Graphene Nanocomposites Templated from Cage-Containing Metal-Organic Frameworks for Oxygen Reduction in Li-O2 Batteries
Authors: Wu Gang 
Published: February 2020

Recently, the physical and chemical properties of carbon, such as crystal structure, morphology, and defect chemistry, have been demonstrated to significantly impact the ORR activity of carbon-based cathode in Li-O2 batteries.(1, 2) Compared to other carbon catalysts studied, transition metal based nitrogen-doped nanocarbon materials (e.g., graphene, tube, onion-like carbon) have been known as the most promising class of non-precious metal ORR catalysts in various electrolytes for electrochemical energy conversion and storage devices,(3-7) including nonaqueous Li-O2 batteries.(8) When nitrogen atoms are doped into carbon lattices, electronic and geometric structures of carbon will be significantly modified. This doping process leads to non-uniform distribution of the spin and atomic charge density, which is very important to oxygen adsorption and activity enhancement.(9) Meanwhile, the resulting defects and disorders can become active sites to adsorb O2 and Li+, increasing the density of reaction sites and thus enhancing the discharge voltage as well as capacity of Li-O2 batteries.(10) Recently, i
n situ formed nitrogen-doped graphene nanostructures were derived from heteroatom polymers (polyaniline, a nitrogen/carbon precursor) via a high-temperature approach and are found to be crucial for the ORR activity enhancement in Li+-O2 nonaqueous electrolyte.(8) These highly graphitized nanocarbons (e.g., nanotube, fiber, onion-like carbon, and graphene) seem to provide a robust matrix for hosting the active sites.(11) The catalyst activity and durability have been directly linked to the carbon morphology and structures in situ formed during the catalyst synthesis via high-temperature treatments.
Here, in an effort to develop high-performance ORR catalysts, we have been interested in investigating the templating effect of cage-containing metal-organic frameworks (MOFs) on the formation of novel nanocarbon composites and on the resulting ORR activity in Li-O2 batteries. Due to the accessibility of metal-cation centers, the variety of building blocks, and the high micropore surface area, MOF is able to offer unique opportunities in synthesizing efficient ORR catalysts.(12) However, to date, the studied MOFs as precursors for catalyst synthesis are limited to commercially available microporous ones. The use of specially designed MOFs with unique structures for preparing high-performance ORR catalysts remains unexplored.
In this work, we report the first demonstration of a newly-developed Co containing MOF material with giant polyhedral cages (cage size ~1.8 nm) as a template in preparation of transition metal-nitrogen-carbon ORR catalysts, especially for nonaqueous Li-O2 battery cathodes. As a result, a Fe-based catalyst dominant by nitrogen-doped graphene/graphene tube (tubular graphene) nanocomposites was prepared through heat-treating a novel nitrogen/carbon precursor dicyandiamide (DCDA) in the presence of iron species. The in situ formation of nitrogen-doped graphene/graphene tube templated by the MOF may provide a new route for preparation of carbon nanocomposites for electrochemical energy applications.
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