bluuebunny/crossref_metadata_2025 huggingface dataset:
High-Performance Thin-Film Transistors Enable By Metal Oxide Semiconductor with Polyaniline Doing in the Channel Layer
Authors: Lee Eun Goo , Lee Jaehak , Lee Sung-Eun , Na Hyun-Jae , Kim Kyungho , Kim Youn Sang
Published: December 2020

Thin-film transistors(TFTs) using metal oxide-based semiconductors have attracted much attention because of their high carrier mobility, excellent optical transparency, low process temperature and solution process. In particular, indium oxide(InO
x
) is capable of aqueous routes for environmentally friendly and shows excellent carrier transport due to high carrier concentrations. Despite these advantages, pristine InO
x
films are generally polycrystalline structures, making it difficult to apply to flexible electronics, and it is necessary to adjust the carrier density to improve stability. Moreover, such a polycrystalline thin film does not exhibit uniform threshold voltage and field-effect mobility, making it difficult to enlarge the device. To address this problems, InO
x
was doped with inorganic materials such as Ga, Zn, Sn and Sc. Since the thin film transistors using these amorphous metal oxide semiconductor lowers the carrier density, it adversely affects carrier mobility.

In order to solve the doping problem of the conventional inorganic material that lowers the carrier mobility, we proposed solution-processed polyaniline doping in the channel layer of metal oxide semiconductors. As a result, depending on the polymer concentration inside the metal oxide semiconductor, the crystallinity of the InO
x
film decreases and the carrier concentration is controlled, affecting electrical properties such as field-effect mobility, on / off ratio, onset voltage and hysteresis. These results show the possible applicability to the industrial field as well as a significant meaning in improving the properties with polymer doping in the solution-processed metal oxide semiconductor at low temperatures.
Welcome Remarks - H03: Thin Film Transistors 15 (TFT 15)
Authors: Kuo Yue
Published: December 2020

Welcome to H03: Thin Film Transistors 15 (TFT 15)! Take a moment to listen to welcome remarks from the lead organizer, Yue Kuo.
High Electron and Low Heat Transports of 1D Atomic Defect Tunnels Stabilized in Tungsten Oxide Epitaxial Films
Authors: Kim Gowoon , Feng Bin , Sheu Yu-Miin , Cho Hai Jun , Ikuhara Yuichi , Ohta Hiromichi
Published: December 2020

The coexistence of high electron conduction and low heat conduction are essentially important to realize efficient thermal management materials such as thermoelectric materials. Although introducing 0D point defects (Fig. a) or 2D layers (Fig. c) is known as an effective way to reduce the heat conduction, there is a dilemma that the electron conduction is also reduced due to that electrons and phonons are scattered simultaneously by impurities, defects, and boundaries. Here we show that introducing a 1D atomic defect tunnel is an excellent solution to solve this dilemma. (Fig. b) In this structure, the probability of that phonon hits the 1D atomic defect tunnel or wire is much higher than that of introducing 0D defect or dot event though 1D defect density is low. In order to verify this hypothesis experimentally, we selected oxygen deficient tungsten oxide epitaxial film as the candidate material. The WO
x
films (2.7 < x < 3.0) were heteroepitaxial grown by the pulsed laser deposition technique on (001) and (110) LaAlO3 single crystal substrates under oxygen atmosphere. The oxygen pressure was precisely controlled from 2 to 13 Pa during the deposition. As a result, we found that 1D atomic defect tunnels can be introduced in tungsten oxide epitaxial films by oxygen removal. In order to visualize the atomic arrangement of the WO
x
films, high-angle annular dark-field (HADDF) scanning transmission electron microscopy (STEM) was performed. The resultant WO
x
epitaxial films contain 1D atomic defect tunnel in the in-plane direction and the density increased with decreasing x. The thermal conductivity (κ) of the films drastically decreased from ~7 W m-1 K-1 to ~1.5 W m-1 K-1 in order to minimize κ with increasing 1D defects. The κ values of WO
x
films are close to that of amorphous WO
x
, which shows minimum κ. On the other hand, the electrical conductivity (σ) in the in-plane direction of WO
x
film dramatically increased from ~10-1 S cm-1 to ~103 S cm-1. Since the oxygen removal reduces the valence state of tungsten, carrier electron concentration increases with increasing 1D defects. The introduction of 1D defect tunnels does not affect the electron propagation but selectively suppresses only the phonon propagation most likely due to the fact that mean free path is much shorter for the carrier electron compared to the phonon. Thus, high electron conduction and low heat conduction coexist in oxygen-deficient tungsten oxide epitaxial films. The present finding would be useful to design efficient thermal management materials such as thermoelectric materials.

