The temperature dependent morphological evolution and its effect on the electrochemical supercapacitive properties of Ni(OH)₂ thin films have been systematically investigated. A temperature dependent growth mechanism model is proposed for the changes in microstructure. Different nanostructures of Ni(OH)₂ thin films such as nanoplates, stacked nanoplates, nanobelts and nanoribbons have been fabricated by varying the deposition temperature. An X-ray diffraction study discloses the orientations of different nanostructures and the formation of nanocrystalline β-Ni(OH)₂. Further, these Ni(OH)₂ nanostructures demonstrate excellent surface properties like uniform surface morphology, good surface area, pore volume and uniform pore size distribution. The electrochemical supercapacitive properties of Ni(OH)₂ nanostructures have been investigated by cyclic voltammetry, charge–discharge and electrochemical impedance spectroscopy techniques. The electrochemical studies of the Ni(OH)₂ samples show an obvious influence of surface properties on the pseudocapacitance. The maximum specific capacitance of 357 F g⁻¹ was evaluated for nanoplates at a scan rate of 5 mV s⁻¹. Furthermore, all these Ni(OH)₂ samples show good long-term cycling performances in KOH electrolyte. The Ragone plots ascertain good power and energy densities of all Ni(OH)₂ nanostructured samples. Subsequently, electrochemical impedance measurements for the different nanostructures of Ni(OH)₂ electrodes are assessed indicating that the Ni(OH)₂ nanoplates structured electrodes are suitable for good capacity electrochemical supercapacitors.

Interlocked cubelike <mml:math display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>Mn</mml:mtext></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>Mn3O4 thin films have been prepared by a simple and low temperature chemical bath deposition method. These interlocked cubelike <mml:math display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>Mn</mml:mtext></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>Mn3O4 thin films are further converted into nanoflakes of birnessite <mml:math display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>MnO</mml:mtext></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>MnO2 using voltammetric cycling in aqueous <mml:math display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>Na</mml:mtext></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mtext>SO</mml:mtext></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>Na2SO4 electrolyte. The process is dynamic potential activated, which causes the formation of sheet-shaped nanoflakes. The films are characterized by X-ray diffraction, field-emission-scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform IR spectrum, and wettability test. Impedance spectroscopy studies revealed that charge-transfer resistance of the birnessite <mml:math display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>MnO</mml:mtext></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>MnO2 structure has a lower value than that of the <mml:math display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>Mn</mml:mtext></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>Mn3O4 structure. The effect of different numbers of potential cycles on structure, surface morphology, valence states, and contact angles has been investigated. During the cycling process, the supercapacitance of manganese oxide increased by more than 10 times. The maximum supercapacitance achieved at <mml:math display="inline"><mml:mrow><mml:mn>5</mml:mn><mml:mspace width="0.5em"></mml:mspace><mml:mtext>mV</mml:mtext><mml:mspace width="0.5em"></mml:mspace><mml:msup><mml:mi mathvariant="normal">s</mml:mi><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>5mVs−1 is <mml:math display="inline"><mml:mrow><mml:mn>223</mml:mn><mml:mspace width="0.5em"></mml:mspace><mml:mi mathvariant="normal">F</mml:mi><mml:mspace width="0.5em"></mml:mspace><mml:msup><mml:mi mathvariant="normal">g</mml:mi><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>223Fg−1 . The effect of scan rate on the specific capacitance of birnessite <mml:math display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>MnO</mml:mtext></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>MnO2 electrode has been studied.

In present investigation, we have prepared a nanocomposites of highly porous MnO₂ spongy balls and multi-walled carbon nanotubes (MWCNTs) in thin film form and tested in novel redox-active electrolyte (K₃[Fe(CN)₆] doped aqueous Na₂SO₄) for supercapacitor application. Briefly, MWCNTs were deposited on stainless steel substrate by “dip and dry” method followed by electrodeposition of MnO₂ spongy balls. Further, the supercapacitive properties of these hybrid thin films were evaluated in hybrid electrolyte ((K₃[Fe(CN)₆ doped aqueous Na₂SO₄). Thus, this is the first proof-of-design where redox-active electrolyte is applied to MWCNTs/MnO₂ hybrid thin films. Impressively, the MWCNTs/MnO₂ hybrid film showed a significant improvement in electrochemical performance with maximum specific capacitance of 1012 Fg⁻¹ at 2 mA cm⁻² current density in redox-active electrolyte, which is 1.5-fold higher than that of conventional electrolyte (Na₂SO₄). Further, asymmetric capacitor based on MWCNTs/MnO₂ hybrid film as positive and Fe₂O₃ thin film as negative electrode was fabricated and tested in redox-active electrolytes. Strikingly, MWCNTs/MnO₂//Fe₂O₃ asymmetric cell showed an excellent supercapacitive performance with maximum specific capacitance of 226 Fg⁻¹ and specific energy of 54.39 Wh kg⁻¹ at specific power of 667 Wkg⁻¹. Strikingly, actual practical demonstration shows lightning of 567 red LEDs suggesting “ready-to sell” product for industries.