Electrochemically converting CO₂ into value-added chemicals is a promising approach to mitigate anthropogenic carbon emissions, yet largely limited to short-chained C₁–C₃ products. Herein, we demonstrate a tandem artificial synthesis of biodegradable polyhydroxybutyrate (PHB) plastic from CO₂ building blocks. Batch synthesis of defects-enriched Bi catalyst is firstly demonstrated by plasma bombardment and following in situ electrochemical reduction, which delivers a HCOOH Faradaic efficiency above 80% at tunable concentration from 2 to 250 mM, an energy efficiency up to 41%, and a single-pass carbon conversion efficiency up to 60%. Annular dark field and second electron microscopic analysis, density functional theory (DFT) calcualtions, coupled with H-type and solid-state electrolyzer assessments, point out the vital role of defective and/or stepped Bi surface sites in promoting CO₂-to-HCOOH conversion. Thereafter, as-synthesized high-purity HCOOH is used as the sole carbon source for C-chain growth within microbial fermentation reactor with Ralstonia eutropha, where activated formate dehydrogenase and increased metabolites related to Calvin–Benson–Bassham cycle are found to be responsible for the enhanced polyester accumulation.