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Performance optimization of polymer electrolyte fuel cells (PEFCs) require precise design and knowledge of the porous catalyst layer (CL) structure. Furthermore, understanding the changes to this structure due to degradation is important for performance retention and durability. Hence, there is a need to characterize these porous networks using advanced imaging techniques with multi-scale resolution. Scanning electron microscopy (SEM) have typically been used for characterizing CL as it provides high contrast and high-resolution imaging.[1] However, SEM alone is limited to 2D visualization, requiring considerable sample handling and preparation, and can suffer from edge effects and large depth of focus; making segmentation and interpretation of pores difficult. By combining SEM with focused ion beam (FIB) milling, a three-dimensional visualization can be acquired by eroding a layer of material using an ion beam and imaging the newly exposed surface to obtain a stack of 2D images through the thickness of the material. In contrast, the use of X-ray computed tomography (XCT) allows for three-dimensional imaging with symmetric resolution by volumetric imaging and is non-intrusive where the sample is intact post imaging. Studies focusing on PEFC degradation using XCT have primarily addressed macro-scale changes with recent work also contributing operando imaging to obtain water distribution changes within the gas diffusion layer (GDL) using micro-XCT. [2-6] However, the dynamic and locally variable nature of catalyst layer degradation demands the use of correlative imaging procedures for nano-XCT analysis. In the present work, a correlative workflow is developed for X-ray tomographic imaging of cathode catalyst layer degradation featuring four-dimensional (3D plus time) operando imaging and ex situ nano-scale imaging (Fig. 1). 4D operando imaging shows high water content observed under the lands and near the surface of the cathode GDL. Macro imaging results show that the catalyst layer solid area fraction is gradually reduced by 25% with crack formation and severe corrosion at the edges of the solid phase. In addition, the thickness is found to decrease by 50%, a similar trend to the measured electrochemical surface area. Localized degradation features such as nano-scale cracks and internal pore-size distribution changes are resolved using nano-XCT and tracked by 3+1D imaging at different stages of degradation. Porosity loss quantified by XCT on the order of 40% from beginning-of-life to end-of-life with reduction in connected pore fraction are observed as well as an increase in average pore size by 50%. The effect of changes at the nano-scale on diffusion properties are also calculated and an empirical model is proposed for these types of catalyst layer structures where Knudsen effects are dominant. Overall, the new findings from this work demonstrate important linkages between CL structure, transport properties, liquid water flooding, and fuel cell performance in the context of degradation. The workflow and analysis methods could henceforth be contemplated when employing computational modeling and catalyst layer design studies where correlation to fuel cell performance and durability is desired. Acknowledgements Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program. We also thank 4D LABS at Simon Fraser University for technical assistance during FIB/SEM liftout procedure. References [1] A. Tavassoli et al. J. Power Sources 322 (2016) 17-25 [2] R.T. White et al. J. Power Sources 350 (2017) 94-102 [3] Y. Singh et al. J. Power Sources 345 (2017) 1–11 [4] D Ramani et al. J. Electrochem. Soc. 165 (2018) F3200-08 [5] Y. Singh et al. J. Electrochem. Soc. 164 (2017) F1331-41 [6] R.T. White et al. Sci. Rep. 9 (2019) 1843 Figure 1: a) Wedge shaped MEA sample imaged by micro-XCT for morphological quantification and screening of the cathode catalyst layer to find a representative area to perform FIB liftout before subsequent imaging using nano- XCT. b) 2D cross-sectional planes virtually extracted from the 3D volume in (a). c) FIB lift-out needle attached to the sample using Pt deposition for sample lift-off; the lifted sample is then placed on a pin tip. d) 3D rendering of the selected catalyst layer region imaged using nano-XCT with porosity volume segmentation shown in orange. Figure 1

A measurement of the double-polarization observable E for the reaction $$\gamma p\rightarrow \pi ^0 p$$ γ p → π 0 p is reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility in Bonn using the Bonn frozen-spin butanol (C$$_4$$ 4 H$$_9$$ 9 OH) target, which provided longitudinally-polarized protons. Circularly-polarized photons were produced via bremsstrahlung of longitudinally-polarized electrons. The data cover the photon energy range from $$E_\gamma =600$$ E γ = 600 to 2310 MeV and nearly the complete angular range. The results are compared to and have been included in recent partial wave analyses.

