bluuebunny/crossref_metadata_2025 huggingface dataset:
(Invited) Phase and Stacking of 2D Materials in Heterostructures and Dislocated Nanoplates Impact Their Properties and Applications
Authors: Jin Song
Published: February 2020

The interesting and tunable properties of layered metal dichalcogenides (MX2) and other layered materials heavily depend on their phase and layer stacking. We discovered that chemical exfoliation of layered MX2 nanosheets and the simultaneous conversion from the semiconducting 2H polymorph to the metallic 1T polymorph grants greatly enhanced activity for electrocatalytic hydrogen evolution reaction (HER). We have also developed layer-controlled vertical heterostructures of MX2 by van der Waals epitaxy that have strong electronic interactions across the heterojunctions that can be useful for solar energy conversion. We further show and explain how the layer stacking and physical properties of MX2 are influenced by screw dislocations. A one-to-one correlation of atomic force microscopy and high- and low-frequency Raman spectroscopy of many dislocated WSe2 nanoplates reveals variations in the number and shapes of dislocation spirals and different layer stackings that are determined by the number, rotation, and location of the dislocations. These lead to three class of behaviors: plates with triangular dislocation spirals have non-centrosymmetric stacking that gives rise to strong second-harmonic generation, plates with hexagonal dislocation spirals form the bulk 2H layer stacking commonly observed, and plates containing mixed dislocation shapes have intermediate non-centrosymmetric stackings with mixed properties.
(Invited) Low-Temperature Growth of Two-Dimensional Layered Materials Toward Phase-Engineered Hybrid Films
Authors: Chueh Yu-Lun
Published: February 2020

Novel condensed matter systems can be understood as new compositions of elements or old materials in new forms. According to the definition, various new condensed matter systems have been developed or are under development in the recent years. 2D layered materials, including graphene, transition metal dichalcogenides (TMDs) allow the scaling down to atomically thin thicknesses and possess unique physical properties under dimensionality confinement. Chemical vapor deposition (CVD) process is the most popular approach for all kind of 2D materials due to its high yield and quality. Nevertheless, the need for high temperature and the relatively long process time within each cycle hinders for commercial development in terms of production cost. However, the transfer procedure has become one of the major limitations of the overall performance. In the first part of my talk, I will present several approaches, including ultra-fast microwave heating, plasma selective reaction, controlled segregation process by laser irradiation and metal vapers-assisted growth processes developed in my lab these years to directly grow graphene with controllable thicknesses on arbitrary substrates without any extra transfer process. In the 2nd part of my talk, I will also introduce the development of various methods, including laser irradiation assisted-selenization (LIAS) process, fast microwave annealing and plasms enhance selenization processes on different two-dimensional transition metal dichalcogenides materials in my lab. The detailed formation mechanisms, microstructures and physical performances as well as its applications in gas sensors, photodetector and water splitting behaviors based on these two 2D materials were reported and investigated.
Optimizing a Long-Range Highly Ordered Plasmonic Nanoarray Pattern for Surface-Enhanced Raman Scattering
Authors: Kasani Sujan , Zheng Peng , Wu Nick
Published: February 2020

Surface-enhanced Raman scattering (SERS) is one of the most promising sensing techniques as it is non-invasive, non-destructive, and can reveal the spectral “fingerprints” of molecules. However, it remains a great challenge to fabricate large-scale, highly reproducible SERS substrates. In this work, we have developed a novel method to fabricate a long-range, highly ordered, plasmonic gold nano-ring array pattern based on a modified nanosphere lithography technique. The optical properties of gold nanoring array have been comprehensively studied and optimized for SERS by tuning the geometric parameters of the nanoring arrays using a combination of finite-difference time-domain (FDTD) simulations and cleanroom fabrication techniques. The optimized gold nanoring array has been further demonstrated for SERS biosensing.
(Invited) Ion Intercalation and High Temperature Behavior of 2D Materials
Authors: Hu Liangbing
Published: February 2020

I will discuss our recent progress on engineering of 2D materials, with a focus on metal ion intercalation and ultra-high temperature treatment (up to 3000-3500K). We developed a nano-battery platform that allows in situ measurement of optical, electrical and structure changes of 2D materials during ion intercalations. We used the nano-battery setup to investigate various properties of 2D materials upon ion intercalation, including Li+, Na+ and K+. Dramatic increases in both transmittance and conductivity were reported, which leads to the highest figure of merit (FOM) for transparent conductor applications. We also extended the knowledge to printed reduced graphene oxide network toward large-scale applications. In a separated topic, I will discuss our effort in the past three years on high temperature material behavior for reduced graphene oxide (RGO) networks. High-temperature stability of RGO networks allows us to uniquely engineer them for a range of emerging applications, such as 3D printed rapid heaters and highly conductive RGO paper as lightweight battery current collectors.

 

Keywords: Ion intercalation, Optoelectronics, Nano batteries, 2D materials, 3000K, 3D printing, High conductivity

[1] Chen, Y.; Fu, K.; Zhu, S.; Luo, W.; Wang, Y.; Li, Y.; Hitz, E.; Yao, Y.; Dai, J.; Wan, J.; Danner, V.; Li, T.; Hu, L.* Reduced Graphene Oxide Films With An Ultra-High Conductivity as Li-ion Battery Current Collectors, Nano Letter, Accepted

[2]Yao, Y.; Fu, K.; Yan, C.; Dai, J.; Chen, Y.; Wang, Y.; Zhang, B.; Hitz, E.; Hu, L.* 3D Printable High-Temperature and High-Rate Heaters, ACS Nano, Accepted

[3] Bao, W.; Pickel, A.; Zhang, Q.; Chen, Y.; Yao, Y.; Wan, J.; Fu, Kun.; Wang, Y.; Dai, J.; Zhu, H.; Drew, D.; Fuhrer, M.; Dames, Chris.; Hu, L. Flexible, High Temperature, Planar Lighting with Large Scale Printable Nanocarbon Paper, Advanced Materials, Accepted, Front Inside Cover.

