Sustainable electrochemical reduction of CO2 into chemical products, in particular hydrocarbons and oxygenates, could provide an alternative to mankind’s current use of fossil fuels. A complete solar-driven system based on photovoltaic cells coupled to a CO2 electrolysis cell will be discussed. Optimizing the efficiency of CO2 electrolysis requires minimizing potential losses in all aspects of the device including the cathode, anode, electrolyte, and membrane. Selective production of hydrocarbon and oxygenate products requires management of multi-electron transfer reactions. Finally, the efficiency under illumination critically depends on the coupling of the PV and electrolysis elements.
Optimization of the components, as well as the overall design, of an aqueous phase CO2 electrolyzer cell enables an energy conversion efficiency for hydrocarbons and oxygenates in the 20-30% range to be achieved (overall efficiency to all products can be >50%). A nanostructured IrOx anode was synthesized by electrodepositing IrO2 nanoparticles on ZnO nanorods template and a nanotube IrOx anode was obtained by removing ZnO template. IrOx nanotube anode is employed here to drive the oxygen evolution reaction in the neutral pH range of CO2-saturated bicarbonate buffer (6.8 – 7.4). The IrOx anode has only a 300 mV overpotential at 10 mA cm-2 and can be used for months. High rate electrodeposition of Cu on Ag is used to product a dendritic bimetallic “nanocoral” cathode which maintains high selectivity to hydrocarbons and oxygenates (~60%) and has multi-day stability. Importantly, the high selectivity to hydrocarbons and oxygenates is retained at high concentrations of bicarbonate electrolyte, which enables ohmic losses in the overall cell to be decreased.
Use of power matching electronics enables coupling to series-connected solar cells and to high efficiency 4-terminal tandem cells, and overall solar to hydrocarbon and oxygenate efficiencies of 3-4% (0.3-1 sun illumination) and >5% (1 sun) are achieved, respectively . Notably, these values exceed that of natural photosynthesis (<1%) and approach those of devices, which make simpler and potentially less valuable C1 products such as CO and formic acid. Prospects for further improvements through the use of cathodes with lower overpotentials and by integrating more efficient solar cells will be discussed.
This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.
Gurudayal et al., Energy. Env. Sci. 2017 10, 222-2230.