Nano-cubes for energy storage

A key factor for meeting increasing energy demands is to enhance the energy conversion of renewable resources as well as integrate high performance energy storage devices [1]. These appliances have drawn significant research interest as they are vital electronic components [2]. In this regard, supercapacitors are one of the most important members of the energy storage family owing to their outstanding electrochemical properties. Foremost among these properties are fast charging-discharging rates, high power densities and long cycle lives [3]. In accordance with this purpose, impressive efforts have been carried out on the development of nano engineered and advanced materials which are key for the development of highly productive supercapacitor devices.

From a materials point of view, the materials should match the key requirements; such as superior electrochemical properties and easy compatibility, to be conducive towards large scale commercial operations [4]. To improve the electrical and electrochemical performance of simple oxide materials, mixed transitional metal oxides have received attention due to their demonstration of high specific capacitance with relatively low costs, compared with traditional metal oxides. However, most of the leading transitional metal oxide materials pseudo-capacitive performance is limited with inherently low electrical conductivity [5].

Among these transitional metal oxides, NiMnO₃ is a binary metal oxide material with a high potential in high-performance supercapacitors with remarkable specialties. Theoretically, NiMnO₃ exhibits preferable electrical conductivity and a greater specific capacitance than those of other transition metal oxides (MnO₂, NiO). However, creating a coherent synergistic effect to improve the properties of materials is one of the most interesting competences of nano sciences. Substitution of abundant ions (Zn, Cu, Mn, Ni) to traditional metal oxides results in more interesting peculiarities than their analogues. Likewise, the electrical properties of NiMnO₃ can be advanced by the synergistic effect of Fe doping. This effect possibly originates from trans-location of some of the Mn atoms with Fe ions, resulting a coherent synergistic effect between Ni-Fe-Mn causing better conductivity. The fact remains that an ideal supercapacitor material should also have large specific surface area. According to the results of our experiments, Fe doping on NiMnO₃ increases the porosity of the material, which directly increases the surface area and thus increases the pseudo-capacitive performance. The morphological structure of the material is crucial for those desired features. Furthermore, in order to obtain equal size square plates composing a cubic structure, the synthesis method is vitally important. The composite nano-material has been synthesized through a single-step hydrothermal method. Briefly, Fe doped NiMnO₃ powder has been synthesized with a modification in the method described in the literature [6]. In order to clarify the Fe doping effect on the morphological and electrochemical properties of the material, Fe doped NiMnO₃ powder was synthesized with different Fe molar concentrations varying from 0.001?M Fe to 5?M Fe. Interestingly, these perfectly organized structures cannot be obtained without optimum Fe doping rate which was found to be 0.5?M. As Fe doping concentration increases, the morphology shifts towards cubic to amorphous structures. Similarly, when the reaction is carried out without the hydrothermal method, spherical structures are observed instead of cubic structures. Additively, the hydrothermal method provides the opportunity to fabricate nano-materials for large scale operations by enabling high pressure with low temperature synthesis. The resulting morphology indicates the enlarged specific surface area with a good distribution of twin size square plates. Enlarged specific surface area has a direct effect on the improved electrochemical properties by increasing the active sites. Thus, improvement in the charge storage performance. Furthermore, increased pseudo-capacitive performance results from the increase of the porosity in the material by Fe doping. It is appropriate to say that an optimum level of Fe doping enhances the specific capacitance. Advanced analysis of the composite nano-material was taken by X-Ray Diffraction (XRD). XRD analysis results have verified the Fe doped NiMnO₃ existence in further detail.

The Field Emission Scanning Electron Microscope (FE-SEM, Carl Zeiss Supra 55) image shown on the cover of this issue demonstrates the equal dimensional sizes of the square plates composing a greater cubic structure, obtained by the hydrothermal method. As can be seen from the SEM image, homogeneous porosity is observed between the square plates having a thickness of approximately 3–4?nm and an average edge length of 100?nm, forming the cubic structure of Fe doped NiMnO₃. Our study has demonstrated that the as-synthesized material possesses advanced features and is suitable for energy applications.

Nano-cubes for energy storage

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