bluuebunny/crossref_metadata_2025 huggingface dataset:
(Invited) Intrinsically-Stretchable, Transparent Thin Film Transistors
Authors: Tong Kwing , Liang Jiajie , Pei Qibing
Published: February 2020

Thin-film field effect transistor (TFT) is a fundamental component behind morden electronic devices including displays, sensor arrays, microprocessers, and identification tags. Here, we report the development of new conductor, semiconductor, and dielectric materials that exhibit elastomeric deformability and solution processability. A silver nanowire based screen printing ik was formulaed to print silver nanowire electrodes with conductivity as high as 47K S/cm which are embedded in an elastomer matrix to preserve the electrical conductivity at large strains. A semiconductive single-walled carbon nanotube network was employed as the channel material, and a polyurethane-co-polyethylene oxide as elastomeric dielectric. Intrinsically stretchable TFTs were fabricated using these new electronic materials. The fabrication processes involved the preparation of the source, drain and gate electrodes, semiconductor layer, dielectric layer, and substrate by solution-based techniques under ambient conditions. The resulting devices exhibited a mobility of ~30 cm2V-1s-1, on/off ratio of 103-104, switching current >100 mA, transconducance >50 mS, and relatively low operating voltages. The devices could be stretched by up to 50% strain while retaining a high mobility >10 cm2V-1s-1. No significant loss in electrical property was observed after 500 cycles of repeated stretching to 20% strain. The TFTs were used to drive organic light-emitting diodes with brightness controllable from 0 cd/m2 (off) to 200 cd/m2 (a typical display brightness). The approach and results represent an important progress toward the development of stretchable active matrix displays and other stretchable electronic devices.
Oxide TFT with Sog Gate Insulator
Authors: Hwang Sang Ho , Hong Min-Taek , Lee Jong Mo , Jang Hee jin , Yun Eui Jung , Bae Byung Seong , Kim Myeong Seok , Huh Hyun Sue
Published: February 2020

Oxide TFT (thin film transistor) shows higher mobility and less leakage current than amorphous silicon TFT. In this presentation, we investigated solution processed gate insulator for the oxide TFT. The solution process does not require the use of expensive vacuum equipment, and also has advantages of simple process, high productivity and low cost.

Less defects and trap centers are required for the gate insulator and the conventional material is SiO2 for which vacuum equipments are used to deposit it. Since the gate insulator is deposited on the active layer for the top gate structure, active layer can be deteriorated by plasma ion bombardment during deposition of the gate insulator.

Plasma damage exerted on the active layer during the deposition of the gate insulator have an adverse effect on the TFT characteristics. On the other hand, solution process does not use plasma, therefore, it can remove plasma damage on the active layer. And also, solution process is inexpensive and highly productive process.

In this experiments, we used spin on glass material for the solution process of the gate insulator. We investigated the effect of dilution of the SOG with IPA (isopropyl alcohol). It was coated on a p-type Si wafer by spin coating with 3500 rpm. For the diluted solution, we coated several times for the desired thickness of the gate insulator. Hard bake was done for 20 min at 170℃ after soft-bake for 20 min at 85℃. Final cure was done for 1 hour at 420℃ in nitrogen ambient. Figure 1 shows the current densities of the SOG insulator for the various dilutions of SOG solution. The leakage currents were reduced for the diluted solution.

Figure 1
Improved Properties of Oxide Thin-Film Transistor By Electrohydrodynamic-Jet Printing
Authors: Kwack Young-Jin , Choi Woon-Seop
Published: February 2020

Oxide Semiconductors have been paid attention to suitable TFTs for OLED because of higher mobility than a-Si TFTs, lower leakage current than LTPS TFTs and even low cost. These oxide thin films can be obtained by simple solution process and applied to printed electronics. Printing process is a way to reduce cost and process time effectively over tranditional vacuum process and photo-lithography by direct patterning. Nevertheless, semiconductor films, in most study of solution processed oxide TFTs, are fabricated by spin coating, which is inadequate for electronics industry. For this reason, researches for various printing methodes on oxide TFTs are needed certainly. Electrohydrodynamic-jet(EHD-jet) technique is avilable to form a fine pattern by electric fields between metal stage and tiny orfice tip. We opimized EHD-jet printning conditions for ZTO solution and fabricated with addition patterning technique ZTO TFT sucessfully showing better and stable features than spin coated one.

The effects of speed and temperature of stage on printed pattern width was examined. As the stage temperature accelerated vaporization rate of solvent, printed pattern width was narrower and thicker. Thus, it is essential to optimize printing conditions for any solutions to obtain stable printing state. EHD-jet printed TFTs and patterned TFTs showed similar electrical characteristics with spin coated ZTO TFT but much improved contact resistance and leakage current. We investigated the origin of low leakage current based on the overlap of semiconductor with source and drain electrodes and related fringe effect. Electrical characteristic of ZTO TFT show a mobility of 6.9 cm2/Vs, an on-to-off current ratio of 4.7 x 108, a threshold voltage of 7V and a subthreshold-swing of 0.4 V/dec. Hysteresis behavior and passivation effect were also characterized.
Molecular Dynamics Simulations of Silicene Monolayer Growth
Authors: Cherukara Mathew Joseph , Narayanan Badri , Harder Ross , Sankaranarayanan Subramanian K. R. S.
Published: February 2020

