Ionics DOI 10.1007/s11581-014-1100-9
ORIGINAL PAPER
Highly active Pd/WO3-CNTs catalysts for formic acid electrooxidation and study of the kinetics Chun’an Ma & Yanxian Jin & Meiqin Shi & Youqun Chu & Yinghua Xu & Wenping Jia & Qiaohua Yuan & Jiabin Chen & Huiling Pan & Qiuwei Dai
Received: 8 September 2013 / Revised: 28 January 2014 / Accepted: 3 March 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract The new Pd/WO3-CNTs catalysts are prepared for formic acid electrooxidation in direct formic acid fuel cells (DFAFCs). According to XRD, TEM, and HRTEM results, WO3 particles are covered or overlapped with Pd particles, which have a uniform and narrow size distribution due to the highly dispersion of WO3-CNTs. The electrochemical results show significantly enhanced electrocatalytic performances for formic acid oxidation on Pd/WO3-CNTs catalysts, especially its dramatically improved stability and excellent tolerance to CO poisoning, which is mainly ascribed to the interaction between Pd and WO3. Therefore, Pd/WO3-CNTs catalysts show the great potential as less expensive and more efficient electrocatalyst for DFAFCs. Additionally, the kinetic parameters such as the charge transfer parameter and the diffusion coefficient of formic acid electrooxidation on 20 %Pd/20 %WO3-CNTs were obtained. Keywords Pd nanocatalysts . WO3-CNTs hybrid . Formic acid . Electrooxidation
Introduction In recent years, direct formic acid fuel cells (DFAFCs) system has been attracting considerable attention as promising sources of clean energy targeted for portable applications C. Ma (*) : Y. Jin : M. Shi : Y. Chu : Y. Xu State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310032, China e-mail:
[email protected] Y. Jin : W. Jia : Q. Yuan : J. Chen : H. Pan : Q. Dai School of Pharmaceutical and Chemical Engineering, Taizhou University, Linhai 317000, China
[1–3]. As a liquid fuel, formic acid has its many advantages, such as higher electrochemical activity, no toxicity, and lower crossover through Nafion membrane, compared with methanol [4]. Pd-based nanocatalysts are considered better catalysts for formic acid oxidation in DFAFCs compared with Pt-based nanocatalysts, because Pd-based catalysts exhibit higher catalytic activity where the formic acid can be oxidized directly into CO2 without formation of poisoning intermediates [5, 6], whereas the oxidation of formic acid on Pt catalysts are in line with the main self-poisoning dehydration reaction pathway [6]. However, it is accepted that the Pd catalysts show a poor long stability during the oxidation of formic acid. The addition of some metals or modifiers such as Sn [7], Co [8], Pb [9], TiO2 [10], and P [11] to Pd catalyst has improved the catalytic activity and stability. Despite these promising results, more work is needed to improve the catalytic activity of Pd-based catalysts and their stability to meet the demand of the DFAFCs. Tungsten oxides (WO3) have aroused much attention and been studied in various fields including photoelectrocatalytic processes, electrochromic devices, dye-sensitized solar cells, and gas sensors [12–16]. WO3 was also investigated with interest as electrocatalyst promoter for fuel cells, which have been approved to be able to enhance the overall catalytic activity to a large degree. For example, Pt and PtRu catalysts supported on WO3 had a significantly high activity for the electrooxidation of methanol [17, 18] and formic acid [19–21]. The results showed that WO3 had a good assistant electrocatalytic effect. Specially, WO3 has excellent CO tolerance [22], and hydrated WO3 can form hydrogen bronzes (HxWO3) that effectively facilitate dehydrogenation of small organic molecules [23]. Other important properties of WO3 films in mixed-valent (WVI,V) state include high rates of charge (electron and proton) propagation [24–27]. Recently, it has been reported by Kulesza group that the electrochemical enhancement effect of WO3 nanorods on Pd nanoparticles
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during oxidation of formic acid may be caused by an increase of the catalytic active surface area, mutual interactions, or by the availability of fast and reversible redox transitions involving not only HxWO3 and WO3 but also oxygen transfers between WO3 and WO3−y [23]. However, WO3 has lower specific surface area and conductivity, which has restricted its applications with noble metals. As we know, carbon nanotubes (CNTs) are a promising catalyst support for fuel cells because of their unique properties such as high specific surface area, electrical conductivity, and good thermal and chemical stability [28, 29]. Hence, if CNTs could be modified by WO3, CNTs can play multiple roles as a remarkable catalyst support candidate. On one hand, CNTs can promise WO3 particles to achieve a well distribution, which could get more interfaces between WO3 and noble metals. On the other hand, CNTs will be favorable to enhance the conductivity and corrosion stability. To the best of our knowledge, there are no reports on the WO 3 -CNTs hybrid used as a catalyst support for the electrooxidation of formic acid. Therefore, we design a new catalyst for formic acid electrooxidation, using WO3-CNTs hybrid as support for Pd nanoparticles. The physicochemical properties and electrochemical activities of the catalysts for formic acid electrooxidation were studied. The reasons for the enhanced oxidation activity were discussed as well. In addition, the kinetic parameters under the quasi-steady-state conditions at the catalyst electrode were investigated.
