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Acid-Stable CoWO4/WO3-Microrod Coated by a Thin Carbon-Layer as Efficient Pt Co-Catalysts for Methanol Oxidation and Oxygen Reduction

Jiahuan Li ,  Jiahao Xie ,  Xinyu Wang ,  Ying Dai ,  Xiaoqin Xu ,  Jin Liu ,  Zhuang Cai ,  Xin Meng ,  Jinlong Zou

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Citation:  Jiahuan Li, Jiahao Xie, Xinyu Wang, Ying Dai, Xiaoqin Xu, Jin Liu, Zhuang Cai, Xin Meng, Jinlong Zou. Acid-Stable CoWO4/WO3-Microrod Coated by a Thin Carbon-Layer as Efficient Pt Co-Catalysts for Methanol Oxidation and Oxygen Reduction[J]. Chinese Journal of Structural Chemistry, 2022, 41(7): 220705. doi: 10.14102/j.cnki.0254-5861.2022-0104 shu

Acid-Stable CoWO4/WO3-Microrod Coated by a Thin Carbon-Layer as Efficient Pt Co-Catalysts for Methanol Oxidation and Oxygen Reduction

English

  • INTRODUCTION

    Throughout the history, energy is always the essential factor for development.[1] At present, most of countries (including developed and less-developed countries) still use traditional fossil fuels such as oil, coal and natural gas to meet the electricity demand.[2] The consumption of resources, the deterioration of environment, and the global climate change have forced us to find clean and pollution-free energy sources.[1] Direct methanol fuel cells (DMFCs) as a viable alternative have considerable advantages including low cost and friendly environment, consistent with the sustainable development strategies.[2, 3] It is known that Pt-based materials are generally used in DMFCs as both methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) catalysts.[3-5] In the case where the roles of Pt-based catalysts (monoatomic Pt or Pt nanoparticle (NPs)) are still irreplaceable (such as MOR), we should do more to explore the catalyst with a low Pt loading.[6-10]

    Transition metal compounds have long been served as catalyst/co-catalyst to increase the Pt utilization to gain superior MOR/ORR activity. Tungsten (W) compounds including CoWO4 and WO3 have shown the promising activities in many fields including photocatalysis (photocatalytic degradation or hydrogen evolution),[11-14] photodetectors,[15] fuel cells,[16] supercapacitors,[17, 18] and sensors (acetone or NH3 gas sensing).[19, 20] It has been reported that transition metal oxides (TMOs) can help Pt-based catalyst to reduce the CO poisoning in the MOR process,[21] because the strong metal-support interaction (SMSI) between Pt and TMOs can greatly improve the MOR activity and stability.[22, 23] Among various TMOs, WO3 usually exhibits a high oxygen storage capacity, a good corrosion resistance in acid medium, and a variable W valence, which can facilitate the adsorption of OHads on the Pt surface to promote the CO oxidation during MOR.[21, 22]

    The poor conductivity of WO3 limits its charge-transfer capacity, and therefore, it is interesting to combine WO3 with the highly-conductive materials such as carbon materials to further improve the catalytic activity and CO resistance of Pt-based catalysts. Georgieva et al. prepare the graphite-supported Pt/TiO2/WO3 catalyst by using deposition method, which is 5 times more active than Pt/WO3 for MOR under visible light.[24] Madhu et al. use W18O49 nanowires grown on carbon paper as the Pt support/co-catalyst for proton exchange membrane fuel cell, and Pt/W18O49/carbon paper composite with the three-dimensional structure shows the higher ORR activity and CO toxicity resistance than Pt/C.[25] Ye et al. prepare the Pt/WO3-C catalyst by using the microwave pyrolysis method, which can improve the Pt NPs distribution and electrochemical surface area to enhance the MOR activity.[26] Note that the high activity is also attributed to the uniform dispersion of WO3 on carbon.[26] Furthermore, it is worth to note that wolframite-type CoWO4 belonging to a monoclinic crystal structure is a well-known electroactive material.[24-26] The nano-structure of CoWO4 usually has the high thermal stability and electrical conductivity.[27] However, its charge transfer resi tance s still needs to be lowered to improve the catalytic stability and activity. Therefore, the combination between W-compounds (WO3/CoWO4) and conductive carbon may be a feasible way to prepare the Pt support/co-catalyst.

