Reduced Surface Area for the Oxygen Reduction Reaction in Porous Electrode via Electrical Conductivity Relaxation.

Oxygen reduction reaction (ORR) performance of porous electrode is critical for solid oxide fuel cells (SOFCs). However, the effects of gas diffusion on the ORR in porous media need further investigation, although some issues, such as nonthermal surface oxygen exchange, have been attributed to gas diffusion. Herein, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) with various porosity, pore radii and gas permeability were investigated via the electrical conductivity relaxation method and analysed using the distributed of characteristic time (DCT) model. The ORR is revealed with three characteristic times, which are gas diffusion, oxygen exchange via the surface corresponding to small pores, and oxygen exchange to large pores. Gas diffusion delays the oxygen surface exchange reaction, resulting in very low chemical oxygen surface exchange coefficient compared with that obtained with dense sample under the assumption that all the surfaces are active for the ORR. Reduced surface area is thus defined to quantitatively represent the gas diffusion effects. The reduced surface area increases with increasing gas permeability, demonstrating the importance of electrode engineering for fast gas transport. Moreover, reduced surface area is suggested for replacing the specific surface area to calculate the electrode polarization resistance using the ALS model.

Adatom-Driven Oxygen Intermixing during the Deposition of Oxide Thin Films by Molecular Beam Epitaxy.

Thin film deposition from the vapor phase is a complex process involving adatom adsorption, movement, and incorporation into the growing film. Here, we present quantitative experimental data that reveals anion intermixing over long length scales during the deposition of epitaxial Fe2O3 and Cr2O3 films and heterostructures by oxygen-plasma-assisted molecular beam epitaxy. We track this diffusion by incorporating well-defined tracer layers containing 18O and/or 57Fe and measure their redistribution on the nanometer scale with atom probe tomography. Molecular dynamics simulations suggest potential intermixing events, which are then examined via nudged elastic band calculations. We reveal that adatoms on the film surface act to "pull up" subsurface O and Fe. Subsequent ring-like rotation mechanisms involving both adatom and subsurface anions then facilitate their mixing. In addition to film deposition, these intermixing mechanisms may be operant during other surface-mediated processes such as heterogeneous catalysis and corrosion.

A new open-path eddy covariance method for nitrous oxide and other trace gases that minimizes temperature corrections.

Low-power, open-path gas sensors enable eddy covariance (EC) flux measurements in remote areas without line power. However, open-path flux measurements are sensitive to fluctuations in air temperature, pressure, and humidity. Laser-based, open-path sensors with the needed sensitivity for trace gases like methane (CH4 ) and nitrous oxide (N2 O) are impacted by additional spectroscopic effects. Corrections for these effects, especially those related to temperature fluctuations, often exceed the flux of gases, leading to large uncertainties in the associated fluxes. For example, the density and spectroscopic corrections arising from temperature fluctuations can be one or two orders of magnitude greater than background N2 O fluxes. Consequently, measuring background fluxes with laser-based, open-path sensors is extremely challenging, particularly for N2 O and gases with similar high-precision requirements. We demonstrate a new laser-based, open-path N2 O sensor and a general approach applicable to other gases that minimizes temperature-related corrections for EC flux measurements. The method identifies absorption lines with spectroscopic effects in the opposite direction of density effects from temperature and, thus, density and spectroscopic effects nearly cancel one another. The new open-path N2 O sensor was tested at a corn (Zea mays L.) field in Southwestern Michigan, United States. The sensor had an optimal precision of 0.1 ppbv at 10 Hz and power consumption of 50 W. Field trials showed that temperature-related corrections were 6% of density corrections, reducing EC random errors by 20-fold compared to previously examined lines. Measured open-path N2 O EC fluxes showed excellent agreement with those made with static chambers (m = 1.0 ± 0.3; r2  = .96). More generally, we identified absorption lines for CO2 and CH4  flux measurements that can reduce the temperature-related corrections by 10-100 times compared to existing open-path sensors. The proposed method provides a new direction for future open-path sensors, facilitating the expansion of accurate EC flux measurements.

