Graphene Micro-Supercapacitors

February 27, 2013

Supercapacitors have become useful energy storage devices. I've reviewed some aspects of supercapacitors in two previous articles (Flow Batteries, July 18, 2012 and Carbon Nano-Onion Supercaps, August 26, 2010). It's always worthwhile reviewing the equations describing the simplest capacitor, the parallel plate capacitor.

 C = κεoA/d Q = C V I = ∂Q/∂t W = (1/2) C V2 V = Vo e(-t/RC) Q = C Vo e(-t/RC) I = (Vo/R) e(-t/RC) where C is capacitance, κ is the dielectric constant, εo is the permittivity of free space, A is the area of the capacitor plates, d is the plate separation, Q is the charge, V is the voltage, Vo is the initial voltage, I is the current, W is the stored energy, t is time, and R is the load resistance.

Since my brain is tuned for picofarads (10-12 F), I use 8.854 x 10-12 farads per meter for εo. If you use farads as the units of capacitance, the stored energy will have units of joules. A watt is a joule per second.

You can see from these equations that large plate area, small plate separation and high dielectric constant lead to high capacitance; and, thereby, high energy storage at high voltage. In electric double-layer supercapacitors, the dielectric layer is the interface between conductive electrolyte layers.

This very thin dielectric interface, combined with the use of very high area carbon electrodes made from activated charcoal, gives capacitors of many farads in a few cubic inches. Electrochemical decomposition of the electrolyte, however, limits operation to about 3 - 5 volts.

 Structure of a conventional supercapacitor.There are separate ionic liquids that are negatively and positively charged. One disadvantage of current materials is that these ionic liquids decompose at voltages above a few volts.(Via Wikimedia Commons, modified).

So, how does the energy density of supercapacitors compare with other energy stores? As the table shows, there's still considerable room for improvement. The 0.07 kJ/gram estimate is for commercial supercapacitors at this time.

 Principle Type kJ/gram Capacitor Supercapacitor 0.07 Electrochemical Lead Acid Battery 0.1 Electrochemical NiMH Battery 0.22 Mechanical Flywheel 1 Electrochemical Lithium Battery 2.5 Chemical (Combustion) Ethanol 30 Chemical (Combustion) Gasoline 46.9 Chemical (Combustion) Hydrogen 143

Many university laboratories have been doing research on improving supercapacitor energy storage density. Since graphene is the miracle material of the moment, it's no surprise that some of this research is about using graphene in supercapacitors. A considerable advance in the state of the art has been achieved by scientists from the UCLA NanoSystems Institute, and they've published the results of their study in a recent issue of Nature Communications.[1-3]

Richard Kaner, a professor of Chemistry and Biochemistry, Inorganic Chemistry, and Materials Science and Engineering, and research associate, Maher F. El-Kady, have developed a process for fabrication of graphene micro-supercapacitors.

In the same tradition as Ernest Rutherford's doing experiments with string and sealing wax, they fabricated these with a LightScribe DVD burner.[1-2] The laser imaging in the DVD burner is quite precise, allowing a precise definition of device features.[2] This deviation from traditional photofabrication is explained by Kaner, as follows:
"Traditional methods for the fabrication of micro-supercapacitors involve labor-intensive lithographic techniques that have proven difficult for building cost-effective devices, thus limiting their commercial application... Instead, we used a consumer-grade LightScribe DVD burner to produce graphene micro-supercapacitors over large areas at a fraction of the cost of traditional devices. Using this technique, we have been able to produce more than 100 micro-supercapacitors on a single disc in less than 30 minutes, using inexpensive materials."[2]

 UCLA micro-supercapacitors, as produced by Kaner and El-Kady's DVD burner process(UCLA Photograph.)[2]

In the UCLA process, a layer of plastic is glued to a DVD, and the plastic is then coated with a graphite oxide dispersion in water.[2] The graphite oxide is transformed by the laser light into graphene.[2] The capacitors are formed as interdigitated electrodes of graphene topped with an ionogel electrolyte. Close electrode spacing reduces the diffusion path for the electrolyte ions, so the micro-supercapacitors can charge at a higher rate than other capacitors.[2]

Measurements demonstrate a power density of about 200 W-cm-3, which puts these at the top rank of supercapacitor energy density.[1] The micro-supercapacitors also show excellent cycling stability which is much better than micro-batteries; and they can be charged a hundred to a thousand times faster than batteries.[2] They also have the advantage that they are highly bendable and twistable.[2]

References:

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