Hierarchical nanostructures: Design for sustainable water splitting

Clean and sustainable hydrogen generation renders a magnificent prospect to fulfill the humans’ dream of rebuilding energy supplying systems that work eternally and run without pollution. Water electrolysis driven by a renewable resource of energy, such as wind and solar, is a promising pathway to achieve this goal, which requires highly active and cost-effective electrode materials to be developed. In this comprehensive review, we introduce the utilization of hierarchical nanostructures in electrocatalytic and photoelectrochemical applications. The unique emphasis is given on the synthetic strategies of attaining these hierarchical structures as well as to demonstrate their corresponding mechanisms for performance improvement. Rather than simply discussing all the methods that can be used in nanofabrication, we focus on extracting the rules for structural design based on highly accessible and reliable methods. Examples are given to illustrate the versatility of these methods in the synthesis and manipulation of hierarchical nanostructures, which are concentrated on nonprecious transition metals or their alloys/ compounds. Through this study, we aim to establish valuable guidelines and provide further insights for researchers to facilitate their design of more efficient water splitting systems in the future.