bluuebunny/crossref_metadata_2025_split huggingface dataset:
(Invited) Overview of ALD Synthesized MoS2 Thin Films
Authors: Baumgart Helmut , Zeng Min , Zhang Kai , Chen Xin , Lin Pengtao , Li Qiliang 
Published: February 2020

Two-dimensional (2-D) layered transition metal dichalcogenides, such as MoS2, WSe2, WS2 and MoSe2 have received a lot of attention recently due to their significant thermal, mechanical, electrical and optical properties. Molybdenum Disulfide (MoS2), one of the transition metal dichalcogenides of interest for 2-D electronic systems, exhibits not only good thermal stability and high melting point, but also excellent light absorption, fast photo-response and sizeable band gap properties. Therefore, these unique properties make MoS2 a promising material for various electronic devices, photodetectors, sensors and catalysis applications.
Many techniques have been developed for the synthesis of MoS2 films on different substrates in recent years, such as pulsed laser deposition (PLD), chemical vapor deposition (CVD). However, very little is known about atomic layer deposition (ALD) synthesis of MoS2 films there are very only few references. ALD technique exhibits self-limiting atomic layer reactions in each introduced ALD cycle. It can accurately control film layer thickness stoichiometry, composition, uniformity, and sharp interface. Furthermore, ALD also can be used to deposit conformal film onto very complex structures.
Here we report on a large-area synthesis of MoS2
films on native oxide covered and hydroxyl terminated Si substrates by using the Savannah 100 ALD system from Ultratech/Cambridge Nanotech. Molybdenum hexacarbonyl (Mo(CO)6) and dimethyldisulfide (CH3SSCH3, DMDS) are employed as the chemical ALD precursors for Molybdenum and Sulfur, respectively. Generally 20 sccm N2 was used as a carrier gas for the precursors. The growth temperature was set at 200, 230 and 250 oC. The crystal structure and chemical composition of WSe2 films were identified by using X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The film morphology was characterized by field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The optical properties of ALD MoS2 films were analyzed by Raman spectroscopy and femtosecond transient absorption spectroscopy.
(Invited) Guided Nanowire Optoelectronics
Authors: Joselevich Ernesto 
Published: February 2020

The large-scale assembly of NWs with controlled orientation on surfaces remains one challenge toward their integration into practical devices. During the last few years we reported the growth of perfectly aligned, millimeter-long, horizontal NWs of GaN [1], ZnO [2], ZnSe [3], ZnTe [4], CdSe [5] and other materials, with controlled crystallographic orientations on different planes of sapphire [1-5], SiC [6], quartz [7], and spinel [8]. The growth directions and crystallographic orientation of the NWs are controlled by their epitaxial relationship with the
substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. As a proof of concept for future applications, we demonstrated the massively parallel “self-integration” of NWs into circuits via guided growth [9]. Here we will show how guided nanowires with complex morphologies and heterostructures can be used for the bottom-up fabrication of nano-optoelectronic devices, including photodetectors, photodiodes and photovoltaic cells [10]
(Invited) UVPL Imaging of 4H-SiC Wafers Along the Device Processing Chain
Authors: Kallinger Birgit , Kaminzky Daniel , Edokam Francis , Berwian Patrick , Friedrich Jochen , Schöck Johannes , Bauer Anton 
Published: February 2020