[4] Lacey, S.; Wan, J.; Wald Cresce, A.; Russell, S.; Dai, J.; Bao, W.; Xu, K.; Hu, L. Atomic Force Microscopy Studies on Molybdenum Disulfide Flakes as Sodium-Ion Anodes, Nano Letters, 2015, 15, 1018.

[5] Wan, J.; Gu, F.; Bao, W.; Dai, J.; Shen, F.; Luo, W.; Han, X.; Urban, D.; Hu, L. Sodium-ion Intercalated Transparent Conductors with Printed Reduced Graphene Oxide Networks, Nano Letter, 2015, 15, 3763

[6] Wan, J.; Bao, W.; Liu, Y.; Dai, J.; Shen, F.; Zhou, L.; Cai, X.; Urban, D.; Li, Y.; Juangjohann, K.; Fuhrer, M.; Hu, L. In Situ Investigations of Li-MoS2 with Planar Batteries, Advanced Energy Materials, 2014, 1, 1401742.

[7] Bao, W.; Wan, J.; Han, X.; Cai, X.; Zhu, H.; Kim, D.; Ma, D.; Munday, J.; Drew, D.; Fuhrer, M.; Hu, L. Approaching the Limits of Transparency and Conductivity in Graphitic Materials through Lithium Intercalation, Nature Communications, 2014, 5, 4224.
(Invited) Enhanced Self-Assembly of Crystalline, Large-Area and Periodicity-Tunable TiO2 Nanotube Arrays on Various Substrates for Technological Applications
Authors: Ho Johnny C.
Published: February 2020

Due to their superior physical properties, titanium dioxide (TiO2) nanotube arrays are one of the most investigated nanostructure systems in materials science until now. However, it is still a great challenge to achieve damage-free techniques to realize controllable, cost-effective and high-performance TiO2 nanotube arrays on both rigid and flexible substrates for different technological applications. In this work, we demonstrate a unique strategy to achieve self-assemble crystalline, large-area and regular TiO2 nanotube arrays on various substrates via hybrid combination of conventional semiconductor processes. Besides the usual applications of TiO2 as carrier transport layers in thin film electronic devices, we demonstrate that the periodic TiO2 nanotube arrays can show the effect of optical grating with large-area uniformity. Specifically, the fabricated nanotube geometries, such as the tube height, pitch, diameter and wall thickness, as well as the crystallinity can be reliably controlled by varying the processing conditions. More importantly, utilizing these nanotube arrays in perovskite solar cells can further enhance the optical absorption leading to the improved power conversion efficiency. In contrast to other typical template-assisted fabrication approaches, we employ the soft template here, which would enable the construction of nanotube arrays without any significant damage associated with the template removal. Furthermore, without the thermal restriction of underlying substrates, these crystalline nanotube arrays can be transferred to mechanically flexible substrates by using the simple one-step method, which can expedite these nanotubes for potential utilizations in other application domains.
(Invited) SURMOFs and CCNCs As Novel Tuneable Materials for Electronic Devices and Applications
Authors: Redel Engelbert
Published: February 2020

Here we present a particularly interesting class of surface-anchored (metal-organic frameworks andcoordination network compounds) materials SURMOFs and (conductive coordination network compounds) CCNCs for the fabrication e.g. of 1D (Photonic Band Gap) PBG materials [1,2] as optical sensors, low-k dielectric thin films [3],optoelectronic/electrochromic switchable coatings/devices [4] microelectronic devices, e.g. RRAM devices [5] or Hybrid-Materials NPs@SURMOFs for future artificial Light-Harnessing (LH) assemblies and photocatalysts.[6]

Since these materials are highly porous and the size of their pores are highly adjustable, they can be further functionalized and further tailor to electrically load and unload small ions (optoelectronic applications) or act as photon traps, which are able to efficiently absorb, transport and harness light energy over an artificial LH antenna array.[7] An outlook will be given for highly conductive and porous CCNCs thin film materials as well as further electronic characterization of SURMOFs and CCNCs.

References

[1] J. Liu, E. Redel, S. Walheim, Z. Wang, V. Oberst, J. Liu, S. Heissler, A. Welle, M. Moosmann, T. Scherer, M. Bruns, H. Gliemann, C. Woell Chemistry of Materials (ACS) 2015, 27, 1991-1996.

[2] J. P. Best, J. Michler, J. Liu, Z. Wang, M. Tsotsalas, X. Maeder, S. Roese, V. Oberst, J. Liu, S. Walheim, H. Gliemann, P. G. Weidler, E. Redel and C. Woell Appl. Phys. Lett. 2015, 107, 101902.

[3] E. Redel, Z. Wang, S. Walheim, J. Liu, H. Gliemann, C. Woell, App. Physics Lett. 2013, 103, 091903 [4] J. Liu, W. Zhou, S. Walheim, Z. Wang, P. Lindemann, S. Heissler, J. Liu, P. G. Weidler, T. Schimmel, C. Woell, E. Redel Optics Express (OSA) 2015, 23, 13725-13733.

[5] Z. Wang, D. Nminibapiel, P. Shrestha, J. Liu, K. P. Cheung, W. Guo, P. G. Weidler, H. Baumgart, C. Woell, E. Redel (Back Cover Issue 1/02 2016) ChemNanoMat (Wiley-VCH) 2016, 2, 67-73.

