Angstrom-sized tungsten carbide promoted platinum electrocatalyst for effective oxygen reduction reaction and resource saving

Abstract

Angstrom-sized tungsten carbide dots (WC^dot), WC rods (WC^rod) and 10 nm-sized WC particles (WC^nano) are synthesized and used as platinum (Pt) electrocatalyst supports for efficient oxygen reduction reaction (ORR). The concentration of the tungsten source and formula structure of the carbon source (ion-exchange resin) control the size and shape of the WC materials. The angstrom-sized WC^rod promoted Pt electrocatalyst (Pt/WC^rod) shows slightly higher activity than the 10 nm-sized WC^nano promoted electrocatalyst (Pt/WC^nano), although the WC^rod costs only one tenth the W amount of the WC^nano, indicating resource saving. In addition, the Pt/WC^rod shows higher ORR efficiency and stabilization than the commercial Pt/C. The electron transfer from WC to Pt is believed to account for the excellent performances of the Pt/WC. The significance of the work is that less WC and less Pt can be used to achieve the same or higher ORR performances, and the synthesis method can also be used to synthesize other angstrom-sized materials.

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1. Introduction

Oxygen electro-reduction reaction (ORR) has slow kinetics,^1,2 which requires an electrocatalyst with large loading of noble metals to increase the cathode current, thus causing high cost for low temperature fuel cells. Transition metal carbides can be used as catalyst promoters to promote activity and stability of the noble metal based electrocatalysts through the synergistic effect^3–12 due to electron transfer from carbides to noble metals. The addition of carbides facilitates ORR and can reduce loading of the noble metals to some extent.^13–21

Carbide particle size is a critical factor to determine its extent of promotion effect on noble metal electrocatalyst.^22,23 It is thought that atomic level mixture of carbide and noble metal in electrocatalyst would lead to their full interactions. Precursors of both carbon and metal affect the metal carbide particle size. Traditional synthesis method through carburization of tungsten oxide (WO₃) leads to micro-sized tungsten carbide (WC) particles. Mixing carbon powder and ammonium metatungstate solution leads to 40 nm-sized WC particles.²⁴ Mixing solution of glucose (carbon source) and ammonium metatungstate leads to about 30 nm-sized WC particles.^25,26 Mixing surfactant and tungsten precursor can further reduce WC particle size to about 15 nm.^27,28 Ion-exchange resin as carbon source can reduce WC particle size to about 5 nm.^29,30 To further reduce WC particle size to atomic level is a great challenge.

Herein, we report synthesis of WC materials with at least one-dimension of around 1 nm (angstrom-scale) through an ion-exchange method. Pt particles were then loaded on these WC materials and tested as electrocatalysts for ORR. Results show that the 1 nm-sized WC promoted electrocatalyst has higher activity than 10 nm-sized one, despite that the former only has one tenth the WC content of the latter. The ORR stability of the angstrom-sized WC promoted electrocatalyst is also excellent. This work indicates that nanometer-sized materials can be further reduced in size for higher activity and resource saving.

2. Experimental

2.1 Chemistry material

Polyacrylic weak-base anion-exchange resin (D201×1 resin, Hebi Power Resin Factory, China). Ammonium metatungstate (AMT, A.R., Tianjin Jinke Fine Chemicals, China). Chloroplatinic acid solution (H₂PtCl₆, A. R., Sinopharm Chemical Reagent Co., Ltd) and 20 ml glycol (A.R., Tianjin FuyuFine Chemicals Co., Ltd, China). Nafion (DuPont, USA). All chemicals were of analytical grade and used without further purification.

2.2 Synthesis of angstrom- and 10 nm-sized WC

A 10 g amount of polyacrylic weak-base anion-exchange resin was impregnated by 100 ml of 0.3 mmol L⁻¹ ammonium metatungstate (AMT) solution about 6 h, then separated, dried at 80 °C and calcined in N₂ atmosphere at 800 °C for 1 h. When the sample was cooled to room temperature, it was grinded to obtain the angstrom-sized WC material with rod shape (denoted as WC^rod).

Repeating the above process but change the AMT concentration to 3 or 0.06 mmol L⁻¹, 10 nm-sized WC (denoted as WC^nano) and angstrom-sized WC with dot shape (denoted as WC^dot) were obtained.