The market for power electronic devices is highly competitive among devices made of silicon (Si), silicon carbide (4H-SiC) and gallium nitride (GaN) – devices with best performance and reliability at lowest cost will win the race. The price of SiC devices depends on material costs and device production yield, the latter one is limited by technological and structural defects. The device performance is a matter of device design and process technology, and the device reliability is often limited by structural defects. Hence, a characterization method for structural defects in substrates, epiwafers and partially processed wafers is a keystone for development of competitive SiC power electronic devices. Photoluminescence imaging with excitation in the ultraviolet range (UVPL imaging) is an established and powerful technique for such process-accompanying measurements of SiC wafers and we will review its current status in our talk.
In this paper we will compare different UVPL imaging systems available on the market in terms of UV excitation parameters and their PL detectors and discuss their defect detection capabilities. We will present the visibility of structural defects in substrates and epiwafers with regard to their defect-specific PL wavelength as well as excitation wavelength and intensity. SiC epiwafers can be optically stressed by using high excitation intensity, i.e. the conversion of basal plane dislocations to stacking faults can be provoked in unprocessed material as shown in figure 1. This “optical stress test” can be used for material quality testing prior to device processing as well as for scientific questions such as the velocities of dislocations. Furthermore, we will present UVPL images of partially processed SiC wafers revealing technologically induced structural defects. In case of 6.5 kV PIN diodes, the UVPL measurements allow even for prediction of bipolar device degradation.
UVPL imaging provides fast, non-destructive defect characterization on full-wafer area and allows for targeted process optimization in epitaxial growth and device processing as well as for process control measurements. Defects can be tracked from substrate/epiwafer to different stages of device production. Hence, UVPL imaging is of great importance for cost reduction and reliability improvement of SiC power electronic devices.

Figure 1
Evaporation of Water from Gas Diffusion Layers
Authors: Büchi Felix N , Lal Sreeyuth , Lamibrac Adrien 
Published: February 2020

Evaporation of water from gas diffusion layers (GDL) is an important process for water management in polymer electrolyte fuel cells. At temperatures around 80 °C and locally under-saturated conditions, evaporation can have a significant impact on the GDL saturation.
However, little is known about the evaporation rates of water in gas diffusion layers. First ex-situ results at room temperature and for low saturations have been reported in [1]. But for a better understanding of the evaporation rates under real operating temperatures, generic data is required at different temperatures and saturation levels.
Using a special setup, which is applicable also for X-ray imaging of water saturation, water surface and GDL structure, evaporation rates are determined for well-controlled gas speeds (over the surface of the GDL),, temperatures of up to 80 °C and relevant liquid saturations. Figure 1 shows data (including reproducibility) for Toray TGPH 060 material as function of temperature up to 80 °C, at a capillary pressure of 10 mbar and gas velocity of 6 m/s.
[1] I. V. Zenyuk, A. Lamibrac, J. Eller, D. Y. Parkinson, F. Marone, F. N. Büchi, and A. Z. Weber, J. Phys. Chem. C, 2016, 120 (50), pp 28701–28711
Figure 1: Evaporation rates from Toray TGPH 060 gas diffusion layer material a) as function of temperature at a gas velocity of 6.0 m/s; b) as function of gas velocity on the GDL surface at 60 °C; both for a dry gas stream and capillary pressure of 10 mbar; data shown based on preliminary set-up.

Figure 1
A Novel Oems Method to Monitor Direct Ethanol Fuel Cells in Operando
Authors: Hong Yu-Hao , Zhou Zhi-You , Zhan Mei , Wang Yu-Cheng , Sun Shi-Gang 
Published: February 2020

On-line electrochemical mass spectrometry (OEMS) is a powerful analytic tool for electrochemical reactions. To prevent water into ultra-high vacuum, the inlet of mass spectrometer needs to be covered by a hydrophobic film, which makes it to detect only gaseous and volatile species. Herein, we developed a new OEMS technique based on differential vacuum liquid introduction system through a capillary, so that low-volatile species can be detected effectively. Due to small size of the capillary, the MS technique can in-operando
monitor the working status of different components of fuel cells by changing the positions of the capillary. We detected ethanol, acetaldehyde, and acetic acid from a direct ethanol fuel cell, and found significant difference in their concentrations when the sampling inlet capillary was inserted into catalyst layer or flow field.
The result on catalyst layer shows that about 0.3M ethanol in catalyst layer was used, half was converted into acetic acid, the other half was acetaldehyde. However, when we positioned the sampling capillary into graphite flow field outlet. The generated acetic acid was detected at concentration ~0.86 mM at 45 mA cm-2
. This great difference in concentration in catalyst layer or into flow field indicates the mass transfer is not good of the MEA and the serpentine flow field. In addition, we observed considerable O2 crossover from cathode to anode to chemically oxidize ethanol into acetic acid, thus lead to ethanol waste and formation of mixed potential.

