Unraveling processes in an alkaline water electrolysis cell with reference electrodes and electrochemical impedance spectroscopy

A zero-gap full cell for alkaline water electrolysis is equipped with two reference electrodes. The anode (NiO mesh), the cathode (Ni mesh), and the diaphragm are investigated simultaneously by analyzing the polarization curves and utilizing electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT) analysis. The shares of cell voltage are 44 % of the cathode, 23 % of the anode and 33 % of the diaphragm at 0.4 A/cm2. EIS and DRT analysis focus particularly on the diaphragm, revealing a complex behavior that cannot be described merely as an ohmic resistor as it is traditionally done. EIS shows that the total resistance of the diaphragm comprises an ohmic contribution and a polarization behavior. This behavior is attributed to a funnel effect of OH- ions through the diaphragm. The effect results in a change in the concentration profile near the diaphragm, leading to concentration polarization. To gain deeper insights, two equivalent circuit models with a focus on the diaphragm are developed to describe the behavior of the full cell. Both models showed that the diaphragm cannot be described by a simple ohmic resistance, and the resistance estimated by analyzing the polarization curve is a sum of multiple resistances.