Oxygen reduction reaction via the 4-electron transfer pathway on transition metal hydroxides

Carbon-supported Co(OH)2 and Ni(OH)2 catalysts are prepared to examine the mechanism of oxygen reduction reaction (ORR) on hydroxide catalysts. ORR via the 4-electron transfer pathway on a hydroxide undergoes oxidation of hydroxide by O2 to form oxyhydroxide, followed by electrochemical reaction of oxyhydroxide to regain hydroxide. β-Ni(OH)2 has the same crystal structure and lattice parameters as β-Co(OH)2, but it exhibits a poorer catalytic activity toward ORR than β-Co(OH)2 at a low temperature. The poor catalytic activity of Ni(OH)2/C can be attributed to the difficulty in Ni(OH)2 oxidation and the slow kinetics of NiOOH electroreduction to Ni(OH)2. The catalytic activity of the Ni(OH)2/C catalyst is significantly improved through elevating the operation temperature because Ni(OH)2 oxidation to NiOOH and NiOOH electroreduction are improved at a high temperature. A model of ORR via the 4-electron transfer pathway on transition metal hydroxides is suggested and discussed.

Research highlights
► The reaction of hydroxides with O2 to form oxyhydroxides during oxygen oxidation reaction (ORR) plays an important role in ORR on metal hydride catalysts.
► The electrochemical redox reaction between metal oxyhydroxide and hydroxide is an important step reaction in ORR on metal hydroxide catalysts.
► A new mechanism of ORR on metal hydroxides is suggested.