Thermodynamics of Counterion Release Is Critical for Anion Exchange Membrane Conductivity

Article abstract of DOI:10.1021/jacs.8b03979

As the field of anion exchange membranes (AEMs) employs an increasing variety of cations, a critical understanding of cation properties must be obtained, especially as they relate to membrane ion conductivity. Here, to elucidate such properties, metal cation-based AEMs, featuring bis(norbornene) nickel, ruthenium, or cobalt complexes, were synthesized and characterized. In addition, isothermal titration calorimetry (ITC) was used to probe counterion exchange thermodynamics in order to understand previously reported differences in conductivity. The ion conductivity data reported here further demonstrated that nickel-complex cations had higher conductivity as compared to their ruthenium and cobalt counterparts. Surprisingly, bulk hydration number, ion concentration, ion exchange capacity, and activation energy were not sufficient to explain differences in conductivity, so the thermodynamics of metal cation-counterion association were explored using ITC. Specifically, for the nickel cation as compared to the other two metal-based cations, a larger thermodynamic driving force for chloride counterion release was observed, shown through a smaller ΔH_tot for counterion exchange, which indicated weaker cation-counterion association. The use of ITC to study cation-counterion association was further exemplified by characterizing more traditional AEM cations, such as quaternary ammoniums and an imidazolium cation, which demonstrated small variances in their enthalpic response, but an overall ΔH_tot similar to that of the nickel-based cation. The cation hydration, rather than its hydration shell or the bulk hydration of the membrane, likely played the key role in determining the strength of the initial cation-counterion pair. This report identifies for the first time how ITC can be used to experimentally determine thermodynamic quantities that are key parameters for understanding and predicting conductivity in AEMs.

Full text of DOI:10.1021/jacs.8b03979

Article
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Thermodynamics of Counterion Release Is Critical for Anion
Exchange Membrane Conductivity
†
‡
‡
,†
Michael T. Kwasny, Liang Zhu, Michael A. Hickner, and Gregory N. Tew*
†Department of Polymer Science and Engineering, University of Massachusetts Amherst, Amherst, Massachusetts 01003, United
States
‡
Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United
States
*
S Supporting Information
ABSTRACT: As the field of anion exchange membranes
AEMs) employs an increasing variety of cations, a critical
(
understanding of cation properties must be obtained,
especially as they relate to membrane ion conductivity. Here,
to elucidate such properties, metal cation-based AEMs,
featuring bis(norbornene) nickel, ruthenium, or cobalt
complexes, were synthesized and characterized. In addition,
isothermal titration calorimetry (ITC) was used to probe
counterion exchange thermodynamics in order to understand
previously reported differences in conductivity. The ion
conductivity data reported here further demonstrated that
nickel-complex cations had higher conductivity as compared to
their ruthenium and cobalt counterparts. Surprisingly, bulk hydration number, ion concentration, ion exchange capacity, and
activation energy were not sufficient to explain differences in conductivity, so the thermodynamics of metal cation−counterion
association were explored using ITC. Specifically, for the nickel cation as compared to the other two metal-based cations, a larger
thermodynamic driving force for chloride counterion release was observed, shown through a smaller ΔH for counterion
tot
exchange, which indicated weaker cation−counterion association. The use of ITC to study cation−counterion association was
further exemplified by characterizing more traditional AEM cations, such as quaternary ammoniums and an imidazolium cation,
which demonstrated small variances in their enthalpic response, but an overall ΔH similar to that of the nickel-based cation.
tot
The cation hydration, rather than its hydration shell or the bulk hydration of the membrane, likely played the key role in
determining the strength of the initial cation−counterion pair. This report identifies for the first time how ITC can be used to
experimentally determine thermodynamic quantities that are key parameters for understanding and predicting conductivity in
AEMs.
INTRODUCTION
cation under alkaline conditions, has garnered more attention,
resulting in the synthesis of a number of different cationic
species, the most common of which are summarized in Figure
■
Producing highly efficient and chemically stable anion exchange
membranes (AEMs), as cost-effective replacements for proton
exchange membranes (PEMs), is a significant challenge due to
18,19
1
.
The main categories of cations demonstrated in AEMs
2
0−25
26−28
include ammonium-,
lium-,
cies.
phosphonium-,
imidazo-
and metal-complex spe-
Some cations, namely the quaternary ammonium,
an incomplete understanding of the fundamental properties and1
2
1,24,29−31
32,33
sulfonium-,
parameters that lead to enhanced performance in AEMs.
