Durability Of Polymer Electrolyte Fuel Cells Research Articles

Ionomer Binder-Dependent Degradation Behavior of Cathode Catalyst Layers in PEFCs

Recently, short-side chain (SSC) ionomer binders have been reported to improve the initial performance of polymer electrolyte fuel cells (PEFCs) by giving more uniform ionomer coats on the catalyst surface, particularly in a highly graphitized carbon support, compared with the long-side chain (LSC) ionomer binders in catalyst layers. The uniform coating property of the SSC ionomer binder improves the performance of the PEFCs by increasing the proton conductivity of the catalyst layer, particularly under low humidification conditions. The study on the SSC ionomer binder has been actively carried out from the perspective of the improvement of initial performance of PEFCs. However, the study on the SSC ionomer binder from the perspective of the durability of PEFCs has hardly been performed. In this study, the effect of structural characteristics of ionomer binders on the durability of PEFCs was investigated. In particular, the degradation behavior of different ionomer-based catalyst layers was observed with different humidification conditions. The cathode catalyst layers were fabricated by using three types of ionomer binders with the side chain length and ion exchange capacity (IEC) value as variables. The durability behavior of the PEFC MEA was studied using the catalyst and catalyst support accelerated degradation test (ADT) protocols with different humidification conditions. The catalyst used for the preparation of the MEA was Pt/C (TEC10E50E) and the ionomer binder was LSC (IEC 1.00), SCC (IEC 1.02, 1.39). The ADT was conducted according to the DOE protocol (catalyst ADT: between 0.6 and 1.0 V, 50 mV/s, catalyst support ADT: between 1.0 and 1.5 V, 500 mV/s). The degradation phenomena of the cathode catalyst layer were measured by electrochemical analysis. The change of the electrochemically active surface area (ECSA) of Pt and that of the proton conductivity of the catalyst layer were measured. The corrosion of the catalyst support was measured by measuring the catalyst layer thickness change by SEM. The detailed analysis of degradation behavior of MEAs depending on the ionomer binders and RH conditions will be discussed.

(Invited) Experimental and Physically Based Modelling Analysis of Electrochemical Impedance in PEMFC to Interpret Performance Degradation Causes

The required target for mass deployment of PEMFC in different applications is still not fully achieved, because of a severe performance decay, determined by the interaction of several degradation mechanisms [1]. The comprehension of such mechanisms and the influence of operating condition stressors is still not satisfactory to accurately predict fuel cell lifetime during real operation. Especially the components structural and functional alteration in time, observed by ex-situ techniques, can be hardly associated to a distinct contribution to performance loss. Electrochemical impedance spectroscopy is a promising in-situ technique to distinguish the contributions of different degradation phenomena, but nowadays data analysis is widely performed modelling the fuel cell through simplified electrical circuits that do not permit to achieve a solid physical interpretation of performance degradation causes. A physically based modelling approach, recently explored in the literature [2,3], can cover this gap, however it is not fully consolidated yet and the required effort to develop appropriate tools limits today its utilization to modelling experts. The present work aims at presenting the effectiveness of an approach that combines experimental and physically based modelling analysis of electrochemical impedance in investigating performance degradation occurring in different technologies [4-7]: HT-PEMFC, Low-Pt PEMFC, DMFC, PGM-free PEMFC. The general methodology is presented, highlighting the possible onset of different regimes, where distinct phenomena prevail. The continuum model based on macro-homogenous assumption, solves mass and ionic conservation coupled with Stefan-Maxwell diffusion and Ohm’s law; electrochemical reactions are described with Butler-Volmer kinetics. EIS is solved according to the approach reported in [5]. Subsequently some relevant cases are discussed in details, emphasizing the influence of heterogeneous degradation [5] and local mass transport [7]. One analysis reported in this work is the effect of heterogeneity of ageing on EIS, which determine an increase of the cathodic charge transfer resistance that is not ascribed to a loss of electrochemical active surface, but to the uneven distribution of the reaction rate. Another analysis reported in this work consists in the simulation of EIS with commercial CFD codes: the effect of 3D geometrical features was analyzed, e.g bends in the flow field, flow bypass between channels, interdigitated flow pattern. [1] Boroup, R., & al. (2007). Scientific aspects of Polymer Electrolyte Fuel Cell durability and degradation. Chemical Reviews, 107, 3904-3951. [2] Baricci, A., Zago, M., & Casalegno, A. (2014). A quasi 2D model of a High Temperature Polymer Fuel Cell for the interpretation of impedance spectra. Fuel Cells, 14, 926-937. [3] Kulikovsky, A.A., (2012). A physical model for catalyst layer impedance. Journal of Electroanalytical Chemistry, 669, 28-34 [4] Bresciani, F., Casalegno, a., Zago, M., & Marchesi, R. (2013). A parametric analysis on DMFC anode degradation. Fuel Cells, 14, 386–394. [5] Baricci, A., Zago, M., & Casalegno, A. (2016). Modelling analysis of heterogeneity of ageing in high temperature polymer electrolyte fuel cells: insight into the evolution of electrochemical impedance spectra. Electrochimica Acta, 222, 596–607. [6] Casalegno, A., Baricci, A., Bisello, A., Odgaard, M., Serov, A., & Atanassov, P. (2017). Insight into degradation mechanism in non-precious metal composite catalysts for PEMFC. 7th FDFC, Stuttgart. [7] Baricci, A., Mereu, R., Messaggi, M., Zago, M., Inzoli, F., & Casalegno, A. (2017). Modeling analysis of flow field geometrical features in Polymer Electrolyte Fuel Cells porous media. 7th FDFC, Stuttgart.

Transient Cell Reversal Due to Anode Hydrogen Starvation in PEM Fuel Cells - Experimental and Modeling Study

