Urine Treatment in a Stacked Membraneless Direct Urea Fuel Cell with Honeycomb-like Nickel–Molybdenum Bimetal Phosphide as the Anodic Electrocatalyst

The aim of this study was to synthesize a novel catalyst for urea oxidation and to test a stacked membraneless direct urea fuel cell (DUFC) with raw urine as fuel. The molybdenum nickel phosphides on nickel foam (MoNiP/NF) were synthesized using a combined hydrothermal and phosphating method. The honeycomb-like MoNiP/NF catalyst with a Mo/Ni molar ratio of 0.50 (i.e., MoNiP/NF-0.50) showed the highest electrocatalytic activity for urea oxidation among different catalysts. The stacked membraneless DUFC was constructed using the MoNiP/NF-0.50 electrode as anode and a gas diffusion cathode. With the electrode spacing of 5 mm and 6 electrode pairs, the stacked membraneless DUFC had a maximum voltage of 0.55 V and a power density of 0.115 mW cm–2 at the external resistance of 1000 Ω, at which the power output was 5.32 times higher than that in the individual membraneless DUFC. The urea removal reached 65.8% in the cell at the external resistance of 100 Ω within 192 h. The excellent performance of the cell could be attributed to the high activity of the MoNiP/NF-0.50 catalyst, small electrode spacing, and multiple electrode pairs. Our results should provide promising potential for scalable DUFC with efficient urine treatment.