▎ 摘　　要

Free-standing N-doped graphene (NG2N1O) sheets with 2.3 at. % of nitrogen and residual oxygen content were synthesized using low-pressure microwave plasma. A composite made with Ni(OH)(2) and NG2N1O was prepared by the hydrothermal route. Physicochemical characterizations evidenced the formation of crystalline beta-phase of Ni(OH)(2) nanoplates interconnected with graphene nanosheets. The electrochemical results of N-graphene electrodes evidenced very good supercapacitive response with a high rate capability of 97%, negligible charge transfer resistance of 0.05 Omega cm(2) and very low time constant of 50 ms. The specific capacity of the Ni(OH)(2) + NG2N1O composite increased 20% compared to Ni(OH)(2) (107 mAh g(-1) vs. 86 mAh g(-1), respectively) and the rate capability was 75% at current density of 10 A g(-1), higher than Ni(OH)(2) which retained only 34.4%. The composite showed excellent stability, by retaining 92% of its initial specific capacity after 4000 charge-discharge cycles. Furthermore, electrochemical impedance spectroscopy evidenced that graphene decreased the charge transfer resistance and diffusional contributions while enhancing the capacitive behaviour and the high-frequency response of the electrodes. An asymmetric cell was assembled using activated carbon as negative electrode and the composite as positive electrode. The cell displayed good capacitive response in a potential window of 1.8 V, in aqueous electrolyte, stored a maximum energy density of 38.64 W h kg(-1) at a power density of 450 W kg(-1) and retained 16 W h kg(-1) at a power density of 4.7 kW kg(-1). (C) 2019 Elsevier Ltd. All rights reserved.