Edge enriched cobalt ferrite nanorods for symmetric/asymmetric supercapacitive charge storage

For maximum and efficient charge storage and delivery, porous nanorods with active sites can offer a unique set of advantages. Herein, hierarchically porous (0.2-200 nm) cobalt ferrite (CoFe2O4) nanorods are hydrothermally prepared by tuning oleic acid concentration (100-500 mu L with subsequent annealing (400 degrees C). These hierarchical pores (with edges and cavities) raise the specific surface area of CoFe2O4 nanorods to 154 m(2)g(-1). In addition, 80% of CoFe2O4 sites become accessible boosting the capacitance to a high value (460.5 Fg(-1) at 1.0 Ag-1), while retaining excellent cyclic stability (95.8% for 5000 cycles). Maximum pseudocapacitive contribution from these electrochemically active sites is ensured by rapid diffusion of electrolytic OH- ions (5.8 x 10(-8)cm(2)s(-1)). Moreover, on assembling these nanorods as a positive electrode in asymmetric configuration (graphene nanoribbons as negative electrode), the cell exhibits an outstanding energy density (33.5 Wh kg(-1) at 727.8 W kg(-1)), almost double of its symmetric (16.5 Wh kg(-1) at 288 W kg(-1)) counterpart. (C) 2018 Elsevier Ltd. All rights reserved.