Isotope Exchange Measurements Research Articles

Enhancing oxygen exchange kinetics of solid oxide fuel cell cathode: Unleashing the potential of higher order Ruddlesden-Popper phase surface modification

Tailoring the electrode surface represents a promising strategy to enhance electrochemical performance while preserving base material integrity. Achieving appropriate surface coverage with catalytic active oxide material is critical for efficient oxygen transport at the triple phase boundary (TPB). To further explore this approach, the perovskite structure La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) is decorated with a higher order Ruddlesden-Popper phase Pr4Ni3O10+δ (PNO). This combination is being investigated using electrical conductivity relaxation (ECR) technique to study the oxygen exchange kinetics. Samples with varying surface coverage of PNO are examined in a temperature range of 650 °C–850 °C, with a step change in pO2 between 0.1 atm and 0.21 atm. The chemical diffusion coefficient, Dchem, remains invariant across all the samples, however, the surface exchange coefficient, kchem, varies with the surface coverage of PNO. Notably, the coated sample with a PNO loading content of 0.28 mg cm−2, referred to as PNO5, demonstrates a significant enhancement of nearly two orders of magnitude in kchem compared to bare LSCF at 650 °C. This substantial improvement is ascribed to the increased active sites of ORR within the TPB region and the facilitated electron access through tunneling from LSCF to the coated phase. Pulse isotopic exchange (PIE) measurements on the fractionated powders confirm the fast surface exchange kinetics of PNO. Such remarkable oxygen exchange characteristics encourage the use of PNO, along with LSCF, as potential candidates for SOFC cathodes.

Opposites Attract: Electrostatically Driven Loading of Antimicrobial Peptides into Phytoglycogen Nanocarriers.

Antimicrobial peptides, such as GL13K, have a high binding selectivity toward bacterial membranes, while not affecting healthy mammalian cells at therapeutic concentrations. However, delivery of these peptides is challenging since they are susceptible to proteolytic hydrolysis and exhibit poor cellular uptake. A protective nanocarrier is thus proposed to overcome these obstacles. We investigate the potential to employ biodegradable phytoglycogen nanoparticles as carriers for GL13K using a simple loading protocol based on electrostatic association rather than chemical conjugation, eliminating the need for control of chemical cleavage for release of the peptide in situ. Both the native (quasi-neutral) and carboxymethylated (anionic) phytoglycogen were evaluated for their colloidal stability, loading capacity, and release characteristics. We show that the anionic nanophytoglycogen carries a greater cationic GL13K load and exhibits slower release kinetics than native nanophytoglycogen. Isotope exchange measurements demonstrate that the antimicrobial peptide is entrapped in the pores of the dendritic-like macromolecule, which should provide the necessary protection for delivery. Importantly, the nanoformulations are active against a Pseudomonas aeruginosa clinical isolate at concentrations comparable to those of the free peptide and representative, small molecule antibiotics. The colloidal nanocarrier preserves peptide stability and antimicrobial activity, even after long periods of storage (at least 8 months).

Exchange of Adsorbed Pb(II) at the Rutile Surface: Rates and Mechanisms.

The dynamics of Pb(II) at mineral surfaces affect its mobility in the environment. Pb(II) forms inner- and outer-sphere complexes on mineral surfaces, and this adsorbed pool often represents a large portion of the bioaccessible Pb in contaminated soils. To assess the lability of this potentially reactive adsorbed Pb(II) pool at metal oxide surfaces, we performed Pb(II) isotope exchange measurements between dissolved Pb(II) enriched in 207Pb and natural isotopic abundance Pb(II) adsorbed to rutile at pH 5, 6, and 7. We find that ∼95% of the adsorbed lead is exchangeable. An initially fast exchange (<1 h) is followed by a slower exchange that occurs on a time scale of hours to days. Pb LIII-edge extended X-ray absorption fine structure spectra indicate that similar binding mechanisms are present at all pH values and Pb(II) loadings, implying that differences in exchange rates across the pH range examined are not attributable to changes in the coordination environment. The slower exchange at pH 5 may be associated with interparticle and intraparticle diffusion resulting from particle aggregation. These findings demonstrate that the dissolved Pb(II) pool can be rapidly replenished by adsorbed Pb(II) if this pool is drawn down incrementally by biological uptake or a shift in chemical conditions.

