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−N 4 active site has broken down to Co−N 2 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 O ads on the pyropolymer active sites. The specific geometrical adsorption of molecular oxygen with respect to the plane of the Co−N x 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.