Polymer Electrolyte Fuel Cell Cathode Research Articles

Temperature Dependence on Oxygen Reduction Reaction for Carbon-Supported Pd-Core/Pt-Shell Electrocatalysts

Introduction Polymer electrolyte Fuel Cells (PEFCs) have been developed for the electric device applications such as residential fuel cell cogeneration system and fuel cell vehicles. PEFC cathode catalyst requires an excess amount of platinum due to the degradation of oxygen reduction reaction (ORR) activity at acidic environment and slow reaction kinetics at low temperature. Therefore, it is important to reduce the use of platinum in the catalyst and improve its catalytic activity as well as durability. Core-shell structure is one of the solutions to achieve a higher catalytic activity, higher durability and less use of platinum. However, the factor of controlling ORR activity in core-shell catalyst still has not well understood such as potential/temperature-dependent surface structures and oxidation states of platinum. Convection voltammetry using rotating disc electrode (RDE) is one of the usable ways for evaluating ORR activity of cathode catalyst materials for PEFCs. However, the correlation between ORR activity and the electronic as well as local structure of these materials casted on RDE is not fully clear yet. In this paper, we report the temperature dependency of ORR activity of TEC10V30E Pt/C catalysts (T.K.K.) and Pd-core/Pt-shell/C catalysts from 25 oC to 60 oC, combining RDE measurements and Operando X-ray absorption spectroscopic measurements (XAS). We investigated the correlation with the electronic structure / local structural parameters of the Pt/C catalyst and investigated the factors that govern the oxygen reduction reaction activity on the RDE surface from the viewpoint of temperature dependency. In this research, we synthesized the 1 mono-layer platinum shell on the carbon-supported palladium (Pt-Pd/C) by copper under potential deposition method and investigated its ORR activity at different temperatures and the state of platinum by rotating disk electrode (RDE) measurements and operando X-ray absorption fine structure (XAFS) respectively. The effect of oxygen coverage at Pt surface on ORR activity was mainly discussed. Experimental A model electrode was prepared by applying a catalyst ink prepared from TEC 10 V 30 E Pt / C (TKK) to the surface of glassy carbon RDE. The 1 mono-layer platinum shell on the carbon-supported palladium (Pt-Pd/C) by copper under potential deposition method. The electrochemical cell was constructed using a model electrode fabricated in the working electrode, a Pt mesh for the counter electrode, a reversible hydrogen electrode for the reference electrode, and 0.1 M HClO4 aq for the electrolyte. Electrochemical surface area was calculated by CV measurement under N2 saturated condition and area specific activity value was determined by LSV measurement under N2 and O2 saturated conditions. Measurements were carried out at 10, 25, 40, 50, 60 ºC. For this model electrode, operand XAS measurement of Pt LIII -edge and Pt LII -edge were carried out by using the beamline BL37XU/BL01B1 in SPring-8 (Japan). The measurements were performed by a fluorescent method using a temperature variable type Operand cell that can directly measure the electrocatalyst applied to RDE (Fig. 1). The number of Pt 5d orbital vacancies was calculated from the obtained XANES spectrum, and the local structure of Pt was analyzed from the EXAFS spectrum. The oxygen reduction activity performance in the electrochemical measurement was evaluated from the area specific activity value at 0.9 V of the Pt / C catalyst at 10, 25, 40, 50, 60 ºC. The oxygen reduction activity of the model electrode is determined by the electrochemical parameters related to the oxygen reduction reaction such as the oxygen coverage. Results and discussion According to RDE results, Pt-Pd/C has a potentially higher ORR specific activity than the conventional Pt/C at low temperature (< 50℃) while its catalytic activity has suddenly deteriorated over 60℃. Linear sweep voltammetry showed the coverage of oxygen species at Pt surface dramatically increased at high temperature in 1.1 V (vs. RHE). This oxygen species could reduce the ORR by blocking active sites for additional O2 adsorption. We also found the rapid increase of Pt 5d orbital vacancy by X-ray absorption spectra with the temperature, indicating that Pt on Pd/C was more easily oxidized over 60℃ than Pt/C.

A Comprehensive Physical Impedance Model of Polymer Electrolyte Fuel Cell Cathodes in Oxygen-free Atmosphere

Electrochemical impedance spectroscopy (EIS) is an indispensable tool for non-destructive operando characterization of Polymer Electrolyte Fuel Cells (PEFCs). However, in order to interpret the PEFC’s impedance response and understand the phenomena revealed by EIS, numerous semi-empirical or purely empirical models are used. In this work, a relatively simple model for PEFC cathode catalyst layers in absence of oxygen has been developed, where all the equivalent circuit parameters have an entire physical meaning. It is based on: (i) experimental quantification of the catalyst layer pore radii, (ii) application of De Levie’s analytical formula to calculate the response of a single pore, (iii) approximating the ionomer distribution within every pore, (iv) accounting for the specific adsorption of sulfonate groups and (v) accounting for a small H2 crossover through ~15 μm ionomer membranes. The derived model has effectively only 6 independent fitting parameters and each of them has clear physical meaning. It was used to investigate the cathode catalyst layer and the double layer capacitance at the interface between the ionomer/membrane and Pt-electrocatalyst. The model has demonstrated excellent results in fitting and interpretation of the impedance data under different relative humidities. A simple script enabling fitting of impedance data is provided as supporting information.

