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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Scattering evidence of positional charge correlations in polyelectrolyte complexes

Polyelectrolyte complexation plays an important role in materials science and biology. The internal structure of the resultant polyelectrolyte complex (PEC) phase dictates properties such as physical state, response to external stimuli, and dynamics. Small-angle scattering experiments with X-rays and neutrons have revealed structural similarities between PECs and semidilute solutions of neutral polymers, where the total scattering function exhibits an Ornstein–Zernike form. In spite of consensus among different theoretical predictions, the existence of positional correlations between polyanion and polycation charges has not been confirmed experimentally. Here, we present small-angle neutron scattering profiles where the polycation scattering length density is matched to that of the solvent to extract positional correlations among anionic monomers. The polyanion scattering functions exhibit a peak at the inverse polymer screening radius of Coulomb interactions, q* ≈ 0.2 Å –1 . This peak, attributed to Coulomb repulsions between the fragments of polyanions and their attractions to polycations, is even more pronounced in the calculated charge scattering function that quantifies positional correlations of all polymer charges within the PEC. Screening of electrostatic interactions by adding salt leads to the gradual disappearance of this correlation peak, and the scattering functions regain an Ornstein–Zernike form. Experimental scattering results are consistent with those calculated from the random phase approximation, a scaling analysis, and molecular simulations.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Photoelectrochemically Self Improving Si/GaN Photocathode: Figure 1d Raw Data

Fig1d results were obtained using a 3-electrode configuration with IrOx as the counter electrode and saturated calomel electrode as the reference electrode. Two Si/GaN photocathodes were compared. Sample #1 (named as S#1) was the as-received sample, and sample #2 (named as S#2) was CA tested for 4 hours under same testing conditions described in Fig.1a. The PEC performance (J-V curve) was recorded for both samples. Then a photodeposition of Pt was performed on both samples, each loading time was set to be 5 minutes, then the J-V curve was recorded to track the changes of the performance. The Pt photodeposition was stopped until the J-V curve won't shift (the maximum onset potential was achieved). The results show that, S#1 needs at least 20 minutes of Pt photodeposition to achieve its best PEC performance while S#2, which is after CA testing, only needs 5 minutes of Pt photodeposition to achieve its best PEC performance.