Due to the significant development work by major automotive OEMs, fuel cell electric vehicles (FCEVs) have become much closer to a commercial reality. While the overall system cost reduction is still much needed to warrant commercial success, further improvement in system robustness could ensure better customer experience and wider market adoption. Certain automotive fuel cell operations, e.g., cold/freeze start-up, idling at low power, start-up after long shut-down periods, can lead to conditions that produce fuel cell damage if not managed properly. Drawing power under such conditions can sometimes lead to hydrogen starvation in certain cells of a stack, which can cause the cell voltage to become negative (i.e., cell reversal), leading to anode carbon corrosion and electrode damage.1 Depending on the nature of the root cause and the system control strategy, the reversal events can last from less than 1 second to a few minutes. Though relatively longer cell reversal (minute to tens of minutes) has been often reported and well understood,2 study on transient cell reversal (<1 second) has been rare. In this work, we use a 53-cm2 single cell to study cell reversal events that last 1 second or less and investigate actual anode damage both experimentally and by modeling. Anode hydrogen starvation was created by maintaining a constant hydrogen flow into the anode, while stepping the current density from 0.02 A/cm2 up to 0.6 A/cm2 for a given time (e.g., 1 second). The hydrogen flow is sufficient for 0.02 A/cm2, but is less than the stoichiometric requirement at 0.6 A/cm2. Fig.1 shows cell voltage and current transient profiles during a cell reversal event. It is seen that cell voltage drops from 0.87V to -0.96V when the current density was increased from 0.02 A/cm2 to 0.6 A/cm2. The anode H2 flow was kept constant at 0.3 stoics at 0.6 A/cm2 current density. Upon the increase of current density, cell voltage drops quickly to a first plateau at ~0.65V (Fig.1), which is believed to be due to polarization of cathode oxygen reduction reaction (ORR), while H2 in the anode compartment can still sustain the current for a very short period of time. After this initial plateau, cell voltage drops precipitously to a second plateau at ~-0.96V, where oxygen evolution reaction (OER) as well as carbon oxidation reaction (COR) are sustaining the reversal current (0.6 A/cm2). Although H2 is still being fed to the anode at a flow rate of 67 sccm, the hydrogen oxidation reaction (HOR) has been deactivated due to an oxidized Pt surface at elevated anode potential, which was corroborated by the H2 concentration measurement in anode exhaust using the high-speed mass spectrometer. Furthermore, due to the fast change in voltage, the capacitive current (due to double-layer capacitance) and pseudo-capacitive current (due to desorption Pt-H and formation of Pt-OH) also contribute to the overall reversal current. Thus, the true damage to anode during transient cell reversal has to be accessed by taking into account of oxidation of available hydrogen, the capacitive and pseudo-capacitive current, as well as the oxygen evolution reaction. A kinetic model was, therefore, developed to describe the transient cell reversal by considering the above kinetic and capacitive current contributions. The model was built based on the conservation of charge which couples with hydrogen transport from flow-field to the anode catalyst layer during the transient process. It adopts the Pt-oxide growth model for the evolution of Pt-oxide coverage,3 and utilizes the previously measured kinetics for the relevant electrode reactions.3-4 Additionally, hydrogen oxidation reaction is made to be impeded at high electrode potentials, corresponding to the experimental observation. The resulting model equations are solved for transient anode potential, Pt-oxide coverage, and contributing reaction currents under a given cell reversal condition. In this presentation, we will discuss the details of both the experimental and modeling work in understanding the contributors to the overall reversal current during a transient cell reversal event. References P. T. Yu, W. Gu, J. Zhang, R. Makaharia, F. T. Wagner, and H. A. Gasteiger, Polymer Electrolyte Fuel Cell Durability, F. N. Buchi, M. Inaba, and T. J. Schmidt, Editors, Springer, New York, NY, 2009. T. R. Ralph, S. Hudson, and D. P. Wilkinson, ECS Trans., 1 (8), 67 (2006). S. Arisetty, Y. Liu, W. Gu, and M. F. Mathias, ECS Trans., 69 (17), 273 (2015). W. Gu, P. T. Yu, R. N. Carter, R. Makharia, and H. A. Gasteiger, Modeling and Diagnostics of Polymer Electrolyte Fuel Cells, Modern Aspects of Electrochemistry 49, U. Pasaogullari, C.-Y. Wang, Editors, Springer, New York, NY, 2010. Figure 1

Chemical and Structural Investigation of Pt-Ni Extended Surface Catalyst Electrodes

Typical carbon-supported platinum (Pt) nanoparticle catalysts suffer from having only moderate specific activities relative to bulk Pt, as well as activity losses caused by various degradation processes. [1] Extended surface Pt catalysts are a promising alternative to the current state-of-the-art catalysts, particularly if high surface areas are achieved. Recently, we have developed Pt-Ni extended surface catalysts with surface areas >90 m2/gmPt and improvements in mass activity, specific activity, and durability. [2] However, as-synthesized materials are susceptible to dissolution of the Ni from the core of the Pt-Ni nanowire (NW) and therefore exhibit serious performance losses due to Ni ions binding to proton sites in the ionomer. Through a series of post-synthesis treatments involving annealing and subsequent acid leaching to optimize material structure and composition, the NW specific activity and durability were significantly improved. [3] Current efforts focus on performance optimization of the catalyst layer in membrane electrode assemblies (MEAs), which in addition to detailed information on the as-synthesized and treated catalysts, also requires investigation of these materials when part of an electrode. Structure, composition, and morphology were determined with initial 2D characterization of the NWs and electrodes using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) via scanning transmission electron microscopy (STEM), and X-ray photoelectron spectroscopy (XPS), as well as synchrotron techniques including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies. Beyond these methods, transmission X-ray microscopy (TXM) allowed for high resolution, non-destructive, element-specific analysis of the electrode structure with 2-D and 3-D visualization. Catalyst composition/structure and electrode fabrication conditions were varied to explore opportunities and limitations of in situ characterization of the electrodes with these low Pt loadings extended surface catalysts. R. Borup, J. Meyers, B. Pivovar, Y.S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J.E. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K.I. Kimijima, N. Iwashita, Scientific aspects of polymer electrolyte fuel cell durability and degradation, Chemical Reviews, 107 (2007) 3904-3951.S.M. Alia, B.A. Larsen, S. Pylypenko, D.A. Cullen, D.R. Diercks, D. R.; K.C. Neyerlin, S.S. Kocha, B.S. Pivovar, Acs Catal 4(2014) 1114-1119.S.M. Alia, C. Ngo, S. Shulda, M.-A. Ha, A.A. Dameron, J.N. Weker, K.C. Neyerlin, S.S. Kocha, S. Pylypenko, B.S. Pivovar, Exceptional Oxygen Reduction Reaction Activity and Durability of Platinum–Nickel Nanowires through Synthesis and Post-Treatment Optimization, ACS Omega, 2 (2017) 1408-1418.

Nanocomposite TiO2-f-MWCNTs as durable support for Pt in polymer electrolyte fuel cells

Durability is a major issue and has been the growing focus of research for the successful commercialization of polymer electrolyte fuel cells (PEFCs). Corrosion of carbon support is one of the major forms of Pt/C degradation and affects cell performance during prolonged operation. In the present study, TiO2 nanoparticles are incorporated in functionalized multi-walled carbon nanotubes (MWCNTs) to form TiO2-f-MWCNT nanocomposite-supported Pt which improves the durability of PEFC. Pt/TiO2-f-MWCNT electrocatalyst with different compositions has been prepared by a colloidal method, and their morphological and microstructural properties were investigated. Optimum ratio of TiO2-f-MWCNT-supported Pt shows improved overall cell performance than that of f-MWCNT-supported Pt. Accelerated stress test (AST) shows Pt/TiO2-f-MWCNT electrocatalyst possesses superior electrochemical activity and long-term stability for oxygen reduction in relation to Pt/f-MWCNT. High activity and durability is observed for TiO2-f-MWCNTs as catalyst support through its interaction with Pt and retains more than 75% of the initial electrochemical activity in PEFCs even after 200 h of AST.

