One−Step Synthesis of Popcorn−Carbon/Co₃O₄ Composites as High−Performance Supercapacitor Electrodes

Abstract

In this study, a novel assisted liquid−phase plasma electrolysis was developed to realize one−step synthesis of popcorn biomass−derived porous carbon/cobalt tetroxide (popcorn−carbon/Co₃O₄) composites, effectively improving the structural stability and conductivity of Co₃O₄. The phase structure, morphologies, chemical composition, and weight ratio of the as−prepared popcorn−carbon/Co₃O₄ composites were systematically analyzed. The results of X−ray diffraction (XRD), Raman spectrometer, Fourier infrared spectrometer (FTIR), X−ray photoelectron spectrometer (XPS), and thermogravimetry analyzer (TG) proved the synthesis of the popcorn−carbon/Co₃O₄ composites. Co₃O₄ nanoparticles were distributed relatively uniformly on the popcorn−carbon surface. The electrochemical properties of the popcorn−carbon/Co₃O₄ composite electrode materials were analyzed for exploring the influence of different Co/C ratios on the electrochemical properties of composites. The results showed that the popcorn−carbon/Co₃O₄ composite electrode materials prepared under 200:1 mass ratio of Co(NO₃)₂·6H₂O and popcorn−carbon possessed a specific capacitance and specific capacity of almost 1264 F/g (594 C/g) at a current density of 1 A/g, exhibiting a better electrochemical property. The efficient, fast, and novel assisted liquid−phase plasma electrolysis provides a new method for the preparation of composite electrode materials on the supercapacitors.

1. Introduction

With the development of science and technology, the demand for energy across the whole of society is becoming higher and higher. At present, traditional fossil energy occupies a large proportion in the entire energy supply, but the greenhouse gas generated by the fossil energy causes certain harm for the environment. Therefore, green and clean energy has attracted more and more attention from researchers [1,2]. However, if the electrical energy converted by the above energy cannot be used in time, the excess electrical energy must be stored with the energy storage devices [3]. Therefore, it is an important research focus for the current scientific researchers to develop the energy storage devices with stable properties [4].

The electrode materials play a particularly important role in the charging and discharging process of electrochemical energy storage. Among them, the transition metal oxide Co₃O₄ is widely used as the electrode material of supercapacitors due to its ultra−high theoretical specific capacitance (~3560 F/g) [5]. However, the low conductivity and the poor structural stability of pure Co₃O₄ during charging and discharging reduces its specific capacitance and cycle stability, limiting its application as the electrode material in the supercapacitors [5,6]. At present, an effective strategy is to combine Co₃O₄ with the materials with good conductive property to improve Co₃O₄ overall performance. Carbon materials possess good conductivity, and combining them with Co₃O₄ can effectively improve the structural stability and conductivity of Co₃O₄. Qiu et al. prepared a novel Co₃O₄/nitrogen−doped carbon composite by annealing a cobalt−based metal−organic framework precursor, exhibiting good electrochemical behavior [7]. Sun et al. successfully synthesized carbon/Co₃O₄ composite mesoporous hollow spheres, then composited it with graphene to prepare a composite film. The unique sandwich structure and mesoporous structure could provide more channels for the transport of the electrolyte, thus, displaying better electrochemical performance [8]. Sun et al. selected the cotton−derived carbon fiber as the biological carrier and carbon source, and a core−shell tubular composite (NCM) composed of two−dimensional nanosheets coated with porous carbon fibers was prepared in an ethanol solution reaction system. The results showed that the electrochemical property of NCM was much higher than that of pure Co₃O₄, indicating that porous carbon fiber as a template and carrier of Co₃O₄ was beneficial to reduce the agglomeration of carbon fiber and improve the electron transfer between the active material and the electrolyte [9]. In addition, ramie−derived carbon with a porous hollow tube structure was obtained by pre−carbonization, and then a Co₃O₄@C@Co₃O₄ tubular composite material (CNM−1) with a high specific capacitance was prepared by the solvothermal post−carbonization method. It showed that CNM−1 displayed a specific capacitance of 1280.6 F/g at 1 A/g in the three−electrode system, and still maintained a capacity of 96.89% after 15,000 cycles [10]. Tao et al. deposited Co₃O₄ on the carbon cloth via an electrochemical oxidation method, and the specific capacitance of the sample could reach 226.1 C/g [11]. Du et al. synthesized Co₃O₄ nanosheets on the graphene by a hydrothermal method. The specific capacitance of graphene/Co₃O₄ composite material prepared was 894 F/g at a current density of 1 A/g [12].

