bluuebunny/crossref_metadata_2025_split huggingface dataset:
Composite of Two-Dimensional Titanium Carbonitride Mxene and Nano-Sulfur As Cathode for Li-S Batteries
Authors: Naguib Michael , Wang Hui , Liang Chengdu , Nanda Jagjit 
Published: February 2020

In 2011, a new family of two-dimensional, 2D, transition metal carbides and carbonitrides called MXenes were produced for the first time by etching atomically thin metal layers from ternary layered materials called MAX phases. The latter have a composition of Mn+1AXn, where M stands for early transition metal, A is a metal that belongs to groups 13 and 14 in the periodic table, X is carbon/nitrogen, and n = 1, 2, or 3. So far, nine different MXenes were reported MXenes were reported, viz., Ti3C2, Ti2C, Nb2C, V2C, (Ti0.5,Nb0.5)2C, (V0.5,Cr0.5)3C2, Ti3CN, Ta4C3, and Nb4C3. In addition, many more are predicted to be stable. Since the synthesize process of MXenes is carried in F-contained aqueous systems, the surfaces of the as synthesized material are terminated with a mixture of moieties including O, OH, and F. Bare MXenes are predicted to be metallic conductors while terminated ones are predicted to be semiconductors with relatively small band gaps. Titanium carbonitride is predicted to be an exception since it is predicted to metallic even after termination.
In this work we used a simple mixing approach to prepare Ti3CN/nano S composite with a high sulfur content of ~75 wt.%, and high mass loading. Analysis for their performance in Li-S batteries and the effect of mass loading in addition to the composite material characterization will be presented.
Rational Design of Cathode Structure in Li-S Batteries
Authors: Chen Liwei 
Published: February 2020

As one of the most intensely investigated technologies in the electrochemical energy storage beyong Lithium-ion, lithium-sulfur (Li-S) batteries have observed rapid improvements in their properties in recent years. This presentation reports recent progresses in the understanding and development of Li-S cathode design.
One of the technological bottlenecks in Li-S batteries is the insulating nature of elementary sulfur and the discharging products Li2S2 and Li2S. The slow charge transfer process in the cathode structure thus imposes restrictions on the degree of sulfur utilization, initial capacity and the rate performance.The nano-size effect in Li-S cathodic charge transfer has been thoroughly investigated. S nanoparticles with diameters below 50 nm were fabricated via a membrane-assisted precipitation method. With conductive PEDOT coating, these S nanoparticles show high specific capacity and improved cycle stability. The study indicated that nanosized sulfur required shorter charge diffusion distance and thus showed higher capacity (Sci.Rep.2013,3,1910). Further understanding of the nano-size effect needs better size-controlled S materials, which were obtained using the sulfur-amine chemistry recently developed (Chem. Commun., 2014,50,1202). A series of mono-dispersed S nanoparticles with diameters ranging from 5 nm to 150 nm were employed to investigate the size-dependent charge transfer process and the influence on device performance. Experimental results revealed much improved specific capacity, rate performance and cycle stability upon the reduction of S nanoparticle size. Importantly, the 5 nm S particles showed an initial capacity of 1670 mAh/g, which is the theoretical limit of sulfur (Nano Lett. 2015, 15, 798). This work demonstrates that the high theoretical performance can indeed be realized. The understanding of S cathode charge transfer will help future cathodic design in the Li-S technology.
References:
1. Hongwei Chen, Changhong Wang, Weiling Dong, Wei Lu, Zhaolong Du and Liwei Chen* “Mono-dispersed Sulfur Nanoparticles for Lithium-Sulfur Batteries with Theoretical Performance”
Nano Letters
, 15, 798-802 (2015)

2. Changhong Wang, Hongwei Chen, Weiling Dong, Jun Ge, Wei Lu*, Xiaodong Wu, Lin Guo and Liwei Chen*,”Sulfur-amine Chemistry Based Synthesis of Multi-walled Carbon Nanotube-Sulfur Composites for High Performance Li-S Batteries”,
Chem. Commun.
, 50, 1202 – 1204 (2014)