A measurement of the double-polarization observable E for the reaction \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\gamma p\rightarrow \pi ^0 p$$\end{document}γp→π0p is reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility in Bonn using the Bonn frozen-spin butanol (C\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_4$$\end{document}4H\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_9$$\end{document}9OH) target, which provided longitudinally-polarized protons. Circularly-polarized photons were produced via bremsstrahlung of longitudinally-polarized electrons. The data cover the photon energy range from \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$E_\gamma =600$$\end{document}Eγ=600 to 2310 MeV and nearly the complete angular range. The results are compared to and have been included in recent partial wave analyses.

Understanding of degradation mechanisms present in polymer electrolyte fuel cells (PEFCs) is important to continue the integration of this clean energy technology into everyday life. Further comprehension of the interaction between various components during fuel cell operation is also critical in this context. In this work, a four-dimensional operando X-ray computed tomography method is developed for combined visualization of all PEFC components as well as transient water distribution residing in the cell, which results as a by-product of the electrochemical reaction. Time resolved, identical-location visualization through degradation stages is uniquely enabled by the non-invasive and non-destructive qualities of this method. By applying an accelerated stress test that targets cathode catalyst layer (CCL) corrosion, novel observations resulting from morphological changes of the CCL such as reduction in the water volume in the adjacent gas diffusion layer, CCL crack formation and propagation, membrane swelling, as well as quantification of local carbon loss is achieved. Additionally, insight into features that contribute to reduced fuel cell performance is enabled by the use of this specialized imaging technique, such as increased membrane undulation causing delamination and separation of the CCL from the microporous layer, which greatly affects liquid water pathways and overall device performance.

Durability is a major lever for commercial success of proton exchange membrane fuel cells (PEMFCs). The introduction of OER catalyst to the PEMFC anode has been established as a material based mitigation strategy for reversal events caused by gross fuel (i.e. H2) starvation. We investigated the degradation of two different OER based reversal tolerant anodes during short-term recurring reversal operation to mimic field occurrence of reversal events realistically. PEMFC failure during normal operation can be observed whereas OER activity during reversal operation is unaffected. This result is in contrast to findings for commonly applied prolonged reversal accelerated stress tests (ASTs) and indicates an OER catalyst recovery effect for short and recurring reversal events. Combining the developed AST with cyclic voltammetry, electrochemical impedance spectroscopy and hydrogen pump, tests failures during normal operation is mainly assigned to hydrogen oxidation mass transfer increase indicating carbon corrosion and structural change within the anode catalyst layer. Consequently, the developed combination of AST and further characterization methods enables in situ distinction between catalyst and structural degradation, highlighting to be a good basis to investigate future aspects regarding anode degradation caused by cell reversal.

Data on the reaction γ p → ω p with ω → π 0 γ , taken with unpolarized or polarized beams in combination with an unpolarized or polarized proton-target, were analyzed within the Bonn–Gatchina (BnGa) partial wave analysis. Differential cross sections, several spin density matrix elements, the beam asymmetry Σ, the normalized helicity difference E, and the correlation G between linear photon and longitudinal target polarization were included in a large data base on pion and photo-induced reactions. The data on ω photoproduction are used to determine twelve N ⁎ → N ω branching ratios; most of these are determined for the first time.

The polarization observables T , E , P , H , and G in photoproduction of η mesons off protons are measured for photon energies from threshold to W = 2400 MeV (T), 2280 MeV (E), 1620 MeV ( P , H ), or 1820 MeV (G), covering nearly the full solid angle. The data are compared to predictions from the SAID, MAID, JuBo, and BnGa partial-wave analyses. A refit within the BnGa approach including further data yields precise branching ratios for the Nη decay of nucleon resonances. A Nη-branching ratio of 0.33 ± 0.04 for N ( 1650 ) 1 / 2 − is found, which reduces the large and controversially discussed Nη-branching ratio difference of the two lowest mass J P = 1 / 2 − -resonances significantly.