The above research provides the theoretical guidance for the preparation of carbon−based Co₃O₄ composites with good performance. In this study, popcorn biomass−derived carbon and Co₃O₄ were composited to prepare a supercapacitor electrode material with both the porous structure of carbon materials and the high specific capacitance of Co₃O₄. At present, the methods of combining carbon materials with Co₃O₄ mainly include the hydrothermal reaction, the electrochemical deposition, and the chemical vapor deposition, but these methods have complicated processes, high costs, or insufficient binding of Co₃O₄ on the biomass−derived carbon matrix [13,14]. Therefore, it is urgent to seek an efficient, environmentally friendly, and low−cost technology for realizing a composite of biomass−derived porous carbon and Co₃O₄. In this study, a novel assisted liquid−phase plasma electrolysis [15,16,17] was proposed to realize the high−efficiency composite of biomass−derived porous carbon and Co₃O₄. Assisted liquid−phase plasma electrolysis was developed on the basis of chemical vapor deposition (CVD), electrodeposition, and traditional cathode plasma electrolysis. Compared with the traditional electrolysis, the assisted liquid−phase plasma electrolysis process can produce the thermal, mechanical, and physical−chemical effects, obtaining a unique microstructure of composites [15]. The plasma effect generated in the assisted liquid−phase plasma electrolysis process increases the number of nucleation points and promotes the uniformity of nucleation, thereby achieving an efficient, rapid, and one−step synthesis of Co₃O₄ on biomass−derived porous carbon.

In this study, assisted liquid−phase plasma electrolysis was used to prepare Co₃O₄ on the popcorn biomass−derived porous carbon (popcorn−carbon) to realize the synthesis of popcorn−carbon/Co₃O₄ composites. The influence of different concentration ratios of Co(NO₃)₂·6H₂O and popcorn−carbon on the phase composition, morphologies, and electrochemical properties of popcorn−carbon/Co₃O₄ composites were systematically investigated. The results showed that the prepared popcorn−carbon/Co₃O₄ composite electrode materials with the 200:1 mass ratio of Co(NO₃)_2·6H₂O to popcorn−carbon possessed a specific capacitance and specific capacity of as high as ~1264 F/g (594 C/g) at a current density of 1 A/g, exhibiting better electrochemical properties. The novel assisted liquid−phase plasma electrolysis technology adopted in this study realized the efficient, rapid, and one−step synthesis of popcorn−carbon/Co₃O₄ composites, and provides a new method for the preparation of composite electrode materials on the supercapacitors.

2. Experiment

2.1. Experimental Reagents

Cobalt nitrate·hexahydrate (Co(NO₃)₂·6H₂O, 99%, AR, Macklin), sodium hydroxide (NaOH, 99%, AR, Sinopharm), and sodium nitrate (NaNO₃, 99%, AR, Sinopharm) were purchased from Beijing Sinopharm Chemical Reagent Co. Ltd., Beijing, China. Potassium hydroxide (KOH, pure superior grade, Sinopharm) was purchased from Qingdao Qingke Sail Biotechnology Co., Ltd., Qingdao, China.

2.2. Preparation of Popcorn Biomass−Derived Porous Carbon (Popcorn−Carbon)

First, the popcorn that was washed and dried was carbonized at 500 ℃ for 2 h at a heating rate of 5 K/min in a nitrogen atmosphere. Herein, the used popcorn means the well−known popped, white, and edible food made from the corn kernels. Second, the above pre−carbonized carbon and KOH were mixed at a mass ratio of 1:3 and further carbonized in a tube furnace at 800 °C for 2 h in a nitrogen atmosphere. Finally, the obtained samples were washed several times with hydrochloric acid, ethanol, and deionized water in sequence, then dried and ground. The samples obtained were the popcorn biomass−derived porous carbon (popcorn−carbon).

2.3. Synthesis of Popcorn Biomass−Derived Porous Carbon/Cobalt Tetroxide Composites (Popcorn−Carbon/Co₃O₄)

In current work, three sets of contrasting experiments were carried out. The samples with a mass ratio of Co(NO₃)₂·6H₂O and biomass−derived carbon of 100:1 were named as popcorn−carbon/Co₃O₄−1, the samples with a mass ratio of Co(NO₃)₂·6H₂O and biomass−derived carbon of 200:1 were named as popcorn−carbon/Co₃O₄−2, and the samples with a mass ratio of Co(NO₃)₂·6H₂O and biomass−derived carbon of 300:1 were named as popcorn−carbon/Co₃O₄−3. Taking the popcorn−carbon/Co₃O₄−1 samples as an example, the specific preparation process of the popcorn−carbon/Co₃O₄−1 composites was as follows: First, 0.1 g NaOH, 1.0 g NaNO₃, and 2.0 g Co(NO₃)₂·6H₂O were mixed and dissolved in 50 mL deionized water. Then, the above mixture was added dropwise into Co(NO₃)₂·6H₂O aqueous solution of 250 mL. Finally, 0.02 g popcorn−carbon was added into the above mixed solution and sonicated for 30 min, and the precursor solution for preparing popcorn−carbon/Co₃O₄ composites was obtained. In the assisted liquid−phase plasma electrolysis process, the graphite electrode was selected as the anode, the U−shaped copper electrode was selected as the cathode, and the above precursor solution was the deposition solution. The high−frequency switching DC power supply was turned on, and the deposition solution was treated for 180 s at a deposition voltage of 250 V. Finally, the popcorn−carbon/Co₃O₄ composite powders were collected from the deposition solution by the vacuum filtration system, and dried for 2 h in a vacuum oven at 80 °C. For comparison, pure Co₃O₄ was also prepared via assisted liquid−phase plasma electrolysis without popcorn−carbon under similar processing procedures, as above−mentioned. The schematic of the preparation process for the popcorn−carbon/Co₃O₄ composites is shown in Figure 1.

