Prepared MnO₂ with different crystal forms as electrode materials for supercapacitors: experimental research from hydrothermal crystallization process to electrochemical performances

Abstract

The aim of this study was to prepare manganese dioxide with different crystal forms through hydrothermal treatment of MnSO₄·H₂O–KMnO₄ precursors at various precursor ratios, temperatures, time periods, and pH values. The effect of the preparation parameters on the crystal structure and electrochemical performance of MnO₂ was also studied. The conditions for the preparation of each crystal of MnO₂ were experimentally optimized. The results indicate that the varied parameters affect the crystal form of MnO₂; the ratio of the precursor and reaction temperature are the significant factors affecting the crystal form of MnO₂, and the reaction time and pH value mainly affect the integrity of crystallization. Moreover, the presence of the cations K⁺ and NH₄⁺ assists the crystallization of α-MnO₂. δ-MnO₂ and amorphous MnO₂ have higher specific capacitance. The specific capacitance is determined not only by the crystal form, but also by the parameters of hydrothermal treatment.

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1 Introduction

Supercapacitors can be categorized into electric double-layer capacitors and faradaic pseudocapacitors based on the mechanism of charge storage.¹ The former store energy by forming electric double layers between the electrode and the electrolyte, whereas the latter make use of highly reversible chemical intercalation and deintercalation of electrolytic ions at the surface, or in-phase of electroactive species or redox reactions of electrolytic ions at the electrode to store electric charges.² The charging and discharging behaviours of faradaic pseudocapacitors are quite similar to those of electric double-layer capacitors; however, in the case of electric double-layer capacitors, the electrode potential is proportional to the loaded electric charges, the voltage is linear to the applied time, and the detected current is almost constant.³ It has been proven⁴ that faradaic process can not only prolong the working time, but also increase the specific capacitance of the capacitor. Due to both the interactions of the electrolytic ions with the surface and in-phase of electroactive species, the energy density stored in this process will be 10–100 times higher than that stored via only a superficial process.⁵

As electrode materials of supercapacitors, metallic oxides are believed to have higher faradaic pseudocapacitance rather than the electric double-layer capacitance formed at the surface of the carbon electrode. The faradaic pseudocapacitance can be induced not only at the interface of the electrode|electrolyte, but also inside metallic oxides via fast and reversible redox reactions; therefore, the energy is stored in a three-dimensional space, which improves the utilization rate of materials to provide a relatively high specific capacitance and energy density.⁶

Manganese dioxide (MnO₂) as an electrode material for supercapacitors is of great potential. In faradaic pseudocapacitors, the merits of the MnO₂ electrode material are as follows: high theoretical specific capacitance, wide potential window, rich reserves, environmental friendliness, etc. However, the low conductivity (10⁻⁵ to 10⁻⁶ S cm⁻¹) and unstable structure of MnO₂, leading to poor electrochemical cyclability, limit its application. To overcome these problems, many researchers have tried to design a nano-MnO₂ structure to improve the electrochemical behaviour. It has been reported that the electrochemical performance of MnO₂ depends on the surface area, morphology, and crystal structure and its features.^7,8

The mechanism of energy storage by MnO₂ materials is generally of two types. First is via surface adsorption–desorption: H⁺ or basic ions C⁺ (Li⁺, Na⁺, and K⁺) adsorb at the surface of electrode to store charges. Second is via electron–proton transfer: charges is stored via fast and reversible redox reactions, via which H⁺ or basic ions C⁺ (Li⁺, Na⁺, and K⁺) intercalate in or deintercalate out of both the surface materials and materials inside the crystal structure. Reaction equations are given as follows (eqn (1-1) and (1-2)):

(MnO₂)_surface + M⁺ + e⁻ ↔ (MnO₂·M)_surface (1-1)

