5792 Chem. Commun., 2011, 47, 5792–5794 This journal is c The Royal Society of Chemistry 2011

Cite this: Chem. Commun., 2011, 47, 5792–5794

Palladium–nickel alloys loaded on tungsten carbide as platinum-free

anode electrocatalysts for polymer electrolyte membrane fuel cellsw

Dong Jin Ham,ab

Chanho Pak,bGang Hong Bae,

aSuenghoon Han,

aKyungjung Kwon,

b

Seon-Ah Jin,bHyuk Chang,

bSun Hee Choi

cand Jae Sung Lee*

a

Received 8th September 2010, Accepted 28th March 2011

DOI: 10.1039/c0cc03736b

The strong interaction between PdNi alloys and WC makes

PdNi/WC a novel Pt-free electrocatalyst for the anode hydrogen

oxidation reaction of polymer electrolyte membrane fuel cells

with activity and stability comparable to those of the conven-

tional Pt/C catalysts.

A critical barrier in dissemination of polymer electrolyte

membrane fuel cells (PEMFC) is the extreme dependence on

platinum as both anode and cathode electrocatalysts. Platinum

has excellent electrochemical performance and high stability,

yet it is expensive and of limited supply. There are several

promising alternative materials reported recently as cathode

catalysts for oxygen reduction.1 On the anode side for hydrogen

oxidation reaction, however, the research has focussed on

reducing the amount of Pt2 and a few studies on non-Pt anode

electrocatalysts have reported disappointing performance so

far.3 Because the electrocatalysts account for the highest cost

factor for PEMFC stack, it is urgently needed to find new,

high-efficiency and low-cost non-Pt anode electrocatalysts.

Here we propose palladium–nickel alloys in contact with

tungsten carbides (PdNi/WC) as a Pt-free anode catalyst for

PEMFC. Pd has shown a higher exchange current density (i0)

than that of Pt4 for hydrogen oxidation reaction (HOR).

However, the widespread use of Pd has been hampered by

serious dissolution in acidic conditions as found in PEMFC

(pH o 2).5 Thus, satisfactory electrochemical activities and

stabilities have not been reported for membrane electrode

assemblies (MEA) containing Pd electrocatalyst.6 Tungsten

carbides have been known for their resemblance to Pt in

catalytic activity.7 However, it has a very low electrocatalytic

activity relative to Pt despite its desirable properties such as

high CO tolerance and excellent electrochemical stability in

acidic conditions.8 By combining PdNi alloy and WC, we have

achieved excellent performance and stability comparable to

those of Pt catalyst in the anode reaction of PEMFC.

All Pd, PdNi alloys and Pt were loaded on WC or carbon by

the polyol process to obtain 20 wt% precious metal loading.

According to XRD patterns shown in Fig. 1A, WC showed

the tungsten monocarbide phase with sizes >40 nm (Fig. S1,

ESIw). The Pd/WC and PdNi/WC catalysts showed sharp

peaks corresponding to Pd(111) and Pd(200) in addition to

WC peaks.6 The Pd(111) peak was shifted to a higher angle by

0.02–0.051 due to alloy formation when Ni was added (atomic

ratio Pd :Ni = 3 : 1, 3 : 2) as in Fig. 1B for WC and Fig. S2wfor carbon. When Pd, Pd3Ni and Pd3Ni2 were loaded on WC,

the mean particle sizes of Pd and its alloys determined from

HRTEM images were 1.6, 1.8 and 1.9 nm, respectively, and

they showed a fairly uniform dispersion on WC (Fig. 1C and

D). Element mapping by EDAX in Fig. 1C and Fig. S3w showsthat the occurrence of Pd and Ni coincides with W, indicating

the selective deposition of Pd and Ni on WC. The mean

particle sizes of various electrocatalysts determined by

HRTEM are summarized in Table S1w. Pd and its alloys form

nanoparticles of similar sizes (o2 nm) on WC (Fig. 1C and D)

and carbon (Fig. S4w). Considering the much larger surface

Fig. 1 (A) XRD of (a) pure WC, (b) Pd/WC, (c) Pd3Ni/WC and (d)

Pd3Ni2/WC. (B) XRD peak for Pd(111) of (a) Pd/WC, (b) Pd3Ni/WC

and (c) Pd3Ni2/WC. (C) HRTEM dark field images and EDAX

element mapping for Pd, Ni and W of Pd3Ni2/WC. (D) HRTEM

dark field image of Pd3Ni2/WC in high magnification.

