Processes and Their Limitations in Oxygen Depolarized Cathodes: A Dynamic Model-Based Analysis

Abstract

Oxygen depolarized cathodes (ODCs) are key components of alkaline fuel cells and metal-air batteries or of chlor-alkaline electrolysis, but suffer from limited oxygen availability at the reaction zone. Dynamic analysis is a highly suitable approach to identify the underlying causes, especially the limiting steps and process interactions in such gas diffusion electrodes. Herein, a one-dimensional, dynamic, three-phase model for analyzing the oxygen reduction reaction in silver-based ODCs is presented. It allows for a detailed evaluation of the electrochemical reaction, the mass transport processes, and their interaction. The model also reveals that the depletion of reactant oxygen in the liquid electrolyte is caused by the current-dependent change of the gas-liquid equilibrium as the limiting subprocess. The phase equilibrium, in turn, depends on the slow mass transport of water and hydroxide ions in the liquid phase. Key parameters are the location or size of the gas-liquid interface within the electrode. Profiles of local concentrations and partial pressures of different species reveal a steep gradient of oxygen in the liquid phase, but no limitation in oxygen mass transport in the gas phase. Dynamic simulations with potential steps allowed the identification of different time constants to separate overlapping processes. Accordingly, the mass transport of water and hydroxide ions was identified as the slowest process that strongly influenced the dynamic response of all species, including oxygen, and of the current. The characteristic time constant of the mass transport of water and hydroxide ions across the liquid phase within the ODC is determined to be τ ${}_{\mathrm{mt},H{}_{2}O}$ ≈0.176 s, whereas the time constant of oxygen mass transport into the reaction zone is several magnitudes smaller: τ ${}_{\mathrm{mt},O{}_2}$ ≈1.70×10^-6 s. Finally, a sensitivity analysis confirms that the overall performance is best improved by adjusting mass transport properties in the liquid phase.

Keywords: chlor-alkali electrolysis; computational chemistry; dynamic modeling; electrochemistry; interfaces.