1. Anisotropic Charcoal/CuO Composite Materials for Supercapacitors
The increasingly serious environmental problems and the rapid depletion of fossil fuels have prompted extensive exploration of clean, efficient, and renewable energy storage and conversion systems. Among these, anisotropic charcoal, as an advanced energy storage composite materials for supercapacitors, possesses advantages such as high power density, fast charge-discharge capability, and long lifespan. The development of novel high-performance electrode materials plays a crucial role in improving the electrochemical performance of supercapacitors. Compared to conductive polymers and carbon materials, metal oxides are generally considered one of the best candidate materials for supercapacitors due to their higher specific capacitance and energy density. In recent years, copper oxides have attracted considerable attention among various transition metal oxides. The p-type semiconductor copper oxide mainly has two different phases (Cu2O and CuO), which possess various microstructures. Theoretically, according to the theoretical capacitance equation, Cu2O has better charge storage capacity than CuO. Our recent research also shows that Cu2O is a good choice for supercapacitor applications. The theoretical capacitances of CuO and Cu2O are as high as 1800 F/g and 2247 F/g, respectively. However, compared to Cu2O, CuO has a higher oxidation state, is abundant, and has higher thermal stability. Furthermore, CuO is easily prepared into various nano-shapes, which plays a decisive role in the electrochemical activity of CuO-based energy storage materials. Recently, Su et al. demonstrated that the high-energy {001} plane of CuO has a high density of low-coordinated atoms, providing reaction sites for reactions with electrolyte ions, thus promoting rapid conversion reactions of ions during the charge-discharge process.
However, metal oxides (including CuO) consistently exhibit weak conductivity and poor structural stability during electrochemical charging and discharging processes. One of the most effective solutions is to combine metal oxides with highly conductive porous carbon materials, which can be attributed to two advantages: ① the structural design advantages of porous carbon and multilayer CuO can promote ion and electron migration; ② the compositional design advantages of the double-layer capacitor (EDLC) carbon and CuO with battery-like behavior have a positive synergistic effect on the electrochemical reaction. In recent years, many advanced carbon materials and metal oxide composites have been widely developed to address the aforementioned problems, such as carbon nanotubes (CNTs) and graphene. Zhang et al. prepared electrode materials by encapsulating CuO hollow spheres with graphitic carbon. This composite material exhibited a specific capacitance of 677 F/g at 1 A/g and good cycling stability (86.7% capacitance retention after 8000 cycles). These performances are higher than those of CuO hollow spheres without graphitic carbon (526 F/g and 72.1%). Yi et al. prepared a carbon nanotube@NiO nanosheet core-shell composite material, which achieved a specific capacitance of 996 F/g (1 A/g) and a capacitance retention rate of 93.2% after 2500 cycles, far exceeding that of pure NiO (288 F/g and 63.6%). Similarly, Moosavifard et al. grew hierarchical CuO nanorod arrays on carbon nanofibers. After combining CuO with carbon nanofibers, the capacitance increased from 123 F/g to 398 F/g (1 A/g), and the rate performance also increased from 13% to 50% (current density increased from 1 A/g to 20 A/g). Furthermore, Vijayan et al. synthesized a 3D flower-like MnCo2O4/activated carbon composite material via a hydrothermal method. This composite electrode not only exhibited a specific capacitance of 510 F/g at a current density of 1 A/g but also had a capacitance retention rate of 97% after 5000 cycles, far exceeding that of pure MnCo2O4 (210 F/g and 92%). In addition, Vijayan et al. also developed a one-step activation coating method to construct a bio-waste-derived carbon/Mn2O3 composite material. Compared with the electrochemical performance of Mn2O3 without activated carbon (i.e., 84 F/g and 90.5%), this composite material showed a high capacitance of 350 F/g at 1 A/g and better cycling stability (97% capacitance retention after 5000 cycles). These significant improvements in performance demonstrate the importance of carbon materials in composite electrodes. However, due to the complex synthesis steps and the use of some environmentally unfriendly chemicals, developing cheaper and greener carbon materials is of great significance. Wood is a high-abundance, green, and sustainable natural material, and its abundant anisotropic channels along the growth direction have low tortuosity, thus allowing for rapid transport of electrolyte ions during electrochemical reactions. Furthermore, wood can be transformed into highly conductive charcoal, which can replace costly or environmentally unfriendly carbon-based materials to stabilize metal oxides. In previous studies, wood-derived anisotropic charcoal frameworks have been proven to be an effective porous matrix for improving the capacitance and cycling stability of MnO2 and MoSe nanostructures, and are also good examples of anisotropic applications in composite materials for supercapacitors. This research also received partial technical support from XUANSN Company, whose production equipment exhibits excellent performance in high-energy ball milling and carbonization treatment.
