energies
Article
Effect of Carbon Nanoadditives on Lithium
Hydroxide Monohydrate-Based Composite Materials
for Low Temperature Chemical Heat Storage
Xixian Yang 1,† , Shijie Li 1,2,† , Hongyu Huang 1, *, Jun Li 3 , Noriyuki Kobayashi 3
and Mitsuhiro Kubota 3
1
2
3
*
†
Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of
Sciences, No. 2 Nengyuan Rd., Wushan, Tianhe District, Guangzhou 510640, China;
[email protected] (X.Y.); [email protected] (S.L.)
University of Chinese Academy of Sciences, Beijing 100049, China
Department of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya-shi, Aichi 464-8603, Japan;
[email protected] (J.L.); [email protected] (N.K.); [email protected] (M.K.)
Correspondence: [email protected]; Tel.: +86-20-870-48394
These two authors contribute equally to this work.
Academic Editor: Rui Xiong
Received: 5 April 2017; Accepted: 3 May 2017; Published: 6 May 2017
Abstract: 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·H2 O-carbon nano
thermochemical composite materials were successfully synthesized. The thermochemical composite
materials LiOH·H2 O/CNSs (2020 kJ/kg), LiOH·H2 O/MWCNTs (1804 kJ/kg), and LiOH·H2 O/AC
(1236 kJ/kg) exhibited obviously improved heat storage density and higher hydration rate than
pure LiOH·H2 O (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.
Keywords: nanoparticles; energy storage and conversion; lithium hydroxide monohydrate;
carbon nanoadditives
1. 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
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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 [5–10]. In order
to efficiently store low temperature thermal energy, the inorganic hydrate LiOH·H2 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·H2 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 [14–16], 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·H2 O,
LiOH·H2 O/CNSs, LiOH·H2 O/MWCNTs, LiOH·H2 O/AC) were prepared. Among these samples,
pure LiOH·H2 O and activated carbon-modified LiOH·H2 O (LiOH·H2 O/AC) were used as control
groups to obviously show the advantage of carbon nanomaterial-modified LiOH·H2 O.
2. Materials and Methods
2.1. 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 N2 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
CH4 (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/H2 (300 mL/100 mL)
mixed reducing gas was introduced into the furnace for 2 h. After reduction, Ar/H2 mixed gas was
exchanged to Ar/CH4 (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
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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.
2.2. 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,
Waltham, MA, USA). LiOH·H2 O, LiOH·H2 O/CNSs, LiOH·H2 O/MWCNTs, LiOH·H2 O/AC were used
as raw substances. Then, LiOH and LiOH/CNSs, LiOH/MWCNTs, and LiOH/AC were synthesized
by the decomposition of LiOH·H2 O, LiOH·H2 O/CNSs, LiOH·H2 O/MWCNTs, LiOH·H2 O/AC in
a horizontal tubular quartz furnace with Ar gas at 150 ◦ 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
N2 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.
3. Results and Discussion
3.1. The Microstructure Characterization of Lithium Hydroxide Monohydrate-Based TCMs
Figure 1 shows the XRD patterns of LiOH·H2 O, LiOH·H2 O/CNSs, LiOH·H2 O/MWCNTs, and
LiOH·H2 O/AC, as well as the CNS, MWCNT, and AC samples. As shown in Figure 1, the diffraction
peaks at around 20◦ , 22◦ , 30◦ , 32.19◦ , 33.64◦ , 34.84◦ , 37.07◦ , 38.83◦ , 40.06◦ , 41.61◦ , 43.49◦ , 49.37◦ , 51.36◦ ,
52.47◦ , 55.15◦ , 55.70◦ , 56.92◦ , 62.15◦ , 63.13◦ , 64.55◦ , 65.47◦ , 66.22◦ , 68.35◦ and 71.34◦ , respectively, were
attributed to LiOH·H2 O. In addition, the diffraction peaks at around 25◦ and 43◦ were attributed to
graphitic carbon [26].
