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Composite sPEEK-TPyP membranes development
for portable applications
a, R. Pedicini a, I. Gatto a, E. Passalacqua a,
A. Carbone a,*, A. Sacca
A. Romeo b, L. Monsù Scolaro b, M.A. Castriciano c
a
Istituto di Tecnologie Avanzate per l'Energia “Nicola Giordano”, via S. Lucia Sopra Contesse 5, 98126 Messina, Italy
Dipartimento di Scienze Chimiche, University of Messina Viale Ferdinando Stagno D'Alcontres n.31, 98166
Villaggio S. Agata, Messina, Italy
c
Istituto per lo Studio dei Materiali Nanostrutturati, c/o Dipartimento di Scienze Chimiche Viale Ferdinando Stagno
D'Alcontres n.31, 98166 Villaggio S. Agata, Messina, Italy
b
article info
abstract
Article history:
Composite membranes based on sulphonated Polyetheretherketone (sPEEK) and 5,10,15,20-
Received 9 February 2015
tetra(4-pyridyl)porphyrin (TPyP) were developed for portable applications. A sulphonation
Received in revised form
degree of 65% and different loadings (0e5 wt%) of TPyP porphyrin were used. Spectroscopic
28 July 2015
studies (UVeVis and fluorescence emission), ionic exchange capacity, water uptake,
Accepted 29 July 2015
structural and morphological analyses were carried out. A good interaction between the
Available online xxx
filler and the polymer was found, probably due to specific supramolecular interactions
among the nitrogenous atoms present in the periphery of the porphyrin ring and sulphonic
Keywords:
groups of polymer. The membrane with the lowest loading (~1 wt%) of TPyP, shows slightly
Sulphonated PEEK
lower water uptake and unaltered l values than the reference membrane, resulting in quite
TPyP
unaltered proton conductivity also in anhydrous conditions. Electrochemical tests were
PEFC
performed in a PEFC 25 cm2 single cell at room temperature and anhydrous hydrogen/
Portable applications
humidified air. A power density of 93 mW/cm2 was found in anhydrous conditions for
sample with the lowest loading of TPyP.
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
Introduction
Fuel cells are clean and highly efficient electrochemical systems for energy conversion, that are fed by hydrogen in
advanced systems with the highest performance, or by alternative fuels (e.g. methanol or ethanol) with significant
reduction in performance [1]. Among the existing typologies of
fuel cells, the system that has attracted most interest is the
Proton Exchange Membrane Fuel Cell (PEMFC) due to its wide
range of power density (1 10 3e100 kW), simplicity of
components and its friendly user operating conditions.
A polymer electrolyte membrane fuel cell (PEMFC) is
composed of a membrane able to conduct protons, between
the anodic and cathodic compartments, separating two catalytic electrodes where the fuel oxidation and the comburent
reduction take place.
* Corresponding author. Tel.: þ39 090624273.
E-mail address: alessandra.carbone@itae.cnr.it (A. Carbone).
http://dx.doi.org/10.1016/j.ijhydene.2015.07.159
0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Carbone A, et al., Composite sPEEK-TPyP membranes development for portable applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.159
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Due to the high energy density with respect to the traditional batteries, fuel cells are particularly suitable for the
fabrication of portable systems, more efficient and less bulky
than currently available systems.
The Portable Fuel Cell systems tend to use either DMFC
(direct methanol fuel cells) or PEFC (polymer electrolyte fuel
cells fed by hydrogen) technology, even if the DMFC are
nowadays the most diffused.
However, several problems to be solved, such as catalyst
poisoning during methanol oxidation reaction and methanol
crossover through solid-polymer-electrolyte membrane,
limit their commercialization [2]. The most used polymer
electrolyte membrane is Nafion®, that has high proton
conductivity, chemical and mechanical stability, but suffers
a high methanol permeability, that drastically reduces the
performance. To solve this problem, associated to a high
production cost of this polymer, the research aims at
developing low-cost and highly efficient proton conducting
membranes to be used as polymer electrolytes for portable
fuel cells fed with hydrogen and air under air-breathing
conditions.
