High-power-density alcohol fuel cells can relieve many of the daunting challenges facing a hydrogen energy economy. Here, such fuel cells are achieved using CsH2PO4 as the electrolyte and integrating into the anode chamber a Cu-ZnO/Al2O3 methanol steam-reforming catalyst. The temperature of operation, ~250°C, is matched both to the optimal value for fuel cell power output and for reforming. Peak power densities using methanol and ethanol were 226 and 100  mW/cm^2, respectively. The high power output (305  mW/cm^2) obtained from reformate fuel containing 1% CO demonstrates the potential of this approach with optimized reforming catalysts and also the tolerance to CO poisoning at these elevated temperatures

Boysen, Dane A.

Chisholm, Calum R. I.

Haile, Sossina M.

Uda, Tetsuya

Electrochemical and Solid-State Letters

English

Uda, T

Boysen, DA

Chisholm, CRI

Haile, SM

Kyoto University Research Information Repository

Alcohol fuel cells at optimal temperatures

Caltech Authors - Main

Electrochemical and Solid-State Letters, 9 6 A261-A264 2006 A261Alcohol Fuel Cells at Optimal Temperatures
Tetsuya Uda,a Dane A. Boysen,b Calum R. I. Chisholm,b and Sossina M. Hailez
Department of Materials Science, California Institute of Technology, Pasadena,
California 91125, USA
High-power-density alcohol fuel cells can relieve many of the daunting challenges facing a hydrogen energy economy. Here, such
fuel cells are achieved using CsH2PO4 as the electrolyte and integrating into the anode chamber a Cu-ZnO/Al2O3 methanol
steam-reforming catalyst. The temperature of operation, 250°C, is matched both to the optimal value for fuel cell power output
and for reforming. Peak power densities using methanol and ethanol were 226 and 100 mW/cm2, respectively. The high power
output 305 mW/cm2 obtained from reformate fuel containing 1% CO demonstrates the potential of this approach with optimized
reforming catalysts and also the tolerance to CO poisoning at these elevated temperatures.
© 2006 The Electrochemical Society. DOI: 10.1149/1.2188069 All rights reserved.
Manuscript received December 7, 2005. Available electronically March 31, 2006.
1099-0062/2006/96/A261/4/$20.00 © The Electrochemical SocietyIn recent years, significant attention has been directed to hydro-
gen gas as a fuel. However, major barriers remain to the implemen-
tation of a hydrogen-based energy economy, not the least of which is
the absence of viable hydrogen storage and delivery technologies.
Alcohols have stored energy densities several times that of standard
compressed hydrogen e.g., 1 L of methanol and ethanol are ener-
getically equivalent to 4.4 and 5.8 L, respectively, of hydrogen gas
compressed at 350 atm and thus may address many of the chal-
lenges facing hydrogen as a fuel. Like hydrogen, alcohol fuels have
the potential for being produced from renewable resources and
thereby mitigating the production of carbon dioxide, a major green-
house gas.1 Yet, unlike hydrogen, they enjoy a well-developed fuel
delivery infrastructure second only to gasoline.
Implementation of alcohol fuels in fuel cells, energy conversion
devices that combine the benefits of zero regulated emissions and
high efficiency, requires that the fuel cell electrolyte be impermeable
to the fuel. In addition, moderate temperature operation is desirable
to minimize CO poisoning at the anode and to enhance fuel electro-
oxidation kinetics. The proton conducting electrolyte, CsH2PO4,
meets exactly these requirements, and the authors have successfully
demonstrated elsewhere hydrogen/oxygen fuel cells based on this
electrolyte with good long-term stability at operational temperatures
of 250°C.2 The present work was undertaken with the aim of
developing high power-density CsH2PO4-based fuel cells operating
on methanol. A key innovation involves the incorporation of a steam
reforming catalyst for the conversion of methanol to hydrogen and
CO2 directly in the fuel cell anode chamber.
Experimental
The overall fuel-cell/reformer structure employed is shown in
Fig. 1. The fuel cell portion, the fabrication of which is detailed
elsewhere,3 consisted of a thin layer of CsH2PO4 26-77 m thick
sandwiched between two electrocatalyst layers comprised of a mix-
ture of CsH2PO4, carbon black, electrocatalyst particles, and a fugi-
tive pore-former. This component was, in turn, sandwiched between
two porous stainless steel supports which served as gas diffusion
electrodes. The electrocatalyst in the cathode was Pt 7.7 mg/cm2,
50 mass % in the form of Pt black and the remainder supported on
