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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Production and characterization of extracellular cellulases from a native strain of Trichoderma harzianum using wheat straw from the Mexicali valley as the carbon source. Producción y caracterización de celulasas Extracelulares a partir de cepa nativa de Trichoderma harzianum usando como fuente de Carbono paja de trigo del valle de Mexicali

The purpose of this study was to produce cellulase enzyme from a native strain Trichoderma harzianum, isolated from crop soil. Cellulase production was carried out by two different methods of fermentation, submerging and solid state fermentation. Mandels mineral medium was used as a source of nitrogen and ammonium sulfate. Dry and ground wheat straw was used in fermentation as a carbon source. Cellulase activity was determined using the 3,5-dinitrosalicylic acid (DNS) method. The results showed that wheat straw has the potential to be used as a substrate in the production of cellulases. Trichoderma harzianum showed enzymatic activity of 0.125 FPU / ml (filter paper units) and 1. 35 U / ml CMC (carboxymethyl cellulose units) by solid state fermentation (FSS) after 96 hours of fermentation. The optimum pH for the two substrates was determined by testing with citrate buffers (pH 4 to 6), phosphates (pH 6 to 7) and tris-HCL (pH 8 to 9). To determine the optimum temperature the range of 30 ° C to 80 ° C was investigated. Storage tests were performed every seven days over a period of 6 weeks. The optimum activity pH on paper and CMC was found to be 6. The optimum activity temperature is 70 ° C for PF and 80 ° C for CMC. El propósito de este estudio fue producir enzima celulasa a partir de una cepa nativa Trichoderma harzianum, aislada de suelo de cultivo. La producción de celulasa se realizó por dos métodos diferentes de fermentación, sumergida y fermentación en estado sólido. Se utilizó medio mineral Mandels y como fuentes de nitrógeno urea y sulfato de amonio. Se utilizó paja de trigo seca y molida en las fermentaciones como fuente de carbono. La actividad de la celulasa se determinó por medición de azúcares reductores al papel filtro y a la carboximetilcelulosa usando el método del ácido 3,5- dinitrosalicílico (DNS). Los resultados mostraron que la paja de trigo tiene el potencial para usarse como sustrato en la producción de celulasas. Trichoderma harzianum mostró actividad enzimática de 0.125 FPU/ml (unidades de papel filtro) y 1.35 U/ml a la CMC (unidades de carboximetilcelulosa) por fermentación en estado sólido (FSS) después de 96 horas de fermentación. El pH óptimo para los dos sustratos se determinó realizando pruebas con buffers de citratos (pH 4 a 6), de fosfatos (pH 6 a 7) y tris- HCL (pH 8 a 9). Para la determinación de la temperatura óptima se manejó el rango de 30°C a 80°C. Las pruebas de almacenamiento se realizaron cada siete días en un periodo de 6 semanas. Se encontró que el pH óptimo de actividad en papel y CMC es de 6. La temperatura óptima de actividad es de 70°C para PF y de 80°C para CMC.

