In Situ Transition of a Nickel Metal–Organic Framework on TiO2 Photoanode towards Urea Photoelectrolysis

Photoelectrochemical (PEC) urea splitting is of great significance for urea wastewater remediation and hydrogen production with low energy consumption simultaneously. Nickel hydroxides as electrocatalysts have been widely investigated for urea electrolysis. However, it is an open question how to synthesize highly catalytic Ni(OH)2for the PEC urea splitting. Herein, we take advantage of the instability of metal–organic frameworks (MOFs) to perform an in situ synthesis of Ni(OH)2catalysts on the surface of TiO2nanorod arrays. This transformed Ni(OH)2(T-Ni(OH)2) possesses a superior PEC catalytic activity for water/urea splitting in comparison to the Ni(OH)2prepared by the impregnation method. The in situ transition of a Ni-MOF is accomplished through an electrochemical treatment under AM1.5G illumination in a KOH-and-urea mixed electrolyte. The specific transition mechanism of Ni-MOFs is the substitution of ligands with OH−in a 1 M KOH electrolyte and the successive phase transition. The T-Ni(OH)2@TiO2photoanode delivers a high photocurrent density of 1.22 mA cm−2at 1.23 VRHE, which is 4.7 times that of Ni(OH)2@TiO2prepared with the impregnation method. The onset potential of T-Ni(OH)2@TiO2is negatively shifted by 118 mV in comparison to TiO2. Moreover, the decline of photocurrent during the continuous test can be recovered after the electrochemical and light treatments.Keywords:photoelectrochemical urea splitting;urea oxidation reaction;titanium dioxide;metal–organic framework;in situ transition