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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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Efficient Iridium Catalysts for Formic Acid Dehydrogenation: Investigating the Electronic Effect on the Elementary β-Hydride Elimination and Hydrogen Formation Steps

We report herein a series of Cp*Ir complexes containing a rigid 8-aminoquinolinesulfonamide moiety as highly efficient catalysts for the dehydrogenation of formic acid (FA). The complex [Cp*Ir(L)Cl] (HL = N-(quinolin-8-yl)benzenesulfonamide) displayed a high turnover frequency (TOF) of 2.97 × 10 4 h –1 and a good stability (>100 h) at 60 °C. Comparative studies of [Cp*Ir(L)Cl] with the rigid ligand and [Cp*Ir(L')Cl] (HL' = N-propylpypridine-2-sulfonamide) without the rigid aminoquinoline moiety demonstrated that the 8-aminoquinoline moiety could dramatically enhance the stability of the catalyst. The electron-donating ability of the N,N'-chelating ligand was tuned by functionalizing the phenyl group of the L ligand with OMe, Cl, and CF 3 to have a systematical perturbation of the electronic structure of [Cp*Ir(L)Cl]. Experimental kinetic studies and density functional theory (DFT) calculations on this series of Cp*Ir complexes revealed that (i) the electron-donating groups enhance the hydrogen formation step while slowing down the β-hydride elimination and (ii) the electron-withdrawing groups display the opposite effect on these reaction steps, which in turn leads to lower optimum pH for catalytic activity compared to the electron-donating groups.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Impact of Porous Transport Layer Morphology on the Performance of Proton Exchange Membrane Water Electrolyzers with Ultra-Low Iridium Loadings

Reducing Ir loadings in proton exchange membrane water electrolyzer anodes is critical for lowering capital expenses. Loading reduction could be achieved by improving the Ir activity via doping/alloying and/or the development of advanced microstructures. However, the anode porous transport layer (PTL) is a comparatively simple component whose properties also impact Ir utilization. Therefore, well-designed PTLs may also enable reduced Ir loadings. In this work, we survey eight PTLs from various manufacturers to observe their impact on cell performance at low (0.4 mg Ir cm -2 ) and ultralow (0.1 mg Ir cm -2 ) Ir loadings. The PTLs were characterized by their microstructural properties, including porosity, particle size distribution, and pore size distribution. Electrochemical cell performance was correlated to PTL morphology, and it was found that PTLs with lower porosities and smaller particle and pore radii enabled good performance even at ultralow Ir loadings. 1000-h durability testing indicated that using lower porosity PTLs can significantly improve durability behavior. A runaway voltage phenomenon was observed during durability testing of cells with ultralow Ir loadings, which was caused by increases in both anode and cathode overpotentials. Furthermore, we observed that the beginning of test performance of 0.1 mg Ir cm -2 cells correlates to the 1000-h degradation rates of 0.4 mg Ir cm -2 cells, suggesting that for the Ir catalyst used in this work, short-term testing at ultralow loadings can be used as an indicator of long-term degradation at higher loadings.

08 HYDROGEN↗
DOE OSTI2020

H 2 Evolution at an Electrochemical “Underpotential” with an Iridium-Based Molecular Photoelectrocatalyst

Molecular platforms capable of both photochemical H 2 evolution and electrochemical catalyst regeneration hold promise for the efficient generation of solar fuels. Systems studied to date require a significant electrochemical overpotential (η), so no photon energy is being chemically stored. A detailed study of the parameters that govern the electrochemical overpotential of molecular photoelectrocatalysts is presented. By understanding how tuning the solvent, catalyst structure, and acid pK a affect the kinetics and thermodynamics of the potential- and light-driven H 2 evolution reaction, conditions were identified wherein the solvento complex [Cp*Ir(bpy)(NCCH 3 )][PF 6 ] 2 (Cp* is pentamethylcyclopentadienyl; bpy is 2,2'-bipyridine) mediates photoelectrocatalytic H 2 formation at an electrochemical “underpotential” (η < 0) driven by visible light. Under 460 nm illumination and in the presence of the weak organic acid H-PhTMG + (2-phenyl-1,1,3,3- tetramethylguanidinium), the catalyst facilitates H 2 evolution with Faradaic efficiencies up to 90% and overpotentials as low as –90 mV. As a result, mechanistic studies enabled the construction of graphical “maps” that can guide the choice of catalyst and conditions to minimize overpotential, applicable to both photoelectrocatalysts and dark electrocatalysts.

