The effect of 3D carbon nanoadditives on lithium hydroxide monohydrate based composite materials for highly efficient low temperature thermochemical heat storage

Article abstract of DOI:10.1039/c8ra00269j

Lithium hydroxide monohydrate based thermochemical heat storage materials were modified with in situ formed 3D-nickel-carbon nanotubes (Ni-CNTs). The nanoscale (5-15 nm) LiOH·H₂O particles were well dispersed in the composite formed with Ni-CNTs. These composite materials exhibited improved heat storage capacity, thermal conductivity, and hydration rate owing to hydrogen bonding between H₂O and hydrophilic groups on the surface of Ni-CNTs, as concluded from combined results of in situ DRIFT spectroscopy and heat storage performance test. The introduction of 3D-carbon nanomaterials leads to a considerable decrease in the activation energy for the thermochemical reaction process. This phenomenon is probably due to Ni-CNTs providing an efficient hydrophilic reaction interface and exhibiting a surface effect on the hydration reaction. Among the thermochemical materials, Ni-CNTs-LiOH·H₂O-1 showed the lowest activation energy (23.3 kJ mol^-1), the highest thermal conductivity (3.78 W m^-1 K^-1) and the highest heat storage density (3935 kJ kg^-1), which is 5.9 times higher than that of pure lithium hydroxide after the same hydration time. The heat storage density and the thermal conductivity of Ni-CNTs-LiOH·H₂O are much higher than 1D MWCNTs and 2D graphene oxide modified LiOH·H₂O. The selection of 3D carbon nanoadditives that formed part of the chemical heat storage materials is a very efficient way to enhance comprehensive performance of heat storage activity components.

Full text of DOI:10.1039/c8ra00269j

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The effect of 3D carbon nanoadditives on lithium
hydroxide monohydrate based composite materials
for highly efficient low temperature
Cite this: RSC Adv., 2018, 8, 8199
thermochemical heat storage
a
c
Shijie Li,ab Hongyu Huang,
Jun Li, Noriyuki Kobayashi,c Yugo Osaka,d
*
*
Zhaohong Hea and Haoran Yuana
Lithium hydroxide monohydrate based thermochemical heat storage materials were modified with in situ
formed 3D-nickel-carbon nanotubes (Ni-CNTs). The nanoscale (5–15 nm) LiOH$H2O particles were well
dispersed in the composite formed with Ni-CNTs. These composite materials exhibited improved heat
storage capacity, thermal conductivity, and hydration rate owing to hydrogen bonding between H2O and
hydrophilic groups on the surface of Ni-CNTs, as concluded from combined results of in situ DRIFT
spectroscopy and heat storage performance test. The introduction of 3D-carbon nanomaterials leads to
a considerable decrease in the activation energy for the thermochemical reaction process. This
phenomenon is probably due to Ni-CNTs providing an efficient hydrophilic reaction interface and
exhibiting a surface effect on the hydration reaction. Among the thermochemical materials, Ni-CNTs–
LiOH$H2O-1 showed the lowest activation energy (23.3 kJ molꢀ1), the highest thermal conductivity
(3.78 W mꢀ1 ꢀ1) and the highest heat storage density (3935 kJ kgꢀ1), which is 5.9 times higher than that
K
of pure lithium hydroxide after the same hydration time. The heat storage density and the thermal
conductivity of Ni-CNTs–LiOH$H2O are much higher than 1D MWCNTs and 2D graphene oxide
modified LiOH$H2O. The selection of 3D carbon nanoadditives that formed part of the chemical heat
storage materials is a very efficient way to enhance comprehensive performance of heat storage activity
components.
