Electrochemical performance of nanocrystalline nickel/gadolinia-doped ceria thin film anodes for solid oxide fuel cells

Transcription

1 Available online at Solid State Ionics 178 (2008) Electrochemical performance of nanocrystalline nickel/gadolinia-doped ceria thin film anodes for solid oxide fuel cells Ulrich P. Muecke a,, Kojiro Akiba b,1, Anna Infortuna a,2, Tomas Salkus c,3, Nataliya V. Stus d,4, Ludwig J. Gauckler a,5 a ETH Zurich, Department of Materials, Nonmetallic Inorganic Materials, Wolfgang-Pauli-Str. 10, CH-8093 Zurich, Switzerland b Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, S8-11, , O-okayama, Meguro-ku, Tokyo, , Japan c Faculty of Physics, Vilnius University, Sauletekio al. 9/3, LT Vilnius, Lithuania d Department of Chemistry, Kyiv National Taras Shevchenko University, 64, Volodymyrska str., UA-01033, Kyiv, Ukraine Received 13 April 2007; received in revised form 10 September 2007; accepted 2 October 2007 Abstract Nickel oxide/ce 0.8 Gd 0.2 O 1.9 x (NiO/CGO) films were deposited by spray pyrolysis and pulsed laser deposition on polished CGO electrolyte pellets. The thicknesses of the as-deposited films were nm. The sprayed films showed a homogeneously distributed nano-grain sized microstructure after annealing in air whereas the PLD films exhibited a texture with elongated columnar grains oriented perpendicular to the substrate surface. The electrochemical performance of the Ni/CGO cermet thin film anodes was measured in a symmetrical anode/electrolyte/anode configuration in a single gas atmosphere setup by impedance spectroscopy. The polarization resistance of 60/40 vol.% Ni/CGO spray pyrolysed films decreased with decreasing grain size and was 1.73 and 0.34 Ω cm 2 for grain sizes of 53 and 16 nm, respectively, at 600 C in 3% humidified 1:4 H 2 :N 2. The activation energy was 1.45 and 1.44 ev in the temperature range of C. The performance of the 49/51 vol.% Ni/CGO PLD cermet film was comparable to the spray pyrolysis films and was 0.68 Ω cm 2 at 600 C with an activation energy of 1.46 ev. The electrochemical performance was similar to stateof-the-art thick film anodes and the Ni/CGO thin film cermets are promising candidates as electrodes in micro solid oxide fuel cells Elsevier B.V. All rights reserved. Keywords: Nickel; Ceria; Anode; Nanocrystalline; SOFC; Impedance spectroscopy; Polarization 1. Introduction Corresponding author. Tel./fax: addresses: ulrich.muecke@mat.ethz.ch (U.P. Muecke), a.akiba@mtl.titech.ac.jp (K. Akiba), anna.infortuna@mat.ethz.ch (A. Infortuna), tomas.salkus@ff.vu.lt (T. Salkus), n_stus@univ.kiev.ua (N.V. Stus), ludwig.gauckler@mat.ethz.ch (L.J. Gauckler). 1 Tel./fax: Tel.: ; fax: Tel.: ; fax: Tel.: Tel.: ; fax: Micro solid oxide fuel cells (μ-sofcs) operating at temperatures of C are currently under investigation as a battery replacement for portable electronic devices [1 3]. These devices consist of a fuel cell membrane supported on a micromachinable substrate material. The thickness of the anode/electrolyte/cathode membrane is in the micron range, requiring thin film preparation methods for the electrodes and the electrolyte with layer thicknesses of not more than several hundred nanometers each. The advantages expected from this design are low ohmic resistances in the cell, fast start-up times and reduction of the operating temperature from C of state-of-the-art SOFC systems to C. Thin film electrodes are also indirectly used in state-of-the-art SOFCs whenever anode [4] or cathode [5] functional layers are used [6]. Sol gel [7,8], sputtering [9,10], spray pyrolysis [11 16] or pulsed laser deposition [17,18] can be used to prepare electrode or electrolyte thin films. Thin film electrolytes [19,20] are intensively studied for the use in anode supported SOFC systems or for micro SOFCs [21,22]. Thin film cathodes were studied as model electrodes with defined geometry [23], as dense interlayer to modify the /$ - see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.ssi

