Synthesis and characterisation of novel materials for energy applications

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Jarvis, Abbey ORCID: https://orcid.org/0000-0003-4421-904X (2021). Synthesis and characterisation of novel materials for energy applications. University of Birmingham. Ph.D.

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Abstract

In this thesis a range of different systems have been investigated including perovskite phases and perovskite type phases such as Ruddlesden-Popper materials. In addition apatite materials have also been investigated. These materials have been of particular interest due to appealing properties for energy applications, in particular for solid oxide fuel cells. A particular focus has been on iron oxide systems due to the low cost and high natural abundance of iron. Oxyanion doping and fluorination has been investigated for the perovskite related systems, while exsolution of Cu has been investigated for apatite systems, with the aim to improve the properties of these materials.

Perovskite (SrFeO\(_{3-δ}\), Sr\(_{0.5}\)Ba\({0.5}\)FeO\(_{3-δ}\) and BaFeO\(_{3-δ}\),) and Ruddlesden-Popper materials (Sr\(_4\)Fe\(_3\)O\(_{10-δ}\) and Sr\(_4\)Fe\(_2\)(Cu/Co)O\(_{10-δ}\)) were doped with various oxyanions (sulfate, borate, phosphate, chromate and carbonate). It was found that a range of oxyanions could be successfully incorporated into these perovskite and Ruddlesden-Popper phases. Interestingly, X-ray diffraction and thermogravimetric analysis suggests carbonate stabilises the cubic perovskite Ba-Fe-O system at low synthesis temperatures. The perovskite systems, SrFeO\(_{3-δ}\) and Sr\(_{0.5}\)Ba\(_{0.5}\)FeO\(_{3-δ}\) and all Ruddlesden-Popper systems were tested in dry and wet N\(_2\), and conductivity measurements were performed in order to test suitability for use as cathodes in solid oxide fuel cells. Following the successful incorporation of oxyanions into perovskite phases, oxyanion incorporation into Ruddlesden-Popper materials also showed oxyanions could be successfully incorporated into the structure. The Ruddlesden-Popper phases showed promising results when heating in wet N\(_2\) indicating potential for future studies into the use in conjunction with a proton conducting electrolyte. However, low conductivity values were reported for these Ruddlesden-Popper phases, which could be improved by replacing some Fe for Cu/ Co. The most promising oxyanion doped Ruddlesden-Popper material was found to be Sr\(_4\)Fe\(_{3-x}\)S\(_x\)O\(_{10-δ}\), therefore further doping strategies (Cu/ Co doping) were carried out for these materials. Cu/ Co incorporation successfully improved the stability and conductivity of these materials for SOFC applications.

Further oxyanion studies were investigated to prepare “0201-1201” type layered oxide phases with this doping strategy allowing the successful synthesis of new phases, Sr\(_{4.5}\)Fe\(_2\)(S/Cr)\(_{0.5}\)O\(_{9±δ}\). Neutron diffraction indicated successful incorporation of SO\(_4\)\(^{2-}\) and CrO\(_4\)\(^{2}\)- with X-ray absorption spectroscopy additionally supporting the incorporation of tetrahedral Cr\(^{6+}\) as observed for Sr\(_4\)Fe\(_{3-x}\)(S/Cr)\(_x\)O\(_{10-δ}\) phases. Although low conductivity, the stability of materials in dry and wet N\(_2\) indicates potential for use as SOFC cathode materials particularly for proton conducting SOFCs if further optimised.

In addition to doping on the cation sites, fluorination of the Ruddlesden-Popper phases, Sr\(_4\)Fe\(_{2.75}\)(S/Cr)\(_{0.25}\)O\(_{10-δ}\) was investigated. Successful fluorination of Sr\(_4\)Fe\(_{3-x}\)(S/Cr)\(_x\)O\(_{10-δ}\) was shown through X-ray diffraction with a large cell expansion with increased F content. Sr\(_4\)Fe\(_{2.75}\)Cr\(_{0.25}\)O\(_{10-δ}\) was found to incorporate higher F contents than the sulfate doped analogues attributed to the ability to reduce not only iron, but also chromium. Using a low temperature synthesis method, fluorinated Ruddlesden-Popper materials are not only successfully synthesised, but also have the additional ability to incorporate a range of F contents. This work highlights interesting materials with the potential for use in fluoride-ion batteries.

Potential solid oxide fuel cell electrolyte materials were also investigated following the successful incorporation of oxyanions into perovskite and perovskite type electrode materials. These materials include oxyanion doped perovskite type Ba\(_3\)(Y/Tm)\(_2\)Ti\(_{1-x}\)(S/P)\(_x\)O\(_{8+y}\) phases. Ba\(_3\)(Y/Tm)\(_2\)Ti\(_{1-x}\)(S/P)\(_x\)O\(_{8+y}\) phases were successfully prepared, showing stabilisation of the cubic perovskite structure for Ba\(_3\)Tm\(_2\)Ti\(_{1-x}\)(S/P)\(_x\)O\(_{8+y}\) phases. These oxyanion doped systems were found to have improved conductivities compared with the undoped phases.

Following the successful stabilisation of potential SOFC materials through oxyanion doping, an alternate doping strategy was considered. Exsolution was used in order to exsolve copper nanoparticles onto the surface of apatite materials. Due to the appealing properties of apatite phases as electrolyte materials for solid oxide fuel cell applications, a selection of Si/Ge/P apatite phases were investigated. All apatite phases were doped with Cu and heated in 10% H\(_2\)/ 90% N\(_2\). After heat treatment, materials were analysed with X-ray diffraction and scanning electron microscopy showing the successful exsolution of Cu particles on the surface of the apatite structure. Future studies will investigate the potential of such phases for use as anodes in solid oxide fuel cells.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):