Effect of Manganese Porphyrin Covalent Immobilization on Electrocatalytic Water Oxidation and Oxygen Reduction Reactions

Complexes of first row transition metals are a promising class of tunable and inexpensive catalysts for electrochemical energy applications. Although considerable efforts have been devoted to the activity studies, little attention has been paid to the effects of different immobilization modes on reaction mechanisms. In this work, we studied the influence of covalent immobilization on the performance of Mn tetraphenylporphyrin in oxygen evolution (OER) and oxygen reduction (ORR) reactions. Ligation of the complex to carbon surface was attained via potentiostatic electroreduction of porphyrin diazonium salt with the following metalation and electrodeposition time was found to be a convenient tool to control the amount of electrochemically active catalyst on the electrode. Cyclic voltammetry suggests that the increase of porphyrin surface concentration upon prolonged electrodeposition shortens average Mn–Mn distance and proportionally enhances probability of at least two metal atoms simultaneously participating in a catalytic process. Optimization of organic layer density has profound effect on the catalyst performance in ORR in alkaline medium. 5 min electrodeposition furnishes the best catalyst, which features the 4-electron pathway being predominant at low overpotentials where the noncovalent counterpart shows selectivity to H₂O of ∼50%. What is more, overall catalytic current at −0.79 V vs NHE was 2.4 times higher for covalently immobilized porphyrinate. Electrokinetic measurements and impedance spectroscopy suggest that the reaction proceeds via formation of Mn^II intermediate with stepwise O₂ reduction to H₂O₂ and then to H₂O. Similar effects were observed in acidic electrolyte. The OER rate is less sensitive to immobilization mode and mainly depends on the amount of accessible catalyst.