Highly Stable, Low Gas Crossover, Proton-Conducting Phenylated Polyphenylenes

Article abstract of DOI:10.1002/anie.201703916

Two classes of novel sulfonated phenylated polyphenylene ionomers are investigated as polyaromatic-based proton exchange membranes. Both types of ionomer possess high ion exchange capacities yet are insoluble in water at elevated temperatures. They exhibit high proton conductivity under both fully hydrated conditions and reduced relative humidity, and are markedly resilient to free radical attack. Fuel cells constructed with membrane-electrode assemblies containing each ionomer membrane yield high in situ proton conductivity and peak power densities that are greater than obtained using Nafion reference membranes. In situ chemical stability accelerated stress tests reveal that this class of the polyaromatic membranes allow significantly lower gas crossover and lower rates of degradation than Nafion benchmark systems. These results point to a promising future for molecularly designed sulfonated phenylated polyphenylenes as proton-conducting media in electrochemical technologies.

Full text of DOI:10.1002/anie.201703916

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Highly Stable, Low Gas Cross-Over, Proton-Conducting
Phenylated Polyphenylenes
Authors: Michael Adamski, Thomas J. G. Skalski, Benjamin Britton,
Timothy J. Peckham, Lukas Metzler, and Steven Holdcroft
This manuscript has been accepted after peer review and appears as an
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using the Digital Object Identifier (DOI) given below. The VoR will be
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201703916
Angew. Chem. 10.1002/ange.201703916
Link to VoR: http://dx.doi.org/10.1002/anie.201703916
http://dx.doi.org/10.1002/ange.201703916

10.1002/anie.201703916
Angewandte Chemie International Edition
COMMUNICATION
Highly Stable, Low Gas Cross-Over, Proton-Conducting
Phenylated Polyphenylenes
Michael Adamski, Thomas J. G. Skalski, Benjamin Britton, Timothy J. Peckham, Lukas Metzler, and
Steven Holdcroft*
Abstract: Two classes of novel sulfonated phenylated
linkages between phenyl rings along the polymer
backbone.[18][23][24]
polyphenylene ionomers are investigated as polyaromatic-based
proton exchange membranes. Both types of ionomer possess
high ion exchange capacities yet are insoluble in water at
elevated temperatures. They exhibit high proton conductivity
under both fully hydrated conditions and reduced relative
humidity, and are markedly resilient to free radical attack. Fuel
cells constructed with membrane-electrode-assemblies
containing each ionomer membrane yield high in situ proton
conductivity and peak power densities that are greater than
obtained using Nafion reference membranes. In situ chemical
stability accelerated stress tests reveal that this class of the
polyaromatic membranes allow significantly lower gas crossover
and lower rates of degradation than Nafion benchmark systems.
These results point to a promising future for molecularly-
designed sulfonated phenylated polyphenylenes as proton-
conducting media in electrochemical technologies.
Recently, we reported the synthesis of a well-defined,
branched, sulfonated polyphenylene homopolymer (sPPP-H+)
using pre-sulfonated monomers.[24] Membranes cast from this
polymer exhibited high proton conductivity and ex situ stability to
oxidative degradation (as determined by 1H NMR). When
employed as a membrane and/or ionomer in the catalyst layer of
a fuel cell, sPPP-H+ supported a power density comparable or
exceeding that of Nafion®, the archetypal PFSA ionomer.
However, while sPPP-H+ membranes remain intact in H2O at
RT, they swelled excessively at higher temperatures, thus
limiting research to in situ durability.
