Determination of N and O‐atoms and N2 (A) Metastable Molecule Densities in the Afterglows of N2‐H2, Ar‐N2‐H2 and Ar‐N2‐O2 Microwave Discharges

Early afterglows of N₂‐H₂, Ar‐N₂‐H₂ and Ar‐N₂‐O₂ flowing microwave discharges are characterized by optical emission spectroscopy. The N and O atoms and the N₂ (A) metastable molecule densities are determined by optical emission spectroscopy after calibration by NO titration for N and O‐atoms and measurements of NO and N₂ band intensities. If an uncertainty of 30% is estimated on N‐atomic density, an inaccuracy of one order of magnitude is obtained on the O and N₂ (A) densities. In N₂‐(0.05‐2.5%)H₂ and Ar‐(1‐50%)N₂‐(0.05‐2.5%) H₂ gas mixtures, the O‐atoms are coming from O₂ impurities in the discharge. Concentrations of N and O‐atoms and of N₂ (A) densities are compared to the ones obtained in Ar‐(5‐50%)N₂‐(0.2‐2.5%) O₂ gas mixtures in which a controlled amount of O₂ is added. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of N‐carbamoylamino acid nitrosation by {NO + O2} in the solid‐gas phase. Effects of NOx speciation and kinetic evidence for a multiple‐stage process

Nitrosation of N‐carbamoylamino acids (CAA) by gaseous NO + O₂, an interesting synthetic pathway to amino acid N‐carboxyanhydrides (NCA), alternative to the phosgene route, was investigated on N‐carbamoyl‐valine either in acetonitrile suspension or solventless conditions, and compared to the classical nitrosating system NaNO₂ + CF₃COOH (TFA), the latter being quite less efficient in terms of either rate, stoichiometric demand, or further tractability of the product. The rate and efficiency of the NO + O₂ reaction mainly depends on the O₂/NO ratio. Evaluation of the contribution of various nitrosating species (N₂O₃, N₂O₄, HNO₂) through stoichiometric balance showed the reaction to be effected mostly by N₂O₃ for O₂/NO ratios below 0.3, and by N₂O₄ for O₂/NO ratios above 0.4. The relative contribution of (subsequently formed) HNO₂ always remains minor. Differential scanning calorimetry (DSC) monitoring of the reaction in the solid phase by either HNO₂ (from NaNO₂ + TFA), gaseous N₂O₄ or gaseous N₂O₃, provides the associated rate constants (ca. 0.1, 2 and 10⁸ s^?1 at 25°C, respectively), showing that N₂O₃ is by far the most reactive of these nitrosating species. From the DSC measurement, the latent heat of fusion of N₂O_3, 2.74 kJ · mol^?1 at ?105 °C is also obtained for the first time. The kinetics was investigated under solventless conditions at 0°C, by either quenching experiments or less tedious, rough calorimetric techniques. Auto‐accelerated, parabolic‐shaped kinetics was observed in the first half of the reaction course, together with substantial heat release (temperature increase of ca. 20°C within 1–2 min in a 20‐mg sample), followed by pseudo‐zero‐order kinetics after a sudden, important decrease in apparent rate. This kinetic break is possibly due to the transition between the initial solid‐gas system and a solid‐liquid‐gas system resulting from water formation. Overall rate constants increased with parameters such as the specific surface of the solid, the O₂/NO ratio, or the presence of moisture (or equivalently the hydrophilicity of the involved CAA), however without precise relationship, while the last two parameters may directly correlate to the increasing acidity of the medium. Copyright © 2007 John Wiley & Sons, Ltd.     

DC Townsend Discharge in Nitrogen: Temperature‐Dependent Phenomena

Low‐current Townsend discharge in nitrogen has been studied in the temperature range of T = 100–300 K in a semiconductor‐gas‐discharge structure. It was found that the sustaining voltage U_S increases with time when a current is passed through the structure at low T. This effect was not observed at room temperature. A hypothesis is put forward that a film of a neutral phase of nitrogen is formed on the electrodes under cryogenic discharge conditions. The presence of the condensed thin‐film phase leads to a decrease in the secondary electron emission from the electrode and to a corresponding increase in U_S. A possible mechanism of the phenomenon is associated with the formation of large neutral aggregates in the form of [N⁺₂(N₂)_n]^– in the gas discharge volume. The condensation of these aggregates seems to yield a phase that is comparatively stable at cryogenic temperatures (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Piperidine‐appended imidazolium ionic liquids as task‐specific catalysts: computational study,synthesis, and multinuclear NMR

