Double-network thermocells with extraordinary toughness and boosted power density for continuous heat harvesting

Abstract

•Double networks with a synergy effect are designed for high-performance thermocells•Stretchable thermocells show extraordinary toughness even higher than cartilages•Robust networks are capable of loading high concentrations of electrolytes•The thermocell reaches the highest-level power density in quasi-solid thermocells Thermocells have recently attracted increasing interest in the field of the Internet of Things owing to their capacity to continually power wearable electronics by converting low-grade heat to electricity. However, liquid thermocells face leakage risk of electrolytes, while quasi-solid thermocells crosslinked by physical networks encounter the challenges of poor mechanical properties and low power densities. Inspired by topologically entangled multi-networks in muscles and cartilages, this work designs double chemically crosslinked networks with a synergy effect to address these challenges. This thermocell not only shows large stretchability, impressive strength, notch insensitivity, and extraordinary toughness, which is even higher than that of cartilages, but also reaches the supreme grade of power density in quasi-solid thermocells. We believe that this work will change the landscape of high-performance thermocells and benefit the realization of sustainable self-powered wearable electronics in the era of the internet of things. Thermocells hold great potential for continually harvesting waste heat to power ubiquitous electronics, but their integration is hampered by leakage risk and poor mechanical properties of liquid electrolytes. Although some physically crosslinked networks have been introduced to prepare leak-free quasi-solid thermocells, they are easy to break because of extremely small fracture energies of about 10 J m−2 and limited by low output power densities of 0.01–0.06 mW m−2 K−2. Herein, this study designs double chemically crosslinked networks to address the mechanical challenges while also improving power density. The thermocell shows a large stretchability of 217%, impressive strength of 1,190 kPa, notch insensitivity, extraordinary toughness of 2,770 J m−2, and a boosted output power density of 0.61 mW m−2 K−2. It works stably even when being sliced and stretched. This study breaks the mechanical limitations of thermocells and will benefit the realization of sustainable self-powered wearable electronics in the era of the internet of things. Thermocells hold great potential for continually harvesting waste heat to power ubiquitous electronics, but their integration is hampered by leakage risk and poor mechanical properties of liquid electrolytes. Although some physically crosslinked networks have been introduced to prepare leak-free quasi-solid thermocells, they are easy to break because of extremely small fracture energies of about 10 J m−2 and limited by low output power densities of 0.01–0.06 mW m−2 K−2. Herein, this study designs double chemically crosslinked networks to address the mechanical challenges while also improving power density. The thermocell shows a large stretchability of 217%, impressive strength of 1,190 kPa, notch insensitivity, extraordinary toughness of 2,770 J m−2, and a boosted output power density of 0.61 mW m−2 K−2. It works stably even when being sliced and stretched. This study breaks the mechanical limitations of thermocells and will benefit the realization of sustainable self-powered wearable electronics in the era of the internet of things. 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Acta. 2017; 225: 482-492Crossref Scopus (52) Google Scholar Therefore, quasi-solid thermocells require a rational design on crosslinked networks, which should not only ensure stretchability to adapt to dynamic interfaces and mechanical toughness to maintain physical integrity but also should allow high-concentration electrolytes to optimize the conductivity and improve power density.22Sun J.Y. Zhao X. Illeperuma W.R.K. Chaudhuri O. Oh K.H. Mooney D.J. Vlassak J.J. Suo Z. Highly stretchable and tough hydrogels.Nature. 