Research On Transport Characteristic And Performance Optimization Of Mea In Pemfc

Membrane electrode assembly（MEA）is the specific site where electrocatalytic reaction occur in proton exchange membrane fuel cell（PEMFC）,and plays a role in the transport of multiphase mass during the electrochemical reaction process,and has a decisive impact on the performance,stability,and life of PEMFC.Precious metal platinum（Pt）is currently the best catalyst choice for MEA in practical application due to its excellent catalytic activity and stability,and is also the key to cost control of PEMFC.However,when the amount of Pt decreases to a certain extent,the oxygen transport channel at the catalytic active site decreases and the oxygen transport resistance increases,leading to a sharp decline in the power generation performance of MEA,especially during high current density operation.How to reasonably design and prepare MEA structure that is conducive to efficient transport for water and gas is an important issue in promoting the commercial development of PEMFC.Starting from the gas transport characteristic of MEA and the structure design of three-phase boundary（TPB）,catalyst layer（CL）,and microporous layer（MPL）,this paper studies the gas transport mechanism in the micro-nano porous structure of electrode materials,and analyzes the resistance and voltage loss of gas transport in MEA,designs TPB with a hole structure and an ordered tower structure CL to enhance gas transport,and patterned MPL to improve water management capability,providing a theoretical basis and practical basis for achieving high-performance MEA.This paper mainly includes the following four aspects of research work:（1）Research on gas transport characteristic of MEA under different current densities.In MEA,the reaction gas must pass through gas diffusion layer（GDL）and reach the catalytic active site inside CL to participate in the reaction.Therefore,quantifying the gas transport resistance of MEA operating at different current densities is crucial for developing high-performance PEMFC.The total oxygen transport resistance(R_tot)of MEA operating at different current densities is measured by establishing a transport model and combining it with the limiting current density.According to the variation pattern of R_tot,the working range of MEA is divided into dry and wet regions,and the inherent relationship between the two is analyzed.When in the dry region,R_tot is less affected by humidity and mainly depends on the electrode material itself,which remains almost constant.In the wet region,due to the plenty product water,R_totincreases sharply.Based on the experimental results,an empirical formula for the R_tot,i at a specific current density is proposed.Moreover,the voltage loss estimated based on the R_tot,i is reasonably close to the actual measured value.This study provides a simple method for determining the R_tot of MEA operating in the full current range.（2）Constructing TPB with a hole structure to enhance reaction activity.The electrocatalytic reaction occurs at TPB where the catalyst,reaction gas,and ionomer closely intersect.The TPB structure depends on the interaction between ionomer and catalyst.Due to the strong adsorption and confinement effect of sulfonic acid groups on Pt,ionomer is prone to accumulate around the catalyst,causing poisoning and burial of Pt active site,resulting in a decrease in oxygen reduction activity and obstruction of local oxygen transport.By using polystyrene sulfonic acid as a masking agent to protect the catalyst and then removing it,the distribution of ionomer on the Pt surface is successfully adjusted,resulting in TPB with a highly active pore structure.Compared with conventional TPB-CL,hole TPB-CL has the uniform distribution of ionomer and the rich pore structure,which is conducive to the release and the local oxygen transport of Pt catalytic site,and the catalytic activity per unit mass of Pt has increased by approximately 10%.This work indicates that the addition of masking agent can effectively regulate the distribution of ionomer on the catalyst surface,thereby improving the TPB structure and enhancing the performance of PEMFC.（3）Constructing an ordered tower structure CL to enhance gas transport performance.Conventional structure CL is the disordered porous structure formed by randomly stacking massive nanoscale TPB aggregate.The disordered stacking process of electrode material leads to high tortuosity and low interconnectivity of pore structure,which seriously hinders the transport of reaction gas in CL.Based on template transfer printing technology,an ordered tower structure CL is designed and prepared using an ordered pore structure alumina template.Unlike the disordered pore structure of conventional structure CL,tower structure CL constructs the rich ordered pore structure between the towers,improving the tortuosity and interconnectivity of gas transport channel,which is conducive to the transport of reaction gas and the exposure of catalyst catalytic site.Compared to conventional structure CL-MEA,tower structure CL-MEA has smaller charge transfer impedance and mass transport impedance in actual operation,with a peak power increase of about 19%.This work indicates that constructing ordered pore structures in CL can enhance gas transport performance,thereby enhancing the power generation performance of PEMFC.（4）Constructing a patterned structure MPL/CL interface to improve water retention performance.The interface between MPL and CL is a transition zone for the transport of multiphase mass between components,which seriously affects the power generation performance and stability of MEA.Among them,the structural difference on the surface of MPL determines the interface state of MPL/CL.An MPL with grid grooves on the surface is designed by using screen printing technology to analyze the impact of patterned MPL/CL interface structure on multiphase mass transport in MEA and its adaptation scenarios.Unlike flat MPL,there are numerous groove structures at the patterned MPL/CL interface,which are conducive to water storage and do not affect the electronic conduction at the interface.Compared to flat MPL-MEA,patterned MPL-MEA exhibit better water retention and lower proton conductivity resistance during low humidity operation,such as the Ohmic resistance is reduced by 23%at 50%relative humidity and 500 m A cm^-2 current density,demonstrating better cell performance.This work indicates that patterned structure on the surface of MPL are beneficial for moisturizing the electrode and reducing Ohmic resistance,and has significant advantage in low humidity application scenarios.