418824 Thermodynamic and Achievable Efficiencies of Solar-Driven Electrochemical Conversion of Water and Carbon Dioxide to Transportation Fuels

Monday, November 9, 2015: 3:15 PM
355D (Salt Palace Convention Center)
Meenesh R. Singh, Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, CA and Alexis T. Bell, Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Lab, Berkeley, CA

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.

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