F Elbaz-Poulichet

The Carnoules mine. Generation of As-rich acid mine drainage, natural attenuation processes and solutions for passive in-situ remediation

The former Carnoules Pb-Zn mine (Department of Gard, France) has produced 1.5 Mt of solid waste containing pyrite and associated metals and metalloids including arsenic and thallium. The tailings are stored behind a dam. Upon oxidation they generate acid (pH≤3) water containing up to 350mg/l of arsenic, 750 to 2700mg/L of iron, sulphate (2000-7500mg/L) and a few mg/L of heavy metals (Pb, Zn, Cd). The water surges at the bottom of the dam forming the Reigous Creek.

For the last ten years, the physicochemical, biological, mineralogical and hydrological characteristics of the site have been monitored by the Hydrosciences laboratory with the aim of understanding the processes responsible for the release of pollutants in the tailing stock and of reducing pollution in the mining creek. The results of this study indicate that, unlike at other sites, pyrite oxidation takes place at the bottom of the tailing stock at the entry of oxygenated underground water. The bacterial populations that are present in the tailings and may catalyse the oxidation reactions were identified. About 20 to 30% of the arsenic initially present in the spring water in the form of As(III), precipitates with Fe in less than 30 meters. This leads to the formation of ochre precipitates containing 20% of arsenic around bacterial structures. Several organisms (bacteria and euglena) that mediate Fe and As oxidation in the creek and thus contribute to accelerating arsenic retention in the solid phase were identified. On the basis of these results and pilot plant experiments, solutions for passive in-situ bioremediation are proposed.

INTRODUCTION

The history of mining activity has resulted in a significant waste legacy, which represents a threat for the environment. Major accidents, most often resulting from the breaching of an effluent retaining dike, can cause major pollution accidents like in England where the outburst of the Wheel Jane tin mine in 1992 resulted in severe contamination of the Carnon River and the Fal Estuary by toxic metals [START_REF] Younger | The contribution of science to risk-based decision-making: lessons from the development of full-scale treatment measures for acidic mine waters at Wheal Jane, UK[END_REF]. More recently, the breaking of a dam retaining wastes from the Aznallcollar mine in Spain resulted in the contamination of the Guadiamar River [START_REF] Achterberg | Impact of Los Frailes mine spill on riverine, estuarine and coastal waters in southern Spain[END_REF], an affluent of the Guadalquivir, which drains the Donana national park (UNESCO world reserve).

These well-publicised disasters should not mask the silent reality of chronic and long lasting environmental pollution generated by mine drainage. In many cases mining waste can continue to adversely affect the environment for centuries after the abandonment of the mine itself, contaminating the soils in the immediate vicinity but also being dispersed by both surface and ground waters. Although precise quantification of the scale of this pollution is difficult, [START_REF] Johnson | Acid mine drainage remediation options : a review[END_REF] estimated that, in 1989, approximately 19,300 km of rivers and 72,000 ha of lakes and reservoirs throughout the world were severely affected by mining effluents.

Mine drainage results from the oxidation of pyrite and other sulphide minerals generally associated with coal and metal-bearing mineral deposits. They are most often acidic and contain high concentrations of metals and metalloids (including arsenic)

The Carnoules mining site (town of Saint Sebastien d'Aigrefeuille, France) provides an example of the unwanted legacy of mining activity. After a review of the history of the site including critical episodes, we summarize the studies that have been carried out for decades to better understand the link between chemical, biological and hydrodynamic processes which control the generation of acid mine drainage (AMD). We then examine a potentially low cost method of passive in-situ bioremediation of water.

THE HISTORY OF THE CARNOULES MINE (SAINT SEBASTIEN D'AIGREFEUILLE, FRANCE)

The exploitation

The extraction of Zn-Pb at Carnoules (Fig. 1 Subsequently, the mining company cleaned the site and reinforced the dam.

After receiving official authorisation (departmental decree), the mine officially closed on October 24, 1963. It has generated a total of 1 500 000 tons of wastes, split into two tailings of respectively 460 000m 3 and 115 000 m 3 , the first being located at the source of the Reigous Creek.

From mine closure (1963) to now

In 1963, the residents of the Amous River basin formed an association. Their fight was legitimated by the publication of a report by [START_REF] Faucherre | Rapport sur l'analyse des eaux de l'Amous[END_REF] attesting that the river water was toxic for both animals and humans. The Penarroya mining company rejected any responsibility by arguing that water pollution had already been present before 1951. In 1968, the company sold the site to a Belgian notary.

In 1970, a quantitative model of the acidification process, and the mechanism of dissolution and deposition of the chemical elements was proposed by Michard and Faucherre (1970). To our knowledge, this was the first scientific publication on the consequences of the oxidation of the sulphide wastes from the Carnoules mine.

In September 1976, the tailings partially collapsed due to a violent Mediterranean thunderstorm. This was followed in October 1976 by the sudden evacuation of the 100 000 m 3 of water initially contained in a lake that had formed in the tailing stock. The accident was responsible for the pollution of water and soil in the entire Amous valley.

The accident also highlighted a juridical imbroglio where both the administrative authorities and Pennaroya and later Metaleurope tried to pass on to each other responsibility for the former mining installations and for the cost of the security works that needed to be undertaken to prevent further dissemination of the pollution. In 1980, in the absence of any consolidation of the tailings, another thunderstorm resulted in further pollution of the downstream river network.

In 1982-1983, the wastes were gathered in a single tailing stock with a capacity of about 550 000 m 3 , behind a concrete dam (Fig. 1). The tailings were covered with an impermeable layer of clay, then arable soil and vegetation. However, this did not prevent acidic and As-rich water (pH=2-3) from emerging at the base of the dam and continuing to pollute the Reigous Creek and the Amous River. In addition, due to corrosion of the concrete by the wastes, the pipes channelling the water under the tailing stock exploded. This resulted in the formation of slashes, exposing the pyrite to air and water. In This material is deposited in the middle of and across the upstream part of The Reigous Creek at the site of its natural spring (Fig. 1).

The surrounding runoff waters are collected by concrete ditches running along the borders of the tailing surface. These waters flow into a small shaft and then down the gradient to the Reigous Creek through a concrete pipe that crosses the tailings.

Rainwater is collected along the surface of the tailings by the same system; then ditches drain the water laterally.

The site has been monitored by the HydroSciences Laboratory for more than 10 years. The equipment comprises 17 piezometers in the tailings, automatic systems for continuous water discharge acquisition and a microcosm for aerobic water treatment. This intensive instrumentation has enabled documentation of the main hydro-bio-geo-chemical characteristics of the site.

The tailings stock: a reverse oxidation zone

The general hydrochemistry and aquifer hydrodynamics have been broadly characterised by [START_REF] Koffi | Reverse oxidation zoning at a mine tailings stock generating arsenic-rich acid waters (Carnoulès, France)[END_REF]Casiot et al., (2003a). The clay cover and the low permeability of the waste material (10 -7 m/s) considerably limit infiltration of rainwater into the tailings. During extreme rainy events, sinkholes formed in the tailing but even after refilling, AMD generation continues in absence of an oxidation zone at the surface. Variations in the water table, calculations of the water balance and flow modelling indicate subsurface water inputs and water flow along the bedrock within a more permeable sandy horizon. The primary region of oxidation is thus located at the base of the tailings,

where the oxygen rich rainwater can penetrate directly.

The AMD is mainly generated in the northern part of the tailings. The mineralogy of the precipitates is reported in [START_REF] Morin | Bacterial immobilization and oxidation of arsenic in acid mine drainage (Carnoulès creek, France). XANES and XRD evidence of As(V)-or As(III)-Fe(III) gels and tooeleite[END_REF]. In winter, the precipitation of Fe(III)-As(III) predominates, in the form of nano-crystals of tooeleite, a very rare mineral. In summer, the precipitate is amorphous and contains mainly Fe(III)

and As(V). The reasons for this seasonal difference are not completely understood, but cannot be linked to water temperature, which remains about the same (14-16°C) in the vicinity of the source. Laboratory experiments showed that the oxidation processes that take place in Reigous Creek are mediated by bacteria (Casiot et al, 2003b, Fig.3). In this context, seasonal variations in community structures may well play a role in determining the nature of the precipitate as also suggested by laboratory experiments [START_REF] Morin | Bacterial immobilization and oxidation of arsenic in acid mine drainage (Carnoulès creek, France). XANES and XRD evidence of As(V)-or As(III)-Fe(III) gels and tooeleite[END_REF]Casiot et al., 2003b, Bruneel et al., 2003[START_REF] Duquesne | Immobilization of arsenite and ferric iron by Acidithiobacillus ferroxidans in acid mine drainage[END_REF]. Reigous and the confluence with the Amous River. This decrease is related to (i) dilution with non-polluted seepage and (ii) coprecipitation of metals and As with Fe oxides that precipitate all along the Reigous Creek and at the Amous-Reigous confluence [START_REF] Casiot | Sorption and redox processes controlling arsenic fate and transport in a stream impacted by acid mine drainage[END_REF].

