BA Boukamp

D Pérez-Coll

E Perry Murray

E Siebert

G Amow

J Newman

JM Ralph

K Kammer

K. Kammer

KK Hansen

LA Chick

M Juhl

NL Robertson

NQ Minh

P Fabry

S-I Hashimoto

SB Adler

X Granados

Crossref

An EIS study of La2 − x Sr x NiO4 +  δ SOFC cathodes

La2 − x Sr x NiO4 +  δ materials were investigated as cathodes for the electrochemical reduction of oxygen on a Ce1.9Gd0.1O1.95 (CGO10) electrolyte using cone-shaped electrodes together with electrochemical impedance spectroscopy. The area-specific resistance (ASR) of the La2 − x Sr x NiO4 +  δ nickelates towards the reduction of oxygen is equal to the ASR of perovskites; however, it is not as low as for the best Fe–Co-based perovskites. The lowest ASR is found for the compound La1.75Sr0.25NiO4 +  δ with an ASR of 23.8 Ωcm2 measured on a cone-shaped electrode in air at 600 °C. It is suggested that difference in oxide ionic conductivity of the nickelates is the main cause for the different activity of the nickelates towards the electrochemical reduction of oxygen

Kammer Hansen, Kent

Online Research Database In Technology

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 17, 2017An EIS study of La2-aEuro parts per thousand x Sr (x) NiO4+ (delta) SOFC cathodesKammer Hansen, KentPublished in:IonicsLink to article, DOI:10.1007/s11581-009-0331-7Publication date:2009Link back to DTU OrbitCitation (APA):Kammer Hansen, K. (2009). An EIS study of La2-aEuro parts per thousand x Sr (x) NiO4+ (delta) SOFCcathodes. Ionics, 15(3), 325-328. DOI: 10.1007/s11581-009-0331-7 1An EIS study of La2-xSrxNiO4+δ SOFC cathodes.  K. Kammer* Fuel Cells and Solid State Chemistry Division Risø National Laboratory for Sustainable Energy Technical University of Denmark DK-4000 Roskilde, Denmark  Abstract: La2-xSrxNiO4+δ materials were investigated as cathodes for the electrochemical reduction of oxygen on a Ce1.9Gd0.1O1.95 (CGO10) electrolyte using cone-shaped electrodes together with electrochemical impedance spectroscopy (EIS). The area specific resistance (ASR) of the La2-xSrxNiO4+δ nickelates towards the reduction of oxygen is equal to the ASR of perovskites; however, not as low as for the best Fe-Co based perovskites. The lowest ASR is found for the compound La1.75Sr0.25NiO4+δ with an ASR of 23.8 Ωcm2 measured on a cone-shaped electrode in air at 600oC. It is suggested that difference in oxide ionic conductivity of the nickelates is the main cause for the different activity of the nickelates towards the electrochemical reduction of oxygen.  Introduction: Environmental friendly production of heat and electricity is highly desirable. One way to achieve this is the use of a solid oxide fuel cell (SOFC) [1]. However, one major obstacle that must be overcome before commercialisation of the SOFC is a lowering of the operation temperature [2]. In order to make this possible new electrode materials are needed. On the cathode side several alternatives exist to                                                  * e-mail: kent.kammer.hansen@risoe.dk  2the classical LSM/YSZ (LSM = Strontium substituted Lanthanum Manganite, YSZ = Yttria Stabilised Zirconia) SOFC cathode. Among the alternative cathodes Co and Fe-Co based perovskites reveal the highest activity towards the reduction of oxygen [3]. However, an alternative to perovskite based cathodes could be nickelates with the K2NiF4 structure [4, 5]. It has been shown that the nickelates has an activity in the same order of magnitude as manganese based perovskites [4, 5]. In this work a series of La2-xSrxNiO4-δ nickelates (x = 0.00, 0.05, 0.15. 0.25 and 0.35) are synthesised and investigated using the cone-shaped electrode technique together with EIS. The cone-shaped electrode technique is a convenient method when different electrode materials are to be compared. The use of point (or cone-shaped) electrodes were first suggested by Fabry and Kleitz [6]. As the name indicates the working electrode is shaped like a cone, and the cone-shaped electrode is pushed towards the electrolyte with a small weight.    