New Insights into CO<sub>2</sub> Electroreduction in Acidic Seawater
Peng Chen, Haifeng Shen, Min Zheng, Mietek Jaroniec, Yao Zheng, Shi‐Zhang Qiao
Abstract
The electrochemical CO 2 reduction reaction (CO 2 RR) is of great importance to produce valuable chemicals. In conventional alkaline and “acid + salts”-based CO 2 RR, 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 CO 2 RR without salt addition. The Faradaic efficiencies (FEs) and partial current densities of C 2+ on typical copper in the “H 2 SO 4 in raw seawater” electrolyte are comparable with those for conventional “KOH in DI water” and much higher than those for “H 2 SO 4 + 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·H 2 O species, which improve interactions with *CO 2 –, while Cl – anions enhance the adsorption strength of key CO 2 RR intermediates (namely, *CO on copper, *COO – on silver, and *OCHO on tin). Through these interactions with water molecules and CO 2 RR intermediates, such free but functional ions in seawater play a highly important role in promoting selectivity and activity for CO 2 RR, as well as SPCE in acidic seawater. Furthermore, using acidic seawater as an alternative CO 2 RR electrolyte has significant economic and ecological benefits compared with traditional alkaline DI water electrolytes.