CO2 Reduction to CO and Methanol in Solid Oxide Electrolysis Cell:
Experimental and DFT Studies
Neetu Kumaria, M. Ali Haidera, Pankaj Tiwaria, Nishant Sinhab and S. Basua
a
b
Indian Institute of Technology, Delhi, India, 110016
Dassault Systemes, Galleria Commercial Tower, 23 Old Airport Road, Bangalore 560008
Corresponding Author e-mail ID: [email protected]
Abstract
Carbon dioxide (CO2) utilization as a potential feedstock to produce renewable fuels, like carbon
monoxide, methanol, methane, formic acid etc., is limited by thermodynamic stability of this linear
molecule and sluggish reaction kinetics of CO2 reduction. Reactions involving the reduction of
CO2 to hydrocarbon molecule are endogenic. Electrochemical route is being explored where
endogenic reaction step can be tuned with applied electric potential. In order to improve the
kinetics, high temperature solid oxide electrolysis (SOE) of CO 2 was the suggested technology.
A solid oxide electrolysis cell (SOEC) was fabricated with YSZ as an electrolyte and LSM as
anode material. Cathode material of CuO-GDC-YSZ composite, was utilized for CO2 reduction
reaction in high temperature SOEC. Physical characterization of CuO-GDC-YSZ cathode was
performed by X-Ray diffraction (XRD), and scanning electron microscopy (SEM). Electrochemical
characterizations of SOEC were performed at 750 ºC and 800 ºC, using impedance spectroscopy
(IS) and current voltage (I-V) measurements in different ratio of CO2/CO. The ohmic as well as
polarization losses were observed to be reduced by increasing the percentage of CO in the
reaction atmosphere (CO2/CO) on the cathode material. This might be because of the faster
reaction kinetic of CO oxidation as compared to the CO2 reduction kinetics. As we increase the
CO percentage in from 0% to 90% in inlet gas the open circuit voltage were increased from 0.11
V to 0.98 V respectively. Chrono amperometry (CA) were performed at certain potential and
corresponding reduction current was observed to be constant. Products of CO2 reduction reaction
were analysed in gas chromatogram (GC). CO2 reduction was studied in presence water. Due to
co-electrolysis of CO2 and H2O some hydrocarbon products like CH4, methanol were expected to
formed on the cathode side of cell. 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 CO 3δspecies 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 ΔEbinding = -36.0 kJ/mole.
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 on stoichiometric and reduced ceria surface. While on the stoichiometric surface,
COOH dissociation COOH→CO+OH (ΔEact = 55.6 kJ/mole, ΔH = 5.7 kJ/mole) is likely to be
difficult as compared to rest of the elementary steps, whereas on the reduced surface the
energetics of the same step were significantly lowered (ΔEact = 47.4 kJ/mole, ΔH = -80.4 kJ/mole).
In comparison, hydrogenation of methoxy, H3CO+H→H3COH, appears to be relatively difficult
(ΔEact = 58.7 kJ/mole) on the reduced 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 have mixed ionic-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. 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.