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.     

Proton Conductivity and Thermal Properties of Ba(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>)<Subscript>2</Subscript>

We have grown single crystals of barium dihydrogen phosphate and studied its thermal transformations during heating to 500°C and its electrotransport properties. Ba(H₂PO₄)₂ (Pccn) has been shown to undergo no phase transitions up to its dehydration temperature. The thermal decomposition of Ba(H₂PO₄)₂, accompanied by dehydration, involves two steps, with maximum rates at ~265 and 370°C, and results in the formation of barium dihydrogen pyrophosphate and barium metaphosphate, respectively. The total enthalpy of the endothermic dehydration events is–244.6 J/g. Using impedance spectroscopy, we have studied in detail the proton conductivity of polycrystalline and single-crystal Ba(H₂PO₄)₂ samples in a controlled atmosphere. Adsorbed water has been shown to have a significant effect on the proton conductivity of Ba(H₂PO₄)₂ up to 130°C. The proton conductivity of the Ba(H₂PO₄)₂ single crystals has been shown to be anisotropic. The conductivity anisotropy correlates with specific structural features of the salt. Higher conductivity values, 3 × 10^–9 to 2 × 10^–7 S/cm in the range 60–160°C, have been observed in the [100] crystallographic direction, exceeding the conductivity along [010] by an order of magnitude. The activation energy for proton conduction is 0.80 eV.     

Synthesis and properties of sulfonated polyimide–polybenzimidazole copolymers as proton exchange membranes

Chemical stability of polymer electrolyte membranes (PEMs) is the key factor affecting the lifetime of fuel cells. It is greatly desirable to develop the PEMs with both high proton conductivity and excellent chemical stability. In this study, a series of sulfonated polyimide–polybenzimidazole copolymers (SPI-co-PBIs) are synthesized via random condensation polymerization of 1,4,5,8-naphthalene tetracarboxylic dianhydride, 4,4′-bis(4-aminophenoxy)biphenyl-3,3′-disulfonic acid, and an amine-terminated polybenzimidazole oligomer. The ion exchange capacities of the resulting SPI-co-PBIs are in the range 1.90–2.47 meq g^?1. Under fully hydrated condition, the SPI-co-PBI membranes show higher proton conductivities than Nafion112. It is found that the incorporation of a small fraction of PBI moiety into the polyimide structure resulted in significant improvement in radical oxidative stability. For example, the SPI-co-PBI-19/1 containing 5 mol % PBI moiety shows only 0.6 wt% weight loss after being soaked in the Fenton’s reagent (3 % H₂O₂ + 3 ppm FeSO₄) at 80 °C for 150 min, whereas the corresponding benzimidazole group-free sulfonated polyimide is completely dissolved in the Fenton’s reagent at 80 °C for 140 min. The SPI-co-PBI membranes also show excellent hydrolytic stability due to the highly stable ladder structure of the benzimidazobenzisoquinolinone linkages.     

High temperature proton exchange membranes prepared from epoxycyclohexylethyltrimethoxysilane and amino trimethylene phosphonic acid as anhydrous proton conductors

High temperature anhydrous proton exchange membranes based on phosphonic acid were prepared from epoxycyclohexylethyltrimethoxysilane (EHTMS) and amino trimethylene phosphonic acid (ATMP) by sol–gel process. The structures and properties of membranes with different phosphonic acid content were extensively characterized by FTIR, TG-DSC and XRD. Their proton conductivity under dry condition was also investigated under different temperature. The results show that the proton conductivity of the prepared membranes strongly depends on temperature, and the proton conductivity ranges from 8.81 × 10⁻⁵ S cm⁻¹ at 20 °C to 4.65 × 10⁻² S cm⁻¹ at 140 °C under anhydrous condition. It indicates that the increasing temperature is favorable for congregating of the grafted–PO₃H₂ and increasing of the proton mobility. In addition, from the results of AFM images, it was confirmed that the continuous distribution of phosphonic acid groups is favorable for the formation of the proton transport channel, which can significantly enhance the proton conductivity of the membranes.     

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.     

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

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.     

