High Current Region Research Articles

Effect of unevenly-distributed V pits on the optical and electrical characteristics of green micro-light emitting diode

In this study, we investigated the effect of different-size V pits on the opto-electrical performance of different-size InGaN-based green (520 nm) light-emitting diodes (LEDs). The size of V pits varied from 57 to 250 nm. Two different-size LEDs (10 and 300 µm) were fabricated with flip-chip structures. It was shown that at 4 A cm−2, the forward voltages of 300 µm-size and 10 µm-size LEDs were in the range of 2.32–2.82 V and 2.23–2.53 V, respectively. Compared with the LEDs with small V pits, the LEDs with large V pits produced higher output power at the high current region (>50 A cm−2), but lower output power at the low current region (<20 A cm−2). The 300 µm-size LEDs displayed maximum external quantum efficiency (EQE) at 2 A cm−2, whereas the 10 µm-size LEDs exhibited peak EQE at current densities ⩾13 A cm−2. For all samples, the electroluminescence (EL) wavelengths were blue shifted with increasing current density. Unlike the 300 µm-size LEDs, the 10 µm-size LEDs exhibited a large deviation in the light output, EQE, and EL wavelength. The 10 µm-size LEDs revealed larger S than the 300 µm-size LEDs. The backscattered electron (BSE) and monochromatic cathodoluminescence (CL) results showed that for all samples, V pits were unevenly distributed across the whole wafer. Based on the CL and BSE results, the effect of different V pit sizes on the performance of green micro-LEDs at the low current region is described and discussed.

The Effect of Flow Field Property in Toluene Direct Electro-Hydrogenation Electrolyzer

Introduction Recently, the utilization of renewable energy has been expected to decrease the carbon dioxide emission. For effective utilization of the renewable energy that fluctuates and uneven distributes, the large-scale storage and transportation of hydrogen as secondary energy is important technology. Toluene (TL)/ methylcyclohexane (MCH) organic hydride system is the candidates of energy carrier, and have the advantages of efficiency, safety and handling. We have studied the high-efficient electrolytic direct-hydrogenation of toluene with water decomposition for the organic hydride system as an efficient energy carrier process.1 In these studies, the importance of mass transfer in porous carbon flow field for TL/MCH has been clarified. In this study, we have investigated the effect for electrochemical property on the thickness of the porous carbon flow field to improve mass transfer for the cathode reaction in the toluene direct electro-hydrogenation electrolyzer. Experimental Figure 1 shows the schematic drawing of toluene direct electro hydrogenation electrolyzer. A DSE® electrode (De Nora Permelec ltd) for the oxygen evolution reaction, a carbon paper (29BC, thickness=235 μm or 10BC, thickness=385 μm, SGL carbon ltd.) applied 0.5 mg-PtRu cm-2 of PtRu/C (TEC61E54, TKK) catalyst with ionomer, and Nafion 117® (Du Pont) were used for the anode, the cathode, and the proton exchange membrane (PEM), respectively. The carbon paper as the flow field was loaded 0.02 mg-Pt cm-2 of Pt particles by