MnO₂ + M⁺ + e⁻ ↔ MnOOM (1-2)

where M⁺ stands for Li⁺, Na⁺, or K⁺. During tunnel deintercalation, it has been proven by Toupin et al.⁹ via XPS analysis that there is valent variation of Mn⁴⁺/Mn³⁺. Kuo etc. et al.¹⁰ using the in situ XRD method found periodical changes in the crystal lattice of MnO₂ during the deintercalation process. Both the abovementioned reports laid the foundation for the second energy storage mechanism of MnO₂. Furthermore, Song et al.¹¹ proposed the energy storage mechanism via lattice defects. Normally, the energy storage mechanism of amorphous materials is superficial adsorption and desorption and that of crystal materials is electron–proton transfer.^12,13 Therefore, the electrochemical performance of MnO₂ materials depends on the crystal structure, surface morphology, and grain size, among which, the effect of crystal structure on the electrochemical performance of manganese dioxide and its composites cannot be neglected.^12,14

Zhang et al.¹⁵ prepared MnO₂ with different kinds of crystal structures and morphologies via redox reactions of potassium permanganate and sodium nitrite using a hydrothermal process. They also found that nano-α-MnO₂ ball with a low crystalline degree demonstrated a very high specific capacitance of 200 F g⁻¹. Rusi et al.¹⁶ also prepared α-MnO₂ with a low crystalline degree, but through electrochemical deposition in a manganese acetate tetrahydrate electrolyte. The prepared α-MnO₂ characterized by cyclic voltammetry test in the Na₂SO₄ electrolyte at a scan rate of 1 mV s⁻¹ showed a high specific capacitance of 238 F g⁻¹ and good stability; after cycling for 1900 times, the capacitance retention was 84%. Myeongjin Kim et al.¹⁷ also prepared graphene oxide (GO)/MnO₂ via one-step hydrothermal crystallization as compared to reduced graphene oxide (rGO) prepared by reducing GO with hydrazine hydrate and phosphate acid; needle-shaped MnO₂ grew on reduced graphene oxide to form H-rGO/MnO₂ and S-rGO/MnO₂. In cyclic voltammetry test, the needle-shaped MnO₂ loaded onto reduced graphene oxide forming nano-composites of H-rGO/MnO₂ shows a high specific capacitance of 327.5 F g⁻¹ in a 1 M NaSO₄ electrolyte at a scan rate of 10 mV s⁻¹. Previous researchers have achieved ideal results with beautiful nano-architectures and excellent electrochemical performance with the aim of enhancing the capacity via few specific technologies, for example, heterogeneous doping,^18–24 morphological design and control,^25–30 crystal structure design and selection,^31–33 and so on, which are indeed effective in enhancing the capacity and conductance of MnO₂. However, no systematic research on the hydrothermal crystallization process of powder MnO₂ considering all operating parameters and electrochemical performances for different crystal forms of MnO₂ has been reported to date. Herein, MnSO₄·H₂O–KMnO₄ was applied as a precursor. By varying the ratio of the precursor, reaction temperature, time, and pH value, α-MnO₂, β-MnO₂, and δ-MnO₂ were prepared. The effect of cationic dopant on the crystal form of MnO₂ was also examined. Finally, the electrochemical properties of prepared MnO₂ with different crystal forms were studied for its applications in supercapacitors.

2 Experiments

2.1 Preparation of MnO₂

All the chemicals used were of analytical grade. Non-purified polycrystalline MnO₂ was simply prepared via one-step hydrothermal synthesis. The classical synthesis procedures are as follows: different designed amounts of potassium permanganate (KMnO₄) were weighed and dissolved in 60 mL distilled water. When it was fully dissolved, the pH value of the solution was adjusted by adding 1 M H₂SO₄ or NH₃·H₂O. After the addition of MnSO₄·H₂O to the above mentioned solution, a black precipitate was formed. The solution was then stirred at room temperature for 30 min and then transferred into a 100 mL Teflon reactor, which was later placed in an incubator for hydrothermal reaction. The reactor was removed after different reaction times and then naturally cooled down to room temperature. The filtered residue was washed with distilled water till the filtrate became neutral. Finally, the solid product was frozen and dried. By changing the ratio of MnSO₄·H₂O : KMnO₄, time duration of the hydrothermal process, pH value, and amount of additive cations, powder MnO₂ with different crystal forms was obtained.

