卷 52, 编号 7 (2016)

The kinetics of nickel reduction and morphological changes in Ni–10Sc1CeSZ composite anodes in intermediate-temperature solid oxide fuel cells (SOFC) are studied using the Raman spectroscopy technique with the help of application of optically transparent single crystal solid electrolyte membranes and also the thermogravimetric analysis technique. It is shown that the first reduction cycle differs considerably from all the further ones, which is related to morphological changes of nickel grains occurring during the first reduction cycle. A general scheme of occurrence of the process is suggested in studies of model cells using the Raman spectroscopy technique and also in the case of thermogravimetric analysis of powders; it explains the causes for significant differences between the total duration of the process as measured using different techniques. The results of the work can be used for optimization of the mode of initial reduction of the anodic SOFC electrode.

The effect of the degree of dispersion and the ratio of initial components of metalloceramic composites based on Ni and Sc₂O₃-stabilized ZrO₂ (Ni/ScSZ) on the kinetics of sintering, conductivity, and polarization resistance of the corresponding anodes in solid-oxide fuel cells (SOFC) is studied. The composites are prepared from nano- and submicrosized powders of NiO and ScSZ (10.5 mol % Sc₂O₃) containing particles with the average size of 0.02–0.33 μm. Anode composites of three types differing in the ratio of initial components (NiO-ScSZ) with different degrees of dispersion: micro-micro, nano-micro, and nano-nano are studied. Due to the ratio of particle sizes, the anodic composites of the nano-nano type demonstrate the preferential electronic conduction (the percolation threshold) starting from the Ni content of about 35 vol %, in contrast to the other two types of anodic composites for which this threshold is achieved at 30 vol %. The lowest polarization resistance is typical of anode composites with the Ni content of about 40 vol %. The use of one or both components in the nanosized state makes it possible to decrease the anodic polarization up to two times. It is demonstrated that an active cermet anode for SOFC can be fabricated in the form of a planar three-layer structure Ni/ScSZ-ScSZ-Ni/ScSZ prepared from nanosized powders by the tape casting technique and cosintering.

For materials based on ferrites and manganites with Са²⁺ and Ва²⁺ cations substituted into А sublattice, the functional properties are studied and the prospects as electrode materials for solid-oxide fuel cells are assessed. The electronic conductivity of materials based on La_0.5A_0.5Mn_0.5Ti_0.5O_3–δ is shown to decrease with the increase in the ionic radius of alkali-earth substituent; however, for La_0.5Ва_0.5Mn_0.5Ti_0.5O_3–δ and La_0.5Ва_0.5Fe_0.5Ti_0.5O_3–δ, the appearance of n-conduction is observed during reduction, which may provide adequate conductivity under anodic conditions. Under the conditions of fuel cell operation, the thermal expansion coefficients of these materials are (13.0–13.5) × 10^–6 K^–1. The thermal and chemical expansion increases with the increase in the radius of alkali-earth cation; the latter value does not exceed 0.2%, which is acceptable for preparation of electronic layers. The transition of oxygen through membranes based on materials studied is determined to the large extent by the kinetics of surface exchange which depends on the rate of delivery of oxygen vacancies to the surface. Doping of ferrites with chromium or titanium decreases the electronic and ionic conductivity; however, the presence of substituents in В sublattice makes it possible to stabilize the perovskite phase in a wide range of р(О₂), decrease the thermal and chemical expansion, and prevent to the large extent the ordering of oxygen vacancies, which allows one to consider these materials as the candidates for electrodes in symmetrical solid-oxide fuel cells.

The Ba_0.5Sr_0.5Co_0.8–xW_xFe_0.2O_3–δ (х = 0–0.1) materials prepaMIECred by partial substitution of cobalt in BSCF with tungsten were studied. The tungsten solubility limit in the structure of cubic perovskite BSCF was shown to be ~2%. The doping with the highly charged W⁶⁺ (2%) cation improved the functional properties of BSCF: it increased the oxygen permeability and membrane stability in the CO₂-containing atmosphere and suppressed the cubic–hexagonal perovskite polymorphic transition. This stabilizes high oxygen fluxes during long-term stability tests.

The work describes the methods of manufacturing single cells of solid oxide fuel cell (SOFC) with thin–film YSZ and CGO electrolytes and also with the bilayer YSZ/CGO electrolyte. Formation of YSZ and CGO films on the supporting NiO–YSZ anode of SOFC was carried out using the combined electron–ionic–plasma deposition technique. The microstructure and phase composition of the formed coatings are studied and also comparative analysis of electrochemical characteristics of single fuel cells with different electrolytes is performed. It is shown that the maximum power density of 1.35 W/cm² at the temperature of 800°C is obtained for the cell with bilayer YSZ/CGO electrolyte. However, the highest performance at lower working temperatures (650–700°C) is characteristic for the fuel cell with single–layer CGO electrolyte; its power density is 600–650 mW/cm².

A complex study of regularities of oxidative reactions is carried out in subsurface layers of current collectors of solid oxide fuel cells manufactured of Crofer 22APU or Crofer 22H stainless steel. The methods of scanning electron microscopy, energy dispersive X-ray analysis, and Raman spectroscopy are used to study the distribution of the main steel elements in subsurface layers of current collectors as dependent on the operation time under the conditions of a cathodic chamber of solid oxide fuel cells. A mechanism of the process is suggested and contact resistance between the current collector and LSM cathode is calculated using the model of a Schottky barrier for a metal–semiconductor junction.

Composite membranes consisting of polyvinyl alcohol and disperse particles of polyantimonic acid with different ratios of components were studied. Processes related to the relaxation and proton transport in a heterogeneous system were considered. The proton transport in the membranes was shown to occur both within the particle (along the chains of alternating wcapacitorater molecules and oxonium ions), forming macrodipoles, and in the intercrystallite space in the polymer matrix. The relaxation was described by the two-layer Maxwell–Wagner capacitor with an effective relaxation time. The relaxation activation energy obtained from the dielectric loss data coincided with the proton conductivity activation energy in the frequency-independent region, which points to the same nature of ion transport.

New cathode and anode materials for fuel cells with an electrolyte based on alkali carbonate melts have been studied. The regions of the orthorhombic and rhombohedral phases in the LaFe_1–yCo_yO₃ + x/2Li₂O system in air and in contact with molten (Li_0.68K_0.32)₂CO₃ electrolyte were investigated. The electric conductivity was analyzed in the range 300–1020 K. The electrocatalytic activity in oxygen reduction was analyzed for the new cathode materials. A method for introducing the Al₂O₃ additive in the anode material was suggested. The polarization characteristics of the porous gas-diffusion electrodes were determined.