improved durability and cost-effective components for new ... · pdf fileduramet deliverable...

FP7 Project No. 278054 – DURAMET

DURAMET Deliverable Report D.6.1– Data-set on monopolar configuration stack assembling and testing for portable applicationsincluding specific performance, durability and accelerated test results. 30/11/2014–Version 4

Improved Durability and Cost-effective Components for New Generation Solid PolymerElectrolyte Direct Methanol Fuel Cells

Collaborative Project - FCH JU GRANT AGREEMENT N° 278054SP1-JTI-FCH.2010.4.4 Components with advanced durability for Direct Methanol Fuel Cells

Start date: 01.12.2011 – Duration: 36 monthsProject Coordinator: Antonino Salvatore Aricò – CNR-ITAE

DELIVERABLE REPORT

D6.1 – DATA-SET ON MONOPOLAR CONFIGURATION STACK ASSEMBLING AND TESTING FOR PORTABLE APPLICATIONSINCLUDING SPECIFIC PERFORMANCE, DURABILITY AND ACCELERATED TEST RESULTS

Due Date 30 November 2014

Author(s)

O. Barbera, A. Stassi, G. Giacoppo, C. D’Urso, V. Baglio, A.S. Aricò (CNR-ITAE)M. Sgroi, F. Zedde (CRF)M. Schuster ( Fumatech)

Workpackage WP6Workpackage leader CRFLead Beneficiary CRFDate released by the leadbeneficiary 30 November 2014

Date released by Coordinator 30 November 2014

DISSEMINATION LEVEL

PU Public XPP Restricted to other programme participants (including the Commission Services)

RE Restricted to a group specified by the consortium (including the Commission Services)

CO Confidential, only for members of the consortium (including the Commission Services)

NATURE OF THE DELIVERABLE

R Report X






DELIVERABLE REPORT



Author(s)




DISSEMINATION LEVEL





R Report X






DELIVERABLE REPORT



Author(s)




DISSEMINATION LEVEL





R Report X



P Prototype

D Demonstrator

O Other

SUMMARY

Keywords DMFC,Fuel cell monopolar stack; printed circuit boards; passive mode

Abstract

The present deliverable aims to provide a data‐set for the monopolar DMFCmini-stack configuration in terms of design, assembling and testing. This fuelcell monopolar stack, operating in passive mode, was designed for portableapplications. The device consisted of 10 cells, with output power of 1.20 W.Two printed circuit boards were used to clamp and support the MEAs and toelectrically connect the electrodes. The normalised power density for thepassive mode operation ministack ( natural convection, air breathing) was 25mWcm-2 and appeared suitable for portable applications.

REVISIONS

Version Date Changed by Comments

0.1 03/11/2014 V. Baglio First draft

0.2 10/11/2014 F. Zedde Implementation

0.3 20/11/2014 F. Zedde Revision

0.4 24/11/2014 A.S. Aricò Revision



D.6.1 – DATA-SET ON MONOPOLAR CONFIGURATION STACK ASSEMBLING

AND TESTING FOR PORTABLE APPLICATIONS INCLUDING SPECIFIC

PERFORMANCE, DURABILITY AND ACCELERATED TEST RESULTS

INTRODUCTION

The activity carried out in WP6 is aimed at assessing the new developed components in direct methanolfuel cell (DMFC) short stacks for testing under realistic operating conditions.

