Electrocatalytic CO₂ reduction reaction (CO₂RR) in acidic media is a promising approach to overcome the unavoidable formation of carbonates in alkaline or neutral electrolytes. However, the proton-rich environment near the catalyst surface favors hydrogen evolution reactions (HER), leading to lower energy efficiency of the desired products, especially in industrial-level current densities. Here, quaternary ammonium cationic surfactant (cetyltrimethylammonium bromide (CTAB)) was introduced into acidic electrolyte to modulate the interfacial microenvironment, which greatly enhanced CO₂ electroreduction to formic acid (HCOOH) at the Bi/C nanoparticles electrode. Using a Bi/C nanoparticles electrode with CTAB added, constant production of formic acid was enabled with a cathodic energy efficiency of > 40% and maximum FE_HCOOH (FE = Faradaic efficiency) of 86.2% at −400 mA·cm⁻² over 24 h. Combined with in-situ attenuated total reflection Fourier transform infrared spectroscopy, the concentration of *OCHO intermediates significantly increased after CTAB modification, confirming that the hydrophobic interface microenvironment formed by dynamic adsorption of positively charged long alkyl chains on Bi/C nanoparticle electrodes inhibited HER and improved the selectivity of CO₂RR to HCOOH.