palladium; oxygen reduction; rotating disc; spectroscopic ellipsometry; atomic force microscopy
Oxygen reduction reaction (ORR) is one of the most extensively studied electrochemical reactions due to its importance in fuel cells technology. Palladium based electrocatalysts have attracted increasing interest for oxygen reduction reaction in both Biotin Hydrazide and alkaline solutions. The activity of bare Pd, as being close to Pt, the most active of all pure metals for ORR, has been studied in details on various Pd single crystals in acid media . It was shown that Pd(100) is the most active of all Pd low index surfaces, exceeding the activity of Pt(110), as the most active among low index Pt surfaces . Detailed investigation of the activity of Pd single crystals or polycrystalline Pd, Pd(poly), for ORR in alkaline media is missing.
ORR has been widely studied on various carbon supported Pd catalysts in both acid and alkaline solutions. ORR on electrodeposited Pd coatings on glassy carbon proceeded mainly through four-electron reaction pathway in both acid and alkaline solutions, and the reaction mechanism was similar to that on bulk Pd  and . The carbon-supported Pd nanoparticles have shown high activity towards ORR in alkaline solution, comparable to the activity of Pt/C . Cubic palladium nanoparticles of predominantly (100) orientation have shown the enhanced electrocatalytic activity towards oxygen reduction in both acid and alkaline media as compared to spherical Pd nanoparticles or bulk Pd . In addition, it has been reported that the modification of Pd(111) and carbon supported Pd nanoparticles themselves by Pt monolayer contributed to the enhancement of the catalytic activity for ORR in acid solutions in comparison with the reaction on Pt(111) and Pt nanoparticles .