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图1 DEG的结构与电学性能
通讯作者简介
周晓峰
华东师范大学
周晓峰，华东师范大学，副教授。于2002年6月和2007年6月获华东师范大学电子工程系学士学位和硕士学位，2012年6月获中国科学院上海微系统与信息技术研究所博士学位。2007年至2018年中国科学院微系统与信息技术研究所分别担任助理研究员和副研究员。2019年起至今担任华东师范大学集成电路科学与工程学院副教授，主要致力于微纳电子机械系统(MEMS/NEMS)、微纳传感器技术和微能源采集技术的研究。2020年，与香港城市大学王钻开教授合作提出了一种体效应的固液界面发电机，成功突破过往液滴发电机发电功率密度不高的瓶颈，相关研究成果以共同第一作者身份在Nature期刊上发表。在Nature、Science Advances、National Science Review和Nano Energy等顶级学术刊物上发表论文 70余篇; 申请中国发明专利 14 余项，其中 10项已获得授权，另外还获得了 4项美国专利授权。
王钻开
香港城市大学
王钻开，香港城市大学机械工程系讲座教授，教育部“国家人才计划”讲座教授，香港青年科学院创始成员，国际仿生工程学会Fellow。2000年毕业于吉林大学，获机械工程学士学位，2003年毕业于中国科学院上海微系统与信息技术研究所，获微电子学硕士学位，2008年在伦斯勒理工大学获得机械工程博士学位，2008-2009年在美国哥伦比亚大学生物医学工程系进行博士后研究，2009年进入香港城市大学任教。主要研究方向为仿生拓扑机械系统和微观传递现象等。曾获2022香港研资局高级学者奖、2021年青山科技奖、2020科学探索奖（香港首届）、第35届世界文化理事会特别青年嘉奖（2018）、香港城市大学杰出研究奖以及国际仿生学会杰出青年奖。培养的博士生获得香港青年科学家奖（2人）、美国材料学会杰出研究生金奖和银奖、上银优秀机械博士论文银奖等，多人入选国家海外高层次人才引进计划。
期刊简介
Droplet旨在成为跨学科的高水平学术交流平台，展示液滴和气泡相关领域的前沿研究成果，推进国际科研传播与合作。
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Making use of water droplets as a sustainable green energy source
利用液滴作为一种可持续的绿色能源
Yanqin Yang, Chengkuo Lee
https://doi.org/10.1002/dro2.15
摘要：摩擦纳米发电机(TENG)和液体-固体发电装置发展的历史里程碑。
2012年，人们发明了第一个摩擦纳米发电机（TENG），通过摩擦和静电感应的耦合效应将机械能转化为电能。此后，人们为提高这些能量采集器的输出功率密度做出了大量努力，图1a总结了这些里程碑式的成果，其最大功率密度为10 MW/m2。在TENG领域取得的重要成就中，基于液体-固体的TENG（L-S TENG）为人们从海洋、雨滴、潮汐等采集水能铺平道路。如图1b所示，2013首个被报道的L-S TENG 是利用液体作为一个接触层进行接触和分离模式的TENG。在过去的几年里，包括液体类型、接触表面形态和性质以及系统结构的研究都在液体能量采集技术中被报道，目的是为了更好地设计能量采集系统。尽管有这些优化，L-S TENG的最大功率密度仍然是有限的（11.7 W/m2），因为电荷只是通过界面效应产生和转移。除了TENG技术，水电流和反向电润湿是两种有代表性的水能采集技术，但它们的输出也受到界面效应和复杂操作的限制。
为了应对这一挑战，Wang等人在2020年提出了一种基于液滴的发电机（DEG），它具有类似于晶体管的新型结构，用于液滴能量采集。这种新型DEG通过将氧化铟锡衬底上的聚四氟乙烯（PTFE）薄膜与一个在PTFE薄膜表面的小铝电极相结合而成。这种DEG的瞬时功率密度比其表面没有铝电极的同类产品高几个数量级。提升的超高功率密度归因于体效应，即在PTFE表面上撞击的水滴的扩散使得水滴、两个电极和PTFE之间形成了一个闭环的电系统。因此，电荷在体闭环电系统中定向流动，而不是在固液界面。同时，DEG的出色性能也依赖于介质材料（PTFE）较高的表面电荷密度，这是通过连续的液滴撞击（1.6×104秒）对PTFE表面进行预充电而实现的，这样，在饱和的表面电荷密度下，进一步的液滴能量收集可以更有效率。在L-S发电机的这项重要工作之后，Wu等人提出了一种均匀电润湿辅助电荷注入（h-EWCI）方法，该方法允许超高的负电荷密度，从而将功率密度提高到162 W/m2。在发明了DEG之后，动态工作机制和单电极工作模式已经得到了研究，以便能够全面了解DEG并实现多用途应用。除了长的预充电时间（1.6 × 104 s）仍然是商业化道路上的一个障碍外，DEG技术在输出功率密度方面显示了其前景。现在，张楠（Nan Zhang）及其合作者在Droplet期刊上撰文指出，通过系统性地对流体力学和电路系统进行建模和优化，可以实现提高瞬时功率密度和减少充电时间。
本篇由华东师范大学（ECNU）和香港城市大学（CityU）的研究团队撰写的文章中，DEG的输出性能由介质材料、介质材料的厚度、液滴离子浓度和外部负载决定。虽然团队使用了与Xu等人的研究中相似的器件结构和材料，本研究中的作者也研究了介质层厚度和液滴离子浓度的最佳参数，以实现超高的输出密度以及缩短充电时间，实现了2.03 kW/m2的最大瞬时功率密度，同时缩短了6.36秒的充电时间。这是在DEG甚至所有水能采集器中实现的最高输出性能（图1）。
Figure 1
