High Current Density Range Research Articles

Influence of Ionomer Chemistry on Oxygen Permeability in Hydrocarbon-Based Proton-Exchange Membrane Fuel Cells

The pursuit of alternatives to perfluorinated sulfonic acid (PFSA) materials is of particular interest in light of the global and European mid-term goals to avoid fluorinated materials, and the technical target to operate proton-exchange membrane (PEM) fuel cells at temperatures of 120°C. Non-fluorinated hydrocarbon ionomers offer improved chemical stability at temperatures above 100°C, are less environmentally hazardous, exhibit lower gas crossover, and have the potential to reduce production costs.1 Recent years have seen significant progress in the development of hydrocarbon-based membranes and electrodes, with performance on par with that of state-of-the-art PFSA-based fuel cells under optimized conditions.2 However, a performance gap between hydrocarbon- and PFSA-based PEM fuel cells persists under application-relevant conditions, necessitating an understanding of the limitations that arise. Mass transport-related phenomena, particularly at the ionomer/catalyst interfaces, play a crucial role in controlling PEM fuel cell performance.3 While extensive studies have been conducted for PFSA-based fuel cells to deconvolute the local from the total transport resistance of the cathode catalyst layer, no such studies have been conducted for hydrocarbon ionomers. In this study, we investigated gas transport resistance and its components in hydrocarbon-based PEM fuel cells using H2- and O2 limiting current methods and ex-situ investigations in ionomer thin film behavior, while varying the ionomer's ion-exchange capacity. Our results show that local transport resistances are similar to those measured with PFSA-based cells with respect to different ionomer to carbon mass ratios (I/C). However, higher transport-induced resistances are observed under the potential formation of liquid water in the range of high current densities. We attribute these effects to the more pronounced water uptake of the ionomer thin film compared to the PFSA based reference. Thus, reducing ionomer swelling while maintaining the necessary proton conductivity of the ionomer thin film is crucial for next-generation hydrocarbon PEM fuel cells.References(1) H. Nguyen, C. Klose, L. Metzler, S. Vierrath and M. Breitwieser, Adv. Energy Mater., 2022, 12, 2103559.(2) H. Nguyen, F. Lombeck, C. Schwarz, P. Heizmann, M. Adamski, H. F. Lee, B. Britton, S. Holdcroft, S. Vierrath and M. Breitwieser, Sustainable Energy Fuels, 2021, 5, 3687–3699.(3) T. Schuler, A. Chowdhury, A. T. Freiberg, B. Sneed, F. B. Spingler, M. C. Tucker, K. L. More, C. J. Radke and A. Z. Weber, J. Electrochem. Soc ., 2019, 166(9), F3020.

Power Generation Charactristics of Polymer Electrolyte Fuel Cells with Electrocatalysts Supported on SnO2 in High Current Density Range

Introduction Considerable efforts have been made for FCEV commercialization, for which sufficient performance and durability have to be achieved1. SnO2-supported electrocatalysts with carbon backbone have a potential to achieve both high catalytic activity and high cycling durability2-3. However, it is necessary to further optimize cell performance up to high current densities for such alternative electrocatalysts. By controlling the thickness of the seal material and optimizing the Nafion content of the electrocatalyst layer, successful reduction of various overvoltages and improved power generation performance can be achieved, especially in the high current density region4. This study aims to further improve cell performance at high current densities by varying electrocatalyst layer fabrication conditions and selecting suitable settings for the MEAs. Experimental Using mesoporous carbon (MC) as a support backbone and SnO2 as a support surface layer on the cathode side5, membrane-electrode-assemblies (MEAs) with Pt/Sn0.92Nb0.08O2/MC electrocatalysts were fabricated. On the anode side, a standard Pt/C catalyst (TEC10E50E, Tanaka Kikinzoku Kogyo, Japan) was used. Cell performance and overvoltages up to high current densities were measured by varying the size of electrocatalyst particles. Start-stop cycle tests up to 60,000 cycles were also conducted. Results and discussion By ball milling before the Pt loading process in the preparation of the SnO2-supported electrocatalyst Pt/Sn0.98Nb0.02O2/MC, various overvoltages could be reduced, and cell performance became almost comparable to that of the standard Pt/C catalyst. Figure 1 shows the comparison of IV characteristics and Figure 2 shows the comparison of microstructures of electrocatalyst layers. Ball milling resulted in a more appropriate microstructure of the electrocatalyst layer due to the homogenization of the electrocatalyst particle size. As shown in Figure 3, the ECSA retention after 60,000 start-stop cycles was about 50%. Even when MC was used as the support framework, high durability could be achieved due to the presence of SnO2, which was further improved by ball milling. Without ball-milling, the retention was about 45%.In the future, we aim to further improve cell performance by controlling the porous microstructure and water management of the electrocatalysts layer. We will fabricate cells under various conditions, conduct performance tests, and analyze overvoltages. The start-stop and load cycle durability will also be evaluated. Based on these efforts, the latest results of cell performance and durability tests will be presented. Acknowledgment This paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References Takahashi, T. Ikeda, K. Murata, O. Hotaka, S. Hasegawa, Y. Tachikawa, M. Nishihara, J. Matsuda, T. Kitahara, S. M. Lyth, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 169, 044523 (2022).Nakazato, D. Kawachino, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, K. Sasaki, J. Electrochem. Soc., 165, F1154 (2018).Matsumoto, M. Nagamine, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, K. Sasaki, J. Electrochem. Soc., 165, F1164 (2018).Ogawa, Y. Inoue, K. Yamamoto, M. Yasutake, Z. Noda, S. M. Lyth, J. Matsuda, M. Nishihara, A. Hayashi, K. Sasaki, ECS Trans., 109 (9), 241 (2022).Inoue, M. Yasutake, Z. Noda, S. M. Lyth, M. Nishihara, A. Hayashi, J. Matsuda, K. Sasaki, ECS Trans., 109 (9), 413 (2022). Figure 1

