Mixed transition metal oxides and hydroxides are at the forefront of materials research for alkaline electrocatalysis due to high catalytic activity and low anticipated cost in energy conversion technologies. Recent density functional theory (DFT) calculations for the alkaline oxygen evolution reaction (OER) on FexNi100-xO(H)y electrocatalysts implicate Fe(IV) with a high-spin 3d4 configuration as part of a synergistic dual catalyst site (Fe(IV)-Ni(IV)). Experimental evidence for Fe(IV) remains ambiguous, with hard x-ray absorption spectroscopy (Fe K-edge) and Mössbauer spectroscopy providing evidence for and against Fe(IV) formation. Efforts to probe metal 3d-oxygen 2p hybridization provide conflicting evidence on the relationship between d-electrons, valency, and M-O covalency for relevant transition metal oxides (e.g., Fe2O3, NiO, LaFeO3, LaNiO3). Further, the extent of metal 3d-oxygen 2p orbital covalency in FexNi100-xO(H)y electrocatalysts likely shifts during OER. A coupled proton-electron transfer mechanism might exist, but non-concerted proton-electron transfer mechanisms have also been proposed. In short: The d-state electronic structure of Fe (and Ni) in these oxides/hydroxides, including formalized oxidation state and metal 3d-O 2p covalency, must be unambiguously determined experimentally to fully delineate OER mechanism(s) and propel catalyst materials development. Our research combines coordinated soft x-ray spectroscopic, electrochemical, and theoretical investigations of the electronic structure of FexNi100-xO(H)y and other AxB100-xO(H)y transition metal oxides to delineate and advance consistency between electronic structure and proposed theories of metal-oxygen covalency, metal oxidation state, and proton-electron transfer reaction mechanisms.
Hypothesis: When an FexNi100-xO(H)y electrocatalyst is under applied potential in the Faradaic region of the oxygen evolution reaction, a population of Fe atoms will transition from a Fe(III) 3d5 to a Fe(IV) 3d4 electron configuration. The Fe atoms will have a low level of 3d orbital mixing with O 2p, as evidenced by DFT-validated spectral features and experimentally seen in cutting-edge soft x-ray spectroscopy approaches. Objective 1: Use and further develop cutting-edge soft x-ray spectroscopy to describe the electronic and chemical structure at the Fe and O atoms, including accepted formal oxidation state and extent of orbital mixing or covalency, with Fe(II), Fe(III), and Fe(IV) reference compounds. Known reference materials for Fe, Ni, and O that represent a range of valence, coordination environment, phase, and metal d-oxygen p orbital hybridization will be used to correlate experimentally-obtained spectra with theoretical projected density of states. In situ cell design and hardware will be pursued. Objective 2: Determine Fe, O, and Ni electronic and chemical structure for as-synthesized and post-electrochemistry FexNi100-xO(H)y electrocatalyst materials. FexNi100-xO(H)y experimental electrocatalyst materials will be explored with soft x-ray spectroscopy, and electrochemical analysis will be used to link electronic band structure results from experiment and theory. Objective 3: Develop a computational framework to describe the FexNi100-xO(H)y electrocatalyst electronic structure. Computational and theoretical modeling will create computed spectra, describe kinetic and thermodynamic scaling relationships, propose relevant reaction mechanism(s), and iteratively enable determination of electrocatalyst band structure.
Future work will extend to relevant and related AxB100-xO(H)y transition metal oxides. This research will contribute new knowledge about the chemical and electronic structure of FexNi100-xO(H)y (and similarly structured) electrocatalysts, with a particular focus on the active catalyst and probing the existence of a Fe(IV) oxidation state. This project will provide a scientific basis for the alkaline electrocatalysis research community to improve and direct catalyst development and design, including shedding light on active site identification, reaction mechanism, and correlations between as-synthesized catalyst properties and operando electronic band structure, with impacts to electrocatalysis, battery materials, chemical conversion, fossil fuel upgrading, polymer conversion/recycling, and water treatment.
|Effective start/end date||9/1/20 → 8/31/23|
- Basic Energy Sciences: $750,000.00