Received 10th January 2018
Accepted 16th February 2018
DOI: 10.1039/c8ra00269j
rsc.li/rsc-advances
uses reversible chemical reactions to store and release thermal
energy, facilitates the efficient utilization of thermal energy due
to its high heat storage density.10 Based on heat storage working
1. Introduction
As an important part of clean and efficient utilization of alter-
native energy, the development of thermal energy storage
technology has become increasingly important in recent years
owing to the incremental consumption of fossil energy and the
emission of greenhouse gases.1,2 These technologies are divided
into three main types: sensible heat storage,3,4 latent heat
storage,5,6 and thermochemical heat storage.7,8 All of these
technologies participate in solving the supply and demand
mismatch of thermal energy, and improve energy efficiency.9
Among these technologies, thermochemical heat storage, which
temperature, the thermochemical heat storage technology
could be divided into two parts: high temperature heat storage
and low temperature heat storage.9,10 As core components of
these technologies, numerous thermochemical materials
(TCMs) have been selected. For instance, metal hydroxides,
metal hydrides, and metal carbonates are typically used as
TCMs for high-temperature thermochemical heat storage,
whereas inorganic salt hydrates and salt ammoniate are
considered as promising candidates for low-temperature ther-
mochemical heat storage due to their different decomposition
temperature.11–16 For efficiently storing low-temperature
thermal energy, the inorganic hydrate lithium hydroxide mon-
ohydrate (LiOH$H2O), which possesses high energy density
(1440 kJ kgꢀ1) and mild reaction process, was selected as
a potential material.17 However, similar to other inorganic
hydrates18,19 pure LiOH$H2O exhibit low hydration rate and
thermal conductivity;15,17 moreover, its heat storage density
decreases aer hydration, thereby seriously limiting the appli-
cation of the material. Hence, composite TCMs with high heat
aKey Laboratory of Renewable Energy, Guangdong Provincial Key Laboratory of New
and Renewable Energy Research and Development, Guangzhou Institute of Energy
Conversion, Chinese Academy of Sciences, No. 2 Nengyuan Rd, Wushan, Tianhe
District, Guangzhou 510640, P. R. China. E-mail: lisj@ms.giec.ac.cn; huanghy@ms.
giec.ac.cn; hezh@ms.giec.ac.cn; yuanhr@ms.giec.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing 100049, PR China
cNagoya University, Furo-cho, Chikusa-ku, Nagoya-shi, Aichi 464-8603, Japan. E-mail:
Junli@energy.gr.jp; kobayashi@energy.gr.jp
dKanazawa University, Kanazawa-shi, Ishikawa-ken 920-1192, Japan. E-mail:
y-osaka@se.kanazawa-u.ac.jp
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storage density and high thermal conductivity are of consider- nickel foam as catalyst.35 First, nickel foam was placed in
ꢁ
able synthetic value.
a tubular furnace, which was heated to 700 C, and lled with
3D carbon nanomaterials (3D-carbon nanotube sponge and Ar/H2 (300 mL/100 mL) mixed reducing gas for 2 h. Aer
arrays20,21), which exhibit large surface area, low bulk density, and reduction, Ar/H2 mixed gas was replaced with Ar/C2H4 (400 mL/
chemical stability,22–24 are widely used in various elds, such as 100 mL), the temperature was increased to 550 ꢁC, and the
electronics25,26 and catalysis24,27 as typical carbon nanomaterials. In system was allowed to react for 20 min. Aer the temperature
addition, these materials offer excellent hydrophilic property aer decreased to 25 ꢁC, the 3D Ni skeleton CNTs (Ni-CNTs) was
the introduction of surface oxygen groups. However, under normal obtained. Then, aer being oxidized by 5% O2 for 2 h at 250 ꢁC,
conditions, traditional one-dimensional or two-dimensional the as-prepared 3D Ni-CNTs were composited with LiOH$H2O
carbon nanomaterials are selected and used for latent heat by impregnation method. First, 0.5 g LiOH$H2O was dissolved
thermal energy storage.2,28,29 3D carbon nanomaterials are rarely in 1 mL deionized water under vigorous stirring. Subsequently,
used for thermal energy storage,30 especially thermochemical heat 3D nanocarbon were added in the LiOH aqueous solution at
storage. In our previous work, 1D carbon nanomaterials,31 room temperature, and the mixture was stirred continuously for
MWCNTs were used to modify LiOH$H2O; heat storage perfor- 4 h. Aerward, the products were withdrawn and vacuum
mance was markedly enhanced, but the thermal conductivity freeze-dried. The obtained materials with different LiOH$H2O
needed improvement. Hence, 2D-carbon nanomaterial15,32 gra- content (14 wt%, 23 wt%, 39 wt%, 100 wt%) were named as Ni-
phene oxide was selected and, which markedly improved the CNTs–LiOH$H2O-1, Ni-CNTs–LiOH$H2O-2, Ni-CNTs–LiOH$H2-
energy storage performance of LiOH$H2O and Mg(OH)2. Graphene O-3, and LiOH$H2O, respectively.
oxide also positively affected thermal conductivity to a certain
extent. Hydrophilic substances, such as lithium chloride (LiCl),
13X-zeolite, and NaY-zeolite, were also used to enhance the heat
storage performance and obtain expected results.33 Kato et al.
