bluuebunny/crossref_metadata_2025 huggingface dataset:
(Invited) Engineering of Optical Activity of Plasmonic Nanospirals
Authors: Huang Zhifeng
Published: February 2020

Chirality, describing an asymmetric property that an object cannot be superimposed onto its mirror image, naturally exists from the macro to atomic scale. Recently, optical activity has been imposed onto plasmon, paving the way for utilizing surface plasmon resonance to develop chirality-related applications in pharmaceutical and agriculture production, food quality control, disease diagnosis and medical treatment, as well as environment protection and sustainable development. It is a fundamental prerequisite to understand optical principle and engineer optical activity, which is still in its infancy. In this talk, we will report on a flexible tailoring of optical activity in the UV and visible regime. Optical activity stems from the helicity of plasmonic nanospirals that are generated by glancing angle deposition (GLAD), and the flexible optical engineering is ascribed to the facile control over helical structures using GLAD.
(Invited) Flexible Transparent Conductors Based on Metallic Nanomesh
Authors: Ren Zhifeng
Published: February 2020

Materials are often optically transparent but not electrically conducting like glass, or conducting but not transparent like metals, or transparent and conducting like indium-doped tin oxide (ITO) but not flexible. Flexible transparent conductors are required for many potential applications like foldable displays, impact resistant screens, etc., which demands transparent conductors very flexible. Nanowires such as silver, carbon nanotubes, etc., has been laid on flexible transparent polymers to achieve high electrical conductivity, but the contact resistance between the wires prevent the conductivity from being high enough. At the same time, graphene has also been studied as flexible transparent conductors without great success due to the large decrease of transparency when high enough conductivity is needed.

We discovered metallic nanomesh network exhibiting super flexibility, transparency, and conductivity by grain boundary masking and depsoition. These conductors exhibit sheet resistance lower than 10 ohms per square, transparency above 85%, stretchability more than 300%, bendability to a radius less than 1 mm, and can be repeatedly stretched for more than 100,000 times without fatigue. Further more, these materials are also very resistant to scratches.
Amorphous Cobalt-Iron Hydroxide Nanosheet Array Electrocatalyst for Efficient Electrochemical and Photoelectrochemical Oxygen Evolution
Authors: Zhang Xingwang , Liu Wei , Liu Hu
Published: February 2020

Water splitting by electrolysis or solar-driven photoelectrochemical (PEC) reaction has been broadly recognized as one of the most promising chemical method for sustainable energy conversion and storage systems. However, the overall efficiency of water splitting is largely limited by the kinetically sluggish oxygen evolution reaction (OER). Efficient electrocatalysts are required to enhance OER by lowering the overpotential and accelerating the kinetics. Herein, we report amorphous cobalt-iron hydroxide (CoFe-H) nanosheet array synthesized by a facile electrodeposition method as a highly efficient OER catalyst for both electrochemical and photoelectrochemical water splitting. The nanosheet array maximized the electrochemically active surface area, meanwhile the amorphous features with abundant defects offered rich active sites for OER. Therefore, the amorphous CoFe-H nanosheet array exhibited superior electrochemical activity toward OER. More importantly, we designed CoFe-H/BiVO4 core-shell architecture for PEC-OER system. Due to the high efficient OER activity of amorphous CoFe-H nanosheets, CoFe-H/BiVO4 photoanodes also exhibited superior PEC-OER performance. Our work provides a promising approach to fabricate highly efficient electrocatalysts for electrochemical and solar-driven photoelectrochemical water splitting.
Advances in AlGaN/GaN HEMT Surface Passivation
Authors: Koehler Andrew D. , Tadjer Marko J. , Anderson Travis J , Chojecki P. , Hobart Karl D , Kub Francis J
Published: February 2020

The high efficiency and power density of AlGaN/GaN high electron mobility transistors (HEMTs) can be limited by parasitic charge trapping at sites on the semiconductor surface. Trapped electrons at the surface deplete the two-dimensional electron gas (2DEG), resulting in effects known as current collapse, increased dynamic on-resistance, DC-RF dispersion, among others. A SiN layer is conventionally implemented as surface passivation for GaN HEMTs, however the properties of this SiN layer and the impact of this layer on current collapse depend drastically on the deposition conditions of this film. Although passivation of GaN HEMTs has been studied extensively, the specific details behind the SiN passivation deposition are not frequently provided in literature. In this work, we investigate several passivation techniques and study their performance under dynamic switching conditions, under off-state stress, and implement hyperspectral electroluminescence imaging to identify electric field nonunifornities. We investigate plasma enhanced chemical vapor deposition (PECVD) SiN, a common GaN HEMT passivation method, and demonstrate how to optimize the deposition conditions to effectively passivate the HEMT. Generally, a mixed frequency PECVD SiN process is used, where a combination of low frequency (LF) e.g. 100-360 kHz and high frequency (HF) 13.56 MHz plasmas are used to create the SiN film. However, the LF step could create undesirable damage to the surface of the AlGaN barrier from N+ ion bombardment, degrading device performance. We developed a bilayer PECVD process, where a 10 nm thick layer is deposited using HF plasma only in order to protect the AlGaN surface, followed by a HF/LF mixed frequency SiN deposition. Annealing the 10 nm thick HF layer densified the film and resulted in a HEMT with significantly improved current collapse. On the other hand, SiN layers grown in situ during MOCVD growth of the GaN/AlGaN layers have also been demonstrated to serve as an effective protective layer for subsequent HF/LF PECVD SiN deposition, yielding HEMTs with low current collapse. Atomic layer epitaxy (ALE) deposition of a thin (10 nm) AlN layer is evaluated, and is shown to serve well as a passivation layer. This is particularly advantageous, since the thin layer is beneficial for reducing gate capacitance for high frequency operation. Finally, a nanocrystalline diamond (NCD) layer is incorporated on the surface of the GaN HEMT, primarily to improve thermal management, but this also is effective as a surface passivation layer, mitigating surface charge trapping. Effective passivation of the AlGaN/GaN HEMT surface is essential for mitigating current collapse, and we demonstrate recent advancements in passivation technology.
Direct Numerical Simulation of Particle Segregation and Dynamics in Inertial Microfluidics By Discrete External Boundary Force-Lattice Boltzmann Method
Authors: Huang Yi , Liu Zhaohui , Zheng Chuguang , Qian Shizhi
Published: February 2020