The development of poly-aromatic based membranes
plays a key role as an alternative to Nafion® membranes [3e7].
A wide literature exists on this class of polyelectrolyte but
some problems should be overcome such as a low proton
conductivity at reduced relative humidity, the mechanical
stability and lifetime [8]. One of the most used polymer
membrane is based on sulphonated polyetheretherketone (sPEEK) with a sulphonation degree in the range 40e70% [9e14].
Compared to the membranes reported in the state-of-the-art,
the alternative membranes exhibit reduced cost (by a factor of
about 40); better mechanical properties; improved stability to
radical species; reduced hydrogen crossover; similar unit area
resistance.
In addition to the above mentioned characteristics,
nowadays, the research is moving on the development of
membranes containing nitrogenous groups that promotes
specific acid-base interactions between functional aminosulphonic groups, by improving the proton conductivity
[15e17]. In fact, the sulphonic acid groups interact with the
nitrogenous base by forming hydrogen bridges, protonation of
the nitrogen sites and polysalts formation. In this framework,
to the best of our knowledge, only a report has been addressed
to a class of membranes functionalized with porphyrins as
nitrogenous sources [18]. Porphyrins are a class of versatile
molecules and porphyrin-based architectures have diverse
potential applications as biomimetic models, as functional
materials for the transport of energy, charge, molecules, and
ions [19] as well as hydrogen ion sieve structure [20e23].
Furthermore, it is widely reported in literature, that porphyrins, due to their chemical structure, are able, in particular
experimental condition, to self-aggregate forming extended
supramolecular network [24]. With this aim, the ability of
sulphonated polyetheretherketone (sPEEK) membranes to
specifically interact with 5,10,15,20-tetra(4-pyridyl)porphyrin
(TPyP) was exploited and its proton path for the conduction
mechanism for applications in portable devices was
investigated.
Experimental
Polymer sulphonation
A Polyetheretherketone (Victrex PF450) was functionalized in
concentrated sulphuric acid according to a standardized procedure, reported elsewhere [25], in order to obtain a sulphonation degrees (SD) of 65%.
Membrane preparation
Membranes were prepared with a doctor-blade standardized
procedure [25] Composite membranes were further prepared
using the sulphonated polymer. The 5,10,15,20-tetrakis(4pyridyl)-21H,23H-porphine (TPyP) was purchased from
Aldrich Chemical Co. and used without further purification.
The porphyrin was added to the polymer in different weight
percentages 0.77, 1.5 and 5wt% in DMAc solution. In order to
better solubilize the porphyrin, the resulting mixture was
stirred for 1 h before to be added to the polymeric solution. All
the membranes were dried at 80 C for 3 h then detached from
the glass by impregnation with H2O. A thermal treatment was
carried out at 120 C for 16 h, An acid treatment was carried
out at first in 1 M H2SO4 at 60 C and then in water at the same
temperature, in order to purify the membranes of any residual
solvent and activate groups for proton ion exchange for the
coordination of water molecules. Membranes with a thickness
ranging from 50 to 70 mm were obtained.
Polymers and membranes X-ray analyses (XRD)
The X-ray powder diffraction (XRD) analyses were performed
by using a Philips X-ray automated diffractometer (model
PW3710) with Cu Ka radiation source. The 2q Bragg angles
were scanned between 5 and 100 2q.
UVeVis and fluorescence
UV/Vis spectra were obtained on a HewlettePackard mod.
8453 diode array spectrophotometer directly on the membranes. Fluorescence measurements were carried out on a
Jasco mod.FP-750 spectrofluorimeter.
SEMeEDX analyses
A field emission Scanning Electron Microscope (Philips mod.
XL30 S FEG) was used to investigate the cross-section
morphology of the membranes. A gold coating was used to
avoid sample charging and to permit electronic conduction.
Samples for cross-section were prepared dipping the membrane in liquid Nitrogen and breaking the samples to have a
perfect fracture.