Pyrazine-Functionalized Donor–Acceptor Covalent Organic Frameworks for Enhanced Photocatalytic H2 Evolution with High Proton Transport

The well-defined 2D or 3D structure of covalent organic frameworks (COFs) makes it have great potential in photoelectric conversion and ions conduction fields. Herein, a new donor–accepter (D–A) COF material, named PyPz-COF, constructed from electron donor 4,4′,4″,4′″-(pyrene-1,3,6,8-tetrayl)tetraaniline and electron accepter 4,4′-(pyrazine-2,5-diyl)dibenzaldehyde with an ordered and stable π-conjugated structure is reported. Interestingly, the introduction of pyrazine ring endows the PyPz-COF a distinct optical, electrochemical, charge-transfer properties, and also brings plentiful CN groups that enrich the proton by hydrogen bonds to enhance the photocatalysis performance. Thus, PyPz-COF exhibits a significantly improved photocatalytic hydrogen generation performance up to 7542 µmol g⁻¹ h⁻¹ with Pt as cocatalyst, also in clear contrast to that of PyTp-COF without pyrazine introduction (1714 µmol g⁻¹ h⁻¹). Moreover, the abundant nitrogen sites of the pyrazine ring and the well-defined 1D nanochannels enable the as-prepared COFs to immobilize H₃PO₄ proton carriers in COFs through hydrogen bond confinement. The resulting material has an impressive proton conduction up to 8.10 × 10⁻² S cm⁻¹ at 353 K, 98% RH. This work will inspire the design and synthesis of COF-based materials with both efficient photocatalysis and proton conduction performance in the future.     

Transport properties and modification of potassium dihydrogen orthophosphate

We have studied the thermal behavior, proton conductivity, and structural properties of composite proton electrolytes based on KH₂PO₄, the mixed salt K_0.97Cs_0.03(H₂PO₄)_0.97(HSO₄)_0.03, and silicon dioxide with a pore size of 70 Å in a wide composition range. The results demonstrate that the proton conductivity of the (1–x)KH₂PO_4–xSiO₂ (x = 0.1–0.5) composite systems increases by more than two orders of magnitude, reaching 3 × 10^–3 S/cm at a temperature of 225°C. The increase in conductivity is due to the formation of a disordered amorphous state of the salts as a result of partial KH₂PO₄ dehydration and the formation of K₄H₆(PO₄)₂P₂O₇ as an intermediate product. In the composites based on the highly conductive, disordered K_0.97Cs_0.03(H₂PO₄)_0.97(HSO₄)_0.03 mixed salt, close in composition to KH₂PO₄, heterogeneous doping causes no increase in conductivity, and the conductivity decreases with increasing doping level, which is caused by dispersion of the salt and the dehydration process, leading to the formation of K₄H₆(PO₄)₂P₂O₇ and KPO₃.     

Characterization of mechanochemically synthesized MHSO4–H4SiW12O40 composites (M = K,NH4, Cs)

Anhydrous proton conductive MHSO₄–H₄SiW₁₂O₄₀ (MHS–STA) composites were successfully synthesized using mechanochemical treatment. 80MHS·20STA (mol%) composite, for example, showed very high anhydrous proton conductivity above 10^?3 S cm^?1 in a temperature range from 160 to 60 °C under ambient pressure. From the X-ray diffraction study, it was confirmed that the mechanochemical treatment induced chemical interactions via ion-exchange between M⁺ ion in MHS and H⁺ ion in STA. Furthermore, phase-transition of raw substances, such as melting, dehydration and superprotic phase-transition, was suppressed in mechanochemically synthesized MHS–STA composites, indicating that improvement of anhydrous proton conductivity for MHS–STA composites is caused by the changes in protic conduction behavior.     

Fabrication of films of hydrogen uranyl phosphate tetrahydrate and their use as solid electrolytes in electrochromic displays

We have found that the high proton conductivity of hydrogen uranyl phosphate tetrahydrate HUO₂PO₄·4H₂O (HUP) is sufficient to enable an HUP/H_xWO₃ electrochromic cell to function as fast as a cell with an acidic solution electrolyte. Switching times down to 0.3 s were found. The fabrication procedure to obtain the stable and uniform films of HUP required has been optimized, and dense sintered films up to 6 cm in diameter have been produced.     

