PEFC Cathode Research Articles

Relationship between synthesis conditions and ORR activity of PtCo nanowire/C catalysts for PEFC cathodes

Nanowire (NW) structures have attracted attention as durable catalysts for the oxygen reduction reaction (ORR) due to their resistance to aggregation, unique morphology, and efficient electron conduction. However, NWs often suffer from limited structural stability, and there is a lack of research on their performance in membrane electrode assembly (MEA) studies. In this study, PtCo NW/C catalysts with various amounts of oleylamine (OAm) were prepared, and their oxygen reduction reaction (ORR) activity and durability were investigated by MEA and RDE testing conditions. XRD analysis and TEM observations showed that the prepared PtCo NWs were successfully alloyed,and PtCo NW alloys were observed. CV and LSV results showed that the PtCo NW/C OAm100 catalyst with 2 h of acid treatment had the best ORR activity. The I-V test results demonstrated that the PtCo NW/C OAm100 catalyst with 2 h of acid treatment had higher cell voltage, and cell performance was also excellent. It was found that the ORR activity of PtCo NW was improved by adding the appropriate amount of OAm.

The Formation Mechanism of Ordered Mesoporous Carbon with Network-Structure: A Novel Support Material for Pt-Based Catalysts in PEFC Cathodes

INTRODUCTIONTo achieve carbon neutrality in the upcoming sustainable society, it is crucial to utilize hydrogen as a fuel in the transportation sector. Fuel cell vehicles are expected to play an important role in the future hydrogen society, but there are still challenges to be overcome before their widespread adoption. For example, Pt/C-based catalysts used in PEFC cathodes have an issue with poisoning of the Pt due to direct contact with the ionomer, especially during low-load operation. Recently, a new approach using accessible-pore carbons has been proposed, where Pt is placed at an appropriate position within the nanopores of a carbon support to suppress ionomer poisoning of Pt [1]. In order to realize this concept, it is essential to design a mesoporous carbon support that possesses nanopores with appropriate size and proper ordering. We have developed an ordered mesoporous carbon with a hierarchical network structure (Net-OMC) consisting of small OMC particles of several tens of nm in size, which are connected to form a network structure with well-developed primary pores. The Net-OMC exhibited highly ordered nanopores on the surface of the OMC primary particles, with a pore size ca. 5 nm and an interpore distance of ca. 8 nm. Pt nanoparticles were loaded selectively within the Net-OMC nanopores by the selective Pt-deposition technique developed by our research group. Pt/Net-OMC catalyst exhibited higher mass activity and durability for the ORR in comparison with those for a commercial Pt/CB catalyst [2]. In this study, the formation mechanism of Net-OMC and the relationship between the catalytic properties of Pt/Net-OMC and its pore structure was investigated in detail.EXPERIMENTALNet-OMC powder was synthesized with resol-nonionic surfactant micelles as a carbon source and structure-directing agent. Resol-F127 (nonionic surfactant) micelles were synthesized by mixing phenol, formaldehyde, water and F127 with sodium hydroxide. After hydrothermal treatment of the mixture, the resulting solid was filtered and washed thoroughly and dried under vacuum. The obtained powder was carbonized at 700 ºC followed by annealing at 1000 ºC. Pt was deposited on the Net-OMC powder by selective deposition techniques. Cyclic voltammetry and linear sweep voltammetry were carried out in N2-saturated and O2-saturated 0.1 M HClO4 solution at 25 ºC using the conventional rotating disk electrode (RDE) technique. Potential step cycling measurements simulating load cycling between 0.6 V and 1.0 V vs. RHE were carried out according to the FCCJ protocol. Coating of the catalyst on a glassy carbon disk substrate was carried out by an electrospray (ES) coating technique developed by our group [3]. N2 adsorption measurements at liquid nitrogen temperature were carried out. The morphology of the Pt/ns-OMC was observed by scanning transmission electron microscopy (STEM).RESULTS AND DISCUSSIONSTEM images for the ordered mesoporous carbon particles with network structure (Net-OMC) with varying concentration during hydrothermal synthesis are shown in Figure 1. In Fig. 1(a), the network structure formed by connection of OMC primary nanoparticles with developed macropores is clearly observed. The OMC nanoparticles were tightly connected via a thick necking structure. From the high magnification image (Fig. 1(a), inset), highly ordered nanopores were observed on the surface of the OMC particles. The OMC primary particle size and the connecting neck diameter changed with the resol micelle concentration during the hydrothermal treatment. When the concentration of the micelle solution was decreased, both the primary particle size and necking length decreased. At a concentration of 1.8 mmol l-1, the formation of monodisperse particles with roughly the same size from the resol-F127 micelle particles was observed, as estimated by dynamic light scattering analysis. Thus, it is believed that the Net-OMC structure is formed through the self-assembly of micelle particles during the hydrothermal synthesis. The controllable network structure of Net-OMC is expected to provide a favorable porous structure of the catalyst layer for the MEA measurement even during high-load operation.

