Enhanced proton conductivity of Nafion-azolebisphosphonate membranes for PEM fuel cells

Article abstract of DOI:10.1039/c9nj03405f

Fuel cells are among the cleaner alternatives of sustainable energy technologies, where their proton exchange membranes continue to be a key component with many challenges and opportunities ahead. In this study, different indazole-and benzotriazolebisphosphonic acids were prepared and incorporated into new Nafion-doped membranes up to a 5 wt% loading. The new membranes were characterised, and their proton conductivities were evaluated using electrochemical impedance spectroscopy. Membranes with a 1 wt% loading showed better proton conductivities than Nafion N-115 at all temperature and under relative humidity conditions studied. In these conditions, the best value was observed for the membrane doped with [hydroxy(1H-indazol-3-yl)methanediyl]bis(phosphonic acid) (BP2), with a proton conductivity of 98 mS cm^-1. Activation energy (E_a) values suggests that both Grotthuss and vehicular mechanisms are involved in the proton conduction across the membrane.

Full text of DOI:10.1039/c9nj03405f

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Enhanced proton conductivity of Nafion-azolebisphosphonates
membranes for PEM fuel cells
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
a
Fátima C. Teixeira, Ana I. de Sá, António P. S. Teixeira and C. M. Rangel*
DOI: 10.1039/x0xx00000x
Fuel cells are among the cleaner alternatives of sustainable energy technologies, where their proton exchange membranes
continue a key component with many challenges and opportunities ahead. In this work, different indazole- and
benzotriazolebisphosphonic acids were prepared and incorporated into new Nafion doped membranes up to 5 wt%
loading. The new membranes were characterised, and their proton conductivities evaluated using Electrochemical
Impedance Spectroscopy. Membranes with 1 wt% loading showed better proton conductivities than Nafion N-115, at all
temperature and relative humidity conditions studied. In these conditions, the best value was observed for the membrane
-
1
doped with hydroxy(1H-indazol-3-yl)methanediyl]bis(phosphonic acid) (BP2), with a proton conductivity of 98 mS cm .
Activation energy (E ) values suggest that both Grotthuss and vehicular mechanisms are involved in the proton conduction
across the membrane.
a
condensation, which, consequently, limit the operation of the
membrane to temperatures up to the boiling point of water.
At high temperature, such drawbacks are surpassed, and
PEMFCs can increase their performance, and present higher
energy efficiency, but they can also show lower thermal
1
. Introduction
The world’s demands for energy are still growing in a context
of a reduction of fossil fuel reserves and an increase of
environmental problems. Cleaner, sustainable, renewable and
environmentally-friendly sources for energy systems are
crucial challenges of the 21st century. Most countries have
already established an increase of their energy production
from renewable sources. Therefore, zero emission energy
systems have attracted more attention and fostered increased
efforts and investments. But most renewable energy sources
are low density sources and remain unstable and intermittent
stability and dehydration of the membrane with loss of proton
13-19
conductivity.
The membrane is regarded as a key material
affected by its chemical and structural stability, its
permeability to fuel and oxidant, as well their humidity and
temperature conditions, and consequently their proton
conduction. Commercial membranes for PEMFC are being
made of organic polymers with acidic functionalities (including
sulfonic, carboxylic and phosphonic acid groups), but their
proton transport conduction still depends on the presence of
the conducting water or other electrolyte content, which
1
-4
production sources.
Among the cleaner energy technologies, fuel cells became an
alternative to the use of fossil fuels sources of energy. Fuel
cells are electrochemical devices that convert the chemical
energy stored in a fuel directly and efficiently to electrical
energy, with high efficiency and low environmental impact.
Proton exchange membrane fuel cells (PEMFC) are promising
conversion devices amongst the fuel cells, due to their
properties, including their high power-to-weight ratio and high
5-18
usually limit their operation to temperatures below 90 °C.
The most studied and used PEM are Nafion membranes, which
have excellent chemical stability and high proton conductivity.
This membrane is a hydrophobic perfluorosulfonated polymer,
with sulfonic acid groups, capable of donating protons, whose
transport is carried out through water molecules content,
20-26
5
-18
which limits its operation to 80 °C.
Most of the early
power density.
studies were carried out on membranes with sulfonic acid
groups as proton carriers, but some studies showed that
phosphonic acid can act as better proton carrier since their
A drawback of the known PEM technology is the strong
dependence on the water content of the membrane due to
their operation under water-assisted proton conduction. This
causes several concerns regarding water and heat
management, with slow electrode reaction kinetic and water
-1
proton transfer has a lower energy penalty (37.2 kJ mol ) than
-1
sulfonic acid groups (69.9 kJ mol ). Also, the phosphonic acid
group can have both proton donor and acceptor properties,
via its self-ionization, enhancing their proton transport
through the membrane. Phosphonic acids have better
oxidation resistance and thermal stability than sulfonic acid
groups, allowing their operations at high temperature. This
a.
