Eletrocatalisadores de In3+-Bi2O3 com aplicação na redução seletiva de CO2 à ácido fórmico

Abstract

Undoped and doped bismuth oxide (Bi2O3) samples with different concentrations of indium ions (In3+) were synthesized for application as electrocatalysts in carbon dioxide electrochemical reduction reactions into formic acid (HCOOH). The materials were prepared in two different synthesis time: 2 minutes (route I) and 8 minutes (route II), by microwave hydrothermal method (MH), which promotes the direct and rapid heating of reactant medium and control of morphology and particle size with energy economy. The results demonstrated that the incorporation of In3+ promotes a transition of crytal structure and morphology of Bi2O3 nanostructures. Structural characterizations via XRD and Rietveld refinements confirmed the obtention of a mixture of cubic (γ-Bi2O3) and monoclinic (α-Bi2O3) phases, indicating that the incorporation of In3+ into the lattice promotes the stabilization of monoclinic phase. Raman spectra confirmed the obtaining of both cubic and monoclinic phases for the pure and doped samples by the presence of its characteristic bands, corroborating the XRD results. SEM images showed that the pure samples of both routes exhibited irregular rod-shaped morphologies while In3+ doped Bi2O3 materials presented flower-like morphologies composed by two-dimensional nanosheets agglomerates on the surface of the rods. Surface area analyses showed that the observed modification in the morphology resulted in an increase in the porosity of the materials for doped samples obtained by route I. Electrocatalytic tests and electrochemical characterizations were carried out on samples synthesized by route I and demonstrated that the sample doped with 8% of In3+ exhibited the highest performance, achieving a current density of -250 mA cm-2 and Faradaic efficiency of 96.9% for HCOOH. Electrochemical impedance spectroscopy measurements indicated a reduction of charge transfer resistance from 2708.5 Ω cm2 for Bi2O3 to 12.1 Ω cm2 for 8% In3+-doped Bi2O3. XPS spectra analyses indicated the stabilization of the 8% In3+ doped catalyst by the preservation of signals attributed to bismuth even after the electrocatalysis reactions, indicating a structural and electronic synergistic effect caused by the doping process, leading to enhanced activity, selectivity, and stability of the electrocatalyst.