Anodic deposition of NiO_x water oxidation catalysts from macrocyclic nickel(ii) complexes

Molecular complexes have been found to be excellent precursors for the deposition of catalytically active metal oxide films. Here, three macrocyclic Ni(ii) amine complexes have been used for the electrochemical deposition of NiO_x films from either a borate buffer solution (pH 9.2) or more basic conditions (pH = 12.9). The cyclic voltammetry of the complexes in both electrolytes shows very similar features and indicates the deposition of a catalytically active nickel oxide film. The NiO_x films have been characterised using infrared (IR) and Raman spectroscopy complemented by scanning electron microscopy. Testing of the water oxidation activity at pH 9.2 and 12.9 showed that the films deposited from macrocyclic Ni(ii) complexes exhibit similar catalytic activity to those derived from Ni²⁺ salts. The macrocyclic complexes offer the advantage of greater solubility and solution stability over a wider range of deposition conditions. At pH 9.2, the catalytic activity of the NiO_x films was significantly higher when using a borate buffer and, in addition, the films were more active at pH 12.9 than at pH 9.2. The NiO_x films deposited from molecular complexes were also found to show electrochromic properties. The oxidation of water by these films was enhanced by visible light. Water oxidation currents were observed to increase by ∼20% under simulated solar radiation.