Historical milestones of the development of triboelectric nanogenerators (TENGs) and liquid–solid-based electricity generators.(a) The peak power density roadmap of TENG. (b) The peak power density roadmap of liquid–solid-based electricity generators including TENG and the droplet-based electricity generators (DEGs). (c) Versatile applications of liquid–solid-based TENG (including conventional TENG and DEG).
高输出的重要关注点是DEG中产生的电荷量要充足，并且在扩散的液滴接触到上层电极时能够迅速释放。作者为DEG的工作机制提出了一个类似的电路模型，DEG可以被视为三个电容器之间的电荷转移，例如水-PTFE（CPTFE）、水-PTFE界面（CEDL1）和水-上电极界面（CEDL2）。CEDL1和CEDL2的厚度比CPTFE的厚度小几个数量级，因此CPTFE可以被视为一个电荷源，其达到饱和表面电荷密度的充电时间以及可释放的电荷由PTFE层的厚度来调节。这个模型提供了一个易于理解的方法，可以轻松地理解所有DEG的工作机制。
有三个重要的参数来评估DEG的输出性能，例如，Voc、 Isc和P，它们分别表示为 和。作者总结了介质材料厚度和液滴离子浓度对DEG输出性能的影响。首先，介质材料的厚度不仅影响CPTFE和时间常数，而且当厚度足够大时，通过减少电场强度的中间作用，会使PTFE上的表面电荷Q减少。其次，液滴浓度与EDL的形成高度相关，因此表面电荷Q也可能受到影响。液滴浓度也决定了内部阻抗Rw以及时间常数t。第三，除了介质材料的厚度和液滴离子浓度外，功率密度也可能受到外部负载的影响。因此，为了获得DEG的最佳输出性能，所有这三个参数都需要考虑。在这项研究中，当电介质层的厚度为300μm，离子浓度为200mM，外部负载为51kΩ时，报告的最大功率密度约为2.03kW/m2。
此外，充电时间方面，随着厚度的增加，来自一个撞击液滴的PTFE上的存储电荷量增大，而PTFE上的饱和电荷减少，有助于减少撞击次数和缩短充电时间。因此，本研究中实现了缩短6.36秒的充电时间。
N. Zhang等人为研究人员提供了一个明确的方向，即通过控制介质材料、介质材料厚度、液滴浓度和外部负载来提高DEG的性能。除了本研究关注的最佳的介质材料厚度和液滴离子浓度外，开发具有高电荷存储能力的新型材料以进一步提高DEG的输出性能也是很有前景的。当然，本研究所报告的工作机制和电路模型对于开发基于液体-固体的发电装置，实现更高的功率密度和实用性至关重要。如图1c所示，液态固体发电装置的应用已经涵盖了许多领域，如海洋、潮汐、雨滴等的能量采集器，以及自供电的物理和化学传感器。高功率密度DEG或其他液体-固体发电装置的进一步发展可以提供无缝解决方案，以保持这些领域的持续繁荣。
上下拉动翻看英文原文
In 2012, the first triboelectric nanogenerator (TENG) was invented to convert mechanical energy into electricity via the coupling effects of triboelectrification and electrostatic induction. Since then, extensive efforts have been devoted into increasing the output power density of these energy harvesters, and the milestones have been summarized in Figure 1a with a maximum power density of 10 MW/m2.2 Among the exciting achievements in TENG field, the liquid–solid-based TENGs (L–S TENG) are paving their ways for harvesting water energy from oceans, raindrops, tides, and so on. As shown in Figure 1b, the first L–S TENG was reported in 2013, which utilized the liquid as one contact layer for a contact-and-separation mode TENG.8 In the past few years, the investigations including liquid types, contact surface morphologies, and properties, as well as system architectures are reported in the liquid energy harvesting technology aiming for a better system design. Despite these optimizations, the maximum power density of L–S TENGs is still limited (11.7 W/m2) because the charges are only generated and transferred by interfacial effect. In addition to TENG technology, hydro-voltaic and reverse electrowetting are two of the representative technologies developed for water energy harvesting while their outputs are also restricted by the interfacial effect and complicated operations.
Figure 1
