bluuebunny/crossref_metadata_2025 huggingface dataset:
Non-Pressure Joining Method for Zn-Al Solder by Pre-Ultrasonic Bonding
Authors: Tanisawa Hidekazu , Takahashi Hiroki , Kato Fumiki , Koui Kenichi , Sato Shinji , Murakami Yoshinori , Sato Hiroshi
Published: February 2020

Introduction

In this paper, we propose non-pressure soldering method during reflow process for Zn-Al solder by breaking oxide film on its surface in advance of heating. SiC power semiconductors can operate at a temperature of 200 °C or more. The temperature difference between junction of the semiconductors and ambient air makes effective heat dissipation, therefore the cooling system that occupies a large volume in an semiconductor power module can be minimized. If we use these advantages, we have to use high melting point solder such as Zn-Al alloy solder. But the solder, which is usually provided as preformed sheet, have stiff natural oxide film on its surface, therefore applying pressure to the preformed sheet during reflow process is necessary to break the oxide film for secure soldering. New proposed method don’t need these way. The reflow process becomes simplified.

Method and Result

Effectiveness of the joining method is described with Fig. 1. This figure shows fracture surfaces of samples after shear test. The samples are attempted to join a circuit pattern and a SiC dummy chip by soldering of Zn-Al preformed solder. The sample (a) is not given ultrasonic bonding before reflow. The preformed sheet of the sample (b) is attached to both the chip and the circuit pattern by ultrasonic bonding in advance of reflow. Then the two samples are heated in the same reflow process and evaluated based on shear test. As a result of the test, The chip of the sample (a) was easily peeled off. On the other hand, shear strength of (b) is 60.4 MPa on average. The reason of different results is described below. Since the natural oxide film was broken by ultrasonic bonding, and a newly formed surface is exposed, the solder and both the chip and the circuit pattern are mutually joined. When the sample is heated, the solder in the preformed sheet melts and overflows from the bonding surface. Therefore the chip and the circuit pattern are joined by soldering. Without breaking oxide film like (a) in advance of reflow, solder joint doesn’t occur. The proposed soldering method has a potential to be low-cost and simple process for packaging of SiC power semiconductors.

Acknowledgment

This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), "Next-generation power electronics/Consistent R&D of next-generation SiC power electronics", and "the Novel Semiconductor Power Electronics Project Realizing Low Carbon Emission Society" (funding agency: NEDO).

Figure 1
(Invited) Investigation on the Carbon Supply in the Top Seeded Solution Growth of SiC Bulk Crystal
Authors: Jeong Seong Min , Ha Minh-Tan , Yoon Ji-Young , Lee Ji Eun , KIM Byeong-Guen , Lee Myung Hyun , Kim Younghee
Published: February 2020

Top Seeded Solution Growth (TSSG) method is assumed to be a promising bulk growth technique for high quality SiC substrate. Generally, the sources of Si and C in the TSSG method are Si chunk and Graphite crucible, respectively. The crystal growth through TSSG method is achieved by the precipitation of solid SiC from C dissolved a Si melt on a seed crystal located at the top of the liquid. The C solubility in pure Si liquid is extremely low, so that metallic solvents such as Cr, Ti and Fe were added to increase the C solubility in Si liquid. Consequently, the growth rate of SiC crystal through the TSSG method has been reached up to 2 mm/hour. However, as increasing C solubility, the loss of the graphite crucible became another critical issue. Because the C source in the TSSG method is the graphite crucible, the reservoir of Si liquid, the increase of C solubility means the decrease of the crucible durability. The graphite crucible durability influences not only on the thickness of the grown crystal, but also on the temperature distribution in the reactor.

In this study, we introduce our recent progress on the carbon supply in the TSSG method to produce a reliable long-term growth technique. The interface between graphite crucible and Si liquid was first studied from point of thermodynamics. Considering the reaction between graphite crucible and Si liquid, several crucible structures with different materials and designs were suggested for the improvement of TSSG method based on theoretical understanding.
TEM Study on Microstructure of Stacking Fault Nucleation Sites in 4H-SiC P-I-N Diodes
Authors: Nakamura Yu , Frushou Tomoaki , Yamamoto Shigehisa
Published: February 2020

Silicon carbide(SiC) is an attractive material for high-power applications. However, a number of crystal defects in SiC substrates and epitaxial layers limit commercialization of SiC-based devices. For example, the expansion of Shockley-type stacking faults(SFs) produced by dissociation of basal plane dislocations(BPDs)　in 4H-SiC epitaxial layers cause the degradation(forward voltage drop) of p-i-n diodes under forward bias. Since it has been found that the nucleation sites of SFs are pre-existing BPDs, it is necessary to eliminate the BPDs in epitaxial layers. There are two types of nucleation sites of the SFs in different shapes. One is the “line source”, and the other is the “point source”. The line source is the basal plane screw dislocations which parallel to the off cut direction with Burgers vector 1/3[11-20] in the epitaxial layers. Their shape can be clearly observed by electroluminescence or X-ray topography as a straight line. On the other hand, the dislocation structure of the point source is not well known. To eliminate the point source of the SFs, it is necessary to know more about microstructure of their nucleation sites and origin. In this work, we investigated the microstructure of the “point source” and its origin in p-i-n diodes using photoluminescence imaging and transmission electron microscopy. As a result, we found a short basal plane screw dislocation segment(about 300 nm) along [-12-10] direction at the nucleation site of the SF located on a little above the epilayer/substrate interface. The BPD segment was converted from a part of the threading edge dislocation(TED). The TED/BPD/TED structure would be formed during epitaxial growth or subsequent thermal treatment.
Investigation of Penetration Depth and Defect Image Contrast Formation in Grazing Incidence X-ray Topography of 4H-SiC Wafers
Authors: Yang Yu , Guo Jianqiu , Goue Ouloide Yannick , Raghothamachar Balaji , Dudley Michael , Chung G , Sanchez E , Maning I
Published: February 2020

