Desenvolvimento de eletrodos de difusão gasosa baseado em nanofolhas de SnO2 com desempenho melhorado na redução de CO2 em HCOOH

Abstract

The concentration of gaseous pollutants like CO₂ has significantly increased due to anthropogenic activities. It is urgent to develop effective methods, such as the electrochemical route, to mitigate environmental damage caused by the inadequate disposal of CO2. However, challenges persist, such as the low activity, selectivity, and stability of electrocatalysts, as well as the low solubility of CO₂ in water, resulting in current densities below -30 mA·cm⁻². In this regard, the use of gas diffusion electrodes in a flow electrochemical cell eliminates the need to dissolve CO₂ in an aqueous electrolyte and enables achieving current densities above -200 mA·cm⁻². Therefore, the present study investigated the effect of different synthesis routes of SnO₂ on its physicochemical properties (structure, morphology, and oxidation state) and their electrochemical activity in the reduction of CO₂ to HCOOH. The materials were synthesized by hydrothermal method at different pHs, and the gas diffusion electrodes were prepared by spray-coating. Characterization techniques demonstrated that the SnO₂-NS and SnO₂-NP samples presented a tetragonal crystalline structure in the rutile phase of SnO₂, where the sample synthesized at alkaline pH showed a higher degree of crystallinity and nanosheet morphology (SnO₂-NS), while the sample synthesized at neutral pH showed a nanosphere morphology (SnO₂-NP). The samples were subjected to preliminary photocatalysis tests in the ultraviolet region to evaluate their activity, as CO₂ reduction reactions are more complex. The synthesized materials showed excellent performance in the reduction of CO₂ to HCOOH using a flow electrochemical cell, with the SnO₂-NS sample presenting a faradaic efficiency (~90%) superior to the SnO₂-NP sample. The SnO₂-NS sample, when applied under optimized conditions, presented a faradaic efficiency of 98% with a current density greater than -220 mA·cm⁻². The improved catalytic performance of the SnO₂-NS sample is likely related to its two-dimensional morphology and the presence of tin in two oxidation states, Sn²⁺ and Sn⁴⁺, which resulted in a decrease in the charge transfer resistance of the electrode. Thus, it can be observed that the morphology of SnO₂ affects its performance in the electrochemical reduction of CO₂, with the SnO₂ nanosheet morphology being the sample with the best performance for this application.