For 15 min, Nickel foam (110 PPI, 1 mm thickness, 350 g m⁻², 1.0 × 1.5 cm⁻²) was submerged in a solution of hydrochloric acid, ethanol, and deionized water to remove any oxides or impurities from the substrate surface [32]. After drying, they were weighed, and their weights were recorded. The deposition of manganese dioxide was carried out using linear sweep voltammetry in a three-electrode system (The French BioLogic electrochemical workstation utilized nickel foam as the working electrode, saturated Ag/AgCl as the reference electrode, and Pt as the counter electrode) by using a mixture of 0.06 mol L⁻¹ of Mn(CH3COO)₂ and Na₂SO₄ as the experimental precursors, as shown in Fig. 1 below. Four sets of samples were set up and scanned for two rounds with a voltage window of 0–1 V at the rates of 40, 60, 80 and 100 mV/s, respectively. After deposition, the samples were cleaned and dried in an airbox at 60°C for 12 h. Weighing and recording the weights of the samples. The difference ∆m between the first and second weighing represents the electrodes’ active material mass, as stated in Table 1 below.

Scanning electron microscope (SEM, Zeiss ULTRA 55 SEM) and transmission electron microscope (TEM, FEI Tecnai G2 F20) were used to observe the microscopic morphology of the prepared MnO₂ films.

Chronoamperometry (CA), cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) were used to evaluate the electrochemical performance of the MnO₂ thin film electrode in a three-electrode system (Each MnO₂ electrode served as the working electrode, saturated Ag/AgCl served as the reference electrode, and Pt served as the counter electrode in the French BioLogic electrochemical workstation) at room temperature, using 1 M Na₂SO₄ solution as the electrolyte. According to the galvanostatic charge/discharge curve and cyclic voltammetry curve, obtained from the test, the following Eqs. (1) and (2) were used to compute the materials’ mass specific capacitance Cm and area specific capacitance Cs , respectively.

(1) Cm=QV⋅m=IgΔtΔV⋅m

In Eq. (1), Ig represents the mass current density for charging and discharging, Δt represents the time required for charging and discharging, m represents the mass of the MnO₂ electrode sheet, and ΔV represents the voltage change during completing charging and discharging.

(2) Cs=∫IdVsΔvV

In Eq. (2), s represents the area of the MnO₂ electrode sheet, Δv represents the scan rate, V represents the scan interval, and ∫Idv represents half of the area of the CV curve integral.

The Faradaic redox reactions of sodium ions embedded and de-embedded on the surface of MnO₂ during the charging and discharging process are shown in Eq. (3) below.

(3) MnO2+Na++e−⇌MnOONa

Scanning electron microscopy (SEM) pictures of electrodes generated at various scan rates are shown in Fig. 2. Firstly, it can be observed that the manganese dioxide films showed a loose porous structure at lower scan rates. This might be because the water in the solution was electrolyzed to produce H₂ and O₂ at a high voltage (close to 1 V), and the Mn²⁺ near the bubbles could not reach the holes produced by the electrode oxidation to MnO₂. In fact, it was observed that bubbles were generated in the electrolytic bath during the electrode deposition preparation. Secondly, the decrease in pores with increased scan rate was due to the fact that the electrode was kept for a longer time at high voltages with lower scan rates, while more bubbles were generated by the electrolysis of water in the solution. Besides, cracks on the film surface gradually increased with increasing scan rates due to uneven growth caused by the shorter growth time of the manganese dioxide nanomaterials at higher scan rates.

In electron microscopy images at high magnification, the electrode samples at high scan rates showed a densely packed nanorod morphology with overlapping associations. This structure facilitated the rapid transfer and transport of electrons and ions, resulting in an increase in the number of active sites for electrochemical reactions and an increase in the materials’ specific capacitance. Additionally, at a scan rate of 40 mV/s, the aggregation of nanorods on the MnO₂ electrode surface resulted in the formation of nanospheres which was because the longer electrode deposition time at low scan rates could lead to the growth of nanorods for a long time in a preferred orientation. However, the electrochemical redox reaction occurred mainly at the electrode surface. The active materials at the bottom of this structure were covered and cannot participate in the electrode reaction for capacitance provision. Thus, the material mass utilization was low.

To further understand the microstructure of the microstructure of the manganese dioxide electrode samples prepared, the prepared, the prepared samples were observed by using transmission electron microscopy (the selected samples were made at a scan rate of 80 mV/s for observation), as shown in Fig. 3. It can be observed from TEM Figs. 3a–3c that the manganese dioxide is presented as a densely packed nanorod morphology with overlapping associations. This non-dense three-dimensional structure has a high surface area ratio, which improves ion output from the electrolyte and the kinetics of manganese dioxide electrode reactions [33]. Fig. 3d shows the HR-TEM image of the electrode, where the lattice stripes are clearly visible with an interplanar crystal spacing of 0.24 and 0.21 nm, respectively, corresponding to the crystal faces (210) and (211) [34,35], respectively, which belong to the orthogonal crystal system.

