Direct Spectroscopic Observation of the Structural Origin of Peroxide Generation from Co-Based Pyrolyzed Porphyrins for ORR Applications

Abstract

Pyrolyzed transition metal based porphyrins present an attractive alternative to state of the art Pt-based electrocatalysts for fuel cell applications based on their comparatively low cost. Unfortunately, the large array of precursors and synthetic strategies has led to considerable ambiguity regarding the specific structure/property relationships that give rise to their activity for oxygen reduction. Specifically, considerable debate exists in actual chemical structure of the pyrolyzed reaction centers, and their relationship to membrane-damaging peroxide yield. In this manuscript a comprehensive electrochemical and spectroscopic study of pyrolyzed CoTMPP produced via a self-templating process is presented. The resulting electrocatalysts are not carbon-supported, but are highly porous self-supported pyropolymers. Rotating ring disk electrode measurements showed that the materials pyrolyzed at 700 °C exhibited the highest performance, whereas pyrolysis at 800 °C resulted in a significant increase in the peroxide yield. X-ray photoelectron spectroscopy and Co L and K edge extended X-ray absorption fine structure (EXAFS) studies confirm that the majority of the Co−N4 active site has broken down to Co−N2 at 800 °C. Application of Δμ analysis (an X-ray absorption near-edge structure difference technique) to the in situ Co K edge EXAFS data allowed for direct spectroscopic observation of the geometry of Oads on the pyropolymer active sites. The specific geometrical adsorption of molecular oxygen with respect to the plane of the Co−Nx moieties highly influences the oxygen reduction reaction pathway. The application of the Δμ technique to other transition metal based macrocycle electrocatalyst systems is expected to provide similarly detailed information.