The absorption, desorption, and diffusion of water in Nafion is extremely complex, with literature values for water diffusivity varying over several orders magnitude in some cases. In part, this variation can be attributed to inconsistencies in sample preparation and handling. Changes in water absorption/desorption and diffusion in Nafion membranes can be affected by membrane thickness, annealing conditions, processing methodologies, and water vapor concentration. While these relationships have been studied in great detail for bulk membranes, there has been very little work done on Nafion thin films. It has been well documented that polymer thin films (typically <100nm thick) demonstrate significant material property deviations from bulk values. These deviations are believed to be mediated by interactions at each polymer interface. Understanding these interactions is vital to devising proper models for more complex systems such as fuel cells, which consist of several polymer interfacial interactions, for example, at the triple phase boundary of a membrane-electrode assembly (MEA). These studies will also be beneficial in understanding the behaviors of composite membranes as more complex membrane systems are being realized with the incorporation of various fillers and additives. The work presented here focuses on studying the water absorption and diffusion in Nafion thin films as a function of film thickness (l), keeping other variables consistent. Utilizing specular x-ray reflectivity (SXR) we have measured the extent of swelling and the equilibrium water content as a function of relative humidity and film thickness for films less than 220 nm. We observe that the equilibrium swelling and water content is constant at a set relative humidity for films above 100nm. Films thinner than 100 nm show suppression in equilibrium swelling and water content with decreasing film thickness and approach 65% suppression in both quantities for 20 nm films. To complement the swelling studies, the diffusion of water vapor through the thin film membranes has been studied by employing polarization-modulation infrared reflection-absorption spectroscopy (PMIRRAS). By modeling the growth of the water peak in the IR spectrum as a function of time, we have determined the diffusion coefficient (D) as a function of film thickness. These results show that, along with suppression in swelling, there is a power-law relationship between D and l. More specifically, we find that films in a thickness range of 20 nm to 220nm reveal a diffusion coefficient that is 4-5 orders of magnitude slower than that measured in bulk membranes. Understanding water absorption, desorption, and diffusion of thin Nafion films gives us valuable insight into the interfacial interactions experienced in a working fuel cell and is vital to the development and optimization of Nafion membrane interfaces with respect to water management and conductivity.

The main challenges for the introduction of the polymer electrolyte fuel cell (PEFC) technology in various applications, for instance electric cars, are high cost and insufficient lifetime under application-relevant conditions. Radiation grafted membranes based on sulfonated polystyrene or derivatives thereof offer a potentially cost-effective alternative to commonly used perfluoroalkylsulfonic acid (PFSA) membranes. The preparation involves the electron-beam irradiation of a fluorinated or partially fluorinated base film. A graft copolymer is subsequently formed by introducing the irradiatied film into a monomer solution. Proton conductivity is obtained by sulfonation of the grafted film. The chemical aging of the proton exchange membrane (PEM) by radical intermediates, such as HO*, limits the durability of the fuel cell. Crosslinked radiation grafted membranes show a substantially improved durability. Yet it is of interest to understand the underlying radical attack mechanisms in poly(styrene sulfonic acid) (PSSA) based membranes and identify means to improve the intrinsic chemical stability of the material. We have shown that favorable combinations of grafting monomers, such as alpha-methylstyrene (AMS) or styrene and methacrylonitrile (MAN), yield membranes with enhanced durability. In this contribution, the implications of using multi-monomer grafting systems are discussed: the co-grafting of styrene and MAN, for instance, requires an understanding of the co-polymerization kinetics. Also, relevant characterization techniques have to be adopted to quantify film composition and modification during processing. Furthermore, fuel cell relevant membrane properties, such as ion exchange capacity, water uptake and proton conductivity, are presented. In situ characterization of membranes in the fuel cell show the improved chemical stability of AMS / MAN and styrene / MAN co-grafted membranes over pure styrene grafted membranes. Potential stabilizing mechanisms will be discussed.

