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Lecture 2 Superionic conducting materials for low temperature 300600C solid oxide fuel cells

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Title: Lecture 2 Superionic conducting materials for low temperature 300600C solid oxide fuel cells


1
Lecture 2Superionic conducting materials for low
temperature (300-600C) solid oxide fuel cells
  • Bin Zhu, 1e forskare, docent
  • (KTH, www.kth.se)
  • Professor at Harbin Engine. Univ.
  • Tel.0046-8-7908241 Fax 0046-8-108579
  • emailbinzhu_at_ket.kth.se, binzhu_at_kth.se
  • Homepage www.ket.kth.se/avdelningar/krt

Biomass and fuel cells lecture at Helsinki
Univ. of Tech., 2005-10-11
2
Key concerning points
  • Challenges for conventional FC and solutions?
  • Materials are the key where the FC system built
    on
  • Thus new materials innovations/RD play extremely
    important role! In parallel, conventional FC
    technology to lower the T
  • Material fundamentals and theories

3
Content
From SOFC single phase, single ion (O2-)
conductor
Background and Challenges
Composites Fuel Cell
Composites, nano-comp. Two phase and Hybrid
(Super) ion conductors, Comp.-ionics
4
Fuel cell economy!New century?
5
Who is the best?!
6
Critical challenges
Two bottlenecks fuel and price H2 20-25 years,
non H2-economy Targeting the existing hydrocarbon
fuels is the way to develop marketable FCs PEFC
(H2) , plus Pt resource limit SOFC (H-C fuel) 10
times more cost hinders the marketability and
affordability To develop marketable FC, key
materials
7
Pt PEFC industrialization bottleneck
  • Car of the future may stall at start
  • Publication Date09-July-2005
  • AM US Eastern Timezone Source Business Day
  • Kazuo Okamoto, the new head of research and
    development at Toyota, Japan's biggest car maker,
    says "With the current type of technology, we
    know alreadythat (platinum supplies) will not be
    sufficient."
  • Ironically, if manufacturers are too successful
    in selling vehicles designed to solve the
    shortage of one resource, oil, they may run into
    a shortage of another.

8
Technology vs Market
Hydrocarbon fuels
9
conventional, YSZ
Advanced ITSOFC
Nernst
Tubular
First SOFC
Planar
80s
90s
1900
30s
2000
New materials
breakthrough
Fuel cell RD has experienced several generations
of Scientists efforts, and serious developments
for several decades With huge investment
10
Marketability crucial issue!
  • Future of Sulzer fuel cell venture uncertain
    (2005-09-30) http//www.HyWeb.de/gazette-e
  • "f-cell Award" in gold and 12,500 Euro go to
    Sulzer Hexis AG of Winterthur, Switzerland, for
    its residential SOFC system.
  • But Despite notable achievements in the
    development of residential CHP fuel cell systems
    (see f-cell award for Sulzer below), Sulzer has
    concluded that carrying the risk of further
    significant investments solely is beyond its
    scope and focus. Sulzer thus has decided that
    from 2006 onwards the venture investments will be
    stopped. Sulzer intends to start winding down the
    operations of Sulzer Hexis in a controlled
    manner, if no buyer can be found within short.

11
3000
1000
lt 100
12
Scientists meet ever big challenge the gape
between technology
market!
13
Two ways in developing ILTSOFCs
i) Thin film technology for advanced YSZ ii)
New materials a) Ceria instead of Zirconia b)
others, e.g., provskite oxides (single phase
materials) c) Ceramic composites (two-phase
materials)
14
Table of Content technical line 1
  • Advanced Thin film technology

Build on conventional YSZ materials for ITSOFCs,
above 700oC
15
  • Cathode materials (La,Sr) (Fe,Co)O3-x
  • Conductivity
  • Stability and compatibility with the electrolyte
  • Thin film electrolyte material YSZ (Zr,Y)O2-x
  • Model structures of YSZ
  • Cluster expansion
  • Stable ground states
  • Anode NiO-YSZ composite

16
Cathode is much more concerned to determine the
LTSOFC performances
17
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18
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20
Relative oxygen defect energies
?Hdefect formation Edefect cell
Estoichiometric cell ½ Eoxygen binding
Oxygen defect energies define a measure of
stability of the material.
21
Migration Enthalpies Perovskite ABO3

110 of cubic perovskite
J. Kilner and R. Brook, Solid State Ionics, 6,
237 (1982).
22
Oxygen Ion Diffusion
  • Experiments to models
  • Ionic conductivity

?Hmigration Eactivated Estable
23
Vacancy Diffusion Coefficients
  • Diffusivity D (self or Tracer)
  • Related to Vacancy diffusivity Dv by simple
    relationship

