Oxygen Reduction on Rh(111) and Se-Modified Rh(111) Surfaces

Oxygen reduction is studied at Rh(111) and Rh(111) modified by a Se adlayer. Controlled mass transport was achieved in a flow through cell. Hydrogen peroxide, detected at a second electrode behind the working electrode, amounted to few percent at Rh(111) and for low Se coverages, at higher coverages it amounted up to 40%. Small coverages of Se lead to a decrease of the over potential by 50 mV. The Tafel slope amounts to 80 mV at potentials positive of the hydroxyl reduction peak and nearly 200 mV at potentials below. These values do not change after modification by Se. Hydrogen adsorption is completely suppressed for Se coverages of 0.5 (Se atoms per Rh atoms). In addition, the point of total zero charge was determined to be 0.2 V using the CO charge displacement method.