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DOI: 10.1002/cctc.201402478
Promotion of Mesoporous Vanadium Carbide Incorporatedon Resorcinol–Formaldehyde Resin Carbon Compositeswith High-Surface-Areas on Platinum Catalysts forMethanol ElectrooxidationKui Li,[a, b] Jianbing Zhu,[b, c] Meiling Xiao,[b, c] Xiao Zhao,[b, c] Shikui Yao,[b, c] Changpeng Liu,*[a]
and Wei Xing*[c]
Introduction
Owing to their high specific energy and efficiency, low pollu-tion and cost, long working time, and simple processing atroom temperature, direct methanol fuel cells have been con-sidered as one of the most promising candidates for portableelectrical applications.[1, 2] Despite these advantages, there arestill several obstacles that must be overcome for wide applica-tion of direct methanol fuel cells, including the often undesira-ble activity, durability, and high cost of Pt-based catalysts.[3–7]
Currently, intensive research efforts are mainly focused on de-veloping more active and stable Pt-based electrocatalysts formethanol oxidation reaction (MOR) with enhanced Pt utiliza-tion efficiency.[8–13] One interesting aspect of Pt-based catalystsis the need for effective support materials, which can amplifythe performance of Pt-based nanoparticles by high dispersionand electronic effects.[14] As is well known, ideal support mate-rials should not only offer high conductivity and low cost butalso high stability and a suitable specific surface area.[15, 16] Sofar, Vulcan XC-72 carbon as the support for dispersing noble-
metal catalysts has been widely used in fuel cells. However,the pristine carbon materials are chemically inert and lack suf-ficient binding sites for anchoring metal nanoparticles result-ing in poor dispersion and aggregation of metal nanopar-ticles.[17–25]
Recently, transition-metal carbides have attracted significantattention as promising support materials for fuel cell anodiccatalysts because of their excellent stability and acceptableconductivity and surface area.[26–30] Furthermore, the transition-metal carbides in group 4–6, such as tungsten carbide, molyb-denum carbide, and vanadium carbide, possess high chemicalstability and a strong synergistic effect with Pt, which is veryhelpful for the resistance to catalyst poisoning.[31–38] Lee’sgroup synthesized tungsten carbide microspheres as an elec-trocatalyst support for MOR by a resin method, the supportedPt nanoparticles exhibited higher activity than a commercialPt–Ru/C catalyst.[27] Fu and co-workers researched the synergis-tic effect of WC and Pd on graphene by DFT calculations andX-ray photoelectron spectroscopy, which is favorable for etha-nol electrooxidation.[35] Shen and co-workers first reported thepreparation and use of vanadium carbide as a Pt electrocata-lyst support for the oxygen reduction reaction (ORR) andMOR.[39, 40]
Herein, we developed a novel mesoporous catalyst-supportmaterial of vanadium carbide incorporated on resorcinol–form-aldehyde resin carbon (V8C7@RFC) with an extralarge specificsurface area. The as-prepared electrocatalysts displayed higherelectrocatalytic activity and tolerance for CO poisoning thanthe commercial Pt/C catalyst. These enhanced performanceswere studied deeply from the point of a synergistic effect be-tween V8C7@RFC and the Pt nanoparticles.
Vanadium carbide incorporated on resorcinol–formaldehyderesin carbon (V8C7@RFC) was synthesized as a novel mesopo-rous catalyst-support material by pyrolysis of the resorcinol–formaldehyde resin and NaVO3 mixture. The material’s BET sur-face area was 564 m2 g�1 and thus much higher than that of389 m2 g�1 for the carbon powders yielded by resin carbona-tion. Physical characterization revealed that the supporting ma-terial possesses a mesoporous structure and Pt nanoparticles
are homogeneously dispersed on the V8C7@RFC surface. Elec-trochemical measurements demonstrated that the V8C7-modi-fied Pt catalyst exhibits a negative shift of over 100 mV in theonset potential for COads electrooxidation and a dramaticallyenhanced activity in methanol oxidation reaction. The en-hancement was mainly attributed to the electronic effect be-tween Pt and V8C7 and the mesoporous structure providingideal anchor sites for Pt dispersion.
