Investigating the effect of stack voltage on void fraction, stray currents, and current distribution in an alkaline water electrolysis stack using computational fluid dynamics

Abstract

Stray currents, an undesired phenomenon in bipolar alkaline water electrolyzer (AWE) stacks, deviate from the current pathway into fluid channels and manifolds, thereby limiting the maximum current efficiency. This paper presents a novel and reliable numerical method, based on the finite element formulation, to investigate the hydrodynamic and electrical performance of the stack. A three-dimensional (3D) multiphysics model of a 12-cell AWE stack has been built in COMSOL®, incorporating an electrochemical model and an Euler-Eulerian k-epsilon turbulence model. The numerical model was validated against experimental data, including polarization curves and individual cell voltages, to verify the accuracy of the model. Subsequently, the influence of void fraction on stray currents and current distribution was determined for different stack voltages. The numerical results revealed that stray currents vary inversely with the void fraction. As the stack voltage increased, the higher rate of gas evolution raised the peak void fraction from about 0.13 at 18 V to roughly 0.47 at 26.4 V. The accumulated gas acted as an added resistance on the outlet side, lowering the stray-current fraction from approximately 33% at 18 V to about 11% at 26.4 V. The current distribution was non-uniform along the stack, and as the stray-current losses diminished, the Faradaic efficiency improved from about 67% to roughly 89% at full load. The specific energy consumption, however, reached a minimum of about 58 kWh/kg H2 at around 20 A, indicating that peak efficiency and lowest energy use do not coincide. The model offers useful guidance for the design and operation of larger-scale bipolar AWE stacks.