[6] W. Guo, C. Zhi, C. Yang, T. Neumann, C. Kuebel, W. Wenzel, A. Welle, W. Pfleging, O. Shekhah, C. Woell, E. Redel Nanoscale (RSC) 2016, 8, 6468-6472.

[7] J. Liu, W. Zhou, J. Liu, I. Howard, G. Kilibarda, S. Schlabach, D. Couprey, M. Addicoat, S. Yoneda, Y. Tsutsui, T. Sakurai, S. Seki, Z. Wang, P. Lindemann, E. Redel, T. Heine, C. Woell *VIP ≡ Very Important Paper (Angewandte Front Cover Issue 2015-54/25) Angewandte Chemie International Edition (Wiley-VCH) 2015, 54, 7441-7445.
(Invited) Orthogonal E-Beam Lithography Approach on 2D Organic-Inorganic Halide Perovskites
Authors: Lin Chun-Ho , Cheng Bin , Li Ting-You , Durán Retamal José Ramón , He Jr-Hau
Published: February 2020

Organic-inorganic halide perovskite-based solar cells are the fastest-advancing photovoltaic technology in nowadays. To further increase optoelectronic applications on perovskite, researchers have devoted great efforts to explore two-dimensional (2D) layered halide perovskites very recently. This novel 2D perovskites can tune their optoelectronic properties by changing the thickness and functional organic group, which is not achievable for 3D perovskites. For 2D materials with ultra-thin thickness (usually less than 10 nm), the E-beam lithography is the most important fabrication process to build the nanostructures on materials to realize the nanodevices. However, the solvents used in current E-beam lithography can strongly destroy the perovskites. In this work, we fabricated the photodetector based on 2D (C6H5C2H4NH3)2PbI4 perovskite with nanoscale Au electrodes by using E-beam lithography process with orthogonal solvents. We show this new lithography method didn’t cause any damage to both optical and electrical properties of 2D perovskite. The I-V characteristics and photoresponse of perovskite photodetector were also measured. We believe this breakthrough will fully activate 2D perovskite for nano-applications.
(Invited) Hyperbolic Metamaterials and Their Imaging, Lasing, Sensing Applications
Authors: Rho Junsuk
Published: February 2020

Metamaterials, artificially structured nanomaterials, have enabled unprecedented phenomena such as negative refraction. However, despite the great and promising science, making practical devices based on metamaterials have been one of the most important issues due to the difficulty in fabrication. In this abstract, I will discuss recent development of scalable large-scale nanofabrication efforts to make practical metamaterials-based devices. The example is realizing a super-resolution imaging device of wafer-scale hyperlens, which is the first experimental demonstration of near- to far-field imaging of biological living objects at visible light with resolution beyond the diffraction limit in two lateral dimensions. With proposed hyperlens array, we show the first bio-sample imaging experimental result of super-resolution imaging on hyperlenses by combining to conventional microscopy system for imaging biomolecules. Hippocampal neuron cells are imaged by visible light through the hyperlens array with resolution down to 150 nm, beyond the diffraction limit. The super-resolution imaging result of neuron at sub-diffraction size shows that hyperlens can be used as a useful imaging tools in biology, pathology, medical science and nanotechnology. If time allows, I will discuss recent applications of hyperbolic metamaterials such as nanoscale cavity far beyond diffraction limit and sensing biomolecular matters. Also, such a unique fabrication techniques mentioned in this talk will provide the opportunity to achieve practical metamaterial devices as the significant step making nanoscience to nanotechnology.

Figure 1
Comprehensive Modeling and Simulation of the Effects of Surface Defects on Graphene Chemical Sensors
Authors: Lowenberger Aaron , Li Qiliang
Published: February 2020

Chemical sensors are ubiquitous in the world, with important and broad applications in medicine, industrial process monitoring, automotive and aerospace applications, and the military. Due to its unique electrical properties and an ideal two-dimensional structure, graphene shows great promise as a highly sensitive, low noise sensor material. In this work, we report the results of comprehensive first-principles simulation and modeling of the electrostatic response of graphene when exposed to multiple chemical vapors, including hexane, tetrahydrofuran, toluene, and acetone. The effects of different graphene surface defects, such as vacancy defects, hydrogen defects, oxygen defects and hydroxyl group defects on graphene conduction have been explored, including how these defects affect the response of graphene when exposed to each of the studied chemical vapors. The simulation results were analyzed and compared with experimental data with a focus on understanding the physics and kinetics of the adsorption and desorption of gas molecules on the graphene surface. The results of this modeling study of surface defects on graphene sensors is very important for understanding how to engineer advanced chemical vapor sensors for increased sensitivity and selectivity.
(Invited) SiC MOSFET Reliability and Implications for Qualification Testing
Authors: Lelis Aivars , Green Ron , Habersat Dan
Published: February 2020

