Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acsaem.5c01530

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1039/d5cy00200a

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Language：English   Publishing type：Research paper (scientific journal)  

The fouling of non-targeted biomolecules on sensing surfaces, which can cause a reduction in sensing performance, is a severe problem in immunosensing platforms. The incorporation of hydrophilic polymers on sensing surfaces is effective against antifouling. However, such an approach can reduce the density of the capture antibody, resulting in a decrease in sensitivity and signal output. Here, both high sensitivity and antifouling properties were achieved using a porous-membrane-based immunosensor. This sensor can drastically mitigate the signal reduction due to the introduction of an antifouling moiety by antibody densification in submicron-scaled pores. The ideal ratio of the receptor/antifouling moiety was estimated from numerical modeling. The high sensitivity and antifouling properties of the designed sensor were demonstrated via the detection test of interleukin-6 (IL-6). The proposed sensor exhibited excellent antifouling and high sensitivity with limits of detection of 4.8 and 1.2 pg mL-1 in artificial saliva and serum, respectively. The study findings highlight the potential of membrane-based sensors for practical diagnoses.

DOI： 10.1039/d5lc00031a

PubMed

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1039/D5CC00934K

Web of Science

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DOI： 10.1021/acsami.4c18777

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Abstract

Solid‐state crystalline multimetal oxides have been widely studied for their applications as electrode materials, e. g., in fuel cells, water electrolyzer, lithium‐ion batteries, and metal‐air batteries. Particularly, hydrogen production via water electrolysis is considered a key technology for realizing a sustainable circular carbon society. Enhancing the activity of electrocatalysts is a critical challenge for improving the efficiency of the water electrolysis technology. Thus, strategies for designing prominent catalyst materials have drawn considerable attention. Our group has been conducting research aimed at establishing comprehensive design strategies for the rational and rapid development of highly active catalysts. In this Personal Account, we present our research efforts on enhancing the activity of cost‐efficient nonprecious metal‐based oxide catalysts for the oxygen evolution reaction at the anode of water electrolysis. Specifically, we propose design strategies based on crystal structures of multimetal oxides and demonstrate how these strategies have contributed to the development of highly active electrocatalysts.

DOI： 10.1002/tcr.202400246

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acsanm.4c05978

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Hydrogen/oxygen separation under wet conditions has recently become important in the production of hydrogen.

DOI： 10.1039/d4se01692k

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.surfin.2025.105809

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.memsci.2025.123896

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Abstract

Nanostructured Pt‐based catalysts have attracted considerable attention for fuel‐cell applications. This study introduces a novel one‐pot and low‐temperature polyol approach for synthesizing support‐free, connected nanoparticles with non‐Pt metal cores and Pt shells. Unlike conventional heat treatment methods, the developed support‐free and Fe‐free connected Pd_core@Pt_shell (Pd@Pt) nanoparticle catalyst possesses a stable nanonetwork structure with a high surface area. This approach can precisely control the atomic‐level structure of the Pt shell on the Pd core at a low deposition temperature. The optimized Pd@Pt catalyst with a Pt/Pd atomic ratio of 0.8 and a Pt shell thickness of 1.1 nm exhibits a threefold improvement in oxygen reduction reaction (ORR) mass activity compared to that of commercial carbon‐supported Pt nanoparticle catalyst (Pt/C). Durability evaluation demonstrated 100% retention of specific activity after 10,000 load cycles, owing to the stable nanonetwork and uniform coverage of the Pt shell. In addition, the support‐free, connected core–shell nanoparticle catalyst overcomes the carbon corrosion issues commonly associated with conventional carbon‐supported catalysts while simultaneously improving both ORR activity and load cycle durability. These findings highlight the potential of this innovative approach to develop support‐free catalysts for polymer electrolyte fuel cells and other energy devices.

DOI： 10.1002/advs.202408614

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This study reports nickel-based phosphate nanoparticles as outstanding electrocatalysts for the oxygen evolution reaction and uncovers main factors determining their activities.

DOI： 10.1039/d4na00794h

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Publishing type：Research paper (scientific journal)   Publisher：The Electrochemical Society  

Polymer electrolyte fuel cells (PEFCs) have gained considerable attention due to their high energy density, high conversion efficiency, and wide operating temperatures. The main challenges encountered in the commercialization of PEFCs are the sluggish kinetics and low durability of the cathodic oxygen reduction reaction (ORR) catalyst. The conventional electrocatalyst, platinum (Pt) nanoparticles (2~5nm) dispersed on carbon support, suffer insufficient catalytic activity and low durability owing to enlargement in particle size due to particle coalescence or Ostwald ripening during PEFC operations. In this scenario, the facile fabrication of advanced functional catalysts with improving ORR activity and stability is significant for PEFCs.

Among the various catalysts reported so far in the literature, Pt is an effective metal for catalyzing the ORR reaction due to its intrinsic catalytic performance and stability, however, it's scarcity and high cost limit its large-scale practical application. Compared to nanoparticle counterparts, the anisotropic one-dimensional nanostructure has been explored for higher activity due to its high flexibility, outstanding conductivity, and thermal stability.^1–3 Several strategies have been adopted in the literature to enhance the catalytic activity and the Pt utilization efficiency. One aspect is the atomic scale tuning of the Pt chemical environment through proper alloying with bimetallic or trimetallic elements. In addition, it is essential to construct favorable structures in alloy systems with maximum exposed active sites.

In this work, we synthesized a unique nanostructure of Pt alloy nanowires (PtNiCo NW) through a solvothermal method by reducing the Pt, Ni, and Co precursors in a solvent mixture with reducing agents and surfactants (oleyl amine with W(CO)₆, glucose and CTAB). The atomic scale tailoring of the Pt chemical environment of the catalyst was achieved by varying the alloy composition. The TEM observation indicates that structural changes occurred by changing the reaction conditions of the catalysts; initially, smooth nanowires are formed with a diameter of 2 nm (Fig. 1a) followed by the formation of the rough surface due to an increase in the reduction of Ni over the Pt NWs over time. A porous nanostructure of Pt with Ni (Fig 1b) was fabricated through an acid treatment process.

The electrochemical surface area and ORR activities of the PtNiCo NWs on carbon support were estimated from rotating disk electrode measurements in 0.1 M HClO₄ electrolyte solution. Compared with the smooth PtNiCo NW catalyst, the porous PtNiCo NW catalyst with a rough surface exhibited a higher mass activity of 1.6 ± 0.1 Amg-Pt⁻¹ and an excellent specific activity of 3.7 ± 0.2 mAcm-Pt⁻². The higher specific activity would arise from the increased roughness on the Pt surface through the formation of porous structure and the optimized surface-active sites on the catalyst surface through composition tuning.

Acknowledgment

This presentation is based on the results obtained from the project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan.

References M. Li et al, Science, 2016, 354 (6318), 1414–1419.

X. Tian et al, Science, 2019, 336 (6467), 850–856.

K. Jiang et al, Sci. Adv., 2017, 3(2), e160170.



Fig 1. TEM images of Pt-based nanowires having (a) smooth and (b) rough-porous structures.



