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lectrochemical and gas phase parameters of cathodes for intermediateemperature solid oxide fuel cells
. Lust ∗,1, R. Küngas, I. Kivi1, H. Kurig, P. Möller1, E. Anderson, K. Lust1, K. Tamm,. Samussenko, G. Nurk1
nstitute of Chemistry, University of Tartu, 2 Jakobi Street, Tartu 51014, Estonia
r t i c l e i n f o
rticle history:eceived 30 May 2009eceived in revised form 4 November 2009ccepted 4 November 2009
a b s t r a c t
A series of cyclic voltammetry, chronoamperometry and electrochemical impedance experiments havebeen carried out in order to investigate the effect of cathode composition and porosity on the electrochem-ical characteristics of strontium-doped lanthanum, praseodymium and gadolinium cobaltite cathodes.The impedance responses at different electrode potentials of the half cell and symmetric single cell
vailable online 10 November 2009
eywords:SCOSCOSCOolid oxide fuel cell
setups are compared and analyzed by the equivalent circuit modeling method. The deconvolution ofimpedance spectra for single cell cathode and anode reactions contributions based on the results ofsimultaneous analysis of half cells and symmetric single cells has been made by differential impedancereal part vs. ac frequency plot analysis method. Noticeable influence of cathode chemical composition,meso-porosity and macro-porosity on the electrochemical activity of the oxygen electroreduction hasbeen demonstrated. Seeming activation energy values have been calculated and discussed.
mpedance spectroscopy
. Introduction
Solid oxide fuel cells (SOFCs) are considered as one of the mostromising systems for energy conversion in the near future due toheir very high electrical efficiency (up to 60%) and the possibil-ty to operate with zero emission if H2 as a fuel is used. Takingnto account the possibility to use the residual heat generateduring exothermal fuel electrooxidation reaction (CH4, CH3OH,2H5OH, etc.) the high total chemical energy conversion efficiency
nto electricity and heat together can be expected (∼80–85%),epending on the fuel composition and temperature applied [1–3].owever, wider use of SOFC technology is restrained by highaterial costs and long-term durability problems. Both of these
hallenges have been addressed quite successfully over recentears by the introduction of the so-called intermediate tempera-ure solid oxide fuel cell (IT-SOFC) concept [2–12]. IT-SOFCs makese of low-temperature oxygen ion conductors, i.e. electrolyte,uch as gadolinia doped ceria, Ce0.9Gd0.1O2−ı (CGO), or samaria
oped ceria, Ce0.9Sm0.1O2−ı (CSO), instead of the traditional yttria-tabilized zirconia, ZrxY1−xO2−ı. Thus, allowing reduction of theperating temperature of the cell to below 500–800 ◦C [8–12]reatly enhances the long-term stability of the fuel cell construction
materials and current collectors. Equally importantly, lower oper-ating temperatures allow application of cheaper materials, such asvarious stainless steels, to be employed as potential constructionand current collector materials. Additionally, the issue of findingsuitable sealing materials becomes less crucial [1,2,3,13].
One of the key issues in the IT-SOFC research is the developmentof cathode materials that would possess high electrochemi-cal activity towards the oxygen electroreduction process atreduced operation temperature [3–12]. La0.6Sr0.4CoO3−ı (LSCO),Pr0.6Sr0.4CoO3−ı (PSCO) and Gd0.6Sr0.4CoO3−ı (GSCO) are the mixedelectronic and ionic conductors of ABO3 perovskite structure. Theelectronic conductivity of LSCO, PSCO and GSCO is much higherthan that for Sr-doped LaMnO3 (LSMO), the cathode material mostcommonly used in high-temperature SOFCs [1,13–16]. The mainconcerns regarding the wider use of LSCO and PSCO are the largethermal expansion mismatch with the CGO or CSO electrolyte, aswell as the relatively low tolerance towards reducing environments[1,5,17,18].
Numerous papers on LSCO have been published over the recentyears [3,6–12,17], but a large scatter is present in the resultspublished by different groups. There are only few publications con-cerning the electrochemical behavior of PSCO and GSCO cathodes
[4,8–11].
Oxygen electroreduction kinetics at mixed ionic|electronicconductors and the effect of cathode chemical composition, over-potential and cell geometry have been studied by Adler [19–22] andMaier and co-workers [23,24] in detail. Importance of the chemi-
The X-ray diffraction studies (Fig. 2) indicate that the crys-tallinity of LSCO, PSCO and GSCO nanopowdwes is good, if thesintering temperature Tsint > 1423 K has been applied during 24 h.The crystallographic parameters (Fig. 2) obtained for LSCO and
670 E. Lust et al. / Electrochim
al potential gradient inside solid porous cathode phase (so-calledhemical capacitance) on the charge transfer kinetics and mech-nism for various electrochemical processes has been analyzeds well [22–27]. Moreover, of the papers that do investigate theffect of cell geometry on the reliability of the determination oflectrochemical parameters [14,15], only some [16–18] providexperimental data to validate the conclusions reached by math-matical modeling [26,27].
In this paper, a different approach has been chosen. Aomparison of experimentally obtained results for poroust|Ce0.9Gd0.1O2−ı|Ln0.6Sr0.4CoO3−ı half cells (Ln stands fora, Pr or Gd) is provided, based on which theoreticalxplanations have been given and conclusions drawn. Ni-e0.9Gd0.1O2−ı|Ce0.9Gd0.1O2−ı|Ln0.6Sr0.4CoO3−ı single cells haveeen completed and tested under variable fuel compositionH2 + Ar + H2O vapor) and O2 partial pressure conditions. Differ-nce derivative impedance plot analysis method has been usedor deconvolution of oxygen electroreduction and fuel oxidationrocesses parameters [26,27].
