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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 12: 1915 - 1928

DOI: https://doi.org/10.7536/PC240322

Review

Biomass-Based Ionic Thermoelectric Devices

  • Sai Zheng 1, 2 ,
  • Xiaoyu Guan , 1, 2, * ,
  • Bingyuan Zhang 1, 2 ,
  • Yanxia Zhu 1, 2 ,
  • Dongping Li 1 ,
  • Qingxin Han , 1, 2, * ,
  • Xuechuan Wang , 1, 2, *
Expand
  • 1 College of Bioresources Chemical and Materials Engineering (College of Flexible Electronics), Shaanxi University of Science & Technology, Xi’an 710021, China
  • 2 Institute of Biomass & Functional Materials, Shaanxi University of Science & Technology, Xi’an 710021, China
* e-mail: (Xiaoyu Guan);
(Qingxin Han);
(Xuechuan Wang)

Received date: 2024-03-20

  Revised date: 2024-06-05

  Online published: 2024-06-30

Supported by

National Natural Science Foundation of China(22308210)

Scientific Research Program Funded by Shaanxi Provincial Education Department(23JK0350)

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Abstract

The new ionic thermoelectric material based on biomass has the advantages of high ionic Seebeck coefficient, good flexibility, low cost, green biodegradability, etc., and has broad application prospects in the construction of safe, stable, and efficient flexible wearable thermoelectric devices. In this paper, the preparation methods, thermoelectric principles, and thermoelectric properties of ionic thermoelectric capacitors and ionic thermocells based on biomass materials such as cellulose and gelatin and their latest applications in wearable body heat collection devices, flexible temperature sensors, and self-driven human monitoring systems in the past five years are reviewed. Combined with the current research, we further summarize the difficulties and shortcomings in the research of biomass-based ionic thermoelectric materials, as well as the difficulties and challenges facing the future promotion and application of biomass-based thermoelectric devices. Finally, we propose targeted solution ideas for the existing problems, providing important theoretical guidance and technical references for related research in this field.

Contents

1 Introduction

2 Ionic thermoelectric overview

2.1 Thermoelectric materials

2.2 Ionic thermoelectric effect

2.3 Ionic thermoelectric properties examination index

3 Research progress in biomass-based ionic thermoelectric devices

3.1 Biomass materials overview

3.2 Preparation of biomass-based ionic thermoelectric materials

3.3 Design of biomass-based ionic thermoelectric components

3.4 Functional applications of biomass-based ionic thermoelectric devices

4 Conclusion and outlook

Key words： biomass materials; ionic thermoelectric devices; ionic thermocells; flexible wearable devices

Cite this article

Sai Zheng , Xiaoyu Guan , Bingyuan Zhang , Yanxia Zhu , Dongping Li , Qingxin Han , Xuechuan Wang . Biomass-Based Ionic Thermoelectric Devices[J]. Progress in Chemistry, 2024 , 36(12) : 1915 -1928 . DOI: 10.7536/PC240322

1 Introduction

Low-grade heat (<100 ℃) is widely present in nature and human society, including solar thermal, ocean thermal, geothermal, industrial waste heat, and human body residual heat. This type of low-grade heat has low temperature, low concentration, and low energy, making it difficult to collect and utilize[1⇓-3]. To alleviate the energy crisis, environmental pollution, and global warming caused by the dependence on fossil fuels, thermoelectric conversion technology has become one of the key research directions for scientists exploring new renewable green energy sources, breaking through the limitations of fossil fuels, and achieving sustainable development of energy and the environment[4⇓-6].
Thermoelectric conversion technology, also known as thermoelectric power generation technology, is a green power generation technology that utilizes temperature differences to convert thermal energy into electrical energy, enabling the collection and utilization of low-grade thermal energy. Thermoelectric materials mainly include two major categories: electronic and ionic types, which are functional materials capable of converting thermal energy into electrical energy[7⇓-9]. Traditional electronic thermoelectric materials include metallic alloy conductors, semiconductors, and conductive polymers, typically characterized by high electrical conductivity, but they suffer from issues such as low Seebeck coefficient, certain biotoxicity, and high costs[10⇓-12]. Compared to traditional electronic types, ionic thermoelectric materials have a Seebeck coefficient in the millivolts per Kelvin (mV/K) range, along with advantages such as good flexibility, low cost, and low toxicity and pollution, offering broad application prospects in fields such as thermoelectric generators, self-powered systems, wearable body heat harvesting devices, aerospace, and artificial intelligence[13⇓-15].
Currently, ionic thermoelectric materials are mainly composed of organic polymer matrix and ionic electrolytes, where the matrix materials mainly include synthetic polymer (polyelectrolyte gels) and natural polymers (natural biomass and derived materials). Compared with synthetic polymer materials, natural biomass and derived materials have advantages such as being green, non-toxic, environmentally friendly, low-cost, renewable, and easily biodegradable. Ionic thermoelectric materials constructed with natural biomass matrices like cellulose and gelatin also possess a three-dimensional natural framework advantage. In recent years, ionic thermoelectric devices built using biomass materials as the matrix have shown excellent thermoelectric conversion performance, demonstrating broad application potential in low-grade heat harvesting[16⇓-18].
This review first introduces the research status of ionic thermoelectrics, then focuses on summarizing the preparation methods and application fields of biomass-based ionic thermoelectric devices, and finally summarizes the challenges and deficiencies in the current research on biomass-based ionic thermoelectric materials, offering suggestions and prospects for areas that urgently need to be advanced.

