Surface Exchange Rate Research Articles

Novel oxygen partial pressure relaxation technique for study of oxygen exchange in nonstoichiometric oxides. The model of relaxation kinetics

The development of the theoretical background of a new relaxation technique based on pO2 monitoring at the outlet of the continuous-flow fixed-bed reactor is presented. The model describes the relaxation kinetics of oxygen exchange between the nonstoichiometric oxide and the gas phase in a case when surface reaction is the rate-limiting step. The model takes into account the feedback between the oxygen exchange rate and the oxygen partial pressure in the reactor: the rate is affected by oxygen released or uptaken by the sample. In the frame of material-gas feedback approach the influence of the sweep gas flow rate on the relaxation rate constant is shown. An analytical expression which allows determining correct value of the key material kinetic characteristic (surface exchange rate constant) from experimentally measured relaxation rate constant is derived. The proposed feedback approach has the versatility: the correction of the relaxation rate constant is necessary not only for the novel oxygen partial pressure relaxation method, but also for the other traditional relaxation techniques when oxides possess high oxygen exchange rate that does not allow to maintain constant oxygen partial pressure in the reactor even with high sweep gas flow rate.

An in operando study of chemical expansion and oxygen surface exchange rates in epitaxial GdBaCo2O5.5 electrodes in a solid-state electrochemical cell by time-resolved X-ray diffraction

Chemical expansion and O2 surface exchange rates show a remarkable asymmetric response when the oxygen stoichiometry O5.5±δ crosses δ = 0.

Double perovskite cathodes for proton-conducting ceramic fuel cells: are they triple mixed ionic electronic conductors?

18O and 2H diffusion has been investigated at a temperature of 300 °C in the double perovskite material PrBaCo2O5+δ (PBCO) in flowing air containing 200 mbar of 2H216O. Secondary ion mass spectrometry (SIMS) depth profiling of exchanged ceramics has shown PBCO still retains significant oxygen diffusivity (~1.3 × 10−11 cm2s−1) at this temperature and that the presence of water (2H216O), gives rise to an enhancement of the surface exchange rate over that in pure oxygen by a factor of ~3. The 2H distribution, as inferred from the 2H216O− SIMS signal, shows an apparent depth profile which could be interpreted as 2H diffusion. However, examination of the 3-D distribution of the signal shows it to be nonhomogeneous and probably related to the presence of hydrated layers in the interior walls of pores and is not due to proton diffusion. This suggests that PBCO acts mainly as an oxygen ion mixed conductor when used in PCFC devices, although the presence of a small amount of protonic conductivity cannot be discounted in these materials.

Ceria: Recent Results on Dopant-Induced Surface Phenomena

Redox studies on dense zirconia-doped ceria pellets were carried out by thermogravimetric investigations and dilatometry. Up to 1600 K reduction parameters determined by both methods correspond to each other. At higher temperatures, however, thermogravimetry overestimates the degree of reduction since mass loss is not only due to oxygen exsolution but also to selective evaporation of CeO2 whose vapour pressure is considerably higher than that of ZrO2. As a consequence surface segregation of zirconia occurs in (Ce,Zr)O2−δ pellets leading to a porous surface zone of Ce2Zr2O7 pyrochlore which gradually grows in thickness. Surface enrichment of zirconia is detrimental for splitting CO2 or H2O since re-oxidation temperatures of (Ce,Zr)O2−δ are known to be shifted towards lower temperatures with increasing ZrO2 content. Thus, very harsh reduction conditions should be avoided for the (Ce,Zr)O2−δ redox system. The kinetics investigations comprised the high temperature reduction step (T ≅ 1600 K) and the “low” temperature oxidation reaction with a carbon dioxide atmosphere (T ≅ 1000 K). The reduction kinetics (at around 1600 K and an oxygen activity of 7 × 10−4 in the gas phase) directly yield the (reduction) equilibrium exchange rate of oxygen in the order of 10−7 mol·O/(cm3·s) as the kinetics are surface controlled. The oxidation step at around 1000 K, however, occurs in the mixed control or in the diffusion control regime, respectively. From oxygen isotope exchange in combination with SIMS depth profiling oxygen exchange coefficients, K, and oxygen diffusivities, D, were determined for so-called equilibrium experiments as well as for non-equilibrium measurements. From the obtained values for K and D the (oxidation) equilibrium exchange rates for differently doped ceria samples were determined. Their dependency on the oxygen activity and the nature and the concentrations of a tetravalent dopant (Zr) and trivalent dopants (La, Y, Sm) could be semi-quantitatively rationalised on the basis of a master equation for the equilibrium surface exchange rate.

