Measuring temperature and heat flux is important for regulating any physical, chemical, and biological processes. Traditional thermopiles can provide accurate and stable temperature reading but they are based on brittle inorganic materials with low Seebeck coefficient, and are difficult to manufacture over large areas. Recently, polymer electrolytes have been proposed for thermoelectric applications because of their giant ionic Seebeck coefficient, high flexibility and ease of manufacturing. However, the materials reported to date have positive Seebeck coefficients, hampering the design of ultra-sensitive ionic thermopiles. Here we report an “ambipolar” ionic polymer gel with giant negative ionic Seebeck coefficient. The latter can be tuned from negative to positive by adjusting the gel composition. We show that the ion-polymer matrix interaction is crucial to control the sign and magnitude of the ionic Seebeck coefficient. The ambipolar gel can be easily screen printed, enabling large-area device manufacturing at low cost.

Corn starch and citric acid, two low-cost and abundant materials, were used for establishing a novel screen printable hydrogel for printed electronics applications.Corn starch was modified with citric acid by melt-blending, the so obtained thermoplastic starch was ground to powder and added to a water-starch suspension. Ultrasonication was used to prepare hydrogels of different citric acid concentrations.The most promising hydrogel contained 10 % citric acid by weight, provided an ionic conductivity of (2.30 ± 0.07) mS•cm -1 and appropriate rheological properties for screen and stencil printing. The hydrogel shows superb printability and prolonged stability against degradation. The corn starch hydrogel was used as printable gel polymer electrolyte in fully printed supercapacitors. The specific capacitance of the printed supercapacitor reached 54 F•g -1 . The printable hydrogel polymer electrolyte is easy to produce without in-depth chemical knowledge, is based on widely used and non-toxic materials and may be used as a functional layer in other printed electronics applications such as printed batteries.

The technical application of screen and stencil printing has been state of the art for decades. As part of the subtractive production process of printed circuit boards, for instance, screen and stencil printing play an important role. With the end of the 20th century, another field has opened up with organic electronics. Since then, more and more functional layers have been produced using printing methods. Printed electronics devices offer properties that give almost every freedom to the creativity of product development. Flexibility, low weight, use of non-toxic materials, simple disposal and an enormous number of units due to the production process are some of the prominent keywords associated with this field. Screen printing is a widely-used process in printed electronics, as this process is very flexible with regard to the materials that can be used. In addition, a minimum resolution of approximately 30 µm is sufficiently high. The ink film thickness, which can be controlled over a wide range, is an extremely important advantage of the process. Depending on the viscosity, layer thicknesses of several hundred nanometres up to several hundred micrometres can be realised. The conversion and storage of energy is an important topic, either in the field of renewable energies or the energy supply of the Internet of Things (IoT). This thesis addresses the print production of both device classes. Vertically structured thermoelectric generators (TEGs) for energy conversion and stacked supercapacitors for energy storage are produced by screen printing. Papers I-IV focus on the generation of functional layers of vertically aligned thermoelectric generators. These can convert heat directly into electrical energy. The vertical design was chosen due to the simple application of the device at the heat source. The general feasibility of screen-printed, vertically aligned TEGs was demonstrated. Optimisation of the thermoelectric materials is required, so that the process can be used sensibly. In paper III, the Ni containing model ink was optimised for filling the cavities in the insulator layer. In paper IV the printed thermoelectric generators are modelled. The performance of a set of parameters can be estimated by this model. The high Seebeck coefficient of ionic conductors is used in paper V in so-called ionic thermoelectric supercapacitor (ITESC), a combination of TEG and supercapacitor. Paper VI presents an environmentally friendly supercapacitor with a printable separator based on cornstarch and citric acid, which has a competitive electrochemical performance compared to printed supercapacitors reported elsewhere. In paper VII, some parameters of screen-printed primary Zn/MnO 2 cells are optimised and a printable separator based on cornstarch and lactic acid was successfully tested. "The journey is the reward" Confucius comes closest to german aphorism "Der Weg ist das Ziel". This sentence best describes how I feel about my doctoral thesis, which is now being completed. The journey was longer than I initially thought, bu...

Thermoelectric generators (TEG) directly convert heat energy into electrical energy. The impediments as to why this technology has not yet found extensive application are the low conversion efficiency and high costs per watt. On the one hand, the manufacturing process is a cost factor. On the other, the high--priced thermoelectric (TE) materials have an enormous impact on the costs per watt. In this thesis both factors will be examined: the production process and the selection of TE materials. Technical screen printing is a possible way of production, because this method is very versatile with respect to the usable materials, substrates as well as printing inks. The organic conductor PEDOT:PSS offers reasonable thermoelectric properties and can be processed very well in screen printing. It was demonstrated by prototypes of fully printed TEGs that so--called vertical printed TEGs are feasible using standard graphic arts industry processes. In addition, the problems that occur with print production of TEGs are identified. Finally, approaches to solve these problems are discussed.

Potentially important parameters of printed flexible batteries based on the primary zinc-manganese dioxide (Zn/MnO 2 ) system were investigated using a fractional factorial design of experiments. A novel hydrogel electrolyte made of lactic acid and cornstarch proved the functionality. Using this kind of hydrogel, it was important to prevent unwanted side reactions with the zinc electrode. Accurate control of the electrode mass is absolutely necessary, since the mass of deposited MnO 2 and the mass ratio of MnO 2 :Zn determine the capacity of the cells. With optimised parameter settings the capacity of printed cells was increased to over 100 mAh, which is equivalent to an area-related specific capacity of about 7.3 mAh cm −2 . This is within the range of commercially available button cells, which provided between 125-155 mAh under the same discharge conditions. By means of electrochemical impedance spectroscopy, the cells were examined immediately after assembly. Within three hours the cell could be identified as, in principle, working or defective. As such, precious measurement time can be saved by selecting promising cells prior to characterisation.

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