Oxygen permeability and surface kinetics of composite oxygen transport membranes based on stabilized δ-Bi2O3

Composite ceramic membranes based on the ionic conducting Tm-stabilized δ-Bi2O3 (BTM) and the electronic conducting (La0.8Sr0.2)0.99MnO3-δ (LSM) exhibit among the highest oxygen flux values reported for Bi2O3-based membranes. Here, we use pulse-response isotope exchange (PIE) and oxygen flux measurements to elaborate on limiting factors for the oxygen permeation in BTM - 40-70 vol% LSM composites. Once both phases percolate, between 30 and 50 vol% BTM, the flux is essentially independent of the BTM/LSM volume ratio. The oxygen permeability is under mixed diffusion- and surface control, gradually becoming more bulk-limited with increasing temperature. The oxygen exchange coefficients of BTM-LSM are significantly higher than its constituent phases, revealing that a cooperative surface exchange mechanism enhances the kinetics. Some of the Tm was substituted with Pr to introduce electronic conductivity in BTM. (Bi0.8Tm0.15Pr0.05)2O3-δ (BTP) exhibits higher surface exchange coefficients compared to BTM, but the oxygen flux remains one order of magnitude lower than that of percolating BTM-LSM composites.

Pt nanoparticles embedded in CeO2 and CeZrO2 catalysts for biogas upgrading: Investigation on carbon removal mechanism by oxygen isotopic exchange and DRIFTS

This work investigated the effect of Pt nanoparticles embedded into CeO2 (Pt@CeO2) and CeZrO2 (Pt@CeZrO2) on the carbon removal mechanism for the dry reforming of methane, in comparison to impregnated Pt/CeO2. Morphological and structural characterization by TEM and Raman spectroscopy showed that Pt sintering is suppressed on both structures and the doping with Zr led to the CeZrO2 solid solution formation. A combination of TPR, oxygen isotopic exchange and DRIFTS measurements demonstrated that embedded Pt nanoparticles interact more strongly with ceria than supported Pt, and as a consequence, reactive lattice oxygen becomes more abundant on the catalyst surface, promoting the mechanism of carbon gasification over Pt nanoparticle. The low availability of oxygen species on Pt/CeO2 resulted in higher carbon formation, as demonstrated by TPO analysis. Therefore, the Pt@CeO2 and Pt@CeZrO2 catalysts were more resistant to coke formation due to the higher presence of reactive oxygen species at the metal-support interface, promoting the balance between the rates of carbon formation and carbon gasification over Pt nanoparticle.

Isotope Exchange Measurements of the Interfacial Reaction Rate Constant of Nitrogen on Fe-Mn alloys and an Advanced High-Strength Steel

Precise control of dissolved nitrogen is required for producing Third-Generation Advanced High-Strength Steels (AHSS). Compared with well-documented thermodynamic data, the data on the kinetics of nitrogen dissolution in these steels are limited. The isotope exchange technique (15N–14N exchange reaction) was applied to measure the nitrogen reaction rate at different temperatures, on pure liquid iron, liquid iron containing different levels of manganese, and liquid Fe-Mn-Al-Si steel with a Third-Generation AHSS composition. The increase in the interfacial reaction rate with increased temperature corresponded to an activation energy of 144.2 kJ/mol for pure iron. For Fe-Mn alloy, the interfacial reaction rate increased with the increased temperature and increased manganese content. For liquid AHSS (Fe-Mn-Al-Si), the interfacial reaction rate was approximately one third of the rate for pure liquid iron, at the same temperature, with a corresponding activation energy of 160.5 kJ/mol.

Investigating the effect of a fluoroethylene carbonate additive on lithium deposition and the solid electrolyte interphase in lithium metal batteries using in situ NMR spectroscopy

An in situ NMR study of Li deposition and the SEI on Li metal. Isotope exchange measurements reveal the fast transport properties in the SEI formed with FEC and the accelerated SEI formation rate, in part explaining the homogeneous electrodeposition using FEC additives.

