(202a) Thermodynamic and Achievable Efficiencies of Solar-Driven Electrochemical Conversion of Water and Carbon Dioxide to Transportation Fuels

Thermodynamic and achievable limits of solar-driven electrochemical conversion of water and carbon dioxide to fuels will be presented in this talk. The maximum thermodynamic efficiency of adiabatic electrochemical synthesis of various solar-fuels is in the range of 32-42%. The single, double, and triple junction light absorbers are found to be optimal for the electrochemical load ranges of 0-0.9 V, 0.9-1.95 V, and 1.95-3.5 V, respectively. The attainable solar-to-fuel (STF) efficiencies are obtained using Shockley-Queisser limits of multijunction light absorber and the electrochemical load curves for three different configurations such as photoelectrochemical cells (PECs), tandem photoelectrochemical cells, and PV-electrolyzers. The maximum achievable STF efficiencies of PEC producing syngas (CO:H₂ = 1:3) and hythane (CH₄:H₂ = 2:3) are 25% and 18%, respectively. The quality of solar-fuels such as syngas (using silver) and hythane (using copper) can also be adjusted by tuning the band gaps of triple junction light absorbers. A scheme consisting of tandem double-junction PEC is proposed, where the first PEC produces H₂ and the second PEC utilizes H₂ and CO₂ to make fuels. The STF efficiency of such tandem PECs forming hythane can be 10% higher than the single triple-junction PEC. We also show that PV-electrolyzers can not only be efficient than the PECs but can also provide better control over the quality of solar fuels for the case of varying solar insolation.