On the microstructure of PEM fuel cell catalyst layers

In this work, the microstructure of conventional catalyst layers of PEM fuel cells (PEMFCs) is investigated and correlated to the catalyst layer water sorption and retention properties, electrochemical properties and fuel cell performance. Two types of conventional carbon support were chosen for investigation: Ketjen Black and Vulcan XC-72. The microstructure of carbon supports, Pt/carbon catalyst powders, and 3-component, Pt/carbon/ionomer catalyst layers is studied in order to evaluate the effects of carbon support microstructure and Nafion ionomer loadings on the resultant CL microstructure. The microstructure of the carbon support is found to be a significant factor in the formation of the structure in the 3-component CLs serving as a rigid porous framework for distribution of Pt and ionomer. It is found that deposition of Pt particles on Ketjen Black significantly changes its porosity possibly by forming at the mouth of the support’s micropores, thus affecting its effective microporosity, whereas Pt deposited on Vulcan XC-72 does not significantly affect the support’s microporosity. The co-deposition of ionomer in the CL strongly influences porosity of the catalyst layer, especially for pore sizes < 20 nm, which are ascribed to the pores within primary carbon particles (pore sizes < 2 nm) and to the pores within agglomerates of the particles (pore sizes of 2-20 nm). Ketjen Black and Ketjen Black-based CLs were found to posses higher water sorption and retention capacity than Vulcan-XC-72 based catalyst layers due to carbon microstructure. Electrochemical properties and overall fuel cell performance were found to be strongly dependent on carbon support microstructure, ionomer loading and water sorption and retention capacity of the catalyst layers. It is postulated that carbons with a pore size distribution in the mesoporous range are beneficial for improved Pt utilization, especially, for fuel cell operation in dry conditions. It was also found that in these CLs the ionomer loading can be significantly decreased without significant reduction in performance.