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employed to obtain significant reductions in stack cost. Cheaper membrane materials with high conductivities will also be critical in providing a scalable system, especially if the applicable current densities remain low. If current densities can be increased by an order of magnitude while limiting cell overpotentials and main- taining reasonable faradaic efficiencies, this will lead to an order of magnitude reduction in stack cost for a given production rate of CO2. Table II summaries some of the pros and cons for electroche- mical methods of CO2 capture. Point-Source vs Direct-Air Capture Point-source (e.g., flue gas) capture of CO2 is a more thermo- dynamically advantageous approach compared to DAC. Capture of CO2 from fossil fuel combustion is primarily performed in one of three configurations with different corresponding CO2 concentra- tions: oxy-fuel post-combustion capture (>90% CO2), pre-combus- tion capture (e.g., partial oxidation, 30%–35% CO2), and air post- combustion capture (5%–25% CO2). Minimum work to separate CO2 depends logarithmically on the concentration; the minimum work to separate CO2 from flue gas (15% CO2 in N2) at 300 K is 7 kJ mol−1 CO2. Given practical limitations to process efficiency, 32 kJ −1 cycling,23,27 although overpotentials and faradaic losses present barriers to reaching those limits. Other important thermodynamic limits are set by the absorption reaction enthalpy between CO2 and the aqueous sorbent solution. When an alkaline capture solution is used, e.g., NaOH or KOH, the enthalpy of the exothermic absorption reaction is 109.4 or 95.8 kJ mol−1 CO2, respectively. The enthalpy of absorption/desorption of a typical monoethanolamine solution has been estimated to be approximately 80 kJ mol−1 CO2.65 For the AEM electrochemical cell methods described in Refs. 15, 27, 30, the minimum energy consumption as dictated by the standard electrode potentials of the half-reactions66 is 35.1 and 56.2 kJ mol−1 CO2 for a cell operating fully in the bicarbonate and carbonate mode, respectively. Although theoretically low energy consumption is obtainable using electrochemical approaches, it is also important to consider how a practical system would operate and the energy associated with mass transport (e.g., pumping of the capture solution, delivery of the feed gas mixture). Sorption-based methods are the only commercialized approach for DAC, mainly due to the large volumes of air that need to be processed. Methods based on aqueous sorbents typically regenerate the sorbent using a causticization/thermal regeneration process. The cost of DAC is the subject of much debate, and has been estimated to 64 adoption of renewable energy becomes more widespread. CO2 Transport in Alkaline Membranes Ion-conducting membranes are of great interest in electroche- mical separation processes, as they allow for selective transport of species through a physical barrier. AEMs are particularly relevant for CO2 separation because of their active transport of (bi)- carbonate. As the name implies, AEMs use negatively charged ions (e.g., hydroxide) as the charge carrier, as opposed to PEMs, which use hydrogen cations (see Fig. 5). In general, AEMs are a more nascent technology than PEMs; commercial implementation of PEMs in automotive fuel cell applications has already begun, albeit with some challenges that still need to be overcome.69 The past decade has brought a significant level of interest in alkaline polymer membranes, a subgroup of AEMs, for fuel cells or co-electrolysis of CO2 to form CO as a fuel or chemical precursor. Developments in AEMs for electrochemical applications are discussed in a perspec- tive by Varcoe et al.70 Alkaline membranes in fuel cells.—Alkaline membranes enable the use of non-noble metal catalysts in fuel cell applications, but face complications when air is used as the oxidant, as there is a significant negative influence due to CO2. CO2 neutralizes hydroxide in the membrane via Eqs. 9–12 and reduces catalytic activity and ion conductivity. However, bicarbonate in the membrane will be converted to carbonate and hydroxide and transported out of anode side as CO2 during power generation via Eq. 14. Early studies describe the so-called “self-purging mechanism” in alkaline ex- change membrane fuel cell systems.71–75 Under some conditions, almost all of the CO2 fed from ambient air can be absorbed, transported, and released on the anode side of the alkaline membrane, and a higher degree of self-purging is observed at higher current densities.72,74,76–78 A modeling study by Krewer et al.78 found that temperature has an unclear effect on membrane carbonation during AEM fuel cell operation, and that carbonate formation is heavily favored over bicarbonate at current densities of 0–1500mA cm−2. The same mol CO2 represents a rough lower limit of energetic cost for be $30–1000 per ton of CO2 captured. 67 amine scrubbing, times higher. 4 31,58 with literature showing energetic inputs to be 4–5 A techno-economic analysis by Fasihi et al. based on estimates from companies commercia- lizing DAC technology gave a value of 243–317 kJ mol−1 CO2 for combined electrical and thermal energy requirements, although current values are likely to be higher. According to a thermodynamic analysis of DAC performed in 2011, an energetic cost of greater than 400 kJ mol−1 CO2 was considered potentially counterproductive due to the CO2 intensity of fossil-based electricity sources in the United States at the time.68 However, this value will shift upwards as Membrane separation is typically discussed in reference to post- combustion gases, as it generally requires gas mixtures containing 10% or more CO2.47,59,60 Two key parameters that require optimiza- tion for such systems are membrane permeability and selectivity, as the pressure gradient through the membrane provides the driving force. A number reviews on non-electrochemical, membrane-based methods of CO2 capture from combustion gases have been pub- lished, focusing on conventional polymeric membranes,47 facilitated transport membranes,59 mixed matrix membranes,61 organosilica membranes,62 and membrane contactors as compared to membrane gas separators.60 In terms of an electrochemical separation process, the chief benefit of point source capture is that the relatively high concentration of CO2 allows for higher current densities to be reached at higher faradaic efficiencies in a direct gas separation cell. For a liquid electrolyte absorption step, less gas would need to be flowed through the absorber to saturate the solution, reducing energy costs. However, common flue gas contaminants (e.g., NOx, SO2, soot) would potentially pose additional challenges such as electro- catalyst poisoning or electrolyte degradation. High energy requirements are the primary detriment of DAC compared to point-source capture. Due to the extremely dilute concentration, at least 2500 tons of air need to be processed in order to capture one ton of CO2, so a minimal pressure drop is desired. There are therefore implications for duration of the capture step and methodology to effectively deliver air to the capture medium and sufficiently saturate it with CO2. Lackner et al.63 reviewed the state of the art and challenges in CO2 capture from ambient air. Some of the critical requirements the authors note for air capture are the thermodynamic limits associated with the free energy of mixing CO2 in air at 300 K and the mechanical work required to compress CO2 from 1 to 60 bar isothermally, 22 and 11 kJ mol−1 CO2, respectively. The minimum theoretical separation work for the range of CO2 partial pressures between 0.1 mbar and 1 bar is shown in Fig. 4, with the partial pressure of CO2 in air indicated. However, there are also significant advantages to air capture, namely, that it allows for geographical decoupling from the emission source. This is of particular relevance to the transportation sector, which is still heavily reliant on carbon-based fuels. DAC can also reduce the need to transport CO2 over long distances and enable the production of synthetic fuels in a closed carbon cycle. An additional benefit is the significantly reduced scrubbing requirement of air vs flue gas.64 Electrochemical methods that operate isothermally at low tem- peratures can approach the theoretical separation work shown in Fig. 4 due to removing the inefficiencies associated with thermal Journal of The Electrochemical Society, 2020 167 133504PDF Image | CO2 Separation and Transport via Electrochemical Methods
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