30–50 nm, length of about 0.5–2 μm, and specific surface area > 60 m 2 g −1 ) were purchased from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences. Characterization techniques Transmission electron microscopy (TEM) and high resolution transmission electron microscope (HRTEM) were carried out on the JEOL-2010 microscope at a potential of 200 kV and a current of 103 mA. X-ray diffraction (XRD) was recorded on Philips PW3040/60 X-ray diffractometer with Cu Kα radiation of wavelength λ=0.15,406 nm. Galvanostatic polarization curves were performed in threecompartment electrochemical glass cell with Zahner IM6 electrochemistry workstation at 25 °C. All other electrochemical measurements were carried out with a CHI 730D electrochemistry workstation at 25 °C. Working electrodes were prepared from 4-mm diameter glassy carbon disk electrodes. The catalyst powders were mixed with ethanol and 5 wt% Nafion solution, and then coated on a mirror-polished glassy carbon disk electrode. Pt foil and saturated calomel electrode (SCE) were used as counter and reference electrode, respectively. For the electrochemical measurement of the absorbed CO, CO was purged into the solution for 30 min when the electrode potential was fixed at 0.04 V versus SCE. Then, N2 was bubbled into the solution for 30 min to remove the CO in the solution.
Results and discussion Experimental Physicochemical characterization Sample preparation In the first step, the WO3-CNTs hybrid material was prepared. A given amount of multiwall CNTs was added to an aqueous solution of ammonium tungstate (10 and 20 wt%, respectively) and maintained at 90 °C for 10 h under vigorous agitation. Subsequently, the suspension was filtered, and the solid was transferred to a tubular oven at 400 °C for a heat treatment of 4 h to obtain a stable WO3-CNTs support. In the second step, the Pd/WO3-CNTs catalysts were synthesized (WO3 content 10 and 20 wt%, respectively). First, a calculated amount of 0.01 mol L−1 palladium chloride solution was added into 50 mL of ethylene glycol. Second, a given amount of the above WO3-CNTs hybrid material was ultrasonically stirred until well dispersed in the solution. The obtained suspension was heated in a microwave for 20 s with 20-s interval breaks for several cycles. Finally, the slurry was filtered and dried under vacuum at 90 °C. For comparison, Pd/CNTs (20wt%Pd loading) were also prepared using the same method. The nominal content of Pd in the catalysts was 20 wt%. All solutions were prepared using Millipore Milli-Q water and analytical grade reagents. CNTs (purity > 95 wt%, diameter
Figure 1 depicts the XRD patterns of Pd/WO3-CNTs and Pd/ CNTs catalysts. As shown, the peak located at a 2θ value of about 26° is attributed to the graphite (002) plate of the CNTs, and the other four peaks (40.0°, 46.5°, 68.0°, and 82.0°) are reflections for the face-centered cubic crystal lattice of the
Fig. 1 XRD patterns of Pd/WO3-CNTs and Pd/CNTs samples
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(111), (200), (220), and (311) facets of palladium crystal (JCPDS no. 46-1043) [4]. Furthermore, the main peaks correspond to (002), (020), (200), (−112), (022), (−202), (220), (222), (232), (322), (024), (042), (240), and (420) planes at 2θ=23.1, 23.6, 24.4, 28.6, 33.3, 33.6, 34.2, 41.9, 50.2, 50.8, 53.5, 54.2, 54.8, and 55.9, respectively, matching with the monoclinic WO3 (JCPDS no. 43-1035) [30]. The strong peaks of Pd (111), (200), (220), and (311) facets can be observed on the Pd/WO3-CNTs catalysts, indicating the existence of Pd nanoparticles. In addition, the characteristic peaks for WO3 on both Pd/WO3-CNTs catalysts can be recognized. The intensity of WO3 might be weakened due to the strong diffraction of Pd, compared with that on the corresponding WO3-CNTs supports. Therefore, both Pd/WO3-CNTs catalysts are made of WO3, palladium particles, and CNTs. Figure 2a, d displays the TEM micrographs and the corresponding histograms of 20 %Pd/10 %WO3-CNTs and 20 %Pd/ 20 %WO3-CNTs catalysts. Pd nanoparticles are found to have a uniform dispersion on the WO3-CNTs support with an average particle