the impregnation method for the chemical-hydrogenation catalyst.2 The cathode was hot-pressed on the PEM at 120oC and 0.1 MPa for 3 min to fabricate a membrane cathode assembly. Geometrical electrode area was 11.6 cm-2. Operation temperature of the electrolyzer conducted at 60oC. The anode and cathode compartments were circulated 1 mol dm-3 H2SO4 and 1, 5 and 10% toluene diluted by methylcyclohexane, respectively. The tip of the Luggin capillary of a reversible hydrogen electrode (RHE) was placed near the anode. The electrochemical measurements were conducted for the linear sweep voltammetry (LSV), the electrochemical impedance spectroscopy (EIS), the chronoamperometry (CA) and the current efficiency measurement. Results and discussion Figure 2 shows the cell voltage and the internal resistance of R­1 (see inset of Fig. 2) as a function of the current density curves for two types thickness of flow field with 1, 5 and 10% TL feed at 60oC. Internal resistance of R­1 was obtained from the high frequency intercept of the Nyquist plot of the EIS mesurement. The cell voltage and R­1 of 10BC flow field was lower than that of 29BC at all TL concentration. Especially, the cell voltage of 10BC exhibited high performance about 1.83 V at 400 mA cm-2. Figure 3 shows the iR corrected anode and cathode polarization curves of with 1, 5 and 10% TL feed at 60 oC. The anode polarization curves were not affected for the change of the TL concentration and the type of flow field. On the other hand, the cathode overpotential of 10BC was lower than that of 29BC at all TL concentration. This difference was remarkable at high current density region, and it would be affected by the improvement of TL mass transfer in the flow field. Figure 4 shows the current efficiency with 1, 5 and 10% TL feed at 60oC. At 1% TL feed, obvious difference for current efficiency between 10BC and 29BC were not observed. In the initial stage of current efficiency drop at 5 and 10 % TL feed, the current efficiency of 29BC was higher than that of 10BC. However, in higher current region, 10BC showed higher current efficiency than 29BC. In this experiment, the flow rate for two types of flow field was same. At this condition, the thinner 29BC has higher linear velocity than 10BC, and is easy to supply with toluene to the catalyst layer. Hence, 29BC might exhibit the high current efficiency in lower current density region. Under 90-95% current efficiency, which the hydrogen generation increased, the thicker 10BC exhibited the higher current efficiency than the 29BC. It would be considered that the thinner 29BC is easy to occupy the hydrogen gas in the flow field, and the toluene supply to the catalyst layer was insufficient. Acknowledgements This work was partially supported by the Toyota Mobility Foundation and Core Research for Evolutional Science and Technology (CREST) “innovative reaction” (Funding agency: JST). Anode was supplied by De Nora Permelec Ltd. Reference Mitsushima, Y. Takakuwa, K. Nagasawa, Y. Sawaguchi, Y. Kohno, K. Matsuzawa, Z. Awaludin, A. Kato, and Y. Nishiki, Electrocatalysis, 7, 127 (2016). Nagasawa, Y. Sawaguchi, A. Kato, Y. Nishiki, and S. Mitsushima, Electrochemistry 86, 339 (2018). Figure 1

