Electrocatalytic Reduction of Oxygen to Peroxide on a Microporous Carbon Layer in a Gas Diffusion Electrode: Implications for Scale-up

Abstract

When coupled with renewables, the two-electron oxygen reduction reaction(O2RR) could be a valuable process to generate hydrogen peroxide (H2O2), a versatile and environmentally friendly oxidant[1]. While the electrocatalyst's nature determines the catalytic activity and selectivity (2e- vs. 4e- reduction), the O2 gas mass transport, electrolyte type (e.g., alkaline, neutral, or acidic) and its flow rate, and the electrode configuration (e.g., trickle-bed or gas diffusion (GDE) electrode) are also essential factors for scale-up[2-4]. Typically, operational current densities for O2RR are limited to less than 100 mA cm-2, which is inadequate for industrial applications. The porous gas diffusion electrode (GDE) assembly provides continuous O2 gas delivery at the catalyst/electrolyte interface by overcoming O2 mass transport limitations encountered in trickle-bed or dissolved gas (single-phase) configurations. While the GDE could enable the operation under industrially relevant current densities (i.e., > 100 mA cm-2), for two-phase flow processes, flooding imposes severe challenges in the GDE scale-up and the durability of the process. Here, we present that a microporous carbon-coated gas diffusion electrode could enable very high current density (> 500 mA cm-2) synthesis of alkaline peroxide in a flow reactor with more than 90% faradaic efficiency. The microporous carbon layer provides dual functionality, namely the catalytically active sites for the reaction and controlling the GDE flooding. In addition to the electrode design, the two-phase fluid flow dynamics and electrolyzer operation conditions are significant factors for long-term stability. Although the fluid flow effect on the trickle bed type electrolyzer has been studied [3], no study exists on the systematic investigation of the two-phase mass transfer effect in the GDE-based electrolyzer for peroxide generation with a catholyte layer. The experimental findings and mass transfer modeling highlight further improvements concerning the process scale-up.