Role of electrocatalytic properties of infiltrated nanoparticles in the activity of cathodes of solid oxide fuel cells – A case study of infiltrated La0.8Sr0.2CoxMn1-xO3 (x=0, 0.5, and 1) on Pt electrode

In this paper, critical role of infiltrated nanoparticles (NPs) on the electrocatalytic activity of cathodes of solid oxide fuel cells (SOFCs) for O2 reduction reaction is studied on well-defined and microstructurally clean Pt electrodes using infiltrated La0.8Sr0.2CoO3 (LSC), La0.8Sr0.2Co0.5Mn0.5O3 (LSCM), and La0.8Sr0.2MnO3 (LSM) NPs. The promotion factor, fp, as defined as the ratio of electrode polarization resistance, RE of the electrode prepared by the conventional method to that of the infiltrated electrode measured under identical conditions, is close to unit for the 0.2 mg cm−2 NPs infiltrated Pt electrodes, indicating that the promotion effect of infiltrated perovskite NPs on the electrocatalytic activity of Pt electrodes is very small or negligible under open circuit conditions. However, under dc bias of 100 mV, fp is 35, 18 and 7 for the infiltrated LSC-Pt, LSCM-Pt and LSM-Pt electrodes, respectively. The electrode activity is significantly promoted by the presence of the NPs under cathodic polarization conditions. The electrochemical activity of Pt electrodes with infiltrated NPs depends strongly on the nature of the infiltrated materials with the order of LSC > LSCM > LSM. The higher oxygen surface exchange rate as well as faster oxygen transportation kinetics of the B-site Co-rich perovskite oxides is responsible to the better catalytic activity of the infiltrated Pt cathodes.

Correlation of Structure and Surface Exchange Kinetics on Bismuth-Based Oxides

High temperature cubic phase bismuth oxide has the highest ionic conductivity because of the highly defective fluorite structure, with near unity ionic transference number. However, bismuth oxide has multiple crystallographic polymorphs at different temperature domains and the conductivity of bismuth oxide can vary by orders of magnitude depending on the phase. Although cubic phase bismuth oxide can be stabilized by doping, doped bismuth oxides, such as erbium stabilized bismuth oxide (ESB), undergo an anion ordering transition near 600 °C, leading to a decrease in ionic conductivity. The phase transition and anion ordering in stabilized bismuth oxides provides a unique opportunity to directly correlate crystal structure, conductivity and surface exchange properties on ionically conducting oxides. Here, gas phase isotopic oxygen exchange was performed on Bi2O3 and ESB as a function of exchange time at different temperatures, to determine the structure effect on surface exchange kinetics. Our results suggest that bismuth-based oxides have intrinsic catalytic activity toward oxygen exchange and the doping of erbium affects the surface exchange rate. The relationship between diffusion coefficient (D) and surface exchange coefficient (k) on bismuth-based oxides is explored and possible rate-limiting steps are proposed.

Net ecosystem exchange and energy fluxes measured with the eddy covariance technique in a western Siberian bog

Abstract. Very few studies of ecosystem–atmosphere exchange involving eddy covariance data have been conducted in Siberia, with none in the western Siberian middle taiga. This work provides the first estimates of carbon dioxide (CO2) and energy budgets in a typical bog of the western Siberian middle taiga based on May–August measurements in 2015. The footprint of measured fluxes consisted of a homogeneous mixture of tree-covered ridges and hollows with the vegetation represented by typical sedges and shrubs. Generally, the surface exchange rates resembled those of pine-covered bogs elsewhere. The surface energy balance closure approached 100 %. Net CO2 uptake was comparatively high, summing up to 202 gC m−2 for the four measurement months, while the Bowen ratio was seasonally stable at 28 %. The ecosystem turned into a net CO2 source during several front passage events in June and July. The periods of heavy rain helped keep the water table at a sustainably high level, preventing a usual drawdown in summer. However, because of the cloudy and rainy weather, the observed fluxes might rather represent the special weather conditions of 2015 than their typical magnitudes.