Effect of Solvent on Ionomer Adsorption Behavior of Polymer Electrolyte Membrane Fuel Cell Catalyst

Polymer electrolyte membrane fuel cell (PEMFC) is a promising power source with zero local carbon emissions that may aid in realizing the development of low-carbon societies. Catalyst layers (CL) composed of catalyst composites and an ionomer are key components in PEMFC. To control the structure of the CL, especially for the localization of the ionomer, understand the adsorption behavior on carbon surface is important. The quantitative analysis of ionomer adsorption isotherm is a powerful tool for analyzing the adsorption behavior, however, existing experiments have been carried out in water, and the effects of alcohol are yet to be considered. Because the mixtures of water/alcohol are used in the actual fabrication of PEMFC, the adsorption isotherm using the binary mixtures of water/alcohol as the solvents could provide practical information to optimize the PEMFC performance.Principal project of this study is to investigate and evaluate the effects of the alcohol contents on the interaction between the ionomer (Nafion) and the surface of the carbon support. The control of the interaction between ionomers and catalyst composites is as well as crucial in controlling the PEMFC performance. As the catalyst composites, commercial Vulcan/Pt (CB/Pt) and Pt-loaded polybenzimidazole (PBI) polymer-wrapped Vulcan (CB/PBI/Pt) catalysts were compared. For the detailed Nafion adsorption experiment, using synthesized CB/PBI/Pt and commercial CB/Pt disperse in aqueous ethanol solutions (87 or 20 wt%) with different amounts of Nafion (i.e., carbon-to-ionomer weight ratio, I/C=0.1~1.4). Then the dispersions were filtered with a syringe filter, and the filtrates were measured with 19F-NMR to calculate the amount of adsorption. For 19F-NMR, filtrates were added to trifluoroacetic acid and heavy water used as internal standard and deuterium solvent, respectively. The amount of Nafion was calculated based on the integral ratio of the largest peaks of Nafion ranging from −115 to −130 ppm in reference to the trifluoroacetic acid peak at −75.5 ppm. To prepare the adsorption isotherm, the adsorption amount (Γ [mg·mg−1]) of Nafion was plotted as a function of the equilibrium concentration (C e). The maximum adsorption amount (Γ max [mg·mg−1])) and Langmuir constant (K L) were determined based on the Langmuir equation. This paper will also discuss the difference in CL structure and cell performance at I/C=0.72 which formulation in low (water-) and high ratio alcohol (alcohol-rich) in ink solvent.PBI-coated CB (CB/PBI) was prepared according to our previous reports1, and the adsorption isotherms were measured in water- and alcohol-rich solvents (Figure.1), in which the BET values of 88 and 119 m2·g-1 for CB/PBI and CB/PBI/Pt, respectively, were used for the calculation of Γ’ (mg·m−2) in place of Γ (mg·mg−1). For the first layer adsorption, K L was calculated as 82 and 55 in the water- and alcohol-rich solvents, respectively. Comparing these values with those of CB, clear increase of the K L in the alcohol-rich solvent from 32 (CB) to 55 (CB/PBI) was obtained, whereas the K L values in the water-rich solvent were almost comparable between CB (89) and CB/PBI (82). In addition, the Γ max of the first layer in the alcohol-rich solvents increased from 0.024 mg mg‒1 (CB) to 0.034 mg mg‒1 (CB/PBI), while that in water-rich solvents were comparable between CB (0.043 mg mg‒1) and CB/PBI (0.043 mg mg‒1). Both results clearly indicate the stronger interaction between PBI and Nafion compared to the interaction between CB and Nafion in alcohol-rich solvents, while strong interaction in water-rich solvents was still maintained. A similar trend was also observed for Pt-loaded CB/PBI (CB/PBI/Pt), namely, the K L (43) and Γ’max (0.218 mg·m−2) were larger than those of CB/Pt (K L = 25 and Γ’max = 0.179) in the alcohol-rich solvent, while in water-rich solvent, K L (44) and Γ’max (0.277 mg·m−2) of CB/PBI/Pt were very high similar to CB/Pt (K L = 29 and Γ’max = 0.188 mg·m−2). On the other hand, when we compare Γ’max values of CB/PBI and CB/PBI/Pt, the decrease of the values after Pt loading in the water-rich and alcohol-rich solvents were observed, suggesting the Nafion adsorption onto Pt surface is inferior for PBI systems regardless of alcohol content.Furthermore, the performances of different catalysts were evaluated and discussed to understand the effect of alcohol content on Nafion adsorption and CL structure. Figure. 2 represents a performance curve that maximum power density of MEA having CB/Pt was 580 and 750W cm-2, while that of CB/PBI/Pt was 720 and 810 W cm-2, in alcohol and water-rich solvents, respectively. The high current density range (1.2~2A cm-2) also showed higher potential in water-rich conditions that related to O2 diffusion in CL.Reference 1. S. M. Jayawickrama,T. Fujigaya.et al. Electrochim. Acta. 312, 20 (2019). Figure 1