09 BIOMASS FUELS↗
NASA NTRS1997

Threatened and Endangered Species Survey for Patrick Air Force Base, Florida

A review of previous environmental work conducted at Patrick Air Force Base (PAFB) indicated that several threatened, endangered, or species of special concern occurred or had the potential to occur there. This study was implemented to collect more information on protected species at PAFB. A map of landcover types was prepared for PAFB using aerial photography, groundtruthing, and a geographic information system (GIS). Herbaceous vegetation was the most common vegetation type. The second most abundant vegetation type was disturbed shrubs/exotics. The beach and associated dune vegetation comprised 3.2% of the land area, but was the most extensive natural community within PAFB. A few isolated mangrove communities exist along the Banana River. Seventy-seven species of vascular plants occurred on the dunes, including four species listed by state agencies: spider lily (Hymenocallis latifolia), prickly pear cactus (Opuntia stricta), beach star (Remirea maritima), and inkberry (Scaevola plumien). Surveys of other habitats revealed eighty-four species of vascular plants including two state-listed species: spider lily and prickly pear cactus. Many of these areas are dominated by invasive, exotic species, particularly Brazilian pepper (Schinus terebinthifolius) and Australian pine (Casuarina equisetifolia), and native species of open or disturbed sites such as camphorweed (Heterotheca subaxillaris) and beardgrass (Andropogon spp.). Due to the isolation of PAFB from other natural areas, most exotic plant populations on the base are not an immediate threat to intact native plant communities. Dune habitat was surveyed for the southeastem beach mouse (Peromyscus polionotus niveiventris) by quarterly trapping along eight 100 m transects. No beach mice were found. The limited extent of dune habitat, its fragmented condition, and the isolation of PAFB from extant populations of the beach mouse probably accounts for its absence. Surveys of birds on PAFB found an avifauna characteristic of species that occur in the Indian River Lagoon system. Twenty-five species of waterbirds were observed during quarterly surveys on PAFB, including five species listed as species of special concern by the state of Florida: Snowy Egret (Egretta thula), Little Blue Heron (Egretta caerulea), Tricolored Heron (Egretta tricolo4, White Ibis (Eudocimus albus), and Brown Pelican (Pelecanus occidentalis). The Golf Course was used extensively by almost all species of waterbirds on PAFB. Twenty-two species of shorebirds were observed on PAFB. Although no listed species were observed, the potential exists for several protected species of shorebirds to use the beach at PAFB during some parts of the year. The Airfield runways and associated grass areas were important sites at PAFB for loafing and feeding for some shorebirds. Surveys of rooftop nesting by Least Terns (Stema antillarum) on PAFB found a large colony on a rooftop in the PAFB Industrial Area. This colony produced some independent young. Two rooftop Least Tern colonies reported from previous years were inactive during 1996. A small number of Black Skimmers (Rhynchops nigee attempted to nest at the Least Ten colony but were unsuccessful. Surveys for the gopher tortoise (Gopherus polyphemus) revealed burrows and tortoises only at the Waste Study Site; five burrows and three tortoises were observed. No Florida scrub lizards (Sceloporus woodi), eastern indigo snakes (Drymarchon corais couperl), or diamondback terrapins (Malademys terrapin terrapin) were observed. American alligators (Alligator mississippiensis) were observed on the Golf Course and using ditches, ponds, and areas along the Banana River. The amount of dune habitat could be expanded by not mowing areas adjacent to the dunes to allow dune species to colonize and expand. Planting dune species as part of the beach renourishment project will also increase this habitat. Exotic plants dominate several areas on the base and are used by threatened, endangered, and species of special concern. However, the use of native vegetation in landscaping projects throughout the base would improve habitat for wildlife, and invasive, exotic plants should not be used in any horticultural plantings. Water quality of ponds, ditches, and canals is important for waterbirds; it should be maintained and protected from contamination. Nesting Least Terns are sensitive to disturbance; rooftops used for nesting should be protected from disturbance. Monitoring of Least Tern and Black Skimmer nesting should be continued to determine what roofs are being used and whether nesting is successful. Furthermore, based on the large numbers of waterbirds observed on PAFB, continued monitoring of them is recommended.

Oddy, Donna M.↗
DOE OSTI2026

Hybrid Oscillator-Qubit Quantum Processors: Instruction Set Architectures, Abstract Machine Models, and Applications

