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. 2020 Jul 24;5(30):18975–18986. doi: 10.1021/acsomega.0c02243

Producing “Symbiotic” Reduced Graphene Oxide/Mn3O4 Nanocomposites Directly from Converting Graphite for High-Performance Supercapacitor Electrodes

Yu Gu †, Jian Wu †, Xiaogong Wang ‡, Weijie Liu †,*, Shu Yan †
PMCID: PMC7408257  PMID: 32775899

Abstract

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Almost all existing methods for preparing reduced graphene oxide/Mn3O4 (RGO/Mn3O4) composites are based on the synthetized graphene or graphene oxides (GO), which make them complicated and high-cost processes. Here, we reported a new method, which is able to convert graphite directly to RGO/Mn3O4 composites. Thus, it is simpler, more economical, and productive. The structure of RGO/Mn3O4 inheriting intermediate product GO/MnO2 composites that are formed by the present method is a novel three-dimensional “multilayer steamed bread” nanostructure, which constitutes mutually beneficial “symbiosis”. The nano-Mn3O4 supports the space between RGO layers and further to the combination of RGO to self-assemble into large-sized (>40 μm) nanocomposites. Meanwhile, the formed Mn3O4 particles were small (60 × 10 nm2) in diameter and distributed homogeneously without the use of any template and surfactant. Because the structure and nanosize of composite cause the excellent electrochemical properties, RGO/Mn3O4 electrodes deliver an enhanced specific capacitance of 438.7 F/g at 0.3 A/g and outstanding cyclic stability (77.5% of its initial capacitance is retained after 1000 cycles).

1. Introduction

In recent years, transition-metal oxide pseudocapacitors have attracted wide attention owing to their high capacitance, high energy, and low pollution to the environment.1,2 Among them, Mn3O4 has been studied as a potential electrochemical material because of its excellent physical and chemical properties in ordinary conditions, including abundance, nontoxicity, and low price.2,3 However, on the other hand, its poor electrical conductivity and dense morphology have impeded its application.4 As a result, some researchers have suggested to combine Mn3O4 with graphene to overcome its shortages to be used for chemical absorption, sensors, supercapacitors, and so forth.5−8 Unfortunately, the existing method to produce the uniform composite of Mn3O4 nanoparticles and graphene or graphene oxide (GO) is a very complicated and costly process.9−13

GO, an embellished monomolecular graphene with numerous oxygen-containing functional groups, has attracted much attention of potential usages in various fields in the past decade, including electronics, electrochemistry, sensors, films, and so forth.14−20 Meanwhile, GO can also serve as the precursors in the preparation of reduced GO (RGO), and high-quality GO is vital in controlling the structure, property, and application potential of RGO.20−23 A chemical method is commonly used to prepare GO at the present, owing to the advantage of high processing ability, ease of mass production by low-cost graphite, and good physical and chemical properties after reduction.24

The earliest work about the chemical synthesis of GO was reported by Brodie in 1859, which took more than 4 days.25 Later in 1958, Hummers and Offeman developed their widely used classical method (the Hummers method).26 With the contributions of many other researchers, the mechanism of Hummers method has been extensively explored since then.27,28 Specifically, potassium permanganate (KMnO4) and concentrated sulfuric acid (H2SO4) react to form the main oxidants, manganese heptoxide (Mn2O7) and permanganyl cation (MnO3+), which diffuse between the graphene layers and react with nearby carbon atoms. The oxidants would attack the graphene layers, and the oxidized areas spread randomly over the flakes. Unlike sulfur, manganese does not form any functionality and has a long lifetime on the GO basal planes.28 It was also proved that the products of oxidants were divalent manganese ions (Mn2+) because deionized water was added into the reaction system, and further oxidation of MnO4– with high oxidizing potential occurs.

RGO/Mn3O4 composites were studied widely in supercapacitors and achieved outstanding achievements. The specific research results of RGO/Mn3O4 and its similar composites for supercapacitors are shown in Table 1.34

Table 1. Comparison of the Specific Capacitance and Capacitance Retention among Some Similar RGO/Mn3O4 Composite Supercapacitors.

