Abstract
The manuscript presents results on the influence of external pressure on graphene exfoliation and subsequent 3D structuring by means of liquid-phase exfoliation. In contrast to known and applied exfoliation methods, the current study exploits the enhancement of splitting forces caused by the application of high pressure. The manufacturing pathway allowed to increase the surface area from 750 m2/g (nanoplatelets) to ca. 1100 m2/g (after 3D structuring). Electrochemical studies revealed that the 3D graphene materials were active in the oxygen reduction reaction (ORR). The outstanding ORR activity of 3D structured graphene materials should not be ascribed to heteroatom catalytic centers since such heteroatoms were successively removed upon increasing the carbonization temperature. XPS data showed that the presence of transition metals and nitrogen (usually regarded as catalytic centers) in G-materials was marginal. The results highlight the importance of structural factors of electrodes in the case of graphene-based materials for Zn–air batteries and ORR.
Highlights
The manuscript presents results on the influence of external pressure on graphene exfoliation and subsequent 3D structuring by means of liquid-phase exfoliation
In our previous studies, the liquid phase splitting was supplemented by the 3D structuring of exfoliated graphene flakes14–16. 3D structuring employs a nanopowder, of which particles penetrate the newly formed spaces between the exfoliated graphene flakes, providing a permanent separation of them and avoiding a secondary stacking due to π–π attracting forces
The electrochemical studies of the oxygen reduction reaction (ORR) and Zn–air batteries measurements were carried out with a potentiostat (Autolab, PGSTAT128N, Netherland) and were performed according to methods published in our other article[14,17]
Summary
The manuscript presents results on the influence of external pressure on graphene exfoliation and subsequent 3D structuring by means of liquid-phase exfoliation. Two important features of common electrode materials cannot be ascribed to pristine, i.e. strictly monolayer graphene: low manufacturing cost as well as a well-developed surface area and pore structure. A surface area of 2640 m2/g should be theoretically expected from graphene, real graphene samples, mainly nanoplatelets, exhibit a much less developed surface area of 10–750 m2/g1,2 This fact, along with a relatively high price of typical 2D g raphene[3], turned researchers’ attention to other sources of graphene, i.e. graphite and its exfoliation to less agglomerated structures. Liquid-phase exfoliation is regarded as a scalable method yielding graphene flakes of high quality and purity and low agglomeration[13]. The interflake spaces, even if created, collapse after the removal of the liquid phase To avoid this effect, which practically excludes exfoliated graphene from electrode manufacturing, some additional measures must be undertaken. The current paper addresses this pressure-exfoliation issue and is intending, as a pioneering study, to prove the positive influence of forced liquid exfoliation of graphene nanoplates to less agglomerated graphene structures, which can be structured to 3D electrode materials
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