bluuebunny/crossref_metadata_2025_split huggingface dataset:
Evaluation of Hvpe GaN Layers Grown on Ammonothermal GaN Substrates By Synchrotron X-Ray Topography
Authors: Wu Shuang , Raghothamachar Balaji , Dudley Michael , Freitas Jaime A. , Sochacki Tomasz , Bockowski Michal 
Published: February 2020

For obtaining low defect density GaN substrates with controllable electronic properties for high performance electronic and optoelectronic devices, quasi-bulk growth by hydride vapour phase epitaxy (HVPE) on ammonothermal grown GaN substrates appears to be a feasible approach [1]. Synchrotron X-ray topography (white beam and monochromatic beam) has been employed to characterize dislocation configurations in such GaN substrates that were grown by HVPE on ammonothermal GaN substrate, and then slicing the substrate. Threading dislocations and basal plane dislocations (BPDs) are observed in these GaN substrates. Compared with GaN grown on sapphire and silicon carbide substrates [2], HVPE GaN on ammonothermal GaN [3] is obviously of higher crystalline quality with lower defect densities. Threading screw dislocation (TSD) density was measured to be about 880 cm-2 while threading edge dislocation (TED) density was about an order higher (~ 5824 cm-2). The distribution of BPDs which are induced by deformation, was found to be highly non-uniform with most regions of the wafer nearly BPD-free as well as high densities concentrated near one edge. Also, besides the surface features, like the surface scratches, arrays of threading dislocations can be seen from the transmission and grazing incidence topographs, which likely originate from scratches on the surface of the ammonothermal substrate. Correlation of the defect distributions with the HVPE growth process and their implication for devices will be discussed.
References
[1] J. A. Freitas, Jr., J. C. Culbertson, N. A. Mahadik, T. Sochacki, M. Bockowski, and M. Iwinska, Cryst. Growth Des., 15, 4837 (2015).
[2] Tian. Y, et al. Direct growth of freestanding GaN on C-face SiC by HVPE. Sci. Rep. 5, 10748; doi: 10.1038/srep10748 (2015).
[3] S. Sintonen. Synchrotron radiation x-ray topography of crystallographic defects in GaN [D]. Finland: School of Electrical Engineering, 2014. 15-24
Sensitivity of Breakdown Voltage of Power Transistors to Dopant Impurities
Authors: Zhu Chen , Andrei Petru 
Published: February 2020

To increase the breakdown voltage (BV) and, in the same time, decrease the on-state voltage (OSV) of wide-band gap transistors it is essential to carefully design the doping profile of these transistors in order to decrease the resistivity of the material during the on-state current flow and decrease the impact ionization generation rate in regions with high electric field during off-state. In general, the two problems depend on each other and there is trade-off between the increasing the BV and decreasing the OSV [1]. In this article we will analyze this tradeoff by using the formalism of doping sensitivity functions of the BV and OSV. These functions show how sensitive the BV and OSV are to infinitesimal variations of the acceptor or donor impurities inside the semiconductor. For instance, the doping sensitivity function of the BV, gBV(r), shows how much the BV increases by adding one donor impurity at location r inside the device. Similarly, the doping sensitivity function of the OSV, gOSV(r), shows how much the OSV increases by adding one donor impurity at location r inside the device. As we have shown in [2, 3], the doping sensitivity functions are instrumental in the optimization of WBG power devices.
The doping sensitivity functions of the BV and OSV can be computed efficiently by using the adjoint method. Once these functions are computed they can be coupled with gradient-based optimization methods to estimate the optimum doping profiles for acceptors, Na(r), and donors, Nd(r), in order to increase the BV and decreases the OSV. At the conference we will describe the numerical implementation of the adjoint method for the optimization of WBG power devices and present simulation results for a vertical and horizontal SiC MOSFETs. More importantly, we will show that in general, in wide-band-gap MOSFETs and IGBTs, the doping sensitivity functions of the BV and OSV, gBV(r) and gOSV(r), respectively, and are not linearly dependent. Therefore, from a mathematical point of view, the BV and OSV are quantities that can be controlled independently by changing the doing profiles inside the transistor. Different equivalent ways to define and compute the doping sensitivity functions of BV and OSV will also be presented. The different definitions of the doping sensitivity functions will be compared to each other and the computational complexity will be discussed in each case.
[1] J. Baliga, Fundamentals of Power Semiconductor Devices: Springer, 2008.
[2] P. Andrei, "Using Doping Sensitivity Functions to Optimize Power Transistors," IEEE Workshop on Microelectronics and Electron Devices (WMED), Boise, ID, 2015.
[3] C. Zhu and P. Andrei, “Adjoint Method for Increasing the Breakdown Voltage and Reducing the On-State Resistance in Wide Band”, 231 ECS Meeting, New Orleans, 2017.
[4] P. Andrei and L. Oniciuc, "Suppressing Random Dopant-Induced Fluctuations of Threshold Voltages in Semiconductor Devices," Journal of Applied Physics, vol. 104, Art. No. 104508, 2008.
Novel Fuel Cell MEA Based on Pt-C Deposited by Magnetron Sputtering
Authors: Ostroverkh Anna , Johanek Viktor , Dubau Martin , Kus Peter , Veltruska Katerina , Vaclavu Michal , Fiala Roman , Smid Bretislav , Ostroverkh Yevhenii , Matolin Vladimir 
Published: February 2020

