Evgeny N. Naumovich

Ceria-based materials for solid oxide fuel cells

This paper is focused on the comparative analysis of data on electronic and ionic conduction in gadolinia-doped ceria (CGO) ceramics as well as on the electrochemical properties of various oxide electrodes in contact with ceria-based solid electrolytes. Properties of electrode materials, having thermal expansion compatible with that of doped ceria, are briefly reviewed. At temperatures below 1000 K, Ce0.90Gd0.10O2−δ (CGO10) was found to possess a better stability at reduced oxygen pressures than Ce0.80Gd0.20O2−δ (CGO20). Incorporation of small amounts of praseodymium oxide into Ce0.80Gd0.20O2−δ leads to a slight improvement of the stability of CGO20 at intermediate temperatures, but the difference between electrolytic domain boundaries of the Pr-doped material and CGO10 is insignificant. Since interaction of ceria-based ceramis with electrode materials, such as lanthanum-strontium manganites, may result in the formation of low-conductive layers at the electrode/electrolyte interface, optimization of electrode fabrication conditions is needed. A good electrochemical activity in contact with CGO20 electrolyte was pointed out for electrodes of perovskite-type La0.8Sr0.2Fe0.8Co0.2O3−δ and LaFe0.5Ni0.5O3−δ, and LaCoO3−δ/La2Zr2O7 composites; surface modification of the electrode layers with praseodymium oxide results in considerable decrease of cathodic overpotentials. Using highly-dispersed ceria for the activation of SOFC anodes significantly improves the fuel cell performance.

Ionic transport in oxygen-hyperstoichiometric phases with K2NiF4-type structure

Abstract Results on oxygen permeation through dense ceramics of La2−xSrxNi1−y−zFeyCuzO4+δ (x=0–0.10; y=0.02–0.10; z=0–0.10), LaPrNi0.9Fe0.1O4+δ, La2Cu1−xCoxO4+δ (x=0.02–0.30) and Ln2CuO4+δ (Ln=Pr, Nd) at 973–1223 K suggest two significant contributions to the ionic conductivity of the oxygen-hyperstoichiometric phases with K2NiF4-type structure. The relative role of the first of them, oxygen interstitial migration in the rock-salt-type layers of the K2NiF4-like lattice, increases with increasing temperature; the role of oxygen vacancy diffusion in the perovskite layers increases when temperature decreases. This behavior was attributed to the lower activation energy for ionic conduction via the vacancy diffusion mechanism. The oxygen permeability of the title materials was found to be limited by both bulk ionic conductivity and surface exchange rates and may thus be enhanced by catalytically active layers, including Pt, Ag and praseodymium oxide, deposited on the membrane surface. Oxygen permeability of K2NiF4-type phases exhibiting maximum ionic transport, such as La2Ni0.98Fe0.02O4+δ, La2Ni0.88Fe0.02Cu0.10O4+δ and La2Cu0.90Co0.10O4+δ, is about one order of magnitude lower than that of most permeable perovskite-type materials. Decreasing radii of the rare-earth cations in the A-sublattice of cuprates and nickelates leads to a dramatic decrease in ionic transport, similar to perovskite oxides. Thermal expansion coefficients of the title materials vary in the range (10.1–13.4)×10−6 K−1.

Research on the electrochemistry of oxygen ion conductors in the former Soviet Union. II. Perovskite-related oxides

Abstract The review is devoted to the analysis of experimental results on electrochemical and physicochemical properties of the perovskite-related oxide phases obtained at scientific centers of the former Soviet Union. The main attention is focused on oxides with high electronic conductivity, which are potentially useful as electrodes for high-temperature electrochemical cells with oxygen-ion conducting solid electrolytes and interconnectors of solid oxide fuel cells, and on mixed ionic-electronic conductors for oxygen separation membranes. Along with perovskite-like solid solutions based on LnMO3−δ (Ln is a rare-earth element, Mu2009=u2009Cr, Mn, Fe, Co, Ni) and SrCoO3−δ, properties of the oxide phases Ln2MO4±δ (Mu2009=u2009Cu, Ni, Co) with the K2NiF4-type structure are briefly reviewed.

