Theoretical Investigation of Electron Transfer Mechanisms in Bioinspired Iron-Porphyrin Catalysts for Oxygen Reduction Relevant to Nigerian Fuel Cell Development

The development of fuel cell technologies for distributed power generation in Nigeria requires advances in electrocatalysis, particularly the oxygen reduction reaction at the cathode, which currently depends on expensive and scarce platinum group metals. Iron-porphyrin complexes, bioinspired by the active sites of cytochrome P450 enzymes and haemoproteins, offer a structurally diverse and earth-abundant alternative catalyst platform. However, the theoretical understanding of electron transfer mechanisms governing oxygen reduction in these systems under electrochemical conditions remains incomplete. This study conducted a theoretical investigation of electron transfer mechanisms in a library of synthetic iron-porphyrin catalysts using multi-scale computational methods, combining quantum mechanical calculations, molecular dynamics simulations, and microkinetic modelling to elucidate the catalytic cycle. Density functional theory calculations at the M06-2X/def2-TZVP level were employed to characterise all species along the proposed catalytic pathways, with dispersion corrections and implicit solvation. Non-adiabatic electron transfer rates were calculated using Marcus theory parameterised with quantum chemical data. Twelve novel iron-porphyrin structures were computationally designed by systematic variation of peripheral substituents and axial ligand identity to modulate the iron centre electronic environment. The most promising catalyst design predicted a turnover frequency for oxygen reduction 4.3 times greater than the unsubstituted iron tetraphenylporphyrin benchmark. These predictions were experimentally validated by rotating disk electrode voltammetry for three synthesised candidates. The original theoretical framework developed provides mechanistic insights into the rate-determining step as the second proton-coupled electron transfer and delivers experimentally testable design rules for improved iron-porphyrin oxygen reduction catalysts.

Keywords: iron-porphyrin, oxygen reduction reaction, electron transfer, density functional theory, fuel cell catalysis