Gas diffusion layers and electrodes as dynamic interfaces: Directional capillary comparison and a matched-condition reporting framework

Gas diffusion layers (GDLs) and gas diffusion electrodes (GDEs) are often ranked using dry porosity, permeability and contact angle, although their electrochemical function is determined by an installed, wetted, polarized and history-dependent state. This critical review develops a directional comparison and reporting framework for proton-exchange-membrane fuel-cell cathodes and gas-fed CO 2 -electrolysis cathodes at the porous-backing level. For water moving from the catalyst side toward the channel in a fuel cell, an excessive drainage barrier can promote interfacial accumulation. For electrolyte invading a gas-fed GDE from the catalyst or membrane side, a higher entry barrier can protect gas pathways. At matched compression, liquid chemistry, wetting history and loading protocol, gas demand ( Da g ), directional capillary loading ( Π cap ) and installed strain ( ε c ) organize these comparisons. Bias-dependent breakthrough and phase-specific salt saturation provide complementary diagnostics. Literature anchors connect gas transport and water distribution to mechanical support, electrical conduction and thermal contact. Worked comparisons distinguish diffusivity changes from installed-length effects, while validation tests fix measurement definitions before assessing independent outcomes. The contribution is a practical rule for comparing architectures and reporting their operating conditions, rather than a unique-state description or common numerical safe limit.