Oxygen Permeability of Ce0.8Gd0.2 O 2 − δ ‐ La0.7Sr0.3MnO3 − δ Composite Membranes

(CGO) and (LSM) possess similar thermal expansion coefficients and were thus combined in dual‐phase membranes for oxygen separation. Studies of oxygen permeation through CGO‐LSM composite ceramics, containing similar volume fractions of the phases, showed that the oxygen transfer is limited by the bulk ionic conductivity. The oxygen conduction in the composites depends strongly on processing conditions, decreasing with interdiffusion of the phase components. Blocking oxygen ionic conduction is assumed to be due to formation of layers with low ionic conductivity at the CGO grain boundaries, caused by diffusion of lanthanum and strontium into CGO. The permeation fluxes through CGO‐LSM membranes at high feed‐side oxygen pressures (1–50 atm) exhibit Wagner‐type behavior and exceed significantly the oxygen permeability at lower oxygen pressures.

The stability and mixed conductivity in La and Fe doped SrTiO3 in the search for potential SOFC anode materials

Both physical properties and the level of mixed conduction obtained in La and Fe doped SrTiO3 are widely influenced by composition. In contrast to La free compositions, La containing compositions show high stability against reaction with yttria stabilised zirconia (YSZ) and a closely matching thermal expansion coefficient (∼1×10−5 K−1). Faradaic efficiency measurements for Sr0.97Ti0.6Fe0.4O3–δ and La0.4Sr0.5Ti0.6Fe0.4O3–δ show ionic transference numbers in air between 5 × 10−3 to 4 × 10−2, and 2 × 10−4 to 6 × 10−4 respectively, decreasing with decreasing temperature. The substitution of La for Sr is observed to deplete the level of both ionic and total conductivity obtained in air.

Oxygen permeation through Sr(Ln)CoO3-δ (Ln = La, Nd, Sm, Gd) ceramic membranes

Abstract Oxygen permeability of Sr0.65−xLa0.35CoO3−δ (x=0–0.10), Sr0.65La0.35Co0.90O3−δ and Sr0.7Ln0.3CoO3−δ (Ln=La,xa0Nd,xa0Sm,xa0Gd) ceramics was studied under temperatures of 950–1250 K. The experiments held proved that the transfer of oxygen through the strontium cobaltite-based ceramics was limited both by the rate of oxygen exchange between oxides and gas phase and by ionic conductivity of the materials. It has been established that the permeability of materials studied increases rapidly with increase in oxygen partial pressure. A creation of vacancies in both cation sublattices of ABO3 perovskite results in decreasing oxygen permeation flux. Therewith, cation deficiency in the sublattice B of the cobaltites have a lower influence on oxygen permeability in comparison with ones in the sublattice A. Oxygen permeation flux through Sr(Ln)CoO3−δ ceramics decreases with decreasing mean radius of the cations in the sublattice A. Electrical conductivity of the cobaltites reduces when oxygen partial pressure diminishes or concentration of the cation vacancies increases.

Research on the electrochemistry of oxygen ion conductors in the former Soviet Union. I. ZrO2-based ceramic materials

Abstract Developments of solid electrolytes and mixed conductors based on stabilized zirconia in the former Soviet Union are reviewed. Primary attention is given to experimental data on high-conducting electrolytes, mixed conductors obtained by doping zirconia with transition metal oxides, oxygen exchange and oxygen permeation processes, as well as properties of metal electrodes in contact with the stabilized zirconia.

