Hydrophilic Surface Modification of Nylon 6 Fiber by Incorporating Amino Acids for Hydrovoltaic Water Evaporation Energy Harvesting

Abstract

In recent years, climate change has been accelerating due to the rise in global temperatures caused by increasing fossil fuel consumption. Therefore, there is a growing need to develop eco-friendly and renewable energy sources that do not emit carbon. At the same time, the expansion of the Internet of Things (IoT) market has increased the demand for power sources for small electronic devices. In this context, energy harvesting technologies are gaining attention as carbon-free and sustainable power sources that are well-suited for small electronics. Energy harvesting is a technology that generates electrical power from various ambient energy sources such as solar radiation, vibrations, wind, heat, and water. Among these sources, we’ve focused on water. Water covers approximately 70% of the Earth’s surface and absorbs about 34% of the solar energy received by the earth, making it a massive energy reservoir. However, the types of water-based electric power generation so far were limited to the conventional hydroelectric power generation, which utilizes only the kinetic energy of the water falling from the height. Recently, research on the hydrovoltaic effect, a technology that generates electricity through an interaction between water and nanomaterials has been actively conducted. The hydrovoltaic effect uses various forms of water such as droplet, moisture, flow, wave, etc. and can use latent heat of water not only the kinetic energy of the water. There are several types of hydrovoltaic effects, for instance evaporation-driven, moisture-driven, and solid-liquid relative motion-driven electric effects. Among this, the evaporation-driven hydrovoltaic effect has been actively studied because it provides a continuous DC output and its working mechanism is relatively well understood. However, most of the developed nanomaterials in this field have been fabricated on rigid substrates, making them susceptible to delamination or structural damage when external forces are applied. Furthermore, the device configurations are often limited, which restricts their application environments—typically to situations where the device is immersed in a water reservoir. In this paper, we developed a flexible and substrate-free active material of evaporation-driven hydrovoltaic generator by surface-modifying a nylon 6 fiber film, resulting in improved output voltage and current. Nylon fibers are known for their toughness, high tensile strength, and elasticity. They are also intrinsically hydrophilic, enabling them to absorb water. Nylon 6 fiber surface was functionalized by crosslinking poly-L-lysine (PLL) using glutaraldehyde as crosslinker. Poly-L-lysine (PLL) is a cationic polypeptide of lysine amino acid that has numerous positively charged hydrophilic amino groups. It is commonly used to impart a positive surface charge, particularly for applications such as cell fixation or antibacterial effects. An imine bond, which is also called schiff base, is formed between the aldehyde groups of glutaraldehyde and the amino groups of both amino acid and nylon 6. In this study, PLL grafting on nylon fiber surface was conducted to increase the fiber surface charge, zeta potential and hydrophilicity. First, output comparisons were conducted between devices modified with various amino acids— including glutamic acid, serine, lysine, glycine, poly-L-lysine—on the nylon fiber surface. The output voltage polarity and magnitude varied depending on the charge amount and polarity carried by the grafted amino acids. Furthermore, the origin of the electrical output from fabricated devices was also confirmed through the electrode effect test and it was identified that side reactions between water and the Cu electrode could be neglected. Next, whether crosslinking between PLL and the nylon 6 fiber surface actually occurred was confirmed through Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). In addition, the surface morphology and hydrophilicity were characterized through scanning electron microscopy (SEM) and contact angle measurements. Finally, the optimal crosslinking conditions including reaction process time, temperature and PLL concentration were determined based on output performance. Moreover, the electrical characteristics of fabricated devices were evaluated by measuring the I-V curve and power density, as well as conducting a switch polarity test and a long-term durability test. Through this study, flexible and freestanding evaporation-driven hydrovoltaic generator is developed, which overcome the limitations of devices studied so far, such as rigidity and lack of flexibility and increase the applicability of the device. |본 논문은 poly-L-lysine을 표면에 가교하여 표면특성을 개질한 nylon 6 fiber를 사용하여 모세관 흐름으로 구동되는 수력 발전기를 제작한 논문이다. 최근 들어 물을 이용한 에너지 하베스팅 기술 중 하나인 모세관 흐름 구동 수력발전에 대한 연구가 활발히 이루어지고 있고, IoT 기술의 발전으로 배터리 없이 작은 전자기기에 전원을 공급하는 기술이 필요해지는 상황에서 위 기술이 주목받고 있다. 기존에 연구된 모세관 흐름 구동 수력발전기는 대부분 그래핀, 산화물 나노입자 계열이기 때문에 추가적인 기판이 필요하다. 또한 기판 위의 물질들은 외부 충격에 의해 쉽게 구조가 망가지거나 기판에서 분리된다. 이러한 점들은 실제 디바이스의 응용에 어려움을 끼친다. 이러한 점을 극복하기 위해 본 연구에서는 추가적인 기판이 필요 없고 유연한 고분자 섬유인 나일론 6 섬유를 활성물질로 사용한 모세관 흐름 구동 수력발전기를 개발하였다. 나일론 6 섬유 표면에 양이온성 고분자인 poly-L-lysine(PLL)을 가교 결합하여 표면전하와 zeta potential 을 높여 출력을 높였다. 실제 가교결합 반응이 일어났는지 여부를 FT-IR, XPS 로 확인하였고, 표면 형상과 친수성 정도는 주사전자 현미경과 접촉각 측정실험으로 확인하였다. 또한 가장 높은 출력을 만들어내는 PLL 농도와 가교결합 온도, 가교결합 반응 시간을 최적화하였다. 또한 표면을 개질한 나일론 섬유 필름으로 증발구동 수력발전기를 제작하였고 전류-전압 곡선 및 전력밀도 측정, 출력전압의 장기적 안정성을 테스트하여 디바이스의 전기적 특성을 살펴보았다. 본 연구를 통해 유연하고 추가적인 기판이 필요 없는 증발구동 수력발전 활성물질을 개발하였다.