Fuel Cell Operating Conditions Research Articles

ChemInform Abstract: Platinum Nanocatalysts on Metal Oxide Based Supports for Low Temperature Fuel Cell Applications

AbstractReview: 117 refs.

Synthesis of Nitrogen-doped Niobium Dioxide and its co-catalytic effect towards the electrocatalysis of oxygen reduction on platinum

Carbon materials are commonly used as the support for electrocatalysts due to their high surface area and good electronic conductivity. However, the main issue of carbon support is severe corrosion under fuel cell operation conditions, especially the high potential and oxygen-rich environment at cathodes. Recently, novel support materials such as transition metal oxides present excellent improved corrosion resistance and have been attracting tremendous attention. In this work, nitrogen doped NbO2 (donated as N-NbO2) is synthesized by using a facile and efficient hydrothermal method and then employed as the composite support with XC-72 carbon black, following by loading of Pt nanoparticles to prepare composite catalysts towards oxygen reduction reaction (ORR). The morphology, structure, composition and electrochemical properties were conducted by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and linear sweep voltammetry (LSV). Such Pt/N-NbO2/C catalyst shows excellent catalytic activity, more positive onset potential, higher half-wave potential, and significantly enhanced mass activity (173.44mAmgPt−1) in comparison with Pt/C catalyst (27.22mAmgPt−1), which can be attributed to the doping N atoms and the synergistically tuned electronic structure. Meanwhile, the electrochemical stability of the Pt/N-NbO2/C is desirable in comparison with Pt/C catalyst.

Au/CuO–CeO2 catalyst for preferential oxidation of CO in hydrogen-rich stream: Effect of CuO content

A series of Au/CuO–CeO2 with various amounts of CuO was prepared. CuO–CeO2 was prepared by co-precipitation and Au was loaded by deposition–precipitation. The aim of this study was to develop a catalyst which is highly active and selective in preferential oxidation of CO in hydrogen stream (PROX). These catalysts were characterized by N2-sorption, XRD, TEM, HR-TEM, and XPS. The PROX reaction was carried out in a fixed bed continuous flow reactor with a feed of CO: O2: H2: He = 1.33: 1.33: 65.33: 32.01 in volume ratios. The incorporation of copper ion into ceria lattice increased the oxygen storage capacity of ceria and enhanced the activity of the catalyst. The excess CuO would present on the surface of CeO2. Au particles were well dispersed well on the support and the particle size of gold was in the range of 2 and 4 nm. Au was the active species and CuO played the promoter role. Those CuO presented on the surface of CeO had negative effect for CO oxidation. The selectivity of oxygen reacting with CO increased with increasing CuO content. Adding suitable amount of CuO in Au/CeO2 is beneficial for CO conversion and suppressing hydrogen oxidation in fuel cell operation condition.

Highly Durable and Active PtFe Nanocatalyst for Electrochemical Oxygen Reduction Reaction.

Demand on the practical synthetic approach to the high performance electrocatalyst is rapidly increasing for fuel cell commercialization. Here we present a synthesis of highly durable and active intermetallic ordered face-centered tetragonal (fct)-PtFe nanoparticles (NPs) coated with a "dual purpose" N-doped carbon shell. Ordered fct-PtFe NPs with the size of only a few nanometers are obtained by thermal annealing of polydopamine-coated PtFe NPs, and the N-doped carbon shell that is in situ formed from dopamine coating could effectively prevent the coalescence of NPs. This carbon shell also protects the NPs from detachment and agglomeration as well as dissolution throughout the harsh fuel cell operating conditions. By controlling the thickness of the shell below 1 nm, we achieved excellent protection of the NPs as well as high catalytic activity, as the thin carbon shell is highly permeable for the reactant molecules. Our ordered fct-PtFe/C nanocatalyst coated with an N-doped carbon shell shows 11.4 times-higher mass activity and 10.5 times-higher specific activity than commercial Pt/C catalyst. Moreover, we accomplished the long-term stability in membrane electrode assembly (MEA) for 100 h without significant activity loss. From in situ XANES, EDS, and first-principles calculations, we confirmed that an ordered fct-PtFe structure is critical for the long-term stability of our nanocatalyst. This strategy utilizing an N-doped carbon shell for obtaining a small ordered-fct PtFe nanocatalyst as well as protecting the catalyst during fuel cell cycling is expected to open a new simple and effective route for the commercialization of fuel cells.

A molecular simulation study of chemical degradation and mechanical deformation of hydrated Nafion membranes

A combined modeling approach using kinetic Monte Carlo and molecular dynamics simulations is applied to investigate both chemical and mechanical degradation of Nafion membranes on the molecular level. In different hydration conditions, two major degradation reactions are identified to be the main chain unzipping and the side chain scission. The dissolution process of Nafion is evaluated in terms of weight loss, production emission rate and evolution of functional groups. Further, the complicated structural deformation is preliminarily investigated by imposing linear strain on degraded Nafion membrane. Different craze patterns are compared before and after chemical degradation, and the mechanism of crack propagation is proposed. Finally, prospective applications of our modeling approach are addressed for future studies of membrane degradation phenomena under fuel cell operation conditions.