This tutorial offers a pedagogical guide to hybrid quantum processors that integrate discrete-variable (DV) qubits and continuous-variable (CV) oscillators. Aimed at computer scientists, engineers, and physicists, it provides an overview of the experimental, algorithmic, and architectural aspects of this novel and rapidly developing hardware model. Experimental realizations of this model include superconducting, trapped-ion, and neutral-atom platforms. By combining DV and CV components, hybrid oscillator-qubit processors enable a powerful new paradigm that offers complementary strengths for quantum control, error correction, computation, and simulation. Working toward the goal of a full-stack system connecting applications to CV-DV hardware, we define and formulate abstract machine models and instruction set architectures. These essential abstractions enable codesign of hardware and software, and resource estimation for exploring the potential of current and future hardware for computational and simulation tasks. Using these abstractions, we present both new and existing examples that illustrate the benefits of hybrid CV-DV processors relative to traditional DV-only hardware in computation as well as quantum simulation of physical models. Examples include algorithms for transferring states between DV and CV systems, performing the quantum Fourier transform, and simulation of lattice gauge theories. Relative to qubit-only hardware, the bosonic degrees of freedom natively available in hybrid architectures can substantially reduce the circuit complexity of simulations for physical models containing bosons. A key technique is the extension of quantum signal processing ideas to CV-DV systems. This work is intended to serve as a timely and comprehensive guide to this relatively unexplored yet promising approach to quantum computation and to provide a road map to guide future development.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

Inter-cofactor protein remodeling rewires short-circuited transmembrane electron transfer

Intraprotein electron transfer (ET) requires explicit local control of the environment of cofactors to influence their intermolecular distances, relative orientations, and redox properties. Efficient, longer-range ET often utilizes molecular orbitals of aromatic residues present in the intervening space. Here, revitalization of a vestigial ET pathway in the bacterial photosynthetic reaction center is achieved by scanning with tryptophans to uncover markedly improved routes of electron conduction in a key stabilizing step spanning 15 Å between tetrapyrrole and quinone cofactors. This ET event is maximally enhanced by pairing one or more tryptophans with a threonine to influence quinone binding and/or redox potential. Synergistic effects of these substitutions increase the yield of that ET step to ~95%. Joining these substitutions with mutant residues that improve initial ET steps dramatically enhances transmembrane charge separation via this redesigned version of a pathway that is quantitatively inactive in the native protein-cofactor complex.

Biophysical chemistry↗
DOE OSTI2025

jaxhps: An elliptic PDE solver built with machine learning in mind

Elliptic partial differential equations (PDEs) can model many physical phenomena, such as electrostatics, acoustics, wave propagation, and diffusion. In scientific machine learning settings, a high-throughput PDE solver may be required to generate a training dataset, run in the inner loop of an iterative algorithm, or interface directly with a deep neural network. To provide value to machine learning users, such a PDE solver must be compatible with standard automatic differentiation frameworks, scale efficiently when run on graphics processing units (GPUs), and maintain high accuracy for a large range of input parameters. We have designed the jaxhps package with these use-cases in mind by implementing a highly efficient and accurate solver for elliptic problems with native hardware acceleration and automatic differentiation support.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2022

Scar states in a system of interacting chiral fermions

Here, we study the nature of many-body eigenstates of a system of interacting chiral spinless fermions on a ring. We find a coexistence of fermionic and bosonic types of eigenstates in parts of the many-body spectrum. Some bosonic eigenstates, native to the strong interaction limit, persist at intermediate and weak couplings, enabling persistent density oscillations in the system, despite it being far from integrability.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Soft Lattice and Defect Covalency Rationalize Tolerance of β‐CsPbI 3 Perovskite Solar Cells to Native Defects

Abstract Although all‐inorganic metal halide perovskites (MHPs) have shown tremendous improvement, they are still inferior to the hybrid organic–inorganic MHPs in efficiency. Recently, a conceptually new β‐CsPbI 3 perovskite reached 18.4 % efficiency combined with good thermodynamic stability at ambient conditions. We use ab initio non‐adiabatic molecular dynamics to show that native point defects in β‐CsPbI 3 are generally benign for nonradiative charge recombination, regardless of whether they introduce shallow or deep trap states. These results indicate that MHPs do not follow the simple models used to explain defect‐mediated charge recombination in the conventional semiconductors. The strong tolerance is due to the softness of the perovskite lattice, which permits separation of electrons and holes upon defect formation, and only allows carriers to couple to the low‐frequency vibrations. Both factors decrease notably the non‐adiabatic coupling and slow down the dissipation of energy to heat.

