Titanium dioxide-supported non-precious metal oxygen reduction electrocatalysts

1. Materials synthesis and characterization Catalyst synthesis: Commercially available TiO 2 , with the particle size ranging from 50 to 150 nm and in the B.E.T. surface area from 50 to 100 m 2 /g, was first treated in 0.5 M HCl solution for 24 hours to remove impurities. In a typical approach to the synthesis of the PANI-Fe-TiO 2 catalyst, 2.0 mL aniline was first dispersed in 0.5 M HCl solution. The solution was kept below 10°C while the oxidant (ammonium peroxydisulfate, (NH 4) 2 S 2 O 8 , APS) and transition metal precursors (FeCl 3) were added. The solution was stirred for three hours to allow aniline to fully polymerize and form polyaniline (PANI). Then 1.0 g of HCl-treated TiO 2 was mixed with the PANI. After constant mixing for 24 hours, the suspension containing TiO 2 , polymer, and Fe species was vacuum-dried using a rotary evaporator. After ball-milling the dry powders for 24 hours, the subsequent heat treatment was performed at temperatures ranging from 800 to 1000°C in an inert atmosphere of nitrogen gas for one hour. The heat-treated sample was then pre-leached in 0.5 M H 2 SO 4 (typically using 100 mL of solution per 0.1 g of the catalyst powder) at 80°C for 8 hours to remove unstable and inactive species from the catalyst and then thoroughly washed in de-ionized water. In the final step, the catalyst was heat-treated again under identical conditions to the first heat treatment to reduce the oxygen-containing functional groups on the catalyst surface, thereby enhancing hydrophobicity. The product was a PANI-Fe-TiO 2 non-precious metal catalyst. In order to explore the roles of nitrogen, iron and TiO 2 in ORR catalysis, the performance of various catalysts synthesized in this study has been compared to that of the TiO 2 , heat-treated PANI-coated TiO 2 (denoted as PANI-TiO 2), and PANI-Fe supported on carbon black, Ketjenblack EC-300J (PANI-Fe-C). These samples were subjected to identical heat-treatment and chemical post-treatment steps. ORR activity was electrochemically evaluated with rotating disk electrode (RDE) and catalyst selectivity for the four-electron reduction of oxygen (hydrogen peroxide yield) was determined with rotating-ring-disk electrode (RRDE). RDE/RRDE measurements were performed using a CHI Electrochemical Station (Model 750b) in a conventional three-electrode cell. To avoid any potential contamination of non-precious metal catalyst by platinum, all experiments were carried out with a graphite rod as the counter electrode. A Ag/AgCl …