In the last decade, inertial microfluidics has attracted considerable attention for particle manipulation including particle focus, concentration and separation. To illustrate the detailed underlying mechanism of the particle segregation occurring in inertial microfluidics, dynamics of a single and multiple neutrally buoyant spherical particles in a straight microchannel with a rectangular cross-section under Reynolds numbers ranging from 50 to 200 have been simulated using the Lattice-Boltzmann method with the discrete external boundary force. In a square microchannel, particles are focused to the equilibrium positions, each corner and center of each edge, which is in agreement with the theoretical analysis as well as the experimental observations from the literature. Inertial focusing of multiple spherical particles in rectangular microchannels with different channel aspect ratio over a wide range of Reynolds numbers have also been explored. Trains of particles are formed along the flow direction near the planar equilibrium positions of a single particle. The well-known focusing of the particles at the two centers of the long channel walls occurs at a relatively low Reynolds number, whereas additional stable equilibrium positions emerge close to the short walls with increasing Reynolds number. The equilibrium positions of particles with different sizes have also been studied, and larger particles focus more close to the centerline than the smaller ones. The verified numerical tool provides a new method to simulate the particle segregation and dynamics in inertial microfluidics, uncovers the underyling physics, and provides a design tool for inertial microfluidic devices.
High Photoresponsivity Multilayer MoS2 Thin-Film Transistors with Local Bottom Gate Structure
Authors: Hong Seongin , Kim Ok Jin , Han Gyuchull , Kwon Junyeon , Liu Na , Hong Young Ki , Kim Dong Hak , Omkaram Inturu , Yoon Youngki , Kim Sunkook
Published: February 2020

2D layered transition metal dichalcogenides (TMDs), especially MoS2, has received great attention for next-generation semiconductor devices because their thin film transistors (TFTs) show a nearly ideal subthreshold swing (SS ≈ 70 mV decade-1), high on/off current ratio (I
on/I
off ≈ 108), and high field-effect mobility (µ
eff > 100 cm2 V-1 s-1). In addition, TMDs have the optical characteristic of tuning bandgap from direct to indirect with increasing their layers. Even though photoresponsivity of multilayer MoS2 phototransistor due to its nature of indirect bandgap is lower than that of single layer MoS2, multilayer MoS2 has higher density-of-state and wider spectral response than single layer MoS2.

In order to enhance photoresponsivity of multilayer MoS2 phototransistor, we suggest the phototransistor with local bottom gate structure (LBGS), where the bottom gate length of the device is shorter than the channel length. The underlap regions of between gate and channel work like the series resistance in dark state due to ineffective gate modulation at the region. To demonstrate the enhancement of phototransistor performance using LBGS, we previously studied amorphous silicon TFT with LBGS. The photosensitivity enhancement up to 64 times as compared to the conventional a-Si TFT. And then, we used multilayer MoS2 as a channel material to constitute phototransistor with LBGS. The giant photoreponsivity of 342.6 AW-1 at 2 mW cm-1 shows three orders of magnitude larger than that of global gate multilayer MoS2 TFTs. This result of our experiment and simulation reveals that multilayer MoS2 phototransistor with local bottom gate can be used as various photosensing devices.
(Invited) Electrical and Structural Origin of Self-Healing Phenomena in Pentacene Thin Film Transistors
Authors: Kang Evan S. H. , Zhang Hongbin , Donner Wolfgang , von Seggern Heinz
Published: February 2020

Despite the great potential for flexible large-area applications in organic electronics, electrical and structural instabilities have been widely regarded as an unavoidable weakness of organic semiconductors. Electronic components made of organic molecules are considered more fragile and sensitive to external conditions than their inorganic counterparts and the lifetime of them is shorter than that of conventional devices. Despite these shortcuts it is even more surprising that organic field-effect transistors (FETs) can undergo a self-healing process, i.e. their electrical characteristics can counter-intuitively improve over time, due to the unstable nature of weakly bound molecules. This remarkable phenomenon has never been understood completely.

