Enhanced Intrinsic Electrochemical Capacitance in Carbon Nanotube Yarns for Increasing Electrical Power Generation
Authors: Oh Seongjae , Kim Shi Hyeong 
Published: February 2024

Mechanical energy harvesters, which convert mechanical energy to electrical energy, have gained significant attention as potential replacements for conventional energy devices like batteries and capacitors. Triboelectric and piezoelectric nanogenerators have been researched to improve harvesting performance and to harness them low-power electrical devices[1,2]. For practical use, considering the surrounding environments are necessary because harvesting performance is dependent on surrounding conditions such as frequency of mechanical motion, humidity, and temperature. The ocean, in particular, provides a troublesome environment for conventional mechanical energy harvesters, given its submerged conditions, the presence of various ions, and low-frequency mechanical motion, making it challenging to generate electrical energy[3].
Chemo-mechanical energy harvesters, a novel type of mechanical energy harvester, have been developed to generate electrical energy within ionic environments (i.e., immersed in electrolytes). These harvesters, which consist of working electrodes, counter electrodes, and electrolytes, provide electrical energy from changes in the electrochemical accessible area induced by mechanical energy. Coiled carbon nanotube (CNT) yarn harvesters, a type of chemo-mechanical energy harvester, were reported in 2017 as a promising approach for ocean-specific harvesters.[4] Coiled CNT yarns, a key elements for generating electrical energy, is fabricated from the highly twist insertion in the CNT sheets with large intrinsic electrochemical capacitance (IEC). This densification process provides the stretching-induced large changes in IEC in coiled CNT yarn, thus generating highest frequency-normalized peak power compared to other mechanical energy harvesters. However, the inevitably decreased IEC during densification hindered optimal harvesting performance in coiled CNT yarns.
To address this issue, this study proposes a novel spinning method called longitudinally aligned (LAY) yarn-spinning to enhance IEC in coiled CNT yarns. Compared to the conventional coiled CNT yarn, our coiled CNT yarn named L-coiled CNT yarn provided a large IEC (5.6 F g−1) and large IEC variations (35%) during stretching to 80%. Using this novel structure in in coiled CNT yarn, our chemo- mechanical harvester delivered 2.6 times the open circuit voltage (OCV, 270 mV), 4 times the peak power (540 W kg
−

1), and twice the efficiency (2.15%) of prior Twistron reported in 2017. The experimental structure analysis and computational analysis successfully supported our results. Especially, it is discovered that the enhanced IEC enables a decrease in matching impedance, resulting in more energy-efficient circuits with harvesters. For the practical use, L-coiled CNT yarn harvesters was investigated in an ocean-like environment with a frequency ranging from 0.1 to 1 Hz, it demonstrated the high volumetric power of 1.6 to10.45 mW cm−3. We believe presented approaches including the LAY spinning process, related electrical circuit integration, and lowering matching impedance could be a guidance for future studies.
Reference
[1] Jianlong Wang et al., "Enhancing Output Performance of Triboelectric Nanogenerator via Charge Clamping", Adv. Energy Mater. 2021, 11, 2001669.
[2] Xiaomin Huang et al., "Piezoelectric Nanogenerator for Highly Sensitive and Synchronous Multi-Stimuli Sensing" ACS Nano. 2021, 15,19789-19792.
[3] Minyi Xu et al., "High Power Density Tower-like Triboelectric Nanogenerator for Harvesting Arbitrary Directional Water Wave Energy", ACS Nano. 2019, 13,1932-1939.
[4] S H Kim et al., "Harvesting electrical energy from carbon nanotube yarn twist" Science. 2017, 357, 773.