The presented research is focused on Pt-C composite catalysts for proton exchange membrane fuel cell (PEMFC) applications. Nanostructured Pt-C thin films (thickness 25-150 nm, Fig 1d) were deposited on carbon paper gas diffusion layer (Sigracet 29 BC) substrates by magnetron sputtering method. Total Pt loading in the thin films varied in the range 5.5-30 µg/cm2as determined by XPS. These catalysts were examined for their activity in hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR).
A fuel cell with a membrane electrode assembly (MEA) based on Nafion 212 membrane with our Pt-C anode (Pt loading 5.5-30 µg/cm2) and a commercial state-of-the-art Pt/C cathode (Pt loading 300µg/cm2) generated output power up to 1.0 W/cm2 (Fig 1a), value comparable to MEA with commercial powder anode using much higher Pt loading 200-300 µg/cm2. This result demonstrates significant improvement in platinum utilization for HOR due to high dispersion of Pt provided by the magnetron sputtering deposition method. Similarly, a MEA with the commercial Pt/C cathode and our Pt/C thin film (Pt loading 5.5-30 µg/cm2) as anode yielded stable power up to 0.73W/cm2(Fig 1 b), proving its high efficiency for ORR.
Based on the above findings, a MEA with both magnetron sputtered Pt/C electrodes using total Pt loading 20 µg/cm2 was tested with a 4 cm2 cell operating at 70˚C in H2/O2-feed regime for 24 hours. The platinum content on cathode and anode side was chosen 14.4 µg/cm2 and 5.5 µg/cm2, providing highest efficiency for HOR and ORR, respectively. I-V polarization and power density curves were measured with fresh catalyst as well after a 24-hour duration test without any significant lost of efficiency during this period of time (Fig 1c). The magnetron sputtered Pt-C with fine metal dispersion proved to be a promising catalytic material with high Pt utilization for hydrogen-feed PEMFC applications. An additional significant benefit of the Pt-C composite is its lower requirement with respect to feed humidity.

Figure 1
Development of a Common Differential Fuel Cell Test Fixture and Protocols to Expedite Material Development
Authors: Arisetty Srikanth , Rock Jeffrey , Dabel Jeremy , Lakshmanan Balsu , Kjell Eggen , Kennette Claudette , DeBolt Michael , Kinthali Suresh , Cherat Naajein , Naidu Dodla , Roth Mark , Harper Michael 
Published: February 2020