2.2 X-ray diffraction characterization of MnO₂

X-ray diffraction was used to characterize the phase and weight percentage of manganese dioxides prepared under the conditions of different MnSO₄·H₂O : KMnO₄ ratios, hydrothermal treatment time durations, pH values, and cations doped.

2.3 Assembly of electrodes

(I) Porous nickel foam was used as a current collector and cut into 2.5 × 2 cm² as a matrix of electrode sheet, which was later washed with acetone for 10 min, distilled water for 5 min, 1 M HCl for 20 min, distilled water for 5 min, and anhydrous ethanol for 5 min. After washing, the cut foam was dried in an incubator at 50 °C for eight hours and then pressed and weighed. The weight was obtained as m.

(II) Powered active species, acetylene black, and polyvinylidene fluoride (PVDF) were mixed in the ratio of 8 : 1 : 1. An appropriate amount of N-methyl pyrrolidone (NMP), as a binder, was added to sufficiently wet the powder. Through magnetic stirring and ultrasonication, the powder mixture and the binder were uniformly mixed.

(III) The abovementioned mixture was blade coated on nickel foam and then placed in a drying oven at 50 °C to remove the solvent. The active species-loaded nickel foam was then pressed into an electrode sheet, the weight of which was obtained as m₁; the mass of the active species was calculated from the difference in mass before and after they were coated on nickel foam.

(IV) The electrode sheet was placed in the prepared 1 M Na₂SO₄ electrolyte till it was completely wet; after this, the electrochemical test was carried out.

2.4 Electrochemical test

Ivium potentiostat was used for cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), electrochemical impedance spectroscopy (EIS) analyses of electrode materials. A three-electrode system was applied, in which the active species, platinum wire (Pt), and saturated calomel electrode (SCE) were used as the working electrode, counter electrode, and reference electrode, respectively. The electrolyte applied was 1 M NaSO₄. The potential window for the CV test was 0–0.8 V, and the frequency range for the EIS test was from 100 kHz to 0.01 Hz.

3 Results and discussion

3.1 Factors of hydrothermal process influencing the MnO₂ crystal forms

Fig. 1 shows the XRD patterns of both α-MnO₂ and β-MnO₂.¹⁵ On comparing the graphs of two different crystals, it was observed that the diffraction angles of characteristic peaks of both α-MnO₂ and β-MnO₂ were 12.744° and 56.586°, respectively, via ratio of which, for instance, I_12.744°/(I_12.744° + I_56.586°), the relative amount of α-MnO₂ was obtained.

Fig. 1 XRD patterns of α-MnO₂ and β-MnO₂.

Fig. 2(a) displays the XRD patterns of MnO₂ obtained via hydrothermal synthesis at different ratios of the MnSO₄·H₂O–KMnO₄ precursor. It can be observed from Fig. 2(a) that when the ratio was 3 : 2, the product was β-MnO₂; when the ratio was 1 : 1, α-MnO₂ appeared; when the ratio was 3 : 4.5, the amount of α-phase increased; with a decrease in the amount of MnSO₄·H₂O, the ratio became 3 : 9, and the peaks of the products δ-MnO₂ and trace amount of Mn₃O₄ were observed at the diffraction angles of 36.091°, 36.487°, 38.033°, and 44.425°. The presence of Mn₃O₄ with poor crystallinity might be caused by the little amount formed immediately when MnSO₄·H₂O and KMnO₄ reacted with each other, or by crystal defects.

Fig. 2 XRD patterns of MnO₂ obtained by hydrothermal synthesis (a) at different ratios, (b) different reaction times, (c) different pH values, and (d) different cations doped.

