Energy
Many global challenges are related to energy, be it energy efficiency, alternative fuels or technologies for energy conversion and stroage. Materials are central to these challenges and face continuously increasing requirements.
The efficiency of energy systems can often be improved by the use of either new materials, e.g. lighter materials in the transport industry, or the improvement of existing materials, e.g. the introduction of coatings that can withstand higher temperatures and pressures or that reduce friction.
However, where materials are probably most important is in the development of new technologies for energy conversion and storage. Miniaturized fuel cells require sophisticated membrane materials and multilayer thin film structures. Flexible thin films and nanostructures allow for energy harvesting in solar cells and current battery technologies can be revolutionized by the use of new electrode materials. In all technologies it is not only the inherent materials properties, which are of key importance, but also their processing on the micro- and nano scale.
Listed below are projects that deal with materials and processes for energy applications in the following fields of interest:
Energy Conversion and Storage
We work on the development of efficient microwave-assisted synthesis routes to lithium metal phosphate-based materials for lithium ion batteries. Such an approach was used for the synthesis of LiFePO4 doped with divalent (Mn, Ni, Zn), trivalent (Al) and tetravalent (Ti) metal ions in varying concentrations. In spite of the low synthesis temperature of 180 °C all the as-synthesized powders are highly crystalline. The short reaction times of just a few minutes represent the basis for an efficient and time-saving screening of different types of dopants with respect to optimized electrochemical performance in lithium-ion batteries. The Ni- and Zn-doped LiFePO4 with nominal dopant concentrations of 7 and 2 mol%, respectively, outperformed all the other samples, offering initial specific charge of 168 Ah kg-1 and excellent capacity retention of 97% after 300 full cycles. A discharge rate of 8C still resulted in 152 Ah kg-1 after 50 cycles.
Surface plasmons are special waves that can propagate at the interface of a metal. Plasmonic devices utilize patterned interfaces to manipulate these waves for various applications. While typically plasmons are generated by light, they can also be created by heat. We are studying this effect for modifying the thermal emission (i.e. glow) of a heated metal. Specifically, we examine patterned foils for use in thermophotovoltaics (which converts heat into electricity). The foil absorbs broadband heat from the sun and then re-emits a spectrally narrow beam of thermal emission. If this beam is optimized for and collected by a specific photocell, the sunlight can be efficiently converted into electricity.
Semiconductor nanocrystals, or quantum dots, exhibit optical properties that depend on their size. In particular, their absorption spectrum, which defines the colours of light that they absorb, can be tuned. This has advantages for solar cells, where the nanocrystal size can be selected to optimize the absorption of sunlight. Nanocrystals also exhibit quantum mechanical effects that provide new routes to increase efficiency. For example, they allow the extraction of energy from “hot electrons.” This energy is typically lost in conventional solar cells. We are studying such effects as well as solar cells based on quantum dots.
The growing interest in photovoltaic (PV) devices combined with the need for low-cost processing, have contributed to the quick expansion of organic PVs. In such devices the organic components form nanometer-sized electron-acceptor and electron-donor domains at whose interfaces excitons can dissociate into free charge carriers. This project deals with the fabrication of such devices made of blends of poly(3-hexylthiophene)-block-poly(4-vinylpyridine) (P3HT-b-P4VP) rod-coil block copolymers with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Careful spectroscopic and TEM characterization has shown that devices made from these blends offer not only a higher thermal stability, but also an improved photon-to-current conversion efficiency, which positions the present system among the best-performing PV devices having block copolymers as the major constituent.
Living cells host complex functional nanosystems, i.e. enzymes, which use metabolic energy to separate charge carriers across membranes. Cells also possess complex feedback mechanisms to regulate production and function of these enzymes. We investigate how such nanosystems can be coupled to macroscopic electrodes to harvest electrical power in the micro-Watt regime while keeping the cells alive. Ultimately, one may envision medical implants, such as sensors, pace makers or hearing aids, to be powered with electricity extracted directly from the surrounding tissue rather than batteries.
Materials Developments for Optimised Efficiency
Better understanding of the mechanical behaviour allows designing power generation systems with increased efficiency and higher operational flexibility. Models with improved predictive capabilities require an insight into the mechanisms of deformation and damage in high temperature materials, under conditions of non-isothermal multi-axial cyclic loading and creep-fatigue interaction. Methods for accelerated thermo-mechanical ageing are developed to predict long term properties of new higher temperature materials.
Fiber reinforced polymers show outstanding mechanical and functional properties. Functions like structural damping or others can directly be integrated into the components. To benefit from these interesting possibilities, the selection of fibers and matrix systems as well as the manufacturing processes must strongly be considered during the design of structural components. Our research focuses on the optimisation of the functionality of fiber reinforced polymer structures considering the used materials, the design possibilities, the manufacturing processes and the integration of passive or active functional elements.