Introduction

As one of the most potential energy storage devices, supercapacitors are popular in relevant research field because of their fast charge and discharge rates, high power density, long cycle life, and high safety [1]. However, the development and large-scale application for supercapacitors are greatly limited by its low energy density. According to the energy density formula E = 1/2CV2, it can be seen that developing high specific capacity electrode and broadening the work potential window of supercapacitors could greatly improve energy density. Therefore, it is of great significance to develop cathode materials with higher reversible specific capacity for supercapacitors [2].

Transition metal oxides (NiO [3], MnO2 [4], Co3O4 [5]) are most widely studied and reported cathode materials for supercapacitors for the advantages of high specific capacity, good structural stability, low cost, non-toxicity, and abundant natural resources [6]. Among these transition metal oxides, Co3O4 is considered as an ideal cathode material for supercapacitors due to its high theoretical specific capacity (333.9 mAh g−1), unique nanostructure, and environmental friendliness [7]. However, single metal oxide Co3O4 as cathode material for supercapacitors usually exhibits poor electronic conductivity and cycle life as well as the lower specific capacity (far below the theoretical specific capacity), which limits its large-scale application [8]. Guan et al. [9] prepared needle-like Co3O4 for supercapacitors with the capacity of 26.2 mAh g−1 at 0.1 A g−1 and 70% capacity retention after 1000 cycles at 0.2 A g−1. In order to compensate for the shortcoming mentioned above, the conductivity of Co3O4 can be improved by combining with carbon materials [10], conductive polymers [11], and conductive metal oxide/sulfide [12, 13]. However, the introduction of carbon would greatly decrease the specific capacity of Co3O4. Some reports showed that metal elements doping can adjust the band gap and enhance the conductivity and capacity of electrode materials effectively. Zhang et al. [14] synthesized Fe-doped Co3O4 through hydrothermal followed by a post-heat treatment for supercapacitors with the capacity of 277.3 mAh g−1 and 86.9% capacity retention after 5000 cycles at 20 A g−1. Recently, our group developed Mn-doped Co3O4 mesoporous nanoneedle array via hydrothermal method combined with annealing for supercapacitors, with the specific capacity of 102.1 mAh g−1 at 1 A g−1 and long cycle stability of 104% capacity retention after 10,000 cycles [15]. However, there are little research on the preparation of heteroatom-doped Co3O4 by a more convenient and environmental method. Electrodeposition is often used to build integrate electrodes due to the strong adhesion between the active materials and current collectors [16, 17]. For instance, Huang et al. [18] prepared Al-doped CoS by electrodeposition method for supercapacitors with improved electrochemical activity.

Herein, we have firstly synthesized a novel electrode based on Mo-doped Co3O4 3D nanosheets arrays on nickel foam (NF) through one-step electrodeposition method followed by annealing in air. The calculated bandgap results of Mo-doped Co3O4 showed that its band gap could be modified and its conductivity could be enhanced by Mo doping, resulting in excellent capacity performance. The Mo0.25Co1.25/NF demonstrated a high specific capacity of 128.2 mAh g−1 (923 F g−1) at 1 A g−1 (higher than that of undoped Co3O4, 80.4 mAh g−1) and superior cycle stability with 95.2% capacity retention after 10,000 cycles at 10 A g−1. The assembled hybrid supercapacitor (HSC), Mo0.25Co1.25O/NF//KOH//N-doped carbon/carbon cloth (N-C/CC), exhibited a high specific capacity 81.5 mAh g−1 (586.8 F g−1) at 1 A g−1 and achieved a high-energy density of 64.3 Wh kg−1 at 794.1 W kg−1, as well as excellent cycle stability with 87.7% capacity retention after 10,000 cycles. The great enhancement in electrochemical performance is contributed to the effective doping of Mo element, the porous structure of MoxCo1.5−xO, and the 3D heterogenerous architecture.

