(Invited) Improving the Stability and Transport Properties of a High Concentration of Protons in Phosphate Glass at Intermediate Temperatures
Authors: Omata Takahisa , Ishiyama Tomohiro , Nishii Junji 
Published: December 2024

Fuel cells that operate at around 300 °C are expected as the next-generation fuel cells that can use liquid organic hydrogen carriers, such as methanol and methylcyclohexane, as fuel directly. Because of the lack of available solid electrolytes working at this temperature range, there is no fuel cell systems available at this temperature. Therefore, new electrolytes are currently being extensively investigated.
Since the discovery of proton conductivity in phosphate glasses by Namikawa et al. in 1966, their proton conductivity has been investigated for applications in electrochemical devices such as fuel cells. Because conventional phosphate glasses could not hold proton carriers at temperatures above 250 °C, they were not suitable as an electrolyte for fuel cells operating at 300 °C. The reason why the glass cannot hold proton carriers at elevated temperatures is that the protons in glass exist predominantly as molecular water, which is readily released outside the glass at elevated temperatures. This limitation has recently been overcome by a proton injection method into phosphate glasses termed as alkali-proton substitution (APS), where the sodium ions in glass are electrochemically substituted with protons at high temperature (Fig. 1(a)). [1] A variety of glasses have been discovered that have high proton conductivity using APS (Fig. 1(b)). For examples, 36HO1/2–4NbO5/2–2BaO–4LaO3/2–4GeO2–1BO3/2–49PO5/2 glass exhibits high proton conductivity of 2 mScm−1 at 300 °C. [2] In order to achieve the conductivity higher than 10 mScm-1 required for practical fuel cell applications, it is necessary to further improve the stability and conductivity of proton carriers.
Considering the glass formation range and the proton carrier density, the appropriate composition of the glass is around metaphosphate consisting of chain-type phosphate ions, (PO3
-)n. The deprotonation of glass fabricated using APS occurs through a dehydration-condensation reaction; thus, the migration or diffusion of PO4 tetrahedra is involved. Based on this, suppressing the viscous flow of the glass and its melt is an effective way to avoid deprotonation of the glass and its melt. Cross-linking the chain-type phosphate ions with heteroatoms that form covalent bonds with oxygen atoms is one of the ways to suppress viscous flow. Regarding proton transport, it is comprised of two fundamental processes: dissociation of the O-H bond and migration of the mobile proton; therefore, to enhance proton conductivity, controlling the strength of the O-H bond and migration of mobile protons are necessary. The strength of the O-H bond can be controlled by the bonding character of the P-O bond. The migration of mobile protons can be enhanced by enhancing the proton migration between the chain-type phosphate ions. Recently, it has been found that cross-linking between chain-type phosphate ions by heteroatoms that form a coordination polyhedron possessing non-bridging oxygen facilitates proton migration between phosphate ions. Based on these understanding, proton-conducting phosphate glass electrolytes that achieve 10 mScm-1 at 300 °C are being explored.
The recent understandings mentioned above will be presented with specific examples.

References:

[1] T. Ishiyama, et al., J. Electrochem. Soc., 160 (2013) E143.
[2] T. Yamaguchi, et al., J. Mater. Chem. A 6 (2018) 23628.

Figure 1