Laboratório Nacional de Energia e Geologia, I.P., Estrada do Paço do Lumiar, 22,
649-038 Lisboa, Portugal. E-mail: carmen.rangel@lneg.pt
Departamento de Química, ECT & Centro de Química de Évora, IIFA, Universidade
de Évora, R. Romão Ramalho, 59, 7000-671 Évora, Portugal
1
b.
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endorses the use of phosphonic acid group as proton membranes were recorded on an Alpha-R spectrVoiemw Aerttieclre Ownilitnhe
27-35
conductor on PEMs.
DOI: 10.1039/C9NJ03405F
an Attenuated Total Reflection (ATR) module, from Bruker
-
1
There are other compounds combining acceptor and donor ion Optics. IR spectra were plotted in the range 4000 and 400 cm
-1
carrier capabilities, including some azaheterocycles. Their and the bands wavelengths are quoted in cm .
proton-conducting capability can be attributed to their High-resolution (HRMS) mass spectra analyses were obtained
nitrogen atom amphoteric behaviour. The properties of many by electrospray ionization (ESI) and were performed at the
heterocycles enable them to be used as electrolytes in University of Vigo (Spain), at Unidad de Espectrometria de
3
6-47
materials working at high temperatures.
Masas - C.A.C.T.I., on a MicrOTOF (Bruker Daltonics)
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The development of new materials for fuel cell applications instrument.
and the limitations of commercial membranes prompt us to
synthesize new compounds to work as electrolytes to obtain
Scanning Electron Microscopy (SEM)
membranes with high proton conduction. Our previous studies
have demonstrated that incorporation of arylbisphosphonic Scanning Electron Microscopy (SEM) images of the new
acids into Nafion, results in an enhancement of the proton membranes were obtained on a Philips XL30 FEG (Field
48
conductivity of the doped Nafion membranes.
Emission Gun), with an acceleration voltage of 5–10 kV.
Taking into account the properties of azaheterocycles and Samples were previously coated with a layer of gold to
phosphonic acids, we devise the preparation of new Nafion enhance electrical conduction.
membranes doped with azolebisphosphonates derivatives,
Proton conductivity
which could act as a source of protons and concurrently as
proton acceptor, facilitating the intermolecular transport of Evaluation of the in-plane proton conductivity () of the new
protons through the membrane. To this aim, membranes was performed by EIS, using a Solartron 1250
azolebisphosphonates were prepared, characterized and frequency response analyzer, coupled to a Solartron 1260 FRA
incorporated into Nafion membranes, using a casting method electrochemical interface, and a commercial BT-112 BekkTech
previously implemented in our group. The new BPs doped conductivity cell (Scribner Associates Inc.). The measurements
membranes were characterized and their proton were done over a frequency range from 65 kHz to 5 Hz, with a
conductivities evaluated by electrochemical impedance test signal amplitude of 10 mV. The bulk resistance (R ) of the
b
spectroscopy (EIS).
membranes were accessed through the analysis with the
ZView software (Version2.6b, Scribner Associates) of the
results. The measurements were performed in a Binder KBF
2
. Experimental
1
15 climatic chamber under variable conditions of
temperature (30, 40, 50 and 60 C) and relative humidity (RH)
40, 60 and 80%). The measurements were achieved directly

2.1. Chemicals and Materials
(
All air-sensitive reagents were used under a dry nitrogen
atmosphere. Dry THF was obtained from its distillation from
sodium benzophenone ketyl. Purification by column
chromatography was carried out on silica gel (230–400 mesh)
and a positive pressure of nitrogen was used.
from the temperature-controlled humidity chamber, after 2 h
equilibration period.
The proton conductivity (
1)

) was calculated using the equation
(
(1)
All reagents and deuterated solvents were purchased from
Aldrich. Nafion-115 film was acquired from FuelCell Store and
®
Nafion perfluorinated resin solution, a 20 wt% mixture in
where L - distance between the two electrodes (cm), R – bulk
b
2
resistance (Ω), and A - cross-sectional area (cm ).
lower aliphatic alcohols and water (34%), was purchased from
2
.3. Synthesis of dopants
Aldrich. 1H-Benzotriazole
Aldrich). All the above chemicals and materials were used as
received, with no further purification.
(1) is commercially available
Hydroxy(1-methyl-indazol-3-yl)methanediyl]bis(phosphonic
(
acid) (BP1) and hydroxy(1H-indazol-3-yl) methanediyl]bis
(
phosphonic acid) (BP2) were prepared and characterized
4
9,50
according to Teixeira et al.