Durability of Pt-Co Alloy Polymer Electrolyte Fuel Cell Cathode Catalysts under Accelerated Stress Tests

The durability of Pt-Co alloy cathode catalysts supported on high surface area carbon is investigated by subjecting them to accelerated stress tests (ASTs). The catalysts had different initial Co contents and nanoparticle morphologies: a “spongy” porous morphology for the high-Co (H) content catalyst, and a fully alloyed crystalline morphology for the medium-Co (M) and low-Co (L) content catalysts. The specific activity of the catalysts depends on their initial Co content, morphology and nanoparticle size, and remained higher than 1000 μA/cm2-Pt after 27–50% Co loss. The H-catalyst electrode showed the smallest kinetic overpotentials (ηcs) due to higher initial Pt loading than the other two electrodes, but it had the fastest increase in ηcs with AST cycling due to lower Co retention; the L-catalyst electrode showed higher ηcs due to a lower initial Pt loading, but had a smaller increase in ηcs with aging due to higher Co retention; the M-catalyst electrode showed a similar increase in ηcs with aging, but this increase was due to the combined effects of Co dissolution and electrochemically active surface area (ECSA) loss. The modeled increase in mass transfer overpotentials with aging correlates with the initial Pt loading, ECSA loss and the initial catalyst morphology.

New design of a PEFC cathode separator of for water management

Generally, polymer electrolyte fuel cells (PEFCs) need humidifiers to prevent the drying of the membrane, but this use of humidifiers creates water management issues, such as the flooding/plugging phenomena and decreased system efficiency because of an increase in the electric energy needed for auxiliary equipment. Although most researchers have developed high-temperature membranes that do not need humidifiers, a lot of time is necessary for the development of these membranes, and these membranes drive up costs. Therefore, we propose a new cathode separator design that can recycle water generated by power generation in the same cell and a stack structure that can redistribute water collected in the cathode outlet manifold to drying cells. Because the new cathode separator has a bypass channel from the gas outlet to the gas inlet to transport excess water, a dry part in the gas inlet is supplied with excess water in the gas outlet through the bypass channel even if the PEFC is operated under dry conditions. Excess water in the PEFC stack can be transported from the cell with excess water to the drying cell through the cathode outlet manifold with a porous wall. Therefore, we confirm the influence of the plugging phenomenon in the cathode gas outlet manifold on the cell performance of each cell in the stack. As a result, the cell performance of the new cathode separator design is better than that of the standard separator under the low humidity conditions. We confirm that the plugging phenomenon in the cathode outlet manifold affects the cell performance of each cell in the stack.

Experimental observation of the effect of cleansing agents on the performance of polymer electrolyte fuel cells

Regular maintenance/cleaning of fuel pipeline and system hardware is an essential requirement in fuel cell operation to prevent contamination. An experimental and analytical study is performed to aid the selection of appropriate cleansers to be used as cleaning agents in polymer electrolyte fuel cell (PEFC). Screening tests for several cleansers are carried out during the injection of samples into the PEFC cathode inlet. One proper agent (naphtha) has shown a fully recoverable and minimal effect on the fuel cell performance and as such is determined as the best cleansing agent. Unlike other samples, naphtha does not contain any metallic components such as sodium or potassium in its composition. Furthermore, PEFC can still operate at ∼0.4 V at constant current (1 A/cm2) even with a considerable flow rate (250 μl/min) of the selected cleanser. Detailed analytical analysis of this cleanser is provided by curve fitting the electrochemical impedance spectroscopy data, and evaluation of binary gas diffusion coefficients. It is indicated that performance loss during naphtha exposure is mainly due to the adsorption of contaminants on active Pt sites and an increase in mass transfer resistance.

A New Approach for the Synthesis of Composition-Controlled, Fe-Based O2-Reduction Catalysts for Polymer Electrolyte Fuel Cells