Chu, Weibin↗
DOE OSTI2018

Expanding the breeder’s toolbox for perennial grasses

With the increase in world energy demand and the environmental impacts associated with continued fossil fuel use, there is a great need for the development of alternative and sustainable fuel sources. Switchgrass ( Panicum virgatum L. ), a perennial grass native to North America, is poised to become one of the first dedicated biomass crops deployed on a large scale in the United States in large part due to its inherent high yield, adaptability and perennial nature. Current ‘cultivars’ of switchgrass are essentially undomesticated wild selections from natural populations. Thus, there is considerable potential for improvement of this emerging energy crop. Traditional breeding approaches in switchgrass have been hampered by its self-incompatibility and sensitivity to inbreeding, therefore there is a significant need to develop new tools for improved switchgrass breeding. The proposed research uses a systems biology approach to accelerate switchgrass breeding, to develop its biotechnological improvement through the generation of stress-tolerant varieties, and to establish ways to minimize the potential impact of transgenes on ecosystems. In this proposal, we will develop new breeding tools for perennial grasses and apply these tools toward improving switchgrass. Specifically, we will employ two complementary approaches to overcome the difficulty of breeding a large, polyploid, self-incompatible plant. The first approach will accelerate conventional breeding by developing a method to rapidly create doubled haploid (homozygous) lines for breeding. This approach is based on altering CENH3, a centromere-specific histone, leading to the production of haploid offspring when the centromere mutant is crossed to wild type plants. Subsequent genome doubling results in fully homozygous individuals and overcomes one of the major limitations in breeding self-incompatibile crops. These homozygous lines would allow switchgrass breeders to apply a variety of breeding approaches that have been previously impossible. The second approach is to develop tools and reagents to accelerate the biotechnological improvement of switchgrass including: modules for improved stress tolerance, a test bed to evaluate transgenes in a perennial model grass and a robust gene containment system. In order to maintain a favorable energy balance and avoid competition with food crops, dedicated biomass crops must be grown with minimal inputs (fertilizer and water) on marginal lands that are unsuited to the traditional high input agriculture necessary to produce food. Thus, abiotic stress tolerance and efficient water and nutrient use are of paramount importance. Considerable basic research has been directed at these areas and this knowledge has been used to develop transgenic approaches to greatly improve water and nutrient use efficiency. By applying these approaches to switchgrass we will rapidly increase drought tolerance and nutrient use efficiency. This approach can rapidly introduce these traits in different cultivars allowing breeders to add these traits to their elite germplasm. Since biotechnologically improved switchgrass must ultimately be grown in open fields, they must be designed to minimize the potential impact of the transgenes on non-target ecosystems. The best way to do this is to minimize gene flow from the transgenic plants. Thus, we will develop technology to introduce sterility into the transgenic plants. We will then combine this technology with the stress resistance modules to create switchgrass lines with enhanced resistance to abiotic stresses.