The broad diffraction peaks of LiOH·H2 O/CNSs, LiOH·H2 O/MWCNTs, and LiOH·H2 O/AC
indicated that LiOH·H2 O were well dispersed on CNSs, MWCNTs, and AC. Figure 2a–f provide
the SEM images of CNSs, MWCNTs, LiOH·H2 O, LiOH·H2 O/CNSs, LiOH·H2 O/MWCNTs and
LiOH·H2 O/AC, respectively.
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·H2 O
(Figure 2c) was aggregated with large diameters (300 nm–1 µm). LiOH·H2 O particles were well
supported and dispersed on the surface of the carbon nanospheres (Figure 2d) and carbon nanotubes
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(Figure 2e). Furthermore, no obvious structure deterioration could be observed after the introduction
introduction
of
O. However, the surface of the activated carbon (Figure 2f) was intensively
introduction
of LiOH
LiOH··H
H2the
2O. However, the surface of the activated carbon (Figure 2f) was intensively
of LiOH
·H2 O. However,
surface of the activated carbon (Figure 2f) was intensively covered after
covered
after
the
intervention
of LiOH·H2O.
covered
after
the
intervention
the intervention of LiOH·H2 O. of LiOH·H2O.
Figure
XRD
patterns
MWCNTs,
CNSs,
activated
carbon,
LiOH·H
2O/MWCNTs,
Figure
1.
XRD
patternsofof
ofMWCNTs,
MWCNTs,CNSs,
CNSs,activated
activatedcarbon,
carbon, LiOH
LiOH·H
O,
LiOH·H
2O/MWCNTs,
Figure
1. 1.
XRD
patterns
·H222O,
O,LiOH·H
LiOH·H
2 O/MWCNTs,
2O/CNSs and LiOH·H2O/AC.
LiOH·H
2O/CNSsand
andLiOH
LiOH·H
LiOH·H
LiOH
·H2 O/CNSs
·H22O/AC.
O/AC.
Figure
2. SEM
images of
(a) CNSs,
(b) MWCNTs,
(c) LiOH
·H2 O, (d)
(d) LiOH·H
2 O/CNSs,
Figure
Figure 2.
2. SEM
SEM images
images of
of (a)
(a) CNSs,
CNSs, (b)
(b) MWCNTs,
MWCNTs, (c)
(c) LiOH·H
LiOH·H22O,
O, (d) LiOH·H
LiOH·H22O/CNSs,
O/CNSs,
(e)(e)
LiOH
·
H
O/MWCNTs;
and
(f)
LiOH
·
H
O/AC.
2
2
O/MWCNTs; and
and (f)
(f) LiOH·H
LiOH·H22O/AC.
O/AC.
(e) LiOH·H
LiOH·H22O/MWCNTs;
Figure
33 shows
the
TEM
CNSs,
(b)
MWCNTs,
(c)
··H
Figure
3 shows
the TEM
(a) CNSs,of
MWCNTs,
LiOH
·H2 O, (d) LiOH
·H2 O/CNSs,
Figure
shows
theimages
TEMof images
images
of(b) (a)
(a)
CNSs, (c)
(b)
MWCNTs,
(c) LiOH
LiOH
H22O,
O,
(d)
LiOH
·
H
2
O/CNSs,
(e)
LiOH
·
H
2
O/MWCNTs
and
(f)
LiOH
·
H
2
O/AC.
Compared
with
Figure
3a,
(e)(d)
LiOH
·H·2HO/MWCNTs
and (f)
·H2 O/AC.
3a, it with
couldFigure
be observed
LiOH
2O/CNSs, (e) LiOH
·HLiOH
2O/MWCNTs
and Compared
(f) LiOH·H2with
O/AC.Figure
Compared
3a, itit
could
observed
the
2O nanoparticles with a diameter of 20–30 nm were successfully
could
be
observed
that
the LiOH
LiOH··H
Hwith
2O nanoparticles
a diameter
of 20–30
nm were supported
successfullyon
that
the be
LiOH
·H2 O that
nanoparticles
a diameter with
of 20–30
nm were
successfully
supported
on
the
CNSs
(Figure
3d)
with
clear
particle
structures.
Additionally,
it
on the
3d) with
clear particle
structures.
Additionally,
it was
was also
alsoon
thesupported
CNSs (Figure
3d)CNSs
with (Figure
clear particle
structures.
