(Invited) Electrochemical Extraction and Conversion of CO₂ from Seawater

Capture and conversion of CO₂ from anthropogenic emission is becoming an increasingly important social responsibility as the concentration of atmospheric CO₂ continues to rise above record high levels. To achieve carbon negative conditions in the long term, capture and extraction of CO₂ directly from air and seawater will likely play a much bigger role. The concentration of the present CO₂ in the atmosphere is ~400 ppm, or 0.00079 kg m⁻³. As a result, a large volume of air needs to be processed in direct air capture processes. In contrast, world ocean constitutes the largest carbon sink, absorbing about 40% of anthropogenic CO₂ since the beginning of industrial era with an effective CO₂ concentration of 2.1 mmol kg⁻¹, or 0.095 kg m⁻³ in seawater, which is a factor of 120 times larger than in the atmosphere. Thus, extraction of CO₂ from seawater provides an alternative and unique approach in the global carbon removal technological landscape.

Herein, a new bipolar membrane electrodialysis (BPMED) cell design, in which the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at the electrodes were replaced with reversible redox couple reactions with minimal thermodynamic and kinetic voltage losses, was designed, constructed and evaluated. At an operating current density of 3.3 mA cm⁻², and a seawater flow rate of 37 ml min⁻¹, a record low electrochemical energy consumption of 155.4 kJ mol⁻¹ or 0.98 kWh kg⁻¹ of CO₂ was achieved. The demonstrated BPMED cell also exhibited a high CO₂ extraction efficiency of 71% of total dissolved inorganic carbon (DIC). We also demonstrated, for the first time, the direct coupling between the CO₂ extraction from seawater via BPMED and electrochemical CO₂ reduction (CO₂R) into fuels and chemicals. The outlet CO₂ stream from the BPMED was fed into tandem custom-built vapor-fed GDE cells to first perform the oxygen reduction reaction (ORR) using a nanoparticle Ag-based catalyst followed by CO₂R using Cu or Ag as the electrocatalyst. This configuration allowed us to convert CO₂ from seawater to fuels and valuable chemicals such as carbon monoxide, ethylene, ethanol, and propanol with total Faradaic efficiency (FE) of up to 73% at current densities of 58 mA cm⁻² using CO₂R Cu electrocatalyst and to carbon monoxide with selectivity of up to 95% at current densities of 11.15 mA cm⁻² using CO₂R Ag electrocatalyst.

Figure 1