An effective Pd-Ni2P/C anode catalyst for direct formic acid fuel cells

Article abstract of DOI:10.1002/anie.201308620

The direct formic acid fuel cell is an emerging energy conversion device for which palladium is considered as the state-of-the-art anode catalyst. In this communication, we show that the activity and stability of palladium for formic acid oxidation can be

Full text of DOI:10.1002/anie.201308620

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Angewandte
Communications
DOI: 10.1002/anie.201308620
Electrocatalysis
An Effective Pd–Ni2P/C Anode Catalyst for Direct
Formic Acid Fuel Cells**
Jinfa Chang, Ligang Feng, Changpeng Liu, Wei Xing,* and Xile Hu*
Angewandte
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Abstract: The direct formic acid fuel cell is an emerging energy
conversion device for which palladium is considered as the
state-of-the-art anode catalyst. In this communication, we show
that the activity and stability of palladium for formic acid
oxidation can be significantly enhanced using nickel phosphide
(Ni2P) nanoparticles as a cocatalyst. X-ray photoelectron
spectroscopy (XPS) reveals a strong electronic interaction
between Ni2P and Pd. A direct formic acid fuel cell incorpo-
rating the best Pd–Ni2P anode catalyst exhibits a power density
of 550 mWcmÀ2, which is 3.5 times of that of an analogous
device using a commercial Pd anode catalyst.
assisted ethylene glycol reduction method (see the Supporting
Information for details). The Raman spectra of Ni2P/C
particles display two very distinctive D and G bands at 1331
and 1589 cmÀ1, respectively (Supporting Information, Fig-
ure S1). The D band arises from structural defects in the
graphitic plane, whereas the G band is related to the E2g
vibrational mode of the sp2-bonded graphitic carbons. In the
XRD pattern of the Ni2P/C, the diffraction peaks of Ni2P are
visible (Figure S2). These peaks are not observed in the XRD
pattern of Pd–Ni2P/C; however, the presence of Ni2P was
confirmed by EDS (Figure S3g) and element distribution
maps (Figure S3a–f). Typical TEM images of Ni2P/C and Pd–
Ni2P/C (30 wt% of Ni2P on C) are shown in Figure 1. The
D
irect formic acid fuel cells are considered to be a promising
power source for portable electronic devices. The develop-
ment of active anode catalysts for the oxidation of formic acid
is therefore an active area of research.[1] Pd-based catalysts
have recently drawn attention because they can catalyze the
oxidation of formic acid by a direct path that reduces the
poisoning effect associated with conventional Pt catalysts.[2]
Increasing the activity of Pd can lead to a lower usage of this
rare and costly metal. Several nanostructured Pd catalysts
exhibited a high performance for formic acid oxidation;
however, these catalysts were synthesized under harsh con-
ditions and required cumbersome post-treatments.[2b,3] The
addition of Ni, Co, Fe, P, or N is reported to enhance the
catalytic activity of Pd.[4] Unfortunately, the dissolution or
instability of the promoter elements results in the rapid decay
of catalytic performance.
Herein, we demonstrate that Ni2P can act as a stable
cocatalyst for Pd-catalyzed formic acid oxidation. The Pd–
Ni2P/C anode system shows remarkable catalytic activity and
stability. When integrated in a direct formic acid fuel cell, the
hybrid catalyst gives power density and discharge stability
superior to several state-of-the-art catalysts.
Figure 1. a) HRTEM image of Ni2P/C where the (111) lattice of Ni2P
can be observed, and the carbon support is visible. b) TEM image of
the Pd–Ni2P/C catalyst. c) HRTEM image of Pd–Ni2P/C catalyst; both
Pd(111) and Ni2P(111) lattices can be observed, and the carbon
support is visible. d) Size distribution of the Pd–Ni2P/C catalyst. Scale
bars in (a) and (c) are 5 mm, scale bar in (b) is 50 nm.