2 U.P. Muecke et al. / Solid State Ionics 178 (2008) electrolyte cathode interface [24] or as thin film electrode for miniaturized SOFCs [15,25,26]. Thin film Ni cermet anodes were less studied compared to thin film cathodes and electrolytes. Porous Pt films were used as anode and cathode in miniaturized cells by Huang et al. [2]. Sputtered Ni YSZ cermet films were reported for high temperature SOFC application [27] and for substrate supported medium temperature SOFC [28]. For miniaturized SOFCs, the microstructure of sputtered Ni YSZ cermets was investigated by La O [9], butno electrochemical data was given. Little is known if sufficient electrochemical performance can be reached with thin film anodes. In particular, there is no data available on the influence of microstructure and preparation method on the polarization resistance of thin film anodes. Therefore, Ni CGO thin films with grain sizes in the nanometer range were prepared by spray pyrolysis and pulsed laser deposition and the polarization resistance of the electrodes was measured for different grain sizes. 2. Experimental The preparation and electrical conductivity of the spray [16,29,30] and PLD Ni CGO anodes [31] was investigated earlier. All films used in this study were electronically conductive and the nickel particles in the films formed a percolating nickel network. The total conductivity of the sprayed 60/40 Ni/CGO films after reduction was 1000 S/cm at 600 C [16] and the cross-plane ohmic resistance of the anode films was, therefore, negligible. Symmetric anode/electrolyte/anode cells were prepared to electrochemically characterize the anode films. The cells were tested in a single gas atmosphere setup and the polarization resistance was measured by impedance spectroscopy Electrolyte substrates Electrolyte pellets were fabricated from a Ce 0.8 Gd 0.2 O 1.9 x (99.9%, Praxair, Woodinville, WA, USA) by uniaxially pressing with 28 MPa for 2 min, isostatically pressing with 280 MPa for 3 min and sintering in air at 1600 C for 4 h with heating and cooling rates of 3 C/min up and 5 C/min down. The density of the CGO pellets exceeded 95% of the theoretical density (ρ theoretical =7.29 g/cm 3 ). YSZ (8 mol% yttria doped, TZ-8Y, Tosoh, JP) pellets were prepared the same way except that the sintering temperature was 1500 C and the dwell time 2 h. The density exceeded 99% of the theoretical value (ρ theoretical = 5.85 g/cm 3 ). All pellets were polished on both sides to a surface roughness of r A =20 Å, measured over a scan length of 50 μm. The diameters of the pellets were mm and the thickness of the CGO pellets was 2 and that of the YSZ was 3 mm after polishing. The substrates were cleaned with ethanol prior to thin film deposition Thin film preparation Spray pyrolysis of NiO CGO films A detailed description of the spray pyrolysis setup used to deposit the NiO Ce 0.8 Gd 0.2 O 1.9 x anode thin film was given earlier [16]. The working distance was 39 cm, the air pressure 1.0 bar, the precursor flow rate 2.5 ml/h and the substrate surface temperature 430 C in this study. The deposition time was varied between 60 and 90 min, resulting in films with thicknesses of ± 100 nm. The precursor was prepared by dissolving nickel-(ii)-nitrate hexahydrate (98% purity, Fluka, Buchs, CH), cerium-(iii)-nitrate hexahydrate (99.5%, Alfa Aesar, Karlsruhe, DE) and gadolinium- (III)-chloride hexahydrate (99.9%, Alfa Aesar) with the corresponding stoichiometry in a mixture of 10: 90 vol.% ethanol (Scharlau, Barcelona, ES): tetraethylene glycol (Aldrich, Steinheim, DE). The salts were first completely dissolved in ethanol and tetraethylene glycol was added afterwards. The crystal water content of the salts was verified by thermogravimetry before weighing. The total salt concentration was 0.1 mol/l. The compositions of the films were chosen to result in a Ni/ CGO volumetric ratio of 40/60 and 60/40 after reduction, corresponding to a NiO/CGO weight ratio of 70.2/29.8 and 86.3/13.7, respectively, in the oxidized state. The correct film composition was verified in an earlier study [29]. The anode films with an area of mm 2 were sprayed symmetrically through a 0.1 mm thick Mo mask on both sides of a CGO or YSZ pellet to form a cell. The procedure and geometrical details are outlined in [25]. The a films were X-ray amorphous after deposition and were crystallized by annealing the samples in air at 800 or 1000 C for 10 h with a heating and cooling rate of 3 C/min. The dwell time of 10 h was chosen to establish a stable grain size and to fully crystallize the films [22]. The sprayed films were dense after annealing at 800 or 1000 C in air and porosity was introduced during reduction of NiO to Ni upon the first exposure to a reducing atmosphere, resulting in a (calculated) film composition of 22 vol.% pores, 31 vol.% Ni and 47 vol.% CGO for the 40/60 Ni/CGO and 30 vol.% pores, 42 vol.% Ni and 28 vol.% CGO for the 60/40 Ni/CGO samples Pulsed laser deposition (PLD) of NiO CGO films The PLD target was prepared the same way as the electrolyte pellet except that a 60/40 wt.% NiO/Ce 0.8 Gd 0.2 O 1.9 x (49/ 51 vol.