Raman spectroscopy and thermal gravimetric analysis (TGA) were used to evaluate the thermal and atmosphere stability of Sr1.9VMoO6-δ (SVMO-19), an A-site deficient double perovskite. Motivated by previous reports describing SVMO-19's unprecedented electrical conductivity under reducing atmospheres, studies described in this work determine SVMO-19's stability under conditions commonly encountered in high temperature solid oxide electrolysis and fuel cell applications. Vibrational Raman data show that SVMO-19 is stable up to 1000 °C under reducing, inert, and CO2 containing atmospheres. Under air, however, in situ Raman data show that SVMO-19 phase separates at temperatures ≥ 600 °C. The primary degradation products include a scheelite phase (SrMoO4) as well as a vanadium containing single perovskite, SrVO3, and a vanadium containing pyrochlore Sr2V2O7. TGA measurements suggest that SVMO decomposition in air begins at even lower temperatures (400 °C). TGA data show that SVMO is stable under N2 at temperatures as high as 900 °C, consistent with Raman data. SVMO oxidation kinetics are analyzed using both a simple kinetic model consisting of two independent first-order processes and an Avrami model. The data are better described by the pair of first order processes, but an Arrhenius analysis using both models result in an activation energy (E a) for SVMO degradation between 0.48 and 0.65 eV. Taken together, these findings are considered in the context of properties required by electrode materials used in reversible solid oxide electrochemical cells.

Vibrational sum frequency generation (VSFG) and Langmuir trough surface pressure measurements were used to investigate how the soluble surfactant perfluorooctanoic acid (PFOA) affects dipalmitoylphosphatidylcholine (DPPC) monolayers adsorbed to the air-water interface. Studies were performed as a function of PFOA concentration for DPPC surface coverages of 40 Å2/monomer and 55 Å2/monomer, corresponding to tightly packed and moderately packed monolayers, respectively. VSFG data showed that PFOA does not affect the structure of tightly packed DPPC monolayers, but cooperative adsorption of PFOA to moderately packed DPPC monolayers forces lipid compression, creating highly ordered acyl chains. PFOA adsorption to moderately packed DPPC monolayers also creates a charged interface, evidenced by significant intensity growth in the -OH stretching region as interfacial water is oriented by the resulting electric double layer. Interestingly, this effect is also observed for the tightly packed DPPC monolayers, implying that the surfactant adsorbs to the solvated lipid headgroups without affecting lipid chain conformation. PFOA-DPPC interactions at all DPPC surface coverages are apparent in Langmuir isotherms, where PFOA concentrations as low as 10 μM induce DPPC monolayer liftoff at 130 Å2/monomer rather than ∼90 Å2/monomer observed in the absence of PFOA and a monolayer collapse pressure that is ∼6 mN/m lower than that for a pure DPPC monolayer.

This study employed complementary optical and electrochemical methods to identify mechanisms responsible for the electrochemical removal of carbon from commercial Ni-yttria stabilized zirconia (YSZ) cermet anodes in functioning solid oxide fuel cells (SOFCs) operating at 800 degrees C. Near infrared thermal imaging (NIRTI), Fourier-transform infrared emission spectroscopy (FTIRES), and chronoamperometry (CA) were used to measure changes under an inert atmosphere to a carbon-contaminated anode after an overpotential is applied. NIRTI showed that the thermal processes accompanying the electrochemical oxidation of carbon is symmetrical around the current collector when the device is operated under potentiostatic conditions. FTIRES showed a steep rise of both CO and CO2 produced and similarly steep fall in the amount of CO while the concentration of gas phase CO2 remains relatively constant as the carbon is oxidized. The evolution of the chronoamperometry data together with the NIRTI and FTIRES data show that carbon oxidation is heterogeneous with carbon closest to the current collector being removed at higher currents and carbon further away being electrochemically oxidized at lower currents. These observations suggest electrochemical carbon oxidation processes dominate at higher currents and catalytic carbon gasification by the electrochemical products dominate at lower currents.

