15.1.1 Materials for thin-film solid oxide fuel cells (SOFCs)

Techniques for the preparation of crystalline oxide thin films using physical and chemical vapor deposition techniques have been developed independently of SOFCs because of their importance in many device applications including, dielectric and ferroelectric materials for memory applications (Martin et al., 2010), passive electrodes (Qin et al., 2009), resistive memory (Lin et al., 2008), magnetoresistance (Liu et al., 2010) and superconducting electronics (Claeson et al., 1999). Most of the properties of these films have been studied at ambient temperature or below. In the last decade, however, more effort has been dedicated to the investigation of the high-temperature properties of oxide thin films where phenomena such as changes in stoichiometry, order–disorder, and ionic conductivity become important. The high-temperature behavior is relevant to the performance of devices which interact chemically with their environment via chemical reaction and ion diffusion – for example, chemical sensors, ion transport membranes and SOFCs. The use of thin films in such devices makes possible the effective elimination of bulk ionic diffusion as a rate-limiting process leading to faster overall kinetics and lower operating temperatures. Lowering the operating temperature to ≤ 500°C is of key importance to the successful development of micro SOFCs. For very thin films or films containing very small grains (nano-dimensioned), grain boundaries become important and the overall kinetics become limited by transport across solid–solid interfaces and by reaction at gas–solid interfaces. The presence of such interfaces can either enhance or degrade the overall performance. A general discussion of the importance of interfaces in the context of nano-ionics can be found in the review articles by Maier (2005, 2009).

In addition to potential applications as components in micro-SOFCs, dense oxide thin-film electrodes are of interest for more fundamental studies of reaction mechanisms. Thin films simplify the electrode architecture and eliminate uncertainties associated with porous microstructures found in bulk electrodes (Adler, 2004). Crystalline thin films also make possible the separation of bulk and grain boundaries and direct determination of surface reaction rates; epitaxial films permit the study of the effects of strain and, for anisotropic materials, of property anisotropy. In the following, some of the recent work on the materials aspects of thin-film electrolytes and electrodes is reviewed.

15.2.1 Yttria-stabilized zirconia

A variety of different deposition techniques have been used to make YSZ thin films including dc and rf reactive sputtering from metal alloy targets (Thiele et al., 1991; Wang et al., 1992; Jankowski et al., 2002; La O’ et al., 2004; Kang et al., 2006; Huang et al., 2007; Rey-Mermet and Muralt, 2008; Johnson et al., 2010; Kerman et al., 2011), pulsed laser deposition (PLD) (Muecke et al., 2008b; Noh et al., 2009b; Tarancon et al., 2009; Johnson et al., 2010) atomic layer deposition (ALD) (Shim et al., 2007; Prinz, 2008; Jee et al., 2010), spray pyrolysis (Wilhelm et al., 2005), chemical vapor deposition (Chun and Mizutani, 2001), electron beam evaporation (Baertsch et al., 2004) and solgel techniques (Kosacki et al., 2000; Peters et al., 2009). A review of thin-film deposition methods is given by Beckel (Beckel et al., 2007).

The method of deposition has a significant effect on the microstructure of the films, as does the nature of the substrate (amorphous, polycrystalline or single crystal) and the substrate temperature used during deposition. For example, PLD on amorphous Si₃N₄ gave a columnar microstructure with 10 nm grains at 400°C (Tarancon et al., 2009), whereas PLD on c-cut sapphire single crystals at 600°C gave polycrystalline films with a columnar micro-structure and some degree of (111) orientation (Heiroth et al., 2008, 2010). Deposition on single-crystal MgO at 500°C gave (100)-oriented epitaxial films with a narrow rocking curves (< 1°) (Kosacki et al., 2005). Physical deposition techniques using heated substrates usually give crystalline material whereas those prepared by chemical deposition are initially amorphous and require further heat treatment to crystallize and control the grain size.

In addition to the synthesis of free-standing membranes for application in devices, many investigations have been made of electrolyte films on dense substrates in order to make more fundamental studies of the effects of micro- and nano-structure, epitaxial strain and impurities on the electronic and ionic conductivity. Specific studies of nanoscale effects on the conductivity of YSZ have been made by a number of authors. Kosacki and co-workers studied highly textured (cube-on-cube) YSZ films deposited on MgO by PLD (Kosacki et al., 2004, 2005). For films < 60 nm thick, an increase in the conductivity was observed and attributed to the increased importance of the interface. Dense YSZ films were also prepared by a polymeric precursor spin coating method with grain sizes of 1–400 nm on single-crystal sapphire and polycrystalline Al₂ O₃ substrates. Nanocrystalline samples were reported to show enhanced electrical conductivity compared with micro-crystalline material (Kosacki et al., 2000). Chao et al. (2009) prepared 8–55 nm thin films of YSZ by atomic layer deposition (ALD) and measured inplane oxide ion conductivities as a function of temperature, film thickness and yttria concentration in the film. Higher conductivities were observed for thinner films which were thought to be due to increased oxide conduction along the surface. Increasing the yttrium concentration at the surface also led to higher conductivity.

