From carbon nanotubes to highly adaptive and flexible high-performance thermoelectric generators

Highlights

• Introducing a 3-level structure principle for fabricating the high-performance AF-TEG.

• Large-area modification strategy for numerous p- and n-type CNT thermoelectric legs.

• A high thermoelectric voltage of 1.05 V can be generated by the AF-TEG at ΔT ≈ 44 K.

• Power management scheme regulates the thermoelectric voltage for use in practice.

Abstract

Flexible thermoelectric generators (TEGs) provide a sustainable solution for directly harvesting low-grade heat from various curved surfaces, presenting a promising potential for feeding energy to personal-electronics or internet-of-things systems. Nevertheless, the room-temperature millivolt output of the cutting-edge all-solid-state flexible material-based TEGs (AF-TEGs) is still unappeasable for flexible electronics in practical applications. Here, we report on an AF-TEG consisting of a 3-level structure based on thin-film carbon nanotube (CNT) assemblies that can generate a high voltage (1.05 V) and sufficient outpower (0.95 mW) at moderate ΔT (~44 and 39 K, respectively) for powering an electrochromic device. The advanced nature of the AF-TEG is that conformation is highly adaptive and programmable, showing the surface suitability which can capture waste heat in both in-plane and out-of-plane direction. Moreover, an AF-TEG power management scheme is proposed for powering a commercial LED and an electronic thermometer, which indicate promising potential for practical applications.

Introduction

Renewable energy sources such as waste heat have attracted much attention in response to the current increasing demands for electricity and depletion of fossil fuels [1], [2]. However, most of thermal energy such as solar heat, body heat, and engineering heat diffuses into the ambient atmosphere invalidly. Flexible thermoelectric generators (TEGs) with all-weather thermal conversion capability enable directly harvest such waste heat from curvilinear surfaces into electricity noiselessly, thereby improving energy efficiency through waste heat recovery [3]. In addition, the thermal energy harvesting technology provides a reliable solution for energy requirements in the field of flexible and portable microelectronics because of sustainable power-supply characteristics [4].

Thorough investigations have been implemented in developing high-performance flexible TEGs over recent decades [2], [5], [6], [7], [8], [9], [10]. Among thermoelectrics, ion TEGs with a capacitive configuration, including liquid-state thermocells and ion gelatin, enable produce substantial voltage by harvesting low-grade heat [11], [12]. However, the redox reactions or thermodiffusion that occurs under temperature gradients in these ionic systems usually lead to electrode corrosion, resulting in reduced working life. Besides, the encapsulation and stability are shortcomings in use that need to be addressed. Alternatively, all-solid-state flexible TEGs continuously generate electricity via the migration of electrons or holes without any chemical reactions, which support more reliable use in reality. Compared with the flexible TEGs based on block materials, devices integrated with flexible TE materials have higher curvature adaptability and wearing comfort, and therefore have a broader development prospect. Recently, Zhou and coworker [13] reported a leaf-inspired flexible TEGs fabricated by using p-type PEDOT:PSS film and n-type constantan alloy thin film, which is capable of directly capturing the body's heat energy by vertically standing on the skin. Lv et al. [14] used single-wall carbon nanotube films to build spring-shaped wearable TED with fundamental dual elastomer layers. Furthermore, all-solid-state flexible material-based TEGs (AF-TEGs) can be fabricated using earth-abundant element, for example of carbon, making AF-TEGs inexpensive in the future [7], [15]. However, from the practical perspective of thermoelectric (TE) conversion, the output from 24 to 123 mV or 10 to 173 μW of the state-of-the-art AF-TEG at a ΔT of 5.2–18.5 K, to the best of our knowledge, still hardly meets the practical requirements of microelectronics [4].

The challenges encountered in the sufficient performance of AF-TEGs mainly focus on two aspects: High-density integration of TE legs, and scalable fabrication of high-performance flexible TE materials [3], [7]. First, although high-density integration corresponds to a theoretically linear augment in open-circuit voltage [16], the increased TEG area and resistance associated with internal metal electrodes across the device are still obstruction in real use. Second, advances in integration of massive TE legs enhance the aggressive requirements of the large-area TE materials with robust TE properties [3], but the existing TE materials fabricated using vacuum-assisted filtration, printing technology, and so on are not sufficient to enable the industrial production for high-density integrated generators [17], [18]. In the face of these challenges, a necessary solution is to obtain low-cost and large-area TE materials that are facilely integrated into high-density flexible TE devices.

Carbon nanotubes (CNTs) are especially attractive for the great potential use in AF-TEGs, as their energy bands are readily adjustable, as well as the inherent flexibility in nature and scalable preparation [5], [19], [20]. In addition, the semiconductor characteristic of CNTs can be modified via simple physical adsorption or chemical treatments, such as coating and annealing, which is conducive to achieving n-type characteristic and excellent TE performance [21]. To date, however, most design strategies for CNT-based TEGs still involve alternatively connecting p- and n-type legs via depositing metals or fixing wires [22], [23]. Compared to the bare TE legs, the conformation of the TE leg-metal bridge-TE leg (TMT) offers a limited possibility to promote the multi-pair integration of the AF-TEGs because of its expanded device volume. Especially for the double-interfaces in each TMT unit, electrical performance decay on account of increasing contact resistance severely limits the output power density of the AF-TEGs [5], [24], [25]. Therefore, the traditional TMT geometry capable of power generation is not definitely suitable for CNT-based AF-TEGs. Innovative strategies that optimize the unit configuration rolled into a generator yielding the decreasing metal bridges are urgently needed for an AF-TEG with sufficient power output that sustainably, directly powers microelectronics.

To confront the aforementioned challenges, we propose a 3-level structure principle for constructing the high-performance AF-TEG using CNT films as the precursors. First, we employed large-scale p- and n-type materials modified from thin-film CNTs assemblies in producing 1st-level TE units, in order to form an all-CNT single-interface in individual unit instead of traditional double-interface structure for reducing the resistance. Second, the flexible 2nd-level TE films were built by hot-pressing TE units electrically in series and thermally in parallel to guarantee multiple TE couples for high-density integration. Third, the 2nd-level TE films were further constructed into a highly adaptive, light-weight AF-TEG as 3rd-level structure for realizing ultra-high-power density. Owing to above designs, this AF-TEG comprising ten 2nd-level TE films is capable of twisting or folding into three-dimensional (3D) direction from 2D structure, has ability to harvest diverse waste heat from multiple surfaces, such as human body. This design strategy has the other advantage that it is naturally compliant nature of LEGO-style to add the TE units directly, thereby facilitating a maximized open circuit voltage of 1.05 V at a ΔT of ~44 K. For practical use, our AF-TEG exhibits significant capability in powering an electrochromic device at ΔT ≈ 32 K, demonstrating a beneficial prospect as a self-powered smart window for regulating internal light intensity and temperature by colour-change in the future. We further explored the power management technology based on the AF-TEG: a thermal voltage was converted from 0.35 to ~4 V using a power management technology to successfully power a commercial light emitting diode (LED) and an electronic thermometer. Overall, this truly adaptive AF-TEG provides a propagable and practical solution for the recovery of low-grade heat and energy supply for microelectronics.