Improvement of ORR Activity of Monoclinic Zirconium Oxides by Fe and F Co-addition for PEFC Cathodes

Improvement of ORR Activity of Monoclinic Zirconium Oxides by Fe and F Co-addition for PEFC Cathodes

Degradation Analysis of Pt/Nb–Ti<sub>4</sub>O<sub>7</sub> as PEFC Cathode Catalysts with Controlled Arc Plasma-deposited Platinum Content

Degradation Analysis of Pt/Nb–Ti<sub>4</sub>O<sub>7</sub> as PEFC Cathode Catalysts with Controlled Arc Plasma-deposited Platinum Content

Slot-die-coating operability windows for polymer electrolyte membrane fuel cell cathode catalyst layers

Roll-to-roll (R2R) slot-die coating of polymer electrolyte membrane fuel cell (PEMFC) catalyst layers represents a scalable deposition method for producing 10–20 m2·min−1 of catalyst-coated gas diffusion layers (GDLs). This high-throughput production technique will help lower the cost of PEMFC catalyst layers. The uniformity of the wet layer applied by slot die deposition is affected by process parameters such as substrate speed, vacuum pressure applied at the upstream meniscus, gap between the slot die lips and substrate, ink rheology, and other ink and substrate properties. The set of conditions for producing a defect-free coating with a dilute ink typically requires little to no upstream vacuum pressure, so suitable operating conditions can be found easily through trial and error and operator intuition. However, the higher viscosity of more concentrated inks dramatically shifts the range of settings that result in a homogeneous coating to higher vacuum levels, which are harder to find through hit or miss. A predictive model showing the range of operable conditions decreases material wastage inherent in experimentally searching for suitable parameters. In this study, the defect-free coating parameter window is explored experimentally and theoretically for two concentrations of PEFC cathode inks. Both a full capillary hydrodynamic model and a computationally cheaper viscocapillary model successfully predict the experimentally determined coating window within the experimental and model uncertainty limits for inks with 5.3 wt% and 12.0 wt% solids ink while maintaining the 0.1 mgPt·cm−2Pt areal loading target. This paper demonstrates a viable pathway for meeting the $30/kWnet ultimate cost target of the United States Department of Energy (U.S. DOE) Hydrogen Fuel Cells Technologies Office (HFTO). The concentrated ink lowers the thermal energy and capital expenditure (CapEx) budget of the coating process by decreasing the amount of time, energy, and floorspace required for drying the coating.

Impact of Nickel Ions on the Oxygen Reduction Reaction Kinetics of Pt and on Oxygen Diffusion through Ionomer Thin Films

The effects of dissolved nickel on the oxygen reduction reaction (ORR) kinetics and oxygen transport properties of perfluorosulfonic acid (PFSA) thin films were investigated using rotating disk electrode (RDE) measurements of ORR on a PFSA-coated platinum electrode. The electrochemical characterization in 0.1 M perchloric acid (HClO4) with and without added Ni2+ quantitatively measured the impact of ionic interactions between the Ni2+ cations and sulfonate (SO3 −) anions on oxygen transport through the PFSA thin film. Cyclic voltammetry (CV) curves in deaerated electrolyte showed that Ni2+ cations diffusing through the PFSA thin film interact with the Pt surface altering the hydrogen underpotential deposition and stripping processes and decreasing ORR kinetics. The RDE limiting current results point to reduced permeability of oxygen through PFSA-Ni2+ compared to PFSA-H+. The results indicate that transition metals leached from Pt alloy catalyst may be detrimental not only to the intrinsic ORR kinetics of the PEFC cathode catalyst through loss of the ORR-enhancing transition metal, but may also inhibit the diffusion of oxygen to the catalytic sites and poison the ORR.

Nanostructuring Pt Electrocatalysts Supported on Nanocrystalline SnO2 for Polymer Electrolyte Fuel Cells

Introduction Conventional Pt/C electrocatalysts for PEFCs have a difficulty in durability against start-stop potential cycling, mainly due to degradation of carbon black through electrochemical oxidation especially on the cathode side. SnO2 is known to be stable at a high potential under the PEFC cathode conditions, so that SnO2 is considered as an alternative support material for PEFC cathodes [1-4]. In particular, we have prepared the oxide-core Pt-shell electrocatalyst that can reduce materials cost of PEFCs while developing electrocatalysts with both higher durability and ORR mass activity [3]. In our previous study [5], freeze drying has been successfully applied as an effective procedure for highly dispersing SnO2 nanoparticles on relatively-stable conductive framework, for nanostructuring PEFC electrocatalysts as shown in Fig. 1. Here, the objective of this study is to directly deposit Pt on SnO2 on an atomic level by applying a chemical procedure towards the oxide-core Pt-shell electrocatalyst preparation. Experimental Graphitized carbon black (GCB200, Cabot) was used as the conductive filler of the electrocatalysts acting as the conductive framework. SnO2 was prepared on the conductive filler via the microwave homogeneous precipitation. In order to deposit Pt on the surface of the SnO2, Pt(acac)2 and acetone were added to the Sn-containing solution. Pt/SnO2/GCB was prepared via the freeze drying, and heat treatment was performed to obtain a Pt-based catalyst powder. Nanostructure of the electrocatalysts obtained was observed by FE-SEM and STEM. ICP analysis was also made to quantify Pt loading. Electrochemical characterization of these electrocatalysts was carried out by half-cell measurements. Electrochemical surface area (ESCA) was measured by cyclic voltammetry (CV). Oxygen reduction reaction (ORR) activity was derived from kinetically controlled current density (ik) after the rotating disk electrode (RDE) measurement. Results and discussion We have succeeded to prepare SnO2 nanoparticles with a diameter of about 3 nm on the conductive filler in higher dispersion by applying the microwave homogeneous precipitation and the freeze drying. In addition, it was revealed that Pt nanoparticles were selectively deposited on nanocrystalline SnO2. Furthermore, high-resolution STEM observation revealed that a part of Pt was deposited near an atomic level on the SnO2 surface which could be a Pt-shell of the electrocatalysts, as shown in Fig. 2. Especially in Fig. 2(b), Pt atoms could be distinguished on the nanocrystalline SnO2 particles acting as a core of the electrocatalyst, with a minimized electron-conductive pathway between the Pt catalyst and the GCB conductive framework. Electrochemical properties of such nanostructured electrocatalysts will be reported and discussed. References Y. Nakazato, D. Kawachino, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi and K. Sasaki, J. Electrochem. Soc., 165 (14), F1154 (2018).S. Matsumoto, M. Nagamine, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 165 (14). F1165 (2018).M. Nagamine, Z. Noda, H. Manabe, J. Matsuda, A. Hayashi, and K. Sasaki, ECS Trans., 86 (13), 531 (2018).K. Kakinuma, R. Kobayashi, A. Iiyama, M. Uchida, J. Electrochem. Soc., 165 (15), J3083 (2018).T. Yoshizumi, M. Nagamine, Z. Noda, J. Matsuda, A. Hayashi, and K. Sasaki, ECS Trans., 92 (8), 479 (2019). Figure 1

