1 kg, offered a continuous power output of 25 W at 12 V. Methanol feed in this cell was provided by a Smart Fuel Cell M125 cartridge the size of cigarette packet weighing just

Figure 10. The runtime gap between device power demand and battery

supply capability. [Source: Boston Consulting Group (BGP)]. Reproduced

with permission from Elsevier, Copyright 2007. 154

Table 5. Permanent capacity loss for Li-ion batteries as a function of the

storage conditions. (Reproduced with permission from Elsevier, Copyright

2007. 154 )

Storage

temperature 40% charge 100% charge

0 °C (32 °F) 2% loss after 1 year 6% loss after 1 year



60 °C (140 °F) 25% loss after 1 year 40% loss after 3 months

Source: Battery University.com

Figure 11. Practical volumes of Li-ion battery and DMFC systems.

Reproduced with permission from Elsevier, Copyright 2002. 156






150 g. The cartridge holds 125 cm 3 of methanol, providing an average power output of 20 W with atleast 7 h runtime (see Fig. 13 ). The fuel cartridge in the Smart Fuel Cell from SFC uses 100% methanol. A water management system controls the optimal methanol concentration supplied to the fuel cell. In addition, SFC developed an automated cartridge filling and assembly line for its own methanol fuel cartridges and also established an international network of distributors.

A couple years later, a portable DMFC hybrid system fi tted with an exchangeable 1.5 Ah lithium polymer rechargeable battery (SFC C20-MP) with a power output of 20 W was devel-oped by Smart Fuel Cell AG. 168 This system weighed 2 kg and was fueled by hot-swappable, 500 mL methanol fuel car-tridges. Two SFC C20-MP systems were developed: one was designed for moderate ambient temperature operation (1–35 °C) and operated with neat methanol fuel (normal unit). The other system was designed for high ambient temperature oper-ation (1–50 °C) and operated on a methanol–water mixture

(desert unit). The US Army Communications-Electronics Research Development and Engineering Center (CERDEC) tested these two systems and found that the normal unit had a peak fuel efficiency of 19.1% at 19.6 W average power out-put, whereas the desert unit had a peak fuel efficiency of 19.7% at 20 W average power output. 168 Both systems showed improved reliability and electrical characteristics when com-pared with previous DMFC systems tested by CERDEC. How-ever, it was concluded that further work was needed to continue to improve these systems. 168

In 2008, SFC AG developed a DMFC (carrying a 10 L car-tridge weighing 18.5 lbs) which was claimed to provide the same energy content as 595 lbs of batteries for use in remote video surveillance applications for round-the-clock, unmanned, off-grid power in any location. 169 This DMFC operated a sur-veillance camera for up to 8 weeks without any intervention. SFC AG manufactured and shipped 13,000 DMFC products with 100,000 fuel cartridges for consumer electronics from the year 2002 to 2009. More recent sales information is not pres-ently available. 170

In 2005, Neah Power Systems commercialized small DMFCs using silicon-based membranes in place of the standard polymer-based electrolyte, thus eliminating the issues associated with polymeric membranes typically used in the conventional DMFC systems. 171 The company advertised the design as offering high catalytic surface area while simultaneously maintaining a small form-factor, which is important for producing higher power density. In 2009, Neah Power Systems developed a portable DMFC equipped with silicon-based architecture (porous sili-con, catalyst-supporting electrode structure) and circulating liquid electrolyte capable of generating a power density greater than 180 mW cm −2 . 172 Current status on this high power den-sity portable DMFC is not available.

In 2008, Mechanical Technology Inc., (MTI) developed its third generation “Mobion Technology”, aimed to overcome many of the technical barriers preventing commercialization of DMFCs for handheld and portable electronic devices. The Mobion Technology provided a simplified architecture that eliminated the need for gravity and remix pumps and condensers, while allowing utilization of the high energy density of a 100% methanol feedstream, supplied in hot-swappable cartridges. In 2008, the company unveiled a Mobion external power pack prototype with a removable methanol cartridge ( Fig. 14 ) that could provide 25 Wh of power or up to 25 h of power from each methanol cartridge. 173 Unfortunately, at the end of 2011, MTI had shelved the “Mobion Technology” because the micro fuel cell division had incurred $60 million in losses without produc-ing any commercial products. 174 However, as per 2013 report, MTI was exploring new opportunities to develop the DMFC technology again. 174

In 2004, Toshiba Inc. (Japan) developed a highly compact DMFC, which was claimed to be the smallest DMFC at that time. 175 Total weight was 130 g with a volume of 140 cm 3 including the fuel cell cartridge. The system could generate 1 W of con-tinuous power for approximately 20 h on 25 cm 3 of methanol. The Toshiba system used the water produced in the cathode to

Figure 12. Current/voltage plot for the DMFC cells developed by the

Samsung Advanced Technology Institute (SAIT) in Korea. Reproduced with

permission from Elsevier, Copyright 2002. 35

Figure 13. (a) The miniature DMFC (Smart Fuel Cell C25) compact power

system and (b) laptop computer integrated with miniature DMFC.

Reproduced with permission from Elsevier, Copyright 2003. 167






dilute the methanol supplied to the anode, thereby eliminating the need of predilution and reducing the size of the fuel reservoir. In 2005, Toshiba unveiled two DMFC prototypes, one generating 100 mW and the other generating 300 mW that were integrated into MP3 players with fuel capacities of 3.5 and 10 mL, respec-tively. 176 In 2009, Toshiba launched its fi rst commercial DMFC product called “Dynario™” ( Fig. 15 ). 177 The Dynario™ (with dimensions of 150 × 21 × 74.5 mm 3 and a weight of 280 g) was intended to deliver on-the-go power to digital consumer products. The Dynario™ utilized a high concentration methanol solution and produced 2.5 W, which was suffi cient to charge a typical cell phone battery twice. 175 On a trial basis, Toshiba manufactured 3000 Dynario™ units in 2009 and shipped this product to cus-tomers to gauge their reaction. The cost of this product was $327, which included fi ve 50 mL methanol cartridges and addi-tional methanol cartridges cost $35 each. 178

In 2009, Sony launched a cordless mobile phone charging system powered by a hybrid DMFC/Li-ion battery system. 179 The cordless speaker system was driven by four DMFC single

cells sharing a 270 mL methanol tank, with each DMFC single cell providing a power of 550–600 mW. The system was supple-mented with a Li-ion secondary battery to provide higher power outputs when required. The hybrid system could provide a maximum power output of about 10 W. Assuming 3 h/week use of the cordless speaker system in a home entertainment envi-ronment, the speaker can be powered for a year without refi lling the tank. Although miniature DMFCs have shown progress toward commercialization, further improvements in energy, power density, and miniaturization of the total package are required. Other aspects requiring attention include, increasing stack longevity, optimization of heat/water management, build-out of the fuel cartridges/canister distribution infrastructure, and establishment of new regulations.