24
Oxygen diffusivity in perovskites
n.b there is a relationship between a and the
diffusion coefficient high D high expansion
coefficient a
25
Experimental YSZ Phase Diagram
  • Yttria stabilizes cubic phase at room
    temperature.
  • YO1.5 YZr ½VO??
  • ? - 57.1 YO1.5 ordered phase (Zr3Y4O12)

C. Pascual, P. Duran, J Am Cer Soc 66 1 (1983)
23-27
V. Stubican, et. al. J Am Cer Soc 61 1-2 17-21
(1978).
26
Thin film YSZ for ITSOFC?
  • As a general requirement, area specific
    resistivity (ASR) for the total FC lt 0.5 Wcm2 to
    electrolyte to the ASR ( d/s) lt 0.15Wcm2. Max.
    150 µm for YSZ electrolyte requests about 950 C
    (s 0.1 S/cm). at 10µm, YSZ near 700C, but may
    not guarantee the long-stability performances and
    reproducible results. (B. Steele Imperial College
    calculation the electrolyte thickness not less
    than 50µm .)

27
Material Challenges (lt 600 C )
Key materials-electrolyte-cathode
-anode
So far, conventional single-phase
Y0.08Zr92O2(YSZ) and ion doped ceria
(Sm0.2CeO0.8O2 ) cannot meet demands of LTSOFcs
due to their low conductivity (10-3 S/cm 600C for
doped ceria and lt 10-4 S/cm for YSZ.
28
Table of Content technical line 2
Next genration fuel cell
Build on new multi-functional ceramic
materials For LTSOFCs
29
LT(300 to 650ºC)SOFC Targets Mission
  • Combination of advantages for a new FC
  • Extremely low cost, market-competitive
  • Wide fuel flexibility for various existing fuels
  • the stationary residence and Tractionary
    application
  • Various portables

30
How to realize LTSOFCs Key materials Nanotechnol
ogy Nanocomposites Superionic conductor Hybrid
H/O2- conductor
31
Planar FC architecture
Cerment Or metal-ceramic composite
FCs function depends on macro- (um) and
microscopic (Nano) processes
32
MEA fabricated by anode supported electrolyte
film (screen-printed.
Materials Developments Interconnect Metals (Fe
alloy) Electrolyte thin film, YSZ,
GDC/SDC Ceria-composites Cathode LSCF Binary
metal oxides Cermets Anode Cerments (e.g.
Ni-SDC Ni-Cu-SDC composite)
A dedicated fabrication state of art is The
common feature for current ITSOFCs
33
Many nano-tech approaches/ways have been used to
prepare nano-structured materials for SOFC
electrolyte YSZ (yttrium stabilized zirconia)
34
Sintered to dense film electrolyte not
nano anymore
1200 ?C 5h, gt95
8YSZ film SEM 900 ?C lt100 nm
35
Ion-doping ceria viewed as most promising
alternative for YSZ, single phase and single ion
(O2-) conductor, also struggled for decades, s
800C /YSZ 1000C
36
Wide wet-chemical approaches are used to prepare
nanostructrued ion doped ceria,typically
co-precipitation
  • In this type of reaction two solutions mixed
    together make one product which is insoluble.
    This product comes out of the solution and can be
    seen as a solid.

37
TEM (a) and SEM (b) images
(a)
50 nm
38
GNP approach for Nano-SDCpowders
39
Various SDC (doped ceria) powders phase
2 q
Different G/N, ratio of M (Glycine) /MSDC (a)
1.0, (b) 2.5, (c)1.7 ,(d) 2
40
SDC powder morphology prepared at various
SEM
TEM
SDC powder prepared under different M Glycine /
MSDC ratio (750oC sintering)TEM and SEM photos M
Glycine / MSDC 2
41
Screen-printed ITSOFC I-V characteristics
anode supported SDC electrolyte thin film
anode Ni-SDC, cathode SmSrCo, electrolyte
SDC,20µm
42
Doped ceria problems and challenges
Structure limits the s-ion,
insufficient (e.g. s 10-3 - 10-2 S/cm at 600C )
but 0.1 S/cm requested
Instability in FC anodic condition Ce4
Ce3, e- conduction, poor
mechanical properties, micro-cracks, very
difficult for scaling up
  • What can we do?!!!

43
Nano-Tech???
  • Nano-SOFC Problems
  • HT, causing the nano-materials instable. High
    surface energy drives the nano-particle growing
    larger fast
  • FC strong oxidized and reduced atmospheres,
    critical requests.
  • SOFC???

