Study of Oxygen Reduction Reaction on Polycrystalline Rhodium in Acidic and Alkaline Media.

This study examines the kinetics and mechanism of the oxygen reduction reaction (ORR) on a polycrystalline rhodium electrode (Rh(poly)) in acidic and alkaline media, using rotating disc electrode measurements. This study found that the ORR activity of the Rh(poly) electrode decreases in the order of 0.1 M NaOH > 0.1 M HClO₄ > 0.05 M H₂SO₄ concerning the half-wave potentials. The Tafel slopes for ORR on Rh(poly) in the cathodic direction are 60 and 120 mV dec⁻¹ at low and high overpotentials, respectively, in perchloric acid and alkaline solutions. However, strongly adsorbed sulfate anions hinder the ORR on Rh(poly) in sulfuric acid, leading to higher Tafel slopes. The highest ORR activity of Rh(poly) in an alkaline media suggests the promoting role of the specifically adsorbed OH⁻ anions and RhOH. In all cases, ORR on Rh(poly) proceeds through the 4e-series reaction pathway. [ABSTRACT FROM AUTHOR]