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DOI： 10.1149/ma2024-02412698mtgabs

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Publishing type：Research paper (scientific journal)   Publisher：The Electrochemical Society  

The improvement of the activity and durability of Pt-based electrocatalysts for oxygen reduction reaction (ORR) is essential to realize the widespread uses of polymer electrolyte fuel cells (PEFCs). Our group has developed a carbon-free connected Pt₁–Fe₁ catalyst with a chemically ordered superlattice structure.^1–7 This catalyst consists of a beaded nanonetwork (5–15 nm) by the connection of Pt₁–Fe₁ nanoparticles with high electrical conductivity, thus eliminating the need for a carbon support. The connected Pt₁–Fe₁ catalyst exhibits an ORR specific activity about 10 times higher than a commercial Pt/C catalyst. This catalyst with carbon-free and chemically ordered structures improves the durability against start/stop and load cycles.^1,4 Furthermore, the carbon-free catalyst layer using the connected Pt₁–Fe₁ catalyst with a porous, hollow capsule structure has high porosity (&gt; 80%) and thin thickness (~ 1 μm), which would be beneficial for oxygen transport.

To advance connected nanoparticle catalysts, this study addressed structural control of connected Pt-based catalysts and carbon-free catalyst layers using the developed catalysts. Since Fe ions induce the Fenton reaction and accelerate the degradation of electrolyte polymers, carbon-free and iron-free connected Pt₃–Co₁ catalysts with chemically ordered structures (Figure 1a) were developed using our silica coating method.⁴ The investigation of the structural effects on ORR performances in 0.1 M HClO₄ electrolyte solution indicated that higher ordered degrees of the connected Pt₃–Co₁ catalysts improved ORR specific activity and load cycle durability. The STEM-EDX line mapping results suggested that a highly ordered structure can greatly suppress metal leaching from the catalyst. Furthermore, as shown in Figure 1b, this study investigated the structural effects on fuel cell performances for the carbon-free cathode catalyst layers. Here, the connected Pt₃–Co₁ catalyst with a hollow capsule structure was used, and the structural parameters such as ionomer thickness on capsules, capsule size, etc. were controlled. This presentation will discuss the relationship between catalyst layer structures and fuel cell performances through electrochemical and oxygen transport analyses. The developed connected Pt-based catalysts and the findings obtained in this study will contribute greatly to the design of a carbon-free cathode catalyst layer to achieve enhanced PEFC performances.

<p></p> Acknowledgement: This presentation is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan.

<p></p> References

Kuroki, T. Yamaguchi et al., [1] Energy Environ. Sci., 8, 3545–3549 (2015). [2] J. Electrochem. Soc., 163, F927–F932 (2016). [3] ACS Appl. Energy Mater., 1, 324–330 (2018). [4] ACS Appl. Nano Mater., 3, 9912–9923 (2020). [5] ACS Appl. Energy Mater., 5, 13176–13188 (2022). [6] J. Chem. Eng. Jpn., 56, 2197946 (2023). [7] Jpn. J. Appl. Phys., 63, 048002 (2024).



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DOI： 10.1149/ma2024-02443024mtgabs

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Anion Exchange membrane water electrolyzers are reached the stage of commercialization by the advancement of durable anion exchange membranes. Highly active and stable electrocatalyst for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are essential to achieve energy efficient and cost-effective hydrogen production. Among the two electrochemical reactions, OER is more sluggish as it involves four electron transfers for the generation of an oxygen molecule.^{[ 1 ]} Many non-noble metal oxides-based catalysts are displayed OER activity in alkaline medium, but among them Ni, Fe, and Co based mono/bi/tri metallic oxides/hydroxide/oxyhydroxides exhibit high OER activity.^{[ 2–4 ]} Even though the actual catalytic mechanism is still debated, Ni-Fe oxides have received much attention over all other catalysts because of the synergism between the oxides and oxyhydroxides of Fe and Ni to bring down the OER overpotential.

Several methods were adopted for the synthesis of Ni-Fe oxides. However, most of them lead to the formation of mixed oxides and it affects the overall conductivity of the catalyst. The controlled electrochemical method of preparation possesses, the selective synthesis of conducting Ni-Fe oxides. In this work, we adapted a two-step electrochemical synthesis method to produce free-standing Ni-Fe oxide electrode with high OER activity. At first, a layer of γ–NiOOH was constructed over a cleaned nickel foam by applying a high voltage in 1M KOH. Raman spectra of oxidized Ni foam, represented in Figure 1A show two major peaks at 481 cm^–1 and 560 cm^–1 corresponding to the bending and stretching vibration of γ–NiOOH. Further, the broad peak band between 850 cm^–1 and 1200 cm^–1 is ascribed to the active oxygen species (NiOO^–) in oxyhydroxide.^[5,6] In the second step, a thin Fe layer was electrochemically deposited over the γ–NiOOH. The Raman spectra after Fe deposition also confirmed the existence of γ–NiOOH along with the peak corresponding to FeOOH at around 684 cm^–1.^[7] Cross-sectional SEM images with EDX of Fe deposited γ–NiOOH (γ–NiOOH-Fe) presented in Figure 1B show an oxide layer of ≈250 nm thickness with a uniform distribution of Fe. For a comparison, Fe deposited directly on a cleaned Ni foam (Ni-Fe) was also prepared thorough electrochemical deposition. The OER activity and stability of both catalysts were tested in 1 M KOH. γ–NiOOH-Fe catalyst attained 10 mA cm^–2 at 1.47 V whereas to reach the same current density Ni-Fe took 1.49 V. No degradation in catalytic activity was observed in the case of γ–NiOOH-Fe compared to Ni-Fe catalyst after the accelerated durability test and constant current electrolysis at 10 and 50 mA cm^–2 in 1 M KOH (Figure 1C and D). The above results demonstrate that the γ–NiOOH-Fe catalyst exhibits excellent activity and stability as an anode catalyst for anion exchange membrane water electrolyzer.

Acknowledgement

This presentation is based on results obtained from a project, JPNP14021, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

References J. Song, C. Wei, Z. Huang, C. Liu, L. Zeng, X. Wang and Z. J. Xu, Chem Soc Rev, 49, 2196–2214 (2020).

M. I. Jamesh and X. Sun, J Power Sources, 400, 31–68 (2018)

P. M. Bodhankar, P. B. Sarawade, P. Kumar, A. Vinu, A. P. Kulkarni, C. D. Lokhande, and D. S. Dhawale, Small, 18, 2107572–2107597 (2022).

