Synthesis and characterisation of oxyanion-doped cobalt containing perovskites

In this paper we report the incorporation of borate, silicate and phosphate into La0.6Sr0.4Co0.8Fe0.2O3−δ (LSCF) and Sr0.9Y0.1CoO3−δ (SYC) cathode materials for SOFCs. In the former, an increase in the electronic conductivity was observed, which can be correlated with electron doping due to the oxyanion doping favouring the introduction of oxide ion vacancies. The highest conductivity was observed for\ud La0.6Sr0.4Co0.76Fe0.19B0.05O3−δ, 1190 S·cm-1 at 700ºC, in comparison with 431 S·cm-1 for undoped La0.6Sr0.4Co0.8Fe0.2O3−δ at the same temperature. For Sr0.9Y0.1CoO3−δ series the conductivity suffers a decrease on doping, attributed to any effect of electron doping\ud being outweighed by the effect of partial disruption of the electronic conduction pathways by the oxyanion. In order to investigate the potential of these materials as SOFC cathodes, the chemical compatibility with Gd0.1Ce0.9O1.95 (CGO10) was investigated and no reaction was observed between RT and 1100 ºC for both series. Composites of these cathode materials with 50% CGO10 were examined on dense\ud CGO10 pellets and the area specific resistances (ASR) in symmetrical cells were determined. The ASR values, at 800ºC, were 0.20, 0.08 and 0.11 W·cm2 for\ud La0.6Sr0.4Co0.8Fe0.2O3−δ, La0.6Sr0.4Co0.76Fe0.19B0.05O3−δ and La0.6Sr0.4Co0.78Fe0.195Si0.025O3−δ, respectively. For the SYC materials, the oxyaniondoped compositions also showed an improvement in the ASR values with respect to the\ud parent compounds, despite the lower electronic conductivity in these cases. At 800ºC, the values obtained for Sr0.9Y0.1CoO3−δ, Sr0.9Y0.1Co0.975B0.025O3−δ,\ud Sr0.9Y0.1Co0.975Si0.025O3−δ and Sr0.9Y0.1Co0.975P0.025O3−δ, were 0.09, 0.05, 0.05 and 0.03 W·cm2, respectively. This observation may be due to an increase in ionic conductivity\ud due to oxyanion incorporation leading to the formation of oxide ion vacancies. In addition, the stability of these systems towards CO2 was studied. For La0.6Sr0.4Co0.8(1-\ud x)Fe0.2(1-x)MxO3−δ series, all compositions showed no evidence for reactivity with CO2 between RT and 1000 ºC. On the other hand, for the Sr0.9Y0.1Co1-xMxO3−δ series, some\ud reactivity was observed, although the CO2 stability was shown to be improved on oxyanion doping. Thus these results show that oxyanion doping can have a beneficial effect on the performance of perovskite cobaltite cathode materials

Neutron diffraction structural study of the apatite-type oxide ion conductor, La8Y2Ge6O27: location of the interstitial oxide ion site

Apatite-type rare earth silicates/germanates have attracted considerable interest recently due to their high oxide ion conductivities. Despite evidence in support of a conduction mechanism involving interstitial oxide ions, the exact location of the interstitial oxide ion sites continues to attract controversy. In this paper we report a neutron diffraction structural study for the high oxygen excess compound, La8Y2Ge6O27. The structural model indicates that the oxide ions are located between the GeO4 tetrahedra, leading to significant localised distortions. These results, coupled with recent modelling studies, hence, support the conclusion that oxide ion migration proceeds via these tetrahedra

Enhancement of the conductivity of Ba2In2O5 through phosphate doping

In this paper, we demonstrate the successful incorporation of phosphate into Ba2In2O5, which leads to the conversion from an orthorhombic to a cubic unit cell. The resulting increased oxygen vacancy disorder leads to an enhancement in the oxide ion conductivity at low temperatures. In addition, in wet atmospheres, significant proton conduction is observed

Synthesis and characterization of proton conducting oxyanion doped Ba2Sc2O5

In this paper we report the successful synthesis of the cubic oxyanion containing perovskites, Ba2Sc2-xPxO5+x (x=0.4, 0.5), with the samples analysed through a combination of X-ray diffraction, NMR, TGA, Raman spectroscopy and conductivity measurements. Conductivity measurements indicate a p-type contribution to the conductivity in oxidizing conditions at elevated temperatures, with evidence for proton conduction in wet atmospheres. For the latter bulk conductivities of 5.9 x 10-3 and 1.3 x 10-3 Scm-1 at 500○C were obtained for x=0.4 and 0.5 respectively, comparable to other perovskite proton conductors, while the stability towards CO2 containing atmospheres was improved compared to BaCeO3 based systems.\ud Related Si doped systems have also been prepared, although in this case small Ba2SiO4 impurities are observed. We also provide evidence to suggest that “undoped” Ba2Sc2O5 contains carbonate groups, which accounts for its thermal instability