There are a number of potential reliability issues associated with SiC power MOSFETs, including threshold-voltage stability, gate-oxide reliability, body-diode robustness, short-circuit current robustness, and radiation effects. This talk is primarily focused on threshold-voltage stability and the need for an improved test method to unambiguously separate out good devices from bad ones. Threshold-voltage stability is affected primarily by active charge traps in the near-interfacial region of the insulating gate oxide. Their close proximity to the semiconductor interface leads to a strong time dependence in the direct-tunneling mechanism in response to changes in gate bias. This time dependence is not properly accounted for in the existing test methods for assessing high-temperature gate-bias (HTGB) effects, which allow temporary removal of bias during cool down and significant un-biased delay (up to 96 hours) before the post-stress measurements are performed. However, this delay, introduced to accommodate the practical constraints of industrial testing, renders this test practically meaningless due to the significant recovery that occurs in the charge states of the near-interfacial oxide traps. This difficulty can be overcome by reapplying the gate bias for a brief period of time before measuring. Details of the nature of the near-interfacial oxide traps will be discussed, including their activation energy. All this work will be presented within the context of standards development within JEDEC, and a new SiC power-devices qualification working group.
GaN Lateral High Voltage Photoconductive Semiconductor Switches
Authors: Koehler Andrew D. , Nath Anindya , Khachatrian Ani , Anderson Travis J , Tadjer Marko J. , Mastro Michael A , Hite Jennifer K , Buchner Stephen , Hobart Karl D , Kub Francis J
Published: February 2020

Wide bandgap GaN photoconductive semiconductor switches (PCSSs) have gained recent attention due to high critical electric field strength and high electron saturation velocity to provide high power ultrafast devices. To reduce leakage currents, PCSSs have previously been demonstrated on semi-insulating GaN achieved by Fe compensation doping. However, Fe is known to have strong memory effects, which can redistribute Fe into subsequent films, as well as a narrow window of acceptable doping levels. PCSSs with fast response time are required for applications requiring high repetition rates. C-doped GaN is used to reduce leakage current and improve the photocurrent of lateral PCSSs. The C-doping is performed by simply tuning the growth parameters, such as temperature, pressure and III/V ratio and creates shallow accepter levels, compensating any unintentional n-type doping. In this work, we demonstrate high voltage GaN:C PCSSs with fast response times measured using a picosecond pulsed laser and high breakdown voltages, over 4000 V, measured in a vacuum probe station.
(Invited) Ultrawide Bandgap β-Ga2O3 Thin Films: Growths, Properties and Devices
Authors: Rafique Subrina , Han Lu , Zhao Hongping
Published: February 2020

Ultrawide bandgap (UWBG) gallium oxide (Ga2O3) represents an emerging semiconductor material with excellent chemical and thermal stability up to 1400 oC. It has a band gap of 4.5-4.9 eV, much higher than that of the GaN (3.4 eV) and 4H-SiC (3.2 eV). It exhibits high transparency in the deep ultraviolet (DUV) and visible wavelength region due to its very large bandgap. The monoclinic b-phase Ga2O3 represents the thermodynamically stable crystal among the known five phases (α, β, γ, δ, ε). The breakdown field of β-Ga2O3 is estimated to be 8 MV/cm, which is about three times larger than that of 4H-SiC and GaN. These unique properties make β-Ga2O3 a promising candidate for high power electronic device and solar blind photodetector applications. More advantageously, single crystal β-Ga2O3 substrates can be synthesized by scalable and low cost melting based growth techniques such as edge-defined film-fed growth (EFG), floating zone (FZ) and czochralski methods. For β-Ga2O3 thin film synthesis, both molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) have been demonstrated to produce high quality and controllable doping films but with slow growth rates (2-10 nm/min). Halide vapor phase epitaxy (HVPE) using chloride precursors was demonstrated to grow β-Ga2O3 films with fast growth rates (>5 μm/hr). Recently, we have developed a low pressure chemical vapor deposition (LPCVD) method to grow high quality β-Ga2O3 thin films on both native Ga2O3and c-sapphire substrates with controllable doping and fast growth rates up to 10 μm/hr.

In this talk, we present a study on the defects in β-Ga2O3 thin films grown via LPCVD. The β-Ga2O3 thin films were grown on native β-Ga2O3 substrates and sapphire substrates using high purity gallium and oxygen as the precursors, and argon (Ar) as the carrier gas. The growth temperature ranged between 850 ˚C and 950 ˚C. The β-Ga2O3 thin films were characterized by using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). FESEM images were taken with Helios 650. High resolution transmission electron microscopy (HRTEM) images and selected-area electron diffraction (SAED) were taken using a FEI Tecnai F30 at 300 kV. Bright field, dark field, combined with two-beam condition TEM were used to characterize the defects in the as-grown films. From our studies, β-Ga2O3 thin films grown on (010), (001) and (-201) β-Ga2O3 substrates have shown different growth rates as well as different properties of interfacial defects. Improved LPCVD growths to suppress defects in β-Ga2O3thin films grown on different substrates will be discussed. Room temperature Hall measurements will be performed to understand the dependence of carrier transport on defects.

In summary, the fundamental defect studies are performed on β-Ga2O3 thin films grown via LPCVD on Ga2O3 and sapphire substrates. The results from this study are important knowledge for power device applications.
Estimation of Lattice Strain in 4H-SiC Commercial Wafer by Synchrotron Monochromatic X-ray Topographic Contour Mapping
Authors: Guo Jianqiu , Yang Yu , Raghothamachar Balaji , Dudley Michael
Published: February 2020

Unique and excellent properties of 4H-SiC have rendered a series of successful applications in various energy-related fields such as Electric Vehicle (EV) and Smart Grid. While efforts have been made over decades towards the improvement of crystal quality, the densities of crystalline defects existing in the as-grown crystals are still significant. One of the major issues is the residual stress left by the inhomogeneous distribution of crystal defects, which will lead to lattice bending and lattice strain once the bulk as-grown crystals are sliced into wafers. Therefore, the level of lattice strain and bending equivalently characterizes the level of overall homogeneity of the sliced wafer.

