Solid polymer proton conducting electrolytes for fuel cells
C. de Bonis1, A. D’Epifanio1, B. Mecheri1. S. Licoccia1, A.C. Tavares2
1Department
of Chemical Science and Technology & NAST Center, University of Rome Tor
Vergata, Via della Ricerca Scientifica, 00133 Rome, Italy
2Quebec
Center for Functional Materials, Institut national de la recherche scientifique - Énergie
Matériaux Télécommunications (INRS-EMT), 1650, Boulevard Lionel-Boulet, Varennes
(Québec), J3X1S2, Canada
1. Introduction
There are numerous practical applications based on electrochemical cells that use solid
electrolytes. These include batteries, fuel cells, sensors, electrolysers, water purification,
electrodialysis and seawater desalination 1-7.
An electrochemical cell is a device capable of producing electrical energy from
spontaneous chemical reactions (G<0), or driving chemical reactions through an external
source of electrical energy (G>0). Electrochemical cells contain two electrodes (the anode and
the cathode) and an electrolyte between them. The driving force of an electrochemical cell is the
reactions at the anode (oxidation) and at the cathode (reduction). Oxidation refers to the loss of
electrons by a chemical specie (reductant) and reduction to the gain of electrodes by a chemical
specie (oxidant). In an electrochemical cell the electronic current flowing outside the cell equals
the ionic current flowing in the cell 8.
An electrolyte is a material able of conducting ions and usually is an electrical insulator8.
Electrolytes can be solutions (e.g. KOH, H2SO4), molten salts (e.g. Li2CO3), solid ion conducting
polymers
(e.g
perfluorinated
polymers
bearing
sulfonic
acid
groups
or
benzyltrimethylammonium groups) and ionic crystals (e.g. ZrO2:Y2O3, Na3Zr2PSiO12).
Solid electrolytes conducting O2-, H+, Li+, Na+, Ag+, F-, Cl-, OH- ions have been reported
for many years now. The conductivity range is typically 10-3 S/cm <  < 10 S/cm depending on
the material structure and operating temperature. But for comparison purposes, at room
temperature, the conductivity of a solid electrolyte is inferior to that of liquid electrolytes. For
1
example, (KOH) =0.6 S.cm-1 (30%, 20oC), (H2SO4) =0.82 S.cm-1 (5.2M, 20oC) and (Nafion)
0.07 S.cm-1 (fully hydrated, 20oC).
Solid electrolytes can be used in electrochemical cells as ion exchange membranes to
allow the passage of ionic current between the anode and the cathode placed on opposite sides of
the electrolyte, or in the electrodes when mixed (electronic and ionic conductor) conductivity is
needed5,9. The ionic conductivity  is given by:
𝜎𝑖 = 𝑧𝑖 ∙ 𝐶𝑖 ∙ 𝜇𝑖
(1)
Where zi is the ion charge, Ci is the density of mobile ions and i the mobility of the ions. Thus, a
solid electrolyte has a large number of mobile ions. The ion conductivity is an activated
transport; therefore it increases exponentially as temperature increases:
𝐸
𝜎 = 𝐴𝑒𝑥𝑝(− 𝑅𝑇𝑎 )
(2)
Where A is a proportional constant, Ea is the activation energy, R is the universal gas constant
8.314 J·mol-1.K-1) and T the temperature (in K).
There are two main broad classes of solid electrolytes: crystalline (or ionic) solids and ion
conducting polymers. In crystalline solid electrolytes, ion conductivity occurs by means of ions
hopping through energetically equivalents sites in the crystal structure. High conductivity
requires a large number of mobile ions, or on other words, a large number of accessible empty
sites, either vacancies or interstitial sites. A practical way to increase the density of mobile ions
is by doping the crystalline solid with heterovalent ions forming solid solutions. For example,
replacing three Li+ ions by one Al3+ ions in Li4-3xAlxSiO4 to generate cation vacancies, or by
replacing Zr4+ ions by Y3+ ions in yttria stabilized zirconia to generate anion vacancies. The
activation energy controls the ion mobility. The empty and occupied sites should have similar
potential energies with a low activation energy barrier for ion hopping between neighboring
sites. The ion mobility is thus related to the crystal structure. Ionic solids with a densely packed
crystal structure are characterized by large activation energy (1 eV or higher) and low
conductivity. Ionic solids such as -AgI, RbAg4I5 and Na -Al2O3 known as fast ion conductors,
2
are formed by solid frameworks with open conduction pathways and are characterized by low
values of activation energy, for example 0.03 eV for AgI above 420K. Practical applications of
this class of compounds include, for example, ion selective electrodes (Ag2S for Ag+ and LaF3
for F-), molten salts electrochemical cells (Na -alumina in ZEBRA batteries), oxygen anion
conductors (yttria stabilized zirconia) for solid oxide fuel cells and for oxygen sensors.
Solid polymer electrolytes (SPE) consist of polymer backbones functionalized with high
concentration of fixed ionic charges. The function of the SPE is determined by the charge of the
ion-exchange groups and the nature of the counter ions, and can be classified accordingly10:
a) cation exchange membranes: have anionic charged groups (-COO-, -SO3-, etc.) and cations can
selectively permeate through them;
b) anion exchange membranes: have cationic charged groups (e.g. -NR3+) and anions can
selectively permeate through them;
c) amphoteric ion exchange membranes: contain randomly distributed cationic and anionic
functional groups;
d) bipolar ion exchange membranes: bi-layer membranes with a cation exchange membrane layer
and anion exchange membrane layer;
e) mosaic ion exchange membranes, which have separate domains with cationic and anionic
groups.
But types a) and b) are those used in industrially.
Because of the presence of ionic groups, ion exchange membranes adsorb water
molecules in an extent that depends on the surrounding relative humidity. The electrical
conductivity of ion exchange membranes depends on the concentration, size and charge of the
ions, as well as on the water content, chemical structure and morphology of the membranes. In
particular the mobility of the ions depends on its charge density and on its degree of solvation11.
Ion exchange membranes are used in dehumidification of gases, humidity sensors,
actuators, pervaporation, facilitated transport and in electrochemical processes such as
electrodialysis, brine electrolysis, redox flow vanadium batteries and solid polymer electrolyte
fuel cells10. A significant amount of the work on SPEs is relevant to proton exchange membrane
fuel cells, therefore we targeted this application for this book chapter. Emphasis is given to SPEs
functionalized with sulfonic acid groups.