Perfluorooctanoic acid's (PFOA) effects on human hemoglobin (Hb) at micromolar and submicromolar PFOA concentrations were investigated using time-correlated single photon counting (TCSPC) fluorescence, native mass spectrometry (NMS), and ion mobility spectrometry (IMS). TCSPC results show that tryptophan fluorescence quenching mechanisms in Hb change from Förster resonance energy transfer (FRET) with the heme to charge transfer to the peptide backbone as the PFOA concentration increases. NMS showed 4 lower and 2 higher affinity sites for PFOA interacting with Hb. At concentrations as low as 10 nM, 2 PFOA molecules bind to Hb, leading to destabilization of the complex, loss of an α subunit, and release of heme. Together, these data show that PFOA alters the biophysical properties of human Hb in ways that suggest allosteric inhibition.

Optical emission spectroscopy was used to spatially resolve excited OH (OH*) emission as a function of height along the flame for nitromethane (NM) combusting in both inert and oxidative environments at 34 bar. Emission spectra from OH A2Σ+ → X2Π relaxation show that electronically excited OH* emission persists over the length of a nitromethane flame in air, but emission diminishes abruptly within several millimeters of nitromethane flames burning in inert atmospheres (N2). These findings mark the first reported instances of spatially resolved spectral images of NM combustion at elevated pressures and provide important benchmarks for monopropellant combustion models, where number densities of products and intermediates are often reported as a function of distance from a nominal flame front. Vibrational and rotational temperatures calculated as a function of distance along the flame are compared to Cantera-simulated adiabatic flame temperatures. OH* rotational temperatures appear thermalized, although vibrational temperatures suggest that the radical possesses excess vibrational energy. Additionally, vibrational temperatures are used to infer the formation pathway─chemical vs thermal─of OH* as a function of height within the flame.

A number of mechanistic schemes exist attempting to outline the processes involved in the reforming of carbon in solid oxide fuel cells (SOFCs) using ex situ techniques at temperatures and in environments that are far from those used under operational conditions and therefore not necessarily reflective of true events. I n situ vibrational spectroscopies (Raman and Fourier transform infrared emission, FTIRE) and near infrared thermal imaging (NIRTI) techniques are uniquely suited to probe the processes occurring in SOFCs in real time under operando conditions. In this study these methodologies are coupled to electrochemical data and employed to elucidate and compare the mechanisms underlying the partial oxidation reforming reactions. This process seems to be governed by the reformer’s affinity for the Ni-anode surface. We argue that high affinity for the anode surface reported for O, resulting from the surface dissociation of O 2 , result in slow oxidation of carbon possibly due to competition with the more favorable formation of NiO.

Stabilized Li6.25La3Al0.25 Zr2O12 (cubic LLZO or c-LLZO) is a Li+-conducting ceramic with ionic conductivities approaching 1 mS-cm. Processing c-LLZO so that it is suitable for use as a solid state electrolyte in all solid state batteries, however, is challenging due to the formation of secondary phases at elevated temperatures. The work described in this manuscript examines the formation of one such secondary phase La2Zr2O7 (LZO) formed during sintering c-LLZO at 1000 °C. Specifically, spatially resolved Raman spectroscopy and X-ray Diffraction (XRD) measurements have identified gradients in Li distributions in the Li ion (Li+)-conducting ceramic Li6.25La3Al0.25 Zr2O12 (cubic LLZO or c-LLZO) created by thermal processing. Sintering c-LLZO under conditions relevant to solid state Li+ electrolyte fabrication conditions lead to Li+ loss and the formation of new phases. Specifically, sintering for 1 h at 1000 °C leads to Li+ depletion and the formation of the pyrochlore lanthanum zirconate (La2Zr2O7 or LZO), a material known to be both electronically and ionically insulating. Circular c-LLZO samples are covered on the top and bottom surfaces, exposing only the 1.6 mm-thick sample perimeter to the furnace’s ambient air. Sintered samples show a radially symmetric LZO gradient, with more LZO at the center of the pellet and considerably less LZO at the edges. This profile implies that Li+ diffusion through the material is faster than Li+ loss through volatilization, and that Li+ migration from the center of the sample to the edges is not completely reversible. These conditions lead to a net depletion of Li+ at the sample center. Findings presented in this work suggest new strategies for LLZO processing that will minimize Li+ loss during sintering, leading to a more homogeneous material with more reproducible electrochemical behavior.