% Ni/CGO) powder (99.9%, Praxair) was used. The target was sintered in air at 1300 C for 4 h with 3 C heating and5 Ccoolingrate. The PLD NiO CGO films were deposited at a target to substrate distance of 55 mm, an oxygen partial pressure of 200 mtorr and a substrate temperature of 500 C. 150,000 pulses of a 248 nm eximer laser with 5 J/cm 2 fluence at the target surface were used to deposit 800 nm thick films (PLD workstation, Surface, Hueckelhoven, DE). The cell geometry was the same as for the sprayed films. The anodes were annealed in air at 1000 C for 10 h with 3 C/min heating and cooling rate after deposition, resulting in a dense NiO/CGO film Contacting of the anodes The deposited anodes were electrically contacted by sputtering a30 50 nm Pt or Au film (SDC050, Baltec, Balzers, LI, 180 s, 45 ma, mbar Ar, 5 cm working distance) on the films. A Pt or Au mesh with two Pt or Au wires for current and voltage was

3 1764 U.P. Muecke et al. / Solid State Ionics 178 (2008) attached on the sputtered film with Pt (C3605 P, Heraeus, Hanau, DE) or Au paste (C5754 B, Heraeus). The wires were fixed to the electrolyte pellet outside the electrode area with non-conductive ceramic glue (Feuerfestkitt, Firag AG, Ebmatingen, CH). The temperature of the pellet was measured with a selfmade type S thermocouple (Pt-90 wt.%pt/10 wt.% Rh, Johnson Matthey) attached to the electrolyte pellet with ceramic glue (Firag) Electrochemical characterization by impedance spectroscopy After contacting, the samples were placed in an atmospherically sealed furnace for impedance measurements and heated to 650 C in air to burn out the Pt or Au paste binder and to stabilize the microstructure of the current collectors. The heating rate was 3 C/min and the gas flow rate 60 sccm air. All gas flows were controlled by mass flow controllers (El-Flow, Bronkhorst, Reinach, CH). After 1 h in air, the oven chamber was purged with 500 sccm dry nitrogen for min and hydrogen was added. The hydrogen to nitrogen ratio was 1:4 and the gas was humidified with 3 vol.% water by reacting air with hydrogen inside the oven over a Pt mesh. The total gas flow rate including water vapor was 500 sccm. After 1 h under reducing conditions, impedance spectra of the cells were recorded (SI SI1287, Solartron, GB) in the frequency range from 100 khz to 1 Hz with 10 steps per decade, an ac amplitude of 20 mv and no bias. The samples were then cooled down under reducing conditions with 3 C/min and electrochemically characterized in 50 C steps down to 400 C. The data was analyzed using the ZView software (Scribner Associates, Southern Pines, NC, US). The oxygen partial pressure in the oven chamber was monitored by measuring the potential across a Pt/YSZ/Pt oxygen sensor (Viking Chemicals, Føllenslev, DK). Fig. 1 shows the typical impedance spectra of a symmetrical Ni CGO/CGO/Ni CGO cell at different temperatures. The cell was annealed at 800 C and the Ni/CGO ratio was 60/40 (see below for details). A low frequency arc and the beginning of a high frequency arc were observed for all cells. The high frequency part of the spectrum was attributed to the electrolyte because its resistance was constant between samples, depended on the electrolyte material, scaled linearly with the electrolyte thickness and was independent of dc bias. The measured area specific electrolyte resistance of 3.8 Ω cm 2 at 600 C results, together with the electrolyte thickness of 2 mm, in an electrolyte conductivity of 5.3 S/m. This value is in good agreement with published conductivity values of Ce 0.8 Gd 0.2 O 1.9 x of 1.8 S/m [32] and 3.6 S/m [33]. The low frequency feature was attributed to the electrodes and the polarization resistance of both electrodes was calculated as the difference between low frequency intercept of the impedance curve with the real axis and the electrolyte resistance. The polarization resistance of one electrode then corresponds to one half of the measured polarization resistance because both electrodes were prepared identically. All polarization resistances in the following are given for one electrode (except for the spectra given in Fig. 1). The area specific resistance (ASR) was Fig. 1. Typical impedance spectra of a symmetric anode/cgo/anode cell between 450 and 600 C. The thin film anode had a Ni/CGO volumetric ratio of 60/40 and an average grain size of 16 nm. The numbers mark the peak frequencies in Hz. obtained from the polarization resistance by multiplying with the anode area Structural characterization The surface morphologies and compositions of the films were analyzed with a scanning electron microscope (LEO 1530, Carl Zeiss SMT, Oberkochen, DE). Film thickness and surface roughness were measured with a surface profiler (Alpha Step 500, KLA Tencor, San Jose, CA, USA). 3. Results and discussion The polarization resistance of thin film anode cermets was measured as a function of current collector, electrolyte material, grain size, composition and preparation method Current collector Sputtered platinum and Pt paste were used as current collector for the cells. Platinum itself is an excellent electrocatalyst for the

