Ions Doping And Performance Study Of High Temperature Non-perovskite Structure Proton Conductors

High temperature proton conductor (HTPC) can be used in many fields, such as the electrolyte for the solid oxide fuel cells, materials for the hydrogen permeation, hydrogen sensors, due to its excellent electrical properties. According to the crystal structure, the HTPC can be divided into perovskite and non-perovskite structure HTPC. Perovskite structure HTPC exhibits high electrical conductivity, but poor stability in H2O or CO2 containing atmosphere, or bad sintering property. Non-perovskite structure HTPC shows quite high stability in H2O or CO2 containing atmosphere, together with the relatively low electrical conductivity. Thus in order to increase the conductivity of non-perovskite HTPC such as LaNbO4 and La27WsO55.5-?, the ionic doping treatment will be adopted. The corresponding unitary and binary ionic doping of LaNbO4 and La27WsO55.5-? are studied and investigated to reveal the effect on the conductivity. The related conductivity enhancement mechanism is also discussed based on the results.Unitary ionic doping starting with ionics of the lower valence, equal valence or high valence is used to improve the performance of LaNbO4, and the effect of doping is investigated. The lower valence ionic as Zn can improve the conductivity of LaNbO4 obviously, but the solubility of Zn in LaNbO4 is quite low, and thus the influence on the crystal structure of LaNbO4 is little. A insulating second phase Zn2Nb2O7 will be formed when the Zn concentration excesses its solubility (1 mol.%), which lowers the conductivity and grain sizes. Ionics of the lanthanide elements as Sm, Gd, and Yb have the valence of+2 and+3, which can introduce the oxygen vacancies into the lattice of LaNbO4 and improve the conductivity, but the effect on the structure and conductivity is limited. Mo ionic with the valence as+6 shows high solubility in LaNbO4 and apparent effect on the crystal structure. The tetragonal structure can be stabilized at room temperature, and the conductivity is remarkably increased when 20 mol.% Mo is doped into LaNbO4. But the stability is poor in the reducing atmosphere for Mo doped LaNbO4.The binary ionic co-doping is also attempted to improve the performance of LaNbO4. The results confirm that Ca and Sm co-doping can increase the conductivity and phase transformation temperature of LaNbO4. However the conductivity decreases with the increasing Sm contents, as the lattice parameters decrease by increasing Sm contents. Ce and Yb co-doping can increase the electron and ion conductivity of LaNbO4, which increases with the concentration of Ce and Yb roughly. The stability of Mo doped LaNbO4 can be improved obviously by co-doping with elements such as Si, Ti, Zr, W, which also lowers the conductivity. The solubility of Si, Ti, Zr in LaNbO4 is increased obviously by co-doping with Mo, indicating that Mo exhibits major effect on the crystal structure of LaNbO4. The results indicate that lowering the concentration or preventing the reduction of Mo is the main reason for the improvement of the stability of Mo doped LaNbO4.The water uptake and electrical conductivity of La27WsO55.5 and La27(W0.85Nb0.15)5O55.5-? are investigated. Nb doping shows little effect on the water uptake of La27WsO55.5, however, which will improve the oxide ion and proton conductivity. The modified intrinsic crystal structure may be responsible for the conductivity improvement. The electron conductivity of La27(Wo.85Nb0.15)5055.5-8 in the reducing atmosphere can be improved by Mo doping, but the crystal structure, water uptake as well as the conductivity in the oxidizing atmosphere keep almost constant. The hydrogen permeation properties of La27(W0.85Nb0.15)5O55.5-? are also improved by Mo doping.HTPC as La27(W0.55Nb0.15Mo0.30)5O55.5-? exhibits good hydrogen permeation properties. And the slow surface reaction rate as well as the low electron conductivity are the main limiting factors for the hydrogen permeation process. The surface reaction rates can be fastened with the application of Pt catalyst. The external load circuit also shows obviously effect on the the hydrogen permeation properties.