title: Characterization of NiFe oxyhydroxide electrocatalysts by integrated electronic structure calculations and spectroelectrochemistry.
abstract: NiFe oxyhydroxide materials are highly active electrocatalysts for the oxygen evolution reaction (OER), an important process for carbon-neutral energy storage. Recent spectroscopic and computational studies increasingly support iron as the site of catalytic activity but differ with respect to the relevant iron redox state. A combination of hybrid periodic density functional theory calculations and spectroelectrochemical experiments elucidate the electronic structure and redox thermodynamics of Ni-only and mixed NiFe oxyhydroxide thin-film electrocatalysts. The UV/visible light absorbance of the Ni-only catalyst depends on the applied potential as metal ions in the film are oxidized before the onset of OER activity. In contrast, absorbance changes are negligible in a 25% Fe-doped catalyst up to the onset of OER activity. First-principles calculations of proton-coupled redox potentials and magnetizations reveal that the Ni-only system features oxidation of Ni2+to Ni3+, followed by oxidation to a mixed Ni3+/4+state at a potential coincident with the onset of OER activity. Calculations on the 25% Fe-doped system show the catalyst is redox inert before the onset of catalysis, which coincides with the formation of Fe4+and mixed Ni oxidation states. The calculations indicate that introduction of Fe dopants changes the character of the conduction band minimum from Ni-oxide in the Ni-only to predominantly Fe-oxide in the NiFe electrocatalyst. These findings provide a unified experimental and theoretical description of the electrochemical and optical properties of Ni and NiFe oxyhydroxide electrocatalysts and serve as an important benchmark for computational characterization of mixed-metal oxidation states in heterogeneous catalysts.
authors.affiliation: Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801.
authors.forename: Zachary K
authors.initials: ZK
authors.lastname: Goldsmith
authors.affiliation: Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801.
authors.forename: Aparna K
authors.initials: AK
authors.lastname: Harshan
authors.affiliation: Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706.
authors.forename: James B
authors.initials: JB
authors.lastname: Gerken
authors.affiliation: Institute for Molecular Engineering, University of Chicago, Chicago, IL 60637.|Argonne National Laboratory, Lemont, IL 60439.
authors.forename: Márton
authors.identifier: 0000-0003-1321-9207
authors.initials: M
authors.lastname: Vörös
authors.affiliation: Institute for Molecular Engineering, University of Chicago, Chicago, IL 60637; gagalli@uchicago.edu stahl@chem.wisc.edu shs3@illinois.edu.|Argonne National Laboratory, Lemont, IL 60439.
authors.forename: Giulia
authors.initials: G
authors.lastname: Galli
authors.affiliation: Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706; gagalli@uchicago.edu stahl@chem.wisc.edu shs3@illinois.edu.
authors.forename: Shannon S
authors.initials: SS
authors.lastname: Stahl
authors.affiliation: Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801; gagalli@uchicago.edu stahl@chem.wisc.edu shs3@illinois.edu.
authors.forename: Sharon
authors.identifier: 0000-0002-3782-6995
authors.initials: S
authors.lastname: Hammes-Schiffer
keywords: NiFe oxyhydroxide; density functional theory; electrocatalysis; oxygen evolution reaction; spectroelectrochemistry