Electrocatalytic reduction of CO 2 to produce higher alcohols† Shamsa Munir, abc Amir Rahimi Varzeghani ab and Sarp Kaya * abd Electrodeposited and thermally oxidized copper surfaces have been documented in recent years to produce simple alcohols. In this work, we endeavored to study the electrochemical reduction of CO 2 at different electrodes prepared via the electrodeposition method, namely, Cu–Cu 2 O, Cu–Cu 2 O–ZnO, and Cu–ZnO. In addition, thermally oxidized Cu (Cu-TO) was also investigated. C1, C2, and C3 species were produced on Cu–Cu 2 O–ZnO, Cu–Cu 2 O, and Cu–ZnO. The highest faradaic efficiency (FE of 97.4%) of the liquid products (methanol, formate, n-propanol, acetone) was evidenced on Cu–ZnO. The formation of C3 species with high FE on the Cu–ZnO electrode is attributed to the fast C–C–C coupling at the Cu–Zn interface. On thermally oxidized Cu, the total FE of the liquid products (methanol, formate, ethanol, acetate, n-propanol) was found to be 58.51%, which is considerably closer to the already reported values for these electrodes. Moreover, the Cu–Cu 2 O–ZnO electrode revealed selectivity toward methanol production. Detailed morphological and elemental analyses of the electrode, performed using XPS, Raman spectroscopy, and FESEM, as well as activity measurements to obtain an insight into the mechanistic pathways, reveal that C–C coupling is favored on Cu 0 sites rather than Cu 2 O. Moreover, methanol formation seems to proceed via O coordination of CO 2 to Cu–Cu 2 O surface having (100) facets, whereas C coordination is favored on Cu-TO with (111) exposed faces, resulting in Cu 0 sites. The localized formation of ZnO nanoflowers was observed on Cu–ZnO electrodes after the electrochemical reduction of CO 2 , which is attributed to the mechanistic pathway involving chemical steps, leading to the formation of C3 species. 1. Introduction The electrochemical conversion of CO 2 to liquid fuels is one of the most promising incentives to utilize atmospheric CO 2 whose concentration is predicted to increase to 570 ppm by the end of the century. 1 CO 2 reduction chemistry is fairly rich and various gas- and liquid-phase products could be obtained. For instance, the production of formic acid with high faradaic efficiencies (FEs) using Cu- or Cu-oxide-based electrodes has been reported. 2–4 Alternative systems such as Pd-multiwalled carbon nanotubes 5 have also been utilized for the electro- catalytic conversion of CO 2 to produce formic and acetic acids with FEs of 34.5 and 52.3%, respectively. However, the conver- sion of CO 2 to alcohols is of special interest. The production of methanol, from otherwise wasted CO 2 , has considerable signicance since a large quantity of this chemical is manu- factured worldwide. 6 In addition, the high operating temperature and pressure required for the synthesis procedures demand large energy investments. 7 Therefore, mild and envi- ronmentally friendly methods of CO 2 conversion to methanol have become more signicant. Similarly, the selective produc- tion of ethanol using CO 2 reduction is one of the bigger incentives to develop efficient and cost-effective electrode materials for this reaction. Methanol production using oxidized Cu is reported in one of the earlier studies by Frese et al.; 240% FE for methanol using 0.5 M KHCO 3 at 1.9 V (SCE) has been evidenced. 8 However, the onset potential of methanol formation was around 0.4 V (SCE). 8 Since FEs are determined based on six electron transfers for methanol, values greater than 100% show the involvement of both electrochemical and chemical steps in the reaction mechanism. 8,9 Since then, several studies have documented the formation of methanol with appreciable amounts using Cu-oxide catalysts. 10 The property of copper oxide surfaces to produce liquid fuels have been investigated using core–shell nanoparticles 11 and composites materials. 12–14 Methanol has been reported with 42.7% FE on Cu/CuO core– shell catalysts. 15 The selective production of methanol with a 95% FE is documented by using hybrid CuO/Cu 2 O semi- conductor nanorod arrays 6,16 in a photoelectrochemical setup. The photoelectrocatalytic efficiency is determined by the a Material Science and Engineering, Koç University, Istanbul, Turkey. E-mail: sarpkaya@ku.edu.tr b Koç University T ¨ UPRAS¸ EnergyCenter, Istanbul, Turkey c Chemistry Department, Women University Swabi, Swabi, Pakistan d Chemistry Department, Koç University, Istanbul, Turkey † Electronic supplementary information (ESI) available. See DOI: 10.1039/c8se00258d Cite this: DOI: 10.1039/c8se00258d Received 3rd June 2018 Accepted 19th August 2018 DOI: 10.1039/c8se00258d rsc.li/sustainable-energy This journal is © The Royal Society of Chemistry 2018 Sustainable Energy Fuels Sustainable Energy & Fuels PAPER Published on 22 August 2018. Downloaded on 9/20/2018 12:41:45 PM. View Article Online View Journal