B1138
Journal of The Electrochemical Society, 154 ͑11͒ B1138-B1147 ͑2007͒
0
013-4651/2007/154͑11͒/B1138/10/$20.00 © The Electrochemical Society
Electro-oxidation of Ethanol at Gas Diffusion Electrodes
A DEMS Study
a,z
b,d,
a,b,
c
c
c
V. Rao, C. Cremers, * U. Stimming, * Lei Cao, Shiguo Sun, Shiyou Yan,
c
c
Gongquan Sun, and Qin Xin
a
Department of Physics E19, Technische Universität München, D-85748 Garching, Germany
Bavarian Center for Applied Energy Research (ZAE Bayern), D-85748 Garching, Germany
b
c
Dalian Institute of Chemical Physics, Direct Alcohol Fuel Cell Group, Liaoning Province, Dalian 116023,
China
The ethanol electro-oxidation at gas diffusion electrodes made of different catalysts, Pt/C, PtRu͑1:1͒/C, and PtSn͑7:3͒/C, were
studied by on-line differential electrochemical mass spectrometry in a wide temperature range ͑30–90°C͒ as a function of the
anode potential, the fuel concentration, and catalyst loading. The CO current efficiency ͑CCE͒ of the ethanol oxidation reaction
2
͑
EOR͒ exhibits a maximum at about 0.6 V and decreases rapidly with further increasing potentials. The CCE for the EOR goes
down with the increase in concentration of ethanol. CCE for ethanol oxidation reaction shows a strong increase with increasing
2
catalyst loading. The CCE increases with increasing temperature, exceeding 75% at 90°C, 0.1 M ethanol, and 5 mg/cm Pt
catalyst loading. PtSn/C shows high CCE, like Pt/C. But PtRu/C exhibits very small CCE. Of the intermediates, acetaldehyde is
quite active for further oxidation. But acetic acid is fairly resistant against further oxidation. Our results indicate that the C–C bond
scission observed for the EOR with CCE in excess of 50% has to proceed in parallel with ethanol oxidation to either acetaldehyde
or acetic acid, and not sequentially from acetic acid further on, as acetic acid cannot be oxidized any further.
©
2007 The Electrochemical Society. ͓DOI: 10.1149/1.2777108͔ All rights reserved.
Manuscript submitted May 22, 2007; revised manuscript received July 9, 2007. Available electronically September 10, 2007.
5
Direct oxidation fuel cells ͑DOFCs͒ have recently attracted ma-
ratios between 5 and 2. They reported acetaldehyde as the main
jor attention, as an alternative to hydrogen fuel cells, mainly due to
easier fuel storage and handling. The organic liquids used for
DOFCs are much simpler to handle than gaseous hydrogen and also
in many cases do not require any new distribution infrastructure as
they are already widely available as, e.g., ethanol in its denatured
form. The most researched type of DOFC is direct methanol fuel
cell, DMFC. The methanol has better kinetics of oxidation on the
platinum based catalysts in low temperature range than all other
reaction product, whereas CO only a minor product, without many
2
differences in the product selectivities on Pt–Ru and Pt-black cata-
lysts. Using chromatographic techniques, Hitmi et al. found that at
low ethanol concentrations the main product is acetic acid, whereas
acetaldehyde is the major product at high concentration ͑Ͼ0.1 M͒
during ethanol oxidation on polycrystalline Pt at 10°C. Arico et al.
investigated the electrochemical oxidation of ethanol in a liquid-feed
solid polymer electrolyte fuel cell operating at 145°C, 4 atm anode
pressure, 5.5 atm cathode pressure, and 1 M ethanol. Under these
aliphatic alcohols and is also known to oxidize completely to CO2.
This leads to better performance of DMFCs. But some disadvan-
2
conditions, using 2 mg/cm 60% PtRu/C as anode catalyst, high
tages of methanol are its toxicity and relatively low boiling point.
Also most of the methanol today is produced using natural gas as the
base material, which is not a renewable energy resource. Because of
these shortcomings ethanol, the next alcohol, is considered to be an
option because of being less toxic, high in energy content ͑ethanol:
6
selectivity toward CO formation ͑95%͒ was reported. Fujiwara et
2
al. studied ethanol oxidation for selectivity to CO vs acetaldehyde
2
on electrodeposited Pt and PtRu electrodes using model electro-
chemical cell DEMS and reported that Ru addition helps in forma-
2
tion of more CO and less acetaldehyde.
2
8
kWh/kg, methanol: 6 kWh/kg͒ and its availability from renewable
resources.
However, the oxidation of ethanol to CO2 is much slower in
Camara and Iwasita investigated the effects of ethanol concen-
tration on the yields of CO , acetic acid, and acetaldehyde as
2
electro-oxidation products on the polycrystalline Pt electrode using
Fourier transform-infrared ͑FTIR͒ spectroscopy. They found acetic
acid as a major product at low ethanol concentrations, and CO2
being produced to a minor extent. With increasing ethanol concen-
comparison to methanol, as it requires the scission of a C–C bond.
So ethanol electro-oxidation is associated with the formation of sev-
eral unwanted by-products like acetaldehyde and acetic acid. The
efficiency of ethanol oxidation can be improved by development of
catalysts exhibiting faster kinetics and higher selectivity toward CO2
as product and by optimizing the oxidation conditions.
7
trations, the pathway producing acetaldehyde becomes dominant.
Wang et al. studied the product distribution for EOR systematically
as a function of temperature and concentration in a model DEMS for
Various research groups have made efforts to gain mechanistic
understanding of the ethanol oxidation reaction ͑EOR͒. The reaction
is known to follow a complex multistep mechanism, involving a
number of adsorbed reaction intermediates and by-products result-
8
supported platinum catalyst ͑Pt/C͒. Very low CO formation was
2
reported for EOR at their working conditions. They investigated
Pt/C, Pt Sn/C, and PtRu/C catalysts for EOR and reported that the
3
1
addition of Sn and Ru increases the faradaic activity without any
ing from incomplete ethanol oxidation. The major adsorbed inter-
increase in the CO current efficiency which was reported to be 1%
2
mediates were identified as adsorbed CO and Rads and R-Cads hy-
drocarbon residues, whereas acetaldehyde and acetic acid have been
detected as the main by-products using differential electrochemical
9
in all cases. Lamy’s group at the University of Poitiers has pub-
lished a number of papers about the PtSn based catalyst for
1
0-12
2
3
EOR.
Sn is proposed to activate adsorbed water at lower poten-
mass spectrometry ͑DEMS͒,
infrared spectroscopy,
or
1
3,14
4
tial than Pt, leading to higher activity.
They studied the product
chromatography. Wang et al. studied the relative product distribu-
tion for the EOR in a polymer electrolyte fuel cell operating with
ethanol as the anode feed using on-line mass spectrometry in the
temperature range between 150 and 190°C and water: ethanol molar
distribution of EOR in a fuel cell with high performance liquid
chromatography ͑HPLC͒ and reported 20% CO formation for Pt/C
2
catalyst, which reduced to around 7% in the case of Pt–Sn and
Pt–Sn–Ru based catalyst. The last two catalysts were reported to
1
5
favor acetic acid as the final product.
However, as discussed above, the results about the mechanism of
ethanol oxidation vary widely depending on several parameters like
oxidation in a model electrochemical cell or at a fuel cell membrane
electrode assembly ͑MEA͒, temperature, concentration, etc. Also
*
Electrochemical Society Active Member.
d
Present address: Fraunhofer Institute for Chemical Technology, Division of Ap-
plied Electrochemistry, D-76327 Pfinztal, Germany.
E-mail: vrao@ph.tum.de
z