4 U.P. Muecke et al. / Solid State Ionics 178 (2008) hydrogen oxidation and yields low polarization resistances when used as SOFC anode material [34]. When used as a current collector on a thin film electrode, the Pt/thin film interface and not the film itself might be the electrochemically active regions. A Pt/CGO/Pt cell without sprayed anode was, therefore, prepared to compare the polarization resistance of the Pt current collector alone to that of a sprayed anode with the Pt current collector on top. The Pt only cell consisted of sputtered Pt and Pt paste on each side and was prepared identically as the cells with sprayed anodes except that the sprayed anode was left out. The cells were measured between 450 and 600 C at an oxygen partial pressure of bar (details see below) at 600 C. The polarization resistance of the Pt/CGO interface was approximately twice that of the 60/40 Ni/CGO sprayed anode in the temperature range C (both shown in Fig. 2). At 600 C, the ASR of the Pt current collector was 0.65 Ω cm 2 with an activation energy of 1.47 ev and of the Ni/CGO anode 0.34 Ω cm 2 with an activation energy of 1.44 ev. As the polarization resistance of the cells with Pt current collector was in the order of magnitude of the current collector alone, the correctness of the data was additionally verified by replacing the Pt current collector with an Au current collector in a 60/40 Ni/CGO cell. Gold is known to have a poor electrochemical activity towards hydrogen oxidation and polarization resistances are large compared to Pt [34,35]. No difference was found between the polarization resistances of Ni/CGO anodes with Pt or Au current collector within experimental errors. It can, therefore, be concluded that the measured ASR with a Pt current collector corresponds to the real polarization resistance of the sprayed Ni/ CGO anode and Pt was used as current collector material for all cells. The use of gold current collectors was abandoned due to its poor adhesion to the sprayed anodes Electrolyte material Fig. 2. Polarization resistances of a sputtered Pt/Pt paste electrode compared to a 60/40 Ni/CGO thin film anode with sputtered Pt/Pt paste current collectors. Fig. 3. Polarization resistance of 60/40 Ni/CGO anode thin films on CGO and YSZ electrolytes compared to literature values of Ni/Gd- or Sm-doped ceria anodes on CGO and YSZ [38 40]. The oxygen partial pressure inside the oven chamber was measured with a Pt/YSZ/Pt oxygen sensor during cell characterization. The voltage across the sensor was 1.06 V, close to the theoretical value of V at 600 C [36]. This potential corresponds to an oxygen partial pressure p O2 of bar according to the Nernst equation. The electrolytic domain boundary of micron sized CGO at 600 C is at p O bar [37] and some electronic conductivity of the CGO electrolyte pellets will, therefore, be present. The electronic resistance acts like an electronic resistor in parallel to the anode/electrolyte/anode cell, resulting in smaller apparent polarization resistances measured for the anodes [38,39]. To assess the influence of the electronic conductivity of the CGO on the measured polarization resistances, 60/40 Ni/CGO anodes sprayed on CGO pellets and on YSZ pellets were measured in the temperature range of C under identical conditions. All films were annealed at 1000 C before characterization. YSZ is a purely ionic conductor at an oxygen partial pressure of bar at 600 C and the measured impedance spectra of the anodes on YSZ should not contain any electronic contribution from the electrolyte. Fig. 3 shows the polarization resistances of the Ni/CGO thin films on the different electrolytes. The polarization resistances of the films on CGO and on YSZ were in the same order of magnitude with 1.73 Ω cm 2 and 7.2 Ω cm 2 at 600 C and 101 and 61.5 at 400 C, respectively. However, the activation energy changed from 1.45 ev for the anode on CGO to 0.77 evon YSZ and the polarization curves are crossing each other at 480 C. The activation energies agree well with reported literature values of thick-film Ni/doped ceria anodes prepared by powder sintering on different electrolytes (Fig. 3). Jörger [40] reported 0.9 ev for 60/40 vol.% Ni/CGO anodes on YSZ and Primdahl et al. [41] 0.91 ev for 2/98 vol.% Ni/CGO on YSZ. The use of different electrolyte materials, therefore, changes the apparent activation energies of the anodes. As thin film anodes are likely to be employed in (miniaturized) low-temperature SOFCs with single- or multi-layer ceria/zirconia based electrolytes, all anode thin films were characterized on CGO pellets. This also avoided possible interfacial reactions between