. Experimental
.1. Preparation of LSCO, PSCO and GSCO powders
The porous LSCO, PSCO and GSCO powders for cathodes haveeen synthesized using various synthesis methods, and character-
zed by BET, X-ray diffraction, SEM and AFM.The cathode powders were prepared by the nitrate solu-
ion thermal combustion method (NTDM), using La(NO3)3·6H2O,r(NO3)3·6H2O, Gd(NO3)3·6H2O, Sr(NO3)2 (Aldrich, 99.9%) ando(NO3)2·6H2O (98%, Riedel-de Haën) as precursors and glycine99%, Sigma–Aldrich) as the reducing and complex-forming agent11,18,25–28]. Detailed description of the experimental procedureas been given elsewhere [11,25–28] and only a short overview wille given here. La(NO3)3·6H2O, Co(NO3)2·6H2O, and Sr(NO3)2 whereissolved in MilliQ + water and thereafter the calculated amount oflycine, dissolved in MilliQ+ water, was slowly added under mod-rate stirring to receive a real solution. The solution was heated onhot plate to form a viscous solution and thereafter added drop-ise into a Pt-beaker that was preheated to the temperature range
rom 575 to 675 K. The solvent was quickly evaporated and the cor-esponding reagents reacted autothermally to form the fine sizetructural complex oxide LnSCO nanopowder. For homogenizationnd better time stability, the additional sintering of the nanopow-er materials was carried out at T ≥ 1373 K during 8 h [25–27].
The porous structure of the nanopowders was characterizedy nitrogen adsorption (Brunauer–Emmett–Teller, BET), Langmuir,on-local density functional theory (NLDFT) and correspondingimulation models [12–16,25,27,29–31]. The porosity analysis haseen made at relative pressure p/p0 = 0.2 and the adsorption
sotherms for LSCO, PSCO and GSCO were measured at the boilingemperature of liquid nitrogen (−196 ◦C), using the Nova 1200 Sys-em (Quantachrome or ASAP 2010 system). The isotherms obtainedan be classified for material structure of a medium pore sizeomewhat less or around 2 nm (Fig. 1). However, weak hysteresisetween adsorption and desorption isotherms at p/p0 > 0.2 indi-ates that in addition to the small amount of micropores there areesopores with the pore diameter d > 2.0 nm in the cathode pow-
ers studied [8–11]. Similar data were obtained for LSCO and PSCO.aking into account that the classical Barrett–Joyner–Hallenda
BJH) theory is applicable only for condensation process in meso-ores and Dubinin–Radushkevich (DR), Horvath–Kawazoe (HK)nd Saito–Foley (SF) models, based on the continuous pore fill-ng method, are applicable for micropores only, the pore sizeistribution (PSD) and pore volumes were determined addition-
Fig. 1. Cumulative pore volume and differential pore volume, vs. pore radius, r,dependences for GSCO powder (SBET = 105 m2 g−1).
ally by using NLDFT, assuming the so-called slit-shaped poresmodel for micropores [8–11,25–27]. The resulting powders demon-strated an exceptionally high BET surface area (SBET(PSCO) > 210,SBET(LSCO) > 160, and SBET(GSCO) > 105 m2 g−1) and a distinct peakat 1.60 nm, 1.57 nm or 1.55 nm in the corresponding pore radiusdistributions [11,18,25–27], respectively. However, the BET surfacearea depends noticeably on the sintering temperature and duration.
Fig. 2. XRD data for LSCO (a) and PSCO (b) nano-powders sintered at 1423 K, andfor GSCO at various sintering temperatures, noted in figure (c).
E. Lust et al. / Electrochimica Ac
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NTDM nanopowders sintered at 1423 K 8 h (Fig. 3b), show goodcrystallinity for counter electrode (Pt), electrolyte (CGO) andperovskite-like cathodes as well. The SEM data (Fig. 4) show thatvery well developed macroporous structure of the cathodes, andnearly nonporous electrolyte have been developed.
ig. 3. Cumulative pore volume and differential pore volume vs. pore radius plotsor LSCO cathode (a), and XRD data for LSCO|CGO three-electrode half cell (b).
SCO are in a good agreement with literature data [4]. It was foundhat GSCO is not a single phase material due to the limited solubil-ty of Sr2+ in GdCoO3−ı lattice [4,8–10] (Fig. 2c). In the case of Tsintower than 1273 K it is impossible to obtain the lattice parametersor LSCO and PSCO. Thus, the crystallinity of LSCO, PSCO and GSCOs low and it depends strongly on duration of sintering process atemperatures from 1100 to 1223 K.
For comparison, the traditional solid phase reaction methodas also used to prepare the cathode powders [8–10]. The cor-
esponding compounds (La2O3·6H2O or Pr6O11·6H2O, SrCO3 ando2O3·3H2O) and MilliQ+ water were very well mixed in stabi-
ized ZrO2 mill at rotating speed 150 rpm during 1 h. Thereafterhe paste received was dried and pressed into tablets, using pres-ure 95 MPa cm−2, and heated for solid state reaction during 10 h at473 K. After cooling the material was crushed, ball-milled, pressed
nto pellets and heated during 10 h at 1473 K for second solid stateeaction step, and thereafter for the third reaction step as well.he crystallographic parameters obtained for SSRM powders areimilar for NTDM powders, and thus, are independent of powderreparation method used, if Tsint ≥ 1423 K has been applied for sta-ilization of materials prepared. However, SBET for raw LSCO, GSCOnd PSCO powders prepared by SSRM is noticeably lower (from 10o 63 m g−1) than those for NTDM prepared powders.
