Methanol Tolerant PWA-Pt/C Catalyst with Excellent Electrocatalytic Activity for Oxygen Reduction in

1252Chinese Chemical Letters Vol. 16, No.9, pp 1252-1254, 2005http://www.imm.ac.cn/journal/ccl.htmlMethanol Tolerant PWA-Pt/C Catalyst with Excellent ElectrocatalyticActivity for Oxygen Reduction in Direct Methanol Fuel CellYan Zhuo Lu', Tian Hong LUl2, Chang Peng Liu', Ya Wen TANG?, Wei XING'*'Changchun Institute of Applied Chemistry, Graduate School of the Chinese Academy of Sciences,Chinese Academy of Sciences, Changchun 130022'Environmental Friendship Laboratory, College of Chemistry and Environmental Science, NanjingNormal University, Nanjing 210097Abstract: It was reported for the first time that phosphorictungstenic acid (PWA) could promotethe oxygen reduction reaction (ORR) and inhibit the methanol oxidation reaction at the cathodicPUC catalyst in the direct methanol fuel cell (DMFC). When the weight ratio of PWA to PVC is 1,the composite catalyst increases the reduction current of oxygen by about 38% and decreases theoxidation current of methanol by about 76% compared with that of the Pt/C catalyst.Keywords: DMFC, phosphorictungstenic acid, methanol tolerance, oxygen reduction.There are several problems preventing DMFC from the commercial applications. Oneof them is“'methanol crossover", which causes a mixed potential due to theelectrocatalytic oxidation of methanol at the PUC cathode and even poisoning the PuCcatalyst'. Thus, it is important to find a cathodic electrocatalyst, which possses thegood electrocatalytic activity for ORR and methanol tolerant ability. In this paper, itwas reported that the PWA-PUC composite catalyst shows the above ability.All electrochemical measurements were performed using a Princeton AppliedResearch Model 273A potentiostat/Galvanostat and a three electrode electrochemical cell.The Ag/AgCl electrode and a Pt foil were used as the reference electrode and theauxiliary electrode, respectively. The working electrode was prepared as follows. 1, 3,5,7or10mgPWAandthe5mgofPt/Ccatalystweremixedinthe0.4mLof5wt.%Nafion solution. The mixtures obtained are called as Pt-PWA1/C, Pt-PWA3/C,Pt-PWA5/C, Pt-PWA7/C and Pt-PWA10/C catalyst, respectively. 5 μL of the catalystink prepared was transferred onto the glassy carbon electrode (5 mm in diameter)(GCE).After evaporating the solvent under air, the working electrode was ready to be used.Figure 1 shows the oxygen reduction activity at the different electrodes in the 0.5mol/L H2SO4 solution saturated with oxygen. It can be clearly observed from Figure 1that the onset potentials of ORR at the different electrodes are almost the same, but thelimiting currents are different. The limiting C中國煤化工ctrode is about800μ A at -0.2V. It is the largest among all8% larger thanTYHCNMHG* E-mail:xingwei@ciac.jl.cnYan Zhuo LV et al.1253that at the PU/C electrode.Figure 2 shows the cyclic voltammograms of the 0.5mol/L H2SO4 solution with 1mol/L CH3OH and saturated oxygen at the different electrodes. Two strong peaks for themethanol oxidation in the positive and negative scan directions were observed at the PU/Celectrode (Figure 2, Curve a). However, the oxidation peaks of methanol at otherPt-PWA/C electrodes (Figure 2, Curve b, c and d) are much weaker than that at the P/Celectrode. The peak current at the Pt-PWA5/C electrode is about 76% less than that atthe PtC electrode. Especially, the oxidation peaks of methanol at the Pt-PWA 10/Celectrode almost disappear. It is illustrated that PWA can significantly inhibits theoxidation of methanol at the PUC electrode. From the above results, it can beconcluded that the Pt-PWA5/C catalyst shows the best performance for theelectrocatalytic activity for ORR and the methanol tolerance among all the compositecatalysts with the different weight ratio of PWA and Pt/C.Figure 1 Linear sweeping voltammograms of oxygen saturated in the 0.5 mol/L H2SO4solution at (a) Pt-PWA5/C, (b) Pt-PWA3/C, (C) Pt-PWA1/C, (d) Pt-PWA7/C, (e)Pt/C, (f) Pt-PWA 10/C electrodes.00-200400-800-300 0300 600 900 1200E/mV (vs.Ag/AgC)Rotation speed: 1000 rpm, Scan rate: 10 mV/s.Figure 2 Cyclic voltammograms of the (a)PtC, (b) Pt-PWA1/C, (c) Pt-PWA5/C (d) Pt-PWA10/Celectrodes in the 0.5mol/L H2SO4 solution with 1 molL CH3OH and saturated oxygen..0.5-/a.50.0--1.5-300 0 300 600中國煤化工E/mV( vs.Ag/AgCMYHCNMHGScan rate: 100 mV/s.1254Methanol Tolerant PWA-PUC CatalystThe reason for PWA to promote ORR and inhibit the methanol oxidation is not veryclear so far. Perhaps, PWA possesses the pseudo-liquid behavior, such as high oxygenaffinity2. Thus, PWA would promote ORR. However, methanol, as the relativelylarge molecule is difficult to permeate through PWA to the Pt surface and its oxidation isprevented in certain degree.AcknowledgmentsThe authors are grateful for the financial supports of“973" Project (G2000026408),“863" Projectof Science and Technology Ministry of China (2001AA323060,2003AA517060), the NationalNatural Science Foundation of China (20373068, 20433060)， Foundation of Department ofScience and Technology of Jiangsu Province (BG200302), Industrialization Foundation ofAdvanced Technique of Jiangsu Province (JH02-080) and National“211" Key Project.References1. A. Kuver, K. Potje Kamloth, Electrochim.Acta,1998,43,2527.2. M. Misono, C. R. Acad, Sci. Paris Serie lic. Chimie/Chem, 2000, 3, 471.Received 18 October, 2004中國煤化工MYHCNMHG