Influence of surface terminal groups on the efficiency of two-electron oxygen reduction reaction catalyzed by iron single atoms on Ti3C2Tx (T = Cl, Br, NH) MXene

Abstract

Two Ti₃C₂ MXenes having single atom Fe were prepared from a Ti₃AlC₂ precursor by Al etching using an FeX₂ (X = Cl, Br) Lewis acid as an etching reagent in NaCl/KCl molten salt. After removal of the excess Fe metal using concentrated aqueous HX solution, the resulting MXenes having Cl or Br surface terminations were concomitantly doped with single Fe atoms. A third sample was prepared from Fe(SA)-Ti₃C₂Br_x by exchanging Br with –NH– using LiNH₂ in molten salt. The presence of single atom Fe was determined by aberration corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) with atomic resolution where isolated Fe atoms were observed. These Fe(SA)-Ti₃C₂T_x materials exhibit electrocatalytic activity for the oxygen reduction reaction (ORR), with the selectivity to H₂O₂ depending on the surface termination and the electrolyte pH. Involvement of Fe SA was inferred from the onset potential of the ORR and the Fe(III)/Fe(II) reduction potential and the change of the Tafel slope in this region. Under the best conditions, at pH 13, Fe(SA)-Ti₃C₂(NH)_x exhibits for the ORR an average electron transfer number (n) of 2.5, which corresponds to an H₂O₂ generation efficiency of 75%; these data compare well with those of the best Fe single-atom electrocatalysts.