Multi-length scale thermo-mechanical behavior of ionomer for proton exchange membrane fuel cell

Abstract

Proton exchange membrane fuel cell (PEMFC), which is an electrochemical energy conversion device, has been received attention due to its high-energy conversion efficiency and sustainability. While various applications of PEMFC have been developed, the durability has been one of the challenges for the commercialization of PEMFCs. During operation, membrane electrode assembly (MEA) experiences a wide range of temperature and humidity. Temperature and humidity cycle during operation will cause dimensional change and stress in MEA. Due to repeated stress during hygrothermal cycles, membrane thinning and pinholes of membrane, cracks in electrode and delamination at interface between electrolyte and electrode are developed during operation. To predict and prevent these mechanical failures, mechanical properties of ionomer for PEMFC should be inspected. In this research, thermo-mechanical behavior of ionomer was explored because ionomer was important material as an electrolyte and as a binder in electrode. Intrinsic thermal behavior without substrate was investigated through in-plane thermal strain measurement method. To elucidate thermal behavior of ionomer thin film, thermal expansion of ionomer thin film with various thickness was measured upon annealing conditions. Mechanism of thermal expansion of ionomer was suggested based on transmission electron microscopy and small angle X-ray scattering measurement. Through the experiments, it was explained that nanostructure of ionomer had great effect on mechanical degradation of ionomer membrane and intrinsic thermal behavior of ionomer thin film. Investigation of thermo-mechanical behavior of ionomer could contribute to provide robust MEA and improve durability of PEMFC.