Effect of Catalyst Loading on the Degradation of d-PtCo/C Cathode Catalyst

The Consortium for Fuel Cell Performance and Durability (FC-PAD) was formed by the U.S. Department of Energy Fuel Cell Technologies office (FCTO) in 2015 to improve the understanding of performance and durability issues in polymer electrolyte membrane fuel cells (PEMFCs) and to help meet the DOE 2020 targets. This talk will summarize some of the durability work performed within the consortium on commercial dealloyed-PtCo/C catalyst-based membrane electrode assemblies (MEAs). Commercial MEAs using Nafion®HP membranes, ElystPt300670 (30wt.%PtCo/high surface area C) cathode catalyst, and ElystPt200380 (20wt.%Pt/Graphitized carbon) anode catalyst were obtained from Umicore. The MEAs were assembled in a 5cm2differential cell reported by D. R. Baker et al.1and conditioned extensively before performing beginning of life (BOL) characterization. The conditioning protocol included a recovery step where the cell was operated at high RH and low temperatures and voltages to flush out impurities and maximize the performance.2,3The samples were then subjected to the DOE-recommended square wave catalyst accelerated stress test (AST) which is a 3 sec hold at 0.6V and 3 sec hold at 0.95V at 80oC and 100%RH.4Characterizations were performed after 15,000 and 30,000 cycles of the AST with the recovery protocol applied to the MEA before characterization. Electrochemical characterization included mass activity, electrochemical surface area, polarization curves, Electrochemical Impedance Spectroscopy (EIS), and oxygen transport measurements. The BOL and end-of-test catalysts were also characterized by electron microscopy, energy dispersive X-ray spectroscopy, X-ray absorption spectroscopy, and small-angle X-ray scattering to determine particle size and Pt to Co content. The oxygen reduction reaction mass activity, electrochemical surface area, and performance in the kinetic region of the catalysts degrade with AST cycling. This loss can be attributed to both Co leaching from the catalyst and catalyst particle size increase. The effect of this degradation was most pronounced in the high current region for the MEA with low Pt loading. For example, in Figure 1, the voltage loss at 1A/cm2after 30,000 AST cycles is ~50mV for the 0.15mgPt/cm2MEA and ~200mV for the 0.05mgPt/cm2MEA. This is primarily due to increases in the pressure-independent oxygen transport resistance. The non-Fickian oxygen transport resistance of low-loaded (0.05 mgPt/cm2) MEAs is significantly larger than those of the high-loaded MEAs even at BOL and worsens significantly with ageing of the catalyst. EIS experiments in HelOx (21% O2, balance He) also revealed that there was an increase in the pressure-dependent gas transport during the AST cycling. However, little or no change was observed in the ionomer sheet resistance over the course of the durability testing, independent of the catalyst loading. These results will be compared with recent results reported for pure Pt catalyst-based MEAs subjected to similar tests.5In this talk, the analysis of the cathode catalyst composition and morphology before and after the testing will be presented in detail and correlated to the observed performance losses. Acknowledgements This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Fuel Cells Program Manager: Dimitrios Papageorgopoulos and Technical Development Manager: Greg Kleen). References D. R. Baker, D. A. Caulk, K. C. Neyerlin, and M. W. Murphy, J. Electrochem. Soc.,V156(9), B991-B1003 (2009). https://doi.org/10.1149/1.3152226.Zhang, B. A. Litteer, F. D. Coms, and R. Makharia,. J. Electrochem. Soc., V159(7),F287-F293 (2012). https://doi.org/10.1149/2.063207jes.Zhang et al, U.S Patent Application Publication US 2011/0195324 A1, August 11 (2001).United States Department of Energy Fuel Cells Technologies Office Multi-Year Research, Development, and Demonstration Plan. https://www.energy.gov/sites/prod/files/2017/05/f34/fcto_myrdd_fuel_cells.pdf.G. S. Harzer, J. N. Schwammiein, A. M. Damjanovic, S. Ghosh, and H. A. Gasteiger, J. Electrochem. Soc.,V165(6), F3118-F3131 (2018). https://doi.org/10.1149/2.0161806jes Figure 1

Usage of FEM for synthesis of dead-beat current controller for permanent magnet synchronous motor

PurposePredictive controllers and permanent magnet synchronous motors (PMSMs) got more attention over the past decades thanks to their applicable features. This paper aims to propose and verify a method to design a predictive current controller with consideration of motor characteristics obtained from finite element analysis (FEA).Design/methodology/approachPermanent magnet motor parameters and its maps can be calculated by means of FEA. The model takes into account magnetic saturation and thermal electro-magnetic properties. For each dq current vector and each position, self and mutual inductances are calculated. Based on co-energy method and fundamentals of coordinate transformation dynamic and static, dq inductances are obtained. These are used in classical and modified dead-beat current controller equations.FindingsTo sustain good features of a controller over higher current regions, it is necessary to adapt control law of a dead-beat controller. After its modification, control quality can be superior over classical solution in high saturation regions. The transient simulations of controller and motor give accurate results.Originality/valueCommon predictive current controllers use nominal motor parameters in their equations. The authors proposed a modified dead-beat current controller to improve the control quality. There is no need to apply self-tuning algorithms, and implementation of the controller is not much more complicated than that of the classical controller. Designer of a control system can obtain required data from motor designer; in design process of modern machines such data are often already available. The proposed methodology increases control quality of the presented dead-beat controller.

