bluuebunny/crossref_metadata_2025 huggingface dataset:
Boron and Phosphorous Doping of GeSn for Photodetectors and Light Emitting Diodes
Authors: Frauenrath Marvin , Hartmann Jean-Michel , Nolot Emmanuel
Published: September 2020

With an electrically pumped group IV laser operating at room temperature as a long term goal, we would like to replace the boron and phosphorous doped Ge layers used in previous pin structures with GeSn:B and GeSn:P. We have thus investigated the in-situ doping of GeSn with Ge2H6, SnCl4, B2H6 (p-type) or PH3 (n-type) at 349°C, 100 Torr on Ge Strain-relaxed Buffers, themselves on Si(001) substrates. The Sn content from Wavelength Dispersive X-Ray Fluorescence (WDXRF) was constant around 6.5% for low F(B2H6)/F(Ge2H6) and F(PH3)/2*F(Ge2H6) Mass-Flow Ratios (MFRs). It fell down to 4.6% (GeSn:B) and 5.6% (GeSn:P) for higher MFRs. This drop was likely due to the dopant precursors that catalyzed the Ge Growth Rate component. Substitutional B and P contents were extracted from the differences between real Sn contents from WDXRF and “apparent” Sn contents from X-Ray Diffraction (compressive strain compensation by small B and P atoms). The B substitutional concentrations increased sub linearly with the MFR, reaching values of at most 5.2x1019 cm-3. Meanwhile, the P substitutional concentration increased linearly with the MFR, up to 2.2x1020 cm-3. WDXRF gave us also access to the atomic P concentrations, with values as high as 2.2x1020 cm-3 A cross-hatch along <110> was otherwise present on the surface of the various GeSn:B and GeSn:P layers, which were rather smooth. Only the GeSn:B layers grown at the three highest MFRs exhibited some B and/or Sn surface segregation.
Analysis of Sn Behavior During Ni/GeSn Solid-State Reaction by Correlated X-ray Diffraction, Atomic Force Microscopy, and Ex-situ/In-situ Transmission Electron Microscopy
Authors: Quintero Andrea , Gergaud Patrice , Hartmann Jean-Michel , Delaye Vincent , Bernier Nicolas , Cooper David , Saghi Zineb , Reboud Vincent , Cassan Eric , Rodriguez Philippe
Published: September 2020

A comprehensive study of the impact of Sn content on the Ni/GeSn phase sequence, the morphological and electrical properties evolution during the solid-state reaction (SSR) has been performed. The phase sequence was monitored by in-situ X-ray diffraction. These results were complemented by atomic force microscopy and sheet resistance measurements together with transmission electron microscopy imaging and energy-dispersive X-ray spectroscopy applied to specimens annealed both ex-situ and in-situ. The phase formation and Sn behavior during the solid-state reaction was followed. Sn segregation around grain boundaries hampered atom diffusion, delaying the growth of the Ni(GeSn) phase. In addition, a correlation between the Sn content and the morphological and electrical properties degradation has been observed.
(Invited) Si-Cap-Free Low-DIT SiGe Gate Stack for High-Performance pFETs
Authors: Arimura Hiroaki , Wostyn Kurt , Ragnarsson Lars-Åke , Conard Thierry , Chasin Adrian , Franco Jacopo , Mitard Jerome , Horiguchi Naoto
Published: September 2020

We have demonstrated a metal high-k gate stack with DIT as low as 5.4×1011 cm-2eV-1 at an EOT of 10.9 Å on a planar Si0.7Ge0.3 MOS capacitor test vehicle without using a Si-cap. The key enablers of the DIT reduction are GeO scavenging process converting GeO into SiO and gate stack nitridation processes. Although the final DIT was found to be highly sensitive to the metal gate deposition process, nitridation of HfO2 was found to negate the negative impact from ALD-based TiN/W. Additional MOS capacitor experiments suggested that the role of the nitridation of HfO2 is to reduce the oxygen diffusivity, resulting in suppressing the undesired oxygen diffusion from the ALD-based metal electrode through the high-k, and resultant interface layer regrowth with additional GeO formation. These results imply that the oxygen profile control throughout the gate stack process is the key to the low-DIT SiGe gate stack.
(Invited) Border-Trap Characterization for Ge Gate Stacks with Thin GeOX layer Using Deep-Level Transient Spectroscopy
Authors: Nakashima Hiroshi , Wen Wei-Chen , Yamamoto Keisuke , Wang Dong
Published: September 2020

The deep-level transient spectroscopy (DLTS) measurements for the SiO2/GeO2/Ge gate stacks represent the different pulse height dependences of the border trap (BT) signals for p- and n-MOSCAPs. We propose the tunneling models to/from BT in GeO2 to explain the difference. Based on this model, the BTs in Al2O3/GeOx/p-Ge gate stacks were characterized using DLTS. Through evaluating the gate stacks with different GeOx thickness, the respective BTs in Al2O3, Al2O3/GeOx interface region, and GeOx were detected. The density of BT (N
bt) in Al2O3 (6~9 × 1017 cm-3) is lower than those in GeOx (~2 × 1018 cm-3), and the highest N
bt (~1 × 1019 cm-3) was found in Al2O3/GeOx interface region. Ge p-MOSFETs with Al2O3/GeOx/p-Ge gate stacks were fabricated and analyzed. We confirmed that the ITs and the BTs near to valence band edge of Ge affect the effective mobility of Ge p-MOSFETs in high field region.
Evaluation of Thermal Conductivity Characteristics in Polycrystalline Silicon - The Effect of Nanostructure in the Grains
Authors: Takeuchi Haruki , Yokogawa Ryo , Takahashi Kazuya , Komori Katsuhiko , Morimoto Tamotsu , Sawamoto Naomi , Ogura Atsushi
Published: September 2020