2.4. Characterization of Structure and Properties

2.4.1. Structure Characterization

The phase composition of the popcorn−carbon/Co₃O₄ composites were analyzed by a X−ray diffractometer (XRD, 7000X, Shimadzu, Kyoto, Japan). The surface morphologies of popcorn−carbon/Co₃O₄ composites were characterized using a scanning electron microscope (SEM, JSM−7610F, JEOL, Akishima, Japan) and transmission electron microscope (TEM, JEM−2100F, JEOL, Akishima, Japan). The bonding state of popcorn−carbon/Co₃O₄ composites was analyzed by a Fourier infrared spectrometer (FTIR, Nicolet 6700, Thermo Fisher, Waltham, MA, USA). The order degree of the popcorn−carbon/Co₃O₄ composites was analyzed by a Raman spectrometer (LabRAM HR Evolution, HORIBA, Longjumeau, France). An X−ray photoelectron spectrometer (XPS, ESCALAB 250 XI, Thermo Scientific, Horsham, UK) was used to analyze the combined state of different elements in the popcorn−carbon/Co₃O₄ composites. The weight percentage of Co₃O₄ in the popcorn−carbon/Co₃O₄ composites was explored by a thermogravimetric analyzer (TG, Sta 449F3, Netzsch, Selb, Germany). N₂ adsorption–desorption isotherms and pore distribution plots of popcorn−carbon and popcorn−carbon/Co₃O₄−2 composites were analyzed using physical adsorption equipment (ASAP 2460, Micromeritics, Norcross, GA, USA).

2.4.2. Characterization of Electrochemical Properties

The electrochemical properties of popcorn−carbon/Co₃O₄ composites were characterized using an Electrochemical Workstation (CHI660E, Chenhua, Shanghai, China). In the three−electrode system, the working electrode was foamed nickel coated with the as−prepared popcorn−carbon/Co₃O₄ composites; the counter electrode was a high−purity platinum sheet; the reference electrode was a saturated calomel electrode; and the electrolyte was a 6 M KOH solution. The preparation of the working electrode was displayed as below: 80 wt% of the active material (popcorn−carbon/Co₃O₄ composites), 10 wt% of acetylene black, 10 wt% of polyvinylidene fluoride (PVDF), and an appropriate amount of N−methylpyrrolidone were mixed to obtain a homogeneous slurry. The slurry was then brushed on the Ni foam (~1 × 1 cm²), dried at 60 °C overnight in the vacuum drying oven, and pressed into a thin sheet with mass loading of approximately 2.5 mg/cm² of active material. The specific capacitance of popcorn−carbon/Co₃O₄ composites was calculated according to Equations (1) and (2) [6]:

C_m = (I × ∆t)/(m × ∆V)

(1)

C_q = (I × ∆t)/m

(2)

Among them, C_m (F/g) and C_q (C/g) represents the specific capacitance and specific capacity, respectively; I (A) signified the constant discharge current; Δt (s) denotes the discharge time; ΔV (V) expresses the operating voltage window; and m (g) represents the quality of composites coated on the foamed nickel.

