Electrocatalytic Behaviors Of Membrane Electrode Assemblies For Direct Methanol Fuel Cells

Direct methanol fuel cells(DMFCs)are paid much attention to applications in portable,transportation,as well as stationary due to its higher theoretical power density,lower emissions,rapid start-up and convenient to save and transport the fuel.Membrane electrode assembly(MEA)consisted of anodic electrode,electrolyte and cathodic electrode,is one of important parts.Commercial catalysts for the anode and cathode are Pt/C and Pt Ru/C,respectively.Methanol is oxided to form CO2,proton and electron at the anode.Proton and electron are transferred to cathode by proton exchange membrane and external circuit and form H2 O with O2.So DMFC is an energy device that converts chemical energy of fuel directly to electrical energy without limitation of Carnot cycle.However,some challenges hinder the commercialize of DMFC,such as(i)sluggish kinetics of C H3 OH oxidation and oxygen reduction reaction(ORR)at the cell anode and cathode,respectively,(ii)CH3OH crossover through the polymeric membrane,(iii)the easy dissociation of DMFC from the intermediate of methanol electrooxidation accelerates,(iv)difficulty of water and heat management.According to above issues,the thesis starts with effects of structure optimization of MEA and thermostability of catalyst layer(CL)on the performance of DMFC,then investigates the self-assembly behaviors between Pt/C and Nafion.In addition,an in-situ evaluation for ORR activity is design.The main contents are shown as follow:(1)Investigate the effect of Nafion content in cathodic catalyst layer on performance of DMFC and optimize structure of MEA.The content of Nafion affects proton transfer and water management of CL.With increasing Nafion content,proton conductivity and surface roughness of C L are improved,however,the surface of CL became more hydrophilic to easily flood cathode and pore valume is decreased to increase mass transfer resistance.So CL with 30 wt.% Nafion show the best performance of DMFC with 90 mW cm-2;(2)Investigate the thermostablility of C L through heating treatment.It is found that carbon corrosion is obviously suppressed after adding Nafion.Nafion is nona thin film in C L and lower thermostablility than bulk Nafion membrane,in addition,Pt/C accelerates Nafion dissociation.Performances of DMFC reduce after heating treatment of CL at varying temperature,expecially,at 200? with 69% power density.But ESA of Pt are not changed after heating treatment,suggesting that Nafion at active sites is not dissociated.So Nafion at non-active sites occurs to dissociate and loss proton conductivity and performance;(3)Self-electrochemical reaction occurs by the mechanism of galvanic cell during the preparation of C L consisted of Pt/C and Nafion.During the self-assembly between Pt/C and Nafion,the micro galvanic cell consisted of oxygen in air and oxygen functional groups in the surface of carbon as oxidant,platinum as reductant,carbon as electron pathway and Nafion as proton pathway and PtO as a oxidation product,has been found.The cognition of self-assembly and relationship between Nafion,Pt and carbon is further deepened.The appearance of PtO on the surface of Pt makes a turnover of Nafion structure so that more hydrophilic sulphonic groups directly contact to Pt and triple-phase boundary is extended.(4)An in-situ micro-MEA technique,which could precisely measure the performance of ORR electrocatalyst using Nafion as electrolyte,was designed and compared with regular thin-film rotating-disk electrode(TFRDE)(0.1 M HC lO4)and normal in-situ membrane electrode assembly(MEA)tests by using commercial 20% Pt/C.It successfully mimics the structure of regular MEAs and obtains similar results to a re gular MEA,thus providing a new technique to simply measure the electrode activity without being bothered by complicated fabrication of regular MEA.The traditional TFRDE method only makes the acquisition of data at low current density with being limited by the solubility of O 2 in water.In order to further understand the importance of in-situ measurement,Fe-N-C as a typical oxygen reduction reaction(O RR)free-Pt catalyst was evaluated by TFRDE and micro-MEA and lager deviation between micro-MEA and TFRDE is found.Therefore,it is suggested that the micro-MEA is better in evaluating the in-situ ORR performance.