3
4−39
Optimizing AEM performance is critical for promoting fuel
cells’ widespread adoption as current fuel cell technologies,
which utilize PEMs, require costly precious metal catalysts and
exhibit poor performance across a range of operating
imidazolium, and phosphonium examples, have also been
functionalized with a variety of different moieties to further
improve their alkaline stability. However, changing the
functional groups around the cation also influences how the
cations interact with their counterions and the surrounding
water, resulting in significant modulation of ion exchange
2
−10
conditions.
Though AEMs currently offer several advan-
tages over their PEM counterparts, including cheaper
fabrication, improved oxygen reduction kinetics, and a wider
range of available fuels, most AEMs still suffer from poor cation
chemical stability and a generally low ion conductivity, although
some recent reports have shown improvements in one or both
22,24,26−28
capacity (IEC), water uptake, and ion conductivity.
Given the effects of the cation functional group on membrane
properties and the massive library of potential cations,
1
,11−17
of these areas.
Although both limitations impact AEM performance, cation
chemical instability, which arises from the degradation of the
Received: April 13, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.8b03979
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Journal of the American Chemical Society
Article
Figure 1. Chemical structures of various cations studied for use in AEM applications, where structures in black are amine-based cations, red are
sulfur-based cations, green are phosphorus-based cations, and blue are metal-complex cations.
Table 1. Summary Comparing Reports of Different Cations Highlighting the Difficulty in Comparing between Reports Due to
Changes in Membrane Properties or Experimental Conditions
Polymer
Cation
Conductivtiy (mS/cm)
IEC (mmol/g)
Condition
Reference
Polyethylene
Ammonium
Phosphonium
Ammonium
Sulfonium
40
1.29
0.67
1.2
20 °C, liquid water
22 °C, liquid water
80 °C, liquid water
80 °C,100% RH
46
26
47
33
22
Polysulfone
55
15.6
1.37
1
,48
quantifying the effects that different cations have on AEM
performance becomes critical, but has proven difficult to
accomplish.
developing highly conductive AEMs.
One such parameter,
the conducting ion concentration, or the number of free ions
within the membrane, is important for ion conduction, but is
3
2,40
1
,49
The difficulty in constructing cation-conductivity relation-
ships in these materials arises due to a lack of consensus in the
field about how to directly measure the cation’s impact on AEM
also difficult to study experimentally.
Therefore, comparing
AEMs with different cations, while maintaining similar AEM
properties and conductivity conditions, becomes critical for
isolating the impact of changing the cation identity on
conductivity. Furthermore, understanding the cation’s inter-
actions with the surrounding aqueous environment becomes
necessary so that parameters such as the percentage of free ions
in an AEM can be determined.
Toward that end, our group has previously developed a metal
cation-based AEM platform that has enabled thorough
structure−property relationship studies linking changes in the
identity of the metal-cation to overall AEM performance,
without also altering other AEM properties as described in the
2
4,41−45
conductivity.
This lack of consensus arises because there
is no standard approach toward synthesizing and characterizing
AEMs, resulting in a variety of polymer backbones, topologies
and cations demonstrating a range of properties. As a result, few
reports directly compare different types of cations while
holding other membrane properties constant. AEM properties,
such as IEC and morphology, and experimental conditions,
such as the bulk hydration of the membrane and temperature,
vary from one report to another, so it remains difficult to
benchmark a material’s overall performance. For example, when
reports of functional polyethylenes containing either quaternary
ammonium- or aminophosphonium-based cations are com-
pared (Table 1), the quaternary ammonium AEM appeared to
have better conductivity than its aminophosphonium AEM
34,36,37
previously published work.
In our materials platform,
ring-opening metathesis polymerization (ROMP)-based, poly-
meric membranes containing bis(norbornene) nickel-, ruthe-
nium-, and cobalt-terpyridine cations were synthesized and
characterized, showing that AEMs with nickel complexes
26,46
counterpart, but also had a larger IEC.
Likewise, when
37
comparing polysulfone membranes functionalized with quater-
nary ammonium- or tertiary sulfonium-based cations, the
ammonium-functionalized material again seemed to outper-
form the alternative, except in this case the quaternary
ammonium AEM was tested in liquid water whereas the
sulfonium AEM was characterized under 100% relative
outperformed their ruthenium and cobalt counterparts.
