Effects of Nafion® Dehydration in PEM Fuel Cells

The current dependence of the ohmic resistance of Nafion membranes was examined with different types of humidification: cathodic (ChAd), anodic (CdAh), anodic and cathodic (ChAh) and no humidification at all (CdAd). Data show that for stacks with humidified cathodes (ChAd and ChAh), the resistance is small and relatively insensitive to the presence of the anodic humidification. On the contrary, for stacks with non-humidified cathodes (CdAh and CdAd), the membrane resistance is high and strongly dependent on current and anodic humidification. The kinetics of membrane dehydration was examined by recording the galvanostatic transients of the stack voltage and resistance, after removing the humidification. It was found that the changes in the ohmic resistance ΔR _Ω(t), although significant, cannot explain entirely the observed decay of the stack voltage. To account for the difference, an additional resistive term is introduced ΔR _p(t). Explicit equations were found for the time and current dependence of the two resistive terms ΔR _Ω(t) and ΔR _p(t) after humidification removal. A tentative explanation for the new resistive term was provided using electrochemical impedance spectroscopy (EIS). EIS data obtained at low overpotential show that dehydration of the Nafion present in the cathode catalytic layer results in an increase of the polarization resistance; the apparent deactivation of the cathode electrocatalyst appears to be due to a decrease of the electrochemically active surface area.     

Magnetoresistive sensors with rapidly solidified Permalloy fibers

The microstructure and properties of Permalloy fibers were studied. Both transverse and longitudinal extraordinary magnetoresistive effects have a positive initial slope due to stress-related rotation of moments in the closure domains and to the coherent rotation of moments in the columns. Fiber response was also investigated as a function of the field direction and angle of incidence, fiber twisting and for strong transverse fields. The results are discussed in terms of the existing models, and a new model is proposed to explain the transerse effect. The structures and operating principles of single- and dual-fiber biased sensors based on the longitudinal effects are presented. Distortions due to overbiasing are accounted for by the pinning and expansion of large-area closure domains with moments tilted by the casting stress. The sensitivity-linearity range product is the same for several bias levels, and a family of sensors was obtained using the same fiber     

Magnetic properties of electrodeposited CoFeB thin films and nanowire arrays

A pulse electrodeposition method for preparing stress-free CoFeB thin films is described. The method was optimized to produce amorphous films with soft magnetic properties and the best composition was found to be Co₉₄Fe₅B₁. The optimized conditions were used to obtain arrays of ordered nanowires, using nanoporous alumina membranes as templates. While magnetization measurements demonstrate that the microstructure of the array influences the detailed characteristics of the hysteresis loops, the general pattern of the magnetization curves, characterized by high saturation fields and low squareness, is the same as for crystalline Co arrays obtained in similar conditions. This observation, as well as data obtained from variable temperature measurements, shows that the overall magnetic behavior is determined mainly by the competition of the shape anisotropy and magnetostatic interactions. Whether the wire axis is easy or hard for the array depends upon the array geometry: wire diameter, length and the packing density of the wires. In order to explain these effects, a micromagnetic model was used to calculate the saturation fields for in-plane and out-of-plane magnetized array, as a function of the geometrical parameters. These agree well with the experimental results.     

Efficiency and inductance analysis of a three-region thin-film head

Expressions for the efficiency and inductance of a thin-film magnetic head are derived using an extended transmission-line model. This extension was performed by applying the Paton-Jones unidimensional model to an actual head geometry, with three regions in the longitudinal cross section: a pole tip region, a coil region, and a back protection region. The computer analysis of the efficiency and inductance is used to predict geometry optimization of the head.     

Magnetostatic interactions and coercivities of ferromagnetic soft nanowires in uniform length arrays

First-order reversal curve diagrams have been used to investigate magnetostatic interactions and average coercivity of individual wires in soft ferromagnetic uniform length nanowire arrays. We present a method for identifying these physical parameters on the out-of-plane first-order reversal curve diagrams: the position of the irreversible part on the critical axis is a good approximation to the average value of the nanowire coercivity and the maximum interaction field is equal to the interaction field at saturation. Their dependence upon material (CoFeB and Ni) and nanowire length are presented. The magnetostatic interactions increase linearly with length, in agreement with a model developed previously. The global array coercivity, obtained from magnetization curves, is generally lower than the apparent average coercivity for individual nanowires. This coercivity reduction increases linearly with the magnetostatic interactions. The general shape of the out-of-plane first-order reversal curve diagrams is compared with those obtained from a theoretical moving Preisach model.     

PEM fuel cells as membrane reactors: kinetic analysis by impedance spectroscopy

Proton exchange membrane fuel cells (PEMFC) are considered as electrochemical reactors, performances of which are regarded in the context of the various effects influencing FC output, such as mass transports, kinetic of electrode reactions and charge transfer in polymer electrolyte membrane (PEM). An experimental approach, involving the employment of impedance spectroscopy (IS), which allows a deep insight into the nature of these effects, is discussed and its applications to the different aspects of PEMFC functioning are reported. As examples of the use of IS in PEMFC studies, the investigations of the membrane conductivity and in situ studies of the anode and the cathode processes during FC operation are presented.