Polymer Electrolyte Fuel Cell Operating Conditions Research Articles

(Digital Presentation) Operando x-Ray Fluorescence Spectroscopic Study of in-Plane Cerium-Ion Radical Quencher Distribution in Polymer Electrolyte Membranes

One of the challenges for the widespread use of polymer electrolyte fuel cells (PEFCs) is the suppression of chemical degradation in polymer electrolyte membrane (PEM). This degradation is caused by radical species generated due to the reaction of hydrogen peroxide with impurities in the PEM during PEFC operation. One way to suppress the chemical degradation is to add cerium ions to the PEM as radical quencher, 1) which absorbs the radicals through a redox reaction, thereby suppressing PEM degradation caused by radicals. However, it is known that cerium ions move in the in-plane direction due to the humidity and potential distribution, where the depletion of cerium ions causes PEM degradation.2) Therefore, it is required to elucidate the mechanism of cerium ion transport phenomena in the in-plane direction under PEFC operating conditions. High-energy X-ray fluorescence spectroscopy enables operando measurements of cerium distribution by taking advantage of penetration into the fuel cell inside.3) In this study, we have developed operando measurement cells that can observe the distribution phenomenon of cerium ions by using X-ray fluorescence spectroscopy while controlling the humidity gradient in the in-plane direction. Using the developed method, we analyze the cerium ion transport phenomena under the humidity gradient in the membrane electrode assembly (MEA). For the operando X-ray fluorescence measurements, end-plates with an X-ray transmission window and two gas inlets were used to control two different humidity conditions in the PEM. X-ray was incident at 90 degrees from the MEA plane, and a germanium solid-state detector was positioned perpendicular to the incident X-rays to acquire Ce-Ka fluorescence X-ray from the MEA. The catalyst coated membranes composed of a perfluorosulfonic acid-based cation exchange membrane with cerium-ion substitution, coated with a platinum catalyst layer were used. Operando X-ray fluorescence measurements were performed at BL37XU at SPring-8 (Japan). X-rays with incident energy ranging from 40 to 65 keV were used to scan the cell position and detect Ce-Ka fluorescence X-ray. The cell temperature was set at 60°C. Cerium ions in the MEA were found to migrate from the high humidity side to the low humidity side within a range of millimeters in approximately one hour. The migrated cerium ions tended to diffuse into homogeneous state within in a few hours under the uniform humidity condition. Acknowledgments The authors would like to thank M. Toida (Toyota Motor Corporation) for providing samples and for in-depth discussions. References 1) Endoh, ECS Trans., 16, 1229 (2008).2) Y-H. Lai, K. Rahmoeller, J. Hurst, R. Kukreja, M. Atwan, A. Maslyn and C. Gittleman, Electrochem. Soc., 165, F3217 (2018).3) Y. Orikasa, A. Takezawa, K. Amemiya, Y. Tsuji, T. Asaoka, M. Ohki, O. Sekizawa, and K. Nitta, ECS Trans., 109, 109 (2022).

Variation of Local Structure and Reactivity of Pt/C Catalyst for Accelerated Degradation Test of Polymer Electrolyte Fuel Cell Visualized by Operando 3D CT‐XAFS Imaging

AbstractIn practical applications of polymer electrolyte fuel cells (PEFC), the wide variations of local structures and environment of electrocatalysts bring about complex reaction behaviours inside a membrane electrode assembly (MEA). The variations of local coordination structure and redox response of a Pt/C cathode catalyst in an MEA before and after a typical accelerated degradation test (ADT) were three‐dimensionally visualized by operando Pt LIII‐edge computed‐tomography X‐ray absorption fine structure (CT‐XAFS) imaging under PEFC operating conditions for the first time. The set of operando CT‐XANES and CT‐EXAFS analyses visualized changes in the local coordination structure and redox reactivity of the Pt catalyst by a typical ADT process.

In-Situ Monitoring of Both Pt and Cu Dissolved from Pt-Cu Alloys Using Channel Flow Multi Electrode