(Invited) Oxygen Exchange Kinetics on Technological Versus Model Type Electrodes

The rate of the oxygen exchange at the gas solid interface is of key importance for the performance of solid oxide fuel cells (SOFCs), especially at low temperature, as well as for the overall durability of solid oxide electrolysis cells (SOECs), when operated at high current density. Hence, details of this process are of great technological importance. Whereas simple guidelines like; i) Combine an electrolyte material and an electro-catalyst in the electrode, ii) maximize triple phase boundary length per unit area, iii) maximize surface area of the electro catalyst, iv) use preferably a mixed conducting electro-catalyst, v) maximize ionic conductivity in both phases,... are well established in the field, and have lead to development of well performing electrodes, the rate limiting steps of the exchange process itself are not well established. It is very challenging to elucidate these on the “real” geometrically very complex composite systems. This has spurred interest in studying simpler model electrode systems [1,2,3], for example in the form of ultra-thin dense films (with a well-defined surface area) and also in trying to model electrode performance from a detailed geometrical description of the electrode structure and the material properties deduced from studies of the materials in bulk form. The study of thin film model electrode systems certainly lends insight into the details of the exchange process, but the approach is not without problems, as the model system may differ from the “real” nanostructured composite electrodes in a number of ways, e.g. with respect to surface structure/termination, chemical composition [4], purity and possibly strain, which may all affect the exchange process. In this paper we discuss challenges in predicting performance of real electrodes from studies on model systems. We shall compare trends observed on dense thin films of LSC (La0.5Sr0.5CoO3), LSCF (La0.58Sr0.4Co0.2Fe0.8O3- δ) and LSF (La0.6Sr0.4FeO3) with trends observed when these materials are used in porous composites. Also a model study of a multi-phase nano-structured electrode, prepared by infiltrating a porous CGO backbone structure with “LSC” will be presented, and it is shown that performance can be well accounted for when assuming exchange properties as deduced from LSC in bulk form. Finally, recent results obtained on two other types of simplified model electrode systems are presented. The first one is based on a dense composite of two phases (Fig. 1, A and B) and in the second type a mixed conducting material is studied with and without second phase surface decorations (Fig. 1, D). In both cases, the oxygen exchange is studied using conductivity relaxation (Fig. 1, C). The advantage of these geometries are that they are simpler than the real electrodes, e.g. the TPP can be accurately assessed, and the manufacturing route in many respects lies closer to the one applied on technological electrodes than does the ones typically applied when making ultra-thin films. On a LSF/CGO (La0.6Sr0.4)0.98FeO3-Ce0.9Gd0.1O1.95) composite system with different volume ratios between the two phases (0%, 30 %, 50 %, 70 % CGO) we find that the relaxation time for re-equilibrating the oxygen stoichiometry in the sample after a sudden change in oxygen activity reduces strongly the more CGO is added. This is not only an effect of the increased ionic conductivity in the composite (CGO has a higher ionic conductivity than LSF) but also the surface exchange rate is increased (by up to a factor of 25) relative to the one of the pure LSF sample. Effects of adding small amounts of second phase decorations on the surface on both the composites and the LSF end-member were studied. Results are compared to recent findings in literature [5] and the underlying mechanisms for the increased surface exchange rates are discussed.

Degradation of La0.6Sr0.4Fe0.8Co0.2O3-δ Oxygen Electrodes on Ce0.9Gd0.1O2-δ Electrolytes during Reversing Current Operation

In order to assess the durability of La0.6Sr0.4Co0.2Fe0.8O3 oxygen electrodes in reversible solid oxide cells, current switched ∼1000 h galvanostatic (0.7, 1.0, and 1.5 A/cm2) life tests were performed on symmetrical Ce0.9Gd0.1O2–electrolyte cells at 700°C. Cell operating voltage and resistance, the latter measured by impedance spectroscopy, were monitored throughout. Degradation was minimal for the 0.7 A/cm2 case. For the higher current densities, the cell voltage and resistance increased with time, although the cell appeared to stabilize after ∼500 h in the 1.5 A/cm2 case. Post-test analyses showed no evidence of electrolyte cracking or delamination for any current. However, 3D imaging revealed measureable microstructural coarsening after 1.5 A/cm2 operation that was not present after 0.7 A/cm2 operation. Furthermore, the amount of Sr segregated onto LSCF surfaces was higher for the cells operated with current switching versus the as-prepared and zero-current cells. Analysis of the results suggest that much of the degradation was due to decreased oxygen surface exchange rate due to current-enhanced Sr segregation, with a smaller contribution due to microstructural coarsening. The possibility of extrapolating these accelerated tests to longer times is discussed.