3
2. Proton exchange membranes
A fuel cell is an electrochemical device that converts the energy of a fuel into electricity,
and those using proton exchange membranes (PEM) are among the most promising ones because
of their potential application in portable electronics, stationary and automotive. They operate
between room temperature and 140°C, can use hydrogen, methanol or other liquid fuels. During
operation the fuel is oxidized at the anode generating protons and electrons. The electrons flow
from the anode to the cathode through the external circuit whereas the protons cross the
electrolyte membrane to reach the cathode. The oxygen reduction reaction takes place and water
is produced:
Anode:
H 2 → 2 H + + 2 e−
(3)
Cathode:
2 O2 + 2 H + + 2 e− → 2 H2 O
(4)
Overall:
H2 + 2 O2 → 2 H2 O
(5)
The proton exchange membrane is the core component of PEM fuel cells. To achieve
high efficiency, the membrane must possess the following features: i) high proton conductivity to
support high current with minimal resistive losses; ii) low permeability to reactants; iii) chemical
and electrochemical stability under operating conditions; iv) adequate mechanical strength and
stability; and v) production costs compatible with the intended application.
PEMs can be classified according to their polymer backbone as hydrocarbon membranes,
partially halogenated hydrocarbon membranes, and perfluorocarbon membranes. The most
common cation exchange group used for fuel cells applications is the sulfonic acid group
because it is a very strong acid (apparent pk -6 for –CF2SO3H and 0-1 for alyl/alkyl – SO3H),
although phosphonic acid (pk1=2-3; pk2=7-8) and imidazole are also protogenic groups of
potential interest for operating temperatures above 100C and low relative humidity (RH)10,12.
PEMs rely on the mobility of protons in the aqueous network formed inside the solid
polymer13. Proton transport proceeds through the membrane following two main mechanisms14.
The first is the vehicle mechanism where the proton diffuses together with the vehicle water. The
counter diffusion of unprotonated water allows the net transport of protons. Therefore, the
4
observed conductivity depends on the rate of vehicle diffusion and it can be expressed as a
function of water self-diffusion coefficient (DH2O) that represents a measure of the average
mobility of water in the membrane. The other mechanism is known as Grotthuss mechanism or
“proton hopping” or “structure diffusion”. In this process, the water molecules show pronounced
local dynamics but reside on their sites. The process consists of two steps: (1) proton transfer
from one water molecule to the other by hydrogen bonds; (2) consequent reorientation of water
dipoles that results in the formation of an uninterrupted trajectory for proton migration. The
schematic description of two typical proton conduction mechanisms is shown in Figure 1. The
prevalence of one or the other mechanism depends on the hydration level of the membrane, and
it has been suggested that proton hopping is more significant at high water contents15. The
activation energy for proton conduction in SPEs depends on the water content and typically
decreases from 0.4-0.5 eV for dry membranes (contain only residual water molecules) and 0.1
eV for fully hydrated and swollen membranes16.
Figure 1 - Simplified scheme of the proton transfer in Nafion by the Grotthuss mechanism (solid
lines) and the vehicle mechanism (dotted lines). Adapted from /Reprinted with permission from
[Choi P, Jalani N H and Datta R 2005 Thermodynamics and proton transport in Nafion II. Proton
diffusion mechanisms and conductivity J. Electrochem. Soc. 152 E123–30]17.
5
2.1. Nafion®
The key ionomer currently used in PEM fuel cells applications is Nafion® which is
produced by DuPont. Nafion® is a perfluorosulfonic acid polymer (PFSA). It has a
polytetrafluoroethylene (PTFE) backbone, which confers high chemical inertness, with the side
chains consisting of perfluorinated vinyl polyether ending in sulfonic acid groups, -SO3H, that
give proton exchange capability to the polymer. The chemical structure is shown in Figure 2,
where the values of n, x and y can be varied to produce materials with different equivalent
weights.
Figure 2 – Chemical structure of Nafion® ionomer (downloaded from commons.wikimedia.org Nafion structure.png).
Nafion®, shows excellent proton conductivity (0.09 to 0.12 Scm-1 at 80C and RH
between 34 and 100% RH18) and mechanical strength, as well as high thermal and chemical
stability. Nafion’s structure as function of water content has been the topic of many
investigations and it has been investigated by swelling studies, infrared spectroscopy, small angle
x-ray scattering, and transmission electron microscopy, to name a few19. These studies have
shown that a hydrated membrane contains two phases, an ionic phase that is associated with the
hydrated sulfonic acid groups, and a non-ionic phase that is the perfluorinated matrix. The actual
form of the phases depends on the water content. Several models have been proposed since the
early 1970s, to predict ionic transport properties of Nafion describing the way in which ionic
groups aggregate. These models include the Mauritz-Hopfinger Model20, the Yeager Three Phase
Model21, and the Gierke Cluster Network Model22 . In the cluster network model proposed by
Gierke and Hsu, the structure is an inverted micelle in which the ion-exchange sites are separated
from the fluorocarbon backbone thus forming spherical clusters, connected by short narrow
channels, Figure 3.
6
Figure 3 - Gierke’s cluster network model of Nafion membranes “Reprinted from Journal of
Membrane Science, 13 /3, William Y. Hsu, Timothy D. Gierke, Ion transport and clustering in
Nafion perfluorinated membranes, 101-105, Copyright (1983), with permission from Elsevier.”22
Thus, with increasing water content the clusters grow and form transitory
interconnections with each other. This network of collapsed channels leads to a percolation-type
phenomenon. Gierke and Hsu also used the percolation theory to correlate the electrical
conductivity with the water content of the membrane, expressed as λ, i.e. the number of water
molecules per sulfonic group. According to this theory, there is a critical amount of water
available in the membrane below which ion transport is extremely difficult due to the absence of
extended pathways. The percolation threshold in Nafion is around λ = 2
20
as shown in Figure 4
where conductivity data is plotted against . At low hydration level, i.e. λ in the range 1-2, it is
reasonable to consider that all water molecules absorbed by the Nafion membrane are associated
with the sulfonate heads because of the hydrophobic nature of the backbone and the hydrophilic
nature of the sulfonic groups. Moreover, hydronium ions will be localized on the sulfonate heads
and the conductivity will be extremely low being the amount of water absorbed insufficient for
the formation of a continuous water phase
23
. For λ in the range 3-5, the counterion clusters
continue to grow and, as λ approaches 5, the membrane becomes more conductive because some
counterion clusters may connect, but there is still insufficient water for all clusters to coalesce.
Molecular dynamics simulations indicate that 5 water molecules form the primary hydration
shell for the sulfonic groups and any additional water molecules are not as strongly bound and
thus form a free phase24. For λ ≥ 6, counterion clusters coalesce to form larger clusters and
eventually a continuous phase is formed, and the conductivity threshold is overcome.
7
Figure 4 Variation of the proton conductivity of Nafion as a function of the water content in the
membrane. “Reprinted from L. Carrette L, K.A. Friedrich, U. Stimming, Fuel Cells:
Fundamentals and Applications, Fuel Cells, 1(1), 5-38, Copyright (2001), with permission from
John Wiley and Sons.”4..
Because the proton’s mobility relies on the formation of a continuous aqueous network
inside the ionomer, proton conductivity shows a strong dependence on the hydration level of the
PEM as shown in Figure 4 for Nafion. A humidification system is necessary to keep the
membrane hydrated during fuel cell operation, which represents a major cost of the fuel cell
system25.
