(Invited) Enable Efficient Solar Driven CO2 Reduction with Stable and Highly Active Oxygen Evolution Catalyst in Neutral pH

Abstract

Using electricity generated from solar cells, conversion of water and CO2 to syngas in an electrolytic cell mimics the photosynthesis process and provides a facile pathway to the production of various chemicals and liquid fuels.1 Development of new oxygen evolution reaction (OER) catalysts that are both active and stable in neutral pH is highly desired for such a process to avoid the complication of managing the pH gap between CO2 reduction on the cathode and water oxidation on the anode. Over the last several years, we have investigated various transition metal oxides as potential candidates for OER for artificial photosynthesis, including CoWO4, Li2CoGeO4 and various perovskites.2-4 It has been revealed that these catalysts were not stable in neutral pH and subjected to degradation either thermodynamically or kinetically.In this presentation, we report a new brownmillerite OER catalyst, Sr2GaCoO5 (SGC), with superior performance and durability. At pH 7.0, the catalyst has a specific activity about 7.8-18.3 times higher than that of IrO2 at the applied electrode potentials between 1.53-1.73 V vs. RHE. With a catalyst loading of 1 mg/cm2, the measured overpotential was only 0.38 V at a current density of 10 mA/cm2, which maintained unchanged over the testing period of 72 hrs. A comparative study against Sr2AlCoO5 indicates the excellent stability of SGC is related to the presence of gallium oxide in the crystal structure, which may help prevent SGC from dissolution under the OER operation condition in neutral pH, although detailed mechanistic understanding should be further developed by additional studies in the future. Combining this novel OER catalyst with an anodized silver cathode and a triple junction of GaInP/GaInAs/Ge photovoltaic, the CO2 reduction system demonstrated a solar-to-CO energy conversion efficiency of 13.9% over a period of 19 hrs with no appreciable performance degradation.5 This result has shown that artificial photosynthesis can indeed be more efficient than the natural photosynthesis process for CO2 reduction, and should play an important role in addressing the challenges of global warming and climate change.