A novel synthesis of ultra thin graphene sheets for energy storage applications using malonic acid as a reducing agent

Abstract

The present manuscript reports the novel synthesis of ultrathin graphene sheets employing malonic acid as a reducing agent (GRH-MA) under mild pH conditions. AFM reveals their thickness to be 0.41 ± 0.03 nm. XPS of GRH-MA exhibits the increased intensity of the CC band suggesting the effective reduction of graphene oxide (GO) with enhanced sp² character. Optical, infrared and Raman spectroscopy, the increased C/O ratio in FESEM and TEM analysis also support this observation. Current–voltage (I–V) measurements show about four orders of magnitude higher conductivity for GRH-MA (4.4 S cm⁻¹) as compared to that of GO (3.05 × 10⁻⁴ S cm⁻¹). The reduction of GO by oxalic acid (GRH-Ox) under identical experimental conditions was observed to be less efficient as indicated by optical and Raman spectroscopy, and I–V measurements. The efficient reduction by malonic acid is thus understood by the presence of the active methylene group, which makes it an effective nucleophile. XRD of GRH-MA sheets annealed at 300 °C (GRH-MA300) exhibits the ‘d’ spacing of 0.35 nm. HRTEM analysis of GRH-MA300 also showed a similar ‘d’ spacing of 0.35 ± 0.01 nm with a hexagonal structure, indicating the formation of a more ordered graphitic structure upon annealing. IR analysis of this sample exhibited a significant reduction in the oxygen functionalities and I–V measurements showed a more than 4-fold increase in conductivity as compared to that of GRH-MA. In cyclic voltammetry, GRH-MA shows a fairly high specific capacitance (C_s) of 173 F g⁻¹ which is more than 23-fold higher to that of GO (7.5 F g⁻¹) at 100 mV s⁻¹ in 1 M H₂SO₄. Galvanostatic charge–discharge measurements show the maximum C_s value of 254 F g⁻¹ at 1 A g⁻¹ which is more than an order of magnitude higher to that of GO (18.6 F g⁻¹) and 1.25 times higher to that of GRH-Ox (202 F g⁻¹), respectively. Moreover, 1000 cycles of charge–discharge at 10 A g⁻¹ exhibit fairly good cyclic stability demonstrating its immense potential for energy storage applications.