Electrochemical Characterization of Novel Polyantimonic-Acid-Based Proton Conductors for Low- and Intermediate-Temperature Fuel Cells

The development of novel proton-conducting membrane materials for electrochemical power units, i.e., low temperature fuel cells (FCs), efficiently working up to 300 °C, is a critical problem related to the rapid shift to hydrogen energy. Polyantimonic acid (PAA) is characterized by high conductivity, sufficient thermal stability and can be regarded as a prospective proton-conducting material. However, the fabrication of bulk PAA-based membranes with high proton conductivity remains a challenging task. In the present work, for the first time, the authors report the investigation on proton conductivity of bulk PAA-based membranes in the temperature range 25–250 °C, both in dry air and in moisturized air. Using PAA powder and fluoroplastic as a binder, fully dense cylindrical membranes were formed by cold uniaxial pressing. The structures of the PAA-based membranes were investigated by SEM, EDX, XRD and Raman techniques. STA coupled with in situ thermo-XRD analysis revealed that the obtained membranes corresponded with Sb2O5·3H2O with pyrochlore structure, and that no phase transitions took place up to 330 °C. PAA-based membranes possess a high-grain component of conductivity, 5 × 10−2 S/cm. Grain boundary conductivities of 90PAA and 80PAA membranes increase with relative humidity content and their values change non-linearly in the range 25–250 °C.

Ion transport in hydrated compounds based on crystalline polyantimonic acid

The present work is devoted to the study of crystalline polyantimonic acid (CPA) structural properties in conditions of ion exchange and heat treatment of its substituted Ag+, H+-forms. According to the obtained data of X-ray diffraction qualitative phase analysis, the compounds analyzed crystallize within the framework of the pyrochlore type structure (space group Fd-3m). For phases being isomorphic to this structural type, it was shown that with an increase in the degree of substitution α, a relative intensity redistribution of the group of reflections with even and odd indices and decrease of unit cell parameter a were observed.. The data of thermogravimetric analysis allows to conclude that the thermolysis of CPA’s ion-substituted forms proceeds in a wide temperature range from 297 to 973 K being accompanied by a decrease of samples weight. Applying the Rietveld method, the structural characteristics of CPA and its derivatives were refined, and a model of populating the corresponding metal ions by crystallographic positions within a pyrochlore-type structure was proposed. Using a complex of physicochemical methods (thermogravimetric and X-ray diffraction analyses), the effect of hydration of CPA compounds with a pyrochlore type structure on transport properties was determined. It is shown that with an increase of substitution degree α in samples of CPA’s Ag+, H+-forms in conditions of alternating current the value of the specific conductivity decreases monotonically. An increase of silver ions content in the phases of Ag+, H+-forms leads to the change of proton-binding energy with the crystal lattice. In the condition of elevated temperature in hydrated CPA compounds, charge transfer is performed by silver ions. It was found that the electrical conductivity in the samples analyzed can also rise with a silver ions amount decrease (16d-positions are partially filled with Sb3+).

Ion exchange of H+/Na+ in polyantimonic acid, doped with vanadium ions

The influence of doping of polyantimonic acid with vanadium ions on the ion exchange H+/Na+ kinetics is determined. With the help of the X-ray analysis it is shown that doped compounds H2Sb2-xVxO6-δ · nH2O, where x = 0.14; 0.36; 0.48 have the pyrochlore-type structure. With the increase of the amount of vanadium in a sample there is a decrease of a unit cell parameter. It is determined that there is a reduction of a unit cell frame, formed by antimonic and vanadium ions, and a size reduction of hexagonal cavities, in which protons are located. It is shown that during the ion exchange of H+/Na+ there is a change of the unit cell plane without a symmetry breaking of a crystal structure, which demonstrates solid solution formation. With the increase of the amount of vanadium in the sample there is a decrease of the ion-exchange capacity, which is connected with the decrease of the channel’s section size. The ion exchange H+/Na+ kinetics has been studied with the approximation of the Boyd’s model for the intergrain diffusion. Diffusion coefficients had the order 10−13 m2/s and decreased with the increase of the amount of vanadium in the sample.