A Robust Versatile Hybrid Electrocatalyst for the Oxygen Reduction

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A Robust Versatile Hybrid Electrocatalyst for the Oxygen Reduction Reaction Kun Wang, Yi Wang, Yexiang Tong, Zhangweihao Pan, and Shuqin Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03751 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 11, 2016

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ACS Applied Materials & Interfaces

A Robust Versatile Hybrid Electrocatalyst for the Oxygen Reduction Reaction Kun Wanga, Yi Wangb*, Yexiang Tongc, Zhangweihao Pana, Shuqin Songa* a

The Key Lab of Low-carbon Chemistry & Energy Conservation of Guangdong Province,

School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China. b

School of Chemical Engineering and Technology, Sun Yat-sen University, Zhuhai 519082,

China. c

School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China.

KEYWORDS: WC-FeWO4@FeN-OMC, oxygen reduction reaction, in acidic and basic media, comparable activity, superior stability.

ABSTRACT: Identifying non-precious-metal catalysts with desirable over-all performance for oxygen reduction reaction (ORR) in either acidic or basic media is still a bottleneck. Here, a hybrid material is reported, in which tungsten carbide (WC) and ferberite (FeWO4) are attached to the Fe and N dual-doped ordered mesoporous carbon (WC-FeWO4@FeN-OMC), as a superior performance catalyst for the ORR in either acidic or basic media. In comparison with the frequently used Pt/C ORR catalyst (20 wt.%), our hybrid materials exhibit comparable

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electrocatalytic activity mainly via a 4e ORR process, better stability, and totally tolerance to methanol in either acidic or basic media. These advantages, especially the outstanding stability in acidic media, render the WC-FeWO4@FeN-OMC as a promising potential non-precious-metal ORR catalyst in practical fuel cell applications.


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1.

Introduction Excessive demand for energy has been attracting more and more attention on developing

alternative sustainable highly efficient, environmental friendly and cheap energy systems. The oxygen reduction reaction (ORR) is vital for renewable-energy technologies including fuel cells1,2 and metal-air batteries.3,4 Owing to its sluggish kinetics, high over-potential and poor cycling durability in either acidic or basic media, effective ORR catalysis still remains a major contemporary hurdle. Until now, it has been acknowledged that Pt or Pt-based alloys are the best catalysts for the 4e ORR. However, susceptibility to time-dependent drift, sensitivity to CO and high cost5-7 are the main shortcomings, which leads to the unsuitability of Pt-based electrocatalysts for large-scale applications of fuel cells. Hence, substitute catalysts based on non-precious-metals (NPMs) or metal-free alternatives have been intensively spotlighted. Heteroatom doped carbon materials, for example graphene,8-15 carbon nanotubes16-20 and amorphous carbon,21-28 have been identified to possess outstanding performance for the ORR because the doped atoms can induce uneven charge distribution on their adjacent carbon atoms. The ORR catalysts in basic media have achieved big advance. However, insufficient activity and poor stability of these catalysts in acidic media still exist and have become a limiting factor to the end-use for fuel cells. Therefore, it is a great challenging but necessary issue to develop efficient universal ORR NPMs catalysts in either acidic or basic media. Tungsten carbide (WC) has attracted more and more concerns due to its similar catalytic properties to those of Pt,29-31 possessing both desirable stability in acidic or basic media,32-36 and desirable tolerance to CO and H2S poison.37-38 However, WC alone shows limited activity in electrochemical reactions. Fortunately, the synergistic effect of WC with noble metals suggests that WC, as an accelerating agent for electrocatalyst, can increase the electrocatalytic


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performance of precious metal catalysts. In order to overcome the limitation on the WC electrochemical activity for ORR, lots of work on controlling the size of WC,39 the shape of WC,40 and WC-based nanocomposites have been reported.41 In the present study, we identify a novel hybrid ORR electrocatalyst by attaching the WC and FeWO4 compounds to Fe and N dual-doped OMC (WC-FeWO4@FeN-OMC). This hybrid material is able to promote a 4e ORR electrocatalysis with a desirable activity, superior stability and excellent methanol tolerance in comparison with that of the counterpart Pt/C catalyst (ETEK) in either acidic or basic condition.

