Development Of Energy Technologies Research Articles

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

Through-Plane Cerium Ion Migration and Diffusion Analysis on Plymer Electrolyte Membrane by Operando X-Ray Fluorescence Spectroscopy

High durability is a critical requirement for expanding the use of polymer electrolyte fuel cells (PEFCs). One degradation factor of PEFCs under typical operating conditions is electrolyte membrane degradation. Radical species formed from the reaction between hydrogen peroxide and impurities during fuel cell operation cause significant degradation of the PEM. To scavenge the radicals, cerium species are added to the PEM, which react with the radicals by redoxing the cerium ion [1]. To maintain the effectiveness of the cerium radical quencher in the PEM, the concentration of cerium in the PEM must be maintained during the long-term operation of the PEFC. However, cerium species easily diffuse and migrate out of the PEM [2]. If the cerium ion is depleted, the PEM will be severely attacked by radicals. To prevent such cerium depletion, it is important to understand the cerium migration phenomena. Experimental detection of cerium distribution phenomena through PEM by operando X-ray fluorescence spectroscopy with resolution of several micrometers has been reported [3]. We have also developed operando XRF using focused micron X-ray with submicrometer resolution [4]. In this study, we further develop the analysis method for cerium diffusion through planar PEM with sub-second time resolution. This method is used to estimate the apparent diffusion coefficient of cerium in PEM.MEAs were prepared by a conventional method using a Pt/C catalyst and a cerium-containing polymer electrolyte with a thickness of 12 μm(GORE-SELECT® MEMBRANEM788.12). The active electrode area was 1.0 cm2. Single cells were assembled using custom end plates with an X-ray permeation window. Operando X-ray fluorescence spectroscopy measurements were performed on BL37XU at SPring-8 (Japan). The monochromated X-ray at an energy of 45 keV was focused to less than 500 nm by a Kirkpatrick-Baez mirror. To detect the cerium distribution in MEA, the fluorescence intensity of Ce-Kα was counted with a Ge semiconductor detector by scanning the position of the cell.In the open circuit condition, most of the cerium is present in the PEM, but during cell loading, the cerium moves to the cathode layer. Immediately after the start of 2.0 A cm-2 loading, the intensity of cerium in the cathode layer increases in a few seconds. Simultaneously, a cerium concentration gradient is observed through the MEA. The cerium concentration gradient shows the current density dependence. When the current load is stopped, the concentrated cerium in the catalyst layer diffuses into the PEM. The diffusion coefficient of cerium ion is estimated from the time dependence of cerium concentration in PEM.Acknowledgments: This work is based on results obtained from the project commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. E. Endoh et al, ECS Electrochem. Lett., 2, F73 (2013).S. M. Stewart et al, ECS Electrochem. Lett., 3, F19 (2014).A. M. Baker et al, ECS Trans., 92, 107 (2019).Y. Orikasa et al, ECS Trans., 109 109 (2022).

(Poster Award - Honorable Mention) Ta2O5 Catalyst Support for Polymer Electrolyte Fuel Cells

