Synthesis and Proton Conductivity of the Mixed Cation Phosphate, KCo_1−<i>x</i>H_2<i>x</i>(PO₃)₃ • <i>y</i>H₂O with a with a One-dimensional Tunnel Structure

Abstract

A mixed cation phosphate, KCo 1−x H 2x (PO 3 ) 3 • yH 2 O, with a tunnel structure was synthesized by a coprecipitation method and investigated as a proton conductor. KCo 1−x H 2x (PO 3 ) 3 • yH 2 O formed a solid solution in the range of x = 0-0.18, and the enhancement of the absorption peaks corresponding to the vibration and stretching modes of OH and POH were observed for increasing x. The sample with x = 0.18 exhibited a high proton conductivity in the temperature range of 25-250°C, and the value of 2.6 x 10 −3 Scm −1 was obtained at 250°C. This sample maintained a high proton conductivity of over 10 −3 Scm −1 at 150°C for 9 h, and no electron conductivity was confirmed in a hydrogen or oxygen gas atmosphere.

Tunnel-Structured Phosphate Exhibiting High Proton Conductivity and Thermal Stability over a Wide Intermediate Temperature Range

“…The conductivity of KNi 1– x H 2 x (PO 3 ) 3 ·y H 2 O improved with the proton content. The proton conductivity increased from room temperature to 200 °C and decreased slightly above 225 °C due to the desorption of water of crystallization, which is a similar feature to other tunnel phosphates. − The highest conductivity value of 1.7 × 10 –2 S cm –1 was obtained at 275 °C for the sample with x = 0.18. The electronic conductivity value, σ e , at x = 0.18 obtained from the DC measurement was in the order of 10 –6 S cm –1 , as shown in Figure S4, close to the measurement limit of the instrument.…”

“…Although the positions of the protons in the crystal structure could not be determined in this study, the FTIR spectra suggested that they were located between the PO 4 tetrahedra and the oxygen of the crystal water, the same as other tunnel phosphates. − Since PO 4 tetrahedra tend to form P–O–H bonds with protons, phosphates with the structure of connected PO 4 tetrahedra would have induced an adjacent arrangement of crystal water in the open framework. The distances of K–O(4) and K–O(5) were 2.52 and 2.62 Å, respectively.…”

“…In the stoichiometric sample ( x = 0), no dehydration reaction was observed at 600 °C and a single phase of tunnel phosphate was observed up to 800 °C (see Figures S2 and S3). The irreversible phase transition at around 300 °C that occurred in other tunnel phosphates was not observed for KNi 1– x H 2 x (PO 3 ) 3 ·y H 2 O. − Although the introduction of vacancies at Ni 2+ destabilized the tunnel structure, samples with x > 0 retained the tunnel framework up to 600 °C.…”

“…The lattice parameters were affected by the cation vacancies at the Ni 2+ site, excess protons, and water of crystallization in the unit cell. The lattice parameters of the mixed-cation phosphates tend to decrease with an increase in water of crystallization that forms hydrogen bonding with PO 4 tetrahedra, − and a curvilinear change in this material would reflect the change in water of crystallization for x .…”

“…The distances of K–O(4) and K–O(5) were 2.52 and 2.62 Å, respectively. Water molecules coordinated with the large cations composed of the framework have the potential to be maintained at high temperatures, and the difference in the coordination environment of water molecules can cause the multistep desorption of water of crystallization. − In addition to the difference in the coordination environment of water molecules, the sample would have contained a large amount of water of crystallization at room temperature. Therefore, repulsion between adjacent water molecules would have occurred more easily at room temperature than at high temperature, which should contribute to multistep dehydration.…”