2.

Experimental

2.1 Raw materials There was no any further purification for all adopted reagents of analytical grade. 2.2 Materials preparation Tungsten carbide/ Ordered mesoporous carbon composites (denoted as WC/OMC) were synthesized through a hydrothermal process with ammonium metatungstate (AMT) and glucose as the tungsten precursor and carbon source, respectively.45 In this process, an ordered mesoporous silica (SAB-15) was adopted as the hard template. The WC-FeWO4@FeN-OMC hybrids were prepared as follows: 100.0 mg hemin (C34H32ClN4O4Fe, Aladdin) were dissolved in 50 mL acetic acid solution and mixed with 200.0 mg WC/OMC. After 1 h stirring, the asobtained mixture was then completely condensed at 70oC under vigorous stirring and the obtained powders were treated at 50oC in an oven overnight. Finally, the as-prepared sample was calcinated at 700oC for 3 h in nitrogen with the heating rate being controlled at 5oC/min. For


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comparison, FeN-C was also prepared using the same procedure as those for the WCFeWO4@FeN-OMC but without WC/OMC. 2.3 Physical and chemical characterization Small-angle and wide-angle X-ray diffraction (XRD) measurements were carried out by a D-MAX 2200 VPC diffractometer with Cu Kα radiation (30 kV, 30 mA). Transmission electron microscopy (TEM) images were taken on a JEM-2010 (HR) at 200 kV. Scanning electron microscopy (SEM) and element mapping images were obtained using a Quanta 400FEG. X-ray photoelectron spectroscopy (XPS) tests were conducted by an ESCALAB 250. 2.4 Electrochemical characterization Rotating disk electrode (RDE) The electrochemical tests were performed on an Auto84480 electrochemical station in a home-made electrolyzer, in which the counter electrode was a Pt foil. A glassy carbon (GC) disk (d= 5.0 mm) was used as the substrate of the working electrode. The working electrode was swept at 20 mV/s at different rotating speeds of 400-2000 rpm. In the basic electrolyte solution, Hg/HgO (filled with 0.1 mol/L KOH, with the potential of -0.867 V vs. NHE) acted as the reference electrode. The ORR activity was evaluated in 0.1 mol/L KOH saturated with nitrogen or oxygen at room temperature. In the acidic electrolyte solution, saturated calomel electrode (SCE, filled with saturated KCl solution, its potential is -0.242 V vs. NHE) served as the reference electrode. The ORR activity was evaluated in 0.5 mol/L H2SO4 saturated with nitrogen or oxygen. The catalyst ink preparation was the same as described in our previously published


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paper.47 For NPMs catalyst, the catalyst loading was maintained at 800 µg catalyst/cm2.21,24 For the Pt/C (20 wt. Pt%), the catalyst loading was 30 µg Pt/cm2. To investigate the mechanism of the ORR, Koutecky-Levich (K-L) plots were further obtained from the RDE results. After linear fitting, their corresponding slopes were adopted to obtain the number of transferred electrons (n) due to the K-L equation: i -1 = ik−1 + iL−1

iL = 0.620nFADO 2/3ω1/2ν −1/6CO2 = Bω1/2

(1)

