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Performance of an alkaline-acid direct ethanol fuel cell

L. An, T.S. Zhao*

Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,

Hong Kong SAR, China

a r t i c l e i n f o

Article history:

Received 5 March 2011

Received in revised form

19 April 2011

Accepted 20 April 2011

Available online 8 June 2011

Keywords:

Fuel cell

Direct ethanol fuel cell

Alkaline-acid

Species concentrations

Membrane thickness

Power density

* Corresponding author. Tel.: þ852 2358 8647E-mail address: metzhao@ust.hk (T.S. Zh

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.04.150

a b s t r a c t

This paper reports on the performance of an alkaline-acid direct ethanol fuel cell

(AA-DEFC) that is composed of an alkaline anode, a membrane and an acid cathode. The

effects of membrane thickness and the concentrations of various species at both the anode

and cathode on the cell performance are investigated. It has been demonstrated that the

peak power density of this AA-DEFC that employs a 25-mm thick membrane is as high as

360 mW cm�2 at 60 �C, which is about 6 times higher than the performance of conventional

DEFCs reported in the literature.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction Moreover, this problem is rather difficult to be solved based on

Ethanol is a sustainable, carbon-neutral transportation fuel

source. It is an ideal fuel for direct oxidation fuel cells for

portable and mobile applications, as it offers multiple advan-

tages over hydrogen and methanol, including higher energy

density and ease of transportation, storage and handling.

Hence, direct ethanol fuel cells (DEFCs) have recently received

ever-increasing attention [1e4].

Typically, conventional DEFCs can be divided into two

types in terms of the employed membrane: proton exchange

membrane direct ethanol fuel cells (PEM-DEFCs) and anion

exchange membrane direct ethanol fuel cells (AEM-DEFCs).

Past efforts have been mainly devoted to PEM-DEFCs and

significant progress has been made [5e7]. However, the slug-

gish ethanol oxidation reaction (EOR) kinetics is still a main

barrier that limits the cell performance of PEM-DEFCs.

.ao).2011, Hydrogen Energy P

the acid electrolyte, even with the Pt-based catalysts. On the

other hand, unlike in acid media, the kinetics of both the EOR

and oxygen reduction reaction (ORR) in alkalinemedia become

much faster than that in the acidmedium. It has been recently

demonstrated that when the acid electrolyte was changed to

alkaline one, i.e. AEM, the cell performance could be

substantially improved [8e13]. Although promising, the cell

performance still needs to be substantially improved before

the widespread commercialization. Another important

parameter that limits the performance of DEFCs operating

under both acid and alkaline media is that thermodynami-

cally, their theoretical voltage is low (1.14 V).

Recently, we proposed a new type of DEFC, termed as

alkaline-acid DEFC (AA-DEFC) shown in Fig. 1, which is

composed of an alkaline anode, a membrane and an acid

cathode [14]. The anolyte is an aqueous solution of ethanol

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Na+

Na+

Alkaline Anode Acid Cathode

CEMEtOHOH-

Na+

CH3COONa

H2O2H+

SO42-

Na2SO4

Na+

Loade- e-

Fig. 1 e Schematic of the alkaline-acid direct ethanol

fuel cell.

0 100 200 300 400 5000.0

0.5

1.0

1.5

2.0

Current Density,mA cm-2

Cel

l Vol

tage

,V

0

50

100

150

200

250

Power D

ensity,mW

cm-2

Temperature: 60oC

Fig. 2 e Polarization and power density curves of the

alkaline-acid DEFC [14].

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and sodium hydroxide, while the catholyte is an aqueous

solution of hydrogen peroxide and sulfuric acid. In addition,

a cation exchange membrane (CEM) is employed to conduct

sodium ions. On the anode, ethanol reacts with OH� provided

by NaOH according to [1,15]:

CH3CH2OHþ 5NaOH/CH3COONaþ 4Naþ þ 4e� þ 4H2O (1)

The produced electrons pass through an external electrical

load and arrive at the cathode. In the meantime, Naþ ions as

the charge carrier migrate from the anode to the cathode to

close the internal circuit.

