# Unique Triboelectric Nanogenerator Using Carbon Nanotube Composite Papers
## 1 Introduction  
Renewable energy, such as solar, wind, and geothermal energy, is gaining considerable attention as a solution to the energy and environmental problems that have arisen in recent years. These types of renewable energy are continuously regenerated in nature and can be used indefinitely. This is especially promising for devices used in IoT and other applications because there are cases in which batteries are difficult to replace. Moreover, there are increasing demands for energy harvesters that can acquire energy from nature. Among these energy harvesters, triboelectric nanogenerator (TENG) devices, which are extremely simple in structure and use frictional charging, have been the most actively studied in recent years [1; 2; 3; 4; 5; 6]. In this study, carbon nanotubes (CNTs) are considered to develop new types of TENGs. A TENG is a generator that can effectively convert low-frequency mechanical energy that is wasted in the outside world into electrical energy. For example, TENGs are being developed that weave energy from the ocean as their energy source. This can be achieved independently or in combination with a conventional generator. In the last few years, the operating principle of TENGs has been developed, enabling them to harness power from dynamic blue energy (ocean and river waves) [7; 8; 9; 10; 11]. The operating principle of TENGs involves the repetition of contact and separation between two materials (sheets) with opposite frictional charges, resulting in a periodic change in the potential difference, whichis extracted to generate electricity. Generally, a material that tends to carry a positive frictional charge and a material that tends to carry a negative frictional charge are prepared for TENGs [1,2]. CNTs, which were discovered in 1991 [12], are known for their high chemical stability, mechanical strength, high electrical and thermal conductivity, and metallic and semiconducting nature [13, 14, 15, 16, 17, 18]. Due to the various beneficial characteristics described above, there are great expectations for the practical application of various devices using CNTs [19]. However, because CNTs are generally nanoscale materials with a diameter of only a few nm and a length of only a few \(\upmu\)m, and because most commercial products on the market are in powder form or in water dispersions, they are difficult to handle. This makes it challenging to develop applications for them. Therefore, research is being conducted not only on methods that use CNTs alone but also on their application to devices in the form of composite materials in combination with other materials. Such solutions are referred to as \"CNT composite materials\". By creating a composite material, their handling becomes easier, and the functions of CNTs can be exploited in this form [20, 21, 22, 23]. CNT composite papers [24], which can be easily handled as they are \"familiar materials\" with the various features of CNTs, are currently being developed. These composite papers have attracted much attention as new, unique materials because they have the same processability and deformability as paper while maintaining the various functions of CNTs. Various applications based on the CNT composite papers have already been investigated, including paper dye-sensitized solar cells [25], paper actuators [26], and thermoelectric power-generating papers [27]. Recently, to improve their flexibility and power-generating efficiency, a new type of TENG based on the combination of CNTs and polymers has been proposed [4,5]. Such TENGs have certain usefulness as energy harvesters, but it has been found that issues remain regarding their durability and the complexity of their fabrication. In this context, the CNT composite paper is expected to represent a candidate for new TENG devices. Therefore, the development of a unique TENG device using CNT composite papers is considered in this study. A CNT composite paper can be produced easily using a method derived from the traditional Japanese method of washi paper production [24].