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Design of Heterogeneities and Interfaces with Nanofibers in Fuel Cell Membranes

  • Marta ZatońEmail author
  • Sara Cavaliere
  • Deborah J. Jones
  • Jacques Rozière
Living reference work entry
  • 305 Downloads

Abstract

Many fuel cell membranes are highly heterogeneous systems comprising mechanical and chemical reinforcing components, including porous polymer sheets, nanofibers or nanoparticles, as well as radical scavengers or hydrogen peroxide decomposition catalysts. In the last 10 years, great attention has been devoted to 1D nanomaterials obtained by electrospinning. Several chemistries and compositions from aliphatic or aromatic polymers to metal oxides and phosphates and morphologies from nanofibers to nanotubes have been employed to prepare nanocomposite membranes. Despite the significant advances realized, further improvements in ionomer membrane durability under operation are still required. In particular, it is crucial to control the heterogeneity induced by the nanofiber component and to strengthen the interface between them and the matrix. Specific interactions have been demonstrated to improve the fiber/matrix interface with overall improvement of dimensional and mechanical properties. In this chapter we review the different approaches to fuel cell membrane reinforcement based on electrospun polymers and inorganic nanofibers.

Keywords

Fuel cell Proton exchange membrane Proton conductivity Electrospinning Ionomer Composite membrane 

Abbreviations

(s)PAES

(Sulfonated) Poly(arylene ether sulfone)

(s)PEEK

(Sulfonated) Poly(ether ether ketone)

(s)PEEKK

(Sulfonated) Poly(ether ether ketone ketone)

(s)PES

(Sulfonated) Polyethersulfone

(s)PFEK

(Sulfonated) Poly(fluorenyl ether ketone)

(s)PI

(Sulfonated) Polyimide

(s)PPESK

(Sulfonated) Poly(phthalazinone ether sulfone ketone)

(s)PSU

(Sulfonated) Polysulfone

1D

One-dimensional

3D

Three-dimensional

ADL

Acid doping level

BPPO

Bromomethylated polyphenylene oxide

CDP

Cesium dihydrogen phosphate

CNF

Carbon nanofibers

CNT

Carbon nanotubes

C-PAMPS

Poly(2-acrylamido-2-methylpropane-sulfonic acid)

Cys

Cysteine

DMAc

Dimethylacetamide

DMD

Direct membrane deposition

DMF

Dimethylformamide

DMFC

Direct methanol fuel cell

DMSO

Dimethyl sulfoxide

EW

Equivalent weight

FER

Fluoride emission rate

Gly

Glycine

LSC

Long side-chain

Lys

Lysine

MEA

Membrane electrode assembly

MW

Molecular weight

NT

Nanotubes

OCV

Open-circuit voltage

PA

Phosphoric acid

PAA

Polyacrylic acid

PAN

Polyacrylonitrile

PBI

Polybenzimidazole

PBz

Polybenzoxazine

PEI

Polyetherimide

PEMFC

Proton exchange membrane fuel cells

PEO

Polyethylene oxide

PFSA

Perfluorosulfonic acid

PPA

Polyphosphoric acid

PPSU

Polyphenylsulfone

PTFE

Poly tetrafluoroethylene

PVA

Polyvinyl alcohol

PVB

Polyvinyl butyral

PVDF

Poly vinylidene fluoride

PVDF-HFP

Polyvinylidene fluoride-hexafluoropropylene

PVP

Polyvinylpyrrolidone

RH

Relative humidity

SEM

Scanning electron microscopy

Ser

Serine

sPOSS

Sulfonated polyhedral oligomeric silsesquioxane

sPPO

Sulfonated poly(phenyleneoxide)

sPS

Sulfonated polystyrene

SSC

Short side-chain

sZrO2

Sulfonated zirconia

TEM

Transmission electron microscopy

TEOS

Tetraethylorthosilicate

Tg

Glass transition temperature

vol%

Volume percent

wt%

Weight percent

ZCCH

Zinc-aminotriazolato-oxalate

ZrP

Zirconium phosphate

Introduction

Proton exchange membrane fuel cells (PEMFC) convert the chemical energy of a fuel into electric energy and heat. Fuel cells can generate electricity continuously, with low or zero pollution emission as long as the fuel and oxidant are supplied. The key electrochemical reactions of hydrogen oxidation and oxygen reduction at the electrodes, as well as proton transfer from the anode to the cathode, take place in the core component of the PEMFC – the membrane electrode assembly (MEA) – which consists of an ionomer (proton-conducting polymer) membrane and anode/cathode catalyst layers as schematically represented in Fig. 1. For low-temperature fuel cells (60–90 °C), the fuel is hydrogen (or alcohols as in, e.g., direct methanol or ethanol fuel cells, DMFC, DEFC) and the oxidant is oxygen from the air (or gaseous O2).
Fig. 1

Schematic representation of a proton-exchange membrane fuel cell (PEMFC)

One major factor impeding large-scale commercialization of the PEMFC is the durability of the MEA components, in particular the electrolyte membrane [1, 2, 3]. It must not only be impermeable to the direct transfer of reactants and electronically insulating, but it also needs to demonstrate high proton conductivity while being mechanically and chemically robust during the fuel cell lifetime [4]. Although these requirements have triggered development of many types of fuel cell electrolytes based on polysulfones (PSU), poly(benzimidazoles) (PBI), poly(imides) (PI), or poly(aryletherketones) (PAEK) [5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22], so far the state-of-the-art materials are perfluorosulfonic acid (PFSA) polymers . The PFSA membrane microstructure comprises two well-defined components: ionic clusters with hydrated sulfonic acid groups well percolated and phase separated from the hydrophobic backbone (polytetrafluoroethylene, PTFE) [23, 24]. The heterogeneity of the ionomer structure provides mechanical integrity and high stability in very harsh (electro)chemical environments due to the presence of the PTFE backbone and high proton conduction properties ensured by the presence of ionic domains [25]. The parameter that well describes transport properties of the PFSA ionomers in relation to its mechanical integrity is the equivalent weight (EW), defined as the ratio between the dry mass of the polymer in the acid form in grams and the equivalents of exchangeable groups. The EW can be tuned, and as a first approach, the lower the EW, the higher the proton conductivity of PFSA ionomers. Different membrane properties often closely related to polymer structure heterogeneity in terms of spatial organization of hydrophilic/hydrophobic domains were widely studied [25, 26, 27]. Based on this improved understanding, two main families of PFSA ionomer compositions are now classified: long side-chain (LSC) and short side-chain (SSC) structures (Fig. 2). In the SSC polymer structure, there is no -O-CF2CF(CF3)- unit in the pendant chain, and the length of the perfluoro vinyl ether side-chain is 2 (Aquivion® type) or 4 (3M™ type) CF2 units.
Fig. 2

Chemical structures of LSC – Nafion® structure (a) and SSC perfluorosulfonic ionomers (b) and (c)

One of the main constraints of PEMFC membranes is their loss of proton conductivity at temperatures above 100 °C in a non-pressurized cell. In such membranes, the acidic functionalities of the polymers dissociate when solvated with water allowing proton transport. Thus the high conductivity of proton exchange membranes is directly related to the amount of ionic domains in the polymer structure and to their degree of hydration.

Another factor to be considered for direct fuel cells is the fuel crossover. Methanol crossover occurs by diffusion through the water channels in hydrated PFSA as well as a consequence of electroosmotic drag (active transport among with hydronium ions during DMFC operation). The permeated alcohol is chemically oxidized at the cathode, which causes electrode depolarization or mixed potential resulting in lower performance and lower fuel efficiency of the DMFC, as well as cathode catalyst poisoning [28].