Experimental: Synthesis of the nickel based materials was done using the glycine-nitrate route [7]. In short the metal-nitrates dissolved in water were mixed in the appropriate ratio in a beaker, before glycine was added. The mixtures were then heated on a hot plate until combustion. The resulting powders were then transferred to alumina crucibles and the powders were calcined at 1100oC/12 h in air in a box furnace. The metal nitrates used were the following: La(NO3)3•6H2O (Alfa Aesar, 99.9 %), Sr(NO3)2 (Alfa Aesar, 99 %) and Ni(NO3)2•6H2O (Fluka, 97 %). After calcination the powders were investigated with powder X-ray diffraction (XRD) using a Stoe theta-theta diffractometer equipped with Cukα radiation. The cone-shaped electrodes were made as follows: 7-8 g of powder was pressed in a die with diameter = 10 mm. Thereafter the cylinders were sintered at 1250oC in air in a box furnace for  312 h. Finally the cylinders were machined into cone-shaped electrodes by the use of diamond tools. As an electrolyte a pellet of CGO10 (Rhodia) was used. The CGO10 pellets were made by mixing CGO10 powder with stearic acid and glycerine (both 1 w/w %) in a ball mill together with EtOH. The mixture was ball milled overnight. After drying the mixture was pressed into pellets. The pellets were sintered at 1500oC/2 h in air in a box furnace. After sintering the pellets were polished with diamond paste (1 μm). The thickness of the electrolyte pellets were around 2-3 mm. The electrochemical measurements were done in a set-up described in [8]. The EIS measurements were done using a Solartron 1260 in stand alone mode. The electrochemical measurements were done at temperatures of 800, 700 and 600oC in air. Before the measurements the cone-shaped electrodes were equilibrated at each temperature for 24 h. The EIS were recorded from 1 MHz to 0.05 Hz with five points measured at each decade. An amplitude of 24 mV were used throughout. The PC-DOS program ‘equivcrt’ by B.A. Boukamp [9] was used for fitting of the data. The data were fitted with the circuit Rs(R1Q1)(R2Q2)(R3Q3), where R is a resistance and Q is a constant phase element with the admittance:  (1) Y = Y0(jω/ω0)n,  where Y0 is a constant, ω is the cyclic frequency, and n is an exponent. Y0 and n is found from the fitting.  The contact area of the cone-shaped electrodes was determined by the use of Newman’s formula [10]:   4(2) *41σsRr = ,  where Rs is the intercept with real axis in the impedance plot at high frequency, and σ* is the specific conductivity of the electrolyte. r is the radius of the contact between the cone-shaped electrode and the electrolyte, from which the contact area can be calculated. The calculated contact areas are around 10-4 cm2.  Results: The results from the powder XRD experiments reveal that all the nickelates are of single phase [11]. All the nickelates belong to the tetragonal crystal system. The densities of cone-shaped electrode were around 90 % of the theoretical value. An example of an EIS spectrum can be found in Figure 1. The spectrum consists of three arcs, as validated by the fitting using ‘equivcrt’. Characteristic frequencies are indicated on the spectrum. The total ASR values for the nickelates at 600, 700 and 800oC are listed in Table 1.  The total ASR finds its minimum for the compound with the composition La1.75Sr0.25NiO4+δ with an ASR value of 23.8 Ωcm2 at 600oC. The total ASR values of the nickelates at 600oC are plotted in Figure 2. The results of the fitting of the EIS measurements at 600oC can be found in Table 2 in the form of ASR of the individual arcs together with the n values.   Discussion: The ASR of the investigated nickelates towards the reduction of oxygen is on the level with good perovskite based cathodes [12-16]. This indicates that nickelates with the K2NiF4 structure can be used instead of perovskite based nickelates. However, the ASR of the nickelates is not as low as for the best Fe-Co based perovskite materials [16]. It should be noted that the ASR found on cone- 5shaped electrodes is much higher than the ASR found on planar porous composite electrodes, see i.e. [16, 17]. This is partly due to the fact that the cone-shaped electrodes not are composite electrodes, and partly due to the interface between the electrode-electrolyte, which not is optimized in the case of the cone-shaped electrodes. The geometrical configuration of the cone-shaped electrode limits the number of contact points at the electrode/electrolyte interface. Also the cone-shaped electrode is dense and therefore has a low surface area.  