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Protonic membrane for fuel cell for co-generation of power and ethylene

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   Available online at www.sciencedirect.com Journal of Power Sources 176 (2008) 122–127 Short communication Protonic membrane for fuel cell for co-generationof power and ethylene Zhicong Shi, Jing-Li Luo ∗ , Shouyan Wang,Alan R. Sanger, Karl T. Chuang  Department of Chemical and Materials Engineering, #536, University of Alberta, Edmonton, Alberta T6G 2G6, Canada Received 30 July 2007; received in revised form 14 October 2007; accepted 15 October 2007Available online 26 October 2007 Abstract Yttrium-dopedbariumcerate,BaCe 0.85 Y 0.15 O 3 − α  (BCY15),membranesareproton-conductingelectrolytesforintermediate-temperatureprotonicceramic fuel cells (IT-PCFC), useful for, among other processes, co-production of power and ethylene by dehydrogenation of ethane. BCY15membranes showed good conductivity at intermediate temperatures, 15 and 20mScm − 1 at 700 and 750 ◦ C, respectively. Maximum power densitywas174mWcm − 2 at700 ◦ C,withacorrespondingcurrentdensityof320mAcm − 2 ,usingaC 2 H 6 ,Pt/BCY15/Pt,O 2  fuelcell,withaca.0.5mmthick membrane, producing 34% ethane conversion with 96% ethylene selectivity Comparison of performances using vertical and horizontal set-upsshowed that horizontal set-ups are subject to torsional strain, causing reduced cell performance resulting from even minor leakage at the glass seal.© 2007 Elsevier B.V. All rights reserved. Keywords:  Protonic ceramic fuel cell; Dehydrogenation of ethane; Doped barium cerate ceramic; Glass sealant 1. Introduction Conversion of alkanes to the corresponding alkenes is anindustrially important process due to the high demand for feed-stocks, and in particular for high purity alkenes for manufactureofpolymers.Recently,intermediate-temperatureprotonceramicfuelcells(IT-PCFC)whichsimultaneouslyproducevalue-addedchemicals and electrical power gained much attention for theirhigh energy conversion efficiency and low environment impact[1–3]. Previous research showed that direct hydrocarbon solidoxidefuelcells(SOFC)usingoxygenion-conductivesolidelec-trolytes such as yttrium-stabilized zirconia (YSZ) can generateelectrical power by conversion of hydrocarbon fuel to carbonoxides and water over the anode catalyst [4]. However, conven- tionalYSZ-basedSOFCrequireanoperationtemperatureabove850 ◦ C, which places severe demands on the materials used asinterconnects and sealants. Interestingly, replacing the oxygen ∗ Correspondingauthorat:#536,DepartmentofChemicalandMaterialsEngi-neering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada.Tel.: +1 780 492 2232; fax: +1 780 492 2881.  E-mail address:  jingli.luo@ualberta.ca (J.-L. Luo). ion conductor by proton conductor in the fuel cell systems canbringsignificantpotentialadvantagesforselectiveconversionof hydrocarbons(Fig.1)[5].Firstly,nocarbonoxidesaregenerated intheanodechamber,becausetheproton-conductingelectrolyteonly permits the transfer of protons, and no oxygen is availablefor further reactions of the dehydrogenation product. Secondly,proton conduction implies that water vapor is produced at thecathode,whereitissweptawaybyair,ratherthanattheanode(asin SOFC), where it dilutes the fuel. Thirdly, PCFC can be oper-ated at intermediate temperatures with good electrochemicalperformance, which makes it easier to find suitable connectionand sealing materials for these fuel cell systems.To date, proton-conducting doped perovskites (AB (1 −  x  ) C  x  O (3 − α ) ; A=Ca, Sr, and Ba; B=Ce and Zr; C=Sc, Y and lan-thanides) are the most attractive solid electrolyte candidates foralkanedehydrogenationIT-PCFC.Partialsubstitutionoftetrava-lent B ions by trivalent C ions (acceptor doping) in perovskitescan introduce oxygen ion vacancies (V O •• ). The exposure of thesedopedperovskitestoeitherhumidorhydrogen-containingatmospheresatelevatedtemperaturesresultsintheincorporationof protons by reactions (1) and (2) [6]: H 2 O(g)  +  V O •• + O O × ↔  2OH O • (1) 0378-7753/$ – see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.jpowsour.2007.10.056   Z. Shi et al. / Journal of Power Sources 176 (2008) 122–127   