Silicene, the Si based counterpart to graphene is predicted to demonstrate similar desirable features to graphene, possessing a Dirac cone in the band-structure and Fermi velocities comparable to graphene. It has also been predicted to demonstrate several exotic properties such as the quantum spin Hall effect, chiral superconductivity, giant magnetoresistance, and has the added benefit of being easy to integrate into current industrial device engineering processes. Silicene monolayers have been grown on Ag (111) and Ir (111) substrates and recently a working field-effect transistor has been demonstrated at room temperature. While it is possible to characterize the final monolayer structure, it is impossible to probe experimentally the evolving atomic structure during growth. We use large-scale molecular dynamics (MD) simulations to study the growth of silicene monolayers on Ag (111) and Ir (111) surfaces. Initially, individual Si atoms aggregate to form small clusters containing 3-6 Si atoms with different ring geometries. Over time, these rings merge to form larger clusters, but containing extended line defects and grain structures. Figure 1 A-D shows the growth process on an Ag (111) surface, while Figure 1 E shows the final grown structure on an Ir (111) surface. Interestingly, structures grown on the Ag surface are prone to more defects, in particular extended regions of sub-surface growth, while the structures grown on Ir are relatively defect free. We also explore the effect of different growth conditions such as temperature and deposition flux on the structure of the monolayers. Finally, we study the mechanical and thermal properties of these in-silico grown monolayers and compare their performance to previously reported data for ideal structures. The processing-structure-property correlations we draw will enable experimental processes to be tailored to obtain target device properties.

Figure 1
(Invited) Interfacial Engineering of Two-Dimensional Nanomaterials: Chemical Functionalization and Scanning Probe Characterization
Authors: Wang Qing Hua
Published: February 2020

Atomically thin two-dimensional (2D) materials have recently emerged as promising materials for electronic and photonic devices due to their unique layer-number-dependent properties. Because they are atomically thin, interfacial interactions between 2D materials and their surroundings play an important role in regulating their properties for device applications. The electronic and optical properties of 2D materials can be readily engineered by covalent and non-covalent chemical functionalization methods, and can be characterized with atomic-scale precision using scanning probe microscopies. In this work, we demonstrate the functionalization of MoS2 and WSe2 by covalent and non-covalent chemistries, which result in charge transfer, layer-dependent reactivities, and molecular rearrangements at their surfaces. We characterize the structural, electronic, and optical changes to the 2D materials using ultrahigh vacuum scanning tunneling microscopy (UHV STM), atomic force microscopy (AFM), and Raman and photoluminscence (PL) spectroscopic mapping. By controlling the chemical groups that are used in the functionalization reactions, we can engineer their effect on the interfacial properties of 2D materials, which will be important to the processing of novel electronic and photonic devices.
Low Threshold Single Mode ZnO Nanowire Nanolasers
Authors: Visimberga Giuseppe , Boese Markus , O'Dwyer Colm
Published: February 2020

With a wide bandgap of 3.36 eV at room temperature and large exciton binding energy of 60 meV (excitons being stable up to room temperature), ZnO holds promise for use in blue and ultraviolet optical devices [1], including ultraviolet microlasers. Lasing has been demonstrated with epitaxial and microcrystalline thin films [2], and in arrays of ZnO nanorods [3] and nanowires [4], for example. Usually, the feedback mechanism in nanorod and nanowire-based lasers is related to the formation of Fabry–Perot cavities for longitudinal modes [5], or to the structures ability to couple and facilitate guided modes [6]. Discriminating between the feedback mechanisms in ZnO nanostructures is not an easy issue, especially in microstructures consisting of nanorods with various orientations. Additionally, nanoscale systems also suffer from impure spectral characteristics and multimodal emission. Strategies to improve emission characteristics have seen limited success due to the sensitivity of nanoscale resonator cavities to optical properties dependant on structural shape and details of the crystallinity. Whispering gallery modes and complicated multimodal emission with a spread in gain factors compound the issue of spectrally pure emission in nanoscale systems.

High crystallinity is a prerequisite for short-cavity ZnO nanorod lasers due to expected coexistence of heavy mirror losses and scattering losses caused by the intrinsic structure and surface defects typically affecting ZnO. Here, arrays of vertically aligned ZnO nanorods were fabricated by CVD, with very high structural purity and quality. Characterization of aluminum deposition as surrounding layers on nanorods of 2-5 μm in length and 150-200 nm in width will be presented, and we detail the effect of selectively coated individual nanorods on the waveguiding and emission quality of the nanorod laser. The aluminum deposition alters the hexagonal cavity properties by reducing the guided modes and lowering the gain of the higher order modes so that a better, more spectrally pure emission is possible.

[1] U. Ozgur, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S.-J. Cho, M. Morkoc, J. Appl. Phys.
98, 041301 (2005).

[2] Z. K. Tang, G. K. L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Appl. Phys. Lett.
72, 3270 (1998).

[3] G. Visimberga, C. C. Faulkner, M. Boese and C. O'Dwyer, ECS Trans., 45, 51 (2012).

[4] S. P. Lau, H. Y. Yang, S. F. Yu, H. D. Li, M. Tanemura, T. Okita, H. Hatano, H. H. Hng, Appl. Phys. Lett.
87, 013104 (2005).

[5] Y. Zhang, R. E. Russo, S. S. Mao, Appl. Phys. Lett.
87, 043106 (2005).