Whereas water sorption improves proton conductivity, it also leads to morphological
instability and, at elevated water content, to membrane swelling26. The maximum working
temperature of all Nafion-based fuel cells is limited to 80-90 °C due to the loss of membrane
mechanical strength determined, at higher temperature and large hydration, by the plasticizing
effect of water. Moreover, under dynamic conditions, swelling cycles contribute to mechanical
fatigue.
In fact, one of the key challenges in the design of proton exchange membranes is to retain
high conductivity at low water content3, especially at high temperature25. Overall, operation at
high temperature (> 100 C) is desirable to reduce PEM fuel cells costs and promote its large
scale commercialization: it enhances the reaction kinetics at both electrodes and thus it reduces
the catalyst loading on both electrodes, allows a more efficient utilization of the waste heat, and
simplifies the water and thermal management systems25.
8
Nafion membranes possess an additional major hurdle that inhibits the large scale
commercialization of fuel cells operating with liquid fuels. The unique microstructure of Nafion
ionomer results in a high crossover rate of liquid fuels from the anode to the cathode through the
membrane27. This not only lowers the fuel utilization at the anode but also increases the
overpotential of the cathode, hence lowering the cell performance.
All these drawbacks essentially imply that the Nafion membrane cannot be used "as is"
for fuel cell applications in a wide range of temperature, relative humidity (RH) and liquid fuels.
Substantial effort is being made to develop membranes with appropriate electrochemical, and
other physico-chemical properties at the operating conditions28-30. Different approaches are being
pursuit and include (i) the development of alternative ionomers based on non perfluorinated
polymers31,32, on polyarylene or on aliphatic main chains33-38, (ii) modification of existing
ionomer membranes through the formation of blends and composites29, (iii) synthesis of new
hybrid systems39,40.
2.2 Alternative sulfonated ionomers and membranes
Among the polymers alternative to Nafion, arylene main-chain polymers, such as
poly(ether ketone), poly(ether sulfone), poly(benzimidazole) and poly(phenylene sulfone) as
shown in Figure 5, have been widely investigated34-36,41-44.
(a)
(b)
(c)
9
Figure 5 – Chemical structure of (a) Polyetheretherketone
(http://commons.wikimedia.org/wiki/File:Polyetheretherketone01.png / public domain) ; (b)
Udel Polysulfone (http://commons.wikimedia.org/wiki/File:Polysulfone.svg); and (c) Poly(2,2’m-(phenylene)-5,5’-bibenzimidazole) (adapted from
http://en.wikipedia.org/wiki/File:PolybenzimidPhOester.png (licensing by Creative commons)).
Such polymers are inexpensive and possess high chemical and mechanical stability at
temperatures higher than 90 – 100 C34-36. Moreover, their aromatic structure offers the
possibility of electrophilic and nucleophilic substitutions, to prepare ionomers with desired
features for PEMFC and DMFC applications45. The most important modification regards the
introduction of sulfonic acid moieties to obtain proton-conducting aromatic polymers. Several
methods have been developed to prepare proton-conducting electrolytes, including direct
sulfonation of a polymer backbone, total synthesis from monomer building blocks, and grafting
of functional groups onto a polymer main chain46 In general, the larger the number of sulfonic
acid groups per structural unit, the larger the membranes’ ionic exchange capacity and water
uptake and higher their conductivity. Nevertheless, excessive swelling of the membranes could
lead to a dilution of the charge carriers and to a lower proton conductivity33.
Some variations on Nafion’s hydration scheme are expected for sulfonated polyarylene
membranes. Sulfonated polyarylenes with sulfonic acid groups bound directly to the aromatic
chain have less pronounced hydrophobic/hydrophilic separation with respect to Nafion because
their backbones are less hydrophobic and flexible, and their sulfonic acid groups are less acidic
and therefore, also less polar. As a consequence, narrower channels and a less-connected
network of clusters are present in sulfonated polyetherketones microstructure, resulting in a
higher dependence of the transport properties on water content due to percolation concepts27 A
schematic representation of the microstructure of sulfonated polyetherketone compared with that
of Nafion is reported in Figure 6. As illustrated in Figure 7, high conductivity levels are
achieved only with a high degree of sulfonation resulting into low mechanical properties and
high rate of methanol crossover due to excessive swelling47.
10
Figure 6 - Schematic representation of the microstructures of Nafion and a sulfonated
polyetherketone “Reprinted from Journal of Membrane Science, 185, K.D. Kreuer, On the
development of proton conducting polymer membranes for hydrogen and methanol fuel cells,
29-39, Copyright (2001), with permission from Elsevier.”27
11
Figure 7. Proton conductivity measured at room-temperature proton conductivity of two Dow
membranes, Nafion, two sulfonated poly(arylene ether ketone)s (SPEK and S-PEEKK), and
sulfonated poly(phenoxyphosphazene) (S-POP) as a function of the degree of hydration n; the
number below the compound acronym/ name indicates the equivalent weight of the ionomer).
"Reprinted with permission from Kreuer K-D, Paddison SJ, Spohr E, Schuster M. Transport in
Proton Conductors for Fuel-Cell Applications: Simulations, Elementary Reactions, and
Phenomenology. Chemical Reviews 2004;104:4637-78. Copyright (2004) American Chemical
Society." 47.
Several strategies have been used to overcome the excessive swelling of highly
sulfonated polyarylenes. These include the synthesis of aromatic polymer chains cross-linked
covalently by organic spacers such as α,ω-dihalogenoalkanes48, the use of partially fluorinated
backbones31, placing the protogenic groups on short pendant side chains to increase the
separation between the polymer main chains and the sulfonic acid groups49, or building multiblock
copolymers
using
coupling
reactions
between
hydrophilic
and
hydrophobic
macromonomers29. Polymers with pendant sulfonic acid groups in side chains are in general
more stable against hydrolysis than those with sulfonic acid groups attached directly on the
polymer backbone. In addition, sulfonic acid groups on pendant side chains have a higher degree
of freedom which results in a better phase separation and higher proton conductivity with respect
to the random sulfonated analogues50,51.
12
Hydrophilic-hydrophobic multiblock copolymers are considered an interesting step
forward in the rational design of PEMs. An ideal morphology has been pursued by controlling
the microphase separation in segmented block copolymers where hydrophilic sulfonated polymer
segments form an interconnected 3D- network responsible for efficient proton transport
especially at low relative humidity52-54, while a complementary network of hydrophobic nonsulfonated segments cause a reinforcing effect, preventing excessive swelling in water and
enhancing mechanical properties55,56 Their proton and water transport increase significantly with
increasing block length because the longer block induces a more developed phase separation57.
