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Figure 4. Minimum theoretical separation work vs CO2 partial pressure at T = 300K. * indicates CO2 partial pressure in air. Determined by Wmin = -RT(pbal ln(pbal) + pCO2 ln(pCO2)), where pbal represents the partial pressure of a generic balance gas. primary proton acceptor for the HOR in potassium hydroxide. Various studies have modeled the ion exchange process in alkaline membranes,76,79,85,86 and water management is a key issue.79,87 As shown in Fig. 5, AEM fuel cells have two reactions involving water compared with one for a PEM fuel cell, and sufficient water must be supplied to the cathode to produce hydroxide. A recent review by Ziv et al.88 comprehensively details the developments in under- standing the effect of CO2 on AEM fuel cells. Some studies have examined the use of carbonates to transport charge in AEM fuel cells (i.e., carbonate cycle) in contrast to the more common hydroxide.89–92 In this case, CO2 is fed intentionally on the cathode side at relatively high concentrations to maintain the membrane in (bi)-carbonate form. This potentially allows for higher power densities and improved stability in long-term operation.92 In such a configuration, an increase in the CO2 at the cathode can actually improve cell performance due to an increase in kinetic current.91 One challenge with this route is the reduced ion mobility of carbonate and bicarbonate as compared to hydroxide.80,85 Another area that requires improvement is the selection of catalysts that promote carbonate formation over hydroxide formation.91 Alkaline membranes for CO2 reduction.—A number recent of CO2 co-electrolysis studies also contain relevant information on CO2 transport in alkaline membranes. The main drawback of using liquid electrolytes instead of polymer electrolyte membranes in electro- chemical CO2 reduction is the poor solubility of CO2, which limits applicable current densities and potential scale-up. Polymer electro- lyte membranes have lower gas permeability than electrolyte- impregnated porous media, and thus can be made thinner to reduce Journal of The Electrochemical Society, 2020 167 133504 Figure 5. Schematic of (a) AEM and (b) PEM fuel cell operation. Electro-osmotic drag in the AEM fuel cell occurs in the reverse direction of a PEM fuel cell. study determined that at current densities greater than 500 mA cm−2, the carbonate builds up in the membrane at the anode in an enrichment zone less than 2 μm thick that is critical to transport and release of CO2. Other studies found that the ability to flush (bi)- carbonate from the anode side efficiently as CO2 is a key challenge.74,79 The buildup of carbonate at the anode catalyst layer has also been identified as a source of the high anode overpotential in anion-exchange membrane fuel cells.75,76,80,81 While the intro- duction of CO2 on the cathode side leads to a clear degradation in performance of the cell due to increased ionic and reaction resistance,82 experiments show that the effect is at least partially reversible.75 Zheng et al.83 demonstrated that reducing the cathode gas flowrate and increasing hydration can reduce CO2-related voltage losses in AEM fuel cells. St. John et al.84 investigated the effects of carbonate on the oxygen reduction reaction (ORR) and the HOR for ruthenium/platinum catalysts in alkaline fuel cells. For the HOR and ORR on platinum, authors observed no dependence on the presence of carbonate, indicating the role of water/hydroxide as the ohmic losses.93 Indeed, faradaic efficiencies of CO of >90% have been reported in literature for alkaline membrane CO2-splitting.94–96 Liu et al.94 performed a CO2-electrolysis study using imidazolium- functionalized polymer membranes with silver catalyst at the cathode and iridium dioxide catalyst at the anode, and achieved very high CO selectivity (>95%) and current densities up to 600 mA cm−2. A potassium bicarbonate solution was fed to the anode to assist ion conduction, and water management was identified as a key issue in effective operation. The ratio of CO2:O2 at the anode was close to 2, suggesting carbonate as the charge carrier. Yin et al.96 reported on a cationic polymer membrane co-electrolyzer with gold catalyst at the cathode and iridium dioxide catalyst at the anode that operated with pure water. Unlike the setup by Liu et al.,94 no alkaline solution was fed during operation, but the duration of the experiment was limited to 100 h as opposed to 4000 h. The cell was capable of maintaining current densities of 500 mA cm−2 and CO2 crossover was low (<1 μl min−1·cm−2), suggesting that hydroxide was the main charge carrier.PDF Image | CO2 Separation and Transport via Electrochemical Methods
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