bluuebunny/crossref_metadata_2025 huggingface dataset:
Vertical Structure TFT Using Thin Gate Dielectric
Authors: Kang Seok Jun , Kang In Hye , Kim Seo Gwon , Woo Jae Geun , Yu Eun Seong , Bae Byung Seong
Published: December 2020

Oxide thin film transistor (TFT) is widely used for a display backplane. It can also be used in many applications, such as sensors and wearable devices that require low power consumption. The channel of the existing TFT is formed horizontally, but the structure in this study is formed vertically, so it is more resistant to stretching. For low power consumption, the driving voltage must be lowered. To achieve the goals above, vertical structure TFT is being studied by several groups. In this study, a vertical structure TFT using a thin gate dielectric film was developed. Figure 1 shows the structure of the vertical TFT. As shown in Fig. 1, IGZO was used as an active layer and deposited by RF magnetron sputter. The dielectric between the source/drain and gate was deposited by ALD (Atomic Layer Deposition). Since the channel length is determined by the thickness of the gate electrode, short channel length TFT is formed easily for low driving voltages. In addition, a thin gate dielectric was used to decrease the threshold voltages of the TFT. Since a very thin gate dielectric was used, a low threshold voltage was obtained, which enable low power consumption TFT circuits.

Figure 1
(Invited) Non-Equilibrium Treatment of Molecular Dopants for 2D Materials
Authors: Kiriya Daisuke
Published: December 2020

2D materials gather attention due to their unique and quantum-confined electronic structures applicable to optoelectronics and a platform for physics. One representative 2D material is transition metal dichalcogenides (TMDCs), which consist of a three-atomic-layered structure with a thickness of only ~0.7 nm. Representative TMDCs are MoS2 and WSe2, and both materials are semiconducting materials with the direct-band gap at the monolayer limit. To apply these unique materials for atomically thin electronics and quantum devices, controlling the carrier concentration is important. In this talk, I will present our recent results to control the carrier behaviors of TMDCs using molecules, with a particular focus on the molecules in non-equilibrium environments, starting with the preparation process of a dopant molecule in one (Ref.1). We found a method to prepare a strong electron dopant, benzyl viologen (BV), in a non-equilibrium manner. In the previous works, the BV molecule is synthesized using a strong reducing agent, sodium borohydride (NaBH4), in water. However, the method is inefficient because NaBH4 reacts vigorously with water at the same time. We replaced NaBH4 with indium metal and found a synthetic method to prepare BV dopant molecules under a non-equilibrium convection flow environment. The convection flow continuously synthesizes BV dopant molecules to a high concentration; furthermore, the method eliminates the purification process to use the dopant solution. Additionally, I will show a non-equilibrium dopant molecular assembly on TMDCs (Ref. 2 and 3). BV molecules usually interact immediately with the TMDC surface and assemble uniformly. Because of the hydrophobic character of the BV molecules, it is difficult to apply photoresists to make local carrier modulation at submicron scales. We overcome the issue by using the self-pattern formation process of the BV molecular films on the surface of TMDCs. By applying hydrophobic/hydrophilic repulsive interaction, the BV film destabilizes and shows pattern formation at scales of about ~200 nm periodicity. Our analysis found that the patterns induce pn and metal/semiconductor junctions on TMDCs. This non-equilibrium process would be useful to prepare electronic device structures via the spontaneous process, which is uniformly distributed at large scale.

Keywords: Molecular dopant, Transition metal dichalcogenides, TMDCs, convection flow, molecular assembly, spontaneous pattern formation

References:

1K. Matsuyama, A. Fukui, et al., ChemistryOpen, 2019, 8, 908.

2H. Ichimiya, et al., ACS NANO, 2018, 12, 10123.

3H. Ichimiya, et al., ACS Appl. Mat. Int. 2019, 11,15922.
Quartz-Driven Quartz@WO3 Piezocatalyst for Hydrogen Evolution Reaction
Authors: Lin Meng-Chin , Wu Jyh-Ming
Published: December 2020

Piezocatalyst is one of the most innovative catalysts for hydrogen evolution reaction in these years. It has attracted broad interest due to its ultra-high efficiency and renewability. In this study, we initiatively fabricate single-crystal quartz microrods (Quartz MRs) from rice husks, which possess excellent piezoelectricity. Firstly, the raw rice husks are washed and acidized with HCl solution for several hours, following by calcination at 550℃ for 6 hours to produce highly porous SiO2 (RH-SiO2) that serves as the precursor of quartz MRs. After that, some NaCl and 0.6M NaOH solution are mixed with the as-synthesized RH-SiO2. The final mixture is then poured into the Teflon-lined stainless-steel autoclave, undergoing hydrothermal process at 220℃ for 12 hours. To form the Quartz@WO3 heterostructure, Na2WO4·2H2O and citric acid are dissolved in DI water and the pH value is adjusted to 1.0 under stirring. After the solution is formed, the as-prepared quartz MRs are added into it and sealed in an autoclave, placing in the oven at 120℃ for 3 hours. Finally, the product is calcined to remove residual water and we can obtain the Quartz@WO3 heterostructure. The crystal structures of Quartz MRs and WO3 are well identified by XRD analysis, which corresponds to alpha quartz and monoclinic WO3 respectively. Though gas chromatography (GC) analysis, the hydrogen generation rate of Quartz@WO3 effectively enhances around 53% compared to that of pristine WO3 under ultrasonication as an external mechanical force, this may be associated with the piezoelectric effect from the quartz MRs.
Suppression of Anisotropic Heat Conduction in Ion-Substituted Layered Cobalt Oxides
Authors: Ohta Hiromichi , Cho Hai Jun , Takashima Yugo , Onozato Takaki
Published: December 2020

