Density functional theory study of oxygen migration in molten carbonate

•The process of oxygen migration in MC has been examined using DFT.•A cooperative “cogwheel” mechanism is proposed.•The lower energy barriers indicate that oxygen transfer in MC is fairly easy.•The results imply that ORR in MC modified cathodes is facilitated.

The process of oxygen migration in alkali molten carbonate salts has been examined using density functional theory method. All geometries were optimized at the B3LYP/6-31G(d) level, while single point energy corrections were performed using MP4 and CCSD(T). At TS, a O–O–O linkage is formed and O–O bond forming and breaking is concerted. A cooperative “cogwheel” mechanism as described in the equation of CO42−+CO32−→CO32−⋯O⋯CO32−→CO32−+CO42− is involved. The energy barrier is calculated to be 103.0, 136.3 and 127.9 kJ/mol through an intra-carbonate pathway in lithium, sodium and potassium carbonate, respectively. The reliability and accuracy of B3LYP/6-31G(d) were confirmed by CCSD(T). The calculated low values of activation energy indicate that the oxygen transfer in molten carbonate salts is fairly easy. In addition, it is found that lithium carbonate is not only a favorable molten carbonate salt for better cathode kinetics, but also it is widely used for reducing the melting point of Li/Na and Li/K eutectic MC mixtures. The current results imply that the process of oxygen reduction in MC modified cathodes is facilitated by the presence of MC, resulting in an enhancement of cell performance at low operating temperatures.

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