Resume of final report for the project PSO-F&U DK-SOFC b, Long term SOFC development

Transkript

1 Resume of final report for the project PSO-F&U DK-SOFC b, Long term SOFC development Resume af slutrapport for projektet PSO-F&U DK-SOFC b, Langsigtet SOFC udvikling Project manager : Mogens Mogense n Project responsible institute : Fuel Cells and Solid State Chemistry Department Risø National Laboratory, The Technical University of Denmark Frederiksborgvej 399, Postbox Roskild e CVR-nr.: DK Background, purpose and activities of the project The overall objective of the Danish SOFC (= Solid Oxide Fuel Cell) program is to commercialize Danish SOFC technology. The present one-year project was a part of a multi-year effort (previously funded by The Danish Energy Research Program), which addresses the long term development goals. Two cell generations are under development: A further development of the 2 d generation cell (2G), which at present has an operation temperature of around 800 C, and the development of a new metalsupported cell for operation at approx. 600 C (3G). Some of the cell components aimed for the 3G cell have been tested out on 2G half-cells. Such cells are referred to as 2.5G cells. The purpose of the project was to contribute to the development of cells and stacks that will be competitive with other technologies for electricity production. The final goals are : 1) Decreasing the ASR enough that the cell can operate in the temperature range C. 2) Improving the lifetime of the cells. The degradation should not exceed 0.25 % per 1000 h over 40,000 h. 3) Improving of the robustness of electrodes and cells, both mechanically and chemically, and 4) Development of anodes that can tolerate untreated natural gas. It was clear on beforehand that the final goals could not be reached in this one-year project. The specific activities in this project were : 1) Development of methods for fabrication of improved electrode structure, 2) Characterization of the effect of transition metals in the new electrolyte, 3) Investigation of relations between new processes/process parameters, electrode structure and performance using advanced electron microscopy, 4) Development of individual processes to obtain new dualphase-electrodes (anode for unreformed natural gas and low temperature cathode) based on the already identified materials, 5) Studies of the interfaces between the electrodes and the electrolyte in order to decrease the charge transfer resistance across barriers caused by segregation of impurities, 6) Continued studies of electrode kinetics and the interplay with the fabrication procedure to gain deeper insight into the mechanisms that causes the relative high polarization resistance at 600 C, 7) Writing of papers to international journals based on results from earlier years as well as this project, 8) Acquisition of knowledge by literature studies, informal cooperation and

2 participation in conferences, 9) Fabrication of coupling components, glass seals and possibly other components by IRD on request from Risø, 10) Cell tests under special conditions (e. g. higher pressure) with the purpose of revealing mechanisms that limit the cell performance and durability (IRD), and 11) Provision of improved stee l (TOFC). Some achieved result s Screen printing has been introduced as a new technique for the application of both composite LSM/YSZ (lanthanum strontium manganate / yttria stabilized zirconia) and LSFC/CGO (LaSrFeCo-oxide/CeGd-oxide) cathodes to SOFCs. It is shown that screen printed cathodes may perform better than sprayed cathodes with respect to polarization resistance (Rp) as measured with electrochemical impedance spectroscopy. Several ways to improve the cathode performance have been investigated. Since the results regarding the screen printed are very promising, screen printing is now the technique chosen for future cathode development. The results of the test of a cell