The excessive cost of polymer electrolyte fuel cells (PEFCs) would be significantly reduced if the Pt-based catalysts needed to catalyze the oxygen reduction reaction (ORR) at the cell’s cathode were substituted with inexpensive materials based on abundant metals.1 Such non-noble metal catalysts (NNMCs) have been studied since the 1960s and, in recent years, a refinement of their syntheses has led to initial ORR-activities commensurate with those observed for Pt-based catalysts.2,3 However, this outstanding performance is accompanied by a fast deactivation that remains poorly understood,4 and that is likely related to the NNMC’s composition. For Fe-based catalysts, the latter typically consists of Fe-N4 like moieties that are believed to be responsible for the material’s ORR-activity, along with side phases (e.g., Fe-oxides, carbides and/or nitrides) that do not participate in the catalytic process.5 NNMC deactivation could in terms be caused by the demetallation of the ORR-active Fe-N4 sites, and/or be related to the inactive side-phases, which could catalyze the formation of the aggressive Fenton’s reagent or get leached and poison the cathode’s proton-exchange ionomer.4 To clarify the contribution of these composition-related mechanisms to the overall catalyst deactivation, NNMC synthesis approaches allowing a fine composition control are urgently needed. With this motivation, we have developed a new NNMC preparation method in which polyacrylonitrile is used as the N- and C-precursor, and an inexpensive, thermally decomposable compound as the porosity-inducing agent. Initially, the precursors’ ratios and the temperature of their heat treatment are systematically modified to yield catalysts with different surface N- and (micro)porous-contents. These features are then shown to be only partially related to the catalysts’ ORR-activity (assessed in rotating disk electrode measurements in 0.1 M HClO4 – cf. Figure 1), which for the best performing materials reaches values of ≈ 1.0 A∙gNNMC −1 at 0.8 V vs. RHE. Additionally, the catalytic performance is related to the relative content of Fe-N4 sites and side phases, which is assessed through transmission electron microscopy and X-ray absorption spectroscopy. To investigate this effect in greater detail, the composition of selected catalysts is further modified by including reducing agents in the catalyst synthesis’ acid washing and / or heat treatment steps, as to favor the removal of Fe-based side-phases.6,7 Ultimately, the performance of selected catalysts with different proportions of active and inactive phases is assessed in PEFC tests which shed light on the impact of their composition on catalytic activity and stability. In summary, this contribution will introduce a novel NNMC-synthesis approach leading to catalysts with different, well-defined compositions, allowing to draw correlations between this key property and the materials’ catalytic activity and durability in PEFC cathodes.

Influence of Transition Metal and Synthesis Methodology on the Active Site Density on the Surface of PGM-Free Catalysts

State-of-the-art polymer electrolyte fuel cells (PEFCs) utilize platinum (Pt) or Pt-alloy nanoparticles supported on high surface-area carbon as catalysts for oxygen reduction reaction (ORR) at the PEFC cathode.1 Platinum group metal-free (PGM-free) catalysts have the potential to reduce the high PEFC cost by replacing Pt with far less expensive materials. Over the past two decades, the growing interest in PGM-free catalysts has led to significant improvements in their performance, which is now approaching that of Pt-based catalysts.2–5 Several insights into the critical factors, such as the role of transition metal, catalyst precursors and pore-formers have been studied.3–5However, for further development of PGM-free catalyst understanding the nature of active site(s) in PGM-free catalysts, as well as the determination of the active site density and turnover frequency (TOF) are vital. PGM-free catalysts have little affinity to “standard” molecular probes, such as carbon monoxide and hydrogen sulfide, that effectively poison ORR active sites in PGM catalysts.6,7 Only recently, an ex situ and in situ method for reversibly probing the active sites and estimating the active site density has been proposed.8,9 Malko et al. showed that nitrogen-based molecular probes can poison the active site and then be reductively stripped off in a voltammetric scan.9 In this work, we have studied the effect of the transition metal and synthesis approach (type of precursor and catalyst fabrication method) on the active site. Active site density in catalysts derived from different transition metals was estimated using sodium nitrite (NaNO­2) as probe.9Figure 1a shows the reductive stripping curves for Los Alamos-developed Fe- and Co-derived CM-PANI-C catalysts, with Co exhibiting much higher affinity to the probe than Fe. The effect of different synthesis methodology and precursor with same transition metal was also studied. Reductive stripping of the probe from CM-PANI-Fe-C catalysts with and without Zn added as a pore former occurs at the same peak potentials suggesting the presence of identical active site in both cases (though occurring at a higher concentration in the catalyst synthesized using Zn). On the other hand, a catalyst with atomically dispersed Fe appears to bind the probe more strongly than CM-PANI-Fe-C catalysts (Figure 2b). Active site density estimation provides a quantitative assessment for the effect of the transition metal and synthesis methodology for further development of PGM-free catalyst. Acknowledgments This research is supported by DOE Fuel Cell Technologies Office, through the Electrocatalysis Consortium (ElectroCat). References S. Litster and G. McLean, J. Power Sources, 130, 61 (2004).H. T. Chung, J. H. Won, and P. Zelenay, Nat. Commun., 4, 1922 (2013).H. T. Chung et al., Meet. Abstr. MA2016-02, 2825 (2016).L. Lin, H. T. Chung, X. Yin, U. Martinez, and P. Zelenay, Meet. Abstr. MA2016-02, 2826 (2016).U. Martinez, E. F. Holby, J. H. Dumont, and P. Zelenay, Meet. Abstr. MA2016-02, 2827 (2016).Q. Wang, Z. Zhou, Y. Lai, Y. You, J. Liu, X. Wu, E. Terefe, C. Chen, L. Song, M. Rauf, N. Tian and S. Sun, J. Am. Chem. Soc., 136, 10882 (2014).7. D. Malko, T. Lopes, E. Symianakis, and A. R. Kucernak, J. Mater. Chem. A, 4, 142 (2016).N. R. Sahraie, U. I. Kramm, J. Steinberg, Y. Zhang, A. Thomas, T. Reier, J. Paraknowitsch and P. Strasser, Nat. Commun., 6, 8618 (2015).D. Malko, A. Kucernak, and T. Lopes, Nat. Commun., 7, 13285 (2016). Figure 1

Protection Mechanisms of Inorganic Inhibitors for Localized Corrosion of Stainless Steels at Anodic Polarization

