bluuebunny/crossref_metadata_2025 huggingface dataset:
(Invited) A Revolutionary GaN-on-SiC Heteroepitaxy for High-Frequency Power Transistors
Authors: Chen Jr-Tai , Bergsten Johan , Malmros Anna , Janzén Erik , Thorsell Mattias , Rorsman Niklas , Kordina Olof
Published: February 2020

GaN HEMTs have a great potential to be well suited for the forthcoming 5G communication infrastructures and other millimeter wave applications, where high-power and high-efficient devices are required. For GaN HEMT heterostructures used in high-frequency applications (> 8 GHz), semi-insulating (SI) SiC is the best choice of substrates, owning to its high thermal conductivity and high resistivity, and the maturity in large-wafer mass production (up to 150 mm). Nevertheless, the typical thickness of a GaN layer grown on SiC substrates has to be at least 1.5~2.0 µm to obtain a structural quality manufacturable for device fabrication, due to the large in-plane lattice mismatch with GaN (3.4%). As a result, the thick GaN layer must be doped with acceptor-like impurities like carbon or iron [1] during the epitaxial growth to increase the resistivity, preventing a parallel conduction in the device. However, these impurities introduce deep traps, which capture electrons during the RF operation, rendering a depletion of the channel electrons thus reducing the device current/power density.

We report the realization of a revolutionary heterostructure of III-nitride materials grown on SiC substrates by a unique MOCVD method. Our success in the material growth enabled a new design of HEMT heterostructures requiring no intentional Fe- or C-doping. Excellent DC and RF performances of the highly-scaled GaN-based high electron mobility transistors (HEMTs) using the proposed new heterostructures are demonstrated. The new HEMT heterostructures also open up new opportunities in material growth schemes to further reduce the charge trapping effects that have been prevailing in the field over that last two decades. Consequently, a high RF output power of ~4 W/mm was obtained at a fundamental frequency of 30 GHz in the HEMT devices with a gate length (Lg) of 0.1 µm, biased at VDS= 30 volt. We also found that the thermal resistance of the new HEMT devices exhibits a substantially lower thermal resistance (RTH) of 4.1 oC mm/W, as compared with that of the conventional ones, which is typically around 9~10 oC mm/W [2] The details of the new heterostructures and device characteristics will be presented. Our breakthrough in the material growth has brought the GaN HEMT technology to a new height and will serve as a new route for further improvement.

IV.References

[1] J.-T. Chen et al, Appl. Phys. Lett. 102, 193506 (2013). Impact of residual carbon on two-dimensional electron gas properties in AlGaN/GaN heterostructure.

[2] J. Joh et al, IEEE Trans. Elec. Dev. 56, 2896 (2009) Measurement of Channel Temperature in GaN High-Electron Mobility Transistors.

Figure 1
Lithium Manganese Oxyfluoride As a New Cathode Material Exhibiting Oxygen Redox
Authors: House Robert A. , Jin Liyu , Maitra Urmimala , Roberts Matthew R. , Bruce Peter G.
Published: February 2020

The next generation of high performance Li-ion rechargeable batteries relies on the discovery of new positive electrode materials with a greater capacity to store charge. Generally speaking, highly ordered layered materials, such as LiCoO2 (LCO), LiNi0.85Co0.1Al0.05O2 (NCA) and LiNi0.13Mn0.54Co0.13O2 (NMC), are considered to be the best-in-class. One strategy to increase the capacity has been to explore the ordered Li-Rich materials, Li1+xM1-xO2, where x~1.2, where the greater number of Li ions increases the total theoretical capacity. However, an optimal candidate system for commercialisation within this space has yet to be identified.

In contrast to ordered materials, disordered structures have been largely ignored by the scientific community as they are perceived to exhibit poor Li+ mobility. However, recent research has overturned this understanding and several disordered rock salt structures with competitive energy densities have been proposed (such as Li4Mn2O5, Li2VO2F and Li1.2Mn0.4Ti0.4O2). Could these highly disordered Li-rich materials offer better structural stability and performance than their layered counterparts?

Here, we present the discovery of a new Li-rich disordered rocksalt material, lithium manganese oxyfluoride (LMOF), presented in Figure 1, whose structure appears to remain stable even over 100 charge-discharge cycles. Furthermore, LMOF exhibits a large, reversible capacity to store charge and, interestingly, this appears to be unexplained within the limits of transition metal redox capacity alone, raising the question of how such a large charge storage capacity can be achieved. In this work we fully characterise this new material and specifically we examine the role that oxygen plays in this anomalous charge compensation process.

Figure 1
High Voltage Cobalt Free Cathode Materials for Advanced Lithium Ion Batteries
Authors: Wohlfahrt-Mehrens Margret , Axmann Peter , Gabrielli Giulio , Balasubramanian Prasanth , Mancini Marilena
Published: February 2020

Lithium ion batteries are widely used in many applications and are the most promising energy storage systems for future mobility. Nevertheless, they still require further improvements in order to meet the energy and cost requirements for the next generation of electric vehicles. The development of cobalt free cathode materials with high capacity and high voltage can contribute significantly to further cost reduction of lithium ion batteries.