However, their synthesis is often complex thus increasing the materials cost. Overall, synthetic
approaches based on structure-property relationships of ionomers, represent a very promising
way to obtain more efficient proton-conducting membranes having the desired features for fuel
cell applications31,57,58
Anhydrous proton-conducting electrolytes consist of a more or less inert polymer matrix
that is swollen with an appropriate proton solvent, usually phosphoric acid. These membranes
are appealing for fuel cell operation at temperatures well above 100 °C without the need of
humidification. One of these is poly(2,2’-m-(phenylene)-5,5’-bibenzimidazole) (PBI) which
structure is reported in Figure 8. Non-modified PBI shows very low proton conductivity. Hence,
it is necessary to dope the polymer with sulfuric or phosphoric acid to increase its proton
conductivity59,60. However, acid leaching from the membranes and corrosion of cell components
are some of the problems limiting the performance of fuel cell devices based on such
membranes.
Alternative concepts use amphoteric heterocycles such as imidazole as proton conducting
species ”imbibed” in a polymer matrix. Proton transport occurs through heterocyclic hydrogenbonded networks under both anhydrous and low relative humidity conditions. As for sulfonic
acid based ionomers the ion conductivity depends on the local mobility of the heterocycles
within the polymer films and on the effective concentration of mobile protons in the
membranes12,60-64. The proton conductivity of these systems increases with the addition of strong
acids are added due to the protonation of some of the heterocycles within the polymer matrix64,65.
Recently, ionic liquids have also been proposed for high temperature proton conductors
mainly due to their anhydrous high conductivity and good thermal stability. Nevertheless, the
13
conductivity of ionic liquid based composite membranes is lower than that of the original ionic
liquids. Therefore, only a few groups have reported demonstrations of the ionic liquid based
solid membrane electrolytes in fuel cells66-68.
The composite strategy, where an inorganic phase is dispersed within the ionomeric host
has demonstrated to be another effective way to improve the transport and mechanical properties
of ionomers. Several advantages can be obtained by using composite membranes, such as: i)
improving the self-humidification of the membrane at the anode side; ii) suppressing the fuel
crossover, e.g. methanol in DMFC; iii) improving the mechanical strength of membranes without
excessively sacrificing proton conductivity39,40,69. The solid inorganic compounds can be
classified as inert hygroscopic fillers, proton conductive fillers, hydrophilic and proton
conductive fillers and include: hygroscopic oxides (SiO2, TiO2, SnO2), clays, zeolites, heteropoly
acids, zirconium phosphonates39,70-73. For example, i) hygroscopic fillers e.g. SiO2, TiO2, SnO2
and zeolites, improve the water retention and the dimensional stability of the membranes74,75; ii)
the operation of fuel cells fed with liquid fuels was sucessfully extended to high temperature76,77;
and iii) Beta and Faujasite zeolites improved the proton conductivity and the DMFC
performance of Nafion73,78.
Composite membranes containing exfoliated layered compounds or 1D structures such as
nanotubes or nanorods as fillers are also an effective strategy to improve relevant properties of
electrolytes76,79. The presence of one and two-dimensional nanomaterials, which have
substantially different properties with respect to those of nanometric spherical particles, can
enhance the mechanical strength while acting as a physical barrier to fuel crossover76,80,81.
Performance of composite electrolytic membranes is in fact strongly related to the
polymer/inorganic phase interfacial properties. In detail, the higher the interface interaction
between the polymer and the dispersed particles, the greater is the filler influence on the original
characteristics of the polymer82,83 .
Composite membranes are in general prepared by casting the polymer solution with an
inorganic component. The main disadvantage of such composite systems is related to the fact
that it is very difficult to obtain homogenous systems, where the inorganic particles are well
dispersed in the polymeric matrix. Therefore, in-situ sol-gel synthesis of the inorganic filler in
14
the hydrophilic clusters of the PEM when applicable is a preferred alternative to nanocasting
method73,76 .
Recent reports76,83,84 have succeeded in their endeavour to identify under which
conditions inorganic-organic membranes provide properties superior to those shown by their
polymer-only counterpart, and there is every reason to be optimistic that MEAs based on
nanocomposite membranes have a role to play in liquid feed fuel cells or in the highly strategic
operation conditions of low RH at 110-130 °C. Current hurdles persist: membrane electrical
resistance and long-term durability under fuel cell operation. To boost the membranes’
conductivity, surface functionalization of the inorganic fillers with protogenic groups is being
exploited73,85-87. But, in-depth studies of ageing and degradation under realistic operation
conditions are still needed to enable the synthesis of more advanced materials and alignment
with current targets.
3. Characterization of solid polymer electrolytes
The performance of H2/O2 and liquid feed fuel cell is strongly influenced by the proton
and water transport properties of the PEM. The fuel cell ohmic loss is proportional to the ionic
resistance of the PEM and high conductivity is essential to assure the required performance.
Water molecules in the membrane increase the proton mobility according to the vehicle
mechanism, but a high water uptake by the membrane decreases the density of sulfonic acid
groups or charge carriers88. Therefore, changes in the water content and water mobility have an
impact on the proton conductivity of the membranes78,88
This section provides a short description and application of complementary
characterization tools (proton conductivity measurements, dynamic vapor sorption and
differential scanning calorimetry) used to assess transport properties of PEMs. Although these
are the first properties to be considered when evaluating PEMs for potential use in fuel cells, it
should be stressed that other chemical, morphological, mechanical and thermal properties are
also critical to define the “ideal” electrolyte for fuel cell applications and to study structure–
property relationships. These properties can be studied by means of several characterization
15
techniques including bulk chemical analysis and ion exchange capacity, thermal gravimetric
analysis, transmission electron microscopy, small-angle X-ray scattering, tensile tests, dynamic
mechanical analysis, fuel cell life and Fenton’s tests28,89-91.
3.1
Proton conductivity
The membrane’s proton conductivity is determined by measuring its resistance against
the flow of a direct current or an alternative current at controlled temperature and hydration
level. The conductivity  is calculated through the equation:
𝑙
σ = 𝑅.𝑆
(6)
where l is the distance between the two probe electrodes and S the cross-sectional area of
the membrane.
