Prereduction Governed Microcomposition Tuning of CuBi ₂ O ₄ Switches CO ₂ Electroreduction Selectivity

The microcompositional evolution of electrocatalysts during CO2 reduction critically governs intermediate adsorption; however, simultaneous structural and compositional alterations hinder the isolation of individual factors influencing the product selectivity. To address this challenge, we engineered two morphologically identical model catalysts with subtle compositional distinctions through high-temperature hydrogenation (CuBi2O4-HA) and in situ electrochemical reduction (CuBi2O4-ER) strategies using CuBi2O4 as a precatalyst. Both systems maintained Bi2O3/Bi matrices while exhibiting distinct copper speciation: CuBi2O4-HA predominantly contained metallic Cu0, whereas CuBi2O4-ER retained the partial Cu+ species. Comprehensive in situ characterization coupled with electrochemical analysis revealed that residual Cu+ species significantly enhanced *CO surface coverage and accelerated C–C asymmetric coupling kinetics. Consequently, CuBi2O4-ER achieved a 41.4% Faradaic efficiency for ethylene at −0.87 V versus RHE, with exceptional stability attributed to heterojunction effects. In contrast, CuBi2O4-HA preferentially generated CO and HCOOH. This study systematically decouples morphological and compositional influences, demonstrating that subtle copper valence variations (particularly Cu+ stabilization) fundamentally dictate the C2 pathway selectivity. Our findings establish a paradigm for isolating critical catalytic descriptors through precision engineering of compositionally graded model systems.