Ultrathin porous NiMnO₃ nanosheets on carbon cloth for use as supercapacitor electrode

Supercapacitors, also known as electrochemical capacitors, are a new type of energy storage devices that were developed in the 1980s. The capacitance is stored via the formation of a double layer or faradaic capacitance reactions. The energy storage performance of supercapacitors is intermediate between those of secondary batteries and conventional capacitors. The characteristics of both traditional capacitors and batteries can be determined by assessing their energy storage performance.^1–4 The electrode materials used for supercapacitors include mainly carbon materials, transition metal oxides and hydroxides, and conductive polymer materials. Many researchers have focused on metal oxide electrode materials owing to their good redox properties and abundance.

Researchers have studied several inexpensive metal oxide/hydroxide materials, such as NiO,^5,6 MnO₂,^7,8 Co₃O₄,⁹ Ni(OH)₂,¹⁰ Co(OH)₂,¹¹ and Fe₃O₄.¹² MnO₂ has been considered as one of the most promising electrode materials owing to its affordability, abundance, and high theoretical specific capacitance (1370 F/g). Subramanian et al.¹³ prepared nanostructured MnO₂ that presented the high specific capacitance of 168 F/g when used as supercapacitor electrode material. Moreover, Qu et al.¹⁴ prepared MnO₂ nanorods in neutral aqueous electrolytes with the specific capacitance up to 201 F/g as cathodes for asymmetric supercapacitors. These studies have revealed that the low specific capacitance of MnO₂ was attributed to its low conductivity, which hindered its application as a potential capacitor with high specific capacitance. An effective method of increasing the conductivity of MnO₂ is to combine it with materials with good conductivity. Fan et al.¹⁵ fabricated polypyrrole/MnO₂ on a carbon cloth, and the specific capacitance of this combination reached 325 F/g. Bao et al.¹⁶ prepared a Zn₂SnO₄/MnO₂ composite supercapacitor electrode material with the maximum specific capacitance of 621.6 F/g (which was determined based on the mass of the active substance in MnO₂). Therefore, the interactions between different materials could enhance the synergistic or complementary effects of materials and improve the electrochemical properties of composites.

In this study, we used a simple one-step hydrothermal method to prepare NiMnO₃ nanosheets on a carbon cloth (CC) with high specific capacitance. NiMnO₃@CC materials were prepared using nickel nitrate hexahydrate [Ni(NO₃)₂·6H₂O] and manganese chloride tetrahydrate (MnCl₂·4H₂O). The relationships between the composition, hydrothermal reaction time, and capacitance of NiMnO₃ nanosheets were studied, and the results indicated that the NiMnO₃@CC materials are ideal electrode materials for supercapacitors.

Ni(NO₃)₂·6H₂O, MnCl₂·4H₂O, urea (CO(NH₂)₂), hexamethyltetramine [(CH₂)₆N₄], potassium hydroxide (KOH), acetone, ethanol, and all other reagents were analytically pure. Several 3 × 3.3 cm² carbon cloths were cleaned using acetone, ethanol, and deionized water and were dried for later use. The following instruments were used for the experiments: an electronic balance, a constant-temperature magnetic stirrer, a numeric control ultrasonic cleaning instrument, a 50 ml high-pressure polytetrafluoroethylene-lined reaction kettle, an air drying box, an electric thermostatic drying oven, a field emission scanning electron microscopy (FESEM, JSM-7100F) instrument, an x-ray diffraction (XRD, TD-3500) apparatus, a transmission electron microscopy (TEM, JEM 2010) device, and a CHI660e electrochemical workstation.

Ni(NO₃)₂·6H₂O and MnCl₂·4H₂O were dissolved in 30 ml deionized water at the molar ratios of 4:2, 3:3, and 2:4 (the total molar concentration of the mixtures was 0.2M) under constant stirring. Afterward, 0.09 g CO(NH₂)₂ and 0.21 g (CH₂)₆N₄ were added to the above solution, and the mixture was stirred for 30 min to form a clear mixed solution, which was subsequently added to a 50 ml polytetrafluoroethylene-lined reaction kettle. The carbon cloth was slowly immersed in the solution and later obliquely inserted into the polytetrafluoroethylene-lined reaction kettle. After the system was heated to 120 °C for 6 h, the carbon cloth was removed and cooled to room temperature. Then, the cloth, which was covered with a light-yellow substance, was dried at 80 °C for 6 h. The average mass of the NiMnO₃ nanosheets loaded on the carbon cloth substrate is presented in Table I.