On the cathode, hydrogenperoxide reactswithHþ provided

by H2SO4 and electrons to produce water according to:

2H2O2 þ 2H2SO4 þ 4e�/2SO2�4 þ 4H2O (2)

Hence, Na2SO4 will be produced by combining Naþ and SO2�4 in

the cathode, which can be taken away by the cathode solution

flow. The overall reaction is obtained by combining the EOR

given by Eq. (1) and the hydrogen peroxide reduction reaction

(HPRR) given by Eq. (2), i.e.:

CH3CH2OHþ 5NaOHþ 2H2O2 þ 2H2SO4/CH3COONa

þ 2Na2SO4 þ 8H2O (3)

We tested the above-described AA-DEFC and demonstrated

that the peak power density was as high as 240 mW cm�2 at

60 �C, as can be evidenced from Fig. 2 [14]. The high perfor-

mance of this type of DEFC can be attributed to the following

reasons: 1) the fuel cell has a high theoretical voltage (2.52 V)

rendered by the alkaline anode and acid cathode [14], 2) it

has a low overpotential of HPRR [16e22], and 3) it renders

fast kinetics of the EOR at a lower potential (Eao ¼ 0.74 V)

[9,12,14].

Themainobjective of thisworkwas to investigate the effect

ofmembrane thickness on theperformanceof theAA-DEFC. In

addition,we also investigated the effects of the concentrations

of various species, including hydrogen peroxide, and sulfuric

acid, ethanol, and sodiumhydroxide, on the cell performance.

We show that the AA-DEFC employing a thinner membrane

(25 mm) can yield a peak power density as high as 360mWcm�2

at 60 �C.

2. Experimental

2.1. Membrane electrode assembly

Four membrane electrode assemblies (MEAs), with different

thickness CEMs, were prepared in this work. The CEMs used in

this work were the Nafion series membrane, N211 (25 mm),

N212 (50 mm), N115 (125 mm), and N117 (175 mm), which were

treated as the cation conductors. The procedures of treating

the Nafion membranes included [23,24]: i) immersing them in

10 wt.% NaOH solution; ii) heating them to 80 �C for 1 h; and

iii) washing them by deionized (DI) water several times. The

four CEM-MEAs had the same anode and cathode electrodes

and the same active area of 1.0 cm � 1.0 cm. The anode elec-

trode was formed by following the steps: i) a catalyst ink was

prepared by mixing a homemade PdNi/C with a loading of

1.0 mg cm�2, ethanol as the solvent and 5 wt.% Nafion as the

binder [25]; ii) the anode catalyst ink was stirred continuously

in an ultrasonic bath for 20min such that it waswell dispersed

and iii) the anode catalyst ink was brushed onto a piece of

nickel foam (Hohsen Corp., Japan) that served as the backing

layer. Similarly, on the cathode, the catalyst ink was prepared

by mixing 60 wt.% Pt/C (Johnson-Matthey) with a loading

of 3.9 mg cm�2, ethanol as the solvent, and 5 wt.% Nafion as

the binder [14]. Subsequently, the cathode catalyst ink was

brushedonto a piece of carbon cloth (ETEK, TypeA) that served

as the backing layer to form a cathode electrode.

2.2. Fuel-cell setup and instrumentation

EachMEAwas fixed between an anode and a cathode flowfield.

The both flow fields were made of 316 L stainless steel plate, in

which a single serpentine flow channel, 0.5 mm deep and

1.0 mm wide, was grooved by the wire-cut technique. An

alkaline solution containing ethanol andNaOHwas fed into the

anode flow channel at a flow rate of 2.0 mL min�1 by a peri-

staltic pump, while an acid solution containing H2O2 and

sulfuric acid (H2SO4) was fed into the cathode flow channel at

a flow rate of 2.0 mL min�1 by another peristaltic pump.