Modeling of Transient Platinum Degradation in a Low Pt-Loading PEFC under Current Cycling

Pt degradation is one of the most important aging mechanisms that control the lifespan of automotive polymer electrolyte fuel cells (PEFCs). The consequence of Pt degradation is loss of the electrochemical active surface area (ECA) in cathode catalyst layers (CCLs) of PEFCs. The reduction of ECA increases not only the activation overpotential through reducing sites of oxygen reduction reaction (ORR) kinetics but also the mass transport loss through causing an additional micro-scale oxygen transport resistance in the ionomer film surrounding Pt particles. In this study, a 1D physics-based Pt degradation submodel is coupled into the transient M2 model to study the non-uniform Pt degradation and its impacts on long-term PEFC performance. The performance loss of a low Pt-loading PEFC with Pt degradation, the interactions of Pt degradation with the micro-scale transport resistance, the cause and consequence of non-uniform Pt degradation, as well as a strategy of raising lower current density in current cycling test are quantified. This Pt degradation model is demonstrated to be an effective approach to better understand Pt degradation, performance loss caused by Pt degradation, and mitigation strategies to alleviate Pt degradation, all important for achieving excellent durability of PEFCs.

RRDE Analysis of Ethylene Glycol and Caprolactam Effects on the ORR

Polymer electrolyte fuel cells (PEFCs) are very promising as clean energy sources for future automotive applications. However, the performance and durability of PEFCs can be significantly decreased by the presence of contaminants. Such species may adsorb on the Pt catalyst, penetrate into the ionomer and membrane, or compete with the main reactions, resulting in a decrease of the active surface area, ionic conductivity of the catalyst layer and membrane, and cell performance. Also, H2O2 is a side product of the oxygen reduction reaction (ORR) and its associated yield often increases in the presence of organic contaminants, such as acetonitrile, propene, naphthalene, and methyl methacrylate [1-3]. A larger H2O2 generation rate exerts an undetermined influence on the performance and durability of PEFCs. The formation of H2O2 has been linked to the radical attack decomposition of the Nafion® ionomer, which is accelerated by the presence of Fe2+ and Cu2+ ions [4,5]. The Nafion® ionomer is not only used as an electrolyte membrane, but is also added to the catalyst layer to exploit its binding property and improve catalyst utilization. Consequently, the decomposition of the Nafion® ionomer is a significant risk to PEFC performance and durability. Therefore, it is necessary to investigate the ORR, hydrogen oxidation reaction (HOR), and H2O2 yield in the presence of contaminants to understand their effects on PEFCs. Ethylene glycol (EG) is of interest because it is widely used as a coolant, antifreeze agent, and de-icing solution. De-icing of airport runways and airplanes is the primary source of EG in the environment. EG is also dispersed in the environment by the disposal of products that contain it. Caprolactam is another potential contaminant released by PEFC system materials either as a result of degradation or leaching. It has been detected in many leachates including from polyphthalamide materials that are being considered for use as balance-of-plant structural materials [6,7]. Both EG and caprolactam can poison ORR Pt/C catalysts [8-10], but the H2O2 yield has not been measured. In contrast to the slow ORR kinetics, the HOR kinetics on Pt/C catalysts is so fast that the cell voltage losses are negligible even for very low Pt loadings. However, attention has not been given to the poisoning effects of EG and caprolactam on the HOR. These considerations are important for prevention because tolerance levels are currently missing. The rotating ring/disk electrode (RRDE) is the equipment of choice to reveal the H2O2 yield and reaction pathway. In this presentation, the contaminant impacts of EG and caprolactam on the kinetics of ORR and HOR in acid media using RRDE are reported. Results will emphasize the changes in the electrochemical surface area, ORR and HOR kinetic currents, Tafel slope, H2O2 yield,and reaction pathway as a function of EG and caprolactam concentration. Acknowledgments Authors are grateful to the Office of Naval Research (award N00014-13-1-0463) and the Hawaiian Electric Company for their ongoing support to the operations of the Hawaii Sustainable Energy Research Facility.

Preparation of Ultrathin Palladium Nanosheet and Its Application in Pd@Pt Core-Shell Catalyst for Oxygen Reduction Reaction

High cost, sluggish kinetics, and poor durability of Pt catalysts predominantly hinder the wide-spread commercialization of fuel cells. Pt-based core-shell nanosheet catalyst may resolve these problems listed above because of the following 3 advantages. 1) The Pt content can be greatly reduced via core−shell nanostructures consisting of a Pt shell on appropriate monometallic or alloy cores; 2) Nanosheets have high surface-to-volume ratio and terrace sites. The active sites for the oxygen reduction reaction (ORR) have been suggested to be located on the terrace sites of the nanocrystals.[1] 3) Nanosheets should dramatically reduce Pt dissolution due to few edges and corners with their low coordinate sites. Terraced facets have been reported to be more stable than edges and corners.[2] Here, we present a novel method to synthesize palladium (Pd) nanosheets by a wet-chemical preparation at room temperature (Fig.A). The Pd precursor (Pd(acac)2) is reduced by CO in the presence of decylamine (DA). The protection agent (DA) was easily removed by washing with acetic acid. Using this nanostructure as a core, Pd@Pt core-shell nanosheets were prepared by surface limited redox replacement (SLRR). AFM indicates Pd nanosheets are synthesized with a thickness around 0.9 nm to 1.6 nm and lateral size from tens to several hundred nanometers (Fig.B). After dispersing the Pd nanosheet on a carbon support, Pd@Pt core-shell nanosheets with different Pt shell thickness were prepared, i.e. Pd6.0ML@Pt2.7ML, Pd6.0ML@Pt4.4ML, and Pd6.0ML@Pt5.9ML. The electrochemically active surface area (ECSA) of Pd@Pt nanosheets are 56, 48, 60 m2/g-PGM (90, 66, 77 m2/g-Pt), respectively (Fig. C). The linear sweep voltammetry is applied by using a rotating disc electrode (RDE) in 0.1 M HClO4 at room temperature. The ORR activity is evaluated from jk values at 0.9 V (vs RHE) estimated from Koutecky-Levich Plots. The Pd@Pt nanosheets demonstrated mass activity of 364, 685, 598 A/g-Pt (227, 501, and 492 A/g-PGM), which is much higher than the benchmark Pt/C catalyst (177 A/g-Pt) for the oxygen reduction (Fig. D). This research was supported in part by the “Polymer Electrolyte Fuel Cell Program” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Fig. A) TEM of Pd@DA nanosheets; B) AFM image of Pd nanosheets; C) CV profiles of the Pd@Pt nanosheets recorded in N2-saturated 0.1 M HClO4 solution at a sweep rate of 50 mV/s; and D) The mass activity of Pd@Pt nanosheets and commercial Pt/C. Reference F. J. Perez-Alonso, D. N. McCarthy, A. Nierhoff, P. Hernandez-Fernandez, C. Strebel, I. E. L. Stephens, J. H. Nielsen, I. Chorkendorff, Angew. Chem. Int. Ed. 2012, 51, 4641–4643.F. N. Büchi, M. Inaba, T. J. Schmidt, Polymer Electrolyte Fuel Cell Durability, Springer Science+ Business Media, New York, 2009. Figure 1

A Rapid Ex-Situ alternative for Mechanical Accelerated Stress Testing of Fuel Cells

