1 Introduction

Electrochemical capacitors (ECs) are also known as ultracapacitors, electrical double layer capacitors, or supercapacitors. ECs as an effective way of the energy storage have attracted considerable attention due to the properties of high power density, excellent cycle stability, and rapid charging–discharging rate [1–3]. Various materials have been investigated as the electrode materials in supercapacitors. To date, the most widely used active materials for ECs electrodes include three types, i.e. carbon materials, conducting polymers and transition metal oxides [4–9]. Metal oxides have the advantage of better cycling stability than polymer materials and own higher energy density than conventional carbon materials, but they possess a pivotal inferior of poor conductivity. Among these materials, manganese oxide (MnO2) has attracted significant interest due to its high theoretical specific capacitance, low-cost, abundance and environmentally friendly property [10, 11]. However, the poor conductivity, slow ion transport rate and short cycle limit the specific capacitance of MnO2 [12, 13]. Therefore, further efforts have been devoted to maximize their capabilities and electrochemical performance by incorporating nanoscale MnO2 electrode materials with highly porous and electrically conductive materials to form hybrid or composite nanostructures [5, 14].

In order to obtain MnO2 with the desired morphology and high accessible surface sites to achieve highest theoretical capacitance, extensive efforts have been dedicated to adjust synthesis methods, including hydrothermal synthesis [15], sol–gel process [16], electrodeposition method [17], template method [18], and electrophoretic co-deposition [19]. According to literatures, MnO2 has several morphologies, such as nanowires, nanorods, nanosheets, and hierarchical structures [12, 20, 21]. Among these morphologies, hierarchical materials could facilitate applications in ECs since they can provide large surface areas for reaction, interfacial transport and also shorten diffusion paths [22, 23]. For instance, Zhang et al. introduced NiCo2O4 into MnO2 nanostructured electrodes to synthesize a 3D hierarchical NiCo2O4@MnO2 hybrid which showed a high specific capacitance of 913.6 F g−1 at 0.5 A g−1 [24]. Yu et al. fabricated hierarchical core–shell-like MnO2@MnO2 nanoarchitectures on carbon fibers and obtained a maximum specific capacitance of 244.54 F g−1 at a current density of 0.5 A g−1 [25]. And, Liu et al. synthesized graphene nanoribbon–MnO2 hierarchical hybrid nanomaterials exhibiting a significantly specific capacitance of 305 F g−1 at 0.5 A g−1 [26]. Since the hierarchical pore structure may result in a better performance of supercapacitors, it is necessary to develop novel hierarchical structures directly on conductive substrates using simple, convenient, and cost-effective approaches. Morerecently, researches have been focused on the application of a hydrogen coevolution cathodic electrodeposition followed by oxidation process for the synthesis of high surface MnO2 films [12, 21, 27]. Manganese hydroxide (Mn(OH)2) can be converted to MnO2 by an in situ electrooxidation progress [21] or annealing in the presence of oxygen [12]. Significantly, the hydrothermal process has been proved to be an effective and controllable method to produce MnO2 with various hierarchical nanostructures [20, 28].

In this paper, we demonstrated the synthesis of hierarchical MnO2 nanosheets through a hydrogen coevolution cathodic electrodeposition followed by a facile hydrothermal process, which was first used to convert the Mn(OH)2 to hierarchical MnO2 nanosheets. The aims of designing such MnO2 are listed as the following: (1) the MnO2 nanosheets would relax the transport of ions because of the nanostructures. (2) The pores of the MnO2 would enable fast, reversible Faradaic reactions and provide short ion diffusion paths. (3) The hierarchical MnO2 would obviously enhance the utilization rate of MnO2 material because of large specific surface area. Electrochemical measurements show that the MnO2 nanosheets, tested as an electrode material, exhibit a higher specific capacitance (391 F g−1 at 5 mV s−1 and 335 F g−1 at 0.5 A g−1), and a long-term cycling stability (91.8% capacitance retention after 1000 consecutive cycles). Remarkably, the as-prepared hierarchical MnO2 films exhibit superior pseudocapacitive performance with excellent cycling stability and high-rate capacity during cycling, they are promising materials for high-performance ECs applications.

2 Experiment

2.1 Experimental procedure

2.1.1 Materials

MnSO4, Na2SO4, KMnO4, acetone (AR; Chuandong Chemical Co., Chongqing), Teflon-lined pressure vessel (Jiapeng Chemical Co., Chongqing), ITO (Sn-doped-In2O3, 15 Ω sq−1, 10 × 10 mm), and Pt foil (10 × 10 mm, 2 mm in thickness, 99.99%). All chemicals used in this work were of analytical reagent grade and used as received without further purification.