Molecular modeling of the proton density distribution in a water-filled slab-like nanopore bounded by Pt oxide and ionomer

A slab-like pore model consisting of a thin water slab that is confined by a metal wall on one side and a dense ionomer skin layer on the other side was simulated with classical molecular dynamics. The model mimics thin-film structures of Pt/support (referred to as the metal), water layer, and ionomer phase encountered in cathode catalyst layers of polymer electrolyte fuel cells. The equilibrium proton density near the metal surface is the key variable required to predict the ORR activity and the Pt dissolution rate under fuel cell operating conditions. Here, we explored the equilibrium proton density distribution in the confined water layer as a function of oxide coverage at the metal surface, excess surface charge density, water layer thickness, and ionomer film structure. The effect of ionomer hydrophobicity on the water dynamics in the nanopore was also investigated. The electric dipole field created by the oxide layer on the Pt surface interacts strongly with protons, and concentrates protons at the metal surface. A direct relation was found between the surface proton concentration and the oxide layer dipole moment. Performing simulations with different ionomer structures and pore widths clarified the sensitivity of the proton density distribution to properties that are specific for the structure and distribution of the ionomer phase in catalyst layers.

Application of high velocity oxygen fuel flame (HVOF) spraying to fabrication of La0.8Sr0.2Ga0.8Mg0.2O3 electrolyte for solid oxide fuel cells

La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM) is considered a promising electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs) due to its high ionic conductivity and stability under fuel cell operating conditions. Here we report our findings in investigating the feasibility of using a high velocity oxygen fuel flame (HVOF) spraying process for cost-effective fabrication of dense LSGM electrolyte membranes. The flame and in-flight particle behavior were simulated numerically to optimize the microstructure and phase compositions of the LSGM deposits. The measured gas leakage rate of an LSGM deposit is ∼7 × 10−7 cm4gf−1 s−1. The single cell assembled with 50–55 μm HVOF-sprayed LSGM electrolyte shows open circuit voltage (OCV) of 1.08 V at 800 °C, suggesting that the as-sprayed LSGM deposit is dense enough for direct application as SOFC electrolyte. At 800 °C, the ionic conductivity of the sprayed LSGM deposit is ∼0.04 S cm−1, indicating that the HVOF spraying is a promising process for low-temperature fabrication of dense LSGM electrolyte membranes for IT-SOFCs.

Back Cover: Apparent Oxygen Uphill Diffusion in La0.8Sr0.2MnO3 Thin Films upon Cathodic Polarization (ChemElectroChem 10/2015)

The Back Cover picture illustrates oxygen incorporation and diffusion pathways for Sr-doped lanthanum manganite (LSM) electrodes with mixed ionic electronic conductivity. Novel electric bias-isotope exchange experiments under solid oxide fuel cell operation conditions reveal the interplay between grain/grain boundary diffusion, electrode reaction, and Wagner–Hebb-type polarization effects. More details can be found in the Article by J. Fleig et al. on page 1487 in Issue 10, 2015 (DOI: 10.1002/celc.201500167).

Hydrocarbon-based fuel cell membranes: Sulfonated crosslinked poly(1,3-cyclohexadiene) membranes for high temperature polymer electrolyte fuel cells

High temperature fuel cell membranes based on poly(1,3-cyclohexadiene) were prepared by a Polymerization-Crosslinking-Sulfonation (PCS) approach, and a broad range of membrane compositions were achieved using various sulfonating reagents and reaction conditions. Membranes were characterized for their proton conductivity and thermal degradation behavior. Some of the membranes showed up to a 68% increase in proton conductivity as compared to Nafion under the same conditions (100% relative humidity and 120 °C). Thermogravimetric analysis revealed that these membranes are thermally stable up to 200 °C. High proton conductivity and thermal stability, combined with much lower cost as compared to Nafion®, make these materials potentially interesting as fuel cell membranes, although they are chemically vulnerable under fuel cell operation conditions.

Comparative Study of BaY0.1Ce0.9O3-δ Membrane with BaY0.1Zr0.9O3-δ Protective Layers Synthesized with Spray Pyrolysis and Magnetron Sputtering Methods