44
Nano side effects
  • Tschöpe, SSI, 139 (2001) 267
  • Ionic to e- conduction transition
  • se- increases for nano particle size
  • ea for ion increses with decresing grain size

45
Nano and normal (mm) SDC
True effect? Ion or e-?
46
A case study SDC conductivity Ionic and
electronic in nano-scale significant se-, fr. 10
(600c) as high as 35 (800C) of the total
A case study in DMU G/N 1.7 ratio of M
(Glycine) /MSDC
47
How to realize the LTSOFCs
Nanocomposite combination of advantages from
outstanding nanotechnology and ceramic composite
Nanotech?
  • Several hundreds C,plus strong oxidation and
    reduction , nano-particle driven by high surface
    energy growing fast instability.
  • Challenge!

48
Composite technology make LTSOFC materials
practical
  • Not a single phase (not only doping)
  • Not a simple mixture
  • Bi- or multi-phases (interactions between the
    phases, but not chemically, rather
    physically-composite.)
  • Conductivity not limited by the structure (doping
    concentration, ion kinds etc.), enhanced strongly
    by the composite effect different from the
    single doping phase.
  • Can be multi-kind of ion conduction, functional
    ceramics.

49
Nano-composite tech.make LTSOFC success
  • Solutions
  • Nano-tech. composite - tech. (two-phase), to
    prepare the nano-core nano-layer-shelled
    nano-composite
  • to prevent Ce4 from the reduction.
  • Hybrid H and O2- together, novel
    functional electrolytes.

50
Why composites?
Major ion conducting phononmena and contributions
  • 1. Bulk conduction in the single phase, ion
    doping, YSZ, SDC, LGSM, sMax caused by the doping
    limit
  • grain and grain boundary behavios, the later
    causes the conduction barrier, negative. However,
    in nano-scale, turns to positive effect.
  • 2. Two phases materials conduction between two
    phases, i.e., interfacial conduction is more
    important than the grain conduction, including
    also single phase behavior (ion doping).
  • 3. Both intrinsic (host) ions and extrinsic
    (guest) ions, non-source and transported source
    ions, in-situ gas/FC operation environments
    extremely important for ceria-based materials due
    to non-stoichimistry nature dependence
  • The single phase materials meet the conductivity
    challenge not qualified for ILT range. The
    composites offer the way. More significant
    contributions from interfaces between the two
    phases.

51
Why composites, meet Material demands
  • High s at Lower T
  • High chemical stability
  • Good reproducibility
  • Low cost
  • Easy scale-up and production
  • Plentiful nature resources

52
Advantages of the composites
  • high conductivity
  • Mechanical improvement, soften/plastic ceramics
  • Flexibility in designing and selection of
    materials
  • Flexibility in designing of material function
  • Flexibility in material preparation and device
    fabrication
  • Cost-effectiveness

53
Composite ionics and
nano-ionics
54
Nano-ionics and composite ionics Is there such
ion conduction behavior?
  • Two conductivity maximums
  • Ion doping (a) and interfacial phase (b) behavior

Nano-CeO2-ZrO2 -SiO2, -Al2O3, O2-
diffusivity Grain boundary 3-4 orders in
nano-ZrO2 Higher than bulk, U. Brossmann etc J.
Appl. Phys. 85 (1999) 7646
55
Nano-materials make ion conductors
similaritynano-composite/ionics advances
  • Nano-YSZ and ion doped ceria display unusually
    high ion conductivity than the regular bulk
    materials, the high interfacial grain boundaries
    conduction into account. Highly distributed
    nano-particles create much larger
    boundaries/particle contacts in nano-level,
    changes the ion phenomenon in um-level or macro
    process.

56
A hit from nano-phenomenon
  • In nano-scale, many unusual material properties
    completely different from regular bulk material
    (mm-mm), one is the non-conductor becoming the
    conductor.
  • Ion doping and theory are ineffective. E.g., SDC
    (good O2-conductor) and CeO2 or SiO2
    (non-conducting) in composite having similar
    conducting behaviors in the nano-scale. The
    difference left only matter on the particle size
    and particle surface morphology,
    physicalchemical properties and most importantly
    the interfacial properties between the grains
    (single or bi-components).
  • Ion doping becomes a way to modify and activate
    the surface properties.

57
Is it true? Nano-process, ion conduction mechanism
  • In nano-scale, grain boundaries change from the
    barrier to conducting channel for ion
    transportation with also much lower activation
    energy

Nano-scale
Normal, grian, bulk
Nano-scale, by grain boundaries
58
Nano-tech. brings single phase and two-phase
composite materials in the same
theoretical/functional basis/mechanism
Nano-scale
59
www.nexttech.com
ceria-composite two-phase
Single SDC bulk phase
Nano-SDC phase
60
Nano-composites are basically two-phase
materials XRD studies
Two phase visible SDC-BaCO3
Only SDC phase visible
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