The voltage limit for aqueous asymmetric supercapacitors is usually 2 V, which impedes further improvement in energy density. Here, high Na content Birnessite Na MnO nanosheet assembled nanowall arrays are in situ formed on carbon cloth via electrochemical oxidation. It is interesting to find that the electrode potential window for Na MnO nanowall arrays can be extended to 0-1.3 V (vs Ag/AgCl) with significantly increased specific capacitance up to 366 F g . The extended potential window for the Na MnO electrode provides the opportunity to further increase the cell voltage of aqueous asymmetric supercapacitors beyond 2 V. To construct the asymmetric supercapacitor, carbon-coated Fe O nanorod arrays are synthesized as the anode and can stably work in a negative potential window of -1.3 to 0 V (vs Ag/AgCl). For the first time, a 2.6 V aqueous asymmetric supercapacitor is demonstrated by using Na MnO nanowall arrays as the cathode and carbon-coated Fe O nanorod arrays as the anode. In particular, the 2.6 V Na MnO //Fe O @C asymmetric supercapacitor exhibits a large energy density of up to 81 Wh kg as well as excellent rate capability and cycle performance, outperforming previously reported MnO -based supercapacitors. This work provides new opportunities for developing high-voltage aqueous asymmetric supercapacitors with further increased energy density.
Although the theoretical capacitance of MnO is 1370 F g based on the Mn/Mn redox couple, most of the reported capacitances in literature are far below the theoretical value even when the material goes to nanoscale. To understand this discrepancy, in this work, the electrochemical behavior and charge storage mechanism of K-inserted α-MnO (or KMnO) nanorod arrays in broad potential windows are investigated. It is found that electrochemical behavior of KMnO is highly dependent on the potential window. During cyclic voltammetry cycling in a broad potential window, K ions can be replaced by Na ions, which determines the pseudocapacitance of the electrode. The K or Na ions cannot be fully extracted when the upper cutoff potential is less than 1 V vs Ag/AgCl, which retards the release of full capacitance. As the cyclic voltammetry potential window is extended to 0-1.2 V, enhanced specific capacitance can be obtained with the emerging of new redox peaks. In contrast, the K-free α-MnO nanorod arrays show no redox peaks in the same potential window together with much lower specific capacitance. This work provides new insights on understanding the charge storage mechanism of MnO and new strategy to further improve the specific capacitance of MnO-based electrodes.
Polyaniline (PANI), one of the most attractive conducting polymers for supercapacitors, demonstrates a great potential as high performance pseudocapacitor materials. However, the poor cycle life owing to structural instability remains as the major hurdle for its practical application; hence, making the structure-to-performance design on the PANI-based supercapacitors is highly desirable. In this work, unique core-shell NiCo2O4@PANI nanorod arrays (NRAs) are rationally designed and employed as the electrode material for supercapacitors. With highly porous NiCo2O4 as the conductive core and strain buffer support and nanoscale PANI layer as the electrochemically active component, such a heterostructure achieves favorably high capacitance while maintaining good cycling stability and rate capability. By adopting the optimally uniform and intimate coating of PANI, the fabricated electrode exhibits a high specific capacitance of 901 F g(-1) at 1 A g(-1) in 1 M H2SO4 electrolyte and outstanding capacitance retention of ∼91% after 3000 cycles at a high current density of 10 A g(-1), which is superior to the electrochemical performance of most reported PANI-based pseudocapacitors in literature. The enhanced electrochemical performance demonstrates the complementary contributions of both componential structures in the hybrid electrode design. Also, this work propels a new direction of utilizing porous matrix as the highly effective support for polymers toward efficient energy storage.
3D black mesoporous Li4Ti5O12−δ nanowall arrays with greatly enhanced rate performance are fabricated as promising anodes for advanced lithium-ion microbatteries.
Light-powered fuel-free colloidal motors possess significant potential for practical applications ranging from nanomedicine to environmental remediation. However, current lightpowered colloidal motors often require the incorporation of expensive metals or high concentrations of toxic chemical fuels, which is a severe limitation for their practical applications. Integrating highly ordered and porous materials with a large surface area into colloidal motors is a promising strategy for upsurging their self-propelled speed and adsorption, which will benefit many applications. Here, highly efficient, fuel-free, and light-activated metal organic framework (MOF)-3trimethoxysilyl propyl methacrylate Janus colloidal motors with a hierarchical morphology are reported. These colloidal motors can be driven by UV or visible light, with a self-propelled speed tuned by the light intensity. The speed can be further enhanced by morphology optimization or by the addition of H 2 O 2 as a fuel. The colloidal motors display a superior efficiency in removing heavy metal ions of Hg, which is up to ∼90% within 40 min from the contaminated water, attributed to their high surface area, hierarchical morphology, large number of active sites, and high mobility. This work not only offers a facile approach to incorporate a versatile MOF family into the design of fuel-free and light-powered Janus colloidal motors, but also demonstrates their potential for real-life applications such as environmental remediation.
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