Many test fixtures used for Membrane Electrode Assembly (MEA) development by the fuel cell community have major design drawbacks that both reduce the confidence in data sets and make the sharing of results difficult. A new common hardware design that incorporates the learnings and best practices from various technical leaders is needed to improve build robustness and reduce cell-to-cell variation. Such common hardware can yield improved data quality and accurate comparisons, thus accelerating materials development. Further, creating a common set of differential cell protocols along with corresponding data processing tools can facilitate sharing of results and researcher confidence in data quality. The design objective of the single cell hardware is to ensure MEAs are exposed to uniform conditions during testing of both performance and durability with minimal influence due to the flow field.
In this talk, we will present the design and results from a simplified test hardware that can improve the overall quality of testing and facilitate comparisons from lab-to-lab. The design ensures as much as possible a uniform distribution of all physical quantities such as velocity, pressure, temperature, and oxygen concentration in the flow field. Minimal oxygen concentration gradient across land-channel is ensured in the design with parallel flow fields, having fine land of 0.24 cm, and operating at high (>10) stoichiometry. Pressure drop across the inlet and outlet ports in the hardware is also reduced by designing the ports for minimal restriction. Temperature and compression uniformity is also ensured in the design. The hardware is set to operate with stress control through a pneumatic system integrated into the anode end-plate with the additional capability to regulate compression during operation.
To investigate the uniformity of key paramaters, FEA, CFD, thermal, and ohmic analyses were performed on the designed cell. Hardware replicates will be tested at multiple labs, and an operating procedure will be developed and shared with the community. Assuming successful development, we propose broad adoption of this hardware across the community as a common MEA testing platform.

Figure 1
Using Multi-Scale Modeling to Understand Transports inside PEMFC Under Different Configurations
Authors: Shimpalee Sirivatch , Satjaritanun Pongsarun , Weidner John W. , Hirano Shinichi , Lu Zijie , Shum Andrew , Zenyuk Iryna V. , Ogawa Shohei , Litster Shawn 
Published: February 2020

The objective of this study is to use multi-scale/multi-dimensional model to enhance the development of a fuel cell stack and electrochemical cells with increasing in performance at high current densities. Experimental investigation of oxygen transport has been limited by an inability to resolve water saturation-dependent properties. This work shows the successful in development of a multi-scale calculation technique that incorporates detailed structure of each scale dimension in every component of a fuel cell and simultaneously performed a prediction. The effect of operating conditions under different types of flow-fields, gas diffusion layers, and compression pressures on the transports inside PEMFC will be the primary focus of this work.
Figure 1 gives the idea of multi-scale modeling approach at each component of the fuel cell. The flow-field bipoplar plates and MEA models are calculated using traditional CFD method with exisitng PEMFC model [1-3]. The micro-structure of gas diffusion layer (GDL) and/or microporous layer (MPL) are numerically predicted by Lattice Bolzmann method (LBM) [4-6]. Therfore, during the calcuation, the solutions of each iteration from CFD and LBM are required to simulteneously exchange for the next iteration until all solutions are converged, especially at the interfaces [7]. Figure 2 shows liquid water inside GDL and GDL w/ MPL from the simulation when the macroscopic model is combined with the microscopic model under baseline operation.

References:

S. Shimpalee, D. Spuckler, and J. W. Van Zee J. Power Sources, 167 (2007) 130-138.

S. Shimpalee, M. Ohashi, C. Ziegler, C. Stoeckmann, C. Sadeler, C. Hebling, J. W. Van Zee, Electrochemica Acta, 54 (2009) 2899-2911.

S. Shimpalee, J. of Electrochem. Soc., 161 (2014) E3138-E3148.

U. Frisch, B. Hasslacher, Y. Pomeau, Physical review letters 56 (14) (1986) 1505-1508.

G. R. McNamara, G. Zanetti, Physical Review Letters 61 (1988) 2332-2335.

P. Satjaritanun et al., submitted J. of Electrochem. Soc., (2017).

S. Shimpalee et al., abstract # 2387, Pacific Rim Meeting on Electrochemical and Solid-State Science, October 2, 2016, Honolulu, Hawaii.

Figure 1
The Influence of CO on Hydrogen Evolution
Authors: Banas Charles Joseph , Bonville Leonard J. , Pasaogullari Ugur , Molter Trent M. 
Published: February 2020

Poisoning of platinum catalyst due to adsorption of carbon monoxide (CO) severely hinders the hydrogen oxidation-evolution reactions due to loss of electrochemically active surface area and changes to the reaction kinetics on the electrodes.[1,2] This paper presents electrochemical impedance spectroscopy (EIS) data for hydrogen oxidation and evolution in presence of . The system consists of two isolated 10mil diameter platinum electrodes coated with a thin layer of Nafion electrolyte for ionic conduction. Both electrodes are exposed to the same environment. EIS measurements are taken both before and after 6 hours of CO exposure. At the beginning of testing, two distinct semicircles are visible in the Nyquist plots at both low and high potentials ( and ). Following CO poisoning, EIS measurements show a significant increase in the diameters of both semicircles. While the data obtained at high potential () shows an amplification of the original Nyquist plot, the measurements taken at the lower current () have an intermediary curve between the original two semicircles.
In this work, we will look to understand the mechanisms that are at work when CO is present during the hydrogen oxidation/evolution reaction. The impedance spectra will be reproduced using an equivalent electrical circuit (EEC) model to examine what circuit elements replicate the measured EIS data and what can be the cause the introduction of the third curve in the spectrum.
References

N.R. Elezovic, Lj. Gajic-Krstajic, V. Radmilovic, Lj. Vracar and N.V. Krstajic, Electrochim. Acta, 54, 1375 (2009).