5 1766 U.P. Muecke et al. / Solid State Ionics 178 (2008) YSZ and CGO [42] that can become important if the total electrode thickness is in the nanometer range. However, to avoid misinterpretation of the experimental results due to the electronic conductance of the electrolyte, all films were characterized under identical oxygen partial pressures on CGO pellets with identical thicknesses. The results can then be compared relative to each other without error. Comparing the polarization resistance of thin film electrodes on YSZ to literature values of thick film anodes [40,41,43 47], the thin film electrodes show a comparable performance. The polarization resistances of nm thin film anodes on YSZ are approximately half an order of magnitude larger (7.2 Ω cm 2 compared to 1.1 Ω cm 2 [40] at 600 C) than thick film electrodes with the same composition and thicknesses in excess of 10 μm. State-of-the-art full cells exhibit total interfacial polarization resistances from anode and cathode of 0.2 Ω cm 2 [48]. However, the electrochemical activity of thin film electrodes can be further enhanced by decreasing the grain size Grain size The electrochemical activity of a cermet anode with given microstructure and materials depends on the volume and on the grain size [49]. Smaller grains increase the triple phase boundary length and, as a result, the polarization resistance decreases. The thickness and, therefore, volume of thin films is usually limited by the preparationprocessandtheonlywayto lower the polarization resistance is by decreasing the grain size. Ni CGO cermets with different grain sizes were, therefore, studied to investigate the influence of grain size on the polarization resistance. In a previous study [29], the average grain size of sprayed NiO CGO thin films in the oxidized state was measured as a function of annealing temperature in air. Grain sizes of 16 and 53 nm were found for annealing temperature of 800 and 1000 C for 10 h, respectively. No difference in grain size between the NiO and CGO grains was found. Reduction of the films at 600 C results in a stable biphasic Ni/CGO structure with a percolating Ni phase and a stable CGO framework. Excessive Ni coarsening was observed for smaller grains. Samples with 16 and 53 nm were, therefore, used to measure the polarization resistances as a function of grain size. The Ni/CGO ratio was 60/ 40, the film thickness of the 16 nm sample 500±100 nm and of the 53 nm sample 800±100 nm and the electrolyte material CGO. The oxygen partial pressure was bar (1.06 V). Fig. 4 shows SEM cross section micrographs of the 16 and 53 nm grain size anodes after testing. The films adhered well to the electrolyte surface with the ceramic grains of the 16 nm sample being well attached to the electrolyte surface, even after annealing at 800 C only. Sinter necks between ceramic grains of the film and the electrolyte pellet are clearly visible for the 53 nm grain size sample which was annealed at 1000 C. The ceramic grains within the volume of the films were well connected and formed a stable network. No signs of nickel agglomeration were found within the film or on the film surface. The polarization resistance of the 53 nm grain size anode was 1.73 Ω cm 2 at 600 C with an activation energy of 1.45 ev (Fig. 5) and decreased by half an order of magnitude to 0.34 Ω cm 2 at 600 C for the 16 nm anode (Figs. 1 and 5). The activation energy remained unchanged at 1.44 ev. These values are roughly the same as those obtained from state-of-the-art thick film anodes. It can be concluded that the electrochemical activity of thin film electrodes scales with the grain size and that small grains provides a means to lower the polarization resistance. From a practical application point of view, an optimum between maximized electrochemical activity and a long-term stable cermet microstructure exists. The polarization resistance decreases with decreasing grain size. However, the tendency towards detrimental nickel grain coarsening increases with Fig. 4. SEM cross-section micrographs of 60/40 Ni/CGO anodes with a,c) 16 nm and b) 53 nm grain size after testing.

