In-situ synthesize of graphene-Mn 3 O 4 nanocomposites for high-rate pseduocapacitive electrodes

pseduocapacitive electrodes Danfeng Qiu, Xiao Ma, Jingdong Zhang, Bin Zhao, Zixia Lin 1 Key Laboratory of Radar Imaging and Microwave Photonics (Nanjing Univ. Aeronaut. Astronaut.), Ministry of Education, College of Electronic and Information Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China; dfqiu@nuaa.edu.cn (D.Q.); 2792359285@qq.com (X.M.); zhangjd@nuaa.edu.cn (J.Z.) 2 National Laboratory of Microstructures and School of Electronic Science and Engineering, Nanjing University, Nanjing, China; binzcaptain@gmail.com (B.Z.); 267319441@qq.com (Z.L.) * Correspondence: dfqiu@nuaa.edu.cn; Abstract: Mn3O4 /graphene nanosheets (GNS) composites serve as very excellent electrode materials for supercapacitors. They can fully combine the advantages of two materials such as graphene and metal oxide. Meanwhile, they can improve not only the specific energy and specific power of the materials, but also the cyclic stability of the materials. The results of the cyclic voltammetry and constant current charge discharge test on the composite electrode material have shown that the Mn3O4 /GNS powder sample has good capacitive performance. When the scanning rate is 5~50mV, the specific capacity retention rate of the composite electrode is 80.3% and 88% respectively. Mn3O4 nanoparticles, with the highest ratio of network coated GNS, exhibit a specific capacitance value of 957.6 F g at a current density of 2 A g in 1 M Na2SO4 solution. Besides, its network structure demonstrates high specific capacity and multiplying performance.

In all kinds of pseudopotential electrode materials, ruthenium oxide (RuO2) has been widely studied because of its good conductivity, rapid reversible redox reaction and high specific capacity [20,21].However, due to its price price and toxic features, it has not been widely applied.Therefore, finding a cheap alternative material becomes rather urgent.Manganese oxide enjoys multiple advantages, such as high theoretical specific capacity (1370 F g −1 [22,23]), good environmental compatibility, good oxidation-reduction and charge storage characteristics, high chemical, thermal stability, and low price.It is expected to replace RuO2 in some fields.Meanwhile, MnOx also enjoys some advantages, such as rich material, low cost and friendly environment.However, its electrical conductivity (10 −7 ~10 −8 S cm −1 ) is quite low, which limits its application in the field of supercapacitor.As the cycle life of a single MnOx is still not ideal yet, there are many carbon materials, such as activated carbon、 carbon nanotube, carbon aerogels and graphene etc. have been widely applied to combine with the metal oxides and hence to increase the conductivity of manganese oxide.
Graphene has a unique two-dimensional structure with a single atomic thickness, high specific surface area, excellent electrical and mechanical properties.Therefore, the combination of graphene and Mn3O4 can not only improve the electrical and mechanical strength of Mn3O4, but also bring greater surface/boundary area to the composite.More electrochemical active sites, therefore, are provided in this way.
Manganese oxide loaded with graphene has a good application prospect as the electrode material of supercapacitor.However, the preparation of manganese oxide is very complex.Currently, it is mainly constrained by two aspects, including the immaturity of the process and the high cost.
In this paper, Mn3O4/GNS nanocomposites were prepared by thermal decomposition of graphene and manganese nitrate complex.The organic combination of graphene and manganese oxide nanoparticles can fully realize the synergistic effect of double capacitance and pseudo capacitance.The pore structure of electrode materials is an important factor affecting the capacitance performance.Three dimensional porous Mn3O4/GNS composites were constructed by thermal decomposition of manganese oxide nanoparticles in situ by three-dimensional porous graphene paper.The three-electrode system exhibits a specific capacitance value of 987.6 F g −1 at a current density of 2 A g −1 in 1 M Na2SO4 solution.Such composite exhibits good cyclic stability with a small loss of 5.2% of maximum capacitance over 2000 consecutive cycles.

