bluuebunny/crossref_metadata_2025_split huggingface dataset:
Thermoelectric Properties of Metal-Organic-Framework Thin Films
Authors: Chen Xin , Zhang Kai , Hassan Zeinab Mohamed , Wang Zhengbang , Lin Pengtao , Baumgart Helmut , Redel Engelbert 
Published: February 2020

Bulk Metal-Organic-Framework (MOF) films are designed scaffold-like compounds which are composed by inorganic metal clusters connected with organic linker molecules, forming highly ordered, crystalline and porous structures. These bulk MOF frameworks were initially designed for gas storage due to their high storage capacity inside the pores of MOF bulk materials. However, the application for electrical devices were very limited resulting from their insulating character. Recently, it has been reported that the electrical properties of bulk host MOFs can be modulated by infiltrating guest molecules (metal clusters) inside the porous MOF framework. This renders MOF materials a novel and promising material for microelectronic devices, sensors, catalytically active materials and thermoelectrics.
In our work, oriented SURMOF films and random polycrystalline MOF films Cu3(BTC)2 (BTC: benzene tricarboxylate, known as HKUST-1 MOF, were synthesized by a liquid phase epitaxy (LPE) spray method grown on surface functionalized gold coated silicon substrates and regular non-functionalized silicon substrates covered with a thick 484 nm thermal SiO2 layer respectively. The tetracyano-quinodimethane (TCNQ) guest molecules were infiltrated into the MOFs to modulate the electrical properties of the film. The horizontal Seebeck coefficient parallel to the sample surface of both oriented SURMOF films and random polycrystalline MOF films were measured. The random oriented MOF films exhibit a high positive Seebeck coefficient of 422.32 µV/K at 350 K. However, the horizontal Seebeck coefficient of oriented SURMOF films fluctuates around 0 µV/K. This can be interpreted that these highly oriented SURMOF films exhibit a large anisotropy with no charge carrier transport in horizontal direction parallel to the sample surface, but only carrier transport in vertical direction. The anisotropic nature of SURMOF films were further proven by IV characterization. The electrical conductivity of polycrystalline MOF film in horizontal direction was observed in the temperature range of 260 K to 350 K to calculate power factor (S2δ) of MOF films. The maximum value of power factor was found at 350 K about 1.527 µW/(m2K), which is about one order of magnitude higher than reported one. The thermal conductivity measurement on polycrystalline MOF films indicated the TCNQ infiltration not only contributes to enhance electrical conductivity, but also results in a reduction of thermal conductivity of MOF films. The reduced thermal conductivity can be attributed to enhanced phonon scattering due to TCNQ infiltration. The results demonstrate that random polycrystalline MOF film has a promising application potential in future thermoelectric and electronic devices.
(Invited) Volume Resistive Switching in Metallic Perovskite Oxides Driven By the Metal-Insulator Transition
Authors: Palau Anna , Gonzalez-Rosillo Juan Carlos , Ortega Rafael , Jareño Julia , Coll Mariona , Arndt B , Dittmann Regina , Maggio-Apprile Ivan , Suñé Jordi , Obradors Xavier , Puig Teresa 
Published: February 2020

In recent years, huge efforts have been made in the research community to overcome the limitations in Flash and RAM memories, and consequently, new emerging technologies are competing in the race towards a new paradigm in information storage. In particular, nonvolatile memories based on the Resistive Switching (RS) effect, where two (or more) reversible resistance states can be induced upon application of an electric field, has emerged with excellent performance. This phenomenon has been observed in many oxide systems, in particular in perovskite oxides, which are materials showing outstanding properties giving rise to exotic physical phenomena due to the strong electronic correlation, such as metal-insulator transitions (MIT). This is the case of the metallic perovskite La1-xSrxMnO3 (LSMO), RENiO3 (RE=rare earth) and YBa2Cu3O7(YBCO) family compounds, which are able to display Volume RS effects induced by the MIT and therefore, small changes in carrier concentration can induce huge resistance changes. The mechanism underlying this phenomenon is still unclear although oxygen vacancies mobility plays an important role in the mechanism underneath this phenomenon.
In this presentation we will discuss our studies on bipolar resistive switching of the mentioned perovskite oxides. Switching characteristics have been evaluated by C-SPM and I(V) curves with metal electrodes. Scanning tunneling microscopy and spectroscopy (STM/S), transport and resistive measurements were performed to gain insight into the local density of states of the material for different resistance states. Large resistance rations (102-104), over 100 cycles experiments, multilevel switching and switching dynamics have been evaluated. In addition, we have confirmed the strong influence of different atmospheres on the resistive switching properties of bare LSMO thin films. Remarkably, this influence disappears when the LSMO layer is capped with a CeO2thin layer, which acts as an oxygen reservoir, making this bilayer a proper material choice for encapsulation. A 3-terminal proof of principle device based on Ag/CeO2/LSMO/CeO2/Ag will be presented.
We believe that these results contribute to a better understanding of the physical mechanism behind the robust RS effect of these metallic perovskite oxides films.
(Invited) Requirements for Highly Accurate Multiphysics Modeling of SiC Power MOSFETs and Power Modules
Authors: Grossner Ulrike , Kakarla Bhagyalakshmi , Ziemann Thomas , Muting Johanna , Stark R. , Kovacevic-Badstuebner Ivana 
Published: February 2020