Figure 1
(Invited) Methods for Understanding and Mitigating High Current Density Performance Losses in Low Loaded Pt-Based PEMFCs
Authors: Neyerlin Kenneth Charles , Anderson Lawrence , Chuang Abel , More Karren L. , Ahluwalia Rajesh , Mauger Scott A , Bender Guido , Pivovar Bryan S , Gu Wenbin , Kumaraguru Swami , Kongkanand Anusorn , Kocha Shyam S. 
Published: February 2020

Significant advances in the development of electrocatalysts that exceed the DOE target of 440 mA/mgPt (H2/O2, 0.90V, 80oC, 100% RH, ptotal=150 kPa) for ORR activity have placed proton exchange membrane fuel cells on a promising path towards achieving the DOE target of a 0.1 mgPt/cm2
elec cathode loading by 2020. However, unanticipated voltage losses that manifest at high current density and low Pt loading have prevented the attainment of the 0.125 gPGM/kWrated 2020 target.1 While some of the observed voltage losses for low-loaded Pt electrodes (< 0.1 mgPt/cm2
elec) has been attributed to oxide dependent kinetics that manifest at the lower iR-free potentials (< 0.75V),2 it is believed that a significant portion of this unanticipated loss stems from an oxygen transport resistance local to or associated with electrochemically accessible Pt surface area.2-4 While the exact cause of this phenomenon remains unknown, studies have demonstrated that this loss both: 1) scales with total Pt surface area4 and 2) can be associated with the incorporation of ionomer into the cathode electrode.3 As such, this loss has been termed the local Pt resistance (RO2
Pt).
In this work we have applied a variety of in-situ electrochemical diagnostics across a range of material sets (e.g. electrocatalysts, carbon supports, ionomers, and membranes) in order to understand their impact on high current density operation in low-Pt loaded electrodes. Values derived for RO2
Pt will be compared to those determined from ex-situ measurements in an effort to elucidate the fundamental reasons for the observed performance loss.
Additionally, parallel approaches involving novel and state-of-the-art, electrocatalysts, electrodes and MEA designs aimed at mitigating performance loss at high current density and low Pt loading will be presented.

Acknowledgements

This work was funded through the DOE FC-PAD Consortium and by the U.S. Department of Energy under CRADA #CRD-14-539.

References

1. https://energy.gov/eere/fuelcells /doe-technical-targets polymer-electrolyte-membrane-fuel-cell-components
2. T. A. Greszler, D. Caulk, and P. Sinha, Journal of the Electrochemical Society,
159 (12), F831-F840 (2012).
3. H. Iden, S. Takaichi, Y. Furuya, T. Mashio, Y. Ono, and A. Ohma, Journal of Electroanalytical Chemistry,
694 37-44 (2013).
4. A. Kongkanand and M. F. Mathias, Journal of Physical Chemistry Letters,
7 (7), 1127-1137 (2016).
Quantitative Measurement of Local Fuel Cell Electrode Potentials
Authors: Spendelow Jacob S , Dumont Joseph H , Mukundan Rangachary , Castanheira Luis , Hinds Gareth , Borup Rod L. 
Published: February 2020

Measurement of local electrode potentials in operating fuel cells can provide important information about spatial variations in performance and durability. These measurements are useful for improved understanding of localized effects associated with contaminants, startup/shutdown, electrode irregularities, membrane cracks/holes, and fuel starvation. Accurate measurement of local electrode potentials requires incorporation of multiple reference electrodes into the fuel cell hardware. By connecting external reference electrodes to the outside of the MEA using ionomer salt bridges, local potential measurements can be obtained without disturbing cell operation [1].
Contaminants such as CO can cause strong localized electrode poisoning in operating fuel cells. Incorporation of multiple reference electrodes has enabled measurement of the distribution of anode losses in a fuel cell exposed to CO, as depicted in Fig. 1. Similar techniques can be used to measure and deconvolute losses associated with the anode and cathode. However, robust ionic connectivity between the MEA and the external reference electrodes is required to enable quantitative potential measurements. Excessive impedance in this ion-conducting pathway can result in slow response times, reference potential drift, and non-quantitative measurements. This work describes improved techniques for integrating external reference electrodes, allowing more accurate quantification of local electrode potentials.
Fig. 1. Change in cell voltage and local anode potential (vs SHE) during exposure to CO gas (~400 s) and removal of CO (~1,800 s).
References

E. Brightman, G. Hinds, Journal of Power Sources 267 (2014) 160-170.

Acknowledgements
This research is sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and the U.K. National Measurement System.