Experiment

Treatment of nickel foam

Nickel foam (NF) purchased from Jiangsu Kunshan Guangjiayuan Company. The original specification of nickel foam is 250 mm × 1000 mm × 1.5 mm. Nickel foam was soaked in 3M HCl for 10 min; then rinsed with deionized water for 3 times; then soaked in acetone, ethanol, and deionized water for 15 min; and then rinsed 3 times with anhydrous alcohol and dried at room temperature.

Fabrication of MoxCo1.5−xO/NF

In the experiment, the total molar amount (1.5 mM) of the Co(NO3)2·6H2O and Na2MoO4·2H2O was unchanged. Various integrated electrodes were electrodeposited on NF under the same condition by changing the amount (0.75, 0.5, 0.25, 0.125, and 0 mM) of Na2MoO4. The electrodes were noted as MoxCo1.5−xO/NF (x = 0, 0.125, 0.25, 0.5, and 0.75).

Corresponding molar mounts of Co(NO3)2·6H2O and NaMo2O4·2H2O were dissolved in 25 mL deionized water by constant vigorous stirring (1 h). KCl (2.5 mM) was added into the above solution and stirred for extra one hour as electrolyte. The metal hydroxide precursor was electrodeposited on NF in a standard three-electrode system. The NF (1 × 1.5 cm2), platinum sheet, and the Ag/AgCl were used as the work electrode, the counter electrode, and the reference electrode, respectively. Cyclic voltammetry (CV) was performed at the scan rate of 50 mV s−1 for 60 cycles within a voltage window of −1.2~0.2 V (vs Ag/AgCl). The obtained electrodes were repeatedly rinsed with deionized water and dried at 60 °C for 6 h. Finally, the as-prepared hydroxide samples were annealed at 350 °C for 2 h in air. The loading mass of MoxCo1.5−xO/NF (x = 0, 0.125, 0.25, 0.50, and 0.75) electrodes was 1.8, 1.2, 0.9, 0.7, and 0.5 mg cm−2, respectively.

Characterization

The X-ray diffraction (XRD, Bruker D8-Advance diffractometer) of MoxCo1.5−xO/NF was performed with Cu Kα radiation. The corresponding element valence and distribution of electrodes were investigated by X-ray photoelectron spectrometer (XPS, Thermo-VG Scientific ESCALAB 250X). The morphology of the materials was identified by field emission scanning electron microscopy (FE-SEM, JSM-IT500HR) with energy-dispersive X-ray (EDX) and transmission electron microscopy (TEM, JEOL JEM2100 PLUS).

Electrochemical measurements

The electrochemical tests including cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were performed on an electrochemical workstation (Wuhancorr Test) at room temperature in three-electrode or two-electrode system. The long-term cycling stability was measured with the LAND battery test system (Wuhan Rambo Testing Equipment Co. Ltd). The as-prepared MoxCo1.5−xO/NF was used as the working electrode, platinum sheet was used as the counter electrode, Hg/HgO was used as the reference electrode, and 6 M KOH solution was used as electrolyte, respectively. The hybrid supercapacitor (HSC) was assembled with Mo0.25Co1.25O/NF as cathode and N-C/CC as anode. The cellulose was chosen as separator, and 2 M KOH solution was used as electrolyte in the HSC. The specific capacity of electrodes was calculated according to Eq. (1) [6]:

$$Q = \frac{I\Delta t}{{3.6m}}$$
(1)

Here, Q (mAh g−1) is the specific capacity, I (A) is the discharge current, and \(\Delta t\) (s) and m (g) represent the discharge time and the loading mass of the active material, respectively.

The mass of the cathode and anode was determined according to the following Eq. (2) [6]:

$$\frac{{m_+ }}{{m_- }} = \frac{{Q_- }}{{Q_+ }}\frac{\Delta V}{{\Delta {V_+ }}}$$
(2)

In which, \({m_+ }\)(\({m_- }\)), \({Q_+ }\)(\({Q_- }\)), and \(\Delta {V_+ }\)(\(\Delta {V_- }\)) are the loading mass, the specific capacity, and the absolute value of the discharge potential windows of cathode (anode), respectively. The total loading mass of cathode and anode is about 2.2 mg.