123Fig. 1. Schematic of an intermediate-temperature proton ceramic fuel cell (IT-PCFC) using hydrocarbon as fuel. H 2 (g)  +  2O O × ↔  2OH O • + 2e ′ (2)The proton forms a covalent bond with a lattice oxygen, andmigrates mainly as lone protons jumping between stationaryoxide ions in oxides (Grotthuss mechanism), different from themechanisms using proton transport by vehicles such as OH − ,H 2 O, H 3 O + , and NH 4+ in liquids and in solids with looselybondedvehiclesoropenchannelsorlayers(vehiclemechanism)[1]. The activation energy of proton hopping is at least 0.4eV,which is assigned to activation of host and target oxygen ions[1].Duetoitshighlatticeconstantsandverysmalldeviationfromthe ideal cubic perovskite structure, doped barium cerates showthe highest proton conductivities among the family of proton-conductingperovskites:BaCeO 3  >SrCeO 3  >BaZrO 3  >CaZrO 3 [2].Intensestudieshavebeenconductedintothestabilityofthisclass of materials in a high surface area powder form at hightemperatures in a pure CO 2  atmosphere [7–11]. Unfortunately, the stability of doped and undoped BaCeO 3  is poor in CO 2  atelevated temperatures, though the reaction of BCY15 with CO 2 to form BaCO 3  is very slow in an atmosphere containing a rela-tively low partial pressure of CO 2  [12]. The stability of bariumceratescanbeincreasedwhenCeatomsarepartiallysubstitutedby Zr atoms because BaZrO 3  is chemically more stable thanBaCeO 3  [13]. However, BaZrO 3  has much lower conductivitythan BaCeO 3 , and partial substitution by Zr resulted in lowerproton conductivity.In this paper, we describe a successful approach to applyprotonic fuel cells for co-production of power and value-addedproducts. Using ethane as a fuel, an IT-PCFC generates bothelectrical power and ethylene, with high selectivity, and thisprocess has higher energy conversion efficiency when com-pared with current commercial technologies for direct catalyticdehydrogenation of ethane to ethylene in chemical reactors.We will also describe means for improving electrochemicalperformance, ethane conversion and ethylene selectivity byusing a torsional stress free design set-up and a glass sealantinstead of a ceramic sealant in a horizontal IT-PCFC. We alsofound that conventional sealants were prone to leakage understandard laboratory operational conditions. To our knowledge,only a few prior studies focused on glass–ceramic compositesealants [14,15] and glass sealant [16] for high temperature fuel cells. We will now demonstrate improved sealing of themembrane using glass sealant in IT-PCFC using BCY15 aselectrolyte. 2. Experimental 2.1. Material preparation and characterization Polycrystalline powders of BaCe 0.85 Y 0.15 O (3 − α )  (BCY15)were synthesized by solid-state reactions from stoichiomet-ric BaCO 3  (Sigma–Aldrich, 99+%, A.C.S. reagent), CeO 2  andY 2 O 3  (nano-sized powders, both from Inframent AdvancedMaterials). Twenty-four hours ball-milled raw mixtures werecalcined at 1400 ◦ C for 10h in air. The resulting materials wereball-milled again for 24h, pressed at 30MPa into discs with adiameter of 20mm and a thickness of  ∼ 2mm, and sintered at1550 ◦ C for 10h in air to obtain high-density membranes. Sur-face layers which may have contained impurities were removedby polishing, and the thickness was reduced to about 0.5mm.Platinum paste (Heraeus Inc., CL11-5100) was applied to eachsideofthesintereddiscswithanareaof  ∼ 0.5cm 2 ,andthediscswerecalcinatedat900 ◦ Cfor30minbeforeuseforelectrochem-ical measurements.The yttrium-doped barium cerate series of electrolytematerials, BaCe 0.85 Y 0.15 O (3 − α )  (BCY15) showed the high-est conductivity in the temperature range of 550–800 ◦ C.Hence we used BCY15 as electrolyte. The crystallinity of BCY15 was determined using powder X-ray diffraction (XRD)analysis using Rigaku Rotaflex X-ray diffractometer. Themicro-structure and morphology of BCY15 membrane wasinvestigated using scanning electron microscopy (SEM) tech-nique with Hitachi S-2700 microscope. The micro-structuresand compositions of used ceramic sealant (Aremco 503) andglasssealant(Aremco617),bothscratchedfromthefuelcellset-up after high temperature testing, were determined by SEM andenergy-dispersive X-ray spectroscopy (EDS) analysis (HitachiS-2700). 