Surfactant assisted exfoliation of near infrared fluorescent silicate nanosheets.

Fluorophores that emit light in the near infrared (NIR) are advantageous in photonics and imaging due to minimal light scattering, absorption, phototoxicity and autofluorescence in this spectral region. The layered silicate Egyptian blue (CaCuSi4O10) emits as a bulk material bright and stable fluorescence in the NIR and is a promising NIR fluorescent material for (bio)photonics. Here, we demonstrate a surfactant-based (mild) exfoliation procedure to produce nanosheets (EB-NS) of high monodispersity, heights down to 1 nm and diameters <20 nm in large quantities. The approach combines planetary ball milling, surfactant assisted bath sonication and centrifugation steps. It avoids the impurities that are typical for the harsh conditions of tip-sonication. Several solvents and surfactants were tested and we found the highest yield for sodium dodecyl benzyl sulfate (SDBS) and water. The NIR fluorescence emission (λem ≈ 930-940 nm) is not affected by this procedure, is extremely stable and is not affected by quenchers. This enables the use of EB-NS for macroscopic patterning/barcoding of materials in the NIR. In summary, we present a simple and mild route to NIR fluorescent nanosheets that promise high potential as NIR fluorophores for optical applications.

Evaluation of functional layers thinning of high temperature polymer electrolyte membrane fuel cells after long term operation.

The operation related degradation processes of high temperature polymer electrolyte membrane fuel cell operated with hydrogen-rich reformate gas are studied. CO impurities from the reformate gas are strongly adsorbed by the catalyst surface, leading to poisoning and thus, reduction of the overall performance of the cell. Most of the studies are performed in a laboratory set-up by applying accelerated stress tests. In the present work, a high temperature polymer electrolyte membrane fuel cell is operated in a realistic configuration for 12 000 h (500 days). The fuel cell contains as electrocatalyst Pt in the cathode and a Pt-Ru alloy in the anode. The study of the degradation occurring in the functional layers, i.e. in different regions of cathode, anode and membrane layer, is carried out by scanning electron microscopy, (scanning) transmission electron microscopy and energy dispersive X-ray spectroscopy. We observed a thinning of the functional layers and a redistribution of catalyst material. The thinning of the cathode side is larger compared to the anode side due to harsher operation conditions likely causing a degradation of the support material via C corrosion and/or due to a degradation of the catalyst via oxidation of Pt and Ru. A thinning of the membrane caused by oxidation agents is also detected. Moreover, during operation, catalyst material is dissolved at the cathode side and redistributed. Our results will help to design and develop fuel cells with higher performance.