Degradation mechanisms of proton exchange membrane fuel cell under typical automotive operating conditions

Abstract

Abstract The proton exchange membrane (PEM) fuel cell is an ideal automotive power source with great potential, owing to its high efficiency and zero emissions. However, the durability and life-span limit its large-scale application. Complex automotive operating conditions significantly accelerate fuel cell aging, and result in diverse degradation mechanisms that require comprehensive understanding. This review focuses on three harsh conditions of open-circuit/idling, dynamic load, and startup-shutdown. In-situ and ex-situ accelerated stress tests (ASTs) for the three conditions are summarized in terms of methodology, research objectives, and conditions of application. Reversible decay may arise during ASTs and lead to an over-estimation of the aging state, of which the causes and recovery procedures are emphasized. The degradation mechanisms are elaborated systematically according to parameter characteristics, microstructure, and aging reactions. First, increased gas permeation and a high cathode potential during open-circuit/idling combine to intensify generation of free radicals that cause membrane degradation. Pt degradation and migration are also accelerated, characterized by increased Pt particle growth and precipitation in the membrane. The debate regarding the effect of Pt precipitation on membrane degradation is resolved based on a literature review. Second, dynamic load brings about changes in the thermal/humidity state, altered reactant demand, and potential cycling, which lead to mechanical degradation, gas starvation, and Pt particle growth, respectively. To account for the accelerated particle growth, electrochemical Ostwald ripening and increased Pt dissolution are reviewed. Third, an air/hydrogen boundary appears in the anode under startup-shutdown condition and causes carbon corrosion in the local cathode via the reverse current mechanism. The cathode thereby suffers from severe and non-uniform structural damage and even structural collapse, accompanied by Pt agglomeration and detachment. Meanwhile, difficulties in mass transfer arise because of ionomer redistribution, decreased porosity, and carbon surface hydrophilization. In addition, cold start produces severe damage to component structures. This paper seeks to guide further investigation into improved fuel cell durability via mechanism analysis, condition optimization, control strategy development, structural design of the membrane electrode assembly, and component material development.