Energy Development Organization Research Articles

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

Comparison of MEA Degradation through FCV Actual Driving Test and Load-Cycle Durability Test

Introduction In research and development of high-durability polymer electrolyte fuel cells (PEFCs) for fuel cell vehicles (FCVs), accelerated stress tests (ASTs) are employed to evaluate durability of materials with a short time [1-3]. In order to validate an AST, it is necessary to confirm that the degradation mechanisms in the AST and in actual FCV operation are consistent, and to elucidate quantitative correspondence of their degradation rates. In this study, we analyzed degradation of a membrane electrode assembly (MEA) after FCV real-world driving and compared the degradation factor with that of a load-cycle durability test in a laboratory. Experiments MEA samples were extracted from an FC stack of a Toyota MIRAI 2014 model which was operated in real-world driving of 200,000 km, about 6000 h. MEA performance and electrochemical properties of them were investigated using a small differential cell with a straight flow field. As a comparison, another MEA at beginning of life (BOL) was subjected to a load-cycle durability test following a protocol encouraged by New Energy and Industrial Technology Development Organization (NEDO), Japan. Cell temperature and relative humidity were controlled at 80°C and 100%, respectively. A square-wave potential cycle shown in Figure 1 was adopted. MEA performance was investigated after cycles of 500, 1000, 3000, 10,000, and 30,000. Structures of catalyst layers and catalyst particles were analyzed for the MEAs after driving test and 30,000 cycles of the load-cycle test using scanning electron microscope (SEM), transmission electron microscope (TEM), and X-ray absorption fine structure (XAFS). Results and discussion The performance of the MEA after 200,000 km driving is close to that of the MEA after 30,000 cycles of the load-cycle test as shown in Figure 2. The performance degradation is mainly attributed to degradation of the catalyst layer, and properties of a gas diffusion layer and a proton exchange membrane are unchanged from those of the BOL sample. Degradation mechanisms of the catalyst layers in the driving test and the load cycle test are well corresponding. First, mass activities of PtCo catalyst decay to about a half of the BOL value mainly due to the decrease in electrochemically effective surface areas (ECSAs) as represented in Figure 3. Decrease rates of the ECSAs are consistent with the increase in average particle sizes of the catalyst. The catalyst size distributions of the two samples are also comparable. Then, oxygen transport resistance in the catalyst layers is increased. The resistance of the samples after the driving test and 30,000 cycles of the load-cycle test are twice as large as the BOL value. As shown in Figure 4, the transport resistance obviously exhibits inverse proportion to the catalyst surface area. This relationship represents that the primary factor of the resistance is one at vicinity of catalyst particles including ionomer permeation [4], and increase in it is attributed to decrease in the catalyst surface area. A performance simulation based on a simple 1-dimensioal MEA model supports that the decrease in the ECSA causes most of the MEA performance degradation through the deterioration in the catalyst activity and the oxygen transport resistance. Conclusions The MEA performance degradation after the 200,000 km real-world driving with the system of MIRAI 2014 model is primarily caused by catalyst metal coarsening and is well simulated by NEDO load-cycle durability test of 30,000 cycles. This result has validated the test protocol and should contribute to a target-setting of durability of future MEAs.

Suppression of PEFC Membrane Degradation By Using SnO2 As Electrocatalyst Support

Introduction Long-term durability of polymer electrolyte fuel cells (PEFCs) is required for various applications such as automobiles. It has been suggested that certain ions dissolved from electrocatalyst materials accelerate radical formation and subsequent chemical degradation of polymer electrolyte membrane (PEM) by the Fenton reaction [1]. Membrane degradation increases hydrogen and oxygen permeability and decreases proton conductivity and mechanical strength [2]. These phenomena significantly affect the electrochemical performance and durability of PEFCs. Therefore, in case metal oxides are used as catalyst support, it is necessary to check the dissolution of metal ions and to evaluate their effects on membranes. Our research group has been studying Nb-doped SnO2 (Sn(Nb)O2), showing improved durability of the electrocatalysts [3]. Here in this study, we evaluate the effects of Sn(Nb)O2 on the chemical degradation of PEMs. Experimental A commercial Pt/C catalyst (TEC10E50E, Tanaka Kikinzoku Kogyo, Japan) was used as the anode catalyst, while three types of cathode catalysts were applied and compared: 1) the commercial Pt/C; 2) Pt/MC where Pt was directly deposited on mesoporous carbon (MC); and 3) Pt/Sn(Nb)O2/MC where Pt was deposited on the Sn(Nb)O2 decorated on the MC. The durability test at open circuit voltage (OCV) was performed for 100 h. Before and after the durability test, cell performance, hydrogen crossover current density, and electrochemical surface area (ECSA) were characterized. The degree of membrane degradation was evaluated by measuring fluorine emission rate (FER) by ion chromatography, and membrane thickness and elemental distribution by energy-dispersive x-ray spectroscopy (EDS) coupled with field-emission scanning electron microscopy (FESEM). Results and discussion It was found that the use of Pt/Sn(Nb)O2/MC led to better durability in the OCV durability test. The decrease in OCV using Pt/Sn(Nb)O2/MC was smaller than that using Pt/C and Pt/MC, less than half of that Pt/C, as shown in Figure 1. The membrane thickness with Pt/Sn(Nb)O2/MC after the durability test was the same as that before the durability test, indicating that SnO2 actually suppressed the membrane degradation as shown in Figure 2. EDS elemental analysis confirmed no Sn ion dissolution from the cathode side as shown in Figure 3, also suggesting that SnO2 at the cathode suppressed the membrane degradation. In this presentation, we will quantitatively analyze the membrane degradation in using SnO2 and discuss possible mechanisms. Acknowledgment This paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Hydrogen and Oxygen Permeability through PEFC Membrane and Membrane Electrode Assembly

IntroductionThe permeation of feed gas through a proton exchange membrane (PEM) affects a PEFC performance [1, 2]. The temperature and relative humidity (RH) dependences of the hydrogen and oxygen permeability through the perfluorinated sulfonic-acid (PFSA) membrane were investigated. A membrane is considered to consist of a bulk layer sandwiched between skin layers [3]. In this study, by measuring the hydrogen and oxygen permeation flux through a membrane and plotting the permeation resistance against the membrane thickness, the bulk layer effective diffusion coefficients and the skin layer mass transport coefficients of PEM and membrane electrode assembly (MEA) were examined.ExperimentalNafion™ NR-211, NR-212, and N115 (Chemours) with different thickness (at RH of 50 %), 25.8, 51, and 127 μm, respectively, were used as PEM. Catalyst layers made of Pt-loaded carbon (Pt 50 wt%) and Nafion ionomer (I/C = 1.0) with a thickness of 8.5-10 μm on both sides of PEMs were prepared as MEA. They were installed separately in a Japan Automobile Research Institute (JARI) standard cell whose active area was 5.0 cm×5.0 cm. The cell was placed in the constant temperature and humidity oven (ESPEC, LHL-113) and the relative humidity of the supplied gas was controlled by the bubbler temperature. The hydrogen and oxygen fluxes through a membrane were measured at cell temperatures of 60-90 °C and RH of 0.2-1.0 (moisture content λ = 1.7-10.5) with different partial pressures of nitrogen diluent gas. Permeate gas was quantified by a gas chromatograph (Agilent 990 micro GC, column: MSieve5A, detector: TCD). The schematic drawing of the measurement system is shown in Fig. 1.Results and discussionThe permeate gas flux through a membrane, Nj (j = H, O), is expressed asNj = k pj Δpj , where k pj is the gas permeance and Δpj is the partial pressure difference between the supply side and the permeate side.Fig. 2 shows the temperature and RH dependences of O2 permeance through PEM and Fig. 3 shows the permeation resistance against the membrane thickness. k pO increases with increasing RH and 1/k pj becomes lower at higher temperature. The permeation resistance through MEA tends to be lower than PEM. By assuming that Nafion has a bulk layer and skin layers [3], the bulk layer and skin layer resistances were separated from a slope and an intercept of trend lines in Fig. 3. The permeation resistances through MEA do not have y-axis intercept. The fused layer can be created by hot press process in manufacturing MEA and an x-axis intercept might be a fused layer thickness. It is easier to permeate gas through the fused layer than the skin layer of PEM. The temperature dependences of the bulk layer effective diffusion coefficient, D ej (B), are shown in Fig. 4. D eH (B) and D eO (B) of PEM and MEA are on the same line, with activation energies, E a, of 23.9 and 20.6 kJ/mol, respectively at λ = 6. D ej (B) can be formulated as follows: D eH (B) = (4.26×10-11 + 6.50×10-11RH) exp(-E aH/R (1/T-1/T ref)) m2/s D eO (B) = (8.16×10-12 + 3.39×10-11RH) exp(-E aO/R (1/T-1/T ref)) m2/s T ref = 353.15 K, E aH = 0.65 λ + 20.0 kJ/mol, E aO = 21.2 kJ/mol,Reproduced D eH (B) and D eO (B) are shown in Fig. 5. The skin layer mass transport coefficient, k pj (S), can be obtained from the y-axis intercept of a trend line in Fig. 4 and it shows that only PEM has skin layers. It is necessary to consider the gas permeation through MEA not through PEM in designing PEFCs because using the transport properties through PEM may lead to overestimate the performance.ConclusionsBy measuring the hydrogen and oxygen permeation flux through PEM and MEA in PEFC, the bulk layer effective diffusion coefficients and the skin layer mass transport coefficients of hydrogen and oxygen were obtained. The bulk layer effective diffusion coefficients of PEM and MEA were found to be the same and the temperature and RH dependences of D ej (B) were formulated as a function of temperature and RH. The skin layer mass transport coefficient appears only in PEM.AcknowledgementThis work was supported by the FC Platform Program: Development of design-for-purpose numerical simulators for attaining long life and high-performance project (FY 2020–FY 2023) conducted by the New Energy and Industrial Technology Development Organization (NEDO), Japan.

