1 Development of Cu2O/Cu/graphite composite paper electrodeposition process for supercapacitor electrode design

Supercapacitors are a class of highly promising green energy storage devices that can bridge the power/energy density gap between traditional capacitors and batteries or fuel cells to produce fast charge and discharge energy storage devices with moderate energy density. Supercapacitors have ultra-high power density, long service life, excellent reversibility and wide operating temperature. Recently, the application of supercapacitors in Airbus A380 aircraft and trolleybuses has verified their safety and reliability. According to the energy storage mechanism of supercapacitors, supercapacitors can be divided into electrochemical double layer capacitors (EDLCs) and Faraday pseudocapacitors. Pseudocapacitors that use fast and reversible Faraday reactions for energy storage theoretically have higher specific capacitance than EDLCs because the electron storage mechanism of EDLCs only involves electrostatic adsorption and separation at the interface. Typical metal oxides/oxides, such as RuO₂, MnO₂, NiO, Ni(OH)₂, Co₃O₄, Co(OH)₂, V₂O₅, SnO₂, and Fe₃O₄, are widely considered to be ideal candidates for supercapacitors due to their high specific capacitance, combining high energy density with good electrochemical stability, which is superior to traditional carbon materials and conductive polymers. Among these materials, although RuO₂ and MnO₂ have been widely studied, the study of Cu₂O has gradually become one of the important directions for supercapacitor electrode design in recent years. Cu₂O has unique structural diversity, rich synthesis strategies, and a variety of potential applications such as photoelectric reduction of CO₂ and solar-driven water decomposition. Most importantly, the theoretical specific capacitance of Cu₂O is as high as 2247.6 F/g, which shows broad application prospects in the field of supercapacitors.

In recent years, the development of cellulose paper-based electrodes in the field of ultrathin and ultralight supercapacitors has attracted great attention. Cellulose paper can be used as a green, economical, lightweight, and flexible platform to combine with various electrochemically active substances to prepare green and novel supercapacitor electrodes. The key properties that make cellulose paper an ideal matrix for supercapacitors are that the cellulose surface has a chemical composition that is easily functionalized (such as cellulose with rich hydroxyl groups), and the 3D porous fiber structure of cellulose paper can absorb electrolytes and act as an electrolyte storage to promote ion transport. However, cellulose paper is an insulator. To improve conductivity, cellulose paper is usually integrated with various conductive materials, such as carbon materials (such as carbon nanotubes and graphene), conductive polymers [such as polypyrrole, polyaniline and poly (3,4-ethylenedioxythiophene)], semiconductors and metal materials. So far, many physical and chemical techniques (such as powder metallurgy, plasma spraying, magnetron sputtering, electrodeposition and vapor deposition) have been considered to uniformly deposit active materials onto a variety of different substrates to achieve different functions and uses. Among them, electrodeposition, as a traditional electrochemical coating method, also shows great potential in the field of supercapacitor electrode design. As early as more than 200 years ago, Luigi Valentino Brugnatelli achieved the electrodeposition of gold on the surface of silver, pioneering this field. Electrodeposition involves two typical methods: an electrophoretic process based on a particle suspension and an electrolytic process (also known as electroplating) with a metal salt solution as a precursor. Compared with other techniques, electrodeposition has several advantages: ① fast, ② low cost, ③ good reproducibility, ④ high purity, ⑤ high deposition rate, ⑦ the ability to form coatings on substrates with very different physical and chemical properties, ⑧ the ability to form multilayer films with sizes ranging from nanometers to micrometers, and ⑨ the ability to overcome shape limitations or have the potential to produce complex-shaped parts. Therefore, electrodeposition provides a general and convenient way to achieve controllable coatings of composite materials.

In response to the above problems, this chapter designs a simple, easy-to-use, and cost-effective method (i.e., pencil painting-electrodeposition-electrooxidation): graphite, copper, and copper oxide nanostructures are sequentially deposited on a cellulose paper substrate to prepare a self-supporting, binder-free multilayer core-shell composite paper electrode (labeled as Cu₂O/Cu/GCP). First, graphite is deposited on cellulose paper by pencil painting. This simple method can make cellulose paper conductive and facilitate the subsequent electrodeposition of metallic copper. After depositing metal Cu on the surface of graphite/cellulose paper (GCP) by electrodeposition, the electrodeposited copper surface is electro-oxidized into highly electrochemically active Cu₂O by constant voltage polarization. This method has been proven to effectively convert metals into their oxides or hydroxides. This unique multilayer core-shell structure gives the electrode excellent electrochemical properties. In addition, an asymmetric supercapacitor (ASC) device (labeled as Cu₂O/CU/GCP//CP) was designed and assembled using Cu₂O/Cu/graphite composite as the positive electrode, cellulose paper directly as the separator, and carbon paper as the negative electrode, and its electrochemical properties and further studied its electrochemical performance. This design not only verifies the feasibility of the material combination strategy, but also provides an experimental basis for the subsequent optimization of supercapacitor electrode design.

2 Preparation of Cu2O/Cu/graphite/cellulose composite paper with multilayer core-shell structure

The schematic diagram of the synthesis strategy of the multilayer Cu₂O/CU/GCP electrode is shown in Figure 1. First, a thin layer of graphite was coated on the surface of cellulose paper by a fast, simple and environmentally friendly pencil painting method, and then a Cu layer was deposited on the GCP by cathodic electrodeposition, i.e., Cu²⁺+2e^–→Cu. Finally, Cu₂O was in situ grown on the surface of the copper layer by electro-oxidation (constant voltage polarization) in KOH solution. These processes led to the formation of a quaternary composite material with a hierarchical multilayer core-shell structure.

Figure 1 Schematic diagram of the synthesis process of Cu2O/Cu/GCP

2.1 Mechanical painting and electrodeposition to construct Cu/graphite/cellulose composite paper

Specifically, in this supercapacitor electrode design, the preparation of GCP first used a simple and fast pencil painting method to coat graphite on cellulose paper, that is, using an ordinary 8B pencil to draw 5 times on a piece of cellulose paper to obtain a uniform thin graphite coating. For the preparation of Cu/GCP, the electrochemical workstation (CS350, Wuhan Koster Instrument Co., Ltd.) was used in a three-electrode system to carry out the electrodeposition experiment of metal Cu. The working electrode is GCP, the counter electrode is platinum foil (2 cmx2 cm), and the reference electrode is Ag/AgC1 electrode. The electrolyte is 0.6 mol/L copper sulfate aqueous solution (sulfuric acid is used to adjust the electrolyte pH to 2). The constant current polarization method is used at a constant current of -50 mA for 24 hours. After electrodeposition, the Cu/GCP composite material is washed with distilled water and dried at 25°C.

2.2 In situ growth of pyramid-shaped Cu2O on the Cu surface by electro-oxidation

For the preparation of Cu₂O/Cu/GCP, Cu/GCP is used as the working electrode, platinum foil is used as the counter electrode, Ag/AgC1 electrode is used as the reference electrode, and 6 mol/L KOH solution is used as the electrolyte. Under the condition of constant voltage polarization of 0.3V, pyramid-shaped Cu₂O is in situ grown on the surface of the Cu layer at a specific time. The synthesized Cu₂O/Cu/GCF composite material is rinsed with water and naturally air-dried at 25°C. This method provides a simple and effective construction strategy for supercapacitor electrode design, which helps to improve the electrochemical performance and structural stability of electrode materials.