Here, we expand upon that work in an effort to elucidate why
nickel-complex cations featured the highest chloride ion
conductivity compared to the other cations within the same
materials platform. To accomplish this goal, a series of AEMs
were synthesized containing a poly(ethylene oxide) (PEO)
cross-linker at different metal cation mole fractions, to compare
between nickel-, ruthenium-, and cobalt-complex cations. Ion
conductivity was measured in liquid water, confirming that
nickel-complex cations facilitated higher chloride ion con-
ductivity than ruthenium- or cobalt-complex materials. The
mechanism by which some cations facilitate improved ion
conductivity over others was elucidated through isothermal
titration calorimetry (ITC) that probed the thermodynamics of
3
3,47
humidity (RH) (Table 1).
Because conductivity is
dependent on both the IEC and the bulk hydration of the
membrane, it is difficult to compare AEMs that have differences
in those properties and in the conditions of the conductivity
experiment.
The overall mechanism of conductivity in AEMs is not
completely understood, and identifying parameters that impact
conductivity, especially those related to the cation, is crucial for
B
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Article
a
Scheme 1. Synthetic Scheme for the PEO Series of AEMs
a
The mole fraction, f, of metal cation and DCPD, X and Y respectively, was varied to change the overall metal content without affecting the
crosslinking density. Nomenclature and components are shown.
cation−counterion pair strength for both metal and nitrogen-
based cations. This report, for the first time, details specifically
how changing the cation affected AEM conductivity by
experimentally exploring cation−counterion association ther-
modynamics.
Table 2. Membrane Properties for all AEMs in the PEO
Series
a
b
Sample
Water Uptake (%)
IEC (mmol/g)
Ea (kJ/mol)
0.36RuPEO
0.55RuPEO
0.36NiPEO
0.55NiPEO
0.36CoPEO
0.55CoPEO
169 ± 53
152 ± 25
162 ± 27
193 ± 46
154 ± 12
152 ± 29
0.89
1.12
0.99
1.26
0.99
1.26
15.6
16.2
16.3
16.5
15.1
RESULTS AND DISCUSSION
■
AEM Synthesis and Characterization. To better under-
stand why nickel cations showed improved ion conductivity,
bis(norbornene) metal-terpyridine complexes, containing vary-
ing amounts of either nickel, ruthenium, or cobalt, were
synthesized and copolymerized via ROMP with norbornene,
dicyclopentadiene (DCPD), and a long, hydrophilic cross-
linker poly(ethylene oxide) (PEO), to maintain the same cross-
a
Liquid water uptake values, average of three trials, for AEMs in the
−
Cl form at room temperature, where water uptake = [(m − m )/
m
wet
dry
b
dry]. Theoretical IEC values for the AEMs calculated from the
monomer feed ratio.
34,36,37
link density (Scheme 1).
The dinorbornene function-
alized PEO cross-linker (4 kg/mol, Figure S1), synthesized
following previously reported procedures, was used to improve
the water uptake and mechanical properties of the membranes
50−52
for handling purposes.
at a cationic monomer mole fraction (f) of 0.36 and three with f
0.55 within the network. The IEC for these samples could
Six AEMs were synthesized: three
=
not be determined experimentally due to the presence of the
ester bonds in the PEO that could degrade in the IEC back-
titration procedure. Therefore, the theoretical IEC values,
calculated using the feed ratios of the monomers and ranging
from 0.89 to 1.26 mmol/g, were used for these materials,
although it should be noted that these theoretical values are
known to be an overestimation, as demonstrated from
analogous, non-PEO containing, cross-linked membranes
through back-titration (Scheme S1 and Table S1). Full
nomenclature for these samples is reported in Scheme 1,
where the superscript before the element indicates the metal
cation monomer mole fraction (f) in the network and the
Figure 2. Chloride ion conductivity for the PEO series AEMs in liquid
0.55
water as a function of temperature for each AEM. Sample
CoPEO
was not tested as it was too weak and broke during handling.
“PEO” subscript denotes the presence of the PEO cross-linker.
In order to maintain the same cross-link density between both
sets of membranes, the lower metal content samples had
DCPD at f = 0.19, and the higher metal content samples had no
DCPD, with all six membranes having norbornene and the
PEO cross-linker at f = 0.36 and 0.09, respectively. All samples
were characterized for both water uptake (Table 2) and
mechanical properties (Figure S2 and Table S2).
Ionic Conductivity of Metal Cations. The PEO-
containing samples were characterized for their chloride ion
conductivity in liquid water as a function of temperature.
Conductivity measurements were taken at 20, 30, 40, 60, and
conductivity cell. As expected, all samples demonstrated an
increase in conductivity with temperature. In addition, as the
metal complex content was increased from f = 0.36 to 0.55, the
conductivity increased along with the increase in IEC of the
materials from 0.89−0.99 mmol/g to 1.12−1.26 mmol/g,
respectively (Figure 2 and Table 2). All three membranes with
a lower mole fraction had similar conductivity, contrary to a
previous report that showed that nickel-based AEMs exhibited
better conductivity than cobalt- and ruthenium-based AEMs.
