Journal of Membrane Science 277 (2006) 1–6

Rapid communication

Hybrid membranes for fuel cells based on nanometerYSZ and polyacrylonitrile matrix

Ioan Stamatin a,∗, Adina Morozan a,∗, Keith Scott b, Anca Dumitru a, S. Vulpe a, F. Nastase a

a 3Nano-SAE Research Centre, University of Bucharest, P.O. Box MG-38, 077125 Bucharest-Magurele, Romaniab School of Chemical Engineering and Advanced Materials, University of Newcastle upon Tyne,

Newcastle upon Tyne NE1 7RU, United Kingdom

Received 28 October 2005; received in revised form 28 February 2006; accepted 1 March 2006Available online 18 April 2006

Abstract

A great deal of effort is required to design polymer electrolyte fuel cells (PEFCs) using new polymers and hybrid organic/inorganic compoundsthat can work at higher temperatures. These materials must have lasting thermal stability as well as improved ionic conductivity. Operation atelevated temperatures is desirable for PEFCs systems since they draw high power density in fast electrode kinetics and also for improvement ofCccpfimmm©

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O-tolerance, etc. In the higher range of temperatures (150–200 ◦C), new materials such as solid acids with phase transition to superprotoniconduction or strong solid acids supported on metal oxide systems seem to be an appropriate alternative to polybenzimidazoles (PBI) and otheromposite membranes. In this context, we evaluate a new hybrid matrix resistant at high temperatures, made of pyridine polymer obtained fromolyacrylonitrile (PAN) and nanometric oxides (e.g., zirconium(IV) oxide-yttria stabilized, YSZ) by thermo-oxidative process in a centrifugaleld. These hybrid matrixes aim further developments either to embed solid acids nanoparticles or to design strong solid acids on nanooxides,aking them appropriate for proton exchange membranes. SEM, TEM, XRD and Raman spectroscopy were used to establish the structure andorphology and to characterize the composite membranes. The dependence of the electrical conductivity on temperature, water uptake, andethanol permeability are evaluated.2006 Elsevier B.V. All rights reserved.

eywords: Hybrid membranes; Pyridine polymers; Thermo-oxidation; Centrifugal field

. Introduction

The drawbacks encountered in fuel cells science in the lastentury can now be tackled through advances in nanomaterialsroviding improved membranes and electrodes to amend theirgures of merit [1–4]. For the conversion of chemical energy offuel and an oxidant directly into electrical power with high effi-iency, there is a need to overcome several key factors dependingn fuel cell type: crossover effects, CO-catalyst poisoning in thease of polymer electrolyte fuel cell (PEFC) and direct methanoluel cell (DMFC) to name a few [5,6]. A review of fuel cell fieldselated to PEFCs shows their inability to operate at high tem-eratures [7].

∗ Corresponding authors. Tel.: +40 21 457 48 38; fax: +40 21 457 48 38.E-mail addresses: istarom@polymer.fizica.unibuc.ro (I. Stamatin),

dina.morozan@gmail.com (A. Morozan).

The option to move PEFCs towards higher temperatures(150–200 ◦C) gives benefits of faster electrochemical reactions,reduction of CO catalyst poisoning, improved thermal manage-ment, simplified water management system, working with driedmembranes [1,2,8–12]. For the above reasons, it is desirableto move PEMs technology towards high temperature opera-tion using new proton exchangers based on solid acids [13] orsuperacids supported on nanometric zirconia [14] as alternativesto sulphonated or phosphorilated polyelectrolytes. These mate-rials need a new stable matrix to make them functional withlasting proton conductivity and thermal stability. The hybridmembranes, containing inorganic nanoparticles of solid acidsin polymer matrix, are identified as a remarkable family ofproton conducting solid polymer electrolytes. Polymer com-posite membranes with inorganic additives of nanometric sizewere extensively studied because they are able to operate athigher temperatures than the pure polymers [15–17] as wellas have improved proton conductivity, water retention ability,

376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2006.03.002

2 I. Stamatin et al. / Journal of Membrane Science 277 (2006) 1–6

reactivity related to high surface area and mechanical support[3,4,18,19]. To continue improvement in this field, we proposea new type of hybrid material made of stable ladder polymerobtained from polyacrylonitrile (PAN) by a thermo-oxidativeprocess in centrifugal field. Nanometric oxide selected here (e.g.,zirconium(IV) oxide-yttria stabilized, YSZ) to be embedded inthe polymer matrix is a usual support in the design of a largevariety of solid acids [14]. Beside that, YSZ as ionic conductorwill raise the electrical conductivity by its intrinsic contributionand by concentration over percolation threshold. The polymermatrix made of pyridine ladder polymer has extremely lowelectronic conduction and a good affinity for proton transportthrough its pendant groups such as OH or oxygen; therefore,the conduction mechanism is mainly governed by YSZ. Wecan conclude that the composites PAN-YSZ will give a flexibleand maneuvrable membrane for PEFC where electrical prop-erties are tunable by nanooxide contribution while diffusion,water uptake and other properties are determined by the poly-mer matrix. The structural properties, water uptake, methanolpermeability and electrical conductivity are evaluated and com-pared with other membranes used in PEFCs.

2. Experimental

PAN polymer, in powder form (molecular weight M = 1.6×nsfPtuwiYiapatCaab(

performed in a centrifugal field in air at the same spinning speed.In this range of temperatures, the cyano repeat units fuse in a lad-der pyridine polymer (Fig. 1). Over 400 ◦C, the ladder polymerchains continue to join adjacent to each other giving ribbon-like fused ring polymers, which are precursors for carbon fibers.This method of thermo-oxidation in centrifugal field can providefilms with thickness in the range 10–25 �m.

Samples collected were analyzed by polarized opticalmicroscopy, scanning electron microscopy (FEI-Quanta 400electronic microscope), transmission electron microscopy(Philips CM120ST, Customized Microscope 120 Super Twin),X-ray diffraction (Bruker D8-Advance powder diffractometer,Cu K� radiation – λ = 0.154 nm), Raman spectroscopy (Ramanspectrometer, model 2001, Ocean Optics, using as source GaAslaser diode with power 500 mW) and FT-IR (Thermo-NicoletNexus with 8 cm−1 resolution).

The electrical conductivity measurements were performedwith power source meter Keithley 2400, in a Faraday boxand flat heater with temperature controller. To establish thetransport mechanisms, the membranes, considered as thin films(10–15 �m), are fixed between two electrodes: Ni and intrinsicSi-1 0 0 (asymmetric configuration). Depending on the polariza-tion potential (Si, the positive electrode) on the sample, holescan be injected and from the I–V characteristics, the transportmechanisms can be easily established. Each sample with a giventhickness is stabilized in temperature and I–V characteristicsaecdwharatfwE

w

us

lting

106, determined by the viscosimetric method) and YSZanopowder (ZrO2 contains 3% Y2O3) with 100–120 m2/gurface area and particle size under 100 nm, were purchasedrom Sigma–Aldrich. Dimethylformamide (DMF)–10 wt.%AN solutions (DMF-PAN) were prepared and then mixedogether along with oxide nanopowder, using ultrasonic bathntil homogeneous mixtures are formed. The amount of YSZas 1, 1.5, 2 and 2.5% (w/w) to PAN and sample index-

ng in the following mode: PAN-YSZ-1, PAN-YSZ-1.5, PAN-SZ-2, and PAN-YSZ-2.5. The PAN films without YSZ were

dentified as PAN400. Each mixture was sprayed on Al-foilnd the foil was tightly positioned on the spinneret head,lugged at rpm of 66.6–75 Hz (4000–4500 rpm). The temper-ture on spinneret was held constant at 80 ◦C for drying andhen increased up to 400 ◦C with soaking time of 1 h [20].ontrolling the temperature for solvent evaporation and anppropriate 66.6–75 Hz (4000–4500 rpm), realizes continuousnd pore-free films, which are critical in designing such mem-ranes for fuel cell applications. The thermo-stabilization stagedehydrogenation–cyclization and oxidation) (up to 400 ◦C) was

Fig. 1. The ladder pyridine polymer resu

re measured between −10 and 10 V. The method is accuratenough and gives the possibility to identify whether the ioniconduction of YSZ is altered by thermal treatment. A detailedescription of the method is given in Ref. [21]. The water uptakeas measured from the weight differences between the fullyydrated membranes and the dry membranes. The membranesre equilibrated in distilled water at room temperature for 24 h;emoved from the water and quickly dried with absorbent papernd measured with a thermogravimetric balance to determineheir wet mass (Mwet). The membranes were then dried at 100 ◦Cor 24 h and weighed to determine their dry mass (Mdry). Thenater uptake (amount of absorbed water) was calculated usingq. (1):

ater-uptake (%) = Mwet − Mdry

Mdry× 100 (1)

Methanol diffusion coefficient (DMeOH) was determinedsing an evaporation system. The sample was kept in a thermo-tated enclosure where the diffusion through the film was mea-

from PAN thermo-stabilization process.

I. Stamatin et al. / Journal of Membrane Science 277 (2006) 1–6 3

sured by weighing the permeate using a microgram precisiontorsion balance. For pure methanol and 65 ◦C temperature level,by graphic representation: (Mt)/(M∞) = f (

√t) and DMeOH is

determined using Eq. (2) [22].

