Structure and Dynamics Investigations of Sr/Ca-Doped LaPO₄ Proton Conductors

Abstract

Proton conductors loom out of the pool of candidate materials with great potential to boost hydrogen alternatives to fossil-based resources for energy. Acceptor-doped lanthanum orthophosphates are considered for solid oxide fuel cells (SOFCs) for their potential stability and conductivity at high temperature. By exploring the crystal and defect structure of x% Sr/Ca-doped LaPO 4 with different nominal Sr/Ca concentrations (x = 0−10) with neutron powder diffraction (NPD) and X-ray powder diffraction (XRD), we confirm that Sr/Ca-doped LaPO 4 can exist as self-supported structures at high temperatures during solid oxide fuel cell operation. Thermal stability, surface topography, and size distribution are also studied to better understand the proton conductivity for dry and wet compounds obtained at sintering temperatures ranging from 1200 to 1400 °C using a combination of scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), and electrochemical impedance spectroscopy (EIS). The results confirm that Sr-doped samples exhibit the highest proton conductivity of our samples and illustrate the impact of material design and versatile characterization schemes on the development of proton conductors with superior functionality.

NASICON-Type Lithium-Ion Conductor Materials with High Proton Conductivity Enabled by Lithium Vacancies

“…The small absorption peaks next to the asymmetric stretching vibration peaks at 1198.37 and 1207.05 cm –1 are interpreted as the contribution of a fraction of electrons to the P–O vibrations from the small and highly charged zirconium cations polarizing the P–O band . It can be observed that the absorption band representing the asymmetric stretching vibration of PO 4 is wider in the FTIR spectrum of the treated material than the FTIR spectrum of the original sample, which is due to the increase in bond length in the P–O–H vibration and is related to the presence of protons . Furthermore, the weak absorption peak around 3400 cm –1 represents the stretching vibration of −OH. − Li 2.5 Sr 0.75 Zr 1.…”

“…57 It can be observed that the absorption band representing the asymmetric stretching vibration of PO 4 is wider in the FTIR spectrum of the treated material than the FTIR spectrum of the original sample, which is due to the increase in bond length in the P−O−H vibration and is related to the presence of protons. 58 Furthermore, the weak absorption peak around 3400 cm −1 represents the stretching vibration of −OH. 59−61 Li 2.5 Sr 0.75 Zr 1.…”