Introduction Polymer electrolyte fuel cell (PEFC) is considered to be clean energy conversion device due to its low operating temperature and portability. Previously, Pt is used as cathode catalyst in PEFC, because Pt exhibits high catalytic activity for oxygen reduction reaction (ORR). Since Pt is limited resources and high cost, Pt alloy catalysts should be developed instead of Pt catalysts. Pt-Cu alloys exhibit greater catalytic activity for ORR than Pt, 1 however, Pt-Cu alloys would degrade under PEFC operating conditions. Therefore, we need to clarify dissolution mechanism of Pt-Cu alloys. In our previous work, we established channel flow double electrode (CFDE) for in-situ detection of Pt, and we proposed dissolution mechanism of Pt under simulating PEFC operating condition using CFDE. 2 In this study, we firstly improved CFDE to in-situ detect both Pt and Cu dissolved from Pr-Cu alloys, and then dissolution behavior of Pt-Cu alloys has been investigated by using this technique. Experimental In order to determine detection potential (E CE) of Cu2+ dissolved from Pt-Cu alloys, we prepared various concentration (0, 0.1, 0.5, 1.0 mM) of CuSO4, whose pH was adjusted to around 0.5 by adding H2SO4. Under laminar flow condition of the electrolyte (10 cm s-1), Au electrode was subjected to cathodic polarization at 298 K in Ar-purged these solutions. Potential range of the experiment from open circuit potential (OCP) to −0.2 V vs. SHE at 1 mV s-1. In order to confirm quantitativity of E CE determined by the cathodic polarization experiments, dissolved Cu2+ was detected on downstream Au collector electrode during anodic polarization of upstream Cu working electrode using channel flow cell. Au collector electrode potentiostatically polarized at (E CE=) 0, −0.1, and −0.2 V in Ar-purged 0.5 M H2SO4, when the working electrode anodically polarized from OCP to 1.0 V at 2 mV s-1. Results and Discussion Diffusion limiting current of residual oxygen was observed between 0.2 − −0.1 V in the cathodic polarization curve in 0 mM CuSO4 (Cu2+ free). And then, cathodic current increased by hydrogen evolution reaction (HER) below −0.1 V. On the other hand, cathodic current observed in the solution containing more than 0.1 mM CuSO4 was higher than that in 0 mM CuSO4. This is caused by the following reaction on the Au electrode: Cu2+ + 2e- → Cu. In addition, Cu2+ reduction current almost increased proportionally to Cu2+ concentration below −0.15 V. However, this concentration-dependent current overlapped with HER current. Therefore, quantitative determination of E CE for Cu2+ detection needs further experiments. Cu working current (i WE) exponentially increased with increasing potential from the OCP, and reached almost constant value (0.1 A cm-2). During this anodic polarization, Au collector current (i CE) at E CE = 0, −0.1, and −0.2 V shows similar trend in the i WE. After subtracting residual current from the i CE, we calculated dissolution current (i Diss.) using collection number (N = 0.38). When E CE was 0 V, i WE was not completely match i Diss. during the polarization, which means that E CE = 0 V is not quantitative potential for Cu2+ detection. On the contrary, i WE almost equals to i Diss. at E CE = −0.1, and −0.2 V. Hence, Cu2+ dissolved from Cu is quantitatively detected on Au electrode at E CE < −0.1 V. Reference 1. P. Strasser, S. Koh, and J. Greeley, Phys. Chem. Chem. Phys., 10, 3670 (2008). 2. Z. Wang, E. Tada, and A. Nishikata, J. Electrochem. Soc., 161 (4), F380 (2014).

SPring-8 BL36XU: Synchrotron Radiation X-Ray-Based Multi-Analytical Beamline for Polymer Electrolyte Fuel Cells under Operating Conditions.

We designed and constructed a beamline BL36XU at the 8 GeV synchrotron radiation facility SPring-8 to provide information required for the development of next-generation polymer electrolyte fuel cells (PEFCs) by clarifying the dynamic aspects of structures and electronic states of cathode catalysts under PEFC operating conditions and in the deterioration processes by accelerated durability test protcols. To investigate the mechanism and degradation process for the cathode electrocatalysis in practical PEFCs, we developed advanced time- and spatially-resolved in-situ/operando X-ray absorption fine structure measurement systems and complementary analytical systems (X-ray emission spectroscopy (XES), X-ray diffraction (XRD), X-ray computer tomography (CT) and hard X-ray photoelectron spectroscopy (HAXPES)) and combined them to develop multi-analytical systems at BL36XU. Multi-analytical systems are very powerful for observing spatial-temporal features of the transient processes occurring in complex systems such as PEFCs. This account describes the design, performance, and research results of the BL36XU and multi-analytical in-situ/operando systems.

Operando XAFS Imaging of Distribution of Pt Cathode Catalysts in PEFC MEA.

Three-dimensional imaging using X-ray as a probe is state-of-the-art for the characterization of heterogeneous materials. In addition to simple imaging of sample morphology, imaging of elemental distribution and chemical states provides advanced maps of key structural parameters of functional materials. The combination of X-ray absorption fine structure (XAFS) spectroscopy and three-dimensional imaging such as computed tomography (CT) can visualize the three-dimensional distribution of target elements, their valence states, and local structures in a non-destructive manner. In this personal account, our recent results on the three-dimensional XAFS imaging for Pt cathode catalysts in the membrane electrode assembly (MEA) of polymer electrolyte fuel cell (PEFC) are introduced. The distribution and chemical states of Pt cathode catalysts in MEAs remarkably change under PEFC operating conditions, and the 3D XAFS imaging revealed essential events in PEFC MEAs.

Oxygen Reduction Reaction Properties for Dry-Process Synthesized Pt/TaCx Nanoparticles