P = 4

(DMeOH

πl2

)1/2

(2)

where Mt is the weight at time t, M∞ the weight at saturation,t the time, P the slope from a graphical representation of thefunction mentioned above and l is the sample thickness.

3. Results and discussion

In the thermo-oxidative process, up to 400 ◦C with an appliedcentrifugal field, PAN400 films usually follow the same schemeas in PAN-carbon fibers process [23]. The thermo-oxidative pro-cess in centrifugal field causes the cyano repeat units to formjoined pyridine cycles in long aligned chains as shown in Fig. 1[24]. In agreement with other reports [25], PAN400 membranesproduced by our proprietary methods have the same IR signatureidentified by the existing functional groups (C N at 2330, 2340,2360 cm−1, C N 1234 cm−1, residual N H at 3445 cm−1, C O

F2(a

ig. 2. Multi-scale organization of PAN-YSZ-x hybrid membranes: A representative00× magnification; (b) SEM micrograph (micrometric scale) shows a well alignmensub-micrometric scale) with YSZ particles represented in bright field; (d) TEM imalong with PAN microfibrils alignment.

example for PAN-YSZ-2.5. (a) OM image (micrometric scale), polarized light,t of the microfibrils made of pyridine cycles; (c) low magnification TEM imagege as in (c) with excess of bright field to show the alignment of nanoparticles

4 I. Stamatin et al. / Journal of Membrane Science 277 (2006) 1–6

(aldehydes) at 1740 cm−1, C O (ketones) at 1700 cm−1). Thesame structure and composition have had PAN-YSZ-x with aslightly different face of the PAN400 treated in the same condi-tions, which has been discussed in detail in our previous paper[20]. The major difference was observed in multi-scale organiza-tion, Fig. 2, where PAN-YSZ-x membranes are investigated overmore than three orders of magnification by using successivelyvisual (resolution about 0.1 mm), optical microscopy (resolutionabout 1 �m), SEM that gives morphological information in the100 �m–10 nm range and TEM (resolution in the lattice fringemode: 0.14 nm). For our purpose, we limited the investigationto a reasonable resolution to obtain the maximum informationonly for particle of YSZ distribution and for morphology andtexture of PAN-YSZ-x membranes. On visual inspection, theylook flat, continuous and without contours. In Fig. 2a, opticalmicroscopy (OM) performed directly on PAN-YSZ-2.5 in thereflection mode with polarized natural light shows a specifictexture with undulations, because of the mechanical relaxationprocesses. The SEM micrograph (Fig. 2b) shows a truthfulfibrils structure, well-oriented along with the deposition cylin-der circumference. As expected, a very fine pore distributionis observed under 20–30 nm limits (only appreciative). TEMimages, at low magnification, identify the orientation, distribu-tion and uniformity of YSZ nanoparticles (Fig. 2c and d). Theimages, processed in bright field, show the particle distributionand their coalescence. We observe that all particles are alignedaasltc

sp2aFant(rtRrPtitic

3

s

Fig. 3. XRD patterns for YSZ, PAN-YSZ-2.5, PAN400.

positive polarization on silicon to induce holes injection. Forcomparison, in Fig. 5b is shown a YSZ solid electrolyte [5].

PAN-YSZ-x hybrid membranes show a different behavior. Atfirst sight, at less than 380 K, there is a slight dependence of con-ductivity for increase in temperature. Above 380 K, the electricalconductivity increases by a continuous transition to YSZ beingdependent on concentration. In addition, this transition dependson the activation energy. PAN-YSZ-1 shows a sharper transi-tion than PAN-YSZ-1.5 and PAN-YSZ-2 where the activationprocess ranges from 400 to 500 K. For PAN-YSZ-2.5, due tothe larger number of nanoparticles, the electrical conduction isalso dependent on intergrain contacts and the percolation effect.Over this concentration, PAN-YSZ-x lose mechanical stabilitybecoming brittle with large variation in electrical conduction.In comparison with doped semiconductors, the curves are sim-

long with the polymer fibrils. The finest particles are distributedt interface with polymer fibrils and the larger ones are encap-ulated into the polymer matrix. That is a consequence of thearge range in the initial size distribution: from few nanometerso a maximum of 100 nm. In addition, there are events where theoalescence is observable.