09 BIOMASS FUELS↗
DOE OSTI2018

Expanding the Breeder’s Toolbox for Perennial Grasses

With the increase in world energy demand and the environmental impacts associated with continued fossil fuel use, there is a great need for the development of alternative and sustainable fuel sources. Switchgrass (Panicum virgatum L.), a perennial grass native to North America, is poised to become one of the first dedicated biomass crops deployed on a large scale in the United States in large part due to its inherent high yield, adaptability and perennial nature. Current ‘cultivars’ of switchgrass are essentially undomesticated wild selections from natural populations. Thus, there is considerable potential for improvement of this emerging energy crop. Traditional breeding approaches in switchgrass have been hampered by its self-incompatibility and sensitivity to inbreeding, therefore there is a significant need to develop new tools for improved switchgrass breeding. The proposed research uses a systems biology approach to accelerate switchgrass breeding, to develop its biotechnological improvement through the generation of stress-tolerant varieties, and to establish ways to minimize the potential impact of transgenes on ecosystems. In this proposal, we will develop new breeding tools for perennial grasses and apply these tools toward improving switchgrass. Specifically, we will employ two complementary approaches to overcome the difficulty of breeding a large, polyploid, self-incompatible plant. The first approach will accelerate conventional breeding by developing a method to rapidly create doubled haploid (homozygous) lines for breeding. This approach is based on altering CENH3, a centromere-specific histone, leading to the production of haploid offspring when the centromere mutant is crossed to wild type plants. Subsequent genome doubling results in fully homozygous individuals and overcomes one of the major limitations in breeding self-incompatibile crops. These homozygous lines would allow switchgrass breeders to apply a variety of breeding approaches that have been previously impossible. The second approach is to develop tools and reagents to accelerate the biotechnological improvement of switchgrass including: modules for improved stress tolerance, a test bed to evaluate transgenes in a perennial model grass and a robust gene containment system. In order to maintain a favorable energy balance and avoid competition with food crops, dedicated biomass crops must be grown with minimal inputs (fertilizer and water) on marginal lands that are unsuited to the traditional high input agriculture necessary to produce food. Thus, abiotic stress tolerance and efficient water and nutrient use are of paramount importance. Considerable basic research has been directed at these areas and this knowledge has been used to develop transgenic approaches to greatly improve water and nutrient use efficiency. By applying these approaches to switchgrass we will rapidly increase drought tolerance and nutrient use efficiency. This approach can rapidly introduce these traits in different cultivars allowing breeders to add these traits to their elite germplasm. Since biotechnologically improved switchgrass must ultimately be grown in open fields, they must be designed to minimize the potential impact of the transgenes on non-target ecosystems. The best way to do this is to minimize gene flow from the transgenic plants. Thus, we will develop technology to introduce sterility into the transgenic plants. We will then combine this technology with the stress resistance modules to create switchgrass lines with enhanced resistance to abiotic stresses.

09 BIOMASS FUELS↗
NASA NTRS1973

Coordination and establishment of centralized facilities and services for the University of Alaska ERTS survey of the Alaskan environment

The author has identified the following significant results. Scene 1072-21173 of the Anaktuvuk Pass region of the Brooks Range, Alaska, was studied from the point of view of a resource survey for purposes of land use planning as part of the effort to develop ERTS data processing and interpretation techniques. Other data sources and surface observations were utilized to produce a resource survey of a remote and undeveloped region of Alaska. Three vegetative types are apparent: moist tundra, low brush, and high brush. Watersheds are easily defined on the multispectral imagery. Features related indirectly to economic minerals are discernible from ERTS-1 imagery supported by ground truth data. These include mountains, outwash plains and alluvial deposits, drainage patterns, lineaments and probable bedding planes. This region falls within present land class categories which are not inconsistent with the imperatives of the resources. These land class categories include native village withdrawals, regional deficiency area, national interest study area for possible inclusion in a national system, public interest areas, utility corridor, and state land selection.

Belon, A. E.↗
NASA NTRS2010

A Comparison of Oxidized Carbon Abundances among Comets

Comets contain relatively well preserved icy material remaining from the epoch of Solar System formation, however the extent to which these ices are modified from their initial state remains a fundamental question in cometary science. As a comet approaches the Sun, sublimation of the ices contained in its nucleus (termed " native ices") releases parent volatiles into the coma, where they can be measured spectroscopically. One means of assessing the degree to which interstellar ices were processed prior to their incorporation into cometary nuclei is to measure the relative abundances of chemically-related parent volatiles. For example, formation of C2H6 by hydrogen atom addition (e.g., to C2H2) on surfaces of ice-mantled grains was proposed to explain the high C2H6 to CH4 abundance observed in C/1996 B2 (Hyakutake) [1]. The large C2H6/CH4 abundance ratios measured universally in comets, compared with those predicted by gas phase production of C2H6, establishes H-atom addition as an important and likely ubiquitous process. CO should also be hydrogenated on grain surfaces. Laboratory irradiation experiments on interstellar ice analogs indicate this to require very low temperatures (T approx. 10-25 K), the resulting yields of H2CO and CH3OH being highly dependent both on hydrogen density (i.e., fluence) and on temperature ([2],[3]). This relatively narrow range in temperature reflects a lack of mobility below 8-10 K on the one hand, and reduced sticking times for H-atoms as grain surfaces are warmed above 20 K on the other. The relative abundances of these three chemically-related molecules in comets provides one measure of the efficiency of H-atom addition to CO on pre-cometary grains (Fig. 1).

