Development of Carbon Fiber-Based Ni(OH)2 Electrode for Application in Urea Wastewater Treatment/Hydrogen Production

Abstract

Introduction Urea-containing wastewater is discharged every day from factories, agricultural fertilizers, and human/animal urine, etc. If we do not properly teat it in time, it will cause environmental pollution, health hazards and other problems. Conversely, treating urea-containing wastewater can produce H2 energy source through an electrolysis process [1], which is considered attractive due to its lower electrolysis voltage than that for water splitting. In the past, nickel hydroxide (Ni(OH)2) has been used as a catalyst material for urea oxidation reaction (UOR), because it has several advantages such as cheap and good catalytic effect on urea. However, UOR performance is still not satisfied due to poor material design for electrodes, and thus needs to be improved. In light of this, our research is committed to developing a composite electrode integrating Ni(OH)2, carbon nanotubes (CNTs), and carbon fibers (CFs). The electrode allows to enhance UOR performance, which can simultaneously benefit urea degradation and H2 production. Experimental CFs were used as the electrode substrate, and Ni(OH)2 were deposited on the CFs surface together with carbon nanotubes (CNTs), through a one-step electrophoretic co-deposition method. Afterward, the composite was further experienced a hydrothermal reaction to complete the fabrication of the Ni(OH)2/CNT/CF electrode. SEM, XRD, Raman spectroscopy, and TGA were used to examine the material properties of the composite electrode. A three-electrode electrolytic system composed of the composite electrode, a Pt counter electrode, and a Ag/AgCl reference electrode was set up to treat the urea-containing wastewater. During the treatment, cyclic voltammetry and linear sweep voltammetry were applied to investigate the Ni(II)/Ni(III) redox reaction and UOR. Details about the electrode kinetics were also discussed. Results and Discussion The experimental results show that the Ni(OH)2/CNT/CF electrode has more favorable UOR performance compared to the Ni(OH)2/CF and CF ones. In addition, the α-Ni(OH)2-phase catalyst is found to present better electrocatalytic activity than the β-Ni(OH)2-phase catalyst. This could be due to larger d spacing of (001) plane in α-Ni(OH)2, allowing more inserted water molecules than those in β-Ni(OH)2. Accordingly, it expedites ion transport between the Ni(OH)2 sheets and repeated Ni(II)/Ni(III) transformation during continuous UOR [2], corresponding to better electrochemical performance. On the other hand, doping CNTs is found to benefit the increased reaction area and electrical conduction of the composite electrode. As a result, the required overpotential for UOR is reduced and the reversibility of Ni(II)/Ni(III) redox reaction is improved, leading to more efficient UOR.