In this paper, we explore the syntheses of novel sulfonated
phenylated polyphenylenes using Diels-Alder (D-A)
polymerization reactions with emphasis on molecular design to
Hydrocarbon-based proton exchange membranes (PEMs)
and ionomers, intended for electrochemical applications (fuel
cells, electrolyzers, and water treatment)[1][2] are actively sought
after as alternatives to traditional perfluorosulfonic acid (PFSA)
ionomers[2][3][4] due to their ease of synthesis, low cost, low gas
crossover, high Tg, and fewer environmental concerns.[5][6] Many
different ion-containing polymers have been investigated with
significant focus on those incorporating aromatic groups as part
of the polymer main chain, such as sulfonated derivatives of
poly(arylene ether)s,[7] poly(arylene ether ketone)s,[8][9][10]
poly(arylene sulfone)s,[9] poly(imide)s,[11]
enhance the positive attributes of sPPP-H+. This is
accomplished by incorporation of spacer units, biphenyl and
naphthyl, in the polymer backbone. Optimization of conditions
for synthesis of the polymers is aided by synthetic studies of
oligophenylene model compounds which bear structural
similarities to the analogous polymers, but are simpler to
characterize.[18][23][25] Biphenyl and naphthyl-linked small
molecules SM-B and SM-N were obtained through [4 + 2] D-A
cycloaddition between 3c and linkers 2b or 2c, respectively
(Scheme 1a). Reaction conditions identical to the intended
polymerization conditions were employed in order to confirm the
stability of the desired spacer units at the temperatures
poly(benzimidazole)s,[2][12] and poly(para-phenylene)s.[8][13][14]
However, it is the general consensus that hydrocarbon-based
ionomers to date are inhibited by a greater sensitivity to
oxidative degradation either ex situ (e.g., Fenton’s Reagent test)
and/or in situ (e.g., in PEM fuel cells).[2][8] Recent attention has
therefore focused on the rational design of hydrocarbon
ionomers with enhanced chemical stability.[6][15][16]
necessary to facilitate the D-A reaction.[17][24]
~
-
-
(a)
SO3
SO3
-
SO3
O
i
-O3S
+
R
2
R
2
-O3S
-O3S
SM-X
+
Sulfonated phenylated polyphenylenes (sPPPs) have
been of particular interest as PEMs due to the inherent chemical
and mechanical stability of a fully aromatic backbone.[17][18] Work
in this area, however, had been limited by the challenge of
synthesizing well-defined polymer backbones composed of
sterically-encumbered, rigid, aryl-aryl linkages,[18][19] their limited
solubility in polar solvents,[8][18] and ill-defined molecular
structures as a result of the post-sulfonation technique
+ 4 HNEt3
+
+ 2 HNEt3
3c
R
=
2c
2a
2b
X =
P
B
N
-
commonly employed. These challenges lead to a random
distribution of ionic groups on the multitude of available phenyl
rings,[20][21][22] as well as the uncertainty of the ratio of meta:para
SO3
(b)
-O3S
O
SO3H
SO3H
iii
R
i
ii
+
R
2
n
-
SO3
[*]
M. Adamski, T. J. G. Skalski, B. Britton, Dr. T. J. Peckham, L.
Metzler, Prof. S. Holdcroft
Department of Chemistry
HO3S
HO3S
sPPX-H+
O
-O3S
+
+ 4 HNEt3
1c
Simon Fraser University
8888 University Drive, Burnaby BC, V5A 1S6 (Canada)
E-mail: holdcrof@sfu.ca
(i) PhNO2, oil bath (195° C, 24 h); (ii) 1 M NaOH; (iii) 1 M H2SO4
Supporting information for this article is given via a link at the end of
the document.
Scheme 1. Sulfonated branched oligophenylenes and polyphenylenes.
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10.1002/anie.201703916
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COMMUNICATION
Use of pre-sulfonated monomers allows for the synthesis
of polymers containing four sulfonic acid groups per repeating
unit, with precise control over their positioning.[24] Syntheses
were accomplished through [4 + 2] D-A cycloaddition between
monomer 1c and linkers 2b or 2c to yield sPPB-HNEt3+ and
1 were observed at 95% RH for sPPN-H+ at 30 °C and 80 °C,
respectively. These values are significantly higher than
previously reported sulfonated polyphenylenes[20][24][30][31][32] and
the 79 mS cm-1 (30 °C) and 113 mS cm-1 (80 °C) values
obtained for Nafion NR-211 under identical conditions. sPPB-H+
exhibits proton conductivities of 129 mS cm-1 and 172 mS cm-1
at 30 °C and 80 °C respectively, likewise larger than previously
reported sPPP-H+ and NR-211. Conductivities decline, as
expected, under lower RH,[26][33] due to decreasing membrane
water content.[5] The high proton conductivities of sPPN-H+ is
likely due to its markedly high water uptake which may allow for
a greater connectivity of aqueous domains throughout the
material.[34][35] Comparison of the acid concentrations ([SO3H]):
1.17, 1.43, and 1.55 mmolSO3H/cm3membrane for sPPN-H+, sPPB-
H+, and NR-211;[26] and their proton mobility values (µH+): 2.0,
0.9, and 0.5 × 10-3 cm2 V−1 s−1 at 30 °C, further supports this
assertion (Table S39).[26][36] That is, although the membranes
possess lower acid concentrations than NR-211, their proton
mobilities are much higher (especially for the case of sPPN-H+).