About 30 m away from its source, the Reigous Creek receives contributions from small tributaries. Those coming from the left bank do not drain either the mineralised area or the former mining installations. However, the water coming from the right bank drains abandoned mine galleries and quarries where pyrite is still present.

The increase in Cd, Pb, and Zn, which occurs at station Reigous av.conf., thus probably reflects the inputs from the former mine quarries.

At the confluence with the Amous River (Fig. 1), the Reigous discharge ranges between 0.6 and 20L/s. The Amous discharge ranges between 50 and 150L/s. Upstream from the confluence, the Amous River is not affected by AMD and the concentrations of dissolved metals, arsenic and sulphate, which are generally several orders of magnitude lower than in the Reigous Creek, do not provide evidence of contamination but simply reflect the natural regional background. Downstream from the confluence, the dissolved concentrations in the Amous River increase drastically and the dissolved concentrations of Cd sometimes exceed the French regulatory limit for the conservation of satisfactory biodiversity ([Cd]=1.3 µg/L). [START_REF] Casiot | Sorption and redox processes controlling arsenic fate and transport in a stream impacted by acid mine drainage[END_REF] showed that the increase in pH that occurs where the two rivers meet leads to the precipitation of large amounts of Fe in the form of ferrihydrite which scavenges As(III). However this natural attenuation process is not efficient at high water temperatures. In addition, reductive processes may also seasonally contribute to the reduction of Fe-oxides and to the release of As into solution. Hence, the Amous River can be still considered as seriously affected by mine drainage up to where it meets the Gardon river at Anduze.

Table 1 shows the annual metal fluxes of the Reigous Creek calculated on the basis of monthly sampling.

These fluxes are generally low compared to those transported by the Rhone River to the Mediterranean Sea [START_REF] Guieu | Significance of atmospheric fallout on the upper layer water chemistry of the North-Western Mediterranean[END_REF].

Nevertheless, [START_REF] Ollivier | Interface continnt-océan. Géochimie du Rhône (flux et bilan d'érosion) et transfert d'eaux souterraines en Camargue (Apport des isotopes du radium)[END_REF] showed that the contribution to the total Rhone fluxes increases drastically during flood events that affect the Gardon. To cite one example, the flood event of September 2002 represented respectively 26% and more than 30%

of the particulate load of Sb and Pb transported by the Rhône River at the city of Arles.

Table 1: Metal fluxes in the Reigous Creek. Further work is necessary to optimize the functioning of the treatment process to remove 2 to 7 T/yr of As over a period of about 500 years. Another challenge would be to block the generation of AMD inside the tailings themselves.

Reigous flux (kg/an) SO4

  ) officially started in 1730. At the beginning of the 20 th century, the volume of the mining waste was estimated at 38 000m 3 . Between 1951 and 1963 Pennaroya and then Metaleurop mining companies extensively exploited the ore. In 1961, during exploitation, the breaking of a dam retaining the water was responsible for major pollution of the Reigous Creek and the Amous River.

  Fig. 1: Maps showing the location of the Carnoules site (a), a sketch map of the first 30 meters of the Reigous Creek (COW) showing the locations of the sampling stations.(b), cross-sections (c,e) and map (d) of the Carnoules tailing deposit. (OZ:oxidising zone) Variations in water level are given for December and June 2002.

Fig. 2 .

 2 Fig. 2. Variations in O2 (a), Fe(II) (b) and dissolved As(III) (c) along the Reigous creek. Fe and As concentrations are expressed as a percentage of their concentration in the spring water (suffix s). Total dissolved As varies as As(III). 1, A, C, E, F, 2: Sample locations (cf. Fig. 1b).

bFigure 3 :

 3 Figure 3: Kinetics of Fe (triangles) and As removal (circles); free bacteria cells (open symbols). a bacterial precipitate is present (fixed bacteria) (closed symbols); UV light (solid line); experiments in the dark (dotted line). A is the concentration of Fe(II) or total dissolved As in the aqueous phase and A0 is the concentration in the Carnoules spring water before incubation

Fig. 4 :

 4 Fig.4: pH, dissolved metal, As and sulphate concentrations in the Reigous creek and in the Amous River. The stations on the Xaxis are ranked according to their distance from the Reigous Spring: COWG: 40m, Aval Ru rouge: 300m, Reigous av. conf.: 1120m; Amous 1200: 2300m.

  (2005) recently presented a review of AMD remediation strategies in which they distinguished abiotic or biotic treatments and subsequent active or passive treatments (Fig.5).Active treatments require continuous intervention, for example the supply of chemical products. Passive treatments exploit processes that occur in natural systems. Their maintenance costs are generally lower than for active treatments. Biological processes and bioremediation are generally based on the capacity of microorganisms to catalyse chemical reactions, which immobilise metals and/or neutralise the water acidity.

Figure 5 :

 5 Figure 5: Different AMD remediation strategies (Johnson and Hallberg, 2005).

Figure 6 :

 6 Figure 6: Sketch map of the passive in-situ remediation (PIR) system for the removal of As-rich ochre in Carnoulel AMD.


Rakhi Sood

Sara Cavaliere
email: sara.cavaliere@umontpellier.fr

Deborah J Jones

Jacques Rozière

Electrospun Nanofibre Composite Polymer Electrolyte Fuel Cell and Electrolysis Membranes

Keywords: PEMFC, DMFC, electrospinning, nanofibres, polymer electrolyte/proton conducting membranes, mechanical reinforcement, inorganic materials, crosslinking

Large-scale commercialisation of Proton Exchange Membrane Fuel Cell (PEMFC) technology for automotive and stationary applications demands the development of a robust, durable and cost-effective materials. In this regard, ionomer membranes being present at the core of PEMFCs are required to maintain elevated proton conductivity, high mechanical strength and low gas permeability during the lifespan of the fuel cell. These challenges are addressed by investigating novel nano-structured membrane materials possessing long-range spatial organisation of ionic and hydrophobic domains at the micro-and nano-scales. Electrospinning, a versatile and easily up-scalable tool for the preparation of nanofibrous polymers and ceramics with targeted architectures, is being extensively applied for the development of nanostructured electrolyte membranes. This review describes the most important advances in the use of electrospun materials for the preparation of new generation fuel cell proton conducting membranes. It also highlights the challenges to be overcome and the new directions and future application fields of composite nanofibre-based membranes in the broader context of energy materials.

Introduction

Proton exchange membrane fuel cells (PEMFC) and water electrolysers (PEMWE) are expected to play significant roles in the near future in the electrification of transport, for heat and electricity supply in stationary applications, and as a means of large scale storage of renewable energy by using wind or solar energy to provide power for hydrogen generation by water electrolysis [1,2]. Despite their advanced development, costs must be further reduced, and the durability of the components over time must still be improved [3][4][5]. The Membrane Electrode Assembly (MEA) is the core component of these electrochemical energy conversion devices. The MEA comprises a proton exchange membrane, anode and cathode, as schematically represented in Figure 1. At the core of the MEA is the polymer membrane generally functionalised with either acid or basic groups [6][7][8][START_REF] Jones | Polymer Electrolyte Membrane and Direct Methanol Fuel Cell Technology[END_REF][START_REF] Varcoe | [END_REF][11][12][13][14][15]. The membrane separates the electrodes and prevents direct reaction of reactant gases and transport ions between the electrodes. It also provides mechanical integrity to the assembly. Proton conductivity in polymer electrolyte membranes strongly depends on water content as water molecules act as proton carriers [16]. Materials providing the best association of the above properties of conductivity, chemical stability and mechanical strength are perfluorosulfonic acid (PFSA) ionomers such as Nafion ® , Aquivion ® , 3M Corporation ionomers. The high proton conductivity of PFSA-membranes is correlated with their morphology in which ionic domains are wellpercolated and phase-separated from hydrophobic domains that provide mechanical strength [17]. The importance of the spatial organisation of hydrophilic and hydrophobic domains in determining the conductivity and mechanical properties of ionomer membranes is wellestablished [18,19]. This is influenced by the nature of the polymer backbone and side chain compositions, and the spatial organisation of functional and non-functional groups in random polymers and block copolymers.

In an operating fuel cell or electrolyser, particular constraints or stresses arise that lead to accelerated materials ageing. For example, membrane mechanical failure is accelerated by the swelling and shrinkage that occurs during membrane hydration and dehydration that accompany operation under variable load [20]. Membranes showing little or no change in dimension at different water contents are required, since they will be more robust over time on fuel cell operation and extend MEA lifetime. They also allow the use of thinner membranes which facilitates water management in the cell, and lowers the electrical resistance. These factors have triggered extensive research on the development of improved proton exchange membranes comprising 1) Advanced perfluorosulfonic acid ionomers [21,22]; 2) PFSAionomer composite membranes [23,[START_REF] Scherer | Fuel Cells I[END_REF]; 3) Functionalised polyaromatic or partiallyperfluorinated ionomers and their composite membranes [4,13,[START_REF] Jannasch | [END_REF][26][27][28][29][30][31][32][33][34][35], and4) Ionomers or polymers in combination with strong acids/heterocycles/ionic liquids [3,[36][37][38][39]. Of all these approaches, novel sulfonated polyaromatic polymers, shorter side chain PFSAs, and composite membranes (polymer blends and inorganic-organic membranes) with PFSA or alternative sulfonated polymers have been greatly investigated.

