Cationic carbon nanotube modulates surface fields for general acidic CO₂ reduction with aqueous organic cations

The (bi)carbonate formation in neutral or alkaline media severely limits the carbon and energy efficiency of electrochemical CO₂ reduction reaction (CO₂RR). Alkali metal cations and polymer coatings employed to inhibit hydrogen evolution reaction (HER) during acidic CO₂RR often exacerbate salt precipitation or increase cell resistivity. Here we report cationic carbon nanotubes (CCNTs) as universal additives enabling efficient CO₂RR in strong acids. Using bismuth, silver, and copper oxide catalysts, we attain Faradaic efficiency (FE) of 95%, 100%, and 67% for HCOOH, CO, and C₂H₄ in pH≤1 electrolyte. Notably, while organic cations typically suffer from poor proton shielding and inferior selectivity, CCNT additives enable their use as electrolytes for selective acidic CO₂RR. Their higher solubility permits elevated concentrations, improving stability and energy efficiency. Quantum Mechanics reveals that CCNTs physically mixed with metal catalysts alter the distribution of electric field at the electrolyte/electrode interfaces, impeding HER by suppressing hydronium ions migration while promoting CO₂RR. These findings highlight CCNTs as versatile and effective additives for advancing acidic CO₂RR with improved selectivity, carbon efficiency, energy efficiency, and stability.