In this work, we demonstrate that the self-healing in pentacene FETs is induced by a structural phase transformation and can be triggered by various extrinsic or intrinsic influences. We evaluate in detail the necessary conditions for electrical self-healing in pentacene FETs and reveal the structure-property relation at the heart of this phenomenon: a structural phase transformation from the metastable monolayer phase to the so called “thin film phase”. The transformation basically involves a tilt of the molecules against the surface normal near both interfaces of pentacene thin films (pentacene-dielectric and metal-pentacene) and has a profound influence on the electronic properties of pentacene FETs, which we demonstrate by way of electronic structure calculations using DFT and semi-classical transport theory. Finally, possible external triggers able to initiate the self-healing process are discussed. A case study adopting various fabrication procedures reveals that external triggers such as nitrogen gas or other organic compounds seem to be necessary to initiate the self-healing process.
One Step Synthesis of Au Nanoparticle-Cyclized Polyacrylonitrile Composite Films and Their Use in Organic Nano-Floating Gate Memory Applications
Authors: Cho Se-Phin , Jang Sukjae , Jo Hae-Na , Lee Sang-A , Bae Sukang , Lee Sang Hyun , Hwang Junyeon , Joh Han-Ik , Wang Gunuk , Kim Tae-Wook
Published: February 2020

In this study, we synthesized Au nanoparticles (AuNPs) in polyacrylonitrile (PAN) thin films using a simple annealing process in the solid phase. The synthetic conditions were systematically controlled and optimized by varying the concentration of the Au salt solution and the annealing temperature. X-ray photoelectron spectroscopy (XPS) confirmed their chemical state, and transmission electron microscopy (TEM) verified the successful synthesis, size, and density of AuNPs. Au nanoparticles were generated from the thermal decomposition of the Au salt and stabilized during the cyclization of the PAN matrix. For actual device applications, previous synthetic techniques have required the synthesis of AuNPs in a liquid phase and an additional process to form the thin film layer, such as spin-coating, dip-coating, Langmuir-Blodgett, or high vacuum deposition. In contrast, our one-step synthesis could produce gold nanoparticles from the Au salt contained in a solid matrix with an easy heat treatment. The PAN:AuNPs composite was used as the charge trap layer of an organic nano-floating gate memory (ONFGM). The memory devices exhibited a high on/off ratio (over 106), large hysteresis windows (76.7 V), and a stable endurance performance (>3000 cycles), indicating that our stabilized PAN:AuNPs composite film is a potential charge trap medium for next generation organic nano-floating gate memory transistors.
Multiphysics Simulations of Photoelectron Generation and Transport in Dye Sensitized TiO2 Nanorod-Based Solar Cells
Authors: Li Dailin , Zhang Chuanfa , Peterson Samuel J. , Dong Lifeng
Published: February 2020

Dye-sensitized solar cells (DSSCs) have attracted a lot of attention in scientific research for practical applications during the past decade. Due to unique photoelectrical properties, TiO2 nanorods can enhance the performance of DSSCs. In order to further improve efficiency, a model using COMSOL Multiphysics is developed to reveal carrier transport processes inside TiO2 nanorod-based DSSCs. Modeling electron diffusion through different materials and various parameters provides information on how to improve overall efficiency of solar cells. In this study, an optical and electrical multiphysics simulation of DSSC is presented to explore photoelectron generation and transport using TiO2 NRs formed on a conductive glass substrate. In addition to investigating the effects of length, spacing and diameter of the TiO2 nanorods, ZnO nanorod materials will be investigated for their transport processes. Various dye molecules are also studied in order to enhance photovoltaic properties of TiO2 nanorod-based DSSCs.
Direct Determination of the Barrier Height of Au Contact on p-Type Ultrananocrystalline Diamond/Hydrogenated Amorphous Carbon Composite Films
Authors: Hanada Takanori , Ohmagari Shinya , Zkria Abdelrahman , Yoshitake Tsuyoshi
Published: February 2020

Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films have a specific film structure, wherein a large number of diamond nanograins are embedded in an a-C:H matrix. The UNCD/a-C:H films possess large optical absorption coefficients of more than 106 cm–1 in the photon energy range between 3 and 6 eV. These optical features are profitable for the application of thin-film based solar cell and UV photodetectors. Although the Metal-Semiconductor-Metal (MSM) geometry is suitable for photodetection by using single-layered UNCD/a-C:H film, the property of film/metal interface hasn’t been investigated in detail. In this study, a direct determination of barrier height of metal contact on p-type UNCD/a-C:H film was performed using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). As a result, it was revealed that the down bending occurs on the film/metal interface, which indicates that depletion layer might appear on the surface of p-type UNCD/a-C:H by this structure. Whereas the surface of UNCD/a-C:H films is oxidized, that band-alignment of that does not change. It is possible that that different physical phenomena from that of single crystalline diamond occurs. There are numerous dangling bonds on the surface of UNCD/a-C:H film, therefore it is supposed that the dangling bonds have greatly affected the surface state.
Photoconduction of p-type Ultrananocrystalline Diamond/Hydrogenated Amorphous Carbon Composite Films in Metal-Semiconductor-Metal Geometry
Authors: Hanada Takanori , Ohmagari Shinya , Zkria Abdelrahman , Yoshitake Tsuyoshi
Published: February 2020

Photovoltaics are representative renewable energy devices and the development of the next-generation photovoltaics have attracted considerable attentions. Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films, which possess large absorption coefficients of more than 106 cm−1 at photon energy range between 3 and 6 eV is one of the candidate materials to be employed as photovoltaic devices.