Promotion Effect of Metal Nanoparticle on Surface Exchange Reaction of (La,Sr)MnO3 Film with Different Orientation

Noble metals Pt and Pd decorated on cathode materials can effectively improve the performance of solid oxide fuel cells. Here, we explore the promotion effect of metal nanoparticles on the surface exchange reaction rates of (La,Sr)MnO3 (LSM) as a function of temperature and the LSM orientation. The surface chemical exchange coefficient, Kchem, is investigated using electrical conductivity relaxation (ECR) method for Pt and Pd decorated epitaxial (001)-, (110)-, and (111)-oriented LSM films. Compared with the bare LSM film, larger Kchem is observed with the Pd and Pt nanoparticles decorated LSM films at the temperature in the range from 500°C to 650°C. Pt nanoparticles decorated on (110)-LSM promotes the surface exchange rate to a factor up to 129, about 5 times higher than the Pd decorated (110)-LSM.

In Situ, Non-Contact Studies of Oxygen Exchange Kinetics in Thin Film Mixed Conducting Electrodes

More rapid and durable surface exchange kinetics at the oxygen electrode are desirable for long-term operation of reversible solid oxide cell (R-SOC) devices. Understanding and improving the oxygen exchange kinetics would benefit from measurement techniques that enable in situ and continuous evaluation of both the defect equilibria and kinetics of native electrodes (without potentially blocking or catalytic metal current collectors). The optical transmission relaxation (OTR) technique is one such approach, which relies on the change in optical absorption signatures of thin film electrode materials upon changing their oxygen contents, typically corresponding to changes in transition metal cation valence states. In this work, the OTR method was applied to thin film electrodes of Sr(Ti,M)O3-x (M=Fe, Co) to evaluate both point defect equilibria and surface exchange kinetics over a range of temperatures and oxygen partial pressures. While the thin film defect equilibria derived this way showed reasonable agreement to established bulk defect chemistry, the surface exchange kinetics were additionally compared to those measured by other methods (curvature relaxation, simultaneous ac-impedance spectroscopy, and simultaneous conductivity relaxation). Again reasonable agreement was found, though the rapid aging behavior of the films' surface exchange rate likely was the cause of some discrepancy. Additionally the surface exchange kinetics measured by ac-impedance spectroscopy were slightly slower than those measured optically, a result that may be attributed to a potentially blocking role of porous current collectors in the former approach. The benefits and limitations of the OTR approach will be discussed, including its particular utility in evaluating the impact of electrode microstructure on exchange kinetics.

Effect of Heterointerface on Cathode Performance of Solid Oxide Fuel Cells: (Sm,Sr)CoO3-Based Oxide

Solid oxide fuel cells (SOFCs) offer many advantages such as high energy conversion efficiency and high fuel flexibility. However, their high operating temperatures (700-900 °C) sometimes accelerate the degradation of components and result in a long time to start-up and shut- down time of the systems. Thus, many researchers have tried to reduce the operating temperature of SOFCs. The lowering of operating temperatures leads to the decrease in the reaction rate over electrodes. One possible approach to overcome this issue is the development of cathode materials with high activity for oxygen reduction reaction (ORR). Nowadays, mixed ionic-electronic conductors, a series of ABO3 perovskite-type materials such as (Ln,Sr)CoO3-δ (Ln: Lanthanoid), have attracted much attention as promising cathode materials due to their higher performance in comparison to a conventional cathode of (La,Sr)MnO3+δ. The previous studies on electrode kinetics of (La,Sr)CoO3-δ cathode revealed that the rate-determining step of the electrode reaction was the oxygen exchange process over the surface [1,2]. Recently, the oxygen surface exchange property at the heterointerface between (La,Sr)CoO3-δ and (La,Sr)2CoO4-δ (K2NiF4-type structure) has been studied. In particular, Sase et al. have reported that the rate of oxygen surface exchange was much faster at the (La,Sr)CoO3-δ/(La,Sr)2CoO4-δ heterointerface than at the surface of (La,Sr)CoO3-δ [3]. Yashiro et al. demonstrated high electrochemical performance of the heterointerface-introduced (La,Sr)CoO3-δ-based composite cathode [4]. In this study, then, we focused on (Sm,Sr)CoO3-δ, which has higher performance than (La,Sr)CoO3-δ, and examined the effect of Sm0.5Sr0.5CoO3-δ(SSC113)/SmSrCoO4(SSC214) heterointerface on the cathode performance. The oxide materials with different weight ratios of SSC113 and SSC214 were synthesized by mixing starting materials of metal nitrates in the desired ratios. The resulting composite oxide was mixed with Ce0.9Gd0.1O1.95 (GDC) in a weight ratio of 70:30. The symmetric cells consist of the obtained material as electrodes and GDC as an electrolyte were fabricated. The electrode performance was evaluated in air at 550-800 °C by the electrochemical impedance spectroscopy. The composite material with SSC113:SSC214 = 80:20 wt.% exhibited higher performance than SSC113. This was attributed to the promotion of electrode reaction. The 18O isotope exchange reaction was also carried out for dense pellets of SSC-based materials. The composite pellets of SSC113 and SS214 provided higher surface exchange coefficient than SSC113. These results indicate that the introduction of (Sm,Sr)CoO3-δ /SmSrCoO4 heterointerface is one of the promising ways to design the high performance cathode for SOFCs operative at low temperatures. [1]K. Masuda, A.Kaimai, K. Kawamura, Y. Nigara, T. Kawada, J. Misusaki, H. Yugami, H, Yugami, H. Arashi, Solid State Fuel Cells V, U. Stimming, S. C. Singhal, H. Togawa, W. Lehnert, Editors, PV 97-40, p473, The Electrochemical Society Proceedings Series, Pennington, NJ (1997) [2]T. Kawada, K. Masuda, H. Suzuki, A. Kaimai, K. Kawamura, Y. Nigara, J. Mizusaki, H. Yugami, H. Arashi, N. Sakai, et al., Solid State Ionics, 121, E252 (2002). [3] M. Sase, F. Hermes, K. Yashiro, K. Sato, J. Mizusaki, T. Kawada, N. Sakai, H. Yokokawa, J. Electrochem. Soc., 155, 793 (2008). [4] K. Yashiro, T. Nakamura, M. Sase, F. Hermes, K. Sato, T. Kawada, J. Mizusaki, Electrochem. Solid-State Lett. , 12, 135 (2009).

