Biphenyl appended non-noble metal complexes as electrocatalysts for the electrochemical oxygen reduction reaction

Abstract

The oxygen reduction reaction (ORR) is one of the most important reactions in many electrochemical processes. There has been a growing interest in the replacement of noble metal electrocatalysts for oxygen reduction reaction (ORR). In continuation of these efforts herein we report the design and synthesis of biphenyl appended non-noble transition metal and Zn(II) complexes as electrocatalysts in the aforementioned reaction. In this regard nitrogen-based bidentate ligand, 6-(quinolin-8-yl)-6,7-dihydro-5H-dibenzo[c,e]azepine as Ligand, L was synthesized and used for the preparation of a series of mononuclear Ni(II) and Zn(II) complexes, [ML(X)2] which differ only by the halide ion attached to the respective metal center. All complexes of this series showed tetrahedral geometry regardless of the halide ion and metal center (Ni/Zn). The complexes were characterized by different spectroscopic techniques. The triflate (OTf) complexes of Fe(II) and Cu(II) were also synthesized to evaluate their ORR activity. All the complexes were investigated for their electrocatalytic activity in oxygen reduction reaction (ORR) in which complex 1, [NiL(Cl)2] showed the highest activity with an onset potential of 0.75 V. The remaining complexes also showed significant ORR performance with onset potentials close to that of complex 1.