Rapid Conversion of Co2+ to Co3+ by Introducing Oxygen Vacancies in Co3O4 Nanowire Anodes for Nitrogen Removal with Highly Efficient H2 Recovery in Urine Treatment

Abstract

Urine is a nitrogenous waste biomass but can be used as an appealing alternative substrate for H2 recovery. However, urine electrolysis suffers from sluggish kinetics and requires alkaline condition. Herein, we report a novel system to decompose urine to H2 and N2 under neutral conditions mediated by Cl• using oxygen-vacancy-rich Co3O4 nanowire (Ov-Co3O4) anodes and CuO nanowire cathodes. The Co2+/Co3+ cycle in Co3O4 activates Cl- in urine to Cl•, which rapidly and selectively converts urea into N2. Thus, electron transfer is boosted for H2 production, eliminating the kinetic limitations. The shuttle of Co2+ to Co3+ is the key step for Cl• yield, which is accelerated due to the introduction of Ov. Electrochemical analysis shows that Ov induces positive charge on the Co center; therefore, Co2+ loses electrons more efficiently to form Co3+. H2 production in this system reaches 716 μmol h-1, which is 320% that of non-radical-mediated urine electrolysis. The utilization of Ov-Co3O4 further enhances H2 generation, which is 490 and 210% those of noble Pt and RuO2, respectively. Moreover, urine is effectively degraded in 90 min with the total nitrogen removal of 95.4%, and N2 is the final product. This work provides new insights for efficient and low-cost recovery of H2 and urine remediation.