title: Tracking Catalyst Redox States and Reaction Dynamics in Ni-Fe Oxyhydroxide Oxygen Evolution Reaction Electrocatalysts: The Role of Catalyst Support and Electrolyte pH.
abstract: Ni-Fe oxyhydroxides are the most active known electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolytes and are therefore of great scientific and technological importance in the context of electrochemical energy conversion. Here we uncover, investigate, and discuss previously unaddressed effects of conductive supports and the electrolyte pH on the Ni-Fe(OOH) catalyst redox behavior and catalytic OER activity, combining in situ UV-vis spectro-electrochemistry, operando electrochemical mass spectrometry (DEMS), and in situ cryo X-ray absorption spectroscopy (XAS). Supports and pH > 13 strongly enhanced the precatalytic voltammetric charge of the Ni-Fe oxyhydroxide redox peak couple, shifted them more cathodically, and caused a 2-3-fold increase in the catalytic OER activity. Analysis of DEMS-based faradaic oxygen efficiency and electrochemical UV-vis traces consistently confirmed our voltammetric observations, evidencing both a more cathodic O2release and a more cathodic onset of Ni oxidation at higher pH. Using UV-vis, which can monitor the amount of oxidized Ni+3/+4in situ, confirmed an earlier onset of the redox process at high electrolyte pH and further provided evidence of a smaller fraction of Ni+3/+4in mixed Ni-Fe centers, confirming the unresolved paradox of a reduced metal redox activity with increasing Fe content. A nonmonotonic super-Nernstian pH dependence of the redox peaks with increasing Fe content-displaying Pourbaix slopes as steep as -120 mV/pH-suggested a two proton-one electron transfer. We explain and discuss the experimental pH effects using refined coupled (PCET) and decoupled proton transfer-electron transfer (PT/ET) schemes involving negatively charged oxygenate ligands generated at Fe centers. Together, we offer new insight into the catalytic reaction dynamics and associated catalyst redox chemistry of the most important class of alkaline OER catalysts.
authors.affiliation: Department of Chemistry, Chemical Engineering Division, Technical University of Berlin , Straße des 17. Juni 124, 10623, Berlin, Germany.|Department of Physics, Free University of Berlin , Arnimallee 14, 14195, Berlin, Germany.
authors.forename: Mikaela
authors.initials: M
authors.lastname: Görlin
authors.affiliation: Department of Chemistry, Chemical Engineering Division, Technical University of Berlin , Straße des 17. Juni 124, 10623, Berlin, Germany.
authors.forename: Jorge
authors.initials: J
authors.lastname: Ferreira de Araújo
authors.affiliation: Department of Chemistry, Chemical Engineering Division, Technical University of Berlin , Straße des 17. Juni 124, 10623, Berlin, Germany.
authors.forename: Henrike
authors.initials: H
authors.lastname: Schmies
authors.affiliation: Department of Chemistry, Chemical Engineering Division, Technical University of Berlin , Straße des 17. Juni 124, 10623, Berlin, Germany.
authors.forename: Denis
authors.initials: D
authors.lastname: Bernsmeier
authors.affiliation: Department of Chemistry, Chemical Engineering Division, Technical University of Berlin , Straße des 17. Juni 124, 10623, Berlin, Germany.
authors.forename: Sören
authors.initials: S
authors.lastname: Dresp
authors.affiliation: Department of Chemistry, Chemical Engineering Division, Technical University of Berlin , Straße des 17. Juni 124, 10623, Berlin, Germany.
authors.forename: Manuel
authors.initials: M
authors.lastname: Gliech
authors.affiliation: Institute of Surface Chemistry and Catalysis, Ulm University , Albert-Einstein-Allee 47, 89081, Ulm, Germany.
authors.forename: Zenonas
authors.initials: Z
authors.lastname: Jusys
authors.affiliation: Department of Physics, Free University of Berlin , Arnimallee 14, 14195, Berlin, Germany.
authors.forename: Petko
authors.initials: P
authors.lastname: Chernev
authors.affiliation: Department of Chemistry, Chemical Engineering Division, Technical University of Berlin , Straße des 17. Juni 124, 10623, Berlin, Germany.
authors.forename: Ralph
authors.initials: R
authors.lastname: Kraehnert
authors.affiliation: Department of Physics, Free University of Berlin , Arnimallee 14, 14195, Berlin, Germany.
authors.forename: Holger
authors.initials: H
authors.lastname: Dau
authors.affiliation: Department of Chemistry, Chemical Engineering Division, Technical University of Berlin , Straße des 17. Juni 124, 10623, Berlin, Germany.
authors.forename: Peter
authors.identifier: 0000-0002-3884-436X
authors.initials: P
authors.lastname: Strasser