Nowadays, metal separators have been used for automotive polymer electrolyte fuel cell (PEFC). Stainless steel (SUS) is one of the suitable materials for metal separators [1]. However, localized corrosion could occur due to the potential change during PEFC operations because stainless steel as a separator contacts with acidic solution containing small amounts of chloride and fluoride ions. It is well known that a few percent of molybdenum as an alloy element in SUS316 and SUS447 inhibits pitting corrosion. Hashimoto et al. [2] reported that molybdenum in stainless steel forms molybdenum oxy-hydroxide or molybdate on the active surface sites which are defects formed in the passive film, and this action could bring a homogeneous passive film. The present work has been carried out to discuss the protection mechanisms of molybdenum alloy element and molybdate ions for the stainless steel and to propose the theory of inhibitor development for the stainless steel separators in PEFC.Measurements of anodic polarization curves of SUS304, SUS316 and SUS447 were carried out. The working electrode (WE) was polished with diamond paste as a pretreatment and its minimum particle size was 1 μm. After that, it was ultrasonically cleaned in methanol and distilled water. The surface area of the WE was 0.28 cm2. The counter electrode was platinum and the reference electrode was a saturated KCl/Ag/AgCl (SSE) electrode. Polarization curves were measured by scanning the working electrode potential at 100 mV min-1 from open circuit potential to 1.5 V vs. SSE. The electrolyte solution was 1 mol/dm3 H2SO4aqueous solution (pH 3) which contained 10 ppm NaCl and 30 ppm NaF. Figure 1 shows polarization curves of SUS447 in electrolytes which contain (a) no inhibitor, (b) 1 mmol/dm3 Na2MoO4 and (c) 1 mmol/dm3 Na2MoO4 + 1 mmol/dm3 Na2WO4 as inhibitors. Passive and transpassive regions are observed in all polarization curves and the boundary between passive and transpassive region is approximately 0.8 V vs. SSE. The cathode of PEFC in practical use is between 0.7 V and 0.8 V vs. SSE. It is considered that the potential of cathode is normally in the passive region. However, the current density in the transpassive region should be small to avoid contamination of metallic ions dissolved from stainless steel. In Fig 1, all current density is approximately 5 μA cm-2 in the passive region. The potential of current density in the solution of (c) is ten times smaller than that of (a) in the transpassive region, indicating that the coexistence of Na2MoO4 and Na2WO4has significant inhibitor effect for transpassive dissolution of stainless steel. In this study, protection mechanisms of inorganic corrosion inhibitors are proposed on the basic of thermodynamic data. Reference s [1] J. Imamura, Y. Tarutani, NIPPON STEEL & SUMITOMOMETAL Technical Report, 396 (2013). [2] K. Hashimoto, K. Asami, M. Naka and T. Masumoto, Corros . S ci., 19, 857 (1979). Figure 1

Tracking Degradation Induced Structural and Compositional Changes of a Polymer Electrolyte Fuel Cell Cathode Catalyst Layer Following Voltage Cycling Using Micro-Xct

Degradation of cathode catalyst layers during typical automotive operation is a concern for long-term durability and performance of polymer electrolyte fuel cells (PEFCs). Although many degradation pathways exist, carbon corrosion following significant voltage fluctuations should be considered a primary degradation mechanism due to the role carbonaceous material plays in catalyst layer composition and structure. Following carbon support corrosion, catalytic activity is lost due to removal, isolation, or agglomeration of catalyst particles. Eventual collapse of the weakened carbon support structure can lead to changes in porosity and pore size distribution, thus blocking transport pathways for gas flow and water removal. Furthermore, changes in surface roughness and composition by addition of surface oxidation groups can increase hydrophilicity, increasing the possibility of flooding in the catalyst layer [ 1]. A detailed understanding of the structural and compositional changes that occur following degradation of the catalyst layer is therefore required. To date, imaging of PEFC component degradation, in particular the cathode catalyst layer, has primarily been limited to ex situ techniques and associated qualitative morphological observations [ 2, 3]. Recent developments in lab-based X-ray computed tomography (XCT) systems have allowed for nondestructive in situ imaging of PEFCs [ 4] accomplished by using a unique device fixture design and XCT operation to obtain same location tracking with high quality three-dimensional tomographies over different stages of cathode catalyst layer (CCL) degradation [ 5]. In this work, this technique is expanded to visualize and measure quantitative morphological and compositional changes that occur with degradation of the cathode catalyst layer during voltage cycling, see Figure 1. This analysis is combined with simultaneous tracking of the liquid water distribution in the gas diffusion layer and CCL by specialized operandovisualization, not previously performed for lab-based XCT. The attenuation of X-rays is related to the elemental composition as well as the density of the material being imaged as defined by the Beer-Lambert law. By exploiting this property, novel insight into the local compositional changes of the cathode catalyst layer are investigated and uniquely correlated to the water distribution changes in an operating fuel cell, which has not been previously possible. The acquired compositional and morphological changes are further supplemented with electrochemical diagnostics measurements such as fuel cell polarization curves, electrochemical active surface area (ECSA) and double layer capacitance. This comprehensive study highlights the effect of CCL degradation on overall fuel cell performance. Significant losses in the mass transport regime of the polarization curve are correlated to possible flooding of the catalyst layer and reduced oxygen transport through observation of density increase and reduced thickness. Local material composition changes such as carbon loss, ionomer distribution and platinum loading are also calculated and discussed. Acknowledgements: Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant.