Status of DMFC technology for transportation

Methanol ICEs

Methanol is a simple chemical with excellent combustion properties that makes it an ideal fuel for ICE-driven vehicles. 180 Methanol contains about half the energy density of gasoline. Despite its lower energy content, methanol has a higher octane rating than gasoline ( ∼ 100), which facilitates greater fuel/air compression ratios (10–11 to 1 versus 8–9 to 1 for gasoline engine) and thus higher effi ciencies. 15 In addition, methanol is a safer and more environmentally benign fuel. In fact, methanol vehicles produce lower pollutant emissions, including decreased hydrocarbon emissions as well as decreased levels of NO x , SO 2 , and particulates when compared to gasoline-powered vehicles.

In the late 1980s, automotive companies including General Motors, Ford, Chrysler, Volvo, and Mercedes developed the technology to transform existing engine models into methanol fl exible fuel vehicles (FFVs) for the cost of a few hundred dollars per vehicle. 15 This methanol FFV option is available on a num-ber of car/truck models and allows a vehicle to run on any combination of methanol and gasoline up to M-85, a blend of 85% methanol and 15% unleaded gasoline. In 1997, the number of U.S. methanol FFVs reached 21,000 units. Approximately 15,000 of these vehicles were located in California, where over 100 methanol refueling stations were located. 181 Unfortunately, in the late 1990s, the attractiveness of methanol as a transporta-tion fuel was diminished by the lower price point of gasoline (less than $1.00 per gallon at that time). 182 Since then, gasoline prices have soared, but the methanol FFV has not yet rebounded. In the year 2009, the U.S. Congress passed the Open Fuel Standard Act, mandating that 50% of the vehicles produced in the US market must be compatible with blends up to M-85 and E-85 fuel (a blend of 85% of ethanol and 15% gasoline) by 2012 and that 80% of the vehicles produced should have the fuel option by the year 2015. 166 Recently, the Lotus car company unveiled a “tri-f lex-fuel” vehicle, the Exige-270E, which can run on a mixture of gasoline and methanol as well as gasoline and ethanol. 183

The chemical and physical properties of methanol raise some issues for use in ICEs. Methanol can corrode some of the metals used in modern automotive engines, including aluminum, zinc,

Figure 15. The DMFC external power supply Dynario™ launched by

Toshiba. Reproduced with permission from Elsevier, Copyright 2013. 177

Figure 14. The Mobion DMFC charger from MTI Micro based on Mobion chip

platform (inset) eliminates the need for cumbersome water management

around the micro fuel cell. Reproduced with permission from Elsevier,

Copyright 2009. 173






and magnesium. 184 It can also react with plastics, rubbers, and gaskets, causing them to soften and swell and resulting in even-tual leaks or system malfunctions. Therefore, automotive systems should be specifi cally designed for pure and blended methanol. Cold-start represents another major issue using pure methanol. Cold-start problems can occur because methanol lacks the highly volatile compounds such as butane, isobutene, or propane generally found in gasoline, which help to provide ignitable vapors to gasoline engines even under the most frigid conditions. 185

To further increase effi ciency and lower emissions, methanol fed fuel cell vehicles (FCVs), which are fully qualifi ed as zero emission vehicles (ZEVs) 186 are being examined as an alternative for automotive power. The application and status of methanol-based fuel cell technology for the transportation sector are further discussed below.

DMFC technology to replace automotive ICEs

Both hydrogen and methanol fed FCVs can potentially offer higher efficiency and lower emission levels than either gasoline or pure methanol/blended methanol-powered ICE vehicles. 12 , 15 , 187 Presently, automotive companies are invest-ing most heavily in hydrogen fed PEMFCs for FCVs. PEMFCs have high power density and a low working temperature (about 70 °C) that allows for rapid start-up. The electric effi-ciency is usually 40–60% and the output power can be changed quickly to meet a variable load. As a result of these character-istics, PEMFCs are considered to be promising for transport applications and a large number of companies are actively engaged in their development. 188 – 191 Despite these advan-tages, both on-board and stationary hydrogen storage and distribution present significant challenges. 9 , 10 The primary aim of any on-board fuel storage system is to provide a safe system to deliver sufficient fuel to satisfy the driving range requirement of the vehicle. These conditions must be accom-plished at low cost without adding excessive weight and com-promising the interior volume. At standard temperature and pressure (i.e., in the absence of compression), hydrogen gas requires about 3000 times more space than gasoline for an equivalent amount of energy. 10 Storing under pressure improves the energy density by volume, allowing the tank to be smaller but not lighter. Currently, compressed hydrogen is the preferred solution in most fuel cell powered prototype vehicles, but its usage is far from satisfactory. Currently, hydrogen is pressurized from ∼ 350 to 700 atm in tanks made of new composite light-weight materials, such as carbon fi ber with aluminum/steel/polymer liners, but these materials are very expensive. 10 Even at 700 atm, hydrogen has 4.6 times lower volumetric energy content than gasoline, and the gravimetric energy density comparison is even less favorable (due to the considerable weight of the tanks). Moreover, in contrast to gasoline tanks, a hydrogen tank cannot adopt a shape that best suits the vehicle because the compressed hydrogen tank must be cylindrical to ensure its integrity under high pressure. Hydrogen tanks can therefore pose spatial

integration issues. An intensive research is underway in an effort to find a more safe/convenient way to store hydrogen in fuel cell powered vehicles. 192 – 194

In view of the challenges associated with on-board hydro-gen storage for PEMFC-powered vehicles, some companies have shown interest in developing DMFC-based FCVs. 11 , 195 – 202 As discussed previously, methanol can significantly mitigate on-board storage constraints. Moreover, it can be distributed using the existing gasoline distribution infrastructure with only minor modifications. 11 , 12 In 1998, General Motors Cor-poration (GMC) with its German subsidiary, Opel, introduced a methanol fuel-cell powered car called “Zafira”. 186 The car was four-seat equipped with 50 kW electric motor. However, in 2000, Opel unveiled an updated version of Zafi ra, which run on hydrogen. 183 In the same year, Daimler-Chrysler (Germany) unveiled NECAR 5 powered by a PEMFC, where the hydro-gen was produced by on-board methanol reformation. The NECAR 5 was the first vehicle in which an entire fuel cell sys-tem with a built-in methanol reformer was accommodated within the underbody of a compact car platform (see Fig. 16 ). The car could carry five passengers with their luggage and attained a top speed of 90 mph. 186

In the following year, Ballard Power Systems Inc. (BPSI, Canada) in collaboration with Daimler-Chrysler, developed a 6 kW DMFC system for a one-person vehicle. 11 , 186 In this sys-tem, both the anode and cathode catalysts were impregnated over an oxidized carbon substrate (to increase the wettability of the carbon substrate). The MEAs were fabricated by a con-ventional approach by hot pressing the anode and cathode layers to either side of a solid polymer electrolyte. 196 The details of the stack assembly, design, and performance of this system were not revealed. In 2003, Yamaha (Japan) developed the FC06 prototype DMFC-powered motorcycle with an out-put power of 500 W. 203 In 2007, an improved version of the DMFC-powered two-wheeler (FC-Dii) was introduced with an output power of 1 kW. The cell stack provided a 30%