Electrochemical performance of hydrothermally synthesized N-Doped reduced graphene oxide electrodes for supercapacitor application

In the present study, nitrogen-doped reduced graphene oxide is synthesized using hydrothermal method for high-performance supercapacitor application. The N-doping level is found to be ~7 at.%, and nitrogen atoms remain in pyridinic, pyrrolic and quaternary form. Because of the multiple synergistic effects of good wettability, increased electron density and a great number of active sites, the material shows a remarkably improved electrochemical behavior compared to reduced graphene oxide prepared by the same method. The electrochemical performance was measured both in the alkaline, and acidic electrolyte in two-electrode configuration. In alkaline electrolyte nitrogen doped graphene shows excellent cyclic stability (97% retention after 1000 cycles) and low internal resistance (0.207 Ω), resulting in higher specific capacitance in high current region compared to acidic electrolyte.

Nanostructured core-shell cobalt chalcogenides for efficient water oxidation in alkaline electrolyte

In water splitting, oxygen evolution reaction (OER) requires most active and stable electrocatalysts to overcome their sluggish kinetics thereby improving the device efficiency. In this research work, we developed nanocarbon protected cobalt sulfide, selenide and telluride (core-shell Co9S8@NC, CoSe@NC and CoTe@NC) using solvent and catalyst free auto-pressurized (Swagelok) solid-state thermolysis method and introduced as active and stable OER electrocatalysts in alkaline electrolyte. In 1 M KOH aqueous solution, the nickel foam supported Co9S8@NC shows the highest OER activity with an overpotential of 288 mV at 10 mA cm−2, which is 33 and 68 mV lower than that of CoSe@NC and CoTe@NC electrocatalysts, respectively. In addition, the Co9S8@NC exhibits small Tafel slope of 65 mV dec−1 calculated from the low current density region (10 mA cm−2) and increases to 120 mV dec−1 at high current densities region (100 mA cm−2). All three electrocatalysts show good stability with negligible potential loss at a static OER current density of 10 mA cm−2. The obtained results with electrochemical active surface area revealed that the thin carbon layer coating controls nanostructure formation together with liable utilization and strong protection of active sites from the harsh electrolyte conditions, thereby providing constructive activity and stability.

The Examination and Characterisation of Water Transport in Metal Gas Diffusion Layers for PEMFC Via EIS and Neutron Imaging.

Novel materials for gas diffusion layers (GDLs) in polymer electrolyte membrane fuel cells (PEMFCs) are increasingly under investigation to improve high current performance. Metal gas diffusion layers are of interest as they provide high thermal and electrical conductivity as well as offering the possibility of integration of the flowfield and gas diffusion layer. These properties are all of importance to fuel cells as they should give increased volumetric power density and decrease resistive losses at higher current densities. Some recent studies have shown that these metal GDLs can outperform commercial carbon GDLs in the high current region [1]–[3]. The water dynamics of these types of GDL are poorly understood however, and there has been little or no research on this topic. We have performed a study using neutron imaging to study the movement of water in metal mesh GDLs and used EIS to study the consequent effect of mass-transport resistance at a number of current densities and humidities [4]. We have observed a number of interesting phenomena in the water transport in metal-mesh GDLs, which show quite different behavior to that of the normal carbon papers. When metal mesh is used as the anode and cathode GDL, there appears to be water accumulation in the areas of the channel, which also promotes the back-diffusion of water from the cathode to the anode side of the cell. With a carbon-paper GDL, liquid water accumulates in the land, and there does not appear to be significant back-diffusion compared to the metal mesh GDL. These behaviors also have significant effects on mass-transport and the performance of the cell. The insights provided by the neutron imaging, along with the EIS data will allow us to inform novel and future fuel cell designs.

