A-Site Deficient (Pr0.6Sr0.4)(1-s)Fe0.8Co0.2O3-delta Perovskites as Solid Oxide Fuel Cell Cathodes

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1 Downloaded from orbit.dtu.dk on: Apr 21, 218 A-Site Deficient (Pr.6Sr.4)(1-)Fe.8Co.2O3-delta Perovkite a Solid Oxide Fuel Cell Cathode Kammer Hanen, Kent Publihed in: Journal of the Electrochemical Society Link to article, DOI: / Publication date: 29 Document Verion Publiher' PDF, alo known a Verion of record Link back to DTU Orbit Citation (APA): Kammer Hanen, K. (29). A-Site Deficient (Pr.6Sr.4)(1-)Fe.8Co.2O3-delta Perovkite a Solid Oxide Fuel Cell Cathode. Journal of the Electrochemical Society, 156(1), B1257-B126. DOI: / General right Copyright and moral right for the publication made acceible in the public portal are retained by the author and/or other copyright owner and it i a condition of acceing publication that uer recognie and abide by the legal requirement aociated with thee right. Uer may download and print one copy of any publication from the public portal for the purpoe of private tudy or reearch. You may not further ditribute the material or ue it for any profit-making activity or commercial gain You may freely ditribute the URL identifying the publication in the public portal If you believe that thi document breache copyright pleae contact u providing detail, and we will remove acce to the work immediately and invetigate your claim.

2 Downloaded 3 Jun 21 to Reditribution ubject to ECS licene or copyright; ee /29/156 1 /B1257/4/$25. The Electrochemical Society A-Site Deficient Pr.6 Sr.4 1 Fe.8 Co.2 O 3 Perovkite a Solid Oxide Fuel Cell Cathode K. Kammer Hanen*,z Fuel Cell and Solid State Chemitry Diviion, Riø National Laboratory for Sutainable Energy, Technical Univerity of Denmark, DK-4 Rokilde, Denmark B1257 Five A-ite deficient Pr.6 Sr.4 1 Fe.8 Co.2 O 3 perovkite =.1,.5,.1,.15, and.2 were yntheized uing the glycine-nitrate proce. The perovkite were characterized with powder X-ray diffraction XRD, dilatometry, four-point dc conductivity meaurement, and electrochemical impedance pectrocopy uing cone-haped electrode on a Ce.9 Gd.1 O 1.95 electrolyte. XRD revealed that only the compound with =.1 and.5 were of ingle phae. The other compound contained an additional phae in the form of a Co Fe pinel. The thermal expanion coefficient decreaed ytematically with an increae in. The total conductivity followed the mall polaron hopping proce below a certain characteritic temperature. The total conductivity decreaed with increaing. It wa likewie hown that the polarization reitance found a minimum for the compound with =.5, with a total area pecific reitance more than 3 time lower than the weakly A-ite deficient Pr.6 Sr.4.99 Fe.8 Co.2 O 3 perovkite. 29 The Electrochemical Society. DOI: / All right reerved. Manucript ubmitted May 26, 29; revied manucript received June 29, 29. Publihed Augut 2, 29. The olid oxide fuel cell SOFC i a high temperature generator of heat and electricity. One major obtacle that mut be overcome, if the SOFC i to be commercialized, i lowering of the operation temperature. 1 To make thi poible, new electrode material are needed. On the cathode ide Co- or Fe Co-baed electrode have the highet activity toward the reduction of oxygen in an SOFC, 2 even though example of other ytem with an equal electrochemical activity are known. 3 In thi ene two compoition have been extenively invetigated in the literature, Sm.5 Sr.5 CoO 3 and La.6 Sr.4 Fe.8 Co.2 O An area pecific reitance ASR of.18 cm 2 at 6 C ha been reached by a compoite of Sm.5 Sr.5 CoO 3 and Ce 1 x Gd x O 2 CGO. 4 Thi i till not a high enough performance at 6 C, and better electrode material mut be developed. Recently, even better/lower ASR value have been obtained on a Ba.5 Sr.5 Fe.2 Co.8 O 3 -baed cathode. 1 An ASR of le than.1 cm 2 at 6 C wa obtained. However, electrode made with thi compound are not table over time. 11,12 A few tudie of A-ite deficient Fe Co-baed perovkite have been reported It wa hown that the conductivity depend on the A-ite toichiometry. 