A proton conducting polymer material operating at elevated temperatures under dry conditions has been of interest in recent years for application in PEM fuel cells. Operating fuel cells at elevated temperatures offers advantages in extending fuel impurity tolerance and in system simplification since humidification is not as much of an issue as in typical perfluorinated sulfonic acid proton conducting polymers. Of all the approaches to achieving a high temperature proton conducting polymer membrane, PBI/phosphoric acid has shown significant promise. In this presentation we will summarize some of the recent models, calculations, and experiments we have performed of the transport processes in the PBI/PA system leading towards a better understanding of the durability of this system especially under conditions of high current densities. We have been investigating PBI/PA membranes in electrolyzers to purify and pressurize hydrogen and will summarize our experimental hydrogen pump data with a PBI/PA membrane electrode assembly. 1Qingfeng Li, Jens Oluf Jennsen, Robert F. Savinell, and Niels J. Bjerrum, Progress in Polymer Science, 34(5), 449-477(2009)

Polymer Electrolyte Membrane Fuel Cell (PEMFC) is the most promising power source for next generation transportation and residential applications. For mass commercialization of PEMFC system, one of the main barriers is the high cost of Membrane Electrode Assembly (MEA) using expensive fluoropolymer membrane and Pt catalyst. LG Chem has been developing MEA with hydrocarbon based membrane and ink-jet coating technology. Hydrocarbon membrane has advantages of low price and high thermal stability. Our proprietary multiblock copolymer, which showed high proton conductivity, was improved in water management by morphology control and the hydrocarbon based membrane was optimized for MEA application. Recently, its comparable performance and chemical stability to those of common fluoropolymer membranes increased the potential for an alternative membrane. Ink-jet is a non-impact dot printing technology in which small droplets of ink are jetted from nozzle plate to substrates such as papers and polymers. Drop-on-demand of Pt catalyst ink using ink-jet reduces material loss significantly and enables electrode patterning to open the way of designing the next generation MEA. Using Pt catalyst inks specially formulated for ink-jetting and ink-jet systems with modified print heads, continuous and homogeneous jetting was achieved and successfully applied to MEA electrode manufacture. It was found the ink-jet electrode had very flat and homogenous layer with few crack and played a role in increasing performance. In the presentation, the improvement and further development of materials and MEAs at LG Chem will be discussed.

Ability to tune the electronic and structural properties of nanocatalysts can potentially lead towards the superior catalytic enhancement that was reported for the Pt3Ni(111)-skin surface [1]. Fine tuning of the surface properties is usually done on extended well-defined surfaces in ultra-high vacuum. A number of surface sensitive tools could be utilized such as AES, LEIS and UPS before controlled transfer into real reaction environment. The single and polycrystalline crystalline well-defined surfaces have been used to benchmark the activity range that could be expected on Pt based electrodes. The knowledge accumulated from well-defined systems is further used to engineer nanoscale surfaces with designated composition and morphology. It has been proposed that surface modifications induced by the second/third metal, and consequent catalytic enhancements could occur through the following effects: (1) Electronic effect, due to changes in the metallic d-band center position vs. Fermi level; and (2) Structural effect, which reflects relationship between atomic geometry, and/or surface chemistry, i.e., dissolution – surface roughening. In principle, different near-surface composition profiles have been found to have different electronic structures. Modification in Pt electronic properties alters adsorption/catalytic properties of corresponding materials. The most active systems for the electrochemical oxygen reduction reaction (ORR) are established to be the Pt skin near surface formationThe similar levels of catalytic enhancement have been established for corresponding nanoscale materials. In addition to electronic properties we have found how catalytic activity could be affected by the concentration profiles of nanoscale surfaces. Ability to control surface and near surface catalyst properties enables fine tuning of catalytic activity and their durability.[1] V. Stamenkovic, B. Fowler, B.S. Mun, G. Wang, P.N. Ross, C.A. Lucas, N.M. Markovic, Science 315 (2007) 493-497.[2] V. Stamenkovic, B.S. Mun, K.J.J. Mayrhofer, P.N. Ross, N.M. Markovic, J. Rossmeisl, J. Greeley, J.K. Nørskov, Angewandte Chemie International Edition 45 (2006) 2897.[3] V. Stamenkovic, B.S. Mun, M. Arenz, K.J.J. Mayrhofer, C. A. Lucas, G. Wang, P.N. Ross, N.M. Markovic, Nature Materials 6 (2007) 241.