The Ni2P/C particles were synthesized by a solid phase
reaction. Pd was deposited onto Ni2P/C by a microwave-
[*] J. F. Chang,[+] Prof. Dr. C. Liu, Prof. Dr. W. Xing
State Key Laboratory of Electroanalytical Chemistry, Laboratory of
Advanced Power Sources, Changchun Institute of Applied Chemis-
try, Chinese Academy of Sciences, Changchun 130022 (P.R. China)
E-mail: xingwei@ciac.jl.cn
Dr. L. G. Feng,[+] Prof. Dr. X. L. Hu
Institute of Chemical Sciences and Engineering, Ecole Polytech-
nique Fꢀdꢀrale de Lausanne (EPFL)
ISIC-LSCI, BCH 3305, Lausanne 1015 (Switzerland)
E-mail: xile.hu@epfl.ch
Homepage: http://lsci.epfl.ch
Ni2P nanoparticles can be observed with a finger lattice of
0.221 nm, which corresponds to its (111) lattice (Figure 1a).[5]
After deposition of Pd, the Pd nanoparticles were uniformly
distributed on the Ni2P/C hybrid support with a narrow size
distribution; the average particle size of Pd is about 3.5 nm
(Figure 1b,d), which is an optimal particle size for formic acid
oxidation.[1g,2e,6] Finger lattices of Ni2P and Pd can both be
observed (Figure 1c). The particle size of Pd in other samples
where the wt% of Ni2P on C varied was also about 3.5 nm
(Figure S4).
Typical Pd electrochemical behaviors for all Pd–Ni2P/C
samples were observed in H2SO4 (0.5m; Figure S5). The Pd–
Ni2P/C (30 wt% of Ni2P on C) catalyst shows the largest
electrochemical surface area (ECSA), according to the area
of the hydrogen desorption peaks (Table S1). The more
accurate ECSA obtained from CO-stripping experiments
(Figure S6) were used to calculate the specific activity for all
catalysts. The peak potential of the adsorbed CO is commonly
[+] These authors contributed equally to this work.
[**] The work in CAS is supported by the National Basic Research
Program of China (973 Program, 2012CB215500, 2012CB932800),
the National High Technology Research and Development Program
of China (863 Program, 2012AA053401), the Recruitment Program
of Foreign Experts (WQ20122200077), the National Natural Science
Foundation of China (20933004, 21073180) and the Strategic
priority research program of CAS (XDA0903104). The work at EPFL
is supported by a grant from the Competence Center for Energy and
Mobility (CCEM) in the framework of the Hytech project.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308620.
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used as a tool to compare the antipoisoning ability.[1b,e] The
Pd–Ni2P/C (30 wt% Ni2P) catalyst has the most negative peak
potential (Table S1), thus indicating that this material has the
best anti-poisoning ability.
The activities of Pd–Ni2P/C catalysts with different weight
percentages of Ni2P were compared (see Figure 2 for mass
Figure 3. Linear sweep voltammograms of Pd–Ni2P-30%, PdNi/C,
PdP/C, Pd/C–C and Pd/C–H in H2SO4 solution (0.5m) containing
HCOOH (0.5m) at 50 mVsÀ1
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(Table S4). The stability of catalysts was probed by chro-
noamperometric measurements with stationary and rotating
disk electrodes (Figure S13). The Pd–Ni2P/C-30% catalyst
exhibits the best stability for formic acid oxidation, with the
highest stable currents.
Figure 2. Cyclic voltammograms of Ni2P and Pd–Ni2P with different
amounts of Ni2P in H2SO4 solution (0.5m) containing HCOOH (0.5m)
at 50 mVsÀ1
.
XPS was applied to probe the origin of the promotional
effect of Ni2P. From Pd to Pd–Ni2P/C (Figure 4), the Pd 3d
peaks are shifted significantly, by about 1 eV, to a lower
binding energy. This shift might be attributed to an partial
electron transfer from Ni2P to Pd; this would increase the
electron density of Pd and enhance the penetration of outer-
layer electrons to the inner layer.[7] This results in higher
shielding of the nuclear charge and weakens the binding of 3d
electrons. On the other hand, the Pd 3d peaks are not
substantially shifted from Pd to PdNi/C and PdP/C. These
results suggest that the enhanced activity and stability of Pd–
Ni2P/C might be attributed to a strong electronic interaction
between Pd and Ni2P. In addition to the electronic effect, Ni2P
might directly participate in formic acid oxidation. Recently,
Ni2P was found to be an effective hydrogen evolution
catalyst.[8] Hydrogen adsorption might be facile on Ni2P,
activity; see also Figure S7 for specific activity and Table S2
for an overall comparison). Ni2P alone has almost no catalytic
activity, but it has a strong influence on the activity of Pd. The
optimized loading is 30% Ni2P on C. The Pd–Ni2P/C-30%
catalyst has the best catalytic stability, based on chronoam-
perometric (CA) measurements (Figure S8). Electrochemical
impedance spectroscopy (EIS) and X-ray photoelectron
spectroscopy (XPS) were applied to investigate the influence
of Ni2P loading. Based on EIS analysis (Figure S9), it was
hypothesized that more formic acid is oxidized around the
peak potential of the Pd–Ni2P/C-30% catalyst, probably
owing to reduced passivation at Pd. In the XPS spectra, a shift
in the Pd binding energy was observed (see below); this shift
was the same in all samples, regardless of Ni2P loading
(Figure S10). Thus, the optimal loading of Ni2P likely arises
from a balanced interaction between Pd and Ni2P, rather than
simply from an optimal electronic effect, which would have
been detected by XPS.
