Graphene-Based Nanocomposite Materials for Environmental and Energy Applications

Graphene, a two-dimensional carbon sheet with one atom thickness, is considered the thinnest material. Graphene possess very large surface area, high electrical conductivity, excellent mechanical properties (Young's modulus, breaking strength) and very attractive thermal conductivity. Therefore it is not surprising why this unique carbon nano-structure has inspired huge interest in diverse fields including physics, materials science, chemistry and biology.

Following the initial huge impact of graphene on physics, recent research interest has focused on the use of graphene as building platform for the development of functional nanocomposites. This potential has inspired many possibilities related to energy and environmental aspects involving the controlled introduction of various functional building blocks (molecules, species, nanoparticles, ionic liquids, polymers etc) to graphene. Methods reported so far towards the preparation of functionalized graphene nanocomposites include covalent and noncovalent chemical approaches, chemical electroless deposition, hydrothermal and solvothermal growth, electrochemical and electrophoresis deposition, photochemical reaction, as well as physical deposition and mixing. Currently the most promising approach for the facile, up-scaled synthesis of low-cost graphene-based nanocomposites exhibiting well-defined, controlled functionality involves the use of covalent chemistry strategies. 

Within this context, we successfully attached to functionalized graphene derivatives, various different hyperbranched (namely polyethyleneimine) and/or dendritic (namely diaminobutane polypropylene imine) polymers. The derived novel graphene-polymer nanocomposites were then evaluated for a series of highly-challenging environmental and energy applications. We report the most promising results including the use of the synthesized graphene-polymer nanocomposites as very aatractive adsorbents for CO₂ capture as well as their use for the development of components (membranes) used in electrochemical processes (fuel cells). The covalent attachment of the chosen dendritic and/or hyperbranched polymers to the graphitic framework resulted to a significantly enhanced performance of the resulting hybrid nano-composites. Moreover, we discuss in detail, how the structural parameters of the attached molecules (e.g. quantity and type of functional groups, molecular size etc) were found to critically affect not only the resulting graphene-based nanocomposites; but most importantly their behavior against the evaluated applications. The latter opens the pathway to develop novel nano-structured materials possessing tuned-by-demand properties.

The structural characterization of the synthesized graphene-based nanostructures was performed by a combination of experimental analytical techniques. X-Ray diffraction measurements were used to investigate structural order and basal spacing of layered structures, while electron microscopies (SEM and TEM) revealed details regarding their morphology. Thermal analysis (TGA) measurements as well as Raman, FT-IR and X-Ray photoelectron spectroscopies were employed to identify correspondingly the content and the chemical state of the various introduced species in the final synthesized nanocomposites. The porous characteristics of synthesized nanocomposites were investigated by BET measurements while detailed CO₂ sorption and kinetic measurements were performed under different humidity and relative pressure conditions to evaluate their adsorption capacity.

Acknowledgments
This work was partly supported by the EU funded, Marie-Curie Industry-Academia Partnership and Pathways project entitled “CarbonComp” (Grant Agreement No. 286413).