Continuous Flow Photoelectrochemical Reactor for Solar Conversion of Carbon Dioxide to Alcohols

A newly-designed continuous flow photoelectrochemical reactor (CFPR) was applied to the solar photoelectrosynthesis of alcohols from carbon dioxide (CO2) dissolved in aqueous media. Hybrid p-type CuO/Cu2O semiconductor nanorod arrays, prepared by a three-step synthesis method (sol-gel synthesis, thermal anneal, and cathodic electrodeposition) were used as photocathodes. Scanning electron microscopy and powder X-ray diffraction were used to characterize these hybrid materials at various stages of the preparation protocol as well as before and after being subjected to photoelectrolysis for selected periods of time. The performance of the CFPR with the hybrid photocathode was analyzed in CO2-saturated aqueous bicarbonate solution at −0.3 V vs Ag/AgCl under simulated AM 1.5 solar irradiation for periods up to 5 h. The hybrid photocathode formulation facilitated efficient photoelectron injection to CO2 while its robustness and high photoelectrochemically active surface area enhanced the formation of alcohols. Gas chromatographic analyses of the photoelectrolysis solutions under a flow rate of 5 ml/h revealed ethanol to be the main product followed by isopropanol and methanol. The amounts of alcohol assayed indicated a faradaic efficiency ranging from 75 to 96%. The formation of C-C bonds in the products is a significant finding in this study and a mechanistic hypothesis is presented to account for products such as ethanol instead of methanol as is usually seen. Finally, the CFPR performance was numerically simulated to unravel the influence of flow rate and channel geometry on CO2 conversion and consumption rate.