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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Biphasic Janus Particles Explain Self-Healing in Pt–Pd Diesel Oxidation Catalysts

The addition of Pd to Pt-based diesel oxidation catalysts is known to enhance performance and restrict the anomalous growth of Pt nanoparticles when subjected to aging at high temperatures in oxidative environments. To gain a mechanistic understanding, we studied the transport of the mobile Pt and Pd species to the vapor phase, since vapor phase transport is the primary route for sintering in these catalysts. The results are surprising: there is a 30-fold drop in the effective vapor pressure of Pt in the Pt-Pd catalysts compared to monometallic Pt. At the same time, there is a significant enhancement in the vapor pressure of Pd, compared to PdO, which otherwise has a negligible vapor pressure at the aging temperature. Such behavior cannot be explained simply by alloying Pt and Pd in the metallic phase, or a core-shell morphology where a PdO shell covers a Pt core. Transmission electron microscopic examination of catalysts aged up to 50 h in air at 800 °C shows that the particles exhibit a biphasic “Janus”-like structure. The metal and oxide phases are conjoined, exposing a metal and an oxide face to the gas phase. The high mobility of the Pt and Pd allows them to be partitioned into the metal and oxide phases, in apparent thermodynamic equilibrium. The PdO helps to trap mobile PtO2 and as a result contains high concentrations of Pt oxide, consistent with its role in mitigating the transport of Pt to the vapor phase and preventing the growth of anomalously large particles. In turn, Pt allows Pd to remain metallic, allowing the catalyst to retain both metal and oxide functionality for catalysis. The regeneration of deactivated catalysts typically requires an external input, such as a change in the working environment from reducing to oxidizing or vice-versa. Here, we show that the mobile species, which are primary contributors to catalyst sintering are effectively returned to the active site, hence our use of the term “selfhealing”. The detailed insights into the inner workings of the Pt-Pd diesel oxidation catalysts can help provide clues to the design of robust and durable heterogeneous catalysts.

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

Surface oxygenation of multicomponent nanoparticles toward active and stable oxidation catalysts

Abstract The need for active and stable oxidation catalysts is driven by the demands in production of valuable chemicals, remediation of hydrocarbon pollutants and energy sustainability. Traditional approaches focus on oxygen-activating oxides as support which provides the oxygen activation at the catalyst-support peripheral interface. Here we report a new approach to oxidation catalysts for total oxidation of hydrocarbons (e.g., propane) by surface oxygenation of platinum (Pt)-alloyed multicomponent nanoparticles (e.g., platinum-nickel cobalt (Pt–NiCo)). The in-situ/operando time-resolved studies, including high-energy synchrotron X-ray diffraction and diffuse reflectance infrared Fourier transform spectroscopy, demonstrate the formation of oxygenated Pt–NiOCoO surface layer and disordered ternary alloy core. The results reveal largely-irregular oscillatory kinetics associated with the dynamic lattice expansion/shrinking, ordering/disordering, and formation of surface-oxygenated sites and intermediates. The catalytic synergy is responsible for reduction of the oxidation temperature by ~100 °C and the high stability under 800 °C hydrothermal aging in comparison with Pt, and may represent a paradigm shift in the design of self-supported catalysts.

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

Nickel catalysts for affordable fuel cells

Hydrogen oxidation reactions in hydroxide exchange membrane fuel cells have slow kinetics. Further, switching from platinum group metal (PGM) electrocatalysts to those that are PGM-free is a challenging task as the latter are prone to oxidation. Now, stable and active nickel–molybdenum–niobium catalysts are introduced for this type of fuel cell.

25 ENERGY STORAGE↗
DOE OSTI2022

Enhancing glycerol electrooxidation from synergistic interactions of platinum and transition metal carbides

To effectively utilize glycerol as a fuel for electrochemical fuel cells, it is necessary to optimize catalysts for effective C-C bond cleavage and complete oxidation of reaction intermediates to achieve maximum efficiency. Here, the current work showed that the synergistic interactions of platinum (Pt) with transition metal carbide (TMC) substrates, such as tungsten carbide (WC) and tantalum carbide (TaC), fulfilled these criteria. The TMC-supported Pt catalysts showed higher activity and selectivity for complete glycerol oxidation than commercial 10 wt% Pt/C. In-situ FTIR analysis revealed that 5 wt% Pt/WC was the most effective catalyst among those tested for complete glycerol oxidation at 0.9 V vs RHE. In-situ X-ray absorption fine structure characterization and density functional theory calculations provided additional insight into the synergistic interactions for glycerol oxidation over Pt/TMC catalysts.

