Core-shell Hybrid Nanostructure Based High Performance Supercapacitor Electrode

Ashutosh Kumar Singh (left) and Kalyan Mandal

Authors: Ashutosh Kumar Singh, Kalyan Mandal

Affiliation: Department of Condensed Matter Physics and Material Sciences,
S.N. Bose National Centre for Basic Sciences, Kolkata, India.

Background:

Since last decade, energy crisis has been one of the vital problems in the society due to the excessive use of fossil-fuel resources and environmental pollution. Therefore, the development of very light-weight and environment-friendly proficient energy storage devices has become the priority of the researchers and scientists for satisfying the demand of modern consumer’s hybrid electric and portable electronics devices [1,2]. In this progression, researchers have developed new type of energy storage devices called supercapacitors, also known as electrochemical capacitors which offer high power and eneergy density, high rate capability as well as superb cycle stability as compared to conventional battery and capacitors [3,4].

The supercapacitors are classified in two groups based on their charge storage method. The first group is called pseudocapacitor which involves the redox reactions of the electrode materials at the interface of electrode and electrolyte, whereas the second group is known as electric double layer capacitor which holds charge separation at the interface of electrode and electrolyte [5,6]. By changing the morphology of the electrode materials, one can manipulate the performance of a supercapacitor. The performance and quality of the supercapacitors are very much dependent on the materials and morphology used in the preparation of their electrodes. Recently, many metal oxide based materials (like RuO₂, NiO, Fe₂O₃, MnO₂, Co₃O₄, TiO₂, etc.) have been used for the fabrication of pseudocapacitor electrodes; among them NiO and Fe₂O₃ have been widely used as redox active materials for the fabrication of supercapacitor electrodes of different morphologies, such as Fe₂O₃-nanotube, Fe₂O₃-thinfilm, electrospun Co₃O₄-nanostrtuctures, porous Fe₂O₃-nanostrtuctures, NiO–nanobelts, NiO–nanoballs, NiO–nanoflowers, NiO–nanoflakes. The reasons behind the extensive use of NiO and Fe₂O₃ as supercapacitor electrode materials are: they are very stable in nature, they are non-toxic as well as environment friendly and they are very cheap and easily available.

Aim:

After having so many of supportive properties for being used as electrode materials in supercapacitors, still their reported specific capacitance values are very low compared to their own theoretical specific capacitance value and other metal oxide based electrodes. The only problem restricts them to be used as an electrode material for high performance supercapaitor is their bad electrical conductivity and we all are aware of the fact that electrode material must have high electrical conductivity for high performance supercapacitor.

Scope:

In the recent development process of supercapacitor performance, it has been found that the electrical conductivity could be improved by introducing impurities via doping of one metal oxide material with other metal oxide material. This doping process enhances the charge movement which affects the reactions at the interface of electrode and electrolyte. So far in the literature we have not found any work based on NiO and Fe₂O₃ as mixed component transition metal oxides for supercapacitor electrodes. However, keeping all the above research facts in the mind, still there exist a plenty of remarkable opportunities to enhance the electrochemical properties of NiO and Fe₂O₃ based electrodes.

Unique features of the approach:

Therefore, we report a simple fabrication technique and unique electrochemical properties of the electrode based on core/shell Fe-Ni/Fe₂O₃-NiO hybrid nanostructures (HNs). This core-shell HNs have very high aspect ratio with a porous thin nanolayer of redox active oxides which would provide a very large surface area for redox reactions at the interface of electrode and electrolyte. This would contribute to the enhancement of the ion and electron movement and performance of the supercapacitor. In addition, the core material consists of conductive FeNi nanowires (NWs) which provides the expressway for the electrons to transport to the current collector via core material.

This would automatically improve the rate capability and power density of the supercapacitor. The electrical conductivity of the electrode could be improved by introducing impurities via doping of one metal oxide (NiO) material with another metal oxide (Fe₂O₃) material. The unique feature of this electrode fabrication technique is that it doesn’t contain any extra binder material. As a result, there would be enhancement in the charge transfer kinetics [7]. This kind of fabrication technique could be applied in the fabrication process of electrodes of all energy storage devices in general.

Significant Results:

According to our anticipations, the core/shell Fe-Ni/Fe₂O₃-NiO hybrid nanostructure shows high quality supercapacitive performance in terms of specific capacitance (1415 F/g), energy density (27.6 Wh/kg), power density (10.3 kW/kg), cycling stability (remain 95% of initial specific capacitance after 3000 charge/discharge cycle) and rate capability [7]; these profound results made it a very good and unique alternative for the next generation supercapacitor electrodes.

References:
[1] Patrice Simon, Yuri Gogotsi, "Materials for electrochemical capacitors". Nature Materials, 7, 845 (2008).  Abstract.
[2] John R. Miller, Patrice Simon, Electrochemical Capacitors for Energy Management". Science 321, 651 (2008). Abstract.
[3] Zhibin Lei, Li Lu, X.S. Zhao, "The electrocapacitive properties of graphene oxide reduced by urea". Energy & Environmental Science, 5, 6391 (2012). Abstract.
[4] Sheng Chen, Junwu Zhu, Xiaodong Wu, Qiaofeng Han, Xin Wang, "Graphene Oxide−MnO2 Nanocomposites for Supercapacitors". ACS Nano 4, 2822 (2010). Abstract.
[5] Wei Chen, R.B. Rakhi, Liangbing Hu, Xing Xie, Yi Cui, H.N. Alshareef, "High-Performance Nanostructured Supercapacitors on a Sponge". Nano Letters, 11, 5165 (2011). Abstract.
[6] Raghavan Baby Rakhi, Wei Chen, Dongkyu Cha, H. N. Alshareef, "Nanostructured Ternary Electrodes for Energy-Storage Applications". Advanced Energy Materials, 2, 381 (2012). Abstract.
[7] Ashutosh K. Singh, Kalyan Mandal, "Engineering of high performance supercapacitor electrode based on Fe-Ni/Fe2O3-NiO core/shell hybrid nanostructures". Journal of Applied Physics, 117, 105101 (2015). Abstract.