2.3 Preparation of electrocatalysts

Typically, WC material (60 mg) was put into a mixture, which contained chloroplatinic acid solution (H₂PtCl₆, 5.33 ml, containing 40 mg Pt) and 20 ml glycol. Then, it was ultrasonicated for 30 min to obtain a well-dispersed ink. Its pH was then adjusted to 10 by 1 mol L⁻¹ NaOH/glycol solution. The sample was then treated by a 900 W microwave oven for heating at a 10 s on and 10 s off process for 12 times.^27,31 After that, the resultant catalyst was washed and dried in vacuum at 40 °C for 24 h, and the Pt/WC electrocatalyst was obtained. The Pt particles supporting on WC^nano, WC^rod and WC^dot were denoted as Pt/WC^nano, Pt/WC^rod and Pt/WC^dot, respectively. The Pt contents in these electrocatalysts were 40 wt%. The accurate Pt contents were tested by inductively coupled plasma-atomic emission spectrometry (ICP, USA).

2.4 Synthesis of electrodes

Pt/WC (5 mg) or commercial Pt/C (4 mg, 47.6 wt% Pt, Japan) was dispersed in a solution containing 1.95 ml ethanol and 0.05 ml 5 wt% Nafion with the assistant of ultrasonic and got electrocatalyst ink, respectively. The ink (0.005 ml) was loaded on the surface of a glass carbon electrode (0.25 cm²) and dried naturally. The total Pt content was 0.02 mg cm⁻².

2.5 Electrochemical characterization

The electrochemical properties of the samples were performed in an O₂-saturated 0.1 mol L⁻¹ HClO₄ solution scanned between 0 and 1.1 V (vs. RHE) at a scan rate of 5 mV s⁻¹, 1600 rpm and kept at 25 °C. A reversible hydrogen electrode (RHE) and a Pt foil (1.0 cm²) were used as reference and counter electrodes, respectively.

2.6 Physical characterization

Transmission electron microscopy (TEM) micrographs were obtained with a JEOL-JEM-2010 (JEOL, Japan) operating at 200 kV. The crystal phase of the samples was analyzed by X-ray diffraction (XRD) analysis using an X-ray diffractometer (XRD, Japan, Cu_Kα, λ = 1.54 Å) at a scan rate of 10° min⁻¹ in the region (2θ) from 20° to 80°.

3. Results and discussion

WC^nano, WC^rod and WC^dot were prepared by a facile calcination method, and then Pt was introduced on their surface to form the electrocatalysts. The structure and shape of the catalyst were measured. The ORR properties and the stabilities of the samples were measured and the results showed that the Pt/WC has better performance than the Pt/C. The interaction between the Pt and the tungsten carbide is beneficial for the enhanced ORR ability.

Fig. 1 exhibits the XRD patterns of the WC^nano, WC^rod and WC^dot. The peaks at 31.5°, 35.6° and 48.3° ascribed to the (001), (100) and (101) crystal facets of WC. The peak intensities reduce with the decrease of AMT concentrations, which correspond to the reduction of WC contents and particle sizes. It is obvious that no other W compounds (oxides and W₂C) or W metal exist. Since WC is more efficient than the other W compounds or W metal in exerting promotion effect,^32–35 this synthesis method of WC is highly efficient.

Fig. 1 XRD patterns of the WC^nano, WC^rod and WC^dot.

Fig. 2a is the TEM image of the WC^rod, it can be seen that the width of WC rods is about 1 nm and the length is several nanometers. The WC rods are well-dispersed in carbonized resin. Fig. 2b and its inset are HRTEM images of the WC^rod. It is clear that a rod with width of 1.7 nm, length of 6.0 nm and facet lattice of WC (100) can be observed. Fig. 2c shows the TEM image of the WC^nano, which shows WC particles with the diameters of around 10 nm. Fig. 2d is the TEM image of the WC^dot, the dots are too small to be clearly seen. The WC^nano, WC^rod and WC^dot were synthesized with different AMT concentrations; the above results indicate that the AMT concentration controls the shape and size of the WC materials.

Fig. 2 (a) TEM and (b) HRTEM images of the WC^rod, (c) TEM image of the WC^nano, and (d) TEM image of the WC^dot.