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

     

Preparation and proton conductivity of poly(vinylidene fluoride)/layered double hydroxide nanocomposite gel electrolytes

Layered double hydroxide (LDH) was synthesized in the presence of sodium dodecyl sulfate. X-ray diffraction (XRD) and infrared spectrum revealed that dodecyl sulfate (DS) anions were successfully intercalated into the interlayers of LDH. Poly(vinylidene fluoride)/LDH nanocomposite membranes were prepared by mixing the DS intercalated LDH with poly(vinylidene fluoride) (PVDF) in N,N’′-dimethylformamide solution followed by the solvent evaporation. The nanocomposite membranes were further swollen with a H₃PO₄ solution in ethylene carbonate-propylene carbonate to obtain the proton conducting nanocomposite gel electrolytes. XRD and transmission electron microscope results showed that LDH particles were well-dispersed in the polymer matrix and partially intercalated by polymer chains. The proton conductivity was highly enhanced in the nanocomposite gel electrolyte systems. In the case of the nanocomposite gel electrolyte containing 7.40 wt.% LDH, the proton conductivity increased by about 2.5 times compared to pure PVDF gel electrolyte.     

Characterization of injectable hydrogels based on poly(N-isopropylacrylamide)-g-chondroitin sulfate with adhesive properties for nucleus pulposus tissue engineering

The goal of this work is to develop an injectable nucleus pulposus (NP) tissue engineering scaffold with the ability to form an adhesive interface with surrounding disc tissue. A family of in situ forming hydrogels based on poly(N-isopropylacrylamide)-graft-chondroitin sulfate (PNIPAAm-g-CS) were evaluated for their mechanical properties, bioadhesive strength, and cytocompatibility. It was shown experimentally and computationally with the Neo-hookean hyperelastic model that increasing the crosslink density and decreasing the CS concentration increased mechanical properties at 37 °C, generating several hydrogel formulations with unconfined compressive modulus values similar to what has been reported for the native NP. The adhesive tensile strength of PNIPAAm increased significantly with CS incorporation (p < 0.05), ranging from 0.4 to 1 kPa. Live/Dead and XTT assay results indicate that the copolymer is not cytotoxic to human embryonic kidney (HEK) 293 cells. Taken together, these data indicate the potential of PNIPAAm-g-CS to function as a scaffold for NP regeneration.     

Electrospinning fabrication and electrochemical properties of LiFePO4/C composite nanofibers

LiFePO₄/C composite nanofibers were synthesized by calcination of the [LiOH + Fe(NO₃)₃ + H₃PO₄]/PVP electrospun nanofibers. Polyvinyl pyrrolidone (PVP) was used as the electrospinning template and carbon source. During the calcination [LiOH + Fe(NO₃)₃ + H₃PO₄] were transformed to LiFePO₄ and PVP was decomposed into carbon. The morphology and properties of the as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Brunauer–Emmett–Teller (BET) specific surface area analysis, electrochemical impedance spectroscopy and galvanostatic charge–discharge measurements. The results indicate that the mean diameter of as-prepared LiFePO₄/C composite nanofibers is 179.08 ± 29.66 nm and the BET specific surface area is 66.59 m² g^?1. The addition of carbon does not affect the structure of LiFePO₄, but improves its electrochemical performances. At the current density of 0.2 C, the initial discharge capacity of LiFePO₄/C electrode is 133.6 mAh g^?1 and there is no obvious capacity fading after 100 cycles. The formation mechanism of the LiFePO₄/C composite nanofibers was also proposed.     

Preparation and electrochemical performances of LiFePO4/C composite nanobelts via facile electrospinning

LiFePO₄/C composite nanobelts were synthesized by calcination of the [LiOH + Fe(NO₃)₃ + H₃PO₄]/polyvinyl pyrrolidone (PVP) electrospun nanobelts. PVP was used as the electrospinning template and carbon source. During the calcination, [LiOH + Fe(NO₃)₃ + H₃PO₄] were transformed to lithium iron phosphate (LiFePO₄) and PVP was decomposed into carbon. The morphology and properties of the as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Brunauer–Emmett–Teller (BET) specific surface area analysis, electrochemical impedance spectroscopy and galvanostatic charge–discharge measurements. The results indicate that the mean width of LiFePO₄/C composite nanobelts is 2.50 ± 0.33 μm, the average thickness is about 162 nm and the BET specific surface area is 19.4 m² g^?1. The addition of carbon does not affect the structure of LiFePO₄, but improves its electrochemical performances. At the current density of 0.2 C, the initial discharge capacity of LiFePO₄/C electrode is 123.38 mAh g^?1 and there is no obvious capacity fading after 50 cycles. The formation mechanism of LiFePO₄/C composite nanobelts was also proposed.     

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.     

