bluuebunny/crossref_metadata_2025 huggingface dataset:
Assessment of Factors Controlling the X-ray Penetration Depth in Studies of 4H-SiC using Monochromatic and White Beam Synchrotron X-ray Topography in Reflection Geometry
Authors: Yang Yu , Guo Jianqiu , Goue Ouloide Yannick , Wu Fangzhen , Raghothamachar Balaji , Dudley Michael
Published: February 2020

Synchrotron X-ray Topography has been shown to be a vital tool for the nondestructive characterization of defects in 4H-SiC crystals [1-2]. Techniques utilizing reflection geometry are particularly useful for discerning defects at different depths below the crystal surface. For example, in studying defects in SiC pin diode structures, which typically comprise a buffer layer homoepitaxially grown on a substrate, with the drift layer grown on top of the buffer layer, it is important to be able to discriminate the depth at which particular defect configurations reside. This is particularly important for the characterization of defects resulting from relaxation processes such as interfacial dislocations and half loop arrays [3]. Accurate discernment of the depth of a defect requires precise measurement of the effective penetration depth of the X-ray beam in the diffraction geometry in question. This paper provides an assessment of the factors controlling that penetration depth in grazing incidence, reflection and back reflection geometries. There are generally two approaches adopted depending of the level of perfection of the crystal. In deformed regions (such as around dislocation cores) the penetration depth is simply determined by photoelectric absorption. In perfect regions it is determined by extinction. We will present a comparison between measured penetration depths in various diffraction geometries with those calculated using these two approaches and develop an optimized model to explain our observations.

[1] Muller, G.S., et al., Volume production of high quality SiC substrates and epitaxial layers: Defect trends and device applications. Journal of Crystal Growth, 2012. 352(1): p. 39-42.

[2] Dudley, M., et al., Characterization of 100 mm diameter 4H-Silicon carbide crystals with extremely low basal plane dislocation density, Material Science Forum,2001. 65: p. 353-356.

[3] Wang, H., Dudley, M., et al., Studies of the Origins of Half-Loop Arrays and Interfacial Dislocations Observed in Homoepitaxial Layers of 4H-SiC. Journal of Electronic Materials, 2015. 44(5): p. 1268-1274
(Invited) AlGaN Higher Power Devices
Authors: Jones Kenneth A.
Published: February 2020

The maximum power that GaN and SiC devices can routinely handle with their present designs is ~105 watts. This is determined by the fact that for a given breakdown voltage, VB, the maximum doping level, nmax (for electrons) and therefore the current density, is limited primarily by the critical filed, ξC, and access to the carrier as defined by the donor, ED, or acceptor, EA, depth. To a lesser extent, it is also limited by the carrier – usually the electron – mobility, μn. GaN, and the III-V semiconductors in general, have an advantage over group IV semiconductors in that they usually form continuous alloys so that ξC can be increased incrementally by increasing the fraction of the larger band gap material in the ternary; for the group IV it is Si, SiC, or diamond, and nothing else in between. To a first approximation the energy gap, EG, increases linearly from 3.39 for GaN to 6.2 eV for AlN and ξC from 3.5 to 11.7 V/cm. They actually increase sublinearly with the concentration, and the effects of this will be briefly explored. The carrier of interest is the electron because the depth of the Mg acceptor, which is deep to begin with, increases quite rapidly with the Al concentration. The dopant of interest is the Si donor, which behaves like a hydrogenic atom up to ~80% Al; above this it is thought to form a DX center. ED increases in depth from 32 to ~75 meV up to this Al%, and although this is ~3kT at room temperature, the large density of states in the conduction band enables most of the donors to be ionized. μn does decrease with the Al%, and at 80% Al it is about half the value it is for GaN. However, this deficit is more than made up for by the increased ξC, which has more than doubled. The conductivity also has a higher order dependence on ξC than it does on μn; in the ideal vertical diode it is proportional to ξC
3 and it is only linearly proportional to μn. Utilizing AlGaN creates other issues, especially for the vertical device, which will have to be the structure used because very high power lateral devices would occupy a larger surface area and be more prone to surface breakdown. The higher source and drain contact resistance for AlGaN will be a problem for both lateral and vertical devices, as will be alloy scattering in the 2DEG. It will be difficult to create a vertical structure that can be easily turned on and off be it with a current aperture or a JFET structure. The AlGaN drift region will also contain mismatch dislocations due to its lattice mismatch substrate, and it will be more difficult to confine the majority of dislocations to the interface, as it is done in SiGe devices, because Si and Ge are cubic and have active primary slip systems exposed to a substantial amount of shear. AlN and GaN have the hexagonal wurtzite structure, and neither the primary basal slip system nor the secondary prismatic slip systems have any shear operating on the slip planes so slip, if it occurs, occurs on pyramidal planes which have large critical shear stresses.
(Invited) Quality Improvement and Mapping Analysis of Single Crystal 4H SiC Grown with Purified Beta-SiC Powder Source
Authors: Kim Younghee , Jung Eunjin , Lee Myunghun , Kim Jungyu , Choi Doojin
Published: February 2020

