Secondary batteries and capacitors have a high electromotive force together with a high cyclic property and are capable of fast charging and discharging, using a polymeric compound having a high stability and high reactivity in an acidic ambient, as an electrode active material. The secondary batteries and capacitors include an indole polymeric compound as an electrode active material. The polymeric compound uses a proton as a charge carrier and has a constituent represented by general formula (1):
wherein each R denotes a hydrogen atom, a halogen atom, a hydroxyl group, a carboxyl group, a sulfonic group, a sulfuric acid group, a nitro group, a cyano group, an alkyl group, an aryl group, an alkoxyl group, an amino group, an alkylthio group and an arylthio group, which may be the same or different from each other, and at least one R is a substituent other than a hydrogen atom.