Electrochemical　reduction　of　CO2　to　valuable　fuels　is　appealing　for　CO2　fixation　and　energy　storage.　However,　the　development　of　electrocatalysts　with　high　activity　and　selectivity　in　a　wide　potential　window　is　challenging.　Herein,　atomically　thin　bismuthene　(Bi-ene)　is　pioneeringly　obtained　by　an　in　situ　electrochemical　transformation　from　ultrathin　bismuth-based　metal–organic　layers.　The　few-layer　Bi-ene,　which　possesses　a　great　mass　of　exposed　active　sites　with　high　intrinsic　activity,　has　a　high　selectivity　(ca.　100　%),　large　partial　current　density,　and　quite　good　stability　in　a　potential　window　exceeding　0.35　V　toward　formate　production.　It　even　deliver　current　densities　that　exceed　300.0　mA　cm−2　without　compromising　selectivity　in　a　flow-cell　reactor.　Using　in　situ　ATR-IR　spectra　and　DFT　analysis,　a　reaction　mechanism　involving　HCO3−　for　formate　generation　was　unveiled,　which　brings　new　fundamental　understanding　of　CO2　reduction.　©　2020　Wiley-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim