bluuebunny/crossref_metadata_2025_split huggingface dataset:
(Invited) Simulation of the Effects of As-Grown Defects on GaN-Based Power HEMTs
Authors: Mukherjee Shrijit , Patrick Erin , Law Mark E , Ren Fan , Pearton Stephen J. 
Published: February 2020

With the wide band-gap of Gallium Nitride (GaN) and high mobility of 2 dimensional electron gas(2DEG), AlGaN/GaN high electron mobility transistors (HEMTs) are considered ideal candidates for high speed and power applications ranging from hybrid vehicles to space communication. Developments in the fabrication of GaN-based blue LEDs have been incorporated into the processing of AlGaN/GaN HEMT structures leading to improvements in amount of defects and dislocations resulting from heteroepitaxial growth. But evident degradation of dynamic performance of such devices due to the presence of as-grown defects still remains a subject of major concern. Using FLorida Object Oriented Device Simulator (FLOODS) TCAD, this work looks to analyze the impact of variation of defect properties as well as their spatial distribution and concentration on the steady-state and dynamic behavior of AlGaN/GaN HEMTs.
FLOODS TCAD’s defect modeling capability will be applied to examine how multiple defect levels, of both donor and acceptor-like nature, can influence device performance. Particular emphasis will be on transient switching, such as gate-lag and drain-lag experiments, in consideration of the slow nature observed frequently in literature. While comprehensive investigation of the role of charged defects at the AlGaN/passivation layer interface on performance degradation has already been performed, contribution by as-grown defects, expected to be in significant quantity in the GaN buffer layer, is yet to be investigated. Building on experimental results, which suggest the presence of multiple acceptor-like defects, impact on steady-state current-voltage characteristics will be observed followed by an examination of how such defects will affect dynamic performance by isolating their contribution to transient and switching behavior.
Figure: (a) Drain current against gate voltage at VDS=0.5V for HEMT A with 1 acceptor-like level (nT=1014 cm-3, Et=Ec-0.7eV) and HEMT B with 2 acceptor levels(Level 1: nT=1014 cm-3, Et=Ec-0.7eV; Level 2: nT=1015 cm-3, Et=Ec-0.5eV) (b) Optical pumping like detrapping of electrons, biasing at VDS=1V and VGS=0V. HEMT A(Level 1: nT=1013 cm-3, Ec-0.2eV, τ=1s; Level 2: nT=4x1013 cm-3, Ec-0.5eV, τ=1ms), HEMT B(Level 1: nT=1013 cm-3, Ec-0.2eV, τ=1s; Level 2: nT=1013 cm-3, Ec-0.5eV, τ=1ms)

Figure 1
Miniaturized Redox Flow Batteries for Electronic Applications: CFD Modeling
Authors: Kumar Brijesh , Bhargav Atul , Ruch Patrick 
Published: February 2020

Miniaturized redox flow batteries have seen interest in electronic applications because of their potential to simultaneously deliver electric power and remove heat. For these applications, the flow battery has to be constructed on a side of a computer chip, with components such as flow channels, manifolds, supply tubes, electrodes, membranes and current collectors. There are efforts underway to optimize the design of these components for different objectives such as maximum power density or maximum efficiency. Due to space constraints, design rules employed for classical redox flow batteries are only of limited use in microfluidic systems. Since experimentation with micro-scale components is especially expensive and time-consuming, there is a need to develop computational tools to understand trade-offs in the design and operation of these flow batteries. We have carried out computational fluid dynamics study of redox flow batteries using commercially available software. This paper analyses the effect of manifold and flow channel design on current and voltage distribution in the flow battery.
The anolyte and catholyte fluid flows are modeled using Navier-Stokes equations of mass momentum and energy conservation. Species diffusion through porous electrodes is modeled using species-dependent diffusion coefficients, which are calculated based on the fluid state and the electrode geometry. The electrochemical reactions are modeled using the Butler-Volmer equation with a pre-defined exchange current density. The species concentrations, pressures and temperatures at various parts of the flow battery are solved using a multiphysics simulation software package. We find that the flow channel geometry and manifold design has an important effect on the flow distribution as well as current collection inside the flow battery. It also affects the pressure drop, thereby affecting the pumping power required to keep the battery operational. Based on these findings, we make observations regarding cell design and operation. We expect that these simulations, once validated with experiments, lead to a better understanding of the trade-offs involved in the design and operation of miniature redox flow batteries for electronic applications.
Effect of MoO3 Vapor Flow to Large Area MoS2 Few-Layered Films Growth
Authors: Chen Hung-Yi , Wang Hsiang-Chen , Wu Wen-Wei , Lu Ming-Pei , Lu Ming-Yen 
Published: February 2020