carbon. As the anode electrocatalyst Pt-Ru was employed
5.6 mg/cm2 Pt and 2.9 mg/cm2 Ru, formed of a mixture of
Pt0.5Ru0.5 black comprising 85 mass % of the metals in the anode
and Pt-Ru 2:1 mass ratio supported on carbon. The active areas of
the cathode and anode were 2.3-2.9 cm2 and 1.74 cm2, respectively.
The total measured current and power were divided by the area of
the anode to obtain current and power densities.
The reformer was Cu-ZnO supported on Al2O3, a well known
a Present address: Materials Science and Engineering, Kyoto University, Sakyo,
Kyoto 606-8501 Japan.
b Present address: Superprotonic, Inc., Pasadena, CA 91101, USA
z E-mail: smhaile@caltech.educatalyst system for methanol reforming.4 A composite comprised of
CuO, 30 wt % ZnO 20 wt % Al2O3 CuO, 31 mol % ZnO, 16 mol
% Al2O3 was prepared by co-precipitation from aqueous
solution.5,6 Copper, zinc, and aluminum nitrate were dissolved in
water total metal concentration of 1 mol/L and added to an aque-
ous solution of sodium carbonate 1.1 mol/L. The resulting precipi-
tate was rinsed with deionized water, filtered, and dried in air at
120C for 12 h. The dried powder 1 g was lightly pressed into the
form of a disk, 3.1 mm thick and 15.6 mm in diameter, then cal-
cined at 350C for 2 h. This disk was placed adjacent to the anode-
side gas diffusion electrode see Fig. 1. The copper oxide was re-
duced to metallic copper by exposure to hydrogen for 2 h in the fuel
cell test station prior to fuel cell measurements.
Fuel cells were operated both with and without the reformer, and,
in addition to methanol, using hydrogen, reformate a mixture of H2,
CO, and CO2 in a 74.7:0.985:24.3 volume ratio, and ethanol as the
fuel see Table I. In the case of hydrogen, the fuel was supplied at
a rate of 100 standard cm3/min and was humidified to a water partial
pressure of 0.3 atm by passing through hot water 70°C. Methanol
and ethanol fuels were premixed with water to dilution levels of 43
and 36 vol %, respectively and delivered to the fuel cell using a
vaporizer. In addition, commercial vodka Absolut, Sweden with a
nominal ethanol concentration of 40 vol % was also employed. To
permit direct comparisons, the hydrogen, alcohol, and reformate fu-
els were supplied to the anode at equivalent hydrogen flow rates,
Figure 1. Schematic diagram of an internal-reforming solid acid fuel cell.
A262 Electrochemical and Solid-State Letters, 9 6 A261-A264 2006assuming complete reforming. At the cathode, the gas was supplied
at a rate of 100 standard cm3/min and also humidified to 0.3 atm,
again, by passing the gas through hot water. Cells were operated at
240-260°C. The theoretical or Nernstian open-circuit voltage
OCV under these conditions is 1.15 V, again, assuming complete
reforming for the alcohol fuels.
Results and Discussion
Typical polarization curves obtained from the two different types
of cells, one configured with a reformer and one without are pre-
sented in Fig. 2. The effectiveness of the reformer in enhancing fuel
cell power output is immediately evident. The fuel cell configured
with a reformer delivered a peak power output from methanol al-
most as high as it did from neat hydrogen, 226 vs 270 mW/cm2
Fig. 2A. Furthermore, the OCVs from these two fuels were almost
identical 962 and 976 mV for the hydrogen and methanol, respec-
tively. These values are somewhat lower than the theoretical OCV
because of the difficulties in obtaining reliable seals at such high
temperatures using polymer sealants. Elsewhere it has been demon-
strated that theoretical OCVs can be obtained from solid acids if the
sealant is exposed to less severe conditions.7
In the absence of the reformer, the power output from methanol
dropped to less than half that obtained from hydrogen, 148 vs
335 mW/cm2 at peak power, and the OCV dropped by almost
100 mV from 983 to 879 mV Fig. 2B. Comparing the power out-
puts of the two cells when operated on neat hydrogen, the slightly
lower power from the cell incorporating the reformer is simply a
result of the somewhat thicker membrane. Specifically, the data in
Fig. 2A were obtained from a fuel cell with a 47 m thick electro-
lyte and those of Fig. 2B from a fuel cell with a 34 m thick elec-
trolyte.
The fuel cell operated with reformate fuel in the absence of a
reformer catalyst layer exhibited power output almost comparable
to that of neat hydrogen 305 mW/cm2 at peak power vs
335 mW/cm2. Thus, despite the almost 1% CO in the fuel stream,
there is little to no poisoning of the Pt-Ru catalyst. This is presum-
