Editorial Board

Initially, a stainless steel (SS) substrate was polished with silicon carbide paper and washed with ethanol and deionized (DI) water. Then, a 0.05M NiCl₂ solution was prepared in 40 mL DI water and 0.1 M Na₂SO₄ was added into the NiCl₂ solution under constant magnetic stirring for 300 s at room temperature. Then, a Ni precursor was deposited on the well cleaned SS substrate using a three-electrode setup in the above prepared solution as an electrolyte. The three-electrode set up consisted of SS substrate as working electrode, platinum plate as counter electrode and saturated calomel as reference electrode. To deposit the nickel hydroxide thin film, a -1.0 V/SCE potential was applied to the SS substrate for 300 s. After electrodeposition of the nickel hydroxide thin film, the substrate was rinsed in DI water and dried in natural light.

A 0.01 M potassium permanganate solution was prepared in 40 mL DI water and 0.2 mL methanol was added into the potassium permanganate solution under constant magnetic stirring for 300 s at room temperature. Then, manganese precursor was coated on the well cleaned SS substrate using the three-electrode setup using the prepared solution as an electrolyte. A -1.0 V/SCE potential was applied on the SS substrate for 300 s to deposit the manganese hydroxide thin film. Finally, the thin film of manganese precursor was rinsed in DI water and dried in natural light.

0.01 M nickel chloride, 0.01 M potassium permanganate, and 0.1 M Na₂SO₄ solutions were added into 40 mL DI water under constant magnetic stirring and 0.2 mL methanol was added into it to form the final electrolyte solution. Then, the Ni-Mn DH thin film was prepared on the SS substrate in a final electrolyte solution by applying a -1.0 V/SCE potential for 300 s using a three-electrode setup. After the electrodeposition process, the Ni-Mn DH thin film was rinsed with DI water and designated as Ni-Mn-DH-1.

Surface morphology observation and elemental mapping of material were performed using field-emission scanning electron microscopy (FE-SEM) technique with the help of FE-SEM; SU-70, Hitachi High Technologies. The composition and elemental valence states of Ni-Mn DH thin film were analysed using X-ray photo emission spectroscopy (XPS) technique with the help of XPS; ESCALab Mark II, VG Scientific Ltd. model. The electrochemical charge-storage analyses of thin film electrodes were performed using cyclic voltammetry (CV), and galvanostatic charge-discharge (GCD) techniques in aqueous 1 M KOH electrolyte. Impedance study of thin film electrodes was performed within a frequency range of 0.1 to 100 kHz at 10 mV ac amplitude.

Electrodeposited thin films of manganese precursor, nickel precursor, and manganese nickel double hydroxide are scanned using FE-SEM technique to find surface micro/nanostructure analysis. Fig. 1(a)-(b) shows a surface microstructure similar to that of micro-bushes for the manganese precursor with macropores that are formed at the bottom of micro-bushes structure. Fig. 1(c)-(d) presents nano-spheres surface morphology of nickel precursor, with nano-spheres of nickel precursor that are composed of very fine nano-flakes. While, Fig. 1(e)-(f) depicts very fine nanoparticles of nickel manganese double hydroxide with dimensions less than 30 nm. Thus, the manganese-nickel double hydroxide demonstrates an ultra-fine nanostructure compared with that of individual manganese and nickel precursors. Such an ultra-find nanostructure surface nanomaterial can be highly useful electrode for increasing the electrochemical chargestorage properties of pseudocapacitor.

Fig. 1. 
FE-SEM images of (a), (b) manganese hydroxide (c), (d) nickel hydroxide and (e), (f) nickel-manganese layered double hydroxide thin films.

The chemical composition or elemental mapping of thin films is performed using energy dispersive X-ray spectroscopy (EDS) technique. Fig. 2(a)-(c) show manganese and oxygen elements present in the manganese hydroxide thin film. Nickel hydroxide thin film [Fig. 2(d)-(f)] shows homogeneous distribution of nickel and oxygen elements, and manganese-nickel double hydroxide thin film shows [Fig. 2(g)-(i)] presence of manganese, nickel and oxygen elements. Fig. 3(a) shows comparative FT-IR curves of different thin films recorded for different wave numbers within the range of in 648 to 4000 cm^-1. A broad FT-IR peak in the range of 3680 to 2780 cm^-1 is attributed to stretching vibrations of molecular water and hydrogen bond (-OH) groups [10]. Similarly, the weak bond at 1627 cm^-1 corresponds to bending mode of water molecules [11]. The presence of CO₃²⁻ in the thin film samples is evidenced by its vibration bands from middle to lower wave numbers. The bands observed at 1052, 827, 697 cm^-1 wave numbers are attributed to –CO, -(CO₃)^2- and O-C-O bonds, which are introduced in the material owing to the exposure of thin films to atmospheric carbon and oxygen species. Finally, the band at 961 cm^-1 present in all thin films, which is attributed to metal hydroxide (M-OH) bending modes [12]. Thus, manganese hydroxide, nickel hydroxide, and manganese nickel hydroxide materials are confirmed from FT-IR study of all metal hydroxide thin films.