Iridium Oxide Nanorods Supported on Sb-Doped SnO2 As Highly Active Oxygen Evolution Catalysts for PEM Water Electrolysis

With the increasing installation of renewable energy generation systems such as solar and wind power, efficient energy storage devices are required to overcome the problem of fluctuations in the availability of electric energy. In particular, the splitting of water by electrolysis to produce storable hydrogen and oxygen appears to be one of the most promising approaches for large-scale energy storage. Proton exchange membrane water electrolysis (PEMWE) has received a great deal of attention due to its ability to respond quickly to renewable energy fluctuations and to operate at high current densities. However, a key limitation of PEMWE is the high cost associated with the use of high loadings of noble metal Ir catalysts at the anode electrode, which is necessary due to the slow kinetics of the oxygen evolution reaction (OER). Therefore, reducing the Ir loading at the OER electrode is essential to realize large-scale applications of PEMWE. This requires the development of highly active OER catalysts to accelerate the reaction or reduce the overpotential. Herein, we report a remarkably active OER catalyst of Ir oxide with nanorod-like structure supported on antimony-doped tin oxide (Sb-SnO2), showing an order of magnitude increase in Ir mass-specific activity than a commercial IrOx catalyst at 1.5 V vs. RHE. We also investigated the temperature dependence of the OER activity, as a way to elucidate the mechanism of enhanced catalysis, from both experimental and theoretical perspectives.The Sb-SnO2 support material (Sb dopant content 4 at.%) with fused-aggregate network structure was prepared by flame pyrolysis method.1 The as-prepared support powder was annealed at 700 oC for 2 h in air using a rotary kiln furnace. Sb-SnO2 supported IrOx nanorods (NRs) catalyst was synthesized by a colloidal method.2 The obtained IrOx NRs/Sb-SnO2 powder was heat-treated at 350 °C in N2 atmosphere for 2 h and at 150 °C in 1% H2 (N2 balance) for 2 h. The Ir loading was determined to be 49.3 wt.% by inductively coupled plasma optical emission spectrometry (ICP-OES). The OER activity was evaluated in 0.1 M HClO4 at 25 to 80 oC using the rotating disk electrode (RDE) technique.2,3 From Figure 1a, it can be seen that the Sb-SnO2 support particles present an interconnected network structure, which is considered to be beneficial to enhance the electron conduction and gas diffusion in the electrocatalytic reaction. On the support, the majority of IrOx nanoparticles are shown to have coalesced, forming rodlike structures. Figure 1b shows OER polarization curves (iR-corrected and normalized to the Ir mass) of the IrOx NRs/Sb-SnO2 catalyst compared to commercial IrOx catalyst (TANAKA Precious Metals). It is clear that the IrOx NRs/Sb-SnO2 catalyst significantly outperforms the IrOx reference catalyst over the entire potential range, and the OER onset potential of the IrOx NRs/Sb-SnO2 catalyst is lower. At 1.5 V (Figure 1c), the IrOx NRs/Sb-SnO2 catalyst exhibits a mass activity (MA) of 0.40 A mg-1 Ir, which is about 10 times higher than that of the IrOx reference catalyst. This dramatically enhanced activity makes it possible to significantly reduce the Ir loading on the anode of PEMWE. Experimental observations and density functional theory (DFT) calculations suggest that the nanorod geometry and strong metal-support interactions may play an important role in lowering the energy barrier for the crucial first step of the OER, thereby enhancing the OER activity. Acknowledgement This work was partly supported by the JSPS KAKENHI (23H02059) and by the project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

Relationship between Bubble Generation Behavior and Hydrogen Evolution Performance at High Current Densities during Alkaline Water Electrolysis

Introduction Alkaline water electrolysis (AWE) has attracted considerable attention as a method to produce hydrogen. However, one of the problems with AWE is the increase in overpotential, which is attributed to bubbles covering the electrode surface at high current densities. Therefore, it is important to understand the bubble generation behavior to develop strategies for reducing the overpotential. In this study, we observed bubble generation behavior on Platinum (Pt) electrodes using a high-speed camera [1] and conducted electrochemical measurements to investigate the relationship between the current density/electrode potential and bubble generation behavior. Experiments Pt wires with diameters of 200 μm were used as the working electrodes. The reference and counter electrodes were a reversible hydrogen electrode (RHE) and Ni rods with a diameter of 3 mm, respectively. All measurements were conducted using a three-electrode electrochemical cell with a 2 M KOH aqueous solution as the electrolyte at 303±1K. To minimize the solution resistance, the working and reference electrodes were positioned as close as possible to each other, with a separation distance of approximately 2 mm. To ensure a uniform current-density distribution, a Ni-rod counter electrode was inserted on both sides of the cell. Observation windows were installed on the sides of the cells. Only a 3-mm parallel section of the working-electrode wire was exposed to the electrolyte to observe the electrolytic reaction, while the rest of the wire was covered with PTFE heat-shrink tube and epoxy resin. After appropriate pretreatment, cyclic voltammetry (CV) measurements were performed at a scan rate of 5 mV s-1 to obtain the polarization curves. Electrochemical impedance spectroscopy (EIS) measurements were conducted at −0.2 to −0.8 V vs. RHE with a potential amplitude of 10 mV, over a frequency range of 3×105 to 1 Hz, where 60 repeated measurements were performed for each frequency. The solution resistance was obtained from Cole–Cole plots and corrected for potential losses. The current density (i geo / A cm-2) was normalized by the geometric surface area of the cylindrically shaped working electrode. Constant current electrolysis was performed under twelve conditions of i geo = −0.05 to −1.5 A cm-2. Under each condition, the formation behavior of hydrogen bubbles was photographed using a high-speed video camera. Results and discussion Figures 1 and 2 show the polarization curves at the cathode and anode, respectively [2]. The overvoltage increased with i geo, as evidenced by the shift of the polarization curves from Tafel slope, in the range of −i geo ＜ 0.9 A cm−2 at the cathode and in the range of i geo > 0.6 A cm−2 at the anode. Figure 3 shows Cole–Cole plots obtained at i geo = −1.0 A cm−2 at the cathode and Figure 4 shows Cole–Cole plots obtained at i geo = 1.0 A cm−2 at the anode [2]. Figure 5 shows the hydrogen bubble generation behavior obtained at i geo = −1.0 A cm−2 at the cathode, and Figure 6 shows oxygen bubble generation behavior obtained at i geo = 1.0 A cm−2 at the anode [2]. The Cole–Cole plots indicate the resistance component on the low-frequency side at high current densities in addition to the charge transfer resistance. Moreover, the arc shapes of the resistance components differ between the anode and cathode, suggesting that different mechanisms are responsible for the increase in the overvoltage at high current densities. We believe that these resistances arise from reactant transfer limitations due to the hydrogen and oxygen bubble layers covering the electrode surfaces. Moreover, it was also confirmed that the charge-transfer resistance is inversely proportional to the current density, indicating that the effective electrode surface area does not change over a wide range of current densities. By comparing Figures 5 and 6, it was observed that oxygen bubbles at the anode where a larger overvoltage increase than cathode was measured had larger diameters and were less likely to detach from the electrode. Furthermore, the bubble layer formed on the electrode surface was thicker on the anode than on the cathode. Acknowledgements Part of this study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in the Advancement of Hydrogen Technologies and Utilization Project (JPNP14021), commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Corrosion Behavior of Al Alloy A6061 Jointed with High-Tensile Strength Steel By Friction Stir Welding in NaCl Solution

