Search

To implement successful water management strategies in polymer electrolyte membrane (PEM) fuel cells, we need to understand how the complex heterogenous nature of the gas diffusion layer (GDL) impacts the pore-scale transport of liquid water. Here, we investigated the 3-D pore-scale liquid water distribution within the cathode GDL via in operando synchrotron X-ray tomography during low current density fuel cell operation to capture the early appearance of liquid water pathways. We found that the invasion of liquid water into the GDL only partially filled certain GDL pores. Liquid water wicked along some GDL fibers, which was attributed to the hydrophilic nature of carbon fiber and the presence of pore-scale mixed wettability within the GDLs. Mixed wettability and partial pore filling should be considered in the pore-scale modeling and design of next-generation GDLs.

Mg-RE alloy Electron717 (ZE10A, UNS M11600) has a nominal composition of 1.2wt % Zn, 0.25wt% Zr, <0.5%Zr, 0.007wt%Mn, 0.004wt %Fe, 0.001wt%Ni and balance Mg. The alloy was investigated in three different heat-treated conditions such as (1) as received (AR) (average grain size dav = 7.90 um), (2) solution treated at 525degC for 8 hours (ST8 with dav: 26.24 um) and (3) solution treated at 525degC for 24 hours (ST24 with dav: 41.37 um). Anodic and cathodic polarizations were carried out in three different electrolytes (pH unadjusted) such as: a) 0.6M NaCl, b) 0.5M NaNO3, and c) 0.1 M Na2SO4. The results are compared with commercial purity magnesium with an average grain size of 16 um. The effect of grain size on the corrosion rate was observed for ZE10A alloy in the 0.1 M NaNO3 solution. Increase in the grain size increased the corrosion current density. The grain size effect diminished when polarized to high anodic potentials. Commercial purity magnesium showed lower corrosion current density than the ZE10A specimens in all the solutions tested in this study.

Commercial purity Mg, Mg-RE alloys ZE10A (UNS M11600), and EV31A (UNS M12310) were evaluated in the as-received condition for potential application as anode for Mg-air battery. Anodic and cathodic polarization studies were performed in 0.6 M NaCl, 0.5 M NaNO3 and 0.1 M Na2SO4 electrolytes. Hydrogen evolved during anodic polarization was measured at different potentials. Electrochemical impedance spectroscopy was carried out at open circuit conditions. The results showed that commercial purity magnesium had higher corrosion resistance than the Mg-RE alloy specimens in all the electrolytes tested. All the specimens showed negative difference effect (NDE) where the rate of hydrogen evolution increased with increased anodic polarization. The NDE was more pronounced on ZE10A than on other materials. The rate of hydrogen evolution was about three times higher in NaCl solution than in other two solutions. The results suggest that ZE10A could be a potential anode material for Mg-air battery due to its high anodic activity in spite of the observed NDE.

Several radioactive iodine isotopes are formed as fission products in the nuclear fuel, and retained within the fuel matrix by the fuel cladding as containment. During reprocessing of the used fuels by the pyroprocessing route, a significant fraction of iodine is reported to be retained in the molten salt as iodide (I-). The oxidation of I- is strongly influenced by the Cl-. When Cl- concentration is in excess of I-, the iodide oxidizes to [ICl2]- in two single electron steps via an [I2Cl]- intermediate. The presence of chloride leads to oxidation of I3- to occur at less positive potentials. Chemla effect of type 1 has been observed in the LiCl-KCl molten salt systems wherein the mobility of the K+ is larger than that of Li+ when the fraction of KCl is higher than 0.325 at 723 K. The Chemla effect could lead to accumulation of KCl near the cathode and adversely influence the electrolytic parameters.

The palladium film deposited on a nickel substrate in 1-butyl-1-methylpyrrolidinium dicyanamide using a constant potential electrodeposition method was shown to be composed of nanoparticles under transmission electron microscopy. The electrodeposition was carried out by applying a potential hold of -2.3 V vs Fc/Fc+ for 16 h. The size distribution of palladium nanoparticles was unimodal and narrow with an average size of 6.3 +- 1.6 nm. The Pd nanoparticles were polycrystalline with (111) plane being the most prevalent. The implications of this include the having to reconsider the properties of electrodeposited Pd thin films from ionic liquids and opening further applications of electrodeposition of Pd to include nanoparticle synthesis.

Despite the overwhelming effort to improve the efficacy of resistive random access memory (RRAM), the underlying physics governing RRAM operation have proven elusive. A survey of the recent literature almost universally indicates that the remaining glaring issues center around variability as well as endurance. The initial filamentary "forming" process is often linked to these problems. This work details our recent efforts to bring the forming process under control and the resulting improvements in RRAM viability in hafnia-based devices. We track the forming process via a "forming energy" metric that allows for filament optimization. By removal of all current compliance elements, and their associated parasitics, a targeted forming energy is achieved using ultrashort voltage pulses. By tailoring the forming energy, we show remarkable endurance window control.