The most important consideration when growing single crystal silicon carbide by the physical vapor transport method is to minimize defects. To minimize defects caused by temperature gradient, we used β phase SiC powder, which has a low sublimation temperature, and purified the β phase SiC powder to improve the purity of single crystal SiC. Furthermore, we performed thermodynamic computational simulations based on compositions of purified and non-purified β-SiC powders to study the impact of metallic impurities within SiC powder on the composition of single crystal SiC. We grew SiC at temperatures about 200 °C lower than the previous growth temperature using purified β-SiC powder and mapped the phase change behavior of SiC according to different growth temperatures. Moreover, we compared and analyzed the characteristics of SiC polytype formation and crystallinity according to growth temperature. We compared the distribution of defects and dislocations of single crystal 4H SiC grown from purified and non-purified β-SiC powder to study the impact of source purification on defect generation. We also investigated the effect of metallic impurities on the formation of defects and dislocations through content analysis of metallic impurities.
(Invited) Semiconductor/Oxide Composite Nanowires Supplying White Luminescence
Authors: Ishikawa Fumitaro , Yamamoto Naoki
Published: February 2020

Integration of semiconductor and oxides within a monolithically synthesized nanowire system would have strong potential for the future functional devices. We then challenge to fabricate semiconductor/oxide heterostructure nanowire with molecular beam epitaxy combined with wet-oxidation of compound semiconductor heterostructure system.

We fabricate oxide heterostructure nanowire by selective wet oxidation of GaAs/AlGaAs core-shell type nanowire grown on Si substrate. Applying adequate oxidation condition, we obtain GaAs/AlGaOx semiconductor-oxide composite nanowire as illustrated in the Figure. At an adequate condition, the shell AlGaAs was selectively oxidized but the core GaAs remained sue to its large contrast of oxidation rate.

The figure also shows the result of cathodoluminescence measurement on the sample. The contrast in the NW (the panchromatic CL image) reflects the core-shell structure of the NWs. The spectrum acquired from the 1×1μm2 area involving the NW is also shown in the figure. The spectrum exhibits a sharp peak at 340 nm and a broad peak extending from approximately 400 nm to 700 nm, probably constituting several overlapped peaks. This spectrum is similar to previously reported spectra of Al2O3 nanoparticles. As seen in the figure, the shell layer emitted strong luminescence at 342 nm and 423 nm. Furthermore, the observed emissions were localized, in consistence with luminescence sourced from nanostructured oxides. This suggests the formation of shell oxides with luminescent properties. The observed spectral range varies from violet to a green-yellow regime, possibly reflecting the different arrangements of molecules responsible for the luminescence. In our case, luminescence is observed from ultraviolet (300 nm) to near infrared (700 nm) wavelengths, including the broadened tail of the spectrum.[1] This broad white luminescence might be sourced from the small Ga constituent within the amorphous AlGaOx shell and the wide nanoscale particle size distribution of the shell.

[1] H. Hibi, M. Yamaguchi, N. Yamamoto, and F. lshikawa, Nano Lett. 14, 7024, 2014.

Figure 1
Design of Supported PtCo Electrocatalysts for Pemfcs
Authors: Stassi Alessandro , Gatto Irene , Saccà Ada , Patti Assunta , Monforte Giuseppe , Baglio Vincenzo , Aricò Antonino S.
Published: February 2020

Pt-alloy electrocatalysts have been investigated for operation in both hydrogen and methanol-fed polymer electrolyte membrane (PEM) fuel cells  with the aim of evaluating their performance and resistance to degradation. Nanosized PtCo catalysts with particle size of about 3 nm were prepared by using a sulphite complex route  [1-4]. To achieve a high  degree of alloying a carbothermal reduction was used. This caused the occurrence of a primitive cubic ordered structure.  The surface properties were investigated by X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectroscopy (LE-ISS, 3He+ at 1kV). The formation of a Pt skin layer on the surface of the alloy electro-catalyst was studied.  Oxide supports were evaluated as alternative to carbon black materials with the aim to increase the electrochemical stability. The structural and surface characteristics of the samples as well as the type of catalyst support appear to strongly influence the PtCo catalyst  performance and degradation.  

References

[1]  Aricò, A.S., Stassi, A., Modica, E., Ornelas, R., Gatto, I., Passalacqua, E., Antonucci, V.

Performance and degradation of high temperature polymer electrolyte fuel cell catalysts

Journal of Power Sources, (2008) 178 (2), pp. 525-536.

[2]  Aricò, A.S., Stassi, A., Gatto, I., Monforte, Passalacqua, E., Antonucci, V.

Surface Properties of Pt and PtCo Electrocatalysts and Their Influence on the Performance and Degradation of High-Temperature Polymer Electrolyte Fuel Cells,

J. Phys. Chem. C (2010), 114, 15823–15836.

[3] Baglio, V., D'Urso, C., Sebastián, D., Stassi, A., Aricò, A.S.

PtCo catalyst with modulated surface characteristics for the cathode of direct methanol fuel cells

(2014) International Journal of Hydrogen Energy, 39 (10), pp. 5399-5405.

[4] Stassi, A., Gatto, I., Baglio, V., Passalacqua, E., Aricò, A.S.

Oxide-supported PtCo alloy catalyst for intermediate temperature polymer electrolyte fuel cells

(2013) Applied Catalysis B: Environmental, 142-143, pp. 15-24.