Introduction To develop highly active and durable cathode electro-catalysts for polymer electrolyte fuel cell (PEFCs), core-shell type nanoparticles (NPs) have been actively studied. It is well known that oxygen reduction reaction (ORR) properties of the core-shell NPs greatly depend on the core-materials (transition metal elements (M) and/or Pt-M alloys). Particularly, deactivation of ORR under the PEFC operating conditions (low pH, electrochemical potential fluctuations) through elution of M is serious issue. Therefore, developments of highly-stable core materials under the operating conditions have been needed for practical applications of the core-shell type NPs. Ham et al. reported that early transition metal carbides, such as TaCx and HfCx, show high thermal stability and high corrosion resistance under the PEFC operating condition [1]. In this study, we prepared Pt/TaCx NPs by using arc-plasma deposition (APD) of TaCx followed by electron-beam deposition of Pt in ultra-high vacuum (UHV; ~10-8Pa) and investigated the ORR properties (initial activity and electrochemical stability) of the Pt/TaCx NPs. Experimental Highly-oriented pyrolytic graphite (HOPG) was used as a substrate. First, HOPG was scraped by Scotch tape in air and cleaned by annealing for 30 min in UHV. TaCx NPs are synthesized by using APD (Advanced RICO; APS-1) of Ta at substrate temperatures of 1173K or 1273K under CH4 partial pressure of 0.05 Pa. Subsequently, Pt was deposited on the TaCx NPs/HOPG at 873K by electron-beam deposition in UHV. As a reference, Pt/Ta NPs was fabricated through APD of Ta followed by the electron-beam deposition of Pt at 873K in UHV. The prepared Pt/TaCx and Pt/Ta NPs samples were transferred from UHV to electrochemical (EC) setup without air exposure to avoid sample oxidation and contamination. Cyclic voltammetry (CV) was performed in N2-purged 0.1 M HClO4. After that, linear sweep voltammetry (LSV) for ORR was conducted by using rotating disc electrode (RDE) method in O2-saturated 0.1 M HClO4. ORR activity was evaluated as j k values at 0.9 V vs. RHE: the j k value was estimated from Koutecky-Levich plots and electrochemical surface area (ECSA) of Pt. The electrochemical stabilities of the samples were discussed based on change in the j k values during applying square-wave potential cycles (PCs) between 0.6 (3 s) and 1.0 V (3 s) in O2-saturated 0.1 M HClO4 at a room temperature. Crystal structures, surface morphologies, and micro-structures of the prepared NPs samples were observed by XRD, scanning tunneling microscopy (STM), and scanning transmission electron microscope (STEM) combined with energy dispersive X-ray spectrometry (EDS), respectively. Results and Discussion STM images of the Pt/Ta and Pt/TaCx (1173K & 1273K) NPs were shown in Fig. 1(a). The NPs with average diameter of ca. 6 nm are synthesized on the HOPG substrate, irrespective of the Ta deposition conditions (with or without CH4 partial pressure during APD). Fig. 1(b) shows CVs for the corresponding NPs. Hydrogen (QH; 0.05 ~ 0.35 V) and OH-related (QOH; 0.6 ~ 1.0 V) charges for both the Pt/TaCx NPs are higher than that for the Pt/Ta NPs. The evaluated specific ORR activities (j k / ECSA) of the samples are summarized in Fig. 1(c). Filled and hatched bars are the activity before and after 10k PCs of each samples, respectively. One might notice that both Pt/TaCx NPs exhibited much higher activity than the Pt/Ta NPs even after the 10k PCs. Especially, the Pt/TaCx - 1173K shows highest ORR activity and durability among the samples: the retention rate after 10k PCs was ca. 85% of the initial activity. The results clearly reveal that, as for the core materials, tantalum carbide is effective for enhancing ORR activity as well as durability of the core-shell type Pt-based NPs. Acknowledgement This study was supported by the new energy and industrial technology development organization (NEDO) of Japan.

Structural Kinetics of Cathode Events on Polymer Electrolyte Fuel Cell Catalysts Studied by Operando Time-Resolved XAFS

Operando time-resolved X-ray absorption fine structure (XAFS) analysis is promising to investigate the structural kinetics of cathode events on polymer electrolyte fuel cell (PEFC) electrocatalysts. Here we reviewed our recent reports on operando time-resolved XAFS analysis for PEFC cathode catalysts under PEFC operating conditions. Systematic analysis of a series of operando time-resolved XAFS (XANES and EXAFS) spectra showed details of the structural transformations of Pt cathode catalysts (Pt/C, Pt3Co/C, and Pt3Ni/C) in PEFC, and the rate constants of various structural parameters of the Pt cathode catalysts such as electron transfer from/to Pt (oxidative/reductive electron density change of Pt), Pt–O bond formation/breaking, and Pt–Pt bond breaking/reformation of the cathode catalysts were successfully obtained to discuss the mechanisms of the redox processes of the Pt cathode catalysts and the degradation behaviors of the Pt cathode catalysts in PEFC cells. In this paper, we discuss about structural kinetics of the Pt cathode catalysts in PEFC suggested by operando time-resolved XAFS analysis in the viewpoints of the effects of Pt alloying and degradation.

Heterogeneous Catalytic Reactor for Hydrogen Production from Formic Acid and Its Use in Polymer Electrolyte Fuel Cells

A proof-of-concept prototype of a heterogeneous catalytic reactor has been developed for continuous production of hydrogen via formic acid (FA) dehydrogenation. A laboratory-type polymer electrolyte fuel cell (PEFC) fed with the resulting reformate gas stream (H2 + CO2) was applied to convert chemical energy to electricity. To implement an efficient coupling of the reactor and PEFC, research efforts in interrelated areas were undertaken: (1) solid catalyst development and testing for H2 production; (2) computer modeling of heat and mass transfer to optimize the reactor design; (3) study of compatibility of the reformate gas fuel (H2 + CO2) with a PEFC; and (4) elimination of carbon monoxide impurities via preferential oxidation (PROX). During the catalyst development, immobilization of the ruthenium(II)–meta-trisulfonated triphenylphosphine, Ru-mTPPTS, catalyst on different supports was performed, and this complex, supported on phosphinated polystyrene beads, demonstrated the best results. A validated mathematical model of the catalytic reactor with coupled heat transfer, fluid flow, and chemical reactions was proposed for catalyst bed and reactor design. Measured reactor operating data and characteristics were used to refine modeling parameters. In turn, catalyst bed and reactor geometry were optimized during an iterative adaptation of the reactor and model parameters. PEFC operating conditions and fuel gas treatment/purification were optimized to provide the best PEFC efficiency and lifetime. The low CO concentration (below 5 ppm) in the reformate was ensured by a preferential oxidation (PROX) stage. Stable performance of a 100 W PEFC coupled with the developed reactor prototype was successfully demonstrated.