[a] K. Li, Prof. Dr. C. LiuLaboratory of Advanced Power SourcesChangchun Institute of Applied Chemistry, Chinese Academy of Sciences5625 Renmin Street, Changchun, 130022 (PR China)E-mail : [email protected]
[b] K. Li, J. Zhu, M. Xiao, Dr. X. Zhao, S. YaoUniversity of Chinese Academy of SciencesBeijing, 100039 (PR China)
[c] J. Zhu, M. Xiao, Dr. X. Zhao, S. Yao, Prof. Dr. W. XingState Key Laboratory of Electroanalytical ChemistryChangchun Institute of Applied ChemistryChinese Academy of Sciences5625 Renmin Street, Changchun, 130022 (PR China)E-mail : [email protected]
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Results and Discussion
Characterization of the mesoporous V8C7@RFC support
Mesoporous V8C7@RFC was synthesized by heating the mix-tures of resorcinol–formaldehyde resin (a carbon precursor, de-noted as RFC) and sodium metavanadate (a vanadium precur-sor). V8C7@RFC-R (10 000 R equals 1.25, 2.50, 5.00, and 10.0, andR is the molar ratio of NaVO3 to deionized water). The phasepurity of the product can be confirmed by the XRD patterns.In Figure 1, the XRD patterns of V8C7 are shown achieved from
resorcinol–formaldehyde resin and NaVO3 at 1173 K for 3 h.The peak of C at 43.48 is ascribed to the (1 0) facet of graphite,and the peak at 26.48 for all the samples is designated to(0 0 2) plane reflection of carbon. The diffraction peaks at 2q
values of 37.48, 43.48, 63.08, and 75.68 correspond to the (2 2 2),(4 0 0), (4 4 0), and (6 2 2) facets of a V8C7 crystal, respectively,[33]
and the peak intensity of the V8C7 became sharper with the in-creased concentration of NaVO3 as a result of the increasedparticles size, which can be evidenced by the Debye–Scherrerformula.[41]
The structural properties of the mesoporous supports aresummarized in Table 1. With increasing the concentration ofthe vanadium source, the BET surface area, the pore diameter,and the pore volume of the corresponding V8C7@RFC-R sup-
port present a trend of volcanic-type change, which revealsthe important role of the vanadium precursor on the structureof the final V8C7@RFC support. Notably, [email protected]% pos-sesses the largest BET surface area of 564.45 m2 g�1 (comparedto 216.13 m2 g�1 for Vulcan XC-72 and 389.41 m2 g�1 for carbonnanospheres). Further investigation of the pore structure forthe [email protected]% was conducted. As shown in Figure 2,typical type-IV N2 sorption isotherms (Figure 2 a) were ob-served on the mesoporous [email protected]% sample. Clearly,capillary condensation is observed at the wide relative pres-sures between 0.8 and 1.0, indicating a distribution of mesopo-rous sizes. According to the IUPAC recommendations, the typeof hysteresis loop of the N2 isotherms is intermediate between
Figure 1. XRD patterns of a) C, b) [email protected]%, c) [email protected]%,d) [email protected]%, and e) [email protected]%.
Table 1. Properties of the Vulcan XC-72, RFC, [email protected]%,
[email protected]%, [email protected]%, and [email protected]% supports,respectively.
Samples BET surface area[m2 g�1]
Average poresize [nm]
Pore volume[cm�3 g�1]
Vulcan XC-72 216.1 13.44 0.574RFC 389.4 2.59 [email protected]% 384.6 6.42 [email protected]% 564.4 17.39 [email protected]% 450.7 12.50 [email protected]% 386.3 5.56 0.445
Figure 2. a) Nitrogen adsorption isotherms, b) the corresponding pore-widthdistribution curves, and c) the V/t plots of mesoporous [email protected]%materials.