M. Asnavandi, Y. Yin, Y. Li, C. Sun, and C. Zhao, ACS Energy Lett, 3, 1515–1520 (2018)

O. Diaz-Morales, D. Ferrus-Suspedra, and M. T. M. Koper, Chem Sci, 7, 2639–2645 (2016).

B. S. Yeo and A. T. Bell, The Journal of Physical Chemistry C, 116, 8394–8400 (2012)

J. Xu, B. X. Wang, D. Lyu, T. Wang, and Z. Wang, Int J Hydrogen Energy, 48, 10724–10736 (2023).



Figure 1. (A) The Raman spectra of γ-NiOOH, and γ-NiOOH-Fe (B) Cross sectional SEM- EDX images of γ-NiOOH-Fe, and (C) Linear sweep voltammograms of γ-NiOOH-Fe and Ni-Fe measured before and after the durability test in 1 M KOH at 25 °C.



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DOI： 10.1149/ma2024-02422796mtgabs

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/jacs.4c05740

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A simple two-step electrochemical process for the synthesis of a catalyst with the synergistic combination of γ-NiOOH and FeOOH for efficient and durable oxygen evolution reaction in an alkaline medium.

DOI： 10.1039/d4su00538d

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.iecr.4c02773

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acsami.4c01401

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.desal.2024.117297

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.jpowsour.2023.233962

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Publishing type：Research paper (scientific journal)   Publisher：The Electrochemical Society  

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 (&lt; 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).

References

[1] Tamaki, T., Yamaguchi, T. et al., Energy Environ. Sci., 8, 3545–3549 (2015)

[2] Kuroki, H., Yamaguchi, T. et al., ACS Appl. Nano Mater., 3, 9912–9923 (2020)



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DOI： 10.1149/ma2023-02401960mtgabs

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Publishing type：Research paper (scientific journal)   Publisher：The Electrochemical Society  

Water electrolysis technology, which produces hydrogen through electrolysis of water using electricity derived from renewable energy, is attracting attention as a way to realize a carbon-neutral society. Among water electrolysis systems, anion exchange membrane water electrolysis (AEMWE) can achieve high energy conversion efficiency and quick response to fluctuating power supplies by using thin polymer electrolyte membranes, and operates in an alkaline environment, allowing the use of inexpensive non-precious metals for bipolar plates and electrodes. However, conventional AEMs and ionomers exhibit low durability in high-temperature and alkaline environments, which is a critical issue in realizing high-performance and highly durable membrane–electrode assemblies (MEAs) for AEMWE.

Our group developed high-molecular-weight (M _w) and ether-free, polyfluorene-based anion conducting polyelectrolyte (PFT-Cx-TMA, Cx represent the number of carbon atoms in the alkyl side chain) as shown in Fig. 1a.¹ These polyelectrolytes show high OH⁻ conductivity over 100 mS cm^{− 1}, high chemical durability (stable in 8 M NaOH at 80 °C for 168 h), and high tensile strength (25–42 MPa, comparable to a Nafion membrane). This study applied PFT-Cx-TMA to AEMs and ionomers in MEAs to evaluate the durability against various AEMWE operations (constant current operation and start-stop operation in different voltage ranges), demonstrating MEAs with both high performance and high durability.

Fig. 1b shows the constant current durability of the MEA with PFT-C8-TMA as AEM and ionomer.² This test was performed at the constant current density of 0.2 A cm^{− 2} and 80 °C using a 1 M KOH aqueous solution fed to the anode. Here, to investigate the effects of AEMs and ionomers on MEA durability, conventional precious metal catalysts (anode: IrO₂, cathode: Pt/C or PtRu/C) were used in the MEA. The MEA with PFT-C8-TMA exhibited a current density of 1.0 A cm^{− 2} at 1.79 V and there was almost no voltage change even after the constant current operation for over 100 h. It is noteworthy that the MEAs using PFT-C8-TMA and PFT-C10-TMA demonstrated high performance and high durability not only in the alkaline-fed operation but also in the pure-water-fed operation.

Next, this study investigated the start-stop durability of MEAs using voltage cycles simulating the start-stop operation of water electrolyzer. Here, square-wave voltage cycling of holding at 1.8 V and 0.1 V for 1 min each was used for the start-stop test at 80 °C with a 1 M KOH solution fed to the anode. As a comparison, the MEA was prepared using a commercial AEM (Sustainion X37-50, 50 μm in thickness) and ionomer (Sustaionion XB-7). As shown in Fig. 1c, the MEA using a PFT-C10-TMA AEM (56 μm in thickness) and ionomer was highly durable in the start-stop test compared to the MEA using commercial Sustaionion. During the test, the elution of black solid particles from the catalyst layer and the increase in charge transfer resistance (Rct) were observed in the MEA with Sustaionion, whereas no elution of catalyst materials and no change in Rct were confirmed in the MEA with PFT-C10-TMA. These results suggest that nonlinear mechanical stress via rapid bubble formation and shutdown during start-stop cycles caused the leaching of catalysts, resulting in the performance loss of the MEA with Sustainion. The commercial ionomer (Sustainion XB-7) exhibits a large dimensional change (156%) compared to the PFT-C10-TMA membrane (34%) at 80 °C in a 1 M KOH aqueous solution. Catalyst particles covered with a highly swollen ionomer are easily detached from a catalyst layer by the nonlinear mechanical stress. On the other hand, the PFT-C10-TMA ionomer has a high molecular weight (M _w = 240000 g mol⁻¹), low swelling degree, and well-entangled polymer chains, hence preventing leaching of the catalyst particles.

In summary, this study demonstrates that MEAs using high M _w and ether-free polyfluorene-based electrolytes with both high chemical and mechanical durability have excellent durability for AEMWE operations including constant and dynamic (start-stop voltage cycling) operations.

Acknowledgement

This presentation is based on results obtained from a project, JPNP14021, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

References

(1) S. Miyanishi and T. Yamaguchi, Polym. Chem., 2020, 11, 3812–3820.

(2) R. Soni, S. Miyanishi, H. Kuroki, and T. Yamaguchi, ACS Appl. Energy Mater., 2021, 4, 1053–1058.



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DOI： 10.1149/ma2023-02422069mtgabs

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Publishing type：Research paper (scientific journal)   Publisher：The Electrochemical Society  

Polymer electrolyte fuel cells (PEFCs) have been considered as the promising next-generation energy-conversion technology, owing to their high energy efficiency and clean emissions. However, a sluggish oxygen reduction reaction (ORR) rate and the low durability of conventional catalysts in PEFCs limit the wider commercialization. To address these issues, our group has developed a carbon-free, connected Pt₁–Fe₁ catalyst with a porous hollow capsule structure. This catalyst comprises the nano-sized network formed by the connection of Pt₁–Fe₁ nanoparticles and exhibits an enhanced ORR specific activity (9 times higher than a commercial Pt/C) as well as excellent durability against start-stop operations due to the carbon-free structure.^[1,2] However, during the load cycle operation, iron is dissolved from the catalyst, leading to the loss of ORR activity and the degradation of polymer electrolytes caused by the Fenton reaction.

In this work, carbon-free and iron-free connected Pt_x-Co₁ catalysts with chemically ordered superlattice structures have been developed for the first time, as shown in Fig. 1(a). In addition, the metal compositions and chemically ordered (superlattice) degrees in the catalysts were controlled to investigate the structural effects on ORR activity and durability of the connected Pt_x-Co₁ catalysts.

