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tested. Important findings were that CO2 transfer rate increased with increasing inlet CO2 concentration and increasing current density. At a given current density, there was a peak effective inlet CO2 concentration beyond which the transfer rate was unaffected. Other results were that performance was unaffected by cell temperature and the scaled-up system performed as predicted based on the single cell system. Chin and Winnick51 also developed a steady-state numerical model for the aforementioned system and found good agreement between the simulations and experiments. Further studies by Winnick et al.52,53 developed CO2 separation from air using a molten carbonate electrochemical cell operating at high temperatures (>500 °C). This approach benefits from its insensitivity to relative humidity and fast reaction kinetics. Experiments were run in both a hydrogen mode (i.e., fuel cell) and a driven mode feeding only N2 at the anode. In the driven mode, the reaction at the anode was oxygen evolution from the oxidation of carbonate or bicarbonate, i.e.: 2CO32- « 2CO2 + O2 + 4e- [15] 4HCO- « 4CO +O +2H O+4e- [16] respectively, and ionic current carried by hydroxide and hydrogen ions negatively affects this parameter. In the experiments, higher inlet CO2 concentrations led to higher faradaic efficiencies, but lower fractions of CO2 separated. Using an AEM, air containing 400 ppm CO2 was separated with an energy consumption of 350 kJ mol−1 and 23% faradaic efficiency. Water management was an issue when operating with fully humidified gases, prompting experiments with a room-temperature ionic liquid electrolyte solu- tion containing cesium bicarbonate, which achieved comparable but slightly lower efficiencies. Other studies by Landon and Kitchin15 and Pennline et al.30 employed an AEM in an electrochemical cell to separate mixtures of approximately 50% CO2 in O2 (Eqs. 9–12, 15–16). Only humidified argon was used as a carrier gas on the anode side, resulting in oxygen evolution and CO2 formation from primarily bicarbonate. Both studies employed nickel and platinum as electrocatalysts, and cell potentials of up to 1.2 V were used in order to stay below the water-splitting potential of 1.23 V. The cell setup of Landon and Kitchin15 reached current densities up to 6 mA cm−2 with an average ratio of 3.56:1 CO2:O2 measured at the anode. Authors determined that the investigated membrane conductivity and stability were too low and electrocatalysts were not active enough for practical application in coal power plants. In addition, the gas mixture examined was not analogous to an air-combustion flue gas mixture, but the results compared favorably with previous studies of electro- chemically separating CO2 from breathing gas mixtures.55,56 Rigdon et al.27 examined the use of AEM to separate mixtures of CO2 and O2 in an electrochemical cell (Eqs. 9–12, 15–16). Platinum was used as the electrocatalyst for both the anode and cathode, and the polymer membrane was approximately 57 μm thick. Humidified N2 was supplied to the anode side of the cell, and humidified 50% CO2 in O2 was supplied to the cathode. Similar to the study by Landon and Kitchin,15 CO2 was transported via membrane carbona- tion and evolved at the anode along with O2. Cell potentials of up to 1.5 V were reached at current densities of approximately 2 mA cm−2 with a resulting transference of approximately 0.67 CO2 molecules per electron, indicating a CO2 transport across the membrane via a mixture of carbonate and bicarbonate ions. The dominant transport pathway shifted from carbonate to a carbonate/bicarbonate mixture as the cell potential increased. There are important implications about the quality of CO2 at the outlet of an electrochemical concentration cell system depending on the methodology used. While water can be easily separated from the gas mixture through condensation, gas phase separations are more difficult and costly to address. Inert carrier gases are often used to facilitate gas analysis, but other options would be more practical in a scaled-up system, e.g., recirculation of the anode gas. If carbonate and bicarbonate are neutralized at the anode in an inert atmosphere, the highest ratio of CO2 to O2 possible is 4:1 based on the stoichiometry of Eq. 16. Authors have proposed using the resulting gas mixture for oxy-combustion, resulting in a pure CO2 stream.30 If the cell is operated in a fuel cell mode (H2 at the anode), hydrogen oxidation produces a mixture of CO2, water, and residual H2. Running the cell at low stoichiometries would in theory allow for higher concentrations of CO2 to be obtained, although this would also approach an upper limit due to mass transport limitations and lead to possible degradation in the cell. In spite of this limitation, mixtures of H2 and CO2 could be useful chemical feedstocks, for example, in reverse water gas-shift reactors or methanol synthesis. Use of quinones could theoretically produce a pure stream of CO2 in the same way that electrodialysis methods do, although O2 evolution is still possible.28 Energy consumption of CO pumping with AEMs is still 2 relatively high in the pioneering studies, but may be decreased below the state of the art for absorption through advancements in the catalyst and membrane materials, as well as cell engineering.27 As with most AEM fuel cell studies,57 the majority of electrochemical cell studies reviewed in this work use platinum as the electrocatalyst, but cheaper, platinum-group-metal-free catalysts will need to be 3222 A more recent study by Spinelli et al.54 used thermodynamic system modeling to examine the possibility of retrofitting natural gas and coal power plants with molten carbonate fuel cells for CO2 capture. The study estimated a specific primary energy consumption of approximately 57 kJLHV/mol CO2 avoided, which was signifi- cantly lower than conventional amine scrubbing methods. Li and Li55 investigated the use of an electrochemical membrane cell to remove CO2 from breathing gas mixtures. Authors employed a 1-mm porous polyamide sheet saturated with potassium bicarbo- nate between two nickel screens. The feed gas, consisting of 4% CO2, 56% O2, and 40% N2 was fed to the cathode side, resulting in mostly CO2 with some O2 collected at the anode. Current densities of up to 25 mA cm−2 were tested, at which point the CO2 removal rate became relatively independent of current density. A follow-up study by Xiao and Li56 used the same experimental setup to separate a humidified mixture of 4.8% CO2, 17% O2, and 78.2% N2 on the cathode side with humidified N2 on the anode side and modeled its performance. Experiments found the CO2 removal rate to be independent of feed and carrier gas velocities within the range of 0–1.5 m s−1. Based on simulation and experimental results, transfer of CO2 was mainly controlled by resistances in the electrolyte solution (i.e., adsorption of CO2 at the cathode, migration as carbonates, and evolution at the anode). As described previously, electrochemically-mediated quinones can be used as redox-active CO2 carriers in an electrochemical cell configuration. Gurkan et al.26 characterized potential ionic liquids for their quinone solubility and stability in the presence of CO2. This operated through the same principle as that shown in Eq. 5. The high polarity of the examined quinone enabled more effective CO2 separation by mitigating diffusive back-transport. Watkins et al.28 investigated an electrochemical separation cell containing a quinone liquid-soaked membrane to separate CO2 from a simulated flue gas mixture. The quinones, represented as Q, were used to transfer protons in the overall reaction: 2HCO-+QH « 2CO +2HO+Q+2e- [17] 3222 leading to the net transfer of CO2 from the cathode to the anode side. Platinum was found to be the most effective catalyst in terms of CO2 transport, while palladium and ruthenium catalysts showed signifi- cant O2 due to water splitting at higher potentials. Eisaman et al.16 developed an electrochemical method of CO separation using an ion-conducting membrane. The proof-of-concept study employed either 390-μm-thick cellulose paper saturated with a cesium carbonate solution or a 500-to-600-μm-thick AEM saturated with a potassium carbonate solution. Ideal faradaic efficiencies for pure bicarbonate and pure carbonate transport are 100% and 50%, Journal of The Electrochemical Society, 2020 167 133504 2PDF Image | CO2 Separation and Transport via Electrochemical Methods
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