14 SOLAR ENERGY↗
DOE OSTI2026

Non-ideal stoichiometry and thermochemistry of aqueous iridium oxide nanoparticles in proton-coupled electron transfer and oxygen-atom transfer

Reported here are reactions of aqueous colloidal IrO x nanoparticles (NPs) with proton-coupled electron transfer (PCET) and oxygen-atom transfer (OAT) organic reagents, determining the reaction stoichiometries and thermochemistry. IrO x NPs have attracted much attention for their high electrocatalytic activity, but understanding of their fundamental reaction chemistry is limited. This IrO x NP model system is simple, with UV-vis titrations demonstrating reversible interconversion between predominantly Ir IV and predominantly Ir III NPs. This simplicity allows studies that reveal their complex non-idealities. The NP redox chemistry has a “super-Nernstian” stoichiometry of ∼1.3H + per 1e − transferred during both PCET and OAT reactions, as measured with electrochemistry and chemical methods. Spectroelectrochemistry revealed a broad distribution of surface IrO x –H bond dissociation free energies (BDFEs), becoming weaker as more H is added. Such variation in binding strengths—a non-ideal binding isotherm—is common for surface adsorbates. For IrO x , the variation of BDFE(IrO–H)s is fit well to a Frumkin isotherm with a width of 6.5 kcal mol −1 . For OAT from the reactive oxygen atoms of IrO x NPs, bracketing experiments gave 93 ± 24 kcal mol −1 for the average BDFE(O x Ir–O), with a predicted spread much larger than that for the BDFE(IrO–H). Taken together, the results show the importance of non-ideal stoichiometry and thermochemistry for IrO x NPs, and they open a path to more complete models to understand catalytic redox reactions at such surfaces.

Iridium Oxide Nanoparticles (NPs)↗
DOE OSTI2022

Dominant Energy Carrier Transitions and Thermal Anisotropy in Epitaxial Iridium Thin Films

High aspect ratio metal nanostructures are commonly found in a broad range of applications such as electronic compute structures and sensing. The self-heating and elevated temperatures in these structures, however, pose a significant bottleneck to both the reliability and clock frequencies of modern electronic devices. Any notable progress in energy efficiency and speed requires fundamental and tunable thermal transport mechanisms in nanostructured metals. Here, in this work, time-domain thermoreflectance is used to expose cross-plane quasi-ballistic transport in epitaxially grown metallic Ir(001) interposed between Al and MgO(001). Thermal conductivities ranges from roughly 65 (96 in-plane) to 119 (122 in-plane) W m -1 K -1 for 25.5–133.0 nm films, respectively. Further, low defects afforded by epitaxial growth are suspected to allow the observation of electron–phonon coupling effects in sub-20 nm metals with traditionally electron-mediated thermal transport. Via combined electro-thermal measurements and phenomenological modeling, the transition is revealed between three modes of cross-plane heat conduction across different thicknesses and an interplay among them: electron dominant, phonon dominant, and electron–phonon energy conversion dominant. The results substantiate unexplored modes of heat transport in nanostructured metals, the insights of which can be used to develop electro-thermal solutions for a host of modern microelectronic devices and sensing structures.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Dimetalloylene (M‐E‐M) Complexes of Heavier Main Group Elements Ge, Sn, Pb, Bi via Cleavage of E‐X Bonds (X=N(SiMe 3 ) 2 , O t Bu) with an Iridium Hydride

Abstract Reactions of the Ir V hydride [ Me BDI Dipp ]IrH 4 {BDI=(Dipp)NC(Me)CH(Me)CN(Dipp); Dipp=2,6‐ i Pr 2 C 6 H 3 } with E[N(SiMe 3 ) 2 ] 2 (E=Sn, Pb) afforded the unusual dimeric dimetallotetrylenes ([ Me BDI Dipp ]IrH) 2 ( μ 2 ‐E) 2 in good yields. Moreover, ([ Me BDI Dipp ]IrH) 2 ( μ 2 ‐Ge) 2 was formed in situ from thermal decomposition of [ Me BDI Dipp ]Ir(H) 2 Ge[N(SiMe 3 ) 2 ] 2 . These reactions are accompanied by liberation of HN(SiMe 3 ) 2 and H 2 through the apparent cleavage of an E−N(SiMe 3 ) 2 bond by Ir−H. In a reversal of this process, ([ Me BDI Dipp ]IrH) 2 ( μ 2 ‐E) 2 reacted with excess H 2 to regenerate [ Me BDI Dipp ]IrH 4 . Varying the concentrations of reactants led to formation of the trimeric ([ Me BDI Dipp ]IrH 2 ) 3 ( μ 2 ‐E) 3 . The further scope of this synthetic route was investigated with group 15 amides, and ([ Me BDI Dipp ]IrH) 2 ( μ 2 ‐Bi) 2 was prepared by the reaction of [ Me BDI Dipp ]IrH 4 with Bi(NMe 2 ) 3 or Bi(O t Bu) 3 to afford the first example of a “naked” two‐coordinate Bi atom bound exclusively to transition metals. A viable mechanism that accounts for the formation of these products is proposed. Computational investigations of the Ir 2 E 2 (E=Sn, Pb) compounds characterized them as open‐shell singlets with confined nonbonding lone pairs at the E centers. In contrast, Ir 2 Bi 2 is characterized as having a closed‐shell singlet ground state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Polymer Entrapment Flash Pyrolysis for the Preparation of Nanoscale Iridium-Free Oxygen Evolution Electrocatalysts