2012; 489: 133-136Crossref PubMed Scopus (3166) Google Scholar, 23Zhao X. Multi-scale multi-mechanism design of tough hydrogels: building dissipation into stretchy networks.Soft Matter. 2014; 10: 672-687Crossref PubMed Google Scholar, 24Chen G. Cui Y. Chen X. Proactively modulating mechanical behaviors of materials at multiscale for mechano-adaptable devices.Chem. Soc. Rev. 2019; 48: 1434-1447Crossref PubMed Google Scholar Herein, inspired by the topologically entangled multi-networks in biological tissues, we developed double chemically crosslinked networks to address the mechanical and thermoelectric challenges in current quasi-solid thermocells. The primary design principles include: (1) The first swelling network augments the rigidity while the second network provides stretchability; (2) There is a synergy between different networks to enhance toughness; (3) Robust networks are capable of loading high-concentration electrolytes; and (4) To enlarge the thermopower, the preferred chemical networks are those that have interactions with thermogalvanic ions. Conventional thermocells are composed of liquid electrolytes with redox couples (thermogalvanic ions), including iodide/triiodide,25Zhou H. Yamada T. Kimizuka N. 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Zhou J. Wearable thermocells based on gel electrolytes for the utilization of body heat.Angew. Chem. Int. Ed. Engl. 2016; 55: 12050-12053Crossref PubMed Scopus (138) Google Scholar and ferro/ferricyanide.15Yu B. Duan J. Cong H. Xie W. Liu R. Zhuang X. Wang H. Qi B. Xu M. Wang Z.L. Zhou J. Thermosensitive crystallization–boosted liquid thermocells for low-grade heat harvesting.Science. 2020; 370: 342-346Crossref PubMed Scopus (110) Google Scholar,17Han C.G. Qian X. Li Q. Deng B. Zhu Y. Han Z. Zhang W. Wang W. Feng S.P. Chen G. Liu W. Giant thermopower of ionic gelatin near room temperature.Science. 2020; 368: 1091-1098Crossref PubMed Scopus (159) Google Scholar,27Duan J. Feng G. Yu B. Li J. Chen M. Yang P. Feng J. Liu K. Zhou J. Aqueous thermogalvanic cells with a high Seebeck coefficient for low-grade heat harvest.Nat. Commun. 2018; 9: 5146Crossref PubMed Scopus (132) Google Scholar They are reported to have a thermopower of about 1 mV K−1 in a temperature gradient field. Take the ferro/ferricyanide [Fe(CN)64−/Fe(CN)63−] as an example. For a thermocell added with the redox couple [Fe(CN)64−/Fe(CN)63−], there is a reversible redox reaction Fe(CN)63− + e− Fe(CN)64− in a temperature gradient field (Figure 1A). At the hot side, there is a thermodynamically preferred oxidation reaction Fe(CN)64− → e– + Fe(CN)63−, accompanied by electron transfer to the hot electrode. It leads to an increase in electrochemical potential and a lower electrode potential. While on the cold side, the reduction reaction Fe(CN)63− + e− → Fe(CN)64− is thermodynamically favorable and electrons are attracted from the cold electrode, along with a decrease in electrochemical potential and a higher electrode potential. The reduced Fe(CN)64− ions transfer to the high-temperature electrode via convection, diffusion, and migration, while the oxidized Fe(CN)63− ions are back to the low-temperature electrode, enabling a continuous reaction. Consequently, voltages are generated between two electrodes under the temperature gradient in a quasi-continuous way. To improve the mechanical performance of the liquid electrolytes, we introduced swelling-augmented double networks via a sequential network formation method.28Gong J.P. Katsuyama Y. Kurokawa T. Osada Y. Double-network hydrogels with extremely high mechanical strength.Adv. Mater. 2003; 15: 1155-1158Crossref Scopus (2905) Google Scholar,29Gong J.P. Materials science. Materials both tough and soft.Science. 