(2)

where iL (A) is limiting current, ik (A) is ORR’s kinetic current, n (mol) is the transferred electrons involved in the ORR for each oxygen molecule, F is the Faraday’s constant (96485 C/mol), A (cm2) is the electrode area, DO2 (cm2/s) is the diffusion coefficient of oxygen in 0.1 mol/L KOH solution saturated with oxygen, ω (s) is the rotating speed of electrode in rounds per min (rpm), ν (cm2/s) is the kinematic viscosity of the electrolyte, CO2 (mol/cm3) is the oxygen concentration in 0.1 mol/L KOH saturated with oxygen. B (mA/rpm1/2) is the reciprocal of slopes.4-5 Rotating ring-disk electrode (RRDE) Catalysts and the corresponding electrodes were obtained referred to the literature.47 The ring current (IR) was tested by adopting a Pt ring electrode in either basic or acidic media. The potential of 0.5 V (vs. Hg/HgO) was applied to Pt ring electrode in 0.1 mol/L KOH or 1.2 V (vs. SCE) in 0.5 mol/L H2SO4 saturated with oxygen. The 4e selectivity of ORR was calculated according to Equation (3), and the H2O2 yield was determined by Equation (4):


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n=

4 × ID ID + IR / N

(3)

4−n 2

(4)

%HO −2 =

where ID and IR is the disk and ring current, respectively, and N is the collection efficiency of H2O2 on the Pt ring with the value of 0.37.18,19 The methanol tolerance test was conducted in 0.1 mol/L KOH or 0.5 mol/L H2SO4 solution saturated with oxygen with or without 0.5 mol/L CH3OH by using CV technique. The accelerated aging tests were conducted in 0.1 mol/L KOH or 0.5 mol/L H2SO4 saturated with oxygen by CV, which was obtained by 10000 cycles between 0.6 V-1.0 V (vs. NHE) at 50 mV/s.

3.

Results and discussion The detailed process for the WC-FeWO4@FeN-OMC fabrication is schematically

demonstrated in Figure 1. Tungsten carbide/ordered mesoporous carbon composites (WC/OMC) were synthesized via a hydrothermal process as above described. Next, hemin (C34H32ClN4O4Fe, Aladdin, a metal heterocyclic molecule containing nitrogen, as the Fe and N sources) and WC/OMC were uniformly mixed in acetic acid solution. Subsequently, the solvent was evaporated in a heating and drying process. The as-prepared powders were further heated at 700oC for 3 h in nitrogen atmosphere. In this course, FeWO4 (FeO & WO3) nanoparticles nucleated and were supported on the ordered mesoporous carbon, with the incorporation of both Fe and N species and then into the carbon lattices. The final product is a hybrid composed of WC, FeWO4, and Fe-N co-doped ordered mesoporous carbon.


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The structure and morphology of the WC-FeWO4@FeN-OMC were checked by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The XRD results identify the formation of FeWO4 in the hybrid through the introduction of hemin (Figure S1). On the other hand, a decreased order degree can be found for the obtained WC-FeWO4@FeN-OMC, which can be estimated from the weakened strength of the (10) diffraction peaks that represents the formation of a highly ordered 2D hexagonal mesostructured (Figure S2). To study the pore structure of WC/OMC and WC-FeWO4@FeN-OMC, their N2 adsorption-desorption isotherms curves were obtained (Figure S3). There exist distinct hysteretic loops in the typical IV N2 adsorption-desorption isotherms for both samples, indicating their mesoporous characteristics. The BET surface areas and mesopore volumes of WC/OMC and WC-FeWO4@FeN-OMC are 676 m2/g and 0.43 cm3/g, 409 m2/g and 0.47 cm3/g, respectively. SEM and elemental mapping analysis indicate the existence of C (78.14 at.%), O (18.52 at.%), W (2.69 at.%) and Fe (0.65 at.%) in the WC-FeWO4@FeN-OMC hybrid with homogeneous distributions (Figures 2a-b). High-resolution TEM (HRTEM) reveals that FeWO4 is singlecrystal and the respective crystal lattice fringes of 0.57 nm and 0.36 nm correspond to (100) and (110) atomic spacing of the FeWO4 single-crystal structure (Figure 2d). However, it is difficult to give the HRTEM image of the WC because it may have a bad crystal form resulting from the easy oxidation of WC exposed in the air, although the XRD results identify the existence of WC. As well-known, the XRD characterization is based on the bulk content but the TEM characterization is based on the tiny local-microscopic area. In addition, the atomic ratio of the W to Fe from the following EDS data (inset of Figure 2a) is more than one, which also could claim that parts of W exists in the form of WC.


