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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 901 records · Page 50
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Utilizing ink composition to tune bulk-electrode gas transport, performance, and operational robustness for a Fe-N-C catalyst in polymer electrolyte fuel cell

With lower site density and turnover frequency, platinum group metal (PGM)-free catalysts based electrodes are often greater than 50 mu m thick in order to increase performance across the fuel cell operating range. Consequently, PGM-free electrodes have an additional bulk electrode transport resistance beyond the local or aggregate level transport in thin platinum-based electrodes. In parallel to the development of more active and durable PGM-free catalysts, advancements in understanding the interplay between PGM-free electrode fabrication, bulk-electrode transport, proton conductivity and performance are needed. Here, the relationship between ionic and gas phase transport through the electrode thickness is modified by adjusting electrocatalyst and ionomer flocculation/interaction at the ink level. The influence of the ink composition (water/n-propanol content) is examined via various in-situ electrochemical and ex-situ characterization techniques and the resulting electrode structure/performance relationship contrasted with electrode performance robustness across a range of relative humidity (RH). For the electrocatalyst examined here, a water-rich (82 wt% H2O) ink formulation was favorable for operation at high RH due to improved molecular diffusion through larger electrode pores. In contrast, the improved interactions between ionomer and electrocatalyst enabled a more robust electrode and higher performance during low RH operation for the 50 wt% H2O content ink.

08 HYDROGEN↗
DOE OSTI2017

Improved measurements of two-photon widths of the χ c J states and helicity analysis for χ c 2 → γ γ

Based on 448.1 x 10 6 ψ(3686) events collected with the BESIII detector, the decays ψ(3686) → γχ cJ ,χ cJ → γγ(J = 0,1,2) are studied in this paper. The decay branching fractions of χ c0,2 → γγ are measured to be β(χ c0 → γγ) = (1.93 ± 0.08 ± 0.05 ± 0.05) x 10 -4 and β(χ c2 → γγ) = (3.10 ± 0.09 ± 0.07 ± 0.11) x 10 -4 , which correspond to two-photon decay widths of Γ γγ (χ c0 ) = 2.03 ± 0.08 ± 0.06 ± 0.13 keV and Γ γγ (χ c2 ) = 0.60 ± 0.02 ± 0.01 ± 0.04 keV with a ratio of R = Γ γγ (χ c2 )/Γ γγ (χ c0 ) = 0.295 ± 0.014 ± 0.007 ± 0.027, where the uncertainties are statistical, systematic and associated with the uncertainties of β(ψ(3686) → γχ c0,2 ) and the total widths Γ(χ c0,2 ), respectively. For the forbidden decay of χ c1 → γγ, no signal is observed, and an upper limit on the two-photon width is obtained to be Γ γγ (χ c1 ) < 5.3 eV at the 90% confidence level. Finally, the ratio of the two-photon widths between helicity-zero and helicity-two components in the decay χ c2 → γγ is also measured to be f 0/2 = Γ λ=0 γγ (χ c2 )/Γ λ=2 γγ (χ c2 ) = (0.0 ± 0.6 ± 1.2) x 10 -2 , where the uncertainties are statistical and systematic, respectively.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Highly Coupled Heterobicycle‐Fused Porphyrin Dimers: Excitonic Coupling and Charge Separation with Coordinated Fullerene, C 60

