Hydrothermal synthesis of mesoporous NiMnO3 nanostructures for supercapacitor application: Effect of electrolyte
Graphical abstract
Introduction
It is indispensable to develop alternative sources for energy production cleanly and sustainably to overcome the rapid depletion of fossil fuels and its adverse impact on health and the environment. In this context, the utilization of electrochemical devices like supercapacitors for energy storage and the conversion will be a relevant solution to reduce environmental degradation [1], [2], [3].
There are different energy storage devices such as batteries, fuel cells, ordinary capacitors, and supercapacitors. Fuel cells and batteries can store an extensive amount of energy but suffer from unsatisfactory power density. Ordinary capacitors have an enormous amount of power density, but they have low energy density. Therefore, to meet upcoming clean and green energy demand, the development of lightweight, low-price, excellent performance, flexible, and environmentally benign energy technology is necessary to store and transform energy efficiently [4, 5]. Electrical supercapacitors are passive and static electrochemical energy storage systems with low weight, superior working safety, ultralong cycle span, and rapid intercalation-deintercalation rates. Moreover, electrical supercapacitors exhibit huge energy density compared to an ordinary capacitor and high power delivery rate as compared with secondary batteries and fuel cells [6, 7].
Supercapacitors have been made up of different electrode materials such as carbon-based [8], polymers [9], and transition metal oxides (TMOs) [10]. Supercapacitors based on carbon composites and conducting polymers suffer from unsatisfactory specific capacitance (Cs). However, the interior electrical conductivity of TMOs determines their feasibility for supercapacitor applications. RuO2 is an excellent material for supercapacitor application, but the higher cost and rareness of ruthenium is limiting its commercial production [11]. Therefore, most of the researchers have focused their attention on other TMOs, based on materials involving cost-effective metal elements with multiple oxidation states. Especially, Ni and Mn-based TMOs and mixed TMOs instead of RuO2 owing to their high conductivity, a larger value of Cs, and more active redox reactions [12]. Different TMOs such as NiO [10], MnO2 [13], NiCo2O4 [14], MnFe2O4 [15], NiMnO3 [16], CuCo2O4 [17] and NiMn2O4 [18] have been utilized as active electrode materials for the electrochemical supercapacitor application.
Out of different TMOs, NiMnO3 (NMO) is a suitable choice as an electrode material for electrochemical energy storage applications. It offers certain advantages such as low-price, enormous conductivity, high chemical stability, large operating potential window, and abundance of constituent elements in nature [16]. Researchers are working on improving electrical conductivity, electrochemical performance, and enhancing the Cs of NMO through forming its composites with several materials. Kakvand et al. [19] studied the synthesis of NMO/C through the co-precipitation method and reported Cs of 285 Fg−1 at 1 Ag−1 and 93.5% cycle stability after 1000 cycles. Ge et al. [20] reported synthesis of flower-like NMO/Ni(OH)2 nanocomposites and obtained Cs of 3800 Fg−1 with 83% cycle stability after 1000 cycles at a current density of 1mAcm−2. Sanchez et al. fabricated NMO-reduced graphene oxide nanocomposites as electrode materials for a hybrid energy storage device which displayed a capacity of 91 mAhg−1 at a scan rate of 5mVs−1 [21]. Giri et al. [22] developed NMO/nitrogen-doped graphene nanocomposite, which provided Cs of 523.5 Fg−1, and 82.31% cycle stability for 1000 cycles at a current density of 1 Ag−1.
Among the different methods available for the synthesis of nanostructures, the hydrothermal method has a lot of advantages, such as low operating temperature, easy operation, and simple instrumentation. It facilitates uniform nucleation and growth of the materials [23]. Moreover, the long-term cyclic stability of the electrode material is an important parameter for the practical use of supercapacitors.
The present research article reports hydrothermal growth of rhombohedral NMO nanostructures (NSs). The electrochemical properties of the working electrodes fabricated from NMO NSs are examined in various electrolytes such as 1 M KOH, 1 M NaOH, and 1 M Na2SO4. At a scan rate of 5 mVs−1, the NMONickel foam (NF) working electrode showed a specific capacitance of 435 Fg−1 and 100% cyclic stability for 2000 cycles, in 1 M KOH electrolyte.
Section snippets
Chemicals
Analytical grade chemicals, such as nickel acetate (Ni(CH3COO)2•4H2O, 98%), manganese acetate (Mn(CH3COO)2•4H2O,98%), methanol (CH3OH), potassium hydroxide (KOH), sodium hydroxide (NaOH), and sodium sulfate (Na2SO4), were used in the present research work without further purification.
Synthesis of mesoporous NMO NSs by hydrothermal method
NMO NSs were synthesized by a hydrothermal route. Nickel acetate and manganese acetate were used as the precursors for the synthesis of NMO NSs. Nickel acetate and manganese acetate were separately dissolved in a
XRD study
XRD pattern of NMO NSs is displayed in Fig. 2. The reflections observed at 24.6°, 30.6°, 33.6°, 36.5°, 39.3°, 39.8°, 41.8°, 44.6°, 50.6°, 55.1°, 58.4°, 64.1°, and 65.9° are indexed to (012), (011), (104), (110), (015), (006), (113), (202), (024), (116), (018), (214), and (300) planes, respectively. The XRD study revealed a rhombohedral crystal structure [ICDD No. 00–012–0269] of hydrothermally synthesized NMO NSs, corresponding to space group R. The average crystallite size of the NMO NSs is
CV analysis
Electrolyte plays a vital role in determining the electrochemical performance of electrode material employed for supercapacitor application [26]. CV analysis of the NMONF electrode is performed in a cell containing 1 M KOH, 1 M NaOH, and 1 M Na2SO4 electrolytes at various scan rates of 5 mVs−1 to 100 mVs−1 and by varying potential window range over −0.25 V to 0.45 V. Fig. 5A displays CV plots of the NMONF electrode in (a) 1 M KOH (b) 1 M NaOH and (c) 1 M Na2SO4, respectively. CV plots in Fig. 5
Conclusions
The XRD analysis revealed a rhombohedral crystal structure of the NMO NSs with R-space group. FE-SEM micrographs reflected the mesoporous nature of NMO NSs comprising interconnected nanosheets. The existence of constituent elements such as Ni, Mn, and O in the synthesized NMO NSs is confirmed from the EDAX analysis. The surface wettability study explored the hydrophilic nature of NMONSs. The NMONF electrode delivered a maximum specific capacitance of 435 Fg−1 at a scan rate of 5 mVs−1 in 1 M
CRediT authorship contribution statement
S.D. Dhas: Writing - original draft, Visualization, Data curation. P.S. Maldar: Conceptualization, Methodology, Validation. M.D. Patil: Software, Data curation. K.M. Hubali: Resources. U.V. Shembade: Formal analysis, Investigation. S.B. Abitkar: Formal analysis, Investigation. M.R. Waikar: Software. R.G. Sonkawade: Supervision. G.L. Agawane: Supervision. A.V. Moholkar: Supervision.
Declaration of Competing Interest
1) No conflict of interest we wish to confirm that there are no known conflicts of interest associated with this publication and there is no significant financial support for this work that could have influenced its outcome. 2) Funding – All the sources of funding for the work described in this publication are acknowledged.
Acknowledgments
Author S. D. Dhas gratefully acknowledges Chhatrapati Shahu Maharaj Research, Training and Human Development Institute (SARTHI), Pune, for providing funding through CMSRF, and author Dr. A. V. Moholkar acknowledges DST-SERB for providing funding under project No. [SERB/F/1699/2018–19]. All the authors acknowledge the Special Assistance Program (SAP) [F.530/16/DSA-II/2018 (SAP-1)] and PIFC (Physics Instrumentation Facility centre), Department of Physics for providing characterization facilities
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