Figure 1
A High-Capacity Aqueous Rechargeable Supercapacitors Based on Electrochemical Al3+ Intercalation
Authors: Ai Yuanfei
Published: December 2020

With a three-electron redox-mechanism-based high capacity, Al-ion-based electrochemical battery capacitor systems are expected to provide high capacity, low cost and high safety to meet the increasing energy demands. However, the reversible insertion of Al3+ into electrode materials remains a difficult project. Here, we demonstrate a new strategy to achieve reversible Al3+ insertion in a classical conductive polymeric material namely PEDOT:PSS, in aluminum sulfate aqueous electrolyte system. The freestanding composite film with a hierarchical porous structure is prepared through vacuum-assisted curing of a mixed dispersion containing PEDOT:PSS on carbon cloth substrate. The as-prepared PEDOT: PSS composite electrode exhibits extremely high capacitances of 265 F/g at the current density of 0.2 A/g, and enhanced electrochemical stability in the aqueous Al ions electrolyte. The PEDOT:PSS electrode sustains more diffusion of Al ions with more electric quantity in the Al3+ electrolyte, compared to in traditional acid electrolyte. It also exhibits a lower charge transfer resistance in the aluminum ion electrolyte, rather than in other cations electrolyte, which helps reduce the polarization at the electrode/electrolyte interface and improved the aluminum ion migration. Furthermore, the PEDOT: PSS electrode and an activated carbon anode are assembled as aqueous rechargeable Al ion electrochemical supercapacitor. The discharge capacity of this system exhibits to be 51 mAh/g at the current density of 100 mA/g and displays a high energy density of 43.2 Wh/Kg at the power density of 265 W/Kg, demonstrating a promising aqueous energy storage device with high performance and low cost.
Advanced Experimental Technique to Measure Thermal Conductivity Using Suspended Micro-Island Devices
Authors: Nguyen Anh Tuan , Lee Woochul
Published: December 2020

The invention of suspended micro-island device has enabled researchers to directly measure thermoelectric properties in nanomaterials spanning from nanowires and nanotubes to 2 dimensional materials. The unique capability of this device of probing thermal conductivity at the level of single nanowire and 2D sheet has significantly contributed to expanding knowledge in nanoscale thermal transport. While the suspended micro-island device remains a powerful tool to measure thermoelectric properties, we further advanced experimental techniques using suspended micro-island devices in order to increase success rates in sample preparation as well as to increase accuracy in thermal conductivity measurements. We will explain the detailed structure of the newly designed suspended micro-island devices and our experimental configurations. Further, we will describe advantages and versatile usages of our experimental techniques in various scenarios.
(Invited) Photon Upconversion and Triplet Dynamic Nuclear Polarization in MOFs
Authors: Yanai Nobuhiro
Published: December 2020

Photon upconversion (UC) is the key technology to overcome the thermodynamic efficiency limit of solar energy conversion devices. Triplet-triplet annihilation (TTA)-based UC has drawn increasing attention in recent years, primarily because of its occurrence under low-intensity, noncoherent light excitation. However, the application of TTA-UC has been significantly hampered by the slow diffusion of excited molecules in solid matrices. The use of emitter-based metal–organic frameworks (MOFs) allows the fast triplet exciton diffusion among the regularly aligned emitter chromophores. Besides, near-infrared (NIR)-to-visible (vis) TTA-UC is very important for various applications, however, it remains challenging mainly due to the energy loss during the S1-to-T1 intersystem crossing (ISC) of molecular sensitizers. We circumvent this energy loss by employing a sensitizer with direct singlet-to-triplet (S-T) absorption in the NIR region. We achieve the NIR-to-visible TTA-UC in the solid-state by introducing the S-T absorption sensitizer into emitter-based MOFs.

Nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) are powerful and versatile methods in modern chemistry and biology fields. Nevertheless, they suffer from intrinsically limited sensitivity due to the low nuclear spin polarization at ambient temperature. One of the promising methods to overcome this limitation is dynamic nuclear polarization based on photo-excited triplet (triplet-DNP) that allows the hyperpolarization at room temperature. Previous studies of triplet-DNP have been limited to dense crystalline and amorphous materials, and it remains difficult to hyperpolarize biology-relevant probes. To overcome this limitation, we introduce the chemistry of MOFs to the field of triplet-DNP. The nanoporous structure of MOFs allows the accommodation of polarizing agents as well as other guest molecules. This work paves the way towards the hyperpolarization of various probe molecules at room temperature for imaging applications.
(Keynote) Electromagnetic Field Stimulated MOFs
Authors: Knebel Alexander , Chen Donghui , Hosseinimonjezi Bahram , Santos Jaciara C. C. , Redel Engelbert , Caro Jürgen , Volkmer Dirk , Woell Christof
Published: December 2020

Electromagnetic, external stimuli can be used to switch Metal-Organic Framework (MOF) materials and to achieve control over the pores of these materials on different ways. We designed MOFs with switchable moieties in their side-chains, such as azobenzene, to be switchable under irradiation of UV/Vis light. Additionally, we developed devices to be able to measure the impact of electromagnetic fields in-situ. We focused on surface anchored MOF (SURMOF) thin films, deposited on asymmetric, porous α-Al2O3 supports as gas separation membrane layers. It is possible to control the gas transport properties of the materials using, but also using electric fields, and measure changes gas separation (permeability and selectivity) of binary mixtures in-situ, using the Wicke-Kallenbach technique. The light switchable systems were investigated on their CO2 separation performance: By switching the azobenzene between cis and trans isomers inside the pore windows, it was possible to switch dipolar moments, thus changing the adsorption properties towards the quadrupolar moment of CO2. Using electric fields of 500 V/mm we could show on the prominent example of ZIF-8, a zeolitic imidazolate framework, that it has a strong impact on the gas transport properties, especially looking at the molecular sieving. The mechanistic of the electric field influence on the ZIF is as follows: The electric field distorts the lattice of the MOF, shifting the Zn2+ ions, lowering the symmetry of the unit cell. This effect leads to polarized polymophs of the structure, sharpening the molecular sieving through a stiffening effect. The purification of Propylene/Propane, a highly desired separation for the plastics industry, could be increased through the electric field stimuli by 33%.

