Research On Structure-activity Properties Of PEMFC Multiscale Reaction Interface

The promotion and application of proton exchange membrane fuel cells(PEMFC)will be very beneficial to achieve peak carbon dioxide emissions and carbon neutralization goals.Reducing the cost of proton exchange membrane fuel cells and solving the bottleneck problems of related technologies is the only way for their largescale commercialization.The characteristics of the electrode reaction interface have a great influence on the performance of proton exchange membrane fuel cells.This dissertation focuses on the catalyst utilization and mass transfer structure-activity relationship at the electrode interface of different scales,and has made the following progress:(1)The activity of M-N-C catalysts is enhanced by constructing a protophilic surface environment.Pyrolytic transition metal carbonitride(M-N-C)materials are considered to be the most promising non-platinum replacements for platinum-based catalysts for oxygen reduction reaction(ORR).Given that the first proton-coupled electron transfer step(PCET)of ORR has been demonstrated to be the rate-determining step(RDS)of M-N-C catalysts,we envision that a protic catalyst surface might contribute to the enhanced ORR activity.Here,we propose and implement a polyaniline(PANI)modification strategy to endow the surface of Fe-N-C catalysts with a protophilic environment.The ORR performance was enhanced due to the increased interfacial proton concentration,a positive shift of 20 mV in the half-wave potential under acidic conditions was observed,and the switching frequency tripled(from 0.46 to 1.28 e·s-1·sites-1).Our work starts with ORR kinetics and opens a new avenue for the design of M-N-C ORR catalysts.(2)The introduction of hydrophobic fluorinated multi-walled carbon nanotubes improves the accessibility and utilization of platinum in membrane electrodes.Improving the utilization of platinum-based catalysts is the key to reducing the cost of fuel cells.The close coverage of the platinum by the ionomer increases the mass transfer resistance of oxygen to the platinum sites.Since the cathode reaction generates water,and the electric drag effect promotes the migration of anode water to the cathode,the cathode is prone to flooding,and also hinders the diffusion and transmission of oxygen.In this work,hydrophobic fluorinated multi-walled carbon nanotubes(F-MCNTs)were prepared by high-temperature heat treatment and introduced into the cathode catalytic layer as an additive.F-MCNT optimizes the assembled morphology of ionomers,increases porosity,and modulates the hydrophilicity and hydrophobicity of pores,thereby improving the accessibility and utilization of platinum.(3)The addition of sulfonic acid-functionalized multi-walled carbon nano tube film layers optimizes the electrical drag and reverse diffusion effects of water,and realizes water management in hydrogen-oxygen fuel cells.When a PEMFC is operating,internal water migration can greatly affect performance output,thus requiring better water management.An additional membrane layer containing sulfonic acidfunctionalized multi-walled carbon nano tubes(SO3-MCNT)was sandwiched between a commercial Nafion proton exchange membrane and an anode catalyst layer.The newly added membrane layer is able to optimize the electro-osmotic drag(EOD)and reverse diffusion(BD)effects of water,and accelerate proton transfer,thereby significantly improving the performance of the MEA.The advantage of this method is obvious under the conditions closer to the actual operation of PEMFC.(4)Mitigation of cathode concentration polarization in direct methanol fuel cells using composite proton exchange membranes containing sulfonated multi-walled carbon nanotube layers.A membrane layer containing sulfonated multi-walled carbon nanotubes was loaded on one side of a commercial N212 proton exchange membrane by ultrasonic spraying,and a composite membrane(N212-SO3-MCNT)was obtained.The composite membrane was used to prepare fuel cell membrane electrode(MEA)for direct methanol fuel cell(DMFC)testing.Compared with the membrane electrode samples using ordinary N212 membrane,the performance of the membrane electrode has been significantly improved.Further analysis showed that the introduction of the sulfonated multi-walled carbon nanotube membrane layer reduced the transmembrane water migration from the anode to the cathode,alleviated the flooding of the cathode,thereby reducing the concentration polarization and improving the performance of the membrane electrode.