Water oxidation activity of azo‐azomethine‐based Ni (II), Co (II), and Cu (II) complexes

Abstract

Nickel, cobalt, and copper complexes were synthesized by the reaction of metal acetate salts and azo‐azomethine‐type ligand H2L (H2L = 4‐chloro‐1,2‐bis[2‐hydroxy‐5‐(phenylazo)benzylideneamino]benzene). The complexes were characterized by spectroscopic methods, molar conductivity measurements, and elemental analysis. The complexes were investigated as water oxidizing catalysts by several electrochemical techniques. Our findings revealed that the nature of the central metal ion plays an essential role in the stability of the complexes and their electrocatalytic activity. Although all modified electrodes with complexes showed good activities for water oxidation compared with bare carbon paste electrode, nevertheless, NiL showed a much superior electrocatalytic activity in basic solution in terms of onset potential and Tafel slope. Experiments indicated that at pH = 11, NiOx is probably a heterogeneous catalyst for the oxidizing of water in the presence of NiL. However, about CoL, it was revealed that a high valent cobalt oxo intermediate is active in the electrocatalytic process. On the other hand, field‐emission scanning electron microscope images showed the formation of nanorods on the electrode surface. However, upon our observations, it was difficult to determine the real role of CoL in the water oxidation reaction. Surprisingly, the results indicated that CuL is not stable under electrochemical conditions, and after performing the amperometry for a long time, its electrocatalytic activity decreases.