X-ray topography, particularly synchrotron monochromatic X-ray topography (SMBXT), enables a highly strain-sensitive technique for imaging the near surface regions of single crystals. When a strained crystal is illuminated by synchrotron monochromatic radiation, the crystal does not diffract entirely. In fact, only a limited region fulfills the Bragg’s Law and diffracts. In order for other regions to diffract, the wafer has to be rocked through the perfect Bragg’s condition with tiny increments. The variation in diffraction condition can thus be mapped as a function of position, which is caused by local lattice bending and strain. By recording the same map with the opposite diffraction vector, i.e. rotating the wafer about the original diffraction vector by 180 degree, the strain component can be separated with the bending component so that two individual maps of lattice strain and lattice bending, respectively, can be obtained.

In this paper, we use this method to estimate the local lattice strain and lattice bending as a function of position in a 75mm 4H-SiC commercial substrate wafer. Results are compared with X-ray topographic images recorded simultaneously from the same wafer.

Figure 1
Studies on Doping Concentration Variations in 4H-SiC Substrates Using X-ray Contour Mapping
Authors: Yang Yu , Guo Jianqiu , Raghothamachar Balaji , Chan Xiaojun , Kim Taejin , Dudley Michael
Published: February 2020

4H-SiC is one of the promising wide bandgap material that attracts considerable attention in power electronics industry. Several applications have benefited from the improved performances of devices made of 4H-SiC in terms of its higher breakdown fields, efficiency and reliability as well as excellent physical properties. However, the widespread commercialization of this material is still being hindered by the various defects introduced during the crystal growth which is found to degrade the device performance and lifetime. Stacking faults, for example, have been considered to be one of the most detrimental structural defects. It has been predicted theoretically that high nitrogen doping concentration level (above 2 x 1019cm-3) inside 4H-SiC crystal will increase propensity for formation of stacking faults. These can create quantum wells which can lower the free energy of the whole crystal once the barrier for partial dislocation motion is overcome by thermal energy (achieved by annealing above 1000o C) [1-2]. Moreover, from a perspective of high power electronic applications, other severe issues, such as inhomogeneous resistivity and a large {0001} surface roughness of substrate are characteristic of heavily nitrogen doped SiC crystal [3]. It is known that, the nitrogen incorporation kinetic is anisotropic in different crystallographic directions during PVT growth of SiC. Therefore, better understanding the nitrogen doping distribution inside the material is important. In our experiments, we conducted X-ray topographic contour mapping [4] using synchrotron monochromatic X-ray beam by rocking the heavily doped wafer surface successively with a fixed angular step size around the near-surface diffraction vectors 0008 and -0008 respectively. From these measurements, a strain map can be derived by deconvoluting the lattice parameter variations from the lattice tilt. The nitrogen doping concentration can be calculated from the isotropic lattice strain due to the incorporation of dopants according to the equation from H. Jacobson [5].

[1] Y. Yang, J. Guo, Ouloide Goue, B. Raghothamachar, M. Dudley, G. Chung, E. Sanchez, J. Quast, I. Manning, and D. Hansen, J. Crystal Growth, 452, 32 (2016).

[2] T.A. Kuhr, J. Liu, H. J. Chung et al., Journal of Applied Physics,92, 5863 (2002).

[3] N. Ohtani, M. Katsuno, M. Nakabayashi, T. Fujimoto et al., J. Crystal Growth, 311, 1475 (2009).

[4] S. R. Stock, Haydn Chen and H. K. Birnbaum, Philosophical Magzine A, 53(1), 73(1986).

[5] H. Jacobson, J. Birch, C. Hallin, A. Henry, R. Yakimova, T. Tuomi, E. Janzén, and U. Lindefelt, Applied Physics Letters 82 (21), 3689 (2003).

Figure 1
Effects of GDL Compression on SGL Binder
Authors: Lamibrac Adrien , Büchi Felix N , Eller Jens
Published: February 2020

The gas diffusion layer (GDL) is the key component for efficient mass and heat transport between the flow field structures and the catalyst layer of polymer electrolyte fuel cells (PEFC). The characterization of its transport properties under different compression levels is important to understand the implications of the inhomogeneous GDL compression on the channel-rib scale. While experimental and numerical studies show good agreement for GDLs with solid binder [1], the contributions of the porous binder in SGL type GDLs remain unclear [2, 3].

This work reports on the compression dependent characterization of the GDL morphology, mass and energy transport by means of X-ray tomographic microscopy (XTM) and numerical transport simulations of SGL 24 BA material. X-ray tomographic microscopy images were recorded with a 2.2 um voxel edge length. Though the pores in the binder are not resolved with such voxel dimensions, a specific segmentation procedure of the reconstructed images makes possible to distinguish between fibres, binder and void (see Figure 1, left). The porous binder domains are identified as stiff GDL component, as the volume of the binder domain remains constant for compression levels between 0 – 25 % (see Figure 1, right). In this compression range only the volume of the identified void domain reduces, the fibers deform but the binder domains are not affected. For compression levels above 25 % the binder starts to be compressed and the volume of the binder domains reduces similar as the overall GDL volume. Comparing numerical transport simulations with experimental GDL effective diffusivity and conductivity data, the binder effective transport parameters are determined for different compression levels.

References

[1] J. Becker, R. Flückiger, M. Reum, F. N. Büchi, F. Marone, M. Stampanoni, J. Electrochem. Soc., 156, B1175, 2009.

[2] R. Flückiger, S. A. Freunberger, D. Kramer , A. Wokaun, G. G. Scherer, F. N. Büchi, Electrochim. Acta 54, 551–559, 2008.

[3] A. Rashapov, J. T. Gostick, Transport in Porous Media, 115, 411-433, 2016.