Recently, Peters et al. (2009) made a detailed study of grain size effects in YSZ thin films. Films of 8.3 mol% YSZ, 400 nm thick, were prepared on sapphire by a sol–gel method and grain sizes were systematically adjusted to 5 nm ≤ d ≤ 782 nm by subsequent processing. High-resolution electron microscopy was used to show that the grains and grain boundaries were chemical homogeneous without contamination. Oxygen transport was found to be impeded by the grain boundaries. No evidence for an enhanced conductivity in 400 nm thin films with grain sizes in the range 5 nm ≤ d ≤ 36 nm was observed in contrast to the behavior of very thin films where surface conduction plays a large role. A survey of the previous experiments in more detail than given here can be found in Peters et al. (2009). It is worth noting that the calculated depletion width in YSZ due to space charge effects was estimated by Hertz and Tuller (2009) to be only 0.4 nm which is too small to produce significant electronic conductivity, consistent with the experimental data. In general, the variations observed in the properties of thin films with different thickness and grain size illustrate the critical importance of interfaces in nanoscale materials and is an area that needs further study. Some further aspects of interfaces are discussed in Section 15.2.4.

15.2.2 Doped ceria

The electrolytes studied in most detail after YSZ are based on doped ceria (GDC, SDC and YDC). Ceria electrolytes are an attractive alternative to YSZ because of their higher conductivity; at temperatures of ≤ 500°C, the electronic contribution to the conductivity is expected to be small. GDC has been used in some micro-fuel cell devices (Section 15.5) but most thin-film studies have concerned fabrication and properties. Measurements have been made on both highly epitaxial films and on nanocrystalline and polycrystalline films. The latter have been synthesized by physical deposition techniques with amorphous or polycrystalline substrates held at ambient or low temperatures or by chemical methods at ambient temperature followed by annealing at higher temperature to induce crystallization and grain growth.

Highly oriented GDC (Ce₀.₈Gd₀.₂O_{2 – x}) thin films were grown on single-crystal (001) MgO substrates by PLD (Chen et al., 2003). The films were highly c-axis-oriented with cube-on-cube epitaxy despite the extremely large lattice misfit of > 28%. The electrical conductivity is ionic down to pO₂ = 1 × 10⁻¹⁹ atm at 500–800°C. The activation energy changes from 0.86 eV for as-deposited films to 0.74 eV after annealing and the conductivity increases to a value very close to that of bulk grains in polycrystalline material, indicating the absence of any significant contribution from grain boundaries. Single-crystal epitaxial thin films with similar properties were deposited on NdGaO₃ (NGO) and LaAlO₃ (LAO) substrates by PLD. Jacobson and co-workers studied the microstructure of GDC films grown on different substrates including MgO, YSZ, LAO, NGO and SrTiO₃ (STO) with different film-substrate lattice mismatch ratios (Huang et al., 2004a, 2004b, 2005). The highest film crystallinity was obtained on LAO with a small compressive strain. This single-crystal GDC film shows no columnar grain growth but contains directionally aligned Gd-rich nanoparticles which act to relax strain fields induced during the film growth.

Highly crystalline epitaxial films of Sm_0.2Ce_0.8O_1.9 (SDC) films were grown by PLD on MgO single-crystal substrates but with a buffer layer of STO to provide a proper lattice match. At 700°C, SDC/STO films have an ionic conductivity 7 × 10⁻² S cm⁻¹ with an activation energy of 0.74 eV; an electronic contribution becomes apparent at pO₂ = ~ 10⁻¹⁵atm. These values are comparable to those reported for GDC on NdGaO₃. Similar results were recently obtained for polycrystalline GDC prepared by rf sputtering which remains a pure ionic conductor at 700°C down to 10⁻¹⁷ atm (Podpirka and Ramanathan, 2009).

Yttria-doped ceria is the third system that has received attention (Tian and Chan, 1996, 1997, 1998, 1999, 2002). YDC films containing _0.58 and 4 mole% Y₂O₃ were prepared by e-beam evaporation and by PLD and were deposited on different substrates. Both polycrystalline and textured films were obtained with grain sizes ranging from 50 to 500 nm. The ionic conductivities of the films were dominated by grain boundaries.

Nanocrystalline films on sapphire substrates were prepared by spin coating a substrate with polymeric precursor (Suzuki et al., 2002). The grain size of these films depends upon the annealing temperature and the dopant content; higher dopant levels give smaller grains. The ionic conductivity was found to increase as the grain size decreases, and the activation energy for the ion mobility decreases from 1.3 eV for 36 nm grains to 1.0 eV for 9 nm grains. Chiodelli et al. (2005) prepared thin films of GDC, 80–400 nm thick, by off-axis reactive sputtering on polished polycrystalline alumina and quartz substrates which were held at ambient temperature or heated to 300 or 600°C. Samples were annealed at 800°C before conductivity measurements. On quartz the conductivity is comparable to polycrystalline material but is much lower on alumina, apparently because of the substrate roughness.

Rupp and co-workers prepared nanocrystalline films by both spray pyrolysis and PLD (Rupp and Gauckler, 2006). PLD typically gives columnar microstructures whereas spray pyrolysis give more isotropic grains. Spray pyrolysis onto a sapphire single-crystal substrate held at 310°C, followed by annealing at 800–1100°C, was used to obtain different grain sizes in a stable microstructure. The conductivities decreased with increasing grain size as observed previously by Suzuki et al. (2002). Similar results were obtained for samples with comparable grain sizes on sapphire substrates. A comparison of these results with the data of Suzuki et al., is given by Beckel (Beckel et al., 2007). Very high conductivities were observed in ultrathin films of nanocrystalline GDC with thicknesses comparable to the grain size (20–50 nm) prepared by dc reactive sputtering (Huang et al., 2006a). Conductivities were determined to be 3–4 orders of magnitude higher than those of thicker films (> 500 nm). The unusual properties were attributed to the reduction of cross grain boundary resistance and the segregation of the Gd dopant in the vicinity of grain boundaries.

