Green preparation of low-crystalline CoMoO4·0.9H2O/carbon composite for energy storage applications using a mechanochemical method

Metal oxide/carbon composites have attracted considerable attention owing to their high charge storage capacity and excellent cycling stability. In this work, hierarchical porous carbon loaded with low-crystalline CoMoO4·0.9H2O (CMOC) was prepared via a solvent-free mechanochemical method. Due to the low-crystalline CoMoO4·0.9H2O, the porous structure, and the superior conductivity of carbon, CMOC exhibited good charge storage capacity (340 F g−1) and excellent cycling stability (~ 85% of capacitance retention for 10,000 cycles). Moreover, an asymmetric supercapacitor device (CMOC//AC ASC) was assembled using CMOC as the positive electrode and activated carbon as the negative electrode, thus achieving a high-energy density of 23.1 Wh kg−1 at a power density of 798.1 W kg−1. The mechanochemical method used in the study is simple, cost-effective, and generalizable. This study will provide valuable information for the large-scale preparation of metal oxide/carbon composites as electrode materials for supercapacitors.


Introduction
With the increasing demand for electronic devices and energy, the development of environmentally friendly energy conversion/storage devices with high power and energy density is vital and urgent [1][2][3]. Compared with lithium-ion batteries, supercapacitors have received considerable attention in recent years owing to their high-power density, long-term cycle, and low-cost advantages [4][5][6].
Supercapacitors can be divided into electric double-layer capacitors and pseudocapacitors, and they store energy through charge migration and Faradaic reactions, respectively. Carbon materials are widely used to prepare supercapacitor electrodes because of their rich variety, high specific surface area, and good stability [7,8]. However, pure carbon materials store energy through physical adsorption, thus limiting their specific capacitance and energy density. This limitation can be overcome by preparing composites of metal oxides and carbon materials because the host-guest synergistic effect of metal oxide/carbon materials can greatly increase the cycling stability and energy density of electrode materials [7,9,10].
CoMoO 4 is one of the most common materials for preparing metal oxide/carbon composites owing to its high electrical conductivity, strong electrochemical activity, low-cost, and natural abundance advantages [11][12][13][14]. For example, Xia et al. proposed a facile hydrothermal method to prepare CoMoO 4 /graphene composite (CoMoO 4 /G), and the prepared composite exhibited a high surface-to-volume ratio and large electroactive area. Compared with pure CoMoO 4 , the CoMoO 4 /G composite exhibited lower resistance, good rate capability, and higher cycling stability [15]. Xu et al. prepared reduced graphene oxide-cobalt molybdate (RGO/ CoMoO 4 ) nanocomposite assisted by microwave irradiation for the first time. The prepared nanocomposite exhibited excellent electrochemical performance, which could be attributed to the synergistic effect of the two components [16]. Li et al. synthesized monodispersed CoMoO 4 nanoclusters on ordered mesoporous carbon via the impregnation method. The composite exhibited good wettability, high specific surface area (> 700 m 2 g), and regular mesoporous channels (~ 4 nm), thus resulting in excellent electrochemical performance [17]. Chen et al. prepared a hybrid electrode material, CoMoO 4 /bamboo charcoal (BC), using a one-pot solvothermal reaction and annealing process. The electrode exhibited a specific surface area of 74.4 m 2 g −1 , and the capacitance of the hybrid material was 1.7 times higher than that of the CoMoO 4 precursor. The hybrid electrode exhibited a high specific capacitance of 422.3 F g −1 (0.5 A g −1 ) and good long-term cycling stability [18].
Numerous studies have reported the composites of CoMoO 4 /carbon; however, uniform dispersion of CoMoO 4 nanoparticles on carbon materials to avoid agglomeration remains a challenge. In addition, the preparations of these composites often involve complicated steps, harsh conditions, and waste materials. With the increasing demand for supercapacitor materials in modern society, exploring green and convenient methods to prepare electrode materials is crucial. Thus, in this work, a solvent-free mechanochemical method was used to prepare hierarchical porous carbon loaded with low-crystalline CoMoO 4 ·0.9H 2 O (CMOC).
