A General Electrodeposition Strategy for Fabricating Ultrathin Nickel Cobalt Phosphate Nanosheets with Ultrahigh Capacity and Rate Performance.

Transition-metal phosphates/phosphides possess promising theoretical electrochemical characteristics and exhibit great potential in advanced supercapacitors. Unfortunately, limited by the processing techniques and overall structure, their specific capacity and rate performance are still unsatisfactory. Herein, we report the fabrication of transition-metal phosphate electrodes with an ultrathin sheetlike array structure by one-step electrodeposition at room temperature. As a proof-of-concept, a transition-metal phosphate member of NiCo(HPO4)2·3H2O with an ultrathin nanosheet structure (thickness ∼2.3 nm) was synthesized and investigated. The as-prepared NiCo(HPO4)2·3H2O electrode showcases an ultrahigh specific capacity of 1768.5 C g-1 at 2 A g-1 (the highest value for transition-metal phosphates/phosphides reported to date), superb rate performance of 1144.8 C g-1 at 100 A g-1, and excellent electrochemical stability. Moreover, the transition-metal phosphate nanosheet array can be uniformly deposited on various conductive substrates, demonstrating the generality of our strategy. Therefore, this simple electrodeposition strategy provides an opportunity to fabricate ultrathin transition-metal phosphate nanosheet materials that can be used for energy storage/conversion, electrocatalysis, and other electrochemical energy-related devices.

Hierarchical Nanosheets/Walls Structured Carbon-Coated Porous Vanadium Nitride Anodes Enable Wide-Voltage-Window Aqueous Asymmetric Supercapacitors with High Energy Density.

The energy density of aqueous asymmetric supercapacitors (ASCs) is usually limited by low potential windows and capacitances of both anode and cathode. Herein, a facile strategy to fabricate hierarchical carbon-coated porous vanadium nitride nanosheet arrays on vertically aligned carbon walls (CC/CW/p-VN@C) as anode for aqueous ASCs is reported. The potential window of CC/CW/p-VN@C electrode can be stably extended to -1.3 to 0 V (vs Ag/AgCl) with greatly improved specific capacitance (604.8 F g-1 at 1 A g-1), excellent rate capability (368 F g-1 at 60 A g-1), and remarkable electrochemical stability. To construct ASCs, a Birnessite Na0.5MnO2 nanosheet arrays (CC/CW/Na0.5MnO2) cathode is similarly built. Benefiting from the matchable potential windows and high specific capacitances of the rationally designed anode and cathode, aqueous CC/CW/p-VN@C||CC/CW/Na0.5MnO2 ASCs with a wide voltage window of 2.6 V are fabricated. Moreover, the ASCs showcase an ultrahigh energy density up to 96.7 W h kg-1 at a high power density of 1294 W kg-1, and excellent cycling stability (92.5% retention after 10 000 cycles), outperforming most of previously reported ASCs and even comparable to that of organic electrolyte supercapacitors (SCs). This efficient strategy for fabricating 2.6 V aqueous ASCs suggests a promising research system for high energy density SCs.

Co3O4 Supraparticle-Based Bubble Nanofiber and Bubble Nanosheet with Remarkable Electrochemical Performance.

Hollow nanostructures based on transition metal oxides (TMOs) with high surface-to-volumetric ratio, low density, and high loading capacity have received great attention for energy-related applications. However, the controllable fabrication of hybrid TMO-based hollow nanostructures in a simple and scalable manner remains challenging. Herein, a simple and scalable strategy is used to prepare hierarchical carbon nanofiber (CNF)-based bubble-nanofiber-structured and reduced graphene oxide (RGO)-based bubble-nanosheet-structured Co3O4 hollow supraparticle (HSP) composites (denoted as CNF/HSP-Co3O4 and RGO/HSP-Co3O4, respectively) by solution self-assembly of ultrasmall Co3O4 nanoparticles (NPs) assisting with polydopamine (PDA) modification. It is proved that the electrochemical performance of Co3O4 NPs can be greatly enhanced by the rationally designed nanostructure of bubble-like supraparticles combined with carbon materials as excellent electrodes for supercapacitors. The favorable structure and composition endow the hybrid electrode with high specific capacitance (1435 F g-1/1360 F g-1 at 1 A g-1/5 mV s-1) as well as fantastic rate capability. The asymmetric supercapacitors achieve an excellent maximum energy density of 51 W h kg-1 and superb electrochemical stability (92.3% retention after 10 000 cycles). This work suggests that the rational design of electrode materials with bubble-like superstructures provides an opportunity for achieving high-performance electrode materials for advanced energy storage devices.