2.1.2 Preparation of Mn(OH)2

Mn(OH)2 films were electrodeposited without templates in the solution containing 0.1 M MnSO4 and 0.1 M Na2SO4 at a bath temperature of 25 °C. ITO with a typical area of 1 × 1 cm2 was used as cathode for the growth of Mn(OH)2 film, and Pt foil was used as a counter electrode. Before electrodeposition, ITO and Pt were treated with acetone to remove contaminants. The two electrodes were separated by a distance of 30 mm, and the electrodeposition was carried out by pulse current at a cathodic current density of 0.1 A dm−2. The parameter included pulse interval (80 ms), on time (50 ms) and cycles (1000). The as-electrodeposited films were washed thoroughly with distilled water. The cathodic electrodeposition leads to the formation of Mn(OH)2 as the following reaction [12, 29, 30]:

$$\text{2}{{\text{H}}_{\text{2}}}\text{O }+\text{ 2}{{\text{e}}^{-~}}\to \text{ }{{\text{H}}_{\text{2}}}+\text{ 2O}{{\text{H}}^{-}}$$
(1)
$$\text{M}{{\text{n}}^{\text{2}+}}+\text{ 2O}{{\text{H}}^{-~}}\to \text{Mn}{{\left( \text{OH} \right)}_{\text{2}}}$$
(2)

The coevolution H2 bubbles act as a dynamic template for the formation and growth of porous nanostructure [21] or nanosheet arrays of Mn(OH)2 [27].

2.1.3 Synthesis of hierarchical MnO2 nanosheets

In the subsequent step, MnO2 films were obtained by hydrothermal reaction using the prepared Mn(OH)2 films and KMnO4 at moderate temperature in a Teflon-lined pressure vessel. Briefly, 0.44 g KMnO4 was dissolved in 35 mL distilled water first. Subsequently, the well-mixed aqueous solution of KMnO4 was transferred into a Teflon-lined pressure vessel with the as-deposited Mn(OH)2 films on ITO dipped into the vessel, sealed and heated at 110 °C or 160 °C for 24 h, respectively. The formation of MnO2 is as the following reaction:

$$\text{2Mn}{{\text{O}}_{\text{4}}}^{-}+\text{ 3Mn}{{\left( \text{OH} \right)}_{\text{2}}}\to \text{ 5Mn}{{\text{O}}_{\text{2}}}+\text{ 2O}{{\text{H}}^{-}}+\text{ 2}{{\text{H}}_{\text{2}}}\text{O}$$
(3)

After the reaction was finished, the pressure vessel was cooled to room temperature naturally. The MnO2 films on ITO were washed with distilled water and finally dried at 80 °C in air. In short, the synthesis of hierarchical MnO2 nanosheets was carried out by a two-step approach of a cathodic electrodeposition followed by a hydrothermal treatment. The whole fabrication process for hierarchical MnO2 nanosheets as supercapacitor electrode materials is illustrated in Fig. 1.

Fig. 1
figure 1

Schematic illustration of the fabrication processes for hierarchical MnO2 nanosheets

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2.2 Material characterization and electrochemical test

The prepared films were characterized by X-ray photoelectron spectroscopy (XPS, ESCALab250), transmission electron microscope (TEM, JEM-100CX II), and X-ray diffractometer (XRD, XRD-6000) and scanning electron microscopy (SEM, JSM-7800F) with energy dispersive X-ray spectroscopy (EDX). And, Nitrogen adsorption–desorption isotherms were measured at 77 K on Quadrasorb SI system. Specific surface area and pore volume were calculated by the Brunauer–Emmett–Teller (BET) method. Pore size distribution was estimated from desorption branches of isotherms by the Barrett, Joyner, and Halenda (BJH) method. In addition, inductively coupled plasma atomic emission spectroscopy (ICP, ICAP6300) was used to analyze the actual loading of MnO2 on the conductive substrates.

The electrochemical properties of the MnO2 films were investigated with a CHI660B electrochemical workstation (Chenhua, Shanghai) under cyclic voltammetry (CV), Galvanostatic charge–discharge (GCD) tests, and electrochemical impedance spectroscopy (EIS) measurements in a conventional three-electrode cell. The three-electrode cell configuration consisted of a Pt foil as the counter electrode, a standard calomel electrode (SCE) as the reference electrode, and the products prepared on ITO substrates as the working electrode. A solution containing 0.1 M Na2SO4 served as the electrolyte at 25 °C. CVs were recorded between 0 and 0.8 V vs. SCE at various scan rates ranging from 5 to 100 mV s−1. GCD testing was conducted between 0 and 0.8 V vs. SCE. Then EIS measurements were carried out at the frequencies from 100 kHz to 0.01 Hz.

