Proton Transfer in Molten Lithium Carbonate: Mechanism and Kinetics by Density Functional Theory Calculations

Xueling Lei,Changyong Qin,Kevin Huang

doi:10.1038/s41598-017-07726-3

Xueling Lei, Changyong Qin + Show 1 more

Open Access

https://doi.org/10.1038/s41598-017-07726-3

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Abstract

Using static and dynamic density functional theory (DFT) methods with a cluster model of [(Li2CO3)8H]+, the mechanism and kinetics of proton transfer in lithium molten carbonate (MC) were investigated. The migration of proton prefers an inter-carbonate pathway with an energy barrier of 8.0 kcal/mol at the B3LYP/6-31 G(d,p) level, which is in good agreement with the value of 7.6 kcal/mol and 7.5 kcal/mol from experiment and FPMD simulation, respectively. At transition state (TS), a linkage of O–H–O involving O 2p and H 1 s orbitals is formed between two carbonate ions. The calculated trajectory of H indicates that proton has a good mobility in MC, oxygen can rotate around carbon to facilitate the proton migration, while the movement of carbon is very limited. Small variations on geometry and atomic charge were detected on the carbonate ions, implying that the proton migration is a synergetic process and the whole carbonate structure is actively involved. Overall, the calculated results indicate that MC exhibits a low energy barrier for proton conduction in IT-SOFCs.

Highlights

Yttrium-doped barium zirconate (BZY) is one of the promising proton conducting electrolytes for intermediate temperatures (IT) SOFCs due to its excellent chemical stability and good bulk proton conductivity
The migration energy barrier of 8.0 kcal/mol for local structure extracted from the (Li2CO3)[8] cluster is consistent with the 7.5 kcal/mol obtained from ab initio molecular dynamic simulations
This result is excellent in line with the experimental value of 7.6 kcal/mol observed in a molten carbonate (MC)/BZY composition at 600 °C

Summary

OPEN Proton Transfer in Molten Lithium

Using static and dynamic density functional theory (DFT) methods with a cluster model of [(Li2CO3)8H]+, the mechanism and kinetics of proton transfer in lithium molten carbonate (MC) were investigated. Have been previously reported by other groups and largely enhanced cell performance was observed Such composite electrolytes are easy to fabricate with low cost, which will open a door for developing novel electrolytes for IT-SOFCs. In addition, it is important to point out that the contribution from hydroxide ion (OH−) to the proton conductivity was not considered here, while this was reported to be noticeable in ref. For proton conduction in BZY/MC as described in Eqn (1), the protons produced by surface defect reactions were transferred to the neighboring carbonate-ions (CO32−) at the BZY/MC interface to form HCO3− They will be transported inside of the MC phase. The proton transfer process is local and the reaction is usually at high temperature in a molten carbonate salt, both of which will bring detriments to the results from static DFT calculations in the gas phase using a limited size of cluster model. Long range interactions in the molecular system were estimated by CAM-B3LYP46, but no significant difference was observed

Results and Discussion

Conclusions

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