It has been theoretically predicted that additional energy states are introduced between the bandgap of diamond owing to a large amount of GBs. Here, GBs denote interfaces between UNCD grains and those between UNCD grains and an a-C:H matrix. Doped impurity atoms are preferentially incorporated into GBs. As a result, the production of p-type and n-type conduction with enhanced electrical conductivities is realized by boron (B) and nitrogen (N) doping, respectively.

Thus far, p-type UNCD/a-C:H films were deposited on n-type Si substrates, and it was confirmed that they exhibit typical rectifying behaviors. The UNCD/a-C:H films certainly act as drift layers in heterojunction diodes. Moreover, our recent studies have experimentally proved that UNCD/a-C:H can detect deep-ultraviolet (DUV) light with high external quantum efficiencies and the detectivity for 254 nm monochromatic light was enhanced at low temperatures. As for the photodetection of UNCD/a-C:H, spectral photoresponse measurements suggests that localized mid-gap states act as the generation centers of photocarriers. Although the peculiar photodetection properties might be attributable to GBs, the details were unknown.

In this study, the photovoltaic characteristics of UNCD/a-C:H films are experimentally demonstrated. The photovoltaic characteristics of UNCD/a-C:H films are discussed on the basis of photoconduction spectroscopic analysis data.
An Organic Metal Halide Tandem Solar Cell Embracing Non-Toxic Tin Perovskite
Authors: Minh Tam Hoang , Ilhwan Oh
Published: February 2020

Providing high solar power conversion efficiency organic metal halide perovskite emerged as a promising photovoltaic material for electricity generation in the future. But to date, one of major problems is inevitable toxicity of lead as a component. The non-toxic tin perovskite is a favourable alternative. Furthermore, it provides a low band gap absorber material (1.2eV) to construct tandem cell which absorbs the wide solar spectrum. In this report, by laminating the tin perovskite solar cell with a high band gap perovskite solar cell we successfully fabricate perovskite-perovskite tandem cell adopting non-toxic tin perovskite material. This lamination is not only provided a high efficiency tandem cell but also introduce a mean of protection for tin perovskite from oxidation.
Roll-to-Roll Sputtered ITO/Ag/ITO Multilayers for Highly Transparent and Flexible Electrochromic Applications
Authors: Kim Tae-Ho , Park Sung-Hyun , Kim Doo-Hee , Nah Yoon-Chae , Kim Han-Ki
Published: February 2020

Electrochromism refers to the phenomenon where optical changes in a material occur during electrochemical reactions under an applied voltage. Because of advantages such as high contrast and low operating voltage, electrochromic (EC) devices have been widely used in smart windows, switchable mirrors, optical shutters, and information displays.Such devices are usually fabricated on a rigid glass substrate to permit their use in automobile and building applications, which require long-term stability under climatic conditions. However, the use of soft polymer or textile substrates has increased the versatility of such technology, allowing the fabrication of novel flexible, stretchable, and wearable EC devices. Since the first flexible EC devices using WO3layers were developed in 1995 a wide variety of inorganic and organic materials has been investigated with respect to their compatibility with flexible substrates. However, most EC devices fabricated using these materials showed poor performances, especially in terms of coloration speed and long-term stability, which was mainly due to the electrical and mechanical properties of the flexible substrate. In addition, the large area and cost-effective roll-to-roll (RTR) coating process for transparent electrodes has been demonstrated for the fabrication of low cost flexible EC devices. To date, the most common transparent electrode for EC devices is indium-tin-oxide (ITO) due to its low resistivity, high transparency, and mature mass production via processes such as sputtering. Amorphous ITO(a -ITO) films can easily be coated on a flexible substrate using conventional DC sputtering at ambient temperatures, but the high sheet resistance and brittleness of thick a-ITO films prevents their use for flexible and transparent EC devices. Oxide-metal-oxide(OMO) multilayer films have been extensively reported in flexible optoelectronic devices as an alternative to a-ITOfilms. However, there are few reports regarding flexible and transparent EC devices fabricated using OMO multilayer electrodes. The development of a conductive layer with high electrical conductivity and stable mechanical flexibility is greatly needed for flexible EC device applications. In this paper, we report the electrochromic properties of poly(3-hexylthiophene) (P3HT) films coated on ITO/Ag/ITO (IAI) multilayers deposited on flexible PET substrates using an ambient temperature RTR sputtering system. By continuous sputtering of ITO/Ag/ITO layers on the flexible PET substrate, we realized highly transparent, flexible, and conductive electrodes for flexible and transparent EC devices. We characterized the coloration efficiency, response time, and long-term stability of the P3HT on the flexible substrate compared to the material prepared on glass substrates.
(Invited) Alkaline Water Electrolysis: Achieving High Current Densities
Authors: Carmo Marcelo , Tigges Fabian , Paciok Paul , Lüke Wiebke , Stolten Detlef
Published: February 2020

