Properties of La Sr Co Fe O (LSCF) Double Layer 0.6 0.4 0.2 0.8 32x Cathodes on Gadolinium-doped...

8/14/2019 Properties of La Sr Co Fe O (LSCF) Double Layer 0.6 0.4 0.2 0.8 32x Cathodes on Gadolinium-doped Cerium Oxid

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Solid State Ionics 106 (1998) 255261

Properties of La Sr Co Fe O (LSCF) double layer0.6 0.4 0.2 0.8 3 2 xcathodes on gadolinium-doped cerium oxide (CGO) electrolytes

II. Role of oxygen exchange and diffusion

*B.C.H. Steele , Joong-Myeon BaeCentre for Technical Ceramics , Department of Materials , Imperial College , London SW 7 2 BP, UK

Received 6 March 1997; accepted 4 August 1997

Abstract

The area specic resistivity (ASR) of La Sr Co Fe O cathodes on Gd-doped ceria electrolytes has been0.6 0. 4 0 .2 0 .8 3 2 xmeasured over the temperature range 325725 8C. The ASR values for single layer porous cathodes could be satisfactorilyinterpreted in terms of available data for oxygen surface exchange coefcients. By introducing a thin ( | 1 mm) dense layerof La Sr Co Fe O (LSCF) adjacent to the gadolinium-doped cerium oxide (CGO) electrolyte the resistivity values0.6 0 .4 0 .2 0 .8 3 2 xwere decreased by a factor of 23 times, which could be explained by increasing the effective interfacial contact areabetween the LSCF and GGO. However calculations using data for oxygen self diffusion coefcients indicated much higher

ASR values than determined experimentally for these duplex layer cathode structures. It is necessary, therefore, to invokeenhanced oxygen ion transport through the dense layer which could arise either due to the development of A-site decientLSCF material, or too rapid diffusion via grain boundaries.

Keywords : Oxide cathodes; Oxygen surface exchange; Oxygen self diffusion; Solid oxide fuel cells

2 11. Introduction change kinetics k (cm s ). More recently a publi-

cation by Adler et al. [6] has provided a detailedMore than 10 years ago one of the authors [1] quantitative interpretation of the behaviour of porous

emphasised the importance of oxygen exchange mixed conducting oxygen electrodes which inte-kinetics in determining the magnitude of cathode grates information about the various macroscopicresistivity values. Subsequent publications [25] pathways available for the cathodic reduction of have discussed semi-quantitatively the relative roles oxygen. Evaluation of the Adler model using ex-of microstructural features, oxygen surface and bulk perimental AC impedance data for symmetrical

2 2 2 1diffusion D * (cm s ), and oxygen surface ex- LSCF/ CGO/LSCF cells has shown good agreement

with values predicted by the model using relevantproperties for the LSCF cathode material, providingcredence for this general approach. A number of

* important implications follow from this model. In theCorresponding author. Tel.: 1 44 171 594 6744; fax: 1 44 171584 3194; e-mail: [email protected] context of this paper we shall restrict our discussion

0167-2738/98/$19.00 1998 Elsevier Science B.V. All rights reserved.PI I S 0 1 6 7 - 2 7 3 8 ( 9 7 ) 0 0 4 3 0 - X


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256 B.C . H . Steele , J .- M . Bae / Solid State Ionics 106 (1998) 255 261

to small cathodic overpotentials ( | 100 mV), which observations of Kleitz and coworkers [7,8] usingis the situation prevailing for AC impedance mea- silver electrodes. Most of the overpotential losses,surements (typical signal amplitudes are 50 mV), and therefore, will be associated with the differenceSOFC operation at 0.7 V. between the thermodynamic potential responsible for

The more important consequences of this theoret- electron transport ( 2 m ) and the thermodynamiceical approach include the following: potential responsible for vacancy transport ( m ). Thev

(1.1) The steady state electron and vacancy elec- origin of this gap (see Fig. 1) can be understood bytrochemical potential proles are depicted schemati- considering the equilibrium state of the followingcally in Fig. 1. The change, dm , at the current reaction:ecollector/mixed conducting oxide interface will usu-

?? 2 x1ally be small as the contact is essentially ohmic. ] O 1 V 1 2e O2 o o2Moreover, the change in vacancy electrochemicalpotential, dm , at the mixed conducting oxide/elec-vtrolyte interface can also be small according to the i.e. Dm 5 1/2m 1 m 1 2ms O v e2

Fig. 1. Schematic representation of charge transfer and chemical losses in porous mixed conducting oxide cathodes.


