Oxygen Reduction Reaction Activity of Iron (II) Phthalocyanine Monolayers in Acidic Media Studied Using a Rotating Disk Electrode Technique

Abstract

Metal complexes such as metalloporphyrins and metallophthalocyanines are widely studied in various research fields due to their unique physical and chemical properties. Especially, their catalytic activities for oxygen reduction reaction (ORR) are of great interest because this reaction plays an important role in energy conversion or biological redox processes. Among various metal complexes, Iron (II) phthalocyanine (FePc) is expected as a candidate for four-electron ORR catalysts. However, most of the kinetic studies for ORR have been carried out using heat-treated catalysts of FePc. Unfortunately, their chemical structures become ill-defined after the heat treatment, and thus, the nature of active sites is still unclear. Actually, there is a possibility that four-electron ORR activity is induced by dimeric FePc. The intrinsic activity of monomeric FePc, especially in acidic media, is still under discussion. In this study, well-defined monolayers of monomeric FePc complexes were fabricated on chemically modified Au surfaces using two-step assembly [1]. Their ORR activity was quantitatively measured in acidic media using a rotating disk electrode (RDE) technique. The catalytic behavior of FePc was compared with that of Cobalt (II) tetraphenylporphyrins (CoTPP), which is known as two-electron ORR catalyst.The monolayers of FePc or CoTPP were fabricated on Au surfaces as follows. First, an Au disk electrode with diameter of 10 mm was modified with self-assembled monolayers of pyridine-4-thiol (PySH) or 1,6-diisocyanohexane (C6DI). Next, metal complexes of FePc or CoTPP were immobilized on top of these molecular monolayers through axial ligation with their different terminal groups. The ORR activity of these samples was measured in O2-saturated 0.05 M H2SO4 aqueous solution using the RDE method.Figure shows rotation rate dependences of RDE polarization curves for CoTPP and FePc on C6DI-modified Au. According to the Koutecky-Levich analysis of these curves at 0 V vs. RHE, the electron transfer number was estimated to be 1.9 for CoTPP, indicating that CoTPP indeed catalyzed only two-electron ORR. On the other hand, the estimated transfer number for FePc was 2.4, suggesting that this complex was indeed able to catalyze the four-electron ORR in the acidic condition. Interestingly, the catalytic behavior of FePc on C6DI-modified Au was apparently different from that on PySH-modified Au. In the poster, we will present details about the ORR activities of FePc measured on various monolayer-modified Au electrodes.Reference[1] Toyama T, Sato S, Motobayashi K, Uosaki K, Ikeda K. J Solid State Electrochem. DOI: 10.1007/s10008-019-04461-9.Figure. Rotation rate dependence of RDE polarization curves for FePc/C6DI/Au and CoTPP/C6DI/Au measured in 0.05 M H2SO4 solution, where FePc/C6DI/Au and CoTPP/C6DI/Au denote FePc and CoTPP monolayers immobilized on C6DI-modified Au, respectively. Figure 1