3. Results and Discussion

3.1. XRD Analysis

The phase composition of as−prepared popcorn−carbon/Co₃O₄ composites was analyzed, and Figure 2 shows the XRD patterns of the popcorn−carbon, popcorn−carbon/Co₃O₄−1, popcorn−carbon/Co₃O₄−2, and popcorn−carbon/Co₃O₄−3 composites, respectively. From the XRD spectrum of popcorn−carbon, as shown in Figure 2a, obvious diffraction peaks at 2θ = 20.6°, 43.3° could be observed corresponding to the carbon peak. From the XRD spectrum of the popcorn−carbon/Co₃O₄−1 composites (Figure 2b), it could be seen that there were obvious diffraction peaks at 2θ = 37.3°, 59.2°, and 65.6°, corresponding to the standard spectrum of Co₃O₄ (JCPDS 42−1467), indicating that Co₃O₄ was loaded on the popcorn−carbon surface [5]. Figure 2c shows the XRD spectrum of the popcorn−carbon/Co₃O₄−2 composites. Compared with the popcorn−carbon/Co₃O₄−1 composites, the peak positions were similar to that of the popcorn−carbon/Co₃O₄−1 composites, but the carbon diffraction peak was significantly reduced, indicating that the loading of Co₃O₄ in the popcorn−carbon/Co₃O₄−2 composites was greater than that of the popcorn−carbon/Co₃O₄−1 composites. Similarly, the peak positions of popcorn−carbon/Co₃O₄−3 were similar to those of the popcorn−carbon/Co₃O₄−1 and popcorn−carbon/Co₃O₄−2 composites, and there were obvious diffraction peaks at 2θ = 37.3°, 59.2°, and 65.6°, indicating that Co₃O₄ had been successfully loaded on the popcorn−carbon surface.

3.2. Surface Morphologies

The surface morphologies of the as−prepared popcorn−carbon and popcorn−carbon/Co₃O₄ composites were characterized, as shown in Figure 3. From the surface morphology of the popcorn−carbon (Figure 3a), it could be observed that the surface of popcorn−carbon was relatively smooth, and there was an obvious pore structure, which was conductive to the electron transfer between the active material and the electrolyte, providing a large number of active sites for the subsequent loading of Co₃O₄ [18]. Figure 3b shows the SEM morphology of the popcorn−carbon/Co₃O₄−1 composites. As shown in Figure 3b, Co₃O₄ in the form of nanoparticles was distributed on the popcorn−carbon surface. Co₃O₄ particles were unevenly distributed on the popcorn−carbon surface, and some of the popcorn−carbon surface was exposed. Figure 3c shows the SEM morphology of the popcorn−carbon/Co₃O₄−2 composites. As shown in Figure 3c, the Co₃O₄ nanoparticles were distributed relatively uniformly on the popcorn−carbon surface, and Co₃O₄ nanoparticles were thin and uniform without the extra aggregation. Figure 3d shows the SEM morphology of the popcorn−carbon/Co₃O₄−3 composites. As shown in Figure 3d, the Co₃O₄ nanoparticles were preferentially deposited on the pores of popcorn−carbon, and the particles were larger and agglomerated. In addition, the uneven distribution of Co₃O₄ particles on the popcorn−carbon surface resulted in the exposure of the carbon matrix.

In order to investigate the surface morphology of the popcorn−carbon/Co₃O₄−2 composites in detail, popcorn−carbon/Co₃O₄−2 composites were further characterized by TEM. As shown in Figure 3e, there were many pores on the popcorn−carbon surface, which was consistent with the SEM results (Figure 3a), and these pore structures provided a path for the electron migration between the active material and the electrolyte. Figure 3f shows the TEM morphology of the popcorn−carbon/Co₃O₄−2 composites. As shown in Figure 3f, Co₃O₄ in the form of nanoparticles was evenly distributed on the popcorn−carbon surface. From the selected area electron diffraction (SAED) pattern inset in Figure 3f, it could be observed that the as−prepared composites presented polycrystalline diffraction rings, corresponding to the crystal planes of (222), (311), (220), and (111), which were consistent with the crystal planes of Co₃O₄ [5], indicating that the oxide loaded on the popcorn−carbon surface was Co₃O₄.

