(Invited) Diagnostics of Microstructure and Properties of Polymer Electrolyte Fuel Cell Catalyst Layer

Abstract

Rising levels of greenhouse gas emissions, especially carbon dioxide (CO2), are regarded as one of the causes of global warming in recent years. To develop technologies for reducing CO2emission plays an important role for vehicle manufacturers, and Nissan has been developing zero-emission vehicles (ZEVs) such as battery electric vehicles (BEVs) and fuel cell electric vehicles (FCEVs). The biggest issues to penetrate FCEVs into the market are cost reduction of polymer electrolyte fuel cell (PEFC) power system including hydrogen storage subsystem, and development of hydrogen infrastructure. Related to the former issue, one of the most significant challenges is to reduce platinum group metals (PGM) in fuel cell stacks. PGM are used as electrocatalyst and contained in catalyst layers (CLs) of membrane electrode assemblies (MEAs) in the stack. Various kinds of research activities on PEFC electrocatalyst have been reported so far, indicating high oxygen reduction reaction (ORR) activity and durability mainly measured in aqueous electrolyte systems such as a rotating disk electrode method.1-8 On the other hand, electrocatalyst is mixed with electrolyte to form CLs through deposition and solidification process. From industrial point of view, it is needed that performance of the electrocatalyst should be guaranteed as CL in MEA. CL is a multi-scale component, i.e. micro-scale porous electrode consisting of nano-scale materials (Fig. 1), where electrochemical reaction and mass transport occur simultaneously. Micro and nano structure of CL has been revealed gradually by means of characterization techniques remarkably progressed recently, such as electron microscope. 9-10However, it is still under development to characterize and diagnose structure and properties of CLs as overall and average value. To reduce PGM usage, so far Nissan has been doing research and development on CL based on the strategy to understand the correlation between CL/MEA performance and the consisting materials through properties and structure via key mechanism.11-20In this presentation, diagnostics of PEFC CLs related to activity and mass transport are explained, mainly focusing on the unique analysis Nissan has been developing thus far.