Micro Supercapacitor: fabrication and characterization of an asymmetric device in aqueous electrolyte.

The dream for an energy storage device is have an high energy density together with an high power density. The capacitors and the batteries try to improve at the maximum one of these two quality, but the trade-off can be find thanks to the supercapacitors. These new devices are similar to a capacitors, but instead to have a dielectric between the two electrodes they have an electrolyte: the charge’s storage is possible thanks two different phenomena, the electrical double layer and the pseudo-capacitance. The field of application is increasing: they could be use as capacitors when an high power density is needed, however having an high capacitance they could be also integrated with other devices similarly to batteries. This latter characteristic could be helped also with the scaling down of the dimensions creating micro supercapacitors and the aim of the thesis is try to physically create its. These devices are small and so can easily integrate with other electronic devices, increasing the field of application. The state of the art moves towards higher performance devices, but at the same time could be useful study low cost, health safe and environmentally friendly’s processes and devices. The project starts with the idea of creating wearable devices and for this is chosen a flexible Kapton substrate for the electrodes. Then, these are covered before by thin films, after that an electrochemical deposition of porous 3D gold is carried on to increase the superficial area, and after they are covered with activated material to increase the capacitance exploiting the EDLC and pseudo-capacitance. To increase the power and the energy density, the potential window is risen creating asymmetric devices: one electrode is done by carbons exploiting the effect of the electrical double layer, instead the second is formed by manganese oxide taking advantage from the pseudo-capacitance. The deposition techniques are studied setting different parameters to find the best. For the gold deposition first is use the evaporation process to create an adhesion layer of chromium and crystalline gold and then is apply the electro-plating process to create the dendritic gold. Also the manganese oxide is placed thanks and electro chemical process, instead the carbon particles are fixed with an electro-phoretic deposition. The family of possible electrolyte is very big, but putting as aim the cost, health safe and environmentally friendly the neutral electrolyte Na2SO4 is chosen. The interaction between this and the electrodes’ materials is studied, characterising theme and trying to create devices with optimized performance. In fact, typically the aqueous electrolytes have a limited potential windows due to the electrolysis effect at 1.23 V, but creating asymmetric device is possible increase it up to 1.6 V. Different kind of SCs' configuration are analysed, from the stack, where the two electrodes are one in front to the other, to the planar in which the two are in the same plane. About this last layout is possible try to increase the capacitance creating also interdigitated devices. Furthermore, the use of photolithographic process permits a distance of some micrometres about the two electrodes, creating micro-supercapacitors. Finally planar geometries begin to be analysed trying to find the best trade off with the optimization of the capacitance and the physical fabrication, leaving space for further developments.