The production of $\eta^{\prime}$ mesons in coincidence with forward-going protons has been studied in photon-induced reactions on 12C and on a liquid hydrogen (LH2) target for incoming photon energies of 1.3-2.6 GeV at the electron accelerator ELSA. The $\eta^{\prime}$ mesons have been identified via the $\eta^{\prime} \rightarrow \pi^{0} \pi^{0}\eta \rightarrow 6 \gamma$ decay registered with the CBELSA/TAPS detector system. Coincident protons have been identified in the MiniTAPS BaF2 array at polar angles of $2^{\circ} \le \theta_{p} \le 11^{\circ}$ . Under these kinematic constraints the $\eta^{\prime}$ mesons are produced with relatively low kinetic energy ( $\approx 150$ MeV) since the coincident protons take over most of the momentum of the incident-photon beam. For the C-target this allows the determination of the real part of the $\eta^{\prime}$ -carbon potential at low meson momenta by comparing with collision model calculations of the $\eta^{\prime}$ kinetic energy distribution and excitation function. Fitting the latter data for $\eta^{\prime}$ mesons going backwards in the center-of-mass system yields a potential depth of $V = -(44 \pm 16(stat) \pm 15(syst))$ MeV, consistent with earlier determinations of the potential depth in inclusive measurements for average $\eta^{\prime}$ momenta of $\approx 1.1$ GeV/c. Within the experimental uncertainties, there is no indication of a momentum dependence of the $\eta^{\prime}$ -carbon potential. The LH2 data, taken as a reference to check the data analysis and the model calculations, provide differential and integral cross sections in good agreement with previous results for $\eta^{\prime}$ photoproduction off the free proton.

The first measurements of the beam-target-helicity-asymmetries $E$ and $G$ in the photoproduction of $\omega$-mesons off protons at the CBELSA/TAPS experiment are reported. $E$ ($G$) was measured using circularly (linearly) polarised photons and a longitudinally polarised target. $E$ was measured over the photon energy range from close to threshold ($E_\gamma = 1108$~MeV) to $E_\gamma = 2300$~MeV and $G$ at a single energy interval of $1108 < E_\gamma, Comment: 6 pages, 6 figures

Data on the polarization observables T, P, and H for the reaction gamma p -> p pi(0) are reported. Compared to earlier data from other experiments, our data are more precise and extend the covered range in energy and angle substantially. The results were extracted from azimuthal asymmetries measured using a transversely polarized target and linearly polarized photons. The data were taken at the Bonn electron stretcher accelerator ELSA with the CBELSA/TAPS detector. Within the Bonn-Gatchina partial wave analysis, the new polarization data lead to a significant narrowing of the error band for the multipoles for neutral-pion photoproduction. (C) 2015 The Authors. Published by Elsevier B.V.

Synchrotron based X-ray tomographic microscopy (XTM) is used for imaging and quantifying the redistribution of phosphoric acid (PA) in high temperature polymer electrolyte fuel cells (HT-PEFC) in-operando. The main focus of this work is the redistribution of phosphoric acid under dynamic load conditions. Therefore, two different load cycling protocols were applied and the transient redistribution within the fuel cell components was imaged. XTM, for the first time, revealed that the examined PBI based membrane system exhibits extensive electrolyte migration from cathode to anode under high current density operation. PA flooding of anode gas diffusion layer (GDL) and flow field channels occurred. Implications for technical applications and fuel cell degradation are discussed. Quantification of the migrated electrolyte is made by correlating in-operando grayscale values to ex-situ reference samples.

Photoproduction of $\eta$ mesons from neutrons} \abstract{Results from measurements of the photoproduction of $\eta$ mesons from quasifree protons and neutrons are summarized. The experiments were performed with the CBELSA/TAPS detector at the electron accelerator ELSA in Bonn using the $\eta\to3\pi^{0}\to6\gamma$ decay. A liquid deuterium target was used for the measurement of total cross sections and angular distributions. The results confirm earlier measurements from Bonn and the MAMI facility in Mainz about the existence of a narrow structure in the excitation function of $\gamma n\rightarrow n\eta$. The current angular distributions show a forward-backward asymmetry, which was previously not seen, but was predicted by model calculations including an additional narrow $P_{11}$ state. Furthermore, data obtained with a longitudinally polarized, deuterated butanol target and a circularly polarized photon beam were analyzed to determine the double polarization observable $E$. Both data sets together were also used to extract the helicity dependent cross sections $\sigma_{1/2}$ and $\sigma_{3/2}$. The narrow structure in the excitation function of $\gamma n\rightarrow n\eta$ appears associated with the helicity-1/2 component of the reaction.

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.