Ion exchange capacity (IEC)
An acid-base titration was carried out to determine the
membrane IECm (meqSO3H/mg) and is based on the
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neutralization of Hþ ions, belonging to the acid group eSO3H.
The experimental procedure is described elsewhere [25] The
IEC is calculated using the following formula:
IECm ¼ (Vtit$[M])/mdry
(1)
where:
To determine the interaction between SPEEK and TPyP, IEC
of SPEEK polymer (IECp) within the composite membrane was
determined considering the Polymer Fraction (PF) calculated
from Eq. (2):
(2)
where IECm is the experimental IEC of the membrane.
Water uptake, lambda and dimensional variation
The ability to retain the water of the membrane (wu) is usually
calculated from the difference in weight between the dried
and the wet sample. The wet weight (mwet) is determined after
immersion of the sample in distilled water at room temperature for 24 h, while for the dry weight (mdry), the sample is
dried in a vacuum oven at 80 C for 2 h. The percentage of
water absorbed is given by the following expression:
wu ¼ (mwet
mdry)/mdry 100
(3)
The l value (expressed as moles H2O/moles-SO3H) was
calculated through the water uptake and IEC values ratio, both
expressed in moles:
l ¼ (mol H2O)/(mol SO3H)
(4)
Through this parameter is possible to evaluate the capability of the SO3H groups in the composite membranes to coordinate water molecules.
The dimensional variation (A%) was calculated in terms of
geometrical are percentage using the following expression:
A% ¼ (Awet
Adry)/Adry 100
(5)
Proton conductivity
The conductivity was measured at 30 C and full humidification (100% RH) using a commercial cell (Bekktech). The measure is carried out in the longitudinal direction of the sample
with a four-probe method and DC current, with a potentiostategalvanostat (AMEL mod.2049) and calculated using the
formula:
s ¼ L/RWT
where:
L ¼ 0.425 cm, constant distance between the two Pt
electrodes;
R ¼ resistance in U;
W ¼ sample width in cm;
T ¼ sample thickness in cm.
All the measurements were carried out after an equilibration time of 1 h after reaching the operative conditions. After
measuring the 100%RH the gas was turned to dry and 1 h was
waited before to perform the measurement.
Vtit ¼ titrant volume (ml);
[M] ¼ titrant molarity;
mdry ¼ dry mass of the sample (g);
IECp ¼ IECm/PF
3
(6)
Fuel cell tests
Home-made electrodes were prepared by a spray technique
described in the referenced paper [17,26] and coupled to the
membranes to obtain MEAs. The same Pt loading (0.5 mg/cm2)
in the catalytic layer was used for both anodes and cathodes
and a 50% Pt/C (Alfa Aesar) was used as an electrocatalyst. A
Sigracet-24BC (SGL group) was used as a gas diffusion layer.
Membranes and electrodes were assembled by cold-pressing.
Fuel cell tests, in terms of polarisation curves, were carried
out in a commercial 25 cm2 single cell at 30 C, with fully
humidified H2/air and dry H2/humidified air at 1 absolute bar.
The gas fluxes were fixed at 1.5 and 2 times the stoichiometry
at the current work for hydrogen and air, respectively.
Electrochemical impedance spectroscopy (EIS) was performed by using a potentiostat/galvanostat (AUTOLAB
PGSTAT30) equipped with a frequencies response analyzer
(FRA module) and a 20 A BOOSTER. All impedance measurements were performed in the potentiostatic mode of fuel cell
operation at a constant potential of 850 mV. The impedance
spectra were obtained varying the frequency of the voltage
perturbation signal from 0.1 Hz to 100 kHz, by using amplitude
of 10 mV for the perturbing signal.
The H2 cross-over of the membranes was measured by a
linear sweep voltammetry method, in the same operative
conditions of IeV curves, by varying the cell potential from
0 to 0.8 V with a scan rate of 4 mV/s. The current value at 0.4 V
was used for cross-over calculation.