Herein, we employed the silica-coating method for the catalyst synthesis, which involves the combination of silica coating and high-temperature annealing. As shown in Fig. 1(b), the surface of the Pt_x-Co₁ nanoparticles/SiO₂ sample was coated with thin silica layers to prevent the detachment of Pt_x–Co₁ nanoparticles from the SiO₂ template as well as the large catalyst agglomeration and coalescence during the annealing process. We controlled the chemically ordered degrees of the connected Pt₁–Co₁ and Pt₃–Co₁ catalysts by using different annealing temperatures. The XRD patterns of the catalysts showed the peaks corresponding to L1₀ type and L1₂ type chemically ordered structures for the connected Pt₁–Co₁ and Pt₃–Co₁ catalysts annealed at 600°C, respectively. These two catalysts possessed ordered degrees (S) of ca. 50–60%. The TEM images of the connected Pt_x–Co₁ catalysts showed the formation of connected Pt_x–Co₁ nanonetwork and porous hollow capsule structures.

The electrochemical measurements of ORR activity and durability were conducted in a 0.1 M HClO₄ electrolyte solution. The connected Pt₁–Co₁ and Pt₃–Co₁ catalysts achieved 6- and 10-times higher ORR specific activities than that of Pt/C, respectively. Subsequently, the ORR activity and load cycle durability (0.6 V for 3 s Û 1.0 V for 3 s at 60 °C) of the connected Pt₃–Co₁ catalyst with different ordered degrees (S = 0%, 28%, and 58%) were evaluated. As the ordered degrees in the connected Pt₃–Co₁ catalysts were higher, the ORR specific activity was increased. Also, the retention of the specific activity of the ordered catalyst (S = 58%) was higher than that of the disordered catalyst (S = 0%) after 10,000 load cycles. The STEM-EDX line mapping results showed that 83% of the alloyed Co retained in the catalyst with a higher ordered degree even after 10000 load cycles, whereas the disordered and lower ordered catalysts retained lower Co ratios after the test, indicating the suppression effect of an ordered structure on metal dissolution. In addition, the L1₂ ordered Pt₃–Co₁ structure remained in the connected catalyst (S = 58%) after load cycle test, confirmed by the TEM diffraction analysis.

In summary, we successfully developed the connected Pt_x-Co₁ catalysts with chemically ordered superlattice structures. In addition, we clarified the structural effects of metal composition and ordered degree in connected Pt_x-Co₁ catalysts on ORR activity and load cycle durability. These results provide useful guidelines for the design of advanced ORR catalysts towards high-performance PEFCs.

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).

References

[1] Tamaki, T. Yamaguchi, T. et al., Energy Environ. Sci., 8, 3545–3549 (2015)

[2] Kuroki, H., Yamaguchi, T. et al., ACS Appl. Nano Mater., 3, 9912–9923 (2020)



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DOI： 10.1149/ma2023-02401959mtgabs

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DOI： 10.1080/00219592.2023.2197946

Web of Science

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DOI： 10.1021/acsaem.3c00997

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

Machine learning analysis revealed the importance of structural features involving A-site metals in AxByOz multimetal oxides for their OER activity.

DOI： 10.1039/d3ya00238a

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.macromol.3c00672

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Informa UK Limited  

DOI： 10.1080/00219592.2023.2210195

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acssuschemeng.2c07387

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Language：English   Publishing type：Research paper (scientific journal)  

A highly sensitive immunosensor is developed using membrane pores as the recognition interface. In this sensor, a Cu-free click reaction is used to efficiently immobilize antibodies, and the sensor inhibits the adsorption of nonspecific proteins that degrade sensitivity. Furthermore, the sensor demonstrates rapid interleukin-6 detection in the picogram per milliliter range.

DOI： 10.1039/d2ay02110b

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acssuschemeng.2c03663

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Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

Non platinum based metal phosphide cathode electrodes with a very low Ru loading for sustainable hydrogen generation and futuristic carbon free alkaline water electrolyzers.

DOI： 10.1039/d3se00169e

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Language：Japanese   Publisher：Japan Synchrotron Radiation Research Institute  

DOI： 10.18957/rr.10.6.540

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

Stimuli-responsive star polymers are promising functional materials whose aggregation, adhesion, and interaction with cells can be altered by applying suitable stimuli. Among several stimuli assessed, the potassium ion (K+), which is known to be captured by crown ethers, is of considerable interest because of the role it plays in the body. In this study, a K+-responsive star copolymer was developed using a polyglycerol (PG) core and grafted copolymer arms consisting of a thermo-responsive poly(N-isopropylacrylamide) unit, a metal ion-recognizing benzo-18-crown-6-acrylamide unit, and a photoluminescent fluorescein O-methacrylate unit. Via optimization of grafting density and copolymerization ratio of grafted arms, along with the use of hydrophilic hyperbranched core, microsized aggregates with a diameter of 5.5 μm were successfully formed in the absence of K+ ions without inducing severe sedimentation (the lower critical solution temperature (LCST) was 35.6 °C). In the presence of K+ ions, these aggregates dispersed due to the shift in LCST (47.2 °C at 160 mM K+), which further induced the activation of fluorescence that was quenched in the aggregated state. Furthermore, macrophage targeting based on the micron-sized aggregation state and subsequent fluorescence activation of the developed star copolymers in response to an increase in intracellular K+ concentration were performed as a potential K+ probe or K+-responsive drug delivery vehicle.

DOI： 10.1021/acsomega.2c06763

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DOI： 10.1021/acsaem.2c01067

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DOI： 10.1149/MA2022-02441689mtgabs

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DOI： 10.1021/acsaem.2c02133

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DOI： 10.1016/j.desal.2022.115923

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Publishing type：Research paper (scientific journal)   Publisher：MYU K.K.  

DOI： 10.18494/sam3905

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DOI： 10.26434/chemrxiv-2022-n214g

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DOI： 10.1021/acs.iecr.1c04210

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DOI： 10.1016/j.ijhydene.2022.03.008

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DOI： 10.1002/celc.202101679

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DOI： 10.1021/acsaem.1c02817

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Proton conducting materials suffer from low proton conductivity under low-relative humidity (RH) conditions. Previously, it was reported that acid-acid interactions, where acids interact with each other at close distances, can facilitate proton conduction without water movement and are promising for overcoming this drawback [T. Ogawa, H. Ohashi, T. Tamaki and T. Yamaguchi, Chem. Phys. Lett., 2019, 731, 136627]. However, acid groups have not been compared to find a suitable acid group and density for the interaction, which is important to experimentally synthesize the material. Here, we performed ab initio calculations to identify acid groups and acid densities as a polymer design that effectively causes acid-acid interactions. The evaluation method employed parameters based on several different optimized coordination interactions of acids and water molecules. The results show that the order of the abilities of polymer electrolytes to readily induce acid-acid interactions is hydrocarbon-based phosphonated polymers > phosphonated aromatic hydrocarbon polymers > perfluorosulfonic acid polymers ≈ perfluorophosphonic acid polymers > sulfonated aromatic hydrocarbon polymers. The acid-acid interaction becomes stronger as the distance between acids decreases. The preferable distance between phosphonate moieties is within 13 Å.

DOI： 10.1039/d1cp00718a

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DOI： 10.1039/d1cc03152j

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DOI： 10.1088/2515-7655/abe2f6

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DOI： 10.1021/acs.analchem.1c00250

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DOI： 10.1021/acsaem.0c02710

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DOI： 10.1039/d0se01295e

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society  

DOI： 10.1021/acsaem.0c01938

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DOI： 10.1016/j.memsci.2020.118772

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：John Wiley and Sons Inc  

DOI： 10.1002/celc.202101235

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DOI： 10.1039/D0CY01437K

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Publishing type：Research paper (scientific journal)   Publisher：The Electrochemical Society  

Oxygen evolution reaction (OER) catalysts for polymer-electrolyte water electrolysis (PEWE) requires a high Ir loading of 1–4 mg cm⁻² because a carbon support cannot be used for a PEWE anode where a highly oxidizing potential is applied and most supports including carbon are corroded. Connected nanoparticle catalysts showing electron conductivity and a high surface area without any conducting supports due to the formation of electron-conducting networks. Thus, the connected nanoparticles catalysts (Fig. 1 (A)) are promising for OER. Previously, we have developed connected Ir nanoparticle catalysts coated onto silica with high density for OER in PEWE.^[1]

In this study, we proposed connected Ir-Ru nanoparticle catalysts for OER in PEWE because the alloy of Ir and Ru shows high OER activity in the research of nanoparticle catalysts on conducting supports [2–3]. The catalysts were synthesized as follows. First, connected Ir-Ru nanoparticles were synthesized onto silica template (PDDA/SiO₂) with high density via polyol method using tetraethylene glycol as the reducing agent, and Ru(III) acetylacetonate and Ir(III) acetylacetonate as the metallic precursors (connected Ir-Ru/SiO₂). Then, silica was coated onto connected Ir-Ru/SiO₂ using tetraethyl orthosilicate and ammonia aqueous in ethanol by stirring for 6 hours at room temperature. After silica coating, these nanoparticles were treated in supercritical ethanol at 280–450 ºC for 60–90 min. Dissolution of SiO₂ in 3 M NaOH solution at 85ºC for 3 h made porous hollow structure (connected Ir-Ru capsule). OER performance was evaluated by electrochemical measurement in 0.1 M HClO₄ solution. Membrane electrode assemblies (MEAs) were prepared by CCM method using connected nanoparticle catalysts as the anode catalyst, Pt/C as the cathode catalyst and Nafion® membrane as the electrolyte membrane as shown previously. The Ir loading of the MEAs was below 1 mg cm⁻², while the Pt loading was 0.3 mg cm⁻². Water electrolysis and electrochemical impedance spectroscopy (EIS) were performed at 80 °C with circulating water in the anode flow channel.

Fig. 1 (B) shows TEM image of connected Ir-Ru capsule. Capsule and networks structure of each catalyst were observed by TEM images. Fig. 1 (C) shows OER curves of connected Ir and Ir-Ru catalysts. All synthesized catalysts showed higher OER activity than commercial catalyst. Among them, connected Ir-Ru/SiO₂ had the highest OER activity, although both Ir-Ru catalysts showed lower onset potential than Ir catalysts. Water electrolysis performance of the MEA using connected Ir-Ru/SiO₂ was higher than that of the MEA using connected Ir/SiO₂, as shown in Fig 1 (D).

In conclusion, we successfully synthesized connected Ir and Ir-Ru nanoparticle catalysts. Among them, connected Ir-Ru/SiO₂ showed the highest OER activity and water electrolysis performance. The results showed that connected Ir-Ru nanoparticle catalysts are promising as carbon-free catalysts for water electrolysis.

Acknowledgement

Part of this presentation is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

[1] Y. Sugita, T. Tamaki, H. Kuroki and T. Yamaguchi, Nanoscale Adv., 2, 171 (2020).

[2] T. Audichon, B. Guenot, S. Baranton, M. Cretin, C. Lamy and C. Coutanceau, Appl. Catal. B Environ., 200, 493 (2017).

[3] R. V. Genova-Koleva, F. Alcaide, G. Álvarez, P. L. Cabot, H.-J. Grande, M. V. Martínez-Huerta and O. Miguel, J. Energy Chem., 34, 227 (2019).



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DOI： 10.1149/ma2020-02382474mtgabs

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To realize the widespread uses of polymer electrolyte fuel cells (PEFCs), the improvement of the activity and durability of Pt-based electrocatalysts for oxygen reduction reaction (ORR) is essential. In our group, a carbon-free connected Pt–Fe catalyst with a porous hollow capsule structure has been developed. ^[1–3] This catalyst consists of a nanosized beaded network by the connection of Pt–Fe nanoparticles. The beaded metal network is electrically conductive, enabling the removal of carbon supports form a catalyst layer. The carbon-free cathode catalyst layer using a connected Pt–Fe catalyst exhibits high durability against start/stop cycles, because of the elimination of carbon corrosion problems. In addition, the ORR specific activity of a connected Pt–Fe catalyst is about 9 times higher than that of a commercial Pt nanoparticle catalyst supported on carbon black (Pt/C). Thus, a connected Pt–Fe catalyst provides a high ORR activity and excellent start/stop durability. On the other hand, the dissolution of catalyst metals during a load cycle operation is one of critical issues for Pt-based ORR catalysts. Our previous studies using Pt-alloy nanoparticle catalysts on carbon black demonstrate that chemical ordered (superlattice) structures improve load cycle durability due to the suppression of metal dissolution. ^[4–8] Therefore, as shown in Figure 1A, this study developed a carbon-free connected Pt–Fe catalyst with a high chemical ordering structure to achieve high durability against both start/stop and load cycle operations.

In this study, we proposed a new synthesis method using the combination of a SiO₂ coating and an annealing treatment (Figure 1B) to control chemically-ordered (face centered tetragonal: fct) structures in a connected Pt–Fe catalyst. The SiO₂ layer coated on catalyst surfaces prevents the detachment of Pt–Fe nanoparticles and the large catalyst agglomeration and coalescence during the high-temperature annealing. Using this method, the connected Pt–Fe catalyst having a nano-sized network and a high fct degree (70–80%) was successfully prepared. This catalyst exhibited ca. ten times higher ORR specific activity, compared with that of the Pt/C. Furthermore, as shown in Fig. 1C, the connected Pt–Fe catalyst with the high fct degree (76%) showed the higher retention of the ORR specific activity after 10000 load cycles (0.6 V for 3 s and 1.0 V for 3 s in N₂-saturated 0.1 M HClO₄ solution at 60 °C), compared to the catalyst with the low fct degree (47%). The results of the STEM-EDX line mapping indicated that a highly ordered fct structure greatly contributes to the suppression of metal dissolution from the catalyst.