    In this study, zeolitic imidazolate framework (ZIF)-8@ZIF-67 material is synthesized as carbon and Co sources to prepare the Co-species embedded nitrogen-doped carbon (Co-NC). The mixture of ammonium tungstate and Co-NC is carbonized to obtain the WO3@N-doped carbon layer (WO3@NCL micro-rods) to support the CoWO4 NPs (CoWO4/WO3@NCL-mr). It is the first time to use CoWO4/WO3@NCL-mr catalysts as the Pt-supports/co-catalyst for MOR/ORR. WO3 is grown directionally along the (110) crystal plane to form the micrometer-sized rod. By carbonization at high temperature, WO3-mr is covered by a carbon thin-layer (~5 nm) derived from NC to protect the oxygen vacancies and active sites on the surface of WO3. CoWO4 NPs are derived from the combination of Co and W species, which are firmly anchored on the surface of WO3@NCL-mr. Due to the largely-exposed surface of micro-rod, Pt NPs can be well dispersed on the surface of CoWO4/WO3@NCL-mr to increase the exposure of Pt active sites. As expected, the resistance to CO (MOR) or methanol (methanol penetration, ORR) poisoning, as well as the MOR/ORR activity of Pt-CoWO4/WO3@NCL-mr catalysts, is much higher than those of commercial Pt/C in acidic electrolytes.

    RESULTS AND DISCUSSION

    Composition and Structure Characterizations of CoWO4/WO3@NCL-mr. Morphology and structure of the synthesized samples are elucidated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images. Figure 1a shows the morphology of Co-NC obtained from the carbonization of ZIF-8@ZIF-67 polyhedron at 900 ℃. Figure 1b-1e show the SEM images of CoWO4/WO3@NCL-mr-X (X = 750, 800, 850 and 900), which indicate that the micro-rods orienting along the (110) crystal plane of WO3 are formed. The formation of microrods is ascribed to two reasons including Oswald ripening and Kirkendall effects.[28-30] With the addition of W sources, the dodecahedral structure of Co-NC completely disappears. Co-NC is decomposed to form the carbon layer to coat on the surface of WO3-mr, and the Co atoms are combined with W atoms to generate the CoWO4 NPs. As shown in Figure 1f, CoWO4/WO3@ NCL-mr-850 has the micro-rod shape with a length of ~1.5 um and a diameter of ~0.6 um. In Figure 1g and 1h, the lattice fringes of CoWO4, WO3, and Pt can be clearly observed (HRTEM images). As Figure 1g depicts, there is an obvious carbon thin-layer (~5 nm) coated on the surface of WO3-mr, which can protect the oxygen vacancies and active sites on WO3-mr. The carbon layer is gene rated by the pyrolysis (decomposition) of NC.[31] In Figure 1h, the micro-rod-shaped body is WO3 with the monoclinic structure and attachment of CoWO4 and Pt NPs,[32] and the lattice spacing of 0.37, 0.28, 0.22 nm corresponds to the crystal planes of WO3 (110), CoWO4 (020) and Pt (111), respectively.

    Figure 1

    Figure 1.  SEM images of Co-NC (a) and CoWO4/WO3@NCL-mr-X (X = 750 (b), 800 (c), 850 (d) and 900 (e)); TEM image (f) and high-resolution TEM images (g and h) of Pt-CoWO4/WO3@NCL-mr-850.
    DownLoad: Full-Size Img PowerPoint

    Figure 2 shows the X-ray diffraction (XRD) patterns of the CoWO4/WO3@NCL-mr composites before and after Pt loading. In Figure 2a, four characteristic diffraction peaks of CoWO4/WO3@NCL-mr-X (X = 750, 800, 850, and 900) at around 24.0°, 28.2°, 33.1°, and 34.0° correspond to the (110), (101), (111) and (200) crystal planes of WO3 (JCPDS, 05-0388), respectively. Other peaks can be classified as the wolframite-type monoclinic CoWO4 crystal [space group P2/a (13)] (JCPDS, 15-0867).[33, 34] In Figure 2b, the diffraction signal of Pt can be clearly observed at around 39.7°, corresponding to the (111) crystal plane of Pt (JCPDS, 04-0802). These peaks match well with the standard cards and there are no other impurities, indicating that only WO3 and CoWO4 co-exist in the catalysts, in agreement with TEM results. The XRD patterns do not show a graphic carbon peak, indicating that WO3-mr and/or CoWO4 NPs should be wrapped by the amorphous carbon-layer.

    Figure 2

    Figure 2.  XRD patterns of CoWO4/WO3@NCL-mr-X (a) and Pt-CoWO4/WO3@NCL-mr-X (b) (X = 750, 800, 850 and 900).
    DownLoad: Full-Size Img PowerPoint