Power Generation Charactristics of Polymer Electrolyte Fuel Cells with Electrocatalysts Supported on SnO2 in High Current Density Range

The durability of cathode catalysts in polymer electrolyte fuel cells remains a significant technical issue. Start-stop cycles cause carbon support corrosion and Pt catalyst particle detachment from the support leading to a significant cell degradation. SnO2-supported electrocatalysts using carbon materials as the conducting framework have the potential to achieve both high activity and high durability. However, cell performance in the high current density range needs to be improved to apply them to heavy-duty vehicles. This study selected mesoporous carbon (MC) as the conductive framework because the use of mesopores could also improve load-cycle durability. In this study, the microstructure of the electrocatalyst layers using Pt/Sn0.98Nb0.02O2/MC is controlled to improve the power generation of the cells. In addition, Pt/Sn0.98Nb0.02O2/MC electrocatalyst demonstrates durability improvement against the start-stop cycles.

Impact of self-sputtering in high power impulse magnetron sputtering (HiPIMS) with helium

Conventional magnetron discharge is a widely used technology for many applications. In the last decade, the high current density sputtering regime has been an interesting alternative for tailoring thin film properties. In this paper, we focused on the electrical characterization of the helium magnetron plasma operated at average gas pressure (5 Pa) with a molybdenum target. Optical emission spectroscopy diagnostics also supports this study by providing information on electron density evolution. The analysis of the plasma–surface interaction zone on the target unveiled the physical mechanisms associated with the high current density range (6 A cm−2), corresponding to different discharge regimes. The self-sputtering yield plays a key role in high-power impulse magnetron sputtering discharge operated with helium. The electron density is highly dependent on the presence of a metal.

(Digital Presentation) Effect of the Hydrophilic Layer in Double Microporous Layer Coated Gas Diffusion Layer on PEFC Performance

The polymer electrolyte fuel cells (PEFCs) are commonly used in the vehicle industry, but the relatively higher costs of power generation limit the potential for further application. The PEFCs output power can be mainly undermined by the water management of the membrane electrode assembly (MEA). If the water content balance in the MEA is broken by fast water expelling, the dehydrated membrane significantly raises the proton transport resistance, which results in severe ohmic resistance loss. Oppositely, the slow water removal pace makes the production water stay at the catalyst layer (CL) surface, and the reactants are prohibited from accessing the reaction area and causing high reactants concentration overpotential during the high current density range. To elongate the limiting current density and raise the output power, beneficial water management should keep the MEA from dehydration by using the water generated from electrochemical reactions and can expel additional water from the CL and gas diffusion layer (GDL) interface. To implement the favorable water balance in the cell, the GDL with the microporous layer (MPL) is necessary.Traditional MPL mainly contains carbon black and hydrophobic binder Polytetrafluoroethylene (PTFE), which can guarantee water-unoccupied pathways for gas transport. The produced water can be expelled by the relatively large pores, and the water transportability is mainly controlled by the pore size and hydrophobicity. When the pore size is excessively narrow, the easily gathered water creates flooding between the CL and GDL. Large pores can solve this issue but bring the water stored in the MPL. Thus, a double MPL is developed to achieve better water management when it cannot forwardly enlarge the pore sizes under low and high humidity conditions. A commercially available hydrophobic MPL coated GDL (SGL 22BB) is the standard sample in this study. As for double MPL coated GDL, the hydrophilic layer is coated on the hydrophobic MPL coated GDL. One candidate composition uses Nafion as the hydrophilic binder, TiO2 as the hydrophilicity improvement particles, and the rest of the part is carbon black; the other way applies only Polyvinyl alcohol (PVA) as the hydrophilic binder and mixes with carbon black. Both types of hydrophilic slurry are directly coated on the 22BB, and the maximum pore diameter slightly changed from 45um (SGL 22BB) to a smaller size. These very thin hydrophilic layers modify the surface properties, which can help reduce the surface contact angle and make water easier to be introduced into the hydrophobic MPL.According to the tests, the performance of the double MPL containing PVA binder becomes worse than the Nafion-TiO2 double MPL, even than standard hydrophobic MPL. Due to the strong hydrophilicity of the PVA binder, even though a tiny amount of it is added to the top layer, water accumulation still occurs in the MPL, so the PVA binder is not suitable for the MPL property modification. The double MPL, which applied a Nafion-TiO2 hydrophilic layer, achieves lower oxygen transport resistance under high humidity conditions than the standard hydrophobic MPL. Besides, the appropriate composition of the hydrophilic contents is determined. With the increase of the TiO2 and Nafion content, the significantly enhanced hydrophilicity leads to more water absorption. However, it blocks the gas pathways, showing terrible reactants transport and high concentration over-potential. When the hydrophilic content becomes overly low, it is not enough to afford the water expelling, and water still occupies the MPL and CL interface. The thickness of the top hydrophilic MPL is another critical design parameter. A too thick hydrophilic layer can hold more water and cause a high risk of hampering reactants supply. The moderate thickness of the hydrophilic layer should be less than 5μm, which guarantees the function of the water transport and keeps away from severe water absorption.