In general, the conductivities for nanocrystalline ceria increase with decreasing grain size and are lower than for epitaxial films which approach the value found for bulk crystalline grains. The electronic contribution to the total conductivity is not known in all cases. Overall, the conductivities reported vary by more than two orders of magnitude (10⁻⁴−10⁻² S cm⁻¹ at 600°C) indicating that further work is needed to understand the effect of the deposition method on microstructure, segregation at grain boundaries and impurities.

15.2.3 Lanthanum strontium magnesium gallate

A limited amount of work has been reported on the synthesis of LSGM films. Mathews et al. (2000) used PLD to deposit films of LSGM on quartz and silicon substrates held at room temperature. The as-deposited films were amorphous but on annealing at 700°C gave a single-phase cubic structure. Subsequently, ~ 500 nm thick LSGM films were deposited by PLD on quartz, STO, sapphire, and calcia-stabilized zirconia (CSZ) followed by post-annealing in air at 730°C (Joseph et al., 2002). Stoichiometric films were obtained under high-vacuum conditions but not in an oxygen environment. Highly oriented films of LSGM were obtained on STO but the other films were polycrystalline. The oriented film on STO was less conductive than bulk LSGM by a factor of 10 at 730°C. Ishihara and co-workers also used PLD to make thin films with the composition La_0.8Sr_0.2Ga_0.8Mg_0.15Co_0.0.5O_2.8 on silica and LAO substrates (Mitsugi et al., 2004). Films were amorphous when the substrate temperature was less than 750°C. The films were poly-crystalline on silica and oriented on LAO. The conductivities measured for the films were comparable on both substrates and were very high (1 S cm⁻¹ at 600°C). In more recent studies, LSGM thin films were deposited by PLD on a NiO–Fe₂O₃ substrate which was reduced in situ to a porous Ni–Fe alloy. High power densities were measured for single cells with porous thick-film Sm_0.5Sr_0.5CoO₃ cathodes (Ishihara et al., 2008; Ju et al., 2009, 2010).

15.2.4 Interface effects

One approach to study the influence of surface and interface contributions on the conductivity of very thin films is to generate controlled interfaces by making superlattices of two different materials. The presence of internal interfaces can modify properties through space charge effects, by the presence of disordered regions due to mismatched lattice dimensions of the two phases, and strain when the interfaces are coherent. Space charge effects that lead to a variation in the density of majority charge carriers in the space charge region were first experimentally demonstrated and modeled for nanocrystalline CeO₂ (Chiang et al., 1997; Tschope, 2001; Kim and Maier, 2002). Subsequently the concept was extended to epitaxial hetero-structures prepared by sequential deposition of CaF₂/BaF₂ layers. When the thickness of the layers was reduced to ~ 20 nm while keeping the total thickness constant, the F⁻ conductivity parallel to the interfaces was increased by several orders of magnitude (Sata et al., 2000; Guo and Maier, 2009a, 2009b). In situations where the mobile defect concentration is high, the Debye length is correspondingly short and structural effects (a combination of lattice strain and extended defects due to lattice mismatch) are responsible for any variations in conductivity. The effects of coherent, semi-coherent and disordered interfaces have been studied for a number of different fluorite oxygen ion conductors in superlattices with ZrO. (Azad et al., 2005; Wang et al., 2009) or with RE₂ O₃ oxides (RE = Y, Lu, Sc) (Peters et al., 2007; Korte et al., 2008, 2009; Schichtel et al., 2009, 2010). For example, the multilayer system CSZ (ZrO₂ + 8.7 mol% CaO)/Al₂O₃, represents a system with incoherent interfaces. The total conductivity of the CSZ increases by two orders of magnitude when the thickness of the individual CSZ layers is decreased from 780 to 40 nm. Changes in the activation energy for the interfacial transport suggest that the ionic mobility is much higher in the disordered regions of the incoherent interfaces than in the bulk. In contrast, in multilayer samples of YSZ/Sc₂O₃ with layer thicknesses between 8 and 250 nm prepared on (0001) Al₂O₃ substrates, the interfaces between YSZ, Sc₂O₃, and the substrate are sharp and the layers are highly textured. As the individual layers in the multilayer get thinner, the conductivity of the multilayers decreases due to the presence of compressive stress in YSZ at the YSZ/Sc₂O₃ interfaces. This observation agrees well with the results for YSZ/Y₂O₃ superlattices where tensile misfit strain leads to a conductivity increase. An overview of the effect of strain and disorder is shown in Fig. 15.1, adapted from Janek and co-workers (Korte et al., 2009). The MgO/YSZ system investigated by Kosacki et al. (2005) is also included.

In recent work, Garcia-Barriocanal et al. (2008) reported a very large enhancement (eight orders of magnitude) in the conductivity of superlattices in which 10 nm thick STO films alternated with YSZ layers of thickness between 62 and 1 nm. The epitaxial growth of the YSZ on STO gives a large, tensile strain (7%) in the thin YSZ layers in the plane of the layer. The dc conductivity of the 1 nm YSZ layer showed a record value of 0.014 S/cm at 357 K, with an activation energy of 0.64 eV. These results have been the subject of much debate concerning the ionic or electronic nature of the observed conductivity (Garcia-Barriocanal et al., 2009; Guo, 2009). Janek and co-workers (Schichtel et al., 2009) concluded that the maximum enhancement in ionic conductivity that could be expected from the 7% strain was about 2.5 orders of magnitude and that this could not explain the observed behavior. Cavallaro et al. recently prepared YSZ/STO multilayers by PLD with comparable layer thicknesses to the earlier work but with a different microstructure (YSZ islands in a matrix of STO) (Cavallaro et al., 2010). The absolute value of the conductivity found for these films, along with the apparent activation energies are, however, very similar. The dependence of the electrical conductivity on pO₂^1/4 and the absence of ¹⁸O diffusion along the interfaces led this group to conclude that the enhancement is related to electronic rather than ionic conductivity.