References

Knebel, A. et al. Defibrillation of soft porous metal-organic frameworks with electric fields. Science
358, 347–351; 10.1126/science.aal2456 (2017).

Wang, Z. et al. Tunable molecular separation by nanoporous membranes. Nat. Commun.
7, 13872; 10.1038/ncomms13872 (2016).

Müller, K. et al. Switching Thin Films of Azobenzene-Containing Metal-Organic Frameworks with Visible Light. Chem. Eur. J.
23, 5434–5438; 10.1002/chem.201700989 (2017).

Knebel, A. et al. Azobenzene Guest Molecules as Light-Switchable CO 2 Valves in an Ultrathin UiO-67 Membrane. Chem. Mater.
29, 3111–3117; 10.1021/acs.chemmater.7b00147 (2017).

Tan, N. Y. et al. Investigation of the terahertz vibrational modes of ZIF-8 and ZIF-90 with terahertz time-domain spectroscopy. Chem Commun
51, 16037–16040; 10.1039/c5cc06455d (2015).

Figure 1
Metal-Organic Framework Enhanced Cellular and Humoral Immune Response Against Sepsis Causing Infection
Authors: Luzuriaga Michael , De Nisco Nicole J , Gassensmith Jeremiah J
Published: December 2020

Urinary tract infection (UTI) is the most common adult bacterial infections worldwide, with ~80% of all uncomplicated UTI caused by uropathogenic E. coli (UPEC). While anyone can develop a UTI, women in their 20’s and postmenopausal women are at particularly high risk—indeed, half of all women will develop a UTI in their lifetime. Even with antibiotic therapy, 25-50% of patients will experience recurrence within a year.1 Initial and subsequent UTIs can alter bladder epithelial cell barrier function, which in turn creates an environment more favorable to subsequent infection.2 This results in recurrent UTI, which requires long-term antimicrobial therapy and can reoccur in a patient indefinitely, causing a lifetime of pain, discomfort, and an elevated risk of septicemic death.3 Unfortunately, antibiotic resistance makes treating UTIs less effective. Indeed, 10–25% of uncomplicated UTI patient isolates are resistant to trimethoprim/sulfamethoxazole (TMP/SMX)—the standard-of-care antibiotic regime for UTI.4
,
5 Alarmingly, >70% of postmenopausal recurrent UTI patients are either allergic or resistant to TMP-SMX.5 Given the ubiquity of these infections and the high likelihood of recurrence, the societal cost is estimated to be over $5 billion each year in the US alone.6
A prophylactic or therapeutic vaccine would be an ideal first line of defense against UTIs for high risk groups, but no FDA-approved vaccines against UTI—or indeed any pathogenic E. coli infection—exist.7
Memristive Organic Bismuth Iodides for Neuromorphic Computing with High Performance
Authors: Yang June-Mo , Kim So-Yeon , Park Nam-Gyu
Published: December 2020

We report here synthesis of memristive organic bismuth iodide and its application to neuromorphic computing. AxBiyIx+3y (A = organic monovalent cation) thin films are prepared by spin coating, where structural dimensionality and electrical properties are found to depend on composition. The solid solution synthesized by mixing 2 dimensional (2D) composition with 0 dimensional (0D) one exhibits bipolar switching behavior, where electrical conductivity is significantly lowered as compared to the individual 2D and 0D phase. In addition, activation energy for resistive switching is also substantially lowered by mixing the dimensionality. Memristor is coupled with transistor (1t-1r system), which results in the resistance of 60 MΩ for writing and the operating voltage of 0.6 V alongside linear and symmetric weight update and selectivity. This indicates that the organic bismuth iodide is highly promising for large scale neural network. Furthermore, a crossbar array prepared based on the synthesized organic bismuth iodide shows linear and symmetric weight update. Simulation using MNIST (Modified National Institute of Standards and Technology) data set based on single-layer perceptron model confirms that average of accuracy rate of the synthesized organic bismuth iodide in the crossbar array is close to ideal rate accuracy.
(Invited) Ion Tunable Electronic Materials Systems for Neuromorphic Computing
Authors: Talin Alec Alec
Published: December 2020

Tuning electronic conductance through solid state electrochemical ion insertion has emerged as a promising technology to enable next-generation, ultralow energy computing architectures1-3. Unlike two-terminal non-volatile memory elements, the three-terminal redox transistor decouples the ‘write’ and ‘read’ operations using a ‘gate’ electrode to tune the conductance state through charge transfer reactions involving ion injection into the channel electrode through a solid-state electrolyte. The insertion of ions into the bulk of the channel acts to dope the material through a gradual composition modulation that leads up to thousands of finely spaced conductance levels (synaptic weights) with near-ideal analog behavior. These properties enable low-energy operation without compromising analog performance and non-volatility. However, the strong coupling of ionic and electronic processes sharply challenges our current understanding of solid-state electrochemical systems, particularly at decreasing dimensions and timescales relevant to computing technology. In my talk I will discuss the rich portfolio of challenging, exciting fundamental science questions about ion tunable electronic materials systems and how we can harness these to realize a new paradigm for low power neuromorphic computing.