Layered compounds often show the anisotropic transport properties of both electron and phonon; the electrical/thermal conductivity along with the layer is high and the electrical/thermal conductivity perpendicular to the layer is low. For example, layered Na
x
CoO2, which is known as a promising candidate for oxide thermoelectric material [1], exhibits a rather large thermoelectric power factor along with the CoO2 layer, whereas it shows low electrical conductivity in perpendicular to the CoO2 layer. In addition, the thermal conductivity is large along with the CoO2 layer, whereas it is small in perpendicular to the CoO2 layer. Therefore, the anisotropic electron/phonon transport properties need to be suppressed to improve the thermoelectric figure of merit of Na
x
CoO2. Here we demonstrate that the anisotropy in thermal conductivity κ of Ax
CoO2 (A = Li, Na, Ca, and Sr) can be suppressed by increasing the weight of Ax
. We fabricated Ax
CoO2 epitaxial films with two different crystallographic orientations with respect to the surface normal to the substrates by the Reactive Solid-Phase Epitaxy method [2,3]. We measured the electrical conductivity σ and thermopower S along with the substrate surface whereas we measured the κ in normal to the substrate surface. We extracted the κ parallel to the layers (κ
p) from the comparison of two different crystallographic orientations [4]. The κ
p was 3-6 times larger than that perpendicular to the layers (κ
v). The κ
p decreased with increasing the atomic mass of Ax
though κ
v was insensitive to it; anisotropy of κ was controlled by substituting heavier Ax
-ions [5]. The results of this study will be of great value in understanding fundamental heat transport properties of layered structures as well as developing Ax
CoO2 based thermoelectric materials. In addition, the analysis of the anisotropic thermal conductivity used in this study can be applied to other layered materials.

[1] I. Terasaki et al., Phys. Rev. B
56, R12685(R) (1997).

[2] H. Ohta et al., Cryst. Growth Des.
5, 25 (2005).

[3] K. Sugiura, H. Ohta et al., Appl. Phys. Lett.
88, 082109 (2006).

[4] K. Sugiura, H. Ohta et al., Appl. Phys. Lett.
94, 152105 (2009).

[5] H.J. Cho, H. Ohta et al., Adv. Mater. Interfaces
7, 1901816 (2019).

Figure 1
Non-Stoichiometric Ferroelectric Bismuth Ferrites (Bi25FeO40) Enhancing Photocatalyst Activity in the Acidic Environment
Authors: Wu Guan Hua , Wu Jyh-Ming
Published: December 2020

In this work, non-stoichiometric bismuth ferrites (Bi25FeO40) particles that possess ferroelectric and photocatalysis properties were synthesized by the hydrothermal method. According to our previous work, piezoelectric materials can be employed in numerous applications. When applied to organic dye degradation, the photocatalysis effect is largely enhanced with the assistance of ferroelectricity[1, 2]. The Bi25FeO40 belongs to the I23 space group, which is a non-centrosymmetric piezoelectric crystal structure. The piezoelectricity is investigated by utilizing the piezoresponse force microscope (PFM) to measure its bias to phase relation. The lifetime of electron-hole pairs stimulated by laser could be measured by photoluminescence (PL) and time-resolved photoluminescence (TRPL). Due to its ferroelectricity, when the poling electric field raised, the remnant polarization and lifetime were increased; the intensity of the PL spectrum dropped reversely[2, 3]. This represents the recombination possibility of the electron-hole pairs were decreased by the poling process. Moreover, by adjusting the pH value of the degradation environment, the contact ability of the dye molecules and catalysts was improved. Under the condition of pH=3.5, the Rhodamine B (RhB) dye can be fully degraded in one hour while there is only 16% degraded in one hour of the original RhB solution(pH=4.8). This work demonstrated a photocatalyst assisted by ferroelectric property which could be enhanced through the poling process and prolong the lifetime of the electron-hole pairs. Furthermore, it shows greater degradation ability on RhB dye in acidic environment due to the better contact ability with the dye molecules, which makes it a new choice of acidic industrial wastewater treatment.

References:

Wu, M.-H., et al., Ultrahigh efficient degradation activity of single- and few-layered MoSe2 nanoflowers in dark by piezo-catalyst effect. Nano Energy, 2017. 40: p. 369-375.

Wang, Y.-C. and J.M. Wu, Effect of Controlled Oxygen Vacancy on H2-Production through the Piezocatalysis and Piezophototronics of Ferroelectric R3C ZnSnO3 Nanowires. Advanced Functional Materials, 2020. 30(5): p. 1907619.

Liu, Y.L. and J.M. Wu, Synergistically catalytic activities of BiFeO3/TiO2 core-shell nanocomposites for degradation of organic dye molecule through piezophototronic effect. Nano Energy, 2019. 56: p. 74-81.
Multi-Functional Flexible Sensor Sheets for Iot Society
Authors: Takei Kuniharu
Published: December 2020

Multiple sensing functionalities enable to collect a variety of information for the next class of Internet of Things (IoT) concept such as medical/healthcare, robotics, environment, and infrastructures. In addition to the functionalities, mechanical flexibility allows to attach the devices over non-planar surfaces conformally, resulting in that more information can be monitored precisely. In particular, multi-functional electronic skin (e-skin) can contribute to the applications of robotic skin and infrastructure due to low-cost, macro-scale devices that can cover a variety of objects. In this talk, soft robotics with e-skin functions and air flow mapping for the infrastructure applications are mainly discussed.

