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Backgrounds and Principles of Low-Grade Heat Harvesting

Developing sustainable energy is one of the most promising ways to mitigate the energy crisis caused by fossil fuels and greenhouse gas emissions. Low-grade heat (<100?°C), as a typical wasted energy, exhibits widely in nature, industry, and daily life. Except for geothermal energy, almost all generated heat resources (solar heat, machine heat, body heat, etc.) depend on some factors and cannot meet the continuous energy demand of the modern Internet of Things (IoTs) [1]. In particular, partial solar energy can be converted into electricity by solar cells based on the photovoltaic effect. However, the enormous amount of thermal energy is still not harvested efficiently. Therefore, the development of devices to convert heat into electricity should be the top priority at present.

Among the various technologies, solid-state thermoelectric (s-TE) and liquid thermocell (LTC) are the two dominant energy conversion technologies for high-value-added utilization of low-grade heat [2]. It is worth noting that all thermodiffusion-based cells (TDCs), thermogalvanic effect-based cells (TGCs), and thermoextraction-based cells (TECs) using liquid electrolytes fall into the category of LTCs. With the rapid development of sustainable energy, high power, and energy electronics, new and urgent demands have been placed on energy conversion technologies, such as the integration of TE conversion and storage. Unfortunately, most of the reported devices can only realize the energy conversion from heat to electricity, and external energy storage devices (i.e. capacitors and batteries) are required to store the charge generated from thermoelectrical processes, which increases the cost and complexity of the developed systems. In general, both capacitors and batteries are the leading energy storage technologies. Especially, Li-ion batteries as one of commercial devices are widely used in consumer products due to their high energy supply. In addition, Li-ion batteries have recently been developed to further improve the energy density of batteries and meet certain requirements. However, the sluggish kinetics of electron and ion during the charging and discharging processes may lead to partial energy loss, which increases heat generation and dendrite formation. In addition, some reported failures in electric cars, airplanes, and energy storage power plants prompt us to pay attention to battery thermal management. Electrochemical capacitors, also known as supercapacitors, can output a high power density and complement batteries in some areas due to their low cost, long cycle life, and satisfactory safety. However, many efforts are being made to improve the relatively low energy density of capacitors to meet the growing interest in high energy and power. Therefore, the development of thermoelectrochemical devices with integrated energy conversion and storage, which have fast response, long durability, and high energy/power density, is of great importance [3]. To our knowledge, TE devices can be classified into three different forms, including organic Rankine cycles, traditional TEs, and thermocells. TEs can be further divided into two types based on the electrode materials used: inorganic TEs and organic TEs. Thermocells can be identified as devices based on electric double-layer capacitive (EDLC) mechanism and pseudocapacitive behavior.

1.1 Backgrounds and History

The development history of thermoelectrical devices is a story of discovering energy conversion mechanisms. The demonstration of direct conversion from heat to electricity can date back to 1822. The first physical effect, named "Seebeck effect," was found by German scientist, Thomas Johann Seebeck. As displayed by the timeline in Figure 1.1, two different metal wires were connected to form a current loop in this experiment. When heating one of the junctions while the other junction was maintained cold, there was a magnetic field exhibiting around the circuit. Although Seebeck did not provide a correct explanation for this interesting phenomenon, it did not prevent him from conducting comparative works on many materials, which laid the foundation for later thermoelectrical studies. On the basis of this design, J. C. A. Peltier found that the temperature near the junction can be changed when current flowed through two different metals in 1834, which is also called as "Peltier effect", confirming the TE effect. Until 1838, the feature of the Peltier effect was rationally explained by Heinrich Friedrich Emil Lenz: whether the junction of two different conductors absorbs or releases heat depends on the direction of current flowing through the circuit, and the amount of heating (cooling) is proportional to the magnitude of the current. As a proof of concept, the preformed ice on the junction can be melted into water only by changing the direction of the current.

Both the Seebeck effect and the Peltier effect are discovered on the junction of two different conductors; however, those are still not interface effects. The relationship between the TE effect was not well recognized until the nineteenth century. In 1857, William Thomson (Lord Kelvin) made a comprehensive analysis of the Seebeck effect and the Peltier effect using the thermodynamic principle established by himself. Thomson believed that there was a simple multiple relationship between the Peltier coefficient and the Seebeck coefficient at absolute zero. On this basis, he theoretically predicted a new TE effect, that is, when a current flows through a conductor with uneven temperature, the conductor will not only generate irreversible Joule heat but also absorb or release a certain amount of heat. This new TE effect is also known as the Thomson effect.

Figure 1.1 Historic timeline for the development of thermoelectric devices.

In the 1910s, Edmund Altenkirch proposed a satisfactory theory for TE refrigeration and power generation. In detail, a promising TE material must have the following merits including a large Seebeck coefficient (S, ensure relatively obvious TE effect), high electrical conductivity (s, reduce the generated Joule heat), and low thermal conductivity (?, retain heat near the junction). It is worth mentioning that the relationship among such parameters can be described as: Z = S2s/?, which can be used to evaluate the thermoelectrical performances in practical applications. Since only metals and their compounds were considered important conductors at the time, researchers focused their attention on metals and corresponding alloys, ignoring semiconductor materials. Nevertheless, most metals exhibit a very low Seebeck coefficient (~10?µV?K-1), which not only results in a low conversion efficiency but also limits the development of thermoelectrical devices.

With the rapid development of semiconductor materials in the middle of the twentieth century, researchers found that the Seebeck coefficient of semiconductors was 10 times higher than that of metals. In 1947, the first semiconductor-based TE generator was invented by Telkes, which can deliver a heat-to-current efficiency of 5%. Meanwhile, the first TE refrigeration prototype was fabricated in 1953 by Bi2Te3 and Bi as working electrodes. However, the advantages of semiconductor as TE material are not fully manifested due to the significant difference in conductivity between semiconductors and metals in the time. In the following time, a lot of works are focused on the properties optimization of semiconductors to obtain a high energy conversion efficiency.

In the 2010s, a new class of thermoelectrochemical system, termed LTCs, which includes thermionic capacitors and pseudocapacitive thermocells, was proposed. Typically, the thermoelectrochemical performances of LTCs can be greatly improved by electrode development and electrolyte optimization. For example, Crispin et al. constructed an ionic TE supercapacitor through a remarkably strong ionic Soret effect (thermal diffusivity) using polymeric electrolyte and carbon nanotube (CNT) electrode, which can realize the conversion of heat into stored charge [4]. On the other hand, Liu and Chen et al. have proposed a new ionic TE material based on the synergistic TD and TG effects, demonstrating significant promise for heat-to-current conversion using ions as energy carriers and opening up a new research field for high-performance thermoelectrochemical devices [5]. In 2022, Zhang and co-workers demonstrated a zinc ion thermal charging cell (ZTCC) based on the TD and TE of electrolyte ions. Because of the unique feature of a multivalent charge carrier and the relatively low potential of zinc anode, a high output voltage and large ionic Seebeck coefficient/thermopower can be achieved [6].

Since the 2020s, the amount of research related to TE devices has continuously and dramatically increased in line with the emerging increased demand for sustainable, flexible, high performance, and safe energy conversion-storage devices. Moreover, many advanced materials and devices have been discovered for high-value-added conversion of low-grade heat into stored energy, along with the development of nanoscience and characterization techniques.

1.2 Working Principles of Current Systems

Typically,...