Briefly, 0.872 g Ni(NO₃)₂·6H₂O, 0.594 g MnCl₂·4H₂O, 0.09 g CO(NH₂)₂, and 0.21 g (CH₂)₆N₄ were dissolved in 30 ml deionized water, and the mixture was stirred for 30 min to form a clear solution. Afterward, the solution was added to the transparent solution in the polytetrafluoroethylene-lined reaction kettle, and the carbon cloth was slowly immersed in the mixture until it slowly sank to the bottom of the reaction kettle. The temperature was maintained at 120 °C for 3 h, 6 h, and 9 h, and the solution and cloth were allowed to react. Afterward, the solution was allowed to cool to room temperature. The product, which consisted of a layer of pale yellow material deposited on the carbon cloth, was removed from the reactor, washed with water and ethanol, and dried at 80 °C for 6 h. The average amounts of NiMnO₃ nanosheets loaded on the carbon cloth substrate are listed in Table II.

XRD and FESEM were used to determine the microscopic morphology of the samples. In addition, a CHI660e (Shanghai Chenhua) electrochemical workstation, where the NiMnO₃ nanosheets on carbon cloth, Pt, and Hg/HgO were the working, auxiliary, and reference electrode, respectively, and 6M KOH was the electrolyte, was used to characterize the electrochemical behavior of the NiMnO₃@CC electrodes.

The XRD profile of the sample was obtained in the 2θ range of 20°–80°, as shown in Fig. 1. The major diffraction peaks correspond to the rhomb-centered hexagonal structure of NiMnO₃ (JCPDS card no. 75-2089). Among them, the two strong diffraction peaks at 33.5° and 59.6° corresponded to the (211) and (2-11) planes of NiMnO₃, respectively. In addition, the three relatively weak peaks at 25.3°, 44.6°, and 70.2° were ascribed to the (110), (200), and (422) planes of NiMnO₃, respectively.

As illustrated in Fig. 2, the NiMnO₃ nanosheets with the Ni/Mn molar ratio of 3:3 obtained after 6 h of hydrothermal synthesis. As presented in Fig. 2(a), many sheet-like materials grew on the carbon cloth, and it was determined that they were lamellar and presented a simple structure. Moreover, the NiMnO₃ sheet-like materials were almost perpendicular to the carbon cloth fibers, and the vertical array facilitated the charge transfer and ion diffusion. As illustrated in Fig. 2(b), the sheet-like material consisted of wrinkled porous sheets that were uniform in size and ∼15 nm thick. The lamellar structure of the composites increased their specific surface area, and thus, the reactive area of the active composite material was increased. The synthesized NiMnO₃ materials presented a nanosheet structure; its size was in the range of a few hundred nanometers, and its thickness was very thin according to the TEM image in Fig. 2(c). The selected area electron diffraction patterns [Fig. 2(d)] revealed that the NiMnO₃ nanosheets were polycrystalline. The effects of the particle size, morphology, and structure on the electrochemical properties of the NiMnO₃ nanosheets are very important. All the ultrathin NiMnO₃ nanosheets prepared in this study were conducive to the construction of a highly porous nanostructure, which was very beneficial for avoiding the agglomeration of active materials and increasing the contact area for the solid–liquid reaction. The energy storage mechanism of metal oxide electrode materials is typically based on the reversible Faraday reaction between the electrode materials and electrolyte, which involves the transfer of electrons between them. Therefore, the lamellar structure of the NiMnO₃ nanosheets was more conducive to the insertion and ejection of OH^– groups, which increased the reversible Faraday electron transfer reaction rate and thus could generate high specific capacitances.