Mercury Isotope Fractionation during the Exchange of Hg(0) between the Atmosphere and Land Surfaces: Implications for Hg(0) Exchange Processes and Controls.

Atmosphere-surface exchange of elemental mercury (Hg(0)) is a vital component in global Hg cycling; however, Hg isotope fractionation remains largely unknown. Here, we report Hg isotope fractionation during air-surface exchange from terrestrial surfaces at sites of background (two) and urban (two) character and at five sites contaminated by Hg mining. Atmospheric Hg(0) deposition to soils followed kinetic isotope fractionation with a mass-dependent (MDF) enrichment factor of -4.32‰, and negligible mass-independent fractionation (MIF). Net Hg(0) emission generated average MDF enrichment factors (ε202Hg) of -0.91, -0.59, 1.64, and -0.42‰ and average MIF enrichment factors (E199Hg) of 0.07, -0.20, -0.14, and 0.21‰ for urban, background, and Hg mining soils and cinnabar tailing, respectively. Positive correlations between ε202Hg and ambient Hg(0) concentration indicate that the co-occurring Hg(0) deposition (accounting for 10-39%) in a regime of net soil emission grows with ambient Hg(0). The MIF of Hg(0) emission from soils (E199Hg range -0.27 to 0.14‰, n = 8) appears to be overall controlled by the photochemical reduction of kinetically constrained Hg(II) bonded to O ligands in background soils, while S ligands may have been more important in Hg mining area soils. In contrast, the small positive MIF of Hg(0) emission from cinnabar ore tailing (mean E199Hg = 0.21‰) was likely controlled by abiotic nonphotochemical reduction and liquid Hg(0) evaporation. This research provides critical observational constraints on understanding the Hg(0) isotope signatures released from and deposited to terrestrial surfaces and highlight stable Hg isotopes as a powerful tool for resolving atmosphere-surface exchange processes.

Interstitially Mixed Self-Assembled Monolayers Enhance Electrical Stability of Molecular Junctions.

Electrical breakdown is a critical problem in electronics. In molecular electronics, it becomes more problematic because ultrathin molecular monolayers have delicate and defective structures and exhibit intrinsically low breakdown voltages, which limit device performances. Here, we show that interstitially mixed self-assembled monolayers (imSAMs) remarkably enhance electrical stability of molecular-scale electronic devices without deteriorating function and reliability. The SAM of the sterically bulky matrix (SC11BIPY rectifier) molecule is diluted with a skinny reinforcement (SCn) molecule via the new approach, so-called repeated surface exchange of molecules (ReSEM). Combined experiments and simulations reveal that the ReSEM yields imSAMs wherein interstices between the matrix molecules are filled with the reinforcement molecules and leads to significantly enhanced breakdown voltage inaccessible by traditional pure or mixed SAMs. Thanks to this, bias-driven disappearance and inversion of rectification is unprecedentedly observed. Our work may help to overcome the shortcoming of SAM's instability and expand the functionalities.

Oxygen surface exchange kinetics measurement by simultaneous optical transmission relaxation and impedance spectroscopy: Sr(Ti,Fe)O3-x thin film case study.

We compare approaches to measure oxygen surface exchange kinetics, by simultaneous optical transmission relaxation (OTR) and AC-impedance spectroscopy (AC-IS), on the same mixed conducting SrTi0.65Fe0.35O3-x film. Surface exchange coefficients were evaluated as a function of oxygen activity in the film, controlled by gas partial pressure and/or DC bias applied across the ionically conducting yttria-stabilized zirconia substrate. Changes in measured light transmission through the film over time (relaxations) resulted from optical absorption changes in the film corresponding to changes in its oxygen and oxidized Fe (~Fe4+) concentrations; such relaxation profiles were successfully described by the equation for surface exchange-limited kinetics appropriate for the film geometry. The kchem values obtained by OTR were significantly lower than the AC-IS derived kchem values and kq values multiplied by the thermodynamic factor (bulk or thin film), suggesting a possible enhancement in k by the metal current collectors (Pt, Au). Long-term degradation in kchem and kq values obtained by AC-IS was also attributed to deterioration of the porous Pt current collector, while no significant degradation was observed in the optically derived kchem values. The results suggest that, while the current collector might influence measurements by AC-IS, the OTR method offers a continuous, in situ, and contact-free method to measure oxygen exchange kinetics at the native surfaces of thin films.