2 Overview of Ionic Thermoelectrics

2.1 Thermoelectric Materials

In 1821, the proposal of the Seebeck effect marked the birth of thermoelectric materials and the thermoelectric effect. The Seebeck effect can be simply explained as the movement of charge carriers within a metal conductor or semiconductor from the hot end to the cold end under a temperature gradient, accumulating at the cold end, thereby forming an electric potential difference inside the material. At the same time, under the action of this potential difference, a reverse charge flow is generated. When the thermal motion of the charge flow and the internal electric field reach dynamic equilibrium, a stable thermoelectric potential is formed at both ends of the material.
Traditional thermoelectric materials mainly use inorganic compounds, such as bismuth telluride (Bi2Te3)[19], lead telluride (PbTe)[20], germanium telluride (GeTe)[21], and silicon-germanium (SiGe) alloys[22]. Although these alloy-based semiconductor thermoelectric materials exhibit excellent thermoelectric performance, the cost of thermoelectric materials accounts for more than half of the total cost of a thermoelectric device due to the toxicity, scarcity, and high price of rare elements in the alloys, thus limiting the widespread application of thermoelectric devices. In addition, the mechanical rigidity of inorganic semiconductors and semi-metallic alloys also restricts their use in portable and wearable devices[23-24].
In recent years, new ionic thermoelectric materials based on organic polymers such as polyvinyl alcohol [25], polyacrylamide[26], polyethylene oxide (PEO)[27], waterborne polyurethane[28], bacterial cellulose[29], and gelatin [30] have received extensive attention due to their excellent thermoelectric conversion performance and good flexibility. Among these, biomass materials represented by cellulose and gelatin also possess advantages such as low cost, non-toxicity, good biocompatibility, abundant reserves, renewability, and easy biodegradability, making them promising as new green thermoelectric conversion materials with broad application prospects in fields like flexible wearable devices.

2.2 Ionic Thermoelectric Effect

Due to the differences in charge carriers within materials, the thermoelectric conversion principles of electronic and ionic thermoelectric materials also differ. Electronic thermoelectric materials generate a potential difference due to the thermal migration of electrons or holes inside the material under a temperature gradient, which is known as the Seebeck effect where electrons serve as the energy carriers. In contrast, ionic thermoelectric materials form a potential difference due to the thermal migration of anions and cations inside the material under a temperature gradient; this effect is also referred to as ionic thermal diffusion, as shown in Figure 1a. Besides this method of generating a thermoelectric potential, ionic thermoelectric materials have another way through introducing redox pairs with controllable temperature into the material (such as Fe(CN)64- and Fe(CN)63-). The redox pairs undergo redox reactions on both sides of the electrodes, utilizing the entropy change from the redox reactions to achieve electron transfer, thereby converting thermal energy into electrical energy. This effect is known as ionic thermogalvanic, as illustrated in Figure 1b.
图1 （a）热扩散效应示意图；（b）热电流效应示意图

Fig. 1 (a) Schematic of thermodiffusion effect; (b) Schematic of thermogalvanic effect

2.3 Evaluation Metrics for Ionic Thermoelectric Performance

The thermoelectric performance of ionic thermoelectric materials is typically evaluated by the ionic Seebeck coefficient (S), and its calculation process can be expressed by a formula:
$S=\frac{V}{\Delta T}$
the ionic Seebeck coefficient, which is the ratio of the open-circuit voltage generated at both ends of a material to the temperature difference applied across the material, represents the transport properties of anions and cations within the material and is a fundamental parameter that determines the ionic thermoelectric effect of the material.
In recent years, to comprehensively evaluate the thermoelectric performance of ionic thermoelectric materials, many research efforts have used the dimensionless thermoelectric figure of merit (ZT) at absolute operating temperatures to assess ionic thermoelectric performance. The key parameters of the ZT value can be expressed by a formula:
$ZT=\frac{{{S}^{2}}\mathrm{ }\!\!\sigma\!\!\text{ }T}{\mathrm{ }\!\!\kappa\!\!\text{ }}$
where T is the absolute temperature (K); S is the ionic Seebeck coefficient of the material (mV/K); σ is the ionic conductivity of the material (S/m), a parameter indicating the ease with which anions and cations flow within the material; κ is the thermal conductivity of the material (W/(m·K)), also known as the heat conduction coefficient, a performance parameter for measuring the thermal transmission of the material. When it is difficult to measure the thermal conductivity of thermoelectric materials, the power factor (PF) can also be used to evaluate the ionic transport properties of the material, with its key parameters expressed by the formula:
$PF={{S}^{2}}\mathrm{ }\!\!\sigma\!\!\text{ }$
therefore, excellent ionic thermoelectric materials need to have high ionic Seebeck coefficients and ionic conductivity, while maintaining low thermal conductivity. Compared to traditional alloy-based thermoelectric materials, biomass-based ionic thermoelectric materials possess a Seebeck coefficient of millivolts per kelvin and lower thermal conductivity. Although the ionic conductivity of current biomass-based ionic thermoelectric materials is generally numerically lower than the electronic conductivity of alloy-based thermoelectric materials, there is still tremendous room for improvement. How to balance and optimize the conflict among these three parameters in biomass-based ionic thermoelectric materials, and to increase the ionic Seebeck coefficient while maximizing the ZT value, will be the focus and hotspot of future research.