The durability of polymer electrolyte fuel cells (PEFCs) is one of the main challenges facing the commercialization of automotive fuel cells. The lifetime of the membrane, a critical structural component of PEFCs, is one of the principal obstacles in achieving the fuel cell industry durability targets [1]. Under dynamic automotive operating conditions and duty cycles, the membrane is subjected to chemical and mechanical degradation, which could cause hydrogen leaks and ultimate cell failure. Chemical degradation is linked to the polymer molecular decomposition caused by radical species formed during the fuel cell operation as by-products of electrochemical reactions [2]. On the other hand, mechanical degradation is attributed to the fracture caused by the induced mechanical and hygrothermal stresses in a constrained cell [2]. The US Department of Energy (DOE) introduced standardized in-situ accelerated stress test (AST) protocols [3]. Following the DOE mechanical AST protocol, the membrane mechanical durability under pure humidity cycling was investigated [4]. In this manner, two customized in-situ mechanical AST protocols were utilized by our group to evaluate the mechanical durability of PEFCs, indicating significant decay in mechanical properties, formation of microstructural cracks, and initiation of failure [5]. Despite the valuable outputs of the in-situ studies, the current protocols are time consuming and costly. As hygrothermal fatigue is expected to dominate the membrane mechanical lifetime, an ex-situ mechanical fatigue-creep based AST was recently developed by our group as a more convenient alternative [6]. The proposed ex-situ tensile fatigue-creep test demonstrated in this work is intended to evaluate the mechanical durability of catalyst coated membranes (CCMs) in a fraction of the time required for the conventional in-situ tests [7]. The proposed ex-situ tensile fatigue-creep accelerated stress test (TFC-AST) was conducted on dog bone shaped [8] CCM samples using a dynamic mechanical analyzer (DMA) equipped with environmental chamber. After equilibration at 80°C and 50% RH, as illustrated in Figure 1, a high frequency sinusoidal cyclic tensile load with stress ratio (R) of 0.2 and 6.1 MPa mean stress was applied on the CCM samples to certain fractions of fatigue lifetime (~140,000 cycles) [7]. When compared to the well-defined standard in-situ mechanical ASTs which last about 8 weeks [5], the proposed TFC-AST results in mechanical failure ~400 times faster than the in-situ tests due to higher frequency and magnitude of fatigue and creep loading. Depending on the total lifetime, TFC-ASTs were interrupted at different fractions of the CCM lifetime, i.e. 20%, 40%, 60%, and 80%, and partially fatigued samples were extracted for analysis. The obtained partially degraded CCMs were further studied through mechanical and microstructural techniques. The mechanical properties of the extracted CCMs were investigated via tensile and hygrothermal expansion experiments in the same manner as reported in [9] using DMA. Tensile tests revealed remarkable increase in the tensile strength of the partially degraded samples indicating the alignment of the polymer molecules along the TFC stress direction. CCM thermal and hygral expansions were evaluated by stepwise increase in temperature and relative humidity, respectively. Interestingly, the CCM was found to contract, which is contradictory to the typical hygrothermal expansion behaviour of these materials [5,9]. This behaviour can be attributed to the exclusively tensile loading of the TFC-AST. In addition to the mechanical testing, morphological evolution of the TFC-AST degraded CCMs was also examined and compared with the analogous in-situ mechanical AST degraded CCMs using transmission electron microscopy (TEM). The TEM micrographs provided supplementary evidence regarding the reorientation of membrane molecules along the TFC stress direction. However, the mechanical failure of the specimens was found to be dominated by fatigue, similarly to the corresponding in-situ tests. Figure 1. Schematic of the proposed tensile fatigue-creep accelerated stress test (TFC-AST) protocol for rapid mechanical durability testing of fuel cells. Acknowledgements: This research was supported by Ballard Power Systems and the Natural Sciences and Engineering Research Council of Canada through an Automotive Partnership Canada grant.

Sb or Nb Doped Tin Dioxide Aerogels Based PEFC Cathode

Catalyst supports for Polymer Electrolyte Fuel Cells (PEFC) are currently made up of carbon blacks. This material is however not thermodynamically stable in fuel cell operating conditions and loss of performance is observed with time, especially at the cathode side. To improve PEFC durability and make this technology a credible alternative to conventional power sources, carbon free cathodes were prepared. With a remarkable morphology, aerogels have already proven their ability to efficiently support catalysts for PEFC application [1, 2]. In this study doped tin dioxide aerogels are proposed as alternative support presumably stable in PEFC operating conditions.Antimony and niobium doped tin dioxide aerogels were synthetized using sol-gel route in acidic media from alkoxide precursors. These materials have shown particularly adapted physico-chemical properties [3]. Platinum catalyst supported on doped SnO2 aerogels was prepared by two methods. Method A was based on the impregnation of a platinum salt followed by a reduction under UV and a heat treatment in oxidative or reducing atmosphere. Method EG is a conventional polyol method using ethylene glycol. Electrocatalysts structures and morphologies were investigated by X-ray diffraction and transmission electron spectroscopy. Active Electrochemical Surface Areas (ECSA) and catalytic activities for oxygen reduction reaction (ORR) were measured on Rotating Disk Electrode (RDE). Method A leads to the formation of particularly well dispersed Pt nanoparticles on aerogel surface (Figure 1), whereas filament form was observed after the use of Method EG. Heat treatments have shown direct influence on Pt structure and crystallinity. Highest ECSA was recorded after method A (45 m². mgPt -1) while highest ORR mass activity was measured after method EG (40 mA. mgPt -1). This value is even higher than that of the chosen carbon based electrocatalyst reference, TEC10E40E, measured in the same conditions (23.4 mA. mgPt -1). Half-cell and MEA singlecell test measurements will also be presented to complete the characterization panel. The work presented here is funded by the European Union's Seventh Framework Program for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n325239 (FCH-JU project Nano-CAT) and the French National Research Agency PROGELEC programme, (ANR-12-PRGE-007 project SURICAT). It was supported by Capenergies and Tenerrdis. Figure 1. TEM image of a Pt/Sb doped (10 at%) SnO2 aerogel from method A REFERENCES 1. M. Ouattara-Brigaudet, , S. Berthon-Fabry, C. Beauger, M. Chatenet, N. Job, M. Sennour, Influence of the carbon texture of platinum/carbon aerogel electrocatalysts on their behavior in a proton exchange membrane fuel cell cathode. International Journal of Hydrogen Energy37, 9742-9757 (2012). 2. 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 Cells11, 726-734 (2011). 3. G. Ozouf, C. beauger, Niobium- and antimony-doped tin dioxide aerogels as new catalyst supports for PEM fuel cells, Journal of Materials Science 51, 11, 5305-5320, DOI 10.1007/s10853-016-9833-7 (2016) Figure 1

Electrochemical Fabrication of Gold Nanoparticle Catalyst on CNTs for Glucose Fuel Cells

