Lanthanum Germanate‐Based Apatites as Electrolyte for SOFCs

Abstract

Germanate apatites with composition La10–xGe5.5Al0.5O26.75–3x/2 have been evaluated for the first time as possible electrolytes for solid oxide fuel cells (SOFCs). Different electrode materials have been considered in this study, i.e. manganite, ferrite, nickelates and cobaltite as cathode materials; and NiO–CGO composite and chromium–manganite as anodes. The chemical compatibility and electrochemical performance of these electrodes with La9.8Ge5.5Al0.5O26.45 have been studied by X‐ray powder diffraction (XRPD) and impedance spectroscopy. The XRPD analysis did not reveal appreciable bulk reactivity with the formation of reaction products between the germanate electrolyte and these electrodes up to 1,200 °C. However, a significant cation interdiffusion was observed by energy dispersive spectroscopy (EDS) at the electrode/electrolyte interface, which leads to a significant decrease of the performance of these electrodes.

Effects of La content on the densification, microstructure, and conductivity of doped La10−xGe6O26±δ electrolytes

“…For temperatures above 650°C, the electrical conductivity values of all La x Ge 5.5 Mo 0.5 O 26±δ samples were similar, except for x = 8.75; however, for temperatures below 650°C, the electrical conductivity decreased with the increased La content, although the grain sizes increased slightly with the increased x values and the decreased number of grain boundaries. The activation energy of the samples was estimated to be 1.4–1.7 eV in the low‐temperature region ( T ≤ 700°C) and 0.8–1.0 eV in the higher temperature region ( T ≥ 750°C), indicating a phase transformation from triclinic to hexagonal symmetry . The above phenomena confirm the triclinic structure of the samples revealed in Fig.…”

“…Occupying the smaller of these channels are A site cations, while the larger channel contains oxide ions . The oxide ions in the channels are considered mobile and lead to a high level of oxide ion conductivity …”

“…In spite of their remarkable conductivity at low temperatures, La 10− y Si 6 O 26±δ apatites need to overcome the technical obstacles of requiring a high sintering temperature, usually greater than 1700°C, and a long soaking time before they can be applied in SOFCs . An approach to resolve the barrier is to utilize the germanium‐containing apatites, such as La 10− y Ge 6 O 26±δ, instead of silicon‐based apatites, to reduce the sintering temperature . The replacement of Si ions by Ge ions was found not only to decrease their sintering temperatures, but also to improve their electrical conductivities .…”

“…4 The oxide ions in the channels are considered mobile and lead to a high level of oxide ion conductivity. 5 It was first reported by Nakayama et al [6][7][8] that the oxygen-excess-type Si-based materials of the La 10Ày Si 6 O 26AEd system possess high oxide ion conductivity. The conductivity of lanthanum silicates depends strongly on the doping regime and cation and anion nonstoichiometry.…”

Gallium-doped lanthanum germanates as electrolyte material of solid oxide fuel cells

“…The oxide ions in the channels are considered mobile, leading to a high level of oxide ion conductivity. 3) That the Si-based materials of the La 10¹y Si 6 O 26«¤ system possess high oxide ion conductivity was first reported by Nakayama et al 4)6) The conductivity of lanthanum silicates depends strongly on the doping regime and the cation and anion nonstoichiometry.…”