Two-dimensional (2D) materials are a new material system with interesting and extraordinary physical properties. These characteristics make them have many applications including field-effect transistor (FET), flexible electronics, and optoelectronic devices [
1,2,3
]. Transition Metal Dichalcogenides (TMDs), such as MoS2 and WSe2, are drawn much attentions, because they have direct bandgap if they are monolayer. Acquisition of large area MoS2 flakes using chemical vapor deposition (CVD) method is feasible. Here, we report a method to optimize the uniformity and the size of MoS2 few-layered films. The growth substrates were placed up-side down on the MoO3 source with the separation (d) of 7 to 9 mm during the growth (figure 1a). The resulting films were significantly different (figures 1b-d). Based on the results, the MoS2 films grown when the separation is 8mm have the best uniformity and largest area (figure 1c). The MoS2 films are as large as 1 cm2, which is 3-3.5 times larger than that grown from other separation parameters. The number of MoS2 films was identified to be monolayer from Raman analysis (E1
2g to A1g peak difference is about 20.5 cm-1) and photoluminescence analysis (a peak at ~677 nm). The detailed growth mechanism of large area MoS2 films is discussed in this study.

References

Yin, ZY et al, ACS NANO., 6, 74-80 (2012).

Tsai, ML et al, ACS NANO., 8, 8317-8322 (2014).

Lopez-Sanchez, O et al, Nature Nanotechnology., 8, 497-501 (2013)

Figure 1
Advances in Electrochemical Hydrogen Compression and Purification
Authors: Bouwman Peter Jaime 
Published: February 2020

These are exciting times to be pioneering Electrochemical Hydrogen Compression (EHC) as the call for linking renewable electrical energy and chemical energy storage is becoming stronger. Hydrogen is ideally suited for electrochemical conversion, thanks to its relative abundance in compounds on earth, extremely fast redox reaction and highest energy content by weight. Although we do not produce hydrogen, we are utilising the fast redox reaction to convert hydrogen gas at the membrane interface to protons, and back to hydrogen gas again.
The principles of Electrochemical Hydrogen Compression have been disseminated before and demonstrated the capability to achieve high pressures of 100 MPa single stage (14,503 psi), while simultaneously purifying the hydrogen gas. The company HyET is developing a product that will be suitable for multiple applications, ranging from (home) refuelling hydrogen vehicles to purification and recycling of industrial hydrogen (bio)gas.
Here we present the outcome of the project PHAEDRUS and DONQUICHOTE, where EHC technology was developed and validated as compressor for a Hydrogen Refuelling Station, further compressing the hydrogen gas supplied directly from a PEM electrolyser. Both technologies exhibit dynamic operation, fast response and modularity making them well compatible. The most effective system configuration to capture renewable wind and solar energy and deliver high pressure hydrogen has been investigated.
In addition, we review the purification ability of the EHC technology, extracting hydrogen selectively from mixed gasses in low and high concentrations. Hydrogen gas is typically produced from compounds (e.g. methane), its value is highly determined by its quality and any residual contaminations. The solid membrane was design to hold back high pressure hydrogen, and therefore shows superior permselectivity limiting passage of larger molecules. As for the active hydrogen extraction process, the membrane selection should be viewed in tandem with the catalyst system, which may be affected by specific contaminations.
Technical working principles are elaborated on in this presentation, pointing out key benefits and scientific challenges that were encountered and to some degree resolved, finally discussing the roadmap ahead leading towards very attractive applications.