25 ENERGY STORAGE↗
DOE OSTI2020

Photosynthesis of a Photocatalyst: Single Atom Platinum Captured and Stabilized by an Iron(III) Engineered Defect

Single atom (SA), noble metal catalysts are of interest due to high projected catalytic activity while minimizing cost. Common issues facing many synthesis methodologies include complicated processes, low yields of SA product, and production of mixtures of SA and nanoparticles (NPs). Herein we report a simple, room-temperature synthesis of single Pt-atom decorated, anatase Fe-doped TiO 2 particles that leverages the Fe dopant as an engineered defect site to photodeposit and stabilize atomically dispersed Pt. Both particle morphology and Fe dopant location are based on thermodynamic principles (Gibbs-Wulff construction). CO-DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) reveals absence of bridge-bonded CO signal, confirming atomically dispersed Pt. XAS (x-ray absorption spectroscopy) of both Pt and Fe indicates Fe-O-Pt bonding that persists through catalytic cycling. Mass balance indicates Pt loading on single particles is 2.5 wt% Pt; single Pt-atom decorated nanoparticle yield is 17%. Pt containing particles show more than an order-of-magnitude increased photo-oxidation efficiency relative to particles containing only Fe. High single-atom-Pt yield, ease of synthesis, and high catalytic activity demonstrate the utility and promise of this method. In conclusion, the principles of this photodeposition synthesis allow for its generalizability towards other SA metals of catalytic interest.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Ultrasmooth Epitaxial Pt Thin Films Grown by Pulsed Laser Deposition

Platinum (Pt) thin films are useful in applications requiring high-conductivity electrodes with excellent thermal and chemical stability. Ultrasmooth and epitaxial Pt thin films with single-crystalline domains have the added benefit of providing ideal templates for the subsequent growth of heteroepitaxial structures. Here, we grow epitaxial Pt (111) electrodes (ca. 30 nm thick) on sapphire (α-Al 2 O 3 (0001)) substrates with pulsed laser deposition. This versatile technique allows control of the growth process and fabrication of films with carefully tailored parameters. X-ray scattering, atomic-force microscopy, and electron microscopy provide structural characterization of the films. Various gaseous atmospheres and temperatures were explored to achieve epitaxial growth of films with low roughness. A two-step (500 °C/300 °C) growth process was developed, yielding films with improved epitaxy without compromising roughness. The resulting films possess ultrasmooth interfaces (<3 Å) and high electrical conductivity (6.9 × 10 6 S/m). Finally, Pt films were used as current collectors and templates to grow lithium manganese oxide (LiMn 2 O 4 (111)) epitaxial thin films, a cathode material used in Li-ion batteries. In conclusion, using a solid-state ionogel electrolyte, the films were highly stable when electrochemically cycled in the 3.5-4.3 V vs Li/Li + range.

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

Effect of surface acidity modulation on Pt/Al 2 O 3 single atom catalyst for carbon monoxide oxidation and methanol decomposition

With ideal metal utilization efficiency and homogeneous active sites, single atom catalysts (SACs) have attracted extensive attention in heterogeneous catalysis field. The performance of SACs is highly dependent on the local coordination environment of metal single atoms. However, few works reported the effect of surface acidity modification on the performance of SACs such as Pt SACs which have been widely studied nowadays. In this work, a Pt/Al 2 O 3 SAC has been successfully prepared using a bayerite Al(OH) 3 as support through simple IWI method for Pt loading. Through post-modification of Pt/Al 2 O 3 with acidic WO 3 and basic MgO, the surface acidity of Pt/Al 2 O 3 was tuned. Here, activity of CO oxidation and methanol decomposition on the modified catalysts has been evaluated. It was found that the catalytic performance was highly related to the surface acidity of Pt SACs. Basic MgO modified Pt/Al 2 O 3 performed better in CO oxidation under the reaction condition with H 2 O. Acidic WO 3 modified Pt/Al 2 O 3 showed superior activity in methanol decomposition. Detailed characterization and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study suggested that the positive H 2 O effect on CO oxidation was due to the mitigated formation of bicarbonate species on Pt/Al 2 O 3 and WO 3 /Pt/Al 2 O 3 catalysts and the promoted desorption of carbonate species from MgO/Pt/Al 2 O 3 . The superior methanol decomposition performance on WO 3 /Pt/Al 2 O 3 was owing to its abundant oxygen vacancies and high resistance to carbonate accumulation. This study highlights the importance of surface acidity modulation of Pt SACs in heterogeneous catalysis and suggests that fine-tuning the surface acidity can be an effective strategy in new SACs development.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Single Metal Atom Catalysts Prepared by Diluted Atomic Layer Deposition