3 Advances in Biomass-Based Ionic Thermoelectric Devices

3.1 Overview of Biomass Materials

Biomass materials refer to natural organic materials derived from living organisms such as animals, plants, and microorganisms, which are primarily composed of carbon, hydrogen, and oxygen in terms of chemical composition. Biomass materials are diverse, widely distributed, and abundant in reserves[31⇓-33]. Common biomass materials include wood, bark, straw, animal skin, seaweed, etc., and further processing and extraction can yield derivative materials such as cellulose, lignin, gelatin, chitosan, and alginate.
Compared with synthetic polymer materials, biomass materials are green, non-toxic, biocompatible, renewable, and biodegradable, potentially replacing synthetic polymer materials derived from fossil resources such as petroleum and coal to achieve sustainable development of energy and the environment[34⇓-36]. In addition, natural biomass materials are rich in a large number of active functional groups and have stable chemical structures, which can be used to prepare high-performance biomass-based composite materials by compounding with other materials. They are widely used as pollutant adsorbents[37], food packaging films[38], drug-controlled release carriers[39], biological tissue scaffolds[40], and artificial skin[41], among others.
In recent years, with the strict control of environmental pollution and the proposal of "dual carbon" goals, biomass-based materials have shown great potential in high-value application fields such as flexible electronic sensors[42], battery separators[43], battery electrodes[44], evaporation power generation[45], and ionic thermoelectrics[46]. Particularly, biomass-based materials have received significant attention and research in the field of ionic thermoelectrics. Ionic thermoelectric capacitors[47] and ionic thermoelectric batteries[48] designed using biomass-based ionic thermoelectric materials, represented by cellulose, gelatin, lignin, agarose, chitosan, and sodium alginate, have been widely applied in wearable body heat harvesting devices[49], flexible temperature sensors[50], and self-powered human monitoring systems[51].

3.2 Preparation of Biomass-Based Ionic Thermoelectric Materials

3.2.1 Cellulose-Based Ionic Thermoelectric Materials

Cellulose is a natural high molecular weight polysaccharide compound composed of glucose, with the main functional group being hydroxyl, distributed at various positions along the cellulose chain, which endows cellulose with good hydrophilicity and reactivity. As the most abundant biomass resource in the plant kingdom, cellulose possesses high toughness, strength, biocompatibility, and biodegradability. Its chemical and physical structural stability can support the formation of ionic gels, laying the foundation for the preparation of cellulose-based ionic thermoelectric materials[52⇓-54].
Cellulose chains contain a large number of hydroxyl functional groups, characterized by a negatively charged surface. Li et al.[55] based on the enhanced ionic selectivity of charged molecular chains and the synergistic thermal diffusion effect of ions, first chemically treated natural wood to prepare cellulose membranes with nano-microchannel structures, then used a certain concentration of sodium hydroxide (NaOH) to penetrate the cellulose membrane, resulting in p-type cellulose-based ionic thermoelectric materials. As shown in Figure 2, the negative charge on the surface of cellulose controls the transport of ions along the cellulose direction. The strong ionic selectivity promotes the transport of Na+ ions along the cellulose nanochannels while hindering the directed transport of OH− ions, thereby increasing the difference in thermal mobility between anions and cations. The ion Seebeck coefficient generated by the ion concentration difference at the cold and hot ends can reach 24 mV/K. This ionic selective cellulose membrane has extremely high application potential in temperature sensing and low-grade heat energy harvesting.
图2 天然纤维素的纳米通道及纤维素膜的离子迁移选择性增强的示意图[55]

Fig. 2 Schematic of nano-channels of natural cellulose and selective enhancement of ion migration in cellulose membranes[55]. Copyright 2019, Springer Nature

Ionic liquids have advantages such as low vapor pressure, low thermal conductivity, high ionic conductivity, and high thermal and chemical stability. They have received widespread attention as an excellent ionic source in the preparation of ionic thermoelectric materials[56⇓-58]. Liu et al.[59] used bacterial cellulose as the framework for ionic gel, combining it with 1-ethyl-3-methylimidazolium dicyanamide ([EMIM]DCA) and employing a modified co-solvent evaporation method to prepare a p-type flexible cellulose ionic thermoelectric gel. The polar cellulose matrix induces the dissociation of ionic liquid, where the anions in the ionic liquid form strong hydrogen bond interactions with the hydroxyl functional groups on the cellulose molecular chains, leading to highly selective cation diffusion under a thermal gradient. This cellulose-based ionic thermoelectric gel ultimately exhibits a high ionic Seebeck coefficient of 18.04 mV/K, a high ionic conductivity of 2.88 S/m, and a low thermal conductivity of 0.21 W/(m·K), achieving a ZT value of 1.33 at 298 K, which is already higher than that of traditional Bi2Te3 (with ZT ~1.18 at 303~473 K)[60].
Carboxylated cellulose contains abundant oxygen-containing functional groups, among which carboxyl groups have high reactivity and can form stable coordination with metal cations. Moreover, transition metal chlorides easily form stable complexes in aqueous solutions[61-62]. Chen et al.[63] first prepared a cellulose/polyvinyl alcohol dry film using the freeze-drying method, then soaked the dry film in an aqueous solution of CuCl2-polyethylene glycol for thorough penetration, where Cu2+ ions formed coordination with the carboxyl groups. The free Cu2+ ions not involved in coordination, influenced by the high concentration of Cl− ions in the aqueous environment, further induced the formation of [CuCl4]2− complex ions. Given that [CuCl4]2− complex ions and Cl− ions have higher diffusion coefficients within the cellulose membrane compared to Cu2+ ions, this results in high-quality n-type ionic thermoelectric performance. Ultimately, this cellulose composite film, featuring copper-coordinated carboxylated cellulose, exhibits an ionic Seebeck coefficient of -26.25 mV/K, ionic conductivity of 0.847 S/m, and thermal conductivity of 0.466 W/(m·K), demonstrating promising application prospects in low-grade heat harvesting.