Figure 1
Measurement of Through-Plane Humidity Distribution inside an MEA by Thin Film Sensor
Authors: Minami Ryotaro , Hasegawa Noriyoshi , Araki Takuto 
Published: February 2020

A Proton exchange membrane fuel cell (PEMFC) is attracting attention as a clean and new power source system. However, there are several problems for commercial uses. One of the problems is water management inside the PEMFC. Too much water inside the PEMFC disturbs mass transport and leads to lower power density, on the other hand, proton exchange membrane (PEM) and an ionomer in a catalyst layer (CL) need water to maintain proton conductivity. In addition, water vapor and liquid water flow coexist inside the PEMFC because the operating temperature is lower than 100 degrees. Therefore, understanding water transport inside the PEMFC is difficult and a key to improve the cell performance. Especially, it has recently been noticed that a micro porous layer (MPL), which is inserted between a CL and a gas diffusion layer (GDL), controls the water transport and improve cell performance, but the role of the MPL is not clarified yet.
For understanding water transport through the membrane electrode assembly (MEA), relative humidity (RH) is an important factor. So, we developed a thin film humidity sensor (TFHS), which is capacitive type, for measuring RH inside the MEA. In our previous study[1], width of TFHS was 0.35mm. However, wider sensor prevents the mass transport more. In this study, we developed thinner TFHS whose width was 0.1mm.Capacitive type TFHS was developed with MEMS technology. Fig. 1 shows an overview and schematic cross section of TFHS. The humidity sensor consisted of Parylene, Ni and Au. Parylene, that is polymer material, was applied as an insulating material and a sensing material to obtain a uniform and homogeneous film. In addition, hysteresis of it was very small and a uniform and homogeneous film was evaporated. Ni was deposited as an electrode and it was thin enough to obtain water vapor permeability to sensing material. Au was deposited as reinforcement so as not to break the sensor when inserted inside the PEMFC.
In situ measurements with TFHS was investigated with supplying RH20% and RH60% gas. Flow rate of gases was 200 and 500 NmL/min respectively, and the separator temperature was set to 60oC. In this experiment, TFHS was inserted at the CL/MPL interface under the rib. In situ measurements were conducted three times at the each condition, and reproducible results were obtained. The results implies that water is mainly transported as vapor phase near the MPL. In the RH60 condition, liquid water may start to accumulate under the rib from 1.0 A/cm2 because RH change is almost saturated near the RH100%.
Reference
[1]. J. Tsujikawa, R. Minami, and T. Araki, ECS transactions, 69, 471-476, (2015)

Figure 1
Experimental and Numerical Investigation of Humidity Effect on Performance of PEM Fuel Cells
Authors: Liso Vincenzo , Al Shakhshir Saher , Nielsen Mads Pagh 
Published: February 2020

In this study, we conducted an experimental test to evaluate the performance of a PEM single cell at different humidity level of the reactants (i.e. hydrogen and air). The motivation was to use the test dataset to validate a simple numerical model which predict the I-V curve of the cell when the humidity is varied. We conducted the test using a 50cm2 single cell, five-layer membrane electrode assembly composed of a Nafion 211 membrane and carbon paper as GDL. Relative humidity (RH) was varied alternatively between 50 and 100 percent both at anode and cathode side. We found that the best cell performance was obtained when RH was fixed at 100% both at anode and cathode side. On the other hand, cell performance was particularly affected when relative humidity was reduced at the cathode side. This can be explained by the decrement of water diffusion from the cathode to anode and therefore a loss in electrical conductivity. The test also showed that humidity mainly affects Ohmic losses rather than activation or overpotential losses. Ohmic losses are mainly related to water hydration in polymer electrolyte membranes.
The cell performance model considers only relationships for electro-osmotic drag, membrane ionic conductivity and diffusion coefficient of water in the membrane. Hydraulic permeability was neglected as the pressure difference between anode and cathode side was minimal during the test. Temperature, mass flow rate, and pressure were also important to estimate the water and mass balance. We fitted the model parameters to the test results and find out that cell ohmic resistance was the main relationship to be modulated at different humidity levels. In fact, there is a close link between water uptake, cell resistance, and membrane conductivity and thickness.
The model simplicity was essential because the code was aimed to be implemented in a larger fuel cell system model. The model will come useful to develop a strategy for water management.
Analysis of Ionomer Distribution in Catalyst Layers by Two-Stage Ion-Beam Processing
Authors: Suzuki Takahiro , Koyama Takamasa , Tsushima Shohji 
Published: February 2020