The energy density (E) and power density (P) of the HSC were calculated by the following Eqs. (3) and (4) [7]:

$$E = \frac{1}{2}QV$$
(3)
$$P = 3600\frac{E}{\Delta t}$$
(4)

where Q (mAh g−1), V (V), and \(\Delta t\) (s) represent the specific capacity, potential window, and the discharge time of the HSC, respectively.

Results and discussion

Structure characterization

The synthesis process of MoxCo1.5−xO/NF is depicted in the Fig. 1, which contains a facile electrodeposition and an annealing treatment. Na2MoO4 was selected as the Mo source. As an inorganic salt, KCl can improve the conductivity of the electrolyte.

Fig. 1
figure 1

Schematic diagram of MoxCo1.5−x/NF synthesis

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The phase and structure of MoxCo1.5−xO/NF (x = 0, 0.125, 0.25, 0.5, and 0.75) were determined by the XRD. In the Fig. 2a, there are several obvious diffraction peaks at 19.0°, 31.0°, 36.9°, 38.6°, 44.9°, 55.7°, 59.5°, and 65.4°, which can be indexed to the (111), (220), (311), (222), (400), (422), (511), and (440) crystal planes of Co3O4, respectively (JCPDS card no.74-1656). The peaks at 44.21°, 51.25°, and 75.92° are the strong diffraction peaks of Ni foam. The major peak (311) of all MoxCo1.5−xO/NF (x > 0) electrodes shows a slight shift due to Mo doping. However, the diffraction peaks at 23.32°, 25.70°, and 27.33° attributed to orthorhomhic structure of MoO3 (JCPDS crad no.05-0508) are observed when the Na2MoO4 amount is more than 0.5 mM for MoxCo1.5−xO/NF (x = 0.5 and 0.75). The XRD result of Mo0.25Co1.25O/NF before annealing is presented in Fig. 2b. The peaks at 19.0°, 32.4°, 37.7°, 57.7°, 61.3°, 69.2°, and 71.1° corresponding to (001), (100), (101), (110), (111), (103), and (201) planes of Co(OH)2 (JCPDS card no.03-0913), respectively. Moreover, no diffraction peaks of molybdenum oxide could be detected in Mo0.25Co1.25O samples due to the amorphous molybdenum oxide [19].

Fig. 2
figure 2

a XRD of MoxCo1.5−xO/NF; b XRD of Mo0.25Co1.25O/NF before and after annealing

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The morphologies of all electrodes were observed by SEM. As shown in Fig. 3a–c, the abundant nanosheets of Mo0.25Co1.25O with a thickness about 50nm assemble nanosheet arrays on the top of 3D NF substrate. This structure provides vast active sites for electrochemical reactions. While for the other MoxCo1.5−xO/NF electrodes, all the electrodes are composed of nanoparticles growing on 3D NF skeletons (Fig. S1). When the content of Mo source increases from 0 to 0.25 mM, the morphology of the materials presents the nanosheet array structure consisted of numerous nanoparticles. While the content of Mo source increases to 0.5 and 0.75 mM, the nanoparticles begin to agglomerate and the nanosheet arrays are not obvious. Therefore, according to the SEM results of all MoxCo1.5−xO/NF electrodes, we can determine that the Mo0.25Co1.25O/NF electrode has a better morphology than the other electrodes. Figure 3d shows the SEM mapping of Co, O, and Mo elements in the Mo0.25Co1.25O/NF electrode. Obviously, the Co, O, and Mo elements are homogenous distributed on the whole integrated electrode, and the SEM mapping results show that the atom content for Co, O, and Mo elements is 34.0%, 59.8%, and 6.2%, respectively, which indicates that molybdenum is successfully introduced to the electrode.

