Carbon dioxide (CO₂) reduction reaction (CO₂RR) in acidic media is emerging as a promising strategy to convert CO₂ into fuels and chemicals while achieving high carbon utilization. [1-2] In contrast to conventional neutral or alkaline systems, acidic electrolytes mitigate the formation of (bi)carbonates that otherwise consume CO₂ and lower overall carbon efficiency.[3] However, operating under acidic conditions significantly intensifies the competing hydrogen evolution reaction (HER), necessitating careful control of local pH, interfacial water structure, and ion transport to maintain selectivity toward CO₂RR.[4] The addition of metal cations as supporting electrolytes has been identified as an effective strategy to shift the selectivity from HER towards CO₂RR. Several hypotheses—such as modulation of the local electric field, local pH, and specific interactions—have been proposed to explain cation effects.[5] Yet, studies of organic cations as supporting electrolytes remain scarce and are largely confined to neutral or alkaline systems[6-9], leaving their ­­role in acidic CO₂RR poorly understood.

Here, we investigate organic cations as supporting electrolyte to broaden the understanding of cation effect in acidic systems for CO₂RR. Alkylammonium cations were tested as supporting electrolytes for CO₂RR on Cu at pH=2 in flow cell electrolyzer from -5 to -200 mA.cm^-2. Our results demonstrate that alkylammonium cations can largely prevent HER and favor CO₂RR towards both C₁ and C₂₊ products. The necessity of alkali cation for CO₂RR was excluded via impurities analysis via Inductively Coupled Plasma Mass Spectrometry along with controlled addition of alkali cations. Because alkylammonium are not expected to have specific interaction with CO₂RR intermediate, our findings support the hypothesis that the interfacial electric field generated by cations is sufficient to promote C–C coupling on Cu. Extending this approach to neutral and alkaline systems, we found that at high current densities, CO₂RR performances depend primary on the interfacial field and local pH induced by cations and are surprisingly independent of the bulk pH.

These results contribute to a unified understanding of the electrode–electrolyte interface for CO₂RR under acidic, neutral, and alkaline conditions at industrially relevant current densities. Importantly, we demonstrate that specific chemical interactions between cations and reaction intermediates are not required for C-C coupling. This insight opens new opportunities for the use of organic cations in CO₂RR systems.