Proton exchange membrane fuel cells (PEMFCs) are susceptive to loss of performance due to poisoning by contaminants in the environment and from the fuel cell system. Performance loss can be caused by the decrease in catalytic activity through the adsorption of contaminants on the catalyst surface, or due to the loss of conductivity in the H+-Nafion membrane due to exchange with impurity cations. We are studying three aggressive poisons: airborne sulfur species (SO₂, H₂S and COS), sodium chloride (NaCl), and the glycol-based liquids used as the system coolant and how they poison the materials in the PEMFC. By understanding the extent and process of the poisoning, we have developed electrochemical methods based on voltage cycling to regain the performance of contaminated fuel cells. The methods are materials specific. For instance certain catalysts, such as Pt-alloys, recover their activity at lower voltages than pure Pt catalysts, which we explain in terms of how they adsorb oxygen. Voltage cycling is typically necessary to release the charge on the Pt catalysts, and desorb charged species from their surface.

A low density monolayer tantalum oxy-phosphate gel coating immobilizes platinum nanoparticles on carbon supports. When the resulting nanocomposite is heated under a 10% H2 atmosphere in N2 up to 500 °C, there is no observed change in the crystallite size from the Scherer analysis of the XRD, while at higher temperatures particle ripening can be used advantageously to tune the average Pt particle size, and in turn the electrocatalytic activity. We have shown the utility of this heterogeneous catalyst support system as it produces high mass activity (0.46 A/mg Pt) while also having significantly improved stability (durability) in the harsh environment of the oxygen reduction reaction (ORR) in PEM fuel cells.

Polymer electrolyte fuel cell (PEFC) is defined by the membrane electrolyte based on the polymer membrane. Nowadays, PEFC can divide into the polymer electrolyte membrane fuel cell (PEMFC) and direct methanol fuel cell (DFMC) by the fuel such as hydrogen and methanol, respectively. These fuel cells are about to commercialize for the residential and portable power applications according to the decent efforts from the academia, governments and industry. However, there are still challenges about the durability and cost of the fuel cells for expanding the market size. In a view of the cost, the Pt used mainly as catalyst is considered as a most expensive component and should be reduced by increasing the activity of catalyst or introducing cheaper materials. There are two main approaches to increase the catalyst activity. One is the addition of another metal to form an alloy with Pt and the other is the increase the utilization of Pt nanoparticle by using a novel support. For replacing the Pt, the cheaper non-pt elements like Pd and Ru have been extensively developed as a possible catalyst for PEFC. Recently, in our lab, the novel support for the highly loaded Pt catalyst has been synthesized and applied to the membrane electrode assembly (MEA) for the DMFC. In this presentation, the author would like to introduce the method and development route of ordered mesoporous carbon as novel support and demonstrate the mitigation of Pt amount in the MEA. And then, the author would like to present the very recent effort for removing the Pt in the MEA by non-Pt catalyst for anode and cathode for the intermediate temperature PEMFC, which used a polymer membrane with phosphoric acid at 150 ^oC.