(PEFC&E 2022 Student Poster Award Winner - 1st Place) MEA Performance of Pd@Pt Core-Shell Catalysts Supported on Different Particle Sizes of Mesoporous Carbon

1. Introduction Very high performance, especially on output power density (6 kW/L), for PEFCs in FCVs is targeted for 2030 in Japan [1]. Therefore, we need to improve not only ORR activity, but also oxygen diffusivity of Pt-based cathode catalysts for PEFCs. In this study, highly interconnected mesoporous carbon (MPC) [2] was selected as a support of Pd@Pt core-shell catalyst. The Pd@Pt core-shell catalyst NPs were supported on different particle size of the MPC and cell performance was investigated. 2. Experimental We selected CNovel®MH-18 (central mesopore diameter: 4 nm, SBET: 1,334 m2/g, TOYO TANSO) as a MPC support. As-received MPC with an average size of ca. 2 μm was pulverized to ca. 800, 450, and 250 nm by a wet bead-milling process. The Pd core NPs were loaded on MPCs by an impregnation-thermal reduction method and the Pt shell was formed on the Pd core NPs by a direct displacement reaction method [3]. Characterizations of the MPC, the Pd/MPC cores and the Pt/Pd/MPC catalysts were conducted by N2 gas adsorption, TG-DTA, XRD, XRF, TEM, SEM, CV and the ORR activity of the catalysts was evaluated by the RDE method. MEAs with 1×1 cm2 were fabricated by a decal method using a reinforced membrane (12 µm in thickness, Gore) and a GDL (TORAY). The Pt metal loading at the cathode was 0.1 mg-Pt/cm2. Nafion® DE2020 was used as an ionomer for the catalyst ink preparation (I/C was set to 0.83). I-V performance was evaluated at 80oC, 35-95% RH. H2 gas (418 mL/min.) and air (998 mL/min.) were supplied to the anode and the cathode, respectively, and H2 gas was pressurized to 150 kPa (ambient + 50 kPa) at gas-outlet. 3. Results and Discussion N2 gas adsorption data were analyzed by the t-plot method and the changes in the total, external and internal surface area of the MPCs pulverized for different times are shown in Figure 1. It was shown that the total and the external surface area increased, while the internal surface area decreased with pulverization time. These results suggest that fresh surface was newly formed by the pulverization, and the fraction of the internal surface was decreased. Figure 2 shows cross-sectional TEM images of the Pt/Pd/MPC catalysts using different sizes of MPC supports (ca. 2,000, 800, 450 and 250 nm), which indicates that the catalyst NPs were uniformly supported throughout the interior of MPC regardless of the particle size of MPC. The ECSA of the Pt/Pd/MPC catalysts changed little with the particle size of MPC (ca. 120 m2/g-Pt), while the ORR mass activity at 0.9 V was the highest (1,533 A/g-Pt) when the catalyst NPs were supported on relatively large MPC (800 nm).I-V curves at 80oC, 35% RH of the MEAs using the Pt/Pd/MPC cathode catalysts using different sizes of MPC supports are demonstrated in Figure 3. Interestingly, the cell voltage was higher over the entire current density range for Pd@Pt catalysts using MPCs with larger particle sizes (2000 and 800 nm). This tendency was also observed at a high relative humidity of 95% RH. These results suggest that the Pd@Pt catalyst NPs supported on larger MPC (2,000 and 800 nm) contributed to ORR in the high current density range even when the relative humidity was low. Figure 4 compares the connectivity of mesopores in Ketjen Black EC-600JD (KB-600JD, LION) and CNovel® MH-18 visualized from 3D-TEM observation results. It is clear that the MPC has a higher mesopore’s connectivity than KB-600JD. Figure 5 shows cross-sectional SEM images of the Pt/Pd/MPC. Larger pores were observed inside of the MPC support. Therefore, it is considered that the high connectivity of the mesopores and the presence of larger pores inside of the MPC support contributed to the superior I-V performance observed in MEAs using the catalysts supported on MPCs with particle size of ca. 2,000 and 800 nm. At present, it is not clear why the catalysts supported on smaller MPCs (450 and 250 nm) exhibited inferior I-V performance, though the oxygen diffusion inside MPCs should be easier than that for larger ones.This study was partly supported by NEDO, Japan. The CNovel®MH-18 supports with different size were supplied by Dr. Shodai of TOYO TANSO.