Acknowledgement

The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2011-2014) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement Impact  n°  303452.
Molecular Engineering of Ion-Conducting Polymers for Fuel Cell Membrane Applications
Authors: Lee Woo-Hyung , Mohanty Angela D , Bae Chulsung
Published: February 2020

Ion-conducting polymer membranes (cation exchange and anion exchange) are a key component in clean energy conversion devices (e.g., fuel cells and water electrolysis).  For example, perfluorinated sulfonic acid polymers such as Nafion are state-of-the-art proton-conducting polymers, but they suffer from high cost, mediocre mechanical strength, and moderate operation temperature.  Thus, it is highly desirable to develop readily tunable, high-performance ion-conducting membranes based on low-cost hydrocarbon polymers.

Over the years we have developed a novel polymer functionalization method that allows incorporation of a variety of ionic groups to aromatic polymers.  Using the method we synthesized highly proton-conductive fuel cell membranes containing sulfonate groups with different acidity strengths (fluoroalkylsulfonic, arylsulfonic, and alkylsulfonic acid groups) [1-2].  The effect of these different sulfonate groups of polymers on proton transport properties and fuel cell performance will be discussed.

Recently, anion exchange membrane fuel cells (AEMFCs) have gained significant attention because they can efficiently and cleanly convert the chemical energy stored in fuels directly into electrical energy [3].  Since AEMFCs utilize a solid alkaline electrolyte [i.e. anion exchange membrane (AEM)] to transport hydroxide anions, they can avoid issues previously faced with the concentrated aqueous KOH electrolyte in alkaline fuel cells.  However, significant shortcomings still remain in AEMFCs such as poor long-term alkaline stability and low hydroxide conductivity.  In our continued effort to develop robust, high performance ion-conducting polymers, we prepared styrene-ethylene/butylene-styrene (SEBS)-based AEM materials bearing various ammonium-cation groups.  Utilizing our groups’ recently developed metal-catalyzed synthetic strategy, involving C-H borylation followed by Suzuki coupling [4], we successfully introduced chemically stable ammonium-cation species to the aromatic units of SEBS.  The synthesis, hydroxide transport properties, alkaline stability, and fuel cell performance of these SEBS-based AEMs will be presented.

References:

[1] Y. Chang, G. F. Brunello, J. Fuller, M. Hawley, Y. S. Kim, M. Disabb-Miller, M. A. Hickner, S. S. Jang, C. Bae, Macromolecules
2011, 44, 8458–8469.

[2] Y. Chang, G. F. Brunello, J. Fuller, M. Disabb-Miller, M. Hawley, Y. S. Kim, M. A. Hickner, S. S. Jang, C. Bae, Polym. Chem.
2013, 4, 272–281

[3] G. Couture, A. Alaaeddine, F. Boschet, and B. Ameduri, Prog. Polym. Sci.,
2011, 36, 1521–1557.

[4] A. D. Mohanty, Y. –B. Lee, Liang, Zhu, M. A. Hickner, C. Bae, Macromolecules, 2014, 47, 1973–1980.
FIB+SEM Tomography and Numerical Simulation of Corroded PEM Fuel Cell Cathodes
Authors: Star Andrew G. , Fuller Thomas F
Published: February 2020

Carbon support corrosion in PEM fuel cell cathodes causes severe polarization losses due to mass transport resistance and underutilization of the expensive platinum catalyst. Tomography by FIB+SEM serial sectioning has recently been demonstrated on PEM fuel cell cathodes and the technique is used here to image and generate 3D reconstructions of small volumes (~3μm3). Morphological changes following extensive carbon corrosion were observed and quantified. Direct simulations were performed on the 3D reconstruction generated from SEM data which allowed for calculation of effective transport properties, notably effective diffusion coefficients, porosity, and tortuosity.

Figure 1
Fast X-Ray Tomographic Imaging: Investigation of Freezing Mechanisms in PEFC during Sub-Zero Start-Ups
Authors: Mayrhuber Immanuel , Marone Federica , Stampanoni Marco , Schmidt Thomas J. , Büchi Felix N
Published: February 2020

Starting from subfreezing temperatures is a challenge for polymer electrolyte fuel cells. The product water removal is a problem since ice may form, limiting the gas transport in gas diffusion and catalyst layers.

X-ray tomographic imaging is used to investigate the liquid/solid water phase in the GDL and to deduce information on the mechanism of freezing of the water, which induces a performance drop during freeze starts of PEFC. As freeze starts are transient processes tomographic imaging is tuned for fast acquisition while conserving acceptable image quality. Tomographic scans of 4.9 s are recorded.

As freezing of the super-cooled water is a temperature dependent nucleation process [1,2], the time to freezing of the super-cooled water depends on the cell temperature. The figure shows an isothermal freezing experiment with X-ray imaging and a slice through the tomographic scan of the cathode GDL 30 s before performance drop.