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B.C . H . Steele , J .- M . Bae / Solid State Ionics 106 (1998) 255 261 257

dm /4 5 2 d m 2 d m / 2. [3], L , which denes the change-over from surfaceO e v c2to bulk control. It can be shown that L 5 D / k , andc

This expression indicates that the difference between typically L has a value around 100 mm [3]. As thecdm and dm /2 corresponds to the deviation of thee v optimal grain size in porous electrodes is around 1oxygen chemical potential, and thus the degree of mm, it follows that the kinetics of oxygen surfacechemical oxidation or reduction of the mixed con- exchange will inevitably be the rate controlling stepductor (i.e. non-stoichiometry). As shown in Fig. 1, in the behaviour of oxide cathodes.this chemical redox of the electrode provides the An alternative way of evaluating the relativedriving force for the reaction of the mixed conductor importance of oxygen surface and bulk kinetics is towith O , transport of molecular oxygen in the pores2 determine the extension ( l ) of the triple phasecof the electrode, transport of molecular oxygen in the boundary (TPB) region due to diffusion within thegas phase boundary layer, and ambipolar discussion oxide cathode material. The following expression hasof neutral combinations of vacancies and electrons been derived in recent publications [6,9](or holes) within the mixed conductor. These chemi- ]]] ]

l 5 (1 2 f ) L / t s,cal processes can contribute signicantly to the c cpotential drop in the electrode. A schematic repre- where L ( D / k ) is the critical length parametercsentation of these chemical losses is also depicted in

expressing the relative importance of bulk diffusionFig. 1.and interfacial oxygen exchange, f is the fraction(1.2) Our analysis [6] shows that the non-charge 2 1porosity, r is the tortuosity factor and s cm is thetransfer processes outlined in the previous section dolength of the TPB region per unit area, or the porenot contribute additively to the total cell impedance.surface area for unit volume. Assuming a particleThus ad hoc descriptions of these processes in terms 2 1dimension of 1 mm (s | 20 000 cm ), porosity ( f )5of equivalent circuit elements are generally not valid.0.5, and tortuosity ( t )5 5, then for a typical D / k (1.3) Close to equilibrium the oxygen surface 2 22 1 value of 10 cm (100 mm), the extension of theexchange coefcient k (cm s ) for the reaction:TPB region will be about 5 mm. For a poor ionic

?? 2 x1] O 1 V 1 2e O conductor such as LSM where D / k has a value of 2 o o22 510 at 10008C [6] the extension ( l ) of the TPBccan be considered as an exchange neutral ux 2 8

region only amounts to 4 3 10 m (ie. a few unitsdensity (cf. i , the exchange current density). Ito cells). It is also instructive to examine the enhance-should be emphasised that this reaction is NOT ament to the allowed ionic ux by the incorporationcharge transfer reaction. It follows that the quantity,of a porous electrode (coating) on top of the dense zFk / V , where V is the molar volume of oxygen inm m ion conducting membrane.the oxide, can be viewed as a characteristic electrode

The maximum enhancement is given by [9]:current density ( j ), although the analogy between je e ]]] ]and i , should not be carried too far [6]. However theo L s(1 2 f / t ) 1 f cexpression for j does provide a qualitative means of eevaluating the importance of surface exchange kinet- and for a typical mixed conductor with the valuesics in the behaviour of porous electrode structures given above this expression has a value of about 5.[3,4]. For example, taking a typical value for k , Signicant enhancements can only be obtained if

2 7 2 1 2 210 cm s , yields a current density of 6 mA cm . rapid surface diffusion uxes are invoked. ForIt is thus important to increase the effective surface example if the surface diffusion and oxygen ex-

