Graphite-Conjugation Enhances Porphyrin Electrocatalysis

We synthesize porphyrins that are strongly electronically coupled to carbon electrodes by condensing diaminoporphyrin derivatives onto o-quinone moieties native to graphitic carbon surfaces. X-ray photoelectron and absorption spectroscopies along with electrochemical data establish the formation of a high-fidelity conjugated pyrazine linkage to the surface with preservation of the metalloporphyrin scaffold. Using the O2 reduction reaction (ORR) as a probe, we find that conjugation dramatically enhances the rate of catalysis. A graphite-conjugated Co porphyrin, GCC-CoTPP, displays an onset current density of 10 μA/cm2 at 0.72 V versus the reversible hydrogen electrode, whereas a nonconjugated amide-linked Co porphyrin onsets at 0.66 V. This corresponds to an order of magnitude enhancement in the activation-controlled turnover frequencies for ORR upon conjugation. This work establishes a versatile platform for examining the emergent reactivity of porphyrins strongly coupled to metallic electrodes.