[1]        Y. Ishikawa, et al., J. Power Sources, 179 (2008), pp. 547-552

[2]        T.J. Dursch, et al., Langmuir 28, (2012), pp 1222-1234

Different freezing mechanisms are observed. At low temperatures (i.e. -20 °C) and low current density no water is observed in the GDL. The product water seems to freeze instantly in the catalyst layer (CL) which leads to a slow performance drop with filling of the CL with ice. At the same low temperatures, but higher current densities super-cooled liquid water emerges from the CL and fills part of the macro-porous gas diffusion layer and even penetrates into the channel. In this case the freezing dynamics are different: as long as the water remains liquid, operation continues. Then all water freezes instantly at the same time and operation stops abruptly.

Figure 1
Progress Towards Commercial Solid Hydrogen Storage Systems
Authors: Fisher Allison M , Bennington Stephen
Published: February 2020

Cella Energy has developed an ammonia borane-poly(ethylene oxide) solid hydrogen storage fuel that is being demonstrated as a lightweight high-performance replacement for batteries in a number of premium markets such as powering unmanned aerial vehicles.  Cella’s technology will be suitable for the high energy requirements of “green/electric aircraft” replacing carbon fuels for powering on-board systems with a safety factor that exceeds the capability of compressed hydrogen. 

Development of a viable hydrogen storage system will overcome a critical barrier for the widespread commercialization of hydrogen and fuel cells technologies, an emerging renewable, clean energy industry with potential for growth in stationary, portable, and transportation areas.  One of the most promising approaches to achieve high energy density system goals is to store hydrogen as a solid chemical hydride and release the hydrogen thermally, ideally between 80-150°C.  The process by which the solid hydrogen fuel will be used is conceptually easy:  the fuel is packaged as discrete “pellets” that are in contact with an ignition element such as a heater, insulated, and packed into a cartridge-type container.  Combined with the appropriate electronic controls and filters, this is a hydrogen storage fuel system. 

Several challenges impact the success of these systems significantly.  Simply put, in addition to hydrogen, all solid hydrogen fuels release some quantity of toxic impurities on heating, and many fuels expand rapidly and considerably while releasing a large quantity of heat, which imparts mechanical stress on packaging, among other issues.  In addition, some volatiles are able to oligomerize in the gas phase, which means that if not trapped before leaving the hydrogen fuel cartridge then there is high probability of clogging fuel lines to the fuel cell based energy conversion system, as well as contamination of the fuel cell stack.  This presentation will present an overview of Cella’s efforts to develop solid hydrogen fuel systems, as well as active R&D efforts to understand and optimize solid hydrogen material properties for safe storage and dehydrogenation when fuel is needed.
Platinum Deposited on Boron Doped Diamond Nanoparticles By the Rotating Disc Slurry Electrode (RoDSE) As a Catalyst for Alkaline Fuel Cell
Authors: Acevedo Raul , Cabrera Carlos R
Published: February 2020

Platinum deposited on boron doped diamond is a robust material for use as an electrode. In this work platinum is deposited on boron doped diamond by a method developed in the laboratory, the Rotating Disc Slurry Electrode (RoDSE).

The synthesized catalyst was characterized by Raman spectroscopy, xray diffraction, transmission electron microscopy, cyclic voltammetry and chronoamperometry in a half cell. The catalyst was used in the anode side of a membrane electrode assembly (MEA) of a direct alkaline fuel cell where power density curves were obtained for hydrogen and ammonia on the anode side, oxygen was used on the cathode side.
Effects of Functionalize Carbon Nanofibers Supported Catalysts on Oxygen Reduction Reaction in Alkaline Medium
Authors: Huang Wenjiao , Ahlfield John M. , Kohl Paul A , Zhang Xinsheng
Published: February 2020

Fuel cells are attractive because they provide clean and efficient energy for stationary and portable applications. Among the various types of fuel cells, the proton-exchange membrane fuel cells (PEMFC) have a high energy density and a fast start-up, but the cost of noble-metal catalysts and perfluorinated membranes is high. Anion-exchange membrane fuel cells (AEMFC) are attractive because they have enhanced the oxygen reduction and methanol oxidation kinetics at high pH, which makes the use of non-noble catalysts possible.

Carbon nanofibers (CNFs) have outstanding structural, mechanical and electrical properties, and are widely used as catalyst supports in fuel cells. CNFs are also considered to be good material for the oxygen reduction reaction catalysts in alkaline fuel cells because of their high surface area, thermal stability and electrical conductivity.

In this work, we use CNFs as the support for fuel cell catalysts in hybrid fuel cells, which combine alkaline and acidic conditions at the different electrodes. Functional groups were added onto the CNFs, including carboxyl groups (CNF-OX), carbonyl groups (CNF-CO), hydroxyl groups (CNF-OH) and nitrogen-containing groups (CNF-ON). In previous studies, they were tested them in half-cell experiments and it was found that the catalysts supported on CNF-ON have high ORR activity. The CNF-ON electrodes can change the ORR from the two-electron pathway to the four-electron pathway. Pt/CNFs have been tested in hybrid fuel cell and compared with Pt/C.
Gas Crossover in Polymer Electrolyte Membranes
Authors: Jung Aeri , Kim Min Soo
Published: February 2020

The polymer electrolyte membrane fuel cell system has received a considerable attention as one of powerful candidates for the next generation power sources. Numerous studies have been carried out to develop good performance and high durability systems with PEM fuel cells. As a result, several solutions to enhance the operating characteristics have been formed; however, durability of the system is still required to be improved.

