SEALANTS FOR SOLID OXIDE FUEL CELLS

Transcription

1 b SEALANTS FOR SOLD OXDE FUEL CELLS by T. W. Kueper,. D. Bloom, nd M. Krumpelt Electrochemicl Technology Progrm, Argonne Ntionl Lbortory, 97 S. Css Avenue, Argonne, L 6439 TEL: (78) FAX: (78) The submitted mnuscript hs been uthored by contrctor of the US. Government under contrct NO. W-3-9-ENG-38. Accordingly. the U. S. Governmentretins nonexclusive, roylty-free License to publish or reproduce the published form of tbh contribution, or llow others to do so, for U. S. Government purposes Februry, 996. The 996 EPRUGR Fuel Cell Workshop on Fuel Cell Technology Reserch & Development, The Electric Power Reserch nstitute nd the Gs Reserch nstitute, April 2-3,996, Tempe, AZ.

2 Selnts for Solid Oxide Fuel Cells T. W. Kueper,. D. Bloom, ndm. Krumpelt Electrochemicl Technology Progrm, Chemicl Technology Division Argonne Ntionl Lbortory, 97 S. Css Ave., Argonne, L 6439 Telephone: (78) FAX: (78) ntroduction Bsic requirements for selnt re good bonding to the mterils of interest, chemicl stbility in the operting environment, nd low gs permebility. For high-temperture opertion s in Solid Oxide Fuel Cells (SOFCs), the selnt must lso hve therml expnsion which is resonbly close to tht of the other mterils involved nd must hve some complince, or softness, to llow for some mismtch between the components to be joined (Viewgrph ). n this pper, we discuss fmily of glss-cermic mterils with mechnicl, chemicl, nd electricl properties tht re suitble for these demnding high-temperture pplictions. Our pproch to developing selnt with these properties ws to strt with low-melting glss, with T, between 5 nd 75 C, nd to mnipulte the temperture index of viscosity. The viscosity trget ws lo6 P-s t the operting temperture (Viewgrph 2). We lso used known molr dditivity coefficients to choose composition with the desired coefficient of therml expnsion (CTE) in the temperture rnge below Tg. We studied glsses nd glss-cermics bsed on boron oxide s the primry glss former to obtin low T,. We tested more thn 4 compositions in our five-component (SrO-L,O,-Al,O,- B,O,-SO,) oiide system (Viewgrph 3). Some 2 of these were glsses nd glss-cermic mterils. We were ble to predictbly lter compositions to obtin desired rtio of glss to crystlline mteril. Two specific compositions, Mterils 4 nd 43 ( glss-cermic nd glss, respectively), were eventully chosen s exemplry seling mterils nd were used for further testing by fuel cell developers nd for commercil glss production tests. Results Within our chosen composition rnge, we found therml expnsions rnging between 7 nd 6 x -6/oC (Viewgrph 4). Pure glsses hd expnsions in the 8 to 9 x -6/oC rnge, while glsscermics hd higher expnsion coefficients. Liner regression nlysis ws performed on the expnsion dt to determine first-order effects from the vritions in composition (Viewgrph 5). Lnthn, lumin, silic, nd boron oxide percentges were used s independent vribles. The y-intercept in this nlysis represents the CTE vlue of fictitious pure-stronti glss. We were ble to fit the dt firly well (3 =.86) with this liner model. The best fit gve n intercept of 9.4 x -6/oC for stronti, while effects for ddition of the other oxides were ll slightly negtive, with coefficients rnging from -.7 for lnthn to -.2 for silic.