For the soft robots with e-skin, tactile pressure distribution and temperature of the holding object are simultaneously detected by integrating these sensors on the tip of soft robot finger. Next, for the infrastructure monitoring, flexible air flow sensor array is demonstrated to monitor air flow distribution on the curved surfaces. For the flow sensor, calorimetric methodology is used to monitor the temperature distribution generated by a printed flexible heater. This heat distributions are detected by printed temperature sensors. Integration of these heaters and temperature sensors, as the first proof-of-concept, realizes real-time air flow mapping on the curved surface successfully.
Quasi-High Entropy Oxide Ca(ZrYCeCr)Oxsynthesized By Sol-Gel Process
Authors: Chen Hsuan-Yu , Wu Jyh-Ming
Published: December 2020

Quasi-High Entropy Oxide is composed of Ca, Zr, Y, Ce, and Cr, which are synthesized via sol-gel method at room temperature. These metallic nitrates, including calcium nitrate, zirconium nitrate, yttrium nitrate, cerium nitrate, and chromium nitrate are mixed together to prepare the precursor and stirring for one four. After thoroughly mixing, adding citric acid as a chelating agent to link all kinds of metal ions together as chelates. Then, adjusting the pH value to 7 by adding ammonia solution, and drying at 90oC for 48-hour. After that, the as-synthesized dried powders were annealed at 1100oC for three-hour to obtain the final product. The x-ray diffraction pattern indicates that as-synthesized powders of CaZrYCeCrOxdemonstrated a single-phase fluorite structure with precipitating the second phase of CaZrO3, which exhibited luminescence and ferroelectric properties. Notably, based on the transmission electron microscope images, the as-synthesized CaZrYCeCrO nanoparticles display highly porous morphologies, implying that the CaZrYCeCrO may highly potential for the application of the electrochemical reactions.
WSe2/WOx Heterostructure Growth Via Laser Irradiation with a Metal Absorption Layer
Authors: Hsu Yu-Chieh , Kuo Yao-Zen , Tang Shin-Yi , Chueh Yu-Lun
Published: December 2020

Recently, two-dimensional materials are considered as potential materials for future electronic devices. Moreover, fabrication of two-dimensional heterojunction attract lots of attention. Here, we use a laser with metal-assisted oxidation method to construct WSe2/WO3 heterostructure. In the experiment, a plasma-assisted chemical vapor reduction (PACVR) process was applied to synthesis large area WSe2 layer. A layer of nickel metal was deposited as a first layer which will absorb the heat of laser and oxidize the top WSe2 layer, then a additional oxide layer was deposited between nickel layer and WSe2 layer to prevent the nickel metal layer from getting selenized when we used PACVR process to selenize the amorphous WO3 precursor. In the laser oxidation process, we choose 808 nm wavelength continuous wave laser as the laser source which will not absorb by WSe2 layer cause the laser energy is lower than bandgap of WSe2. Therefore, the nickel metal layer will absorb most of the laser energy then heat the top WSe2 layer under atmosphere. The top area of WSe2 heated by the metal layer will get oxidized into WO3 hence formed a heterojunction with the outer area which was not expose by laser. We also used photolithography to get different pattern of nickel layer which could control and limit the oxidized area by the metal layer but not by the laser exposure area. After the laser oxidation process, this WSe2/WO3 structure was further fabricated into photodetector showed its great possibility in future electronic applications.
Automatic Deposition of Oriented Copper Hydroxide Nanobelt Films for the Heteroepitaxial Growth of Metal-Organic Frameworks
Authors: Linares-Moreau Mercedes , Klokic Sumea , Carraro Francesco , Falcaro Paolo
Published: December 2020

One of the key challenges that the field of metal-organic frameworks (MOFs) faces today is the possibility of processing these materials as thin films to enable their integration into existing technologies [1-4]. This is particularly important for devices that exploit anisotropic properties obtained from highly oriented materials. Based on a previously described procedure to obtain centimetre-scale oriented MOF films [5], we hereby report the optimization and extension of this procedure into an automated, operator-independent method that affords higher yields without compromising the long-range orientation. The procedure is based on the deposition of crystalline copper hydroxide nanobelts that can be aligned to obtain substrates with preferential orientation. The influence of different deposition parameters on the coverage and orientation of the films was studied and tuned in order to obtain reproducible samples with higher yields. With applications in fields such as gas storage, catalysis and light harvesting, among others, these oriented ceramic substrates can be converted on demand to different MOFs by exploiting a rapid heteroepitaxial growth [6]. This automatic method introduces the attractive prospect of the potential implementation of MOFs in large-scale thin-film processing.

Figure 1:

Comparison between manual and automated procedures for the deposition of oriented copper hydroxide films on silicon substrates. Left: Azimuthal angle dependence of intensity profiles of the (020) reflection of Cu(OH)2 at a diffraction angle of 23.8°corresponding to a typical sample obtained by each method, showing the superior alignment of the automatic procedure. Right: Optical images showing the surface coverage and long range (centimetre-scale, top) and micro alignment of the substrate (by polarized-light optical microscopy, bottom).