Synchrotron X-ray Topography has been shown to be a vital tool for the nondestructive characterization of defects in 4H-SiC crystals [1-2]. Techniques utilizing reflection geometry are particularly useful for discerning defects at different depths below the crystal surface. For example, in studying defects in SiC pin diode structures, which typically comprise a buffer layer homoepitaxially grown on a substrate, with the drift layer grown on top of the buffer layer, it is important to be able to discriminate the depth at which particular defect configurations reside. This is particularly important for the characterization of defects resulting from relaxation processes such as interfacial dislocations and half loop arrays [3]. There are generally two approaches adopted depending of the level of perfection of the crystal. In deformed regions (such as around dislocation cores) the penetration depth is simply determined by photoelectric absorption. In perfect regions it is determined by extinction. Extinction contrast predicts particularly shallow penetration depths in these geometries, whereas kinematical theory predicts much deeper penetration depths. However, even under the kinematical diffraction conditions, the penetration depth calculated theoretically by just considering a dislocation as a single line is much smaller than the value measured experimentally. In our study we compared two models involving different diffracting volumes. In one model, the volume is defined by the misorientation larger than the FWHM of the rocking curve in the vicinity of the dislocation core. In another model, the contrast is naturally formed by simulating the photoelectric absorption profile of the diffraction beam according to the local lattice plane tilt by assuming the dislocation image formation is dominant by orientation contrast. We will present a comparison between measured penetration depths in grazing incidence geometry with these two approaches and develop an optimized model to explain our observations.

[1] Muller, G.S., et al., Volume production of high quality SiC substrates and epitaxial layers: Defect trends and device applications. Journal of Crystal Growth, 2012. 352(1): p. 39-42.

[2] Dudley, M., et al., Characterization of 100 mm diameter 4H-Silicon carbide crystals with extremely low basal plane dislocation density, Material Science Forum,2001. 65: p. 353-356.

[3] Wang, H., Dudley, M., et al., Studies of the Origins of Half-Loop Arrays and Interfacial Dislocations Observed in Homoepitaxial Layers of 4H-SiC. Journal of Electronic Materials, 2015. 44(5): p. 1268-1274
Design and Fabrication of Ag / AgCl Composite Reference Electrode Based on Microfluidic Chip
Authors: Chen Li , Zheng Xiaolin , Hu Ning , Liao Yanjian
Published: February 2020

Polydimethylsiloxane (PDMS) has been widely used in chip fabrication. However, PDMS itself is a non-conducting polymer, and the adhesion between metal and PDMS is also really weak. Hence, integrating microelectrodes on PDMS microchips has become a technical bottleneck in many applications such as miniaturized electrochemical (EC) sensing systems.
A new method was presented to integrate Ag/AgCl electrode on microfluidic chips by using a new conductive composite. It was made of the mixture of PDMS and silver powders, so good electrical conductivity was achieved. Two chloridizing methods were selected to fabricate Ag/AgCl reference electrode. In the electrolysis method, Ag-PDMS composite electrode (as anode) was electrolyzed for 24 hours in saturated KCl solution. In the second one, silver chloride powder was uniformly mixed with silver powders and PDMS pre-polymer, and then cured 1 hour at 90 oC. These composite materials could be efficiently shaped as different electrode structures. Its potential response was measured in different chloride solutions, and its stability was also measured in water (10 days) and 3 mol/L KCl solution (14 days), respectively. Electrochemical characteristics of the electrode were also analyzed in potentiometric and amperometric measurements.
SEM pictures showed that the surface of the electrode was fluffy and of rough morphology, which might increase its contacting area with solution, so as to improve the measuring adequacy and accuracy. The electrodes had a fast and stable potential response toward Cl− concentration change.The much tiny potential fluctuation indicated that the composite electrode was stable, which could be served as a good reference for EC measurements. Its CV curves showed its performance was almost as good as a standard reference electrode.
Based on its excellent properties, the composite material can be widely applied in microfluidic field. The new method has many advantages such as simple operation, low cost and no special environmental requirements.
Investigation of MPL Effect on PEFC Cold Start
Authors: Zhou Jiaxun , Xie Xu , Jiao Kui
Published: February 2020

Cold-start (subzero startup) capability of polymer electrolyte fuel cell (PEFC) is of great importance. In this paper, the effects of the micro-porous-layer (MPL), varying start-up temperatures, start-up current density variation on the cold-start operation are investigated. We found PEFC with anode micro-porous-layer (AMPL), compared with one with CMPL, has higher possibility of successful cold start. By analyzing the anode and cathode pressure revolution of PEFC, the effect of MPL on the super cooled water removal and ice formation at different temperatures (-7,-10,-15 and-20℃) are discussed . In addition, we investigate the ice distribution in MEA through the X-ray device, by comparing the initial image when fuel cells shut down (before the produce the water) and final image after failed cold-start. We also explore the negative voltage phenomenon in the initial process of cold-start operation.
Effect of Flow-Field Structure on the Performance of Hydrogen/Bromine Redox Flow Batteries
Authors: Oh Kyeongmin , Ju Hyunchul
Published: February 2020