In recent years, weight reduction of automobile bodies has been effective in reducing carbon dioxide emissions. For this reason, the replacement of steel, which has a large specific gravity, with aluminum alloys and carbon fiber reinforced plastics is being promoted for the right materials in the right places. In this case, an appropriate dissimilar material joining method is needed in terms of ease of assembly, joining performance, and cost. Friction stir welding (FSW) has emerged as a promising joining method for a variety of engineering applications. FSW is a solid-phase joining technique that has been shown to have significant advantages over conventional welding in terms of the strength and quality of the joints produced. However, during joining of metallic materials by FSW, microstructural changes may occur that affect their corrosion degradation. In this study, the corrosion behavior of A6061 jointed with high-strength steel (SPCC980) by FSW in aqueous NaCl solution and the effect of Al alloy microstructural changes on it were investigated.In this study, FSW was performed on A6061 and SPCC980 steel plates, and galvanic corrosion behavior was investigated by immersion tests and surface observations, as well as electrochemical measurements of Al alloys. Immersion tests were performed in 1 M NaCl solution. Micro electrochemical polarization tests of A6061 were also performed to investigate the corrosion behavior.In the immersion test of the galvanic couple of A6061 and SPCC980 steel plates in NaCl solution, galvanic corrosion occurred with the Al alloy as the anode and the steel plate as the cathode, and hydrogen evolution was observed mainly from the BM site on A6061 in the initial stage of immersion. This is due to the hydrogen evolution reaction occurring inside the pitting corrosion of the A6061. The pits was formed near Fe-rich inclusions in the BM site. In the HAZ site, corrosion propagated from Fe-rich inclusions along grain boundaries, but no pitting corrosion was formed. In the NZ site, minor localized corrosion was developed. These results indicate that the susceptibility to localized corrosion and its morphology differ in each site on the A6061. The difference in localized corrosion susceptibility on the A6061 was considered to be caused by the difference in the distribution of Fe-rich inclusions. However, the fact that the size and number of Fe-rich inclusions in each site did not differ suggests that the difference in anodic dissolution activity near Fe-rich inclusions affects pitting corrosion susceptibility in each site. A6061 alloy is a precipitation-strengthened Al alloy reinforced by β'' phase, which is a metastable phase of Mg2Si, and dense precipitation of β'' phase was observed in the BM site. Therefore, the reason for the higher localized corrosion susceptibility of BM is related to the precipitation of the β'' phase.This paper is based on results obtained from a project, JPNP14014, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Multi-Modal Structure and Distribution Analysis of Functional Groups on Carbon Supports for Polymer-Electrolyte Fuel Cells

Although structures of the functional groups formed on the surface of the carbon supports and their distribution on/inside supports have crucial impacts on determining performance of the catalyst electrodes, understanding of the relationship between them remains unclear, because of limitation of experimental methods for quantitative analysis of them with high spatial resolution. It is well-known that functional groups of the catalyst supports affect chemical properties of the supports, such as, redox- and acid-base properties, hydrophobicity, and electronic interaction. Thus, they should have a great influence on various aspects those related to the performance of polymer electrolyte fuel cell (PEFC), such as the activity, mass transportability, and durability of the catalysts and catalyst layers, and manufacturing process of catalysts and catalyst layer. 1,2). Furthermore, recently, in addition to various conventional high-surface area carbon supports, such as, Vulcan and Ketjen black, accessible carbon supports with mesopores which can host Pt particles protects them from direct ionomer adsorption and allow proton and O2 to have reasonable access to Pt particles, have been attracting great attention for improving catalyst utilization, activity, mass transportability, and suppression of Pt dissolution 3-5). The relationship between structure variation of functional groups on various supports and electrode performances should be of interest. The major difficulty for analyzing the functional groups on carbon supports come from the structure complexity of carbon surface (as well as inner part of the carbon particles), inhomogeneous distribution of various functional groups produced in various chemical processes, and multi-component structure of catalyst electrodes with catalysts and ionomers. Against such background, in this study, we have developed a new experimental approach utilizing multi-modal analysis method with x-ray photoelectron spectroscopy (XPS), x-ray absorption spectroscopy (XAS), temperature-programmed-desorption mass spectroscopy (TPD-MS), vibrational spectroscopy, electron tomography (3D-TEM), and cryo transmission electron microscopy (cryo-TEM). By combining suitable experimental probes for each structural feature and each component in multi-component structures, and by analyzing in integrated manner, we are able to understand the complex structure of functional groups on and inside carbon supports. Here, we demonstrate an example of the analysis for Vulcan (VULCAN® XC72, Cabot Corporation) and Pt-supported Vulcan (TEC10V30E, Tanaka Kikinnzoku Kogyo K. K. ). Vulcan is a solid carbon support used for polymer electrolyte fuel cell catalysts. The sample was reduced for about 30 minutes in a 100% hydrogen atmosphere at room temperature. Hard x-ray photoelectron spectroscopy (HAXPES) using synchrotron radiation x-rays with x-ray energy of 8 keV (BL46XU at SPring-8) . Also soft X-ray XAS were performed on BL1N2 at Aichi SR . Figure 1 shows the C1s HAXPES spectrum and the C K-edge XANES spectrum acquired by total electron yield (TEY) method. HAXPES was normalized by the signal intensity of the C-C substrate components (sp2, sp3) of the support at 284.5 eV 6), whereas the C K-edge XANES is normalized by the signal intensity of the 2pz (π*) orbital extending perpendicular to the carbon hexagonal network plane of the C–C substrate component of the support at 285.2 eV 7, 8). The functional groups are clearly observed in both spectra and the signal intensity of the functional group of the Pt catalyst increased with respect to the support alone, indicating that the amount of functional group on the support increased due to the Pt loading process. By combining other experimental results and theoretical simulations, the relationship between structure properties and electrode performance can be discussed. Acknowledgement This work was conducted in FC-Platform project supported by the New Energy and Industrial Technology Development Organization (NEDO). References :1) E. Antolini, Applied Catalysis B: Environmental 88, 1 (2009)2) Y.-J. Wang, N. Zhao, B. Fang, H. Li, X. T. Bi, and H. Wang, Chem Rev 115, 3433 (2015)3)V. Yarlagadda, M. K. Carpenter, T. E. Moylan, R. S. Kukreja, R. Koestner, W. Gu, L. Thompson, and A. Kongkanand, ACS Energy Letters 3, 618 (2018)4)S. Ott, A. Orfanidi, H. Schmies, B. Anke, H. N. Nong, J. Hübner, U. Gernert, M. Gliech, M. Lerch and P. Strasser , Nature Mater., 19, 77 (2020)5)A. Kobayash, T. Fujii, C. Harada, E. Yasumoto, K. Takeda, K. Kakinuma, and M. Uchida, Acs Appl. Energy Mater. 4 2307 (2021)6) S. Kundu, Y. Wang, W. Xia, and M. Muhler, J. Phys. Chem. C, 112, 16869 (2008)7) Y. Murakami, Y. Ota, and T. Okada, Anal. Sci., 39, 67 (2023)8) C. L. Guillou, S. Bernard. F. de la Pena, and Y. L. Brech, Anal. Chem., 90, 8379 (2018) Figure 1