2 6 2 2 1area of a porous cathode at least 100 3 to enable change coefcients have values of 10 cm s and

2 6 2 1current densities to be achieved that are of interest 10 cm s , respectively, then the enhancementtechnologically. factor becomes 45 for the same microstructural

(1.4) For mixed conducting electrodes it is also parameters.important to identify the relative importance of It appears that the charge-transfer of oxygen ionssurface exchange and bulk diffusion kinetics. This across the interface between a mixed conductor and acan be achieved by dening a characteristic length ceramic electrolyte is facile [7,8], and so the optimal


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design for a cathode appears to consist of a thin ( | 1mm) dense layer of the electrocatalyst (e.g. LSCF)next to the electrolyte, followed by a porous layer of the electrocatalyst to collect and enhance the ux of oxygen from the gas to the interfacial region. As-suming a target area specic resistivity (ASR) of

20.15 V cm for the cathode, then [3] it follows that:

2 R 5 0.15 5 L / s 5 L V RT / D*( zF ) .d d m

2 1Thus knowing the oxygen self diffusion D (cm s )value allows the maximum permitted thickness ( L )dof the dense interfacial layer to be calculated.

2. Results and discussion

In a companion paper [10] it was reported thatcathode area specic resistivities (ASR) as low as 0.5

2V cm could be obtained at 520 8C with LSCF/ CGO20 assemblies.

The strategy adopted was to ensure SiO free2electrolyte/electrode interfaces [10], and also to use

*Fig. 2. Oxygen surface exchange ( k ) and selfdiffusion ( D )a double LSCF cathode layer incorporating a thin 0coefcients for La Sr Co Fe O0.6 0 .4 0.2 0.8 3 2 xdense interfacial layer of the mixed conducting

oxide. Preparation and experimental details have

already been provided [10], and the present paper isconcerned with interpreting the ASR data usingavailable information for oxygen surface exchange k

2 1 2 2 1(cm s ) and bulk diffusion D (cm s ) kinetics forLSCF and relevant microstructural parameters.

Using the isotopic exchange diffusion proletechnique [11] on the compositionLa Sr CO Fe O , the following expressions0.6 0.4 0.2 0.8 3 2 xhave been derived [12] for D and k :

2 2 1 D 5 30.2 exp( 2 1.93 eV/ kT ) cm s ........[A]

2 1k 5 0.66 exp( 2 1.09 eV/ kT ) cm s .........[B]

These data are depicted graphically in Fig. 2. Fig. 3. Cathodic reaction pathways for oxygen reduction andinjection into ceramic electrolytes.The general macroscopic pathways available for

oxygen reduction processes on porous cathode/solidelectrolyte structures were already being discussedmore than twenty years ago [13], and can be cates that this can be neglected for the presentsummarised in the accompanying diagram (Fig. 3). system as k values for CGO surfaces are very low.

Reaction pathway (j ): Reaction of molecular Reaction pathway (j ): Dissociative-adsorption of 1 2oxygen with electrolyte surface (a). Data [14] indi- oxygen molecules and surface diffusion towards the


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triple-phase-boundary (TPB) designated (b) in Fig.3.

Reaction pathway (j ): Surface reaction followed3by dissolution and diffusion of oxygen ions towardscathode /electrolyte interface (c). The solid statetransport of oxygen ions would include normal bulk lattice diffusion together with contributions fromgrain boundary and dislocation core pathways if appropriate.

It has already been mentioned above that aquantitative model has been derived [6] to reproduceexperimental data for selected LSCF/CGO struc-tures. However in the present paper the semi-quan-titative approach used in earlier papers [35] hasbeen adopted to illustrate more easily the relative

importance of the various parameters used in thecurrent investigation.