DiSanti, M. A.↗
DOE OSTI2026

Short‐Term Hourly Weather Forecasting Using PredRNN With Image Preprocessing

Global weather forecast models are vital tools with numerous applications, including public safety, agriculture, and transportation. Recent advancements in artificial intelligence (AI) and deep learning (DL) have shown the potential to enhance weather forecasting accuracy and speed. In this study, we developed a short-term hourly weather forecast framework with a wavelet transform function for data preprocessing and a spatiotemporal DL model, PredRNN, for predicting five surface atmospheric variables, including wind speed and direction, mean sea level pressure (MSLP), temperature, and precipitation. The framework demonstrated promising results. It produces global forecasts at 0.25° (∼25 km) with a 1-day lead time RMSE of 1.8 m/s for wind components, 180 Pa for MSLP, and 1.8 K for temperature. Although our model does not surpass state-of-the-art AI weather forecast models across all metrics, it outperforms these models in precipitation forecasting and wind prediction at short lead times and achieves comparable accuracy for MSLP. Its native hourly forecasting capability, together with training on widely accessible GPU hardware, contributes meaningfully to the advancement of accessible DL weather forecasting methods. Our work highlights the importance of integrating temporal components and data transformation techniques to improve the predictability and accuracy of weather forecasts.

Tran, Hoang [Pacific Northwest National Laboratory↗
DOE OSTI2024

On the emerging potential of quantum annealing hardware for combinatorial optimization

Abstract Over the past decade, the usefulness of quantum annealing hardware for combinatorial optimization has been the subject of much debate. Thus far, experimental benchmarking studies have indicated that quantum annealing hardware does not provide an irrefutable performance gain over state-of-the-art optimization methods. However, as this hardware continues to evolve, each new iteration brings improved performance and warrants further benchmarking. To that end, this work conducts an optimization performance assessment of D-Wave Systems’ Advantage Performance Update computer, which can natively solve sparse unconstrained quadratic optimization problems with over 5,000 binary decision variables and 40,000 quadratic terms. We demonstrate that classes of contrived problems exist where this quantum annealer can provide run time benefits over a collection of established classical solution methods that represent the current state-of-the-art for benchmarking quantum annealing hardware. Although this work does not present strong evidence of an irrefutable performance benefit for this emerging optimization technology, it does exhibit encouraging progress, signaling the potential impacts on practical optimization tasks in the future.

96 KNOWLEDGE MANAGEMENT AND PRESERVATION↗
NASA NTRS2009

Ikhana: Unmanned Aircraft System Western States Fire Missions. Monographs in Aerospace History, Number 44

In 2006, NASA Dryden Flight Research Center, Edwards, Calif., obtained a civil version of the General Atomics MQ-9 unmanned aircraft system and modified it for research purposes. Proposed missions included support of Earth science research, development of advanced aeronautical technology, and improving the utility of unmanned aerial systems in general. The project team named the aircraft Ikhana a Native American Choctaw word meaning intelligent, conscious, or aware in order to best represent NASA research goals. Building on experience with these and other unmanned aircraft, NASA scientists developed plans to use the Ikhana for a series of missions to map wildfires in the western United States and supply the resulting data to firefighters in near-real time. A team at NASA Ames Research Center, Mountain View, Calif., developed a multispectral scanner that was key to the success of what became known as the Western States Fire Missions. Carried out by team members from NASA, the U.S. Department of Agriculture Forest Service, National Interagency Fire Center, National Oceanic and Atmospheric Administration, Federal Aviation Administration, and General Atomics Aeronautical Systems Inc., these flights represented an historic achievement in the field of unmanned aircraft technology.