This study demonstrated, for the first time, the synthesis method as shown in Figure 1B to prepare a connected Pt–Fe catalyst with a nanosized network and a high chemical ordering structure. The carbon-free connected Pt–Fe catalyst with an enhanced chemically ordered structure exhibited a high ORR activity as well as improved durability against both start/stop and load cycle operations. The results obtained in this study provide useful guidelines for the design of high-performance ORR catalysts for PEFCs.

Acknowledgements

This work was financially supported by Kanagawa Institute of Industrial Science and Technology (KISTEC) and Core Research for Evolutionary Science and Technology at the Japan Science and Technology Agency (JST-CREST, JPMJCR1543).

References

[1] T. Tamaki, H. Kuroki, T. Yamaguchi et al., Energy Environ. Sci., 8, 3545–3549 (2015).

[2] H. Kuroki, T. Tamaki, and T. Yamaguchi, J. Electrochem. Soc., 163(8), F927–F932 (2016).

[3] H. Kuroki, T. Tamaki, T. Yamaguchi et al., ACS Appl. Energy Mater., 1(2), 324–330 (2018).

[4] B. Arumugam, T. Tamaki, T. Yamaguchi et al., RSC Adv., 4, 27510–27517 (2014).

[5] T. Tamaki, T. Yamaguchi et al., J. Power Sources, 271, 346–353 (2014).

[6] B. Arumugam, T. Tamaki, T. Yamaguchi, ACS Appl. Mater. Interfaces, 4, 27510–27517 (2015).

[7] H. Kuroki, T. Tamaki, T. Yamaguchi et al., Ind. Eng. Chem. Res., 55, 11458–11466 (2016).

[8] T. Tamaki, H. Kuroki, T. Yamaguchi et al., J. Appl. Electrochem., 48, 773–782 (2018).



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DOI： 10.1021/acs.macromol.9b01627

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DOI： 10.1016/j.ijhydene.2019.09.143

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DOI： 10.1021/acs.analchem.9b02489

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DOI： 10.1021/acs.iecr.9b02111

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DOI： 10.1002/cctc.201900960

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DOI： 10.1016/j.cplett.2019.136627

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DOI： 10.4164/sptj.56.100

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DOI： 10.1021/acsaem.8b01525

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DOI： 10.1039/C8CY02232A

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DOI： 10.1039/c8ta08400a

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DOI： 10.1021/acs.iecr.8b02446

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DOI： 10.1016/j.jpowsour.2018.05.013

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DOI： 10.1021/acsaem.8b00790

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Publishing type：Research paper (scientific journal)   Publisher：The Electrochemical Society  

Oxygen evolution reaction (OER) catalysts for polymer-electrolyte water electrolysis (PEWE) suffered from their low surface area because carbon support cannot be used for OER operating at high potential. Connecting metal nanoparticles enable catalysts to conduct electrons through nanoparticle networks without any support materials while keeping their high surface area. Thus, connected nanoparticle catalysts (Fig. 1 (A)) are promising for OER. Previously, we have developed connected Pt-Fe nanoparticle catalysts with hollow capsule structure for oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs).^[1]

In this study, we proposed Ru and Ir nanoparticle catalysts for OER in water electrolysis because Ru and Ir have high OER activity. Ru and Ir nanoparticle catalysts were synthesized as follows. First, Ru and Ir nanoparticles were synthesized on silica template (PDDA/SiO₂) via polyol method using tetraethylene glycol as reducing agent and Ru(III) acetylacetonate or Ir(III) acetylacetonate as the metallic precursors (M/PDDA/SiO₂, M is Ru or Ir). Then, these nanoparticles were treated in supercritical ethanol at 330ºC for 90 min (Ru) or 60 min (Ir) (sc-M/SiO₂). Dissolution of SiO₂ in 3 M NaOH solution at 85ºC for 3 h made porous hollow structure (M capsule). OER performance were evaluated by electrochemical measurement in 0.1 M HClO₄ solution.

Fig. 1 (B) shows TEM image of Ir capsule. Capsule and networks structure of each catalysts were observed by TEM images. All Ru catalysts inactivated for OER at the first CV cycle. OER activity of Ru/PDDA/SiO₂ did not improve by heat oxidation treatment in air at 400ºC. Fig. 1 (C) shows OER curves of Ir catalysts. All Ir catalysts showed stable OER. Among them, Ir/PDDA/SiO₂ had the highest OER activity. Mass activity at 1.48 V of Ir/PDDA/SiO₂ was higher than RuO₂ ^[2], IrO₂ ^[2] and Ir/C­^[3] in previous reports as shown in Fig.1 (D). Heat treatment of Ir/PDDA/SiO₂ at 200ºC further improved.

In conclusion, we successfully synthesized connected Ru and Ir nanoparticle catalysts. Among them, Ir/PDDA/SiO₂ showed the highest OER activity and the activity was further improved by heat oxidation treatment. The results showed that connected Ir nanoparticle catalysts are promising as carbon-free catalysts for water electrolysis.

[1] T. Tamaki, H. Kuroki, T. Yamaguchi et al., Energy Environ. Sci., 8, 3545-3549 (2015).

[2] Y. Lee et al., J. Phys. Chem. Lett., 3, 399-404 (2012).

[3] HN. Nong et al., Chem. Sci., 5, 2955-2963 (2014).





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Solid alkaline fuel cells (SAFCs) have attracted much attention as next-generation energy conversion electrochemical devices. Despite numerous advantages of SAFCs including the usages of various type catalysts and liquid fuels with high energy density, the practical applications of these devices are limited because of the drastic decrease in the cell performance during the high temperature operation under alkaline condition. This low performance of the device is mainly attributed to the severe degradation of membrane-electrode assembly (MEA). Therefore, the development of a durable and stable MEA is critical to improve the cell performances and stabilities in this system.

Here we report the development of highly-durable MEA using a new carbon-free cathode catalyst and an aromatic polyelectrolytes without an ether linkage. As shown in Fig. 1A-a, the carbon-free catalyst consists of a nanosized beaded network formed by the connection of Pt–Fe nanoparticles.^[1–3] The beaded metal network is electrically conductive, enabling the removal of carbon supports from catalyst layer. Hence, carbon corrosion problems can be avoided, leading to high durability against the cyclic start-stop operation of the system.

We successfully demonstrated the high durability of this carbon-free catalyst in alkaline electrolyte solution. In addition, the specific activity of a connected Pt–Fe catalyst for oxygen-reduction-reaction (ORR) is about five times higher than that of a commercial Pt-nanoparticle catalyst supported on carbon black.

Fig. 1A-b represents the chemical structure of new spirobifluorene-based aromatic ionomer with no ether-linkage. This anion-conductive ionomer exhibited high alkaline and oxidative durability because the backbone did not contain ether linkages, heteroatoms, and benzylic C–H bonds. It was also demonstrated that the thin-layer coating (&lt; 1 μm) of spirobifluorene-based ionomer on the anion-exchange membrane (Fig. 1A-c) improved alkaline and oxidative stability, and suppressed the permeability of potassium formate (HCOOK) as fuel.

