To electrodeposit the mesoporous manganese oxide, an electrolyte composed of 6 mM sodium dodecyl sulfate (SDS), 6 mM ethylene glycol (EG) and 10 mM manganese nitrate (Mn(NO₃)₂) in 100 mL of deionized (DI) water was prepared. Then, the mixture was vigorously stirred at 40°C for 30 min. All electrodeposition of manganese hydroxide films was performed at 70°C on indium tin oxide (ITO)-coated glass under constant potential (−1.6 V vs. Ag/AgCl/KCl (3 M)) in a standard three-electrode system with a Pt mesh and Ag/AgCl/KCl (3 M) as counter electrode and reference electrode, respectively. After deposition, the working electrodes were washed with ethanol several times and the heat treatment was performed at 200°C for 3 h. Finally, to remove the remaining surfactant, the synthesized electrodes were immersed in ethanol for 18 h. The synthesized sample was denoted as Mn₂O₃-SDS+EG. For comparison, manganese oxide films were also deposited without EG and SDS or with only SDS. The films were denoted as pristine-Mn₂O₃ and Mn₂O₃-SDS, respectively.

The porous structure was characterized using small-angle X-ray diffraction (SAXRD; RIGAKU, D/MAX 2200V/PC, Cu Kα (1.54 Å) X-ray irradiation source) and transmission electron microscopy (TEM; Philips, CM200). The oxidation states of manganese oxide were obtained via X-ray photoelectron spectroscopy (XPS; Thermo scientific, KAlpha).

Electrochemical properties such as the Tafel plot and cyclic voltammetry (CV) were measured using a three-electrode system with a Pt mesh and Ag/AgCl/ KCl (3 M) as the counter electrode and reference electrode, respectively. The Tafel plots was measured over a potential range of −1.2 V to −1.8 V vs. Ag/AgCl with a scan rate of 10 mV s⁻¹ to study the influence of the surfactants and additives on electro-deposition. CV for the measurement of capacitance was performed in a 0.5 M Na₂SO₄ solution for a sweep between 0 and 0.8 V vs. Ag/AgCl.

The mesoporous manganese oxide film was prepared by following two steps. In the first step, manganese hydroxide was cathodically electrodeposited using an aqueous solution with nitrate ions according to following reactions [34]:

Finally, the manganese oxide was synthesized on the ITO glass by heat treatment at 200°C for 3 h.

For the synthesis of manganese oxide, tree kinds of electrolytes were used; 1) without surfactant (pristine-Mn₂O₃), 2) with only SDS (Mn₂O₃-SDS), and 3) with a mixture of SDS and EG (Mn₂O₃-SDS+EG). To investigate the influence of the surfactants on electrodeposition, the Tafel slopes for three different electrolytes were investigated in the potential range from −1.2 to −1.8 V (vs. Ag/AgCl/KCl (3 M)). As shown in Fig. 1, a similar plateau was observed for the three electrolytes, and the Tafel slopes of all electrolytes were calculated to be about 145 mV dec⁻¹. Therefore, it could be concluded that the electrodeposition reaction was identical in all of the electrolytes, and there was no serious side reaction with the addition of surfactants.

The oxidation states of the synthesized manganese oxide films after calcination at 200°C were evaluated by XPS. The binding energies obtained from XPS analysis were aligned by referencing the C 1s peak to 284.8 eV. As shown in Fig. 2, the Mn 3s spectra for all samples exhibited a spin energy separation of 5.6 eV regardless of the electrolyte. The synthesized manganese oxides after heat treatment at 200°C were confirmed to be Mn₂O₃, which was consistent with the XPS spectra reported by H. Xia et al. [35]. Based on the Tafel slope and XPS results, it was confirmed that the surfactant (SDS) and additive (EG) used in the synthesis of the mesoporous structure did not affect the formation of the manganese oxide films.

Small-angle X-ray diffraction (SAXRD) was used as a general tool to confirm the existence of an ordered mesoporous structure. Fig. 3 shows the SAXRD spectra of the manganese oxide films synthesized from three different electrolytes. The measurements were carried out between 2° and 10° at a scan rate of 0.3° min⁻¹. The inter-pore distance of the deposited films was calculated based on the following Bragg’s law:

where n is a positive integer, λ is the wavelength of the X-ray (Cu Kα = 1.5406 Å), d is the inter-pore distance (in other words, the center-to-center distance between neighboring pores), and θ is Bragg’s angle. In the case of pristine-Mn₂O₃ which was synthesized without surfactant, no SAXRD peaks were observed, which indicated the absence of an ordered mesoporous structure. On the other hand, Mn₂O₃-SDS showed peaks at 2.64°, 3.24°, 4.76°, and 6°, corresponding to d-spacings of 3.34 nm, 2.72 nm, 1.85 nm, and 1.47 nm, respectively. Similarly, Mn₂O₃-SDS+EG films exhibited several peaks at 2.92°, 3.62°, 5.06°, and 6.5°, which were assignable to 3.02 nm, 2.44 nm, 1.74 nm, and 1.36 nm, respectively. These diffraction patterns demonstrated that the electrodeposited Mn₂O₃ displayed a highly ordered mesoporous structure, expecting enhanced electrochemical performance for a supercapacitor.