with LSM/YSZ cathode have shown that the area specific resistance was decreased by more than 30 % at 750 C. Further, a 2.5G cell was fabricated with a screen printed LSFC cathode that was impregnated with extra LSFC afterwards. This resulted in the best SOFC ever tested at Risø reaching a power density of 1.5 W/cm2 at 750 C. A mathematical modeling effort was fruitful in the sense that a model, which has been implemented on a computer, can actually "predict" with a fair accuracy, the actual measured performance of two different LSFC/CGO cathodes. Zirconia co-doped with yttria and scandia has been chosen as the electrolyte for use in future cell generations. Moreover, the incorporation of small amounts of A1203 has been found to be beneficial as a sintering aid and scavenger for traces of silicon. Our usual supplier, Tosoh, has been unable supply these compositions, and therefore, powders from other suppliers have been investigated. The conductivity has been measured in the temperature range C. For a selection of these samples, the durability (i.e. time dependence of the conductivity) has been studied at C. On samples giving promising results, an investigation of the effect of co-doping with several elements has been initiated. A novel instrument, a CAHT- SPM (controlled atmosphere high temperature scanning probe microscope) has been developed in cooperation with the Danish company, DME Danish Micro Engineering A/S. The first phase of this development is about finished, i.e. the instrument has been refined to a state that it is possible to make correlating topographical and electrical pictures of a surface of a heat treated YSZ pellet at 650 C. This kind of picture is to our best knowledge the very first of its kind. At IRD a fabrication route for suitable seal glass powders has been developed. The preparation route includes a pre-reaction step of specific parts of the components in question before mixing with the other precursor chemicals. The final mixture can then be melted using relatively cheap alumina crucibles. IRD fabricates these crucibles themselves. Batches-sizes up to 1 kg of glass powders can be fabricated with the existing laboratory facilities at IRD. IRD has furthermore had a substantial activity in fabricating all ceramic-coupling layers and all standard seal glass powders for single cell tests at Risø and at IRD. Several attempts to test single cells at elevated pressure have been carried out at IRD. The work has shown that this is more challenging than expected; leakages have been allocated mainly to originate from the auxiliary test components rather than from the sealed cell itself. Fabrication of a new test house with less possibility for leakage, e.g. fewer lead-ins, is in progress.

3 ,, A Plansee alloy ITM- 14 has been tested in a stack together with other potential steel interconnects and characterized post-mortem at Topsoe Fuel Cell A/S. The alloy has an overall composition of approximately Cr24Fe65Mo2Sio.s-aMno.a-o.3 and Ti, La, Y, and Zr, all less than 1%. The addition of the latter elements results in the formation of an ODS (oxide dispersed strengthened) alloy, which has promising properties as a future interconnect. Conclusion The work of the project was in broad terms carried out according to the plans. All milestones but no. 11 were met. A new screen printing procedure for electrode fabrication was introduced with great success, and major break-throughs were achieved in this project. Ten papers were published (see below), one patent application was filed, and another patent application was prepared. Baggrund, formål og aktivitete r Målet for det danske SOFC (= Solid Oxide Fuel Cell) program er at kommercialisere dansk udviklet SOFC. Nærværende etårige projekt var et led i en flerårig indsats, som varetager de langsigtede udviklingsmål, hvor der for tiden arbejdes med to generationer af celler : En videreudvikling af 2. generationscellen (2G), der