The scalable and facile preparation of single-atom catalysts remains a critical challenge. Here, in this study, we introduce diluted atomic layer deposition (DALD), a unique approach for synthesizing supported metal catalysts with precisely tunable loadings. Unlike conventional metal deposition by ALD which uses pure metal precursors, DALD employs a diluted precursor mixture, combining organometallic precursors with the corresponding free ligand in controlled ratios. The method enables precise control over metal loadings, allowing the synthesis of structures ranging from nanoparticles to isolated single atoms, as exemplified by Ir, Rh, and Pt on high-surface-area γ-Al 2 O 3 . With its inherent simplicity and exceptional efficiency in metal precursor utilization, DALD represents a highly scalable strategy, unlocking opportunities for integrating single-atom catalysts into industrial processes.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Simulated Start-Stop and the Impact of Catalyst Layer Redox on Degradation and Performance Loss in Low-Temperature Electrolysis

Stress tests are developed that focus on anode catalyst layer degradation in proton exchange membrane electrolysis due to simulated start-stop operation. Ex situ testing indicates that repeated redox cycling accelerates catalyst dissolution, due to near-surface reduction and the higher dissolution kinetics of metals when cycling to high potentials. Similar results occur in situ, where a large decrease in cell kinetics (>70%) is found along with iridium migrating from the anode catalyst layer into the membrane. Additional processes are observed, however, including changes in iridium oxidation, the formation of thinner and denser catalyst layers, and platinum migration from the transport layer. Increased interfacial weakening is also found, adding to both ohmic and kinetic loss by adding contact resistances and isolating portions of the catalyst layer. Repeated shutoffs of the water flow further accelerate performance loss and increase the frequency of tearing and delamination at interfaces and within catalyst layers. These tests were applied to several commercial catalysts, where higher loss rates were observed for catalysts that contained ruthenium or high metal content. These results demonstrate the need to understand how operational stops occur, to identify how loss mechanisms are accelerated, and to develop strategies to limit performance loss.

08 HYDROGEN↗
DOE OSTI2026

Processing iridium foil targets for the production of radioplatinum isotopes

Dissolving noble metals remains challenging because of their high reduction potentials and tendency to passivate as oxides. This barrier is acute for iridium metal targets used to produce platinum radioisotopes, where post-irradiation processing is costly and hazardous with conventional routes. Here, we report a rapid alkali-fusion method that converts Ir metal directly into chloride media compatible with downstream separations. In the process, an Ir foil is fused with Na 2 O 2 and KCl at temperatures above the KCl melting point; the molten salt enhances heat transfer and facilitates O 2− /Na + transport to the Ir surface, overcoming Ir's oxidation resistance. The fused cake is then cooled and leached in 10 M HCl, yielding Ir in solution without specialized microwave digestion or HF. Pt separation from bulk Ir was evaluated using extraction-chromatography resins. A newly formulated ionic-liquid resin containing Cyphos IL 104 (E104) was compared with two commercial resins (TrisKem TK200 and TK201). All resins achieved >90 % Ir recovery. Pt co-recovery depended on resin chemistry: TK200 recovered 85.9 % Pt, TK201 recovered 54.1 % Pt, and E104 recovered 72.9 % Pt. This study introduces a fast, scalable dissolution route for solid Ir targets and demonstrates resin choices that tune Ir/Pt separation, lowering the barrier to routine production and purification of Pt radioisotopes.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2020

Use of in Situ Synchrotron Techniques to Probe the Oxidized Surface of Molybdenum Nitride Oxygen Reduction Electrocatalysis