2
.2. Characterization
2
.3.1. Synthesis of ethyl 2-(1
H
-benzotriazol-1-yl)acetate 2 and
ethyl 2-(2
H-benzotriazol-2-yl)acetate 3
Spectroscopic Characterization
To a solution of 1H-benzotriazole
t
1 (2.00 g, 16.79 mmol) and
Synthesized compounds were characterized by NMR
1
spectroscopy using a Bruker Avance III HD 400 ( H 400 MHz,
KO Bu (2.26 g, 18.47 mmol) in EtOH (50 mL) was added
BrCH CO Et (2.1 mL, 18.47 mmol) and the reaction mixture
31
1
13
31
P 162 MHz) and Bruker Avance II 300 ( H - 300, C – 75, P -
21 MHz) spectrometers. Chemical shifts ( ) are specified in
ppm with coupling constants (J) indicated in Hz.
IR were done on Perkin Elmer FT-IR 1725xIR
2
2
was refluxed for 2 h. Solvent was removed by evaporation, and
1

the resulting residue was washed with H O and extracted with
2
ethyl acetate. The organic solution was washed with brine, and
a
dried over anhydrous MgSO . After filtration, the solvent was
4
spectrophotometer with Fourier Transform. These records
were obtained using KBr discs. Infrared spectra of the
removed in vacuum to obtain an oil. The resulting oil was
2
| J. Name., 2012, 00, 1-3
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purified by column chromatography (silica, 1:1 ethyl 2972, 2734, 2363, 2345, 1944, 1924.00, 1606, V1ie4w5A2rti,cle1O4n3li8ne,
DOI: 10.1039/C9NJ03405F
2 (2.42 g, 70%) as 1381, 1331, 1297, 1253, 1202, 1162, 1100, 1050, 1004, 975,
ether/petroleum ether) to give compound
white solid and compound
Overall yield is 89%.
3 (639 mg, 19%) as white solid. 920, 899, 776, 768, 754, 742, 678, 650, 642, 574, 558, 507,
1
477, 439. H NMR (300 MHz, DMSO-d ):

(ppm) = 5.13 (t, JHP
=
6
5
1
1
Compound
CDCl3): 1.26 (t, J = 7.2, 3H, OCH CH ), 4.25 (q, J = 7.2, 2H, 6-H), 7.47 (t, J = 7.6, 1H, ArH, 5-H or 6-H), 7.91 (d, J = 8.3, 1H,
OCH CH ), 5.42 (s, 2H, NCH ), 7.40 (t, J = 7.8, 1H, ArH, 5-H), ArH, 4-H or 7-H), 7.96 (d, J = 8.3, 1H, ArH, 4-H or 7-H). C RMN
2: mp 81-82 °C (lit., 81-82 °C). H NMR (400 MHz, 10.2, 2H, NCH C(OH)(PO H ) ), 7.33 (t, J = 7.6, 1H, ArH, 5-H or
2 3 2 2

2
3
13
2
3
2
7
4
.46-7.52 (m, 2H, ArH, 6-H and 7-H), 8.09 (d, J = 8.4, 1H, ArH, (75 MHz, DMSO-d6):
 (ppm) = 51.1 (NCH ), 73.1 (t, J = 140.8,
2 CP
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-H). C(OH)(PO H ) ), 112.8 (Ar:C7), 118.5 (Ar:C4), 123.4 (Ar:C5),
3
2 2
5
1
1
31
Compound
3
: mp 120-121 °C (lit., 120-121 °C). H NMR (400 126.5 (Ar:C6), 134.5 (Ar:C7a),145.0 (Ar:C3a). P NMR(121
1.28 (t, J = 7.2, 3H, OCH CH ), 4.27 (q, J = 7.2, MHz, H PO /DMSO-d ): (ppm) = 15.6. MS (ESI): m/z
H, OCH CH ), 5.52 (s, 2H, NCH ), 7.39-7.42 (d, J = 7.8, 2H, ArH, =346(MNa , 9%),324(MH , 100%). HRMS (ESI) m/z calcd for
MHz, CDCl3):


+
2
3
3
4
6
+
2
5
2
3
2
+
-H and 6-H), 7.86-7.89 (m, 2H, ArH, 4-H and 7-H).
C H N O P 324.01450 [MH] , found 324.01441.