Degradation of (La,Sr)(Co,Fe)O3-Δ SOFC Cathodes at the Nanometre Scale and below

For developing solid oxide fuel cells operating at intermediate temperatures (ITSOFCs), metallic materials have become a preferential choice for the interconnect due to their low cost and excellent physical and chemical properties. However the presence of chromium in all commonly used metallic alloys has been found to cause poisoning of the cathode leading to rapid electrochemical performance degradation of the cathodes including one of the most promising (La,Sr)(Co,Fe)O3-δ (LSCF) perovskite oxides [1-3]. Despite the extensive research on the chromium deposition and poisoning processes, careful microstructural studies, especially at the nanoscale, are rare, which can provide valuable information for the fundamental understanding of the Cr poisoning mechanisms required for developing Cr tolerant cathode materials. In this paper, we examine the Cr poisoning mechanisms in (La0.6Sr0.4)0.95(Co0.2Fe0.8)O3-δ (LSCF6428) materials by correlating the bulk electrochemical properties of the cell with their structural and chemical change at multi-scales down to the nanometer level. Cells with LSCF cathodes were prepared, and the effect of Cr poisoning on the electrochemical behavior of the cell was assessed by impedance spectroscopy. The change in nano/microstructure and chemistry due to poisoning were studied in parallel by a combination of advanced ion and electron microscopy techniques including focus ion beam (FIB) tomography, high resolution (scanning) transmission electron microscopy ((s)TEM) and analytical STEM. The systematic combination of bulk and high-resolution analysis on the same cells allows, for the first time, to directly correlate Cr induced performance degradation with subtle and localized structural/chemical changes of the cathode down to the atomic scale. Up to two orders of magnitude reduction in conductivity, oxygen surface exchange rate and diffusivity were observed in Cr poisoned LSCF6428 samples. These effects are associated with the formation of nanometer size SrCrO4; grain boundary segregation of Cr; enhanced B-site element exsolution (both Fe and Co); and reduction in the Fe valence, the latter two being related to Cr substitution in the LSCF phase. The finding that significant degradation of the cathode happens before obvious microscale change points to new critical SOFC degradation mechanisms effective at the nanometer scale and below, which provide new insight for the development of future poisoning resistant electrode materials not only for SOFCs but also for other devices such as solid oxide electrolysis cells.

Degradation of (La0.6Sr0.4)0.95(Co0.2Fe0.8)O3-δ Solid Oxide Fuel Cell Cathodes at the Nanometer Scale and below.