While the characterization of electrocatalytic activity of nanostructured electrocatalysts for fuel cell cathode reaction (i.e., oxygen reduction reaction (ORR)) were mostly based on rotating disc electrode (RDE) measurements, the detailed evaluation of such catalysts by a combination of both RDE and real fuel cell measurements provide valuable fundamental insights into the assessment of the nanostructured activity and stability of the catalysts, The presentation describes the findings of such an investigation of a series of nanoengineered Pt based multimetallic cathode electrocatalyst in proton exchange membrane fuel cells (PEMFCs). Examples of the discussion will focus on one bimetallic catalyst, gold-platinum (AuPt) nanoparticle supported on carbon, and one trimetallic catalyst, platinum-vanadium-iron (PtVFe) nanoparticle supported on carbon. The membrane electrode assembly was prepared using carbon-supported AunPt100-n or PtnVmFe100-n-m nanoparticles with controlled sizes and compositions that were thermally treated under controlled temperature, atmosphere, and time. The electrocatalytic performances of these catalysts in the fuel cells was found to be dependent on the composition and the nanoscale phase properties which are controlled by the thermal treatment parameters. For AuPt/C catalysts, excellent fuel cell performance was observed for the catalysts which are characteristic of an alloyed AuPt phase with a lattice parameter approaching that for an Pt-rich alloy phase. The observed combination of high activity and high durability of the selected AuPt catalysts indicated that the nanoengineered bimetallic/trimetallic catalyst systems, upon further refinement and optimization of the nanoscale phase properties and durability serve as a promising candidate of electrocatalysts for the practical application in fuel cells.

Carbon supported Co-Pd nanocomposites with different surface structure and various ratio of Co/Pd were synthesized by ultrasound assisted polyol process. Sonochemical treatment of palladium acetylacetonate and cobalt acetylacetonate in ethylene glycol in the presence of a carbon support without any other surfactant, reagent for pH adjustment or stabilizer produced 4-7 nm sized CoxPd nanoparticles dispersed on the carbon support. The absence of additional reagents in the sonochemical reaction system makes unsoiled nanoparticles in comparison with conventional wetting methods. The structures of the nanoparticles, pure Pd, Co-Pd alloy and core-shell structures were characterized by XRD and TEM. Electrocatalytic oxygen reduction reaction (ORR) behavior of the materials was measured by rotating disk electrode (RDE) method and compared with pure Pd/C and commercial Pt/C. We obtained a larger mass activity for ORR with Co-Pd mixed phase samples than pure Pd/C and a comparable result with commercial Pt/C. Especially, Co0.25Pd core-shell structure shows a remarkable enhanced activity for ORR.

The need to limit precious metal usage in catalysis has lead to numerous syntheses of core-shell nanostructures that increase catalytic efficiency via decreased loading. Here we report a generalized seed-mediated approach towards creation of multi-metallic core/shell nanoparticles (NPs) with less than 10% Pt in the sub-10 nm size range. First, a surfactant mediated process produced composition and size controlled Pd-TM alloy nanoparticles (NPs). These NPs were used as seeds to grow a 1nm Pt-containing shell, resulting in the desired composition controlled core/shell nanostructures. The mono-dispersity of the seeds was critical to the accurate construction of the shells. These surfactant coated core-shell NPs were further supported on the Ketjen carbon, and were found to be readily “cleaned” by a 99% acetic acid wash. To demonstrate the impact of the core-shell interfacial interaction in fuel cell electrocatalysis, these core-shell/C NPs were evaluated for the Oxygen Reduction Reaction (ORR) in 0.1M Perchloric Acid at 308K. The catalysts show unprecedented core component dependent ORR activity and durability. This study opens up a new vista to exploit core-shell interactions for future development of multi-component NPs with unique properties for catalytic applications. Reference: 1.V.Mazumder, M.Chi, K.L. More, S.Sun. J. Am. Chem. Soc., 2010, 132, 7848.

