Key Takeaways & Executive Findings
- •• The hybrid ion system strategically combines the high-voltage characteristics of Zn2+ redox with the exceptionally fast kinetics of NH4+, significantly boosting thermoelectric performance for low-grade heat harvesting. • The Zn2+/NH4+ co-insertion/thermoextraction mechanism is elucidated, where NH4+ exhibits exceptionally fast migration due to its unique hydrogen bonding diffusion behavior. • The device achieves a record 19.6 mW m⁻2 K⁻2 normalized power density with 72 h continuous operation, demonstrating strong application potential. • The thermal charging cell achieves a high thermopower of 12.5 mV K−1 and a Carnot-relative efficiency of 12.74%, offering a simple and effective strategy for low-grade heat conversion and storage.
Abstract
Zn-based thermal charging devices, utilizing the synergistic effect of ion thermoextraction and thermodiffusion, are able to efficiently convert thermal energy into electrical energy and storage in the devices, making them a highly promising technology for low-grade heat recovery and utilization. However, the low output power density and energy conversion efficiency resulted by the slow diffusion kinetics of Zn2+ hinder their development. Herein, we present a high-performance thermal charging cell design using Zn2+/NH4+ hybrid ion electrolyte, which not only maintains the high output voltage of the Zn-based thermoelectric system, but also significantly enhances the output power density due to the fast diffusion kinetics of NH4+. Based on this strategy, the thermal charging cell displays a high thermopower of 12.5 mV K−1 and an excellent normalized power density of 19.6 mW m−2 K−2 at a temperature difference of 35 K. The Carnot-relative efficiency is as high as 12.74%. Moreover, it can operate continuously for over 72 h when the temperature difference persists, achieving a balance between thermoelectric conversion and output. This work provides a simple and effective strategy for the design of high-performance thermal charging cells for low-grade heat conversion and utilization.
1. Introduction
Low-grade heat (<100 °C) is generated in daily life and industrial production of human society, which is often neglected and wasted because it is difficult to utilize efficiently [1–3]. Conventional solid-state thermoelectric devices (s-TEs) based on the Seebeck effect can realize the direct conversion of heat to electricity [4]. However, the relatively low Seebeck coefficient (~µV K–1) still limits its practical application in areas such as Internet-of-Things (IoTs) sensors and wearable devices [5–7]. The emerging ionic thermoelectric devices (i-TEs), with a thermopower on the order of mV K–1, have overcome the critical limitation of insufficient energy conversion efficiency of s-TEs, offering new possibilities for the utilization of low-grade heat and source-free power supply for low-power devices [8–10].
During last decade, substantial progress has been made in the area of i-TEs. The thermopower of i-TEs has effectively improved by regulating the entropy difference of thermally driven redox reactions or designing nanostructure electrode materials [11–16]. However, the energy conversion efficiency remains relatively low. Moreover, additional energy storage units are often required for practical applications, thereby increasing the complexity of system integration [17–19]. To address these challenges, our group has developed novel zinc-ion thermal charging cells (ZTCCs) combining zinc-ion batteries with the ionic thermoelectric system, which not only improve the thermoelectric conversion efficiency but also integrate energy conversion with energy storage into a single system [20]. Through the thermoextraction of Zn2⁺ (its thermal-driven ejection from the cathode lattice) and its subsequent thermodiffusion (the concentration-gradient-driven diffusion in the electrolyte), coupled with the plating/stripping process on the Zn metal anode, the ZTCCs realized the high-efficiency conversion and storage of low-grade heat into electrical energy. By rationally designing the structure of the VO2 cathode material, a thermopower of 12.5 mV K–1 and an energy conversion efficiency of 0.95% (7.25% of the relative efficiency of Carnot) were obtained for the ZTCCs [21]. Furthermore, by employing a V2O5 cathode material with a higher ionic diffusion rate, the thermopower of the ZTCCs was further enhanced to 23.4 mV K–1 [22]. In addition, Cui et al. introduced the concept of zinc-ion hybrid supercapacitors into ionic thermoelectric systems, selecting a common adsorption-type cathode (commercial activated carbon YP-80F) to match with Zn metal anode, and assembled a new type of zinc-ion thermally charged supercapacitor (ZTCSCs) [23]. Specifically, by utilizing the reversible adsorptio
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Zhiwei Han, Shengliang Zhang, Helang Huang, Jing Wang, Hui Dou, Tianran Zhang, Xiaogang Zhang (2026). A High-Performance Thermal Charging Cell with High Power Density and Long Runtime Enabled by Zn2+ and NH4+ Co-insertion. Nano-Micro Letters. https://doi.org/10.1007/s40820-025-02011-9
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Frequently Asked Questions
What is the main innovation of this thermal charging cell?
The main innovation is the use of a Zn2+/NH4+ hybrid ion electrolyte, which combines the high voltage of Zn2+ redox with the fast kinetics of NH4+, significantly enhancing power density and efficiency for low-grade heat harvesting.
What are the key performance metrics of the device?
The device achieves a thermopower of 12.5 mV K−1, a normalized power density of 19.6 mW m−2 K−2 at a temperature difference of 35 K, and a Carnot-relative efficiency of 12.74%. It can operate continuously for over 72 hours.
How does the Zn2+/NH4+ co-insertion mechanism work?
The mechanism involves the co-insertion and thermoextraction of Zn2+ and NH4+ ions. NH4+ exhibits exceptionally fast migration due to its unique hydrogen bonding diffusion behavior, which enhances the overall kinetics and power density.
What is the significance of this work for low-grade heat recovery?
This work provides a simple and effective strategy to design high-performance thermal charging cells, enabling efficient conversion and storage of low-grade heat (<100°C) into electrical energy, which is often wasted in daily life and industrial processes.
What are the potential applications of this technology?
Potential applications include powering low-power devices such as IoT sensors and wearable devices, as well as integrating with energy storage systems for efficient low-grade heat utilization.
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