Anton Karlsson: Electrohydrodynamic printing of supercapacitors

Abstract

This thesis presents research towards fully printed supercapacitor cells in a stacked electrode configuration. A supercapacitor is an energy storage device similar to a battery, but with higher power density and lower energy density. This, together with a long cycle life, make supercapacitors a suitable device for fast charging and discharging in the range of seconds. Printing of supercapacitors could be useful to implement energy storage in microsystems, small electronic devices.

The core components of a supercapacitor are two electrodes of a conductive material with a high surface area, a separator or gap between the two electrodes, and an electrolyte. Bulk-manufactured supercapacitors use a stacked electrode configuration, but printed supercapacitors typically place the electrodes side-by-side, with both electrodes printed on the same surface. This is due to challenges when printing the different materials on top of each other.

The electrohydrodynamic processes of electrospinning and electrospraying have been developed into printing methods at suitable resolutions for printing of supercapacitor cells of a few cm2. Porous electrodes are printed using electrospraying of graphene oxide. These electrodes are deoxygenated after or during printing using ascorbic acid (vitamin C) and/or alkaline electrolyte to achieve sufficient electrical conductivity and energy storage performance. The electrochemical characteristics of deoxygenated graphene oxide electrodes is measured using cyclic voltammetry, galvanostatic cycling, and electrical impedance spectroscopy. The graphene electrodes, both printed and coated, have good energy and power densities for electrical double layer supercapacitors of these materials. Development of high-resolution electrospinning has been used to print solid separators that soak in the electrolyte while preventing the printed electrodes from short-circuiting. Current collectors and electrolyte have also been printed or dripped using electrohydrodynamic processes. By combining these four methods, stacked electrode supercapacitor cells are fully printed and chemically treated directly in the printing setup. This configuration of electrodes increases the electrode size compared with the side-by-side electrode configuration, which is limited to half the size of the full supercapacitor cell.

The thesis is focused on developing the printing methods as well as the material and electrochemical characterization. The microstructures of the printed materials are viewed and measured using scanning electron microscopy. The graphene oxide deoxygenation is measured using Raman spectroscopy and energy dispersive X-ray spectroscopy. The work of this thesis provides new strategies for printed graphene supercapacitors, in graphene oxide treatments, printing methods, and electrode configuration. The strategies have been developed to enable robust printing in air, without extra post-printing steps.