We focused on the nanostructure size by changing the additional annealing conditions during poly-Si thin film fabrication with keeping the same grain size, and evaluated the thermal conductivity characteristics of the poly-Si thin film by Raman spectroscopy. Laser-power dependent Raman spectroscopy showed the temperature increase caused by laser heating. The temperature increase of each sample is different depending on the poly-Si fabrication process. It is considered that this temperature change is caused by the increase in the phonon scattering sites inside the grain due to change in the nanostructure sizes. Moreover, it was revealed that the sample with lowest thermal conductivity has the medium nanostructure size. It may need to consider not only the size but also the density of the nanostructure to understand these phenomena. In conclusion, we clearly indicated that not only the average grain size but also the size and density of the nanostructures inside grains should be carefully considered to control thermal conductivities.
Thermal Silicidation of Ni/SiGe and Characterization of Resulting Silicide Films Using Raman Spectroscopy and X-ray Diffraction
Authors: Yoo Woo Sik , Kang Kitaek , Ishigaki Toshikazu , Kim Jung Gon , Hasuike Noriyuki , Harima Hiroshi , Yoshimoto Masahiro
Published: September 2020

Thermal silicidation characteristics of Ni/Si1-xGex with various Ge content was studied under different annealing temperatures in the range of 225oC ~ 400oC in N2 ambient. TiN capped Ni/Si1-xGex/Si/SiO2/Si wafers with x values in the range of 0.15 and 0.30 in 0.05 intervals were used. Thermal silicide formation was performed in a stacked hotplate-based annealing system designed for 300mm wafers. For silicide characterization, measurements of spectral reflectance, sheet resistance, Raman spectra as well as X-ray diffraction curves were performed to investigate optical properties, electrical properties, crystallographic phases and reaction mechanisms. Multiwavelingth micro-Raman spectroscopy was found to be very promising as a non-contact, in-line silicidation process monitoring technique. We have studied effects of Ge content and annealing temperature dependence on the electrical and crystallographic properties of the resulting thermal silicides (nickel germanosilicides).
Increased Critical Thickness for Strained SiGe on Ge-on-Si(111)
Authors: Youya Wagatsuma , Alam Md. M , Okada Kazuya , Hoshi Yusuke , Yamada Michihiro , Hamaya Kohei , Sawano Kentarou
Published: September 2020

We fabricate tensile-strained SiGe layers on patterned Ge-on-Si(111) and investigate their surface morphologies and strain states. It is demonstrated that the surface ridge roughness involving crack formation is completely avoided by the patterning of the Ge-on-Si. Resultantly, the 160 nm thick SiGe with a Ge concentration of 75% is pseudomorphically grown on the patterned Ge-on-Si. This thickness largely exceeds the critical thickness of strained SiGe grown a Ge substrate, meaning that the critical thickness can be increased by the patterning. We can, therefore, say that this method is promising for strained SiGe-based spintronics and high mobility device applications.
Strong Room-Temperature Electroluminescence from Ge-on-Si by Precise in-situ Doping Control
Authors: Yamada Kodai , Hoshi Yusuke , Sawano Kentarou
Published: September 2020

We observe strong room-temperature electroluminescence from Ge-on-Si. Ge p-i-n junction is formed within the epitaxial Ge by the precise in-situ doping method and additionally high density P delta doping is performed in the vicinity of the surface to create a low resistivity Ohmic contact. An EL device was mesa-defined by the photo-lithography and following reactive ion etching process in order to suppress leak current. Current-voltage characteristic reveal a good diode rectifying property with an on/off ratio of over 105. The EL spectrum appears at around 2.1 kA/cm2 injected current, and the EL intensity drastically increases with the injection current increase. These results indicate that the Ge-on-Si p-i-n structure has high potential to realize high efficiency Ge lasers monolithically integrated on the Si platform.
Capturing the Effects of Free Surfaces on Threading Dislocation Density Reduction
Authors: Bioud Youcef Ataellah , Boucherif Abderraouf , Patriarche Gilles , Drouin Dominique , Arès Richard
Published: September 2020

The big concern with using silicon as a substrate for making Ge and III-V devices is the dislocation density in the epilayers. Dislocations degrade device performance by trapping the photo-generated carriers, dragging down efficiency. In this manuscript, a Ge/Si substrate is treated with a post epitaxial process to create a region with a high density of nanovoids within the Ge/Si structure, which acts as a free surface that attracts dislocations, facilitating the subsequent annihilation of their threading arms. Nanovoids are formed in the Ge layer by electrochemical porosification, followed by thermal annealing, creating a new configuration referred to as nanovoid-based Ge/Si virtual substrate (NVS).
(Invited) Insights into the UV-C Optical Absorption of AlN Substrates Grown by PVT
Authors: Dalmau Rafael , Britt Jeffrey , Schlesser Raoul
Published: September 2020

The structural and optical properties of 2-inch AlN substrates grown by physical vapor transport were assessed. Ultraviolet transmittance and absolute reflectance spectra were measured spectrophotometrically. Accurate absorption coefficients were calculated with a method accounting for reflection losses. Production of high-quality, macrodefect-free AlN substrates possessing absorption coefficients at 265 nm with values from 30 to 50 cm-1was demonstrated.
Design of Ion-Implanted Junction Termination Extension for Vertical GaN Pin Rectifiers
Authors: Cho Minkyu , Xu Zhiyu , Bakhtiary-Noodeh Marzieh , Detchphrom Theeradetch , Dupuis Russell D , Shen Shyh-Chiang
Published: September 2020