Additionally,
it was
also well-supported
well-supported
on
the
MWCNTs
(Figure
3e),
but
part
of
the
LiOH
·
H
2O nanoparticles were
on the
(Figure
3e), ·but
of the LiOH
·H2Oconnected
nanoparticles
thewell-supported
MWCNTs (Figure
3e),MWCNTs
but part of
the LiOH
H2 Opart
nanoparticles
were
with were
others
connected
with
others
aa clear
The
diameter was
in the
of
nm
connected
withinterface.
others without
without
clear interface.
interface.
The nanoparticle
nanoparticle
thetorange
range
of 50
50which
nm
without
a clear
The nanoparticle
diameter
was in thediameter
range ofwas
50 in
nm
100 nm,
to
100
nm,
which
was
a
bit
larger
than
that
supported
on
the
CNSs.
As
for
the
LiOH
·
H
2O/AC sample
to
100
nm,
which
was
a
bit
larger
than
that
supported
on
the
CNSs.
As
for
the
LiOH
·
H
2O/AC sample
was a bit larger than that supported on the CNSs. As for the LiOH·H2 O/AC sample (Figure 3f),
(Figure 3f),
clear LiOH
3f), no
no
LiOH··H
H22O
O particles
particles could
could be
be observed
observed on
on the
the activated
activated carbons.
carbons. Also,
Also, the
the pure
pure
no(Figure
clear LiOH
·Hclear
2 O particles could be observed on the activated carbons. Also, the pure LiOH·H2 O
LiOH
·
H
2O (Figure 3c) existed in the form of stacked flakes. The LiOH·H2O content of
LiOH·3c)
H2Oexisted
(Figurein 3c)
existedof in
the form
stacked
The LiOH
H2O content
of
(Figure
the form
stacked
flakes.of The
LiOHflakes.
·H2 O content
of ·LiOH
·H2 O/CNSs,
LiOH
LiOH··H
H22O/CNSs,
O/CNSs, LiOH
LiOH··H
H22O/MWCNTs,
O/MWCNTs, LiOH
LiOH··H
H22O/AC
O/AC was
was about
about 50%,
50%, as
as characterized
characterized by
by AAS.
AAS.
LiOH
·H2 O/MWCNTs,
LiOH
·H2 O/AC
was about
50%, as characterized
by AAS.
During the synthesis
During
During the
the synthesis
synthesis of
of LiOH
LiOH··H
H22O/CNSs,
O/CNSs, LiOH
LiOH··H
H22O/MWCNTs,
O/MWCNTs, and
and LiOH
LiOH··H
H22O/AC,
O/AC, intermolecular
intermolecular
of interactions
LiOH·H2 O/CNSs,
LiOH
·
H
O/MWCNTs,
and
LiOH
·
H
O/AC,
intermolecular
interactions
such
2
2
interactions such
such as
as hydrogen
hydrogen bonding
bonding may
may exist
exist between
between the
the additives
additives and
and LiOH
LiOH··H
H22O,
O,
as hydrogen bonding may exist between the additives and LiOH·H2 O, respectively, owing to
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respectively, owing to the presence of oxygen-containing functional groups such as hydroxyl,
the presence of oxygen-containing functional groups such as hydroxyl, carbonyl and carboxyl
carbonyl and carboxyl groups [14,23,27] on the surface of the CNSs, MWCNTs and AC. Therefore,
groups [14,23,27] on the surface of the CNSs, MWCNTs and AC. Therefore, with the proper additives
with the proper additives supplying hydrogen bonding, the composites could show a good ability
supplying
hydrogen
bonding, theofcomposites
a good
ability for
the aggregation
for retarding
the aggregation
LiOH·H2O.could
The show
porosity
structures
of retarding
CNSs, MWCNTs,
AC,
of LiOH
LiOH
·
H
O.
The
porosity
structures
of
CNSs,
MWCNTs,
AC,
LiOH
·
H
O,
LiOH
·H2 O/CNSs,
2 LiOH·H2O/CNSs, LiOH·H2O/MWCNTs, and LiOH·H2O/AC were
2 also measured
·H2O,
by
LiOH
·H2 O/MWCNTs,
and LiOHisotherms.
