Effect of Carbon Nanoadditives on Lithium Hydroxide Monohydrate-Based Composite Materials for Low Temperature Chemical Heat Storage

Carbon nanospheres (CNSs) and multi-walled carbon nanotubes (MWCNTs) as nanoadditives were used to modify lithium hydroxide monohydrate for low temperature chemical heat storage application. The lithium hydroxide monohydrate particles were well dispersed on the nanoscale level, and the diameter of nanoparticles was about 20–30 nm in the case of the carbon nanospheres and 50–100 nm the case of the MWCNTs, as shown by transmission electron microscopy characterization results. X-ray diffraction results indicated that the LiOH·H2O-carbon nano thermochemical composite materials were successfully synthesized. The thermochemical composite materials LiOH·H2O/CNSs (2020 kJ/kg), LiOH·H2O/MWCNTs (1804 kJ/kg), and LiOH·H2O/AC (1236 kJ/kg) exhibited obviously improved heat storage density and higher hydration rate than pure LiOH·H2O (661 kJ/kg), which was shown by thermogravimetric and differential scanning calorimetric (TG-DSC) analysis. It appears that nanocarbon-modified lithium hydroxide monohydrate thermochemical composite materials have a huge potential for the application of low temperature chemical heat storage.


Introduction
In recent years, due to the consumption of fossil energy and global warming, thermal energy storage technologies have become more and more attractive and have been seriously considered as an important part of efficient utilization of alternative energy [1,2].These technologies include three main type: sensible heat storage, latent heat storage and chemical heat storage.All of these technologies play a role in solving the supply and demand mismatching of thermal energy and improve energy efficiency [3].Among these technologies, thermochemical heat storage, which uses reversible chemical reactions to store and release thermal energy, is more suitable for the efficient utilization of thermal energy due to the high heat storage density of thermochemical materials [4].Based on the heat storage working temperature, thermochemical heat storage technology could be generally divided into two parts: high temperature heat storage (200-1100 • C) and low temperature heat storage (<200 • C) [3,4].As one of the core parts of these technologies, a large number of thermochemical Energies 2017, 10, 644 2 of 9 materials (TCMs) have been selected.For instance, metal hydroxides, metal hydrides, and metal carbonates can be used as TCMs for high temperature thermochemical heat storage, while inorganic salt hydrates and salt ammoniate are considered the promising candidates for low temperature thermochemical heat storage due to their different decomposition temperatures [5][6][7][8][9][10].In order to efficiently store low temperature thermal energy, the inorganic hydrate LiOH•H 2 O, which has a high energy density (1440 kJ/kg) and mild reaction process, was selected as the most promising candidate [11].However, just like other inorganic hydrates [12,13], both the hydration rate and thermal conductivity of pure LiOH•H 2 O are still low [9,11], which seriously limits the application of this material.Hence, the preparation of heat storage composite TCMs with strong water sorption and high thermal conductivity is of great importance.Carbon nanotubes (CNTs) and carbon nanospheres (CNSs) are both typical carbon nanomaterials, which exhibit large surface area, high thermal conductivity, low bulk density and chemical stability [14][15][16], and they are widely used in many different fields such as electronics [17,18], catalysis [19,20] and latent heat thermal energy storage [2,21,22].In addition, as a traditional macro carbon material, activated carbon (AC) also shows many good properties such as high adsorption capacity, high stability and low density, which are commonly used for gas adsorption [23] and catalyst synthesis [24].Furthermore, all of these carbon materials are also substances with excellent hydrophilic properties after the introduction of surface oxygen groups.However, so far, carbon nanomaterials are rarely applied in the synthesis of inorganic hydrate-based TCMs.In our present work, in order to investigate the effect of carbon nanomaterials on the thermal energy storage performance of lithium hydroxide monohydrate, four kinds of TCMs (LiOH