.2. Half cell preparation and experimental setup
The supporting electrolyte was prepared by dry-pressingf the commercial CGO powder (99.9%, NexTech Materials,BET = 19.9 m2/g) at 90 MPa for a period of 10 s, followed by a 15 hintering cycle at 1723 K. The resulting tablets were 725 ± 10 �m
n thickness, 19.9 ± 0.1 mm in diameter and 1.47 ± 0.01 g in weight,orresponding to 99.6% of the theoretical density of CGO in agree-ent with our previous data [8–14,25–27].The single phase LSCO and PSCO materials formed by SSRM were
rushed and ball-milled in ethanol and, after adding an organic
ta 55 (2010) 7669–7678 7671
binder (polyvinylalcohol) and solvent (turpentine oil), were screenprinted through a 200 mesh-screen onto one side of the CGO elec-trolyte as a cathode and sintered at T = 1323 K for 5 h. The flat grosssection area was 0.5 ± 0.05 cm2. The LSCO, PSCO and GSCO powderssynthesized using NTDM [25–27] have been used without addi-tional mechanical treatment for the preparation of correspondingcathodes at CGO or CSO electrolyte.
SBET of ready LSCO cathode was 9.6 ± 0.8 m2 g−1 (NTDM) andthe pore size distribution function (i.e. pore diameter) has a max-imum near 27.0 Å (Fig. 3a). The total pore volume was remarkable(Vtot ≈ 4·10−3 cm3 g−1). The nearly similar parameters have beenobtained for PSCO and GSCO cathodes. However, it should be notedthat porosity of the cathodes tested depends noticeably on the sin-tering temperature applied [8–11,25–27]. The specific surface areasSBET from 2 to 6 m2 g−1 (maximal at 1423 K) have been obtainedfor cathodes prepared from SSRM powders, depending strongly onsintering duration and temperature [8–11].
XRD data for cathodes, prepared at T = 1423 K (8 h) from
Fig. 4. SEM images for LSCO sintered 8 h at 1423 K (a) and for LSCO|CGO interface(b).
7672 E. Lust et al. / Electrochimica Acta 55 (2010) 7669–7678
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The assembly was placed inside a vertical furnace (Carbolite VST12/400) and exposed to air or oxygen or oxygen/air mixture. Thus,the experimental construction of the three-electrode half cell wasvery similar to that analyzed by Adler et al. [19,22] and Mogensen
ig. 5. AFM image (a), selected height profile (b), height distribution histogram (c),intered at 1373 K. LSCO surface (e), and distribution functions of particles size (f) a
According to the BET, SEM and AFM data (Figs. 3–5), there areicro-, meso-, and macro- (transport) pores inside the porous
athodes studied. However, for better analysis of the influence oforosity characteristics on the electrochemical results the LSCOathode materials with specific surface area from 2 to 10 m2 g−1
ynthesized by NTDM and SSRM will mainly be discussed.The surface structure of LSCO, PSCO and GSCO was analyzed
sing AFM non-contact mode method. Fig. 5(a–d) demonstrateshat the very rough surface with the value of root mean squareoughness, Rms, higher than 240 nm at Tsint = 1473 K and higherhan 730 nm at Tsint = 1173 K has been formed. The particle sizend area distribution function have been calculated by fitting theFM data by program Gwyddion 2.10 (Fig. 5e). Mainly the bimodalarticle size distribution functions with peaks at dpeak1 = 0.52 �mnd dpeak2 = 0.91 �m (Fig. 5f), and also corresponding area distribu-ion functions with peaks at Speak1 = 0.8 �m2 and Speak2 = 2.2 �m2
Fig. 5g) have been calculated for LSCO. Quite similar particle sizend area distribution functions and Rms values have been obtainedor PSCO and GSCO prepared from NTDM powders, if the same
intering temperature and duration have been used.
In order to have good electrical contact, a thin layer of Pt-pasteas applied onto the cathode material for current collection pur-oses and fired at 1273 K for 1 h. The three-electrode half celleometry and symmetrical two-electrode single cell experimental
-dimensional picture (d) for LCGO cathode, prepared from NTDM nanopowder andrticles surface area (g) obtained by simulation with Gwyddin 2.10 program.
setup are shown schematically in Fig. 6. For asymmetrical three-electrode half cell, the reference electrode was placed inside adrilled hole at the distance of less than 10−2 cm from the work-ing electrode [8–11,25–27]. The surface area of the Pt counter andLuggin-like reference electrodes were 3 and 0.04 cm2, respectively.
Fig. 6. Experimental setups used for asymmetrical three-electrode half cell (a) andsymmetrical two-electrode single cell (b) measurements (WE: working electrode,CE: counter electrode, RE: reference electrode). Other details are given in figure andtext.