Effects of Oxygen Defects in Zr Oxide-Based Thin Films on Oxygen Reduction Reaction in Acidic Media

The use of carbon-supported platinum as cathode catalysts of polymer electrolyte fuel cells has some problems such as high cost and low amount of resources of platinum. Therefore, we have attempted to develop substitute of platinum based on group 4 metal oxides. In the case of Zr oxide-based powder catalysts, we found that zirconium oxide-based compounds with oxygen defects and deposited carbon derived from organometallic precursors had definite activity of oxygen reduction reaction (ORR) [1][2]. However, the ORR activities of these catalysts still remain low. In order to increase the ORR activity drastically, it is necessary to clarify the catalyst design. This requires an understanding of the active sites and the electron conduction path separately, which were not separated in powder catalysts. On the other hand, Sugino et al. investigated the effect of oxygen vacancies and nitrogen doping on the active site based on first-principles calculations [3]. Defects by oxygen vacancies and nitrogen dopants were introduced into the tetragonal ZrO2(101) slab. It was found that the energy barrier of the rate-determining step, or the removal of adsorbed OH, was comparable to (or only slightly lower than) that of the pristine surfaces, indicating that defects play only a minor role in enhancing the ORR activity. In other words, it was suggested that oxygen vacancies and nitrogen dopants contributed to the emergence of ORR activity by enhancing the electrical conductivity of ZrO2 rather than by forming high quality active sites. Simple models are suitable for experimentally confirming the results of theoretical calculations and for understanding the active sites and the electron conduction paths separately. Therefore, we attempted to prepare nitrogen doped ZrOx nanofilms by RF magnetron sputtering method with different targets and atmospheres and to investigate their potential as model electrodes.ZrOx thin films of ca. 3-5 nm thickness were prepared by RF magnetron sputtering method using glassy carbon (GC) plates as substrates and ZrN or Zr metal plates as targets under Ar, Ar+O2 and Ar+N2. The sputtering power, the deposition time, and the substrate heating temperature were 100 W, 10 min, and 300 ℃, respectively. The flow rates of Ar, O2, and N2 gas were controlled to keep the total chamber pressure at approximately 0.6 Pa. When the gases were mixed, the partial pressure ratio was set to 1:1. Electrode prepared with a ZrN target in Ar was denoted as "ZrN_Ar", and electrodes prepared with a Zr target in Ar:O2 and Ar:N2 were denoted as "Zr_Ar+N2" and "Zr_Ar+O2", respectively. A conventional three-electrode cell was used for electrochemical measurements with Reversible Hydrogen Electrode (RHE) as a reference and a GC plate as a counter electrode in 0.5 M sulfuric acid at 30 ℃. The oxygen reduction current density i ORR (geometric area standard) was calculated by subtracting the current in N2 from the current in O2.Figure 1 shows the effect of sputtering condition on potential-ORR current curves of the ZrOx thin film electrodes. For comparison, the result of the GC substrate only is also plotted as "GC only." The ORR activity of the ZrN_Ar was lower than that of the GC only. In addition, the current of the steady-state cyclic voltammogram of the ZrN_Ar was smaller than that of the GC only. This may be due to the electrochemical oxidation of ZrN surface and the formation of an insulating layer without active sites, presumably Zr hydroxide. On the other hand, the onset potential of the Zr_Ar+N2 and the Zr_Ar+O2 for the ORR were higher than that of the GC only and the Zr_Ar+N2 showed highest ORR activity. The steady-state cyclic voltammograms of the Zr_Ar+N2 and the Zr_Ar+O2 were larger than that of GC only, revealing that these electrodes might have high conductivities. In addition, XPS measurements suggest that the Zr_Ar+N2 contains nitrogen and the Zr_Ar+N2 and the Zr_Ar+O2 have oxygen vacancies. These results suggested that their high ORR activities were caused by nitrogen doping and oxygen defects. Therefore, the electrodes prepared by RF magnetron sputtering method may be able to separate the active sites from the electron conduction by changing the sputtering conditions such as film thickness.AcknowledgementThe authors thank the financial support of the New Energy and Industrial Technology Development Organization (NEDO).References(1) A. Ishihara et al., J. Phys. Chem. C, 123, 18150 (2019).(2) M. Chisaka et al., ACS Omega, 2, 678 (2017).(3) S. Muhammady et al., J. Phys. Chem. C, 126, 15662 (2022). Figure 1

Oxygen Reduction Reaction Properties of Dry-Process Synthesized Pt-Non PGM Transition Metal Multi-Component Alloys: Influence of Transition Metal Elements

Introduction Multi-component alloys (MCAs) have been attracted much attention in various engineering application fields. [1] By changing numbers of constituent elements (n) as well as their composition ratios (𝑥) of MCAs, the mixing-entropy (ΔSmix = −𝑅∑(𝑖=1→n) 𝑥𝑖 ln 𝑥𝑖) increases with increasing n, resulting in reduced Gibbs free-energy (ΔG = ΔH - TΔS) of the MCAs, which correlates with interesting bulk material’s properties. Furthermore, the MCAs surfaces possess specific chemical properties, for example, high thermodynamic and chemical stabilities, probably caused by the reduced ΔG, that are much different from those of binary alloys, e.g., Pt-Co or Pt-Ni. Therefore, the MCAs surfaces can be applied for novel nano-materials developments, including electrocatalytic materials. [2] Indeed, our previous study for single-crystal surfaces of the MCAs system of Pt and non-PGM Cr-Mn-Fe-Co-Ni (known as Cantor alloy [3]) demonstrates that enhanced oxygen reduction reaction (ORR) activity at a pristine state and, furthermore, electrochemical structural stability even through the potential cycle (PC)-loading. [4] Thermodynamical and chemical properties of the MCAs surfaces are, in general, a superposition of specific properties of the constituent elements and newly emerging properties that stemming from multi-component alloying. Therefore, catalytic influences of the multiple alloying-elements on ORR properties (activity and structural stability) of Pt-containing MCAs (referred to as Pt-MCAs) surfaces are much complicated, due mainly to the sluggish diffusion and segregation behaviors of the constituent elements. Thus, optimizing the constituent elements (kinds and n) and their composition ratios (𝑥) at the surface vicinity is surely essential for considering an application of the MCAs as practical ORR catalysts. In this study, we synthesized Pt-MCAs on Pt(111) substrate surface with various n and kinds of the constituent elements by controlling vacuum-deposition amounts of the elements and conducted fundamental investigations aimed for high performance cathode alloy catalyst’s developments of polymer electrolyte membrane fuel cells. Experimental In this study, we focused on equi-composition-ratio of the MCAs by picking up 2~5 elements of Cr, Mn, Fe, Co, and Ni as alloying elements of Pt. 10 monolayer (ML)-thick (1 ML = ca. 0.3 nm) MCAs layer was first deposited on the surface-cleaned Pt(111) substrate by using an arc-plasma deposition method at 300 K, and subsequently annealed in vacuum at 773 K for 30 min. Then, 4 ML-thick Pt layer was deposited on the MCAs layer at 300 K and post-annealed at 623 K in vacuum. The samples thus prepared are designated as Pt/MCAs/Pt(111). The atomic-level micro-structures and composition ratios of the surface-vicinity of resulting Pt/MCAs/Pt(111) were characterized by cross-sectional STEM-EDS and XPS. CVs and LSVs were conducted in N2-purged and O2-saturated 0.1 M HClO4 by using an RDE apparatus. ORR activity was evaluated from j k values estimated at 0.9 V vs. RHE by using Koutecky-Levich equation and structural stability (ORR durability) was discussed based upon the activity transitions during applying 5,000 potential cycles (PCs) of 0.6(3s)‐1.0(3s) V vs. RHE in O2 saturated 0.1 M HClO4 at room temperature. Results and Discussion The ORR activity trends under applying the PC loading (a), initial CVs (b), and XPS-estimated surface compositions of Pt/MCAs/Pt(111) are summarized in Fig. 1(a-c), where Cr-Mn-Fe-Co-Ni, Mn-Fe-Co-Ni, Cr-Fe-Co-Ni, and Fe-Co-Ni are adopted for the MCAs. Notably, the ORR activity of Cr containing Pt/Cr-Fe-Co-Ni/Pt(111) (green) decreased remarkably up to 2,000 PCs, compared to Cr-less Pt/Mn-Fe-Co-Ni/Pt(111) (red), Pt/Fe-Co-Ni/Pt(111) (yellow), and both Cr and Mn containing Pt/Cr-Mn-Fe-Co-Ni(Cantor alloy)/Pt(111) (blue). Furthermore, Pt/Cr-Fe-Co-Ni/Pt(111) exhibited increased electric double layer charge in the potential region of 0.4 to 0.6 V at the pristine state (b). Additionally, XPS-estimated relatively smaller and larger composition ratios of the Pt and Cr, respectively, among the tested Pt/MCAs/Pt(111) (c), suggesting that Cr of the MCAs preferentially diffuses and segregates at the surface-vicinity through the heat treatment process of the sample fabrication, which might cause severe decrease in ORR activity at the early stage of the PCs loading (up to 2,000 PCs). [5] At the meeting, correlations between the ORR properties and atomic-level microstructures of Pt/MCAs/Pt(111) surfaces will be discussed in detail. Acknowledgement This study was supported by new energy and industrial technology development organization (NEDO) of Japan, JST SPRING, Grant Number JPMJJSP2114 (Y.C.) and Grant-in-Aid for JSPS Fellows (Y.C.).