A hydrogen/bromine regenerative fuel cell involves complex water and heat transport phenomena during charge and discharge that significantly affect the degree of electrolyte dehydration, electrochemical reactions, proton transport, the concentrations of bromide complexes at the hydrogen electrode, etc. Therefore, developing innovative water and heat management schemes is critical to improve the uniformity of key distributions inside a cell as well as to enhance cell performance and durability. In this work, a three-dimensional, transient, two-phase, non-isothermal hydrogen/bromine fuel cell model is developed by rigorously accounting for two-phase transport and various heat generation mechanisms including irreversible heat, entropic heat, and Joule heating. The model is applied to a 25 cm2 cell in order to precisely investigate water transport and thermal aspects under various charge and discharge conditions. A parallel computing methodology is employed to handle large-scale simulations involving millions of grid points. The large-scale simulations are able to provide extensive multidimensional contours of species concentration, temperature, and current density, assisting in identifying optimal water and thermal management strategies based on a typical geometry of a real-scale hydrogen/bromine cell.
On-Board Hydrogen Powered Proton Exchange Membrane Fuel Cells
Authors: Sankir Mehmet , Kara Kenan , Abdullayeva Nazrin , Demirci Sankir Nurdan
Published: February 2020

One of the main concerns in the commercialization of fuel cell powered electrical vehicles is the on-board storage of hydrogen. Although pressurized vessels used in conventional systems provide higher hydrogen feeding rates, they have disadvantageous due to their extra weight and higher volumes in the systems. In this study, we report direct hydrogen generation from chemical hydrides, which does not require pressurized vessels. Self-dehydrogenation of chemical hydrides is a very slow process. In order to increase hydrogen generation kinetics, highly active and stable catalysts should be used. Although precious catalysts such as platinum and ruthenium meet these demands, their higher cost and recycling problems make them less useful in the fuel cell area.

A novel approach will be presented here for the fast, safe and stable hydrogen generation for fuel cells. This was achieved by circulating the acid and the chemical hydride solutions which were separated by the disulfonated poly(arylene ether sulfone) copolymer membrane. Since the rate of proton transfer is a function of ion exchange capacity (IEC) and scales with the degree of disulfonation of membranes, tailoring the IEC of membranes is crucial during the hydrogen generation. Therefore, membranes with varying sulfonation degrees (25-45 molar %) have been prepared and their hydrogen generation rates have been evaluated. Additionally, it was found that the type of acid and acid concentration influenced the hydrogen generation rates.

The effect of temperature on hydrogen generation performances was also investigated. When temperature exceeded the 80 °C, the hydrogen generation performance of N212 (the state of art membrane, Nafion™) was decreased due to the morphological changes occurred at this temperature. However the performances of our membranes without morphological relaxations continued to increase at higher temperatures. Furthermore, we run a proton exchange membrane fuel cell with our hydrogen generator and demonstrated continuous hydrogen generation more than 400 h. As a result, highly efficient on-board hydrogen generator for electrical vehicles powered by fuel cells presented here completely eliminates the need for separate facilities for producing hydrogen.
Electron-Conducting Perovskites As Oer Electrocatalysts in Alkaline Medium
Authors: Jain Anchal , Ramani Vijay K
Published: February 2020

Hydrogen is an attractive energy carrier and electrolysis of water is an important route to hydrogen generation. Alkaline water electrolysis is preferred over electrolysis in acidic medium due to the possibility of lowering stack costs and enhancing he library of stable electrocatalyst materials available for the electrochemical reactions. The high anode overpotential arising from the sluggish oxygen evolution reaction (OER) has led to significant interest in developing stable and active OER electrocatalysts. IrO2 (state of the art catalyst), RuO2 and PGM-based pyrochlores are suitable catalyst materials that exist today, but there is benefit in finding cost-effective alternatives.

In this study, perovksite oxides of the form La[Ni(1-x-y)CoxFey]O3 (where 0≤x≤1 and 0≤y≤1) were synthesized by the co-precipitation method. Many of these perovskites exhibited electron conductivities greater than 0.1S/cm, eliminating the need to add carbon for OER studies and implying the likelihood of making conducting electrodes without additives with these materials. The electrocatalytic activity for the OER was studied using a rotating disk electrode (RDE) in 0.1M KOH. A decrease in OER activity was observed with increasing cobalt content in perovskites with y=0. A similar trend was observed for perovskites containing iron with x=0. All the perovskite electrocatalysts exhibited higher onset overpotentials than state-of-the-art-IrO2 but many exhibited significantly higher specific activities than IrO2 at 1.6V vs RHE. Specific activities of 0.21 A/m2, 7.08 A/m2 and 1.01 A/m2 at 1.6V vs RHE and 0.05 A/m2, 0.1 A/m2 and 0.07 A/m2 at 1.5V vs RHE were obtained for IrO2, LaNi0.6Co0.4O3 and LaNi0.6Fe0.4O3 respectively.
Alkaline Stability of Poly(phenylene oxide) Based Anion Exchange Membranes Containing Imidazolium Cations
Authors: Wang Zhongyang , Parrondo Javier , Ramani Vijay K
Published: February 2020

