Effect of alkali metal cations on the performance of anion exchange membrane water electrolyzer

Abstract

The performance of anion exchange membrane water electrolyzers (AEMWEs) has been greatly improved with the materials (ionomers and catalysts) developments. However, the lack of understanding of their integrated structure limits further advancement. In this work, we investigated on the catalyst/electrolyte interface by varying the metal cations of the aqueous alkaline electrolyte. Larger metal cation sizes boost the catalytic activity of hydrogen (HER) and oxygen evolution reactions (OER) with double layer capacitance (Cdl) increment while impeding mass transport of their products (H2 and O2). Based on this observation, we proposed an electrical double layer (EDL) model on the catalyst surfaces. For the anode, by the larger cation size, the more cations and OH- molecules are adsorbed to the catalyst surface, and the increased local pH leads to higher OER activity. Simultaneously, the densely packed molecules impede O2 gas diffusion. In the same manner, large cations induce adsorption of a high concentration of H2O molecules to the cathode electrode, suppressing H2 gas diffusion, but lower local pH enhances the HER kinetics. This study presents an in-depth understanding of the catalyst/electrolyte interface, a new perspective on liquid electrolyte design, and further direction for polymer electrolyte design of AEMWEs.