The reactivity of lattice nitrogen within the Ni2Mo3N and NiCoMo3N phases

In this study, the reactivity of bulk lattice nitrogen within the filled β-Mn structured Ni2Mo3N phase has been investigated by application of powder neutron diffraction and heterolytic nitrogen isotopic exchange measurements. In contrast to Co3Mo3N, despite the similarity in the N immediate local environment comprising NMo6 octahedra, its reactivity is found to be limited and this lower reactivity was maintained upon the introduction of a significant proportion of cobalt to yield its filled β-Mn structured CoNiMo3N quaternary nitride counterpart.

Ru/Al2O3 catalyzed CO2 hydrogenation: Oxygen-exchange on metal-support interfaces

The metal-support interfaces of metallic nanoparticles supported on oxide surfaces determine the activated dissociation of CO2 in CO2 hydrogenation. It also guides the catalytic pathway towards either CO2 methanation or reverse water-gas shift (rWGS). In this work, Ru/Al2O3 catalysts with different Ru structural configurations were prepared by controlling the Ru weight loadings, which revealed the structure-dependence of production rates for CO and CH4 formation with different apparent activation energies. Based on the characterization results, two catalyst models were setup: the Ru9/Al2O3 model consisted of an interface of monolayer Ru sites tightly contacted with γ-Al2O3 support, and the Ru35/Al2O3 model represented a relatively larger Ru nanocluster supported on γ-Al2O3. Theoretical calculations of these two models demonstrated that monolayer Ru sites favored the rWGS route with a relatively low energy barrier for both CO2 activation and CO formation steps, while Ru nanoclusters preferred the methanation route energetically. Furthermore, the combination of theoretical calculations and experimental isotope-exchange measurements suggested that the interfacial O species in Ru-O-Al interfaces played a critical role in CO2 activation via oxygen-exchanging with the O atom in the feeding CO2 and consequently incorporation into the final hydrogenation product.

Contribution of Triple-Phase Boundary Reaction in Cathodic Reaction of Solid Oxide Fuel Cell

In a conventional SOFC cathode using mixed ionic and electronic conductor (MIEC), the electrochemical reaction proceeds not only at triple-phase boundaries (TPB: gas/electrode/electrolyte) but also on double-phase boundaries (DPB: gas/electrode). In the MIEC cathode, electrochemical reaction generally occurs within a limited area from electrode/electrolyte interface because the resistance for oxide ion conductivity becomes significant compared to that for surface reaction as the diffusion length of oxide ion becomes longer. Our research group has experimentally clarified the reaction area in a porous MIEC cathode by operando micro X-ray absorption spectroscopy (XAS) measurements (1). In a porous La0.6Sr0.4CoO3-δ electrode under a constant cathodic overpotential of 140 mV at 873 K in 10-2 bar of oxygen partial pressure P(O2), it was found the electrochemical reaction proceeded within 2 µm from the electrode/electrolyte interface. In the higher P(O2), the reaction area would become even shorter. It is in general believed that the total electrochemical reaction in a porous MIEC cathode is dominated by the reaction on DPB and the contribution of the reaction at TPB is insignificant. However, when the electrochemical reaction proceeds only in the vicinity of the electrode/electrolyte interface, as mentioned above, it is considered that the electrochemical reaction at TPB cannot be ignored. In this study, for investigating the contribution of the TPB reaction, patterned thin film electrodes with/without TPB, which were kinds of columnar electrodes, were fabricated. The distributions of electrochemical reaction in the model electrodes were evaluated by operando micro XAS measurements. In a La0.6Sr0.4CoO3-δ patterned electrode without TPB, the electrochemical reaction proceeded within 60 µm from the electrode/electrolyte interface under a constant cathodic overpotential of 190 mV at 973 K in 10-1 bar of P(O2). On the other hand, in the patterned electrode with TPB, the electrochemical reaction proceeded within 40 µm from the interface under the similar experimental conditions. The distribution of electrochemical reaction was decreased by the introduction of TPB, which demonstrated the contribution of the TPB reaction even in the MIEC cathode. The distribution of electrochemical reaction was also evaluated by isotopic exchange experiments and following secondary ion mass spectrometry (SIMS) measurements (2). 18O/16O isotopic exchanges were performed with/without cathodic overpotential, and the isotope exchange profiles were analyzed by SIMS measurements. The electrochemically incorporated 18O was evaluated as the difference between the isotope exchange profiles with/without cathodic overpotential. In the presentation, the contribution of the TPB reaction in the MIEC cathode will be discussed based on the results obtained by isotopic exchange measurements. 1. Y. Fujimaki, H. Watanabe, Y. Terada, T. Nakamura, K. Yashiro, S. Hashimoto, T. Kawada and K. Amezawa, ECS Trans., 2013, 57 (1), 1925-1932. 2. Y. Fujimaki, T. Nakamura, K. Develos-Bagarinao, K. Yamaji, K. Yashiro, T. Kawada, F. Iguchi, H. Yugami and K. Amezawa, ECS Trans., 2015, 68 (1), 623-630.

