Researchers in Michael Gratzel’s group at EPFL in Switzerland recently reported a solar-to-chemical conversion efficiency of 6.5% for the reduction of CO2 to carbon monoxide (CO):
This is the best reported solar-to-CO conversion efficiency for solar-driven CO2 reduction, and was achieved with three series connected perovskite photovoltaic (PV) cells. When the electrical output from the PV cells was coupled to an IrO2 anode (for the oxygen evolution reaction) and a roughened gold cathode (for CO2 reduction), the device achieved stable current densities with high selectivity towards CO for almost 20 hours. Thin film perosvkite PV cells have attracted a lot of attention in recent years due to the significant improvement in efficiency (up to 20% now), and they are a natural choice of PV material to couple with electrochemical reactions due to the ability of a single perovskite cell to produce photovoltages above 1.0 V.
(a) Schematic of the device combining photovoltaics with an electrochemical cell. (b) Generalized energy diagram for converting CO2 into CO with three perovskite solar cells. Image and caption from: http://www.nature.com/ncomms/2015/150611/ncomms8326/abs/ncomms8326.html