The Pd–Ni2P/C-30% catalyst was compared with the
state-of-the-art commercial Pd/C (Pd/C–C) catalyst, home-
made Pd/C (Pd/C–H), PdNi/C and PdP/C catalysts by linear
sweep voltammetry (see Figure 3 for mass activity; see also
Figure S11 for specific activity). Among all these catalysts,
Pd–Ni2P/C-30% exhibits the best activity for formic acid
oxidation. PdP/C and PdNi/C performed slightly better than
Pd/C–H, but are largely inferior to Pd–Ni2P/C. The promo-
tional effect of the Ni2P is therefore much larger than P or Ni
alone. The Tafel slopes of these catalysts were also compared
(Figure S12), and Pd–Ni2P/C-30% has the smallest Tafle
slope (Table S3), which is advantageous for practical appli-
cations. Furthermore, the specific activity of Pd–Ni2P/C-30%
compares favorably with other recently reported catalysts
Figure 4. XPS spectra of the Pd 3d region for Pd–Ni2P/C-30%, PdNi/C,
PdP/C, and Pd/C–H.
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To demonstrate the potential of the Pd–Ni2P/C catalyst in
a direct formic acid fuel cell, the catalyst was integrated at the
anode of a homemade fuel cell. The steady-state polarization
and power-density curves of several catalysts were compared
(Figure 5a,b). Consistent with the results from electrochem-
ical measurements, the Pd–Ni2P/C-30% catalyst exhibits the
highest power density. This power density of 550 mWcmÀ2 is
about 2 times of that of PdNi/C, 3 times of that of PdP/C, and
3.5 times of that of a state-of-the-art Pd/C commercial
catalyst. Moreover, the Pd–Ni2P/C catalyst also shows the
most stable discharge ability at 0.35 V (Figure 5c). These
results confirm the promising activity of Pd–Ni2P/C for direct
formic acid fuel cell.
In summary, a novel Pd–Ni2P/C electrocatalyst was
developed for formic acid oxidation. The catalyst exhibited
excellent activity and stability, and was successfully integrated
into a direct formic acid fuel cell, showing superior perfor-
mance to a state-of-the-art Pd/C catalyst. The work is
a significant step towards the development of more active
and practical catalysts for direct formic acid fuel cells.
Received: October 3, 2013
Published online: November 25, 2013
Keywords: electrocatalysis · formic acid · fuel cells · oxidation ·
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nickel
[1] a) Y. X. Chen, M. Heinen, Z. Jusys, R. J. Behm, Angew. Chem.
2006, 118, 995 – 1000; Angew. Chem. Int. Ed. 2006, 45, 981 – 985;
b) W. Gao, J. E. Mueller, Q. Jiang, T. Jacob, Angew. Chem. 2012,
124, 9584 – 9589; Angew. Chem. Int. Ed. 2012, 51, 9448 – 9452;
c) F. J. Vidal-Iglesias, A. Lꢀpez-Cudero, J. Solla-Gullꢀn, J. M.
Feliu, Angew. Chem. 2012, 124, 1 – 1; Angew. Chem. Int. Ed. 2012,
51, 1 – 5; d) F. J. Vidal-Iglesias, J. Solla-Gullꢀn, E. Herrero, A.
Aldaz, J. M. Feliu, Angew. Chem. 2010, 122, 7152 – 7155; Angew.
Chem. Int. Ed. 2010, 49, 6998 – 7001; e) S. Zhang, Y. Shao, G. Yin,
Y. Lin, Angew. Chem. 2010, 122, 2257 – 2260; Angew. Chem. Int.
Ed. 2010, 49, 2211 – 2214; f) D. Chen, Z. Zhou, Q. Wang, D. Xiang,
N. Tian, S. Sun, Chem. Commun. 2010, 46, 4252 – 4254; g) L. Feng,
X. Sun, C. Liu, W. Xing, Chem. Commun. 2012, 48, 419 – 421;
h) M. K. Debe, Nature 2012, 486, 43 – 51; i) R. F. Service, Science
2002, 296, 1222 – 1224.
[2] a) H. Lee, S. E. Habas, G. A. Somorjai, P. Yang, J. Am. Chem. Soc.
