Solid-state hydrides for CO₂ conversion: thermochemical and mechanochemical pathways

Solid-state hydrides combine hydrogen storage with the direct reduction of CO₂, which opens an alternative route for carbon capture and utilisation. This review examines the use of ionic, complex and transition-metal-based ternary hydrides as reactive media for the conversion of gaseous CO₂ and solid inorganic carbonates into methane, CH₄-H₂ fuel blends, oxygenated compounds and functional carbon materials. First, the thermodynamic basis of hydride-driven reduction is presented, together with the limits of equilibrium screening when kinetic factors control the reaction pathway. Then, thermochemical and mechanochemical activation routes are compared for each hydride class, distinguishing direct hydride transfer from in situ catalytic contributions, and the reduction of spent carbonate sorbents is addressed. Finally, the advantages and limitations of the different hydride classes are summarised, their performance is benchmarked against catalytic Sabatier methanation, and the main challenges related to hydride regeneration, energy efficiency and reactor design are identified.