Bifunctional oxygen electrodes with gradient hydrophilic/hydrophobic reactive interfaces for metal air flow batteries

Abstract Oxygen evolution (OER) and oxygen reduction (ORR) are two critical reactions that occurred at tri-phase reactive interfaces of oxygen electrodes. ORR needs hydrophobic/aerophilic interfaces, while OER requires hydrophilic/aerophobic interfaces. However, traditional oxygen electrodes are fabricated with hydrophobic binders, such as polytetrafluoroethylene, to construct gas diffusion layers for both ORR and OER catalyst layers. This results in scanty tri-phase reactive interfaces for OER. Herein, we propose a gradient hydrophilic/hydrophobic structure to increase tri-phase reactive interfaces. Designed oxygen electrodes exhibit larger tri-phase reactive interfaces and lower overpotentials than traditional ones. The charge voltage of zinc air flow batteries (ZAFBs) reduces by 190 mV, and the energy efficiency of ZAFBs increases by 6% at 50 mA cm−2. Importantly, ZAFBs run lustily beyond 100 h with negligible performance degradation. This study provides a simple and scalable method to construct tri-phase reactive interfaces in bifunctional oxygen electrodes for metal air flow batteries.