Defect-engineered TiO2 nanotube cathode for nitrate reduction to ammonia and upcycling into (NH4)2SO4 in the paired electrolysis system

Wastewater contains significant amounts of NO3- that can be directly electrochemically upcycled into the valuable ammonia products. In this study, based on the density functional theory, a defect-engineered TiO2 nanotube array cathode (Co-BTNA) was developed for NO3- reduction and further upcycling into (NH4)2SO4 in the paired electrolysis system. The doped cobalt and oxygen vacancy defects in the Co-BTNA synergistically facilitated the electrons transfer to NO3- with high selectivity of NH3. Meanwhile, the acid anolyte produced via the electrochemical half-reaction at anode was used to trap NH3 for (NH4)2SO4 production. At a current density of 18 mA·cm−2, 100 % of NO3- removal efficiency and 93.0 % of NH3 selectivity were achieved with NH3 recovery rate of 182.25 g-(NH4)2SO4·d−1·gcat−1 at energy consumption of 27.1 kWh·kg−1(NH4)2SO4. In general, this study proposed a sustainable and efficient approach for upcycling wasted NO3- into ammonia fertilizer without external acid and alkali addition.