Identification of catalytic sites for cerium redox reactions in a metal-organic framework derived powerful electrocatalyst

The development of N-doped carbon materials with highly catalytic activity for catalyzing electrode reactions as alternatives to noble metal is of great importance for the practical implementation of redox flow batteries (RFBs). However, in addition to C-N_x sites, the high structural heterogeneity of N-doped carbon materials often contains metal-N_x sites and graphene encapsulated metal nanoparticle simultaneously, as a result of their metal-assistant synthesis method. Thus it is challenging to identify the predominant active structure within N-doped carbon catalysts for RFBs, which normally receives far less attention in the field of RFB research. Herein, we report a general metal-organic frameworks assisted approach to high-yield growth of nitrogen-doped carbon nanotubes (NCNTs) onto 3D graphite felt (GF) substrates without the requirement of any additional C and N sources or costly and complex CVD systems. The obtained NCNTs-modified GF (NCNT-GF) exhibits high electrocatalytic activity and stability toward cerium redox reactions. Based on the moderate and selective NH₄Cl-purification process and the electrochemical poison experiments, we definitely elucidate the spectator role of atomically dispersed metal-N_x sites and graphene encapsulated metal nanoparticles, and the pivotal role of C-N_x sites in the NCNT-GF materials for cerium redox reactions.