Visualization of Hydrogen Bubbles Using Synchrotron X-Ray CT inside Porous Transport Layer of Direct Toluene Electro-Hydrogenation Electrolyzers

Toluene is a highly versatile solvent commonly used in many industries due to its high energy density and effective hydrogen carrier properties. It can store a considerable amount of hydrogen in a relatively small volume, making it a promising hydrogen carrier for various applications. Toluene's high volumetric energy density also makes it one of the best hydrogen carriers available. Additionally, toluene is easy to handle and transport as it is a liquid at room temperature, and it can be stored and transported like any other liquid fuel. Furthermore, toluene is a safer alternative for hydrogen storage compared to other hydrocarbons due to its low toxicity.Direct toluene-electro hydrogenation electrolyzer is a promising method that enables the simultaneous performance of water electrolysis and toluene hydrogenation in a single system. However, during the toluene direct electrolytic hydrogenation process, the formation of hydrogen bubbles from the electrolysis of water hinders the toluene hydrogenation reaction, leading to a reduced conversion rate of methylcyclohexane (MCH) and Faraday efficiency.In this study, we aimed to understand the conditions for hydrogen bubble generation and transport. We quantitatively evaluated the generation amount and distribution of hydrogen bubbles by visualizing them in an operating electrochemical cell using Synchrotron X-ray CT. Figure 1 displays the distribution of hydrogen bubbles along the porous transport layer (PTL) of the electrochemical cell. These bubbles were visualized using the raw images obtained from Spring-8 and subsequently reconstructed. The cell was operating at a current density of 200 mA/cm2 for a period of 30 minutes with pure water supplied to the anode side. Darker areas in the images indicate regions with hydrogen bubbles, which have a lower density and atomic weight than other substances in the PTL. Therefore, they are more transparent to X-rays and appear darker in the image. Higher current values and longer operation times led to increased hydrogen bubble generation, with a significant increase observed at 30 minutes. By visualizing these hydrogen bubbles, we were able to evaluate the generation distribution, amount, and transport of hydrogen, which can help improve the direct toluene-electro hydrogenation electrolyzer system. Acknowledgement 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 (P14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References Nagasawa K, Sugita Y, Atienza-Márquez A, Kuroda Y, Mitsushima S. Effect of the cathode catalyst loading on mass transfer in toluene direct electrohydrogenation. Journal of Electroanalytical Chemistry 2023:117431. https://doi.org/10.1016/J.JELECHEM.2023.117431.Shigemasa, A. Atienza-Márquez, K. Inoue, S. Jang, F. Reyna-Peña, T. Araki, “Visualization of dragged water and generated hydrogen bubbles in a direct toluene electro-hydrogenation electrolyzer,” J. Power Sources 2023; 554:232304. https://doi.org/10.1016/j.jpowsour.2022.232304.Nagasawa, A. Kato, Y. Nishiki, Y. Matsumura, M. Atobe, S. Mitsushima, “The effect of flow-field structure in toluene hydrogenation electrolyzer for energy carrier synthesis system. Electrochemical Acta 2017; 246:459-65. https://doi.org/10.1016/j.electacta.2017.06.081. Nagasawa, K. Tanimoto, J. Koike, K. Ikegami, S. Mitushima, “Toluene permeation through solid polymer electrolyte during toluene direct electro-hydrogenation for energy carrier synthesis”, Journal of Power Sources, Vol.439, (2019) 22707. https://doi.org/10.1016/j.jpowsour.2019.227070. Figure 1 SPring-8 CT Visualization Reconstructed image after 30 minutes of operation at 200 mA/cm². Figure 1

Enhancement of the ORR Activity and Durability of Pt/C by the Modification with Melamine and N-Containing Aromatic Compounds

The ORR activity and durability of Pt catalysts have to be enhanced for the wider spread of PEFCs. We pay special attention to possibility that surface modification of a Pt catalyst increases its ORR activity. On the Pt surface, Pt-OH species is stabilized too much by network structure of water molecules adsorbed there [1]. The overstabilization of Pt-OH impedes the ORR because this species is an intermediate of ORR. Then, we try to enhance the ORR activity by destabilizing Pt-OH. The perturbation of the water network would destabilize Pt-OH. We try to perturbate the water network by adding an external organic compound to enhance the ORR activity.We tested a wide variety of compounds as an enhancer of ORR of Pt. Although most of compounds tested exhibit no influence of inhibitory effects on the ORR, we found that certain kind of N-containing aromatic compounds -melamine (and tetraazaporphyrin)- enhance the ORR activity of Pt catalysts significantly [2-4]. With the addition of melamine, ORR activity at 0.9 V vs. RHE is increased by 1.6 times although the ECSA decreased (Fig. 1). The enhancement of the ORR activity is reasonably ascribed to the destabilization of Pt-OH, judging from the change of cyclic voltammograms of Pt/C after the addition of melamine.This melamine modification method is also applied for another highly active catalyst (Pt/Pd/C core-shell catalyst) to obtain a further activated catalyst. The mass activity of Pt/Pd/C core-shell catalyst increases by 2.4 times after melamine modification and reaches 3600 AgPt -1 at 0.9 V vs. RHE [5].Melamine also enhances the durability considerably. Under an accelerated durability test (ADT) (80 °C, 0.6 V (3s) - 1.0 V (3s), square wave, 10000 cycles), Pt/C undergoes severe degradation; the ECSA significantly decreases because of the increase in the size of Pt nanoparticles (Fig. 1). However, after the ADT in the presence of melamine, the decrease in ECSA is significantly suppressed [6]. This beneficial effect is due to the suppression of the increase in size of Pt nanoparticles. A melamine polymer (poly-melamine sulfonate)-modified Pt/C also exhibits higher durability than unmodified Pt/C [7]. The suppression of Pt-OH by melamine participates in the enhancement of the durability.In this talk, we introduce discovery of melamine as an ORR enhancer, effect of melamine on the ORR activity/durability of Pt/C, and mechanisms of functions of melamine.This paper is based on the results obtained from projects, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Numerical Simulation of the Two-Phase Flow Dynamics in an Alkaline Water Electrolyzer Considering Difference in Bubble Diameter

