A facile synthesis of high performing MnxOy-rGO hybrid composite for potential supercapacitor application

Abstract

In 2015, the United Nations (UN) has proposed a roadmap to achieve clean, cheap, and renewable energy in their sustainable development goals. As well as the modern technological advancement with zero-emission has been emphasized to upgrade the quality of lifestyles especially in developed countries [1]. As a consequence of the necessity, nowadays smart and heavy-duty electric vehicles have been introduced in front of modern society. Those electrical vehicles are accelerated by a heavy-duty electrical energy storage device (hybrid batteries, supercapacitors) which can be easily changed within a short period and supply required power with efficient mileage too [2].  In the context of energy storage, supercapacitors especially electric double-layer capacitors (EDLC) are good owing to their robust lifetime, longer cycle life, rapid charging-discharging, low maintenance cost, and high power energy storage for multipurpose applications [3].  Usually, carbon-based materials viz. graphene, reduced graphene oxide (rGO), CNT, biomass-derived porous carbon materials, etc. are the most popular candidates of EDLC [4]. Among them, rGO is special for their excellent electrochemical performance attributed to their morphological excellence including the expanded d-spacing and abundant surface area. But their highly agglomerate properties reduce their performance (conductivity, accessible surface area) thus limits their applications [5].

To address this issue, rGO can be synthesized by mixing with other transitional metal or metal oxide which will contribute to the expansion of the rGO layer, and enhance the supercapacitor performance by induces pseudocapacitance thus increasing the charging-discharging rate [6].  Due to the various oxidation states and high theoretical capacitance (1370 Fg^-1) manganese oxides are highly recommended as a high-performance supercapacitor electrode material [7].  Moreover, the natural abundance of manganese is greater than most of the other transitional metals. Hence, the incorporation of Mn_xO_y also attributes to rGO electrode performance by enhancing the surface area, interlayer distance, and charge transfer rate due to the fast-reversible redox reaction [8]. The total electrochemical performance of the Mn_xO_y-rGO electrode highly depends on the morphology and amount of incorporated Mn_xO_y [9]. Interestingly, the presence of high content of Mn_xO_y results in poor performance of the rGO-Mn_xO_y electrode. This might be occurred because of the higher agglomeration of Mn_xO_y in and on between the rGO surface and blocking of rGO pores. As a result, the charge transfer resistance of the electrode material increases. So, it is very crucial to optimize the amount of Mn_xO_y during the preparation of the Mn_xO_y-rGO composite during the synthesis. In this work, the rGO-Mn_xO_y composite was synthesized in a facile way by optimizing the amount of KMnO₄. In brief, different concentration of KMnO₄ was mixed with 0.2 g of GO suspension under vigorous stirring and sonication. 5 mL of glycerin was further added to form precursors for the preparation of Mn_xO_y-rGO. Finally, the mixture was reduced by a one-step hydrothermal reduction process (180°C for 24 h) followed by washing and drying. The electrochemical capacitance performance of all compositions was evaluated using cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) in a two-electrode system with 0.5 M Na₂SO₄ aqueous electrolyte solution. The maximum specific capacitance was found at 280 Fg^-1 at 1 Ag^-1 current density for the rGO-Mn_xO_y composite at the concentration of 0.05mM KMnO₄. The summarized electrochemical data are represented in Table 1.  The total work is expected to contribute to the next generation of hybrid supercapacitor applications.

Table 1.

No.

Conc. of KMnO₄

Electrolyte

Potential  window, (V)

Specific capacitance, C_sp(Fg^-1)

Energy density, E

(Whkg^-1)

Power density,P

(kWkg^-1)

1.     

0.5 mM

Na₂SO₄

1.0

280

9.72

250.0

2.     

0.5 M

Na₂SO₄

1.0

100

3.47

250.0