2.2. The characterization and heat storage performance test
method of LiOH$H2O-based TCMs
investigated the reaction behavior of metal-salt modied Mg(OH)2- Surface topography was measured by eld-emission scanning
based materials; during the heat storage process, LiCl and LiBr electron microscopy (SEM, S-4800, Hitachi Limited). Trans-
acted as catalysts and decreased the activation energy effectively mission electron micrographs (TEM) were obtained with FEI
and substantially improved heat storage performance.5,11,34 More- Tecnai G212 operated at 100 kV and a JEOL JEM-2100F operated
over, the functional groups on the surface of the materials mark- at 200 kV. X-ray diffraction (XRD) analysis was performed on
edly affected the reaction behavior of Mg(OH)2. However, the a D8-advance X-ray diffractometer (Bruker, Germany) with Cu
relationship between heat storage density and particle size, espe- target (40 kV, 40 mA). The scan step size was 0.0167ꢁ, and
cially when materials reached nanoscale, was not discussed. In this counting time was 10.160 s. Nitrogen adsorption–desorption
work, in situ DRIFT spectroscopy technology and chemical reaction was measured at the boiling point of nitrogen (77 K) by
kinetics test were used for an in-depth analysis of the heat storage a Quantachrome QDS-30 analyzer. BET (Brunauer–Emmett–
mechanism, which has not been involved in previous research. Teller) surface area and pore structure were measured by
Moreover, to simultaneously improve the performance of heat nitrogen physisorption under normal relative pressure of 0.1–
storage density, hydration rate, and thermal conductivity, we 1.0. The thermal conductivity of the sample was measured by
synthesized a novel TCM composite of in situ formed 3D-nickel- a DRL-II thermal conductivity tester (Xiangtan Xiangyi Instru-
carbon nanotubes (Ni-CNTs) and LiOH$H2O. Four kinds of ment Co., Ltd.). LiOH$H2O, Ni-CNTs–LiOH$H2O-1, Ni-CNTs–
TCMs were prepared, and the effect of 3D-carbon nanomaterials LiOH$H2O-2, Ni-CNTs–LiOH$H2O-3 were used as raw substance
was extensively investigated. The highest heat storage density of then, LiOH Ni-CNTs–LiOH-1, Ni-CNTs–LiOH-2, and Ni-CNTs–
Ni-CNTs–LiOH$H2O could reach 3935 kJ kgꢀ1, which is 2.2 times LiOH-3 were synthesized by decomposing LiOH$H2O, Ni-CNTs–
higher than that of 1D MWCNTs modied LiOH$H2O (1804 kJ LiOH$H2O-1, Ni-CNTs–LiOH$H2O-2, and Ni-CNTs–LiOH$H2O-3
kgꢀ1) and the thermal conductivity (3.78 W mꢀ1 Kꢀ1) is also much in a horizontal tubular quartz furnace with Ar gas at 150 ꢁC for
higher than MWCNTs modied LiOH$H2O (1.75 W mꢀ1 Kꢀ1
)
3 h. Dehydrated products were cooled to 30 C in an Ar atmo-
31
ꢁ
The heat storage density and the thermal conductivity of Ni-CNTs sphere, and water vapor at a partial pressure of 2.97 kPa and
modied LiOH$H2O are also higher than 2D graphene oxide carried by N2 ow was introduced to the tube for 60 min for
modied LiOH$H2O (1980 kJ kgꢀ1; 1.70 W mꢀ1 Kꢀ1), respec- hydration operation at 30 ꢁC. Aer hydration, the endothermic
tively.15 It indicated that the selection of 3D nano carbon materials heat and temperature of the samples were measured through
as composed part of the chemical heat storage materials is a very Thermogravimetry and Differential Scanning Calorimetry (TG-
efficient way to enhance comprehensive performance of heat DSC) (STA-200, Nanjingdazhan Co., Ltd.), which was also used
storage activity component.