materials capacitor type specific capacity cycling stability refs
RGO/Mn3O4 symmetric supercapacitor 243 F/g at0.5 A/g 82.3% after 1000 cycles (29)
RGO/Mn3O4 symmetric supercapacitor 310 F/g at 1 A/g 100% after 30,000 cycles (30)
graphene/Mn3O4 symmetric supercapacitor 312 F/g at 0.5 A/g 76% after 1000 cycles (27)
graphene/Mn3O4 symmetric supercapacitor 270.6 F/g at 0.2 A/g 91% after 1500 cycles (31)
RGO/Mn3O4 symmetric supercapacitor 228 F/g at 5 A/g 95% after 5000 cycles (32)
RGO/Mn3O4 symmetric supercapacitor 311 F/cm3 at 300 mA/cm3   (33)
graphene/Mn3O4 symmetric supercapacitor 317 F/g at 10 mV/s 100% after 4000 cycles (8)
graphene/polyaniline/Mn3O4 symmetric supercapacitor 500 F/g at 5 mV/s 97% after 3000 cycles (35)
graphene/Mn3O4 symmetric supercapacitor 326.9 F/g 94.6% after 1000 cycles (36)
graphene/Mn3O4 symmetric supercapacitor 457 F/g at 1.0 A/g 91.6% after 5000 cycles (37)
N-dope graphene/Mn3O4/MnO2 symmetric supercapacitor 739 F/g at 0.5 A/g 93.4% after 10,000 cycles (38)
RGO/Mn3O4 symmetric supercapacitor 221.6 F/g at 50 mA/g 97.1% after 1000 cycles (39)
RGO/Mn3O4 symmetric supercapacitor 351 F/g at 0.5 A/g 80.1% after 10,000 cycles (40)
graphene/CeO2/Mn3O4 symmetric supercapacitor 310 F/g at 2 A/g 92.4% after 1000 cycles (7)
RGO/Mn3O4 symmetric supercapacitor 438.7 F/g at 0.3 A/g 77.5% after 1000 cycles this work
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In this article, a new method was introduced, which is able to produce high-grade GO along with in situ formed MnO2 nanoparticles in one-step. Based on the synthesis of “multilayer steamed bread” GO/MnO2 composites, we obtain the same nanostructure of RGO/Mn3O4 composites that are a highly effective type of ultracapacitor materials only by thermal reduction. The properties of the nanocomposites with different mass ratios were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Finally, the possible mechanism of the new method was described as well. The composite electrode achieves an excellent specific capacitance of 438.7 F/g at 0.3 A/g and extraordinary cycling stability with a significant 77.5% capacity retention after 1000 cycles.

2. Results and Discussion

2.1. Structures and Compositions of GMH0, GMH1.3, and GMH2.6

According to the adding amount of H2O2, we named the three samples GMH0, GMH1.3, and GMH2.6, which are shown in Figure 1. The crystal structure and composition of MnO2/GO samples with different mass ratios were analyzed using XRD and Raman spectrometry. Broad XRD peaks of GMH0 were observed indicating that samples’ crystallization is incomplete, and the α-MnO2 (JCPDS 44-0141) particle size is small.41,42 It was also observed that the XRD peaks of α-MnO2 decreased from GMH0 to GMH2.6 and totally disappeared at GMH2.6. In contrast, the XRD peaks of pure GO show the opposite trend (Figure 2b). However, the Raman spectra clearly show that both GMH0 and GMH1.3 contain α-MnO2 and GO (Figure 2a). This is because the peak at about 640 cm–1, labeled A, corresponds to the Mn–O vibration perpendicular to the MnO6 octahedral double chains of MnO, and IG/ID is typical of GO.43 Furthermore, the A/(D + G) peak area ratio can be used to estimate the proportions of GO and MnO2 in the composites. The contents of carbon and manganese elements in GMH0 and GMH1.3 measured using the infrared sulfur–carbon analyzer and flame atomic absorption spectrometer and shown in Figure 2c can also prove that all samples containing lots of GO, GMH0, and GMH1.3 contain MnO2 (Figure 2c). It should be noticed that the observed XRD patterns disagreed with the Raman spectra. In the Raman spectrum, the G band (1590 cm–1) represented the in-plane bond-stretching motion of the pairs of C sp2 atoms (the E2g phonons), and the D band (1350 cm–1) corresponded to the breathing modes of rings or K-point phonons of A1g symmetry. However, the XRD peaks were only affected by the distance between crystal spacing.44 Therefore, we deduced that the α-MnO2 and GO of the compounds formed a symbiosis, which leads to the much larger interlamellar spacing of GO in the samples than pure GO. This explains why the XRD peaks of the pure GO phase disappeared in GMH0. This observation was also supported by the subsequent morphological analyses of SEM and TEM described in the following paragraphs. When the mass percentage of α-MnO2 in the composites decreases, the peak of pure GO (2θ = 10.2) emerged in GMH1.3, which is lower than in GMH2.6. Because of the smaller number of MnO2 particles, a part of graphene basic planes in GMH1.3 collapsed to regular spacing. The XRD and Raman spectrum both identified that GMH2.6 contained no or very little manganese particles. It could be concluded that the composites made in the present study were GO and α-MnO2 composites, and the ratio of GO to α-MnO2 could be adjusted by H2O2 addition.

