Electrochemical CO2 Separation
Electrochemical CO2 separation uses an applied potential to pump CO2 across an electrolyte membrane: O2 reduction at the cathode raises the local pH so CO2 is captured as carbonate or bicarbonate, the anions migrate through the electrolyte, and oxidation at the anode lowers the pH and releases CO2 together with O2. Electrolytes range from high temperature molten carbonates to polymer anion exchange membranes operating near room temperature, and performance is limited by electrolyte conductivity and stability and by electrode kinetics at the gas, electrode and electrolyte interface.
Low temperature anion exchange membrane cell for flue gas (2010)
Pennline et al. argued that molten carbonate cells (above 400 C, corrosive, exposed to SO2 that cannot be removed upstream at that temperature) and earlier bicarbonate-soaked polymer cells (above 2.5 V, where water splitting wastes energy) are impractical for flue gas, and demonstrated a cell with a Neosepta anion exchange membrane between Ni-on-carbon-paper gas-diffusion electrodes (Pennline et al. 2010). Wire screening chose Ni over Pd and Pt for carbonate and bicarbonate decomposition, O2 was pumped alone at 1.2 V, CO2 crossed within minutes of switching to a 50:50 CO2/O2 feed, and transport stopped at 0 V, all below the 1.23 V water splitting threshold. Summary: pennline2010.
Anion exchange membrane cell below the water splitting potential (2010)
Landon and Kitchin characterized a cell with a thin Neosepta anion exchange membrane between a Ni-on-carbon-paper anode and a Pt/C cathode, fed a 49.3% CO2 and 50.7% O2 stream (Landon 2010). Current began near 0.6 V and reached 6 mA/cm2 at 1.2 V, below the 1.23 V water splitting threshold; the membrane contributed only about 30 mV (5.06 Ω cm2), so electrode overpotentials dominated. The anode product had a CO2:O2 ratio of 3.56:1, close to the 4:1 limit for bicarbonate transport, and alternative pathways (direct CO2/O2 electrochemistry, electro-osmotic drag, diffusion) were shown or estimated to be minor. An energy analysis for a 500 MW coal plant concluded that practical rates near 0.5 V with a ratio of at least 3.5 would be needed, so the voltage must roughly halve with better catalysts. Summary: landon2010.
Quinone redox-mediated liquid membrane pump (2015)
Watkins et al. replaced O2 reduction with the proton coupled electron transfer of a quinone couple (2,6-dimethylbenzoquinone/hydroquinone in 1 M NaHCO3) held in a 25 µm porous polypropylene separator between Pt/C coated carbon paper electrodes (Watkins et al. 2015). Hydroquinone oxidation acidifies the anode and releases CO2 from bicarbonate, while quinone reduction raises the cathode pH and captures CO2, ideally one CO2 per electron. With Pt the cell moved CO2 from a CO2/N2/O2 feed at 1.0 to 1.5 V with no O2 in the permeate, whereas Ru and Pd transported CO2 mainly at 2.0 to 2.5 V alongside O2 from water splitting. Qualitative transport efficiencies at 1.0 V depended on pH (57% feed side and 5% permeate side at pH 8.73, 16% permeate at pH 7.20), revealing a trade-off between CO2 uptake and proton driven release. Summary: watkins2015.