As the durability of the system emerges as one of the critical problem, understanding the degradation mechanism in the fuel cell system is the key step for a better system design. Gas crossover has a great influence on the degradation of the membrane electrode assembly. Thus, it is essential to investigate the crossover of related gases through the membrane gas crossover has a negative effect not only on the performance but also on the durability of the system.

In this study, the novel detecting technique to measure accurate gas crossover rate and the effect of various parameters on gas crossover with different gases are investigated experimentally. Experiments are performed with the unit cell which has active area of 25 cm2 with four flow channels. To distinguish the effect of the specific parameter, it is controlled separately in the experimental setup. For measuring the gas crossover rate, mass spectrometer is used in this experiment. Consequently, the effects of temperature, relative humidity condition, hydrogen pressure, and membrane thickness on gas crossover rate in PEM fuel cells are investigated.
Investigation of Durability of MEAs at Higher Temperature
Authors: Kitamura Masahiko , Noda Zhiyun , Matsuda Junko , Hayashi Akari , Sasaki Kazunari
Published: February 2020

PEFCs recently attract much more attention owing to the commercialization of fuel cell vehicles (FCVs). We are interested in the operation of PEFC at high temperature to improve overvoltage loss of reactions occurring in PEFC. In this study, extracting problems on catalytic degradation at higher temperature is particularly focused on, and then durability of MEAs at 105 °C is considered.  

MEAs were prepared using standard catalyst and electrolyte, Pt/Ketjen black catalyst (TEC10E50E, TKK Corp.) and Nafion. For obtained MEAs, current-voltage (IV) performance and durability were studied. Regarding to durability evaluation, two protocols[1] recommended by Fuel Cell Commercialization of Japan (FCCJ), “load cycles” and “start-stop cycles”, were in use. Three different conditions of operation temperature and humidity (80 °C/RH100%, 105 °C/RH57%, and 80 °C/RH57%) were applied.

Regarding to durability analyses using “start-stop cycles”,[2] the increased loss of IV performance was clearly observed at 105 °C/RH57%. To understand degradation mechanism, the structure change of cathode layers was evaluated using FIB-SEM. The cathode layers became thinner in all the cases, but the condition of 105 °C/RH57% led to the thinnest layer. By further three-dimensional evaluation of cathode layers, increased size of pores constructing the catalyst layer was observed. The condition of 105 °C/RH57% resulted in formation of large pores especially near the Nafion membrane. Therefore, we believe such large pore formation derived by carbon corrosion led to collapsing and finally thinning the cathode layer.

On evaluation of “load cycles” durability tests, the operation at 105 °C/RH57% resulted in lower durability than that at 80 °C/RH100%. However, the difference was much smaller comparing to the case in “stop-start cycles”. Regarding to Pt size evaluated by SEM (Figure 1), the size increase of Pt particles was more pronounced in the case of 80 °C/RH100%. The reason most likely comes from enhanced Pt growth under the higher water content. Thus, the decreased IV performance at 105 °C/RH57% involves additional factors such as Nafion degradation at the low humidity condition.

Possible degradation mechanism using two different durability protocols is discussed with macrostructure change in cathode layers.

Reference

[1] A. Ohma et al., ECS Trans, 41(2011) 755.

[2] M. Kitamura et al., ECS Trans, 64 (2014) 755.

Figure 1
Study of Ionomer Adsorption on Carbon Supported Pt Catalyst By the Quartz-Crystal Microbalance (QCM) Method
Authors: Okuyama Takumi , Kinoshita Shinji , Tanuma Toshihiro
Published: February 2020

In polymer electrolyte fuel cells, the three-phase interface in a catalyst layer (CL) is considered to be very important as the electrochemical reaction site. The CL is typically formed using a dispersion of carbon supported platinum catalyst (Pt/C) and ionomer, and the interaction between the ionomer and Pt/C particles probably has an impact on the platinum utilization in the electrode and the dispersion stability.

 In this study, we focused on the adsorptive property of ionomers onto the Pt/C surface. The amount of adsorbed ionomer on a Pt/C catalyst was measured using the quartz-crystal microbalance (QCM) method. The amount was used as an index of the affinity of the ionomer for the Pt/C catalyst surface.

 The Pt/C catalyst particles were immobilized on a QCM sensor. The catalyst-immobilized sensor was placed into a solvent, and was left for a while until the QCM oscillation stabilized. An ionomer solution was then added drop-wise into the QCM cell. The change in resonance frequency of the quatz-crystal before and after adding the ionomer dispersion was recorded. The amount of a perfluorosulfonic acid polymer, Flemion® (Asahi Glass), adsorbed on the Pt/C catalyst surface is plotted against the concentration of the ionomer dispersion in Fig.. The amount was calculated based on the change in the resonance frequency of the QCM sensor. It shows the higher the ionomer concentration, the more ionomer was adsorbed. The relationship between the adsorptive property of various ionomers will be discussed in terms of their chemical properties (IEC, chemical structure etc.). The viscoelastic character of the adsorbed ionomer layers will also be described.