P-Doped SnO2 Powder as a Support for PEFC Cathode

The possibility of commercial P-doped SnO2 was evaluated as a non-carbon support for PEFC cathodes. The effect of heat-treatment under various conditions on the physical properties of the P-doped SnO2 was investigated. The P-doped SnO2 was sensitive by the heat-treatment in reductive atmosphere, 4%H2/Ar. The surface area was hardly affected by the heat-treatment in air or Ar even at high temperature such as 900℃. On the other hand, the conductivity was dramatically affected by the atmosphere during the heat-treatment. The heat-treatment in Ar at higher temperature rather increased in the conductivity, whereas the heat-treatment in air caused the drastic decrease in the conductivity. This difference might be responsible for the formation and extinction of the oxygen vacancies.

Direct preparation of core-shell platinum cathode in membrane electrode assembly catalyst layer for polymer electrolyte fuel cell

Core–shell catalyst has been attracting attention as a low-Pt catalyst for PEFC cathode. However, its mass production method has not yet been established. In this paper, a novel method suitable for continuous production of low-platinum catalyst layer for PEFC is proposed. Catalyst layer of carbon-supported Pd@Pt core–shell catalyst (Pd@Pt/C) is fabricated by using Cu underpotential deposition (UPD) followed by surface-limited redox replacement (SLRR) directly to the porous catalyst layer made of Pd core (Pd/C). The distribution of Pt corresponded well with that of Pd throughout the catalyst layer, indicating that the core–shell reaction occurs in the entire catalyst layer. Pd@Pt/C shows 1.8 times higher mass activity than Pt/C, which is comparable to Pd@Pt/C prepared by conventional microgram-scale method.

Electrocatalysts Supported on Nanocrystalline SnO2 for Polymer Electrolyte Fuel Cells

Introduction Since conventional PEFC electrocatalysts have a difficulty in durability against start-stop and load potential cycling, development of high-performance and high-durability cells for next-generation fuel cell vehicles is desired. SnO2 is known to be stable at a high potential under the PEFC cathode conditions, so that SnO2 is considered as an alternative support material for PEFC cathodes [1-4]. Here in this study, electrocatalysts are prepared using highly graphitized carbon black (GCB), which is a carbon material with an increased degree of graphitization, acting as a conductive framework in the electrocatalyst layers. Nanocrystalline SnO2 particles of several nm in diameter were deposited on the GCB. The use of nanocrystalline SnO2 enables to shorten the electron pathway between the catalyst and the GCB-based conductive framework, down to a few nm. Platinum catalysts were then deposited on SnO2 nanoparticles. The ultimate objective of this study is to develop high-performance cells with high durability by optimizing preparation conditions for nanostructuring. Drying procedure is mainly examined in this study. Experimental Nanocrystalline SnO2 was deposited on GCB (GCD200, Cabot) via the evaporation-to-dryness and the freeze drying. Pt deposition on SnO2 was made using platinum acetylacetonate in order to prepare Pt/SnO2/GCB electrocatalysts. Nanostructure of the electrocatalysts obtained was observed by field-emission scanning electron microscopy (FE-SEM) and scanning transmission electron microscopy (STEM). Thermogravimetry (TG) measurement was applied to quantify SnO2 loading. ICP analysis was made to quantify Pt loading. Average crystallite size of SnO2 and Pt in Pt/SnO2/GCB was measured by XRD. Electrochemical characterization of these electrocatalysts was carried out by half-cell measurements. Electrochemical surface area (ECSA) was measured by cyclic voltammetry (CV), and oxygen reduction reaction (ORR) activity was derived from kinetically controlled current density (ik ) after the rotating disk electrode (RDE) measurements. Results and discussion Figures 1 and 2 show FESEM images of Pt/SnO2/GCB prepared by using the evaporation-to-dryness and the freeze drying, respectively. Figure 1 reveals aggregation of platinum particles, and inhomogeneous distribution was confirmed for platinum particles. On the other hand, as shown in Fig. 2, the platinum particles were homogeneously supported in higher dispersion. The catalytic activity of these electrocatalysts was evaluated by the half-cell measurements. Pt/SnO2/GCB prepared by the evaporation-to-dryness process exhibited ECSA of 68.4 m2g-1, and mass activity of 82.9 Ag-1 at 0.9 VRHE. The electrocatalyst prepared by the freeze drying process exhibited larger ECSA of 80.8 m2g-1, and higher mass activity of 152.5 Ag-1 at 0.9 VRHE. From the results of microstructural observation and the half-cell measurements, the freeze drying process, which enables uniform dispersion of oxide nanoparticles, is suitable for the preparation of the nanocrystalline oxide support, leading to higher electrochemical properties. Acknowledgment Financial support by the Center-of-Innovation program, JST Japan, is gratefully acknowledged.

