Mechanistic Insights and Electrochemical Performance of Ceria Based Materials for Solid Oxide Electrolysis of CO2
Authors: Kumari Neetu , Haider M Ali , Sinha Nishant , Basu Suddhasatwa 
Published: February 2020

Electrocatalytic reduction of CO2 to CO in a high temperature solid oxide electrolysis cell (SOEC) was performed. Gadolinium doped ceria (GDC), was used as an electrocatalyst for CO2 reduction in a SOEC having yttria stabilized zirconia (YSZ) as the electrolyte and lanthanum strontium manganite (LSM) as the oxygen electrode. Physical characterization of CuO/GDC cathode was performed by X-Ray diffraction (XRD), Figure 1(A), and scanning electron microscopy (SEM). Electrochemical impedance spectroscopy (EIS) was utilized to determine the ohmic and polarization losses associated with electrolyte and electrode components of SOEC. Open circuit impedance spectra of GDC cathode in SOEC were obtained at 1023 K with varying ratio of CO2/CO gas in the inlet flow, as shown in Figure 1(B). As the CO2 concentration in the fuel side environment of SOEC was decreased from 100% to 10%, the measured ohmic losses were observed to decrease from 47 to 35 Ωcm2. This might occurred because of the reducing environment in which Ce4+ is partially reduced to Ce3+, resulting in mixed ion-electron conduction leading to higher ionic transport at the electrode-electrolyte interface. Furthermore, open circuit potential (OCP) was observed to be increased from 0.11 V to 0.90 V as the CO2 percentage was decreased from 100% to 90%, possibly due to the same reason. 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 and methanol from CO2 reduction on CeO2(110) surface. CO2 molecule sitting in the vicinity of oxygen vacancy site on the surface, is activated to form bent carbonate CO3
δ- species which dissociates into CO via the incorporation of the oxygen atom into the vacancy. The calculated activation barrier and reaction energy for this redox reaction is 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. DFT calculations showed that the hydrogen atom co-adsorbed on the surface could further 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 adsorbed CO2 or CO could hydrogenate to methanol via formate (HCOO) or carboxyl (COOH) mediated mechanisms. The formate intermediate, produced by the hydrogenation of CO2 (CO2+H→ HCOO), was observed to be more stable with a binding energy of -222.9 kJ/mole on the stoichiometric ceria surface as compared to the carboxyl intermediate Ebinding = -36.0 kJ/mole[1]. Therefore, HCOO is likely to act as a spectator and may not participate in further hydrogenation reaction. Alternatively, carboxyl mediated reaction route involves exothermic reaction steps except only to the dissociation of COOH to CO and OH which was calculated to be endothermic with a reaction energy of 5.0 and 24.4 kJ/mole on stoichiometric and reduced ceria surface respectively. The intrinsic activation barriers of all steps involved in the carboxyl mechanism were calculated and are shown in Figure 1(C). COOH dissociation step was calculated to be of maximum activation barrier (126 kJ/mole) and is likely to be rate determining. The activation energy of this step was calculated to be lowered by ~50kJ/mole on reduced ceria surface. Ceria based materials have been suggested to possess electrocatalytic activity for CO2 reduction which can be further improved by aliovalent metal dopants. Surface of ceria doped with aliovalent dopants such as gadolinium (Gd), praseodymium (Pr) and samarium (Sm), have been suggested to of mixed ionic and electronic conductive nature. Classical molecular dynamic simulations were utilized to determine the oxide ion diffusivity (D) in the Gd doped ceria (GDC) at different temperatures (Figure 1(D) and Table 1). The calculated diffusivity was of 1.15x10-7 cm2s-1 at 1073 K. The activation energy of oxide ion diffusion in GDC was estimated to be of 41.0 kJ/mole. Experimental and theoretical results thus suggest the feasibility of CO2 reduction reaction on ceria surface. Combined with high catalytic activity and fast oxide ion transport, ceria based materials could be a potential candidate for electrocatalytic reduction of CO2.    

Reference

1.  Kumari, N., Sinha, N., Haider, M. A. & Basu, S. CO2 reduction to Methanol on CeO2 (110) Surface: a density functional theory study. Electrochimica Acta 177 (2015) 21–29

Figure 1