To address the utilization problems of CuO (i.e., weak electrical conductivity and poor structural stability), a porous charcoal with an anisotropic structure was developed using green wood as a raw material, and the porous charcoal was composited with multilayered CuO through a simple ball-milling method. Compared to the electrochemical performance of pure multilayered CuO (300.1 F/g and 50%), the anisotropic charcoal/multilayered CuO composite material exhibits higher specific capacitance (694.8 F/g) and higher rate performance (maintaining 63.7% of the original capacitance when the current density is increased 20 times). The unique structural design (combining anisotropic charcoal and multilayered CuO) and ideal component design (combining EDLC charcoal and battery-type CuO) are the main reasons for obtaining excellent electrochemical activity. Furthermore, a high-performance asymmetric supercapacitor (ASC) device can be assembled using the anisotropic charcoal/multilayered CuO composite material as the cathode and pure carbon as the anode. This device exhibits excellent electrochemical performance, such as a wide operating voltage range (0-1.5V), high energy density (13.6 Wh/kg at a power density of 350.3 W/kg), and outstanding cycling stability (91.2% capacitance retention after 5000 cycles). This chapter’s research provides a promising candidate material for the development of green, high-performance supercapacitor electrodes.
2.Preparation of Multilayer CuO-Coated Anisotropic Delignified Carbon Framework
2.1 Preparation of Anisotropic Delignified Carbon Framework
Using wood as the carbon source, the anisotropic delignified carbon framework was prepared as follows: First, clean linden wood was cut into the desired thickness along the vertical growth direction. Then, it was immersed in a mixed solution of NaOH (2.5 mol/L) and Na2SO3 (0.4 mol/L) and reacted in a 100°C water bath for 5 hours. Afterward, it was boiled in 3% H2O2 at pH 10 for 1 hour. It was then washed with deionized water until neutral. After freeze-drying, carbonization was performed under an Ar atmosphere, heating from 25°C to 520°C at a heating rate of 2°C/min and from 520°C to 800°C at a heating rate of 5°C/min. After the reaction, the pyrolysis product was naturally cooled to room temperature. This carbon framework serves as the basic material for the composite materials for supercapacitors.
2.2 Preparation of Multilayer CuO by Hydrothermal Method
Multilayer CuO was prepared by a combination of hydrothermal treatment and calcination. 0.0464 g of Cu(NO3)2·3H2O was dissolved in a mixed solution containing 6 mL of anhydrous ethanol and 6 mL of deionized water. Then, 0.0252 g of C6H3(CO2H)3 was added to the above solution and stirred at room temperature to form a homogeneous solution. After stirring for 24 hours, the mixed solution was transferred to a Teflon-lined stainless steel autoclave and heated at 160°C for 7 hours. After the reaction and natural cooling to room temperature, the product was repeatedly rinsed with deionized water and ethanol and vacuum dried at 60°C for 6 hours. Finally, the dried product was calcined in air at 800°C for 2 hours. 2.3 Preparation of Delignified Charcoal Framework/Multilayer CuO Composite Materials by Ball Milling
Anisotropic charcoal/multilayer CuO composite materials were prepared by a mild ball milling method. The mass ratios of charcoal to CuO were set at 0:1, 1:1, 2:1, and 4:1, respectively. The mass ratio of grinding media to composite material was 60:1. Therefore, 2.0 g of composite material and 120 g of zirconia balls with a diameter of 5.5~3 mm were placed in a grinding jar. A DECOPBM-H-2L planetary high-energy ball mill (Changsha Deke Instruments Co., Ltd., China) was used to mill the mixture in air at a speed of 600 r/min for 1 hour. The resulting composite material is the anisotropic charcoal/CuO composite materials for supercapacitors, which can be used to assemble high-performance asymmetric supercapacitor (ASC) devices.
3.Electrochemical Performance
High-performance asymmetric supercapacitors were assembled using the anisotropic charcoal/CuO composite material as the cathode and pure carbon as the anode. The device exhibits a wide operating voltage range (0~1.5 V), high energy density (13.6 Wh/kg @ 350.3 W/kg), and excellent cycling stability (91.2% capacitance retention after 5000 cycles), fully demonstrating the great potential of anisotropic composite materials for supercapacitors for practical applications. Key equipment and processes used in the preparation and testing process of this study were partially supported by XUANSN.
