Electrical conductivity and structural properties of proton electrolytes based on CsH2PO4 and silicophosphate matrices with low phosphorus content

We have demonstrated that silicophosphate matrices with various P₂O₅ contents can be used to produce medium-temperature highly conductive CsH₂PO₄-based composites. The low-temperature conductivity of all the composites studied is higher than that of the salt at low humidity by up to three and half to four orders of magnitude. The phase transition disappears with increasing additive content. The proton conductivity, thermal characteristics, and phase composition of the salt are shown to depend significantly not only on the composition of the composite but also on that of the silicophosphate matrix. The composites with Si : P = 1 : 0.14 contain disordered CsH₂PO₄ in the range x = 0.1–0.6. Increasing the percentage of the heterogeneous component leads to CsH₂PO₄ amorphization at x = 0.7 and the formation of the CsH₅(PO₄)₂ compound at x = 0.8–0.9. The thermodynamic properties and thermal stability of the composites vary in accordance with this. We have assessed the thermal stability of the electrolytes of various compositions under isothermal conditions at 200–210°C and water vapor content of ?0.6–1% in air in prolonged tests. The thermal stability of the materials is shown to depend significantly on both the percentage of the salt in the composite and the composition of the matrix. We have determined the optimal thermal and transport characteristics of the composites based on silicophosphate gel with lower phosphorus content. This opens up the possibility of using them as membranes in medium-temperature hydrogen fuel cells.     

Synergistic effect of SiO2 on proton conduction and thermal behavior for nanocomposite electrolyte CsH2PO4 fuel cells

Journal of Materials Science: Materials in Electronics - Solid acid composite electrolytes (1?x)CsH2PO4(CDP)/xSiO2 (0?≤?x?≤?0.35) were prepared and...     

超薄CsH_2PO_4/SiO_2复合质子交换膜的制备

采用溶胶-凝胶工艺及旋转涂膜法制得厚度为800nm的CsH2PO4/SiO2复合质子交换膜,利用X射线衍射仪与傅里叶变换红外光谱仪分析该膜的晶相和化学构造。通过扫描电镜观察,确认该膜致密、均匀,并且CsH2PO4晶粒分散于膜中。在80～280℃的温区、低湿度环境下,测定该复合质子交换膜的质子传导性能,其质子传导面阻在160℃时为0.1Ω.cm2。     

Mechanochemically synthesized CsH2PO4–H3PW12O40 composites as proton-conducting electrolytes for fuel cell systems in a dry atmosphere

Abstract

Cesium dihydrogen phosphate (CsH₂PO₄, CDP) and dodecaphosphotungstic acid (H₃PW₁₂O₄₀·nH₂O, WPA·nH₂O) were mechanochemically milled to synthesize CDP–WPA composites. The ionic conductivities of these composites were measured by an ac impedance method under anhydrous conditions. Despite the synthesis temperatures being much lower than the dehydration and phase-transition temperatures of CDP under anhydrous conditions, the ionic conductivities of the studied composites increased significantly. The highest ionic conductivity of 6.58×10^?4 Scm^?1 was achieved for the 95CDP·5WPA composite electrolyte at 170 °C under anhydrous conditions. The ionic conduction was probably induced in the percolated interfacial phase between CDP and WPA. The phenomenon of high ionic conduction differs for the CDP–WPA composite and pure CDP or pure WPA under anhydrous conditions. The newly developed hydrogen interaction between CDP and WPA supports anhydrous proton conduction in the composites.     

Studies on transparent and solid proton conductors based on NH4H2PO4 doped poly(vinyl alcohol)

A novel transparent and anhydrous proton conductor, which can be used in solid electrochromic device (ECD), was prepared by mixing poly(vinyl alcohol) (PVA) with ammonium dihydrogen phosphate (NH₄H₂PO₄). X-ray diffraction, differential scanning calorimeter (DSC), and Infrared spectra were used to characterize the structure of PVA/xNH₄H₂PO₄ composite membrane. Proton conductivity of the composite membranes was studied by the complex impedance method. The proton conductivity of the composite membranes increases with increasing temperature and increases with increasing phosphate doping-level at first and then decreases with increasing phosphate content after a certain value of x. The highest proton conductivity is near the area of x = 0.067. The transmittance of the complex membranes always decreases with increasing doping level of phosphate.     