Investigation of Cs(H2PO4)1 − x (HSO4) x (x = 0.15–0.3) superprotonic phase stability

A detailed investigation of the highly conductive Cs(H₂PO₄)_1?x (HSO₄) _x (x = 0.15–0.3) proton electrolyte, its structural properties, and ageing behavior was carried out using X-ray diffraction, DSC, and impedance and NMR spectroscopy. The high conductivity of electrolytes (～2 × 10^?2 S/cm) remains stable during long-term ageing at 180–200°C due to stabilization of the high temperature phase to lower temperatures. The room temperature ¹H MAS NMR spectrum of (CsH₂PO₄)_1?x (CsHSO₄) _x demonstrates the predominantly highly mobile protons present in these materials with the residual low-mobile protons, which agrees with the XRD data. According to XRD and ¹H NMR data, the cubic phase of Cs(H₂PO₄)_{1 ? x} (HSO₄) _x (x = 0.15–0.3) that stabilizes at room temperature gradually transforms to a low-temperature monoclinic one. The kinetics of the phase transformation for mixed salt depends markedly on the relative air humidity. A possible stabilization mechanism of the Cs(H₂PO₄)_{1 ? x} (HSO₄) _x superionic phase with high proton mobility at low temperatures is discussed.     

Coaxial electrospinning fabrication and electrochemical properties of LiFePO4/C/Ag composite hollow nanofibers

LiFePO₄/C/Ag composite hollow nanofibers were synthesized by calcination of the coaxial electrospun nanofibers with polyvinyl pyrrolidone (PVP) as core and [LiOH + Fe(NO₃)₃ + H₃PO₄]/PVP/AgNO₃ as shell. PVP was used as the electrospinning template and carbon source. During the calcination, LiFePO₄ precursor was transformed to LiFePO₄ while AgNO₃ and PVP were decomposed into silver and carbon. The morphology and properties of the as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy, BET specific surface area analysis, electrochemical impedance spectroscopy and galvanostatic charge–discharge measurements. The results indicate that the mean diameter of as-prepared LiFePO₄/C/Ag composite hollow nanofibers is 154.5 ± 18.6 nm and the BET specific surface area is 119.14 m² g^?1. The addition of silver and carbon does not affect the structure of LiFePO₄, but improves its electrochemical performances. At the current density of 0.2 C, the initial discharge capacity of LiFePO₄/C/Ag hollow nanofibers electrode is 138.71 mAh g^?1, which is higher than that of LiFePO₄/C nanofibers electrode. The improved specific capacity may be attributed to increase electrode conductivity after the introduction of silver. The formation mechanism of the LiFePO₄/C/Ag composite hollow nanofibers was also proposed.     

Synthesis of Substituted Octacalcium Phosphate for Filling Composite Implants Based on Polymer Hydrogels Produced by Stereolithographic 3D Printing

Based on theoretical and experimental analysis of ionic equilibria in buffer solutions, we have found conditions for octacalcium phosphate (OCP), Ca₈(HPO₄)₂(PO₄)₄ ? 5H₂O, synthesis. Succinate-substituted OCP, Ca₈(HPO₄)_2–хSuc_x(PO₄)₄ ? 5H₂O, with x = 0.8–0.9 has been synthesized at pH 5.5 and t = 60°C in 3 h via α-TCP hydrolysis in a 0.25 M succinic buffer solution. The synthesis product is more stable to thermolysis than is pure OCP: the apatite-like product is stable up to 630°C. The succinate@OCP powder has been used as a filler for poly(ethylene glycol)diacrylate-based hydrogel in producing a composite implant by stereolithographic 3D printing.     

New medium-temperature proton electrolytes based on CsH2PO4 and silicophosphate matrices

Silicophosphate gels ranging widely in P₂O₅ content and specific surface area have been synthesized by a sol-gel process. We have demonstrated the possibility of producing medium-temperature high-conductivity systems based on silicophosphate matrices and CsH₂PO₄. The thermal, structural, and transport properties of composite proton electrolytes have been investigated. The results indicate that the electrical conductivity of the composites based on matrices with Si : P = 1 : 0.5 increases by up to three and half or four orders of magnitude and that their proton conductivity is ～10^?3 to 3 × 10^?2 S/cm at temperatures from 90 to 220°C and a water vapor content of ?0.6–1 mol % in air. The additive suppresses the superionic phase transition of CsH₂PO₄. The increase in conductivity at low contents of the heterogeneous component is due to both CsH₂PO₄ dispersion and the presence of protonated centers on the matrix surface. When the mole fraction of the additive exceeds 0.3, the composites contain CsH₅(PO₄)₂, a compound with a lower thermal stability, which is responsible for their high conductivity in a limited temperature range.