Figure 1
Synthesis and Evaluation of Core-Shell Electrocatalysts for Oxygen Reduction Reaction
Authors: Zhu Shangqian , Yue Jeffrey , Qin Xueping , Shao Minhua 
Published: February 2020

The slow kinetics of the oxygen reduction reaction (ORR) at the cathode and high cost of electrocatalysts (commonly Pt and Pt-based alloys) for this reaction are major hurdles that hinder the large scale production of proton exchange membrane fuel cells (PEMFCs).[1] A core-shell structure consisting of an atomically thin layer (up to several atoms thick) of Pt deposited on Pd has been recognized as a promising ORR electrocatalysts to replace bulk Pt-based materials.[2] Ideally, all of the Pt atoms will be fully utilized since they are all on the surface in the case of an atomically thin layer of Pt on a core material. The large scale synthesis of Pd@Pt core-shell catalysts, however, has not been fully realized. This study reported two synthesis methods to prepare gram scale batch of Pd@Pt/C electrocatalysts with the assistance of citric acid.
The first is Cu-mediated-Pt-displacement method involving the displacement of an underpotentially deposited (UPD) Cu monolayer by Pt.[3] In this method, the Cu monolayer deposited on Pd then is displaced by Pt via a surface limited redox replacement (SLRR) reaction: Pd@Cu + PtCl4
2- → Pd@Pt + Cu2+ + 4Cl-. Recent studies have shown that the SLRR reaction is rather complicated and poorly-controlled resulting in formation of 3D Pt islands instead of a uniform overlayer, especially in the large batch size synthesis. By adding citric acid in the displacement reaction, the activity of Pd@Pt/C can be enhanced by 2 times compared with that synthesized without citric acid.
The second method is direct chemcial reduction of PtCl4
2- with the assistance of citric acid without preformation of a Cu UPD layer. Comparing with other reducing agents, the reducing power of citric acid is neither too strong nor too week and allows the reduction reaction to occur at the room temperature, which significantly simplifies the reaction process. In addition, the strong interaction of citric acid with metals may reduce the Pd dissolution and introduce charge transfer from adsorbed citric acid molecules to Pd surface, which can also be used to reduce Pt cations.
Both methods resulted good quality of core-shell catalyts in terms of activity and durability. The Pt activities of Pd@Pt/C preapred by the Cu-mediated-Pt-displacement and chemcial reduction methods were 0.95 and 0.78 A mg-1 at 0.9 V, respectively. Fuel cell performance will be also reported and the role of citric acid will be discussed in the presentation.
References
[1] in: M. Shao (Ed.) Electrocatalysis in Fuel Cells: A Non- and Low- Platinum Approach, Springer, London, 2013.
[2] R.R. Adzic, Electrocatalysis, 3 (2012) 163-169.
[3] R.R. Adzic, J. Zhang, K. Sasaki, M.B. Vukmirovic, M. Shao, J.X. Wang, A.U. Nilekar, M. Mavrikakis, J.A. Valerio, F. Uribe, Top. Catal., 46 (2007) 249-262.
Thermal Modelling and Validation of PEM Fuel Cell System for Automotive Applications
Authors: Liso Vincenzo , Nielsen Mads Pagh , Al Shakhshir Saher 
Published: February 2020