Importantly, that data was collected at 95% RH as opposed to
liquid water, which may be significant. It is possible that the
combination of being tested in liquid water and the higher
water content of these samples promoted increased mobility of
the chloride ions, which diminished the conductivity differences
37
8
0 °C, as shown in Figure 2. The conductivity of sample
CoPEO could not be studied as it was mechanically too weak
0
.55
and fractured when handled as it was inserted into the
C
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5
3
between metal cations. As the metal content increased,
ρ × IEC
1 + WU
0.55
c =
however,
NiPEO showed a larger increase in conductivity
(3)
than 0 RuPEO, consistent with the previously observed trend.
To better understand this conductivity data, the conductivity
values at 80 °C for each sample were plotted as a function of
bulk hydration number (λ), as shown in Figure 3. The
.55
37
Figure 4. Chloride ion conductivity at 80 °C plotted versus ion
concentration for each AEM. Sample 0 CoPEO was not tested as it
was too weak and broke during handling.
.55
Figure 3. Chloride ion conductivity at 80 °C plotted versus bulk
0
.55
where c is ion concentration of the chloride ions in the swollen
membrane, in mmol Cl /mL of membrane and water
hydration number, λ, for each AEM. Sample CoPEO was not tested
−
as it was too weak and broke during handling.
combined, and ρ is the density of the dried sample measured
34,54
0.55
by determining its dried volume and mass.
AEM CoPEO,
again not shown, had an ion concentration of 0.58 mmol/mL.
hydration number, or number of water molecules in the
membrane per counterion, for the samples was calculated using
eqs 1 and 2, and is derived from the bulk hydration of the
membrane:
0
.55
0.55
AEM
NiPEO, similarly to
CoPEO, had the largest ion
concentration at 0.57 mmol/mL, whereas all other samples had
ion concentrations between 0.28 and 0.52 mmol/mL (Figure
0
.55
4
). Though
NiPEO had both the highest ion concentration
λ = 1
000 × WU
and conductivity, ion concentration could not explain all of the
0
.55
M
× IEC
H O
(1)
(2)
conductivity data. For example,
RuPEO had the second
2
lowest ion concentration, 0.32 mmol/mL, but also the second
highest conductivity, 12.3 mS/cm at 80 °C.
Though the relationship between λ and ion concentration
can be further explored through eqs S2 and S3, Figures 3 and 4
ultimately show that these three parameters (λ, ion
concentration, and IEC) were not sufficient to account for
the nickel cation-based AEM’s superior conductivity. Therefore,
mhyd − mdry
WU =
mdry
where WU is the water uptake, MH2 is the molecular mass of
O
water, 18.015 g/mol, IEC is the theoretical IEC of the
membrane, mhyd is the hydrated mass of the AEM, and m is
dry
34,54
0.55
the dry mass of the AEM.
AEM CoPEO, not shown, had
the activation energy of ion conduction, E , which relates to
a
a λ of 67.
kinetics through the Arrhenius equation, was calculated for each
membrane by plotting the natural logarithm of its chloride ion
When the AEM conductivity data was plotted against λ in
Figure 3, it became clear that increasing the metal complex
content, from f = 0.36 to 0.55, decreased the amount of water
molecules per counterion (λ). Because a positive correlation
between increasing AEM cation content and an increased
hydration number is often seen in literature for other AEMs
and PEMs, it was surprising that here the hydration number
conductivity versus 1/T. E was then determined from the slope
a
of the best fit linear regression, using the following form of the
Arrhenius equation, eq 4:
Ea
RT
ln(σCl
−
) = ln(σ ) −
0
(4)
15,54
appeared to decrease with increased IEC for each sample.
where σCl
−
is the ion conductivity, σ is the pre-exponential
0
In this case, the nickel complex showed the smallest decrease in
λ, with a decrease of 7%, whereas ruthenium- and cobalt
complex-based samples showed declines of 29% and 22%,
respectively. All six samples had hydration numbers ranging
from 75 to 105, and while the conductivity and λ of samples
with f = 0.55 appeared to have a positive correlation, that
correlation broke down when comparing AEMs with f = 0.36
factor, R is the gas constant, and T is the absolute temperature
34,55
(Table 2, Figure 5).