Acceptor-doped perovskite-type oxides including BaY0.1Ce0.9O3- d (BYCO) are known for excellent proton conductivity under fuel cell operating conditions and applicable in electrolysers, reactors and sensors. However, chemical stability of this oxide is relatively poor. Originating from oxide’s high basicity BaCeO3 reacts easily with H2O and CO2, forming BaCO3, CeO2 and Ba(OH)2. At the same time zirconia based electrolyte, typically with high grain boundary resistance for protons, exhibit good chemical stability and seems to be attractive as chemical barrier layer for ceria based proton conductor. In this study, BaY0.1Zr0.9O3- d (BYZO) protective layers were synthesized onto BaY0.1Ce0.9O3- d substrate using spray pyrolysis and magnetron sputtering methods, and characterized using electrochemical and microstructural analysis methods. Stability tests at carbon CO2containing environment were conducted. Powder for BYCO substrate was prepared by ultrasonic spray pyrolysis method. Substrates were sintered at 1500 °C in powder bed and afterward polished to obtain flat surfaces, which were coated with BYZO protective layers. Dual layer electrolytes obtained were sintered at 1350 °C between BYCO pellets, then characterized with scanning electron microscopy, x-ray diffraction and FIB-TOF-SIMS methods. Electrochemical characterization was carried out at temperature range from 550 to 800 °C. Arrhenius plots with good linearity for four different systems (uncoated polished BYCO, uncoated unpolished BYCO, magnetron sputtered BYZO at BYCO and spray pyrolyzed BZYO at BCYO) were constructed and activation energies Ea were calculated. Lowest activation energy has been found for the polished uncoated substrate (0,35 eV). For the magnetron sputtered and spray pyrolyzed layers Ea values were 0,37 eV and 0,38 eV, respectively. Highest activation energy was observed for unpolished substrate (0,39 eV), suggesting barium ion deficit in the near-surface layer, which can be removed by polishing. Thus, BYZO films deposited increase slightly the activation energy values compared with non-coated membranes. As expected, total ionic conductivity differs greatly from polished to unpolished and from coated to non-coated membrane. Lowest conductivity was characteristic for uncoated polished membranes with the smallest surface area (also the smallest length of three phase boundary between electrolyte, platinum and gas phase) and highest conductivity was characteristic for uncoated unpolished membrane with the highest surface area. BYZO layer synthesized using spray pyrolysis gave worse conductivity compared with BYZO layer synthesized using magnetron sputtering. This is very likely caused by lower homogeneity and higher grain boundary resistance of the layer prepared using spray pyrolysis. In this study it has been demonstrated, that electrical conductivity depends on the microstructure of studied layers as well as on the interfacial structure of solid-gas-Pt boundary of the membrane. Protective BYZO layers significantly affected the time stability of the BYCO membrane in the CO2 containing environment.

PEM Fuel Cell Electrodes Prepared By Photocatalytically Deposited Pt on TiO2 and Successive Electrospinning

For successful commercialization of low temperature proton exchange membrane (PEM) fuel cells, the system should have a long life and a high performance. Further, to make them cost effective a decrease in Pt loading is also deemed necessary. Pt supported on titania nanoparticles have shown to possess high stability. Unlike standard carbon, TiO2 itself doesn’t corrode in fuel cell operation conditions. Also, TiO2 is shown to form stronger bond with Pt owing to ‘strong metal support interaction’ (SMSI) [1], increasing the stability of the system itself. Due to the oxygen spillover effect, titania also acts as a co-catalyst, enhancing the oxygen reduction reaction [2]. Here, Pt/TiO2 is prepared by reducing a Pt precursor photocatalytically under UV illumination and directly on the support [3]. Recently in another development, electrospinning was found as an efficient way to prepare porous electrodes with very low Pt loading [4]. Electrospinning facilitates the preparation of a variety of fiber networks and architectures thus enabling to achieve porous structures which are required to enhance the sluggish oxygen reduction reaction and decrease the platinum loading in a PEM fuel cell. The versatility of electrospun materials not only depends on the precursor, but also on modifying the solution and process parameters, giving a considerable control over the morphology. In this work, these two things are integrated together. First Nafion containing a small amount of PAA is mixed to form the electrospinning solutions with addition of Pt precursor along with titania in different amounts to obtain various loadings. In the presence of a strong source of UV light the Pt precursor is reduced to Pt and deposits itself on the TiO2. Fig. 1 shows the scanning electron micrograph of the 20 wt.% Pt/TiO2 catalyst held on a fibrous PAA/Nafion structure. The catalyst layer is prepared by electrospinning directly onto the GDL. The obtained catalyst layer is hot pressed on Nafion membrane to fabricate membrane electrode assembly, which will be later studied for H2/O2 fuel cell system.

Yttrium and Nickel Co-Doped BaZrO3 as a Proton-Conducting Electrolyte for Intermediate Temperature Solid Oxide Fuel Cells