3. Hongwei Chen, Weiling Dong, Jun Ge, Changhong Wang, Xiaodong Wu, Wei Lu and Liwei Chen*, “Ultrafine sulfur nanoparticles in conducting polymer shell as cathode materials for high performance lithium/sulfur batteries”,
Scientific Reports
, DOI:10.1038/srep01910 (2013).
(Invited) High Power SiC Power Processing Unit Development
Authors: Scheidegger Robert , Santiago Walter , Bozak Karin E , Piñero Luis R , Birchenough Arthur 
Published: February 2020

The NASA space flight program can be divided into manned and unmanned programs with each contributing and ultimately setting the stage for advanced space transportation systems, future space habitats, and spacecraft that can send humans deep into the solar system. Most spacecraft use solar arrays for the main source of power except for those that had relatively short flight duration, or for deep space probes that were designed for very long flight duration. As the vision for space exploration evolves; spacecraft, space stations, and space habitats power needs increase exponentially thus creating the need for more efficient and revolutionary power technologies. NASA is investing in the most cutting-edge power technologies to enable the human space exploration beyond the earth vicinity.
The NASA Human Exploration Framework Team (HEFT) recommended demonstrating Spacecraft buses with increasing power & decreasing specific mass to enable advanced electric and plasma propulsion spacecraft that will decrease trip times to Mars and beyond. Primary or auxiliary electric propulsion systems provide many potential benefits to the spacecraft developer when compared to chemical propulsion systems. The high specific impulse provided by electric thrusters substantially reduces the amount of propellant required to achieve a given mission. This mass savings can be leveraged in many ways including reduction of spacecraft mass, reduced launch vehicle requirements, increased on-orbit lifetime, or increased payload.
Recent advances in solar array technology that produce more efficient and lighter solar arrays have enabled the notion of high power spacecraft as large as 300kW to MW class. These high power systems will require high power, higher voltage distribution and power converters all requiring high voltage parts.
In response to these technology needs, NASA GRC has been developing a solar electric power processing unit that would operate from a nominal 300VDC spacecraft bus. The unit utilizes 1200V commercial silicon carbide power MOSFETs and high voltage SiC schottky diodes. This developments main objective is to demonstrate the capability of high power, high voltage, and power processing for a 2000-second specific impulse Hall Effect Thruster. To date this development has demonstrated the mass and efficiency gains which using silicon carbide at high voltage provides illustrating the need for radiation hardened space rated components.
(Invited) Radiation-Induced Defect Mechanisms in GaN HEMTs
Authors: Koehler Andrew D. , Anderson Travis J , Specht Petra , Weaver Brad D , Greenlee Jordan D , Tadjer Marko J , Shahin David I , Hobart Karl D , Kub Francis J 
Published: February 2020

Gallium nitride (GaN) high electron mobility transistors (HEMTs) have been recognized as a key enabling technology for power amplifiers and high power converters for next-generation military and civilian systems. The superiority of GaN over Si benefits applications such as wireless communications, and electronic warfare attack, protection, and support systems. Also, GaN HEMTs also are excellent candidates for applications in radiation intensive environments such as counter weapons of mass destruction system survivability and space systems. The objective is to investigate radiation-induced defects on GaN HEMTs, leading to understanding about the defect formation mechanisms and their impact on performance and reliability of GaN HEMTs.
AlGaN/GaN HEMT epitaxial layers were grown on Si substrates by metal organic chemical vapor deposition (MOCVD). The GaN buffer layer is >1 μm and the AlxGa1-xN (x=27%) barrier layer is 17 nm thick. HEMTs were exposed to 2 MeV protons at room temperature in a tandem Van de Graaff accelerator iteratively, up to a fluence of 6 x 1014 H+/cm2. I-V, C-V, and Hall measurements were performed to characterize the radiation-induced changes in HEMT performance, and high-resolution transmission electron microscopy (HRTEM) is used to identify radiation-induced defect mechanisms.
Bombardment of 2 MeV protons causes a uniform distribution of vacancies, interstitials, and Frenkel pairs, with minimal larger damage cascades. The stopping range of 2 MeV is much larger than the epitaxial GaN thickness. The AlGaN/GaN interface of an as-fabricated HEMT has an abrupt junction between the AlGaN and GaN. However, after irradiation, atomic recoil of displaced atoms results in intermixing of the AlGaN and GaN layers. The irradiated HEMT has an increased surface roughness. Since the AlGaN/GaN interface is where the two-dimensional electron gas (2DEG) forms, the surface roughness impacts the 2DEG mobility. The Hall mobility measured on Van der Pauw structures begins to degrade at a fluence of 5 x 1013 H+/cm2. This mobility degradation is attributed to the increase in surface roughness between the AlGaN and GaN layers. In addition, TEM analysis revealed where certain threading dislocations exist, that after proton irradiation, Ni from the Schottky Ni gate diffuses down into the dislocation. This impacts the HEMT performance by changing leakage paths, but also, voids were observed in the Ni gate, where the nickel had diffused into the semiconductor at the gate edges.

