Self-healing catalysis in water

Abstract

Principles for designing self-healing water-splitting catalysts are presented together with a formal kinetics model to account for the key chemical steps needed for self-healing. Self-healing may be realized if the catalysts are able to self-assemble at applied potentials less than that needed for catalyst turnover. Solution pH provides a convenient handle for controlling the potential of these two processes, as demonstrated for the cobalt phosphate (CoP i ) watersplitting catalyst. For Co 2+ ion that appears in solution due to leaching from the catalyst during turnover, a quantitative description for the kinetics of the redeposition of the ion during the self-healing process has been derived. The model reveals that OER activity of CoP i occurs with negligible film dissolution in neutral pH for typical cell geometries and buffer concentrations. solar energy | water splitting | renewable energy storage | self-healing catalysis | cobalt phosphate W ater is a desirable medium for storing solar energy. Chargeseparated states generated by solar capture in semiconductors may be transferred directly or indirectly to catalysts, which rearrange the chemical bonds of water to produce the high-energy products of oxygen and hydrogen (1, 2). Hydrogen may be used directly as a fuel (3, 4) or converted into a liquid fuel with its combination with carbon dioxide via inorganic (5) or hybrid biological-inorganic catalysts (6-8). In addition, renewable hydrogen may be used to derive other energy-intensive products (9) such as ammonia for fertilization (10). Although an atomically simple conversion, the bond rearrangement of two water molecules to hydrogen and oxygen is chemically complex. The reaction encompasses a four-electron process, and this multielectron inventory must be coupled intimately to protons (11) to avoid the high-energy intermediates summarized on the Frost diagram of water ( Fig. 1). Most water-splitting catalysis is performed in concentrated base (12). However, in contradistinction to the harsh conditions of basic water, the ease and simplicity of interfacing catalysts to semiconducting materials in fabricated devices such as buried junctions [e.g., the artificial leaf (13,14)] is facilitated when neutral or near-neutral water is used, which also engenders greater materials stability and lessened liability for the implementation of new technologies. For these reasons, earth-abundant materials for water splitting at neutral and near-neutral conditions is now generally being adopted as a preferred approach for achieving direct photoelectrochemical energy conversion processes (15). Nonetheless, the use of pure water is insufficient to meet future global energy needs. Growth in energy during this century is projected to be driven by the urgent needs of emerging and low-income countries that currently lack access to reliable energy (16). Achieving the decarbonization needed to address climate change in the developing world has provided an imperative for the development of a distributed renewable energy infrastructure...

Operando X‐Ray Absorption Spectroscopy Shows Iron Oxidation Is Concurrent with Oxygen Evolution in Cobalt–Iron (Oxy)hydroxide Electrocatalysts

“…[8] However in that case no concurrent oxidation of Fe was observed and bond length changes due to Co/Feoxidation during the Co 2+/3+ oxidation wave and during the OER were not separated. Since Co is the only element present, it must be part of the active site.I ft he rate-determining step (RDS) in oxygen evolution is OÀOb ond formation, as has been reported for CoO x H y catalysts in neutral pH, [29,41,42] the edge shift could be attributed to the accumulation of nominally Co 4+ intermediates prior to the RDS.I nc ontrast, when the OER is occurring on Co(Fe)O x H y ,F e 3+ is further oxidized while Co 3+ is not. [24] Balasubramanian et al reported Fe oxidation and FeÀOb ond shortening ex situ in anodically polarized Ni(Fe)O x H y films.…”

Operando X‐Ray Absorption Spectroscopy Shows Iron Oxidation Is Concurrent with Oxygen Evolution in Cobalt–Iron (Oxy)hydroxide Electrocatalysts

“…Since Co is the only element present, it must be part of the active site. If the rate‐determining step (RDS) in oxygen evolution is O−O bond formation, as has been reported for CoO x H y catalysts in neutral pH, the edge shift could be attributed to the accumulation of nominally Co 4+ intermediates prior to the RDS. In contrast, when the OER is occurring on Co(Fe)O x H y , Fe 3+ is further oxidized while Co 3+ is not.…”

The Oxidation of Peroxide by Disordered Metal Oxides: A Measurement of Thermodynamic Stability “By Proxy”

“…The redox potential (of formation) of a catalytic species is key to the viability of the self‐healing mechanism. That is, the material must have a high enough redox potential to perform the water oxidation reaction yet a low enough redox potential that the material can reform under the water oxidation conditions . In this description of a self‐healing mechanism, the metal oxide material acts as a transient, or metastable, species which is consumed in the water oxidation reaction (2 MnO x +4 H + ⇌O 2 +2 Mn 2+ +2 H 2 O) and reformed from the solution (Mn 2+ +2 H 2 O⇌MnO x +4 H + +2 e − ).…”