We fabricated the MEA using the in-house made durable catalyst and membrane material, i.e., the carbon-free connected Pt–Fe catalyst, spirobifluorene-based ionomer, and anion conductive pore-filling membrane with durable thin layer. The in-house made MEA, operated at a high temperature of 80 °C, exhibited high power density of 220 mW cm⁻² and high open circuit voltage of about 1.0 V (Fig. 1B), here, a mixture of aqueous solutions of 4M HCOOK and 2 M KOH was used as the fuel. Further, more importantly, the high performance of the MEA retained even after the operation at 80 °C and 0.2 A cm⁻² for 150 h. Therefore, for the first time, we demonstrated a highly durable and stable MEA in alkaline medium for the direct formate SAFCs.

[1] T. Tamaki, H. Kuroki, S. Ogura, T. Fuchigami, Y. Kitamoto, and T. Yamaguchi, Energy Environ. Sci., 8, 3545–3549 (2015).

[2] H. Kuroki, T. Tamaki, and T. Yamaguchi, J. Electrochem. Soc., 163(8), F927–F932 (2016).

[3] H. Kuroki, T. Tamaki, M. Matsumoto, M. Arao, Y. Takahashi, H. Imai, Y. Kitamoto, and T. Yamaguchi, ACS Appl. Energy Mater., 1(2), 324–330 (2018).





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We have developed the connected platinum-iron (PtFe) nanoparticle catalyst with a porous hollow capsule structure as a carbon-free catalyst-layer material for polymer electrolyte fuel cells (PEFCs). ^[1,2] As shown in Fig. 1A, this catalyst consists of a nanosized beaded network by the fusion of PtFe nanoparticles with a crystallite size of 6~7 nm and an L1₀type chemically-ordered (face centered tetragonal) structure. The beaded metal network is electrically conductive, enabling the removal of carbon supports form a catalyst layer. The carbon-free cathode catalyst layer using a connected PtFe catalyst exhibits high durability against start-stop cycles, because of elimination of carbon corrosion problems. In addition, the specific activity of a connected PtFe catalyst for oxygen-reduction-reaction (ORR) is about 9 times enhanced, compared with that of a commercial Pt-nanoparticle catalyst supported on carbon black (Pt/C). On the other hand, inside a cathode catalyst layer, complex phenomena occur during a fuel-cell operation, such as ORR on catalyst surface, diffusion of oxygen molecules, proton conduction via ionomers, etc. Thus, a high performance of PEFC requires not only high ORR activity and stability but also lower resistance of oxygen mass transport.

In this study, the following structural effects on fuel-cell performances for the carbon-free catalyst layers using connected PtFe catalysts were investigated; (i) inner-space in hollow capsule, and (ii) inter-space between capsules (Fig. 1B). The effect of inner-space in hollow capsule was discussed by the comparison of the connected PtFe catalysts with hollow structure or on solid (silica) support. The inter-space between capsules was controlled by the use of different capsule sizes (capsule size = 300 nm, or 500 nm in diameter). The electrochemical analyses of the prepared catalyst layers revealed that the hollow structure and the enlargement of inter-space between capsules improved the IV performance, especially at the high current density region, implying an improved oxygen mass-transport due to a sufficient space for oxygen diffusion in the catalyst layer and/or effective removal of liquid water from the catalyst layer. Moreover, in this study, the structural effect of gas-diffusion layer on fuel-cell performances was also investigated. It was found that the gas diffusion layer with higher gas permeability would facilitate the removal of liquid water from the capsule catalyst layer, resulting in a lower oxygen-transport resistance. In this presentation, by means of structural and electrochemical analyses of the capsule catalyst layers, the relationship between the catalyst-layer nanostructures and the fuel-cell performances will be discussed in details. The knowledge obtained in this study will provide new insights into the design of a carbon-free catalyst layer to achieve an enhanced PEFC performance.

[1] T. Tamaki, H. Kuroki, S. Ogura, T. Fuchigami, Y. Kitamoto, and T. Yamaguchi, Energy Environ. Sci., 8, 3545-3549 (2015).

[2] H. Kuroki, T. Tamaki, and T. Yamaguchi, J. Electrochem. Soc., 163(8), F927-F932 (2016).





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<p>The carbon nanotube interpenetrated porous doped carbon alloy derived from trimetallic ZIF displays excellent oxygen reduction activity in alkaline media.</p>

DOI： 10.1039/c7se00249a

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DOI： 10.1149/2.1611707jes

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DOI： 10.1021/acsami.6b06979

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We developed connected carbon-free platinum-iron (PtFe) nanoparticle catalysts with porous hollow capsule structure (Fig. 1) as oxygen-reduction-reaction (ORR) electrocatalysts for polymer electrolyte fuel cells (PEFCs).^[1] This catalysts consist of a beaded network by connected PtFe nanoparticles with a crystallite size of 6~7 nm and a chemically-ordered (face centered tetragonal) superlattice structure. The beaded network is electrically conductive; thus, carbon-supports can be removed from catalyst layers in PEFCs. We have demonstrated that an MEA prepared using a carbon-free cathode of connected PtFe-nanoparticle catalysts was highly durable against start-up/shut-down operations because of the elimination of carbon-corrosion problems. Moreover, an ORR specific activity of a connected PtFe catalyst is about 9 times higher than that of a commercial Pt-nanoparticle catalyst supported on carbon black (Pt/C).

In this study, the structural effects of a connected PtFe catalyst on an ORR activity were investigated in order to elucidate the factors of its high ORR activity. Note that the connected PtFe catalyst exhibited about twice higher ORR specific activity than the PtFe-nanoparticle catalyst (just nanoparticles without any connection between them) on carbon black. In addition, the ORR specific activity of the connected PtFe nanoparticles with hollow structure (with no supports) was 1.5 times higher than that of the connected PtFe nanoparticles on the electroconductive carbon support. These results indicate that the unique structures of connected nanoparticle catalysts, such as connected beaded network and hollow structure, enhance an ORR activity.

We also have performed the refined structural analyses of a connected PtFe catalyst by using in-situ X-ray absorption spectroscopy (XAS) combined with Rietveld structure refinement with powder X-ray diffraction data, and spherical aberration corrected scanning transmission electron microscope (Cs-corrected STEM). The in-situ XAS analysis disclosed shorter Pt‒Pt bond distance and more Pt 5d vacancy of the connected PtFe catalyst compared with the commercial Pt/C. In addition, the Cs-corrected STEM observations revealed that the beaded network formed by connected PtFe nanoparticles possessed polycrystalline structure. In this presentation, the effects of the geometric and electronic structures of a connected PtFe catalyst on an ORR activity will be discussed in details. The knowledge obtained in this study provides guidelines for the design of ORR catalysts with enhanced ORR activities to achieve a high performance PEFC.

