Towards Fully Synthetic Transition Metal-Nitrogen-Carbon Electrocatalysts for Oxygen Reduction Reaction

Abstract

Electrocatalytic reduction of oxygen as a type of heterogeneous catalysis employs a solid state catalytic phase immobilized on a highly conducting electrode to reduce oxygen dissolved in aqueous electrolytes. Due to the presence of two or more phases in such systems, the role of the solid-liquid (catalyst surface-electrolyte) interface becomes very critical in governing the catalytic activity and rate of reaction. Oxygen reduction reaction (ORR) mostly follows an inner-sphere electron transfer pathway, i.e., bond formation/bond breaking process between the active redox species and the catalyst surface during electron transfer. As the nature of the catalytic surface has a significant effect on these processes, understanding of the catalyst surface and molecular structure is of paramount importance. Extensive research using spectroscopic and electrochemical tools has previously established detailed structure-property relationships of M-N-C catalysts in ORR. 1 However, because of the structural disorder introduced by typically used high-temperature synthesis methods for Fe-N-C catalysts, control on the nature and distribution of the catalytically active sites in these materials is a critical challenge. In this work, we establish the use of fully synthetic, non-pyrolysis routes based on total organic/inorganic synthesis towards generating M-N-C catalysts that incorporate specific active sites and closely resemble the microstructure of their pyrolyzed counterparts. This molecular synthesis route affords a very precise understanding of the catalyst structure and a much greater control on it. The model M-N-C catalysts synthesized are termed as arylated arenes. Arylated arenes are a separate class of molecules that can be synthesized by multi-arylation strategies. 2 Since metal coordinated to nitrogen atoms (MN 4 ) is an established active site for the direct 4-electron reduction of O 2 , our approach aimed to incorporate this moiety in the pyridinic arylated arene designed specifically for this work. A three-step reaction scheme was used for the synthesis of a pyridinic nitrogen-containing arene which was then complexed with Fe 2+ ion. Non-aqueous media were used for different steps and techniques like recrystallization and column chromatography employed for purification of the products. The complexed arene was heterogenized on a 3D graphene support. The materials were studied by NMR spectroscopy and X-ray photoelectron spectroscopy (XPS). The catalysts with and without support were then tested for electrocatalytic properties using a rotating ring-disk electrode along with the control samples. The results observed show remarkable promise as the Fe-coordinated pyridinic arene supported on 3D graphene (Fe-Ar GNS) exhibits better oxygen reduction catalysis as compared to the pyrolyzed Fe-coordinated pyridinic arene (Fe-Ar pyrolyzed) proved by linear sweep voltammetry. This result offers new prospects for non-pyrolytic synthesis routes for M-N-C catalysts in the future. Specific active sites can be incorporated into the catalysts by this method and their density can also be controlled. In this way, a fundamentally novel approach can be used to impact activity of M-N-C catalysts for electrocatalytic applications. References: Artyushkova, K.; Serov, A.; Rojas-Carbonell, S.; Atanassov, P., Chemistry of Multitudinous Active Sites for Oxygen Reduction Reaction in Transition Metal–Nitrogen–Carbon Electrocatalysts. The Journal of Physical Chemistry C 2015, 119 (46), 25917-25928. Suzuki, S.; Yamaguchi, J., Synthesis of fully arylated (hetero)arenes. Chemical Communications 2017, 53 (10), 1568-1582.