Continuous conversion of CO2 to alcohols in a TiO2 photoanode-driven photoelectrochemical system

Abstract

BACKGROUND The recycling of CO2 by photo-electrochemical reduction has attracted wide interest due to its potential benefits when compared to electro-, and photo-catalysis approaches. Among the different available semiconductors, TiO2 is the most employed material in photo-electrochemical cells. Besides, Cu is a well-known electrocatalyst for alcohols production from CO2 reduction. RESULTS In this study, a photo-electrochemical cell consisting on a TiO2 photoanode Membrane Electrode Assembly (MEA) and a Cu plate are employed to reduce CO2 to methanol and ethanol continuously under UV illumination (100 mW·cm-2). A maximum increment of 4.3 mA·cm-2 in current between the illuminated and dark conditions is achieved at -2 V vs. Ag/AgCl. The continuous photo-electrochemical reduction process in the filter-press cell is evaluated in terms of reaction rate (r), as well as Faradaic (FE) and energy (EE) efficiencies. At -1.8 V vs. Ag/AgCl, a maximum reaction rate of r=-9.5 -mol·m-2·s-1, FE=-16.2 % and EE=-5.2 % for methanol, and r=-6.8 -mol·m-2·s-1, FE=-23.2 % and EE=-6.8 % for ethanol can be achieved. CONCLUSIONS The potential benefits of the photoanode-driven system, in terms of yields and efficiencies, are observed when employing a TiO2-based MEA photoanode and Cu as dark cathode. The results demonstrate first the effect of UV illumination on current density, and then the formation of alcohols from the continuous photoreduction of CO2. Increasing the external applied voltage led to an enhanced production of methanol, but decreases ethanol formation. The system outperforms previous photoanode-based systems for the CO2-to-alcohols reactions.