NEW HIGH- CONDUCTING HYBRID CsH₅(PO₄)₂-BUTVAR COMPOUNDS

Highlights

• Polymer (1–x)CsH5(PO4)2-xButvar compounds (x = 0–0.3, x - weight fraction) were first synthesized.

• The optimal synthesis method and composition were determined.

• Highly conductive polymer electrolytes (10^-2 S/cm) with 50–100 μm thick were obtained.

Abstract

Hybrid polymer compounds (1–x)CsH₅(PO₄)₂-xButvar (polyvinyl butyral brand Butvar®) (x = 0–0.3, x - weight fraction), synthesized with ethanol and isopropanol as solvents, studied using the electrochemical impedance method, powder X-ray diffraction analysis (XRD hereinafter), differential scanning calorimetry, and electron microscopy. A more optimal synthesis method was determined and the fundamental possibility of creating new highly conductive thin polymer electrolytes based on CsH₅(PO₄)₂ was shown. The crystal structure of the CsH₅(PO₄)₂ (P2₁/c) salt remains unchanged in the polymer hybrid system, but dispersion and partial amorphization of salt were observed during synthesis. It was found that, to achieve the highest proton conductivity of (1–x)CsH₅(PO₄)₂-xButvar, the most preferable method of synthesis is the precipitation of the salt, from the starting components, in the presence of a solvent common with the polymer. The most optimal is the method of salt precipitation from a polymer solution in ethanol as a common solvent with further tape casting. The thickness of polymer electrolytes is no more than 50–100 μm. Proton conductivity increases by 2 orders of magnitude relative to the initial salt and depends on composition, uniformity of salt distribution in the polymer volume, and the presence of residual humidity during electrolyte synthesis. The highest proton conductivity was 10^-2 S/cm at 130 °C at low humidity conditions. Changes in the electrotransport characteristics of the system are caused by dispersion and partial amorphization of the salt, as well as by the presence of insignificant amounts of residual water molecules on the salt surface under the conditions of the synthesis of polymer compositions.