In a dc method the potential difference across two probe electrodes in contact with the
membrane follows the Ohm’s law over a wide range of current densities, and the resistance can
be determined from the slope of the line E vs j. In the ac method, a periodic small-amplitude ac
signal (voltage or current) is applied and the associated response (current or voltage) coming
from the cell is measured92. The voltage response to a sinusoidal current signal is a sinusoid, at
the same frequency () but shifted in phase (φ):
𝑖𝑡 = 𝑖0 sin(𝜔𝑡)
(7)
𝐸𝑡 = 𝐸𝑖0 sin(𝜔𝑡 + 𝜑)
(8)
The impedance Z is defined as the ratio of the voltage to the current at a given frequency:
𝑍 =
𝐸𝑡
𝑖𝑡
=
E0 sin(𝜔𝑡+𝜑)
(9)
𝑖0 sin(𝜔𝑡)
By applying Euler’s relationship the impedance can be expressed as a complex function,
with a real and imaginary part:
16
𝑍 = 𝑍0 (𝑐𝑜𝑠𝜑 + 𝑗𝑠𝑖𝑛𝜑)
(10)
In an electrochemical system, slow kinetics reactions and diffusion of chemical species
can impede the electron flow. Electrochemical systems can thus be considered analogous to the
resistors, capacitors, and inductors that hinder the flow of electrons in an electrical circuit. In the
case of a simple resistor, the phase shift is zero degrees and the current is in phase with the
voltage. Thus, according to equation 9, the impedance is purely real and independent of the
frequency. For an ohmic resistance Zt=R. Figure 8 shows typical impedance data in the form of
Bode plot obtained for a Nafion 117 membrane using a four probe cell (see below) at 100%
relative humidity. First, the frequency region over which the impedance has a constant value is
identified and the impedance value takes to calculate membrane’s conductivity using equation 6
93
.
Figure 8 – Impedance data recorded for a Nafion 117 membrane at 100% RH. Data was
acquired in the in-plane plane direction and using a four-probe cell93 [Sone et al Sone et al J.
Electrochemical Society, 143 (1996) 1254].
Electrochemical impedance spectroscopy is the most commonly used method to measure
the membranes resistance and to determine its proton conductivity. It is a rapid and accurate
17
method and quite suitable for dielectric materials such as PEMs. The dc method has been
nevertheless used72,94 being the major advantage of this method the straightforward analysis of
the E-j data.
The membrane’s conductivity can be measured perpendicular to the membrane’s
thickness (through-plane conductivity) or along the plane of the membrane (in-plane
conductivity). In addition, measurements can be done using either the four-probe or the twoprobe method. In fact, there is no standard method for measuring the proton conductivity of
ionomers and each method / cell configuration has its own advantages and disadvantages. Figure
9 illustrates some of the conductivity cells reported in the literature.
c
d
Figure 9 – (a) In-plane and (b) through-plane two probe conductivity cells; (c) In-plane
and (d) four-probe conductivity cells95. [(a) and (b) “Reprinted from Journal of Electroanalytical
Chemistry, 622, Soboleva T, Xie Z, Shi Z, Tsang E, Navessin T, Holdcroft S. Investigation of
18
the through-plane impedance technique for evaluation of anisotropy of proton conducting
polymer membranes, 145-152, Copyright (2008), with permission from Elsevier.” 94 [Slade et al
Journal of The Electrochemical Society, 149 (2002) A1556-A1564 for (d)].
During the fuel cell operation, protons move through the cross section of the membrane
thus measurements done under this configuration are more relevant for the practical application.
However, in this configuration, the area of the electrodes ( cm2) is much larger than the distance
between them (given by the membrane thickness,  micrometers), so the cell constant (l/S) is
small and the contribution from the interface formed between the membrane and the electrodes is
large. On the contrary, in the in-plane measurements the cell constant is larger because the
distance between the electrodes is in the order of mm to cm, and the section under interest is the
cross-section of the membrane. The bulk conductivity of the membrane is the dominant element
contributing to the measurement
95
. Conductivity measurements on both directions are
nevertheless important to quantify the effect of morphological anisotropy of PEMs on their
proton conductivity 95-97.
In the two-probe method, the voltage drop is measured across the same two electrodes
where the current flows. Accordingly, the measured impedance (or resistance) includes the
contribution of all components in the current pathway. When determining for example the
membrane resistance from the total cell impedance, all other contributions such as electrodes’
resistance, leads inductance and membrane – electrodes contact resistance should be subtracted
from the total cell impedance95,98. This is done by recording the impedance of the short circuited
and open cells95. With the four-probe method only the bulk membrane resistance is measured
because two different pairs of electrodes are used, and the current flowing and the voltage
sensing are done independently. The current is imposed to the external pair and the voltage drop
along the membrane sectional area is measured using the central pair of electrodes. The effect of
the contact resistance is clearly seen in Figure 10 that shows the variation of the in-plane proton
conductivity of a Nafion 112 membrane exposed to 95% RH and immersed in liquid water, as a
function of the torque applied to the electrodes, for a four-probe and a two-probe configuration.
To a higher torque corresponds a lower interface resistance between the membrane and the
19
electrodes, and the influence of this addition resistance is more important when the conductivity
of the membrane is lower (less hydrated membrane)98.
Figure 10 – Effect of applied torque on the measured proton conductivity of Nafion 112 by fourprobe (,) and the two-probe (,) configurations at 95% RH and 60 °C, and by four-probe ()
and two-probe () methods in the liquid-water state at 60 °C. "Reprinted with permission from
Lee CH, Park HB, Lee YM, Lee RD. Importance of Proton Conductivity Measurement in
Polymer Electrolyte Membrane for Fuel Cell Application. Industrial & Engineering Chemistry
Research 2005;44:7617-26. Copyright (2005) American Chemical Society."98.
Measurements of ion conductivity over a wide range of temperature and relative humidity
are important to conclude on the effect of composition and structure of the new ionomers on the
proton conduction and operational temperature. An example is given in Figure 11 that illustrates
the effect of the filler content and composition on the proton conductivity of Nafion composite
membranes containing TiO2 and propyl sulfonic acid functionalized TiO2 nanoparticles87. The
loading of an appropriate amount of propylsulfonic-functionalized titania allows the preparation
of Nafion-based composite membranes with higher conductivity and dimensional stability than
pristine Nafion up to 140 C.
20
Figure 11 – Arrhenius plot of Nafion, Nafion-TiO2 and Nafion - propyl sulfonic acid
functionalized TiO2 at 100% RH; the numbers in the legend indicate the wt% of filler with
respect to Nafion. (“Reprinted from Journal of Power Sources, 248, Cozzi D, de Bonis C,
D'Epifanio A, Mecheri B, Tavares AC, Licoccia S. Organically functionalized titanium
oxide/Nafion composite proton exchange membranes for fuel cells applications, 1127-1132,
Copyright (2014), with permission from Elsevier) 87
3.2
States of water and water mobility
A critical parameter affecting the performance and proton conduction mechanism of
PEMs is their hydration level and water diffusion coefficient as a function of the water content.
Hence, it is important to study water sorption and diffusion behavior of electrolytes over a wide
range of relative humidity.
3.2.1 Dynamic vapor sorption
Dynamic Vapor Sorption (DVS) is a gravimetric technique which allows the fast and
accurate determination of vapor sorption isotherms and diffusion kinetics. A simplified scheme
of the DVS apparatus is shown in Figure 12.a: the samples are placed in a weighing pan and
21
exposed to a partial pressure- and temperature-controlled environment. To study water
management in solid electrolytes, water is used as sorbate and the electrolyte as sorbent. The
vapor partial pressure around the sample is controlled by mixing saturated and dry carrier gas
steams using electronic mass flow controllers. The temperature is maintained constant, by
enclosing the entire system in a temperature-controlled incubator. By measuring the change in
mass as a function of time up to the equilibrium, a typical diagram shown in Figure 12.b is
obtained.