Introduction Polymer electrolyte fuel cells (PEFCs) operate at low temperatures and have fast startup times, making them suitable for automobile and household power applications. However, due to their low operating temperature, PEFCs require a catalyst. In general, platinum is used as the catalyst and carbon black (CB) is used as a support. Carbon nanotubes (CNTs) have recently attracted attention as an alternative to CB due to their large specific surface area, high mechanical strength, and high electrical conductivity, which collectively would improve the durability of PEFCs. In recent years, glucose fuel cells have been attracting attention. When the diameter of gold nanoparticles is less than 10 nm, they have been reported to be effective as catalysts.1)In this study, the fabrication process for a new gold catalyst supported by CNTs as an anode for glucose fuel cells was examined. Experimental (1) Fabrication of Ni/CNT composite films 2) A Watts nickel plating bath (NiSO4·6H2O, NiCl2·6H2O, H3BO3), polyacrylic acid (MW = 5000), brightener (saccharin Na, 1.4-butynediol) and vapor grown carbon fibers (VGCFs) were used as the basic plating bath, CNT dispersant, and CNTs (Showa Denko). Electrodeposition was carried out under galvanostatic conditions. Reverse electrolysis was carried out to removing Ni deposite on the CNTs. The surface morphologies of the films were observed by field-emission scanning electron microscopy (FE-SEM). (2) Reforming of the CNT surface by plasma treatment Plasma treatment with argon was carried out for 5 min at 10, 50, 100, and 150 W on the Ni/CNT composite plating films. The surface morphologies of the plasma-treated samples were observed by FE-SEM, and the CNT defects were evaluated by Raman spectroscopy. (3) Formation of Au nanoparticles A gold sulfite bath (NaAuCl4·2H2O, Na2SO3·7H2O) was used as the plating bath. Electrodeposition of gold on the Ni/CNT films was conducted under galvanostatic conditions. The surface morphology of the plating was observed using FE-SEM and scanning transmission electron microscopy (STEM). Results and discussion (1) Fabrication of Ni/CNT composite films Ni/CNT composite films with smoother surface morphology were formed than without the addition of brightener (Fig. 1(a) and (b)). The reverse electrolysis process was effective at removing Ni deposits on the CNTs (Fig. 1(c)). The reason is thought to be due to the effect of the brightener. Fig. 2 shows a plot of the ratio of drain to gate voltages, ID/IG, vs. the power used during the plasma treatment. ID/IGincreased with increasing power, indicating that the CNT defects increased with increasing power. (2) Reforming of the CNT surface by plasma treatment Fig. 2 shows a plot of the ratio of drain to gate voltages, ID/IG, vs. the power used during the plasma treatment. ID/IGincreased with increasing power, indicating that the CNT defects increased with increasing power. (3) Formation of Au nanoparticles Compared to non-treated CNTs, smaller gold nanoparticles with sizes of about 10 nm were observed on the plasma-treated CNTs. References 1) M. Pasta, L.Hu, F. Mantia, Y. Cui, Electrochemistry Communications, 1981-84 (2012). 2) S. Arai, A. Fujimori, M. Murai, M. Endo, Materials Letters, 62 35-45 (2008). Figure 1

Structure/property relationship of Nafion XL composite membranes

Perfluorosulfonic-acid (PFSA) ionomer membranes (most commonly Nafion®) are currently the prototypical proton-exchange-membrane in polymer-electrolyte fuel cells (PEFCs), for which durability still represents a technical barrier to their commercialization. In an effort to address the durability demands, PFSA membranes with reinforcement and/or stabilizers have become of great interest as they have demonstrated superior durability in PEFCs compared to their unreinforced analogues. One such particular membrane that is tailored for enhanced durability and commonly employed in PEFCs is Nafion XL, a Nafion-based ionomer membrane with mechanical reinforcement and chemical stabilizers. Despite an increasing number of recent studies demonstrating its improved lifetime in accelerated stress testing (AST), its structure and transport properties have not been investigated in a systematic fashion. In this paper, we report water uptake, dimensional change, conductivity, and mechanical properties of Nafion XL membrane, as well as its strong anisotropy, in comparison to (unreinforced) Nafion 212 membrane. Moreover, water-domain spacing and crystallinity of Nafion XL membrane, determined from small- and wide-angle X-ray scattering (SAXS/WAXS) experiments, are correlated with the measured properties to establish a structure/property relationship, and discussed within the context of composite materials. It is also found that (pre)conditioning of the membrane by heating in water at different temperatures could have significant impacts on its structure/property relationship, in particular, the mechanical stability and conductivity, and their anisotropy, which were related to morphological changes observed from microscopy studies. The findings reported here not only provide a new dataset that can be used for PEFC performance and durability modeling but also benefit the efforts on developing composite ion-conductive membranes.

The Impact of Dynamic Anode Hydrogen Starvation on Proton Exchange Membrane Fuel Cell Performance

Due to high efficiency and environmentally friendly benefits, proton exchange membrane fuel cell (PEMFC) vehicles have widely received attention by automotive manufacturers. With many years of research and development, fuel cell technologies have gradually grown to a point of market introduction. However, cost and robustness are two major challenges that need to be overcome to enable commercial viability of fuel cells in automotive applications. Simplifying the controls hardware, algorithms and software through robust cell designs can reduce the cost and overall complexity of the system. There are a few operating stages1 within a fuel cell system such as start/stop2, anode/cathode starvation, and local anode starvation3 that need careful consideration in developing the controls systems. GM has developed a series of screening protocols to assess how these events impact the electrode performance. Anode dynamic hydrogen starvation can happen when the requested load is higher than the amount of hydrogen being fed to a single or a few cells in the stack. In this work, we have performed a detailed study to simulate these dynamic starvation events in a single cell and investigate the effects of such events on PEM fuel cell performance. Single cells built with 53-cm2 catalyst coated diffusion media (CCDM) membrane electrode assemblies (MEAs) equipped with a high-speed mass spectrometer (HSMS) on the anode side were employed for investigating the cell performance with gas evolution before, during, and after a short dynamic anode hydrogen starvation. Two levels of anode loadings (0.5X and 1X) were screened for their robustness against the undesired anode hydrogen starvation events. Figure 1 shows cell voltage and gas evolution profiles during anode hydrogen starvation from 0.2 A/cm2 up to 0.6 A/cm2 within 90 seconds at 80 °C. At 0.2 A/cm2, the cell voltage was ~0.8V and the gas composition was N2 and unreacted H2. As the current density went up to 0.6 A/cm2, the H2 flow rate is insufficient for the current demanded, and the cell voltage quickly dropped to ~-0.8- -1.0V, which initiated oxygen evolution reaction (OER) followed by carbon oxidation reaction (COR) as the cell voltage kept decreasing. The current density was switched back to 0.2 A/cm2 when the cell voltage hit to -1.5 V. to prevent excessive / uncontrolled damage to the cell in a single event. Three events are shown in Figure 1 to observe the changes of OER and COR on the anode electrode. The peak areas of O2 and CO2 from gas evolution indicate that the OER deactivated while the COR was sustained to meet the current demand. Twelve events at two levels of anode loading were conducted in this test for evaluating the robustness of the anode and cathode electrodes. In this presentation, we will discuss the test protocol to evaluate the robustness of anode electrode against anode hydrogen starvation. The results show that the anode was damaged with increased starvation events depending on starvation duration time, current demand, electrode loadings, and anode electrode materials. Results indicate that a combination of anode electrode material development and engineering design is the best approach to addressing anode hydrogen starvation. Acknowledgement The authors would like to thank Dr. Manish Sinha for his helpful discussion on anode hydrogen starvation in a PEM fuel cell system.