6 U.P. Muecke et al. / Solid State Ionics 178 (2008) Fig. 5. Polarization resistances of 60/40 Ni/CGO anodes with 16 and 53 nm grain size. decreasing grain size [29] and a compromise has to be made between activity and thermal stability Composition The percolation limit of metallic conductivity in Ni/CGO cermets is a function of film composition and, uniquely for thin films, of the grain size to film thickness ratio. If the average grain size of Ni and CGO particles is in the range of the film thickness, the percolation limit approaches the values of two dimensional percolation and 60 vol.% of Ni are necessary for an in-plane percolating Ni phase. However, if the film thickness is larger than the average grain size, the films are comparable to a three dimensional structure and 40 vol.% of Ni are sufficient for metallic percolation [29]. Films with 40/60 and 60/40 Ni/CGO were, therefore, compared to investigate if a reduction of the Ni volume fraction yields an increase in electrochemical performance of the anodes. The samples were annealed at 1000 C for 10 h in air prior to testing. The oxygen partial pressure was bar (1.09 V) at 600 C. The polarization resistances of the samples were, within experimental error, the same and it can be concluded that the Ni to CGO ratio of the anode film within the compositional range studied here has a negligible effect on the performance. Fig. 6. SEM cross-section micrographs of a PLD anode film a) in the oxidized state after annealing at 1000 C and b) after testing under reducing conditions. The electrochemical performance of the PLD electrode was slightly better than that of a sprayed electrode annealed at the same temperature of 1000 C. The polarization resistances were 1.73 and 0.68 Ω cm 2 and the activation energies 1.45 and 1.46 ev for the spray pyrolysis and PLD anode, respectively, at 600 C (Fig. 7) PLD anode NiO CGO films were also prepared by PLD in order to compare their performance to that of sprayed anodes. After annealing in air at 1000 C, the films were dense with a texture of columnar grains perpendicular to the substrate surface (Fig. 6a). However, each column was composed of both NiO and CGO grains and after reduction the microstructure was more isotropic ( Fig. 6b). The microstructure of the PLD electrode after testing was coarser than the sprayed anode (Fig. 4b) and larger pores can be distinguished in the SEM micrographs. The interface between the anode and the electrolyte was well established and the current collector adhered well to the electrode surface. Fig. 7. Polarization resistances of a PLD 49/51 Ni/CGO anode compared to a sprayed 60/40 Ni/CGO anode.

