High Methanol Tolerance Of Carbon-Supported Metal Phthalocyanine Modified Pt Nanoparticles Electrocatalysts For Oxygen Reduction

Direct methanol fuel cell(DMFC) is believed to present the best opportunity for early commercialization of fuel ceils.But the disadvantage of widely used perfluorosulfonic acid(PFSA) membranes and Pt catalysts impedes the further commercilisasion of DMFC.Because the PFSA membrane in a DMFC allows methanol to cross over from the anode to the cathode side leading to methanol electro-oxidation and oxygen reduction occurs at the cathode simutanously which results in mixed potential and additional fuel consuming.Moreover,methanol electro-oxidation will result in Pt poisoning by CO intermediates strongly adsorbed. All these problems reduce the DMFC performance,which could be solved either by using proton exchange membrane with lower methanol permeability or by developing new cathode electrocatalysts with both higher methanol tolerance and higher activity towards ORR than Pt.This work focuses on developing novel cathode catalysts with high methanol tolerance.MN4 macrocycle such as various metal phthalocyanines(MePc) were selected as assisting catalyst of Pt to prepare co-catalysts since MN4 macrocycle is insensitive to methanol but with low activity towards oxygen reduction reaction (ORR) and poor stability.In this dissertation,firstly the methanol-tolerant activity of carbon-supported metal phthalocyanine-Pt co-catalysts(Pt-MePc/C) prepared by impregmation method was evaluated systematically.Consequently the mechanism of methanol-tolerant activity was analysized further.Based on this analysis,a new method as called colloid method was present to synthsize carbon-supported MePc-Pt co-catalysts(MePc-Pt/C),and then the methanol tolerance of such co-catalysts was also evaluated.Finally the catalytic activity towards ORR of co-catalysts synthesied by colloid method was researched in detail,especailly the kinetics of oxygen reduction on MePc-Pt/C co-catalysts.The main experimental results and conclusions are listed as follows:1) Pt-MePc/C(Me=Fe,Co,Ni,Cu) co-catalysts prepared by impregamation method show better methanol-tolerant activity than Pt/C,especially Pt -NiPc/C and Pt-CuPc/C demonstrate the best methanol-tolerant activity.2) The promotion of methanol-tolerant activity of Pt-MePc/C co-catalysts is partly attributed to weak adaptation of carbonaceous intermediates(CO_ads) to Pt surface thus to prevent Pt poisoning.In addition,the decreased apparent activation energy for CO_ads electro-oxidation on Pt due to MePc modification also contributes to the promotion of methanol-tolerant activity.3) NiTsPc-Pt/C and CuTsPc-Pt/C co-catalysts(molar ratio MeTsPc:Pt=1:10) were successfully synthesized by colloid method.From the electrochemical analysis, though electrochemical active area of Pt in colloid co-catalysts is decreased compared with impregnation catalysts,specific activity of methanol electro-oxidation on colloid co-catalysts are higher than impregnation catalysts and Pt/C.Moreover,I_f/I_b is 2.00 and 1.94 for CuTsPc-Pt/C and NiTsPc-Pt/C respectively,which is higher than that of Pt/C and impregnation co-catalysts.These results indicate that colloid co-catalysts have more homogeneous metal phthalocyanine distribution than impregnation ones, so metal phthalocyanine could better protect Pt surface in colloid co-catalysts thus more Pt surface is eased to CO_ads poisonin.The apparent Ea of colloid co-catalysts is similar to that of impregnation co-catalysts,which means such homogeneously modified metal phthalocyanine doesn't change the mechanism of methanol electro-oxidation on co-catalysts.4) Colloid co-catalyst has better catalytic activity towards ORR than Pt/C in the present of methanol.Through kinetics analysis of ORR on co-catalysts,limiting current density j₁ for NiTsPc-Pt/C and CuTsPc-Pt/C is 22.2 and 14.6 mA cm^-2 respectively,both higher than that for bulk Pt,in fact,metal phthalocyanine doesn't transform the route of ORR on Pt.ORR on NiTsPc-Pt/C and CuTsPc-Pt/C nearly conducts via a 4-e process.Moreover,Tafel slope b for NiTsPc-Pt/C and CuTsPc-Pt/C,exchange current density J₀ for NiTsPc-Pt/C and CuTsPc-Pt/C,are all similar to the typical value of oxygen reduction catalyzed by Pt.