Dependence of Proton Conductivity on Cathode Degradation in PEFC

I ntroduction Since the commercialization of FCVs, PEFCs are paid more attention to, and for further improvement of PEFC efficiency, raising the operating temperature is considered as one of the possible methods. Increase in the temperature above 100℃ leads to decrease in relative humidity, resulting in lower proton conductivity. Based on this background, we have been working on durability analyses of PEFC cathode under the low proton conductivity condition. Our study has suggested that the proton conductivity of ionomers in the cathode layer more dominantly influences on the degradation of PEFC cathode than that of electrolyte membranes [1, 2]. Therefore, in this study, dependence of the proton conductivity of ionomers on PEFC cathode degradation has been investigated by controlling the proton conductivity of ionomers. Experimental Three kinds of ionomers with different proton conductivity, AquivionD72>AquivionD83>Nafion, were used in this study. MEAs were made by spraying the slurry containing 46%Pt/KB (TEC10E50E) and each ionomer on the Nafion membrane (Nafion 212). Resulting MEAs were named as AqD72 MEA, AqD83 MEA, and Naf MEA, and evaluated under the high humidity condition (80℃-RH100%) and the low humidity condition (105℃-RH57%), where difference in proton conductivity influences less and more on the fuel cell reaction, respectively. Here, for durability analyses, degradation of carbon derived by the reaction with H2O in the cathode was focused on, and potential cycling between 1.0 and 1.5 V was performed as an accelerated degradation test. Then, changes in IV performance and each overvoltage were evaluated before and after the durability test. Changes in the cathode pore structure were also analyzed by FIB-SEM. Results and discussion Pore volume distribution before and after durability test was analyzed separately for the GDL side and the membrane side. Here, for each MEA, three different parts of the cathode were evaluated by FIB-SEM, and average pore volume with S.D. was analyzed based on the cathode image reconstructed from about 100 FIB-SEM pictures. Under low humidity condition, Naf MEA showed more localized increase in pore volume derived by carbon oxidation degradation at the membrane side than AqD72 MEA and AqD83 MEA, as shown in Figure 1. The reason is probably because only H2O generated by fuel cell reaction can contribute to carbon oxidation reaction in the low humidity condition, and such H2O is inhomogeneously produced only near the membrane side since Nafion has low proton conductivity (0.026 S/cm) comparing to AquivionD72 (0.090 S/cm) and AquivionD83 (0.080 S/cm). Unlike the low humidity condition, the pore volume in the cathode similarly increased on degradation among all MEAs under the high humidity condition. This is most likely because ionomers have enough high proton conductivity (over 0.060 S/cm) in all MEAs under high humidity. Consequently, dependence of proton conductivity on carbon oxidation degradation in the low humidity condition was observed, and controlling the proton conductivity of ionomers was found to be important especially under low humidity.

Electrocatalysts Supported on Nanocrystalline SnO2 for Polymer Electrolyte Fuel Cells

Conventional PEFC electrocatalysts have a difficulty in durability against start-stop potential cycling, mainly due to degradation of carbon black, supporting Pt nanoparticles, through electrochemical oxidation especially on the cathode side. SnO2 support has high electronic conductivity among various oxide supports, and is stable under the strongly acidic PEFC cathode condition. In this study, freeze drying has been successfully applied as an effective procedure for highly dispersing SnO2 nanoparticles on conductive framework, for nanostructuring PEFC electrocatalysts.

Green Hydrogen and Polymer Electrolyte Fuel Cells for Our Future Sustainable Growth

