Development of a Mechanically Flexible 2D-MXene Membrane Cathode for Selective Electrochemical Reduction of Nitrate to N2: Mechanisms and Implications

The contamination of water resources by nitrate is a major problem. Herein, we report a mechanically flexible 2D-MXene (Ti₃C₂T_x) membrane with multilayered nanofluidic channels for a selective electrochemical reduction of nitrate to nitrogen gas (N₂). At a low applied potential of -0.8 V (vs Ag/AgCl), the MXene electrochemical membrane was found to exhibit high selectivity for NO₃^- reduction to N₂ (82.8%) due to a relatively low desorption energy barrier for the release of adsorbed N₂ (*N₂) compared to that for the adsorbed NH₃ (*NH₃) based on density functional theory (DFT) calculations. Long-term use of the MXene membrane for treating 10 mg-NO₃-N L^-1 in water was found to have a high faradic efficiency of 72.6% for NO₃^- reduction to N₂ at a very low electrical cost of 0.28 kWh m^-3. Results of theoretical calculations and experimental results showed that defects on the MXene nanosheet surfaces played an important role in achieving high activity, primarily at the low-coordinated Ti sites. Water flowing through the MXene nanosheets facilitated the mass transfer of nitrate onto the low-coordinated Ti sites with this enhancement of particular importance under cathodic polarization of the MXene membrane. This study provides insight into the tailoring of nanoengineered materials for practical application in water treatment and environmental remediation.