Rational Design for Mn3O4@carbon Foam Nanocomposite with 0D@3D Structure for Boosting Electrochemical Performance

A rational strategy was developed to prepare a nanocomposite with 0D/3D architecture. The composite containing Mn 3 O 4 nanoparticles (0D) and carbon foam (3D) could be applied as an electrode material for supercapacitor by taking advantage of high conductivity of carbon foam (CF) and high pseudocapacitance of Mn 3 O 4 nanoparticles. CF was prepared by a carbonization method using melamine foam, and then Mn3O4 nanoparticles were combined with carbon foam by a one-step hydrothermal method to prepare Mn3O4@CF nanocomposite. The 0D@3D hierarchical structure of Mn3O4@CF nanocomposite using CF as a 3D growing skeleton prevents agglomeration and increases reactive sites of Mn3O4 nanoparticles. In addition, CF as a conductive skeleton shortens the charge transfer path. The synergistic effect between CF and Mn3O4 improves the electrochemical performance of CF. Three Mn3O4@CF composites were prepared by adjusting the mass of the reactants in the processes of hydrothermal reaction. The Mn3O4 nanoparticles are uniformly grown on the CF surface with a diameter of 18 nm. Mn3O4@CF-2 composite has a specic capacitance of 212.8 F/g at a current density of 1 A/g, which is much higher than that of pristine CF (79.1 F/g) and Mn3O4 (112.7 F/g). The cyclic stability of Mn 3 O 4 @CF-2 is retained as 86.1% of initial capacitance after 2000 cycles at the current density of 1 A/g. It proves the feasibility of the as-mentioned strategy and broadens the application of carbon foam in supercapacitor.


Introduction
In recent years, the energy crisis has attracted more and more attention. The sustainable development of new energy vehicles and mobile electronic devices requires energy storage devices to put forward higher requirements in miniaturization and energy density and power density improvement. Supercapacitors, which can far exceed ordinary capacitors and electrochemical cells in energy density and power density respectively, have become a hot spot in energy storage eld. Meanwhile, supercapacitors have the advantages of fast charging and discharging speed, long cycle life, low pollution and high safety [1][2][3][4][5] . The speci c capacitance and conductivity properties of electrode materials play an important role in the electrochemical performance of supercapacitors. Among many electrode materials, carbon foam (CF) with three-dimensional skeleton [7][8][9][10][11][12][13][14][15][16] has an interconnected electronic channel, which greatly improves the electrochemical performance of the electrode compared with that of one-dimensional and twodimensional carbon materials.
As an electrode material, CF has been widely studied. Zhang et al. [6] prepared N-doped 3D porous CF at different temperatures by a one-step carbonization method using melamine foam (MF) as raw material.
By controlling the temperature, it is found that the resistance of CF in the 2 M KOH electrolyte is very low, and the resistance decreases when the carbonation temperature increases. The speci c capacitance can still maintain 96% of the maximum speci c capacitance after 5000 cycles. The symmetrical supercapacitor assembled with CF as electrode material can have the highest energy density of 9.34 Wh/kg at the power density of 6.9 kW/kg. Xiao et al. [9] carbonized the commercial MF to prepare a self-supporting N-doped carbon foam (NCF) with lightweight, interconnected 3D network and rich nitrogen content. On the bene t of