Catalytic reactions in direct ethanol fuel cells

Article abstract of DOI:10.1002/anie.201005745

(Figure Presented) Different anode catalysts (Pt/C, PtRu/C, Pt ₃Sn/C) and operating potentials lead to different product distributions in the anode exhaust of direct ethanol fuel cells, as shown by liquid-state ¹³C NMR spectroscopy (see typical spectrum). Addition of Ru or Sn to Pt/C increases current density, mainly because of enhanced acetic acid production, and the potential dependences of products give clues to reaction pathways of ethanol electro-oxidation.

Full text of DOI:10.1002/anie.201005745

Communications
DOI: 10.1002/anie.201005745
Fuel Cells
Catalytic Reactions in Direct Ethanol Fuel Cells**
In Kim, Oc Hee Han,* Seen Ae Chae, Younkee Paik, Sung-Hyea Kwon, Kug-Seung Lee,
Yung-Eun Sung, and Hasuck Kim
For fuel-cell applications, ethanol is becoming a more
attractive fuel than methanol or hydrogen because it has
higher mass energy density and can be produced in great
quantities from biomass.[1] Additionally, ethanol is less toxic
than methanol and easier to handle than hydrogen.[2,3]
species that are identifiable by their chemical shifts. The
DEFC anode exhaust has been shown to give well-resolved
13C peaks that can unambiguously identify chemical spe-
cies.[14] We have used 13C liquid-state NMR spectroscopy to
identify and quantify the reaction products present in the
liquid anode exhaust of DEFCs that were operated with three
different anode catalysts at various potentials. The results
were used to explain the effect of elements such as Ru and Sn
on the Pt/C anode catalyst and to propose reaction mecha-
nisms of ethanol on Pt-based catalysts.
ꢀ
However, the C C bond in ethanol leads to more complicated
reaction intermediates and products during oxidation,[2–12]
ꢀ
and catalysts must be able to activate C C bond scission for
complete oxidation to CO2. Consequently, much effort has
been made to investigate the reaction mechanisms of direct
ethanol fuel cells (DEFCs) with various analytical meth-
ods.[2–12] Especially the intermediates and products that are
generated during the electrochemical reaction at different
ethanol concentrations and potentials have been investigated
and quantified by chromatographic techniques,[4–6] infrared
reflectance spectroscopy (IRS),[4,6–9] and differential electro-
chemical mass spectrometry (DEMS).[8–10] These studies
revealed that most of the ethanol was oxidized to acetic
acid (AA) or acetaldehyde (AAL) on Pt, but not much to
CO2. Additionally, investigations of ethanol oxidation on
various catalysts showed that alloying Pt with other transition
elements improves the catalytic activity.[6,10,12,13] However,
DEMS is limited to the detection of volatile chemicals, and
IRS requires smooth electrodes with sufficient reflectivity. On
the other hand, liquid-state nuclear magnetic resonance
(NMR) spectroscopy is a straightforward analytical method
which can be applied to an operating fuel cell without any
modification.[14] In liquid-state NMR spectroscopy, peak areas
are linearly proportional to the abundance of chemical
The 13C liquid-state NMR experiments were performed
on DEFCs containing 40 wt% Pt/C, PtRu/C, or Pt3Sn/C
anode catalysts prepared by a polyol method. Full exper-
imental details are described in the Supporting Information.
Figure 1 shows the 13C NMR spectra of the anode exhaust
from the DEFCs with Pt3Sn/C anode catalysts. The spectra
were expanded in the y scale while maintaining the relative
peak heights. The chemical species were assigned to the peaks
in the spectrum according to literature data,[15] and C atoms
that are responsible for 13C NMR signals are underlined. In
the exhaust, the dominant reaction products were AAL (d =
207 ppm), AA (d = 177 ppm), and ethane-1,1-diol (ED, d =
88 ppm) at various potentials. Ethyl acetate (d = 62, 175 ppm)
and ethoxyhydroxyethane (d = 63, 95 ppm) also appeared,
but only in trace amounts and hence were ignored. The
coupling constants of 2.8 and 1.6 Hz between the 13C-labeled
sites were used to distinguish CH2 groups in ethyl acetate and
ethoxyhydroxyethane, respectively. For comparison purposes,
the NMR spectra were also obtained for the DEFCs
containing Pt/C and PtRu/C anode catalysts, and AA, AAL,
and ED were major products detected for all three catalysts.