High-Throughput Synthesis and Characterization of PGM-Free Oxygen Reduction Reaction Electrocatalysts

Unprecedented progress has been made over the past decade in increasing both the oxygen reduction reaction (ORR) activity and durability of platinum group metal-free (PGM-free) polymer electrolyte fuel cell cathode catalysts.1,2 For example, electrocatalytic activities approaching those of platinum have been obtained for classes of catalysts derived from iron salts and metal-organic frameworks and iron salts and carbon-nitrogen-containing polymers such as polyaniline and cyanamide.3-6 With further improvements in these materials and electrodes based on these materials, especially in hydrogen-air performance and long-term performance durability, these materials will become viable for numerous applications including automotive propulsion power. For the general class of pyrolyzed iron-carbon-nitrogen PGM-free materials, it has been determined that variables such as the metal salt, polymer, metal to polymer ratio, as well as the temperature and atmosphere in which the composites are pyrolyzed are important in determining the activity and activity stability of the resulting catalysts.1,2 Changing these variables and testing their effect on the resulting catalyst properties is a very time-consuming process and only a limited portion of the composition and temperature space have been explored for this broad class of materials. This presentation will describe the development and application of high-throughput methodology to explore the effects of these parameters on the activity and fuel cell performance of iron-polyaniline and iron- zeolitic imidazolate framework-derived ORR electrocatalysts. The robotic systems in Argonne’s High-Throughput Research Laboratory have been utilized to synthesize numerous (forty to forty-five) compositions in these two material systems. A multi-channel flow double electrode (m-CFDE) cell was designed and constructed for the simultaneous screening the ORR activity of multiple samples using an aqueous hydrodynamic technique. Unlike commercially-available m-CFDE’s, this cell was designed with removable electrode plugs facilitating the quick change of catalyst on the glassy carbon electrode and also replacement of damaged electrodes. The results from the m-CFDE cell were validated against those obtained using the standard rotating disk electrode technique. The high-throughput structural characterization of the materials using techniques such as X-ray diffraction and X-ray absorption spectroscopy and correlation of the phase and atomic structure will be described. The high-throughput testing of these materials in a 25-electrode array fuel cell from NuVant Systems Inc. will also be described. In addition, the use of in situmulti-sample X-ray absorption spectroscopy to determine the atomic structure of the materials during the pyrolysis step of preparation will be described. Acknowledgements This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the auspices of the Electrocatalysis Consortium (ElectroCat). This research used the resources of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC, also under contract DE-AC-02-06CH11357. References M. Lefèvre, E. Proietti, F. Jaouen, and J.-P. Dodelet, Science, 324 (2009) 71-74.H. Chung, G. Wu, D. Higgins, P. Zamani, Z. Chen, P. Zelenay "Heat-Treated Non-precious Metal Catalysts for Oxygen Reduction " for Electrochemistry of N4 Macrocyclic Metal Complexes, Volume 1: Energy, Editors: J. H. Zagal, F. Bedioui, Springer, 2016.E. Proietti, F. Jaouen, M. Lefèvre, N. Larouche, J. Tian, J. Herranz, and J.-P. Dodelet, Nature Comm., 416 (2011) 1-9.H. Barkholtz, L. Chong, Z. Kaiser, T. Xu, and D.-J. Liu, Catalysts, 5 (2015) 955-965.H. Zhang, H. Osgood, X. Xie, Y. Shao, and G. Wu, Nano Energy, 31 (2017) 331-350.H.T. Chung, D.A. Cullen, D. Higgins, B.T. Sneed, K.L. More, and P. Zelenay, 2016 PRiME Meeting, Honolulu, Hawaii, October 2-7, 2016.

