Various two-dimensional (2D) materials have recently attracted great attention owing to their unique properties and wide application potential in electronics, catalysis, energy storage, and conversion. However, largescale production of ultrathin sheets and functional nanosheets remains a scientific and engineering challenge. Here we demonstrate an efficient approach for large-scale production of V 2 O 5 nanosheets having a thickness of 4 nm and utilization as building blocks for constructing 3D architectures via a freezedrying process. The resulting highly flexible V 2 O 5 structures possess a surface area of 133 m 2 g −1, ultrathin walls, and multilevel pores. Such unique features are favorable for providing easy access of the electrolyte to the structure when they are used as a supercapacitor electrode, and they also provide a large electroactive surface that advantageous in energy storage applications. As a consequence, a high specific capacitance of 451 F g −1 is achieved in a neutral aqueous Na 2 SO 4 electrolyte as the 3D architectures are utilized for energy storage. Remarkably, the capacitance retention after 4000 cycles is more than 90%, and the energy density is up to 107 W·h·kg −1 at a high power density of 9.4 kW kg −1 . KEYWORDS: 2D layers, V 2 O 5 , 3D architectures, high energy density, supercapacitor S upercapacitors, also called electrochemical capacitors or ultracapacitors, are extensively studied as they complement lithium-ion batteries due to their high power density, fast delivery rate, and long lifespan. However, the low energy density of supercapacitors largely obstructs the way of their applications as standalone devices. The energy density (E) of a supercapacitor is determined by its specific capacitance (C) and the cell voltage (V) according to the equation of E = 1/2CV 2 .
1,2Thus, improving the specific capacitance of electrode materials is an efficient way to achieve supercapacitors with a high energy density. Generally, high-capacitance electrode materials possess a high surface area and good electrical conductivity since these properties are strongly related to the electrochemical double layer capacitance or electroactive surface for redox-reactions, resulting in pseudocapacitance within an electrode. To date, a large number of high-surface-area carbonaceous materials such as activated carbon, carbon nanotubes, and graphene have been employed as electrode materials for supercapacitors.3−7 In particular, it was shown that graphene, a 2D monolayer of carbon atoms, is an excellent building block for constructing 3D architectures having improved electrochemical performance advantageous for energy storage devices. 6,8 However, these carbon-based electrochemical double-layer capacitors have a low capacitance, especially at high charge/ discharge rates. Metal oxides and hydroxides overcome these limitations of carbon and commonly exhibit high capacitance for energy storage owing to their more efficient energy storage mechanism, and they have potential to be the electrode materia...
