Water transport analysis during cathode dry operation of anion exchange membrane water electrolysis

The effect of water diffusion through an anion exchange membrane (AEM) on the concentration overpotential (ηconc) during cathode dry operation of AEM water electrolysis was experimentally examined using electrolytic cells with different membrane electrode assemblies (MEAs). The specially designed MEAs were used in the cells to obtain reliable and reproducible data to clarify the influence of membrane thickness (tmem) and porosity of cathode catalyst layer (CL). The relative humidity of generated hydrogen (ϕH2) during electrolysis was also measured based on dew point measurements of the hydrogen. The ηconc analysis for cells with single- and double-AEM MEAs revealed that water diffusion through the membrane was the main contributor to ηconc. The quantitative agreement between ϕH2 data and ηconc revealed that the difference in ηconc between the two types of MEAs is explained by the water concentration difference between anode and cathode via the Nernst equation. The effect of the porosity of the cathode CL on cell performance and on water transport was also examined experimentally. The results revealed that a high-porosity cathode CL tended to keep the cathode in a drier state during electrolysis compared with a low-porosity cathode CL. When ϕH2 is lower than a threshold value in the range from 0.5 to 0.6, the ion conductivity of AEM and ionomer would decrease, and the cell performance would deteriorate due to an increase in cell resistance (Rcell) and/or activation overpotential (ηact).