[1] T. Tamaki, H. Kuroki, S. Ogura, T. Fuchigami, Y. Kitamoto, and T. Yamaguchi, “Connected nanoparticle catalysts possessing a porous, hollow capsule structure as carbon-free electrocatalysts for oxygen reduction in polymer electrolyte fuel cells”, Energy Environ. Sci., 2015, 8, 3545-3549.





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DOI： 10.1149/ma2016-02/38/2383

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DOI： 10.1021/acs.chemmater.6b02230

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DOI： 10.1039/c6ra20784g

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DOI： 10.1021/acsami.5b03137

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DOI： 10.1021/acsami.5b00532

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DOI： 10.1063/1.4917725

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DOI： 10.1016/j.jpowsour.2014.08.005

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DOI： 10.1021/ac5021485

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Polymer electrolyte membrane fuel cells (PEFCs) have attracted considerable attention due to their high efficiency and clean electric generating system. Especially, in terms of the catalytic activity and water management, there is a great demand for operation under high temperature and low relative humidity. Under such situation, we recently succeeded in the development of heat–resistant electrolyte membrane (PI-CLSPES) with high ion exchange capacity (IEC) from porous polyimide (PI) substrates and highly sulfonated polyethersulfones (SPES150: IEC = 4.9meq/g) by using both polymer-filling and thermal cross–linking methods. The PI substrate can be prepared by using the viscolastic phase separation phenomenon: multi-stage solvent replacement of polyammic acid (PAA) cast film into N-methyl-2-pyrrolidone (NMP) and methanol, followed by thermal treatment. We found that the changes in viscosity of PAA solution, thickness of PAA cast film and immersion time afford the PI substrates with various pore sizes. On the other hand, the SPES150 membrane with high IEC is obtained by further sulphonation of sulfonated poly(arylene ether sulfone) (SPES50) in concentrated sulfuric acid, and allowed for impregnation into the pores of the porous PI substrate. The SPES150 inside the PI pores can be cross-linked between the sulfonic acid and phenyl groups by appropriate thermal treatment, and consequently, lead to the high thermal stability and suppression of membrane swelling. Moreover, the pore-filling and cross-linked membranes still retain the high IEC, and displayed the superior proton conductivity under high temperature comparable to that of Nafion. The detailed proton conducting behavior of the polymer electrolyte membranes as well as their structural analysis will be presented.

DOI： 10.1149/ma2014-02/21/1295

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Publishing type：Research paper (scientific journal)   Publisher：The Electrochemical Society  

The polymer electrolyte fuel cells (PEFCs) are excellent candidates as next-generation electric generators for vehicles, residential uses, laptop, etc. However, there are many issues to be solved to enlarge commercially uses. One of issues is related to Pt catalysts for oxygen reduction reaction (ORR) on a cathode electrode. Pt catalysts show low ORR activity and low durability, leading to lower performance in PEFCs. Our earlier studies found the chemically ordered face centered tetragonal (fct) intermetallic Pt-alloy catalysts (L1₀ type super-lattice structure) can satisfy both of higher ORR activity and long-term durability, whereas many of the previously-reported Pt-alloys cannot. It is considered that the high performance of fct-Pt alloy catalysts would result from the chemically-ordered structures, but it is unclear how they work to enhance ORR activity and durability. Thus, in this presentation, we will discuss the relationship between the ORR activities and surface structures in fct-Pt alloys by the detailed characterization of the catalyst’s surface structures.

The fct-PtFeNi catalysts with different atomic compositions of Fe and Ni (Pt₅₀Fe₃₅Ni₁₅, Pt₅₀Fe₂₅Ni₂₅, Pt₅₀Fe₁₅Ni₃₅) were prepared using a simple solid-state impregnation method; the mixture of metal (Pt, Fe, Ni)-salts and carbon blacks were annealed under H₂/N₂ at a temperature of 800^°C. The prepared fct-PtFeNi catalysts exhibit about 3 times higher ORR mass activity, compared with the commercial Pt catalyst. Especially, the mass activity of Pt₅₀Fe₃₅Ni₁₅ is about 0.62 A/mg_Pt; it is higher than that of fct-Pt₅₀Fe₂₅Ni₂₅ and fct-Pt₅₀Fe₁₅Ni₃₅ (about 0.5 A/mg_Pt). To discuss more about the enhancement of the ORR activities in the fct-PtFeNi alloys, the surface structures of the catalysts were examined by in-situ XAFS measurements. The EXAFS analysis of Pt-L₃ edge reveals that the distance of Pt-Pt bonds in the fct-PtFeNi catalysts is about 2.72 nm, which is shorter than that of commercial Pt catalyst. Interestingly, the coordination number of Pt oxides (Pt-O or Pt-OH) in the fct-Pt₅₀Fe₃₅Ni₁₅ is more increased at high potential of 0.8~1.2V, compared with other fct-PtFeNi catalysts with the lower content of Fe. The tendency of the formation of Pt oxides is in accord with that of the ORR activities in the fct-PtFeNi with the different compositions of Fe and Ni. Therefore, it is inferred that fct-PtFeNi with higher content of Fe would form more Pt oxides, leading to a faster reaction rate of oxygen reduction. This study brought out new insights; the surface structures of chemically-ordered fct-Pt alloys, such as the Pt-Pt bond distance and the formation of Pt oxides, strongly affect the ORR activities, as those of the previously-reported disordered Pt-alloys do as well. It is believed that our findings can aid in developing fct-Pt alloys with more enhanced catalytic activity and durability for PEFCs.

DOI： 10.1149/ma2014-02/21/1036

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DOI： 10.1039/c4ra05628k

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DOI： 10.1016/j.memsci.2013.08.001

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DOI： 10.1016/j.jbiosc.2012.12.019

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DOI： 10.1021/jp405088s

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DOI： 10.1246/cl.130794

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DOI： 10.1039/c3ra40920a

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DOI： 10.1016/j.jpowsour.2012.12.064

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DOI： 10.1021/ma3018603

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DOI： 10.1016/j.elecom.2012.09.003

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DOI： 10.1021/jp206552k

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DOI： 10.2494/photopolymer.25.555

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Publishing type：Research paper (scientific journal)   Publisher：The Membrane Society of Japan  

DOI： 10.5360/membrane.37.288

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DOI： 10.1021/ac202629h

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DOI： 10.1021/jp2068717

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DOI： 10.1002/fuce.201000141

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DOI： 10.1016/j.memsci.2011.02.044

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DOI： 10.1021/jp107767d

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DOI： 10.1021/ie101299q

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DOI： 10.1246/cl.2010.736

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DOI： 10.2494/photopolymer.23.571

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DOI： 10.11491/scej.2010f.0.308.0

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DOI： 10.11491/scej.2010f.0.676.0

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DOI： 10.1021/jp810575u

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DOI： 10.11491/scej.2008.0.354.0

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DOI： 10.11491/scej.2008.0.639.0

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DOI： 10.1541/ieejfms.128.559

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DOI： 10.2494/photopolymer.20.239

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DOI： 10.1295/kobunshi.56.84

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DOI： 10.1016/j.ijhydene.2018.01.205

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