The electrochemical performance test diagram of the NiMnO₃@CC electrode material with the Ni/Mn molar ratio of 3:3 obtained after 6 h of hydrothermal synthesis is presented Fig. 3. The cyclic voltammetry (CV) curves of this electrode in the voltage range of −0.2–0.8 V at the scanning rates of 10 mV/S, 20 mV/S, 30 mV/S, 40 mV/S, and 50 mV/S are illustrated in Fig. 3(a). As the scanning rate increased, the redox current gradually increased and the redox peak gradually lagged behind. The electrode presented a strong redox peak and a large integral area. This indicated that a good reversible redox reaction took place on it, and therefore, the electrode presented excellent specific capacitance. This was attributed to the synergistic effect of the NiMnO₃@CC electrode. (1) The prepared product consisted of thin 15 nm sheets with abundant pores. This was beneficial for the contact area between the electrode material and electrolyte and promoted the redox reaction of the active substance. (2) The addition of the MnO₂ layer increased the distance between the layers of the electrode material, which was conducive to improving the diffusion of ions in the electrode material. (3) The staggered deposition of the Ni(OH)₂ and MnO₂ layers during the synthesis of the composite was advantageous for electronic transmission.

The galvanostatic charge–discharge (GCD) curves of the NiMnO₃@CC electrode under different current densities in the range of −0.05–0.45 V are illustrated in Fig. 3(b). The GCD curves presented distinct pseudo capacitance characteristics. Moreover, the charge–discharge behavior of the NiMnO₃@CC electrode was in agreement with their CV curves [Fig. 3(a)]. The following formula can be used to calculate the specific capacitance (C_m) of a single electrode using its discharge current curve:

where I (mA) is the discharge current, Δt (s) is the total discharge time, m (mg) is the mass of active material, and ΔV (V) is the voltage drop during the discharge process. Our calculations indicated that the highest C_m value of the NiMnO₃@CC electrode at the current density of 1 A/g was 2330 F/g. Furthermore, the C_m values of the NiMnO₃ composite at the discharge current densities of 2 A/g, 3 A/g, 5 A/g, 10 A/g, and 20 A/g were 2104 F/g, 2016 F/g, 1940 F/g, 1860 F/g, and 1640 F/g, respectively. Therefore, when the current density increased from 1 A/g to 20 A/g, C_m decreased by 29.7%. This indicated that the NiMnO₃ composite material presented a good capacity-to-high-current ratio at high current densities, which was attributed to its good structural stability and conductivity. In addition, the large ohmic drop and insufficient reaction of the active substances that participated in the redox process led to the decrease in C_m to a certain degree at high discharge current densities. The comparison of the capacitances between our product and other NiMn-based materials in the literature^17–21 is shown in Table III. Evidently, NiMnO₃ nanosheets presented in this study show excellent supercapacitor performance. This can be attributed to the following factors. First, the dense NiMnO₃ nanosheets coated thinly on the substrate can provide more exposed electroactive sites, shortening the ion-diffusion route that enhances the specific capacitance. Second, the NiMnO₃ nanosheets are grown directly on the carbon cloth, thus avoiding the use of a polymer binder for electrode preparation, which ensures good structural integrity and fast electron transport.