A novel enhanced diffusion sampler for collecting gaseous pollutants without air agitation.

A novel enhanced diffusion sampler for collecting gaseous phase polycyclic aromatic hydrocarbons (PAHs) without air agitation is proposed. The diffusion of target compounds into a sampling chamber is facilitated by continuously purging through a closed-loop flow to create a large concentration difference between the ambient air and the air in the sampling chamber. A glass-fiber filter-based prototype was developed. It was demonstrated that the device could collect gaseous PAHs at a much higher rate (1.6 ± 1.4 L/min) than regular passive samplers, while the ambient air is not agitated. The prototype was also tested in both the laboratory and field for characterizing the concentration gradients over a short distance from the soil surface. The sampler has potential to be applied in other similar situations to characterize the concentration profiles of other chemicals.

Mapping electrochemically driven gas exchange of mixed conducting SrTi0.7Fe0.3O3 - δ and Ce0.8Gd0.2O1.9 thin films by 18O tracer incorporation under reducing atmosphere.

Thermally and electrochemically driven 18O tracer exchange experiments in H2/H218O atmosphere were performed on SrTi0.7Fe0.3O3 - Î´ and Ce0.8Gd0.2O2 - Î´ thin films on single crystalline YSZ substrates. Noble metal current collectors were deposited on both films and electrochemically polarized during the exchange experiment. The resulting tracer distribution was analyzed by spatially resolved secondary ion mass spectrometry. Increased tracer fraction near the current collectors was found under cathodic polarization and decreased tracer fraction under anodic polarization. High cathodic bias leads to enhanced n-type electronic conductivity, which increases the extent of the electrochemically active zone.

Electrochemical properties of La0.6Sr0.4CoO3 - δ thin films investigated by complementary impedance spectroscopy and isotope exchange depth profiling.

The oxygen exchange and diffusion properties of La0.6Sr0.4CoO3 - Î´ thin films on yttria stabilized zirconia were analyzed by impedance spectroscopy and 18O tracer experiments. The investigations were performed on the same thin film samples and at the same temperature (400 °C) in order to get complementary information by the two methods. Electrochemical impedance spectroscopy can reveal resistive and capacitive contributions of such systems, but an exact interpretation of the spectra of complex oxide electrodes is often difficult from impedance data alone. It is shown that additional isotope exchange depth profiling can significantly help interpreting impedance spectra by giving reliable information on the individual contribution and exact location of resistances (surface, electrode bulk, interface). The measurements also allowed quantitative comparison of electrode polarization resistances obtained by different methods.

Cross-Validation of the Remarkably High Surface Oxygen Exchange Kinetics of PrBa0.5Sr0.5Co1.5Fe0.5O5+δ: A Combined Thin-Film Mass Relaxation and Bulk Electrical Conductivity Relaxation Study.

In this work, we measure the oxygen kinetic properties of double perovskite PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF), a material widely used as the air electrode in solid oxide electrochemical cells, by mass relaxation (MR) and electrical conductivity relaxation (ECR) experiments. MR studies are carried out using thin films deposited on a gallium phosphate piezocrystal microbalance, and ECR studies are performed using a bulk bar sample with 97% theoretical density. Measurements are performed at 600 °C over the temperature oxygen partial pressure range from 10-4 to 0.21 atm. Despite the differences in experimental formats and surface microstructural features, the ks values extracted from the two methods are found to be in good agreement with one another. The rate constant is found to increase with oxygen partial pressure with a power law dependence, rising from 1.0 × 10-6 cm/s at 3.2 × 10-4 atm to 1.2 × 10-4 cm/s at 0.24 atm, as averaged over the oxidation and reduction directions. The rates in the oxidation direction are observed to be slightly higher than those in the reduction direction for a given pair of pO2 values, suggesting that the final pO2 value controls the overall relaxation behavior. The power law exponent describing the dependence of ks on pO2 is found to be 0.74 ± 0.01. The ECR study of the bulk sample reveals that even with a diffusion length of 1.8 mm, the relaxation process is largely free of diffusion limitations, indicating that PBSCF has the high bulk transport properties required for a double-phase boundary oxidation/reduction pathway.