In recent years, GaN bipolar switches have attracted much research interests for the implementation of high-power and high-voltage applications that laterally-scaled GaN-based power field-effect transistors were not optimally implemented. In the context of vertical power device design, a proper electric field engineering would be of crucial importance to exploit the device performance that could potentially reach the theoretical power switching limit for a given material system. In this report, we studied the effectiveness of the floating guard ring design of a vertical GaN p-i-n (PIN) rectifiers for kV-range applications using a planar device fabrication that involves the use of ion-implantation as a mean of device isolation and the formation of floating guard rings. We found the gap between floating guard ring has major impact on the electric field distribution in vertical PIN rectifiers. As a result, 1 kV of blocking voltage was achieved by 6 floating guard ring design using nitrogen-ion implanted isolation.
(Invited) GaN Homoepitaxial Growth and Substrate-Dependent Effects for Vertical Power Devices
Authors: Hite Jennifer K. , Mastro Michael A , Anderson Travis J. , Gallagher James C. , Ebrish Mona , Freitas Jaime A.
Published: September 2020

Raman and photoluminescence mapping are used to sort GaN native substrates into two categories: those with uniform free carrier concentration and those with large variations in carrier concentration. Further characterization after growing 2-5 µm of homoepitaxial GaN by metal organic chemical vapor deposition shows the variations in the substrate carry into the epitaxial films, as observed with both mapping techniques. This is also supported by low temperature photoluminescence measurements, which show a broadening of the near band edge peak in epitaxy on the inhomogeneous substrates.
(Invited) P-Type and N-Type Channeling Ion Implantation of SiC and Implications for Device Design and Fabrication
Authors: Das Hrishikesh , Sunkari Swapna , Justice Joshua , Malousek Roman , Chochol Jan , Wada Ryota , Kuroi Takashi
Published: September 2020

This work focuses on evaluating and demonstrating channeled p-type and n-type implantations in silicon carbide in a repeatable mass-production environment. Range increase of about 3X is observed using channeled conditions as opposed to normal incident conditions for both Aluminum and Phosphorous. The various advantages enabled by this technology for advanced device designs are highlighted. Super-junction devices targeting the same voltage range can be fabricated using one or two lesser epitaxial regrowth layers.
Characterization of Dislocations in 4H-SiC Single Crystals at the Initial Growth Stage by Synchrotron X-ray Topography
Authors: Ailihumaer Tuerxun , Peng Hongyu , Liu Yafei , Raghothamachar Balaji , Dudley Michael
Published: September 2020

This paper describes the investigation of dislocation behavior during the early stages of PVT-grown 6-inch 4H-SiC crystals by synchrotron monochromatic beam x-ray topography technique. Ray tracing simulation is applied to simulate the dislocation images. Our studies show that most TSDs/TMDs are replicated into the newly grown crystal while most TEDs are generated by either nucleation in pairs at the seed/crystal interface or by redirection of BPDs in the seed crystal. Most BPDs in the newly grown crystal are of screw type with 1/3 [112‾0], which is verified by comparison with the simulated images. TEDs with the same and opposite sign of Burgers vector are found to be deflected onto the same basal plane by the overgrowth of macro-steps and glide towards to the same or opposite directions respectively. TMDs deflected onto the basal plane get dissociated into a and c components, with the a segment undergoing glide to form V-shaped configurations.
Selective-Area Growth of AlInAs Nanowires
Authors: Tai Yoshiki , Gamo Hironori , Motohisa Junichi , Tomioka Katsuhiro
Published: September 2020

A superlattice (SL) resonant tunneling field-effect transistor (RTFET) is one of the candidates for future FETs. However, there is a difficulty in forming high-quality SL with various lattice mismatched systems. The nanowire (NW) structure shows promise in terms of avoiding the misfit dislocation, suggesting that a lattice-mismatched InGaAs/AlInAs SL NW can be formed for the RTFET. We report on the selective-area growth of AlInAs NWs forInGaAs/AlInAs NW RTFETs.
A Simple Approach to Growth Mode of InN and InGaN Thin Films on GaN(0001) Substrate
Authors: Nagai Katsuya , Akiyama Toru , Nakamura Kohji , Ito Tomonori
Published: September 2020

The growth of InN and InGaN thin films on GaN(0001) substrates are simply interpreted by using the macroscopic theory using free energy formula with the aid of empirical interatomic potential calculations as well as ab initio calculations. Our empirical interatomic potential calculations reveal that the system with 2D growth with misfit dislocation (2D-MD) is more stable than that with 2D coherent growth (2D-coherent) for InN films beyond 0.7 monolayers (ML), and the thickness for the stabilization of 2D-MD decreases with In composition. Moreover, the analysis of free energies demonstrate that 2D-MD is favored under In-rich conditions while three-dimensional growth with both islands and misfit dislocations is favored under N-rich conditions. The difference in growth mode is due to the surface energy anisotropy depending on the growth condition. The results suggest the surface energy anisotropy is a crucial factor for determining the growth modes of InN and InGaN on GaN(0001) substrate.
(Invited) Highly Reliable Metal Oxide Thin Film Transistors for Flexible Devices
Authors: Uraoka Yukiharu , Corsino Dianne , Bermundo Juan Paolo , Fujii Mami N. , Uenuma Mutsunori
Published: September 2020