Dissolution and the Consequent Surface Morphology of Pt-Cu Binary Alloys Under Potential Cycling

Polymer electrolyte fuel cell (PEFC) is regarded as clean energy conversion system, however its high cost prohibits the progress of PEFC. Thus, a reduction in the amount of Pt used as cathode catalyst is required. In order to overcome this problem, Pt transition-metal (Pt-M) alloy catalysts have been developed instead of Pt catalysts, however a deterioration of Pt-M alloy catalysts is concerned because both Pt and M are dissolved from Pt-M alloy catalysts under PEFC operating conditions. In this study, potential cycling tests and inductively coupled plasma mass spectroscopy (ICP-MS), field emission-scanning electron microscopy (FE-SEM), and field emission auger electron spectroscopy (FE-AES) were combined, and dissolution behavior of Pt-Cu alloys has been investigated under potential cycling by using these techniques. Pt50-Cu50 (Pt-50 at% Cu) and Pt25-Cu75 were fabricated by arc-melting under an Ar atmosphere. These alloys were subjected to potential cycling tests at 298 K in 0.5 M H2SO4 solution with a two-compartment stationary cell. A double junction silver / silver-chloride electrode was used as reference electrode, and Au coil was used as the counter electrode. A total of 1,000 cycles of potential cycling tests were employed between 0.05 V and 1.4 V vs. SHE at 100 mV s-1. Tests was interrupted every 100 cycles, and the test solution was sampled for quantitative ICP-MS analysis of both Pt and Cu ions dissolved in it. In addition, surface structure and depth profile after potential cycling tests were evaluated by FE-SEM and FE-AES, respectively. FE-AES depth profiles before and after 100 cycle potential cycling confirmed that Pt-enriched layer formed on Pt25-Cu75 surface after potential cycling. Moreover, the amount of dissolved Pt and Cu ions during 1st sequential order of 100-cycles-potential cycling were 1 monolayer (ML) and 2370 ML, respectively. Hence, Pt-enriched layer formed by the large amount of Cu selectively dissolved from Pt25-Cu75. FE-SEM image of Pt25-Cu75 after 100 cycle potential cycling showed numerous pits (d pit = 2 – 5 nm) formed on its surface, and the large amount of Cu dissolution occurs from 2nd to 10th sequential order of 100-cycles-potential cycling. Thus, selective dissolution of Cu seems not to be suppressed in the latter cycles because Cu dissolution continuously occurs through the pits. On the other hand, FE-SEM image of Pt50-Cu50 after 100 cycle potential cycling was extremely flat. Accordingly, dense Pt-enriched layer formed on its surface, and it suppressed further Cu dissolution from the alloy. The authors acknowledge Ookayama Materials Analysis Division, Tokyo Institute of Technology, for assistance with the ICP-MS analysis.

Oxygen Reduction Reaction Activity and Durability for Model Pt Shell Layers on Ir(111) Prepared by Molecular Beam Epitaxy

Introduction The core-shell type nanostructure, that is composed of ultra-thin Pt coating on other transition metal nanoparticles have attracted much attention from the viewpoints of cathode electrode catalysts in polymer electrolyte fuel cell (PEFC). Compressive strain of the Pt-shells induced by lattice mismatch and direct electronic interaction between the Pt shell and core elements influences electronic properties of the Pt shell layers and, thereby, contributing to enhanced oxygen reduction reaction (ORR) activity [1]. However, because of operating conditions of PEFC, i.e., low pH, high positive potentials, ORR activity of the core-shell catalysts decline through dissolution of core elements and rearrangements of surface Pt atoms. Considering standard electrode potentials, iridium is stable among the transition metals and Pt-shell/Ir-core nanoparticles might be stable under the PEFC operating conditions. However, at present, relationship between Pt-Ir bimetallic surface structures and ORR properties has been less reported [2]. In this study, we prepared well-defined Pt/Ir(111) bimetallic surfaces by molecular beam epitaxy (MBE) in ultra-high vacuum (UHV) and studied the ORR activity and durability. Experimental Ir(111) single crystal substrate was cleaned by Ar+ sputtering and annealing at 1273 K in UHV. Various-monolayers(ML)-thick Pt was deposited onto the clean Ir(111) substrate by an electron beam evaporation method at the substrate temperature of 673K. The resulting surface structures were verified by reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) in UHV. The UHV-fabricated samples were transferred to electrochemical evaluation systems set in a N2-purged glove box without air exposure. CV and LSV measurements were conducted in N2-purged and O2-saturated 0.1 M HClO4, respectively. The ORR activity was evaluated from j k values at 0.9 V vs. RHE by using the Koutecky-Levich equation. EC degradation of the Pt/Ir(111) surface was evaluated by applying potential cycles between 0.6 V (3s) and 1.0 V (3s) vs. RHE in the O2-saturated solution at room temperature. Results and Discussion Fig. 1 (a) summarizes CV curves of the MBE-prepared nML-thick Pt/Ir(111) (n=1-4: Pt n ML/Ir(111)) and clean Pt(111). All CV curves of the Pt nML/Ir(111) show symmetrical peaks similar to the clean Pt(111) (so-called butterfly peak) in the region of OH adsorption and desorption (0.6-1.0 V), accompanied with remarkable decrease in hydrogen adsorption and desorption charges (0.05-0.35 V). The CV features are typical for highly-ORR-active Pt-based bimetallic surfaces reported to date, suggesting that the topmost surfaces comprise pure Pt(111) lattice. Fig. 1 (b) presents changes in j k during potential cycles loading between 0.6 and 1.0 V. Initial ORR activity enhancement factors for the 2ML, 3ML and 4ML-thick Pt shell layers can be estimated to be ×24, ×12 and ×8, respectively compared with the clean Pt(111). Because Ir lattice constant is smaller than Pt, the Ir(111) substrate should induce compressive surface strain on the Pt-shell. As a result, the ORR activity is enhanced. Furthermore, because compressive strain of the Pt shell layers should be relaxed with increasing the Pt shell layer thickness, the initial activity enhancement factors decreased with increasing the thickness. After the 5000 potential cycles loading, the enhancement factors for the Pt2ML/Ir(111) decreased to ×2 vs. clean Pt(111) indicating that compressive surface strain is easily released by the potential cycle loading. In contrast, particularly, the initial activity of the 4ML-Pt-shell remains nearly unchanged even after the 5000 cycles loading: the activity is the highest among the other well-defined Pt-M(111) bimetallic systems, e.g., Pt/Pd(111) [3], Pt-skin surfaces of Pt-Ni(111) [4] and Pt-Co(111) [5], reported in our laboratory. The results can be explained by 3D migration of the surface Pt atoms during the potential cycle loading and by release in compressive strain in Pt shell. Therefore, the results suggest that Pt-Ir bimetallic system can be expected to be applied to ORR catalysts for practical use. Acknowledgement This study was supported by new energy and industrial technology development organization (NEDO) of Japan and Grant- in Aid for young scientists (B) from the Japan society for the promotion of science (N. T.).