Expanded PTFE (ePTFE) impregnated with Nafion ® or other PFSA (e.g. Gore-Select ® membrane) leads to reinforced membranes having excellent mechanical properties due to the presence of the ePTFE reinforcement, which allows the preparation of thin membranes (5-30 µm) with low area resistance [40][41][START_REF] Bahar | Ultra-Thin Integral Composite Membrane[END_REF], but also low methanol crossover [START_REF] Lin | [END_REF] that show much higher durability than non-reinforced membranes. Nevertheless, ePTFE reinforced membranes have lower proton conductivity than predicted from the inert polymer volume fraction, which has been attributed to the incompatibility between the highly ionic PFSA and the low-dielectric PTFE, resulting in an ionomer deficient layer [44]. Similarly, inorganic-organic membranes have markedly improved mechanical properties, resulting from the specific interactions between inorganic and organic components and the internal scaffolding role played by the inorganic material, that have enabled fuel cell operation at higher temperatures and lower relative humidity (RH) in many cases in comparison to the corresponding non-modified membranes [45][46][47][48][49][50].

Electrospinning is increasingly recognised as a powerful means of introducing unique phaseseparated architectures into composite proton exchange membranes [51][52][53][54][55]. The electrospinning process leads to mechanically robust fibrous structures with diameters in the nano to micrometre range by utilising the electrical force between a drop of fluid polymer on a capillary tip and a grounded collector. As the intensity of the electric field increases, the hemispherical surface of the drop elongates to form a conical shape known as the Taylor cone.

For a critical value of the applied electric field, the repulsive forces overcome the surface tension of the drop and a jet of the solution is ejected from the tip of the Taylor cone. The solvent evaporates during flight of the jet to the collector, and a mat of solidified continuous, ultra-thin fibres is formed. Figure 2 is a schematic illustration of an electrospinning set-up and the Taylor cone jets issued from the electrified nozzle during the electrospinning process. The electrospinning technique is of particular interest in the development of fuel cell and electrolyser membranes since it allows introduction of a nanophase-separated morphology in composite systems [START_REF] Subianto | Electrospinning for Advanced Energy and Environmental Applications[END_REF]. Indeed, it allows the elaboration of composite membranes with a rather facile means to control of the long-range organisation/distribution/percolation of hydrophilic and hydrophobic domains of the ionomer by adjusting the type of electrospun material, the volume fraction of the nanofibres and the experimental conditions. Additionally, electrospinning can provide uniaxial alignment of the polymer chains within nanofibres, which can result in enhanced mechanical properties [START_REF] Kakade | [END_REF]58] and can promote the formation of interconnected channels which facilitate improvement in proton conductivity [59,60].

All these factors have greatly increased the range of materials that can be prepared through electrospinning and explains the increased interest in the last decade. Figure 4 shows the number of articles published from 2005 to 2015 on electrospun materials for fuel cells (Figure 3 (a)) and electrospun polymer membranes for fuel cells in particular (Figure 3

(b)).

Additionally, other recent articles have dealt with the general aspects and the trends in the development of materials for electrochemical energy storage and conversion devices using electrospinning [53][54][55][START_REF] Cavaliere | Electrospinning for Advanced Energy and Environmental Applications[END_REF][START_REF] Palaniswamy | [END_REF]. This review focusses on the strategies employed for the development of polymer membranes incorporating an electrospun nanofibre component, and the interest as discussed in the recent literature for their application in fuel cells or electrolysers.

The review is broadly organised according to four types of membrane architecture as presented in Figure 4: (a) electrospun mat of inert reinforcing nanofibres embedded in a proton conducting polymer, (b) electrospun mat of nanofibres of a proton conducting polymer embedded in an inert polymer, (c) an inorganic electrospun (proton conducting or non-conducting) nanofibre mat embedded into a (proton conducting or non-conducting) polymer matrix, (d) a cross-linked electrospun polymer mat embedded in a (proton conducting or non-conducting) polymer matrix.

Composite membranes comprising an electrospun mat of uncharged nanofibres embedded in an ionomer matrix

This strategy involves electrospinning a robust non-conducting or a weakly proton conducting material into a nanofibre mat, which acts primarily as a mechanical support, followed by its subsequent embedding (by impregnation, casting etc.) into a proton conducting polymer.

Recent developments in the electrospinning technique have enabled the possibility to control spatial orientation of the electrospun fibres and hence well-aligned and highly ordered architectures can be elaborated, which can further increase the reinforcing effect of the electrospun mat in the composite membrane. An overview of membranes incorporating inert electrospun nanofibres is shown in Table 1. Typical polymers that have been used to produce electrospun reinforcements include polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polystyrene (PS), polysulfone (PSU and polyimide (PI) [63][64][65][66][67][68][69][70][71][72][73][74][75].

Several endeavours have been undertaken specifically to develop fuel cell membranes with reduced methanol crossover for direct methanol fuel cells (DMFC) by introduction of inorganic particles or inert polymers into the ionomer to act as a physical barrier and increase tortuosity [6]. For instance, PVDF, selected for its high resistance to methanol crossover, thermal stability and mechanical strength, has been combined with Nafion ® to obtain blend membranes [76][77][78].

However, the resulting blend membranes were observed to suffer from insufficient mechanical strength, which was considered to be due to the partial miscibility between the two polymers, [79,80] and from high cell resistance, which was attributed to poor compatibility of the membrane and the electrodes. In an attempt to overcome these drawbacks, Choi et al. developed membranes from electrospun PVDF and Nafion ® . The electrospun PVDF mats exhibited high porosity and formed a 3D inter-connected network that was intact within the composite membrane after impregnation with Nafion ® [64], Figure 5 (a). The membranes presented lower proton conductivity than pristine Nafion ® . However, MEAs with the nanofibre composite membrane were reported to show better electrochemical performance than when using Nafion ® 115 at 65 °C in 2M methanol (Figure 5 (b)) which was attributed to improved interfacial contact with the electrodes as well as lower thickness of the composite membranes. In an attempt to enhance the compatibility at the interface between PVDF nanofibres and PFSA ionomer, Li et al. functionalised PVDF nanofibres with Nafion ® [68,70]. After impregnation of a Nafion ®functionalised PVDF electrospun mat with Nafion ® , the resulting membrane was reported to present superior mechanical strength (Young's modulus of 1840 MPa) and methanol-barrier properties (methanol permeability of 0.86 x 10 -6 cm 2 /s) compared to recast Nafion ® with large thickness (Young's modulus of 1280 MPa and methanol permeability of 1.46 x 10 -6 cm 2 /s). Furthermore, its proton conductivity was observed to be higher (60 mS/cm at 20 °C) than that of a Nafion ® membrane comprising a mat of non-functionalised PVDF nanofibres, Nafion ® 117 or recast Nafion ® (36, 42, 22 mS/cm respectively). The direct methanol fuel cell power density with 2M methanol for an MEA with the functionalised PVDF mat was 83 mW/cm 2 at 70 °C (60 and 55 mW/cm 2 respectively in MEAs with Nafion ® 117 and recast Nafion with thickness of about 90 μm). The authors attributed the improved properties to the formation of proton conducting channels on the nanofibre surface [68]. The mechanical properties and proton conductivities of these membranes are shown in Figure 6 respectively. The properties requirements for water electrolyser membranes are in many respects similar to those of fuel cell membranes however since the difference in pressure between anode and cathode sides in a water electrolyser can be several tens of atmospheres and the constraints in terms of membrane mechanical strength are even more stringent. Furthermore, the hydrogen permeability of an electrolyser membrane must be low enough to ensure that the safety threshold concentration of hydrogen in oxygen is not reached at the cathode through hydrogen crossover from the anode. The presence in the membrane of an electrospun nanofibre network of a robust polymer can help in satisfying both of these requirements. Polysulfone nanofibre reinforcements have been embedded in short-side-chain PFSA (Aquivion ® ) membranes of ionomer equivalent weights 700 -1000 mol/g. In each case, the Young's modulus of the reinforced membrane was greater than that of the corresponding non-reinforced membrane, and while the proton conductivity was lower, this was a consequence only of the lower ion exchange capacity of the membrane, and was not compounded by any other effect. The hydrogen crossover of the reinforced membranes was slightly lower than that of Nafion ® [81]. Positive effects in particular on membrane mechanical properties are thus observed for composite membranes incorporating an electrospun nanofibre mat, even in the absence of specific interactions between the reinforcement and the proton conducting polymer. This is the case typically with PSU and PFSA for example; arguably hydrophobic interactions will operate in the case of PVDF reinforced PFSA. Nevertheless, stronger interactions between the reinforcing and matrix components, such as between a basic polymer and PFSA or sulfonic acid functionalised polyaromatic polymer, are expected to provide greater benefit. Further, in such acid-base systems a distinct induction effect is exhibited, by which protonation and deprotonation are promoted, resulting in superior low-energy-barrier proton hopping pathways [82].