Several other experiments were performed using single variables as contrast. The CV and GCD curves of the NiMnO₃@CC electrodes with different Ni/Mn molar ratios obtained after 6 h of hydrothermal synthesis are depicted in Fig. 4. The CV curves revealed that the redox currents of the prepared NiMnO₃@CC electrodes gradually increased as the scanning rate increased and the redox peaks gradually lagged behind. Distinct redox peaks were observed on the CV curves, which were attributed to the redox reaction of the NiMnO₃ materials, as mentioned above. The GCD curves demonstrated that as the current density increased, the charging and discharging times of the NiMnO₃@CC electrodes decreased correspondingly. At the same current density, the charging and discharging times and C_m values of the NiMnO₃@CC electrodes with different Ni/Mn molar ratios were different. The C_m values of the NiMnO₃@CC electrodes with different Ni/Mn molar ratios are presented in Fig. 4(g). As the Ni content of the NiMnO₃ nanosheets increased, the C_m values of the composites increased gradually. The C_m value of the NiMnO₃@CC electrodes with the Ni/Mn molar ratio of 3:3 reached the maximum value at the same current density. When the Ni content of the NiMnO₃ nanosheets was increased, the increase in the thickness of the nanosheets hindered the full contact between the active substances of the composite and electrolyte, which resulted in the decrease in C_m. Moreover, the CV and GCD curves of the NiMnO₃@CC electrodes with the Ni/Mn molar ratio of 3:3 obtained after different hydrothermal reaction times are illustrated in Fig. 5. These curves were consistent with the above-mentioned basic explanation; moreover, the reaction mechanism of these composites was the same, but their C_m values, which are presented in Fig. 5(e), were different. At the same current density, the C_m value of the NiMnO₃@CC electrode with the Ni/Mn molar ratio of 3:3 obtained after 6 h of hydrothermal synthesis was the highest of all composite electrodes analyzed in this study. Comparing the above-mentioned data groups, we found it easy to recommend NiMnO₃. This type of electrode material can be easily manufactured and exhibits better properties than other NiMn-based materials. The C_m value of the NiMnO₃ material with the Ni/Mn molar ratio of 3:3 obtained after 6 h of hydrothermal synthesis was the highest of all analyzed samples. Next, we tested and characterized the electrochemical stability of the corresponding NiMnO₃ material.

The cycle life of supercapacitor electrode materials is very important for their practical applications. The constant current charge–discharge curve of the NiMnO₃@CC electrode at the current density of 10 A/g is presented in Fig. 6(a). As the number of cycles increased, the electrode capacitance gradually decreased. After 800 charge–discharge cycles, C_m remained stable. After 1000 charge–discharge cycles, C_m was 1130 F/g, which was 67.2% lower than the initial value. This indicated that the electrode material presented good cyclic stability. The decrease in C_m could be attributed to the gradual collapse of the original structure and change in crystal shape of the composite during the reaction. These structural changes resulted in the decrease in the specific surface area and capacitance of the electrode. However, in general, NiMnO₃ composite electrodes exhibit good cyclic stability.

The ac impedance spectrum of the NiMnO₃@CC electrode is illustrated in Fig. 6(b), which consists of a semicircle and an oblique line in the high- and low-frequency regions, respectively. The semicircle in the high-frequency region in Fig. 6(b) is the resistance reactance spectrum of the electrochemical reaction of the electrode, and the diameter of the semicircle is the charge transfer impedance, which was approximately 0.1 Ω. This indicated that the material, electrolyte, and contact resistances were very small. The NiMnO₃@CC electrode presented good electrical conductivity, which favored the redox reaction, and thus presented high C_m. The oblique line in the low-frequency area in Fig. 6(b) is the Warburg impedance caused by proton diffusion. The intersection of the high-frequency end of the arc and real axis is the resistance of the solution.

NiMnO₃ nanosheets were hydrothermally synthesized using Ni(NO₃)₂·6H₂O and MnCl₂·4H₂O. The morphology and structure of the synthesized materials were characterized using different methods, and their electrochemical properties were studied in detail. Electrochemical tests revealed that the composition and preparation time of the NiMnO₃@CC materials significantly affected their C_m values. The NiMnO₃@CC electrode with the Ni/Mn molar ratio of 3:3 obtained after 6 h of hydrothermal synthesis presented the best electrochemical performance of all analyzed composites. Its specific capacitance at the current density of 1 A/g was as high as 2330 F/g; moreover, this composite presented excellent multiplier performance and good cycle stability. Therefore, owing to their low initial raw material cost, simple preparation method, and high capacitance, the NiMnO₃@CC electrode presented excellent potential for use as electrochemical capacitor electrode materials. As supercapacitor electrode materials, the NiMnO₃@CC electrode exhibited good charge–discharge capacity and cycle stability. Overall, the NiMnO₃ nanosheets could open new directions for future research and development of supercapacitor electrode materials.

This work was supported by the Scientific Research Fund Project of Yunnan Education Department (Grant No. 2019J0367).

The data that support the findings of this study are available from the corresponding author upon reasonable request.