DMFCs have received worldwide attention because of several intrinsic advantages, such as simplicity ofoperation, high theoretical energy density, easy recharging. Comparing methanol fuel cell with theircounterpart, proton exchange membrane (PEM) fuel cells, DMFCs are simpler in construction and do notneed high pressure hydrogen gas storage delivery or processing. Additional simplification in DMFCs includesair-breathing operation for low power application. However, the sluggish electro-oxidation kinetics and highcrossover rate of methanol are the main factors that hinder the commercialization of DMFCs. During thepast few decades, developments in electro – active materials and PEM technology design promoted theapplication of DMFCs in small portable power devices. To generate an appropriately sized DMFC stack, asuitable number of unit cells should be designed, and the operating conditions optimized to enhance thepower density of the DMFC system. DMFC stacks are classified into two types, based on the configuration ofelectrodes, membrane, and bipolar plates. The first type is a monopolar stack, which is assembled byplacing electrodes of the same polarity on the same side of the membrane and then electrically connectingthe anodes and cathodes in series. Despite the easy supply of reactants, the monopolar stack could beaffected by a high internal resistance because the electric current flows along conductive strips. The secondtype of stack is a bipolar design, which consists of a number of repeating units of membrane electrodeassembly (MEA) and bipolar plate. The bipolar plate separates each MEA and provides an electricalconnection in series between the adjacent cells in the stack. It also supplies reactants to each cell throughflow channels formed on both sides of the plates. The bipolar stack has lower internal resistance and thus,is more adequate for larger stacks than the monopolar design, which is more suitable for portableapplications. Portable power devices with high energy density and easy recharging are becoming importantfor many electronic devices, such as notebook computers, personal digital assistants, power tools andcellular phones. Currently, these devices are powered with current densities in the range of several mA cm-2

by primary and secondary batteries. The discharge time at the maximum rate and functionality of theseelectronic devices are often limited by the quantity of energy that can be stored and carried within them.Advances in the development of portable fuel cells should have a great impact on the use and developmentof modern electronic devices. Passive DMFC can potentially result in high reliability, low cost, high fuel



utilization and high energy density (using a concentrated methanol solution), which are in favour ofportable equipments in future electronic devices.

EXPERIMENTAL

A direct methanol fuel cell mini-stack for portable applications was designed with two strings composed of5 cells connected in series, whereas, the two strings were connected in parallel, the single cell area was4.85 cm2 and the operating conditions were ambient pressure and temperature. Since there was a smallmethanol cross over through the membrane, causing direct combustion with air, the stack was self-heating.For portable applications, the device has to be as simple as possible, thus, it should be operated withoutrecirculation pump or fan to feed methanol and air respectively, assembled with a limited number of parts,light and easy to recharge.

Figure 1: Assembling concept of a string (a) and of the two strings with the reservoir (b)



To meet these requirements, the architecture of the 10-cell stack was conceived as planar, and modular,each pair of current collector and clamping plates unified on a single component, the reactants provision inpassive mode (spontaneous diffusion, natural convection). The stack was composed of two sets of MEAsconsisting of 5 cells each, arranged in a planar configuration. Each MEA was prepared allocating the 5 activeelectrodes on both sides of a single membrane. Two printed circuit boards (PCB) were chosen to clamp andsupport an MEA and to electrically connect the five electrodes by thin gold strips. In correspondence ofeach active area, for air and methanol feeding, five windows were machined on the boards (Fig. 1a).

Figure 2: A view of the assembled modular stack

Two gaskets made of PTFE 250 m of thickness were used to prevent reactants leakage. The two strings offive cells are facing with the Methanol reservoir at the anode, as in Fig 1b, where an exploded view of thestack is shown. Thus, both of the anode sides of the two stacks are exposed to methanol, which is located ina central reservoir. Two gaskets avoid fuel leakage. Fig. 2 shows a complete view of the assembled modularstack. Methanol is transported by convective diffusion from the central reservoir the anodes. While thecathode sides are directly exposed to the ambient air, which spontaneously diffuses to the electrodes bynatural convection. At an operating electrochemical current density of 100 mA cm-2, the fuel utilization isreduced by about 20 % due to the methanol cross – over. This contributes to the stack self-heating throughthe direct catalytic combustion of methanol and air but reduces the fuel efficiency. The methanol cross –over through the membrane has been estimated in the range of 20 mA cm-2 equivalent current densityfrom ex-situ measurements (not shown). The internal temperature of the stack can reach 40 – 50 °C. Thespecific configuration of the window influences the fuel cell performance and heat exchange. The diffusionwindows can be designed with or without internal ribs or holes, and serve to sustain and contact the MEAactive area. This implies that the fluid dynamics of the reactants and the distribution of the clamping forcecan be influenced by the ribs design. To investigate the influence of the diffusion windows geometry on thestack performance, different boards with different hole shape were designed, the selected one is shown in



Fig. 2. The window is characterized by a specific ratios between the open area and MEA contacting surfaceof 63 %. The higher the open surface the higher the diffusion of the reactants and heat exchange. Whereasthe larger the MEA contacting surface the lower the ohmic resistance became.