Imidazolium ionic liquids (IMILs) with a piperidine moiety appended via variable length methylene spacers (with n = 1–4) were studied computationally to assess their potential to act as internal base for N‐heterocyclic carbene (NHC) generation. Proton transfer energies computed by B3LYP/6‐311+G(2d,p) were least endothermic for the basic‐IL with n = 3, whose optimized structure showed the shortest C₂‐H‐‐‐‐N(piperidine) distance. Inclusion of counter anion (Cl or NTf₂) caused dramatic conformational changes to enable close contact between the acidic C₂‐H and the anions. To examine the prospect for internal C₂‐H‐‐‐‐N coordination, multinuclear NMR data (¹H, ¹⁵N, and ¹³C) were computed by gauge independent atomic orbitals–density functional theory (GIAO‐DFT) in the gas phase and in several solvents by the PCM method for comparison with the experimental NMR data for the basic ILs (with n = 2–4) synthesized in the laboratory. These studies indicate that interactions with solvent and counter ion are dominant forces that could disrupt internal C₂‐H‐‐‐‐N coordination/proton transfer, making carbene generation from these basic‐ILs unlikely without an added external base. Therefore, the piperidine‐appended IMILs appear suitable for application as dual solvent/base in organic/organometallic transformations that require the use of mild base, without the necessity to alkylate at C‐2 to prevent N‐heterocyclic carbene formation. Copyright © 2016 John Wiley & Sons, Ltd.     

X‐ray fluorescence analysis of Co,Ni, Pd,Ag, and Au in the scrapped printed‐circuit‐board ash

A method for the quantitative analysis of Co, Ni, Pd, Ag, and Au in the scrapped printed‐circuit‐board ash by X‐ray fluorescence (XRF) spectrometry using loose powder was developed. The printed‐circuit‐board samples were converted to ash pyrolytically in porcelain crucibles by sequential heating using a gas burner and electric furnace, and then were ground with a ball mill. The calibrating standards were prepared by adding the appropriate amounts of NiO powder and aqueous standard solutions containing Co, Pd, Ag, and Au to the base mixtures of Al₂O₃ (5.0 mass%), SiO₂ (49 mass%), CaCO₃ (11 mass%), Fe₂O₃ (3.3 mass%), and CuO (30 mass%) as a matrix. Then, 10 g of the resulting mixtures were dried and homogenized for 90 min with a V‐type mixing machine. Specimens for XRF analysis were prepared from the so‐called loose‐powder method in which powder samples were compacted into a hole (12.0‐mm diameter and 5.0‐mm height) in an acrylic plate and covered with a 6‐µm thickness of polypropylene film. Matrix effects were corrected using the intensity value of Compton scattering for PdKα, AgKα, and AuLβ₂, and that of background scattering at 35.8° (2θ) for CoKα and NiKα. The detection limits corresponding to three times the standard deviation of the blank intensity were 2.5–45 µg g^?1. The proposed method was validated against the pressed‐powder‐pellet method by comparing the calibration curves. Moreover, the concentrations of Co, Ni, Pd, and Ag determined using the proposed XRF method were approximately the same as those resulting from an atomic‐absorption‐spectrometric analysis. Copyright © 2013 John Wiley & Sons, Ltd.     

Vibrational spectroscopy and DFT calculations of N,N′‐dicyclohexylcarbodiimide

Infrared (IR) and Raman spectra were obtained for N,N′‐dicyclohexylcarbodiimide (DCC) in the solid state and in CHCl₃ solution. Structures and vibrational spectra of isolated, gas‐phase DCC molecules with C₂ and C_i symmetries, computed at the B3‐LYP/cc‐pVTZ level, show that the IR and Raman spectra provide convincing evidence for a C₂ structure in both the solid state and in CHCl₃ solution. Using a scaled quantum‐chemical force field, these density functional theory calculations have provided detailed assignments of the observed IR and Raman bands in terms of potential energy distributions. Comparison of solid‐state and solution spectra, together with a Raman study of the melting behaviour of DCC, revealed that no solid‐state effects were evident in the spectra. Copyright © 2010 John Wiley & Sons, Ltd.     

