DFT Study of Oxygen Dissociation in Molten Carbonate

Abstract

Using density functional theory method, we have studied the oxygen dissociation in alkali molten carbonate at the B3LYP/6-31G(d) level. The calculated energies were then verified by MP4 and CCSD(T). A four-formula cluster (M2CO3)4, M = Li, Na, and K was used to describe the molten carbonate. It was found that the adsorption of oxygen to molten carbonate is of a chemical type and leads to the formation of CO5(2-) in MC, which was confirmed for the first time by DFT calculations. The energy barrier for its dissociation is calculated to be 197.9, 116.7, and 170.3 kJ/mol in the (M2CO3)4 cluster, M = Li, Na, and K, respectively. If the reaction of O2 + 2CO3(2-) → 2CO4(2-) is approximated as a one-step reaction, the activation energy is estimated to be 96.2, 15.1, and 68.6 kJ/mol, respectively. The reaction rate is first order to the pressure of oxygen. Surprisingly, the reaction of oxygen dissociation has the lowest energy barrier in sodium carbonate, which is consistent with the recent experimental findings. It is very clear that the molten carbonate salt has directly participated in the ORR process and plays an important role as a catalyst in the cathode of SOFCs. The oxygen reduction has been facilitated by MC and enhanced cell performance has been observed.

Conduction Mechanisms in Oxide–Carbonate Electrolytes for SOFC: Highlighting the Role of the Interface from First-Principles Modeling

“…This clearly indicates that this process in the composite is possible and that it can further be boosted when taking place in the boundary layers between the two phases. In addition, it should be noted that computed barriers in the carbonate phase (0.95 eV) are very close to the values already reported by Lei et al 46 in case of oxygen migration in molten Li 2 CO 3 , Na 2 CO 3 , and K 2 CO 3 (∼1 eV), but that the lowest values are obtained when migration takes place at the interface (0.74 eV), outlining again the key role of the interface in diffusion processes.…”

“…Finally, it is also worth considering the properties of O atom migration at the oxide–carbonate interface because carbonates may enhance the oxygen reduction process in SOFC through a particular mechanism for O migration in the carbonate phase. The migration species could be CO 4 2– ions, whereas the transport mechanism is a cooperative “cogwheel” (or “paddlewheel”) mechanism, involving the breaking and reforming of O–CO 3 2– bonds − (see Scheme -B). It was also suggested that this process can be relevant in the case of SOFCs with composite carbonate–oxide electrolytes .…”

Density functional theory study of oxygen migration in molten carbonate

“…The structures were optimized at the B3LYP/6-31G (d) level and showed very good agreement with the previous report in Ref. [18]. The structure of each transition state was initially indentified using PES scan method, i.e.…”

Role of carbonate phase in ceria-carbonate composite for low temperature solid oxide fuel cells: A review

“…Similar to Zhao's study [89][90][91], Lei et al [113,114] thought that the carbonates are mainly proton conduction sources by formation of intermediate product of HCO 3 À in the ceria-carbonate in fuel cell conduction. Therefore, they used a first principles approach to examine the energetics of H + transfer in CO 3 2À , Li 2 CO 3 crystals and (Li 2 CO 3 ) 8 clusters, in which (Li 2 CO 3 ) 8 cluster was chosen to represent the state of an alkali molten carbonate in normal fuel cell condition.…”

“…Oxygen adsorption on the molten carbonate and charge transfer at the molten carbonate/LSCF interface provides additional ORR pathway, therefore, significantly improves the electrode activity and fuel cell performance. The same group also did a detailed analysis of the oxygen dissociation and migration process in molten carbonate by DFT method . They found that Na 2 CO 3 presented the lowest energy barriers for oxygen dissociation and the lowest activation energy for the above Eq.…”