Figure 16. Mercedes-Benz A-class compact car powered by a methanol-

fueled FC system (although notably not a DMFC). Reproduced with

permission from Mercedes-Benz – Copyright: Daimler AG.






system electrical efficiency in a light-weight, compact, and high power density package. 204

Among hybrid electric vehicles (HEVs), the plug-in HEV has already made an impact in the market. On-board batter-ies provide the primary power, enabling typical driving range of 30–60 km before lengthy (8+ hour) recharging is required. 7 Some companies have therefore examined hybrid DFMC/battery systems to extend range and improve HEV perfor-mance. 7 , 186 Toyota (Japan) developed a 25 kW DMFC system that works in conjunction with a downsized battery pack to power its RAV 4 SUV. In this hybrid system, batteries are con-stantly recharged by the DMFC. Toyota's design draws extra power from the batteries to supplement the fuel cell during acceleration. The batteries also enhance the vehicle by pro-viding instant power, and facilitate the initial warm-up cycle required to bring the fuel cell up to power upon cold start. 186 In the year 1999, Georgetown unveiled a hybrid DMFC-battery system to power a prototype 40-foot transit bus using a 100 kW methanol-fueled phosphoric acid fuel cell. In this system, the fuel cell provided the baseload motive power while the batteries provided surge power and enabled storage of energy provided by regenerative braking. The speed and range of this hybrid bus was comparable to that of standard diesel buses, and it could be refueled quite easily and quickly. Even compared to clean-burning natural gas powered buses, fuel cell/hybrid buses are expected to have less than one-tenth of the hydrocarbon emis-sions and only 2% of the CO emissions as well as virtually no emission of nitrogen oxides or particulate matter (PM). 186

Julich Forschungszeatrum (Germany) developed a com-mercial electric vehicle (EV) powered by a hybridized DMFC system (comprised of 100 single cells with a power output of 1.3 kW) coupled to a bank of Li-ion batteries. 205 Under nor-mal operation, the DMFC drives the electric motor, while the battery boosts the power during start-up or uphill accel-eration. Interestingly, in this DMFC system, the Pt loading level was reduced by 50% compared to other state-of-the-art DMFC electrodes. Furthermore, to ensure a lighter stack, many of the metallic stack materials were replaced with graphite components. A collaboration aimed at scooters as a potential early-adopter market entry point for DMFC tech-nology was formed by Neah Power and EKO (India) to develop a DMFC to charge battery-powered “EV 60” electric scooters. 206 This second-generation electric scooter is marketed in the USA, Europe, and Asia. Smart Fuel Cell AG developed the EFOY DMFC + Li-ion hybrid system for light-weight EVs weighing less than 500 kg as shown in Fig. 17 . 207 The DMFC is equipped with an automatic control system that constantly monitors the battery's charge state. Whenever it drops below a predefined level, the DMFC automatically begins charging the battery. Once the battery is fully charged, the fuel cell automatically returns to standby. This eliminates unfavora-ble operation modes like partial load operation and increases the total effi ciency ratio. The company has successfully proved its reliability and functionality in many off-grid and light electric vehicle applications. The EVOY fuel cartridges are available at almost 1000 sales points in Europe and the US.

In 2008, Oorja Protonics developed the first DMFC on-board charging system, the OorjaPack™, which was designed to power material handling vehicles like forklifts ( Fig. 18 ). 208 The DMFC system helps to continuously replenish the energy depleted by the operation of the vehicle and maintains the battery state of charge during operation between 50 and 80%. Using the hybrid DMFC/battery system eliminates the need for extra batteries and eliminates the downtime associ-ated with periodic battery charging or swap-out. The resulting savings of 30–45 min per day per vehicle results in increased operational efficiency as well as productivity savings.

As discussed above, while progress continues on the devel-opment of DMFC systems for small vehicles or scooters, most

Figure 17. SFC developed EFOY DMFC powered Li-ion hybrid system for

small electrical cars weighing less than 500 kg. Reproduced with

permission from World Electric Vehicle Journal, Copyright 2009. 207

Figure 18. DMFC (Oorja Protonics™) for on-board battery charging for

material handing vehicles. Reproduced with permission from Elsevier,

Copyright 2012. 208






current efforts for large-scale passenger cars and trucks have focused on hydrogen fed PEMFCs.

Key challenges in DMFC technology

Of all the potential applications for DMFC, the portable electronics power market appears to be the most compelling in the short-to-intermediate term. While several portable devices equipped with DMFC can already be purchased in niche commercial markets, widespread commercialization into mass markets such as mobile phone and laptops still faces numerous technical challenges. One of the key challenges for DMFC is increasing the power density and reducing the size of the system. In the transportation sector, although using DMFC technology alleviates the on-board fuel storage issue, the cost, power density, and efficiency of the DMFC system are far from satisfactory when compared against H 2 -based fuel cells. Perhaps the greatest of many critical chal-lenges impeding DMFC commercialization at larger scales is system costs, which are driven by materials and manufactur-ing challenges.

Materials challenges

Sluggish MOR

The MOR on state-of-the-art anode catalysts (such as PtRu supported on carbon or PtRu black) is signifi cantly hindered because of the poor electrokinetics. The rate of MOR in the anode of a DMFC is quite slow as compared to that of the hydrogen oxidation reaction in the anode of the PEMFC. The poor elec-trokinetics of the MOR leads to a high activation overpotential, which in turn, lowers the power density and the efficiency of the cell. To improve the kinetics of the MOR, higher catalyst loadings are used (>2 mg cm −2 ). 125 This obviously increases the cost of the system. Even if the catalyst loading is increased enough to ensure good initial performance, the long-term stability of the PtRu/C catalyst suffers, particularly in acid-based systems, because of the ruthenium dissolution processes. 42 , 45 , 46 , 49 , 85 The gradual dissolution of ruthenium from the anode and its migration to cathode during the operation of DMFC further decreases the kinetics of both the MOR and ORR in the anode and cathode, respectively. 85 , 209

To improve performance and stability of the PtRu catalyst, work has been focused on developing PtRuX-based ternary catalysts, where X is Mo, Sn, Os, or W. 42 , 45 , 49 , 210 Among these ternary catalysts, PtRuMo is reported to be more active for the MOR than bimetallic PtRu. 210 However, the stability of PtRuMo is not known. As discussed in the preceding sections, chemical modifi cation of the support is also being explored as a way to improve catalyst performance and durability. 56 – 60 Of particular note, PtRu catalysts supported on nitrogen modifi ed carbon supports have been shown to exhibit signifi cantly higher perfor-mance and stability when compared to undoped counterparts. 61 – 63 Using nitrogen modifi ed carbon as the supporting material for PtRu in the anode of the DMFC could be helpful in achieving higher power density while simultaneously decreasing the amount of catalyst required.