ably a consequence of the relatively high temperature of fuel cell
operation. The reformate results further suggest that even modest
improvements in the development of a reforming catalyst to more
efficiently catalyze the methanol steam reforming reaction
CH3OH + H2O → 3H2 + CO2 1
may result in methanol fuel cells with power densities comparable to
those obtained from hydrogen fuel cells. This is in stark contrast to
PEM fuel cells, in which direct operation on methanol typically
results in power outputs that are only about 15% of that from
hydrogen.8
Because of the tremendous challenges of steam reforming
ethanol,9 which normally requires temperatures of 350°C or higher
even for the best catalysts,10 power densities obtained from this fuel
were lower than from methanol, as shown in Fig. 3. Nevertheless,
the peak power densities obtained from ethanol 36 vol % in H2O
and even commercial vodka were as high as 100 mW/cm2. Fur-
thermore, the OCVs, 950 mV, were higher than those obtained
from methanol fuel cells in the absence of a reformer, again dem-
onstrating that benefit is derived from the integration of the reformer
Table I. Experimental conditions employed in fuel cell measurement
Cell Reformer Electrolyte thickness, m Operational temperature, °C
1 Yes 77 260
2 Yes 47 260
3 No 26a 240 and 260
4 No 34a 240
a 2.5 vol % SiO2 was added to the CsH2PO4 electrolyte to provide mecha
b This cell also operated using ethanol 36 vol % in H O and vodka 402catalyst layer into the fuel cell anode chamber. Given the presum-
ably high impurity content in the commercial vodka, the results
additionally demonstrate the tolerance of the fuel cell anode catalyst
to impurities at moderate temperatures.
A more quantitative analysis of the performance of these cells
which have slightly different thicknesses and certainly differences
in cathode microstructures can be achieved by estimating the over-
potentials at the anodes, anode. The anode-overpotential the drop in
voltage across the anode-electrolyte interface affects the overall cell
voltage according to
Vcell = Vocv
N
− cathode + iR + anode 2
where Vocv
N
, , i, and R are, respectively, the Nernstian voltage ex-
pected value at open circuit, the overpotential at the electrodes,
the current density, and the ohmic resistance of the cell. For any
given cell, the ohmic drop in voltage as a function of current density
the iR term is the same irrespective of the measurement condi-
tions. Moreover, because all measurements were performed with a
Fuels
Methanol 43 vol %, neat hydrogen
Methanol 43 vol %, neat hydrogenb
thanol 43 vol %, neat hydrogen, reformate H2, 24.3% CO2, 0.985% CO
thanol 43 vol %, neat hydrogen, reformate H2, 24.3% CO2, 0.985% CO
upport to these thin membranes.
ethanol as the fuel.
Figure 2. Cell voltage open symbols and power density closed symbols
curves obtained when using pure hydrogen H2, premixed reformate H2,
24.3% CO2, 0.985% CO, and methanol H2O, 43 vol % CH3OH fuels with
a solid acid fuel cell device comprised of A a CsH2PO4-fuel cell cell 2,
Table I at 260C with a methanol reformer Cu-ZnO/Al2O3 catalyst layer
and B a CsH2PO4-fuel cell cell 4, Table I at 240C without a reformer.s.
Me
Me
nical s
vol %
A263Electrochemical and Solid-State Letters, 9 6 A261-A264 2006fixed oxygen flow rate, the cathodic overpotentials for any given cell
are also identical regardless of the fuel employed. Thus, for any
given cell, if we subtract the cell polarization measured under
hydrogen/oxygen conditions, Vcell
H2
, from the total polarization, Vcell
fuel
,
we obtain a measure of the anode behavior relative to its behavior
under neat hydrogen
Vcell
fuel
− Vcell
H2  anode
fuel
− anode
H2 3
The difference term of Eq. 3 is plotted as a function of current
density in Fig. 4 for methanol fuel cells both with and without a
reformer, and for reformate fuel cells in which a reformer was not
employed. The strong similarity between the behavior of reformate
and hydrogen in these fuel cells, which reflects the excellent CO
tolerance, is particularly evident from this representation of the data,
as is the reproducibility of the anode behavior. There is only a small
linear dependence of anode overpotential on current density. It is
likely that this voltage drop is due simply to gas dilution effects.
Figure 3. Cell voltage open symbols and power density closed symbols
curves obtained from integrated reformer and SAFC power generators
SAFC  solid acid fuel cell using ethanol 36 vol % in H2O and vodka
40 vol % ethanol as the fuel. See Table I for details of fuel cell character-