All AEMs had similar activation
energies, between 15.1 and 16.5 kJ/mol, indicating neither
metal content nor metal center had an effect on the mobility of
the chloride ions within the AEM for chloride conductivity
56
(Table 2).
Thermodynamic Characterization of Metal-Based
and between different f. For example, for the samples with f =
Cations. Because the E of ion conduction is indicative of
a
0
.36, 0 CoPEO had the highest conductivity, followed by
.36
the kinetics of ion mobility and was similar regardless of the
metal center, the thermodynamics of counterion conduction
0
.36
0.36
NiPEO and then
RuPEO, but the λ values showed the
56
opposite trend.
was characterized for the different cations. Counterion release
for conduction depends on the strength of the cation−
counterion association, with a weaker association increasing the
The conductivity at 80 °C was then plotted as a function of
ion concentration, calculated using eq 3 (Figure 4):
D
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Article
bicarbonate solution from zero to two molar equivalents of
bicarbonate to chloride counterion. As the solutions were
titrated, the heat of counterion exchange was monitored
(
Figures 6A and S4−S7) and the change in enthalpy for each
injection (calculated as the area under the heat absorption
spike) was plotted against the molar ratio of NaHCO to initial
3
chloride counterion as a binding curve (Figure 6B). All
titrations were compared to a control monitoring the heat of
dilution of the NaHCO into pure RO water, where the curves
3
in Figure 6 are corrected for this heat of dilution. The peaks in
Figure 6A became larger after injections 6 and 15 because a
larger volume of NaHCO was injected after those points, but
3
the overall ΔH for the reaction continued to decrease as shown
in Figure 6B. Traditionally, the binding curve obtained would
be fit to determine a binding constant, however, the nickel
complex did not produce a curve that could be fit, as it was too
noisy, most likely due to the low amount of heat
Figure 5. Chloride conductivity for PEO-containing AEMs as a
function of temperature, where the E was calculated from the slope of
a
5
9,60
the linear regression.
exchanged.
Therefore, the total change in enthalpy,
ΔH , for all three titrations was calculated by summing each
tot
data point in the plot (Table 3), as each data point represented
1
,49
free ion concentration and ion conductivity.
Therefore, the
association strength between each metal cation and its
corresponding chloride counterion was examined with a
counterion exchange experiment using isothermal titration
calorimetry (ITC) to determine if the thermodynamics of
counterion exchange could account for the trends in
conductivity. ITC is used for measuring the heat exchange
upon binding between two species, making it the method of
Table 3. ITC Data for Each Metal Complex Showing the
Total Change in Enthalpy for the Overall Exchange Reaction
Sample
ΔHtot (cal/mol HCO3−)
Bis(terpy)Ni
Bis(terpy)Ru
Bis(terpy)Co
586 ± 41
1,003 ± 48
1,120 ± 42
57,58
choice for this study.
However, because ITC required the
analyte to be soluble in an aqueous medium, neither the AEMs
nor the metal complex monomers could be characterized.
Therefore, three model, analogous bis(terpyridine) metal
complexes were synthesized from nonfunctional terpyridine,
bis(terpy)Ni, bis(terpy)Ru, and bis(terpy)Co, as shown in
Scheme S2, to isolate the metal center’s effect on the strength
of the cation−counterion pair. The association strength was
quantified by measuring the enthalpic response of breaking the
initial ion pair between the metal center and its chloride
counterions by adding bicarbonate counterions to form new
ion pairs.
the ΔH for a single injection. Interestingly, the spontaneous
exchange (negative ΔG) for all metal complexes was
endothermic, with positive ΔH values, indicating that a
tot
positive change in entropy, ΔS, drove the reaction (eq 5).
ΔG = ΔH − TΔS
(5)
The change in entropy should be similar for all three
reactions since the same number of principle molecules were
involved in each exchange and the same exchange occurred for
each titration. In addition, all experiments were run at the same
temperature, 25 °C. Therefore, differences in the measured
To probe the thermodynamics of cation−counterion
association, the metal complexes in the chloride form were
dissolved in RO water then titrated with an aqueous sodium
ΔH could be correlated to differences in the overall change in
tot
energy, ΔG, of the reaction, using eq 5. For the exchange,
−
Bis(terpy)Ni had a ΔH of 586 cal/mol HCO , about half as
tot
3
Figure 6. Counterion binding curves for bis(terpyridine)-metal complexes. (A) Raw ITC data for each metal complex stacked on the y-axis for
clarity, average of two trials, which has been corrected for the heat of dilution of the NaHCO solution into water and (B) integration data plotted as
3
a binding curve showing the decrease in the change in enthalpy for each injection as the molar ratio of NaHCO increased. Complexes contained
3
either nickel (green), ruthenium (black), or cobalt (magenta). Samples were run in Milli-Q water at 25 °C exchanging chloride counterions for
bicarbonate.