From in-Situ to in-Operando Evaluation of SOFC Cathodes for Enhanced ORR Activity and Durability

To study the SOFC cathode oxygen reduction reaction (ORR) mechanism and the effects of ambient gas composition on ORR, we used in-situ 18O isotope exchange techniques to probe the exchange of H2O and CO2 with two of the most common SOFC cathode materials, (La0.8Sr0.2)0.95MnO3-x (LSM) and La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF). A series of temperature programmed isotope exchange measurements were performed to comprehensively study the interaction of H2O and CO2 with the cathode surface as a function of temperature, oxygen partial pressure, and contaminant gas concentration. The experimental data are summarized in a Temperature-PO2 diagram to visualize the dominant reactions at each temperature and PO2 for the two cathode materials. We further report on the development of a novel in-operando 18O isotope exchange technique by modifying our heterogeneous catalysis system for operation and measurement under electrochemical polarization. The system allows ORR investigations under real time, real life, operating conditions. We will present our studies on the effect of cathodic polarization on LSM and LSCF. The studies show the effects of operating voltage, temperature, and oxygen partial pressure on the oxygen exchange coefficient. We will also present our model for the understanding and analysis of the isotope oxygen exchange profiles under in-operando conditions.

From in-Situ to in-Operando Evaluation of SOFC Cathodes for Enhanced ORR Activity and Durability

To study the SOFC cathode oxygen reduction reaction (ORR) mechanism and the effects of ambient gas composition on ORR, we used in-situ 18O isotope exchange techniques to probe the exchange of H2O and CO2 with two of the most common SOFC cathode materials, (La0.8Sr0.2)0.95MnO3-x (LSM) and La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF). A series of temperature programmed isotope exchange measurements were performed to comprehensively study the interaction of H2O with the cathode surface as a function of temperature, oxygen partial pressure, and contaminant gas concentration. We further report on the development of a novel in-operando 18O isotope exchange technique by modifying our heterogeneous catalysis system for operation and measurement under electrochemical polarization. The system allows ORR investigations under real time, real life, operating conditions. The studies show the effects of operating voltage, temperature, and oxygen partial pressure on the oxygen exchange coefficient. We will also present our model for the understanding and analysis of the isotope oxygen exchange profiles under in-operando conditions.

(Invited) From in-Situ to in-Operando Evaluation of SOFC Cathodes for Enhanced ORR Activity and Durability

To study the SOFC cathode oxygen reduction reaction (ORR) mechanism and the effects of ambient gas composition on ORR, we used in-situ 18O isotope exchange techniques to probe the exchange of H2O and CO2 with two of the most common SOFC cathode materials, (La0.8Sr0.2)0.95MnO3-x (LSM) and La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF). A series of temperature programmed isotope exchange measurements were performed to comprehensively study the interaction of H2O and CO2 with the cathode surface as a function of temperature, oxygen partial pressure, and contaminant gas concentration. The experimental data are summarized in a Temperature-PO2 diagram to visualize the dominant reactions at each temperature and PO2 for the two cathode materials. We further report on the development of a novel in-operando 18O isotope exchange technique by modifying our heterogeneous catalysis system for operation and measurement under electrochemical polarization. The system allows ORR investigations under real time, real life, operating conditions. We will present our studies on the effect of cathodic polarization on LSM and LSCF. The studies show the effects of operating voltage, temperature, and oxygen partial pressure on the oxygen exchange coefficient. We will also present our model for the understanding and analysis of the isotope oxygen exchange profiles under in-operando conditions.