Figure 1.

Figure 1

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Schematics of the procedure of fabricating RGO/Mn3O4 composites and the formula of each sample.

Figure 2.

Figure 2

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(a) Raman spectra of GMH0, GMH1.3, and GMH2.6. (b) XRD patterns of GMH0, GMH1.3, and GMH2.6. (c) C and Mn contents measured for different samples using the infrared sulfur–carbon analyzer and flame atomic absorption spectrometer.

The chemical structure of GO/MnO2 was further investigated by X-ray photoelectron spectroscopy (XPS), as shown in Figure 3. The XPS survey spectrum of GMH0 showed clearly C, Mn, and O elements. The separation of peak energy between the Mn 2p1/2 and Mn 2p3/2 is 11.7 eV, which agrees with classical XPS peaks of MnO2.45 The C 1s spectrum was deconvoluted into five peaks assigned to C=C, C–OH, C–O–C, C–OH, and HO–C=O. The C–OH, C–O–C, C–OH, and HO–C=O peaks attribute to the oxygen-containing group of GO.46 Also, the C=C is the chemistry state of GO base sheets.47 The chemical interaction of MnO2 nanoparticles and GO gives rise to the Mn–O–C peaks in the XPS O 1S spectrum.

Figure 3.

Figure 3

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XPS spectra of the GMH0 composite. (a) Survey spectrum, (b) Mn 2p, (c) C 1s, and (d) O 1s.

The microstructures of the composites are analyzed by the image of SEM. First, we analyze GMH0 treating without ultrasonication. We could see that many MnO2 nanoparticles joined with GO together. Upon closer inspection, we found that the combination of GO and MnO2 shows three kinds of location relation. The area in which nanoparticles MnO2 generate on the surface of GO is labeled letter A, as shown in Figure 4a. In area B, we could observe that very thin GO covers on the MnO2. Finally, we label the rest of area by letter C, where smooth and occasionally undulating graphene appeared but no nanoparticles were found. We can infer that most nanoparticles exist between GO layers compared with MnO2 on the surface.

Figure 4.

Figure 4

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(a) SEM image of GMH0 without sonication. (b–d) SEM images of GMH0.

To further analyze the structural relationship between MnO2 and GO, we ultrasonically treated GMH0 for 2 h. It was found that agglomerated nanoparticles of the surface fell out under the action of ultrasonication, and the GO surface with full of pleats fell out entirely (Figure 4b), which were similar to areas C, as shown in Figure 4a. Except for GO, we found vaguely that densely packed nanoparticles were underneath GO (Figure 4b,c). Together with XRD, Raman spectra (Figure 2a,b), SEM images (Figure 4c,f), and TEM (Figure 4d), we conclude second that small-size nanorob MnO2 (60 × 10 nm2) is homogeneously generated in the GO layers. In addition, the formation of MnO2 not only further “exfoliated” GO but also acted with the GO layers to self-assemble into the large size (>40 μm) but nanostructured composites. Meanwhile, after the action of ultrasonication for 2 h, the nanostructure remains intact under the force of MnO2 joint. It can also be seen that this nanostructure is very stable.