3.3. Composition Analysis

Based on the above analysis, the popcorn−carbon/Co₃O₄−2 composites prepared with a mass ratio of Co(NO₃)₂·6H₂O to popcorn−carbon of 200:1 displayed a good phase structure and surface morphology. Therefore, the compositions of the popcorn−carbon and popcorn−carbon/Co₃O₄−2 composites were further analyzed. Figure 4a shows the Raman spectra of the popcorn−carbon and popcorn−carbon/Co₃O₄−2 composites. As shown in Figure 4a, the D and G characteristic peaks of two samples were observed at 1326 cm⁻¹ and 1571 cm⁻¹, respectively, indicating the existence of carbon in the popcorn−carbon and popcorn−carbon/Co₃O₄−2 composites. In addition to the D and G characteristic peaks, the Raman spectrum of the popcorn−carbon/Co₃O₄−2 composites showed an A_1g vibration peak of Co₃O₄ at 664 cm⁻¹, indicating that Co₃O₄ particles were loaded on the popcorn−carbon surface [19]. Figure 4b shows the FTIR spectra of the popcorn−carbon and popcorn−carbon/Co₃O₄−2 composites. As shown in Figure 4b, the peak at 3445 cm⁻¹ corresponded to the stretching vibration of the –OH group of popcorn biomass−derived porous carbon or remained water [20]. In addition, the peaks of two samples at 1635 cm⁻¹, 1380 cm⁻¹, and 1104 cm⁻¹ were ascribed to the stretching vibrations of C=O, C–O, and C–O–C in the popcorn−carbon, respectively [20,21]. Unlike the FTIR spectrum of popcorn−carbon, there were two peaks at 662 cm⁻¹ and 568 cm⁻¹ for the popcorn−carbon/Co₃O₄−2 composites, corresponding to the tensile vibration modes of tetrahedral coordinated Co²⁺−O and octahedral coordinated Co³⁺−O of Co₃O₄, respectively, indicating the existence of Co₃O₄ in the popcorn−carbon/Co₃O₄−2 composites [22]. Figure 4c shows the XPS spectrum of popcorn−carbon/Co₃O₄−2 composites. As shown in Figure 4c, popcorn−carbon/Co₃O₄−2 composites consisted of C, O, and Co elements, further indicating the existence of Co₃O₄ in the as−prepared popcorn−carbon/Co₃O₄−2 composites. In order to analyze the weight of Co₃O₄ in the as−prepared popcorn−carbon/Co₃O₄−2 composites, the popcorn−carbon and popcorn−carbon/Co₃O₄−2 composites were analyzed by a thermogravimetric analyzer. As shown in Figure 4d, the weight of popcorn−carbon decreased slowly before 450 °C, and reduced rapidly after 450 °C until burn−out, mainly due to the oxidation reaction of carbon. For the popcorn−carbon/Co₃O₄−2 composites, the entire weight loss was relatively slow, and the final weight retention rate was 74.8%, that is, the weight proportion of Co₃O₄ in the popcorn−carbon/Co₃O₄−2 composites was 74.8%. Figure 4e,f shows the N₂ adsorption–desorption isotherms and BJH pore distribution plots of the popcorn−carbon and popcorn−carbon/Co₃O₄−2 composites, respectively. The two samples exhibited different isotherm types, indicating the different capillary condensation and pore structure. For pristine popcorn−carbon, it was classified as a typical type I isotherm, according to the International Union of Pure and Applied Chemistry (IUPAC) classification. The isotherm typically increased with increasing relative pressure P/Po at 0.00–0.017, implying the microporous features, which was mainly at 1 nm (Figure 4f). Compared to pristine popcorn−carbon, the popcorn−carbon/Co₃O₄−2 composites illustrated typical type IV isotherm with a hysteresis loop from 0.45 to 1.0, indicating the existence of mesopores, which were mainly at 2 nm and 4 nm (Figure 4f). Figure 4e,f shows that pristine popcorn−carbon had high S_BET = 1255.3 m²/g and pore volume = 0.60 cm³/g compared to the popcorn−carbon/Co₃O₄−2 composites (S_BET = 205.6 m²/g and pore volume = 0.29 cm³/g).

3.4. Electrochemical Properties

The electrochemical properties of the as−prepared popcorn−carbon and popcorn−carbon/Co₃O₄ composites were investigated. Figure S1a in the Supplementary Materials shows the cyclic voltammetry (CV) curves of popcorn−carbon. As shown in Figure S1a, the CV curves of popcorn−carbon exhibited a nearly rectangular shape at a scan rate of 5 mV/s to 200 mV/s, and each curve remained almost the same shape, indicating a good rate capability. In addition, as the current density increased, the peak area also increased, but there was no obvious redox peak, indicating that popcorn−carbon was an ideal electric double−layer capacitor. Figure S1b shows the galvanostatic charge/discharge (GCD) curves of popcorn−carbon at different current densities. As shown in Figure S1b, the GCD curves of popcorn−carbon were close to two straight lines, displaying an almost symmetrical triangular shape, suggesting a typical electric double−layer capacitance behavior [23]. According to Equation (1), the specific capacitances calculated were 130 F/g, 129 F/g, 119 F/g, 111 F/g, 106 F/g, and 100 F/g at current densities of 0.5 A/g, 1 A/g, 2 A/g, 5 A/g, 10 A/g, and 20 A/g, respectively. The area of all electrodes tested was ~1 × 1 cm², and the mass of active material was ~2.5 mg/cm². It could be observed that the specific capacitances decreased as the current density increased. It was because the ion utilization rate was high at the low current density. However, when the current density increased, the ion movement was limited due to the constraint of time, resulting in the decrease of specific capacitance [24,25].

Figure 5a shows the CV curves of Co₃O₄ at different scan rates, and it existed obvious oxidation and reduction peaks near 0.35–0.45 V and 0.05–0.2 V, respectively. Figure 5b shows the GCD curves of Co₃O₄ at different current densities, and the specific capacitances and specific capacities calculated were 533 F/g (250 C/g), 521 F/g (245 C/g), 469 F/g (220 C/g), 419 F/g (197 C/g), 356 F/g (167 C/g), and 277 F/g (130 C/g) at current densities of 0.5 A/g, 1 A/g, 2 A/g, 5 A/g, 10 A/g, and 20 A/g, respectively. The area of all electrodes tested was ~1 × 1 cm², and the mass of active material was ~2.5 mg/cm².