(a)
Figure 12.
(b)
Illustration of the DVS apparatus interfaced with a personal computer
(http://www.thesorptionsolution.com/Products_DVS_Advantage_Instrument.php;
Reprinted
with permission from Surface Measurements Systems” (a) and kinetics of water adsorption of a
typical Nafion membrane at 25°C and different partial pressures (b).
The amount of water uptake (WU) by the sample exposed to a defined partial pressure
can be thus obtained using equation 11:
WU aw 
m(eq ) aw  mdry
(11)
mdry
where m(eq)aw is the mass of the sample at the equilibrium at a defined water activity (aw)
and mdry is the dry mass of the sample.
A sorption isotherm is the graphic representation of WU values: it describes the
relationship between the water content of the electrolyte and water activity at constant
22
temperature. Water is a small molecule and polar adsorptive therefore its adsorption mechanism
is influenced by water’s affinity to the adsorbent’s surface. Hence, the shape of the isotherm also
reflects the hydrophilicity/ hydrophobicity of the surface. IUPAC proposed a classification for
water sorption isotherms as illustrated in Figure 13 99,100.
Each isotherm shape is ascribed to a material with specific hydrophilic characteristics.
Type I is characteristic of very hydrophilic materials. Type II and type IV isotherms are
characteristic of moderate hydrophilic materials. Adsorbents showing a type IV are hydrophilic
as well. Adsorbents with low hydrophilicity will give rise to a type III and type V isotherms. The
type VI is typical of hydrophilic material with multiple sorbent–water interactions and stepwise
sorption while type VII isotherm is characteristic of very hydrophobic materials.
Figure 13.
IUPAC classification of adsorption isotherms for materials with different
hydrophilicity. (Adapted from Microporous and Mesoporous Materials, 114, Ng, E.; Mintova S.
Nanoporous materials with enhanced hydrophilicity and high water sorption capacity, 1-26,
Copyright (2008), with permission from Elsevier) 100
3.2.1.1.
Determination of the diffusion coefficients from DVS measurements
DVS measurements allow evaluating the water diffusion coefficient through electrolyte
materials. Assuming that the water sorption can be described by a fickian behavior, the water
diffusion coefficient, D, can be calculated from the relation between mass variation and the time
23
of water vapor exposure up to equilibrium
101
. This relation is obtained by combining the first
Fick’s law (Eq. 12) (describing the transfer of solute atoms per unit area in a one-dimensional
flow) and the conservation of mass relationship (Eq. 13) and expressed by the second Fick’s law
(Eq. 14):
J  D
C
x
(12)
C
J

t
x
(13)
C
2 J
 D 2
t
x
(14)
where J is the amount of substance flowing per unit area per unit time, C is the
concentration, and x is the position. Assuming a constant diffusivity and that the water activity is
constant across the membrane/vapor interface (c=c∞ at x ± d/2), the solution of the equation (14)
gives the normalized mass change as a function of the time:
Mt
4

M d
D t
(15)

where Mt is the amount of water adsorbed at time t, M∞ is the amount of water adsorbed
at equilibrium and d is the sample thickness.
By plotting Mt/M∞ for a sample exposed to a certain partial pressure P/P0 (i.e., water
activity) as a function of the square root of time (Figure 14) and by fitting the curve to the
equation 15, D can be obtained. This equation is valid for values of Mt/M∞ < 0.4, where the plot
of Mt/M∞ against t1/2 is linear102,103.
24
Figure 14.
Typical plot of Mt/M∞ versus t1/2 at a given value of water activity (aw). "Adapted
with permission from Mecheri B, Felice V, Zhang Z, D’Epifanio A, Licoccia S, Tavares AC.
DSC and DVS Investigation of Water Mobility in Nafion/Zeolite Composite Membranes for
Fuel Cell Applications. The Journal of Physical Chemistry C 2012;116:20820-9. Copyright
(2012) American Chemical Society." 78
Figure 15 shows the water diffusion coefficient values measured by DVS at 25 C of a
Nafion membrane as a function of the water activity. As shown in the Figure, D increases with
water content in the membrane at low aw and reaches a maximum in the 0.3 to 0.4 aw range. The
increase in D in this aw range is due to the fact that water is less tightly associated with the
sulfonic acid sites of Nafion as water content increases. At higher water activities, D decreases
with increasing aw due to the occurring of a water aggregation process that provides kinetic
limitations of the adsorption of water on the polymer matrix78.
25
Figure 15. Diffusion coefficient (D) values of a recast Nafion membrane as a function of water
activity at 25 °C. "Adapted with permission from Mecheri B, Felice V, Zhang Z, D’Epifanio A,
Licoccia S, Tavares AC. DSC and DVS Investigation of Water Mobility in Nafion/Zeolite
Composite Membranes for Fuel Cell Applications. The Journal of Physical Chemistry C
2012;116:20820-9. Copyright (2012) American Chemical Society." 78
3.2.1.2 Determination of the different states of water
From the water sorption measurements it is also possible to obtain information about
water mobility (and consequently the proton transport) by investigating the state of water in
electrolytes. In fact, specifically designed models can be applied to the sorption isotherms in
order to get insights on water transport properties of electrolytes.
For instance, conventional dual mode sorption models (Langmuir-type) are effective to
describe isotherms with a concave towards the activity axis, while engaged species induced
clustering model (Flory-type) has been highly successful in modeling isotherms in polymers with
a convex to aw axis104. Multimode sorption models (Park-type) are particularly suited to fit
sigmoidal isotherms, which are the most common isotherm shapes among ionomers105.
26
By applying the multi-mode model proposed by Park to the sorption isotherms, the
presence of three different mechanisms in the sorption process can be hypothesized:
a) specific adsorption at low water activity, described by the Langmuir model;
b) non-specific adsorption, described by the Henry’s law;
c) water clustering at high water activity.
All these contributions can be formulated in the following equation:
WU 
a L K L aW
n
 K H a w  nK A aW
1  K L aW
(16)
where aL is the specific site capacity, KL is an affinity constant, KH is the Henry’s law
coefficient, KA is the aggregation equilibrium constant, and n is the aggregate size.
A distinct population of water adsorbed in the membrane can be associated to each
adsorption mechanism: specific adsorbed water (WSA), non-specific adsorbed water (WNSA) and
clustered water (WC). Each water population is described by the terms constituting Eq. 16, as
follows:
WSA 
a L K L aW
1  K L aW
(17)
WNSA  K H a w
(18)
WC  nK A aW
(19)
n
In Figure 16 it is reported the typical result of the curve fitting of a polymer membrane
sorption isotherm, where the adsorbed water was separated into the three contributions, so that
the sum of WSA, WNSA, WC matched the experimental isotherm data.
