Tungsten-based carbides as anode for intermediate-temperature fuel cells

Article abstract of DOI:10.1149/1.3604780

Tungsten carbides were prepared from ammonium paratungstate via temperature-programmed carburization under flowing a gaseous mixture of CH ₄H₂ to employ as anode catalysts in fuel cells consisting of CsH₂PO₄SiP₂O₇-based composite electrolyte operative at 200C. The resulting materials were characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The heat-treatment at high temperatures promoted the reduction and carburization of tungsten component. The single phase of WC was observed for the samples subjected to the carburization at and above 800°C. The single cell employing the catalyst prepared at 850C attained the best performance. The anode material containing the metallic W exhibited low stability under the power generation condition. With nickel or cobalt additives, the carburization of tungsten species was initiated at low temperatures. The samples with the additives heat-treated at high temperatures were composed of several tungsten carbides including WC. When these samples were applied as anode catalysts, the additive species lowered the cell performance. These results indicated that the WC phase was the most effective electrocatalyst for the hydrogen oxidation.

Full text of DOI:10.1149/1.3604780

B1072
Journal of The Electrochemical Society, 158 (9) B1072-B1075 (2011)
0013-4651/2011/158(9)/B1072/4/$28.00 The Electrochemical Society
C
V
Tungsten-based Carbides as Anode for Intermediate-
Temperature Fuel Cells
,z
Hiroki Muroyama, Koji Katsukawa, Toshiaki Matsui, and Koichi Eguchi
*
*
*
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku,
Kyoto 615-8510, Japan
Tungsten carbides were prepared from ammonium paratungstate via temperature-programmed carburization under flowing a gase-
ous mixture of CH4=H2 to employ as anode catalysts in fuel cells consisting of CsH2PO4=SiP2O7-based composite electrolyte
operative at 200ꢀC. The resulting materials were characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The
heat-treatment at high temperatures promoted the reduction and carburization of tungsten component. The single phase of WC
was observed for the samples subjected to the carburization at and above 800ꢀC. The single cell employing the catalyst prepared
at 850ꢀC attained the best performance. The anode material containing the metallic W exhibited low stability under the power
generation condition. With nickel or cobalt additives, the carburization of tungsten species was initiated at low temperatures. The
samples with the additives heat-treated at high temperatures were composed of several tungsten carbides including WC. When
these samples were applied as anode catalysts, the additive species lowered the cell performance. These results indicated that the
WC phase was the most effective electrocatalyst for the hydrogen oxidation.
C
V
2011 The Electrochemical Society. [DOI: 10.1149/1.3604780] All rights reserved.
Manuscript submitted April 7, 2011; revised manuscript received June 3, 2011. Published July 1, 2011.
Fuel cells are one of the promising energy conversion devices
due to their high efficiency and low emissions. Recently, polymer
electrolyte fuel cells (PEFCs) have attracted a great deal of attention
for stationary and transportation applications, and have been devel-
oped for commercialization. Operating temperature of PEFCs is
limited up to 100ꢀC because water plays an important role in proton
conduction in perfluorosulfonic acid membrane electrolytes. Such
low operating temperatures induce some issues, e.g. CO poisoning
of Pt electrocatalysts, and complicated fuel processing and water
management. Thus, electrolytes operating above 100ꢀC under low
humidified conditions are expected to overcome these problems,
and considerable efforts have been directed for the development of
such electrolyte materials.1–8
We have reported a new proton-conductive electrolyte of
CsH2PO4=SiP2O7-based composite. The CsH2PO4=SiP2O7 compos-
ite with a molar ratio of 1=2 exhibited 44 mS cmꢁ1 at 266ꢀC under
30% H2O=Ar atmosphere.3 In this composite, the solid acid of
CsH2PO4 reacted with a part of SiP2O7 at the contacting interface to
form CsH5(PO4)2, which served as a new proton-conductive phase.
Fuel cell employing this composite electrolyte was operated suc-
cessfully at around 200ꢀC, indicating its high stability under fuel
cell operating conditions.6 For such fuel cells, the platinum catalyst
supported on carbon (Pt=C) was used as an electrocatalyst as for
PEFCs.
programmed carburization and evaluated their catalytic performance
as the anode in fuel cells operating at 200ꢀC. Temperature-
programmed carburization is known as a suitable process for the
preparation of the carbide compounds with high surface area.17,22 In
addition, additive effects of Ni and Co components on the carburiza-
tion reaction and the catalytic activity of tungsten-based carbides
were also investigated.