3.2.2 Gelatin-Based Ionic Thermoelectric Materials

Gelatin is a biomacromolecule prepared by the thermal denaturation of collagen, with its amino acid side chains containing a large number of active groups such as amino, carboxyl, and hydroxyl groups, which can provide crosslinking points for multiple non-covalent crosslinks. At the same time, gelatin has characteristics such as non-toxicity, good biocompatibility, low cost, and ease of solution processing, so it is often selected as a polymer matrix[64⇓-66].
Previous research on ionic thermoelectrics has mainly focused on aqueous solution systems of acids or bases, with liquid ionic thermal batteries facing challenges in encapsulation and the risk of electrolyte leakage[67-68]. Han et al.[69] chose organic gelatin as the matrix, preparing a gelatin-based solid-state ionic thermoelectric gel by introducing the redox couple KCl and K4Fe(CN)6/K3Fe(CN)6. As shown in Figure 3, KCl, decoupled by the COO− groups on the gelatin chains, exhibits p-type thermoelectric behavior dominated by K+ thermal diffusion. At the hot electrode, [Fe(CN)6]4− easily loses electrons to be oxidized into [Fe(CN)6]3−, injecting electrons into the hot electrode; at the cold electrode, [Fe(CN)6]3− readily gains electrons to be reduced back to [Fe(CN)6]4−, leaving holes at the cold electrode. The generated electrochemical potential is consistent with the thermal diffusion of KCl. Therefore, due to the synergistic effect of ionic thermal diffusion and the thermogalvanic effect, this gelatin-KCl-K4Fe(CN)6/K3Fe(CN)6 thermoelectric gel shows an ionic Seebeck coefficient of 17 mV/K, suitable for assembling wearable human body heat harvesting devices. Quasi-solid-state ionic thermoelectric materials have high thermoelectric power and can be designed as flexible, wearable thermoelectric devices, but they also suffer from poor thermal stability, narrow operating temperature ranges, and low power density. Based on this, Li et al.[70] introduced glutaraldehyde into the ion gel system of gelatin-KCl-K4Fe(CN)6/K3Fe(CN)6, where the Schiff base reaction between glutaraldehyde and gelatin molecules forms strong covalent bonds, enhancing the thermal stability of the ionic gel, expanding the operating temperature range from 9 ℃ to 23 ℃, and after freeze-drying, forming a three-dimensional porous structure that facilitates ionic diffusion. The introduction of glutaraldehyde increased the ionic Seebeck coefficient of the original ionic gel to 24 mV/K.
图3 明胶中离子通过热扩散与氧化还原反应协同作用示意图[69]

Fig. 3 Schematic of the synergistic action of ions in gelatine through thermal diffusion and redox reactions[69]. Copyright 2020, AAAS

Current research on ionic thermoelectrics mainly focuses on improving thermoelectric performance by utilizing the interactions between ions and the matrix, but there is a lack of studies on methods to regulate the interactions between ions. Li et al.[71] proposed a strategy to enhance thermoelectric performance through an ionic entanglement effect, introducing CF3SO3K and CH3SO3K organic salts into gelatin hydrogels. Since CF3SO3− and CH3SO3− anions exhibit binding forces through H—F, F—O, and H—O bonds, forming entanglements between the anions, this significantly slows down the thermal diffusion coefficient of anions, widening the difference in thermal diffusion coefficients between anions and cations, thereby increasing the ionic Seebeck coefficient to 28 mV/K. The quasi-solid-state gelatin−CF3SO3K-CH3SO3K obtained through this strategy has great potential for powering IoT sensors.

3.2.3 Other Biomass-Based Ionic Thermoelectric Materials

Lignin is a highly branched, amorphous, three-dimensional natural polymer composed of complex phenolic and aromatic compounds. Lignin is a by-product obtained from the separation of cellulose through chemical treatment of natural wood[72], and due to its low toxicity, eco-friendliness, good biocompatibility, and easy enzymatic degradation, it has been widely used in tissue engineering, drug delivery, energy storage materials, and biosensors[73⇓-75]. Muddasar et al.[76]prepared lignin/polyvinyl alcohol composite films with vertically aligned structures using a template method and directional freeze crystallization. After infiltrating the synthesized lignin-derived films with KOH aqueous solution, as shown in Figure 4, the terminal hydroxyl or phenolic groups of lignin were converted into anionic hydroxyls after soaking in KOH solution, forming a double electric layer due to the attraction between the negatively charged channels and K+, leading to the formation of freely mobile K+and OH−ions that are hindered in mobility after bonding in the electrolyte. The tested ion Seebeck coefficient was 5.71 mV/K and the ionic conductivity was 5.15 S/m. Its low cost and sustainability make it promising for large-scale energy harvesting and wearable devices.
图4 木质素基复合膜的离子选择性示意图[76]

Fig. 4 Schematic of the ion selectivity of lignin-based composite membranes[76]. Copyright 2023, Wiley

Agarose is a linear polysaccharide, primarily extracted and isolated from large marine algae, and due to its excellent gel properties, stability, and biocompatibility, it has been widely used in food processing, drug-controlled release materials, and energy storage electrolyte materials[77⇓-79]. The agarose matrix contains a large number of hydroxyl groups and sugar rings, providing active sites for water storage and gel formation. Based on this, Li et al.[80] designed a sodium dodecylbenzenesulfonate ionic agarose gel, where sodium dodecylbenzenesulfonate provides small-sized cations Na+ and amphiphilic benzenesulfonate groups. As shown in Figure 5, the hydrophilic end of the benzenesulfonate group induces the formation of spherical micelle structures in the agarose solution and connects with the agarose chains, while the hydrophobic alkyl chain ends point towards the center, forming transport channels for Na+. The negatively charged benzenesulfonate groups become part of the gel matrix, promoting the decoupling of anions and cations and increasing the thermal diffusion difference between anions and cations, thereby achieving an ionic Seebeck coefficient of 41.8 mV/K. Due to its advantages of easy solution processing and flexibility, it can be used as a wearable device for low-grade heat harvesting.
图5 琼脂糖基凝胶多孔结构和Na+输运通道的示意图[80]

Fig. 5 Schematic of the porous structure and Na+ transport channels of agarose-based gels[80]. Copyright 2023, Wiley

