Universität Stuttgart

The conversion of waste heat into electrical current by a thermoelectric converter can significantly contribute to a more sustainable usage of our resources. The p-type misfit-layered [Ca2CoO3-δ][CoO2]1.62 is known for its promising conversion efficiency, which yet needs to be improved significantly for commercial applications. The efficiency of a material increases with the Figure of Merit ZT=σα^2/κ, with Seebeck coefficient α, electrical conductivity σ, and thermal conductivity κ. The aim of this thesis is to provide a better understanding of the electrical and the thermal properties of the complex [Ca2CoO3 δ][CoO2]1.62 and to use this understanding to improve the efficiency of converters. Accordingly, (i) the increase of ZT via cation substitution is shown; (ii) a better understanding of the electrical transport above room temperature is developed; (iii) the effect of stoichiometric defects and secondary phases on the thermoelectric properties is investigated. Finally, (iv) [Ca2CoO3 δ][CoO2]1.62 - CaMn0.97W0.03O3 δ - converters are fabricated and the efficiency is increased by a suitable converter design. More specifically, the unexplored influence of Ru and In substitution on the thermoelectric properties of the polycrystalline [Ca2CoO3 δ][CoO2]1.62 is investigated. While In does not have a positive effect, Ru for Co substitution increases ZT up to 20 %. This increase stems from a strong reduction of the thermal conductivity - which is probably induced by resonance scattering - while the decrease of the power factor α^2 σ is minor. The electrical transport mechanism of pure and Ru-substituted [Ca2CoO3 δ][CoO2]1.62 between room temperature and 800 K so far lacks a coherent theoretical model. Surprisingly, the framework of Anderson localization, which was developed to describe conduction in an impurity band of semiconductors, can be applied to the oxide. The Anderson model assumes that transport happens via charge-carrier hopping in a random Coulomb potential. For [Ca2CoO3 δ][CoO2]1.62, charges are considered to hop between Co sites in the CoO2 layer, while the random potential originates from interactions with the mismatched Ca2CoO3 δ layer. The presence of the ionized Ru atoms further alters the Coulomb potential, which increases the activation energy of the transport behavior. This understanding might contribute to the development of better theoretical models for the prediction of the thermoelectric properties of substituted [Ca2CoO3 δ][CoO2]1.62 compounds. A further improvement of the materials efficiency can be achieved by systematic introduction of stoichiometric defects and impurity phases. Here, the unexplored influence of the Co/Ca ratio on the thermoelectric properties of [Ca2 wCoO3 δ][CoO2]1.62, and the effect of Co3O4 impurity phase are investigated. It is shown that an increasing Co/Ca ratio in the [Ca2 wCoO3 δ][CoO2]1.62 phase leads to a larger figure of merit ZT induced by a strong resistivity drop. The decrease of resistivity stems from additional p-type charge carriers created by the formation of Ca vacancies. The Co3O4 impurity phase increases the thermal conductivity of the composite samples and leads to a reduction of ZT when the volume fraction of the Co3O4 phase is increased from 1% to 3%. Hence, the best figure of merit is expected close to the upper phase boundary of the [Ca2 wCoO3 δ][CoO2]1.62 phase. Not only the figures of merit of the materials, but also the design of a thermoelectric converter determines the device efficiency. In a converter, a p-type and a suitable n-type thermoelectric material are connected electrically in series and thermally in parallel. Here, [Ca2 wCoO3 δ][CoO2]1.62 is combined with the n-type CaMn0.97W0.03O3-δ and the device efficiency is improved by a variation of the ratio A_p/A_n of the cross section areas of the legs. The good agreement between the experimental values and the predictions of the compatibility model show the high quality of the fabricated devices and the value of the model for the optimization of the converter design. The adjustment of A_p/A_n improves the power output and the efficiency of the converters, where the best volume and area power densities exceed published high temperature values. The achieved efficiency of 1.08 % at a temperature of 1085 K at the hot side is close to the theoretical expected efficiency and can be further improved via ZT.

Hydrogen as an energy carrier is very versatile in energy storage applications. Developments in novel, sustainable technologies towards a CO2-free society are needed and the exploration of all-solid-state batteries (ASSBs) as well as solid-state hydrogen storage applications based on metal hydrides can provide solutions for such technologies. However, there are still many technical challenges for both hydrogen storage material and ASSBs related to designing low-cost materials with low-environmental impact. The current materials considered for all-solid-state batteries should have high conductivities for Na+, Mg2+ and Ca2+, while Al3+-based compounds are often marginalised due to the lack of suitable electrode and electrolyte materials. In hydrogen storage materials, the sluggish kinetic behaviour of solid-state hydride materials is one of the key constraints that limit their practical uses. Therefore, it is necessary to overcome the kinetic issues of hydride materials before discussing and considering them on the system level. This review summarizes the achievements of the Marie Skłodowska-Curie Actions (MSCA) innovative training network (ITN) ECOSTORE, the aim of which was the investigation of different aspects of (complex) metal hydride materials. Advances in battery and hydrogen storage materials for the efficient and compact storage of renewable energy production are discussed.

Universität Stuttgart

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