G.A. Camara, E.A. Ticianelli, S. Mukerjee, S.J. Lee and J. McBreen, 149(6), A748 (2002).

Figure 1
States of Water in Thermally Annealed Recast Nafion® Films and Impact on Fuel Cell Performance
Authors: Ren Xiaoming , Gobrogge Eric A. , Beyer Frederick 
Published: February 2020

The presence of water in polymer electrolyte materials has substantial impact on the materials’ morphology, mechanical and transport properties, and thus dedicate the materials' performance in many important applications in energy production and conversion devices such as fuel cells and electrolyzers. In this study, the states and distributions of water in recast H+-Nafion® ionomer films thermally annealed at a temperature up to 120oC were determined from attenuated total internal reflection FTIR (ATR-FTIR) spectra in the O-H stretching region which were analyzed using principle component and multivariant curve resolution methods. The morphologic changes of the films indicated by the ATR-FTIR experiments are further confirmed by the small-angle X-ray scattering (SAXS) experiments. It is concluded that a Nafion film annealed at a higher temperature results in a more robust polymer matrix surrounding the ionic clusters, and thus exert a stronger restriction on the amount of water uptake in each one of the ionic clusters. The restricted amount of water in the ionic clusters limits the extent of the hydrogen bonding of the water clusters residing in the ionic clusters upon the ionomer film water uptake. These results could lend useful insights on the nature of water in the polymer electrolyte materials, and on the effects of thermal annealing treatment used in fabricating electrodes from the catalyst inks containing the ionomer solution. The impacts of the cathode catalyst layer annealing treatment on the hydrogen/air fuel cell performance are also presented.
Rotating Ring-Disk Electrode Voltammetry Considering the Quasi-Four Electron Reduction of Oxygen for Fe/N/C Catalsyts
Authors: Nabae Yuta , Muthukrishnan Azhagumuthu , Wu Yun 
Published: February 2020

Understanding the kinetics of the oxygen reduction reaction (ORR) for fuel cell applications is quite important but difficult because the four-electron pathway is often overestimated by including a quasi-four electron pathway that consists of the formation and reduction of H2O2. To solve this problem, here we demonstrate a novel analysis method with experimental data over a Pt-free Fe/N/C cathode catalyst. In this study, H2O2 voltammetry was conducted separately to evaluate the rate constant of the H2O2 reduction more accurately, and the obtained data were combinatorally analyzed with those from the ORR experiments. First, experimental and mathematical modification of the conventional Damjanovic approach was performed, and then the effect of the catalyst loading density was carefully studied by utilizing a novel reaction model with consideration of the quasi-four-electron pathway to avoid overestimation of the four-electron pathway kinetic parameters. Figure 1 shows the reaction current, I
1, I
2 and I
3, evaluated with different analysis methods. With the conventional Damjanovic-Hsueh approach (left), the evaluated reaction currents are very noisy and do not correspond to the measured current. After the experimental and mathematical modification (middle), the noise in the analysis results are much reduced. Furthermore, by considering the quasi-four electron reduction, the overestimation of the four-electron pathway can be avoided (right). In the most overestimated case, the percentages contribution of four-electron pathway over the Fe/N/C catalyst was estimated as 85% with the modified Damjanovic approach, while that from the newly proposed model is 51%. This method will be applicable for many other cathode catalysts and will facilitate understanding the nature of the ORR.
Figure 1. The comparison of conventional Damjanovic-Hsueh approach, modified Damjanovic approach and newly proposed model for ORR over Fe/N/C catalyst.