Pt/C is widely used as polymer electrolyte fuel cells (PEFC) electrocatalysts. The cathodic environment of PEFC is prone to high acidity and high potential, which can degrade cell performance due to carbon corrosion 1. Therefore, electrocatalysts with oxide support such as SnO2 supported on carbon-based materials have been developed, achieving high durability. Further improvement of their catalytic performance is desirable, using a Pt-based alloy catalyst and a mesoporous carbon (MC) 2,3. However, studies using SnO2 as a support have suggested that the low melting point of Sn caused alloying of Pt and Sn and/or melting of metallic Sn, causing degradation of catalytic activity 3. From these reasons, here, we focus on tantalum (Ta) with much higher melting point to fabricate Ta2O5/MC electrocatalysts support, which can support Pt alloy catalysts to achieve both high durability and high activity. Ta2O5 was prepared by a sol-gel method using Ta-ethoxide, and Pt-Co alloy nano-particles were deposited via the acetylacetonate method 4. The amount of Ta2O5 on the MC was set to be 30 and 60 wt.%, and the durability was evaluated by varying the amount of Ta2O5. The molar ratio of Pt to Co was set at 3:1. The microstructure of the electrocatalyst was observed by field-emission scanning electron microscopy (FESEM) and scanning transmission electron microscopy (STEM), and the alloy was evaluated by XRD measurements. Half-cell tests were made to evaluate electrochemical performance and durability (load cycle and start-stop cycle). From the XRD measurements, the Pt peaks were shifted and the Ta2O5 peaks were confirmed, indicating that Pt-Co alloys were formed without forming the alloy of Pt and Ta. FESEM observation revealed that Ta2O5 particles were impregnated with a diameter of about 5 nm, and Pt-Co alloy particles were impregnated with a diameter of 2 to 3 nm. Some of catalyst particles were found to be impregnated on Ta2O5, but no significant selectivity to Ta2O5 was observed. Both Pt effective surface area (ECSA) and oxygen reduction reaction (ORR) activity of the prepared catalysts were higher than of Pt/C, confirming the positive effect of Pt-Co alloy and MC on activity enhancement. Figure 1 shows that the load potential cycle durability was improved using MC, and the start-stop cycle durability was improved by using Ta2O5. In particular, the electrocatalysts with a lower loading of Ta2O5 exhibited higher durability under load potential cycling and subsequent higher ECSA compared to the Pt-Co/C electrocatalyst. Acknowledgment This paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References Ohma, K. Shinohara, A. Iiyama, T. Yoshida, and A. Daimaru, ECS Trans, 41(1), 775 (2011).Matsumoto, M. Nagamine, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, and K.Sasaki, J. Electrochem. Soc., 165 (14), F1164 (2018).Inoue, M. Yasutake, Z. Noda, S. M. Lyth, M. Nishihara, A. Hayashi, J. Matsuda, and K. Sasaki, ECS Trans., 109 (9), 413 (2022).Hayashi, H. Notsu, K. Kimijima, J. Miyamoto, and I. Yagi, Electrochem. Acta., 53, 6117 (2008). Figure 1

Oxygen Reduction Reaction Activity of Platinum Nanosheets Derived from Layered Platinic Acid

Platinum nanoparticles are mainly utilized as the cathode catalyst for polymer electrolyte fuel cells. However, the utilization of platinum for the oxygen reduction reaction (ORR) is insufficient and one must tackle the trade-off between high surface area and high stability. We have previously shown that metallic nanosheets such as ruthenium nanosheets exhibit high electrocatalytic activity and stability owing to the two-dimensional structure with an atomic scale thickness.1, 2 Platinum nanosheets, if available, should also have the potential to increase the utilization of the atoms and durability. While a few studies have reported the synthesis of platinum nanosheets,3 the thickness of the nanosheets is more than 1 nm, and the utilization of platinum has so far been lower than that of the Pt nanoparticles.In this study, we report double-layered platinum nanosheets derived from layered platinic acid.4 Layered platinic acid was synthesized through a solid-state-reaction and subsequent acid treatment. From the layered platinic acid, platinum oxide nanosheets were obtained as a colloid via chemical exfoliation. The thickness of the platinum oxide nanosheet was ~0.9 nm, consistent with the thickness of a single PtO6 octahedron. The platinum oxide nanosheets on silicon wafer were topotactically reduced to platinum nanosheets by mild thermal treatment in hydrogen. The thickness of the platinum nanosheet was ~0.5 nm, corresponding to a two atomic layer platinum nanosheet.Carbon supported platinum nanosheets were prepared by the reduction of platinum oxide nanosheets supported on carbon. The electrochemically active surface area (ECSA) of the carbon supported platinum nanosheets was 100 to 120 m2 (g-Pt)-1, which was considerably larger than that of 3 nm platinum nanoparticle (80 m2 (g-Pt)-1). In addition, the platinum nanosheet showed 1.7 times higher mass activity towards the oxygen reduction reaction compared to that of 3 nm platinum nanoparticles.Various methods for the topotactic reduction of platinum oxide nanosheets on carbon was studied. In addition to the mild thermal treatment in hydrogen gas, electrochemical reduction was also conducted. The ECSA of the platinum nanosheet prepared by electrochemical reduction at constant potential was 100 to 120 m2 (g-Pt)-1, which was almost same as the platinum nanosheet reduced by the mild thermal treatment in hydrogen. The specific activity was 1.2 to 1.3 times higher than that of the platinum nanosheet reduced by the mild thermal treatment in hydrogen. Therefore, the platinum nanosheet reduced by electrochemical reduction showed high mass activity for ORR.AcknowledgementThis work was supported as part of the “Polymer Electrolyte Fuel Cell Program” and the FC-Platform projects from the New Energy and Industrial Technology Development Organization (NEDO) of Japan (20001201-0, 22101136-0).