Surface tension, conductivity, and dynamic light scattering (DLS) measurements were used to examine the surface and bulk solution behaviors of three members of the PFAS family, perfluorooctanoic acid (PFOA), perfluorobutanesulfonic acid (PFBS), and the potassium salt of perfluorooctanesulfonic acid (PFOS). Measurements were carried out in solutions having variable (acidic) pH and in solutions buffered to pH = 8.0. Surface tension data show traditional soluble surfactant behavior, and results illustrate that PFOA, PFBS, and PFOS surface activity depends sensitively on solution phase pH. The tightly packed monolayers formed by PFOA in mildly acidic solutions imply that the surface pH of PFOA solutions is several units lower than bulk. Results from conductivity experiments generally show increasing conductivity with increasing bulk solution surfactant concentration. In pH = 8.0 solutions, changes in conductivity slope with surfactant concentration suggest the onset of micelle formation at concentrations <1 mM, markedly lower than reported in literature. In general, apparent critical micelle concentrations (CMCs) determined from conductivity data agree with similar predictions made from surface tension results. DLS measurements show that at concentrations close to the predicted PFAS CMCs, objects with diameters <= 10 nm start to form. However, unlike micelles, these objects continue to grow with increasing bulk solute concentration. These aggregates form structures having diameters of 50-150 nm. Aggregate size shrinks modestly as solution phase temperature increases, and this behavior is reversible. Cryo-EM images of PFOA solutions confirm a broad distribution of particles, supporting the DLS measurements. Findings reported in this work represent the first evidence that these three EPA-regulated PFAS surfactants form aggregates rather than micelles in solution. Findings also begin to reconcile differences in reported surface behaviors that have led to CMC predictions in the literature varying by more than an order of magnitude.

Optical emission spectroscopy was used to measure rovibrationally resolved spectra of water formed during nitromethane combustion at elevated pressures in both oxidizing and inert atmospheres. Complementary kinetic models were used to predict flame temperatures, product distributions, and product formation rates, and results were compared with experimental observations. Experiments were carried out at pressures of 27.4 bar and 34.2 bar in inert atmospheres (referred to as monopropellant conditions) and air (referred to as bipropellant conditions). Dispersed emission shows many rovibrational transitions with the strongest occurring between 13,000 and 14,500 cm(-1). These lines are primarily assigned to the relaxation from water's (3,0,1) vibrationally excited state to its vibrational ground state. Weaker progressions in this same region are assigned to water relaxation from the (1,0,3) and (2,2,1) vibrationally excited states. Data collected under mono- and bipropellant conditions showed very similar relative intensities of individual rovibrational lines. The spectra were fit using a simulated temperature of 2,500 K +/- 500 K for both mono- and bipropellant conditions. Ex situ FTIR spectra of NM exhaust confirms the presence of H2O in both mono- and bipropellant combustion. Interestingly, analyses of these same spectra also show significant amounts of CO in monopropellant exhaust, but no detected CO in bipropellant exhaust. These latter findings show higher CO/CO2 concentration ratios compared to simulations, motivating the need for refined models describing nitromethane monopropellant combustion.

Independent methods show that sub-microMolar concentrations of perfluorooctanoic acid (PFOA), a member of the PFAS family of "forever chemicals", change the properties of DPPC vesicle bilayers. Specifically, calorimetry measurements show that PFOA at concentrations as low as 0.1 nM lowers DPPC's gel-liquid crystalline transition enthalpy by several J/g without changing the transition temperature (T gel-LC), and dynamic light scattering (DLS) data illustrate that PFOA markedly broadens the size distribution of DPPC vesicles. Furthermore, DLS results from PFOA-containing, DPPC vesicle solutions also contain smaller objects having diameters of 30-50 nm. Close inspection of cryo-EM images reveals that DPPC vesicles formed in the presence of PFOA are multilamellar and the smaller objects have a clear bilayer structure similar to niosomes. A consequence of these PFOA-induced changes to DPPC bilayer structure is that the bilayers themselves are more susceptible to secondary solute accumulation. Time resolved emission measurements of Coumarin 152 (C152) report that C152 is 3-fold more likely to partition into the bilayer's acyl chain, hydrophobic interior when PFOA is present, and fluorescence lifetimes from C152 partitioned into the polar region of the lipid bilayer show evidence of PFOA-induced membrane hydration below T gel-LC.