The structure of the ion-exchange resin (D201×1) and concentration of AMT (or ratio of resin to AMT) may also play roles on the sizes and shapes of WC particles. Fig. 3 shows the structural formula of the ion-exchange resin, which has long-chain. It can be seen that a high concentration of AMT results in agglomerated WC chain (WC^nano); a medium concentration of AMT results in discontinuous WC chain (WC^rod); and a low concentration of AMT results in dotted distribution of WC (WC^dot). The effect of heating temperature and time on morphology of WC was also studied. We changed the heating process to 1100 °C for 2 h and 550 °C for 1 h, and got WC^rod and WO₃^rod respectively. It is found that both the particles have the similar sizes and shapes to the WC^rod that is synthesized at 800 °C for 1 h (TEM images are not shown). The results indicate that the structure of resin and ratio of resin to AMT decide the size and shape of WC^rod.

Fig. 3 The formula of D201×1 resin (n = 2000) and effect of composition of resin and ratio of resin to AMT on the size and shape of WC.

Pt/WC electrocatalysts were prepared by loading Pt particles on the as-synthesized WC materials. Fig. 4 shows XRD patterns of the Pt/WC^nano, Pt/WC^rod and Pt/WC^dot electrocatalysts. Except the peaks ascribed to WC, the peaks at 39.8°, 46.2° and 67.5° are ascribed to the (111), (200) and (220) crystal facets of Pt, respectively. It shows that the Pt peaks of Pt/WC^rod and Pt/WC^dot are wider than that of Pt/WC^nano, indicating that the Pt particles in Pt/WC^rod and Pt/WC^dot are smaller than those in Pt/WC^nano.

Fig. 4 XRD patterns of the Pt/WC^nano, Pt/WC^rod and Pt/WC^dot.

Fig. 5 shows TEM images of the Pt/WC^nano, Pt/WC^rod and Pt/WC^dot electrocatalysts. Insets in Fig. 5a, c and d are the corresponding Pt particle size distributions that are randomly selected (the number of Pt particles is 100). Due to that these WC materials have different WC contents (leading to different densities or specific surface areas), Pt particles on these WC materials show different distributions and sizes. It can be seen that Pt distribution on WC^rod is moderate, on WC^nano is dense, and on WC^dot is sparse. The average Pt diameters are calculated as 2.3 nm on WC^rod, 3.4 nm on WC^nano and 2.4 nm on WC^dot, respectively. The results are matched with the XRD results (Fig. 4). Fig. 5b is the HRTEM image of the Pt/WC^rod, displaying the WC (100) and Pt (111) lattices. The inset EDS pattern of Fig. 5b confirms the existence of C, W and Pt elements (the existence of Cu and Cr element are ascribed to the bottom bush). The WC particles are very difficult to be discerned from Pt particles, since they are overlapped and have the similar sizes; moreover, the angstrom-sized WC has too low degree of crystallization to be identified from its crystal lattices. However, the Pt particles on WC–C composite should be as follows: Some Pt particles are on surface of WC, some are around WC and some are on surface of C.³⁶ In addition, Pt particles in commercial Pt/C are uniformly dispersed and the average Pt diameter is calculated to be 2.7 nm from its TEM image (not shown).

Fig. 5 (a) TEM and (b) HRTEM images of the Pt/WC^rod, (c) TEM image of the Pt/WC^nano, and (d) TEM image of the Pt/WC^dot. Inset of (b) is the corresponding EDS pattern; insets of (a), (c) and (d) are the corresponding Pt particle size distributions.

The ORR performances of the Pt/WC^nano, Pt/WC^rod, Pt/WC^dot and the commercial Pt/C electrodes were performed in 0.1 mol L⁻¹ HClO₄ solution (which is O₂-saturated) with the scan rate of 5 mV s⁻¹, at 25 °C, 1600 rpm (Fig. 6). Fig. 6a exhibits that the onset potentials of them are in the following sequence: Pt/WC^rod = Pt/WC^nano (+1.03 V) > Pt/WC^dot (+1.02 V) > Pt/C (+1.01 V). The half-potential (E_1/2) values of them are in the following sequence: Pt/WC^rod (+0.888 V) > Pt/WC^nano (+0.880 V) > Pt/WC^dot (+0.861 V) > Pt/C (+0.849 V). Both the onset potentials and E_1/2 values of ORR on all the Pt/WC electrocatalysts are higher than those of the Pt/C, which suggests the doped WC significantly reduces ORR over potential.