Chitosan, a polysaccharide composed of glucosamine molecules obtained by deacetylation of chitin, has a regular molecular chain with highly reactive —NH2 functional groups. Chitosan possesses characteristics such as biodegradability, biocompatibility, cell affinity, bioactivity, and non-toxicity, making it widely used in various fields including artificial tissue materials, drug sustained-release materials, and medical absorbable materials[81⇓-83]. Chen et al.[84] designed a multifunctional polymer matrix composed of chitosan and polyvinyl alcohol, using CuCl2 as an ionic dopant for ionic thermoelectric materials. The coordination between chitosan and Cu2+ ions constrains the cations, while the polycationic electrolyte repels the cations, limiting the thermal diffusion rate of Cu2+ ions. Additionally, free Cu2+ ions in aqueous solution coordinate with high concentrations of Cl− ions to form [CuCl4]2− complex ions, further enhancing the anion thermal diffusion rate. This ultimately achieves an ionic Seebeck coefficient of −28.4 mV/K and an ionic conductivity of 4.05 S/m. The synergistic effect of different types of ions and polymers holds significant research value in the design of multifunctional ionic thermoelectric materials.
Sodium alginate is a natural polysaccharide byproduct extracted from brown algae such as kelp or sargassum after the extraction of iodine and mannitol. Sodium alginate possesses excellent stability, solubility, viscosity, and safety, and has been widely used in the food industry, pharmaceuticals, and flexible sensing fields[85⇓-87]. Hsiao et al.[88] synthesized SA/PVA/PEG hydrogels using sodium alginate, polyvinyl alcohol, and polyethylene glycol through a freeze-thaw method, and adjusted the thermoelectric and mechanical properties of the hydrogels by soaking them in solutions of different concentrations of sodium tetrafluoroborate (NaBF4). According to the counterion condensation theory[89], some Na+ ions condense along the negatively charged sodium alginate polymer chains, leading to an increased friction between BF4− ions and the polymer chains, while the Na+ ions not involved in condensation have less friction with the polymer chains. Therefore, free Na+ ions have a higher thermal mobility than BF4−, resulting in the SA/PVA/PEG/NaBF4-1.5 M hydrogel achieving an ionic Seebeck coefficient of 66.8 mV/K and an ionic conductivity of 3.14 S/m. Due to its low cost, easy preparation, and excellent mechanical and thermoelectric properties, it can be used as a potential material for wearable devices.

3.3 Design of Biomass-Based Ionic Thermoelectric Devices

3.3.1 Ionic Thermoelectric Capacitors

Thermoelectric materials based on the thermal diffusion effect generate a thermal voltage through ion thermomigration induced by a temperature gradient, but ions cannot reach the external circuit through the interface between the electrolyte and the electrode. Therefore, thermoelectric materials based on the thermal diffusion effect are typically designed as capacitors, where the charges generated at the electrolyte-electrode interface can be stored in capacitive electrode materials, thereby converting thermal energy into electrical energy and outputting it to the external circuit[90⇓-92].
Ionic thermoelectric capacitors designed with ionic thermoelectric materials exhibit two modes of operation: ionic charging and electronic discharging, which can be further divided into four stages. As shown in Figure 6, taking a p-type ionic thermoelectric capacitor as an example, the first stage is thermal ionic charging, where under the influence of a temperature gradient, cations and anions accumulate near the cold and hot electrodes respectively, forming an ionic concentration difference, thus generating and gradually increasing a voltage between the two electrodes; the second stage is electronic discharging, during which, with an external load connected between the two electrodes, electrons spontaneously transfer from the hot electrode through the external circuit and the load, to compensate for the voltage imbalance caused by ionic accumulation; the third stage is reverse ionic charging, after disconnecting the external load and turning off the heat source, in the absence of the temperature gradient, the accumulated cations and anions at both ends diffuse back to their original state, while the electrons, due to the disconnection of the external circuit, remain on the electrodes, generating a voltage in the opposite direction to that in Stage I. The fourth stage is reverse electronic discharging, when the external load is reconnected, the electrons previously accumulated on the cold electrode flow back to the original electrode through the external circuit and the load, producing a current in the opposite direction to that in Stage III. Among these four stages, Stages II and IV both achieve the conversion of thermal energy into electrical energy. Therefore, based on the cyclic nature of these four stages, ionic thermoelectric capacitors can sustainably convert thermal energy into electrical energy.
图6 p型离子热电电容器的工作模式示意图

Fig. 6 Schematic of the operating mode of a p-type ionic thermoelectric capacitor

Biomass-based ionic thermoelectric materials constructed ionic thermoelectric capacitors exhibit high output current and power. Ke et al.[93] developed a novel gelatin-based ionic thermoelectric capacitor with supramolecular structure by utilizing the multiple non-covalent interactions between ionic liquid [EMIM]DCA and gelatin molecular chains. The output power of the capacitor varies with the load, reaching its maximum when the external resistance is close to the internal resistance of the gel. Under a temperature difference of 10 K and an external resistance of 2 KΩ, a single ionic thermoelectric device can generate a pulsed output power of 0.14 μW and a closed-loop current of 4.5 μA, with its maximum output power being approximately 45 times that of a single conductive polymer (PEDOT: PSS) composite organic thermoelectric module[94]. Wu et al.[95] designed an ionic thermoelectric supercapacitor based on CaBC/NaCl cellulose gel, which, under a temperature difference of 5.5 K and a discharge resistance of 20 KΩ, has a corresponding relative conversion efficiency of about 0.002%. When the external load resistance is 100 KΩ, the maximum power density collected in a single thermal cycle is 3.07 J/m2.
Table 1 summarizes the specific thermoelectric performance parameters of some biomass-based ionic thermoelectric materials based on the thermal diffusion effect. The designed biomass-based ionic thermoelectric capacitors have received considerable attention, but the intermittent operation mode reduces the current density and thermoelectric conversion efficiency, also limiting the application of ionic thermoelectric capacitors in energy storage and device power supply.
表1 基于热扩散效应的生物质基离子热电材料的热电性能概括