13 Likewie, it wa hown that the thermal expanion coefficient TEC decreae when the A-ite toichiometry i lowered. From powder X-ray diffraction XRD it wa concluded that the A-ite deficient perovkite were rhombohedral and ingle phae. 13 Reult of the electrochemitry of the A-ite deficient La.6 Sr.4 1 Fe.8 Co.2 O 3 were reported in Ref. 15. The activity wa trongly dependent on. Thee A-ite deficient perovkite conited of two cubic perovkite phae and one pinel phae. 15 The trongly A-ite deficient perovkite La.6 Sr.4.9 Fe.8 Co.2 O 3 ha been invetigated a porou compoite electrode with Ce.9 Gd.1 O 1.95 CGO1 with relatively good reult, a an ASR value of.19 cm 2 wa obtained on a CGO1 electrolyte at 6 C. 14 Thi i better than the ASR value obtained on the toichiometric La.6 Sr.4 Fe.8 Co.2 O 3 perovkite. 9 Even though the perovkite wa trongly A-ite deficient, the perovkite wa not table together with yttria-tabilized zirconia. In thi tudy a erie of A-ite deficient Pr.6 Sr.4 1 Fe.8 Co.2 O 3 perovkite =.1,.5,.1,.15, and.2 wa yntheized and invetigated a SOFC cathode by uing electrochemical impedance pectrocopy EIS and cone-haped electrode. The ue of cone-haped electrode i very convenient when tudying SOFC electrode The contact area of the cone-haped electrode can be determined uing Newman formula 2 * Electrochemical Society Active Member. z kkha@rioe.dtu.dk 1 r = 4R 1 where R i the intercept at the real axi at high frequency and i the pecific conductivity of the electrolyte. r i the radiu of the contact between the cone-haped electrode and the electrolyte, from which the contact area can be calculated. The polarization reitance on cone-haped electrode i much higher than the polarization reitance on porou planar electrode. Fe Co-baed cathode with Pr are of particular interet a they perform better than Fe Co-baed cathode with other A-ite cation. 21 Experimental Synthei of the Pr.6 Sr.4 1 Fe.8 Co.2 O 3 perovkite wa done uing the glycine-nitrate route. 22 Aqueou olution of the metal nitrate were mixed in an appropriate ratio in a beaker. Glycine wa then added. The olution were then heated on a hot plate until they ignited. The reulting powder were tranferred to alumina crucible and were calcined at 11 C/12 h in air in a box furnace. The following tarting compound were ued a metal nitrate: Pr NO 3 3 Alfa Aear, 99.9%, Sr NO 3 2 Alfa Aear, 99%, Fe NO 3 3 Alfa Aear, 98%, and Co NO 3 2 Alfa Aear, 99.8%. To evaluate the phae purity of the calcined powder, powder XRD uing a Stoe diffractometer equipped with Cu K radiation wa performed. The diffractogram were recorded in the range 2 : 2 8. The bar for the dilatometry and conductivity meaurement and the cylinder for the fabrication of the cone-haped electrode were made a follow: The powder were preed in appropriate die bar and cylinder and intered at 125 C/12 h in a box furnace in air. The bar were then machined into mm bar and the cylinder were machined into cone-haped electrode. The conehaped electrode had a bae of 7.5 mm and ide with an angle of 45. The denity of the bar wa determined with the Archimede method. The CGO1 pellet ued for the electrochemical meaurement wa made a follow: CGO1 powder Rhodia wa mixed with tearic acid and glycerine in a ball mill with ethanol. After drying the powder wa preed to a dik in an appropriate die and intered at 15 C/2 h in air in a box furnace. The tearic acid wa added a a lubricant and the glycerine wa added a a binder. The dilatometry meaurement were performed by uing a Netzh 42 dilatometer in the temperature range of 3 1 C with heating and cooling ramp of 2 C/min. The bar were kept at 1 C for 2 h before cooling down. The four-point dc conductivity meaurement were alo performed in air uing a Keithley micro-ohmmeter together with a peronal computer PC controlling the ohmmeter and the furnace temperature. An in-houe PC Linux program, wa ued for the collection of the data temperature and reitance. The