2.2. Fuel cell system fabrication In our fuel cell set-up, platinum wires and meshes were usedat both electrodes as output terminals and current collectors.The test station was designed for operation at temperatures ashigh as 900 ◦ C with the fuel cell in either vertical or horizon-tal orientation. As described previously, the anode and cathodegas chambers were formed by placing the membrane electrodeassembly (MEA) between concentric pairs of alumina tubes,and the assembly was heated in a Thermolyne F79300 tubularfurnace [17]. All the above components were assembled and the assembly was supported using a stainless steel support, whichwas set on a frame with a rotational axis so the fuel cell set-upcouldbeoperatedinhorizontalorverticalorientation.Theinnertube of each compartment extended from outside the heatedreaction zone to a position close to the respective electrode of the cell. The outer perimeter of each outer tube was sealed tothe membrane electrolyte with a thin layer of ceramic sealant(Aremco 503) at the cathode side, or with a thin layer of glasssealant (Aremco 617) at the anode side. To cure the sealants,  124  Z. Shi et al. / Journal of Power Sources 176 (2008) 122–127  the furnace temperature was increased at a rate of 0.5 ◦ Cmin − 1 to 110 ◦ C, where it was kept for 2h and then was increasedto 871 ◦ C at a rate of 1 ◦ Cmin − 1 and kept for 20min. Thenthe furnace temperature was decreased to a set point for con-ducting the test. Nitrogen (Praxair, Grade 4.8) was fed into theanode chamber and extra dry oxygen (Praxair, Grade 2.6) wasfed into the cathode chamber during sealant curing. After thecell had stabilized at the selected temperature, ethane fuel wasintroduced into the anode chamber. The system was maintainedunder these conditions for 30min to stabilize before conductingmeasurementsateachselectedtemperature.PureC 2 H 6  (Praxair,Grade 2.0) and extra dry O 2  were introduced into the anode andcathode compartments via the inner tubes, and after reactions inthe fuel cell gases exited via the gas outlets in the outer tubes.All gas flows in fuel cell tests were controlled using mass flowcontrollers.The Thermolyne F79300 tubular furnace had a uniform tem-perature zone ( ± 0.6 ◦ C) in the central part of the cell. Thetemperature of the zone where the MEA was located duringtests was monitored using a calibrated thermocouple to ensureconsistency of readings in each test. 2.3. Electrochemical characterization Cell open circuit voltage (OCV) was monitored as a func-tion of time on stream. Data were recorded with a Solartron SI1287 electrochemical interface and SI 1260 frequency responseanalysis instruments. When a steady OCV was reached, elec-trochemical impedance spectrum (EIS) measurements wereperformed in the frequency range 0.1Hz to 1MHz and acamplitudeof10mVtodeterminethecellresistanceandconduc-tivity of BCY15 electrolyte. Potentiodynamic measurements ata scanning rate of 5mVs − 1 were conducted to determine thecell current–voltage curves from which the current density andpower density were calculated. 2.4. Tests of ethane conversion and ethylene selectivity The outlet gases from the anode chamber were analyzedonline using a HP5890 Gas Chromatography with a packedcolumn (OD: 1/8 in.; length: 2m; Porapak QS) and a thermalconductivity detector (TCD). 3. Results and discussion 3.1. High temperature fuel cell set-up A series of tests was conducted on a cell in horizontal ori-entation using, for example, sleeves containing a controlledatmosphere. These tests showed that performance improvedwhentheatmospheresurroundingthesealcontainednooxygen,indicating that leakage of external air through the seal affectsthe fuel cell performance. Thus we sought means to preventformation of leaks through the seal.We found that when the fuel cell set-up had vertical orienta-tion, glass sealant could be beneficially used instead of ceramicsealant. In addition, in vertical orientation there was less tor-sional strain, and consequently less propensity to cracking thesealant. Fuel cell performance was thereby improved by reduc-ingthepropensityforleakageofoxygenintotheanodechamber.Apartfromtheinfluenceofdifferentsealants,wealsowantedtodetermine if there were additional effects of cell orientation onfuel cell performance. Therefore, the fuel cell set-up had a rota-tional axis so that the fuel cell could be tested in either verticalor horizontal orientation. 