There is a growing interest in using anion exchange membranes (AEMs) as separators in alkaline membrane fuel cell (AMFCs) and in other energy conversion and storage systems such as redox flow batteries (RFBs), alkaline water electrolyzers (AWEs) and reverse electrodialysis (RED) cells. The most commonly used cation group in AEMs is the benzyl trimethylammonium cation. However, it had been shown that quaternary ammonium-based AEMs are sensitive towards Hofmann elimination (1) and direct nucleophilic elimination reactions (2) that result in loss of ion exchange capacity (IEC) and ionic conductivity. To solve the alkaline stability issue inherent to quaternary-ammonium-group-containing AEMs, several alternative cations, including imidazolium, benzimidazolium, guanidinium, phosphonium and metal cations have been proposed and investigated.

Imidazolium-based AEMs have drawn researchers’ interests mainly because of their high hydroxide ion conductivities. In addition to 1-methylimidazole and 1,2-dimethylimidazole, a series of modified imidazole bases, namely 1-heptyl-2-methyl-1H-imidazole and 1-dodecyl-2-methyl-1H-imidazole, were synthesized, wherein these modified bases contained a long alkyl chain at the N-3 position . Anion exchange membranes (AEMs) were prepared with derived imidazolium cations either grafted onto the benzyl position of poly(phenylene oxide)(PPO) or affixed using a hexyl (6-carbon) spacer chain. AEMs with cations affixed to the benzyl position were synthesized by bromination of PPO followed by reaction with the imidazole bases. AEMs with the hexyl spacer were synthesized by Friedel-Crafts acylation of 6-bromo-1-hexanoyl chloride on PPO, followed by reduction of the ketone group and reaction with the imidazole bases. The alkaline stability of the various imidazolium-cation-containing AEMs was evaluated by: 1) measuring the change in IEC after immersion in 1M KOH at 60°C for up to 14 hours; and 2) using 1-D and 2-D NMR spectroscopy to investigate any changes in chemical structure. Both NMR and IEC experiments showed that PPO functionalized with modified imidazolium cations (with long alkyl chain at the N-3 position) did not exhibit better alkaline stability than benchmark AEMs made with 1-methylimidazole. However, the use of a six-carbon spacer did improve AEM alkaline stability when the AEM was prepared directly using 1-methylimidazole and 1,2-dimethylimidazole, but not when the AEM was prepared with the modified imidazolium cations (with a long alkyl chain at the N-3 position).

References

1. C. G. Arges and V. Ramani, Journal of The Electrochemical Society, 160, F1006 (2013).

2. S. Chempath, B. R. Einsla, L. R. Pratt, C. S. Macomber, J. M. Boncella, J. A. Rau and B. S. Pivovar, The Journal of Physical Chemistry C, 112, 3179 (2008).
Bilayer Cathode Structure to Improve Ice Tolerance in PEM Fuel Cells
Authors: Lei Chao , Chuy Carmen , Ngo Tran , Wong Sharon , McDermid Scott
Published: February 2020

In polymer electrolyte fuel cells, the freezing of product water deteriorates fuel cell performance at sub-zero temperatures. When ice forms in the membrane electrode assembly (MEA), catalyst sites are blocked from oxygen resulting in large portions of the MEA to become inactive during Freeze Start- up (FSU).

Improving the ice tolerance of the CCM is important to increase the robustness of the MEA to Freeze-Start up failure mechanisms. It is believed that the cathode catalyst layer is one of the key sub-components for ice tolerance, which is the ability of the MEA to accumulate ice before the MEA becomes inoperable. Isothermal Constant Current (ICC) measurements in units of (C/cm2) are commonly used to indicate this capability.

ICC data shows a clear correlation of ice tolerance with thickness of the catalyst layer. However, many of the control factors which can be used to improve ice tolerance in MEAs by increasing the thickness of the catalyst layer are limited by design constraints related to material cost and performance trade-offs. It would be difficult to achieve catalyst layer thickness >20um without seriously impacting performance under hot conditions or mass transport at high current densities.

Using a carbon/ionomer layer adjacent to the cathode catalyst layer allows to tune-in cathode attributes related to water management and ice distribution under freezing conditions, such as: thickness, porosity, hydrophilicity, interfaces and electric resistance. By changing attributes of the carbon bilayers separately from the cathode catalyst layer, the impact to the ORR reaction under normal or hot/dry operating conditions and mass transport region can be minimized.
Toughened Membrane/Catalyst Layer Interface with Mechanical Nano-Fastener for Hydrocarbon Membrane Based Polymer Electrolyte Membrane Fuel Cell
Authors: Kim Hee-Tak , Yuk Seongmin
Published: February 2020

Polymer electrolyte fuel cells (PEMFCs) have been spotlighted as one of the promising eco-friendly energy technologies for stationary and automotive applications owing to zero CO2 emission, high energy density and moderate operation conditions. In this technology sector, polymer electrolyte membrane, one of the key components of PEMFC, has been intensely studied for several decades. Conventionally,perfluorinated sulfonic acid (PFSA) membranes like Nafion are used due to their high proton conductivity and mechanical stability. However, their high cost has been pointed out as a significant drawback interrupting mass commercialization of fuel cell electric vehicle. In this regard, hydrocarbon (HC) membranes, as cheaper alternatives, have been intensively studied in replacing PFSA membrane.

