Full Length ArticleFluoroalkylated nanoporous carbons: Testing as a supercapacitor electrode
Graphical abstract
Introduction
This decade has witnessed a significant increase in the consumption of energy storage devices, including supercapacitors (SC). They play an important role in the energy storage and conversion systems. However, the recognized drawbacks of the existing commercial recharging devices are their still low energy storage capacities. This parameter depends on an electrode material for the most. Recently, with the increasing volume of scientific activity in this field, researchers have focused on the improvement of electrode materials for SC. Conductive carbons as an electrode material for SCs provide beneficial properties: large specific surface areas, high conductivities, and high electrochemical stabilities in addition to their environment-friendly sustainable sources and inexpensive production [1]. In particular, for such purpose were proposed surface-doped advanced activated carbons and/or graphene [2], [3].
Numerous studies were carried out through electrode material fine-tuning, considering conducting polymers [4], [5], [6] and different forms of carbon [7], [8], [9]. Besides, a large diversity of carbonaceous materials has been proposed and studied. Now, they seem to be the most proper for application in SC [10]. An actual task of current studies is enchaining of their electrochemical properties. One of the convenient methods for such purpose is the surface doping with fluorine [11]. Typically, innovative fluorinated carbon materials possess the potential of providing a hydrophobic surface while maintaining chemical passivity and advanced porosity [12], [13]. On this background, tuning C/F ratio with fluorine groups fixed to carbon and increasing the stability of such fluorine groups are of great importance for the preparation of fluorinated carbon materials of different structural origin having advanced macroscale properties, for example, tribological, mechanical and electromagnetic, including capacitive ones [14], [15], [16], [17], [18], [19], [20].
When Liu et al. introduced the fluorinated carbon CFx cathodes in 2014 [21], their improved properties have attracted lots of attention and currently, the use of the fluorinated carbons remains one of the most prominent ways of material-mediated low capacity problem-solving.
Technically, direct fluorination of carbon materials and the thermal decomposition of fluorinated polymers are known techniques of fluorocarbons preparation. Their application results in the carbon surface coverage with groups that have high amphiphobicity and high resistance to oxidation [12], [13], [21], [22], [23].
Considering fluorinated polymers, it is clear that in contrast to expensive and poorly soluble polytetrafluorethylene [22], the use of polyvinylidene fluoride, which is a cheaper polymer well soluble in organic solvents, could be much more effective.
Indeed, one can effectively produce fluorinated carbon materials for selective adsorption and electrochemical applications [24], [25], [26], [27], [28], [29], [30], [23] by thermal, radiative, chemical and mechanochemical carbonization of polyvinylidene fluoride [24], [25], [26], [27], [28], [29], [30], [23], [31], [32], [33], [34], [35]. Despite this apparent success, there are several drawbacks. For example, the de(hydro)fluorination lowers the fluorine content in the prepared fluorinated carbons [27], [30], [31], [36], [37] while evaporation of polymer chains causes unwanted gases production [38]. Practically all known methods used for the preparation of fluorinated carbons materials are faced with serious technical difficulties and strictly speaking are tedious. In particular, they require the use of free fluorine or plasma. Here we propose a simple and effective direct fluoroalkylation technique for the functionalization of the inner surface of nanoscale porous carbons, which is described in [39]. We characterize these materials as a new class of fluorinated carbons (Fluocar® F materials) and demonstrate their utility as supercapacitor electrodes with an improved capacitance.
Section snippets
Materials
Norit® granular activated carbon 830 W (designated “830 W”) prepared by carbonization and activation of natural coal was supplied by Electrochemservice (the official distributor of Norit® in Ukraine) and used as an initial material. DLC Supra 30 (designated “Supra 30”) activated carbon was selected as a known reference material for electrochemical studies [40]. Deashed 830 W was repeatedly washed with HCl, HF, and double-distilled water (DDW). In this study, all required acids and salts were
Results and discussion
According to Table 1, elemental analysis revealed the addition from 0.09 to 0.72 mmol g−1 of F, higher for more elevated temperatures of fluoroalkylation as expected. We suppose that the highest F content is due to an intensive thermal decomposition of carboxylic (Cb) surface groups that promote the HFC homolysis.
The remaining ash content in the parent 830 W was found to be ∼1.9 mass%. After fluoroalkylation, it was noticeably reduced to 0.2–0.3 mass% and will have a negligible influence on the
Conclusions
We propose a novel direct method of fluoroalkylation for chemical modification of nanoporous carbon electrode materials as a perspective way to improve supercapacitors. The materials obtained this way all possess excellent thermal and chemical stability. The surface fluoroalkylation at 400–500 °C provides up to ∼ 2 mass% of grafted fluorine, which is approx. 1 mmol g−1 of F (mostly in form of CFx-groups). The treatment temperature had a direct influence on the resulting textural parameters and
Acknowledgments
All authors express their gratitude to entrepreneur Vasyl Prusov for the labware. V.V.L. thanks to the National Scholarship Programme of the Slovak Republic managed by Slovak Academic Information Agency (SAIA), n.o., and funded by the Ministry of Education, Science, Research and Sport of the Slovak Republic, grants in 2015, ID number 14511, and in 2017, ID number 20917; and International Visegrad Fund for scholarship ID number 51810574 in 2018. O.Yu.B. thanks to the National Scholarship
Conflict of interest
A. Zaderko, V. Prusov, and V. Diyuk are listed as inventors of the World patent publication WO/2016/072959, Method for carbon material surface modification by the fluorocarbons and derivatives.
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