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 τmt,0.166667emnormalH2normalO ≈0.176 s, whereas the time constant of oxygen mass transport into the reaction zone is several magnitudes smaller: τmt,0.166667emnormalO2 ≈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.

The Key Role of Water Activity for the Operating Behavior and Dynamics of Oxygen Depolarized Cathodes

“…For both systems the reaction zone, facing the gas‐liquid interface, decreases in size with increasing current densities due to the depletion of oxygen. Similarly to what is presented in our previous work, no oxygen associated limitation takes place due to the mass transport in the gas phase, therefore all oxygen mass transport limitation can be attributed to the liquid phase. In the system with the stagnant electrolyte, the reaction zone shrinks to almost zero at the mass transport governed current density of j ≈1.3 kA m −2 .…”

“…In our previous work dealing with ODC operated at 80 °C, we presented a higher value of S interface =134 m 2 m −2 and a slightly higher value for z fa of 11.5 μm . On the one hand this deviation may originate from temperature dependent system characteristics like viscosity or wettability.…”

“…In addition, the fourth section is given by a liquid diffusion layer beyond the electrode. The equations of the validated model have been introduced in our previous work and are shortly summarized in the following (cf. Table ).…”

“…The subsequent relaxation of the current density occurring in the range of seconds is dominated by the mass transport of water and hydroxide ions between the reaction zone and the electrolyte bulk phase . The corresponding time constant can be approximated with equation (34), derived from Fick's second law: …”

“…The influence of variations in PTFE and the silver loadings are discussed in . In our previous model‐based study we explored the limiting processes of ODC . We discovered that the consumption of water and the production of hydroxide ions induced by the electrochemical ORR lead to an enrichment of hydroxide ions at the gas‐liquid interface within the liquid electrolyte.…”