Figure 1: Left) 3D rendering of a ternary segmented SGL 24BA gas diffusion layer at 0 % compression ; green: fibres, red: binder, white: void; Right) relative volume fractions of the components of the ternary segmentation as function of compression.

Figure 1
Introducing Microporous Layer (MPL) Attached Catalyst Coated Membrane for PEFCs
Authors: Tanuma Toshihiro
Published: February 2020

A brand-new membrane electrode assembly (MEA) fabrication method was conceived. The performance of the MEA by using an MPL attached catalyst coated membrane (CCM) was equivalent to that by using the gas diffusion electrode (GDE) method, indicating that the coated interface between the catalyst layer and the MPL plays a significant role in achieving better MEA performance.

For the last few years, we have been doing research on hydrophilic MPLs. We have already reported (1, 2) that the MEA using a hydrophilic MPL showed much better performance in a wide range of pressure and humidity conditions than that using a conventional hydrophobic MPL. We also found that the MEA using the GDE method on the hydrophilic MPL showed the highest cell voltage compared to the MEAs using the decal method and the membrane coating method. In the case of the MEA using the GDE method on the hydrophilic MPL, the catalyst layer (CL) is integrated into the MPL to form a firm interface between the CL and the hydrophilic MPL. We suggested that this interface allows product water and oxygen gas to transport more smoothly (3).

In order to examine the significance of the interface between the CL and the MPL in this work, we used an MPL attached CCM as illustrated in Fig.1, which was fabricated by hot-pressing a cathode CL coated hydrophilic MPL to the membrane with an anode CL. The hydrophilic MPL consists of carbon fiber and ionomer. The MEA employing the MPL attached CCM showed equivalent performance to that by using the gas diffusion electrode (GDE) method as shown in Fig. 2. This result clearly indicates that the coated interface between the CL and the MPL is more favorable than the hot-pressed interface. The MPL attached CCM is easy to handle because it shows little dimensional change despite employing a non-reinforced membrane. As any GDL substrate is applicable to this MPL attached CCM, this is a unique and promising method for MEA fabrication.

References

1. T. Tanuma and S. Kinoshita, Energy Procedia, 28, 12 (2012).

2. T. Tanuma, M. Kawamoto and S. Kinoshita, Journal of The Electrochemical Society, 164, F499 (2017).

3. T. Tanuma and S. Kinoshita, Journal of The Electrochemical Society, 161, F94 (2014).

Figure 1
Investigating the Effect of Non-Uniform Microporous Layer Intrusion on Oxygen Transport in Dry and Partially Saturated Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layers
Authors: Wong Andrew , Bazylak Aimy
Published: February 2020

State-of-the-art gas diffusion layers (GDL) used in polymer electrolyte membrane (PEM) fuel cells have a bi-layered structure comprising of a carbon fiber substrate and a microporous layer (MPL). Addition of the MPL has been shown to improve the fuel cell performance [1-2]. Earlier studies have also shown that the configuration of the MPL relative to the GDL has an effect on gas transport through the combined diffusion medium [3-4]. This study investigates the effect of the MPL intrusion and morphology on the oxygen transport in PEM fuel cell GDLs. In particular, we investigate the effect of varying MPL layer characteristics, both interior and exterior to the carbon fiber substrate in dry and saturated conditions, on the oxygen diffusivity and liquid water saturation at breakthrough conditions. Stochastic generation methods were utilized to create GDLs with varying MPL intrusion depths and morphologies. The generated morphologies of the non-uniform MPL layers were validated using reconstructed microscale X-ray-computed tomography.

Pore network modelling was applied to simulate the oxygen transport behavior within these stochastically generated materials. The liquid water saturation was calculated using an invasion percolation algorithm. Three distinct scenarios were studied: varied MPL intrusion depth, varied MPL outer layer thickness, and constant MPL thickness with varied intrusion depth. In controlling the MPL intrusion, it was found that ~50-60 µm is the critical MPL thickness beyond which the saturated oxygen effective diffusion coefficient rapidly increases and the substrate saturation rapidly decreases (Figure 1). This study suggests that when the GDL is invaded by liquid water through cracks in the MPL, preferential MPL configurations exist for improving oxygen transport and minimizing substrate saturation. The results of this study can be used to guide the continued improvement of MPL fabrication.

References

[1] J. Lee, R. Yip, P. Antonacci, N. Ge, T. Kotaka, Y. Tabuchi, and A. Bazylak, “Synchrotron Investigation of Microporous Layer Thickness on Liquid Water Distribution in a PEM Fuel Cell,” J. Electrochem. Soc., vol. 162, no. 7, pp. F669–F676, Apr. 2015.

[2] Z. Qi and A. Kaufman, “Improvement of water management by a microporous sublayer for PEM fuel cells,” J. Power Sources, vol. 109, no. 1, pp. 38–46, 2002.

[3] C. Chan, N. Zamel, X. Li, and J. Shen, “Experimental measurement of effective diffusion coefficient of gas diffusion layer/microporous layer in PEM fuel cells,” Electrochim. Acta, vol. 65, pp. 13–21, 2012.

[4] N. Zamel, J. Becker, and A. Wiegmann, “Estimating the thermal conductivity and diffusion coefficient of the microporous layer of polymer electrolyte membrane fuel cells,” J. Power Sources, vol. 207, pp. 70–80, 2012.