Fuller, E. J.; El Gabaly, F.; Leonard, F.; Agarwal, S.; Plimpton, S. J.; Jacobs-Gedrim, R. B.; James, C. D.; Marinella, M. J.; Talin, A. A., Li-Ion Synaptic Transistor for Low Power Analog Computing. Advanced Materials
2017,
29.

Fuller, E. J.; Keene, S. T.; Melianas, A.; Wang, Z. R.; Agarwal, S.; Li, Y. Y.; Tuchman, Y.; James, C. D.; Marinella, M. J.; Yang, J. J.; Salleo, A.; Talin, A. A., Parallel programming of an ionic floating-gate memory array for scalable neuromorphic computing. Science
2019,
364 (6440), 570.

Li, Y. Y.; Fuller, E. J.; Asapu, S.; Agarwal, S.; Kurita, T.; Yang, J. J.; Talin, A. A., Low-Voltage, CMOS-Free Synaptic Memory Based on LixTiO2 Redox Transistors. Acs Applied Materials & Interfaces
2019,
11 , 38982.
Electrochromic Properties of Metallo-Supramolecular Polymers and the Display Device Application
Authors: Higuchi Masayoshi
Published: December 2020

Metallo-supramolecular polymers were synthesized by the complexation of transition metal ions including Fe(II), Ru(II), and Os(II) with ditopic organic ligands such as bisterpyridines. The obtained polymers were easily processed to the film state by spray-coating due to the amorphous nature of polymers. The polymers have specific color such as blue, red, and yellow, based on the metal-to-ligand charge transfer (MLCT) absorption in the complex moieties of polymer. The author and coworkers found reversible electrochromism in the polymer film, triggered by the electrochemical redox of the metal ions in the polymers. We confirmed high durability to the repeated color changes more than 100,000 times. Fe(II)-based metallo-supramolecular polymer (polyFeL1), which was prepared by the 1:1 complexation of Fe(II) ions and bis(terpyridyl)benzene (L1), exhibited high coloration efficiency (263.8 cm2/C). When tris(terpyridyl)benzene (L2) and Fe(II) ions were further added to a polyFeL1 solution, the linear polymer structure of polyFeL1 was hyperbranched. Interestingly, the hyperbranched structure has enhanced the coloration efficiency of the polymer film up to 383.4 cm2/C. We could introduce two metal species (Fe(II) and Ru(II)) in the polymer by simple mixing of Fe(II), Ru(II), and L1 at the molar ratio of 0.5:0.5:1. The obtained heterometallo-supramolecular polymer displayed multi-color electrochromism by changing the applied potential, because the redox potential of Fe(II)/(III) and Ru(II)/(III) are different. We succeeded in fabricating electrochromic devices using the polymers. Owing to the amorphous nature of polymers, flexible electrochromic sheet devices were also fabricated. The nonvolatile display devices with both high durability and abundant color variation will be attractive for wide applications containing smart windows, wearable IoT displays, and digital signages.
Strong Coupling State on Plasmonic Lattice Structures Under Electrochemical Potential Control
Authors: Minamimoto Hiro , Hayashi Takahiro , Murakoshi Kei
Published: December 2020

The collective oscillation of free electrons in the metal nanostructures induced by the visible light illumination is recognized as the localized surface plasmon (LSP). The coherent light energy exchange between excitons of molecules and LSP can modulate not only the absorption characteristic but also the individual energy decay. This reversible light energy exchange process between LSP and molecular exciton is called as the strong coupling regime. Although the strong coupling can allow use the light energy efficiently, the relative short lifetime of LSP due to the energy dissipation via the radiative damping limits the application. On the other hand, the surface lattice plasmon (SLR) mode which is excited on the two dimensionally arranged metal nanoparticles shows the longer excited lifetime because the diffractive light scattered from each particle is used for the excitation of the LSP at neighboring nanoparticle, resulting in the suppression of the scattering loss. As we demonstrated in our previous study, we have successfully achieved the fine electrochemical structure control method for obtaining the longest lifetime of the SLR mode. From these backgrounds, in this study, we have fabricated the Au lattice structure on a conductive glass and to support organic dye molecules on it to achieve the strong coupling regime. Through the extinction and fluorescence measurements, we have successfully observed the peak splitting which was originated from the formation of the strong coupling. In addition, by the combination of electrochemical method, we have confirmed our successful results for the reversible tuning of the coupling strength. From all experiments, we have successfully estimated the number of the molecules contributing to the strong coupling state.

[1] H. Minamimoto et al., Phys. Chem. C 2018, 122, 14162.
Enhanced Electrochromic Performances of Polyhedral Oligomeric Silsesquioxane (POSS) Modified Viologen Derivatives
Authors: Park Jong Seung , Pande Gaurav K. , Choi Jun Hong , Han Seong-Hun
Published: December 2020

Polyhedral oligomeric silsesquioxane (POSS) is an inorganic silica nanocage surrounded by organic functional groups. Due to the presence of organic peripheries around the rigid case, functionalized POSS has often exhibited outstanding properties, thus finding useful for constructing porous inorganic-organic hybrid networks.