3 Results and discussion

Figure 2 shows the XRD patterns of prepared Mn(OH)2 by cathodic electrodeposition. Excluding strong peaks from In2O3 (JCPDS card no.06-0416), the weaker diffraction peaks Mn(OH)2 (JCPDS card no. 08-0171) are observed. Since Mn(OH)2 is easily oxidized in air and affected by strong signal from In2O3, there is lack of clear peaks. The XRD patterns suggest that the sample is in poor crystalline state.

Fig. 2
figure 2

XRD patterns of prepared Mn(OH)2 by cathodic electrodeposition

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In Fig. 3a, EDX analyses revealed that the MnO2 film consisted of manganese and oxygen, and no other chemical elements were detected except elements of ITO substrate. XPS for the MnO2 film is given in Fig. 3b. The binding energy peaks of 2p3/2 and 2p1/2 are located at 642.3 eV and 653.9 eV, respectively, which can be assigned to Mn2p3/2 and Mn2p1/2 of Mn4+ in pure MnO2 [27, 31]. Figure 3c shows the XRD pattern of the MnO2 film on the substrate. There are several peaks assigned to In2O3 in ITO substrate. A careful examination of all the manganese oxide XRD patterns enabled us to find out that the chemical compound was most likely ε-MnO2 (JCPDS card no. 12-0141) which has low crystallinity and that the peaks could not be indexed. Therefore, EDX, XPS, and XRD analyses jointly indicated that the prepared MnO2 films consisted of manganese dioxide without appreciable impurity of other elements.

Fig. 3
figure 3

a EDX, b XPS of Mn2p, and c XRD of obtained MnO2 films at 110 °C

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Figure 4a shows the morphology of the as-electrodeposited Mn(OH)2 without hydrothermal oxidation. It can be seen that as-electrodeposited Mn(OH)2 has free-standing and homogeneous nanosheet array structure with highly uniform surface, which are consistent with the previous reports [27]. In the homogeneous nanosheet arrays, the nanosheet thickness varied from 20 to 50 nm. Figure 4b shows SEM image of the synthesized MnO2 film after hydrothermal oxidation at 110 °C. This MnO2 is presented as thick MnO2 sheets with an average thickness of 80 nm, which consisted of developing nanosheets on its surface. Compared with Mn(OH)2 nanosheets (Fig. 4a), the hierarchical MnO2 sheets also freely stand on ITO substrate, but the thickness of MnO2 nanosheets increased and the surface changed. As showed in Fig. 4c, compared with Fig. 4b, the MnO2 possesses a cluster structure, possibly resulting from the self-decomposition of KMnO4 at 160 °C. The hydrothermal reaction of the decomposition of KMnO4 can be expressed as [32]:

Fig. 4
figure 4

SEM images of (a) the as-electrodeposited Mn(OH)2, the obtained MnO2 at (b) 110 °C, (c) 160 °C, and (d) TEM image of the obtained MnO2 after hydrothermal oxidation at 110 °C

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$$\text{4Mn}{{\text{O}}_{\text{4}}}^{-}+\text{ 2}{{\text{H}}_{\text{2}}}\text{O}\to \text{4Mn}{{\text{O}}_{\text{2}}}+\text{ 4O}{{\text{H}}^{-}}+\text{ 3}{{\text{O}}_{\text{2}}}$$
(4)

Clearly, there is no hierarchical structure compared with the SEM image of obtained MnO2 at 110 °C. The hierarchical MnO2 sheets were also observed by TEM image at 110 °C in Fig. 4d. TEM observation further reveals that the uniform MnO2 sheets are assembled by MnO2 nanosheets with a thickness of 2–4 nm. The desirable porous structure on MnO2 sheets surface was obtained due to the random self-assembling and the unparallel nanosheets [33]. The unique hierarchical MnO2 nanosheets are different from that of hollow sea-urchin shaped MnO2 hierarchical nanostructures which were prepared via the hydrothermal reduction of KMnO4 [28, 34]. The nanopores could afford high specific surface area for electrochemical energy storage and decrease the electrolyte ion transportation path. Consequently the hierarchical porous structure is in favor of the enhancement of supercapacitors performance [35].