Photochemical Preparation of Pt Electrocatalysts on SnO2 Support for PEFCs

In polymer electrolyte fuel cells (PEFCs), Pt nano-particles dispersed on carbon black (Pt/C) are generally used as electrocatalysts. Reducing the amount of expensive Pt is the important issue for the implementation of low-cost PEFCs. Since the oxygen reduction reaction (ORR) occurs only on the surface of the Pt nano-particles, core-shell catalysts in which Pt monolayer shell is loaded on metallic nano-particles core are effective in reducing the Pt usage. Although such core-shell catalysts with Au or Pd cores exhibit very high activity compared with the commercial Pt/C, their durability still remains as a challenge (1). The cost reduction effect is also limited if we use expensive metal cores. According to previous studies, the durability of the PEFC electrocatalyst can be improved by using SnO2 as a electrocatalyst support which has thermochemical stability under the PEFCs cathode conditions (2-4). Therefore, it is, in principle, possible to develop electrocatalyst with both higher durability and ORR activity by employing such stable oxide as the core instead of noble metal. Here, the objective of this study is to deposit Pt on SnO2 at an atomic level by applying a photochemical procedure. Experimental Vapor grown carbon fiber (VGCF-H) was applied as the conductive backbone of the electrocatalysts. SnO­­2 was prepared on the VGCF-H (SnO2/VGCF-H) by the ammonia co-precipitation method. As a procedure to deposit Pt on the SnO2 nano-particles at an atomic level, photochemical procedure was applied to reduce Pt ions by irradiating ultraviolet (and visible) light having energy exceeding the band gap energy of oxide semiconductors. Since this photochemical reaction occurs only on semiconductors, the selective Pt deposition on SnO2 is possible. In addition, we selected methanol and formic acid as sacrificial reagents to capture electron holes for the photochemical reactions. Nanostructure of the obtained electrocatalysts was observed by STEM. We also made ICP analysis to quantify Pt loading. Electrochemical characterization of these electrocatalysts was carried out by the half-cell measurements. Electrochemical surface area (ESCA) was measured by cyclic voltammetry (CV), and oxidation reduction reaction (ORR) activity was derived from kinetically controlled current density (ik ) after the rotating disk electrode (RDE) measurement. Results and discussion STEM image of Pt/SnO2/VGCF-H prepared by irradiating ultraviolet (UV) light for 30 minutes in methanol is shown in Figure 1. This micrograph shows Pt particles deposited with a diameter of about 1nm on SnO2 surface. Although the Pt loading increased with UV irradiation time, the Pt particle diameter increased and ECSA tended to decrease. When formic acid was used, the photochemical reaction was promoted, and more Pt was deposited with the same UV irradiation time, compared with methanol. We have succeeded to homogeneously coat Pt on the oxide support and to improve both ECSA and mass activity by shortening the UV irradiation time.

Investigation of Oxygen Transport Resistance in M-ACLS Electrodes for PEFC Cathodes

1.Introduction Significant reduced use of platinum catalyst is essential for full-scale implementation of polymer electrolyte fuel cells (PEFCs). Toshiba has developed an alternating catalyst layer structure (ACLS) catalytic electrode fabricated by a sputtering process and is studying its application in PEFCs.1 The ACLS cathode has been modified and previous studies have reported good power generation characteristics and high durability at a platinum loading of 0.15 mg cm-2 using a modified ACLS (M-ACLS).2 The power generation characteristics of PEFCs strongly depends on oxygen transport in the catalyst layer in addition to the catalytic activity. Because the catalyst layer structure of the M-ACLS electrode is characterized by nanosheet platinum, which differs from commonly used nanoparticulate Pt/C, it is necessary to understand the oxygen transport capability in order to reduce the platinum loading. In recent years, measurement of oxygen transport resistance by the limiting current method using a PEFC single cell is almost established.3,4 In this presentation, we report the result of investigating the oxygen transport resistance of the M-ACLS electrode using the limiting current method ２． Experimental A catalytic electrode precursor containing platinum and pore-forming material was deposited on a commercially available gas diffusion layer substrate by using a sputtering device, and then the pore-forming material was removed to prepare the M-ACLS anode and the M-ACLS cathode. The electrode structure was observed by SEM. Various properties such as the specific surface area of the platinum catalyst, the roughness factor (the ratio of the platinum surface area to the electrode area), and power generation characteristics were evaluated with reference to the Fuel Cell Commercialization Conference of Japan protocol. Furthermore, the limiting current was obtained in temperature range of 50 °C to 80 °C using two different kinds of diluted oxygen gas (oxygen dry mole fraction 0.01, dilution gas N2 or He). Transport resistance was separated by utilizing the difference in oxygen diffusibility between the two kinds of dilution gasses or pressure dependency.3,4 ３．Results and discussion Figure 1 shows an example of a cross-sectional SEM photograph of a M-ACLS catalytic electrode laminated with a porous platinum sheet, which is found to have a high porosity without carrier. Figure 2 shows a typical I-V curve using the limiting current method. It can be seen that the value of the limiting current density changes, reflecting the difference in oxygen diffusibility in the two kinds of dilution gases. In this presentation, we report the oxygen transport resistance analysis results of M-ACLS cathode, oxygen transport resistance per unit platinum surface area, and superiority of M-ACLS catalytic electrode to over nanoparticulate Pt/C. (1) W. Mei, T. Fukazawa, Y. Nakano, Y. Akasaka, K. Naito, ECS Trans., 50(2), 1377 (2012) (2) W. Mei, T. Fukazawa, T. Yang, N. Yoshinaga, Y. Kanai, ECS Trans., 69(17), 755 (2015) (3) T. Mashio, A. Ohma, S. Yamamoto and K. Shinohara, ECS Trans., 11(1), 529 (2007) (4) N. Nonoyama, S. Okazaki, A. Z. Weber, Y. Ikogi, T. Yoshida, J. Electrochem. Soc., 158 (4), B416 (2011) Figure 1

Influences of Chloride and Fluoride Ions on Localized Corrosion of Stainless Steels at Anodic Polarization