Factors Improving Lithium Sulfur Battery Performance with Mesoporous Carbon-Sulfur Cathode by Mixing Vinylene Carbonate Electrolyte with Fluoroethylene Carbonate

1. IntroductionLithium-ion batteries (LIB) are used in a variety of applications, including portable devices such as smartphones. More recently, the development of rechargeable batteries is expected for applications such as next-generation electric mobility, requiring energy storage devices with higher energy density than conventional LIB, and lithium-sulfur (Li-S) batteries are attracting attention among these applications. To realize Li-S batteries with high energy density and safety, we have focused on mesoporous carbon-sulfur cathodes and carbonate-based electrolytes. In previous studies, we have reported that Li-S batteries using vinylene carbonate (VC) and fluoroethylene carbonate (FEC) as electrolyte solvents exhibit superior rate characteristics than using only VC solvent 1). This study investigates the factors that contribute to the improved performance when using a FEC-VC mixed electrolyte.2. Experimental method(1) We assembled the cells using 1.0 M lithium bis(trifluoromethanesulfonyl)imide / VC (Only VC electrolyte) or 1.0 M lithium bis(trifluoromethanesulfonyl)imide / FEC: VC (FEC-VC electrolyte). The cell was pre-cycled at 0.1C in the voltage range of 1 - 3 V, and then discharged at 10 C or 0.1 C and charged at 0.1 C for three cycles. After discharging and charging, each cell and electrode were evaluated using electrochemical impedance spectroscopy (EIS) or XPS. (2) Radical polymerization of VC with 2,2'-azobis(isobutyronitrile) (AIBN) was made by mixing VC and AIBN (VC radical polymer). Anionic polymerization of VC with lithium ethoxide (LiOEt) was also obtained by mixing VC and LiOEt (VC anionic polymer). These polymers were analyzed using XPS and MALDI-TOFMS. (3) The cathode materials after discharging and charging were stripped from the current collector. The powder was analyzed by APCI-TOFMS. (4) The cathode after 10 cycles was analyzed by STEM-EELS.3. Results and discussionWe conducted EIS on Li-S batteries charged and discharged with Only VC electrolyte or FEC-VC electrolyte at 10 C or 0.1 C, respectively. As a result, in the case of Only VC electrolyte, the interfacial resistance increased significantly after charge-discharge at 10 C when compared to that after charge-discharge at 0.1 C. On the other hand, when using FEC-VC electrolyte, the increase in interfacial resistance was suppressed. In addition, XPS investigation of the electrodes after discharging and charging showed that the SEI film thickness increased significantly after discharging and charging at 10 C when using Only VC electrolyte, whereas the increase in SEI film thickness was suppressed when using FEC-VC electrolyte. These suggest that when using FEC-VC electrolyte, a good SEI that suppresses electrolyte decomposition more is formed, and thin and low-resistance SEI is maintained even after rapid discharge, thereby improving rate performance. In addition, the XPS C1s spectra observed on the electrode film showed similar profiles to those of the VC radical polymer when using Only VC electrolyte, and to those of the VC anionic polymer when using FEC-VC electrolyte. Therefore, these polymers were used as models for the assumed SEI organic components when using respective electrolytes and were investigated by MALDI-TOFMS. As a result, the molecular weight of the monomer unit of the polymer was 250 for the VC radical polymer, while it was 60 for the VC anionic polymer. The results of the APCI-TOFMS investigation on the cathode materials after discharging and charging showed that the components with higher molecular weight decreased as using the electrolyte solvent with a higher mixing ratio of FEC to VC. Therefore, we considered that the mixing of FEC makes the SEI organic components less bulky and denser, which suppresses the continuous formation of SEI kinetically and reduces the interfacial resistance. We conducted further investigation about SEI that could suppress electrolyte degradation more when using FEC-VC electrolyte. STEM-EELS investigation of the cathode after 10 cycles of charge-discharge showed that the SEI layer is divided into two layers when using FEC-VC electrolyte, and the SEI inner layer is mainly composed of S, F and Li. The XPS results also revealed the presence of LiF and Li2Sx (x=2-8) inside the SEI. Therefore, it is suggested that when using FEC-VC electrolyte, the formation of a layer that mainly composed of LiF and Li2Sx (x=2-8) inside the SEI acts as a barrier between the electrode and the electrolyte, which can suppress excessive decomposition of the electrolyte.AcknowledgementsThis research was partly performed as a subcontract (JPNP15005) from the New Energy and Industrial Technology Development Organization (NEDO) (contractor: GS Yuasa Corporation; subcontractor: Kansai University).Reference1) K. Kishida et al., The 60th Battery Symposium in Japan, The Committee of Battery Technology, The Electrochemical Society of Japan, p. 2D04 (2019).

