title: In Situ Observation of Active Oxygen Species in Fe-Containing Ni-Based Oxygen Evolution Catalysts: The Effect of pH on Electrochemical Activity.
abstract: Ni-based oxygen evolution catalysts (OECs) are cost-effective and very active materials that can be potentially used for efficient solar-to-fuel conversion process toward sustainable energy generation. We present a systematic spectroelectrochemical characterization of two Fe-containing Ni-based OECs, namely nickel borate (Ni(Fe)-B(i)) and nickel oxyhydroxide (Ni(Fe)OOH). Our Raman and X-ray absorption spectroscopy results show that both OECs are chemically similar, and that the borate anions do not play an apparent role in the catalytic process at pH 13. Furthermore, we show spectroscopic evidence for the generation of negatively charged sites in both OECs (NiOO(-)), which can be described as adsorbed "active oxygen". Our data conclusively links the OER activity of the Ni-based OECs with the generation of those sites on the surface of the OECs. The OER activity of both OECs is strongly pH dependent, which can be attributed to a deprotonation process of the Ni-based OECs, leading to the formation of the negatively charged surface sites that act as OER precursors. This work emphasizes the relevance of the electrolyte effect to obtain catalytically active phases in Ni-based OECs, in addition to the key role of the Fe impurities. This effect should be carefully considered in the development of Ni-based compounds meant to catalyze the OER at moderate pHs. Complementarily, UV-vis spectroscopy measurements show strong darkening of those catalysts in the catalytically active state. This coloration effect is directly related to the oxidation of nickel and can be an important factor limiting the efficiency of solar-driven devices utilizing Ni-based OECs.
authors.affiliation: Materials for Energy Conversion and Storage (MECS), Department of Chemical Engineering, Faculty of Applied Sciences, Delft University of Technology , 2628 BL Delft, The Netherlands.
authors.forename: Bartek J
authors.initials: BJ
authors.lastname: Trześniewski
authors.affiliation: Leiden Institute of Chemistry, Leiden University , P.O. Box 9502, 2300 RA Leiden, The Netherlands.
authors.forename: Oscar
authors.initials: O
authors.lastname: Diaz-Morales
authors.affiliation: Materials for Energy Conversion and Storage (MECS), Department of Chemical Engineering, Faculty of Applied Sciences, Delft University of Technology , 2628 BL Delft, The Netherlands.
authors.forename: David A
authors.initials: DA
authors.lastname: Vermaas
authors.affiliation: Netherlands Organization for Scientific Research (NWO), The European Synchrotron Radiation Facility (ESRF) , CS40220, 38043 Grenoble Cedex 9, France.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Longo
authors.affiliation: Netherlands Organization for Scientific Research (NWO), The European Synchrotron Radiation Facility (ESRF) , CS40220, 38043 Grenoble Cedex 9, France.
authors.forename: Wim
authors.initials: W
authors.lastname: Bras
authors.affiliation: Leiden Institute of Chemistry, Leiden University , P.O. Box 9502, 2300 RA Leiden, The Netherlands.
authors.forename: Marc T M
authors.initials: MT
authors.lastname: Koper
authors.affiliation: Materials for Energy Conversion and Storage (MECS), Department of Chemical Engineering, Faculty of Applied Sciences, Delft University of Technology , 2628 BL Delft, The Netherlands.
authors.forename: Wilson A
authors.initials: WA
authors.lastname: Smith