Fig. 3
figure 3

a–c SEM images and d corresponding element mapping of Mo0.25Co1.25O/NF

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The morphology of Mo0.25Co1.25O/NF was further characterized by TEM. Figure 4a, b show the ultra thin Mo0.25Co1.25O nanosheets with wrinkles. Moreover, there are many nanopores in the nanosheets, which result from the release of H2O molecules during the heat treatment process at high temperature [20, 21]. From the high-resolution TEM (HRTEM) shown in Fig. 4c, the lattice fringes of Mo0.25Co1.25O/NF nanosheets are measured to be 0.249, 0.206, and 0.142 nm, which well match with the (311), (400), and (440) planes of Co3O4 (JCPDS:74-1656), respectively. The selected area electron diffraction (SAED) pattern of Mo0.25Co1.25O/NF (Fig. 4d) shows ring patterns which indicate their polycrystalline structure and each diffraction ring can also be well indexed into crystal faces of Co3O4.

Fig. 4
figure 4

a, b TEM images, c HRTEM, and d SAED pattern of Mo0.25Co1.25O/NF

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The elemental composition and valence state of Mo0.25Co1.25O/NF were investigated by XPS. The survey spectrum (Fig. 5a) shows that Mo0.25Co1.25O/NF consists of Mo, O, Co, Ni, and C elements, in which the element Ni and C derive from NF and the conductive tape used to fix samples, respectively. The Co 2p XPS spectrum (Fig. 5b) has two main peaks and their satellites peaks. The fitting peaks at 779.75 and 794.85 eV are corresponded to Co3+, while the peaks at 780.3 and 796.1 eV are related to Co2+, respectively. The result indicates that cobalt of Mo0.25Co1.25O exists in the form of Co3O4. However, these binding energy values of Co3+and Co2+ are a little different from those of pure Co3O4 reported in the literature [15, 22], which is probably due to the charge redistribution of the materials in the presence of Mo atoms.

Fig. 5
figure 5

XPS spectrum of Mo0.25Co1.25O/NF (a), XPS survey spectra of Co 2p (b), Mo 3d (c), and O 1s (d)

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As shown in Fig. 5c, the significant spectrum of Mo 3d has two main peaks located at 235.1 eV (Mo 3d3/2) and 231.8 eV (Mo 3d1/2). The fitting peaks at 235.1 and 231.8 eV correspond to Mo6+, which prove the successful introduction of Mo source [23, 24]. The fitting peaks of O 1s spectrum (Fig. 5d) at 529.5 eV (O1), 531.2 eV (O2), and 533.2 eV (O3) are related to M–O, OH−, and absorbed water in the Mo0.25Co1.25O/NF electrode, respectively [25].

Electrochemical performance

The CV curves of all MoxCo1.5−xO/NF electrodes at 5 mV s−1 with the potential window of 0~0.6 V (vs Hg/HgO) are shown in Fig. 6a. For the Mo0Co1.5O/NF electrode, a pair of low intensity redox peaks at 0.446 V and 0.247 V can be attributed to the Co2+/Co3+ and Co3+/Co4+ redox process, respectively [26]. With the addition amount of Mo increases from 0 to 0.25 mM, the potential positions of these redox peaks gradually shift negatively and positively, meaning that the redox reaction is getting easier due to the introduction of Mo. However, when the amount of Na2MoO4 increases from 0.25 to 0.75 mM, the redox peak potential begins to increase compared with Mo0.25Co1.25O/NF electrode. It indicates that the reaction of the MoxCo1.5−xO/NF electrode becomes difficult when the Na2MoO4 is added more, and the Mo0.25Co1.25O/NF electrode needs the minimum energy compared to the other electrodes.

Fig. 6
figure 6

CV curves (a) and rate performance (b) of MoxCo1.5−xO/NF electrodes, CV curves (c) and GCD curves (d) of Mo0.25Co1.25O/NF electrode, EIS curves of MoxCo1.5−xO/NF electrodes and the equivalent circuit (e), and cycling performance of Mo0.25Co1.25O/NF electrode (f)

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To exclude the effect of NF on capacity, the bared NF was also tested under the same conditions (Fig. S2a). Compared to all MoxCo1.5−xO/NF electrodes, the bare NF almost exhibits no integrated area, indicating the low capacity contribution from the NF (Fig. S2a).