Online Mass Spectrometric Monitoring of Carbon Corrosion in PEMFC Electrodes Subjected to Accellerated Stress Testing

Polymer electrolyte membrane fuel cells (PEMFCs) are considered to be almost market-ready for automotive applications. The main obstacles which decelerate a broad application arise from the high costs for the production of membrane electrode assemblies (MEAs). Another aspect, which is related to operation costs of a fuel cell system, is its lifetime. The lifetime of PEMFCs especially in automotive applications, which include alternating load conditions and frequent start-stop-cycles, is still too short to make them more attractive vehicle power sources than combustion engines. Regarding the lifetime of PEMFCs the US DoE set the durability target of 5000 h by 2017 (with less than 10% loss of initial performance) for automotive applications [1]. Carbon (C) support corrosion was identified as one main contributing mechanism to the degradation of PEMFCs i.e. membrane electrode assemblies (MEAs) [2]. It is known that C support corrosion is dominantly an issue of the fuel cell cathode. It leads to (i) decrease of electrochemically active surface area and (ii) damage of porous electrode structure causing a deterioration of mass transport [2]. Both named phenomena contribute to a decrease of reaction rate and hence fuel cell performance. Beside the applied support material fuel cell operating conditions have a significant influence on C support corrosion rate e.g. high cathode potential conditions during fuel cell start/stop accelerate C support corrosion rates tremendously compared usual equilibrium operating conditions [2-3]. There is still a lack of understanding on how different operating conditions affect the C support corrosion rate. For this reason carbon corrosion rates of PEMFC cathodes were measured by means of inline mass spectrometry. The method is based on a measurement of the release i.e. generation rate of CO2 at the cathode fuel cell exhaust. A correlation to the C corrosion rate is possible since CO2 is known to be final product of the carbon corrosion reaction. The C corrosion rate of fuel cell cathodes was investigated for different conditions. Here, the investigations were focused on cathode potential. On one hand, corrosion rates for cathode potentials corresponding to usual operating potentials were probed and compared to each other. On the other hand, accelerated stress tests (ASTs) [4-5] involving potential cycling at unusually high cathode potentials were applied in order to simulate fuel cell start/stop conditions. It was found that the C corrosion rate during the simulated start/stop conditions is roughly 30-60 times higher compared to usual fuel cell operating conditions. Another focus was put on a separate measurement of C corrosion rate in the catalyst layer (CL) and gas diffusion layer (GDL). From the results of these investigations it can be concluded that the applied AST conditions dominantly accelerate carbon corrosion rates in the CL and hardly in the GDL. [1] US DOE, Multi-Year Research, Development and Demonstration Plan (2012) [2] P.T. Yu et. al, Carbon-Support Requirements for Highly Durable Fuel Cell Operation in F.N. Büchi et. al, Polymer Electrolyte Fuel Cell Durability, Springer (2009). [3] X.-Z. Yuan et. al, A review of polymer electrolyte membrane fuel cell durability test protocols, Journal of Power Sources. 196 (2011), 9107–9116. [4] Cell Component Accelerated Stress Test and Polarization Curve Protocols for PEM Fuel Cells, http://www.uscar.org/commands/files_download.php?files_id=348 (2013). [5] New Energy and Industrial Technology Development Organization (NEDO) - Development of PEFC Technologies for Commercial Promotion - PEFC Evaluation Project - Cell Evaluation and Analysis Protocol Guideline (2014). Figure 1

Prediction Method for PEMFC Cathode Kinetic Losses Induced By Contaminants

Proton exchange membrane fuel cells (PEMFCs) are attractive energy conversion devices that are expected to be durable but are exposed to a variety of contaminants either present in air and hydrogen or released by fuel cell system materials, which may be deleterious to the cell performance [1-3]. In most if not all cases, the catalyst is affected by the contaminant. Only a small fraction of the large number of likely contaminants has been investigated. For instance, only 21 ambient air species from a list of more than 200 candidates were recently studied [4]. Furthermore, fuel cell contamination tests require significant resources and may last several ten or a few hundred of hours even if accelerated conditions achieved with higher contaminant concentrations are used. As a result, there is a need to develop a simple method to predict the impact of contaminants on fuel cell performance. A prediction method for the performance loss associated with contamination has not been suggested for PEMFCs. However, the use of a molecule dipole moment was proposed to correlate the contaminant effect on the half wave potential of the oxygen reduction reaction [5]. Only 5 contaminants were considered and data were obtained with a thin film catalyst layer electrode mounted in a conventional three-electrode cell. Tests were completed with an aqueous electrolyte solution maintained at room temperature. These operating conditions are quite different than those used to operate a PEMFC with a solid electrolyte and the presence of a gas diffusion layer covering the catalyst layer. The adsorption energy has also been used as a predictor of catalytic performance [6]. The PEMFC performance database resulting from separate tests performed with 21 airborne contaminants [4] is re-examined with the purpose of exploring the usefulness of two correlations between the kinetic loss at steady state and a contaminant physico-chemical parameter. The contaminant dipole moment is first considered thus significantly extending the range of the previous study [5] from 5 to 20 species. Data are also correlated with the contaminant adsorption energy on Pt. The database contains impedance spectra acquired before, during and after a temporary contamination period. The use of impedance spectroscopy during the galvanostatic experiments was more economical because only a single experiment was needed to assess the different cell performance losses. The analysis focuses on kinetic cell performance losses because Pt is a common catalyst used for many processes [7]. Therefore, it was expected that a significant amount of information would be available for physico-chemical parameters characterizing interactions between Pt and selected contaminants including the adsorption energy. The extraction of a surrogate parameter from impedance data representing the kinetic losses and resulting correlations with the dipole moment and the contaminant adsorption energy on Pt will be discussed. The adsorption energy correlation is also related to the O2 adsorption energy on Pt. The impact of contaminant chemical and electrochemical reactions on the Pt surface adsorbate coverage is also considered.ACKNOWLEDGMENTS Authors are indebted to the United States Department of Energy for funding (award DE-EE0000467). The authors are grateful to the Hawaiian Electric Company for their ongoing support to the operations of the Hawaii Sustainable Energy Research Facility. REFERENCES 1. J. St-Pierre, in Polymer Electrolyte Fuel Cell Durability, F. N. Büchi, M. Inaba, and T. J. Schmidt, Editors, p. 289, Springer, New York (2009).2. B. M. Besancon, V. Hasanov, R. Imbault-Lastapis, R. Benesch, M. Barrio, and M. J. Mølnvik, Int. J. Hydrogen Energy, 34, 2350 (2009).3. J. St-Pierre, M. Angelo, K. Bethune, J. Ge, S. Higgins, T. Reshetenko,M. Virji, and Y. Zhai, Electrochem. Soc. Trans., 61 (23), 1 (2014).4. J. St-Pierre, Y. Zhai, and M. S. Angelo, J. Electrochem. Soc., 161, F280 (2014).5. M. S. El-Deab, F. Kitamura, and T. Ohsaka, J. Electrochem. Soc., 160,F651 (2013).6. M. M. Montemore and J. W. Medlin, Catal. Sci. Technol., 4, 3748 (2014). 7. B. C. Gates, Chem. Rev., 95, 511 (1995).