7 1768 U.P. Muecke et al. / Solid State Ionics 178 (2008) PLD anodes, therefore, present an alternative to sprayed thin films. The microstructures of the sprayed and PLD anodes were quite similar, which also resulted in comparable polarization resistances. 4. Summary and conclusion Ni/CGO cermet thin film anodes for the application in miniaturized solid oxide fuel cells were prepared by spray pyrolysis and pulsed laser deposition. The electrochemical performance of the films was evaluated by impedance spectroscopy on symmetrical pellet cells in a single gas atmosphere setup. The particle and pore distribution of the sprayed electrodes was homogeneous after reduction and the electrodes adhered well to the electrolyte. The PLD anodes were columnar after annealing and became more homogeneous after reduction with a larger porosity than the sprayed anodes. The polarization resistance of 60/40 vol.% Ni/CGO sprayed anodes decreased with decreasing grain size from 1.73 to 0.34 Ω cm 2 for grain sizes of 53 and 16 nm, respectively, at 600 C. The electrode performance was independent of composition for sprayed anodes for Ni to CGO ratios of 60/40 and 40/60. The polarization resistance of a 49/51 vol.% Ni/CGO PLD anode was 0.68 Ω cm 2 at 600 C. It can be concluded that the electrode performance is mainly determined by the grain size and microstructure of the cermets. The polarization resistance of nano-grained thin film electrodes reached those of state-of-the-art thick film cermet anodes. The loss of electrochemically active volume during the reduction of the thickness from the micron- to the nanometer range can be compensated by decreasing the grain size. The low polarization resistances makes these films promising candidates as anodes for miniaturized SOFC in the operating range of C. Acknowledgements Financial support from BFE under project number , from KTI under project number DCPP-NW, from the European Union within the REAL-SOFC project and from ETH Zurich is gratefully acknowledged. N.V.S. is grateful for the financial support of the SNSF under grant no PI0I /1. References [1] A. Bieberle-Hütter, D. Beckel, U.P. Muecke, J.L.M. Rupp, A. Infortuna, L.J. Gauckler, mstnews 4-05 (2005) 12. [2] H. Huang, M. Nakamura, P. Su, R. Fasching, Y. Saito, F.B. Prinz, J. Electrochem. Soc. 154 (1) (2007) B20. [3] U.P. Muecke, D. Beckel, A. Bieberle-Hütter, S. Graf, A. Infortuna, J.L.M. Rupp, J. Schneider, P. Müller, A. Bernard, L.J. Gauckler, submitted to Advanced Functional Materials (submitted for publication). [4] A.A.E. Hassan, N.H. Menzler, G. Blass, M.E. Ali, H.P. Buchkremer, D. Stöver, Adv. Eng. Mater. 4 (3) (2002) 125. [5] V.A.C. Haanappel, J. Mertens, D. Rutenbeck, C. Tropartz, W. Herzhof, D. Sebold, F. Tietz, J. Power Sources 141 (2) (2005) 216. [6] D. Stover, H.P. Buchkremer, S. Uhlenbruck, Ceram. Int. 30 (7) (2004) [7] M. Gaudon, C. Laberty-Robert, F. Ansart, L. Dessemond, P. Stevens, J. Power Sources 133 (2) (2004) 214. [8] P.G. Keech, D.E. Trifan, V.I. Birss, J. Electrochem. Soc. 152 (3) (2005) A645. [9] G.J.O. La, J. Hertz, H. Tuller, Y. Shao-Horn, J. Electroceram. 13 (1 3) (2004) 691. [10] L.S. Wang, S.A. Barnett, J. Electrochem. Soc. 139 (4) (1992) [11] D. Perednis, L.J. Gauckler, Solid State Ion. 166 (3 4) (2004) 229. [12] D. Perednis, O. Wilhelm, S.E. Pratsinis, L.J. Gauckler, Thin Solid Films 474 (1 2) (2005) 84. [13] O. Wilhelm, S.E. Pratsinis, D. Perednis, L.J. Gauckler, Thin Solid Films 479 (1 2) (2005) 121. [14] D. Perednis, L.J. Gauckler, J. Electroceram. 