3 With scnning electron microscopy (SEM), crystlliztion could usully be determined by the observtion of fceted precipittes in the glss mtrix. X-ry diffrction showed tht the squenched mterils contined the high-temperture phse of LBO,, while ground nd sintered smples contined the low-temperture phse of this compound. n both cses, bout 5% strontium content ws detected in the crystlline mteril. Diffuse peks from the glss regions were lso present. Viscosities were clculted from bending-bem tests, in which bems of the glss nd glsscermic mterils sg under their own weight when held t constnt elevted temperture. The viscosities clculted from these experiments were found to obey logrithmic reltionships with respect to temperture (Viewgrph 6). The compositions within our selnt system cn be mnipulted to provide suitble viscosity for opertion tempertures between bout 7 nd "c. Oxygen concentrtion cells using Mteril 4 s selnt between zirconi pieces were tested in the temperture rnge 8-OOO"C (Viewgrph 7). Here, furnce ir ws the oxidnt nd flowing, humidified, 6% H, in He ws the fuel. The cell EMF ws mesured t 8,9,nd C.All EMF'S were 99% of the theoreticl vlues clculted by the Nernst eqution nd were stble for periods of t lest 3 dys. During the test, the cell ws thermlly cycled between 4 nd C five times nd between room temperture nd Conce. The EMF'Sfter therml cycling styed within -2 mv of previous vlues. These experiments show tht ny gs lekge through the selnt is quite smll in comprison to norml gs flow rtes. To quntify the permetion rte, simple mesurements using Mteril 4 were mde during the cell test t ech of the three tempertures (Viewgrph 8). For these mesurements, the flow of gs ws stopped, nd the EMF ws mesured s function of time. For clcultion purposes it ws ssumed tht the chnge in cell EMF ws due solely to the permetion of moleculr oxygen through the sel; since other sources of lekge re possible, this should provide n upper limit to the selnt permetion coefficient. The clculted vlues of K (in mm cm3/s-cm cm-hg) rnged from 2 x t looo"c,nd did not chnge with therml cycling. These vlues re t 8 C to 5 x comprble to those of vitreous qurtz, nd we cn conclude tht our mterils re excellent selnts with respect to oxygen permetion. No significnt chnges were seen in these seling mterils when tested t high tempertures in ir for periods up to 5 h. n humidified hydrogen, however, Mteril 4 hd smll weight chnge, bout.5%, in the first 6 h of exposure. The weight loss t first continued with dditionl exposure time; fter totl exposure time of 2 h, weight loss ws.9%. No further chnges in weight were observed with longer times. n series of tests in high humidity (4% H,O t looo"c),weight losses were recorded for vrying selnt compositions. We found tht smples higher in lumin, stronti, nd lnthnum borte hd better stbility, s did those with lower silic mounts (Viewgrph 9). We mesured the electricl conductivity of these mterils t different tempertures nd oxygen prtil pressures, nd concluded tht they exhibit only low, purely ionic conductivity (Viewgrph ). We lso checked the selnt conductivity s function of time in simulted fuel

4 cell conditions, to determine if ny mobile ctions from the fuel cell mterils would be driven into the selnt nd increse conductivity (Viewgrph ). We determined tht the selnt my experience slight chromium diffusion from the interconnect, but tht conductivity vlues remined quite low. We used inductively coupled plsm-tomic emission spectroscopy (CP-AES) to check for vritions from nominl compositions in the ctul smples. Devitions from nominl compositions (in percent of the nominl mount of ech component) were obtined for six different compositions (Viewgrph 2). Mterils G42, G43, G45, nd G47 re low-lnthn, high-silic compositions compred to the two versions of Mteril K. The generl trend is for the low-silic compositions to be deficient in strontium nd luminum, while the high-silic mterils re deficient in boron oxide. These trends re probbly the result of segregtion in the melt. Segregtion is not expected in full-scle, stirred-melt processing. Production of the glss-cermic selnt mterils in commercil quntities hs proved difficult. Two mnufcturers hve filed to supply btches of Mteril 4 without mjor chnges in the properties of the mteril, including therml expnsion vritions nd chnges in hightemperture voltility. The two-phse nture of the mteril mkes conventionl stirred-melt processing difficult, nd excessive crystlliztion complictes the quenching process. Thus, modified processes my be necessry to provide uniform lrge-scle production of these mterils. Smples of Argonne-produced mterils hve been sent to severl fuel cell mnufcturers with successful results (Viewgrph 3). Mterils 4 nd 43 hve shown improvements over other seling mterils in terms of dhesion, mechnicl complince, nd chemicl stbility for vriety of pplictions nd mterils. Conclusion Our glss-cermic mterils pper to hve suitble combintion of properties for hightemperture pplictions, such s solid oxide fuel cells. Their low glss trnsition tempertures nd low temperture index of viscosity led to wide temperture rnge in which the selnt is soft nd yet mechniclly stble. Accommodtion of lrge CTE mismtches hs been demonstrted. The selnt CTEs hve been tilored to desired vlues for mtching to vriety of cermic nd metl components within the low-temperture rnge when the selnt is rigid. t hs lso been shown tht their stbility nd gs permebility re suitble for pplictions in which gses with widely vrying oxygen prtil pressures must be seprted for extended periods of time. These mterils hve been successful in tests by fuel cell developers; however, lrge-scle production of the glss-cermics needs further effort. Acknowledgment This reserch ws sponsored by the U.S. DOE Morgntown Energy Technology Center under Contrct No. W-3-9-ENG-38 nd by the Electric Power Reserch nstitute.

