H/D Isotope Effects Reveal Factors Controlling Catalytic Activity in Co-Based Oxides for Water Oxidation

Abstract

Understanding the mechanism for electrochemical water oxidation is important for the development of more efficient catalysts for artificial photosynthesis. A basic step is the proton-coupled electron transfer, which enables accumulation of oxidizing equivalents without buildup of a charge. We find that substituting deuterium for hydrogen resulted in an 87% decrease in the catalytic activity for water oxidation on Co-based amorphous-oxide catalysts at neutral pH, while 16O-to-18O substitution lead to a 10% decrease. In situ visible and quasi-in situ X-ray absorption spectroscopy reveal that the hydrogen-to-deuterium isotopic substitution induces an equilibrium isotope effect that shifts the oxidation potentials positively by approximately 60 mV for the proton coupled CoII/III and CoIII/IV electron transfer processes. Time-resolved spectroelectrochemical measurements indicate the absence of a kinetic isotope effect, implying that the precatalytic proton-coupled electron transfer happens through a stepwise mechanism in which electron transfer is rate-determining. An observed correlation between Co oxidation states and catalytic current for both isotopic conditions indicates that the applied potential has no direct effect on the catalytic rate, which instead depends exponentially on the average Co oxidation state. These combined results provide evidence that neither proton nor electron transfer is involved in the catalytic rate-determining step. We propose a mechanism with an active species composed by two adjacent CoIV atoms and a rate-determining step that involves oxygen–oxygen bond formation and compare it with models proposed in the literature.

Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives

“…A common starting point is the involvement of equivalent reactive moieties in both molecular and heterogeneous WOCs at the atomic level. 14 Specifically, the surface of a finite particle offers localized molecular centers for catalytic reactions, while some key molecular motifs are essentially surface cut-outs of solids. These structural similarities clearly link both fields for a mutual transfer of insight, which is beneficial and indispensable for optimizing the performance and durability of WOCs in general.…”

“…Furthermore, a reduction of the activity by 10% was observed when 18 O isotope tests were performed, which served as evidence for the direct coupling of Co IV -oxo species during the RDS. 14 2.1.2. Hematite.…”

Key activity descriptors of nickel-iron oxygen evolution electrocatalysts in the presence of alkali metal cations

“…This is explained by an increase in the electrolyte pH in the presence of larger cations due to the higher basicity of the corresponding alkali hydroxide. In recent findings by Pasquini et al 63 the catalytic rate of a CoO x catalyst was found to be controlled by the levels of oxidized Co 4+ at any defined OER overpotential. A similar scenario could prevail in our Ni-Fe catalyst, although, we do not find as strong correlation with the population of oxidized Ni (3+δ)+ during steady-state conditions and the OER activity.…”

CryoEXAFS: X-ray absorption spectroscopy station with cryogenic or in-beam operando electrochemistry sample conditions at BESSY II

“…• In-situ characterization of materials under catalysis conditions with low-temperature data collection (freeze-quench approach, Klingan et al, 2018;Pasquini et al, 2019, Bergmann et al, 2020Smith et al, 2017) • Operando XAS on films for water oxidation reaction (OER) under electrochemistry conditions at room temperature (Smith et al, 2017;González-Flores et al, 2018) • XAS on dilute-solution (1 mM) protein samples at cryogenic conditions (Reschke et al, 2019) • Rapid-scan EXAFS (redox transitions and catalytic processes during cyclic voltammetry) • EXAFS as a function of chemical composition and/or temperature dependent EXAFS (Kesavan et al, 2020) 3 Technical data (a) XAS experiments at cryogenic (cryostat) or operando (electrochemistry) conditions can be performed in parallel using two energy-resolving detectors. (b) Rapid data readout/transfer (e.g.…”