The electrochromic devices (ECDs) offer noticeable color changes due to the electron transfer and redox reaction. The electrochromic materials play a critical role in determining the optical response and absorption intensity of the ECDs. Various electrochromic materials have been investigated, including inorganic metal oxides, viologens, and conducting polymers. Among these, viologen derivatives have been studied in depth owing to high electron-accepting capability and facile synthetic routes. Besides, viologen based ECDs have found various applications, such as smart windows, electronic paper, and the famous antiglare rearview mirrors developed by Gentex. However, critical weaknesses of viologens include low coloration efficiency, slow switching speed, and reduced long-term stability. Furthermore, the durability in the repeated operation of ECDs is limited in non-complementary device structures.

Herein, we present the modification of viologen derivatives with the incorporation of POSS, in which the structural rigidity of POSS fillers and the synthetic versatility of viologen compounds are combined to ensure improved electrochromic performances. The solid-state ECDs are fabricated by sandwiching the single-layered ion gel containing POSS-modified viologens between two transparent electrodes. The resulting electrochromic behaviors are compared with those of conventional viologens, in terms of cyclic voltammetry, spectroelectrochemistry, and kinetic stability. Furthermore, the preparation of flexible ECDs using silver nanowire-PEDOT:PSS composite electrode is presented, and stable electrochromic operation in the repeated redox transitions is demonstrated on various plastic substrates.
Adaptation and Validation of a Pore-Network Model for Tracking Water in PEM Fuel Cell Electrodes
Authors: Allen Jeffrey S , Tajiri Kazuya , Alofari Karrar , Alam Shahriar , Medici Ezequiel
Published: December 2020

Liquid water in PEM fuel cell porous transport layers, also known as gas diffusion layers, has been extensively studied in order to understand how to remove product water and reduce mass trasnport losses. A variety of pore network models and imaging techniques have been developed and successfully used to model water transport in these porous layers. Mass transport losses also occur in the electrodes (catalyst layers) due to water where there is little understanding of how water interacts with the ionomer films and percolates through the pore space. In order to better understand liquid water movement, holdup, and evaporation in catalyst layers a pore network model has been adapted for use on PEM fuel cell electrodes. Network models are built on an abstraction of geometric features. Therefore, ex-situ characterization tests are needed to determine if the abstraction is valid. This talk focuses on the electrode network model development, characterization, and parametric studies examining the interplay between production of heat and water with transport through the catalyst support, pore space and ionomer films.
Two-Dimensional, Two-Phase, Non-Isothermal Model Ofproton Exchange Membrane Fuel Cell
Authors: Monteverde Videla Alessandro Hugo Antonio , Rizvandi Omir Babaie , Fino Debora
Published: December 2020

Fuel cells are part of the environmental and transportation solution of our future. With the creation of new performant catalysts [1], the use of less noble metals in the catalyst layer [2] and possible replacement of the Pt [3], it is essential to develop multiphysic models with high degree of detail for a complete optimization of the system. In this work, we develop a simple and accurate two-dimensional model for the resolution of the effects of anode and cathode flows and GDLs, catalyst layers and membrane. Brinkman equations are used to model flow distribution in GDLs and CLs. Maxwell-Stefan equations are used to model transport of gas species in GDLs and CLs by advection and diffusion mechanisms, and electrochemical reactions in the CL interfaces are modeled by modified Butler-Volmer equations. Furthemore liquid water, and dissolved water are included in the model. Conservation of liquid saturation is included in the cathode layers because of its effect on the gas transport and the cell performance

due to flooding specially at high current densities. Equations are solved numerically with the commercial finite element package, COMSOL Multiphysics v5.5. Fully-coupled solution is used with Newton iterations and direct MUMPS solver.

Figure 1a. Polarization curve computed by the model. Figure 1b. Distributions of liquid water saturation at cell voltage 0.4 V; anode and cathode sides are at the top and bottom, respectively.

Keywords

: PEM, Fuel Cell, CFD, Two-phase, Saturation, FEM, flooding

Stamenkovic, V. R.; Mun, B. S.; Mayrhofer, K. J.; Ross, P. N.; Markovic, N. M. J. Am. Chem. Soc. 2006, 128 (27), 8813−8819.

Thompson, S.T., James, B.D., Huya-Kouadio, J.M., Houchins, C., DeSantis, D.A., Ahluwalia, R., Wilson, A.R., Kleen, G., Papageorgopoulos, D.. J. Power Sources 399, 304–313 (2018)

H.A. Monteverde Videla, S. Ban, S. Specchia, L. Zhang, J. Zhang. Carbon, 76 (2014), pp. 386-400

Figure 1
Two-Phase Dynamics and Hysteresis in the PEM Fuel Cell Catalyst Layer with the Lattice-Boltzmann Method
Authors: Grunewald Jonathan B , Goswami Navneet , Mukherjee Partha P. , Fuller Thomas F.
Published: December 2020

