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30 Chapter 5. Electrochemical Studies of Non-Aqueous-based CO2 Reduction vs. Li+/Li, with stable current density of 1 mA/cm2 observed over 6 hours. Tests described here attempted to translate this setup to a 2-electrode pressurized Swagelok cell, with Pt/Teflonized carbon paper working electrode (cathode) and Mg anode. In this configuration, the inclusion of a reference electrode was not possible, so electrolysis was carried out under constant current and the working potential allowed to drift throughout the experiment; a low current density of 0.06 mA/cm2 was chosen so as not to stress the cells kinetically in these proof-of-concept studies. Representative experimental data are shown in Figure 5.1; CO2 reduction was observed under a range of conditions, confirmed by mass spectrometry (Figure 2) and quantitative analysis was carried out by evaluating the in-situ pressure change assuming ideal gas behavior and the stoichiometry of Equation 5.1 (summarized in Table 5.1 below, at 1 atm CO2): Average Electrodes Electrolyte Temperature working solvent / K potential at 50 μA / V CO2 reduction rate / mol hr-1 Faradaic efficiency (FE) / % Pt/C cathode, Mg anode MeCN 298 2.55 1.8 × 10-6 94 Pt/C cathode, Mg anode MeCN 273 3.69 1.3 × 10-6 70 Pt/C cathode, Mg anode MeCN 253 4.85 8.6 × 10-7 46 Pt/C cathode, Mg anode BTMA-TFSI 298 5.16 1.6 × 10-6 88 Mg cathode, Mg anode MeCN 298 0.90 4.6 × 10-7 25 Pt/C cathode, Mg anode MeCN, no 298 2.82 1.4 × 10-7 7 catalyst Table 5.1: Summary of CO2 reduction experiments under 1 atm CO2. Electrolytes contained 0.1 M Mg(OTf)2 and 0.5 mM catalyst (unless stated otherwise); MeCN also included 0.1 M TBA-HFP. The reaction represented by Equation 5.1 appears to proceed well at 298 K under similar conditions to those described by described by the Kubiak group (cf. Ratiff et al., 1986–1988; Lewis, 1993) (they report a FE of 98±3%; Sampson & Kubiak, 2016). A small degree of H2 evolution was also evident in the DEMS data, but typical H2 concentrations observed were <1% of the total gas volume so this is not considered significant and presumably corresponds to less-than-perfectly anhydrous conditions. Variable-temperature experiments in MeCN suggest Arrhenius behavior down to –20◦C, with decreasing FE and increasing overpotential as the temperature decreases. Interestingly, at 298 K reduction of CO2 occurs at similar rates in MeCN and BMTA-TFSI ionic liquid (although at greater overpotential in the ionic liquid); low-temperature experiments were unsuccessful for BMTA-TFSI as the pure liquid freezes at 7◦C. It was noted that the catalyst appears more stable in ionic liquid solution (by eye, discoloring in MeCN over a period of hours while persisting in BMTA-TFSI for several days). The catalytic effects of both Pt electrode and solution-phase catalyst are readily evident from Table 5.1, and this suggests that productive efforts can be made to optimize these materials in future work.PDF Image | ISRU Challenge Production of O2 and Fuel from CO2
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