The integrated area of all MoxCo1.5−xO/NF (x = 0, 0.125, 0.25, 0.5, 0.75) electrodes are calculated to be 1.488, 3.798, 4.388, 4.301, and 3.571, respectively, manifesting that the Mo0.25Co1.25O/NF electrode has the highest specific capacity. The HOMO and LUMO values of all MoxCo1.5−xO/NF electrodes were investigated based on Eqs. (5) and (6), in which \(E_{onset}^{ox}\) and \(E_{onset}^{red}\) stand for the onset oxidation peak potential and the reduction peak potential. Eox (ferrocene) is the oxidation peak (0.38 V vs Ag/AgCl) of ferrocene [27]. All potentials (vs Hg/HgO) were converted to values vs Ag/AgCl by adding 0.054 V. The HOMO values of all MoxCo1.5−xO/NF (x = 0, 0.125, 0.25, 0.5, and 0.75) were estimated to be −4.841, −4.860, −4.753, −4.853, and −4.839 eV, respectively. The LUMO values of all MoxCo1.5−xO/NF (x = 0, 0.125, 0.25, 0.5, and 0.75) were also evaluated to be −4.798, −4.829, −4.748, −4.836, and −4.820 eV, respectively. In addition, the calculated bandgaps (EgCV) of all MoxCo1.5−xO/NF (x = 0, 0.125, 0.25, 0.5, and 0.75) electrodes based on Eq. (7) are −0.043, −0.031, −0.005, −0.017, and −0.019 eV, respectively. The above results indicate that the additional electronic effect of the composites is caused when the Mo6+ is introduced into Co3O4 (Mo0Co1.5O), which leads to the decrease of the bandgaps of all MoxCo1.5−xO/NF (x > 0) electrodes. Notably, Mo0.25Co1.25O/NF exhibits the lowest bandgap due to the optimal Mo6+ ions doping.

$${\text{HOMO}} = - \left[ {E_{{\text{onset}}}^{{\text{ox}}} - {E_{\text{ox(ferrocene)}}}} \right] - 4.8$$
(5)

[27]

$${\text{LUMO}} = - \left[ {E_{{\text{onset}}}^{{\text{red}}} - {E_{\text{ox(ferrocene)}}}} \right] - 4.8$$
(6)

[27]

$$E_g^{CV} = HOMO - LUMO$$
(7)

[27]

As shown in Fig. 6b, the Mo0.25Co1.25O/NF electrode exhibits the better rate performance (83.2%) than other MoxCo1.5−xO/NF electrodes (83.0%, 81.9%, 80.6%, and 55.3%) for x = 0.75, 0.5, 0.125, and 0, respectively.

The Mo0.25Co1.25O/NF electrode exhibits a pair of peaks at 0.355 V and 0.224 V related to the faradaic reaction of Co3O4 in KOH electrolyte at 5 mV s−1 (Fig.6c). Moreover, even at a high scan rate of 50 mV s−1, the CV curves of Mo0.25Co1.25O/NF electrode still have obvious redox peaks and keep the similar shape to that at 5 mV s−1. The Mo source only improved the stability of the MoxCo1.5-xO/NF (x > 0) electrodes, but did not participate in the redox reaction related to OH−. The corresponding redox reactions are presented in the Eqs. (8) and (9) [12, 30].

$${\mathrm{Co}}_3{\mathrm O}_4+\mathrm{OH}^-+{\mathrm H}_2\mathrm O\leftrightarrow3\mathrm{CoOOH}\,+\,\mathrm e^-$$
(8)
$$\mathrm{CoOOH}\,+\,\mathrm{OH}^-\leftrightarrow\,{\mathrm{CoO}}_2+\,{\mathrm H}_2\mathrm O\,+\,\mathrm e^-$$
(9)

