Photoelectrochemical Studies on Earth Abundant Pentanickel Polyoxometalates as Co-Catalysts for Solar Water Oxidation

The present contribution reports results from photoelectrochemical studies on anatase TiO2 photoanodes for solar water oxidation in the presence of an earth abundant homogeneous pentanickel silicotungstate molecular water oxidation catalyst K10H2[Ni5(OH)6(OH2)3(Si2W18O66)]·34H2O (Ni5-POM). Highly robust anatase TiO2 thin films were prepared in situ on fluorine-doped tin oxide glass (F : SnO2 or FTO) by spin coating and thermal annealing. The as-prepared films were characterized by scanning electron microscopy, energy dispersive X-ray analysis, and UV-vis spectroscopy. Under simulated solar irradiation, a maximum photocurrent of 0.20 mA cm−2 was obtained from FTO/TiO2 photoanodes in the presence of 20 μM Ni5-POM-0.050 M pH 9.0 borate buffer electrolyte at 0.70 V vs. Ag/AgCl (3.0 M KCl), which was a 2.3 times increase in photocurrent as compared to that without using the Ni5-POM catalyst. The use of the catalyst also significantly shifted the onset potential of water oxidation to a much negative potential value with respect to −0.2 V vs. Ag/AgCl. As verified by electrochemical impedance spectroscopy, the above results could be attributed to the effect of the catalyst on decreasing electron–hole recombination and facilitating the hole utilization rate on the semiconductor TiO2–electrolyte interface upon light illumination. Dissolved oxygen produced from the FTO/TiO2–Ni5-POM system was detected with an oxygen sensor. This is the first report on the use of the homogeneous water oxidation catalyst Ni5-POM as a functional co-catalyst for photoelectrochemical water oxidation with semiconductor photoanodes