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H1 (at 0.5<P/P0<0.8) and H3 (at P/P0>0.8). The hysteresisloop is relatively broad, indicating a wide distribution of poresizes, which ranged from 15 to 40 nm (Figure 2 b). The t-plotanalysis (Figure 2 c) also reveals that almost all of the surfacearea is contributed to the primary mesopores.[42, 43]
As discussed above, the NaVO3 amount has a significanteffect on the structure of the V8C7@RFC-R supports. The mor-phology change of the corresponding V8C7@RFC-R was investi-gated by SEM as shown in Figure 3 a–e. A phase transition
(e.g. , the spherical structure collapsed into nanoparticles andbulk) was observed by increasing the amount of NaVO3.[27] InFigure 3 c, under the optimized conditions, the nanospherescollapsed completely into nanoparticles and there is no bulk,which led to the largest surface area and pore diameter, asshown in Table 1.
The electrical conductivity of the mesoporous V8C7@RFC-Rpowder was studied by electrochemical impedance spectros-copy, and the results were used to evaluate the effect of R onthe overall resistance of the V8C7@RFC-R supports.[44, 45] InFigure 4, the Nyquist spectra are shown. It can be observedthat the overall resistance of the V8C7@RFC-R supports de-creased by increasing NaVO3. As previously mentioned,[46] theformation of vanadium carbide in the calcination process canpromote the electrical conductivity of V8C7@RFC-R supports.
Physicochemical characterization of the electrocatalysts
Pt particles were deposited onto the mesoporous RFC,V8C7@RFC-R and Vulcan XC-72 through a polyol reduction pro-cess.[47] In Figure 5, the XRD patterns of the Pt/RFC, Pt/
V8C7@RFC-R, Pt/Vulcan XC-72, and Pt/C-JM electrocatalysts areshown. The crystalline structure of the Pt nanoparticles is re-vealed by all of the XRD patterns, which clearly exhibit diffrac-tion peaks that can be indexed to the face-centered cubic (fcc)crystalline Pt, namely, (111), (2 0 0), and (2 2 0). In the XRD pat-tern of Pt/V8C7@RFC-R, the peaks from V8C7 are invisible, proba-bly because they were buried under the strong and broadpeaks of Pt. From the Scherrer formula, the sharper peaksreveal that the as-prepared catalyst possesses large particlesizes. As the salt concentration increases, the peaks graduallybecome less sharp, which means that the Pt nanoparticles onthe support material are smaller.
In Figure 6, the TEM images of Pt/V8C7@RFC-R are shown,from which one can clearly see that Pt nanoparticles are sosmall that the particles aggregated on the support materials’surfaces.
In Figure 7, the TEM images of the Pt/[email protected]% andcommercial Pt/C-JM catalysts are presented. It is clearly ob-served that the Pt nanoparticles have been deposited homo-
Figure 3. SEM images of a) RFC, b) [email protected]%, c) [email protected]%,d) [email protected]%, and e) [email protected]%. Scale bars = 1 mm (a, b, d, e) ;500 mm (c).
Figure 4. Nyquist plots of a) RFC, b) [email protected]%, c) [email protected]%, d) [email protected]%, and e) [email protected]%, obtained by apply-ing a sine wave with an amplitude of 1.00 mV over the frequency rangefrom 100 kHz to 10 mHz. The inset shows the Nyquist plots in the high-fre-quency range. im = Imaginary, re = real.
Figure 5. XRD patterns of a) Pt/[email protected]%, b) Pt/[email protected]%,c) Pt/[email protected]%, d) Pt/[email protected]%, e) Pt/C, f) Pt/Vulcan XC-72,and g) Pt/C-JM.
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genously onto the [email protected]% support. According tothe size-distribution histograms, the Pt/V8C7@RFC-R (R equals0.250%) and Pt/C-JM catalysts possess mean particle sizes ofapproximately 3.40 nm and 2.83 nm, respectively. The differ-ence in the average Pt particle size between the two catalystswas so small that the particle size effect on the catalytic per-formance can be neglected. The high-angle annular dark-fieldscanning transmission electron microscopy (STEM, Figure 8 aand b) and the elemental mapping images (Figure 8 c–e) of thePt/[email protected]% catalyst further confirmed that the Ptnanoparticles were dispersed homogeneously onto the entiresurface of the [email protected]%. In addition, the clear signalof the element V and C demonstrated that V8C7 dispersed uni-formly onto the resorcinol–formaldehyde resin carbon. InFigure 9, the high-resolution TEM images of Pt/RFC and Pt/V8C7@RFC-R (R � 10 000 = 0.250, 1.00) are displayed, which clear-ly reveal the crystal lattices of V8C7 (2 2 2) and Pt (111).