The charge time and discharge time of Mo0.25Co1.25O/NF electrode at the same current density are almost equal (Fig. 6d), which indicates that the Mo0.25Co1.25O/NF has better faradaic reaction reversibility and rapid I-V response. The calculated specific capacity of Mo0.25Co1.25O/NF electrode is 128.2 mAh g−1 (923 F g−1) at 1 A g−1. The cycling performance of all MoxCo1.5−xO/NF electrodes at 10 A g−1 for 1000 cycles is also investigated (Fig. S2b). The capacity retention of MoxCo1.5−xO/NF (x = 0, 0.125, 0.25, 0.5, 0.75) is 61.7%, 89.4%, 100.6%, 84.2%, and 80.2% at 10 A g−1 after 1000 cycles, respectively. Obviously, the cycle stability of MoxCo1.5−xO/NF electrodes is significantly improved when the Mo is added into Co3O4/NF [19, 31].

Figure 6e demonstrates that each MoxCo1.5−xO/NF electrode presents a small charge transfer resistance (Rct) corresponding to semicircular arc in the high-frequency region and diffusion resistance relate to the slope of line in the low-frequency region [28, 29]. The EIS curves of all MoxCo1.5−xO/NF electrodes were fitted by using ZView software. The results reveal that the Rct values of Mo0Co1.5O/NF, Mo0.125Co1.375O/NF, Mo0.25Co1.25O/NF, Mo0.5Co1O/NF, and Mo0.75Co0.75O/NF electrodes are 4.196, 1.996, 0.618, 1.615, and 2.518 Ω, respectively (Table S1). Obviously, the Mo0.25Co1.25O/NF electrode exhibits the minimum Rct and diffusion resistance among all MoxCo1.5−xO/NF electrodes. This result further verifies that the appropriate doping of Mo source in Co3O4 greatly changes its electronic state and accelerates the electron transfer rate. Therefore, Mo0.25Co1.25O/NF exhibits the smallest bandgap and the best electrochemical performance.

The cycling stability of as-prepared Mo0.25Co1.25O/NF electrode was measured by GCD for 10,000 cycles at 10 A g−1 (Fig. 6f). Initially, the capacity of Mo0.25Co1.25O/NF gradually increases due to the activation of electrode. The capacity retention starts to decrease when it reaches 3500 cycles, which is caused by the microstructure change of the electrode. The capacity retention of the Mo0.25Co1.25O/NF electrode is 95.2% after 10,000 cycles. The Mo0.25Co1.25O/NF electrode has a larger Rct and diffusion resistance after 10,000 cycles (Fig. S3), which is probably caused by the slight change in microstructure of the active materials during 10,000 cycles. Therefore, as perior electrochemical material, the Mo0.25Co1.25O/NF in this work is more competitive than the most reported electrode materials listed in Table 1.

Table 1 Brief capacity and cycling performance list of cobalt- and molybdenum-based materials for SCs in literature
Full size table

Figure S4 shows the SEM images of Mo0.25Co1.25O/NF electrode after 10,000 cycles. The microstructure of the Mo0.25Co1.25O/NF electrode remains stable without obvious change even after 10,000 cycles. The SEM mapping results show that the atom content for Co, O, and Mo elements is 33.2%, 60.7%, and 6.1%, respectively, which is consistent with the original result.

It is believed that the following aspects may be accounted for the high electrochemical performance and excellent cycling stability of Mo0.25Co1.25O/NF electrode for supercapacitors. Firstly, the binder-free feature and Mo source doping can enhance the conductivity of as-prepared electrode, which facilitates the fast charge transportation. Secondly, the nanosheetarrays provide numerous active sites for redox reactions. Thirdly, the interconnect nanosheet arrays on NF provide a stable structure and shorten the diffusion path of ions and electrons. Finally, Co element is responsible for providing high capacity and Mo element is responsible for improving the stability of the electrode [31, 32].