The Raw Materials and Synthesis Method of Lithium Hydroxide Monohydrate-Based TCMs
CNSs were prepared according to the method described in Reference [25].Firstly, 0.66 g resorcin (Aladdin Ltd., Shanghai, China), 0.6 g Pluronic F-127 (Shanghai yuanye Ltd., Shanghai, China) and 876 µL formaldehyde solution (concentration 37%, Tianjin damao chemical reagent company, Tianjin, China) were added to 800 mL deionized water under vigorous stirring at 5 • C for 30 min.Then, this mixture was refluxed at 80 • C for 8 h.The solution was separated by centrifugation and then the phenol formaldehyde resin was obtained.After that, the phenol formaldehyde resin was moved into the tubular furnace and calcinated at 400 • C under N 2 atmosphere for 3 h, following which the temperature was raised to 800 • C and maintained for 2 h.After being cooled naturally to room temperature, the obtained product was oxidized by nitric acid (Guangzhou chemical reagent company, Guangzhou, China) for 2 h under reflux and continuous stirring at 120 • C. Finally, CNSs were obtained.The carbon nanotubes were synthesized by the catalytic chemical vapor deposition method using CH 4 (Huate gas Ltd., Foshan, China) as a carbon source and nickel foam (Shanghai Zhonghui Foam Aluminum Co., Ltd., Shanghai, China) as a catalyst.Firstly, nickel foam was moved into the muffle furnace and oxidized at 700 • C for 1 h under an air atmosphere.Then, the nickel foam was placed into the tubular furnace and the temperature was raised to 700 • C and Ar/H 2 (300 mL/100 mL) mixed reducing gas was introduced into the furnace for 2 h.After reduction, Ar/H 2 mixed gas was exchanged to Ar/CH 4 (400 mL/100 mL), the temperature was raised to 750 • C and the mixture was allowed to react for 20 min.After the temperature had decreased to 25 • C, the obtained substance was then also oxidized by nitric acid for 2 h under reflux and continuous stirring at 120 • C. Finally, carbon nanotubes (MWCNTs) were obtained.The activated carbon was supplied by Sinopharm Chemical Reagent Co., Ltd.(Shanghai, China) and oxidized by nitric acid for 2 h under reflux and continuous stirring at 120 • C. The as-prepared carbon nanospheres, carbon nanotubes and activated carbon were composed with lithium hydroxide monohydrate (Aladdin Ltd., Shanghai, China) by the impregnation method.Firstly, 0.5 g lithium hydroxide monohydrate was dissolved in 1 mL deionized water under vigorous stirring, and then 0.5 g carbon nanospheres, carbon nanotubes and activated carbon were added to the lithium hydroxide aqueous solution at room temperature and stirred continually for 4 h, separately.After that, the products were taken out and vacuum freeze-dried.

The Characterization and Heat Storage Performance Test Method of Lithium Hydroxide Monohydrate-Based TCMs
The surface topography was measured by field-emission scanning electron microscopy (SEM, S-4800, Hitachi Limited, Tokyo, Japan).Transmission electron micrographs (TEM) were obtained with an FEI Tecnai G212 operated at 100 kV and a JEOL JEM-2100F (Japan Electron Optics Laboratory Co., Ltd., Tokyo, Japan) operated at 200 kV.X-ray diffraction (XRD) analysis was performed on a D8-advance X-ray diffractometer (German Bruker, Karlsruhe, Germany) with Cu target (40 kV, 40 mA).The scan step size was 0.0167 • and the counting time was 10.160 s.Nitrogen adsorption-desorption was measured at the boiling point of nitrogen (77 K) using a Quantachrome QDS-30 analyzer (Quantachrome Instruments, Boynton Beach, FL, USA).Brunauer-Emmett-Teller surface