Influence of aqueous electrolytes on the electrochemical behavior of nitrogen-doped graphene hydrogel electrodes for supercapacitors

Abstract

Supercapacitors (SCs) have drawn much attention owing to their unique superiorities. The selection of electrode materials is significantly crucial as it influences SC performance. Defects such as low specific capacitance and poor rate performance need to be settled urgently to achieve enhanced SC performance. Here, we have developed an effective and facile strategy to prepare a binder-free nitrogen-doped graphene hydrogel (NDGH) as an electrode material. The electrochemical performance of NDGH was measured in three different electrolytes (2 M KOH, 1 M Na₂SO₄ and 1 M H₂SO₄), and it showed superior performance in 2 M KOH. NDGH had an outstanding specific capacitance of 224.1 F g⁻¹ at 1 A g⁻¹ and retained 80.7% of the capacitance at 10 A g⁻¹. Meanwhile, NDGH possessed good cycling stability, with 93% capacitance retention after 6000 cycles. The contributions of surface-controlled and diffusion-controlled processes to NDGH capacitance were analyzed in different electrolytes. Compared with 1 M Na₂SO₄ (52%) and 1 M H₂SO₄ (13%), superior surface-controlled contribution was observed in the 2 M KOH (73%) electrolyte at 10 mV s⁻¹. The symmetric supercapacitor (SSC) consisting of NDGH delivered an energy density of 14.3 W h kg⁻¹ at a power density of 582.8 W kg⁻¹ in the KOH electrolyte. These excellent electrochemical performance parameters could be achieved due to the synergistic effect of the multistage pore structure, nitrogen doping and binder-free hydrogel, which allow abundant electrochemical reactions to take place at the interface between the electrode materials and the electrolyte. This strategy may be further broadened to prominently enhance the specific capacitance and rate performance of graphene oxide.