Methanol crossover

DMFCs experience signifi cantly lower OCVs ( ∼ 0.7 to 0.8 V versus ∼ 1.2 V theoretical) and lower power density than H 2 -PEMFCs. The lower OCV and performance in the lower cur-rent density region is mainly attributed to two factors, methanol crossover from the anode to cathode through the Nafi on mem-brane and the poor kinetics of the MOR as discussed above. 211 , 212 Methanol crossover creates a mixed potential in the cathode, and also reduces the methanol utilization in the anode, thereby reducing the voltage of the DMFC by as much as 400 mV. 213 Methanol crossover occurs mainly due to the structure of the Nafion membrane. The Nafion consists of hydrophobic main chains and hydrophilic side chains containing ionic sulfonic acid (–SO 3 H) groups, where the latter groups cluster together to form ionic channels. While the fl ow of water through ionic channels helps to carry the protons and thus permits high pro-ton conductivity (desirable), it also permits the fl ow of methanol across the membrane from the anode to cathode. The formation of wider ionic channels facilitated by the aliphatic polymer structure of Nafion leads to high methanol permeability. Because of this limitation, researchers have modifi ed the Nafi on type membranes and also developed new types of polymeric membranes to meet the additional membrane criteria needed for high-performance DMFC applications. These criteria include reduction of methanol crossover, maintenance of optimum pro-ton conductivity, and an increase in the chemical stability. 86 – 91 , 93

Important progress has been made in addressing these basic needs. For example, researchers from the Loss Alamos National Laboratory developed a new set of membranes from the poly(arylene ether sulfone) family (branded as 6F and 6F-CN), which showed higher initial DMFC performance than Nafi on ( Fig. 19 ), 214 possibly due to reduced methanol crossover. Fur-thermore, such membranes showed only slight performance

Figure 19. DMFC performance obtained with four different membranes

(Nafi on 117, 6F-40, 6F-CN-35, and BPSH-35) at an ambient air pressure

and at the temperature of 80 °C. Reproduced with permission from Los

Alamos National laboratory – FY progress report, 2003. 214






degradation even after 700 h of durability testing. Recently, blended membranes consisting of an acid polymer and a basic polymer with similar aromatic backbones have emerged as a promising technology to reduce methanol crossover and achieve higher DMFC power density compared to Nafi on. 215 – 218 The blended membrane approach involves the tethering of an N-heterocycle group to an aromatic polymer like poly(sulfone) (PSf) or poly(ether ether ketone) (PEEK) to obtain a polymer with basic (rather than acidic) character. This polymer is then blended with an aromatic acid polymer such as sulfonated poly(ether ether ketone) (SPEEK) or sulfonated poly(sulfone) SPf. As an example, a 60 μm thick blended SPEEK/poly(sulfone)-amino benzimidazole (PSf-ABlm) membrane has been shown to exhibit signifi cantly lower methanol crossover than Nafi on 115 (thick-ness 125 μm). 219 However, the proton conductivity of the blended membrane is lower than that of the Nafi on membrane. Although the proton conductivities of blended membranes tend to be lower, the lower thickness of these membranes enables lower total ionic resistance, resulting in lower voltage loss and higher power density than cells based on Nafi on 115 or Nafi on 117 mem-branes. Because of the lower methanol crossover, blended mem-branes exhibit higher OCV and allow the operation of DMFC with higher methanol concentrations, thereby enhancing DMFC power density. 218 In addition, blended membranes exhibit excel-lent chemical, thermal, and mechanical stabilities and are much less expensive than Nafi on-based membranes.

As an alternative to blended membranes, alkaline membranes also show lower methanol crossover and can offer improved elec-trochemical kinetics. Commercially available AEMs include MORGANE-ADP and an ammonia-based AEM membrane from Tokuyama Co. Japan. Recent literature reports show that DMFCs made with Tokuyama alkaline membrane (fed along with NaOH/KOH solution) exhibited higher OCV and comparable or slightly higher performance than acid-based DMFCs. 47 , 48 The ionic mobil-ity of OH − is relatively low compared to that of H + ; thus it is gener-ally required to feed an alkaline liquid (KOH/NaOH solution) along with methanol to improve its OH − ionic mobility during the DMFC operation. However, reaction of the OH − ions with the CO 2 produced during the cell operation can result in the formation of carbonate and bicarbonate ions. 220 , 221 Formation of these ions pose a detrimental effect to the performance of a cell by means of (i) blocking the active sites of the electrodes through the forma-tion of metal carbonate crystals (most commonly as Na 2 CO 3 or K 2 CO 3 ), 221 (ii) depletion of the OH − ions concentration at the anode, thereby decreasing the MOR kinetics, 138 and (iii) incorpo-ration of carbonate/bicarbonate ions into the membrane, thereby decreasing conductivity and creating an unfavorable pH gradient across the membrane. 222 The development of AEMs with higher ionic conductivity and good chemical stability may open up prom-ising avenues for lower-cost, higher-performance DMFCs.

Ruthenium dissolution from PtRu catalyst in the anode

During DMFC operation, ruthenium dissolution takes place from the PtRu catalyst in the anode; it can subsequently migrate through the CEM and redeposit in the cathode, which decreases

the activity of both anode and cathode and in turn affects cell performance and durability. 85 , 122 Ruthenium dissolution occurs during the normal DMFC operation, where the anode experiences potentials around 0.3–0.5 V versus DHE, even though the thermodynamic reversible potential for ruthenium oxidation is significantly higher ( E ° Ru/Ru(OH)3 = 0.74 V and E ° Ru(OH)3/RuO2.H2O = 0.94 V versus reversible hydrogen elec-trode). 223 Although ruthenium has a relatively high thermody-namic reversible oxidation potential, the observed ruthenium dissolution in the lower potential region (0.3–0.5 V versus DHE) of the anode is often attributed to the low Ru(OH) 3 activity in the catalyst layer and the relatively high operating tempera-ture of the cell. 85 Occasionally, DMFC anodes can experience potentials greater than 0.7 V versus DHE, for example during deep discharging or under short circuit. Such situations trigger even greater ruthenium dissolution. 224 Piela et al. reported ruthenium leaching from the anode and migration to the cath-ode even when the DMFC stack was operated under prehumidi-fi ed conditions. 85 Under this prehumidifi ed condition, part of the ruthenium species (mostly likely RuO 2 ) in the PtRu anode might be relatively loosely bound and easily susceptible to disso-lution and migration. Typical PtRu catalysts are not perfectly homogeneous; ruthenium normally exists in various forms including nonalloyed ruthenium metal, ruthenium oxides (RuO 2 ), and ruthenium oxyhydroxide (RuO x H y ) in addition to being alloyed with Pt to form PtRu solid solutions. 225 It has been found that these oxide/oxyhydroxide species (RuO 2 and RuO x H y ) are more easily dissolved than alloyed Ru metal during repeti-tive potential cycling (RPC) in 0.1M H 2 SO 4 solution. 226 , 227