This paper describes the use of a new polymer entrapment flash pyrolysis (PEFP) method for making nanoscale yttrium ruthenate (Y 2 Ru 2 O 7-δ ) electrocatalysts. Here, this approach effectively reduced the synthesis temperature of phase-pure pyrochlore catalysts from 1000 °C to 550 °C, and greatly suppressed the sintering of catalyst particles. The supported nanocrystalline Y 2 Ru 2 O 7-δ catalysts showed enhanced activity towards oxygen evolution reaction (OER) in acidic electrolyte and were stable at 1.50 V for the comparative study (>20 h) under the current density of 10 mA/cm 2 geo in chronopotentiometry testing. This is equivalent to an overpotential value of 270 mV, about half of that of the reference IrO 2 catalyst. X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM) analysis showed that the high-surface-area Y 2 Ru 2 O 7-δ catalyst had an oxygen-deficient structure. This study provides a route to the synthesis of fine ceramic (or oxide)-based electrocatalysts for making high-performing electrocatalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Comparison of model predictions with measured proton-induced production of nickel and iridium isotopes

Calculations were performed to support method development for simultaneous production of Ni and Ir isotopes. Here, the work scope included development of the physical target, irradiation configuration, and post-irradiation radiochemical separations. The assumptions of predictive models previously developed were refined based upon the precise experimental configuration selected, including Os target material and a set of stacked targets for concurrent production of Ni and Ir isotopes. Model predictions of the reaction cross sections with EMPIRE 3.21 and implied isotopic yields are compared with proton beam irradiation measurements to refine model parameters and guide future experiments.

Isotope Production↗
DOE OSTI2023

Advances in arene alkylation and alkenylation catalyzed by transition metal complexes based on ruthenium, nickel, palladium, platinum, rhodium and iridium

Over the past several years, significant advancements have been made for transition metal catalyzed arene alkylation and alkenylation that operate by metal mediated arene C–H activation. These catalytic processes provide a route for arene alkylation and alkenylation that is complementary to traditional acid catalyzed reactions (e.g., Friedel-Crafts reactions and zeolite-based arene alkylations). Catalysts from Groups 8, 9 and 10 have dominated new developments. As a result, this monograph will overview advancements for catalytic arene alkylation and alkenylation for hydrocarbon substrates.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Single-step selective oxidation of methane to methanol in the aqueous phase on iridium-based catalysts

In this work, one-step conversion of methane to methanol using molecular oxygen as the oxidant in the presence of CO in aqueous solutions on copper or palladium promoted Ir-ZSM-5 catalyst is first reported. The addition of a second metal to Ir-ZSM-5 promotes the catalyst activity, while product selectivity can be tuned either to methanol on IrCu-ZSM-5 or to formic acid exclusively on IrPd-ZSM-5. Most effective is the combination of the three metal species together. Approximately 1200 μmol/g cat methanol, or ~23.4 mol of methanol per mol of Ir, are formed on the IrCuPd trimetallic system (methanol selectivity ~80 %) at 150 °C in 1 h. Our results also demonstrate that atomically dispersed Ir(I)(CO) 2 species formed in the presence of CO can activate the C–H bond of methane to methyl species at temperatures below 150 °C. The good stability in cyclic operation is an additional attribute, rendering this type of catalyst a “front-runner” in future catalyst development for direct methane-to-liquid oxygenates.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Electronic and magnetic properties of iridium-based novel Heusler alloys

We report half-metallicity and magnetism including exchange splitting are the most significant physical parameters to predict and design a candidate material for spintronic applications. We report here an ab-initio investigation on chemical formation and dynamical stability along with electronic structure and magnetic properties of Ir 2 Cr (Si, Ge) and IrRhCr (Si, Ge) Heusler alloys. The negative formation and cohesive energies with positive phonon dispersions confirm the stabilities of these alloys. Electronic structure calculations reveal that Ir 2 Cr (Si, Ge) and IrRhCrSi alloys are half-metallic ferromagnets with unprecedented exchange splitting. In addition, IrRhCrGe also shows semi-metallic nature. All of these materials follow Slater Pauling rule with large magnetic moments and 100% spin-polarization. With Cr bearing the majority of the local magnetic moment and exchange splitting, a ferromagnetic state is more stable than a nonmagnetic state. The electronic charge distribution and population analysis confirm mixed ionic and covalent bonding. The magnetocrystalline anisotropy energy, with the easy magnetization along the [1 1 1] direction, is significantly high in Ir 2 CrGe. Elastic constants such as shear (G), bulk (B), Young’s moduli, and Poisson’s ratio indicate that the IrRhCrSi and IrRhCrGe alloys are mechanically stable, and Ir 2 CrSi and Ir 2 CrGe are mechanically unstable. The Pugh’s (B/G) and Poisson’s ratios confirm that the stable alloys are ductile.

36 MATERIALS SCIENCE↗