Electroreduction of Carbon Dioxide to Formate by Homogeneous Ir Catalysts in Water

Electroreduction of CO2 to formate is one of the approaches for converting electrical energy into chemical energy and is expected to be a method for storing unevenly distributed renewable sources. However, a high performance of the electroreduction system including catalysts is indispensable for establishing the energy storage system. Furthermore, it is desirable to convert CO2/formate more efficiently than hydrogen production by water electrolysis. In this paper, we demonstrate the electroreduction systems of CO2 to formate by the systematic combination of iridium (Ir) catalysts and various electrodes in aqueous solutions without any organic additives. It was found that the combination of the Pt black electrode and the Ir catalyst having a picolinamide type ligand gave a faradaic efficiency of formate (95%) and current density (7.2 mA cm(-2)) with -0.49 V vs SHE at pH 8.3. Moreover, we demonstrated that electroreduction of CO2 proceeded efficiently even with a lower potential (-0.44 V vs SHE) than the theoretical potential of water electrolysis (pH 8.3). Electrochemical measurements, the electro-kinetic analysis, and H-1 NMR spectroscopy suggested that Ir hydride (Ir-H) as a key intermediate for electroreduction of CO2 was formed not through reaction with hydrogen produced by electrolysis but rather by two-electron reduction of [Cp*Ir-III(OH2)(L)](n+) (n = 1 or 2). The selective and efficient electroreduction system is attributed to easy generation and strong CO, reduction ability of the Ir-H intermediates in water.