Referring to the structural composition, the XRD patternshow, beside PAN400 which is a typical well-oriented ladderolymer with a peak at 2θ = 17◦, an amorphous part aroundθ = 21◦ (indexed by arrows in Fig. 3) which is in very smallmount in comparison with their area. On comparison withig. 2b, the two contributions correspond to fibrils structuresnd to an amorphous part, which is in the inter-fibril con-ections. The PAN-YSZ-2.5 pattern does not map entirelyhe YSZ structure; only peaks corresponding to (1 1 1) and2 2 0) are observed. That is, once again the YSZ particlesun along the fibrils giving the appropriate texture and orien-ation and no destruction or composition change takes place.aman spectrum, Fig. 4, confirms these observations by a

elative shift for peak at 807–750 cm−1 and coincident withAN 400. This effect is possible due to shadowing effect fromhe PAN layer, which coats each YSZ particle. The remain-ng lines show that the ladder polymer has a complex struc-ure but could not be resolved in details. In conclusion, theres no change in composition and structure of the YSZ parti-les.

.1. Electrical conductivity

In Fig. 5a are summarized the electrical measurements foramples clamped between two electrodes, Ni and Si-1 0 0 with

Fig. 4. Raman shift of YSZ, PAN-YSZ-2.5, PAN400.

I. Stamatin et al. / Journal of Membrane Science 277 (2006) 1–6 5

Table 1Water uptake and methanol diffusion coefficient for hybrid membranes

Water uptake (%) DMeOH (cm2/s) (±0.2 × 10−10) at 65 ◦C DMeOH (cm2/s) at 80 ◦C References

PAN-YSZ-1 38.88 4.8 × 10−10 – –PAN-YSZ-1.5 39.01 5.0 × 10−10 – –PAN-YSZ-2 39.78 5.3 × 10−10 – –PAN-YSZS-2.5 39.71 5.4 × 10−10 – –Nafion 117 30% 4.9 × 10−6 3.30 × 10−6 [26,27]PBI 25–35a – ∼2.7 × 10−8 [28]Sulphonated polyphosphazene 38 – 1.55 × 10−6 [26]Phosphonated polyphosphazene 14 – 2.10 × 10−7 [26]SPEEK 20–120a – 1.32 × 10−7 to 1.45 × 10−7a [29]

a For various degrees of sulphonation.

ilar showing a transition from intrinsic conduction (due to theresidual charges in PAN400) to an extrinsic conduction due tothermal energy activation of the ionic mechanisms from the YSZparticles. We remark that by extension of the plot from Fig. 5bto lower temperature (line c), the measurements have been per-formed up to 600 K due to the polymer matrix stability. Over thisrange of temperature, the polymer matrix continues the conver-sion in carbon fibers and loses its properties as a ladder pyridinepolymer.

3.2. Water uptake and methanol diffusion

Table 1 shows results of water uptake and methanol diffu-sion, with respect to comparison with few materials used inFC as membranes. If water uptake for PAN-YSZ-x has val-ues comparable to Nafion, PBI or sulphonated polyphosphazeneseries, we observe that it is independent of YSZ concentration.In consequence, water uptake is dependent on the microstructurepyridine matrix and its intrinsic microporosity. Probable largerpores will give rise to larger amount of water uptake. Referringto methanol diffusion coefficients PAN-YSZ-x membranes showa very good behavior related to other membranes. PAN-YSZ-xhave a diffusion coefficient lower by up to four orders in magni-

F[

tude than Nafion and other membranes. Although the compari-son is at different temperatures due to its stability, PAN will stillkeep low diffusion coefficients. In addition, we remark that thediffusion coefficient is independent of YSZ concentration.

4. Conclusions

The new type of hybrid membranes open up a new oppor-tunity in fuel cells science and technology. Pyridine polymersobtained from a thermo-oxidative process in centrifugal field andnanometric YSZ embedded into the polymer matrix are an alter-native to develop membranes stable at high temperatures. Theconduction mechanism in membranes is mainly ionic and gov-erned by ionic component. A percolation threshold is observedin the electrical dependence of concentration when tempera-ture reaches saturation. Around 2.5 wt.% nanooxides YSZ andother nanoparticles can be inserted in the matrix for solid acidsand superacids. Water uptake is not dependent of YSZ con-centration as well as on methanol diffusion coefficient. A lowdiffusion coefficient makes these hybrid composites appropriatefor decreasing methanol crossover effects. The ladder polymersresulting from PAN in thermo-oxidative processes with cen-trifugal field prove to be good competitors in designing newtype of membranes for fuel cells. It is expected that the reducednanoparticle size (to 10–15 nm) will lead to improved membranec

atpa

R

ig. 5. (a) Electrical conductivities for PAN-YSZ-x, (b) reference data for YSZ5], (c) data extrapolated from (b) to lower temperatures.

haracteristics.PAN-YSZ-x hybrid membranes are very flexible and can be

rranged in different geometries from planar to tubular. When theemperature exceeds the processing temperature, PAN400 sam-le and PAN-YSZ-x hybrid membranes have the same behaviors in carbon fibers and lose up to 40% of weight.

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