Performance and Durability of Carbon-Free Ionomer-Free PEFC Cathode Using Pt Nanosheet Catalyst

Polymer electrolyte fuel cells (PEFCs) have been attracting attention as highly efficient power sources and have been commercialized for use in residential power plants and automobiles. However, further widespread adoption requires the development of low-platinum catalysts. Additionally, for installation in heavy duty vehicles such as large trucks, high durability at high temperatures is necessary. Pt nanosheets (Pt(ns)), consisting of several atomic layers of Pt with a flat structure, possess high mass activity and durability due to the high specific surface area and low surface energy [1]. Conventional PEFC electrodes consisting of carbon-supported platinum (Pt/C) catalyst and ionomer suffer from degradation due to dissolution/redeposition, Ostwald ripening, and agglomeration of Pt, corrosion of the carbon support, and Pt poisoning by ionomer. Therefore, using unsupported Pt(ns) for an electrode that consists of neither carbon nor ionomer is expected to eliminate the degradation factors, resulting in a highly active and durable electrode. The purpose of this study is to develop highly active and durable electrodes by using Pt(ns) and eliminating degradation factors by making them carbon-free and ionomer-free.Polycrystalline Pt(ns) was synthesized by interlayer oxidation of graphene, as described elsewhere [2]. Catalyst ink was prepared, and cathode catalyst layers were created by dropping or spraying it onto carbon paper. A membrane-electrode assembly (MEA) was made by hot-pressing the cathode catalyst layer together with an anode made of Pt/C onto an electrolyte membrane. Power generation was conducted at 80˚C by supplying H2 to the anode and O2 or air to the cathode, each humidified under ambient pressure. Durability was evaluated by repeatedly applying cyclic potentials between 1.0 V and 1.5 V, supplying N2 to the anode and cathode, and examining changes in the cyclic voltammogram.The I-V curves of MEAs with ionomer-free carbon-free Pt(ns) cathodes fabricated using drop-coating and spray-coating methods and measured using air and oxygen as the cathode supply gases are shown in Fig. 1. As demonstrated in the figure, the activation overpotential at low current density was high, suggesting a low Pt utilization. The shape of the I-V curve of MEA using the spray method at high current density indicated that the concentration overpotential was suppressed under oxygen conditions but was high under air conditions. On the other hand, the concentration overpotential of the MEA using the drop method was high even under oxygen conditions. This may be attributed to the small spaces between the restacked nanosheets, which inhibited diffusion.A start-stop simulated potential cycle test was conducted for the MEA using ionomer-free carbon-free Pt(ns) cathode. The CV curves measured after a certain number of cycles are shown in Fig. 2. As shown in the figure, the change in the shape of the CV was small, and the ECSA remaining rate of the electrode was above 80% even after 8000 cycles, whereas that of the Pt/C cathode was less than 50% after 4000 cycles. This suggests that carbon-free ionomer-free Pt(ns) electrode has the potential as highly durable low Pt electrodes.AcknowledgementPart of this paper is based on results obtained from a project, JPNP20003, subsidized by the New Energy and Industrial Technology Development Organization (NEDO).

