Overview



Overview

Replacing petroleum-powered vehicles with hydrogen-powered vehicles requires a number of technological breakthroughs, but perhaps the greatest challenge is storing enough hydrogen on board to achieve an adequate driving range. The FreedomCAR and Fuel Partnership—a collaboration between the Department of Energy (DOE), the U.S. Council for Automotive Research, and energy companies has developed on-board hydrogen storage targets with the goal of "achieving similar performance and cost levels as current gasoline fuel storage systems." In order to provide a 300-mile driving range between refueling without unacceptably bulky storage vessels, the DOE has established targets for year 2010 of storing 0.045 kg hydrogen per liter of storage system volume and 0.06 kg hydrogen per kg of storage system mass. Longer-term targets are even more demanding: 0.081 kg hydrogen per liter, and 0.09 kg hydrogen per kg of storage mass by the year 2015.

One of the approaches to accelerate discovery of new materials is to applying high-throughput methodologies of combinatorial materials science. Enabling high throughput measurement tools for determining hydrogen absorption characteristics will accelerate discovery of new materials for hydrogen storage. Recently DOE’s Office of Hydrogen, Fuel Cells and Infrastructure Technologies expressed interest in the high-throughput methodology by organizing High-Throughput/Combinatorial Techniques in Hydrogen Storage Materials R&D Workshop. In the 2008 DOE solicitation the “approaches for high throughput/combinatorial synthesis, screening and testing” was specifically mentioned.

In our program we address the needs for new metrology tools, which will allow efficient high-throughput search for novel hydrogen storage materials. Ideally, we want to have the following capabilities:

• Synthesis of combinatorial libraries where an array (continuous or discreet) of small size samples varies either in composition, structure or other parameters for any material of current interest for hydrogen storage.

• Parallel or high-throughput measurements of the arrays for their response to hydrogen from which the amount of hydrogen in the material as a function of pressure, temperature and time could be extracted.

Concerning the synthesis, our combinatorial arrays are deposited as continuous films, mostly with compositional variations, but also with other varying parameters such as microstructure, thickness or composition of catalytically surface. Concerning the measurements, our approach is as follow:

• We develop different indirect measurements that ensure good spatial resolution, such as IR emissivity or micro-Raman. The indirect measurements rely on the following dependences: change in hydrogen content results in changes of physical properties; changes in physical properties affecting the measured signal; thus changes in the signal correlate with the hydrogen content. However, without appropriate calibrations the indirect measurements can be used only quantitatively, e.g., as a screening tool.

• We develop calibration methods of the indirect measurements. For the calibration we want to measure the content of hydrogen while acquiring indirect signal. Measuring the hydrogen content includes (1) high-sensitivity volumetric Sievert-type measurement and (2) Prompt Gamma Activation Analysis (PGAA) neutron beam activated spectroscopic measurement.

• Finally, we hope to do quantitative measurements of in-situ hydrogenation on combinatorial libraries, either in the form of a continuous film or a discrete array of small size samples. Eventually, we will conduct the combinatorial measurements by using (1) calibrated indirect methods or (2) direct PGAA measurements, which will be performed at NIST Center for Nuclear Research.

Research Projects

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Introduction

Synthesis

IR emissivity

Raman and FTIR

PGAA

PCT measurements

Mg-based alloys

TEM

Introduction

Main focus of our program is on high-throughput measurements for hydrogen storage materials. Our main goal is to develop measurements that will not only provide screening capability for a variety of different materials according to their responses to reaction with hydrogen gas, but also give the much-needed information on the amount of hydrogen in the material and on the structural characteristics of the reaction.

High-throughput measurements require combinatorial libraries; synthesis of such libraries is an important part of our research program. Our focus is primarily on thin films with continuously varying compositions – the compositional spreads. The hydrogenation of metal gradient thin films depends strongly on their (micro)structure and local composition; our interest is in creating different microstructural states ranging from amorphous to nano-crystalline to equilibrium phases. Extensive characterization of the lateral gradient as well as of the composition as a function of depth after deposition is of primary importance.

Another important objective of our program is careful structural characterization of hydrogen storage material by using transmission electron microscopy (TEM). In many cases, e.g., nano-scale structure in thin films, TEM is the only tool able to provide correct structural information on phases, morphologies and defects. Air and moisture sensitivity of many of hydrogen storage materials makes TEM studies very difficult; our new Gatan specimen holder with retractable tip will allow such studies by transferring TEM samples from a glove box to a microscope column without exposing to ambient environment.