area and pore structure were measured by nitrogen physisorption under the normal relative pressure of 0.1-1.0.The thermal conductivity of the sample was measured by a DRL-II thermal conductivity tester (Xiangtan Xiangyi Instrument co., Ltd., Xiangtan, China).After hydration, samples were dissolved in hot concentrated hydrochloric acid (Guangzhou chemical reagent company, Guangzhou, China).After cooling to room temperature, the solution was diluted with ultrapure water and analyzed for the lithium content by atomic absorption spectroscopy (AAS, Thermo Scientific S4AA, Thermo Fisher Scientific,  C for 3 h.Dehydrated products were also cooled to 30 • C in the Ar atmosphere, and water vapor with 2.97 kPa of partial pressure carried with N 2 flow was introduced into the tube for 60 min as a hydration operation at 30 • C.After the hydration operation, the endothermic heat and temperature of the samples were measured with the thermo gravimetric and differential thermal analyzer (STA-200, Nanjingdazhan co., Ltd., Nanjing, China), which was also used for measuring the weight change during the dehydration step at the same time.Each TG-DSC measurement was repeated three times in order to ensure correctness.From the SEM analysis, it was confirmed that highly uniform CNSs (Figure 2a) with diameters of 200 nm were successfully synthesized, and MWCNTs (Figure 2b) with diameters of around 100 nm were also successfully prepared.Before the doping of the carbon additives, the bulk LiOH•H 2 O (Figure 2c) was aggregated with large diameters (300 nm-1 µm).LiOH•H 2 O particles were well supported and dispersed on the surface of the carbon nanospheres (Figure 2d) and carbon nanotubes (Figure 2e).Furthermore, no obvious structure deterioration could be observed after the introduction of LiOH•H 2 O.However, the surface of the activated carbon (Figure 2f) was intensively covered after the intervention of LiOH•H 2 O.

The Microstructure Characterization of Lithium Hydroxide Monohydrate-Based TCMs
Energies 2017, 10, 644; doi: 10.3390/en10050644 4 of 9 introduction of LiOH•H2O.However, the surface of the activated carbon (Figure 2f) was intensively covered after the intervention of LiOH•H2O.3a, it could be observed that the LiOH•H2O nanoparticles with a diameter of 20-30 nm were successfully supported on the CNSs (Figure 3d) with clear particle structures.Additionally, it was also well-supported on the MWCNTs (Figure 3e), but part of the LiOH•H2O nanoparticles were connected with others without a clear interface.The nanoparticle diameter was in the range of 50 nm to 100 nm, which was a bit larger than that supported on the CNSs.As for the LiOH•H2O/AC sample (Figure 3f), no clear LiOH•H2O particles could be observed on the activated carbons.Also, the pure LiOH•H2O (Figure 3c 3a, it could be observed that the LiOH•H2O nanoparticles with a diameter of 20-30 nm were successfully supported on the CNSs (Figure 3d) with clear particle structures.Additionally, it was also well-supported on the MWCNTs (Figure 3e), but part of the LiOH•H2O nanoparticles were connected with others without a clear interface.The nanoparticle diameter was in the range of 50 nm to 100 nm, which was a bit larger than that supported on the CNSs.As for the LiOH•H2O/AC sample (Figure 3f), no clear LiOH•H2O particles could be observed on the activated carbons.Also, the pure LiOH•H2O (Figure 3c 1.As can be seen from Table 1, it was clear that the composed LiOH•H 2 O-based TCMs exhibit different textures.Due to the larger BET surface area of the carbon nanoadditives, the specific surface area of LiOH•H 2 O/CNSs (276 m 2 /g) and LiOH•H 2 O/MWCNTs (140 m 2 /g) was higher than that of LiOH•H 2 O/AC (84 m 2 /g) and pure LiOH•H 2 O (15 m 2 /g).According to the SEM and TEM characterization results, it can be concluded that high specific surface area was also an important factor that lead to the form of nanoscale dispersion of LiOH•H 2 O particles.
