Homogeneous Water Oxidation Catalyzed by First‐Row Transition Metal Complexes: Unveiling the Relationship between Turnover Frequency and Reaction Overpotential

Abstract

The utilization of earth‐abundant low‐toxicity metal ions in the construction of highly active and efficient molecular catalysts promoting the water oxidation reaction is important for developing a sustainable artificial energy cycle. However, the kinetic and thermodynamic properties of the currently available molecular water oxidation catalysts (MWOCs) have not been comprehensively investigated. This Review summarizes the current status of MWOCs based on first‐row transition metals in terms of their turnover frequency (TOF, a kinetic property) and overpotential (η, a thermodynamic property) and uses the relationship between log(TOF) and η to assess catalytic performance. Furthermore, the effects of the same ligand classes on these MWOCs are discussed in terms of TOF and η, and vice versa. The collective analysis of these relationships provides a metric for the direct comparison of catalyst systems and identifying factors crucial for catalyst design.

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

“…In the following section, we present a short survey of progress on molecular OER/ORR catalysts to complement the preceding sections with selected examples. For more detailed discussions on the topic, we kindly refer readers to recent representative reviews. ,,,, …”

“…Inspired by nature’s OEC, the development of molecular water oxidation catalysts (WOCs) has witnessed considerable progress over the past two decades. , In recent years, design and development paradigms for molecular WOCs have shifted from noble transition metals toward earth-abundant third-row late TMs, although the earlier ones are still being investigated for their mechanisms that can provide guidelines for designing the latter. ,, In addition to mechanistic studies focusing on the trapping of intermediates via freeze-quenching, quasi- in situ methods have proven essential for the understanding of molecular WOCs and intermediates under real-time operational conditions. This creates the need to develop appropriate in situ / operando or coupled spectroscopic techniques to determine the active site mechanisms.…”

“…This creates the need to develop appropriate in situ / operando or coupled spectroscopic techniques to determine the active site mechanisms. In the following, we present recent examples of in situ mechanistic studies on molecular OER catalysts, which is still an area of emerging research (for an overview of earlier years, we kindly refer readers to previous reviews ,, ).…”

“…17−19 However, the structural integrity of homogeneous molecular catalysts generally faces dramatic challenges under harsh reaction conditions. 14,20,21 As for heterogeneous catalysts, their high stability and facile recyclability render them more feasible for many practical applications. 16,22 OER/ORR processes are generally assumed to occur mainly on the surface of heterogeneous compounds due to their widely accessible and exposed catalytically active centers.…”

“…To date, two main categories of catalysts are being developed for OER/ORR, namely, homogeneous molecular and heterogeneous catalysts. Despite the fundamental differences in their synthetic design and operational conditions, both categories have emerged as promising candidates for OER/ORR applications. − Homogeneous molecular catalysts are considered versatile candidates for OER/ORR, as they allow for the tuning of mono- and multimetal sites via ligand design and related coordination sphere interactions. − Additionally, homogeneous molecular catalysts offer a unique opportunity to understand detailed synergistic mechanisms and turnover pathways via an adjustable molecular spectroscopic toolbox. − However, the structural integrity of homogeneous molecular catalysts generally faces dramatic challenges under harsh reaction conditions. ,, As for heterogeneous catalysts, their high stability and facile recyclability render them more feasible for many practical applications. , …”