New Insights into CO2 Electroreduction in Acidic Seawater

Abstract

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.