Nafion membranes annealed at high temperature and controlled humidity: structure, conductivity, and fuel cell performance

The relationship between electrical and morphological properties of annealed Nafion samples is investigated by X-ray diffraction (XRD), small angle X-ray scattering (SAXS), atomic force microscopy (AFM), and impedance spectroscopy. Experimental data reveal that the heat treatment at high temperature (T ∼130–140°C) with low relative humidity (RH ∼0%) results in significant changes of Nafion such as increased crystallinity and decreased average distance of hydrophilic domains. Such effects were practically absent when the same heat treatment was carried out at high RH ∼100%. The effects of annealing with controlled RH were reflected in the polymer electrolyte fuel cell (PEFC) tests in which the measured performance was markedly reduced for Nafion samples annealed at low RH. Such a feature was related to decreased microstructural stability, water sorption and proton conductivity of the annealed membrane. The observed effects are relevant to evaluate degradation of Nafion during both fuel cell assembly and harsh PEFC operating conditions. Moreover, the experimental results contribute to advance the understanding of Nafion’s properties at high temperature for the development of high-performance ionomer membranes.

Sn02 Aerogels: Towards Performant and Stable PEFC Catalyst Supports

Keywords: Fuel Cells, Catalysts support, Metal oxides, Sol-gel, Aerogels. Carbon blacks supported Pt, currently widely used as electrocatalysts in Polymer Electrolyte Fuel Cells (PEFC) are thermochemically unstable in PEFC operating conditions. This is especially true at the cathode side where, on top of relatively elevated temperature (80°C) and acidic conditions, both the potential and the relative humidity may be high. The resulting carbon oxidation is partially responsible for the PEFC performance decrease observed over time. Hence, long term durability still needs to be improved in order to consider PEFC as credible alternatives to conventional power sources for automotive, stationary or portable applications. Tin dioxide has already been studied as an alternative catalyst support to replace carbon (1, 2). We will show how SnO2 aerogels may even be more promising candidates. Due to their remarkable nanotexturation, aerogels have indeed already proven their ability to efficiently support catalyst for PEMFC application (3). Our SnO2 aerogels were obtained from supercritical CO2 drying of gels synthetized following an acid catalyzed sol-gel route, starting from metal alkoxides precursors. Nb or Sb doping were realized adding the corresponding dopant alkoxide to the sol. The nanoscale morphology can be controlled during the sol-gel synthesis route and is retained after drying. The effect of doping with Nb5+ or Sb5+ on the structure and the morphology of the material was investigated by XRD, SEM and nitrogen sorption. Our materials showed reasonable specific surface areas (80-90 m²/g). The bimodal narrow pore size distribution, centered on around 25 and 45 nm after calcination at 600 °C, is particularly well adapted to the foreseen application. Such morphology (see Figure 1) was directed to i) favor both good dispersion and high activity of the catalyst (Pt) and ii) efficiently manage the water produced at the cathode during the oxygen reduction reaction (ORR). All Sb doped samples exhibited an impressive improvement of electronic conductivity, reaching 1 S/cm for 5 at.% Sb. This is representing a 5 orders of magnitude increase compared to pure SnO2. This level of conductivity is almost equivalent to that of carbon Vulcan XC-72R measured in the same conditions (4 S/cm). These properties make SnO2: Sb a very promising material as catalyst support for PEFC cathode. Our latest results, including platinum deposition and electrochemical activity, will be discussed with respect to the foreseen application. This work is funded by the FP7 NANOCAT European program (SP1-JTI-FCH.2012.1.5). REFERENCES 1. A. Masao, S. Noda, F. Takasaki, K. Ito, K. Sasaki, Carbon-Free Pt Electrocatalysts Supported on SnO2 for Polymer Electrolyte Fuel Cells. Electrochemical and Solid State Letters 12, B119-B122 (2009). 2. K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, M. Watanabe, Synthesis and electrochemical characterization of Pt catalyst supported on Sn0.96Sb0.04O2-delta with a network structure. Electrochimica Acta 56, 2881-2887 (2011). 3. M. Ouattara-Brigaudet, C. Beauger, S. Berthon-Fabry, P. Achard, Carbon Aerogels as Catalyst Supports and First Insights on Their Durability in Proton Exchange Membrane Fuel Cells. Fuel Cells 11, 726-734 (2011). Figure 1

