High-performance asymmetric supercapacitor made of NiMoO4 nanorods@Co3O4 on a cellulose-based carbon aerogel

In this study, a new nanoporous material comprising NiMoO4 nanorods and Co3O4 nanoparticles derived from ZIF-67 supported by a cellulose-based carbon aerogel (CA) has been successfully synthesized using a two-step hydrothermal method. Due to its chemical composition, the large specific surface and the hierarchical porous structure, the NiMoO4@Co3O4/CA ternary composite yields electrodes with an enhanced specific capacitance of 436.9 C/g at a current density of 0.5 A/g and an excellent rate capability of 70.7% capacitance retention at 5.0 A/g. Moreover, an advanced asymmetric supercapacitor (ASC) is assembled using the NiMoO4@Co3O4/CA ternary composite as the positive electrode and activated carbon as the negative electrode. The ASC device exhibits a large capacitance of 125.4 F/g at 0.5 A/g, a maximum energy density of 34.1 Wh/kg at a power density of 208.8 W/kg as well as a good cyclic stability (84% after 2000 cycles), indicating its wide applicability in energy storage. Finally, our results provide a general approach to the construction of CA and MOF-based composite materials with hierarchical porous structure for potential applications in supercapacitors.


Introduction
In recent years, in the light of environmental degradation and an increasing energy demand, the development of various new, renewable and clean energy conversion and storage devices has attracted wide attention [1][2][3][4]. Among them, supercapacitors are promising candidates for energy storage owing to their advanced charge/discharge properties, the high power density and their long life cycles [5,6]. Based on the charge storage mecha-nism, supercapacitors can be classified into electrical double layer capacitors (EDLCs), the capacitance of which originates from the electrostatic adsorption of reversible ions at the electrode/electrolyte interface, and pseudocapacitors, for which the capacitance arises from reversible Faradaic reactions correlating with electroactive species [7,8]. Compared to EDLCs, pseudocapacitors can provide a much higher specific capaci-tance as a result of rapid reversible redox reactions [9,10]. Recently, advanced electrode materials based on transition metal molybdates such as NiMoO 4 [11], CoMoO 4 [12], MnMoO 4 [13] and FeMoO 4 [14] with suitable oxidation states and unique electrochemical properties are regarded as very promising materials for pseudocapacitors [15,16]. Particularly, NiMoO 4 has been widely applied in high-performance pseudocapacitors due to its enhanced electrochemical properties resulting from the high electrochemical activity of the Ni ion and the superb electrical conductivity of the Mo ion [17][18][19]. Unfortunately, despite the fact that NiMoO 4 has a high theoretical capacitance, its widespread practical application in supercapacitors is still restricted due to its low practical capacitance as well as the poor rate performance and wettability. Therefore, the construction of an integrated hierarchical porous nanoarchitecture by combining two metal oxides is a brilliant way to greatly enhance the overall electrochemical performance owing to synergistic effects [20]. For example, Li et al. synthesized 3D hybrid Co 3 O 4 /NiMoO 4 nanowire/nanosheet arrays on a carbon cloth, which exhibited a capacitance of 3.6 F/cm 2 at 3 mA/cm 2 , a capacitance retention of 82% and an increase of the current density from 3 to 15 mA/cm 2 [21]. Cai et al. reported a facile two-step hydrothermal method to synthesize unique 3D Co 3 O 4 / NiMoO 4 core/shell nanowire arrays on Ni foam, and the resulting Co 3 O 4 /NiMoO 4 hybrid electrode exhibited an areal capacitance of 5.7 F/cm 2 at a current density of 30 mA/cm 2 [22]. Zhang et al. described the fabrication of 3D hierarchical Co 3 O 4 / NiMoO 4 flower-like hybrid arrays on Ni foam with a high specific capacitance of 636.8 C/g at a current density of 5 mA/cm 2 and a capacitance retention of 84.1% after 2000 cycles [23]. Metal-organic frameworks (MOFs) with high porosity and tunable functionality are ideal sacrificial templates to synthesize metal oxides [24][25][26]. As a MOF derivative, Co 3 O 4 derived from the zeolitic imidazolate framework-67 (ZIF-67) is considered to be a good electrode material. It does not only maintain the original shape of the MOF, but also has a porous structure, which can yield a graded porous structure when combined with NiMoO 4 . Consequently, such a hierarchical porous nanoarchitecture can not only increase the specific surface area but also provide 3D pathways for fast electrolyte ion diffusion and electron transport.