With the current needs towards achieving significantly higher power density and energy efficiency of power conversion, the industry is highly interested in replacing mature Silicon devices with the emerging Silicon Carbide (SiC)-based technology in the new generation of power electronics systems. Multi-physics modelling represents a very important design step used to evaluate and confirm the potential of SiC devices in a wide-range of power electronics applications before any prototypes are constructed. It enables a detailed assessment of SiC technology with respect to different performance aspects including dynamics, thermal capabilities, and robustness against abnormal operating conditions. As a prototypical example, this paper presents a highly accurate modelling procedure for analyzing short-circuit (SC) capabilities of a commercial SiC power MOSFET (80mΩ, 1.2kV). First, the actual MOSFET structure is modeled in Synopsys Sentaurus TCAD tool suite and calibrated to match the I-V and C-V MOSFET characteristics, taking into account the mobility degrading effects and interface states at the SiC/SiO2 interface. In the next step, TCAD mix-mode simulations are performed using the developed physical model to gain a better understanding of the design parameters like cell pitch, channel length and p-base doping concentration and their impact on the MOSFET SC and dynamic characteristics considering temperature dependencies and design constraints. Accordingly, a trade-off analysis between improved SC capabilities and a good MOSFET dynamic performance will be discussed, which can be further used as a guideline for developing an optimized MOSFET structure. Moreover, a detailed three-dimensional electromagnetic model of the SC measurement setup will be presented in order to identify the influence of the circuit parameters on the current profile of the Device-Under-Test (DUT) during the SC events and to optimize the measurement setup. For a comparison between the measured and simulated MOSFET waveforms, coupling of electrical and electromagnetic domains will be performed in LTSpice, Saber and TCAD mix-mode tool. As a result, this paper will demonstrate the accuracy of three device models side-by-side: the compact device model implemented in LTSpice, the behavioral model of Saber, and the TCAD numerical model. Finally, the paper emphasizes the requirements for a very accurate and efficient multi-domain modelling procedure that can be integrated in the overall development process of SiC power MOSFETs.
Fig. 1 A comprehensive analysis of short-circuit (SC) capabilities of a commercial SiC power MOSFET (80mΩ, 1.2kV) based on a multi-physics modelling approach.

Figure 1
Investigation on Degradation of HT-PEM Membrane-Electrode-Assemblies for Fuel Cells Under Start-Stop-Cycling
Authors: Tullius Vietja , Rastedt Maren , Büsselmann Julian , Schonvogel Dana , Pilinski Nadine , Wagner Peter , Dyck Alexander 
Published: February 2020

