The design of more effective polynuclear catalysts for the Oxygen Reduction Reaction (ORR) is focused on lowering overpotentials, increasing Faradic efficiencies, and improving selectivity. We have synthesized catalysts with two cobalt porphyrins and four rhodium-based “molecular clips” using coordination-driven self-assembly. This synthetic approach is advantageous because of its modularity wherein structures may be tuned by selecting different building blocks and employing the same self-assembly conditions. Our approach allows for the rapid synthesis of these architectures in high isolated yields. Structural and reactivity studies provided a deeper understanding of the role of the catalyst’s geometry in the ORR. Catalytic efficacy was probed using electrochemical methods. The selectivity of these catalysts changes for the kinetically difficult 4e-/ 4H+ reduction of O2 to H2O over the kinetically more accessible 2e-/2H+ reduction to H2O2 with changing geometries of the prisms. Furthermore, we have demonstrated significant cofacial enhancements as measured by the observed standard rate constant ks over a mononuclear analogue. The geometries of the catalysts play an important role in the catalytic reactivity of these prisms, specifically the selectivity towards H2O over H2O2, which for certain architectures rivals the selective of state-of-the-art platinum catalysts.

Coordination-driven self-assembly of cofacial porphyrin prisms as electrocatalysts for oxygen reduction

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ACS Fall 2022

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