We present a series of Co-free, Li-rich Li1+xNi0.5Mn1.5O4 (0<x<1) compounds obtained via a low-temperature synthesis method. Chemically lithiated Li1+yMn1.5Ni0.5O4 (LMNS) (0< y < 1) with spherical particle morphology and high with tap densities (2.2 – 2.5 g/cm3) have been synthesized from coprecipitated mixed nickel manganese hydroxides as precursors. Figure 1 shows the charge/discharge curve of a sample with the composition Li1.8Ni0.5Mn1.5O4. The electrochemical mechanism in the low-potential region and the involved phase transitions, studied by different techniques including HR-TEM and XRD, will be discussed in more detail together with the electrochemical behavior under different operative conditions.

The obtained lithium‐nickel‐manganese oxide samples have been tailored with respect to composition and crystal structure in order to reach high capacities (up to 250 mAh g-1) with good cycling stability. In addition, full cell data with Li1+xNi0.5Mn1.5O4.0 (0<x<1) compounds as cathode materials and either graphite or silicon/carbon composites as anodes will be presented. LiNi0.5Mn1.5O4/Li1.3Ni0.5Mn1.5O4 blend cathodes with adapted extra lithium can be used to compensate irreversible capacity losses of silicon based anodes in full cells [1]. The cycle life of full cells can be further improved by optimizing morphology, particle size distribution and additional surface coating. The influence of electrode preparation and selection of electrolyte additives on long life stability will also be discussed.

[1] G. Gabrielli, M. Marinaro, M. Mancini, P. Axmann, M. Wohlfahrt-Mehrens, J. Power Sources 351 (2017) 35

Figure 1
(Invited) New Battery Chemistries enabled by Advanced Aqueous, Non-aqueous and Hybrid Electrolytes
Authors: Xu Kang
Published: February 2020

Electrolyte/electrode interphase is the key component that supports reversible battery chemistries to operate at potentials far from the thermodynamic equilibria. Although interphases should exist universally in any advanced electrochemical devices with cell voltage >3 V, we only started to understand it on the Li-based anode surfaces in non-aqueous electrolytes. The knowledge about interphasial chemistry and formation mechanism accumulated in the past 2 decades enables us to modify, control and even tailor-design new interphases for various battery chemistries with unprecedented freedom. We have departed from solely dependence on electrolyte solvent as interphasial building-blocks, while recent adventures have even taken us far beyond the demarcation separating non-aqueous and aqueous regimes. In this talk, I will summarize these recent efforts and try to shed light on the future attempts of designing interphases for new chemistries.
Aerosol Chemical Vapor Deposition of One-Dimensional Cu-Doped TiO2 Nanostructured Anodes for High-Rate Lithium Ion Batteries
Authors: Kacica Clayton Thomas , Biswas Pratim
Published: February 2020

Energy storage has become increasingly important in recent years due to the rapidly increasing number of electric vehicles (EVs) in use and expanded utilization of intermittent renewable energy sources. Improvements in longevity, energy density, and high rate capability are needed for the next generation of batteries. Lithium-ion batteries (LIBs), the most widely commercialized rechargeable batteries are being used in EVs and some pilot grid-scale storage facilities.

One avenue for enhancing the performance of LIBs is the development of novel electrode materials. One such material, titanium dioxide (TiO2) exhibits very high stability during cycling and is capable of ultra-high rate charging and discharging. However, a major drawback to TiO2 is its relatively low electrical conductivity, which limits its performance. Using nanostructured, one-dimensional morphologies the issue of low conductivity can be mitigated. Additionally, metal-doping of TiO2 with various metallic cations such as zinc, copper, or niobium has been shown to increase conductivity as well.

Aerosol chemical vapor deposition (ACVD) is an atmospheric pressure technique used to deposit nanostructured metal oxide thin films. In ACVD, a vapor phase organometallic precursor is fed to a reactor and undergoes thermal degradation forming metal oxide monomers. The monomers nucleate to form particles which undergo condensational growth before depositing onto a heated substrate, where the particles sinter into various morphologies. Controlling the rates of these processes using various system parameters leads to the deposition of one-dimensional, columnar structures. Additionally, by introducing two precursors simultaneously doped materials can be synthesized, with the composition controlled by the relative flowrates into the reactor.

In this work, we aim to investigate the effect of Cu-doping on the properties and performance of nanostructured TiO2 negative electrodes in LIBs. To achieve this, Cu-doped TiO2 films with compositions between 0-10 at% are synthesized on stainless steel using ACVD and directly utilized as anodes in LIBs. In order to determine the effect of Cu-doping on battery performance a rate capability test is performed by cycling the batteries at progressively higher rates. Additionally, long-term cycling is performed to determine the stability of the electrodes. Additionally, characterization of the lithium diffusion coefficient is performed using both cyclic voltammetry (CV) and galvanostatic intermittent titration technique (GITT). Finally, density function theory (DFT) simulations are performed in order to validate the experimentally measured values for the lithium diffusion coefficient and the electrical conductivity.
Improving the Cycling Performance of Lithium Ion Batteries Silicon/Graphite Anodes Using PI Binder
Authors: Oh Jeonghun , Jin Dahee , Jo Hearin , Park Jinkyu , Yoon Byeolhee , Choi Junyoung , Kim Seokwoo , Ryou Myung-Hyun
Published: February 2020

As the market for Electric Vehicles (EV) and Energy Storage System (ESS) grows, there is a growing demand for Lithium secondary batteries with high energy density.

However, graphite (372 mAh g-1 for LiC6), a commercially available anode active material, is not suitable for large scale battery system such as EV or ESS due to its low theoretical capacity.