Conventional solid oxide fuel cells (SOFCs) based on yttria-stabilized zirconia (YSZ) require high operating temperatures in the 800-1000 °C range. Such high operating temperatures hinder the commercialization of SOFCs due to degradation and high cost issues. On the other hand high temperature proton conductors (HTPCs), due to their lower activation energy for proton conduction, can achieve high conductivity at relatively lower temperatures compared to their oxygen ion conducting counterparts. BaZrO3 related HTPC electrolyte materials possess good chemical stability and high bulk conductivity. However, due to its high refractory nature, fabrication of fully dense electrolyte films is very challenging. The poor sinterability of BaZrO3based materials leads to a large number of resistive grain boundaries, thus lowering the total proton conductivity. Previous reports have shown that NiO can function as an effective additive for enhancing the densification of BaZrO3 based materials [1, 2]. These studies did not focus on controlling the stoichiometry so as to accommodate Ni at a specific site in the perovskite structure. Contrary to the previous NiO sintering aid studies, the present work focuses on substitution of Zr with Ni in the perovskite B-site in a targeted fashion. Y-doped BaZrO3 (BZY) is chemically stable at fuel cell operating conditions and shows the best protonic conductivity when Y-content is 20 mol% [3]. In this work, the sinterability of 20 mol% Y-doped BaZrO3 (BZY20) was significantly improved by partial substitution of Zr with Ni in the perovskite B-site. The effect of Ni-doping on the shrinkage behavior of BZY has been identified through dilatometric measurements. The SEM analysis showed a pore-free dense microstructure for 4 mol% Ni-doped BZY (BaZr0.76Y0.2Ni0.04O3-δ, BZYNi04) and therefore electrochemical characterization studies were carried out on this material. The total conductivity of BZYNi04 in wet air was observed to be 0.0037 Scm-1 at 600 °C. The open circuit voltage (OCV) values recorded for BZYNi04 pellet fuel cell were 0.96, 0.99, 1.01, 1.03 and 1.034 V at 700, 650, 600, 550 and 500 °C respectively indicating that there is not much electronic conductivity. Ni-doped BZY electrolyte films were fabricated on an optimized NiO-BZY anode using co-pressing and co-firing process. Subsequently, a full fuel cell was fabricated with PrBaCo2O5+δ (PBCO) and BZY composite cathode. The ease of film electrolyte fabrication of low thickness assisted by the good sinterability of BZYNi04 coupled with employing an anode functional layer aided in attaining a superior power density output. Peak power density values of 428 and 340 mWcm-2 were obtained at 700 and 650 °C respectively when humidified hydrogen (≈ 3% H2O) and static air were used as the fuel and oxidant respectively. To the best of our knowledge, the power performance obtained in this work is the best reported yet for BaZrO3based proton conducting SOFCs. References P. Babilo, S. M. Haile, J. Am. Ceram. Soc. 88 (2005) 2362.J. Tong, D. Clark, M. Hoban, R. O’Hayre, Solid State Ionics 181 (2010) 496.K. D. Kreuer, S. Adams, W. Munch, A. Fuchs, U. Klock, J. Maier, Solid State Ionics 145 (2001) 295.

Hydrocarbon-based polymer electrolyte cerium composite membranes for improved proton exchange membrane fuel cell durability

Hydrocarbon-based cerium composite membranes were prepared for proton exchange membrane fuel cell applications to increase oxidative stability. Different amounts of cerium ions were impregnated in sulfonated poly(arylene ether sulfone) (SPES) membranes and their physicochemical properties were investigated according to the cerium content. Field-emission scanning electron microscopy and inductively coupled plasma analyses confirmed the presence of cerium ions in the composite membranes and 1H NMR indicated the successful coordination of sulfonic acid groups with the metal ions. Increasing amounts of cerium ions resulted in decreases in the proton conductivity and water uptake, but enhanced oxidative stability. The oxidative stability of the composite membranes was proven via a hydrogen peroxide exposure experiment which mimicked fuel cell operating conditions. In addition, more than 2200 h was achieved with the composite membrane under in situ accelerated open circuit voltage (OCV) durability testing (DOE protocol), whereas the corresponding pristine SPES membrane attained only 670 h.

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

The Power of in-Situ Studies: How X-ray Absorption Spectroscopy Can Help to Develop PEMFC Catalysts with Increased Activity and Stability

Low and high temperature polymer electrolyte membrane fuel cells (LT- and HT-PEMFC) are promising systems in a future sustainable energy scenario, where renewable energies can be implemented by a power-to-gas strategy. The supply of hydrogen or methanol produced by intermittently available wind and solar energy will then be converted into electricity on demand using a combination of water electrolysis and fuel cells. However, in order to make this dream a viable reality, costs need to be dramatically reduced and the material’s stability significantly improved. In our opinion, this is only possible after detailed insights into the reaction and degradation mechanisms in these systems have been obtained, so that a knowledge-based design of specifically tailored materials becomes possible. State-of-the-art catalysts in fuel cells are Pt(-M) with M=Ru, Co nanoparticles with particle sizes ranging between 2 nm and 5 nm and metal loadings between 20 and 80 wt.% supported on a high surface area carbon material. Recent research focuses also on functionalized carbons, ordered mesoporous and templated carbons, as well as carbon nanotubes (CNT) and even graphene. However, carbonaceous materials are very prone to corrosion, when subjected to the harsh operation conditions, in particular at the cathode side of the fuel cell. That is why current efforts in our group and others focus on the search for alternative support materials, such as electron-conducting oxides and polymers. Oxide-supported catalysts are well-known in heterogeneous catalysis, so that a closer collaboration between the two communities appears desirable in the future. X-ray absorption spectroscopy (XAS) is a technique optimally suited to studies under fuel cell operation conditions, as neither UHV conditions in the experiment, nor long-range order in the sample are required. It can be applied to monitor the geometric structure of the catalytically active nanoparticles directly following particle oxidation and growth during operation. With the delta µ XANES method and complementary theoretical FEFF8 calculations, it is furthermore possible to identify adsorbed species on small Pt nanoparticles and follow their change in coverage and their competition for free adsorption sites elucidating the reaction mechanism. Since diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) has been applied manifold in heterogeneous catalysis for the study of oxidic support materials and – in contrast to XAS – is applicable in the lab, we decided to design a combined DRIFTS-XAS in-situ cell to investigate nanoparticle-support interactions in different gas atmospheres. This talk will highlight how sophisticated in-situ techniques, such as X-ray absorption spectroscopy, but also others, will help with rational materials design. When the reaction mechanisms can be unraveled, the catalyst can be tailored and hopefully lesser fractions of the costly noble metals will be required. Getting more insight into degradation phenomena should help to optimize the relevant operation conditions in order to avoid those stressing the catalyst unnecessarily. We will give examples of the anode and cathode side of a HT-PEM fuel cell and also show first results measured in our DRIFTS-XAS in-situ set-up.