Additionally, the cell temperature was measured with

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a thermocouple located at the anode current collector, and two

electrical heating rods were installed in the cell fixtures to

control the operating temperature. An Arbin BT2000 (Arbin

Instrument Inc.) was employed to measure the polarization

curves. The internal resistance of the cell was measured by the

built-in function of the Arbin BT2000. Anode polarization data

for these fuel cells were obtained employing an Hg/HgO (MMO)

reference electrode. Cathode polarization data were derived by

subtracting anode polarization values from the respective cell

polarization data.

3. Results and discussion

3.1. Effect of hydrogen peroxide concentrations

Fig. 3a shows the polarization curves with different hydrogen

peroxide concentrations ranging from 1.0 M to 6.0 Mwhen the

sulfuric acid concentration was fixed at 1.0 M and the anode

solution was composed of 3.0 M ethanol and 5.0 M NaOH. It

can be seen that in the low current density region (below

100 mA cm�2), the cell voltage decreased with an increase in

hydrogen peroxide concentration, but at high current densi-

ties the cell voltage first increasedwith the hydrogen peroxide

b

a

0 100 200 300 400 5000.0

0.5

1.0

1.5

2.0

Cel

l Vol

tage

,V

Current Density,mA cm-2

1M 2M 4M 6M

Temperature: 60oCAnode: 3M EtOH+5M NaOH,2mL min-1

CEM thickness: 175 mCathode: H2O2+1M H2SO4,2mL min-1

0 100 200 300 400 500-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Pote

ntia

l,V v

s. M

MO

Current Density,mA cm-2

1M 1M 2M 2M 4M 4M 6M 6M

Fig. 3 e Effect of H2O2 concentrations on cell performance.

(a) Polarization curves. (b) Anode and cathode potentials.

concentration and then decreased. The 4.0-M hydrogen

peroxide concentration exhibited the highest performance. At

low current densities, the decrease in the cell voltage with the

hydrogen peroxide concentration can be mainly attributed to

H2O2 crossover from the cathode to the anode as the Nafion

membranes are also permeable to H2O2 [26]. The permeated

H2O2 can react with ethanol and thus form the mixed poten-

tial at the anode, which increases the anode potential, thus

decreasing the voltage. Increasing the hydrogen peroxide

concentration at the cathode can cause a higher rate of H2O2

crossover, leading to an increase in the mixed potential at the

anode, thereby increasing the anode potential, as evidenced

by the measured anode potentials shown in Fig. 3b. As

a result, the cell voltage decreasedwith the hydrogen peroxide

concentration at low current densities. At high current

densities, the underlying mechanisms leading to the perfor-

mance behavior is more complex. When the hydrogen

peroxide concentration is increased from 1.0 M to 4.0 M,

although the anode potential slightly increases with the

hydrogen peroxide concentration as a result of the H2O2

crossover, the mass transport at the cathode are also

enhanced, reducing the concentration polarization loss.

Moreover, the reduced polarization loss not only compensates

the mixed potential in the anode resulting from the H2O2

crossover but also improves the cell performance. Therefore,

the cell voltage improved with increasing the hydrogen

peroxide concentration from 1.0 M to 4.0 M. When the

hydrogen peroxide concentration was further increased to

6.0 M, the cathode potential did not changemuch as hydrogen

peroxide concentration in the cathode catalyst layer (CL) is

sufficient to ensure the H2O2 transport. On the other hand, the

high hydrogen peroxide concentration causes rather serious

H2O2 crossover, leading to the increased anode potential and

thus decreasing the cell performance, as shown in Fig. 3b.

Therefore, the cell voltage was degraded when the hydrogen

peroxide concentration was increased to 6.0 M, resulting in an

optimal hydrogen peroxide concentration that can yield the

highest performance at high current densities.

