Effects of slurry properties on anode cermets for solid oxide fuel cells

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PowderTechnology 88 (1996) 173-178

Effect of slurry properties on anode cermets for solid oxide fuel cells S. Sddhar, U.B. Pal Massachusetts Institute of Teehnotogy, 77 Massachusetts Avenue, Cambridge, MA 02! 39 USA

Received21 September1995; revised I Febvm~ 1996

Al~tract The influence of the initial slurry preperties on the resulting Ni-YSZ ( y'~rla-stshiiizedzirconia) cermet structure that constitete~ the solid oxide fuel cell anode has been examined. Rheological study of the slutr~ '-::s been conducted as a function of powder concentration. The powder consisted of Ni (80 wt.%) and YSZ (20 wt,%) particles, and the s!urry was prepared by mixing the powders in polyvinyl alcohol (PVA) solution, This suspension was shown to Ig pseudoplastic without a yield point and a model based on the Ostwald- de Waele power law has been developed. Porous structures resulting frvrn slurry coating, sintering and infiltration were examined utilizing microprobe. scanning electron microscopy (gEM) and X-ray techniques. The resulting structure was shown to be inadequate when the slurry suspensions contained less than 12 or more than 13 vol.% of the powders. Keywords: SlunT;Pseudopiesfic;Settling:Cezw.et;Solidoxidefuelcell; Electrode

1. Introduction A porous cermet of Ni and YSZ (yttria-stabilizedzircouia) constitutes the anode in state-of-the-art solid oxide fuel cells (SOFC). The role of ~he structure is to provide current collectors (Ni particles) and electrochemical charge transfer sites (three phase interfaces between Ni. YSZ and pore). Furthermore, the anode often assumes the role of a catalyst for various chemical reactions. To maintain the polarization losses at acceptable levels, it is imperative that the Ni and YSZ particles form continuous paths that allow ionic and electronic migration from the electrolyte/cermet interface through the entire cermet [ ! ]. In addition, there has to be enough contact points between YSZ and Ni particles to Wovide charge-transfer sites. Processing techniques for depositing these electrodes include screen printing or tape casting a layer of NiO and YSZ over the electrolyte followed by reduction of the NiO to Ni [2]. These processes are sensitive to the particle size of the NiO because the contact area between the particles changes during the course of the reduction process. Furthermore, the reduced Ni particles often have a core of NiO. As a result, these electrodes have higher sheet resistance. A Combination of slurry coating and electrochemical vapor deposition (EVD) [3] or physical vapor deposition (PVD) is often used as an alternative method to depe6it the cermet. 0032-5910/96/$15.00 O 1996ElsevierScienceS.A.All tightste~e~ed PIIS0032-5910(96)03114-2

These processes result in stable cermets, however they also involve expensive vapor deposition processes. Slurry coating is an economically favorable method that has shown to give reproducible results in terms of porv6ity and layer thickness [4,5]. The process consists of coating die electrolyte-substrate with the slurry and, after drying, subjecting it to a number of sintering cycles. The initial slurry and coating/ drying process has a great degree of influence on the final porosity and panicle dis~hnfiou. Hence it is imperative that the properties of the slurry are closely monitosud. The advantage of studying and controlling theological properties are twofold. Firstly, the optimal slerry/powder concentration and dispersion technique can be established and secondly, changes in sluny properties dining sU3ragedue to evaporation of water and settling of heavy particles (Ni) can be monitmed and the col~¢ntrations alKI dispersions can be altered to obtain the required theological properties for repredocibly depositing the desired cermet stroctme.

