Ultra‐Long Bi Nanowires Coupled With Tapered Si Microwires for Selective Photoelectrochemical CO ₂ ‐to‐Formate Conversion

Photoelectrochemical CO 2 reduction offers a promising route to valorize CO 2 into value‐added chemicals; however, achieving both high efficiency and selectivity remains challenging. The synthesis of ultra‐long Bi 2 O 3 nanowires that are electrochemically transformed into metallic Bi nanowires is reported, which exhibit highly efficient and stable CO 2 reduction when integrated into electrochemical and photoelectrochemical systems. Rietveld refinement and Halder–Wagner analyses quantify oxygen vacancy formation during the reduction of Bi 2 O 3 to Bi, and theoretical mechanistic studies reveal that these vacancies stabilize the key OCHO intermediate, playing a crucial role in attaining high CO 2 ‐to‐formate selectivity. The ultra‐high aspect ratio of Bi 2 O 3 ‐derived Bi nanowires maximizes the density of electrochemically active sites and facilitates rapid electron transport, collectively contributing to superior CO 2 reduction performance. Consequently, the Bi nanowires achieve Faradaic efficiencies above 95% for electrochemical formate production across a wide potential window, with hydrogen evolution effectively suppressed. Motivated by a leaf inspired network, the coupling of Si microwires with Bi nanowires enables efficient charge transfer while preserving light harvesting, functioning as a co‐catalyst without blocking incident photons. As a result, the integrated Bi nanowire‐tapered Si microwire photoelectrode demonstrates efficient and selective solar‐driven CO 2 ‐to‐formate conversion with outstanding activity and long‐term stability under simulated sunlight irradiation.