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Currently, one-dimensional nanostructured binary metal oxides attract a great attention in supercapacitors (SCs) application due to their rapid charge transportation. In this respect, different nanostructures of FeCo 2 O 4 are designed by simply tuning the reaction temperature in hydrothermal synthesis. These nanostructures are directly grown on flexible stainless steel mesh and further applied as binder-free electrodes for SCs. The systematic study is carried out to confirm the relation between surface characteristics and electrochemical properties of FeCo 2 O 4 thin film. Among different nanostructures, FeCo 2 O 4 nanowire arrays exhibit hierarchical mesoporous structure and demonstrate good surface properties including high surface area and appropriate pore volume. As a consequence, relatively high specific capacitance of 1963 F g − 1 is obtained for the FeCo 2 O 4 nanowire electrode. Further, asymmetric SC is fabricated using nanowired-FeCo 2 O 4 and nanoparticulated-MnO 2 thin films as negative and positive electrodes with neutral Na 2 SO 4 electrolyte. Impressively, the MnO 2 //FeCo 2 O 4 cell could be successfully cycled in a wide voltage window of 2.0 V, which can achieve a specific capacitance of 218 F g − 1 and energy density of 43 Wh kg − 1 . In addition, the SCs exhibit improved capacitance with cycling, which is attributed to opening of micro-pores occurred by frequent ion transport. NPG Asia Materials (2017) 9, e419; doi:10.1038/am.2017.145; published online 18 August 2017 INTRODUCTIONResearchers and engineers have always focused on the development of efficient methods for energy production to improve the quality of human lives. 1,2 Fossil fuels are the major energy sources that are being used in the world today to fulfill the energy demand. However, the excessive consumption of fossil fuels leads to air pollution due to release of toxic gases such as CO 2 , CO, NO 2 and SO 2 . Consequently, obtaining energy from clean and safe renewable energy resources (like solar, wind and ocean energy) to reduce the air pollution is getting important. However, the energy obtained from renewable energy resources is uncertain and inconsistent as it depends upon environmental conditions. To overcome the difficulties, energy storage technology for storing the renewable energy should be developed. 3,4 Since the last decade, the industry and research institutes have made significant effort to develop inexpensive and proficient energy storage devices. 5 Presently, Li-ion batteries (LIBs) and supercapacitors (SCs) are considered as the promising energy storage devices because of their excellent electrochemical capabilities. 6 Especially, SCs have garnered more attention than LIBs due to their outstanding electrochemical

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