Although, Silicon (Si, 4200 mAh g-1 for Li22Si5 or Li4.4Si) has a higher theoretical capacity than graphite, it has not been successfully implemented in commercialized LIBs due to suffering from large volume changes (> 300 %) during alloying and dealloying. Moreover, the stresses accompanying these changes cause mechanical failure and result in severely degraded electrochemical performances of Si electrodes.

Consequently, Si/C composite anodes (Si/C anodes), wherein Si constitutes only a portion of the active material, have been proposed as an alternative to pure Si anodes. This approach exhibits certain advantages, since even if the theoretical anode capacity increases infinitely, the total theoretical capacity of the battery soon reaches saturation due to the limited theoretical capacity of the cathode.

Herein, we used PVdF and P84 binder in high-loading Si/C anodes (~4.0 mg cm-2) and investigated their electrochemical and mechanical properties. Specifically, mechanical properties such as electrochemical stability, cycling performance and rate capability were evaluated using cyclic voltammetry, with the corresponding data recorded using a battery cycler.

The mechanical properties of the above anodes were evaluated using Surface And Interfacial Cutting Analysis System (SAICAS).
Facile Synthesized Nanoporous Si/C Anode for High Power and Energy Applications
Authors: Min Chan-Woo , Nazir Aqsa , Park Chan-Jin
Published: February 2020

As the development of electric vehicles (EVs) is progressing recently, it is urgent to develop high energy density batteries. Currently, Li-ion batteries (LIBs) have been mainly used for EVs. In LIBs, the most representative commercial anode material for LIBs is graphite. Unfortunately, its low theoretical capacity (372 mAh g−1) and rate capability is not sufficient to be used in emerging applications, such as EVs and large-scale ESS. Si is a promising anode material for LIBs due to its large theoretical capacity (3600 mAh g-1 for Li15Si4), low working potential (< 0.4 V vs. Li/Li+), low cost, large amount resources, low toxicity, and environmental friendliness. However, Si based anodes in LIBs have several drawbacks. During alloying with Li, Si exhibits large volume expansion of >300%, leading to crush of the active materials and capacity drop. Si electrode also has low intrinsic electrical conductivity compared with graphite. In addition, solid electrolyte interface (SEI) layer is not stable when Si is exposed to organic electrolytes, leading to the consumption of electrolyte and low Coulombic efficiency. Recently, studies on using a buffer layer coating have been conducted to alleviate the stress induced by volume change of Si. Recently, we reported an ultra-simple Mg-thermal-reduction method for the production of mass-scalable coral-like Si powders with a high surface area, and 3-dimensionally (3D)-interconnected Si structures. It is noticeable that simple Mg-reduction process can be used at low cost in ambient air.

In the present work, we employed carbon coating to improve the conductivity of the previously obtained porous nano-sized Si electrode and the stability of SEI layer. We prepared a nano-porous Si/C composite through a facile sol-gel route with various amounts of carbon from citric acid precursor. The morphology and phase purity of the synthesized Si/C composites were analyzed by XRD, FE-SEM, TEM, Raman spectroscopy, and BET analysis. Furthermore, electrochemical characteristics of Si/C electrode was analyzed by electrochemical impedance spectroscopy (EIS), cyclic voltammetry, and galvanostatic charge-discharge test. The Si/C electrode exhibited a long cycling stability, good rate capability, and high reversible capacity. In particular, the Coulombic efficiency in the first cycle was improved significantly. This superior performance of the electrode is attributed to the improvement in electrical conductivity, Li-ion accessibility, stability of SEI layer, and the suppression of volume expansion owing to porous nano-sized Si and carbon coating.
Li[Ni0.865Co0.120Al0.015]O2 Cathode with Compositional Gradation for High-Energy and Long-Life Lithium-Ion Batteries
Authors: Park Kang-Joon , Park Geon-Tae , Sun Yang-Kook
Published: February 2020

Lithium-ion batteries (LIBs) are considered to be the most promising power sources for electric vehicles (EVs) because of their high energy densities and long cycle lives. However, major impediments for general public acceptance of EVs are their cost, durability, and driving range.1,2 Following the successful application of the hybrid scheme to the NCM cathodes,3-6 we developed a core–shell with concentration gradient Li[Ni0.865Co0.120Al0.015]O2 cathode material (CSG NCA) with a Ni-rich core to maximize the discharge capacity and a Co-rich particle surface to provide structural and chemical stability. Compared to the conventional NCA cathode with a uniform composition, the gradient NCA cathode exhibits improved capacity retention and better thermal stability. Even more remarkably, the gradient NCA cathode maintains 90% of its initial capacity after 100 cycles when cycled at 60 °C, whereas the conventional cathode exhibits poor capacity retention and suffers severe structural deterioration. The superior cycling stability of the gradient NCA cathode largely stemmed from the gradient structure combines with the Co-rich surface, which provides chemical stability against electrolyte attack and reduces the inherent internal strain observed in all Ni-rich layered cathodes in their charged state, thus providing structural stability against the repeated anisotropic volume changes during cycling. The high discharge capacity of the proposed gradient NCA cathode extends the driving range of electric vehicles and reduces battery costs. Furthermore, its excellent capacity retention guarantees a long battery life. Therefore, gradient NCA cathodes represent one of the best classes of cathode materials for electric vehicle applications that should satisfy the demands of future electric vehicles.

References

U.S. Department of Energy, EV Everywhere Grand Challenge Blueprint, 31 January 2013.