XPS analysis was used to survey the surface composition ofthe catalysts and the electronic structure of Pt atoms, which isbelieved to play a key role in the improved electronic conduc-tivity and the binding strength to metal nanoparticles. In Fig-ure 10 A (a–d), the V 2p binding energy of Pt/V8C7@RFC-R shifts
positively with increasing V8C7 concentration. As shown in Fig-ure 10 B (f), the Pt 4f signal of Pt/C-JM exhibits one doubletwith a Pt 4f7/2 binding energy of 71.62 eV and a Pt 4f5/2 bindingenergy of 74.99 eV. Compared to the Pt/C-JM spectrum (Fig-ure 10 B (f)), the Pt 4f spectrum of Pt/V8C7@RFC-R (Fig-ure 10 B (a–e)) displays a negative shift of one doublet with itsPt 4f7/2, Pt 4f5/2 binding energy. The data are compared inTable 2. Overall, the negative shift in the Pt 4f binding energy
Figure 6. TEM images of a) Pt/RFC, b) Pt/[email protected]%, c) Pt/[email protected]%, d) Pt/[email protected]%, and e) Pt/[email protected]%. Scalebars = 100 nm (a); 50 nm (b–e).
Figure 7. TEM images and the corresponding particle-size distribution histo-grams of a, b) Pt/C-JM and c, d) Pt/[email protected]% catalysts. Scalebars = 20 nm.
Figure 8. a, b) STEM images and c–e) the corresponding elemental mappingimages of the Pt/[email protected]% catalyst. Scale bars = 100 nm.
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and the positive shift in the V 2p binding energy for Pt/V8C7@RFC can support the existence of an electron interaction
between Pt and V8C7. This strong interaction between themetal and the support is significant for improving the per-formance of the electrocatalyst.[23, 35]
Electrochemical characterization of the electrocatalysts
As CO species are the main poisoning intermediate duringmethanol electrooxidation, a good catalyst for methanol elec-trooxidation should possess excellent CO electrooxidizing abili-ty, which can be determined by CO stripping voltammetry. InFigure 11, the CO stripping voltammogram curves are shownfor the Pt/V8C7@RFC-R, Pt/RFC, Pt/Vulcan XC-72, and Pt/C-JMcatalysts. Clearly, the onset potential and peak potential of COoxidation for Pt/V8C7@RFC-R were more negative than that forpure Pt/RFC, demonstrating that the addition of V8C7 is benefi-cial for CO oxidation. Among the Pt/V8C7@RFC-R catalysts(Table 3), Pt/[email protected]% presents the lowest onset po-
tential for CO oxidation (351.7 mV), and the value is 70 mVlower than that of the Pt/RFC catalyst (421.5 mV). The CO oxi-dation peak potential for Pt/[email protected]% (451.5 mV) isalso lower than that of Pt/C (496.4 mV). The results reveal thatPt/[email protected] 0/000 may own the best catalytic stability forMOR as a result of its excellent tolerance to CO poisoning.
The electrocatalytic performances of the Pt/Vulcan XC-72,Pt/C-JM, Pt/RFC, and Pt/V8C7@RFC-R catalysts for MOR wereevaluated by cyclic voltammogram measurements. As shownin Figure 12, the Pt/[email protected]% catalyst has a highercatalytic activity for methanol electrooxidation than the others,with a peak current activity of 725.6 mA mgPt
�1, 2.1 timeshigher than that of Pt/Vulcan XC-72. The order of the maxi-
Figure 9. HRTEM images of a) Pt/RFC, b) Pt/[email protected]%, and c) Pt/[email protected]%. Scale bars = 5 nm.
Figure 10. XPS patterns of a) Pt/[email protected]%, b) Pt/[email protected]%,c) Pt/[email protected]%, d) Pt/[email protected]%, e) Pt/RFC, and f) Pt/C-JMfor A) the V 2p and B) Pt 4f binding energies, respectively.