The analysis indicates that when the ratio of MnSO₄·H₂O : KMnO₄ is reduced, MnO₂ with a β-phase and mixtures of β- and α-phase and α-phase and δ-phase is obtained. There are three reasons for the formation of these products: first, K⁺ intercalates into the crystal lattice of MnO₂ to support the lattice and form a larger [2 × 2] tunnel structure. Second, K⁺ preferentially adsorbs at the surface of [MnO₆]; this decreases the surface energy of [MnO₆], leading the co-edge of [MnO₆] unit to form a [2 × 2] tunnel structure. Third, from the viewpoint of crystallization, with a constant quantity of KMnO₄ but reduced quantity of MnSO₄·H₂O, the degree of supersaturation of the crystallized product decreased, which influenced the nucleation of the crystal to form MnO₂ with different phases. The slower the reaction, the lower the supersaturation level; under this condition, α-MnO₂ is more likely to be formed. With the increasing amount of KMnO₄, when MnSO₄·H₂O : KMnO₄ reaches 3 : 9, δ-MnO₂ is obtained via the hydrothermal treatment at 180 °C for 5 h. The formed δ-MnO₂ with a larger surface area and stratified structure was shaped via the rearrangement of [MnO₆] units induced by an increase in the concentration of potassium ions.

Based on the abovementioned discussion, the summary can be drawn as follows: in the MnSO₄·H₂O–KMnO₄ system, if MnSO₄·H₂O : KMnO₄ ≥3 : 2, the product of hydrothermal crystallization is β-MnO₂. If the ratio of MnSO₄·H₂O to KMnO₄ falls between 3 : 4.5 and 1 : 1, the product is a mixture of β- and α-MnO₂. If MnSO₄·H₂O : KMnO₄ falls between 3 : 12 and 3 : 9, the product is δ-MnO₂. Furthermore, [MnO₆] unit in β-MnO₂ forms single-stranded octahedron via the common edge and extends along the crystal axis to form a [1 × 1] tunnel structure. However, the channel is very small (0.189 nm in diameter) with few crystal defects and water to support the movement of cations. However, the [MnO₆] unit in α-MnO₂ forms single- or double-stranded octahedron via the common edge and [1 × 1] and [2 × 2] tunnel structures, between which the [1 × 1] channel is hollow, but small (0.189 nm in diameter), and the [2 × 2] channel with a diameter of 0.46 nm is helpful for basic ions (such as K⁺) to intercalate and deintercalate. Interestingly, δ-MnO₂, with a special stratified structure with an interspacing of 0.7 nm, is beneficial for the free transportation of charged ions, protons, electrons, etc., which further enhances the transfer coefficient of electrons for ion exchange or redox reactions, leading to no damage of the crystal stratified structure.

Fig. 2(b) displays the XRD results of the effect of time duration of hydrothermal treatment on the crystal forms of MnO₂. It can be observed from Fig. 2(b) that the relative amounts of β-MnO₂ are 72.9%, 68.4%, and 64.6% for 5 h, 10 h, and 15 h treatment, respectively. The amount of β-MnO₂ decreases with the increasing treatment time; this indicates that the mixture formed in the first three hours has poor thermal stability and, therefore, continuously transforms into β-MnO₂ under treatment. Therefore, hydrothermal treatment of 3 h will be preferred for forming α-MnO₂ with good crystallinity. During the hydrothermal treatment, the reaction time was proven to have a significant influence on the growth of nano-materials; moreover, Yang et al.¹⁴ have reported that the growth of crystal and its morphology were controlled by a curing process. As the curing time increases, large grains continue to grow completely, but small grains gradually disappear to feed the growth of large grains due to different solubilities of large and small grains, which was driven by reduction of surface energy. Within 3–5 h, the transformation from α- to β-phase can occur due to a relatively higher K⁺ concentration induced by re-crystallization.

Fig. 2(c) shows the XRD results of crystal forms of MnO₂ prepared at different pH values. It can be observed from Fig. 2(c) that the product is a mixture of 66.7% β-MnO₂ and α-MnO₂ with good crystallinity at a pH value of 2.940 adjusted by 1 mol L⁻¹ H₂SO₄; however, the product at neutral is 43.4% β-MnO₂ with poor crystallinity, whereas the β-MnO₂ content in the product becomes 37.2% with good crystallinity at a pH value of 10.822 adjusted by 1 mol L⁻¹ NH₃·H₂O. The percentage of product under different pH conditions indicates that when the concentration of cations is higher than the concentration of H⁺ ions, the product is mainly α-MnO₂ with a [2 × 2] tunnel structure. It has been reported that by adjusting the pH value, the optimum matched degree of hydrolysis can be controlled;³⁴ this further clarifies that the pH value mainly affects the solubility of the precursor, causing the presence of [MnO₆] unit in different aggregates with various solubilities and degrees of hydrolysis. Therefore, this is consistent with the experimental result that acidic environment is helpful for the growth of β-MnO₂, but basic environment is beneficial for the formation of α-MnO₂. With the MnSO₄·H₂O : KMnO₄ ratio of 1 : 1, a temperature of 150 °C, reaction time of 5 h, and a pH value ranging from 2 to 10, the mixtures of β- and α-MnO₂ were obtained as the products of the hydrothermal process.