Vibrational sum frequency generation (VSFG), Fourier transform infrared (FTIR) absorbance, and Raman scattering were used to investigate the water structure at the gypsum (CaSO42H(2)O)-air and gypsum-water interfaces under ambient thermal and atmospheric conditions. Results show that water structurally embedded in the gypsum matrix consists of two different populations. One population has decoupled -OH bonds with one bond oriented in-plane, while the second water population has a more traditional, normal mode structure with the molecular C-2 symmetry axis aligned along the surface normal. Based on previously reported molecular dynamics simulations, we propose that surface water molecules having the decoupled -OH bonds (population 1) result from strong hydrogen bond donation to gypsum's sulfate oxygens, whereas water molecules in the second population sample a more symmetric environment that shifts water's vibrational frequencies to similar to 3400 cm(-1). Gypsum samples in contact with bulk water show significantly diminished VSFG signals with no observable new features. This result is attributed to loss of VSFG response due to the signal transmission into the bulk water and to an absence of surface-induced structure in the adjacent aqueous phase, consistent with a mineral surface near its point of zero charge. Removal of the bulk aqueous phase results in VSFG spectra that are very similar to those measured prior to water immersion, even when the aqueous phase consisted of D2O although relative intensities of individual bands change following exposure to D2O. These results imply that some fraction of gypsum's surface structural water remains associated with the substrate and does not readily exchange with an adjacent phase. Furthermore, these findings provide direct molecular-level insight into previous force microscopy studies that proposed the existence of tightly bound surface waters at the gypsum/aqueous interface.

Accumulated carbon (also termed coke) formed by hydrocarbon fuels on solid oxide fuel cell (SOFC) anodes blocks electrocatalytic sites at triple-phase boundaries, impedes transport through the porous electrode, and can react with nickel (Ni) to further degrade electrode performance. These effects are mitigated in the presence of oxygen-containing reformers such as H2O, CO2, and O2. However, the mechanism responsible for carbon abatement by reforming agents remains speculative, with many models proposed but little direct, experimental evidence to support them. In this work, we use operando near-infrared thermal imaging and Fourier transform infrared emission spectroscopy to expand on previous operando Raman spectroscopic studies that examined carbon gasification of a precoked Ni-YSZ membrane electrode assembly. The work presented in this article demonstrates significant differences in the gasification of carbon by H2O compared to similar concentrations of O2. These differences include spatially homogeneous cooling over the anode under humidified Ar corresponding to the endothermic gasification of carbon with H2O versus spatially heterogeneous heating over the anode under O2 that is localized near the gas entry port of the anode chamber. The anode surface temperature differences observed between H2O- and O2-driven gasification are discussed within the context of product evolution and the impact on the SOFC electrochemical performance.

Resonance enhanced Second Harmonic Generation (SHG) was employed to assess if conduction band electrons in silicon (Si) will promote molecular adsorption of ambient species and how such adsorption depends on temperature. Experiments were performed with three types of Si (n-doped or n-Si, p-doped or p-Si, and undoped Si) at temperatures between 18 and 260 degrees C and under atmospheres of dry N-2 and dry (cylinder) air. All Si types were covered with a 2-4 nm thick native oxide layer. Under N-2, all Si types behave similarly, with SHG intensity [I(2 omega)] diminishing with increasing temperature. This effect was reversible and attributed to electron-phonon scattering. In the presence of O-2, I(2 omega) from n-Si at room temperature is enhanced significantly. Neither p-doped Silicon (p-Si) nor undoped Si show similar effects at room temperature, with I(2 omega) being independent of gas phase composition. At temperatures >= 175 degrees C, all three Si types behaved similarly with no dependence on atmospheric O-2 content. Varying the amount of O2 above n-Si at room temperature and measuring I(2 omega) suggested that O-2 adsorption to n-Si could be described with a Langmuir isotherm and an adsorption energy of -0.13 +/- 0.05 eV. Increasing n-Si's oxide thickness (to 600 nm) rendered the substrate insensitive to ambient gas phase composition. Taken together, these findings support a description of Si's surface electronic structure that is controlled by n-Si conduction band electrons backbonding into the pi* orbitals of adjacent O-2 and imply that these conduction band electrons can affect adsorption despite the presence of a native oxide film.