+
sPPN-HNEt3 , respectively (Scheme 1b). A detailed synthesis of
each compound is outlined in the Supporting Information (SI).
Gel permeation chromatography (GPC) analyses indicated a Mw
+
of 175,000 Da (Mw/Mn = 1.56) for sPPB-HNEt3 , and 329,000 Da
+
(Mw/Mn = 2.33) for sPPN-HNEt3 . Successful polymerizations
were confirmed by 1H NMR spectroscopic analysis, using the
triethylammonium cations as internal probes. The expected
integration ratios between the methyl protons (36 H), methylene
protons (24 H), and the polymer aromatic backbone protons
(sPPB-HNEt3+ 40 H; sPPN-HNEt3+ 38 H) were observed.
Polymer acidic forms sPPB-H+ and sPPN-H+ were cast
into membranes from DMSO solutions (5% w/w) and dried at
85 °C overnight. Water uptake and swelling ratios are
summarized in Table S37. Unlike sPPP-H+, both polymers were
insoluble in DI H2O at 80 °C. sPPB-H+ displayed considerably
lower water uptake and swelling values than sPPN-H+, but
higher than Nafion NR-211.[26]
Figure 1. (a) Proton conductivity of polymer membranes at 30 and 80 °C and;
(b) the respective log plot.
Fenton’s reagent is commonly employed as a preliminary
ex-situ accelerated degradation test for studying PEM oxidative
stability due to its ability to generate oxygen-containing free
radicals in solution.[27][28] After exposure to Fenton’s reagent (1 h,
80 ºC), membranes displayed no observable mass loss (0.69 ±
0.71% and 0.09 ± 0.62% for sPPB-H+ and sPPN-H+
respectively), and no changes in chemical structure (1H NMR),
indicating a markedly high chemical resilience to free radical
attack. In contrast, phenylated, sulfonated polyarylene ethers
displayed mass losses of up to 20% and eventual dissolution
under these conditions.[29]
Mechanical strength measurements show that sPPB-H+
has superior tensile strength and Young’s modulus to NR-211
(59.6 ± 1.4 MPa and 1331 ± 29 MPa vs. 17.3 ± 0.4 MPa and 270
± 17 MPa, respectively), but lower elongation at break (17.5 ±
1.3% vs. 148.3 ± 3.6%, respectively) when measured in dry
state. Similar performances were noted for both sPPN-H+ and
sPPP-H+ (Table and Figure S48). Fully hydrated membranes
showed decreases in tensile strength and Young’s modulus,
with minor increases in elongation at break (except in the case
of sPPN-H+). In both conditions, membranes were notably
robust, flexible, and not brittle, and data compare well to
previously published mechanical strength measurements of
post-sulfonated phenylated polyphenylenes.[20]
Titration experiments show that sPPB-H+ and sPPN-H+
possess IECs of 3.19 ± 0.05 meq. g-1 and 3.28 ± 0.06 meq. g-1,
respectively, compared to theoretical values of 3.46 meq. g-1 and
3.54 meq. g-1, respectively. These IECs are slightly lower than
observed for sPPP-H+ membranes (3.47 meq. g-1 experimental,
3.70 meq. g-1 theoretical) due to the increase in equivalent
weight caused by incorporation of the biphenyl and naphthyl
moeities.