To conduct additional research on the sample surface’s oxidation state and chemical makeup, we characterized the MnO₂-sv80 through the XPS technique. The results are shown in Fig. 4. Fig. 4a shows the Mn2p energy level diagram of the MnO₂-sv80. The two peaks with spin splitting energy difference of 12.8 eV in the diagram are typical MnO₂ characteristic peaks, corresponding to the 2p1/2 (641.7 eV) and the 2p3/2 (653.9 eV) of the MnO₂, respectively [36]. Fig. 4b presents the O1s energy level diagram of the MnO₂-sv80, with a center peak at 529.8 eV, corresponding to the Mn-O-Mn bond; the peak at 531.4 eV corresponds to the C=O bond, which may be due to CO₂ in the air.

Fig. 5a shows each electrode sample’s constant current charge/discharge curves a with varying current densities (1 A g⁻¹, 2 A g⁻¹, 5 A g⁻¹, 10 A g⁻¹, 20 A g⁻¹). To begin, it is discovered that each electrode’s charge/discharge curves resemble equilateral triangles, which indicates good electrode reversibility. Secondly, there is a voltage drop when each electrode is discharged. The reason for the IR drop is the self-induction electromotive force, which is generated during the conversion process from charging to discharging between the equivalent series resistance of the electrodes and inductive coupling. Additionally, MnO₂-sv80 has a significantly longer discharge time than the other electrodes, indicating the highest specific capacitance. The specific capacitance of each electrode with varying current densities can be computed using GCD curves and Eq. (1), as shown in Fig. 5b. The MnO₂-sv80 electrode has an overlapping-associated nanorod structure characterized by a large specific surface area, which can provide more Na⁺ embedding/de-embedding sites. As a result, the MnO₂-sv80 electrode obtained an ultra-high specific capacitance of 531.4 F g⁻¹, and it also had a high specific capacitance of 223.2 F g⁻¹ and a capacitance retention of 42% at a current density of 20 A g⁻¹. However, at a current density of 1 A g⁻¹, the specific capacitance of the MnO₂-sv40 electrode samples was only 220 F g⁻¹, which was because MnO₂ had a longer deposition time at low scan rates, resulting in an increase in film thickness, and even the selective growth of MnO₂ nanosphere structure on the surface of this electrode. However, the electrode reaction occurred mainly on the surface of the electrode sheet. The covered MnO₂ inside could not participate in the electrode reaction and then could not provide specific capacitance, which led to extremely low utilization of material mass. Moreover, all samples’ specific capacitance falls as the current density increases, owing to the active materials’ inability to complete their reaction at high current densities. We obtained the non-toxic and non-polluting MnO₂ electrode material was prepared using a simple linear sweep voltammetry procedure by electrodeposition on a three-dimensional porous nickel foam substrate. Then, the higher specific capacitance was obtained without extra doping or a secondary processing process (see Table 2 for a comparison with previous research).

Additionally, cyclic voltammetry is a critical technique for determining the electrochemical properties of materials. Fig. 5c shows at the scan rate of 5 mV/s, the cyclic voltammetry curves of those samples were prepared at different scan rates. It can be seen that all samples have a pair of obvious redox peaks around 0.7–0.9 V. The cyclic voltammetry curve of MnO₂-sv80 is approximately rectangular curve shape and has good symmetry plus, the ratio of the absolute values of the oxidation and reduction peaks is closer to 1. This indicates that this electrode material’s charging and discharging process is dynamic and reversible. Moreover, the CV curve of the sample with an 80 mV/s scan rate has the largest area, indicating that the sample has a higher area specific capacitance. As illustrated in Fig. 5d, the area specific capacitance of each electrode sample can be calculated using the CV curve and Eq. (2). At first, as the scan rate increased, the active material deposition became smaller, and the thin film electrode thickness became thinner. Thus, the MnO₂ nanomaterial could fully contact and react with the electrolyte, which improved the utilization of the active material mass. Then, as the scan rate further increased, the electrode specific capacitance decreased due to the low MnO₂ deposition despite the high mass utilization rate. Thus, as the scan rate increased, the electrode area specific capacitance increased first and then decreased due to the synergistic effect of these two factors. However, the mass of active material increases as the specific capacitance decreases. This is due to inadequate reaction and low utilization, mainly caused by surface intercalation or electric double-layer adsorption.