In a search for $\omega$ mesic states, the production of $\omega$-mesons in coincidence with forward going protons has been studied in photon induced reactions on $^{12}$C for incident photon energies of 1250 - 3100 MeV. The $\pi^0 \gamma$ pairs from decays of bound or quasi-free $\omega$-mesons have been measured with the CBELSA/TAPS detector system in coincidence with protons registered in the MiniTAPS forward array. Structures in the total energy distribution of the $\pi^0 \gamma$ pairs, which would indicate the population and decay of bound $\omega~^{11}$B states, are not observed. The $\pi^0 \gamma$ cross section of 0.3 nb/MeV/sr observed in the bound state energy regime between -100 and 0 MeV may be accounted for by yield leaking into the bound state regime because of the large in-medium width of the $\omega$-meson. A comparison of the measured total energy distribution with calculations suggests the real part $V_0$ of the $\omega~^{11}$B potential to be small and only weakly attractive with $V_0(\rho=\rho_0) = -15\pm$ 35(stat) $\pm$20(syst) MeV in contrast to some theoretical predictions of attractive potentials with a depth of 100 - 150 MeV., Comment: 13 pages, 8 figures

The excitation function and momentum distribution of $\eta^\prime$ mesons have been measured in photon induced reactions on $^{12}{}$C in the energy range of 1250-2600 MeV. The experiment was performed with tagged photon beams from the ELSA electron accelerator using the Crystal Barrel and TAPS detectors. The data are compared to model calculations to extract information on the sign and magnitude of the real part of the $\eta^\prime$-nucleus potential. Within the model, the comparison indicates an attractive potential of -($37 \pm 10(stat)\pm10(syst)$) MeV depth at normal nuclear matter density. Since the modulus of this depth is larger than the modulus of the imaginary part of the $\eta^\prime$-nucleus potential of -($10\pm2.5$) MeV, determined by transparency ratio measurements, a search for resolved $\eta^\prime$-bound states appears promising., Comment: 13 pages, 6 figures

The photoproduction of $\omega$ and $\eta^\prime$ mesons off carbon and niobium nuclei has been measured as a function of the meson momentum for incident photon energies of 1.2-2.9 GeV at the electron accelerator ELSA. The mesons have been identified via the $\omega \rightarrow \pi^0 \gamma \rightarrow 3 \gamma$ and $\eta^\prime\rightarrow \pi^0 \pi^0\eta \rightarrow 6 \gamma$ decays, respectively, registered with the CBELSA/TAPS detector system. From the measured meson momentum distributions the momentum dependence of the transparency ratio has been determined for both mesons. Within a Glauber analysis the in-medium $\omega$ and $\eta^\prime$ widths and the corresponding absorption cross sections have been deduced as a function of the meson momentum. The results are compared to recent theoretical predictions for the in-medium $\omega$ width and $\eta^\prime$-N absorption cross sections. The energy dependence of the imaginary part of the $\omega$- and $\eta^\prime$-nucleus optical potential has been extracted. The finer binning of the present data compared to the existing data allows a more reliable extrapolation towards the production threshold. The modulus of the imaginary part of the $\eta^\prime$ nucleus potential is found to be about three times smaller than recently determined values of the real part of the $\eta^\prime$-nucleus potential, which makes the $\eta^\prime$ meson a suitable candidate for the search for meson-nucleus bound states. For the $\omega$ meson, the modulus of the imaginary part near threshold is comparable to the modulus of the real part of the potential. As a consequence, only broad structures can be expected which makes the observation of $\omega$ mesic states very difficult experimentally., Comment: 10 pages, 8 figures

Synchrotron based X-ray tomographic microscopy was used to image the redistribution of phosphoric acid in HT-PEFC due to electrolyte migration from cathode to anode. The acid migration rate, transference number of the hydrogen phosphate ion and flooding of the anode gas diffusion layer (GDL) was analyzed for MEAs with different membrane acid doping levels (24–36 mgcm−2) and membrane materials (imbibed m-polybenzimidazole (PBI) and polyphosphoric acid (PPA) processed p-PBI). The most influential factors for the acid migration rate are current density and the amount of free acid in the membrane. High doping level of the membrane and current density above 0.4 A cm−2 significantly increase the migration rate. From the migration rates apparent transference numbers for the hydrogen phosphate anions in the order 10−5 to 10−4 are calculated at the high current densities. Besides the membrane properties, also the influence of the microstructure of the porous transport layers was analyzed. Most probably cracks in the catalyst and microporous layers facilitate the migration of acid into the anode GDL., Journal of the Electrochemical Society, 163 (8), ISSN:0013-4651, ISSN:1945-7111