Pd electrodeposited in aqueous-based solutions suffer from hydrogen embrittlement which could be avoided using ionic liquids. Herein, electrodeposition of Pd in 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]) was performed at 110 oC. Three types of Pd species are found having distinct reduction potentials from cyclic voltammograms (CVs). On electroreduction by potentiostating in-situ etched Ni at -1.4 V vs Pd pseudo-reference electrode, Pd was deposited into flat structures. Energy dispersive spectroscopy (EDS) showed that the Pd deposits were strongly associated with carbon containing molecules. The carbon containing molecule is most probably strongly coordinating imidazolium cations.

UT Arlington is researching the impact high pulsed loading has on high power electrochemical energy storage devices. Fundamental effects on aging due to high pulsed loading are being investigated by studying the capacity fade and changes in the cells electrochemical impedance [1]. Experiments have been performed in which a 3Ah LiNixCoyAl1-x-yO2 cell is discharged at rates as high as 83C (250 A) in a 10 kHz pulsed fashion for 140s. A 1C recharge procedure is used between discharge cycles. The experimental results obtained when cycling at elevated rates are compared to results obtained from a second identical cell which is charged using a 1C constant current (CC) - constant voltage (CV) procedure and discharged at a constant 1C rate. This paper describes the experiments performed and shows the impact high pulsed loading has on the aging and capacity fade of a high power lithium-ion battery after fifty cycles have been performed.

A 3Ah lithium-ion battery, LiNixCoyAl1-x-yO2, has been repeatedlyrecharged at an elevated C rate in a pulsed fashion to understand theimpact this has on cell lifetime. A peak charge rate of 33C, ~100 A, isapplied in 3 s increments with 6 s of idle time between pulses. Betweenrecharges, the cell is discharged at a constant current (CC) 1C rate. Cellfatigue is determined using electrochemical impedance spectroscopy (EIS)and capacity measurements. During each elevated recharge, the cellsvoltage, current, and temperature are monitored using high speed dataacquisition systems. EIS measurements are made before and after each1C-CC discharge. A second cell of the same type is also cycled on apotentiostat using a 1C-CC, constant voltage (CV) recharge and 1C-CCdischarge. The goal is to compare the aging of the two cells in order tounderstand the differences introduced from elevated recharge rates.

Accurately measure the transient details of switching memristive switch is crucial to the elucidation of the switching mechanism. Such high-speed measurements often plagued by artifacts. Here we describe a measurement technique capable of capturing Set/Reset characteristics of memristive switches with high accuracy. It can accurately measure the transient current during the Set/Reset operation with rise time as short as 2 ns. The circuit is designed to cycle through (Set/Reset) and sense (read the state) rapidly to enable the study of endurance. The sense circuit is able to measure currents as low as 30 pA yielding accurate measurement of the resistance in the off state up to 1.6 G. Pt/Ta2O5/Cu memeristive switches are examined that exhibit ON and OFF state resistance (Ron and Roff) ratio of >104 and endurance cycles of >6x104.

Nitrogen functionalized ordered mesoporous carbon (NOMC) was synthesized and decorated with Pt nanoparticles using a modified polyol process. After characterizing porosity and surface chemistry, their activity towards oxygen reduction reaction was studied using a thin-film rotating disk electrode and as a cathode in a proton exchange membrane fuel cell. In both tests, lower specific activity but higher mass-specific activities were found for Pt/NOMC compared to Pt/Vulcan carbon.

Nitrogen functionalized ordered mesoporous carbon was synthesized using pyrrole as the precursor and SBA-15 as the template, and its physical and electrochemical properties were investigated. Highly ordered mesoporus carbon with high nitrogen content (C/N = 8.3) was obtained. Physical properties were elucidated using transmission electron microscopy, Brunauer-Emmett-Teller N2 adsorption, attenuated total reflectance fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The resulting material was found to have a specific capacitance of 182.5 F/g. Also, the oxygen reduction reaction activity and corrosion resistance was superior to that of Vulcan XC-72R.

A modified polyol process was used to deposit platinum nanoparticles on nitrogen functionalized ordered mesoporous carbon. The pH was adjusted to control the platinum size and loading. Several physical characterization tools were used to determine the size, dispersion and structure of the catalyst and support including X-ray Photoelectron Spectroscopy, X-ray Diffraction, and high resolution transmission electron microscopy. The electrochemically active area for the oxygen reduction reaction was computed by cyclic voltammetry. Platinum deposited on nitrogen functionalized ordered mesoporous carbon was found to have higher dispersion and utilization than platinum deposited on Vulcan XC-72R.