The mass of MnO2 was obtained by ICP [36]. Figure 5a illustrates a comparison of CV curves of the MnO2 nanosheets at 110 °C and 160 °C at a fixed scan rate of 20 mV s−1. The shape of CV curve at 110 °C presents typical pseudocapacitive behavior. The area of the CV curves is directly proportional to the charge, and is finally proportional to the capacitance [37]. EIS analysis is employed to further gain a deep insight into the electrochemical properties of hierarchical MnO2 nanosheet electrodes. Figure 5b compares Nyquist impedance spectra for the MnO2 prepared at different temperatures. It can be found that the Nyquist plots consist of semicircle arcs at high frequency and straight lines at low frequency. It is widely acknowledged that the semicircle presents the electrochemical reaction impedance of the film electrode and the straight line displays the diffusion of the electroactive species [38]. A bigger semicircle means larger charge-transfer resistance, and higher slope indicates higher diffusion rate. The inset of the equivalent circuit fitting the impendence curve of the MnO2 is displayed in Fig. 5b, where the R s is the bulk electrolyte resistance, R ct is Faradic charge-transfer resistance, R w is the ion diffusion resistance in electrolyte, and C F is the electrochemical capacitance. The comparison chart illustrates that the MnO2 at 110 °C has a smaller R ct than 160 °C and the MnO2 at 110 °C has a straight slopping line, meaning that it has a smaller R w, leading to a better electrochemical character than the MnO2 at 160 °C. Clearly, we could draw a conclusion that MnO2 nanosheets at 110 °C have superior specific capacitance value. Therefore, the temperature of 110 °C is the suitable temperature to synthesize the MnO2 nanosheets.

Fig. 5
figure 5

a Cyclic voltammograms at 110 °C and 160 °C and b Nyquist plots for MnO2 electrodes at the frequencies from 100 kHz to 0.01 Hz at 110 °C and 160 °C. The inset shows the equivalent circuit

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Figure 6 presents the Nitrogen adsorption–desorption isotherms and pore size distribution for the MnO2 nanosheets hydrothermally oxidated at 110 °C. BET surface area measurement reveals that the MnO2 nanosheets have a BET surface area of 53.031 m2 g−1, much larger than the previous reports [20, 34]. The pore size distribution, calculated from the adsorption branch of the nitrogen sorption isotherm, reveals that the average pore size is ~2.0 nm. Such unique MnO2 nanosheet arrays with high specific surface area could provide large active sites and fast charge transfers at electrode–electrolyte interface, ensuring the high electrochemical capacity for the MnO2 nanosheets electrode.

Fig. 6
figure 6

Nitrogen adsorption-–desorption isotherms (a) and the pore size distribution curve from the adsorption branch (b) of the MnO2 nanosheets hydrothermally oxidated at 110 °C

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To explore the advantages of MnO2 hydrothermally oxidated at 110 °C as electrodes for ECs, we studied their performance by preforming CV measurements, GCD tests, and EIS. Figure 7a shows the CV curves of the MnO2 electrode in 0.1 M Na2SO4 at different scan rates (5–100 mV s−1). The shape of CV curves is close to a well-defined rectangular within the potential range of 0–0.8 V, indicating the ideal electrical double-layer capacitance behavior and fast charging–discharging process characteristic. As the scan rate increases, the shape of the CV curves remains unchanged, demonstrating an excellent rate capacitive performance [39, 40]. The specific capacitance (F g−1) was calculated from CV curve at different scan rates using the following equation [41]:

Fig. 7
figure 7

a CV curves of the hierarchical MnO2 nanosheets at different scan rates, b GCD curves of the hierarchical MnO2 nanosheets at different current densities, c cycle performance of the hierarchical MnO2 nanosheets, and d the specific capacitance of MnO2 at different scan rates and different current densities

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$${{C}_{\text{s}}}=\frac{\int{I\,\text{d}v}}{m\cdot v\cdot \Delta V},$$
(5)

where I is the response current (A), m is the mass of the MnO2 (g), ν is the scan rate of CV curves (V s−1), and △V is the potential window width (V). The specific capacitance of the MnO2 nanosheets arrays is calculated at different scan rates and different current densities, as showed in Fig. 7d. The specific capacitance of the MnO2 nanosheets arrays is calculated to be 391 F g−1 at 5 mV s−1 and 335 F g−1 at 0.5 A g−1. The higher specific capacitance results from the highly porous structure which facilitates ion transfer into the MnO2 electrode [42]. We can see the specific capacitance gradually decreases with increase of the scan rates and the current densities. The decrease in capacitance suggests that parts of the surface of the electrode are inaccessible at high scan rates. In that case, the Na+ ion will have difficulty in permeating into the bulk of the electrode at high scan rates, causing the reduced diffusion time. Furthermore, the material in the deep pore has little impact on the pseudocapacitance [43–45].