8 12 3 7 2
2
.3.2. Synthesis of 2-(1
(490 mg, 2.388 mmol) (1 eq.) in aqueous NaOH
solution (10 M) (4 mL) was stirred at reflux for 1.5 h. The A mixture of carboxylic acid
H
-benzotriazol-1-yl)acetic acid 4
2.3.5. Synthesis of (2-(2
H-benzotriazol-2-yl)-1-hydroxyethane-
1
,1-diyl)bis(phosphonic acid) BP4
Compound
2
5
(100 mg, 0.564 mmol) and
mixture was cooled, followed by its acidification with 10% thionyl chloride (1 mL) was kept under reflux for 3 h. Thionyl
aqueous HCl solution. The aqueous solution was extracted chloride was removed under reduced pressure to give a
with EtOAc and the combined organic extracts were washed residue with the corresponding acyl chloride. This crude with
with brine. The resulting organic solution was dried over the acyl chloride was dissolved in dry THF and 0.38 mL of
anhydrous MgSO and filtered. The solvent was removed to tris(trimethylsilyl)phosphite (1.129 mmol) was added dropwise
4
afford
a
solid which was recrystalized from ethyl and the reaction proceeded for 1h at room temperature. The
(389 mg, 92%) as solvent was removed under reduced pressure and the
acetate/petroleum ether to give compound
4
5
2
1
white solid. mp 220-221 °C (lit., 212-213 °C). H NMR (400 resulting crude was stirred with methanol for 1 h. After that,
MHz, MeOD): 5.59 (s, 2H, NCH ), 7.43 (t, J = 7.6, 1H, ArH, 5-H solvent was removed under reduced pressure, and the
or 6-H), 7.56 (t, J = 8.0, 1H, ArH, 5-H or 6-H), 7.71 (d, J = 8.4, resulting crude was washed with ethyl ether and acetone to

2
1
H, ArH, 4-H or 7-H), 7.99 (d, J = 8.4, 1H, ArH, 4-H or 7H).
give compound BP4 as a white powder (110 mg, 61%). mp 183
-
1
°
C. max (KBr) (cm ): 3600-2100, 3331, 3223, 3013, 2967, 2786,
364, 2346, 1647, 1571, 1502, 1450, 1428, 1384, 1364, 1334,
3 (286 mg, 1.392 mmol) (1 eq.) in a 10 M aqueous 1289, 1221, 1195, 1152, 1130, 1140, 1036, 977, 882, 772, 754,
2
.3.3. Synthesis of 2-(2H-benzotriazol-2-yl)acetic acid 5
2
Compound
NaOH solution (3 mL) was stirred at reflux for 1.5 h. The 710, 680, 642, 628, 573, 560, 503, 459. H NMR (300 MHz,
mixture was cooled, followed by its acidification with 10% DMSO-d6): (ppm) 5.20 (t, JHP 10.1,
aqueous HCl solution. The aqueous solution was extracted 2H,NCH C(OH)(PO H ) ), 7.40-7.46(m, 2H, ArH, 5-H and 6-H),
1

=
=
2
3 2 2
3
1
with EtOAc and the combined solution were washed with 7.89-7.94 (m, 2H, ArH, 4-H and 7-H). P NMR(121 MHz, H PO /
3
4
1
3
brine. The resulting organic solution was dried over anhydrous DMSO-d6):
 (ppm) = 14.6. C RMN (75 MHz, DMSO-d ): δ
6
MgSO and filtered. After solvent removal under vacuum, it (ppm) = 58.2 (NCH ), 72.7 (t, J = 143.8, C(OH)(PO H ) ), 118.0
4
2
CP
3 2 2
was obtained a solid which was recrystalized from ethyl (Ar:C4,C7), 126.5 (Ar:C5,C6), 143.6 (Ar:C3a,C7a). MS (ES): m/z
+
+
acetate/petroleum ether to give compound 5 (240 mg, 97%) as =346(MNa , 9%), 324(MH ,100%). HRMS (ESI) m/z calcd for
1
white solid. mp 223-225 °C. H NMR (400 MHz, MeOD):
+
 5.60 C H N O P 324.01450 [MH] , found 324.01475.
8 12 3 7 2
(s, 2H, NCH ), 7.41-7.44 (m, 2H, ArH, 5-H and 6-H), 7.85-7.87
2
(m, 2H, ArH, 4-H or 7-H).