Figure 5c shows the CV curves of the popcorn−carbon/Co₃O₄−1 composites. As shown in Figure 5c, the curves maintained nearly the same shape under different scanning rates, and there were obvious oxidation and reduction peaks near 0.35–0.45 V and 0.05–0.2 V, respectively. The related reversible oxidation−reduction reactions are expressed as follows:

Co₃O₄ + H₂O + OH⁻ = 3CoOOH + e⁻

(3)

CoOOH + OH⁻ = CoO₂ + H₂O + e⁻

(4)

Figure 5d shows the GCD curves of the popcorn−carbon/Co₃O₄−1 composites. As shown in Figure 5d, according to Equations (1) and (2), the specific capacitances and specific capacities calculated were 878 F/g (413 C/g), 694 F/g (326 C/g), 613 F/g (288 C/g), 522 F/g (245 C/g), 464 F/g (218 C/g), and 370 F/g (174 C/g) at current densities of 0.5 A/g, 1 A/g, 2 A/g, 5 A/g, 10 A/g, and 20 A/g, respectively. The area of all electrodes tested was ~1 × 1 cm², and the mass of active material was ~2.5 mg/cm². Compared with pure Co₃O₄, the specific capacitances of the as−prepared popcorn−carbon/Co₃O₄−1 composites increased to a certain extent, indicating that Co₃O₄ was successfully loaded on the surface of the popcorn−carbon, which was also consistent with the results of XRD, SEM, TEM, Raman, FTIR, XPS, and TG. Similarly, as the current density increased, the specific capacitances of the popcorn−carbon/Co₃O₄−1 composites decreased to a certain extent.

Figure 5e shows the CV curves of the popcorn−carbon/Co₃O₄−2 composites. As shown in Figure 5e, it displayed a similar CV curve shape compared to the popcorn−carbon/Co₃O₄−1 composites, and the curves kept the same shape under different scan rates, displaying a better rate capability. Figure 5f shows the GCD curves of the popcorn−carbon/Co₃O₄−2 composites. As shown in Figure 5f, it could be observed that a larger platform appeared during the discharge process. The specific capacitances and specific capacities calculated were 1412 F/g (664 C/g), 1264 F/g (594 C/g), 1037 F/g (487 C/g), 853 F/g (401 C/g), 740 F/g (348 C/g), and 626 F/g (294 C/g) at current densities of 0.5 A/g, 1 A/g, 2 A/g, 5 A/g, 10 A/g, and 20 A/g, respectively. The area of all electrodes tested was ~1 × 1 cm², and the mass of active material was ~2.5 mg/cm². Compared with popcorn−carbon/Co₃O₄−1 composites, the specific capacitances of the as−prepared popcorn−carbon/Co₃O₄−2 composites were greatly improved. Similarly, the specific capacitances of the popcorn−carbon/Co₃O₄−2 composites decreased to a certain extent with the increase in current density.

Figure 5g shows the CV curves of the popcorn−carbon/Co₃O₄−3 composites. As shown in Figure 5g, compared to the popcorn−carbon/Co₃O₄−1 and popcorn−carbon/Co₃O₄−2 composites, it exhibited a similar CV curve shape, and there were obvious oxidation and reduction peaks near 0.35–0.45 V and 0.05–0.2 V, respectively. Figure 5h shows the GCD curves of the popcorn−carbon/Co₃O₄−3 composites. The specific capacitances and specific capacities calculated were 957 F/g (450 C/g), 864 F/g (406 C/g), 738 F/g (347 C/g), 586 F/g (275 C/g), 500 F/g (235 C/g), and 400 F/g (188 C/g) at current densities of 0.5 A/g, 1 A/g, 2 A/g, 5 A/g, 10 A/g, and 20 A/g, respectively. The area of all electrodes tested was ~1 × 1 cm², and the mass of active material was ~2.5 mg/cm². Compared with pure Co₃O₄, the specific capacitances of the as−prepared popcorn−carbon/Co₃O₄−3 composites increased to a certain extent, mainly due to the excellent electrical conductivity of popcorn−carbon during the charging and discharging process.