Long Term Effects of Airborne Contaminants in PEMFC Cathode

Air pollution is a challenge for the commercialization of proton exchange membrane fuel cells (PEMFCs) [1]. Recently, twenty-one species were selected from a list of more than 200 airborne pollutants (US Environmental Protection Agency database) for single cell accelerated tests [2]. Results showed that most contaminants differently affected cell performance. Seven of these twenty-one species were down selected for detailed studies based on quantitative criteria [2]. The seven contaminants are acetonitrile, acetylene, bromomethane, iso-propanol, methyl methacrylate, naphthalene and propene [2]. All seven contaminants can adsorb on the Pt surface, reduce the effective electrochemical active area for the oxygen reduction reaction (ORR), and even impact the ORR pathway and increase the formation of hydrogen peroxide (H2O2) [3] that promotes ionomer decomposition [4]. Acetonitrile also decreases the proton conductivity of the membrane. The larger H2O2 production rate determined with a rotating ring/disc electrode maintained at 30 °C is not expected to be representative of a PEMFC. Therefore, it is currently unclear if long term operation of a PEMFC exposed to a contaminant will lead to faster catalyst layer ionomer and membrane degradation.A long term test with acetonitrile injected in the cathode was conducted. Acetonitrile was selected because it has several impacts on cell performance and promotes the formation of H2O2. Electrochemical impedance spectroscopy (EIS) was intermittently used to obtain information on cell processes with minimal disturbance to the test. Cyclic voltammetry (CV) was applied before and after the long term test to measure the decrease in electrochemically active Pt sites. Transmission electron microscopy (TEM) was employed to analyze Pt particles agglomeration and Pt deposition in the electrolyte membrane. Inductively coupled plasma mass spectroscopy (ICP-MS) was utilized to detect Pt dissolution in exhaust water. Ion chromatography (IC) was exploited to measure the F– emission rate in the exhaust water and identify other ions including NH4 +. Scanning electron microscopy (SEM) was used to investigate thickness changes of catalyst layers and membrane.Figure 1 shows the cell voltage and high frequency resistance responses during the long term acetonitrile contamination experiment. Initially, the cell voltage at 0.4 A cm–2 was 0.667 V and the high frequency resistance (HFR) was ~0.1 Ω cm2. Within the 300 h pre-poisoning period, the cell voltage decreased with a degradation rate of 1.18 x 10–4 V h–1 and the HFR stabilized at 0.088 Ω cm2 at 220 h. During the first 20 h of acetonitrile exposure, the cell voltage dropped by 0.13 V and the HFR increased by 0.021 Ω cm2. Interestingly, the cell performance was stable during the following ~600 h even though the HFR continued its slow increase. The cell voltage eventually decreased by ~0.02 V during the last 400 h of exposure to acetonitrile. For the recovery phase, the cell voltage and HFR were respectively restored to 0.551 V and 0.109 Ω cm2 during the first 20 h. After that period, an accidental interruption occurred leading to a 4 min OCV event that subsequently increased the voltage and decreased the HFR. After that event, the cell voltage resumed its decay with a degradation rate of 1.40 x 10–4 V h–1. At around 425 h of the recovery period, a second interruption occurred resulting in a cell voltage jump of ~0.02 V which was followed by degradation with a similar rate after the first interruption. The long term exposure to acetonitrile appears to mitigate the regular degradation of cell performance; despite the initial performance loss due to acetonitrile adsorption on Pt and the continuing HFR increase attributed to the effect of acetonitrile reaction products on ionomer electrolyte. The OCV operation may also accelerate the cell performance recovery.The presence of long term contaminant effects was ascertained by focusing on Pt catalysts and ionomer electrolyte degradation. The Pt catalysts’ degradation was probed by comparing EIS, CV, ICP-MS and TEM results with control samples. The ionomer degradation was explored by comparing EIS, IC and SEM results with control samples.ACKNOWLEDGMENTSThis work is supported by the Department of Energy (DE-EE0000467). Authors are also grateful to the Hawaiian Electric Company for their ongoing support to the operations of the Hawaii Sustainable Energy Research Facility.REFERENCES1. J. St-Pierre, in Polymer Electrolyte Fuel Cell Durability, F. N. Büchi, M. Inaba, and T. J. Schmidt, Editors, p. 289, Springer (2009).2. J. St-Pierre, M. S. Angelo, and Y. Zhai, J. Electrochem. Soc., 161, F280 (2014).3. J. Ge, J. St-Pierre, and Y. Zhai, Electrochim. Acta, 134, 272 (2014).4. A. Bosnjakovic and S. Schlick, J. Phys. Chem. B, 108, 4332 (2004). Figure 1

Characterization of Pt Catalysts Supported on Ta-SnO2 with Fused Aggregated Network Structure

Polymer electrolyte fuel cells (PEFCs) are attractive power generation systems for electric vehicles and residential co-generation systems. Platinum-based catalysts supported on carbon black (CB) have electronically conductive paths with gas transport paths and are suitable for catalysts for PEFCs. Cathode catalysts reach potentials of over 1.5 V during startup/shutdown, where the carbon undergoes degradation due to its intrinsic thermodynamic instability [1,2]. Such degradation problems should somehow be overcome to achieve long-term durability of PEFCs. SnO2 and TiOx are promising candidates for cathode catalyst supports with long-term durability at high potentials [3,4]. We have recently characterized Pt catalysts supported on Nb-SnO2 (Pt/Nb-SnO2), which had a CB-like fused aggregated network structure and distinctive stability with respect to high potential exposure, by half-cell and membrane electrode assembly (MEA) measurements [5-9]. The ceramic supports with these structures were able to supply both electronically conductive paths and gas diffusion paths in the catalyst layers, resulting in high cathodic activity with durability for the PEFCs. In the present study, we found that Pt supported on Ta-SnO2 (Pt/Ta-SnO2) has even higher electrical conductivity than that of Pt/Nb-SnO2, by half cell and MEA measurements. We synthesized the Pt catalysts supported on the Ta-SnO2 support by the flame combustion method and colloidal method; details of the methods and characterization were described in earlier papers [5-9].The Ta-SnO2 support powders (BET surface 45 m2 g-1; crystallite size 16.7 nm) were determined to be single-phase rutile-type SnO2 from the X-ray diffraction analysis. A transmission electron microscopic (TEM) image (Fig. 1) showed that the support was composed of fused aggregated network structures, and that the Pt nanoparticles (particle sizes 3.1 ± 0.6 nm; interparticle distances 9.4 nm; loaded amount 15.8 wt%) were uniformly dispersed on the support. The electrical conductivity of the Pt/Ta-SnO2 was ca. 4 times higher than that of Pt/Nb-SnO2. The electrochemically active surface area (ECSA) of Pt/Ta-SnO2, which was estimated from the cyclic voltammograms (CVs, Fig. 2), was 66.4 m2 g-1, similar to that of Pt/Nb-SnO2 (65.6 m2 g-1). The kinetically controlled current density and mass activity at 0.85V of Pt/Ta-SnO2 (Fig. 3) were approximately equal to those of Pt/Nb-SnO2 and were higher than that of a commercial Pt/CB catalyst (TEC10E50E, Tanaka Kikinzoku Kogyo Co.). Changes in the ECSA as a function of the logarithm of the number of potential step cycles (0.9-1.3 V, 30 s, Fig. 4) showed that the stability of Pt/Ta-SnO2 was the same as that of Pt/Nb-SnO2 and was superior to those of both a commercial Pt/CB and commercial Pt catalyst supported on graphitized carbon black (Pt/GCB, TEC10EA50E, Tanaka Kikinzoku Kogyo Co.). We also found by MEA evaluation that the ohmic resistance using the Pt/Ta-SnO2 cathode was lower than that using Pt/Nb-SnO2 (Fig. 5), and that the performance using a Pt/Ta-SnO2 cathode was superior to those using either cathodes of Pt/Nb-SnO2 or Pt/CB (Fig. 6). We consider that both the low resistivity and the high Pt utilization of Pt/Ta-SnO2 led to both the lowering of the ohmic resistance and the improvement of catalytic activity in the catalyst layer. We conclude that the Pt/Ta-SnO2catalyst is an attractive candidate for PEFC cathodes for fuel cell vehicles. Acknowledgement This research 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, and the JSPS KAKENHI grant Number B24350093.