size of 5–11 nm, centered in 8.1 nm at 20 %Pd/10 %WO3-CNTs catalyst. Similarly, Pd particles disperse with an average particle size of 5–13 nm, centered in 8.6 nm at 20 %Pd/ 20 %WO3-CNTs catalyst. In HRTEM image of Fig. 2e, the continuous ordered lattice fringes are well resolved. The lattice spacing is calculated to be ~0.222 and ~0.263 nm, corresponding to the distance of the (111) facet of Pd nanoparticles and the (220) facet of monoclinic WO3, respectively. The values are in Fig. 2 TEM images of a 20 %Pd/ 10 %WO3-CNTs and d 20 %Pd/ 20 %WO3-CNTs. Particle size histograms of b 20 %Pd/10 %WO3-CNTs and c 20 %Pd/20 %WO3-CNTs. e HRTEM image of 20 %Pd/20 %WO3-CNTs. f EDX image of 20 %Pd/20 %WO3-CNTs
excellent agreement with the standard distance value of (111) facet of Pd (0.2245 nm) and the standard distance of the (220) facet of monoclinic WO3 (0.2623 nm). Therefore, WO3 domains are most likely to be covered or overlapped with Pd nanoparticles, which is helpful to the assistant electrocatalytic effect of WO3 with Pd noble metal. Also, W peak appears in the EDX spectrum (Fig. 2f), which confirms that element W exists inside the bulk phase of 20 %Pd/20 %WO3-CNTs catalyst. Electrochemical characterization The electrochemical features of different electrodes were analyzed in 0.5 M H2SO4, as shown in Fig. 3. The hydrogen adsorption and desorption peaks in the potential interval of −0.2–0 Von 20 %Pd/10 %WO3-CNTs and 20 %Pd/20 %WO3CNTs electrodes are larger than those on 20 %Pd/CNTs (Pd loading = 0.2 mg cm−2) electrode. This indicates that Pd/ WO3-CNTs catalysts have larger electrochemical surface areas (ESA). The characteristic voltammetric peaks on WO3-CNTs support can be observed (curve 4), suggesting the redox transitions involving simultaneous injection or removal of electrons and protons, which has been elaborated by Kulesza group [23]. Furthermore, it is found that the hydrogen desorption peaks for the two Pd/WO3-CNTs catalysts shift obviously toward lower potentials, which means that the presence of WO3 can weaken the adsorption strength of hydrogen on the Pd surface. This is probably attributed to the hydrogen
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Fig. 3 Cyclic voltammograms in 0.5 M H2SO4 solution for Pd/WO3CNTs and Pd/CNTs electrodes. Scan rate 50 mV s−1. Inset is the magnified image of curve 4
spillover effect of WO3 [22]. In addition, both the electrodes are observed to deliver reduction peaks of about 0.5 V, which can be attributed to the reduction of the oxide formed on the Pd during the forward scan [31]. The cyclic voltammetry (CV) and LSV results for Pd/WO3CNTs, Pd/CNTs, and the WO3-CNT electrodes in 0.5 M H2SO4 containing 1 M HCOOH are shown in Fig. 4a, b. Evidently, there is no anodic peak at the WO3-CNTs electrode, indicating that WO3-CNTs support has no electrocatalytic activity for formic acid oxidation. Obviously, a large peak near 0.2 V and a very small peak near 0.5 V for all electrodes can be observed on 20 %Pd/CNTs catalyst electrodes. Whereas, the current densities of the main anodic peak of formic acid increase sharply at 20 %Pd/10 %WO3-CNTs and 20 %Pd/20 %WO3-CNTs (59.1 and 92.6 mA cm−2, respectively), which are 1.8 and 2.8 times as large as that of 20 %Pd/CNTs (32.6 mA cm−2). The onset potential of formic acid oxidation at the 20 %Pd/20 %WO3-CNTs is almost the same as that at the 20 %Pd/10 %WO3-CNTs; however, both are more negative than that at Pd/CNTs catalyst electrode (Fig. 4b). Fig. 4 a Cyclic voltammograms in 1 M HCOOH + 0.5 M H2SO4 solution for Pd/WO3-CNTs and Pd/CNTs electrodes. Scan rate of 50 mV s−1. b Linear sweep voltammograms in 1 M HCOOH + 0.5 M H2SO4 solution for Pd/WO3-CNTs and Pd/CNTs electrodes. Scan rate of 20 mV·s−1