Binary phase shaping for selective single‐beam CARS spectroscopy and imaging of gas‐phase molecules

We report on mode‐selective single‐beam coherent anti‐Stokes Raman scattering spectroscopy of gas‐phase molecules. Binary phase shaping (BPS) is used to produce single‐mode excitation of O₂, N₂, and CO₂ vibrational modes in ambient air and gas‐phase mixtures, with high‐contrast rejection of off‐resonant Raman modes and efficient nonresonant‐background suppression. In particular, we demonstrate independent excitation of CO₂ Fermi dyads at ∼1280 and ∼1380 cm⁻¹ and apply BPS for high‐contrast imaging of CO₂ jet in ambient air. Copyright © 2010 John Wiley & Sons, Ltd.     

N‐Methylbenzoazacrown ethers with the nitrogen atom conjugated with the benzene ring: the improved synthesis and the reasons for the high stability of complexes with metal and ammonium cations

An improved method for the synthesis of formyl derivatives of N‐methylbenzoazacrown ethers is proposed. They are prepared in up to 68% yields over fewer steps and with a much shorter time required for the last step. The stability constants of complexes formed by N‐methylbenzoazacrown ethers and their structural analogs with alkali metal, alkaline‐earth metal and ammonium cations were determined by ¹H NMR titration in CD₃CN. High stability of complexes of N‐methyl derivatives of benzoazacrown ethers is demonstrated, comparable with or even exceeding the stability of benzocrown‐ether complexes and markedly exceeding the stability of complexes of phenylazacrown ethers with the same macrocycle size. The structures of azacrown ethers and their complexes with Ba(ClO₄)₂ were studied by X‐ray diffraction. A high degree of pre‐organization of N‐methylbenzoazacrown ethers toward the formation of complexes with metal and ammonium cations was noted, which is due to the clear‐cut pyramidal geometry of the nitrogen atom and the orientation of the lone electron pairs (LEPs) of most heteroatoms towards the centre of the macroheterocycle. Copyright © 2009 John Wiley & Sons, Ltd.     

Enols of 2‐nitro‐ and related 2‐substituted malonamides

The structures of 2‐substituted malonamides, YCH(CONR¹R²)CONR³R⁴ (Y = Br, SO₂Me, CONH₂, COMe, and NO₂) were investigated. When Y = Br, R¹R² = R³R⁴ = HEt; Y = SO₂Me, R¹–R⁴ = H and for Y = CONH₂ or CONHPh, R¹–R⁴ = Me, the structure in solution is that of the amide tautomer. X‐ray crystallography shows solid‐state amide structures for Y = SO₂Me or CONH₂, R¹–R⁴ = H. Nitromalonamide displays an enol structure in the solid state with a strong hydrogen bond (O^…O distance = 2.3730 Å at 100 K) and d(OH) ≠ d(O^…H). An apparently symmetric enol was observed in solution, even in appreciable percentages in highly polar solvents such as DMSO‐d₆, but K_enol values decrease on increasing the solvent polarity. The N,N′‐dimethyl derivative is less enolic. Acetylmalonamides display a mixture of enol on the acetyl group and amide in non‐polar solvents, and only the amide in DMSO‐d₆. DFT calculations gave the following order of pK_enol values for Y: H > CONH₂ > COMe ≥ COMe (on acetyl) ≥ MeSO₂ > CN > NO₂ in the gas phase, CHCl₃, and DMSO. The enol on the C?O group is preferred to the aci‐nitro compound, and the N? O? H^…O?C is less favored than the C?O? H^…O?C hydrogen bond. Copyright © 2009 John Wiley & Sons, Ltd.     

Theoretical studies on 2‐(5‐amino‐3‐nitro‐1,2,4‐triazolyl)‐3,5‐dinitropyridine (PRAN) and its derivatives