The production of hydrogen via water electrolysis is currently recognized as the only option to store gigawatt electrical energy coming from renewable energy sources such as wind and sunlight. However, commercially available alkaline electrolyzers are still limited to low current densities (around 0.5 Acm-2), primarily due to its internal resistance losses. In order to reach the excellent current densities of acidic polymer electrolyte membrane (PEM) water electrolysis (ranging between 2 and 4 Acm-2), it is crucial to tailor electrolyte construction and cell design in order to overcome the high ohmic losses of conventional diaphragms and archaic cell designs used in alkaline electrolysis. Here, we demonstrate for the first time alkaline electrolysis achieving current densities as high as 2 A.cm-2 at 2 volts, not only well overcoming the performances of lab-scale and commercial alkaline electrolyzers, but also closely matching the performance of state-of-the-art PEM water electrolysis.
Investigation of a Correlation Between PEFC Operation Conditions and Electrocatalysts Degradation
Authors: Hayashi Akari , Kitamura Masahiko , Noda Zhiyun , Sasaki Kazunari
Published: February 2020

One of the strategies to further increase PEFC performance is elevation of the operation temperature, reducing the reaction overvoltage of oxygen reduction reactions. On the other hand, durability of materials used in PEFC is assumed to be lowered at the higher temperature condition. In this study, understanding degradation phenomena of electrocatalysts at the higher temperature is particularly focused on in order to develop durable electrocatalysts at the elevated temperature.

MEAs were prepared using the standard catalyst and electrolyte, Pt/Ketjen black catalyst (TEC10E50E, TKK Corp.) and Nafion, respectively. Three different conditions of operation temperature and humidity (80 °C/RH100%, 105 °C/RH57%, and 80 °C/RH57%) were applied. Durability of MEAs in three conditions was studied through the potential cycles between 0.6 and 1.0 V assuming “load cycles” of FCVs.[1], [2] 

Since Pt dissolution/re-precipitation is assumed to occur during the cycles, the changes in activation overvoltage and ECSA were carefully examined under three conditions. After 20000 potential cycles, activation overvoltage and ECSA increased and decreased, respectively, the most at 105 °C/RH57%. On the other hand, such degradation was much suppressed under the operation at 80 °C/RH57%. In order to understand the difference in degradation phenomena, the amount of water molecules was considered. With the presence of a large amount of water molecules, the rate of reducing ionized Pt (Pt2+) into Pt metal is assumed to be smaller. Then, in the case of 80 °C/RH57%, the amount of water molecules was smallest among three conditions, leading to fast and reversible reduction of Pt2+to Pt metal. Therefore, Pt particle growth was believed to be suppressed. However, if two factors, the amount of water and temperature, are considered, the temperature factor most likely influences more on Pt particle growth. 

Reference

[1] A. Ohma et al., ECS Trans, 41 (2011) 755.

[2] M. Kitamura et al., ECS Trans, 69 (2015) 701.
Effect of Catalyst Distribution on Macroscopic MASS Transport Properties of a FUEL CELL Catalyst Electrode: Pore-Scale Modeling Approach
Authors: Salomov Uktam , Asinari Pietro
Published: February 2020

The work elucidates the effect of micro-morphology parameter. i.e. catalyst distribution, on macroscopic mass transport properties of a catalyst electrode of a PEM fuel cell. Indeed, the micro-morphology parameters have significant effect on transport properties, such as permeability, tortuosity. Pore-scale analysis of fluid flow has been done using Lattice Boltzmann method. Besides other morphological parameters, like porosity, clusterization, geometry those are common for a porous medium, the mass transport properties of a porous electrode of a fuel cell is governed by catalyst particle deposition distribution. This fact enables to control flow field inside of the most vulnerable membrane electrode assembly compartment. Redistribution of catalyst might be required to design novel MEA more robust prone to degradation processes, especially for transport application. In particular, the influence of a catalyst loading on permeability has been studied. It is explored that increasing the catalyst loading definitely improves transport parameter, but this improvement has maximum effect up to certain (ωPt≈0.2) value of catalyst loading. Moreover, how permeability changes depending on a distribution of the same amount catalyst towards membrane has been considered. Pore-scale simulation of different catalyst layer configuration with regards to catalyst deposition shows that redistribution of catalyst in exponential decaying towards membrane allows three times improve permeability of catalyst electrode compared to conventional/homogeneous distribution.
Ionomer Layer Design of the Electrode Using PGM-Free Electrocatalysts for Anion Exchange Membrane Fuel Cells
Authors: Asazawa Koichiro , Takano Aoi , Nishino Eriko , Yamaguchi Susumu , Miyake Junpei , Uchida Makoto , Miyatake Kenji
Published: February 2020

An anion exchange membrane fuel cell (AEMFC) using hydrazine hydrate liquid fuel has been investigated1 and the concept vehicle was exhibited at the 42nd and 43rdTokyo Motor Show.