Quasi-one-dimensional Mo chains for efficient hydrogen evolution reaction

Structural modulation of catalytic nanostructures and fundamental understanding of their active sites at the atomic scale are important for predicting and improving the catalytic properties of nanostructures. Here, we prepared quasi-one-dimensional (1D) metal molybdenum (Mo) chains confined in atom-thick molybdenum disulfide (MoS2), referred henceforth as Mo/MoS2 nanosheets, and evaluated their hydrogen evolution reaction (HER) properties. The experiment and theoretical calculations show that the quasi-1D Mo chain with unsaturated coordination exhibits high HER activity. The unsaturated Mo sites in the chains increase the carrier density and facilitate the diffusion of hydrogen along the chains, mimicking an atomic scale reactor which leads to an experimentally observed enhanced catalytic performance. Within the framework of Volmer-Tafel model, the calculated kinetic barrier for H2 evolution is only 0.48 eV for Mo/MoS2, which is significantly lower than that for the Pt (111) surface (∼0.8 eV). In particular, Mo/MoS2 nanosheets supported on reduced graphene oxide (Mo/MoS2/RGO) outperformed commercial Pt on glassy carbon (Pt/C) in the practically meaningful high-current region (>140 mA cm−2) in 0.5 M H2SO4 solution and (15 mA cm−2) in 1.0 M NaOH solution, demonstrating that the Mo/MoS2/RGO could potentially replace Pt catalysts in practical HER systems. Additionally, this study provides crucial insights into the role of active centers in catalysis through a model-structure-performance relationship, thus pointing the way to a commercially viable technology for HER.

Synchronized, periodic, and chaotic dynamics in spin torque oscillator with two free layers

A phase diagram of the magnetization dynamics is studied by numerically solving the Landau-Lifshitz-Gilbert (LLG) equation in a spin torque oscillator consisting of asymmetric two free layers that are magnetized in in-plane direction. We calculated the dynamics for a wide range of current density for both low and high field cases, and found many dynamical phases such as synchronization, auto-oscillation with different frequencies, and chaotic dynamics. The observation of the synchronization indicates the presence of a dynamical phase which has not been found experimentally by using the conventional electrical detection method. The auto-oscillations with different frequencies lead to an oscillation of magnetoresistance with a high frequency, which can be measured experimentally. The chaotic and/or periodic behavior of magnetoresistance in a high current region, on the other hand, leads to a discontinuous change of the peak frequency in Fourier spectrum.

Application of Carbon Felt as a Flow Distributor for Polymer Electrolyte Membrane Fuel Cells

In this work, carbon felt is employed as a flow distributor in polymer electrolyte membrane fuel cells and the fuel cell performance of this design is compared to that of a conventional graphite serpentine channel design and copper metal foam. Experimental results show that carbon felts as flow distributors offer advantages over other flow distributors in terms of enhanced rates of mass transfer due to their levels of high porosity and strong convective flow. As a result, fuel cells with carbon felt as flow distributors perform well and especially in high current regions. In addition, the effects of the hydrophilicity of carbon felt on fuel cell performance are evaluated. The hydrophobic surface of raw carbon felt is rendered hydrophilic via ozone treatment. Ozone-treated hydrophilic carbon felt helps remove water from the hydrophobic gas diffusion layer, decreasing mass transfer resistance. However, the formation of oxygen functional groups by ozone treatment increases the levels of ohmic resistance. Thus, it is necessary to develop highly conductive and hydrophilic carbon felt as a flow distributor for high performance of polymer electrolyte membrane fuel cell.

A Feasibility Study Comparing Two Commercial TIG Welding Machines for Deep Penetration

Developing a deep penetration TIG welding technique to produce welds of equal quality to the industrial standard practise of laser-based welding techniques has the potential to lower production complexity and cost. Higher currents levels are required to increase penetration depth in conventional TIG welding but this results in excessive weld bead width amongst other detrimental effects. However, through K-TIG and A-TIG techniques these detrimental effects can be circumnavigated. Prior experimental work on weld pool dynamics in conventional TIG welding in higher current regions has been sparse as TIG welding enhanced through novel techniques provides the best quality welds. This paper is an early feasibility study for novel deep penetration welding techniques motivated by observations made during research done at The University of Sheffield where unexpected activity in the weld pool was identified during TIG welding with a VBC IE500DHC between 300A – 1000A. This current range is labelled the ‘Red Region’. Understanding the fluid dynamics of the molten metal in the weld pool at the ‘ Red Region' current level will help in the creation of novel techniques for deep penetration TIG welding. Addressing this, this paper compares the quality of welds produced between 100A and 200A on 316 Stainless Steel by two industrially leading welding machines; the Miller Dynasty 350 and the VBCie 500DHC.