Indium Tin Oxide as Catalyst Support for PEM Fuel Cell: RDE and MEA Performance

Introduction Conventional carbon supports suffer from catastrophic failure during start-up/shut-down events, which result in high cathode interfacial potentials, which, in turn, lead to carbon corrosion and catalyst degradation. Carbon corrosion is a major problem in fuel cells because it leads to loss of catalyst activity and collapse of electrode structure. Nissan, as an automotive OEM, has always approached the issue of catalyst support durability at the materials level, rather than at the systems-level, since system-level mitigation is an added cost to the balance of plant (BOP), and presents one additional factor that can fail during operation. NTCNA and IIT have been working on the development of high-surface-area, corrosion-resistant non-carbon supports, and we have previously demonstrated a class of highly stable non-carbon supports, e.g. ruthenium-titanium oxide (RTO) (1). In this work, we present the results of our work on a second class of non-carbon supports - tin-doped indium oxide (ITO). Materials and Methods RDE Testing – RDE tests were performed in a 3-compartment electrochemical cell containing 0.1 M HClO4 electrolyte. A glassy carbon disk (0.196 cm2) uniformly covered with Pt/C or Pt/ITO catalyst served as the working electrode. The counter and reference electrodes were Pt foil and RHE, respectively. Membrane Electrode Assembly (MEA) Fabrication - Catalyst inks were prepared by mixing Pt/C or Pt/ITO with water, n-propanol, and a Nafion ionomer dispersion. The mass-based ionomer/support (I/S) ratio in the ink was kept constant at 0.9. The cathode catalyst layers were formed on GDLs using an automated robotic spray system. MEAs (25 cm2) were prepared by hot pressing cathode GDEs (0.35 mgPt/cm2) and commercial anode GDEs (0.4 mgPt/cm2) onto Nafion® NR211 membranes. Results and Discussion Figure 1 shows the RDE performance of Pt/ITO vs. a commercially available Pt/C benchmark catalyst of comparable Pt particle size. The ECA obtained for Pt/ITO (22 m2/gPt) was lower than Pt/C (45 m2/gPt) due to the low surface area of ITO which causes Pt particle agglomeration. Even with a lower ECA, Pt/ITO shows promising performance since its mass activity of ~150 mA/mgPt matches that of the Pt/C benchmark catalyst. However, we have repeatedly observed that Pt/ITO performs very poorly in MEA testing. Figure 2 shows a typical H2/O2 polarization curve obtained for Pt/ITO at a high Pt loading of 0.35 mg/cm2. The only difference between the two MEAs is the cathode catalyst. Other than the dramatic difference between the two polarization curves, it can also be seen that Pt/ITO shows a very high HFR (high frequency resistance) of ~220 mΩ·cm2. The HFR represents the ohmic resistance due to proton transport in the membrane and the electronic contact resistances in the cell. The high HFR is attributed to the poor electronic conductivity of the ITO support since the two MEAs in Figure 2 have the same membrane, cell hardware, and cell components. It is hypothesized that hydroxylated species form on the ITO surface under fuel cell operating conditions. Hydroxide and oxy-hydroxide species may readily form on the surface of ITO due to hydrolysis reactions. Hydroxylated species such as In(OH)3 have low solubilities, hence they may remain adsorbed on the ITO surface, forming a passivating layer that will increase ohmic resistance (2). Furthermore, the Pourbaix diagram for indium shows that under fuel cell conditions, the formation of In3+ is favorable, and it is possible that In3+ can act as a poison that covers the Pt active sites. This is supported by the changes observed in the CV profile in MEA (loss of Hupd features and resistive behavior), suggesting some changes in the chemical properties of ITO and Pt poisoning. The promising RDE results did not translate to good performance in MEA, and this is hypothesized to be caused by cross-over hydrogen in MEA. H2 crossing over from anode to cathode may accelerate the formation of hydroxylated species. Experiments are underway to verify this.

