Free-standing N-Graphene as conductive matrix for Ni(OH)2 based supercapacitive electrodes

Abstract

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. Physico-chemical characterizations evidenced the formation of crystalline β-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 Ω cm2 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−1vs. 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.