2014; 344: 161-162Crossref PubMed Scopus (253) Google Scholar The basic rule is that, the first network is crosslinked by a neutral monomer (e.g., acrylamide [AM]) and a charged monomer (e.g., 2-acrylamide-2-methylpropane sulfonic acid [AMPS]).23Zhao X. Multi-scale multi-mechanism design of tough hydrogels: building dissipation into stretchy networks.Soft Matter. 2014; 10: 672-687Crossref PubMed Google Scholar,28Gong J.P. Katsuyama Y. Kurokawa T. Osada Y. Double-network hydrogels with extremely high mechanical strength.Adv. Mater. 2003; 15: 1155-1158Crossref Scopus (2905) Google Scholar,30Gong J.P. Why are double network hydrogels so tough?.Soft Matter. 2010; 6: 2583-2590Crossref Scopus (1405) Google Scholar The first network is then stretched by customizable swelling behavior and is thus rigid for robust support and energy dissipation. Meanwhile, the second stretchable network synergistically enhances the toughness.30Gong J.P. Why are double network hydrogels so tough?.Soft Matter. 2010; 6: 2583-2590Crossref Scopus (1405) Google Scholar Also, the gel networks are preferred to have interactions with thermogalvanic ions, which is favorable to enlarge the thermodynamic entropy difference and to enhance thermopower.15Yu B. Duan J. Cong H. Xie W. Liu R. Zhuang X. Wang H. Qi B. Xu M. Wang Z.L. Zhou J. Thermosensitive crystallization–boosted liquid thermocells for low-grade heat harvesting.Science. 2020; 370: 342-346Crossref PubMed Scopus (110) Google Scholar,17Han C.G. Qian X. Li Q. Deng B. Zhu Y. Han Z. Zhang W. Wang W. Feng S.P. Chen G. Liu W. Giant thermopower of ionic gelatin near room temperature.Science. 2020; 368: 1091-1098Crossref PubMed Scopus (159) Google Scholar We first evaluated the intermolecular interaction between the monomer units and thermogalvanic ions based on UV spectra analysis.27Duan J. Feng G. Yu B. Li J. Chen M. Yang P. Feng J. Liu K. Zhou J. Aqueous thermogalvanic cells with a high Seebeck coefficient for low-grade heat harvest.Nat. Commun. 2018; 9: 5146Crossref PubMed Scopus (132) Google Scholar For the Fe(CN)64− (aqueous K4Fe(CN)6 solution), its UV absorbance band shifts from 218 to 215 nm with the addition of AMPS (Figure 1B). While the absorbance band location of Fe(CN)63− (aqueous K3Fe(CN)6 solution) remains almost unchanged after the addition of AMPS (Figure 1C). The different UV band shifts indicate that the AMPS units have stronger intermolecular interaction with Fe(CN)64− than with Fe(CN)63−, which also means more AMPS are prone to bond with Fe(CN)64− (Figure 1D).27Duan J. Feng G. Yu B. Li J. Chen M. Yang P. Feng J. Liu K. Zhou J. Aqueous thermogalvanic cells with a high Seebeck coefficient for low-grade heat harvest.Nat. Commun. 2018; 9: 5146Crossref PubMed Scopus (132) Google Scholar Therefore, a polymer network of P(AM-co-AMPS) bonds with Fe(CN)64− more easily rather than with Fe(CN)63−. It enlarges the difference of the ions’ mobility, i.e., the entropy difference between Fe(CN)64– and Fe(CN)63–.27Duan J. Feng G. Yu B. Li J. Chen M. Yang P. Feng J. Liu K. Zhou J. Aqueous thermogalvanic cells with a high Seebeck coefficient for low-grade heat harvest.Nat. Commun. 2018; 9: 5146Crossref PubMed Scopus (132) Google Scholar Finally, the enlarged entropy difference of the redox couple [Fe(CN)64–/Fe(CN)63–] is beneficial to boost thermopower (Seebeck coefficient).15Yu B. Duan J. Cong H. Xie W. Liu R. Zhuang X. Wang H. Qi B. Xu M. Wang Z.L. Zhou J. Thermosensitive crystallization–boosted liquid thermocells for low-grade heat harvesting.Science. 2020; 370: 342-346Crossref PubMed Scopus (110) Google Scholar,17Han C.G. Qian X. Li Q. Deng B. Zhu Y. Han Z. Zhang W. Wang W. Feng S.P. Chen G. Liu W. Giant thermopower of ionic gelatin near room temperature.Science. 2020; 368: 1091-1098Crossref PubMed Scopus (159) Google Scholar,27Duan J. Feng G. Yu B. Li J. Chen M. Yang P. Feng J. Liu K. Zhou J. Aqueous thermogalvanic cells with a high Seebeck coefficient for low-grade heat harvest.Nat. Commun. 2018; 9: 5146Crossref PubMed Scopus (132) Google Scholar Besides, for the first network of P(AM-co-AMPS), the anionic monomer causes high osmotic pressure in the aqueous solution and thus the coiled chains in the polymerized network can be stretched during swelling.28Gong J.P. Katsuyama Y. Kurokawa T. Osada Y. Double-network hydrogels with extremely high mechanical strength.Adv. Mater. 2003; 15: 1155-1158Crossref Scopus (2905) Google Scholar The swelling behaviors and the stretching states of the network can be tailored by varying the mass ratio of AMPS and AM (Figure S1). Here, we chose a mass ratio of 2:1, in which the high swelling ratio was about 27:1. This not only ensures sufficient stretching of the network but also avoids rupture caused by excessive swelling. In addition, the stretched network is relatively rigid and advantageous to dissipate energy.23Zhao X. Multi-scale multi-mechanism design of tough hydrogels: building dissipation into stretchy networks.Soft Matter. 2014; 10: 672-687Crossref PubMed Google Scholar,30Gong J.P. Why are double network hydrogels so tough?.Soft Matter. 2010; 6: 2583-2590Crossref Scopus (1405) Google Scholar The second network is loosely crosslinked by the neutral monomer AM and offers stretchability. It is worthwhile to note that, we designed the same monomer unit in the first and the second network, which is different from conventional double networks composed of a pure polyelectrolyte network and the second neutral network. The same monomer units in two networks are advantageous to reduce the interfacial energy, to provide a synergy between them, and thus to increases energy dissipation to improve toughness.31Zhao M. Li W. Laves phases formed in the binary blend of AB4 Miktoarm star copolymer and A-homopolymer.Macromolecules. 2019; 52: 1832-1842Crossref Scopus (40) Google Scholar The Fourier transform infrared (FTIR) spectrum of the freeze-drying hydrogel indicates the successful synthesis of the polyelectrolyte double network (Figure S2). The bands located at 1,680–1,550 and 1,037 cm−1 belong to the amide group of AM and the sulfonic acid group of AMPS, respectively.32Lei Z. Wu P. A supramolecular biomimetic skin combining a wide spectrum of mechanical properties and multiple sensory capabilities.Nat. Commun. 2018; 9: 1134Crossref PubMed Scopus (286) Google Scholar After completing the solvent exchange with redox electrolyte solutions, the final double-network thermocells are homogenous, transparent, stretchable, and tough (Figure 1E). We first investigated the influence of the electrolyte concentration on the thermoelectric efficiency. Thermopower (Se) was measured on a self-made temperature gradient platform (Figure S3; Note S1). For the redox couple [Fe(CN)64–/Fe(CN)63–] in a 0.4 M aqueous solution (nearly saturated), the thermopower was 1.4 mV K−1, which is in line with previous reports (Figure S4).12Yang P. Liu K. Chen Q. Mo X. Zhou Y. Li S. Feng G. Zhou J. Wearable thermocells based on gel electrolytes for the utilization of body heat.Angew. Chem. Int. Ed. Engl. 2016; 55: 12050-12053Crossref PubMed Scopus (138) Google Scholar,17Han C.G. Qian X. Li Q. Deng B. Zhu Y. Han Z. Zhang W. Wang W. Feng S.P. Chen G. Liu W. Giant thermopower of ionic gelatin near room temperature.Science. 2020; 368: 1091-1098Crossref PubMed Scopus (159) Google Scholar For the hydrogel after the solvent exchange in a 0.4 M [Fe(CN)64–/Fe(CN)63–] solution, thermopower increased to 1.5 mV K−1 (Figure S5). The improved thermopower is mainly due to the fact that AMPS units have a stronger intermolecular interaction with Fe(CN)64− than with Fe(CN)63−, as indicated by the UV spectra.15Yu B. Duan J. Cong H. Xie W. Liu R. Zhuang X. Wang H. Qi B. Xu M. Wang Z.L. Zhou J. Thermosensitive crystallization–boosted liquid thermocells for low-grade heat harvesting.Science. 2020; 370: 342-346Crossref PubMed Scopus (110) Google Scholar,27Duan J. Feng G. Yu B. Li J. Chen M. Yang P. Feng J. Liu K. Zhou J. Aqueous thermogalvanic cells with a high Seebeck coefficient for low-grade heat harvest.Nat. Commun. 