Role of Hydrogen Bonding in Phase Change Materials

Article abstract of DOI:10.1021/acs.cgd.9b01541

Phase change materials (PCMs), which melt in the temperature range of 100-230 °C, are a promising alternative for the storage of thermal energy. In this range, large amounts of energy available from solar-thermal, or other forms of renewable heat, can be stored and applied to domestic or industrial processes, or to an organic Rankine cycle (ORC) engine to generate electricity. The amount of energy absorbed is related to the latent heat of fusion ( "H_f) and is often connected to the extent of hydrogen bonding in the PCM. Herein, we report fundamental studies, including crystal structure and Hirshfeld surface analysis, of a family of guanidinium organic salts that exhibit high values of "H_f ?, demonstrating that the presence and strength of H-bonds between ions play a key role in this property.

Full text of DOI:10.1021/acs.cgd.9b01541

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Article
The Role of H-bonding in Phase Change Materials
Karolina Matuszek, Vijayraghavan R, Mega Kar, and Douglas R. Macfarlane
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b01541 • Publication Date (Web): 27 Dec 2019
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The Role of H-bonding in Phase Change Materials
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Karolina Matuszek, R. Vijayaraghavan, Mega Kar, Douglas R. MacFarlane*
Monash University, School of Chemistry, Clayton, VIC 3800 (Australia)
Keywords: Thermal energy storage, phase change materials, guanidinium organic salts,
ionic liquids, hydrogen bonds
Phase change materials (PCMs) which melt in the temperature range of 100-230 °C, are
a promising alternative for the storage of thermal energy. In this range, large amounts of
energy available from solar-thermal, or other forms of renewable heat, can be stored and
applied to domestic or industrial processes, or to an Organic Rankine Cycle (ORC) engine
to generate electricity. The amount of energy absorbed is related to the latent heat of
fusion (ΔHf) and is often connected to the extent of hydrogen bonding in the PCM. Herein,
we report fundamental studies, including crystal structure and Hirshfeld surface analysis,
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of a family of guanidinium organic salts that exhibit high values of ΔHf, demonstrating that
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the presence and strength of H-bonds between ions plays a key role in this property.
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Introduction
Intermittent energy supply is one of the main issues with most renewable energy
sources and resolution of this issue requires inexpensive and efficient energy storage
technologies. Thermal energy storage (TES) is one of the options, for example using
phase change materials (PCMs).1-2 PCMs are classified as materials in which heat is
absorbed when the material undergoes a phase transition, usually melting, and the heat
can be released upon crystallization or solidification.2 The amount of energy needed to
melt the material, the heat of fusion (ΔHf), is one of the primary properties of a PCM. The
melting point of a PCM determines its application, and for many industrial applications, or
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for input to an Organic Rankine Cycle (ORC) engine, this needs to be in the range of 100-
250 °C. Recognizing that the energy collection efficiency of solar thermal technology is
substantially higher (as high as 70%) than photo-voltaic cells, interest has grown rapidly
in thermal energy storage in recent years.3
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Commercial PCMs include inorganic salt hydrates, paraffins, fatty acids/esters, alloys
of metals, or eutectics of organic and inorganic compounds.2, 4-7 Nevertheless, each of
these materials suffers specific drawbacks: paraffins are volatile, flammable and possess
low thermal conductivities, inorganic salt hydrates have phase separation problems,
suffer supercooling and are corrosive in many cases, while organic and inorganic eutectic
materials have transition temperatures too low for many applications.8-12 Recently,
researchers are focusing on improving some of the required properties of a PCM or on
developing new families of materials.13-19 A summary of literature data on PCMs with
melting points between 100-230 °C and their properties are presented in Table S1.