Fig. 4. Area specic resistivities of single and double layer LSCFInspection of Table 1 indicates that D / k ( L )c cathodes on etched CGO20(SSC) electrolytes. A log 2 scale is2 3 2 4values are in between 10 and 10 cm (1 mm) and provided for comparison between the two data sets.so much of the LSCF particles (1 mm size) next tothe electrolyte can be involved in carrying the

2 1oxygen ux to the electrolyte interface. It follows high oxygen surface exchange coefcients k (cm s )that reaction pathways, j and j , (Fig. 2), are both measured for LSCF mixed conductors. These collec-2 3involved in the cathodic current, and for this situation length values provide good agreement for thetion a very simplistic model [35] provides the single layer electrode ASR between the measured

2following expression for the cathode ASR value: (e.g. 0.18 V cm at 7008C) and calculated (0.24

2

V cm ) values. There is also satisfactory agreement R 5 R 3 w /(2 3 l ), between the activation energies for the k valuesc s h(1.16 0.1 eV) and electrode resistivities (1.2 6 0.1 eV)

where l is the collection length. As R 5 RTV / to support the assumption that the rate determiningh s m( zF )2k , values of R can be calculated for various step in the cathode kinetics is the surface exchangeecollection lengths (Table 1). reaction. These results provide another example of

Examination of Table 1 indicates that it is neces- the drawbacks in interpreting electrode kinetic datasary to propose collection lengths ( l ) of around in terms of classical electrochemical concepts [5].c10100 mm to account for the observed single layer Also included in Fig. 4 are the data for the doublecathode ASR values reported in Fig. 4. These are layer electrodes, and inspection shows that thereasonable values taking into account the relatively resistivities are decreased by approximately 23

Table 1Calculations of area specic resistivity values using available surface exchange and diffusion data

2 2 1 2 1 2 2 2T (K) D cm s k cm s D / k mm R V cm w mm l mm R V cm R V cms h c d

2 9 2 6973 3.3 3 10 1.5 3 10 22 4.75 1 100 0.024 1.058

2 9 2 6973 3.3 3 10 1.5 3 10 22 4.75 1 10 0.24 1.058

2 9 2 6973 3.3 3 10 1.5 3 10 22 4.75 1 1 2.4 1.058

2 10 2 7873 2.2 3 10 3.4 3 10 6 18.91 1 100 0.09 14.82

2 10 2 7873 2.2 3 10 3.4 3 10 6 18.91 1 10 0.94 14.82

2 12 2 8773 7.8 3 10 5.2 3 10 1 109 1 100 0.54 365

2 12 2 8773 7.7 3 10 5.2 3 10 1 109 1 10 5.45 365


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times. The simple model [4] would predict a de- The use of dense mixed conducting layers tocrease of around 2 as all the electrolyte/ electrode improve the performance of SOFC cathodes hasinterfacial region is now available to carry the often been mentioned in the literature (eg. [1719])oxygen ux into the electrolyte. However the simple and the present contribution provides a furthercubic grain model probably overestimates the grain example. Finally it should be noted, that as a generalarea actually in contact with the electrolyte, and so it principle, dense mixed conducting layers (e.g. dopedis not surprising that the enhancement in cathode UO , ITO, etc) can also act as diffusion barriers.2conductivity can be up to 3 times. Such an application allows the relatively poor

Assuming this general approach to be satisfactory, electrocatalyst, doped LaMnO , to be replaced by the3then it is necessary to calculate the contribution ( R ) more catalytically active doped LaCoO , whichd 3to the cathode resistance due to the thin dense mixed reacts when placed in direct contact with YSZ.conducting layer. The value of R is given by:d

2* R 5 L V RT / D ( zF ) ,d d m 3. Conclusions

(where Ld is 0.5 micron) . . . [C] and appropriate The area specic resistivities (ASR) of single layervalues are listed in Table 1. Clearly, using the porous LSCF cathodes have been measured and themeasured self diffusion values, relatively large val- values attributed to control of the oxygen ux by the2ues (e.g. 1.058 V cm at 7008C) for the double-layer oxygen surface exchange reaction. The incorporationcathode are calculated whereas the measured values of a dense thin ( | mm) layer next to the electrolyte2are around 0.18 V cm . In fact it is necessary to reduced the ASR values by a factor of 23. Topostulate diffusion coefcients greater than account for this improvement in the cathodic be-2 8 2 2 110 cm s , to ensure that the dense layer does not haviour it was necessary to propose that oxygen ionmake a signicant increase to the cathode resistivity. mass transport through the dense layer was enhancedIt is clear, however, that the thin dense layer of by either the formation of A-site decient LSCF orLSCF adjacent to the electrolyte is reducing rather by rapid diffusion via grain boundaries.

than increasing the electrode resistivity, and so thepartial oxygen ion conductivity must be higher thanthat calculated using the data in equation [A].