5 ~~ - FGURE DESRED PROPERTES FOR SEALANT Strong, impervious, thermlly cyclble bonds Coefficients of therml expnsion (CTEs) well-mtched to those of the SOFC mterils Accomodtes CTE mismtch between SOFC nd mnifolding mterils by viscous flow Chemicl comptibility = with SOFC components = with cell gses High electricl resistivity ~ Argonne Ntionl Lbortory / Electrochemicl Technology Progrm AUWJ

6 FGURE 2 MECHANCAL COMPLANCE MNMZES STRESS L = E AccAT L Cermic J CY cn be minimized by: -E& - A d - AT & or T, \ ooooc ---- Time Requires blnce between low softening temperture nd dequte viscosity t the operting temperture Agronne Ntionl Lbortory/Chemicl Technology Division

7 E i % c m x m v) =w S n z cn W cn cn c3 z v) E m Q) E3 X P 8- W - o 3 m Cm. e C > Q) Q) z G, mm S m V.-s n ml C m C W cn m cn - z W cn pe 3 33 m i7i

8 FGURE 4 THERMAL EXPANSON OF GLASS-CERAMC SEALANT S WELL=MATCHEDTO YSZ 2 Mteril K YSZ (Coors ZDY-4) Softening point Differentil expnsion./././ /./* t? 4 4 H d 2 / -2 2 r r ,_ 5 J* e i Temperture, "C Argonne Ntionl Lbortory / Electrochemicl Technology Progrm AUCM

9 FGURE 5 THERMAL EXPANSON AS A FUNCTON OF COMPOSTON FTS LNEAR MODEL Clculted dditivity coefficients per mole % of component oxides (xl6/ C): Coefficient Stndrd Error B A L23.7 ntercept Si Expression cn be used to predict CTE s for compositions similr to Mterils 4 nd 43 with resonble ccurcy Agronne Ntionl L.bortory/Chemicl Technology Division

10 FGURE 6 THESE SEALANTS CAN COVER A RANGE OF SOFTENNG TEMPERATURES Viscosities were mesured by the sgging bem method FT dsldt.oo rl(p-s).: Temperture, "C 9 3. M bg L3 *. (ds/dt) b*'d K- glss-cermic 42, 43, 47 - glsses n v) l4 9. n > v -,R,,d y 7.,,,, ' ooorr, K" Adding crystlline phse reduces the temperture index of viscosity 8394 Agronne Ntionl LbortoryKhemicl Technology Division

11 - FGURE 7 SEALNG ABLTY OF GLASS-CERAMC WAS CONFRMED N ELECTROCHEMCAL TESTS Commercil, stbilized zirconi tube Pt wires f r- t! U Pt wires E LL Air 5 / Thermocouple > f coupon Glss-cermic selnt U n U n Y n v c. U U n U n 2 Time, dys These mesured 8 C EMFs re 99% of 9 C theoreticl ooooc Agronne Ntionl LbortoryKhemicl Technology Division