Constantly forward-moving performance and durability targets have driven the design of novel heterogeneous fuel cell membranes. First of all membrane thickness was reduced down to one tenth or one twentieth of that of first-generation Nafion®-like membranes in order to decrease the electrical resistance and improve water back transport. Thus the >200-μm-thick PFSA membrane of the 1990s gave a way to an ultrathin composite membrane system in which the well-documented heterogeneity of the ionomer membrane, at the microscale level, is further enhanced by incorporation of a mechanical [29] and/or chemical [30, 31] reinforcement. Reinforcement processing and membrane casting are usually separated steps, which create countless possibilities for different material designs. The nature of the ionomer or functionalized polymer, the chemical and mechanical reinforcing components, as well as the membrane thickness and any gradient of composition across it can be possibly developed or tuned for an engineered fuel cell membrane. For PFSA, ionomer development continues to be pursued in the direction of high proton conducting materials [32, 33].

These or other newly developed polymers/ionomers can be further associated with different types of mechanical supports in order to achieve membranes with outstanding performance and durability. The main role of the mechanical reinforcement in such a membrane design is to mitigate structural aging of the material and to extend MEA lifetime by constraining dimensional changes of highly conducting but mechanically fragile ionomer membranes. In contrast to thin self-standing pristine ionomer membranes, the reinforced electrolyte is able to withstand compression stress created by hydration and dehydration cycling that accompanies operation under variable load. Furthermore, some of the supports can be or can contain radical scavengers, which prevent radical attack and the ensuing ionomer defragmentation.

Although the role of a mechanical support in macrocomposite membranes is clear, parameters such as material choice, desired architecture, and most importantly the interface between the support substrate and ionomer are all crucial for fuel cell performance and durability. Membrane reinforcements are often prepared using thermostable and mechanically robust polymers or inorganic materials [34, 35, 36, 37, 38, 39, 40]. The latter bring enhanced water retention, which significantly facilitate fuel cell operation at low relative humidity and high temperature conditions. Some of the most well-established substrates are expanded PTFE sheet from Gore® Fuel Cell Technologies [29], laser-drilled polysulfone or polyimide from Giner Electrochemical Systems [41], or polysulfone/microglass fiber fleece [42]. These materials fulfill their role as mechanical reinforcements; however, the use of each of them poses unique challenges: high processing cost, low flexibility in architecture design, and poor interface between substrate and ionomer to name but a few. Electrospinning with its versatility is attracting attention for the potential it has of introducing targeted architectures and interfaces into composite membrane systems [43, 44, 45]. Indeed, the variety of morphologies that can be achieved, from solid, hollow, porous, or core-sheath nanofibers or tubes to inorganic or hybrid fibers embedding nanoparticles, gives a tremendous freedom in engineering new materials. Furthermore, electrospinning technology allows control of the crucial parameters of porosity, fiber diameter, and their distribution in the matrix, which in terms of elaboration of composite membrane means the possibility of fine tuning interface and spatial organization between inert and conducting phases as well as pore interconnectivity. Many researchers have recently recognized these emerging opportunities for individualized products.

The aim of this chapter is to describe the most relevant advances in the use of electrospun materials for the preparation of heterogeneous nanocomposite proton-conducting membranes. For the sake of clarity, the chapter is organized into three sections based on material design criteria:
  • Composite membranes with an electrospun inorganic component embedded in a polymer/ionomer matrix (paragraph 2, approach C in Fig. 3)

  • Composite membranes with electrospun ionomer nanofibers embedded in a polymer/ionomer matrix (paragraph 3, approach A in Fig. 3)

  • Composite membranes with electrospun polymer fibers embedded in an ionomer matrix (paragraph 4, approach B in Fig. 3)

Fig. 3

Types of membrane architectures based on electrospun materials: (a) inert material/ionomer surrounding a 3D interconnected nanofiber web of a proton-conducting polymer (paragraph 3), (b) proton-conducting polymer matrix surrounding an electrospun web of nanofibers of an inert or proton-conducting or cross-linked electrospun polymer (paragraph 4), (c) electrospun (proton-conducting or inert) inorganic web embedded in a (proton-conducting or inert) polymer matrix (paragraph 2)

For a detailed explanation of the electrospinning technique and principles, the reader is invited to refer to published books or reviews [46, 47]. The effect of electrospinning parameters on nanofiber membrane properties and performance is still difficult to evaluate and rationalize due to their numerous compositions and architectures. However, some endeavors may be found in literature [48].

Composite Membranes with Electrospun Inorganic Nanofibers Embedded in a Polymer/Ionomer Matrix

As discussed above, among the challenges to tackle for proton exchange membrane fuel cells are retention of proton conductivity at high temperature and low relative humidity and suppression of direct crossover of the fuel to the cathode. Other concerns of great importance are mechanical stress in particular during wet-dry cycling and the free radical-induced chemical degradation of the polymer structure. The formation of radical species during fuel cell operation occurs through the decomposition of hydrogen peroxide in the presence of trace metal ions originating from the corrosion of system components. In particular hydroxyl, hydroperoxyl, and superoxide (HO, HOO, O 2) radicals attack vulnerable polymer sites, which leads to structure defragmentation and membrane thinning and consequent MEA failure.

In this context, inorganic materials such as metals and metal oxides, zeolites, metal hydrogen phosphates, and heteropoly acids are of special interest when it comes to composite membrane development. The main advantages of adding an inorganic component are the improved proton conductivity [17, 49], water retention, thermal stability, and reduced fuel crossover in the resulting hybrid inorganic/organic systems [5, 50]. Additionally, some inorganic additives are recognized as effective radical scavengers or hydrogen peroxide decomposition catalysts [4]. Embedded in the polymer structure, they efficiently mitigate radical attack and consequent ionomer defragmentation and significantly increase fuel cell durability [51]. Finally, some inorganic moieties with specific morphologies and interaction with the matrix ionomer can play an essential role in improving mechanical strength and dimensional stability of composite membranes.

A range of approaches for the incorporation of inorganic moieties has been employed [52]. Among them electrospinning not only allows the preparation of 1D inorganic nanostructures with tuned compositions and morphologies [53, 54, 55, 56, 57] but also offers the possibility to build and control interfacial interactions between membrane components. It should be emphasized that the properties of hybrid membranes largely depend on the nature of the polymer matrix bearing acidic (e.g., PFSA) or basic groups (e.g., polybenzimidazole). However, no less important to successful material design are the homogenous dispersion and/or orientation of inorganic species in the ionomer, their morphology/shape control, and their propensity to favor interactions between organic/inorganic constituents. This section treats all these aspects as well as the development of novel hybrid inorganic-organic membranes.

The strategy to improve membrane performance in dry and hot operating conditions through incorporation of hygroscopic metal oxide particles such as SiO2, TiO2, SnO2, and ZrO2 has been widely studied. The key challenges of this approach are the homogeneous distribution of the inorganic filler, the reproducibility of membrane processing, and the membrane homogeneity. All the issues can be addressed by electrospinning of inorganic additives.

For instance, the approach of highly dispersed silica fibers embedded in electrospun sPEEK polymer nanofibers was used by Lee et al. [58] to develop a PFSA-sPEEK/SiO2 composite material for medium-temperature (>100 °C) fuel cells. Clearly the purpose of this work was to improve membrane water retention by incorporation of a hygroscopic oxide. Silica was formed in situ, during nanofiber formation, and was uniformly distributed through the sulfonated poly(ether ether ketone) web that was impregnated with Nafion® ionomer to prepare a dense membrane with good adhesion between the nanofiber and ionomer components. The presence of hygroscopic silica increases the water uptake capacity of the composite system, whereas sPEEK nanofibers provide its mechanical integrity and significantly reduced membrane swelling in the membrane in-plane directions.