The interpretation of the EIS data can be done accordingly to the mechanism suggested for perovskite based electrodes by Siebert et al. [18] and others [19]. In this mechanism the high frequency arc is suggested to be due to transfer of oxide ions at the electrode-electrolyte interface. The near-equivalent capacity of this arc can be calculated using the following equation [20]:  (3) Cω = R(1-n)/n nY /10 ,  Where R is a resistance, and Y0 and n is defined as in equation (1). The near-equivalent capacity of the high frequency arc is around 0.5 μFcm-2 for all the compounds at 600oC, strongly suggesting that this arc is due to a double layer effect. The near-equivalent capacity is in close agreement with the value found for a double layer capacitance in [21]. The n value for this arc is about the same for all the nickelates, indicating that it is the same reaction that gives rise to this arc on the different nickelates. With respect to the magnitude of the high frequency arc in the impedance spectra no clear trend are observed for this arc with x. This could be because the magnitude of the high frequency arc depends on the micro-structure of the cone-shaped electrodes and therefore does not reflect true material properties of  6the electrode materials. The medium frequency arc seems to vary systematically with the amount of strontium in the structure as it decreases with increasing strontium content until x = 0.25 where after it increases again. That the medium frequency arc is absent for the compound with x = 0.25 is because it is not possible to separate the medium frequency arc from the low frequency arc in the fitting. No detailed study of the mechanism, for the reduction of oxygen on this type of cathode materials, has been made. However, if the same mechanism as for perovskites is valid then the medium frequency arc is due to diffusion of oxide anions in the bulk of the electrode [18, 19]. This indicates that the ionic conductivity of the La2-xSrxNiO4-δ materials find its maximum for the compound with x = 0.25 as the magnitude of the medium frequency arc is lowest for this compound. When x is increased the amount of interstitial oxygen decreases and the compound with x = 0.25 has almost no interstitial oxygen [22]. This might lead to a high mobility of oxide ions in these layers. At x = 0.35 no more interstitial oxygen is present, and the conduction of oxygen must proceed via oxide ion vacancies in the structure. For the compound with x = 0.35 only a low amount of oxide ion vacancies will be present in the structure leading to a low oxide ionic conductivity. The low frequency arc is for perovskites normally attributed to a slow redox reaction at the surface of the electrode [18, 19]. This arc decreases when x is increased from 0.00 to 0.05, and it is of approximately the same magnitude as the compound with x = 0.15. When x is further increased the magnitude of the low frequency arc increases. This indicates that the redox reaction occurs most easily on the cathodes with x = 0.05 and 0.15. This could be due to a high oxide ion conductivity of these materials. However, it does not explain why the magnitude of this arc increases when going from x = 0.15 to 0.25, where the oxide ion conductivity also increases. This indicates that another parameter plays a role in the  7bond breaking of oxygen. This parameter could be the amount of Ni(II) as the amount of Ni(II) decreases with increasing x. It should be mentioned that the electronic conductivity also depends on the amount of strontium as it decreases with increasing x [11]. A lowered electronic conductivity might slow down the redox reaction. However, this can not explain why the intermediate materials are more active than the strontium poor materials, and it suggests that the electronic conductivity is of less importance for the redox reaction on this type of materials. The n values are strongly dependent on the composition of the electrode material. However, this might reflect the quality of the EIS data. The near-equivalent capacity of this arc can be calculated as for the high frequency arc. The values are given in Table 3 for a temperature of 600oC. The values all fall into the range depicted for a chemical capacitance in [23].   Conclusion: The La2-xSrxNiO4+δ materials have a performance equal to good perovskite SOFC cathodes. The ASR is mainly determined by the oxide ionic conductivity of the materials. The intermediate compound La1.75Sr0.25NiO4+δ has the lowest ASR at 600oC.  