To date, Ni foam [27], copper grid [28] and titanium mesh [29] have been mostly selected as collectors, whereas the high cost of these materials limited their practical application. Carbon aerogel (CA) has been considered an ideal supporting material to hybridize with electroactive materials because of its low cost, easy fabrication, large surface area, interconnected porosity and high electrical conductivity [30,31]. Due to its micro/mesoporous 3D morphology with large open pores, it offers more space to grow electroactive materials, efficiently reducing the internal resistance and enhancing the rate capability. Therefore, novel hybrid nanorods and nanoparticles of the electroactive metal oxides incorporated into a porous, conductive 3D network of CA could be promising electrode materials for supercapacitors.
Based on the above considerations, we present a simple scalable strategy to fabricate an integrated NiMoO 4 @Co 3 O 4 hierarchical porous structure aligned on CA, which is derived from a cellulose precursor, to be applied in an advanced asymmetric supercapacitor (ASC). The NiMoO 4 nanorods originated from ZIF-67 and were uniformly grown on CA frameworks to support the loading with Co 3 O 4 polyhedral nanocrystals. The hierarchical porous structure of the composite made of NiMoO 4 nanorods and Co 3 O 4 nanoparticles derived from ZIF-67 on the CA skeleton (NiMoO 4 @Co 3 O 4 /CA) provides a reaction interface adequate for shortening of the ion diffusion length and effectively buffering the volume change in the electrochemical reaction process. More importantly, the specific surface area is enlarged for such a porous structure, facilitating reactions of active substances and improving the electrochemical performance. Our results indicate an improved energy and power density and cycle stability of the corresponding ASC device (called NiMoO 4 @Co 3 O 4 /CA//AC).

Results and Discussion
The synthesis procedure of the NiMoO 4 @Co 3 O 4 /CA composite is illustrated in Figure 1. First, CA was obtained by carbonization of the cellulose aerogel precursor, which was produced from microcrystalline cellulose (MC). Second, the produced CA was used as the backbone for the growth of NiMoO 4 nanorods employing a hydrothermal method followed by heat treatment. By this approach, NiMoO 4 /CA composites were obtained, in which the NiMoO 4 nanorods uniformly filled the 3D network of CA, providing plenty of sites for coupling with ZIF-67. Third, ZIF-67 was in situ crystallized on the surface of the NiMoO 4 /CA skeleton by a hydrothermal method. Finally, after the pyrolysis of the NiMoO 4 @ZIF-67/CA precursor at 350 °C for 2 h under air atmosphere, the NiMoO 4 @Co 3 O 4 /CA composite was obtained.
The morphology of CA, NiMoO 4 /CA, NiMoO 4 @ZIF-67/CA and NiMoO 4 @Co 3 O 4 /CA was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure S1 in Supporting Information File 1, after the pyrolysis process, the volume of CA is drastically reduced compared to that of the precursor cellulose hydrogel and aerogel. The network of intertwined nanofibers of CA shown in Figure 2a was maintained from the cellulose aerogel ( Figure S2, Supporting Information File 1). The diameter of the nanofibers was about 20-50 nm. The SEM top-view   Figure 2b indicating a homogenous distribution of interconnected nanofibers within the CA sample. The magnified SEM image (inset of Figure 2b) reveals that each carbon nanofiber of CA is surrounded by plenty of the NiMoO 4 nanorods. Figure 2c confirms that the CA network is well hybridized with NiMoO 4 . This morphology supports the subsequent deposition of ZIF-67. Figure 2d and Figure 2e show that ZIF-67 with the characteristic dodecahedral morphology has been uniformly grown on the surface of the NiMoO 4 nanorods and inside the voids of the NiMoO 4 /CA composite.

image of NiMoO 4 is shown in
ZIF-67 is processed into Co 3 O 4 by an annealing process reported previously [32]. The resulting Co 3 O 4 exhibits a nest-like structure and a porous morphology, and the dodecahedral structure is largely kept with no apparent collapse. The TEM image in Figure 2f clearly shows that the Co 3 O 4 particles have preferably grown on the surface of the NiMoO 4 nanorods with a nestlike morphology. This unique hierarchical porous architecture of the NiMoO 4 @Co 3 O 4 /CA composite is characterized by three types of network structure. The first is a filamentous network with little bundles formed by interconnected nanofibers of CA, which could provide diffusion channels for electrolyte ions and could be a conductive substrate to serve as a backbone; the second consists of interlaced NiMoO 4 nanorods and acts as a bridge in the ternary hierarchical structure, which offers a large surface area for loading of active materials and contributes to the pseudocapacitance; the third is given by the Co 3 O 4 nanoparticles derived from ZIF-67, which have nanosized channels and cavities.