Figure 2 shows the relative quantities of the major organic
chemicals in the anode exhaust of the DEFCs with different
anode catalysts at different potentials. For the DEFC with
Pt/C anode catalyst, the NMR peak areas of the reaction
products were monotonically depleted with increasing oper-
ating potential above 0.1 V versus the standard hydrogen
electrode. Thus, more oxidation products were produced from
the fuel when the DEFC was operated at a higher current and
a lower potential. However, the addition of Ru or Sn to Pt
caused variations in the NMR spectral patterns. Production of
AA dramatically increased. Subtracting the product popula-
tions for Pt/C from those for PtRu/C and Pt3Sn/C (dotted lines
in Figure 2) separates the contributions of Ru or Sn from
those due to Pt/C. For example, the enhanced AAL and ED
production on PtRu/C and Pt3Sn/C compared to on Pt/C was
almost zero at 0.1 V and slightly increased above 0.2 V. In
contrast, AA production was greatly enhanced and different
production behaviors were observed depending on the anode
catalysts. On the PtRu/C anode catalysts, AA production
[*] Dr. I. Kim,[+] Dr. O. H. Han, Dr. S. A. Chae, Dr. Y. Paik, S.-H. Kwon
Analysis Research Division, Daegu Center
Korea Basic Science Institute, Daegu, 702-701 (Korea)
Fax: (+82)53-959-3405
E-mail: ohhan@kbsi.re.kr
Dr. O. H. Han
Graduate School of Analytical Science and Technology
Chungnam National University, Daejeon, 305-764 (Korea)
Dr. I. Kim,[+] Dr. H. Kim
Department of Chemistry, Seoul National University
Seoul, 151-747 (Korea)
Dr. K.-S. Lee,[$] Dr. Y.-E. Sung
School of Chemical and Biological Engineering
Seoul National University, Seoul, 151-744 (Korea)
[+] Present address: SB LiMotive Co., Ltd., Yongin (Korea)
[$] Present address: Fuel Cell Center, Korea Institute of Science and
Technology, Seoul (Korea)
[**] This work was supported by KBSI grants (K29030 and K30030) to
O.H.H.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005745.
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Angew. Chem. Int. Ed. 2011, 50, 2270 –2274

Figure 2. Populations of the reaction products in the anode exhaust
versus operating potential of fuel cells containing Pt/C, PtRu/C, or
Pt3Sn/C anode catalyst. The dotted lines indicate the populations of
the reaction products versus operating potential after subtracting the
values of the fuel cell with Pt/C anode catalyst.
CH3CH2OH þ H2O ! CH3COOH þ 4 Hþ þ 4 eꢀ
ð4Þ
Figure 1. Representative 13C liquid-state NMR spectra of the anode
exhaust collected at different operation potentials for a DEFC
containing 40 wt% Pt3Sn/C anode catalyst.
Additionally, these equations show that the production of
AA and ED, but not AAL, may be greatly influenced by the
reaction of water on the catalysts. In fact, oxygenated species,
such as OH, that were adsorbed on the catalysts from the
reaction of water, influence the oxidation of CO-like species
on PtRu or Pt3Sn catalysts.[6,12,13,16] The potential dependences
of the water reaction and adsorption-site competition
between water and fuel or its reaction intermediates were
reported.[2,6] Therefore, the drastically increased production
of AA at 0.4 V on PtRu/C might be due to enhanced reaction
drastically increased at 0.4 V. On the other hand, AA
production was dominant on Pt3Sn/C over a wider potential
range and reached a maximum at 0.2 V.
The electric energy that is stored in an ethanol molecule
can be regarded as 12eꢀ according to Equation (1).
C2H5OH þ 3 H2O ! 2 CO2 þ 12 Hþ þ 12 eꢀ
ð1Þ
ꢀ
of water with reaction intermediates, such as COCH3, that
were adsorbed on the surface of the catalysts (Scheme 1).