The emphasis of this study is the investigation of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFC) of various manufacturers with simulated start-stop cycles. This study is part of the national project QUALIFIX, which is funded by the German Ministry of Economy and Energy. Its purpose is to develop a framework to log, revise and improve the quality of fuel cells, its system components, as well as its manufacturing chain. Coherent test procedures enable the comparability of single fuel cells of various manufacturers. Furthermore, these framework conditions should enhance the lifetime of fuel cells, as well as minimize the production costs in an early stage.
In this study all tested membrane electrode assemblies (MEAs) have been investigated under identical test conditions using simulated Start-Stop (S/S) cycles. Consequently, the MEAs have been started up under defined conditions and operated for twelve hours at a constant current density of 0.3 A/cm². Following this, the MEAs have been shut down for another twelve hours and have been allowed to cool down to room temperature. Furthermore, all MEAs have been held under a nitrogen atmosphere at 30 °C during the stop phases before the next startup procedure has been performed. Figure one presents the fuel cell voltage at 0.3 A/cm² as function of time for one examinee after 500 h, correlating to 21 S/S cycles.
As a result, a continuous cell degradation rate of -33 µV/h could be observed. Next, degradation for HT-PEMFC MEAs have been evaluated by an extensive in situ electrochemical characterization, which has been executed before and after 21 S/S cycles. This characterization includes among the voltage curve, electrochemical impedance spectra, cyclic as well as linear sweep voltammetry. Additionally, all MEAs have been analyzed through ex situ characterizations, including, among others, micro computed tomography
(µ-CT), analysis of product water via ion chromatography (IC), as well as the determination of the phosphoric acid content, once S/S cycling has been stopped. Simultaneously, fresh MEAs of the same batch have been tested with the same ex situ methods. This allows to determine the ‘begin of life’ (BoL) condition in a pristine state.
As mentioned above, an unprecedented benchmarking of MEAs of various manufacturers has been performed with respect to HT-PEMFC degradation. Moreover, the results will be compared to the ex situ findings of the BoL-Analysis.
Figure 1: Voltage as function of time of one MEA under start stop cycles for 500 hours

Figure 1
Nanostructure of HT-PEFC Electrodes Investigated with Scattering Methods
Authors: Holderer Olaf , Khaneft Marina , Lin Yu , Liu Shuai , Feoktystov Artem , Kruteva Margarita , Zorn Reiner , Lehnert Werner 
Published: February 2020

Small angle scattering with X-rays (SAXS) and neutrons (SANS) is used as a powerful tool to investigate different components of fuel cells or even groups such as membrane electrode assemblies (MEAs) (1,2). In particular, the structure of electrode layers and membrane electrode assemblies of High Temperature Polymer Electrolyte Fuel Cells (HT-PEFCs) is studied in this contribution with neutron and x-ray scattering (1). Electrode layers in different stages of preparation, pure and doped with different amounts of phosphoric acid have been studied with SAXS. Structural changes have been observed with the addition of phosphoric acid, which is used in this type of HT-PEFC as proton conducting electrolyte.
Details of the phosphoric acid distribution and possible void distribution in electrode layers have been obtained with contrast variation experiments in small angle neutron scattering, where the different parts of the sample can be highlighted by exchanging hydrogen with deuterium, here in the phosphoric acid.
An outlook will point at the strength of quasielastic neutron scattering experiments to shed light on the proton diffusion on different length scales in the range of 0.1-10 nm, as a microscopic equivalent to the macroscopic conductivity, and it will address the question of what can be retrieved from a multicomponent system such as the different parts of a fuel cell.

M. Khaneft, O. Holderer, O. Ivanova, W. Lüke, E. Kentzinger, M.S. Appavou, R. Zorn, W. Lehnert, Fuel Cells
16 (2016), 406 – 413

O. Holderer, O. Ivanova, B. Hopfenmüller, M. Zamponi, W. Maier, A. Majerus, W. Lehnert, M. Monkenbusch, R. Zorn, Int. J. Hydrogen Energy
39 (2014) 21657 – 21662
19F NMR Studies of 3M Ionomer Adsorption on Carbon Supports and Pt/C Catalysts
Authors: Cantillo Nelly M. , Li Luyue , Peng Jing , Goenaga Gabriel A. , Zawodzinski Thomas A. 
Published: February 2020

In this study, the adsorption properties of water-dissolved 3M 825 equivalent-weight (EW) ionomer and carbon (C) supports or carbon-supported platinum (Pt/C) catalysts were studied with 400 MHz 19F Nuclear Magnetic Resonance Spectroscopy (NMR). The supports and catalyst materials (with and without ionomer addition) were also analyzed with Brunauer-Emmet-Teller (BET) nitrogen adsorption-desorption, transmission electron microscopy (TEM) and X-ray diffraction (XRD).
3M ionomer/carbon/Pt interactions were also explored through differential scanning calorimetry (DSC). Using these methods, a series of samples with and without ionomer addition were analyzed to explore the different interactions between components (Pt/Carbon, and Carbon/ionomer).
Preliminary DSC results are shown in Figure 1 for the 3M ionomer, ionomer/Pt, ionomer/Vulcan-carbon and the catalyst layer (CL) samples, where the CL is composed of ionomer/Vulcan-carbon/Pt. The 3M ionomer and ionomer/Vulcan-carbon samples, on the other hand, exhibit a weak and broad endothermic peak between 50 and 120°C. This process has been associated with the combination of structural changes in ionic cluster, and water volatilization in PFSA ionomers.1Both experiments with Pt present showed an exothermic feature around 95°C and 135°C for the ionomer/Pt and the CL samples, respectively.
The adsorption behavior of 3M ionomer on the carbon and catalyst samples was explored using 19F NMR. Figure 2 shows an NMR spectrum of the 3M PFSA ionomer in water solution after base correction. This figure also includes the chemical shifts and peak assignments. The strongest peak at -76.55 ppm corresponds to the internal reference (trifluoro acetic acid, CF3COOH) in solution. The ratio of the most prominent 3M ionomer peak (-124.13 ppm) associated with the CF2 group in the backbone and the internal reference peak was used for the analytical study, a similar approach to previous studies of Nafion.2–4