For more efficient hydrogen production, predicting the characteristics of bubble flow inside a water electrolyzer is critical since bubbles affect ion transport, and electrode coverage by bubbles influence overpotentials and the entire electrolysis performance. Transport of bubble significantly depends on its size. In this study, a CFD model of an alkaline water electrolyzer which considers difference in sizes of generated bubbles was developed. The model treats bubbles of different sizes as different gas phases, which are independent of each other. The total bubble mass source for a given current density is divided into phases of each bubble size according to experimental visualizations. Drag, virtual mass, lift, wall lubrication, and turbulent dispersion forces are considered as interaction between gas and liquid phase.The following three sizes of bubbles were considered: diameters d = 36 μm (small), 75 μm (medium), and 208 μm (large). The entire calculation domain represents the anode side of a zero-gap monopolar alkaline water electrolyzer. Figure 1 shows the volume fraction fields of the three gas phases (i.e., bubble sizes) at a constant current density of 0.6 A/cm2. The bubbles are generated on the electrode surface at the z=0 plane. As the liquid phase forms a circulating flow inside the electrolyzer, small bubbles are transported along the circulating downstream flow due to smaller buoyancy force compared to their viscous drag. In contrast, large bubbles flow out directly to the upper outlet because buoyancy force dominates. This result points out the need to consider the difference in bubble diameter for designing flow field inside alkaline water electrolyzer.[Acknowledgements]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 (P14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO).Fig. 1. Total gas phase volume fraction field obtained from the sum of the volume fraction field for each size bubble, and its experimental visualization. Figure 1

Carbon-Free, Connected Core-Shell Nanoparticle Catalysts with Advanced Oxygen-Reduction Performances for PEFCs

As the world faces diminishing non-renewable energy sources and a surge in global pollution, establishing a hydrogen and low-carbon economy has become crucial. Polymer electrolyte fuel cells (PEFCs) hold great promise as a sustainable energy source with the potential for high efficiency, and clean power generation in both stationary and mobile applications. However, the conventional platinum nanoparticle catalysts on carbon support (Pt/C) used for the oxygen reduction reaction (ORR) at the cathode shows sluggish ORR activity, leading to high cost and inferior PEFC performance. Moreover, the carbon-supports used for the Pt catalysts are susceptible to degradation caused by carbon-corrosion during start-stop operations, which can severely compromise the overall performance of the PEFCs. Hence, in order to significantly enhance the commercial viability of PEFCs, considerable progress must be made in terms of cost, performance, and durability.Previously our group has reported carbon-free, connected Pt-alloy catalysts to address the above issues. This catalyst is composed of connected Pt-alloy nanonetworks with high electronic conductivity and exhibits enhanced ORR activity, high start-stop durability, and thin catalyst layers. Specifically, this catalyst exhibited 9 times higher specific activity than that of the commercial Pt/C, making it an excellent candidate for cathode catalyst applications.[1,2] However, high-temperature annealing process is required to develop connected networks between the Pt-alloy nanoparticles, resulting in larger crystallite sizes and reduced surface areas. This limitation has prevented connected Pt-alloy catalysts from achieving significant improvement in mass activity.In this work, we have proposed a novel synthesis method that enables the development of connected core-shell nanoparticle catalysts without any high-temperature treatment. This method eliminates the particle growth and exhibit a high surface area, making it highly effective for ORR applications. Here, a carbon-free catalyst with a core-shell nanonetwork structure having a non-Pt (Pd) core and Pt shell was developed. Additionally, the relationship between the thickness of the Pt-shell layer and ORR activity was investigated.Connected core-shell catalysts with Pd nanoparticles as core and Pt as atomic shell (Pd@Pt) were synthesized in a one-pot polyol process that does not require high-temperature annealing as shown in Fig. 1. The formation of connected Pd@Pt core-shell nanonetworks (< 10 nm) was confirmed by TEM and STEM-EDX analysis. Also, the structure of Pt shell on core metals was successfully tuned by controlling the reaction temperature and time, precursor ratio, etc. Here, the number of the Pt layers on the surface of the connected Pd@Pt core-shell catalysts were varied from 1–6 atomic layers and the results show that the ORR mass activity was notably enhanced in the catalyst with a moderate shell thickness ranging from 2.5 to 3.5 Pt atomic layers. This suggests that connected Pd@Pt catalyst with a moderate Pt layer achieves both high surface area and ORR specific activity. The high surface area of the developed catalysts is attributed to a new synthesis method that does not use high temperature annealing and a low Pt core-shell structure. The improvement in ORR specific activity can be attributed to the more suitable d-band center position of the Pt shell, which is influenced by strain and ligand effects resulting from the Pd core. The ORR performance of the carbon-free connected Pd@Pt catalyst showed 2–4 times enhancement in electrochemical surface area (ECSA) and 1.5–3 times improvement in ORR mass activity compared to the connected Pt-alloy catalysts prepared using high-temperature annealing. Further, high durability of the connected core-shell catalyst against load cycles (0.6 ↔1.0 V) at 60 °C in acidic electrolyte solution was also demonstrated, indicating that a high stability nanonetwork connected by Pt atomic shell.In summary, we have successfully demonstrated a new synthesis method for the formation of connected core-shell nanoparticles by Pt atomic shell without high-temperature annealing and prepared a carbon-free, connected Pd@Pt core-shell catalyst with advanced ORR performance, highlighting its great potential as a cathode catalyst. Acknowledgement The part of this presentation is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

First-Principles Calculations of Fluoride-Ion Migration in Fluorine-Ion Battery Electrolyte Material K2BiF5

Fluoride-Ion migration in K2BiF5 was studied using first-principles static and dynamic calculations. Due to presence of edge sharing BiF7 chain along b axis in this structure, one dimensional Fluoride-Ion migrations alongBiF7 chain strongly suggested. Our calculation revealed not only this Fluoride-Ion migrations along BiF7 chain, but also F ion migrations between BiF7 chain were observed. First-principles calculations of the electronic density of states indicate that K2BiF5 has a large band gap of about 4.8 eV, making it suitable as an ionic conductor. The Fluoride-Ion conduction in the b-axis direction was strongly suggested by the presence of BiF7 chains sharing a ridge in the b-axis direction in the structure of this material, but the F-ion conduction path analysis by first-principles molecular dynamics shown in Fig. 1 indicates that the F-ions are not only in the b-axis direction and in BiF7 chains, but are also active betweenBiF7 The active Fluoride-Ion are determined by the elastic band method. The activation energy determined by the elastic band method is 0.19 eV within a BiF7 chain, whereas it is 0.25 eV between BiF7 chains, indicating that F ions can conduct with the same activation energy as within a BiF7 chain.AcknowledgmentsThis work was supported by the RISING3 (JPNP21006) projects from the New Energy and Industrial Technology Development Organization (NEDO), Japan. Figure 1

Dimensionless Moduli Governing Under-Rib Transport Phenomenon in Polymer Electrolyte Fuel Cell

