F.M.B. Marques

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.

Oxygen transport in Ce0.8Gd0.2O2−δ-based composite membranes

Abstract Gadolinia-doped ceria electrolyte Ce 0.8 Gd 0.2 O 2− δ (CGO) and perovskite-type mixed conductor La 0.8 Sr 0.2 Fe 0.8 Co 0.2 O 3− δ (LSFC), having compatible thermal expansion coefficients (TECs), were combined in dual-phase ceramic membranes for oxygen separation. Oxygen permeability of both LSFC and composite LSFC/CGO membranes at 970–1220 K was found to be limited by the bulk ambipolar conductivity. LSFC exhibits a relatively low ionic conductivity and high activation energy for ionic transport (∼200 kJ/mol) in comparison with doped ceria. As a result, oxygen permeation through LSFC/CGO composite membranes, containing similar volume fractions of the phases, is determined by the ionic transport in CGO. The permeation fluxes through LSFC/CGO and La 0.7 Sr 0.3 MnO 3− δ /Ce 0.8 Gd 0.2 O 2− δ (LSM/CGO) composites have comparable values. An increase in the p-type electronic conductivity of ceria in oxidizing conditions, which can be achieved by co-doping with variable-valence metal cations, such as Pr, leads to a greater permeability. The oxygen ionic conductivity of the composites consisting of CGO and perovskite oxides depends strongly of processing conditions, decreasing with interdiffusion of the phase components, particularly lanthanum and strontium cations from the perovskite into the CGO phase.

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.

Cathode materials for intermediate temperature SOFCs

Abstract This work examines the electrical transport properties and electrode performance of two ceramic cathode materials (La0.84Sr0.16Co0.3Fe0.7O3 – LSCFe7 and La0.84Sr0.16Co0.7Fe0.3O3 – LSCFe3) prepared by solid state reaction from oxides and carbonates. Point-type electrodes pressed against the electrolyte material (GCO – Gd2O3 doped CeO2) were used to study the intrinsic performance of these electrode–electrolyte combinations. Pt electrodes were also used for comparison. For both ceramic cathodes a significant decrease in the cell ohmic contribution with increasing cathodic overpotentials suggests that the electrochemical reaction spreads from the triple phase boundary line to the electrode surface, a typical feature of mixed conducting electrode materials. Overall, the steady state cathodic polarization measurements performed at 800°C showed the LSCFe3 sample to be the best cathode material. LSCFe3 also had a higher conductivity than LSCFe7 (in the range 200 to 800°C), thus showing an overall highly promising performance.

Perovskite-like system (Sr,La)(Fe,Ga)O3−δ: structure and ionic transport under oxidizing conditions

Abstract The maximum solid solubility of gallium in the perovskite-type La 1− x Sr x Fe 1− y Ga y O 3− δ ( x =0.40–0.80; y =0–0.60) was found to vary in the approximate range y =0.25–0.45, decreasing when x increases. Crystal lattice of the perovskite phases, formed in atmospheric air, was studied by X-ray diffraction (XRD) and neutron diffraction and identified as cubic. Doping with Ga results in increasing unit cell volume, while the thermal expansion and total conductivity of (La,Sr)(Fe,Ga)O 3− δ in air decrease with gallium additions. The average thermal expansion coefficients (TECs) are in the range (11.7–16.0)×10 −6 K −1 at 300–800 K and (19.3–26.7)×10 −6 K −1 at 800–1100 K. At oxygen partial pressures close to atmospheric air, the oxygen permeation fluxes through La 1− x Sr x Fe 1− y Ga y O 3− δ ( x =0.7–0.8; y =0.2–0.4) membranes are determined by the bulk ambipolar conductivity; the limiting effect of the oxygen surface exchange was found negligible. Decreasing strontium and gallium concentrations leads to a greater role of the exchange processes. As for many other perovskite systems, the oxygen ionic conductivity of La 1− x Sr x Fe 1− y Ga y O 3− δ increases with strontium content up to x =0.70 and decreases on further doping, probably due to association of oxygen vacancies. Incorporation of moderate amounts of gallium into the B sublattice results in increasing structural disorder, higher ionic conductivity at temperatures below 1170 K, and lower activation energy for the ionic transport.