The electrochemical properties of the as−prepared Co₃O₄ and popcorn−carbon/Co₃O₄ composites under three Co/C mass ratios were compared. Figure 6 shows the CV and GCD curves of the as−prepared Co₃O₄ and popcorn−carbon/Co₃O₄ composites with different Co/C mass ratios. As shown in Figure 6a, the CV curves of Co₃O₄, popcorn−carbon/Co₃O₄−1, popcorn−carbon/Co₃O₄−2, and popcorn−carbon/Co₃O₄−3 composites were similar in shape, and there were obvious oxidation and reduction peaks. However, obviously, the CV curve area of the popcorn−carbon/Co₃O₄−2 composites were larger than those of the popcorn−carbon/Co₃O₄−3 and popcorn−carbon/Co₃O₄−1 composites, and the CV curve area of Co₃O₄ was the smallest. Figure 6b shows the GCD curves of Co₃O₄, popcorn−carbon/Co₃O₄−1, popcorn−carbon/Co₃O₄−2, and popcorn−carbon/Co₃O₄−3 composites at a current density of 1 A/g. As shown in Figure 6b, the four curves were similar in shape, and the platform appeared during the discharge process. The specific capacitances and specific capacities of Co₃O₄, popcorn−carbon/Co₃O₄−1, popcorn−carbon/Co₃O₄−2, popcorn−carbon/Co₃O₄−3 composites at a current density of 1 A/g were 521 F/g (245 C/g), 694 F/g (326 C/g), 1264 F/g (594 C/g), 864 F/g (406 C/g), respectively. Compared with the specific capacitance of Co₃O₄, the specific capacitances of three composites were all greatly improved, and the specific capacitance of popcorn−carbon/Co₃O₄−2 composites was the highest, which was consistent with the results of the CV curves (Figure 6a). This was mainly attributed to the fact that the Co₃O₄ nanoparticles in the popcorn−carbon/Co₃O₄−2 composites were more evenly distributed on the popcorn−carbon surface. In addition, the synergistic effect originated from the large specific surface area, the excellent electrical conductivity of the popcorn biomass−derived porous carbon, and the faradaic behavior of Co₃O₄.

In order to understand the dominant storage mechanism in the composite electrode (capacitive or faradaic/battery−type), the parameter b needs to be calculated. The relationship between the current density (i) and scan rate (ν) could be described by the following equations [26,27]

$$\mathrm{i}\left(\mathsf{\nu }\right)={\mathrm{i}}_{\mathrm{cap}}+{\mathrm{i}}_{\mathrm{dif}}=\mathrm{a}\times {\mathsf{\nu }}^{\mathrm{b}}$$

(5)

$$\log \left(\mathrm{i}\right)= \log \left(\mathrm{a}\right)+\mathrm{blog} \left(\mathsf{\nu }\right)$$

(6)

where a and b are variable parameters. Parameter b values could be estimated from the slope of the log(i) vs. log(ν) plot. The calculated parameter b values at 0.37 V were 1.00, 0.58, 0.72, 0.67, and 0.65 for popcorn−carbon, Co₃O₄, popcorn−carbon/Co₃O₄−1, popcorn−carbon/Co₃O₄−2, and popcorn−carbon/Co₃O₄−3 electrodes, respectively, as shown in the inset of Figure 6c. The b value of the popcorn−carbon was 1.00, indicating the fast surface redox reaction and charge/discharge process inherent to electric double−layer capacitors (EDLC). The b values of the popcorn−carbon/Co₃O₄ electrodes were in 0.5–0.8, indicating diffusion−controlled kinetics with dominant faradaic (battery−type) behavior. In addition, the popcorn−carbon/Co₃O₄−2 composites exhibited a superior cycling stability with 86.2% capacitance retention for 5000 cycles at 10 A/g, as shown in Figure 6d.

Most of the previous research achieved relatively high specific capacitance for Co₃O₄/carbon composites as electrodes for supercapacitors, and the results are summarized in

Table 1. Comparison of the electrochemical performance of Co₃O₄/carbon electrodes.

Composite	Electrode Configuration	Current Density (A/g)	Electrolyte	Capacitance (F/g)	Ref.
Graphene/Co3O4	Three-electrode	1.0	6 M KOH	570	[5]
Co3O4/RGO	Three-electrode	1.0	2 M KOH	1138.11	[28]
Co3O4@NGC	Three-electrode	1.0	3 M KOH	688.3	[29]
3DPC/Co3O4	Three-electrode	1.0	3 M KOH	423	[19]
Co3O4/rGO−C	Three-electrode	1.0	6 M KOH	709.1	[30]
Co3O4−rGO	Three-electrode	0.5	6 M KOH	1260.6	[31]
Co3O4/RGO	Three-electrode	1.0	2 M KOH	271.7	[32]
Co3O4 NCs	Three-electrode	1.0	6 M KOH	865	[33]
Co3O4@SrGO	Three-electrode	1.0	1 M KOH	406	[34]
popcorn− carbon/Co3O4	Three-electrode	1.0	6 M KOH	1264	Current work

. The above synthesized methods mainly included ball−mill, hydrothermal, and dipping. The above preparation processes were relatively complex and time−consuming, and the as−prepared composites needed to be annealed for several hours at 400–600 °C in order to obtain the targeted transition−metal oxide. In this study, assisted liquid−phase plasma electrolysis is efficient and rapid and can realize one−step synthesis of composites. In addition, the as−prepared popcorn−carbon/Co₃O₄ composites exhibited a specific capacitance of 1264 F/g (594 C/g) at 1 A/g and remained at 86.2% of the initial capacity at 10 A/g for 5000 cycles.