Leached ruthenium species migrate through the membrane to the cathode and contaminate the pure-Pt cathode catalyst. There are several views on the deposition and nature of ruthe-nium species in the cathode. Chung et al. 228 found that ruthenium was accumulated at the surface of the cathode; their time-of-fl ight secondary ion mass spectroscopy (TOF-SIMS) results suggested that the ruthenium accumulates as RuO x rather than metallic ruthenium. Another study showed that the migrated ruthenium aggregated at the interface between the membrane and the cathode. 229 Ruthenium contamination in the cathode typically leads to voltage losses in a DMFC on the order of 40–60 mV by reducing the ORR activity and the cathode's ability to handle methanol crossover. 85 , 225 Although detailed investigations have been conducted on ruthenium dissolution and its migra-tion through CEMs to the cathode of acid-based DMFCs, no reports are available on the ruthenium dissolution and its migration through AEMs in alkaline-based DMFCs.

Measures to mitigate ruthenium dissolution

Ruthenium dissolution issues in DMFC anodes can be addressed (i) by developing PtRu structures with intrinsically greater stability, and/or (ii) by stabilizing PtRu by means of intro-ducing external additives or by modifying the carbon support on which the PtRu resides. It has been demonstrated that face cen-tered cubic (fcc) structured PtRu solid solutions are more stable than PtRu with RuO x H y and hexagonal close packed (hcp)






structured PtRu solid solutions. 209 , 227 Lee et al. showed that the electrochemical stability of PtRu in the presence of methanol is enhanced when the degree of metallic alloying of the PtRu increases. 230 In a similar fashion, Hyun et al. prepared 60 wt% PtRu(1:1) catalysts with different degrees of alloying that were subjected to RPC for 40 cycles. 231 It was found that the PtRu cata-lysts with the highest degree of alloying were more stable than the PtRu catalysts with lower degrees of alloying. In other studies, researchers used metal oxides such as SiO 2 , TiO x , and WO x to stabilize the PtRu catalyst to increase long-term stability of both the PtRu alloy and PtRuO x H y . 232 , 233 These metal oxides were shown to help to prevent Ru dissolution and aggregation/agglomeration of the PtRu nanoparticles in the anode. Cabello-Moreno et al. compared the stability of PtRu supported on carbon versus unsupported PtRu and found that the former exhibited higher stability than the latter. 234 Kang et al. reported that PtRu supported on carbon nanofi bers provided better sta-bility than commercial catalysts based on carbon blacks. 55 While their study did not extract direct evidence for the effect of the support, metal dissolution tests reinforced the hypothesis that the support likely plays a key role in catalyst stabilization. In a closely related study, PtRu nanoparticles deposited onto 1-aminopyrene (1-AP)-functionalized multiwalled carbon nano-tubes (MWCNTs) (PtRu/1-AP-MWCNTs) showed higher elec-trocatalytic activity toward the MOR and signifi cantly improved stability compared to PtRu supported on conventional acid treated MWCNTs as well as on carbon black. 235 The increased stability of PtRu/1-AP-MWCNTs was attributed to strong attach-ment of PtRu on MWCNTs via 1-AP interlinks. Interestingly, PtRu catalysts impregnated/deposited on nitrogen-modifi ed carbon supports [PtRu/C (N-doped)] have exhibited higher sta-bility and signifi cant mitigation of Ru dissolution when compared to undoped counterparts during long-term DMFC opera-tion. 61 – 63 The higher stability of PtRu on N-doped carbon has been attributed to the lone pair of electrons on the sp 2 orbital (pyridinic) nitrogen sites in the plane of carbon rings, which immobilizes the PtRu particles more firmly.

Manufacturing challenges

MEA fabrication

DMFC power density and performance are largely dictated by the structure of the MEA. The most widely used fabrication techniques for the mass production of the MEAs, such as the CCM and DTM methods present certain advantages and disad-vantages in this regard. Although the CCM method provides a good interfacial contact between the catalyst layer and the electrolyte membrane, this method is limited by membrane swelling and wrinkling problems, resulting in deformation or cracking of the catalyst layer due to rapid volume changes of the membrane. To minimize the dimensional changes caused by the standard CCM method, Park et al. 236 presoaked their mem-branes in ethylene glycol (to induce swelling) and subsequently sprayed the catalyst slurry on the preswollen membranes. MEAs fabricated by this method showed better fuel cell performance when compared to conventional MEAs due to the reduction in

stress of the membrane and catalyst layer by the preswelling process. Importantly, the spraying process must be carried out in several steps to avoid sedimentation of the catalytic particles. Since this is time consuming, the spraying method may not be suitable for high catalyst loading. 237

In the DTM method, the catalyst layers are deposited on Tefl on decal substrates and are subsequently transferred to a dry membrane during the hot-pressing of the MEA. The catalyst lay-ers in MEAs fabricated by DTM tend to exhibit relatively poor interfacial contact with the membrane. 122 Because of this prob-lem, DTM MEAs experience higher ohmic resistance during DMFC operation. Moreover, it is diffi cult to control the porosity and the thickness of the catalyst layer due to the dehydration of the membrane during the decal transfer process and possible sin-tering of the catalytic nanoparticles occur because of the heat load that is applied. 238 In the DTM, the ionomer segregation (ionomer in the catalyst layers migrates and form a skin layer in the side of the catalyst layer, which is closer to the surface of the Tefl on substrates due to hydrophobic interaction between iono-mer and Tefl on) usually occurs on one side during the high-temperature MEA hot-pressing process (160–210 °C). 100 , 239 The formation of ionomer segregation in the catalyst layer acts as a barrier for reactant mass transport during the fuel cell opera-tion. 100 Recently, researchers overcame the ionomer segregation problem by introducing buffer layers composed of carbon powder and Nafi on ionomer in between the catalyst layers and the Tefl on decal substrates during the MEA fabrication process. 110 , 240 The overall fabrication process associated with conventional DTM is rather complex and attempts are being made to simplify it for large-scale MEA production. Cho et al. and Krishnan et al. 110 , 241 have demonstrated a simplifi ed DTM process that involves trans-fer of the catalyst layer to the membrane at temperatures as low as 110–140 °C without any material modifi cation or performance deterioration. The schematic diagram of the conventional high-temperature and low-temperature DTM is shown in Fig. 20 .

