bluuebunny/crossref_metadata_2025_split huggingface dataset:
(Invited) Nanostructured Conjugated Materials in Metal-Organic Frameworks
Authors: Kitao Takashi , Uemura Takashi 
Published: December 2020

As conjugated polymers play crucial roles in advanced nanoscale devices, future applications require a deeper understanding optoelectrical properties of conjugated polymers in low-dimensional assemblies. Confinement of polymer chains in nanoporous materials is a feasible method to control the polymer assemblies because this approach can prevent the entanglement and conformational disorder of polymer chains in the bulk state. The regulated and tailor-made pore characteristics of metal-organic frameworks (MOFs) allow the achievement of precise molecular assemblies in the nanopores, permitting fundamental studies on polymer properties that, until now, have remained elusive.1 Here, we report that the hybridization of MOFs and polythiophene (PTh) provides a great insight into inherent optoelectrical properties of PTh as well as functional nanomaterials for electronics and sensors.2 Controlled assembly of PTh donor and acceptor molecules is a key structure for photovoltaics and photocatalysis. We also demonstrated that unique interfaces between PTh and acceptor molecules, such as fullerene and titanium oxide, mediated by MOFs, enabled remarkable photoelectronic properties superior to conventional PTh systems.3

Other than conjugated polymers, a variety of conjugated materials, including graphene nanoribbons, could be prepared in MOFs, providing distinctly properties to those prepared in the bulk state because of the formation of specific assemblies.4

REFERENCES
1. Chem. Soc. Rev. 2017, 46, 3108.
2. (a) J. Am. Chem. Soc.,
2015, 137, 5231. (b) Angew. Chem. Int. Ed.,
2016, 55, 708. (c) Polym. Chem., 2017, 8, 5077. (d) Eur. Polym. J., 2018, 109, 162. (e) J. Am. Chem. Soc., 2019, 141, 19565.
3. (a) Nat. Commun., 2018, 9, 1660. (b) J. Phys. Chem. C, 2018, 122, 24182.
4. J. Am. Chem. Soc.,
2020
, 142, 5509.

Figure 1
Analysis of Pristine and Tcnq-Loaded MOF FILMS By Contactless Microwave Dielectric Spectroscopy Technique
Authors: Amoah Papa K. , Hassan Zeinab Mohamed , Redel Engelbert , Baumgart Helmut - , Obeng Yaw 
Published: December 2020

Metal-organic frameworks (MOFs) are porous and crystalline Coordination Network Compounds (CNCs), also referred to as porous coordination polymers (PCPs), which consist of inorganic metal (or metal/oxo) clusters connected by organic linkers. Surface Anchored Metal-organic-framework (SURMOF) films consist of quasi-liquid phase epitaxially grown highly ordered crystalline MOF films. For this study we used MOF Cu3(BTC)2 (BTC: benzene tricarboxylate) films, also known as HKUST-1 (Hong Kong University of Science and Technology-1), which were infiltrated with tetracyano-quinodimethane (TCNQ) guest molecules to modulate the charge transport characteristics. HKUST-1 MOF thin films have been grown using a liquid phase epitaxy (LPE) spray method in which the metal-containing solution [1 mmol Cu(OAc)2] and the linker solution [0.1 mmol BTC] are sprayed alternately on modified silicon substrates. Subsequently, the samples were immersed into an ethanoic TCNQ solution.
We report on localized charge transfer processes at microwave frequencies of porous metal-organic framework (MOF) HKUST-1 Cu2(BTC)3 samples. As part of this study we will compare measurements done in the HKUST-1 pristine form of the material with open pores and HKUST-1, where the pores have been infiltrated with tetracyano-quinodimethane (TCNQ), resulting in a hybrid structure with enhanced electrical conductivity and a significant measurable effect in the microwave propagation characteristics [1]. We present data and analysis from an evaluation of pristine and loaded HKUST-1 SURMOF films using the contactless broadband microwave dielectric spectroscopy (BDS). The technique enables the accurate operando evaluation of the electrical and magnetic properties of materials without compromising the kinetic conditions of the experiment. The BDS method is uniquely suitable for investigating charge transfer reactions, regardless of the nature of the charge carriers, i.e., electrons and holes [2]. As such, these material state-sensitive parameters could be used to monitor charge transfer changes in the SURMOF and SURMOF-TCNQ hybrid analytes.
[1] G. M. Espallargas and E. Coronado, “Magnetic functionalities in MOFs: from the framework to the pore”, Chem. Soc. Rev., 2018,47, 533, DOI: 10.1039/c7cs00653e
[2] A. M. Wernbacher et al, Operando Electrical Conductivity and Complex Permittivity Study on Vanadia Oxidation Catalysts, J. Phys. Chem. C 2019, 123, 8005−8017, DOI: 10.1021/acs.jpcc.8b07417
Subfilamentary Networks and Their Dynamics As Source of Cycle-to-Cycle Variability in Reram Devices
Authors: Dittmann Regina , Hensling Felix V. E. , Rieck Jan Lennart , Valenta Richard , Bäumer Christoph 
Published: December 2020