Table 1 Summary of thermoelectric properties of biomass-based ionic thermoelectric materials based on thermal diffusion effect

Base material Transmit ions Seebeck mV/K Ionic conductivity S/m Heat conductivity W/(m·K) ZT year
Cellulose NaOH 24 — 0.48±0.03 — 2019[55]
CMC-Na NaCl 17.1 2.68 — — 2021[47]
BC [EMIm]DCA 18.04 2.88 0.21 1.33 2021[59]
BC NaCl −27.2 20.42 0.318 — 2022[95]
GEL [EMIm]DCA 2.83 2.29 — — 2022[93]
CS/PVA CuCl2 −28.4 4.05 0.487 2.42 2023[84]
AG Na:DBS 41.8 0.0182 — — 2023[80]
SA/PVA/PEG NaBF4 66.7 3.14 — — 2023[88]
Lignin/PVA KOH 5.71 5.15 0.195 0.25 2023[76]

3.3.2 Ionic Thermal Battery

Based on the thermal current effect, ionic thermoelectric materials differ from ionic thermal diffusion in that temperature-induced redox reactions occur between the electrolyte and the electrode, and electron exchange can directly reach the external circuit through the interface of the electrolyte and the electrode to form a current. Therefore, its thermoelectric conversion is a continuous process, and the generated voltage can directly power various electronic devices. A simple ionic thermoelectric cell can be prepared with two electrodes and a quasi-solid-state electrolyte containing redox couples, which has the advantages of simple preparation, continuous current output, and long-term stable operation, laying the foundation for the multifunctional design and large-scale application of ionic thermoelectric devices[96⇓-98].
Currently, commonly used redox pairs for biomass-based ionic thermoelectric materials include Fe2+/Fe3+[99], [Fe(CN)6]3−/[Fe(CN)6]4−[100], and I−/I3−[101]. These are combined with biomass and derived materials to construct quasi-solid-state ionic thermal batteries. For example, Zong et al.[102] designed a thermoelectric gel prepared from bacterial cellulose and FeCl2/FeCl3, which was then assembled into a cellulose hydrogel ionic thermal battery using a composite electrode sheet of carbon fiber paper and copper. By connecting six thermoelectric units in series and encapsulating them with flexible polydimethylsiloxane material, an open-circuit voltage of about 2 V could be generated at a temperature difference of 50 K, with an external circuit current reaching 950 μA and a maximum power of 535 μW. This ionic thermal battery could charge a 470 μF capacitor to ~2 V in 6 s. Wu et al.[103] introduced K4Fe(CN)6/K3Fe(CN)6 and guanidine hydrochloride (GdmCl) into a nanocellulose/polyacrylamide composite hydrogel matrix, achieving an ionic Seebeck coefficient of 3.84 mV/K and an ionic conductivity of 10.85 S/m, as shown in Figure 7. A simple ionic thermal battery device designed by connecting 15 units in series, with a temperature gradient of 65 K applied between the cold and hot ends, resulted in an equilibrium thermal voltage of approximately 3.35 V, a maximum current of 0.6 mA, and a power output of 0.23 W. The voltage generated by the series device could directly power electronic devices such as calculators, thermometers, fans, and LEDs, demonstrating the significant potential for low-grade heat energy harvesting.
图7 由15个单元串联的高功率离子热电池供电装置[103]

Fig. 7 High-power ionic thermocells power supply device consisting of 15 units connected in series[103]. Copyright 2023, American Chemical Society

The synergistic effect of the electronic Seebeck effect and ionic thermal diffusion can also achieve sustainable output with high thermoelectric performance, as shown in Figure 8. He et al.[104] realized continuous thermally-induced power generation through the coupling of electronic and ionic thermal migration by introducing the ionic liquid BMIM:Cl into porous carbonized grapefruit peel. This ionic thermoelectric material achieved an ionic Seebeck coefficient of 32.7 mV/K and an ionic conductivity of 31.2 S/m. A single simple ionic thermocell generated a voltage of 0.65 V under a temperature gradient of 20 K, and could continuously output voltage and current for over 3000 minutes, demonstrating excellent operational durability. By connecting five ionic thermocells in series, they were able to directly power an electronic thermometer and hygrometer under a temperature difference of 20 K.
图8 电子-离子耦合传输的可持续供电离子热电池[104]

Fig. 8 Sustainably powered ionic thermocells with coupled electron-ion transport[104]. Reproduced with permission from Kuan Sun, published 2023, SpringerLink

Biomass-based ionic thermal batteries are simple to manufacture, cost-effective, and sustainable, holding great potential for continuous power supply to electronic devices. However, due to the current and voltage still being at a relatively low level, they cannot yet match commercial generators and batteries. Therefore, enhancing the thermal stability of biomass-based ionic thermal batteries, to fully utilize large temperature difference scenarios such as solar thermal energy and machine waste heat, and achieve continuous high-energy power generation, is key to realizing the large-scale application of these batteries.