Figure 1
Nano-Scale Simulation of Platinum Particle Dissolution, Re-Deposition, and Transport
Authors: Ogawa Shohei , Yarlagadda Venkata , Kongkanand Anusorn , Litster Shawn
Published: February 2020

Dissolution and redeposition of platinum (Pt) in the catalyst layer of polymer electrolyte fuel cells (PEFCs) results in a loss of electrochemically active surface area and increases in the overpotential for the oxygen reduction reaction, reducing efficiency.1 Typically, carbon supported Pt (Pt/C) catalyst exhibits Pt particles both on the external surface of the carbon support as well as within the internal micro-pores and smaller mesopores of the support that are known to be inaccessible to ionomer. Experimental evidence suggests that there may be preferential exchange of Pt between external and internal Pt.2,3Here, we apply a computational model of the ionomer coated catalyst surface in combination with a Pt particle embedded within a water-filled micro-pore. To evaluate the transfer of Pt between internal and external Pt, we have developed a three-dimensional computational model to simulate proton and Pt ion transport using a Poisson-Nernst-Planck model in combination with Pt dissolution and redeposition as well as Pt oxide coverage models (Figure 1a). Our results suggest that the external Pt particle in contact with the incomer increases in a diameter, whereas the Pt particle in the pore reduces its diameter. The Figure 1 c) shows the change of the diameter of the external and internal particles with the electrode potential cycling.

This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.

Figure 1
Understanding Binary Interactions and Aging Effects in Catalyst Layer Inks for Controlled Manufacturing
Authors: Dixit Marm , Hatzell Kelsey B
Published: February 2020

Controlled manufacturing and processing of multi-material electrodes and catalyst layers for energy storage and conversion applications is important for achieving low-cost and durable systems. Typically electrodes are coated using solution-based or wet- methods to achieve high aspect ratio thin films. The processing conditions as well as the ink formulation affect the ultimate material arrangment and electrode properties. Currently, ink formulation primarily focuses on macro-scale process-specific optimization (i.e. viscosity and surface/interfacial tension) rather than understanding local and nano-scale interactions between in constituents in an ink. Herein, we investigate local interactions between carbon and polymer components in a carbon based ink. We specifically investigate the role of active material surface chemistry plays on aggregation, stability, and sedimentation dynamics in multi-component inks1,2. The material arrangement and organization in a cast thin film is discerned from both shear and elongational flow processing methods. Finally, we introduce novel direct and indirect (rheological ) techniques to probe shear induced transformationd in inks or viscoelastic materials3,4.

[1] Holdcroft, S. (2013). Chemistry of Materials, 26(1), 381-393.

[2] Takahashi, S., Shimanuki, J., Mashio, T., Ohma, A., Tohma, H., Ishihara, A.,& Miyazawa, A. (2017). Electrochimica Acta, 224, 178-185.

[3] Hatzell, K. B., Eller, J., Morelly, S., Tang, M., Alvarez, N. J., & Gogotsi, Y. (2017). Faraday Discussions

[4] Hatzell, K. B., Boota, M., & Gogotsi, Y. (2015). Chemical Society Reviews, 44(23), 8664-8687.
Understanding Binary Interactions in Fuel-Cell Catalyst-Layer Inks
Authors: Berlinger Sarah A , McCloskey Bryan D , Weber Adam Z.
Published: February 2020

The catalyst layer (CL) in a fuel cell comprises the majority of the stack cost. Therefore, for mass commercialization of fuel cells, it is critical to understand the formation process of the CL, thereby informing the design and optimization of next-generation CLs.

CLs are made from precursor inks that are colloidal dispersions of a catalyst (typically platinum on carbon), an ion-conducting polymer (traditionally perfluorinated sulfonic acid or PFSA, the most widely used of which is Nafion), and a solvent traditionally composed of different low-weight alcohols and water. Ink properties directly impact the resultant morphology, transport, and electrochemical properties of the CL.[1] While there has been much work on ink composition, the research to date has been almost entirely empirical, resulting in limited understanding of fundamental species interactions in the ink and their effect on the resultant CL structure. Furthermore, though the structure of nafion in solution has been extensively studied,[2-4] there has been little work on applying this information to decribe interactions with other ink components.

In this study, we create a series of inks composed of commerical PFSA dispersions of different equivalent weights and side chains, a variety of solvents, and model compound particles. Effects of solvent composition, ionomer loading, and processing conditions are investigated on both the solution phase and the resulting cast solid layers including ionomer thin films and micrometer layers with and without solid particles. Using microscopy, rheology, and dynamic light scattering, we explore the aggregation behavior and fundamental forces governing the behavior of these ionomer/particle, particle/solvent, and solvent/ionomer interactions. Using these insights, design metrics and governing design rules are elucidated, which can help in fabrication of next-generation CLs.

Acknowledgements

This work was mainly funded under the Fuel Cell Performance and Durability Consortium (FC PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231. S.B also acknowledges support from National Science Foundation Graduate Research Fellowship under Grant No. DGE 1106400.

References

1. Holdcroft, S., Fuel Cell Catalyst Layers: A Polymer Science Perspective. Chemistry of Materials, 2014. 26(1): p. 381-393.

2. Lee, S.-J., et al., Solution properties of nafion in methanol/water mixture solvent. Polymer, 2004. 45(8): p. 2853-2862.

3. Welch, C., et al., Nafion in Dilute Solvent Systems: Dispersion or Solution? ACS Macro Letters, 2012. 1(12): p. 1403-1407.

