A new C=C embedded porphyrin sheet with superior oxygen reduction performance

C₂ is a well-known pseudo-oxygen unit with an electron affinity of 3.4 eV. We show that it can exhibit metal-ion like behavior when embedded in a porphyrin sheet and form a metal-free two-dimensional material with superior oxygen reduction performance. Here, the positively charged C=C units are highly active for oxygen reduction reaction (ORR) via dissociation pathways with a small energy barrier of 0.09 eV, much smaller than that of other non-platinum group metal (non-PGM) ORR catalysts. Using a microkinetics-based model, we calculated the partial current density to be 3.0 mA/cm² at 0.65 V vs. a standard hydrogen electrode (SHE), which is comparable to that of the state-of-the-art Pt/C catalyst. We further confirm that the C=C embedded porphyrin sheet is dynamically and thermally stable with a quasi-direct band gap of 1.14 eV. The superior catalytic performance and geometric stability make the metal-free C=C porphyrin sheet ideal for fuel cell applications.