475202 Transition Metal-Nitrogen-Carbon Electrocatalysts for Oxygen Reduction Reaction
The makeup and structure of the active site/sites of the Platinum Group Metal-free (PGM-free) ORR electro-catalysts, including geometry (coordination) and chemistry (composition and oxidation state) remain contentious even after 50 years of research. There is an emerging agreement that iron and nitrogen functionalities, displayed on the surface if the carbonaceous substrate/support, govern ORR activity. This is often combined with a broadly accepted hypothesis that micro-porous surface area plays a critical role forming the active site. Candidate structures participating in ORR include multitudes of nitrogen defect motifs in the carbon matrix of different degrees of graphitization, with metal incorporated as metal nano-particles, corresponding (native) oxides and/or as atomically dispersed, oxidized metal species, linked (coordinated) to nitrogen defects in carbonaceous matrix in a variety of possible configurations. These consist of in-plane defects such as graphitic nitrogen and Fe-coordinated to three or four nitrogen atoms and a plurality of possible edge sites such as pyridinic, pyrrolic, quaternary and Fe-N2/Fe-Nx sites. In this study, we have analyzed more than 45 distinct M-N-C electrocatalysts synthesized from three different families of precursors, such as (i) N-containing polymer, (ii) N-chelating macrocycles and (iii) N-containing small organic molecules (amines). Catalysts were evaluated and analyzed structurally using exactly the same protocol for deriving structure-to-property relationships. We have identified possible active sites participating in different ORR pathways: (1) metal-free electrocatalysts support partial reduction of O2 to H2O2; (2) pyrrolic nitrogen acts as a site for partial O2 reduction to H2O2; (3) pyridinic nitrogen displays catalytic activity in reducing H2O2 to H2O; (4) Fe coordinated to N (Fe-Nx) serves as an active site for 4e- direct reduction of O2 to H2O. The ratio of the amount of pyridinic and Fe-Nx to the amount of pyrrolic nitrogen serves as a rational design metrics of M-N-C electrocatalytic activity in oxygen reduction reaction occurring through the preferred 4e- reduction to H2O.
There are two main hypotheses in the literature: one claiming that nitrogen functionalities on/in carbon-based support are directly responsible for their ORR activity and the second suggesting that nitrogen groups serve as the coordinating environment for metal ions, serving as reactive centers for the ORR. Theoretical calculations have confirmed that incorporation of nitrogen atoms in the carbon matrix enhances its electronic properties, but nitrogen atoms are not catalytically active towards a direct reduction of oxygen. Our mechanistic and structural studies have shown that multiple (yet similar) chemical functionalities most likely co-exists in practical M-N-C catalysts, displaying engagement in various steps of the ORR and thus obscuring the “simple model” of the “most active “active site”.
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