Acknowledgements: Colleagues at the Fuel Cells and Solid State Chemistry Division are thanked for fruitful discussions. Financial support from Energinet.dk through PSO-R&D-project no. 2006-1-6493 is gratefully acknowledged.  References:  [1]  N.Q. Minh, T. Takahashi, ‘Science and technology of ceramic fuel cells’, Elsevier Science B.V. (1995) [2] N.Q. Minh, J. Am. Ceram. Soc., 76 (1993) 563 [3]  J.M Ralph, C. Rossignol, R. Kumar, J. Electrochem. Soc., 150 (2003) A1518  8[4] G. Amow, S.J. Skinner, J. Solid State Electrochem., 10 (2006) 538 [5] D. Pérez-Coll, A. Aguardo, M.J. Escudero, P. Núñez, L. Daza, J. Power Sources, 178 (2008) 151 [6] P. Fabry, M Kleitz, Electroanal. Chem. Interfac., Electrochem., 57 165 (1974) [7]  L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas, G.J. Exarhos, Matt. Lett., 10 (1990) 6 [8] M. Juhl, M. Mogensen, T. Jacobsen, B. Zachau-Christiansen, N. Thorup, E. Skou, in Solid Oxide Fuel Cells IV, PV-95-1, p. 554, the Electrochemical Society Proceedings Series, Pennington, NJ (1995) [9] B.A. Boukamp, Solid State Ionics, 20 (1986) 31 [10] J. Newman, J. Electrochem. Soc., 113 (1996) 501 [11] Vibe L.E. Simonsen, Linda Nørskov, Anke Hagen, K. Kammer Hansen, accepted for publication in J. Solid State Electrochem. [12]  S.-I. Hashimoto, K. Kammer, P.H. Larsen, F.W. Poulsen, M. Mogensen, Solid State Ionics, 176 (2005) 1013 [13]  S.-I. Hashimoto, K. Kammer, F.W. Poulsen, M. Mogensen, J. Alloys and Compounds, 428 (2007) 256 [14]  K. Kammer, L. Mikkelsen, J.B. Bilde-Sørensen, J. Solid State Electrochem., 10 (2006) 934 [15] K.K. Hansen, K.V. Hansen, Solid State Ionics, 178 (2007) 1379 [16] K. Kammer, Solid State Ionics, 177 (2006) 1047 [17] E. Perry Murray, M. J. Sever, S. A. Barnett, Solid State Ionics, 148 (2002) 27 [18] E. Siebert, A. Hammouche, M. Kleitz, Electrochimica Acta, 40 (1995) 1741 [19] L.O. Jerdal, ‘The Oxygen Electrode Process in Solid Oxide Fuel Cells’ Ph.D. thesis NTNU, Norway (1998)  9[20] T. Jacobsen, B. Zachau-Christiansen, L. Bay, S. Skaarup, in Proceedings of the 17th International Symposium on Materials Science; High Temperature Electrochemistry: Ceramics and Metals, ed. F.W. Poulsen et al., Roskilde, Denmark, pp. 29 (1996) [21] N.L. Robertson, J.N. Michaels, J. Electrochem. Soc., 138 (1991) 1494 [22] X. Granados, J. Fontcuberta, M. Vallet-Regi, M.J. Sayagués, J.M Gonzáles-Calbet, J. Solid State Chem., 102 (1992) 455 [23] S.B. Adler, J.A. Lane, B.C.H. Steele, J. Electrochem. Soc., 143 (1996) 3554  10Table captions:  Table 1. Total ASR values for the five nickelates at 600, 700 and 800oC in air.  Table 2. ASR and n values of the individual arcs for the five nickelates at 600oC in air. High is high frequency arc, medium is medium frequency arc and low is low frequency arc.  Table 3. Chemical capacitance of the five nickelates at a temperature of 600oC.  11Table 1. Temperature x = 0.00 x = 0.05 x = 0.15 x = 0.25 x = 0.35 600oC 54.4 Ωcm2 24.9 Ωcm2 24.8 Ωcm2 23.8 Ωcm2 63.4 Ωcm2 700oC 7.19 Ωcm2 3.19 Ωcm2 3.69 Ωcm2 3.89 Ωcm2 9.51 Ωcm2 800oC 1.78 Ωcm2 0.65 Ωcm2 0.89 Ωcm2 1.15 Ωcm2 2.51 Ωcm2  Table 2.   High Medium Low Compound ASR n ASR n ASR n x = 0.00 3.76 Ωcm2 0.77 33.0 Ωcm2 0.24 17.6 Ωcm2 0.72 x = 0.05 6.53 Ωcm2 0.80 6.97 Ωcm2 0.39 11.4 Ωcm2 0.70 x = 0.15 7.53 Ωcm2 0.91 5.33 Ωcm2 0.81 11.9 Ωcm2 0.42 x = 0.25 3.42 Ωcm2 0.81 - - 20.4 Ωcm2 0.50 x = 0.35 9.74 Ωcm2 0.84 32.5 Ωcm2 0.49 21.0 Ωcm2 0.64   Table 3.  compound x = 0.00 x = 0.05 x = 0.15 x = 0.25 x = 0.35 Cchem 5.34 mFcm-2 130 mFcm-2 18.0 mFcm-2 1.02 mFcm-2 43.0 mFcm-2 12Figure captions:  Figure 1. EIS spectrum of a La1.95Sr0.05NiO4 cone-shaped electrode measured in air at 600oC. Circles are measured data. Solid line is fitted data. The EIS data were fitted with the circuit Rs(R1Q1)(R2Q2)(R3Q3).  Figure 2. The variation of the total ASR with x at 600oC in air.  13Figure 1. Zreal / Ωcm20 5 10 15 20 25-Zimag / Ω cm201234563.1 KHz 0.16 Hz 14Figure 2. x in La2-xSrxNiO4+δ0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40ASR / Ω cm2203040506070600oC

An EIS study of La2-aEuro parts per thousand x Sr (x) NiO4+ (delta) SOFC cathodes

http://orbit.dtu.dk/ws/files/4331998/Kammer_hansen_Ionics.pdf

An EIS study of La2-aEuro parts per thousand x Sr (x) NiO4+ (delta) SOFC cathodes

Abstract

Similar works

Full text

Available Versions

Crossref

Online Research Database In Technology