Abstract: This study developed a high-performance electrocatalyst for the electrochemical reduction of CO₂ to formate, thereby promoting CO₂ resource utilization and contributing to carbon neutrality. A self-supported bismuth (Bi) metal catalyst was fabricated via constant-current electrodeposition utilizing carbon cloth as the conductive substrate. By varying the electrodeposition current density, three catalyst samples exhibiting distinct microstructures and electrocatalytic performance were obtained. Among these, the catalyst comprising uniformly distributed nanoparticles with an average diameter of approximately 5 nm and exhibiting a dense, compact structure demonstrated significantly superior catalytic activity compared with the other two samples. Electrochemical performance tests revealed that this catalyst achieved a Faradaic efficiency of up to 95.49% for formate production at −1.1 V (vs RHE) in an H-type electrolytic cell. Furthermore, it maintained stable operation for over 30 h at −1.0 V (vs RHE) with a current density of −87 mA·cm⁻² without significant performance degradation. In a flow cell, the catalyst operated continuously for 15 h at a high current density of −150 mA·cm⁻², with the Faradaic efficiency for formate production consistently exceeding 80%, indicating excellent catalytic performance and operational stability. To comprehensively investigate the outstanding performance of the catalyst, electrochemical impedance spectroscopy and electrochemical surface area analyses were conducted. The catalyst exhibited the lowest charge transfer resistance of only 2.12 Ω and a high electrochemical surface area value of 77.11 cm², indicating enhanced electron transfer capability and a greater density of electrochemically active sites. Further characterization of the catalyst employing X-ray diffraction and high-resolution transmission electron microscopy was conducted before and after the electrochemical reaction. During the electrochemical CO₂ reduction process, structural reconstruction occurs on the Bi surface, resulting in the formation of a Bi₂O₂CO₃ phase. This phase was validated by distinct diffraction peaks in the X-ray diffraction patterns and well-defined lattice fringes observed in the high-resolution transmission electron microscopy images, confirming that Bi₂O₂CO₃ is the active species responsible for catalysis. These findings indicate that, during CO₂ reduction, metallic Bi undergoes an in situ phase transition to Bi₂O₂CO₃, which plays a crucial role in achieving high formate selectivity and efficiency. Compared with conventional Bi-based catalysts, such as those derived from metal-organic frameworks or coated materials, the self-supported structure developed in this study offers significant advantages. The elimination of conductive binders or dispersants reduces interfacial resistance and mass transport barriers, thereby improving overall electron and reactant transport efficiencies. Notably, this study established a clear structure–activity relationship by tuning the electrodeposition current density. At a low current density (−5 mA·cm⁻²), the Bi particles exhibited poor dispersion, whereas a high current density (−30 mA·cm⁻²) resulted in significant particle agglomeration and uneven film thickness, both of which negatively impacted catalytic activity and stability. Among the catalysts synthesized at varying current densities, only the catalyst produced at a moderate current density (−15 mA·cm⁻²) exhibited uniformly distributed nanoparticles, excellent conductivity, and high product selectivity, thereby making it the most efficient catalyst of the series. In conclusion, this study successfully developed a highly efficient and stable self-supported Bi-based electrocatalyst system and established a controllable and reproducible fabrication strategy via precise control of the electrodeposition parameters. These findings provide valuable theoretical insights and practical guidance for the design of high-performance CO₂ electrocatalysts via electrodeposition.