Energies 2017, 10, 644; doi: 10.3390/en10050644 5 of 9 respectively, owing to the presence of oxygen-containing functional groups such as hydroxyl, carbonyl and carboxyl groups [14,23,27] on the surface of the CNSs, MWCNTs and AC.Therefore, with the proper additives supplying hydrogen bonding, the composites could show a good ability for retarding the aggregation of LiOH•H2O.The porosity structures of CNSs, MWCNTs, AC, LiOH•H2O, LiOH•H2O/CNSs, LiOH•H2O/MWCNTs, and LiOH•H2O/AC were also measured by nitrogen adsorption-desorption isotherms.The Brunauer-Emmett-Teller (BET) surface area, pore volume and average pore size are shown in Table 1.As can be seen from Table 1, it was clear that the composed LiOH•H2O-based TCMs exhibit different textures.Due to the larger BET surface area of the carbon nanoadditives, the specific surface area of LiOH•H2O/CNSs (276 m 2 /g) and LiOH•H2O/MWCNTs (140 m 2 /g) was higher than that of LiOH•H2O/AC (84 m 2 /g) and pure LiOH•H2O (15 m 2 /g).According to the SEM and TEM characterization results, it can be concluded that high specific surface area was also an important factor that lead to the form of nanoscale dispersion of LiOH•H2O particles.

The Heat Storage Performance Test of Lithium Hydroxide Monohydrate-based TCMs
Comparative performance tests of pure LiOH•H2O, LiOH•H2O/AC, LiOH•H2O/CNSs and LiOH•H2O/MWCNTs were carried out, the results of which are shown in Figure 4.It was found that the reaction rate of lithium hydroxide and water vapor was slow and the conversion of LiOH to LiOH•H2O was only about 42% after 1 h hydration, which was calculated by the approximately 18% mass loss of H2O, as shown in Figure 4a.It was also found that the endothermic heat value of LiOH•H2O was only about 661 kJ/kg.Figure 4b shows the DSC curve of CNS-modified LiOH•H2O.It could be seen that after 1 h hydration of LiOH/CNSs, LiOH was fully hydrated to LiOH•H2O and, furthermore, the heat storage density of this sample normalized by LiOH•H2O content could reach 2020 kJ/kg.This value of LiOH•H2O contained in LiOH•H2O/MWCNTs also reached a high level (1804 kJ/kg), as shown in Figure 4c.As for the LiOH•H2O/AC sample, the heat storage density of   4. It was found that the reaction rate of lithium hydroxide and water vapor was slow and the conversion of LiOH to LiOH•H 2 O was only about 42% after 1 h hydration, which was calculated by the approximately 18% mass loss of H 2 O, as shown in Figure 4a.It was also found that the endothermic heat value of LiOH•H 2 O was only about 661 kJ/kg.Figure 4b  surface atoms would obviously increase; therefore, the crystalline field and binding energy of the internal atoms were different from that of the surface atoms, which have many dangling bonds due to the lack of adjacent atoms.So, owing to the unsaturated bonds in atoms, nanoparticles show better thermodynamic properties [28,29].Meanwhile, due to the increase of surface atoms and their existing hydrophilic functional groups, a greater amount of H 2 O and LiOH was able to react, which in turn improved the heat storage performance of the composite.Furthermore, according to the TEM characterization results, the reason that the heat storage density of LiOH•H 2 O/CNSs was higher than that of LiOH•H 2 O/MWCNTs may be due to the smaller particle size of LiOH•H 2 O (20-30 nm) that existed in LiOH•H 2 O/CNSs compared to that in LiOH•H 2 O/MWCNTs (50-100 nm).It could be speculated that smaller size nanoparticles could make a greater contribution to the enhancement of heat storage density of TCMs.Additionally, after the addition of CNSs, MWCNTs and AC to LiOH•H 2 O, the thermal conductivity of these composed TCMs became higher than that of pure LiOH•H 2 O (shown in Figure 5).Presently, the synthesis of carbon nanoadditive-modified, LiOH•H 2 O-based thermochemical materials has not yet been fully developed, and the heat storage density of the inorganic hydrate could be further improved by controlling its hydrophilic property and particle size.