AcknowledgementsA possible explanation for the better than expectedmass transport properties of the dense LSCF is to

The authors gratefully acknowledge nancial sup-suggest that the electrostatic spray assisted vapourport by the Korean government and Johnson Mattheydeposition (ESAVD) fabrication technique is re-Co. in the UK.sulting in an A-site decient LSCF. The enhanced

electrode performance already reported [15,16] forthe A-site decient material may be due to the

Referencespresence of an increased anion vacancy concen-tration which would be associated with larger values

[1] B.C.H. Steele, J.A. Kilner, P.F. Dennis, A.E. McHale, M.*for D . An alternative explanation could involve an0van Hemert, A.J. Burggraaf, Solid State Ionics 1819 (1986)enhanced grain boundary contribution due to the1038.

relatively small grain size of the thin layer associated [2] B.C.H. Steele, Mater. Sci. Eng. B13 (1992) 79.with the EASVD fabrication route. Indeed signicant [3] B.C.H. Steele, Solid State Ionics 75 (1995) 157.oxygen transport via grain boundaries in LSCF [4] B.C.H. Steele, Solid State Ionics 8688 (1996) 1223.

[5] B.C.H. Steele, Solid State Ionics 94 (1997) 239.samples was observed [12] at lower temperatures18 16 [6] S.B. Adler, J.A. Lane, B.C.H. Steele, J. Electrochem. Soc.when examining O / O isotopic diffusion proles.

143 (1996) 3554.It should also be noted that with a 50 mV signal the [7] M. Kleitz, T. Kloidt, L. Dessemond in: F.W. Poulsen et al.

*value of D would be not be expected to change (Eds.) Proc. 14th Ris Int. Symp. on Mat. Sci., Ris National0signicantly across the dense LSCF layer. Lab. Roskilde, Denmark, 1993, p. 89.


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[8] M. Kleitz, L. Dessemond, T. Kloidt, M.C. Steil in M. Dokiya [14] J.D. Sirman, J.A. Kilner in Proc. 17th Ris Intern. Symp. onet al. (Eds.), SOFC IV, Proc-Vol. 951, Electrochem. Soc., Mat. Sci., F.W. Poulsen et al. (Eds.) Ris Nat. Lab., Roskilde,New Jersey, 1995, p. 527. Denmark, 1996, p. 417.

[9] H. Deng, M. Zhou, B. Abeles, Solid State Ionics 74 (1994) [15] D. Waller, J.A. Lane, J.A. Kilner, B.C.H. Steele, Mater. Lett.75. 27 (1996) 225.

[10] J.-M. Bae, B.C.H. Steele, Solid State Ionics 106 (1998) (this [16] D. Waller, J.A. Lane, J.A. Kilner, B.C.H. Steele, Solid Stateissue). Ionics 8688 (1996) 767.

[11] R.J. Chater, S. Carter, J.A. Kilner, B.C.H. Steele, Solid State [17] H.H. Mobius, B. Rholand, Rev. Energ. Primaire 2 (1966) 27.Ionics 5356 (1992) 859. [18] D.B. Meadowcroft, Nature 226 (1970) 847.

[12] S.J. Benson, R.J. Chater, J.A. Kilner, to be published in Proc. [19] P. Van Den Berghe, H. Tannenberger; US Patent No.Vol 9724, Ionic and Mixed Conducting Ceramics III, Paris, 4 130 693, 19.12.1978.Aug 31Sept 5, 1997, Electrochem. Soc., New Jersey, 1997.

[13] S. Pizzini in: Fast Ion Transport in Solids, W. van Gool (Ed.)North Holland, 1973, p. 461.

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