For the improvement of the AEMFCs, technological breakthrough is necessary for anion exchange membranes. Especially, it is important to overcome the counteracting relationship of high ion conductance and low fuel permeability.

In addition, technology to optimize triple-phase interface formed by catalyst and ionomer is important to improve the performance.

Through many efforts to control the triple-phase interface, state-of-the-art proton exchange membrane fuel cell electrode that consists of platinum group electrocatalysts and perfluorinated ionomer has been investigated. However, the catalyst utilization is still insufficient. In addition, it is more challenging to control the triple phase interface for PGM-free electrocatalysts and anion exchange ionomers.

Our approach to develop the advanced AEMFC is using anion conductive quaternized aromatic multiblock copolymers, poly(arylene ether)s (QPEs)2as shown in Fig.1 and controling the interface and PGM-free electrocatalysts both for anode and cathode.

In this preliminary study, we focus on ionomer layer design for anode and cathode evaluated by ionomer thickness covered on the electrocatalysts. (TEM image shown in Fig.2)

MEA consisting of the QPE membranes and ionomers was fabricated with PGM-free electrocatalysts and subjected to performance evaluation. The MEA performance of direct hydrazine fuel cells will be shown and discussed in the meeting.

Figure captions

Fig. 1. Representative structure of QPEs.3

Fig. 2. TEM Image of ionomer layer (red line) on the cathode catalysts.

References

[1] K. Asazawa, K. Yamada, H. Tanaka, A. Oka, M. Taniguchi, and T. Kobayashi, Angew. Chem. Int. Ed., 46, 8024, 2007.

[2] M. Tanaka, K. Fukasawa, E. Nishino, S. Yamaguchi, K. Yamada, H. Tanaka, B. Bae, K. Miyatake, and M. Watanabe, J. Am. Chem. Soc., 133, 10646–10654, 2011

[3] R. Akiyama, N. Yokota, E. Nishino, K. Asazawa, and K. Miyatake, submitted.

Acknowledgements

This work was supported by CREST, JST.

Figure 1
(Invited) Genetic Algorithms and DFT for Accelerated Design of Nanoalloys
Authors: Jennings Paul C. , Lysgaard Steen , Tripkovic Vladimir , Hansen Heine A. , Vegge Tejs
Published: February 2020

Binary and ternary alloys often show improved activity, selectivity or durability as electrocatalysts compared to elemental catalysts. For practical applications, nanoparticle catalysts are required to maximize utilization of precious metals such as platinum. Theoretical simulations based on density functional theory (DFT) have helped rationalize experimental observations and suggested new design directions for experimentalists.1 Commonly, theoretical studies are, however, limited to small clusters or extended surfaces under periodic boundary conditions. Although DFT simulations of nanoparticles with hundreds of atoms have become possible in recent years,2 prediction of properties such as morphology, atomic distribution and catalytic activity of nanoalloys remains a challenge.

Here, we report recent theoretical work on the global optimization of atomic distribution within nanoalloy catalysts using genetic algorithms and we predict their activity using density functional theory. We investigate the activity of Pt-Pd-Au binary and ternary alloys for the oxygen reduction reaction (ORR) and the stability of these ternary alloys under ORR conditions. Our findings suggest subsurface Pd, in binary Pt-Pd films, results in improved activity over pure Pt, while addition of small amounts of Au will hinder Pt dissolution and might also enhance activity. We study the ORR activity of 2 nm ternary Pt-Au-M Mackay icosahedral core-shell nano-particles in order to identify core elements, which increase activity and particle stability through strain and ligand effects.

Acknowledgements

Financial support from the European Commission under the FP7 Fuel Cells and Hydrogen Joint Technology Initiative grant agreement FP7-2012-JTI-FCH-325327 for the SMARTcat project is gratefully acknowledged.

References

1. I. E. L. Stephens, A. S. Bondarenko, U. Grønbjerg, J. Rossmeisl and I. Chorkendorff, Energy Environ. Sci., 2012, 5, 6744.

2. L. Li, A. H. Larsen, N. A. Romero, V. A. Morozov, C. Glinsvad, F. Abild-Pedersen, J. Greeley, K. W. Jacobsen and J. K. Nørskov, J. Phys. Chem. Lett., 2013, 4, 222–226.
(Invited) Engineering Modeling of PEM Water Electrolysis: A Survey
Authors: Bensmann Boris , Hanke-Rauschenbach Richard
Published: February 2020

The conversion of electric energy into pressurized hydrogen by means of water electrolysis is currently discussed as promising option to handle intermittent electricity supply that occurs with the increasing use of renewable energy sources. Subsequently, this hydrogen could be (1) either reconverted into electricity, or (2) converted into other energetic molecules, such as methane (power to gas) or liquid fuels, or (3) utilized as a feed stock for the chemical industry.

Besides conventional alkaline water electrolysis, a water electrolysis technology based on a Polymer Electrolyte Membrane (PEM) is discussed since several years, which is the focus of the present contribution. The particular advantages of PEM water electrolysis are higher attainable current densities, higher voltage efficiency, and higher load flexibility. Current disadvantages include far lower technical maturity, shorter life time, and higher specific costs compared to alkaline water electrolysis.