In this work, a Lattice-Boltzmann-Method (LBM) model for simulating hysteresis in a proton exchange membrane (PEMFC) electrode is presented. One of the main challenges hindering study of the cathode catalyst layer (CCL) in PEMFCs is the lack of understanding of two-phase transport and how it affects electrochemical performance. Researchers have typically used high level approximations that oversimplify the microstructure of the CCL—these are known as macrohomogenous models. However, as the field has progressed, the flaws in these idealizations are being revealed, especially in areas of improving our understanding of flooding as well as catalyst layer hysteresis. Previously, the microstructure details needed to build an accurate mesoscale model have eluded researchers; however, with advances in tomography and focused-ion-beam scanning-electron-microscopy (FIB-SEM), creating these representations has become possible. Using LBM with these representations, the difficult problem of catalyst layer capillary hysteresis can be examined. In two-phase capillary hysteresis, both the equilibrium saturation position as well as absolute value depends on the wetting history. Based on the models, it is ascertained that at lower capillary numbers, the liquid begins to undergo capillary fingering – only above a capillary pressure of 5 MPa, a regime change into stable displacement is observed. As capillary fingering does not lead to uniform removal of liquid, the prediction is that because high capillary pressures are needed to change to the regime of stable displacement, wicking will not be an effective means of water removal.

Figure 1
Carbon Nano-Tube Based Nano-Porous Layer in Membrane-Electrode Assemblies for Polymer Electrolyte Membrane Fuel Cells
Authors: Kim Jaeyeon , Kim Hyeok , Song Hyeonjun , Im Dasom , Jeong Youngjin , Park Taehyun
Published: December 2020

In the membrane-electrode assemblies(MEAs) for polymer electrolyte membrane fuel cells(PEMFCs), there is a micro-porous layer(MPL) which is placed between gas-diffusion layer(GDL) and catalyst-coated layer(CCL). It roles as a buffer layer between GDL and CCL, where it helps removing the byproduct (Water) and connects electrically to conduct the generated electrons outside of the electrochemical reaction sites. However, commercial MPLs are normally composed of nano-particulated bulk carbon, which are hot-pressed in the manufacturing process. Therefore it is normally brittle and their thickness is micro-scale. This feature sometimes causes the structural breakdown by the byproduct or cracked by repeated pressurization of hydrogen or air. In addition, the micro-thick layer is not proper to remove the byproduct because such thick structure would rather disturb removing the byproduct water from the inside of the CCL.

This study therefore developed the alternative of the conventional MPL. It is called “carbon nano-tube sheet” which is composed of well-networked multi-walled carbon nano-tubes. Thus it shows higher electrical conductivity, mechanically stronger, and thinner than conventional MPLs. It was employed as a new MPL and the thickness effect on the electrochemical performance was explored. As a result, the MPL thickness of 0, 15, 30, 100 μm was investigated. The electrochemical polarization characteristics showed that the 15 μm thick MPL has the highest performance, meaning it is the optimized thickness. 0 thick (No MPL) and 100 μm thick MPL showed lower electrochemical performance than that of 15 μm thick MPL. For no MPL case, it is attributed that the contact between CCL and GDL was poor because of no buffer layer, while the case of thicker MPL is attributed to the disturbance of the water removal and poor diffusivity of the reactant into the CCL. Their corresponding electrochemical impedance spectra were also investigated and the results also showed the same effects (Electrical connection and removal of water).
Study on the Nanostructure of Nafion Ultra-Thin Film at Pt/C Catalyst Layer in PEMFC: Molecular Dynamics Simulation Approaches
Authors: Kang Haisu , Kwon Sung Hyun , Lee Seung Geol
Published: December 2020

In this study, the hydrated Nafion film in the CL of the cathode for PEMFC is investigated using molecular dynamics simulation (MD) method, exhibiting different structural characteristics on the Pt and carbon surfaces. First, it is found that water molecules, hydronium ions, and sulfonate groups are highly concentrated at the interfacial region between the Nafion phase and Pt surface, whereas Nafion backbone chains are present in a high concentration at the interface between Nafion phase and carbon surface. Second, it is also found from pair correlation function analysis that the water molecules and sulfonate groups in the hydrated Nafion phase are more associated with the Pt surface compared to the carbon surface, which is due to their strong attractive interactions with the Pt surface that makes the dimension of hydrated Nafion phase 4–7% thinner on the Pt surface. Third, it is observed from water-occupied volume analysis that water molecules on the carbon surface can form a large-size water phase between the Nafion phase and carbon surface because the Nafion-carbon interface is not tightly integrated due to their weak interaction. In these structural characteristics, it is demonstrated that the water diffusion and proton vehicular diffusion are suppressed in the interfacial region of the Pt surface due to the highly packed structures in water phase as well as polymer phase in addition to the strong molecular interaction with the Pt surface, whereas the proton hopping diffusion is enhanced due to the well-developed organized water phase via hydrogen bonding network.
Investigation of Liquid Water Formation in PEMFC By Direct Simulation and Operando X-Ray Computed Tomography
Authors: Satjaritanun Pongsarun , Shimpalee Sirivatch , Hirano Shinichi , Sasabe Takashi , Naito Hiroshi , Hirai Shuichiro , Zenyuk Iryna V.
Published: December 2020

Direct modeling-based Lattice Boltzmann Method (LBM) [1-3] combined with in-operando X-ray computed tomography (CT) is used to investigate the liquid water formation in the porous layers of polymer electrolyte membrane fuel cell (PEMFC). Studies of the local saturation of liquid water, heat transfer, species transport, and electrochemical kinetics with platinum metal group (PGM) and PGM-free catalyst layers are explored under different cell operating conditions. The detailed structure of dry (i.e., GDL, MPL, and CL) and wet samples (i.e., liquid saturation) used in this study were obtained by in-operando X-ray CT, which allows for accurate visualization of water transport in operating PEMFCs. The overall predictions were compared and validated with experimental data [4, 5]. Figure 1 shows the comparison examination of liquid formation on the detailed structure of porous layer with PGM and PGM-free catalyst layers. The results reveal that the liquid water evolution/saturation inside the GDL samples are different when the cell operating conditions and transport properties are changed. More results will be presented and discussed.

