An electrical energy storage device structure comprises a first conductive sheet, a second conductive sheet and an electrolyte sheet placed between the first conductive sheet and the second conductive sheet. In the device, at least one of the first conductive sheet and the second conductive sheet comprises a layer of carbon nanoparticles. The carbon nanoparticle layer is arranged to be adjacent to the electrolyte sheet. The carbon nanoparticles may include both high aspect ratio carbon nanoparticles and low aspect ratio carbon nanoparticles. The device is flexible and at least partially transparent.