for measuring weight change during dehydration step. Each TG-
DSC measurement was repeated three times in order to ensure
correctness. The activation energy of dehydration performance
of all samples was calculated using Ozawa method,36 which is
applicable for calculating the activation energy of thermal
decomposition and dehydration reaction. By using the Ozawa
2. Experimental
2.1. Raw materials and synthesis method of LiOH$H2O-
based TCMs
The 3D-carbon nanotubes were synthesized by catalytic chem- method, the following equation can be obtained based on the
ical vapor deposition method with C2H4 as carbon source and reaction rate expression and the Arrhenius's equation:
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ꢀ
ꢁ
b
T2
R
A
E 1
ln
¼ ln
ꢀ
E f ðaÞ R T
In this equation, E is the activation energy (kJ molꢀ1), b is the
heating rate (K sꢀ1), T is temperature (K), R is the ideal gas
constant (J (mol K)ꢀ1), A is a pre-exponential factor, a is the
dehydration conversion, and f(a) is a function of dehydration
conversion. During the calculation of activation energy, the
dehydration conversion was maintained at 70%. The heating
rates were 3 K minꢀ1, 7 K minꢀ1 and 10 K minꢀ1, and the
activation energy was obtained from the slope (ꢀE/R) of this
equation.
3. Results and discussion
3.1. Microstructure characterization of LiOH$H2O-based
TCMs
Fig. 2 SEM images of (a) LiOH$H2O, (b) Ni-CNTs–LiOH$H2O-1, (c) Ni-
CNTs–LiOH$H2O-2 and (d) Ni-CNTs–LiOH$H2O-3.
Fig. 1 shows the XRD patterns of LiOH$H2O, Ni-CNTs–
LiOH$H2O-1, Ni-CNTs–LiOH$H2O-2, and Ni-CNTs–LiOH$H2O-3
samples. As shown in Fig. 1, the diffraction peaks at around 30ꢁ,
32.19ꢁ, 33.64ꢁ, 34.84ꢁ, 37.07ꢁ, 38.83ꢁ, 40.06ꢁ, 41.61ꢁ, 43.49ꢁ,
49.37ꢁ, 51.36ꢁ, 52.47ꢁ, 55.15ꢁ, 55.70ꢁ, 56.92ꢁ, 62.15ꢁ, 63.13ꢁ,
64.55ꢁ, 65.47ꢁ, 66.22ꢁ, 68.35ꢁ and 71.34ꢁ, respectively, were
attributed to LiOH$H2O, whereas the diffraction peaks at
around 25ꢁ were attributed to graphitic carbon.37 Meanwhile,
the diffraction peaks at around 45ꢁ, 52ꢁ, and 77ꢁ could be
assigned to metal nickel. It could be clearly seen that the
diffraction peaks of LiOH$H2O were sharp and strong in the
LiOH$H2O sample, but the diffraction peaks of LiOH$H2O
became weaker and more diffused when 3D carbon nano-
material Ni-CNTs were added. Hence, the composites of Ni-
CNTs and LiOH$H2O were successfully synthesized, and the
addition of Ni-CNTs resulted in the extensive dispersion of
LiOH$H2O particles in the composite materials.
3, respectively. As shown in Fig. 2a, before the addition of 3D-
carbon nanoadditives, the bulk LiOH$H2O was aggregated
with a large diameter (300 nm to 2 mm). Aer the modication
of Ni-CNTs, no obvious change was observed in the regular
structure for Ni-CNTs–LiOH$H2O-1 (Fig. 2b) and Ni-CNTs–
LiOH$H2O-2 (Fig. 2c). This nding indicates that the interven-
tion of LiOH$H2O did not lead to structural deterioration at
a mass ratio < 23% and that LiOH$H2O was highly dispersed in
the composite materials. This result agrees well with the XRD
result. When the content of LiOH$H2O reached 39% (Ni-CNTs–
LiOH$H2O-3), several LiOH$H2O particles could be seen on the
surface of Ni-CNTs (Fig. 2d).
Fig. 3 shows the TEM images of Ni-CNTs–LiOH$H2O with
different contents of LiOH$H2O. In Fig. 3, LiOH$H2O nano-
particles with a diameter around 5 nm were successfully
dispersed on Ni-CNTs (Fig. 3b) with clear particle structure.
