The hydrogen rainbow on the chemist's palette: emerging Earth-abundant Fe-based catalysts for low-carbon H2 production

The global transition toward sustainable energy systems requires efficient, low-carbon routes for hydrogen production. The sustainability of hydrogen as an energy carrier is intrinsically linked to its production methods, which are traditionally classified using color-coded terminology. Although hydrogen gas is naturally colorless, it is categorized into distinct “colors” based on its manufacturing pathways and associated environmental footprints. Iron-based catalysts are highly attractive candidates for multi-feedstock H2 production technologies due to their low cost, Earth abundance, and environmental compatibility. Furthermore, their ability to exhibit multiple oxidation states and diverse active phases makes them highly versatile systems and substitutes for less sustainable noble transition metals. This review comprehensively evaluates the application of prominent iron-based catalysts developed over the past decade across the entire spectrum of H2 generation technologies. It highlights key representative systems, focusing on high-oxidation-state metal oxides like benchmark hematite (Fe2O3) while also detailing advanced iron phosphides, carbides, and sulfides engineered to drive efficient water splitting. These catalysts serve as stable semiconductor materials for promoting photoelectrochemical, electrochemical, and thermochemical water splitting in green H2 production. They also play a crucial role in biomass pyrolysis and gasification by enhancing tar reforming and improving syngas quality to yield renewable bio-hydrogen. Finally, they are central to hydrocarbon reforming (including steam, dry, and partial oxidation methods) and methane pyrolysis/cracking, where they facilitate high-temperature reactions, mitigate carbon fouling and boost gray/blue and turquoise H2 production, respectively. In addition, iron-based oxygen carriers are known to promote coal gasification and chemical looping processes, enabling efficient fuel conversion that underpins traditional black and brown hydrogen production schemes. This review consolidates diverse chemical routes, underscoring the central role played by iron-derived catalysts in bridging the traditional and emerging hydrogen economies while advancing scalability, efficiency, and alignment with global clean energy goals.