The development of active and stable earth-abundant catalysts for the oxygen reduction reaction (ORR) is needed for widespread, economic development of fuel cell technologies. Designing and optimizing these non-platinum group metals is challenging, however, because they are susceptible to composition and structure changes, including dissolution, oxidation, and corrosion, both in air and under reaction conditions. To identify the active surface, and thus understand the properties that affect activity, the catalyst surface must be characterized in situ. Herein, we utilize a grazing incidence electrochemical cell to investigate in situ composition and morphology changes of a molybdenum nitride (Mo-N) thin film catalyst using grazing incidence x-ray absorption spectroscopy (GI-XAS) and x-ray reflectivity (XRR). In rotating ring disk electrode measurements, we find that the activity, selectivity, stability, and capacitance of the Mo-N catalyst is dependent on the maximum potential to which it has been exposed. Specifically, the overpotential required to reach -2 mA cm-2geo decreases by over 90 mV when the maximum potential is increased from 0.3 to 0.8 V vs RHE (Figure 1). Because the Mo-N oxidizes rapidly in air, ex situ characterization methods including x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry can provide only limited insight into these in situ catalyst changes. Using in situ GI-XAS measurements at applied potentials between 0.3 and 0.9 V vs RHE, however, we are able to determine that the surface of the film oxidizes and becomes more amorphous when exposed to increasingly higher potentials (Figure 1). Furthermore, the surface remains oxidized on the order of several hours when returned to "ORR relevant potentials" (< 0.6 V vs RHE), indicating that this surface-oxidized nitride is the active surface for ORR. Using in situ XRR measurements, we find that there is no change in surface roughness at potentials below 0.7 V vs RHE, but the film roughens significantly at 0.8 V vs RHE, correlating with ex situ measurements of Mo dissolution at this potential (Figure 1). We therefore conclude that the intrinsic activity of the Mo-N catalyst increases when exposed to potentials up to 0.7 V vs RHE, while above that potential activity enhancements are due to the exposure of more active sites through dissolution. The in situ electrochemical surface-sensitive x-ray characterization as used here is a promising methodology for understanding and leveraging surface dynamics to improve the performance of non-traditional catalysts.

Kreider, Melissa↗
DOE OSTI2024

Pd-Ru pair on Pt surface for promoting hydrogen oxidation and evolution in alkaline media

Hydrogen oxidation reaction in alkaline media is critical for alkaline fuel cells and electrochemical ammonia compressors. The slow hydrogen oxidation reaction in alkaline electrolytes requires large amounts of scarce and expensive platinum catalysts. While transition metal decoration can enhance Pt catalysts’ activity, it often reduces the electrochemical active surface area, limiting the improvement in Pt mass activity. Here, we enhance Pt catalysts’ activity without losing surface-active sites by using a Pd-Ru pair. Utilizing a mildly catalytic thermal pyrolysis approach, Pd-Ru pairs are decorated on Pt, confirmed by extended X-ray absorption fine structure and high-angle annular dark-field scanning transmission electron microscopy. Density functional theory and ab-initio molecular dynamics simulations indicate preferred Pd and Ru dopant adsorption. The Pd-Ru decorated Pt catalyst exhibits a mass-based exchange current density of 1557 ± 85 A g –1 metal for hydrogen oxidation reaction, demonstrating superior performance in an ammonia compressor.

25 ENERGY STORAGE↗
DOE OSTI2020

Selective Electrocatalysis Imparted by Metal-Insulator Transition for Durability Enhancement of Automotive Fuel Cells.

Repetitive start-up and shut-down events in polymer electrolyte membrane fuel cells for automotive applications lead to serious corrosion of the cathode due to an instantaneous potential jump that results from unintended air leakage into the anodic flow field followed by a parasitic oxygen reduction reaction (ORR) on the anode. Here we report a solution to the cathode corrosion issue during the start-up/shut-down events whereby intelligent catalyst design is used to selectively promote the hydrogen oxidation reaction (HOR) while concomitantly suppressing the ORR on the anode. Platinum thin layers supported on hydrogen tungsten bronze (Pt/HxWO3) suppressed the ORR by converting themselves into an insulator following exposure to oxygen, while selectively promoting the HOR by regaining metallic conductivity following subsequent exposure to hydrogen. The HOR-selective electrocatalysis imparted by a metal-insulator transition in Pt/HxWO3 demonstrated a remarkably enhanced durability of membrane electrode assemblies compared to those with commercial Pt/C catalysts.

Jung, Sang-Mun↗
DOE OSTI2020

In Situ Strain Evolution on Pt Nanoparticles during Hydrogen Peroxide Decomposition

Fundamental understanding of structural changes during catalytic reactions is crucial to understanding the underlying mechanisms and optimizing efficiencies. Surface energy and related catalytic mechanisms are widely studied. However, the catalyst lattice deformation induced by catalytic processes is not well understood. Here, we study the strain in an individual platinum (Pt) nanoparticle (NP) using Bragg coherent diffraction imaging under in situ oxidation and reduction reactions. When Pt NPs are exposed to H 2 O 2 , a typical oxidizer and an intermediate during the oxygen reduction reaction process, alternating overall strain distribution near the surface and inside the NP is observed at the (111) Bragg reflection. In contrast, relatively insignificant changes appear in the (200) reflection. Density functional theory calculations are employed to rationalize the anisotropic lattice strain in terms of induced stress by H 2 O 2 adsorption and decomposition on the Pt NP surface. Our study provides deeper insight into the activity-structure relationship in this system.