Effects of Relative Humidity and Particle Phase Water on the Heterogeneous OH Oxidation of 2-Methylglutaric Acid Aqueous Droplets.

Organic aerosols can exist as aqueous droplets, with variable water content depending on their composition and environmental conditions (e.g., relative humidity (RH)). Recent laboratory studies have revealed that oxidation kinetics in highly concentrated droplets can be much slower than those in dilute solutions. However, it remains unclear whether aerosol phase water affects the formation of reaction products physically and/or chemically. In this work, we investigate the role of aerosol phase water on the heterogeneous chemistry of aqueous organic droplets consisting of 2-methylglutaric acid (2-MGA), measuring the reaction kinetics and the reaction products upon heterogeneous OH oxidation over a range of RH. An atmospheric pressure soft ionization source (direct analysis in real time, DART) coupled with a high-resolution mass spectrometer is used to obtain real-time molecular information on the reaction products. Aerosol mass spectra show that the same reaction products are formed at all measured RH. At a given reaction extent of the parent 2-MGA, the aerosol composition is independent of RH. These results suggest the aerosol phase water does not alter reaction mechanisms significantly. Kinetic measurements find that the effective OH uptake coefficient, γeff, decreases with decreasing RH below 72%. Isotopic exchange measurements performed using aerosol optical tweezers reveal water diffusion coefficients in the 2-MGA droplets to be 3.0 × 10-13 to 8.0 × 10-13 m2 s-1 over the RH range of 47-58%. These values are comparable to those of other viscous organic aerosols (e.g., citric acid), indicating that 2-MGA droplets are likely to be viscous at low humidity. Smaller γeff at low RH is likely attributed to the slower diffusion of reactants within the droplets. Taken together, the observed relationship between the γeff and RH is likely attributed to changes in aerosol viscosity rather than changes in reaction mechanisms.

Effect of Nb Doping on Hydration and Conductivity of La27W5O55.5−δ

Hydration properties and electrical characteristics of the high‐temperature proton conductor La27(W0.85Nb0.15)5O55.5−δ are investigated by means of thermogravimetry, impedance spectroscopy, and the electromotive force (EMF) method as a function of temperature, water vapor, and oxygen partial pressures, as well as isotope exchange measurements in order to elucidate the mechanism and thermodynamics of protons formation and transport. The highest proton conductivity, 1.3 × 10‐3 S/cm, is achieved at 700°C in wet O2. Proton self‐diffusion coefficients are estimated from thermogravimetric measurements of hydration and conductivity data. Comparison of the conductivity characteristics between nominally pure and Nb‐substituted materials reveals that the ionic conductivity increases and the activation energy decreases with Nb doping. These differences are discussed to reflect changes in the structure promoting ionic transport rather than changing the concentration of defects to any large extent.