Introduction

Ever-increasing energy requirements determine search for new energy technologies and proton electrolytes with a number of significant properties. The new proton conducting membrane materials have to be developed because conventional sulfonated organic polymers, such as Nafion, fail to operate above 100 °C or in a dry atmosphere. So there are some technical difficulties associated with low-temperature fuel cells, for example the need of simplified water management system, of enhanced electrochemical kinetics and high carbon monoxide-tolerance. It is known that a family of acid salts with the general formula M_nH_m(AO₄)_p – (di) hydrogen phosphates, sulfates, selenates of alkali metals (n, m, p - integers) has a high proton conductivity in the temperature range of 100–250 °C. The uniqueness of the salts of this family is associated with the existence of superionic phases with high proton conductivity, up to ~ 6 × 10^-2 at T = 250 °C (CsH₂PO₄), due to the presence of a disordered network of hydrogen bonds and excess structural crystallographic positions for proton transfer. The high proton conductivity of M_nH_m(AO₄)_p compounds manifests itself in the region of moderate temperatures and low humidity and reaches values comparable to those in the melt. High conductivity is associated with the transfer of structural protons. The acid salts of alkali metals of the MHSO₄, MH₂PO₄ and MH₅(PO₄)₂ families are most suitable for use as membranes in electrochemical devices and medium-temperature fuel cells. In accordance with the values of conductivity and temperature range, the membrane of CsH₂PO₄ and its composites undoubtedly has the greatest potential for use [1], [2], [3], [4]. The first data on hydrogen fuel cells with a CsH₂PO₄ membrane appeared in 2004–2005 [5], [6], [7]. Fuel cells based on CsH₂PO₄ have a number of advantages over low-temperature fuel cells with a polymer electrolyte of the Nafion type [8], [9]. One of the possible candidates for use in a medium-temperature fuel cell as a membrane is CsH₅(PO₄)₂ - cesium pentahydrogen diphosphate [10], [11], [12], [13]. At room temperature, CsH₅(PO₄)₂ has a monoclinic structure P2₁/c with unit cell parameters a = 10.879 Å, b = 7.768 Å, c = 9.526 Å, β = 96.60°, Z = 4 [10]. Strong hydrogen bonds directed along the coordinate axes have a significant effect on the formation of the crystal structure and properties of the compound. Four hydrogen bonds are practically equivalent, though one is shorter; moreover, all hydrogen atoms participate in the formation of strong hydrogen bonds (d (O … O) < 2.7 Å). The crystal structure of CsH₅(PO₄)₂ contains infinite layers of hydrogen-bonded phosphate tetrahedra located parallel to the (1 0 0) plane, {[H₂PO₄]}_∞. The layers are stitched together through electrostatic interaction with Cs atoms and hydrogen bonds. It was found that, despite the high content of protons, the conductivity of single crystals is low, does not exceed 10^-4 S/cm at temperatures up to 145 °C, and falls below 10^-8 S/cm at T < 90 °C. Low conductivity with high activation energy, Ea ≈ 2 eV, is associated with the presence of a strong 3-dimensional network of hydrogen bonds, which impedes proton transfer along any crystallographic direction due to the high energy of defect formation. The inequality of hydrogen bonds determines the presence of anisotropy in the conductivity of CsH₅(PO₄)₂ single crystals [14], [15]. The specificity of the salt structure suggests the possibility of a phase transition to a superionic state. But there is only an amount of indirect data on the presence of a superionic phase transition at temperatures close to melting point. CsH₅(PO₄)₂ polycrystals have a higher proton conductivity, exceeding single crystals by up to three orders of magnitude, which is associated with the formation of a pseudo-liquid layer, at the crystallite boundary, caused by the adsorption interaction of residual water molecules with phosphate groups of the salt. It was shown that the increased conductivity of the layers is due to the high mobility of protons and the weakening of the network of hydrogen bonds [15], [16]. The activation energy of polycrystals’ conductivity is 1.05–1.1 eV. The electrotransport properties of CsH₅(PO₄)₂ polycrystals were modified using the heterogeneous doping method [15], [16]. A noticeable effect of the grain surface on the conductivity of the salt allows us to expect a further increase in conductivity upon heterogeneous doping of CsH₅(PO₄)₂ with highly dispersed oxides and salts [17], [18], [19]. A number of nanocomposites with high proton conductivity, reaching 10^-3 S/cm at temperatures of 140 °C, have been synthesized with the introduction of highly dispersed silica and tin pyrophosphate. A fuel cell with relatively high discharge characteristics operating in an atmosphere of high humidity was investigated based on a CsH₅(PO₄)₂- SiP₂O₇ membrane [13], [17]. The structural properties and proton conductivity of the nanocomposites based on CsH₅(PO₄)₂ and the metal organic coordination polymer Cr-MIL-101 (chromium terephthalate III) were studied [20]. Composites (1–x)CsH₅(PO₄)₂– xCr-MIL-101 showed high proton conductivity at medium temperatures (80–140° C) in the absence of chemical interaction between the components. The conductivity of CsH₅(PO₄)₂, like other physicochemical characteristics of the salt in the polymer matrix, significantly changes when the salt fills the pore space of Cr-MIL-101, as a result of not only dispersion, but also the amorphization of the salt. In such composite systems the highest proton conduction properties result from the enhanced proton mobility and concentration along an extended oxide surface and from the facilitation of nanodispersed state of the corresponding acid salts by the nanoporous ceramic additive. The production of flexible hydrophobic thin-film membranes with high proton conductivity at medium temperatures is of significant interest for the creation of different electrochemical devices. It is of course important to obtain an extremely fine electrolyte with acceptable mechanical integrity, high ionic conductivity and long service life.

This work is devoted to a new hybrid polymer system, the method of its synthesis, the study of the structural and electrotransport properties of (1–x)CsH₅(PO₄)₂-xButvar. Polymer (1–x)CsH₅(PO₄)₂-xButvar systems of different compositions were synthesized through a method of casting using different solvents and different methods of salt introduction into the polymer composition. The present work demonstrates the successful preparation of solid polymer proton electrolytes composed of CsH₅(PO₄)₂ and Butvar® B98. These hybrid compounds possess remarkable proton conductivity at temperatures up to 140 °C in dry atmosphere, which surpasses the properties of some investigated materials.

Polyvinyl butyral (Butvar® B98), belongs to a family of polyvinyl butyral resins. Changing the concentration of the starting components and the reaction conditions during the Butvar synthesis makes it possible to obtain polymers with different ratios of acetal, hydroxyl, and acetate groups. The presence of three different functional groups ensures high adhesion to various types of surfaces. The polymer is resistant to alkalis and acids. Butvar, characterized by high chemical resistance, thermal stability, high resistivity and extremely low water absorption, which can significantly affect the properties of polymer composite systems [21], was chosen as a polymer heterogeneous matrix. The main process of thermal degradation of polyvinyl butyral, with a weight loss of up to 89% is observed in the range 406 °C–460 °C. Butvar has been successfully used for the synthesis of highly conductive hybrid polymer electrolytes based on CsH₂PO₄ [22], [23].