Ni and/or Co nanoparticles as catalysts for oxygen reduction reaction (ORR) at room temperature

Unsuccessful attempts in using Ni and/or Co nanoparticles as catalysts for oxygen reduction reaction (ORR) are reported. Several synthetic approaches such as encapsulation of nanoparticles in dendrimers, coating by polyethylene glycol and surface modification by anthraquinone were carried out to isolate the nanoparticles from aqueous corrosive media and to prevent their agglomeration at very low and very high pH. In all the attempts described, negative results towards ORR were consistently obtained. On the contrary, in the case of cobalt incorporated in a polypyrrole matrix (Co–Ppy–C) a catalytic activity for oxygen reduction both in acidic and alkaline medium was observed. The data indicate that neither Ni nor Co surface sites, when not coordinated to the matrix nitrogen, are electroactive towards oxygen reduction, pointing out the crucial role of the heteroatom associated with (coordinating) transition metals in the ORR catalytic process. We believe that our data are in agreement with literature although the reported data scarcity does not make the full conclusion possible. All results were obtained at room temperature, the catalytic activity of the synthesized materials at elevated temperatures are unknown.