In recent work, electrospun poly[2,2'-(m-phenylene)-5,5'-dibenzimidazole] (PBI) webs have been embedded into low (700 g/mol) equivalent weight short-side-chain Aquivion ® . Furthermore, PBI fibres with very fine diameters (120 nm) were obtained by co-axial electrospinning with PBI in the core and an external solvent sheath. The effect of the PBI mat thickness on membrane conductivity and mechanical properties was investigated, and it was concluded that for thickness ratio of around 1 (PBI mat) to 3 (overall membrane thickness) the membrane mechanical properties showed significant improvement over those of non-reinforced EW 700 g/mol Aquivion ® (elastic modulus increased from ca. 40 MPa to ca 160 MPa), while the membrane conductivity still exceeded 30 mS/cm at 110 °C, 50 % RH (160 mS/cm at 80 °C, 95 % RH). PBI-reinforced Aquivion ® membranes have shown exceptional stability and durability in fuel cell tests designed to accelerate chemical and mechanical degradation mechanisms, including open-circuit-voltage (OCV) hold testing at 85 °C and 13 % RH, and wet-dry cycling at OCV (Figure 7) [START_REF] Jones | Reinforced Ionomer Membranes[END_REF]. These results are attributed to ionic cross-linking with partial proton transfer from the ionomer to the basic sites of PBI nanofibres in the electrospun web, which improves the interface and the cohesion between the mat and the ionomer. Furthermore, heterocyclic polymers such as PBI are known to have antioxidant properties and thus the reinforcement may also provide chemical stabilisation by scavenging species such as peroxy free radicals that are produced during fuel cell operation and which attack the PFSA polymer in ways that are increasingly well understood. If this is borne out, this represents the first example of single additive to a membrane contributing to improvement of both mechanical and chemical stability.

In the studies discussed above, the electrospun nanofibre mats of inert polymers were combined with a PFSA ionomer. Lee et al. combined inorganic proton conducting sulfonic acid/phosphonic acid-functionalised silicate glass electrolytes synthesised from 3glycidyloxypropyl trimethoxysilane (GPTMS)/orthophosphoric acid (H3PO4) or a GPTMS/ 3trihydroxysilyl-1-propanesulfonic acid (THPSA) mixture and an electrospun polyimide nonwoven, KFM-NT ® [66,67]. The flexibility of the composite membranes was improved over that of the silicate glass type membrane (Figure 8) [67], and the proton conductivity at 80 °C and 80 % RH of the GPTMS/THPSA = 1/9 composite membrane (166 mS/cm) was higher than that of Nafion ® (70 mS/cm) or sulfonated polysulfone (37 mS/cm), which the authors attributed to the presence of a highly interconnected network of densely concentrated acid groups (IEC ca. 2.47 mmol/g, sulfonated polysulfone: 1.56 mmol/g; Nafion ® : 1.03 mmol/g) and large amount of chemically adsorbed water for the composite membrane.

Ballengee et al. have elaborated composite membranes by simultaneous electrospinning of charged (Nafion ® ) and inert components followed by post-electrospinning processing steps which result in different types of composite membrane architectures (ionomers fibres embedded into an inert matrix or vice versa) depending on the post-treatment (Figure 9) [84][85][86]. The details of this approach will be further addressed in the section 3 below.

Composite membranes comprising an uncharged polymer and an embedded 3D inter-connected mat of ionomer nanofibres

This approach involves electrospinning an ion conducting material into a 3D interconnected network nanofibre mat that is impregnated typically with an inert component to obtain composite membranes. Here, the electrospun fibres provide the pathways for proton conduction while the inert component acts as the primary mechanical support of the system. Sanders et al.

were the first to explore the possibility of utilising electrospraying and electrospinning to prepare proton conducting membranes incorporating Nafion ® [87]. Since then, various research groups have investigated the elaboration of PFSA ionomer nanofibres using electrospinning.

Table 2 presents the results of different investigations associated with the electrospinning of PFSA based polymers.

It is well-established that the electrospinnability of a polymer solution critically depends on entanglements of the polymer chains, which in turn depends on the molecular weight of the polymer as well as its concentration in the solution [88]. Perfluorosulfonic acid ionomers such as Nafion ® , and short-side-chain (SSC) Aquivion ® , 3M ionomers, etc., owing to the electrostatic interactions that result from their chemical structure comprised of a hydrophobic polymer backbone and side chains terminating in sulfonic acid groups, exhibit formation of aggregates or micelles in aqueous media and non-aqueous media [89]. This self-assembly behaviour of PFSA ionomers results in a low degree of chain entanglement and generally PFSA tends to electrospray as beads rather than electrospin into fibres. The addition of a high molecular weight carrier polymer and increase of the ionomer concentration in dispersion are means to overcome this situation.

Several carrier polymers such as PEO [90][91][92], PVA [90], PVP [93] and PAA [94] have been used to facilitate the electrospinning of PFSA ionomers. Nevertheless, PEO remains the one most commonly used due to its low quantities required to obtain uniformly-sized nanofibres of PFSA polymers, the importance of which will be discussed now. Laforgue et al. electrospun 5 wt% Nafion ® dispersion containing PEO (Mw = 200 kDa) [90]. The resulting morphology of the deposited Nafion ® changed from spheres, obtained using 4.8 wt% of PEO, to nanofibres with diameter between 80 and 180 nm when using 16.7 wt% PEO. The high content of PEO in the Nafion ® nanofibres tended to lower the proton conductivity [91]. Pintauro et al. studied electrospinning of PFSA polymers with different side chain length and equivalent weight (Nafion ® EW 1100 [95], Liquion ® 1115 [96], 3M EW 825 and 733 [91]) and investigated a range of experimental parameters, including air humidity, solvent, molecular weight of carrier polymer, applied voltage, polymer solution concentration and flow rate. These authors observed that bead-free nanofibres of 3M PFSA could be prepared using merely 1 wt% of PEO (Mw = 300 kDa) by using a higher total polymer concentration of 10 wt% rather than the 5 wt% used in previous works. Figure 10 presents SEM micrographs along with the diameter distribution histograms of electrospun mats prepared from 10 wt% solution containing different wt% ratios of 3M PFSA (EW 825)/PEO [91]. Furthermore, an increase in the molecular weight of the carrier polymer from 300 kDa to 1,000 kDa enabled bead-free nanofibres with only 0.3 wt% PEO resulting in nanofibres with average diameter around 150 nm. By use of very high molecular weight PEO (8,000 kDa), Elabd et al. prepared Nafion ® nanofibres with 400 nm average diameter with only 0.1 wt% PEO [97]. Dynamic light scattering provided evidence for interactions between the Nafion ® and high Mw PEO. All these examples showed that an increase in the solution viscosity facilitates the deposition of PFSA nanofibres using a very low weight percent of carrier polymer. However, clearly the electrospinning of such solutions is not possible above a critical viscosity, where the surface tension is higher than the applied electrostatic force. Subianto et al. further studied the impact of the side chain length on the electrospinning of PFSA ionomers by comparing short-side-chain Aquivion ® EW 830 and longside-chain Fumion ® EW 900 [98?99]. At a given ionomer equivalent weight, the former has a higher volume concentration of sulfonic acid groups, and is more crystalline, than long side chain (LSC) PFSA. It was observed that the effect of the molecular weight of PEO was more marked when electrospinning the LSC ionomer, with a sharp transition between bead formation when using PEO of Mw 400 kDa, and deposition of fibres with PEO of Mw 1,000 kDa, whereas a continuous evolution from beaded to uniform fibres was observed when using PEO of increasing molecular weight on electrospinning SSC Aquivion ® . It was concluded that this is a result of weaker inter-chain interactions in LSC PFSA, which leads to a dispersion of lower viscosity. This assumption was further confirmed by the fact that using the same total polymer concentration of 20 wt% for both ionomers, the amount of PEO required for formation of electrospun fibres is significantly lower for Aquivion ® . Under these conditions Fumion ® fibres are 350-600 nm in diameter, while Aquivion ® forms thinner fibres with narrow size distribution (250-300 nm), which may be attributed to a more densely packed conformation of SSC ionomer during electrospinning, as well as to the higher PFSA:PEO ratio which favours thinner fibres.

