Ion Transport Control in Alkaline Water Electrolyzers at Elevated Temperatures Revealed by Molecular Simulations and Multiphysics Modeling

Alkaline water electrolyzers (AWEs) offer a carbon dioxide-emission-free route to convert renewable electricity into hydrogen. Improving their efficiency requires a deep understanding of transport and kinetics within an electrolytic cell. By combining molecular simulations of ion diffusivities with detailed multiphysics simulations, we report here a surprising finding that neglecting the temperature dependence of ion transport in an AWE could lead to erroneous predictions of current density reducing with increasing temperature. This is because ion mobilities reduce with temperature, leading to severe transport limitations if diffusivities are considered to be independent of temperature. We model ion diffusion, migration, and convection via the Nernst-Planck formulation, fluid dynamics via the continuum Euler-Euler approach, along with electrode kinetics at the Butler-Volmer level of theory. We carry out molecular dynamics simulations to determine the temperature dependence of OH − and K + diffusivities and show that considering temperature-dependent diffusivities leads to the current density increasing with temperature. We also demonstrate that gas bubble formation and OH − concentration gradients lead to reduced electrolyte conductivity near the electrodes. The higher levels of cathodic gas production, as compared to those at the anode, lead to asymmetries in the concentration profiles. We provide physical understanding regarding these observations, including that at high cell potentials, transport limitations play a crucial role in modulating AWE performance. Overall, our findings provide critical insights into the interplay between ion transport, temperature, and electrochemical performance, which are crucial for accurate multiscale simulations and modeling-based optimization of such devices.