3D imaging↗
DOE OSTI2023

Impacts of PTL coating gaps on cell performance for PEM water electrolyzer

The anode porous transport layer (PTL) is a critical component of polymer electrolyte membrane (PEM) water electrolyzers, providing electrical conduction and water/oxygen transport to the anode catalyst layer. Platinum group metal (PGM) coatings are applied to titanium PTLs to prevent oxidation and preserve conductivity. To implement PEM water electrolysis on a large scale, it is essential to minimize the cost of mass-manufactured cell components while ensuring that defects in materials and components do not adversely affect the performance of the cell. In this study, we created gaps in the Pt coatings that are typically applied to PTLs to understand their effects on cell performance and inform on fabrication tolerances. The PTL coatings were fabricated by sputter deposition of Pt, intentionally leaving uncoated regions in either large patches or thin strips. For either geometry, the cell performance decreased with the increasing fraction of the uncoated regions. However, PTLs containing multiple thin uncoated strips caused a less severe performance effect than PTLs with continuous uncoated regions of the same relative total area. Through analysis of cell performance, impedance, and 3D charge transport modelling, here we demonstrate that both the anode catalyst layer and membrane play a role in distributing current to mitigate the impact of coating defects at a short length scale. These results allow to draw conclusions about the tolerance requirements with regards to PTL coating uniformity in terms of both the size and total area of defects.

08 HYDROGEN↗
DOE OSTI2020

Mechanism of Electrochemical Oxidation of Nitroxide Radicals in Ethaline Deep Eutectic Solvent

Deep eutectic solvents (DESs) are emerging as promising electrolytes for electrochemical energy storage applications. Electroactive nitroxide-radical-containing organics can be dissolved in DESs to facilitate redox reactions; however, mechanistic know-how of their charge transfer kinetics at the electrode surface is rather limited. Here, we investigate the mechanism underlying the electrochemical oxidation of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-hydroxy-2,2,6,6-tetramethyl-piperidine-1-oxyl (4-hydroxy-TEMPO). Using polarization measurements on a platinum rotating disk electrode and micro-electrode, we show that the anodic charge transfer coefficient (a) for one-electron transfer oxidations of TEMPO and4-hydroxy-TEMPO approaches 0.9 in DES as well as in aqueous electrolytes, i.e., a significant deviation fromα≈0.5 expected for symmetric redox behavior. To explain this observation, a two-step oxidation mechanism is proposed wherein the nitroxide-containing species undergo fast charge transfer at an electrode surface followed by slow rate-limiting desorption of the adsorbed oxidized species. Numerical simulations are reported to characterize how the proposed two-step mechanism manifests intransient cyclic voltammetry behavior of the 4-hydroxy-TEMPO oxidation reaction, and good agreement with experiments is noted.

25 ENERGY STORAGE↗
DOE OSTI2020

High-Performance Pt–Co Nanoframes for Fuel-Cell Electrocatalysis

Pt-based alloy catalysts are promising candidates for fuel-cell applications, especially for cathodic oxygen reduction reaction (ORR) and anodic methanol oxidation reaction (MOR). The rational design of composition and morphology is crucial to promoting catalytic performances. Here, we report the synthesis of Pt–Co nanoframes via chemical etching of Co from solid rhombic dodecahedra. The obtained Pt–Co nanoframes exhibit excellent ORR mass activity in acidic electrolyte, which is as high as 0.40 A mg Pt –1 initially and 0.34 A mg Pt –1 after 10 000 potential cycles at 0.95 V RHE . Furthermore, their MOR mass activity in alkaline media is up to 4.28 A mg Pt –1 and is 4-fold higher than that of commercial Pt/C catalyst. Furthermore, experimental studies indicate that the weakened binding of intermediate carbonaceous poison contributes to the enhanced MOR behavior. More impressively, the Pt–Co nanoframes also demonstrate remarkable stability under long-term testing, which could be attributed to the negligible electrochemical Co dissolution.

25 ENERGY STORAGE↗