Fully solution-processed amorphous InZnO (a-IZO) thin-film transistors (TFTs) were demonstrated utilizing solution-based channel, gate insulator, and electrodes with a maximum fabrication temperature of 300°C. Particularly, a single layer of a-IZO was used as both the channel and the source/drain electrode layer by selectively tuning the role of a-IZO as a semiconductor or a conductor through photo-assisted treatments. By employing a self-aligned TFT structure, the a-IZO electrodes were functionalized by UV irradiation and excimer laser annealing (ELA). The fully solution-processed a-IZO TFTs exhibited high performance with an average mobility of up to 38 cm2 V-1 s-1, which surpasses those of previously reported approaches for fully solution-processed oxide TFTs. Moreover, the overall device performance, including subthreshold swing of 225 mV dec-1 and on-voltage of -0.4 V, is comparable to those of vacuum-processed oxide TFTs.
TCAD Simulation of a 3D NAND Memory Utilizing In-Ga-Zn-Oxide: "3D OS NAND" with 4 V Drive, High Endurance and Density
Authors: Kunitake Hitoshi , Kimura Hajime , Tsuda Kazuki , Godo Hiromichi , Murakawa Tsutomu , Sawai Hiromi , Baba Haruyuki , Sasagawa Shinya , Ikeda Takayuki , Yamazaki Shunpei
Published: September 2020

We have evaluated, through TCAD simulation, 3-dimensional oxide-semiconductor NAND (3D OS NAND), a novel vertically-stacked 2T-1C memory using cylindrical c-axis aligned crystalline In-Ga-Zn-Oxide (CAAC-IGZO) channels. Taking advantage of low off-leakage of CAAC-IGZO field-effect transistors (CAAC-IGZO FETs), the memory does not use a metal-oxide-nitride-oxide-silicon (MONOS) structure for writing. This memory architecture is potentially capable of achieving a memory density and a 10-year retention similar to NAND flash, operating with a supply voltage of 4 V or below, while overcoming the endurance challenge of NAND flash.
(Invited) Improved Copper Electrode Integration for Thin Film Electronics on Glass
Authors: Kim Hoon , Zhu Bin , Huang Ming-Huang , Vaddi Rajesh , Manley Robert G.
Published: September 2020

We demonstrate single layer Cu interconnects using thin CuMn alloy as a temporary adhesion layer. After short annealing at 300°C for 60 s of a 10 nm CuMn alloy with 500 nm of Cu film, CuMn alloy layer is converted to pure Cu and the Mn is reacted with glass to form MnOx serving as an adhesion layer. After optimization of Mn concentration, alloy thickness, annealing environment, time and temperature, a resistivity lower than 1.8 μΩ·cm of the electrode was achieved. We also evaluated the effect of glass surface treatment to confirm the sensitivity of this process depending on substrate preparation conditions. This process has been shown to be very stable for different acid treatments on display glasses. It is also compared to a Ti/Cu stack as a reference.
Edge-Directed LTPS Via Flash Lamp Annealing Using a Cr Adhesion Layer for Improved Wettability
Authors: Packard Glenn , Rosenfeld Adam , Hum Matthew , Manley Robert G. , Hirschman Karl D.
Published: September 2020

PMOS TFTs built on flash lamp annealed polycrystalline silicon are presented. A thin layer of chromium is used as an adhesion promoter to prevent randomized voids from forming, allowing predictable grain growth in an elongated morphology oriented away from silicon mesa edges. Operational TFTs were realized, revealing that the Cr under-layer does not result in a significant conductance in parallel with the channel, and effectively serves as a barrier to glass contaminants. The methodology supports the production of TFTs with a high degree of electrical uniformity. Comparisons are made between electrical characteristics of transistors with grains oriented in the same direction as the channel carrier pathway versus those with perpendicular orientation; the former demonstrating approximately ten percent higher channel mobility. Process details and representative electrical characteristics are described.
Photoconductive Solution Processed ZnO Quasi-superlattice Films
Authors: Buckley Darragh , Inguva Saikumar , McNulty David , Zubialevich Vitaly Z. , Parbrook Peter , Gity Farzan , Hurley P. K. , O'Dwyer Colm
Published: September 2020

Photoconductance is reported from single layer and 20 layer quasi-superlattice films of ZnO grown by solution processing and spin coating. At sub-band (532 nm) excitation, significant and stable photoconducrrent is observed and can be modulted by laser power density.
(Invited) Optimizing Material Systems for All-Inkjet-Printed Organic Thin-Film Transistors
Authors: Jiang Chen , Nathan Arokia
Published: September 2020

All-inkjet-printed organic thin-film transistors enjoy advantages of low-cost printability and mechanical flexibility, thereby enabling newly emerging application areas. Recent developments have shown devices with low operating voltages and steep subthreshold slopes, using a bottom-gate bottom-contact structure. However, it would be also interesting to understand the optimization of the underlying materials system. This study investigates the composition and associated solvents for semiconductor inks, as well as the necessity of encapsulation. Basically, we compare the semiconductor inks of 6,13-bis(triisopropylsilylethynyl)pentacene with and without polystyrene binder, and find the importance of the polymer binder in lowering trap state density. Comparing semiconductor inks of different boiling points, e.g. toluene (low) and anisole (high), suggests that using a solvent with a high boiling point can enhance semiconductor crystallization. Using encapsulation with a fluoropolymer CYTOP is essential to reduce the trap state density. These results are important for further development of novel all-inkjet-printed organic thin-film transistors.
(Invited) Display and LSI Applications of Oxide Semiconductor LSIs (OS LSIs) Using Crystalline In–Ga–Zn Oxide (IGZO): Applications Related to Coronavirus COVID-19 Pandemic
Authors: Onuki Tatsuya , Okamoto Yuki , Aoki Takeshi , Matsuzaki Takanori , Kozuma Munehiro , Kunitake Hitoshi , Motoyoshi Ryosuke , Kimura Hajime , Yamane Yasumasa , Sasagawa Shinya , Yamazaki Shunpei
Published: September 2020