Determination of Surface Recombination Velocity From Current–Voltage Characteristics in SiC p-n Diodes

Surface recombination velocity on mesa sidewalls of SiC p-n diodes with various surface passivation conditions was evaluated from the device-size-dependent preexponential factor of recombination current ( ${J}_{\text {0rec}}$ ). The diodes passivated by SiO2 with postoxidation nitridation were dipped into HF to eliminate a shunt current, which is evoked by the nitrided SiO2 layer and disturbs the analysis of the recombination current. For accurate determination of the surface recombination velocity, an effective recombination zone width was precisely derived taking account of the distribution of carrier density in the depletion layer. The surface recombination velocity of the diodes without any passivation and with the postoxidation nitridation (NO annealing at 1250 °C for 70 min) was determined as $\text {1.2}\times \text {10}^{\text {7}}$ and $\text {6.0}\times \text {10}^{\text {5}}$ cm/s, respectively, which indicates that the postoxidation nitridation can reduce the surface recombination by a factor of about 20. We confirmed that TCAD simulation could reproduce the current–voltage characteristics by utilizing the extracted parameters. In addition, an evaluation method was proposed to determine the surface recombination velocity from the high-current region, where diffusion current is dominant.

Measurement Methodology for Accurate Modeling of SiC MOSFET Switching Behavior Over Wide Voltage and Current Ranges

This paper presents two novel measurement methods to characterize silicon carbide (SiC) MOSFET devices. The resulting data are utilized to significantly improve the extraction of a custom device model that can now accurately reproduce device switching behavior. First, we consider the $I_{\text{d}}- V_{\text{ds}}$ output characteristics of power devices such as SiC transistors. These are typically measured using traditional curve tracers, but the characterization of the high-voltage and high-current (HVHC) region is very challenging because of device power compliance and self-heating. In this paper, we introduce a measurement technique that overcomes self-heating and derives the HVHC region from switching waveforms. The switching transient characteristics of devices are used to determine drain current $(I_{\text{d}})$ as a function of drain–source voltage $(V_{\text{ds}})$ in the HVHC range. Second, we consider another challenging characterization area: measurement of nonlinear capacitances when device is turned on. These capacitance characteristics of on-state devices are important for correcting disagreements between simulations and measurements in turn-off switching transient waveforms and cannot be measured using a conventional capacitance-voltage meter. We introduce S-parameter measurements as an effective method to obtain the capacitance characteristics of both off-state devices and on-state devices. These novel measurement techniques have been applied to the modeling of a SiC device. The extracted device model, a modified version of the popular Angelov−GaN high-electron-mobility transistor model, shows significant improvement in terms of the accuracy of switching waveforms of devices over a wide range of operating conditions.

Porous Nickel-Phosphorous Electrode Synthesized By Fast-Simple Electrodeposition Process for Efficient Overall Water Splitting