LiOH$H2O particle size showed a growth trend with increasing
LiOH$H2O content in Ni-CNTs–LiOH$H2O composite materials
(Fig. 3d and f). When LiOH$H2O content reached 39% (Fig. 3f),
the LiOH$H2O nanoparticle size could increase to 15 nm, which
is a bit bigger than the other two composite materials. Pure
LiOH$H2O (Fig. 2a) existed in the form of stacked akes and
showed the largest particle size (300 nm to 2 mm). The addition
of Ni-CNTs can effectively induce nanoscale LiOH$H2O disper-
sion, and the resulting particle size is markedly smaller than
that of pure LiOH$H2O. During the synthesis of Ni-CNTs–
LiOH$H2O composite materials, intermolecular interaction,
such as hydrogen bonding, may exist between Ni-CNTs and
LiOH$H2O owing to the presence of oxygen-containing func-
tional groups, such as hydroxyl, carbonyl, and carboxyl
groups,38–40 on the surface of Ni-CNTs. Therefore, proper addi-
tives supplied hydrogen bonding could show good ability for
retarding the aggregation of LiOH$H2O. The porosity structures
of LiOH$H2O were also measured by nitrogen adsorption–
desorption. The BET specic surface area, pore volume, and
average pore size are shown in Table 1. Ni-CNTs–LiOH$H2O
Fig. 2a–d provide the SEM images of LiOH$H2O, Ni-CNTs–
LiOH$H2O-1, Ni-CNTs–LiOH$H2O-2, and Ni-CNTs–LiOH$H2O-
Fig. 1 XRD patterns of LiOH$H2O, Ni-CNTs–LiOH$H2O-1, Ni-CNTs–
LiOH$H2O-2, Ni-CNTs–LiOH$H2O-3.
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curve of 3D-carbon nanomaterial modied LiOH$H2O. Aer 1 h
of hydration of the Ni-CNTs–LiOH$H2O composite, LiOH was
fully hydrated to LiOH$H2O, and the heat storage density of Ni-
CNTs–LiOH$H2O-1 normalized by LiOH$H2O content can reach
3935 kJ kgꢀ1. High heat storage densities of 3505 kJ kgꢀ1 and
2782 kJ kgꢀ1 were observed in LiOH$H2O contained in Ni-
CNTs–LiOH$H2O-2 (Fig. 4c) and Ni-CNTs–LiOH$H2O-3
(Fig. 4d), respectively. Therefore, compared with pure LiOH
and with the same duration of the hydration reaction LiOH, and
H2O molecule can be fully converted to LiOH$H2O owing to the
introduction of Ni-CNTs, the hydration reaction rate of Ni-
CNTs–LiOH$H2O was substantially improved. On the one hand,
owing to the formation of hydrophilic functional groups on the
surface of Ni-CNTs during preparation, H2O adsorption became
easier and provided a completely different reaction interface
between LiOH and water molecules. On the other hand, Ni-
CNTs–LiOH$H2O showed ultrahigh heat storage density
exceeding that of pure LiOH$H2O owing to the existence of
hydrophilic functional groups41 and increased specic surface
area, which markedly enhanced the dispersion of LiOH$H2O
nanoparticles and the contact surface area with water mole-
cules. The low specic surface area of pure LiOH$H2O may exert
a negative effect on heat storage density. When the particle size
reached nanoscale, the amount of surface atoms evidently
increases; moreover, the crystalline eld and binding energy of
internal atoms were notably different from those of surface
atoms, which possessed numerous dangling bonds owing to the
lack of adjacent atoms. Hence, the unsaturated bonds in atoms
show that nanoparticles present enhanced thermodynamic
property.42,43 Meanwhile, due to increased number of surface
atoms and the existence of hydrophilic functional groups, more
H2O and LiOH can react, and therefore, heat storage perfor-
mance can be improved. Furthermore, according to SEM and
TEM characterization results, the heat storage density of Ni-
CNTs–LiOH$H2O was higher than that of LiOH$H2O possible
because of the smaller particle size of LiOH$H2O (5–15 nm) in
Ni-CNTs–LiOH$H2O than that in pure LiOH$H2O (300 nm to 2
mm). Thus, small-sized nanoparticles can notably contribute to
the enhancement of heat storage density for composite mate-
rials; as the particle size expands, the heat storage density of
LiOH$H2O decreases. Furthermore, aer the addition of Ni-
CNTs to LiOH$H2O, the thermal conductivity of composite
materials evidently increased and exceeded that of pure
LiOH$H2O (Fig. 5) owing to the high thermal conductivity of Ni-
CNTs. The highest thermal conductivity of Ni-CNTs–LiOH$H2O
can reach 3.78 W mꢀ1 Kꢀ1, which is 2.2 times higher than that
of pure LiOH$H2O.
Fig.