photocathode↗
DOE OSTI2011

Solar Energy Research Center Instrumentation Facility

SOLAR ENERGY RESEARCH CENTER INSTRUMENTATION FACILITY The mission of the Solar Energy Research Center (UNC SERC) at the University of North Carolina at Chapel Hill (UNC-CH) is to establish a world leading effort in solar fuels research and to develop the materials and methods needed to fabricate the next generation of solar energy devices. We are addressing the fundamental issues that will drive new strategies for solar energy conversion and the engineering challenges that must be met in order to convert discoveries made in the laboratory into commercially available devices. The development of a photoelectrosynthesis cell (PEC) for solar fuels production faces daunting requirements: (1) Absorb a large fraction of sunlight; (2) Carry out artificial photosynthesis which involves multiple complex reaction steps; (3) Avoid competitive and deleterious side and reverse reactions; (4) Perform 13 million catalytic cycles per year with minimal degradation; (5) Use non-toxic materials; (6) Cost-effectiveness. PEC efficiency is directly determined by the kinetics of each reaction step. The UNC SERC is addressing this challenge by taking a broad interdisciplinary approach in a highly collaborative setting, drawing on expertise across a broad range of disciplines in chemistry, physics and materials science. By taking a systematic approach toward a fundamental understanding of the mechanism of each step, we will be able to gain unique insight and optimize PEC design. Access to cutting-edge spectroscopic tools is critical to this research effort. We have built professionally-staffed facilities equipped with the state-of the-art instrumentation funded by this award. The combination of staff, facilities, and instrumentation specifically tailored for solar fuels research establishes the UNC Solar Energy Research Center Instrumentation Facility as a unique, world-class capability. This congressionally directed project funded the development of two user facilities: TASK 1: SOLAR DEVICE FABRICATION LABORATORY DEVELOPMENT The space allocated for this laboratory was shell space that required an upfit in order to accommodate nano-fabrication equipment in a quasi-clean room environment. This construction project (cost $279,736) met the non-federal cost share requirement of $250,000 for this award. The central element of the fabrication laboratory is a new $400,000+ stand-alone system, funded by other sources, for fabricating and characterizing photovoltaic devices, in a state-of-the-art nanofabrication environment. This congressionally directed project also included the purchase of an energy dispersive x-ray analysis (EDX) detector for a pre-existing transmission electron microscope (TEM). This detector allows elemental analysis and elemental mapping of materials used to fabricate solar energy devices which is a key priority for our research center. TASK 2: SOLAR ENERGY SPECTROSCOPY LABORATORY DEVELOPMENT (INSTRUMENTATION) This laboratory provides access to modern spectroscopy and photolysis instrumentation for characterizing devices, materials and components on time scales ranging from femtoseconds to seconds and for elucidating mechanisms. The goals of this congressionally directed project included the purchase and installation of spectroscopy and photolysis instrumentation that would substantially and meaningfully enhance the capabilities of this laboratory. Some changes were made to the list of equipment proposed in the original budget. These changes did not represent a change in scope, approach or aims of this project. All of the capabilities and experiments represented in the original budget were maintained. The outcome of this Congressionally Directed Project has been the development of world-class fabrication and spectroscopy user facilities for solar fuels research at UNC-CH. This award has provided a significant augmentation of our pre-existing instrumentation capabilities which were funded by earlier UNC SERC projects, including the Energy Frontier Research Center UNC EFRC, funded by the US Department of Energy Office of Basic Energy Sciences. Equipment funded by this congressional award has provided important new capabilities for UNC SERC and has greatly facilitated collaborative research by many multi-institutional teams in the six partner institutions of the UNC EFRC, including Duke University, North Carolina Central University, and North Carolina State University. This state-of-the-art instrumentation has allowed us to design cutting-edge experiments that provide insight into the molecular structure and dynamics of materials and components for solar energy conversion under real working conditions. This research has resulted in ten publications already published or in preparation that acknowledge support from DOE EERE for this congressionally directed project.

14 SOLAR ENERGY↗
DOE OSTI2012

Final LDRD report :

The distinction between electricity and fuel use in analyses of global power consumption statistics highlights the critical importance of establishing efficient synthesis techniques for solar fuels those chemicals whose bond energies are obtained through conversion processes driven by solar energy. Photoelectrochemical (PEC) processes show potential for the production of solar fuels because of their demonstrated versatility in facilitating optoelectronic and chemical conversion processes. Tandem PEC-photovoltaic modular configurations for the generation of hydrogen from water and sunlight (solar water splitting) provide an opportunity to develop a low-cost and efficient energy conversion scheme. The critical component in devices of this type is the PEC photoelectrode, which must be optically absorptive, chemically stable, and possess the required electronic band alignment with the electrochemical scale for its charge carriers to have sufficient potential to drive the hydrogen and oxygen evolution reactions. After many decades of investigation, the primary technological obstacle remains the development of photoelectrode structures capable of efficient conversion of light with visible frequencies, which is abundant in the solar spectrum. Metal oxides represent one of the few material classes that can be made photoactive and remain stable to perform the required functions.

Kronawitter, Coleman X.↗
DOE OSTI2013

Direct Solar Hydrogen Production Using High Temperature Solid Oxide Photoelectrochemical Cells

Photolytic water splitting using semiconductor materials has received increasing scientific interest in recent years. The key component in this technology is the photoelectrochemical (PEC) cell, which can absorb solar light to directly generate hydrogen from liquid water at low temperature (below 100°C). The objective of this project was to investigate a photocatalytic method for hydrogen production from steam at high temperature (~ 800°C) using solid oxide PEC cells (SOPCs). In a SOPC, the cathode material of a traditional solid oxide fuel cell (SOFC) is replaced by or mixed with a photo-sensitive semiconductor material such as TiO 2 . The ceramic PEC cell would be operated at high temperature with the TiO 2 surface exposed to concentrated solar energy. At room temperature, the magnitude of the bandgap energy of TiO 2 is relatively large such that photoelectrochemical excitation of TiO 2 requires ultraviolet light.