This paper shows structures and features of a field-effect transistor (FET) fabricated using a c-axis aligned crystalline In–Ga–Zn oxide (CAAC-IGZO), which is a typical example of crystalline oxide semiconductor (OS) ceramics, as a channel material, and applications of the CAAC-IGZO FET up to this point. In this study, we propose an AI accelerator as novel LSI using an OS (OS LSI) and make a detail description of the AI accelerator.
(Invited) The Interplay of Conformation and Electronic Structure in Metal Organic Frameworks
Authors: Pramudya Yohanes , Kozlowska Mariana , Heidrich Shahriar , Krstić Marjan , Haldar Ritesh , Neumann Tobias , Schlöder Tobias , Zhang Qiang , Symalla Franz , Hassan Zeinab Mohamed , Liu Xiaojing , Heinke Lars , Wöll Christof , Wenzel Wolfgang
Published: September 2020

Metal organic frameworks (MOFs) are a fascinating class of materials that is currently investigated in the context of many applications. The large size of the chemical space of these systems, comprising organic linkers connected by metal centers, precludes an exhaustive experimental search for optimal materials. For this reason, a variety of modeling tools, ranging from structure-generators to electronic structure methods has been applied to understand and increasingly predict their function. Here we review some recent studies, where computational methods were used to elucidate MOF properties, such as electroluminescence, refraction index, and charge transports. As these methods become more predictive, they enable computational screening of MOF materials.
Synthesis and Supercapacitive Properties of Ni/Co-S Derived from MOF-74
Authors: Pang Xiancai , Gao Xiaolei , Li Xu , Yu Jianhua , Zhang Qian , Sui Lina , Dong Lifeng
Published: September 2020

Supercapacitors have been considered as one of the most promising energy storage devices for developing renewable energy with increasing demands of energy and environmental protection. Binary transition metal sulfides have captured tremendous interests in the field of supercapacitors due to intrinsically high electrical conductivity and superior capacitive properties. In this work, MOF-74 with Ni, Co and different ratios of Ni/Co (2:3, 1:1, 3:2) are synthesized by a facile solvothermal method, and then adopted as templates to derive Ni3S4, Co3S4 and NiCo sulfides by a heat treatment and subsequently solvothermal sulphidation. Electrochemical measurements indicate that the obtained rod-like sulfides have excellent capacitance properties. Notably, Co3S4 exhibits a high specific capacitance of 1843 F g-1 at 1 A g-1, which decreases to 930 F g-1 at 20 A g-1 with a retention of circa 50%. While NiCo-S derived from NiCo-MOF-74 (Ni/Co=2:3) displays superior specific capacitance of 1910 F g-1 and 1160 F g-1 at 1 A g-1 and 20 A g-1, respectively, with a retention of above 60%. It shows that NiCo-S has higher specific capacitance and superior rate capability, which can be attributed to synergistic effects accompanying with the introduction of Ni.
Synthesis of Metal-Organic Framework-Derived Carbon Supported CoFe2O4 with Enhanced Oxygen Reduction Reaction Catalytic Properties
Authors: Li Zongzhao , Sui Jing , Zhang Qian , Yu Liyan , Dong Lifeng
Published: September 2020

Fe2Co-MOF is prepared through a simple hydrothermal reaction and calcination at 600oC for 2 h under N2 atmosphere. Then heating at 300oC for 2 h under air, a new composite catalyst CoFe2O4@carbon nanorods (CoFe2O4@CNR) is obtained. The composite structure can effectively alleviate the agglomeration of metal particles, make transition metal nanoparticles uniformly dispersed on graphitized carbon under the action of non-covalent bonds. Compared with traditional spinel structure compound CoFe2O4, the CoFe2O4@CNR nanocomposite shows excellent oxygen reduction reaction performances with an onset potential of -0.2 V vs. Ag/AgCl, a half-wave potential of -0.34 V, and a diffusion-limited current density of 5.05 mA.cm-2, which are better than CoFe2O4 with obvious superior difference of 100 mV, 90 mV and 3.07 mA.cm-2, respectively.
Hybrid Methodology for Efficient on the Fly (Re)Parametrization of Proton Exchange Membrane Fuel Cells Electrochemical Model for Diagnostics and Control Applications
Authors: Kravos Andraž , Kregar Ambrož , Katrašnik Tomaž
Published: September 2020

Successful parametrization and re-parametrization of the models used in PEMFC observer applications is instrumental to assure ideal control of the system. To increase ease of parametrisation and accuracy of parameter determination, this paper presents framework of twin analytical ansatzes for modelling PEMFC response in time and frequency domains sharing same set of calibration parameters. Owing to thermodynamically consistent modelling basis of the twin models, which are based on electrochemical model with state-of-the-art extrapolation capabilities and replication of the experimental data with one set of calibration parameters, they are valid in all current density regions. Furthermore, unique sharing of the calibration parameters enables unprecedented enrichment of the dataset that can be used to determine values of model’s calibration parameters with higher certainty or enhances identification of individual calibration parameters that are otherwise harder to be uniquely determined. Additionally, proposed hybrid methodology also enables a significant reduction in the measurement time and enable re-parametrization on the fly.
Investigation of Liquid Water Distribution under Temperature Gradient in PEFC Using in-situ X-ray Visualization
Authors: Komiyama Takahiro , Sasabe Takashi , Naito Hiroshi , Kawamura Katsuyuki , Suzuki Toshiyuki , Hirai Shuichiro
Published: September 2020