High ΔHf in a PCM can arise from the presence of strong hydrogen bonds in the
structure.14, 20-22 The best and simplest example of this hypothesis is ice Ih, which melts
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with a notably high energy uptake of 334 J/g or 6 kJ/mol. At temperatures above -100 °C
ice forms in the hexagonal structure (Ih) presented in Figure 1a, based on a highly
organized lattice structure with a hydrogen bond between every pair of oxygens, creating
an H-bonded network. The partial disruption of this network results in a high ΔHf.23-24 Ice
Ih exists in the P63/mmc space group, with angles H-O-H of about 106.6°, and around
180° between O-H-O, ensuring very strong H-bonds.25 Although, ΔHf for ice is high at 334
J/g26, water itself still contains a significant number of strong hydrogen bonds after melting
(Figure 1b). Therefore, ΔHf only reflects the effect of small changes in the hydrogen bond
lengths and angles on melting. Ice would be a very good PCM, but unfortunately its low
melting point limits its application in a thermal energy storage context.
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H
b)
a)
O
1.0
Å
2.8 Å
H
180o
1.8 Å
H
H
O
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Figure 1. a) crystal structure of hexagonal ice (Ih)27, b) hydrogen bonds in water.25
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Organic salts, emerging from the field of ionic liquids (ILs), composed of an organic
cation and an organic or inorganic anion, may offer a promising PCM alternative.
Recently, a number of organic salts/ionic liquids have been investigated as PCMs and
their high ΔHf values have also been connected to the existence of hydrogen bonds.16-17,
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20-21, 28
Our group has recently described a family of pyrazolium salts as phase change
materials.29 Organic salts are usually non-flammable, chemically and thermally stable,
and relatively non-volatile with good heat transfer properties.30 Moreover, a wide variety
of possible combination of cations and anions offer the opportunity to fine tune the melting
point and ΔHf of these salts for suitable applications. One particularly interesting family of
organic salts from a structural point of view are those based on the guanidinium cation;
these were previously shown by our group to exhibit the desired melting point range of
100-230 °C for solar-thermal applications.21 Specifically, the organic salt guanidinium
mesylate ([Gdm][CH3SO3]) was shown to have high ΔHf (190 J/g). The molecular origins
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that lead to this high ΔHf need to be investigated and understood more deeply, since this
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could lead to the design of new PCMs with superior properties.
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The guanidinium cation (Figure 2) is a good model to study H-bonding in organic salts.
It is a planar molecule with D3h symmetry and contains three NH2 groups, giving six
hydrogen bonding sites. [Gdm]+ has extraordinary thermodynamic stability and high
basicity resulting from delocalization of six π-electrons and energetic barrier to rotation
and therefore is classified as having Y-aromaticity.31-33 Due to their remarkable properties,
salts based on the guanidinium cation have previously been extensively investigated via
crystal structures34-38, theoretical studies39 and they have also been considered for
applications in perovskite solar cells40, plastics and explosives.41
Figure 2. Resonance stabilized guanidinium cation, and anions used in this study.
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Here we present an extensive study of the family of guanidinium organic salts
expanding the scope of materials beyond our previous publication21 to include
alkylsulfonate, hydrogensulfonate, tetrafluoroborate, halides and nitrate salts and
carrying out an in-depth crystal structure and Hirshfeld surface analysis to probe the
molecular origins of ΔHf. The salts are known in the literature, but most have not
previously been investigated in regard to the structural origins of their thermal storage
properties.
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Results and discussion
Thermal properties study
The thermal properties of the guanidinium organic salts, such as melting point, solid-
solid transitions (if present), were examined using DSC (Differential Scanning
Calorimetry) and the results are summarized in Table 1 and Table S2. All of the
guanidinium salts studied here were solid at room temperature, with melting points
ranging from 100 to 230 (± 2) °C, and with heats of fusion ranging between 75 and 190 J
g-1 (± 5%). In the cases of [Gdm][CF3SO3], [Gdm][C2H5SO3], [Gdm][CH3C6H4SO3],
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[Gdm][NO3] and [Gdm][BF4], a solid-solid transition was observed prior to melting; in most
contexts this is an undesirable behavior as some of the overall heat of fusion is being
absorbed at these lower temperatures.
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Table 1. Thermal properties of organic salts based on the guanidinium cation.