3.2. Phase composition and micro-structure of BCY15 BCY15 perovskite was prepared using a solid-state reac-tion method. Its bulk structure and micro-structure, electricalconductivity and proton transport numbers were determined.XRD analysis proved that BCY15 powder calcined at 1400 ◦ Cfor 10h comprised a single perovskite phase with good crys-talinity [18]. SEM analysis (Fig. 2) showed that BCY15 membrane was very dense and formed by intimate contactBCY15 particles of 2–7  m size, which favored fast conductingprocessbysuppressingtheimpedanceofgrainboundaryregions[2]. 3.3. Conductivity of BCY15 Based on its high oxygen defect concentration, which incor-porate protons upon exposure to humidity or H 2 -containingatmospheres [19], BCY15 membranes provided good con- ductivity: 12, 15 and 20mScm − 1 at 650, 700 and 750 ◦ C,respectively (Fig. 3). Thus the components of the reaction mix- tures and surface intermediates in the anode (H 2 , surface Hspecies) and cathode (H 2 O) compartments provided the nec-essary protons. The activation energy (  E  a ) of the mobility of protonic defects is 0.53eV, which is typical of perovskite-typeprotonic conductors [1]. The ion conductivity of BCY15 in intermediate-temperature region is comparable with that of 8%yttria stabilized zirconia (8YSZ), the most used electrolyte cur-rentlyusedinSOFC,whichisintherangeof10and15mScm − 1 at 700 ◦ C [20,21]. Therefore, BCY15 is a promising candidate for use as electrolyte in intermediate-temperature fuel cells. Fig. 2. Cross-sectional scanning electron microscopy (SEM) of BaCe 0.85 Y 0.15 O (3 − α )  (BCY15) sintered in air at 1550 ◦ C for 10h.   Z. Shi et al. / Journal of Power Sources 176 (2008) 122–127   125Fig. 3. Conductivity–temperature curves for BaCe 0.85 Y 0.15 O (3 − α )  (BCY15)ceramic membrane tested at fuel cell operating temperatures and open circuitcondition.  E  a  is activation enthalpy of the mobility of protonic defects. Although BCY is a mixed ion conductor [22], oxygen ion conductivity is sufficiently low that it does not affect cell per-formance under the present operating conditions [18]. 3.4. Enhanced performance of vertically oriented IT-PCFC  C 2 H 6 –O 2  IT-PCFC at a vertical orientation can deliverenhanced performance when using Pt/BCY15 (thick-ness=0.44mm)/Pt membrane electrode assembly (MEA)and glass sealant (Aremco 617) applied on the anode side. Theflow rates of C 2 H 6  and O 2  are both kept at 100cm 3 min − 1 during testing. At 650 ◦ C, the maximum power density wasenhanced to 116mWcm − 2 and the corresponding currentdensity was 235mAcm − 2 in a vertical IT-PCFC (Fig. 4), Fig. 4. Current density–voltage (open markers) and current density–power den-sity curves (solid markers) of a C 2 H 6 –O 2  IT-PCFC with vertical orientation,having BCY15 membrane and Pt paste as both anode and cathode electrodes at650 ◦ C (squares) and 700 ◦ C (triangles). The flow rates of C 2 H 6  and O 2  wereboth 100cm 3 min − 1 . much higher than that of 21mWcm − 2 and 58mAcm − 2 in ahorizontal orientation using ceramic sealant [18]. Accordingly, the ethane conversion and ethylene selectivity improved to30% and 97%, respectively. At 700 ◦ C, the maximum powerdensity and the corresponding current density improved to174mWcm − 2 and 320mAcm − 2 , respectively, again muchhigher than that of 56mWcm − 2 and 164mAcm − 2 for thehorizontal set-up. Also, both ethane conversion and ethyleneselectivity were enhanced to 34% and 96%, respectively .  Thehigh ethane conversion and ethylene selectivity showed theadvantage of an IT-PCFC reactor for C 2 H 6  conversion to C 2 H 4 ,compared with catalytic oxidative dehydrogenation of ethaneto ethylene on chromium oxide or nickel oxide based catalysts[23,24].It is noteworthy that no acetylene was detected in the effluentfrom the anode chamber. The divergence from 100% selectivitythereforearosefromformationofcokeandotherunidentifiedby-products. Consequently, we are investigating modifications tothe catalyst and its environment with the objectives of reducingcoking while avoiding formation of unwanted by-products. Inparallel,wearemodifyingoperatingparameterstoenhancecon-versiontowardcommerciallyviablelevelswithoutdeleteriouslyaffecting selectivity. 