Considering our future sustainable growth, renewable energies should be the primary energy. In that way we need technologies to store and transport renewable energies. The Green Hydrogen is the hydrogen from water using renewable energies. The environmental impact factor(EIF) was defined as the ratio of the quantity of materials produced by energy consumption of mankind to the natural movement on the earth. Fig.1 shows the relation between the impact factor and the energy consumption density. By the comparison of environmental impact factor, the Green Hydrogen could keep our environment more than 2 orders of magnitude cleaner compared to that of fossil fuels (1). With using Green Hydrogen, we could produce clean electricity through fuel cells without any pollutants. The Green Hydrogen Energy System might be the final energy system for the sustainable growth of human beings.Polymer electrolyte fuel cells are expected for the residential and transportable applications, due to their high power density and low operating temperature. Low temperature fuel cells have theoretically higher efficiency compared to higher temperature fuel cells such as SOFCs. Polymer electrolyte fuel cells are expected for the residential and transportable applications, due to their high power density and low operating temperature. Many ENEFARMs (micro CHP) are operating and fuel cell vehicles are also commercially available in Japan. However, the estimated amount of Pt reserve is limited and its cost is high. The instability of Pt cathode and carbon might be the big problems to improve the stability of the present PEFC system. A stable non-precious metal oxide cathode with stable metal oxide support might be the final goal for the cathode of PEFC for fuel cell vehicles. In the future energy system fuel cells should be operated at higher efficiency such as 60 %(HHV) since their theoretical efficiency is very high. To get this high efficiency, fuel cells should be operated at 0.9 V or higher. To get this high operation voltage, their operation temperature might be higher than 120 oC for future PEFCs. At these high potential and temperature Pt and carbon are no more stable. We need new materials, such as metal oxides that are stable in acid and oxygen atmosphere.We have reported that partially oxidized group 4 and 5 metal carbonitrides and organometallic complexes are stable in an acid solution and have definite catalytic activity for the oxygen reduction reaction (ORR) (2,3). In this paper we will report our recent advancement of the group 4 and 5 metal oxide catalyst with a metal oxide support without carbon.The ORR activity of the TixNbyOz + Ti4O7 is higher than that of the Ti4O7, indicating that the TixNbyOz might have active sites for the ORR. The highest onset potential of the TixNbyOz +Ti4O7 was over 1.1 V vs. RHE (2). No degradation of the ORR performance of TixNbyOz + Ti4O7 was observed during both start-stop and load cycle tests. The ORR activity depended on the heat treatment temperature. Especially we got the highest ORR activity in reducing atmosphere at 800oC. Ti3+ on the surface might affect the ORR reaction. The ORR activity of ZrOx catalyst that was made by the arc-plasma deposition depended on the thickness of the catalysis layer. The maximum ORR current was obtained at the thickness of 2 nm. The tunneling current might help the electron conduction for ORR.Considering these factors we are going to improve the ORR activity of group 4 and 5 metal oxide cathodes with oxide support. The authors wish to thank to the New Energy and Industrial Technology Development Organization (NEDO) for their financial support. REFERENCES1) K. Ota, et al., Advances in Hydrogen Production, Storage and Distribution, p.32, Woodhead publishing, UK (2014)2) K. Ota, et al., Polymer Electrolyte Fuel Cells 17, (ECS Trans., 80(8), 717-714 (2017).3) K. Ota, et al., Polymer Electrolyte Fuel Cells and Electrolyzers 18, (ECS Trans. 86(13)), 549-558 (2018) Figure 1

静電スプレー（ES）法によるPEFC 用低白金担持 Pt/Ta-SnO 2 カソードのセル性能の改善

静電スプレー（ES）法によるPEFC 用低白金担持 Pt/Ta-SnO 2 カソードのセル性能の改善

Oxygen Reduction Activity of TiO2 Single Crystals in Acidic Media

Low temperature fuel cells have theoretically higher efficiency compared to higher temperature fuel cells such as SOFCs. Polymer electrolyte fuel cells are expected for the residential and transportable applications, due to their high power density and low operating temperature. Many ENEFARMs (micro CHP) are operating and fuel cell vehicles are also commercially available in Japan. However, the estimated amount of Pt reserve is limited and its cost is high. The instability of Pt cathode and carbon might be the big problems to improve the stability of the present PEFC system. A stable non-precious metal oxide cathode with stable metal oxide support might be the final goal for the cathode of PEFC for fuel cell vehicles. In the future energy system fuel cells should be operated at higher efficiency such as 60 %(HHV) since their theoretical efficiency is very high. To get this high efficiency, fuel cells should be operated at 0.9 V or higher. To get this high operation voltage, their operation temperature might be higher than 120 oC for future PEFCs. At these high potential and temperature Pt and carbon are no more stable. We need new materials, such as metal oxides that are stable in acid and oxygen atmosphere.We have reported that partially oxidized group 4 and 5 metal carbonitrides and organometallic complexes are stable in an acid solution and have definite catalytic activity for the oxygen reduction reaction (ORR) (1-8). In this paper we will report our recent advancement of the group 4 and 5 metal oxide catalyst with metal oxide support without carbon.All electrochemical measurements were performed in 0.1 mol dm-3 H2SO4 at 30 oC with a 3-electrode cell. Chronoamperometry (CA) was performed from 0.2 to 1.2 V vs. RHE under O2 atmosphere to obtain ORR current. The ORR current density was normalized by the electric charge of the double layer capacitance under N2 atmosphereThe ORR activity of the TixNbyOz + Ti4O7 is higher than that of the Ti4O7, indicating that the TixNbyOz might have active sites for the ORR. The highest onset potential of the TixNbyOz +Ti4O7 was over 1.1 V vs. RHE. No degradation of the ORR performance of TixNbyOz + Ti4O7 was observed during both start-stop and load cycle tests. Therefore, the group 4 and 5 oxide base cathodes had superior ORR activity and durability compared to Pt cathode under the cathode conditions of PEFCs.There are some difference in ORR activities of TiOx which treated in reductive atmosphere and oxidative atmosphere at high temperature. Ti3+ on the surface may be the active site for the ORR reaction.TiOx which was treated in reducing atmosphere at high temperature showed mostly the 4 electron reaction for ORR. The reaction rate of 4 electrons was 10 to 30 times faster than that of 2 electrons and 5 times faster than that of Pt.The ORR activity of ZrOx catalyst that was made by the arc-plasma deposition depended on the thickness of the catalysis layer. The maximum ORR current was obtained at 2 nm. The tunneling current might help the electron conduction for ORR.Considering these factors we are going to improve the ORR activity of group 4 and 5 metal oxide cathodes with oxide support. The authors wish to thank to the New Energy and Industrial Technology Development Organization (NEDO) for their financial support. REFERENCES1) A. Ishihara, Y. Shibata, S. Mitsushima, K. Ota, Journal of The Electrochemical Society, 155, B400-B406 (2008).2) A. Ishihara, M. Tamura, Y. Ohgi, M. Matsumoto, K. Matsuzawa, S. Mitsushima, H. Imai, K. Ota, Journal of Physical Chemistry, ser. C, 117, 18837-18844 (2013). 3) A. Ishihara, M. Chisaka, Y. Ohgi, K. Matsuzawa, S. Mitsushima, K. Ota, Physical Chemistry Chemical Physics, 17, 7643-7647 (2015).4) N Uehara, A. Ishihara, M Matsumoto, H. Imai, Y. Kohno, K. Matsuzawa, S. Mitsushima, K. Ota, Electrochimica Acta, 182, 789-794 (2015).5) A. Ishihara, M. Hamazaki, M. Arao, M. Matsumoto, H. Imai, Y. Kohno, K. Matsuzawa, S. Mitsushima, and K. Ota, Journal of The Electrochemical Society, 163 (7) F603-F609 (2016)6) K. Ota, Y. Tamura, T. Nagai, K. Matsuzawa, S. Mitsushima, A. Ishihara, ECS Transactions, 75(14), 875-883 ( 2016)7) M. Chisaka,, A. Ishihara, H. Morioka, T. Nagai, S. Yin, Y. Ohgi, K. Matsuzawa, S. Mitsushima, K. Ota、ACS Omega, 2, 678-684 (2017)8) K.Ota, T.Nagai, Y.Tamura, M.Arao, M.Matsumoto, K.Matsuzawa, H.Imai, T.Napporn, S.Mitsushima, A. Ishihara, ECS Transactions, 80(8), 717-714 (2017).