Platinum thin films have been deposited by atomic layer deposition (ALD) using MeCpPtMe3 and oxygen as precursors. The nucleation phase of ALD Pt thin film deposition has been found to follow a Volmer-Weber island growth mechanism resulting in Pt nano dots and discontinuous films. Regular monolayer by monolayer ALD growth kicks in once the Pt film reaches a critical thickness of about 15 nm. Beyond the nucleation phase ALD Pt proceeds at a linear deposition rate of 0.5Aa/cycle. ALD growth of platinum was extended to nanoporous templates and platinum nanotubes were synthesized using porous alumina templates.

MOS capacitors have been used as depth-dependent probing tools for MOS-DLTS studies of ALD HfO2 on s-Si/SiGe/Si composite samples. Surface nitridation prior to HfO!2 deposition was carried out to improve the HfO2/s-Si interface by suppressing hafnium silicate interlayer formation. The overall effects of nitridation on the HfO2/s-Si interface and the s-Si/SiGe bulk region with different strained Si layer thicknesses were studied. The relationship between DLTS peak amplitude and pulsing time indicates the emission source in bulk region is related with Ge decorated dislocations and in the interface region it originates from the HfO2/s-Si interface. In the s-Si/SiGe bulk region, nitridation reduces the trap concentration and shifts trap energy level toward mid-gap. For the interface, nitridation shifts trap energy level toward valence band and reduces hole capture cross section significantly.

Multiple co-axial nanotube structures of various transition metal oxides were successfully synthesized by Atomic Layer Deposition (ALD) techniques utilizing nanoporous anodic aluminum oxide (AAO) as templates. In order to fabricate free-standing tube-in-tube nanostructures successive ALD nanotubes were grown on the interior walls of the AAO nanochannels. The co-axial nanotubes were alternated by sacrificial spacers of ALD Al2O3, to be chemically removed to release the nanotubes from the AAO template. This synthesis may be extended to fabricate n-times tube-in-tube nanostructures of different materials with applications in multi sensors, broad band detectors, nanocapacitors and photovoltaic cells by growing specific sequences of insulating, semiconducting and conducting ALD layers.

MOS capacitors were fabricated on strained Si layers epitaxially grown on Si0.8Ge0.2/Si substrates and bulk Si with HfO2 as high-k material to investigate electrical properties and oxide/sSi interface quality. The performance of high-k dielectric material significantly depends on the interface quality between the ALD dielectric and the strained silicon substrate due to oxygen diffusion from HfO2 and Ge diffusion from the SiGe buffer layer. The electrical and structural properties were investigated by performing C-V and I-V measurements, surface roughness measurements by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Samples were tested with varying film thickness of strained Si capping layers (232, 928 and 1982 Aa). To investigate interface quality, samples were nitrided in a rapid thermal annealing (RTA). It was found that the interface quality of strained Si and ALD dielectric improves by nitridation, which prevents oxygen diffusion and acts as a diffusion barrier layer between the HfO2 and the out-diffused Ge. In contrast to sSi this is not the case for non strained relaxed bulk Si. The results show that the performance of device depends on the type of substrate used and the type of surface treatment performed before ALD deposition of HfO2.

HfO2 films were deposited using atomic layer deposition (ALD) at temperatures ranging from 100oC to 250oC. We report detailed nanoindentation measurements on the pure amorphous, mixed-phase and polycrystalline phase of HfO2. We use nanoindentation techniques for determining the important mechanical properties such as hardness and effective modulus. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and atomic force microscopy (AFM) to determine the structural properties of the various phases of these ALD HfO2 films. We measured the effective modulus to be 370 GPa for amorphous low temperature ALD HfO2 and 240 GPa after crystallization with rapid thermal annealing (RTA). Similarly, the hardness measurements revealed a value of 18 GPa for amorphous HfO2 and a lower value of 15 GPa following the transition temperature from amorphous to polycrystalline HfO2.

The challenges of reducing gate leakage current and dielectric breakdown beyond the 45 nm technology node has shifted engineers' attention from SiO2 to higher dielectric constant materials also known as high-k materials such as HfO2 and Al2O3. Therefore, it is important to investigate the electrical and mechanical properties of these materials that are projected to replace SiO2. In this study, HfO2 and Al2O3 have been deposited by atomic layer deposition (ALD) on Si wafers. Using continuous stiffness method (CSM) Nanoindenter(r) XP, we report elasto-mechanical properties of HfO2 and Al2O3 on Si. HfO2 has a hardness and modulus of 9-10 {plus minus} 2 GPA and 200 {plus minus} 40 GPA respectively. In addition, the hardness and modulus of Al2O3 were determined to be respectively 10-11 {plus minus} 2 GPA and 200 {plus minus} 40 GPA.