In this work, a set of derivatives of 2‐(5‐amino‐3‐nitro‐1,2,4‐triazolyl)‐3,5‐dinitropyridine (PRAN) with different energetic substituents (?N₃, –NO₂, –NH₂, –NF₂) have been studied at the Becke, three‐parameter, Lee–Yang–Parr/aug‐cc‐pvdz, Becke, three‐parameter, Lee–Yang–Parr/6‐31G(d), Becke, three‐parameter, Perdew 86/6‐31G(d), and Becke three‐parameter, Perdew–Wang 91/6‐31G(d,p) levels of density functional theory. The gas‐phase heats of formation were predicted with isodesmic reactions and the condensed‐phase HOFs were estimated with the Politzer approach. The effects of different functionals and basis sets were analyzed. –N₃ and –NO₂ greatly increase while –NH₂ and –NF₂ slightly decrease heats of formation. An analysis of the bond dissociation energies and impact sensitivity shows that all compounds have good stability. The crystal densities (1.82–2.00 g/cm³) computed from molecular packing calculations are big for all compounds and that of the –NF₂ derivative is the largest. All derivatives have higher detonation velocity and detonation pressure than PRAN. Compounds 3 and 4 (R = NO₂ and NF₂) have better performance than hexahydro‐1,3,5‐trinitro‐1,3,5‐trizine and the performance of 4 is quite close to that of 1,3,5,7‐tetranitro‐1,3,5,7‐tetraazacyclooctane, they are promising candidates of high energy compounds and worth further investigations. Copyright © 2012 John Wiley & Sons, Ltd.     

Conformational analysis of N‐phenyl‐ and N‐trifyl‐4,4‐dimethyl‐4‐silathiane 1‐sulfimides

N‐Substituted 4,4‐dimethyl‐4‐silathiane 1‐sulfimides [R = Ph ( 1 ), CF₃ ( 2 )] were studied experimentally by variable temperature dynamic NMR spectroscopy. Low temperature ¹³C NMR spectra of the two compounds revealed the frozen ring inversion process and approximately equal content of the axial and equatorial conformers. Calculations of the 4‐silathiane derivatives 1 , 2 and the model compound [R = Me ( 3 )] as well as their carbon analogs, the similarly N‐substituted thiane 1‐sulfimides [R = Ph ( 4 ), CF₃ ( 5 ), Me ( 6 )] at the DFT/B3LYP/6–311G(d,p) level in the gas phase and in chloroform solution using the PCM model at the same level of theory showed a strong dependence of the relative stability of the conformer on the solvent. The electronegative trifluoromethyl group increases the relative stability of the axial conformer. Copyright © 2010 John Wiley & Sons, Ltd.     

Catalysis by hydrogen chloride in the gas‐phase elimination kinetics of 2‐phenyl‐2‐propanol and 3‐methyl‐1‐buten‐3‐ol

A homogeneous, molecular, gas‐phase elimination kinetics of 2‐phenyl‐2‐propanol and 3‐methyl‐1‐ buten‐3‐ol catalyzed by hydrogen chloride in the temperature range 325–386 °C and pressure range 34–149 torr are described. The rate coefficients are given by the following Arrhenius equations: for 2‐phenyl‐2‐propanol log k₁ (s^?1) = (11.01 ± 0.31) ? (109.5 ± 2.8) kJ mol^?1 (2.303 RT)^?1 and for 3‐methyl‐1‐buten‐3‐ol log k₁ (s^?1) = (11.50 ± 0.18) ? (116.5 ± 1.4) kJ mol^?1 (2.303 RT)^?1. Electron delocalization of the CH₂?CH and C₆H₅ appears to be an important effect in the rate enhancement of acid catalyzed tertiary alcohols in the gas phase. A concerted six‐member cyclic transition state type of mechanism appears to be, as described before, a rational interpretation for the dehydration process of these substrates. Copyright © 2007 John Wiley & Sons, Ltd.     

Design and development of a controlled pressure/temperature set‐up for in situ studies of solid–gas processes and reactions in a synchrotron X‐ray powder diffraction station

A novel set‐up has been designed and used for synchrotron radiation X‐ray high‐resolution powder diffraction (SR‐HRPD) in transmission geometry (spinning capillary) for in situ solid–gas reactions and processes in an isobaric and isothermal environment. The pressure and temperature of the sample are controlled from 10^?3 to 1000 mbar and from 80 to 1000 K, respectively. To test the capacities of this novel experimental set‐up, structure deformation in the porous material zeolitic imidazole framework (ZIF‐8) by gas adsorption at cryogenic temperature has been studied under isothermal and isobaric conditions. Direct structure deformations by the adsorption of Ar and N₂ gases have been observed in situ, demonstrating that this set‐up is perfectly suitable for direct structural analysis under in operando conditions. The presented results prove the feasibility of this novel experimental station for the characterization in real time of solid–gas reactions and other solid–gas processes by SR‐HRPD.     