This paper reports on a measurement of the double-polarization observable G in $\pi^0$ photoproduction off the proton using the CBELSA/TAPS experiment at the ELSA accelerator in Bonn. The observable G is determined from reactions of linearly-polarized photons with longitudinally-polarized protons. The polarized photons are produced by bremsstrahlung off a properly oriented diamond radiator. A frozen spin butanol target provides the polarized protons. The data cover the photon energy range from 617 to 1325 MeV and a wide angular range. The experimental results for G are compared to predictions by the Bonn-Gatchina (BnGa), J\"ulich-Bonn (J\"uBo), MAID and SAID partial wave analyses. Implications of the new data for the pion photoproduction multipoles are discussed.

New data on the polarization observables T, P, and H for the reaction γp→pπ0 are reported. The results are extracted from azimuthal asymmetries when a transversely polarized butanol target and a linearly polarized photon beam are used. The data were taken at the Bonn electron stretcher accelerator ELSA using the CBELSA/TAPS detector. These and earlier data are used to perform a truncated energy-independent partial wave analysis in sliced-energy bins. This energy-independent analysis is compared to the results from energy-dependent partial wave analyses.

The first measurement of the helicity dependence of the photoproduction cross section of single neutral pions off protons is reported for photon energies from 600 to 2300 MeV, covering nearly the full solid angle. The data are compared to predictions from the SAID, MAID, and BnGa partial wave analyses. Strikingly large differences between data and predictions are observed, which are traced to differences in the helicity amplitudes of well-known and established resonances. Precise values for the helicity amplitudes of several resonances are reported.

Near threshold omega meson photoproduction is studied with the CBELSA/TAPS experiment at the ELSA accelerator of Bonn University. Single as well as double polarization observables are measured using either linear or circular polarized photon beams and a longitudinal polarized proton target., Proceeding for NSTAR 2011 conference

The first measurement is reported of the double-polarization observable G in photoproduction of neutral pions off protons, covering the photon energy range from 620 to 1120 MeV and the full solid angle. G describes the correlation between the photon polarization plane and the scattering plane for protons polarized along the direction of the incoming photon. The observable is highly sensitive to contributions from baryon resonances. The new results are compared to the predictions from SAID, MAID, and BnGa partial wave analyses. In spite of the long-lasting efforts to understand {\gamma}p -> p{\pi} 0 as the simplest photoproduction reaction, surprisingly large differences between the new data and the latest predictions are observed which are traced to different contributions of the N (1535) with spin-parity J^P = 1/2^- and N (1520) with J^P = 3/2^- . In the third resonance region, where N (1680) with J^P = 5/2^+ production dominates, the new data are reasonably close to the predictions., Comment: Submitted for publication in PRL

It is shown that microchannel plates (MCPs) tend to act as if they were discrete stage electron multipliers with a fixed number of stages or dynodes if a plausible assumption is made regarding the behavior of the secondary electrons emitted from the semiconducting sidewalls of tubular channel electron multipliers under the grazing incidence conditions predominantly encountered in these multipliers. The shape of the resultant predicted gain-voltage transfer characteristic for the MCP fits well with experimental data, confirming the assumption made and permitting the use of curve matching techniques to determine such important MCP parameters as the average number of active dynodes, the gain per stage, the crossover potential for the MCP wall material, the transit time through the multiplier, etc.

Experimental measurements of the lateral (i.e., radial) spreading of photoelectrons from a multialkali photocathode in an unfocused image dissector tube have shown that (a) the distribution of the radial emission energies is approximately Maxwellian and (b) the radial emission energies are smaller than expected based on the difference between the input photon energies and the long wavelength threshold energy of the multialkali photocathode ( congruent with1.4 eV). Both results are favorable with respect to the predicted electron optical image transfer properties of proximity focused image tubes. The observed data indicate that at least a 40-lp/mm or a 1600-lp/40-mm image field diameter should be resolvable for white light input to commercially available tubes-and that this resolving power should increase to at least 60 lp/mm for input wavelengths longer than approximately 600 nm and decrease to 30 lp/mm or less for input wavelengths shorter than approximately 400 nm.