Analysis of Species Concentrations in Bifunctional Air Electrodes Using Mathematical Modeling and Simulation

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Metal-air secondary batteries using aqueous alkaline solutions are attracting attention as a next-generation large-scale energy storage system. However, the large overpotential of the bifunctional air electrode is a challenge for its practical application. In order to reduce the overvoltage, one approach is to optimize the design of the electrode to enhance both electrochemical as well as transport properties within. We have previously estimated the oxygen diffusion resistance in bifunctional air electrodes with different thicknesses and electrolyte solutions from the difference in the steady state potential under air and oxygen flow. We identified that the reaction zone for the oxygen reduction and evolution reactions concentrated on the gas and electrolyte sides of the catalyst layer, respectively. 1 On the other hand, it is difficult to analyze local reaction rates in more detail since gas diffusion electrodes have a complex structure with a mixture of solid, gas, and liquid. In this study, we analyzed the local conditions in gas diffusion electrodes by combining digital simulations using finite difference methods and electrochemical measurements. The electrolyte solution's distribution significantly affects the simulation result since the electrolyte solution works as both an ionic conductiont and an obstacle to oxygen transport.2 Therefore, the electrolyte solution distribution in the electrode was determined by impedance analysis using a transmission line model, and a simulation model was constructed based on this impedance analysis.Figure 1 shows the Nyquist plot of a gas diffusion electrode at OCV. A transmission line-type frequency dependency is observed in the high-frequency region. The impedance simulated with an equivalent circuit where the ion transport resistance in the electrode does not change in the thickness direction shows a relatively significant difference from the measured one. On the other hand, the impedance simulated with an equivalent circuit with the linear decrease in the ionic conductance from the electrolyte to gas diffusion layer sides showed good agreement with the measured one. This result indicates that the amount of electrolyte in the gas diffusion electrode decreases from the electrolyte side to the gas diffusion layer side. Simplifying the pore structure with the porosity and average pore size obtained with the Barrett-Joyner-Halenda (BJH) method and the tortuosity calculated from the Bruggeman’s equation, the electrolyte thickness was calculated to be 2.1 and 1.1 nm at the electrolyte and gas sides, respectively. The local conditions in the electrode obtained with a one-dimensional simulation model based on the obtained electrolyte solution distribution will be presented at the site. Acknowledgment This work was financially supported by RP-LEAD with DFG (JPJSJRP20221602 and SCHR 1756/1-1).