S.-T. Myung, F. Maglia, K.-J. Park, C. S. Yoon, P. Lamp, S.-J. Kim, Y.-K. Sun, ACS Energy Lett.
2017, 2, 196.

Y.-K. Sun, S.-T. Myung, B.-C. Park, J. Prakash, I. Belharouak, K. Amine, Nat. Mater.
2009, 8, 320.

Y.-K. Sun, Z. Chen, H.-J. Noh, D.-J. Lee, H.-G. Jung, Y. Ren, S. Wang, C. S. Yoon, S.-T. Myung, K. Amine, Nat. Mater.
2012, 11, 942.

B.-B. Lim, S.-J. Yoon, K.-J. Park, C. S. Yoon, S.-J. Kim, J. J. Lee, Y.-K. Sun, Adv. Funct. Mater.
2015, 25, 4673.

6. D.-W. Jun, C. S. Yoon, U.-H. Kim, Y.-K. Sun, Chem. Mater.
2017, 29, 5048.
(Invited) Photophysics and Electronic Structure of Metal-Organic Frameworks
Authors: Shustova Natalia
Published: February 2020

The development of porous crystalline materials such as metal–organic frameworks (MOFs) could add a new dimension to a wide number of applications including supercapacitors, electrodes, or thermoelectrics. From this perspective, the understanding and tailoring of the electronic properties of MOFs are key fundamental challenges that could unlock the full potential of these materials. In the presentation, I will focus on the fundamental insights responsible for the electronic properties of bimetallic systems in which the second metal incorporation occurs through (i) metal replacement in the framework nodes, (ii) metal node extension, and (iii) metal coordination to the organic ligand. Microwave conductivity, X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy, powder X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, pressed-pellet conductivity, and theoretical modeling were imployed to shed light on the key factors responsible for the tunability of MOF electronic structures. Experimental prescreening of MOFs was performed based on changes in the density of electronic states near the Fermi edge, which was used as a starting point for further selection of suitable MOFs. As a result, we demonstrated that the tailoring of MOF electronic properties could be performed as a function of metal node engineering, framework topology, and/or the presence of unsaturated metal sites while preserving framework porosity and structural integrity. These studies unveil the possible pathways for transforming the electronic properties of MOFs from insulating to semiconducting, as well as provide a blueprint for the development of hybrid porous materials with desirable electronic structures.
Seebeck Coefficient Measurements of Pristine and Tcnq Loaded HKUST-1 Thin Films
Authors: Hassan Zeinab Mohamed , Chen Xin , Zhang Kai , Redel Engelbert , Baumgart Helmut
Published: February 2020

Seebeck meaurements have been performed on i) quasi-liquid epitaxially highly oriented HKUST-1 Surface Anchored Metal-Organic Framework (SURMOF) films and for comparison on ii) random polycrystalline HKUST-1 thin films; pristine as well as loaded with tetracyanoquinodimethane (TCNQ).

However, the horizontal Seebeck coefficient of highly oriented (001) SURMOF films is practically around 0 µV/K as there is no measurable horizontal carrier transport parallel to the SURMOF surface. Only in the polycrystalline and pristine HKUST-1 thin films exhibit a high and positive Seebeck coefficient of 422.30 µV/K at 330 K. For the TCNQ loaded polycrystalline HKUST-1 thin films a lowering of the Seebeck coefficient of 388.80 µV/K at 330 K.

The high Seebeck coefficients of these random polycrystalline MOF films demonstrates promising application potential of MOF films in future thermoelectric and electronic devices working highly efficient in the RT (Room Temperature) region.

X. Chen, Z. Wang, Z. M. Hassan, P. Lin, K. Zhang, H. Baumgart, E. Redel ECS Journal of Solid State Science and Technology 2017 6(4), 1-4.

Figure 1
Dipolar Molecular Rotors in Surface-Anchored Metal Organic Frameworks
Authors: Hamer Sebastian
Published: February 2020

The functionalization of surfaces with molecular machines is highly interesting due to the vast ﬁeld of applications. Transferring the function of individual molecules to a macroscopic scale by bringing a multitude of them to act collectively is essential for materializing new technologies. For this purpose, dipolar rotors can be put into a supra molecular structure to switch the light transmission of a material by changing their orientation. A high three-dimensional order can be achieved by using rotors as linkers in surface-anchored metal organic frameworks (SURMOFs).1-3 Crafted SURMOF thin films allow the alignment of the dipolar rotors in an external electric ﬁeld.4 Making use of this behaviour, the transmission of light can be modulated with the application of the field and be investigated towards its uses in optical technologies. In this project, different organic linker molecules with permanent dipole moments are synthesized and used in SURMOF fabrication. Dielectric loss spectroscopy of the SURMOF thin films gives information about the dipole-dipole interaction within the layers and the alignment of rotors along the applied electric field.5 Devices are being constructed with a transparent top electrode for electric field application. Information gathered from the behaviour of first generation linker molecules will refine the molecule designs for the next generation so that the optical and electronic properties of the material can be tuned as desired.

Shekhah, H. Wang, S. Kowarik, F. Schreiber, M. Paulus, M. Tolan, C. Sternemann, F. Evers, D. Zacher, R. A. Fischer, C. Woll, J. Am. Chem. Soc.
2007, 129, 15118-15119.