In this work, (NH 4 ) 6 Mo 7 O 24 ·4H 2 O, Co(NO 3 ) 2 ·6H 2 O, and glucose were mixed and then ball-milled. The mixture was treated at a high temperature to obtain CMOC. The obtained CMOC exhibited a hierarchical porous structure, two different sizes of nanoparticles, and low-crystalline CoMoO 4 ·0.9H 2 O, endowing the composite with high specific capacitance, good rate capability, and excellent cycling stability. The asymmetric supercapacitor prepared with CMOC as a positive electrode, and activated carbon as a negative electrode exhibited excellent performance. This work shows that the mechanochemical method has great application prospects in the preparation of supercapacitor electrode materials.

Preparation of CoMoO 4 ·0.9H 2 O/carbon composites (CMOC)
Ammonium molybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 ·4H 2 O); cobalt nitrate hexahydrate (Co(NO 3 ) 2 ·6H 2 O), with a molar ratio of 1:7; and glucose (of a certain mass) were added into the stainless steel ball mill jar and ball-milled at 600 rpm/ min for 2 h (mass of stainless steel balls was 40 g, and total mass of reactants was 2 g). The samples obtained after ball milling were placed in a tube furnace and calcined in a nitrogen atmosphere for 2 h at a calcination temperature of 350 °C and heating rate of 5 °C min −1 . After the obtained samples were naturally cooled to room temperature, final samples were obtained and labeled as CMOC-x (x is the ratio of the total mass of (NH 4 ) 6 Mo 7 O 24 ·4H 2 O and Co(NO 3 ) 2 ·6H 2 O to the mass of glucose, x = 1, 2, 4, and 8).

Characterization
The X-ray diffraction (XRD) spectra was collected with Rigaku D/Max 2500 V/PC X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm). The data of specific surface area and pore size distribution were collected with a Micromeritics ASAP 2420 specific surface area analyzer at 77 K. The morphology and microstructure were characterized using a field emission scanning electron microscope (FESEM, JSM-6700F, JEOL) and transmission electron microscope (TEM, FEI Tecnai G2 f20s -twin D 573). The surface chemical composition information was obtained by energy-dispersive X-ray spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB250).

Results and discussion
In this study, CMOC composites were prepared via a ball milling-calcination method. In the ball milling process, (NH 4 ) 6 Mo 7 O 24 ·4H 2 O, Co(NO 3 ) 2 ·6H 2 O, and glucose crystals were evenly mixed and pulverized; CoMoO 4 ·xH 2 O and byproducts were obtained. The by-products obtained included NH 4 NO 3 and free water. These substances were uniformly wrapped in glucose. During the calcination step, the following reactions occurred: the dehydration and carbonization of glucose, the partial dehydration of CoMoO 4 ·xH 2 O, the thermal decomposition of NH 4 NO 3 , and the volatilization of adsorption water. Finally, the gas and water produced in the reactions were removed from the reaction system, and CMOC composites were prepared. The possible nucleation mechanism of CoMoO 4 ·0.9H 2 O is as follows: the presence of water in (NH 4 ) 6 Mo 7 O 24 ·4H 2 O and Co(NO 3 ) 2 ·6H 2 O led to the localized formation of a high-concentration solution of molybdate and cobalt ions after ball milling. The collision of these ions resulted in the formation of CoMoO 4 ·xH2O, which was encapsulated by glucose. During the calcination process, the carbon derived from glucose impeded the complete dehydration and Ostwald ripening, ultimately leading to the formation of nanoscale CoMoO 4 ·0.9H 2 O. The composition, morphology, and electrochemical properties of CMOC were investigated.
X-ray diffraction (XRD) was conducted to determine the phase structure of the products. As shown in Fig. 1, the broad peak of CMOC-1 that appeared between 20 and 40° could be attributed to the amorphous carbon derived from glucose.