Figure 1
The Effect of Carbon-Based Substrates onto Non-Precious and Precious Electrocatalytic Centers
Authors: Sreekuttan M. Unni , Campos-Roldan Carlos A. , Mora-Hernandez Juan M. , Luo Yun , Estudillo-Wong Luis A. , Alonso-Vante Nicolas 
Published: February 2020

Novel catalysts based on precious and non-precious catalytic centers, as well as support materials are currently under investigation all over the world. The improvement of oxygen reduction reaction (ORR) on Pt-based catalysts is strongly related to its dispersion and interaction with the substrates. [1],[2] The tolerance to small organics is also influenced by the electronic modification of the catalytic center itself or induced by the substrate.[3] A variety of carbon-based substrates such as carbon nanotubes (CNT), graphite (HOPG), graphene, carbon nanohorns (CNH),[4] various graphitic domains of MWCNT, and non-carbonaceous based materials, such as metal oxides (Titania, doped-Titania, Yttria, Ceria, etc.) [5],[6]can be used as both catalyst and catalyst support materials.
On the way to further explore the substrate effect on non-precious electrocatalyst, CoSe2was supported on Carbon, CNH and nitrogen doped nanohorn (NCNH). The activity of non-Pt catalysts was found to be strongly dependent on the surface composition of substrate materials. The improved interaction of modified substrate and the metal centers (either precious or non-precious) modify the active reaction centers of the electrocatalyst, which in turn boost the catalytic activity towards molecular oxygen reduction with minimum overpotential. The analysis indicates that the improved activity measured in half-cells was validated in microfluidic fuel cells.
Acknowledgements
Authors acknowledge the partial support of the European Union’s Seventh Framework Programme (FP7/2007-2013) for the Fuel Cell and Hydrogen Joint Technology Initiative under grant agreement nr 303492 CathCat. SM U acknowledges Raman-Charpak fellowship, JMMH, J.M.M.-H., C.A. C.-R., and L.A. E-W acknowledge financial support from CONACYT-Mexico Nr.: MX-351743; MX-561206; MX-234729.

[1] Y. Luo, N. Alonso-Vante, Electrochim. Acta, doi: 10.1016/j.electacta.2015.04.098.
[2] J. Ma, A. Habrioux, Yun Luo, G. Ramos-Sanchez, L. Calvillo, G. Granozzi, P. B. Balbuena, N. Alonso-Vante, J Mater. Chem. A, Submitted; J. Ma, A. S. Gago, W. Vogel, N. Alonso-Vante, Chemcatchem, 2013, 5, 701-705.
[3] J. Ma, A.S. Gago, W. Vogel, N. Alonso-Vante, Tailoring and Tuning the Tolerance of a Pt Chalcogenide Cathode Electrocatalyst to Methanol, ChemCatChem, 5 (2013) 701-705.
[4] Sreekuttan M. Unni, J. M. Mora-Hernandez, Sreekumar K., N. Alonso-Vante, ChemElectroChem, Submitted.
[5] C.Liu , Y.Wei and K.Wang, Chem.Comm., 2010, 46, 2483-2485.
[6] Y. Luo, A. Habrioux, L. Calvillo, G. Granozzi, N. Alonso-Vante, ChemCatChem, doi: 10.1002/cctc.201500130.
Helox Study of Transport Limitations in Non-PGM Cathodes for PEM Fuel Cells
Authors: Pavlicek Ryan , Leonard Nathaniel , Calabrese Barton Scott , Mukerjee Sanjeev 
Published: February 2020

