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.
Metal complexes composed of redox-active pyridinediimine (PDI) ligands are capable of forming ligand-centered radicals. In this Forum article, we demonstrate that integration of these types of redox-active sites with bioinspired secondary coordination sphere motifs produce direduced complexes, where the reduction potential of the ligand-based redox sites is uncoupled from the secondary coordination sphere. The utility of such ligand design was explored by encapsulating redox-inactive Lewis acidic cations via installation of a pendant benzo-15-crown-5 in the secondary coordination sphere of a series of Fe(PDI) complexes. Fe(PDI)(CO) was shown to encapsulate the redox-inactive alkali ion, Na, causing only modest (31 mV) anodic shifts in the ligand-based redox-active sites. By uncoupling the Lewis acidic sites from the ligand-based redox sites, the pendant redox-inactive ion, Na, can entice the corresponding counterion, NO, for reduction to NO. The subsequent initial rate analysis reveals an acceleration in anion reduction, confirming this hypothesis.
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