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Understanding the factors that control microstructure formation in catalyst layers (CLs) of polymer electrolyte fuel cells is of vital importance for improving the operation of these cells. Here, we employ, for the first time, coarse-grained molecular dynamics simulations to perform a structural analysis of the microphase segregation occurring during the fabrication process of CLs. Our mesoscale simulations provide insights into the structural correlations and dynamical behavior of different phases in the catalyst layer composite. This versatile computational study, moreover, rationalizes how the solvent used in catalyst layer fabrication influences the evolution of stable agglomerated conformations. In this realm, we evaluate dispersion media with distinct dielectric properties in view of capabilities for controlling the sizes of carbon/Pt agglomerates and ionomer domains and the resulting pore network topology. These insights are highly valuable for the structural design of catalyst layers with optimized performance and stability.

Reported results of coarse-grained molecular dynamics simulations rationalize the effect of water on the phase-segregated morphology of Nafion ionomers. We analyzed density maps and radial distribution functions and correlated them with domain structures, distributions of protogenic side chains, and water transport properties. The mesoscopic structures exhibit spongelike morphologies. Hydrophilic domains of water, protons, and anionic side chains form a random three-dimensional network, which is embedded in a matrix of hydrophobic backbone aggregates. Sizes of hydrophilic domains increase from 1 to 3 nm upon water uptake. At low water content, hydrophilic domains are roughly spherical and poorly connected. At higher water content, they convert into elongated cylindrical shapes with high connectivity. Further structural analysis provides a reasonable estimate of the percolation threshold. Radial distribution functions from coarse-grained and atomistic molecular dynamics models exhibit a good agreement. Water cluster size distributions from coarse-grained molecular dynamics and dissipative particle dynamics are consistent with small angle x-ray scattering data. Moreover, we calculated the water diffusivity by molecular dynamics methods and corroborated the results by comparison with pulsed field gradient NMR.

We here describe the fabrication of superelastic conducting sheath-core fi bers that are as thin as 40 µm, characterize the properties of these downsized sheath-core fi bers, and show that they could be deployed as capacitors and sensors whose realized performance is either usefully highly sensitive or usefully insensitive to applied strains that can exceed 300%. Even when smaller in diameter than a human hair, these sheath-core fi bers, which comprise a carbon nanotube sheath on a rubber core, can be reversibly stretched by 800%, while undergoing a 72-fold increase in fi ber conductivity and a desirably low resistance change of 12%. The present downsizing has reduced the cross-sectional area of such strain sensors by a factor of 400, and correspondingly decreased the applied forces needed for their practical deployment by about the same amount. We show that Pt-containing carbonnanotube-sheath, rubber-core fi bers provide amperometric biosensors for glucose, whose response is insensitive to a 45% stretch, as well as supercapacitor electrodes that can be stretched 300% without signifi cantly changing capacitance. Variants of these sheath-core fi bers were woven as wires to transport current in highly uniaxially or biaxially stretched textiles, and deployed as high sensitivity, capacitance-based sensors for measuring the contraction of giant stroke artifi cial muscles.While many exciting previous advances have provided highly stretchable structures for energy harvesting, energy storage, sensing, and external transmission of sensor responses, [4][5][6][7][8][9][10][11][12][13][14][15][16][17] most of these strategies work by embedding relatively rigid microdevices having these functionalities into an elastomeric structure. [ 4,5 ] Important targeted applications are for such needs as wearable devices that monitor human body fl uids, like sweat, and sensor systems for morphing mechanical structures, like aircraft wings and robots. [18][19][20][21][22] For stretchable electronics applications, micrometer-scale elastic conducting fi bers with high electrical conductivity, large reversible elastic strain, and high quality factor ( Q R = percent strain/percent resistance change) are desired, [23][24][25][26] where the percent resistance change for a given strain range is defi ned using the difference between maximum and minimum resistances divided by the resistance at lowest strain. Conducting elastomers have been fabricated by methods such as incorporating conducting particles in rubbers [27][28][29] or attaching sheets of conducting nanofi bers, [6][7][8][9] graphene, [ 30,31 ] or other conductors to the surface of a rubber sheet. [ 32 ] Although strains exceeding 700% have been achieved in reversible elastic fi bers, their diameters are in the range of Downsized diameter superelastic conducting fi bers are needed for electronic interconnects having a strain-independent conductance, sensitive strain sensors enabling giant stroke ranges, artifi cial muscles, and energy storage and chemical sensing fi bers whose perform...

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Gas diffusion electrodes ͑GDEs͒ containing a graded distribution of Nafion were prepared and characterized, and their performance as fuel cell cathodes compared to GDEs possessing a uniform distribution of Nafion. Cyclic voltammetry, electrochemical impedance spectroscopy ͑EIS͒, and porosimetry are used to characterize the variations in electrochemical properties, ionic conductivity, and microstructures. The cathodic performance was improved over uniform electrodes at intermediate and high levels of polarization when the Nafion content in the GDE was higher toward the catalyst layer/membrane interface and lower toward the catalyst layer/carbon paper interface since this maximizes proton transport in the GDE in the region of greatest ion flux and maximizes porosity in the region of greatest gaseous flux, respectively. Fuel cell performance is much poorer when the gradient of Nafion content is reversed, i.e., highest at the catalyst layer/carbon paper interface since this distribution disfavors proton and gas transport in the regions where they need to be maximized.