Experimental
Tungsten carbide was obtained by temperature-programmed reac-
tion as follows.22 Ammonium paratungstate, (NH4)10W12O41ꢂ5H2O,
(APT, Ardrich) was loaded into an alumina boat and placed into a
quartz tube inside a furnace. Temperature was raised up to 200ꢀC with
a supply of pure nitrogen and subsequently up to target temperatuꢁre1s
of 650–900ꢀC in 50% CH4=H2 with a heating rate of 0.2ꢀC min
.
The sample was heat-treated at the desired temperature for 5 h and
then exposed to 2% O2=N2 for 6 h after cooling to room tempera-
ture. The resultant tungsten compound was denoted as APT-T
(T ¼ 650–900), which was prepared by the heat-treatment at TꢀC. A
mixture of carbon black (Vulcan XC72R) and the obtained tungsten
compound in a weight ratio of 1.5:1 was prepared for electrochemi-
cal measurements and represented as APT-T=C (T ¼ 650–900).
Commercial WC (Aldrich) mixed with carbon black was also used
as an electrocatalyst and denoted as commercial WC=C.
Many studies on development of electrode materials with low Pt
loading or without Pt metal have been extensively conducted so far
due to high cost of noble metals. Among them, carbides of transition
metals have been paid attention as prospective alternative electroca-
talysts, since molybdenum and tungsten carbides showed catalytic
activity comparable to noble metals for several reactions.9–13 More-
over, considerable efforts have been devoted to improving insuffi-
cient catalytic activity of these materials for hydrogen oxidation.
Several preparation methods have been tried to enlarge the catalytic
surface area of carbide compounds.14–22 Addition of other metal
species to molybdenum carbide or tungsten carbide has been also
studied to enhance the catalytic activity. The hydrogen oxidation
was facilitated by the formation of oxycarbide species in cobalt-
tungsten carbide.23 These approaches, however, were less effective
for the enhancement of catalytic activity. These carbide materials
have been examined in the low temperature range below 100ꢀC,
while the use of carbides at higher temperature can be considered as
a possible factor for the promotion of catalytic reactions. In this
study, therefore, we prepared tungsten compounds via temperature-
For the synthesis of tungsten carbide containing nickel or cobalt
species, Ni(NO3)2ꢂ6H2O or Co(NO3)2ꢂ6H2O and APT were used as
starting materials. The metal nitrate hydrate and ATP were dis-
solved in distilled water. The carbon black was also added to the so-
lution in a weight ratio of 1.5:1 to total metal components (W, Ni,
Co). This solution was stirred during evaporation to dryness. The
resulting precursor was carburized by temperature-programmed
reaction as mentioned above. APTMxC-T (M ¼ Ni, Co; x ¼ 0.01,
0.5; T ¼ 650, 750, 850) denotes tungsten compound containing M
component with a molar ratio of 1:x, which was heat-treated at TꢀC.
Crystal structure of the samples was analyzed by X-ray diffrac-
tion (XRD, Rigaku, Rint 1400 X-ray diffractometer). Crystallite
size of the tungsten compounds was evaluated by Sherrer equation.
Specific surface area was measured by BET method with N2 adsorp-
tion (Shimadzu, Gemini 2375). Binding energy of W 4f for the sam-
ples was obtained by X-ray photoelectron spectroscopy (XPS) with
Mg Ka radiation (Shimadzu, ESCA-3400). Ag 3d electron binding
energy corresponding to metallic silver was referenced at 368.3 eV
for calibration.
Performance of a single cell employing the resultant electrocata-
lyst (APT-T=C, APTMxC-T, and commercial WC=C) was evaluated
at 200ꢀC. For the fabrication of anode, the resultant catalyst was
*
Electrochemical Society Active Member.
