Том 53, № 7 (2017)

The possibility is discussed of estimating the values of conductivity and dielectric permeability of three-phase composites using the modified mixing equation similar to the equation suggested earlier for two-phase systems. It is shown that the choice of theoretical mixing equation parameters for three-phase composites of the conductor–electrolyte–insulator type can be controversial; variants of solving this problem are suggested. According to the equation, three percolation thresholds can exist in this three-phase system; their position can be estimated on the basis of the maximum of dielectric permeability. The value of the maximum is determined by the ratio of conductivities of the conducting and dielectric phases. The equation can be extended to the case when interaction exists between the individual phases of the composite and a highly conducting layer is formed at the interface between these phases. The calculation of three-phase composites of the conductor–composite solid electrolyte–insulator type is calculated for the first time. It is shown that the mixing equation also allows calculating electric properties of composites in this case.

The characteristics of low-temperature hydrogen–oxygen (air) fuel cell (FC) with cathodes based on the 50 wt % PtCoCr/C and 40 wt % Pt/CNT catalysts synthesized on XC72 carbon black and carbon nanotubes (CNT) are compared with the characteristics of commercial monoplatinum systems 9100 60 wt % Pt/C and 13100 70% Pt/C HiSPEC. It is shown that the synthesized catalysts exhibit a high mass activity, which is not lower than that of commercial Pt/C catalysts, a high selectivity with respect to the oxygen reduction to water, and a significantly higher stability. The characteristics of PtCoCr/C and Pt/CNT were confirmed by testing in the hydrogen—oxygen FCs. However, when air was used at the cathode, especially in the absence of excessive pressure, a voltage of FC with the cathode based on PtCoCr/XC72 is lower as compared with the commercial systems. Probably, this is associated with the transport limitations in the structure of trimetallic catalyst synthesized on XC72 carbon black due to the absence of mesopores. This drawback was eliminated to a large extent by raising the volume of mesopores as a result of application of mixed support (XC72 + CNT) and the use of only CNT for the synthesis of the monoplatinum catalyst. However, this did not eliminate another drawback, namely, a low platinum utilization coefficient in the cathode active layer as compared with that measured under the model conditions in the 0.5 M Н₂SO₄ solution. Therefore, further research is required to improve the structure of the catalytic systems, which are synthesized both on carbon black and nanotubes, while maintaining their high stability and selectivity.

Entire carbon nanofiber mats (carbon nanofiber paper) based on polyacrylonitrile pyropolymer composite were prepared by the preliminary oxidation (stabilization) of the initial polymer at 250–350°C in air and following pyrolysis at 800–1200°C under vacuum. The mats were tested as cathodes in a fuel cell on polybenzimidazole membrane. Properties of the pyropolymers which were obtained by polymer carbonization could be significantly changed by the addition of specific additives to polyacrylonitrile and also by changing thermal treatment. Particularly, the addition of Ketjen Black® or Vulcan® XC72 carbon blacks and polyvinyl pyrrolidone during electrospinning step resulted in increase of material electrical conductivity and inner porosity, which is important for improving fuel cell performance. Depending on oxidation and pyrolysis temperature, the physical properties of platinated carbon nanofiber paper and the efficiency of a fuel cell on polybenzimidazole membrane significantly change.

Metal molybdates MMoO₄ (M = Ca, Sr) and their composites with vanadium oxide V₂O₅ were synthesized. An X-ray diffraction analysis confirmed that the obtained molybdates were single-phase, and the heterogeneous systems were two-phase. The temperature dependences of the total conductivity of the composites were studied. The ion transport numbers in the {CaMoO₄ · xV₂O₅} composites (x = 1–30 mol %) were studied by the EMF method. The conductivity of the composites at x ≤ 5 mol % was shown to be ionic. The conductivity of the composites was described using the mixing equation.

A new method of obtaining gastight ceramic based on CaZr_0.95Sc_0.05O_{3 – δ} is presented. The microstructure and electric properties of the obtained samples, same as the behavior of composite electrodes in contact with this electrolyte, are studied for application of the obtained results in the technology of formation of electrochemical devices. The design of bilayer electrodes is suggested, in which the materials tested as the functional layer were layered lanthanum nickelate La2NiO_{4 + δ} and substituted lanthanum nickelate La_1.7Ca(Sr,Ba)_0.3NiO_{4 + δ} in combination with the electrolyte components of Ce_0.8Sm_0.2O_{2 – δ} and BaCe_0.89Gd_0.1Cu_0.01O_{3 – δ}. The collector layer used was lanthanum nickelate–ferrite LaNi_0.6Fe_0.4O_{3 – δ} and manganite La_0.6Sr_0.4MnO_{3 – δ} that are characterized by high electron conductivity, low layer resistance and are close by their values of coefficient of linear thermal expansion to the materials of functional layers. Electrochemical activity of the obtained electrodes are compared with the characteristics of composite electrodes based on lanthanum ferrite–cobaltite La_0.6Sr_0.4Fe_0.8Co_0.2O_{3 – δ} and deficient lanthanum manganite La_0.75Sr_0.2MnO_{3 – δ}.

Cathode materials in the form of Li_{1 + x}(Ni_yMn_zCo_{1 – y – z})_{1 – x}O_{2 – δ} (0 ≤ x ≤ 0.2, 0.2 ≤ y ≤ 0.6, 0.2 ≤ z ≤ 0.4) core–shell nanoparticles coated with a thin carbon shell were synthesized by thermal destruction of metal-containing compounds in oil and studied. The results of element analysis, X-ray diffraction analysis, scanning electron microscopy, X-ray photoelectron spectroscopy, and electrochemical tests of cathodes based on the obtained complex oxides in model cells were presented. The complex oxide Li_1.2Ni_0.2Mn_0.4Co_0.2O_1.9 was the most promising composition because the loss of capacity after 50 cycles was 4% at a current density C/2 and an operating potential of 3.0–4.4 V relative to E (Li/Li⁺). When the current density in discharge increased sixfold (3 C), the loss of capacity was 14% relative to the value obtained at a discharge current C/2 at voltages 3.0 to 4.4 V.

Heterogeneous systems based on the proton–conducting oxide of La_0.95Sr_0.05ScO_{3 – δ} with Cu, Fe, Ni, Pd, La_0.9Sr_0.1MnO_{3 – δ} considered as potential materials of solid oxide fuel cell (SOFC) electrodes are synthesized. Chemical interaction between individual components of composite materials is studied, dependences of thermal and chemical expansion of the electrolyte and composites are obtained, conductivity of electrodes is measured under the conditions of SOFC operation.