Power Generation Performance of Polymer Electrolyte Fuel Cells with Electrocatalysts Supported on SnO2 in High Current Density Range

1. Introduction Considerable efforts have been made for FCEV commercialization, where both performance and durability have to be achieved1.SnO2-supported electrocatalysts with carbon backbone have a potential to achieve both high catalytic activity and high cycling durability2-3, but it is necessary to further optimize electrochemical performance up to high current densities for such alternative electrocatalysts in the future. The objective of this study is to examine cell performance at high current densities by varying electrocatalyst layer fabrication conditions and selecting suitable gas diffusion layer and sealant thickness for the MEAs. 2. Experimental Membrane-electrode-assemblies (MEAs) were fabricated using a Pt/C standard catalyst (Tanaka Kikinzoku Kogyo, TEC10E50E) for both electrodes, and cell performance up to 2.5 A/cm2 were examined by varying sealant thickness and gas diffusion layers on the cathode side. Using mesoporous carbon (MC) as a support backbone and SnO2 as a support surface layer on the cathode side, cells with Pt/Sn0.92Nb0.08O2/MC electrocatalysts were fabricated. Cell performance and overvoltages up to high current densities were evaluated by varying ionomer ratio and heat treatment temperature in electrocatalyst layer fabrication. Cells prepared under optimized conditions were fabricated and subjected to a 60,000-start-stop cycle tests. 3. Results and discussion The cell with the standard Pt/C catalyst exhibited the lowest concentration overvoltage and the highest performance when the GDL was 22BB (SGL Carbon), the GDL thickness was 225μm, and the sealant thickness was 100 µm on the cathode side, as shown in Fig. 1 (measured at full humidification at 80°C). When the 22BB GDL with microporous layer (MPL) is used, the cell performance was found to be improved by compressing GDL with a thinner sealant.For cells with the alternative SnO2-based catalyst, cell performance reached a maximum when the ionomer was 8 wt.% of the catalyst, when the GDL was 22BB, and when the sealant thickness was 100μm. Furthermore, as shown in Figs. 2 and 3, the activation overvoltage was minimized, and cell performance was maximized when the heat treatment temperature of tin oxide loading was 600°C. Under the optimized conditions, the ECSA retention after 60,000 start-stop voltage cycles was 45% as shown in Fig. 4. This confirms that even when MC is used as the catalyst support backbone, the cycle durability is improved by applying tin oxide.In the future, we aim to further improve the catalytic activity by modifying parameters such as heat treatment temperature and loadings in catalyst preparation to reduce activation overvoltage. In addition, to suppress the increase in concentration overvoltage, we have to improve water management. Based on these efforts, latest results of cell performance will be reported in this presentation. Acknowledgments A part of this study was supported by the New Energy and Industrial Technology Development Organization (NEDO). References Takahashi, T. Ikeda, K. Murata, O. Hotaka, S. Hasegawa, Y. Tachikawa, M. Nishihara, J. Matsuda, T. Kitahara, S. M. Lyth, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 169, 044523 (2022).Nakazato, D. Kawachino, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, K. Sasaki, J. Electrochem. Soc., 165 (14), F1154 (2018).Matsumoto, M. Nagamine, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, K. Sasaki, J. Electrochem. Soc., 165 (14), F1164 (2018). Figure 1

Power Generation Performance of Polymer Electrolyte Fuel Cells with Electrocatalysts Supported on SnO2 in High Current Density Range

The durability of conventional Pt/C catalysts is still an important technical issue for polymer electrolyte fuel cells (PEFCs). SnO2-supported electrocatalysts with carbon materials as electronic conductive frameworks have the potential to achieve both high catalytic activity and high durability against carbon support corrosion. However, it is still necessary to optimize cell preparation conditions to improve electrochemical cell performance especially in the high current density range. Here, PEFCs with such electrocatalysts are prepared by varying the preparation conditions of the catalyst layer in the membrane electrode assembly (MEA) and a combination of gas diffusion layers (GDLs) and sealant thicknesses. Microstructural optimization is still needed for improving cell performance and minimizing overvoltages.

Ferroelectric Nanogap-Based Steep-Slope Ambipolar Transistor.

The subthreshold swing (SS) of metal-oxide-semiconductor field-effect transistors is limited to 60mV dec-1 at room temperature by the Boltzmann tyranny, which restricts the scaling of the supply voltage. A nanogap-based transistor employs a switchable nanoscale air gap as the channel, offering a steep-slope switching process. Meanwhile, nanogaps featuring even sub-3 nm can efficiently block the current flow, exhibiting the potential for tackling the short-channel effect. Here, an electrically switchable ferroelectric nanogap to construct steep-slope transistors, is exploited. An average SS of 15.9mV dec-1 across 5 orders and a minimum SS of 13.23mV dec-1 are obtained in the high current density range. The transistor exhibits excellent performance with near-zero off-state leakage current and a maximum on-state current of 202 µA µm-1 at VDS = 0.5V. In addition, the transistor can turn off with either a positive or negative increase in the gate voltage, exhibiting ambipolar characteristics.