Figure 1
Structural and Chemical Analysis By Transmission Electron Microscopy of Pt-Ru Membrane Precipitates in Proton Exchange Membrane Fuel Cell Aged Under Reformate
Authors: Escribano Sylvie , Guetaz Laure , Henry Philemon A. , Jacques Pierre-André
Published: February 2020

Carbon supported platinum-ruthenium (Pt-Ru/C) nanoparticles are used as anode catalysts in PEMFCs operated under reformate owing to their good carbon monoxide tolerance. The stability of these catalysts during fuel cell operation is still not well known. In this work, we have studied by transmission electron microscopy (TEM) the microstructural evolution of a membrane/electrode assembly after a 1,000 h ageing test under reformate (26 ppm of CO) representative of a stationary application. The analyses clearly show evidence of Ru dissolution from the anode with the observation of Pt-Ru precipitates in the membrane and of small Ru precipitates in some areas of the microporous layer near the anode. This Ru dissolution happens despite a rather low estimated anode overpotential (around 0.2V). The distribution of the Pt-Ru membrane precipitates was found to be similar to that observed in aged MEAs with pure Pt catalysts, i.e. a precipitation band several micrometres away from the cathode catalyst layer made of the largest precipitates along with smaller precipitates between the band and the anode catalyst layer. The structure and the chemistry of the membrane precipitates were accurately analysed 1. The high resolution TEM images and EDS (Energy Dispersive X-Ray Spectroscopy) Pt, Ru elemental maps show that the largest precipitates display a singular flower shape consisting of a Pt-rich face-centred cubic ( fcc) crystallographic structure core surrounded by a Ru-rich hcp (hexagonal close-packed) crystallographic structure shell (Fig. 1 a and b). These results suggest a precipitate formation mechanism (Fig. 1c) where the Ru reduction is catalysed by Pt within the membrane. Moreover, the localization of the precipitation band near the cathode seems to indicate that the Pt in the precipitates comes from the dissolution of cathodic Pt/C and that both Pt and Ru ions are reduced by the hydrogen crossover.

In addition, the degradation of a MEA aged in a stack under 10 ppm of CO was studied. The MEA degradation was analysed in different zones of the MEA surface area; near the gas inlet, outlet and in the middle. As the durability tests were applied with continuous registration of current density maps thanks to a S++ ® current scan line device, the observed degradation heterogeneities has been correlated to the local conditions.

1  P. A. Henry, L. Guétaz, N. Pelissier, P.-A. André, S. Escribano, J. Power Sources 275 (2015) 312-321

Aknowledgement : This work was supported by the European Union’s Seventh Framework Program (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n°256776 (FCH-JU project Premium Act).

 

Figure 1
Molecular Dynamics Study of Proton Transport in Modeled Water Cluster Structure of Polymer Electrolyte Membrane
Authors: Mabuchi Takuya , Tokumasu Takashi
Published: February 2020

The polymer electrolyte membrane (PEM) is one of the principal components for polymer electrolyte fuel cells. In the nanoscopic structure of the membrane, ion clusters are formed by water molecules gathered vicinity of sulfonate groups which are hydrophilic parts of Nafion membrane, and proton transport (PT) are largely attributed to the nanoscopic structure of polymer membranes and water aggregations. Thus, it is critical to understand the PT mechanisms through polymer membrane and clarify an important link between the membrane nanostructure and the PT properties. In the bulk aqueous solution, it is observed that proton mobility is roughly 5 times higher than other cations such as K+ which have an ionic radius similar to the hydronium ions. This property of PT is attributed to a combination of proton hopping, known as the Grotthuss mechanism, and vehicular mechanism where protons diffuse in solution as a hydrated form (e.g. hydronium ions). Many research efforts have been focused on understanding the PT mechanism using theoretical basis for the proton hopping models such as the empirical valence bond (EVB) approaches which allow PT phenomena to be simulated within a molecular dynamics (MD) framework. In PEM, it is considered that proton hopping transport still takes place although the water dynamic property in the bulk aqueous solution is inhibited by a lack of bulk-like water structure due to the phase separation in PEM. Since the phase separation is considered one of the primary factors affecting its performance, the morphological properties of PEM have been studied experimentally and been characterized by proposing the cluster models (e.g., the cylinder model, the lamellar model, and the cluster-rod model) which reasonably fit the experimental scattering spectra. However, a detailed relationship between the morphological features and PT is still under debate. In this study, atomistic MD simulations have been performed to study the effects of water cluster structure on PT properties. The systems were built to closely approximate the proposed hydrophilic cluster structures (the cylinder model and the lamellar model) that are the most major morphological models of Nafion. The PT properties were estimated in terms of diffusion coefficient and proton pathway using a modified EVB model developed based on the previous study of the two-state EVB model by Walbran et al. The diffusion coefficients in each dimension were calculated and correlated with the cluster size as well as the type of cluster models. It was found that proton diffusion is strongly affected by the cluster model and its size, which can be associated with the proton pathway in each cluster model. The influence of the sulfonate groups on the PT properties were also investigated. Our simulation results provide insight into quantitative information about PT properties in atomic level.
Synthesis of High Pd-Loaded Pd/C Catalyst Using Ultrasonic Dispersions for the Direct Formic Acid Fuel Cells
Authors: Tsujiguchi Takuya , Onishi Ryo , Osaka Yugo , Kodama Akio
Published: February 2020

