No abstract

The potential of calcium-doped layered perovskite compounds, BaNd 1– x Ca x InO 4– 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, BaNdInO 4 . The highest total conductivity of 2.6 × 10 –3 S cm –1 was obtained in the BaNd 0.8 Ca 0.2 InO 3.90 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% H 2 O) was 1.3 × 10 –4 S cm –1 . The incorporation of water vapor decreased the activation energies of the bulk conductivity of the BaNd 0.8 Ca 0.2 InO 3.90 sample from 0.755(2) to 0.678(2) eV in air. The saturated BaNd 0.8 Ca 0.2 InO 3.90 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 BaNd 1– x Ca x InO 4– x /2 is a fast proton conductor.

Oxides with proton conductivity have a great potential for applications in environmental energy technology. Despite the Ba-Ce 0.4 Zr 0.4 Y 0.2 O 3−δ (BCZY) perovskites being well-known proton conductors, it is a challenge to determine the optimal operating temperature range where the energy applications benefit most from this unique property. The protonic transport properties strongly depend on crystal structure and local distortions in the participating cation coordination sphere, according to related temperatures and gas feed. The transport and crystallographic properties of BCZY were simultaneously studied by impedance spectroscopy (IS) and synchrotron X-ray diffraction (S-XRD). A strong correlation between conductivity and the lattice parameter, corresponding in principle to a cubic symmetry, was observed, mainly between 400 and 700°C. The protonic conductivity range was analyzed by the H/D isotopic effect on the impedance spectra, which helped to identify protonic conduction as the governing transport mechanism below 600°C, while the transport via oxygen vacancies dominates above this temperature. In order to assess the real crystallographic structure, the simultaneous refinement of laboratory XRD and neutron diffraction (ND) patterns was performed. According to this, BCZY changes from rhombohedral symmetry below 400°C to cubic at 600°C in a second-order phase transition. Complementary quasielastic neutron scattering (QENS) enables us to determine a protonic jump length of 3.1 Å, which matches the O−O distances in the octahedral oxygen coordination sphere around the cations. These results support the protonic self-diffusion through proton hopping between intraoctahedral O sites as the main transport mechanism up to 600°C.