n‐Type Conductivity in Gadolinia‐Doped Ceria

The ionic and n-type conductivities of ceria-based electrolytes doped with gadolinium (10 and 20 cation %) or gadolinium and praseodymium (2% Pr and 18% Gd) were studied by impedance spectroscopy in air, between 573 and 1,273 K, and by constant frequency (10 kHz) conductivity measurements as a function of the oxygen partial pressure (p{sub O{sub 2}}) formed between 1,073 and 1,273 K with air at the reversible electrode. From data obtained with different experimental techniques it was concluded that estimates for the electronic conductivity of all compositions were consistent and that all materials behaved in a rather similar manner. Estimated n-type conductivities suggest that Pr-doped materials have a slightly lower electronic conductivity and a larger electrolytic (and ionic) domain, but overall differences are quite small. The model behavior used in analyzing experimental results is discussed based on the existing knowledge of the defect chemistry of ceria-based electrolytes, and a range of working conditions is identified where the ionic conductivity can be assumed constant and the electronic conductivity proportional to p{sub O{sub 2}}{sup {minus}1/4}.

Ionic conductivity of La(Sr)Ga(Mg,M)O3−δ (M=Ti, Cr, Fe, Co, Ni): effects of transition metal dopants

Abstract Oxygen-ion conductivity of the perovskite-type solid solutions (La,Sr)Ga1−zM2O3−δ (M=Ti, Cr, Fe, Co; z=0–0.20), LaGa1−y−zMgyMzO3−δ (M=Cr, Fe, Co; y=0.10–0.20, z=0.35–0.60) and LaGa1−zNizO3−δ (z=0.20–0.50) was studied using the techniques of oxygen permeation, Faradaic efficiency, ion-blocking electrode and the e.m.f. of oxygen concentration cells. Oxygen-ion transference numbers vary from 2×10−6 to 0.98 throughout the series and p-type electronic conductivity increases with increasing transition metal content. Substitution of Ga with higher valence cations (Ti, Cr) decreases ionic conductivity whereas small amounts of Fe or Co (∼5%) increase ionic conductivity. For higher transition metal contents, lower levels of oxygen-ion conductivity and an increase in the activation energy, EA, for ionic transport, from 60 (5%-doped) to 230 kJ/mol (>40%-doped) are observed. In heavily doped phases, EA tends to decrease with temperature and, above 1170 K, values are similar to the undoped phase suggesting that an order–disorder transition takes place. Factors affecting the observed ionic conductivity trends are discussed.

Oxygen Ionic and Electronic Transport in Apatite-Type Solid Electrolytes

The oxygen ionic conductivity of apatite-type La 9.83 Si 4.5 Al 1.5-y Fe y O 26±δ (y = 0-1.5), La 10-x Si 6-y Fe y O 26±δ (x = 0-0.77; y = 1-2), and La 7-x Sr 3 Si 6 O 26-δ (x = 0-1) increases with increasing oxygen content. The ion transference numbers, determined by faradaic efficiency measurements at 973-1223 K in air, are close to unity for La 9.83 Si 4.5 Al 1.5-y Fe v O 26+δ and La 10 Si 5 FeO 26.5 , and vary in the range 0.96-0.99 for other compositions. Doping of La 9.83 (Si, Al) 6 O 26 with iron results in an increasing Fe 4+ fraction, which was evaluated by Mossbauer spectroscopy and correlates with partial ionic and p-type electronic conductivities, whereas La-stoichiometric La 10 (Si, Fe)O 26+δ apatites stabilize the Fe 3+ state. Among the studied materials, the highest ionic and electronic transport is observed for La 10 Si 5 FeO 26.5 , where oxygen interstitials are close neighbors of Si-site cations. Data on transference numbers, total conductivity, and Seebeck coefficient as a function of the oxygen partial pressure confirm that the ionic conduction in Fe-substituted apatites remains dominant under solid oxide fuel cell operation conditions. However, reducing p (O 2 ) leads to a drastic decrease in the ionic transport, presumably due to a transition from the prevailing interstitial to a vacancy diffusion mechanism, which is similar to the effect of acceptor doping. Iron additions improve the sinterability of silicate ceramics, increase the n-type electronic conductivity at low p(O 2 ), and probably partly suppress the ionic conductivity drop. The thermal expansion coefficients of apatite solid electrolytes in air are (8.8-9.9) X 10 -6 K -1 at 300-1250 K.

La2Zr2O7 formed at ceramic electrode/YSZ contacts

Strontium-doped LaCoO3 or LaMnO3 materials have been studied for use as cathodes for solid oxide fuel cells (SOFCs). This choice relies on the required properties and competitive cost. However, formation of reaction products under typical electrode-firing conditions may affect the performance of SOFCs. La2Zr2O7 was detected at YSZ/electrode interfaces. This reaction product was synthesized from powders and characterized to obtain a better understanding of its effects on cell performance. Its structural, thermal, and electrical properties are reported.