Doped Germanate‐Based Apatites as Electrolyte for Use in Solid Oxide Fuel Cells

Abstract

Apatite ceramics, known for their good electrical conductivities, have garnered substantial attention as an alternative electrolyte for solid oxide fuel cells (SOFCs). However, studies focusing on the electrochemical performances of SOFCs with apatities as electrolytes remain rare, partly due to their high sintering temperature. In this study, the effects of Mg2+, Al3+, Ga3+, and Sn4+ dopants on the characteristics of La9.5Ge6O26 ± δ are examined and their potential for use as SOFC electrolytes evaluated. The results indicate that La9.5Ge5.5Al0.5O26 is stabilized into a hexagonal structure, while the La9.5Ge5.5Sn0.5O26.25, La9.5Ge5.5Ga0.5O26, and La9.5Ge5.5Mg0.5O25.75 ceramics reveal triclinic cells accompanied with the second phase La2Sn2O7 or La2GeO5. The study further demonstrates that a high sintering temperature is needed for both the La9.5Ge5.5Mg0.5O25.75 and the La9.5Ge5.5Sn0.5O26.25 ceramics, and the worst electrical conductivity among the examined systems appears in the La9.5Ge5.5Ga0.5O26 ceramic. The La9.5Ge5.5Al0.5O26 ceramic is accordingly selected for cell evaluation due to its ability to reach densification at 1,350 °C, its good electrical conductivity of 0.026 S cm–1 at 800 °C, and its acceptable thermal expansion coefficient of 10.1 × 10–6 K–1. The maximum power densities of the NiO‐SDC/La9.5Ge5.5Al0.5O26/LSCF‐SDC single cell are found to be respectively 0.22, 0.16, 0.11, and 0.07 W cm–2 at 950, 900, 850, and 800 °C.

A perspective on low-temperature solid oxide fuel cells

“…67 Germanium-containing analogs, La 10Ày Ge 6 O 26AEd were also investigated. 68 La 2 Mo 2 O 9 also provides good conductivity, but its incompatibility with electrode materials and its stability under redox cycling, are serious concerns. 69 Melilite structure La 1.54 Sr 0.46 Ga 3 O 7.27 was reported to provide competitive conductivity over the 600-900 1C temperature range.…”

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

“…There have been several doping studies on lanthanum germanates . In our previous study, the structures and electrical conductivities of La 9.5 Ge 5.5 Me 0.5 O 26±δ (Me: Mg, Al, Ga, Sn, Fe, Mn, Ti, Ni, Mo, Nb, and W) were examined . Although all the germanates were densified at temperatures below 1500°C, aliovalent doping of Nb, W, and Mo on the Ge site marked with the highest electrical conductivity among the compositions evaluated.…”

“…8,16,[18][19][20][21][22][23][24] In our previous study, the structures and electrical conductivities of La 9.5 Ge 5.5 Me 0.5 O 26AEd (Me: Mg, Al, Ga, Sn, Fe, Mn, Ti, Ni, Mo, Nb, and W) were examined. [25][26][27] Although all the germanates were densified at temperatures below 1500°C, aliovalent doping of Nb, W, and Mo on the Ge site marked with the highest electrical conductivity among the compositions evaluated. In this study, the effects of La content on the densification, microstructural evolution, and electrical conductivity of La x Ge 6Ày Me y O 26AEd (Me: Nb, W, Mo) electrolytes were investigated through density measurement, X-ray diffraction (XRD), scanning electron microscopy (SEM), and conductivity measurement.…”