Visualization of Oxygen Bubbles on a Flat Ionomer-Coated Platinum Electrode

To improve the performance of polymer electrode membrane electrolyzers, it is necessary to grasp oxygen transport from the reaction site. This paper presents a visualization study which examines the effect of ionomers on the behavior of oxygen bubbles generated from the catalyst covered with ionomers. For the sake of visualization, a flat electrode was fabricated on a glass plate as shown Fig.1 (a). The reaction area was plated with platinum and covered entirely with ionomer using a spin coater.As shown Fig.1 (b), oxygen bubbles generated on ionomer surface were successfully captured with a high-speed camera. As shown Fig.1 (c), (d) the post visualization by electron microscopy with EDX showed that there were no significant cracks and damages on the ionomers. In other words, oxygen bubbles are generated on the ionomer surface without an apparent three-phase boundary (i.e., catalyst-ionomer-water). This suggests oxygen generated on the Pt surface was transported through the ionomer layer. The three-phase boundary may not be mandatory for the catalyst layer. The results presented in this research can provide better understanding of oxygen transport properties inside catalyst layer, thereby contributing to the future optimization of the structural design of catalyst layers. 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).Fig.1 (a) Flat electrode on a glass plate, (b) Figure of the oxygen bubbles when electrolyzing, Element mapping of (c) Platinum and (d) Fluorine Figure 1

FC-Platform Activities and Post Analysis of Durability Tested MEA

FC-Platform, specialized in PEFC's analysis and evaluation, has been launched as a NEDO's funded project. FC-Platform consists of 5 groups of the electrochemical evaluation, simulation, materials analysis, materials informatics and management, involves several institutions. The numbers of various materials research themes have been also running in the NEDO's project. FC-Platform provides better directions of improving for these materials research through analysis and evaluation of their materilas. Moreover, Materials informatics (MI) have been applyed to the development of thier matrials in several themes. Visualizing and predicting phenomena in the PEFCs are essential for realizing high performance and durability at a material level. Therefore, we are focusing on advanced visualization techniques and trans-scale stack modeling development. We also had post analysis and evaluation of the MEAs under long term road-cycle test.This work was supported by a NEDO FC-Platform project commissioned by th e New Energy and Industrial Technology Development Organization (NEDO).

PEFC Electrocatalysts Using Ta-Based Support on Mesoporous Carbon

Introduction Cathode catalysts of polymer electrolyte fuel cells (PEFCs) are subjected to a severe environment with strong acidity and high potential, leading to catalyst degradation[1]. In particular, the degradation of electrocatalysts in fuel cell vehicles is often caused by oxidative corrosion of their carbon support during start-stop cycles, and by dissolution and re-deposition of platinum during load cycles[2]. Our research group has developed electrocatalysts using Nb-doped SnO2 on carbon support, achieving high start-stop potential durability[3]. The use of Ti and Sn as metal supports instead of carbon has also been considered as a fundamental solution against start-stop related degradation[4]. However, catalytic activity has to be improved, compared to the standard Pt/C catalysts. Here in this study, catalysts using mesoporous carbon (MC) support are prepared using metallic Ta with a high melting point. Parameters such as loading and heat treatment conditions are varied to tailor higher performance and durability of PEFC electrocatalysts. Experimental Ta/MC supports were prepared by steam hydrolysis of an ethoxide reagent on mesoporous carbon (CNovel®, TOYO TANSO) with mesopores of 10 nmΦ. The Pt/Ta/MC catalysts was then prepared by decorating with Pt using the acetylacetonate (acac) method[5] . The microstructure of the prepared Ta/MC supports and the Pt/Ta/MC catalysts was observed using scanning transmission electron microscopy (SU9000, Hitachi High-Tech Corp., Japan). Electrochemical characterization of the catalysts was performed by half-cells: Pt electrochemical surface area (ECSA) was evaluated by cyclic voltammetry (CV), and oxygen reduction reaction activity (ORR activity) was evaluated from the activation-dominant current value by rotating disk electrode (RDE) measurements. The catalysts durability was evaluated by applying potential cycles simulating start-stop and load cycles. Results and discussion As shown in Fig. 1, microstructural observation of the catalyst supports by STEM confirmed that small Ta particles were uniformly supported, and that the Ta particles were impregnated into the mesopores of MCs. EDS elemental analysis was performed to calculate the residual oxygen content. It was found that the reduction of Ta2O5 to Ta proceeded by high-temperature heat treatment in an H2 atmosphere. The initial activity and durability of the prepared Pt/Ta/MC catalysts were evaluated by half-cell electrochemical measurements. ECSA and mass activity were 78.5 m2/g and 179.2 A/g, respectively, showing initial activity approaching that of the standard Pt/C catalyst. Regarding durability, high load cycle durability was confirmed. In this presentation, the microstructure and electrochemical properties of the Pt/Ta/MC catalysts will be presented as alternative cathode catalysts. Acknowledgment This paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).