In recent years, nanofiber production via electrospinning has gained importance because of superior properties of submicron fibers. In this study, the effect of molecular weight, concentration of solution, electric conductivity, surface tension and solution viscosity of the polymer solution on the roller electrospinning of PVA nanofibers was investigated. One nonspinnable and two spinnable polymer species were studied. The effect of polymer concentration and solution viscosity on the electrospinning process throughput, fiber diameters and quality of nanofiber layers was measured. According to the results there is a significant difference in rheological behavior of nonspinnable and spinnable polymer solutions. Electric conductivity and surface tension of the solutions did not influence both throughput and fiber diameter significantly. Whereas molecular weight has an important effect on the spinnability, concentration of the solutions has not. On the contrary, concentration influences the process throughput considerably and properties of nanofibers and nanofiber layers to some extent. POLYM. ENG. SCI.,

Over its lifetime in a fuel cell electric vehicle, a polymer electrolyte membrane fuel cell inevitably suffers from certain duration of dry operational conditions, where significant performance losses of the fuel cell take place. In this study, we investigate the activity changes of the fuel cell after a prolonged degradation protocol under dry operational condition, followed by various recovery procedures under wet conditions. The utilization of diluted air on the cathode side is found to be advantageous for the recovery due to the superior heat and water management. This more efficient recovery protocol allows the deconvolution of reversible and irreversible voltages losses after dry operations. A subsequent mechanistic study reveals an irreversible decrease of the effective ionomer coverage on the catalyst particles, while the proton conductivity of the catalyst layer drops. These observations point towards ionomer structural changes caused by the dry conditions. This is confirmed by post-mortem analysis via scanning electron microscope, showing clearly that ionomer redistributes and migrates, an additional mechanism which leads to the performance losses. Overall, the degradation mechanisms seem to be mitigated by higher ionomer content in the catalyst layer, while the investigated surface modification of carbon support shows minor sensitivities.

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Applying nitrogen-modified carbon support in proton exchange membrane fuel cells (PEMFCs) has been attracting increasing interest due to the resulting performance enhancement. In the present study, we attempted to uncover the origin and gain a deeper understanding of the different N-modification processes, whose influences are responsible for the improvement. By utilizing chemically modified Ketjenblack supports comprising altered fraction of N-functionalities, we investigated the underlying mechanism of the drastically reduced voltage losses under fuel cell operation conditions. In all, we demonstrate the key role of support modification induced by ammonia in strengthened support/ionomer interactions and alter physico-chemical properties of the carbon support contributing towards enhanced MEA performance. With the use of X-ray photoelectron spectroscopy, we show unambiguous evidences that not all N modified surfaces yield the desired performance increase. Rather, the latter depends on a complex interplay between different electrochemical parameter and catalyst properties. We want to emphasize the ionomer/support interaction as one important factor for enhanced ionomer distribution and present a prove of a direct interaction between the ionomers´ sidechains and N-functional groups of the support.

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