materials Article Zn- and Ti-Doped SnO 2 for Enhanced Electroreduction of Carbon Dioxide Katarzyna Bejtka 1, * ,† , Nicolò B. D. Monti 1,2,† , Adriano Sacco 1 , Micaela Castellino 2 , Samuele Porro 2 , M. Amin Farkhondehfal 1 , Juqin Zeng 1 , Candido F. Pirri 1,2 and Angelica Chiodoni 1   Citation: Bejtka, K.; Monti, N.B.D.; Sacco, A.; Castellino, M.; Porro, S.; Farkhondehfal, M.A.; Zeng, J.; Pirri, C.F.; Chiodoni, A. Zn- and Ti-Doped SnO 2 for Enhanced Electroreduction of Carbon Dioxide. Materials 2021, 14, 2354. https://doi.org/10.3390/ ma14092354 Academic Editor: Benjamin Solsona Received: 30 March 2021 Accepted: 27 April 2021 Published: 1 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Center for Sustainable Future Technologies @POLITO, Istituto Italiano di Tecnologia, Via Livorno 60, 10144 Turin, Italy; nicolo.monti@iit.it (N.B.D.M.); adriano.sacco@iit.it (A.S.); Amin.Farkhondehfal@iit.it (M.A.F.); juqin.zeng@iit.it (J.Z.); fabrizio.pirri@iit.it (C.F.P.); angelica.chiodoni@iit.it (A.C.) 2 Department of Applied Science and Technology, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Turin, Italy; micaela.castellino@polito.it (M.C.); samuele.porro@polito.it (S.P.) * Correspondence: katarzyna.bejtka@iit.it † These authors contributed equally. Abstract: The electrocatalytic reduction of CO 2 into useful fuels, exploiting rationally designed, inexpensive, active, and selective catalysts, produced through easy, quick, and scalable routes, represents a promising approach to face today’s climate challenges and energy crisis. This work presents a facile strategy for the preparation of doped SnO 2 as an efficient electrocatalyst for the CO 2 reduction reaction to formic acid and carbon monoxide. Zn or Ti doping was introduced into a mesoporous SnO 2 matrix via wet impregnation and atomic layer deposition. It was found that doping of SnO 2 generates an increased amount of oxygen vacancies, which are believed to contribute to the CO 2 conversion efficiency, and among others, Zn wet impregnation resulted the most efficient process, as confirmed by X-ray photoelectron spectroscopy analysis. Electrochemical characterization and active surface area evaluation show an increase of availability of surface active sites. In particular, the introduction of Zn elemental doping results in enhanced performance for formic acid formation, in comparison to un-doped SnO 2 and other doped SnO 2 catalysts. At −0.99 V versus reversible hydrogen electrode, the total faradaic efficiency for CO 2 conversion reaches 80%, while the partial current density is 10.3 mA cm −2 . These represent a 10% and a threefold increases for faradaic efficiency and current density, respectively, with respect to the reference un-doped sample. The enhancement of these characteristics relates to the improved charge transfer and conductivity with respect to bare SnO 2 . Keywords: electrochemical CO 2 reduction; doped SnO 2 catalyst; mesoporous; oxygen vacancy; HCOOH production 1. Introduction Climate change causes are attributed by scientific community to the increased produc- tion of carbon dioxide (CO 2 ) by anthropogenic activity. CO 2 is in fact one of the principal greenhouse gases which are contributing to global warming, and in order to minimize the continuous growth of its atmospheric concentration, it can be used as raw material to obtain products with high energy value. This can be achieved via various processes including electrochemical CO 2 reduction with the use of proper catalysts. Among many products that can be obtained (depending on the catalyst characteristics, the reaction conditions and the electrolyte used), the CO 2 reduction reaction (CO 2 RR) to carbon monoxide (CO) or formic acid (HCOOH) is up to now the economically most viable process that can challenge conventional production routes [1]. In particular, formic acid with its low toxicity, availability, and convenient handling results in a great range of applications in agriculture and chemical industry fields [2]. Moreover, it is considered as Materials 2021, 14, 2354. https://doi.org/10.3390/ma14092354 https://www.mdpi.com/journal/materials