Carbon-supported platinum catalyst electrodes: Characterization by transmission electron microscopy, X-ray absorption spectroscopy, and electrochemical half-cell measurement on a phosphoric acid fuel cell

An electrode for a phosphoric acid fuel cell (PAFC) was prepared by the deposition of carbon-supported platinum catalysts on a PTFE (Polytetrafluoroethylene)-treated carbon paper, followed by a passivation in flowing nitrogen at 300°C. The electrode performance was tested in phosphoric acid electrolyte at 190°C by using a three-electrode half-cell system with either a Pd/H or a dynamic hydrogen reference electrode. The structure of platinum clusters on the catalyst powders, the fresh electrode, and the electrode after six test cycles was characterized by transmission electron microscopy (TEM) and X-ray absorption spectroscopy. Both the catalyst powders and the fresh electrode have the same white line intensity and extended X-ray absorption fine structure (EXAFS) on the Pt L_III edge, indicating that the morphology and electric properties of the platinum clusters were not altered during electrode fabrication. However, after the electrochemical tests, some of the platinum clusters on the electrode were aggregated. The average Pt-Pt coordination number in the Pt clusters increased from 5.5 to 6.1 while no change in the Pt-Pt bond distance (2.77 Å) was observed. TEM analysis also shows an increase in the average Pt particle size from 32 to 45 Å and a broadening in particle size distribution when comparing the fresh catalyst with the Pt-containing powders scraped off from the used electrode. In addition, a slight decrease in the normalized white line intensity characterizing the Pt clusters on the electrode was observed after the electrochemical tests. The decrease in the white line intensity could be due to a growth of Pt clusters and/or a reduction of partially oxidized Pt clusters on the electrode during the electrochemical tests.