However, until now, the challenge in adopting cost-effective HC membrane for PEMFCs has been the poor interfacial adhesion between catalyst layers (CLs) and HC membrane, which causes the membrane to delaminate easily, losing efficiency with use. Here, we present scalable mechanical nano-faster featured by three-dimensional interlocked interfacial structure between HC membrane and PFSA-based CL as a novel strategy to tackle the interfacial issue. It is realized by forming nano-porous skins on the both side of HC membrane and successively fiiling the pores with PFSA ionomer with scalable wet coating methods. The interlocking interface tightly binds the HC membrane and CL owing to its highly-interlocked ball and socket joint structure. The interfacial adhesion is dramatically enhanced by 37-fold with the nano-fastener. The membrane electrode assembly (MEA) with the three-dimensional interlocking interface exhibits 17 times higher durability than that with flat interface, paving a way to realize highly robust and cost-effective HC membrane-based PEMFCs for automotive use.
External Reinforcement of Hydrocarbon Membrane By Combining Toughened Catalyst Layers and Interlocking Interfaces for High Mechanical Robustness of PEMFC
Authors: Yuk Seongmin , Lee Dong Hyun , Choi Sungyu , Doo Gisu , Kim Hee-Tak
Published: February 2020

For decades, in polymer electrolyte membrane fuel cell (PEMFC) field, hydrocarbon (HC) membranes have been constantly studied to replace perfluorinated sulfonic acid membranes owing to their low cost and high fuel efficiency. However, adopting HC membranes to practical PEMFCs has been not successful due to their poor mechanical stability which causes a mechanical failure with generating pin-hole under repeated volume expansion/shrinkage during cell operation. Conventionally, the problem has been addressed by inserting a porous mechanical supporter in HC membrane which is denoted as ‘internal reinforcement’. However, the introduction of the inert support decreases proton conduction and increases membrane cost.

Here we present an external reinforcement of HC membrane as a new strategy to enhance mechanical durability of HC membrane. It features the incorporation of mechanically tough porous fibrous network into catalyst layers and the strong connection of HC membrane and the toughened catalyst layers with introducing an interlocking interface. The mechanically toughened catalyst layers and interfaces can effectively mitigate the volume change of HC membrane, lowing the membrane failure. Under an accelerated humidity cycling test, the externally reinforced HC membrane exhibits an enhanced durability compared to un-reinforced counterpart. Furthermore, contrary to the internal reinforcement, the external reinforcement strategy does not cause any loss of proton conductivity of HC membrane. Therefore, the external reinforcement coupled with toughed catalyst layers and interfaces can provide an effective way to enhance durability with preserving the proton conductivity of pristine HC membrane.
High-Performance Polymer Electrolyte Membrane Fuel Cells Based on Fe/N/C ORR Catalyst
Authors: Wang Yu-Cheng , Lai Yu-Jiao , Zhou Zhi-You , Sun Shi-Gang
Published: February 2020

One of the barriers hindering the commercialization of polymer electrolyte membrane fuel cell (PEMFC) is the high cost, in large part due to the expensive Pt-based catalyst for both the cathodic oxygen reduction reaction (ORR) and anodic hydrogen oxidation. The ORR requires more Pt catalyst than anodic hydrogen oxidation because of the sluggish kinetics. Therefore, ternary Fe/N/C material, as a non-Pt catalyst for ORR, is being studied intensively as one of the most promising alternatives for expensive Pt-based catalysts. However, the catalytic activity of Fe/N/C materials is considerably inferior to that of Pt-based catalyst in acidic medium. What’s more, the stability of Fe/N/C is poor and the decay mechanism remains unclear.

Here, we found that iron salts can also greatly influence the ORR activity of Fe/N/C, and the use of Fe(SCN)3 instead of FeCl3 can nearly double the ORR activity. The improvement may be correlated with S doping and high surface area. The Pmax of PEMFC using this improved catalyst reached 0.94 and 1.03 Wcm-2 at 1 and 2 bar back-pressure, respectively. Moreover, we find that oxidized environment in PEMFC, mainly caused by oxygen, high voltage and hydrogen peroxide, can deteriorate Fe/N/C catalyst.

Figure 1
Iron-Polypyrrole Electrocatalyst with Remarkable Activity and Stability for Oxygen Reduction Reaction
Authors: Tran Thanh-Nhan , Song Min Young , Yu Jong-Sung
Published: February 2020

