(Digital Presentation) On-Chip Micro-Supercapacitors of 3D Carbon Nanotube/Gold Nanocomposites through Silicon-Based MEMS Technology
Authors: Du Kang , Lu Pai 
Published: August 2024

The high-power and high-frequency response characteristics of supercapacitors have enabled their promising application in fasting charging/discharging scenarios such as power grid stabilization and Internet of Things (IoT). The widespread use of portable electronic devices and the evolution of the IoT have underscored the growing demand for the ongoing miniaturization and enhanced energy autonomy of existing circuit components [1]. Micro-supercapacitors (MSCs) have surfaced as a promising solution to meet these demands, offering superior power density and cycle life compared to similar battery products [2]. However, integrating MSCs with electronic circuits presents significant challenges, often serving as a roadblock to comprehensive system miniaturization. To cater to the requirements of practical application scenarios, particularly in System-in-Package (SiP) and System-on-Chip (SoC) contexts, the fabrication of MSCs requires the use of silicon-based semiconductors or Micro-Electro-Mechanical Systems (MEMS) technology [3]. This strategic approach not only enhances performance but also ensures shorter interconnection distances, more compact dimensions, and increased energy and power density across all components. Emphasizing high power density, elevated energy density, and responsive high-frequency behavior is crucial for the optimal characteristics of MSCs in future applications. As for electrode materials, carbon, which is abundant and cost-effective, has spurred extensive research into various nanostructured carbon-based materials for Electric Double-Layer (EDL) MSCs, including activated carbon, carbon nanotubes (CNTs), carbide-derived carbon, and onion-like carbon [4]. Additionally, transition metal oxides such as ruthenium oxide and manganese oxide, along with conductive polymers like polypyrrole and polyaniline, have proven useful as materials for pseudocapacitive micro supercapacitors [5]. Notably, CNTs fabricated through Chemical Vapor Deposition (CVD) are unequivocally compatible with MEMS microfabrication technology. In the context of EDL MSCs, a dense CNT network structure boosts energy density but impedes ion transport, thereby reducing power density. Conversely, a sparse CNT network structure facilitates ion transport but compromises high-frequency response due to fewer charge transfer paths. Therefore, the development of CNT-based MSCs tailored to practical application needs a more thorough investigation into CNT network structures and their current collector designs.
Here, we report the fabrication of on-chip MSCs that are fully compatible with Silicon-based MEMS technology. Figure 1 illustrates the fabrication and characterization process of on-chip MSCs utilizing silicon-based MEMS technology, showing distinct surface morphologies of b-Si nanostructures after CNT growth and Au layer deposition. Figure 2 presents the electrochemical performance of the on-chip MSCs. The CV curves exhibit an ideal rectangular shape at a scan rate of 1 V/s. Even at a high scan rate of 100 V/s, the CV curve maintains a quasi-rectangular shape with minimal distortion, confirming the ultrafast response performance (Fig. 2 (a-b)). The phase angles of the on-chip MSCs are measured to be -68.9° and -66.2° in 1.0 M Na2SO4, and -64.1° and -70.8° in 0.5 M H2SO4 at 120 Hz (Fig. 2 (c)). The absence of a prominent semicircle in the Nyquist plots at high frequencies region implies fast electron transfer and ion diffusion within CNT/Au nanocomposite electrodes. Furthermore, the equivalent series resistances (ESR) are 0.604 Ω·cm2, 0.201 Ω·cm2, 0.504 Ω·cm2, and 0.126 Ω·cm2 for Si-CNT-20 and Si-CNT-20-Au MSCs (Fig. 2 (d)), indicating excellent interfacial contact between the CNT layer and Au current collector and good electrode conductivity. Fig. 2(e) shows that the MSCs deliver a specific capacitance (CA
) of 0.049 mF/cm2, 1.041 mF/cm2, 0.101 mF/cm2, and 1.368 mF/cm2 in 1.0 M Na2SO4 and 0.5 M H2SO4 at 120 Hz, respectively. The relaxation time constant (τ0
) is calculated to be as short as 1.65 ms for the MSCs (Si-CNT-20-Au), underscoring rapid ion diffusion within electrodes in 0.5 M H2SO4 electrolyte (Fig. 2 (e)).
In conclusion, we have successfully used b-Si as the scaffold structure and 3D CNT/Au nanocomposites as the active material for on-chip MSCs application. Significantly, the incorporation of this silicon-based 3D CNT/Au nanocomposite electrode presents promising prospects for the application of MSCs in varied domains, including wearable electronics, IoT devices, and sensor networks.

References

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[2] G. S. Gund, J. H. Park, R. Harpalsinh, M. Kota, J. H. Shin, T. I. Kim, Y. Gogotsi, H. S. Park, Joule, (2019), 3, 164.
[3] C. Lethien, J. Le Bideau, T. Brousse. Energy Environ. Sci., (2019),12, 96-115.
[4] Z. Fan, N. Islam, and S. B. Bayne, Nano Energy, (2017), 39, 306-320.
[5] J. Wei, X. Li, H. Xue, J. Shao, R. Zhu, H. Pang, Adv. Mater. Interfaces., (2018), 5(9), 1701509.

Acknowledgement

Financial support from the Innovation Norway through an EEA Grant under Grant No. 2021/336905, is acknowledged.

Figure 1