A NiO positive electrode for aqueous hybrid capacitors was prepared by a new galvanostatic anodization method. A thin sheet of heat-treated Ni was galvanostatically oxidized in aqueous 85% H3PO4 containing 0.5 M NH4F. During the anodization, the thickness of the Ni metal sheet decreased from 10 to 2 mu m and 2-to 3-mu m-thick NiO layers were formed on both sides of the Ni metal sheet. A hybrid capacitor comprising the anodized NiO positive electrode, an activated carbon negative electrode, and aqueous 10 M KOH exhibited a maximum specific energy of 16 W h kg(-1) and a specific power of 22.2 kW kg(-1). (C) 2015 Elsevier B.V. All rights reserved.