[6] H. Zhou, M. Wissinger, J. Fallert, R. Hauschild, F. Stelzl, C. Klingshirn, H. Kalt, Appl. Phys. Lett.
91, 181112 (2007).
(Invited) Low-Voltage Complementary Electronics from Ion Gel-Gated Vertical Van Der Waals Heterostructures
Authors: Cho Jeong Ho
Published: February 2020

Isolation of ultrathin layered materials with disparate electronic properties has prompted their integration into van der Waals heterostructures with unique properties for electronics. By employing ion gel dielectrics, we demonstrate here low-voltage complementary circuits comprised of n-type and p-type van der Waals heterojunction vertical field-effect transistors (VFETs). The n-type and p-type VFETs were fabricated from MoS2 and WSe2 layers on graphene, respectively, with gating achieved by a top ion gel dielectric. Due to the high specific capacitance of the ion gel dielectric, the work function of the underlying graphene and consequently the energy barrier for charge injection into the MoS2 and WSe2 layers are widely tuned at low operating voltages. The resulting VFETs possess high on-state current densities (> 3000 A cm-2) and on/off current ratios (> 104) in a narrow voltage window (< 3 V). In addition to enhancing VFET device metrics at reduced operating voltages, the high specific capacitance of the ion gel dielectric allows unprecedented regimes of charge transport to be accessed including p-channel phenomena in graphene-MoS2 vertical heterostructures. Furthermore, since the ion gel-gated VFETs possess low threshold voltages, a low-power complementary logic inverter is demonstrated using n-type and p-type VFETs. Overall, these results demonstrate the feasibility of vertical van der Waals heterostructures for high-performance, low-voltage electronics.
Self Assembled ZnO Nanowire Field Effect Transistors
Authors: Matei Elena , Florica Camelia , Costas Andrea , Enculescu Monica , Enculescu Ionut
Published: February 2020

Electrodeposition of ZnO from aqueous baths may be employed as a straightforward path in obtaining in a cheap way nanostructures with well tailored morphology. Nanowires are a particular type of nanostructure which can be obtained in this way with or without employing a template. Further these nanowires can be employed as channel in field effect transistors. However the fabrication of such electronic devices based on nanowires is a difficult and time consuming task due to the challenge in providing the nanostructures with electrical contacts. Usually techniques such as electron beam lithography or focused ion beam induced metallization are employed for this goal [1].

We present here an alternative to these approaches in which simple electrodeposition on interdigitated electrodes can lead to the formation of high quality field effect transistors with pure or doped ZnO nanowire channel. The applications of such nanostructure based devices are numerous most being related to sensing applications where one makes use of the high surface to volume ratio and a cheap and easily scalable method of fabrication represents an important advantage.

References

[1] C. Florica, E. Matei, A. Costas, M.E.T. Molares, I. Enculescu, Field Effect Transistor with Electrodeposited ZnO Nanowire Channel, Electrochim. Acta. 137 (2014) 290–297. doi:10.1016/j.electacta.2014.05.124.
(Invited) Multi-Functional Flexible Healthcare Sesnors and Integrated Circuits
Authors: Takei Kuniharu
Published: February 2020

The concepts of trillion sensor network and Internet of Things (IoT) require a variety of sensors that can be economically attached on any surfaces. To address this requirement, printed flexible electronics is one of solutions to integrate multifunctional macro-scale sensor networks without increasing fabrication cost compared to the Si-based sensors. To show a proof-of-concept of the printed and flexible devices, in this talk, a printed multi-functional flexible healthcare patch will be presented. The device contains a temperature sensor, a electrocardiogram sensor, an acceleration sensor, and an ultraviolet sensor on a flexible plastic substrate, which are fabricated by all printing methods. Especially, inorganic nanomaterials mixed with organic materials were used to achieve the high performance, air-stability, and mechanical flexibility. Furthermore, for the future flexible signal processing circuits, digital and analog circuits will be also demonstrated. It should be noted that those circuits are still fabricated by using a standard semiconductor lithography technique due to difficulity of material integration using printing methods. Those multi-fuctional sensors and flexible circuits are successfully demonstrated as the first proof-of-concepts. Although more developments to realize printed fully flexible device systems are required, we believe that the demonstrations of the macro-scale flexible sensor sheets  may contribute to move forward to realizing the next generation electronics.
(Invited) Towards High-Power All-Solid-State Batteries
Authors: Kühnel Ruben-Simon , Duchêne Léo , Roedern Elsa , Remhof Arndt , Battaglia Corsin
Published: February 2020

While the size of electronic components has shrunk dramatically over the last few decades, the footprint of autonomous electronic devices is today often limited by the battery. Miniaturization of batteries is challenging in particular when using liquid electrolytes based on organic solvents. All-solid-state batteries employing solid-state electrolytes promise to facilitate miniaturization and integration. Current developments of solid-state electrolytes for lithium ion batteries focus mainly on oxide- and thiophosphate-based compounds, but have not yet enabled the fabrication of a competitive all-solid-state battery.