As previously observed for Nafion ® [96], electrospinning SSC Aquivion ® from solvents such as dimethylformamide, DMF, and dimethylacetamide, DMAc, resulted in a more stable polymer jet and produce nanofibres with good homogeneity. Their high dielectric constant and dipole moment enhance solution conductivity.

The problem of self-assembly/aggregation is not pronounced for sulfonated polyaromatic hydrocarbon polymers, which completely dissolve in various aprotic polar solvents such as DMF, DMAc, DMSO, etc. The possibility of electrospinning these ionomers depends solely on physical parameters such as molecular weight and the polymer volume fraction in solution. By carefully adjusting these parameters along with other processing parameters such surface tension of the solvent, distance between the source electrode and the target substrate, temperature, humidity, solution flow rate, applied voltage etc., electrospinning of these sulfonated polymers can be favoured over their electrospraying [88,100]. Various sulfonated aromatic polymer such as sulfonated polystyrene, sulfonated poly (arylene ether sulfone), sulfonated poly (ether ether ketone) have been electrospun into nanofibres without the use of any carrier polymer [100][101][102][103][104][105][106][107].

Several authors have reported that electrospun ionomers and sulfonated polymers present higher conductivity that dense membranes of the same polymer, and have suggested that this is due to a more ordered arrangement of proton conducting channels with longer channel lengths formed due to preferential orientation of polymer chains [59,60,97,101,102]. One of the most striking examples is the work of Elabd et al. who determined the proton conductivity of single Nafion ® nanofibre (Figure 11 (a)) [97]. Values of 1.5 S/cm, which are an order of magnitude higher than the bulk membrane conductivity, were observed along single fibres of diameter 400 nm. Interestingly, proton conductivity was found to be strictly related to the nanofibre diameter i.e. fibres with diameters >2 µm presented the conductivity similar to that of bulk Nafion ® (∼ 0.1 S/cm) whereas fibres with diameters <1 μm exhibited a sharp increase in the conductivity with decreasing diameter as shown in Figure 11 (b). X-ray scattering provided evidence for the preferential alignment of inter-connected ionic aggregates along the fibre axis direction. This is an important observation that should be better exploited in the design of future fuel cell or electrolysis membranes. Indeed, when the same ionomer is used in the fibrous mat and in the matrix (e.g. poly (phthalazinone ether sulfone ketone) [108], sulfonated polyimide [59]), proton conductivity, swelling resistance and mechanical and thermal stability are increased and gas permeability decreased compared to the corresponding cast membranes, demonstrating the effect of « fiberisation ».

Choi et al. initiated the development of composite proton conducting membranes of

PFSA/sulfonated aromatic polymer nanofibres impregnated with inert crosslinkable monomer [91,95,104,109]. Electrospun PFSA was impregnated with a UV-cross-linkable polyurethane resin using the following procedure: 1) Electrospinning of the ionomer into a nanofibre mat; 2) Densification of the mat under applied pressure to increase the volume density of the nanofibres; 3) Welding of the intersecting fibres thereby generating a 3D inter-connected pathways for proton conduction by exposure of the mat to vapours of a suitable solvent or by annealing; 4) Impregnation of the mat with an inert polymer [91,95]. The dependence of the proton conductivity at 80 °C on RH, and stress-strain curves are shown in Figure 12. The nanofibre-reinforced membranes demonstrated better mechanical properties than homogeneous membranes, which is attributed to the mechanical strength of the nanofibre morphology of the PFSA polymer as well as the reinforcing effect of the polyurethane. In addition, the proton conductivity of the composite membranes was higher than that of Nafion ® 112, which was ascribed to their higher charge density as well as expected formation of inter-connected 3D network of proton channels. Furthermore, gravimetric water swelling was lower than expected, based on the volume fraction of ionomer. Similar approaches have been employed by others to prepare composite membranes comprising electrospun ionomers with the pores filled by inert or charged polymers [59,[110][111][112][113][114][115][116][117][118][119][120][121], and an summary overview is given in Table 3.

As mentioned in the previous section, composite membranes elaborated by simultaneous electrospinning of both charged and inert polymers is another interesting approach (Figure 9) [84][85][86]122,123]. Dual nanofibre mats of Nafion ® and polyphenylsulfone (PPSU) were fabricated and two types of composite membranes were obtained using different postelectrospinning treatments (Figure 8): 1) PPSU nanofibres dispersed in Nafion ® matrix obtained by thermally annealing the Nafion ® part of the dual mat, which then flows into the PPSU interfibre void space without destroying the PPSU fibre structure; 2) Nafion ® nanofibres dispersed in PPSU matrix by exposing the PPSU part of the dual mat to chloroform vapour followed by thermal annealing resulting in filling of empty space between Nafion ® nanofibres without harming the Nafion ® fibre structure [84]. These membrane types presented similar proton conductivities that depended only on the Nafion ® content, while the second membrane type presented superior mechanical properties.

Composite membranes based on inorganic electrospun mats embedded in a polymer matrix

Inorganic oxide and solid acid components such as metal oxides, zeolites, metal hydrogen phosphates and heteropolyacids have been incorporated in ionomer membranes, leading to an improvement in the proton conductivity under low RH [27,45,124] and a means of limiting the methanol crossover by increasing tortuosity [3,6,125]. Additionally, careful selection of inorganic materials based on their properties and morphologies can play a vital role in finetuning other properties, in particular increased mechanical strength and lower dimensional swelling [START_REF] Scherer | Fuel Cells I[END_REF]. In this context, the electrospinning of inorganic materials could be very valuable since it allows fabrication of homogeneously sized 1D nanostructures with tuned compositions and morphologies [126][127][128][129][130][131].

The incorporation of porous metal oxide (TiO2, CeO2 and ZrO1.95) nanotubes produced by electrospinning into a Nafion ® membrane was an effective approach to enhance the conductivity under dry but also fully humid conditions [129,131]. Indeed, it was considered that the electrospun oxides increased membrane water retention, and also enhanced water diffusion due to their tubular morphology. At 18 % RH and 80 °C, the Nafion-metal oxide nanotube composite membranes with optimum nanotube loading presented remarkably improved PEMFC performance as compared with a commercial Nafion ® 212 membrane (power density of Nafion-metal oxide nanotube composite membranes: 641 mW/cm 2 for the one with 1.5 wt% TiO2 nanotube content, 449 mW/cm 2 for the one with 0.5 wt% CeO2 nanotube content, 546 mW/cm 2 for the one with 1.5 wt% ZrO2 nanotube content and 186 mW/cm 2 for Nafion ® 212 membrane). Furthermore, at 80 °C and various RH values, the Nafion-TiO2 nanotube composite membranes always exhibited higher performance in compared to Nafion ® 212 and Nafion-TiO2 nanoparticle membranes, owing to the positive effect of the 1D hollow morphology on the water retention capacity of TiO2.

Thiam et al. developed a composite membrane of Nafion ® and electrospun palladium-silica nanofibres [126]. The role of palladium in these membranes is to improve their water retention capacity and reduce their permeability towards methanol [132,133]. Figure 13 displays SEM images of electrospun Pd-silica nanofibres that show a smooth surface and a narrow distribution of fibre diameter and homogeneous dispersion of the nanofibres within the Nafion ® matrix. A marked reduction in the methanol permeation was observed for the composite membranes owing to blockage of methanol transport channels by the Pd/SiO2 nanofibres along with significant increase in membrane water uptake and proton conductivity, due to assistance by Si-OH groups in facilitating the construction of a 3D hydrogen bonding pathway for proton diffusion throughout the membrane. The composite membrane with 3 wt% fibre content presented the best performance with proton conductivity of 129 mS/cm and methanol permeability of 8.36 x 10 -7 cm 2 /s in comparison to Nafion ® 117 with 98.3 mS/cm conductivity and 12.30 x 10 -7 cm 2 /s methanol permeability. Also, the presence of Pd nanoparticles is believed to help in the oxidation of permeated methanol and permit the diffusion of hydrogen allowing to keep a high conductivity [133].

An innovative route to the preparation of composite membranes using an electrospun inorganic mat involved utilisation of "reactive" co-axial electrospinning [128]. In this approach, the synthesis of the inorganic material occurs in situ, during the simultaneous electrospinning of the reactants from a coaxial needle. A zirconium precursor (zirconium propionate) and a phosphorus source (phosphorus pentoxide) were electrospun into a mat using a coaxial needle, the core and the sheath of the needle being fed by separate solutions. The reaction between components was initiated at the core/shell interface in the fibres that were then calcined and further treated with phosphoric acid to produce a zirconium hydrogen phosphatezirconium oxide nanofibre mat. The composition throughout the fibres was determined to be homogeneous, indicating that reaction had propagated throughout the core/shell regions.

Impregnation of this mat with Aquivion ® SSC PFSA afforded a composite membrane with greater elastic modulus and yield point and increased proton conductivity than pristine, noncomposite Aquivion ® membranes [128].