2008, 130, 5406 – 5407; b) V. Mazumder, S. Sun, J. Am. Chem. Soc.
2009, 131, 4588 – 4589; c) Z. Bai, L. Yang, Y. Guo, Z. Zheng, C.
Hu, P. Xu, Chem. Commun. 2011, 47, 1752 – 1754; d) S. Patra, B.
Viswanath, K. Barai, N. Ravishankar, N. Munichandraiah, ACS
Appl. Mater. Interfaces 2010, 2, 2965 – 2969; e) E. Antolini,
Energy Environ. Sci. 2009, 2, 915 – 931.
[3] a) X. X. Wang, J. D. Yang, H. J. Yin, R. Song, Z. Y. Tang, Adv.
Mater. 2013, 25, 2728 – 2732; b) V. Mazumder, M. F. Chi, M. N.
Mankin, Y. Liu, O. Metin, D. H. Sun, K. L. More, S. H. Sun, Nano
Lett. 2012, 12, 1102 – 1106; c) C. Liao, Z. D. Wei, S. G. Chen, L. Li,
M. B. Ji, Y. Tan, M. J. Liao, J. Phys. Chem. C 2009, 113, 5705 –
5710.
Figure 5. a) Steady-state polarization, b) power-density curves and
c) discharge curves at 0.35 V for Pd–Ni2P/C-30%, PdNi/C, PdP/C and
Pd/C–C as anode catalysts for the direct formic acid fuel cell with
formic acid (3m) at 308C. The flow rate of formic acid was
200 mLminÀ1 and the flow rate of O2 was 500 mLminÀ1
.
[4] a) C. Du, M. Chen, W. Wang, G. Yin, ACS Appl. Mater. Interfaces
2010, 2, 105 – 109; b) C. Du, M. Chen, W. Wang, G. Yin, P. Shi,
Electrochem. Commun. 2010, 12, 843 – 846; c) X. M. Wang, Y. Y.
Xia, Electrochem. Commun. 2008, 10, 1644 – 1646; d) L. L. Zhang,
Y. W. Tang, J. C. Bao, T. H. Lu, C. Li, J. Power Sources 2006, 162,
177 – 179; e) J. Jia, R. Wang, H. Wang, S. Ji, J. Key, V. Linkov, K.
Shi, Z. Lei, Catal. Commun. 2011, 16, 60 – 63; f) B. M. Leonard, Q.
Zhou, D. Wu, F. J. DiSalvo, Chem. Mater. 2011, 23, 1136 – 1146;
which might accelerate formic acid oxidation in a fashion
similar to the hydrogen spill-over effect.[9] Ni2P might also
activate water, producing -OHads to oxidize CO and other
poisoning intermediates adsorbed at adjacent Pd sites
through the so-called bifunctional mechanism.[1a,g] More
work is required to test these mechanistic hypotheses.
Angew. Chem. Int. Ed. 2014, 53, 122 –126
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
125

.
Angewandte
Communications
g) G. Yang, Y. Chen, Y. Zhou, Y. Tang, T. Lu, Electrochem.
Commun. 2010, 12, 492 – 495.
[7] D. J. Ham, C. Pak, G. H. Bae, S. Han, K. Kwon, S.-A. Jin, H.
Chang, S. H. Choi, J. S. Lee, Chem. Commun. 2011, 47, 5792 –
5794.
[8] E. J. Popczun, J. R. McKone, C. G. Read, A. J. Biacchi, A. M.
Wiltrout, N. S. Lewis, R. E. Schaak, J. Am. Chem. Soc. 2013, 135,
9267 – 9270.
[9] a) K. W. Park, K. S. Ahn, Y. C. Nah, J. H. Choi, Y. E. Sung, J.
Phys. Chem. B 2003, 107, 4352 – 4355; b) L. Feng, Z. Cui, L. Yan,
W. Xing, C. Liu, Electrochim. Acta 2011, 56, 2051 – 2056.
[5] Y. Lu, X. L. Wang, Y. J. Mai, J. Y. Xiang, H. Zhang, L. Li, C. D.
Gu, J. P. Tu, S. X. Mao, J. Phys. Chem. C 2012, 116, 22217 – 22225.
[6] a) L. L. Zhang, T. H. Lu, J. C. Bao, Y. W. Tang, C. Li, Electro-
chem. Commun. 2006, 8, 1625 – 1627; b) N. Cheng, R. A. Webster,
M. Pan, S. Mu, L. Rassaei, S. C. Tsang, F. Marken, Electrochim.
Acta 2010, 55, 6601 – 6610.
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