Faradic side reactions at Ti3C2Tx-based cathode for ammonium extraction studied in a novel PG-MCDI system

This study addresses the challenge of efficient ammonium recovery from wastewater, a key issue in global nitrogen cycle management. A novel capacitive deionization (CDI) system, which innovatively integrated a gas diffusion membrane and a proton exchange membrane, called PG-MCDI, was firstly reported to realize NH4+ recovery from wastewater. This system was specifically designed to regulate the cathode side reaction, particularly oxygen reduction reaction (ORR), to control the local pH near cathode surface to maintain 8 - 10. The pH change can affect the NH4+/NH3 equilibrium, resulted in a speciation change from NH4+ to NH3 & sdot;NH3 will thus be extracted from the feed solution through gas diffusion membrane. Further, Ti3C2Tx-based particles were developed with enhanced NH4+ adsorption capacity through graphite C doping and a macroporous structure, alongside proper ORR performance reinforced by redox couple TiX+/Ti4+ to keep high interfacial pH. This dual-functional approach significantly improved the NH4+ removal capacity. The integrated PG-MCDI exhibited excellent NH4+ removal capacity of 278 mg N g-1 and energy consumption of 0.73 kWh kg- 1N at 1.5 V, which are superior to other electrochemical systems. The PG-MCDI system assembled with Ti3C2Tx-based cathode can be routinely employed as an effective strategy to address the energy and environmental issues of recovering NH4+ from wastewater.