In this study a validated model for a PEM fuel cell system is developed to actively control the supplied reactants and to manage water within the cell structure and correctly provide system cooling. Due to the substantial difference in response times of each of the system volumes, a model of each components system is developed for analysis and controller design to achieve thermal tracking while adequately rejecting system disturbances.
Control-oriented transient models have been developed to account for the formation of liquid water within the gas channels or within both the channels and the GDL, however the advantages of having a validated model will help achieving adequate humidity regulation. Due to the notoriously slow response of humidity transducers, especially near saturated conditions, this is an advances the field of fuel cell reactant pre-treatment. In developing this strategy, a critical step will be accomplished by properly selecting the controller references used for temperature feedback. Although this may at first seem like a simple step, the selected temperature references have a profound impact on the resulting thermal and humidity regulation. If not properly considered, the system response could be unnecessarily slow or produce an undesirable excursion in humidity. The establishment of this relationship allows for accurate dynamic voltage estimation under a range of operating conditions, and is a necessary step towards further understanding physical phenomena.
Resonance-Stabilized Guanidinium-Tethered Poly(Phenylene)s
Authors: Lee Kwan-Soo , Fujimoto Cy , Kim Yu Seung 
Published: February 2020

Anion exchange polymer electrolyte is a key element for alkaline membrane fuel cell (AMFC) applications. In fuel cells, the polymer electrolytes require alkaline stable covalently tethered cationic group to conduct hydroxide ion. Benzyltrimethyl ammonium cationic group has been mostly often employed in anion exchange polymer electrolytes because it is reasonably basic, stable and easily synthesized. However, the demands on more stable and less catalyst poisoning cationic groups are growing.
One promising stable cationic groups that potentially can be tethered in the polymer backbone is guanidinium. However, benzyl guanidinium tethered polymers prepared via chloromethylation and subsequent cationization of the halomethyl group showed a limited stability in high pH conditions due to the nucleophilic degradation of the benzylic carbon. In 2011, we had developed a new synthetic method to synthesize guanidinium tethered polymer via activated fluorophenyl-amine reaction.1 This new synthetic method allows to prepare phenyl guanidinium (instead of benzyl guanidinium via conventional method) tethered polymer. The direct connection between phenyl group and guanidinium can stabilize the cationic group by whole resonance structure. However, continuing study indicates that the poly(arylene ether) polymer that we employed undergoes ether-cleavage degradation under high pH conditions.2

In this study, we report the phenyl guanidinium tethered ether-free poly(phenylene)s. Activated fluorophenyl-amine reaction was used to synthesize the poly(phenylene)s. The alkaline stability of the polymer is exceptional: no chemical structural degradation was found during 1000 h in 0.5 M NaOH at 80°C. The tailored poly(phenylene) anion exchange ionomers is well suited for the ionomeric binders, particularly for hydrogen oxidation reaction in alkaline membrane fuel cells.
References:
1 D. S. Kim, A. Labouriau, M. D. Guiver, and Y.S. Kim, Chem. Mater. 2011, 23, 3795-3797.
2 C. Fujimoto, D. S. Kim, M. Hibbs, D. Wrobleski, Y.S. Kim, J. Memb. Sci. 2012, 423, 438-449.
PdFe Nanoparticles for Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells
Authors: Lee Kug-Seung , Kang Yun Sik 
Published: February 2020

Carbon-supported PdFe nanoparticles (NPs) were prepared with composition-controlling via a modified chemical synthesis and a heat-treatment at high temperature under a reductive atmosphere. The synthesis combined the polyol reduction method and hydride reduction method, which was used to obtain monodispersed PdFe NPs. In addition, to induce structural modifications, the as-prepared PdFe NPs received heat-treatment under a reductive atmosphere. Results of structural characterizations including highresolution powder diffraction (HRPD), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) analysis, indicated that heat-treated PdFe NPs exhibited a higher degree of alloying and surface Pd atomic composition compared with as-prepared ones. Furthermore, new crystalline phases were detected after heat-treatment. Thanks to the structural alterations, heat-treated PdFe NPs showed ~3 and ~18 times higher mass- and area-normalized oxygen reduction reaction (ORR) activities, respectively than commercial Pt/C. Single cell testing with heat-treated PdFe catalysts exhibited a ~2.5 times higher mass-normalized maximum power density than the reference cell. Surface structure analyses, including cyclic voltammetry (CV), COad oxidation, and XPS, revealed that, after heat-treatment, a downshift of the Pd d-band center occurred, which led to a decrease in the affinity of Pd for oxygen species, resulting in more favorable ORR kinetics.
Peald-TiN Protective Coatings for Metallic Bipolar Plates of PEMFC
Authors: Yun Eun-Young , Kim Da-Young , Lee Han-Bo-Ram , Kwon Se-Hun 
Published: February 2020