No peaks corresponded to Co-Mo oxides. This phenomenon might be attributed to the low content of metal oxides in the composite. The XRD curves of CMOC-2 and CMOC-4 could be indexed to low-crystalline CoMoO 4 ·0.9H 2 O (ICDD, card no. 14-0086) [19,20]. CMOC-2 and CMOC-4 did not contain the common α-or β-CoMoO 4 with high crystallinity because the presence of glucose-derived carbon hindered the process of crystal growth. During the ball milling process, glucose and CoMoO 4 ·xH 2 O were properly mixed; then, the mixed sample was dehydrated to form a viscous polymer and carbonized during the calcination process, thus hindering the flow of metal salts and preventing CoMoO 4 ·0.9H 2 O from growing into large crystals. The XRD pattern of CMOC-8 (with the minimal carbon content) confirmed the above view, which showed obvious peaks at 26.5°, 27.2°, and 28.4° assigned to the (002), ( 1 12), and ( 3 11) planes of CoMoO 4 , respectively (ICDD, card no.21-0868). Therefore, glucosederived carbon mixed with the synthesized composites via the mechanical ball milling method promoted the formation of low-crystalline CoMoO 4 ·0.9H 2 O. The electrochemical capacitive properties of CMOC-2 and CMOC-4 were enhanced owing to their low crystallinity, consistent with previous study [21].
To further confirm that the prepared samples were composites of CoMoO 4 and amorphous carbon, Raman characterization was performed (Fig. 2). The Raman spectra of the samples showed visible peaks at 350, 670, 810, 877, 932, 1360, and 1580 cm −1 . Among the peaks, the broad peak at 350 cm −1 could be assigned to the Mo-O-Co bond [22]. The peaks at 670, 810, 877, and 932 cm −1 corresponded with MoO 3 [23,24]. Additionally, the peaks at 1360 and 1580 cm −1 could be attributed to the D and G bands of amorphous carbon, respectively. Moreover, the higher the mass ratio of metal oxides to amorphous carbon, the higher the ratio of the intensity of CoMoO 4 relevant peaks to that of carbon relevant peaks. Therefore, the XRD and Raman analysis results confirmed that the samples were the composites of guest CoMoO 4 nanoparticles and amorphous carbon.
To further explore the mechanism behind the binding of CoMoO 4 ·0.9H 2 O to amorphous carbon, the morphologies of the composites were examined using SEM. Figure 3 a and d show that CMOC exhibits porous structures. The structures resulted from the pore-forming agent (water vapor, nitrogen, and oxygen) generated during the calcination process of mixtures. In addition, no agglomerated CoMoO 4 ·0.9H 2 O particles were observed, indicating that the CoMoO 4 ·0.9H 2 O particles were uniformly distributed on the porous carbon. TEM was used to further measure the particle size of CoMoO 4 ·0.9H 2 O. The results showed two different particle sizes of the composite: ~ 15 nm (Fig. 3b and c) and ~ 3 nm (Fig. 3e and f). As shown in Fig. 3b, the  (Fig. 3c). The lattice fringes with a spacing of 0.31 nm were observed, which were assigned to CoMoO 4 ·0.9H 2 O. Moreover, the oxides of nanoparticles with smaller sizes were observed in the region where the carbon layers were thicker (Fig. 3f). These smaller sizes of nanoparticle oxides might be because thicker carbon layers restricted the growth of nanoparticle oxides more strongly. The TEM images of CMOC-1, CMOC-4, and CMOC-8 are shown in Figure S1. Co-Mo oxides were hardly observed in CMOC-1; however, Co-Mo oxides were agglomerated in CMOC-4 and CMOC-8. Therefore, porous carbon with uniform distribution of CoMoO 4 ·0.9H 2 O nanoparticles that showed the best capacitor performance according to electrochemical tests could only be prepared only when the ratio of the total mass of metal salts to the mass of glucose was equal to 2. In addition, it can be seen from the elemental mapping of the composite that Co, Mo, O, and C elements were uniformly distributed, indicating that CoMoO 4 ·0.9H 2 O and carbon are tightly bound.
