A Hybrid Molecular Photoanode for Efficient Light Induced Water Oxidation

A hybrid photoanode, made of a multilayered heterostructured WO3/BiVO4 semiconductor and a molecular water oxidation catalyst Ru(tda)(py-pyr)2 (Ru-WOC, where tda is [2,2’:6’,2’’-terpyridine]-6,6’’-dicarboxylato and py-pyr is 4-(pyren-1-yl)-N-(pyridin-4-ylmethyl)butanamide), is described. Both elements are linked by a highly conductive carbon nanotubes fibre film (CNTf), which acts as charge transfer and anchoring platform, where the catalyst is attached through pi-pi stacking interactions. Photoelectrochemical characterization of the resulting electrodes shows that the full photoanode WO3/BiVO4/CNTf/Ru-WOC outperforms the bare WO3/BiVO4 electrode at a potential range of 0.3 V to 0.8 V vs NHE at pH 7, with current densities enhanced by 0.05 to 0.29 mA/cm2. Bulk electrolysis experiments and oxygen gas measurements prove that the enhanced photocurrent is due to the catalytic water oxidation reaction. Detailed electrochemical impedance spectroscopy (EIS) analysis is used to investigate the role of the multiple layers involved in the process. The CNTf/Ru-WOC interface is responsible of increasing charge accumulation and also reducing recombination phenomena. On one hand, the CNTf is able to hold the charge coming from the light absorbed by the WO3/BiVO4 semiconductor, as shown by the high capacitive values observed for a WO3/BiVO4/CNTf electrode in the whole range of studied potentials (0.15-0.85 V vs NHE). On the other hand, the Ru-WOC transfers the charge to the solution by performing fast water oxidation catalysis. This is supported by the low resistivity shown by the full WO3/BiVO4/CNTf/Ru-WOC electrode at low potentials (E < 0.5 V vs NHE). The robustness and high catalytic rate of the Ru-WOC ensures the proper performance of the hybrid photoelectrode device. The latter is particularly important as it opens an avenue of opportunities to improve the performance of photoanodes for the water oxidation reaction based on the easy modification of ligands of the molecular catalyst to tune its structural, electronic and catalytic properties. This is a unique advantage with regard to the commonly used catalysts based on metal oxide or oxy(hydroxides), with limited tunability.