High Energy-Density Supercapacitor, Enabled By Carbon Nanostructures
Authors: Carley Christopher Scott , Espinoza Danny J , Reyes-Rodriguez Jose Luis , Ahn Ethan C 
Published: December 2020

Objective:

A supercapacitor is an emerging energy storage technology that can exhibit capacitances of many orders of magnitude higher than traditional dielectric capacitors, owing to the atomically-small interplanar distance (i.e., the Debye length) between the electrolyte ions and the electronic charges [1]. However, a supercapacitor technology that can surpass the energy-density (in Wh/kg) of conventional batteries or fuel cells while still preserving its well-known advantages of long life-cycle, robust thermal operating range, and high power-density (in W/kg) has not yet been demonstrated.
Carbon nanostructures, including single-walled carbon nanotubes (SWCNTs) and graphene (or reduced graphene oxide, rGO), possess the great potential to advance the energy storage technology due to their excellent thermal and electrical conductivity, flexibility while maintaining strength, high surface area-to-mass ratio, and elemental abundancy [1]. A previous work by Niu and co-workers [2] demonstrated the rGO-based micro-supercapacitors (i.e., having rGO as the interdigitated electrodes) with enhanced specific capacitances. Although a preliminary test was performed on how such device parameters as electrode width would impact the overall supercapacitor performance, a physical, specific insight was still not provided on how the biggest existing challenge of modern supercapacitor – namely, relatively low energy-density, can be addressed. In this work, we carefully designed a device fabrication process to develop the electric double layer capacitor (EDLC)-type [3] micro-supercapacitors and investigated both carbon nanostructures (Fig. 1 for SWCNT (a) and rGO (b) solutions) as electrodes. The great tunability of these carbon nanostructures (by surface functionalization, chemical reduction, etc.) will lead us to systematically investigate the effect of varying design parameters on the energy-density aspect of the supercapacitor performance.

New Results:

We adopted both experimental and theoretical approaches to best study the carbon nanostructure-based micro-supercapacitor devices. Experimentally, our device fabrication process is much simpler than what is shown in the previous work [2], thus enabling us to fabricate and test a greater number of prototype devices of varying electrode dimensions (Fig. 2 for a representative photomask design with electrode width and space of 400 mm). The device fabrication process includes photolithography to pattern the interdigitating electrode structure, dropwise deposition of SWCNT or rGO, polymeric coverage of electrode ends to protect Au contacts, Au etching via iodine gas, and deposition of H3PO4/PVA gel electrolyte (Fig. 3 for the detailed fabrication process flow). It is important to note that in this work, the carbon nanostructures were deposited by using a hypodermic syringe (Fig. 4). This ensures ease of electrode deposition because otherwise some other expensive experimental techniques would have been used (e.g., electrophoretic deposition [2]).
To verify the validity of our devices and further guide our experimental efforts, we created a model to simulate the rGO/CNT-based micro-supercapacitors. Two models were created in this work (one with MATLAB and another with PSpice), allowing us to compare and confirm our simulation results (Fig. 5 for the circuit diagram). To accurately model the supercapacitor device, a simple capacitor model was first created to serve as the foundation, and then we used data from the capacitor to create a look-up table based on the capacitance-voltage curve of the capacitor [4-5]. From these models we were able to collect the output power, current and voltage of the supercapacitor device. Fig. 6 shows the simulated energy-density and power-density plot of the supercapacitor based on rGO (a) and SWCNT (b) electrodes.

Significance:

Currently available renewable energy sources supply only a very small fraction of the electricity consumed in the nation. For example, only 1.7% of about 4 trillion kWh of electricity was generated from photovoltaics in 2019 [6]. This is mainly due to a great challenge in enhancing the solar energy conversion efficiency while still keeping the affordable price [7]. High energy-density supercapacitors developed in this work will serve as the next-generation energy storage device to greatly complement the existing energy harvesting or conversion technologies by dealing with the intermittent nature of renewable energy sources.

References:

[1] G. Xiong et al., "A Review of Graphene-Based Electrochemical Microsupercapacitors", Electroanalysis 26, 2014, 30-51.
[2] Z. Niu et al., "All-solid-state flexible ultrathin micro-supercapacitors based on graphene", Advanced Materials 25, 2013, 4035-4042.
[3] M. Aslani, "Electrochemical Double Layer Capacitors (Supercapacitors)", http://large.stanford.edu/courses/2012/ph240/aslani1, 2012.
[4] J. Patrik, and B. Andersson "Comparison of Simulation Programs for Supercapacitor modeling", Chalmers University of Technology, http://webfiles.portal.chalmers.se/et/MSc/AnderssonJohanssonMSc.pdf, 2008
[5] C. Fărcaş et al., "Modeling and simulation of supercapacitors", SIITME, IEEE, 2009.
[6] US EIA, "What is U.S. electricity generation by energy source?", https://www.eia.gov/tools/faqs/faq.php?id=427&t=3, 2020,
[7] B. Parida et al., "A review of solar photovoltaic technologies", Renewable and sustainable energy reviews 15, 2011, 1625-1636.

Figure 1