Synthesis

IR emissivity (Leo)

Emissivity is a material's ability to emit thermal radiation; the emissivity varies with temperature, but also with chemistry and physical properties of a material. In particular, infrared emissivity depends on electrical resistivity (higher resistivity results in higher emissivity). The sorption of hydrogen in metals and alloys causes in new scattering centers and eventually leads to a modified density of states, which leads to a less metallic character and therefore an increase in the electrical resistance or change of conductivity from metallic to semiconducting or insulating. Thus emissivity is expected to increase with the increase of hydrogen in a material.

Based on these principles we’ve constructed an experimental setup to measure in-situ changes in infrared (IR) emissivity of hydrogenated/desorbed samples. Our work is focused on studying thin films, either of constant composition or of varying compositions, thicknesses etc. The samples are set in a hydrogenation chamber on a heating stage, and IR emission images are collected through a sapphire window by an IR camera, which employs a 256x256 array of InSb diodes. The camera has peak sensitivity at a wavelength of 5 micrometers, but is able to detect over an integrated range of 1.0 to 5.5 μm. In our recent work we demonstrated sensitivity of the method for compositional and structural variations and suggested that it can be used as a screening tool for combinatorial R&D of hydrogen storage materials. The method is a complementary technique to the optical method; however the IR measurements do not require optical transparency of the hydrogenated material and can thus potentially be used for a larger class of materials.

Raman and FTIR

The focus of our research is to develop a bench-top characterization technique to rapidly screen thin-film samples for interesting hydrogen storage materials. The bench-top technique would ideally be something that is readily available to most labs; in our case we are focused on the optical properties of the materials as measured by IR emissivity, Fourier Transform Infra-Red spectroscopy, and Raman spectroscopy. These measurements are considered to be in-direct measurements of the hydrogenation status of samples as they measure a secondary property (ie IR emission or its phonon modes).

For introductions to Raman and F.T.I.R. spectroscopy see:









PGAA

In order to render these measurements quantitative we are attempting to calibrate the optical response of the material with a direct measurement of its hydrogen content, prompt gamma activation analysis (PGAA). PGAA is a spectroscopic measurement of the mount of hydrogen contained within a sample, the principle of which is to bombard the samples with neutrons and monitor the gammas emitted as a function of energy. By performing the direct and in-direct techniques on identical samples with identical conditions it is hoped that we can establish the limits of detection for the in-direct method, and perhaps eventually quantify the amount of hydride present.

For more information on PGAA please follow these links:

The CSTL division website for PGAA



The NCNR website for PGAA



Pressure-Composition-Temperature (PCT) measurement

Pressure-Composition-Temperature (PCT) measurement provides scientists with a source of important information related to thermodynamical properties of hydrides. PCT measurement is generally performed at isothermal conditions, where the pressure in a function of concentration of hydrogen in a hydride is collected at a constant temperature (Fig. 1). As a result, PCT measurement is also known as Pressure-Composition-Isotherm (PCI). Thermodynamic properties of different hydride systems, such as enthalpy or entropy, can be compared using their PCT data.

A fully-automatic, and computer-controlled volumetric apparatus (known as Sieverts apparatus, or PCT apparatus) has been installed in our lab to perform PCT measurement (Fig.2 and 3b). It is capable of measuring isotherms from 25 to 500oC to a maximum hydrogen pressure of 100 bar.

On-going projects at MSEL Hydrogen Storage Materials Program include the modification of PCT apparatus for small volume/mass samples, and combination of PCT measurement with other methods for measuring hydrogen capacity (In-situ PCT with Raman spectroscopy or in-situ PCT with infrared (IR) imaging measurement).

Mg-based alloys (Leo)

Mg and Mg-based alloys are of great interest for the hydrogen storage research due to the high gravimetric density of hydrogen in MgH2; the improvement of thermodynamics and kinetics through alloying and structural modification has been studied intensively. Hydrogenation of Mg and Mg-transition metal (TM) films has been also studied for their improved hydrogenation kinetics and for mirror switching properties; in particular Mg, Mg-Ni, Mg-Ti films were recently studied in detail. These studies provide a good reference for the combinatorial measurements of our group.