Fig. 2(d) illustrates the XRD results of the crystal forms of MnO₂ prepared with different amounts of cations doped. The products obtained under the conditions of a precursor ratio of 1 : 1, 180 °C, and 5 h were mixtures of β- and α-MnO₂. However, the crystal form started to transform when the cations were doped into the system. In detail, [MnO₆] octahedron with a common edge forms β-MnO₂ with a [1 × 1] single-stranded channel along the C axis, which later transforms to a double-stranded channel. The double-stranded octahedron shares a common apical angle with the single-stranded octahedron to form α-MnO₂ with a [1 × 1] + [2 × 2] tunnel structure. Besides this, K⁺, H⁺, NH₄⁺, and H₂O can support the large crystal lattice without damage, enlarge the path of cations and electrons, and enhance the transport coefficient. The main contribution to the improvement is likely to be the different surface energy of [MnO₆] octahedrons, which caused the variation of the linking forms among the octahedrons units leading to the appearance of crystal defects, finally, causing the formation of channels and vugular channels with different unit cell parameters.

3.2 Electrochemical characterizations

Using cyclic voltammetry curves to calculate the specific capacitance, the value of charges is obtained by integrating the CV curve. Based on the weight of the active species on the electrode, the specific capacitance can be calculated using the eqn (2-1):where V is the electrode potential (V), I is the responding current (A), ν is the scanning rate (mV s⁻¹), and m is the mass of the electroactive materials (g).

The calculation of the specific capacitance via GCD curves is shown in eqn (2-2):

where I is the charging–discharging current, m is the mass of the electroactive materials, Δt is the time for discharging, and ΔE is the voltage variation during charging or discharging. The electrochemical properties of various crystal forms of MnO₂ are shown in Fig. 3.

Fig. 3 (a) CV curves of α-MnO₂ under different reaction conditions; (b) CV curves of α- and β-MnO₂ under different reaction condition; (c) CV curves of polymorphous MnO₂ at 10 mV s⁻¹; and (d) GCD curves of polymorphous MnO₂ at a current density of 0.5 A g⁻¹.

Fig. 3(a) shows the cyclic voltammetry curves of α-MnO₂ prepared under various conditions at a scan rate of 10 mV s⁻¹. It can be observed that all eight α-MnO₂ samples display quasi-rectangular CV curves without obvious redox peaks between 0 and 0.5 V and 0 and 0.8 V against the SCE reference electrode; this indicates relatively good capacitive characteristics.³⁵ In particular, the α-MnO₂ sample prepared at 100 °C held for 5 h exhibits the highest absolute responding current within the commonly wide potential window from 0 to 0.8 V against the SCE reference electrode; this suggests the better capacitance characteristics under the control of potential limit of hydrogen and oxygen evolution in an aqueous electrolyte.

Fig. 3(b) and (c) show the cyclic voltammetry curves of α- and β-MnO₂ and δ-MnO₂, respectively, prepared under various conditions at a scan rate of 10 mV s⁻¹. The existence of both electric double-layer capacitance and faradaic pseudocapacitance can be realized through quasi-rectangular CV curves and minor redox reaction peaks in all the samples in both Fig. 3(b) and (c).

The quasi-rectangular CV curves indicate the presence of adsorption and desorption processes at the interface of electrode|electrolyte (electric double layers). In Fig. 3(b), in particular, the CV curve of the α + β-MnO₂ sample, prepared at a 1 : 1 ratio of precursor and a pH value of 2.940 and hydrothermally treated at 150 °C for 5 h, shows the largest closed area among all the α + β-MnO₂ samples, suggesting better electrochemical property in the potential window of 0–0.8 V.