2. Material and experimental procedure The filamentary Ni powder used was INCO 287 with a diameter of 2.2-3.3 pan and a length of 30-50/an. "lids powder type has Igen used in earlier work [4,5] due to its excellent conducting prot_~.~es and good sintering capabili-

174

s. Sridhar, U.K Pal/Powder Technology88(1996) 173-178

tics for achievingcontrolled porosity. YSZ was obtained from TOSOH-Ceramic Divisionand consisted of 13.47 wt.% Y~O3 doped ZrO2. Particle analysis showed the particle size distribution to be between 9 and 0.2 pin with over 90% of the particles being smaller than 3 /tm. The polyvinyl alcohol (PVA) solution was prepared by heating deionized water containing 6 wt.% PVA powder (Dupont, EIvano175-15 ) to 80 °C with continuous stirring. The slurry solutions were prepared by dispersing known mixtures of Ni and YSZ powders (Ni constituted 80 wt.% of the powders) in a known volume of PVA solution. The powder was added in four batches. Between each addition the dispersion was milled in a SWECO-Vibromill for 5 rain. After all the powder was added, the dispersion was milled for 66 min. Milling for longer times did not result in any change in the theological properties. Dispersion by manual stirring required much longer times to achieve the same rheological properties. No grinding media was used since this would cause the soft filamentary Ni particles to break. Rheological characterization of the slurry at various powder to PVA solution ratios were conducted at room temperature with a Brookfield viscometer, model DV-II +. A small sample adapter was used to hold 8 ml of the slurry to be tested. For the viscosity range encountered, spindle #21 was most applicable. Due to the fast settling rate of the Ni particles, the viscosity had to he measured immediately (within 1 rain) after the milling process. Measurements were made while ramping from the lowest to highest shear rate and then ramping back to the lowest. At high shear rates, measurements were made after 10 spindle rotations. However, due to the t'~st settling rate this could not he done at the low shear rates ( corresponding to low rotation speeds ), hence the measurements were taken after 40 s. After milling, the slurry was immediately coated on one side of a YSZ-electrolyte substrate. The coating was then passed through a blade, adjusted at a constant height to achieve an even coating after which it was left to dry for 24 h. The coated substrate was then sintered at 1100 °C for 17 h under an atmosphere of forming gas (N, with 5% Hz) with 5% H,_O followed by cooling under an atmosphere of pure forming gas. To increase the YSZ content especially at the electrolyte/cermet interface and obtain long time adherence, the substrate was then placed inside a vacuum chamber and infiltrated with a solution containing 30 vol.% YSZ. and 70 vol.% of a deionized water and 4N HNO3 mixture. Finally the coating was sintered at 1200 °C for 7 h in forming gas. The final cermet structure after this operation usually contained 60 vol.% Ni which is above the limit (30 vol.%) required according to the percolation theory for the cermet electronic conductivity to be close to that of Ni [ 6]. The YSZ in the initial powder and more so the YSZ in the infiltration solution contributes to the stability of the cermet and its adherence to the electrolyte. Due to incompatible thermal expansion, Ni will not by itself adhere to YSZ substrates at high operating temperatures of the SOFC (8001000°C).

3. Results and discussion 3.1. Rheologicalproperties

Viscosity as defined by Skelland [7] is non-Nowtonian, that is, dependent on shear rate for most suspensions and especially so for multi-phase slurry solutions with tendencies for settling.

,/d, / The deviation from Newtonian behavior, for example, the dependence o f ¢) on shear rate, w i l l characterize the solution

as being pseudoplastic, dilatant, Bingham, etc. Moreover, time dependency and yield stress phenomenon may be present. Figs. 1 and 2 show the measured rheological properties of Ni-YSZ powders suspended in a PVA solution. The powder mixture consisted of 80 wt.% Ni and 20 wt.% YSZ. Volume fractions of powder, Xp, between 0.091 and 0.132 were studied.

~o. £

•

t: o

i 20

o

t ) 40 ~o shear Rate (lhs)

i 80

i lOO

Fig. I. Shearstressvs. shearrate fordifferentvolumefractions(Xp)of NiYSZpo',,,der suspendedin a PVAsolution.

8000

]~=0.10t ] I'--A'--Xp=I3.112 I

7ooo

I"N-xp<'l=t I

=o0

'

I'-*-xP=°'t=

I

lOOO

a

~ tO

20

30

40

50

ah~w ~ (11/=) Fig. 2. Viscosity vs. shearrate for different volume fractions of power (Xp). shown in Fig. I.