Tm ᴼC
± 2 ᴼC
208
ΔHf
J g-1 ± 5%
ΔHf
ΔSf
Material
kJ mol-1 ± 5% J mol-1 K-1
[Gdm][CH3SO3]21
[Gdm][C6H5SO3]
[Gdm][HSO4]21
[Gdm][CF3SO3]21
[Gdm][C2H5SO3]21
190
29
62
210
138
30
62
100
133
21
56
115i, 160
101i, 160
6i, 130
22i, 83
5, 91
150d
1, 27
4, 14
1, 21
14
3, 63
10, 32
3, 42
31
[Gdm][CH3C6H4SO3]21 175i, 227
[Gdm]Cl
188
[Gdm]Br
190
107
15
33
[Gdm]I
200
75
14
29
[Gdm][NO3]21
[Gdm][BF4]
30i, 217
101i, 173
11i, 174
2i, 103
1, 21
1, 15
4, 43
2, 34
d – decomposition or evaporation after melting
i – solid-solid transition
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Crystal structures
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Crystal structures of a wide variety of guanidinium organic salts were accessed from
the Cambridge Crystallographic Data Centre (CCDC)34-38, 42-46 and analyzed with a focus
on the nature and extent of hydrogen bonding. A detailed discussion of each structure is
presented in the Supplementary Information. Additionally, the Hirshfeld surface analysis
was used, which helps to visualize packing modes, shapes and intermolecular
interactions in crystals, and also enables quantification of them via a fingerprint plot. The
Hirshfeld surfaces were created using the CrystalExplorer tool-kit, developed by
Spackman and co-workers.47-48
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Hirshfeld surface analyses conducted for guanidinium salts, based on crystal structures
found in the database, are presented in Figure 3a. The surface is color-coded from blue
to red, visualizing the distances between the atoms inside the surfaces to the nearest
atom outside. The longer the distance between the atoms, the darker the blue color shown
on the surface. On the other hand, particularly close interactions are colored red. It is
these interactions that we hypothesize as being disrupted during melting, and having the
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main contribution to the ΔHf value. Comparing the Hirshfeld surfaces of guanidinium
organic salts, it can be seen that these red spots have different sizes. They can be further
evaluated in the two-dimensional fingerprint plot (Figure 3b). This plot is a 2D histogram
representing the fraction of the surface points having each value of de and di, coloring the
points according to frequency as blue (few points), through green to red (representing
many points).48 The axes represent di (distance from surface to the nearest atom inside
the surface) and de (distance from the surface to the nearest atom outside the surface),
therefore, showing the distances between atoms from neighboring molecules. From
Figure 3b it can be seen that range of interactions occurring in the [Gdm][HSO4] (1.73 to
~4.4 Å) and [Gdm][BF4] (1.9 to ~4.4 Å) are the narrowest among all studied salts.
Additionally, [Gdm][HSO4] has the shortest H-bond (H··O = 1.73 Å) which is represented
by two spikes; this bond connects two bisulfate anions, as also described above.
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b)
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Figure 3. a) Hirshfeld surface analysis of guanidinium organic compounds, b) fingerprint
plots of guanidinium organic compounds.
Examining the extended crystal structures of the guanidinium organic salt studied here,
it can be noticed that each of the salts presents a unique motif in the solid phase, Figure
4 and Figures S1-S12. In all of them, H-bonding between cations and anions appear to
be the main driving force for crystal packing.
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Figure 4. Crystal structure of [Gdm][CH3SO3], a) one ion pair and hydrogen bonding
structure (black dots), b) lattice structure view down the b axis.
Most of these salts have six hydrogen bonds, connecting [Gdm]+ and the anions (Table
2). Usually three cations are connected to three anions, creating a two-dimensional
hydrogen bonded sheet. At the same time, these materials produce a range of ΔHf values,
from 14 to 30 kJ mol-1, indicating that each of these hydrogen bonds has different strength
(2.3-5.0 J per H-bond). It has been demonstrated previously that the longer the H-bond
the weaker it is and, additionally, a longer and weaker H-bond is more likely to deviate
from linearity.25, 49 Taking a closer look into N-H··X bonds in guanidinium salts we can
notice a pattern. A graph, Figure 5, of ΔHf vs. bond length demonstrates that the shorter
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the bond length, the higher is the ΔHf. The shortest average bond length was noted for
[Gdm][C6H5SO3] of 2.76 Å. In general, the alkylsulfonates salts, being in the range of 2.76
– 2.95 Å, possess higher ΔHf than other non-sulfonate based salts. On the other hand,
H-bonds in the halides are the longest among the studied salts, and therefore the resulting
ΔHf is much lower (14 - 15 kJ mol-1).
