Graphene-Based Assemblies As Electrode Materials for Supercapacitors
Authors: Duclairoir Florence , Banda Harish , Perie Sandy , Daffos Barbara , Taberna Pierre-Louis , Dubois Lionel , Crosnier Olivier , Simon Patrice , Lee Daniel , De Paepe Gaël 
Published: February 2020

Reduced graphene oxide (RGO) based electrochemical double-layer capacitors (EDLC), have been extensively studied as good power capability can be achieved but they suffer from low capacitances (~100-150 F/g), as the reduced graphene sheets partially restack through π- π interactions limiting the resulting adsorption active surface area.[1] To avoid this restacking different paths have been followed in the literature: using graphene aerogels or expanded graphene structures.[2] In this latter case, an intercalate or pillar is used to space out the graphene layers and recover active surface area. Different nature of pillars or intercalates such as carbon nanotubes, carbon blacks, diamine, aromatic, metallic ions or big macrocyles have been described.[2,3]
The results, that will be presented, depict the researches devoted to the development and study of graphene based expanded assemblies designed to be tested as electrode materials for supercapacitors. The material development involves the bridging of graphene oxide derived sheets using alkyl diamine as linkers (Fig. 1a).[3] The production of graphene galleries of height matching that of different alkyl chain length pillars was evidenced, notably with XRD (Fig. 1b). The purpose of this design was to enhance the specific capacitance by limiting graphene sheet restacking and optimizing the inter-sheet distance (d-spacing) or pore size with respect to the ion size, as was previously evidenced with other carbon materials.[4] These materials (named RPs) have been tested electrochemically with various tetraalkylammonium tetrafluoroborate salts in acetonitrile. This approach resulted in adsorption active surface area tuning, yielding clear evidence of the electrolytic ions entering the inter-layer galleries. Indeed only ions with diameter smaller than the d-spacing access the inter-layer galleries.[5a] Ex-situ solid-state NMR analysis were performed to further demonstrate the ionic species adsorption at the material surface. Further optimization dealing with pillar amount or nature and material density tuning showed the high interest of this re-aggregation limitation method, as gravimetric and volumetric capacitances 4 times higher than that of RGO (230 F/g and 210 F/cm3 respectively) have been achieved (Fig. 1c).[5b]
Figure 1: a) Scheme differentiating reduced graphene oxide RGO and pillared graphene structures; b) XRD diffractograms obtained for pillared graphene prepared diamines with different chain lengths (5, 6 and 8 C atoms); c) Volumetric capacitance and power capability tests obtained on a pillared material compared to RGO.

References

[1] a) Raccichini, et al. “The Role of Graphene for Electrochemical Energy Storage” Nat. Mater. 2015, 14, 271. b) Stoller et al. “Graphene-Based Ultracapacitors” Nano Lett. 2008, 8 , 3498. c) Whitby et al. “Chemical Control of Graphene Architecture: Tailoring Shape and Properties” ACS Nano 2014, 8, 9733.
[2] a) Hu et al. “Ultralight and highly compressible graphene aerogels” Adv. Mater. 2013, 25, 2219. b) Sun et al. “Porous graphite oxide pillared with tetrapod-shaped molecules” Carbon 2017, 120, 145. c) S. Park et al. “Graphene Oxide Papers Modified by Divalent Ions—Enhancing Mechanical Properties via Chemical Cross-Linking” ACS Nano 2008, 2, 572. d) Kim et al. “Three-dimensional pillared metallomacrocycle–graphene frameworks with tunable micro- and mesoporosity” J. Mater. Chem. A 2013, 1, 8432.
[3] Lee et al., “Tunable Sub-Nanopores of Graphene Flake Interlayers with Conductive Molecular Linkers for Supercapacitors” ACS Nano 2016, 10, 6799.
[4] Largeot et al. “Relation between the Ion Size and Pore Size for an Electric Double-Layer Capacitor” J. Am. Chem. Soc. 2008, 130, 2730. b) Merlet et al. “Highly Confined Ions Store Charge More Efficiently in Supercapacitors” Nat. Commun. 2013, 4, 2701.
[5] a) Banda et al. «Ion Sieving Effects in Chemically Tuned Pillared Graphene Materials for Electrochemical Capacitors” Chem. Mater. 2018, 30, 3040. b) Banda et al. “Sparsely Pillared Graphene Materials for High-Performance Supercapacitors: Improving Ion Transport and Storage Capacity”, ACS Nano 2019, 13, 1443.

Figure 1