One major obstacle for the implementation of redox-based memristive devices in non-volatile memory or neuromorphic computing is the large cycle-to-cycle and device-to-device variability [1]. Here, we combine different complementary analysis methods such as atomic force microscopy (AFM), thermography and operando photoelectron emission spectroscopy (PEEM) [2] to monitor the filament shape and position in memristive SrTiO3 cells and correlate it to the observed variability of the resistance values. Our operando PEEM analysis revealed that some devices exhibit cycle-dependent variations in the shape of the conductive filament or in the oxygen vacancy distribution at and around the filament [3]. In other cases, even the location of the active filament changes from one cycle to the next. We propose that both effects originate from the coexistence of multiple (sub-)filaments observed by transmission electron microscopy [4] and that the active, current-carrying filament may change from cycle to cycle. Besides this, thermal imaging enabled us to monitor the position of the filament during repeated cycling. During cycling of some devices, we observed a movement of the filament towards the edge of the device that goes along with a drift of the low resistive state. As soon as the filament reaches the boarder, the resistance value stabilizes and remains constant during the following cycles. By modifying the interface at the top electrode, we could engineer the position of the filament and its stability against cycling. Our studies thereby represent the scientific basis, rather than prior purely empirical engineering approaches, for developing stable memristive devices.

References

[1] R. Dittmann and J. P. Strachan, Perspective article,
APL Mater. 7, 110903, 2019
[2] C. Bäumer et al., Nature Commun. 7, 12398 2016
[3] C. Bäumer et al., ACS Nano 11 (7), 6921, 2017
[4] H. Du et al., Chem. Mat. 29, 7, 3173, 2017
Electrical Characteristics of Phase Change Memory By Adding HfO2 Layer between Phase Change Materials and Heater Layer
Authors: Yun Hee Ju , Choi Seok , Choi Byung Joon 
Published: December 2020

Phase change memory (PCM) is a next-generation memory storing data with a large difference in resistance between crystalline and amorphous of phase change materials. PCM has outstanding properties such as scalability, high operation speed and non-volatility. However, conventional PCM has some issues to overcome, such as high reset current and set/reset asymmetry. Among them, poor adhesion between the phase change materials and SiO2, interfacial layer dielectric, which can cause problems during memory operation. Also, reset behavior takes higher current consumption due to its melt-quenching process, so improving the thermal efficiency is necessary. The thermal conductivity and interfacial properties between the heater and the phase change materials affect nucleation, so that electrical properties may change by inserting the interfacial layer.
In this study, we examine the effects of the interfacial layer between GeTe and heater on crystallization and electrical properties. 1.5nm-thick HfO2 layer was deposited with atomic layer deposition (ALD) on the bottom electrode contact surrounded by SiO2, and then 20nm-thick GeTe was deposited thereon with ALD. The diameter of bottom electrode contact is varied from 120 to 2000 nm. TiN as top electrode was grown by sputtering. The electrical properties were characterized with varying pulse width and bias polarity. GeTe-based PCM exhibits the abrupt set and reset switching, while more gradual transition is observed in the device with HfO2 insertion layer. Additionally, GeTe film was not grown without insertion layer, whereas, ALD growth of GeTe film on HfO2 layer was facile with improved adhesion. As a result, HfO2 insertion layer affects both adhesion and the electrical properties of PCM.
Finite Element Model Based Determination of Local Membrane Conductivity of Polymer Electrolyte Fuel Cells
Authors: Schuller Arnaud , Eller Jens 
Published: December 2020

