A Viable Hydrogen Storage and Release System Based on Cesium Formate and Bicarbonate Salts: Mechanistic Insights into the Hydrogen Release Step

Abstract

Abstract Aqueous solutions of cesium formate and bicarbonate represent an effective hydrogen storage–delivery couple that undergoes either release or take up of hydrogen in the presence of {RuCl 2 ( m TPPTS) 2 } 2 (TPPTS=triphenylphosphine trisulfonate) and excess m TPPTS ligand, with no other additives required. Cesium salt solutions offer the advantage of improved volumetric and gravimetric H 2 density compared to their sodium and potassium analogs, owing to their high water solubility. Details of the equilibrium between formate and bicarbonate, which constitutes an important parameter for the applicability of this H 2 storage/release cycle, were determined. H 2 production is readily tunable by controlling the operating pressure. This behavior was also rationalized through the identification of catalytic intermediates under various conditions. High concentration formate and bicarbonate solutions were used during the tests and the bidirectional catalytic system could be recycled without loss of activity or replacement of solvent. A tentative mechanism is proposed for the formate dehydrogenation step. Among the identified hydride species, the pentacoordinated [RuH(H 2 O)(TPPTS) 3 ] complex was indispensable for promoting the formate dehydrogenation reaction.