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In the case of the H-bond angle (N-H··X), the relationship (Figure 5) is not as clear,
however materials with angles closer to linearity tend to have higher ΔHf. The bond
closest to linearity was observed for [Gdm][CF3SO3] at174° and this material also exhibits
high ΔHf = 27 kJ mol-1. Again, the highest deviation from linearity was obtained for halide
salts (160 – 157°) thereby resulting in a lower ΔHf. Surprising results were obtained for
[Gdm][HSO4], with average H-bond angles of 150°. Taking a closer look into the crystal
structure it can be noticed that H-bonds between cation and anion ie. [Gdm]+ and [HSO4]-
are very weak in the range of 133 – 153°, yet H-bonding between hydrogensulfonate
anions is very strong (2.62 Å, 174°). In fact, this bond is even stronger than the bonds
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observed in [Gdm][C6H5SO3] (2.76 Å, 172°). This suggest that the strength of H-bonds
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between anions plays a key role in the ΔHf value observed for [Gdm][HSO4].
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Interestingly, ice has a much lower ΔHf than these salts, in terms of kJ mol-1, albeit the
H-bonds are short (2.76 Å) and angles almost linear (179°) suggesting their high strength.
So why is ΔHf so low in this case? This may be explained by estimating the degree of
disorder (Dd) introduced during melting using the Boltzmann equation, ΔSf = R ln(N),
where ΔSf is the entropy of fusion, R is the ideal gas constant, and N expresses the ratio
of maximum configurations per molecule in the disordered phase, compared to the
ordered phase. Table 2 lists the degree of disorder Dd on melting for all of the compounds
studied.50 The obtained value for water (N =14) is much lower than for [Gdm][C6H5SO3]
(N = 1762), thus the latter begins to explore a much greater range of configurations in the
liquid state due to a stronger disruption of the H-bond lattice on melting. When ice melts,
water still contains many strong, bent hydrogen bonds and hence the structure remains
relatively constrained. Figure 6 plots the measured △Hf data as function of Dd. A clear
correlation is observed, with a suggestion of an upper bound being approached in
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[Gdm][C6H5SO3] and [Gdm][CF3SO3]. This is to be expected, since the degree of disorder
in the liquid phases of these compounds is likely to be similar when all of the H-bonding
structure is eventually disrupted, for example at elevated temperatures.
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To summarize, ΔHf depends on the hydrogen bonding present in the crystal structure,
being a function of not only their number, but also the strength of the hydrogen bonds
(length and deviation from linearity), and most importantly the extent of disruption of the
crystal phase on melting.
Table 2. Thermal properties, degree of disorder and crystal structure analysis of organic
salts based on guanidinium cation.
△Hf,
kJ mol-1
30
no of
H-b
6
J per H- Av. angle Av. length
Material
Dd1, N
1762
1459
1815
199
bond
N-H··X, ° N-H··X, Å
[Gdm][C6H5SO3]
[Gdm][CH3SO3]
[Gdm][CF3SO3]
[Gdm][CH3C6H4SO3]
[Gdm][HSO4]
[Gdm][NO3]
5.0
172
169
174
166
149
165
2.76
2.92
2.99
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[Gdm][C2H5SO3]
[Gdm][BF4]
[Gdm]Br
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[Gdm]Cl
[Gdm]I
Ice Ih
1 Degree of disorder – estimated from, ΔSf = R ln(N)
2 Sum of the heat of solid-solid transition and fusion
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Figure 5. The relationship between ΔHf and N-H··X hydrogen bond length and angle, (X
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= O, Cl, Br, I).
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Figure 6. The relationship between ΔHf and degree of disorder.