Polymer Electrolyte Fuel Cells (PEFCs) are an essential technology for future fast refill, long range and clean mobility. However, for better market penetration, their efficiency still needs to be improved. An important aspect, leading to different kinds of losses is the water management. Indeed, there exists a humidity gradient between the gas inlet and gas outlet in PEFCs with active areas of some hundred cm². This can lead to uneven membrane conductivity over the cell area which results in current density inhomogeneities, resulting in lower cell efficiency and shorter lifetime. Therefore, the measurement of localized membrane conductivity in technical cells is of great importance. Different to the typical invasive membrane conductivity measurement techniques, such as segmented flow field plates or shunt resistor boards, we propose a non-invasive method [1] which is able to provide localized information about the ionic conductivity in the membrane using electrical impedance tomography (EIT). EIT is based on the relationship that exists between one object’s conductivity distribution and the surface potential that one could measure when an alternating current is applied to the object. The objects surface potential is measured during the injection of an alternating current between two electrodes at the remaining electrodes (see Figure 1). Once enough injection-measurement pairs have been acquired, the objects inner conductivity can be reconstructed.
This presentation will discuss the results of a numerical feasibility study of the application of EIT for the determination of the membrane conductivity in PEFCs. A finite element model (FEM) based representation of a simplified fuel cell geometry is used to solve the surface potential distribution as function of the membrane conductivity distribution and current injection. Using a minimization approach the membrane conductivity distribution is adjusted such that the mismatch between the surface potential of the FEM solution and synthetic measurement data is reduced. In order to simplify the problem and reduce the degree of freedoms in the minimization formulation, so far, the conductivity distribution in the membrane is estimated by 1D interpolations from cell inlet to cell outlet (see Figure 2). Finally, the working principle of the method is experimentally demonstrated using fuel cell relevant materials.
[1] 2019P04382EP patent application

Figure 1
Water Distribution in Fuel Cell Gas Channels Using a Mechanistic Discrete Particle Model
Authors: Niblett Daniel , Niasar Vahid J. , Holmes Stuart 
Published: December 2020

Water removal and accumulation in fuel cell gas channels is a complex phenomenon which is a consequence of the strength of the capillary forces and inertial forces (1 < Re < 1000). The presence of water in the channel restricts the access of oxygen to diffuse through the gas diffusion layers to the catalyst layer. The air supplied exchanges momentum to the droplets and water clusters, causing detachment when the adhesion force is overcome.
Water emerging from discrete locations as droplets interact with both the channel walls and each discrete water cluster, depending on the configuration and operating conditions. Complexity increases as multiple detachment and coalescence events occur over large channel lengths and time scales. Modelling these systems with traditional method computational fluid dynamic (CFD) methods is computationally expensive so we have developed a new mechanistic discrete particle model to handle the liquid water two-phase flow.
Taking inspiration from interface resolving volume of fluid (VoF) simulations of water flow in gas channels, we use analytically equations determined by the equilibrium contact angle in different configurations on the channel wall to predict detachment times and coalescence events. We compare the prediction of water area coverage ratio and channel saturation to the VoF simulations and can also estimate dynamic pressure drop.
Using this model, we are able to investigate the dynamic effects of liquid water accumulation and distribution in the channel, with often periodic detachment events arising from the formation of liquid plugs. Consequently, the model is able to investigate the effect of the channel wall wettability, from hydrophilic to hydrophobic and the impact that has on the distribution and accumulation of water in the system.
We compare the speed of this model against the VoF simulations to show that this type of modelling of two-phase flow in the channel can enable higher resolution accuracy when coupled with continuum scale approaches. Furthermore, with this approach it is possible to investigate the effect of: channel dimensions, liquid water flow rate and spatial location of liquid water emergence. This should allow for designed fuel cell structures to be evaluated.