Experimental
GO was synthesized from natural graphite powders (universal grade, 99.985%) based on the Hummers' method with slight modifications [24,25].The as-prepared GO was stripped for 3 minutes in air at 300 °C, and then treated for 3 hours in Ar protection at 900 °C.Then, GNS was obtained.In the synthesis of typical Mn3O4/GNS nanocomposites, 537 mg of Mn (NO3)2 aqueous solution (50%) was mixed with 50 ml ethanol.Besides, 28 mg of GNS was added to the solution for 15 minutes.The suspension is mixed with a magnetic stirrer in the hood combined with the continuous evaporation of the ethanol in the solution.Dry Mn (NO3)2 / GNS composites were collected and treated at 200 °C for 10 h, and Mn (NO3)2 was converted to Mn3O4.The relevant mechanisms can be explained as follows: Based on weight, there is about 80% of Mn3O4/GNS is Mn3O4.During the control experiments, a Mn(NO3)2 aqueous solution (50%) was heated in the air under 200 °C for 10 h to prepare the simplex Mn3O4 sample.The obtained samples were explored via TEM, namely transmission electron microscopy, SEM, namely scanning electron microscopy and XRD, namely X-ray diffraction.Base materials (10 wt.%), which were mixed with N, N di-methyl-acetamide (∼10 ml).were used to fabricate the Mn3O4/GNS electrode.Afterwards, the final mixture obtained was stirred under the room temperature for around 24h using a magnetic stirrer (1200 rpm).
Subsequently, a think graphite sheet (specific area of 1 × 1 cm 2 ) was used to coat the final product.The electrodes being prepared later got dried under 60 °C in the hot air oven for around 3h.For the fabricated electrodes, there was around 1.2 mg cm −2 mass loading of active materials over the graphite sheet.The same electrochemically active material/binder (PVDF)/conductive carbon ratio (80:10:10) was also used for the preparation of pure Mn3O4 and GNS for the control experiment and hence to make an accurate electrochemical properties of the resultant electrodes.It can be seen from the XRD analysis chart of the composite powder in Fig. 1a that the major components of graphene and GNS/ Mn3O4 have low crystallinity.The diffraction peaks occurring on the all the reflections of the samples are consistent with the standard pattern of the tetragonal Mn3O4 (JCPDS file no.80-0382).There was none other impurity or phase being found in these patterns.For the multiplayer graphene of the GNS sample, the broad peak at around 25 °C was a kind of diffraction peak.There are two typical peaks of both samples which are at around 1350 and 1585 cm −1 being assigned to D and G brands of graphene correspondingly, suggesting that there were a multitude of defects in the GNS.Besides, according to fig.2c and 2d, it can be seen that the size of Mn3O4 is around 20-100 nm.Mn3O4 nanoparticles are dispersed on the surface of the graphene to form an interconnected junction.Meanwhile, for the trip of manganese oxide crystals, the graphene is of great importance.The pore structure of the 3D network skeleton effectively limits the size of the manganese oxide nanoparticles.At the same time, the pore of the graphene has been further expanded through the gas generated from the decomposition of manganese nitrate.The synergistic effect between graphene and manganese oxide was obvious.This not only improves the conductivity of materials, but also accelerates the diffusion of anion and cation.The specific capacitance of Mn3O4/GNS nanocomposites is larger and reversible.

Results and Discussion
Fig. 3b presents the charging and discharging curves of Mn3O4/GNS nanocomposites.
The calculated specific capacitance at is 957.6, 905.2, 854.6, 802.3,765.2F g −1 at the current density of 2, 5, 10, 20, 50 A g −1 respectively, which are proved by a large number of cyclic tests (Fig. 3c).After 1000 cycles of charge and discharge under current density of 20 A g −1 , the specific capacitance did not fail significantly, which was still 763.8F g −1 .That suggests that it retained 95.2% of the original activity.This is due to the synergistic effect of Mn3O4 and graphene during the electrochemical testing, which inhibits the loss of specific capacitance because of the large crystallization of the other side.In light of this, regardless of the increase of specific capacitance or the prolongation of cycle life, the combination of Mn3O4 and graphene has played a good role during the promotion.From Fig. 3d, the sample Mn3O4/GNS has a second-highest capacity retention rate in 3 samples while the capacity of the sample is the highest.The sample of graphene has the highest capacity retention rate.However, the capacity is far lower than that of the Mn3O4/GNS sample.This is due to the supporting effect of graphene on the overall morphology of the powder, making the composite powder form a good network structure, and the specific surface area and mesoporous porous content of the powder are greatly improved.
The synthesis mechanism of Mn3O4/GNS nanocomposites is shown in Fig. 4. By means of solution coprecipitation, manganese nitrate can uniformly adhere to the surface of graphene.Then, the thermal decomposition was conducted, decomposing the manganese nitrate into Mn3O4 nanocomposites.This method of in-situ thermal decomposition ensures that Mn3O4 nanoparticles can be firmly embedded in the surface and channel of graphene to achieve good electrical contact and thus to improve the electrochemical properties of manganese oxide in the process of charge and discharge.
Afterwards, the thermal decomposition was conducted.It can be found through the SEM and XRD analysis that the obtained compound is rather complex.It has been shown from the electrochemical tests that graphene and Mn3O4 composite electrodes have better electrochemical capacitance than graphene.In the constant current charge discharge test, the specific capacitance of the composite material is about 760 F g −1 under the high current charge while the discharge is up to 20 A g −1 .Meanwhile, the specific capacitance retention rate of the 2000 cycles is as high as 95%.The CV curve of the composites were approximately rectangular, indicating that the composites had good reversibility.

Fig. 2a and
Fig.2a and 2bshow that there are obvious lamellar structures and wrinkles at the edge of the graphene sheet.The natural flexural lamellar structure of graphene and the formed fold structure increase the specific surface area of the prepared three

Fig. 3a shows
Fig.3ashows the cyclic voltammetry (CV) curves of as prepared samples of Mn3O4/GNS nanocomposites.The redox peaks appear in the cyclic voltammetry curves of Mn3O4/GNS are near O.23 and 0.45 V, indicating that the capacitor is mainly a pseudopotential, which mainly stores charge by two-dimensional and quasi

Fig. 3 .
Fig. 3. (a) CV curves of Mn3O4/GNS with different scan rates; (b) charge/discharge curves of Mn3O4/GNS at various current densities; (c) the specific capacitance of composites with different current densities; (d) cycling performances of composites at current density of 20 mA g −1 .