Oxide-Core Pt-Shell Electrocatalysts for PEFCs: Photochemical Preparation Using SnO2 Nanoparticles

Core-shell electrocatalyst, in which a Pt atomic layer covers a nano-size core of a different metal, is attractive in reducing Pt usage for PEFC cathode electrocatalyst. Whilst such core-shell electrocatalysts exhibit very high mass activity for oxygen reduction reaction (ORR), their cost and durability still remain as a challenge. If inexpensive and thermochemically-stable conductive oxide could be applied to the core instead of a noble metal, cost reduction and durability improvement of such core-shell electrocatalysts may be expected. In this study, we try to develop oxide-core Pt-shell electrocatalysts by directly depositing Pt on SnO2 at an atomic level via a photochemical preparation procedure. SnO2 cores with a diameter down to ca. 3 nm could be prepared by comparing various preparation procedures of SnO2 and types of carbon-based materials acting as conductive fillers. Electrochemical activities of such oxide-core Pt-shell electrocatalysts are characterized and discussed.

Potentiostatic and Potential Cycling Dissolution of Polycrystalline Platinum and Platinum Nano-Particle Fuel Cell Catalysts

The dissolution of Pt in aqueous electrolytes has been studied for over forty years, most recently in the context of understanding the observed loss in electrochemically-active surface area (ECA) of cathode electrocatalysts in polymer electrolyte fuel cells. Despite extensive research, there are many unresolved issues regarding the dissolution of nano-particle Pt, such as the source of the observed potential dependence of potentiostatic and potential cycling dissolution rates. To help resolve these issues, in this paper we present results of measurements of the concentration of dissolved Pt and Pt dissolution rates for carbon-supported platinum nano-particles (Pt/C) in dilute perchloric acid, as a mimic of the PEFC cathode environment, as a function of potential and upper potential limit of potential cycling. Also presented, for comparison, are results of similar studies on polycrystalline platinum. In situ Pt LIII X-ray absorption spectroscopy was used to determine the extent of oxidation, the coordination environment, and loss of Pt from the Pt nano-particles to elucidate the mechanism of Pt dissolution. Based on the correlation of these studies with those presented in the literature, mechanisms for Pt dissolution under potentiostatic and potential cycling conditions are proposed.

Factors Which Affect Oxygen Reduction Activity of Niobium Added Titanium Oxides as Non-Platinum Cathodes for Polymer Electrolyte Fuel Cells