Solid‐phase epitaxy of InOx Ny alloys via thermal oxidation of InN films on yttria‐stabilized zirconia

The characteristics of InO_x N_y alloy films prepared via thermal oxidation of InN epitaxial films with In‐ or N‐polarities grown on nearly lattice‐matched, yttria‐stabilized zirconia (YSZ) substrates are investigated. The InN films were oxidized to InO_x N_y with a gradual change in O/N composition by annealing in air. Structural analysis revealed that the temperature for phase transition from wurtzite structure depends on the polarity of InN, and N‐polar InO_x N_y films can retain their wurtzite structure even at higher temperatures compared with the case of In‐polar films. Furthermore, changes in the valence band structure and optical characteristics of the InO_x N_y alloys take place via thermal oxidation. These results indicate that InO_x N_y grown via thermal oxidation of N‐polar InN on YSZ can be considered as an alloy semiconductor for optoelectronic devices. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rotational disorder in 2‐piperidyl‐5‐nitro‐6 ‐methylpyridine: structural phase transition and its vibrational characteristics

X‐ray diffraction (XRD) studies have shown that 2‐piperidyl‐5‐nitro‐6‐methylpyridine, C₁₁H₁₅N₃O₂, undergoes a structural phase transition at T = 240 K. The room temperature structure is tetragonal, space group I4₁/a, with the unit‐cell dimensions a = 13.993(2) and c = 23.585(5) Å. The pyridine ring takes trans conformation with respect to the piperidine unit. While pyridine is well ordered, the piperidine moiety shows apparent disorder resulting from a libration about the linking N C bond. The low‐temperature phase is monoclinic, space group I2/a. Contraction of the unit‐cell volume by 2.3% at 170 K enables the C H···O linkage between the molecules of the neighbouring stacks. As result, the asymmetric unit becomes bi‐molecular. The thermal librations of the piperidine and methyl groups become considerably reduced at 170 K and nearly fully reduced at about 100 K. The IR spectra and polarised Raman spectra agree with the X‐ray structure and confirm the disorder effect on the piperidine ring. The assignment of the bands observed was made on the basis of DFT chemical quantum calculations. Copyright © 2008 John Wiley & Sons, Ltd.     

Determination of N and O – Atom and N2(A) Metastable Molecule Densities in the Afterglows of N2 and N2‐O2 Microwave Discharges

Early afterglows of N₂ and N₂‐O₂ flowing microwave discharges are characterized by optical emission spectroscopy. The N and O atom and N₂(A) metastable molecule densities are determined by optical emission spectroscopy after calibration by NO titration for N‐atoms and measurements of NO and N₂ band intensities for O‐atoms and N₂(A) metastable molecules. By using N₂ tanks with 50 and 10 ppm impurity, it is determined in the afterglow an O‐ atom impurity of 150‐200 ppm. Variations of the N and O‐atom and N₂(A) metastable molecule densities are obtained in the early afterglow of N₂–(9·10^–5–3·10^–3)O₂ gas mixtures. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The hydrogen‐bond effect on the high pressure behavior of hydrazinium monochloride

The first high pressure study of solid hydrazinium monochloride has been performed by in situ Raman spectroscopy and synchrotron X‐ray diffraction (XRD) experiments in diamond anvil cell (DAC) up to 39.5 and 24.6 GPa, respectively. The structure of phase I at room temperature is confirmed to be space group C2/c by the Raman spectral analysis and Rietveld refinement of the XRD pattern. A structural transition from phase I to II is observed at 7.3 GPa. Pressure‐induced position variation of hydrogen atoms in NH₃⁺ unit during the phase transition is attributed to the formation of N―H…Cl hydrogen‐bonds, which play a vital role in the stability and subsequent structural changes of this high energetic material under pressure. This inference is proved from the abnormal pressure shifts and obvious Fermi resonance in NH stretching mode of N₂H₅⁺ ion in the Raman experiment. Finally, a further transition from phase II to III accompanied with a slight internal distortion in the N₂H₅⁺ ions occurs above 19.8 GPa, and phase III persists up to 39.5 GPa. Copyright © 2015 John Wiley & Sons, Ltd.     

