Abstract: Embodiments of the invention relate to energy storage devices, e.g., capacitors and batteries, that may include a composite article of elongated conductive structures embedded in a polymer matrix. In some embodiments, a liquid containing ionic species may be dispersed within the polymer matrix of the article. The liquid may contact the elongated conductive structures within the polymer matrix. When the composite article is used as an energy storage device, the large surface area at the interface between the elongated conductive structures and the liquid can provide high energy storage. Embodiments of the invention enable storing energy using a composite article that exhibits both high and low temperature stability, high cyclic repeatability, and mechanical flexibility. The composite article can also be non-toxic, biocompatible and environmentally friendly. Thus, the composite article may be useful for a variety of energy storage applications, such as in the automotive, RFID, MEMS and medical fields.