References:

[1] Bétard, A.;Fischer, R.A. Chemical reviews
2011, 112, 1055

[2] Falcaro, P.; Ricco, R.; Doherty, C.M.; Liang, K.; Hill, A.J.; Styles, M.J. Chemical Society Reviews
2014, 43, 5513

[3] Stassen, I.; Burtch, N.; Talin, A.; Falcaro, P.; Allendorf, M.; Ameloot, R. Chemical Society Reviews
2017, 46, 3185

[4] Liu, J.; Wöll, C. Chemical Society Reviews
2017, 46, 5730

[5] Falcaro, P.; Okada, K.; Hara, T.; Ikigaki, K.; Tokudome, Y.; Thornton, A. W.; Hill, A. J.; Williams, T.; Doonan, C.; Takahashi, M.; Falcaro, P. Nature materials
2017, 16, 342.

[6] Ikigaki, K.; Okada, K.; Tokudome, Y.; Toyao, T.; Falcaro, P.; Doonan, C.J.;Takahashi, M. Angewandte Chemie 2019, 131, 6960.

Figure 1
(Invited) Three-Dimensional Nanostructuring of Metal-Organic Frameworks for Energy Applications
Authors: Tricoli Antonio
Published: December 2020

Metal-organic frameworks (MOFs) are a growing family of microporous materials that are attracting significant research and industrial interest, thanks to their unique features including a tuneable and narrow intrinsic pore size distribution, and very high surface area. MOFs are also being increasingly used as a precursor for the preparation of well-dispersed metal-containing carbon electrodes. A major challenge remains the three-dimensional structuring of MOFs into hierarchical functional morphologies and their integration as active elements in electrical material systems, due to their usually low electronic conductivities. Here, we present some flexible strategies for the structuring of MOFs into three-dimensional morphologies and their application for energy storage. In particular, we focus on lithium–sulfur (Li–S) batteries as a promising system for the next-generation of high energy density batteries. Despite progress, the energy density of current Li–S technologies is still below that of conventional intercalation-type cathode materials, due to limited stability and utilization efficiency at high sulfur loading.

We demonstrate a highly performing free-standing monolithic electrode architecture for Li–S batteries with superior electrochemical stability and energy density [1]. The electrode layout consists of a highly conductive three-dimensional network of N,P co-doped carbon with well-dispersed metal-organic frameworks nano-domains of ZIF-67 and HKUST-1. The hierarchical monolithic 3D carbon networks provides an excellent environment for charge and electrolyte transport as well as mechanical and chemical stability. The electrically-integrated MOF nano-domains significantly enhance the sulfur loading and retention capabilities by inhibiting the release of lithium polysulfide specifies as well as improving the charge transfer efficiency at the electrolyte interface. Our optimal 3D carbon-HKUST-1 electrode architecture achieves a very high areal capacity of > 16 mAh cm−2 with a capacity retention of 82 % at 0.2C for over 300 cycles, providing an attractive candidate material for future high energy density Li–S batteries. We believe that our approaches provide some promising directions for the engineering of functional MOF morphologies for electrochemistry.

References

Liu, B.; Bo, R.; Taheri, M.; Di Bernardo, I.; Motta, N.; Chen, H.; Tsuzuki, T.; Yu, G.; Tricoli, A. Nano Lett.
2019, 19, (7), 4391-4399.
(Invited) Reduced Thermal Conductivity of Tcnq and F4-TCNQ Infiltrated HKUST-1 Due to Suppression of Linker Flexural Phonon Modes
Authors: DeCoster Mallory E , Jung Sangeun S , Hassan Zeinab Mohamed , Babaei Hasan , Tiernan Emma M. , Tomko John , Wilmer Chris , Redel Engelbert , Giri Gaurav , Hopkins Patrick
Published: December 2020

MOFs are of great interest for applications in gas storage, separation, catalysis and more recently, thermoelectrics (TE) due to their extremely high porosity (surface area), large chemical and mechanical tailorability defined by the choice of the metal node, the organic ligand, and infiltrated guest molecules, and low thermal conductivities. In particular, new emergent properties, such as electronic conductivity and energy transfer, have been achieved by infiltrating MOF pores with specialized ‘guest’ molecules. The thermal transport in infiltrated MOFs is critically important for understanding the efficiency of both gas adsorption and thermoelectric applications. Understanding the impacts on thermal transport upon MOF infiltration with an electrically conductive guest molecule is critical for realizing an efficient TE material. This work experimentally investigates the impacts on the thermal properties of HKUST-1 MOFs infiltrated with charge accepting TCNQ and F4-TCNQ guest molecules. We provide experimental evidence paired with Molecular Dynamics (MD) results to describe the thermal transport mechanism as a loading-dependent structural change within the MOF. Since HKUST-1 exhibits negative thermal expansion (NTE), we note the unusual phenomena that the material softens as it becomes denser with increasing temperatures. We show this is achieved by which the ligands take on a contorted configuration as they absorb more energy at higher temperatures. Upon infiltration, the MOF softens, however the softening (and therefore reduction in phonon group velocity) cannot be completely responsible for the large reduction in thermal conductivity that is experimentally observed (~71% at room temperature). Spectral Energy Density (SED) and MD results indicate that localized phonon modes from the adsorbates act as additional scattering mechanisms to reduce the thermal conductivity. Further, experimental evidence of the existence of low frequency ZA-like flexural modes (usually found only in 2D materials) within the highly porous 3D MOF structure likely contributes to a large portion of the thermal conductivity, and is suppressed upon infiltration with a guest molecule, causing the large reduction in thermal conductivity.
(Invited) In-Memory Computing with Memristor Circuit Primitives
Authors: Graves Catherine Elizabeth
Published: December 2020

