Abstract
Electrical double-layer capacitors (EDLCs), commonly known as supercapacitors, store energy through reversible adsorption of electrolytic ions on the charged electrode surfaces.1 Extensive work on carbide-derived carbons (CDCs), ACs and templated carbons revealed that the sub-micro pores (< 1 nm) contribute significantly to the overall charge storage.2,3 Carbon materials with uni-modal pore size distribution curves (PSD) are essential for such a focused analysis.4 However, the difficulties in synthesizing such carbon materials and obtaining their PSDs reliably through gas sorption have hampered the progress.5 Meanwhile, research on graphene derivatives for charge storage in SCs has shown promising advances owing to good surface areas and electrical conductivities. However, much of the current research is devoted to improving the material porosity with a focus on PSDs, similar to the research on ACs.6 Surprisingly, little attention has been devoted towards exploring the possible confinement of the electrolytic ions into the unique layered structures of graphitic materials. An analysis of materials with varied inter-graphene sheets distance (d-spacing) in SCs would assess the possibility of such confinement.
In this context, this study is aimed at understanding the consequences of interlayer spacing on electrochemical charge storage. A class of graphene-like materials with varying d-spacing have been synthesized using alkyl diamines cross-linking and are characterized for their application in SCs. Electrochemical analysis of the materials in SCs using electrolytes containing tetraalkylammonium cations of different sizes (alkyl: ethyl, propyl, butyl and hexyl) showed limitations of ion adsorption in different materials. The PSD curves of the materials show no presence of sub-micro pores (< 1 nm) and thus relate the observed ion limitation to d-spacing. Hence, this work demonstrates that the inter-layer galleries in graphene like materials indeed confine electrolyte ions based on the size constrictions. A correlation between the d-spacing and the electrolyte ion sizes is established and a condition for maximum capacitances versus d-spacing for a specific electrolyte is being investigated.
[1] Simon, P.; Gogotsi, Y. Nat Mater 2008, 7 (11), 845–854.
[2] Merlet, C.; Péan, C.; Rotenberg, B.; Madden, P. A.; Daffos, B.; Taberna, P.-L.; Simon, P.; Salanne, M. Nat. Commun. 2013, 4, 3701-3707.
[3] Raymundo-Piñero, E.; Kierzek, K.; Machnikowski, J.; Béguin, F. Carbon 2006, 44 (12), 2498–2507.
[4] Kondrat, S.; Pérez, C. R.; Presser, V.; Gogotsi, Y.; Kornyshev, A. A. Energy Environ. Sci. 2012, 5 (4), 6474–6479.
[5] Béguin, F.; Presser, V.; Balducci, A.; Frackowiak, E. Adv. Mater. 2014, 26 (14), 2219–2251.
[6] Li, C.; Shi, G. Adv. Mater. 2014, 26 (24), 3992–4012.