A study of mass transfer limitations in a non-PGM cathode MEA were undertaken by studying the limiting current as a function of flow rate with varying oxygen carrier gases. Previous reports have indicated that the mass transfer coefficient for oxygen in the gas phase and electrolyte phase can be elucidated by studying the limiting current behavior in the presence of carrier gases of varying molecular weight. [1] We apply this approach to a non-PGM cathode using helium, nitrogen, and argon as carrier gases. Under conditions of low fractional O2 conversion, mass transfer limitations were found to exist mainly in the catalyst layer and flow field, not in the gas diffusion layer. Combined with mercury porosimetry studies to elucidate the macropore size distribution, this data is modeled using a previously-develop code [2] to provide an estimate of the gas-phase mass transfer coefficient, and by extension the fraction of hydrophobic, gas-phase pores in the catalyst layer. 1. T. V. Reshetenko and J. St-Pierre, J. Electrochem. Soc., 161, F1089 (2014). doi:10.1149/2.1021410jes. 2. N. D. Leonard, K. Artyushkova, B. Halevi, A. Serov, P. Atanassov, and S. Calabrese Barton, J. Electrochem. Soc., submitted (2014).

Figure 1
PEM Electrolysis: Ready for Impact
Authors: Mittelsteadt Cortney K 
Published: February 2020

The need for energy storage has never been so pressing; due to their increasing proliferation and the intermittent nature of wind and solar power sources, many regional grids can no longer accept more of these sources. Electrolysis of water using polymer electrolyte membranes (PEMs) has long held the promise of large-scale energy storage, needing only the simplest of reactants, water. More importantly it is perfectly suited for storing excess energy from these sources as they can handle tremendously high ramp rates as well as many start-stop cycles. The adoption of PEM electrolyzers has been impeded due to the headwinds of high capital cost, (including expensive high precious metal content), poor efficiency, and the empty promises made by the hydrogen economy from many years. Numerous incremental improvements, and increasing economies of scale over the past 10 years have led to very large decreases in the cost of PEM electrolysis. The complexity of the electrolyzer stack and precious metal content have been decreased, while simultaneously increasing lifetime. Increasing operating temperature has had the largest impact on efficiency and output, while improved membrane supports have allowed for higher differential pressures; reducing the need and cost of ancillary systems (see Figure 1). These advances have had the combined effect of reducing the cost of hydrogen from PEM electrolysis by nearly an order of magnitude, allowing it to approach the cost of pumped hydro, the standard in large-scale energy storage. Subsequently, whereas only a few years ago there were no commercial PEM electrolyzer stacks in the 1 MW range, today no fewer than five companies are at or near launch of MW electrolyzer systems.
Figure 1. Total cost of hydrogen as a function of plant size and operating current

Figure 1
Gas Permeation Study in Thin and Ultra-Thin Ionomer Films
Authors: Weber Adam Z , Tesfaye Meron , McCloskey Bryan D 
Published: February 2020

