Rapid Production of Mn3O4/rGO as an Efficient Electrode Material for Supercapacitor by Flame Plasma

Benefiting from good ion accessibility and high electrical conductivity, graphene-based material as electrodes show promising electrochemical performance in energy storage systems. In this study, a novel strategy is devised to prepare binder-free Mn3O4-reduced graphene oxide (Mn3O4/rGO) electrodes. Well-dispersed and homogeneous Mn3O4 nanosheets are grown on graphene layers through a facile chemical co-precipitation process and subsequent flame procedure. This obtained Mn3O4/rGO nanostructures exhibit excellent gravimetric specific capacitance of 342.5 F g−1 at current density of 1 A g−1 and remarkable cycling stability of 85.47% capacitance retention under 10,000 extreme charge/discharge cycles at large current density. Furthermore, an asymmetric supercapacitor assembled using Mn3O4/rGO and activated graphene (AG) delivers a high energy density of 27.41 Wh kg−1 and a maximum power density of 8 kW kg−1. The material synthesis strategy presented in this study is facile, rapid and simple, which would give an insight into potential strategies for large-scale applications of metal oxide/graphene and hold tremendous promise for power storage applications.


S1. Modified Hummers method
For oxidation step of synthesis, 23 mL of concentrated H2SO4 is placed into a 500 mL roundbottom flask and chilled to 0 °C using an ice bath. The natural flake graphite (1 g) and sodium nitrate (0.5 g) are added to the acid and stirred. 3g of KMnO4 is added slowly and allowed to dissolve, with the temperature is closely monitored so as not to allow the mixture to go above 10 °C. This mixture is then allowed to react at 35 °C for 30 min, after which distilled water (50 ml) is added. The mixture is stirred for 15 min at 98 °C at which point an additional 140 mL of DI water is added. Shortly after the dilution with 140 mL of water, 10 ml of 30% H2O2 is added to the mixture resulting in a brilliant yellow color along with bubbling. The mixture is allowed to settle for at least 5 h after which the clear supernatant is decanted. The remaining mixture is centrifuged and washed with a total of 500 mL of 5% HCl solution followed by 1.5 L of DI water to remove the acid. The resulting solid is dried at 35 °C for 24 h.

S2. Preparation of activated graphene
The activated graphene was prepared via a modified microwave exfoliated method which has been reported by Rodney S. Ruoff previously. [1] Briefly, graphite oxide (GO) cake was synthesized from purified natural graphite (SP-1, Bay Carbon, Bay City, MI, USA) by the Hummers method. Microwave exfoliated graphite oxide (MEGO) was prepared by irradiating the GO in a microwave oven (GE) in ambient conditions. Upon microwave irradiation, the very large volumetric expansion of the GO cake yields a black and fluffy MEGO powder. The as-prepared MEGO powder was then dispersed and soaked in aqueous KOH solution, followed by filtration and drying, to form the MEGO/KOH mixture for chemical activation. The MEGO/KOH mixture was then put in a tube furnace under flowing argon at a pressure of about 400 Torr, and heated at 800 °C for one hour to enhancing their specific surface area.

S3. Mass measurement and calculation
The obtained MnO2/GO film was filtered and dried under vacuum at 35 °C. Finally, the MnO2/GO film were quickly passed through a naked flame from a burning alcohol lamp to obtain Mn3O4/rGO. After dried the Mn3O4/rGO ground into fine powder curing，the samples were directly used as an electrode, without adding any polymer binder or carbon black: the composite were pressed between two nickel foams (1 × 1 cm 2 ) to form an electrode. The loading mass of the Mn3O4/rGO active materials can be obtained by measuring the weight change of the samples before and after the process of preparation electrode.

S4. The calculation of CV and GCD
Cyclic voltammetry (CV), galvanostatic charging/discharging (GCD), and electrochemical impedance spectroscopy (EIS) of the as-prepared samples were investigated on an electrochemical station (CHI760). The mass specific capacitance (C) were calculated from the CV and GCD curves according to the equations (1) and (2) below, respectively.
(1) (2) where i is the applied current, v is the scan rate, Δt is the discharge time, ΔU is the operating voltage window, m is the mass of the active material.

S5. The calculation of GCD of an ASC
The mass specific capacitance (C), mass specific energy density (E) and mass power density (P) were calculated from the GCD curves according to the equations below. (3) where i is the applied current, Δt is the discharge time, ΔU is the operating voltage window, m is the total weight of the positive and negative electrodes active material.

S6.
Asymmetric supercapacitors were constructed using Mn3O4/rGO as a positive electrode, activated graphene (AG) as a negative electrode, and 0.1 M NaSO4 as an electrolyte. To balance the charge storage, the masses of the positive and negative electrodes were optimized according to the following equation: (6) According to the CV test at 20 mV s -1 , the optimized mass ratio between Mn3O4/rGO and AG should be m+/m− = 0.50449.    Table S1 Comparison of the fabrication process and electrochemical performance of the as manganese oxide material in this work with those reported in previous literatures.

Materials
Specific capacitance (F g -1 ) Current density/ scan rate reference