Polymer electrolyte fuel cells (PEFCs) are one of promising electric generating devices. It is necessary to understand the relationships between the water distribution and the temperature distribution in gas diffusion layers (GDLs) to improve the performance at high current density. To reveal the effects of cell temperature on liquid water accumulation inside PEFCs, in-situ X-ray measurement and numerical simulation were performed at 40, 60, 70, and 80°C. The measurement was carried out using a fuel cell having a serpentine-type flow field, ribs and flow channels of 300 μm width, and 1.5×3.0mm2 active area. Current density was kept 1.5 A/cm2 for 300 secs in all cases. Both the experimental results and the numerical results showed that the liquid water distribution in GDLs strongly depends on cell temperature. In the case of lower temperature, liquid water accumulated in a region near CL and MPL. While in the case of higher temperature, liquid water accumulated in a region near rib. In addition, the experimental results indicated that the vapor crossover depended on existence of liquid water in MPL, and it was shown that the amount of vapor crossover increased at lower temperature condition.
Modeling Impedance Caused by Ohmic Losses in High Surface Area Carbons in Polymer Electrolyte Fuel Cells
Authors: Darling Robert M. , Burlatsky Sergei F.
Published: September 2020

An analytical model for a blocking porous electrode is derived and used to predict how ohmic resistance inside micropores that host catalyst nanoparticles affects impedance. This paper revisits a widely used approach for estimating ionic conductivities of catalyst layers in fuel cells. An interfacial term associated with resistance in micropores is derived and added to the classical expression for the impedance of a blocking porous electrode at low frequency. The revised model predicts a physically realistic rise in the real component of the impedance at low frequency when reasonable pore dimensions and conductivities are used as inputs.
Effect of Reaction Products on the PEFC Catalyst Ink Property and Catalyst Layer Quality
Authors: Uemura Suguru , Sakai Katsunori , Sasabe Takashi , Matsumoto Hidetoshi , Sugimori Hidekazu , Shinohara Kazuhiko , Hirai Shuichiro
Published: September 2020

The catalyst layer is a critical component of proton exchange membrane fuel cells. Previous studies suggest that several reaction products were generated in the catalyst ink mixing process and it can effect on the structure of the catalyst layer. In this study, by identifying the reaction product contained in the catalyst ink and measuring those quantitative concentration, the effects on the catalyst ink and the catalyst layer were investigated. As a result, it was shown that acetaldehyde can affect the catalyst ink properties and the catalyst layer quality (micro-scale crack formation).
X-ray Imaging of Water Distribution in Cathode Electrode of PEFC and Control of Liquid Water Transport by Electrode Perforation
Authors: Nishida Kosuke , Funaoka Ryoichi , Furukawa Tatsuki , Kono Yudai
Published: September 2020

To alleviate water flooding in cathode electrodes of polymer electrolyte fuel cells (PEFCs), the modified electrode/channel structure with electrode perforation and channel hydrophilization has been proposed. This study investigated the liquid water distribution in the modified electrode with penetration grooves of an operating PEFC using X-ray imaging, and evaluated the effect of groove size on the water removal inside the electrode. The influence of the combinational electrode/channel structure on the cell performance was also demonstrated by the constant-current operation test. It was noted that the electrode perforation effectively reduces the water accumulation in the cathode GDL. An increase in the groove width enhances the in-plane water discharge towards the large groove. The combinational structure with electrode perforation and channel hydrophilization improves the limiting current density due to the positive through-plane water removal from the porous media to the flow channel.
Influence of Proton Activity Gaps between Electrodes on Open-Circuit Potential of H2/H2 and H2/Air Cells
Authors: Yoshida Toshihiko , Hojo TaKemasa , Jozuka Tetsuya , Matsuda Toshihiko , Danilovic Nemanja , Weber Adam Z. , Suzuki Toshiyuki
Published: September 2020

In polymer electrolyte fuel cells, proton activity thermodynamically correlates with the relative humidity (RH) as well as water uptake (λ) of the ionomer electrolyte. Proton activity differences between the two electrodes arise from RH control, and the activity terms of the Nernst equation could become significant. We investigated the effect of proton activity on open-circuit potentials (OCPs) and cell performance by controlling the RH of H2/H2 and H2/air cells, and on the kinetic current (ik
) using a rotating disk electrode (RDE). In the H2/H2 cell, OCPs were caused by the RH difference between electrodes; thermodynamic analysis was performed to rationalize the experimental OCP by correlating the RH and λ. The H2/air cell performance was low owing to the high resistance and low exchange current density at RH < 30%. The RDE measurements showed that ik
decreased with increasing concentrations (proton activities) of HClO4, H2SO4, and CF3SO3H (TFMSA).
Nature-Inspired Flow-Fields and Water Management for PEM Fuel Cells
Authors: Trogadas Panagiotis , Cho Jason I.S , Bethapudi Viswanath Sasank , Shearing Paul , Brett Dan , Coppens Marc -Olivier
Published: September 2020