E
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Figure 7. Nitrogen-based cations used for ITC characterization arranged in order of increasing ΔHtot from left to right.
large as bis(terpy)Ru and bis(terpy)Co, at 1003 and 1120 cal/
surrounding aqueous environment (Figure 7 and Table S3).
The titration of these cations demonstrated that increasing the
alkyl chain length of the cation’s substituents appeared to have
little effect on counterion binding. This was surprising since
increasing the hydrophobicity of the cation was expected to
increase the strength of the ion pair as it becomes less water-
−
mol HCO , respectively.
3
Because the nickel complex had a smaller ΔH , but likely a
tot
similar TΔS term, the ΔG of counterion exchange for the nickel
complex was more negative than for the other metals,
suggesting that there was more of a thermodynamic driving
force for chloride ion release, a parameter that has previously
been difficult to measure for AEMs and PEMs. Additionally, the
soluble, but that was not the case in this instance as ΔH was
tot
40,63
small for all three cations and similar for BEtA and BMeA.
lower ΔH indicated that less energy needed to be absorbed
Interestingly, the ΔH values for the quaternary ammonium
tot
tot
by the nickel system to facilitate counterion exchange, strongly
suggesting that nickel had weaker initial binding to its chloride
cations, either BMeA or TMA, and the imidazolium cation,
EMI, correlate well with their respective AEM conductivity
values from literature. These cations have been characterized
extensively in AEMs, and in almost every case when they were
compared in the same report the quaternary ammonium
outperforms the imidazolium with higher conductivity
6
1
counterion. Weaker initial counterion binding and a stronger
thermodynamic driving force for counterion release should
increase the number of free ions and increase the AEM’s
1
,49
conductivity.
Given that ion release is critical to any
34,41,43,64,65
conductivity mechanism, and that the percentage of free ions is
directly correlated to conductivity, this thermodynamic driving
force for ion release appears to be the prevailing factor for the
values.
Similarly, the quaternary ammonium cations
were also able to release their chloride counterion more easily
than the imidazolium, as demonstrated by lower ΔH values.
tot
1
,62
nickel-based AEM’s superior ion conductivity.
This data further indicates that the differences in the ΔH
tot
Thermodynamic Characterization of Nitrogen-Based
Cations. Characterizing the thermodynamic driving force for
counterion release in metal cations led to a quantitative
understanding of conductivity trends in the corresponding
AEMs. Therefore, the same technique was used to study more
traditional nitrogen-based cations in order to investigate the
general applicability of ITC toward a wider range of cations.
Consequently, five nitrogen-based cations, tetramethylammo-
nium (TMA), benzyltrimethylammonium (BMeA), benzyltrie-
thylammonium (BEtA), benzyltributylammonium (BBuA),
and 1-ethyl-3-methylimidazolium (EMI), were studied using
ITC (Figure 7). All five nitrogen-based cations were
characterized following the same procedure used for the
metal cations. The heat of counterion exchange was monitored
values obtained from the exchange titrations in ITC can be
related to the cation’s ultimate ability to conduct its counterion
in an AEM, both for these more traditional, nitrogen-based
cations as well as for the metal cations.
Interpreting ITC Data in terms of Hydration. The
differences, or lack of differences, among cations in the
thermodynamics of cation−counterion association captured
by ITC were most likely related to the cation hydration on the
molecular level, as opposed to the bulk hydration number
obtained from the swollen membrane. It is important to note
that the ion hydration relates to the number of water molecules
directly interacting with the cation, which is a separate
parameter from the hydration shell, which corresponds to the
number of water molecules surrounding the cation, but not
66
(Figures S8A and S9A), with the change in enthalpy for each
necessarily directly interacting with it. In general, less
hydrated cations have increased association strength with
their counterions, which manifests as an increase in the
magnitude of the enthalpic response during counterion
injection plotted against the molar ratio in Figures S8B and
S9B. Interestingly, the titration of the nitrogen-based cations
produced curves more similar to the bis(terpy)Ni titration
curve, suggesting that the nickel cation had counterion
exchange behavior more similar to the nitrogen-based cations
67
exchange as measured by ITC. Furthermore, larger cations
tend to have less hydration than smaller ions, as water
molecules surrounding the cation interact more with each other
and less with the cation, due to lower electrostatic forces and
than to the other metal cations. The ΔH was again calculated,
tot
and the results in Table S3 show that the nitrogen-based
cations were also overall endothermic reactions, yet still
spontaneous, meaning that entropy dominated their counterion
exchange as well. All nitrogen-based cations demonstrated low
34,66,68,69
more delocalized charge associated with the larger ion.
