Investigations of ultra-low air stoichiometries in hydrogen fuel cells via operando current and oxygen content cartography and multiphysics simulations

Hydrogen fuel cell stacks suffer from considerable parasitic power loss due to the operation of air blowers and compressors. To improve the efficiency of the system, it is thus critical to explore operations under low air stoichiometries, but severe voltage fluctuations typically occur in these conditions. Herein, we combine high-resolution current cartography, computational fluid dynamics, and electrochemical simulations to characterize the water and gas interactions under low cathode stoichiometries in a 50 cm 2 quadruple serpentine flow field. The oxygen flow rate and local oxygen content profiles are calculated from the current density profiles, providing a new characterization lens. Under low oxygen stoichiometry (1.01), the cell voltage decays, as low current density and oxygen flow rate fronts propagate from the exhaust to the centre of the cell. This is attributed to a reduction in the electrochemically active area due to water build-up. Furthermore, at low air stoichiometries (1.23 and 1.13), severe voltage fluctuations are observed. These fluctuations are accompanied by a sharp decrease in local current density, together with oxygen contents as low as 3% and 1%, at the exhaust. Computational fluid dynamics and electrochemical fuel cell simulations for low air stoichiometries reveal that, depending on the gas diffusion layer permeability, under-rib convection can occur at the edges of the flow field. These lead to oxygen depletion, water build-up, and low current densities in the centre of the channels. These create rapid voltage fluctuations under low air stoichiometries due to the slow transport of water and oxygen diffusion limitation in nitrogen.