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The XPS results gives the atomic percentages at the sample surface of C (79.03 at.%), O (11.99 at.%), W (1.27 at.%), N (5.38 at.%) and Fe (1.1 at.%) for the WC-FeWO4@FeN-OMC hybrid (Figure 3a). The high content of O element exists due to the oxidation state of WC exposed in the air, which can also be proved from the W4f spectrum of WC/OMC in Figure 3f. The spectrum of high resolution N1s of the hybrid can be deconvoluted into different signals, including 398.6, 399.7, 399.8, 401.0, and 404.6 eV, which correspond to pyridinic N (38.9 at.%), pyrrolic N (23.1 at.%), Fe-N (7.6 at.%), graphitic N (24.8 at.%), and other N (5.3 at.%), respectively (Figures 3b-c). Among them, pyridinic N is acknowledged to be catalytically active for the ORR. The high resolution Fe 2p3/2 spectrum of the hybrid includes Fe0 and oxidized iron at 707.5, 709.4, and 710.9 eV. They correspond to Fe or Fe-N, Fe2+ and Fe3+ (Figure 3d), respectively. Furthermore, the high resolution W4f spectrum suggests that the WC/OMC and WC-FeWO4@FeN-OMC consist of WC and WO3-x states (Figures 3e and S5). The peak of W4f spectra of WO3 in the hybrid is negatively shifted relative to that of WC/OMC (Figure 3f), which can be explained by the electron transfer between FeO and WO3 after the formation of FeWO4 (FeO & WO3). One strange phenomenon could be noticed that the Fe peak is quite small and difficult to separate from the background with respect to the W peak, though the atoms ratio of Fe to W is nearly even (Figure 3a). This is because that the intensity of XPS peak is not only related to the species content, but also is directly dependent on the sensitivity factor (SF) for each element. Here, SF of W (2.75) is higher than that of Fe (2.00).52 In order to further clarify the formation of Fe-N bonding not Fe-C bonding, the high resolution C1s have been shown (Figure S4). Obviously, N is successfully doped into the C lattice, which could be proved by the C1s (287.2 eV, C-N) and N1s (pyridinic N, pyrolic N, graphitic N). On the other hand, Fe-C bonding (Fe3C) is not observed from the high resolution C1s spectrum. Also, through the comprehensive


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analysis of the high resolution C1s, Fe2p and N1s spectra, it could identify that Fe-N bonding is formed in the WC-FeWO4@FeN-OMC hybrid. As well-known, Fe3C is active for enhancing ORR. In fact, its enhancing mechanism for ORR may be due to that it can greatly improve the graphitic degree of carbon support. Because the Fe3C formed in the high-temperature pyrolysis process is not stable, and easy to be decomposed into Fe and highly oriented carbon in the low temperature, which could be identified by the improved graphitic degree after the introduction of hemin (Figure S6), further leading to the enhancing ORR performance. To assess the ORR activity of the WC-FeWO4@FeN-OMC hybrid in acidic media, we first examined its electrocatalytic properties in 0.5 mol/L H2SO4 saturated with nitrogen or oxygen by the technique of CV at 20 mV/s (Figure 4a). FeN-C and WC/OMC were also evaluated as the contrast objects (Figure S7). In the nitrogen-saturated solution, all samples present no obvious characteristic peaks in CV curves. Whilst, one can see that there are both reduction and oxidation peaks at about 0.4 V, which could be resulted from the redox couple of hydroquinone-quinone.46 In 0.5 mol/L H2SO4 saturated with oxygen, they present well-defined reduction peaks for the ORR. There exists an obvious ORR peak at 0.42 V in the the CV of the WC-FeWO4@FeNOMC displays. One can observe that the ORR onset potential is distinctly shifted to the highpotential direction with respect to that of FeN-C and WC/OMC. The high ORR activity at WCFeWO4@FeN-OMC is also judged from its onset potential (0.53 V vs. SCE) (Figure 4b), which is very approximate to the corresponding value of Pt/C (0.57 V). Base on the review results (Table S1), the ORR activity of WC-FeWO4@FeN-OMC is one of NPMs electrocatalysts with the highest reported activities in acidic media. 48-49 As known, the ORR can take place through the following two different processes. One is a 4e pathway, in which oxygen reacts with e and H+ or hydroxyl to produce water. The other is a 2e pathway with OOH- as an intermediate.42-44