Carbon formation remains the primary degradation mechanism for solid oxide fuel cells (SOFCs) operating on carbonaceous fuels. The mechanisms for the remediation of carbon (C) induced degradation via electrochemical gasification and reforming using O 2(g) and H 2 O (g) was studied using Near Infrared Thermal Imaging (NIRTI), Fourier Transform Infrared Emission Spectroscopy (FTIRES), chronoamperometry/chronopotentiometry (CA/CP), and mass spectrometry (MS). Carbon removal follows a stepwise mechanism, first oxidizing surface carbon to CO (g) , and subsequently to CO 2(g) . CO (g) oxidation requires a catalytic surface to form CO 2 which plays a key role in removing C via the reverse Boudouard chemistry. NIRTI reveals spatially heterogenous chemistry and suggests a specific role of surface oxygen species. These species form from dissociative adsorption and non-faradaic oxide flux through the electrolyte, as well as O 2 transport limited processes occurring due to high O 2 utilization. C removal from electrochemical oxidation and steam spatially homogeneous compared to O 2 , due in part to the respective active surface species, and their respective transport limitations. Under O 2 C removal is appears incomplete, despite electrochemical results. These experiments clarify the mechanisms responsible for remediation of C on SOFC anodes and highlight the need of spatially resolved techniques to study SOFCs under operating conditions.

Conductometric titration is shown tobe a technique capableof quickly, accurately, and inexpensively quantifying sulfate concentrationsin ground and surface water samples. Conductometric titrations were used to measure sulfateconcentrationsin ground and surface water samples taken from land reclaimed afteropen-air coal mining. Sulfate concentrations ranged from 460 mg/Lin surface water upstream of the former coal mine's locationto almost 3500 mg/L in groundwater sampled at the spoil site. Datafrom the titration measurements were benchmarked against EPA-approvedion chromatography (IC) measurements and results agreed to within & PLUSMN;3.6% (averaged over 36 samples) with a range of +10.4 and -11.3%.To test the generality of conductometric titration as a method formeasuring dissolved constituents in environmental aquatic systems,additional measurements testing for chloride were performed with surfacewater samples collected from four different sites in south centraland southwest Montana. Chloride concentrations ranged from 2.2 to12 ppm. Based on measurements with control samples prepared in thelaboratory, the environmental sample measurements are believed tobe accurate to within & PLUSMN;6.4%. These conductometric titration studieshighlight the technique's simplicity, accuracy, cost effectiveness,and potential to produce rapid results. Additional analyses suggestthat even simpler, non-species-specific conductivity data can providean on-site, rapid assessment of sulfate levels in ground and surfacewater when historical speciation data are available.

Carbon fouling (or coking) is one of the primary degradation mechanisms that leads to performance loss and eventual failure in high temperature, solid oxide electrochemical cells (SOCs). Accumulated carbon blocks electrocatalytic sites at three phase boundaries, impedes transport through porous electrode structures and can react with common electrode materials such as Ni to form carbides that disintegrate in a process described as metal dusting. Operando Raman studies examining carbon remediation through gasification showed surprising results. (J. Phys. Chem. C, 119 (2015) 7637.) When comparing carbon removal from Ni-YSZ cermet anodes at 750˚C by gas phase H 2 O, O 2 , and CO 2 , experiments measuring intensity of the carbon ‘G’ band, Raman studies showed that H 2 O was most effective, removing all observable carbon in a matter of seconds. Similar amounts of O 2 required approximately one minute to remove detectable carbon, and CO 2 required even longer and never removed all of the observable carbon. These results were supported by changes in the SOC open circuit potential. These Raman measurements, however, sampled only a single location on the anode. Work described in this presentation shows the anode surface chemistry to be considerably more heterogeneous than might be inferred from the Raman data. A combination of near-IR thermal imaging (NIRTI), Fourier transform IR emission spectroscopy (FTIRES), voltammetry and downstream mass spectrometry (MS) exhaust analysis have begun to spatially resolve processes associated with carbon gasification from coked Ni-cermet anodes at 800˚C, and show that carbon gasification proceeds through a sequence of steps involving both surface and gas phase reactions.