A nature-inspired solution methodology is used to address flooding issues at high relative humidity (RH) in proton exchange membrane fuel cells (PEMFCs). The water management strategy is based on the mechanism employed by certain lizards, residing in arid environments, to drink water. A thin graphite plate with laser engraved capillaries is installed on our best performing lung-inspired flow-field based PEMFCs with 4 generations (N) and its performance at 100% RH is compared against commercial serpentine flow-field and lung-inspired flow-field (N = 4) based PEMFCs. Initial results demonstrate significant improvement in the performance and operating range of the lung-inspired flow-field based PEMFC when implementing this water management mechanism.
Development of an Oxygen Mass Transport Coefficient Measurement and Separation Method for Proton Exchange Membrane Fuel Cells
Authors: Buehre Lena Viviane , Suermann Michel , Hanke-Rauschenbach Richard , Bensmann Boris , Bethune Keith , St-Pierre Jean
Published: September 2020

In this work, we use a method to separate the total oxygen mass transport coefficient into molecular, Knudsen, and ionomer contributions. Therefore, limiting current density measurements are carried out as a function of the diluent gas (He, N2, CO2), temperature (30, 50, 80°C), relative humidity (50, 75, 100%), and oxygen concentration (1, 3, 5, 7%) using state of the art membrane electrode assemblies with three platinum loadings (0.05, 0.1, 0.15 mg/cm2). As expected, the molecular diffusion coefficient is independent of the platinum loading, but increases with temperature to a varying degree depending on the humidity level. On the other hand, the Knudsen diffusion coefficient increases with increasing electrochemical active surface area and temperature, and with decreasing relative humidity. The separation procedure includes a novel feature to isolate the ionomer mass transport resistance. Its interpretation as well as the method’s reliability are critically questioned using operating condition dependencies.
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: September 2020

To spread polymer electrolyte fuel cells, improving the cell performance is required. The cell performance depends on various factors. One of the factors that lowers cell performance is proton transport resistance in catalyst layers. Catalyst layers have multiscale structures which influence on proton transport resistance. In this study, to investigate the relationship between structures of catalyst layers and cell performances, mass transport and chemical reactions are calculated in the 3D catalyst layer models. The information about the ionomer thickness dependence on the diffusion coefficient of protons based on the molecular dynamics simulation is introduced to transport calculation in order to analyze nanoscale structure influence on the cell performance. As a result, we have found that the output voltage increases over the whole range of current density with increasing ionomer/carbon ratio considering ionomer thickness dependence. This result suggests that the nanoscale structure in catalyst layer has a large influence on the cell performance. Furthermore, cell performance analysis with ionomer thickness distribution based on experimental values is conduced in this study. The result suggests that contribution of nanoscale proton transport characteristic increases by improving the ionomer distribution model.
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: September 2020

The structure of cathode catalyst layer (CCL) is critically important for improved performance, durability, and stability of polymer electrolyte fuel cells (PEFCs). In this research, two types of CCL are designed to do comparison, one is flat structure, and the other is 3D structure. Inkjet printing method was used in this research to fabricate the structures of cathode catalyst layer, then the MEA was taken to do electrochemical evaluation. The results showed that electrochemical properties of CCL with 3D structure were better than the flat one through I-V curve and impedance. Overall, it is demonstrated that with a 3D structure, it is possible to achieve favorable morphological and microstructural characteristics.
NO/NO2 Contamination of HT-PEM Fuel Cells – A Degradation Study with Evaluation of Regeneration Strategies
Authors: Schonvogel Dana , Büsselmann Julian , Wagner Peter , Dyck Alexander
Published: September 2020

High temperature PEM fuel cells (HT-PEMFCs) are characterized by enhanced tolerances towards reaction gas impurities like CO. However, impacts of NOx stemming from ambient air are not completely understood yet. This study provides the analysis of performance changes in dependence of several operational modes after cathode feed contamination using 10 ppm of NO or NO2. Open circuit voltage, N2 supply, cyclic voltammetry, and an increased temperature of 180°C were used to investigate cell degradation and to evaluate their usability of regenerating the cell. NO and NO2 contamination resulted in voltage loss rates greater than 350 µV h-1. This dramatic impact on the HT-PEMFC was verified here to be irreversible without any regenerating effect by the applied cell modes. This evidences that less a suitable operation strategy but more an engineering approach in view of appropriate filter systems is a way to solve NOx contamination issues.
High Efficiency Operation Conditions in Polymer Electrolyte Fuel Cell System with Hydrogen Circulation
Authors: Ma Yulei , Yamaguchi Kazuhiro , Kageyama Miho , Kawase Motoaki
Published: September 2020

Water management is an essential problem to achieve PEFC’s maximum performance. Hydrogen circulation is one of the methods to humidify the cell with water produced in oxygen reduction reaction (ORR), while the overall hydrogen utilization can be 100%. Additionally, by controlling the operating conditions, such as gas flow direction, one-pass conversion, and humidifying temperature, the cell performance can be improved. Water transport distribution along the gas channel was analyzed by numerical simulation, and the competition of the proton transfer resistance, the oxygen transfer resistance, and the reaction resistance were discussed quantitatively. The results indicate that the countercurrent mode attains better performance due to higher moisture content at high humidification conditions, and the co-current mode can be chosen at low humidification condition. The one-pass conversion higher than 0.30 is required to humidify the cell sufficiently.
Combined Two-phase Co-flow and Counter-flow in a Gas Channel/Porous Transport Layer Assembly
Authors: Beale Steven B. , Andersson Martin , Weber Norbert , Marschall Holger , Lehnert Werner
Published: September 2020