GCD curves of MnO2 were further measured as shown in Fig. 7b. A high linear and symmetric nature is observed, illustrating a better electrochemical capacitive characteristic and superior reversible redox reaction between alkali cations (Na+) and the MnO2 nanosheets [33]. The specific capacitance is calculated according to the following equation [46]:

$${{C}_{\text{s}}}=\frac{I\cdot \Delta t}{m\cdot \Delta V}.$$
(6)

where I is the discharge current (A), △t is the discharge time (s), m is the mass of MnO2 (g), and △V is the potential drop (V) during the discharging. The specific capacitances are calculated from the galvanostatic discharge processes at current densities of 0.5, 1, 2, and 5 A g−1 are 335, 261, 229, and 202 F g−1, respectively; and the C s retention ratio is 77.2% when the current density is increased 10 times to 5 A g−1. The noticeable improvement of the specific capacitance of MnO2 film is attributed to the unique hierarchical porous architecture which could shorten diffusion paths for electrolyte ions, significantly enhancing the intercalation of ions and the utilization rate of electrode material. The hierarchical MnO2 nanosheets can provide larger surface area for more available electroactive sites. Moreover, the nanopores are able to absorb the electrolyte and shorten the diffusion paths, assuring efficient contact between active materials and electrolytes [35]. However, the capacity decreases with the increase in applied current density, which was caused by the combination of large voltage (IR) drop and distributed resistance of the pores [19, 47].

Excellent cycling stability is vital for EC operations. Herein, the long-term cycle stability of MnO2 nanosheets electrode is evaluated by repeating the charging–discharging test at a current density of 5 A g−1 for 1000 cycles, as presented in Fig. 7c. After 1000 consecutive cycles, the discharge capacitance maintains at around 91.8% of the initial value, showing a higher electrochemical cycling stability. Table 1 shows the electrochemical properties of the as-synthesized hierarchical MnO2 nanosheets and the recent reported data in the literature for comparison purposes. It can be seen that the higher capacitance and improved cycling stability are attributed to the hierarchically porous structure, which could hinder the corruption of MnO2 more validly [48].

Table 1 Comparison of the electrochemical properties between the as-synthesized hierarchical MnO2 nanosheets and the recent reported data in the literature
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To get more information of the synthesized MnO2 films, Fig. 8 is presented to compare the MnO2 after electrochemical oxidation (EO) (Fig. 8a) and hydrothermal oxidation (HO) at 110 °C (Fig. 8b). Figure 8a has an inhomogeneous and unordered MnO2 film, which can result in high ion transport resistance and decrease the specific capacitance. As expected, the MnO2 nanosheets after HO are uniformly deposited and homogeneously aligned on the ITO, which has a highly open and porous structure in contrast to the MnO2 after EO. As showed in Fig. 8b, the forming porous structure can endow the fast diffusion of electrolyte ions into pores and reduce the transport length of ions and ion transport resistance, leading to a better rate performance of supercapacitors [52]. As shown in Fig. 8c, d, the MnO2 after HO has a higher specific capacitance and capacitance retention. Obviously, the MnO2 after HO has a superior cycling stability and can receive higher specific capacitance.

Fig. 8
figure 8

SEM images of MnO2 after (a) electrochemical oxidation (EO) and (b) hydrothermal oxidation (HO). Electrochemical properties of the MnO2 after EO and HO: (c) CV curves and (d) the life cycle stability at 0.1 V s−1

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4 Conclusions

In summary, the hierarchical MnO2 nanosheets were synthesized through a hydrogen coevolution cathodic electrodeposition followed by a hydrothermal oxidation process. These nanosheets provide a large electrochemically active surface area for fast and reversible Faradic reaction and exhibit a specific capacitance (391 F g−1 at 5 mV s−1 and 335 F g−1 at 0.5 A g−1). Moreover, the hierarchical MnO2 nanosheets retain 91.8% of its initial capacitance even after 1000 cycles of galvanostatic charge–discharge process. The high-performance MnO2 nanosheets can be attributed to the highly accessible specific surface area and reduced diffusion length of ions. Such results not only demonstrated the hierarchical MnO2 nanosheets are promising materials for supercapacitor devices, but also presented a hybrid approach for fabricating nanostructure.