Solid electrolytes based on Na3PO4 doped with S,Se, Mo,W

The cubic form of Na₃PO₄ forms extensive ranges of solid solutions, which are stable from room temperature to the melting point, by the replacement mechanism, Na + P = Y : Y = S, Se, Mo, W. In all four cases, the conductivity increases markedly with increasing Y content and is attributed to the increase in number of mobile sodium ion vacancies. Partial phase diagrams, lattice parameters and conductivity Arrhenius parameters are given for each system     

DSC and conductivity studies on PVA based proton conducting gel electrolytes

An attempt has been made in the present work to prepare polyvinyl alcohol (PVA) based proton conducting gel electrolytes in ammonium thiocyanate (NH₄SCN) solution and characterize them. DSC studies affirm the formation of gels along with the presence of partial complexes. The cole-cole plots exhibit maximum ionic conductivity (2.58 × 10⁻³ S cm⁻¹) for gel samples containing 6 wt% of PVA. The conductivity of gel electrolytes exhibit liquid like nature at low polymer concentrations while the behaviour is seen to be affected by the formation of PVA-NH₄SCN complexes upon increase in polymer content beyond 5 wt%. Temperature dependence of ionic conductivity exhibits VTF behaviour.     

Phase Transition Study of Thermal Dependence of Soft Mode Frequency,Dielectric Constant and Dielectric Tangent Loss Properties in CsH2PO4 (CDP) and CsD2PO4 (DCDP) Crystals

Journal of Low Temperature Physics - Modified two-sublattice pseudospin lattice coupled-mode (PLCM) model Hamiltonian by adding third- and fourth-order anharmonic interactions, extra...     

Hydrolysis of tetracalcium phosphate in H3PO4 and KH2PO4

The activity product of tetracalcium phosphate (TTCP, Ca₄(PO₄)₂O), was determined at 37°C, and the hydrolysis of TTCP was investigated in 0.01–0.1 mol l^–1 H₃PO₄ and KH₂PO₄ solutions by means of calcium and phosphorus analyses, X-ray diffraction and infrared analysis. The activity product, defined as K _sp=(Ca²⁺)⁴ (PO ₄ ^3– )² (OH^–)², was 37.36 as pK _sp, which was smaller than that previously reported (42.4). TTCP easily hydrolysed to form calcium-deficient apatite (Ca-def OHAp, Ca_5–x (HPO₄) _x (PO₄)_3–x (OH)_1–x ), or dicalcium phosphate dihydrate (DCPD, CaHPO₄2H₂O), depending on the initial phosphate concentration. With 0.1 mol l^–1 H₃PO₄, TTCP hydrolysed to form DCPD within several minutes. In 0.025 mol l^–1 H₃PO₄ and 0.1 mol l^–1 KH₂PO₄, TTCP hydrolysed to form Ca-def OHAp through DCPD. In the latter solution, a small amount of octacalcium phosphate (OCP, Ca₈(H₂PO₄)₂(PO₄)₄5H₂O), was detected as an intermediate product. In 0.025 mol l^–1 KH₂PO₄, TTCP hydrolysed directly to form Ca-def OHAp. In 0.01 mol l^–1 H₃PO₄, hydrolysis of TTCP was not completed, although Ca-def OHAp was only a product. Thus the final product and the degree of hydrolysis depended on the pH and the overall Ca/P ratio in the reaction system. The rate of Ca-def OHAp formation seemed to be controlled by the dissolution rate of TTCP rather than the crystallization rate of the OHAp.     

Intermediate temperature proton electrolytes based on cesium dihydrogen phosphate and poly(vinylidene fluoride-co-hexafluoropropylene)

Proton conductivity, morphology, phase composition and mechanical properties of (1-x)CsH₂PO₄-xp(VDF/HFP) (x?=?0.05–0.25, weight ratio) polymer electrolytes were investigated for the first time. The chemical interaction of the organic matrix and acid salt was not observed and crystal structure of CsH₂PO₄ was preserved. A method for the synthesis of thin membranes with uniform distribution of the components was proposed. Thin flexible membranes with uniform distribution of sub-micrometer CsH₂PO₄ particles in the polymer membranes and improved hydrolytic stability were obtained firstly by using a bead mill. The mechanical strength of the hybrid polymer compounds was determined using the Vickers microhardness measurements. Proton conductivity in the (1-x)CsH₂PO₄-xp(VDF/HFP) electrolytes decreases monotonically with x increase due to the «conductor–insulator» percolation. Nevertheless, the values of proton conductivity remain sufficiently high, and along with small thickness, flexibility, improved mechanical and hydrophobic properties, it makes polymer electrolytes based on CsH₂PO₄ promising for membranes of medium-temperature fuel cells.