4 nm HfO2 films are deposited on silicon-on-insulator (SOI) with various amounts of lattice strain and thickness by atomic layer deposition (ALD). After post deposition annealing (PDA), the samples are studied by Rutherford backscattering spectroscopy (RBS), high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), and x-ray photoelectron spectroscopy (XPS). The as-deposited HfO2 film shows good stoichiometry and thickness uniformity. HRTEM images show that while HfO2 films on non-strained SOI becomes poly-crystalline after PDA at 600 oC, HfO2 films on strained SOI still keep amorphous. The strain SOI also suppresses the IL growth during PDA. The EELS and XPS results confirm the interdiffusion across the HfO2/Si interface. The XPS data also show that formation of Hf-O-Si bonds depend on the SOI lattice strain and thickness. The SOI thickness is critical to reduce the formation of silicate in the IL.

The University of Texas at Arlington (UTA) has performed experiments in which high power 3Ah LiNixCoyAl1-x-yO2batteries have been studied at high C rates. The first of three identical cells has been discharged using a 10 kHz (50% duty cycle), high C rate (100C/300 A) pulsed discharge procedure and a 1C constant current (CC) - constant voltage (CV) recharge procedure. The second cell has been discharged using a 1C CC procedure and recharged using a 111 mHz (33% duty cycle), high C rate (33C/100 A) pulsed recharge procedure. The third cell, the control cell, has been cycled using a 1C CC discharge procedure and 1C CC-CV recharge procedure. The results of these experiments enable the impact each respective high rate procedure has on cell aging to be better understood. The experimental procedures and methods will be discussed in detail and the results of the experiments will be presented.

A 3Ah lithium-ion battery, LiNixCoyAl1-x-yO2, has been repeatedlyrecharged at an elevated C rate in a pulsed fashion to understand theimpact this has on cell lifetime. A peak charge rate of 33C, ~100 A, isapplied in 3 s increments with 6 s of idle time between pulses. Betweenrecharges, the cell is discharged at a constant current (CC) 1C rate. Cellfatigue is determined using electrochemical impedance spectroscopy (EIS)and capacity measurements. During each elevated recharge, the cellsvoltage, current, and temperature are monitored using high speed dataacquisition systems. EIS measurements are made before and after each1C-CC discharge. A second cell of the same type is also cycled on apotentiostat using a 1C-CC, constant voltage (CV) recharge and 1C-CCdischarge. The goal is to compare the aging of the two cells in order tounderstand the differences introduced from elevated recharge rates.

An experimental setup has been designed to measure air-water capillary pressure curves for gas diffusion layers (GDLs) used in polymer electrolyte membrane fuel cells at elevated temperatures. Experiments were conducted at various temperatures from 25 to 85{degree sign}C for Toray 120 with 0% and 10% PTFE by weight. In untreated samples the results suggest that GDL wettability actually changes with temperature once the temperature dependence of the surface tension has been accounted. In treated samples, however, the wettability remains constant with temperature. This behavior is in agreement with the temperature dependent values of water contact angle on PTFE and graphite.

SBA-15 was treated with different concentrations of tetraethyl orthosilicate (TEOS) in the presence of HCl, which catalyzed the hydrolysis and condensation of TEOS, resulting in additional silica to be deposited onto SBA-15, modifying its structure. Although the pore size distribution (PSD) plot showed that wall thickness of SBA-15 changed upon treatment with TEOS, the pore size of the resulting nitrogen doped mesoporous carbon (NMC) did not change significantly. The NMCs were increasingly structurally disordered with the additional TEOS treatment. When platinum nanoparticles (Pt NPs) were deposited on them, uniform particle size and distribution was obtained. Among the NMCs, the most disordered sample showed the lowest activation and mass-transfer overpotentials in a laboratory scale proton exchange membrane fuel cell.

MOS capacitors have been used as depth-dependent probing tools for MOS-DLTS studies of ALD HfO2 on s-Si/SiGe/Si composite samples. Surface nitridation prior to HfO¬2 deposition was carried out to improve the HfO2/s-Si interface by suppressing hafnium silicate interlayer formation. The overall effects of nitridation on the HfO2/s-Si interface and the s-Si/SiGe bulk region with different strained Si layer thicknesses were studied. The relationship between DLTS peak amplitude and pulsing time indicates the emission source in bulk region is related with Ge decorated dislocations and in the interface region it originates from the HfO2/s-Si interface. In the s-Si/SiGe bulk region, nitridation reduces the trap concentration and shifts trap energy level toward mid-gap. For the interface, nitridation shifts trap energy level toward valence band and reduces hole capture cross section significantly.