Fig. 2. 
EDAX elemental mapping of (a)-(c) manganese precursor, (d)-(f) nickel precursor and (g)-(i) nickel-manganese layered double hydroxide thin films.

Fig. 3. 
Comparison (a) FTIR spectra of different thin films. Narrow scan XPS profiles of (b) Mn 2p, (c) Ni 2p and (d) O 1s energy states of different elements present in the nickel-manganese double hydroxide thin film.

XPS technique was used to find oxidation state of the elements in the nickel-manganese double hydroxide thin film. Fig. 3(b) shows Mn 2p XPS profile of manganese with the doublet of Mn 2p_3/2 and Mn 2p_1/2 at 642.7 and 653.8 eV binding energies, respectively. The energy difference between Mn 2p doublets of 11.1 eV indicates that manganese is present with Mn⁺³ state in the nickel-manganese double hydroxide thin film [13]. Ni 2p XPS profile [Fig. 3(c)] in Ni-Mn double hydroxide shows spin-orbit doublets of Ni 2p_3/2 and Ni 2p_1/2 at the binding energies of 855.9 and 873.8 eV, respectively with a 17.9 eV energy difference and two satellite shake up peaks at the higher energy side [14]. It is attributed to Ni⁺² oxidation state of nickel in Ni-Mn double hydroxide thin film [14]. O 1s profile [Fig. 3(d)] shows that metal hydroxide bond is present in the material.

Freshly prepared nickel hydroxide, manganese hydroxide, and nickel manganese double hydroxide thin films were evaluated for electrochemical charge-storage in aqueous 1M KOH within a potential window of 0 to + 0.5 V/AgCl. Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) techniques were used in this regard. The CV curves from 5 to 100 mVs^-1 scan rates of manganese hydroxide in Fig. 4(a) show semi-rectangular shapes without obvious redox peaks, which are related to reversible redox reactions between manganese and hydroxyl (OH^-) ion shown by the following equation.

Fig. 4. 
Cyclic voltammetry and galvanostatic charge-discharge curves of (a), (d) manganese hydroxide, (b), (e) nickel hydroxide and (c), (f) nickel-manganese double hydroxide thin films measured in 1 M KOH.

However, strong reduction and oxidation peaks are seen in the CV curves of nickel hydroxide [Fig. 4(b)] or Ni-Mn double hydroxide [Fig. 4(c)], which is related to reversible electron exchange between nickel and hydroxyl ion (OH-), is represented by the following reaction [8].

The current density peak and integrated area under the CV curve of Ni-Mn double hydroxide thin film is significantly higher compared with that of the current density of corresponding CV curve of manganese hydroxide and nickel hydroxide thin films. This signifies that the charge-storage capacity of Ni-Mn double hydroxide thin film is higher. Furthermore, shapes of CV curves of each electrodes are consistent without considerable change in their pattern, which indicate suitable capacitive characteristics of thin films. GCD study was performed for evaluating the specific capacitance of each thin film in 1 M KOH. GCD profiles of manganese hydroxide [see Fig. 4(d)] and nickel hydroxide [Fig. 4(e)] at different current densities show less charging-discharging times than that of corresponding GCD profiles of the Ni-Mn double hydroxide thin film [Fig. 4(f)], which is consistent with the CV study. The GCD profiles show non-linear behaviour, which is related with reversible redox reaction processes of thin film during charging/discharging cycle as shown by equation (1) and (2). Furthermore, the initial potential drop of the discharge profile of manganese hydroxide is slightly higher than that of nickel hydroxide and the Ni-Mn double hydroxide. It is proportional to the intrinsic resistance of the electrode, which adversely affects the charge-storage kinetics of the material during the charging and discharging process.

The specific capacitance versus current density plot [Fig. 5(a)] of the Ni-Mn double hydroxide thin film shows (obtained from discharging profiles of GCD curves) a maximum value of 1364 Fg^-1 at 3 mAcm^-2 compared with that of 815 and 120 Fg^-1 for nickel hydroxide and manganese hydroxide, respectively. The increased specific capacitance of the Ni-Mn double hydroxide is associated with the increase in redox reactions owing to the incorporation of two transition metals compared with that of single metals in nickel hydroxide and manganese hydroxide. The capacitance of Ni-Mn double hydroxide is higher than that reported by Chen et al. [15] (736.3 Fg⁻¹) for a-MnO₂, and Vidhya et al. [8] for Ni(OH)₂ (1038 Fg⁻¹). Cyclic lifetime of Ni-Mn double hydroxide thin film in 1M KOH was evaluated for successive 3000 cycles at 10 mAcm^-2 current density [See Fig. 5(b)]. It shows 94% capacitance retention after successive cycling of the thin film. The electrochemical impedance spectroscopy plots in Fig. 5(c) show higher capacitive features for the nickel hydroxide and manganese hydroxide electrodes, because their imaginary impedance is higher compared with that of manganese hydroxide.

Fig. 5. 
(a) Comparative specific capacitances of different thin films. (b) Cyclic stability and (c) comparative impedance spectra of nickel hydroxide, manganese hydroxide and nickel manganese double hydroxide.