Water electrolysis is an electrolysis reaction of water, and it is a technology that can produce a large amount of hydrogen environmentally friendly. The purposes of water electrolysis are responding to current hydrogen demand and storing electric energy produced from renewable energy such as solar or wind power to solve their intermittency. Water electrolysis reaction is divided into two half reactions; hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The theoretical equilibrium potential of the water electrolysis is 1.23 V, but the electrolysis reaction of the actual water requires much higher voltage due to the slow kinetics of oxygen and hydrogen reaction. Therefore, in order to increase hydrogen production efficiency, it is necessary to develop a catalyst having high activity and low cost for OER and HER. At present, transition metal based materials such as iron (Fe), cobalt (Co), and nickel (Ni) are mainly studied as high efficiency and low cost catalysts for oxygen and hydrogen evolution reactions for alkaline water electrolysis. Those transition metals are being studied in the form of various compounds such as oxide, carbide, nitride and phosphide. Among various type of compounds, transition metal phosphides (TMP) have drawn much attention for use in the alkaline water electrolysis system. These TMP catalysts have relatively good catalytic activity and stability for both OER and HER, and have high electrical conductivity compared to oxide based catalyst. In addition, TMP catalysts can be synthesized using non-toxic precursors as compared to nitride and carbide based catalysts. However, since most of TMP catalysts requires several hours to days for synthesis process, including high temperature heat treatment processes, the efficiency problem has not been solved. In this study, we have developed a method for producing nickel-phosphorus electrodes for high efficiency oxygen and hydrogen evolution reaction by fast-simple electrodeposition method. The synthesis procedure of the Ni-P electrodes ends in a few seconds at room temperature. High cathodic current density was applied to promote the generation of hydrogen bubbles and the deposition of Ni-P, thereby obtaining a highly porous structure. Hydrogen bubbles played an important role in forming the porous Ni–P layer by acting as self-dynamic templates. The surface area of highly porous Ni-P (HP Ni-P) was 50 times larger than that of the flat Ni electrode. The HP Ni-P exhibits remarkable electro-catalytic activity and stability towards the HER and OER in alkaline solutions. For the HER, overpotential at 10 mA/cm2 was 180 mV, which is slightly higher than that of Pt/C, but HP Ni-P required a lower overpotential than that of Pt/C at the high current region. For the OER, HP Ni-P shows higher catalytic activity than IrO2 at the entire current region; overpotential at 10mA/cm2 was 287 mV. The durability of HP Ni-P was also superior to the precious metal-based catalysts (Pt/C, IrO2) and Ni electrode in the long-term chronopotentiometry test. This result is ascribed to increase in surface area by hydrogen bubble generation and electronic structure change of Ni. Electronic structure change may lead to change in adsorption energy of intermediate adsorbates such as H*, OH*, O* and OOH*, which are participated in the steps of HER and OER, and can promote the kinetics of reactions in alkaline media. Figure 1

Shockley‐Read‐Hall and Auger Recombination in Blue InGaN Tunnel‐Junction Light‐Emitting Diodes

In this study, performance and characteristics of blue InGaN tunnel‐junction light‐emitting diode (TJ LED) are investigated theoretically. Simulation results show that, dissimilar to the single LED, Shockley‐Read‐Hall (SRH) and Auger recombination are critical issues influencing the output performance of blue TJ LEDs in both low and high current regions. In realizing high‐performance TJ LEDs, to pursue the advantages of both satisfactory crystalline quality and suppressed SRH recombination loss, a simplified structure with single quantum well and without electron‐blocking layer is proposed as the unit LED structure that is utilized to stack the TJ LED. Simulation results indicate that the TJ LED with simplified unit LED structure possesses good performance at high output power, which provides a practical and cost‐effective way for the fabrication of TJ LEDs, especially when more unit LEDs need to be cascaded.

Efficient Hydrogen Evolution Reaction Catalysis in Alkaline Media by All-in-One MoS2 with Multifunctional Active Sites.

MoS2 becomes an efficient and durable nonprecious-metal electrocatalyst for the hydrogen evolution reaction (HER) when it contains multifunctional active sites for water splitting derived from 1T-phase, defects, S vacancies, exposed Mo edges with expanded interlayer spacings. In contrast to previously reported MoS2 -based catalysts targeting only a single or few of these characteristics, the all-in-one MoS2 catalyst prepared herein features all of the above active site types. During synthesis, the intercalation of in situ generated NH3 molecules into MoS2 sheets affords ammoniated MoS2 (A-MoS2 ) that predominantly comprises 1T-MoS2 and exhibits an expanded interlayer spacing. The subsequent reduction of A-MoS2 results in the removal of intercalated NH3 and H2 S to form an all-in-one MoS2 with multifunctional active sites mentioned above (R-MoS2 ) that exhibits electrocatalytic HER performance in alkaline media superior to those of all previously reported MoS2 -based electrocatalysts. In particular, a hybrid MoS2 /nickel foam catalyst outperforms commercial Pt/C in the practically meaningful high-current region (>25 mA cm-2 ), demonstrating that R-MoS2 -based materials can potentially replace Pt catalysts in practical alkaline HER systems.