2
.4. Membrane Preparation
2
1
.3.4. Synthesis of (2-(1
,1-diyl)bis(phosphonic acid) BP3
(200 mg, 1.129 mmol), H PO
H-benzotriazol-1-yl)-1-hydroxyethane-
®
Membranes were prepared by casting Nafion /DMAc
48
solutions, based on our previous works, using different
quantities of BP dopants. The initial Nafion solution (20 wt%)
was dried at 40 °C under reduced pressure until a dry residue
was obtained. A new 10 wt% solution of Nafion was prepared
by dissolution of the previously dried Nafion in the required
amount of N,N-dimethylacetamide (DMAc).The desired
amount of BP dopant was added to this solution and the
mixture was stirred in an ultrasonic bath, during 1-5 h, to
ensure complete dissolution of dopants. The resultant
solutions were casted on a Petri dish with 5 cm diameter and
slowly evaporated to remove most of the solvent, until
A mixture of a carboxylic acid
4
3
4
(85%) (0.24 mL, 3.387 mmol) and chlorobenzene (1 mL) was
heated to 100-110 °C with stirring. After dissolution of all
solids, 0.4 mL of PCl (4.516 mmol) was added dropwise and
3
the reaction mixture was stirred at 100-110 °C for 4 h. After
cooling, chlorobenzene was decanted and 9 M aqueous HCl
solution (2 mL) was added to the residue. The obtained
solution was refluxed for 3 h. After that, the solvent was
removed and acetone and methanol were added to precipitate
a white solid. This was washed with acetone and dried to
afford compound BP3 as a white powder (117 mg, 32%). mp
obtaining
a
homogeneous membrane. The resulting
-1
1
90-191 °C. max (KBr) (cm ): 3450-2200, 3261, 3114, 3061,
membranes were fully dried in a vacuum oven at 60 °C,
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followed by their annellation at 140 °C for 2 h. The new The synthesized ester compounds
Journal Name
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DOI: 10.1039/C9NJ03405F
membranes were removed from the Petri dish and were hydrolysed on basic conditions to afford the corresponding N-1
submitted to a hot-pressing at 120 °C and 10 MPa for 10 and N-2 benzotriazole carboxylic acid derivatives (compounds
minutes, to improve their mechanical robustness and to avoid
4 and 5, respectively), as white solids in excellent yields
their shattering. After that, the membranes were activated by (Scheme 1). The NMR spectra showed the same pattern as the
a sequential treatment by boiling them in H O solution (3%), ester derivatives, confirming the presence of the expected
2
2
washing with hot deionized water, boiling in a 0.5 M sulfonic compounds.
acid solution, and washing again with hot deionized water,
0
1
2
3
4
5
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t
during 1 h for each step. Subsequently, the membranes were
stored in deionized water until their use.
N
i) KOBu/EtOH
N
N
N
N
+
N
N
N
N
H
ii) BrCH CO Et
2
2
O
O
The prepared membranes were labelled as Nafion/BP
respectively, where indicates the specific BP used, and
specifies the wt% of dopant.
n-x,
EtO
1
2
3
n
x
OEt
NaOH
NaOH
N
N
N
N
3
. Results and discussion
N
N
O
O
HO
4
3
.1. Synthesis and Characterization of dopants
5
OH
i) SOCl
ii)[(CH
2
New Nafion doped membranes were prepared using the
i) H PO
3 4
(85%)/PCl
3
3
)
3
SiO]
3
P
ii) HCl (9 N)
iii)MeOH
synthesized BP1-4 as dopants (Fig. 1).
N
N
N
N
N
N
HO
HO
PO H
OH
3
2
PO H
3
2
N
N
N
N
PO3H2
PO3H2
N
N
OH
PO
OH
H
2
3
O P
3 2
PO H
3 2
H
N
N
OH
PO H
H O P
N
H O P PO H
2
3
N
2
3
3
2
3
2
Me
H
BP4
H2O3P
BP3
BP1
BP2
BP3
BP4
Scheme 1 Synthesis of benzotriazolebisphosphonic acids BP3 and BP4.
Fig. 1 Structural formula of the bisphosphonic acids used as dopants.
The synthesis of the bisphosphonic acid derivatives were
carried out from the corresponding benzotriazolecarboxylic
Hydroxy(1-methyl-1H-indazol-3-yl)methanediyl]bis(phosphonic
acid) BP1 and hydroxy(1H-indazol-3-
(
)
acids
methods (Scheme 1). The N-2 isomer bisphosphonate BP4
were synthesized by the reaction of carboxylic acid with
4 and 5. Their synthesis were performed using different
yl)methanediyl]bis(phosphonic acid) (BP2) were prepared from
the corresponding carboxylic acid derivatives and
characterized following previous studies, according to Teixeira
5
thionyl chloride, to produce in situ the corresponding acyl
chloride, which reacted with tris(trimethylsilyl)phosphite and
the resulting product further underwent a subsequent
methanolysis (Scheme 1). Since this mild method does not
allow the synthesis of the N-1 isomer bisphosphonate BP3, this
BP was synthesized by the classical method, by reaction of the
4
9,50
et al.
