Development of Carbon Nanotubes Based Nanohybrid Materials for Chemical Heat Storage

Heat chemical storage of waste energy from industrial processes is a promising technology in view of a more efficient use and saving of primary energy sources1. The development of new heat storage materials (HSMs) is of fundamental relevance to improve efficiency of energy storage systems like Chemical Heat Pumps (CHPs) based on the reaction MgO/Mg(OH)2. The priority requirements for a HSM are: high reaction performance, material stability in a medium range of temperatures (200-350 °C), high thermal conductivity. In the present study novel nanohybrid materials (NHMs) made of Carbon Nanotubes (CNTs) and Magnesium Hydroxide (MH) nanoparticles were synthesized through a deposition-precipitation (DP) method. Before DP synthesis, CNTs have been functionalized with a HNO3 gas-phase treatment to improve their compatibility with MH phase. The DP reaction was carried out using Mg(NO3)2 as Mg precursor and NH4OH as precipitating agent. The sensible variables analysed were DP time, temperature, base solution addition rate, MH/CNT ratio and CNTs' functionalization degree. The obtained NHMs were analyzed by Dehydration/Hydration (DH) Cyclic Thermogravimetrical Analysis (TGA), Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and BET analysis. The optimized NHM performances are shown in Fig. 1 compared to that of pure MH by DH cyclic TGA. The NHM shows higher dehydration/hydration conversion and hydration rate in comparison to pure MH. Indeed, the CNTs' functionalization treatment allows a better dispersion of MH particles among the tubes tangles thus avoiding MH nanoparticles sintering (as inferred by SEM analysis in Fig. 2) explaining the higher reaction conversions. The dehydration temperature (at which energy storage begins) is also decreased at ~200 °C. Moreover, the functionalization treatment improves the hydrophilic character of the CNTs, implying an increase in both hydration rate and conversion. [1] Y. Kato, F. Takahashi, A. Watanabe, Y. Yoshizawa, Appl. Therm. Eng. 21 (2001) 1067-1081.