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Historical milestones of the development of triboelectric nanogenerators (TENGs) and liquid–solid-based electricity generators. (a) The peak power density roadmap of TENG. (b) The peak power density roadmap of liquid–solid-based electricity generators including TENG and the droplet-based electricity generators (DEGs). (c) Versatile applications of liquid–solid-based TENG (including conventional TENG and DEG).
To meet this challenge, Wang et al. have proposed a droplet-based electricity generator (DEG) with a novel transistor-like architecture for droplet energy harvesting in 2020. This novel DEG was constructed by combining polytetrafluoroethylene (PTFE) film on an indium tin oxide substrate with a small aluminum electrode fabricated on the surface of the PTFE film. The instantaneous power density of this DEG was several orders of magnitude higher than its counterpart without the aluminum electrode on the surface. The boosted ultrahigh power density is attributed to the bulk effect where the spreading of an impinged water droplet on the PTFE surface enabled the formation of a close-loop electrical system between the droplet, two electrodes, and PTFE. Therefore, the charges flow directionally in the bulk close-loop electrical system rather than at the solid–liquid interface. Meanwhile, the outstanding performance of DEG also relied on the high surface charge density of the dielectric layer (PTFE), which is achieved by pre-charging the PTFE surface via continuous droplet impinging (1.6 × 104 s) so that further droplet energy harvesting could be more efficient with the saturated surface charge density. Following this significant work in L–S electricity generator, Wu et al have proposed a homogeneous electrowetting-assisted charge injection (h-EWCI) method to allow an ultrahigh negative charge density so as to enhance the power density up to 162 W/m2. After the invention of DEG, the dynamic working mechanisms and single electrode working modes have been investigated to enable comprehensive understanding and versatile applications of DEGs. With the hydrophobic treatments and/or hydrophobic structures, the resulted dielectric materials have shown even higher output.18, 37 Except to that the long pre-charging time (1.6 × 104 s) remains as a hurdle on the road for commercialization, the DEG technology shows its promise in terms of output power densities. Now, writing in Droplet, Nan Zhang and colleagues show that boosted instantaneous power density and reduced charging time can be realized via systematically modeling and optimizing the hydrodynamic and circuit system.