Figure 1
Monitoring Carbon Corrosion in Anode of a PEM Fuel Cells Under Fuel Starvation Using a Hydrogen Reference Electrode and on-Line Mass Spectrometry
Authors: Park Sunjin , Lim Katie Heeyum , Lee Woong hee , Jeong Yoonjae , Kim Hansung 
Published: December 2020

Polymer electrolyte membrane fuel cells (PEMFCs) has attracted great attention in transportation and stationary applications with zero emissions and high efficiency compared to conventional combustion engines. However, various factors limit the lifespan of PEMFCs, which remain a major obstacle to large-scale commercialization. In particular, corrosion of the carbon support is one of the main causes of fatal and irreversible deterioration of PEMFCs performance.
If the hydrogen fuel supply is insufficient to maintain current demand due to poor water management or fuel depletion at the anode, carbon corrosion may occur on the anode side. In this fuel starvation condition, reactions that take place in place of hydrogen oxidation, such as water electrolysis and oxidation of the carbon support, compensate for the current shortage.
The exact deterioration mechanism of PEMFCs in fuel starvation is investigated by simultaneously monitoring the potential behavior from the anode and the exhaust gas using hydrogen reference electrodes and in situ online mass spectrometry. Interestingly, water electrolysis starts at a lower potential range and carbon oxidation occurs at a higher range as an alternative reaction to hydrogen oxidation. Thus, carbon corrosion, which is detrimental to cell performance, can be reduced by extending the duration of the water electrolysis reaction. In this regard, the introductions of the graphitized carbon support and water electrolysis catalyst to the anode are effective in alleviating damage due to fuel starvation by making a dominant alternative reaction to continue electrolysis of water. And this alternative reaction is Monitored with a hydrogen reference electrode and using on-site online mass spectrometry.
Operando Analysis of Cation Migration in PEMFC Electrodes Using Nanoscale X-Ray Computed Tomography
Authors: Braaten Jonathan , Kongkanand Anusorn , Litster Shawn 
Published: December 2020

Polymer electrolyte membrane fuel cells (PEMFCs) are beginning to be accepted as the electrification technology of choice for long-range, high-power vehicles due to their clean operation, high efficiency, and the ability to independently scale their range and power. However, PEMFC vehicles are still not cost-competitive with internal combustion engines, largely because of the high cost of the platinum (Pt) used as the electrocatalyst in the cathodes of PEMFCs (1). Recent efforts have succeeded in reducing the Pt content of PEMFC cathodes by using high-performance Pt alloys like PtCo or PtNi, which have improved catalytic activities over pure Pt (2, 3). While these alloys achieve higher power densities with smaller amounts of Pt in beginning-of-life performance, they experience significant degradation over time, including loss of Pt to particle coarsening and a Pt band and ionization and dissolution of the less-stable alloying element (i.e. Co or Ni). These contaminant cations are absorbed by the ionomer, replacing protons on the sulfonate groups and inhibiting proton conduction and affecting oxygen transport, leading to reduced performance at high current densities (4). While the overall loadings of these cations with respect to the total number of sulfonate groups in the membrane is low, these cations are suspected to migrate during operation, causing high local loadings of contaminant cations in the cathode catalyst layers.
The focus of this work was the operando analysis of contaminant cation migration and accumulation using the nanoscale X-ray computed tomography (nano-CT) imaging technique. We designed a miniature nano-CT conducive fuel cell hardware with a Kapton-sealed X-ray viewing port, allowing us to monitor the temporal concentrations of contaminant cations (i.e. Co2+ and Cs+) at the catalyst layer/membrane interface using 2D absorption contrast imaging during cell polarization. The preliminary results from our method have demonstrated back-and-forth cation migration and accumulation under hydrogen pumping conditions, confirming the mobility of these cations under constant current application and demonstrating promise in the operando evaluation of their impact on proton and oxygen transport.
This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.
References:

S. T. Thompson, B. D. James, J. M. Huya-Kouadio, C. Houchins, D. A. DeSantis, R. Ahluwalia, A. R. Wilson, G. Kleen and D. Papageorgopoulos, Journal of Power Sources, 399, 304 (2018).

H. A. Gasteiger, S. S. Kocha, B. Sompalli and F. T. Wagner, Applied Catalysis B: Environmental, 56, 9 (2005).

H. R. Colón-Mercado and B. N. Popov, Journal of Power Sources, 155, 253 (2006).

T. Greszler, T. Moylan and H. Gasteiger, Modeling the impact of cation contamination in a polymer electrolyte membrane fuel cell (2009).

