Investigation of structure and conductivity of LaNb0.9Mo0.1O4.05 based materials
Author(s)
Han, Yidong
Type
Thesis
Abstract
In the search for electrolyte materials for solid oxide fuel cells (SOFCs), LaNbO4 based materials have attracted great attention due to their competitive ionic conductivity and good chemical stability. LaNbO4 with acceptor substituents (Ca/Sr) on La sites exhibits pure proton conductivity, which can be maintained to high temperatures across a wide range of oxygen partial pressures under humidified conditions. However, the conductivity is hindered by the low solubility of the substituents on La sites and a transition from monoclinic phase to tetragonal phase is unavoidable at intermediate temperatures, leading to potential mechanical failures of the devices. Another system, LaNbO4 with donor substituents (W/Mo) on Nb sites, possessing modulated structures, avoids those two drawbacks and exhibits pure oxygen ion conductivities. However, the effects of modulated structures on their electrochemical performances are still unclear.
This work shows that Mo substituted LaNbO4 possesses a 2D incommensurately modulated structure and the modulated structures are eliminated by slightly substituting Gd or W on La or Nb sites, respectively. The modulated structures have negative effects on the electrochemical performance, as the activation energy of an unmodulated structure is about 0.9 eV lower than that of a modulated one. With increasing the content of substituents, either the increased porosity or the emergence of a secondary phase decreases the conductivities.
This work also investigates the stability of LaNb0.9Mo0.1O4.05 under different atmospheres. The results show that the oxidation state of Mo is stable under oxygen, air and nitrogen; however, under vacuum or hydrogen, Mo is reduced and the structure of LaNb0.9Mo0.1O4.05 changes. X-ray absorption spectroscopy results show that these changes are accompanied by changes in the coordination environments of Nb sites. The electrochemical impedance spectroscopy data show that LaNb0.9Mo0.1O4.05 annealed under hydrogen exhibits proton and electronic conductivity, which could be utilised as the anode for SOFCs.
This work shows that Mo substituted LaNbO4 possesses a 2D incommensurately modulated structure and the modulated structures are eliminated by slightly substituting Gd or W on La or Nb sites, respectively. The modulated structures have negative effects on the electrochemical performance, as the activation energy of an unmodulated structure is about 0.9 eV lower than that of a modulated one. With increasing the content of substituents, either the increased porosity or the emergence of a secondary phase decreases the conductivities.
This work also investigates the stability of LaNb0.9Mo0.1O4.05 under different atmospheres. The results show that the oxidation state of Mo is stable under oxygen, air and nitrogen; however, under vacuum or hydrogen, Mo is reduced and the structure of LaNb0.9Mo0.1O4.05 changes. X-ray absorption spectroscopy results show that these changes are accompanied by changes in the coordination environments of Nb sites. The electrochemical impedance spectroscopy data show that LaNb0.9Mo0.1O4.05 annealed under hydrogen exhibits proton and electronic conductivity, which could be utilised as the anode for SOFCs.
Version
Open Access
Date Issued
2024-08-22
Date Awarded
01/12/2024
License URL
Advisor
Skinner, Stephen
Sengodan, Sivaprakash
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)