PEM Fuel Cell Impedance at Open Circuit

Abstract

A physical model for PEM fuel cell impedance Z at open circuit potential (OCP) is developed and analytical expression for Z is derived. The OCP impedance is a sum of the cathode catalyst layer (CCL) and the gas–diffusion layer (GDL) impedances connected in series. The GDL impedance differs from the Warburg impedance, which is often used in modeling of fuel cell electrodes. A key parameter determining the OCP impedance spectrum is the Newman’s dimensionless reaction penetration depth ε. In PEMFCs, ε is large, which makes the GDL impedance a small “invisible” additive to the CCL impedance. In a solid oxide fuel cell (SOFC) anode, ε is small, and the GDL impedance forms a well–resolved separate arc in the impedance spectrum at the OCP. In a real PEM fuel cell, a true OCP regime cannot be achieved due to hydrogen crossover through the membrane. Impedance measurements at zero current in the load yield an equivalent current density of hydrogen crossover through the membrane.