We recently reported the discovery of a superionic phase at room temperature in the class of lithium amide-borohydrides, enabling ionic conductivities comparable to values of common organic liquid electrolytes. We demonstrate good rate performance up to 5C and stable cycling over 400 cycles at 1C in a lithium titanate half-cell battery configuration, indicating high bulk and interfacial stability. Our results demonstrate the potential of lithium amide-borohydrides as solid-state electrolytes for scalable high-power lithium ion batteries.
Photovoltaic Cell Using p-WSe2/n-MoS2 Heterojunction for Optical Switching
Authors: Ahn Jongtae , Kim Jin Sung , Lim June Yeong , Yu Sanghyuck , Im Seongil
Published: February 2020

Two-dimensional (2D) semiconductor materials with discrete bandgap become important because of their interesting physical properties and potentials toward future nanoscale electronics. Semiconductor p-n junctions are essential building blocks for electronic and optoelectronics devices. Conventional lateral p-n heterojunction diode had several problems. Lateral p-n diode has quasi neural region so it’s efficiency is lower than vertical p-n diode. Here we fabricated vertical structure 2D dichalcogenide-based p-n heterojunction diode circuits for Photovoltaic-induced switching applications using p-WSe2 and n-MoS2 nanosheets. Moreover we modulate the thickness of flakes for enhancing the device performance. Our p-n diode PV cell displayed an instantaneous voltage of 0.3 V as Voc due to the difference between conduction and of MoS2 and valence band of WSe2. Furthermore based on such PV effects we successfully demonstrated PV optical switching, logic gating, and finally the gate switching of an IGZO field effect transistor.

Figure 1
Effects of Asymmetric Local Joule Heating on Silicon Nanowire-Based Devices and Their Applications
Authors: Ho Hsiang-Hsi , Tsai Wei-Che , Hong Liang-Zheng , Lyu Cheng-Han , Chien Wen-Ching , Hsu Hsun-Feng
Published: February 2020

With rapid advancement of semiconductor manufacturing and measurement techniques, silicon nanowires (Si NWs) have been produced by several methods and successfully incorporated in solar cells, light emitting diodes and photodetectors due to their unique properties, such as high surface-to-volume ratio, quantum confinement effect and high crystal quality.

In this study, silicon nanowires were fabricated by metal assisted–chemical etching (MacEtch) combined with nanosphere lithography (NSL) and subsequently used as building blocks for parallel silicon nanowire-based devices using alternating current (AC) or direct current (DC) dielectrophoresis alignment across Pt electrodes. The role of Si NWs/Pt metal contacts and its effect on electrical and photosensing properties of the devices were investigated.

The results show that the devices exhibited rectifying current-voltage characteristics randomly after the strong AC electric field was applied across electrodes for NWs alignment. On the other hand, the rectifying behavior of the devices can be observed as the devices were measured in the wide voltage range after the weak DC electric field was applied across electrodes for NWs alignment, and furthermore the rectification direction can be controlled by the voltage sweep direction. This phenomenon can be associated with the increased Schottky barrier height on the high-temperature anode side as the asymmetric Joule heating effect occurs instantly in the electrical measurement process.

The photosensing properties of the rectifying device were further investigated. By utilizing the increased Schottky barrier height in reverse-biased mode under white light illumination, the high speed and high photoresponse can be achieved due to the efficient electron-hole separation by strong built-in electric field in the depletion region.
The Novel Way to Inspection Technique for SiC Substrate by Applying Stress Effects
Authors: Sakata Yoshitaro , Terasaki Nao , Nonaka Kazuhiro
Published: February 2020

Silicon carbide (SiC) has attracted much attention as an important material for high temperature, high power and high frequency devices, because it has advantages compared to silicon (Si) devices. However, SiC substrates are made the same way with Si substrates in production manufacturing.

In production site, fine polishing techniques, such as a chemical mechanical polishing treatment (CMP), are some of the most important base techniques used for the semiconductor manufacturing. Mechanical interaction in the form of friction occurs between the abrasive and the substrate surface during polishing, which may cause formation of latent flaws on the silicon carbide (SiC) substrate surface. Fine polishing-induced latent flaws may become obvious during a subsequent cleaning process if SiC substrates are corroded away by chemical interaction with the cleaning liquid. Latent flaws thus reduce product yield.

Recently, many nondestructive inspection systems that use light scattering from foreign matter particles for their detection have been made commercially available for use in production sites. These systems use laser incident light and a photo-detector (e.g. a photomultiplier tube) for detecting light scattering from foreign particles with high sensitivity. Inspection systems based on Rayleigh scattering and Mie scattering can detect particles smaller than the laser wavelength. However, these systems detect all light scattering as foreign matter particles on the surface. Therefore, these system cannot discriminate between latent flaws and particles. In addition, these inspection techniques cannot detect defects under the surface of substrates.

We propose a novel inspection technique for fine polishing-induced latent flaws by combining the light scattering method with stress effects, referred to as the stress-induced light scattering method (SILSM, Y. Sakata, et al, Review of Scientific Instruments, Vol. 85, No. 8, p. 083303-1 (2014)). In this method, samples are deformed by an actuator and stress effects are induced around the tips of latent flaws. Due to the photo elastic effect, the refractive index of the material around the tip of a latent flaw is changed. This changed refractive index is in turn detected by a cooled CCD camera as variations in the light scattering intensity.

In this report, latent flaws under the surface of SiC substrate are detected non-destructively by applying SILSM, and the utility of SILSM will be evaluated as a novel inspection technique for the substrate. In addition, the results detected by SILSM is compared with the results of conventional laser raman spectrometry and photo-luminescence results, respectively. And latent flaws under SiC substrate surface is evaluated from various directions.