Fig. 6 (a) ORR curves of the electrodes at the initial cyclic voltammetry cycles in O₂-saturated 0.1 mol L⁻¹ HClO₄ solution with the scan rate of 5 mV s⁻¹ at 25 °C, 1600 rpm, (b) the corresponding mass activities of the electrocatalysts; inset of (a) is the cyclic voltammograms (CV) of the electrocatalysts in N₂-saturated 0.1 mol L⁻¹ HClO₄ solution at 25 °C with the scan rate of 50 mV s⁻¹.

Fig. 6b displays the kinetic currents of the electrocatalysts calculated from the experimental data by the mass transport correction for rotating disk electrode:³⁷

i_k = i_di/(i_d − i) (1)

where i is the experimentally resulted current, i_d is the tested diffusion-limited current and i_k is the mass-transport-free kinetic current. The mass activity (i_m) is determined from the calculation of i_k by eqn (1), and which is normalized to the introduced Pt amount. The mass activities at +0.9 V for the electrocatalysts were investigated and the results were displayed in Table 1. It is clear that the mass activities of the electrocatalysts are in the following sequence: Pt/WC^rod (236.1 mA mg_Pt⁻¹) > Pt/WC^nano (219.7 mA mg_Pt⁻¹) > Pt/WC^dot (147.6 mA mg_Pt⁻¹) > Pt/C (119.5 mA mg_Pt⁻¹). The results indicate that the addition of WC greatly improves the ORR activity of Pt electrocatalysts. Therein, the mass activity of the typical Pt/WC^rod is 2.0 times that of Pt/C. The ORR mass activities of the self-made Pt/WC^rod at 0.9 V or 0.85 V were also compared with other Pt/WC catalysts in literatures, as shown in Table 2. It can be seen that the Pt/WC^rod has the similar activity to Pt/WC (5 nm-sized WC),³⁶ but much higher activity than the common WC (more than 5 nm in diameter) promoted Pt catalysts.^13,14,39

Table 1 The EASAs and mass activity of the electrocatalysts

Electrocatalyst	Pt mass content^a	im at 0.9 V (mA mgPt^−1)	EASA (m^2 gPt^−1)	im/EASA (mA mPt^−2)
a The data were determined by inductively coupled plasma-atomic emission spectrometry (ICP).
Pt/WC^rod	37.7%	236.1	62.4	3784
Pt/WC^nano	37.5%	219.7	40.1	5479
Pt/WC^dot	38.2%	147.6	57.1	2585
Pt/C	47.6%	119.5	51.4	2325

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Table 2 ORR activity comparison of the electrocatalysts

Electrocatalyst	im at 0.9 V (mA mgPt^−1)	im at 0.85 V (mA mgPt^−1)
Pt/WC^rod (self made)	236.1	620
Pt/WC (5 nm-sized WC)^36	247.7
Pt/WC (5–10 nm-sized WC)^14	207.4
Pt/WC^39	Too low	287
Pt/WC^13	Too low	181

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The inset of Fig. 6a displays the cyclic voltammograms (CV) of the electrocatalysts in N₂-saturated 0.1 mol L⁻¹ HClO₄ solution at 25 °C with the scan rate of 50 mV s⁻¹. In all the CV curves, the peaks in the potential range of 0–0.25 V are come from the adsorption and desorption of hydrogen on the Pt surface. The peaks above 0.6 V in the anodic scan are due to oxidation of Pt, while peaks between 0.9 V and 0.5 V in the cathodic scan are due to reduction of PtO. The active surface areas (EASAs) of the electrocatalysts could be calculated by eqn (2):³⁸