aDepartment of Chemical Engineering and Division of AdvancedNuclear Engineering, Pohang Accelerator laboratory (PAL),Pohang University of Science and Technology (POSTECH),San 31, Hyoja-dong, Pohang 790-784, Republic of Korea.E-mail: jlee@postech.ac.kr; Fax: 82 54 279 8299;Tel: 8254 279 2266

b Energy lab., Emerging Technology Center, Samsung AdvancedInstitute of Technology (SAIT), Samsung Electronics Co. LTD,San 14-1, Nongseo-dong, Giheung-gu, Yongin, -si, Gyounggi-Do,446-712, Republic of Korea

c Beamline Research Division, Pohang University of Science andTechnology (POSTECH), San 31, Hyoja-dong, Pohang 790-784,Republic of Koreaw Electronic supplementary information (ESI) available. See DOI:10.1039/c0cc03736b

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area of Vulcan XC-72R carbon than WC (240 vs. 76 m2 g�1),

the formation of similar particle sizes on WC could be taken as

evidence of stronger interaction of Pd and Pd alloys with WC

than with carbon.

Fig. 2A and Fig. S5w show the cyclic voltammograms. Pure

WC and Pd/C showed low electroactivities in the hydrogen

adsorption–desorption region. However, when Pd was

combined with WC, strong redox currents were observed.

Furthermore, introduction of Ni led to enhanced anodic and

cathodic peaks. In addition, Pd3Ni2/WC results in a larger

electrochemical area than that of Pd3Ni/WC in Fig. S5w. All

Pd-containing catalysts showed single peak potentials at

�0.120, �0.119 and �0.119 V (vs. Ag/AgCl) for Pd/WC,

Pd3Ni/WC and Pd3Ni2/WC, respectively. These peak

potentials could show an advantage in oxidizing hydrogen

due to lower over-potentials than those of Pt/C, which show

various anodic current peaks at higher potentials. The electro-

chemically active surface area (EAS) of each catalyst was

evaluated from the peak area of the voltammograms in

Fig. 2A and S5w and the results are presented in Table 1.9

Pd/WC, Pd3Ni/WC and Pd3Ni2/WC showed 62, 72 and 83%

EAS values relative to Pt/C (Johnson-Matthey), the state-of-

the-art anode catalyst. Considering the EAS of lab-made Pt/C

is 86% vs. Pt/C (JM), we can state that the activity of Pd and

PdNi alloys on WC is comparable to that of Pt/C. Carbon-

supported electrocatalysts, Pd/C or Pd3Ni/C, all showed low

activity. This leads us to conclude that an effective synergistic

effect is demonstrated between Pd-based metals and WC for

hydrogen oxidation reaction (HOR).

The impressive HOR activity of PdNi/WC was also

observed in the more practical single cell test. As shown in

Fig. 2B, Pt/C (JM) exhibited the highest maximum power

density (Pmax) of 314 mW cm�2. Relative to this reference

catalyst, Pd/WC, Pd3Ni/WC and Pd3Ni2/WC showed 67, 84

and 89% of the activity for Pt/C (JM). Thus the performance

of Pd3Ni2/WC is better than Pt/C (lab-made), and only slightly

less than the best Pt/C (JM). All carbon supported Pd or Pd

alloy catalysts showed very low activity. Thus again, the

unique synergistic effect between Pd and WC, and the alloy

effect of Pd and Ni have been nicely demonstrated. Note also

that Pt and Pd are equally active on WC, whereas only Pt is

active on carbon.

The stability of the electrocatalysts was probed by comparing

maximum current density from the I–V polarization curve

before and after continuous operation at constant current over

50 h (P50 h). The results are shown in Table 1, Fig. S6 and

Fig. S7w. The PdNi alloys on WC lost 16–17% of their initial

activities during this period. The stability was worse than Pt/C

(JM, 10% loss), but better than those of Pt/C (lab-made, 23%

loss) and Pd/C (33% loss). Thus, WC seems to stabilize Pd

particles on its surface better than carbon support.

The longer term stability of Pd3Ni2/WC was tested by the

V–t galvanostatic method at open circuit voltage (OCV) state

with constant current of 0.1 A cm�2 in the single cell as shown

in Fig. 2C. The OCV of Pd3Ni2/WC was well preserved

without any sign of deactivation for 183 h of operating time,

and the voltage at 0.1 A cm�2 showed a mild maximum value

at 107 h and a slight voltage decrease afterwards (inset of

Fig. 2C). Fig. 2C compares the trend of voltage change for

several electrocatalysts at 0.1 A cm�2 with operation time.

Commercial Pt/C (JM) catalyst displayed excellent durability

including rapid cell activation, and a voltage drop of only

1.4% was observed after 187 h. Pt/C (lab-made) also showed

high initial activity, but a serious voltage drop was seen from

50 h, and drop of 15% was observed after 177 h from the

initial maximum value. For Pd/C, the cell voltage increased to

the maximum value during the initial 75 h, and then, the

voltage was rapidly reduced. Thus it is clear that Pd/C has not

only low activity, but also poor stability. The Pd3Ni2/WC

catalyst showed a comparable long-term stability to Pt/C

(JM), which is even better than Pt/C (lab-made).