Alkali-metal and ammonium hydrogen sulfates, selenates and phosphates constitute one of the largest families of solid proton electrolytes. These compounds undergo a phase transition from a low-temperature phase to a superionic phase, characterised by a dynamically disordered hydrogen-bond network. Such compounds are of particular interest since they are proton conductive at temperatures in a range (200-300 °C) where few materials are available and yet membrane development from them is difficult since they are inorganic salts. Recently, an interesting approach to produce a highly proton conducting nanofibre mat of caesium dihydrogen phosphate, CsH2PO4 (CDP) has been described [134], whereby CDP was thermally treated at a temperature higher than its super-protonic phase transition temperature, leading to its dehydration and partial polycondensation as per following reaction:

CsH2PO4(s) → CsH2-2xPO4-x (s) + xH2O (g), (0 < x ≤ 1)
The resulting CDP-polymeric material consisting of chains of condensed phosphates was watersoluble and could be electrospun without a carrier polymer. SEM micrographs of freshly synthesised CDP particles, polymeric CDP and electrospun polymeric CDP are shown in Figure 14, where it may be seen that a highly inter-connected CDP-polymer nanofibre mat was formed.

Its proton conductivity of 8×10 -3 S/cm at 250 °C is close to that of the bulk. Although means have to be found to occlude the residual porosity without loss of the high temperature conductivity, the approach is one route to electrolyte membrane development for intermediate operating temperatures.

Yao et al. prepared electrospun highly conductive sulfated zirconia S-ZrO2 combined with a cross-linked poly (2-acrylamido-2-methylpropane-sulfonic acid, C-PAMPS [116]. The proton conductivity of the membranes increased with decreasing fibre diameter due to increase in the number density of S-ZrO2 fibres, which in turn increased the proportion of ionic channels, water uptake and ion exchange capacity (IEC). It also increased with S-ZrO2 volume fraction owing to fibre induced inter-connected channels capable of anchoring water molecules and providing facile hopping pathways for proton transfer. S-ZrO2/C-PAMPS hybrid membranes comprising 85 nm fibres and 30 % fibre content shows proton conductivity of 3.4×10 -1 S/cm at 100 °C, much higher than that of previously reported C-PAMPS films or composite Nafion ® membranes comprising S-ZrO2 nanoparticles.

Another promising concept, developed by Laberty-Robert et al. [135][136][137], involves electrospinning of combined organic-inorganic precursor solutions to elaborate hybrid membranes which mimic the nanostructuration of Nafion ® (i.e. phase separation between hydrophobic and hydrophilic domains at the scale from few nanometres up to several micrometres). The resulting hybrid membranes consists of an inorganic constituent ensuring proton conductivity and an organic constituent ensuring the mechanical robustness of the system. For instance, electrospinning of a sol-gel-based solution containing PVDF-HFP (polyvinylidenefluoride-hexafluoropropylene) and (2-(4-chlorosulfonylphenylethyltrichlorosilane (CSPTC) with tetraethylorthosilicate (TEOS) gave rise to an organic-inorganic hybrid membrane [136]. Such membrane presented a conductivity of 15 mS/cm at 120 °C under 50 % relative humidity and modulus much higher than that of Nafion ® above 80 °C. The intriguing properties of the organic-inorganic membrane is attributed to its microstructure consisting of bundled fibres (corresponding to assembly of small polymer fibres) surrounded by functionalised silica domains.

Composite membranes of cross-linked electrospun mats embedded in a polymer matrix

Cross-linking of polymer chains is a well-known strategy to produce robust and functional proton conducting membranes with enhanced thermal stability, mechanical properties, and reduced solubility [20]. Two typical cross-linking routes include (1) polymerisation of multifunctional monomer units with partial cross-linking during polymerisation (2) crosslinking of preformed polymer chains. The latter method allows facile processing and may be brought about through reaction along the polymer chain with bi-functional small molecule cross-linkers, a process which generally proceeds through mild and well-defined chemistry, but results in material that must be further purified to remove either catalyst or unreacted small molecules. Alternatively, cross-linking may be triggered externally by the application of heat, light, or pressure to generate highly reactive intermediates that undergo non-selective bond formation in an uncontrolled and ill-defined manner.

To date, only the second of the above routes has been explored to some extent in association with an electrospinning deposition step, where the cross-linked mat was the reinforcing or the ion conducting material. In particular, the polymer solution was mixed with a cross-linker molecule followed by electrospinning into a nanofibre mat and subsequent thermal treatment to induce the reaction [120,137,138] or the polymer solution was electrospun into a nanofibre mat exposed to a cross-linker either in vapour or liquid form under required conditions [65,[140][141][142][143]. The latter of the above approaches was employed to prepare composite membranes of PFSA and polyvinylalcohol for DMFC and PEMFC applications. PVA is low cost and has lower methanol permeability than Nafion ® by two orders of magnitude, but is water soluble, which results in limited dimensional stability of composite membranes reinforced with electrospun PVA [107]. To limit this solubility, electrospun PVA mats were exposed to glutaraldehyde vapour to induce cross-linking between -OH side groups. Impregnation of such cross-linked PVA nanofibre mats with Nafion ® resulted in reduction in methanol permeability by an order of magnitude and enhanced mechanical strength in comparison to non-modified Nafion ® membranes with similar thickness [65,140,141].

Cross-linking of PFSA ionomers has been much investigated, although little reported on in the open literature, to avoid the problem of excessive swelling and mechanical deterioration of the corresponding membranes [3,125,144]. Cross-linked sulfonated polyaromatic membranes often suffer from brittleness due to the presence of covalent networks, and reduced proton conductivity due to utilisation of ionic functions in forming the cross-links. For this matter, electrospinning can help by providing a cross-linked ionomer membrane possessing good conducting properties along with reduced brittleness owing to flexible, interconnected network structure of the electrospun ionomer mat.

Anion-exchange membranes (AEMs) for alkaline fuel cells usually contain tetramethylammonium cationic fixed charge groups on a polymer backbone. To increase their ion conductivity the IEC should be increased by increasing the concentration of positively charged functional groups, which causes poor mechanical properties; the membrane suffers from excessive swelling when fully hydrated and polymer brittleness when dry. The development of novel AEMs passes through novel polymer chemistries and membrane architectures. Anion exchange membranes based on cross-linked electrospun fibres have been prepared by dual electrospinning [145][146][147]. For example, chloromethylated polysulfone (CMPSF) and PPSU have been co-electrospun from separate spinnerets, and the obtained mat submitted to partial cross-linking of the chloromethyl groups with an aliphatic diamine, followed by softening of the PPSU by solvent exposure to obtain a defect-free membrane [146].

Residual chloromethyl groups of the fibres were converted into tetramethylammonium ion exchange sites. The resulting composite membrane presented hydroxide ion conductivity of up to 65 mS/cm at 23 o C in water, with insolubility in water due to the presence of cross-links despite its high IEC (3.1 mmol/g).

Phosphoric acid doped polybenzimidazole (PBI) membranes provide high proton conductivity at ambient relative humidity up to 160 °C and beyond, and are a viable and durable electrolyte for high temperature PEMFC (HT-PEMFC) [148,149]. Acid-imbibed PBI is prepared by immersion of PBI membranes in acid solutions. The uptake of phosphoric acid depends on the temperature of the acid bath and its concentration. Alternatively, H3PO4-PBI membranes are elaborated through a process whereby a polybenzimidazole polymer is prepared by a polycondensation reaction from its monomer components in polyphosphoric acid (PPA). After casting of the PBI-PPA solution, controlled hydrolysis of PPA to phosphoric acid leads directly to PBI membranes containing high quantities of H3PO4. In a third route, a membrane is cast from PBI dissolved in a mixture of phosphoric and polyphosphoric acids, followed by controlled hydrolysis of the PPA component. The membrane mechanical properties are strongly impacted by the number of acid molecules per PBI repeat unit (acid doping level), and it is important to find a means of improving mechanical strength and creep resistance for high acid doping level PBI membranes.

In one approach, polybenzimidazole and polybenzoxazine (PBz), a cross-linking agent, were electrospun [139] to give a PBz-modified PBI nanofibre mat which was thermally cross-linked via ring-opening addition reaction of benzoxazine groups, which allowed subsequent impregnation of the mat with PBI. The composite membrane resulted in high acid doping levels (≈ 13) and dimensional stability upon acid doping. Figure 15 presents the stress-strain curves and the fuel cell performance (H2/O2, 150 °C) of MEAs with the composite membranes with various wt% of PBz in comparison to neat PBI membrane. The membrane containing 10 wt% of cross-linking agent shows a 3-fold increase in Young's modulus (6570 ± 660 MPa) in comparison to neat PBI membrane, as well as a 34 % increase of the fuel cell performance for the corresponding MEA (maximum power density of 670 mW/cm 2 ).