3 Downloaded 3 Jun 21 to Reditribution ubject to ECS licene or copyright; ee B T/ o C Rel. int. /% σ Τ / Scm -1 *K 1e+6 1e =.1 =.5 =.1 =.15 = θ /degree Figure 1. Powder XRD pattern of the compound Pr.6 Sr.4.95 Fe.8 Co.2 O 3 recorded in air at room temperature. ample were heated and cooled with a ramp of 5 C/min. The temperature range panned wa 5 1 C. At every 5 C the ample were kept at temperature for 2 h. The reitance wa meaured every fifth minute. The characterization of the cone-haped electrode were done in a etup decribed in Ref. 23. EIS wa ued to characterize the cone-haped electrode. The EIS wa meaured in the range of 1 MHz.5 Hz with five point meaured at each decade. An amplitude of 36 mv wa ued throughout. An amplitude of 36 mv wa needed to obtain data of ufficient quality. The meaurement were done at 8, 7, and 6 C in the given order. The cone were equilibrated at temperature for 24 h before the meaurement. 24 h were ufficient for equilibration. The meaurement were done in air. For the EIS meaurement a Solartron 126 in tand-alone mode wa ued. The EIS data were treated in the PC- DOS program equivcrt by Boukamp. 24 Three RQ component were ued in the fitting. Q i a contant phae element with the admittance Y = Y j / n 2 where Y i an admittance, i the angular frequency, j i the imaginary unit, and n i the frequency exponent. Reult A reult from the powder XRD i found in Fig. 1. From powder XRD only the ample with =.1 and.5 are ingle phae. The Table I. Unit cell parameter of A-ite deficient Pr.6 Sr.4 1 Fe.8 Co.2 O 3 perovkite meaured with powder XRD in air at room temperature. A Å B Å C Å e+4,5,1,15,2,25,3,35 1 / T1/K Figure 2. Color online Total conductivitie a a function of temperature for the A-ite deficient Pr.6 Sr.4 1 Fe.8 Co.2 O 3 perovkite. The conductivity drop with increaing. The conductivitie have been corrected uing the Bruggeman aymmetric model. 25 other ample contain an additional phae in the form of a Co Febaed pinel phae. A determined from XRD, the amount of the pinel phae increae with increaing. The maximum amount of foreign phae i around 1% for =.2. The unit cell parameter are found in Table I. The unit cell parameter how an irregular change with. The peak are rather broad. Thi i probably due to an inhomogeneou ditribution of the cation. For the intered bar a denity in exce of 92% of the theoretical denity wa obtained. The reult of the dilatometry meaurement are found in Table II. Oberve that the TEC decreae with decreaing toichiometry on the A-ite. The conductivity of the ample i alo dependent on the A-ite toichiometry. The conductivity a a function of temperature i plotted in Fig. 2. The conductivity i highet for the compound with =.1 at all temperature. For all the ample an increae in conductivity with an increae in temperature, until a certain temperature after which the conductivity decreae, i oberved. The maximum in conductivity i found for the compound with =.1, and it give a value of 42 S cm 1 at 52 C. An example of an EIS pectrum i een in Fig. 3, together with the reult of the fitting. The pectrum conit of three arc. The variation in the total ASR with at a temperature of 6 C i found in Fig. 4. The total ASR find a minimum for the compound with =.5 with an ASR of 1.72 cm 2 at 6 C. The total ASR value at the three meaured temperature are found in Table III. The contribution to the total ASR of the individual arc at 6 C are found in Table IV, together with the n value. The n value are the frequency exponent given in Eq. 2. Dicuion The A-ite deficient Pr.6 Sr.4 1 Fe.8 Co.2 O 3 perovkite differ from the A-ite deficient La.6 Sr.4 1 Fe.8 Co.2 O 3 perovkite. 15 In A-ite deficient lanthanum-baed perovkite, two perovkite phae are preent along with a Fe Co-baed pinel Table II. TEC value of A-ite deficient Pr.6 Sr.4 1 Fe.8 Co.2 O 3 perovkite in air from 1 to 9 C. TEC 1 6 K

4 Downloaded 3 Jun 21 to Reditribution ubject to ECS licene or copyright; ee B1259 -Z imag / Ω phae. The two perovkite phae in the A-ite deficient lanthanumbaed perovkite are two phae with different amount of La, Sr, Fe, and Co. In praeodymium-baed perovkite, only one perovkite phae i preent along with a Co Fe-baed pinel phae. The variation in the TEC with i ytematic, a the TEC decreae almot linearly with increaing A-ite deficiency. Thi i probably due to the impuritie with the pinel phae, and becaue cobalt i lot from the perovkite phae 16 Fe-baed perovkite have much lower TEC than Fe Co-baed perovkite. 17 The change in TEC could alo be due to a change in the point defect concentration. The TEC value found for the A-ite deficient Pr-baed perovkite are higher than the TEC for CGO electrolyte. 