Taking into account that each type of adsorbed water is characterized by different
mobility, we correlated the different water population to the water mobility degree in the
membrane.
Being strongly bound to specific sites, the specific adsorbed water is characterized by low
mobility, whereas the dissolved water molecules (Henry population) have higher mobility. Then,
the growth of water clusters reduces the mobility of the water aggregates. As a consequence,
27
among the three types of water population, the non-specific adsorbed water is characterized by
the highest mobility.
Figure 16. (a) Typical curve fitting (Park’s model) of experimental sorption isotherm data
(Nafion membrane at T=25 °C) and the corresponding fitting parameters; (b) Variation of the
three types of water population in the membrane with the water activity. "Adapted with
permission from Mecheri B, Felice V, Zhang Z, D’Epifanio A, Licoccia S, Tavares AC. DSC
and DVS Investigation of Water Mobility in Nafion/Zeolite Composite Membranes for Fuel Cell
Applications. The Journal of Physical Chemistry C 2012;116:20820-9. Copyright (2012)
American Chemical Society."
The amount of each type of adsorbed water was normalized to the total water content in
the membranes, as follows:
 W [ SA] 
W [ SA]
 100
WTOT
 W [ NSA] 
 W [C ] 
(20)
W [ NSA]
 100
WTOT
(21)
W [C ]
 100
WTOT
(22)
As  parameters were defined, W[SA], W[NSA] and W[C] represent the “specific
adsorbed water degree”, “non specific adsorbed water degree”, and “clustered water degree”,
28
respectively. As shown in Figure 16.b specific adsorbed water dominates at low relative
humidity, non specific adsorbed water at intermediate values of RH, and clustered water
dominates at high relative humidity. These variations are consistent with those found for D and
reported in Figure 15. The θW[NSA] parameter thus represents the “water mobility degree” and
allows to compare different electrolytes in terms of water mobility: the higher θW[NSA], the
greater is expected to be the water mobility in the electrolyte72,78.
As already mentioned, the analysis of water sorption isotherms of ionomers is of
paramount importance for the final fuel cell performance and scientific literature in this field is
mainly based on adsorption properties of water on perfluorinated polymer, in particular Nafion
which is the state-of-the-art material106. Sorption isotherms of most common perfluorinated
ionomers can be indeed described by Park’s model. However, the involvement of five adjustable
parameters (see Eq. 16) makes the chemico-physical interpretation not always clear for ionomers
which microstructure is considerably different from that of Nafion, as in the case of polyaromatic
polymers (see Fig. 6). The less pronounced hydrophobic/hydrophilic separation in polyaromatic
polymers compared to Nafion makes the distinction among specific adsorbed water, non-specific
adsorbed water and clustered water (WC) quite difficult.
As an alternative to the multimode Park’s model, the sorption behavior of the membranes
can be analyzed and interpreted on the basis of the dual mode sorption model proposed by
Feng107. The model is based on the Guggenheim–Anderson–de Boer (GAB) multilayer sorption
theory 108-110 and, at variance with GAB model which considers all sorption sites equivalent, the
Feng model is based on the assumption that the sorption sites can be divided in two different
types, one being the polymer matrix region and the other the microvoid region (specific sorption
sites).
According to this model, the water content in the membranes can be described using Eq.
23:
WU  C p
k ' aw
( A'1)k ' aw
 Cp
1  k ' aw
1  ( A'1)k ' aw
(23)
29
where Cp is the weighted mean value of the polymer sorption capacity; k’ and A’ are
temperature-dependent constants. k’ provides a measure of the interaction between water and the
polymer matrix; k’ values lower than 1 indicate very weak interaction between water and
polymer matrix, the higher the k’ value, the greater the hydrophilicity of the polymer. A’
represents the difference between the interaction of a microvoid and the first molecule adsorbed
on it and that of a microvoid and the molecules adsorbed beyond the first molecule in the
multilayer, thus it provides a measure of the affinity between water and the polymer microvoid.
A' values close to 1 correspond to a polymer in a rubbery state without microvoids; the higher A',
the greater the dependence of sorption on microvoids and affinity of specific sites to water.
Feng’s model requires only three parameters: Cp, the weighted mean value of the
sorption capacity of the polymer to water, k’, the affinity between water and the polymer matrix
(hydrophobic region), and A’, the affinity between water and the polymer microvoid
(hydrophilic domains). Rather than discriminating between the different states of water in the
membrane, the values and the comparison of Feng’s parameters allow getting a deep insight on
chemical nature, as well as on polymer microstructure.
Figure 17 shows the water adsorption isotherms of Nafion and sulfonated polysulfone
(SPS) together with the result of a typical curve fitting with Feng’s model. The figure shows the
very good match between the experimental data and the fit curves and the corresponding fitting
parameters of all samples are summarized in the inset table. Cp, k’, and A’ parameters of the
unfilled Nafion membranes are in good agreement with previous papers. Both polymers showed
low k’ values, indicating that sorption in the polymer matrix region is negligible. Hence,
microvoid sorption is predominant, as expected in the case of ionomer systems in which water
associates through the sulfonic acid groups, and Cp represents the monolayer sorption capacity in
the microvoid region (specific adsorption)83,111.
30
Figure 17. Curve fitting of experimental adsorption isotherm data of the unfilled Nafion and SPS
membranes at T = 25 °C. SE=Standard Error. Adapted from Mecheri B, Felice V, D'Epifanio A,
Tavares AC, Licoccia S. Composite Polymer Electrolytes for Fuel Cell Applications: FillerInduced Effect on Water Sorption and Transport Properties. ChemPhysChem 2013;14:3814-21,
Copyright (2013), with permission from John Wiley and Sons. 83.
The comparison between the fitting parameters of unfilled Nafion and SPS indicated that
Cp was higher for SPS than Nafion, whereas A’ parameter shows the opposite trend. Differences
in Cp and A’ for Nafion and SPS are ascribed to differences in microstructures of the two
ionomer.
Both Nafion and SPS phase separate in hydrophilic and hydrophobic domains. The
hydrophobic domains consist of the perfluorinated and polyaromatic backbone for Nafion and
SPS, respectively. The hydrophilic domains arise from the sulfonic acid groups (-SO3H) which
are responsible for bonding with water molecules. Hydrophobic/hydrophilic separation is more
pronounces in the case of Nafion, as depicted in Figure 6. The greater tortuosity of the
hydrophilic domains in SPS than in Nafion makes water phase in SPS lower interconnected than
in Nafion, thus explaining higher Cp and lower A’ values of SPS compared to those of Nafion.