Raman spectroscopy insights into the a- and d-phases of formamidinium lead iodide (FAPbI3)

Solar perovskites have received phenomenal attention and success over the past decade, due to their high power conversion efficiencies (PCE), ease of fabrication and low cost which has enabled the prospect of them being a real commercial contender to the traditional silicon technology. In one of the several developments on the archetypal MAPbI3perovskite absorber layer, FAPbI3was found to obtain a higher PCE, likely due to its more optimum band gap, with doping strategies focusing on the inclusion of MA+/Cs+cations to avoid the unfavourable phase transformation to a photoinactive phase. To better understand the phase change from the photoactive cubic (Pm3¯m) black (a) phase to the unwanted photoinactive (P63/mmc) yellow (d) phase, we make use of variable temperature Raman spectroscopy to probe the molecular species and its relationship to the inorganic framework. We show for the first time there to be no Raman active modes for the a phase up to 4000 cm-1, which can be correlated to thePm3¯mcubic symmetry of that phase. Our detailed studies suggest that previous reports of the observation of Raman peaks for this phase are likely associated with degradation reactions from the localised laser exposure and the formation of Raman active lead oxide. In addition, we have identified water as a contributing factor to the transformation, and observed a corresponding signal in the Raman spectra, although confirmation of its exact role still remains inconclusive

Considering the suitability of symmetrical cell testing in developing electrodes for solid oxide fuel cells via a study of different lanthanum nickelate cathode materials

Developing solid oxide fuel cells (SOFCs) with improved performance and lifetime continues to attract research attention from around the world. One important focus in this field is the synthesis of new air electrode materials that can replace the state-of-the-art lanthanum cobaltite-type phases. A host of materials with a wide range of properties has resulted. However, the means and metrics by which promising cathode materials are best characterised are not widely agreed upon within the literature and this can often complicate comparisons between studies. One common approach to conducting analysis of electrodes is to employ so-called ‘symmetrical cell’ tests which aim to isolate the performance of a specific electrode material under open-circuit conditions. However, despite the prevalence of symmetrical cell testing in the literature, there are some widely accepted limitations of the approach (e.g. limited to study at equilibrium conditions). In this work, a selection of air electrode materials with a wide range of properties were studied in both symmetrical and single cell testing set-ups. This case-study was conducted to identify the correlation between the two approaches and to understand how successful the symmetrical cell testing approach is in identifying favourable electrode materials. The results show that, whilst symmetrical-cell testing can be used to identify open circuit behaviours, the comparison between polarisation resistance at open circuit and performance under polarisation is not always perfectly correlated. Crucially, while the symmetrical cell test can provide some guidance in determining whether a new material may show promise, it highlights the need for more detailed studies to understand material performance under polarised conditions

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. SrFe 0.90Si0.10O3-δ and SrFe 0.85Si0.15O3-δ were analysed by neutron powder diffraction, and the data confirmed the cubic cell, with no long range oxygen vacancy ordering. Mössbauer 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% Ce0.9Gd0.1O 1.95 were therefore examined on dense Ce0.9Gd 0.1O1.95 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. © 2013 The Royal Society of Chemistry.Peer Reviewe

Curvature, hybridization, and STM images of carbon nanotubes

The curvature effects in carbon nanotubes are studied analytically as a function of chirality. The pi-orbitals are found to be significantly rehybridized in all tubes, so that they are never normal to the tubes' surface. This results in a curvature induced gap in the electronic band-structure, which turns out to be larger than previous estimates. The tilting of the pi-orbitals should be observable by atomic resolution scanning tunneling microscopy measurements.Comment: Four pages in revtex format including four epsfig-embedded figures. The latest version in PDF format is available from http://fy.chalmers.se/~eggert/papers/hybrid.pd

Roadmap on Li-ion battery manufacturing research

Growth in the Li-ion battery market continues to accelerate, driven primarily by the increasing need for economic energy storage for electric vehicles. Electrode manufacture by slurry casting is the first main step in cell production but much of the manufacturing optimisation is based on trial and error, know-how and individual expertise. Advancing manufacturing science that underpins Li-ion battery electrode production is critical to adding to the electrode manufacturing value chain. Overcoming the current barriers in electrode manufacturing requires advances in materials, manufacturing technology, in-line process metrology and data analytics, and can enable improvements in cell performance, quality, safety and process sustainability. In this roadmap we explore the research opportunities to improve each stage of the electrode manufacturing process, from materials synthesis through to electrode calendering. We highlight the role of new process technology, such as dry processing, and advanced electrode design supported through electrode level, physics-based modelling. Progress in data driven models of electrode manufacturing processes is also considered. We conclude there is a growing need for innovations in process metrology to aid fundamental understanding and to enable feedback control, an opportunity for electrode design to reduce trial and error, and an urgent imperative to improve the sustainability of manufacture