For example, a previous report studied the binding of various
70
metal chloride salts to polyoxometalates using ITC. As the
+
+
+
+
ΔH values, where BEtA and BMeA had similar ΔHtot of 207
metal was changed between Na , K , Rb , and Cs , the heat
produced by binding increased as the size of the cation
increased. Given how increased ion size also results in reduced
hydration, the increased enthalpic response was related to the
decreased hydration of the ions creating stronger ion pairs. The
relationship between ion hydration, ion pair strength, and the
magnitude of the enthalpic response measured by ITC,
regardless of whether it was exothermic or endothermic, can
tot
and 212, respectively, which were lower than BBuA’s ΔH of
tot
−
−
2
87 cal/mol HCO , TMA’s ΔH of 350 cal/mol HCO , and
3
tot
3
−
EMI’s ΔH of 481 cal/mol HCO (Figure 7).
tot
3
Counterion exchange experiments with BMeA, BEtA, and
BBuA probed the effect of cation hydrophobicity and steric
crowding on counterion association, as those parameters could
affect how the cation interacts with the counterion and the
F
DOI: 10.1021/jacs.8b03979
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Figure 8. Illustration relating the ion hydration to the ion pair association strength and the enthalpic response to breaking and forming ion pairs as
measured by ITC, where green indicates the cation and red indicates the anion.
be understood through Figure 8, which most likely hydration
phenomenon most likely accounts for the changes in ITC
measured here. As the hydration of the cation decreases, the
strength of the ion pair association between the cation and its
initial counterion increases, making it more difficult to perform
the counterion exchange, as expressed by a larger, more
behaved more similarly to the nitrogen-based cations than to
the other metal cations as its hydration level was more similar
to the nitrogen-based cations.
CONCLUSIONS
■
Commonly studied parameters such as IEC, λ, ion concen-
endothermic ΔH .
tot
tration, and E were not sufficient to fundamentally explain the
a
Because BMeA, BEtA, and BBuA all had low ΔHtot and it is
known that tetramethylammonium and tetrabutylammonium
analogues have similar levels of cation hydration, these three
cations further establish the connection between enthalpic
response measured by ITC and cation hydration. Despite the
increased hydrophobicity among the cations, a similar enthalpic
response indicated that cation hydration had a greater impact
on the enthalpic response than the cation’s hydrophobicity.
Furthermore, when comparing TMA, BMeA, and EMI, BMeA
observed conductivity data, so the three metal cation complexes
were evaluated for their counterion association thermody-
namics. Using ITC, the nickel cation complex’s release of their
chloride counterions was found to be more energetically
favorable than in the case of the ruthenium and cobalt cation
complexes, due to cation hydration. Nitrogen-based cations also
71
had ΔH values more similar to that of the bis(terpy)Ni
tot
cation, indicating that the nickel cation behaved more similarly
to the nitrogen-based cations than the other metal cations.
Taken as a whole, ITC revealed that the thermodynamic
driving force for counterion release appears to be a universal
parameter for understanding cation behavior in ion exchange
membranes. Moreover, these studies further implied that the
cation hydration, as opposed to the standardly measured bulk
membrane hydration, has a strong influence on cation−
counterion association thermodynamics, which accounts for
had a slightly lower ΔH than TMA, which was slightly lower
tot
than EMI. These slight differences could also be explained by
small differences in cation hydration. The stabilizing effect of
the benzyl group on the cation for BMeA would allow the
counterion to dissociate more, increasing the hydration of the
cation as compared to TMA. Furthermore, TMA should have
greater hydration than EMI due to the increased size and
delocalization of the charge on the imidazolium ring. As
illustrated in Figure 8, this decreased hydration from BMeA to
TMA to EMI would result in an increase of the initial ion pair
strength with the chloride counterion, causing the larger
the significant differences in the measured ΔH for counterion
tot
exchange. In the future, more comprehensive studies about the
thermodynamics of cation−counterion associations, including
ΔG and ΔS, will likely produce more insight into the
underlying parameters critical for ion conducting membranes.
enthalpic penalty (larger endothermic ΔH ) measured by ITC
tot
for EMI over TMA, and TMA over BMeA (Figure 7).
