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Article

Electrical Energy Storage from Low-Grade Heat Using Reduced Graphene Oxide–Carbon Nanotube Composite Materials

1
Henan International Joint Laboratory of New Civil Engineering Structure, School of Intelligent Construction and Civil Engineering, Luoyang Institute of Science and Technology, Luoyang 471023, China
2
Henan Engineering Research Center of Water Quality Safety in the Middle-Lower Yellow River, Henan Green Technology Innovation Demonstration Base, Luoyang Institute of Science and Technology, Luoyang 471023, China
3
Henan Key Laboratory of Green Building Materials Manufacturing and Intelligent Equipment, Luoyang Institute of Science and Technology, Luoyang 471023, China
4
Leeds Joint School, Southwest Jiaotong University, Chengdu 611756, China
*
Authors to whom correspondence should be addressed.
Materials 2025, 18(20), 4807; https://doi.org/10.3390/ma18204807
Submission received: 29 August 2025 / Revised: 11 October 2025 / Accepted: 19 October 2025 / Published: 21 October 2025
(This article belongs to the Section Carbon Materials)

Abstract

The conversion of low-grade heat into storable electrical energy using nanoporous carbon materials represents an efficient energy harvesting strategy. In this study, a reduced graphene oxide (RGO) and carbon nanotube (CNT) composite with a rich microporous structure was synthesized. A symmetrical thermoelectric cell was constructed to harvest thermal energy. The application of a temperature difference (ΔT) generated a stable equilibrium voltage (Us), which scaled linearly with ΔT. The resulting thermoelectric coefficient (UsT) increased markedly with the carbon nanotube (CNT) content, underscoring the effectiveness of CNT incorporation for improving thermoelectric properties. It also shows a non-monotonic dependence on KCl concentration, first increasing and then decreasing, with a maximum value of 4.17 mV/°C achieved in 0.1 M KCl using the RGO-5%CNTs electrode. When connected to an external load, the discharge voltage and current decay rapidly before stabilizing within seconds. Circuit analysis reveals that the incorporation of CNTs reduces internal resistance and increases the equivalent capacitance. Although instantaneous discharge power declines quickly, the addition of CNTs elevates its initial value and slows the decay rate. Both the average output power and thermoelectric conversion efficiency improve with increasing ΔT and are further enhanced at higher CNT content. Overall, the RGO-CNT composite demonstrates significantly superior thermoelectric performance compared to pure RGO.
Keywords: RGO-CNTs composite; electric double layer; low-grade heat; thermoelectric conversion cell; thermoelectric coefficient RGO-CNTs composite; electric double layer; low-grade heat; thermoelectric conversion cell; thermoelectric coefficient

Share and Cite

MDPI and ACS Style

Yang, Z.; Xu, Y.; Sun, S.; Zhang, Y.; Li, X.; Zhao, Y.; Hao, X.; Xue, C.; Guo, D.; Li, J.; et al. Electrical Energy Storage from Low-Grade Heat Using Reduced Graphene Oxide–Carbon Nanotube Composite Materials. Materials 2025, 18, 4807. https://doi.org/10.3390/ma18204807

AMA Style

Yang Z, Xu Y, Sun S, Zhang Y, Li X, Zhao Y, Hao X, Xue C, Guo D, Li J, et al. Electrical Energy Storage from Low-Grade Heat Using Reduced Graphene Oxide–Carbon Nanotube Composite Materials. Materials. 2025; 18(20):4807. https://doi.org/10.3390/ma18204807

Chicago/Turabian Style

Yang, Zhe, Yijia Xu, Shuocheng Sun, Yujia Zhang, Xiaolu Li, Yan Zhao, Xusheng Hao, Caige Xue, Dening Guo, Jia Li, and et al. 2025. "Electrical Energy Storage from Low-Grade Heat Using Reduced Graphene Oxide–Carbon Nanotube Composite Materials" Materials 18, no. 20: 4807. https://doi.org/10.3390/ma18204807

APA Style

Yang, Z., Xu, Y., Sun, S., Zhang, Y., Li, X., Zhao, Y., Hao, X., Xue, C., Guo, D., Li, J., & Wang, J. (2025). Electrical Energy Storage from Low-Grade Heat Using Reduced Graphene Oxide–Carbon Nanotube Composite Materials. Materials, 18(20), 4807. https://doi.org/10.3390/ma18204807

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