·H2 O/AC The
wereBrunauer-Emmett-Teller
also measured by nitrogen
adsorption-desorption
nitrogen
adsorption-desorption
(BET)
surface area, pore
isotherms.
The
Brunauer-Emmett-Teller
surface
area,
volume
and
pore
size
volume and
average
pore size are shown (BET)
in Table
1. As can
be pore
seen from
Table
1, average
it was clear
that
theare
shown
in
Table
1.
As
can
be
seen
from
Table
1,
it
was
clear
that
the
composed
LiOH
·
H
O-based
TCMs
2
composed LiOH·H2O-based TCMs exhibit different textures. Due to the larger BET surface
area of
exhibit
differentnanoadditives,
textures. Due tothe
thespecific
larger BET
surface
areaof
of the
carbon
nanoadditives,
the carbon
surface
area
LiOH
·H2O/CNSs
(276 m2the
/g) specific
and
surface
of LiOH·H2(140
O/CNSs
m2higher
/g) andthan
LiOH
·H2 O/MWCNTs
(140 m
wasand
higher
than
LiOH·area
H2O/MWCNTs
m2/g)(276
was
that
of LiOH·H2O/AC
(842 /g)
m2/g)
pure
2/g). According
LiOH
·H2O (15
thepure
SEMLiOH
and TEM
results, ittocan
concluded
that
of LiOH
· H2 m
O/AC
(84 m2 /g)toand
·H2 Ocharacterization
(15 m2 /g). According
thebe
SEM
and TEM
that high specific
surface
was also an
factor
that lead
to the
form
of nanoscale
characterization
results,
it canarea
be concluded
thatimportant
high specific
surface
area was
also
an important
factor
dispersion
of LiOH
H 2O
particles.dispersion of LiOH·H2 O particles.
that
lead to the
form ·of
nanoscale
Figure3. 3. TEM
TEM images
images of
of (a)
(a) CNSs,
CNSs, (b)
(b) MWCNTs,
MWCNTs, (c)
(d) LiOH·H2O/CNSs,
Figure
(c) LiOH·H
LiOH·H2O,
2 O, (d) LiOH·H2 O/CNSs,
2O/MWCNTs and (f) LiOH·H2O/AC.
(e)
LiOH·H
(e) LiOH·H2 O/MWCNTs and (f) LiOH·H2 O/AC.
Table 1. Texture parameters of carbon nanoadditive-modified thermal chemical materials CNSs,
Table 1. Texture parameters of carbon nanoadditive-modified thermal chemical materials CNSs,
MWCNTs, AC, LiOH·H2O/CNSs, LiOH·H2O/MWCNTs, LiOH·H2O/AC and pure LiOH·H2O.
MWCNTs, AC, LiOH·H2 O/CNSs, LiOH·H2 O/MWCNTs, LiOH·H2 O/AC and pure LiOH·H2 O.
Samples
BET Surface Area (m2/g) Pore Volume (mL/g) Average Pore Size (nm)
Samples
Pore Volume
Average 4.81
Pore Size (nm)
BET Surface
CNSs
381Area (m2 /g)
0.25 (mL/g)
MWCNTs
343
0.61
8.95
CNSs
381
0.25
4.81
AC
324
0.75
2.71
MWCNTs
343
0.61
8.95
AC
324
0.75
2.71
2O/CNSs
276
0.18
2.64
LiOH·H
LiOH·2H
276
0.18
2.64
2 O/CNSs
LiOH·H
O/MWCNTs
140
0.31
8.81
LiOH·H2 O/MWCNTs
140
0.31
8.81
84
0.03
2.54
LiOH·H2O/AC
LiOH·H2 O/AC
84
0.03
2.54
Pure
LiOH·H2O
15
0.06
1.75
Pure LiOH·H O
15
0.06
1.75
2
3.2. The Heat Storage Performance Test of Lithium Hydroxide Monohydrate-based TCMs
3.2. 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
Comparative
performance
tests
pure
LiOH
H2 O, are
LiOH
·H2 O/AC,
LiOH
H2 O/CNSs
LiOH
·H2O/MWCNTs
were carried
out,ofthe
results
of ·which
shown
in Figure
4. It ·was
found thatand
LiOH
·
H
O/MWCNTs
were
carried
out,
the
results
of
which
are
shown
in
Figure
4.