26 However, ASR / Ωcm e+4 4e+4 6e+4 8e+4 1e+5 Z real / Ω Figure 3. Color online EIS of the compound Pr.6 Sr.4.9 Fe.8 Co.2 O 3 at 6 C in air. Circle are meaured data. Solid line i fitted data. Characteritic frequencie are marked in(pr.6 Sr.4 ) 1- Fe.8 Co.2 O 3-δ Figure 4. Total ASR value a a function of meaured in air at 6 C with EIS. The uncertaintie on the contact area are around 1 15%. Table III. Total ASR value for A-ite deficient Pr.6 Sr.4 1 Fe.8 Co.2 O 3 at three different temperature in air meaured with EIS. The value are given in cm 2. 6 C C C compoite electrode made by the A-ite deficient perovkite and CGO might be of ue a cathode on a CGO electrolyte. The electronic conductivity of the compound change from a metallic to a emiconductor-like behavior when the temperature i lowered, that i, the electronic conductivity increae until a certain temperature i reached after which the electronic conductivity decreae. The conductivity behavior a a function of temperature below a certain characteritic temperature indicate that the conductivity i due to the mall polaron hopping mechanim 27 with the following relationhip = A/kT v exp E a /kt 3 where A i a material contant, k i Boltzmann contant, and E a i the activation energy. The exponent v i 1 for the adiabatic mall polaron hopping proce and 3/2 for the nonadiabatic mall polaron hopping proce. The equation with v=3/2 fit the meaured data bet, in contrat to what ha been oberved by Tai et al. 28,29 but in agreement with Hanen and Hanen 15 for imilar material. The activation energy E a of the total conductivity, below the temperature of maximum conductivity, i around.1 ev for all the compound. The decreae in the total conductivity above a certain characteritic temperature can be explained a follow. When the temperature i increaed, the perovkite loe oxygen. When oxygen vacancie are creted in the perovkite tructure, a decoupling of the overlap between the d-orbital of the tranition metal and the p-orbital of oxygen i the reult. Thi can explain the lowered conductivity at high temperature. 28,29 Not only do the oxygen vacancie hinder the polaron hopping but they alo alter the other defect concentration which lead to a lowering of the concentration of charge carrier. Thi i the main effect. The calculated temperature of maximum conductivity i alo higher than the oberved temperature of maximum conductivity. The calculated temperature can be found uing the equation E a = kt max 4 The calculated temperature of maximum conductivity i around 115 K for all the compound, compared to the oberved temperature of maximum conductivity of approximately 8 K. Thi i alo an indication that the oxygen toichiometry i lowered upon heating of the perovkite. The conductivity behavior found in thi tudy i almot the ame a found in the literature. 13,15 The variation in the total conductivity with can be undertood a follow. When i increaed, cobalt i lot from the perovkite phae, a oberved for A-ite deficient lanthanum-baed perovkite 15 and confirmed by XRD. Fe Co-baed perovkite have a higher conductivity than Fe-baed perovkite. 3 Thi therefore lower the conductivity. Alo, the preence of a pinel lower the conductivity a Co Fe-baed pinel have a much lower conductivity than Fe Co-baed perovkite. 31 Oxide ionic conductivity change a a function of. In the literature it wa hown that a lowered A-ite occupancy greatly influence the oxygen toichiometry, oxygen ion vacancy, and cation vacancy concentration for manganite-baed perovkite. 32 However, no literature data on the defect chemitry of the A-ite deficient Fe Co-baed perovkite have been given. In principle, a lowered A-ite occupancy can lead to the creation of both oxide ion vacan- Table IV. Contribution to the total ASR of the individual arc for A-ite deficient Pr.6 Sr.4 1 Fe.8 Co.2 O 3 meaured at 6 C in air with EIS. The value are given in cm 2. 6 C ASR n ASR n ASR n ASR n ASR n High Medium Low

