DBD plasma-tuned functionalization of edge-enriched graphene nanoribbons for high performance supercapacitors
Introduction
Breakthroughs in rechargeable battery and supercapacitors are urgently needed to meet the rapidly growing energy demands and the development of sustainable economy [[1], [2], [3]]. Generally, the mechanism of rechargeable battery is attributed to the progress of cations intercalation/de-intercalation within the electrode materials, which significantly limits the charge/discharge rate owing to the slow diffusion of cations. Supercapacitors, divided into electrochemical double-layer capacitors (EDLC) and pseudocapacitors according to their reaction mechanism, can deliver superior rate performance than batteries because of their surface reactions of electrode materials [4]. The charge storage of EDLC is mainly attributed to the electrosorption of ions on carbon material surface, which makes its electrochemical performance limited by the specific surface area of the electrodes [5,6]. To enhance the energy density and rate performance of electrode materials, many efforts have concentrated on the preparation of carbon/transition metal oxide composites, which have both high pseudocapacitance and high power density provided by metal oxides and carbon nanostructures, respectively. However, the poor compatibility between the carbon materials and the metal oxides is a tough challenge to be solved [7]. Alternatively, introducing oxygenated functionalization into the carbon skeleton is a simple and efficient strategy to enhance the electrochemical performance of electrode material. Different from EDLC, a large amount of pseudo-capacitance can be produced by the redox reactions of oxygen functional groups on the carbon materials in a basic electrolyte [[8], [9], [10]].
Apart from oxygenated functionalization, the presence of edges in nanomaterial is also beneficial for the enhancement of capacitance [11]. Shi et al. [12] have demonstrated that graphene with abundant edges delivers much higher specific capacitance than basal plane owing to the massive dangling bonds on its edges. These dangling bonds can boost and accelerate the electrosorption of ions in the electrolyte to improve the capacitance. Hence, it is feasible to prepare edge-enriched carbon nanomaterials, such as graphene nanoribbons and graphene quantum dots [[13], [14], [15]], to obtain a higher capacitance by taking advantaging of their intrinsic edge sites. Preparation approaches of graphene nanoribbons (GNRs) with the oxidation unzipping of carbon nanotubes were built by several groups [16,17]. The prepared GNRs possess not only a large number of edge sites but also abundant oxygen functional groups, which contribute significant pseudocapacitance. But the electrical conductivity and rate performance become worse because of too much oxygen functional groups [18]. Therefore, the preparation of GNRs with appropriate oxygen functional groups for excellent electrochemical properties remains a big challenge.
Recently, plasma has attracted increasing attention in the field of preparation and surface modification of carbon nanomaterials [19]. Different from solid, liquid, and gas, plasma is sorted as the fourth state of matter, which can produce many reactive species (ions, unbound free electrons, radicals, and other excited species) [20]. Moreover, plasma can be applied to process or modify materials, such as plasma exfoliation, deposition, doping, etching and other surface treatments [[21], [22], [23], [24], [25]]. Additionally, plasma technology is an environmentally friendly and low energy-consumption process, which is out of use solvent and nonproducing exhaust chemical wastes [26]. In particular, due to the wide pressure range, DBD plasma is very competitive for diverse industrial applications, especially in the scale production of graphene based materials [27].
Herein, we propose a simple and novel strategy to tune oxygenated functionalities of edge-enriched GNRs by DBD plasma. Due to the edge-enriched structure and suitable oxygen functional groups, the GNRs electrode delivers superior specific capacitance (229 F g−1 at 0.5 A g−1) and high rate performance (retain 56.7% at 50 A g−1) in 6 M KOH aqueous electrolyte. Furthermore, a symmetric device assembled by as-prepared electrode exhibits an energy density of 12.65 Wh kg−1 and good electrochemical stability in 1 M Na2SO4 aqueous electrolyte.
Section snippets
Preparation of carbon materials
Graphene oxide nanoribbons (GONRs) were prepared by the unzipping of carbon nanotubes (CNTs) under oxidative condition [28]. Briefly, 1.0 g CNTs were mixed with 130 ml concentrated H2SO4 and treated ultrasonically for 30 min. The resulting production was mechanically stirred for 1 h. Subsequently, KMnO4 (5 g) was carefully added into the mixture and reacted for 1 h and then at 70 °C for 2 h. After that, the resulting product was carefully added into the mixed solution of 500 ml ice water and
Results and discussion
The synthetic procedures are illustrated in Scheme 1. Briefly, the GONRs with abundant edge defects and oxygen functional groups were obtained via chemical oxidation unzipping of CNTs. After freeze drying, the GONRs were subsequently treated in a DBD plasma reactor for different time to prepare P-GNRs samples. Actually, the unstable and stable oxygen functional groups of GONRs can be eliminated and maintained respectively by controlling the DBD plasma treating time. The morphologies of the
Conclusions
In summary, we demonstrate that DBD plasma is capable of removing the oxygen-containing groups on the graphene oxide nanoribbons and/or transforming these unstable oxygen functional groups into more stable ones. The formed unique structures allow these materials with excellent performance in supercapacitor. Moreover, owing to the additional pseudo-capacitance contributed by oxygen functional groups and edge-enriched structure, the as-obtained material shows high specific capacitance (229 F g−1
CRediT authorship contribution statement
Yinzhou Song: Conceptualization, Investigation, Writing - original draft. Zongbin Zhao: Resources, Writing - review & editing, Supervision, Funding acquisition. Xuguang Liu: Validation, Funding acquisition. Yongzhen Yang: Validation, Funding acquisition. Changyu Leng: Writing - review & editing. Han Zhang: Formal analysis. Jinhe Yu: Investigation. Lulu Sun: Investigation. Xuzhen Wang: Resources, Writing - review & editing. Jieshan Qiu: Resources, Supervision.
Acknowledgments
The authors acknowledge financial support from the National Natural Science Foundation of China (Grant No. 51672033, U1610255 and U1703251).
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