Introduction The performance of polymer electrolyte fuel cell (PEFC) can be considerably improved by promoting the under-rib convection or cross flow between adjacent gas channels, e.g., applying the serpentine and staggered partially narrowed flow fields[1]. Although a lot of efforts have been devoted to analyze the under-rib transport phenomenon, an analytical model which quantitively demonstrates the dominant factors of the under-rib processes has not been established. In this study, a theoretical analysis of the relations between oxygen reduction reaction (ORR) and mass transfer capacity is carried out. The dimensionless moduli which combine physical properties, flow field dimensions and operating conditions are derived, which provides a theory for optimization of flow field and operating condition design for PEFC. Theoretical Analysis The water is assumed to only exist in vapor phase in a steady-state isothermal cell. The gas diffusion layer (GDL) is considered as uniform porous medium, in which Darcy’s law is applicable. The even velocity and oxygen mass fraction profiles in through-plane direction inside GDL is considered. The ORR was experimentally proven as a first order reaction of oxygen partial pressure according to our previous study,[2] and is regarded as a surface reaction, so that the oxygen consumption rate can be expressed as Eq. (1). The ORR rate constant, which is the function of the cathode overpotential or electromotive force, is assumed to be fixed across the rib.The theoretical model was derived from one-dimensioned mass balance of oxygen, which exhibits that the under-rib process is dominated by 3 dimensionless moduli, Péclet number Pe, Thiele number φ and the ratio of oxygen mass fractions θ at two sides of the under-rib GDL, which were defined by Eqs. (2)–(4). Péclet number represents the ratio of oxygen convection rate to oxygen diffusion rate, which is determined by the pressure difference between adjacent channels. Thiele number represents the ratio of oxygen diffusion resistance to reaction resistance. The average oxygen mass fraction can thus be calculated from Eq. (5). Results and Discussion The theoretical model was compared with the computational fluid dynamics (CFD) simulation in the computational domain shown in Fig. 1, where the GDL was placed between two parallel gas channels, instead of being laid under the gas channel, and the active area was only located under the rib. In the CFD simulation, the velocity and pressure distributions were calculated by solving Navier-Stokes equations, and the component distribution dependencies on density, diffusivity and viscosity were also considered. Fig. 2 shows the under-rib average current density at different geometries and operating conditions. The dimensionless moduli calculated from the inlet conditions offer an accurate current density prediction, which demonstrates these moduli dominate the under-rib transport phenomenon.The typical operating condition gives φ = 1.5 and Pe = 4, where the model provides the average current density with high confidence. The model provides lower oxygen mass fraction than the CFD results especially in case of θ = 1, which exists in the staggered partially narrowed flow field. The reason can be attributed to the interface between gas channel and GDL, where the low velocity cannot eliminate the mass transport resistance, so that the model overestimated the oxygen mass fraction at rib boundaries, as shown in Fig. 3.The effects of dimensionless moduli on the current density are exhibited in Fig. 4. In case of θ = 1, when Pe is lower than 5, the under-rib convection cannot obviously boost the under-rib oxygen concentration, which was also reported in our previous study[1]. On the other hand, in case of θ< 1 and Pe > 0, although little increase on Pe remarkably improves the under-rib mass transfer which explains the good performance of serpentine flow field, the benefits from the under-rib convection reduces when Pe is high. Additionally, since oxygen cannot be supplied sufficiently from the gas channel, higher φ gives lower under-rib oxygen concentration. Conclusions The theoretical model for describing the under-rib transport phenomenon was established and verified by CFD calculation, which was found to be dominated by 3 dimensionless moduli. The model demonstrates that the Péclet number is required to be large enough in partially narrowed flow fields to maximize the profits from the under-rib convection. Acknowledgment This work was supported by the FC-Platform Program: Development of design-for-purpose numerical simulators for attaining long life and high performance project (FY 2020–FY 2023) conducted by the New Energy and Industrial Technology Development Organization (NEDO), Japan. Reference [1] Y. Ma et al., ECS Trans., 109 (9), 171–197 (2022).[2] M. Kawase et al., ECS Trans., 75(14), 147–156 (2016). Figure 1

Investigating the Reaction Mechanism of Iridium Oxides for PEM Water Electrolysis by Operando Soft X-Ray Absorption Spectroscopy

With the increase in fossil energy consumption and the attendant pronounced climate changes, the call for sustainable energy forms has become urgent and has attracted more and more attention.1 Hydrogen (H2), generated by the electricity-derived water electrolysis, is now considered the promising energy carrier for conversion between electricity and chemical energy.2 The most developed water electrolysis technology up to date is the proton-exchange membrane (PEM) water electrolysis, where commercial Pt/C works as the cathodic catalyst for hydrogen evolution reaction (HER) and noble metal-based materials serve as the anodic catalyst for oxygen evolution reaction (OER).3 Considering the sluggish four-electron reaction kinetics of OER and the harsh acidic environment of PEM water electrolysis, the selectivity of high-efficient OER catalysts is narrowed down to Ir-based catalysts because of their superior activity, long-term stability and corrosion resistance against acidic environment.4 However, the scarcity of Ir on earth has impeded the wide application of PEM water electrolysis and has driven the research of cost-efficient OER catalysts. In this study, we thoroughly investigated several amorphous IrOx catalysts with different degree of crystallinity (SA103, SA58, SA14.6 and SA58) and revealed the relation between their structures and OER activities. Besides, BET surface areas, X-ray diffraction (XRD), Transmission electron microscopy (TEM), scanning TEM (STEM), PDF, and XAS were measured to thoroughly understand the catalytic properties.XRD patterns in Figure 1(a) showed that compared with the well-crystallized IrO2, SA58 sample showed slightly broad peak, while the diffraction peaks were much broad of other samples, indicating their amorphous structures. The amorphous structure of IrOx was characterized through atomic pair distribution function (PDF) analyses as shown in Figure 1(b). Different from the typical tetragonal symmetry of crystal IrO2, the combination of orthorhombic symmetry and monoclinic symmetry was found in the amorphous IrOx. Further, OER activities of all samples were investigated in 0.1 M HClO4 and the current density was calculated against the BET surface areas to examine the intrinsic catalytic activity (Figure 1(c)). The crystallized SA58 sample showed the lowest activity, while SA3.5 sample outperformed all other amorphous oxides. From PDF, it was found that the highest OER active SA3.5 sample exhibited the highest monoclinic phase ratio content. The catalytic structure properties were further evidenced by the XAS results. EXAFS results showed different electronic environment of monoclinic phase samples and further fitting results were in good agreement with the PDF analyses. Moreover, O K-edge results showed the formation of µ2-O (O-O) bond at around 528.7 eV, which was previously observed by Nong, H. N. et. al. through operando O-K edge measurements during OER process.5 It was concluded that that the low symmetry of monoclinic phase in amorphous samples resulted in the structure defection, and thus lead to large amount of active electrophilic OI - species, which boosting the OER activity. This finding provided fundamental understanding of the amorphous iridium oxides and could promisingly shed light on the future OER catalyst design. Acknowledgements This work is based on results obtained from a project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References Götz, M.; Lefebvre, J.; Mörs, F.; McDaniel Koch, A.; Graf, F.; Bajohr, S.; Reimert, R.; Kolb, T., Renewable Power-to-Gas: A technological and economic review. Renew. Energy 2016, 85, 1371-1390.Dincer, I.; Acar, C., Review and evaluation of hydrogen production methods for better sustainability. Int. J. Hydrogen Energy 2015, 40 (34), 11094-11111.Carmo, M.; Fritz, D. L.; Merge, J.; Stolten, D., A comprehensive review on PEM water electrolysis. Int. J. Hydrogen Energy 2013, 38 (12), 4901-4934.An, L.; Wei, C.; Lu, M.; Liu, H.; Chen, Y.; Scherer, G. G.; Fisher, A. C.; Xi, P.; Xu, Z. J.; Yan, C.-H., Recent Development of Oxygen Evolution Electrocatalysts in Acidic Environment. Adv. Mater. 2021, 33 (20), 2006328.Nong, H. N.; Falling, L. J.; Bergmann, A.; Klingenhof, M.; Tran, H. P.; Spöri, C.; Mom, R.; Timoshenko, J.; Zichittella, G.; Knop-Gericke, A.; Piccinin, S.; Pérez-Ramírez, J.; Cuenya, B. R.; Schlögl, R.; Strasser, P.; Teschner, D.; Jones, T. E., Key role of chemistry versus bias in electrocatalytic oxygen evolution. Nature 2020, 587 (7834), 408-413. Figure 1

Iron Phosphates as Highly Efficient Precatalysts for Oxygen Evolution Reaction and Their in Situ Electrochemical Conversion