Thiam et al. developed a composite membrane of Nafion® and electrospun palladium-silica nanofibers (Pd/SiO2) with the aim of reducing methanol crossover in DMFC [53]. A mixture of tetraethoxysilane, polyvinylpyrrolidone, and Pd nanoparticles was electrospun and the nanofiber web impregnated with the ionomer. The role of Pd nanoparticles in the composite membrane is to facilitate the oxidation of permeated methanol [59]. The final Pd/SiO2/Nafion® composite membrane indeed demonstrated excellent fuel barrier properties. Furthermore, a significant increase in membrane water uptake and proton conductivity was related to the participation of Si-OH groups in the construction of 3D proton conduction pathways. In other studies, in order to improve the efficiency of proton transport, SiO2 nanofibers were synthesized with immobilized amino acids such as cysteine (Cys), serine (Ser), lysine (Lys), and glycine (Gly) [60]. The proton conductivity of pristine and composite membranes decreased in the following order: Nafion®-Cys (242 mS/cm at 80 °C) > Nafion®-Ser > Nafion®-Lys > Nafion®-Gly > Nafion®.

Another strategy that introduces SiO2 nanoparticles in Nafion® bulk was based on the dip-coating of an electrospun polyimide (PI) nonwoven web in SiO2/polyetherimide (PEI) [61]. The presence of silica particles interconnected by PEI in the PI nonwoven substrate enabled its facile impregnation with PFSA ionomer and significantly improved its mechanical strength. The composite membrane demonstrated higher water retention and suppressed dimensional change between water-swollen and dry states. In another approach, a PI nonwoven web was integrated into a proton-conducting silicate glass electrolyte fabricated via in situ sol-gel synthesis of 3-trihydroxysilyl-1-propanesulfonic acid/3-glycidyloxypropyl trimethoxysilane mixtures [62]. The high proton conductivity of the resulting glass electrolyte in nonhumidified conditions makes of this material an interesting alternative membrane type for further investigation.

In another study, electrospun sulfonated zirconia (sZrO2) nanofibers were combined with a cross-linked poly(2-acrylamido-2-methylpropane-sulfonic acid) (C-PAMPS) [63]. sZrO2/C-PAMPS hybrid membranes with 30% fiber content showed exceptionally high proton conductivity of 340 mS/cm at 100 °C. Clearly, continuous sZrO2 nanofibers served as interconnected channels capable of anchoring water molecules and providing facile hopping pathways for proton transfer. Moreover, thinner fiber diameters gave rise to higher proton conductivities most probably due to increased surface area and density of sZrO2 nanofibers.

A different strategy toward novel thermostable high-performing inorganic-organic membranes is the incorporation of ordered mesoporous solids. For instance, mesoporous metal oxide (TiO2, CeO2, and ZrO1.95) nanotubes (NT) have been embedded into a Nafion® membrane to increase water retention in dry conditions [64]. The tubular structure in which metal oxide particles form a porous shell as displayed in Fig. 4 was achieved by calcination of metal precursors homogeneously dispersed in electrospun polyacrylonitrile nanofibers.
Fig. 4

SEM images of TiO2 TNT (a), CeO2 NT (c), ZrO1.95 NT (e) and TEM images and corresponding lattice fringes (inset) of TiO2 TNT (b), CeO2 NT (d), ZrO1.95 NT (f) (Reprinted with permission from [64]. Copyright (2014) American Chemical Society)

This unique architecture of inorganic nanotubes not only increased water retention capability but also enhanced water diffusion in composite PFSA-based membranes. Indeed the amount of the water strongly bound to sulfonic acid groups was found to be two times higher in the nanotube-filled membranes than in the Nafion® 212 membrane at subzero temperature. Similarly, the water self-diffusion coefficient of TiO2 NT/Nafion® was remarkably high in comparison with pristine Nafion® (3.527 10−9 and 2.003 10−9 m2/s, respectively). Such improved water capacity and diffusion resulted in low ohmic and mass transport resistance of composite materials and led to higher PEMFC performance than with Nafion® 212. For instance, the maximum power density values of MEAs operating at 18% RH and 80 °C were 641 mW/cm2 with a 1.5 wt% TiO2 NT membrane, 449 mW/cm2 with a membrane comprising 0.5 wt% CeO2 NT, 546 mW/cm2 with a membrane containing 1.5 wt% ZrO2 NT, and 186 mW/cm2 with Nafion® 212. The best results in terms of performance in both dry and fully humidified conditions were obtained with the membrane containing TiO2 nanotubes. Further studies revealed that nanotubes with smaller diameter (and thus high surface area) demonstrated greater water retention, which the authors related to an expanded ionic cluster size in the Nafion® ionomer [65]. Also the durability of an MEA prepared using a hybrid nanofiber membrane was much improved, which was ascribed to enhanced water back diffusion [65, 66].

We earlier proposed the use of zirconium phosphate/zirconium oxide (ZrP/ZrO2) in the form of nanofibers, rather than nanoparticles, to introduce a reinforcing effect in a composite membrane [56]. The preparation of a nanofiber architecture required the use of “reactive” coaxial electrospinning: a zirconium precursor and a phosphorus source were spun together from separate solutions in order to delay formation of zirconium phosphate gel. The synthesis of the inorganic material occurs in situ, at the core/shell interface in the jet. The resulting web was then calcined, treated with phosphoric acid, and finally impregnated with SSC PFSA ionomer to form a composite membrane. It was demonstrated that the fiber length and high aspect ratio provide an extended interaction with the proton-conducting matrix. ZrP prevented membrane dehydration at elevated temperature and ensured high proton conductivity. Indeed, the ZrP/ZrO2 enriched membrane demonstrated improved elastic modulus, yield point, and proton conductivity in comparison with pristine PFSA membranes [56].

Solid acids are of particular interest due to their high proton conductivity (1–100 mS/cm) at the intermediate temperature range (200–300 °C). These compounds undergo a phase transition from a low-temperature phase to a superprotonic phase, characterized by a dynamically disordered hydrogen-bond network. We proposed a different approach to electrospinning a highly interconnected proton-conducting fiber web from an aqueous solution of thermally treated cesium dihydrogen phosphate, CsH2PO4 (CDP) [67]. CDP heat-treated at a temperature higher than its superprotonic phase transition temperature undergoes dehydration and partial polycondensation, and its dissolution in water leads to a viscous solution, which can be electrospun without a carrier polymer. SEM micrographs of freshly synthesized CDP particles, polymeric CDP, and electrospun polymeric CDP are shown in Fig. 5. The CDP-polymer nanofiber web showed a maximum proton conductivity of 80 mS/cm at 250 °C.
Fig. 5

SEM micrographs of (a) CDP, (b) polymeric CDP, and (c) electrospun polymeric CDP (Reprinted from [67] with permission of The Royal Society of Chemistry)

Carbon nanofibers (CNFs) are also receiving attention as a component of fuel cell membranes especially for direct methanol fuel cells [68, 69]. CNFs possess high aspect ratio and specific surface area and can be easily functionalized. For instance, Liu et al. used sulfonated CNF to create a hydrogen bonding interaction between the sulfonated fibers and sPEEK [69]. CNFs sheared into short length could be uniformly dispersed in composite membranes to generate tortuous methanol permeation pathways as illustrated in Fig. 6.
Fig. 6

The proposed mechanism of proton transport and methanol crossover prevention of sPEEK and sulfonated CNF/SPEEK composite membranes (Reprinted from [69]. Copyright (2017), with permission from Elsevier)

Choi et al. combined the electrospinning of sPEEK with a carbon nanotube (CNT) forest [70]. The resulting aligned and interconnected nanofiber web was compressed and exposed to DMF vapor to eliminate the porosity, giving rise to dense and hierarchically organized CNT/sPEEK membranes with improved mechanical stability and performance over recast Nafion® and sPEEK membranes.

