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 O release and a more cathodic onset of Ni oxidation at higher pH. Using UV-vis, which can monitor the amount of oxidized Ni in situ, confirmed an earlier onset of the redox process at high electrolyte pH and further provided evidence of a smaller fraction of Ni in 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.

Recent Progress on Multimetal Oxide Catalysts for the Oxygen Evolution Reaction

“…[40,66,67,69,[71][72][73][74][75] Among them, X-ray absorption spectroscopy (XAS) has widely been used because it can provide information about the oxidation states and local structure of transition metal ions under OER conditions. [40,69,71,75] Görlin et al performed a kinetic study of Ni-Fe LDH under OER conditions based on quantitative analysis of in situ differential electrochemical mass spectrometry (DEMS) and XAS. [69] The combined DEMS and XAS analyses distinguished the injection charge depending on the reactions for which it was consumed, namely the oxidation of transition metals or water molecules, as shown in Figure 3d.…”

Fe‐CoP Electrocatalyst Derived from a Bimetallic Prussian Blue Analogue for Large‐Current‐Density Oxygen Evolution and Overall Water Splitting

“…Up to date, tremendous achievements have been acquired in the development of earth‐abundant element‐based electrocatalysts, including transition‐metal oxides/hydroxides,6 chalcogenides,7 phosphides,8 nitrides,9 and carbides 10. Among them, metal phosphides have been extensively investigated as high‐performance electrocatalysts for HER, showing low overpotential and small tafel slope in the acidic media 11.…”

Chemical Recognition of Active Oxygen Species on the Surface of Oxygen Evolution Reaction Electrocatalysts

“…For the Fe‐containing films, the potential–pH dependence becomes close to −90 mV/pH for pH >12.5. This value, characteristic of a super‐Nernstian potential–pH shift corresponding to the exchange of 2 electrons and 3 protons (ratio of H + /e − involved equals to 1.5),17, 25 indicate a different deprotonation mechanism for Fe‐containing film. Hence, depending on the deprotonation process, different oxygen species are formed on the surface of the films that show, or not, specific interaction with TMA + cation, as visualize by Raman spectroscopy (Supporting Information, Figure S8).…”

“…Interestingly, amorphous structures lie at the frontier between heterogeneous and homogeneous catalysts but the exact nature of the active site as well as their interaction with the adsorbed water molecules remain unclear. Moreover, even though agreement has been largely reached recognizing that deprotonation is key for the formation of the so‐called active oxygen species,15, 16, 17 their exact nature and reactivity remains elusive, owing to their transient nature and the lack of proper characterization techniques 18…”