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

• Published in: Electrochimica acta 2020-02, Vol.334, p.135592, Article 135592
• Main Authors: Upadhyay, Kush K., Bundaleska, N., Abrashev, M., Bundaleski, N., Teodoro, O.M.N.D., Fonseca, I., de Ferro, André Mão, Silva, Rui Pedro, Tatarova, E., Montemor, M.F.
• Format: Article
• Language: English
• Subjects: Activated carbon; Aqueous electrolytes; Asymmetric supercapacitor; Beta phase; Charge transfer; Electrochemical impedance spectroscopy; Electrodes; Energy storage; Flux density; Frequency response; Graphene; High-frequency response; Low pressure; Microwave plasmas; N-doped graphene; Ni(OH)2–N-graphene composite; Nickel compounds; Nitrogen; Organic chemistry; Oxygen content; Time constant
• ISSN: 0013-4686; 1873-3859
• DOI: 10.1016/j.electacta.2019.135592

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. [Display omitted] •Plasma driven highly controlled free standing N-doped graphene sheets showed high frequency response.•N-doped graphene- Ni(OH)2 composite showed improved electrochemical performance and low time constant.•Asymmetric design showed supercapacitive behaviour up to 1.8 V in aqueous electrolyte.