Original organic-inorganic hybrid membranes have been developed in Laberty-Robert’s group [71, 72] with a design intent to mimic Nafion® with its phase separation between hydrophobic and hydrophilic domains at the nano- and macroscale (see paragraph 1). The hybrid membranes prepared by electrospinning a sol-gel-based solution containing PVDF-HFP (polyvinylidene fluoride-hexafluoropropylene) and 2-(4-chlorosulfonylphenyl)ethyltrichlorosilane with tetraethylorthosilicate (TEOS) [72] showed proton conductivity of 15 mS/cm at 120 °C and 50% RH as well as an exceptional modulus above 80 °C. These properties were related to the particular microstructure of the organic-inorganic membrane, consisting of bundles of assembled small polymer fibers surrounded by functionalized silica domains.

In conclusion, all composite membranes comprising metal oxide 1D nanomaterials are characterized by improved water retention and fuel cell performance in low RH and high temperature conditions. Furthermore, the presence of some of the inorganic components reviewed so far enhanced water back diffusion and mechanical stability of the composite membranes. This long list of benefits provided by inorganic additives should be completed by radical scavenging activity and mitigation of chemical membrane decomposition [4].

The effective approach to mitigate PFSA membrane degradation is the incorporation of inorganic radical scavenger species such as SnO2, TiO2, CeO2, or MnO2. The scavenging ability of different additives largely depends on the rate constant of the redox reaction of the radical quencher with the hydroxyl radical HO•, as well as on the rapidity of regeneration of its active sites. Fast kinetics of hydroxyl radical quenching makes ceria one of the most efficient radical scavengers. Other factors such as ceria morphology, its distribution in the polymer matrix, or stability in highly acidic environment can be modified and further improved using electrospinning. Ketpang et al. investigated the influence of incorporation of mesoporous cerium oxide nanotubes on composite membrane durability [66]. The CeO2 NT/Nafion® not only outperformed pristine Nafion® membrane when operating under hot and dry conditions but also exhibited remarkable durability. After 100 h of accelerated stress test in open circuit voltage (OCV) conditions, the fluoride (product of the PFSA membrane decomposition) emission rate (FER) of the composite membrane was 20 times lower than that of the commercial PFSA membrane. Beyond the possibility of tuning ceria morphology, electrospinning was developed as a strategy for optimal ceria distribution within the polymer matrix. This approach was first considered in our work [31], in which we developed a thin protective composite layer of PFSA nanofibers embedded with CeOx that was incorporated into the MEA at the desired anode/cathode interface [73]. The lifetime of MEAs with asymmetric composite membranes comprising such a PFSA/CeOx layer was eight times longer than an unmitigated MEA, and the FER and OCV decay were significantly reduced. Interestingly, this approach was more effective when the PFSA/CeOx enriched side of the membrane was oriented to the anode side. This finding was related to enhanced regeneration of active Ce3+ sites in the reductive environment and to the partial dissolution of CeOx at the anode interface and further migration of cerium ions through the membrane. In contrast, cerium species created after CeOx dissolution at the cathode side are potentially easily leached from the MEA and so do not contribute to the prevention of membrane degradation.

Indeed, the main problem associated with the integration of CeO2 into PFSA membranes is the dissolution and migration of cerium ions into both catalyst layers of the fuel cell . This instability of ceria in PFSA ionomer prompted further study, and a new approach of ceria immobilization on an electrospun polymer web has been investigated. The purpose of the electrospun support is to suppress the leaching of free radical scavengers. The additives were co-dissolved or dispersed in the polymer solution used for nanofiber web preparation. The integration of nanofiber-supported ceria in PFSA membranes was claimed in recent patent application [74]. The effectiveness of this solution has been confirmed also by recent work from Breitweiser et al. where PVDF-HFP nanofibers embedding CeO2 nanoparticles were directly electrospun onto gas diffusion electrodes and impregnated with a Nafion® dispersion [75]. The resulting reinforced membrane after 100 h aging showed at least three times lower voltage decay rate (0.39 mV/h) compared to that of a Gore-Select® membrane (1.36 mV/h). Furthermore, energy dispersive X-ray spectroscopy did not reveal any significant migration of cerium into the catalyst layers during degradation after 100 h, thus corroborating that the nanofiber web provided anchorage to ceria.

Composite Membranes with Electrospun Ionomer Nanofibers Embedded in a Polymer/Ionomer Matrix

In this approach, fuel cell composite membranes comprise a highly proton-conducting nanofiber web usually impregnated with an inert component to provide its mechanical stability. In such a scenario, the nanofibers, as a novel ion transport material, have to be very well interconnected in order to ensure a continuous proton-conducting path through the membrane.

The membrane micro-/nanostructure can be greatly influenced by the processing methods used. The electrospinning technique offers an opportunity to influence polymer chain conformation and organization at the nano-/microscale, thus determining membrane properties such as water uptake, proton transport (refer to section “Introduction”), or thermal and mechanical stability (refer to section “Composite Membranes with Electrospun Polymer Fibers Embedded in an Ionomer Matrix”). In the context of the approach of electrospun ionomer nanofibers embedded in a polymer/ionomer matrix, the elaboration of a nanofiber web with an adequate proton conduction pathway is crucial. Indeed, the first study on entirely electrospun/sprayed sulfonated poly(ether ether ketone ketone), sPEEKK, fiber mats exhibited a channel-shaped network of ionic groups and, according to small-angle X-ray scattering results, improved phase separation compared to dense membranes [76]. PFSA and sPI polymers showed fast ion transport [77, 78, 79, 80] when spun into 1D nanomaterials. Numerous investigations were performed to shed more light on the phenomena occurring in the polymer structure during the electrospinning process (polymer discharging and fiber formation). It was suggested that long-range ordered arrangements of polar groups in the polymer chains form proton-conducting channels [76, 77, 78, 79, 80]. Sulfonated polyimide nanofibers were unidirectionally aligned using electrospinning [79, 80, 81], giving rise to ultra-high single-fiber proton conductivity values >1 S/cm at 30–90 °C and 95% RH. These fibers displayed a separation of hydrophobic and hydrophilic domains on the wall and in the core of the fiber, respectively, with the formation of a quasi-one-dimensional narrow conduction pathway that facilitated proton transport (Fig. 7). Additionally, since the polymer chains within the nanofiber were oriented in the axial direction, the mechanical strength of the nanofibers was significantly improved. The authors related such directional properties to the electrostatic forces between the collector and the electric charge present on a given nanofiber [79]. Similarly, outstanding proton conductivity (1.5 S/cm) was described for a single Nafion® nanofiber with diameter of 400 nm [77]. Interestingly, electrospinning of Nafion® fibers with diameters >2 μm did not present any advantage in terms of proton conductivity as the measured values were similar to that of bulk Nafion® (∼100 mS/cm). However, once the fiber diameter decreased to less than 1 μm, a sharp increase in the conductivity has been observed. Once again, these results were linked to a confinement effect in thinner fibers, which assists the alignment of the ionic domains in the longitudinal direction, a conclusion supported by information deduced from X-ray scattering experiments. Similar results were reported on ionomer nanowires which were incorporated into a micro-fuel cell [78].
Fig. 7

