Inhibition of Surface Chemical Moieties by Tris(hydroxymethyl)aminomethane: A Key to Understanding Oxygen Reduction on Iron–Nitrogen–Carbon Catalysts

The effect of tris(hydroxymethyl)aminomethane (Tris) on the activity of metal–nitrogen–carbon (M–N–C) platinum metal group-free (PGM-free) catalysts for oxygen reduction reaction (ORR) is studied. Electrochemical tests performed in the acidic and alkaline electrolyte and comparison between electrochemical observations for metal-free and metal-containing catalysts along with density functional calculations showed the multiplex mechanism of oxygen reduction in which numerous species perform partial and full reduction of oxygen to peroxide or water, respectively. The important role of protons in the mechanism of oxygen reduction is highlighted.