Ion conductivity in La2Hf2O7 and La2Mo2O9 as potential good electrolyte in SOFCs

What is it about?

This work aims to unravel the generation of ion conductivity of two potential electrolyte materials in SOFCs including La2Hf2O7 and La2Mo2O9, that caused by the intrinsic defects of materials. We utilize the computational calculation method to simulate material structures and study their electrical and electronic properties especially induced by one of the most important intrinsic defect oxygen anion Frenkel pair defect. A-Fr defect is pair defect consist one vacancy and one interstitial atom, in which the migration of the interstitial atom determines the ion conductivity of the electrolyte. In La2Hf2O7, we have find three migration sites for the interstitial atom formed by a-Fr pair defect in different position in the lattice. These migration sites share close energy barrier for oxygen atom and they can compose a continuous migration path throughout the electrolyte that result in good ion conductivity of La2Hf2O7. La2Mo2O9 shows a little bit different mechanism. The structure of La2Mo2O9 shows a low symmetry of crystal structure with abundant vacancies and intrinsic mobile oxygen atoms. Therefore, the ion conductivity originates from the short-range disorder in lattice with assistance of a-Fr pair defects. In addition, we apply thermodynamics calculation to predict the working temperature of La2Hf2O7 and La2Mo2O9, which supports previous experiment results. It is also proposed that La2Mo2O9 has a lower working temperature than La2Hf2O7 due to the more possible migration paths based on the highly mobile structure of La2Mo2O9 with low symmetry.

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Why is it important?

The fossil fuels are still the main the power supply in present earth, which are widely applied in the daily life and consuming very quickly. Due to the serious air pollution caused by fossil fuels, new energy supply system with low air pollution and high energy density are the urgent problem that need to be solved. Under the demand for new green power supply system, solid oxide fuel cells(SOFCs) attract scientists’ attention based on simple chemical reaction with high energy efficiency, which is environmentally friendly with no air pollution. To actualize a high performance SOFCs, the choice of electrolyte, which should be highly ion conductive with high thermal stability, is dominating factor. To screen out more potential electrolyte candidates, comprehension of the generation of ion conductivity is very significant and beneficial. Ion conductivity means ion transportation in lattice and the migration path is usually supplied by the intrinsic defects of materials. Previous works have proved that a-Fr pair defect is important in affecting ion conductivity, which will be our main focus in this work. By understanding the mechanism of how a-Fr pair defect determine the ion conductivity, it will be easier to guide further material design and synthesis. Moreover, the working temperature prediction will also help us to screen out different appropriate electrolyte materials in various temperature range. In conclusion, our detailed work supplies mechanism of generation of ion conductivity of materials in novel microscopic aspect, which is important for understanding working mechanism of electrolyte materials and beneficial for discovering more potential materials for next generation high performance SOFCs.

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