Integration of nickel-cobalt double hydroxide nanosheets and polypyrrole films with functionalized partially exfoliated graphite for asymmetric supercapacitors with improved rate capability

High-rate asymmetric supercapacitors (ASCs) made of abundant and low-cost electrode materials and operating in safe aqueous electrolytes can be attractive for electrochemical energy storage. Here, we design a new type of ASC by using pseudo-capacitive nanomaterials, Ni-Co double hydroxide (Ni-Co DH) nanosheets and polypyrrole (PPy) films, for the cathode and anode, respectively, which were integrated with a functionalized partially exfoliated graphite (FEG) current collector. Benefiting from the "super highway" for fast electron/ion transportation in hybrid systems, the as-prepared electrodes exhibit superior rate capability (2442 and 2039 F g(-1) at 1 and 50 A g(-1), with 83.5% retention for Ni-Co DH; 560 and 441 F g(-1) at 1 and 50 A g(-1), with 79% retention for PPy). The assembled ASC displays a high specific capacitance (261 F g(-1) at 1 A g(-1)) and excellent rate capability; 77% of its initial capacitance can be retained when the current density increases 30 times from 1 to 30 A g(-1). An energy density of 61.3 W h kg(-1) can be achieved by the ASC at 0.65 kW kg(-1). Even at an ultra-high power density of 19.5 kW kg(-1), the ASC can still deliver a high energy density of 47.2 W h kg(-1). Through careful control of charges which can be stored in the anode and cathode, the cycling stability of the ASC is much improved, and 91% capacitance retention can be achieved after 5000 charge/discharge cycles. These features demonstrate a new avenue for developing high-performance pseudo-capacitive electrodes and rational assembly strategies for high power/energy density charge storage devices with good cycling stability.