Oxide ion transport in Lanthanum-rich apatite and melilite structures

Schultze, Tim Konrad; Martin, Manfred (Thesis advisor); Lüchow, Arne (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022

Abstract

Lanthanum-rich apatite and melilite structures are highly conductive oxygen ion solidstate electrolyte materials at intermediate temperatures (500 − 800 ◦C), which creates the opportunity for lower operation temperatures in energy conversion units. This work is aiming at a deeper understanding of the interplay between structure, composition and ion migration, in order to improve the oxygen ion conductivity of the mentioned materials and transfer the gained knowledge to other structures. Apatites: To give answers to questions regarding the influence of oxygen content and doping on the migration in apatite materials, the compositions La10Si18O27, La9.33Si6O26, La8B2Si6O26 and La8Sr2Si6-yXyO26 were considered (B = Mg, Ca, Sr, Ba). Migration was investigated for interstitialcy and vacancy mechanisms along the highly conductive La-tunnels along the c-axis, applying density functional theory. Our calculations show that the interstitialcy mechanism is the predominant migration mechanism for all compositions with a minimum in migration barrier for Sr doping. Migration barriers along the ab-plane, between the hexagonal La-tunnels, were discovered as the rate determining step for oxygen ion conductivity in lanthanum apatites and further investigated. To improve migration along the ab-plane Si-site doping was considered for the composition La8Sr2Si6-yXyO26 with X = Al, Fe, Ga, Ge, In, Mg. Out of all dopants, Al and Fe showed a decrease in the migration barriers in ab-direction, while increasing barriers in c-direction. Experimental investigations with electrochemical impedance spectroscopycould partially confirm a beneficial trend for Sr doping, Fe doping and co-doping. Furthermore, high defect formation energies were determined, which render formation of defects by thermal excitation unlikely. Instead, it is assumed, that the concentration and type of defect is entirely depended on the composition.Melilites: With density functional theory, different migration mechanisms such as vacancy and interstitial were tested against the proposed interstitialcy mechanism. Our calculations conclude that only the interstitialcy mechanism can explain high oxygen ion conductivity. Furthermore, the influence of the La/Sr ratio in La1+xSr1-xGa3O7+0.5x was analysed with regards to migration in order to build an energy model for kinetic Monte Carlo simulations. Overall, La coordination of oxygen interstitial sites is more stable compared to Sr coordination, due to trapping of oxygen ion interstitials by Sr. Two migration paths were identified and a comprehensive energy model for site and migration energies was developed. Applying kinetic Monte Carlo simulations, we found an increase of oxygen ion conductivity and decrease of activation energy, which is in good accordance with experimental data from literature. Subsequently, various Ga-site dopants were investigated on its effect on the oxygen ion conductivity in La1.5Sr0.5Ga3-yXyO7.25 (X = B, Al, In, Si, Sc, Zn). The density functional theory analysis of site and migration energies for the doped systems identified only Al as a dopant with beneficial behaviour. The existing energy model was augmented and subsequent kinetic Monte Carlo simulations revealed a slight increase in conductivity and decrease in activation energy with increasing dopant fraction of Al. Experimental investigations with electrochemical impedance spectroscopy could partially confirm a beneficial trend for Al doping. Furthermore, high defect formation energies were determined, which render formation of defects by thermal excitation unlikely. Instead, it is assumed, that the concentration and type of defect is entirely dependent on the composition.

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