Figure 1
In Situ TEM Study of the Coarsening of Carbon Black Supported Pt Nanoparticles in Hydrogen
Authors: Simonsen Søren Bredmose , Wang Yan , Jensen Jens Oluf , Zhang Wenjing 
Published: February 2020

The control of sizes and shapes of nanostructures is of tremendous importance for the catalytic activity in electrochemistry and in catalysis more generally. However, due to relatively large surface free energies, nanostructures often sinter to form coarser and more stable structures that may not have the intended physicochemical properties.
Pt is known to be a very active catalyst in several chemical reactions and for example as carbon supported nanoparticles in fuel cells.
The presentation focusses on coarsening mechanisms of Pt nanoparticles supported on carbon black during exposure to hydrogen. By means of in situ transmission electron microscopy (TEM), Pt nanoparticle coarsening was monitored in 6 mbar 20 % H2/Ar while ramping up the temperature to ca. 900 °C. Time-resolved TEM images directly reveal that separated ca. 3 nm sized Pt nanoparticles in the pure hydrogen environment are stable during constant temperature ramping by 10°C/min up to ca. 800 °C. The coarsening above this temperature is fully dominated by the particle migration and coalescence mechanism. This is contrary to supported Pt nanoparticles in oxygen, where the coarsening is fully dominated by Ostwald ripening. For agglomerated Pt nanoparticles, coalescence events were observed already at ca. 200 °C. The temperature-dependency of particle sizes and the observed migration distances are consistent with simple early models for the migration and coalescence.
Performance Improvements in High-Temperature PEM Fuel Cells
Authors: Steenberg Thomas , Hjuler Hans A. , Jensen Jens Oluf , Li Qingfeng , Cleemann Lars Nilausen 
Published: February 2020

The work presented here focuses on recent results obtained on degradation of PBI membranes used in membrane electrode assemblies (MEAs) for high temperature polymer electrolyte fuel cells (HTPEM) that operates in the temperature range 140-180 °C. The testing is done at 160 °C using either pure hydrogen or reformate of different compositions.
Pure platinum versus PtCo alloys have been studied. The latest developments show that it is possible to reduce the Pt loading, increase the platinum utilization and achieve a long lifetime. We have shown that cells based on a thermally cured membrane proved a degradation rate of as little as 0.5 μV/h over an extended period of time. This is, to the authors’ knowledge, lower than what is ever reported for HTPEM.
The degradation mechanisms over time have been studied using SEM and TEM characterization tools. It has been shown that the Pt nanocatalyst particles grow from approx. 3 to 9 nm over 17,000 hours on the cathode side, while the anode is far less affected.
The durability of HTPEM can now be considered similar to low temperature PEM as shown in Fig. 1. Here, more than 15,000 hours is demonstrated in single cells at 300 mA/cm2. The degradation rate is around 4 μV/h for approx. 13,000 hours.
The high operating temperature makes it possible to make commercial fuel cell systems using methanol (methanol-water mixtures) as fuel. The HTPEM cells can tolerate CO impurities up to more than 3 vol-% without significant losses. Several demonstration projects have been made, including a Fiat 500 equipped with a 5 kW methanol based fuel cell system. This car is in every day operation at a catering company in Denmark. Another project is a street sweeper with the same type and size of system as shown in Fig. 2.