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nd co-workers [12]. The cell configuration shown in Fig. 6(a) isssentially an application of the three-electrode configuration usedn aqueous electrochemistry to the three-electrode solid state sys-ems [8–12,25–28,32]. By placing the reference electrode (RE) in
drilled hole as close to the working electrode (WE) as possi-le, the ohmic potential drop (IR) caused by the solid electrolyteesistance is minimized and the applied electric potential, to a firstery rough approximation, corresponds to the potential at the WEs. Pt|porous Pt|O2 reference electrode. By increasing the surfacerea of the porous Pt counter electrode (i.e. anode, CE), one guar-ntees that the capacitance of the CE does not contribute to theathode capacitance measured [18,20,24,25,26,32–35]. Along withany others [8–12,25–28], Chung et al. [16] and Adler [19–22]
howed that such cell geometry minimizes the polarization con-ribution of CE, and thus is suitable for fundamental research ofhe electrochemical properties of SOFC cathode materials at differ-nt electrode potentials. For single cell studies the setup given inig. 6(b) was used, where the anode and cathode with same flatross section areas have been deposited onto the electrolyte pelleti.e. the symmetrical two-electrode configuration).
The impedance complex plane plots (Z′′,Z′, i.e. so calledyquist plots) were measured using potentiostat/galvanostat
Solartron 1287) and frequency response analyser (Solartron 1260)12–16,25–27,32–36] (jZ′′ and Z′ are the imaginary and real partsf the impedance, respectively, and j = √−1). The ac frequency fas varied from 1 × 106 to 1 × 10−1 Hz and the ac voltage appliedas 5 mV. The impedance spectra were recorded at 10 points perecade.
. Results and discussion
.1. Cyclic voltammetry and chronoamperometry data
Cyclic voltammetry data (Fig. 7a and b) show high currentensities (j) for LSCO, GSCO and PSCO prepared from NDTManopowders, whereas noticeably lower current densities haveeen obtained in the case of SSRM powders based cathodes. Currentensity vs. potential, E, plots depend strongly on the cathode chem-
cal composition, and j is highest for LSCO, i.e. for a cathode havingowest atom mass of A position atom in ABO3 perovskite com-ounds. (The flat gross section area has been used for calculation ofhe current density values [8–11,25–27].) The chronoamperometryata also confirm this conclusion (Fig. 7c). The current densities forDTM-LSCO and NDTM-PSCO cathodes are 3–4 times higher than
or corresponding SSRM powders based cathodes.Chronoamperometry curves obtained indicate that the shape
f the j,t-curves depends on SBET, T and E as well. At small timest < 2.0 s) |j| increases with time. At T ≤ 773 K the cathodic currentensity is noticeably higher for the cathodes prepared from NTDMowders. The increase in the cathodic current density with timet t ≤ 1 s can be explained by extending the active reaction zonerom the open surface area to the porous surface of mixed con-ucting cathode. The increase in concentration of the “chargedxygen” species with increasing the negative cathode potential willmprove the catalytic activity, and corresponding decrease in thealues of seeming activation energy, Eact, being in a good agree-ent with experimental results discussed later in the impedance
nalysis section.The Tafel-like overpotential �,ln j-curves (Fig. 8a) have been cal-
ulated from the j,t-curves at t > 600 s, if the stable values of j have
een established at fixed E and T (in order to obtain �, E has beenorrected for the ohmic potential drop, i.e. IR drop, where R has beenbtained from the impedance data at ac frequency f → ∞). Accord-ng to these calculations, the values of transfer coefficient, ˛c, higherhan 0.5 indicate the mixed kinetics (i.e. in addition to slow elec-
Fig. 7. Cyclic voltammogramms (scan rate 50 mV s−1) for LSCO|CGO half cell at var-ious temperatures (noted in figure) (a) and for LSCO|CGO (1), PSCO|CGO (2) andGSCO|CGO (3) half cells at 973 K (b). Chronoamperometry data for LSCO|CGO (1; 1′),PSCO|CGO (2; 2′), and GSCO|CGO (3; 3′) at 873 K (1; 2; 3) and 973 K (1′; 2′; 3′) (c).
tron transfer, also the slow Oads− or Oads mass transfer (diffusion)
seems to be a rate-determining step) for less macroporous cath-odes. The exchange current density j0, obtained from the Tafel-likeplots (Fig. 8b), increases with temperature, and SBET. The seemingTafel constant (intercept at ln j = 0; Fig. 8c) depends strongly on thecathode composition and increases with the increase of porosity(not shown for shortness).
3.2. Nyquist plots
According to the impedance data (Nyquist plots, given inFigs. 9 and 10, and impedance modulus, |Z|, and phase angle,ı, vs. log f plots, Fig. 11), the ohmic series resistance Rex of thesystem (bulk electrolyte + contact + Pt wire resistances) was deter-mined at very high-frequency (Rex ≡ Z(ω → ∞) with ω = 2�f) at E = 0.Comparison of Z′′,Z′ plots (Figs. 9 and 10) for SSRM and NTDMpowders based cathodes indicates that the shape of impedancespectra depends somewhat on the chemical composition, butmainly on the method used for preparation of the raw cathodepowders [12–16,25–27]. The influence of the chemical composi-tion of the electrolyte on the shape of Z′′,Z′-plots is comparativelysmall (not shown for shortness) [8–11,25–27]. The lowest Rex
has been obtained for LSCO|CGO half cell with a cathode pre-pared from NTDM powder (SBET = 147 m2 g−1), and the highest Rex
for LSCO|CGO prepared from SSRM powder (SBET = 10 m2 g−1). The
same tendency of cathode activity is valid for PSCO and GSCOelectrodes. Thus, the high-frequency series resistance, Rex (contactresistance, depending on the contact area between electrolyte andcathode), depends strongly on the particles diameter of the rawcathode powders (being noticeably lower for NTDM powders com-
7674 E. Lust et al. / Electrochimica Acta 55 (2010) 7669–7678
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Fig. 10. (a) Experimental Z′′ ,Z′ (Nyquist) plots for CGO|LSCO (NTDM powder;SBET = 147 m2 g−1) at T = 873 K at various potentials (vs. Pt|porous Pt|O2) noted in fig-
ig. 8. Tafel-like overpotential vs. ln j curves for LSCO|CGO (1; 1 ), GSCO|CGO (2; 2 ),
nd PSCO|CGO (3; 3′) half cells at 873 K (1; 2; 3) and 973 K (1′; 2′; 3′) (a); exchangeurrent density (b) and Tafel constant (c) vs. temperature dependences for LSCO|CGO1), GSCO|CGO (2) and PSCO|CGO (3) half cells.