Interestingly, cation hydration could also explain the
EXPERIMENTAL SECTION
■
differences in ΔH observed for the metal cations. Metal-
tot
Monomer Synthesis and Membrane Preparation. The
homoleptic bis(terpyridine) metal complexes and the norbornene
functionalized telechelic PEO were synthesized according to previous
complex cations are influenced by their metal-ligand field
bonding and spin state, either low spin or high spin, where the
spin state relates to the overall energy profile of the metal
3
4,36,37,50,51
reports.
In order to keep the cross-link density consistent,
7
2
the cationic metal complexes were polymerized in the presence of
norbornene, dicyclopentadiene (DCPD), and the hydrophilic PEO.
The metal-containing monomer, dicyclopentadiene, norbornene, and
telechelic PEO were dissolved in a methanol/chloroform mixture. A
solution of Grubb’s second generation catalyst (G2) in chloroform was
added and the solution was stirred vigorously for up to 1 min. The
solution was transferred to a preheated (40 °C) aluminum pan
(diameter of ∼7 cm and depth of ∼1.5 cm) on a hot plate set to 40 °C.
The pan was then covered with a glass jar (diameter of ∼7.5 cm and
depth of ∼9 cm) to slow evaporation of the solvent. After 1 h at 40 °C,
the cover was removed and the temperature remained at 40 °C for
another hour, after which the temperature was raised to 70 °C for one
more hour. The membrane was then cooled and transferred to a glass
jar where it was swollen in 100% methanol, followed by 70% aqueous
methanol, and then 30% aqueous methanol for at least 6 h for each
solution. Finally, the membrane was swelled in 100% RO water for at
least 12 h and the resulting membrane was stored in fresh RO water
until characterization unless otherwise stated. For further information
complex. Comparing the high- and low-spin states of an
octahedral complex, like the metal complexes used here, shows
that the high-spin state results in an increased charge
73
localization within the center of the cationic complex.
Specifically, for the complexes studied here, the diamagnetic
bis(terpy)Ru is in the low-spin state, whereas the paramagnetic
bis(terpy)Ni complex is in the high-spin state, meaning that its
37
charge is increasingly localized in the center of the complex.
Furthermore, while octahedral Co(II) complexes can either be
in the high- or low-spin state, it has been shown that
bis(terpy)Co complexes are primarily low-spin with chloride
7
4
counterions.
When in an aqueous medium, the increased charge
localization for bis(terpy)Ni could cause more water molecules
to directly interact with the complex, increasing the hydration
of the metal complex cation. This would also explain why it
G
DOI: 10.1021/jacs.8b03979
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
regarding monomer and membrane synthesis, refer to the Supporting
Information.
ACKNOWLEDGMENTS
■
This work was funded by the NSF NRT program DGE-
545399 and the GAANN Fellowship DoED P200A150276.
Nonfunctional terpyridine-metal complexes were synthesized by
dissolving 2,2′:6′,2″-terpyridine and the desired metal chloride salt in
methanol. The solution was stirred overnight after which the solvent
was removed, and the resulting powder was dried under vacuum
producing quantitative yields, unless otherwise stated. For further
information on metal complex synthesis, refer to the Supporting
Information.
Characterization. Liquid water uptake was measured for all AEMs
in the chloride form at room temperature. The fully hydrated AEM
was removed from liquid water and the surface was blotted to remove
excess surface water not absorbed into the membrane. The mass of the
AEM was then recorded immediately and the membrane was placed
back into water for 5−10 min. This process of weighing the hydrated
membrane was repeated 3−5 times until consistent masses were
obtained. They were then dried for 24 h in a vacuum at 50 °C, and the
dried AEM was weighed for its dehydrated mass.
1
ITC data was obtained at the biophysical characterization core
facility within the Institute for Applied Life Sciences (IALS) at
The University of Massachusetts Amherst. The authors thank
Ms. Kelly McLeod for providing the norbornene-functionalized
PEO. The authors also thank Mr. Nicholas Posey for his
assistance in preparing the manuscript. M.A.H. acknowledges
the Corning Foundation for fellowship support and the NSF
DMREF program under award CHE-1534326.
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̃
3
(
(
NaHCO (∼10 mM) was titrated into the aqueous cation solution (all
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3
(
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(
(
1
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̃
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(
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(
AC) impedance spectroscopy three times during the hour. This was
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(
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(
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ORCID
Liang Zhu: 0000-0003-4551-0963
Gregory N. Tew: 0000-0003-3277-7925
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Notes
The authors declare no competing financial interest.
H
DOI: 10.1021/jacs.8b03979
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
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DOI: 10.1021/jacs.8b03979
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX