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Chemical Engineering Journal 187 (2012) 367– 371

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

j ourna l ho mepage: www.elsev ier .com/ locate /ce j

eolite applications in fuel cells: Water management and proton conductivity

ei Hana, Siu Ming Kwana, King Lun Yeunga,b,∗

Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, PR ChinaDivision of Environment, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, PR China

r t i c l e i n f o

rticle history:eceived 20 October 2011eceived in revised form 13 January 2012ccepted 21 January 2012

eywords:

a b s t r a c t

Zeolites are increasingly used in fuel cell, from fuel conversion and conditioning to membrane additivesas well as support for electrocatalysts. Zeolites’ ionic conductivity and capacity for water adsorption andretention were exploited to create zeolite and zeolite-PFSA proton conducting membranes for PEMFC.HZSM-5 micromembranes of different Si/Al ratios (from ∞ for Sil-1 to 30) were fabricated in a regulararray on silicon followed by assembly and test for fuel cell performance. HZSM-5 with high aluminum

EMFCicro fuel cellicroreactorafionZSM-5

content exhibits good proton conductivity and better PEMFC performance that approaches that of NafionMEA. A rational approach in the design of structured HZSM-5-PFSA composite membrane was demon-strated. The PFSA was confined in subnanoliter volumes within zeolite sleeves to obtain self-humidifyingmembrane and enhanced PEMFC performance (i.e., up to nine-fold in MPD) at elevated temperatures (i.e.,up to 100 ◦C) and dry conditions. Higher water retention and improved membrane thermomechanicalproperties are believed to be responsible.

. Introduction

Proton exchange membrane fuel cell (PEMFC) operated at highemperatures (i.e., 150–200 ◦C) has the advantages of faster elec-rochemical kinetics, greater catalyst tolerance to CO poisoning,impler water and heat managements and even the possibilityf using non-precious metal catalyst [1]. Thus, a more efficientnd simpler fuel cell design and operation can be realized with aignificant cost saving. However, the proton transport in perflu-rosulfonic acid (PFSA) membrane relies on liquid water and thisonstrains their practical operating temperature below 80 ◦C [2–4].his is exacerbated by water imbalances caused by electro-osmosisrag and back diffusion in the PFSA membrane. There is there-ore active research in developing membranes that could retainnd manage water through self-humidification process [5–7]. Kimnd co-workers [7] employed PtY zeolite catalysts in their zeolite-FSA membrane. The Pt provided catalytic site for water generationhile the zeolite absorbed the water and thus helped keep theembrane hydrated during high temperature fuel cell operation.lthough crucial for high temperature operation, the proton con-

uction mechanism in PFSA membrane at low degrees of hydration

s still poorly understood [8].

∗ Corresponding author at: Department of Chemical and Biomolecular Engineer-ng, Clear Water Bay, Kowloon, Hong Kong, PR China. Tel.: +852 23587123;ax: +852 23580054.

E-mail address: kekyeung@ust.hk (K.L. Yeung).

385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2012.01.102

© 2012 Elsevier B.V. All rights reserved.

Nano-, micro- and macro-composite membranes with inor-ganic or inorganic–organic particles dispersed in PFSA had beenextensively studied by various authors [9–11]. The best compos-ite membranes used particles that are hydrophilic and have highproton conductivity. It had been speculated that these particlesact as water reservoirs and help retain the loosely bonded waterwithin the PFSA ionic domains enabling their operation at hightemperatures. Zeolites are known for their capacity to adsorb andretain water. Also, zeolites’ three-dimensional crystalline frame-work of tetrahedral SiO4 and AlO4 structural units creates periodicinterconnecting channels and spaces that host unique chemistry.The insertion of [AlO4]5− into the [SiO4]4+ framework results inexcess negative charge that have to be counterbalanced by protonsor cations (i.e., organic and inorganic). Cations at extra-frameworksites are easily exchanged and their migration is responsible forthe observed ionic conductivity of zeolites. The ionic conductivityof zeolites was determined by experiments and model calcula-tions [12–16]. Some zeolites such as hydrated tin-mordenite canhave ionic conductivity (e.g., 0.1 S cm−1) approaching that of PFSA[12,13]. Calculation using semi-classical transition state theory(SC-TST) [14], QM-Pot [15] and DFT [16] methods indicated thatamong the zeolites investigated (i.e., HY, H-chabazite, HZSM-5), theHZSM-5 had the highest proton mobility due to the flexibility of itsframework lattice.