Where E refers to the potential (V) and I refers to the measured current density (mA cm⁻²); I₀, E₁ and E₂ refer to the current density and potentials at the initial and terminal points of hydrogen desorption peak, which are marked in the Fig. 6a; a₀ is the Pt loading normalized to the electrode surface area (mg_Pt cm⁻²), v₀ is the scan rate (mV s⁻¹), Q is the electric quantity in the hydrogen desorption peak and normalized to Pt loadings (C g_Pt⁻¹), and Q_H the hydrogen adsorption electric quantity for a smooth polycrystalline Pt (Q_H = 2.10 C m⁻²).²⁷ The calculated EASAs are also summarized in Table 1. They are in the following order: Pt/WC^rod (62.4 m² g_Pt⁻¹) > Pt/WC^dot (57.1 m² g_Pt⁻¹) > Pt/C (51.4 m² g_Pt⁻¹) > Pt/WC^nano (40.1 m² g_Pt⁻¹). The order is completely consistent with the Pt size order: Pt/WC^rod (2.3 nm) < Pt/WC^dot (2.4 nm) < Pt/C (2.7 nm) < Pt/WC^nano (3.4 nm). Both the Pt/WC^rod and Pt/WC^dot have higher EASAs than the Pt/WC^nano, which is due to the lower WC contents in both the WC^rod and WC^dot. As has been pointed out above, lower WC content means lower density or higher specific surface area, leading to uniform dispersion of Pt particles. However, the WC^rod has higher WC content than the WC^dot, but the EASA of Pt/WC^rod is a little higher than the Pt/WC^dot. The reason is that strong interaction existing between WC and Pt³ benefits dispersion of Pt particles too. More WC content means more interaction with Pt, favouring the introduced Pt particles in smaller size to some extent. Therefore, to obtain higher EASA of Pt/WC electrocatalyst, the WC content should be moderate.

The values of the i_m/EASA (i_m normalized to the EASA) are also displayed in Table 1. It is clear that with the increase of WC contents (carbon powder < WC^dot < WC^rod < WC^nano), the values of i_m/EASA of the electrocatalysts increase (Pt/C < Pt/WC^dot < Pt/WC^rod < Pt/WC^nano), proving the promotion effect of WC on Pt. It is worth noting that although the WC^nano has higher promotion effect than WC^rod, the Pt/WC^rod has higher Pt mass activity than the Pt/WC^nano. The reason is that the Pt/WC^rod has more EASA than the Pt/WC^nano (Table 1). Therefore, the promotion effect from WC and the EASA from Pt decide the ORR activity. Consequently, the WC content could be carefully adjusted to get higher Pt mass activity.

In order to investigate the ORR stability of the typical Pt/WC^rod electrocatalyst, the 1^st and 1000^th ORR curves cycles of the Pt/WC^rod and Pt/C are tested in an O₂-saturated 0.1 mol L⁻¹ HClO₄ solution and the results are shown in Fig. 7. After the 1000 cycles, the corresponding E_1/2 values decreased 9 mV for Pt/WC^rod and 15 mV for Pt/C (Fig. 7a), respectively. The corresponding mass activities (Fig. 7b) at +0.9 V of them were computed by eqn (1) and the results are displayed in Table 3. It can be seen that the Pt/WC^rod electrocatalyst has the activity retention of 90.4% while the Pt/C has only 77.6% after the 1000 cycles. The inset of Fig. 7a shows that the EASAs have the similar retentions as the ORR mass activities after the 1000 cycles. These findings indicate excellent electrocatalytic stability of the Pt/WC^rod.

Fig. 7 (a) ORR curves of the Pt/WC^rod and Pt/C at the 1^st and 1000^th cycles in O₂-saturated 0.1 mol L⁻¹ HClO₄ solution with the scan rate of 5 mV s⁻¹ at 25 °C, 1600 rpm, (b) the corresponding mass activities at the 1^st and 1000^th cycles; inset of (a) is the CV curves of the electrocatalysts in N₂-saturated 0.1 mol L⁻¹ HClO₄ solution at 25 °C with the scan rate of 50 mV s⁻¹ after the ORR curves of 1^st and 1000^th cycles.

Table 3 The ORR stability comparison of the Pt/WC^rod and Pt/C electrocatalysts

Electrocatalyst	CV cycle	im at 0.9 V (mA mgPt^−1)	Activity retention
Pt/WC^rod	1^st	236.1	90.4%
	1000^th	213.5
Pt/C	1^st	119.5	77.6%
	1000^th	92.7

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It is known that electrons can transfer between carbides and the loaded noble metals.^3,38 Fig. 8 shows XPS spectra of the Pt 4f on Pt/C and Pt/WC^rod. Obviously, negative shift occurred in XPS spectra after the introduction of WC^rod. The result indicates that there is electron-donating (transfer) from WC to Pt, which, on one hand, enhancing the ORR activity due to improved electron cloud density, and on the other hand, improves its stability due to the interaction force between WC and Pt.