Temperature dependence (at 95% relative humidity, RH) and (b) RH dependence (at 90 °C) of proton conductivity of sPI nanofibers prepared using different electrospinning conditions (applied voltage, V2, of 0.5, 1.0, and 3.0 kV), a cast sPI membrane, and Nafion® membrane (Reproduced from [80] with permission of The Royal Society of Chemistry)

Undoubtedly, molecular orientation within the polymer chains is a critical factor in determining the intrinsic proton conductivity of nanofibers. Although most studies focus on the preparation of highly conducting fibers for a specific application, there has been some research on the influence of electrospinning conditions on fiber properties. For instance, polymer orientation in electrospun nanofibers can be induced by electric field [82, 83, 84], nature of the collector [85], solvent relaxation time [83], as well as electrospinning solution and polymer properties. Orientation itself is a result of two competing processes, namely, the extensional forces, which orient polymer chains along the filament direction, and orientation relaxation. The latter is strictly connected to the polymer flexibility, its molecular weight, and glass transition temperature (Tg). In other terms, molecular orientation parallel to the fiber axis occurs during electrospinning; however, chain relaxation usually caused by residual solvent promotes the return of the polymer structure to the isotropic state, unless chain relaxation is hindered (e.g., through the use of charged collectors) [84]. Two main conclusions could thus be drawn. The electrospinning process itself induces molecular orientation of the ionic domains of the ionomers along the fiber, which can be further enhanced by addition of a polar solvent [86]. Furthermore, ionomers may be able to retain such created orientation through ionic bonding between domains, thus locking in the oriented structure and preventing relaxation.

After this brief overview of the properties of ionomer nanofibers, the following part will focus on the preparation of fibrous ionomer substrates and composite membranes from them. As already mentioned, the principal role of an electrospun ionomer web in this approach is to ensure high proton conductivity in heterogeneous membranes. Due to their outstanding properties, PFSA ionomers are undoubtedly the most used electrolytes in fuel cell applications and therefore excellent candidates for nanofiber preparation. However, the electrospinning of these materials is very challenging [87, 88]. Indeed, PFSA exhibits formation of aggregates or micelles in aqueous and nonaqueous media [89]. As a colloidal dispersion, PFSA ionomers lack adequate entanglement of the polymer chains, indispensable property for the electrospinning of a polymer solution. PFSA thus tends to give rise to electrosprayed particles rather than electrospun fibers. It should be noticed that chain entanglement also closely depends on the molecular weight of the polymer and its concentration in the electrospinning solution [90]. This hurdle was overcome by the addition of high molecular weight polymer carriers such as polyethylene oxide (PEO) [91, 92, 93], polyvinyl alcohol (PVA) [93], polyvinylpyrrolidone (PVP) [94, 95], polyacrylic acid (PAA) [96], and polyacrylonitrile (PAN) [97]. Among these, PEO is the most commonly used, due to its good compatibility with PFSA dispersions and low amount required to obtain uniformly sized electrospun nanofibers. The incorporation of such carrier polymers increases the entanglement of polymer chains as well as the solution viscosity. Therefore, the choice of an appropriate carrier with adequate molecular weight can significantly improve the outcome from electrospinning. On the other hand, the main drawback of this approach is the lower proton conductivity of the composite fibers due to the presence of nonconducting carrier, strongly interacting with the ionomer and probably disrupting the conductive pathways it formed. For instance, Laforgue et al. electrospun 5 wt% Nafion® dispersion with 200 kDa PEO [93]. At this molecular weight, 5 wt% of PEO was not enough for Nafion® nanofibers to form. Fibers with diameters of 80–180 nm could only be collected when the PEO content in the electrospinning solution was increased to 16 wt%, which significantly lowered the proton conductivity [92].

Two principal approaches can be followed to limit this undesirable effect. The first involves electrospinning using a high molecular weight carrier, which significantly reduces the concentration needed for fiber formation. In such a scenario, Nafion® nanofibers can be electrospun to a very high volume fraction [77]. Another approach consists of reducing the amount of polymer carrier required by using short side-chain and low EW ionomers. The influence of the side-chain length on the electrospinning process was investigated in earlier work in our group [98, 99]. Interestingly, the effect of the molecular weight of PEO was very pronounced for electrospun LSC ionomer, which demonstrated a sharp transition between bead formation (when PEO of molecular weight Mw 400 kDa was applied) and fiber deposition (when using PEO of Mw 1000 kDa). On the other hand, the morphology of electrospun SSC ionomers changed progressively from beads to uniform fibers when using PEO of increasing molecular weight. Finally, the total amount of polymer carrier required for fiber stretching was found to be significantly lower for SSC ionomers, other conditions being equal. These observations were interpreted in terms of weaker interchain interactions in LSC PFSA, which leads to a dispersion of lower viscosity. Pintauro et al. also investigated electrospinning of LSC and SSC PFSA polymers with different equivalent weights, namely, Nafion® EW 1100 [100], EW 1115 [101], and 3M™ EW 825 and 733 [92]. These authors observed that bead-free nanofibers of 3M PFSA could only be achieved using 1 wt% of PEO (Mw = 300 kDa) and 10 wt% of ionomer dispersion. However, increasing the molecular weight of the carrier from 300 to 1000 kDa resulted in bead-free nanofibers with only 0.3 wt% PEO. Finally, electrospinning of both LSC and SSC ionomers using solvents with high dielectric constant and dipole moment (e.g., dimethylformamide, DMF, and dimethylacetamide, DMAc) resulted in a more stable polymer jet and nanofiber homogeneity [101].

In contrast to PFSA ionomers, most sulfonated polyaromatic hydrocarbon polymers can be spun into nanofibers without addition of a carrier, since they do not form colloidal dispersions but solutions in common aprotic solvents such as DMF, DMAc, dimethyl sulfoxide (DMSO), etc. The spin ability of these polymers is simply related to their molecular weight and their volume fraction in the electrospinning solution [90, 102]. Indeed, it was demonstrated that by carefully adjusting polymer physical properties along with processing parameters, nanofibers of sulfonated polysulfone (sPSU), sulfonated polystyrene (sPS), sulfonated poly(arylene ether sulfone) (sPAES), and sulfonated poly(ether ether ketone) (sPEEK) could be elaborated [76, 80, 102, 103, 104, 105, 106, 107, 108]. Sulfonated polyimides containing fluorinated groups to enhance their solubility in N,N-dimethylformamide were also successfully electrospun into nanofibers [80].

The next step after the elaboration of such proton-conducting nanofibers in the form of a two-dimensional web is their integration into a matrix to form a dense composite membrane. The incorporation of nanofibers with extremely large surface area, remarkable mechanical strength, and proton conductivity are expected to confer unique properties to the final materials. The resulting membrane characteristics do not simply result from the sum of the individual contributions of their components but also from the synergy created by an extensive nanofiber/matrix interface (see also paragraph 4). Changing the types of interactions between membrane components, the surface energy and the existence of labile bonds can lead to modification of membrane properties including fuel permeability, ionic conductivity, chemical, mechanical, and thermal stability. Indeed, using the same ionomer (e.g., poly(phthalazinone ether sulfone ketone)) in the fiber web and in the matrix [109] significantly increased proton conductivity, swelling resistance, and mechanical and thermal stability and decreased gas permeability compared to the corresponding “homogeneous” cast membranes. A similar strategy was employed for composite membranes based on sulfonated polyimide where aligned sPI nanofibers were embedded into a matrix of the same polymer, showing improved proton conductivity, durability, and gas barrier properties in comparison with non-reinforced sPI cast membranes (see paragraph 4). It is worth noting that the authors avoided the dissolution of the nanofiber mat while impregnating it in the matrix of the same polymer, by dissolving the latter either at high temperature [109] or in a non-protonated form [81]. Chemical cross-linking will be another option further discussed in paragraph 4.