Energies 2017, 10, 644; doi: 10.3390/en10050644 6 of 9 LiOH•H2O was lower than that of LiOH•H2O/MWCNTs and LiOH•H2O/CNSs, and reached 1236 kJ/kg (Figure 4d).It was indicated that at the same duration of the hydration reaction, due to the addition of CNSs, MWCNTs and AC, LiOH was fully reacted with H2O molecules and converted to LiOH•H2O compared with pure LiOH.In other words, the hydration reaction rate of LiOH•H2O/CNSs, LiOH•H2O/MWCNTs and LiOH•H2O/AC were greatly enhanced.On one hand, the existing hydrophilic functional groups on the surface of CNSs, MWCNTs and AC could make H2O adsorption easier and provide a completely different reaction interface between LiOH and the water molecules.On the other hand, the reason that LiOH•H2O/CNSs and LiOH•H2O/MWCNTs composed TCMs showed ultrahigh heat storage density, higher than that of LiOH•H2O/AC and pure LiOH•H2O TCMs, could be due to their higher specific surface area, which substantially enhanced the dispersion of LiOH•H2O nanoparticles and increased the contact of the surface area with the water molecules.Also, the low specific surface area of LiOH•H2O/AC and pure LiOH•H2O may be the reason for their lower heat storage density.When the particle size reached the nanoscale, the amount of surface atoms would obviously increase; therefore, the crystalline field and binding energy of the internal atoms were different from that of the surface atoms, which have many dangling bonds due to the lack of adjacent atoms.So, owing to the unsaturated bonds in atoms, nanoparticles show better thermodynamic properties [28,29].Meanwhile, due to the increase of surface atoms and their existing hydrophilic functional groups, a greater amount of H2O and LiOH was able to react, which in turn improved the heat storage performance of the composite.Furthermore, according to the TEM characterization results, the reason that the heat storage density of LiOH•H2O/CNSs was higher than that of LiOH•H2O/MWCNTs may be due to the smaller particle size of LiOH•H2O (20-30 nm) that existed in LiOH•H2O/CNSs compared to that in LiOH•H2O/MWCNTs (50-100 nm).It could be speculated that smaller size nanoparticles could make a greater contribution to the enhancement of heat storage density of TCMs.Additionally, after the addition of CNSs, MWCNTs and AC to LiOH•H2O, the thermal conductivity of these composed TCMs became higher than that of pure LiOH•H2O (shown in Figure 5).Presently, the synthesis of carbon nanoadditive-modified, LiOH•H2O-based thermochemical materials has not yet been fully developed, and the heat storage density of the inorganic hydrate could be further improved by controlling its hydrophilic property and particle size.

Conclusions
To sum up, carbon nanoadditive-modified, LiOH•H2O-based thermochemical materials were synthesized, characterized and well applied for low temperature chemical heat storage.The existence of carbon nanomaterials (CNSs and MWCNTs) in the LiOH•H2O-based composite TCMs cause the LiOH•H2O particles to disperse into the nanoscale level and form different sizes of particles.The hydration reaction rates of the composed materials were greatly improved and the heat storage density of LiOH•H2O in the composed TCMs reached 2020 kJ/kg and 1804 kJ/kg, respectively, which were much higher values than that of LiOH•H2O/AC (1236 kJ/kg) and pure LiOH•H2O (661 kJ/kg).This may due to the hydrophilic property, high specific surface area and the nanoparticle size effect of the composed thermochemical materials.Finally, the additives also showed certain a positive effect on the thermal conductivity of the composed TCMs.This may due to the hydrophilic property, high specific surface area and the nanoparticle size effect of the composed thermochemical materials.Finally, the additives also showed certain a positive effect on the thermal conductivity of the composed TCMs.