Significant advances in the performance of polymer-electrolyte fuel cells (PEFCs) can be attributed to perfluorinated sulfonic-acid (PFSA) ionomers like Nafion and 3M-ionomer. They serve both as electrolyte membranes and as conductive binders in electrode structures. In electrode structures, ionomers are major contributors to unexplained large mass-transfer over potentials in PEFCs [1, 2]. Thin (< 1 µm) and ultra-thin (< 0.025 µm) ionomer films in the catalyst layer serve as ionic charge carriers to catalytic sites. Low resistance to gas transport for increased supply of reactants to catalytic sites is required for optimal performance. However, increase in transport resistance of thin-film ionomers is observed indicating deviation from bulk (> 1 µm thickness) ionomer properties [2]. This resistance is further exasperated with decrease in platinum loading, which is an essential consideration for decrease in cost of PEFCs [3]. Much has been done to shed light on the aberration of ionomer thin-film from bulk in other properties such as proton conductivity, morphology and water uptake. This has allowed many to attribute the increase in local mass transfer resistance at the catalyst layer to the decrease in permeability of thin-film ionomers as compared to that of bulk. However, direct measurement of thin-film ionomer membranes has not been done.
Substantial amount of work has been done to study gas transport and permeation in bulk ionomer films [4]. However, much remains to be explored in thin and ultra-thin ionomer films. In this talk, we present data for gas permeation in thin and ultra-thin PFSA films. Thin ionomer films are supported on well studied, highly permeable rubbery poly(dimethylsiloxane) (PDMS). O2, N2 and H2 permeability of the ionomer film is measured using a constant-volume, variable-pressure permeation system. Experimental results and variations of permeability of thin films from bulk films as a function of temperature and thickness will be presented. In addition, to tackle the complex and not fully understood effect of water in gas transport, gas permeability as a function of humidity will also be presented.
Acknowledgement
We would like to thank Norman Su for providing assistance in permeation system assembly and operation. This work made use of facilities at the Biomolecular Nanotechnology Center at University of California, Berkeley and the Molecular Foundry at Lawrence Berkeley National Laboratory. This work was funded in part by the University of California Chancellor's Graduate Fellowship and the Assistant Secretary for Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231.
References
[1]Nonoyama, N. et al (2011). Journal of Electrochemical Society. 158(4)
[2]Gresler, T. et al (2012). ). Journal of Electrochemical Society. 159(12)
[3]Weber, A. et al (2012). Journal of Materials Chemistry A. 2,17207
[4] Kocha, S. et al (2006).AIChE Journal. 52(5)
Thermo-Stable Non-PFSA Cation-Conductive Fluoropolymers for Medium-Temperature PEMFCs
Authors: Xu Shaoyi , Jiang Rong , Gao Yong 
Published: February 2020

Medium-temperature PEMFCs operated at a temperature between 100 and 180 oC can be used as an alternative to phosphoric acid fuel cells—avoiding the leach and loss of phosphoric acid electrolytes from the cells during fuel cell operations. At an elevated temperature, Pt catalysts are less sensitive to carbon monoxide poisoning—allowing the use of low-quality hydrogen fuels produced from reforming processes. However, the thermal stability of perfluorosulfonic acid (PFSA, e.g., NafionTM) membranes and PFSA catalyst binders is the bottleneck problem for raising the operation temperature of PEMFCs above 100 oC. It was reported that NafionTM membranes can only have a lifespan of several hundred hours under humidified conditions at 130 oC.
Recently, we designed, synthesized and tested a group of non-PFSA fluoropolymers as cation-conductive electrolytes via a novel C-C coupling reaction. Heteroaromatic groups can be covalently attached to the side chains of a PFSA—with no acid or base labile sulfonyl groups. The protonated form of such fluoropolymers can transport protons and can be fabricated into thin films. An example of non-PFSA polymers examined in our laboratories is shown in the enclosed figure. Many of non-PFSA fluoropolymer membranes showed ion conductivities, mechanic strengths, and electrochemical properties, compatible to NafionTM membranes, but with much improved thermal stabilities. Some preliminary test results of a prototype membrane are shown in table 1:

Table 1. Comparison of our prototype membrane vs. commercial N-117 membrane.

Thickness (µm)
In-Plane Conductivitya
(in de-ionized water, 25 oC)
IEC (meq/g)
Water uptake
(%,25 oC)

N-117
183
77 mS/cm
0.9
17

Our Prototype
193
71 mS/cm
0.7
18

*Four-electrode AC impedance measurements.
Most remarkably, our prototype membrane showed much improved thermal stability against N-117. Within three hours, a commercial N-117 membrane quickly lost its ion conductivity in a heated water bath of 150 oC in a pressure reactor, while our prototype membrane showed no sign of the reduction of its conductivity after five months.

Figure 1