The dramatic rise of data-intensive workloads has revived special-purpose hardware and architectures for continuing improvements in computational speed and energy efficiency. While traditional CMOS ASICs deliver some performance gains, typically by limiting data movement or implementing “in-memory computation”, such approaches still suffer from low power efficiency. Therefore, new proposals leveraging non-volatile memristive devices for in-memory computation are highly attractive in a variety of application domains. While originally developed for as digital (binary) high density non-volatile memories, memristive devices have demonstrated a wide range of behaviors and properties – such as a wide range of tunable analog resistance and non-linear dynamics – which motivate their use in novel functions and new computational models. Many recent in-memory compute studies have focused on crossbar circuit architectures, demonstrating their application for neural networks, scientific computing and signal processing. However, other circuit primitives – such as content addressable memories (CAMs) and combined systems such as crossbar arrays and non-linear elements– have shown further promise for mapping a diverse range of complimentary computational models such as finite state machines, pattern matching, hashing algorithms and Hopfield networks for tackling optimization problems. In this talk, I will review the different exciting opportunities for in-memory computational primitives levering non-volatile memristive devices and their circuits and architectures for enabling low power, high-throughput computation in a variety of application domains. I will also highlight the different materials and device requirements which steer material stack design and fabrication for these diverse circuits and application domains. Finally, I will show recent lab demonstrations of various applications mapped to these in-memory computational circuit primitives based on memristor devices, and give an outlook on performance.
Gallium Telluride Based Phase Change Materials for Neuromorphic Photonic Applications
Authors: Bychkov Eugene
Published: December 2020

Neuromorphic photonics intensively uses classical phase-change materials based on GeTe-Sb2Te3 (GST) composition line either to develop on-chip non-volatile photonic memories [1] or artificial synapses for complex integrated photonic systems capable of the image recognition [2]. The GST materials were originally designed for DVD and Blu-ray disks and not necessarily fully optimized for neuromorphic applications. As a possible alternative, we will present here structure and properties of gallium telluride based systems. The structural studies using hard x-rays and ab initio molecular dynamics modeling will be compared with corresponding results for GST phase-change alloys.

[1] C. Rios, M. Stegmaier, P. Hosseini, et al. Nature Photonics 9 (2015) 725.

[2] J. Feldmann, N. Youngblood, C. D. Wright, et al. Nature 569 (2019) 208.
Simulated Annealing Based on Memristors for Efficient Solution of Optimization Problems
Authors: Yang Ke , Yang Yuchao
Published: December 2020

Optimization problems are ubiquitous in scientific research, engineering as well as daily lives. However, solving a complex optimization problem often requires excessive computing resource and time, and faces challenges in easily getting trapped into local optima. Here, we propose a memristive optimizer hardware based on Hopfield network. A single memristor crossbar is used to store the weight parameters of fully-connected Hopfield network and adjust the network dynamics in situ. Furthermore, we harness the intrinsic nonlinearity of memristors within the crossbar to implement an efficient and simplified annealing process for the optimization. Solutions of continuous function optimizations as well as combinatorial are experimentally demonstrated, indicating great potential of the transiently chaotic memristive network in solving optimization problems in general.
Macroscopic Modeling of Liquid Water in Channel of Jari Cell Based on Detailed Two-Phase CFD
Authors: Tanaka Shoichi , Takayama Tsutomu , Tsukamoto Takayuki , Motegi Haruki , Takayama Ryo , Komiyama Keisuke , Yoneda Masakazu
Published: December 2020

In this work, we modeled liquid water behavior in channels based on detailed two-phase CFD for application on Full-Cell scale multiphysics simulation. In our work, in order to execute efficient simulations in full-cell scale, all relevant transport phenomena in PEFC (mass, chemical species, and heat) are considered macroscopic, where the phenomena are coupled with electrochemical reactions. In order to apply this method for conditions in which liquid water is important, two-phase fluid models should be constructed properly.

In order to model two-phase flow in channels, we evaluated the relationship between liquid water volume fraction, pressure drop, and liquid water velocity in macroscopic control volume with detailed CFD using OpenFOAM. The calculation target is the JARI cell channel and mesh size is 50 micrometer order. We used the interIsoFoam for the solver, which uses the VOF method for two-phase flow and geometric surface reconstruction method for surface capturing. The relationship is expressed by the table data that shows the relationship between the liquid phase volume fraction and liquid phase velocity or pressure loss multiplier. The table data shows a good representation of the below.

Dependency of pressure drop on the water volume fraction

Dependency of drainage droplet size on the gas flow rate

Dehydration speed

In order to evaluate the obtained models, we calculated PEFC simulations with the models and validated the calculations.