Results and discussion
In order to test the dispersion of porphyrin in the polymeric
matrix, the membranes were first characterized in terms of
XRD profile. In Fig. 1 is reported the comparison among the
XRD patterns of pristine SPEEK membrane, composite membranes with different amount of tetra(4-pyridyl)porphine
(TpyP, hereafter) and TPyP powder. The spectra show the
typical amorphous profile of the sulphonated polymer centred
at about 18 2q, that remains quite unaltered after the functionalization of the membrane with TPyP. The absence of TPyP
peak is a clear indication that porphyrin is present in the
membranes in its monomeric form or as small oligomers/
aggregates.
Due to their structural features, i.e. the extended planarity
and the presence in periphery of the ring of specific groups,
the porphyrins are able to induce van der Waals, p e stacking
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Fig. 1 e XRD patterns of developed membranes compared
to TPyP.
and hydrophobic interactions, that allow, in appropriate
experimental conditions, the formation of dimers, trimers,
small oligomers or larger aggregates [27]. The aggregation
phenomena for this class of molecules are accompanied by
characteristic changes in their spectroscopic properties, i.e.
bathochromic or hypsochromic shifts of the absorption and
emission bands, Beer law's deviation, quenching of the luminescence. The photophysical properties of TPyP in its monomeric form have been reported in organic solvents [28,29] as
well as the modulation of the protonation behaviour and of
the self-aggregation [30,31]. To get better inside into the aggregation state of the TPyP porphyrin into membranes, spectroscopic investigation on the composites were performed.
Figs. 2 and 3 show the UVeVis and fluorescence emission
spectra of SPEEK-0.77% TPyP. The position of the absorbance
as well as the emission bands remain almost unchanged for
all the investigated samples. Due to high porphyrin
Fig. 2 e Extinction spectra of SPEEK-0.77% TPyP (solid line)
and, after immersion in 1 M NaOH aqueous solution
(dashed line), T ¼ 298 K. Inset extinction value after cyclic
dipping into acidic (1 M HCl) and alkaline solutions.
concentrations it was not possible to detect the B-band typical
for this chromophores and it was taken into account the position of the Q-bands (500e700 nm) which have a lower
extinction coefficient with respect to the B-band. The extinction spectra of the membranes show the presence of 2 Qbands centred at 595 and 645 nm, respectively The fluorescence emission spectrum shows two bands centred at 662 and
718 nm. According to literature, these spectroscopic behaviour, especially the presence of 2 Q bands indicate a D4h geometry for the porphyrin ring, pointing out the occurring, into
the membranes, of the porphyrin core protonation [30] Independently, from the thickness membranes and from the
porphyrin loads, the chromophores embedded into membranes exhibit good stability and chemical resistance to
thermal annealing and acidic treatment. In fact, the spectroscopic features before and after the treatments remain unchanged for all investigated samples. Furthermore,
protonation and deprotonation of the porphyrin can be easily
obtained by simple immersion of the membrane in an alkaline
solution. As reported in Fig. 2, dipping the membranes into
alkaline aqueous solutions the green colour of the samples
turns instantaneously to red and its extinction spectrum is
reported in Fig. 2. The presence of 4 Q-bands in the extinction
spectrum centred at 518, 552, 590, 645 nm respectively, clearly
indicate a D2h geometry for the porphyrin ring, pointing out
the occurrence of the porphyrin core deprotonation. This evidence is supported by the presence of two emission bands
centred at 650 and 716 nm in the fluorescence spectrum [30].
Porphyrin protonation can be restored and the systems switch
fast and reversibly upon repetitive cycling by dipping into base
and acidic solutions (inset of Fig. 2). The switching properties
are extremely reproducible and the membranes remain stable
for months.
SEM analyses were performed to check the morphology of
composite membrane compared to pristine SPEEK one (Fig. 4).
In both cases a dense and homogeneous structure was highlighted, meaning that no agglomeration of TPyP occurred,
even for membrane with the highest TPyP content (reported in
Fig. 4b).