TEM images of the deposited manganese oxide films prepared with surfactants are shown in Fig. 4. Compared with featureless and dense structure of manganese oxide fabricated without surfactant (Fig. 4a), a highly ordered mesoporous structure was clearly observed in the manganese oxide films electrodeposited with surfactants as a templating agent. This ordered mesoporous structure could be very helpful in increasing the accessible electrochemical surface area. The inter-pore distance was estimated to be 2.5–3.5 nm from the TEM images in Fig. 4, which was consistent with the small-angle XRD results. The highly ordered mesoporous structures synthesized with surfactants were thus confirmed using SAXRD and TEM. It has been known that EG can modify the construction of micelle structure of surfactant, leading to formation of rod-like array. Tan et al. [36] fabricated the two-dimensional mesoporous nickel hydroxide films by electrochemical deposition in dilute surfactant solutions in the presence of EG. In the solutions, EG gradually enlarges the spherical micelle volume of SDS as an expander, resulting in modification of micelle shape from the sphere to rod-like hexagonal structure. The unique mesoporous structure derived from the stable cylinder micelles can provide a high specific surface area, pore volume and a uniform mesopore size distribution, thus expecting the enhanced specific capacitance.

To investigate the electrochemical properties of the ordered mesoporous manganese oxides, cyclic voltammetry (CV) was conducted at scan rates of 10, 40, 60, 80, 100, and 200 mV s⁻¹ in the potential range of 0–0.8 V using a 0.5 M Na₂SO₄ aqueous solution. Because all samples were electrodeposited at the identical conditions (at −1.6 V vs. Ag/AgCl/KCl (3 M) until reaching 200 mC) and the currents were almost similar as confirmed from Tafel slope measurement in Fig. 1, the deposited amounts of manganese oxide in each film was almost same. The deposited amount of manganese oxide was calculated from the weight difference between bare electrode (ITO coated glass) and manganese oxide coated electrode using a micro balance. As shown in Fig. 5, all CV plots regardless of scan rate exhibited a roughly rectangular plateau with broad quasireversible redox peaks. In cases of ordered mesoporous manganese oxides, the areas of CV were much higher than that of the non-porous sample (pristine-Mn₂O₃). which was attributed to their high electrochemical surface area. These results indicated an increase in accessible sur face sites for supercapacitive behavior in the ordered mesoporous samples relative to the non-porous sample.

Fig. 6(a) shows the CV plots of each sample at 200 mV s⁻¹. Based on the CV plots, the specific capacitance of the deposited films was calculated using the following equation:

where I is the current, dt is the scan time interval, m is the sample mass, ΔV is the potential range of each scan, and C is the specific capacitance of the synthesized electrode, respectively. The ordered mesoporous samples exhibited higher electrochemical performance than the non-porous sample, as shown in Fig. 6(a). The specific capacitances at 10 mV s⁻¹ were calculated to be 32.13 F g⁻¹, 51.25 F g⁻¹, and 72.04 F g⁻¹ for pristine-Mn₂O₃, Mn₂O₃-SDS, and Mn₂O₃-SDS+EG, respectively. These results indicate that the ordered mesoporous samples exhibited higher capacitance due to the higher electrochemically active surface area and well-developed porous structure. In addition, an ordered structure can increase the ionic mobility between the electrolyte and electrode surface by reducing the ionic diffusion path [37]. Interestingly, manganese oxide synthesized with a mixture of SDS and EG (Mn₂O₃-SDS+EG) exhibited much higher specific capacitance than manganese oxide synthesized with only SDS (Mn₂O₃-SDS) at all scan rates investigated. This result can be explained by the well-developed mesoporous structure with the addition of EG, as mentioned above. For all samples, the specific capacitance decreased with an increase of scan rate, as shown in Fig. 6(b). It is well known that a low scan rate allows sufficient electrolyte penetration into pores, leading to a more efficient contact between ions in electrolyte and the active materials. At high scan rates, in contrast, the high ion consumption rate (reaction rate) compared with the diffusion rate results in a low capacitance [38]. Among the synthesized samples, Mn₂O₃-SDS+EG exhibited the best electrochemical activity for a supercapacitor in the wide range of scan rate, which was attributed to an increase in the numerous active sites and enhanced accessibility of electrolyte caused by the unique mesoporous structure derived from cylindrical micelles.