Porphyrin Aggregation under Homogeneous Conditions Inhibits Electrocatalysis: A Case Study on CO₂ Reduction

Metalloporphyrins are widely used as homogeneous electrocatalysts for transformations relevant to clean energy, environmental remediation, and sustainable organic synthesis. It is common knowledge that metalloporphyrins are prone to aggregation due to π–π stacking of their planar structures, but surprisingly the influence of aggregation on homogeneous electrocatalytic performance has not been investigated previously. Herein, we present three structurally related iron meso-phenylporphyrins whose aggregation properties are expected to be different in commonly used N,N-dimethylformamide (DMF) electrolyte. Both spectroscopy and light scattering are used to provide evidence of extensive porphyrin aggregation under conventional electrocatalytic conditions. Using the electrocatalytic reduction of CO₂ to CO as a test reaction, cyclic voltammetry reveals an inverse dependence of kinetics on catalyst concentration for all three porphyrins. The inhibition at higher catalyst concentrations extends to bulk performance, where up to 75% of the catalyst at 1 mM is inactive compared to at 0.25 mM. We additionally report how aggregation is perturbed by organic additives, axial ligands, and porphyrin redox state. Periodic boundary calculations are used to gain additional insights into aggregate stability as a function of metalloporphyrin structure. Finally, we generalize the aggregation phenomenon by surveying metalloporphyrins with different metals and substituents. The takeaways of this study are that homogeneous metalloporphyrin catalysts can aggregate severely even in well-solubilizing organic electrolytes, that aggregation can be easily modulated through choice of experimental conditions, and that the extent of aggregation must be considered for accurate catalytic benchmarking.