its excellent structural exibility and high porosity, NCF can withstand up to 80% of the compression strain without signi cantly reducing its volume after 100 cycles. When exposed to the current density of 1 mA/cm 2 and the electrolyte of 5 M LiCl, the NCF electrode material shows the area capacitance of 332 mF/cm 2 and the mass ratio capacitance of 52 F/g. All-solid symmetric supercapacitor devices with NCF electrode can resist 60% strain without signi cant electrochemical performance change. Although MF carbonization can obtain CF with excellent mechanical properties and very low resistance, the further application of CF is greatly limited because CF still has a low speci c capacitance. In order to overcome this limitation, two main strategies are as follows: one is to design the material structure and prepare the hierarchical composite as nano-material@micron-material. Cheng et al. [10] combined 2D carbon nanosheets with CF by dipping and annealing at high temperature. The performance of supercapacitors based on this composite was signi cantly improved, and the speci c capacitance at the current density of 1 A/g and 10 A/g was as high as 364 F/g and 321.86 F/g, respectively. Another is to obtain additional pseudo-capacitance by means of compositing with pseudocapacitance material, so as to improve the speci c capacitance of the material. Wang et al. [11] used the in-situ growth of NiCo 2 O 4 nanosheets on CF to obtain a 2D/3D composite material (CF-NiCo 2 O 4 ) with excellent electrochemical properties. In this study, we chose the second way to composite CF with pseudocapacitance materials and obtain higher speci c capacitance. There are many considerations in the selection of pseudocapacitance materials. Manganese is abundant in nature, and its oxide has excellent performance in cost, environmental friendliness, high voltage window and theoretical speci c capacitance value reaching 1370 F/g [17][18][19][20][21][22][23] . It is widely adopted in the eld of supercapacitor electrode materials. However, due to the characteristics of easy agglomeration of manganese oxides in the preparation process and the disadvantage of poor conductivity of manganese oxides, the speci c capacitance of manganese oxides is often less than the theoretical value in practical applications. In order to solve these problems, on the one hand, the nanometer material of manganese oxide could be prepared to increase the speci c surface area; on the other hand, the composite with carbon material can improve its conductivity. An et al. [24] and Wang et al. [25]  nanoparticles with 3D CF to provide high capacitance.
In this paper, 3D CF was rstly prepared by a carbonization method through MF and used as the growth skeleton, and 0D Mn 3 O 4 nanoparticles were then grown on the surface of CF by a one-step hydrothermal method to prepare Mn 3 O 4 @CF composites with different mass ratios. In order to determine whether the composite was synthesized successfully, many characterization methods were used to analyze the composition and molecular structure, surface Raman scattering signal, valence state, microstructure and its electrochemical performance. The relationship among constituent, microstructure and properties was comprehensively explored. The in uence of the structure design in CF and Mn 3 O 4 composites on the electrochemical performance was analyzed. The results show that CF and Mn 3 O 4 can play a synergistic role through the structural design, and greatly improve the overall electrochemical performance.