    The chemical compositions of Pt-CoWO4/WO3@NCL-mr catalysts are characterized by X-ray photoelectron spectroscopy (XPS) (Figure 3 and Figure S1-S7). As expected, Co 2p, Pt 4f, W 4f, O 1s, C 1s and N 1s are observed in the catalysts. The high-resolution XPS spectra of Co 2p (Figure 3a and Figure S1) exhibit two peaks at around 780.9 (Co 2p3/2) and 796.5 (Co 2p1/2) eV, corresponding to the characteristic peaks of Co2+.[35, 36] The other two peaks at around 786.3 and 802.8 eV are the satellite peaks for Co 2p3/2 and Co 2p1/2 (marked as "sat."), respectively.[35, 37, 38] Figure 3b shows that the Pt species has been successfully deposited on the CoWO4/WO3@NCL-mr support. The Pt 4f spectra can be classified as the double peaks from the spin-orbit splitting of Pt 4f7/2 and Pt 4f5/2 (Figure 3b and Figure S2). The strongest doublet (at around 71.0 and 74.2 eV) belongs to the metallic Pt (Pt0), while the other doublet (at around 72.4 and 76.0 eV) corresponds to the PtO (Pt Ⅱ).[36] The main peaks are basically ascribed to Pt0, indicating that the SMSI between Pt and CoWO4/WO3@NCL-mr can well protect the Pt active sites from deactivation (oxidation).[36] In addition, the percentage contents of Pt0 and PtO in Pt/C and Pt-CoWO4/WO3@NCL-mr-X (X = 750, 800, 850 and 900) are shown in Figure S3, indicating that Pt0 is the main species in these catalysts. The catalytic activity of Pt is mainly determined by Pt0, followed by Pt Ⅱ.[36] Figure 3c and Figure S4 are the high-resolution XPS spectra of W4f. The spectra of W 4f indicates the presence of W in two chemical states (CoWO4 and WO3), and two sets of double peaks corresponding to W 4f7/2 and W 4f5/2 (W6+) can be observed.[35, 37, 38] The peaks at 35.2±0.1 and 35.8±0.1 eV correspond to the W 4f7/2, while the peaks at 37.4±0.2 and 38.3±0.1 eV correspond to the W 4f5/2.[35] Therefore, the XPS results confirm the successful formation of CoWO4 and WO3 or CoWO4/WO3 heterojunction (as shown in TEM image), which can promote the charge transfer to improve the ORR activity.

    Figure 3

    Figure 3.  High-resolution XPS spectra of Co 2p (a), Pt 4f (b), W 4f (c), O 1s (d), C 1s (e) and N 1s (f) for Pt-CoWO4/WO3@NCL-mr-850.
    DownLoad: Full-Size Img PowerPoint

    Figure 3d and Figure S5 show the deconvolution peaks of O 1s spectra. The three peaks correspond to O2- ion in WO4 root, WO3, and the oxygen ions in -OH adsorbed on the surface of the materials, which are located at around 530.1 (O2-) and 531.38 (W-O) and 532.73 eV (-OH), respectively.[34] The high-resolution C 1s spectra (Figure 3e and Figure S6) show three peaks including C1, C2, and C3 peaks located at around 284.17, 285.6, and 288.8 eV, respectively. Generally, peak C1 is assigned to the C-C bond corresponding to the amorphous carbon of NCL;[37] peak C2 is attributed to the C-O/C-N, which indicates the existence of N-doped carbon skeleton;[39] and peak C3 corresponds to the O-containing group of O-C=O. The hydrophilic O-groups on the carbon skeleton can promote the adsorption and reduction of oxygen.[40]

    Figure 3f and Figure S7 show the high-resolution spectra of N1s, indicating the successful doping of N atoms. The four deconvolution peaks of N 1s are located at around 398.4 (N1), 400±0.3 (N2), 400.7±0.4 (N3) and 403.7 eV (N4), which can be assigned to pyridine-N, pyrrole-N, graphite-N, and oxide-N, respectively.[40-42] As shown in Figure 4a, with the increase of temperature, the N2 contents in Pt-CoWO4/WO3@NCL-mr catalysts decrease due to the instability of N2 at high temperatures.[43] As previously reported, N1 and N3 are deemed to play important roles in enhancing the ORR catalytic activity.[41-43] N1 can reduce the energy barrier of O2 adsorption and increase the first electron transfer rate during ORR.[42] N3 can increase the electrical conductivity of catalyst to facilitate the charge transfer.[43] The doping of N atoms in the carbon framework makes the surrounding of carbon atoms more positively charged, which may also facilitate the coating and even-distribution of Pt NPs on the surface of CoWO4/WO3@NCL-mr. As shown in Figure 4b, the proportions of N1+N3 in Pt-CoWO4/WO3@NCL-mr-X (X = 800 and 850) even exceed 75%, which has a positive impact on the enhancement of ORR activity.

