Graphene Micro Planar Super-Capacitors For Energy Storage
While there is currently an enormous effort being poured into next generation chemical battery technology, including various forms of lithium ion batteries, supercapacitors represent the next wave of energy storage technology, promising vastly superior performance. Ionic’s technology represents a solution to the problems that have inhibited the widespread adoption of supercapacitors as energy storage devices.
Batteries and capacitors seem similar as they both store and release electrical energy. However, there are crucial differences between them that impact their potential applications, due to their functional differences.
The potential energy in a capacitor is stored in an electric field, where a battery stores its potential energy in a chemical form.
The technology used in chemical storage batteries currently yields greater energy densities (capable of storing more energy per weight) than capacitors.
However, a battery typically discharges its energy more slowly than a capacitor because there is a lag associated with the chemical reaction necessary to transfer the chemical energy into electrical energy.
A capacitor on the other hand stores the electrical energy directly on the plates so the discharging rate for capacitors is directly related to the conduction capabilities of the capacitors plates.
A capacitor can discharge and charge faster than a battery because of this energy storage method.
The primary uses for supercapacitors to date have been in combination with batteries or for specialised applications where the specific characteristics of supercapacitors presented an advantage over batteries. The applications include:
- Transport in buses, light rail and cars used alongside batteries or traditional petrol / diesel engines
- Energy harvesting and grid electricity applications where they are used to buffer electricity supply in conjunction with other energy storage systems
- Electronics applications with fluctuating loads where supercapacitors are used to stabilise the power supply from batteries
So, to date, while supercapacitors offer many advantages, their low energy per weight has inhibited widespread application as an energy storage device.
Ionic’s graphene micro planar supercapacitors can deliver all the benefits of supercapacitors in a much smaller, lighter device.
In 2015, the Ionic / Monash research team published ground-breaking research titled Miniaturized Supercapacitors: Focused Ion Beam Reduced GO Supercapacitors with Enhanced Performance Metrics  in volume 5 of the Journal of Advanced Energy Materials (cover shown in graphic).
That research demonstrated how micro-supercapacitor architectures with planar geometry provide several advantages, such as the ability to control and reduce the distances ions travel between two electrodes, easy integration to microdevices and the potential of being extended into 3 dimensions without compromising the interelectrode distances.
Focused ion beam (FIB) technology was used to directly write miniaturized planar electrode systems of reduced GO (FIB-rGO) on films of GO. Using optimized ion beam irradiation, the research team created interdigitated FIB-rGO electrode designs with 40 μm long and 3.5 μm wide fingers with ultra-small interelectrode spacing of 1 μm.
These devices demonstrated a large capacitance (102 mF/cm²), ultra-small time response (0.03 ms), low equivalent series resistance (0.35 mΩ/cm2), and retain 95% of the capacitance after 1000 cycles at an ultrahigh current density of 45 mA/cm². (See table below for comparison figures).
These performance metrics showed extraordinary improvements in a number of measures of supercapacitor performance compared to existing reports. The improvements were due to the miniaturized electrode dimensions and minimal damage to the graphene sheets.
These results were the first step toward largescale fabrication of arrayed, planar, high-performance micro-supercapacitors with a small environmental footprint.
While some competing micro-supercapacitors can provide high power densities, comparisons (see table below) demonstrate that Ionic’s miniaturised devices provide enhanced energy density while maintaining high power density.
This is the figurative holy grail in supercapacitor research.
The table below shows comparable performance against the best competing supercapacitors as presented in independent, peer-reviewed research from the University of California Los Angeles .
|Parameters||Ionic’s Technology||Best Competitors||What this means|
|Response time (ms)||0.033||19||Rapid power surge and faster charging|
|Energy density (Wh/cm³)||0.173||0.002||Sustain high power for longer life|
|Capacitance (mF/cm²)||102||2.314||More energy in same volume|
|Equivalent series resistance (mΩ cm2)||0.35||3600||Less losses|
Since this early publication, Ionic has continuing research in the field and has made a number of advances over the past 18 months to bring this technology closer to market. This has included significant progress on manufacturing methods that will drive economic, large scale production of planar super capacitor devices.