Fig. 8 XPS spectra of the Pt 4f on Pt/WC^rod and Pt/C.

4. Conclusions

Angstrom-sized WC rods, WC dots and 10 nm-sized WC particles have been synthesized. The AMT concentration and formula structure of the ion-exchange resin control the size, shape and content of WC materials. The higher WC content leads to higher promotion effect on Pt electrocatalyst, but too high WC content results in large Pt particles due to reduced specific surface area. On the other hand, the lower WC content leads to smaller Pt particles, but too small WC content results in less promotion effect on Pt and slightly increased Pt sizes (or reduced EASAs) because of reduced interaction between WC and Pt. The WC^rod with moderate WC content leads to both considerable promotion effect on Pt and EASA of Pt electrocatalyst. The typical Pt/WC^rod shows higher Pt mass activity for ORR than the Pt/WC^dot (with less WC content), Pt/WC^nano (with more WC content) and Pt/C (without WC). The activity of Pt/WC^rod (236.1 mA mg_Pt⁻¹) is 2.0 times that of commercial Pt/C (119.5 mA mg_Pt⁻¹), indicating potential economic value. It is significant that the Pt/WC^rod shows slightly higher ORR activity than the Pt/WC^nano, but the WC^rod costs only one tenth of amount of W precursor of the WC^nano during the synthesis process, indicating resource saving. Although the cost saving from reduction of the WC may be negligible in the total cost of the catalyst, the method could be used to synthesize other angstrom-sized materials to reduce consumption and cost and benefit sustainable development of the world. Moreover, the Pt/WC^rod electrocatalyst possesses much higher ORR stability than commercial Pt/C. The efficient electron transfer between the two materials is believed to increase the electron cloud density of Pt and the linkage between WC and Pt, leading to enhanced ORR activity and stability of the Pt/WC electrocatalysts.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China for Youths (No. 21407065, 21506079), Natural Science Foundation of Jiangsu Province for Youths (BK20140533), China Postdoctoral Science Foundation (No. 2014M551520, 2014M560399), Jiangsu Postdoctoral Science Foundation (1401143C) and Jiangsu University Scientific Research Funding (14JDG164).

