Development of efficient and economic water oxidation catalysts (WOCs) remains a crucial bottleneck on the way to artificial photosynthesis applications. Over the past few decades, WOC research has turned into a fascinating interdisciplinary field that ranges from bio-inspired molecular design over nanomaterials and thin films to solid materials tuning. Under the umbrella of WOC optimization, advanced in situ/operando analytical techniques are being developed as increasingly powerful tools to elucidate the controversial discussions about the molecular or nanoscale nature of many WOCs. More and more of these approaches also enable the monitoring of possible key intermediates as an essential prerequisite for proposing catalytic mechanisms. This review is organized in three main parts: first, recent highlights outline frontiers in WOC development, such as the benefits of connecting molecular WOCs with solids along with the introduction of molecular concepts into heterogeneous WOC research. Next, a brief overview of emerging in situ/operando approaches demonstrates new options for monitoring WOC transformations. Finally, selected monitoring studies over the entire WOC dimensionality spectrum illustrate interesting cases of catalytic border crossings as new input for WOC construction.
Single-atom catalysts with maximum metal utilization efficiency show great potential for sustainable catalytic applications and fundamental mechanistic studies. We here provide a convenient molecular tailoring strategy based on graphitic carbon nitride as support for the rational design of single-site and dual-site single-atom catalysts. Catalysts with single Fe sites exhibit impressive oxygen reduction reaction activity with a half-wave potential of 0.89 V vs. RHE. We find that the single Ni sites are favorable to promote the key structural reconstruction into bridging Ni-O-Fe bonds in dual-site NiFe SAC. Meanwhile, the newly formed Ni-O-Fe bonds create spin channels for electron transfer, resulting in a significant improvement of the oxygen evolution reaction activity with an overpotential of 270 mV at 10 mA cm−2. We further reveal that the water oxidation reaction follows a dual-site pathway through the deprotonation of *OH at both Ni and Fe sites, leading to the formation of bridging O2 atop the Ni-O-Fe sites.
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