Ahmet Kusoglu

Unexplained transport resistances for low-loaded fuel-cell catalyst layers

For next-generation polymer-electrolyte fuel cells, material solutions are being sought to decrease the cost of the cell components, and, in particular, the amount of catalyst, without sacrificing performance and lifetime. However, as recently shown, this cannot be achieved in practice due most likely to limitations caused by the ionomer thin-film surrounding the catalyst sites, where confinement and substrate interactions dominate and result in increased mass-transport limitations. Mitigation of this issue is paramount to the future commercial viability of polymer-electrolyte fuel cells.

Correlating Humidity-Dependent Ionically Conductive Surface Area with Transport Phenomena in Proton-Exchange Membranes

The objective of this effort was to correlate the local surface ionic conductance of a Nafion 212 proton-exchange membrane with its bulk and interfacial transport properties as a function of water content. Both macroscopic and microscopic proton conductivities were investigated at different relative humidity levels, using direct-current voltammetry and current-sensing atomic force microscopy (CSAFM). We were able to identify small ion-conducting domains that grew with humidity at the surface of the membrane. Numerical analysis of the surface ionic conductance images recorded at various relative humidity levels helped determine the fractional area of ion-conducting active sites. A simple square-root relationship between the fractional conducting area and observed interfacial mass-transport resistance was established. Furthermore, the relationship between the bulk ionic conductivity and surface ionic conductance pattern of the Nafion membrane was examined.

Confinement-driven increase in ionomer thin-film modulus.

Ion-conductive polymers, or ionomers, are critical materials for a wide range of electrochemical technologies. For optimizing the complex heterogeneous structures in which they occur, there is a need to elucidate the governing structure-property relationships, especially at nanoscale dimensions where interfacial interactions dominate the overall materials response due to confinement effects. It is widely acknowledged that polymer physical behavior can be drastically altered from the bulk when under confinement and the literature is replete with examples thereof. However, there is a deficit in the understanding of ionomers when confined to the nanoscale, although it is apparent from literature that confinement can influence ionomer properties. Herein we show that as one particular ionomer, Nafion, is confined to thin films, there is a drastic increase in the modulus over the bulk value, and we demonstrate that this stiffening can explain previously observed deviations in materials properties such as water transport and uptake upon confinement. Moreover, we provide insight into the underlying confinement-induced stiffening through the application of a simple theoretical framework based on self-consistent micromechanics. This framework can be applied to other polymer systems and assumes that as the polymer is confined the mechanical response becomes dominated by the modulus of individual polymer chains.

Numerical Investigation of Mechanical Durability in Polymer Electrolyte Membrane Fuel Cells

William B. Johnson Gore Fuel Cell Technologies Follow this and additional works at: http://engagedscholarship.csuohio.edu/enme_facpub Part of the Mechanical Engineering Commons Publishers Statement

Electrochemical/Mechanical Coupling in Ion-Conducting Soft Matter

Mechanical and electrochemical phenomena exhibit many interesting multidirectional couplings in ion-exchange soft matter due to their intrinsic material physiochemical states and responses to environmental stressors. In this Perspective, such coupling is explored in terms of recent studies with a focus on the degradation of polymer-electrolyte fuel-cell membranes. In addition, (electro)chemical-mechanical coupling of ion-conducting polymers in other applications is also introduced, as there is a research need to explore the interactions between these often wrongly assumed disparate fields in order to optimize, exploit, and discover new technologies and applications.

In Situ Method for Measuring the Mechanical Properties of Nafion Thin Films during Hydration Cycles.

Perfluorinated ionomers, in particular Nafion, are an essential component in hydrogen fuel cells, as both the proton exchange membrane and the binder within the catalyst layer. During normal operation of a hydrogen fuel cell, the ionomer will progressively swell and deswell in response to the changes in hydration, resulting in mechanical fatigue and ultimately failure over time. In this study, we have developed and implemented a cantilever bending technique in order to investigate the swelling-induced stresses in biaxially constrained Nafion thin films. When the deflection of a cantilever beam coated with a polymer film is monitored as it is exposed to varying humidity environments, the swelling induced stress-thickness product of the polymer film is measured. By combining the stress-thickness results with a measurement of the swelling strain as a function of humidity, as measured by quartz crystal microbalance (QCM) and X-ray reflectivity (XR), the swelling stress can be determined. An estimate of the Youngs modulus of thin Nafion films as a function of relative humidity is obtained. The Youngs modulus values indicate orientation of the ionic domains within the polymer films, which were confirmed by grazing incidence small-angle X-ray scattering (GISAXS). This study represents a measurement platform that can be expanded to incorporate novel ionomer systems and fuel cell components to mimic the stress state of a working hydrogen fuel cell.

Discovery of a new isomeric state in

We report on the observation of a new isomeric state in

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Ni: Evidence for a highly-deformed proton intruder state.

Ni. We suggest that the newly observed state at 168(1) keV above the first 2

Water Uptake in PEMFC Catalyst Layers

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