New Insights into CO₂ Electroreduction in Acidic Seawater

The electrochemical CO2 reduction reaction (CO2RR) is of great importance to produce valuable chemicals. In conventional alkaline and “acid + salts”-based CO2RR, the aqueous electrolyte normally needs to be refreshed due to the gradually more neutral feature of pH during electrolysis operation. Therefore, both solutes and deionized (DI) water in electrolytes are required to be regenerated regularly. In this work, acidic seawater (pH < 2) was used as a low-cost but efficient electrolyte for CO2RR without salt addition. The Faradaic efficiencies (FEs) and partial current densities of C2+ on typical copper in the “H2SO4 in raw seawater” electrolyte are comparable with those for conventional “KOH in DI water” and much higher than those for “H2SO4 + salts” systems. Moreover, single-pass carbon efficiencies (SPCEs) in acidic seawater are significantly higher than the values in alkaline DI water. Such an abnormal phenomenon was also demonstrated for CO and HCOOH generation on typical silver and tin catalysts, respectively. In situ Raman spectroscopy and controlled experiments revealed that metal (denoted as M) cations in seawater ensure a higher concentration of M·H2O species, which improve interactions with *CO2–, while Cl– anions enhance the adsorption strength of key CO2RR intermediates (namely, *CO on copper, *COO– on silver, and *OCHO on tin). Through these interactions with water molecules and CO2RR intermediates, such free but functional ions in seawater play a highly important role in promoting selectivity and activity for CO2RR, as well as SPCE in acidic seawater. Furthermore, using acidic seawater as an alternative CO2RR electrolyte has significant economic and ecological benefits compared with traditional alkaline DI water electrolytes.