Fuel cells are attractive power sources with high conversion efficiencies and low pollution. The high cost, low activity and poor stability of many existing Pt-based cathode catalysts for oxygen reduction reaction (ORR) constitute a major area of problems in the commercialization of fuel cells. This presentation reports our recent findings on the correlation between the lattice structural properties and the electrocatalytic performance for a series of nanostructured trimetallic nanoparticle catalysts prepared by nanoengineered synthesis and processing routes. One example involves thermal treatment to manipulate the surface and strain properties of PtmM1nM2100-m-n (M1, M2 = Ni, Co, V, Fe, W, etc) nanoparticle electrocatalysts. In addition detailed characterization of the lattice structures using XRD and XAS techniques, the electrocatalytic activities were compared with those obtained with commercially-available platinum catalysts. The results revealed new insights into the understanding of how the strain and surface properties of the trimetallic nanoparticle catalysts influence the electrocatalytic activity and stability. Implications of the findings to the design of highly active and stable trimetallic catalysts for fuel cell applications will also be discussed.

The catalytic nature of gold has been found to be tunable through the control of nanoparticle size, the combination of matrix materials, and the nanostructures of composites. As a previous research, catalytic activities of the Au/AlPO₄ nanocomposites were examined for oxygen-reduction reaction (ORR). It is important to distinguish the alteration in the size or surface facets of gold from the combinational modification with aluminum phosphate. In this research, aluminum phosphates are deposited on gold catalysts with simple electrode geometry, and the activity of gold catalyst with/without aluminum phosphate is examined. The ratio of gold surface facets can affect the ORR activity. Using the underpotential deposition of lead on gold, it is observed that the surface facets of gold are unchanged during the formation of aluminum-phosphate overlayer on gold catalysts. More feasible reactions existed within the region of approximately 0.8 – 1.0 V (vs. reversible hydrogen electrode) in the alkaline media. These feasible reactions are associated with hydrogen peroxide intermediates, and the involved mechanisms will be discussed in this talk. [1] Y. Park, B. Lee, C. Kim, J. Kim, S. Nam, Y. Oh, and B. Park J. Phys. Chem. C 114, 3688 (2010). [2] Y. Park, B. Lee, C. Kim, J. Kim, and B. Park, J. Mater. Res. 24, 140 (2009). Corresponding Author: Byungwoo Park: byungwoo@snu.ac.kr

Natural complexes of transition metal-N4 macrocycles, such as Fe-porphyrins and Fe-phthalocyanines, have been considered as oxygen reduction catalysts for low-temperature fuel cells. So far, the promising Fe-porphyrin-based Fe-N4 catalysts have not yet demonstrated an acceptable performance in the oxygen reduction reaction (ORR). This is primarily because the simple blending or chemical functionalization of Fe-porphyrins with carbon supports (carbon black or carbon nanotubes) results in a low density of active catalytic sites and poor electrical, thermal, mechanical contacts with the carbon supports. Very recently it was suggested, based on results of first-principles density-functional theory (DFT) calculations, that the Fe-N4 unit could be effectively incorporated in a large scale into graphenes by defect engineering, but without losing its original chemical activities [1,2]. The seamless connection of Fe-N4 with the sp2 graphitic network may be suitable for overcoming the current ORR limitations of the Fe-porphyrin-based technology.Here we report results of total energy and electronic structure analyses of Fe-N4 embedded carbon nanotubes and their formation mechanisms based on DFT calculations. We reveal that the special nanotubes can provide efficient pathways for ORR, comparable to those of Pt catalysts. We will also compare the theoretical findings with experiment results of Fe-porphyrinic nanotubes, including x-ray and ultraviolet photoemission spectra and cyclovoltammetry and rotating disk electrode analyses for their ORR performance [3]. [1] Y.-H. Kim, Y. Y. Sun, W. I. Choi, J. Kang, and S. B. Zhang, Phys. Chem. Chem. Phys. 11, 11400 (2009).[2] W. I. Choi, S.-H. Jhi, K. Kim, and Y.-H. Kim, Phys. Rev. B 81, 085441 (2010).[3] D. H. Lee, W. J. Lee, W. J. Lee, Y.-H. Kim, and S. O. Kim, submitted (2010).