XPS was used to characterize the elemental composition of CMOC (Fig. 4). The full spectrum of XPS showed the existence of C, Co, Mo, and O elements in the sample (Fig. 4a). Figure 4b shows the XPS high-resolution energy spectrum of C 1 s, and its peaks at binding energies were 284.6, 285.5, and 287.5 eV, which can be attributed to sp 2 hybrid carbon, sp 3 hybrid carbon, and a carbon-oxygen double bond, respectively [15]. The XPS spectra of Co 2p (Fig. 4c) showed absorption peaks at binding energies of 781.5 and 797.4 eV, corresponding to Co 2p 3/2 and Co 2p 1/2 , respectively, indicating that the presence of Co 2+ in the composite. Moreover, the satellite peaks of Co 2p 3/2 and Co 2p 1/2 can be observed at the binding energies of 786.9 and 803.1 eV, respectively [20]. The XPS characteristic peaks of Mo 3d appeared at 232.4 and 235.6 eV, corresponding to Mo 3d 3/2 and Mo 3d 5/2 , respectively (Fig. 4d), indicating that Mo element in the sample was + 6 valence [25]. The XPS characteristic peaks of O 1 s ( Figure S2) appeared at binding energies of 530.4 and 531.4 eV, corresponding to the metal-oxygen bond and adsorbed water, respectively [17].
Nitrogen adsorption-desorption tests were performed to determine the surface area and the pore diameters of CMOC ( Fig. 5a and b). The adsorption and desorption isotherms of CMOC exhibited the characteristics of type IV isotherms with a typical H4 hysteresis loop, and no saturated adsorption plateau was observed, indicating irregular pore structures. The BET method was used to calculate the specific surface areas of CMOC-1, CMOC-2, CMOC-4, and CMOC-8, and their values were 2.8, 7.4, 16.6, and 17.7 m 2 g −1 , respectively, indicating that the higher the content of glucose-derived carbon, the smaller the specific surface area of the composite. This phenomenon might be attributed to the small specific surface area of the glucose-derived carbon [26], resulting in a smaller specific surface area of the composite than that of pure metal oxides. The CMOC-4 and CMOC-8 composites had both micropores and mesopores, while CMOC-1 and CMOC-2 had only mesopores and smaller pore volumes, indicating that the glucose-derived carbon blocks the micropores and partial mesopores of the metal oxides. The above results showed that glucose-derived carbon reduced the specific surface area of metal oxide/carbon composites from two perspectives. However, the specific capacitance of the composites was higher than that of pure metal oxides. This phenomenon might be because carbon doping made the composites more conductive than pure metal oxides.
The electrochemical properties of CMOC were examined through CV, GCD, and EIS tests in an aqueous KOH (2 M) electrolyte. Figure 6a shows the CV behaviors of CMOC-1, CMOC-2, CMOC-4, and CMOC-8 at a scan rate of 100 mV s −1 and a voltage window of 0-0.5 V. All electrodes showed a pair of clear redox peaks, which corresponded to the Co 2+ /Co 3+ redox reaction. The plausible redox reactions may be inferred as follows [20]: At the same scan rate, the peak area of CMOC-2 was larger than those of other samples, thus exhibiting the highest specific capacitance in all samples. The CV curve of  Figure 6b shows the GCD curves of CMOC-1, CMOC-2, CMOC-4, and CMOC-8 at a current density of 1 A g −1 and a voltage window of 0-0.4 V. All the GCD curves showed symmetric shape, indicating that the CoMoO 4 electrode materials exhibited high Coulombic efficiency. Among them, the curve of CMOC-2 showed the longest discharging time, which was consistent with the CV results. Based on the discharging times of CMOC-1, CMOC-2, CMOC-4, and CMOC-8, the specific capacitances of the four electrodes were calculated to be 275, 340, 335, and 310 F g −1 at a current density of 1 A g −1 , respectively. Table 1 is the comparison of the specific capacitance of CMOC-2 and some of other previously reported electrode materials. Figure 6c shows the CV curves of the CMOC-2 electrode at different scan rates ranging from 10 to 100 mV s −1 . Symmetrical redox peaks were observed at all scan rates, indicating that CMOC-2 electrode material exhibited low Fig. 6 a, b CV and GCD plots of CMOC-x; c, d, e CV, GCD, and Nyquist plots and the equivalent electric circuit model of single electrode CMOC-2; f cycling performance and coulombic efficiency of single electrode CMOC-2 obtained by GCD curves at 10 A g −1 resistance and good electrochemical reversibility. As the scan rate increased, the peaks