Enhanced Performance of La0.8Sr0.2FeO3-δ-Gd0.2Ce0.8O2-δ Cathode for Solid Oxide Fuel Cells by Surface Modification with BaCO3 Nanoparticles.

Recently, Fe-based perovskite oxides, such as Ln1-xSrxFeO3-δ (Ln = La, Pr, Nd, Sm, Eu) have been proposed as potential alternative electrode materials for solid oxide fuel cells (SOFCs), due to their good phase stability, electrocatalytic activity, and low cost. This work presents the catalytic effect of BaCO3 nanoparticles modified on a cobalt-free La0.8Sr0.2FeO3-δ-Gd0.2Ce0.8O2-δ (LSF-GDC) composite cathode at an intermediate-temperature (IT)-SOFC. An electrochemical conductivity relaxation investigation (ECR) shows that the Kchem value of the modified LSF-GDC improves up to a factor of 17.47, demonstrating that the oxygen reduction process is effectively enhanced after surface impregnation by BaCO3. The area-specific resistance (ASR) of the LSF-GDC cathode, modified with 9.12 wt.% BaCO3, is 0.1 Ω.cm2 at 750 °C, which is about 2.2 times lower than that of the bare cathode (0.22 Ω.cm2). As a result, the anode-supported single cells, with the modified LSF-GDC cathode, deliver a high peak power density of 993 mW/cm2 at 750 °C, about 39.5% higher than that of the bare cell (712 mW/cm2). The single cells based on the modified cathode also displayed good performance stability for about 100 h at 700 °C. This study demonstrates the effectiveness of BaCO3 nanoparticles for improving the performance of IT-SOFC cathode materials.

Nonstoichiometry, Defect Chemistry and Oxygen Transport in Fe-Doped Layered Double Perovskite Cobaltite PrBaCo2-xFexO6-δ (x = 0-0.6) Membrane Materials.

Mixed conducting cobaltites PrBaCo2-xFexO6-δ (x = 0-0.6) with a double perovskite structure are promising materials for ceramic semi-permeable membranes for oxygen separation and purification due to their fast oxygen exchange and diffusion capability. Here, we report the results of the detailed study of an interplay between the defect chemistry, oxygen nonstoichiometry and oxygen transport in these materials as a function of iron doping. We show that doping leads to a systematic variation of both the thermodynamics of defect formation reactions and oxygen transport properties. Thus, iron doping can be used to optimize the performance of mixed conducting oxygen-permeable double perovskite membrane materials.

High-Temperature 2D Optical Relaxation Visualizes Enhanced Oxygen Exchange Kinetics at Metal-Mixed Conducting Oxide Interfaces.

Solid-state heterointerfaces are of interest for emergent local behavior that is distinct from either bulk parent compound. One technologically relevant example is the case of mixed ionic/electronic conductor (MIEC)-metal interfaces, which play an important role in electrochemistry. Metal-MIEC composite electrodes can demonstrate improved catalytic activity vs single-phase MIECs, improving fuel cell efficiency. Similarly, MIEC surface reaction kinetics are often evaluated using techniques that place metal current collectors in contact with the surface under evaluation, potentially altering the response vs the native surface. Techniques enabling direct and local in situ observation of the behavior at and around such heterointerfaces are needed. Here, we develop a spatially resolved optical transmission relaxation (2D-OTR) method providing continuous evaluation of local, high-temperature, controlled atmosphere defect kinetics across a ∼1 cm2 sample area simultaneously in a contact-free manner. We apply it to observe the spatial variance of oxygen incorporation and evolution rates at ∼525-620 °C, in response to step changes in oxygen partial pressure, on MIEC SrTi0.65Fe0.35O3-x films as a function of distance from porous Pt and Au layers. Using this model geometry, we find significant enhancements in kinetics adjacent to the metals that decay over a few millimeter distance. To extract kinetic parameters, we fit the short-term optical data (initial portion of relaxations) with an exponential decay function appropriate for surface-exchange-limited kinetics, yielding apparent surface exchange coefficients (kchem) with spatial resolution, decreasing with distance from the metal. To understand the kinetic processes governing the complete (long-term) optical relaxations, we performed COMSOL simulations, which demonstrated that a combination of laterally varying kchem and in-plane diffusion controls the observed kinetics over the full time range. Further support for spatially varying kchem comes from demonstrations of changing surface and bulk chemistry vs distance from the metal-MIEC interface, by X-ray photoelectron and optical absorption spectroscopies, respectively. Although microporous Pt and Au are not excellent electrodes in isolation, both metals exert a synergistic effect on the oxygen surface exchange rate in the presence of the mixed conducting film.