Fig. 3 e Fluorescence emission spectrum of SPEEK-5% TPyP
SPEEK-0.77% TPyP (solid line) and, after immersion in 1 M
NaOH aqueous solution (dashed line), T ¼ 298 K.
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Fig. 4 e SEM analysis of a) SPEEK membrane, b) SPEEK-5%TPyP membrane.
To verify the influence of TPyP on the proton exchange
properties of SPEEK, the experimental (IECm) and the calculated IECp were compared and reported in Fig. 5.
The IECm decreases with the increase of porphyrin
amount, because the TPyP has not own exchangeable protons.
In addition, the ability of SPEEK polymer to form interactions
with nitrogenous groups is known and elsewhere reported
[25]. Also with the TPyP introduction, specific interactions
between sulphonic and nitrogenous groups are present, as
highlighted from IECp data. This values were calculated by
considering the nominal amount of TPyP introduced in the
membranes and the obtained values are related to the SPEEK
polymer in the composite membranes. As reported in Fig. 5,
the IECp is still lower than pristine SPEEK membrane, indicating that interactions occurred and limit the proton exchange capacity.
The behaviour of water uptake (w.u.) and dimensional
variation (area %) is shown in Fig. 6, as a function of the TPyP
content.
In accordance to IEC behaviour, both w.u. and A% decrease
by increasing filler loading, meaning that the TPyP interaction
with the sulphonic groups of the PEEK reduces the swelling of
the polymeric matrix. An interesting trend is highlighted in
Fig. 7, where is reported the proton conductivity as a function
of the lambda values. Because of the lambda values is
Fig. 5 e IEC behaviour as a function of TPyP loading.
Fig. 6 e Water uptake and area variation as a function of
TPyP amount.
generally related to the hydration of sulphonic groups of the
polymer, the capability to maintain unaltered the properties
to coordinate water in composite membranes is an important
feature to be considered. As expected, the proton conductivity
increases with the increase of l values and for samples SPEEK-
Fig. 7 e Proton conductivity as a function of lambda values.
Please cite this article in press as: Carbone A, et al., Composite sPEEK-TPyP membranes development for portable applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.159
6
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Table 1 e Proton conductivity values at different
operative conditions.
Table 2 e Electrochemical data at 100%RH H2/air.
Membrane
OCV Rs (EIS) H2 cross-over PD @ 0.7 V
V
Ucm2
at 0.4 V
mW/cm2
2
700 mV
mA/cm
SPEEK
SPEEK-0.77%TPyP
SPEEK-1.5%TPyP
SPEEK-5%TPyP
0.985
1.000
0.999
1.000
Proton conductivity at
30 C, mS/cm
Membrane
SPEEK
SPEEK-0.77%TPyP
SPEEK-1.5%TPyP
SPEEK-5%TPyP
l, mol
H2O/SO3H
100%RH
13
13
12
11
42
36
26
7
Dry
<0.01
10
<0.01
<0.01
0.64
0.65
0.82
1.98
1.2$10
1.2$10
3.1$10
9.2$10
4
5
4
5
65
80
50
24
0.77% TPyP and pristine SPEEK recast the proton conductivity
is similar due to the same l value (l ¼ 13). Moreover, only the
SPEEK-0.77% TPyP membrane shows an appreciable value of
proton conductivity in dry H2 condition (see Table 1), this
behaviour could mean that this membrane is less sensitive to
dehydration phenomena.
Because the setup of the measurement is different from
fuel cell, in which the membrane is constrained in the MEA,
in this kind of measurement the sample is more available to
hydration changes and can prompt respond to these
changes. The SPEEK-0.77 TPyP sample is the only that
maintains a suitable hydration to reach a proton conduction,
due to its intrinsic properties. The other samples showed a
drop in the proton conduction with a reported
value < 0.01 mS/cm, under the instrument sensitivity limit,
due to a rapid de-hydration.