p.t. har driftstemperatur ved ca. 800 C, og udvikling af en ny metal-supporteret celle til drift ved ca. 600 C (3G). Arbejdet er rettet mod 1) at minimere den arealspecifikke modstand ved en given temperatur, 2) at sænke driftstemperaturen uden at forøge cellestakkens arealspecifikke modstand, 3) at forbedre robustheden af celler og komponenter, og 4) at afdække driftsrelevante mekanismer som en væsentlig forudsætning for kommende teknologigennembrud. En vigtig del af linjen i projektet er opbygning af knowhow med tilhørende patenter, der kan sikre industrielle samarbejdspartnere en fordelagtig position i fremtiden. Der fokuseres på følgende problemer: 1) Nedbringelse af den arealspecifikke indre modstand, ASR, så SOFCstakken kan operere ved C, 2) Forbedring af cellernes levetid, så degraderingen er mindre end 0,25 % pr timer over timer, 3) Forbedring af cellernes/elektrodernes robusthed, både mekanisk og kemisk, og 4) Udvikling af elektroder, der tåler ubehandlet naturgas (dvs. elektroder, som tåler udfældning af kul og tåler den svovl, der er i naturgas). Konkret var formålet med dette projekt at bringe os nærmere til disse langsigtede mål, som det dog på forhånd var klart, at vi ikke kunne nå i det etårige projekt. De specifikke aktiviteter var: 1) Udvikling af metoder til fremstilling af bedre grænsefladestrukturer i elektrode/elektrolytgrænseregionen, 2) Karakterisering af effekten af overgangsmetaller i den nye elektrolyt, 3) Undersøgelse af sammenhænge mellem nye processer/procesparametre, elektrodestruktur og ydelse vha. avanceret elektronmikroskopi (FEG-SEM - nanoskala), 4) Udvikling af særskilt proces til nye tofase-elektroder (anode til ureformeret naturgas og lavtemperaturkatode) på grundlag af de nu identificerede materialer, 5) Studier af grænsefladerne mellem elektroderne og elektrolytten med henblik på at mindske modstanden for ladningsoverførsel over barrierer forårsaget af segregeringer af sporstoffer, 6) Fortsatte studier af elektrodekinetik i samspil med fremstillingsproceduren med henblik på en bedre forståelse af de mekanismer, der forårsager den relativt høje elektrodemodstand omkring 600 C, 7) Skrivning af artikler til internationale tidsskrifter på grundlag af resultater fra såvel tidligere år som indeværende projekt, 8) Hjemtagning af viden gennem litteraturstudier, uformelt samarbejde og deltagelse i konferencer og workshops, 9. Fremstilling af

4 d koblingskomponenter og glaspakninger m.v. hos IRD efter behov,10) Celletests med specielle betingelser (f.eks. højere tryk) med henblik på opklaring af mekanismer, som begrænser cellens ydeevne og levetid (IRD), og 11) Tilvejebringelse af bedre stål (TOFC). Nogle af de opnåede resultate r Silketryk er blevet indført som en ny fremstillingsteknik både for kompositte LSM/YSZ (lanthan strontium manganate / yttria stabiliseret zirconia) og LSFC/CGO (LaSrFeCo-oxide/CeGd-oxide) katoder til SOFC. Flere måder til at optimere silketryksprocessen er gennemført. Det er vist, at silketryk kan give så meget bedre katoder end sprøjtemaling, at det er besluttet at anvendesilketryk som elektrodepålægningsteknik i det fremtidige udviklingsarbejde. Den areal-specifikke modstand af en celle med en silketrykt LSM/YSZ-katode fandtes at være 30 % lavere en almindelig 2G celler ved 750 C. Også en 2.5G celle blev fremstillet med en silketrykt LSFC/CGO-katode, der endvidere var blevet efterimprægneret med yderligere LSFC. Cellen er den bedste, der nogensinde er målt på Risø, idet effekttætheden var 1,5 W/cm2 ved 750 C. En indsats på matematisk modellering var vellykket, idet en model, der blev implementeret på en computer, kunne "forudsige" den målte celleopførsel med rimelig nøjagtighed for celler med to forskellige LSFC/CGO-katoder. Den fremtidige elektrolyt er valgt til at være zirconia co-dopet med scandia og yttria. En yderligere tilsætning af små mængder A1203 virker gavnligt ved at fremme sintringen af zirconia-pulveret. Vores