Efficient oxygen reduction to alkaline peroxide at current densities up to 500 mA cm-2 on a gas diffusion electrode with hydrophobic carbon microporous layer: Influence of fluid dynamics and reactor operation on scale-up

The two-electron electrochemical O2 reduction reaction (2e-ORR) on a gas diffusion electrode (GDE) using clean electricity is an attractive process for generating the green oxidant hydrogen peroxide (H2O2). Electrolyte flooding through the porous catalyst and diffusion layers is one of the major factors inhibiting the operational stability and longevity under industrially relevant current densities (>100 mA cm(-2)). This study reports for the first time that optimizing the two-phase flow and the mode of reactor operation brings about a remarkable improvement in the high current density operation (up to 500 mA cm(-2)) on a scalable (20 cm2 geometric area) and commercially available Sigracet 39BB GDE with the microporous carbon layer (MPL) acting as the catalyst layer. The interacting effects of current density, gas and liquid flow rates and the flooding threshold limit for the differential liquid/gas pressure were determined. While high peroxide Faradaic efficiency (FE > 95 %) was attained in a gas flow-by mode during short-term experiments, long-term stability was only established when the gas was switched to the flow-through or quasi-flow-through mode of operation. Under the latter conditions, at 500 mA cm(-2) operational stability of more than 20 h was demonstrated with a peak FE of 97 % in 1.0 M KOH, at 293 K, 1 atm. It was found that optimizing the reactor operation has a profound effect during scale-up, and developing novel catalytic materials for 2e-ORR to alkaline peroxide is not warranted since a low-cost microporous carbon-based GDE is sufficient for stable and efficient operation.