The high electrochemical stability of WC based catalysts

was also confirmed by linear sweep voltammetry as shown in

Fig. 2D conducted under the same experimental conditions as

CV including 1 M H2SO4 electrolyte. Pd/C showed anodic

currents from +0.150 V, which belongs to the usual operating

potential range of the anode (o0.406 V). Thus the surface of

Pd on carbon are easily changed to oxygen-containing soluble

species in the operating conditions. However, the oxidation

Fig. 2 (A) CV of (a) Pd3Ni/WC, (b) Pd/WC, (c) Pt/C (JM), (d) Pd/C

and (e) WC. (B) Single cell performances of (a) Pt/C (JM), (b) Pd3Ni2/

WC, (c) Pd3Ni/WC and (d) Pd/WC. (C) V–t curves in the single cell at

0.1 A cm�2 of (a) Pt/C (JM), (b) Pd3Ni2/WC, (c) Pt/C (lab-made) and

(d) Pd/C (inset Pd3Ni2/WC) at (a) open circuit voltage (OCV) and (b)

0.1 A cm�2. (D) Linear sweep voltammetry of (a) Pd3Ni2/WC, (b) Pd/

C (lab-made), (c) Pd3Ni/WC, (d) Pd/WC and (e) WC.

Table 1 Electrochemical activity in half- and single-cell tests andinitial long-term stability in single cells after 50 h

ElectrocatalystHalf cell

Single cell

EASa/m2 g�1 Pmax/mW cm�2 P50 h/mW cm�2

Pt/C (commercial JM) 51.0 314 283Pt/C (lab-made) 43.8 265 205Pt/WC 31.1 224 198Pd3Ni2/WC 42.4 280 232Pd3Ni/WC 36.7 263 221Pd/WC 31.7 211 172Pd3Ni/C 6.4 148 111Pd/C 5.4 114 76Pure WC 2.1 — —

a EAS (electrochemical active surface area) was calculated by

QH/[Pt or Pd] � CML with CML = 210 mC cm�2 for Pd and Pt.

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5794 Chem. Commun., 2011, 47, 5792–5794 This journal is c The Royal Society of Chemistry 2011

potentials of Pd and PdNi alloys on WC were dramatically

shifted to higher potential regions above +0.430 V, which

were out of the applied potential region. The oxidation

potential of WC was also shifted to a higher anodic potential

region by combining with these metals (Fig. S14w); from

+0.373 V (pure WC) to +0.430 V (Pd/WC) or +0.435 V

(PdNi alloy/WC). The stability of WC itself was investigated

by measuring XPS spectra before and after 20 cycles of CV

tests in the range of�0.2 to 0.9 V (vs.Ag/AgCl, 3 MNaCl). As

shown in Fig. S8w, WC loaded with Pd or PdNi alloys did not

show much change in XPS spectra after CV. However, the

XPS spectrum of pure WC changed to that of the oxidized

form (WO3). These results are consistent with the previous

linear sweep voltammetry results related with WC stability.10

XPS did not give any Ni signal (Fig. S12w), probably due to its

low concentration and surface segregation of Pd in PdNi alloy.

However, Ni/WC showed stability in repeated CV (Fig. S13w).These results have demonstrated the electrochemical stability

of WC loaded with Pd or Pd alloy.

As mentioned, a well-known limitation of Pd catalysts as

fuel cell electrodes is its vulnerability to dissolution in acidic

conditions as has been confirmed for Pd/C. Yet, by combining

with WC, the stability of Pd increases dramatically. This could

be attributed to a strong interaction of Pd or Pd alloys with

WC. The selective deposition of Pd and Pd alloys on WC

shown in Fig. 1C and S3w could be considered as evidence of

the strong interaction. From the selective deposition of Pd and

PdNi alloys on WC, an electronic interaction between them is

also expected.11 In X-ray absorption near edge spectroscopy

(XANES) analysis shown in Fig. 3A and B (also in Fig. S9 and

Table S2w), the white-line area of W increased by 4.8, 7.8 and

7.9% for Pd/WC, Pd3Ni/WC and Pd3Ni2/WC, respectively,

relative to pure WC. Since the area is directly related to d-band

vacancy, the results indicate that, when Pd and PdNi alloys

are loaded on WC, electron transfer takes place from the

W d-band to the metals. The transferred electrons could

be related with the enhanced electrocatalytic activities by

affecting the electron density of the partially-filled Pd d-band,

which would affect the kinetics of hydrogen oxidation by

reducing activation and adsorption energies of hydrogen.12

These electrons could contribute to the higher electroactivity

of the metallic Pd than hydroxide or other oxygen containing

Pd phases.13

The electronic state of Pd was probed by XPS in Fig. 3C and

S10w. The peaks at 335.4 and 340.7 eV correspond to Pd 3d5/2and Pd 3d3/2 of metallic Pd (Pd0), respectively, and those at