3 TEM images of (a) Ni-CNTs–LiOH$H2O-1, (c) Ni-CNTs–
LiOH$H2O-2, (e) Ni-CNTs–LiOH$H2O-3, and HRTEM images of (b) Ni-
CNTs–LiOH$H2O-1 (d) Ni-CNTs–LiOH$H2O-2 and (f) Ni-CNTs–
LiOH$H2O-3.
samples show different textures. Owing to the addition of Ni-
CNTs with different mass ratio, Ni-CNTs–LiOH$H2O shows
a larger specic surface area than pure LiOH$H2O (15 m2 gꢀ1).
According to SEM and TEM characterization results, high
specic surface area is an important cause of the nanoscale
dispersion of LiOH$H2O particles.
3.2. Heat storage performance test on LiOH$H2O-based
TCMs
The heat storage performance test of pure LiOH$H2O, Ni-CNTs–
LiOH$H2O-1, Ni-CNTs–LiOH$H2O-2 and Ni-CNTs–LiOH$H2O-3
were carried out and shown in Fig. 4. The Y axis is the
amount of heating per unit time and mass (W gꢀ1), and the X
axis is temperature. The area of the curve is proportional to the
change of enthalpy, as for heat storage materials it stands for
the heat storage density. The pure LiOH$H2O heat storage
density was only approximately 661 kJ kgꢀ1 due to the slow
reaction rate of LiOH and water vapor. In the hydration reac-
tion, only about 42% of LiOH can be converted to LiOH$H2O
aer 1 h of hydration, as calculated by approximately 18% mass
loss of H2O shown in Fig. 4a. By contrast Fig. 4b shows the DSC
Table
1 Texture parameters of carbon nanoadditives modified
In situ DRIFT test was conducted to investigate the effect of
hydrophilic functional groups of 3D-carbon nanotubes on the
hydration/dehydration reaction of LiOH$H2O. Raw materials
LiOH$H2O and the Ni-CNTs–LiOH$H2O composite were
exposed to a ow of N2 (300 mL minꢀ1) at 150 ꢁC to decompose
for 24 h. Then, the as-prepared samples LiOH and Ni-CNTs–
LiOH were placed in the in situ reactor of an FT-IR spectrometer.
The reactor was vacuumed and purged using a He ow. The in
situ DRIFT test started with the hydration reaction of LiOH and
H2O and nished with dehydration reaction of LiOH$H2O.
composed thermochemical materials and pure LiOH$H2O
Surface area
Pore volume
Average pore
size (nm)
Samples
(m2 gꢀ1
)
(mL gꢀ1
)
Ni-CNTs
146
119
99
62
15
0.30
0.20
0.11
0.14
0.06
8.17
6.79
6.76
5.62
1.75
Ni-CNTs–LiOH$H2O-1
Ni-CNTs–LiOH$H2O-2
Ni-CNTs–LiOH$H2O-3
Pure LiOH$H2O
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Fig. 4 TG-DSC curves of as-synthesized samples: (a) pure LiOH after 1 h hydration, (b) Ni-CNTs–LiOH-1 after 1 h hydration, (c) Ni-CNTs–LiOH-
2 after 1 h hydration, and (d) Ni-CNTs–LiOH-3 after 1 h hydration.
Fig. 6a shows the in situ DRIFT spectroscopy of the hydration
Fig. 6b shows the in situ DRIFT spectroscopy of dehydration
reaction of LiOH and H2O. The bands in the range of 3800– reaction of LiOH$H2O obtained from 1 h hydration of LiOH.
2200 cmꢀ1 are always assigned to the stretching vibrations of When LiOH$H2O was heated to 60 C, the peak intensities of
ꢁ
structural OH groups and physical adsorbed H2O molecules.44 In OH in structural H2O (3679 cmꢀ1 and 1573 cmꢀ1) started to
the spectra, the peak at around 3679 cmꢀ1 and 1573 cmꢀ1 could be decrease. When the temperature exceeded 70 ꢁC, the trend
attributed to the stretching vibrations (nOH) and bending vibrations became more evident. Meanwhile, the peaks of OH (3235 cmꢀ1
(bOH) of the structure water in LiOH$H2O, respectively.45 Besides, and 1644 cmꢀ1) in physical adsorbed H2O appeared and
broad peaks in the 2842–3423 cmꢀ1 range were centered at around increased from 60 ꢁC to 80 ꢁC, indicating the form of free water.