The degradation of intermediate temperature solid oxide fuel cell (ITSOFC) cathodes has been identified as a major issue limiting the development of ITSOFCs as high efficiency energy conversion devices. In this work, the effect of Cr poisoning on (La0.6Sr0.4)0.95(Co0.2Fe0.8)O3-δ (LSCF6428), a particularly promising ITSOFC cathode material, was investigated on symmetrical cells using electrochemical impedance spectroscopy and multiscale structural/chemical analysis by advanced electron and ion microscopy. The systematic combination of bulk and high-resolution analysis on the same cells allows, for the first time, direct correlation of Cr induced performance degradation with subtle and localized structural/chemical changes of the cathode down to the atomic scale. Up to 2 orders of magnitude reduction in conductivity, oxygen surface exchange rate, and diffusivity were observed in Cr poisoned LSCF6428 samples. These effects are associated with the formation of nanometer size SrCrO4; grain boundary segregation of Cr; enhanced B-site element exsolution (both Fe and Co); and reduction in the Fe valence, the latter two being related to Cr substitution in LSCF. The finding that significant degradation of the cathode happens before obvious microscale change points to new critical SOFC degradation mechanisms effective at the nanometer scale and below.

(Invited) Dense Gas Separation Membranes - Current Status and Future Challenges

The application areas of oxygen transport membranes (OTM) and the potential benefits they can provide will be described. The importance of close thermal integration with the process to which the oxygen is supplied will be elucidated by balance-of-plant studies and techno-economical evaluations in two different applications; biomass gasifiers and cement production. The status of the technology will be presented with emphasis on results from several national and EU-funded membrane development projects. There are still several challenges to overcome before use of the technology will be wide spread. One is the required upscale of the technology another one is ensuring the required reliability and durability of the membranes. Recently developed methods for characterizing the mechanical properties of OTM materials and components at operating conditions and a theoretical analysis of stresses encountered during operation will be presented and based on these strategies to ensure fail-safe design/operation. An OTM material should ideally have high ionic and electronic conductivity as well as good chemical stability under both oxidizing and reducing conditions. Further, the mechanical properties (strength, toughness, creep rate) must match the requirements of the application. Candidate materials involve a broad class of Fe-, Co- and Ti-based perovskites. Recent results on selected materials from this broad class of materials will be presented. Challenges with reaching both the required transport properties and the required mechanical reliability with such single phase perovskites has spurred renewed interest in dual phase membranes where a good ionic conductor (typically based on zirconia or ceria) is combined with a second phase providing electronic (or mixed) conductivity. Our most recent results on such dual phase membranes will be presented involving both zirconia and ceria based systems. On both tubular and planar LSF/CGO membranes high oxygen fluxes >10 Nml min-1 cm-2 (850°C) have been demonstrated between air on one side of the membrane and a strongly reducing “syngas” mixture (CO/H2/H2O/CO2) on the other side. The tubular membranes were prepared by dip coating on an extruded low cost MgO support and the planar ones by phase inversion tape casting providing a highly oriented pore structure in the support layer. The processes limiting the flux in these systems were investigated in some detail by conductivity relaxation. Whereas the addition of CGO to LSF has the expected effect of increasing the ionic transport in the bulk it also had a more surprising effect of enhancing the surface exchange rate. Possible explanations for this will be presented. Finally, selected results on zirconia based composite membranes optimized for high pO2 applications will be presented including ZnO-ZrO2 and (MnCO)3O4 -ZrO2 membranes. For the latter system (MnCo2O4/(Y2O3)0.01(Sc2O3)0.10(ZrO2)0.89) fluxes around 1 Nml min-1 cm-2 at 850°C have recently been demonstrated between air and a nitrogen purge (pO2~10-3 atm.) using a ~10 μm thick supported membrane.

Mechanisms of Performance Degradation of (La,Sr)(Co,Fe)O3-δ Solid Oxide Fuel Cell Cathodes

Symmetric cells with porous La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) electrodes on Gd0.1Ce0.9O1.95 (GDC) electrolytes were aged at 800°C for 800 hours in ambient air. Electrochemical impedance spectroscopy (EIS) measurements performed periodically at 700°C showed a continuous increase of the polarization resistance from 0.15 to 0.34 Ω·cm2. Three-dimensional (3D) tomographic analysis using focused ion beam–scanning electron microscopy (FIB-SEM) showed negligible changes due to the ageing, suggesting that the observed resistance increase was not caused by electrode morphological evolution. However, an increased amount, by a factor of 3, of a water-soluble Sr rich surface phase on the aged LSCF electrode was detected by an etching procedure coupled with inductively coupled plasma-optical emission spectrometry (ICP-OES). The electrochemical analysis in combination with the microstructural parameters determined by FIB-SEM was used to examine the effect of Sr segregation on the rate of oxygen surface exchange, based on the Adler-Lane-Steele (ALS) model.