Electron–hole conduction in Pr-doped Ce(Gd)O2−δ by faradaic efficiency and emf measurements

Abstract Non-negligible electrode polarisation in faradaic efficiency and emf electrochemical cells used for transport number determination results in apparent ion transference numbers which are lower than the true values. However, appropriate modifications of these techniques combined with use of electrodes having a high polarisation resistance, enable a precise determination of even minor electronic contributions to the total conductivity of solid electrolytes. Experimental modification of the faradaic efficiency method, taking into account the electrode polarisation resistance, is proposed and verified using Ce 0.80 Gd 0.18 Pr 0.02 O 2− δ ceramics. Substitution of 2% gadolinium cations in the lattice of Ce 0.80 Gd 0.20 O 2− δ solid electrolyte with praseodymium was found to increase the p-type electronic conductivity by 2.5–4 times, while the activation energy for the electron–hole transport decreases from 145 to 125 kJ mol −1 . The oxygen ion transference numbers of Ce 0.80 Gd 0.18 Pr 0.02 O 2− δ in air vary in the range 0.996–0.970 at 873–1223 K, decreasing with increasing temperature. No significant effect of co-doping with praseodymium on the ionic conductivity, crystal lattice and thermal expansion of ceria solid electrolyte was found.

Transport properties and stability of Ni-containing mixed conductors with perovskite- and K2NiF4-type structure

Abstract The total conductivity and Seebeck coefficient of a series of Ni-containing phases, including La2Ni1−xMxO4+δ (M=Co, Cu; x=0.1–0.2) with K2NiF4-type structure and perovskite-like La0.90Sr0.10Ga0.65Mg0.15Ni0.20O3−δ and La0.50Pr0.50Ga0.65Mg0.15Ni0.20O3−δ, were studied in the oxygen partial pressure range from 10−18xa0Pa to 50xa0kPa at 973–1223xa0K. Within the phase stability domain, the conductivity of layered nickelates is predominantly p-type electronic and occurs via small-polaron mechanism, indicated by temperature-activated hole mobility and p(O2) dependencies of electrical properties. In oxidizing conditions similar behavior is characteristic of Ni-containing perovskites, which exhibit, however, significant ionic contribution to the transport processes. The role of ionic conduction increases with decreasing p(O2) and becomes dominant in reducing atmospheres. All nickelate-based phases decompose at oxygen pressures considerably lower with respect to Ni/NiO boundary. The partial substitution of nickel in La2Ni(M)O4+δ has minor effect on the stability limits, which are similar to that of La0.90Sr0.10Ga0.65Mg0.15Ni0.20O3−δ. On the contrary, praseodymium doping enhances the stability of La0.50Pr0.50Ga0.65Mg0.15Ni0.20O3−δ down to p(O2) values as low as 10−17–10−10xa0Pa at 1023–1223xa0K.

Oxygen ionic conductivity of Ti-containing strontium ferrite

Abstract The total electrical conductivity, oxygen permeability, Faradaic efficiency and thermal expansion of perovskite-type Sr 0.97 Fe 0.80 Ti 0.20 O 3− δ were studied. The oxygen permeation through strontium ferrite–titanate ceramics was found to be limited by both bulk ionic conduction and surface exchange rates, similar to other Fe- and Ti-containing perovskites. Increasing iron content in the system Sr 0.97 (Ti,Fe)O 3− δ leads to a considerable increase in oxygen ionic and electronic conductivities, and thermal expansion. The ion transference numbers of Sr 0.97 Fe 0.80 Ti 0.20 O 3− δ were determined by the Faradaic efficiency measurements, and estimated from the oxygen permeability data. Typical values of the transference numbers vary in the range from 4×10 −3 to 1.5×10 −2 at 973–1223 K, decreasing with reducing temperature. The oxygen ionic conductivity of Sr 0.97 Fe 0.80 Ti 0.20 O 3− δ is close to the conductivity of the Zr 0.92 Y 0.08 O 1.96 solid electrolyte. The average thermal expansion coefficient of strontium ferrite–titanate, calculated from dilatometric data, is (13.8±0.1)×10 −6 K −1 at 300–780 K and (27.0±0.4)×10 −6 K −1 at 780–1040 K.