Progress of ultrafine noble metal nanocatalysts regulated by confining engineering for water electrolysis

Water electrolysis has been considered a promising technology for producing sustainable green hydrogen fuel energy. Noble metal nanocatalysts have exhibited widely recognized activities for electrocatalysis. However, the high cost of producing hydrogen by noble metal limits the widespread adoption of this fuel. The key to reducing the usage lies in reducing the size of noble metals to the atomic scale to offer rich active sites and unique electronic structures to enhance their catalytic activity. Generally, the charge transfer between the host support and guest materials by confining engineering can improve the catalytic activity, as well as overcome the aggregation and/or structural degradation of guests. In contrast to previous reviews involving electrochemistry under confinement, or confined nonprecious metal-based catalysts, our review offers a scientific analysis of the structure-activity relationship in the development of platinum group metals via confinement engineering for water splitting. Our discussion covers diverse confinement strategies including space confinement, space-electrostatic confinement, and space-coordination confinement with a special focus on the advances in the coordination-confinement (pyrolysis) strategy for ultrafine noble metal nanostructures. Finally, we put forward future development directions including hydrogen spillover path, in situ characterization, and electrical double layer atomic/molecule structure analysis in confined metal-based electrocatalysts for water electrolysis.