Fluorine Modification Promoted Water Dissociation into Atomic Hydrogen on a Copper Electrode for Efficient Neutral Nitrate Reduction and Ammonia Recovery

Abstract

Electrocatalytic nitrate reduction to ammonia (NITRR) offers an attractive solution for alleviating environmental concerns, yet in neutral media, it is challenging as a result of the reliance on the atomic hydrogen (H*) supply by breaking the stubborn HO-H bond (∼492 kJ/mol) of H₂O. Herein, we demonstrate that fluorine modification on a Cu electrode (F-NFs/CF) favors the formation of an O-H···F hydrogen bond at the Cu-H₂O interface, remarkably stretching the O-H bond of H₂O from 0.98 to 1.01 Å and lowering the energy barrier of water dissociation into H* from 0.64 to 0.35 eV at neutral pH. As a benefit from these advantages, F-NFs/CF could rapidly reduce NO₃^- to NH₃ with a rate constant of 0.055 min^-1 and a NH₃ selectivity of ∼100%, far higher than those (0.004 min^-1 and 9.2%) of the Cu counterpart. More importantly, we constructed a flow-through coupled device consisting of a NITRR electrolyzer and a NH₃ recovery unit, realizing 98.1% of total nitrogen removal with 99.3% of NH₃ recovery and reducing the denitrification cost to $5.1/kg of N. This study offers an effective strategy to manipulate the generation of H* from water dissociation for efficient NO₃^--to-NH₃ conversion and sheds light on the importance of surface modification on a Cu electrode toward electrochemical reactions.