Systematic study of transition-metal (Fe, Co, Ni, Cu) phthalocyanines as electrocatalysts for oxygen reduction and their evaluation by DFT

In this work, a facile approach is reported to prepare a series of transition-metal phthalocyanines (TMPc) supported on graphitized carbon black (TMPc/GCB, TM: Fe, Co, Ni and Cu) as oxygen reduction reaction (ORR) electrocatalysts, via pi-pi interaction self-assembly. Through transmission electron microscopy (TEM), Raman spectroscopy and UV spectroscopy, it was found that TMPc was coated on graphitized carbon black with non-aggregated morphology. The catalytic activity, both in terms of the onset potential (0.98 V to 0.76 V) and half-wave potential (0.90 V to 0.55 V) follows the trend of FePc/GCB > CoPc/GCB > CuPc/GCB > NiPc/GCB. However, the catalytic durability follows the decreasing order of NiPc/GCB > CoPc/GCB > FePc/GCB > CuPc/GCB. To better elucidate the ORR catalytic mechanism for TMPc/GCB, we employed density functional theory (DFT) calculations and drew the following conclusions: (i) the -O-2 adsorption is the major step to determine the ORR catalytic activity; (ii) the way O-2 is adsorbed on TMPc is the key point affecting the Tafel slope; (iii) the -H2O2 desorption determines the transfer electron number; and (iv) the -OH desorption and the central metal atom removal leads to the damage affecting catalytic durability.