Figure 1
Engineering Metal-Organic Framework-Derived PGM-Free Catalysts for Oxygen Reduction in Acidic Media
Authors: Wu Gang 
Published: February 2020

To significantly reduce the cost of proton exchange membrane (PEM) fuel cells, current Pt must be replaced by platinum-metal-group (PGM)-free catalysts for the oxygen reduction reaction (ORR) in acid [1-5]. We presented a new class of high-performance atomic iron carbon catalysts through controlled chemical doping of iron ions into zinc-zeolitic imidazolate framework (ZIF), a type of metal-organic framework (MOF). The novel synthetic chemistry enables accurate size control of Fe-doped ZIF catalyst particles with a wide range from 20 to 1000 nm without changing chemical properties, which provides a great opportunity to increase the density of active sites that is determined by the particle size. We elucidated the active site formation mechanism by correlating the chemical and structural changes with thermal activation process for the conversion from Fe-N4 complex containing hydrocarbon networks in ZIF to highly active FeNx sites embedded into carbon. A temperature of 800oC was identified as the critical point to start forming pyridinic nitrogen doping at the edge of the graphitized carbon planes. Further increasing heating temperature to 1100oC leads to increase of graphitic nitrogen, generating possible synergistic effect with FeNx sites to promote ORR activity. The best performing catalyst, which has well-defined particle size around 50 nm and abundance of atomic FeNx sites embedded into carbon structures, achieve a new performance milestone for the ORR in challenging acidic media including a half-wave potential of 0.85 V vs RHE. This has been approaching to Pt and holds great promise for future PEM fuel cells.
References:
(1) H. Zhang, H. Osgood, X. Xie, Y. Shao and G. Wu, Nano Energy, 31, 331-350, 2017.
(2) G. Wu, A. Santandreu, W. Kellogg, S. Gupta, O. Ogoke, H. Zhang, H.-L. Wang and L. Dai, Nano Energy, 29, 83–110, 2016.
(3) S. Gupta, S. Zhao, O. Ogoke, Y. Lin, H. Xu and G. Wu, ChemSusChem, 10, 774–785, 2017.
(4) X.-J. Wang, H. Zhang, H. Lin, S. Gupta, C. Wang, Z. Tao, H. Fu, T. Wang, J. Zheng and G. Wu, Nano Energy, 25, 110–119, 2016.
(5) X. Wang, Q. Li, H. Pan, Y. Lin, Y. Ke, H. Sheng, M. T. Swihart and G. Wu, Nanoscale, 7, 20290-20298, 2016.
Multilayer Hierarchical Nanofibrillar Electrodes with Tuneable Lacunarity with 2D like Pt Deposits for PEMFC
Authors: Ercolano Giorgio , Farina Filippo , Cavaliere Sara , Jones Deborah J. , Rozière Jacques 
Published: February 2020

Here we present a novel approach to electrocatalysts layer design for PEMFC. Pt conformally electrodeposited around surface of carbon nanotubes growth on a self-standing nanofibrous electrode (NFE). We found that by combining two cost-effective and up-scalable techniques, electrospinning and electrodeposition it is possible to produce a layered electrode with tuneable lacunarity and engineer the surface area distribution along the electrode thickness and in turn the Pt content distribution.
It is possible to obtain an electrocatalyst with outstanding Pt exploitation and high durability while minimizing fabrication complexity[1]. We produced hierarchical 3D electrocatalyst layers with tuneable morphology that can be used directly in a membrane electrode assembly, allowing the complete exploitation of their highly porous structure, in a similar fashion to NSTF[2] or other 3D macroscopic assembly techniques. These structured electrocatalyst layers have high electrical conductivity for fast charge transport, and engineered density for efficient reactant mass transportation. This method allowed us to obtain self-standing nanofibrous electrodes with ultra-low Pt loading (0.05 mg/cm 2), that show very high activity towards the oxygen reduction reaction and electrochemical surface areas as high as 150 m2/g. A detailed microscopic, structural and electrochemical characterisation of the complete electrodes, including their application as the cathode in a proton exchange membrane fuel cell is presented.
[1] G. Ercolano, F. Farina, S. Cavaliere, D.J. Jones, J. Rozière, J. Mater. Chem. A 5 (2017) 3974-3980.
[2] M.K. Debe, A.K. Schmoeckel, G.D. Vernstrom, R. Atanasoski, J. Power Sources 161 (2006) 1002–1011.
Acknowledgement: This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 700127 INSPIRE. This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme.