Pyrazolium Phase-Change Materials for Solar-Thermal Energy Storage

Article abstract of DOI:10.1002/cssc.201902601

Thermal energy storage technology utilizing phase-change materials (PCMs) can be a promising solution for the intermittency of renewable energy sources. This work describes a novel family of PCMs based on the pyrazolium cation, that operate in the 100–200

Full text of DOI:10.1002/cssc.201902601

Accepted Article
Title: Pyrazolium Phase Change Materials for Solar-Thermal Energy
Storage
Authors: Karolina Matuszek, R. Vijayaraghavan, Craig M. Forsyth,
Surianarayanan Mahadevan, Mega Kar, and Douglas Robert
MacFarlane
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To be cited as: ChemSusChem 10.1002/cssc.201902601
Link to VoR: http://dx.doi.org/10.1002/cssc.201902601
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10.1002/cssc.201902601
ChemSusChem
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Pyrazolium Phase Change Materials for Solar-Thermal Energy
Storage
Karolina Matuszek[a], R. Vijayaraghavan[a], Craig M. Forsyth[a], Surianarayanan Mahadevan[b], Mega
Kar[a] and Douglas R. MacFarlane*[a]
Abstract: Thermal energy storage technology utilizing phase change
materials (PCMs) can be a promising solution for the intermittency of
renewable energy sources. This work describes a novel family of
PCMs based on the pyrazolium cation, that operate in the 100-200°C
temperature range, offering safe, inexpensive capacity and low
supercooling. Thermal stability and extensive cycling tests of the most
promising PCM candidate, pyrazolium mesylate (Tm=168 ± 1°C,
ΔHf=160 J g-1 ± 5%, ΔHtotalv=495 MJ m-3 ± 5%) show potential for its
use in thermal storage applications. Additionally, this work discusses
the molecular origins of the high thermal energy storage capacity of
these ionic materials based on their crystal structures, revealing the
importance of hydrogen bonds in PCM performance.
from ΔHf using density and also allows for any sensible heat
component (Qsens).
Previous studies of vacuum tube solar-thermal collectors show
that temperatures in the region of 100-200°C are routinely
achievable.[2] This indicates that a PCM having its melting
transition between 100-200°C would be ideal for thermal energy
storage for this application. A number of PCMs are described in
the literature that can operate between 100-200°C. However,
most of these are inadequate for practical application. Inorganic
salt hydrates (eg. MgCl2*6H2O Tm=111°C)[3], typically offer only
low total volumetric energy storage, (ΔHtotal = 423 MJ m-3), are
v
usually corrosive and suffer phase separation; sugar alcohols (eg.
galactitol Tm=187°C, ΔHtotal =735 MJ m-3),[4] which suffer from
v
significant supercooling (80°C for galactitol) and a cyclability
study shows that they oxidize very quickly;[3-5] high density
Introduction
polyethylene, HDPE, (Tm=133°C, ΔHtotal =398 MJ m-3) which is
v
thermo-oxidatively degraded during cycling[6] and lipid derived
monoamides[7] Diamides[8], with good thermal properties
(Tm=150°C, ΔHf =200 J g-1) have been described, but there is no
study of their extended cycling, confirming their stability. Also,
some dicationic salts have been described (e.g. 1,4-bis(3-
vinylimidazolium-1-yl) bromide, Tm=135°C, ΔHtotalv =568 MJ m-3)[9],
nevertheless the synthesis cost of these is expected to be high
and there is a doubt about thermal stability due to the presence
of a halide ion. On the other hand guanidinium mesylate
(Tm=208°C, ΔHf =190 J g-1)[10] is promising for high-temperature
applications and aliphatic fatty diamides offer good thermal
properties, (Tm=140°C, ΔHf =200 J g-1)[11] and stability on cycling,
Renewable energy has the ultimate capacity to resolve the
environmental and scarcity challenges of the world’s energy
supplies. However, both the utility of these sources and the
economics of their implementation are strongly limited by their
intermittent nature. Therefore, inexpensive means of energy
storage needs to be part of the design. Thermal energy storage
(TES) offers numerous benefits to various energy generation and
consumption scenarios, as it can be used to directly supply
thermal energy where needed in industrial and domestic
settings.[1] All materials are capable of storing thermal energy
during heating in form of “sensible heat” , the quantity (Qsens) being
determined by the specific heat of the material, the temperature
change and the amount of material. On the other hand, in a phase
change materials (PCM), latent heat storage occurs when the
material undergoes a phase transition. Most commonly the phase
transition used in PCMs is melting, so the energy is stored in the
liquid state and can be released upon crystallization or
solidification. The heat of fusion (ΔHf), is one of the primary
properties of a PCM, while its melting point determines its
applications. In many applications the total volumetric energy
v
but lack of data obstruct complete ΔHtotal calculations.
Additionally, there are only a few materials available in the market
in this temperature range, for example biobased PureTemp 151[12]
(Tm = 151°C, ΔHtotalv = 575 MJ m-3) and organic PulseICE A164[13]
(Tm = 164°C, ΔHtotalv = 423 MJ m-3). Further data on this literature
summary is provided in Table S1. In our view the significance of
the field of application demands a much broader range of
tuneable materials.
Hence, we describe a novel family of organic salts that have a
strong potential as PCMs for this temperature range and
investigate one of them in extensive detail, demonstrating thermal
properties, thermal cycling and stability, safety and viable
economics. We also probe the molecular origins of the high phase
change energy of these materials by investigation of the
interactions in their crystal structures and Hirshfeld surface
analysis, the disruption of which requires absorption of substantial
amounts of energy during melting.
stored (ΔHtotal , MJ m-3) is the key parameter; this is calculated
v
[a]
[b]
Dr. Karolina Matszek, Dr. R. Vijayaraghavan, Dr. Craig M. Forsyth,
Dr. Mega Kar, Prof. Douglas R. MacFarlane
School of Chemistry
Monash University
Clayton, VIC 3800, Australia
E-mail: Douglas.MacFarlane@monash.edu
Dr. Surianarayanan Mahadevan
Chemical Engineering Department
Central Leather Research Institute
Adyar, Chennai 6000-20, Tamilnadu, India
Results and Discussion
Organic salts are good PCMs candidates as they can be highly
chemically and thermally stable, non-flammable, and relatively
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/cssc.201902601
ChemSusChem
FULL PAPER
non-volatile, with good heat transfer properties.[14] Additionally, a
wide spectrum of cations and anions offer the opportunity to tailor
depending on the anion used, providing the opportunity to adjust
the system for the temperature characteristics for different parts
the salt to have optimum characteristics, in particular melting point, of the world, and therefore it can serve as a toolbox during the
for the target application and location. Here we present a group
of new organic salts which are based on the pyrazolium cation
([Pzy][A]) (Figure 1). According to the X-ray analysis, each
material shows a unique crystal motif in the solid phase, with
hydrogen bonding and Coulombic interactions between cation
and anion being main driving forces in the crystal packing. The
pyrazolium salicylate salt, however, differs markedly in that the
proton transfer is not complete and the obtained material is a co-
crystal. A more detailed analysis of all of the crystal structures is
presented below and in the ESI.
design of a multigeneration system. For example, [Pzy][C6H5SO3]
(Tm = 137°C) and [Pzy][C2H5SO3] (Tm = 113°C) may be useful in
other contexts.
Table 1. Thermal properties of pyrazolium organic salts as thermal energy
storage materials.
Tm,
°C ± 1°C
ΔHf,
J g-1 ± 5%
Qsens
,
ΔHtotal,
MJ m-3 ±5%
Material
J g-1 ± 5%
[Pzy][CH3SO3]
[Pzy][C6H5SO3]
[Pzy][C2H5SO3]
168
137
113
59b, 95b,
147
160
105
108
17b, 24b,
27
151
116
79
495
333
275
The thermal properties of the pyrazolium organic salts, including
melting points, solid-solid phase transitions (where observed),
heat of fusion (ΔHf) and heat capacity, were examined using
differential scanning calorimetry (DSC), as summarized in Table
1 and Table S2. All studied materials were solids at room
temperature, with melting points between 91 and 168 (± 1)°C, and
ΔHf between 40 and 160 J g-1 (± 5%). For [Pzy][CF3SO3], two
solid-solid phase transitions were also observed at 59 and 95 (±
1)°C, with associated heats of transition, respectively ΔHf = 17
and 24 J g-1 ± 5%; in most contexts this is an undesirable
behaviour as some of the overall heat of fusion is being absorbed
at these lower temperatures. Table 1 also lists the sensible heat
(Qsens) that can be stored in the material calculated from the
measured heat capacities (Table S3 and Figure S1). This is then
combined in Table 1 with ΔHf to calculate a total thermal energy
stored. This is also converted, using the density, into practical
[Pzy][CF3SO3]
114
324
[Pzy][BF4]
Pzy-Sal acid*
[Pzy][HSO4]
[Pzy]Cl
133
91
40
134
115
80
-
-
-
-
321
-
122
112
68
-
* Co-crystal of pyrazole and salicylic acid; Qsens – sensible heat calculated
from heat capacity within the temperature range from 333 to (Tm - 5)K, a –
calculated density from crystal structure; b – solid-solid transition; c –
calculated from the sum of ΔH for all transitions; DSC procedure: heating
rate 10 ⁰C min-1, three cycles performed, results taken from second cycle.