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To further study the electrocatalytic process of the WC-FeWO4@FeN-OMC in acidic solution, the linear sweeping curves were acquired on the RDE (Figure S8). The K-L plots present linear relation and their fitting lines are almost in parallel, implying that ORR follows a first-order reaction kinetics and involves close electron transfer number for various applied potentials.13 To prove the ORR mechanism of the hybrid, we employed a RRDE, where the generated H2O2 could be quantitatively analyzed. The measured H2O2 yields are below ~8 % from -0.1 to 0.5 V, with n of ~3.9 (Figures 4c-d). It agrees well with the result of the K-L plots, implying the ORR at WC-FeWO4@FeN-OMC in acid primarily undergoes a 4e pathway. The durability of the WC-FeWO4@FeN-OMC hybrid was evaluated by using the electrochemical accelerated aging process by round-scanning the potential from -0.2 to 0.8 V at 20 mV/s in 0.5 mol/L H2SO4 saturated with oxygen.50-51 After 10000 continuous potential cycles, a small negative change of ~105 mV is observed in the half-wave potential E1/2 (Figure 5a), which shows superior durability to that of Pt/C (~188 mV negative shift; see Figure 5b). Considering that, methanol crossover can cause a deteriorated ORR performance, our hybrid was also checked for methanol tolerance. We examined the CV (Figure S9) and RDE (Figures 5c-d) curves in the presence of 0.5 mol/L methanol. The ORR activity at Pt/C is drastically decreased induced by methanol oxidation reaction (MOR), but the WC-FeWO4@FeN-OMC showed no distinct activity degradation. The outstanding tolerant capability to CH3OH can be resulted from the obviously decreased ORR potential than that for MOR at [email protected] results imply that the WC-FeWO4@FeN-OMC can be adopted as an substitute NPMs catalyst with desirable versatile performance for ORR in acidic solution. This is associated with (i) the synergistically enhanced electrochemical activities between WC, FeWO4 and FeN/OMC for the ORR and (ii) the increasing graphitic degree of OMC after Fe introduction.


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On the other hand, we also measured the ORR in basic media. CVs of various catalysts in 0.1 mol/L KOH saturated with nitrogen or oxygen were acquired to investigate the ORR performance of the catalysts (Figure S10, 6a). In the solution saturated with nitrogen, all catalysts show featureless CV curves. Comparatively, in 0.1 mol/L KOH saturated with oxygen, they present clear reduction peaks for the ORR. There exists an obvious reduction peak for ORR at -0.07 V in the CV curve for the WC-FeWO4@FeN-OMC catalyst (Figure 6a). One can observe that the ORR onset potential is distinctly shifted to the high-potential direction with respect to that of FeN-C (-0.12 V) and WC/OMC (-0.16 V) (Figure S10). To further check the ORR performance of the samples in basic solution, the ORR polarization curves on FeN-C, WC/OMC, WC-FeWO4@FeN-OMC and Pt/C were obtained by RDE and the results are shown in Figure 6b. FeN-C and WC/OMC have the ORR onset potential of -0.01 V and -0.09 V, and limiting current density of -4.30 mA/cm2 and -4.40 mA/cm2, respectively. Interestingly, WC-FeWO4@FeN-OMC exhibits significantly higher activity with the ORR onset potential of 0.04 V, which is close to Pt/C in terms of onset potential. These results suggest that WC, FeWO4 and FeN-OMC in WC-FeWO4@FeN-OMC hybrid play a combined role in enhancing the ORR performance in basic media. The polarization curves and corresponding K-L curves at the WC-FeWO4@FeN-OMC hybrid and RRDE results at 1600 rmp were obtained, as shown in Figures 6c-f. The current density (j) at various rotating speeds (ω) and pre-set potentials is used for plotting K-L curves through plotting j-1 against ω-1/2 based on Equations (1) and (2). As shown in Figure 6d, there exists a good linearity for all the K-L plots at different applied constant potentials. Moreover, one can observe that after all the fitting lines are parallel at various potentials. This gives an indicator that that the ORR on WC-FeWO4@FeNOMC is first-order reaction at the investigated potentials. Among principle factors for ORR


