Abstract: This study aims to develop a high-performance electrocatalyst for the electrochemical reduction of CO2 to formate, thereby promoting CO2 resource utilization and contributing to the achievement of carbon neutrality. A self-supported bismuth (Bi) metal catalyst was fabricated using a constant-current electrodeposition method, with carbon cloth serving as the conductive substrate. By regulating the electrodeposition current density, three catalyst samples with distinct microstructures and electrocatalytic performances were obtained. Among them, the catalyst composed of uniformly distributed nanoparticles with an average diameter of approximately 5 nm and featuring a dense and compact structure demonstrated significantly superior catalytic activity compared to 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, and operated stably for over 30 hours at a current density of -63 mA?cm-2 without significant performance degradation. In a flow cell, the catalyst maintained continuous operation for 15 hours at a high current density of -150 mA?cm-2, with the formate Faradaic efficiency consistently exceeding 80%, indicating excellent catalytic performance and operational stability. To gain deeper insight into the origin of the catalyst’s outstanding performance, 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, suggesting enhanced electron transfer capability and a greater abundance of electrochemically active sites. The catalyst was further characterized before and after the reaction using X-ray diffraction and high-resolution transmission electron microscopy. During the electrochemical CO2 reduction process, structural reconstruction occurred on the Bi surface, resulting in the formation of a Bi2O2CO3 phase. This phase was evidenced by distinct diffraction peaks in the X-ray diffraction patterns and well-defined lattice fringes observed in high-resolution transmission electron microscopy images, confirming Bi2O2CO3 as the actual catalytically active species. These findings indicate that during CO2RR, metallic Bi undergoes an in-situ phase transition into Bi2O2CO3, which plays a key role in achieving high formate selectivity and efficiency. Compared with conventional Bi-based catalysts such as coated or metal-organic framework -derived 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 the overall electron and reactant transport efficiency. More importantly, this study established a clear structure–activity relationship by tuning the deposition current density. At a low current density (-5 mA?cm-2), the Bi particles were poorly dispersed, whereas a high current density (-30 mA?cm-2) resulted in severe particle agglomeration and uneven film thickness, both of which were detrimental to catalytic activity and stability. Only the catalyst synthesized at a moderate current density (-15 mA?cm-2) exhibited uniformly distributed nanoparticles, excellent conductivity, and high product selectivity, making it the most efficient catalyst among the three. In conclusion, this study not only successfully developed a highly efficient and stable self-supported Bi-based electrocatalyst system but also established a controllable and reproducible fabrication strategy by precisely adjusting electrodeposition parameters. These findings provide important theoretical insights and practical guidance for the design of high-performance CO2 electrocatalysts via electrodeposition methods.