Coupled FEM and CFD Modeling of Structure Deformation and Performance of PEMFC Considering the Effects of Membrane Water Content

Based on a coupled finite element method (FEM) and computational fluid dynamics (CFD) model, the structural deformation and performance of a proton exchange membrane fuel cell (PEMFC) under different membrane water contents are studied. The water absorption behavior of the membrane is investigated experimentally to obtain its expansion coefficient with water content, and the Young’s modulus of the membrane and catalyst (CL) are obtained through a tensile experiment. The simulation results show that the deformation of the membrane increases with water content, and membrane swelling under the channel is larger than that under the rib, forming a surface bump under the channel. The structural changes caused by the membrane water content have little effect on the performance of PEMFC in the low-current density range; while its influence is significant in the medium- and high-current density range. A medium membrane water content value of 12 achieves the best fuel cell performance due to the balance of membrane resistance and mass transport.

Behavior of high current density pulsed magnetron discharge with a graphite target

Conventional magnetron discharge with a graphite target is a technology used worldwide to deposit thin films for a large range of applications. In the last decade, the high current density sputtering regime stands out as a very interesting alternative allowing the tailoring of coating properties. The peak power density normalized to the target area can exceed 107 W m−2, leading to an important ionization of the sputtered atoms. In this paper we focused on the electrical characterization of a magnetized plasma operated at average gas pressure (5 Pa; Ar and He) with a graphite target. A cross-correlation with a high-speed gated camera and optical emission spectroscopy measurements of the plasma evolution is also given. The analysis of the plasma–surface interaction zone on the target unveiled the physical mechanisms associated with the high current density range (1.8–32.5 A cm−2), corresponding to several regimes of discharge. For graphite, it will be demonstrated that the gas rarefaction induced by the vapor wind is negligible due to its low sputtering yield. Thus, the gas recycling is the dominant mechanism sustaining the discharge, even for the higher discharge current regime when a spot is present. Spokes and other instabilities were also identified and are discussed.

Relationship between Evaluation of Oer Performance and Oxygen Bubble Generation Behavior in Alkaline Water Electrolysis

Introduction Alkaline Water Electrolysis (AWE) is anticipated as a mean of producing hydrogen as an energy carrier for sustainable development goals. One of the problems in alkaline water electrolysis is the rise in anode overvoltage at the high current density range. To further improve the efficiency of hydrogen production, grasping the bubble generation behavior and development of electrode materials based on it are important. In this study, using nickel electrodes with different diameters, the formation behavior of oxygen bubbles on the electrodes was observed with a high-speed video camera, and the relationship between the current density / electrode potential and the bubble formation behavior was investigated. Experimental Working electrode were Ni wire (NiW) (two types with diameters of 100 and 200 μm). A reference and a counter electrode were a reversible hydrogen electrode (RHE) and Ni rods (diameter 3 mm), respectively. All measurements were performed with a three-electrode electrochemical cell [1] at 303±1K with KOH aqueous solution. To reduce the solution resistance as much as possible, the working electrode and the reference electrode were placed as close as possible (about 1 mm). To equalize the current density distribution, Ni rods counter electrode were inserted from both sides of the cell. Observation windows were fitted on the sides of the cell. The working electrode was exposed only the horizontal part about 3 mm where we want to observe the electrolytic reaction, and the other parts were covered by PTFE heat-shrink tube and epoxy resin. After appropriate pretreatments, cyclic voltammetry (CV) measurement was performed at a scan rate of 5 mV s-1 to obtain polarization curves. Electrochemical impedance spectroscopy (EIS) measurement was performed at 1.5-1.8 V vs. RHE with a potential amplitude 10 mV, a frequency range of 1-106 Hz and 20 repeated measurements for each frequency. The solution resistance was obtained from the Nyquist plots, and the potential loss was corrected. The current density i geo / A cm-2 was normalized by the geometric surface area of the cylindrical shaped working electrode. Constant current electrolysis was performed under five conditions of i geo = 0.05-1.0 A cm-2. Under each condition, the formation behavior of oxygen bubbles was photographed with a high-speed video camera. From the obtained photographed images, the diameters of hundreds of oxygen bubbles detached from the electrodes were analyzed to calculate the Sauter mean diameter d 32. Results and discussion Figure 1 shows the polarization curves of the Ni wire electrodes with different diameters. In the range of i geo < 0.3 A cm-2 where the logarithmic value of the current density i geo changed linearly with respect to the potential, there was no significant difference in an OER performance among the two types of electrodes. In the range of i geo > 0.3 A cm-2, the overvoltage increased with i geo regardless of the electrode diameter, and it was confirmed that the smaller the diameter of the electrode was, the smaller the overvoltage at the same current density was, especially in the high current density range.Figure 2 shows the relationship between current density and overvoltage. It was confirmed that the overvoltage increased as the current density increased, and the smaller the surface curvature of the electrode was, the larger the overvoltage was.Figure 3 shows the relationship between the current density and oxygen bubble diameter. The larger the current density was, the larger the oxygen bubble diameter was, and the oxygen bubble diameter was in proportion to the 1/3 power of i geo. From the in-situ observation of the oxygen bubble generation behavior on the electrode with a high-speed video camera [2], the oxygen bubbles generated on the electrode coalesced and detached from the upper end of the Ni wire electrode placed horizontally. From the above, it was considered that the larger the surface curvature of the electrode was, i.e. the smaller the electrode diameter was, the smoother the detachment of oxygen bubbles from the electrode was. It is implied that large curvature of electrode surface contributed to the smaller overvoltage. Acknowledgements A part of this study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in Advancement of Hydrogen Technologies and Utilization Project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Reference [1] Y. Kojima, R. Misumi, M. Kaminoyama, S. Mitsushima, 43rd Electrolysis Technology Debate, 14 (2019)[2] H. Ikeda, Y. Kojima, R. Misumi, M. Kaminoyama, S. Mitsushima, 22nd Society of Chemical Engineers Student Presentation, D08 (2020) Figure 1

