The role of unsaturated coordinated sites of electrocatalysts in electrochemical energy conversion:A review

Abstract

Facing the exhausting of fossil fuel and the objective of carbon neutrality, electrocatalytic processes can convert natural wind, solar, or tidal energy into chemical energy through hydrogen evolution reaction (HER), oxygen evolution reaction (OER), CO₂ reduction reaction (CO₂RR), nitrogen reduction reaction (NRR), and other reactions. Clearly, efficient electrocatalysts are the key to achieving the green energy with a low carbon footprint and the high value-added products. A remarkable electrocatalyst should possess abundant surface active sites, superb intrinsic activity, effective carrier transport performance, and stable functionality in electrolytes with hash pH value for enhanced energy conversion efficiency. In this review, we summarized the routes of construction of efficient active sites at the surface of electrocatalysts in the reported literatures, and concluded the positive correlations between the intrinsic activity of active sites and the unsaturated coordinated electronic structure of them. Taking the electrochemical hydrolysis (HER/OER) as the examples, various reported electrocatalysts were listed to illustrate this kind of positive correlations based on the structure and mechanisms research during HER/OER processes in acidic or basic environments. The theoretical investigation results were intercompared to verify the positive relationship of the active sites and the unsaturated coordinated sites through whose interaction with hydrolysis intermediates, as well as the charge and mass transfer dynamics. Finally, we discussed some of the challenges that lie ahead in the development of efficient transition metal-based electrocatalysts and the related mechanistic studies. This work will provide some guidelines for the design and preparation of electrocatalysts with a high level of intrinsic activity.