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Fig. 3 CO2-from-seawater prototype performance. (a) Normalized current density (total current per total active membrane area per acid solution flow rate in units of mA cm2 lpm1), (b) Efficiency of CO2 extraction, (c) CO2 flow rate (slpm), and (d) Energy (kJ mol1 (CO2)) as functions of the pH of the output acid solution for various solutions and flow rates. polarization of 0.83 V per BPM is required to induce significant water dissociation at room temperature, with higher voltages required at higher current densities.27 From Fig. 2b, we see that the dissociation of water in the BPMs results in acidification (from H+ ions) and basification (from OH ions) of alternating compartments in the BPMED membrane stack. Adjacent to each BPM is an AEM that allows anions in the input seawater (primarily Cl ions) to be transported across the membrane towards the positively-charged anode. In this way, the combined action of the BPMs and AEMs under the applied voltage is to add H+ ions and Cl ions to the acidified compartments, and to replace Cl ions with OH ions in the basified compartments. At each end of the membrane stack is a CEM that separates the alternating stack of BPMs and AEMs from the electrode compartments. As shown in Fig. 2b, this means that Na+ and other cations from the anode compartment are transported across a CEM from the anode compartment into the adjacent base compartment, and that Na+ and other cations are trans- ported across a CEM from the acid compartment adjacent to the cathode compartment into the cathode compartment. As is well known, increasing the H+ ion concentration of seawater shifts the equilibrium of the carbonate buffer system (via the reactions CO32 + 2H+ $ HCO3 + H+ $ CO2 + H2O) such that most of the DIC is in the form of dissolved CO2 for pH values less than 6 (compared to natural seawater with pH values of 8.1–8.3, for which most of the DIC is in the form of HCO3 ions). Using standard equilibrium relations for seawater, one can show that at a pH of 6, 5, and 4, approximately 50%, 91% and 99% of the DIC is in the form of dissolved CO2.14,15 Given the concentration of DIC in seawater (typically 2.2–2.5 mM),14 the equilibrium between CO2 gas dissolved in the acidified seawater and directly above the acidified solution means that the partial pressure of CO2 gas above the solution must be less than 0.08 atm to allow continuous extraction of the dissolved CO2. For this reason (assuming pure CO2 gas is desired as output, thus elimi- nating the possible use of a sweep gas to extract the CO2 from solution), we use the membrane contactors and vacuum pump shown in Fig. 2 to vacuum strip the CO2 from the acidified solution. It is important to note that the extraction of CO2 from seawater described in this manuscript works in a fundamentally different manner than the extraction of CO2 from KHCO3/ K2CO3 solutions previously investigated in our lab.3,4 The previous work on the extraction of CO2 from KHCO3/K2CO3 solutions was performed for solutions with concentrations in the range of 0.125M–2M, for which HCO3 and CO32 were the primary anions present in solution. In these experiments, CO2 was extracted by transporting HCO3 ions and CO32 ions across AEMs into adjacent solutions that were acidified via the opera- tion of BPMs, with the HCO3 and CO32 ions effectively acting View Online Energy Environ. Sci. This journal is a The Royal Society of Chemistry 2012 Downloaded by University of Oxford on 26 March 2012 Published on 06 February 2012 on http://pubs.rsc.org | doi:10.1039/C2EE03393CPDF Image | Co2 Extration from Seawater
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