Application of biopolymers in thin layer capacitors and high power batteries

Electrochemical capacitors, also known as supercapacitors, are devices capable of storing a large amount of electrical charge and ultrafast release of stored energy, i.e. characterized by very high power density. Thus, they are predominantly used in applications requiring large bursts of power to stabilize the power supply or in combination with renewable energy sources. They consist of two porous electrodes pre-wetted with an electrolyte and a separator inserted in between; in the simplest approach, the electric charge is accumulated in an electrical double-layer, which upon applying of external voltage, forms at the electrode/electrolyte interface. The ever-growing energy demand and climate changes propel the research towards new and more ecologically friendly alternative solutions for the development of sustainable energy storage and conversion devices still characterized by good electrochemical performance and operating in a wide range of temperatures. One of the major drawbacks of conventional electrochemical capacitors is the presence of the electrolyte in liquid form which presents a possibility of its leakage from the device but also requires complicated procedures of safe encapsulation. To overcome this hindrance one can combine the electrolyte and separator in one functional material working simultaneously as an ions reservoir and physical barrier between the electrodes which is a so-called gel-polymer electrolyte (GPE). Such a combination will not only eliminate the electrolyte in liquid form but also minimize the volume of the introduced solution, having a positive effect on the weight of the device and the potential manufacturing costs. In this work, cellulose was proposed as a main component of the GPE in combination with agarose and in-situ polymerized norepinephrine subsequently used in electrochemical capacitors impregnated with different aqueous electrolytes addressing the drawbacks of conventional devices, e.g. low energy density, environmental impact. Incorporating more than one polymer matrix within the GPE allowed for improving the mechanical properties of the obtained hydrogels due to the formation of an interpenetrating polymer network (IPN). In the course of the entire doctoral thesis, various electrolytes were examined, which arose from the current challenges posed to electrochemical capacitors, i.e. improving their parameters including, above all, energy density, environmental impact, but also increasing the spectrum of their applications, e.g. in miniaturized devices and in-vivo systems. In this regard, the important achievements of this work include the use of polyoxometalates in this role, also as redox electrolytes, and the incorporation of these compounds into the designed new gel-polymer matrices with improved strength parameters and high absorption capacity.