2.2. Thermal Reduction and the Formation of Mn3O4

The thermal-reduced samples were named RGMH0, RGMH1.3, and RGMH2.6. The XRD patterns of the samples manifest that α-MnO2 was transformed to Mn3O4, and GO was reduced to RGO (Figure 5a). The changing rules on the XRD patterns of RGMH0, RGMH1.3, and RGMH2.6 are similar to those of GO/MnO2 (Figure 2b). Hence, we infer that the nanostructure of RGO/Mn3O4 inherits the GO/MnO2, and the macro size of composites did not change after low-temperature heating (Figure 5b). Moreover, we found that the abundant nano-Mn3O4 prevents the graphene to come in contact with each other, as observed from XRD (Figure 5a). The theories of secondary electron image and back-scattered electron image are different. The former only presents the details on the surface because the low-energy incident electron is almost impenetrable. In comparison, the latter has a larger average atomic number of Z and brighter image, and the high-energy incident electron promotes the penetration ability to the micron level. In this study, the back-scattered electron contrast images (Figure 5d,f,h) show the Mn3O4 plane projection of three-dimensional distribution of each sample in the absence of Mn3O4 particles on the surface (Figure 5c,e,g). In other words, Mn3O4 homogeneously remains in the RGO layers. Next, we conclude third that GO/MnO2 turns to RGO/Mn3O4 during the heat treatment. Moreover, the macrostructure and microstructure of the composites are unchanged. Figure 5i directly shows that the shape and size of Mn3O4 are the same to MnO2 of the intermediate products. The atomic ratio of element Mn and C (0.81) is very close to Mn3O4 (0.75) (Figure 5j).

Figure 5.

Figure 5

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(a) XRD patterns of RGMH0, RGMH1.3, and RGMH2.6. (b,c) SEM secondary electron image of RGMH0. (d) SEM back-scattered electron image of RGMH0. (e,f) SEM secondary electron and back-scattered electron image of RGMH1.3. (g,h) SEM secondary electron and back-scattered electron images of RGMH2.6. (i) TEM images of RGMH0. (j) Energy spectrum analysis of GMH0.

Chemical bonding detail states of RGO/Mn3O4 nanocomposites are further investigated by XPS spectrometry, which are shown in Figure 6. The survey spectrum of RGMH0 identifies that Mn, C, and O elements exist in nanocomposites. Energy separation between two peaks of Mn 2p1/2 and Mn 2p3/2 locating at 653.10 and 641.50 eV, respectively, is computing to be about 11.60 eV, which attributes to Mn3O4.48 The weak peaks of M 2p spectrum at 642.90 eV correspond to Mn3+ ions in Mn3O4.49 The peak of the C 1s spectrum (Figure 6b) located at 284.3, 286.40, and 288.50 eV represents C–C bond, C–OH, and O–C=O, respectively. The two peaks at 531.70 and 530.10 eV of O 1s spectrum correspond to oxygen in defects and Mn–O–C, respectively. The Mn–O–C band indicates strong chemical interaction between Mn3O4 nanoparticles and RGO.39

Figure 6.

Figure 6

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XPS spectra of the RGMH0 composite. (a) Survey spectrum, (b) C 1S, (c) Mn 2p, and (d) O 1s.

2.3. Mn(VII) Converts to Mn(II) in the Oxidation Process

The origin stage of our experiment only contains K+, H+, MnO4–, and SO42– ions. According to the color of the wastewater (Figure 7b), the MnO4– ion does not exist in the wastewater.50−52 The XRD patterns of sediment and graphite show that the sediment contains no other insoluble substances, except the partially oxidized graphite (Figure 7a). Hence, we guess that MnO4–, the main oxidizing agent, was fully reduced to another colorless soluble ion. During the experiment (measurement of Mn2+ ions in wastewater), the color of wastewater turned purple (Figure 7c). From the above analysis, we conclude that MnO4– converts to Mn2+. The equation is as follows

2.3. 1

Figure 7.

Figure 7

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(a) XRD patterns of partially oxidized graphite. (b) Wastewater was collected during the GO filtration from the reaction system. (c) Pictures of color test of Mn2+ ions.

2.4. Formation Mechanism of RGO/Mn3O4

Based on the above results and other relevant studies, we propose a reaction mechanism to interpret the formation process of the special nanostructures composed of RGO and Mn3O4. We divide the method of converting bulk graphite to RGO/Mn3O4 into four steps. In the first step of preoxidation, sulfuric acid reacts with graphite to form a sulfuric acid graphite intercalation compound (stage-1 GIC-H2SO4). In the second step of oxidation, the graphite oxide is synthesized when KMnO4 is added slowly. The third step begins when the mixture of water and H2O2 acts with the reaction system, which converts graphite oxide into GO. Meanwhile, MnO2 is synthesized between the GO layers, which is called the synthesis stage. The last step is converting GO/MnO2 to RGO/Mn3O4.

In addition, another oxidation process also occurs in this step. In the first step, the graphite blended with concentrated sulfuric acid is initially jet-black but it slowly turns deep blue as the reaction occurs. During the color variation, sulfuric acid is inserted into the layers of graphite to form stage-1 GIC-H2SO4.44 The theoretical chemical reaction rate of stage-1 GIC-H2SO4 depends on the electrochemical potential of the concentrated sulfuric acid in the reaction (Figure 8).54 In the experimental condition of modified Hummers’ method, stage-1 GIC-H2SO4 is formed within 3–5 min.

