Hydrogen Storage and Release via Carbon Dioxide Hydrogenation to Formate Salts under High-Pressure Conditions with Ir Complex and Subsequent Formic Acid Dehydrogenation

Abstract

To investigate the potential of formic acid (FA) as a hydrogen carrier, we examined hydrogen storage and production through formate salts generated via CO₂ reduction under supercritical fluid conditions. Formate salts were synthesized using Cp*Ir homogeneous catalysts to reduce CO₂ under supercritical conditions (CO₂: 12 MPa; H₂: 0.5 MPa; total 12.5 MPa), achieving turnover frequency (TOF) of 10,240 h⁻¹ and a turnover number (TON) of 20,480 within 2 h at 50 °C. The maximum formate concentration reached 0.81 mol/L after 18 h. The resulting formate salt solution (0.81 mol/L) was subsequently converted into FA (0.50 mol/L) with a 96 % yield by exchange of the cation (K + to H +) using an ion exchange resin. FA was then dehydrogenated to regenerate hydrogen, achieving a FA conversion exceeding 98 %. This process yielded an overall hydrogen recovery rate of over 90 %. These findings demonstrate a viable method for hydrogen storage and regeneration using formate salts as an efficient hydrogen carrier for future energy applications.