3.2. Effect of sulfuric acid concentrations

The polarization curves for different sulfuric acid concentra-

tions ranging from 0.5 M to 2.0 M at a fixed hydrogen peroxide

concentration of 4.0 M are presented in Fig. 4a, which shows

that the cell performance slightly increased with the sulfuric

acid concentration. Similarly, in order to gain insight into the

mechanisms leading to the behavior, the respective anode

and cathode potentials were also measured, respectively, and

are shown in Fig. 4b. It can be seen that the anode potential

remained almost the same for different sulfuric acid concen-

trations but the cathode potential slightly increased with the

sulfuric acid concentration. As a result, it can be concluded

that the improved cell performance with increasing the

sulfuric acid concentration ismainly attributed to the increase

in the cathode potential. This is because an increase in the

sulfuric acid concentration leads to a higher Hþ concentration,

accelerating the hydrogen peroxide reduction reaction [18].

Therefore the kinetic loss resulting from the hydrogen

peroxide reduction reaction can be reduced as the sulfuric

a

b

0 100 200 300 400 500-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Pote

ntia

l,V v

s. M

MO

Current Density,mA cm-2

0.5M 0.5M 1 M 1 M 2 M 2 M

0 100 200 300 400 5000.0

0.5

1.0

1.5

2.0

Cel

l Vol

tage

,V

Current Density,mA cm-2

0.5M 1 M 2 M

Temperature: 60oCAnode:3M EtOH+5M NaOH,2mL min-1

CEM thickness: 175 mCathode: 4M H2O2+H2SO4,2mL min-1

Fig. 4 e Effect of H2SO4 concentrations on cell performance.

(a) Polarization curves. (b) Anode and cathode potentials.

0 100 200 3000.0

0.5

1.0

1.5

2.0

Cel

l Vol

tage

,V

Current Density,mA cm-2

1M 3M 5M

Temperature:60oCAnode: EtOH+5M NaOH,2mL min-1

CEM thickness: 175 mCathode: 4M H2O2+1M H2SO4,2mL min-1

Fig. 5 e Effect of ethanol concentrations on cell

performance.

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acid concentration increases, so that the cathode potential

and cell performance can be upgraded.

3.3. Effects of ethanol concentrations

We also investigated the effect of ethanol concentrations on

the cell performance when the NaOH concentration was fixed

at 5.0 M and the cathode solution was composed of 4.0 M

hydrogen peroxide and 1.0 M sulfuric acid, as shown in Fig. 5.

It can be seen that in the low current density region, the cell

voltage increased with increasing the ethanol concentration,

but at high current densities the cell voltage first increased

with the ethanol concentration and then decreased.

In general, for a given anode catalyst, the anode potential

depends on the local concentrations of both ethanol and

hydroxyl ions in the anode CL. A change in either ethanol or

hydroxyl ions concentrationswill lead to a change in the other

[10e12]. At low current density region, the solutions with

1.0-M and 3.0-M ethanol cause the relatively poor kinetics as

a result of insufficient ethanol concentrations in the anode CL.

Hence, increasing the ethanol concentration to 5.0 M can

maintain the concentrations of both ethanol and hydroxyl

ions at an appropriate level and thus yield the highest cell

voltage. However, the appropriate concentrations of both

ethanol and hydroxyl ions in the anode CL are changed as

a result of the increased consumption of reactants at high

current densities. For a given NaOH concentration of 5.0 M,

there is an optimal ethanol concentration of 3.0 M, yielding

the best cell performance. The reason for this behavior is

explained as follows. Too low ethanol concentration will

result in the slow electrochemical kinetics and the high

concentration loss, and thus the decrease in the cell perfor-

mance; too high ethanol concentration will cause too low

hydroxyl ion concentration at the active surfaces, leading to

the difficulty in the adsorption of hydroxyl on the active site,

hence the electrochemical kinetics is lowered and thus the

cell performance is reduced.