The well-defined porous structure of nest-like Co 3 O 4 provides many active sites for charge storage, thus improving the contact between electrode and electrolyte and facilitating the transport of electrons during the redox reactions [33]. Such a hierarchical structure can effectively enlarge the specific surface area of Faradaic reactions and shorten the diffusion pathways for the fast ion transfer, thus increasing the performance of the supercapacitor. The crystal structure of CA, NiMoO 4 , the Co 3 O 4 nanoparticles derived from ZIF-67 and the NiMoO 4 @Co 3 O 4 / CA composite was examined using X-ray powder diffraction (XRD) as shown in Figure 3a. For CA, a broad diffraction peak is observed at about 22.8°, which can be attributed to the (120) planes of amorphous carbon. The five well-defined diffraction peaks appearing at 2θ values of 14. To investigate the chemical composition and the valence states of the NiMoO 4 @Co 3 O 4 /CA nanocomposite, X-ray photoelec-tron spectroscopy (XPS) was performed and the results are shown in Figure 4. According to Figure 4a, the elements Co, Ni, Mo, O and C can be clearly identified in the spectrum of the NiMoO 4 @Co 3 O 4 /CA composite. The C 1s core-level spectrum can be deconvolved into three peaks, which correspond to the C-C (284.8 eV), C-OH (286.3 eV) and O=C-O (288.4 eV) bonds (Figure 4b) [34]. In Figure 4c, two peaks are observed at 780.9 and 796.6 eV corresponding to Co 2p 3/2 and Co 2p 1/2 , respectively, indicating that the NiMoO 4 @Co 3 O 4 /CA composite electrode material contains both Co 3+ and Co 2+ [35]. The peaks at 787.1 and 802.8 eV with a spin-energy separation of 15.7 eV can be attributed to the shake-up satellite peaks of Co 2+ [36]. Figure 4d shows the Ni 2p spectrum where two characteristic peaks appear at 856.5 and 874.3 eV along with two shake-up satellite peaks with a spin-energy separation of 17.8 eV, corresponding to the Ni 2p 3/2 and the Ni 2p 1/2 levels of Ni 2+ [37,38]. The Mo 3d core-level spectrum (Figure 4e) shows two main peaks at 232.4 and 235.5 eV corresponding to the Mo 3d 5/2 and Mo 3d 3/2 levels of Mo 6+ , respectively [39]. Figure 4f shows the core-level spectrum of O 1s. It can be divided into two main peaks at 530.4 and 531.2 eV, which are attributed to typical metal-oxygen bonds and oxygen ions of low coordination numbers at the surface, respectively [40]. The XPS results further indicate that the NiMoO 4 @Co 3 O 4 /CA sample contains C, Co, Ni, Mo and O atoms.
The N 2 adsorption/desorption isotherms and the pore size distribution curves of the CA, NiMoO 4 /CA and NiMoO 4 @Co 3 O 4 / CA samples are shown in Figure 5. Both the CA and the NiMoO 4 /CA samples exhibit typical type-IV curves with distinct H 3 -type hysteresis loops suggesting the existence of mesopores. The calculated Brunauer-Emmett-Teller (BET) specific surface areas (S BET ), total pore volumes and average diameters are listed in Table 1. From Figure 5c, it can be clearly seen that the NiMoO 4 @Co 3 O 4 /CA sample is characterized by a combination of type-IV and type-I isotherms, indicating the presence of micro-and mesopores with monolayer-multilayer adsorption. Furthermore, two distinct pore distribution curves are observed in the inset of Figure 5c revealing a hierarchical porosity: micro/mesopores smaller than 5 nm and meso/macropores with diameters of 20-60 nm. As shown in Table 1, the NiMoO 4 @Co 3 O 4 /CA sample has the largest S BET of 334.47 m 2 /g and V total of 0.8 cm 3 /g, much higher than that of CA (87.7 m 2 /g, 0.3 cm 3 /g) and NiMoO 4 /CA (94.9 m 2 /g, 0.4 cm 3 /g). The hierarchical porosity and large surface area of the NiMoO 4 @Co 3 O 4 /CA composite could be attributed to the aggregation and gathering of ZIF-67 particles on the porous skeleton of NiMoO 4 /CA forming such a porous structure, which is in agreement with the SEM images shown in Figure 2. On the one hand, the micropores mainly originate from the Co 3 O 4 nanoparticles derived from ZIF-67. These enlarge the specific surface area and increase the number of active sites for charge storage. On the other hand, the meso/macropores consist of interconnected nanofibers of CA and interlaced NiMoO 4 nanorods, which facilitate the inclusion of electrolytes into the particles and enhance the number of diffusion channels. Thus, we conclude that the obtained NiMoO 4 @Co 3 O 4 /CA electrode may serve as a new multi-functional material for energy conversion and storage applications [41,42].