Our projects involving Mg-based alloys are:

• Rapidly solidified Mg-Ni alloys with Ni=0.5 to 2 at%. In this work we are interested in the effect of extended solubility of Ni in Mg on hydrogenation.

• Mg-TM metals films: For a number of transition metals Mg is immiscible with TM, e.g., TM=V, Cr. Our goal is to engineer a two-phase nano-scale microstructure in which Mg grains will be storage units for hydrogen, whereas a TM phase will act as fast diffusion path to deliver or remove hydrogen from MgH2. Hydrogenation properties of the Mg-TM films are studied by different method.

TEM (Leo)

Finding correlations between structure/microstructure and property is a key for understanding and developing new materials. In many cases, which include materials for hydrogen storage, only the powerful combination of imaging, diffraction and analytical methods of transmission electron microscope (TEM) can provide a correct picture of structural details. Most of the hydrogen storage materials are challenging for TEM studies for the following reasons: (a) extreme sensitivity to air and moisture, e.g., all complex hydrides; (b) difficulties of preparing TEM samples, e.g., ball-milled alloys; (c) difficulties of preserving hydride structure during the specimen preparation and in the microscope. In our program TEM is used for the following projects:

• TEM characterization is used systematically for combinatorial films. Considering that for many materials the deposition process occurs in far-from-equilibrium condition, often the deposited films look “x-ray amorphous”, while in fact having a complex nano-scale structure.

• With the recently acquired Gatan’s TEM holder we will be able to transferring TEM samples from a protective environment of a glove box to a microscope column without exposing a sample to air. With this procedure, microstructural details of many air-sensitive materials, e.g., borohydrides or nanoscaffolded structures, will be studied.

People

Leonid (Leo) Bendersky, Ph.D.

NIST staff member since 1987;

Leader of the MSEL Hydrogen Storage Materials Program;

Expertise: Phase transformations, Crystallography, TEM, Thin films; Combinatorial methods of hydrogen storage materials and functional oxides.

Jason Hattrick-Simpers, Ph.D.

PhD thesis: Combinatorial Investigation of Magnetostrictive Materials;

Current position at NIST: National Research Council Fellow;

Research project: Development of combinatorial in-situ measurement techniques to monitor the absorption/desorption of hydrogen in hydrogen storage materials. The particular emphasis is on being able to spatially map the sorption properties of thin-film composition spread samples via optical techniques such as IR emissivity, F.T.I.R., and Raman spectroscopy. The goal is to render these techniques quantitative by calibrating them with an absolute measurement of the hydrogen present in the sample with a technique such as Prompt Gamma Activation Analysis (PGAA). PGAA is a direct measurement of the presence of hydrogen in a sample, which is done by bombarding the sample with neutrons and monitoring the gamma particles that are emitted as a function of energy.

Expertise: Development of combinatorial samples and their characterization techniques. Library design is a key aspect of combinatorial material science; careful consideration must be given to the physical property being measured and the method by which the property is to be measured. For instance, when measuring the resistance of a gas sensor material in the presence of different gases a larger change in resistance is desired. Therefore it is desirable to deposit individual compositions discretely on top of a pre-patterned device electrode configuration with large areas to maximize S/N. In the case of measuring the dielectric constant of a material in microwave frequency ranges, the measurement can be done with high accuracy and high spatial resolution. Therefore a continuous spread sample covering a large composition region can be deposited. Finally, if the mechanical response of a material under external stimuli (i.e. temperature, magnetic field, etc.) is to be measured then the composition spread sample can be deposited on a thinned wafer, such as a bimorph cantilever array. Here a laser spot can be positioned on the tip of the cantilever and the movement of its reflection can be monitored, allowing the stress state of the material to be determined.

Chun Chiu, Ph.D.

PhD thesis: Controlled Mechano-chemical Synthesis and Properties of Nanostructured Hydrides in Mg-Al-H and Mg-B-H Systems;

Current position at NIST: Researcher Associate;

Research project: (1) Develop/processing of lightweight compounds for hydrogen storage material: (a) Rapid solidification (RS) of Mg-Ni alloy; (b)Producing both bulk and RS Al-Cu-Li quasicrystals; (2) Develop Sieverts apparatus for measuring hydrogen content in: (a) Bulk material (e.g., samples in lump, chip, granule, or powder form); (2) Sample with small volume/mass (e.g., thin film); (3) In-situ PCI measurement with Raman, IR or PGAA.