Highly active and durable chemically ordered Pt–Fe–Co intermetallics as cathode catalysts of membrane–electrode assemblies in polymer electrolyte fuel cells

A cathode catalyst with enhanced activity for the oxygen reduction reaction and high durability is greatly required in polymer electrolyte fuel cells (PEFCs). We report that chemically ordered intermetallics, Pt2FeCo/C with L10 ordered structure and Pt6FeCo/C with L12 ordered structure, achieve both higher activity in solution and higher durability as the cathode catalyst of a membrane–electrode assembly (MEA) under actual PEFC operating conditions, compared with a commercial Pt/C. The mass activity of Pt2FeCo/C is about twice as high as that of a commercial Pt/C, and Pt6FeCo/C has a slightly higher mass activity than a commercial Pt/C in solution. These intermetallics were evaluated for durability as the cathodes of MEAs. During the load cycle durability test, the electrochemical surface area (ECSA) of Pt2FeCo/C is retained, while that of Pt6FeCo/C decreases, although less than a commercial Pt/C. The retention of ECSA is consistent with smaller changes in the particle size, observed with transmission electron microscopy, of the intermetallics than of a commercial Pt/C. X-ray diffraction patterns after the durability test show that while some part of ordered intermetallics is transformed into pure Pt, the chemically ordered structure of Pt2FeCo/C is largely retained. Highly active and durable ordered intermetallics are promising as cathode catalysts of PEFCs.

Corrosion Resistance of Pt-Skin Layer on Pt-Co Alloy Surface in Simulated PEFC Environment

Polymer electrolyte fuel cells (PEFCs) have attracted attention as a power source of fuel cell vehicles (FCVs). Pt nanoparticle is used for the cathode catalyst of PEFCs. However, Pt catalyst has problems associated with durability. In particular, submonolayer dissolution of Pt catalyst during PEFC operating conditions causes a cell performance loss of PEFCs. In order to decrease the Pt usage, Pt-Co alloy catalyst has been introduced because of it’s the highest catalytic activity among various Pt alloys. A layer formed by dealloying of Co and following Pt condensation in acid treatments is called Pt-skin layer. The Pt-skin layer, which is formed on the Pt-Co surface, contributes to catalytic activity and corrosion resistance of Pt-Co alloy catalyst. Because dissolution of Pt-skin layer on Pt-Co alloy surface causes not only particle growth similar to the case of Pt catalyst, but also degradation of the catalytic activity by Co dissolution, it is important to reveal the corrosion resistance of Pt-skin layer. Dissolution behavior of Pt-skin layer on Pt-Co alloy surface in simulated PEFC cathode environments was investigated in this study. Pt-Co alloy (molar ratio 1:1) obtained by arc melting was used as specimen. After polishing down to 0.25 mm diamond paste, it was heat-treated under vacuum at 1273 K for 24 hours to remove the work-affected layer, and cooled in the furnace. The surface was immersed in naturally aerated 0.5 M H2SO4 at 298 K for 1 hour as the dealloying treatment. 100 cycles of cyclic voltammetry (CV) at 10 mV·s-1 in the potential range of 0.6 ~ 1.4 V vs. SHE were performed in naturally aerated 0.5 M H2SO4 solution at 298 K as the dissolution test. Au was used as the counter electrode, and Ag/AgCl (3.33 M KCl) as the reference electrode. The cross section of specimen was observed by STEM and analyzed by EELS. The solution after CV was analyzed by ICP-MS. Figure 1 shows the cross-sectional STEM-HAADF images of Pt-Co electrode surfaces and EELS spectrums before and after the dealloying treatment. Peaks of Co were observed in the energy loss of 790 ~ 800 eV. Before dealloying treatment, Intensity of Co peak at the point of 2 nm from the surface was almost the same as that at 0 nm. Whereas, after that treatment, Co peak at 2 nm was lower than that of 0 nm. This demonstrates that thickness of Pt-skin layer is at least within 2 nm was formed on the Pt-Co alloy surface. Fig. 2 shows cyclic voltammogram of pure Pt and Pt-Co alloy in 0.5 M H2SO4. The CV of Pure Pt and Pt-Co alloy showed the oxide formation peak around 0.9 V and 1.1 V respectively. Furthermore, amount of reduction charge between 0.6 ~ 1.4 V calculated from CV of Pt-Co alloy (389 mC·cm-2) was lower than that of pure Pt (524 mC·cm-2). These results mean that oxide formation of Pt-skin layer is suppressed than that of pure Pt. Amount of Pt ions dissolved from pure Pt during CV was 343 nm·cm-2, and that from Pt-Co alloy surface was 182 nm·cm-2. Thus, amount of Pt ions dissolved from Pt-skin layer was about a half of that from pure Pt. Pure Pt was heat-treated in the same condition as the Pt-Co alloy. This indicates that Pt-skin layer on Pt-Co alloy surface have higher corrosion resistance than pure Pt. Previous study on the dissolution mechanism of Pt reported that Pt dissolution follows the oxide formation.1 Based on this mechanism, suppression of the oxide formation leads to high corrosion resistance of Pt-skin layer. 1. Y. Sugawara, T. Okayasu, P. Yadav, A. Nishikata, and T. Tsuru, J. Electrochem. Soc., 159 (11), F779-F786 (2012). Caption list: Figure 1 Cross-sectional STEM-HAADF images and EELS spectrums (a) before and (b) after dealloying treatment. 3 points shown in image a specimen were analyzed by EELS. Figure 2 Cyclic voltammogram of pure Pt and Pt-Co alloy at 10 mV·s-1 in naturally aerated 0.5 M H2SO4 at 298 K.

