Abstract
Bioinspired electrodes have been constructed by physiabsorption of two air stable iron porphyrin complexes, one bearing an imidazole coordination and the other bearing a thiolate coordination. To control the electron transfer (ET) rate to these O-2 reducing electrocatalysts, the complexes were immobilized on edge plane graphite electrode and alkyl thiol self-assembled monolayer (SAM) modified Au electrodes with varying chain lengths of the thiols Catalyst immobilized SAM modified surfaces were characterized using surface enhanced resonance Raman spectroscopy (SERRS), and their electrocatalytic O-2 reduction properties were investigated using rotating ring disc electrochemistry (RRDE). While the imidazole bound complex showed increase in partially reduced oxygen species (PROS) on decreasing ET rate, the thiolate bound Complex showed the opposite trend, that is, the value of PROS reduced on decreasing the ET rate. SEARS coupled to rotating disc electrochemistry (SERRS-RDE) technique helps gain insight into the O-2 reduction mechanism. The results obtained indicate. that while the imidazole bound iron porphyrin complex reduces O-2 through an inner sphere mechanism using a high-spin (HS) Fe-II species, the thiolate ligated complex shows an inner sphere as well as outer sphere mechanism using a HS Fe-II and low-spin (LS) Fe-II species, respectively. The PROS formation by a HS Fe-II species of this thiolate bound complex increases with decreasing ET rates while that of a LS Fe-II species decreases with decreasing ET rates.