Notes and references

1. J. Ge, J. St-Pierre and Y. Zhai, Electrochim. Acta, 2014, 134, 272–280.
2. M. F. Li, L. W. Liao, D. F. Yuan, D. Mei and Y. X. Chen, Electrochim. Acta, 2013, 110, 780–789.
3. G. Cui, P. K. Shen, H. Meng, J. Zhao and G. Wu, J. Power Sources, 2011, 196, 6125–6130.
4. L. Wang, T. Du, J. Cheng, X. Xie, B. Yang and M. Li, J. Power Sources, 2015, 280, 550–554.
5. Z. X. Yan, M. M. Zhang, J. M. Xie, J. J. Zhu and P. K. Shen, Appl. Catal., B, 2015, 165, 636–641.
6. L. Elbaz, C. R. Kreller and N. J. Henson, J. Electroanal. Chem., 2014, 720–721, 34–40.
7. C. He and P. K. Shen, Nano Energy, 2014, 8, 52–61.
8. C. Tang, D. Wang, Z. Wu and B. Duan, Int. J. Hydrogen Energy, 2015, 40, 3229–3237.
9. Y. Oh, S. K. Kim, D. H. Peck, J. Jang, J. Kim and D. H. Jung, Int. J. Hydrogen Energy, 2014, 39, 15907–15912.
10. S. B. Yin, L. X. Yang, L. Luo, F. Huang, Y. H. Qiang, H. W. Zhang and Z. X. Yan, New J. Chem., 2013, 37, 3976–3980.
11. L. M. Jiang, H. G. Fu, L. Wang, W. Zhou, B. J. Jiang and R. H. Wang, RSC Adv., 2014, 4, 51272–51279.
12. Z. S. Li, S. Ji, B. G. Pollet and P. K. Shen, Chem. Commun., 2014, 50, 566–568.
13. N. R. Elezović, B. M. Babić, L. J. Gajić-Krstajić, P. Ercius, V. R. Radmilović, N. V. Krstajić and L. M. Vracar, Electrochim. Acta, 2012, 69, 239–246.
14. C. He, H. Meng, X. Yao and P. K. Shen, Int. J. Hydrogen Energy, 2012, 37, 8154–8160.
15. I. J. Hsu, Y. C. Kimmel, Y. Dai, S. Chen and J. G. Chen, J. Power Sources, 2012, 199, 46–52.
16. C. Liang, L. Ding, C. Li, M. Pang, D. Su, W. Li and Y. M. Wang, Energy Environ. Sci., 2010, 3, 1121–1127.
17. V. Kiran, K. Srinivasu and S. Sampath, Phys. Chem. Chem. Phys., 2013, 15, 8744–8751.
18. A. C. Garcia and E. A. Ticianelli, Electrochim. Acta, 2013, 106, 453–459.
19. X. B. Gong, S. J. You, X. H. Wang, Y. Gan, R. N. Zhang and N. Q. Ren, J. Power Sources, 2013, 225, 330–337.
20. C. K. Poh, S. H. Lim, Z. Tian, L. Lai, Y. P. Feng, Z. Shen and J. Y. Li, Nano Energy, 2013, 2, 28–39.
21. P. Justin, P. H. K. Charan and G. R. Rao, Appl. Catal., B, 2014, 144, 767–774.
22. Z. Yan, H. Meng, P. K. Shen, R. Wang, L. Wang, K. Shi and H. G. Fu, J. Mater. Chem., 2012, 22, 5072–5079.
23. Z. X. Yan, H. Wang, M. M. Zhang, Z. F. Jiang, T. Jiang and J. M. Xie, Electrochim. Acta, 2013, 95, 218–224.
24. F. P. Hu and P. K. Shen, J. Power Sources, 2007, 173, 877–881.
25. Y. Jin, D. Liu, X. Li and R. Yang, Int. J. Refract. Met. Hard Mater., 2011, 29, 372–375.
26. J. L. Lu, Z. H. Li, S. P. Jiang, P. K. Shen and L. Li, J. Power Sources, 2012, 202, 56–62.
27. Z. Yan, F. Li, J. Xie and X. Miu, RSC Adv., 2015, 5, 6790–6796.
28. L. Borchardt, M. Oschatz, S. Graetz, M. R. Lohe, M. H. Rümmeli and S. Kaskel, Microporous Mesoporous Mater., 2014, 186, 163–167.
29. R. Wang, C. Tian, L. Wang, B. Wang, H. Zhang and H. Fu, Chem. Commun., 2009, 21, 3104–3106.
30. Z. Yan, M. Zhang, J. Xie and P. K. Shen, J. Power Sources, 2013, 243, 336–342.
31. Z. Q. Tian, S. P. Jiang, Y. M. Liang and P. K. Shen, J. Phys. Chem. B, 2006, 110, 5343–5350.
32. J. D. Oxley, M. M. Mdleleni and K. S. Suslick, Catal. Today, 2004, 88, 139–151.
33. J. C. Kim and B. K. Kim, Scr. Mater., 2004, 50, 969–972.
34. S. V. Pol, V. G. Pol and A. Gedanken, Adv. Mater., 2006, 18, 2023–2027.
35. S. Chouzier, P. Afanasiev, M. Vrinat, T. Cseri and M. Roy-Auberger, Solid State Chem., 2006, 179, 3314–3323.
36. Z. Yan, G. He, M. Cai, H. Meng and P. K. Shen, J. Power Sources, 2013, 242, 817–823.
37. B. Lim, M. J. Jiang, P. H. C. Camargo, E. C. Cho, J. Tao, X. M. Lu, Y. M. Zhu and Y. N. Xia, Science, 2009, 324, 1302–1305.
38. Z. Yan, G. He, P. K. Shen, Z. Luo, J. Xie and M. Chen, J. Mater. Chem. A, 2014, 2, 4014–4022.
39. N. R. Elezovic, B. M. Babic, P. Ercius, V. R. Radmilovic, L. M. Vracar and N. V. Krstajic, Appl. Catal., B, 2012, 125, 390–397.

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