In another approach, ionomer fibers have been embedded into non-functionalized, non-proton-conducting polymers. Pintauro’s group developed composite membranes containing PFSA or sulfonated aromatic polymer nanofibers impregnated with an inert cross-linkable monomer [92, 100, 104, 110]. In order to ensure a proton-conducting interconnected network, the mat was densified to increase the volume density of the nanofibers and weld them. Such membranes demonstrated better mechanical properties than monocomponent membranes, due to the mechanical strength of the nanofiber structure as well as the reinforcing effect of the inert robust polymer matrix. In addition, the proton conductivity was outstanding due to the formation of an interconnected 3D network of proton channels. The dependence of the proton conductivity at 80 °C on RH and the experimental stress-strain curves are shown in Fig. 8.
Fig. 8

(a) Dependence of in-plane proton conductivity on relative humidity at 80 °C of EW 733 PFSA and EW 825 PFSA nanofiber composite membranes embedded within cross-linked NOA 63 and Nafion® 212; (b) Stress-strain curves of wet membrane samples at 25 °C; a: EW 733 PFSA homogeneous membrane, b: EW 825 PFSA homogeneous membrane, c: EW 733 PFSA nanofiber network membrane (0.70 fiber volume fraction), d: EW 825 PFSA nanofiber network membrane (0.73 fiber volume fraction), and e: UV-cross-linked NOA 63 (Adapted from [92] with permission of The Royal Society of Chemistry)

This approach has been widely employed by many other research groups as summarized in Table 1 [58, 63, 79, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120].
Table 1

Overview of composite membranes comprising electrospun nanofibers of proton-conducting electrolytes and a matrix of inert polymer

Electrospun material

Matrix material

Membrane processing

Proton conductivity (mS/cm)

References

3M™ ionomer EW 733

Norland Optical Adhesive 63

Web impregnation

160 (80 °C; 80% RH)

[92]

3M™ ionomer/PPSU

PPSU/3M™ ionomer

Dual electrospinning

93 (120 °C; 50% RH)

[121]

sPES

Nafion®

Web impregnation

87.5 (25 °C; 95% RH) for 70 wt% nanofiber content

[118]

sPAES/sPOSS

Norland Optical Adhesive 63

Web impregnation

94 (30 °C; 80% RH) for 70 vol% nanofiber content

[111]

(60/40 w/w)

sPAES

Norland Optical Adhesive 63

Web impregnation

86 (25 °C; 100% RH) for 70 wt% nanofiber content

[104]

sPFEK/PES

Co-electrospinning layer by layer

61 (80 °C; 100% RH)

[115]

sPI

sPI

Web impregnation

ca. 100 (80 °C; 98% RH) for 10 wt% nanofiber content

[122]

sPS

Nafion®

Web impregnation

180 (80 °C; 100% RH)

[116]

sPS/PEO

Vinyl-terminated poly (dimethylsiloxane)

Web impregnation

100 (25 °C; 98% RH)

[119]

(70/30 w/w)

sPEEK/PVB

sPEEK/PVA

Web impregnation

13.5 (60 °C)

[120]

(70/30 w/w)

(65/35 w/w)

sPEEK/PVB

sPEEK/PVA

Web impregnation

38 (120 °C)

[123]

(70/30 w/w)

(65/35 w/w)

PVDF/PVA

Chitosan

Web impregnation

23 (25 °C; 100% RH)

[124]

sPPESK/ZCCH

Co-electrospinning followed by hot pressing

82 (160 °C)

[106]

sPPESK EW 500

sPPESK EW 580

Web impregnation

186.4 (80 °C; 100% RH)

[109]

sPEK EW 1380

PEK

Dual electrospinning

112 (80 °C; 100% RH)

[125]

The methods initially developed to embed nanofibers into a matrix are impregnation/casting with a polymer (or prepolymer followed by cross-linking) to fill the voids of porous webs [45]. Recently, a single step strategy based on simultaneous electrospinning of charged and inert polymers has been developed, referred to as “dual electrospinning” [101, 121, 125, 126, 127]. This approach does not require a further impregnation step as the matrix polymer is already present within the dual electrospun composite, and instead membrane processing is finalized by hot pressing of the nanofiber web. Moreover, this method is not limited by dispersion/compatibility issues that often plague blended membrane systems. Ballengee et al. demonstrated the versatility of dual electrospinning by designing two distinct membrane structures: (1) Nafion® nanofibers embedded in inert/uncharged polyphenylsulfone (PPSU) and (2) a Nafion® film reinforced by a PPSU nanofiber network [126]. Both membranes were prepared from the same dual nanofiber mat of Nafion® and PPSU, which was submitted to different posttreatments. Both membrane types presented similar proton conductivities directly related to the Nafion® content, while the membrane type (1) presented superior mechanical properties.

Composite Membranes with Electrospun Polymer Fibers Embedded in an Ionomer Matrix

Another complementary strategy for preparation of composite nanofibrous electrolytes for PEMFC involves embedding a nanofiber polymer web into an ionomer matrix that ensures the proton conductivity of the system. The electrospun web is thus a robust nonconducting polymer, mainly playing the role of mechanical support in an approach at one level similar to that of ePTFE impregnated with PFSA (e.g., Gore-Select® membrane; see paragraph 1) [29, 128, 129, 130]. However, in recent years the use of proton-conducting materials or polymers functionalized with acidic or basic moieties has been developed, instead of an inert polymer, so demonstrating the versatility of the approach and the panoply of possible materials declinations and associations (as schematically depicted in Fig. 3). To accompany the increased complexity and multifunctionality of the composite nanofiber-reinforced membranes, their preparation process, initially based on simple impregnation or casting, has evolved to be further combined with other methods such as dual electrospinning, in situ functionalization and pore filling, and direct membrane deposition.

Polymers that have been used to produce electrospun reinforcing mats include polyvinylidene fluoride (PVDF), polystyrene (PS), polyvinyl alcohol (PVA), polysulfone (PS), and polyimide (PI) [57, 62, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143]. When these polymers (e.g., PVDF) were associated with Nafion® in blend membranes, poor mechanical strength and high cell resistance were observed [144, 145, 146, 147, 148]. However, when PVDF was introduced in form of electrospun fibers embedded in Nafion®, the properties of the resulting membranes were improved. MEAs comprising these nanofiber-based membranes demonstrated better DMFC performance than those using Nafion® 115 at 65 °C in 2 M methanol, which was ascribed to improved interfacial contact with the electrodes and lower membrane thickness [132].

In general, the introduction of a nanofiber mat of robust inert polymers has been demonstrated to be beneficial in particular on membrane mechanical properties, even in the absence of specific interactions between the reinforcement and the proton-conducting polymer. Arguably, the interactions operating in these composite systems are hydrophobic. A step further has thus been the optimization of the compatibility of the interface between fibers and matrix with chemical functionalization that leads to increased durability of the membranes.