Current energy issues have prompted demand for the innovations in constructing a clean and sustainable energy platforms. One of examples for energy carrier to actualize a zero carbon society is hydrogen, which is a promising carbon-free fuel with an excellent weight energy density. Hydrogen production through alkaline water electrolysis using renewable energy resources (light and wind)-derived electricity is an attractive ecofriendly and cost-effective technology because cheap nonprecious electrocatalysts can be employed in alkaline media. However, oxygen evolution reaction (OER) at the anode side in the water electrolysis is basically slow and prevents the widespread applications of water electrolysis. Thus, highly efficient and cheap electrocatalysts for OER are highly desirable.Iron (Fe)-based compounds have recently received much attention as OER electrocatalysts because Fe is a quite earth-abundant metallic element that is a very cheap and non-toxic. Therefore, Fe is a promising candidate as electrocatalyst for OER. Due to the insufficient OER activity on simple Fe oxides, previous studies have attempted to enhance the activity by incorporating Fe and other metallic elements.[1,2] Recently, we established a novel comprehensive structural descriptor for the OER on Fe-based simple and multimetal oxides;[3] i.e., shorter minimum Fe–O bond length in the crystalline structures led to higher OER activity regardless of elemental composition, Fe–O coordination number, and crystal category. Thus, this descriptor enabled us to quickly develop excellent OER catalyst, Ba0.65Ca0.35Fe12O19, comprising extremely short Fe–O bond lengths.[4] Furthermore, we found that post-spinel CaFe2O4 catalyzes OER via "multi-iron-site mechanism" involving a reaction intermediate with O–O direct formation on multiple Fe atoms, and CaFe2O4 consequently displayed outstanding OER activity despite the long Fe–O bond length.[5] However, we speculate that OER activity of Fe-based oxides cannot be further enhanced based on crystal structures, and it necessitates to employ another type of compounds.In this study, we report the OER performances of Fe-based phosphates in alkaline media. We selected three kinds of Fe-based phosphates with distinct crystal structures: trigonal FePO4, trigonal Fe3O3(PO4), and monoclinic Fe4(P2O7)3. The electrochemical measurements and post-characterizations identified the Fe-based phosphates as efficient precatalysts that can be electrochemically converted into highly active Fe-based compounds. Notably, the OER performances of the electrochemically converted Fe-based compounds overcame those of the above-mentioned most active Fe-based multimetal oxides and any previously reported crystalline OER electrocatalysts. Therefore, the Fe-based phosphates are promising candidates as OER precatalysts, which can be in situ converted into excellent OER electrocatalysts, and have further advantages of low price, environmental friendliness, and easy production.[6] The part of this paper is based on results obtained from a project, JPNP14021, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). This paper was funded in part by the JSPS KAKENHI Grant-in-Aid (18H01786), the JST PRESTO (JPMJPR15S3) and CREST (JPMJCR16P3) programs, the Tokuyama Science Foundation, and the “Creation of Life Innovative Materials for Interdisciplinary and International Researcher Development” program of the MEXT. The authors thank Dr. Takeshi Aihara in Tokyo Institute of Technology for fruitful discussions.

Microscale Simulation of Carbon Support Structure Degradation in Polymer Electrolyte Fuel Cell

Understanding the mechanism of Polymer electrolyte fuel cell (PEFC) degradation plays an important role in improving fuel cell performance and durability. For the PEFC system, the structure of catalyst layers (CLs) is the critical component that determines cell performance. The PEFC degradation due to the reduction of carbon on the CLs has been investigated in previous studies [1]. In our past studies, we focused on the actual heterogeneous porous structure of catalyst layer. From these results, it was found that mass transport performance of oxygen and proton in heterogeneous porous catalyst layer, which consists of carbon support, ionomer and void space, is much lower than that of theoretical prediction in homogeneous porous media [2-5]. In addition, it depends on the structure of carbon black, the coating structure of ionomer and the local distribution of these materials. Thus, it is important to understand and to predict the carbon support structure change by degradation because this structural degradation strongly affect the mass transport performance. In this work, the effect of CLs porous carbon support degradation on cell performance is investigated by microscale simulation.The CLs structures, consisting of porous carbon, ionomer, and Pt, are created by our previous study multi-block method [2,6]. In our simulation, we focus on studying the effect of CLs' beginning-of-life (BOL) and end-of-life (EOL) structure in the degradation process. The CLs creation procedure is illustrated in Fig. 1. Firstly, CLs structure are created by packing CB and covered ionomer and Pt. The amount of ionomer and Pt are assumed as constant during the degradation process. Therefore, the amount of carbon is reduced in EOL structure. In this study, various CLs structures with decreasing of carbon volume are constructed. Furthermore, the mechanical stress distribution under compression condition was simulated in each structure to understand the effect of framework of carbon on local porosity in catalyst layer. The effect of carbon corrosion in CLs is investigated by calculating the cell performance of PEFC of each CLs structure by ORR mass transport and electrochemical reaction calculation with our previous method [6].The cell performance is significantly decreased by reducing carbon. The degradation mechanism of PFEC is explained by degradation of not only effective surface area but also the porous structure change which affect oxygen diffusion performance. Moreover, compared with BOL structure the apparant thickness of covered ionomer layer is higher in EOL structures because of carbon corrosion and constancy of ionomer. Hence, the higher oxygen diffusion resistance in secondary pore and ionomer layer occur in CLs degradation structures. The effect of this dynamic structure change on cell output performance will be reported in this presentation.Acknowledgements:This research was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan, grant number P20003-20001327-0.

Pt-Ta-Co Electrocatalysts for Polymer Electrolyte Fuel Cells

Introduction Electrocatalysts with both high catalytic activity and high potential cycling durability are required for polymer electrolyte fuel cells. It is of technological interest to develop electrocatalysts using mesoporous carbon as a support framework and Pt-Ta-Co as a catalyst. Fuel cells using this novel electrocatalyst have to be prepared to evaluate their electrochemical performance. Here in this study, we aim to examine polymer electrolyte fuel cells using these new electrocatalysts by evaluating the current-voltage characteristics and various overvoltages under different electrocatalyst layer preparation conditions such as ionomer and solid contents. Experimental A novel electrocatalyst (Pt0.7Ta0.2Co0.1/MC) was prepared by loading Pt-Ta-Co catalyst onto mesoporous carbon as a support framework by the acac method [1]. MEAs with Pt/C (TEC10E50E, Tanaka Kikinzoku Kogyo, Japan) as the anode electrocatalyst layer and a new electrocatalyst as the cathode electrocatalyst layer were prepared by spray printing. The cathode electrocatalyst layer was deposited with various ionomer and solid contents, and fabrication conditions of other cells were based on previous conditions [2]. The MEAs prepared were subjected to performance tests and overvoltage separation. Their cell performance was analyzed by subsequent microstructural observation using field emission scanning electron microscopy (FE-SEM). Results and discussion Figure 1 shows cell voltage of MEAs with the new electrocatalyst with various ionomer contents under operating conditions of 80°C and 100% relative humidity. In the preparation of the cathode electrocatalyst layer using the new electrocatalyst, it was found that relatively high performance was obtained at the solid content of 3.5 wt.% and ionomer content of 35 wt.%. The solid content of 3.5 wt.% was found to exhibit higher performance than that of 5 wt.%. Although the new electrocatalyst was highly active on the half-cell level, its electrochemical performance in a single cell has still lower than that of the cell fabricated using the standard Pt/C (TEC10E50E) for both electrodes. The concentration overvoltage was particularly high, as shown in Fig. 2, and microstructural observations revealed issues related to the microstructure of the electrocatalyst layer. Further improvement of cell performance has to be made through detailed analysis and optimization of the microstructure of the electrocatalyst layers such as their porosity. Acknowledgments This paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References A. Hayashi, H. Notsu, K. Kimijima, J. Miyamoto, and I. Yagi, Electrochim. Acta, 53, 6117–6125 (2008).T. Ogawa, Y. Inoue, K. Yamamoto, M. Yasutake, Z. Noda, S. M. Lyth, J. Matsuda, M. Nishihara,Hayashi, and K. Sasaki, ECS Trans., 109 (9), 241 (2022). Figure 1