3.2.2 Differential scanning calorimetry
31
The Differential Scanning Calorimetry (DSC) is a thermoanalytical technique which
monitors heat effects associated with phase transitions and chemical reactions as a function of
temperature112-114. It consists in measuring the difference in heat flow between the sample and a
reference at the same temperature, the temperature of both sample and reference being increased
at a constant rate. The heat flow difference between the sample and the reference can be either
positive or negative, whether the process is endothermic or exothermic. The result of a DSC
experiment is a curve of heat flux versus temperature or versus time. The area enclosed between
the trend line and the base line is a direct measurement for the amount of heat, ΔH, needed for
transformation. Useful information can be obtained by DSC analysis of polymer samples such as
its degree of crystallinity (from the ratio of the heat of fusion of a polymer sample and the
enthalpy of a 100% crystalline sample), specific heat, the purity of the polymer and occurring of
oxidation, cross-linking, chain breakage.
As far as water management of electrolytes is concerned, DSC provides information on
states of water and water mobility through the electrolyte material. Focusing on ionomer
electrolytes, three different categories of water can be discerned by recording DSC thermograms
at subzero temperatures:
1) non-freezable bound water (WNF), strongly bound to the ionic groups present in the
polymer. This type of water is characterized by the fact that it does not crystallize even when the
swollen sample is cooled down to -100°C. These are water molecules in close proximity to an
ionic group like in hydration shells are highly polarized and unable to crystallize. WNF does not
yield characteristic thermal transition in DSC analysis.
2) freezable bound water, weakly polarized. This type of water crystallizes at
temperature lower than 0°C.
3) freezable unbound water, crystallizes at 0°C
The freezable water (WF), being more loosely bound, has higher mobility than the non
freezable water and it is expected to give a more significant contribution to the proton transport
mechanism.
By performing a DSC analysis in the range between -50°C and 10°C, the freezable water
can be quantified from the endothermic peak below 0°C. An example of DSC thermograms
32
obtained from two different polymer electrolyte membranes in the range -50°C and +10°C
showing an endothermic peak ascribed to the melting of freezable water is reported in Figure 18.
Figure 18. DSC thermogram of (a) an unfilled Nafion membrane, and (b) a composite
Nafion/zeolite membrane. "Reprinted with permission from Mecheri B, Felice V, Zhang Z,
D’Epifanio A, Licoccia S, Tavares AC. DSC and DVS Investigation of Water Mobility in
Nafion/Zeolite Composite Membranes for Fuel Cell Applications. The Journal of Physical
Chemistry C 2012;116:20820-9. Copyright (2012) American Chemical Society." 78
The percentage of freezable water in the sample can obtained from the following formula:
 A
1 
WF (%)  
100
 H W md 
ry


(24)
where A is the area of the endothermic peak, ΔHw is the enthalpy of melting for bulk
water (333 J g-1) and mdry is mass of the dried sample
The degree of freezable water, θF, can be defined normalizing the freezable water content
to the total WU which can be measured gravimetrically (for instance, by DVS).
F 
WF
 100
WU
(25)
33
As above mentioned, a higher degree of mobile water corresponds to higher proton
conductivity and, since only WF yields thermal transitions similar to bulk water, its content in
the membrane can be discerned from total WU by using DSC 72,78.
Figure 19 shows the variation of F with the filler content for Nafion – zeolite composite
membranes. Zeolites are aluminosilicates cations with relatively free to move along the cavities
of the framework. Moreover, they have a very high specific surface area which results in a high
water sorption capacity, further facilitating the ion transport. As the zeolite content increased, F
values increased up to a maximum value then decreased at highest zeolite content. These
findings indicate that the zeolite likely contributed to the enhancement of the water mobility
degree in the composite membrane, which was related to its high water sorption capacity and to
the introduction of porosities at the polymer/filler interface. However, the reduction of this effect
over ca. 4 wt.% zeolite content, suggest the formation of dead-end porosities which hinder water
mobility78.
Figure 19 – Variation of F as a function of the zeolite content for Nafion – Faujasite composite
membranes. "Adapted with permission from Mecheri B, Felice V, Zhang Z, D’Epifanio A,
Licoccia S, Tavares AC. DSC and DVS Investigation of Water Mobility in Nafion/Zeolite
Composite Membranes for Fuel Cell Applications. The Journal of Physical Chemistry C
2012;116:20820-9. Copyright (2012) American Chemical Society."78
4. Summary and outlook
34
Solid electrolytes are materials capable of conducting ions. They are used in many
electrochemical devices including batteries, sensors, electrolysers and fuel cells.
Proton exchange membrane fuel cells are considered attractive power sources for portable
applications, in-situ power generation and for automotive. Nevertheless, these systems still suffer
from limitations that need to be addressed to compete with batteries, fossil fuels and internal
combustion engines. Polymer electrolyte membranes are one of the limiting elements of this
technology. Nafion®, a perfluorinated sulphonic acid ionomer, is the most widely used
electrolyte for both hydrogen and liquid-fed proton exchange membrane fuel cells due to its high
proton conductivity, chemical and mechanical stability. A unique feature of Nafion is the
microphase separation between the hydrophobic backbone and the hydrated sulfonic acid
domains, resulting in the formation of wide and well separated water channels for the proton
transport. Nafion membranes show a strong dependence of proton conductivity on the
membrane’s hydration level and are permeable to liquid fuels. In the first case, the fuel cell
system needs an expensive humidification auxiliary system to keep the membranes hydrated. In
the second case the fuel cell efficiency is dramatically reduced. Therefore, the development of
alternative polymer electrolyte membranes with high proton conductivity in a wide range of
temperature and hydration conditions, mechanical robustness, chemical and electrochemical
stability, low cost, and low fuel permeability remains a critical challenge for advancing fuel cell
technology.
Hydrocarbon membranes are potential candidates to replace Nafion. Significant efforts
are being done to develop novel ionomers consisting of hydrocarbon backbones and pending side
chains with terminal sulfonic acid groups to mimic Nafion’s unique morphology. Structure –
properties relationships are fundamental to learn on the dependence of the transport properties on
the membranes’ composition, morphology and water content, and to design better electrolytes.
Proton conductivity is a fundamental property of a proton exchange membrane. When
evaluating potential electrolytes for fuel cells, their proton conductivity is usually measured
under controlled temperature and relative humidity. There is not yet a standard method for
measuring the membranes’ proton conductivity, but measurements on both directions of the
membrane (in-plane and through-plane) could give valuable information on the membranes’
anisotropy. Proton conductivity of electrolytes depends on their hydration level hence it is
35
important to study water sorption and water diffusion over a wide range of relative humidity. The
water states and water diffusion in electrolytes cab be assessed by Dynamic vapor sorption and
differential scanning calorimetry. Excellent correlation has been found between proton
conductivity and degree of mobile water determined by the two mentioned methods.
Advances in proton exchange membranes for fuel cells will likely contribute to the
development of other related fields including electrodialysis (water purification and treatment)
and redox flow batteries for energy conversion.
Acknowledgments
The authors would like to thank Maria J.V.R. Paulo (INRS-EMT) for helping with the editing of
this chapter.
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