It was reported that the electrooxidation of formic acid could undergo through two parallel pathways [3]. The first oxidation peak (near 0.2 V) is ascribed to the direct oxidation of formic acid into CO2 on the particle surface that is free of adsorbed poisonous intermediates (e.g., CO) (so-called the direct path), whereas the second peak (near 0.5 V) is attributed to the oxidation of CO species adsorbed on the particle surface that arise from the non-Faradaic dissociation of formic acid (so-called indirect path) [32]. The ratio of the current density of the first anodic peak (Ja) to the cathodic peak (Jc) essentially reflects the fraction of the catalyst surface that is not poisoned by CO adsorption and can be used to measure the catalyst tolerance to CO poisoning [32]. The Ja/Jc ratio at Pd/CNTs catalyst is 0.90, suggesting that initially about 10 % of the catalyst surface was poisoned by CO adsorption. While the Ja/Jc ratio at 20 %Pd/10 %WO3-CNTs is 0.95, indicating that the latter has less CO poisoning than the former. However, the Ja/Jc ratio at 20 %Pd/ 20 %WO3-CNTs is 0.99, exhibiting excellent tolerance to CO poisoning. This implies that the voltammetric currents predominantly arise from the direct oxidation of formic acid into CO2. The results illustrate that WO3 can promote the oxidation of formic acid on the Pd catalyst through the direct pathway. Figure 5a shows the chronoamperometric curves of the three catalysts at 0.4 V in a 0.5 M H2SO4 solution containing 1 M HCOOH. In these curves, an initial rapid decrease in the current density was observed for all of the catalysts, demonstrating the poisoning of the electrocatalysts [4, 31]. The current densities on 20 %Pd/10 %WO3-CNTs and 20 %Pd/ 20 %WO3-CNTs are larger in comparison with that on Pd/ CNTs within a 1-h time frame. The 20 %Pd/20 %WO3-CNTs catalyst is able to maintain the highest current density all the time. Therefore, the results further demonstrate that the addition of WO3 can greatly improve the activity and stability of the catalyst for the electrooxidation of formic acid. In order to evaluate the stability of the catalysts, the galvanostatic polarization curves for 20 %Pd/20 %WO3-CNTs and 20 %Pd/CNTs electrodes are studied in 0.5-M H2SO4 solution containing 1 M HCOOH at different current densities, as
Ionics Fig. 5 a Chronoamperometric curves of Pd/WO3-CNTs and Pd/ CNTs electrodes in 0.5 M H2SO4 containing 1 M HCOOH at 0.4 V. b Galvanostatic polarization curves for 20 %Pd/20 %WO3CNTs (curves 1, 2, and 3) and 20 %Pd/CNTs electrode (curves 4, 5, and 6) in 0.5 M H2SO4 containing 1 M HCOOH solution at different current densities. Galvanostatic polarization curves for 20 %Pd/20 %WO3-CNTs (curve 7) and 20 %Pd/CNTs electrodes (curve 8) in 0.5 M H2SO4 solution without formic acid
shown in Fig. 5b (curves 1–6). For comparison, the galvanostatic polarization curves for 20 %Pd/20 %WO3-CNTs (curve 7) and 20 %Pd/CNTs (curve 8) catalysts in 0.5M H2SO4 solution without formic acid were also tested. Only one steady potential above 1.2 V are found on curves 7 and 8, which can be attributed to O2 evolution. However, significant differences are observed on curves 1–6, where formic acid electrooxidation takes place. For 20 %Pd/20 %WO3-CNTs electrode (curves 1–3), the potential remains at ~0 V for 3,600 s and 3,300 s when the current densities are 1.5 and 7.5 mA cm−2, respectively. After the current density being increased to 15 mA·cm−2, the potential keeps at 0 V for 2,400 s, then rises up and maintains at 0.65 V for the next 1,200 s. Combined with the CV results, the steady potential of ~0 and ~0.65 V should be attributed to the formic acid electrooxidation through the direct path and indirect path. However, the steady potential is higher, and the residence time is shorter on 20 %Pd/CNT electrode (curves 4–6) than that on 20 %Pd/20 %WO3-CNTs electrode (curves 1–3) at the same current density. From our galvanostatic polarization results, the 20 %Pd/20 %WO3-CNTs electrode was testified to be more stable than the 20 %Pd/CNTs electrode at all same current densities; therefore, in our study, the addition of WO3 was proved to be able to improve the stability of the Pd catalyst for formic acid electrooxidation. In order to evaluate the poisoning effect of CO, COstripping cyclic voltammograms of the adsorbed CO at the different catalysts were investigated, as shown in Fig. 6. There is a well-defined stripping peak of COad at the potential of approximately 0.84 Von Pd/CNTs, illustrating that CO can be strongly adsorbed on the Pd surface [33]. Whereas on 20 %Pd/ 10 %WO3-CNTs and 20 %Pd/20 %WO3-CNTs catalyst electrodes, the anodic peaks of the adsorbed CO are located at about 0.76 and 0.71 V, respectively, which are 80 and 130 mV more negative than that at the Pd/CNTs catalyst electrode. That is an indication that the addition of WO3 is helpful in weakening the CO adsorptive bond on the Pd active sites. The