3235 cmꢀ1. This peak (3235 cmꢀ1) and another peak at the lower The intensity of the peaks gradually decreased owing to easy
band (1644 cmꢀ1) were the nOH and bOH of OH, respectively, in the
physical adsorbed H2O.44,46 In Fig. 6a it could be also observed that
when the hydration reaction was ready for start, the weak peaks,
which stand for OH in structure H2O and physical adsorbed H2O
molecules have been existed. This nding may be due to the
reaction of LiOH and residual physical adsorbed H2O on the
surface of LiOH. With prolonged hydration reaction, the peak
intensities of structural OH (3679 cmꢀ1 and 1573 cmꢀ1) markedly
increased at 15 min hydration. These peaks slowly increased
during 15 min to 120 min hydration reaction, indicating the
continuous reaction of LiOH and water steam and the decrease in
hydration reaction rate. During this reaction, no obvious change
could be observed for the peak intensities of OH (3235 cmꢀ1 and
1644 cmꢀ1) in physical adsorbed H2O because of the steady water
steam ow in the in situ reactor. Aer the hydration reaction, the
reactor was vacuumed and purged using a dry He ow, then
LiOH$H2O was heated at a rate of 0.5 ꢁC sꢀ1 under the control of
temperature-programmed technology.
Fig. 5 Thermal conductivity of LiOH$H2O, Ni-CNTs–LiOH$H2O-1,
Ni-CNTs–LiOH$H2O-2 and Ni-CNTs–LiOH$H2O-3.
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Fig. 6 (a) The in situ DRIFT spectroscopy of the hydration reaction of LiOH; (b) the in situ DRIFT spectroscopy of dehydration reaction of
LiOH$H2O; (c) the in situ DRIFT spectroscopy of hydration reaction of Ni-CNTs–LiOH; (d) the in situ DRIFT spectroscopy of dehydration reaction
of Ni-CNTs–LiOH$H2O.
desorption and blowing away of physical adsorbed water at Owing to the effect of hydrogen bonding, the adsorption of H2O
elevated temperature.
on the surface of Ni-CNTs–LiOH was considerably enhanced,
Fig. 6c shows the in situ DRIFT spectroscopy of hydration facilitating an easier and more rapid reaction of H2O and LiOH
reaction of Ni-CNTs–LiOH and H2O. During hydration reaction, than before.
the peak intensities of OH in structure H2O (3679 cmꢀ1 and
Fig. 6d presented the in situ DRIFT spectroscopy of dehy-
1573 cmꢀ1) showed a marked increase with 5 min hydration dration reaction of Ni-CNTs–LiOH$H2O. When Ni-CNTs–
ꢁ
because of the existing hydrophilic groups, such as C–OH and LiOH$H2O was heated to 50 C, the peak intensities of OH in
C]O, on the surface of 3D-carbon nanotubes. Aer 20 min, the structural H2O (3679 cmꢀ1 and 1573 cmꢀ1) started to decrease;
peaks were virtually unchanged, which indicating that the beyond 60 ꢁC, LiOH$H2O was continuously dehydrated until the
hydration reaction rate of LiOH and water steam was enhanced, structure of H2O was fully lost. Meanwhile, the peaks of OH
exceeding that of pure LiOH. The peak intensities of OH (3181 cmꢀ1 and 162ꢁ5 cmꢀ1) in physical adsorbed H2O appeared,
(3181 cmꢀ1 and 1625 cmꢀ1) in physical adsorbed H2O did not increased from 50 C to 90 C, and eventually decreased; this
ꢁ
notably change. The band at around 3520 cmꢀ1 can be assigned trend was ascribed to the easy removal of H2O at elevated
to stretching vibrations, whereas the band at around 1400 cmꢀ1 temperature. The formed water could also produce hydrogen
was attributed to the bending vibrations of C–OH.44 The band at bonding with C]O groups (1720 cmꢀ1) on the surface of Ni-
1720 cmꢀ1 was assigned to C]O groups. When the water steam CNTs, thereby causing a shi of the C]O band to a lower
owed into the reactor, the wavenumber of C]O groups shied position (1710 cmꢀ1). During the dehydration reaction froꢀm1
from 1720 cmꢀ1 to 1710 cmꢀ1 because of the formation of 30 C to 120 C, the intensities of C]O and C–OH (3520 cm
hydrogen bonding2 between C]O groups and adsorbed H2O. and 1400 cmꢀ1) peaks showed no obviously change, indicating
ꢁ
ꢁ
8204 | RSC Adv., 2018, 8, 8199–8208
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Fig. 7 The activation energy of dehydration reaction of (a) LiOH$H2O, (b) Ni-CNTs–LiOH$H2O-1, (c) Ni-CNTs–LiOH$H2O-2, (d) Ni-CNTs–
LiOH$H2O-3.