2-(1H-benzotriazol-2-yl)-1-hydroxyethane-1,1-
diyl]bis(phosphonic acid) (BP3) and [2-(2H-benzotriazol-2-yl)-1-
hydroxyethane-1,1-diyl]bis(phosphonic acid) BP4 where
prepared by similar processes from the starting material
benzotriazole. Commercially available benzotriazole was
[
(
)
1
corresponding carboxylic acid isomer
4 with phosphorus
used as staring material. This compound performed a
trichloride and phosphoric acid, at chlorobenzene, followed by
acidic hydrolysis, resulting in the expected BP3 (Scheme 1).
These benzotriazole BPs were characterized using IR, NMR, MS
and HRMS which confirmed the proposed structures. Both
bisphosphonic acid BP3 and BP4 present a very large band
nucleophilic substitution reaction, with the bromoester
t
BrCH CO CH CH as reagent, in the presence of KO Bu as base,
2
2
2
3
53
using a method optimized by us, to afford a mixture of N-1
) and N-2 ( ) regioisomers (Scheme 1). These isomers were
separated by column chromatography, to afford the pure ester
derivatives and
(2
3
-
1
between 3600 and 2100 cm , corresponding to the stretching
band of O-H and hydrogen bond, with maxima at 2734 and
2
3.
-
1
N-1 and N-2 regioisomers were identified through their NMR
data, in agreement with the observed molecular symmetry or
asymmetry of N-2 derivatives or N-1 derivatives, respectively,
and the N-1 and N-2 substituted benzotriazole regioisomers
2
780 cm , respectively, due to P-OH stretching band. The
characteristic bands of phosphonic group were observed
-
between 1221 and 1100 cm , and between 1050 and 882cm ,
58-62
for P=O and P-OH stretching bands, respectively.
1
-1
5
1,54-57
1
1
NMR data reported in the literature
: the H NMR spectra
The H NMR spectra support the proposed structures of BP3
and BP4, showing a triplet at 5.13 and 5.20 ppm (JH,P ≈10 Hz),
of N-1 isomer present four distinctive signals at the aromatic
region of the spectra, while the spectra of N-2 isomer only
show two different signals, due to their structural symmetry.
respectively, for NCH protons, due to their coupling with the
2
phosphorus atoms of two phosphonic acid groups attached to
3
1
the same carbon (Fig. 2 (a)). The P NMR spectra of BP3 and
4
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4
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5
5
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5
5
5
5
5
5
6
BP4 (Fig. 2 (b)) show a singlet at 15.6 and 14.6 ppm, New Nafion doped membranes were prepared by a casting
View Article Online
48
DOI: 10.1039/C9NJ03405F
respectively, proving that both phosphorus atoms of method based in our previous studies, using the prepared
bisphosphonic acid group are chemically and magnetically bisphosphonates BP1 BP4, with 1 wt% and 5 wt% loadings.
The C NMR spectra confirm the presence of Prepared membranes are indicated schematically in Fig. 3.
–
49,53
13
equivalent.
two bisphosphonic groups bonded to the C(OH) carbon atom
of BP3 and BP4, which is observed as a triplet at 73.1
(J =140.8 Hz) and 72.7 ppm (J =143.8 Hz), respectively, due
CP CP
to the coupling of this carbon atom with the 2 equivalent
phosphorous nuclei, confirming the proposed structures with
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
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8
9
0
1
13
bisphosphonate group (Fig. 2 (c)). H and C NMR spectra of
benzotriazole moiety present the similar pattern, varying
according to the symmetry or asymmetry structure of each
regioisomer: the N-1 regioisomer showed different signals for
all protons and carbon atoms, in agreement with the
molecular asymmetry of these regioisomers, with the N-2
regioisomers displaying one signal for each two atoms, in
agreement with their structural symmetry (Fig. 2).
Fig. 3 Membranes prepared by casting using 1 wt% and 5 wt% dopant
loadings.
31
1
(b) P NMR
(
a) H NMR
Casting of Nafion/DMAc solutions with the specific dopant and
its loading, on a Petri dish, lead to the formation of the desired
membranes, which were successively submitted to hot-
BP3
BP4
pressing at 120 C and 10 MPa, for 10 minutes, in order to
increase their mechanical robustness, and to do not brittle
during their activation.
The new membranes were analyzed by ATR FT-IR
spectroscopy. Their spectra showed very strong bands near
-1
200 and 1144 cm , characteristic of Nafion polymer, due to
1
22,63-65
the S=O stretching and C-F wagging bands, respectively.
These Nafion bands are much more intense than those of
phosphonic acid groups of dopants and, due to the small wt%
quantity of dopant used, the Nafion bands overlap the
phosphonic acid groups bands and only discrete differences
are observed in spectra of new membranes (Fig. 4).