337.6 and 342.3 eV originate from the Pd 3d5/2 and Pd 3d3/2 of

oxidized Pd (Pd2+).14 The fraction of Pd0 and Pd2+ was

quantified in Table S3w from the XPS peak areas. The metallic

Pd fraction is higher in Pd/WC than Pd/C catalysts, and a

further increase is observed in PdNi alloy/WC. There seems to

be an additional electron transfer from Ni to Pd due to the

difference of electronegativity.14 A good correlation between

the maximum power densities obtained in single cell and the

fraction of metallic Pd0 is nicely demonstrated in Fig. 3D.

Thus, the electron transfer from W to Pd (and from Ni to Pd)

produces a large amount of metallic Pd on WC, which is

responsible for high catalytic activity by reducing adsorption/

dissociation energies for hydrogen oxidation.

In summary, PdNi alloy loaded on tungsten monocarbide

(WC) provides a novel Pt-free electrocatalyst for the anode of

PEMFC. Electrochemical activity and stability comparable to

those of commercial Pt/C catalyst have been demonstrated in

single cell operation. The strong interaction between selec-

tively deposited Pd or PdNi and WC contributes to electron

transfer from W and Ni to Pd, producing metallic Pd0 species,

which is responsible for the high electroactivity and impressive

stability. The excellent properties together with its relatively

inexpensive price and abundance make PdNi/WC a strong

candidate for replacing Pt as an anode catalyst for PEMFC.

This work has been supported by Samsung Electronics Co.

Ltd, RIST/Posco, and Hydrogen R&D Center sponsored by

the Ministry of Education, Science and Technology of Korea.

Notes and references

1 R. Bashyam and P. Zelenay, Nature, 2006, 443, 63.2 H. A. Gasteiger, J. E. Panels and S. G. Yan, J. Power Sources,2004, 127, 162.

3 D. J. Ham and J. S. Lee, Energies, 2009, 2, 873.4 S. A. Grigoriev, E. K. Lyutikova, S. Martemianov andV. N. Fateev, Int. J. Hydrogen Energy, 2007, 32, 4438.

5 S. Uhm, H. J. Lee, Y. Kwon and J. Lee, Angew. Chem., Int. Ed.,2008, 47, 10163.

6 Y. Lee, S. Han and K. Park, Electrochem. Commun., 2009, 11,1968.

7 R. B. Levy and M. Boudart, Science, 1973, 181, 547.8 D. J. Ham, Y. K. Kim, S. H. Han and J. S. Lee, Catal. Today,2008, 132, 117.

9 (a) B. I. Podlovchenko and E. A. Kolyadko, J. Electroanal. Chem.,1987, 224, 225; (b) Z. Bai, L. Yang, L. Li, J. Lv, K. Wang andJ. Zhang, J. Phys. Chem. C, 2009, 113, 10568.

10 E. C. Weigert, A. L. Stottlemyer, M. B. Zellner and J. G. Chen,J. Phys. Chem. C, 2007, 111, 14617.

11 M. P. Humbert, C. A. Menning and J. G. Chen, J. Catal., 2010,271, 132.

12 (a) T. Ghosh, M. B. Vukmirovic, F. J. Disalvo and R. R. Adzic,J. Am. Chem. Soc., 2010, 132, 906; (b) S. J. Yoo, H. Park, T. Jeon,I. Park, Y. Cho and Y. Sung, Angew. Chem., Int. Ed., 2008, 47,9307.

13 Y. Suo and I. Hsing, Electrochim. Acta, 2009, 55, 210.14 (a) S. Y. Shen, T. S. Zhao, J. B. Xu and Y. S. Li, J. Power Sources,

2010, 195, 1001; (b) Y. Diao, R. Yan, S. Zhang, P. Yang, Z. Li,L. Wang and H. Dong, J. Mol. Catal. A: Chem., 2009, 303, 35.

Fig. 3 (A), (B) X-Ray absorption edges of (a) pure WC, (b) Pd/WC

and (c) Pd3Ni/WC. (C) The XPS results for Pd 3d of (a) Pd/C, (b)

Pd3Ni/C, (c) Pd/WC, (d) Pd3Ni/WC and (e) Pd3Ni2/WC. (D) The

correlation between maximum power densities and proportions of

metallic Pd (Pd0).

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