that the physical–chemical property of Ni-CNTs is steady within nanoparticles, which mainly result in decreased activation
the total dehydration temperature range. The dehydration energy.48,49 In summary, the addition of 3D-carbon nano-
temperature of Ni-CNTs–LiOH$H2O was approximately 10 ꢁC material Ni-CNTs can not only enhance water absorption at the
lower than that of pure LiOH$H2O; this nding is in good LiOH particle surface but also decrease activation energy.
agreement with the results of the TG-DSC test.
Owing to the addition of Ni-CNTs, LiOH$H2O are dehydrated
As shown in Fig. 7, the activation energies of the dehydration more easily, and the reaction mechanism of composited TCMs
reaction of (a) LiOH$H2O, (b) Ni-CNTs–LiOH$H2O-1, (c) Ni- may deviate from that of pure LiOH$H2O because of the surface
CNTs–LiOH$H2O-2, and (d) Ni-CNTs–LiOH$H2O-3 as obtained effect of 3D Ni-CNTs–LiOH$H2O nanocomposite materials
using the Ozawa method are 52.1, 23.3, 25.8, and 38.2 kJ molꢀ1
,
during the thermochemical reaction process.
respectively.36 The activation energies of composite TCMs were
markedly lower than that of pure LiOH$H2O due to the surface
effect of nano-LiOH$H2O composite TCMs. The specic surface
area or surface-to-volume ratio, which changes with particle
size, depends on activation energy.47 By combining the SEM,
TEM, and BET characterization results, it could be found that as
the Ni-CNT content increased, the specic surface area of the
composite TCMs also increased, whereas the particle size of
LiOH$H2O decreased. The activation energy of LiOH$H2O
dehydration reaction in composite TCMs showed similar trend
to the particle size variation of LiOH$H2O. This trend can be
attributed to diminished particle size, leading to increased
surface-to-volume ratio and molar surface energy of
4. Conclusions
For investigating the effects of in situ formed 3D-carbon nano-
additives (Ni-CNTs) on the thermal performance of low-
temperature LiOH$H2O-based composites as thermochemical
heat storage materials, four kinds of LiOH$H2O-based
composite TCMs were successfully constructed and character-
ized. Owing to the addition of 3D-carbon nanoadditives, the
nanoscale (5–15 nm) LiOH$H2O particles were well dispersed in
the composite with Ni-CNTs. The heat storage capacity and
thermal conductivity of the composite materials were markedly
improved. Meanwhile, the hydration rate was enhanced due to
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the hydrogen bonding formed between H2O and hydrophilic
groups on the surface of Ni-CNTs, as shown by the combined
results of in situ DRIFT spectroscopy characterization and the
heat storage performance test. The activation energy for the
thermochemical reaction process notably decreased aer the
addition of Ni-CNTs possibly because Ni-CNTs provide efficient
hydrophilic reaction interface and exhibit surface effect in the
hydration reaction. Among these TCMs, Ni-CNTs–LiOH$H2O-1
showed the lowest activation energy (23.3 kJ molꢀ1), highest
thermal conductivity (3.78 W mꢀ1 Kꢀ1), and highest heat
storage density (3935 kJ kgꢀ1), which is 5.9 times higher than
pure lithium hydroxide aer the same hydration duration. The
heat storage density and the thermal conductivity of Ni-CNTs–
LiOH$H2O are great higher than 1D MWCNTs and 2D graphene
oxide modied LiOH$H2O. The selection of 3D carbon nano-
additives as composed part of the chemical heat storage mate-
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Conflicts of interest
11 H. Ishitobi, K. Uruma, M. Takeuchi, J. Ryu and Y. Kato,
Dehydration and hydration behavior of metal-salt-modied
materials for chemical heat pumps, Appl. Therm. Eng.,
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There are no conicts to declare.
Acknowledgements
This work was supported by Science and Technology Planning
Project of Guangdong Province, China (2016A050502040),
Research project of the Chinese Academy of Sciences (QYZDY- 13 D. A. Sheppard, M. Paskevicius and C. E. Buckley,
SSW-JSC038) and the National Natural Science Foundation of
China (No. 51406209).
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