δ (ppm)
δ (ppm)
13
(
c) C NMR
BP3
BP4
The membranes were also analysed by SEM microscopy. The
micrographs of selected membranes are presented in Fig. 5. All
membranes present a homogeneous and compact surface,
without any visible pores. Their surface seems to have small
honeycomb wrinkles, formed during the drying process of the
membrane. The new membranes present similar morphology
among them but different from the Nafion membrane, with
some roughness at their surface, suggesting a different
composition and the incorporation of the BPs dopants
throughout the membrane.
δ (ppm)
Fig. 2 The H (a), P (b) and C NMR (c) spectra of dopants.
1
31
13
3
.2. Membrane Preparation
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6
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DOI: 10.1039/C9NJ03405F
ARTICLE
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1
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9
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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
a) Nafion
(b) Nafion
Nafion/BP1-1.0
Nafion/BP1-5.0
Nafion/BP2-1.0
Nafion/BP3-1.0
Nafion/BP4-1.0
Nafion/BP2-5.0
Nafion/BP3-5.0
Nafion/BP4-5.0
E
4000.0
3600
3200
2800
2400
2000
1800
1600
1400
1200
1000
800
600
373.9
000.0
3600
3200
2800
2400
2000
1800
1600
1400
1200
1000
800
600
373.9
1/cm
4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200
1000 800
600
373.9
4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200
1000 800
600
373.9
Wavenumber / cm-1
Wavenumber / cm-1
Fig. 4. ATR FT-IR spectra of Nafion doped membranes with 1 wt% (a) and 5 wt% (b) loadings.
Nafion
Nafion/BP1-1.0
Nafion/BP3-1.0
Nafion/BP4-1.0
Nafion/BP2-1.0
Fig. 5 SEM images of Nafion membranes doped with 1.0 wt% BP1- BP4.
3.3. Proton conductivity of Nafion membranes
with 5 wt% loading. An exception was observed for BP4, with a
slight increment of proton conductivity, up to 10%, with the
increase of the wt% loading. The elongated form of this dopant
can allow a better stacking of different molecules of dopants in
the polymer, facilitating the formation of a more compact and
better structured dopant network in the membrane, reducing
the structural perturbation caused by the incorporation of a
greater amount of dopant in the Nafion polymer structure. A
decrease on the proton conductivity with the increase of the
phosphonic acid dopant wt% loading has been already
observed by the authors when using aromatic bisphosphonic
The proton conductivity of prepared membranes was
evaluated by Electrochemical Impedance Spectroscopy, at
different temperatures (30, 40, 50 and 60 °C) and relative
humidity (40, 60 and 80% RH) (Fig. 6), since these are key
parameters on the performance of the fuel cells.
All membranes present better proton conductivity than Nafion
membrane with values up to 1.51 fold higher, when evaluated
at the same temperature and RH conditions, with the
exception of the membrane prepared with 5 wt% loading of
BP3, which proton conduction network seems more affected
by the properties of this dopant at this wt% loading, moving it
away from optimal proton conduction, and showing slightly
lower proton conductivity value than Nafion (Fig. 6).
48
acid derivatives.
The proton conductivities of all membranes were comparable
between them, when evaluated at the same temperature and
RH conditions. But, generally, the membrane with BP2 as
dopant showed better conductivities, especially at more
elevated RH conditions. The best value was observed for
The membranes prepared with a 1 wt% loading showed, in
general, better proton conductivity than membranes prepared
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6
membrane with BP2, 1 wt% loading, at 80% RH, with a proton Also, for the same membrane, the highest protonViceownAdrtiuclcetOivnliitnye
-
1
DOI: 10.1039/C9NJ03405F
conductivity of 98 mS cm . This compound has a nitrogen observed at 40% RH is lower than the lowest proton
atom at the ring bonded to a hydrogen atom, allowing it to conductivity observed with 60% RH, and the same is observed
participate in the proton transport by accepting the proton between 60% RH and 80% RH. When at the same temperature,
during its movement throughout the membrane. Regarding this increase is comparatively higher between 40% and 60% RH
the ring moiety, the results suggest that it is not a key feature conditions, than between 60% to 80% RH conditions. The
for proton conductivity, since there is no clear order on the observed results suggest that relative humidity conditions play
proton conductivities of membranes containing indazole or an important role on the proton conductivity, within the
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
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2
3
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0
1
2
3
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5
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7
8
9
0
benzotriazole derivatives.
studied conditions, for these membranes.
