(Photo) electrochemical water oxidation at anodic TiO2 nanotubes modified by electrodeposited NiFe oxy-hydroxides catalysts

Abstract

Anodic TiO2 nanotubes are potential candidates for industrial scale and stable photoelectrochemical water oxidation. Major roadblocks for the realization of this technology however are the limited light absorption, as this material usually absorbs only in the UV, and the sluggish water oxidation kinetics at the electrode/electrolyte interface. Various efforts have nonetheless demonstrated absorption and photocurrent generation with visible light while a typical solution for the slow water oxidation is electrode modification via oxygen evolution catalysts; NiFe (oxy) hydroxides in particular are among the highest turn-over efficiency and low-cost catalysts. Studies about loading NiFe (oxy) hydroxides onto anodic TiO2 nanotubes by electrodeposition methods are rarely reported and the influence of the loaded catalysts on the photoanode performance is not fully understood. In this work, NiFe (oxy) hydroxides are first electrodeposited on Au substrates; the as-deposited catalysts indicate enhanced oxygen evolution catalytic activity. The same deposition method is then applied at various TiO2 nanotube systems. Specifically, mouths, walls and whole bodies of anodic TiO2 nanotubes are electrodeposited with NiFe (oxy) hydroxides and the corresponding photo response to water oxidation are investigated. We show that at relatively low applied potential (≤1.5 V vs. RHE) the NiFe (oxy) hydroxides layer significantly reduces photon absorption and facilitates interfacial recombination, leading to a decreased photocurrent density. In contrast, at relatively high bias (>1.5 V vs. RHE), the oxygen evolution is enhanced due to the intrinsic electro-catalytic properties of the loaded catalysts. Synergistic effect are not found in the three NiFe (oxy) hydroxide films deposited at TiO2 nanotube systems.