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catalysts, the preferred number of transferred electrons (n) should be four. For further insight, one can distinguish from Figure 6d that the corresponding fitting plots are linear ones, which is almost in parallel with the theoretical plot of the 4e ORR processat the investigated potentials and the n value is estimated to be 3.9-4.1, indicating that the ORR on the WC-FeWO4@FeNOMC catalyst proceeds in a 4e transfer path in basic media. The 4e reaction selectivity of ORR on the WC-FeWO4@FeN-OMC catalyst was further evaluated by RRDE technique. Figures 6e-f show the RRDE results on WC-FeWO4@FeN-OMC in 0.1 mol/L KOH saturated with oxygen. In light of RRDE curves, the n value and H2O2 yields could be calculated according to Equations (3) and (4), respectively. The n value is almost four from -0.75 to -0.05 V, which is in agreement with the RDE results (Figure 6d). Moreover, the very low ring current suggests that a quite low amount of H2O2 was produced. Therefore, the WC-FeWO4@FeN-OMC hybrid can catalyze ORR via an intrinsic 4e transfer process in basic solution. The long-term durability of the WC-FeWO4@FeN-OMC electrocatalyst is deemed to be an essential factor for ORR performance. The RDE curves of WC-FeWO4@FeN-OMC (Figure 7a) shows no obvious decay in the ORR activity after accelerating aging process by continuously performing 10000 potential cycles from -1.0 V to 0.1 V at 20 mV/s in 0.1 mol/L KOH saturated with oxygen. Comparatively speaking, there is an obvious deterioration in ORR catalytic activity for Pt/C through the same accelerating process. After the round-potential-scanning, E1/2 and limiting current density is almost not changed for these two ORR electrocatalysts (Figure 7b). Consequently, the WC-FeWO4@FeN-OMC hybrid possesses a comparable durability to Pt/C. The desired ORR catalytic materials should possess satisfying inertness to the adopted fuel in PEMFCs (e.g., CH3OH for direct methanol fuel cell) because fuel crossover exists. To check the tolerance to CH3OH, CV and RDE curves were conducted in 0.1 mol/L KOH saturated with


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oxygen in absence or presence of 0.5 mol/L CH3OH, respectively (Figures 7c-d, Figure S11). It is found that the WC-FeWO4@FeN-OMC electrocatalyst exhibits a high tolerance to methanol. Comparatively, there is an obvious deterioration in the ORR performance for Pt/C because of the electrocatalytic capability of Pt/C to both ORR and CH3OH oxidation.

4. Conclusion In a word, we have successfully synthesized the WC-FeWO4@FeN-OMC hybrid as one of the high performance universal ORR electrocatalysts in either acidic or basic media, which exhibits a desirable ORR activity, prior stability and excellent tolerant capability to CH3OH in comparison to commercial Pt/C catalyst, potentially making this hybrid an appealing NPMs catalysts for ORR. The synergistic effect of nanomaterials broadens the channels to advanced energy materials.