Symmetrical redox peaks in both Fig. 3(b) and (c) indicate good reversible faradaic processes.³⁶ In Fig. 3(c), the minor redox peaks for δ-MnO₂ are induced by storage of charges through free tunnel deintercalation of electrolytic ions (hydrated Na⁺, 0.358 nm in radius) within a two-dimensional stratified structure with a 0.7 nm spacing of layers. Generally, the more sufficient the redox reactions occurring through electrons and electrolytic ions with electrode crystals, the higher the reaction peaks; the faster the reactions, the sharper the peaks in the CV curves. Therefore, the CV curves of δ-MnO₂ show a quick response (sharp peak) to current steering, as shown in Fig. 3(c), indicating a fast reaction resulting from small inner resistance; however, the CV curves of β-MnO₂ show irregular shapes with a small closed area, but better rectangular shape, which indicates good capacitive properties.

To obtain more information on how suitable the products are for electrode materials of supercapacitors, galvanostatic charge–discharge tests were performed on different crystal forms of MnO₂ in 1 M Na₂SO₄ at 0.5 A g⁻¹, as illustrated in Fig. 3(d). The plots obtained are nearly linear, indicating that the prepared MnO₂ have good pseudocapacitive characterization. The specific capacitances were 286, 111.6, 26.8, and 3.1 F g⁻¹ for δ-, α-, α + β-, and β-MnO₂, respectively. Obviously, δ-MnO₂ exhibits best specific capacitance. α-MnO₂ has a 1D structure with (2 × 2) and (1 × 1) tunnels,³⁷ whereas δ-MnO₂ has a 2D layered structure.³⁸ Protons can be intercalated into the wide (2 × 2) tunnels (∼4.6 Å) of α-MnO₂ and in between the layers (∼7 Å) of 2D δ-MnO₂. The presence of narrow (1 × 1) tunnels in β-MnO₂ makes it unsuitable for capacitor application because it cannot accommodate cations during charge–discharge cycling.³⁷

The cyclic voltammetry curves of δ-MnO₂ prepared at a 1 : 1 precursors ratio at 100 °C held for 2 h, were studied at the scan rates of 5, 10, 20, and 30 mV s⁻¹, as shown in Fig. 4(a), which displays the specific capacitances of 280, 270, 268, and 242 F g⁻¹, respectively, according to the eqn (2-1). The well-presented quasi-rectangular CV curves can also be observed. With the increase in scan rate, the shape of the CV curve deforms; this suggests a conjunct influence of both electric double-layer and pseudofaradaic capacitances.^36,39 At 30 mV s⁻¹, the capacitance retention was still 86.4%; therefore, the product had very good capacitive performance and cycling stability.

Fig. 4 (a) CV curves of δ-MnO₂ at different scan rates; (b) GCD curves of δ-MnO₂ electrodes at 0.5 A g⁻¹; (c) the Nyquist plots of the δ-MnO₂ electrodes in open potential; and (d) cycling life of the MnO₂ electrodes at 3 A g⁻¹.

The galvanostatic charge–discharge (GCD) tests of δ-MnO₂ were further considered to examine the electrochemical performance, as shown in Fig. 4(b). It can be seen from Fig. 4(b) that the GCD curves present symmetrical triangles, which suggest good charging–discharging reversibility, and little IR drops (caused by interfacial potential of different contacting phases, E = IR, determined by current passing through the inner resistance). The specific capacitances can be obtained from Fig. 4(b) via eqn (2-2), which are 571 F g⁻¹, 286 F g⁻¹, and 140 F g⁻¹ at the current densities of 0.2 A g⁻¹, 0.5 A g⁻¹, and 1 A g⁻¹, respectively. It has been reported that with an increase in current density, the storage of electrolytic ions and electrons in the crystal of MnO₂ drops. Most electrolyte ions and electrons can only approach the surface via adsorption–desorption to store energy, which have small specific capacitance.⁴⁰