In our simulator, in order to estimate the performance of PEFC cells taking all relevant transport phenomena (mass, chemical species, and heat) is solved on the coarse-grained mesh, coupled with electrochemical reactions. The mass transport equation is solved by Darcy’s law, using the distribution of the equivalent hydraulic parameter in flow channels estimated by detailed gas phase CFD. Here mass transport is coupled with heat and chemical species transport equations. Those all transport phenomena are coupled with electrochemical reactions in the MEA. Transport of chemical species and water through the MEA is also considered. Many engineering models are employed in order to consider various transport phenomena, like water up-take into electrolytes and effective transport of oxygen from the gas phase to reaction sites in catalyst layers, etc. Electrochemical reactions are modeled by the Butler-Volmer equation, in the manner of lumped parameter models.

The calculation using the derived two-phase model well reproduce the below phenomena.

Various Overpotentials difference between drive conditions

Drainage speed difference depending on gas flow rates

Liquid water difference between under the rib and the channel

Oxygen depletion depending on stoichiometry and liquid water distribution
Microscale Investigation of the Transport Characteristics in Surface-Treated Gas Diffusion Layers with Gradient Porosities for Fuel Cell Applications
Authors: Liu Jiawen , Chung Sung-Jae , Akbar Ali , Um Sukkee
Published: December 2020

In the present study, microscale transport phenomena in gas diffusion layers (GDLs) with gradient porosities are investigated statistically. A series of GDLs are randomly generated with linear and nonlinear gradient porosities at a 95% confidence level to reflect the heterogeneous microstructures. Straight-cylindrical carbon fibers with a uniform diameter are randomly arranged layer-by-layer and then treated with various polytetrafluoroethylene (PTFE) loadings. Furthermore, the reactant transport phenomena throughout the GDLs are simulated using lattice Boltzmann method (LBM). Then, the corresponding mass transport characteristics (i.e., permeability, tortuosity, and effective diffusion coefficient) are predicted in a series of GDL samples. The predicted data reveal an inverse relationship between permeability and PTFE loading. Moreover, the results show that GDLs with gradient porosities are more favourable to the reactant gas diffusion, leading to larger permeability than those of GDLs with uniform porosities. Besides, electrical and thermal conductivities of GDLs are computed in both in-plane and through-plane directions, and show good agreement with published experimental data. These statistical results show that the microscopic transport characteristics of PTFE-treated GDLs with gradient porosities are strongly affected by porosity distribution and PTFE loading. The proposed model can be utilized to optimize GDL structures and investigate the effect of PTFE on microscale transport characteristics of GDLs.
Multiscale Simulation of Proton Transport in the Catalyst Layer with Consideration of Ionomer Thickness Distribution
Authors: Matsuda Tomohiro , Kobayashi Koichi , Mabuchi Takuya , Inoue Gen , Tokumasu Takashi
Published: December 2020

To spread polymer electrolyte fuel cells (PEFCs) widely, which have many advantages, such as low environment load and high energy conversion efficiency, design and optimization of cathode catalyst layers (CLs) are necessary. CLs have inhomogeneous microstructures where Pt catalysts covered with ionomer thin films are supported by carbon particles. Electrons and protons transport to Pt surface through carbon particles and ionomer thin films while oxygen molecules diffuse in void and permeate through ionomer thin film to reach the Pt surface. Therefore, analyzing and understanding of mass transport of proton and oxygen in CLs are important for improving fuel cell performance. However, since an ionomer thin film on the Pt surface has a thickness of about 4 to 10 nm,1 detail understanding of the relationship between the transport mechanism within the nanostructures of ionomers and the fuel cell performance remains an issue. Therefore, we have focused on the analysis of transport phenomena using mesoscale numerical simulations by considering the effects of nanostructures on mass transport obtained from molecular dynamics (MD) simulations. Kobayashi et al.2 have studied the thickness dependence of ionomer thin films on proton transport using MD simulations and introduced these effects into the mesoscale simulations. The authors found that the self-diffusion coefficients of proton (D
H
+) show a peak at the ionomer thickness of 7 nm, in which the value of D
H
+ is almost two times larger than that in the bulk membrane, leading to the high proton conductivity that is 1.6 times larger than the bulk membrane. These results suggest that the influence of ionomer thickness at the nanometer scale on the performance of PEFCs is significant. However, in the mesoscale simulations, the resulting cell performance (i.e., I-V curve) show similar results between the systems with and without consideration of the thickness effects because the thickness distributions obtained from experiments was not sufficiently reproduced using the hydrophilic adhesion model for ionomer distribution in the mesoscale simulation. In this study, we develop a new method to improve the non-uniform ionomer distribution that reproduces the experimental thickness distribution. Furthermore, we take into account the oxygen transport results obtained from MD simulation in the mesoscale simulations to understand the effects of ionomer nanostructures on the oxygen transport at larger scale in CLs and the fuel cell performance.

Reference

Lopez-Haro, M. et al. Three-dimensional analysis of Nafion layers in fuel cell electrodes. Nat. Commun.
5, 1–6 (2014).

Kobayashi, K., Mabuchi, T., Inoue, G. & Tokumasu, T. Nano/Microscale Simulation of Proton Transport in Catalyst Layer. ECS Trans.
92, 515–522 (2019).
(PEFC&E 2nd Place Best Poster Winner) Simulation of Reaction and Mass Transport in PEFC Cathode Catalyst Layer Fabricated by Inkjet Printing Method
Authors: Wei Yuting , Park Kayoung , Inoue Gen , Kimura Naoki , Tsuge Yoshifumi
Published: December 2020

Polymer electrolyte fuel cell (PEFCs) is a potential power source for new-generation automobiles like fuel cell vehicles that run on hydrogen energy, it has been studied a lot for last decades in order to use as the power sources of automotive. The most important characteristics of PEFCs are low operation temperature and quick start and shutdown. However, the properties of PEFCs are still limited due to its high cost and insufficient durability and performance.

