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Controlled Synthesis of Two-Dimensional Nanomaterials for Energy Storage and Conversion

Final Report Summary - 2DMATER (Controlled Synthesis of Two-Dimensional Nanomaterials for Energy Storage and Conversion)

Driven by the increasing high-energy demands, environmental issues and limited fossil resources, the global rush on graphene and related 2D materials is clearly indicating the expectations and the potential of these materials for new applications. One of the great challenges in this area is to develop high-performance lithium ion batteries, fuel cells and electrochemical capacitors that can efficiently convert chemical energy directly into electrical energy. In the 2DMATER project, strategies for the synthesis and processing of various 2D nanosheets across a broad range of inorganic, organic and polymeric materials with molecular-level or thin thickness through both the top-down exfoliation of layered materials and the bottom-up assembly of available molecular building blocks have been developed. Furthermore their application in energy storage or generating devices such as supercapacitors, batteries and fuel cells are tested and optimized. Selected highlights across the different types of materials are i) By using alternating currents, ultrafast delamination of graphite has been achieved to produce high-quality graphene flakes with large dimensional size ( 1-5 µm, 70 %), high yield (1-3 layers, 75 %), low defect density (a C/O ratio of 21.2) and outstanding hole mobility of 430 cm2V-1s-1. Especially, the production rate exceeds 20 g h-1 in laboratory tests that demonstrate the potential to bridge the gap between bench-scale studies and commercial applications. In addition, such high-quality graphene sheets also exhibit good solution-processability in various solvents, which offers great ease for the development of printable inks and other graphene-based composites. ii) The graphene inspired approach of 2D porous polymers resulted a new class of ultralight and porous carbon materials that are associated with high strength-to-weight and surface-area-to-volume ratios for various potential applications. All these 2D porous polymers can be further pyrolyzed under inert atmosphere and high temperature to prepare heteroatoms doped 2D porous carbons and metal oxides embedded 2D carbon nanosheets which show great potential as catalysts for oxygen reduction reaction, as electrodes for supercapacitors, and as organic cathodes for Li-ion batteries. iii) Particularly, in alkaline electrolyte, our nitrogen-doped carbon-based metal-free catalysts show the highest ORR activity (half-wave potential of 0.85 V versus reversible hydrogen electrode with a low loading of 0.1 mg cm-2) in alkaline media among all reported metal-free catalysts. Moreover, when used for constructing the air electrode of zinc-air battery, our metal-free catalyst outperforms the state-of the-art platinum-based catalyst. iv) Moreover, we demonstrated the construction of a novel class of high-performance in-plane interdigital Microsupercapacitors (MSCs) based on micropatterning of methane plasma reduced graphene films with a nanoscale thickness of 6~100 nm. The resulting micro-supercapacitors showed a power density of 1193 W cm 3 that was higher than electrolytic capacitors, and an energy density of 3.1 mWh cm 3 that was comparable to lithium thin-film batteries, in association with superior cycling stability. Such microdevices allowed for operations at ultrahigh rate up to 1000 V s 1, three orders of magnitude higher than conventional supercapacitors. v) Over the last two years, we have carried out pioneering work on the interfacial synthesis of 2DSPs and 2DPs. We reported a cm2-sized single-layer 2DSP (~1.8 nm) with good internal order based on host-guest enhanced interactions with a monomer consisting of a tris(methoxynaphthyl)-substituted truxene spacer and a naphthalene diimide substituted with N-methyl viologenyl moieties as donor and acceptor monomers. Notably, the first macroscopically crystalline 2DPs based on Schiff-base polycondensation of 5,10,15,20-tetrakis(4-aminophenyl)21H,23H-porphine-Co(II) and 2,5-dihydroxyterephthalaldehyde at air-water and water-chloroform interfaces were. Moreover, the surfactant bilayers of perfluorocarboxylic acid served as template to guide the co-assembly of block copolymer micelles (polystyrene-b-poly(ethylene oxide) and aniline (or pyrrole) precursors in aqueous solution, achieving the synthesis of 2D conducting polymer nanosheets (polyaniline and polypyrrole) with highly ordered mesoscale porous structures and good crystallinity. Here, the interface between surfactant and water phase provides a critical 2D confinement geometry for the controlled polymerization of aniline or pyrrole monomers. Additionally, using the metal-organic coordination chemistry, we achieved the synthesis of the first crystalline monolayer 2DSP (0.7 nm in thickness) comprising triphenylene-fused nickel bis(dithiolene) complexes at an air/water interface.
Such 2DSP can homogeneously cover glassy carbon electrodes and exhibited excellent electrocatalytic activity for hydrogen production in water splitting with a Tafel slope of 80.5 mV decade-1, onset overpotential of 110 mV and overpotential of 333 mV to reach a current density of 10 mA cm-2, which exceed the performance of comparable molecular catalysts and even graphene-based metal-free catalysts. This work paved a promising way for the further development of freestanding organic 2D materials bearing functions in energy technologies. Also, for the first time, we report the synthesis of a 2D conjugated COF based on olefin (C=C) linkages using the Knoevenagel condensation reaction. In this work, employing 1,4-phenylenediacetonitrile and three-armed aromatic aldehyde as the key monomers, the Knoevenagel polycondensation reaction successfully leads to the formation of a 2D poly(phenylenevinylene) framework (2DPPV) with a crystalline layered structure and a surface area of 472 m2 g−1, as well as a defined optical band gap of 2.10 eV and a lowest unoccupied molecular orbital (LUMO) of −3.45 eV. Upon thermal and activation treatments, it can be facilely converted into porous carbon nanosheets with large specific surface areas of up to 880 m2 g−1, which exhibit an excellent electrochemical performance as supercapacitor electrodes and electrocatalysts for the oxygen reduction reaction (ORR), representing an economic non-template approach to 2D porous carbon materials for energy-related applications.