Rozière et al. have developed a means of cross-linking PBI membranes in a highly acid swollen state by incorporating a cross-linking molecule into a solution of PBI in phosphoric acidpolyphosphoric acid [150]. Such membranes typically have an acid doping level of 16-25, and present a conductivity of 0.18 S/cm at 160 o C. By electrospinning the PBI-PPA solution containing the cross-linker, following by thermal curing, cross-linked PBI nanofibre reinforcements are obtained that are insoluble in DMAc and that can thus be used to reinforce highly acid doped PBI membranes. This approach has enabled improvement of the mechanical properties improved by a factor 25 compared to non-reinforced non-cross-linked acid doped polybenzimidazole membranes [151]. The current/voltage characteristics of MEAs incorporating an electrospun cross-linked PBI reinforced cross-linked PBI membrane are 640 mV at 0.2 A/cm 2 (hydrogen/air, atmospheric pressure) with a maximum power density of 500 mW/cm 2 . Such membranes are of applicability not only in stationary fuel cell applications, but also as APUs or range extenders for electric vehicle applications. Applying a range extender protocol over a 1000 hour period led to a voltage loss of <20 µV/h.

Concluding remarks and future trends

Fuel cells are an integral part of future technologies for electrochemical energy conversion and storage, alongside water electrolysers, batteries and redox flow systems. With the Mirai fuel cell vehicle now commercialised by Toyota in major regions of the world and Hyundai and Honda following suit, fuel cells are a reality and already contribute to the move from fossil fuels for transport. Nevertheless, costs must be reduced and durability further improved, and the development of new materials with alternative compositions and novel architectures will help to meet these challenges. This review has shown that electrospinning is showing great promise in the generation of materials for proton exchange membranes. This technique demonstrates great versatility and reproducibility, and it leads to the preparation of nanofibres with controlled size and morphology, as well as the possibility of exploiting different processing methodologies including coaxial and multiple needle electrospinning to generate designed structures.

Furthermore, electrospinning will allow the up-scaled production of fibre materials for ionomer membranes with the perspective of industrial application.

The original morphologies, compositions and degrees of alignment of electrospun nanofibres lead to several advantageous features in composite fuel cell membranes based , amongst which, the possibility of reducing membrane thickness and area resistance, without losing mechanical strength. Issues that can arise in preparation of blend membranes by casting, such as interface and compatibility between different polymer components, may be overcome by dual needle electrospinning. Disassociating functions of mechanical strength and ion transport, and controlling phase separation by separating polymer phases or associating non-miscible polymers at designed length scales allow elaboration of ionomer membranes with high proton conductivity and high mechanical strenght. Further improvements are achievable by ionically or chemically cross-linking the electrospun and matrix components to further increase fuel cell lifetime. These accomplishments that lie mainly in the field of membranes for PEMFC are expected to allow the rapid development of complementary fuel cell technologies, such as alkaline and high temperature PEM fuel cells. Despite these achievements, the full realisation of the potential of electrospinning in ionomer membranes is probably yet to come, as they are under assessment for industrialisation. Some challenges still remain, including the feasibility of electrospinning certain polymers, and attaining nanofibres of uniform and sufficiently low diameter, as well as better understanding of the correlation between morphology of electrospun material and processing parameters.

Interfaces can be further adjusted and improved by associating reactive plasma, atomic layer deposition, electron beam or other surface modification techniques with electrospinning. A further challenge remains in the handling of ultrathin electrospun polymer nanofibre mats, but which can surely be overcome as the full benefits are further proven. An extension of the use of nanofibre based composite membranes and separators is also foreseen in other fields of electrochemical energy conversion and storage. A first step is their use in proton exchange membrane water electrolysers, and lithium ion batteries [53] while redox flow cells would also benefit from the approaches described in this review. In perspective, the realisation of an integrated membrane electrode assembly process fully based on electrospinning is an ambitious but realistic goal. Table 1. Overview of proton conducting membranes of inert polymer based electrospun nanofibres and proton conducting electrolyte as matrix.

Figures and table captions

Table 2. Details of the electrospinning process and parameters associated with PFSA based polymers.

Table 3. Overview of proton conducting membranes comprising electrospun nanofibres of proton conducting electrolytes and a matrix of inert polymer.

Figure 1 .

 1 Figure 1. Schematic representation of a Membrane Electrode Assembly (MEA).

Figure 2 .

 2 Figure 2. Schematic illustration of a typical electrospinning set-up and the Taylor cone.

Figure 3 .

 3 Figure 3. Number of published research articles in the period of 2005-2015 concerning (a).

Figure 4 .

 4 Figure 4. Types of membrane architectures based on electrospun materials (a). a proton

Figure 5 :

 5 Figure 5: Nafion ® -impregnated electrospun composite PVDF membranes: (a). SEM image of the cross-section of the composite membrane containing 0.4 g of Nafion ® ; (b). polarisation curves for the composite membranes with various weight contents of Nafion ® and of Nafion ®

Figure 6 .

 6 Figure 6. Nafion ® membrane with Nafion-functionalised PVDF nanofibres, Nafion ® membrane with non-modified PVDF nanofibres (Nafion-CM2), commercial Nafion ® 112 and recast Nafion ® (Nafion-RC) membranes: (a). Stress-strain curves; (b). Proton conductivities at

Figure 7 :

 7 Figure 7: Wet-dry cycling at open-circuit-voltage at 85 °C and 13 % RH for (a). PBI nanofibrereinforced chemically stabilised Aquivion ® EW 700 membrane; (b). chemically stabilised Aquivion ® EW 790 membrane (reference 83).

Figure 8 .

 8 Figure 8. Photographs showing mechanical flexibility of sulfonic acid functionalised silicate

Figure 9 .

 9 Figure 9. Fabrication of two types of nanofibre-composite Nafion ® /poly(phenyl sulfone)

Figure 10 .

 10 Figure 10. SEM images of electrospun mats from 10 wt% solutions with different PFSA/PEO

Figure 11 :

 11 Figure 11: (a). SEM micrograph of a single high-purity Nafion ® nanofibre bridging two

Figure 12 .

 12 Figure 12. (a). Dependence of in-plane proton conductivity on relative humidity at 80 °C for EW 733 PFSA and EW 825 PFSA nanofibre composite membranes and for Nafion ® 212;

Figure 13 .

 13 Figure 13. SEM micrographs of: (a). Palladium-silica (Pd-SiO2) nanofibres; (b). Nafion ®

Figure 14 .

 14 Figure 14. SEM micrographs of (a) CDP, (b) polymeric CDP and (c) electrospun polymeric

Figure 15 .

 15 Figure 15. (a). Nanofibre-reinforced PBI composite membranes (PBI-CM-NF-PBz wt%) and

Figure 1 :

 1 Figure 1: Schematic representation of a Membrane Electrode Assembly (MEA).

Figure 2 :

 2 Figure 2: Schematic illustration of a typical electrospinning set-up and the Taylor cone.

Figure 3 :

 3 Figure 3: Number of published research articles in the period of 2005-2015 concerning (a).

Figure 4 :

 4 Figure 4: Types of membrane architectures based on electrospun materials (a). a proton

Figure 5 :

 5 Figure 5: Nafion ® -impregnated electrospun composite PVDF membranes: (a). SEM image of the cross-section of the composite membrane containing 0.4 g of Nafion ® ; (b). polarisation curves for the composite membranes with various weight contents of Nafion ® and of Nafion ®

Figure 6 :

 6 Figure 6: Nafion ® membrane with Nafion-functionalised PVDF nanofibres, Nafion ® membrane with non-modified PVDF nanofibres (Nafion-CM2), commercial Nafion ® 112 and recast Nafion ® (Nafion-RC) membranes: (a). Stress-strain curves; (b). Proton conductivities at

Figure 7 :

 7 Figure 7: Wet-dry cycling at open-circuit-voltage at 85 °C and 13% RH for (a). PBI nanofibrereinforced chemically stabilised Aquivion ® EW 700 membrane; (b). chemically stabilised Aquivion ® EW 790 membrane (reference 83).

Figure 8 :

 8 Figure 8: Photographs showing mechanical flexibility of sulfonic acid functionalised silicate

Figure 9 :

 9 Figure 9: Fabrication of two types of nanofibre-composite Nafion/poly(phenyl sulfone)

Figure 10 :

 10 Figure 10: SEM images of electrospun mats from 10 wt% solutions with different PFSA/PEO

Figure 11 :

 11 Figure 11: (a). SEM micrograph of a single high-purity Nafion ® nanofibre bridging two

Figure 12 :

 12 Figure 12: (a). Dependence of in-plane proton conductivity on relative humidity at 80 °C for EW 733 PFSA and EW 825 PFSA nanofibre composite membranes and for Nafion ® 212;

Figure 13 :

 13 Figure 13: SEM micrographs of: (a). Palladium-silica (Pd-SiO2) nanofibres; (b). Nafion ®

Figure 14 :Figure 15 :

 1415 Figure 14: SEM micrographs of (a) CDP, (b) polymeric CDP and (c) electrospun polymeric

  ammonium chloride) (PDDA) Poly-(trimethyl hexamethylene terephthalamide) (PTHTP) Poly(phenyleneoxide) (PPO) Poly(amide-co-imide)-poly(trimellitic anhydride chloride-co-4,4'-methylenedianiline) (PAI-

Table 1 :

 1 Overview of proton conducting membranes of inert polymer based electrospun nanofibres and proton conducting electrolyte as matrix.

