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Abstract: Inorganic perovskites exhibit many important physical properties such as ferroelectricity, magnetoresistance and superconductivity as well their importance as Energy Materials. Many of the most important energy materials are inorganic perovskites and find application in batteries, fuel cells, photocatalysts, catalysis, thermoelectrics and solar thermal. In all these applications, perovskite oxides, or their derivatives offer highly competitive performance, often state of the art and so tend to dominate research into energy material. In the following sections, we review these functionalities in turn seeking to facilitate the interchange of ideas between domains. The potential for improvement is explored and we highlight the importance of both detailed modelling and in situ and operando studies in taking these materials forward.

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Abstract: A general method to carry out the fluorination of metal oxides with poly(tetrafluoroethylene) (PTFE, Teflon) waste by spark plasma sintering (SPS) on a minute scale with Teflon is reported. The potential of this new approach is highlighted by the following results. i) The tantalum oxyfluorides Ta3O7F and TaO2F are obtained from plastic scrap without using toxic or caustic chemicals for fluorination. ii) Short reaction times (minutes rather than days) reduce the process time the energy costs by almost three orders of magnitude. iii) The oxyfluorides Ta3O7F and TaO2F are produced in gram amounts of nanoparticles. Their synthesis can be upscaled to the kg range with industrial sintering equipment. iv) SPS processing changes the catalytic properties: while conventionally prepared Ta3O7F and TaO2F show little catalytic activity, SPS‐prepared Ta3O7F and TaO2F exhibit high activity for photocatalytic oxygen evolution, reaching photoconversion efficiencies up to 24.7% and applied bias to photoconversion values of 0.86%. This study shows that the materials properties are dictated by the processing which poses new challenges to understand and predict the underlying factors.

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Abstract: In this work the suitability of nanostructured ferrite materials with the general formula AFe2O4 (where A is a divalent cation) for photocatalytic applications is investigated. Spinel ferrite MgFe2O4 nanoparticles and macroporous CaFe2O4 sponge structures were produced by microwave-assisted syntheses in high-boiling organic solvents and subsequent calcination in air. The elemental composition of the products was monitored by energy dispersive X-ray spectroscopy and the synthesis procedures were optimized to ensure an ideal stoichiometry of the products. Phase purity of the products was confirmed by calcination studies combined with diffraction experiments and by a wide variety of spectroscopic techniques. The morphology of the ferrite materials is characterized by electron microscopy, gas physisorption and mercury intrusion porosimetry. Regarding the electronic band structure of ferrites, a vast dissent is found in published literature. This is addressed by a thorough characterization of the electronic structure using photoelectrochemical measurements, X-ray based spectroscopic techniques, and by a detailed interpretation of their optical absorption spectra. The determined band positions suggest that CaFe2O4 is suitable for photocatalytic hydrogen evolution under visible light, while MgFe2O4 is not. Nevertheless, both phases remain inactive in hydrogen evolution test reactions as well as other photocatalytic experiments. X-ray based spectroscopy suggests that the presence of a transition metal with d5 electronic configuration causes a strong discrepancy between the fundamental electronic band gap and the one determined by optical spectroscopy. The Fe3+ crystal field orbitals involved in the ligand-to-metal charge transfer excitations that are responsible for the absorption of visible light are highly localized at the Fe3+ centers. The weak orbital overlap causes a low mobility of excited charge carriers explaining the inactivity in photocatalysis. Additional to the optical and photocatalytic properties, the magnetism of the synthesized materials is investigated by Mössbauer spectroscopy and SQUID magnetometry. While CaFe2O4 exhibits antiferromagnetic behavior, the MgFe2O4 nanoparticles exhibit a tunable magnetization, that depends on crystallite size and cation inversion and is therefore adjustable by post-synthetic calcination. First attempts towards the synthesis of magnetic NiFe2O4 and MnFe2O4 nanoparticles were made, to extend the scope of magnetic nanoparticles that can be synthesized via the microwave-assisted reaction. Attempting to combine the optical and magnetic characteristics of ferrites with other chemical functionalities in a composite material, phase-pure MgFe2O4 nanoparticles were immobilized on functionalized, ordered-mesoporous SiO2 and organosilica host networks.

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Abstract: Solid state batteries have attracted extensive attention, but the lithium penetration through the solid electrolyte remains a critical barrier to commercialisation and is not yet fully understood. In this study, the 3D morphological evolution of cracks with deposited lithium were tracked as they penetrated through the solid electrolyte during repetitive plating. This is achieved by utilising in-situ synchrotron X-ray computed tomography with high spatial and temporal resolutions. Thin-sheet cracks were observed to penetrate the solid electrolyte without immediate short-circuiting of the cell. Changes in their width and volume were quantified. By calculating the volume of deposited lithium, it was found that the lithium was only partially filled in cracks, and its filling ratio quickly dropped from 94.95% after the 1st plating to ca. 20% after the 4th plating. The filling process was revealed through tracking the line profile of grayscale along cracks. It was found that lithium grew much more slowly than cracks, so that the cracks near the cathode side were largely hollow and the cell could continue to operate. The deposited lithium after short circuit was segmented and its distribution was visualised. DVC analysis was applied to map local high stress and strain, which aggregated along cracks and significantly increased at areas where new cracks formed.

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Abstract: CdTe is currently the largest thin-film photovoltaic technology. Non-radiative electron–hole recombination reduces the solar conversion efficiency from an ideal value of 32% to a current champion performance of 22%. The cadmium vacancy (VCd) is a prominent acceptor species in p-type CdTe; however, debate continues regarding its structural and electronic behavior. Using ab initio defect techniques, we calculate a negative-U double-acceptor level for VCd, while reproducing the VCd1– hole–polaron, reconciling theoretical predictions with experimental observations. We find the cadmium vacancy facilitates rapid charge-carrier recombination, reducing maximum power-conversion efficiency by over 5% for untreated CdTe—a consequence of tellurium dimerization, metastable structural arrangements, and anharmonic potential energy surfaces for carrier capture.