A new facile template-free method is presented to synthesize Fe-treated N-doped carbon (Fe/N-C) catalysts for oxygen reduction reaction (ORR) by employing a synthesis protocol of pyrolysis-leaching-stabilization (PLS) sequence of polypyrrole in the presence of ferric source, which serves dual purposes of an oxidant for pyrrole polymerization and an iron source. Each step in the PLS sequence is assessed in detail in terms of related structural properties of resulting carbon catalysts, and their effects on ORR activities are elaborated to confirm the validity of the current synthesis protocol. It is found that as-prepared carbon catalyst exhibits outstanding high catalytic activity in both alkaline and acidic conditions. The carbon catalyst prepared at pyrolysis temperature of 900 oC (FePPyC-900) shows remarkably high ORR activity with onset potential of 0.96 V (vs. RHE), which is similar to that of Pt/C, whereas the half-wave potential (E1/2) of FePPyC-900 is 0.877 V, which is more positive than that of Pt/C at the same catalyst loading amount in alkaline condition. Furthermore, the FePPyC-900 catalyst also illustrates exceptionally high activity in acidic condition with onset and half-wave potentials, which are almost comparable to those of the state-of-the-art Pt/C catalyst, which is rarely observed for non-Pt based carbon catalyst. In addition, the FePPyC-900 catalyst displays much better stability and methanol tolerance than the Pt/C and exhibits four electron transfer pathway in both alkaline and acidic conditions. Such extraordinary high ORR activity and stability the FePPyC samples can be attributed to the implementation of extra stabilization step in addition to conventional sample preparation steps of pyrolysis and subsequent leaching in current PLS synthesis protocol as well as to the use of highly conducting PPy as a single precursor of carbon and nitrogen in the presence of Fe.
Preparation of Iridium-SnO2/VGCF Electrocatalysts for Water Electrolysis
Authors: Anai Hiroki , Matsuda Junko , Noda Zyiyun , Tachikawa Yuya , Hayashi Akari , Ito Kohei , Sasaki Kazunari
Published: February 2020

Introduction

Hydrogen energy has been paid more attention since the commercialization of FCVs. In order to further distribute FCVs, efficient production of hydrogen fuel becomes important. Water electrolysis is one possible method to produce hydrogen without emitting any carbon dioxide. Moreover, water electrolysis is the best technology to successfully use excess renewable energy, which is often hard to use due to the strong energy fluctuation. With this method, renewable energy can be converted into hydrogen, which can be stored for a long time and used anytime in any places.

Among various water electrolysis systems, the system based the polymer electrolyte has many advantages. Easy and rapid starting/stopping of the system is possible. Also, there exist many similar points to the PEFC system, and the technologies developed for PEFC can be shared. On the other hand, anode catalysts must be newly developed for the electrolysis cell since the carbon support dispersing Pt nanoparticles in PEFC cannot be used under high potential like nearly 2.0 V. Therefore, in this study, novel iridium-based SnO2/VGCF electrocatalysts were developed. Since SnO2, mostly working as a support material for iridium-based catalysts, is highly durable even under such high potential, iridium-based SnO2/VGCF is most likely a good candidate for anode catalysts. Therefore, the aim of this study is development and electrochemical evaluation of iridium-based SnO2/VGCF electrocatalysts.

Experimental

Sn0.98Nb0.02O2/VGCF was synthesized through an ammonia co-precipitation method, using SnCl2・2H2O and NbCl5. Then, IrO2(Ir) nanoparticles were decorated on Sn0.98Nb0.02O2/VGCF via the method that the aqueous solution mixture containing Sn0.98Nb0.02O2/VGCF and H2IrCl6・nH2O was completely evaporated while stirring, followed by the heat treatment at 440 oC in air. Actual amounts of Sn0.98Nb0.02O2 and IrO2were determined based on weight loss by VGCF obtained from TG-DTA. Electrocatalysts were further analyzed by STEM and XPS

Results and discussion

The composition of IrO2/Sn0.98Nb0.02O2/VGCF was determined to be 23 wt.% of IrO2, 58 wt.% of Sn0.98Nb0.02O2, and 19 wt.% of VGCF. From STEM observation, IrO2 (or Ir) particles were successfully observed on the surface of SnO2. Furthermore, in order to clarify either IrO2 or Ir, Ir 4f binding energy was analyzed by XPS and is shown in Figs. 2 and 3. When the electrocatalyst was synthesized in air, iridium-based catalysts most likely exist as IrO2 according to the previous report[1] as seen in Fig. 2. On the other hand, when it was made under 5% H2-N2, the catalysts mostly stay as Ir metal particles as shown in Fig. 3. Electocatalytic activity on water electrolysis is further discussed with these newly prepared electrocatalysts.

References

[1] D.Xu, P.Diao, T.Jin, Q.Wu, X.Liu, X.Guo, H.Gong, F.Li, M.Xiang, Y.Ronghai, ASC Apple. Mater. Interfaces 7, 16746 (2015).

Figure 1
Modeling PGM-Free Active Site Structures: Activity and Durability
Authors: Holby Edward F. , Martinez Ulises , Chung Hoon T , Zelenay Piotr
Published: February 2020

Two of the largest hurdles facing platinum group metal-free (PGM-free) oxygen reduction reaction (ORR) electrocatalysts for polymer electrolyte fuel cell (PEFC) cathodes are the need for improved activity and increased durability. Quantum chemical modeling (in particular, density functional theory, DFT, coupled with thermochemical models) can be utilized to better understand both activity and durability of these materials at the atomic scale. In this work, we apply quantum chemical modeling approaches to study the activity of N-coordinated Mn, Fe, Co, and Ni containing zig-zag edge sites. Both Metal-N4 as well as Metal2-N5edge structures are considered. Spontaneous *OH ligand modification in which the *OH intermediate is persistently bound to metal centers above a relatively low potential is found to be applicable to both structures but only for certain metal species. Experimentally determined trends in activity as a function of metal species fit well with theoretical predictions based on the spontaneous *OH ligand modification hypothesis. In addition, strain-modification of structures and resultant shifts in activity are explored. It is found that, due to anisotropic changes in binding energies with strain, this approach can lead to minor improvements of the computationally obtained ORR activity descriptor.