ared with SSRM materials) used for preparation of the cathodes.ccording to the data in Figs. 9 and 10, the higher values of totalolarization resistance Rp have been obtained for cathodes pre-ared from SSRM powders (Rp = Rex − R0, where R0 = Z′(ω → 0) ishe low-frequency series resistance, obtained by extrapolation ofhe Nyquist plot to the condition Z′′ = 0 if ω → 0) (Fig. 10).
Differently from cathodes prepared from NTDM powderFig. 10a), the Nyquist plots in the case of SSRM powder based cath-des (Fig. 10b) (SBET for powders from 10 to 63 m2 g−1) at f < 20 kHzan be divided into two subcircuits: so-called medium-frequencyrocess (arc 1) with polarization resistance, RMF, and low-frequency
rocess (arc 2) with polarization resistance, RLF. For cathodes basedn NTDM powders, there is only one slightly depressed semicir-le in the Z′′,Z′-plot (Fig. 10a), and the shape of Nyquist plots isearly independent of the chemical composition of the cathode
ig. 9. Nyquist plots for SSRM based LSCO|CGO (1), PSCO|CGO (2) and GSCO|CGO3) half cells at T = 873 K and E = −0.1 V vs. porous Pt|O2 (marks: experimental spec-ra; solid lines: calculated spectra fitted according to Warburg impedance (EC III inig. 12); and dotted line: fitting by Warburg impedance with addition of Gerishermpedance).
ure (solid lines: fitting according to EC IV). (b) Nyquist plots at T = 873 K and E = −0.1 Vfor CGO|LSCO half cells prepared from raw cathode powders with various specificsurface areas SBET (m2 g−1): (1) 147 (NTDM); (2) 63 (SSRM); (3) 35 (SSRM); and (4)10 (SSRM). Insets show the enlarged curves 1, 2, and 3.
used. However, Rp for NTDM-GSCO and SSRM-GSCO materials isnoticeably higher than those for PSCO and LSCO based cathodes.For all systems studied, Rp decreases with increasing T and |E|.Systematic analysis of the Nyquist (Figs. 9 and 10), Bode (Fig. 11)(calculated from the impedance data after subtraction of the veryhigh-frequency series ohmic resistance values), and also log Z′′ vs.log f plots shows that the characteristic frequency fmax (frequencyof the maximum in the Nyquist plot) only weakly increases with thecathode potential applied (Fig. 11 c), but fmax decreases noticeablywith increasing the O2 electroreduction temperature for all cath-odes studied (not shown for shortness). The characteristic relationtime �max (equal to (2�fmax)−1) obtained from the low-frequencypart of the Z′′,Z′-plots depends noticeably on the chemical compo-sition and meso-porosity of the cathode (not shown for shortness).The values of �max are nearly independent of the electrolyte compo-sition if the same cathode material (synthesized under the identicalconditions, using SSRM or NTDM) has been used.
The phase angle ı vs. log f plots for more compact cathodes atT ≤ 773 K (Fig. 11b) show that there is a kinetically mixed processat macroheterogeneous cathode surface at f > 10 Hz with |ı| > 30◦
[35,36]. (Adsorption/absorption characterized with |ı| = 90◦, masstransfer with |ı| = 45◦, and charge transfer with |ı| = 0◦ can be therate determining steps.) However, at f → 0, |ı| → 0◦ shows that theslow charge transfer, i.e. electroreduction step of adsorbed and dis-sociated Oads is the main rate determining step. The shape of thephase angle and impedance modulus |Z| vs. log f plots depends on
the cathode porosity, and |Z| decreases noticeably with the increaseof specific surface area of the raw powder used for preparation ofthe cathodes. In the region of frequency from 10 to 100 Hz, thephase angle depends noticeably on temperature, indicating the
E. Lust et al. / Electrochimica Ac
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ues of ˛W are somewhat lower than 0.5 [12,13,16,25–27,35]. Thediffusion resistance RD (Fig. 13) and the low-frequency chargetransfer resistance R2 decrease with increasing temperature and|E|, if E ≤ −0.2 V, and in the sequence GSCO, PSCO and LSCO. Very
Fig. 12. Equivalent circuits used for fitting the calculated with experimental
ig. 11. Impedance modulus (a) and phase angle (b) vs. as frequency plots forSCO|CGO (NTDM; SBET = 105 m2 g−1) at E = −0.05 (V vs. porous Pt|O2) and tempera-
ures, noted in figure. The log(−Z′′) vs. frequency plots for PSCO|CGO (SSRM powder)t T = 773 K and E , noted in figure (c). (Solid lines: fitting by EC III in Fig. 12).
nfluence of the mass transfer rate in the porous structure on theate of the cathode processes.