A new mechanistic framework to predict OCS fluxes from soils

Abstract. Estimates of photosynthetic and respiratory fluxes at large scales are needed to improve our predictions of the current and future global CO2 cycle. Carbonyl sulfide (OCS) is the most abundant sulfur gas in the atmosphere and has been proposed as a new tracer of photosynthetic gross primary productivity (GPP), as the uptake of OCS from the atmosphere is dominated by the activity of carbonic anhydrase (CA), an enzyme abundant in leaves that also catalyses CO2 hydration during photosynthesis. However soils also exchange OCS with the atmosphere, which complicates the retrieval of GPP from atmospheric budgets. Indeed soils can take up large amounts of OCS from the atmosphere as soil microorganisms also contain CA, and OCS emissions from soils have been reported in agricultural fields or anoxic soils. To date no mechanistic framework exists to describe this exchange of OCS between soils and the atmosphere, but empirical results, once upscaled to the global scale, indicate that OCS consumption by soils dominates OCS emission and its contribution to the atmospheric budget is large, at about one third of the OCS uptake by vegetation, also with a large uncertainty. Here, we propose a new mechanistic model of the exchange of OCS between soils and the atmosphere that builds on our knowledge of soil CA activity from CO2 oxygen isotopes. In this model the OCS soil budget is described by a first-order reaction–diffusion–production equation, assuming that the hydrolysis of OCS by CA is total and irreversible. Using this model we are able to explain the observed presence of an optimum temperature for soil OCS uptake and show how this optimum can shift to cooler temperatures in the presence of soil OCS emission. Our model can also explain the observed optimum with soil moisture content previously described in the literature as a result of diffusional constraints on OCS hydrolysis. These diffusional constraints are also responsible for the response of OCS uptake to soil weight and depth observed previously. In order to simulate the exact OCS uptake rates and patterns observed on several soils collected from a range of biomes, different CA activities had to be invoked in each soil type, coherent with expected physiological levels of CA in soil microbes and with CA activities derived from CO2 isotope exchange measurements, given the differences in affinity of CA for both trace gases. Our model can be used to help upscale laboratory measurements to the plot or the region. Several suggestions are given for future experiments in order to test the model further and allow a better constraint on the large-scale OCS fluxes from both oxic and anoxic soils.

Fundamental Impact of Humidity on SOFC Cathode ORR

Although solid oxide fuel cells (SOFC) have demonstrated excellent performance, the durability of SOFCs under real working conditions is still an issue for commercial deployment. In particular cathode exposure to atmospheric air contaminants, such as humidity, can result in long-term performance degradation issues. Therefore, a fundamental understanding of the interaction between water molecules and cathodes is essential to resolve this issue and further enhance cathode durability. To study the effects of humidity on the oxygen reduction reaction (ORR), we used in-situ 18O isotope exchange techniques to probe the exchange of water with two of the most common SOFC cathode materials, (La0.8Sr0.2)0.95MnO3±δ (LSM) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF). In this experiment, heavy water, D2O (with a mass/charge ratio of m/z = 20), is used to avoid the overlapping of H2O and the 18O2 cracking fraction, which both provide a peak at m/z = 18. A series of temperature programmed isotope exchange measurements were performed to comprehensively study the interaction of water with the cathode surface as a function of temperature, oxygen partial pressure, and water vapor concentration. The results suggest that water and O2 share the same surface exchange sites, leading to competitive adsorption. Our findings show that water prefers to exchange with LSCF at lower temperatures, around 300–450°C. For LSM, O2 is more favorable than water to be adsorbed on the surface and the presence of O2 limits water exchange. The experimental data are summarized in a Temperature-PO2 diagram to help visualize how the exchange of water on each material depends on the operating conditions.

Quantitative Evaluation of Electrochemically Active Area in an SOFC Cathode by Oxygen Isotopic Exchange Measurements of a Model Patterned Electrode

