The effect of H 3 PO 4 as a poison in high temperature polymer electrolyte fuel cells using polybenzimidazole (PBI) membranes was studied as a function of phosphoric acid loading, potential, and temperature. In this work, for the first time, extensive in-operando X-ray absorption spectroscopy investigations were carried out on Pt/C fuel cell cathode catalysts at different temperatures and H 3 PO 4 concentrations at varying fuel cell voltages. Under in-operando conditions, significant H 3 PO 4 anion coverage of the Pt nanoparticles is observed. The Δμ-XANES analysis shows that the O(H)/H adsorption onset potential increases/ decreases with temperature and that this is a result of phosphate anions being driven off the surface at high temperatures (170 °C). With initial coadsorption of H and O(H), the phosphate anions move into registry with the Pt, whereas random adsorption is observed when only phosphate anions are present on the Pt surface. By varying the temperature and the fuel cell potential, the adsorption geometry of the phosphoric acid anion changes with coverage, but in all cases, the anions block Pt sites and reduce the oxygen reduction reaction (ORR) rate.
A versatile electroless plating procedure for the fabrication of rhodium nanomaterials was developed, leading to deposits consisting of loosely agglomerated metal nanoparticles. By using carbon black as the substrate, supported rhodium nanoparticle clusters were obtained. In combination with ion track etched polymer templates, the deposition protocol allowed the first direct synthesis of rhodium nanotubes. Polymer dissolution provided access to well defined, supportless and free-standing rhodium nanotubes of nearly cylindrical shape, 300 nm opening diameter, 28 mm length and 50 nm wall thickness. The characterization by SEM, TEM, EDS and XRD confirmed the purity of the deposit, displayed a small particle size of approximately 3 nm and revealed gaps in the range of a few nanometers between the rhodium particles. BET analysis verified the presence of pores of <5 nm. To evaluate the electrocatalytic potential of the rhodium nanotubes, they were applied in the amperometric detection of hydrogen peroxide. Compared to classical nanoparticle-based sensing concepts, improved performance parameters (sensitivity, detection limit, and linear range) could be achieved.
Sb‐doped SnO2 (ATO) is used as an alternative support material to replace carbon in the highly corrosive environment of a fuel cell cathode. Two ATO powders with different morphologies are decorated with Pt nanoparticles and afterwards used as the cathode catalyst. The commercial ATO powder exhibits crystallites in the nanometer range, while the home‐made ATO powder, which was synthesized by ultrasonic spray pyrolysis, consists of polycrystalline hollow spheres. The spheres have diameters in the micrometer range and are composed of individual nanocrystallites. The unusual morphology of the home‐made ATO offers nano‐ and microporosity at the same time and opens up new possibilities for the controlled design of electrode structures in low‐temperature polymer electrolyte fuel cells. Both materials are characterized by XRD, SEM, and TEM and tested in a single cell set‐up. While almost no current is gained from the membrane electrode assembly with the commercial ATO support, the cell with the home‐made ATO achieves a mediocre performance. This higher activity, however, is obtained with approximately half the Pt content compared to the catalyst with the commercial support. The different behaviours of both ATO powders can therefore mainly be attributed to differences in the specific support morphology.
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