Non-PGM M-N-C ORR Catalysts: Structure, Morphology and Reactivity
Since 2000 UNM has been developing an original method of catalyst synthesis that is based on templating of mono-dispersed of hierarchically structured silica particles: the Sacrificial Support Method (SSM). This method allows producing catalysts with an open-frame morphology at the meso-scale and at the same time eliminates the “formal” support, thus the entire catalyst is the “active material”.2 During the pyrolysis the support ensures the catalyst dispersion and is responsible for the resulting high-surface area. After the pyrolysis, silica (the sacrificial support) is removed by dissolving in KOH or HF, resulting in self-supported M-N-C catalyst. We have demonstrated catalysts made by SSM derived from porphyrins2-5, heteroamines6,8 and polymers.7 We have studied these catalysts intensely by XPS2 and EXAFS/XANES4 and have made suggestions on the ORR mechanism.5,8
Exceptionally active catalysts were developed based on the SSM and derived from Fe salt and N-containing amines or even charge-transfer organic salts that are not soluble in water or usual organic solvents. The catalyst nitrogen precursors do not form any complexes with the transition metal and thus defy the standard assumption that the transition metal is being bond by the nitrogen-containing species at the precursor stage and “survives” the pyrolysis conditions. In the case of the new generation UNM non-PGM catalyst, the silica support, the N-C precursor and the iron salt are mixed mechanically by ball-milling. There is no dissolution or impregnation step at all. The cathode catalysts, obtained in this mechano-chemical synthesis meet DOE-EERE non-PGM targets of performance in PEMFC single MEA tests: 0.8 V at 0.1 A at 1.5 Bar total pressure (H2/O2test).
There is two strictly different classes of M-N-C catalyst depending on the precursor, but even to a larger extend to the “temperature-time trajectory” of the pyrolytic treatment: (i) low-temperature catalysts in which the nitrogen is predominantly of pyrrolic type and (ii) high-temperature catalysts that show abundance of pyridinic nitrogen. The first type of M-N-C is notable for ORR catalyzed by 2x2e- ORR mechanism (with hydrogen peroxide as detectable intermediate) and the second type can be optimized to attain to a 4e-, direct ORR mechanism. With increase of the pyrolysis temperature (or time) above certain critical value most of the nitrogen moieties are being transformed into graphitic type and the catalyst loses activity.
Understanding of the nature of active sites in non-PGM catalysts is difficult due to high heterogeneity of such materials. SSM provides a unique opportunity to study the structure and reactivity of M-N-C catalysts as there is no “support effect” and the activity exceeds the state-of-the-art in non-PGM. All the M-N-C catalysts display plurality of active sites that have reactivity in ORR. Some of these sites support oxygen reduction to peroxide, some are active in peroxide reduction to water, some are active in both reactions and some may support a 4e- reduction of oxygen to water. In the actual catalyst a plurality of such moieties is presents and they all contribute to the integral activity. In this work we introduce the approach of interpreting the structure of the active sites as defects in graphene. Detailed spectroscopic and morphology analysis revealed correlations between chemical species and ORR activity providing sufficient information on the nature of active sites. We will report on the role of the in-plane Fe-N4 and Fe-N3 defects; Fe-N2 and Fe-N2+2edge defects; metal-less N and O moieties as well as reduced metal and metal oxide nano-phases and will show their correlation with the catalyst activity in ORR individual reactions. Relative contributions of these different active sites to the overall cathode process will be discussed.
Acknowledgments: This work is supported by DOE-EERE Fuel Cell Technology Program: “Development of Novel Non Pt Group Metal Electrocatalysts for PEMFC” (S. Mukerjee, PI).
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