However, ED, which may be similarly influenced by the water
reaction, did not behave in the same manner as AA with
respect to the quantity produced at various potentials. The
potential dependence of the enhanced ED population on
Pt3Sn/C or PtRu/C behaved differently from that of the
enhanced AA population and was very similar to that of the
enhanced AAL population. These different behaviors of ED
and AA production clearly indicate that not only the water
dissociation reaction but also other factors that affect all of
the pre-reactions in Scheme 1 are controlling factors for
generation of the reaction products. Thus, the potential
dependences of all of the reaction-controlling parameters,
such as the adsorption strength of the reaction intermedi-
ates[3,6,12,16] and the water reaction,[6,12,13,17] are expected to
differ for each catalyst. Another possible explanation is that
However, conversion of ethanol into CO2 is rather low in
DEFCs.[4,5,7,10] In reality, AAL and AA were the major
oxidation products,[4,5,7,10] and ED was also a major product
detected by NMR spectroscopy in this study. Therefore, the
amounts of the individual products reflect the reaction
pathways and the effectiveness of the catalysts. Energy
corresponding to 2eꢀ is converted during generation of
single AAL or ED molecules, while 4eꢀ are converted in
the generation of single AA molecules, as summarized in
Equations (2)–(4).
CH3CH2OH ! CH3CHO þ 2 Hþ þ 2 eꢀ
CH3CH2OH þ H2O ! CH3CHðOHÞ2 þ 2 Hþ þ 2 eꢀ
Angew. Chem. Int. Ed. 2011, 50, 2270 –2274
ð2Þ
ð3Þ
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Communications
Scheme 1. Ethanol electrochemical reaction pathways on Pt-based catalysts, proposed on the basis of our NMR data and previous reports.[2–13,17–20]
See text for detailed explanation. The reaction steps marked with red stars are those in which adsorbed hydroxy groups (OH)ads are involved and
catalytic sites are regenerated due to reaction with (OH)ads. The chemical species highlighted in yellow were observed in this study, and those
highlighted in brown in previous studies using other analytical techniques.
ED and AAL are intermediates to be converted into AA. On
the other hand, AA behaved like a final product in this case.
In fact, in Figure 2 the overall patterns of the increased
amounts of AA, AAL, and ED (similar amounts and leveling
of the enhanced production of AAL and ED at potentials
above 0.2 V, and the wider variation in the enhanced AA
production versus the potential) also suggest that AAL and
ED are not final products but intermediates to be converted
into AA, even in the presence of enhanced reaction activity
due to Sn or Ru.
Pt/C (Figure 3). Comparison of the NMR results and the
electrochemical data indicated that the current density was
For integrated interpretation of the catalytic reactions, the
NMR results were correlated with electrochemical character-
ization of the catalysts, such CO-stripping voltammetry[16] and
linear-sweep voltammetry of ethanol oxidation in Figure 4,
and the unit cell performance (Figure S4, Supporting Infor-
mation). The onset potential of 0.41 V for CO stripping with
PtRu/C catalyst was lower than that of 0.65 V for Pt/C and
was consistent with the enhanced AA production at 0.4 V for
PtRu/C. Compared to the Pt/C and PtRu/C catalysts, the
onset potential and peak width of the Pt3Sn/C catalyst were
lower and broader in the voltammetric data for CO strip-
ping[16] and ethanol oxidation. The current and power also
increased for the Pt3Sn/C catalyst in the unit cell performance
(Figure S4, Supporting Information), resulting in more effi-
cient ethanol oxidation on Pt3Sn/C compared to PtRu/C or
Figure 3. Current generated from each reaction product in the anode
exhaust versus operating potential of fuel cells containing Pt/C,
PtRu/C, or Pt3Sn/C anode catalysts.
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to vary with the anode catalyst and experimental conditions,
such as operating potential and temperature. The major
pathways under any particular conditions can be studied
kinetically and/or by observing intermediates adsorbed on the
catalysts.