Graphical abstract

     

Growth imperfections in NH4H2PO4-KH2PO4 mixed crystals by X-ray topographic analysis

X-ray topographs of ammonium dihydrogen phosphate (ADP)—potassium dihydrogen phosphate (KDP) mixed crystals with various compositions were taken by the Lang transmission method. Growth imperfections such as growth bands, misorientations, lattice deformations at the boundary between two growth sectors and inclusions etc., present in the crystals were analysed. Dislocations were identified by using the g.b criterion.     

Crystal structure studies of mixed system of NaH2PO4 and KH2PO4 with H3BO3

X-ray powder analysis of solid solution of NaH₂PO₄ and KH₂PO₄ with H₃BO₃ was carried out. Both the systems were observed to be tetragonal. The crystallographic data are reported.     

Characterization of plasticized PMMA-LiBF4 based solid polymer electrolytes

Polymer electrolyte films prepared from poly(methyl methacrylate) and LiBF₄ with different concentrations of plasticizer (DBP) are described. The formation of polymer-salt complex has been confirmed by FTIR spectral studies. The temperature dependence of conductivity of polymer films seems to obey the VTF relation. Values of conductivities of the polymer complexes are presented and discussed.     

Effects of carbonate on hydroxyapatite formed from CaHPO4 and Ca4(PO4)2O

Carbonated hydroxyapatites were formed via reactions in NaHCO₃/NaH₂PO₄ solutions from a mixture of particulate tetracalcium phosphate (TetCP) and anhydrous dicalcium phosphate (DCPA). Reactions were followed by determinations of pH and ion concentrations. The solids formed were analyzed by XRD and FTIR. Rates of heat evolution were established by isothermal calorimetry. Reactions in the absence of NaH₂PO₄ did not reach completion within 24 h. Constitution of reactants to achieve a DCPA-to-NaHCO₃ ratio of 1, in conjunction with the presence of NaH₂PO₄ as a buffer, was found to be optimal for formation of apatite with no remaining reactant. The amount of carbonate incorporated in this apatite was 4–5 wt%. Calorimetry indicated the reaction mechanism to depend on the bicarbonate concentration in solution. The presence of NaH₂PO₄ was found to increase the reaction rate but decrease the extent of carbonate uptake.     

Low thermal expansion materials: a comparison of the structural behaviour of La0.33Ti2(PO4)3, Sr0.5Ti2(PO4)3 and NaTi2(PO4)3

Variable temperature neutron powder diffraction data have been used to examine the structural mechanism of unusual thermal expansion behaviour in La_0.33Ti₂(PO₄)₃ (LaTP). These data are compared to those we have recently reported for the related systems NaTi₂(PO₄)₃ (NaTP) and Sr_0.5Ti₂(PO₄)₃ (SrTP), which differ in the extent of occupation of the extra-framework MI (La, Sr, Na) cation sites. The root cause of the unusual behaviour is ascribed to the differing expansivities of the occupied and vacant MI sites. This leads to cooperative rotations of TiO₆ and PO₄ polyhedra, which can be used to quantify and rationalise the overall anisotropic thermal expansion behaviour.     

Phosphors based on NaZr2(PO4)3-type calcium and strontium phosphates activated with Eu2+ and Sm3+

Ca_0.5Zr₂(PO₄)₃:Eu²⁺, Sr_0.5Zr₂(PO₄)₃:Eu²⁺, and Ca_0.5Zr₂(PO₄)₃:Eu²⁺, Sm³⁺ orthophosphates prepared through precipitation using sol-gel processes are analogs of NaZr₂(PO₄)₃ (NZP) and crystallize in space group R $\bar 3$ . Their crystallographic parameters determined by X-ray diffraction are consistent with the interatomic distances extracted from EXAFS data. Their luminescence spectra obtained under excitation in the range 300?C400 nm contain emission bands between 425 and 525 nm. Substitution of the larger sized cations Eu²⁺ and Sm³⁺ for Ca²⁺ shifts the emission bands to shorter wavelengths and reduces their width because of the decrease in the effect of the crystal field. Analysis of the spectra indicates that Eu²⁺ occupies two types of crystallographic sites (independent interstitial sites of different sizes and shapes in the NZP framework structure). Codoping with Eu and Sm has ensured luminescence with chromaticity coordinates approaching those of white light: (x = 0.27, y = 0.34).