Figure 8.

Figure 8

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Schematics of the first step of conversion of bulk graphite into stage-1 GIC-H2SO4.

As reported, the oxidation process consists of two sequential steps, which can be partitioned by the liquid media in each step55

Oxidation-1 step

2.4. 2
2.4. 3

Oxidation-2 step

2.4. 4
2.4. 5

The chemical reaction mechanism of the second step is more complicated. The KMnO4 is dissolved slowly into the mixture and transformed to manganese heptoxide (Mn2O7) and manganese trioxide ion (MnO3+) in the medium of concentrated sulfuric acid. Because of the high wettability of graphite with MnO3+ in sulfuric acid and the larger interval distance of stage-1 GIC-H2SO4, the two types of manganic acids act as the oxidizing agents to intercalate relatively easily into the gallery of stage-1 GIC-H2SO4.44,54,55 Because of oxidation, stage-1 GIC-H2SO4 is densely covered with various oxygen-containing functional groups, such as epoxide, hydroxyl, carboxyl, and carbonyl groups, at both edges and basal planes of the layers. Meanwhile, the manganese of manganic acids is reduced to Mn2+. At this stage, the positively charged Mn2+ and MnO3+ favorably bind, via an electrostatic force, with the O atoms of the negatively charged oxygen-containing functional groups on the GO sheets, which is the basis for the homogeneous generation of nano-MnO2 in the GO layers. In the oxidation-1, the stage-1 GIC is converted into oxidized graphite (Figure 9), which is defined as pristine graphite oxide (PGO). Certainly, in the nearly 100% H2SO4 solvent, the above-mentioned compounds exist predominantly in their nonionized forms while water was added into acid, ionization takes place.53,56

Figure 9.

Figure 9

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Schematics of the conversion process of stage-1 GIC-H2SO4 into PGO.

When the reaction circumstance is changed by the deionized water, the third step starts. MnO4–, the main oxidant of oxidation-2, is formed in an acidic aqueous medium, which can be easily seen from the solvent color.46 In the stage of oxidation-2, the oxidant agent selectively oxidizes the edge and atomistic defects of PGO.28,46 Furthermore, the epoxide groups introduced in oxidation-1 are hydrolyzed in the aqueous acidic media to form additional hydroxyl groups.54 Because our experiments were conducted at room temperature, the oxidation of MnO4– with GO proceeds very slowly.35 In contrast, the formation of MnO2 shows more activity. Along with the addition of water, MnO3+ and Mn2O7 are immediately converted to MnO4– (eqs 4, 5). Next, dissociative MnO4– reacts with Mn2+ (eq 6) to form abundant nuclei within short time. These nuclei act as anchor sites for the crystal growth. In addition, the space of graphene layers restricts the size growth of MnO2 particles (Figure 10). Because of the above reasons, MnO2 nanoparticles are uniformly generated in the graphite oxide layers. According to classical Hummers’ method, the graphite oxides exposed to water are exfoliated to GO through a series of chemical transformations.46 In our experiment, the generation of MnO2 nanoparticles enlarges the graphite oxide layer spacing to dozens of nanometers, which is far larger than the lamellar spacing formed in classical Hummers’ method and is thus called further “exfoliation”. Moreover, the “exfoliated” GO layers, which are not randomly dispersed to water in our experiment, bond with MnO2 to form a novel three-dimensional “multilayer steamed bread” nanostructure. Because GO/MnO2 was heated, the oxygen-containing chemical bonds on the surface of GO in GO/MnO2 are broken, causing small molecules of water and carbon dioxide to escape.

Figure 10.

Figure 10

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Schematics of the third step from PGO into the GO + MnO2/oxidant-2 step.

In the last step, because GO/MnO2 was heated, the oxygen-containing chemical bonds on the surface of GO are broken, causing small molecules of water and carbon dioxide to escape. Meanwhile, nanosize MnO2 thermally converted to nanosize Mn3O4. The intermediate product GO/MnO2 transforms into the final product RGO/Mn3O4. In addition, the structure and property of composites remain unchanged.