Cho et al. 110 compared the DMFC performance obtained from MEAs made from CCM and low-temperature DTM (Decal-IC) using two different concentrations of methanol (0.5 and 1M) as shown in Fig. 21 . It can be seen from fi gure that the low-temperature DTM MEA yielded higher power density (130 mW cm −2 ) than the CCM MEA (120 mW cm −2 ) when using 0.5M methanol. However, at 1M methanol, the low-temperature DTM MEA exhibited lower power density than the CCM MEA, because in the former, MEA methanol crossover appears to be serious. Further investigation is required to mitigate the methanol crossover problem in low-temperature DTM MEAs.

Manufacturing cost

DMFC system cost remains one of the largest impediments to full-fl edged commercialization across all power levels. DMFC manufacturing cost is higher than both PEMFC and Li-ion batteries because of the need for relatively high loadings (>2 mg cm −2 ) of expensive catalytic materials (PtRu and Pt) and expensive Nafi on-type membranes ($600–$1200 m −2 ). As per the report of one Korean company in 2007, 242 the manufactur-ing cost for a 20 W DMFC for portable laptop power laptop






was estimated to be $333, which is ten-fold higher than the cost of manufacturing a Li-ion battery pack with the same power output. In recent years, significant progress has been made to lower the cost and increase the lifetime of the DMFC sys-tem. 126 , 140 , 243 The Department of Energy (DOE) published updated technical targets for portable DMFC systems to be achieved in the year 2015. These targets include 35% system effi ciency and 5000 h lifetime at a cost of $5 and $7 W −1 for sys-tems of 100–250 W and 10–50 W in size, 244 respectively. Several companies like SFC and MTI have already achieved more than 5000 h lifetime test for their prototype DMFC products, but other details such as system cost are not presently available.

Environmental/Societal/Policy Issues

Environmental impact

A transition to DMFC technologies will produce environmen-tal and societal impacts based on the costs, emissions, recycling potential, and toxicity associated with methanol and methanol

Figure 20. Schematic diagram of conventional high-temperature and low-temperature DTM. Decal-IC refers to right-side low-temperature decal process,

where I and C indicate the outer ionomer and carbon layers, respectively. Decal-I corresponds to the same method as decal-IC except for the carbon layer

coating. Reproduced with permission from Elsevier, Copyright 2009. 110

Figure 21. Comparison of DMFC performance of the MEAs fabricated by

CCM and low-temperature DTM (Decal-IC). Reproduced with permission

from Elsevier, Copyright 2009. 110






production. In addition, the displacement of other fuels associ-ated with a transition to methanol will also have signifi cant envi-ronmental and societal impacts. The degree of these impacts will depend primarily on two factors – the amount and type of fuel that is displaced, and the amount of methanol required for the adopted technologies. The environmental implications will be significantly broader if DMFC technologies requiring large amounts of methanol (like vehicles) are adopted versus technologies that use relatively less methanol (like consumer electronics).

Methods of methanol production

Methanol can be produced using a number of different feed-stocks and techniques (see Refs. 15 and 245 for a more complete study of methanol production conditions and methods). Accord-ing to the Methanol Institute, methanol is the most basic alcohol and “can be made from virtually anything that is, or ever was, a plant. This includes common fossil fuels – like natural gas and coal – and renewable resources like biomass, landfill gas, and even power plant emissions and CO 2 from the atmosphere.” 245 The feedstock and method of methanol production must be considered when evaluating the environmental impact of expanding methanol-based technologies and here we will dis-cuss only commonly used production methods and chemical recycling of CO 2 to produce methanol.

Converting the methane (CH 4 ) found in natural gas to methanol (CH 3 OH) is currently the most widely used and cost-effective method of methanol production. 15 , 245 , 246 It is likely this trend will continue in the near- and mid-term as companies are building, expanding, or restarting natural gas-based metha-nol production plants (mostly along the gulf coast) because of the recent natural gas boom in the US. 247 The typical natural gas to methanol plant utilizes a two-step process to produce methanol. First, the natural gas feedstock is converted into a synthesis gas (syngas) that consists of carbon monoxide (CO), carbon dioxide (CO 2 ), and hydrogen (H 2 ). Syngas is typically produced through the catalytic reforming of methane and steam by the following reaction 245 :

( )+ → + + (14)

The ratio of the components in the syngas depends on sev-eral factors including the reaction temperature, pressure, and water to methane ratio (H 2 O/CH 4 ). The reaction is endother-mic and becomes more effi cient with increases in temperature and/or decreases in pressure. 15 The second step in the process is the catalytic synthesis of methanol from the syngas 245 :

+ + → + + (15)

This step is highly exothermic and most methanol produc-tion plants use the excess heat to generate some of the electric-ity used in the production process.

While the combination of steam reforming and catalytic syn-thesis from natural gas feedstock is the most common route to

produce methanol, each of these steps can be accomplished through a number of different methods and utilizing a variety of feedstocks. Currently, methanol is made almost exclusively using a two-step process where syngas production is the fi rst step, 15 and the relative amounts of CO, CO 2 , and H 2 in the syn-gas stream vary depending on the feedstock. The syngas compo-sition can be expressed using S , a number that depends on the number of moles ( n ) of each species within the gas stream 15 :

( )( )

−=

+ (16)

The ideal value for S is 2, which would produce a perfectly balanced second stage reaction – one possessing neither a defi -ciency ( S < 2) nor an excess ( S > 2) of hydrogen. The composi-tion of the syngas can be manipulated by steam reforming the feedstock or by adding an external source of H 2 (for S < 2) or CO 2 (for S > 2) during the methanol-producing reaction. 15

From an emissions standpoint, values of S at or above 2 are better because they do not produce excess CO 2 . In fact, in the case of methanol production from natural gas via the route detailed in Eqs. (14) and (15) , the value of S is 3 and excess CO 2 (from a waste stream or the environment) can be added and con-sumed to produce a greater quantity of methanol. Conversely, syngas produced from coal via a gasifi cation process yields an S value below 2. In this case, the excess CO 2 produced in the syngas reaction is either consumed by adding H 2 or separated (and released as part of an emission stream). 15

Bio-feedstocks have also been examined as a renewable source for methanol production. The S -value for syngas pro-duced from bio-feedstocks varies depending on the feedstock composition but is typically below 2 (Ref. 248 ) and can be manipulated using the same methods used for fossil-fuel feed-stocks. 15 , 245 , 248 When discussing production of methanol from bio-feedstocks, it is also important to consider the possibility of food crop displacement. Already biofuels have had a signifi -cant impact on food prices in some areas because they com-pete for the same agricultural resources. 17 This would likely also be the case if mass quantities of methanol were produced from biomass.

In addition to the methods of methanol production discussed here, there are many other methods under investigation (see Refs. 15 and 245 for a more complete list) that have the poten-tial to increase production effi ciency and/or decrease emis-sions associated with production. Unfortunately, methanol production, like most industrial processes, is optimized for cost savings rather than for minimizing environmental impacts – a situation that is likely to change only with enactment of stricter emission regulations.