Zeolites in form of particles and powders have been

added and dispersed in various proton conducting poly-mers to prepare composite membranes including zeolite-PFSA[7,17], zeolite-sulfonated polyetherketone [18–20] and zeolite-polyfluoroethylene [21,22]. However, the increased transport

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68 W. Han et al. / Chemical Engin

esistance, particle aggregation and phase separation are majoroncerns [23,24]. This study presents a rational approach to theesign and incorporation of zeolites for water management androton transport in a PEMFC that harmonizes the forms and func-ions of the membrane components to create a proton-exchange

embrane that affords high proton flux and can tolerate highemperature and low humidity operations. This includes struc-ured compositing of PFSA by subnanoliter confinement in regularrray of zeolite-coated pores fabricated on stainless steel foils25] and microfabrication of zeolite proton-conducting micromem-rane array [26,27].

. Experimental

.1. Materials and chemicals

The p-type Si(1 0 0) wafers supplied by the Nanoelectron-cs Fabrication Facility of HKUST were used for the fabricationf zeolite proton conducting membranes. Zeolites were pre-ared from fumed silica (99.8%), tetraethyl orthosilicate (98%),luminum sulfate (+98%) and tetrapropylammonium hydroxide1 M) from Sigma–Aldrich and sodium hydroxide (98%) from BDHhemicals. Sodium nitrate (99%, Fisher), potassium nitrate (99%,DH) and ammonium nitrate (98%, Nacalai) were used in zeolite

on-exchange. Photolithography method was used to prepare a0 �m-thick stainless steel mesh with hexagonal array of 110 �moles. The stainless steel mesh was used to prepare zeolite-PFSAtructured composite membranes. The Nafion PFSA resin (10 wt.%n water) and the 1,2-propanediol (99%) solvent used for mem-rane casting were purchased from Sigma–Aldrich. The Nafion 117embrane and the Pt/C catalyst were purchased from E-Tek.

.2. Zeolite proton conducting membranes

The general procedure for preparing microfabricated zeoliteroton conducting membrane and zeolite-PFSA structured com-osite membrane are shown in the process diagrams in Fig. 1and b, respectively. The detailed membrane fabrication proceduresere reported in prior works [25–27]. The zeolite proton-

onducting membrane consists of an array of self-supported zeoliteicromembranes fabricated on silicon wafer [28,29]. Briefly, the

ilicon wafer was cleaned, photoresist-coated, photolithographednd etched to create the pattern shown in Fig. 1a. The etchedicrowells were cleaned and seeded with a monolayer of 100 nm

PA-Sil-1 seeds, followed by hydrothermal zeolite regrowth from synthesis solution with molar composition of 80 SiO2:yAl2O3:1PA2O:10 Na2O:40,000 H2O. The seeding and regrowth proceduresere adopted from zeolite membrane synthesis [30–35] and zeolite

abrication in microsystem [36–40]. Selective etching removed theemaining silicon supporting the deposited zeolite layer to obtainreestanding micromembranes. Ozone treatment was employed forhe removal of organic structure directing agent (i.e., TPA+) trappedithin the zeolite pores [41]. The TPA+ removal was monitored

nd leak test, single gas permeation and on occasion gas separa-ion were used to assess the membrane quality [42]. The HZSM-5roton conducting membrane was obtained by successively ion-xchanged with 0.5 M NaNO3, KNO3 and NH4NO3, followed by heatreatment in air to convert NH4ZSM-5 to HZSM-5.

The zeolite-PFSA structured composite membranes were fabri-ated on stainless steel mesh (SSM) with 110 �m diameter holesFig. 1b). The stainless steel mesh was first seeded with a mono-

ayer TPA-Sil-1 seeds using (3-mercaptopropyl)trimethoxysilanes organic linkers. The zeolite was then grown from a synthesisolution containing 80 SiO2:4 Al2O3:1 TPA2O:10 Na2O:40,000 H2Ot 150 ◦C for 48 h. The TPA+ were removed by air calcination at

Journal 187 (2012) 367– 371

550 ◦C and the zeolites were ion-exchanged and treated to obtainHZSM-5. A commercial 10 wt.% Nafion resin suspension was dilutedwith water to give a 5 wt.% suspension. The casting solution wasprepared by mixing 1 volume resin suspension with 2 volume 1,2-propanediol. The Nafion resin was casted on the zeolite-coatedSSM, dried in a vacuum oven at 80 ◦C overnight. The casting proce-dure was repeated to obtain a final composite membrane thicknessof 175 �m.

The membranes were examined under Olympus BH2 opticalmicroscope and BX41 fluorescence microscope. Higher magnifica-tion images of the membrane were obtained by JEOL JSM-6300Ffield-emission scanning electron microscope equipped with energydispersive X-ray spectroscopy (EDXS). Analyses were also doneusing X-ray diffractometer (XRD, Philips PW1830), X-ray pho-toelectron spectroscopy (XPS, Physical Electronics PHI 5600),time-of-flight secondary ion mass spectroscopy (ToF-SIMS, ION-TOF GmbH TOF-SIMS V) and differential scanning calorimeter (DSC,TA Q1000).

2.3. Membrane-electrode assembly (MEA)

The MEA was prepared by the glue method described by Zhaoand co-workers [43]. Nafion resin suspension was used as glue tobind the MEA. Porous stainless steel plates coated with 100 nmthick gold layer were used for gas diffusion and current collector. Aslurry of 10 wt.% Pt/C was coated on the plate surface and the mem-brane was sandwiched between two pieces of the coated electrode.The MEAs were tested in a homemade cell. Two sets of mass flowcontrollers and pressure regulators fed ultrahigh purity H2 and O2to the cell at the anode and cathode, respectively. Prior studies haveshown that catalysts are sensitive to pretreatment [44,45]. The MEAwas allowed to stabilize and the electrical properties of the cellwere measured at different feed flow rates and temperatures. Apair of humidifiers was used to measure cell performance at dif-ferent degree of feed humidification. Performance comparison wasmade with standard Nafion MEAs prepared by hot-press method.Nafion 117 membrane was sandwiched between two pieces of thecoated porous stainless steel electrode/diffuser and hot-pressed at130 ◦C for 3 min.

3. Results and discussion

3.1. Zeolite proton conducting membrane

Inorganic membranes have the advantages of better thermaland chemical stabilities than polymeric PFSA membrane. How-ever, mechanical brittleness and low proton conductivity at lowto intermediate temperature remain important obstacles. Manyzeolites exhibit ionic conductivity [12–16,46,47] and methods oftheir fabrication into ultrathin films and membranes are widelyreported [48–51]. Fig. 2a shows an electron micrograph of themicrofabricated zeolite proton conducting membrane. The HZSM-5 micromembranes are freestanding and have a thickness of 6 �m.The zeolite is well-intergrown and display a preferred (1 0 1)orientation. Each micromembrane measures 250 �m × 250 �mgiving the zeolite proton conducting membrane an overall area of3.06 mm2 for proton transport.

Fig. 2b plots the I–V and I–P curves for HZSM-5 (Si/Al = 23 and 16,determined by XPS) and Nafion 117 MEAs. The HZSM-5 MEA oper-ated under dry condition had poor performance due to hydrogencrossover and low proton conductivity. Once the water generated

by fuel crossover hydrated the zeolite, the current flow increasesand quickly reaches a steady state. Simultaneously, the hydrogenpermeance dropped from 2.3 × 10−4 mol m−2 s−1 Pa−1 (dry mem-brane) to none in the fully hydrated membrane. Nafion 117 MEA

W. Han et al. / Chemical Engineering Journal 187 (2012) 367– 371 369

Fig. 1. Schematic diagrams of preparation procedures of (a) HZSM

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ig. 2. (a) SEM image and schematic drawing (inset) of HZSM-5 proton conductingembrane and (b) plots of I–V and I–P of Nafion and HZSM-5 (a) Si/Al = 23 and (b)

i/Al = 16 at room temperature and 100% R.H.