In the next step, the thermal stability of [Pzy][CH3SO3] was
investigated in depth using accelerating rate calorimetry (ARC).
This is a thermal analysis method that allows detailed assessment
of the safety of a material at elevated temperatures. The
instrument heats the sample in small steps of temperature,
searching at each step for heat flows or pressure changes that
would indicate thermal runaway. ARC experiments (Figure S12-
S14) show that [Pzy][CH3SO3] is stable up to 275°C; before that,
no temperature or pressure changes were observed. Additionally,
only a very small self-heat rate (less than 0.08°C min-1) was noted
at 275°C. This result implies that [Pzy][CH3SO3] has no tendency
to enter thermal runaway under operating conditions below 275°C
and the very low self-heating rate thereafter allows ample time for
cooling circuits or other safety measures to be activated. By
comparison, the well-known thermal stability issues within lithium
batteries, which are already being used to store renewable energy,
arise from the fact that the combined active materials enter
thermal runaway at temperatures below 120°C[15] and notably this
problem becomes more significant in large format battery packs
due to heat retention and build up within the pack. Further
analysis of the high temperature stability of [Pzy][CH3SO3] by
thermogravimetric analysis coupled to Fourier-transform infrared
spectroscopy is described in the supplementary information
(Figure S11); this data indicates that a release of small amounts
of water (see Scheme S1 in the ESI) is one of the main first events
in the extended “open-pan” treatment of these materials at
elevated temperatures. Overall, this data indicates that these
materials can be considered safe for use at temperatures in the
100-200°C range and certainly are safer under conditions of fire
than lithium battery storage packs.
v
units of energy stored per unit volume of material, ΔHtotal in MJ
m-3. Within this family of salts, [Pzy][CH3SO3], has the most
suitable characteristics for the solar thermal application (Tm = 168
± 1°C and high values of ΔHf = 160 J g-1 ± 5% and ΔHtotalv = 495
MJ m-3). Consequently, [Pzy][CH3SO3] was chosen here for
further investigation as a practical material, as discussed below.
Additionally, Table 1 offers a wide range of melting points,
Figure 1. Organic salts studied in this work, presented as one ion pair, derived
from their crystal structures. Pzy-Salicylic acid is a co-crystal, crystal structure
revealed lack of proton transfer.
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10.1002/cssc.201902601
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To further confirm the stability of [Pzy][CH3SO3], long-term cycling
tests were performed using a 5 gram-scale differential thermal
analysis (DTA) apparatus (Figure S2) that allows the temperature
within the materials to be measured continuously. The sample is
contained in a closed (but not hermetically sealed) borosilicate
glass tube during this experiment. [Pzy][CH3SO3] (5 g) was
heated and cooled 130 times, in the temperature range of 120-
175°C. Figure 2 presents the first (black curve) and the last (red)
DTA heating and cooling cycle of [Pzy][CH3SO3]; the shoulders
around 160-165°C indicate the melting and freezing transitions.
The DSC trace of the material (blue) is overlaid to indicate the
corresponding DSC features. The first and last DTA cycles are
overlapped showing that the material melts and crystallizes over
the same temperature range. The DSC measurements of ΔHf
performed after cycling confirmed that the value, if anything, was
slightly increased, likely due to improved crystal formation in the
slow cooling (Figure S5). Moreover, the NMR (Figure S4) analysis
of the material carried out before and after cycling did not show
any change in the structure, confirming the chemical stability.
Additionally, very little supercooling was observed during cycling
(<10°C lower than melting), which is a common problem among
known PCMs.[16] This can be explained in terms of the Coulombic
interactions which dominate in these materials and which still exist
in the molten organic salt. These forces create a semi ordered
arrangement of cations and anions close to each other in that
liquid state, that allows the material to rapidly re-arrange into a
well-organized structure when the temperature drops below the
thermodynamic melting point.