Comparison of different performance recovery procedures for polymer electrolyte membrane fuel cells

To both extend system lifetime and enable reliable performance benchmarking, recovery procedures are of great importance to distinguish and mitigate the reversible and irreversible performance degradation of polymer electrolyte membrane fuel cells. In this work, three common recovery procedures available in the literature (JRC-based protocol, DOE-based protocol, and overnight rest) being part of a load cycling durability test are characterized by polarization curves and electrochemical impedance spectroscopy. Such direct comparison using same conditions and material has not been published so far. To compare the relative recovery of the three recovery procedures the recovery related to the last operation period, to the beginning of the whole test, and the non-recovered performance loss within each operation period are assessed. Moreover, the mechanisms leading to the various recovery effects are analyzed. Generally, with the contribution of gas purging to water removal, all three recovery procedures reduce greatly mass transfer resistance and recover most of the performance loss in the high current density range. At lower current density, the three procedures differ substantially. In the case of JRC-based protocol, the kinetic losses are recovered by the reduction of Pt oxides and structure change of ionomer by reducing the cathode potential and fuel cell temperature, respectively. The overnight rest results in similar recovery of the performance loss. The DOE-based protocol leads to relatively low recovery of losses in the kinetic region of the polarization curves. Additionally, the effect of the cell operating history is considered.

CuCoO2 electrodes for supercapacitor applications

Transition metal oxides are best research materials used for energy saving in electrochemical supercapacitor applications. CuCoO2 electrodes are introduced for supercapacitor applications. The pristine and different variations of NaOH concentrations have been investigated. CuCoO2 electrode specific capacitance is 274, 336, 342F/g at 10 mV/s from CV and 208, 218, 250F/g at 0.5 A/g from GCD. 81.36% capacity has been maintained after 5000 cycles in high current density range. Therefore, these transition metal oxide CuCoO2 electrodes are very gifted for future energy storage applications.

Recombination pathways and hole leakage behavior in InGaN/GaN multiple quantum wells with V-shaped pits

Abstract Hole transport in InGaN/GaN multiple quantum wells (MQWs) LEDs has been proposed to be insufficient due to relatively large effective mass and low mobility. Here, for the first time, we have direct observed hole overflow from MQWs and enter into InGaN/GaN superlattices (SLs) layer in V-pits-containing GaN-based green LEDs at room temperature. To suppress the hole leakage, an n-AlGaN hole blocking layer (HBL) is inserted between SLs layer and MQWs. The experimental show an interesting and baffling result is that the HBL-containing sample not only exhibits severe hole leakage in high current density range, but also has higher external quantum efficiency (EQE). Numerical simulation reveals that the HBL can change the transportation path of carriers, forcing more electrons injecting from c-plane into the MQWs instead of from V-pit sidewalls. This will increase the carrier concentration in the MQWs, and eventually lead to higher EQE. Meanwhile, less electrons transported through V-pit allowed more holes injecting into deeper layers via V-pit sidewalls, and electroluminescence emission from the SLs layer became stronger when HBL was inserted.

Development and performances of a 0.5 kW high-pressure alkaline water electrolyser

The purpose of this research paper is to describe the characteristics and electrochemical performances of a pressurized alkaline water electrolysis short stack (5-cells, 0.5 kW) operated at 80 °C, from atmospheric pressure up to 100 bars. Expanded grids of metallic nickel covered with specific porous catalytic structures have been used as working electrodes. A polysulfone-based diaphragm with a high ionic conductivity has been specifically designed for operation in pressurized alkaline water electrolysis cells. I–V polarization curves have been recorded at current densities up to 1000 mA/cm2, at temperatures up to 80 °C and under pressures up to 100 bars. The water electrolysis efficiency of this short-stack has been determined. A specific energy consumption of ca. 4.4–4.5 kWh/Nm3 has been obtained in the high current density range. Durability tests have been performed on the short stack over 1000 h. A limited degradation rate <5 μV/h has been recorded over that period of test.