References:

P. Satjaritanun, J. W. Weidner, S. Hirano, Z. Lu, Y. Khunatorn, S. Ogawa, S. E. Litster, A. D. Shum, I. V. Zenyuk, S. Shimpalee, Journal of The Electrochemical Society, 164, (2017) E3359-E3371.

P. Satjaritanun, S. Hirano, A. D. Shum, I. V. Zenyuk, A. Z. Weber, J. W. Weidner, S. Shimpalee, Journal of The Electrochemical Society, 165, (2018) F1115-F1126.

P. Satjaritanun, S. Hirano, I. V. Zenyuk, J. W. Weidner, N. Tippayawong, S. Shimpalee, Journal of The Electrochemical Society, 167, (2020) 013516.

S. J. Normile, D. C. Sabarirajan, O. Calzada, V. D. Andrade, X. Xiao, P. Mandal, D. Y. Parkinson, A. Serov, P. Atanassov, I. V. Zenyuk, Materials today energy, 9, (2018) 187-197.

F. Akitomo, T. Sasabe, T. Yoshida, H. Naito, K. Kawamura, S. Hirai, Journal of Power Sources, 431, (2019) 205-209.

Figure 1
Towards Gas Distribution Layers
Authors: Csoklich Christoph , Steim Roland , Schmidt Thomas J. , Buechi Felix N.
Published: December 2020

Today, gas diffusion layers (GDLs) are stochastic, carbon based porous materials consisting of carbon fibres (~ 10 µm Ø) in which diffusion is the dominating transport mode for the gas phase. The stochastic nature of these materials requires bulk descriptors for (anisotropic) pore/throat size distributions, bulk electronic/thermal conductivity, tortuosity, effective diffusion coefficients asf.1
,
2 Here, we introduce a novel two-layered structure that not only offers unique design possibilities but also exceeds in performance tests conventional GDL materials under various operating conditions.

The investigated GDL material consists of a gold-coated PET fabric (Sefar). Its two-layered design resembles a combination of GDL and microporous layer (MPL). The simpler, regular structure (Fig. 1) increases the effective diffusivity – both in-plane and through-plane - as compared to Toray 60 carbon paper and shows no notable local variation. Furthermore, only one layer of throats exists for product water to emerge, thus lowering the required break through pressure and leading to faster water removal. The structure and transport properties can be finely adjusted to the fuel cell geometry or the desired operating conditions.

The material and structural properties were investigated ex situ as well as for fuel cell operation. The electrochemical performance was compared to commercial Toray 60 carbon paper (with and without wet proofing/with and without MPL) in a wide parameter range (50-80°C TCell, 60-100 % RHCath, 1.5-2 bar).

Especially at high cathode gas humidity and colder operating temperatures, the new material outperforms the conventional GDL material and reaches higher current densities (up to 200 mA/cm2 or 100 mV gain, cf. Fig. 2). It seems that this structure still distributes the feed gas well enough across the catalyst layer and removes product water straight to the channel. The detailed water transport modes were investigated by operando 4D-X-ray tomographic microscopy. The results show a dry structure over a wide range of current densities, and the crucial alignment between fibres and ribs: only in closed off pores below a rib, water accumulated.

These new, woven GDL materials show the possibilities of deterministic, designed structures to controllably tune single transport properties and improve water management in PEFC GDLs. Furthermore, transport properties as e.g. the electrical conductivity are adjustable even locally. The simpler structure might also narrow the gap between modelling and validation due to the regular structure and easy accessible material properties.

References

[1] Mathias, M. F., Roth, J., et al, Handbook of Fuel Cells, 3, 517–537 (2010).

[2] Rosen, T., Eller, J., et al, J. Electrochem. Soc., 159, F536-F544 (2012).

Figure 1
Understanding Degradation Phenomena in HT-PEM Fuel Cells Using Micro-Computed Tomography
Authors: Schmies Henrike , Schonvogel Dana , Büsselmann Julian , Wagner Peter , Dyck Alexander
Published: December 2020

In times of transition from conventional energy-converting techniques to new technologies for the generation of power and electricity, polymer electrolyte membrane fuel cells (PEMFC) play a crucial role. Especially high temperature (HT)-PEMFCs are operated at around 160 °C and can be fuelled using reformate gas from methanol or natural gas reformers allowing to be used for both, mobile and stationary applications. Typically combined heat and power systems or range extenders are in the current focus of the industry.