    Figure 4

    Figure 4.  Percentage contents of N2 (a) and N1+N3 (b) in Pt-CoWO4/WO3@NCL-mr-X (X = 750, 800, 850 and 900).
    DownLoad: Full-Size Img PowerPoint

    MOR Activities of Pt-CoWO4/WO3/ZNC-CNT Catalysts. Figure 5a shows that the cyclic volammetry (CV) curves of Pt-CoWO4/WO3@NCL-mr catalysts have obvious hydrogen adsorption/desorption regions, and Pt-CoWO4/WO3@NCL-mr-850 has the largest H-region area. ECSAs of the catalysts are calculated by using Equation (1), and the results are presented in Table S1. As shown in Table S1, the highest ECSA of 225.8 m2 gPt-1 is obtained by Pt-CoWO4/WO3@NCL-mr-850, which is approximately 1.97 times higher than that of Pt/C (114.47 m2 gPt-1). It is generally recognized that the larger the ESCA value, the better the MOR catalytic activity. Therefore, the MOR performance of Pt-CoWO4/WO3@NCL-mr-850 catalyst is superior to that of Pt/C. MOR activities of the catalysts tested in 0.5 M H2SO4 + 0.5 M CH3OH between -0.2 and 1.0 V are shown in Figure 5b. Pt-CoWO4/WO3@NCL-mr-850 shows the highest mass activity (2208 mA mgPt-1) among all of the catalysts, which is 3.34 times higher than that of Pt/C (659.4 mA mgPt-1). The mass activities of Pt-CoWO4/WO3@NCL-mr-900 (1700.4 mA mgPt-1) and Pt-CoWO4/WO3@NCL-mr-800 (1038.9 mA mgPt-1) are also higher than that of Pt/C. As shown in Table S2, the mass activity of Pt-CoWO4/WO3@NCL-mr-850 is much higher than those of most of recently-reported Pt-based catalysts. These results imply that the special structure (the micro-rod shape and N-doped carbon layer) of CoWO4/WO3@NCL-mr can increase the exposure of Pt active sites to improve the MOR activity, and the synergy and interfacial interaction between CoWO4 and WO3 can also increase the catalytic activity.[27, 44]

    Figure 5

    Figure 5.  CV curves tested in 0.5 M H2SO4 media (a); CV curves tested in 0.5 M H2SO4 + 0.5 M CH3OH (b); Time-current curves at a potential of 0.67 V (c); Nyquist plots measured in 0.5 M H2SO4 + 0.5 M CH3OH for Pt/C and Pt-CoWO4/WO3@NCL-mr-X (X = 750, 800, 850 and 900) catalysts (d).
    DownLoad: Full-Size Img PowerPoint

    Chronogalvanometry (CA) test is an effective method to evaluate the electrocatalytic stability under the steady-state condition. Figure 5c shows the CA curves of Pt-CoWO4/WO3 @NCL-mr and Pt/C catalysts. After 3600 s, the current densities of Pt/C and Pt-CoWO4/WO3@NCL-mr-850 are 112.04 and 1023 mA mgPt-1, respectively. Therefore, the MOR stability of Pt-CoWO4/WO3@NCL-mr-850 is better than that of Pt/C. Figure 5d shows the Nyquist plots and the equivalent circuit diagram (inset). The semi-circular Nyquist plots indicate that the electrode reactions involved are governed by the charge transfer step.[45] The diameter of the arc represents the charge transfer resistance (Rct). The smaller the diameter, the smaller the resistance, and the faster the charge transfer. As Pt-CoWO4/WO3@NCL-mr-850 (Rct of 77.77 Ω) has smaller diameter of arc than Pt/C (89.02 Ω), it has the fastest charge transfer speed, implying that the hydrogen tungsten bronze (HxWO3) with a high electrical conductivity may be generated on WO3 by combining with hydrogen ion.[46] Moreover, the oxygen vacancies on WO3 can facilitate the hydrogen ion diffusion for HxWO3.[47, 48]

    To identify the CO tolerance of Pt-CoWO4/WO3@NCL-mr during MOR, CO stripping tests are performed. In Figure 6, the onset potential of CO oxidation on Pt/C catalyst appears at around 0.54 V, while that for Pt-CoWO4/WO3@NCL-mr catalysts has significant negative shifts. Therefore, Pt-CoWO4/WO3@NCL-mr catalysts have high CO oxidation activity (CO poisoning tolerance). Compared with Pt/C, CO on Pt-CoWO4/WO3@NCL-mr catalysts is more easily removed (oxidized) to release the active sites. As reported previously, W species can provide abundant oxygen species for CO removal.[45, 49] In addition, the SMSI between Pt and support greatly promotes the exposure of Pt active sites to thus improve the CO oxidation and MOR activity. In the previous reportes, HxWO3 may be generated by the spillover effect (desirable) to improve the CO oxidation and charge transfer.[50-52] Note that the CO mobility on different crystal planes may be affected by the number and distribution of the exposed crystal planes,[53] which accordingly affects the CO tolerance of Pt-CoWO4/WO3@NCL-mr catalysts. CoWO4/WO3 heterojunction attached on the micro-rod structure of WO3 with abundant oxygen vacancies allows rapid oxygen transfer along the interfacial channel, thereby enhancing the redox interfacial activity with the generation of surface-adsorbed hydroxyl, which facilitates the quick regeneration (CO elimination) of the occupied Pt active sites to form a clean catalyst for MOR.