One further method of methanol production is worth noting – that of production by chemically recycling CO 2 . This process requires H 2 , which can be generated through electrolysis of water using any energy source. For a fully renewable process, a renewable energy source like geothermal, solar, or wind energy can be used to produce the necessary H 2 . The second step in the process is the reaction of H 2 with CO 2 (from an industrial waste stream or the environment) to produce methanol. 15 , 249






Carbon Recycling International opened the fi rst commercial plant to produce renewable methanol in 2011. Named after Nobel Laureate George Olah, the plant is located beside the Svartsengi Power Station in Iceland and uses power and waste CO 2 from the geothermal power plant to produce renewable methanol. The plant produces around 2 million liters of metha-nol per year but is currently under expansion to increase pro-duction capacity to 5 million liters per year. The production process is completely renewable and the sole byproduct is oxygen. 249 From an environmental standpoint, renewable methanol is the best methanol production option.

Environmental toxicity of methanol

Produced by a number of natural and human-related activi-ties, methanol is already a part of the earth's environment. Methanol exhibits a low toxicity, and effects related to environ-mental exposure are negligible under normal circumstances. 250 Methanol is not known to accumulate in soil or water sources because it rapidly degrades through both photo-oxidation and biodegradation in aerobic and anaerobic environments. 15 , 251

As with any comparable fuel source, an increase in methanol use increases the chances for an accidental release of methanol into the environment during production, transportation, or storage. Although a fuel leak of any kind is never desirable, methanol possesses a few important advantages that set it apart from traditional fossil fuels. Compared to gasoline or diesel fuel, which contain many toxic and carcinogenic compounds that degrade very slowly in the environment, methanol exhibits low toxicity and rapid degradation – both qualities present a sig-nifi cant advantage in a spill. 15 , 251

In addition, methanol is completely miscible in water, 252 which would allow for rapid dilution and dissipation to nontoxic levels in the case of a large spill into a water source. The remain-ing dilute methanol would then quickly be degraded to CO 2 and water by microorganisms in the environment. 15 , 251 As has been evidenced by several oil spills in recent history, the immiscible nature of oil in water causes widespread and long-lived contam-ination, which is often devastating to the local environ-ment. 253 – 257 Based on these advantages, methanol is viewed as a less toxic and more environmentally friendly fuel.

Environmental and societal considerations for DMFC vehicles

Emissions from the transportation sector are an area of global concern because typical transportation emissions con-tain several harmful substances including VOCs, PM, sulfur dioxide (SO 2 ), carbon monoxide (CO), and various nitrogen oxides (NO x ). Due to the hazardous nature of these emissions many local, 258 national, 259 – 263 and international governing bodies have set forth regulations with increasingly stringent requirements for transportation emissions. These policies have driven interest in alternative fuels and technologies, and DMFC vehicles are only one of many alternatives under exploration.

On the emissions front, a DMFC vehicle is predicted to be far superior to the typical gasoline or diesel ICE vehicle because DMFC vehicles are expected to be ZEVs 186 – or vehicles that do

not emit any pollutants from the tailpipe during operation. Other alternatives that also reduce emissions compared to typi-cal ICE vehicles include hydrogen-powered FCVs (H 2 -FC), EVs, and hybrids. Both H 2 -FC vehicles and EVs are also considered ZEVs, 264 putting them on par with DMFC vehicles on the direct (tailpipe) emissions front. Hybrids, with their combination of battery power and internal combustion produce approximately 80% less tailpipe pollutants than a comparable ICE vehicle. 265

Despite the clear emissions advantage of DMFC vehicles and other alternative technologies, the gradual changes to emission standards has left the door open for options that are technologically easier to achieve and still produce some emis-sion reductions. One such option – that of using methanol in an ICE – either through blending methanol with gasoline for FFVs or using pure methanol fuel (M100) would provide a rel-atively easy method of reducing emissions.

Methanol forms no PM upon combustion and emits lower VOCs and NO x than gasoline. According to the Environmen-tal Protection Agency (EPA), hydrocarbon emissions can be reduced by up to 80% when M100 is substituted for gasoline in the ICE, however actual emissions would vary based on engine design. 266 In reality, despite the fact that methanol combustion is inherently cleaner, it is unlikely that it would yield such a high emissions reduction compared to gasoline due to emission con-trol technologies which have already made signifi cant emission reductions and are constantly being developed, allowing tradi-tional fuels to compete with alternative fuels like methanol. 15

While emission reductions in the transportation sector are an important part of the equation, infrastructure will also play a signifi cant role in any fuel or technology transition. The transportation sector relies on a vast infrastructure – one that is currently built around gasoline and diesel fuel. Adoption of any alternative fuel or technology will likely depend on the ease with which it integrates into the existing fuel infrastruc-ture system, and here, methanol and DMFC vehicles have an advantage.

Because methanol is a liquid fuel it can utilize the same onboard storage (gas tanks) and fueling systems used in the current gasoline- and diesel-based transportation sector, with slight modifications. Fueling stations would also require modifications, namely the conversion of existing double-walled underground gasoline or diesel fuel storage tanks and installation of new piping and dispenser pumps compatible with methanol. Based on cost estimates, 267 10% of the nearly 200,000 gasoline fueling stations in the US could be con-verted to supply methanol for around $1 billion. 186 Com-pared to the more than $10 billion invested every year to maintain the US current fueling system, and the investment of over $12 billion by the oil industry to introduce reformu-lated gasoline in the US, 186 the investment for conversion to methanol is well within the scope of possibility.

While it is unlikely that fueling infrastructure would be upgraded specifi cally for DMFC vehicles, a more likely scenario is the introduction of a methanol-based FFV which could tip the balance and initiate an infrastructure transition to accommo-date methanol-based fuel. 15 Therefore, the possible near-term






transition to methanol-ICE FFVs could help eventually facili-tate a future transition to DMFC vehicles. This is a signifi cant advantage that DMFC vehicles hold over H 2 -fed FCVs. Due to the necessary high pressures or low temperatures needed for H 2 storage, infrastructure development for this technology would be much more expensive than a similar transition to methanol. Indeed, there are currently only 12 hydrogen fueling stations in the US (most located in California), 268 while the number of methanol fueling stations in the US (again mostly in California) is an order of magnitude higher. 266

It is important to note that many of the methanol fueling stations in the US were established as part of a methanol f lex fuel (mostly M85 or 85% methanol) program in California in the 1990s. At the time methanol was introduced, petroleum prices were rapidly falling, which effectively eliminated the economic incentive to switch to methanol. Despite the ulti-mate failure of methanol to enter the fuel market on a broader scale, the program demonstrated that methanol is a viable, and cleaner (than gasoline) transportation fuel. 18 It is likely that the success of methanol as a transportation fuel will con-tinue to be tied to petroleum prices. In our opinion, the recent steep price hikes and falls in the oil and gasoline mar-kets which have led to a high degree of price uncertainty cou-pled with the surge in natural gas (most common feedstock for methanol) in the US may be enough to tip the scale to favor methanol.