2018; 9: 5146Crossref PubMed Scopus (132) Google Scholar It enlarges the entropy difference of the ferro/ferricyanide redox couple and thus improves thermopower.15Yu B. Duan J. Cong H. Xie W. Liu R. Zhuang X. Wang H. Qi B. Xu M. Wang Z.L. Zhou J. Thermosensitive crystallization–boosted liquid thermocells for low-grade heat harvesting.Science. 2020; 370: 342-346Crossref PubMed Scopus (110) Google Scholar,17Han C.G. Qian X. Li Q. Deng B. Zhu Y. Han Z. Zhang W. Wang W. Feng S.P. Chen G. Liu W. Giant thermopower of ionic gelatin near room temperature.Science. 2020; 368: 1091-1098Crossref PubMed Scopus (159) Google Scholar,27Duan J. Feng G. Yu B. Li J. Chen M. Yang P. Feng J. Liu K. Zhou J. Aqueous thermogalvanic cells with a high Seebeck coefficient for low-grade heat harvest.Nat. Commun. 2018; 9: 5146Crossref PubMed Scopus (132) Google Scholar We tested the double-network thermocell with different concentrations of the redox couple, where the maximum thermopower was observed in the sample added with the highest concentration of [Fe(CN)64–/Fe(CN)63−] (0.4 M, Figure 2A). This is probably because of the fact that the high concentration is favorable to enlarge the entropy difference of the redox couple. Many previous reports also observed the maximum thermopower at the high concentration of the redox couple.15Yu B. Duan J. Cong H. Xie W. Liu R. Zhuang X. Wang H. Qi B. Xu M. Wang Z.L. Zhou J. Thermosensitive crystallization–boosted liquid thermocells for low-grade heat harvesting.Science. 2020; 370: 342-346Crossref PubMed Scopus (110) Google Scholar,17Han C.G. Qian X. Li Q. Deng B. Zhu Y. Han Z. Zhang W. Wang W. Feng S.P. Chen G. Liu W. Giant thermopower of ionic gelatin near room temperature.Science. 2020; 368: 1091-1098Crossref PubMed Scopus (159) Google Scholar,27Duan J. Feng G. Yu B. Li J. Chen M. Yang P. Feng J. Liu K. Zhou J. Aqueous thermogalvanic cells with a high Seebeck coefficient for low-grade heat harvest.Nat. Commun. 2018; 9: 5146Crossref PubMed Scopus (132) Google Scholar Meanwhile, the effective ionic conductivity (σeff) of the thermocell increased from about 1 to 5 S m−1 when the [Fe(CN)64−/Fe(CN)63−] concentration increased from 0.05 to 0.4 M. Moreover, the addition of extra electrolytes may facilitate the transport of redox ions in the polyelectrolyte network. For example, the effective ionic conductivity reaches about 12 S m−1 with the addition of 3M NaCl (Figure 2B). However, further increasing the NaCl concentration results in a slight decrease in the effective ionic conductivity, probably because the extremely high concentration impedes the effective ionization and conduction of redox ions. On the other hand, the maximum thermopower appears in the sample after solvent exchange with 0.4 M [Fe(CN)64−/Fe(CN)63−]/1 M NaCl. This is probably due to synergy of the thermogalvanic effect of the redox [Fe(CN)64−/Fe(CN)63−] couple and the Soret effect of the mobile ions (Na+, Cl−, etc.).17Han C.G. Qian X. Li Q. Deng B. Zhu Y. Han Z. Zhang W. Wang W. Feng S.P. Chen G. Liu W. Giant thermopower of ionic gelatin near room temperature.Science. 2020; 368: 1091-1098Crossref PubMed Scopus (159) Google Scholar,33Cheng H. He X. Fan Z. Ouyang J. Flexible quasi-solid state Ionogels with remarkable Seebeck coefficient and high thermoelectric properties.Adv. Energy Mater. 2019; 9: 1901085Crossref Scopus (86) Google Scholar Nevertheless, such an increase is very small, and thus, the contribution of the Soret effect is far from comparable to that of the thermogalvanic effect. It is worthwhile to note that, there is no obvious shrinkage or expansion of the double-network thermocell at high concentrations of electrolytes, confirming the robustness of the double chemically crosslinked networks. Here, we chose a sample with the maximum effective ionic conductivity (minimum resistivity) for further characterization. This quasi-solid thermocell is crosslinked by double chemical networks with interconnected m