3.4. Effect of NaOH concentrations

The effect of NaOH concentrations on the cell performance

was also investigated, as shown in Fig. 6. It can be seen that

there exists an optimal NaOH concentration of 5.0 M, yielding

the best cell performance. The reason for this phenomenon is

explained as follows. Generally, the alkalinity of the anode

environment not only affects the electrochemical kinetics, but

also the transfer of species to the anode [27,28]. Hence,

increasing the NaOH concentration can enhance the kinetics

of EOR. On the other hand, the solution with too high NaOH

concentration (7.0 M) will lead to too low ethanol concentra-

tion in the anode CL, causing the high concentration loss and

thus the decreased cell performance. Consequently, the

competition between the favorable effect of the faster EOR

and the adverse effect of the increased concentration loss

results in an optimal NaOH concentration (5.0 M) that gives

the best cell performance for a fixed ethanol concentration

(3.0 M), which is in agreement with the result of our previous

work in an AEM-DEFC [12].

3.5. Effect of membrane thickness

The effect of themembrane thickness on the cell performance

was investigated and the results are shown in Fig. 7. It can be

200

250

300

350

400

450

500

25 m 50 m 125 m

IR,m

Ohm

1.40

1.45

1.50

1.55

1.60

1.65

1.70O

CV,V

Fig. 8 e Effect of membrane thickness on the open-circuit

voltage and internal resistance.

0 100 200 3000.0

0.5

1.0

1.5

2.0

Cel

l Vol

tage

,V

Current Density,mA cm-2

3M 5M 7M

Temperature:60oCAnode: 3M EtOH+NaOH,2mL min-1

CEM thickness: 175 mCathode: 4M H2O2+1M H2SO4,2mL min-1

Fig. 6 e Effect of NaOH concentrations on cell performance.

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seen when the cell discharged, the effect of the membrane

thickness on cell performance can be divided into two distinct

regions. In the low current density region (<200mA cm�2), the

cell with a thicker membrane gave a better performance,

whereas in the high current density region, the cell with

a thinner membrane yielded a better performance. As shown

in Fig. 8, the change in membrane thickness leads not only to

a change in the rate of species crossover, but also to a change

in the internal resistance of the cell. On one hand, due to the

higher rate of species crossover, a thinner membrane tends to

cause the larger mixed potentials, degrading the cell perfor-

mance. On the other hand, the use of a thinnermembranewill

yield a smaller internal resistance, which tends to increase the

cell performance. The results presented in Fig. 7 suggest that

the predominant factor affecting the cell performance of this

AA-DEFC varied with current density. Under the OCV and low

current density conditions, Fig. 7 indicates that the mixed

potential problem associated with the thinner membrane

(25 mm) is more serious. At the moderate and higher current

densities, however, the effect of the internal resistance

becomes predominant; thus the use of the thinner membrane

yielded a higher cell performance. It should bementioned that

the rate of species crossover decreases with increasing

Fig. 7 e Effect of membrane thickness on cell performance.

current density. This might also contribute to the improved

performance with the use of the thinner membrane at higher

current densities [29]. It should also be mentioned that there

are four species in this system, resulting in the complicated

species crossover phenomenon. The parallel work underway

is focused on the amount of the species crossover and their

effects on the cell performance.

4. Concluding remarks

In this work, we investigated the effects of membrane thick-

ness and the concentrations of various species, including

hydrogen peroxide, sulfuric acid, ethanol, and sodium

hydroxide, on the performance of the alkaline-acid direct

ethanol fuel cell. We have shown that hydrogen peroxide

concentration, ethanol concentration, NaOH concentration,

and membrane thickness, have significant influences on the

cell performance. In particular,wedemonstrated that thepeak

power density of the AA-DEFC with a thinner membrane

(25 mm) was as high as 360 mW cm�2, which is about 6 times

higher than the performance of conventional DEFCs reported

in the literature. It should be mentioned that although this

present fuel cell is promising in terms of the cell performance,

some fundamental issueswith respect to this present fuel cell,

such as the hydrogen peroxide decomposition and species

crossover, merit extensive future research.

Acknowledgments

The work described in this paper was fully supported by

a grant from the Research Grants Council of the Hong Kong

Special Administrative Region, China (Project No. 623010).

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