Fig. 4(c) illustrates the Nyquist plot of δ-MnO₂, commonly composed of a half circle at high frequency and a straight line at low frequency. This combination indicates that the electrode process was controlled by electron transport (kinetics of electrochemical reaction) and diffusion of reactant(s) or product(s) of electrode reactions.⁴¹ At high-frequency range, the plot met the Z′ axis at R_s, which is the resistance of solution, including inner resistance of the electrode material, resistance of diffusion of electrolytic ions, and contact resistance between the active species and current collector.^42,43 The resistance of solution shown in Fig. 4(c) is 1 Ω. The radius of the half circle at high frequency indicates a small resistance of electron transfer (R_ct) in the Faraday's process and easy passage of electrons through electrodes for energy storage.^44,45 It can also be observed from Fig. 4(c) that at low frequency, a straight line presents diffusion impedance, Z_w, with the angle between 45° and 90°, illustrating pseudocapacitive reaction under diffusion control; a line with an angle of 45° indicates diffusion control and that with an angle of 90° indicates an ideal pseudocapacitive reaction.

Apart from specific capacitance, the cycling life of the material for supercapacitors is also an important aspect.⁴⁶ Fig. 4(d) displays the specific capacitance at a current density of 3 A g⁻¹ for δ-MnO₂ prepared at the precursor ratio of 3 : 9, hydrothermally treated at 180 °C for 5 h. Generally, the fading of capacitance retention resulted from the irreversible product Mn(OH)₂, which destroyed the structure of MnO₂ and resulted in a decrease in reactive sites at the surface of electrodes, leading to an increase in inner resistance. The reduced reactive sites at the surface, causing a low utilization rate of electrode materials, lead to a drop of specific capacitance. However, in Fig. 4(d), capacitance retention of 91.8% can be observed after 1000 cycling times, and the specific capacitance only dropped by 8.6% after 1000 cycling times, presenting a relatively high cycling life.

4 Conclusions

Manganese dioxide with different crystal forms was prepared by varying the ratio of the precursor MnSO₄·H₂O–KMnO₄, reaction temperature, time, and pH values. The electrochemical performances of the prepared MnO₂ were studied as well. The conclusions are as follows:

(1) In the MnSO₄·H₂O–KMnO₄ system, MnO₂ with a controlled crystal form can be obtained via hydrothermal treatment from the following parameters:

α-MnO₂; MnSO₄·H₂O–KMnO₄ = 1 : 1, T = 100 °C, t = 5 h, pH at neutral;

β-MnO₂; MnSO₄·H₂O–KMnO₄ = 3 : 2, T = 180 °C, t = 5 h, pH at neutral;

δ-MnO₂; MnSO₄·H₂O–KMnO₄ = 3 : 8, T = 100 °C, t = 3 h, pH at neutral.

(2) Cationic dopants K⁺ and NH₄⁺ are beneficial for the formation of α-MnO₂.

(3) Specific capacitances of the prepared MnO₂ are in the following order: δ-MnO₂ > amorphous MnO₂ > α-MnO₂ > α + β-MnO₂ > β-MnO₂. Therefore, the optimal parameters for hydrothermal crystallization of powder MnO₂ as a starting material for capacitor electrode plate are MnSO₄·H₂O–KMnO₄ = 3 : 8, T = 100 °C, t = 3 h, and neutral pH.

(4) Specific capacitance of δ-MnO₂ displays a specific capacitance of 571 F g⁻¹ at a current density of 0.2 A g⁻¹ and a capacitance retention of 91.4% after cycling for 1000 times.

(5) The prepared MnO₂ crystal powders from the MnSO₄·H₂O–KMnO₄ system have relatively small equivalent series resistance, which is helpful for the transport of electrolytic ions and electrons. These observations display their potential capability as electrode materials for supercapacitors; however, it is necessary to significantly enhance their conductivity using heterogeneous doping during the fabrication of electrode plates from crystal powder for their practical applications in supercapacitors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to thank the National Natural Science Foundation of China (NSFC Grant no. 21503146), the Yangtze Scholars and Innovative Research Team in Chinese University (IRT-17R81), and the Innovative Research Team of Tianjin Municipal Education Commission (TD12-5004) to support the work.

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