Electrocatalytic activity of metalloporphyrins grown in situ on graphene sheets toward oxygen reduction reaction in an alkaline medium

A series of non-noble-metal catalysts for oxygen reduction reaction (ORR), based on metal 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin (M-THPP, M: Fe3+, Co2+, Ni2+, Mn2+) grown on poly(sodium-p-styrenesulfonate) modified reduced graphene oxide (PSS-rGO), were fabricated using an in situ solvothermal synthesis method. The morphology of the M-THPP/PSS-rGO was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Ultraviolet-visible (UV-vis) absorption spectroscopy and X-ray photoelectron spectroscopy (XPS) techniques were utilized to analyse the unusual interactions between the metalloporphyrins and graphene sheets. Electrochemical measurements using rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques were employed to study the catalytic activity and the mechanism of the oxygen reduction reaction on the as-synthesised M-THPP/PSS-rGO catalysts in an alkaline medium. The half-wave potential for the ORR on the CoTHPP/PSS-rGO catalyst was found to be around -0.22 V vs. SCE, which was much higher than those on the other M-THPP/PSS-rGO catalysts and similar to that on Pt/C (-0.20 V vs. SCE). RDE and RRDE results show that the ORR process proceeds mainly via an almost 4-electron pathway on CoTHPP/PSS-rGO. The catalyst stability tests disclose that the CoTHPP/PSS-rGO is much more stable than the other M-THPP/PSS-rGO composites. The assembled CoTHPP/PSS-rGO catalyst possesses high activity, good long-term stability, excellent tolerance to the crossover effect of methanol and a facile 4-electron pathway for ORR, which could be used as a promising Pt-free catalyst in an alkaline direct methanol fuel cell.