RESULTS AND DISCUSSION

The components of the monopolar fuel cell stack were manufactured and assembled as described in thefollowing. The core part of the stack is composed of two sets of membrane electrode assemblies.

Membrane electrode assemblies (MEAs) were prepared at CNR – ITAE by hot – pressing the electrodes ontothe membrane at 130 °C for 1.5 min and 20 kg cm-2. The electrodes consisted of carbon cloth backings,diffusion and catalytic layers. A PtRu catalyst was employed at the anode; it was mixed with 15 wt%ionomer and deposited by a doctor blade technique onto gas diffusion layer (0.36 mm in thickness).Whereas, a Pt catalyst was utilized for cathode fabrication; the catalysts were mixed with 15 wt % ionomerand deposited by a doctor blade onto gas diffusion layers (0.28 mm in thickness). A FUMATECH F-1850membrane was used as the electrolyte. The final arrangement of the planar stack is shown in Fig. 3.

Figure 3: Final arrangement of monopolar stack

Preliminary results were obtained using a Potentiostat – Galvanostat AUTOLAB PGSTAT302. The monopolarstack was connected to the AUTOLAB, then the methanol reservoir was filled with 10 cc of 5M methanolsolution and the cathode side was exposed to the ambient. By imposing a current density thecorresponding potential has been recorded. Polarisation curves are shown in Fig. 4.



The total series resistance for this stack was about 278 m, this value, normalised by the geometric surfacearea of the electrodes, corresponds to 0.54 cm2 at room temperature. This is not significantly differentthan that observed in the literature for a DMFC single cell operating under passive mode. Thus, thecontribution of the monopolar set – up to the ohmic resistance is not relevant, such comparison reveals thesuitability of this stack design for portable applications. The fuel cell stack achieved a maximum power of1.2 W at 1.2 A, matching the target of the project established in MS16 (1 W).

Figure 4: Polarisation and power curves for the monopolar stack under passive mode operation (room temperature, airbreathing, MeOH natural convection).

This corresponds to a power density of 25 mW cm-2, which is similar to best performances reported in theliterature for passive-mode DMFC mini-stacks based on Nafion 115 membrane. This value is lower thanthat obtained by using a single cell based on a graphite hardware under the same operating conditions



(passive mode) due to a lower cell resistance of this latter (0.32 cm2). The maximum performance underambient temperature passive mode operation with the graphite single cell is 35 mW cm-2 (Fig. 5) that is the70% with respect to the project target (MS13).

Figure 5: Polarisation and power density curve for the graphite-based single cell under passive mode operation (roomtemperature, air breathing, MeOH natural convection).

DURABILITY TEST

Before the discharging time-test, a series of polarization curves at different temperatures and % of moisturewas acquired in order to select the specific conditions for the ministack test. This polarization set isreported in Fig. 6. The discharging time-test and the polarization curves for the monopolar stack wereperformed in controlled environment using a 2M methanol solution for fuel. With respect to thepolarization curves shown in the previous section, a lower power output was initially obtained. This isexplained by the fact that the selected concentration of the fuel is lower than that used for theperformance tests. However, a lower concentration generally ensures better durability to the stackcomponents, because, it reduces the crossover of methanol through the membrane. On the other hands, itleads to a lower power output from the stack. A climatic cell, Angelantoni Challange 600, was used for themeasurements (Fig. 7). This was set at 50 °C and 50% moisture. This temperature is reached inside thesystem during the ministack operation as consequence of the self-heating due to methanol cross-over. Theaim of this set of measures was to evaluate the stack durability and its performance over time.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 50 100 150 200 250 300 350Current density / mA cm-2

Cel

l pot

entia

l / V

0

5

10

15

20

25

30

35

40

Pow

er d

ensi

ty /

mW

cm

-2

single cell passive mode



Figure 6: Polarization and power curves acquired at different temperature and moisture levels (controlled environment inclimatic chamber, MeOH natural convection).

