Why impedance of the gas diffusion layer in a PEM fuel cell differs from the Warburg finite-length impedance?

•A physics-based model for GDL impedance in a PEM fuel cell is developed.•An analytical expression for the GDL impedance differs from the Warburg finite-length impedance.•The difference is due to finite double layer capacitance of the catalyst layer the GDL is attached to.•In the limit of zero double layer capacitance, the GDL impedance reduces to the Warburg result.

We report a simple physics-based model leading to an analytical expression for impedance of the gas-diffusion layer (GDL) in a PEM fuel cell. We show that this expression differs from the classic Warburg finite-length impedance due to large double layer capacitance Cdl of a porous catalyst layer the GDL is attached to. It follows that the Warburg solution is obtained for the transport layer attached to a planar electrode with vanishingly small Cdl.