    Figure 6

    Figure 6.  CO stripping voltammograms of Pt/C and Pt-CoWO4/WO3@NCL-mr-X (X = 750, 800, 850 and 900) catalysts.
    DownLoad: Full-Size Img PowerPoint

    As shown in reactions 1-4, CO can be independently adsorbed on CoWO4 (CoWO4-COads) and then oxidized by the surface-adsorbed hydroxyl (CoWO4-(OH)ads).[46, 47] As reported previously, the rearrangement of Co2+ ions in CoWO4 and the high proportion of Co2+-ion pairs with a shorter nearest-neighbor distance favor the methanol (including CO) oxidation via the Langmuir-Hinshelwood (L-H) pathway.[32] Therefore, Pt-CoWO4/WO3@ NCL-mr catalyst has a high resistance to CO poisoning and an outstanding MOR stability. The reaction processes on Pt-CoWO4/WO3@NCL-mr catalysts during the anodization are shown below (reactions 1-9). The hydrogen ions in Pt-H are transferred from Pt to WO3 to form HxWO3 to remove hydrogen, which is called the spillover effect.[54, 55] According to reactions (8) and (9), the formation of HxWO3 makes the dehydrogenation of methanol more effective and helps to remove the adsorbed intermediates during MOR.

    $ \mathrm{C}\mathrm{o}\mathrm{W}{\mathrm{O}}_{4}+\mathrm{C}{\mathrm{H}}_{3}\mathrm{O}\mathrm{H}\to \mathrm{C}\mathrm{o}\mathrm{W}{\mathrm{O}}_{4}–\mathrm{C}{\mathrm{O}}_{\mathrm{a}\mathrm{d}\mathrm{s}}+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-} $

    		(1)

    $ \mathrm{C}\mathrm{o}\mathrm{W}{\mathrm{O}}_{4}+{\mathrm{H}}_{2}\mathrm{O}\to \mathrm{C}\mathrm{o}\mathrm{W}{\mathrm{O}}_{4}–{\mathrm{H}}_{2}{\mathrm{O}}_{\mathrm{a}\mathrm{d}\mathrm{s}} $

    		(2)

    $ \mathrm{C}\mathrm{o}\mathrm{W}{\mathrm{O}}_{4}–{\mathrm{H}}_{2}{\mathrm{O}}_{\mathrm{a}\mathrm{d}\mathrm{s}}\to \mathrm{C}\mathrm{o}\mathrm{W}{\mathrm{O}}_{4}–{(\mathrm{O}\mathrm{H})}_{\mathrm{a}\mathrm{d}\mathrm{s}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} $

    		(3)

    $ \mathrm{C}\mathrm{o}\mathrm{W}{\mathrm{O}}_{4}–(\mathrm{C}{\mathrm{O})}_{\mathrm{a}\mathrm{d}\mathrm{s}}+\mathrm{C}\mathrm{o}\mathrm{W}{\mathrm{O}}_{4}–(\mathrm{O}{\mathrm{H})}_{\mathrm{a}\mathrm{d}\mathrm{s}}\to \mathrm{C}\mathrm{o}\mathrm{W}{\mathrm{O}}_{4}+\mathrm{C}{\mathrm{O}}_{2}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} $

    		(4)

    $ \mathrm{P}\mathrm{t}\mathrm{ }+{\mathrm{H}}_{2}\mathrm{O}\mathrm{ }\to \mathrm{ }\mathrm{P}\mathrm{t}–{\left(\mathrm{O}\mathrm{H}\right)}_{ads}+{\mathrm{H}}^{+}+{4\mathrm{e}}^{-} $

    		(5)

    $ \mathrm{P}\mathrm{t}–{\left(\mathrm{C}\mathrm{H}\mathrm{O}\right)}_{ads}+\mathrm{ }\mathrm{P}\mathrm{t}–{\left(\mathrm{O}\mathrm{H}\right)}_{\mathrm{a}\mathrm{d}\mathrm{s}}\to \mathrm{ }2\mathrm{P}\mathrm{t}\mathrm{ }+\mathrm{ }\mathrm{C}{\mathrm{O}}_{2}+\mathrm{ }2{\mathrm{H}}^{+}+\mathrm{ }2{\mathrm{e}}^{-} $

    		(6)

    $ \mathrm{C}{\mathrm{H}}_{3}\mathrm{O}\mathrm{H}\mathrm{ }+{\mathrm{H}}_{2}\mathrm{O}\mathrm{ }\to \mathrm{ }\mathrm{C}{\mathrm{O}}_{2}+\mathrm{ }6{\mathrm{H}}^{+}+\mathrm{ }6{e}^{-} $

    		(7)

