Development of novel, durable materials for energy harvesting devices, where these devices comprise an array of small capacitors where each such capacitor has one fixed, planar "bottom" electrode, and a second, variable-area "top" electrode realized by a compliant conductor pressing against a dielectric covering the first electrode. This energy converter/harvester module is complemented by an energy storage module involving a battery array.
Fig.1a: Capacitive harvester principle: As the capacity is changed due to deformation of the flexible electrode charges are drained at different voltage levels thus generating electrical energy from mechanical motion. In comparison to parallel plate capacitive harvesters in air or vacuum, energy density can be increased greatly due to the high-k dielectrics which will be developed for that purpose.
Fig. 1b: Effective electrode area increases with pressure (top left to bottom right). Source: FhG IZM
One objective of the project is to reduce the mechanical deformation energy to a minimum which is required to increase the effective electrode area to a maximum, thus increase the efficiency of mechanical to electrical conversion.
Fig. 2: A dedicated low-power electronic circuit is required for high efficiency energy conversion. Different conversion cycles can be implemented depending on the capacitor characteristics and the complexity of the circuit. Source: FhG IZM
Conventional parallel plate capacitor harvesters with varying plate distance show only middling change in capacity and they take advantage of the charge constrained circuit (Fig. 2, left).By contrast, Matflexend can realize large capacitance changes such that Cmin/Cmax < 0.1 and thus realize a higher electrical energy gain in the voltage constrained mode (right) at the same maximum capacity Cmax and same maximum voltage Vmax. To minimize the cost of the electrical circuit and harvester packaging, MATFLEXEND harvesters will be operated at a low voltage (ca. 40 V).
Fig. 3: Processing and deposition methods for low cost production have to be developed for the new capacitor and battery materials such that they have the required parameters and interfaces properties. Source: FhG IZM
Batteries are needed to buffer and smoothen the harvested enegy. Two battery configurations will be tested: the conventional configuration with stacked electrodes (right) and a coplanar (side b side) electrode arrangement (left) which offers advantages in terms of processing and mechanical flexibility (bending).
Fig.4: Thin and partially flexible rechargeable batteries will provide buffer storage for energy autarkic, energy harvesting applications. Source: FhG IZM
For both of the above battery architectures, fabrication will be greatly simplified by a novel printable electrolyte and separator, to be developed in the project. Here, low vapour pressure liquid electrolyte is held inside a high porosity, open-pore structure of a polyHIPE (polymer based high internal phase emulsion, Fig.5).
To increase the power and pulse performance of the battery, 3D electrode structures will be investigated which increase the reactive electrode surface and reduce the diffusion path of the lithium ions. Due to the micrometer dimensions of these structures, they cannot be fabricated by printing or dispensing in the present art. Electrophoretic Deposition (EPD) will therefore be developed to achieve such small lateral dimensions.
Fig. 5: Printable electrolytes for lithium ion batteries will be developed based on polyHIPE and ionic liquids: conventional polyHIPE. Source: University Vienna / IMPERIAL College London
Metal foils and laminates will be investigated as encapsulation materials for the Lithium batteries and harvesters, to achieve long term stable operation.
The project is structured as 10 Workpackages, the ones relevant to Research+Innovation (R+I) are shown below.
MATFLEXEND Project Flyer