Thursday, 27 July 2017
Grand Ballroom East (The Diplomat Beach Resort)
Electrocatalytic reduction of CO2 to CO have been performed in a high temperature solid oxide electrolysis cell (SOEC). Solid oxide cell was fabricated using ceramic cathode, Ce0.9Pr0.1O2-δ (PDC), infiltrated into yettria stabilized zirconia (YSZ) scaffold. Lanthanum strontium manganite (LSM) as anode was brush coated on dense YSZ electrolyte. Physical characterization of infiltrated PDC cathode was performed by X-Ray diffraction (XRD) and scanning electron microscopy (SEM). XRD pattern indicated the phase purity of PDC cathode, Figure 1(a). The porous cathode and dense electrolyte layers of thickness 600 µm and 700 µm respectively, were perfectly adhered as suggested by the SEM image of Figure 1(b). Electrochemical characterizations of SOEC were performed at 750 ºC, using impedance spectroscopy (IS) and current voltage (I-V) measurements. Open circuit impedance spectra of SOEC were obtained with varying ratio of CO2/CO gas in the inlet flow, as shown in Figure 1(c). As the CO concentration in the fuel side environment of SOEC was increased from 0% to 90%, the measured ohmic losses were observed to decrease from 3.5 to 1.5 Ωcm2. This might occurred because of the reducing environment in which Ce4+ may partially be reduced to Ce3+, resulting in mixed ion-electron conduction leading to higher ionic transport at the electrode-electrolyte interface. Similarly, the polarization resistances were decreased by increasing the CO content in reaction atmosphere (CO2/CO) on the cathode side. This might be because of the faster reaction kinetic of CO oxidation as compared to the CO2 reduction kinetics. Furthermore, open circuit potential (OCP) was observed to be increased from 0.06 V to 0.90 V as the CO2 percentage was decreased from 100 to 10%, possibly due to the same reason. Chronoamperometry (CA) were performed at certain a potential and corresponding reduction current was observed to be constant. Products of CO2 reduction reaction were analysed in gas chromatogram (GC). In order to understand the reaction mechanism of CO2 reduction on ceria, density function theory (DFT) calculations were performed to study the formation of CO from CO2 reduction on CeO2 surface. CO2 molecule adsorbed on the neighbor site of oxygen vacancy on the reduced surface, was activated and form a bent carbonate CO3δ- like structure which ultimately dissociates into CO via the incorporation of the oxygen atom into the vacancy. The activation barrier and reaction energy for this redox mechanism, calculated from DFT studies, were of 258.9 kJ/mole and 238.6 kJ/mole respectively. The effect of lateral interactions were studied by performing calculations for the same reaction step on two oxygen vacancy (di-vacancy) on 2x2 supercell unit. The activation barrier and reaction energy on a di-vacancy were significantly reduced to 134.3 and 127.3 kJ/mole respectively. Furthermore, the hydrogen atom co-adsorbed on the surface could assist the CO2 dissociation reaction. In the presence of a hydrogen atom the dissociation reaction occurs in two exothermic steps: CO2+H→COOH, COOH→CO+OH. The intrinsic activation barriers of two hydrogen assisted elementary steps were 39.0 and 47.4 kJ/mole (Figure 1D) respectively, which were significantly lower than the barrier of redox mechanism of CO2 dissociation reaction. The experimental and theoretical studies could suggest a feasibility of CO2 reduction reaction on ceria based materials.