Chronoamperometry is an important technique used to study the electrode reaction process and the stability of electrode materials. Fig. 5e shows the CA curves of each MnO₂ electrode sample. As can be seen from the figure, the electric currents of all samples are rapidly decreasing. The electrode reaction process is mainly influenced by diffusion at the same time, thereafter, the decline is gradually slowed down until it stabilizes. Among them, the sample with a scan rate of 80 mV/s has a relatively flat decline and a high current density, which indicates its relatively sensitive electrochemical response and relatively stable electrochemical performance. This is attributed to the relatively stable interlacing-associated nanorod structure of the MnO₂-sv80 sample.

Fig. 5f shows the long-term cycling performance of each electrode. The cyclic charge/discharge characteristics were obtained through 5000 times charging and discharging in the 1 mol·L⁻¹ Na₂SO₄ electrolyte at a current density of 2 A g⁻¹. It can be observed that the specific capacitance of each electrode at the preliminary stage gradually decreases as the charging frequency increases. This is owing to the cyclic charging and discharging process, in which sodium ions are adsorbed on the surface and embedded into MnO₂ through ion channels, forming MnOONa and transforming manganese from tetravalent to trivalent. Thus, further embedding will form divalent MnOONa₂. Thus, further embedding will form divalent MnOONa₂, leading to the collapse of the manganese dioxide material structure. Also, Mn²⁺ dissolving in the electrolyte results in an irreversible capacity decay. In fact, we observed that the sodium sulfate solution turned brown during the testing phase. Additionally, the MnO₂-sv80 electrode exhibits the highest cycle retention rate, up to 85.4 percent, which is attributed to its interlaced nanorod structure with a high specific surface area, which results in increased stability while minimizing the effect of electrode volume expansion during the cyclic charging and discharging process, thereby increasing the electrode’s cycle retention rate.

To further investigate the electrode materials’ electrochemical characteristics, we employed electrochemical impedance spectroscopy (EIS) to determine their charge transfer kinetics and chemical process. The electrode’s electrochemical reaction process is depicted in Fig. 6a, and it may be classed as either Faradaic or double-layer capacitive. Additionally, the faradaic process generates charge transfer resistance (R_ct) and Warburg impedance (Z_w). Into a result, the reaction process may be abstracted as comparable circuits, as seen in Fig. 6b, where R_Ω represents the resistance between the electrode material and electrolyte, C_d represents double-layer capacitance, Z_f represents the Faradaic impedance produced for the redox reactions and Z_f = R_ct + Z_w. Fig. 6b shows the frequency variation curve of the electrical impedance modulus of the electrode. Electrical impedance is less than ten ohms in the high-frequency range and up to in the low-frequency region, which is characteristic of capacitive behavior. The magnified views in Figs. 6c and 6d show the Nyquist plot of each MnO₂ thin film electrode with its high-frequency area. At the high-frequency intercept on the real axis, the equivalent series resistance (ESR) is composed of the ionic resistance of the electrolyte, the contact resistance between the electrolyte and the electrode, and the resistance of the active material [40]. When the Warburg impedance is between the middle and high-frequency ranges, the plot of the impedance spectroscopy is a semicircle, and the intercept between the semicircle and the real axis is R_Ω, which can be analyzed in combination with the equivalent circuit. The low impedance module on the curve corresponds to the high-frequency region. The changing cycle in the high-frequency region is so short that the material transfer does not occur in time. Hence, the role of Z_w in the equivalent circuit can be ignored. And in the high-frequency region, capacitor C_d shortens the circuit between R_ct and Z_w. Thus, the impedance is only R_Ω. This situation corresponds to the intersection of this curve on the X-axis, As a result, the intercept size of the curve on the X-axis is equal to R_Ω. Additionally, the diameter of the circle represents the charge transfer resistance (R_ct) [41]. At low frequencies, the time of substance transfer is adequate, thus being dominant. The Warburg (Z_w) impedance increases with decreasing frequency, and the curve depicts a straight line with a slope near one. The smaller Warburg (Z_w) impedance of MnO₂-sv80 accelerates the ionic diffusion in the active materials and electrolytes, as well as the transfer of electrons and ions, having a positive impact on the reaction of the Faradaic processes, which can provide a larger specific capacitance [42]. Table 3 below shows values of R_ct and R_Ω which are determined by fitting the curves and the corresponding circuit. The smaller R_ct and R_Ω of MnO₂-sv80 with the smaller loaded fraction of active substance is due to the poor electronic conductivity of MnO₂, and the thinner electrode thickness and larger electrode surface area further reduce the electrode impedance.