Direct Observation of Active Nickel Oxide Cluster in Nickel-Borate Electrocatalyst for Water Oxidation by in Situ O K-Edge X-ray Absorption Spectroscopy

In situ O K-edge X-ray absorption fine structure (XAFS) spectroscopy was applied to investigate the electronic and structural change in the nickel-borate (Ni-B<inf>i</inf>) electrocatalyst during the oxygen evolution reaction (OER). An absorption peak was observed around 528.7 eV at 1.0 V versus Ag/AgCl in a potassium borate aqueous solution, which relates with the formation of nanoscale order domains of edge-sharing NiO<inf>6</inf> octahedra in the Ni-B<inf>i</inf> electrocatalyst. XAFS spectra were measured with variation of the electrode potential from 0.3 up to 1.0 V. The measured absorption peaks suggest that the quantity of NiO<inf>6</inf> octahedra increased in correlation with the OER current; however, when the potential was changed downward, the XAFS absorption peak assigned to NiO<inf>6</inf> octahedra remained constant, even at the electrode potential for no OER current. This difference implies that the water oxidation catalysis proceeds at the domain edge of NiO<inf>6</inf> octahedra. The XAFS technique provides the first successful direct probing of the active species in the Ni-B<inf>i</inf> electrocatalyst during electrochemical reaction.