Surface Exchange Reaction of Mixed Conductive La0.65Ca0.35FeO3-δ during Oxygen Evolution and Incorporation as Traced by Operando X-ray Photoelectron Spectroscopy.

High oxygen permeability of mixed conductive La0.65Ca0.35FeO3-δ (LCF) is applicable to pure oxygen gas generators and cathodes for solid oxide fuel cells, etc.; however, lower surface exchange reactions at temperatures below 800 °C reduce permeability. To understand the microscopic surface reaction mechanism, operando soft X-ray photoelectron spectroscopy of an LCF film surface was conducted during the evolution and incorporation of oxygen. LCF film was prepared on yttria-stabilized zirconia and a current was applied throughout the film at ∼600 °C. From operando X-ray photoelectron spectra, surface oxide species involved in the surface exchange reaction obviously appeared on the film during the evolution of oxygen from the surface. The number of surface oxide species abruptly decreased during incorporation of oxygen. By applying the current from a negative to positive value, the numbers of surface oxide species and ligand holes near Fe3+ ions on the surface both significantly increased. The results infer that ligand holes in the Fe 3d-O 2p hybrid orbitals correspond to active reaction sites at which surface oxide species change to oxygen molecules. Increasing the number of active reaction sites is key to improving oxygen evolution of mixed conductive oxides.

Significant mercury efflux from a Karst region in Southwest China - Results from mass balance studies in two catchments.

Karst regions have long been recognised as landscapes of ecological vulnerability, however the mass balance and fate of mercury (Hg) in karst regions have not been well documented. This study focused on the largest contiguous karst area in China and investigated Hg mass balance in two catchments, one with high geological Hg (Huilong) and the other representative of regional background Hg (Chenqi). The mass balance of Hg was calculated separately for the two catchments by considering Hg in throughfall, open field precipitation, total suspended particulate matter (TSP), litterfall, fertilizer, crop harvesting, air-surface Hg0 exchange, surface runoff and underground runoff. Results show that litterfall Hg deposition is the largest loading (from atmosphere) of Hg in both catchments, accounting for 61.5% and 38.5% of the total Hg input at Huilong and Chenqi, respectively. Air-surface Hg0 exchange is the largest efflux, accounting for 71.7% and 44.6% of the total Hg output from Huilong and Chenqi, respectively. Because both catchments are subject to farm and forest land use, cultivation plays an important role in shaping Hg fate. Mercury loading through fertilizer was ranked as the second largest input (28.5%) in Chenqi catchment and Hg efflux through crop harvest was ranked as the second largest output pathway in both Huilong (27.0%) and Chenqi (52.9%). The net Hg fluxes from the catchments are estimated to be 1498 ± 1504 µg m-2 yr-1 and 4.8 ± 98.2 µg m-2 yr-1. The significantly greater magnitude of net Hg source in Huilong is attributed to higher air-surface Hg0 exchange. The output/input ratio of Hg in this study was much greater than has been reported for other forest or agricultural ecosystems and indicates that the karst region of Southwest China is a significant source of atmospheric Hg. The results of this study should be considered in the development of pollution control policies which seek to conserve fragile karst ecosystems characterised by high geological background of Hg.

Global estimates of dry ammonia deposition inferred from space-measurements.