15.3.1 Platinum

In order to obtain suitably structured electrodes with a large triple phase boundary length for use with thin-film electrolytes, porous Pt fabricated by sputtering has been most frequently used. Porous platinum has good catalytic activity but unfortunately the microstructure at operating temperature is not stable, grain growth occurs depending on time, temperature and film thickness which reduces the length of the triple phase boundary, and eventually the performance degrades. As an example, Muecke et al. (2008b) observed accelerated grain growth of platinum under hydrogen with associated performance loss. The average grain and pore size of the film on the anode side was several times larger than on the cathode side. Similar studies by Kerman et al. (2011) found that overall performance showed extreme sensitivity to Pt anode porosity and microstructure. The poor thermal stability of platinum films probably will require the use of anode materials with more stable microstructures. Some improvement in stability has been observed for Pt-Ni alloys (Wang et al., 2008).

ALD has been used as an alternative to sputtering to deposit Pt films on electrolytes (Jiang and Bent, 2007; Jiang et al., 2008a). Using (methylcyclo-pentadienyl) trimethylplatinum (MeCpPtMe₃) and oxygen as precursors, Pt films were deposited on YSZ with thicknesses of ≤ 30 nm. Peak power densities were achieved for ALD Pt anodes with only one-fifth of the platinum loading relative to dc-sputtered Pt anodes. The sputtered and ALD Pt films had very different microstructures, which accounted for the difference in their performance.

15.3.2 Cermets

In conventional fuel cells, electrodes can be thick, porous composites, with large triple phase boundary lengths. In micro-SOFCs, the equivalent thin-film composite electrode of a metal with an ionically conducting ceramic phase will be needed. The ceramic network in the composite can limit metal grain growth as observed in conventional SOFC Ni/YSZ anodes. A number of groups have investigated the formation of both precious metal (Ag, Pt) cermets with YSZ and Ni cermets with both YSZ and GDC. Barnett and co-workers employed reactive magnetron sputtering to deposit Ag/YSZ cermet films from Ag and Zr/Y targets in Ar–O₂ mixtures (Wang and Barnett, 1992, 1995; Wang et al., 1992). Evaporation of Ag was observed during long-term annealing at 750°C, but at lower temperatures, they are potential cathodes with low resistivity. Thin-film composite electrodes with nanosized grains of Pt and YSZ were prepared by reactive sputtering onto single-crystal YSZ by Hertz and Tuller (2007). Area-specific electrode polarization resistances of < 500 Ω cm² were achieved at 400°C in a dense thin-film electrode. By turning on and off the platinum deposition, a thin-film stack of Pt–YSZ/YSZ/Pt–YSZ with a total thickness of ~ 1 μm was constructed.

Nickel cermet anodes have been prepared on YSZ and on GDC. Tsai and Barnett used dc reactive magnetron sputtering of Ni–Zr–Y targets in Ar–O₂ mixtures to form porous Ni–YSZ films with grain sizes of ~ 35 nm which gave low specific area resistances (0.15–0.35 Ω cm.) at 750°C in 97% H. + 3% H₂O (Tsai and Barnett, 1995). La O’ et al. prepared Ni–YSZ cermets by rf sputtering (La O’ et al., 2004). The films were porous with microstructures consisting of nanosized columnar grains and showed no cracks after treatment at 600°C under a reducing atmosphere. Also by using reactive rf sputtering, Jou and Wu prepared porous Ni/YSZ films on YSZ and evaluated their performance as the anode with a La_0.7Sr_0.3MnO₃ thick-film cathode (Jou and Wu, 2008). The maximum power density at 700°C was lower than that obtained using a screen-printed Ni–YSZ thick anode.

Ni/YSZ cermet films have also be made by PLD from NiO/YSZ targets followed by reduction of the NiO in hydrogen. The oxide films deposited at 700°C contain very small grains (~ several nm) which results in nickel agglomeration on reduction. Post-annealing the films at temperatures in the range of 800–1200°C to induce some grain growth was found to be necessary to obtain a stable microstructure (Noh et al., 2009a, 2010). PLD was also used to deposit NiO/GDC films on YSZ, SiO₂/Si and sapphire substrates held at temperatures from 25°C to 800°C. Upon reduction of the NiO phase in H₂ at 600°C, a stable three-phase, three-dimensional interconnecting microstructure was obtained. Higher reduction temperatures led to coarsening and segregation of the Ni to the surface of the film. Ni grain growth can be minimized by annealing in air at 1000°C after deposition (Infortuna et al., 2009). Muecke et al., measured the electrochemical performance of nanocrystalline Ni–GDC anodes with a thickness of 500–800 nm prepared by spray pyrolysis (Muecke et al., 2008a, 2008c). The polarization resistance at 600°C decreased from 1.73 to 0.34 cm² when the grain size was decreased from 53 to 16 nm, respectively. The polarization resistances of the Ni–GDC sprayed thin films were comparable to those prepared by PLD and were similar to state-of-the-art thick films.

In summary, platinum anodes, while convenient, are unlikely have sufficiently stable microstructures for long-term performance stability. As discussed earlier, several recent studies have shown that cermets may provide a good alternative for use with both YSZ and GDC electrolytes as indeed they do in full-scale SOFCs. Recently, oxides with both electronic and ionic conductivity in the reducing atmosphere at the anode and which are catalytically active have been discovered and evaluated as alternatives to metalcermets (Tao and Irvine, 2003; Huang et al., 2006b). Whether materials of this type can be developed with sufficient catalytic activity to operate at the lower temperatures (350–500°C) needed for thin-film devices is an open question.

