J.M. Porras-Vázquez

Low temperature crystal structures of apatite oxygen-conductors containing interstitial oxygen

Oxygen-stoichiometric La(9.33) square(0.67)(Si(6)O(24))O2 and oxygen-excess La(8.65)Sr(1.35)(Ge(6)O(24))O(2.32) and La(8.65)Sr(1.35)(Si(6)O(24))O(2.32) oxy-apatites have been structurally characterized at low temperatures by the Rietveld method. Oxygen-interstitial distribution has been studied at 15 K for La(9.33) square(0.67)(Si(6)O(24))O2 and La(8.65)Sr(1.35)(Ge(6)O(24))O(2.32) by time-of-flight neutron powder diffraction and at 4 K for La(8.65)Sr(1.35)(Si(6)O(24))O(2.32) by constant-wavelength neutron powder diffraction. The low temperature structural study was undertaken in order to distinguish between the effects of static disorder, originated mainly from the presence of interstitial oxygens, and the anisotropic thermal vibrations. At such low temperatures, the influence of the anisotropic thermal vibrations is minimised. This structural study has firmly established the existence of interstitial oxygens in these materials, which may be useful as electrolytes for solid oxide fuel cells.

Synthesis and characterisation of oxyanion-doped manganites for potential application as SOFC cathodes

In this paper we report the successful incorporation of borate and phosphate into CaMnO3 and borate into La1−ySryMnO3−δ. For CaMnO3, an increase in the electronic conductivity was observed, which can be correlated with electron doping due to the oxyanion doping favoring the introduction of oxide ion vacancies (as well as the higher valence of P5+ compared to Mn4+ in the case of phosphate doping). The highest conductivity at 800 °C was observed for CaMn0.95P0.05O3−δ, 43.0 S cm−1, in comparison with 7.6 S cm−1 for undoped CaMnO3 at the same temperature. For La1−ySryMnO3−δ the conductivity suffers a decrease for all compositions on borate doping, attributed to a reduction in the hole (Mn4+) concentration. In order to investigate the potential of these materials as SOFC cathodes, the chemical compatibility with Gd0.1Ce0.9O1.95 (CGO10) has also been investigated. For the calcium manganites, the lowest temperature examined without reaction was 900 °C, with minor amounts of Ca4Mn3O10 observed at 1000 °C. Composites of these cathode materials with 50% CGO10 were examined on dense CGO10 pellets and the area specific resistances (ASR) in symmetrical cells were determined. The ASR values, at 800 °C, were 1.50, 0.37 and 0.30 Ω·cm2 for CaMnO3, CaMn0.95B0.05O3−δ and CaMn0.95P0.05O3−δ, respectively. For the lanthanum strontium manganites, the B-doped compositions showed an improvement in the ASR values with respect to the parent compounds, despite the lower electronic conductivity. This may be due to an increase in ionic conductivity due to borate incorporation leading to the formation of oxide ion vacancies. Thus these preliminary results show that oxyanion doping has a beneficial effect on the performance of perovskite manganite cathode materials, and suggests that this doping strategy warrants further investigation in other perovskite cathode systems.

Investigation into the effect of Si doping on the performance of SrFeO3−δ SOFC electrode materials

In this paper we report the successful incorporation of silicon into SrFeO3−δ perovskite materials for potential applications as electrode materials for solid oxide fuel cells. It is observed that Si doping leads to a change from a tetragonal cell (with partial ordering of oxygen vacancies) to a cubic one (with the oxygen vacancies disordered). Annealing experiments in 5% H2/95% N2 (up to 800 °C) also showed the stabilization of the cubic form for the Si-doped samples under reducing conditions, suggesting that they may be suitable for both cathode and anode applications. In contrast to the cubic cell of the reduced Si doped system, reduction of undoped SrFeO3−δ leads to the formation of a brownmillerite structure with ordered oxide ion vacancies. SrFe0.90Si0.10O3−δ and SrFe0.85Si0.15O3−δ were analysed by neutron powder diffraction, and the data confirmed the cubic cell, with no long range oxygen vacancy ordering. Mossbauer spectroscopy data were also recorded for SrFe0.90Si0.10O3−δ, and indicated the presence of only Fe3+ and Fe5+ (i.e. disproportionation of Fe4+ to Fe3+ and Fe5+) for such doped samples. Conductivity measurements showed an improvement in the conductivity on Si doping. Composite electrodes with 50% Ce0.9Gd0.1O1.95 were therefore examined on dense Ce0.9Gd0.1O1.95 pellets in two different atmospheres: air and 5% H2/95% N2. In both atmospheres an improvement in the area specific resistance (ASR) values is observed for the Si-doped samples. Thus the results show that silicon can be incorporated into SrFeO3−δ-based materials and can have a beneficial effect on the performance, making them potentially suitable for use as cathode and anode materials in symmetrical SOFCs.