Carbon Corrosion in PEM Fuel Cells during Drive Cycle Operation

PEM fuel cells (PEMFCs) show great promise to increase the fuel efficiency for transportation applications; however, for this application, they must show performance and durability with the requirements for transportation. For transportation applications, the fuel cell will be subjected to frequent power cycling. For example, the DOE/Fuel Cell Tech Team (FCTT) protocol for durability includes load cycling from 0.02 A/cm2 to 1.2 A/cm2 every 0.5 min. The cathode catalyst and catalyst layer have been shown as susceptible to degradation causing loss of performance due to both loss of kinetics for the oxygen reduction reaction and loss of mass transport. Catalyst support-carbon corrosion can result in thinning of the catalyst layer contributing to degradation in performance. To examine the effect of power cycling in situ on carbon corrosion and electrode degradation, we directly measured the catalyst support degradation by measuring CO2 in the cathode outlet by NDIR (Non-Dispersive Infra-Red) while operating a single-cell fuel cell. CO2 present in air was removed by a lime bed prior to introduction to the fuel cell. We operated with a modified DOE/FCTT durability protocol using controlled voltage, and varied the potential limits to explore the effects of the upper potential limit, lower potential limit, the potential step size and time at potential. The upper potential limit was varied from 0.95 to 0.55V; the lower potential limit from 0.40V to 0.80V, with times ranging from 0.5 min to 5 min. The corrosion of three different types of carbon were explored, high surface area (E), vulcan (V), and graphitized (EA). The catalyst support carbon corrosion occurs under normal fuel cell operating conditions and is exacerbated by the voltage cycling inherent in these steps in potential. A series of carbon corrosion spikes during potential cycling is shown in Figure 1 for E-type carbon, varying the upper potential from 0.95 V to 0.60V while keeping the lower potential constant at 0.40V. Sharp spikes in the carbon corrosion rate are observed during a step increase in cell potential with the magnitude of the spikes decreasing as the high cell potential is reduced from 0.95 V to 0.6 V. The carbon corrosion rate at high cell potential (0.95V) decreases with time at potential, indicating formation of passivating carbon surface oxides. Carbon corrosion was measured during the drive cycle measurements for all three types of carbon, with the relative carbon corrosion rates of E>V>EA. The series of step potential carbon corrosion spikes where the potential was varied for the lower potential from 0.40 V to 0.60V while keeping the upper potential constant at 0.95V shows similar results in terms of the carbon corrosion. The magnitude of the spikes decrease as the lower cell potential is raised. These results indicate that the size of the step in potential has a more significant impact on the carbon corrosion rate than does the absolute value of the potential for normal cathode operating potentials. The peak in CO2 evolution occurs when the cell potential increases from high power operation to low power near open circuit. This correlates with when CO2 evolution is observed during cyclic voltammograms, which occurs during the positive sweep at ~ 0.55 to 0.60 V. The evolution of this CO2 peak suggests that oxygen is adsorbed onto the carbon and/or CO is formed on the Pt surface. During long-term operation, a reduction in catalyst layer thickness is observed during drive cycle operation, which can be due to the loss of carbon through carbon corrosion or possibly due to compaction; both effects likely lead to a loss of void volume. This reduction in thickness includes a sharp decrease in catalyst layer thickness within the first 100 hours of operation (30%), eventually reaching ~50% of its thickness after 1000 hours. Most of this reduction in electrode thickness does not appear to be directly due to carbon corrosion as there is little evidence for carbon corrosion from microscopic analysis, especially during the early stages of operation, where the thickness reduction is substantially more than what should be due to carbon corrosion. These results show that carbon corrosion occurs during normal potential operation, and is exacerbated by potential variations during operation. To minimize the carbon corrosion, the size of the steps in potential should be minimized. For a more stable catalyst, another requirement is for the Pt particles to be stabilized on graphitized carbon supports. Acknowledgments Funding for this work is from DOE EERE FCTO, Technology Development Manager Nancy Garland Figure 1

Activity, Stability and Degradation of Carbon Supported Palladium (Pd/C) Fuel Cell Electrocatalysts for the Oxygen Reduction

For large-scale commercialization of polymer electrolyte membrane fuel cells (PEMFCs), a significant reduction or, ultimately, replacement of platinum as cathode catalyst for the oxygen reduction reaction (ORR) is of great interest. As a first step, the similarity of palladium and platinum makes Pd an often-discussed alternative material for the electrocatalytic reduction of oxygen. Moreover, apart from Pt, Pd is located on the very top of the well-known volcano plot for the ORR activity of pure metals published by Nørskov et al. (1). However, materials for the oxygen reduction in acidic media have to address both high catalytic activity and long-term stability. Especially voltage cycling stability is an important factor for technical applications. Here, a high tolerance towards dynamic fuel cell operating conditions is required. Although Pd has been known to be less stable against dissolution compared to Pt, e.g., shown in a recent study by Cherevko et al.(2), a comparative study about the effect of Pd particle size is yet outstanding. In this work, a systematic study of both ORR activity and stability against degradation during voltage cycling is conducted on Pd nanoparticles supported on carbon black (Pd/C), on unsupported Pd-black, and on conventional Pt/C as comparison, using commercial catalysts with various particle sizes. First, the activity for the ORR is evaluated in a rotating (ring) disk electrode experiment, indicating a 5-7 fold lower ORR mass activity at 0.85 V vs. the reversible hydrogen electrode (VRHE) of morphologically similar Pd/C compared to Pt/C. However, the ORR mass activity is a function of the initial electrochemically active surface area (ECSA) of the studied Pd catalysts, with a rather strong dependency for small ECSAs and little dependence at higher ones. Accelerated voltage cycling between 0.5 and 1.0 VRHE is conducted in order to correlate catalyst degradation properties with respect to initial ECSA. To monitor the degradation process, the electrochemically active surface area of the Pd catalysts is evaluated frequently during the accelerated voltage cycling. Additional information on the ECSA degradation is gathered by comparing particle size distributions, determined by transmission electron microscopy (TEM), of pristine catalyst material to that of cycled electrodes. Figure 1 shows cyclic voltammograms taken at various times during accelerated voltage cycling of a 40 wt.% Pd/C catalyst. A clear trend towards degradation in both Hupd (<0.4 VRHE) and (hydr-)oxide (>0.55 VRHE) region can be observed. Adversely, currents in the electrochemical double layer region (0.4 VRHE ≤ E ≤ 0.55 VRHE) stay unaffected, indicating a loss of active Pd surface without significant corrosion of the carbon support over the measured timescale. Both the ORR activity and the voltage cycling stability depend on the initial ECSA in opposing directions. In this work, we thus address the question, whether Pd catalysts with initially large particle size may offer substantially better corrosion stability without significant loss in ORR mass activity.