If any material is to be considered for large-scale energy storage
applications, its cost plays a key role alongside technical
properties. An economic analysis of the preparation costs of
[Pzy][CH3SO3] including raw materials (pyrazole and
methanesulfonic acid) and processing was carried out, yielding a
cost of about 4000 USD per tonne. By comparison, the typical
cost of commercially available ambient temperature PCMs is in
the range of 3800 USD per tonne for paraffin products and 7000
USD per tonne for salt hydrates (note both of these PCM families
are for <100°C application and are not useful in the present
context).[17] Returning to our materials, this cost equates to 27
USD per kWh of stored (thermal) energy. This compares very
favourably with current wholesale prices for large scale battery
installations used for renewable energy storage (~300 USD per
kWh for lithium batteries[18]). In many cold climates domestic and
commercial energy use can be greater than 50% as heat,
therefore the PCM represents an inexpensive way of storing
energy.
Figure 2. Longterm cycling of [Pzy][CH3SO3]. Black curve represents the first
DTA cycle and the red curve the last. Blue: DSC traces of the fresh material
before cycling.
presents as highly ordered lattice structure with a hydrogen bond
between every pair of oxygen atoms, creating an H-bonded
network and resulting in a notably high energy uptake of 333 J g-
1 or 6 kJ mol-1 during melting.[20] This energy uptake involves only
disruption of the H-bond network in ice, not its full destruction. The
energy of each of the two hydrogen bonds per molecule in ice is
estimated to be around 26 kJ per mol (H-bonds), dropping by just
3 kJ per mol (H-bonds) during melting.[20c] This serves to
demonstrate just how substantial the energy embodied within an
H-bond can be in the context of a PCM. With this ice-like
hypothesis in mind, we probed the crystal structures of our
materials to assess the occurrence and strength of H-bonding.
The details of interactions in the crystal structures of
[Pzy][CH3SO3] can be discerned by closer inspection of Figure 3A
in which we see two ion pairs, and also Figure 3B, which show an
expanded view of the structures, the repeating unit cell
represented by the rectangular box. To further probe these crystal
structures, the Hirshfeld surface analysis is a useful tool to
visualize and quantify intermolecular interactions in the
crystals.[21] The Hirshfeld surface for [Pzy][CH3SO3], as shown in
Figure 3C, divides space within the crystal structure between
each molecule and color-codes the surface according to how
close the atoms are to their neighbours. Particularly close
interactions are coloured red, thereby identifying in three
dimensions the regions of strong connections. It is these
connections that we hypothesize as being disrupted during
melting, and the elevated value of ΔHf reflects the energy required.
These “hot-spots” can be further evaluated in the fingerprint plot
of Figure 3D, which show the distances to each point on the
Hirshfeld surface, color-coded according to the interaction
involved and its frequency. This dramatically reveals the presence
of the strong O··H-N hydrogen bonds, as indicated by the two
spikes at very short interaction distances corresponding to the H-
bond distances. These bonds are highlighted as dotted lines in
Figure 3A. [Pzy][CH3SO3] forms a hydrogen bonded cyclic cluster
between two ion pairs, showing the importance of hydrogen
bonding as an important factor beyond the electrostatic
interactions between ions that are common to all of these salts.
We have also delved into the fundamental origins of the high ΔHf
values exhibited by these compounds. Generally, the heat of
melting is taken up by a material to overcome the forces of
interaction between the molecules in the crystal and to excite the
various motional degrees of freedom of the molecules. A high
heat of fusion material is one in which there are strong forces
holding the molecules together in the crystalline form, but not so
strong that the melting point is too high. One molecular interaction
which can potentially contribute to high values of ΔHf is the
10, 19]
hydrogen bond.[9e,
The archetypical example of this
hypothesis is ice. In its crystalline form, hexagonal ice (Ih)
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10.1002/cssc.201902601
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and ethanesulfonate anions. Though H-bonds can be detected,
their angles are far from linearity. The triflate salt [Pzy][CF3SO3] is
a plastic crystal, which will strongly decrease the measured ΔHf.
Additionally, the strongly electron withdrawing -CF3 group will also
affect the electrostatic interactions between the cation and anion.
