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

The microcompositional evolution of electrocatalysts during CO 2 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 (CuBi 2 O 4 -HA) and in situ electrochemical reduction (CuBi 2 O 4 -ER) strategies using CuBi 2 O 4 as a precatalyst. Both systems maintained Bi 2 O 3 /Bi matrices while exhibiting distinct copper speciation: CuBi 2 O 4 -HA predominantly contained metallic Cu 0, whereas CuBi 2 O 4 -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, CuBi 2 O 4 -ER achieved a 41.4% Faradaic efficiency for ethylene at −0.87 V versus RHE, with exceptional stability attributed to heterojunction effects. In contrast, CuBi 2 O 4 -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 C 2 pathway selectivity. Our findings establish a paradigm for isolating critical catalytic descriptors through precision engineering of compositionally graded model systems.