Proton Conduction Mechanism Of Ultrathin Perfluorosulfonic Acid Polymer Film In Proton Exchange Membrane Fuel Cell Catalyst Layers

Although great progress has been made,the commercialization of proton exchange membrane fuel cell（PEMFC）is still hindered by the high system cost.To overcome this bottleneck and realize the ultra-low platinum（Pt）loading,it is necessary to increase the catalyst utilization which requires making full use of the active sites on Pt surfaces.However,the inadequate research on the ultrathin perfluorosulfonic acid polymer（Nafion）film in catalyst layers has become a missing part of the principle of three-phase boundary formation,which limits the improvement of catalyst layers.To solve the underestimated issues,this paper systematically carried out a research on the proton conduction mechanism of ultrathin Nafion films in the catalyst layers.To simulate the aggregation process of ionomers in catalyst layer,self-assembly technology is used to gain nano-scale Nafion thin films with precise and controllable thickness on Si O₂ model substrate.The relationships between proton conduction behavior within ultrathin Nafion films and film thickness,temperature,equivalent weight,non-precious metal ion contamination,colloidal morphology in Nafion solution are studied.The final target is to explore and establish the microstructure model of nanoscale Nafion thin films through micro-characterization technologies.It is found that the loss of phase separation at the nanoscale Nafion thin films（～50 nm）results in a low proton conductivity which is one order of magnitude lower than that of bulk membranes（～10-100 um）.It is found that increasing the thickness of the thin films or using Nafion ionomers with lower EWs value can increase the proton conductivity of Nafion thin films.And by replacing iso-propanol with n-butanol which has lower dielectric constant as the dispersion media of Nafion ionomer,the Nafion thin films possess higher proton conductivity.Furthermore,along with the operation of fuel cell,non-precious metal alloy particles will decay and dissolve in the acidic environment,the leaching out metal ions in turn occupy the cation sites on Nafion thin films,resulting in a further reduction of the proton conductivity.This study provides a guidance for optimizing the structure of ultrathin Nafion film and improving the proton conduction in catalyst layers of PEMFC.