-G. Gu, A. Pfriem, S. Hamsch, H. Breitwieser, J. Wohlgemuth, L. Heinke, H. Gliemann, C. Wöll, Microporous and Mesoporous Materials
2015, 211, 82-87.

L. Zhuang, A. Terfort, C. Wöll, Coordination Chemistry Reviews
2016, 307, 391-424.

Zheng, M. E. Mulcahy, D. Horinek, F. Galeotti, T. F. Magnera, J. Michl, J. Am. Chem. Soc.
2004, 126, 4540.

D. Horansky, L. I. Clarke, J. C. Price, T.-A. V. Khuong, P. D. Jarowski, M. A. Garcia-Garibay, Physical Review B
2005, 72, 014302.

Figure 1
(Invited) Progress Towards the Design of Metal-Organic Frameworks with Targeted Thermal Conductivities
Authors: Wilmer Christopher E , Babaei Hasan , Sezginel Kutay , Asinger Patrick A
Published: February 2020

In practice, the usefulness of metal–organic frameworks (MOFs) for many gas storage applications depends on their ability to rapidly dissipate the heat generated during the exothermic adsorption process. MOFs can be precisely designed to have a wide variety of architectures which can allow tuning their thermal conductivity. In this work, we use molecular dynamics simulations to investigate the effect of pore geometry, gas loading, and interpenetration on the thermal conductivity of MOFs. We find that larger pores and increased gas loading reduce thermal conductivity. We also find that the addition of a parallel thermal transport pathway yields a thermal conductivity nearly the sum of the two constituent frameworks. This relationship holds for a variety of interpenetrating MOFs with different atomic masses and a wide range of interframework interaction parameters. We show that both the strength and range of interactions between constituent frameworks play a significant role in framework mobility as well as framework coupling which can result in deviation from this relationship. We propose a simple model to predict thermal conductivity of the interpenetrated framework based on thermal conductivities of individual frameworks and an interframework interaction parameter which can account for this deviation.
(Invited) Leaving the Simple Models behind: How Surface Mounted Metal-Organic Frameworks (SURMOFs) Grow
Authors: Terfort Andreas
Published: February 2020

While metal-organic frameworks (MOFs) have been developed to expose many interesting properties, their utilization in devices often relies on the deposition at surfaces: This bonding does not only provide efficient mechanical contact, as it is e.g. useful for quartz-crystal microbalance (QCM) measurements, but also electrical contact, e.g. for electrochemical or even electronic applications. In addition, the deposition at solid surfaces provides means of directed localization and orientation of the MOF materials.

Many methods for the deposition of MOFs have been developed, the most common of which rely on the direct growth of the frameworks onto the surfaces [1]. The effectiveness of this approach has been demonstrated for a handful of systems, leading even to demonstrators e.g. for photovoltaic devices. Nevertheless, the deposition of surface-attached MOFs (SURMOFs) fails to produce well-defined surface layers, but rather results in heterogeneous and/or disordered systems.

In this talk we are going to present strategies on how SURMOF films can be deposited successfully by keeping certain parameters, such as deposition temperature [2], involved concentrations and exposure times within specific limits. The success of the respective combination of parameters can be rationalized be understanding and conjoining principles from coordination chemistry and surface chemistry [3], which typically are well-known in the respective communities but rarely taken into account by the other communities.

References:

[1] J.-L. Zhuang, A. Terfort, C. Wöll, Coord. Chem. Rev. 307 (2016) 391-424.

[2] X-J. Yu, J.-L. Zhuang, J. Scherr, T. Abu-Husein, A. Terfort, Angew. Chem. Int. Ed. 29 (2016) 8348-8352.

[3] J.-L. Zhuang, M. Kind, C.M. Grytz, F. Farr, M. Diefenbach, S. Tussupbayev, M.C. Holthausen, A. Terfort, J. Am. Chem. S
oc. 137 (2015) 8237-8243.
Visualization Study about Effects of Local Structure of Micro Porous Layer By X-Ray Radiography
Authors: Naito Hiroshi , Yoshida Toshihiko , Kawamura Katsuyuki , Sasabe Takashi , Hirai Shuichiro
Published: February 2020

Water management inside PEFC is one of the most important matter of studies for the improvement of the performance. Although it is considered that the clarification of transport mechanism of water inside PEFC will guide further development of GDL, there is a divergence of report about water transport mechanism in the past literatures. In the study of Deevanhxay et al.1, for instance, it is reported that there are clear liquid water paths from catalyst layer ( CL ) to GDL through MPL. While in the study of Naito et al.2, such a paths are not observed, and they suggests that water transport to GDL from CL is vapor form. Although they used different GDL, there seems to be consistent parameters governing these differences. As such, this study focuses on local MPL thickness, presence of crack and distribution of PTFE, and discusses correlation between these parameters and a mechanism of water transport inside GDL. Figure represents derived interfaces inside MEA and liquid water distribution at 1.5A/cm2. In the cathode side, it is found that MPL thickness is not uniform but has relatively large undulation. It is also shown that most of liquid water accumulates inside GDL, while in a thinner MPL region marked by dashed circle, clear liquid water accumulation exists. In the presentation, we will show more comprehensive and detailed mechanisms by focusing on effects of crack presence and PTFE distribution using a result of CT measurement of MEA.

Deevanhxay, T. Sasabe, S. Tsushima, S. Hirai, J. Power Sources, 230, 38 (2013).

