Evolution of bismuth oxide catalysts during electrochemical CO2 reduction

Bismuth is a promising electrocatalyst for active and selective CO2 electroreduction to formate. Herein, we investigated the evolution of various Bi-based catalysts (13-Bi2O3 nanoparticles, BiOBr nanosheets, Bi2O2CO3 nanosheets, and large alpha-Bi2O3 and 13-Bi2O3 particles) during reaction. Apart from the alpha-Bi2O3 particles, all the electrocatalysts reach the same Faradaic efficiency (93 +/- 2%) and current density during potentiostatic operation, and their morphology changed to nanosheets. This change in morphology can be linked to the formation of Bi2O2CO3 layers, which are prone to reduction to metallic Bi. The final phase and morphology depend on the size of the Bi precursor. Quasi-in situ XPS suggests that a Bi2O2CO3 contribution persists on the surface even for the reduced catalysts. At a current density of 200 mA/cm2, all catalysts reduce to metallic Bi without forming a welldefined sheet morphology. Only for the Bi2O3 nanoparticles can a FE above 90% be maintained.