isks membrane failure if operated under dry conditions. The tests

ere therefore done at room temperature with fully humidifiedydrogen and oxygen feeds of 2.5 cm3 min−1 each. The plots showhe cell voltage are lower than the 1.23 V, the ideal potential voltageor a H2/O2 fuel cell. This is due to irreversible losses from activation,

-5 and (b) HZSM-5-PFSA proton conducting membranes.

ohmic and concentration polarizations in the assembled MEAs. Thelower ohmic losses of Nafion 117 MEA are expected, because ofthe better contact between the membrane, catalyst and electrodein the hot-pressed MEA. It delivered a maximum power density(MPD) of 12.6 mW cm−2. The HZSM-5a (Si/Al = 23) MEA with MPDof 8.8 mW cm−2 displays severe activation polarization as shown inFig. 2b, but has a significantly lower ohmic polarization comparedto Nafion 117 MEA.

Increasing the aluminum content of the zeolite proton con-ducting membrane, alleviated losses from activation polarizationresulting in a higher open circuit voltage (OCV) as shown in theI–V plot of HZSM-5b (Si/Al = 16) MEA. The HZSM-5b MEA per-formed better than Nafion 117 MEA with a MPD of 13.0 mW cm−2.It displays similar activation and ohmic losses, but not the severeconcentration polarization shown by Nafion 117 MEA. The higheractivation polarization of HZSM-5a with lower aluminum contentcompared to Nafion 117 is due to the activated transport of pro-tons in the zeolites. The transport mechanisms and pathways inPFSA and zeolite are different. It is generally accepted that protonmigrate as hydronium ions along nanometer-sized, hydrated chan-nels formed by the sulfonated side chains of PFSA [52]. In zeolites,the protons migrate through rigid, subnanometer channels andcages via Grotthuss and vehicle transport mechanisms [10]. Vehi-cle transport is only important at high temperatures (ca. 200 ◦C)and can be ignored in this study. Grotthuss transport is rapid and isdominant at temperatures below 120 ◦C. The residual water in thezeolite lowers the barrier for intersite proton hopping [10].

3.2. Zeolite-PFSA structured composite membrane

Zeolite was used to retain water in a zeolite-PFSA structuredcomposite membrane shown in Fig. 3a. The fluorescence imageshows PFSA confined within a regular array of photoetched holesin stainless steel foil coated with a thin layer of zeolite. The holeshave an hourglass shape due to the anisotropic etching that gives anopening diameter on the surface of 112 �m that gradually taperedinward to 76 �m. The SSM was uniformly cladded with a thin

zeolite layer that acts as electrical insulator. The zeolite was alsochosen to provide passive water management by active retentionof water in PFSA. The structured composite membrane was assem-bled by both glue (HZSM-5-PFSA MEA-1) and hot-press methods

370 W. Han et al. / Chemical Engineering Journal 187 (2012) 367– 371

Fig. 3. (a) Fluorescence microscopy image and schematic drawing (inset) of HZSM-5d

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folds the power density of Nafion 117 MEA over the temperaturerange of 25–100 ◦C. Calculation result shows that the ohmic resis-tances of HZSM-5-PFSA MEA-2 are 0.31, 0.28 and 0.85 � cm2 at

-PFSA proton conducting membrane and (b) plots of MPD vs. temperature ofifferent MEAs under dry feed and flow rate of 10 cm3 min−1.

HZSM-5-PFSA MEA-2). This allows comparison to zeolite protononducting membrane and Nafion 117 MEAs. However, Nafion17 MEA rarely survived operation above 50 ◦C under dry feedonditions. The membrane suffered burn damages from hotspotsenerated by hydrogen crossover. Therefore, a humidified Nafion17 MEA at 50 ◦C was instead used for comparison.

