Boosting the supercapacitor performances of activated carbon with carbon nanomaterials
Graphical abstract
Introduction
Being able to combine the high energy storage capability of conventional batteries with the high power delivery capability of conventional capacitors [1,2], supercapacitors (also known as ultracapacitors or electrochemical capacitors) have been developed for a wide range of applications such as consumer electronics, medical electronics, electrical vehicles, electrical utilities, and military defense systems [3,4]. However, the performances of state-of-the-art supercapacitors need to be improved in order to satisfy the rapidly increasing performance demands for these applications. Electrode materials play an important role in determining the performances of supercapacitors and thus have gained considerable research efforts in recent years [5].
Although a variety of materials including carbonaceous materials, metal oxides, metal nitrides, and conjugated polymers have been comprehensively researched as the electrode materials for supercapacitors [[6], [7], [8]], activated carbon (AC) is still the predominant material employed to fabricate electrodes for commercial supercapacitor products, which is largely due to the distinct advantages of AC including high surface area, low cost, commercial availability, and well-established production technologies [9,10]. Nevertheless, the performances of currently available supercapacitor products are limited by the properties of conventional AC electrodes that are fabricated in the industry, usually by combining AC with a binder and a conductivity additive (i.e., carbon black, CB) [11]. First, for example, the similarity in particle size distributions between AC and CB implies the lack of a so-called“particle-mixing-interaction” [12], resulting in a low packing density for the electrodes and thus a poor volumetric performance for the capacitors [13,14]. In fact, it has been emphasized that enhancing the packing (and mass loading) of electrodes is significantly important for electrochemical energy storage devices, including supercapacitors, to be useful for commercial applications [15,16]. Second, the electrical conduction network of a conventional AC electrode is actually formed based on the aggregation of CB particles. Thus, in this system, the isolation of CB aggregates, loose contact between CB aggregates and AC particles, and direct point-to-point contact between AC particles usually indicate a poor electrical conduction network for the electrode [11]. In addition, AC has low electrical conductivity, low meso-/macroporosities, and low electrolyte accessibility [17,18]. All of these factors result in a poor rate capability for the electrodes and thus limited power characteristics for the capacitors. Third, during the charge/discharge cycling of a conventional AC- and CB-based supercapacitor, AC suffers from noticeable expansion/contraction [19] and CB from severe agglomeration [11], both inevitably causing an increased resistance for the electrodes and thus a poor cycle life for the capacitors. It has been stressed, therefore, that control of the dimensional change of AC and the agglomeration of CB upon cycling is necessary to ensure a long cycle life for the capacitors [20]. Consequently, it is very important to overcome the abovementioned shortcomings of conventional AC electrodes in order to boost the performances of the state-of-the-art supercapacitors to meet the performance demands for their applications.
On the other hand, owing to their unique properties of excellent electrical conductivity, thermal conductivity/stability, and mechanical strength, carbon nanomaterials, including carbon nanotubes (CNT) [21], carbon nanofibers (CNF), and graphene [22], have been intensively studied as electrode materials for supercapacitors [4,7,23]. Unfortunately, drawbacks associated with these nanomaterials, such as high cost, low packing, difficulty in mass production, and incompatibility with the electrode manufacturing process presently used in the industry, seriously hinder their practical utilizations for supercapacitors [24]. Alternatively, attempts have been made to incorporate carbon nanomaterials with AC to fabricate composite electrodes for pursuing their practical applications for supercapacitors. In this regard, CNT [25,26], CNF [27,28], and graphene [29,30] have been individually investigated for this purpose. However, performances of the resultant composite electrodes are limited, which is largely attributed to the inefficient effects of these carbon nanomaterials when separately employed in the composites.
In this work, we incorporated conventional high-surface-area AC and conductivity additive (i.e., CB) with carbon nanomaterials (i.e., CNT and CNF), through a simple slurry process, to develop a new class of multi-component nanocomposite electrodes for high-performance supercapacitors. Owing to the synergistic effects from the filling of CB, wrapping of CNT, bridging of CNF, and excellent conductivities of CNT and CNF (forming an effective packing and electrical conduction network) as well as the synergistic effects from the high microporosity of AC and high meso-/macroporosities of CNT and CNF (forming a three-dimensional (3D) hierarchical porous structure), the resultant nanocomposite electrodes exhibited superior performances over the conventional AC electrodes. The final optimized AC/CB/CNT/CNF quaternary nanocomposite electrode showed a high packing density (0.63 g cm−3), a high capacitance (104.9 F g−1, 66.1 F cm−3), a high rate capability (capacitance retained 77.5% at 80 A g−1 vs. 0.5 A g−1), a high energy density (23.5 W h kg−1, 29.6 W h L−1), a high power density (80.7 kW kg−1, 101.7 kW L−1), and a long cycle life (capacitance retained 91.4% after charging/discharging at 10 A g−1 for 30000 cycles), significantly outperforming current supercapacitor technology [31]. In particular, the enhanced volumetric performances of our nanocomposite electrodes indicate their significance in practical applications for commercial supercapacitors [15,16]. Thus, the nanocompositing approach developed in the present work would significantly benefit the supercapacitor industry, utilizing commercially available materials, through a simple slurry process, to produce highly capacitive nanocomposite electrodes on an industrial scale for manufacturing high-performance supercapacitor products at low costs. More importantly, this approach can also be extended to mass-produce high-performance nanocomposite electrodes for other energy-related devices including, for example, batteries, fuel cells, and solar cells.
Section snippets
Synthesis of nanocomposite electrodes
Nanocomposite electrodes were prepared by following a slurry procedure similar to that currently used in the supercapacitor industry. Briefly, the slurries were prepared by mixing predetermined amounts of AC (YP–50F, Kuraray), CB (Super C45A, Timical), CNT aqueous dispersion (5 wt%, diameter: 7–9 nm, length: 1–2 μm, Cnano), and CNF aqueous dispersion with an aqueous binder solution. The CNF aqueous dispersion was made by firstly treating a CNF powder (diameter: 50–100 nm, length: 5–50 μm,
Morphologies and packing of nanocomposites
Morphologies of composite electrodes were characterized by SEM. As shown in Fig. 1a, the AC/CB binary compositing system of a conventional ACE was formed by the filling of CB particles into the voids created between AC particles, where CB particles aggregated together to form the electrical conduction network of the system. Essentially, this system is characteristic of isolation of CB aggregates, loose contact between CB aggregates and AC particles, and direct point-to-point contact between AC
Conclusion
In summary, using a simple slurry process to combine conventional high-surface-area AC and conductivity additive (CB) with carbon nanomaterials (CNT and CNF), we have developed a new class of nanocomposite electrodes for high-performance supercapacitors. Incorporating flexible CNT and rigid CNF into the composites was able to significantly improve the properties of the resultant nanocomposite electrodes. Specifically, packing densities and capacitive performances of the composite electrodes
CRediT authorship contribution statement
Fang Cheng: Investigation, Formal analysis, Writing - original draft. Xiaoping Yang: Investigation, Methodology. Shuangpeng Zhang: Investigation, Validation. Wen Lu: Conceptualization, Funding acquisition, Project administration, Resources, Supervision, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by The Special Significant Science and Technology Program of Yunnan Province (grant number: 2016HE001-2016HE002). Fang Cheng gratefully acknowledges support from Yunnan University Innovative Research Fund for Graduate Students (grant number: YDY17088).
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