3.4 Functional Applications of Biomass-Based Ionic Thermoelectric Devices

3.4.1 Wearable Body Heat Harvesting Device

The continuous basal metabolism of the human body produces a large amount of heat, and the main part of the body for heat dissipation is the skin. When the ambient temperature is lower than the body temperature, the excess heat is dissipated through the skin via radiation, conduction, and convection, making human waste heat a valuable low-grade thermal energy[105⇓-107]. Novel ionic thermoelectric materials, mostly based on organic materials, have flexibility and stretchability similar to human skin. Among them, biomass-based ionic thermoelectric materials, with advantages such as biocompatibility, biodegradability, safety, and non-toxicity, have broad application prospects in the construction of wearable body heat harvesting devices.
To improve the thermoelectric output performance of biomass-based ionic thermoelectric devices under small temperature differences, as shown in Figure 9, Li et al. [108] designed a three-dimensional hierarchical Au/Cu electrode on a gel system of gelatin-KCl-K4Fe(CN)6/K3Fe(CN)6 to enhance the output voltage and power of the thermal battery. By series-connecting 24 thermal batteries and encapsulating them into a wearable body heat harvesting device, a voltage of 2.8 V and an output power of 68 µW can be achieved under a temperature difference of 10 K. The wearable body heat harvesting device generates electricity by collecting heat when worn on the human forearm. In an indoor environment during summer, with a temperature difference of about 3.9 K between the human body and the environment, a stable voltage of 1.04 V can be generated, allowing the wearable body heat harvesting device to directly drive an electronic watch without the need for additional voltage boosters. Han et al. [109] also optimized the gel composition by introducing graphene in the gel system of gelatin-KCl-K4Fe(CN)6/K3Fe(CN)6, thereby enhancing ionic thermopower and output power density. A wearable device assembled from 20 gel thermoelectric generator units in series could drive an electronic watch with the aid of an amplifier, utilizing the temperature difference between the human body and the environment. Hu et al. [110] used cellulose solvents, benzyltrimethylammonium hydroxide (BzMe3NOH) and tetramethylammonium hydroxide (Me4NOH), to dissolve cellulose and serve as electrolytes, preparing p-type and n-type cellulose ionic thermoelectric hydrogels. Using carbon cloth as the electrode material, a thermoelectric generator was constructed by connecting 10 pairs of p-n junctions in series and further encapsulated. When wrapped around the human wrist, under a temperature difference of approximately 13 ℃ between the human body and the environment, it could generate a voltage of 420.6 mV.
图9 三维分层电极设计提高离子热电池输出功率用于可穿戴体热供电装置[108]

Fig. 9 Three-dimensional layered electrode design improves ionic thermocells power output for wearable body heat-powered devices[108]. Copyright 2022, Wiley

At present, biomass-based wearable thermoelectric devices show potential application prospects in collecting human body waste heat to continuously power electronic devices. However, the low output power makes it difficult to drive high-power devices, which is still a key factor limiting the wide application of biomass-based wearable body heat harvesting devices. Therefore, how to improve the thermoelectric output power is a major focus and hotspot of future research.

3.4.2 Flexible Temperature Sensor

In recent years, hydrogel-based flexible temperature sensors utilizing resistive[111], capacitive[112], and thermochromic sensing[113] have been extensively studied. However, due to the fact that the resistance and capacitance of hydrogels are not only influenced by temperature but also easily affected by environmental factors such as strain, humidity, and gases, there exists a potential for multiple signal interferences in the application of flexible temperature sensors[114]. Novel ionic thermoelectric materials can generate a potential difference through selective thermal diffusion of ions or thermally induced redox reactions, thereby outputting a single electrical signal and reducing the impact of other environmental factors, providing new ideas for the research on flexible temperature sensing. Among them, biomass-based ionic thermoelectric materials, with their advantages of high flexibility, low cost, and excellent thermoelectric performance, offer new avenues for the design of flexible temperature sensors.
To construct a simple, efficient, sensitive, and stable flexible temperature sensor, Zong et al.[115] introduced redox pairs of K4Fe(CN)6/K3Fe(CN)6 and FeCl2/FeCl3 into bacterial cellulose hydrogels through a simple immersion method to obtain p-type and n-type ionic thermoelectric gels. Low-cost copper electrodes were alternately connected to the top and bottom of the p-n thermoelectric gels, which were then encapsulated with epoxy resin, ultimately assembled into a 3×3 thermoelectric array. By inducing temperature changes in different parts of the thermoelectric array, electrical signal changes were generated. Heating "T", "E", and "C" shaped heat-conductive glass plates, the thermoelectric array could sense corresponding voltage signals. Yin et al.[116] designed a bacterial cellulose-based thermal battery using K4Fe(CN)6/K3Fe(CN)6 and LiBr as ionic sources, and then constructed a touch-responsive device from an array composed of multiple thermoelectric systems. The thermoelectric device was connected to a boost converter, which was then connected to the corresponding LED lights. Utilizing the temperature difference formed by human touch to output electrical energy, when touching the thermoelectric devices at different positions of the matrix, the corresponding LED lights would light up. This touch-responsive thermal sensing device can be applied in scenarios such as elevator buttons. Han et al.[117] used the cellulose derivative polyquaternium-10 (PQ-10) as the polymer matrix and NaOH as the ionic source to prepare a cellulose-based ionic thermoelectric gel, and designed a highly sensitive flexible temperature sensor array. As shown in Figure 10, the working principle of this flexible temperature sensor is based on the potential difference generated between the sensing electrode and the reference electrode under a temperature gradient. By further integrating multiple thermal sensor arrays, a smart glove was assembled, which could successfully detect the temperature and touch position of contacted objects by monitoring the voltage of all sensing nodes.
图10 超灵敏的柔性温度传感器矩阵结构及温度感知的智能手套[117]

Fig. 10 Ultra-sensitive flexible temperature sensor matrix structure and temperature-aware smart gloves[117]. Reproduced with permission from Dongyan Xu, published 2023, Wiley

At present, flexible temperature sensor arrays constructed with biomass-based ionic thermoelectric materials exhibit high flexibility, high thermoelectric power, and high sensitivity. By monitoring changes in voltage signals, it is anticipated that more flexible and ingenious flexible temperature sensors can be designed in the future, which can then be applied to temperature sensing in fields such as healthcare, robotics, and intelligent environmental interaction.