The present NMR method, which examined the chemicals
in the liquid exhaust from the anode, could not be used to
detect the chemical species that were adsorbed on the
catalysts or gas-phase products such as CO2, CH4, and
CH3CH3. However, generation of these gas-phase products
was insignificant[4,5,7,10,17] and the products in the liquid
exhaust were in semi-equilibrium with the adsorbed species
on the catalysts. Quantitative analyses of the reaction
products, as demonstrated in this study, will provide informa-
tion that can be used to overcome the hindrances to the
commercialization of fuel cells. The variation parameters for
the analyses include fuel-cell operating conditions such as
potential, fuel concentration, and reagent flow rates. Fuel
cells that are prepared with different components such as
catalysts or electrolyte membranes can also be analyzed. The
quantitative analyses also can supply data for more detailed
theoretical studies of reaction intermediates and products in
the fuel cells.[4,6]
In summary, the dependence of product populations on
potential, measured by NMR spectroscopy, is clearly different
on each catalyst, especially for AA. The increased current of
the DEFCs that were prepared with the Pt3Sn/C or PtRu/C
anode catalyst was mainly due to enhanced production of
AA, which was the greatest for Pt3Sn/C. ED is a major
product detected by NMR spectroscopy in addition to the
previously reported major products AA and AAL. With the
maximum AA population for Pt3Sn/C or PtRu/C, the
potentials were consistent with the onset potentials of CO-
stripping and ethanol-oxidation voltammetry. The different
dependences on potential for AA and ED production suggest
that the water dissociation reaction was not the only major
controlling factor for the AA and ED production, and AAL
and ED behave as intermediates to be converted into AA. All
products that were identified in the liquid exhaust by NMR
spectroscopy in this study and previous reports on ethanol
oxidation led to the proposed ethanol reaction pathways. The
NMR experiments demonstrated here can be carried out in a
much shorter time with probes that are equipped with smaller
NMR coils[21] and better sensitivities. The NMR analyses can
be performed on-line and automated if the fuel cell is adapted
to an LC-NMR-type spectrometer.[22] In terms of hardware,
integrating the NMR analyses with other on-line analyses
such as differential electrochemical mass spectrometry
(DEMS) and in situ FTIR of the reaction products in an
operating fuel cell could be highly desirable to improve
investigation of the fuel-cell reaction mechanism.
Figure 4. CO-stripping voltammograms (top) and linear-sweep voltam-
mograms for ethanol oxidation (bottom) on each catalyst.
enhanced after adding Sn or Ru to Pt/C because of increased
AA production, which was strongly dependent on the
potential and the metal that was added to Pt/C. Over a
wider potential range and at the lower onset potential than on
the other catalysts, the present NMR data for enhanced AA
production on Pt3Sn/C catalyst are consistent with the CO-
stripping and linear-sweep ethanol-oxidation voltammetry
data. The onset potential for the CO-stripping and/or ethanol-
oxidation voltammetry agreed with the potential at which the
enhanced AA population reaches a maximum for Pt3Sn/C or
PtRu/C, and corresponds to the optimal population and
distribution of the oxygenated species, such as OH, that were
adsorbed on the Pt3Sn/C or PtRu/C catalysts for AA
production at the onset potential.
In Scheme 1, in the initial steps of the ethanol adsorption
ꢀ
ꢀ
and oxidation, ethanol is adsorbed on Pt as Pt OCH2 CH3
[6,9,18]
ꢀ
ꢀ
and Pt CHOH CH3, as reported previously.
Once
ꢀ
ꢀ
ꢀ
intermediate species such as Pt CO CH3 and Pt CO are
formed, they are oxidized to CH3COOH or CO2 by reaction
with an oxygen-containing species, such as OH, from the
dissociative adsorption of water on Ru, Sn, and Pt.[5,6,13,19,20]
CO species on Pt were detected by FTIR spectroscopy,[4,6–9]
and some traces of CH4 and CH3CH3 were detected in
previous studies.[4,9,17] In this NMR study, the main products,
AA, AAL, and ED, were detected with minor products such
as CH3COOCH2CH3 and CH3CH(OH)OCH2CH3 produced
in homogeneous reactions. CH3CH(OCH2CH3)2 is expected
to be produced by a homogeneous reaction between ethanol
and CH3CH(OH)OCH2CH3 when the concentration of
CH3CH(OH)OCH2CH3 is high. The ethanol reaction path-
ways in Scheme 1 are proposed by combining all of the above
results. The major pathways from ethanol to AA are expected
Received: September 14, 2010
Revised: November 11, 2010
Published online: February 8, 2011
Keywords: electrochemistry · ethanol oxidation · fuel cells ·
.
NMR spectroscopy · reaction mechanisms
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