Redox additive electrolyte assisted promising pseudocapacitance from strictly 1D and 2D blended structures of MnO2/rGO
Introduction
Human life is now increasingly depended upon machineries which require nonstop energy supply. Dependency on fossil fuels to power these machineries has impacted negatively due to decaying of these fossil fuels [[1], [2], [3], [4], [5], [6], [7]]. Development of electrochemical energy storage devices like supercapacitors can resolve many issues for sustainable energy supply [[8], [9], [10], [11]]. Generally, these supercapacitors found limited applications such as in hybrid vehicles and power stabilizers in contrast to Li-ion batteries due to their low charge storage capability [[12], [13], [14], [15], [16], [17]]. Fortunately, their charge storage capability can be improved utilizing faradaic activity wherein charges can be stored via adsorption on to the electrodes as well as inside the interstitial sites commonly termed as pseudocapacitance [[18], [19], [20]]. Environmental friendliness and redox reaction capabilities of transition metal oxides such as ruthenium oxide (RuO2) [21], nickel oxide (NiO) [[22], [23], [24]], cobalt oxide (Co3O4) [[25], [26], [27]] and manganese dioxide (MnO2) [[28], [29], [30]] make them favorable electrode materials for pseudocapacitors. Easy availability and attainment of one dimensional (1D) tunneled structure of MnO2 offers short diffusion path for ions or charges, facilitating the charge storage capability [[31], [32], [33], [34], [35], [36]]. Hence, it arouses the need to explore the capacitive behaviour of MnO2. Researchers have explored the excellent capacitive behaviour of MnO2 nanorods [37,38]. Anyhow, MnO2 exhibits low conductivity and to compensate this, it should be coated with suitable conducting material especially carbonaceous materials. Graphene oxide is the most preferred one for its confined 2D structures which could offer high surface area with improved conductivity [39,40].
However, systematic synthesis of nanomaterials of ultra-finesse morphology with high surface energy suitable for charge storage is still ambiguous. To resolve such issues, a novel systematic approach is presented to get strictly 1D structures of MnO2 and 2D structures of reduced graphene oxide (rGO) with ultra-morphological precision by optimizing a redox reaction maintained at hydrothermal condition. The capacitive performance of the synthesized material when tested as a pseudocapacitor electrode could be further complemented with redox additive, potassium ferricyanide (K3Fe(CN)6) into the KOH electrolyte. Notably, recent studies have showcased an improved capacitive performance using (K3Fe(CN)6) as redox additive to the electrolyte [41,42]. Here, a redox additive electrolyte is employed to extract a promising charge storage capability from the unique blend of 1D and 2D structure. The ease of synthesis technique as presented could potentially be utilized for commercialized production of high performing supercapacitor electrodes. Present work is a continuation of the previous reported works on MnO2 nanostructures based promising functionality [[43], [44], [45], [46], [47], [48], [49], [50]].
Section snippets
Chemicals
Graphite fine powder, Sodium Nitrate (NaNO3), Sodium Nitrite (NaNO2), Hydrochloric acid (HCl), Sulphuric acid (H2SO4), Hydrogen peroxide (H2O2) Potassium Hydroxide (KOH), Potassium Permanganate (KMnO4), Potassium Ferricyanide (K3Fe(CN)6), N-Methyl-2-pyrrolidone (NMP), Super P carbon and Polyvinylidene Fluoride (PVDF) with ultra-pure quality were used from Sigma Aldrich.
1D MnO2 and 2D rGO nanocomposite
Hummers technique was followed with little variations to obtain fine graphene oxide (GO) powder [51,52]. At first, graphite
Structure analysis
Fig. 1(a) represents the XRD pattern of the MnO2/rGO nanocomposite. JCPDS card number 44-0141 corresponds peak at 12.77, 18.05, 25.6, 26.5, 28.7, 36.5, 37.5, 38.8, 41.4, 41.9, 49.83, 56.13, 60.15, 65.28, 69.48 and 72.8 (2θ values) revealing the tetragonal α phase of MnO2 for the material prepared at 0.3 M H2SO4. The miller indices (110), (200), (220), (310), (400), (211), (330), (420), (301), (411), (600), (521), (002), (541) and (312) for each increasing 2θ values with presence of no other
Conclusions
Here, a promising hydrothermal route is developed to control the redox reaction between oxidizing and reducing agents in way to create strictly 1D structures of MnO2 with the help of proton admission. The reaction was successfully optimized in such an extent to incorporate fine 2D structure of rGO over 1D MnO2. The synergistic effect prevailed in the nanocomposite as a pseudocapacitor electrode tuned well with the addition of redox content K3Fe(CN)6 to achieve one of the most promising
Declaration of Competing Interest
None.
Acknowledgements
The author R. Rameshbabu thanks the Chilean National Agency for Research and Development (ANID-FONDECYT), Project No: 3190087 for the financial support. Also, the author Niraj Kumar thanks Uttaranchal University, Dehradun, India for providing seed money grant to carry out this research work under its Division of Research & Innovation.
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