Since oxygen reduction reaction (ORR) at the cathode is the dominant reaction in PEFCs, oxygen proton and electron have to be transferred smoothly in porous catalyst layer. Under normal operating conditions, oxygen transfer is the dominant factor of limiting current density, therefore, oxygen transfer must be increased from the gas channel to the Pt surface through various porous media and various components.

On the other hand, inkjet printing has been considered to be an economical and easy scale-up technology for the micro-scale patterning and fabrication as one of the on-demand methods in a variety of field because it allows for the patterning of various ink. In addition, it can reduce the process time, cost and the toxic waste created during the manufacturing process. Also, this method can be easily applied to perform ink composition studies. Especially for fuel cell applications, inkjet printing resolves many of the problems associated with previous methods of catalyst deposition by allowing a uniform distribution of catalyst ink onto the surface of GDL or membrane and by loading a very low amount of catalyst.

So in this research, in order to increase the performance of polymer electrolyte cells, the oxygen diffusion resistance was tested based on the difference of surface of catalyst layer in cathode by using inkjet method. Firstly, we mixed carbon supported Pt catalyst, distilled water and 20wt% Nafion solution and NPA together and then use homogenizer to disperse the liquid, then we got the catalyst ink. The Second step was fabrication of cathode catalyst layer (CCL) by using inkjet method. Third step was to observe the morphology for CCL with catalyst ink, in order to know the length and the height of a drop of ink, we made a sample, and then we took this sample to observe through laser microscope. When we got the basic information about morphology of CCL, we fabricated catalyst layer by inkjet, and took it to do the MEA evaluation and characterization to investigate their electrochemical properties, such as I-V curves, limiting current densities and impedance. And in order to compare with the experimental results, a theoretical model of the 3D structure was also circulated and discussed. From this model and measured data, it was confirmed that the better electrochemical properties results from 3D structure of catalyst layer in cathode.
Fabrication and Evaluation of Lightweight Bipolar Plates for an Direct Ethanol Alkaline Polymeric Electrolyte Membrane Fuel Cell
Authors: Rojo Osorio Santiago , Sanchez Saenz Carlos Ignacio
Published: December 2020

Increasing awareness of climate change and the dangers of fossil fuels have spurred a need for faster integration of renewable energy sources onto the electrical grid. Direct Ethanol Alkaline Polymeric Electrolyte Membrane Fuel Cells (DE-APEMFC) have been develop as an alternative to this need, specially in Colombia, where the Ethanol Production is bigger than Hydrogen. Besides the operational Temperature make this Technology more attractive to be implemented in the Non Interconnected Zones of the Country. The objective of this study was to create lightweight DE-APEMFC by replacing traditional bipolar plates with combinations of polypropylene (PP), graphite or niquel.

This study involved the fabrication of bipolar plates following a process of electroless coating over polypropylene and the tests to determine their properties in comparison with the DOE Technical Targets for Electrolyte Fuel Cell Components. After bipolar plates testing, qualitative conclusions were made based on the performance of each cell. Then, Fuel cell with be assembled with the developed bipolar plates to determine the effect on the performance measured with polarization curves.The flow channel design was selected from results of previous works where it was determined that for the ethanol serpentine flow field ensure better mass transfer coefficients in the anode, and interdigitated flow field for the cathode ensure better performance for the cell.
Optimization of Mass Transport within Direct Formic Acid Fuel Cell Catalyst Layer via Pore Formers
Authors: Lam Steven , Bixby Meghan Marie , Rice Cynthia A.
Published: December 2020

Direct formic acid fuel cells (DFAFC) offer a potentially sustainable alternative to the batteries in portable devices due to their higher charge capacity, high efficiency, and smaller size. However, due to the two-phase flow in the anode catalyst layer and the small pore size between agglomerates in the catalyst layer (~20 nm), mass transport limitations plague the performance of the DFAFC. To maximize the performance, the two-phase transport of the reactant (formic acid) and the product (carbon dioxide) must be optimized. The incorporation of a pore forming template during catalyst layer fabrication and subsequent removal can increase the porosity between catalyst/ionomer agglomerates and enhance mass transport into and out of the anode catalyst layer.

In 2002, a 3.4x enhancement in current density at 60°C of 3.4x was reported due to pretreatment with 1M methanol.[1] The methanol swelled the anode catalyst layer thereby enhancing mass transport. This methanol pretreatment was found to be only temporal. In 2012, the addition of the pore forming template lithium carbonate (Li2CO3) has been shown to form ~10 mm voids through the thickness of the ~10 mm catalyst layer after acid leaching out the Li2CO3.[2] The formic acid electrooxidation charge transfer resistance was reduced for 17.5 wt% pore former additions, at the detriment of the catalyst layer’s triple-phase boundaries.

The present study aims to increase the nano-range porosity of the anode catalyst layer to enhance two-phase flow, while maintaining the integrity of the triple-phase boundaries. The alternative templating agent is magnesium oxide (MgO) with an average size of only ~50 nanometers. MgO has been shown to increase mass transport of methanol.[3] The current work shows improved catalytic activity due to the increase in the porosity.

Ha, S.; Rice, C.A.; Masel, R.I. and Wieckowski, A., Methanol conditioning for improved performance of formic acid fuel cells. 2002, 112, 655-659.