This research was partially supported by AIST-AMRI budget for Exploratory Research and we gratefully acknowledged this budget with deepest appreciation.
(Invited) Electroosmotic Flow-Driven Ion Current Rectification and Negative Differential Electrolyte Resistance in Nanofluidics
Authors: Yeh Li-Hsien
Published: February 2020

Ion current rectification (ICR) refers to an asymmetric diode-like current-voltage behavior when switching the polarity of applied voltage bias on a nanopore, and negative differential resistance (NDR) refers to a negative conductance behavior, that is, the current decreases with increasing applied voltage bias. Previous studies have shown that the former can be found in a charged nanopore, which has at least one side with dimension comparable to its electric double layer; therefore, the symmetry of ionic concentrations across a nanopore can be broken. The latter can also be achieved in a charged nanopore, which is filled with an aqueous solution with higher salt conductivity and an organic solution with lower salt conductivity, by the competition between the electroosmotic flow (EOF) and the pressure-driven flow. In this talk, I will show the first experimental result that both the ICR and NDR phenomena can be observed in a charged nanopore without conventional rigorous conditions such as overlapped electric double layer, external pressure, and organic solutions. Results show that both the ICR and NDR phenomena depend highly on the solution pH and solution properties. In addition to experimental observations, the model comprising the coupled Poisson-Nernst-Planck (PNP) and Navier-Stokes (NS) equations are employed to demonstrate that both the ICR and NDR phenomena results mainly from the EOF-driven mechanism.
(Invited) Engineering Nonlinear Electrokinetic Flows at Polarizable Interfaces
Authors: Gagnon Zachary
Published: February 2020

Electrokinetic phenomena involve the interaction between applied fields, ions, fluid flows, and particles. When combined with microfluidics, these force fields can produce new and exciting applications. Our lab is interested in understanding and exploiting the interaction between external fields and charged microfluidic surfaces and polarizable fluid interfaces. Recently, we have developed new ways to harness and engineer these flows to exhibit intelligent responsiveness to specific biomolecules. In this talk, I will present our experimental and theoretical findings on how aqueous liquid interfaces polarize and electrohydrodynamically respond to externally applied electric fields. We will show an experimental system we have developed to study and quantify these flows. We then describe a variety of ways to harnesses these interfacial flows for applications in biosensing, nanoemulsions, and biomolecule separation and concentration.
Numerical Study on the Heat Transfer Performance of Non-Newtonian Fluid Flow in a Manifold Microchannel Heat Sink
Authors: Li Si Ning , Zhang Hong Na , Li Qian , Zhang Meng , Li Xiao Bin , Li Feng Chen
Published: February 2020

For the past few decades, with the rapid development of micro-electro-mechanical- systems (MEMS) and Lab-on-a-chip, the electronic devices continue to be miniaturized and highly integrated. Meanwhile, on account of miniaturization and integration, it essentially brings in high heat flux, which makes heat transfer enhancement to be a hot topic in these fields. With the heat flux rising, the traditional cooling method becomes unable to meet the demands of heat removal in the highly integrated devices. As one of the most promising heat exchange technologies with high efficiency, the microchannel heat sinks (MHS) was proposed. For a long time, modifying the microchannel structures is a promising way to enhance heat transfer, such as using manifold microchannel (MMC). Except modifying the microchannel structures, changing mediums in MHS could also contribute to better flow distribution and heat transfer performance. This paper concentrates on the synergy effect of non-Newtonian fluids and MMC heat sink on heat transfer enhancement. Numerical simulation of the non-Newtonian fluid in MMC is carried out in order to obtain its flow and heat transfer performance. The power-law non-Newtonian fluids are chosen as the cooling medium while water is a reference medium. The traditional microchannel (TMC) heat sink as an important reference model aims at evaluating the performance of the MMC heat sink through analyzing temperature distribution, power-law non-Newtonian fluids and so on. Numerical simulations are conducted for Newtonian and non-Newtonian fluid at a fixed heat flux on the heating wall. It is found that, in MMC heat sink, the non-Newtonian fluid characteristics could result in a significant variation in heat transfer coefficient with increasing inlet velocity. Moreover, the introduction of pseudoplastic fluid (shear-thinning fluid) leads to an outstanding increase of heat transfer performance comparing with those of Newtonian and dilatant fluids (shear-thickening fluid).

Keywords:

Manifold microchannel heat sinks; non-Newtonian fluids; heat transfer performance;
Relation Between Current Density Distribution in Land-Channel Direction and Presence of Liquid Water in Proton Exchange Membrane Fuel Cell Flow Channel
Authors: Karani Jash , Shrivastava Udit N , Tajiri Kazuya
Published: February 2020

Since most of the PEMFCs have land-channel geometry in their flow field, the distance between the flow channel to the catalyst layer is not uniform. The distance is longer from the flow channel to the catalyst layer under the land area than to the catalyst layer under the channel area. This difference causes the non-uniform current density in land-channel direction, and the distribution largely depends on the operating conditions. As the inlet gas humidity in the operating condition decreases, the trend in the current density distribution changes from high local current under the channel to relatively uniform, and to higher local current under the land (Figure 1). This trend is due to the combined effects of (1) non-uniform oxygen transport resistance, (2) non-uniform liquid water transport distance causing the local flooding under the land area at wet condition, and (3) the tendency of faster membrane dryout under the channel area at dry condition.

In order to investigate the distributions of various parameters in the land-channel direction, a small-scale segmented cell with about 300-micron resolution was developed by the authors’ group, and successfully measured the current and high-frequency resistance distribution in the land-channel direction. Distribution of oxygen transport resistance from the flow channel to the catalyst layer was also measured using limiting current technique.