Dynamic force balance model considering tapering effect in gas metal arc welding

This paper proposes a metal transfer model describing the dynamic growth and detachment process of the molten drop at the electrode tip in gas metal arc welding under an Ar-rich shielding gas. The proposed model simulates the metal transfer including the growth of the molten drop as well as the formation and break-up of the liquid bridge between the electrode tip and the molten drop based on the forces exerted on the molten and the second order displacement equation. The tapering effect occurring at the side of the electrode tip is considered to enhance the prediction performance in the high-current region above the transition current. Simulation results are compared with experimental results reported in the available literatures and they show good agreement from the low to high current region.

(Invited Plenary) Fuel Cell Stack Development in Toyota

After commercializing MIRAI in 2014, Toyota is focusing more on further cost reduction, and improvement in performance, durability and reliability, which are still challenging for wide-spreading commercialization enabling PEFC technology for next generation FCVs. To enhance PEFC performance directly impacts towards cost reduction due to reduce an amount of PEFC stack materials. Significant R&D efforts and advancements have been made towards filling the gap to the target on PEFC performance. For example, catalyst research aims not only to improve ORR activity itself but also to enhance mass transport performance in the high current region. The mass transport performance depends greatly on its microscopic structure of electrodes. Electrodes are fablicated from catalyst ink, which is a multi-component system, consisting of catalyst carbon, ionomer, and solvent. We employed several visualization techniques to unveil the structure of catalyst ink and electrodes respectively and discuss the relationship between the structure of catalyst ink and electrodes, and its performance. The technical prospects and R&D approaches for the remaining technical challenges on durability and reliability for the next generation FCVs will be also discussed.

Enhanced performance based on a hybrid cathode backing layer using a biomass derived activated carbon framework for methanol fuel cells

Direct methanol fuel cells (DMFCs) suffer from performance reduction due to mass transport losses incurred at high current regions. In this work, we report the use of activated carbon (AC), prepared from factory waste-tea, as cathode microporous layer in the membrane electrode assembly (MEA) of a DMFC, alleviating this mass transport effect. This biomass based AC framework, when tested under fuel cell operating conditions provided 0.25V @ 300mAcm−2, enhancing cell performance by 33% over standard electrodes at 70°C. During uninterrupted durability testing, this electrode displayed exceptional stability in mass transport dominated region, with loss of 15mVday−1, compared to 25mVday−1 loss for the standard. Sample and electrode characterization measurements reveal that pore size distribution/particle size characteristics coupled with hydrophobic nature of the synthesized activated carbon, contributed to the performance improvement.

Differences in the nanoscale electrical properties of GaN films grown on sapphire and ZnO substrates by molecular beam epitaxy.

Gallium nitride (GaN) films were grown on sapphire and zinc oxide (ZnO) single crystal substrates using plasma-assisted molecular beam epitaxy. As ZnO for GaN have a better lattice match, the coverage ratio of the GaN (002) plane on the ZnO substrate was significantly higher by about 45%. According to conducting atomic force microscopy and scanning surface potential microscopy measurements, the surface of GaN films grown on the ZnO substrate had two excellent physical characteristics: (a) an 18% reduction of the high contact current region, and (b) a highly uniform work function distribution. Therefore, for future applications in GaN-based light-emitting diodes, the use of ZnO as a substrate will prolong the luminescence lifetime and enhance the luminescent monochromaticity.