Abstract
The potential of calcium-doped layered perovskite compounds, BaNd1–xCaxInO4–x/2 (where x is the excess Ca content), as protonic conductors was experimentally investigated. The acceptor-doped ceramics exhibit improved total conductivities that were 1–2 orders of magnitude higher than those of the pristine material, BaNdInO4. The highest total conductivity of 2.6 × 10–3 S cm–1 was obtained in the BaNd0.8Ca0.2InO3.90 sample at a temperature of 750 °C in air. Electrochemical impedance spectroscopy measurements of the x = 0.1 and x = 0.2 substituted samples showed higher total conductivity under humid environments than those measured in a dry environment over a large temperature range (250–750 °C). At 500 °C, the total conductivity of the 20% substituted sample in humid air (∼3% H2O) was 1.3 × 10–4 S cm–1. The incorporation of water vapor decreased the activation energies of the bulk conductivity of the BaNd0.8Ca0.2InO3.90 sample from 0.755(2) to 0.678(2) eV in air. The saturated BaNd0.8Ca0.2InO3.90 sample contained 2.2 mol % protonic defects, which caused an expansion in the lattice according to the high-temperature X-ray diffraction data. Combining the studies of the impedance behavior with four-probe DC conductivity measurements obtained in humid air, which showed a decrease in the resistance of the x = 0.2 sample, we conclude that experimental evidence indicates that BaNd1–xCaxInO4–x/2 is a fast proton conductor.
Highlights
Acceptor-doped oxides with a wide variety of crystal structures have exhibited significant proton conductivity when measured under humid gas atmospheres.[1−7] It has been proposed that these ceramic proton conductors can be applied to improve the performance of solid oxide fuel cell (SOFC) devices, developing the proton conducting ceramic fuel cell (PCCFC) device
SOFCs typically operate at high temperatures (600−1000 °C) as the ionic conductivity of the electrolytes reaches acceptable values in this range, depending on the composition.[2−12] Proton conducting ceramics offer the advantage of fast ion conduction at lower temperatures, simplifying the device engineering.[13−16] Among the structures that exhibit proton conduction, the acceptor-doped alkaline earth cerates and zirconates are most common and intensively investigated.[2,3,8−12,16] To date, acceptor-doped BaCeO3 and BaZrO3 perovskite materials have been reported to show high proton conduction at intermediate-high temperatures (400− 700 °C)
X-ray diffraction (XRD) patterns of the as-sintered BaNd1−xCaxInO4−x/2 (x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) pellet samples, as shown in Figure 2, confirmed that a single phase of a monoclinic crystal structure with the P21/c symmetry formed in all cases except for the 30% calcium-substituted sample
Summary
Acceptor-doped oxides with a wide variety of crystal structures have exhibited significant proton conductivity when measured under humid gas atmospheres.[1−7] It has been proposed that these ceramic proton conductors can be applied to improve the performance of solid oxide fuel cell (SOFC) devices, developing the proton conducting ceramic fuel cell (PCCFC) device. The crystal structure of BaNdInO4 consists of alternative stacking of rare earth oxide (Nd−O) units and perovskite (Ba6/8Nd2/8InO3) units with an edge-facing mode between the slabs as shown in Figure 1.17,18 Several cations such as strontium,[19] barium,[20] and calcium[21] have been used to substitute the Nd site in order to create oxygen vacancies and improve the electrochemical properties of these systems. Among all of these BaNdInO4-related oxides, Received: December 21, 2020 Revised: February 25, 2021 Published: March 10, 2021
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