Hydrogen mass transport resistance changes in a high temperature polymer membrane fuel cell as a function of current density and acid doping

Abstract

High temperature polymer electrolyte membrane fuel cells (HT-PEMFC) have phosphoric acid doped membranes. Acid in the membrane is mobile and tends to move out of the membrane depending on the acid doping. The migration of acid (when the doping is high) towards the anode at high current density >0.4Acm−2 causes gas diffusion layer (GDL) and catalyst flooding which thereby results higher hydrogen transport resistance. Thus, it is important to determine the acid doping level, which is optimal. In this study, transient changes in hydrogen mass transport is investigated as a function of doping level and current density. Three doping levels 11, 8.3 and 7 molecules of H2PO4 per PBI repeat unit are investigated. Electrochemical impedance spectroscopy (EIS) was modified to a single frequency measurement and time constant are calculated for resistance change with current density using a linear fit. The time constants are 2.0 ± 0.5, 3.4 ± 0.3, 8.2 ± 0.2 min for low and 2.5 ± 0.8, 4.9 ± 0.3 and 4.5 ± 0.2 min for high current densities, for the respective doping levels. The resistance decreases at high and increases at low current densities for all the doping levels with a varying time constant. This change in time constant is attributed to low doping level having lower capillary pressure to push the acid from reaching GDL pores from the membrane and/or catalyst layer.