With climate change becoming more and more pressing, a switch of energy production from fossil fuels to renewable energy sources is necessary, which leads to big challenges for our today’s society. To meet these challenges, not only the widespread expansion of renewable energies is necessary, but also of reliable and efficient energy storage, which guarantees a reliable energy supply on demand and a stable power grid. Nowadays, different energy storage devices are available for different areas of operation, the most prominent are lithium-ion batteries (LIBs) and electrical double layer capacitors (EDLCs). This thesis is dedicated to the investigation of EDLCs. The first part of this work focuses on the development of alternative electrolytes for high voltage EDLCs. Electrolytes based either on adiponitrile (ADN) or the mixture of acetonitrile (ACN) and ethyl isopropyl sulfone (EiPS) are investigated for their use in high voltage EDLCS. The second part discusses the thermal investigation of EDLCs. On the one side, the interaction of activated carbon (AC) with alternative electrolytes based on the solvents propylene carbonate (PC), butylene carbonate (BC), 3-cyano propionic acid methyl ester (CPAME) or ADN is investigated via thermogravimetric analysis (TGA). On the other side, an in-situ simultaneous thermal analysis (STA) cell is used for the investigation of thermal and electrochemical properties of EDLCs under various temperature and voltage conditions. Here the electrochemical performance of an EDLC is associated with the heat flow generated during operation.