z E-mail: muroyama.hiroki.5c@kyoto-u.ac.jp
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Journal of The Electrochemical Society, 158 (9) B1072-B1075 (2011)
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ultrasonically dispersed in 1-methyl-2-pyrrolidone (Wako Pure
Chemical Industries) to form paste. Subsequently, the obtained paste
was applied on teflon-coated carbon paper (Toray) and heated to
dryness. The loading of electrocatalyst was fixed at 2.5 mg cmꢁ2. A
commercial Pt=C supported on carbon paper (BASF Fuel Cell, Inc.,
phosphoric acid fuel cell, Pt loading: 1.0 mg cmꢁ2) was used for
cathode. The composite electrolyte of CsH2PO4=SiP2O7 was pre-
pared as mentioned in previous reports.3,6 Membrane electrode as-
sembly (MEA) was fabricated by uniaxial pressing of the composite
powder with electrodes (diameter: 13 mm, thickness: ca. 1.3 mm,
electrode area: 0.283 cm2). Then, the resulting MEA was heat-
treated at 220ꢀC for 1 h and placed in apparatus. Gaseous mixtures
of 30% H2O=H2 and 30% H2O=O2 were supplied to anode and cath-
ode, respectively, with a flow rate of 50 ml minꢁ1. A current-voltage
(I-V) characteristic was examined using a potentiostat (Solartron
1287 potentiostat). For a single cell employing APT-T=C, the I-V
curves were iteratively recorded to evaluate the stability of catalysts.
After the first I-V measurement (1st), the single cell was held under
30% H2O=N2 atmosphere for 1 h, and subsequently the measure-
ment was carried out again (2nd). This sequence of operation was
repeated in some cases.
formation of metallic W. Upon heating beyond this temperature, a
weight gain was observed corresponding to the carburization of
tungsten species and the deposition of graphitic carbon. Diffraction
peaks of deposited carbon were not observed in the XRD pattern of
all samples in the present study.
XPS measurements were carried out to examine electronic state
of tungsten. The XPS spectra of W 4f for APT-T (T ¼ 650–800) are
shown in Fig. 2. The spectrum of APT-650 consisted of three peaks
at 32.8, 35.4, and 37.7 eV, which was quite analogous to that of
bulk WO3 reduced at 500ꢀC in hydrogen.24 This report indicates
that tungsten species were in the oxidation states of WO3, W2O5,
and WO2. Considering the diffraction pattern was identical to that
of WO2 phase, the surface of APT-650 was partially oxidized. The
APT-700 sample exhibited four specific peaks at 31.3, 33.4, 35.6,
and 37.8 eV attributable to W(0) 4f7=2, W(0) 4f5=2, W(VI) 4f7=2, and
W(VI) 4f5=2, respectively.24–26 This result revealed the existence of
WO3 as well as metallic W on the surface. For the samples heat-
treated at 750–900ꢀC in a gaseous mixture of CH4=H2, two noticea-
ble peaks corresponding to tungsten carbide were observed in the
ranges of 31.6–31.7 and 33.7–33.8 eV. Judging from the broad peak
at 36.5–38.7 eV in the spectra of these samples, tungsten species in
the oxidation state was present on the surface. The tungsten oxide
might be ascribable to partial oxidation of the tungsten carbide by
the exposure to diluted oxygen gas at the final processing stage.
Figure 3 shows the I-V characteristics of a single cell employing
the resultant electrocatalyst at 200ꢀC. The measurement was repeated
for the evaluation of the stability. For all cells, open circuit voltage
achieved (0.87–0.98 V) was comparable to that for the system using
CsH2PO4=SiP2O7 composite electrolyte with Pt=C electrodes under
the same condition reported previously.6 The single cell with
APT-850=C exhibited the best performance among the samples
investigated. Thus, the WC catalyst was more active for the hydrogen
oxidation than WO2 and W, which were main tungsten species in
APT-650=C and APT-700=C catalysts, respectively. The heat-treat-
ment at 750ꢀC under a CH4=H2 atmosphere led to insufficient carbu-
rization of tungsten component, resulting in the slightly lower
performance at 1st measurement as compared with the heat-treatment
at 850ꢀC. The APT-900=C catalyst did not attain the activity level of
the APT-850=C, although the WC phase was observed as the major
one in these catalysts. The carburization at higher temperature is
expected to promote agglomeration of WC particle and concomitant
carbon deposition over the catalyst. For the single cell employing
Results and Discussion
Influence of carburization temperature on the properties of tung-
sten compounds.— The XRD patterns of the tungsten compounds
synthesized by the heat-treatment of APT with a supply of 50%
CH4=H2 are shown in Fig. 1. Single phases of WO2 and metallic W
were observed in the diffraction patterns of APT-650 and APT-700,
respectively. As the heat-treatment temperature was further raised,
tungsten carbide species such as WC and W2C were formed. The
diffraction pattern of APT-750 suggested the presence of W as well
as W2C and WC, whereas the samples heat-treated at and above
800ꢀC were composed of single phase of WC. This change in tung-
sten compounds approximately agreed with the result of thermogra-
vimetric analysis reported previously.22 According to this literature,
the starting material of APT was decomposed to WO3 at 350ꢀC,
accompanied with evolution of water and ammonia. With elevating
temperature under 50% CH4=H2 atmosphere, WO3 was gradually
reduced to WO2 in the range of 660–690ꢀC. Subsequently, the TG
profile exhibited a maximum weight loss at 760ꢀC, indicating the
Figure 1. XRD patterns of (a) APT-650, (b) APT-700, (c) APT-750, (d)
APT-800, (e) APT-850, and (f) APT-900.