Figure 3
Figure 3 shows the TEM images of (a) CNSs, (b) MWCNTs, (c) LiOH•H 2 O, (d) LiOH•H 2 O/CNSs, (e) LiOH•H 2 O/MWCNTs and (f) LiOH•H 2 O/AC.Compared with Figure 3a, it could be observed that the LiOH•H 2 O nanoparticles with a diameter of 20-30 nm were successfully supported on the CNSs (Figure 3d) with clear particle structures.Additionally, it was also well-supported on the MWCNTs (Figure 3e), but part of the LiOH•H 2 O nanoparticles were connected with others without a clear interface.The nanoparticle diameter was in the range of 50 nm to 100 nm, which was a bit larger than that supported on the CNSs.As for the LiOH•H 2 O/AC sample (Figure 3f), no clear LiOH•H 2 O particles could be observed on the activated carbons.Also, the pure LiOH•H 2 O (Figure 3c) existed in the form of stacked flakes.The LiOH•H 2 O content of LiOH•H 2 O/CNSs, LiOH•H 2 O/MWCNTs, LiOH•H 2 O/AC was about 50%, as characterized by AAS.During the synthesis of LiOH•H 2 O/CNSs, LiOH•H 2 O/MWCNTs, and LiOH•H 2 O/AC, intermolecular interactions such as hydrogen bonding may exist between the additives and LiOH•H 2 O, respectively, owing to
shows the DSC curve of CNS-modified LiOH•H 2 O.It could be seen that after 1 h hydration of LiOH/CNSs, LiOH was fully hydrated to LiOH•H 2 O and, furthermore, the heat storage density of this sample normalized by LiOH•H 2 O content could reach 2020 kJ/kg.This value of LiOH•H 2 O contained in LiOH•H 2 O/MWCNTs also reached a high level (1804 kJ/kg), as shown in Figure 4c.As for the LiOH•H 2 O/AC sample, the heat storage density of LiOH•H 2 O was lower than that of LiOH•H 2 O/MWCNTs and LiOH•H 2 O/CNSs, and reached 1236 kJ/kg (Figure 4d).It was indicated that at the same duration of the hydration reaction, due to the addition of CNSs, MWCNTs and AC, LiOH was fully reacted with H 2 O molecules and converted to LiOH•H 2 O compared with pure LiOH.In other words, the hydration reaction rate of LiOH•H 2 O/CNSs, LiOH•H 2 O/MWCNTs and LiOH•H 2 O/AC were greatly enhanced.On one hand, the existing hydrophilic functional groups on the surface of CNSs, MWCNTs and AC could make H 2 O adsorption easier and provide a completely different reaction interface between LiOH and the water molecules.On the other hand, the reason that LiOH•H 2 O/CNSs and LiOH•H 2 O/MWCNTs composed TCMs showed ultrahigh heat storage density, higher than that of LiOH•H 2 O/AC and pure LiOH•H 2 O TCMs, could be due to their higher specific surface area, which substantially enhanced the dispersion of LiOH•H 2 O nanoparticles and increased the contact of the surface area with the water molecules.Also, the low specific surface area of LiOH•H 2 O/AC and pure LiOH•H 2 O may be the reason for their lower heat storage density.When the particle size reached the nanoscale, the amount
To sum up, carbon nanoadditive-modified, LiOH•H 2 O-based thermochemical materials were synthesized, characterized and well applied for low temperature chemical heat storage.The existence of carbon nanomaterials (CNSs and MWCNTs) in the LiOH•H 2 O-based composite TCMs cause the LiOH•H 2 O particles to disperse into the nanoscale level and form different sizes of particles.The hydration reaction rates of the composed materials were greatly improved and the heat storage density of LiOH•H 2 O in the composed TCMs reached 2020 kJ/kg and 1804 kJ/kg, respectively, which were much higher values than that of LiOH•H 2 O/AC (1236 kJ/kg) and pure LiOH•H 2 O (661 kJ/kg).

BET Surface Area (m 2 /g) Pore Volume (mL/g) Average Pore Size (nm)
The Heat Storage Performance Test of Lithium Hydroxide Monohydrate-Based TCMs Comparative performance tests of pure LiOH•H 2 O, LiOH•H 2 O/AC, LiOH•H 2 O/CNSs and LiOH•H 2 O/MWCNTs were carried out, the results of which are shown in Figure