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5.4 Process B: Oxidation of CO32- to O2 and CO2 31 Variable-pressure investigations were also carried out (at 0.1 psi CO2, 298 K). Unfortunately, a pressure increase was observed during the experiments (inconsistent with Equation 5.1) using either MeCN or BMTA-TFSI solvent; this is attributed to electrolyte decomposition occurring (generating H2) instead of CO2 reduction, suggesting that the concentration of CO2 in electrolyte solution is not sufficient for catalytic reaction at this lower pressure. Tafel experiments (at 50 μV/s) on Pt/C-Mg cells with MeCN electrolyte at 298 K indicated this cell configuration could operate at around 0.6 mA/cm2 before significant non-linear overpotential response (i.e., about twice the current density used in the CO2 reduction experiments described here, roughly consistent with the electrolysis current density observed for constant potential electrolysis in the different cell configuration used in the literature). 5.4 Process B: Oxidation of CO32- to O2 and CO2 In order for O2 generation to occur as anticipated by Equation 5.1, an anhydrous source of CO32- ions in solution is required (as, presumably, water oxidation will take preference in aqueous systems). This is not a trivial pre-requisite as simple carbonate salts are only sparingly soluble in common organic solvents (e.g., MeCN). Initial efforts therefore focused on screening ionic liquids with carbonate salts of lithium, sodium and cesium. In general, solubility of these salts appeared to be less than 0.1 M in all ionic liquids attempted and did not vary much with the cation, despite reports in the literature of high solubility for Cs2CO3 in certain ionic liquids (Jorapur & Chi, 2005). Solubility was notably increased for Li2CO3 (although still saturated around 0.1 M) upon addition of around 0.1 M trimethylneo-pentylammonium fluoride, suggesting that the salt metathesis product (i.e., trimethylneo-pentylammonium carbonate) has higher solubility than the alkali salt. Hence, ammonium carbonates might be a better choice as a carbonate source (and, hence, alkylammonium cations as spectator ions in a full electrochemical cell), however these are not readily available commercially. In general, linear alkylammonium-containing ionic liquids (i.e., BTMA-TFSI and DEMA-OTf) had greater solubility for carbonate salts than those with cyclic cations; DEMA-OTf (with an N-H containing cation) displayed the greatest solubility behavior of all, potentially indicating a preferential interaction between this H-bond donor cation and the CO32- anion. An alternative ionic liquid incorporating the choline cation (with a pendant O-H group) was investigated in this context and did, indeed, appear to have similarly improved solvating properties. However, the relatively high melting point (20◦C) limits its practical use as a solvent, thus requiring blending with DMA for testing. Carbon and LTO electrode exhibited poor wetting with BTMA-TFSI solvent so this was blended with MeCN for testing. Attempted O2-evolution cell tests are summarized in Table 5.2 below.PDF Image | ISRU Challenge Production of O2 and Fuel from CO2
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