at the anode and cathode shifted to higher and lower potentials, respectively, indicating the surface-controlled electrochemical process of the CMOC-2 electrode. Figure 6d shows the GCD curves of CMOC-2 at different current densities. The specific capacitances of CMOC-2 were 340.0, 333.5, 318.0, 295.5, 274.0, and 262.5 F g −1 at the current densities of 1, 2, 4, 6, 8, and 10 A g −1 , respectively. Figure 6e shows the Nyquist plot of the CMOC-2 electrode, which has been fitted with the equivalent circuit provided in the inset of Fig. 6e. The intersection with the real axis (0.81 Ω) at high frequency represented the resistance of the electrolyte and the internal resistance of the CMOC-2 electrode. The excellent conductivity of the CMOC-2 electrode might be attributed to the superior electronic conductivity of carbon. The small semicircle in the high-frequency region indicated the fast charge transfer rate between the electrolyte and electrode surface, which might result from the high hydrophilicity and the porous structure of the electrode materials. The slope angle of the straight line at the low-frequency region was larger than 75°, indicating that the CMOC-2 exhibited excellent capacitive behavior. Figure 6f shows the cycle stability of the CMOC-2 electrode. The electrode cycle stability was initially decayed in the first 2500 cycles and then maintained a constant value (~ 85% of the initial reversible capacity) at a current density of 10 A g −1 . The decay process of capacitance could be attributed to the detachment of weakly bonded materials after cycling.
The charge stored by the electrode comes from two aspects: diffusion reaction and capacitive reaction. Diffusion reactions include bulk pseudocapacitive reactions and battery reactions, that is, redox reactions that occur when ions intercalate or deintercalate from bulk materials. Capacitive reactions include electric double layer reactions and surface pseudocapacitive reactions, that is, adsorption/desorption reactions and redox reactions on or near the surface of electrode materials. The current of the diffusion reaction is proportional to the square root of the scan rate, while the current of the capacitive reaction is linearly related to the scan rate. Therefore, the contribution of the diffusion reaction and the capacitive reaction in the process of charge storage can be judged by the relationship between the current and the scan rate.
The energy storage mechanism of CMOC-2 electrode was explored by the following equation: where i p is the peak current, v is the scan rate, and a and b are the adjustable values. The relationship between log(v) and log(i) of the CMOC-2 electrode is shown in Fig. 7a. The values of b determined by calculating the slopes of the anodic and cathodic peaks are 0.71 and 0.83, respectively, indicating that CMOC-2 electrode exhibited both battery and pseudocapacitive properties. The percentages of the capacitive and diffusion contributions were further quantified by the following equation: where v is the scan rate, k 1 and k 2 are the adjustment factors, and k 1 v and k 2 v 1/2 correspond to capacitive and diffusioncontrolled effects, respectively. The capacitive contributions are 77.6%, 81.9%, 85.5%, 87.1%, and 88.4% at the scan rates of 1, 2, 3, 4, and 5 mV s −1 , respectively (Fig. 7b). This suggested that the capacitive contribution played a dominant role in the total capacity, and a Faradaic redox reaction occurred mainly on the surfaces of the CMOC-2 nanostructures.  [28] 367, 5 A g −1 88% after 1000 cycles CoMoO 4 /C nanorod [29] 451.6, 1 A g −1 93.8% after 2500 cycles CoMoO 4 /rGO [29] 336.1, 1 A g −1 -CoMoO 4 ·0.75H 2 O/PANI composite [30] 380, 1 A g −1 90.4% after 1000 cycles CoMoO4@RGO nanocomposites [17] 856. To further evaluate the potential applications of the CMOC-2 electrode, an asymmetric supercapacitor (CMOC//AC ASC) device was assembled with the CMOC-2 electrode as the positive electrode and an activated carbon (AC) electrode as the negative electrode. The CV curves of the AC electrode display no discernible redox peaks at all examined scanning rates ( Figure S3a). Specifically, at low scanning rates, the CV curves feature symmetrical rectangles, indicating that charge storage in the AC electrode occurs via the double-layer capacitance mechanism. As the scanning rate increases, the shape of CV curves changes, signifying partially reversible electrochemical processes on the AC electrode at high scanning rates and full reversibility at low scanning rates. Furthermore, Figure S3b illustrates the GCD curves of the AC electrode. The GCD curves take on a symmetrical triangle shape at all current densities, thus corroborating the assertion that the AC electrode stores charge via double-layer capacitance. Specific capacitances of the AC electrode at current densities of 1, 2, 4, 6, 8, and 10 A g −1 were computed as 236. 