Shell-Core Type Proton Conducting TiP₂O₇-Based Solid Electrolytes

Abstract

Shell-core type TiP2O7-based electrolytes were synthesized by a low-temperature sol-gel method, and their conductivities were investigated by ac-impedance spectroscopy. The samples heat-treated at 500°C showed higher proton conductivities of 3.8 × 10-2 - 1 × 10-2 Scm-1 at intermediate temperatures (100 - 300°C). The conductivity decreased by reheat-treatments at 600°C and 700°C due to a decrease in the amount of the conductive amorphous shells. However, the mechanical strength of the sample pellet significantly increased by the reheat-treatment. The sample reheat-treated at 600°C kept high conductivities of 8.8 × 10-3 - 1 × 10-3 Scm-1 in a range of 100°C - 300°C.

Mn2+-Doped CeP2O7Composite Electrolytes for Application in Low Temperature Proton-Conducting Ceramic Electrolyte Fuel Cells

“…[1][2][3][4][5][6] In previous works, 4,7 many authors have reported high ionic conductivity in cerium pyrophosphates in dry atmosphere, however, we have shown that the ionic conductivity of CeP 2 O 7 -based materials in unhumidified air is very low (<10 −7 S cm −1 at <200 • C) but their ionic conductivity increases significantly (more than 5 order of magnitude) on their exposure to moisture. Several approaches have been proposed for the design of phosphate-based anhydrous proton conducting electrolytes in 120-400 • C temperature range.…”

Phase Composition and Long-Term Conductivity of Acceptor Doped Ce(PO3)4and CeP2O7with Variable P/Metal Ratio and of CeP2O7-KH2PO4Composite

“…Onoda et al 19 has reported a value of about 10 −6 S • cm −1 at 400 • C in air with 4.2% H 2 O for the conductivity of Ce(PO 3 ) 4 doped with 1 mol% Gd, but this large difference may be associated with the higher calcination temperature, the higher measuring temperature, lower pH 2 O, and lower degree of doping used by Onoda et al 19 Excess P m O n at the grain boundaries has been suggested by various groups 14,16,[27][28][29] to be the reason behind the high proton conductivities reported for some pyrophosphates, as this could explain the many orders of magnitude difference in conductivity determined for nominally similar compositions. There is consensus as to the fact that the calcination and sintering temperatures can significantly affect the transport properties due to loss of P. 2,3,9,14,15 Two studies that have attempted to vary and quantify the metal to P ratio by X-ray fluorescence (XRF), 2,9 suggested that P deficiency can seriously harm the conductivity, whereas P excess has a small influence. For accurate quantification of the XRF signal though, appropriate calibration standards are necessary in order to account for interactions of the X-rays with the matrix.…”

“…A number of studies have shown that the transport properties of pyrophosphates are very sensitive to the synthesis method and thermal history of the samples, 2,3,9,14,15 which affect the P content and the phase composition of the investigated powders. Nevertheless, a detailed examination of the phase and composition development with temperature, humidity and over extended time periods is often lacking, which may be the reason behind the large differences in the transport properties reported by different groups.…”

“…Furthermore, despite the numerous publications dealing with the conductivity of pyrophosphates and polyphosphates, it is still disputed whether the proton conductivity is associated with point defects incorporated in the bulk of the materials or with the presence of amorphous secondary phases residing at the grain boundaries. A number of studies have shown that the transport properties of pyrophosphates are very sensitive to the synthesis method and thermal history of the samples, 2,3,9,14,15 which affect the P content and the phase composition of the investigated powders. Nevertheless, a detailed examination of the phase and composition development with temperature, humidity and over extended time periods is often lacking, which may be the reason behind the large differences in the transport properties reported by different groups.…”

“…2,9,15 Also, higher annealing temperatures generally result in reduced P content in the samples and reduced conductivities. 8,14,16,17 The most commonly followed synthesis route involves the use of excess H 3 PO 4 in the starting mixture, a tactic that entails the risk of forming a P-rich shell around the pyrophosphate particles that will influence the transport properties. The conductivity is then usually measured in the temperature range 100-300 • C and found to undergo a maximum at around 150-200 • C. On the other hand, a monotonically thermally activated behavior has been observed by Nalini et al 8,17 in the temperature range 300-1000 • C for undoped and acceptor doped ZrP 2 O 7 and TiP 2 O 7 .…”