    $ \mathrm{P}\mathrm{t}–{\left(\mathrm{C}\mathrm{H}\mathrm{O}\right)}_{\mathrm{a}\mathrm{d}\mathrm{s}}+\mathrm{ }\mathrm{P}\mathrm{t}–{\left(\mathrm{O}\mathrm{H}\right)}_{\mathrm{a}\mathrm{d}\mathrm{s}}+\mathrm{ }\mathrm{W}{\mathrm{O}}_{3}\to \mathrm{ }2\mathrm{P}\mathrm{t}\mathrm{ }+\mathrm{ }\mathrm{C}{\mathrm{O}}_{2}+{\mathrm{H}}_{\mathrm{x}}\mathrm{W}{\mathrm{O}}_{3} $

    		(8)

    $ {\mathrm{H}}_{\mathrm{x}}{\mathrm{W}\mathrm{O}}_{3}\to {\mathrm{W}\mathrm{O}}_{3}+\mathrm{x}{\mathrm{e}}^{-}+\mathrm{x}{\mathrm{H}}^{+} $

    		(9)

    ORR Activities of Pt-CoWO4/WO3@NCL-mr Catalysts. In Figure 7a, CV tests for Pt-CoWO4/WO3@NCL-mr and Pt/C catalysts are performed in a 0.5 M H2SO4 solution saturated with O2 at 50 mV s-1. All of the catalysts show obvious oxygen reduction peaks.[56] As shown in Figure 7b, Pt-CoWO4/WO3@ NCL-mr-850 has higher half-wave (0.46 V) and onset (0.54 V) potentials than Pt/C (0.41 and 0.50 V). The onset potentials, half-wave potentials and limiting current densities of Pt-CoWO4/WO3@NCL-mr-850 and Pt/C catalysts are listed in Table S3. The promising ORR activity is mainly derived from the cooperation between Pt and Co2+ (CoWO4). The surface Co2+ as active sites for ORR favors the oxygen adsorption on the catalyst surface to improve the oxygen catalysis. Moreover, the ORR-related intermediate (-OH) on catalyst surface (Co2+) can immediately react with H+ to form H2O molecule to release the active sites.[43, 57] It is also reported that abundant oxygen vacancies on WO3 can facilitate the distribution of Pt NPs to enhance the adsorption of O2 molecules onto the surface adjacent to the exposed Pt active-sites.[56-58] Furthermore, the strong interaction between WO3 and NCL (with various N species) can greatly smoothen the charge transfer to enhance the ORR activity.[58, 59]

    Figure 7

    Figure 7.  CV curves of Pt-CoWO4/WO3@NCL-mr-X (X = 750, 800, 850 and 900) and Pt/C (a) catalysts; LSV curves of Pt-CoWO4/WO3@NCL-mr-850 and Pt/C catalysts in O2-saturated 0.5 M H2SO4 solution at 1600 rpm (10 mV s−1) (b); LSV and calculated K-L plots at different potentials (insets) of Pt-CoWO4/WO3@NCL-mr-850 (c) and Pt/C (d) catalysts in O2-saturated 0.5 M H2SO4 solution at different rotation rates (10 mV s−1); RRDE tests of the cathodic catalysts (e); Tafel plots of Pt-CoWO4/WO3@NCL-mr-850 and Pt/C catalysts (f).
    DownLoad: Full-Size Img PowerPoint

    Figure 7c and 7d show the linear sweep voltammetry (LSV) curves measured from 400 to 2500 rpm and K-L curves (inset). As the rotation speed increases, the limiting current (j) increases as the oxygen diffusion from electrolyte to the electrode surface increases. All of the electron transfer number (n) is close to 4.0, indicating that both Pt-CoWO4/WO3@NCL-mr-850 and Pt/C follow the effective 4e- ORR pathway. The rod-like structure (surface) of WO3@NCL-mr can serve abundant Pt-binding sites to promote the exposure of Pt active sites to boost the ORR process. The rotating risk disk electrodes (RRDE) result in Figure 7e further confirms the 4e- ORR pathway on Pt-CoWO4/WO3@ NCL-mr-850. In the potential range of 0.0-0.38 V, H2O2 yields of Pt-CoWO4/WO3/ZNC-CNT-850 change from 7.2 to 12.0%. In 0.5 M H2SO4 solution, the n values of Pt-CoWO4/WO3@NCL-mr-850 and Pt/C vary between 3.8 and 3.9 (Figure S8), indicating that both of them possess a high catalytic-selectivity for converting O2 into OH- via a 4e- ORR pathway. Figure 7f shows the Tafel polarization curves in the low and high potential regions. The lower slope usually suggests that the catalyst has a faster ORR kinetics at the electrocatalytic interface at lower potentials.[60] In the two potential regions, the Tafel slopes of Pt-CoWO4/WO3@NCL-mr-850 are 12 and 179 mV dec-1, which are slightly better than those of Pt/C (38 and 184 mV dec-1). Thereafter, the heterogeneous interfaces among Pt, CoWO4 and WO3 can eagerly construct smooth pathways for charge transfer (4e- ORR process),[61] which contributes to a promising ORR kinetics on Pt-CoWO4/WO3@NCL-mr-850.[60]