15.4.1 (La,Sr)MnO₃

The perovskite lanthanum strontium manganite (LSM) is the most important electrode material in large-scale SOFCs and remains the practical choice for operation in the 700–900°C range because of its high electrical conductivity, high electrochemical activity for the O₂ reduction reaction, compatibility with YSZ at operating conditions and long-term performance stability (Jiang, 2008).

In the context of micro fuel cells, operating at lower temperature, LSM has not been studied in much detail. Holme et al. (2008) used ALD to prepare a 95 nm thick, dense cathode, on 100 nm thick film of dense YSZ as electrolyte supported on a Si₃ N₄/Si as described in Section 15.5. Porous platinum was used as the anode. Only a limited power density was obtained for this cell and the authors concluded that while ALD is a good route to high-quality films, even thin LSM cathodes are limited by slow bulk diffusion at 450°C.

In contrast, a considerable effort has been made to use dense films to understand the mechanistic aspects of LSM electrode performance. Early experiments had problems with the preparation of crack-free films (Gharbage et al., 1994, 1995) but in 1996 Mizusaki and co-workers (Mizusaki et al., 1996) successfully deposited 1–2 μm thick films of La_0.63Sr_0.27MnO₃ on polycrystalline partially stabilized YSZ by PLD. Electrochemical measurements at 700–900°C showed that the reaction kinetics were controlled by chemical diffusion of oxygen through the oxide layer. Ioroi et al. (1997) used electrochemical vapor deposition to prepare dense LSM electrodes on YSZ. Impedance measurements showed that the electrode resistance was proportional to the film thickness, and the inverse of the electrode resistance was proportional to the electrode surface area, showing again the importance of bulk diffusion. In an extensive series of experiments (Brichzin et al., 2000, 2001, 2002; Fleig, 2003; Fleig et al., 2008), Maier, Fleig and co-workers used thin-film microelectrodes of LSM on YSZ to showed that at 800°C, the electrode resistance scales with the area of the microelectrode indicating that diffusion through the electrode area rather that the length of the triple phase boundary is rate determining. The electrode polarization also increases linearly with thickness, confirming the bulk path. On application of an anodic bias potential, however, the resistance scales with the inverse of the microelectrode diameter, indicating a change in the mechanism from bulk to surface due to a decrease in the oxygen vacancy concentration. The role of triple phase boundaries was also studied by Horita and co workers who prepared a square LSM mesh with ~ 2 μm lines separated by ~ 9 μm on YSZ with a Pt anode (Horita et al., 1998, 2000, 2001). The application of a potential leads to oxygen incorporation. Switching the gas from ¹⁶O₂ to ¹⁸O₂ followed by quenching and SIMS analysis revealed the active areas for oxygen incorporation. Measurements were made either on the surface of the LSM pattern on YSZ or on the YSZ after removing the LSM. Under cathodic polarization, high ¹⁸O concentrations were found on the surface of the LSM and as peaks at the edges of the O₂/LSM/YSZ three-phase boundary. After removal of the LSM, ¹⁸O was also observed in the region where the LSM had been attached but only at high cathodic polarization. For large cathodic bias, the increased vacancy concentration in LSM results a strong contribution of the bulk path to the oxygen reduction rate in accordance with the results obtained from the microelectrode measurements.

15.4.2 (La,Sr)CoO_{3 – x}

Lanthanum cobaltate substituted with strontium to various degrees is probably the most extensively studied thin-film system. In early work, Kawada et al. (1999) carried out oxygen isotope exchange experiments with a dense _0.5 ^m thick film of La₀.₆Sr₀.₄CoO_{3 – x} deposited on a Ce₀.₉Ca₀₁O₁₉ polycrystalline substrate by PLD. The isotope exchange profile was measured from the surface into the electrolyte by SIMS. No gradient in the oxygen isotope concentration inside the film was observed because the oxygen diffusion through the La_0.6Sr_0.4CoO_{3 – x} film is so fast. A similar experiment was carried out by Mims et al., for La..₅Sr_0.5CoO_{3 – x} deposited as oriented ~ 300 nm thick films on single-crystal YSZ by PLD (Mims et al., 2000). Isotope exchange and SIMS depth profiling showed a uniform ¹⁸O concentration in the film and a significant barrier to transport across the solid–solid interface perhaps due to reaction of YSZ with LSC. In a series of papers, Yang et al., reported measurements of the surface exchange rate and interfacial transport by AC impedance measurements for La_0.5Sr_0.5CoO_{3 – x} on 100-oriented YSZ and electrical conductivity relaxation for La_0.5Sr_0.5CoO_{3 – x} deposited on LAO, all films were prepared by PLD (Chen et al., 2000, 2002; Yang et al., 2000, 2001a, 2001b). The electrical conductivity relaxation data were used to obtain the surface exchange rates assuming that the kinetics were surface-limited as shown by the ¹⁸O₂ exchange results. As synthesized, the film was epitaxial with a smooth surface (rms roughness ~ 5 Å). After prolonged heating at 900°C, the surface exchange rate increased substantially due to a change in the thin-film surface morphology (rms surface roughness of 70 A) and a decrease in grain size. The change is too large to be due to an increase in surface area and must reflect a change in surface activity due to the increase in number of grain boundaries. The surface reaction rates were determined also from the low-frequency feature observed in the impedance spectra (Fig. 15.2) and are in the range of those measured by ECR; again higher surface reaction rates were obtained on films with smaller grains and more interfaces. The results obtained from four different films are shown Fig. 15.2; substantial variations are observed for films with different degrees of perfection. For La_0.6Sr_0.4CoO_{3 – x} films on GDC, Sase et al. found that the reaction rate on an electrode with 30–50 nm grains was larger by half an order of magnitude compared with 300–500 nm grains (Sase et al., 2006).