Western States fire missions↗
DOE OSTI2022

Visualization of defect induced in-gap states in monolayer MoS 2

Atomic-scale intrinsic defects play a key role in controlling functional electronic properties of two-dimensional (2D) materials. Here, we present a low-temperature scanning–tunneling microscopy and spectroscopy investigation of a common point-defect in monolayer molybdenum disulfide (MoS 2 ). We employ a sample preparation method in which the film surface is never exposed to air so that the native dangling bonds surrounding the defects in the film are preserved. Molybdenum vacancies are identified by their three characteristic in-gap resonances by combining scanning–tunneling measurements with parallel Green’s function-based theoretical modeling. The relative energy shifts between the various in-gap states allow us to identify a relative charge difference between two of the observed vacancies. The role of the substrate on the band structure of the defective MoS 2 monolayer is unveiled. Our study highlights the effects of the substrate on the in-gap states of common defects found in MoS 2 providing a pathway in designing and optimizing 2D materials for electronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Functional Connectivity of Red Chlorophylls in Cyanobacterial Photosystem I Revealed by Fluence-Dependent Transient Absorption

External stressors modulate the oligomerization state of photosystem I (PSI) in cyanobacteria. The number of red chlorophylls (Chls), pigments lower in energy than the P700 reaction center, depends on the oligomerization state of PSI. Here, we use ultrafast transient absorption spectroscopy to interrogate the effective connectivity of the red Chls in excitonic energy pathways in trimeric PSI in native thylakoid membranes of the model cyanobacterium Synechocystis sp. PCC 6803, including emergent dynamics, as red Chls increase in number and proximity. Fluence-dependent dynamics indicate singlet–singlet annihilation within energetically connected red Chl sites in the PSI antenna but not within bulk Chl sites on the picosecond time scale. These data support picosecond energy transfer between energetically connected red Chl sites as the physical basis of singlet–singlet annihilation. The time scale of this energy transfer is faster than predicted by Förster resonance energy transfer calculations, raising questions about the physical mechanism of the process. Our results indicate distinct strategies to steer excitations through the PSI antenna; the red Chls present a shallow reservoir that direct excitations away from P 700 , extending the time to trapping by the reaction center.

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

Defect Chemistry and Hydrogen Transport in La/Sr-Based Oxyhydrides

Oxyhydrides in the series La 2 – y Sr y LiH 1 + y O 3 – y are attractive materials for solid-state hydride electrolytes. However, their chemical stability is poor, and thus far only La 2 LiHO 3 (y = 0) has been demonstrated in a working fuel cell. Using a first-principles approach to study defect chemistry and stability, we find that La 2 LiHO 3 can be stabilized over a range of H-rich, O-poor chemical conditions, while Sr 2 LiH 3 O has low stability. Here, defects are critical for ionic transport and for stability, particularly in Sr 2 LiH 3 O, in which anion antisites and H vacancies readily form. In fact, we show that the ground-state structure of Sr 2 LiH 3 O is inherently disordered. La 2 LiHO 3 has low conductivity, which we connect to the high formation energy of defects such as VO and OH that are necessary for hydride conduction. When Sr 2 LiH 3 O is grown under carefully selected conditions, V H + defects can be made prevalent; these defects give rise to much higher ionic conductivity than can be obtained in La 2 LiHO 3 . In general, we show that the choice of synthesis conditions is vital when seeking to optimize the stability and ionic conductivity of these oxyhydrides, thereby tailoring them for use in solid-state hydrogen fuel cells.

08 HYDROGEN↗