Scalable flame spray-derived bismuth catalysts enable efficient CO$_2$ electrolysis revealed by FIB-SEM nano-tomography and ICP-MS EDX profiling

CO$_2$ electrolysis is a promising route to convert CO$_2$ into value-added chemicals. Here, we use flame spray pyrolysis (FSP) as a rapid and scalable method to synthesize Bi$_2$O$_3$ nanospheres and evaluate their formate production in a gas diffusion electrode flow electrolyzer. Under reaction conditions, Bi$_2$O$_3$ converts in situ into bismuth subcarbonate, delivering >90% formate Faradaic efficiency across −0.6 to −1.6 V vs. reversible hydrogen electrode (RHE), reaching a formate partial current density of −700 mA.cm$^{−2}$ at −1.8 V. This performance is nearly twice that of electrochemically deposited dendritic Bi$_2$O$_3$. To elucidate the origin of this high performance, a combined tomography approach based on focused ion beam-scanning electron microscopy (FIB-SEM) nano-tomography and inductively coupled plasma-mass spectrometry (ICP-MS) EDX cross-sectional profiling was introduced to track catalyst layer morphology and electrolyte-induced flooding. The FSP-derived catalyst forms a compact layer near the microporous layer, confining flooding to the catalyst microporous interface and improving operational stability. This work provides mechanistic insight and characterization guidelines for designing durable, high-performance electrodes for CO$_2$ electrolysis.