Expertise: Four years experience in nanostructured hydrogen storage materials, including mechano-chemical synthesis and characterization of synthesized materials; Microstructural characterization in polycrystalline and nanocrystalline materials, and relationship between structure and properties of materials; Conventional and unconventional manufacturing processes such as casting, heat treatment, powder metallurgy and ball milling technique.

(d) Personal statement

Chun Chiu completed his MASc at the University of Waterloo, Canada in 2003. Between January to June 2003, he studied nanostructural materials as a research assistant in the research group of Dr. Z. Wronski at Materials Technology Laboratory, CANMET, NRC, Canada. He obtained his PhD degree at the University of Waterloo in 2007. He is currently a guest researcher at Materials Science and Engineering Laboratory (MSEL), National Institute of Standards and Technology (NIST) under the guidance of Dr. Leonid A. Bendersky. His research interests include properties of intermetallic alloys, powder metallurgy and synthesis and properties of nanostructural hydrogen storage materials.

Ke Wang, Ph.D.

PhD thesis: Micromechanisms of ductile bulk metallic glasses;

Current position at NIST: Researcher Associate;

Research project: TEM investigation of bulk and thin film hydrogen storage materials;

Expertise: Microstructure analysis of materials.

Zhuopeng Tan, Ph.D.

Ph.D thesis: Formation And Piezoelectricity Of Self-Assembled PbTiO3-CoFe2o4 Nanostructural Films

Current position at NIST: Researcher Associate;

Research project: (1) Study metal-mediated-crystallization (MMC) in FeTi/Pd system. (2) Study effects of catalysts on hydrogenation of metal hydrides using combinatorial methods and IR imaging.

Expertise: Pulsed laser deposition, electron beam evaporation, spin coating, photolithogrphy, scanning electron microscopy, energy dispersive using x-ray, x-ray diffraction, atomic force microscopy (AFM) and AFM based techniques, transmission electron microscopy sample preparation

(d) Personal statement

I am a fresh graduated Ph.D from University of Maryland at College Park and currently doing research on H2 storage projects at NIST. I love to do research on materials science related projects.

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Group

Photo

Description: (a) Education and PhD thesis; (b) Current position at NIST; (c) Research project and expertise; (d) Personal statement

Collaborations

At NIST

Dr. E. Heilweil, Physics Laboratory, NIST

Project: Application of IR emissivity imaging and FTIR to combinatorial screening of hydrogen storage materials

Dr. J. Maslar, Chemical Science and Technology Laboratory, NIST

Project: Application of Raman spectroscopy to in-situ characterization of hydrogen storage materials

Drs. G. Downing, L. Cao, R. Lindstrom, E. Mackey, R. Paul, R. Greenberg, Analytical Chemistry Division, Chemical Science and Technology Laboratory, NIST

Project: Application of Prompt Gamma Activation Analysis (PGAA) to combinatorial characterization of thin film hydrogen storage materials

Drs. H. Wu, T. Udovic, T. Yildrim, NIST Center for Neutron Research, NIST

Project: TEM characterization of hydrogen storage materials

Outside NIST

Prof. E. Rabkin, Dept of Materials Engineering, Technion-Israel Institute of Technology

Project: Effect of rapid solidification on hydrogenation of Mg-Ni alloys

Prof. J.-C. Zhao, Dept. of Materials Science and Engineering, Ohio State University

Project: Novel Mg-based Laves phases for hydrogen storage

Drs. B. Chao and K. Young, Ovonic Materials Division, Energy Conversion Devices ECD, Inc.

Project: Laves phases for NiMH batteries, Hydrogen storage materials

Dr. M. Au, Savannah River National Laboratory (SRNL)

Project: Spectroscopic characterization of Boron Hydrides

Prof. B. Dam, Condensed Matter Physics, Vrije Universiteit, Amsterdam, Netherlands

Project: TEM structural characterization of Mg-Ti films







approaches for high throughput/combinatorial synthesis, screening and testing of storage materials and other novel characterization techniques that can aid in and be applied to the efficient discovery of new materials.

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