Hybrid supercapacitor device

The hybrid supercapacitor (HSC) was assembled by employing Mo0.25Co1.25O/NF as the cathode and N-C/CC as the anode (noted as Mo0.25Co1.25O/NF/KOH//N-C/CC), and the schematic diagram is shown in Fig. 7a. The electrochemical performance of N-C/CC is showen in Fig. S5. Their CV curves of HSC at 5 mV s−1 in a three-electrode system are presented in Fig. S6a. Different potential was also applied to determine the optimized operating voltage of the HSC, as shown in Fig. S6b. The shape of CV curves (Fig. 7b) shows quasi-rectangular, which indicates that the capacity contribution of HSC mainly comes from the capacitive behaviour. With the current density increasing from 1 to 20 A g−1, there are no obvious voltage platform in GCD curves (Fig. 7c). As shown in Fig. 7d, the calculated specific capacity of HSC based on Eq. (1) is 81.5, 73.1, 62.7, 53.3, 46.1, and 42.5 mAh g−1 at the current density of 1, 2, 5, 10, 15, and 20 A g−1, respectively.

Fig. 7
figure 7

a Schematic of the HSC (Mo0.25Co1.25O/NF//N-C/CC), b CV curves, c GCD curves, d rate performance, e cycling performance and insert is the light display, and f Ragone plots of the HSC

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The cycling performance of the HSC is shown in Fig. 7e. The capacity retention of the HSC is 87.7% after 10,000 cycles. Meanwhile, two HSCs connected in series can light up a white LED for 3.5 min. The result proves that the assembled HSC has a superior electrochemical performance than the most reported supercapacitors assembled with Co/Mo-based composite materials [7, 8, 19, 33].

Figure 7 f is the relationship between energy and power density of the HSC. The HSC achieved high-energy density of 64.3 Wh kg−1 at 794.1 W kg−1 and high power density of 13.79 kW kg−1 at 25.2 Wh kg−1, which is comparable to the most reported cobalt and molybdenum based materials previously, such as Cu-doped Co3O4@CC//Fe2O3@CC (57.1 Wh kg−1 at 749.75 W kg−1) [7], Mo-Co-S/CC//AC (14.68 Wh kg−1 at 369 W kg−1) [19], CoMoO4@NiMoO4//AC (28.7 Wh kg−1 at 267 W kg−1) [24], CC@CoMoO4@NiCo-LDH//AC (30.7 Wh kg−1 at 16000W kg−1) [33], and CoMoO4/MnO2/NF//M-CNTF (62.9 Wh kg−1 at 984 W kg−1) [34].

Conclusions

In summary, we have successfully introduced Mo atoms into Co3O4 to construct the integrated electrode (MoxCo1.5−xO/NF) composed of porous nanosheet arrays through a facile one-step electrodeposition methods followed by an annealing treatment. The optimized Mo0.25Co1.25O/NF electrode demonstrates a high specific capacity of 128.2 mAh g−1 (923 F g−1) at 1 A g−1 and 95.2% capacity retention after 10,000 cycles. The assembled Mo0.25Co1.25O/NF//KOH//N-C/CC HSC device delivers a high-energy density (64.3 Wh kg−1 at 794.1 W kg−1) and the capacity retention of 87.7% after 10,000 cycles. In addition, two series of HSCs light up a white LED for 3.5 min. The improvement of electrochemical performance is mainly attributed to the following aspects. Firstly, the band gap and conductivity of Co3O4 can be adjusted by Mo doping; secondly, the 3D hierarchical structucture has many interconnected nanosheets of MoxCo1.5−xO and porous nanosheets can provide large specific surface area, more active sites, and shorten ion diffusion paths. This work proposes a facile, low cost, and environment friendly strategy of electrodeposion for preparing high-performance electrodes in energy storage systems.

Original statement

  1. 1.

    Mo-doped Co3O4 integrated electrodes were prepared by electrodeposition and annealing.

  2. 2.

    The effect of Mo doping on the bandgap of Co3O4 and electrochemical properties was studied.