Tailoring the density of states of Ni(OH)2 with Ni0 towards solar urea wastewater splitting

Abstract

Solar urea wastewater splitting is capable of producing hydrogen and degrading the urea pollutant simultaneously. Nickel hydroxide (Ni(OH)₂) has been recognized as an effective cocatalyst for the urea oxidation reaction (UOR). But the lack of an efficient preparation method and a suitable Ni(OH)₂ based cocatalyst limits the performances of solar urea wastewater splitting. Herein, a potential-cycling method is developed with a high-purity nickel plate serving as the counter electrode and nickel source in a three-electrode configuration. Spherical Ni⁰-doped Ni(OH)₂ nanoparticles are successfully synthesized on the surface of TiO₂ nanorod arrays. The photocurrent density of TiO₂/Ni⁰:Ni(OH)₂ can reach 0.56 mA cm⁻² at 1.23 V_RHE in 1 M NaOH and 0.33 M CO(NH₂)₂ mixed electrolyte under AM1.5G illumination, which is 1.75 and 1.93 times those of TiO₂/Ni(OH)₂ deposited using a normal potentiostatic method with nickel salt solution and pristine TiO₂, respectively. Ni⁰ doping can significantly decrease the charge transfer resistance and provide a more favorable distribution of density of states of Ni(OH)₂ for the UOR. Furthermore, Ni⁰:Ni(OH)₂ decorated TiO₂ photoanodes exhibit good photocurrent retention during 12 h continuous testing. This work expands the preparation technique of urea catalysts and the strategy for developing highly efficient nickel-based catalysts.