Electrochemical Scanning Tunneling Microscopic Observations of Pt Nanostructures Prepared by Ultra-High Vacuum Deposition Methods

Background Carbon-supported Pt (Pt/C) is widely used as electrocatalysts for polymer electrolyte fuel cells (PEFC). However, in highly acidic environments under applying electro-potentials, e.g., PEFC operating conditions [1], catalytic activity of Pt/C is gradually degraded due to aggregation and dissolution of Pt nanoparticles as well as corrosion of carbon support. Therefore, electrochemical (EC) structural stability of Pt nanoparticles on carbon support is an important subject for practical application of the Pt/C catalyst. However, in-situ, real-time observation of degradation process of the practical Pt/C catalyst is generally difficult. In this study, we prepared two types of model nanostructures of Pt to investigate dynamic behavior in EC environments. One is Pt nanoparticles on highly-oriented pyrolytic graphite (HOPG) (Pt/HOPG) prepared by arc-plasma deposition (APD) method. Another is molecular beam epitaxially (MBE) grown Pt monolayer on Au(111) (Pt/Au(111)). In-situ observations for both Pt nanostructures by using electrochemical scanning tunnelling microscopy (EC-STM) are conducted under applying electro-potentials. Experimental Sample preparations: All samples were fabricated in ultra-high vacuum (UHV). Mass of Pt was estimated by a quartz microbalance installed in deposition chambers. (1)APD-Pt/HOPG：Pt nanoparticles were deposited by an arc-plasma gun on 773K-HOPG substrate at arc-voltage of 100V.(2)MBE-Pt1ML/Au(111)：1ML thick Pt epitaxial layer was formed on Au(111) at room temperature by MBE [2]. EC-STM observations: EC-STM observations were conducted in 0.1M HClO4. A tip (Pt-Ir) was approached to the sample surfaces at applying potential of Ework=+0.4V (vs. RHE). Then, the potential was increased up to +1.0V and then decreased back to +0.4V. EC-STM images were collected at fixed several voltages during the potential sweep. EC-STM observations were also conducted at +0.4V after applying potential cycles between +0.6V and + 1.2V. Applied potentials are presented with respected to RHE. Results and discussion Fig.1 shows EC-STM images for Pt nanoparticles (0.404μg/cm2) on HOPG recorded at specific voltages. At +0.4V (a), The Pt nanoparticles were highly oriented on HOPG. The Pt nanoparticles diffused on the substrate with increasing applied voltages, and at +1.0V (b), roughness in height increased relative to the image recorded at +0.4V, revealing that EC-oxidation of the Pt nanoparticles by the positive potential sweep. After negative potential swept from +1.0V to +0.4V (c), a mean particle size was slightly increased. The result suggests that reduction of the oxidized Pt nanoparticles formed by the +1.0V potential sweep correlates with aggregation of the Pt nanoparticles. Acknowledgement This work was supported by New Energy and Industrial Technology Development Organization (NEDO) of Japan.

Durability of Pt/Graphitized Carbon Black Catalysts Under Gas Exchange Start-up Operating Conditions for Fuel Cell Vehicles

In polymer electrolyte fuel cells (PEFC), Pt nanoparticles on high surface area carbon black (CB) supports are used as the cathode catalyst. However, it has been found that Pt/CB catalysts are degraded under PEFC operating conditions due to a combination of processes, which include loss of electrochemically active surface area (ECA) due to agglomeration or dissolution of Pt nanoparticles, and corrosion of the CB supports1). During the start-up of fuel cell vehicles (FCVs), the cathode potential can momentarily climb to more than 1.5 V due to the “reverse current mechanism”, which significantly accelerates the carbon corrosion2). In order to reduce the cost of PEFCs, it is necessary to reduce the amount of Pt by improving the durability, especially for the cathode catalysts. To mitigate the corrosion of the carbon support, the use of graphitized carbon blacks (GCB), for which there is a high degree of graphitization, was found to be effective3).In this study, we investigated the cathode catalyst durability during a start-up process like that in actual FCV operation. Pt/graphitized carbon black catalysts were tested in this practical start-up process, i.e., the anode gas was successively cycled between air, hydrogen, and nitrogen. First, air was flowed to the anode and cathode for 90 s. Second, H2 gas was flowed to the anode to start up the fuel cell for 90 s. Finally, N2 gas was flowed to the anode and cathode for 60 s. These three steps are defined as one cycle. The effect of the start-up process on the cell performance was evaluated using three types of catalysts (commercial 50 wt% Pt/GCB and commercial 30 wt% Pt/GCB supplied by Tanaka Kikinzoku Kogyo K.K., and 30 wt% n-Pt/GCB prepared in house by the nanocapsule-method). The average Pt particle sizes of 50 wt% Pt/GCB, 30 wt% Pt/GCB and n-Pt/GCB were 3.4 ± 0.7 nm, 3.3 ± 0.5 nm and 2.6 ± 0.4 nm, respectively. All of the catalysts were tested for 1000 cycles. The I-V performances were evaluated under O2 and air at 65 °C with 100% RH before and after the durability test, which consisted of given numbers of start-up cycles (N). Cyclic voltammetry (CV) was also examined to estimate the ECA under N2 at the cathode and H2at the anode at 65 °C and 100% RH every 200 cycles.Figure 1 shows the changes of ECA values for each catalyst. Before the durability test, n-Pt/GCB showed the highest ECA. This is due to the highly uniform dispersion of small Pt particles on the surface of the GCB support by the nanocapsule method. After 1000 cycles of durability testing, the ECA advantage of n-Pt/GCB was maintained.Figure 2 shows the changes in mass activity at 0.85 V before and after the durability test for each catalyst. The 30 wt% Pt/GCB and the n-Pt/GCB showed nearly the same initial mass activity. However, n-Pt/GCB exhibited a higher mass activity than that of Pt/GCB after 1000 cycles.From these electrochemical measurements, it was found that n-Pt/GCB had higher performance and durability compared with those for both commercial Pt/GCB catalysts. It is considered that the both Pt/GCB easily underwent agglomeration and/or increase of particle size by electrochemical Ostwald ripening, because the Pt particles were situated quite closely together. However, the particle agglomeration of n-Pt/GCB was relatively suppressed due to the uniformity both of the Pt dispersion and of the Pt particle size, as described in our previous report4). We also investigated the degradation of the cathode catalysts in detail by using Raman spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and the glancing incidence X-ray diffraction (GIXD) method.This work was supported by funds for the “Research on Nanotechnology for High Performance Fuel Cells (HiPer-FC)” project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

