Charge-Transfer Mechanism in Oxygen Reduction over Co Porphyrins: Single-Site Molecular Electrocatalysts to Macromolecular Frameworks

Insight into the operational principles of heterogeneous molecular electrocatalysts is indispensable for the design of low-cost cathodic materials for fuel cells. Herein, we report a mechanistic study of oxygen reduction reaction (ORR) catalyzed by Co tetraphenylporphyrin (CoTPP) in covalent and noncovalent immobilization modes. It was found that the noncovalently immobilized catalyst displays a low ORR rate and moderate selectivity to the 4e(-) pathway of 39%. In contrast, covalent grafting boosts the ORR current by a factor of 1.6 and improves the contribution of the 4e(-) pathway to 47%. The combination of in situ spectroscopy and electrokinetic studies shows that that the molecular-level ORR mechanism involves O-2 adsorption as a rate-determining step and (CoTPP)-T-II as a resting state of the catalyst. Furthermore, a recently developed variable-frequency square wave voltammetry (VF-SWV) was employed for the direct electrochemical imaging of heterogeneous charge-transfer rates for the Co-III/Co-II transformation. We determined that the covalently grafted complex forms an extended macromolecular framework featuring a net of porphyrin-to-porphyrin bonds. Such an architecture enables high equilibrium charge-transfer rates k(0)(Co-III/Co-II) onto the CoTPP centers of up to 200 s(-1) accompanied by a strong outbound propagation of electrons across the surface layer. In contrast, noncovalently immobilized complex behaves mostly as a continuum of noninteractive sites with low electron transfer rate constant k(0)(Co-III/Co-II) < 1 s(-1) and minimum intermolecular electron hopping. Based on these experimental results, a macromolecular ORR mechanism revolving around the mutually balanced fluxes of charges and reactants was established. Thus, the performance of a molecular electrocatalyst could be conveniently controlled via the adjustment of the surface layer structure.