While the HT-PEMFC technology has made great progression in the last decades and started its way into commercialization, it still has to undergo more stages of advancement in order to become a real alternative for power supply in various sectors. One major hindrance of the technology is their insufficient lifetime accompanied by a lack of knowledge about external factors causing system degradation. Therefore, all cell components need a detailed degradation analysis which contributes to fundamental understanding.1-3

In this work, we investigate the components of the HT-PEMFC such as the bipolar plates (BPPs) and the membrane electrode assembly (MEA) regarding degradation phenomena during cell operation with use of contaminated air. By applying µ-computed tomography

(µ-CT), these components are analysed in detail with the aim to reveal structural and morphological changes on the micro meter scale that occur during the life time.

If the cell is operated with traces of contaminants (e. g. 10 ppm ammonia in the cathodic air stream) for a certain time period under constant load conditions, µ-CT investigation helps to identify degradation of the individual components. Figure 1 shows a reconstructed µ-CT image of a typical MEA with the gas diffusion layers (GDL) and the anode and cathode separated by the membrane. Analysis of thicknesses using single cross-sectional images of the MEA reveals that the catalyst layers do not change, while the membrane thickness decreased around 15 µm after operation with trace amounts of NH3. While this study shows local degradation of the MEA, we will investigate the BPP and its porosity after application in single cell HT-PEMFC under various operating conditions using µ-CT in another study. In combination we aim to present the opportunities of employing µ-CT as a technique to visualize and better understand degradation phenomena in HT-PEM fuel cell components.

Søndergaard, T.; Cleemann, L. N.; Becker, H.; Steenberg, T.; Hjuler, H. A.; Seerup, L.; Li, Q.; Jensen, J. O., Journal of The Electrochemical Society
2018,
165, (6), F3053-F3062.

Büsselmann, J.; Rastedt, M.; Klicpera, T.; Reinwald, K.; Schmies, H.; Dyck, A.; Wagner, P., Energies (Open Access Journal)
2020,
13, (3), 567.

Kannan, A.; Li, Q.; Cleemann, L. N.; Jensen, J. O., Fuel Cells
2018,
18, (2), 103-112.

Figure 1
Water Balance Measurements for a Single Proton Exchange Membrane Fuel Cell Using a Hot Wire Anemometer
Authors: Al Shakhshir Saher , Berning Torsten
Published: December 2020

Water balance data for a single proton exchange membrane fuel cell with an active area of 50 cm2 were measured with a hot wire anemometer, placed in the anode outlet stream. The measured hot wire voltage, E, is divided by the calibration signal, E_0, that was taken for a dry hydrogen stream of the same molar flow as the anode outlet at the same temperature. Because water vapor has a much lower thermal conductivity than hydrogen, the recorded hot wire voltage is always lower than the calibration signal, and there is a clear correlation between the relative voltage signal E/E0 and the fuel cell water balance. The relative voltage single can be easily computed at any point in time to thus provide a real-time signal of the fuel cell water balance.

This data was for a variety of operating conditions. The results show the same trends that were observed by other researchers who used the water knock-out method, only with higher accuracy. A decrease in the anode inlet relative humidity decreases the water balance value, rd
, significantly while the cathode inlet relative humidity only slightly affects the measured water balance. The membrane-electrode-assembly in this work was provided by Hydrogen South Africa – Catalysis (HySA Catalysis).

Figure 1
High O2 Permeability Ionomers for Improved Fuel Cell Performance
Authors: Macauley Natalia , Spinetta Magali , Zhong Sichen , Yang Fan , Lousenberg Robert D , Zenyuk Iryna V. , Qi Yongzhen , Jankovic Jasna , Pedram Sara , Xu Hui
Published: December 2020

In a proton exchange membrane (PEM) fuel cell, the local oxygen transport across the ionomer film in the catalyst layer has a significant impact on electrode performance especially at high current density.1 It is therefore crucial to use ionomers that have higher oxygen permeability than the baseline Nafion. In this work, novel ionomers with increased oxygen permeability have been synthesized by copolymerization of perfluoro-2,2-dimethyl-1,3-dioxole (PDD) with perfluoro(4-methyl-3,6-dioxaoct-7-ene) sulfonyl fluoride (PFSVE) and a ter-monomer. PDD is the main source of higher permeability due to its bulky structure, PFSVE provides ionic conductivity and the amount of the ter-monomer is adjusted to achieve high yields without compromising the equivalent weight (EW). The ring structure of PDD creates additional open space within the ionomer structure for improved gas permeability. Some of the newly developed ionomers have up to five times higher permeability than Nafion, which should result in a significant improvement of fuel cell performance, mainly at high current densities. Ionomers with different PDD content and equivalent weight have been studied to establish the correlation between ionomer properties and MEA performance. Local oxygen resistance, ionomer sheet resistance, ionomer coverage, and SO3
- group coverage will be evaluated and correlated to electrode performance. The performance and durability of the electrodes using high permeability ionomer will be correlated to the ionomer interaction with catalyst particle.

This work will provide a comprehensive understanding of interactions among Pt, carbon, ionomer and their impact on the electrode structure and fuel cell performance and durability. The attained information will be used to improve fuel cell electrode design.

Acknowledgement: The project is financially supported by the Department of Energy’s Fuel Cell Technology Office under the Grant DE-SC0018597.

References:

1. Baker et al, J. Electrochem. Soc. 156, B991 (2014).