Figure 7: Climatic chamber “AngelantoniChallenge 600” used for this set of measurements











In Fig. 8, a series of polarization curves obtained at different times during the discharging time-test isreported. Each polarization curve was acquired before starting the controlled current measurement.It is observed that the stack performance decreases as a function of the time.

Figure 8: Polarization and power curves acquired at different times during the discharging time-test (controlled environment,50°C 50% moisture, MeOH natural convection, air breathing).

Part of the performance loss is recoverable and it should be due to the cathode flooding in absence offorce air convection as well as to the effect of methanol-cross over.

The discharging vs. time plot is reported in fig. 9. A voltage drop is observed after 40 hrs.



Figure 9: Discharging time-test, current vs. time in red, potential vs. time in blue and power vs. time in red (controlledenvironment, 50°C 50% moisture, MeOH natural convection, air breathing).



Figure 10: Single cells potential at different times (current steps, controlled environment, 50°C 50% moisture, MeOH naturalconvection, air breathing).

A series of histograms for the cell potentials in the ministack is shown in Fig.10. For several cells, thepotential decay is more marked than for the others. The representative case is due to the potential decay ofthe first cell of the stack. At starting time the cell 1 has lower potential respect all the others cell,proceeding in the test, at 12 and 24 e cell 1 has about half the value of voltage with respect to the othercells. At 36 hours the worst situation happens, the cell 1 is in reversal of voltage. A mechanical damage wasobserved in the PCB hardware that may have caused a fuel infiltration in the cathodic compartment of thestack. Fig. 11 shows the observed mechanical damages.



Figure 11: monopolar stack defects pictures

In Fig. 11 we n note some imperfection in the monopolar stack, pictures 1 and 2 show, in the red circle, apassage of light between the fuel reservoir layer and the gasket, evidence the uneven sealing.

1 2

3

5

4

6



Similar imperfections are shown in pictures 3,4 and 5. Some free space is observed also between the twogaskets that seal the MEA layer. In picture 6, external cracks are highlighted, probably due to the tighteningof the stack. These imperfections may allow fuel leakage and can justify the problems observed during thetime test.

CONCLUSIONS

The fuel cell ministack stack achieved a maximum power of 1.2 W at 1.2 A, operating under passive mode,room temperature, air breathing, natural convection, with 5 M methanol matching the target of the projectestablished in MS16 (1 W).

The achievement of a low contact resistance required the need to assembly the ministack at a compressionof 1 Nm. Unfortunately, this was not compatible with the PCB hardware adopted for the ministack causingan initial mechanical damage that propagated during testing. The mechanical damage also propagated aseffect of the temperature increase as recorded during operation.

Notwithstanding this aspect, the ministack was tested for several hundred hours in total under differentoperating conditions at Messina, Montecatini and Turin (Italy). In particular, it was demonstrated during theDuramet workshop and progress meeting in Montecatini, Italy. However, the best performance, that waslarger than the project target, was just obtained at the beginning of tests since the compression causeddamages propagating with time. Individuation of mechanical robust PCBs is in due course and we expectthat proper hardware components can substantially reduce the degradation with time while maintaininggood compressions as necessary to keep low the series resistance.

Maybe a solution for these problems could be to select more though end plates made of metals likealuminum or stainless steel (this would require proper electrical insulation of the tie rods) or compositepolymeric materials containing for example glass fibers. This would ensure a greater tightening torque.

improved durability and cost-effective components for new ... · pdf fileduramet deliverable...

Documents