Defect engineering in metal-organic frameworks for enhanced hydrogen storage: Unlocking hidden active sites – A review

While hydrogen is a promising clean energy carrier to achieve carbon neutrality, its widespread adoption has been hindered by the absence of efficient, safe, and scalable hydrogen storage solutions. Metal–organic frameworks (MOFs) have emerged as a new class of materials with tunable pore structures, high surface areas, and versatility in structure that recently have been explored for hydrogen storage. The performance of pristine MOFs is, however, limited by weak hydrogen–framework interactions and lack of active adsorption sites. To solve such problems, defect engineering has emerged as a promising approach by building engineered defects such as open metal sites, hierarchical porosity, hidden active centers, and enhanced transport pathways. This review discusses defect engineering strategies to improve hydrogen storage in MOFs, including hydrogen adsorption mechanisms, synthesis methods, defect chemistry, active-site generation, and defect-induced transport occurrences. The importance of integrating experimental characterization and computational modelling for understanding defect behaviour is emphasized. Recent advances in materials informatics, machine learning, and AI-assisted discovery of defect-engineered MOFs are also discussed. Lastly, the main difficulties in defect control, stability, scaling up, and predictive design are presented, along with future prospects for the development of next-generation MOFs for sustainable hydrogen-energy applications.