175

S, Sridhnr. U.R Pal/Powder Technology 88 (1~6) 173-178

7OOO

A

.400

Table I Paralnete~ for the Oslwald-de Waalemodelevaluatedfloraexperir~ntal dala

! 350 ~

Xp

R l 250~ ~

S~

ter~

i=

200 150] lm t4

i 10

, 20

t ~0

i 40

ffaew I~lm 0/*)

Fig. 3. Thixotropicbehaviorfor slurcysolutionwithXp= 0.112, Arrows indicate the direction that the shearrate was ramped.

Some extremely low and high viscosity values have been omitted in Fig. 2 in order to maintain a better scale on the graph. It is readily seen in Fig. I that no yield stress phenomenon is present for any of the powder concentrations studied. Moreover, since the slopes decrease as the shear t .ieincreases the fluid can be characterized as pseudoplastic (shear thinning). This behavior can also be deduced from Fig, 2 where the viscosity is seen to decrease with shear rate. Most of the samples displayed some degree of hysteresis when the shear rate was ramped back. The nature of the hysteresis is shown in Fig. 3. The type of hysteresis was consistent for all the samples that showed time dependency. Of the many existing models for l~eudoplastie fluids [7] the Ostwald-de Wanle model, described by Eq. (2). and also called the power law model, is the most commonly obeyed and seems to give a reasonably good prediction of the theelogical properties of the Ni-YSZ powder slurry. ~dy!

(2)

where "ris the shear stress, K is the consistency factor and n is the flow behavior index which changes from unity to zero as pseudoplastieity increases. From Eqs. ( I ) and (2) the viscosity can be written as: de m ~/=/~) whereto=n-1

0.52 0.57 0.52 0.54 0.68 0.74

g (dynes/cm2)

m

93.8 84.0 52.9 47.9 11.3 6.51

- 10.53 -0.49 -0.47 -0.46 -0.31 -0.26

K

r,/C

(cP)

(m).tcP)

9286 8800 5380 4833 lit6 645

0.961 0.973 0.994 0.993 0.997 0,994

n-I -0AS -0.43 -0.48 -0.46 -0.32 -0.26

that n and m are decreasing functions of Xp whereas K is increasing. As a first approximation, the data has been fitted as a linear function with respect to the volume fraction of I~rlficles,Xp. Eq. (3) can thus be written as a function of Xp, namely:

Ioi~. o

0,132 0,128 0.121 0.112 0.10l 0.091

n

(3)

By plotting the log of viscosity or shear stress versus shear rate the validity of the model can be tested and n. m and K can be evaluated. Table I shows the values obtained through least square fitting of the data in Figs. 1 and 2. Note that m should equal (n - 1) and g ( in eP) shoeld equal/¢ × 100. Table I indicates

"~= ( - 20.4 X 103) + (22.3 X 104 X Xp) ( ~ ) ~'s-~'~xxp (4) 3.2. Characterizationoffmaleermetstrucmre

Several slurries with powder volume fraction between 0.09 and 0.135 were deposited and sintered according to the technique described in the previous section. The blade for the initial coating process was adjusted so that the final film was 60-70/~m thick. An environmental SEM was used to characterize the deposited films using energy dispersive X-ray spectroscopy. Several areas of 300×250 p,m2 were examined. The results showed that. at lower powder concentrations, cermet free areas leave the electrolyte substrate directly visible. Increasing the water/PVA content in the slurry (for Xp
S. Sridhar. U.R Pal / Powder Technology 88 (1996) 173-178

176

,= ~ii~~

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~!i~

/i I

i!i!i~!i!ii~!~!i~!i~ %ii ¸i~ ~ : i i~ i!i~i ~i~i !@i~i ~!!!~ii l i !iii:~ ~, i~ ¸¸ , !!~ii :i i i ~i!ili%~i!!i!!iiili`

/ ¸ i

!i~ i ~i~!!!/il

Fig. 4. (a) SEM image of cermet processed from a slurry with Xp= 0.09; (b) X-ray map of Ni (left-hand side) and Zr (right-hand side).

Fig, 6. Cross-sectionalarea of a cermet processed from a slurry with Xp~ 0.09. (a) Xoraymapof Ni; (b) X-mynmpof Zr.

i

Fig, 5, (a) SEM image of cm'n~t processed from a sluny wilh Xp~ 0.13; (b) X-ray map of Ni ( ieft-h~d side) and Z¢ ( right-hired side).

