The aim of this thesis has been to explore the potential of aqueous immobilized K2CO3 as a possible electrolyte for co-electrolysis of CO2 and water at approx. 200 °C. This has been done by exploring the properties of pure K2CO3 (aq) and immobilized K2CO3 (aq) as well as the properties of the matrix materials SrTiO3 and TiO2. Conductivity measurements of aqueous K2CO3 have been performed using the van der Pauw technique and a specially designed sample holder with Pt wires as electrodes. The conductivity at ambient temperature was found to increase with concentration up to 30-40 wt% K2CO3 (aq), and decline at higher concentrations. Furthermore, the conductivity of 5-30 wt% aqueous K2CO3 was measured up to 180-200 °C at 30 bar. The highest conductivity measured was 1.34 S/cm at 172 °C for 30 wt% K2CO3(aq). The conductivity was found to increase with concentration and also temperature for 5-15 wt% K2CO3 (aq). For 20 and 30 wt% K2CO3(aq) the conductivity increased with temperature up to 150 and 180 °C, respectively, and then a drop was observed, due to precipitation. The activation energy was obtained by fitting the data to a modified Arrhenius equation and was found to be in the range 0.14-0.17 eV. Furthermore, the activation energy was found not to be concentration dependent. Thermodynamic equilibrium calculations were conducted using the program FactSage. The calculated concentration limits agreed with the observed experimental limits. It was found that in CO2 atmosphere the main species in a 10 wt% K2CO3 (aq) solution are K+ and HCO3-. The water partial pressure as well as the amount of water vapour at different temperatures, pressures and K2CO3 (aq) concentrations was also calculated using FactSage. K2CO3 (aq) was immobilized in both SrTiO3 and TiO2. It was found that a loss of conductivity occurred when K2CO3 (aq) was immobilized, however, by increasing the porosity of the matrix the conductivity could be increased and the loss decreased. Archie’s law, with a cementation exponent of 1.6, was found to give a very good fit of the measured conductivity as a function of open porosity at ambient temperature. The conductivity of K2CO3 (aq) immobilized in SrTiO3 or TiO2 was measured at both ambient and elevated temperatures and pressures. It was found to increase with temperature and concentration unless precipitation occurred. The highest conductivity value measured was 0.21 S/cm at 130 °C for 30 wt% K2CO3 (aq) immobilized in SrTiO3, the same value was also obtained for 10 wt% K2CO3 (aq) immobilized in SrTiO3 at 175 °C. The highest conductivity value of K2CO3 (aq) immobilized in TiO2 was 0.19 S/cm at 210 °C for 20 wt% K2CO3 (aq). The activation energy of K2CO3(aq) immobilized in TiO2 or SrTiO3 was found to be in the same range as the activation energy of pure K2CO3 (aq) and was not concentration dependent. For measurements in CO2 atmosphere only TiO2 could be used without any problems. Using SrTiO3 resulted in undesired formation of the insulator SrCO3. No loss of conductivity was observed for 10 wt% K2CO3 immobilized in TiO2 when changing the atmosphere from N2 to CO2. K2CO3 (aq) immobilized in TiO2 shows good promise as a potential electrolyte for co-electrolysis of CO2 and water at 200 °C.
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Tullmar, Peter Blennov, Holtappels, Peter, Bonanos, Nikolaos
Department of Energy Conversion and Storage, Technical University of Denmark, 2013