The formulation of appropriate physico-chemical models for solid polymer electrolysis can help to contribute to the solution of the above mentioned problems. In the present contribution state of the art of modelling of solid polymer electrolyzers is summarized and discussed.
(Plenary) Electrochemical Diagnostics and Modeling in Developing the PEMFC Cathode
Authors: Kongkanand Anusorn , Yarlagadda Venkata , Garrick Taylor Reed , Moylan Thomas E , Gu Wenbin
Published: February 2020

One key challenge to enable a wide spread adoption of proton-exchange membrane fuel cell vehicles is the reduction of Pt use to a level comparable to the incumbent internal combustion engine vehicles (<5 gPM/vehicle). Great progress in recent years in improving the activity and durability of Pt-based catalysts, particularly Pt alloy catalysts, has enabled a reduction of Pt loading in the cathode approaching 0.15 mgPt/cm2 or about 15 gPt/vehicle. However, as we attempt to go below 10-15 g/vehicle, a large performance loss is often observed at high-current density (high power).

In the cathode, where oxygen molecules meet protons and electrons to produce water, one must create as much interface as possible in order to minimize the transport resistances. As Pt loadings are reduced, performance losses due to these transport phenomena become more localized to the Pt and ionomer interface, posing new challenges to diagnose and understand these electrodes.

In this talk, we will discuss how we use in-situ electrochemical diagnostics and modeling to understand the performance of low-Pt electrodes, identify their key limiting factors, and guide our catalyst layer development. Diagnostics used to quantify the kinetic terms, proton conduction, oxygen transport resistance, and Pt-ionomer interaction will be introduced. Moreover, because non-noble metals leach out from Pt alloy catalysts and can replace protons in the ionomer, we will also consider the resulting impact on low-Pt electrode performance.

This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.
Synthesis and Characterization of High Performance Pd-Metal Oxides (Fe3O4, Fe2O3 and CeO2 nanorods) for the Oxidation of Organic Molecules in Alkaline Media
Authors: Solis-Tobías J.E. , Díaz-Guillén J.a. , Meléndez-González P.C. , Sanchez-Padilla N. M. , Pérez-Hernández R. , Alonso-Lemus I.L. , Rodriguez-Varela F.J.
Published: February 2020

In this work, novel Pd-(metal oxide)/C electrocatalysts were synthesized and evaluated as anodes for the oxidation of ethanol, methanol, ethylene glycol, glycerol and isopropanol in alkaline electrolyte. The metal oxides proposed to enhance the catalytic activity of Pd nanoparticles were Fe3O4, Fe2O3 and cerium oxide nanorods (CeO2-NR) pyrolized at 200 and 400 °C. The Pd-(metal oxide)/C anodes, as well as a Pd/C electrocatalyst as comparison, were synthesized using NaBH4 as reducing agent and Vulcan XC-72 as the support. The electrochemical characterization revealed a higher performance of the Pd-(metal oxide)/C electrocatalysts compared to Pd/C for all the oxidation reactions, at several concentrations of the fuels. Among the novel electrocatalysts, that containing CeO2-NR thermally treated at 400 °C (Pd-CeO2-NR400/C) showed the highest catalytic activity. Only in the case of the Ethylene Glycol Oxidation Reaction, Pd-Fe2O3/C showed a performance as high as that of Pd-CeO2-NR400/C.
Effect of Properties of Hydrophilic Microporous Layer (MPL) on PEFC Performance
Authors: Tanuma Toshihiro , Kawamoto Masako
Published: February 2020

The effect of the microporous layer (MPL) on membrane electrode assemblies (MEAs) has been investigated. The performance of an MEA employing an MPL with a larger pore volume was better under 100%RH condition, which we reported in 2015 (1). Under 30%RH, no distinct relationship was found between the pore volume of the MPL and the MEA performance. However, the MEA employing an MPL with a larger median pore diameter showed higher cell voltage.

 

We reported that the MEA using a hydrophilic MPL showed much better performance in a wide range of pressure and humidity conditions than that using a conventional hydrophobic MPL. We applied our technique of preparing a carbon fiber dispersion to the fabrication of MPLs using carbon particles and carbon fibers with various fiber diameters. Properties of carbon materials and MPLs are shown in Table 1. The pore size distribution (Fig. 1) was measured using mercury intrusion porosimetry.

Figure 2 shows the surface SEM images of the MPLs: The MPL made of carbon black (Vulcan XC) has the smallest pore diameter (0.068 μm) and that made of carbon fiber (VGCF-H) has the largest pore diameter (0.77 μm). Although carbon fiber CF-X has the largest fiber diameter of 250 nm, the MPL made of it has the smallest pore volume. This is probably because fibers of CF-X are straight and longer than other carbon fibers, tending not to become tangled and form larger pores.