Ammonia (NH3), as an alkaline gas, contributes substantially to atmospheric nitrogen deposition, which can cause biodiversity loss, water eutrophication and soil acidification. Advances in the application of satellite observations allow us to gain deeper insights into atmospheric NH3 concentrations at large spatial scales. A new satellite-based methodology is proposed for estimating dry NH3 deposition with consideration of bi-directional NH3 exchange. We estimate the global dry NH3 deposition for nine years (2008-2016) by using the Infrared Atmospheric Sounding Interferometer Instrument (IASI) NH3 retrievals. Satellite-based dry NH3 deposition is in general consistent with measured dry NH3 deposition over the monitoring sites (R2 = 0.65). Global dry NH3 deposition over 8 kg N ha-1 is mainly distributed in the Eastern China, Northern and Central Pakistan, and Northern India. An annual increase rate of 0.27 and 0.13 kg N ha-1 y-1 in dry NH3 deposition during 2008-2016 occurs in Eastern China and Sichuan Basin, which are the major Chinese agricultural regions. The NH3 compensation point is high during warm months, and can be above 1 µg m-3 such as in Eastern China, implying the importance of considering the NH3 compensation points for estimating dry NH3 deposition. We find, if the upward NH3 flux was ignored, it will cause 11%, 17%, 5% and 3% overestimation in dry NH3 deposition in Eastern China, Northern India, Eastern United States and Western Europe, respectively. This study presents the potential of using the satellite retrievals to estimate the large-scale dry NH3 deposition, and the methodology is able to provide temporally continuous and spatially complete fine-resolution datasets.

Accelerated Hydrogen Diffusion and Surface Exchange by Domain Boundaries in Epitaxial VO2 Thin Films.

Electronic phase modulation based on hydrogen insertion/extraction is kinetically limited by the bulk hydrogen diffusion or surface exchange reaction, so slow hydrogen kinetics has been a fundamental challenge to be solved for realizing faster solid-state electrochemical switching devices. Here we accelerate electronic phase modulation that occurs by hydrogen insertion in VO2 through vertically aligned 2D defects induced by symmetry mismatch between epitaxial films and substrates. By using domain-matching epitaxial growth of monoclinic VO2 films with lattice rotation and twinning on hexagonal Al2O3 substrates, the domain boundaries naturally align vertically; they provide a "highway" for hydrogen diffusion and surface exchange in VO2 films and overcome the limited rates of bulk diffusion and surface reaction. From the quantitative analysis of the deuterium (2H) isotope tracer exchange, it is confirmed that the tracer diffusion coefficient (D*) and surface exchange coefficient (k*) were increased by several orders of magnitude in VO2 films that had domain boundaries. These results yield fundamental insights into the mechanism by which mobile ions are inserted along extended defects and provide a strategy to overcome a limitation to switching speed in electrochemical devices that exploit ion insertion.

Mercury emission from industrially contaminated soils in relation to chemical, microbial, and meteorological factors.

The Minamata Convention entered into force in 2017 with the aim to phase-out the use of mercury (Hg) in manufacturing processes such as the chlor-alkali or vinyl chloride monomer production. However, past industrial use of Hg had already resulted in extensive soil pollution, which poses a potential environmental threat. We investigated the emission of gaseous elemental mercury (Hg0) from Hg polluted soils in settlement areas in the canton of Valais, Switzerland, and its impact on local air Hg concentrations. Most soil Hg was found as soil matrix-bound divalent Hg (HgII). Elemental mercury (Hg0) was undetectable in soils, yet we observed substantial Hg0 emission (20-1392 ng m-2 h-1) from 27 soil plots contaminated with Hg (0.2-390 mg Hg kg-1). The emissions of Hg0 were calculated for 1274 parcels covering an area of 8.6 km2 of which 12% exceeded the Swiss soil remediation threshold of 2 mg Hg kg-1. The annual Hg0 emission from this area was approximately 6 kg a-1, which is almost 1% of the total atmospheric Hg emissions in Switzerland based on emission inventory estimates. Our results show a higher abundance of Hg resistance genes (merA) in soil microbial communities with increasing soil Hg concentrations, indicating that biotic reduction of HgII is likely an important pathway to form volatile Hg0 in these soils. The total soil Hg pool in the top 20 cm of the investigated area was 4288 kg; hence, if not remediated, these contaminated soils remain a long-term source of atmospheric Hg, which is prone to long-range atmospheric transport.