Strong interactions between the reinforcing and matrix components, such as between basic polymer nanofibers and PFSA or sulfonated polyaromatic polymers have thus been employed to further improve composite membrane properties. In such acid/base systems, a distinct induction effect takes place, by which protonation and deprotonation are promoted, resulting in superior low-energy-barrier proton hopping pathways [149]. In our group we have exploited acid/base and ionic cross-linking in PBI-PFSA composite membranes. Poly[2,2′-(m-phenylene)-5,5′-dibenzimidazole] (PBI) webs were prepared by electrospinning and embedded into low equivalent weight (EW = 700 g/mol) short side-chain Aquivion®. The membrane mechanical properties showed significant improvement over those of non-reinforced Aquivion® of the same EW: Young’s modulus increased from ca 40 MPa to ca 160 MPa. The membrane proton conductivity exceeded 30 mS/cm at 110 °C, 50% RH and 160 mS/cm at 80 °C, 95% RH. PBI/Aquivion® membranes have shown exceptional stability and durability in fuel cell tests designed to accelerate chemical and mechanical degradation mechanisms, including open circuit voltage hold testing at 85 °C and 13% RH and wet-dry cycling at OCV [150]. These results are ascribed to the ionic cross-linking with proton transfer from the ionomer to the surface basic sites of electrospun PBI nanofibers. Additionally, the PBI nanofiber web may also confer chemical stabilization (see paragraph 2) to the composite membrane.

Other acid/base interactions between electrospun fibers and matrix have been exploited in composite membranes, such as sPEEK and chitosan [151] or polydopamine-modified graphene oxide [152]. They result in the improvement of dimensional stability and the proton conductivity. A systematic study on the effect of different types of interfacial interactions (acid/acid, acid/base, acid/neutral, base/base, base/neutral) between electrospun nanofibers and secondary polymers was reported [149]. Among them, acid/base interaction was confirmed as the most effective in enhancing the membrane tensile strength and proton conductivity in hydrated and anhydrous conditions. Further proof of the crucial role of ionic interactions is the improvement in mechanical and proton transfer properties when SSC PFSA was reinforced by pure polysulfone (PSU) nanofibers (weak hydrophobic interaction) [143] and 1,2,3-triazole-functionalized PSU [153] (strong acid/base interaction) [154]. Figure 9 clearly shows the lack of compatibility between PSU fibers and PFSA after drying the membrane (b) in comparison with the strong interaction achieved when the same fibers are functionalized (b).
Fig. 9

Details of SEM micrographs of PFSA membranes embedding (a) 1,2,3 triazole functionalized and (b) bare PSU [154]

Other work consisted of the functionalization of the fibers with acidic groups similar to those present in the ionomer. For example, PVDF nanofibers were functionalized with Nafion® [134, 136] and impregnated into a Nafion® matrix. The resulting membrane presented superior mechanical strength (Young’s modulus = 1840 MPa) to recast Nafion® (Young’s modulus = 1280 MPa). Its proton conductivity (60 mS/cm) surpassed that of Nafion® 117 or recast Nafion® (42 and 22 mS/cm, respectively). This result was attributed to the aggregation of Nafion® chains on the nanofiber surface inducing the formation of proton-conducting channels. The power density in DMFC and H2/O2 single cells for an MEA based on the functionalized PVDF fiber/Nafion® membrane was higher than with MEAs comprising Nafion® [134]. The presence of Nafion®-PVDF nanofibers also reduced the methanol permeability of the membrane allowing DMFC operation with 5 M methanol.

In other studies, polybenzimidazole nanofibers were doped with phytic acid and embedded into Nafion® [155]. The resulting composite membranes showed higher proton conductivity than a pristine recast Nafion® membrane at 80 °C and 40% RH (3.1 mS/cm vs 1.2 mS/cm) that was attributed to the formation of 3D network nanostructures able to effectively transfer protons and water through acid-condensed layers at the fiber/matrix interface. Gas barrier and mechanical properties were also improved upon the introduction of PBI-phytic acid fibers enabling the preparation of ultrathin membranes (<5 μm). The fuel cell performance of MEA bearing the composite membrane was better than that of the recast Nafion®-based MEA, especially at low relative humidity, which was ascribed to the lower membrane resistance and gas barrier permeability than the pristine PFSA membrane; however, the long-term stability and possible elution in wet operation conditions are unknown.

In general, true blend formation between two different polymers exists only in very narrow composition domains and more usually in polymer mixtures separation of phases occurs at the macroscopic level. An elegant alternative is using a network of electrospun nanofibers to organize and control this inherent phase separation. For example, composite membranes consisting of proton-conducting sulfonated polyaromatic polymers including sulfonated polyether ether ketone (sPEEK) and polyimide (sPI) [156] nanofibers were embedded in ionomers including the same sPEEK [120, 157] and sPI [80, 81, 158] and Nafion® [159]. sPEEK nanofibers were incorporated into Nafion® giving rise to a membrane with improved thermal stability, proton conductivity, and lower dimensional swelling [157]. sPI electrospun nanofibers and nanofiber-based fuel cell membranes have been investigated in particular by Kawakami’s group [79, 80, 81, 158, 159]. Gas barrier properties (Fig. 10a), proton conductivity (Fig. 10b), and oxidative and hydrolytic stability were improved compared to corresponding non-modified ionomer membranes (see paragraph 3).
Fig. 10

(a) Oxygen gas permeability at various relative humidity at 80 °C and (b) relative humidity dependence of proton conductivities on the nanofiber composite and recast Nafion® membranes (Reprinted from [159]. Copyright (2017), with permission from Elsevier)

The advances in fuel cell membranes and the versatility of electrospinning led also to novel process methods for their preparation. For instance, Thiele’s group is direct membrane deposition (DMD) (Fig. 11a) [160] of electrospun poly(vinylidene fluoride-co-hexafluoropropylene) nanofibers onto gas diffusion electrodes followed by inkjet printing of a Nafion® ionomer dispersion into the pores. This approach omits a membrane annealing step which is generally considered to be critical for membrane long-term stability. At 120 °C and 35% RH, the power density of the DMD H2/air fuel cell was about 1.7 times higher than that of the reference fuel cell with Nafion® HP membrane (Fig. 11b). Ionic and charge transfer resistance are lower than those of the Nafion® HP membrane due to the membrane architecture and its low thickness (12 μm). Voltage decay below 0.8 mV/h was observed in 100 h accelerated stress test, in agreement with literature values for significantly thicker reinforced membranes.
Fig. 11

(a) Schematic workflow of the production of a directly deposited composite membrane (not to scale). (b) Polarization data comparing the Nafion HP membrane fuel cell to the directly deposited composite membrane fuel cell. The operation conditions were 1.5/2.5 H2/air, atmospheric pressure (Reprinted from [160]. Copyright (2017), with permission from Elsevier)

In another approach, polymer fibers (poly(vinylidene fluoride-co-chlorotrifluoro-ethylene)) were used as initiator for copper-mediated surface-initiated atom transfer radical polymerization of 4-styrene sulfonic acid sodium salt. This allowed the acidic functionalization of the fibers providing proton conductivity of the system while filling the pores with the polymerized material to achieve a dense membrane [161]. This method led to highly proton-conducting and durable membranes.

Besides ionic cross-linking, chemical cross-linking is widely used to increase mechanical strength and dimensional stability of polymer electrolyte membranes [162]. It has been used in PFSA-based membranes and to a larger extent in sulfonated polyaromatic membranes; even if when ionic functions are utilized in forming the cross-links, the proton conductivity can be reduced. The use of cross-linking in electrospun fibers can be beneficial by confining and limiting this undesirable effect, as well as to avoid their solubilization during the impregnation step. It can be performed via the introduction of bifunctional molecular cross-linkers or by the application of heat, light, or pressure. The cross-linker molecule can be incorporated into the polymer solution before electrospinning [120, 163, 164] or put in contact with the already electrospun web either in vapor or liquid form [133, 165].