Lanthanum Nickelate Infiltration into Porous Gd-Doped Ceria As Cathode for Solid Oxide Fuel Cells

To increase Solid Oxide Fuel Cells performances, the optimization of the oxygen electrode is mandatory as its activation overpotential is higher than the one of the hydrogen electrode. One way to increase the cathode electrocatalytic properties and to decrease its polarization resistance is to increase the Triple Phase Boundaries zone, where the oxygen is reduced. A promising solution to achieve this goal is to disperse an oxygen reduction catalyst on the surface of a porous ionic conductor. The lanthanum nickelate La2NiO4+δ (LNO) has remarkable surface exchange reaction rate and oxygen diffusion coefficient. However, when the electrode is shaped by classical screen printing technique, it displays higher polarization resistance (≈ 1 Ω.cm²) than the standard lanthanum cobaltite-based materials (≈ 0.1 Ω.cm² for La0.6Sr0.4Co0.2Fe0.8O3-δ). In this study, cathodes were prepared by infiltration of lanthanum nickelate into a Gd-doped ceria (GDC) backbone sintered on 8YSZ electrolyte pellets. The influence of the preparation parameters of the composite electrodes on their electrochemical activity will be presented. The optimization of the preparation parameters led to a large decrease of the polarization resistance, down to 0.1 Ω·cm² at 600 °C. Using the Adler-Lane-Steele model, the impedance diagrams were fitted, allowing the determination of the surface exchange rate and ionic conductivity of the electrode: the obtained values well agree with those previously reported in the literature. Using this infiltration process, single cells were made starting from commercial half cells (Ni cermet/8YSZ), the cathode being the composite LNO/GDC. Voltammetry measurements showed power density higher than 1 W.cm-2 at 800°C.

Surface Oxygen Exchange Properties of Sr Doped La2NiO4+δ as SOFC Cathode: Thin-Film Electrical Conductivity Relaxation Investigation

Roddlesden-Popper Phases, such as La2NiO4, have been proposed as promising intermediate temperature SOFC cathodes, due to their high surface oxygen exchange and bulk diffusivities. However, relatively low electrical conductivity of LNO phase is a concern for its application. It has been shown that Sr doping is an effective way to improve LNO’s electrical conductivity, but it is important to investigate the effect of such doping on other properties such as oxygen surface exchange coefficient. In this study, Sr doped lanthanum nickelates i.e., La2-xSrxNiO4+δ (0≤x≤0.4) thin and dense films on dense thick GDC (~1mm) substrate, were prepared by using spray-modified pressing method. The surface oxygen transport kinetics was investigated by electrical conductivity relaxation (ECR) technique. Since the thickness of thin films is ~15μm, much less than the characteristic length of LNO, the oxygen transport kinetics are controlled by the surface exchange steps and the oxygen ion concentration is uniform in the film at any time during the ECR process. Such situation enables a more accurate fitting results compared to the traditional fitting procedure based on thick pellets, because only k value is the unknown parameter. The fitted k for LNO is 4.02×10-5cm/s at 0.2atm 700oC, which decreases with lowered oxygen partial pressure. Sr doping harmed the surface exchange kinetics, k of Sr40 is about one order of magnitude smaller than undoped one. Interstitial oxygen and Ni oxidation state are suggested to be predominant roles in determining the surface kinetics. Both Sr doping and lowered oxygen partial pressure lead to the loss of interstitial oxygen and the oxidation of Ni2+, resulting the lowered surface exchange rate.

Ion Transport and Surface Kinetics in Oxygen Ion Conducting Nanocrystalline Oxide Thin Films