.3. Fitting of the impedance data
The theoretical analysis of the cathode processes [18–28] showshat, to a first approximation, the arbitrary chosen equivalentircuit (EC) I presented in Fig. 12(a) can be used for fitting thealculated with experimental impedance spectra (chi-square func-ion �2 ≤ 6 × 10−4 and weighted sum of squares �2 < 0.1 have beenchieved). In EC I (Fig. 12), Rex ≡ Z1(ω → ∞) is the total very high-requency series resistance of the system (practically independentf E); Cgb and Rgb are the high-frequency grain boundary capaci-ance and resistance, respectively; CPE1, R1, CPE2 and R2 are the
edium-frequency and low-frequency constant phase elements
nd charge transfer resistances, respectively. The CPE impedanceCPE = A−1(jω)−˛, where A is a CPE coefficient and ˛ is a fractionalxponent [32–36]. For fitting the impedance spectra, the Zview.2 software has been used [36]. Differently from the less porousSRM powders based cathodes at lower temperature (T ≤ 823 K)
ta 55 (2010) 7669–7678 7675
(Figs. 9 and 10), for more porous NTDM cathodes there is novery well separable semicircles in the region of high-frequencies(f > 1000 Hz) because the so-called grain boundary resistance Rgbhas very low values and EC I simplifies to EC II, indicating that thetransfer of charged oxygen particles in the electrolyte as well asat the cathode|electrolyte phase boundary is quick. Thus, the high-frequency block can be omitted in the case of NTDM cathodes atT ≤ 823 K) (Fig. 10).
A noticeably better fit of the Z′′,Z′- and Bode plots for NTDMbased cathodes at T ≤ 823 K has been obtained by using the EC IIIin Fig. 12, where the low-frequency CPE2 has been replaced to thegeneralized finite length Warburg element (GFW) for a short cir-cuit terminus model [25–27,35]. The GFW impedance is expressedas ZGFW = RD tanh[(iωL2/D)
˛W ]/(iωL2/D)˛W , where RD is the lim-
iting mass transfer (diffusion) resistance, L is the effective diffusionlayer thickness, D is the effective diffusion coefficient of a par-ticle and ˛w is a GFW fractional exponent [12–16,25–27,32–36].The quite small values of chi-square function, �2 < 2 × 10−4, andweighted sum of squares, �2 < 0.03, have been established. Therelative residuals obtained for EC III are also low and have a ran-dom distribution in the whole frequency region studied. Therefore,it seems that the second (low-frequency) arc at T ≤ 823 K charac-terises the kinetically mixed, both charge transfer and mass transfer(generalized diffusion-like) limited processes (|ı| < 15◦) as the val-
impedance data. Rex is the high-frequency series resistance of the system (bulkelectrolyte + contact + Pt wire resistances); CPE1, R1, CPE2 and R2 are the medium-frequency and low- frequency constant phase elements and charge transferresistances, respectively, Cgb and Rgb are the high-frequency grain boundary capac-itance and resistance, respectively; Ws is a short circuit Warburg impedance for thecathode reaction; W2 is a short circuit Warburg impedance for the anode reaction.
7676 E. Lust et al. / Electrochimica Acta 55 (2010) 7669–7678
Fc2
lalavIhiwagtmttfcdknsp
twstrrhcltonTsudfredid
3d
u
ig. 13. Mass transfer resistance vs. electrode potential dependences at T = 773 Kalculated according to the equivalent circuit III (Fig. 12) for GSCO (1, 1′), PSCO (2,′) and GSCO (3.3′) prepared from NTDM powders (1–3) and SSRM powders (1′–3′).
ow R1 values (≤0.5 � cm2 at |E| > 0.05 V) have been calculated. Inddition to EC III, modified EC including additionally the finite-ength type Gerisher Impedance (discussed by Boucamp et al. [37]nd Adler [21,22]) has been tested, but no decrease in �2 and �2
alues was detected, compared with corresponding values for ECII. Thus, based on the suggestion given in [34–36], the �2 valueave to decrease at least one order, if additional element has been
ntroduced, and therefore the more detailed analysis of fitting dataill not be discussed in this paper. Taking into account the notice-
ble influence of cathode porosity on the impedance data, theas phase and surface diffusion steps are probably more impor-ant [37] than the vacancy transport (diffusion) inside the porous
ixed-conducting cathodes. However, the dependence of RD onhe cathode potential applied (Fig. 13) indicates complicated massransfer mechanism (migration, semi-infinite diffusion, surface dif-usion). Therefore it is incorrect to calculate the effective diffusiononstant values from the Warburg-like impedance data. For moreetailed analysis of mass transfer rate on the cathode reactioninetics, the experimental data for cathodes with different thick-ess and macro-porosity are inevitable. Taking into account thepace limitations, these problems will be discussed in our futureublication.
The impedance spectra for more porous cathodes at T > 823 K,o a first approximation, can be simulated by the EC IV (Fig. 12)ith the chi-square function �2 ≤ 6 × 10−4 and weighted sum of
quares �2 < 0.1 (Rex is the total very high-frequency series resis-ance (practically independent of E); CPE2 and R2 are the totaleaction constant phase element and charge transfer resistance,espectively. Cgb, Rgb as well as R1 and CPE1 are negligible due to theigh open porosity and large contact area between electrolyte andathode, the medium/frequency processes are quick, and only theow-frequency adsorption/absorption, mass transfer and chargeransfer processes are rate determining steps for NTDM based cath-des at higher temperature. For less porous cathodes at T ≥ 873 K, aoticeably better fit of impedance data has been achieved by EC III.he small chi-square function values �2 < 2 × 10−4 and weightedum of squares �2 < 0.03 have been established. The relative resid-als obtained for this circuit are very low and have a randomistribution in the whole frequency region studied. The mass trans-er (diffusion) resistance RD and the low-frequency charge transferesistance R2 are higher than for NTDM based cathodes, how-ver, similarly to NTDM based cathodes, the resistances mentionedecrease with increasing temperature and |E|. For all systems stud-
ed, there is a small maximum in the RD vs. E as well as R2 vs. Eependences near E = −0.1 V (vs. Pt|O2).
