Synthesis, Thermal Stability, and Proton-Conductivity of the Mixed-Cation Phosphate with One-Dimensional Tunnels Formed by PO4 Tetrahedral Rings
Authors: Hirose Kazuhiko , Matsuda Yasuaki 
Published: December 2024

The Benitoite-type phosphate, KMg1-x
H2

x
(PO3)3·yH2O, is a promising material as a solid electrolyte for automotive fuel cells due to its high proton conductivity from room temperature to 500°C1). The Benitoite-type KMg1-x
H2

x
(PO3)3·yH2O has a layered structure stacking the units formed by the units formed by P3O9 ring and those composed of (KO6)(MgO6) octahedral units. The P3O9 ring forms one-dimensional tunnels with a cage large enough to occupy H2O(or H3O+), which would provide proton conduction pathway. However, the correlation between the local structure and proton conductivity of this material has not yet been investigated. In this study, with the aim of further improving proton conductivity, we synthesized Benitoite-type phosphate with new compositions by introducing protons as charge compensation by replacing Mg2+ with Li+.
KMg1-x
Li
x
H
x
(PO3)3·yH2O were synthesized by a coprecipitation method. In the X-ray diffraction patterns for the samples with x = 0 - 0.3, the formation of the single phase of Benitoite-type structure with the space group P-6c2 was confirmed. Both the a and c axes tend to decrease with x in these compositions reflecting the formation of a solid solution. Fig.1 shows FT-IR spectra of KMg1-x
Li
x
H
x
(PO3)3·yH2O. In the FT-IR spectra, the absorption peaks corresponding to the vibrational modes of POH and OH became relatively stronger for x, suggesting an increase in the proton contents in the samples. The lattice shrinkage by replacing Li+(0.76 Å) with an ionic radius larger than Mg2+(0.72 Å) would be caused by increasing of hydrogen bonds in the structure. In the TG/DTA measurement, the dehydration reactions from room temperature and 150°C were observed, and the water content increased with increasing x. This is suggested that the acidity of the framework is enhanced by the introduced protons, causing water molecules tend to incorporate into the crystal structure as Lewis bases. The shape of the pellet with x > 0.1 was not retained after retained under 80% relative humidity at room temperature.The conductivity of samples was measured by using AC impedance method. The impedance plots for the sample depict a part of semicircle due to the grain boundary and spike caused by the electrode contribution. The total resistance was determined from the intersection of the spike and the real axis, and the conductivity was calculated. The proton conductivity of the sample with x = 0.1 was higher than that of x = 0 under a humidified N2 gas flow (P
H2O = 3 kPa). the sample with x = 0.1 showed high proton conductivity exceeding 10-3 S cm-1 from room temperature to 500 ℃, and the highest value of 1.9×10-2 S cm-1 was observed at 200°C under a humidified N2 gas flow( P
H2O = 3 kPa). Only the sample with water of crystallization exhibited high conductivity, which suggests domination of proton diffusion via water of crystallization. The conductivity of the Li-substituted Benitoite tends to show higher proton conductivity than Mg-deficiency sample at the same proton content.
Reference
1) Y. Matsuda, et al, Dalton Trans., 50, 7678 (2021).
Acknowledgement
This research was partially supported by Kanto Branch of the Electrochemical Society of Japan, JSPS KAKENHI Grant Number 24K08584 and 21K05246.

Figure 1