Photoanode Discovery in the Ni-Sb Oxide System for Durable Oxygen Evolution

Development of solar fuels generators remains challenged by the visible light activity and durability of photoanodes that evolve oxygen from water to enable sustainable and scalable fuel synthesis. High throughput exploration of the Ni-Sb-O system for stable solar fuels photoanodes revealed maximum photoactivity at Ni/(Ni+Sb) = 0.50. X-ray diffraction patterns of materials with Ni concentrations greater than 0.33 did not match any previously known phase. In this work, we explore this new phase space by a combination of experimental characterization and theoretical calculations. By a symmetry analysis of the observed XRD peaks, we suggest a disordered crystal structure in the <i>P63-mmc</i> spacegroup. Cross sectional TEM measurements confirm the stoichiometry of M₄O_7±1, which is oxygen-deficient compared to the rutile NiSb₂O₆ observed in an adjacent phase field. XAS measurements suggest the antimony to be in a +5 oxidation state in samples ranging from 0.66 to 0.33 Ni, whereas the oxidation states of Ni appear lower in the new phase compared to NiSb₂O₆. These semiconductors are n-type with ~0.1 cm2/Vs mobility according to Seebeck and Hall effect measurements. We identify a family of structures near the convex hull of the <i>ab initio </i>phase diagram motivating further characterization of these new metastable structures. The observation of dispersive bands in the electronic structure of the known rutile NiSb₂O₆ phase necessitates further study of where this desirable property also exists in the family of new structures, and will be pursued via theory and experiment. Overall, the Ni-Sb-O system serves as a good test system for designing bulk and surface compositions for co-design of materials for operational stability, oxygen evolution activity, and light harvesting.