In the condition of acidic solution

2.4. 6

At last, we discuss the effect of H2O2 in deionized water on the MnO2 formation. After the second step reaction with deionized water containing H2O2, the MnO3+ and Mn2O7 react with deionized water to form MnO4– (eqs 4, 5). Next, MnO4– reacts with H2O2 in the acidic solution to form Mn2+ (eq 7). Hence, we can control the MnO4– concentration in the acidic aqueous solution by adjusting the dosage of H2O2. According to eq 6, the molar quantity of MnO4– decides the production of MnO2. After the reaction, the redundant Mn2+ and H2O2 can be completely removed by centrifugal washing. The ratio of GO and MnO2 in GO/MnO2 finally decided the content of Mn3O4 in RGO/Mn3O4.

2.4. 7

3. Electrochemical Experiments

The main features of the “multilayer steamed bread” architecture are as follow: first, RGO supported by Mn3O4 nanoparticles has high specific surface areas, open porosity, and highly conducting, which satisfy double-layer capacitance requirements.57 Furthermore, its highly conductive nature is able to provide fast charge exchange with nickel foam (current collector). Second, the process of the faradaic redox pseudo-capacitance includes the surface adsorption, and the electrolyte ions react with active substance of the electrode. The space of RGO layers enables the intercalation of the electrolyte, the ions of which directly contact with nanoparticle Mn3O4. Nanosize Mn3O4 and RGO enhance its electronic conductivity and increase the number of redox points. The approach has the advantage of dual mechanisms, which lead to high performance of the electrochemical supercapacitor.

3. 8

For exploring the potential applications of in situ nanocomposites, supercapacitor electrodes were characterized by cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD). The tests were performed in 6 M KOH electrolyte and used a classical three-electrode system, under a potential window of 1.6 V (−0.8 to +0.8 V). First, in order to comparison purposes, nano-MnO2, GMH0, and RGMH0 were fabricated as electrodes to test (Figure 11). Because the electrical conductivity of RGO is much higher than GO and manganite, the electrical conductivity of RGMH0 is highest in the three samples (Figure 11c). The CV curves and GCD curves (Figure 12a,b) clearly show that the capacitance of RGMH0 is higher than nano-MnO2 and GMH0. The capacitance of GMH2.6 is higher than nano-MnO2, which can be attributed to pseudocapacitive contribution of GO and nanostructure of nanocomposites. Second, the results shown in figure indicate that the capacitance of the supercapacitor increases with the ratio of Mn3O4/RGO. According to the area of the CV curve (Figure 12a) and eq 8, we could infer that RGMH0 is the largest capacitance, followed by RGMH1.3 and RGMH2.6. The capacitance of composites (RGMH0 and RGMH1.3) combines double layer and pseudocapacitive contribution, which is different from the ideal rectangular shape of double-layer capacitors. Two sharp peaks come from the process of the pseudocapacitive material, which are easily distinguished. The redox peaks are the reason that there are the redox transitions of Mn between (Mn2+/Mn3+) and (Mn3+/Mn4+) in an alkaline medium (eq 9). This behavior like the electrode that porous Mn3O4 nanocrystal graphene electrodes.58 Instead, the redox peaks of RGMH1.3 and RGMH2.6 are smaller and smaller. The decreasing content of Mn3O4 is the reason for the weak peak during the ion exchange process. Another key factor is that the space between RGO, without being supported by enough Mn3O4 nanoparticles, collapse, reduces the specific surface area of RGO and wraps Mn3O4. The Mn3O4 wrapped by RGO slowly reacts with electrolyte. Especially, RGMH2.6 hardly ever include Mn3O4, which only belongs to double-layer capacitance. Moreover, during the process of GO being thermally reduced to RGO, parts of the GO could convert back to graphite without nanoparticles in our experiment. This is the main reason that electrochemical performance of RGMH2.6 is well below the theoretical value of graphene. The maximum values of capacitance for RGMH0 are 517.4 F/g at 10 mV/s, which are greater than RGMH1.3 (283.3 F/g) and RGMH2.6 (114.3 F/g) at the same scan speed. The electrochemical performance of RGMH0 nanocomposite could be ascribed to the interaction of the nanostructure of nano-Mn3O4 and RGO, where open-layered and well-dispersed Mn3O4 nanowire can provide sufficient ions to the Mn3O4. Finally, from the above analysis, we can know that the total capacitance of RGMH0 and RGMH1.3 is contributed by the double layer and pseudocapacitor, and the capacitance of RGMH2.6 only contain double-layer capacitance. Because the condition of chemical test and the electrode preparation of three samples is identical, we could consider that the value of double-layer capacitance of three samples is same. The value of pseudocapacitor of RGMH0 is the difference value of RGMH0 and RGMH2.6, which is 403.1 F/g. Similarly, the pseudocapacitor of RGMH1.3 is 169 F/g.