Electrochemical measurements were performed to explore the applicability of the NiMoO 4 @Co 3 O 4 /CA electrode in a potential supercapacitor. The synthesized samples were used as working electrodes for electrochemical analysis. The electro-chemical experiments were conducted by using a three-electrode testing system in a 2.0 M KOH solution. The reference electrode and the counter electrode were a saturated calomel electrode (SCE) and a platinum electrode, respectively. Figure 6a shows the cyclic voltammetry (CV) curves of the NiMoO 4 @Co 3 O 4 /CA electrode at voltage scan rates of 2.5-50 mV/s. All the CV curves shown in Figure 6a consist of a pair of strong redox current peaks, which can be attributed to the redox reaction between M 2+ and M 3+ (M represents Ni and Co elements) and Co 3+ and Co 4+ , as described by the following equations [43]: (1) Therefore, the coexistence of Ni and Co provides multiple redox reactions for the electrochemical process. The peak currents of the CV redox peaks increased linearly with v 1/2 demonstrating the battery behavior of NiMoO 4 @Co 3 O 4 /CA [44,45]. The specific capacities of NiMoO 4 @Co 3 O 4 /CA become diffusion-controlled in the high scan rate range (above 30 mV/s) and the peak current (i) response is proportional to the square root of the scanning rate (i ∼ v 1/2 ) owing to the diffu-sion-controlled battery behavior. This is further confirmed by the linear change of the current of the CV redox peaks as a function of v 1/2 ( Figure S3, Supporting Information File 1).
To further evaluate the charge storage ability of the electrodes, galvanostatic charge/discharge (GCD) measurements were carried out within a potential window of 0-0.4 V at various values of the current density. Figure 6b shows the GCD curves of the NiMoO 4 @Co 3 O 4 /CA composite at different values of the current density. A distinct plateau occurred during each discharge process in accordance with the battery characteristics derived from the CV results. Based on the GCD curves, the spe-   . At a high current density of 5.0 A/g, the specific capacitance of the NiMoO 4 @Co 3 O 4 /CA composite was still as high as 308.9 C/g, surpassing many metal oxides under the same conditions. For comparison, the NiMoO 4 /CA electrode was tested under the same conditions yielding a capacitance of 339.5 C/g at 0.5 A/g. Hence, the NiMoO 4 @Co 3 O 4 /CA electrode exhibited a higher specific capacitance (436.9 C/g) than the electrode of pure NiMoO 4 /CA (339.5 C/g) at a current density of 0.5 A/g ( Figure S4, Supporting Information File 1). The high capacitance of the NiMoO 4 @Co 3 O 4 /CA composite may be explained as follows. Firstly, the CA scaffold acts as a conducting pathway in the NiMoO 4 @Co 3 O 4 /CA composite due to its excellent electrical conductivity and the 3D carbon nanonetwork. Secondly, NiMoO 4 nanorods with extremely high specific capacitance are considered ideal spots for the nucleation and the growth of ZIF-67 particles. Lastly, the Co 3 O 4 particles derived from ZIF-67 possess a nest-like structure and a porous morphology, which yields a large surface area, rich active sites and shorter diffusion paths for the Faradaic reaction.
In Figure 6d, in the low-frequency region, the NiMoO 4 @Co 3 O 4 /CA composite shows a higher phase angle than the Co 3 O 4 particles derived from ZIF-67, indicating that the NiMoO 4 @Co 3 O 4 /CA electrodes impede the diffusion less strongly, which is attributed to the faster transport of electrons and the charge transfer resistance (R ct ). Fitting the electrochemical impedance spectroscopy (EIS) plots based on the equivalent circuit model (inset of Figure 6d), reveals a solution resistance (R s ) of the NiMoO 4 @Co 3 O 4 /CA composite of 1.3 Ω. This value is smaller than that of the Co 3 O 4 particles derived from ZIF-67 with 1.5 Ω, demonstrating that the electrical conductivity is enhanced upon to the introduction of NiMoO 4 /CA. The cyclic stability of NiMoO 4 @Co 3 O 4 /CA was tested over 5000 cycles at 0.5 A/g. The capacitance retention still reaches 82.7%, while the capacitance retention of Co 3 O 4 /CA derived from ZIF-67 is only 47.1% as shown in Figure S5, Supporting Information File 1. This indicates that the NiMoO 4 @Co 3 O 4 /CA electrode has an outstanding cyclic stability, which is very important for ASC applications.