Proton Exchange Membrane Fuel Cell (PEMFC) is an electrochemical device that convert the chemical energy of reactants directly to electrical energy. The water-based, acidic polymer membrane usually used as its electrolyte. Therefore, the bipolar plate, as one of the most important components in PEMFC system, is required to be high corrosion resistance in the acidic condition. Currently, researchers have tried to develop metallic bipolar plate using a stainless steel. In case of using stainless steel, a passivation or protective coatings layers should be applied to improve its stability in the harsh environment. However, the general intrinsic defects (columnar structures, pinholes, pores, discontinuities) in the protective films, which can greatly affect its high performance. Therefore, a passive thin film with low defects and stable property was needed to be developed.
In this work, TiN thin films were deposited on 316L stainless steel as the protective layer by using plasma enhanced atomic layer deposition (PEALD). For comparison, tetrakis-dimethylamido-titanium (TDMAT) and titanium tetrachloride (TiCl4) were used as different Ti sources, respectively. The microstructure, chemical composition and the corrosion behavior of TiN protective layer deposited by different sources were investigated by 4-point probe, XRD, FESEM, AES and potentiodynamic polarization equipment. The SEM results showed that the uniform TiN protective layers were successfully deposited on the 316L stainless steel substrate by both two sources. and the FCC crystal structure was confirmed by the XRD analysis. In addition, the TDMAT-deposited TiN has lower the corrosion current density (Icorr) and higher corrosion potential (Ecorr) than those of TiCl4-deposited TiN, which could be inferred that the TDMAT-deposited TiN will have a higher corrosion resistance and protective efficiency during the fuel cell application.
A Study on Degradation of Membrane Electrode Assembly By Operating Conditions of Fuel Cell Vehicle
Authors: Yoo Seung Eul , Kim Myong-Hwan , Goo Youngmo 
Published: February 2020

In generally, degradation of membrane & electrode assembly(MEA) according to driving conditions of fuel cell electric vehicle focused on analyzing the state of catalyst, membrane, gas diffusion layer before and after durability evaluation . At present, the exact mechanism for degradation of membrane electrode assembly(MEA) according driving conditions of fuel cell vehicle does not hold and the mechanism were approximately the level that analogize the whole out of a part.
In case of the degradation of MEA by operating modes, it was shown on three sources; (1) the loss of apparent catalytic activity according to the time(ηa), (2) conductivity loss by decreasing ion conductivity per the time, (3) the loss of rate of mass transfer according to the time. In other words, the equation about degradation of MEA can be seen that the basic form can be created by analyzing the polarization according to time.In this study, the 10 driving modes were devised by cell voltage behavior according to driving conditions of fuel cell vehicle. The operating modes were basically designed by high current, middle current and low current. Also, these modes were considered about the most commonly used driving power and time variation in accordance with the current change. The result of analyzing the MEA degradation by current/voltage variation was as below.
The activation overpotential shows a tendency to be rapidly decreased in early state and decrease or increase gradually without reference to operating conditions. The ohmic overpotential tends to decrease in proportion to the time. OCV(Open Circuit Voltage) shows the result of decreasing rapidly in high current mode than low current mode. In this paper, we conducted an analysis of the equation for degradation of MEA according to the time, current/voltage variation.
In conclusions, the system of fuel cell vehicle must be operated in high current range in order to prevent degradation of the MEA in similar load change condition. It is considered to be occurred in oxidation of electrode catalyst under high voltage condition.