In this study a new segmented cell is developed. Using the transparent cathode plate the behavior of liquid water in cathode flow channel can be observed, and simultaneously the current density distribution is measured in land-channel direction. With this cell the relation between the current density distribution in the land-channel direction and the presence of liquid water is investigated. First, it is studied that at what gas humidity and at what overall current density the liquid water starts emerging from the gas diffusion layer surface to the flow channel. This provides us the information about the dependency of the liquid water emergence location on the current density distribution is studied. Then, how the presence of liquid water in the flow channel changes the current distribution is studied.

Figure 1
The Non-Monotonic Surface Charging Behavior of Platinum: Implications for Electrocatalysis in PEFC
Authors: Huang Jun , Zhang Jianbo , Eikerling Michael
Published: February 2020

In electrochemical systems, metal surface charging phenomena dictate the strength of electrostatic interactions between the electrified electrode and ions in solution. Historically, the potential of zero charge (pzc) of the metal has been employed to parameterize the surface charging relation [1][2].

However, for a Pt electrode, classical radiotracer experiments of Frumkin and Petrii revealed a non-monotonic charging behavior upon increasing the metal phase potential, with consecutive transitions from a negative to a positive surface charging region and further to another negative surface charging region [3]. Garcia-Araez et al. later reported a similar ‘negative-positive-negative’ charging relation derived from laser-pulsed experiments [4]. This alternating charging behavior defies the role of the pzc as a sufficient descriptor of electrode charging phenomena. The situation calls for an overhaul of the charging relation that must be derived from first principles. This study presents a refined structural model for electrified interfaces, as shown on the left side of Figure 1. The model accounts explicitly for charging effects caused by the formation of a surface oxide layer and a layer of oriented water molecules. An analytical expression for the charging relation is derived.

The charging relation is parameterized using experimental data for a Pt surface as well as DFT calculations. It exhibits the three different charging regions observed in the experiments cited above. To illustrate the impact of the modified charging relation, it is employed to study the oxygen reduction reaction in nanoporous electrodes of polymer electrolyte fuel cells.

References

[1]. Frumkin, A.; Gorodetzkaya A. Z. Phys. Chem. 1928, 136, 451.

[2]. Petrii, O. A. Zero charge potentials of platinum metals and electron work functions (Review). Russ. J. Electrochem. 2013, 49, 401.

[3]. Frumkin, A. N.; Petrii, O. A. Potentials of zero total and zero free charge of platinum group metals. Electrochim. Acta. 1975, 20, 347.

[4]. Garcia-Araez, N.; Climent, V.; Feliu, J. Potential-dependent water orientation on Pt (111), Pt (100), and Pt (110), as inferred from laser-pulsed experiments. Electrostatic and chemical effects. J. Phys. Chem. C. 2009, 113, 9290.

Figure 1
Effect of Temperature on Platinum Dissolution from PEFC Catalysts
Authors: Wang Zhongqi , Tada Eiji , Nishikata Atsushi
Published: February 2020

The durability of polymer electrolyte fuel cells (PEFCs) is a big concern for its applications like FCVs to be commercialized on a large scale. In particular, the instability of Pt-based cathode catalyst has been discussed intensively. Platinum (Pt) dissolution is recognized as one of the degradation mechanisms for carbon supported Pt catalysts (Pt/C) in PEFCs. It has been found that repeated potential cycles accelerate Pt dissolution.[1-3] Fundamental studies in aqueous media revealed preliminary behaviors of Pt dissolution during one potential cycle. The up-to-date data were mostly obtained near room temperature, and a dominant cathodic dissolution during reduction of Pt oxide was understood. As reported by Fuller et al.,[4] however, degradation of Pt/Cs at an elevated temperature near 60 to 80 °C, whereat a PEFC normally operates, was severer than that at room temperatures. Thus, it is necessary to evaluate Pt dissolution from Pt/Cs at such temperatures.

In prior studies,[5-6] we compared Pt dissolution at 25 °C and 65 °C by inductively coupled plasma mass spectrometry (ICP-MS) and a channel flow double electrode (CFDE). The experiments had some difficulties, but the results showed a ca. 5-time enhancement from 25 to 65 °C. The cathodic dissolution was enhanced because larger amount of Pt oxide was formed at 65 °C. The effect of temperature on the anodic dissolutions and the place exchange process remains unclear yet.

In this study, we evaluate Pt dissolution under temperatures ranging from 20 to 80 °C. We test a series of commercial Pt/C catalysts produced by TKK that have Pt loadings from 20 to 70 wt% and average Pt particle size from ca. 1.5 to 5 nm. We limited the potential region between 0.6 and 1.4 V, which is closely related to the operation region of a PEFC cathode. We use ICP-MS to evaluate to overall dissolutions and a CFDE to analyze the dissolution behavior in one potential cycle. The attached Figure showed a typical CFDE data presenting the current on a working electrode loading a 30wt% Pt/C (TEC10E30A) under potential cycling (I
WE) and the current recorded on a collector electrode (I
CE) at 80 °C in 0.5 M H2SO4. The I
CE helps to understand the dissolution of Pt4+ and Pt2+ complex during one potential cycle. Similar analysis will be discussed at other temperatures like 20, 40, and 80 °C in the presentation.