Table 2. Binding energies (BE) of Pt/Vulcan XC-72, Pt/RFC, Pt/[email protected]%, Pt/[email protected]%, Pt/[email protected]%, and Pt/[email protected]% for V 2p and Pt 4f7/2 and Pt 4f5/2.
Catalysts BE [eV]V 2p Pt 4f7/2 Pt 4f5/2
Pt/C-JM – 71.62 74.99Pt/RFC – 71.55 74.86Pt/[email protected]% 519.02 71.46 74.78Pt/[email protected]% 519.13 71.24 74.61Pt/[email protected]% 519.24 71.12 74.46Pt/[email protected]% 519.35 71.00 74.39
Table 3. Summary comparing Pt/Vulcan XC-72, Pt/C-JM, Pt/RFC and Pt/V8C7@RFC-R catalysts for COads electrooxidation in a 0.5 m H2SO4 solution.
Catalysts Potential toward COads electrooxidation [mV]Peak Onset
Pt/Vulcan XC-72 592.5 540.6Pt/C-JM 565.6 450.0Pt/RFC 496.4 421.5Pt/[email protected]% 481.3 377.4Pt/[email protected]% 451.5 351.7Pt/[email protected]% 455.7 357.1Pt/[email protected]% 456.2 356.6
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mum stable peak current is Pt/[email protected]%>Pt/[email protected]%>Pt/[email protected]%>Pt/[email protected]%>Pt/RFC. The detailed information is shown in Table 4.Moreover, the peak potential of the methanol electrooxidationshifted slightly toward the left with increasing the amount ofV8C7. The results indicate that V8C7 has a clear synergistic effectwith Pt for MOR and can effectively enhance the catalyst activi-ty. The change trend of the onset potential and the peak po-tential with the increase of the V8C7 ratio agrees well with thatof the COads electrooxidation (Figure 11 and Table 3).
From Table 5, the electrochemical surface area (ECSACO) ofthe Pt/V8C7@RFC catalyst is approximately 62.53 m2 gPt
�1, whichis smaller than that of the Pt/C-JM catalyst (72.13 m2 gPt
�1). Thisis in accordance with the fact that the Pt/V8C7@RFC catalysthas a slightly larger particle size than the Pt/C-JM catalyst.However, to obtain the intrinsic activity for MOR, the electrodecurrent was normalized by the ECSACO of Pt calculated fromthe COads stripping-voltammetry results. The specific activity ofthe catalysts is shown in the inset in Figure 12. The order ofthe maximum stable peak current is : Pt/V8C7@RFC>Pt/RFC>Pt/C-JM>Pt/Vulcan XC-72. Furthermore, the Pt/V8C7@RFC elec-trode displays a 2.43-fold higher stable peak current density
Figure 11. Comparison of the COads stripping voltammetry curves betweena) Pt/Vulcan XC-72, b) Pt/C-JM, c) Pt/RFC, d) Pt/[email protected]%, e) Pt/[email protected]%, f) Pt/[email protected]%, and g) Pt/[email protected]% inmass activity.
Table 4. Summary comparing Pt/Vulcan XC-72, Pt/C-JM, Pt/RFC, Pt/[email protected]%, Pt/[email protected]%, Pt/[email protected]%, and Pt/[email protected]% electrodes for MOR in a 1 m CH3OH + 0.5 m H2SO4
solution.
Catalysts Potential toward MeOHelectrooxidation [mV]
Massactivity
Peak Onset [mA mgPt�1]
Pt/Vulcan XC-72 621.1 482.2 234.2Pt/C-JM 625.1 445.3 344.4Pt/RFC 626.3 450.3 298.6Pt/[email protected]% 620.2 420.1 345.2Pt/[email protected]% 619.9 415.5 725.6Pt/[email protected]% 617.1 408.3 493.2Pt/[email protected]% 613.4 400.6 316.2
Table 5. Summary comparing Pt/Vulcan XC-72, Pt/C-JM, Pt/RFC, and Pt/V8C7@RFC catalysts for ECSACO, relative stabilities, and decay rate.
