Duncan P. Fagg

Synthesis and electrical characterisation of doped perovskite titanates as potential anode materials for solid oxide fuel cells

This work reports the synthesis and electrical characterisation over a range of oxygen partial pressures (10–20–1 atm) of the A-site deficient perovskites Sr1–3x/2LaxTiO3–δ, with a view to establishing their potential as anode materials for solid oxide fuel cells. Single phase samples were observed for synthesis in air for 0≤x≤0.6, and the materials remained phase pure for both high and low oxygen partial pressures at the measurement temperature of 930 °C. Good electrical conductivity, which increased with increasing La content, was observed on reduction in low oxygen partial pressures, with values as high as 7 S cm–1 [ P(O2)= 10–20 atm], similar to values observed for the related system, Sr1–x/2Ti1–xNbxO3–δ, examined previously. The conductivity of the fully reduced samples showed metallic character from 100 to 930 °C. As the oxygen partial pressure was raised, the conductivity dropped, showing an approximate [P(O2)]–1/6 dependence for porous samples. New samples, Sr1–y/2–3x/2LaxTi1–yNbyO3–δ, with both La and Nb substitutions, were also studied, and these phases showed similar electrical behaviour. Further results for the Sr1–x/2Ti1–xNbxO3–δsystem are presented and compared with the La doped systems.

The effect of cobalt oxide sintering aid on electronic transport in Ce0.80Gd0.20O2−δ electrolyte

Abstract Additions of 2 mol% CoO 1.333 into gadolinia-doped ceria (CGO) solid electrolyte considerably improve sinterability and make it possible to obtain Ce 0.8 Gd 0.2 O 2− δ ceramics with 95–99% density at 1173–1373 K. The effect of cobalt oxide on the total electrical conductivity in air is negligible if the sintering is performed at 1173 K, although p-type electronic conduction measured at 900–1200 K increases with doping by 10–30 times. When increasing the sintering temperature up to 1773 K, grain growth in Co-containing CGO ceramics is accompanied with a decrease in both ionic and electron-hole transport. The oxygen ion transference numbers under oxygen/air gradient vary in the range 0.89–0.99. The n-type conductivity measured by the ion-blocking technique is lower for Co-containing materials than for undoped CGO, suggesting that the electrolytic domain can, to some extent, be enlarged by cobalt oxide additions. The relative role of both p- and n-type electronic contributions to the total conductivity of CGO increases with increasing temperature. The results show that Co-doped materials can still be used as solid electrolyte for intermediate-temperature electrochemical applications, when the operation temperature is 770–970 K.

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.

Stability and mixed ionic–electronic conductivity of (Sr,La)(Ti,Fe)O3−δ perovskites

Abstract Doping of cation-stoichiometric and A-site-deficient perovskites La x Sr 1− x Ti 1− y Fe y O 3− δ and La x Sr 0.90− x Ti 0.60 Fe 0.40 O 3− δ (0≤ x ≤0.45, 0.40≤ y ≤0.80) with lanthanum leads to a greater stability with respect to reduction in H 2 -containing atmospheres and the reaction with yttria-stabilised zirconia (YSZ). Partial oxygen ionic and p-type electronic conductivities in air as well as thermal expansion decrease with lanthanum content, whilst the effect on the n-type conduction in reducing environments is opposite. The ion transference numbers of (Sr,La)(Ti,Fe)O 3− δ , determined by faradaic efficiency measurements in air at 800–1000 °C, vary from 1×10 −4 to 4×10 −2 , increasing when temperature increases. Deficiency of the A sublattice and increasing iron concentration in the B sublattice result in a higher thermal expansion and greater reducibility; the reactivity with YSZ can be suppressed by the creation of A-site vacancies. The ionic and p-type electronic conductivities were both found to decrease with A-site deficiency and to increase with Fe content. The average thermal expansion coefficients of (Sr,La)(Ti,Fe)O 3− δ in air at 100–850 °C are in the range (10.6–21.5)×10 −6 K −1 .

P-Type Electronic Transport in Ce0.8Gd0.2O2 − δ: The Effect of Transition Metal Oxide Sintering Aids

Small (2 mol%) additions of cobalt, iron and copper oxides into Ce0.8Gd0.2O2 − δ considerably improve sinterability of ceria-gadolinia (CGO) solid electrolyte, making it possible to obtain ceramics with 95–99% density and sub-micron grain sizes at 1170–1370 K. The minor dopant additions have no essential effect on the total and ionic conductivity, whilst the p-type conduction in the transition metal-containing materials at 900–1200 K is 8–30 times higher than that in pure CGO. The oxygen ion transference numbers of the Co-, Fe- and Cu-doped ceramics, determined by the modified e.m.f. technique under oxygen/air gradient, are in the range 0.89–0.99. The electron-hole contribution to the total conductivity increases with temperature, as the activation energy for ionic conduction, 78 to 82 kJ/mol, is significantly lower than that for the p-type electronic transport (139–146 kJ/mol). The results show that CGO sintered with such additions can still be used as solid electrolytes for intermediate-temperature electrochemical applications, including solid oxide fuel cells (SOFCs) operating at 770–970 K, but that increasing operation temperature is undesirable due to performance loss.