3.5. Enhancement of performance of IT-PCFC  When using the vertical design of the C 2 H 6 –O 2  IT-PCFC,two of the factors contributing to the improvement of cell per-formance compared to other tests were (i) excellent sealingperformanceofglasssealant,and(ii)thinnerBCY15membrane(thickness=0.44mm).We found in preliminary experiments that several ceramicsealants were unsuitable for use with ca. 0.5mm BCY15membranes, as they were very prone to cracking, detachedfrom the membranes, or compromised the physical integrityof the membrane surfaces. As discussed in the literature,glass sealant heated to 850 ◦ C can provide an excellent gas-tight seal [15]. We have now shown using SEM micrograph of glass sealant coating formed after heating to 871 ◦ C thata well-bonded contiguous and dense layer is formed with-out any holes or cracks (Fig. 5A). EDS analysis identifiedthe bar-shaped crystalline phase (point 1 in Fig. 5A) asCaSiO 3  (Fig. 5B), and the composition of non-crystallinephase (point 2 in Fig. 5A) was Na 2 O–K 2 O–CaO–SiO 2  com-posites (Fig. 5C). The more amorphous phase extendedbetween the crystallites to form a more dense seal. Bansaland Gamble showed that a glass sealant of composition35BaO–15CaO–5Al 2 O 3 –10B 2 O 3 –35SiO 2  (mol%) does notfully crystallize even after long term heat treatment at750–900 ◦ C[16].Whencalcined,theceramicsealantcomposed of alumina phosphates was shown by EDS (Fig. 6B) to be a porous assemblage of small particles 2–7  m (Fig. 6A), which resulted in detrimental effects caused by continuous leakage of small amounts of air into the anode chamber, producing unde-sirable carbon oxides. Therefore, it is preferable to use calcinedglass sealant which forms a dense and gas impermeable coatingsuitable for fuel chambers of hydrocarbon IT-PCFC. However,  126  Z. Shi et al. / Journal of Power Sources 176 (2008) 122–127  Fig. 5. (A) Scanning electron microscopy (SEM) and (B and C) energy-dispersive X-ray spectroscopy (EDS) analysis of glass sealant (Aremco 617)scratched from the fuel cell set-up after fuel cell tests at high temperature. (B)EDS of point 1 in Fig. 7A. (C) EDS of point 2 in Fig. 7A. as shown in Section 3.4, the glass sealant cannot be subjected to torsional strain.EIS of a single SOFC often reveals electrolyte resistance,indicatedbytheintersectionontherealaxis,andelectrodepolar-ization resistance indicated by the succeeding semi-circle [25].When using a thin (0.44mm) BCY15 membrane instead of a1mm BCY15 membrane as electrolyte and Pt paste as elec-trodes at both sides in an IT-PCFC, the electrolyte resistanceswere reduced to 3.2 and 2.8  cm 2 at 650 and 700 ◦ C (Fig. 7), respectively,thussignificantlyimprovingcellperformance.Therise in electrode polarization resistance from 2.7 to 4.2  cm 2 tested at 650 ◦ C first and then at 700 ◦ C is caused by coke accu-mulation over the anode when heating from 650 to 700 ◦ C andduring stabilizing at 700 ◦ C, as is well known from hydrocar-boncrackingreactions.Thereforeitisimportanttofindeffectiveelectro-catalystsfordehydrogenationofC 2 H 6  withoutcokingina C 2 H 6 –O 2  IT-PCFC. It is also important to develop supportedthinner film proton-conducting electrolytes to further improvecell performance. Fig. 6. (A) Scanning electron microscopy (SEM) and (B) overall energy-dispersive X-ray spectroscopy (EDS) analysis of ceramic sealant (Aremco 503)scratched from the fuel cell set-up after high temperature test.Fig. 7. Electrochemical impedance spectra (EIS) of a C 2 H 6 –O 2  IT-PCFChaving a vertical orientation with BCY15 membrane as electrolyte (thick-ness=0.44mm) and Pt paste as both anode and cathode electrodes at 650 (opensquares) and 700 ◦ C (solid squares). The flow rates of C 2 H 6  and O 2  were both100cm 3 min − 1 . 4. Conclusions Proton-conducting perovskite BaCe 0.85 Y 0.15 O 3 − α  (BCY15)membranes showed good conductivity of 15 and 20mScm − 1 at 700 and 750 ◦ C, respectively. A vertically oriented C 2 H 6 –O 2
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