The medium-frequency feature observed in the impedance spectra is associated with an electrode–electrolyte interface resistance due to reaction of LSC with YSZ at the deposition temperature. The interface resistance of La_0.6Sr_0.4CoO₃ on YSZ is almost independent of pO₂, and increases with time according to a parabolic rate law due to formation of a strontium-rich layer between the electrode and the electrolyte (Sase et al., 2005). In contrast, a La_0.7Sr_0.3CoO_{3 – x} film deposited on YSZ by rf magnetron sputtering with the substrate at ambient temperature did not show this feature and no interfacial reactions were observed below 530°C. Two features were observed in the impedance spectra; a low-frequency semicircle due to the surface reaction and a Warburg-like feature due to diffusion (Ringuede and Fouletier, 2001; Ringuede and Guindet, 1997).

The oxygen vacancy concentration in a dense film varies with the pO₂ or applied voltage. This accumulation or depletion of electrical charge is detected in an impedance measurement as a large capacitance sometimes referred to as a ‘chemical capacitance’ (Jamnik and Maier, 1999, 2001). The equilibrium oxygen vacancy concentration for a dense film of La_0.6Sr_0.4CoO_{3 – x} on polycrystalline CGO was determined from this chemical capacitance (Kawada et al., 2002; Masuda et al., 1997) and found to be much smaller than the values determined for bulk samples by thermo-gravimetry (Mizusaki et al., 1989). More recently, further measurements were made on films with the same composition but a different microstructure (20–200 nm grains, YAG laser). The vacancy concentration determined from the capacitance measurements was again smaller than the bulk value but not so small as for the previous sample prepared using a Kr–F laser. Similar results were obtained for La_0.5Sr_0.5CoO_{3 – x} PLD films on GDC, SiO₂ and MgO substrates using a XeCl excimer laser (Hemmi et al., 2009). Clearly, microstructural effects significantly modify the thermodynamic properties. Nanostructured films show similar effects.

15.4.3 (La,Sr)CoO_{3 – x} nanoparticles

Since thin films containing grain boundaries are inherently nanostructured, deliberately engineering nanostructures is of interest. Two groups have successfully done this by different methods and obtained very high performance electrodes. One approach is to use what have been referred to as vertically aligned nanoporous thin films prepared by off-axis PLD (Yoon et al., 2007, 2009; Wang et al., 2010). This technique produces thin films less than 1 μm thick that are nanoporous and contain very small particles (8 nm in the example shown in Fig. 15.3 for La_0.5Sr_0.5CoO_{3 – x}). The high surface area (80 μm⁻¹) results in an electrode with very low area-specific resistance (0.09 O cm² at 600°C) measured with a symmetrical cell with a polycrystalline GDC electrolyte. The chemical capacitance deduced from the impedance data indicates a very low oxygen vacancy concentration and is comparable to the results of Kawada et al. (2002) discussed above. Table 15.1 shows a comparison of results.

The large difference in the enthalpy of vacancy formation for the film relative to the bulk material is striking. It is also interesting that these nano-structured films appear to be quite stable at least over a period of 240 h (Wang et al., 2011).

Peters et al. (2008) did similar experiments on nanoporous (La_0.5Sr_0.5) CoO_{3 – x} thin-film cathodes (film thickness, 200–300 nm; grain and pore size ~ 50 nm). The films were made by metal–organic deposition and deposited on YSZ or GDC electrolytes. Low polarization resistances were observed on both electrolytes and the polarization resistance of the LSC/YSZ interface remained constant for 100 h at 500°C. In related work, La_0.6Sr_0.4CoO_{3 – x} thin-film electrodes were deposited on YSZ substrates by PLD at different temperatures (Januschewsky et al., 2009). The decrease in the film crystallinity that occurs when the deposition temperature is lowered is accompanied by an increase in the oxygen exchange rate. For X-ray amorphous electrodes deposited between 340°C and 510°C, polarization resistances as low as 0.1 O cm² at 600°C were obtained. The amorphous films also exhibited the best stability of the polarization resistance with time.

15.4.4 Epitaxial films: effect of strain

Unusual effects are observed in epitaxial films when the cell constants of the film are constrained by the lattice constants of the substrate and the films are as a result in either tensile or compressive strain. Typically this is only observed for quite thin films (typically < 100 nm but the thickness depends on the lattice mismatch) before the strain is relaxed by the introduction of defects such as edge dislocations. Both experimental and theoretical studies have been made for LSC cathode materials. Kushima et al., employed first-principles calculations (DFT + U) to obtain a microscopic description of the mechanisms by which strain influences oxygen vacancy formation as well as oxygen adsorption on LaCoO₃ (Kushima et al., 2010). A significant lowering in the oxygen-vacancy formation energies in the bulk and surface were found as the tensile strain was increased. An experimental study of a 40 nm thick strained film of La_0.5Sr_0.5CoO_{3 – x} on SrTiO3 using synchrotron X-ray diffraction showed the formation of an ordered phase at 650 K consisting of La and Sr cations in planes parallel to the surface associated with coherent expansion in the c-direction of about 5% (Donner et al., 2011). This chemical ordering is ascribed to the epitaxial strain imposed by the substrate. The origin of the chemical ordering in LSC films was assessed using density-function-theory-based ab initio methods (GGA + U) on LaCoO₃ and La₀.₈₇₅Sr₀.₁₂₅CoO_{3 – x}. A key factor that drives the A-site ordering is the presence of a higher concentration of vacancies in the strained film than are present in bulk samples under the same conditions. A high concentration of oxygen vacancies was also observed for thin strained films of La_0.8Sr_0.2CoO_{3 – x} (20, 45, 130 nm) deposited on YSZ single crystals with intermediate 5 nm GDC layers to prevent interfacial reactions (La O’ et al., 2010). The films were evaluated by electrochemical impedance spectroscopy using patterned microelectrodes. The oxygen non-stoichiometries in the LSC films was estimated by using the chemical capacitance, and are approximately 100 times greater than those of bulk LSC at pO₂ = 1 atm, whereas they are about five times higher at 10⁻⁴ atm. The surface exchange rates determined from the impedance data are much higher than observed in the bulk, presumably as a result of the high oxygen vacancy concentration in the films. Clearly, epitaxial strain has a profound effect on the thermo-kinetic behavior of these cathode materials.