In recent years, direct formic acid fuel cells, DFAFCs, have received considerable attention for the power sources of the small electronic devices due to their high power density. Considering to the easy handling of the liquid fuel, we have considered that DFAFC can be promising power sources for stationary or transportation as well as the PEFC. However, the current power density of DFAFC has still far from that of PEFC. Therefore, the improvement of the power density is required. Moreover, improvement of the stability of the Pd catalyst is also required since Pd catalyst is poisoned by CO or CO species. In order to achieve high power density and good stability, a numerous catalyst supports, which contribute to the dispersion of the Pd catalyst and inhibit the Pd poisoning due to the interaction between Od and the supports, have been presented. This approach is quite available to solve the disadvantage of the DFAFC, however, it causes the increase of thickness of the anode catalyst layer since the Pd is usually loaded to the support around 20 wt% or less. Thick catalyst layer caused a mass transport limitation of the anode which significantly affects a performance of the DFAFC, therefore, the high Pd-loaded supported catalyst is required. When high Pd-loaded catalyst is prepared, the Pd nanoparticles is easy to agglomerate on the support resulting in the decrease of the electrochemical surface area. In this study, in order to fabricate the high Pd-loaded supported catalyst with well dispersions, we focused on the ultrasonic dispersions of the precursor and the catalyst supports before the chemical reduction. PdCl2 and the Vulcan XC-72 were used as the precursor and the supports, and the effects of the intensity and the time of the ultrasonication on the formic acid oxidation reaction, FAOR, activity, the BET surface area, ECSA and the crystallite diameter measured by XRD were investigated by fabricating Pd/C contained 20 wt % of Pd. From these results, the higher FAOR is obtained when the ultrasonication was conducted at appropriate intensity and time comparing the conventional Pd/C fabricated without ultrasonication. Based on the optimum ultrasonication conditions, Pd/C catalyst contained 50 wt% and 80 wt% of Pd catalyst were prepared. It was found that the Pd/Cs fabricated with ultrasonic dispersions showed much superior FAOR activity comparing to the Pd/Cs fabricated without ultrasonication although they showed a slight decrease of the FAOR activity per unit weight of Pd comparing to that of 20wt% Pd/C fabricated with ultrasonication. Form these results, the ultrasonic dispersion is quite important and the effective for the fabrication of high Pd-laded Pd/C catalyst.
Activity and Durability of Iridium Nanoparticles in Oxygen Evolution
Authors: Alia Shaun M , Kocha Shyam S , Pivovar Bryan S
Published: February 2020

Electrolysis is near parity as an energy storage process when compared to the cost of competing technologies, including batteries, pumped hydro, and compressed air.[1] When coupled with renewable energy sources, such as wind and solar, electrochemical water splitting can also produce hydrogen using a nearly carbon free pathway.[2] Iridium nanoparticles are commonly used as electrocatalysts in acidic electrolyzers due to moderate stability and activity in the oxygen evolution reaction. The activity and durability of iridium nanoparticles has been studied in rotating disk electrode half-cells to develop methods for establishing electrochemical baselines.

Mercury underpotential deposition has been used to determine the surface areas and specific activities of iridium catalysts.[3] This evaluation method has proved beneficial to those used previously since similar surface areas are obtained prior to and following exposure to high potential and the irreversible oxidation of surface iridium. Through mercury adsorption, the intrinsic activities of oxygen evolution catalysts can be evaluated and used to direct future electrocatalyst development.

The activities of iridium nanoparticle have been evaluated over a wide range of potentials in the kinetic region and transport limitations generally become a factor at potentials greater than 1.5 V vs a reversible hydrogen electrode. Initial optimization of electrode coating processes has further provided a robust benchmark in oxygen evolution. Accelerated stress tests have been applied in rotating disk electrode half-cells using potential cycling and potential holds. At moderate potential, activity losses are generally attributed to losses in surface area by nanoparticle agglomeration. Although bubble formation may destabilize the catalyst layer, similar experiments in hydrogen peroxide and formic acid oxidation have suggested that this loss is unique to the higher potentials in oxygen evolution. At elevated potential, heavy activity losses are observed and are increasingly due to iridium dissolution into the electrolyte.

This study establishes rotating disk electrode testing protocols for iridium in oxygen evolution. The observed losses in accelerated stress tests have significant implications on acidic electrolyzers, particularly at low iridium loadings.

[1] K. Harrison, M. Peters, in: U.S. Department of Energy (Ed.), http://www.hydrogen.energy.gov/pdfs/review13/pd031_harrison_2013_o.pdf, 2013.

[2] K. Harrison, M. Peters, C. Ainscough, in: U.S. Department of Energy (Ed.), http://www.hydrogen.energy.gov/pdfs/progress13/ii_a_2_harrison_2013.pdf, 2013.

