CO2 electrolysis in seawater: calcification effect and a hybrid self-powered concept

Abstract

Oceans are regarded as a sink for anthropogenic CO₂; as such seawater provides an attractive electrolyte for electrochemical CO₂ reduction to value-added carbon-based fuels and chemical feedstocks. However, the composition of seawater is inherently complex, containing multiple cations and anions that may participate in the CO₂ electroreduction reaction. Herein, examination of a nanoporous Ag electrocatalyst in seawater reveals a significant influence of calcium ions on the electrochemical CO₂ reduction performance. Under the applied cathodic potential and in the presence of CO₂, calcium ions in the seawater result in calcium carbonate deposition onto the nanoporous Ag, reducing active sites for CO₂ electroreduction. Mitigation of calcification would promote a stable CO₂ electrolysis in seawater. A first proof-of-concept self-powered hybrid CO₂ electrolysis is demonstrated by the coupling of a Mg anode to a nanoporous Ag cathode in 0.6 M NaCl or seawater. A spontaneous oxidation of a Mg alloy at the anode drives cathodic reduction of AgCl to nanoporous Ag, which electrocatalytically reduces CO₂ to CO. Combining galvanic and electrolytic properties in a single electrochemical cell offers a general approach for designing hybrid self-powered electrolysers. Strategies to overcome calcification such as removal of calcium from the seawater and development of anti-calcifying electrocatalysts are needed to promote practicability of seawater as an electrolyte in CO₂ electroreduction technology.