99 GENERAL AND MISCELLANEOUS↗
DOE OSTI2022

Long-Term Stability Metrics of Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting, one of the most promising technologies for clean hydrogen generation, has drawn considerable attention over the past few decades. Achieving simultaneous highly efficient and stable unassisted PEC water splitting has been the “holy grail” in clean and renewable fuel generation. State-of-the-art photoelectrodes have shown relatively high efficiencies (∼10–20%). Still, their stability is limited due to photoelectrode chemical instability, electrolyte resistance, mass transfer issues, and an often unoptimized experimental setup. In this work, we present a framework and a set of protocols for conducting long-term stability experiments and further provide details on several critical factors such as light source calibration, choosing the right counter electrode, the configuration of the PEC cell, and photoelectrode sample preparation.

08 HYDROGEN↗
DOE OSTI2025

CV4Quantum: Reducing the Sampling Overhead in Probabilistic Error Cancellation Using Control Variates

Quasiprobabilistic decompositions (QPDs) play a key role in maximizing the utility of near-term quantum hardware. For example, Probabilistic Error Cancellation (PEC) (an error mitigation technique) and circuit cutting (which enables large quantum computations to be performed on quantum hardware with a limited number of qubits) both involve QPDs. Computations based on QPDs typically incur large sampling overheads that grow exponentially with the number of error-terms mitigated or number of circuit-cuts employed, limiting their practical feasibility. In this work, we adapt the control variates variance reduction technique from the statistics literature in order to reduce the sampling overhead in QPD-based computations. We demonstrate our method using simulation experiments that mimic a realistic PEC scenario. In our experiments, we observed a more than 50% reduction in the number of samples needed to achieve a given precision, in more than 50% of the PEC-based estimations performed in the study when using our approach. We discuss how future research on constructing good control variates can lead to even stronger sampling overhead reduction.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1993

Design and specification of a centralized manufacturing data management and scheduling system

As was revealed in a previous study, the Materials and Processes Laboratory's Productivity Enhancement Complex (PEC) has a number of automated production areas/cells that are not effectively integrated, limiting the ability of users to readily share data. The recent decision to utilize the PEC for the fabrication of flight hardware has focused new attention on the problem and brought to light the need for an integrated data management and scheduling system. This report addresses this need by developing preliminary designs specifications for a centralized manufacturing data management and scheduling system for managing flight hardware fabrication in the PEC. This prototype system will be developed under the auspices of the Integrated Engineering Environment (IEE) Oversight team and the IEE Committee. At their recommendation the system specifications were based on the fabrication requirements of the AXAF-S Optical Bench.

Farrington, Phillip A.↗
NASA NTRS1996

Application of AWE for RCS Frequency Response Calculations Using Method of Moments

An implementation of the Asymptotic Waveform Evaluation (AWE) technique is presented for obtaining the frequency response of the Radar Cross Section (RCS) of arbitrarily shaped, three-dimensional perfect electric conductor (PEC) bodies. An Electric Field Integral Equation (EFIE) is solved using the Method of Moments (MoM) to compute the RCS. The electric current, thus obtained, is expanded in a Taylor series around the frequency of interest. The coefficients of the Taylor series (called 'moments') are obtained using the frequency derivatives of the EFIE. Using the moments, the electric current on the PEC body is obtained over a frequency band. Using the electric current at different frequencies, RCS of the PEC body is obtained over a wide frequency band. Numerical results for a square plate, a cube, and a sphere are presented over a bandwidth. A good agreement between AWE and the exact solution over the bandwidth is observed.