5 Downloaded 3 Jun 21 to Reditribution ubject to ECS licene or copyright; ee B126 Table V. Chemical capacitance of A-ite deficient Pr.6 Sr.4 1 Fe.8 Co.2 O 3 at 6 C in air meaured with EIS. cie and cation vacancie in the perovkite tructure, but thi ha never been oberved for perovkite; alo, the powder XRD data ugget that exce Fe Co i expelled from the perovkite tructure when i increaed. If, however, both anion and cation vacancie are created in the perovkite tructure, thi probably lower the oxide ionic conductivity due to the attraction between cation and anion vacancie. The magnitude of the ASR i very high on cone-haped electrode compared to porou compoite electrode of the ame material. Thi i partly becaue cone-haped electrode are not compoite electrode, and partly due to the interface between the electrode electrolyte, which i not optimized in the cone-haped electrode. The geometrical configuration of the cone-haped electrode limit the number of contact point at the electrode electrolyte interface. Alo, the cone-haped electrode i dene and therefore ha a low urface area. The interpretation of the EIS data can be done according to the mechanim uggeted and confirmed by Siebert et al. 33 We are well aware of other mechanim uggeted in the literature, but thi mechanim fit the data bet. In thi mechanim the high frequency arc i due to the exchange of oxide ion at the electrode electrolyte interface. The equivalent capacity of thi arc can be calculated uing the equation 34 C = R 1 n /n Y 1/n 5 The magnitude of the equivalent capacity of thi arc i between.5 and 1 F cm 2 for all the compound, uggeting that thi arc i due to a double-layer effect. 35 The n value are all cloe to.9 for all the compound, which again indicate that it i the ame type of reaction that occur on all the compound. The medium frequency arc i normally attributed to the diffuion of oxide anion through the bulk of the electrode. 33 Thi arc find it minimum for the compound with =.5, indicating that thi compound ha the highet oxide ionic conductivity. The magnitude of the low frequency arc i alo dependent on, the magnitude being mallet for the compound with =.5. The low frequency arc i due to a low redox reaction at the urface of the electrode. 33 One poible explanation for thi behavior could be the lowering of the urface egregation of A-ite cation, increaing the amount of the catalytic active pecie Fe and Co on the urface of the electrode material. That the magnitude of the low frequency arc increae with a further decreae in i perhap due to the lo of cobalt from the perovkite phae. The equivalent capacity of thi arc can be calculated for the high frequency arc uing Eq. 5. The value are given in Table V. They all fall in the range for a chemical capacitance, a depicted in Ref. 36. Furthermore it eem like the magnitude of the three arc i coupled; they all decreae or increae in the ame manner with a change in. Thi parameter could either be the oxide ionic conductivity or the urface egregation of praeodymium, or perhap both. The electrochemical behavior of the A-ite deficient Pr-baed perovkite i markedly different from the behavior of the A-ite deficient La-baed perovkite. The effect of A-ite deficiency i much le pronounced for the Pr-baed perovkite than for the Labaed perovkite. 8 Thi could be due to le urface egregation for the praeodymium-baed perovkite than for the lanthanum-baed perovkite. Praeodymium perhap fit better in the perovkite tructure than lanthanum, lowering the importance of A-ite deficiency. It could alo be due to the ability of praeodymium to C low F cm change it oxidation tate. It could be that praeodymium ha ome catalytic activity in itelf toward the reduction of oxygen. Thi ha been hown for other ytem. 37 Concluion The TEC decreae almot linearly with, probably due to the lo of cobalt from the perovkite phae. The total conductivity decreae a a function of increaing. The effect of A-ite ubtoichiometry on the activity of praeodymium-baed perovkite toward the reduction of oxygen i reflected in a lowered ASR for the compound with =.5. Thi i thought to be due to lowering of the urface egregation of praeodymium. Acknowledgment Colleague at the Fuel Cell and Solid State Chemitry Diviion are thanked for fruitful dicuion. Financial upport from Energinet.dk through PSO-R&D project no i gratefully acknowledged. Technical Univerity of Denmark aited in meeting the publication cot of thi article. Reference 1. N. Q. Minh, J. Am. Ceram. Soc., 76, J. M. Ralph, C. Roignol, and R. Kumar, J. Electrochem. Soc., 15, A G. Xia and M. Liu, Adv. Mater. (Weinheim, Ger.), 14, C. Xia, W. Rauch, F. Chen, and M. Liu, Solid State Ionic, 149, Y. Liu, W. Rauch, S. Zha, and M. 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