weakened strength of CO adsorption on the catalyst prevents the accumulation of poisoning intermediates. Thus, more active sites are available for the formic acid electrooxidation via the direct pathway, which results in a remarkable enhancement of the electrocatalytic activity and stability. According to the previous studies, the mechanism of formic acid electrooxidation on Pd at lower potential is mainly through the direct path, and at higher potential is through indirect path. At the lower potential, the oxidation of formic acid is through the path of R1 and R2: HCOOH + Pd → Pd–COOH + H+ + e (R1) and Pd–COOH → Pd + CO2 + H+ + e (R2) [34]. A great quantity of hydrogen could adsorb onto the surface of Pd during the electrooxidation of formic acid. Thus, a lot of Pd active sites could be occupied, which hinder the adsorption of formic acid molecules. However, in the presence of WO3, the adsorbed hydrogen on Pd surface could spill over onto the WO3 surface and forms HxWO3[22], and then free the active Pd sites for further chemisorption of formic acid molecules and make the dehydrogenation of formic acid molecules adsorbed on Pd sites more effective. Subsequently, HxWO3 can be readily oxidized to release WO3, which facilitates further spillover of hydrogen.
Fig. 6 CO stripping cyclic voltammograms on different electrodes in 0.5 M H2SO4 with a scan rate of 50 mV s−1
Ionics Fig. 7 Linear sweep voltammograms of 1 M HCOOH + 0.5 M H2SO4 solution on a 20 %Pd/20 %WO3-CNTs and b 20 %Pd/CNTs electrodes at different scan rates
The above process is repeated, which can accelerate the electrooxidation of formic acid. Kinetics study Figure 7 shows the linear sweep voltammograms of 20 %Pd/20 %WO3-CNTs and 20 %Pd/CNTs electrodes at various scan rates in 0.5 M H2SO4 solution containing 1 M HCOOH. The main peak is observed to shift positively as the scan rates increase. The mechanism of formic acid electrooxidation through the direct path is shown in R1 and R2. The rate-determining step is the mass-transfer process [35] or reaction 1. The values of the peak potential are proportional to the log v of scan rates as the following equation for the completely irreversible system [34, 38]. dE p 2:3RT ¼ V ⋅dec−1 ′ dlogv 2αn F
ð1Þ
Where n′ is the number of electrons transferred in the rate-determining step and α is the charge transfer Fig. 8 a Plot of the peak potential versus log v for 1.0 M HCOOH in 0.5 M H2SO4 solution at the 20 %Pd/20 %WO3-CNTs and 20 %Pd/CNTs electrodes. b Plot of the peak current density versus the square root of scan rates for 1.0 M HCOOH in 0.5 M H2SO4 solution at 20 %Pd/20 %WO3CNTs and 20 %Pd/CNTs electrodes
coefficient. The dependence of E p on the log v is shown in Fig. 8a. The slope of the lines on 20 %Pd/ 20 %WO3-CNTs and 20 %Pd/CNTs electrodes is 96.8 and 100.5 mV·dec−1, respectively; therefore, the value of αn′ can be obtained by Eq. (1). The calculated values of αn′ are 0.305 and 0.294, respectively. The rate-determining electron transfer is a one-electronprocess, n′ = 1; thus, α should be 0.305 and 0.294 on 20 %Pd/20 %WO3-CNTs and 20 %Pd/CNT electrodes, respectively. The curves of the peak current density versus the square root of the scan rate are shown in Fig. 8b. The linear relationship between the peak current density and the square root of the scan rate indicates that the oxidation of formic acid on 20 %Pd/20 %WO3-CNTs is a diffusion-controlled process [36, 37]. Additionally, the chronocoulometric measurement has been performed on 20 %Pd/20 %WO3-CNTs electrode, and the corresponding Q-t1/2 plot is illustrated in Fig. 9. The appearance of linear portions in the chronocoulometric plot implies existence of the effectively diffusional type patterns [23].
