Efficient purification and chemical energy recovery from urine by using a denitrifying fuel cell

Urine is a major biomass resource, and its excessive discharge would lead to severe aquatic nitrogen pollution and even eutrophication. In this study, we designed an innovative denitrifying fuel cell (DFC) under illumination to purify urine and convert its chemical energy into electricity. The central ideas include the following: 1) on the anode, chlorine radicals (Cl) and hydroxyl (HO) radicals were induced to react with amine or ammonia in urine into N₂, and to mineralize organics into CO₂, respectively; 2) on the cathode, NO₂^- or NO₃^- generated in the cell was selectively reduced to N₂ and tiny NH₄⁺ by Pd/Au/NF; 3) NH₄⁺ was further oxidized to N₂ by Cl according to process 1), then the total nitrogen (TN) was ultimately removed by a continuous redox loop between anode and cathode; 4) the separation and migration of charges were strengthened by a self-bias poly-Si/WO₃ photoanode. Result indicated that the DFC showed an efficient yield of electricity and almost completely N-removing properties: power density of 2.24 mW cm^-2, total nitrogen and total organic carbon (TOC) removal efficiency, respectively 99.02% and 50.76% for artificial urine; and power density of 2.51 mW cm^-2, TN and TOC removal efficiency, respectively 98.60% and 54.55% for actual urine. The study proposes a potential and environment-friendly approach by using novel DFC to purify urine and generate electricity.