Space-Confined Electrocatalysis for Water Splitting: Synthesis, Performances, and Reaction Mechanism of Nanomaterials Toward High-Performance Catalysis.

Water splitting represents a sustainable and environmentally benign approach for green hydrogen generation and future clean energy solutions. In this context, space-confined synthesis has emerged as a powerful strategy for engineering high-performance electrocatalysts. Recent studies have demonstrated that low-dimensional nanomaterials synthesized via confinement techniques exhibit enhanced electrocatalytic properties. The confined microenvironment imparts superior electrical conductivity, structural stability, and active site accessibility, while also facilitating elevated catalytic activity and offering potential for scalable production in practical energy applications. In this review, we first present mechanistic insights into nanoconfinement-enhanced water splitting electrocatalysis and characterization techniques including in-situ/operando analysis for confined electrocatalysts, emphasizing how the confined architectures from one to three dimensions (1D-3D) regulate electronic structures, facilitate reactant adsorption, and reduce energy barriers. We then outline nanoscale confinement strategies, including in-situ and postsynthetic approaches using diverse host materials such as carbon nanotubes (CNTs), metal-organic frameworks (MOFs), and MXenes, along with advanced methods for controlling particle dispersion and size. Next, we summarize recent progress in confined electrocatalysts for water splitting, highlighting density functional theory (DFT)-guided design and structure-property relationships. Finally, we address current challenges and future opportunities in synthesis control, in-situ characterization, and scalable deployment for practical hydrogen production.