Figure 2. XPS spectra of W 4f for (a) APT-650, (b) APT-700, (c) APT-750,
and (d) APT-800.
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Journal of The Electrochemical Society, 158 (9) B1072-B1075 (2011)
Figure 4. XRD patterns of (a) APTNi0.01C-650, (b) APTNi0.01C-750,
(c) APTNi0.5C-650, (d) APTNi0.5C-850, (e) APTCo0.5C-650, and (f)
APTCo0.5C-850.
phases of metallic W and tungsten carbide were detected regardless
of the additive species, while the sample without additives exhibited
a single phase of WO2 as shown in Fig. 1a. Moreover, as can be
seen in Figs. 4a and 4c, an increase in the amount of additives led to
rapid carburization of the tungsten component. These results indi-
cated that the addition of nickel and cobalt facilitated the reduction
and carburization reactions. The samples heat-treated at 750 and
850ꢀC were composed of W2C as well as WC despite the heat-treat-
ment at relatively high temperatures. Considering graphitic carbon
was deposited at 850ꢀC over the samples with the large amount of
nickel or cobalt, methane should be decomposed to carbon by the
additive components before the reaction with tungsten species.
Thus, the carburization of tungsten species to form WC could not
proceed completely at high temperatures. The XRD patterns also
clarified that the added nickel and cobalt species existed as metallic
Ni and Co3W3C, respectively. Hereafter, when the characteristics of
the samples with additives were evaluated, the APTM0.01C-650
(M ¼ Ni, Co) catalysts containing metallic W were excluded due to
instability in the fuel cell operating condition as mention above.
The crystallite size of the tungsten carbide observed in Fig. 4
was calculated by Sherrer equation as summarized in Table I. The
crystallite size tended to increase either with increasing heat-treat-
ment temperature or with increasing amount of additive component.
The grain growth was more noticeable for nickel than cobalt.
Electronic state of tungsten, nickel, and cobalt elements was
examined by XPS measurement. No specific peaks were detected in
Ni 2p and Co 2p spectra due to a small amount of these components.
The W 4f spectra of all samples showed definite and broad peaks
attributed to carbide and oxide species, respectively. Difference
between the spectra of the sample with and without additive species
was not remarkable.
Figure 3. I-V characteristics of a single cell employing (a) APT-650=C,
(b) APT-700=C, (c) APT-750=C, (d) APT-850=C, and (e) APT-900=C at
200 ꢀC.
APT-850=C, the maximum power density was 4.1 mW cmꢁ2, which
was 7.3% of that in the case of Pt=C anode (56 mW cmꢁ2).6 As a
result, the WC catalyst was much inferior in the activity to the Pt cat-
alyst even at 200ꢀC. According to the previous report, electrochemi-
cal measurement at 50ꢀC revealed that hydrogen adsorption on WC
surface was rate-determined by slow dissociation of hydrogen mole-
cule.10,18 Thus, it was considered that the high operating temperature
of 200ꢀC could not significantly enhance the rate of this step. Note
that the cell performance decreased during the consecutive measure-
ment for APT-700=C and APT-750=C containing metallic W. The
proton conductive phase of CsH5(PO4)2 in the electrolyte was in the
molten state under the operating condition. Considering the electroca-
talysts was exposed to the acidic molten salt, the corrosion of W
metal by the electrolyte should be responsible for the deterioration of
performance. On the other hand, the single cells exhibited highly-re-
producible I-V curves for APT-650=C, APT-850=C, and APT-900=C.
This result clarified that the electrocatalysts consisting of WO2 and
WC were stable under this operating condition. Consequently, the
WC phase was the most favorable among tungsten compounds with
respect to the activity for hydrogen oxidation and the stability in
acidic condition at 200ꢀC.