Polymer electrolyte fuel cells are expected for the residential and transportable applications, due to their high power density and low operating temperature. Many ENEFARMs (micro CHP) are operating and fuel cell vehicles are also commercially available in Japan. However, the estimated amount of Pt reserve is limited and its cost is high. The dissolution of Pt cathode might be the final problem to be solved related to the stability in the present PEFC system. Additionally, the instability of carbon support is also a big problem especially for fuel cell vehicles. Carbon including graphite is thermochemically unstable at room temperature in air or oxygen containing atmosphere. A stable non-precious metal oxide cathode with stable metal oxide support might be the final goal for the cathode of PEFC for fuel cell vehicles. In the future energy system fuel cells should be operated at higher efficiency such as 60 %(HHV) since their theoretical efficiency is very high. To get this high efficiency, fuel cells should be operated at 0.9 V or higher. To get this high operation voltage, their operation temperature might be higher than 120 oC for PEFCs. At these high potential and temperature Pt and carbon are no more stable. We need new materials, such as metal oxides that is stable in acid and oxygen atmosphere. We have reported that partially oxidized group 4 and 5 metal carbonitrides and organometallic complexes are stable in an acid solution and have definite catalytic activity for the oxygen reduction reaction (ORR) (1-7). In this paper we will report our recent advancement of the group 4 and 5 metal oxide catalyst with metal oxide support without carbon. The precious-metal-free and carbon-free cathodes based on oxides (titanium-niobium oxides mixed with Ti4O7 (TixNbyOz + Ti4O7) showed the superior durability (5). The ORR activity of the TixNbyOz + Ti4O7 is higher than that of the Ti4O7, indicating that the TixNbyOz might have active sites for the ORR. The highest onset potential of the TixNbyOz +Ti4O7 was over 1.1 V with respect to reversible hydrogen electrode. No degradation of the ORR performance of TixNbyOz + Ti4O7 was observed during both start-stop and load cycle tests. Therefore, we successfully demonstrated that the precious-metal and carbon-free oxide-based cathodes had superior durability under the cathode conditions of PEFCs. All electrochemical measurements were performed in 0.1 mol dm-3 H2SO4 at 30 oC with a 3-electrode cell. Chronoamperometry (CA) was performed from 0.2 to 1.2 V vs. RHE under O2 atmosphere to obtain ORR current. The ORR current density was normalized by the electric charge of the double layer capacitance under N2 atmosphere In order to qualify the role of Nb oxide for ORR, we used TiO2-Nb (Nb; 0.5, 1 and 5atm%) rods as working electrodes. In reducing atmosphere treatment, TiO2-Nb (0.5atm%) rod had best ORR activity that was heat-treated at 800 oC. However TiO2-Nb(5%) showed the best result in air treatment at 800oC. The heat-treatment temperature and the atmosphere are important to get high ORR activity. In order to study the effect of crystal planes of TiO2, we used the single crystal of TiO2-Nb (0.5 atm.% Nb). The ORR activity depended on the crystal plane significantly. The activity might be affected by many factors. We are looking for the important factor for ORR. The authors wish to thank to the New Energy and Industrial Technology Development Organization (NEDO) for their financial support. REFERENCES 1) A. Ishihara, Y. Shibata, S. Mitsushima, K. Ota, Journal of The Electrochemical Society, 155, B400-B406 (2008). 2) A. Ishihara, M. Tamura, Y. Ohgi, M. Matsumoto, K. Matsuzawa, S. Mitsushima, H. Imai, K. Ota, Journal of Physical Chemistry, ser. C, 117, 18837-18844 (2013). 3) A. Ishihara, M. Chisaka, Y. Ohgi, K. Matsuzawa, S. Mitsushima, K. Ota, Physical Chemistry Chemical Physics, 17, 7643-7647 (2015). 4) N Uehara, A. Ishihara, M Matsumoto, H. Imai, Y. Kohno, K. Matsuzawa, S. Mitsushima, K. Ota, Electrochimica Acta, 182, 789-794 (2015). 5) A. Ishihara, M. Hamazaki, M. Arao, M. Matsumoto, H. Imai, Y. Kohno, K. Matsuzawa, S. Mitsushima, and K. Ota, Journal of The Electrochemical Society, 163 (7) F603-F609 (2016) 6) K. Ota, Y. Tamura, T. Nagai, K. Matsuzawa, S. Mitsushima, A. Ishihara, ECS Transactions, 75(14), 875-883 ( 2016) 7) M. Chisaka,, A. Ishihara, H. Morioka, T. Nagai, S. Yin, Y. Ohgi, K. Matsuzawa, S. Mitsushima, K. Ota、ACS Omega, 2, 678-684 (2017)

Electrochemical Characterization of MEAs with Different Pt-Loading for the Efficient Use of Pt