It
was found
2
the reaction
rate of lithium hydroxide and water vapor was slow and the conversion of LiOH
to
that
the·H
reaction
rate of
lithium
hydroxide
and water
vapor
slow and
theapproximately
conversion of18%
LiOH
LiOH
2O was only
about
42% after
1 h hydration,
which
waswas
calculated
by the
to mass
LiOHloss
· H2 O
about 42%
after 4a.
1 h It
hydration,
was the
calculated
by theheat
approximately
of was
H2O,only
as shown
in Figure
was also which
found that
endothermic
value of
18%
mass
loss
of
H
O,
as
shown
in
Figure
4a.
It
was
also
found
that
the
endothermic
heat
LiOH·H2O was only
2O. It of
2 about 661 kJ/kg. Figure 4b shows the DSC curve of CNS-modified LiOH·Hvalue
LiOH
·Hbe
was that
onlyafter
about
kJ/kg. Figure
4b showsLiOH
the DSC
of CNS-modified
LiOH
·H2 O.
could
1 h661
hydration
of LiOH/CNSs,
wascurve
fully hydrated
to LiOH·H
2O and,
2 Oseen
thethat
heatafter
storage
of this
sample normalized
LiOH
·H2O
content could
reach
It furthermore,
could be seen
1 h density
hydration
of LiOH/CNSs,
LiOHby
was
fully
hydrated
to LiOH
· H2 O
2020
kJ/kg. This value
of LiOH
·H2density
O contained
in sample
LiOH·Hnormalized
2O/MWCNTsby
also
reached
high level
and,
furthermore,
the heat
storage
of this
LiOH
·H2 O acontent
could
(1804
kJ/kg),
as shown
in Figure
4c. As
the LiOH·H
the heat storage
densitya of
reach
2020
kJ/kg.
This value
of LiOH
·H2for
O contained
in2O/AC
LiOH·sample,
H2 O/MWCNTs
also reached
high
level (1804 kJ/kg), as shown in Figure 4c. As for the LiOH·H2 O/AC sample, the heat storage
Energies 2017, 10, 644
Energies 2017, 10, 644; doi: 10.3390/en10050644
6 of 9
6 of 9
density of LiOH·H2 O was lower than that of LiOH·H2 O/MWCNTs and LiOH·H2 O/CNSs, and
reached
(Figurethan
4d). that
It was
that at the same
of the hydration
reaction,
LiOH1236
·H2OkJ/kg
was lower
of indicated
LiOH·H2O/MWCNTs
and duration
LiOH·H2O/CNSs,
and reached
due1236
to the
addition
CNSs,
MWCNTs
fullyofreacted
with Hreaction,
2 O molecules
kJ/kg
(Figureof4d).
It was
indicated and
that AC,
at theLiOH
same was
duration
the hydration
due to and
converted
to LiOH
·H2 OMWCNTs
compared
with
In other
hydration
reaction
rate of
the addition
of CNSs,
and
AC,pure
LiOHLiOH.
was fully
reactedwords,
with Hthe
2O molecules
and
converted
to·H
LiOH
·
H
2
O
compared
with
pure
LiOH.
In
other
words,
the
hydration
reaction
rate
of
LiOH
O/CNSs,
LiOH
·
H
O/MWCNTs
and
LiOH
·
H
O/AC
were
greatly
enhanced.
On
one
hand,
2
2
2
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
existing hydrophilic
on thedifferent
surface ofreaction
CNSs, MWCNTs
and AC could
H2 Othe
adsorption
easier andfunctional
provide agroups
completely
interface between
LiOHmake
and the
H
2O adsorption easier and provide a completely different reaction interface between LiOH and the
water molecules. On the other hand, the reason that LiOH·H2 O/CNSs and LiOH·H2 O/MWCNTs
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·H2 O/AC and pure
composed TCMs showed ultrahigh heat storage density, higher than that of LiOH·H2O/AC and pure
LiOH·H2 O TCMs, could be due to their higher specific surface area, which substantially enhanced the
LiOH·H2O TCMs, could be due to their higher specific surface area, which substantially enhanced
dispersion of LiOH·H2 O nanoparticles and increased the contact of the surface area with the water
the dispersion of LiOH·H2O nanoparticles and increased the contact of the surface area with the
molecules.
