UNM has developed the Sacrificial Support Method (SSM) as a main approach for templated synthesis of hierarchically structured electrocatalysts materials. In this method the catalysts precursors are being absorbed on, impregnated within or mechanically mixed with the support (usually mono-dispersed or meso-structured structured silica), thermally processed (pyrolyzed) and then the silica support is removed by HF to result in open frame structure of a “self-supported” material that consists of the catalysts only. This paper will discuss the applictaion of SSM for the synthesis of Platunum Group Metal-free (PGM-free) electroctalysts for polymer electrolyte fuel cells (PEMFC). <br/>The makeup and structure of the active site/sites of the 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. This lecture will attempt to address rational design metrics of M-N-C electrocatalyst based on a broad experimental set of data.<br/>There are two main hypotheses in the literature: (i) nitrogen functionalities on/in carbon-based support are directly responsible for their ORR activity and (ii) nitrogen groups serve as the coordinating environment for metal ions, serving as reactive centers. 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 ORR.

Platinum group metal-free electrocatalysts derived by sacrificial support method

254th National Meeting (2017)

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