Catalysts ECSACO
[m2 gPt�1]
Relative stability[%]
Decay rate[% h�1]
Pt/Vulcan XC-72 57.25 12.42 43.79Pt/C-JM 72.13 15.40 42.30Pt/RFC 53.31 9.26 45.37Pt/V8C7@RFC 63.53 19.82 40.09
Figure 12. A) Electrocatalytic performances of a) Pt/Vulcan XC-72, b) Pt/C-JM,c) Pt/RFC, d) Pt/[email protected]%, e) Pt/[email protected]%, f) Pt/[email protected]%, and g) Pt/[email protected]% electrodes for methanol electrooxida-tion. B) Specific activities of Pt/Vulcan XC-72, Pt/C-JM, Pt/RFC, and Pt/V8C7@RFC.
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than the Pt/C-JM electrode. However, the specific activity ofthe Pt/RFC electrode is slightly higher than that of the Pt/C-JMelectrode as a result of its tiny ECSACO. A chronoamperometrytest to demonstrate the long-time performance of the catalystin terms of methanol electrooxidation was conducted for7200 s at 0.6 V. The results (in Figure 13) demonstrate that Pt/V8C7@RFC has a higher initial and stable current density than
the Pt/C-JM electrode. We can normalize the current by the ini-tial current to investigate the decay rate for the catalysts, thusevaluating the catalytic stability. The decay rate shown inTable 5 for the Pt/V8C7@RFC electrode is approximately40.09 % h�1, which is slightly lower than that for the Pt/C-JMelectrode (42.30 % h�1). Compared to the Pt/Vulcan XC-72 elec-trode (43.79 % h�1), the results show that the Pt nanoparticlesdeposited on V8C7@RFC material performed much better thanon Vulcan XC-72. Additionally, the decay rate of the Pt/RFCelectrode is 45.37 % h�1, which is much higher than that of Pt/V8C7@RFC. The conclusion above indicates that V8C7 can pro-mote the stability of the Pt catalyst for MOR as a result of theenhanced CO tolerance.
The combined results of XPS, the CO stripping-voltammetryspectrum, and methanol oxidation curves demonstrate thatthere is a strong interaction between Pt and V8C7, which canenhance the catalytic performance of the Pt/V8C7@RFC catalysttoward MOR. The electron transfer from V8C7 to Pt can notonly modify the electronic structure of Pt to enhance the scis-sion of methanol molecules on the Pt active sites, but it alsoincrease the electron density of the surface Pt, which is benefi-cial to weakening the adsorption of intermediate oxygen-con-taining species such as CO on the Pt surface.[35] In addition, asillustrated in Scheme 1, the electron transfer from V8C7 to Pt in-creases the content of oxidized species at the interface be-tween V8C7 and the Pt nanoparticles. Hence, the intermediatespecies adsorbed onto the Pt sites during MOR can be effec-tively removed by the adjoined�OH active species on the V8C7
surface by a bifunctional mechanism.[48] Furthermore, the peakand onset potential of COads oxidation for Pt/V8C7@RFC cata-lysts shifts negatively as the amount of the vanadium source
increases, indicating that V8C7 enhances the activity of metha-nol oxidation. However, there are certain catalysts, such as Pt/[email protected]% and Pt/[email protected]%, the performanceof which did not fit the trend, which may result from the ag-gregation of Pt nanoparticles, as shown in Figure 6.