Pt-Sn(Oxidized Shell)/C and Pt-Sn(Reduced)/C as Cathode Catalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells: Catalyst Performances and Characterization

To address the problems (high price, insufficient total activity and deactivation) related to usage of pure Pt as cathode catalysts for oxygen reduction reactions (ORR) in polymer electrolyte fuel cells (PEFC), carbon-supported Pt-Sn(oxidized shell)/C and Pt-Sn(reduced)/C were used as membrane electrode assembly (MEA) cathode catalysts in PEFC. I-V-load curves in the initial load-cycles showed an increase of the performance for Pt-Sn(oxidized shell)/C, but no increase in the performance was observed with Pt-Sn(reduced)/C and standard Pt/C catalysts. The observed increase in the performance for the Pt-Sn(oxidized shell)/C catalyst may be due to an synergy effect of Pt-nanoparticle core and SnOx-cluster shell formed on the Pt nanoparticle surface. In-situ XAFS at Pt LIII-edge and Sn K-edge under the PEFC operating conditions showed the absence of electron transfer from Sn to Pt and also no changes in the Sn state, indicating its stability. The in situ XAFS for the Pt-Sn(oxidized shell)/C also revealed that Pt nanoparticles made Pt-O bondings at higher voltages than 1.0 V vs. RHE, but the coordination number of Pt-O bonds was much smaller than that for Pt/C. Thus, the Pt-Sn(oxidized shell)/C is a promising candidate as a MEA cathode catalyst in a PEFC system.

Platinum–titanium alloy catalysts on a Magnéli-phase titanium oxide support for improved durability in Polymer Electrolyte Fuel Cells

The robustness of the cathode catalysts used in polymer electrolyte fuel cells (PEFCs) is one of the major factors that determines their durability. In this work, a new class of corrosion-resistant catalyst, Pt–Ti alloy nanoparticles deposited on nano-sized sub-stoichiometric titanium oxide (TiOx), was prepared, and the durability of a Pt–Ti/TiOx cathode under conditions of fuel cell operation was evaluated. Cell performance under a constant current density for >900 h was examined to demonstrate the practical stability of the Pt–Ti/TiOx MEA under PEFC operating conditions. To investigate the effect of high potentials on cathode catalyst activity, a potential cycling test between 1.0 V and 1.5 V vs. a hydrogen anode was applied to the MEA. The results indicated that the electrochemical surface area (ECA) of the Pt–Ti/TiOx MEA is much more stable than that of a conventional Pt/XC72 MEA, and there is almost no loss of ECA even after 10,000 potential cycles. In addition, there was almost no change in the internal resistance of the MEA. TEM analyses of the potential-cycled MEA clearly revealed the excellent stability of Pt nanoparticles supported on TiOx particles.

Gas Transport Properties in Gas Diffusion Layers: A Lattice Boltzmann Study

AbstractThe lattice Boltzmann method is applied to the investigations of the diffusivity and the permeability in the gas diffusion layer (GDL) of the polymer electrolyte fuel cell (PEFC). The effects of the configuration of water droplets, the porosity of the GDL, the viscosity ratio of water to air, and the surface wettability of the GDL are investigated. From the simulations under the PEFC operating conditions, it is found that the heterogeneous water network and the high porosity improve the diffusivity and the permeability, and the hydrophobic surface decreases the permeability.

Verification of Measurement Method of Current Distribution in Polymer Electrolyte Fuel Cells

The aim of the present work is to establish a new technique that can measure current distribution in a Polymer Electrolyte Fuel Cell (PEFC) without contact with the object. When a fuel cell is operated, a magnetic field is caused around the fuel cell by a generated current. The magnetic field strength is measured with a magnetic sensor, and then it is converted to the value of the generated current with the Biot-Savart's law. This technique was verified under the PEFC operating conditions with a specific separator, which could measure real currents in the PEFC. The current distribution determined from a magnetic field is proportional to the real current distribution measured with the specific separator. It appeared that the present technique was useful for measuring the current distribution in PEFCs.