Electrocatalysis of Oxygen Reduction with Elctropolymerized Metallo-Corroles

Sunday, 28 May 2017: 11:40
Grand Salon C - Section 16 (Hilton New Orleans Riverside)
L. Elbaz (Bar-Ilan University), A. Friedman (Bar Ilan University), and Z. Gross (Technion - Israel Institute of Technology)
In recent years we have shown very high electrocatalytic activity with metallo-corroles, specifically with cobalt- and iron-2,3,7,8,12,13,17,18-octabromo-5,10,15-tris(pentafluorophenyl) corrole [M(tpfcBr8)]. Both showed activity comparable with some of the stat-of-the-art molecular catalysts for ORR in acid and alkaline solutions. These very promising results encouraged us to continue developing other corrole-based catalysts in order to enhance the overall activity and shift the reaction mechanism towards the desirable direct 4-electron mechanism. In this work we have taken the bio-mimetic approach of joining catalytic centers together in order to obtain synergistic effects which could be translated to improved activity, selectivity and stability. Cobalt tris (4-aminophenyl) corrole (CoTAPC) were synthesized. The aminophenyl substituents on the corrole ring allowed us to polymerize these corroles using electrochemistry. The CoTAPC was electropolymerized at various polymerization conditions directly on glassy carbon electrodes. We have shown that we can control the growth, structure and density of this polymer by changing the conditions at which the polymerization reaction takes place. The final product was fully characterized using, FTIR, Raman spectroscopy and microscopic techniques. In agreement with previous studies with amino-substituted porphyrins, the obtained polymerization mechanism is similar to that of polyaniline. Indeed, FTIR-ATR analysis shows transition of the primary amine substitutes to secondary and tertiary amine, indicating the formation of dihydrophenazine and phenazine unit linkage respectively.

The electrocatalytic oxygen reduction reaction activity of the polyCoTAPC was studied in both acidic and alkaline conditions. In acidic solution, the number of electrons participating in the reaction (n) was improved by 0.3 electrons and the half-wave potential (E1/2) shifted by 120 mV more positive when compared to the monomer (CoTAPC), while in alkaline solution the n increased by 0.43 electrons and the E1/2 shifted by 200 mV more positive compared to CoTAPC. These shifts are attributed to the unique polymeric structure which allows both sharing of electrons and tandem electrocatalysis by at least two catalytic sites. Another outcome of these unique structures, is the catalytic site density which can possibly compensate for the inherently low turn-over frequency usually associated with non-precious metal catalysts in general.