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Figure 1. Synthesis protocol of the WC-FeWO4@FeN-OMC hybrid.


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Figure 2. (a) Field emission SEM image of the WC-FeWO4@FeN-OMC hybrid (inset is the EDS analysis of the whole area in image a); (b) C, O, W and Fe elemental mappings of the whole region in Figure 2a; (c) TEM image of the WC-FeWO4@FeN-OMC; (d) HRTEM image of the blue marked area in (c) (inset is the corresponding electron diffraction patterns).


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Figure 3. (a) XPS survey of the WC-FeWO4@FeN-OMC; (b) The high-resolution XPS spectrum of N1s; (c) The contents of different N types in the WC-FeWO4@FeN-OMC; (d) The high-resolution XPS spectrum of Fe2p3/2; (e) The high-resolution XPS spectrum of W4f; (f) The high resolution XPS spectrum of W4f of the WC-FeWO4@FeN-OMC and the WC/OMC.


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Figure 4. (a) CV curves of the WC-FeWO4@FeN-OMC for the ORR in nitrogen- or oxygensaturated 0.5 mol/L H2SO4 at 20 mV/s; (b) RDE voltammograms in an oxygen-saturated 0.5 mol/L H2SO4 at room temperature (1600 rpm, 20 mV/s) for the FeN-C, WC/OMC and WCFeWO4@FeN-OMC; (c) RRDE results and (d) electron transfer number and H2O2 yield on the WC-FeWO4@FeN-OMC in an oxygen-saturated 0.5 mol/L H2SO4 (1600 rpm, 20 mV/s) for the WC-FeWO4@FeN-OMC.


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Figure 5. Comparison on the cyclic voltammograms of (a) the WC-FeWO4@FeN-OMC and (b) commercial Pt/C before and after 10000 cycles in an oxygen-saturated 0.5 mol/L H2SO4; RDE voltammograms for (c) the WC-FeWO4@FeN-OMC and (d) commercial Pt/C in an oxygensaturated 0.5 mol/L H2SO4 in the absence or presence of 0.5 mol/L methanol.


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Figure 6. (a) CVs of WC-FeWO4@FeN-OMC for ORR in nitrogen- or oxygen-saturated 0.1 mol/L KOH at 20 mV/s; (b) RDE results in oxygen-saturated 0.1 mol/L KOH (1600 rpm, 20 mV/s) for FeN-C, WC/OMC and WC-FeWO4@FeN-OMC; (c) RDE results for WCFeWO4@FeN-OMC at different rotation speeds (20 mV/s); (d) The K-L plots at applied constant potentials; (e) RRDE results and (f) the electron transfer number and H2O2 yield on the WCFeWO4@FeN-OMCin oxygen-saturated 0.1 mol/L KOH solution (1600 rpm, 20 mV/s).


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Figure 7. RDE voltammograms (1600 rpm, 20 mV/s) for the WC-FeWO4@FeN-OMC (a) and the commercial Pt/C (b) initial and after 10000 round-potential-scanning in oxygen-saturated 0.1 mol/L KOH; RDE voltammograms (1600 rpm, 20 mV/s) for the WC-FeWO4@FeN-OMC (c) and Pt/C (d) in oxygen-saturated 0.1 mol/L KOH or 0.1 mol/L KOH+0.5 mol/L methanol.


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ASSOCIATED CONTENT Supporting Information. Characterization data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Shuqin Song: [email protected]. *Yi Wang: [email protected]. Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT Authors would like to thank the financial support of the Sino-Greek Science and Technology Cooperation Project (2013DFG62590), the National Natural Science Foundation of China (Grant No. 21575299, 21576300, 21276290), and Guangdong Province Nature Science Foundation (2014A030313150).


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Graphical Abstract


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A Robust Versatile Hybrid Electrocatalyst for the Oxygen Reduction

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