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Massive production of practical metal or alloy based electrocatalysts for electrocatalytic CO2reduction reaction is usually limited by energy-extensive consumption, poor reproducibility, and weak adhesion on electrode substrates. Herein, we report the ultrafast thermal shock synthesis and porosity engineering of free-standing Cu–Bi bimetallic nanofoam electrocatalysts with 3D hierarchical porous structure and easily adjustable compositions. During the thermal shock process, the rapid heating and cooling steps in several seconds result in strong interaction between metal nanopowders to form multiphase nanocrystallines with abundant grain boundaries and metastable CuBi intermetallic phase. The subsequent porosity engineering process via acid etching and electroreduction creates highly porous Cu–Bi structures that can increase electrochemically active surface area and facilitate mass/charge transfer. Among the Cu–Bi nanofoam electrodes with different Cu/Bi ratios, the Cu4Bi nanofoam exhibited the highest formate selectivity with a Faradaic efficiency of 92.4% at −0.9 V (vs reversible hydrogen electrode) and demonstrated excellent operation stability.