Also, theAlso,
low the
specific
surfacesurface
area of
LiOH
·H2 O/AC
and pure LiOH·H O may be the
water molecules.
low specific
area
of LiOH
·H2O/AC and pure LiOH·H2 2O may be
reason
for
their
lower
heat
storage
density.
When
the
particle
size
reached
the nanoscale,
the amount
the reason for their lower heat storage density. When the particle size reached
the nanoscale,
the
of surface
would
obviously
increase; therefore,
the crystalline
field and
binding
energy of
amountatoms
of surface
atoms
would obviously
increase; therefore,
the crystalline
field
and binding
the internal
were different
fromdifferent
that of the
surface
whichatoms,
have many
bonds
energy ofatoms
the internal
atoms were
from
that ofatoms,
the surface
whichdangling
have many
duedangling
to the lack
of adjacent
atoms.
owing to
the unsaturated
in atoms, nanoparticles
show
bonds
due to the
lack So,
of adjacent
atoms.
So, owing tobonds
the unsaturated
bonds in atoms,
nanoparticles
show better
thermodynamic
properties due
[28,29].
Meanwhile,
due
to theatoms
increase
better
thermodynamic
properties
[28,29]. Meanwhile,
to the
increase of
surface
andoftheir
surface
atoms andfunctional
their existing
hydrophilic
functional
greater
amount
of Hto
2Oreact,
and LiOH
existing
hydrophilic
groups,
a greater
amountgroups,
of H2 Oaand
LiOH
was able
which in
was
able
to
react,
which
in
turn
improved
the
heat
storage
performance
of
the
composite.
turn improved the heat storage performance of the composite. Furthermore, according to the TEM
Furthermore, according
to the
TEMthat
characterization
results,density
the reason
that the
storage density
characterization
results, the
reason
the heat storage
of LiOH
·Hheat
2 O/CNSs was higher
of LiOH·H2O/CNSs was higher than that of LiOH·H2O/MWCNTs may be due to the smaller particle
than that of LiOH·H2 O/MWCNTs may be due to the smaller particle size of LiOH·H2 O (20–30 nm)
size of LiOH·H2O (20–30 nm) that existed in LiOH·H2O/CNSs compared to that in
that existed in LiOH·H2 O/CNSs compared to that in LiOH·H2 O/MWCNTs (50–100 nm). It could be
LiOH·H2O/MWCNTs (50–100 nm). It could be speculated that smaller size nanoparticles could make
speculated
that smaller size nanoparticles could make a greater contribution to the enhancement of heat
a greater contribution to the enhancement of heat storage density of TCMs. Additionally, after the
storage
density
of TCMs.
Additionally,
the ·addition
CNSs,conductivity
MWCNTs and
AC to
LiOH·H2 O,
addition
of CNSs,
MWCNTs
and AC after
to LiOH
H2O, the of
thermal
of these
composed
the TCMs
thermal
conductivity
of
these
composed
TCMs
became
higher
than
that
of
pure
LiOHof
· H2 O
became higher than that of pure LiOH·H2O (shown in Figure 5). Presently, the synthesis
(shown
in
Figure
5).
Presently,
the
synthesis
of
carbon
nanoadditive-modified,
LiOH
·
H
O-based
carbon nanoadditive-modified, LiOH·H2O-based thermochemical materials has not yet been2 fully
thermochemical
materials
not density
yet been
developed,
andcould
the heat
storage
density by
of the
developed, and
the heat has
storage
of fully
the inorganic
hydrate
be further
improved
controlling
its hydrophilic
property
and particle
size.
inorganic
hydrate
could be further
improved
by controlling
its hydrophilic property and particle size.
Figure 4. TG-DSC curves of as-synthesized samples: (a) pure LiOH after 1 h hydration, (b) LiOH/CNSs
after 1 h hydration, (c) LiOH/MWCNTs after 1 h hydration, and (d) LiOH/AC after 1 h hydration.