Figure 1
Reaction Selectivity for Oxygen Reduction of N-Doped Graphene Nanoclusters
Authors: Matsuyama Haruyuki , Akaishi Akira , Nakamura Jun 
Published: February 2020

Nitrogen-doped graphene exhibits high oxygen reduction reaction (ORR) activity [1]. Recently, it has been theoretically revealed that nitrogen atoms prefer to locate near the zigzag edge of graphene [2,3]. Therefore, many theoretical researchers have paied attention to the nitrogen atom existing at the graphene edge. In this study, we focus on nitrogen site-dependence of ORR activity for the graphene nanocluster having edges with finite sizes.
We investigated the ORR activity for the hexagonal nitrogen-doped graphene nanoclusters having zigzag or armchair edges, using first-principles calculations within the density functional theory. Electrocatalytic activities were evaluated on the basis of the computational hydrogen electrode model proposed by Nørskov et al. [4] In general, the ORR mainly proceeds in two pathways: For the two-electron pathway (2e-), oxygen molecule (O2) is reduced to hydrogen peroxide (H2O2), and for the direct four-electron pathway (4e-), the final product is water (H2O). H2O2 for the 2e- pathway might erode a carbon based electrocatalyst material itself, leading to low durability. Therefore, we also play up the selectivity for the 4e-pathway.
Fig.1(a) shows a model of the graphene nanocluster. The doping sites are denoted by blue circles with numbers. Numbers are allocated from the edge toward in-plane. A nitrogen atom is substituted in each doping site. Models are labeled as X-Y, where symbols X and Y denote doping sites of nitrogen atoms and reaction sites of ORR, respectively. Fig.1(b) shows the energy diagrams for the 4e- and the 2e- pathways for the model 5-6. The maximum potentials are 0.70 V and -0.42 V for the 4e- and the 2e- pathways, respectively. This means the 4e- pathway prefers to occur up to 0.70 V, and the 2e- pathway does not occur unless the backward voltage is applied. Therefore, the model 5-6 shows extremely high selectivity for the 4e- pathway. In almost all reaction sites, the maximum potentials for the 4e- pathway are higher than those for the 2e- pathway. It is noted that the high selectivity for the 4e- pathway is assured not just at the edge but inside the cluster. The effects of edge structures, cluster sizes, and doping configurations on the ORR activity will be discussed in the presentation

References:

[1] K. R. Lee, K. U. Lee, J. W. Lee, B. T. Ahn, S. I. Woo, Electrochem. Commun. 12, 1052 (2010).
[2] S. F. Hung, K. Terakura, T. Ozaki, T. Ikeda, M. Boero, M. Oshimaj, J. Ozaki, S. Miyata, Phys. Rev. B 80, 235410 (2009).
[3] Y. Uchida, S. Gomi, H. Matsuyama, A. Akaishi, J. Nakamura, J. Appl. Phys. 120, 214301 (2016).
[4] J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. Phys. Chem. B 108, 17886 (2004).

Fig.1. Model of the graphene nanocluster and doping sites of nitrogen atoms and free energy diagram for ORR for the model 5-6. Value of UMax are referenced to the standard hydrogen electrode (SHE).

Figure 1
(Invited) First Principles Studies of Nitrogen Electrochemistry and the Electrocatalytic Properties of Ultrathin (hydroxy)Oxide Films
Authors: Zeng Zhenhua , Chun Hee-Joon , Kubal Joseph , Chang Kee-Chul , Markovic Nenad M , Greeley Jeff 
Published: February 2020

Advances in the theoretical understanding of interfacial electrochemistry have, over the past decade, permitted the extension of periodic Density Functional Theory studies, which have traditionally been applied to probe chemistry at gas/solid interfaces, to electrochemical systems where potential-dependent structural and chemical phase transformations occur at liquid/solid interfaces. Indeed, such techniques have been employed to study a surprisingly wide class of electrochemical processes, ranging from electrocatalysis to corrosion.
In this talk, we will discuss how these strategies can be applied to predict the detailed, atomic-scale properties of monolayer (hydroxy)oxide base metal films on precious metal and precious metal alloy substrates as a function of applied potential. We will describe the analysis of Moire patterns, variable oxidation states, and three-phase boundaries of these films, and we will demonstrate how these results may be used to predict the shape and oxidation states of supported two-dimensional nanoislands as a function of electrochemical conditions. We will next discuss how the three-phase boundaries may promote hydrogen evolution rates in alkaline environments, and we will close with some perspectives on the possibilities for electrocatalyst design offered by these systems.
If time permits, we will also discuss a new approach, based on a combination of DFT and kinetic Monte Carlo simulations, that permits direct simulation of polarization curves as a function of sweep rate and that rigorously predicts intrinsic reaction pathways for both electroreduction of nitrates and NO in acidic environments.