Energy storage in chemical bonds and the concept of “solar fuels” is a realistic alternative for sustainable and carbon-neutral energy systems. Solar fuels mimic natural photosynthesis in using solar energy to convert readily available sources, water and carbon dioxide, into high energy fuels. Target reactions include water splitting into hydrogen and oxygen and reduction of carbon dioxide to carbon monoxide, other oxygenates, or hydrocarbons. Catalysts for water oxidation, efficient, stable, cheap first-row catalysts working at very low overpotentials are highly desired; cobalt-based catalysts represent an alternative. On the other side, important challenge in catalysts for reduction of CO2 is the suppression of the concomitant evolution of hydrogen, which is a dominant side reaction from aqueous electrolytes, and a relatively high kinetic preference toward reduction of CO2 compared to H+/H2O is required; cobalt macrocyclic complexes act as effective electrocatalysts for CO2 reduction, although the maximum reported Faradaic Efficiency to CO is 65%. In this scenario, a cobalt molecular catalyst immobilized onto carbon electrodes has been found to be a very selective electrocatalyst for CO2 reduction to CO (Faradaic Efficiency 93%) in a purely aqueous electrolyte solution, with an efficiency and selectivity that compares favorably to the best molecular-based electrocatalyst in the literature. The same complex is an efficient catalyst for water oxidation to oxygen under neutral conditions, with a 92% Faradaic Efficiency, lowering the onset potential in 150 mV compared to previous reported cobalt-molecular catalysts. By combining the two reactivities in a electrolysis cell in CO2 saturated NaHCO3 aqueous solutions pH 7, we demonstrate the overall splitting of CO2/H2O to CO/O2. The surface-functionalized assembly is sufficiently stable under reaction conditions during long-term controlled potential electrolysis. This single cobalt compound catalyzes the electrochemical reduction of CO2 to CO at the cathode and water oxidation to O2 at the anode. The resulting electrolysis cell is simple, cheap, robust and very efficient. We have performed a number of experiments to demonstrate the molecular species are indeed the active catalytic sites. This relevant finding suggests the use of this cobalt catalyst in conjunction with semiconductor for photoelectrochemical solar fuels production.

Single cobalt molecular catalyst for splitting CO2 and H2O into CO and O2

251st National Meeting (2016)

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