The research team based at the East China Normal University (ECNU) and City University of Hong Kong (CityU) reports that the output performance of DEG is determined by the dielectric materials, the thickness of dielectric materials, the droplet ion concentration, and the external impedance. Even using the similar device structure and materials as reported in Xu et al., the authors in this study have investigated the optimal parameters of dielectric layer thickness and droplet ion concentration to enable ultrahigh output density as well as shorting charging time. A maximum instantaneous power density of 2.03 kW/m2 was realized, accompanied with a shortened charging time of 6.36 s. This is the highest output performance achieved in DEGs and even in all water energy harvesters (Figure 1).
The important concern of high output is that the amount of generated charges in DEG should be sufficient and can be released rapidly when the spreading droplet contacts the upper electrode. The authors have proposed a similar circuit model for the DEG working mechanism where the DEG can be treated as the charge transfer between three capacitors, for example, water-PTFE (CPTFE), water-PTFE interface (CEDL1), and water-upper electrode interface (CEDL2). The thickness of CEDL1 and CEDL2 are several orders of magnitude smaller than that of CPTFE, thus the CPTFE can be treated as a charge source, in which the charging time to reach a saturated surface charge density as well as the available charges to be released are modulated by the thickness of PTFE layer. This model provides a friendly approach to easily understanding the working mechanism of all DEGs.
There are three important parameters to evaluate output performance of DEG, for example, Voc, Isc, and P. They have been expressed as , , and , respectively. The authors have summarized the impacts of dielectric material thickness and droplet ion concentration to the output performance of DEG. Firstly, the thickness of dielectric materials not only influences the CPTFE and time constant but also brings reduced surface charge Q on PTFE through the intermediate of the decreased electric field intensity when the thickness is larger enough. Secondly, the droplet concentration is highly related with the formation of EDL so that the surface charge Q could be influenced as well. The droplet concentration also determines the internal impedance Rw as well as the time constant t. Thirdly, the power density could also be influenced by the external impedance besides dielectric material thickness and droplet ion concentration. Therefore, to obtain the optimal output performance of DEG, all the three parameters are required to consider. In this study, the maximum power density reported is about 2.03 kW/m2 when the thickness of dielectric layer is 300 μm, the ion concentration is 200 mM, and the external impedance is 51 kΩ.
Furthermore, speaking of the charging time, it is reported that as the thickness increase, the amount of stored charge on PTFE from one impinging droplet is enlarged while the saturated charge on PTFE is reduced, contributing to reduced impinging times and a short charging time. Thus, a shortened charging time of 6.36 s is achieved in this study.
N. Zhang et al. provide an explicit direction for researchers to enhance the performance of DEG by controlling the dielectric materials, dielectric materials thickness, droplet concentrations, and external impedance. Besides the efforts of investigating optimal dielectric material thickness and droplet ion concentration in this study, it is promising to develop novel materials with high charge storage capability to further advance the output performance of DEG. Certainly, the working mechanism and circuit model reported in this study are crucial in the development of liquid–solid-based electricity generators towards higher power density and practicality. As indicated in Figure 1c, the applications of liquid–solid-based electricity generators have covered many fields, such as energy harvesters from oceans, tides, raindrops, and so on, as well as self-powered physical and chemical sensors. Further advancements of high-power-density DEGs or other liquid–solid-based electricity generators could provide seamless solutions to keep these fields continuously blooming.