Naito, T-C. Jao, T. Yoshida, T. Sasabe, K. Kawamura and S. Hirai, ECS Trans., 80, 425 (2017).

Figure 1
Structure, Properties, and Degradation of Ultrathin Ionomer Films in Catalytic Layers of Fuel Cells Using Atomic Force Microscopy
Authors: Morawietz Tobias , Handl Michael , Friedrich Kaspar Andreas , Hiesgen Renate
Published: February 2020

The performance of polymer electrolyte fuel cells and electrolysers depends on the nanostructure of its components. Mainly the nanostructured electrodes determine cell performance and degradation. In working fuel cell electrodes, the ionomer films that encapsulate the Pt/C agglomerates fall within the range of ultrathin films. In recent years, an oxygen transport resistance has been identified as a major hurdle in the development of low-loaded MEAs and the ionomer layer has been postulated to be the reason for additional resistance.

The quantitative investigation of the nanostructure of fuel cell electrodes, especially the analysis of the correct dimensions of the ionomer component has proven to be exceptionally difficult. Using electron beam-based techniques that need vacuum the ionomer is drying and significantly shrinking, in addition to a low contrast between ionomer and carbon components. Also cryogenic TEM analysis may alter the sample by use of alcoholic solvents that have an input on structure, despite of beam damage.

Atomic force microscopy (AFM) has the advantage to work in humid environment and at elevated temperatures, close to operational conditions. Using material-sensitive tapping mode, the high contrast between the ionomer- and the Pt/C phase in adhesion force mapping allows studying the distribution and thickness of the ionomer films that cover the Pt/C agglomerates (Figure 1c). With electronic current mapping, the free Pt-surface can be evaluated (Figure 1a,b).

In this contribution, the analysis of cross-sections of Nafion- and Aquivion–based fuel cell electrodes by material-sensitive AFM will be presented. Analysis of pristine electrodes delivers the distribution of the ionomer film thickness, small ionomer clusters, and larger agglomerates. A thickness distribution of the ionomer films ranging from roughly 4 to 20 nm was retrieved (Figure 1d). After operation, significant thinning of the ionomer films depending on location within the membrane-electrode-assembly and preparation was found. Differences of the swelling behavior of the ionomer films, and IR analysis prior and after operation were used as a measure for ionomer degradation caused by radical attack. A dependence of macroscopic cell degradation rate on initial electrode ionomer film thickness was observed.

For further determination of the properties of such ultrathin films model layers of Nafion®, and Aquivion® PFSA were examined. Conductive AFM allowed investigations of through- and in-plane conductivity in dependence of the film thickness. Significant differences in conductivity for films below ~10 nm thickness were detected.

Figure 1
Mitigation of Humidification Requirements for Proton Exchange Membrane Fuel Cells Using Inorganic Ligand Surface Chemistry on Platinum and Platinum Alloy Electrocatalysts
Authors: Angelopoulos Anastasios P.
Published: February 2020

Computational and experimental investigations have previously revealed that the surface chemistry of metal nanoparticles used as fuel cell electrocatalysts can influence both wetting by water as well as the morphology of deposited solid polymer electrolytes. However, the impact of nanoparticle surface chemistry on practical fuel cell operation at high current load is unknown. Mass-transport and ohmic overpotentials are found to dominate performance at high load, hindering the impact of significant recent advances in electrocatalytic activity during practical operation. Prior investigations with operating fuel cells have typically focused on altering the distribution and content of ionomer in the electrode as well as the introduction of hydrophilic agents but often neglect the role of metal nanoparticle surface chemistry. In this investigation, we contrast the electrocatalytic and practical fuel cell performance of commercial carbon-supported Pt nanoparticles to carbon-supported and unsupported Pt and Pt-alloy nanoparticles synthesized via a metal-redox scheme that utilizes stabilizing inorganic ligands to alter the nanoparticle surface chemistry. 119Sn Mossbauer spectroscopy, X-ray Absorption, and X-ray Scattering are used to characterize ligand surface chemistry and structure. The catalysts are also characterized with respect to their Pt surface atom coordination using electrochemical stripping of Bi and Ge adatoms. In addition, the surface wettability of the various catalyst components (support, ionomer, metal nanoparticles) is differentiated using dynamic contact angle analysis. We determine that hydrogen/air fuel cell operation at high load is well-correlated to the surface chemistry of the nanoparticles. For a given surface atom coordination and ORR mass activity, catalysts produced via hydrophilic surface ligands permit the electrodes to operate under substantially drier conditions. Comparable single cell maximum power of 0.5 W/cm2 is realized at 40% inlet gas humidification for nanoparticles synthesized utilizing hydrophilic surface ligands versus 80% when an industrial precipitation approach is used for Pt nanoparticles ~3 nm, independent of ionomer loading. A remarkable swap in polarization behavior with humidification is observed. Correlations of fuel cell humidification requirements to alternate nanoparticle morphologies with varying surface atom coordination, inorganic ligand structures, and wetting behavior are described.

Figure 1
Injection Molding of Carbon Nanotube-Filled, Polymer Composite Bipolar Plates
Authors: Sander Meredith L , Pritts James T , Young Brian A , Santamaria Anthony D. , Hollinger Adam S
Published: February 2020

Bipolar plates are one of the primary components of polymer electrolyte membrane fuel cells. These plates deliver reactants to electrodes, remove reaction byproducts, transfer heat, provide mechanical stability, and conduct electrical current. While graphite is the most commonly used material for fabrication of bipolar plates, the machining costs associated with graphite are a major drawback to mass production. In addition, graphite-based plates make the greatest contribution to the weight of the fuel cell stack. For this reason, electrically-conductive polymer composites have been explored as an alternative to the traditional metal alloys commonly used for fuel cell bipolar plates [1]. Polymer composites can reduce both the weight and cost of the bipolar plate, while still providing sufficient electrical properties.