Reference

Z. Wang, E. Tada, and A. Nishikata, J. Electrochem. Soc., 161, F380 (2014).

A.A. Topalov, S. Cherevko, A.R. Zeradjanin, J.C. Meier, I. Katsounaros, and K.J.J. Mayrhofer, Chem. Sci., 5, 631 (2014).

P. Jovanovič, A. Pavlišič, V.S. Šelih, M. Šala, N. Hodnik, M. Bele, S. Hočevar, and M. Gaberšček, ChemCatChem, 6, 449 (2014).

W. Bi and T.F. Fuller, J. Electrochem. Soc., 155, B215 (2008).

Z. Wang, E. Tada, and A. Nishikata, J. Electrochem. Soc., 163, F1-F3 (2016).

Z. Wang, E. Tada, and A. Nishikata, ECS Trans., 69, 255 (2015).

Figure 1
Direct Simulations of Coupled Transport and Reaction on Nano-Scale X-Ray Computed Tomography Images of Platinum Group Metal-Free Catalyst Cathodes
Authors: Ogawa Shohei , Komini Babu Siddharth , Chung Hoon T , Zelenay Piotr , Litster Shawn
Published: February 2020

Polymer electrolyte fuel cells (PEFCs) have been developed for automotive, residential, and portable power applications. Typically, PEFC stacks use platinum (Pt) or Pt alloys as the oxygen reduction reaction (ORR) catalyst in the cathode. Although Pt performs well, its raw material cost is significant and limits future cost reductions. As an alternative, platinum group metal-free (PGM-free) catalyst may have the potential to significantly reduce PEFC costs. Despite recent advances in PGM-free catalyst ORR activity,1 the volumetric activity remains well below that of Pt cathodes and results in thick electrodes with significant transport losses.

To advance PGM-free catalyst electrodes, it is important to understand the coupling between electrochemical reactions, transport phenomena, and microstructure in the catalyst layer. Here, we expand this understanding using a combination of nano-scale X-ray computed tomography (nano-CT) and direct simulations on the CT images. Nano-CT imaging was done for cyanamide-polyaniline-iron PGM-free catalyst cathodes with three different Nafion® loadings (35, 50 and 60 wt%). Using Zernike phase contrast we differentiated the solids (Nafion®, carbon, and iron) and the pore phase,2 and by staining the electrode with cesium, we distinguished the Nafion® phase by absorption contrast in a second scan. The two data sets were overlaid to complete the structure of the whole cathode.3 In other words, at each voxel we have the volume fractions of pore, Nafion®, and carbon catalyst phases. To illustrate, Figure 1a shows the Nafion® fraction over the simulation domain where the Nafion® had agglomerated and was heterogeneously distributed. The spatially-resolved volume fraction data is then input to the simulation of oxygen, proton, and electron transport as well as the local ORR volumetric current. Figure 1b shows the ionic potential and current streamlines in Nafion® phase resulting from the distributed reaction.

Acknowledgement

The authors gratefully acknowledge the support the support of the technology development manager Nancy Garland and funding from the US Department of Energy, the Office of Energy Efficiency and Renewable Energy, Office of Fuel Cell Technologies. The nano-CT instrument was acquired with the support of a Major Research Infrastructure award from the National Science Foundation under Grant No. 1229090.

References

1. G. Wu, K. L. More, C. M. Johnston, and P. Zelenay, Science, 332, 443–447 (2011).

2. S. Komini Babu, H. T. Chung, G. Wu, P. Zelenay, and S. Litster, ECS Trans., 64, 281–292 (2014).

3. S. Komini Babu, H. T. Chung, P. Zelenay, and S. Litster, ECS Trans., 69, 22–33 (2015).

Figure 1
Novel Electrocatalyst for Hydrogen Oxidation in Alkaline Media
Authors: Serov Alexey , Kim Yu Seung , Odgaard Madeleine , Halevi Barr , Atanassov Plamen
Published: February 2020

Anion Exchange Membrane Fuel Cells (AEMFCs) are fast growing alternative to Proton Exchange Fuel Cell (PEMFC) technology. It was shown that several classes of non-platinum group metal (non-PGM) catalysts outperformed platinum in the oxygen reduction in alkaline media.[1] In contrast the published data on alternative to platinum anodes for hydrogen oxidation reaction in alkaline media is scarce.[2]One of the possible non-PGM catalysts for the HOR in alkaline media are Ni- and Pd-based materials. We have prepared Ni-Mo-Cu catalysts that shows HOR activities in the rotating disk electrode (Figure 1). The preliminary results on MEA tests show that the Ni-based alloys as an anode have shown extremely high open-circuit voltages (OCVs) in both oxygen and air (up to 1.1V), which is significantly higher compared to PGM-free anodes in PEMFCs. However, the effort in integration of those materials into MEA with most appropriate ionomer and MEA fabrication technique is needed. In this talk, we will discuss the preparation and characterization of the novel electrocatalysts for HOR in alkaline media.

Figure 1. HOR on Ni-Mo-Cu catalysts in 1M KOH saturated with H2, 1600RPM, 10 mV s-1.

Acknowledgements: Department of Energy, Hydrogen Oxidation Reaction in Alkaline Media, Control Number: 0966-1624, Award Number: DE-EE0006962 (PI A. Serov).

[1] M. H. Robson, A. Serov, K. Artyushkova, P. Atanassov, Electrochim. Acta, 90, 2013, 656–665.

[2] A. Serov, C. Kwak, Applied Catalysis B: Environmental, 91, 2009, 1–10.