In a conventional SOFC porous electrode consisting of mixed ionic and electronic conductor, the electrochemical reaction does not homogenously occur, and the reaction faradaic current decreases with the distance from the electrode/electrolyte interface. It is generally believed that the reaction distribution in the electrode is basically determined by the material properties, e.g. chemical diffusion coefficient D and surface reaction rate k, as well as the electrode microstructure, e.g. porosity, tortuosity, and particle size. However, so far, the contribution of D and k to the formation of the electrochemically active area was not quantitatively confirmed in an experimental manner. In this work, we experimentally evaluated the electrochemically active area in an La0.6Sr0.4CoO3-δ (LSC64) electrode by using 18O/16O isotopic exchange experiments and the following second ion mass spectroscopy (SIMS) analysis, and the contribution of D and k was discussed. Conventional SOFC porous electrodes have very complicated microstructures. For quantitative evaluation of the electrochemically effective area while eliminating the influence of structural complexities of the electrode, a columnar-type electrode (Fig. 1 (a)) is useful. However, this columnar electrode is difficult to be fabricated in practice. Therefore, we proposed the patterned film electrode, which is schematically shown by Fig. 1 (b). In this electrode, an Al2O3 insulating layer is partially inserted between electrode and electrolyte, and oxide ions can pass only through the electrode/electrolyte interface where the insulating layer does not exist. 16O/18O isotopic exchange experiments were performed with the patterned thin film electrode. The electrode was heat-treated at 973 K under P(16O2) = 1 bar for 1 hour and thereafter under the same pressure of 18O2 for 5 minutes. The dashed curve in Fig. 2 shows the observed profile of the oxygen isotopic ratio as a function of the distance from the electrode/electrolyte interface. The remarkable decrease of 18O concentration was found near the interface. This is because 16O can diffuse from electrolyte to electrode while surface-exchanged 18O can diffuse from electrode to electrolyte through the interface. Similar isotope exchange experiments were performed with the patterned thin film electrode under polarization. Cathodic bias of 220 mV were applied under P(O2) = 1 bar at 973K. The observed profile of the oxygen isotopic ratio was given by the solid curve in Fig. 2. The 18O concentration was gradually increased with approaching the electrode/electrolyte interface and drastically decreased in the vicinity of the interface. The increase of 18O was observed within about 20 μm from the interface. Such an increase of 18O near the interface is considered to be caused by electrochemical oxygen incorporation. This suggests that the electrochemically active area was approximately 20 μm from the electrode/electrolyte interface in the case of the LSC64 patterned thin film electrode under P(O2) = 1 bar at 973 K. In the presentation, the results obtained under various P(O2) and temperatures will be also presented, and the contribution of D and k to the formation of electrochemically active area will be discussed. Figure 1

Fundamental Impact of Humidity on SOFC Cathode Degradation

Exposure of solid oxide fuel cell (SOFC) cathodes to atmospheric air contaminants, such as humidity, impacts the oxygen reduction reaction (ORR) mechanism and can result in long-term performance degradation issues. Therefore, a fundamental understanding of the interaction between water molecules and the cathode is essential to develop improved performance cathodes with enhanced durability. To study the effects of humidity on ORR, we used in-situ 18O isotope exchange techniques to probe the exchange of water with two of the most common SOFC cathode materials, (La0.8Sr0.2)0.95MnO3-x (LSM) and La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF). In these experiments, heavy water, D2O (with a mass/charge ratio of m/z=20), was used to avoid the overlapping of the H2O and the 18O2 cracking fraction, which both have a peak at m/z=18. A series of temperature programmed isotope exchange measurements were performed to comprehensively study the interaction of water with the cathode surface as a function of temperature, oxygen partial pressure, and water vapor concentration. The results suggest that H2O and O2 share the same surface exchange sites. Therefore, the presence of H2O competes with O2 for the available surface sites. Our findings show that H2O prefers to exchange with LSM and LSCF at a lower temperature range, around 300-450°C. For LSM, O2 is more favorable than H2O to be adsorbed on LSM surface and the presence of O2 limits H2O exchange with the LSM surface. For LSCF, H2O has two different exchange mechanisms, each dominating in a different temperature region. The experimental data are summarized in a Temperature-PO2 diagram to visualize the dominant reactions at each temperature and PO2 for the two cathode materials.

Quantitative Evaluation of Electrochemically Active Area in an SOFC Cathode by Oxygen Isotopic Exchange Measurements of a Model Patterned Electrode

The electrochemically active area in an SOFC cathode was quantitatively evaluated from 18O/16O isotope exchange with/without polarization and following isotopic distribution analysis by second ion mass spectroscopy. For this purpose, we proposed a patterned dense thin film electrode modelling a columnar SOFC cathode, and was fabricated it with La0.6Sr0.4CoO3-δ on a Ce0.9Gd0.1O1.95-δ electrolyte. Compared with the results under open circuit condition, the increase in the isotopic ratio of 18O was observed near the electrode/electrolyte interface when a cathodic bias was applied. Such an increase in the 18O isotopic ratio was considered due to the electrochemical oxygen incorporation. The electrochemically active area was evaluated as approximately 25 μm in the La0.6Sr0.4CoO3-δ patterned thin film electrode under cathodic polarization of 220 mV in P(O2) = 1 bar at 973 K.