In prior efforts, compression-molded, carbon fiber epoxy composites have shown promise to achieve the electrical conductivities needed for fuel cell bipolar plates [2]. While compression molding can be used for production of high volumes, injection molding is most ideal for the levels of mass production required for full scale commercialization of fuel cells. The main objective of this study is to investigate the suitability of injection molding polymer composites with sufficient electrical conductivities. Polymer composites based on nylon were developed for injection molding of bipolar plates. Carbon fiber was added to nylon at weight percentages ranging from 10 to 50%. The highest conductivity was obtained by direct injection molding. Results show that the percolation threshold for carbon fiber in nylon occurs around 25 wt%. Multi-walled carbon nanotubes were added to the direct injection-molded samples at weight percentages ranging from 2 to 4% to investigate the synergistic effects of adding multiple conductive fillers [3]. The addition of MWCNTs increases conductivities to United States Department of Energy technical targets for bipolar plate electrical conductivity (> 100 S/cm). Samples fabricated with MWCNTs and carbon fiber are shown to achieve up to 400% improvement in conductivity over samples with carbon fiber alone. The addition of carbon black as a third conductive filler, as well as the use of a polymer blend, may further increase electrical conductivity.

[1] Mighri F, Huneault MA and Champagne MF. (2004) Electrically conductive thermoplastic blends for injection and compression molding of bipolar plates in the fuel cell application. Polymer Engineering & Science 44: 1755-1765.

[2] Hwang IU, Yu HN, Kim SS, et al. (2008) Bipolar plate made of carbon fiber epoxy composite for polymer electrolyte membrane fuel cells. Journal of Power Sources 184: 90-94.

[3] Shen L, Wang FQ, Yang H, et al. (2011) The combined effects of carbon black and carbon fiber on the electrical properties of composites based on polyethylene or polyethylene/polypropylene blend. Polymer Testing 30: 442-448.

Figure 1
Mechanical Properties of Perfluorinated Sulfonic Acid Polymer Electrolytes and Their Effect on RH Cycling Durability
Authors: Okuyama Takumi , Watabe Hiroyuki , Hirai Takeshi , Saito Susumu , Shimohira Tetsuji
Published: February 2020

Low resistive electrolyte membranes are important to improve fuel cell performance and they are indispensable for fuel cells, especially for automotive application, which are used under elevated temperature and low humidity conditions. There are some approaches to lowering membrane resistance in fuel cell operation. From a view point of the membrane design, increasing the ion-exchange capacity1, 2 and employing a thin membrane 3 are promising ways to decrease resistance. Although both are effective in improving fuel cell performance, they are not desirable in terms of mechanical durability in fuel cell operation. In reality, reinforced membranes made of perfluorinated sulfonic acid (PFSA) polymer and ePTFE have been widely used in fuel cell sytem3. However, relationship between mechanical properties of PFSA polymers and mechanical durability of unreinforced PFSA membranes is not yet well understood.

In this study, focusing on the backbone and cluster structure of the polymer, we synthesized PFSA polymer electrolytes with various mechanical properties and high proton conductivity as shown in Fig. 1, and investigated their mechanical properties and mechanical durability when used in a membrane electrode assembly. Various polymer properties were obtained, including work of fracture, ratio of yield stress to breaking stress, water uptake, and creep characteristics, which were compared with the results of RH cycling test. Though we couldn’t explain the results of RH cycling test using a single mechanical property, we found a clear relationship between a combination of four mechanical characteristics and the mechanical durability.

Acknowledgement

This research was partially funded by ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan).

References

S. Kinoshita, T. Tanuma, K. Yamada, S. Hommura, A. Watakabe, S. Saito, and T. Shimohira, 226th Meeting of The Electrochemical Society, 1195 (2014).

S. Kinoshita, T. Shimohira, A. Watakabe, S. Hommura, S. Saito, T. Tanuma, and K. Yamada, 26th PRiME, 2414 (2016).

B. Kienitz, J. Kolde, S. Priester, C. Baczkowski, and M. Crum., ECS Transactions, 41(1), 1521 (2011).

Figure 1
Effect of Cobalt Ion Exchange and Thermal Pretreatment of Ionomer Thin Films on Conductivity, Water Uptake and Swelling
Authors: Eskandari Hamideh , Ozhukil Kollath Vinayaraj , Karan Kunal
Published: February 2020

It is known that Cobalt (Co) can leach out from the Pt-Co alloy catalysts causing degradation of long-term fuel cell performance. Although the fundamental mechanism of performance loss is not fully understood, it has been hypothesized that the Co ion interacts with the nanothin ionomer films in the fuel cell catalyst layer by exchanging with the protons and thereby adversely affecting the performance. However, the effect of Cobalt ion, specifically and the transport properties of nanothin fluorinated ionomers films, in general is not fully known.