	Electrospun	Solution	Matrix			Ref
	Material	properties	Material	Details	(mS/cm)	
		17 wt% in	Nafion ®	Pores of electrospun PVDF mat	2.25 (65 °C)	NA
		Acetone/DMAc		impregnated with Nafion ®
		(5/5 w/w)
		15 wt% in	Nafion ®	Membranes for Microbial Fuel Cells	NA	4.9 mW/m 2
		Acetone/DMF				Nafion ®
		(5/5 w/w)				content=0.4g
		19 wt% in	Sulfonated-PS	Thermal polymerisation of styrene	NA	NA
		Acetone/DMAc		monomer using BPO, NMP and DVB
		(2.3/1 w/w)		cross-linker in the pores of PVDF mat
				at 110 °C followed by sulfonation of
				the PS/PVDF membrane; similar
				performance of membrane with 15 wt%
	PVDF			sulfonated PS to that of Nafion ® in
				MEA test
						700 mW/cm 2 in
		17 wt% in DMF	Nafion ®	Functionalisation of PVDF nonwoven	82 (95 °C;	H2/O2 single cell
				with Nafion ® chains using 3-step	95 % RH)	at 65 °C; 122
				reaction route followed by		mW/cm 2 in
				impregnation with Nafion ®		DMFC at 70 °C
						with 5M methanol
						as fuel
						770 mW/cm 2 in
						H2/O2 single cell
		17 wt% in DMF				at 70 °C; 464
			Nafion ®	Functionalisation of PVDF nonwoven	106 (95 °C;	mW/cm 2 in
				with Nafion ® chains, then grafting with	95 % RH)	DMFC at 70 °C
				PSSA followed by impregnation with		with 5M methanol
				Nafion ®		as fuel
			SPAES	Copolymer based on acrylonitrile and		607 mW/cm 2 in
	PAN	10 wt% in	copolymer with	Methyl methacrylate monomers (94:6)	164 (80 °C;	H2/O2 single cell
		DMSO	50 % degree of	synthesized followed by stabilization in	100 % RH)	at 70 °C (100 %
			sulfonation	an oxidizer at 240 °C		RH)
					25 (30 °C;
	PI	NA	SA/PA-	In situ sol gel synthesis of SA/PA	60 % RH)
			functionalized	functionalized silicate glass electrolytes	for PA-	NA
			silicate glass	inside pores of PI nonwoven;	membranes
			electrolyte	appreciable improvement in the
				membrane bendability	166 (80 °C;
					80 % RH)
					for SA-
					membranes
	PTHTP	22 wt% in	PDAC-	LbL deposition of polyelectrolytes on	7 (RT; 100 %	NA
		DMF	sulfonated PPO	the nanofibre surface by spray coating	RH)
						642 mW/cm 2 in
	PAI-PTM	DMF	SPAES (50 %	Blend of PAI-PTM electrospun and	110 (80 °C;	H2/O2 single cell
			degree of	resulting nonwoven impregnated with	100 % RH)	at 70 °C (100 %
			sulfonation)	SPAES copolymer		RH)

Table 2 :

 2 Details of the electrospinning process and parameters associated with PFSA based polymers.

					
	Electrospun	Carrier Polymer/	Solvent	Details	(mS/cm)	Ref
	Material	Additives
				Beaded nanofibres
				obtained above 13	8.7-16
	Nafion ®	5-28.6 wt% PVA	IPA/Water	wt% PVA and	(equilibrated in
		Mw: 2.0x10 5		defect free	water for 24 hrs;
				nanofibres	RT
				obtained at 28.6	measurements)
				wt% PVA
				Defect free fibres	3.5-5.9
	Nafion ®	9-16.7 wt% PEO	IPA/Water	obtained at 16.7	(conditions same
		Mw: 1.1x10 5		wt% PEO	as above)
				Beaded-
				nanofibres
	Nafion ®	5-66 wt% PAA	IPA/Water (3/1)	obtained above 12	NA
		Mw: 4.5x10 5		wt% PAA and
				defect free fibres
				obtained at 25
				wt% PAA
	Nafion ®	2-5 wt% PEO Mw: 5x10 6 g/mol	Ethanol/Water	Fibres with of ~150 nm average diameter	NA
				Well-formed
				nanofibre mats
	Nafion ®	1-2 wt% PEO	IPA/Water (3/1)	with diameter	NA
		Mw: 300 kDa		between 300 and
				900 nm by using
				1wt% PEO
				High purity
	Nafion ®	0.1-2 wt% PEO	Methanol	nanofibres with	1500 (30 °C; 90
		Mw: 8000 kg/mol		400 nm diameter	% RH)
				by using 0.1 wt%
				PEO
				Defect-free
	PFSA ionomer	8-50 wt% PVP	DMF	nanofibre mats	NA
	pellets	Mw: 1.3x10 6		with diameter
				between 100 and
				400 nm by using
				8wt% PVP
		1 wt% PEO		Nanofibre	97 (80 °C; 95 %
	Aquivion ®	Mw: 6000-1.0x10 6	DMAc/H2O	diameter around	RH)
		Da	(99/1)	300 nm	66 (120 °C; 95 %
					RH)
		1.5 wt% PEO		Short side chain	58 (80 °C; 95 %
	Fumion ®	Mw: 6000-1.0x10 6	DMAc	ionomers present	RH)
		Da		higher	54 (120 °C; 95 %
				crystallinity	RH)

Table 3 :

 3 Overview of proton conducting membranes comprising electrospun nanofibres of proton conducting electrolytes and a matrix of inert polymer.

	Electrospun	Solution	Matrix	Details		Ref
	Material	properties	Material		(mS/cm)
				Densified SPAES mats
	SPAES	25 wt% in	NOA 63	imbibed with NOA 63	86 (25 °C; 100 %	104
		DMAc		followed by UV cross-	RH) for 70 wt%
				linking of NOA	SPES content
		20 wt% of 825
		EW PFSA in
		propanol/water
		(2/1 w/w)		Defect-free PFSA mats
				imbibed with NOA 63
		42.5 wt% of 733		followed by UV cross-
	PFSA ionomer	EW PFSA in	NOA 63	linking of NOA;	160 (80 °C; 80 %	91
	(3M corporation)	methanol/water		Removal of carrier	RH) for PEM
		(4/1 w/w) for		polymer by boiling in	with
		0.3-1 wt %		1M H2SO4 solution and	733 EW PFSA
				in DI water for one hour
		PEO as carrier		each time.
		polymer (Mw:
		10 6 Da)
				Bead-free SPES
				nanofibre mat
	SPES	25 wt% in DMF	Nafion ®	impregnated with	87.5 (25 °C; 95 %	119
				Nafion ® solution;	RH) for 70 wt%
				Significant reduction in	SPES content
				nanofibre diameter due
				to sulfonation of SPES
	Poly (ether	20 wt% in		Increase in average
	sulfone) (PES)	DMAc with		diameter and decrease in
	and PFSA resin	0-12 wt% Non-		hydrophobicity of
	(PFSA-Na) Blend	solvent NS	-	electrospun PES/PFSA-	NA	105
	(1/1 w/w)	(Ethanol or		Na nanofibres with
		IPA)		increasing NS content
	SPAES and	40 wt%. in 2-	NOA 63	Densified and welded	94 (30 °C; 80 %	110
	sPOSS Blend	butoxyethanol		mat imbibed with NOA	RH) for 70 vol%
	(60/40 w/w)			63 followed by UV	nanofibre fraction
				cross-linking of NOA
		20 wt% of 660
		EW PFSA in		Dual mat obtained by
	PFSA ionomer	propanol/water		co-electrospinning of 72
	(3M corporation)	(2/1 w/w); 0.3		vol% PFSA and 28
	and	wt % PEO (Mw:	-	vol% PPSU solutions	93 (120 °C; 50 %	86
	PPSU	10 6 Da)		and flowing of PFSA followed by softening	RH)
		Sol. of PPSU in NMP/acetone		ionomer into PPSU inter-fibre space
		(4/1 w/w)
				Co-electrospun
				sandwiched structure
		30 %w/v. of		composed of SPEEK
	SPEEK	SPEEK in		nanofibres as top and
	and	DMAc	-	bottom layers and PES	61 (80 °C; 100 %	114
	PES			nanofibres in the centre;	RH)
				Densification, LbL

Acknowledgement Funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. 306682 is acknowledged. Acid Doping Level (ADL)

Abbreviations