Reddy, C. J.↗
NASA NTRS1998

Application of Model Based Parameter Estimation for RCS Frequency Response Calculations Using Method of Moments

An implementation of the Model Based Parameter Estimation (MBPE) technique is presented for obtaining the frequency response of the Radar Cross Section (RCS) of arbitrarily shaped, three-dimensional perfect electric conductor (PEC) bodies. An Electric Field Integral Equation (EFTE) is solved using the Method of Moments (MoM) to compute the RCS. The electric current is expanded in a rational function and the coefficients of the rational function are obtained using the frequency derivatives of the EFIE. Using the rational function, the electric current on the PEC body is obtained over a frequency band. Using the electric current at different frequencies, RCS of the PEC body is obtained over a wide frequency band. Numerical results for a square plate, a cube, and a sphere are presented over a bandwidth. Good agreement between MBPE and the exact solution over the bandwidth is observed.

Reddy, C. J.↗
NASA NTRS2008

Lessons Learned From Atomic Oxygen Interaction With Spacecraft Materials in Low Earth Orbit

There have been five Materials International Space Station Experiment (MISSE) passive experiment carriers (PECs) (MISSE 1-5) to date that have been launched, exposed in space on the exterior of International Space Station (ISS) and then returned to Earth for analysis. An additional four MISSE PECs (MISSE 6A, 6B, 7A, and 7B) are in various stages of completion. The PECs are two-sided suitcase to size sample carriers that are intended to provide information on the effects of the low Earth orbital environment on a wide variety of materials and components. As a result of post retrieval analyses of the retrieved MISSE 2 experiments and numerous prior space experiments, there have been valuable lessons learned and needs identified that are worthy of being documented so that planning, design, and analysis of future space environment experiments can benefit from the experience in order to maximize the knowledge gained. Some of the lessons learned involve the techniques, concepts, and issues associated with measuring atomic oxygen erosion yields. These are presented along with several issues to be considered when designing experiments, such as the uncertainty in mission duration, scattering and contamination effects on results, and the accuracy of measuring atomic oxygen erosion.

Banks, Bruce A.↗
NASA NTRS2002

Higher Order, Hybrid BEM/FEM Methods Applied to Antenna Modeling

In this presentation, the authors address topics relevant to higher order modeling using hybrid BEM/FEM formulations. The first of these is the limitation on convergence rates imposed by geometric modeling errors in the analysis of scattering by a dielectric sphere. The second topic is the application of an Incomplete LU Threshold (ILUT) preconditioner to solve the linear system resulting from the BEM/FEM formulation. The final tOpic is the application of the higher order BEM/FEM formulation to antenna modeling problems. The authors have previously presented work on the benefits of higher order modeling. To achieve these benefits, special attention is required in the integration of singular and near-singular terms arising in the surface integral equation. Several methods for handling these terms have been presented. It is also well known that achieving ~he high rates of convergence afforded by higher order bases may als'o require the employment of higher order geometry models. A number of publications have described the use of quadratic elements to model curved surfaces. The authors have shown in an EFIE formulation, applied to scattering by a PEC .sphere, that quadratic order elements may be insufficient to prevent the domination of modeling errors. In fact, on a PEC sphere with radius r = 0.58 Lambda(sub 0), a quartic order geometry representation was required to obtain a convergence benefi.t from quadratic bases when compared to the convergence rate achieved with linear bases. Initial trials indicate that, for a dielectric sphere of the same radius, - requirements on the geometry model are not as severe as for the PEC sphere. The authors will present convergence results for higher order bases as a function of the geometry model order in the hybrid BEM/FEM formulation applied to dielectric spheres. It is well known that the system matrix resulting from the hybrid BEM/FEM formulation is ill -conditioned. For many real applications, a good preconditioner is required to obtain usable convergence from an iterative solver. The authors have examined the use of an Incomplete LU Threshold (ILUT) preconditioner . to solver linear systems stemming from higher order BEM/FEM formulations in 2D scattering problems. Although the resulting preconditioner provided aD excellent approximation to the system inverse, its size in terms of non-zero entries represented only a modest improvement when compared with the fill-in associated with a sparse direct solver. Furthermore, the fill-in of the preconditioner could not be substantially reduced without the occurrence of instabilities. In addition to the results for these 2D problems, the authors will present iterative solution data from the application of the ILUT preconditioner to 3D problems.