Additive effect of nickel and cobalt species on the properties of
tungsten-based compounds.— The typical XRD patterns of the tung-
sten-based compounds containing nickel or cobalt are shown in
Fig. 4. For both systems with additives, the diffraction patterns
revealed that reduction and carburization of tungsten species were
promoted with increasing heat-treatment temperature, as in APT-T
samples. In the case of the heat-treatment at 650ꢀC, crystalline
The observed crystalline phases and maximum power density of
the single cell for the samples are also summarized in Table I. The
result of APT-850=C is shown for comparison. The maximum
power density for all electrocatalysts investigated was not specifi-
cally related with the crystallite size of the major phase. The cell
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Journal of The Electrochemical Society, 158 (9) B1072-B1075 (2011)
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electrolyte and these carbide catalysts for anode at 200ꢀC. X-ray dif-
fraction and X-ray photoelectron spectroscopy revealed that reduc-
tion and carburization of tungsten species proceeded readily during
heat-treatment at higher temperatures. The additive nickel or cobalt
component promoted the formation of tungsten carbide species, and
mixtures of tungsten-based carbide phases were confirmed in the
samples with additives. The best single cell performance was
achieved by using APT-850=C consisting of WC. Accordingly, the
key factor for high catalytic activity is the fabrication of single-
phase WC.
The current density for a single cell using APT-850=C as an an-
ode electrocatalyst was lower than that for the conventional Pt=C.
However, the obtained APT-850 was more active than commercial
WC sample for hydrogen oxidation due to higher surface area and
smaller crystallite size. Consequently, it is important to synthesize
fine and highly-dispersed WC particles for the enhancement of cata-
lytic activity.
Table I. Crystallite size and phase, and maximum power density
of single cell for APTMxC-T (M 5 Ni, Co; x 5 0.01, 0.5; T 5 650–
850) and APT-850=C.
Maximum power
Crystallite
density=mW cmꢁ2
Sample
sizea=nm
Crystalline phase
APTNi0.01C-750 15 (W2C)
APTNi0.01C-850 16 (WC)
APTNi0.5C-650 19 (W2C)
APTNi0.5C-750 20 (W2C)
APTNi0.5C-850 24 (WC)
APTCo0.01C-750 12 (W2C)
APTCo0.01C-850 12 (WC)
W2C, WC
W2C, WC
1.6
1.7
W2C, WC, Ni
W2C, WC, Ni, C
W2C, WC, Ni, C
W2C, WC
0.27
0.50
0.58
1.9
1.8
1.6
0.42
0.31
4.1
W2C, WC
APTCo0.5C-650 12 (W2C) W2C, WC, Co3W3C
APTCo0.5C-750 13 (W2C) W2C, WC, Co3W3C, C
APTCo0.5C-850 19 (WC) W2C, WC, Co3W3C, C
APT-850/C
12 (WC)
WC
aThis value was calculated from the strongest diffraction peak, which
was attributed to the compound in parenthesis.
Acknowledgment
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science
and Technology, Japan.
performance for the APTM0.01C-T (M ¼ Ni, Co, T ¼ 750, 850) sam-
ples was comparable despite the heat-treatment temperature and the
additive species. The increase in amount of the additive components
lowered power density. These additives promoted the formation of
carbon, which covered active sites of the catalyst. In addition, it was
noted that the cell performance for APTCo0.5C-750 and APT-
Co0.5C-850 was much lower than that for APTCo0.5C-650. Consid-
ering the peak intensity of Co3W3C phase increased with the heat-
treatment temperature as shown in Fig. 4, the catalytic activity of
this compound should be low for hydrogen oxidation. The maxi-
mum power density for all samples containing the additive species
was not as high as that of APT-850=C. The additives gave rise to
the formation of W2C and Co3W3C as well as WC, while APT-
850=C was composed of the single phase of WC. Thus, it could be
concluded that the WC phase was the most effective for hydrogen
oxidation among the tungsten carbide species observed in the pres-
ent study. This conclusion agreed with the previous literature, which
mentioned that W2C was less active than WC for electrochemical
hydrogen oxidation in a H2SO4 solution at room temperature.18
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Conclusions
We prepared tungsten-based compounds from ammonium
paratungstate via temperature-programmed carburization. Fuel cell
performance was evaluated by employing the CsH2PO4=SiP2O7
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