Proton conductivity measurements were performed using
electrochemical impedance spectroscopy (EIS) under relative
humidities (RH) ranging from 30% to 95%, at both 30 °C and
80 °C (Fig. 1). Maximum values of 222 mS cm-1 and 268 mS cm-
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10.1002/anie.201703916
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COMMUNICATION
Onto 33 ± 2 µm sPPB-H+ and 80 ± 4 µm sPPN-H+
membranes, catalyst layers containing PFSA ionomer and 0.4
mg Pt·cm-2 were deposited by ultrasonic spray coating using a
Sono-Tek ExactaCoat SC.[37] No delamination was observed
before or after fuel cell operation. These were mounted as fuel
cells and conditioned in situ, displaying highly repetitive
operation for 25 hours of RH cycling (Fig. S41). At 80 °C with
zero backpressure, sPPB-H+ and sPPN-H+ membrane-
electrode-assemblies (MEA) displayed peak power densities of
1237 and 927 mW·cm-2, which are 56% and 17% greater than
that obtained using N212 reference MEAs (Fig. 2). Using H2/air,
these MEAs displayed peak power densities of 587 and 445
mW·cm-2, which are 29% larger and similar, respectively, to
N212 (Fig. S43). Using H2/O2 and H2/air, favourable
lifetime compared to N211. Nafion 211 cells completely failed at
153 h, whereas the sPPB-H+ cell provided polarization curves
(Fig. S46) after 400 h, exhibiting a final OCV of 0.71 V and only
a 31% decrease in in situ conductivity, which is still 21% greater
than a N211 fully-conditioned cell at beginning-of-life (Fig. S47).
Figure 3. Accelerated combined chemical/mechanical stress test via an open
circuit voltage hold at 30% RH, 90 °C, H2/Air, zero backpressure. The spikes
represent times where the OCV stress test was interrupted to obtain
polarization curves and gas cross over current densities.
comparisons to N211 were found accounting for differences in
membrane thicknesses and gas diffusion layer (GDL)
optimization (Figs. S43 & S44). In both cases, the in situ
membrane resistances (insets), measured during operation by
the iR-drop method and verified with high-frequency resistance
measurements, were significantly lower than the N212 MEA
reference, which is atypical for hydrocarbon membranes.[38] In
situ conductivities, accounting for differences in membrane
thicknesses, were 170 ± 21 and 261 ± 22 mS·cm-1, for sPPB-H+-
or sPPN-H+-based MEAs, which are 111 and 223% larger than
N212 at 80 °C, respectively (Fig. S42b).
Figure 2. In situ polarization (left axis, solid), power density (right axis,
open), under H2/O2. Conditions were 80 °C, 100% RH, 0.5/1.0 slpm
anode/cathode gas flows, zero backpressure.
1400
N212, 50 µm
sPPB, 33 µm
sPPN, 80 µm
1.0
0.8
0.6
0.4
0.2
0.0
1200
1000
800
600
400
200
0
In summary, the syntheses of two new sulfonated
oligophenylenes SM-N and SM-B were demonstrated, leading to
the synthesis of their respective sulfonated polyphenylenes
+
sPPB-HNEt3+ and sPPN-HNEt3 . The pre-sulfonation technique
affords full retention of sulfonic moieties following D-A
polymerization, and polymers obtained possess high molecular
weights. Exchange to active acidic forms afforded sPPB-H+ and
sPPN-H+, which were cast into membranes for further
characterization, and displayed excellent tensile strength,
Young‘s moduli, and modest elongation at break. EIS analysis
revealed exceptional proton conductivities, even under reduced
RH. Both polymers displayed remarkable fuel cell performance
under non-optimized conditions, with sPPB-H+ maintaining a
high conductivity even after 400 h of accelerated stress testing.
0
500
1000
1500
2000
2500
3000
Current Density, j (mA·cm-2)
Acknowledgements
An in situ chemical stability accelerated stress test (AST)
consisting of a high-temperature, low-RH potential hold at open-
circuit voltage (OCV) was performed, comparing sPPB-H+ with a
N211 reference (Fig. 3). Using H2/air, initial OCVs of sPPB-
H+/N211 were 0.965/0.942 V. Losses at 1, 10, 50, and 100 h
were 2/66, 29/181, 55/231, and 111/271 mV, respectively (Table
S45). In addition, the H2 gas crossover for sPPB-H+ was
substantially lower than N211, e.g. 0.5 vs 3.8 mA/cm2 at 42 h
(Fig. S45). As shown in Fig. 3, the N211 cell showed signs of
failure after 100 h, with H2 crossover currents approaching 100
mA/cm2, while the sPPB-H+ cell exhibited 12 mA/cm2 crossover
current after 100 h accelerated degradation. An OCV of 0.71
was maintained for the sPPB-H+ cell after 400 h, whereas the
N211 cell fell below 0.7 V after 100 h accelerated degradation,
suggesting that the sPPB-H+ membrane cell exhibited a 4x
This work was supported by NSERC and made use of facilities
in 4D LABS, SFU.
Keywords: PEM • Poly(phenylene) • Fuel cell • Proton
Conducting • Polymer
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