Free-Standing Hollow Electrospun Carbon Nanofiber Web As High-Performance Electrode for Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFBs) as large-scale energy storage devices have considered being competent for the utilization of renewables to address their intermittency and fluctuation characteristics [1]. Although there are significant merits and some demonstrations with the power scale up to mega-kilowatt level have been proved, the broad market penetration of VRFBs is still hampered by their relatively high cost [2, 3]. To address this issue, the approaches to improve the cell performance, including the power densities and energy efficiencies, are highly required. Since boosting the cell performance will result in a reduced stack size, which will reduce the cost of the stack repeatable components, such as bipolar plates, separators, and electrodes [4]. Generally, the cell performance is highly related to the porous electrodes that provide the electrochemical reaction sites and transport the species. The conventional carbon electrodes are suffered from the poor electroactivity, thereby leading to a low energy efficiency.In this regard, we here developed a free-standing hollow electrospun carbon nanofiber web (hECNFW) as a high-performance electrode for VRFBs. The hECNFW was fabricated using co-axial electrospinning of poly(acrylonitrile) shell solution and styrene-co-acrylonitrile core solution and subsequently carbonized at high temperature, as illustrated in Figure 1a. Contrary to the conventional carbon electrode consisting of carbon fibers with a diameter of several micrometers, the hECNFW was constituted by nanofibers with a diameter of 200-500 nm. Hence, the specific surface area was largely increased, thereby boosting the electrochemical kinetics for vanadium reaction. Meanwhile, some nitrogen-containing functional groups were remained in the matrix of carbon nanofibers although they were carbonized at temperature as high as 1000oC. The existence of the nitrogen residue was confirmed by X-ray photoelectron spectroscopy. The nitrogen groups in the carbon surface modified the adsorption behavior of vanadium ions and accelerated the conversion of vanadium ions among different chemical valences. Finally, the surface of hECNFW became more hydrophilic, which was beneficial for the adsorption of the vanadium ions.The battery performance of VRFBs assembled with the hECNFW was preliminarily evaluated. As shown in Figure 1b, the battery can be operated at a high current density range. More importantly, at a current density of 200 mA cm-2, the battery achieved an energy efficiency of 83.7%, which was much higher than that of results reported previously. The performance can be further improved by optimizing the assembly work between the thin electrode and membrane. This work is under progress. Meanwhile, further characterizations, including the graphitization, the surface state and microstructure of hECNFW, will be presented at the meeting.The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R).

Reaction of hydrogen evolution on Co−Mo (W) and Ni−Re electrolytic alloys in alkaline media

The mathematical processing of experimental data is performed for the hydrogen evolution reaction (HER) in alkaline media on Co−Mo and Ni−Re cathodes obtained by electrodeposition. It is shown that it is necessary to take into account the possibility of occurrence of the stages of discharge (Volmer), electrochemical desorption (Heyrovský), and recombination of adsorbed hydrogen atoms (Tafel) in HER description on Co−Mo electrodes. Hydrogen evolution on Ni−Re cathodes is well modeled under the assumption of the Volmer–Heyrovský route (discharge and the following electrochemical desorption), but the contribution of diffusion of Hat into the cathode material is significant in the range of high cathodic current densities.

Effect of strengthened road vibration on performance degradation of PEM fuel cell stack

The vehicular fuel cell stack is unavoidably impacted by the vibration and shock in the real-world due to the road unevenness. However, influences of vibration on fuel cell stack have yet to be investigated completely. In this paper, the performance of a fuel cell stack is experimentally studied in terms of gas-tightness, voltage degradation, AC impedance spectra, polarization curve and characteristic parameters in polarization curve through long-term strengthened road vibration tests, in order to investigate the influences of road-induced vibration on performance degradation of fuel cell stack. The vibration tests are carried out on a six-channel multi axial simulation table with the vibration excitation spectra originally derived from the strengthened road of the ground prove. During the vibration test, several kinds of performance test including gas-tightness test, AC impedance diagnosis and polarization curve test are conducted at regular intervals. After the vibration test, the gas leakage rate of anode reaches 1.73 times of the initial value. The open circuit voltage and rated voltage decreases by 0.90% and 3.58%, respectively. Meanwhile, the performance of individual cell voltage uniformity becomes worse distinctly. With the increase of vibration duration, the ohmic resistance obtained from AC impedance diagnosis ascends approximately linearly and presents a growth of 5.36% ultimately. An improved empirical fuel cell polarization curve model is adopted to fit the current–voltage data and estimate the characteristic parameters which decide the shape of polarization curve. It is noted that the limiting current density declines distinctly and the mass transfer loss increases mainly at the range of high current densities. The results indicate that the long-term strengthened road vibration condition exerts a significant influence on the durability of fuel cell stack.