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Fig. 9 Chronocoulometric response of 20 %Pd/20 %WO3-CNTs electrodes. Potential steps (0.1 s) were from −0.2 to 0.8 V. Electrolyte 1 M HCOOH in 0.5 M H2SO4
In the case of a completely irreversible reaction, the peak current in amperes is as follows [38]: 1 1 1 jp ¼ 2:99 105 n αn′ 2 C ∞ D 2 v 2
ð2Þ
where jp is the peak current density measured in amperes per square centimeter, C∞ is the formic acid concentration in the solution in moles per cubic centimeter, D is the diffusion coefficient in square centimeters per second, v is the sweep rate in volts per second, and n is the number of electrons transferred in the sum of the reactions. Thus, the value of D was obtained from the slope in Fig. 8b by using Eq. (2). D is the measurement of the charge-transport rate within the liquid film near the electrode surface. In this case, the value of D is 3.6×10−7 cm2·s−1 on 20 %Pd/CNTs electrode. Also, we can get the value of D as 4.18×10−6 cm2·s−1 on 20 %Pd/20 %WO3-CNT electrode, which is larger than that on 20 %Pd/ CNTs electrode, suggesting the charge-transport rate within the liquid film near the 20 %Pd/20 %WO3-CNTs electrode surface is much higher.
Conclusions This study demonstrates that Pd nanoparticles that supported on WO3-CNTs hybrid have significantly enhanced the electrocatalytic performances for the formic acid oxidation. According to the XRD, TEM, and HRTEM results, Pd/ WO3-CNTs are composed mainly of Pd, WO3 nanoparticles, and CNTs, and WO3 domains are most likely to be covered or overlapped with Pd nanoparticles. Moreover, Pd particles have a relatively narrow distribution of particles in a range of 5–11 and 5–13 nm, respectively. The results of electrochemical analysis illustrate that the addition of WO3 can
improve both the activity and the stability for the formic acid electrooxidation. So, the occurrence of WO3 can influence on the activity of catalyst by two ways: (1) WO3 can decrease the adsorption strength of intermediate such as COad on Pd and can prevent the accumulation of poisoning intermediates, which promotes the oxidation of formic acid in the direct pathway; (2) the hydrogen spillover effect of WO3 will accelerate the dehydrogenation of HCOOH on Pd and will lead to the higher rates of formic acid electrooxidation on the Pd/ WO3-CNTs than on the Pd/CNTs. In addition, the kinetic parameters such as the charge transfer parameter (α) and the diffusion coefficient (D) of formic acid electrooxidation on the catalyst electrodes have been investigated. The results suggest that the chargetransport rate within the liquid film near the 20 %Pd/20 %WO3-CNTs electrode surface is much higher than that on 20 %Pd/CNTs electrode surface. Acknowledgments This work was supported by the International Science and Technology Cooperation Program of China (no. 2010DFB63680), the National Natural Science Foundation of China (nos. 21376220 and 21106133), Zhejiang Provincial Natural Science Foundation of China (no. LQ12B03003), Science and Technology Plan Project of Zhejiang Province (no. 2012C37028) and Taizhou College Students' Research Projects (no.13XS27).
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