Next-Generation Frameworks for Hydrogen Production: A Comparative Review of MOFs, COFs, and HOFs.

Hydrogen is a key energy vector in the shift toward carbon-neutral energy systems, such as the global hydrogen economy. Of the possible production pathways, catalytic water splitting (photocatalytic, electrocatalytic, and thermos-catalytic) has drawn particular attention for its potential in producing green hydrogen. Recently, framework materials such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and hydrogen-bonded organic frameworks (HOFs) have shown enormous potential as precisely tunable catalytic platforms due to their large surface areas, structural flexibility, and tunable electronic properties. Here, we review MOFs, COFs and HOFs for hydrogen production applications, with a focus on structure-property-performance relationships. MOFs provide a range of metal active sites and porosity, but have low conductivity and low stability in water. COFs, thanks to their extended π-conjugation and strong covalent bonds, have better charge mobility and visible-light absorption, making them promising photocatalysts. While less studied, HOFs offer benefits in terms of their easy preparation and reversible structure but suffer from weak intermolecular forces. We explore the advances in band gap engineering, heterojunction formation, cocatalysts, and defect engineering, in terms of key performance indicators of catalytic activity, including turnover frequency (TOF), quantum efficiency, and overpotential. Finally, key issues such as stability, scalability, and standardization, and emerging trends, including machine learning-aided discovery of materials and integrated solar hydrogen systems, are assessed. Of the three types, COFs and MOF-derived catalysts hold the most promise for scalable green hydrogen generation.