Preparation of CF
CF was prepared by a direct carbonization method. MF was cut as 1 cm × 2 cm × 2 cm, and washed with deionized water and ethanol for several times. MF plates were dried in a furnace at 60 ℃ for 48 h. The covered alumina crucible and owing nitrogen were used to carbonize the dry MF with the heating rate of 10 ℃/min. The temperature was rstly kept at 400 ℃ for 1 h, and then kept at 800 ℃ for 2 h. After naturally cooling to room temperature, CF is obtained.

Preparation of Mn 3 O 4
Mn 3 O 4 was prepared by a hydrothermal method. Mn(CH 3 COO) 2 (60 mg) was added into 18 ml anhydrous ethanol, and it was completely dissolved by magnetic stirring for 30 min. The solution was transferred to a high-pressure reactor (25 mL, lined with polytetra uoroethylene). The hydrothermal reaction was conducted for 3 h at a constant temperature of 150 ℃. After the hydrothermal reaction, it was cooled to room temperature naturally, and the samples could be collected by centrifuge. The samples were washed with anhydrous ethanol and deionized water for 3 times, and kept in the drying oven at 60 ℃ for 24 h. Finally, Mn 3 O 4 monomer was obtained.

Preparation of Mn 3 O 4 @CF composites
Mn 3 O 4 @CF composites were prepared by a one-step hydrothermal method ( Fig. 1.). Mn(CH 3 COO) 2 (30 mg) was added into 18 ml anhydrous ethanol, and it was completely dissolved by magnetic stirring for 30 min. The solution containing four pieces of CF were transferred to the high-pressure reactor. The reactor was placed in a constant temperature drying oven, and the hydrothermal reaction was conducted at 150 ℃ for 3 h. After the hydrothermal reaction, it was cooled to room temperature. The samples were washed with anhydrous ethanol and deionized water for 3 times, and kept in the drying oven at 60 ℃ for 24 h. Finally, Mn 3 O 4 @CF composite (Mn 3 O 4 @CF-1) was obtained. In the similar process, Mn(CH 3 COO) 2 with 60 mg and 90 mg was added in, and the obtained materials were denoted as Mn 3 O 4 @CF-2 and Mn 3 O 4 @CF-3, respectively.
We used the electrochemical workstation (Shanghai ChenHua Instrument Co., LTD, CHI 760E) to test the electrochemical performance of the material. The capacitive performance of the material was studied by analyzing the CV curves of the same working electrode at different scanning rates and the CV curves of different working electrodes at the same scanning rates. The speci c capacitance and cyclic performance of the material were studied by analyzing the GCD curve of the working electrode under different current density, the GCD curve of different working electrode under constant current density, and the GCD curve of multiple charge-discharge under constant current density. The equivalent circuit diagram of the electrode system was obtained by tting EIS impedance spectrum, and the kinetic parameters of the electrode system were estimated. XPS analysis was carried out on the surface of Mn 3 O 4 @CF-2. As shown in Fig. 3, the XPS spectrum of each element and the XPS full spectrum (Fig. 3a)  very favorable for electron transport during redox reaction [27] . As shown in Fig. 3d has a strong interaction with CF [28] . As shown in Fig. 3e, it veri es the chemical state of Mn in Mn 3 O 4 @CF-2. There are two sharp peaks at the binding energy of 641.2 eV and 652.8 eV corresponding to Mn 2p 3/2 and Mn 2p 1/2 , respectively. The splitting energy between the two peaks is 11.6 eV, which is consistent with Mn 3 O 4 in the previous report [29] . By analyzing the XPS spectrum of Mn 3 O 4 @CF-2, it is proven that Mn 3 O 4 @CF-2 was successfully synthesized and CF was the N-doped carbon material.  Fig. 5a-d). When the mass of Mn(CH 3 COO) 2 was 60 mg, a large number of Mn 3 O 4 nanoparticles are grown uniformly on the surface of CF (Fig. 5b- (Fig. 6). It can be seen that Mn 3 O 4 @CF-2 contains very uniform Mn 3 O 4 nanoparticles with the size of 18 nm, which could improve the electrochemical performance.

Results And Discussion
The EDX energy spectra of Mn 3 O 4 @CF-2 were obtained to analysis the element type and the distribution of each element (Fig. 7). It can be con rmed that the elements of Mn 3 O 4 @CF-2 are C, N, O and Mn. These elements are uniformly distributed on the CF skeleton, which strongly supports the above analysis results of XRD, Raman and XPS, and further con rms that CF and Mn 3 O 4 constitutes in the composite.
The GCD curve of CF at different current densities, presenting a symmetrical triangle (Fig. 8a). Figure 8b shows the CV curve of CF at different scanning rates, whose curve shape is approximately rectangular. It can be seen that CF has good characteristics of EDLC capacitance. Figure 8c shows the GCD curve of   thus reducing the diffusion resistance of electrolyte ions between the electrolyte and the electrode material. In addition, CF as the conductive skeleton of the composite can also enhance the overall conductivity. As shown in Fig. 10b It is found that the speci c capacitance of the composite was signi cantly higher than that of CF and Mn 3 O 4 . The speci c capacitance of Mn 3 O 4 @CF-2 is the largest at the current density of 1 A/g, reaching 212.8 F/g. CF as the skeleton can improve the conductivity of the composite, which greatly improves the electrochemical performance. There are two reasons for this improvement in electrochemical performance. First, 3D CF is used as the growing matrix to prevent the agglomeration of Mn 3 O 4 nanoparticles. Mn 3 O 4 nanoparticles are uniformly loaded on CF, which greatly increase the contact area and active sites with the electrolyte, and provide high pseudocapacitance for the electrode material. Second, using CF as the conductive skeleton can effectively improve the overall conductivity of the composite. The results prove the possible application of Mn 3 O 4 @CF for supercapacitor. It should be explored more real application in supercapacitor devices. Figure 1 Schematic illustration of the synthesis for Mn3O4@CF.