4. Kim, Y.S., et al., Origin of Toughness in Dispersion-Cast Nafion Membranes. Macromolecules, 2015. 48(7): p. 2161-2172.
Three-Dimensional Water Distribution and Interfacial Effects through Simultaneous Neutron and X-Ray Tomography
Authors: LaManna Jacob M , Yue Yang , Trabold Thomas A , Fairweather Joseph D. , Hussey Daniel S , Baltic Elias , Jacobson David L
Published: February 2020

It is often difficult to fully resolve non-destructively both liquid water and solid structure within an operating fuel cell. Both neutrons and X-rays offer non-destructive probes to understand internal structure of objects of interest. By combining these two probes it is possible to obtain three-dimensional information on liquid water distribution and its interaction with the material structure and interfaces. The NIST neutron imaging facility offers the ability to perform simultaneous neutron and X-ray tomography by orienting a micro-focus X-ray tube orthogonal to the neutron beam. A small, 0.36 cm2, fuel cell was developed specifically to facilitate simultaneous neutron and X-ray imaging and was operated at high flow rates to avoid water slug formation in the gas channels. The presentation will describe initial results and ongoing efforts in simultaneous tomography of fuel cells to further understanding of interfacial effects and overall water distribution. Shown in the figure are three-dimensional volume renderings of the composite neutron and X-ray tomography reconstructions giving water distribution with respect to fuel cell hardware and material interfaces. This work was partially supported by GM through an Academic Partnership Single Project Agreement with Rochester Institute of Technology.

Figure 1
Low Pt Content Proton Exchange Membrane Fuel Cell Stack
Authors: Matolin Vladimir , Fiala Roman , Dubau Martin , Ostroverkh Anna , Lavkova Jaroslava , Kus Peter , Vaclavu Michal , Matolinova Iva
Published: February 2020

Pt doped cerium oxide thin films with a nano-porous structure was prepared by reactive rf magnetron deposition on a fuel cell gas diffusion layer [1] coated with nitrogenated amorphous carbon films (CNx) as an interlayer. We show that the structure and morphology of the cerium oxide-CNx composites depend on the oxygen and nitrogen concentration in working gas used for catalyst deposition. Considerable plasma erosion of the carbonaceous interlayer accompanied by the formation of highly porous carbon/cerium oxide bilayer systems is reported. Oxygen plasma etching of the carbon interlayer occurring simultaneously with cerium oxide film growth is considered to be the driving force for the formation of cerium oxide film nanoporosity with large active surface.

Thin film techniques permits to grow the catalyst film characterized by highly dispersed platinum [2] showing high hydrogen oxidation activity and stability. These new materials may help to substantially reduce the demand for expensive noble-metals in proton exchange membrane fuel cell (PEMFC) catalytic applications.

We prepared 50 cm2 MEAs with 6 μg Pt/cm2 anode and 300 μg Pt/cm2 cathode catalysts. The MEAs activity were measured in 10 cell hydrogen/air water cooled PEMFC stack, developed in our laboratory. Efficiency measurements performed at different pressures, temperatures, and hydrogen flow conditions were compared with performance of standard reference MEAs. We showed that MEAs made by assembling thin film Pt-CeOx/CNx anodes, containing 6 μg Pt per 1 cm2 only, and reference cathodes can be used for fabrication of state of the art low Pt content PEMFC stacks delivering high specific power. It is shown that the almost Pt free anode technology represents promising solution for future fabrication of PEMFC stacks using Pt free cathodes under development.

Reference

[1] Fiala, R; Figueroba, A; Bruix, A; Vaclavu, M; Rednyk, A; Khalakhan, I; Vorokhta, M; Lavkova, J; Illas, F; Potin, V; Matolinova, I; Neyman, KM; Matolin, V, High efficiency of Pt2+- CeO2 novel thin film catalyst as anode for proton exchange membrane fuel cells, Appl. Catal. B-Environ., 197 : 262–270, 2016.

[2] Dvorak, F; Camellone, MF; Tovt, A; Tran, N-D; Negreiros, FR; Vorokhta, M; Skala, T; Matolinova, I; Myslivecek, J; Matolin, V; Fabris, S, Creating single-atom Pt-ceria catalysts by surface step decoration, Nat. Commun., 7 (Feb): Art. No. 10801, 2016.
Influence of Cathode Catalyst Layer Ionomer on Air-Cooled, Open-Cathode Fuel Cells
Authors: Atkinson Robert W. , Garsany Yannick , Rodgers Joseph A. , Hazard Matthew W. , Stroman Richard O'Neil , Gould Benjamin D.
Published: February 2020

Application of a proton exchange membrane fuel cell (PEMFC) in a planar, open-cathode configuration can significantly reduce system weight and complexity compared to a conventional fuel cell stack with closed cathodes. This decrease in device weight makes an array of open-cathode fuel cells a compelling power source for unmanned aerial vehicles because vehicle performance is highly dependent on weight. Forfeiting the ability to precondition incoming air to the cathodes leaves the fuel cells vulnerable to dry-out. Our previous studies on open-cathode fuel cells demonstrate that low porosity gas diffusion media facilitate a lower cell temperature and improve cell hydration but cell performance is still limited by significant ohmic resistance related to dehydration by evaporation at moderate cell temperatures (~40°C). Short-side-chain (SSC) ionomers with high ion exchange capacity have shown promise in closed cathode fuel cells by improving proton conductivity, especially during low RH operation, compared to their long-side-chain counterpart, Nafion. In this work, we study the relationship between cathode catalyst layer ionomer and cell polarization behavior in ambient conditions that have been observed to exacerbate dry-out and flooding in open-cathode fuel cells. Catalyst-coated membranes (CCMs) are prepared in-house with an automated, ultrasonic spray, layer-by-layer deposition method. Catalyst layers are formed with ionomers of variable equivalent weight and pendant side chain length and the catalyst layer proton conductivity is measured as a function of these properties. We use cyclic voltammetry and electrochemical impedance spectroscopy (EIS) to characterize the catalyst layers and performance losses in various ambient conditions.