A schematic diagram of the ASC device (called NiMoO 4 @Co 3 O 4 /CA//AC), which was fabricated using the NiMoO 4 @Co 3 O 4 /CA composite as the positive electrode and activated carbon (AC) as the negative electrode, is shown in Figure 7a. To determine the best working voltage window, the CV curves of the AC and the NiMoO 4 @Co 3 O 4 /CA electrodes have been measured at a scan rate of 5.0 mV/s as shown in Figure 7b. Obviously, the AC electrode works well in a voltage window between −1.0 and 0 V, while the NiMoO 4 @Co 3 O 4 /CA electrode works at 0-0.6 V with capacitive behavior. Thus, the total extent of the voltage window of the ASC device reaches 1.6 V, which is the sum of the voltage window of the AC and the NiMoO 4 @Co 3 O 4 /CA electrodes. Figure 7c exhibits the CV curves of the NiMoO 4 @Co 3 O 4 /CA//AC ASC at different scan rates from 2.5 to 50 mV/s. Obviously, the overall capacitance of the NiMoO 4 @Co 3 O 4 /CA//AC ASC device is a combination of the Faradaic pseudocapacitance and the capacitance of the EDLC [46]. The shape of the CV curve displays the characteristics of hybrid ASCs, and there is no obvious distortion with increasing scanning rate. This indicates a reliable behavior of the capacitance of the ASC device. As demonstrated in Figure 7d, the GCD curve was also evaluated to further discuss the performance of the ASC device at up to 1.    [48], MNMO//AC [49], CNM-1//AC [50]. The inset in Figure 7e and Figure S6 in Supporting Information File 1 show a green LED powered by the assembled ASC device. Furthermore, the ASC device shows an outstanding cyclic performance at a current density of 0.5 A/g. As shown in Figure 7f, the capacitance retention is 84.0% of the initial value after 2000 cycles. These results demonstrate that the NiMoO 4 @Co 3 O 4 /CA//AC ASC is a very promising candidate for practical applications in high-power energy devices.

Characterization
The crystalline structure of the prepared samples was characterized by XRD (D8 Advance, Bruker) using Cu Kα radiation (λ = 0.15406 nm) over a scan range of 5-80°. XPS (Thermo escalab 250Xi, Thermo fisher) measurements were performed using the monochromatized Al Kα radiation at 1486.6 eV. The surface morphology of the samples was observed by SEM (JSM-6701F, JEOL) at an accelerating voltage of 200 kV. TEM images and EDS mappings were recorded using a high-resolution TEM (JEOL JEM-2100) operated at an acceleration voltage of 200 kV. The pore size distribution, mean pore diameter, total pore volume and specific surface area were measured by a N 2 adsorption analyzer (Micromeritics ASAP 2020 instrument) using the BET nitrogen adsorption/desorption technique. The pore volume was calculated from the adsorption data according to the Barrett-Joyner-Halenda (BJH) theory. The total pore volume was determined from the amount of N 2 adsorbed at a relative pressure of P/P 0 = 0.99.

Electrochemical measurements
The electrochemical properties of the NiMoO 4 @Co 3 O 4 /CA electrodes were measured in a three-electrode testing system. CV, GCD and EIS were performed on the electrochemical workstation (CHI760D, Shanghai, China) in 2.0 M KOH aqueous solution. The working electrode materials were prepared by mixing the obtained sample, carbon black, acetylene black and polytetrafluoroethylene (PTEF) emulsions at a mass ratio of 80:7.5:7.5:5.0. The homogeneous slurry was coated on Ni foam substrates (1 cm × 1 cm) and dried at 80 °C for 12 h. The electrodes loaded with the hybrid were then pressed at 5.0 MPa. The SCE and platinum electrodes were used as the reference and counter electrodes, respectively. The specific capacities were calculated from the GCD curves using C = IΔt/m, where I is the charge-discharge current, Δt is the discharge time, and m is the mass loading of the electroactive material. The ASC device was assembled using NiMoO 4 @Co 3 O 4 /CA as the positive electrode and AC as the negative electrode. The details of preparation and characterization of the ASC device are described in the Supporting Information File 1. The cyclic stability tests were conducted on a LAND battery test system (LAND CT-2001A) at room temperature.

Supporting Information
Supporting Information File 1 Details of the preparation of the ASCs, photographs and SEM images of the cellulose aerogel, electrochemical tests and the LED photograph of the ASCs device.