12 FGURE 8 PERMEABLTY OF GLASS-CERAMC S SMLAR TO THAT OF QUARTZ Permebility constnt, K, clculted from AEMF/At upon periodiclly shutting of fuel gs flow q= flux of oxygen A= re t= time pl= finl p2 (clc d from EMF) p2= initil p2 d= thickness Temp., 8 9 O C K, mm-cm3/(s.cm.cmhg) 2.9 x x x -7 - crystlline qurtz mm-cm3(stp)/(s.cm.cmhg) -. t 7OOC = Agronne Ntionl LbortoryK hemicl Technology Division

13 ~ STABLTY N HGH HUMDTY FGURE 9 Slight boric cid voltility ws observed in long-term tests in humidified hydrogen for Mteril K. A series of tests ws then performed using high humidity (4% H,O t C) nd high surfce re selnt smples. Weight losses in these conditions were mesured for vrious mterils. Most weight loss occurred during the first 5 hr of the tests. Mteril K wt. loss:.96% over 25 hr. Reduced Voltility Compositions: Smples high in lumin hd slightly reduced wt. loss (.7 % in 75 hr) Low-silic, high stronti smples hd significnt improvements (. % over 256 hr) Smples high in lnthnum borte were the most stble (less thn.3% wt. loss during 62 hr test fter initil tretment) These trends cn be used with knowledge of softening nd expnsion trends to rrive t selnts with incresed stbility for high-humidity pplictions. Agronne Ntionl Lbortory/Cltemicl Technology Division

14 FGURE THESE SEALANTS HAVE NO ELECTRONC CONDUCTVTY At given temperture, there is no chnge in conductivity with oxygen prtil pressure (Le., they re purely ionic conductors) Argonne Ntionl Lbortory / Electrochemicl Technology Progrm AUCO?

15 CONDUCTVTY OF SEALANT UNDER VOLTAGE GRADENT ~~~ ~~~ During opertion s mnifold sel, mobile ctions my be driven into the selnt under voltge grdients, ffecting conductivity. ~ ~ These conditions were simulted by pplying V to Mteril K while bonded between two lnthnum chromite electrodes t C:.45 - <3E >. = s Time (hours) nitil conductivity decrese probbly due to formtion of interfcil phse high in Sr nd C Grdul conductivity increse my be due to Cr migrtion into selnt These vlues re sufficiently insulting for the ppliction Agronne Ntionl Lubortory/Cliemicl Technology Division

16 FGURE 2 7 Agronne Ntionl Lubortory/Chemicl Technology Division

17 FGURE 3 SEALANTS ARE SUCCESSFUL N TESTS BY OTHER RESEARCH GROUPS Mteril 4 BZr3/Alurnin bonding: Proper bonding, complince, nd chemicl stbility in hydrogen probe ppliction where no other selnt ws stisfctory during 2 yers of testing. Zr2/metl interconnect bonding t 9 C: Shows proper bonding/softening behvior nd therml expnsion comptbility. Problems with powder ppliction/porosity. Zr2/LSM bonding t C: Shows proper bonding nd softening behvior nd therml expnsion comptbility. Mteril 43 Oxygen-conducting cermic/stinless steel: Excellent bonding, mechnicl nd chemicl stbility in fuel cell/oxygen seprtion pplictions t 65 C Agronne Ntionl Lbortory/Chemicl Technology Division

18 b SUMMARY FGURE 4 A fmily of selnt mterils bsed on borte glsses hs been developed. The chemicl nd thermo-mechnicl properties of these selnts mke them superior to other vilble mterils in number of pplictions. Strong, dense bonds re developed to metls nd cermics for use t 8 C to C Therml expnsion nd softening behvior cn be controlled to llow therml cycling without frcture with expnsion-mismtched mterils Selnts re stble in oxidizing nd reducing conditions, nd re good electricl insultors These mterils hve been successful in tests by fuel cell developers nd requests for dditionl quntities continue Commercil production of these glss-cermic mterils hs proved difficult nd my require specil equipment Agronne Ntionl Lubortory/Cliemicl Technology Division