Supercapacitors have gained popularity as future energy storage solutions due to their simple architecture, long cycling life, pulse power supply, and high dynamic charge propagation.¹ They are generally categorized as Electrical Double Layer Capacitors (EDLCs) or pseudo-capacitors.² EDLCs store energy as electrostatic charge on the electrodes’ surfaces; their efficiency is thus limited by the electrochemically active surface area available to the electrolyte.³ Whereas, pseudo-capacitors utilize fast and reversible faradaic processes due to electro-active species localized on the electrode surface to enhance current density.^{1, 4} Concentration of electro-active charged species and electro-chemical efficiency governs the efficiency of these supercapacitors. Recently, carbon-based materials (such as CNTs, graphene foams, graphene aerogels) have received research interest as promising candidates for supercapacitors due to their high electrochemical surface area, high electrical conductivity and accessible surface active sites for modification with electro-active species.⁵ Graphene has emerged as an ideal EDLC electrode material due to its superior electrical properties and high theoretical surface area.⁶ Current graphene electrodes are either pinned to a two-dimensional (2D) surface (such as graphene foams) or are tethered to a 2D substrate (such as carbon nanowalls). Graphene based hybrid-nanomaterials that leverage graphene’s outstanding surface-to-volume ratio coupled with pseudo-capacitive species for high efficiency supercapacitors have not been demonstrated to date.

Here we demonstrate that direct synthesis of three-dimensional (3D) out-of-plane fuzzy graphene (3DFG) on Si nanowire (SiNW) mesh template (NT-3DFG) provides an ideal surface for functionalization with electro-active species to develop EDLC and pseudo-capacitive supercapacitors. We report fine control over 3DFG flake size and edge density by varying the partial pressure of CH₄ precursor, plasma enhanced chemical vapor deposition (PECVD) synthesis process time, and PECVD synthesis temperature (Figure 1.A and B).⁷ Through electron microscopy (SEM and TEM) and Raman spectroscopy a distinct increase in single-to-few layer out-of-plane graphene edges is observed. High density of graphene edges presents high concentration of active sites for surface functionalization through surface doping, such as that through treatment with HNO₃. Post treatment, the electrical conductivity of NT-3DFG is observed to be as high as 2400 S m^-1. Leveraging on graphene’s outstanding surface-to-volume ratio, we have developed a material capable of attaining electrochemically active surface area of ca. 1017 m² g^-1. The introduction of electro-active species on this high surface area material renders specific capacitance as high as 87 ± 8 F g^-1 (Figure 1.C). This value is further enhanced by decorating the template with metal oxide nanoparticles. The reported novel graphene based hybrid-nanomaterial shows great potential to be utilized in next-generation energy storage solutions.

Figure 1. Characterization of NT-3DFG. (A) SEM image of NT-3DFG synthesized with increasing partial pressure of CH₄ and increasing PECVD process time. (B) Effect of increasing PECVD process temperature on NT-3DFG synthesized under 25 mTorr partial pressure of CH₄ for 10 min. (C) Cyclic voltammetry characterization of NT-3DFG before and after HNO₃ treatment. (I) Representative cyclic voltammograms presenting double layer capacitance of NT-3DFG. Blue, yellow and red denote Au working electrode, NT-3DFG synthesized under 25 mTorr partial pressure of CH₄ for 30 min and NT-3DFG synthesized under 25 mTorr partial pressure of CH₄ for 90 min, respectively. Dashed lines represent respective samples after 2-hour HNO₃ treatment. (II) Current density as a function of scan rate. Blue, yellow and red denote Au working electrode, NT-3DFG synthesized under 25 mTorr partial pressure of CH₄ for 30 min and NT-3DFG synthesized under 25 mTorr partial pressure of CH₄ for 90 min, respectively. Closed and open square represent samples before and after 2-hour HNO₃ treatment, respectively.

References:

1. Zhang LL, Zhao X. Carbon-based materials as supercapacitor electrodes. Chemical Society Reviews 2009, 38(9): 2520-2531.
2. Conway BE. Electrochemical supercapacitors: scientific fundamentals and technological applications. Springer Science & Business Media, 2013.
3. Miller JR, Simon P. Electrochemical capacitors for energy management. Science magazine 2008, 321(5889): 651-652.
4. Aricò AS, Bruce P, Scrosati B, Tarascon J-M, Van Schalkwijk W. Nanostructured materials for advanced energy conversion and storage devices. Nature materials 2005, 4(5): 366-377.
5. Pandolfo A, Hollenkamp A. Carbon properties and their role in supercapacitors. Journal of power sources 2006, 157(1): 11-27.
6. Liu C, Yu Z, Neff D, Zhamu A, Jang BZ. Graphene-based supercapacitor with an ultrahigh energy density. Nano letters 2010, 10(12): 4863-4868.
7. Garg R, Rastogi SK, Lamparski M, de la Barrera S, Pace GT, Nuhfer NT, et al. Nanowire-Mesh Templated Growth of Out-of-Plane Three-Dimensional Fuzzy Graphene. ACS nano 2017, 11(6): 6301-6311.