Mechanistic pathways and kinetic studies of oxygen reduction reaction (ORR) at Covalent Triazine Frameworks (CTFs)

Herein, four different metal-free nitrogen containing Covalent Triazine Frameworks (CTFs) based on their applied monomers (DCP, DCBP, mDCB and pDCB) are synthesized via a classical ionothermal synthesis route (ZnCl2, 400/600 degrees C). These materials are fully characterized and their electrochemical activities for ORR are tested and compared to each other including Carbon Super P and Pt black as standards in 0.1 M KOH. While DCP provides similar catalytic activity to Carbon Super P showing mostly a 2e- process (n=2.95) with high H2O2 formation of 52.6 %, the other three CTFs (DCBP, mDCB and pDCB) possess higher ORR activities, surprisingly even much higher limiting current densities than Pt black, proving that O2 is mainly reduced via direct 4epathway since n values are in the range of 3.52 to 3.62 and the detected H2O2 values are in the range of 19-23.9 %. Among the studied CTFs, mDCB reaches a limiting current density of -5.61 mA cm-2 (1.21 mA cm-2 larger than that for Pt black, -4.40 mA cm-2) with 0.11 V larger onset potential compared to Pt black. The significant electrochemical performances of the CTF materials in ORR via a 4e- process are correlated to the high specific surface areas (up to 2500 m2 g-1), large pore volumes (up to 2.05 cm3 g-1) and the largest total N-graphitic/ quaternary contents as well as micro-mesoporous structure properties.