istics.
Figure 4. Difference of anode overpotential anodefuel –anodeH2  as a function of
current density between supplying hydrogen H2 and methanol or reformate
fuel using a solid acid fuel cell Table I, cells 1-4. Data obtained are for
methanol fuel in the absence of a reformer Table I, cells 3 and 4, methanol
fuel in the presence of a reformer Cu-ZnO/Al2O3 catalyst layer Table I,
cells 1 and 2, and premixed reformate fuel H2, 24.3% CO2, 0.985% CO in
the absence of a reformer Table I, cells 3 and 4.Figure 4 also clearly shows that the performance of the methanol
fuel cell incorporating the reformer is comparable to that of the
reformate operated fuel cell at low current densities less than
400 mA/cm2. This suggests that the reformer converts methanol to
hydrogen and CO2 at a rate that is comparable to the rate of elec-
trochemical hydrogen consumption in the fuel cell. At higher current
densities, the overpotential increases sharply. We attribute this to
kinetic limitations of the reforming reaction. That is, for current
densities of 400 mA/cm2 and greater, the rate of conversion of
methanol over the reforming catalyst is not fast enough to keep pace
with the rate at which hydrogen is consumed at the fuel cell anode.
In contrast, in the absence of the reformer, the overpotentials for the
methanol fuel cells are high at all current densities, ranging from
0.1 V under open circuit conditions to 0.2-0.3 V at 600 mA/cm2.
Thus, even at 250°C, methanol cannot be adequately oxidized
and/or reformed on the Pt-Ru anode catalyst so as to yield equilib-
rium concentrations of protons at the fuel cell anode. In conven-
tional PEM fuel cells, open-circuit voltages are typically even lower
than the 0.88 V obtained here in the absence of a reformer. In
those fuel cells, not only is electrocatalysis slow, but there are also
losses in voltage due to the methanol permeation across the mem-
brane methanol cross-over. Furthermore, the methanol concentra-
tion used in those cells is low to reduce the permeation rate of
methanol typically 4 vol % in H2O and also contributes to the
low OCV.
The power outputs obtained here from alcohol-fueled
CsH2PO4-fuel cells under ambient pressures is competitive with the
highest power densities reported for polymer electrolyte based fuel
cells even under high pressure conditions. For example, a peak
power density of 335 mW/cm2 has been achieved from a direct
methanol PEM fuel cell at high temperature 120°C and pressure
1.8-5.0 atm,11 and this value remains essentially the record in the
open literature.8 Similarly, a peak power density of 110 mW/cm2
has been achieved for a direct ethanol fuel cell operated at 140°C
and 4.0-5.5 atm.12 Note that Bjerrum and co-workers have argued
that DMFCs operating at power densities as low as 200 mW/cm2 at
0.5 V would be competitive with direct hydrogen fuel cells operat-
ing at 500-600 mW/cm2 as a consequence of the overall system
simplication.13 The results reported here meet this goal at ambient
pressures, albeit at high Pt and Ru loadings. Efforts are presently
underway to reduce the precious metals content of these solid acid
fuel cells so as to render them truly competitive, and to improve the
flow geometries so as to achieve the high power densities implied by
the cells operated on reformate fuel.
While it has not been demonstrated here, a combined SAFC and
methanol steam reforming catalyst can, in principle, be configured
for tight thermal integration between the exothermic fuel cell reac-
tions and the endothermic reforming reaction. Such a configuration
would provide a key advantage for methanol fueled SAFCS over
PEM fuel cells operated on externally reformed methanol. This is
because in an external reforming system, 59 kJ of heat must be
provided to reform one mole of methanol according to Eq. 1. This
amount is equivalent to the heat of combustion effectively, the en-
ergy content of 0.24 moles of hydrogen. Thus, the integrated design
presented here is preferable over both direct methanol and reformed
methanol systems based on polymer electrolyte membrane fuel
cells.
Acknowledgments
This work is funded by the National Science Foundation, Divi-
sion of Materials Research.
The California Institute of Technology assisted in meeting the publication
costs of this article.
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Alcohol Fuel Cells at Optimal Temperatures

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Alcohol Fuel Cells at Optimal Temperatures

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