Interactions Between Liquid-Water and Gas-Diffusion Layers in Polymer-Electrolyte Fuel Cells

Over the past few decades, a significant amount of research on polymer-electrolyte fuel cells (PEFCs) has been conducted to improve performance and durability while reducing the cost of fuel cell systems. However, the cost associated with the platinum (Pt) catalyst remains a barrier to their commercialization and PEFC durability standards have yet to be established. An effective path toward reducing PEFC cost is making the catalyst layers (CLs) thinner thus reducing expensive Pt content. The limit of thin CLs is high gas-transport resistance and the performance of these CLs is sensitive to the operating temperature due to their inherent low water uptake capacity, which results in higher sensitivity to liquid-water flooding and reduced durability. Therefore, reducing PEFC's cost by decreasing Pt content and improving PEFC's performance and durability by managing liquid-water are still challenging and open topics of research. An overlooked aspect nowadays of PEFC water management is the gas-diffusion layer (GDL). While it is known that GDL's properties can impact performance, typically it is not seen as a critical component. In this work, we present data showing the importance of GDLs in terms of water removal and management while also exploring the interactions between liquid-water and GDL surfaces. The critical interface of GDL and gas-flow-channel in the presence of liquid-water was examined through systematic studies of adhesion forces as a function of water-injection rate for various GDLs of varying thickness. GDL properties (breakthrough pressure and adhesion force) were measured experimentally under a host of test conditions. Specifically, the effects of GDL hydrophobic (PTFE) content, thickness, and water-injection rate were examined to identify trends that may be beneficial to the design of liquid-water management strategies and next-generation GDL materials for PEFCs.

Durability analysis of polymer-coated pristine carbon nanotube-based fuel cell electrocatalysts under non-humidified conditions

A polymer electrolyte fuel cell (PEFC) that shows high durability at elevated temperature and under non-humidified conditions is strongly demanded for the next generation PEFCs. Here we show the importance of using pristine carbon nanotubes (CNTs) as catalyst supports for fuel cell (FC) durability. For that purpose, two different membrane electrode assemblies (MEAs) using pristine CNTs and commercial carbon black (CB) as carbon supports were coated by polybenzimidazole on which platinum (Pt) nanoparticles were deposited. Notably, the CNT-based MEA exhibited ∼4 times higher durability compared to the CB-based MEA. The ex situ monitoring of the fabricated electrocatalysts before and after the durability testing based on transmission electron microscopy and X-ray diffraction measurements revealed higher structural stability of the CNTs compared to the CB in their MEAs. The pristine structure of the CNTs has been revealed to be very important for the high PEFC durability. This is the first report that shows the durability of the pristine CNT-based PEFC at high operating temperature and under non-humidified conditions, which are the targets of the next generation PEFCs with very high performance.

Full-Cell Simulation of Cathodic Degradation in Polymer Electrolyte Fuel Cell System at Start-up, with and without Control Method to Reduce Degradation

Overview Fuel-Cell vehicles (FCVs) powered by Polymer-Electrolyte Fuel Cells (PEFCs) are under development in several car manufacturers for commercial release in near future. Durability of PEFCs is one of most important issues regarding production cost of FCVs. Degradation of membrane-electrode-assembly (MEA) takes place during start-up process and limits life-span of PEFCs [1,2].In this study, full-cell scale numerical investigation of degradation process of at start-up of PEFCs is carried out. Time-dependent behavior of degradation process is calculated, and distribution of amount of carbon corrosion can be predicted. Reduction of degradation by discharging corrosion current can also be simulated. Numerical Modeling A Model for cathodic corrosion is implemented based on our simulation software for PEFCs. An important feature of this software is that macroscopic models are applied for microscopic phenomena in the MEA such as electrochemical reactions and proton/water transport. These models are coupled with multi-dimensional heat and mass transport equations, including effective parameters for gas diffusivity and permeability in the GDL and equivalent hydraulic diameter in the flow channel.During start-up process, the fuel-cell is divided into power-source and load region by H2/air front. Carbon corrosion reaction (COR) takes place in cathode at load region. A kinetic model for electrode reactions including both normal power-source reactions and corrosion reactions is implemented, coupled with multi-dimensional heat, mass and species transport equations. Simulation of control method for reduction of degradation can be done by applying time-dependent external current conditions. Results As a performance test, numerical investigation of start-up process of 29.16cm2test cell with single serpentine flow anode/cathode channel was carried out. Anode inflow gas is assumed to be air with 25% humidity, then switched to hydrogen with 25% humidity at 0.05s. Back pressure is assumed to be 1atm, and operation temperature is 353.15K.In order to simulate discharge control, following two external current conditions are considered.(a) Without control. Output current is fixed to be 0.0A.(b) With control. Output current is changed to 0.2A/cm2at 0.1s.Time-dependent behavior of corrosion (bold) and power-source (thin) currents is shown in Fig.1. In case (a), two currents are exactly the same since output current is zero. The corrosion current increases at the beginning of start-up process, and decreases to zero when start-up is completed. Significant reduction of corrosion current can be seen in case (b). Distribution of corrosion current in case (a) at 0.1s and 0.2s is shown in Fig.2. As H2/air front proceeds, the area of corrosion become narrower, while corrosion current become more concentrated. The same distribution in case (b) at 0.2s is shown in Fig.3, which indicates significant reduction of corrosion. Final distribution of the ratio of carbon corrosion (left) in cathode catalyst layer is shown in Fig.4. Here, initial carbon loading is assumed to be 2.0mg/cm2. Degradation concentrates around the outlet of anode channel, because of long period of corrosion and concentration of corrosion current. In case (a), according to the middle of shown range, the estimated ratio of carbon corrosion after 10000 times of start-up is an order of 1-10%. This corrosion can be reduced by a factor of about 1/5 by discharge control. References Y.Ishigami et.al., J.Power.Sources, 196 (2011), 3003-3008J.P.Meyers and R.M.Darling, J.Electrochem.Soc., 153 (2006), A1432-A1442