Introduction The reduction of Pt-loading particularly in PEFC cathode layers is essential for cutting the cost of PEFCs and further spreading PEFC technology. Nowadays, several approaches of reducing Pt-loading have been reported, and then the importance of optimizing the microstructure of cathode layers including carbon supports has been confirmed.1-2 Therefore, the objective of our research is to develop low Pt-loading PEFCs with high efficiency by optimizing the mount of both Pt and carbon. As a first step, MEAs with varied Pt-loading in cathode layers were prepared with a standard catalyst, TEC10E50E, and a common spray-printing method. Then, the minimum amount of Pt, which is necessary for cathode layers, was evaluated though the detailed electrochemical characterization. Experimental MEAs with varied Pt-loading in cathode layers, such as 0.05, 0.10, 0.15, 0.20, 0.30, and 0.40 mg/cm2, were prepared, while the amount of Pt in anode layers were kept to 0.30 mg/cm2. Then, their performances were evaluated by measuring the IV response, impedance, and Hupd electric charge. Also, the cross-sections of cathode layers were observed by FIB-SEM. Results and discussion From IR-corrected IV curves, activation overvoltage was separately analyzed. Activation overvoltage was clearly proportional to Pt-loading in the region of 0.05 - 0.15 (0.20) mg Pt/cm2, but did not change much in 0.15 (0.20) - 0.40 mg Pt/cm2. Therefore, further with ECSA analyses, the minimum amount of Pt needed for the cathode layer was determined to 0.15 - 0.20 mg Pt/cm2 in this experimental condition. Interestingly, concentration overvoltage increased with decrease in the amount of Pt. In this case, since the amount of carbon decreased, the thickness of cathode layers resulted in decreasing. Possible reasons for this phenomenon can be explained in Figure 1. Although the shorter pathway for fuel diffusion is expected to reduce concentration overvoltage, the blockage of the pathway by water produced during the fuel cell reaction should also be considered. If the same amount of water is produced in catalyst layers with the different thickness, the path is more easily blocked in the thinner layer, leading to higher concentration overvoltage. Therefore, optimization of the cathode layer thickness is also an important factor to design better PEFCs. Accordingly, we will report the approach to develop highly efficient MEAs using the minimum amount of Pt and also reducing concentration overvoltage through controlling the amount of carbon. References (1) M. Lee, M. Uchida, D. A. Tryk, H. Uchida, M. Watanabe, Electrochim, Acta, 56 4783 (2011). (2) K. Takahashi, K. Kakinuma, M. Uchida, J. Electrochem. Soc. 163.(10) F1182 (2016) Figure 1

Analysis of Oxygen Reduction Reaction Rate in a PEFC Utilizing an Ultra-Thin Sputter-Deposited Cathode

Introduction Optimizing a cathode catalyst layer (CL) of a PEFC is required for efficient use of Pt since Pt electrocatalysts account for a large percentage of the stack cost. The oxygen reduction reaction (ORR) rate is a function of the oxygen partial pressure and the cathode electromotive force (emf).[1, 2] The proton potential and the oxygen partial pressure are not constant throughout the cathode thickness due to the transport resistances. In modeling the PEFC cathode, formulation of the intrinsic ORR rate which is not affected by the transport resistances is required.[2] In our previous study, the proton transport resistance in the through-plane direction still remained under low humidity conditions even if the CL thickness was reduced to 3 μm.[3] In this study, Pt was sputter-deposited directly on a microporous layer (MPL) made of carbon black and PTFE, thus preparing an ultra-thin cathode without ionomer. For the purpose of the analysis of the intrinsic ORR rate, effectiveness of the ultra-thin sputter-deposited cathode was investigated. Experimental A two-layer MEA consisting of a Nafion membrane (DuPont NR-212) and an anode CL made of Pt/carbon black (Vulcan XC-72) catalysts covered with ionomer was prepared. The weight ratio of Pt/C was 0.5, the Pt loading was 0.30 mg/cm2, and the ionomer/carbon black was 0.74. As for a cathode, Pt was sputter-deposited on an MPL coating carbon paper using sputtering apparatus (AOV, SPAD-4240UM) having a 0.032 m3 deposition chamber. 20 W of 13.56 MHz radio frequency power was supplied to generate plasma for sputtering. The distance between the Pt target and the substrate was 130 mm and the diameter of the target was 50.8 mm. Pt was deposited at 5.0 Pa in a 20 cm3(STP)/min of Ar flow. The substrate temperature was 100 ˚C. The deposition time was 1, 2, 3, and 10 min for varying Pt amount, yielding the Pt loading of 0.0013–0.013 mg/cm2. This cathode was stacked with the two-layer MEA. The geometric active area was 50 mm × 50 mm. H2 and O2 were humidified in bubblers at 75 ˚C. H2 was supplied to the anode at a flow rate of 600 cm3/min, and O2 was supplied to the cathode at a flow rate of 300 cm3/min. The cell was operated at 80 °C and the pressure at the cell outlet was 1 atm. Results and Discussion Fig. 1 shows polarization curves measured using ultra-thin cathodes. Fig. 2 shows Tafel plots in cases of usual Pt/C CLs. The apparent Tafel slope was 0.036–0.049 V depending on the CL thickness of 4.0–24.2 μm. IR-corrected cell voltage was obtained using the membrane resistance measured as high frequency resistance. At a same current density, as the CL becomes thicker, the ohmic proton potential drop through the cathode becomes greater due to the proton transport resistance in the through-plane direction. As a result, the cathode emf at a CL–GDL boundary becomes higher than the membrane-IR-corrected cell voltage. The true cathode emf is the higher at the higher current density. That is why the apparent Tafel slope increases by increasing the CL thickness. The observed Tafel slopes of both types of cathodes are shown in Fig. 3. The observed Tafel slope in case of the sputter-deposited cathode was located on a trendline exhibited by the apparent Tafel slopes obtained using usual Pt/C cathode where the CL thickness equals zero. This indicates that the Tafel slope which was not affected by the transport resistances was obtained to be 0.317 V. It corresponds to a transfer coefficient of 0.240. Conclusions An ultra-thin cathode was prepared by sputter deposition of Pt directly on an MPL for the analysis purpose. The observed Tafel slope on this cathode was equal to that at the zero cathode CL thickness expected from a trendline of the apparent Tafel slopes measured with the usual Pt catalyst supported on carbon black. This indicates that the intrinsic ORR rate which was not affected by the transport resistances was obtained by using the proposed Pt-sputtered cathode.