Conclusions
In this study, we have investigated guanidinium organic salts as PCMs, and probed the
molecular origins of high ΔHf in some of the salts. The guanidinium cation has been
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chosen as a model due to its hydrogen-bonding potential and known phase-change
properties. Each of the guanidinium salts studied presents a unique crystal structure
where H-bonding between cation and anion is the main driving force for crystal packing.
Our study reveals that ΔHf depends on the hydrogen bonding present in the crystal
structure, not only via their concentration but also their strength (length and deviation from
linearity). In compounds of similar H-bonding, the extent of H-bond disruption on melting
becomes a dominant factor. Thus, an ideal PCM in families of materials such as these is
indicated to be one in which many strong H-bonds are present in the crystalline phase
and which are readily disrupted on melting due to H-bond angle constraints and the
impact of motional degrees of freedom. We are currently investigating the structure of the
liquid state of some of these compounds in more detail via molecular dynamics
simulations and neutron diffraction measurements and these will be reported in a future
paper.
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Experimental Section
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Materials
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Materials used for this study: guanidine carbonate salt (Sigma-Aldrich, 99%),
methanesulfonic acid (Sigma-Aldrich, 99.5%), sulfuric acid (Univar, 95-98%),
benzenesulfonic acid (Sigma-Aldrich, 97%), p-toluenesulfonic acid (Unilab),
tetrafluoroboric acid (AlliedSignal, 50%), hydrochloric acid (Univar, 32%), hydrobromic
acid (Merck, 47%), guanidinium iodide (Sigma-Aldrich, ≥99%), nitric acid (Merck, 70%).
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Synthesis
Firstly, to ensure proper acid to base ratio in the final product a standard titration
analysis was performed. For this purpose, 0.1 M solutions of acids and guanidinium
carbonate in water were prepared. 10 ml of 0.1 M guanidinium carbonate solution was
titrated with 0.1 M solution of the acid. For each addition of 0.2 ml of acid, the pH of the
solution was measured. The inflection point was calculated based on a plot (pH vs volume
of acid). An exact stoichiometric amount of acid was calculated from the inflection point
and used for the synthesis. In the next step [Gdm][CH3SO3], [Gdm][C6H5SO3],
[Gdm][CH3C6H5SO3], [Gdm][Cl], [Gdm][Br], [Gdm][NO3], and [Gdm][BF4] were prepared
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in water by reacting guanidine carbonate salt with the required amount of the
corresponding acid on a 10 g scale for 4 h to ensure completion. Subsequently, water
was removed under vacuum and elevated temperature, and then the solid product was
dried (0.2 mbar, 50 °C, 6h). Materials were obtained with yield in the range of 97 – 99 %.
The purity of materials was confirmed by 1H NMR.
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Thermal properties
Phase transition temperatures (melting - Tm, solid-solid Ts-s) 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 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 peak maximum and ΔHf as area
of the peak.
Crystallization
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[Gdm][CH3SO3] (36 g) was recrystallized from a methanol-hexane mixture (300 ml: 10
ml), giving colorless cubic shaped crystals. The procedure was repeated 3 times to obtain
pure material for thermal stability and cyclability study. In each step the amount of
methanol-hexane was adjusted to the yield from previous step. 21 g of crystalline material
was obtained with total yield was 58 %.
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Hirshfeld surface analysis
Hirshfeld surface analysis was conducted 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 (http://hirshfeldsurface.net/CrystalExplorer), Perth.
[48, 49]) Crystal structures of studied materials were obtained from literature through
CCDC database.
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
AUTHOR INFORMATION
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Corresponding Author
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*Douglas.MacFarlane@monash.edu
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Author Contributions
The manuscript was written through contributions of all authors. All authors have given
approval to the final version of the manuscript.
Funding Sources
The U.S. Army Research Laboratory's Army Research Office (ARO) is gratefully
acknowledged for supporting these studies (W911NF-17-1-0586).
Notes
Any additional relevant notes should be placed here.
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Phase change materials are of growing interest as an approach to storing solar-thermal
energy at the domestic and small-scale enterprise levels. Their energy storage capacity
depends significantly on the enthalpy of melting of the material. In this paper we explore
the extent to which strong H-bonding in the crystalline state contributes towards high
enthalpy of melting.
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