Airline regulation of methanol for consumer electronics

Methanol is a toxic, fl ammable liquid and its use in DMFC-based consumer electronics aboard airlines is subject to regula-tion. The international standard for carry-on allowances for methanol fuel cartridges on airlines was set by the Dangerous Goods Panel (DGP) of the International Civil Aviation Organi-zation (ICAO) and went into effect on January 1, 2007. Prior to rulemaking, the DGP analyzed several safety tests performed on micro fuel cells and fuel cell cartridges for consumer elec-tronics. The tests provided data on how these devices performed at high altitude and under a number of different mechanical and electrical loading conditions. 269 The global standard, detailed in Part 8 of the ICAO Technical Instructions for the Safe Trans-port of Dangerous Goods by Air, 270 was developed from analysis of these tests and allows passengers to carry and use an approved fuel cell with an installed methanol cartridge and up to two spare cartridges. Each cartridge may contain up to 200 mL of methanol. 271

Many countries around the world have incorporated the international standard into their passenger allowance regula-tions including Canada, China, Japan, the United Kingdom, and the United States, 272 and no countries or airlines have filed variations to the rule with the ICAO. 270 In the US, the Department of Transportation (DoT) issued a ruling to match the international standard for methanol micro fuel cells and cartridges in 2008, 272 however, the 200 mL maximum vol-ume allowance per cartridge exceeds the 100 mL maximum allowance limit on liquids established by the Transportation

Security Administration for carry-on bags. 273 Until now, the discrepancy between the two regulations in the US has not caused any signifi cant problems, although it is likely that broad adoption of DMFC-powered consumer electronics would force reconsideration of the liquid allowance standards.

Policy and DMFC

Until now, methanol and DMFC have not had a signifi cant market impact because economics favor existing technologies (batteries and oil/gasoline). Policies are often used to shift economics to favor development or adoption of new or differ-ent technologies, and there are several different policy options that might promote a shift toward methanol and DMFC-based technologies. Many countries institute policies that would promote specifi c technologies, while others prefer to enact policies that would enable a range of technologies within a specifi c area (i.e., clean energy).

Policy options that would impact methanol and DMFC technologies include, but are not limited to, reducing subsi-dies for fossil fuels (existing global fossil fuel subsidies are approximately USD $550 billion annually), 274 federal funding for research and development (broadly for clean energy tech-nologies or specifi cally for methanol/DMFC technologies), increasingly strict industrial and vehicular emission regula-tions, establishment of a tax credit for CO 2 or other emission mitigations/reductions, and enactment of a carbon, emissions, or energy tax.

Most developed countries already provide some funding for research and development of clean energy technologies, how-ever, the amount and specifi c allocation varies drastically from country to country and even from year to year as budgets and technologies change and evolve. Similarly, most developed countries have also established emission regulations, many of which mandate signifi cantly decreasing emissions over a ten- to fi fty-year time-span. 259 – 263 While emission regulations do not specifi cally drive methanol or DMFC technologies, the ability to reduce emissions through methanol production (renewable methanol) and the fact that DMFC vehicles provide emission reductions compared to conventional gasoline-ICE vehicles means emission regulations can benefi t these methanol-based technologies.

Changing the tax code is the other broadly used policy instru-ment that could promote a shift toward DMFC technologies. This can be accomplished in two ways – either through a tax incentive or through establishment of a new tax. Both routes have been uti-lized in governments around the world, and already there are some tax credits for mitigating or reducing emissions (like those given for hybrids, FFVs, EV, etc.). There has also been signifi cant interest in – and in some places progress toward – the enactment of a carbon tax in recent years, 275 – 280 however, wide-spread public support is not yet suffi cient to drive this policy shift on a global scale. It is possible that with increasing climate change, a carbon tax or other tax on energy or emissions may gain the necessary support for enactment, which would also help in driving DMFC and other clean-energy technologies forward.






Summary

In this review, the technological status, as well as the envi-ronmental and policy aspects of DMFCs have been examined. In DMFCs, the direct conversion of the chemical energy stored in liquid methanol to electrical energy eliminates the complex processes of conversion of methanol into hydrogen. Methanol fuel is less expensive (per unit energy) and provides signifi -cantly higher volumetric and gravimetric energy density when compared to compressed hydrogen (at 1000 bar) and even liq-uid hydrogen. Methanol fuel embodies ten times higher specifi c energy density than the state-of-the-art Li-ion batteries. For the above reasons, the DMFC has become an intriguing technology for the portable electronic industry as well as in the transporta-tion sector.

A number of organizations are actively developing DMFCs for portable electronics devices such as cell phones and laptop computers and are already entering into niche commercial mar-kets. Marketing forecasts indicate that the DMFC market may reach $1.1 billion by 2016, and account for 85% of the portable fuel cell market. Current DOE targets anticipate continued improvement of portable DMFC technology to attain 5000 h system lifetime with a cost of $5 W −1 and $7 W −1 for systems of 100–250 W and 10–50 W in size, respectively. Signifi cant com-mercial penetration can be anticipated if these targets are suc-cessfully reached. While these portable applications are not likely to signifi cantly impact the global energy sustainability picture, they provide an important initial market for fuel cell technology.

As far as the transportation sector is concerned, the hydrogen-fed PEMFC is currently the most widely used vehicular fuel cell technology because of numerous advantages, includ-ing high power density, higher electric efficiency (40–60%), and low working temperature, which allows rapid vehicular start-up and load cycling. However, the commercialization of hydrogen-powered PEMFC vehicles suffers from challenges due to on-board storage constraints (even at 1000 bar, hydro-gen takes up about 5 times more space than an equivalent amount of gasoline), lack of a consumer hydrogen distribu-tion infrastructure, and hydrogen production constraints. These factors present major barriers for entry into the com-mercial market. As a potential alternative, DMFC technology circumvents many of the hurdles associated with hydrogen-based transportation fuel cells since the high energy density of liquid methanol resolves on-board storage constraints and it can be distributed using the existing gasoline distribution infrastructure with only minor modifi cations. DMFC vehicles, like H 2 -powered FCVs, offer significant efficiency improve-ments and emission reductions relative to gasoline-powered ICEs. In the early 2000s, several companies actively engaged in the development of DMFC and DMFC-Li-ion battery hybrid powered vehicles. Although DMFC system performance appears to be satisfactory, particularly when hybridized with batter-ies in HEVs, full-f ledged adoption and commercialization of DMFC in the transportation sector remain largely distant and is impeded by a number of daunting factors, including

the typically lower efficiency and power density as well as the higher cost of DMFCs as compared to hydrogen-based fuel cells. Probably because of these reasons, most companies have concentrated their efforts on vehicles with on-board storage of pure hydrogen-fed PEMFCs. However, new breakthroughs in DMFC performance (particularly increases in power den-sity and efficiency, coupled with cost reductions) could even-tually make DMFC vehicles a compelling option.

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