A dual bubble layer model for reactant transfer resistance in alkaline water electrolysis

Alkaline water electrolysis (AWE) can only operate efficiently at a relatively low current density of up to 0.6 A cm–2 because bubbles cover the electrode surface at high current densities. Generally, the upper limit of current density in an electrochemical reaction is determined by the mass transfer of the reactant. Here, we propose a dual bubble layer model for the reactant transfer resistance of the oxygen evolution reaction in AWE. Using simple cylindrical nickel electrodes of different diameters, we conducted electrochemical impedance spectroscopy and microscopic observations with a high-speed video camera. The relationship between the polarization curve of the oxygen evolution reaction in AWE and the mass transfer resistance of hydroxide ions was investigated. At current densities above 0.5 A cm‒2, the bubble layer completely covered the electrode surface. This bubble layer consisted of two layers: an inner one formed by numerous primary bubbles generated on the electrode, and an outer one formed by coalesced large bubbles. The outer layer induces an additional solution resistance, Rsb. Changes in Rsb under various conditions were successfully explained by the thickness and void fraction of the bubble layer, conductivity of the solution, and tortuosity through the gap between bubbles. The inner layer resists the diffusion of hydroxide ions, and the resistance to hydroxide ion transfer through it (Rd) was modeled using the Warburg impedance element with a finite-length diffusion boundary. The charge-transfer resistance at the electrode surface (Rct) is roughly inversely proportional to the current density. This result indicates Rct is less sensitive to bubble coverage and almost all electrode surfaces are active in electrochemical reaction. The results showed that Rd increases with current density as much as Rct when the current density exceeds 1.0 A cm‒2. Up to a high current density of 2.5 A cm‒2, the change in polarization curve (ΔE/Δi) could be quantitatively explained by the sum of the bulk solution resistance, Rs0, additional solution resistance Rsb in the gaps between large bubbles, diffusion resistance Rd in the inner bubble layer, and charge-transfer resistance Rct at the electrode surface.