In the same RH conditions, the proton conductivity of each Similar results, with better values than Nafion, are reported in
membrane increased, as expected, with the increment of the some
66-70
temperature (Fig. 6) demonstrating that proton conductivity is azoleheteroaromatics (see for instance, ref.
works
using
phosphonate
derivatives
or
,
and
thermally activated. From 30 to 60 °C, in most cases, the references cited therein) which, despite the use of different
proton conductivity increases between 50% to 70%, but could conditions, confirm the quality of present results and showed
reach 86% more for the membrane with BP1 with 1.0 wt% that the devised strategy of using derivatives with,
loading, at 40% RH, or could be only 34% for BP2 with 5 wt% simultaneously, a phosphonate group and an azoleheterocycle
loading and 40% RH.
moiety, result in a favourable system to increase the proton
An increment of RH promotes an marked increase of proton conduction on Nafion doped membranes.
conductivity. For the same temperature, proton conductivity
at 80% RH is near 3 fold higher than at 40% RH conditions.
1
.0 wt% BP
1.0 wt% BP
1.0 wt% BP
40% RH
60% RH
80% RH
5
.0 wt% BP
5.0 wt% BP
5
.0 wt% BP
40% RH
60% RH
80% RH
Fig. 6 Comparison of in-plane proton conductivity vs reciprocal temperature at 80%, 60% and 40% RH for Nafion and Nafion membranes doped with 1.0 wt%
BP1-BP4 and 5.0 wt% BP1-BP4 loading.
The activation energy (E ) required for the proton transport
a
ln

= ln 0
(2)
throughout the membranes is estimated quantitatively by the where E - activation energy; 0 - pre-exponential factor; R -
Arrhenius equation (2), using the temperature-dependent ideal gas constant; T - absolute temperature.
a
proton conductivity,
This journal is © The Royal Society of Chemistry 20xx
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4
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4
4
4
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6
significant increase on the proton conductivViietwieAsrticloefOnlainlel
DOI: 10.1039/C9NJ03405F
The mechanism of the proton conductivity across the membranes studied. The best proton conductivity was
membrane is usually identified by the activation energy (Ea). observed, at 80% RH and 60 °C conditions, for membrane with
Two different mechanisms of proton conduction are BP2 with 1 wt% loading, with a proton conductivity of 98 mS
-
1
described: the Grotthuss mechanism, with an E between 9.65 cm .
a
-1
and 38.59 kJ mol , and the vehicular mechanism, with an E of The evaluation of the activation energy of the new membranes
a
-1 5,35,71-75
6.4 kJ mol .
1
suggests that both vehicular and Grotthuss mechanisms
The E of the studied membranes are indicated at Table 1. The coexist in proton transport across the membranes under all RH
a
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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9
0
1
2
3
4
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7
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9
0
1
2
3
4
5
6
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8
9
0
new membranes showed similar or slightly lower activation conditions studied.
energies than Nafion membrane, suggesting that proton
conduction demands an approximate energy in the studied
conditions. The membranes with a 5 wt% loading, in general,
Conflicts of interest
showed a slightly lower Ea.
The assessed Ea values suggest that both vehicular and
There are no conflicts to declare
Grotthuss mechanisms coexist in these membranes under all
5
,35,71-75
studied RH conditions.
Acknowledgements
Table 1. Activation energy required for proton conduction in Nafion N-115 and Nafion
doped membranes.
This research was supported by the Project “PEMFC-SUDOE” -
SOE1/P1/E0293 which is co-financed by the European Regional
Development Fund in the framework of the Interreg Sudoe
programme. We also acknowledge to HERCULES Laboratory
-
1
Activation energy Ea (kJ mol )
Membranes
4
0% RH
60% RH
80% RH
(
University of Évora), for providing access to ATR FT-IR
facilities.
Nafion N-115
Nafion/BP1-1.0
Nafion/BP2-1.0
Nafion/BP3-1.0
Nafion/BP4-1.0
Nafion/BP1-5.0
Nafion/BP2-5.0
Nafion /BP3-5.0
Nafion/BP4-5.0
17
19
17
18
17
19
10
17
16
16
16
17
16
16
14
15
17
16
16
16
17
17
17
16
16
16
15
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7
2
Apart from the membrane doped with BP3 at 5% loading,
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1
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, 75-83.
2
0
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View Article Online
DOI: 10.1039/C9NJ03405F
Enhanced proton conductivity of Nafion-azolebisphosphonates membranes for PEM
fuel cells
a
a
b
a
Fátima C. Teixeira, Ana I. de Sá, António P. S. Teixeira and C. M. Rangel*
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Graphical Abstract
Azolebisphosphonates were prepared and used as dopants to obtain new Nafion doped membranes, which exhibited higher
proton conductivities than Nafion.
D
O
P
A
N
T
HO
HO
N
N
PO3H2
PO3H2
PO3H2
PO3H2
N
N
N
N
OH
OH
PO3H2
N
N
N
N
H2O3P PO3H2
H2O3
BP3
P
Me
H
BP4
S
BP1
BP2
1