Polymer electrolyte fuel cells (PEFCs) are environmentally friendly generators and operate at relatively low temperatures, so they have been put into practical use as power sources for automobiles. A carbon plate is generally used for the separator of PEFC. However, there are problems in using carbon separator such as low mechanical strength and cost, thus metal separators are used for automotive PEFC. Stainless steels are considered to be suitable metal among several candidates for material of metal separator because it is highly corrosion resistant, high workable and relatively reasonable. Since the inside of the PEFC becomes particular corrosive environments, dissolution of metallic ions due to localized corrosion of stainless steel is feared. Typical conditions of the particular corrosive environments assumed in this research are as follows, chloride ion concentration: 10 ppm, fluoride ion concentration: 30 ppm, pH 3, electrode potential: 0.8 V vs. SSE, temperature: 80 degree centigrade. A schematic diagram of the anodic polarization curve of stainless steels is shown in Fig. 1. The anodic polarization curve is divided into three potential regions and they are defined as a passive region, a first transpassive region, and a second transpassive region. Anodic polarization curves of SUS 447 in three kinds of solutions are shown in Fig. 2. The following three solutions were used for measurements. A: pH 3 1 M H2SO4 (80℃) without halide ions, B: 1 M H2SO4 containing 10 ppm NaCl, C: 1 M H2SO4 containing 30 ppm NaF. The anodic polarization curve of SUS 447 in solution A without halide ion shows the passive region and the second transpassive region at nobler potential than 1.2 V vs. SSE. The anodic polarization curve of SUS 447 in the solution B containing chloride ions shows the first transpassive region and the current increase above 0.8 V vs. SSE is observed. If the potential of the PEFC cathode is assumed to be 0.7 ~ 0.8 V vs. SSE, it is near the boundary between the passive region and the first transpassive region, and metallic ions dissolution may occur. Furthermore, in the anodic polarization curve in SUS447 in a solution C containing fluoride ions, the current value in the first second transpassive regions is much larger than that in the solution B, indicating the acceleration of the dissolution rate. It is considered that corrosion protections such as surface treatment and inhibitor addition are necessary when stainless steels are used in the particular corrosive environment in PEFC. In this study, we investigated the influences of chloride and fluoride ions on localized corrosion of stainless steel by electrochemical measurements, and discuss the corrosion protection method of stainless steel as a metal separator of PEFC. Figure 1

Investigation of Oxygen Transport Resistance in M-ACLS Electrodes for PEFC Cathodes

Modified alternating catalyst layer structure (M-ACLS) is a promising low-platinum technology for application in polymer electrolyte fuel cells (PEFCs). Because the catalyst layer structure of an M-ACLS electrode is characterized by nanosheet platinum, which differs from the commonly used Pt/C, it is necessary to understand the oxygen transport capability. In this study, the oxygen transport resistance was investigated using limiting current measurement. It is found that, compared with Pt/C, M-ACLS cathodes exhibited far lower oxygen transport resistance per unit platinum surface area.

In-plane and through-plane non-uniform carbon corrosion of polymer electrolyte fuel cell cathode catalyst layer during extended potential cycles

The impact of electrochemical carbon corrosion via potential cycling durability tests mimicking start-stop operation events on the microstructure of the cathode catalyst layer in polymer electrolyte fuel cells (PEFCs) is investigated using focused ion beam (FIB) fabrication without/with the pore-filling technique and subsequent scanning electron microscope (SEM) observations. FIB/SEM investigations without pore-filling reveals that the durability test induces non-uniform cathode shrinking across the in-plane direction; the thickness of the catalyst layer decreases more under the gas flow channel compared to the area under the rim of the flow field. Furthermore, FIB/SEM investigations with the pore-filling technique reveal that the durability test also induces non-uniform cathode shrinking in the through-plane direction; the pores in the area close to the membrane are more shrunken compared with those close to the microporous layer. In particular, a thin area (1–1.5 μm) close to the membrane is found to be severely damaged; it includes closed pores that hinder mass transport through the catalyst layer. It is suggested that uneven carbon corrosion and catalyst layer compaction are responsible for the performance loss during potential cycling operation of PEFCs.

Unsupported Pt-Ni Aerogels with Enhanced High Current Performance and Durability in Fuel Cell Cathodes.

Highly active and durable oxygen reduction catalysts are needed to reduce the costs and enhance the service life of polymer electrolyte fuel cells (PEFCs). This can be accomplished by alloying Pt with a transition metal (for example Ni) and by eliminating the corrodible, carbon-based catalyst support. However, materials combining both approaches have seldom been implemented in PEFC cathodes. In this work, an unsupported Pt-Ni alloy nanochain ensemble (aerogel) demonstrates high current PEFC performance commensurate with that of a carbon-supported benchmark (Pt/C) following optimization of the aerogel's catalyst layer (CL) structure. The latter is accomplished using a soluble filler to shift the CL's pore size distribution towards larger pores which improves reactant and product transport. Chiefly, the optimized PEFC aerogel cathodes display a circa 2.5-fold larger surface-specific ORR activity than Pt/C and maintain 90 % of the initial activity after an accelerated stress test (vs. 40 % for Pt/C).

