Tuning Hole Accumulation of Metal Oxides Promotes the Oxygen Evolution Rate

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Non-covalent ligand-oxide interaction promotes oxygen evolution

Strategies to generate high-valence metal species capable of oxidizing water often employ composition and coordination tuning of oxide-based catalysts, where strong covalent interactions with metal sites are crucial. However, it remains unexplored whether a relatively weak "non-bonding" interaction between ligands and oxides can mediate the electronic states of metal sites in oxides. Here we present an unusual non-covalent phenanthroline-CoO₂ interaction that substantially elevates the population of Co⁴⁺ sites for improved water oxidation. We find that phenanthroline only coordinates with Co²⁺ forming soluble Co(phenanthroline)₂(OH)₂ complex in alkaline electrolytes, which can be deposited as amorphous CoO_xH_y film containing non-bonding phenanthroline upon oxidation of Co²⁺ to Co^3+/4+. This in situ deposited catalyst demonstrates a low overpotential of 216 mV at 10 mA cm^-2 and sustainable activity over 1600 h with Faradaic efficiency above 97%. Density functional theory calculations reveal that the presence of phenanthroline can stabilize CoO₂ through the non-covalent interaction and generate polaron-like electronic states at the Co-Co center.

IrOx @In2 O3 Heterojunction from Individually Crystallized Oxides for Weak-Light-Promoted Electrocatalytic Water Oxidation

Multi-field coupling, especially photo-assisted electrocatalysis, has recently been studied to further improve the oxygen evolution reaction (OER). In this study, an n-type cubic In₂ O₃ semiconductor is employed for the first time to load IrO_x species (Ir-In₂ O₃ mass ratio: 17.6 %). Consequently, the IrO_x @In₂ O₃ heterojunction, which exhibits outstanding OER performance promoted by weak-light irradiation, is formed. Notably, IrO_x (approximately 1.7 nm in size) and In₂ O₃ are observed to crystallize independently during heterogeneous nucleation with no Ir atoms doped in the In₂ O₃ lattice. This avoids Ir loss and ensures the full exposure of all Ir-based sites. The IrO_x @In₂ O₃ heterojunction exhibits enhanced electrocatalytic water oxidation with overpotential values of 190 and 231 mV at current densities of 10 and 50 mA cm^-2 , surpassing all IrO_x -based catalyst results reported to date. Nano-sized IrO_x on the surface, irradiated by the weak-light beam of LED-365 (1.8 mW cm^-2 ), can be fully activated as an OER site. Moreover, the overpotential is further reduced to 176 and 210 mV to deliver the corresponding current. This work is anticipated to aid in the design of more efficient multi-field coupling OER systems.