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Journal of The Electrochemical Society, 2020 167 133504 Figure 3. Membrane arrangements for electrodialysis/electrolysis setups: (a) CEM-AEM24,29 and (b) CEM-CEM.24 Red text is exclusive to the system in Ref. 24. voltages of 2.1 V, CO2 was captured from the air and released in a pure stream at an energetic cost of 383 kJ mol−1 CO2. Sodium (bi)-carbonate.—The use of BPM electrodialysis for CO2 capture from flue gas was explored by Nagasawa et al.19 with three membrane configurations of BPM and CEM (see Fig. 2a); BPM and AEM (see Fig. 2b); and BPM, CEM, and AEM (see Fig. 2c). Units were arranged in stacks of ten, and sodium bicarbonate and sodium chloride solutions were used in the feed and recovery compartments, respectively. Under a constant current density of 17mA cm−2, faradaic efficiencies of 40%–50% were observed for the two configurations involving CEMs and a value 30% was observed for the BPM-AEM configuration. Leakage of protons through the CEM and hydroxide ions through the AEM were determined to be significant detrimental factors for efficiency. The BPM-CEM con- figuration had the best performance in terms of energy requirement per CO2 recovered. Other findings were that specific energy requirement was reduced with more units in a stack and less distance between membranes. At reduced current densities of between 2.4–9.5mA cm−2, higher current densities were associated with higher specific energy requirements and higher CO2 recovery rates. As can be observed in Table I, the study achieved one of the lowest minimum energy requirements of 92 kJ mol−1 CO2 at a low current density of 2.4 mA cm−2. Further development of the BPM electrodialysis approach was presented in a study by Iizuka et al.,21 which examined the effects of numerous factors on specific power consumption and faradaic efficiency under steady-state conditions. This was in the same membrane arrangement as shown in Fig. 2a. In general, power consumption was favored by higher sodium concentrations in the feed solution, lower current densities, and a higher number of cells. Faradaic efficiency was favored by higher sodium concentrations in the feed solution, higher current densities, and higher flow rates. Both power consumption and faradaic efficiency were favored by higher extents of CO2 absorption and higher extents of CO2 recovery. A cost analysis determined electricity cost and membrane cost to dominate the economics of the overall process. Datta et al.20 developed an electrochemical pH-swing process for CO2 capture from flue gas, employing resin-wafer electrodeioniza- tion and a process liquid of monosodium dihydrogen phosphate and disodium hydrogen phosphate. The system contained alternating CEM and BPM (see Fig. 2a) in a buffer solution to form diluate and concentrate chambers. This system distinguishes itself from other electrodialysis approaches by including ion-exchange resin beads (resin wafers) in the diluate chambers to promote ion transfer and improve pH control. Up to 80% CO2 was captured with purity exceeding 98% from a 15% CO2 in N2 inlet gas. Enhancement of reaction kinetics was a key challenge in this method of CO2 capture. A novel membrane electrodialysis and electrolysis system was investigated by Mehmood et al.,29 which examined both a CEM- AEM and a CEM-CEM arrangement to produce sodium hydroxide from aqueous sodium chloride (see Fig. 3). Protons were generated at the anode through the hydrogen oxidation reaction (HOR) and hydroxide was generated at the cathode through the hydrogen evolution reaction (HER). Sodium ions combined with hydroxide in the alkaline compartment to form sodium hydroxide, which was used in turn to capture CO2 as sodium (bi)-carbonate. Recirculation or treatment of the sodium (bi)-carbonate was not explored in this study. In the second arrangement, the AEM was replaced by a CEM to transport protons to the acidic chamber, which granted improved performance due to the generally higher stability and conductivity of CEMs compared to AEMs. Under optimized conditions, cell voltage was 1.25 V at 50 mA cm−2, leading to a specific energy consump- tion of 367 kJ mol−1 CO2. Another variation of the BPM electrodialysis CO2 capture process was described by Jiang et al.25 with the side process of amino acid production. The BPM electrodialysis unit consisted of three repeating BPM, AEM, and CEM units (see Fig. 2c). Experiments were performed under constant current density condi- tions from 20–50 mA cm−2 with inlet gas concentrations between 10%–30% CO2 in N2. Faradaic efficiencies of up to 87% were reached for the highest inlet concentration of CO2 case. Reported energy consumption was relatively high at 1109–1505kJ mol−1 CO2, but the process simultaneously produced methionine and sodium hydroxide, which are potentially valuable chemical products. Alternative approaches using liquid electrolytes.—Novel elec- trochemical approaches have been proposed based on seawater electrolysis and mineral calcination,40–42 ionic liquids,26,43–45 or amines paired with a transition metal.22,23 Rau40,41 proposed the use of a calcium carbonate solution in tandem with electrolysis of a saline water solution (seawater) to capture atmospheric CO2 as calcium bicarbonate, similar to that process shown in Fig. 3 without ion-selective membranes. This was accomplished through the following electrochemically-driven overall reaction: CaCO3 + 2H2O + CO2 1O2 + H2 + Ca(HCO3)2 [4] 2 with calcium carbonate being split at the anode and calcium ions moving to the cathode via electromigration. Calcium carbonate was deemed an interesting storage medium for CO2 due to its relativePDF Image | CO2 Separation and Transport via Electrochemical Methods
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