Figure 1
Elucidating the Mass Transport Properties of Additively Manufactured Electrodes Using Spatially Resolved Simulation
Authors: Beck Victor A , Ivanovskaya Anna N , Chandrasekaran Swetha , Forien Jean-Baptiste , Baker Sarah E , Duoss Eric B , Worsley Marcus A. 
Published: December 2020

Flow through electrodes are the core, active component in a number of electrochemical systems including fuel cells and flow batteries. Controlling fluid flow and mass transport is a core engineering challenge in these systems and has a dramatic impact on cell power, utilization, and efficiency. Most advanced electrode materials are composed of microscale, disordered porous media. The length scales of these materials allow for very large surface area but at the cost of low flow speeds, low surface mass transfer rates, high pressure drops, and added complexity and cost from the need to use external flow distribution systems like flowfields. The multiscale control over electrode architecture enabled by using additively manufactured electrodes could provide a potential resolution to these trade-offs. To elucidate the mass transport of these electrodes, we use resolved, continuum computation to analyze the fluid flow and species transport in direct ink-write printed, carbon aerogel electrodes. We determine the mass transfer correlations (Sh ~ Pea) across two different lattice aerogel geometries and validate these results against the experimentally manufactured and tested flow-through electrodes operated at limiting current. In contrast to conventional electrodes, the mesoscopic length scales in our electrodes lead to finite Reynolds numbers and we find an increase in the mass correlation exponent (a) as inertial effects become important. We analyze the local mass transfer coefficients in the electrode and correlate regions of enhanced mass transfer with secondary flows in the electrode. Our work provides a promising new pathway to increased mass transfer rates in flow-through electrodes.
LLNL-ABS- 810719
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
Performance and Durability of Mn-Based Platinum Group-Metal Free Catalyst in Membrane Electrode Assemblies
Authors: Yang Fan , Stracensky Thomas , Zhong Sichen , Chen Mengjie , Guo Lin , Wang Yun , Wu Gang , Wang Guofeng , Xu Hui 
Published: December 2020

Proton exchange membrane fuel cells (PEMFCs) have drawn increasing attention in automotive and residential applications. To significantly reduce the cost of the PEMFC stacks, current platinum-group metal (PGM) catalysts must be replaced by PGM-free catalysts. Although Fe-based PGM-free catalysts (Fe-N-C) have exhibited promisingly high activity for the oxygen reduction reaction (ORR)1, they suffer from insufficient stability. Additionally, a major issue associated with Fe-containing catalysts is that dissolved iron ions can facilitate Fenton’s reaction thus forming a strong radical known to degrade the ionomer and membrane2.
Mn-based catalysts have recently been developed, aiming to improve the catalyst durability and mitigate the Fenton’s reaction3. However, the catalyst stability and integration into membrane electrode assemblies (MEAs) have yet to be studied. In this work, density function theory (DFT) has been applied to study the active sites and their degradation mechanism. Computational fluid dynamic (CFD) modeling was also applied to study water, oxygen and proton transport in the electrode. Electrode fabrication optimizations were conducted based on the guidance from the CFD results. The optimizations include ionomer content, electrode porosity and MEA fabrication approaches It has been found that Mn-based catalysts showed improved MEA durability compared to Fe-based catalysts due to the higher carbon graphitization degree, reduced de-metallization and mitigation of peroxide radical formation. However, there is still a significant performance degradation compared to PGM-based MEAs. X-ray absorption has been applied to examine the catalyst and corresponding MEAs before and after durability. The degradation is primarily attributed to the oxidation and agglomeration of active sites.

Acknowledgement: The project is financially supported by the Department of Energy’s HFTO under the Grant DE-EE0008075.

Reference:
1. Wu, K.L. More, C.M. Johnston, P. Zelenay, Science, 332, 443, 2011.
2. F Coms, H Xu, T. McCallum, C. Mittelsteadt, ECS Trans. 64, 389, 2014.
3. Li, J.; Chen, M.; Cullen, D. A.; Hwang, S.; Wang, M.; Li, B.; Liu, K.; Karakalos, S.; Lucero, M.; Zhang, H.; Lei, C.; Xu, H.; Sterbinsky, G. E.; Feng, Z.; Su, D.; More, K. L.; Wang, G.; Wang, Z.; Wu, G., Atomically dispersed manganese catalysts for oxygen reduction in proton-exchange membrane fuel cells. Nature Catalysis
2018,
1 (12), 935-945.