Energies 2017, 10, 644; doi: 10.3390/en10050644
7 of 9
Figure 4. TG-DSC curves of as-synthesized samples: (a) pure LiOH after 1 h hydration,
(b) LiOH/CNSs after 1 h hydration, (c) LiOH/MWCNTs after 1 h hydration, and (d) LiOH/AC after
1h
hydration.
Energies
2017,
10, 644
7 of 9
Figure
Thermal
conductivity
of CNSs,
AC, LiOH·H
·H22O/CNSs,
LiOH
·H2 O/
2 O, LiOH
Figure 5. 5.
Thermal
conductivity
of MWCNTs,
CNSs, MWCNTs,
AC,
LiOH·H
O, LiOH·H
2O/CNSs,
MWCNTs and LiOH·H2 O/AC.
LiOH·H2O/MWCNTs and LiOH·H2O/AC.
4.
4. Conclusions
Conclusions
To
·H22O-based
O-based thermochemical
thermochemical materials
materials were
were
To sum
sumup,
up,carbon
carbonnanoadditive-modified,
nanoadditive-modified,LiOH
LiOH·H
synthesized,
characterized
and
well
applied
for
low
temperature
chemical
heat
storage.
The
existence
synthesized, characterized and well applied for low temperature chemical heat storage. The
of
carbon of
nanomaterials
(CNSs and
MWCNTs)
in the LiOH
·HLiOH
composite
TCMs TCMs
cause
2 O-based
existence
carbon nanomaterials
(CNSs
and MWCNTs)
in the
·H2O-based
composite
the
LiOH
to disperse
into theinto
nanoscale
level andlevel
formand
different
sizes of particles.
2 O particles
cause
the·HLiOH
·H2O particles
to disperse
the nanoscale
form different
sizes of
The
hydration
reaction
rates
of
the
composed
materials
were
greatly
improved
and
the
heat
storage
particles. The hydration reaction rates of the composed materials were greatly improved and
the
density
of
LiOH
·
H
O
in
the
composed
TCMs
reached
2020
kJ/kg
and
1804
kJ/kg,
respectively,
2
heat storage density
of LiOH·H2O in the composed TCMs reached 2020 kJ/kg and 1804 which
kJ/kg,
were
much higher
values
thathigher
of LiOH
·H2 O/AC
(1236ofkJ/kg)
pure LiOH
kJ/kg).
2 O (661
respectively,
which
werethan
much
values
than that
LiOHand
·H2O/AC
(1236·H
kJ/kg)
and
pure
This
may
due
to
the
hydrophilic
property,
high
specific
surface
area
and
the
nanoparticle
size
effect
of
LiOH·H2O (661 kJ/kg). This may due to the hydrophilic property, high specific surface area and the
the
composed
thermochemical
materials.
Finally,
the
additives
also
showed
certain
a
positive
effect
on
nanoparticle size effect of the composed thermochemical materials. Finally, the additives also
the
thermal
conductivity
the composed
TCMs.
showed
certain
a positiveofeffect
on the thermal
conductivity of the composed TCMs.
Acknowledgments: This
This work
work was
was supported
supported by
bythe
theNational
NationalScience
ScienceFoundation
Foundationof
ofChina
China(No.
(No.51406209).
51406209).
Acknowledgments:
Author
Author Contributions:
Contributions: Xixian
Xixian Yang
Yang and
and Shijie
Shijie Li
Licontributed
contributed equally
equally to
tothis
thiswork.
work. They
They contributed
contributed to
to the
the
material
synthesis,
data
analysis
and
paper
writing;
Hongyu
Huang
and
Jun
Li
contributed
to
the
material
material synthesis, data analysis and paper writing; Hongyu Huang and Jun Li contributed to the material
characterization; Noriyuki Kobayashi and Mitsuhiro Kubota contributed to the design of the experiment.
characterization; Noriyuki Kobayashi and Mitsuhiro Kubota contributed to the design of the experiment.
Conflicts of Interest: The authors declare no conflict of interest.
Conflicts of Interest: The authors declare no conflict of interest.
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