3, 224.6, 213.6, 206.4, 200.8, and 195.0 F g −1 , respectively, based on the discharging time of the GCD curves. In Figure S4a, the Nyquist plot of the AC electrode reveals the presence of an intercept and a semicircle in the high-frequency region as well as an oblique line in the low-frequency region. The EIS fitting curve shows that the AC electrode has an internal resistance (Rs) and a charge transfer resistance (Rct) of 0.80 Ω and 1.12 Ω, respectively, indicating low resistance and fast charge transfer kinetics. The Nyquist plot also exhibits an oblique line in the low-frequency region, which is associated with the capacitive performance. The proximity of the oblique line of the AC electrode to the y-axis suggests an enhanced capacitive behavior, which can explain the impressive rate capability of the AC electrode. Additionally, Figure S4b shows a phase angle of 81.5° for the AC electrodes, which supports their superior capacitive behavior and corroborates the findings of the EIS analyses. Figure 8a shows the CV curves of the ASC measured at a scan rate of 100 mV s −1 . The shapes of the curves were maintained without polarization reactions in the potential window of 0-1.6 V. Therefore, the most suitable potential window was 0-1.6 V. Figure 8b depicts the CV curves of the ASC device at the scan rate from 10 to 100 mV s −1 at a potential window of 0-1.6 V. The shapes of the curves revealed the combination of pseudocapacitive behavior and EDLC properties in the ASC device. In addition, the shapes of the CV curves remained unchanged when the scan rates became higher, indicating that the ASC exhibited a high electron transport rate and excellent rate capability. Figure 8c shows the GCD curves of the ASC device at various current densities ranging from 1 to 10 A g −1 . At the current densities of 1, 2, 4, 6, 8, and 10 A g −1 , the specific capacitances of the ASC calculated from the discharging time were 65.1, 47.4, 37.2, 35.9, 27.6, and 24.4 F g −1 , respectively. Figure 8d shows the cycle performance of the CMOC//AC ASC device measured at a current density of 10 A g −1 . After 10,000 cycles, the specific capacitance of the ASC device retained 87.3% of its initial capacitance. The decay process of capacitance could be attributed to the detachment of weakly bonded materials after cycling. The retained specific capacitance confirmed that the device exhibited good stability and high performance during long charging-discharging cycles. In addition, the CMOC//AC ASC exhibited energy densities of 23.1, 16.9, 13.2, 12.8, 9.8, and 8.7 Wh kg −1 at the power densities of 798.1, 1603.2, 3191.4, 5484.7, 6402.9, and 8010.2 W kg −1 , respectively. The energy density and power density of the CMOC//AC ASC device observed in this study are comparable to other previously reported ASCs, such as

Conclusions
In this study, CMOC composites were prepared via a simple ball milling-calcination method. The glucose-derived carbon contributed to the homogeneous dispersion of low-crystalline CoMoO 4 ·0.9H 2 O. The superior electronic conductivity of carbon enhanced the electrochemical performance of the CMOC electrode. Thus, the CMOC electrode exhibited a high capacitance (340.0 F g −1 at 1 A g −1 ) and excellent cycling performance (∼85%, 10,000 cycles). The ASC supercapacitor assembled with CMOC and AC electrodes exhibited an energy density of 23.1 Wh kg −1 at a power density of 798.1 W kg −1 . This work proposes a feasible and productive strategy for preparing bimetallic oxide/carbon composite for supercapacitors and other energy storage devices.
Author contribution Liangyu Liu designed this study and wrote the main manuscript text. Peng Liu, Yong Zhou, Bin Yang, Bing Li, and Xiaoyang Liu revised the article.
Funding This work was supported by the National Natural Science Foundation of China (No. 22171101).

Data availability
The data are incorporated into the article and are available on request.

Declarations
Ethics approval Not applicable.

Competing interests
The authors declare no competing interests.