A plot of MPD in Fig. 3b shows zeolite-PFSA MEA can generateix times the power output of Nafion 117 MEA at room tempera-ure without the need of humidification. Tests were also done atigher temperatures, and the experiment often lasted at least fif-een days for each MEA. Fig. 3b shows that Nafion 117 MEA gavetable performance at 50 ◦C with fully saturated H2 and O2 feeds.he HZSM-5-PFSA MEA-1 was prepared by glue method outper-ormed Nafion 117 MEA even under dry feed condition at elevatedemperatures. The OCV and MPD of Nafion 117 MEA and HZSM--PFSA MEA-1 in Fig. 4 shows the former has a higher OCV of.98 V compared to 0.91 V of the latter reflecting the better con-act between the electrode–catalyst–membrane layers preparedy hot-press method. Both MEAs displayed activation polarization,ut HZSM-5-PFSA MEA-1 has significantly lower ohmic resistancei.e., 0.43 � cm2 vs. Nafion 117 MEA’s 1.69 � cm2). This gave thetructured composite membrane a conductivity of 0.04 S cm−1. Its understood that the main proton transport route is through theonfined PFSA rather than the zeolite wall coatings. The HZSM-5-

FSA MEA-1 suffered a loss in performance at high temperaturesFig. 3b), and inspection of the MEA found delamination caused by

mismatch in the thermal expansion of the materials.

Fig. 4. OCVs and MPDs of different MEAs at room temperature without humidifica-tion.

The HZSM-5-PFSA MEA-2 prepared by hot-press method didnot suffer from delamination and thus, generated more powerand displayed better tolerance to high temperatures as shown inFig. 3b. The HZSM-5-PFSA MEA-2 has an OCV of 0.94 V and MPDof 385 mW cm−2 at room temperature. It can supply two- to-nine-

Fig. 5. (a) DSC curves of HZSM-5-PFSA and casted PFSA membranes and (b) SEM-EDXS data of zeolite-PFSA membrane.

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5, 50 and 100 ◦C, respectively. Even at the highest operating tem-erature, its ohmic resistance is only half that of Nafion 117 MEA.he conventional way of preparing zeolite-PFSA membrane is byasting a mixture of zeolite powder and PFSA suspension. Thisften results in phase separation and uneven particle dispersion7,17–24]. It also creates a discontinuous phase that increases tor-uosity and transport resistance, thus resulting in lower protononductivity [53–55]. Depositing the zeolites on the pore wallvoids this problem, while still maintaining a large interfacial con-act area (ca. 6000 m−1) between PFSA and zeolite for effectiveater management and greater tolerance for high temperature

peration.Confinement of PFSA within rigid pores was also observed to

mprove the strength and dimensional stability of the membrane.t significant decreased the risks of distortion and damage dur-ng harsh operations. PFSA crystallization within confined spaceestricts crystal growth and orientation resulting in a dramatichange in thermal properties and proton mobility. Indeed, DSCeasurements in Fig. 5a show the structured composite mem-

rane has a glass transition temperature of 184 ◦C that is higherhan casted PFSA membrane (i.e., 125 ◦C). This could be the resultf anchorage of the polymer chain within the porous zeolite andossibly the constrained growth of oriented PFSA crystal withinhe narrow pores. Fig. 5b is a SEM picture of the confined PFSA with

superimposed F/S elemental ratio obtained from EDXS. Fluorinenriched near the wall suggests realignment of PFSA chains underonfinement. This could lead to formation of aligned ion channelsor faster proton transport that resulted in the superb performancef the MEA prepared from the structured composite membranes.

. Concluding remarks

This work explored the use of zeolites for proton conduct-ng membranes for PEMFC. The high proton mobility in HZSM-5

as exploited for zeolite proton conducting membrane. Micro-abrication and micromachining techniques were used to preparehe freestanding HZSM-5 micromembranes on silicon support andssemble the unit into a working MEA. It compared well withafion 117 MEA under room temperature operation and low feed

ates common in micro fuel cell. The limitation of the membraneesign and the relative brittleness of zeolites remained importantbstacles. Alternatively, HZSM-5 was employed in the structuredomposite membrane for water regulation in PFSA. A rationalpproach was used to bring the various elements together to opti-ize their forms and functions. We successfully demonstrated that

t is possible through confinement to achieve enhanced PEMFC per-ormance even under harsh operating conditions, simultaneouslytrengthen the membrane and reducing the PFSA amount in theomposite membrane. High temperature operation would enableEMFC to tolerate lower H2 purity and allows more practical hydro-en production systems to be explored [56–60].

cknowledgments

The authors gratefully acknowledge financial support from theong Kong Research Grant Council and Innovation & Technologyund.

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