It is well known that MOR on Pt includes two key reactionsteps, i.e. , the initial dehydrogenation of the adsorbed metha-nol and the sequential oxidation of the poisoning intermedi-ates such as COads. The CO-like species generated in MOR canbe strongly adsorbed onto the Pt surface, and thus, the activePt sites become occupied, resulting in a severe decrease in thereaction kinetics. Therefore, weakening the Pt�CO adsorptionbond strength and/or providing sufficient active �OH speciesare essential to enhance the resistance to COads poisoning andthus to facilitate MOR. To this end, the synthetic effect, elec-tronic effect (or ligand effect) and bifunctional mechanism alleffectively work to inhibit the COads poisoning in MOR.[49] Interms of Pt/V8C7@RFC, the enhanced activity can be analyzedaccording to the synthetic effect and the electronic effect. Asshown in Scheme 1, the illustration of the synergistic effect ofPt and V8C7@RFC for methanol oxidation demonstrate a bettertolerance to CO poisoning and excellent performance formethanol electrooxidation.[35]
Conclusions
A novel mesoporous carbon vanadium carbide (V8C7) was de-veloped by a route based on heating a mixture of a resorci-nol–formaldehyde resin and a sodium metavanadate salt. Theoptimized concentration of NaVO3 was 0.250 � 10�4. A mesopo-rous V8C7@RFC-supported Pt nanoparticle (Pt/V8C7@RFC) cata-lyst exhibited a larger activity, more negative electrooxidationpeak potential, and slower degeneration rate than a commer-cial Pt/C-JM catalyst for methanol electrooxidation. The Pt/V8C7@RFC catalyst displayed a more negative onset potential
Figure 13. Comparison of the stabilities of Pt/Vulcan XC-72, Pt/C-JM, Pt/RFC,and Pt/V8C7@RFC electrodes for MOR. The stability tests were conducted ata potential value of 0.6 V vs. SCE.
Scheme 1. Illustration of the synergetic effect of Pt and V8C7@RFC for metha-nol oxidation.
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and peak potential toward COads oxidation, which had a betterperformance for antipoisoning by CO. The improved per-formance is mainly the result of the larger surface area of themesoporous V8C7@RFC support, coupled with a synergistic be-havior between Pt and V8C7 that can facilitate oxidation of in-termediates. The work demonstrates that the mesoporousV8C7@RFC is a potential candidate support material in directmethanol fuel cells.
Experimental Section
Chemicals
Sodium metavanadate and resorcinol were obtained from AladdinChemistry Co., Ltd, China. An aqueous formaldehyde solution, eth-ylene glycol, H2SO4, and ethanol were purchased from the Shang-hai Chemical Factory (Shanghai, China) and were used as receivedwithout further purification. Nafion ionomer (5 wt %) was obtainedfrom Aldrich. Commercial state of-the-art 20 wt % Pt/C (JohnsonMatthey Company, HiSPEC 3000) was used as the benchmark forcomparison and was denoted as Pt/C-JM. Ultrapure water (Milli-pore, 18.2 MU cm) was used throughout all of the experiments.
Synthesis of V8C7@resorcinol–formaldehyde resin carbon(V8C7@RFC) supports
The V8C7 crystal was synthesized by heating mixtures of resorcinol–formaldehyde resin (a carbon precursor) and sodium metavanadate(a vanadium precursor). The process diagram of the preparation ofthe support materials is summarized in Scheme 2, and the stepswere as follows. In a typical process,[27] as shown in Scheme 2,a mixture containing sodium metavanadate (33.9 mg, relative tothe molar ratio of distilled deionized water from 0 to 1.00 � 10�4)and resorcinol (1.2234 g) was dissolved in distilled deionized water(20 mL) and magnetically stirred for 30 min. The solution washeated at 367 K while aqueous formaldehyde solution (1.640 mL)was added and stirred under reflux. After 24 h, the suspension wasvacuum-dried at RT and milled to obtain a brownish-redVO3
�@resorcinol–formaldehyde resin powder. Finally, the Na-VO3@resorcinol–formaldehyde resin was processed by high-tem-perature heating at 1173 K for 1 h in an Ar gas atmosphere and 2 hin a combination gas flow (90:10 Ar/H2). Afterward, the free carbonwas removed from the V8C7 surface, and the V8C7@RFC was ob-tained after grounding into a powder and washing with deionizedwater.
Synthesis of the Pt/V8C7@RFC-R catalysts
Pt/V8C7@RFC-R catalysts with a Pt loading of 20 wt % were synthe-sized through a process with ethylene glycol as the reducingagent. First, a portion of 20 mg of V8C7@RFC-R (or Vulcan XC-72)was suspended in ethylene glycol solution (20 mL), and a 332 mLH2PtCl6 solution (15.067 mgPt mL�1) was added. Then, the mixturewas heated at 423 K for 3 h. Subsequently, the suspension was fil-tered and washed with deionized water and then dried at 353 Kfor 10 h to obtain the Pt/V8C7@RFC catalysts (or Pt/Vulcan XC-72).