(Invited) Semi-Automated System for Electrodeposition of Pt Monolayer Shell on Refractory Metal Core Fuel Cell Electrocatalysts Directly on Gas Diffusion Layer

According to the US Department of Energy (DOE), reducing the cost and improving the durability of the polymer electrolyte membrane fuel cells (PEMFCs) are the main challenges that hinder their commercialization [1]. The largest portion of the cost of the fuel cell stacks (49%) is attributed to the large amount of Pt on the cathode side, because of the sluggish kinetics of the oxygen reduction reaction (ORR) [1, 2]. Reduction of the amount of the Pt contained in the PEMFCs stacks is required for large scale automotive applications, both for reasons of cost and Pt supply limitations. In order to challenge the researchers all over the world, and to boost the implementation of the PEMFCs in transportation, DOE has set demanding technical targets for development of fuel cell electrocatalysts for transportation applications [1]. It has recently been demonstrated that the mass and specific activities, and as well as the durability of the Pt monolayer (ML) type electrocatalysts exceed the DOE targets for 2020, which renders them as the most advanced type of fuel cell electrocatalysts [3, 4]. In this class of electrocatalysts, a surface limited redox replacement (SLRR) strategy is used in order to deposit a Pt ML shell on another transition metal (or alloy) nanoparticle core [3]. A pre-deposited Cu ML on respective nanoparticles at underpotentials (UPD) is galvanically displaced by Pt in a consecutive step, upon immersion of the sample in Pt2+ solution. Thus, a full Pt utilization is achieved in the Pt ML core-shell electrocatalysts, and their excellent activity and performance stability has been demonstrated in either rotating disk electrode (RDE) or MEA studies [3-5]. Although, those catalysts showed outstanding performance, their reproducible synthesis in large scale remained unresolved. In this contribution we are reporting on novel fast and facile methodology for fabrication of PEMFC cathodes with ultra-low PGM loading. We have designed and developed a semi-automated system for electrodeposition of Pt ML core-shell fuel cell electrocatalysts, utilizing the SLRR strategy, directly on gas diffusion layer (GDL). The system is software controlled and allows fabrication of highly active and durable FC electrodes with geometric area of up to 500 cm2 in a couple of hours, starting from raw materials (chemical salts, and commercial GDLs). The system is consisting of: (i) electrochemical cell that allows fabrication of large electrodes with industrial relevance; (ii) pneumatic control unit with respective electronic control that governs the exchange of different electrolyte solutions from the de-aerated stock solutions to the cell in a particular order; (iii) electronic data acquisition board from National Instruments that allows the LabView software to control step by step all the processes; and (iv) BioLogic potentiostat-galvanostat that controls the refractory alloy electrodeposition, Pd displacement, and Cu UPD and Pt ML deposition processes. The electrodeposition of PtML/Pd/W(Mo)Ni catalysts directly on the GDL is realized in three sequential steps. In the first step, refractory WNi or MoNi alloy nanoparticles are co-electrodeposited directly on functionalized GDL by applying a constant current or pulse deposition protocol, from solution containing Na2WO4 or Na2MoO4, NiSO4, and sodium citrate. In the consecutive step, the solution in the cell is replaced by PdCl2 solution, and Ni is partially displaced by Pd. As a result, WNi (MoNi) core nanoparticles are covered with thin (2-3 monolayers) Pd shells. In the third step, Pt ML is deposited on Pd/WNi (Pd/MoNi) nanostructures in “one-cell configuration” [6] from solution containing both Pt2+ and Cu2+ ions. At the end of the deposition process, the electrode is rinsed with nanopure water, and after drying is ready for assembly. Thus, highly active, inexpensive and durable PtML/Pd/WNi (PtML/Pd/MoNi) electrocatalysts with ultra-low PGM loading are directly electrodeposited on up to 500 cm2 GDL. The features of the system will be discussed along with the electrodes characterization and MEAs performance at real fuel cell operating conditions. Acknowledgment Work at Brookhaven National Laboratory is supported by US Department of Energy, Division of Chemical Sciences, Geosciences and Biosciences Division, under the Contract No. DE-AC02-98CH10886. References http://energy.gov/eere/fuelcells/downloads/fuel-cell-technologies-office-multi-year-research-development-and-22.H. A. Gasteiger, S. S. Kocha, B. Sompalli and F. T. Wagner, Applied Catalysis B-Environmental, 56, 9 (2005).R. R. Adzic, Electrocatalysis, 3, 163 (2012).K. Sasaki, H. Naohara, Y. Choi, Y. Cai, W.-F. Chen, P. Liu and R. R. Adzic, Nature Communications, 3, 1115 (2012).R. Adzic, S. Bliznakov, and M. Vukmirovic. US Patent: Core-Shell Fuel Cell Electrodes,US2015/0017565 A1, Jan. 15, 2015.M. Fayette, Y. Liu, D. Bertrand, J. Nutariya, N. Vasiljevic and N. Dimitrov, Langmuir, 27, 5650 (2011).