Unsupported Pt‐Ni Aerogels with Enhanced High Current Performance and Durability in Fuel Cell Cathodes

AbstractHighly active and durable oxygen reduction catalysts are needed to reduce the costs and enhance the service life of polymer electrolyte fuel cells (PEFCs). This can be accomplished by alloying Pt with a transition metal (for example Ni) and by eliminating the corrodible, carbon‐based catalyst support. However, materials combining both approaches have seldom been implemented in PEFC cathodes. In this work, an unsupported Pt‐Ni alloy nanochain ensemble (aerogel) demonstrates high current PEFC performance commensurate with that of a carbon‐supported benchmark (Pt/C) following optimization of the aerogel's catalyst layer (CL) structure. The latter is accomplished using a soluble filler to shift the CL's pore size distribution towards larger pores which improves reactant and product transport. Chiefly, the optimized PEFC aerogel cathodes display a circa 2.5‐fold larger surface‐specific ORR activity than Pt/C and maintain 90 % of the initial activity after an accelerated stress test (vs. 40 % for Pt/C).

(Invited) High-Performance PGM-Free Electrocatalysts for the Polymer Electrolyte Fuel Cell Cathode

The activity of oxygen reduction reaction (ORR) catalysts often determine performance of polymer electrolyte fuel cells (PEFCs) as measured by their power output, open circuit voltage, and fuel conversion efficiency. Currently, Pt-nanoparticle catalysts, either supported on high surface-area carbons or prepared in a form of contiguous thin layer on conductive or non-conductive supports, represent the state of the art in ORR electrocatalysis at the PEFC cathode. However, the high and variable price and scarceness of Pt have limited its widespread implementation in the -temperature fuel cells to date, especially in automotive transportation. Under these circumstances, platinum group metal-free (PGM-free) ORR catalysts have received growing attention in recent years as a possible replacement for Pt-based formulations. The progress achieved since the development of the first nature-inspired electrocatalysts of oxygen reduction in the seminal work by Jasinski in the 1960s (Nature 201, 1212, 1964), which mostly happened through the broad implementation of the high-temperature synthesis approach, makes replacement of Pt in ORR electrocatalysts with earth-abundant elements, such as Fe, Co, N, and C, a realistic possibility. In this this presentation, we will summarize recent progress in research targeting development of high-performance PGM-free catalysts for oxygen reduction reaction (ORR) at Los Alamos National Laboratory. Two approaches will be discussed in a greater detail: (i) the approach involving fine-tuning of the catalyst porosity and surface area using pore-forming compounds and (ii) the method specifically focusing on the development of atomically dispersed transition metal moieties and avoiding the formation of transition metal-rich nanoparticles during the heat treatment of catalyst precursors. We will demonstrate the impact that porosity/surface area optimization, through the use of either pore formers (cyanamide, ZnCl2) precursor templating (metal organic frameworks) or both, can have on the fuel cell performance of PGM-free catalysts. We will also show how modifications to the electrode structure through the change in the ionomer content and ionomer equivalent weight can lead to substantial improvements in the fuel cell performance at both low- and high current densities (aerial power density of more than 0.50 W/cm2 in H2-air testing at 80°C). Finally, we will recapitulate the challenges still facing PGM-free research that, in spite of all the progress achieved in recent years, is yet to produce materials capable of competing with the incumbent Pt-based catalysts in terms of oxygen reduction activity, performance durability, and cost (specifically, when extended to the overall cost of a fuel cell stack). Acknowledgement Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged.

Zirconium Oxynitride-Catalyzed Oxygen Reduction Reaction at Polymer Electrolyte Fuel Cell Cathodes.

Most nonplatinum group metal (non-PGM) catalysts for polymer electrolyte fuel cell cathodes have so far been limited to iron(cobalt)/nitrogen/carbon [Fe(Co)/N/C] composites owing to their high activity in both half-cell and single-cell cathode processes. Group IV and V metal oxides, another class of non-PGM catalysts, are stable in acidic media; however, their activities have been mostly evaluated for half-cells, with no single-cell performances comparable to those of Fe/N/C composites reported to date. Herein, we report successful syntheses of zirconium oxynitride catalysts on multiwalled carbon nanotubes, which show the highest oxygen reduction reaction activity among oxide-based catalysts. The single-cell performance of these catalysts reached 10 mA cm–2 at 0.9 V, being comparable to that of state-of-the-art Fe/N/C catalysts. This new record opens up a new pathway for reaching the year 2020 target set by the U.S. Department of Energy, that is, 44 mA cm–2 at 0.9 V.

Durability of Unsupported Pt-Ni Aerogels in PEFC Cathodes

The commercial success of polymer electrolyte fuel cells (PEFCs) depends on the development of Pt-based oxygen reduction reaction (ORR) catalysts with greater activity and stability to reduce the amount of expensive noble metal per device. To advance toward this goal, we have tested a novel class of unsupported bimetallic alloy catalysts (aerogels) as the cathode material in PEFCs under two accelerated stress test conditions and compared it to a state-of-the-art carbon-supported benchmark (Pt/C). The investigated Pt3Ni aerogel shows little degradation under high potential conditions (> 1.0 V) which can occur during fuel starvation and start-up/shut-down of the cell. If tested under the same conditions, the Pt/C benchmark displays significant losses of electrochemical surface area and ORR activity due to carbon support corrosion as observed in cross section and transmission electron microscopy analysis. When testing the durability upon extended load cycling (0.6–1.0 V), Pt3Ni aerogel demonstrates less stability than Pt/C which is related to the severe Ni leaching from the alloy under such conditions. These findings highlight the advantages of using unsupported ORR catalysts in PEFCs and point to the reduction of non-noble metal dissolution as the next development step.