Figure 1
Fine Tuning of the ORR Performance-Durability for Pt-Based Electrocatalysts
Authors: Lopes Pietro Papa , Li Dongguo , Lv Haifeng , Strmcnik Dusan , Paulikas Paul Arvydas , Markovic Nenad M , Stamenkovic Vojislav
Published: February 2020

The progress in the ORR electrocatalysis over the last two decades has been achieved primarily through the fundamental understanding of processes that are taking place at well-defined surfaces. For instance, the nature of active sites is a common topic in the literature, however, despite decades of persistent studies aimed to reveal the fundamentals, insight at atomic level is still lacking. Properties such as surface crystallographic orientation, morphology, composition and defects are yet to be assigned to the correlation between the atomic structure and catalyst activity. For the first time, we report on the atomic structure that has been investigated in combination with durability. The ultimate precision that goes beyond a part per million of a single atomic layer has been achieved in determining electrocatalytic properties of the ORR catalysts. Obtained knowledge is of paramount importance in design of advanced highly functional nanoscale materials. Surfaces of Pt-based materials have been characterized by AES, LEED and UPS, which was followed by controlled transfer to electrochemical, in-situ FTIR and STM cells. These findings have been further used to optimize a unique RDE coupled ICP-MS system. Such effort has led towards the design and synthesis of nanoscale materials with superior electrocatalytic properties. Fine tuning of the surface properties induced unprecedented improvements in functionality of real world catalyst for the ORR in fuel cells that will be reported for the first time.
Development of Low Cost High-Temperature Polymer Electrolyte Fuel Cell Membrane-Electrode-Assemblies for Combined Heat and Power Plants in Single Family Homes
Authors: Schenk Alexander , Gamper Stefan , Grimmer Christoph , Pichler Birgit Elvira , Bodner Merit , Weinberger Stephan , Hacker Viktor
Published: February 2020

High-temperature polymer electrolyte fuel cells (HT-PEFCs) are a suitable technology for decentralized small scale electricity and heat production. HT-PEFCs do not require hydrogen infrastructure and are characterized by a simpler and therefore less expensive system compared to available fuel cell systems. In combination with a reformer unit, HT-PEFC systems offer an efficiency gain over conventional combustion of hydrocarbons, such as natural gas. In near future, HT-PEFCs will be even more effective and efficient when renewable biofuels and hydrogen are widespread available.

We present an efficient HT-PEFC based combined heat and power (µ-CHP) system for the provision of electrical energy and hot water in single family households (see Table 1). Due to the optimized design and layout of the fuel cell based µ-CHP system and the respective manufacturing processes of the catalysts and the membrane electrode assemblies (MEA), the demand for cost effective and greenhouse gas efficient energy at customer level is addressed.

Major efforts were devoted to the establishment of scalable catalyst deposition methods, which enable a loss-free utilization of precious metals. By using appropriate multimetallic catalyst systems at anode and cathode, the precious metal loading was reduced by approx. 20% in comparison to commercially available electrodes without compromising performance (see Figure 1) [1]. Furthermore, by introducing post-preparation treatments, the stability of the catalysts was enhanced over commercial Pt/C. The activity and stability of the catalyst systems were evaluated ex situ by means of cyclic voltammetry and accelerated stress tests using a rotating disk electrode (RDE) setup. Furthermore, the catalyst systems were characterized in situ by means of polarization curves, continuous operation, accelerated stress tests and electrochemical impedance spectroscopy measurements at single cell, stack and at system level (see Figure 1).

Acknowledgment

Financial support was provided by The Climate and Energy Fund of the Austrian Federal Government and The Austrian Research Promotion Agency (FFG) through the program Energieforschung (e!Mission).

[1] A. Schenk, C. Grimmer, M. Perchthaler, S. Weinberger, B. Pichler, C. Heinzl, C. Scheu, F.-A. Mautner, B. Bitschnau, V. Hacker, Platinum–cobalt catalysts for the oxygen reduction reaction in high temperature proton exchange membrane fuel cells – Long term behavior under ex-situ and in-situ conditions, J. Power Sources. 266 (2014) 313–322.

Figure 1
Gold-Supported Cerium-Doped NiOx Catalysts for Water Oxidation
Authors: Ng Desmond , García-Melchor Max , Bajdich Michal , Chakthranont Pongkarn , Kirk Charlotte S , Vojvodic Aleksandra , Jaramillo Thomas F
Published: February 2020

The development of high-performance catalysts for the oxygen evolution reaction (OER) is critical for cost-effective conversion of renewable electricity to fuels and chemicals. Here we report the significant improvement in the OER activity of electrodeposited NiOx films resulting from the combined effects of using gold as a metal support and cerium as a dopant. This NiCeOx-Au catalyst delivers high OER activity in alkaline media, and is among the most active OER electrocatalysts reported to date. Based on theoretical modelling coupled with experimental observations, we ascribe the activity to a combination of electronic, geometric and support effects, where highly active under-coordinated sites at the oxide support interface are modified by the local chemical binding environment and by doping the host Ni oxide with Ce. The NiCeOx-Au catalyst is further demonstrated in a device environment by pairing it with a nickel-molybdenum hydrogen evolution catalyst in a water electrolyser, which delivers 50 mA consistently at 1.5 V over 24 hours of continuous operation. This work deepens the understanding of complex multi-component catalytic systems by showing how electronic, geometric and support effects can be combined to enhance catalytic activity.