Der Einfluss der Adsorption auf die Sauerstoffreduktionsreaktion : perfluoralkylierte Phosphorsäurederivate als Modellverbindungen zur Untersuchung der Anionenadsorption an Platin

Abstract

In this work, the influence of anion-adsorption on the oxygen reduction reaction (ORR) at platinum electrocatalysts is evaluated. By chemical modification of the electrolyte composition, polarization-losses for the ORR shall be reduced in order to enhance power-densities in high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). This was studied systematically for the first time by substitution of hydroxyl groups in phosphoric acid by perfluorinated alkyl chains. Perfluoro-alkylated phosphonic- and phosphinic acids proved to be both electro-chemically and thermally stable enough to serve as potential HT-PEMFC electrolytes. Compared with H3PO4 (PA), these acids exhibit substantially weaker adsorption tendencies on the surface of a platinum-based electrocatalyst. Consequently, overpotentials for the ORR on a flat platinum surface are reduced by 75%. The number of hydroxyl groups within the acid molecules and the negative charge density of their corresponding anions were identified as the main descriptor for their adsorption behavior, whereas the length of the perfluorinated side chain did not affect the ORR kinetics negatively. The activation barrier for the ORR on a flat platinum surface was reduced by 40% by adding bis(pentafluoroethyl)-phosphinic acid (BPPA) to the host electrolyte instead of PA. Under the same experimental conditions, the charge transfer resistance for the ORR decreased by more than three orders of magnitude. Electro-catalytic activities for the ORR at Pt3Co/C- and Pt/C-electrocatalysts were found to be significantly higher in perfluorinated electrolytes than in PA. In the pres-ence of BPPA, an enhancement of 85% was achieved. After accelerated aging proce-dures, the electro-chemical active surface area (ECA) decreased significantly. However, electro-catalytic activities for the ORR were still three orders of magnitude higher compared with PA under the same experimental conditions. The ORR-selectivity at Pt3Co/C- and Pt/C was measured to be dependent on the nature of the adsorbed species. The rate of peroxide production was correlated with the steric demand of the anionic species, with larger anions yielding more hydrogen peroxide. A HT-PEMFC single cell setup was successfully operated with mixtures of PA and 20% perfluoroalkyl-substituted phosphonic acids. Compared to operation with PA as an electrolyte, the performance was enhanced significantly