3. 9

Figure 11.

Figure 11

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(a) CV curves of RGMH0, GMH0, and nano-MnO2 at scan rate of 10 mV/s. (b) GCD curves of RGMH0, GMH0, and nano-MnO2 at 0.3 A/g. (c) EIS of RGMH0, GMH0, and nano-MnO2 at 0.3 A/g.

Figure 12.

Figure 12

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(a) CV curves of RGMH0, RGMH1.3, and RGMH2.6 at a scan rate of 10 mV/s. (b) RGMH0 at scan rates from 10 to 100 mV/s. (c) GCD curves of RGMH0, RGMH1.3, and RGMH2.6 at 0.3 A/g. (d) GCD curves of RGMH0. (e) Specific capacitance vs scan rate of RGMH0. (f) Cycling stability at 100 mV/s of RGMH0.

To get more information about the potential RGO and Mn3O4 nanocomposite supercapacitors, we carried out GCD measurements in the 6 M KOH at selected current density between 0.3 and 2 A/g and in the potential window range of −0.8 to +0.8 V. During the C/D steps, the charge curve of RGMH0 is almost symmetric to its discharge curve with a slight curvature (Figure 12d), which indicates the pseudocapacitive contribution along with the double-layer contribution. It is corresponding with the CV curve (Figure 12a). The curve (Figure 12c) manifests double layer and pseudocapacitance of RGMH1.3 and RGMH0. Cs is calculated as Cs = I × Δt/(ΔV × m) from the discharge curves, where I is the constant discharge current, Δt is the discharge time, and ΔV is the potential drop during discharge. The maximum value calculated from the curves is 438.7 F/g (based on the total mass of RGO and Mn3O4), which corresponds to the current density of 0.3 A/g. However, we found that tendency of the RGMH0 capacitance decreases with the scan rate or current density increasing. The possible reason for the capacitance decreasing is the electrolyte penetration in pores and defects of the structure, thus the number of ions is less available near the surface of the electrode. Then, during the annealing process of GO and MnO2 composite, the oxygen-containing chemical bonds on the surface of GO transform to small molecules of water and carbon dioxide to escape and the volume change of MnO2 converting to Mn3O4. It causes to weaken the nanostructure, which also affects its electronic conductivity. Moreover, the deformation of nano-Mn3O4 during the oxidation process may have caused the increasing number of defects formed in the connection point of Mn3O4 and RGO. The produced connecting defects could cause decreasing of the electronic and ionic conductivity, thus having great influence on the rate capability both of pseudocapacitive and the double-layer response. Furthermore, the capacitance stability of the RGMH0 composite evaluated is 77.5% after 1000 cycles at 100 mV/s (Figure 12f), which is ascribed to its structure that is further damaged.

4. Conclusions

RGO/Mn3O4 nanocomposites were prepared from natural graphite via modified Hummers’ method at room temperature without the use of templates or surfactants and simple thermal reduction at 190 °C. The method is a facile, low-cost, and industry-oriented technique. The integration of RGO and the needle-like Mn3O4 crystals endowed such composites with excellent electrochemical behaviors that are useful as electrode materials for supercapacitors. The electrochemical results show high performance including excellent specific capacitance and remarkable capacitance stability. The electric double-layer capacitance of RGO and the pseudocapacitance of nano-Mn3O4 between RGO layers has been achieved simultaneously in the “multilayer steamed bread”-like nanostructure. Moreover, the RGO and Mn3O4 of the nanostructure are beneficial for each other.

5. Experimental Section

5.1. Chemicals

Graphite powder (1000 mesh, 98%, Jinan) was purchased from NORMIC Co., Ltd. Analytically pure sulfuric acid (H2SO4, 98%), potassium permanganate (KMnO4), hydrogen peroxide (H2O2), concentrated phosphoric acid (H3PO4, 85%), sodium phosphate dibasic dihydrate (Na2HPO4·2H2O), ammonium persulfate [(NH4)2S2O8], and silver nitrate (AgNO3) were all obtained from Aladdin Industrial Corporation. All materials were used without further purification.