YSZ is compensated with the YSZ rich infiltration solution. However, if porosity near the electrolyte substrate is low after the first sintering dee to a high Ni powder concentration, the

YSZ solution that is intended for infiltrati:~ndoes not reach the cermet/electrolyte interface. The second sintering causes further Ni sintering resulting in an almost compact Hi layer at the interface (see Fig. 8). Since the thermal expansion coefficients of Hi ( = 14 om/cm K) and YSZ ( = 9 cm/cm K) am incompatible the resulting adhesion between the cermet and the substrate is low. Indeed some of the corrects made from slurries with Xp>0.13 pealed off from the substrates in the form of a thin compact film. Fig. 9 essentially summarizes Eq, (4) and defines the region of desired powder concentration: O.12
4, C o n c l u s i o n s

The rbeological behavior cf slurries for SOFC anode cermets have been investigated for volume fractions of powder, Xp, bet ween 0.09 and 0.132. The powder consisted of 80 wt.% Ni and 20 wt.% YSZ. The aluny is pseudoplastic and agrees with the Ostwald-de Waele power law. Some degree of time

S. Sridhar, U.R Pal /Powdcr Technorogyga (1996) 173-178

Fig. 7. Cross-sectionalar~a of a ¢~llnetprocessed from a s|uny with gp= 0.112, (~t)X-raymapof Ni; (b) X-rayleapof Zr.

Fig. 8+ Crms.s=ctio~ area of a c ~ p~-e~ed ~ Xp=0,132. (a) X-mymapofNi; (b) X-raymapof Zr.

dependency is seen but it is hard to draw further conclusions, since this may be due to the aligning of the filamentary Ni particles or simply settling of the same. Slurries containing Xp=0.09, or less, caused large areas on the substrate to be without cermet. These uncovered areas decreased in size as the powder amount was increased and above Xp=0.12 most of the substrate was covered. At Xp = 0.132 and higher concentrations, settling and sintering of Ni and depletion of YSZ at the electrolyte substrata/cermet interface caused peer adherence.

A F K m

n r v

area (dynes/era 2) force (dynes) consistency factor (dynes/cm 2 or c+P) n-I flow behavior index regression factor velocity (em/s)

a~

w~

"t O.~I

5. List of symbols

177

0.1

0.11

0.12

6.13

0+14

0-15

Xp X~g,9. Slmy V i ~ t t y at diltemmt sh~m"ram as a fmmtion of ~ n mo~ing to Eq. (4) andits effmtcmcermet~

Xp y

volume fraction powder separation (era)

Greek letters

,{

viscosity(cP) shear stress (dyacs/em 2)

0+~

S. Sridhur, U.B. Pal/Powder Technology 88 (1996) 173-178

178

Acknowledgements Financial support by the Electric P o w e r R es ear ch Institute ( E P R I ) is gratefully a c k n o w l e d g e d .

References [ I ] T Kawada, N. Sakai, H. Yokokawa and M Dokiya, S~did State hmics, 40/4-1(1990) 402.

[2 ] P.H. Middleton, M.E. Seierslenand B.C.H. Steele,in S.C. $inghal (ed.), Proc. 1st Int. Syrup. on Solid Oxide Fuel Cells, The Electrochemical

Society, Penninglon. NL 1989, pp. 90-98. [3 ] A.O. lsenberg and G.E. Zymboly, US Pa:¢m No. 4 582 766 (1986). [4] R.E. Jenscn, US Patent No. 4 971 830 (1990). [ 5 ] K.C. Chou, S. Yuan and U.B. Pal.in S.C. Singhal and H. Iwahara ( eds. ), Proc. 3rd Int Syrup. on Solid Oxide Fuel Cellx, The Electrochemical Society, Pennington, NJ, 1993, pp. 431--443. [6] N.Q. Minh, J, Ceram $oe., 76 ([993) ~fi3 [ 7] AH.P. Skelland. Noa.Newtonian Flow and Heat Transfer, Wiley, New York. 1967.