sædvanlige leverandør, Tosoh, kunne ikke levere denne sammensætning, så andre leverandører blev valgt. Den specifikke ledningsevne for et udvalgt antal elektrolytprøver er målt i temperaturintervallet C. For et godt pulver blev virkningen af co-doping med flere relevante metaloxider undersøgt i temperaturintervallet C. En hel ny type instrument, en CAHT- SPM (controlled atmosphere high temperature scanning probe microscope), er udviklet på Risø i samarbejde med det danske firma, DME Danish Micro Engineering A/S, i Herlev. Den første fase af denne apparatudvikling er nu ved at være slut, idet mikroskopet er blevet bragt til et niveau, hvor det er muligt at optage billeder ved 650 C, der viser en klar korrelation mellem overfladestrukturen og variationen af den elektriske modstand hen over overfladen af en YSZ-pille. Så vidt vi ved, dette den første gang sådanne billeder er optaget. IRD har udviklet en bedre fremstillingsprocedure for glaspulver til forseglingsglas. Fremgangsmåden indebærer først et reaktionstrin med nogle af de involverede kemiske forbindelser. Slutblandingen kan smeltes i relativt billige alumina digler, som IRD selv laver. Batch størrelser op til 1 kg kan fremstilles hos IRD. Videre har IRD fabrikeret alle keramiske koblingslag til celletests på Risø og IRD. Adskillige forsøg med tryksætning af celletests er udført af IRD, men dette har vist sig vanskeligere end antaget. Gaslæk henføres hovedsagelig til indføringer af rør i celletesthuset. Et forbedret celletesthus er under konstruktion. En rustfri stållegering fra Plansee, ITM- 1 4, er blevet afprøvet i en cellestak og undersøgt post-mortem. Legeringen, der har den omtrentlige sammensætning Cr24Fe65MoZSio.s-aX1no.z-o.3 og med Ti, La, Y, og Zr alle under 1%. Indholdet af de sidstnævnte grundstoffer resulterer i dannelsen af en ODS (oxide dispersed

5 strengthened) legering, der har lovende egenskaber som et fremtidigt interconnectmateriale. Konklusion Arbejdet i projektet har i store træk fulgt planen. Alle milepæle på nær nr. 11 blev nået. En ny silketryksmetode blev indført, og flere væsentlige gennembrud fandt sted i dette projekt. Der er blevet publiceret ti artikler. En patentansøgning er indleveret og en anden er udarbejdet. List of publication s 1. SØgaard, M. ; Hendriksen, P. V. ; Jacobsen, T ; Mogensen, M., Modelling of the polarization resistance from surface exchange and diffusion coefficient data, in Proc. (cd-rom). 7. European solid oxide fuel cell forum, Lucerne (CH), 3-7 Jul Bossel, U. (ed.), (European Fuel Cell Forum, Oberrohrdorf, 2006) B K. Kammer, `Studies of Fe-Co based perovskite cathodes with different A-site cations', Solid State Ionics, (2006 ) 3. K. Kammer, L. Mikkelsen, J.B. Bilde-Sørensen, `Electrical and Electrochemical Characterisation of Lao.99Fe1_XNiXO3-S Perovskites', J Solid State Electrochem (2006) 4. Hansen, K. V. ; Norrman, K ; Mogensen, M, TOF-SIMS studies of yttriastabilised zirconia. Surf. Interface Anal. (2006) 38, Norrman, K. ; Hansen, K V. ; Mogensen, M, Time-of-flight secondary ion mass spectrometry as a tool for studying segregation phenomena at nickel-ysz interfaces. J. Eur. Ceram. Soc. (2006) 26, M. Mogensen, J HØgh, K. V. Hansen, and T. Jacobsen, A Critical Review of Models of the H2/H20/Ni/SZ Electrode Kinetics, Proc. of SOFC X, Nara, Japan, June S.-I. Hashimoto, K. Kammer, F.W. Poulsen, M. Mogensen, 'Conductivity and Electrochemical Characterisation of PrFel_,NiXO3_8 at high Temperature', J. Alloys and Compounds, (2007) 8. K.K. Hansen, K. V. Hansen, 'A-site deficient (Lao.6Sro.a)SFeo.gCoo.z03-s perovskites as SOFC cathodes', submitted to Solid State Ionics 9. K. K Hansen, M. SØgaard, M. Mogensen, ' Gd0,6Sr0.4Feo.gCoo.203-5' : A Novel type of SOFC Cathode', acceptedfor publication in Electrochem. Solid-State Lett. (2007) 10. K. V. Hansen, C. Sander, M. Mogensen, `Controlled atmosphere high temperature SPM for electrochemical measurements', J. Phys. Conf. Series (2007) 61,