Chapter 13 - Mass transport in the cathode

Abstract

Flow field, gas diffusion layer (GDL), microporous layer (MPL), and catalyst layer (CL) are the main components for mass transport in a proton-exchange membrane fuel cell (PEMFC) cathode. The flow field, also called flow distributor or flow channel, is typically embedded in a bipolar plate. The GDL is a highly porous layer that connects the flow field and the CL. The MPL is composed of carbon agglomerates, commonly integrated between the GDL and CL to achieve good contact and a smooth interface between them. The CL consists of catalysts, catalyst support, and ionomer, wherein the electrochemical reactions occur. These components are compacted together for conducting electrons, delivering oxygen, and removing the product water. Oxygen enters the gas flow field and permeates through the GDL, MPL, and the pores and ionomer films of the CL to reach the catalyst surface for reactions. The structures of these components are required to uniformly distribute the oxygen with a low transport resistance. Water produced on the catalyst surface needs to be expelled through the CL, MPL, and GDL and removed in the flow field by airflow. Water management is significant for PEMFC performance because excessive water will block the oxygen transport paths, while a low water content will dehydrate the ionomer electrolyte and membrane, reducing their proton conductivity. Electrons are produced in the anode CLs and conducted to the cathode via the external circuit. In the future, the power density of PEMFCs needs further improvement for wider commercialization. Therefore, novel architectures of the cathode components emerge to enhance the mass transfer in PEMFC. This chapter introduces important mass transfer mechanisms in the cathode and give an outlook on future routes of the cathode development.