Bauskar, A.S. and Rice, C.A., Impact of anode catalyst layer porosity on the performance of a direct formic acid fuel cell. 2012, 62, 36-41.

Huang, Q.; Jiang, J.; Chai, J.; Yuan, T.; Zhang, H.; Zou, Z.; Zhang, X. and Yang, H., Construction of porous anode by sacrificial template for a passive direct methanol fuel cell. 2014, 262, 213-218.
Characterizing Electrochemical and Thermal Properties of VDF Blended 3M PFSA Ionomer with Chemically Bonded Polyoxometalate Additives
Authors: Kim ChulOong (Christoph) , Kuo Mei-Chen , Lindell Matthew , Thaler Arne , Jiang Ruichun , Gittleman Craig S , Yandrasits Michael , Herring Andrew M.
Published: December 2020

Proton exchange membranes (PEMs) are considered as the most promising material for fuel cell automobile applications due to their high proton conductivity and power density. The radical attacks with peroxide formation cause lower durability, which is a major challenge to further commercialize fuel cell vehicles. Cerium and manganese cations and oxides have been implemented as radical scavenging agents to overcome the challenge from radical attacks. However, the additives tend to not stay on the initial location but move during the operation due to migration and diffusion. The migration results in cluster formation and eventually the additives may dissolve and leave the membrane system. Our previous work modifying commercial fluroelastomers with chemically bonded polyoxometalate resulted in a membrane with radical scavenging properties while inhibiting additive clustering. Advancement in PEM durability with additives has implications not only for the light-duty fuel cell vehicles but also for medium and heavy-duty vehicles applications, which would aid the widespread of commercialization of fuel cell vehicles.

This work focuses on characterizing the blended perfluorinated sulfonic acid (PFSA) ionomer membrane with the chemically bound radical scavenger to enhance the durability of the membrane. Proton conductivity of the membrane was characterized under controlled relative humidity and temperature with electrochemical impedance spectroscopy (EIS). Ionic Exchange capacity of two or more proton conductive sites was measured using titration method. Correlation between water uptake and membrane properties is discussed and measuring using dynamic vapor sorption (DVS). Thermal stability was investigated through thermalgravimetric analysis (TGA).
Nanostructure of a Catalyst Ink Investigated by Contrast-Variation Small-Angle Neutron Scattering
Authors: Yoshimune Wataru , Harada Masashi
Published: December 2020

One of the major aims when investigating catalyst inks for polymer electrolyte fuel cell applications is to understand the relationship between their macroscale physical properties and nanostructure. Catalyst inks are complex colloidal dispersions consisting of catalyst particles, ionomers, and water–alcohol mixtures. Some of the ionomer is adsorbed onto the surface of the catalyst particles. Due to the electrostatic repulsion observed between the particles covered with the ionomer, the ink possesses high dispersion stability. Meanwhile, the role of the remaining ionomer in the dispersion is unclear to date. To clarify the effect of the non-adsorbed ionomer on the physical properties of a catalyst ink, we have established a method to elucidate the amount of non-adsorbed ionomer in the catalyst ink using contrast-variation small-angle neutron scattering (CV-SANS). CV-SANS measurements revealed that the volume fraction of the non-adsorbed ionomer in catalyst ink monotonically increases upon increasing the weight ratio of ionomer/carbon. It was found that the non-adsorbed ionomer in the catalyst ink acts as a typical ionomer dispersion, which can be described by the Huggins equation at high shear viscosity. Our findings may serve as useful guidelines for the preparation of catalyst inks, which is a crucial step for the construction of rationally designed membrane electrode assemblies used for polymer electrolyte fuel cell applications.
(Invited) Redox-Active Electrolyte Capacitors Based on Modified Ionic Liquids
Authors: Wang Yanyu , Xie Han Jin , Gélinas Bruno , Rochefort Dominic
Published: December 2020

The use of redox-active species is a very powerful approach to increase the energy density of electrochemical capacitors via the contribution of faradaic charge storage. The redox centers selected for this purpose must provide rapid electron transfer reactions to maintain high charge and discharge rates and be stable in both oxidation states to ensure long cycle life. Transition metal complexes such as ferrocene fulfill these requirements and their electrochemistry is well studied. However, the efficient incorporation of such redox centers in the capacitor is not necessarily straightforward. The dissolution of electroactive compounds is the electrolyte is a simple approach but is strongly dependent on the compound’s solubility. The adsorption of the redox center on the surface of activated carbon may provide a high local concentration but for a limited number of cycles due to gradual losses in the electrolyte. Finally, the covalent binding of redox centers on carbon may restrict the access to micropores, thereby decreasing the double-layer charging.

Our approach to overcome these limitations and provide an efficient utilization of redox-active centers in electrochemical capacitors is to rely on ionic liquids (ILs) structures modified with a redox center. As such, ILs are viewed here as a vehicle for the redox center. Because imidazolium and triflimide structures are very stable, highly miscible in several polar organic solvent and easy to modify, they have been selected to bear ferrocene electroactive groups. The structure of such redox-active ionic liquids (RAIL) are shown in Fig. 1. This contribution will show an overview of the properties of redox-active ionic liquids for energy storage in electrochemical capacitors, how they can be efficiently applied in these devices and, importantly, how they can represent an advantage over the more limited off-the-shelf ionic liquids.

Figure 1