3.4.3 Self-Powered Human Monitoring System

The main approaches to achieving self-powered human monitoring currently are based on piezoelectric nanogenerators[118], thermoelectric generators[119], and triboelectric nanogenerators[120] that directly convert physiological signals into electrical signals. However, traditional self-powered sensors face limitations such as limited material selection, complex manufacturing processes, and poor material flexibility. In recent years, new ionic thermoelectric materials, mostly based on organic hydrogel materials, can be used to form self-powered human motion monitoring systems[121] and self-powered electronic skins[122] through thermoelectric conversion. Among them, biomass-based ionic thermoelectric materials, with their simple manufacturing process, good flexibility, stability, and biocompatibility, provide new inspiration for the development of novel self-powered human monitoring systems.
To achieve a wearable human behavior monitoring system driven by ionic thermoelectrics, Chen et al.[123]designed a carboxylated bacterial cellulose/polyacrylamide dual-network ionic thermoelectric hydrogel using bis(trifluoromethanesulfonyl)imide lithium as the ionic donor. The stretching of the hydrogel changes its internal network structure, hindering ion transport and thus causing a change in thermovoltage, which can be used for detecting human behaviors such as finger flexion. By amplifying the input voltage of the thermoelectric hydrogel through a voltage converter and then connecting it to a wireless signal transmitter, power is supplied to the wireless signal transmitter, thereby realizing wearable self-powered human behavior monitoring and real-time wireless transmission. Li et al.[124]designed a gelatin/polyvinyl alcohol ionic gel thermoelectric device with K4Fe(CN)6/K3Fe(CN)6as the redox pair, as shown in Figure 11. By integrating the thermoelectric gel patch into a mask, the patch can utilize the temperature difference between the surrounding environment and the heat generated by human respiration to convert physiological data into electrical signals. These signals can be wirelessly connected to a terminal, and ultimately, breathing patterns can be identified through frequency analysis, enabling real-time monitoring of issues such as "suffocation" and "asthma" during the breathing process, and achieving simple self-powered respiratory monitoring. Recently, Li et al.[125]prepared a wearable bacterial cellulose gel-based ionic thermobattery using K4Fe(CN)6/K3Fe(CN)6as the redox pair. This battery can use the irregular surface of the skin as a heat source and, through effective thermoelectric conversion, detect the current change signals output by activities such as finger bending and gripping, realizing a wearable self-powered strain sensing system.
图11 自供电的实时人体呼吸监测装置[124]

Fig. 11 Self-powered real-time human breath detection device[124]. Copyright 2022, American Chemical Society

At present, biomass-based ionic thermoelectric hydrogels have shown potential for application in wearable self-powered sensors, used for collecting different physiological motion signals of the human body and transmitting them wirelessly in real time. As biomass-based ionic gels exposed to air for a long time can easily lose moisture, the decrease in water content affects the thermoelectric performance and sensing accuracy of the material. Therefore, achieving the stability and high precision of sensing, and meeting the adaptability to multiple environmental conditions, still requires further exploration and development.

4 Conclusions and Prospects

This paper summarizes the preparation and application of various biomass-based ionic thermoelectric devices. The flexibility, low cost, and green renewability of natural biomass materials have promoted the development of ionic thermoelectric material polymer matrices from synthetic polymers to natural polymers, in response to the era's theme of green, economic, low-carbon, and environmental protection. Current biomass-based ionic thermoelectric devices show great potential in applications such as flexible wearable devices, but there are still shortcomings.
(1) Biomass-based ionic thermoelectric devices have low ionic conductivity, leading to low device efficiency. Future research directions should not only focus on the improvement of ionic Seebeck coefficient but also emphasize the optimization of ionic conductivity. On one hand, the ionic conductivity can be improved by designing special structures of the matrix materials, such as constructing internal channels with anisotropic structures through directional freezing technology or stretch-induced crystallization, which shortens the migration path of ions inside the material and enhances the transport performance of ions. On the other hand, the conductivity can also be increased through the strategy of electron-ion coupled transport, for example, by using the graphitization process of biomass materials to construct ionic thermoelectric materials with a drift current effect, achieving synergistic ionic-electronic thermoelectric effects, and thereby designing high-efficiency, high-output power ionic thermoelectric devices.
(2) The output power of biomass-based ionic thermoelectric devices is insufficient, which severely limits their application in continuously powering electronic devices. In addition to the performance of the thermoelectric materials themselves, the properties and structure of the electrode materials also play a critical role in electrical output. Therefore, it is possible to improve the output by selecting high-conductivity inert electrodes, such as silver, platinum, and gold. Furthermore, special electrode structural designs can be used to enhance the output power, for example, three-dimensional hierarchical porous carbon electrodes, which increase the electrolyte/electrode interface activity through the high specific surface area of porous carbon, thereby improving the electrochemical performance of the electric double-layer capacitor; another example is redox electrodes, which effectively increase the mass transfer rate of ions and the redox processes on the electrode surface by introducing redox pairs onto the electrode surface, further enhancing the efficiency of thermoelectric conversion.
(3) There is still tremendous potential for the multifunctional and multidisciplinary applications of biomass-based ionic thermoelectric devices. Therefore, it is essential to fully leverage the advantages of biomass materials, such as being green, safe, biodegradable, and biocompatible, and explore their innovative applications in the field of self-powered biosensors, for example, developing self-powered wearable vital sign monitoring devices for monitoring vital signs like heart rate, blood pressure, body temperature, skin conductance, and body composition. In the future, further development could focus on self-driven implantable devices in organisms, such as neural stimulation devices, pacemakers, and physiological regulation equipment, which are high-value applications.

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