Abstract: The invention broadly encompasses energy storage devices or systems and more specifically relates to methods of enhancing the performance of electrochemical double layer capacitors (EDLCs), or supercapacitors or ultracapacitors, and devices formed therefrom. In some embodiments, the invention relates generally to energy storage devices, such as EDLCs that use phosphonium-based electrolytes and methods for treating such devices to enhance their performance and operation. Embodiments of the invention further encompass phosphonium-based electrolytes comprised of phosphonium ionic liquids, salts, and compositions employed in such EDLCs.