Synthesis and characterisation of Ni–SrCe0.9Yb0.1O3−δ cermet anodes for protonic ceramic fuel cells

Abstract Cermet anode material with a proton-conducting ceramic phase, Ni–SrCe 0.9 Yb 0.1 O 3− δ , was synthesised by combustion from mixtures of molten nitrates and urea followed by sintering and reduction. Co-pressing the anode combustion powder on “green” SrCe 0.9 Yb 0.1 O 3− δ electrolyte and co-firing produced symmetrical anode/electrolyte/anode assemblies with planar electrodes of thickness ca. 100 μm. The addition of Co(NO 3 ) 2 ·6H 2 O to the electrolyte as sintering aid lowered the sintering temperature to 1250 °C, thereby allowing high anode porosity while retaining good anode/electrolyte adherence. The anode microstructure is composed of a uniform distribution of submicron Ni and perovskite particles. Analysis of the symmetrical assemblies by a.c. impedance spectroscopy indicates that the electrode polarisation resistances are composed of at least two contributions. Stability issues concerning the Ni–SrCe 0.9 Yb 0.1 O 3− δ anodes in reducing atmospheres are discussed.

Development of novel anodes for solid oxide fuel cells

Abstract Ni cermet anodes pose considerable problems for SOFC operation in natural gas fuels, particularly with regard to carbon deposition due to hydrocarbon cracking. Oxide anodes offer a good alternative, particularly if a material combining good electronic and ionic transport properties can be utilised. In our search for alternative anode materials, we have investigated fluorite-based systems containing reducible early transition metal dopants. The extent of phase stability has been investigated by solid-state chemical techniques and electrical properties have been investigated by ac impedance techniques as a function of both temperature and oxygen partial pressure. The NbZrYO system has been found to provide a good model system exhibiting reasonable electrical properties. Niobium pentoxide exhibits a wide range of solid solubility in the yttria-zirconia cubic fluorite system and the fluorite structure is retained under reducing conditions. Electronic conductivity increases as niobium concentration increases; however oxide-ionic conductivity decreases with extent of niobium substitution. The defect chemistry of this system, which determines the electrical properties, is dominated by the high concentration of oxide vacancies necessary to stabilise the cubic structure, hence electronic conductivity exhibits a P(O2)-1/4 dependence on oxygen partial pressure.

Electrical characterization of highly Titania doped YSZ

The defect fluorite region of the ternary system ZrO2-Y2O3-TiO2 encompasses compositions which offer both, good electronic and oxygen ion conductivity which enable good catalytic activity for the direct oxidation of methane in a solid oxide fuel cell (SOFC). The electrical properties of compositions YxTiyZr1−(x+y)O2−x/2 (with x=0.15, 0.2, 0.25 and y=0.15, 0.18) were characterised in order to find the composition with highest ionic and electronic conductivity. High titanium dopant concentrations (Y) of 15 and 18 atom%, near the solubility limit of Ti4+ in the fluorite structure, have been introduced to achieve a high electronic conductivity at low oxygen partial pressure. The yttrium content x has been varied between 15 and 25 atom% to find the fluorite composition with the highest ionic conductivity for each titanium level.In the pO2-range from 0.21 to 10−13 atm the conductivity is predominantly ionic and constant over that range. The maximum ionic conductivity is 0.01 Scm−1 for the compositions, which contain 15 atom% yttrium. Substantial electronic conductivity is introduced into the system at low oxygen pressures below 10−13 atm via reduction of Ti4+ ions to Ti3+. The maximum electronic conductivity of 0.2 Scm−1 at 930 °C has been measured for a sample with 18 atom% titanium. The slope of all log(σ) vs. log(pO2) plots follows a pO2−1/4-dependence.

Electrochemical behaviour and degradation of (Ni,M)/YSZ cermet electrodes (M=Co,Cu,Fe) for high temperature applications of solid electrolytes

Abstract Homogeneous powders of alternative (Ni,M)/YSZ cermets were prepared by combustion synthesis to obtain homogeneous mixtures of submicrocrystalline powders. These powders were used to prepare symmetrical cells cermet/YSZ/cermet, with porous cermet layers and a dense electrolyte. The cermets were reduced in H2, and tested as potential fuel electrodes for high temperature electrochemical applications. The best performance was found for Ni1−xCox/YSZ cermets but small fractions of Cu might also play a positive effect. Changes in capacitance suggest that this corresponds to cermets with enhanced microstructures. Lower firing conditions are probably needed to avoid the degradation of cermets containing Cu or Fe.

The importance of phase purity in Ni–BaZr0.85Y0.15O3−δ cermet anodes – novel nitrate-free combustion route and electrochemical study

A novel, nitrate-free, combustion method has been developed to prepare Ni–BaZr0.85Y0.15O3−δ (Ni–BZY) cermet anodes for proton ceramic fuel cells. Nickel acetate and 30% H2O2 were used as starting precursors for the combustion reaction into which pre-prepared BZY powders were dispersed. The advantages of this nitrate-free combustion method have been demonstrated by comparison to a more common nitrate/glycine combustion route. The results demonstrate that use of the nitrate-free precursors is essential to avoid partial decomposition of the pre-prepared BZY phase. Employment of the more common nitrate-based precursors, due to their acidic nature, results in removal of Ba from the perovskite phase and formation of Ba(NO3)2, subsequently leading to the presence of BaY2NiO5 in the final product. The impact of Ni–BZY phase purity on resultant polarisation behaviour has been assessed as a function of water vapour and oxygen partial pressures for electrodes of comparable microstructure. Partial decomposition of the perovskite phase limits performance by increasing the higher frequency polarisation resistance and this phenomenon is suggested to be associated with impaired proton transport in the oxide cermet phase. The novel actetate–H2O2 combustion method here described may be of interest for the formation of other ceramic oxide materials, offering economical advantages over more classical nitrate-based combustion routes, as well as significant environmental benefits due to the avoidance of releasing NOx gases.