Palladium‐catalyzed cyclization of 2‐alkynyl‐N‐ethanoyl anilines to indoles: synthesis,structural, spectroscopic,and mechanistic study

This study reports a facial regio‐selective synthesis of 2‐alkyl‐N‐ethanoyl indoles from substituted‐N‐ethanoyl anilines employing palladium (II) chloride, which acts as a cyclization catalyst. The mechanistic trait of palladium‐based cyclization is also explored by employing density functional theory. In a two‐step mechanism, the palladium, which attaches to the ethylene carbons, promotes the proton transfer and cyclization. The gas‐phase barrier height of the first transition state is 37 kcal/mol, indicating the rate‐determining step of this reaction. Incorporating acetonitrile through the solvation model on density solvation model reduces the barrier height to 31 kcal/mol. In the presence of solvent, the electron‐releasing (–CH₃) group has a greater influence on the reduction of the barrier height compared with the electron‐withdrawing group (–Cl). These results further confirm that solvent plays an important role on palladium‐catalyzed proton transfer and cyclization. For unveiling structural, spectroscopic, and photophysical properties, experimental and computational studies are also performed. Thermodynamic analysis discloses that these reactions are exothermic. The highest occupied molecular orbital?lowest unoccupied molecular orbital gap (4.9–5.0 eV) confirms that these compounds are more chemically reactive than indole. The calculated UV–Vis spectra by time‐dependent density functional theory exhibit strong peaks at 290, 246, and 232 nm, in good agreement with the experimental results. Moreover, experimental and computed ¹H and ¹³C NMR chemical shifts of the indole derivatives are well correlated. Copyright © 2015 John Wiley & Sons, Ltd.     

Rigorous solution of a Hubbard model extended by nearest‐neighbour Coulomb and exchange interaction on a triangle and tetrahedron

The Hubbard model extended by both nearest‐neighbour (nn) Coulomb correlation and nearest‐neighbour Heisenberg exchange is solved rigorously for a triangle and tetrahedron. All eigenvalues and eigenvectors are given as functions of the model parameters in a closed analytical form. For fixed electron numbers we found a multitude of level crossings, both in the ground state and in the excited states in dependence on the various model parameters. By coupling an ensemble of clusters to an electron bath we get the cluster gas model or the cluster gas approximation, if an extended array of weak‐interacting clusters is considered. The grand‐canonical potential Ω (μ, T, h) and the electron occupation N (μ, T, h) of the related cluster gases were calculated for arbitrary values (attractive and repulsive) of the three interaction constants. For the cluster gases without the additional interactions we found various steps in N (μ, T = 0, h = 0) higher than one. The reason is the degeneration of ground states differing in their electron occupation by more than one electron. For the triangular cluster gas we have one such degeneration point. For the tetrahedral cluster gas two. As a consequence, we do not find areas with one electron in the μ‐U ground‐state phase diagram of the triangular cluster gas or with one, two and five electrons in the case of the tetrahedral cluster gas. The degeneration point of the triangular cluster gas can not be destroyed by an applied magnetic field. This holds also for the lower degeneration point of the tetrahedral cluster gas. Otherwise, the upper degeneration point breaks down at a critical magnetic field h_c. The dependence of h_c on U shows a maximum for strong on‐site correlation. The influence of nn‐exchange and nn‐Coulomb correlation on the ground‐state phase diagrams is calculated. Whereas antiferromagnetic nn‐exchange breaks the degeneration points of the tetrahedral cluster gas partially only, a repulsive nn‐Coulomb correlation lifts the underlying degeneracies completely. Otherwise both ferromagnetic nn‐exchange and attractive nn‐Coulomb interaction stabilise the degeneration points. The consequences of the cluster gas results for extended cluster arrays are discussed.     

Quantifying phosphoric acid in high‐temperature polymer electrolyte fuel cell components by X‐ray tomographic microscopy

Synchrotron‐based X‐ray tomographic microscopy is investigated for imaging the local distribution and concentration of phosphoric acid in high‐temperature polymer electrolyte fuel cells. Phosphoric acid fills the pores of the macro‐ and microporous fuel cell components. Its concentration in the fuel cell varies over a wide range (40–100 wt% H₃PO₄). This renders the quantification and concentration determination challenging. The problem is solved by using propagation‐based phase contrast imaging and a referencing method. Fuel cell components with known acid concentrations were used to correlate greyscale values and acid concentrations. Thus calibration curves were established for the gas diffusion layer, catalyst layer and membrane in a non‐operating fuel cell. The non‐destructive imaging methodology was verified by comparing image‐based values for acid content and concentration in the gas diffusion layer with those from chemical analysis.