(NH4)3PW12O40-H3PO4 composites as efficient proton conductors at intermediate temperatures

doi:10.1016/j.jmst.2019.06.022

J. Mater. Sci. Technol. ›› 2020, Vol. 37: 128-134.DOI: 10.1016/j.jmst.2019.06.022

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(NH₄)₃PW₁₂O₄₀-H₃PO₄ composites as efficient proton conductors at intermediate temperatures

Xiaoxiang Xu^a^b^c^d^*(), Shunhang Wei^a^d

^a Yongxing Special Materials Technology Co., Ltd, Huzhou 313005, China
^b Clinical and Central Lab, Putuo People’s Hospital, Tongji University, Shanghai 200060, China;
^c School of Engineering, Huzhou University, Huzhou 313000, China
^d School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China

Received:2019-05-29 Revised:2019-06-17 Accepted:2019-06-26 Published:2020-01-15 Online:2020-02-10
Contact: Xu Xiaoxiang

Abstract

Abstract:

(NH₄)₃PW₁₂O₄₀ and (NH₄)₃PW₁₂O₄₀-H₃PO₄ composites have been synthesized by precipitation method. Their phase compositions, thermal stability and morphologies have been investigated. The synthesized composites maintain the same structures as pure (NH₄)₃PW₁₂O₄₀ where phosphoric acid is preserved in residual space of the “spongy crystals” of (NH₄)₃PW₁₂O₄₀. FT-IR spectra confirm the strong interactions between phosphoric acid and Keggin ions. Pristine (NH₄)₃PW₁₂O₄₀ shows poor conductivity in air at high temperatures and strongly depends on water molecules for proton transport. The composites exhibit a much higher conductivity compared with pure (NH₄)₃PW₁₂O₄₀. The highest conductivity achieved is 0.14 S/cm at 170 °C where continuous channels based on phosphoric acid for proton transportation are probably established. Such high conductivity of (NH₄)₃PW₁₂O₄₀-H₃PO₄ composites implies promising applications in fuel cells and other electrochemical devices.

Key words: Proton conductor, Phosphotungstic acid, Intermediate temperature, Keggion ions

Xiaoxiang Xu, Shunhang Wei. (NH₄)₃PW₁₂O₄₀-H₃PO₄ composites as efficient proton conductors at intermediate temperatures[J]. J. Mater. Sci. Technol., 2020, 37: 128-134.

Figures/Tables 9

Fig. 1. XRD patterns of NP0, NP3, NP5 and NP10, standard pattern for (NH4)3PW12O40 (JCPDS: 00-050-0305) is also included for comparison. Main peak around 26° is enlarged on the right. Dashed line is a guide for the eye.

Table 1 Unit cell parameters calculated from XRD patterns (standard deviation is included in the parenthesis).

Sample	Symmetry	Lattice parameter, a (Å)	Volume (Å³)
NP0	Cubic	11.6729(19)	1590.5(5)
NP3	Cubic	11.7033(18)	1603.0(4)
NP5	Cubic	11.7140(22)	1607.4(5)
NP10	Cubic	11.718(3)	1609.0(7)

Fig. 2. Unite cell parameters as a function of H3PO4 to (NH4)3PW12O40 molar ratio: (a) cell parameter a; (b) cell volume V.

Fig. 3. (a) Thermogravimetric analysis (TGA) of NP0, (b) TGA of NP3, NP5 and NP10, (c) XRD patterns of NP0 after TGA and (d) XRD patterns of NP10 after TGA. Standard patterns for W12PO38.5 (JCPDS: 00-041-0369) and (NH4)3PW12O40 (JCPDS: 00-050-0305) are also included for comparison.

Table 2 Weight loss of all samples in different temperature regions.

Sample	Weight loss below 100?°C	Weight loss between 100?°C and 300?°C	Total weight loss
NP0	0.98%	0.99%	1.97%
NP3	2.34%	2.53%	4.87%
NP5	0.23%	3.24%	3.57%
NP10	1.22%	5.62%	6.84%

Fig. 4. Field emission SEM images of freshly prepared sample powders: (a, b) NP0; (c, d) NP3; (e, f) NP5; (g, h) NP10.

Fig. 5. TEM images of NP10 at different magnifications.

Fig. 6. FT-IR spectra of all sample powders (85?wt% H3PO4 is also included for comparisons).

Fig. 7. (a) Conductivity (σ) of NP0 in air and in wet air (humidified at 293?K) and (b) conductivity of NP3, NP5 and NP10 in air (T: temperature).

(NH₄)₃PW₁₂O₄₀-H₃PO₄ composites as efficient proton conductors at intermediate temperatures

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