Graft-Type Anion-Conducting Electrolyte Membranes Having Poly(2-imidazoliumylstyrene) Prepared By Radiation-Induced Grafting for Hydrazine Hydrate Fuel Cells

Introduction The anion-conducting electrolyte membrane fuel cells (AEMFCs) have attracted considerable interest due to their significant advantages over proton conducting electrolyte membrane fuel cells (PEMFC). Because AEMFCs operate under alkaline environments, non-precious metals can be used as active catalysts (e.g. nickel, cobalt and iron) and electrochemical reactions are more facile than in acidic medium. Further advantages include amongst others is the fuel flexibility. The hydrazine hydrate, which has higher energy densities than H2, is one of the powerful options. However, there are still some challenging issues in the practical application of AEMFC. One of the key challenges is to synthesize an alkaline durable anion-conducting electrolyte membrane (AEM), which can survive high temperature and highly basic fuel cell operating conditions.1,2 In recent years, we have developed imidazolium-type AEMs by radiation-induced grafting for direct hydrazine hydrate fuel cells (DHFCs) and founded that copoly(N-vinylimidazolium/styrene)-grafted AEM shows moderate alkaline stability; the AEMs degraded via the initial β-elimination of the imidazolium unit.3 In this work, we synthesized the AEM based on a poly(p-(2-imidazoliumyl)styrene (2STZ)) grafts by radiation-induced grafting to exclude β-hydrogen from the structure. Experiment The 2STZ was provided from Shikoku Kasei Kogyo, Japan. This was first mono N-alkylated by sodium hydrate and methyl or propyl iodide in a standard manner, yielding methylated (MeSTZ) and propylated (PrSTZ) monomers.4 The poly(ethylene-co-tetrafluoroethylene) (ETFE) films were irradiated with a 60Co γ-ray source (JAEA Takasaki, Gunma, Japan) at room temperature in argon atmosphere with an absorption dose of 80 kGy. The pre-irradiated ETFE films were immediately immersed into the argon-purged monomer solution consisting of 33 vol% of MeSTZ or PrSTZ in 1,4-dioxane. The imidazole groups in the grafted-ETFE were N-alkylated using alkyl iodide. The resultant AEMs (an iodide form) were immersed in 1 M HCl - dioxane (1 : 1 volume ratio) solution at 60°C for 24 h to convert into a Cl- form, followed by the ion-exchange reaction with OH- in 1 M KOH solution at room temperature for 16 h. The solution for each ion exchange reaction was changed 3 times during the process to give the OH- forms of the AEMs (Me-MeSTZ, Me-PrSTZand Pr-PrSTZ) (Scheme 1). The ionic conductivity was measured by two-point probe AC impedance spectroscopy at 100% relative humidity in N2-purged deionized water. The ion exchange capacity (IEC) was measured using standard back titration methods. The alkaline stability of AEMs was evaluated by monitoring the change in conductivity of AEMs in 1 M KOH at 80°C. Results and discussion The poly(MeSTZ)- and poly(PrSTZ)-grafted ETFE films with grafting degrees of 78 and 43% were prepared. The N-alkylation yields were 80 (Me-Me), 82 (Me-Pr), and 77% (Pr-Pr). Me-MeSTZ, Me-PrSTZ and Pr-PrSTZ with IECs of 1.39, 1.55 and 1.00 mmol g-1 showed ionic conductivities of 83, 104, and 135 mS cm-1at 60°C, respectively. Figure 1 shows the normalized conductivities of the AEMs on the basis of the initial conductance as a function of immersion time. The Me-MeSTZ and Pr-PrSTZ AEMs showed similar profiles for conductivity decreases, while unsymmetrical Me-PrSTZ showed good alkaline stability. The conductivity loss was mainly caused by imidazolium ring opening degradation reaction,5 which triggered by the attack of hydroxide ion at the 2-position of imidazolium ring. Thus, these results indicated that the steric protection of 2-position is important, however, too much protection causes steric repulsion of alkyl chains attached on the nitrogen of imidazolium ring, resulting in the unstable manner in alkaline conition. Accordingly, the p-(2-imidazoliumyl)styrene-graft with appropriate N-alkyl substituents is promising AEM for alkaline durable direct hydrazine fuel cells. Acknowledgement This work was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) from the Japan Science and Technology Agency (JST).