5.2. Synthesis of RGO/Mn3O4

The new process was developed in the current work for synthesis of RGO/Mn3O4, and the details of which are graphically shown in Figure 1. It can be seen that graphite powder (0.1 g) was slowly added to concentrated H2SO4 (10 mL) at room temperature while being continuously stirred. After 30 min, KMnO4 (0.45 g) was slowly added to the mixture under stirring while the mixture was cooled to 5 °C in an ice bath. Afterward, the mixture was heated back to 35 °C stirred for 3 h, and during the period, the color of the mixture was changed from black to green. The mixture was then dropwise added into deionized water (100 mL) containing 3% H2O2 under intense stirring at temperatures between 35 and 55 °C. The sediments were obtained and were centrifugally washed (4000 rpm) with deionized water until pH 7 was achieved in order to remove SO42– ions and extra metal ions. The solutions were sonicated for 2 h and then centrifuged at 4000 rpm for 40 min. The obtained sediments were dehydrated by using a freeze dryer, and one-tenth of the bottom sediments was taken out to remove agglomerated MnO2 (the agglomerated MnO2 of the GO surface fell off and deposited on the bottom in the function of centrifugal force). The dried samples of the GO/MnO2 powders were thermally treated at 190 °C in argon flow for 10 h and then cooled to room temperature. The final composites were called RGO/Mn3O4.

5.3. Fabrication the Electrochemical Measurement of RGO/Mn3O4

The RGO/Mn3O4 or GO/MnO2 and MnO2 mixed with polyvinylidene fluoride at a mass ratio of 95:5. Appropriate amount of N-methyl-2-pyrrolidone was added into the mixture under stirring to form a homogeneous and high-fluidity suspension liquid. Nickel foam that was used as current collectors (1.0 × 1.0 cm2) was soaked overnight in the suspension liquid until its color was turned black. After taking it out from the liquid, the nickel foam was dried at 80 °C for 10 h in a vacuum furnace and then cooled in vacuum to room temperature. The formed electrodes were then immersed into 6 M KOH solution for further tests.

The electrochemical properties of the electrodes prepared in this work were tested at room temperature by using a classical three-electrode system consisting of a platinum foil, a saturated calomel electrode, and a 6 M KOH solution as the counter electrode, the reference electrode, and the electrolyte, respectively. The CVs were measured using an Alalis CHI660B electrochemical workstation (Shanghai Alalis Instrument Company, China), and GCD was detected using a CHI1140C electrochemical workstation (Shanghai CH Instrument Company, China).

5.4. Measurement of Mn2+ Ions in Wastewater

Except for decreasing the additive amount of KMnO4 and removing the H2O2, the new experiment was basically identical to the above described (Section 5.2). Briefly, graphite powder (0.1 g) was slowly added to concentrated H2SO4 (10 mL) at room temperature stirring. After 30 min, KMnO4 (0.01 g) was slowly added to the mixture. After the reaction was completed, wastewater was collected during the GO filtration from the reaction system. The sediments were centrifugally washed (4000 rpm) with deionized water and dehydrated by a freeze dryer. Then, 6 mL of the wastewater was diluted to 294 mL and stirred at 50 °C for 10 min. The diluted wastewater was then added with 15 mL of concentrated H3PO4, 5 g of Na2HPO4·2H2O, 3.5 g of (NH4)2S2O8, and 3 mL of 2% AgNO3 solution under stirring. The resulting mixture was boiled for 1 min.

5.5. Characterization

The crystallographic structures of GO/MnO2 and RGO/Mn3O4 samples were analyzed by powder XRD measurements (X’PertPro, PANalytical B.V Company, Netherlands) with Cu Kα radiation (λ ≈ 1.54 Å). Their metallography and particle distribution were observed using SEM equipped with an SSX-550 field-emission SEM meter (Zeiss Microscopes Company, Germany) and TEM equipped with a G20 TEM meter (FEI Company, the USA). Their Raman spectra were measured using an HR800 Raman spectrometer at 633 nm (Horiba Jobin Yvon LabRAM). Chemical compositions were analyzed by using an SC-144DR infrared sulfur–carbon analyzer (LECO, The USA) and a Z-2300 flame atomic absorption spectrometer (Hitachi, Japan). The components of composites are identified XPS (ESCALAB 250, Thermo VG Company, the USA).

Acknowledgments

The National Natural Science Foundation of China (no. 51704065), China Postdoctoral Science Foundation (no. 2018M630297), and the Fundamental Research Funds for the Central Universities of China (no. N170204013).

The authors declare no competing financial interest.

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