We have studied the effect of cation(Cobalt) on spin-coated nanothin ionomers on its key properties – (i) proton conduction and water uptake which directly affects the electrochemical performance of fuel cell catalyst layers, and (ii) swelling behavior, which is an indicator of mechanical strength and thereby the durability of the ionomer. Electrochemical impedance spectroscopy (EIS) of films on interdigitated elecrodes for conductivity measurements, quartz crystal microbalance (QCM) for water uptake, and environmental ellipsometry for swelling measurement was employed, Quantification of exchanged Cobalt in nanothin films is non-trivial problem and we have used a set of x-ray techniques (EDX, XPS) for determination of cobalt in the cation exchanged films.

In this work conductivity of the different ionomer thin film (~ 30 nm) were tested before and after Cobalt exchange at different temperatures (30-80 °C) and humidity (40-90 % RH) conditions. It was seen that the conductivity of the ionomer thin films decreased after Cobalt exchanged but the proton conduction of some of the non-commercial ionomers is not as strongly affected by Co ion exchange as that of the state-of-the-art Nafion ionomer. We also investigated the effect of ionomer thin film pretreatment on the conductivity of the cobalt exchanged ionomer. It was found that the the samples annealed at 160 °C and then cation exchanged by exposure to cobalt solution showed less reduction in conductivity than unannealed cation exchanged ionomer films.

The presentation will share the details of the film preparation and characterization as well as the results for the conductivity, water uptake and swelling of the ionomer films.

Figure 1
Durability and Degradation of Direct Dimethyl Ether High Temperature Polymer Electrolyte Membrane Fuel Cells
Authors: Vassiliev Anton , Reumert Alexander Kappel , Jensen Jens Oluf , Aili David
Published: February 2020

Dimethyl ether (DME) combines high energy density with low toxicity and good electro-oxidation kinetics due to the absence of carbon-carbon bonds. It has therefore been identified as an attractive fuel for various fuel cell types, including the high temperature polymer electrolyte membranes (HT-PEM) fuel cells utilizing phosphoric acid doped polybenzimidazole (PBI) membranes [1, 2].

The HT-PEMFC technology has been intensively developed during the last two decades, and recently lifetimes exceeding 9000 h with degradation rates as low as 0.5 μV h-1 have been achieved at 160 °C and 200 mA cm-2 with hydrogen and air [3]. This type of cell was originally developed for a direct methanol fueled system [4], but lifetimes exceeding a few hundred hours have yet not been reported in this mode. We recently demonstrated that this is, at least partly, due to severe chemical incompatibility between methanol and the phosphoric acid within the polymer membrane. It results in formation of organic phosphoric acid ester derivatives, which not only reduces the conductivity of the membrane but also promotes the acid loss [5].

This work presents durability studies of direct DME HT-PEM fuel cells, and extends to a discussion about the predominating degradation mechanisms responsible for the limited lifetimes. The cells were operated at a constant current load at 160 or 200 °C and periodically characterized by polarization curves and electrochemical impedance spectroscopy. Supported by studies on a model system of phosphoric acid and DME, a main degradation pathway is suggested, where methyl phosphates are formed within the membrane. In analogy with the direct methanol cells, this reduces the proton conductivity of the membrane and promotes the acid loss, which leads to gradually increasing internal resistance.

[1] J.O. Jensen, A. Vassiliev, M.I. Olsen, Q.F. Li, C. Pan, L.N. Cleemann, T. Steenberg, H.A. Hjuler, N.J. Bjerrum, J. Power Sources, 211 (2012) 173-176.

[2] J. Neutzler, G. Qian, K. Huang, B. Benicewicz, J. Power Sources, 216 (2012) 471-474.

[3] T. Søndergaard, L.N. Cleemann, H. Becker, D. Aili, T. Steenberg, H.A. Hjuler, L. Seerup, Q. Li, J.O. Jensen, J. Power Sources, 342 (2017) 570-578.

[4] J.S. Wainright, J.T. Wang, D. Weng, R.F. Savinell, M. Litt, J. Electrochem. Soc., 142 (1995) L121-L123.

[5] D. Aili, A. Vassiliev, J.O. Jensen, T.J. Schmidt, Q. Li, J. Power Sources, 279 (2015) 517-521
(Invited) The Effects of Water Binding and Morphology on Proton Transport in Polymer Membranes
Authors: Hickner Michael
Published: February 2020

The effects of water binding and morphology will be reported in the context of designing new proton exchange membranes (PEMs). While much has been learned about the function of proton exchange membranes by studying perfluorosulfonic acid and sulfonated aromatic polymers, new generations of proton-conducting polymers for aqueous applications are being developed. Key to these advancements have been block copolymers and random copolymers with new sulfonic-acid bearing headgroups.

Sulfonated aromatic block copolymers are a known route to high-performing proton exchange membranes. Compared to their random sulfonated copolymer counterparts, the organization of the sulfonate groups in a concentrated ionic domain has demonstrable benefits to proton conductivity. However, the relative humidity effects on the specific headgroup chemistry still often lead to lower conductivity than desirable at low RH.

Using FTIR, we have explored the water binding in aromatic random copolymers with a number of new sulfonate groups. Our data shows that superacid groups on aromatic backbones can give comparable conductivities to perfluorosulfonic acid membranes. The interaction of these headgroups with water is one of the important factors for high proton conductivity – even though their morphologies can be much different than block copolymers or perfluorosulfonic acid materials.

This talk will discuss the morphology and water binding factors that influence the proton conductivity of a number of unique aromatic polymer-based proton exchange membranes.

Figure 1