Hierarchical anions at the electrode-electrolyte interface for synergized neutral water oxidation

The ion-enriched electrode-electrolyte interface is a unique characteristic of electrocatalytic reactions that can effectively tune the local chemical environment. In this study, we discovered a strong synergy between borate and fluoride anions for water oxidation in neutral conditions. Under an optimal ratio of the two anions, the water oxidation current significantly outperforms the individual counterparts by almost an order of magnitude. This synergy was greatly attributed to the hierarchical anions at the interface, with borate approaching the surface and fluoride extending out, as revealed by in situ surface-enhanced Raman spectroscopy and molecular dynamics simulation. This hierarchical anion arrangement created two kinetics regions, with one borate-dominating region to lower the onset potential and one fluoride-dominating region to sustain a larger current. Using the synergized electrolyte, a low-cost and facilely synthesized Co(OH)2 electrode could achieve an outstanding performance of 100 mA/cm2 at 1.791 V versus RHE under pH = 7.87 with a 2-week-long durability.