A novel high-pressure strand burner with the ability to maintain a continuous feed of a liquid monopropellant is used to investigate the linear burning rates of liquid nitromethane as a function of chamber pressure. A new procedure for obtaining linear burning rates is presented. The present study investigates that pressure dependence in a subset of the low-pressure region, ranging from 3 to 8 MPa, which corresponds to a region with significant discrepancies in existing literature. The linear burning rate of liquid nitromethane is known to have a pressure dependence that follows Saint Robert's Law in select pressure regions. The results fall between the upper and lower limits of existing literature with a pressure exponent of n = 1.106. To complement the experimental results, numerical simulations of a simple 1D burner-stabilized flame were conducted with three different nitromethane mechanisms. Similar to the experimental results, the simulations showed significant discrepancies for the predicted flame temperatures and heat release rate for nitromethane combustion in an inert environment. The combined experimental and modeling results highlight the need to further investigate & COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Time-resolved fluorescence spectroscopy in combination with differential scanning calorimetry (DSC) was used to study the chemical interactions that occur when l-phenylalanine is introduced to solutions containing phosphatidylcholine vesicles. Studies reported in this work address open questions about l-Phe's affinity for lipid vesicle bilayers, the effects of l-Phe partitioning on bilayer properties, l-Phe's solvation within a lipid bilayer, and the amount of l-Phe within that local solvation environment. DSC data show that l-Phe reduces the amount of heat necessary to melt saturated phosphatidylcholine bilayers from their gel to liquid-crystalline state but does not change the transition temperature (Tgel-lc). Time-resolved emission shows only a single l-Phe lifetime at low temperatures corresponding to l-Phe remaining solvated in aqueous solution. At temperatures close to Tgel-lc, a second, shorter lifetime appears that is assigned to l-Phe already embedded within the membrane that becomes hydrated as water starts to permeate the lipid bilayer. This new lifetime is attributed to a conformationally restricted rotamer in the bilayer's polar headgroup region and accounts for up to 30% of the emission amplitude. Results reported for dipalmitoylphosphatidylcholine (DPPC, 16:0) lipid vesicles prove to be general, with similar effects observed for dimyristoylphosphatidylcholine (DMPC, 14:0) and distearoylphosphatidylcholine (DSPC, 18:0) vesicles. Taken together, these results create a complete and compelling picture of how l-Phe associates with model biological membranes. Furthermore, this approach to examining amino acid partitioning into membranes and the resulting solvation forces points to new strategies for studying the structure and chemistry of membrane-soluble peptides and selected membrane proteins.

In this study, 50%-50% Ni-8YSZ and Ni-BCZY27 composites were exposed to CH 4 in the absence and presence of H 2 O. Carbon deposition at 750 °C and carbon removal were estimated by exposure to steam using operando vibrational emission spectroscopy and thermal imaging. Carbon removal reveals more CO 2 + CO formation from Ni-8YSZ compared to Ni-BCZY27, suggesting more carbon on the surface of the former. Carbon removal from composites exposed to CH 4 alone indicates limited formation and fast depletion of CO over Ni-BCZY27. Over Ni-8YSZ, CO and CO 2 depletion is gradual and sustained, suggesting carbon formation over Ni-BCZY27 is restricted and full oxidation readily occurs. Methane cracking over Ni-8YSZ is accompanied by more cooling compared to Ni-BCZY27. Wet reforming results in similar cooling over Ni-8YSZ and Ni-BCZY27. The results are discussed in the context of recent work using operando spectroscopies of fuel utilization over Ni-8YSZ SOFCs.