An advanced composite sorbent with high thermal stability and superior sorption capacity without hysteresis for a better thermal battery

Abstract

Efficient thermal energy harvesting utilizing composite sorbents has great potential for cost-effective energy storage applications; however, serious hysteresis and average sorption capacity are persistent bottlenecks for low-heat-loss and high-power-density thermal energy storage. Herein, three strategies for the synthesis of stable composites of ammonium chloride and expanded natural graphite treated with sulfuric acid (ENG-TSA) are reported. Among the three kinds of composites obtained by immersing ENG-TSA and NH₄Cl in water (EWNs), the composite with the simplest mechanical stirring treatment (EWN-Mec) presented the highest thermal stability and could even maintain about 90% initial mass of NH₄Cl after being heated at 180 °C for 40 h. The XRD patterns and FTIR spectrum of only EWN-Mec exhibit all main diffraction peaks and characteristic wavenumbers of ENG-TSA and NH₄Cl, which demonstrate that magnetic stirring and ultrasonic treatment have a negative effect on the valid distribution of NH₄Cl particles in the composites as they destroy the organized layers of ENG-TSA and reduce the dispersive force. SEM images indicate that the NH₄Cl particles in the range of 38–177 nm are distributed in the lamellar structure of ENG-TSA in EWNs. EWN-Mec was chosen for further ammonia sorption tests due to its higher global thermal stability, better distribution of NH₄Cl particles and lower cost of fabrication and future applications as compared to the case of other EWNs. Its ammonia sorption characteristics indicate multi-stage sorption, negligible hysteresis, and a superior sorption capacity of 2.41 g g⁻¹, which is 96% higher than the highest ammonia sorption capacity reported to date. The resulting highest theoretical energy storage density of EWN-Mec (up to 1514 W h kg⁻¹) is comparable to that of a state-of-the-art lithium-ion battery. Furthermore, thermodynamic and kinetic models were established to better understand the mechanism of the combination of solid adsorption and chemical reaction. The simple production process indicates that EWN-Mec can be easily engineered into devices for high-power-density and low-cost thermal energy storage applications.