Physical characterization
TEM, high-resolution TEM (HRTEM) and STEM and elemental map-ping analyses were conducted on a TECNAI G2 operating at200 kV. SEM measurements were performed with an XL30 ESEMFEG field-emission scanning electron microscope to determine themorphology. XPS measurements were performed on a KratosXSAM-800 spectrometer with an MgKa radiation source. XRD meas-urements were performed with a PW1700 diffractometer (PhilipsCo.) using a CuKa (l= 0.15405 nm) radiation source. The XRD pat-terns obtained were analyzed with Jade 5.0 software to remove thebackground radiation.
The textural and morphological features of the various carbon sup-ports and catalysts prepared were determined by nitrogen physi-sorption at 77 K in a Micromeritics ASAP 2020. Textural propertiessuch as the specific surface area pore volume and pore size distri-bution were calculated from each corresponding nitrogen adsorp-tion-desorption isotherm, applying the BET equation and the Bar-rett–Joyner–Halenda (BJH) and t-plot methods.
Electrochemical measurements
Electrochemical measurements were performed with an EG&Gmode 273 potentiostat–galvanostat and a conventional three-elec-trode test cell. The catalyst ink was prepared by ultrasonically dis-persing a mixture containing catalyst (5 mg), ethanol (950 mL), and5 wt % Nafion solution (50 mL). Next, a 5 mL volume of the catalystink was transferred with a pipette onto a precleaned glassy carbondisk (diameter = 4 mm) as the working electrode. A Pt foil and a sa-turated calomel electrode were used as the counter and the refer-ence electrodes, respectively. All of the potentials were relative tothe saturated calomel electrode, unless otherwise noted. To acti-vate and clean the catalyst surface, the working electrodes werecycled from �0.2 V and 0.958 V at a scan rate of 50 mV s�1 ina 0.5 m H2SO4 solution until a stable response was obtained. Theelectrochemical impedance spectra were recorded at 10 points perdecade over the frequency range from 100 kHz to 10 mHz. The am-plitude of the sinusoidal potential signal was 5 mV. To evaluate theactivity of the catalysts for MOR, cyclic voltammetry measurements
Scheme 2. Process for the formation of the V8C7@RFC nanospheres and nanoparticles.
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were performed at RT between �0.2 V and 0.958 V in an electrolytesolution containing 1 m CH3OH and 0.5 m H2SO4, with scan rates of50 mV s�1. To estimate the stability of the catalysts, the chronoam-perometric experiments were performed in the same solution witha potential of 0.6 V. The ECSACO and the tolerance to CO poisoningwere estimated by the CO stripping test, assuming that the Cou-lombic charge required for the oxidation of the CO monolayer was420 mC cm�2. All electrolyte solutions were de-aerated by high-purity nitrogen for at least 20 min prior to each measurement.
Acknowledgements
This work was financially supported by the National Natural Sci-ence Foundation of China (21373199), the National High Technol-ogy Research and Development Program of China (863 program,2012AA053401, 2013AA051002), the National Basic Research Pro-gram of China (973 Program, 2012CB215500, 2012CB932800),and the Jilin Province Science and Technology Development Pro-gram (20100102), the “Strategic Priority Research Program” ofthe Chinese Academy of Sciences (XDA09030104).
Keywords: electrochemistry · mesoporous materials ·platinum · supported catalysts · vanadium
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Received: June 26, 2014Published online on && &&, 0000
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FULL PAPERS
K. Li, J. Zhu, M. Xiao, X. Zhao, S. Yao,C. Liu,* W. Xing*
&& –&&
Promotion of Mesoporous VanadiumCarbide Incorporated on Resorcinol–Formaldehyde Resin CarbonComposites with High-Surface-Areason Platinum Catalysts for MethanolElectrooxidation Platinum–vanadate interaction: The
promotion of mesoporous vanadiumcarbides on Pt nanoparticles for metha-nol electrooxidation is investigated sys-tematically with a special focus on the
electronic effect and the bifunctionalmechanism, which can effectively en-hance the tolerance to CO-like inter-mediates, thus improving the activityfor methanol oxidation.
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