Repurposing discarded porphyrin waste as electrocatalysts for the oxygen reduction reaction

The oxygen reduction reaction (ORR), presently known as the key bottleneck in the mass-scale implementation of fuel cells (FCs), typically relies on the exploitation of scarce and expensive platinum group metals (PGMs). Meanwhile, as a substitute for PGMs, transition metal-nitrogen carbons (TM-Nx-C) are proving to be reliable electrocatalysts (ECs) in which atomically dispersed TMs coordinated with nitrogen are integrated into the carbon matrix. Such TM-Nx coordination already exists in metal-porphyrins making them suitable precursors for TM-Nx-C. Adler-Longo method is the standard recipe for meso‑tetraphenyl porphyrin realizes ca. 20 % yield whereas the residual polypyrromethenes, structurally resembling open porphyrin rings, are often wasted. Herein, the possibility of upcycling waste polypyrromethenes into efficient TM-Nx-C for ORR is demonstrated. A comprehensive structural and morphological characterization is provided, and the electrocatalytic activity towards ORR in an alkaline environment is discussed using Fe and Mn as TMs. The EC synthesized from pure porphyrin precursor at 600 °C (FeTPP_600) had the best performance recording 0.972 and 0.852 V vs RHE for Eonset and E1/2. Mixing porphyrins with their synthetic waste (ratio of 1:4) and pyrolyzing it at 800 °C (FeTPP/Waste(1:4)_800) still exhibits appreciable kinetics with similar results (0.977 and 0.853 V vs RHE for Eonset and E1/2).