[3] S.P. Kounaves, J. Buffle, Journal of The Electrochemical Society, 133 (1986) 2495-2498.
Fundamental Studies of Ion Sorption, Diffusion and Transport in Polyelectrolyte Membranes
Authors: Kamcev Jovan , Paul Donald R. , Freeman Benny D.
Published: February 2020

Charged polymer membranes are widely used for water purification applications involving control of water and ion transport, such as reverse osmosis and electrodialysis, as well as applications such as fuel cell membranes.  Efforts are also underway worldwide to harness separation properties of such materials for energy generation in related applications such as reverse electrodialysis and pressure retarded osmosis.  Improving membranes for such processes would benefit from more complete fundamental understanding of the relation between membrane structure and ion sorption, diffusion and transport properties in both cation and anion exchange membrane materials. Ion-exchange membranes often contain strongly acidic or basic functional groups that render the materials hydrophilic, but the presence of such charged groups has a substantial impact on ion (and water) transport properties.  This presentation will discuss aspects of chemistry, thermodynamics, and transport that are or may be at play in such membrane processes.  Our long-term goal is to develop and validate a common framework to interpret data from both electrically driven and concentration gradient driven mass transport in such polymers and to use it to establish structure/property relations leading to rational design of membranes with improved performance.  Questions of interest include how to properly formulate sorption and transport relationships, how to decouple effects of fixed charge density and water content on sorption and transport properties, how fixed charges affect mobility of counterions and coions, etc.  Ion sorption and permeability data were used to extract information about salt diffusion coefficients in charged membranes.  Techniques have been developed to determine concentrations of both counter-ions and co-ions in polymers.  Salt permeability, sorption and ionic conductivity data have been combined to determine individual ion diffusion coefficients in neutral, cation exchange and anion exchange polymers.  The use of models to correlate and, in many cases, predict the experimental data is discussed.
Autostack - CORE - Industry Led European Consortium to Develop Next Generation Automotive Stack Hardware
Authors: Martin André , Jörissen Ludwig
Published: February 2020

Most of international automotive OEMs have announced plans for the commercialization of fuel cell vehicles in the next years. While this is a clear signal for the functional readiness of fuel cell technology in automotive application, durability, efficiency, power density and cost of the fuel cell stack need further advancements in years to come.

“Auto-Stack Core” is a European funded collaborative project establishing a coalition with the objective to develop best-of-its-class automotive stack hardware with superior power density and performance while using components manufactured in an industrial scale and meeting commercial target cost. The project consortium combines the collective expertise of European automotive OEMs, component suppliers, system integrators and research institutes thus removing critical disconnects between stakeholders.

The technical concept is based on the Auto-Stack assessments carried out under an FCH JU Grant from 2010 to 2012 andreflects the system requirements of major OEMs. It suggests a stack platform concept with the aim to substantially improve economies of scale and reduce critical investment cost for individual OEMs by sharing the same stack hardware for different vehicles and vehicle categories.

Technical stack development is set up in 3 evolutions. So far stack evolution 1 has been designed, built and tested. The stack is based on components ready for manufacturing on an industrial scale. Metallic bipolar plates have been used to fulfill volume and cost targets. The following results have been achieved in evolution 1:

 

Achievements

Evolution 1

Final Specification

Evolution 3

Power
94 kW (continuous)   
99 kW peak for 30 sec
98 KW (continuous)
118 kW peak 30 sec

Power density
2.7 kW×l-1 (continuous)
2.9 kW×l-1 peak for 30 sec
2.8 kW×l-1 (Continuous)
3.4 kW×l-1 peak for 30 sec

Freeze-start capability
Demonstrated from -20 °C
From -25 °C

Specific stack cost
47.32 €×kW-1 @ 30 000 p.a.
< 40.00 €×kW-1 @ 120 000 p.a.

According to a benchmark study of state-of-the-art stack development including innovative material and component solutions, the stack specifications for evolution 3 are within the most demanding in the industry.

Industrial participation in the project and lead of critical work packages by industrial partners will ensure compliance of project objectives with industrial needs and allow direct implementation and utilization of the results. The project is of strategic importance for the competitiveness of the European supply industry and provides a critical contribution to supporting the commercialization of fuel cell and hydrogen technologies in Europe.

The work reported has been supported by the Fuel Cell and Hydrogen Joint Undertaking under Grant Agreement 325335.

Figure 1
Engineering Modeling of PEM Water Electrolysis
Authors: Hanke-Rauschenbach Richard , Bensmann Boris
Published: February 2020

The conversion of electric energy into pressurized hydrogen by means of water electrolysis is currently discussed as promising option to handle intermittent electricity supply that occurs with the increasing use of renewable energy sources. Subsequently, this hydrogen could be (1) either reconverted into electricity, or (2) converted into other energetic molecules, such as methane (power to gas) or liquid fuels, or (3) utilized as a feed stock for the chemical industry.

Besides conventional alkaline water electrolysis, a water electrolysis technology based on a Polymer Electrolyte Membrane (PEM) is discussed since several years, which is the focus of the present contribution. The particular advantages of PEM water electrolysis are higher attainable current densities, higher voltage efficiency, and higher load flexibility. Current disadvantages include far lower technical maturity, shorter life time, and higher specific costs compared to alkaline water electrolysis.

The formulation of appropriate physico-chemical models for solid polymer electrolysis can help to contribute to the solution of the above mentioned problems. In the present contribution state of the art of modelling of solid polymer electrolyzers is summarized and discussed.