Electrocatalytic CO2 Reduction on Nanosized Ag Dendrites Electrodeposited on Indium Doped SnO2 Electrodes Immersed in Aprotic Medium

Abstract

Electrochemical reduction of CO2 by means of metallic nanostructures (i.e. Ag, Au, Cu), sp2 carbon derivatives (i.e. graphite oxide, graphene) and their combinations is considered as a promising pathway for aiding to decrease the effects of the global warming. However, the electrochemical reduction of CO2 contained in aqueous medium requires large overpotentials with a significant competition with the H2 evolution reaction, thus resulting in low faradaic efficiencies. On the other hand, reactivity of metallic nanostructures is strongly dependent from the size and the aspect ratio in nanoparticles and nanorods, respectively, as well as from the crystalline faces exposed to the electrolytic medium. In order to overcome this trouble, we studied the electrocatalytic CO2 electroreduction in aprotic medium by employing nanosized Ag dendrites, which were previously electrodeposited on In-doped SnO2 (ITO) electrodes by means of a double-pulse potential (DPP) technique (deposition conditions were defined by means of cyclic voltammetry (CV) technique). These experiments were carried out in a three-electrode cell containing aqueous 0.1M KNO3+1mM AgNO3. In this sense, the presence of dendritic nanostructures on the silver-modified ITO electrodes (ITO/Ag) was confirmed by scanning electron microscopy. CV results showed that the electrical charge associated to the Ag dissolution is clearly dependent from the growth time (duration ~ms) and independent from the nucleation time (duration ~sec). This result demonstrated that the Ag deposition is kinetically-controlled by one-electron transfer. Furthermore, CV results demonstrated that the thermodynamic potential for the Ag reduction E°Ag+|Ag was observed at -0.32 vs. Ag|AgCl (3M NaCl) during the first scan but, it becomes less cathodic at E°Ag+|Ag= -0.08 vs. Ag|AgCl (3M NaCl) during the successive scans; thus confirming that the dendritic shape of the Ag deposits is promoted immediately after some seeds of metallic atoms are confined on the ITO surface. Complementarily, electrochemical CO2 reduction in a three-electrode cell containing propylene carbonate + 0.1M Bu4NPF6 was confirmed on the as-prepared ITO/Ag electrodes by means of linear voltammetry (LV). In this way, two reduction peaks were localized at -0.3 and -0.5V vs. Ag|AgCl (0.01M AgNO3 + 0.1M Bu4NCl in MeCN), which can be associated to the semireactions 1 and 3, respectively. Electrochemical contribution from the ITO substrate was absent during the CO2 reduction reaction. (1) CO2 + e- → CO2 · - (slow)(2) CO2 + CO2 · - → (CO2)2 · - (3) (CO2)2 · - + e- → -OOC-COO· -