15.4.5 Heterostructured interfaces

Sase et al., investigated large-grained polycrystalline (La₀.₆Sr_0.4)CoO_{3 – x} (113) with (La,Sr)₂CoO₄ (214) grains as a secondary phase by ¹⁸O exchange and SIMS analysis and by impedance spectroscopy. The oxygen surface exchange rate was found to dramatically increase at the interface between the two phases (Sase et al., 2008b; Yashiro et al., 2009). The observation was confirmed for a thin-film structure in which PLD was used to deposit La_1.5Sr_0.5CoO₄ on La_0.6Sr_0.4CoO₃ on a polycrystalline GDC substrate (Sase et al., 2008a). After ¹⁸O isotope exchange, the SIMS images (Fig. 15.4) show clearly that fast oxygen incorporation occurs along the hetero-phase interface as found in the ceramic samples. Similar results were obtained recently for ~ 85 nm thick La_0.8Sr₀.₂CoO_{3 – x} films prepared by PLD on YSZ single crystals with GDC as the buffer layer (Crumlin et al., 2010). The films were subsequently decorated by depositing layers of (La_0.5Sr_0.5)₂CoO_4+x of ~ 0.1, ~ 0.8, ~ 5, ~ 15 nm thickness on top. Partial coverage (< 15 nm) on the perovskite film leads to the formation of nano-islands of 214 on 113 and a large number of interfacial 113/214 boundaries on the film surface. Impedance measurements showed that the oxygen reduction rate was dramatically enhanced (~ 3–4 orders of magnitude above bulk 113) for partial coverage. Increasing LSC214 the thickness to ~ 15 nm led to a fully dense surface layer with a 214 reaction rate. While the mechanism for the enhancement in reaction rate is not yet understood, clearly the interfaces are important.

Thin films of several other perovskite oxide compositions have been investigated, including La_{1 – x}Sr_xCo_{1 – y}Fe_yO_{3 – x} (Baumann et al., 2006a, 2007), Ba_0.5Sr_0.5Co₀.₈Fe₀.₂O_{3 – x} (Baumann et al., 2006b; Burriel et al., 2010b), Sm_0.5Sr_0.5CoO_{3 – x} (Baumann et al., 2008) and SrTi_{1 – x}Fe_xO_{3 – x} (Jung and Tuller, 2008, 2009), but are not discussed here.

15.4.6 Anisotropic materials

In addition to perovskite oxides, other oxide structure types that have anisotropic bulk properties have been investigated for application as thin-film cathodes. The main ones are the K₂NiF₄ structure type exemplified by La₂NiO₄ and the A-site-ordered double perovskites AA″B₂O_5+x.

15.5.1 Silicon

General processing schemes for SOFCs on Si/Si₃N₄ have been described by several authors and there are a number of variations (Jankowski et al., 2002). The method used by Huang et al. (2007) is briefly described and illustrated in Fig. 15.6. A 500-nm thick silicon nitride film was deposited on both sides of a Si wafer by low-pressure chemical vapor deposition to provide an insulating layer and to prevent reaction of Si with YSZ. One side of the Si wafer was coated with photoresist using a mask, then exposed and developed. The exposed silicon nitride was removed by reactive ion etching and the residual photoresist stripped off. Then 50 nm YSZ was deposited on the other side by rf sputtering at 200°C. Windows in the Si were etched with 30% KOH at 85–90°C and silicon nitride on both sides removed by reactive ion etching. Finally, by using physical masks, 80 nm porous Pt layers were patterned on both sides of the electrolyte as cathode and anode. Other variations of this scheme have been used to produce more complex structures such as with a corrugated thin-film electrolyte (Su et al., 2008) and with a nickel grid support (Rey-Mermet and Muralt, 2008; Johnson et al., 2010).

A variety of film deposition methods have been used to deposit the electrolyte layers, the most common being reactive sputtering from an Y–Zr metal target but pulsed laser deposition (Tarancon et al., 2009), atomic layer deposition (Shim et al., 2007) and pulsed injection metal–organic chemical vapor deposition have also been used (Garbayo et al., 2010).