Fink, P. W.↗
NASA NTRS2011

Characteristics of Planar Monopole Antenna on High Impedance Electromagnetic Surface

This paper presents for the first time measured characteristics of a planar monopole antenna placed directly on a high impedance electromagnetic surface or artificial magnetic conductor (AMC). The return loss and radiation patterns are compared between the antenna in free space, and when placed directly on a perfect electrical conductor (PEC), and on the AMC. The antenna measured in free space has a wide pass band from 3 to 10 GHz. The return loss for the antenna on the PEC is nearly all reflected back and the return loss for the antenna on the AMC has a 10 dB bandwidth from 7.5 to 9.5 GHz. The gain of the antenna in free space, on PEC and on AMC is 1, -12 and 10 dBi, respectively. This indicates that the AMC is working properly, sending all the radiation outward with little loss.

Scardelletti, Maximilian C.↗
DOE OSTI2012

Titanium and Magnesium Co-Alloyed Hematite Thin Films for Photoelectrochemical Water Splitting

Using a combination of density functional theory calculation and materials synthesis and characterization we examine the properties of charge-compensated Ti and Mg co-alloyed hematite thin films for the application of photoelectrochemical (PEC) water splitting. We find that the charge-compensated co-alloying results in the following effects: (1) It enhances the solubility of Mg and Ti, which leads to reduced electron effective mass and therefore increased electron mobility; (2) It tunes the carrier density and therefore allows the optimization of electrical conductivity; and (3) It reduces the density of charged defects and therefore reduces carrier recombination. As a result, the Ti and Mg co-alloyed hematite thin films exhibit improved water oxidation photocurrent magnitudes as compared to pure hematite thin films. Our results suggest that charge-compensated co-alloying is a plausible approach for engineering hematite for the application of PEC water splitting.

08 HYDROGEN↗
DOE OSTI2018

Perovskite Solar Cells for Photoelectrochemical Water Splitting and CO 2 Reduction.

Photoelectrochemical (PEC) water splitting is inspired from the natural photosynthesis process in plants, where solar energy is captured and converted into hydrocarbons, glucose, and oxygen. Hydrogen generation from PEC cells through water splitting is a well-established concept. However, primary challenges include improvement of the water splitting efficiency and stability. Perovskite solar cells are revolutionary due to their facile fabrication, their high open circuit voltage which is advantageous to drive solar assisted water splitting reaction with a single photoabsorber. In photovoltaic-electrocatalyst (PV-EC) is the simplest configuration and has the highest potential for commercialization. In this approach, the water splitting anode and cathode is connected with a multijunction solar cell or few solar cells in series to split water without an external bias. Perovskite solar cells would help to reduce the complexity and fabrication cost of carbondioxide reduction tandem cell due to its high open circuit voltage.

Gurudayal, None↗
DOE OSTI2015

Semiconductor-based photoelectrochemical water splitting at the limit of very wide depletion region

In semiconductor-based photoelectrochemical (PEC) water splitting, carrier separation and delivery largely relies on the depletion region formed at the semiconductor/water interface. As a Schottky junction device, the trade-off between photon collection and minority carrier delivery remains a persistent obstacle for maximizing the performance of a water splitting photoelectrode. Here, it is demonstrated that the PEC water splitting efficiency for an n-SrTiO 3 (n-STO) photoanode is improved very significantly despite its weak indirect band gap optical absorption (α < 10⁴ cm⁻¹), by widening the depletion region through engineering its doping density and profile. Graded doped n-SrTiO 3 photoanodes are fabricated with their bulk heavily doped with oxygen vacancies but their surface lightly doped over a tunable depth of a few hundred nanometers, through a simple low temperature re-oxidation technique. The graded doping profile widens the depletion region to over 500 nm, thus leading to very efficient charge carrier separation and high quantum efficiency (>70%) for the weak indirect transition. As a result, this simultaneous optimization of the light absorption, minority carrier (hole) delivery, and majority carrier (electron) transport by means of a graded doping architecture may be useful for other indirect band gap photocatalysts that suffer from a similar problem of weak optical absorption.

77 NANOSCIENCE AND NANOTECHNOLOGY↗