Investigation into the effect of Si doping on the performance of Sr1−yCayMnO3−δ SOFC cathode materials

In this paper we report the successful incorporation of silicon into Sr1-yCayMnO3-δ perovskite materials for potential applications in cathodes for solid oxide fuel cells. The Si substitution onto the B site of a (29)Si enriched Sr1-yCayMn1-xSixO3-δ perovskite system is confirmed by (29)Si MAS NMR measurements at low B0 field. The very large paramagnetic shift (~3000-3500 ppm) and anisotropy (span ~4000 ppm) suggests that the Si(4+) species experiences both Fermi contact and electron-nuclear dipolar contributions to the paramagnetic interaction with the Mn(3+/4+) centres. An improvement in the conductivity is observed for low level Si doping, which can be attributed to two factors. The first of these is attributed to the tetrahedral coordination preference of Si leading to the introduction of oxide ion vacancies, and hence a partial reduction of Mn(4+) to give mixed valence Mn. Secondly, for samples with high Sr levels, the undoped systems adopt a hexagonal perovskite structure containing face sharing of MnO6 octahedra, while Si doping is shown to help to stabilise the more highly conducting cubic perovskite containing corner linked octahedra. The level of Si, x, required to stabilise the cubic Sr1-yCayMn1-xSixO3-δ perovskite in these cases is shown to decrease with increasing Ca content; thus cubic symmetry is achieved at x = 0.05 for the Sr0.5Ca0.5Mn1-xSixO3-δ series; x = 0.075 for Sr0.7Ca0.3Mn1-xSixO3-δ; x = 0.10 for Sr0.8Ca0.2Mn1-xSixO3-δ; and x = 0.15 for SrMn1-xSixO3-δ. Composites with 50% Ce0.9Gd0.1O1.95 were examined on dense Ce0.9Gd0.1O1.95 pellets. For all series an improvement in the area specific resistances (ASR) values is observed for the Si-doped samples. Thus these preliminary results show that silicon can be incorporated into perovskite cathode materials and can have a beneficial effect on the performance.

Ti-doped SrFeO3 nanostructured electrodes for symmetric solid oxide fuel cells

Nanostructured electrodes of Sr0.98Fe1−xTixO3−δ are evaluated as both cathode and anode for solid oxide fuel cells. The electrodes are prepared by a low-cost and simple procedure based on spray-pyrolysis deposition on a porous Ce0.8Gd0.2O1.9 (CGO) layer. A homogenous coating layer of electrode catalyst nanoparticles is formed on the CGO backbone surface in a single deposition-firing step. Sr0.98Fe0.8Ti0.2O3−δ (SFT0.2) exhibits high efficiency operating as both cathode and anode with polarization resistance values of 0.1 Ω cm2 in air and 0.07 Ω cm2 in humidified H2 at 700 °C. An electrolyte supported cell with 300 μm thick La0.9Sr0.1Ga0.8Mg0.2O3−δ electrolyte and SFT0.2 symmetric electrodes shows maximum power densities of 700 and 140 mW cm−2 at 800 and 600 °C, respectively.

Improving the efficiency of layered perovskite cathodes by microstructural optimization

Low-temperature solid oxide fuel cells require the use of cathodes with improved performance. In this context, microstructural optimization is fundamental in order to obtain more efficient and stable materials. However, most of the current fabrication techniques involve multiple steps and therefore they are not suitable for industrial applications. This report describes alternative strategies to prepare PrBaCo2O5+δ (PBC) cathodes by using a simple and economic spray-pyrolysis deposition (SP) method. Three different electrode configurations have been tested: (i) PBC prepared by spray-pyrolysis on dense CGO pellets, (ii) PBC deposited onto porous CGO backbone layers and (iii) submicrometric PBC powders prepared from freeze-dried precursors and deposited by a screen-printing process. The second approach is an alternative to the traditional wet infiltration method with a number of advantages, such as a shorter preparation time and simplicity of implementation at industrial scale. Reduced values of polarization resistance (Rp) are obtained at 600 °C, 0.027 Ω cm2 for SP electrodes on porous CGO backbones, in comparison to 0.22 Ω cm2 for submicrometric powder cathodes by screen-printing. Moreover, SP electrodes demonstrate improved stability with stable Rp values at 650 °C over time. A Ni–CGO anode-supported cell with an SP cathode achieves a remarkable peak power density of 0.95 W cm−2 at 600 °C in comparison to 0.6 W cm−2 for the cell with a screen-printed cathode.

Metal-Doping of La5.4MoO11.1 Proton Conductors: Impact on the Structure and Electrical Properties

La5.4MoO11.1 proton conductors with different metal doping (Ca2+, Sr2+, Ba2+, Ti4+, Zr4+, and Nb5+) have been prepared and structurally and electrically characterized. Different polymorphs are stabilized depending on the doping and cooling rate used during the synthesis process. The most interesting results are obtained for Nb-doping, La5.4Mo1- xNb xO11.1- x/2, where single compounds are obtained in the compositional range 0 ≤ x ≤ 0.2. These materials are fully characterized by structural techniques such as X-ray and neutron powder diffraction and transmission electron microscopy, which independently confirm the changes of polymorphism. Scanning electron microscopy and impedance spectroscopy measurements in dry/wet gases (N2, O2, and 5% H2-Ar) showed an enhancement of the sinterability and electrical properties of the materials after Nb-doping. Conductivity measurements under very reducing conditions revealed that these materials are mixed ionic-electronic conductors, making them potential candidates for hydrogen separation membranes.