.4. Seeming activation energy vs. electrode potentialependences
The capacitive parts of the impedance spectra at f ≤ 20 kHz weresed to determine Rp from the difference between the intercepts
Fig. 14. Arrhenius-like seeming activation energy vs. electrode potential depen-dences for the LSCO|CGO (1; 1′), PSCO|CGO (2; 2′) and GSCO|CGO (3′) half cells,synthesized by nitrate method (1; 2) and solid state reaction method (1′; 2′; 3′).
of the very low and high-frequency parts of the spectra with theZ′-axis of Nyquist plots (Figs. 9 and 10). Comparison of the datashows that the total polarization resistance increases for LSCO withdecreasing SBET, and in the sequence LSCO, PSCO and GSCO, which iscaused by higher mass transfer resistance RD inside the less porouscathode. The nonlinear shape of seeming activation energy Eact vs.potential plots (Fig. 14) indicates the change in the ratio of the ratelimiting processes with increasing the cathode negative potential.At lower T, the value of Eact obtained from Z′′,Z′-plots is in a reason-able agreement with the value of ED obtained from the diffusionresistance RD vs. temperature plots. Thus, the slow mass transferprocess at lower T and |E| characterises mainly the total electrore-duction process, i.e. generalized diffusion-like limited process ofthe electrochemically active oxygen particles into the reaction cen-tre, being the slowest step in the total reaction. The lowest valueof Eact was obtained for most porous NTDM based PSCO cathode athigher negative potentials (Fig. 14).
3.5. Single cell measurements
Electrochemical impedance spectra for the Ni-Ce0.9Gd0.1O2−ı|Ce0.9Gd0.1O2−ı|La0.6Sr0.4CoO3−ı single cell (notedas Ni-CGO|CGO|LSCO) are shown in Fig. 15. According to the com-parison of best-fit values for the �2 parameter, the EC V in Fig. 12can be used for fitting the calculated spectra to the experimentalimpedance data. Influence of the cell voltage, E, on the rateof electrochemical processes is very well visible in Fig. 15(b),indicating that the total polarization resistance decreases withdecreasing E (open circuit potential, EOCV, is equal to 0.91 V forNi-CGO|CGO|LSCO single cell).
An alternative to traditional measurement methodology[8–11,23–25] the method of the difference of derivative of theimpedance real part, ∂Z′, and imaginary part, ∂Z′′, vs. log f pro-posed by Barfod et al. [38] and Jensen et al. [39] was used for thedeconvolution of the real as well as imaginary impedance compo-nents into anodic and cathodic reactions contributions. Thus, thequantities of ∂Z′
fuel and ∂Z′oxidant have been defined as:
∂Z ′fuel =
∂|Z ′H2+Ar|
∂ log f−
∂|Z ′H2
|∂ log f
,
∂Z ′oxidant = ∂|Z ′
air|∂ log f
− ∂|Z ′oxygen|
∂ log f(1)
corresponding to ∂Z′ of fuel (H2 + Ar or H2) and oxidant (pureO2 or air) composition variation experiments, respectively. Basedon the single cell analysis (Figs. 15 and 16), the frequency range
′ ′′
for which the magnitude of |Z|, Z or Z is affected upon switchingfrom hydrogen and argon fuel mixture to pure hydrogen (underconstant oxidant flow) corresponds to the frequency dependenceof the anodic (i.e. fuel oxidation) process. Similarly, by changingthe oxygen partial pressure in the cathode compartment, one can
E. Lust et al. / Electrochimica Acta 55 (2010) 7669–7678 7677
Fig. 15. (a) Nyquist plot for Ni-CGO|CGO|LSCO single cell at open circuit potentialand at 873 K. Filled symbols denote experimental data, black lines are the complexnonlinear least-squares fit by using EC V (Fig. 12). The impedance of model subunitsiaA
iglpatcpOdataeclba
cc
Tf
Fig. 16. ∂Z′ vs. log f plots (a) and ∂Z′′ vs. log f plots (b) for Ni-CGO|CGO|LSCOsymmetrical single cell upon gas composition variation. Closed symbols correspondto the fuel variation experiment, i.e. ∂Z′
fuel or ∂Z′′fuel characterize a shift from
10% H2 + 87% Ar + 3% H2O to 97% H2 + 3% H2O on the anode side at a constant airflow of 100 mL/min. Open symbols correspond to the oxidant variation experiment,i.e. ∂Z′
oxidant or ∂Z′′oxidant characterize a shift from air to pure oxygen on the
cathode side at constant 97% H2 + 3% H2O flow of 100 mL/min. All measurementswere conducted at open circuit conditions at 873 K. (c) ∂Z′
oxidant vs. frequencycurves (c) for LSCO|CGO|Pt half cellfor the oxidant variation experiment from O2 toair at T = 773 K and cathode potentials (vs. Pt|O2), given in figure.
s shown separately along with the corresponding fmax. Nyquist plots (b) and phasengle vs. log f plots (c) for Ni-CGO|CGO|LSCO at T = 873 K and at different cell voltagesE (from 0 to 0.8 V; the direction of increase is shown in figure). Fuel: 10% H2 + 87%r + 3% H2O at 100 mL/min. Oxidant: air at 100 mL/min.