With regards to the durability hurdle of PGM-free electrocatalysts, a structurally sensitive and computationally accessible descriptor is explored. First-principles molecular dynamics derived knock-on displacement threshold energy may serve as a high-throughput, corrosion-mechanism-agnostic descriptor of active site structure durability that does take into account the kinetics of local bond breaking. Further comparisons between computational predictions and experimental findings are required to ascertain the applicability of this simplified durability descriptor for non-PGM active site structures. Current progress and future directions will be discussed.
(Invited) Core-Shell Nanosheet Catalysts: Enhanced Activity and Durability for Oxygen Reduction and Hydrogen Oxidation Reactions
Authors: Sugimoto Wataru , Takimoto Daisuke , Ohnishi Tomohiro , Nutariya Jeerapat , Shen Zhongrong , Ayato Yusuke , Mochizuki Dai , Boulineau Adrien , Demortiere Arnaud
Published: February 2020

Development of Pt-based catalysts with higher activity and durability is essential for implementation of cost-competitive polymer electrolyte membrane fuel cells. Nanoparticle catalysts with core-shell, skeleton, or skin-type structures have shown to outperform Pt nanoparticles. However, complications arise when such catalysts are exposed to harsh operation conditions of the fuel cell where Pt and the alloying metal undergoes consecutive oxidation/reduction and dissolution, de-activating the catalyst. Compared to nanoparticles, extended surfaces (bulk electrodes and nanostructured thin films) are more stable, but suffers from low surface area.

In this study, we describe the concept of “nanosheet catalysts” with surface area, activity, and durability that are higher than conventional core-shell nanoparticles for the oxygen reduction reaction as well as the hydrogen oxidation of reformate fuel. Our approach utilizes metallic Ru nanosheets [1] with atomic-scale thickness prepared via thermal reduction of exfoliated RuO2nanosheets [2,3]. The metallic Ru nanosheet is used as a core for the synthesis of two-dimensional Ru-core@Pt-shell catalysts.

Sub to a few monolayer Pt shell was successively formed on metallic Ru nanosheets with monoatomic thickness via galvanic displacement reaction between Cu and Pt2+. The electrochemically active Pt surface area of Ru-core@Pt-shell nanosheets with 1.5-4.5 monolayer Pt-shell was 112-151 m2 (g-Pt)‒
1. This corresponds to Pt nanoparticles with 1-1.5 nm of diameter. A carbon supported core-shell nanosheet catalyst with a 3.5 monolayer Pt-shell (Ru@Pt-3.5ML(ns)/C) showed 4.5 times higher activity than the benchmark Pt/C catalyst for the oxygen reduction reaction with a slower degradation rate even at high potentials. For the anode reactions, Ru@Pt-1.5ML(ns)/C had 2 times higher hydrogen oxidation activity in pure H2 as well as 300 ppm CO containing H2, and better stability against potential cycling. The developed nanosheet catalysts should provide the high utilization of Pt towards catalytic reactions and concurrently the stability of extended surfaces, offering a practical solution to the trade-off issue.

This work was supported in part by the “Polymer Electrolyte Fuel Cell Program” from the New Energy and Industrial Technology Development Organization (NEDO), Japan.

[1] K. Fukuda and K. Kumagai, e-Journal Surf. Sci. Nanotechnol., 12, 97 (2014).

[2] W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami and Y. Takasu, Angew. Chem. Int. Ed., 42, 4092 (2003).

[3] K. Fukuda, H. Kato, J. Sato, W. Sugimoto and Y. Takasu, J. Solid State Chem., 182, 2997 (2009).

Figure 1
High-Temperature Synthesized PGM-Free Oxygen Reduction Reaction Catalyst
Authors: Chung Hoon T , Cullen David A. , Sneed Brian , Lin Ling , Yin Xi , Martinez Ulises , Purdy Geraldine M , More Karren L. , Zelenay Piotr
Published: February 2020

Platinum group metal-free (PGM-free) oxygen reduction reaction (ORR) electrocatalysts have demonstrated substantial improvements in electrochemical cell and fuel cell performance in the last decade. However, the durability of these catalysts is still insufficient for the implementation of these catalysts in polymer electrolyte fuel cells (PEFCs), particularly in automotive power systems. Recently, carbon oxidation has been reported as a main cause for the destruction of ORR active sites in heat-treated metal-nitrogen-carbon (M-N-C) PGM-free catalysts (1). This result suggests that in order to improve their durability, M-N-C catalysts should be synthesized at as high a temperature as possible to maximize the graphitic content thereby making the catalyst more oxidation resistant. An optimum heat-treatment temperature for the best ORR activity of M-N-C catalysts depends on many factors, including the synthesis approach, type of precursors used, and the precursor ratio. One more factor to account for, based on the reasoning above, is the need to maximize the heat-treatment temperature without sacrificing catalyst activity.

