Favorable Ion Packing in Porous Carbon for High Gravimetric Capacitance Supercapacitors
Authors: Wang Xuehang , Chen De 
Published: February 2020

There is an increasing need for high power density to keep the stability and sustainability of the grid energy storage, as well as to meet the demand for high-power portable devices. Supercapacitor (SC) is an attractive energy storage option for high power application. However, the moderate energy densities of the SCs limit their wide-spread use(1, 2). The existing strategies to achieve high capacitance and hence the specific energy demands micropores to match the ion size(3) and narrow pore size distribution (PSD) for the micropores(4). However, limited type of the carbon materials could offer effectively controlled narrow PSD in microporous region. Moreover, the ion transportation resistance in the micropores is much higher than in the mesopores, leading to lower output power and worse rate capability.
In this work, the high gravimetric capacitance of supercapacitor with good rate capability was achieved by a combine modeling and experimental approach(5). A large-scalable method was utilized to prepare a series of high-surface area (up to 3400 m2/g) and mesopores-rich carbon nanosponge (CNS) with varied PSD by tuning carbonization and activation conditions. The porous structure of the typical CNS is shown in Fig. 1a&b. The prepared materials were tested in carbon-ionic liquid (IL) SCs, and the gravimetric capacitances of SCs were varied from 121 to 290 F/g in neat ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) electrolyte (Fig. 1c). We developed a model of ion packing and demonstrated, quantitively, the significant impact of geometric factor (PSD and ionic size) on the ion-distribution in the pores and hence the gravimetric capacitance. The model predicts satisfactorily the gravimetric capacitance of the CNSs synthesized in our work (Fig. 1c) as well as the reported carbon materials in literatures. The model reveals that the capacitance is determined by the correlation between PSD and an ion-packing function which described the discipline of the ion-distribution in different pore sizes. Based on our model, systematic strategies for rational design of various carbon materials (both micro- and mesoporous and allows broad PSD) and ionic liquids to achieve ultrahigh gravimetric capacitance can be proposed. The mesopore-rich CNS with desired PSD promoted by our model delivered the capacitance up to 290 F/g at 20 oC, which is among one of the highest recorded DL capacitance(2). A better ion-packing at 60 oC results an ultrahigh gravimetric capacitance of 387 F/g of CNS in ionic liquid. The expected specific energy of the full-packaged cell was comparable to nickel–metal hydride (NiMH) batteries(1), while the cell still delivered good rate capability and good stability.
References
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5. X. Wang, H. Zhou, E. Sheridan, J. C. Walmsley, D. Ren and D. Chen, Energy Environ. Sci., 2016, DOI: 10.1039/c5ee02702k.

Figure 1