3.5. Deposition Mechanism of Popcorn−Carbon/Co₃O₄ Composites

Based on the above analysis, it could be observed that the popcorn−carbon/Co₃O₄ composites prepared by combining Co₃O₄ with popcorn biomass−derived porous carbon possessed a high specific capacitance. The plasma arc generated during the assisted liquid−phase plasma electrolysis process played a key role in the formation of popcorn−carbon/Co₃O₄ composites [35]. Therefore, it was of great significance to investigate the physical/chemical reactions in the assisted liquid−phase plasma electrolysis to establish the deposition mechanism of the popcorn−carbon/Co₃O₄ composites. The schematic of the deposition process of the popcorn−carbon/Co₃O₄ composite is shown in Figure 7. Co(NO₃)₂·6H₂O and NaOH produced Co(OH)₂ during the mixing process, and the Co²⁺ ions produced by the ionization of unreacted Co(NO₃)₂ moved to the cathode under an electric field and gathered near the cathode, where it reacted with the retained OH^_ after the H₂O ionization to form Co(OH)₂. The Co(OH)₂ in the precursor solution and the Co(OH)₂ produced in the electrolyte were dehydrated to form Co₃O₄ under the thermal and mechanical effects of the plasma arc, and finally, Co²⁺ ions were deposited on the surface of popcorn−carbon in the form of Co₃O₄. The reactions involved in this process can be expressed as follows:

Co(NO₃)₂ + 2NaOH → Co(OH)₂ + 2NaNO₃

(7)

Co(NO₃)₂ → Co²⁺ + 2NO₃⁻

(8)

2H₂O + 2e⁻ → H₂ + 2OH⁻

(9)

Co²⁺ + 2OH⁻ → Co(OH)₂

(10)

$$\mathrm{Co}(\mathrm{OH}{)}_2 \stackrel{\mathrm{plasma} \mathrm{discharge}}{\to } {\mathrm{Co}}_3{\mathrm{O}}_4+{\mathrm{H}}_2\mathrm{O}$$

(11)

The assisted liquid−phase plasma electrolysis realizes the high−efficiency, rapid, one−step deposition of popcorn−carbon/Co₃O₄ composites, and provides a new method for the preparation of composite electrode materials on the supercapacitors.

4. Conclusions

Assisted liquid−phase plasma electrolysis was developed to realize the one−step synthesis of popcorn−carbon/Co₃O₄ composites. XRD analysis showed that there were obvious Co₃O₄ diffraction peaks at 2θ = 37.3°, 59.2°, and 65.6° for the as−prepared popcorn−carbon/Co₃O₄ composites. Co₃O₄ nanoparticles were distributed on the surface of the popcorn−carbon pores, especially for the popcorn−carbon/Co₃O₄−2 composites, and the Co₃O₄ nanoparticles were thin and evenly distributed on the surface of a popcorn−carbon pore structure. It could be seen from the Raman spectrum that the peak at 664 cm⁻¹ corresponded to the typical A_1g vibration mode of Co₃O₄. According to the FTIR spectrum, there were two peaks at 662 cm⁻¹ and 568 cm⁻¹, corresponding to the tensile vibration modes of tetrahedral coordinated Co²⁺−O and octahedral coordinated Co³⁺−O of Co₃O₄, respectively. XPS analysis indicated that the popcorn−carbon/Co₃O₄−2 composites consisted of C, O, and Co elements, and the weight of Co₃O₄ in the popcorn−carbon/Co₃O₄−2 composites accounted for 74.8%, according to the thermogravimetric analysis. The electrochemical properties of the popcorn−carbon/Co₃O₄ composites were analyzed in the three−electrode system. The results showed that the as−prepared popcorn−carbon/Co₃O₄−2 composite electrode materials possessed a specific capacitance and specific capacity of ~1264 F/g (594 C/g) at a current density of 1 A/g and a superior cycling stability with 86.2% capacitance retention for 5000 cycles at 10 A/g, exhibiting a better electrochemical property, followed by the popcorn−carbon/Co₃O₄−3 composite electrode materials. The novel assisted liquid−phase plasma electrolysis developed in this study realizes the efficient, rapid, and one−step synthesis of the popcorn−carbon/Co₃O₄ composites, and provides a new method for the preparation of composite electrode materials on the supercapacitors.