In [Pzy][HSO4] the structure contains a strong H-bond between
[HSO4] anions, which is linear and short (178°, 1.87 Å). ΔHf of this
material is however low, indicating that in the liquid state the
strong H-bonds are not highly disrupted. In the crystal structure of
[Pzy]Cl, interactions between chloride atoms and hydrogens from
cation are present, with longer distances than in the other salts;
this results in lower ΔHf.
Conclusions
In summary, we have described the properties of a family of novel
phase change materials that offer a suite of physical properties
ideal for low cost storage of thermal energy from solar at a
domestic or industrial level. With melting points between 100 and
168°C such materials would be suitable for delivering energy into
a multi-use cascade that optionally could include an organic
Rankine cycle engine to generate electricity and then utilize the
waste heat in hot water and space heating. Among the studied
salts, the best properties were achieved for pyrazolium mesylate
(Tm=168 ± 1°C, ΔHf=160 J g-1± 5%, ΔHtotalv=495 MJ m-3 ± 5%),
which provides safe, inexpensive capacity, high efficiency
collection and storage of solar energy, comparable to commercial
materials. Investigation of the crystal structures of the materials
supports an “ice-like” hypothesis for the origins of their large
values of the heat of melting in strong, H-bonding. Neutron
diffraction studies of the liquid structure are on the way to further
understand the H-bonding in the liquid state of these salts.
Nonetheless, the structures developed here still fall short of the
strength and frequency of bonding in the 3-dimensional H-bonded
lattice of ice, suggesting that further crystal engineering of these
organic salts may yet reveal even higher values of ΔHf.
Figure 3. Crystal structure and Hirshfeld surface analysis of [Pzy][CH3SO3]. A)
two ion pairs connected by hydrogen bonds presented as dotted lines. B)
expanded, multi-unit cells views. C) Hirshfeld surface analysis where red
regions represent contact distances shorter than the sum of van der Waals
(vdW) radii and white to blue represent contact distances equal or longer than
the sum of the vdW radii, D) two-dimensional fingerprint plot, derived from the
Hirshfeld surface (the blue color variation represents the frequency of the O-H
interactions while the grey represents the other interactions).
The crystal structure details and Hirshfeld surface analysis for the
other pyrazolium compounds can be found in the ESI.
Experimental Section
A summary of the H-bonding data derived from analysis for all of
the compounds is provided in Table S4 and from this we see that
the [Pzy][CH3SO3] embodies a strong hydrogen bond structure in
which four H-bonds are created between two ion pairs of relatively
short length, but with angles ranging from 160° to 169°. The H-
bonds between ions are strengthened and stabilized by H-bonds
from the second ion pair, creating therefore the H-bond cage. This
type of cyclic cluster where ring closure is energetically favoured
makes its breakage also energetically costly, producing a high
ΔHf. In [Pzy][C6H5SO3] the H-bonds are also short and relatively
linear (160-169°). However, only one oxygen in the anion is
responsible for creating the network; this lowers the overall
hydrogen bond strength and therefore results in lower ΔHf than
that for [Pzy][CH3SO3]. [Pzy][C2H5SO3] at first glance is very
similar to [Pzy][CH3SO3] giving the impression that the ΔHf should
also be similar. On closer examination of the crystal structure the
difference between these two materials is considerable. In
[Pzy][C2H5SO3] there are two different types of pyrazolium cations
Materials
Materials used for this study were obtained as follows: pyrazole (Sigma-
Aldrich, 98%), methanesulfonic acid (Sigma-Aldrich, 99.5%),
ethanesulfonic acid (Sigma-Aldrich, 70% wt% in H2O), sulfuric acid (Univar,
95-98%), triflic acid (Sigma-Aldrich, 98%), benzenesulfonic acid (Sigma-
Aldrich, 97%), tetrafluoroboric acid (AlliedSignal, 50%), hydrochloride
(Univar, 32%), salicylic acid (Chem-Supply, 99%), methanol (Scharlau,
HPLC grade, 99.9%), and hexane (MERCK, 99%).
Synthesis
Pyrazolium organic salts were prepared in water (30mL) by mixing
pyrazole with the corresponding acid at 1:1 molar ratio on a 10 g scale.
Reaction was carried out for 2 h to ensure completion. Subsequently,
water was removed under vacuum at elevated temperature, and then the
solid product was dried (0.2 mbar, 50°C, 6h). Materials obtained were of
white colour and yield in the range of 97–99 %. Materials were
recrystallized from saturated solution in methanol with a few drops of
hexane in a laboratory freezer (-20°C) yielding colourless crystals.