Nanocrystalline oxide thin films, e.g., doped oxides of zirconia and ceria films, exhibit extraordinary electrical and electrochemical properties that are of great interest for applications such as solid oxide fuel cells (SOFCs). In nanocrystalline doped oxide thin films fabricated by atomic layer deposition (ALD) or pulsed laser deposition (PLD), interfacial properties are often found to dominate the electrochemical properties. For example, grain boundary plays a crucial role in ionic transport and surface exchange reactions. Ionic transport across grain-boundaries(GBs) is known to be hindered due to an electrical potential barrier built up by defect segregation, while exchange reaction is enhanced near the GBs due to the higher concentration of oxide ion vacancies. Such exotic electrochemical properties of oxide thin films are closely related to the distribution of defects near GBs. Along these lines, we investigated the defect structure near GBs in doped oxide thin films as well as the surface exchange and the ionic transport inside the films associated with grain size and the grain boundary density. We report on the atomic scale study of the defect structure near a single GB of yttria-stabilized zirconia (YSZ) [1]. We show significant oxygen deficiency due to segregation of oxide-ion vacancies near the grain-boundary core with half-width of 0.6 nm, which corroborates well with electron energy loss spectroscopy (EELS) measurements. Additionally, we recently conducted near-atomic-scale EELS study on gadolinia-doped ceria (GDC) thin films [2]: Homogeneous distribution of dopants was observed in as-deposited films, while strong segregation of dopant was shown in the annealed (1100°C) film a width of 1.2 nm to 1.5 nm and an enhancement factor of 1.9. It has been reported that grain boundaries at the electrode(cathode)/electrolyte interface are of great importance in enhancing the oxygen reduction kinetics in SOFCs. Oxide-ion incorporation is greatly facilitated by these surface grain boundaries [3]. Thin-film electrolytes with high surface grain-boundary density consistently show higher exchange current density, and therefore, smaller activation loss. These experimental observations are supported by secondary ion mass spectroscopy (SIMS) and electrochemical impedance spectroscopy (EIS) studies on doped zirconia and doped ceria thin films [3,4]. Polycrystalline YSZ and YDC were annealed in an 18O2 environment so that the exchange reaction between 16O2 inside YSZ and YDC and 18O in the environment can take place. Surface mapping using NanoSIMS clearly indicates the preferential enrichment of 18O along grain boundaries. The activation resistance at the cathode interface is significantly reduced by an electrolyte surface featuring a high density of grain boundaries that facilitates high surface exchange rates. In parallel, surfaces modified by compositionally graded cation doping layers of ~1 nm facilitate enhanced oxide-ion vacancy concentration and improve the power density of the thin-film SOFC by lowering the activation loss [5]. It is known that grain boundaries consisting of grain boundary core and the adjacent space charge layer pose a blocking effect on cross-boundary transport of ions. We show that thermal annealing, which in turn facilitates defect segregation and redistribution, plays an important role on the ionic conductivity of oxide thin films [2,6]. Nanocrystalline thin films of YDC and GDC were fabricated and then post-annealed for grain growth. Interestingly, as the grain size increases, ionic transport is indeed hindered, showing higher activation energy and lower ionic conductivity than the as-deposited films, which have the smallest grains. We speculate that the uniform distribution of dopants may attenuate the space charge layer barrier height and maintain high ionic conductivity, while the annealed films have fully developed space charge layer, which impedes ionic transport. In summary, the grain size and the grain boundary effects have a huge impact on surface exchange and ionic transport. We show that fundamental understanding and surface engineering of grain structures in oxide thin films may have significant implications in improving the electrochemical performance of thin film SOFCs, and therefore, widening their applications. References J. An, J. S. Park, A. L. Koh, H. B. Lee, H. J. Jung, J. Schoonman, R. Sinclair, T. M. Gür, F. B. Prinz, Sci. Rep., 3, 2680 (2013)J. Bae et al., in-preparation J. H. Shim, J. S. Park, T. P. Holme, K. Crabb, W. Lee, Y. B. Kim, X. Tian, T. M. Gür, F. B. Prinz, Acta Mater., 60, 1-7 (2012)Y. B. Kim, J. S. Park, T. M. Gür, F. B. Prinz, J. Power Sources, 196, 10550-10555 (2011)C. C. Chao, Y. B. Kim, F. B. Prinz, Nano Lett., 9, 3626-3628 (2009)J. An, J. Bae, S. Hong, B. Koo, Y. B. Kim, T. M. Gür, F. B. Prinz, Scr. Mater., 104, 45-48 (2015)

La2NiO4+δ infiltrated into gadolinium doped ceria as novel solid oxide fuel cell cathodes: Electrochemical performance and impedance modelling

This paper is devoted to the study of composite cathodes of La2NiO4+δ infiltrated into a Gd-doped ceria backbone. Porous Gd-doped ceria backbones are screen printed onto yttria-stabilized zirconia or Gd-doped ceria dense electrolytes, and infiltrated with a La and Ni nitrate solution (2:1 stoichiometry ratio). The influence of the preparation parameters on the polarization resistance, such as the concentration of the infiltration solution, the amount of infiltrated phase, the annealing temperature, the thickness of the electrode, and the nature of the electrolyte, is characterized by impedance spectroscopy performed on symmetrical cells. The optimization of these parameters results in a decrease of the polarization resistance down to 0.15 Ω cm2 at 600 °C. Using the Adler-Lane-Steele model, the modelling of the impedance diagrams leads to the determination of the ionic conductivity as well as the surface exchange rate of the infiltrated electrode.