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abundance and low cost. The concept was tested at a laboratory scale, and the electrochemically treated seawater absorbed CO2 at a rate approximately 3 times that of untreated seawater. However, energy input was orders of magnitude higher than that expected for a commercial electrolyzer. The author noted the additional significant energy penalty of regeneration if concentrated CO2 is desired as the end product, which would be partially offset by reduced supply and transportation costs if the reactant were recycled. A later study by Rau et al.42 investigated a similar process using silicates and found an experimental energy expenditure of 426–481kJ mol−1 CO2 removed. Reinhardt et al.43 wrote a perspective focusing on CO2 capture using electrogenerated nucleophiles. In such a process, carbon in the CO2 reversibly binds with an electrochemically reduced quinone (Q) within a liquid electrolyte on the cathode and is released on the anode as a pure gas. One benefit of these novel electrolytes is enhanced tunability of the chemistry, which can theoretically reduce the potential differences between the capture and release step.43 This can be implemented in a system that either utilizes electrolyte pumping or electromigration across a static liquid electrolyte, with the following generic overall reaction: Direct separation from a gas mixture via an electrochemical cell.—Other existing literature in the field describes electrochemical cells that directly separate gas-phase CO2 via electromigration of carbon-containing ions across a physical barrier. The electroche- mical cell approach offers a distinct advantage over conventional membrane separation processes,47 namely that it allows for high- selectivity concentration from relatively dilute mixtures through active electrochemical pumping of carbon-containing ions and does not require large pressure gradients for operation. In fact, such a method opens possibilities of pumping against concentration gra- dients (i.e., electrochemical compression), as has already been demonstrated for hydrogen.48 However, there are still significant challenges in operation when the feed gas is extremely dilute, such as the high energetic costs of delivering large volumes of gas to the cell and limitations in applicable current density. In the first reported devices for electrochemical CO2 separation, the electrochemical reactions occur on electrodes separated by an aqueous carbonate solution supported in a thin asbestos matrix. In such a device, water is produced accompanied by the generation of electrical energy (fuel cell-like operation) and the transfer of CO2 from the cathode to the anode side. The National Aeronautics and Space Administration explored electrochemical methods for separa- tion of CO2 in the early 1970s to scrub CO2 from spacecraft cabins.49,50 These early studies note a number of benefits an electrochemical process has over a cyclic adsorption process, which include continuous operation, low CO2 partial pressure capabilities with low system weight, concentrated CO2 without air contamina- tion, premixed H2 and CO2 at relevant ratios for downstream processing, elimination of mechanical CO2 compressors, and low temperature operation with low flows. The electrochemical reactions hypothesized were: -- O2+2H2O+4e «4OH [9] 2 O H - + C O 2 « H 2 O + C O 32 - [10] As a product of the following two reactions: -- CO2+OH «HCO3 [11] HCO- + OH- « H O + CO2- [12] 323 H2+2OH- 2H2O+2e- [13] H 2 + C O 32 - H 2 O + C O 2 + 2 e - [14] - 2- Q + 2e + 2CO2 « Q(CO2)2 were: [7] [5] carrier in a proof-of-concept batch-type cell and achieved pumping of CO2 at <1% levels to pure concentrations at atmospheric pressures with a transference of 0.427 moles CO2 per electron mole (solution CO2 release step, 0.5 for the ideal case). Poor stability in O2 was a critical issue for the system that would preclude operation with ambient air. Stern et al.22,23 developed an electrochemically-mediated amine regeneration system to capture CO2 from flue gas. The study used polyamines as the CO2 sorbents and copper electrodes to produce cupric ions that assist in displacing CO2. The relevant reactions Scovazzo et al.44 developed an aqueous redox active CO 2 A(aq) + mCO2 « A(CO2)m(aq) [6] 2A(CO ) (aq)+Cu(s) « CuA2+ (aq)+2CO (g)+2e- m2m 2/m2 CuA2+ (aq) + 2e- « Cu(s) + 2 A(aq) [8] 2/m m with A representing a generic amine molecule. Pure CO2 was used to saturate the sorbent, and the authors achieved a faradaic efficiency of 42% operating at 2.5mA cm−2.23 The authors also reported a specific energy consumption of 100kJ mol−1 CO2 for their system.22,23 Tuning the chemistry to optimize binding constants for with reactions 9–12 taking place on the cathode side and reactions 13–14 taking place on the anode side. Wynveen et al.49 designed and fabricated a one-man, self-contained electrochemical system for CO CO and reduce the potential difference between the capture and 2 43 2 separation and concentration, capable of removing 1.9 kgCO2/day from air. Humidified CO2 and O2 were fed at the cathode and H2 was fed at the anode, which were separated by a cesium carbonate electrolyte at 21 °C–29 °C. The authors also tested rubidium carbonate and cerium carbonate electrolytes with less success. The electrocatalyst used was platinum, which exhibited better perfor- mance than palladium. The primary figure of merit for system performance was transfer index, or mass of CO removed over mass release process is the subject of ongoing work. Legrand et al.46 proposed a membrane capacitive deionization technique to separate CO2, in which ions in a CO2-containing deionized water solution are removed via an anion exchange or cation exchange membrane and stored in the electrical double layer of a carbon electrode. The experimental cell operated in batch mode at very low current densities of 0.02–0.06 mA cm−2 and feed concentrations 15%, 30%, and 100% CO2 in N2 were tested. Authors reported low energy requirements of approximately 40 kJ mol−1 CO2, but also reported low amounts of CO2 exchange (<9%) −9 2 of O2 consumed, which was affected by current density and CO2 mol/s∙gcarbon. In addition, captured CO2 was desorbed back to the solution rather than to a concentrated gas stream, making direct comparison with other methods difficult. Authors acknowledged the necessity of an additional membrane contactor for separating liquid and gas phases to generate pure streams of CO2. partial pressure. In typical operation, 50% or more of the inlet CO2 stream was transferred to the H2 stream, and the system showed no signs of deterioration after 260 d of operation. A follow-up study by Winnick et al.50 furthered the concept with a scale-up of the technology. The system was tested with one, three, and 90 cells, each with an aqueous carbonate solution (either potassium or cesium carbonate) suspended in an asbestos matrix with platinum catalyst at the electrodes. The system operated in the same configuration as described previously.49 Partial pressures of CO2 up to 12 mbar and current densities up to 32 mA cm−2 were and very low capture CO2 rates of 6∙10 Journal of The Electrochemical Society, 2020 167 133504PDF Image | CO2 Separation and Transport via Electrochemical Methods
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