Structural and mechanistic studies of nickel-borate thin-film oxygen evolving electrocatalysts

Abstract

Increases in global energy demand and rising levels of atmospheric carbon dioxide demand renewable alternatives to fossil fuels as the primary energy sources of the 21st century. Solar energy is by far the most abundant renewable energy resource, yet its widespread use has been hampered by a lack of suitable methods to store energy from sunlight in a cheap and efficient manner. Solar driven water splitting is a promising method of storing solar energy, but a critical bottleneck in developing efficient photoelectrochemical (PEC) water splitting systems lies in the kinetic sluggishness of the water splitting reactions, particularly the oxygen evolution reaction (OER). In this thesis the structural and mechanistic underpinnings for the activity of a promising nickel-based oxygen evolving catalyst (OEC) are discussed. The catalyst is particularly attractive as a result of the simplicity of its preparation as a thin film from aqueous borate-buffered solutions of Ni₂ . Electrochemical and in situ X-ray absorption near-edge structure (XANES) studies of this nickel-borate (Ni-Bi) catalyst indicate that upon initial electrodeposition, Ni centers in the film exist predominantly in the +3 oxidation state and the as-deposited material is largely inactive towards the OER. Catalytic activation is achieved by anodization of the as-deposited material in concentrated borate buffer, pH 9.2, a process which serves to oxidize the nickel centers to a mixed-valence Ni(II/IV) state. Extended X-ray absorption fine structure (EXAFS) spectroscopy studies indicate that Ni-Bi is comprised of nanometer-sized clusters of edge sharing NiO₆ octahedra. A structural transformation is observed during anodization that is akin to that observed in the [beta]-NiOOH-[gamma]-NiOOH transformation, challenging the long-held view that the phase that is the most catalytically active towards the OER is the all-Ni(III) [rho]-NiOOH. Electrokinetic studies indicate that the as-deposited Ni-Bi exhibits a Tafel slope close to 2.3 x 2RT/F, consistent with a turnover-limiting electron transfer (ET) from the geometrically distorted low-spin d⁷ Ni(III) state. Upon anodization to the mixed valence Ni(III/IV) state and elimination of geometric distortion, ET from the resting state becomes more facile resulting in a low Tafel slope of 2.3 x RT/2F, indicative of a rapid two-electron pre-equilibrium followed by a rate limiting chemical step, likely O₂ formation. Anodized Ni-Bi also exhibits an inverse third order dependence in proton activity and inverse first order dependence in borate anion. This kinetically-relevant two-electron, three-proton proton-coupled electron transfer (PCET) equilibrium prior to rate limiting O₂ formation forms the mechanistic basis for the pHdependent difference in activity between Ni-Bi and its cobalt-based analog, which contrarily mediates oxygen evolution via a kinetically-relevant one-electron, one-proton PCET transformation. The difference in catalytic O₂ evolution mechanism is a principal factor in the determination of the overall solar-to-fuels efficiency of PEC water splitting systems.