Gas Crossover and Supersaturation in Advanced Alkaline Water Electrolysis

Abstract

Conventional alkaline water electrolyzers operate with a finite-gap between the cathode and anode [1]. The ohmic resistance induced by this finite-gap results in energy losses. Therefore, the zero-gap assembly is considered an attractive configuration to perform advanced alkaline water electrolysis. However, gas crossover is enhanced in the zero-gap configuration by supersaturated hydrogen concentrations in the vicinity of the electrode. To avoid excessive gas crossover there is a limiting minimal operational current density.Different transport mechanisms can contribute to gas crossover through the porous separator, namely diffusion, convection, and electrolyte mixing. For well-balanced pressures and industrial flow rates, diffusion is the main contributor to gas crossover [2].Local supersaturation at the diaphragm interface significantly contributes to gas crossover [2]. To confirm this hypothesis, in-situ gas analysis experiments were conducted in an electrolysis setup equipped with in-line gas chromatography that could be operated with negligible influence of convective transport mechanisms to evaluate the diffusive transport in both zero- and finite-gap configurations.There is a clear distinction in gas crossover between zero- and finite-gap designs. In Figure 1 is shown that for the zero-gap assembly, hydrogen crossover rates were found to increase with current density, evidencing that supersaturation at the diaphragm interface plays an important role on gas crossover. In contrast, using a finite-gap configuration, gas crossover was significantly reduced and approached equilibrium diffusion rates, proving that gas crossover is mainly driven by diffusive hydrogen transport. Therefore, advanced alkaline electrolysis could be operated at higher current densities by applying a zero-gap configuration and reducing diaphragm thickness, but gas crossover leads to higher hydrogen in oxygen levels at low power load.[1] J. Brauns, T. Turek. “Alkaline water electrolysis powered by renewable energy: A review”. Processes, vol. 8, no. 2 (2020).[2] M. T. de Groot, J. Kraakman, R. Lira Garcia Barros. “Optimal operating parameters for advanced alkaline water electrolysis”. International Journal of Hydrogen Energy, vol. 47, no. 82 (2022) 34773-34783. Figure 1