 

MEAs having an MPL made of four different carbon materials (Vulcan, 24PS, CF-X, and VGCF-H) were prepared, and polarization curves were measured under the conditions of 80 ºC, 30%RH. The IR free cell voltage (@0.1, 0.5, 1.0 and 2.0 A/cm2) for the four MEAs are plotted against the median pore diameter of the MPL in Fig. 3. The larger the pore diameter of the MPL, the higher the cell voltage becomes. The MEA employing an MPL made of VGCF-H, which has the largest median pore diameter, shows the best performance under the dry condition of 30%RH. This is probably because there is little liquid water in the MPL under the dry condition, and the cell voltage is largely dependent on the oxygen diffusivity of the MPL: The larger median pore diameter of the MPL, the better oxygen diffusivity and the higher the cell voltage.

 

References

1.　T. Tanuma and M. Kawamoto, ECS Transactions, 69, 1323 (2015).

Figure 1
Air Breathing PEM Fuel Cells in Aviation
Authors: Flade Steffen , Stephan Thomas , Thalau Oliver , Burberg Torben , Schirmer Johannes , Kallo Josef
Published: February 2020

Aviation industry and governing bodies have set out to significantly reduce the environmental footprint of aviation. The visions brought forward request such low emissions that continuous optimization and advancement of current technology alone are not sufficient. This has led research groups to explore disruptive paths of innovation for aircraft and a trend to the electrification of onboard systems in more electric aircraft. Providing the required electric power using fuel cells and batteries not only promises the reduction of carbon emissions and noise but also allows for system level optimizations using synergy effects from a multi-functional system-integration that aircraft design could benefit from. At the same time using fuel cells in aircraft environments provides great challenges towards system design. System weight and size constraints require the use of air breathing fuel cells in commercial aircraft and airliners. This results in operation at low pressure and temperature which in term confronts the system design with two problems. First the low air density makes cooling of power systems difficult especially considering the low operating temperatures of PEM Fuel Cells. Second the low ambient temperature means the ingested air will heat up much more than during ground operation and in term will remove more water from the fuel cells, making water management and fuel cell operation strategies more important and complicated than in other applications. Also ambient conditions very rapidly change in air, adding even more complexity to the application.

Taking into consideration possibilities of advanced aerodynamic concepts like distributed propulsion and propulsion/wing vortex interaction, that electric propulsion could enable, Fuel cells could be of even larger benefit for aviation when considering a larger time-frame. All-electric aircraft could potentially be emissions-free and very low noise, thus enabling sustainable, environmentally friendly aviation. While not yet feasible for large passenger aircraft, application of such systems is currently being researched in aircraft like the HY4 enroute to this goal. This presentation will discuss the challenges of fuel cell operation in-flight, as well as introduce the concepts for multifunctional fuel cell APU replacements for A320 type aircraft and drive-trains for all-electric aircraft. Finally results from ground- and flight-testing of the all-electric fuel cell powered HY4 aircraft will be discussed.
Effect of Sulfate Contaminant on ORR Activity and PEMFC Performanc
Authors: Dinh Huyen N , Bender Guido , Christ Jason M , Zack Jason W. , McGovern Leah , Przywara Jacob , Carey Christian
Published: February 2020

Lower cost materials for stack hardware and system components help reduce the overall cost of the automotive and stationary fuel cell systems and make these competitive in the market. However, low-cost system component materials need to provide similar function, performance and durability. Intelligently selecting low cost materials for application in polymer electrolyte membrane fuel cell (PEMFC) systems requires understanding the potential adverse effects that system contaminants may have on the fuel cell performance and durability.

There are many prospective balance of plant (BOP) materials that can be used in fuel cell systems. Families of material, based on input from OEMs, fuel cell system manufacturers and other attributes like cost, physical properties, have been studied. These include structural materials, elastomers for seals and (sub)gaskets, and assembly aids (adhesives, lubricants).

The contaminants from the BOP materials were leached out via an accelerated aging procedure. The leachates obtained from these plastics were a mixture of organics, inorganics, and ions. The sulfate anion was one of the anions identified in the leachates and was chosen as the model compound for further studies. Sulfate anions can also come from membrane degradation. Ex-situ electrochemical measurements were carried out to understand the impact of sulfate anion concentration on oxygen reduction reaction (ORR). The ORR scan rate and direction (low to high potential and vice versa) were studied to determine the effect of the initial state of the Pt surface on sulfate contamination. In-situ fuel cell experiments were also carried out. Sulfate anion was introduced to a working fuel cell to determine their effect on fuel cells performance. Our previous work showed that sulfate anion did not result in performance loss when it was infused into the cathode at 0.2 A/cm2. The cathode potential was greater than 0.8 V at this current density. It is postulated that sulfate contamination is potential dependent and that sulfate adsorption only occurs when Pt is in its metallic state. In the current study, the effect of potential on sulfate contamination will be studied by infusing sulfate into the cathode at several constant current densities. Several in-situ diagnostics such as infusion, cyclic voltammetry, electrochemical impedance spectroscopy, and I-V curves were carried out to better characterize the contaminant effects of sulfate anion. The goal is to better understand the contamination mechanisms of specific species so that mitigation strategies can be determined.

The authors would like to acknowledge funding from the U.S. Department of Energy EERE Fuel Cell Technologies Office, under Contract No. AC36-08GO28308 with the National Renewable Energy Laboratory and collaborations with colleagues at GM. Structural plastic materials and leachates were provided by GM for this study.