This paper considers a detailed numerical analysis of combined liquid-gas co-flow in a gas channel, with liquid-gas counter-flow in a porous transport layer, as is typically found on the cathode side of a polymer electrolyte fuel cell. The geometry is obtained by digital reconstruction of nano-computer tomography images. From this, the domain is tessellated with an unstructured castellated or octree mesh, upon which the equations of mass and momentum are solved by means of a volume of fluid method. Liquid water is produced from an electrode where gaseous oxygen is simultaneously consumed by electrochemical reduction; Liquid-gas counter flow in the porous transport layer results in liquid drops being entrained in co-flow in the gas channels and convected by the gas downstream.
Development of an Open-Source Solver for Polymer Electrolyte Fuel Cells
Authors: Zhang Shidong , Beale Steven B. , Shi Yan , Janßen Holger , Reimer Uwe , Lehnert Werner
Published: September 2020

In this study, a three-dimensional, steady-state, non-isothermal, two-phase-flow computational fluid dynamics model is developed for polymer electrolyte fuel cells. It is implemented into an open-source library. All major transport processes, heat and mass transfer, electron/proton transfer, electrochemical reactions are taken into account and solved simultaneously. A two-phase Eulerian-Eulerian algorithm is applied to describe flow in the gas channels on the cathode side of the cell. The present model is validated experimentally, by means of polarization curves and local current density distributions. Fairly good agreement is obtained for a variety of current densities. It is observed that the local current density distributions change as a function of gas humidity.
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: September 2020

Direct formic acid fuel cells (DFAFC) offer a potentially sustainable alternative to the batteries in portable devices due to their higher charge capacity, higher 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, liquid) and the product (carbon dioxide, gas) must be optimized. By incorporating 15 and 25 wt% MgO, the electrochemical surface area (ECSA) of the catalyst layer increased from 21.8 cm2 g-1 to 62.7 and 85.8 cm2 g-1, respectively. Furthermore, at higher potentials the porous catalyst layers improved cell performance by 86%. These improvements in ECSA and cell performance are possibly due to an enhancement of the triple-phase boundary.
Pore-Filled PEMs from Poly(Phenylene Sulfonic Acid)s and Electrospun Poly(Phenylene Sulfone) Fiber Mats
Authors: Hossain M. , Shang Z. , Wycisk Ryszard , Pintauro Peter N.
Published: September 2020

A novel, composite, non-PFSA-based fuel cell membrane has been fabricated using a pore filling technique. The membrane consists of a mechanically stabilizing skeleton from an electrospun poly(phenylene sulfone) (PPSU) fiber mat and a thermally crosslinkable poly(phenylene sulfonic acid) (cPPSA) proton conducting ionomer that fills the interfiber voids. cPPSA copolymer was synthesized using Ullmann coupling copolymerization of 4,4-dibromobiphenyl 3,3-disulfonic acid with 1,4-dibromobenzene-2,5-disulfonic, followed by grafting a certain fraction of backbone sulfonic acid groups with biphenyl linker. The PPSU fiber mat was electrospun from NMP/acetone solution. Pore-filling was carried out by pouring a solution of cPPSA in methanol over the mat, followed by heating at 70oC to evaporate solvent. The cPPSA was crosslinked by an additional heating step, in a vacuum oven at 210oC for 5 hours. The resultant membrane had excellent proton conductivity, 5 times greater than that of Nafion® 211 in the 40-90% RH range at 80oC.
(Invited) Liquid Chromatography/Mass Spectrometry Analysis of Effluent Water from Perfluoroalkylsulfonic Acid (PFSA) Membrane Degradation Studies at Accelerated Durability Test Conditions
Authors: Yandrasits Michael , Komlev Andrei , Kalstabakken Kyle , Lindell Matthew , Kurkowski Michael
Published: September 2020

Perfluoroalkylsulfonic acid (PFSA) ionomer membranes degrade under accelerated testing conditions such as open circuit voltage (OCV). Analyzing effluent water for fluoride ion release rate is a primary method for evaluating the membrane degradation rate. However, many proposed degradation reaction pathways should result in the release of small molecule polymer fragments. Liquid chromatography/mass spectrometry (LC/MS) methods are well suited to analyze for these fragments and provide insight into the degradation reactions. Accelerated OCV durability tests were conducted on membrane electrode assemblies made with 3M Ionomer or Nafion™ membranes. Effluent water was analyzed for fluoride, sulfate, and trifluoroacetic acid by ion chromatography (IC) and other polymer fragments by LC/MS. The results of this study imply measurable degradation occurs at the tertiary fluoride on the polymer backbone or the ether oxygen connecting the side chain to the backbone. In Nafion™, these structural features also exist in the ionomer sidechain.
Measurement of Proton Conductivity and Water Diffusivity of Nafion Membrane at Subzero Temperatures
Authors: Tajiri Kazuya , Shrivastava Udit , Verma Saumitra
Published: September 2020

Objective of this study is to obtain the water transport characteristics of Nafion at subzero temperature for automotive application. To achieve this objective, three different properties of the membrane are quantified: water content, proton conductivity, and diffusivity. Water content is computed from measuring the difference in weight between the dry membrane and humidified membrane at each relative humidity and subzero temperature. To measure the weight a special environmental cell was developed and integrated with the microbalance. To quantify the proton conductivity and the water diffusivity, a conductivity measurement electrochemical cell with segmented electrode is designed and fabricated to measure the high frequency resistance across the membrane at test conditions. An equation is developed for water content in terms of relative humidity and temperature. Equations for proton conductivity and water diffusion coefficient are presented in terms of water content and temperature. These equations are developed from experimental data obtained between 243K and 303K at different relative humidity.