The latter strategy was employed to cross-link polyvinyl alcohol (PVA) fibers to be embedded into PFSA. PVA was chosen because of its low methanol permeability; however, it is water soluble, which precludes the application of these membranes in low-temperature fuel cells [166]. To reduce solubility, electrospun PVA webs were exposed to glutaraldehyde vapor to induce cross-linking between their -OH side groups and were then impregnated with Nafion®. The resulting composite membrane possessed enhanced mechanical and methanol barrier properties in comparison with non-modified Nafion® membranes with similar thickness [133].

Nanofiber mats of polyaromatic ionomers such as sulfonated polyether ether ketone (sPEEK) were used as reinforcement in PFSA matrices [167]. In order to render possible the use of highly functionalized, highly proton-conducting sPEEK for the fibers, thermally induced cross-linking [168, 169] was applied. The resulting composite membranes presented reduced swelling and increased tensile properties.

This approach was also applied to non-fluorinated polymer-based electrolyte membranes [170]. Electrospun and cross-linked bromomethylated poly(2,6-dimetyl-1,4-phenylene oxide) (BPPO) fibers were embedded into sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (sPPO), and the effect of pore-filled sPPO with different ion exchange capacities was investigated. Upon cross-linked fiber introduction, the membrane water swelling was reduced from 90% to 20% and the proton conductivity increased from 30 to 80 mS/cm. These results were ascribed not only to the mechanical reinforcement provided by the fibrous mat but also to the affinity between hydrophilic conducting sPPO and hydrophobic reinforcing BPPO having the same backbone structure.

Compañ et al. developed an approach to prepare reinforced DMFC composite membranes, based on the use of sPEEK blended with additives in both nanofibers and matrix [120]. A web of a blend of sPEEK with hydrophobic polyvinyl butyral (PVB), enhancing dimensional stability and methanol barrier, was embedded in a blend of sPEEK with a hydrophilic PVA, ensuring high proton conductivity [120]. Thermal induced cross-linking performed at different temperatures was investigated in both components. The reinforcing effect of the nanofibers was more significant in matrices with lower cross-linking degrees. Furthermore, sPEEK-based nanofibers contributed to the proton conductivity of the membrane. Polarization curves on MEAs using these sPEEK-based nanocomposite membranes demonstrated that these systems are effective for DMFC application [123].

Cross-linking reactions were also used in nanofiber membranes for high-temperature PEMFC operating around 160–200 °C. The conventional electrolyte used is a polybenzimidazole (PBI) membrane doped with phosphoric acid (PA) [18, 171, 172]. Several routes can be employed for the acid doping process, including immersion of preformed PBI membranes in acid solutions or polycondensation of PBI monomer components in polyphosphoric acid (PPA) or PA/PPA mixtures, followed by the controlled hydrolysis of the latter. Acid doping level (ADL = number of acid molecules per PBI repeat unit) strongly affects the membrane proton conductivity at the expenses of its mechanical properties. Electrospinning combined with chemical cross-linking is one of the strategies used to improve mechanical strength keeping high acid doping levels.

Highly acid-doped (ADL = 16–25) and proton-conducting membranes (180 mS/cm at 160 °C) were obtained by incorporating a cross-linker and, crucially, a compatibilizer, into a solution of PBI in a PA/PPA mixture [173]. By electrospinning the PBI-PPA solution containing the cross-linking molecule, followed by thermal treatment, cross-linked PBI nanofiber webs were obtained. Such reinforcements were insoluble in DMAc and could thus be embedded into highly acid-doped PBI membranes, leading to membranes with mechanical properties improved by a factor 25 compared to pristine acid-doped PBI membranes [174]. The H2/air (atmospheric pressure) fuel cell polarization curves of MEAs incorporating a cross-linked PBI membrane reinforced with an electrospun cross-linked PBI mat presented 640 mV at 0.2 A/cm2 with a maximum power density of 500 mW/cm2. The high durability of these membranes extends their application from stationary to range extenders for electric vehicle applications. Indeed, when applying a range extender protocol over a 1000-h period, the voltage loss was only <20 μV/h.

In other examples, PBI was electrospun with the cross-linking agent polybenzoxazine (PBz) [175]. The resulting PBz-modified PBI nanofiber web was thermally cross-linked allowing its impregnation with PBI, giving rise to a composite membrane with high ADL (≈13) and dimensional stability: Young’s modulus (6570 MPa) presented a threefold increase in comparison with the pristine PBI membrane. Furthermore, the current/voltage characteristics of the corresponding MEAs significantly improved (34% increase of maximum power density to 670 mW/cm2).

Concluding Remarks and Future Trends

Clean, efficient, and fuel-flexible fuel cell technology is beginning to contribute to the energy transition as commercial systems become available for back-up and nomad uses, as well as stationary and, crucial to greenhouse emission control, automotive applications. In particular, PEMFC with minimized pollutant emission, fast start-up, and quick response to power demands has gained increasing attention in the transport sector. Indeed, fuel cell vehicles are now commercialized in major regions of the world by Toyota, Hyundai, and Honda. Nevertheless, issues of cost and durability still need to be overcome, and thus novel materials with alternative designs and compositions are being developed for a next generation of fuel cell components.

This chapter provides an overview of the most pertinent research in the area of electrospun nanomaterials and their application in fuel cell proton exchange membranes. Electrospinning offers the possibility to create nanofiber structures with desired size, morphology, porosity, alignment, architecture, and composition, which has a significant impact on the final features of composite membranes. Furthermore, electrospinning allows the upscaled production of nanofibers with the perspective of industrial application. Chemistry and phase separation in these nanofiber-based membranes is strongly dependent on the choice of the nanofiber material and its functionalization. Thus, the interaction between membrane components at the macro and molecular scale can be designed according to membrane preparation conditions. Indeed, separating functions of mechanical strength and ion transport and controlling phase separation allow the preparation of ultrathin composite membranes with outstanding proton conductivity, mechanical strength, and stability. Further improvements on membrane durability are achievable by introduction of ionic or chemical cross-linking between electrospun and matrix components as well as by the incorporation of antioxidant species confined within the fibers. Many advances have been realized also on the preparation of the final composite dense membrane from nanofibrous mats, from impregnation and casting to dual electrospinning, direct membrane deposition, and reactive polymerization methods. Novel degrees of freedom are thus reached in the design of complex composite systems with effective and upscalable approaches [176, 177]. All these achievements in the field of proton exchange composite membranes have the potential to be transferred to other fuel cell or related technologies, such as alkaline and high-temperature proton exchange membrane fuel cells, water and CO2 electrolysis, and redox flow batteries.

Despite all the progress here presented in employing electrospinning process for membrane development, there are still many areas to explore and challenges to face. Better understanding of the correlation between nanofibers/webs morphology, electrospinning process and membrane processing will facilitate the scale-up of membranes comprising nanofiber webs and support the transition from laboratory to new component products for energy conversion devices.

With regard to the realization of composite membranes, a key parameter will be the fine control and design of the heterogeneity and the interface between the fiber mat and the polymer matrix that we have demonstrated being crucial for performance and durability. The functionalization of the electrospun materials beyond the conventional chemical methods will lead to the combination of electrospinning with different deposition methods including atomic layer deposition and reactive plasmas, still further widening the versatility and the potentiality of the approach.

Notes

Acknowledgments

Funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007–2013) / ERC Grant Agreement n. 306682 and from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 671465 VOLUMETRIQ is gratefully acknowledged. The Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme.

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  • Marta Zatoń
    • 1
    Email author
  • Sara Cavaliere
    • 1
  • Deborah J. Jones
    • 1
  • Jacques Rozière
    • 1
  1. 1.Institut Charles Gerhardt Montpellier, UMR CNRS 5253, Agrégats Interfaces et Matériaux pour l’EnergieUniversité de MontpellierMontpellier Cedex 5France

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