[Pzy][CH3SO3] for DTA cycling was recrystallized 3 times.
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10.1002/cssc.201902601
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Nuclear magnetic resonance (NMR)
1H and 13C NMR measurements were conducted using a Bruker Avance
III 400; 1H at 400.13 MHz, 13C at 100.6 MHz.
attached to nitrogen or oxygen were located in the difference Fourier map
and were generally refined without restraint (see below).
Variata:
Thermogravimetric analysis (TGA)
[Pzy][MeSO3]: The structure was refined as a pseudo-merohedral twin
(twin law 1 0 0 0 -1 0 0 0 -1).
[Pzy][CF3SO3]: The CF3 group was modelled as rotationally disordered
over two positions, with occupancies fixed at 0.80:0.20. The N-H distance
was restrained with d(N-H) = 0.91(2) Å.
Thermal stability was determined by thermogravimetric analysis using a
Perkin Elmer Pyris 1 TGA with Pyris Software over a range of 30−550 °C
under N2 (30 mL min-1), either at a heating rate of 10 °C min-1 or during
isothermal hold at various temperatures.
TGA-FTIR spectroscopy
[Pzy][Salicylic acid]: The N-H and O-H distances were restrained with
d(N-H) = 0.91(2) Å and d(O-H) = 0.86(2) Å respectively.
Hirshfeld surface analysis
Experiments were conducted using a PerkinElmer STA8000 and frontier
Fourier transform infrared spectrometer instruments at a heating rate of
10°C min-1, or during isothermal hold at various temperatures.
Accelerating Rate Calorimetry (ARC)
Hirshfeld surface analysis was constructed using CrystalExplorer (S. K.
Wolff, D. J. Grimwood, J. J. McKinnon, D. Jayatilaka and M. A. Spackman,
CrystalExplorer 2.1, (2007) University of Western Australia
The thermal run away and intrinsic safety were investigated using a
Columbia Scientific Industries (now known as Thermal Hazard
Technology) Austin, Texas, accelerating rate calorimeter (ARC). ARC
works on a “heat-wait-measure” principle and records self-heat rate as a
function of temperature and time vs. temperature and pressure vs.
temperature profiles. The experiments were carried out with sample weight
of 0.5 g, loaded into the ARC sample bomb (titanium), connected to
thermocouples and pressure transducer and enclosed in a shielded
environment. The samples were then subjected to a heat-wait-search
mode of measurement in the temperature range from ambient to 500 °C.
The outputs were recorded and analysed.
(http://hirshfeldsurface.net/CrystalExplorer), Perth.)[21]
.
Acknowledgements
The U.S. Army Research Laboratory's Army Research Office
(ARO) is gratefully acknowledged for supporting these studies
(W911NF-17-1-0586). DRM is grateful to the ARC for funding
under the Australian Laureate Fellowship Scheme (Grant
FL120100019).
Differential scanning calorimetry (DSC)
Phase transition temperatures (melting, freezing, solid-solid) and
thermodynamic data (heat of fusion ΔHf, heat capacity Cp) were
determined using a DSC TA Q200 calorimeter (TA Instruments). The
equipment was calibrated using standards of indium (TA instruments, Tm
= 156°C, ΔHf = 28.45 J g-1) and cyclohexane (Sigma-Aldrich, Tm = 8°C).
Measurements were performed under nitrogen atmosphere in triplicate
with sample size 3-8 mg and heating rate 10°C min-1. Presented values
were taken from the second run of the DSC cycle. Melting point (Tm) was
considered as the peak maximum and ΔHf determined from the area of the
melting peak.
Keywords: thermal energy storage • phase change materials •
ionic liquids • pyrazole • renewable energy sources
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Entry for the Table of Contents
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Pyrazolium organic salts provide
safe, inexpensive capacity, high
efficiency collection, low supercooling
(>10oC) and high volumetric storage
of solar energy (495 ±5% MJ m-3).
Karolina Matuszek, R. Vijayaraghavan,
Craig M. Forsyth, Surianarayanan
Mahadevan, Mega Kar and Douglas R.
MacFarlane*
Page No. 1 – Page No. 6
Pyrazolium Phase Change Materials
for Solar-Thermal Energy Storage
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