    Figure S9 shows the CA and methanol resistance tests for Pt-CoWO4/WO3@NCL-mr-850 and Pt/C. These two tests are the important index for evaluating the ORR activity of catalysts. Under the same conditions, the stability of Pt-CoWO4/WO3@NCL-mr-850 is better than that of Pt/C. The protective effect of the N-doped carbon layer (NCL) can maintain the stability of the co-catalytically active-sites on CoWO4 (Co2+).[62] The synergistic effects between Pt and Co2+, as well as spillover effects of the generated HxWO3, can accelerate the methanol dehydrogenation to release Pt active sites to thus improve the charge transfer and methanol tolerance (ORR stability) on Pt-CoWO4/WO3@NCL-mr-850.

    Possible Mechanisms for MOR and ORR on Pt-CoWO4/WO3@NCL-mr Catalysts. Scheme 1 shows the MOR and ORR processes on Pt-CoWO4/WO3@NCL-mr catalysts. During the MOR process, methanol molecules can be easily adsorbed by O-groups (OH/COOH/COC) on the surface of Pt-CoWO4/WO3@NCL-mr, which are then oxidized by the dispersed Pt active sites. Meanwhile, the hydrogen overflow effect between Pt and WO3 facilitates the formation of HxWO3 on the WO3 surface to enhance the dehydrogenation of methanol molecules by utilizing the oxyphilic nature of oxides,[63] which can facilitate to release the occupied Pt active-sites to further adsorb the methanol molecules. In addition, WO3-mr (HxWO3) and surface-adsorbed hydroxyl (CoWO4-(OH)ads) can enhance the removal of intermediates to improve the resistance to CO poisoning (MOR stability) in acidic media.[34, 46, 47] For ORR, the micro-rod-shaped CoWO4/WO3 @NCL promotes the dispersibility of Pt NPs to relieve the self-aggregation of Pt, and prevent the loss of Pt active sites in acid media to obtain a better synergy between Pt and CoWO4 (Co2+). The carbon layer (NCL) covered on the WO3-mr surface can improve the stability of WO3, and provide the binding sites for CoWO4 NPs (Co2+). The interface between the mixed CoWO4/WO3 crystals can greatly promote the charge transfer during ORR.[44, 59]

    Scheme 1

    Scheme 1.  Schematic diagram of MOR and ORR processes on Pt-CoWO4/WO3@NCL-mr catalysts.
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    CONCLUSION

    In summary, we propose a simple route for the synthesis of CoWO4/WO3@NCL-mr as MOR/ORR co-catalyst/support. Oxygen vacancies on the CoWO4/WO3 can effectively adjust the surface electrons to thereby provide a stable anchoring location and surface for Pt attachment. The mass activity of Pt-CoWO4/WO3@NCL-mr-850 (2208 mA mgPt-1) is much higher than that of Pt/C (659.4 mA mgPt-1). Meanwhile, the electrochemical stability of Pt-CoWO4/WO3@NCL-mr-850 is better than that of Pt/C in acidic media. The micro-rod structure of CoWO4/WO3@ NCL-mr exposes more Pt active sites to enhance the MOR activity/stability. In addition, HxWO3 generated on WO3 is beneficial to improve the CO poisoning tolerance and MOR catalytic activity of Pt-CoWO4/WO3@NCL-mr-850 catalyst. Due to the strong interaction between Pt and carrier, Pt-CoWO4/WO3@NCL-mr-850 exhibits comparable ORR activity and stability to those of Pt/C. These findings provide a new strategy to prepare the microrod-structured catalysts with high electrocatalytic-activity by increasing the surface area and oxygen vacancies simultaneously.

    EXPERIMENTAL

    Synthesis of ZIF-8 and ZIF-8@ZIF-67 Materials. For the synthesis of ZIF-8, 6.16 g (75 mmol) of 2-methylimidazole was dissolved in 150 mL of methanol, which was then added to a solution with 5.95 g (20 mmol) of Zn(NO3)2·6H2O and 150 mL of methanol. The obtained white suspension was stood for 24 h at room temperature (25 ℃), and then was centrifugally washed with methanol for several times. The resulting white sample was dried overnight at 60 ℃ to obtain the product ZIF-8.[40, 64]

    For the synthesis of ZIF-8@ZIF-67, 0.5 g of the synthesized ZIF-8 was dispersed in 100 mL of methanol, and 5.82 g (20 mmol) of Co(NO3)2·6H2O was dissolved in 100 mL of methanol. The above two solutions were thoroughly mixed together. Then, 6.16 g (75 mmol) of 2-methyl imidazole dissolved in 100 mL of methanol was poured into the above mixture to obtain the purple suspension with intense stirring at room temperature (25 ℃) for 24 h. After stirring, the suspension was stood for 24 h at 25 and the resulting precipitate was centrifuged and washed with methanol for at least three times. The purple sample was dried overnight at 60 ℃ to obtain the final product ZIF-8@ZIF-67.[40, 64]