The performances of silicon-based devices have been discussed in a recent excellent review and are only briefly summarized here with some updates (Evans et al., 2009b). As noted by Evans et al., it is very difficult to make comparisons of device performance since different temperatures and fuel conditions have been used. The electrolyte layers differ in thickness ranging from 50 to 150 nm and the membrane areas range from 20 × 20 μm to 240 × 240 μm; corrugated nickel grid supported membranes have been made in mm sizes. In general higher performance is obtained for thinner electrolyte layers provided that they are pinhole free to avoid reduction in the open-circuit voltage and the addition of a GDC layer helps charge transfer on the cathode side leading to better performance. High power densities have been observed for corrugated membranes (677 mWcm⁻² on a 600 × 600 μm membrane at 400°C) and most recently a value of 1037 mWcm⁻² was reported for a 160 × 160 μm membrane with 100 nm of YSZ and Pt electrodes at 500°C (Kerman et al., 2011). In that study, the overall stability during 12 h of continuous use was investigated. The power density was reduced by ~ 50% at 400°C after 12 continuous hours of testing due to coarsening of the Pt electrodes particularly the anode. In the majority of cells that have been evaluated platinum electrodes have been used. More recently, efforts have been made to introduce complex oxides such as La_0.32Sr_0.68CoO₃ (Rey-Mermet and Muralt, 2007) and La₀.₆Sr₀.₄Co₀.₈Fe_0.2O₃ (Xiong et al., 2009; Lai et al., 2011) into silicon-based cells to address the problem with Pt stability.

15.5.2 Foturan®

Foturan® is a photostructurable glass ceramic (Mikroglas Chemtech, n.d.) that has been used at ETH as a substrate for micro-SOFCs. Micropatterning is achieved by exposing through a shadow mask with holes 200 ^m in diameter to UV light (319 nm), while the rest of the sample is covered by a mask. During the exposure, the reactions Ce^3 + + hv → Ce^4 + + e′ and Ag⁺ + e′ → Ag occur. The UV exposure is followed by annealing at 500 and 600°C, when the reduced silver ions agglomerate and grow to larger nuclei at 500°C. At 600°C, the glass crystallizes around the silver nuclei forming lithium metasilicates (Livingston et al., 2007). The crystalline regions can then be removed by etching with aqueous hydrofluoric acid; the crystalline regions etch 20 times faster than the glassy matrix. Muecke et al., fabricated a micro-SOFC consisting of a 550 nm thick YSZ electrolyte (prepared by PLD) with a 50 nm thick sputtered Pt anodes deposited on a Foturan® substrate. Platinum paste, sputtered platinum and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ deposited by spray pyrolysis were used as cathodes. The arrangement is shown in Fig. 15.7 for a cell with a LSCF cathode. The Pt anode and the LSCF cathode form nanoporous electrodes that adhere well to the electrolyte. The YSZ electrolyte has a columnar structure with elongated grains perpendicular to the substrate. The power output from the cells was found to be limited by pinholes in the electrolyte and the electrode polarization resistance, but not by the resistance of the electrolyte. The best results were obtained with a LSCF cathode. Sputtering an additional 200 nm layer of YSZ on top of the PLD layer and using platinum electrodes gave a higher open-circuit voltage (1.06 V) and a maximum power density of 152 mWcm⁻² at 550°C.

15.5.3 Nickel

Several micro-SOFC devices have been constructed based on nickel anode supports. In one approach (Kang et al., 2006), a nanostructured Ni substrate was constructed from an anodic aluminum oxide template following a previously reported two-step replication process (Masuda and Fukuda, 1995). The template has pores 20 nm in diameter on one surface which increase to 200 nm at about 70 nm below this surface. The pores were filled with polymethylmethacrylate (PMMA) to obtain the negative replica and the aluminum oxide removed by dissolution in aqueous sodium hydroxide. The final structure was obtained by electroplating Ni into the negative structure followed by removal of PMMA with acetone. A thin-film SOFC was fabricated using on this substrate by reactively sputtering 200 nm of YSZ followed by sputtering a porous Pt layer. The final structure was 12 mm in diameter and 20 μm thick. The cell was operated on 3% H₂ at 370–550°C and had a maximum power output of 7  mW/cm² at 400°C. The same group also reactively sputtered a thin film of Y–Zr alloy on the surface of an AAO substrate. The volume expansion on oxidation of the alloy resulted in a dense gas tight YSZ thin film. The fuel cell performance was, however, modest, mostly due to the development of cracks due to the poor mechanical properties of AAO at high temperature (Park et al., 2006).

Ignatiev and co-workers used a lithographic approach to build a thin-film cell on a ~ 10 μm nickel foil substrate (Chen et al., 2004; Ignatiev et al., 2008). A 1–2 μm thick film of YSZ was deposited on the nickel foil by PLD. The Ni foil substrate was then converted into a porous anode by photolithographic patterning and etching with FeCl₃. The electrolyte is polycrystalline when deposited on a polycrystalline substrate, and was (100)-textured when deposited on a textured nickel foil substrate. A nickel cermet layer was added to the foil to increase the length of the triple phase boundary and a porous La_0.5Sr_0.5CoO₃ cathode was deposited by PLD. The resultant fuel cell achieved a maximum power density of 40 mW/cm² at 515°C.

Most recently, another and simpler approach was developed that uses a nickel substrate but without lithography or etching (Joo and Choi, 2008). A porous nickel film was prepared by screen printing a layer of nickel oxide onto a ceramic substrate. After reduction in hydrogen at 700–750°C a porous nickel film was obtained (22 μm thick) which was easily removed from the substrate. The free-standing film was mounted on a nickel ring and 2 μm of GDC deposited on top by PLD (substrate temperature 500°C) followed by PLD deposition of porous La_0.7Sr_0.3CoO_{3 – x} at ambient temperature (see Fig. 15.8). Contact to the LSC film was made with Pt. The cell was tested on 97%H₂, 3% H₂O at 450°C of 26 mW/cm². The open-circuit voltage was lower than expected, possibly due to pin holes in the electrolyte layer.

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