Reversible hydrogen storage using CO ₂ and a proton-switchable iridium catalyst in aqueous media under mild temperatures and pressures

Green plants convert CO ₂ to sugar for energy storage via photosynthesis. We report a novel catalyst that uses CO ₂ and hydrogen to store energy in formic acid. Using a homogeneous iridium catalyst with a proton-responsive ligand, we show the first reversible and recyclable hydrogen storage system that operates under mild conditions using CO ₂, formate and formic acid. This system is energy-efficient and green because it operates near ambient conditions, uses water as a solvent, produces high-pressure CO-free hydrogen, and uses pH to control hydrogen production or consumption. The extraordinary and switchable catalytic activity is attributed to the multifunctional ligand, which acts as a proton-relay and strong π-donor, and is rationalized by theoretical and experimental studies.