dentify the electroreduction reaction contribution to Z′ of the sin-le cell [38,39]. Comparison of the ∂Z′ vs. log f plots and ∂Z′′ vs.og f plots for Ni-CGO|CGO|LSCO (Fig. 16a and b) with the oxidantartial pressure variation experiments (not shown for shortness)nd potential variation experiments for half cells (Fig. 16c) suggesthat the process of highest characteristic frequency (fmax ≈ 5 Hz)orresponds to an anodic process, whereas the lower frequencyrocess (fmax ≈ 0.8 Hz) can be assigned to the electroreduction of2 [8–11,18–24,27]. Therefore, the differences of correspondingerivatives established for single cells characterize the oxidationnd reduction processes with different characteristic relaxationime constants, respectively. Furthermore, the values of the char-cteristic relaxation frequency for the single cells obtained byquivalent circuit modeling are in an excellent agreement with theharacteristic relaxation frequency values obtained by the ∂Z′ vs.og f plot analysis (Figs. 9, 10, 15 and 17). However, the differenceetween fmax values is quite low and the exact separation of anodend cathode processes contribution is complicated [25–27,32–36].
In Fig. 17 the ∂Z′ vs. ∂ log f plots for Ni-CGO|CGO|LSCO singleell are shown, calculated at different cell voltages, Efix, and atonstant fuel and oxidant compositions as follows:
∂Z ′ = ∂Z ′(EOCV)∂ log f
− ∂Z ′(Efix)∂ log f
(2)
he influence of cell voltage is very well visible mainly at lowerrequency (f < 1.0 Hz), where electroreduction of O2 is the main lim-
Fig. 17. ∂Z′ vs. log f plots for Ni-CGO|CGO|LSCO single cell upon oxidant varia-tion conditions from pure oxygen to air at T = 873 K, constant fuel (10% H2 + 87%Ar + 3% H2O; flow 100 mL min−1) and oxidant (air, 100 mL min−1) compositions andat different cell voltages (V, noted in figure).
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[J. Electrochem. Soc. 154 (2007) B1325.
678 E. Lust et al. / Electrochim
ting step. The similarity of the corresponding data for half cellsFig. 16c) and single cells (Fig. 16a, b and Fig. 17) is also evident.hus, influence of the cell voltage on the fuel oxidation process ismall.
Temperature variation experiments allowed the Arrhenius acti-ation energy values for the cathodic process to be determined as.36 eV at E = 0. These values calculated for LSCO based single cellre in a very good agreement with our data for half cells discussedbove, as well as with the results of Horita et al. [33], who observedvalue of 1.32 eV for charge transfer limitation under comparablexperimental conditions. Moreover, the experimentally observedpparent reaction order of 0.22 of oxygen partial pressure (theo-etical value: 0.25 [40]) suggests the low-frequency process can bessigned to a slow charge transfer step at the three phase boundaryTPB) of the cathode.
Experimentally observed activation energy values of 0.93 and.22 eV for the anodic charge transfer resistance, R2, and thearburg-like mass transfer resistance, RD, respectively, suggest
hat the anodic contribution is due to slow charge transfer pro-esses at the TPB, coupled probably by surface diffusion of adsorbedydrogen species [41]. The fact that RD was found to be practically
ndependent of applied cell voltage provides further support forhe involvement of non-charged intermediate species in the anodiclectrooxidation process of hydrogen in Ni-CGO|CGO|LSCO singleells [18–28,38–42].
. Conclusions
In the case of cathodes, prepared from the raw power syn-hesized using solid state reaction method, the kinetically mixedrocess (slow mass transport, adsorption and electron transfertages) takes place in cathode|electrolyte systems at temperaturesess than 823 K, if air has been used as an oxidant.
The value of seeming activation energy, Eact, decreasing with thencreasingly negative cathode potential, and the transfer coefficientc > 0.5 indicate that, in addition to the slow electron transfer pro-ess (reduction of molecular oxygen), the adsorption/absorptionnd mass transfer processes of electrochemically active particlesn solid cathode material or at the internal micro–meso–porousathode surface have some influence on the O2 electroreductioninetics in agreement with the fitting results of the impedancepectra.
Activation energy, Eact, depends noticeably on the chemicalomposition and porosity of the raw powder, used for prepara-ion of the cathodes, and Eact noticeably decreases with increasinghe specific surface area of the cathode studied. The high-requency series resistance and grain boundary resistance for theathode|electrolyte system are very low, if the cathode has beenrepared from the nanopowder (SBET > 100 m2 g−1) synthesized bysing the nitrate solution thermal decomposition method.
In order to obtain an estimate of how these cathode materialsehave in real operating conditions, the symmetrical two-electrodeingle cell experimental setup has been completed and tested. Bysing the method of difference derivative of the impedance realnd imaginary parts vs. log frequency plots for three-electrode halfells combined with fuel and oxidant composition variation exper-
ments for single cells at different cell voltages, the impedancepectra for symmetric single cell can be deconvoluted even whenhe simple two-electrode configuration is used for measurements.hus, the three-electrode half cell and two-electrode single celloth can be considered as complementary to one another, and
[
[
[
ta 55 (2010) 7669–7678
should be used jointly in order to gain a thorough insight into theelectrochemical properties of SOFC cathodes.
Acknowledgements
This study was partially funded by Elcogen Ltd. and the EstonianScience Foundation Grant 7791.
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