Mass transport properties of catalyst layers (CLs) for proton exchange membrane fuel cells (PEMFCs) strongly depends on its structure. The ionomer is a proton-carrier material in the CLs. On the other hand, the gas transfer is prevented by the ionomer coated on a catalyst. Ionomer distribution in the CLs, therefore, have gathered much attention. Some papers have suggested methods to analyze the ionomer distribution [1-4]. However, quantitative validation is not enough so far. In addition, these methods especially focused on a microscopic local ionomer distribution. From the point of view of mass transfer, overall through-plane ionomer distribution is important as well as the microscopic distribution. In this study, a novel visualization method of the overall through-plane ionomer distribution in the CLs by using a two-stage ion-beam processing is suggested.
The first stage of the two-stage ion-beam processing is the formation of flat and smooth cross-section by a broad ion beam. The second stage is the selective removal of the ionomer in the catalyst layer by a focused ion beam. Scanning ion microscopic (SIM) images were obtained after the first and second stage. Smoothing filter was applied on the obtained SIM images. The SIM image after the second stage was then conducted threshold by Otsu method and a carbon distribution map was obtained. A pores distribution map is obtained from a combined image of the SIM image after the first stage and the carbon distribution map. The threshold was determined by a p-tile method with ionomer to carbon ratio (I/C). Ionomer distribution was extracted from the carbon and pores distribution maps.
The ionomer distribution of the catalyst layer with I/C 2.0 was analyzed as shown in Figure 1. A catalyst coated membrane was fabricated by a doctor-blading and decal-transfer method [5]. Lower side in the figures is the interface with a polymer electrolyte membrane (PEM) and upper edge is the bottom in the fabrication process before decal-transferring. The through-plane ionomer distribution is not uniform. Thin ionomer layer was formed at the interface with the PEM. This can be because of deposition of ionomer during the drying process.
Acknowledgments
This work was supported by JSPS KAKENHI Grant Number 15H03932.
References
[1] K. More, R. Borup, K. Reeves, ECS Trans., 3 (2006) 717-733.
[2] L.Guétaz, M. Lopez-Haro, S. Escribano, A. Morin, G. Gebel, D. Cullen, K. More, R. Borup, ECS Trans., 69 (2015) 455-464.
[3] S. Komini Babu, H.T. Chung, P. Zelenay, S. Litster, ACS Applied Materials & Interfaces, 8 (2016) 32764-32777.
[4] D. Susac, V. Berejnov, A.P. Hitchcock, J. Stumper, ECS Trans., 41 (2011) 629-635.
[5] T. Suzuki, H. Tanaka, M. Hayase, S. Tsushima, S. Hirai, Int. J. Hydrogen Energy, 41 (2016) 20326-20335.

Figure 1
(Invited) Fuel Cells for Naval Applications
Authors: Anderson Michele L 
Published: February 2020

The high efficiency and scalability of fuel cell systems make them attractive for a range of Naval applications that span sensors and unmanned systems to microgrids and ships. Anticipated benefits include significant endurance enhancements that extend range and on-station mission capability and reduced use of fossil fuels to increase energy independence and energy security. Prior Navy and other agency investments in fuel cell stack technology, reactant storage and processing systems, and balance of plant components has enabled the design, fabrication and integration of practical fuel cell systems. These systems have demonstrated superior performance to other technologies and the ability to fill capability gaps that other technologies are unable to address. This paper will discuss the status of fuel cell systems for Naval applications and the challenges and opportunities for Fleet and Marine Corps implementation.