14 (2) (2005) 103. [15] D. Beckel, A. Dubach, A.R. Studart, L.J. Gauckler, J. Electroceram. 16 (3) (2006) 221. [16] U.P. Muecke, N. Lüchinger, L. Schlagenhauf, L.J. Gauckler, Thin Solid Films (in press). [17] L.R. Pederson, P. Singh, X.D. Zhou, Vacuum 80 (10) (2006) [18] J. Jong Hoon, C. Gyeong Man, Solid State Ion. 177 (11 12) (2006) [19] L.C. De Jonghe, C.P. Jacobson, S.J. Visco, Ann. Rev. Mater. Res. 33 (2003) 169. [20] J. Will, A. Mitterdorfer, C. Kleinlogel, D. Perednis, L.J. Gauckler, Solid State Ion. 131 (1 2) (2000) 79. [21] J.L.M. Rupp, L.J. Gauckler, Solid State Ion. 177 (26 32) (2006) [22] J.L.M. Rupp, A. Infortuna, L.J. Gauckler, Acta Mater. 54 (2006) [23] J. Fleig, F.S. Baumann, V. Brichzin, H.R. Kim, J. Jamnik, G. Cristiani, H.U. Habermeier, J. Maier, Fuel Cells 6 (3 4) (2006) 284. [24] J.M. Bae, B.C.H. Steele, Solid State Ion. 106 (3 4) (1998) 247. [25] D. Beckel, U.P. Muecke, T. Gyger, G. Florey, A. Infortuna, L.J. Gauckler, Solid State Ion. 178 (2007) 407. [26] J.L. Hertz, H.L. Tuller, J. Electrochem. Soc. 154 (4) (2007) B413. [27] K. Hayashi, O. Yamamoto, Y. Nishigaki, H. Minoura, Denki Kagaku 64 (10) (1996) [28] L.S. Wang, S.A. Barnett, Solid State Ion. 61 (4) (1993) 273. [29] U.P. Muecke, S. Graf, U. Rhyner, L.J. Gauckler, Acta Mater (in press), doi: /j.actamat [30] U.P. Muecke, G.L. Messing, L.J. Gauckler, Thin Solid Films (in press). [31] A. Infortuna, U.P. Muecke, A. Harvey, L.J. Gauckler, to be submitted (submitted for publication). [32] B.C.H. Steele, Solid State Ion. 129 (1 4) (2000) 95. [33] D.K. Hohnke, Solid State Ion. 5 (1981) 531. [34] R. Baker, J. Guindet, M. Kleitz, J. Electrochem. Soc. 144 (7) (1997) [35] C. Lu, W.L. Worrell, J.M. Vohs, R.J. Gorte, J. Electrochem. Soc. 150 (10) (2003) A1357. [36] S.W. Zha, C.R. Xia, G.Y. Meng, J. Appl. Electrochem. 31 (1) (2001) 93. [37] M. Gödickemeier, Mixed Ionic Electronic Conductors for Solid Oxide Fuel Cells, dissertation ETH vol , Swiss Federal Institute of Technology, Zurich (1996). [38] M. Liu, H. Hu, J. Electrochem. Soc. 143 (6) (1996) L109. [39] M. Liu, A. Joshi, Characterization of mixed ionic-electronic conductors, in: T.A. Ramanarayanan, H.L. Tuller (Eds.), 180th Meeting of the Electrochemical Society, The Electrochemical Society, Phoenix, Arizona, 1991, pp [40] M.B. Jörger, CuO-CGO Anodes for Solid Oxide Fuel Cells, dissertation ETH no , Swiss Federal Institute of Technology, Zurich (2004). [41] S. Primdahl, Y.L. Liu, J. Electrochem. Soc. 149 (11) (2002) A1466. [42] A. Tsoga, A. Naoumidis, A. Gupta, D. Stöver, Mater. Sci. Forum (1999) 794. [43] Z. Xie, W. Zhu, B. Zhu, C. Xia, Electrochim. Acta 51 (15) (2006) [44] S. Suda, M. Itagaki, E. Node, S. Takahashi, M. Kawano, H. Yoshida, T. Inagaki, J. Eur. Ceram. Soc. 26 (4 5) (2006) 593. [45] T. Ishihara, T. Shibayama, H. Nishiguchi, Y. Takita, Solid State Ion. 132 (3 4) (2000) 209. [46] X. Zhang, S. Ohara, R. Maric, K. Mukai, T. Fukui, H. Yoshida, M. Nishimura, T. Inagaki, K. Miura, J. Power Sources 83 (1 2) (1999) 170. [47] S. Wang, M. Ando, T. Ishihara, Y. Takita, Solid State Ion. 174 (1 4) (2004) 49. [48] Q.L. Liu, K.A. Khor, S.H. Chan, J. Power Sources 161 (1) (2006) 123. [49] F. Hiroshi, I. Mitsuhiro, Y. Koichi, Electrochem. Solid-State Lett. 10 (1) (2007) B16.