Correlation of Time-Dependent Oxygen Surface Exchange Kinetics with Surface Chemistry of La0.6Sr0.4Co0.2Fe0.8O3-δ Catalysts.

The Sr segregation at the surface of a perovskite La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) oxygen electrode is detrimental to the electrochemical performance and durability of energy conversion devices such as solid oxide fuel cells. However, a quantitative correlation of degradation of the oxygen surface exchange kinetics with Sr precipitation formation at the LSCF surface is not clearly understood yet. Herein, the correlation of the time-dependent degradation mechanisms of the LSCF catalysts with respect to Sr segregation phenomenon at the surface were investigated at 800 °C for a prolonged annealing time (∼800 h) by combining in situ electrochemical measurements, and ex situ chemical and structural analyses at the multiscale. The in situ monitored surface exchange coefficient (kchem) was found to drastically drop by ∼86% over the 800 h, and it was accompanied by the formation of Sr-containing secondary phases on the bulk LSCF surface, as expected. However, the estimated coverage of Sr segregation on the LSCF surface was only ∼15%, even after 800 h of aging time, showing significant deviation from the kchem degradation rate (∼86%). The surface chemistry evolution at the clean surface area, which is believed to be electrochemically active, was further analyzed on the nanoscale. The quantified results showed that the Sr elemental fraction of the A-site at the outermost surface of the LSCF samples was becoming deficient from ∼4.0 at 0 h to ∼0.27 at 800 h annealing. Interestingly, the time-dependent behavioral tendencies between kchem degradation and surface Sr fractional changes were highly analogous. Thus, our results suggest that this Sr deficiency at the clean surface region more dominantly impacts the degradation process rather than an electrochemical activity passivation by the SrOx precipitates, which has been shown to be a major degradation mechanism of LSCF performance.

Emergence of Rapid Oxygen Surface Exchange Kinetics during in Situ Crystallization of Mixed Conducting Thin Film Oxides.

The oxygen surface exchange kinetics of mixed ionic and electronic conducting oxides (MIECs) play a critical role in the efficiency of intermediate-to-high-temperature electrochemical devices. Although there is increasing interest in low-temperature preparation of MIEC thin films, the impact of the resultant varied degrees of crystallinity on the surface exchange kinetics has not been widely investigated. Here, we probe the effect of crystallization on oxygen surface exchange kinetics in situ, by applying an optical transmission relaxation (OTR) approach during annealing of amorphous films. OTR enables contact-free, in situ, and continuous quantification of the oxygen surface exchange coefficient ( kchem); we previously applied it to Pr xCe1- xO2-δ and SrTi1- xFe xO3-δ thin films. In this work, the OTR approach was successfully extended to other mixed conducting thin film compositions for the first time (i.e., perovskite SrTi0.65Co0.35O3-δ and Ruddlesden-Popper Sr2Ti0.65Fe0.35O4±Î´), as well as to Pr0.1Ce0.9O2-δ, enabling quantification of the kchem of their native surfaces and comparison of the behavior of films with different final crystal structures. All thin films were prepared by pulsed laser deposition at 25 or 700-800 °C and subject to subsequent thermal treatments with simultaneous OTR monitoring of kchem. The surface roughness, grain size, and crystallinity were evaluated by scanning probe microscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Fluorite Pr0.1Ce0.9O2-δ films grown at 25 °C did not exhibit an increase in kchem after annealing, as they were already crystalline as grown at 25 °C. For all other compositions, OTR enabled in situ observation of both the crystallization process and the emergence of rapid surface exchange kinetics immediately upon crystallization. Perovskite SrTi0.65Co0.35O3-δ and Ruddlesden-Popper Sr2Ti0.65Fe0.35O4±Î´ thin films grown at 25 °C exhibited at least 1-2 orders of magnitude enhanced kchem after annealing compared with highly crystalline thin films grown at 800 °C, indicating the benefits of in situ crystallization.