In this work we have found that for the high-temperature synthesis of active M-N-C catalysts, the most important factors are the selection of the nitrogen precursor, adjustment of the precursor ratio, and the choice of the synthesis route. Without fine-tuning of the synthesis conditions, the ORR activity of the PGM-free catalysts usually decreases when heat-treated above 900° C. By selecting an appropriate synthesis route and catalyst precursors we have been able to perform synthesis at 1100° C and obtain catalysts that show high RDE activity (E½
of 0.81 V vs. RHE, Figure 1). In this presentation, we will summarize the electrochemical and fuel cell performance of these catalysts, with particular emphasis on their durability.

Acknowledgement

Financial support of this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged. Microscopy research was supported through a user project at ORNL’s Center for Nanophase Materials Sciences, which is an Office of Science User Facility.

Reference

Choi Chang Hyuck, Claudio Baldizzone, Jan-Philipp Grote, Anna K. Schuppert, Frédéric Jaouen, and Karl J. J. Mayrhofer, Stability of Fe-N-C Catalysts in Acidic Medium Studied by Operando Spectroscopy. Angew. Chem. Int. Ed.
54, 1 (2015).

Figure 1
Method for Studying High Temperature Aqueous Electrochemical Systems: A Self Pressurized Autoclave
Authors: Holm Thomas , Dahlstrøm Per Kristian , Sunde Svein , Harrington David A. , Seland Frode
Published: February 2020

Understanding oxidation kinetics and reaction mechanism at elevated temperatures is paramount to making efficient fuel cells that can run at temperatures higher than 100oC. We describe a method that allows for studies of aqueous electrochemistry at temperatures above the normal boiling point of water, and this system is used to study potential fuels for direct fuel fed fuel cells. Two alcohols with very different partial pressures at temperatures above 100oC are used in this study: methanol and glycerol. A self-pressurized glass autoclave heated in an oil bath is used for this purpose, and a model of the autoclave is shown in Fig. 1a. Slow ramping of the temperature allows for efficient data acquisition by using cyclic voltammetry up to 140oC, exemplified by Fig. 1b, and kinetic parameters such as onset potentials, Tafel slopes and activation energies are found and discussed for the relevant fuels. For methanol oxidation, dissociative water adsorption plays a key limiting role in the total reaction of methanol to CO2 at all temperatures. For glycerol oxidation, some partial oxidation products do not require an oxygen donor in the form of adsorbed water for their formation. This is deemed to play a fundamental role giving a large drop in overpotential at temperatures above 110oC, and making glycerol a potential fuel cell candidate at these temperatures. Therefore, a partial oxidation of glycerol in a fuel cell offers a potential method for a combined fuel cell and value-adding process for abundant glycerol.

Figure 1
Phase Stability and Electrochemical Characterization of Solid Oxide Fuel Cell Cathodes Containing K2NiF4 Structured Oxides
Authors: Cetin Deniz , Poizeau Sophie , Gopalan Srikanth
Published: February 2020

K2NiF4 -structured oxides, e.g. La2NiO4, exhibit facile kinetics for oxygen surface exchange and comparable electrical conductivity to perovskite-type oxides. Even though they exhibit great potential for replacing the perovskite-type alternatives in SOFC cathodes due to their superior oxygen exchange kinetics, K2NiF4 type-oxides suffer from phase stability issue when in contact with doped-cerias, commonly used as the ionic conducting component in many composite cathode materials.

In this study, we reveal a very effective methodology to stabilize the chemical composition of composite cathodes containing K2NiF4 -structured oxides containing doped-ceria. Phase stability studies were conducted to verify the retention of the K2NiF4 -structured oxides after high temperature processing of the electrode. Electrical conductivity measurements, dilatometry analysis, and X-ray diffractometry as functions of temperature, oxygen partial pressure were conducted over wide compositional windows of interest to identify new high performance cathode compositions.

Button cells featuring the compositions identified through these studies were prepared via tape casting and sintering. The electrochemical performance of these button cells were analyzed through polarization modelling and impedance spectroscopy in the temperature range 700 to 900°C and cathode side oxygen partial pressures between 5x10-2 and 1 atm. The effect of cathode microstructure and thickness on the cell performance is also discussed.
Effect of Chromium Poisoning on Performance of Sr-Doped LaMnO3 (LSM)-Based Cathode in Anode-Supported Solid Oxide Fuel Cells
Authors: Wang Ruofan , Würth Manuel , Pal Uday Bhanu , Gopalan Srikanth , Basu Soumendra Nath
Published: February 2020

Ni-YSZ anode-supported planar solid oxide fuel cells (SOFCs), comprised of a Sr-doped LaMnO3 (LSM) + yttria-stabilized zirconia (YSZ) cathode active layer and a LSM cathode current collector layer, are fabricated. Electrochemical performance tests of the cells with and without chromia-forming alloy interconnect in the temperature range 700 - 800°C are conducted. Humidified hydrogen (97% H2 - 3% H2O) is used on the anode side, and oxygen and water vapor contents on the cathode side are varied, in order to control the rate of Cr vaporization. The cathode performances with and without the Cr environment are analyzed by estimating the polarization losses at the cathode. This is done with the help of impedance spectroscopy measurements and curve-fitting the experimentally measured voltage vs current density (V-i) plots to a polarization model. Microstructures of the cathode cross sections are observed before and after the tests to quantify the effects of Cr deposition. Based on these experimental and analytical results, the effects of Cr poisoning on performance of LSM-based cathode under various conditions are evaluated, and mechanisms of Cr poisoning are proposed.