    Synthesis of CoWO4/WO3@NCL-mr. ZIF-8@ZIF-67 was carbonized at 900 ℃ to prepare the Co-NC composite. The obtained Co-NC was placed in a 5.0 M sulfuric acid (H2SO4) solution and stirred overnight, and then washed with deionized water until pH of the cleaning solution reached neutral. Co-NC was then mixed with 0.80 g of ammonium tungstate and 50 mL of deionized water, and the mixture was continuously stirred at 80 ℃ for 10 h under a water bath condition, and then dried at 80 ℃. The dried mixture was separately heated from room temperature to 750, 800, 850, or 900 ℃ (5 ℃ min-1) under N2 atmosphere. The precursor was kept at the final temperature for 4 h, and then naturally cooled to room temperature (25 ℃) in a N2 atmosphere. The detailed preparation steps are shown in Scheme 2. The final samples were labeled as CoWO4/WO3@NCL-mr-X (X = 750, 800, 850, and 900). Material characterizations including XRD, XPS, N2 adsorption/desorption isotherms, SEM, and TEM were introduced in the 'Supporting Information'. Methods for CV, LSV, electrochemical impedance spectroscopy, RRDE and CA tests were also introduced in the 'Supporting Information'.

    Scheme 2

    Scheme 2.  Synthesis route for CoWO4/WO3@NCL-mr and Pt-CoWO4/WO3@NCL-mr composites.
    DownLoad: Full-Size Img PowerPoint

    ACKNOWLEDGMENTS: We acknowledge the support by National Natural Science Foundation of China (52070074, 21806031 and 51578218) and LongJiang Scholars Program (Young Scholar, Q201912). The authors declare no competing interests.
    COMPETING INTERESTS
    For submission: https://mc03.manuscriptcentral.com/cjsc
    Supplementary information is available for this paper at http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0104
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  • Figure 1  SEM images of Co-NC (a) and CoWO4/WO3@NCL-mr-X (X = 750 (b), 800 (c), 850 (d) and 900 (e)); TEM image (f) and high-resolution TEM images (g and h) of Pt-CoWO4/WO3@NCL-mr-850.

    下载: 全尺寸图片 幻灯片

    Figure 2  XRD patterns of CoWO4/WO3@NCL-mr-X (a) and Pt-CoWO4/WO3@NCL-mr-X (b) (X = 750, 800, 850 and 900).

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    Figure 3  High-resolution XPS spectra of Co 2p (a), Pt 4f (b), W 4f (c), O 1s (d), C 1s (e) and N 1s (f) for Pt-CoWO4/WO3@NCL-mr-850.

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    Figure 4  Percentage contents of N2 (a) and N1+N3 (b) in Pt-CoWO4/WO3@NCL-mr-X (X = 750, 800, 850 and 900).

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    Figure 5  CV curves tested in 0.5 M H2SO4 media (a); CV curves tested in 0.5 M H2SO4 + 0.5 M CH3OH (b); Time-current curves at a potential of 0.67 V (c); Nyquist plots measured in 0.5 M H2SO4 + 0.5 M CH3OH for Pt/C and Pt-CoWO4/WO3@NCL-mr-X (X = 750, 800, 850 and 900) catalysts (d).

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    Figure 6  CO stripping voltammograms of Pt/C and Pt-CoWO4/WO3@NCL-mr-X (X = 750, 800, 850 and 900) catalysts.

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    Figure 7  CV curves of Pt-CoWO4/WO3@NCL-mr-X (X = 750, 800, 850 and 900) and Pt/C (a) catalysts; LSV curves of Pt-CoWO4/WO3@NCL-mr-850 and Pt/C catalysts in O2-saturated 0.5 M H2SO4 solution at 1600 rpm (10 mV s−1) (b); LSV and calculated K-L plots at different potentials (insets) of Pt-CoWO4/WO3@NCL-mr-850 (c) and Pt/C (d) catalysts in O2-saturated 0.5 M H2SO4 solution at different rotation rates (10 mV s−1); RRDE tests of the cathodic catalysts (e); Tafel plots of Pt-CoWO4/WO3@NCL-mr-850 and Pt/C catalysts (f).

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    Scheme 1  Schematic diagram of MOR and ORR processes on Pt-CoWO4/WO3@NCL-mr catalysts.

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    Scheme 2  Synthesis route for CoWO4/WO3@NCL-mr and Pt-CoWO4/WO3@NCL-mr composites.

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  • 发布日期:  2022-07-18
  • 收稿日期:  2022-05-22
  • 接受日期:  2022-06-12
  • 网络出版日期:  2022-06-20
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