Abstract Porphyrin dimers have been widely explored and studied owing to their importance in photosynthetic systems. A vast variety of dimers linked by different groups and at different angles have been synthesized and studied; however, the means by which to synthesize rigidly fused porphyrins with direct conjugation of the chromophores remains limited. Such a class of porphyrins may possess interesting properties that unconjugated or stacked dimers may not exhibit. In this study, bisbenzimidazole‐fused porphyrin dimers and their mono‐ and bis−zinc derivatives are synthesized and characterized. As a consequence of excitonic coupling, these dimers exhibit a split Soret band irrespective of the metal ion in the porphyrin cavity. Steady‐state fluorescence and excitation spectra followed by femtosecond transient absorption spectral studies of the heterometallated dimer, (free‐base and zinc porphyrin) reveals the occurrence of efficient singlet‐singlet energy transfer (>95 % efficiency and rate constant >10 12 s −1 ) within the dyad. Further, donor‐acceptor conjugates were formed by metal‐ligand axial coordination of phenyl imidazole functionalized C 60 and were characterized by a variety of physicochemical techniques. Excited state charge separation from both singlet and triplet excited states of ZnP in the conjugates has been established. The lifetime of the final charge‐separated state was in the 30–40 μs range revealing charge stabilization. Interestingly, no charge separation in the conjugate derived from the heterometallated dimer was observed wherein excitation transfer dominated the process. The present study brings out the importance of the rigid π‐spacer connecting porphyrin dimers in governing the energy and electron transfer events when coupled with an electron acceptor.

Chemistry↗
DOE OSTI2022

Demonstration of no catalytical activity of Fe–N–C and Nb–N–C electrocatalysts toward nitrogen reduction using in–line quantification

Ammonia (NH 3 ) production via the electrochemical nitrogen reduction reaction (NRR) is a promising method for sustainable generation of this important chemical. Efforts are ongoing in finding an efficient, stable, and selective catalyst that will enable the reaction. However, progress is hindered in the field due to lack of reproducibility, most likely a consequence of reports of false-positive results due to improper measurement control and methods. In this study, we explore the NRR activity of a promising class of single atom catalysts, transition metal-nitrogen-carbon (M-N-C) electrocatalysts. Using a state-of-the-art in-line ammonia quantification methodology, with detection limit as low as 1 ppb for ammonia, we show that single atom Nb and Fe embedded in a stable carbon and nitrogen framework do not electrochemically reduce N 2 to NH 3 . Critically, this demonstrates that our experimental setup with in-line sequential injection analysis successfully excludes ammonia contamination from the gas supply and atmospheric sources, allowing for thorough and high-throughput examination of potential NRR catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Phonon and Thermodynamic Properties in YNi{sub 2}B{sub 2}C and LuNi{sub 2}B{sub 2}C from First-Principles Study

The thermodynamic properties of the tetragonal RNi{sub 2}B{sub 2}C (R=Y, Lu) are investigated by means of the density-functional theory (DFT) and density-functional perturbation theory (DFPT) within the quasiharmonic approximation (QHA). The phonon dispersion curves and phonon total and partial density of states are studied, and it is found that the phonon branches can be divided into three obvious regions: a low-frequency region (LFR), an intermediate-frequency region (IFR), and a high-frequency region (HFR). The temperature dependence of the various quantities including isothermal bulk modulus, thermal expansion coefficient, and heat capacity is obtained, and the variation features of these quantities with temperature are discussed in detail.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Dynamic Recrystallization and Grain Refinement of Fe-P-C-Si and Fe-P-C-Si-N Steels

Grain refinement is an effective technique to improve the mechanical properties of steels. In the present work, single-pass hot compression experiments have been conducted on two different compositions of high phosphorous steels to study the microstructural evolution and ferrite grain refinement at various strain rates and deformation temperatures, i.e., 0.01-10 s{sup −1} and 750-1050 °C, respectively. Optical metallography has been employed to understand the physical processes that take place during hot deformation process. The results indicate that when these compositions of high phosphorous steels are worked at relatively low temperatures in the intercritical regions, a ferrite grain size of 5-7 µm is obtained. It is observed that the grain size decreases with an increase in strain rate and with the decrease in deformation temperature. Based on the values of stress exponent (n) obtained in the present work, dislocation creep is identified as the deformation mechanism. The activation energies for deformation of these two types of high phosphorous steels have been calculated. The effect of the alloying elements on the stress–strain curve, microstructure, and grain refinement has been discussed.

36 MATERIALS SCIENCE↗