To verify the single cell performance useful for portable
applications, the composite membranes, were tested in the
operative conditions of low temperature and pressure and
compared to recast SPEEK. In Fig. 8, the polarisation curves at
30 C, 1 ab bar and full hydrated gases are shown. Since the
useful voltage range, for a real application, is between 0.6 and
0.7 V, the IeV curves are cut off at 200 mA/cm2. The performance of composite membranes are in accordance to the
previous chemical-physical and ex-situ electrochemical data,
in fact the performance decreases with the increase of TPyP
amount. The SPEEK-0.77% TPyP shows slight higher performance than SPEEK recast membrane.
All the investigated samples present good OCV values
(Table 2), confirming the dense structure of the membranes.
The increase of Rs values is due to TPyP introduction which
interacts with sulphonic groups limiting the protonic transport. Despite the Rs value of sPEEK-0.77%TPyP is higher than
the recast one, the performance increase is due to the lowest
H2 cross-over, confirmed also from OCV data. The power
density at 0.7 V is reported in Table 2, it is still evident the
benefit of the small amount of TPyP (0.77%) in the polymer
matrix, in fact a power density of 80 mW was reached against
65 mW of SPEEK.
To simulate the real application in portable devices where
gases are not humidified, operative conditions were changed
in dry H2/100%RH air, supposing a hydrated cathode for the
atmospheric humidity (air-breathing) and the water reaction
formation. The polarisation curves are reported in Fig. 9.
Also in this case the trend is similar to the full humidification. The performance slightly increased despite the
dry condition of the anode. This behaviour could be probably caused by a not good water management in full humidification operative conditions, that affects the
performance.
Also in these operative conditions the OCV data remains
good, the H2 cross-over is reduced of one order of magnitude,
in particular for sPEEK-0.77%TPyP respect SPEEK membrane,
and the power density is increased, with the highest value of
93 mW/cm2 at 0.7 V for membrane containing 0.77wt% TPyP
(Table 3).
Anyhow, the SPEEK-0.77% TPyP membrane represents the
best compromise between chemicalephysical properties and
Fig. 8 e IeV curves comparison at low T, P and 100%RH H2/
air.
Fig. 9 e IeV curves comparison at low T, P and dry H2/100%
RH air.
Please cite this article in press as: Carbone A, et al., Composite sPEEK-TPyP membranes development for portable applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.159
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Table 3 e Electrochemical data at dry H2/100%RH air.
Membrane
OCV Rs (EIS) H2 cross-over PD @ 0.7 V
V
Ucm2
at 0.4 V
mW/cm2
2
700 mV
mA/cm
SPEEK
SPEEK-0.77%TPyP
SPEEK-1.5%TPyP
SPEEK-5%TPyP
0.985
1.018
0.985
1.000
0.59
0.78
0.75
2.55
2.9$10
2.4$10
2.8$10
1.0$10
3
4
3
4
85
93
65
29
electrochemical performance and it could be considered a
good candidate to be used in a stack for a real portable
application.
Conclusions
Composite membranes based on sulphonated Polyetheretherketone (sPEEK) and 5,10,15,20-tetra(4-pyridyl)
porphyrin (TPyP) were developed for portable applications. A
sulphonation degree of 65% and different weight percentages
(0e5%) of TPyP porphyrin were used for membranes realization. A complete chemical-physical and electrochemical
characterization was carried out. The TPyP introduction does
not modify the structural and morphology properties of the
polymer while some changes occurs in the IEC, water uptake,
dimensional variation and lambda values. This behaviour is
attributed to the formation of interactions between the sulphonic groups of the polymer and the nitrogenous group of
the TPyP. This interaction with a hydrogen sieve material such
as TPyP, produces a not aggregated form of the TPyP but a
good interaction with the polymeric matrix with a network
formation. The proton conductivity of the membrane with the
lowest loading (0.77wt% TPyP) does not change respect the
pristine SPEEK reference, despite a reduction of water uptake
and area % and is maintained in the same order of magnitude
in dry conditions. In addition, during fuel cell operation at low
temperature, pressure and dry hydrogen, this sample supplies
the best power density. The properties of this membrane
configuration renders it a good candidate for a real application
in portable devices.
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