First Graphene - Super Capacity Batteries

 

Advances in supercapacitors are delivering generational changes in energy-storage options. They are increasingly demonstrating that they can compete against standard chemical batteries in a range of markets.

A supercapacitor is a double-layer capacitor that has very high capacitance but low voltage limits. Supercapacitors store more energy than electrolytic capacitors and they are rated in farads (F).  Supercapacitors store electrical energy at an electrode–electrolyte interface.  They consist of two metal plates, which only are coated with a porous material known as activated carbon. As a result, they have a bigger area for storing much more charge.

 

Supercapacitors have traditionally been made with activated carbon but the issue with this material is that the pores within the activated carbon have poor interconnectivity such that they cannot achieve their potential maximum energy density.  The BEST™ Battery Project with Swinburne University addresses this issue with the use of graphene oxide and lasers that create well-interconnected nanopores that enable the energy density to improve by up to 10x.  

When a supercapacitor is being charged, current (or energy) flows into the supercapacitor and it fills just like filling a glass with water, where the final charged voltage is analogous to the water level in the glass, and the energy stored is analogous to the volume of water in the glass. When connected to a load the energy flows out of the capacitor as a current, transferring the energy stored in the supercapacitor to work done in the load. The lights light, the music sounds, the wheels turn.  This is something like our glass of water with a small tap at its base letting the water out.  The slowly reducing water level is analogous to the voltage across the supercapacitor slowly decreasing as the energy is drained off.

This is different from a battery, where energy is stored in the chemical process of the battery, and the voltage across its terminals does not change greatly during the discharge and recharge cycles.  In a supercapacitor the recharge energy is stored as ions differentiating within its structure and in effect being stored based on their polarity on the supercapacitor’s positive and negative terminals. The more ions that are collected the higher the voltage across the terminals.

Internally the supercapacitor consists of very fine layers of a synthesized graphene oxide film, filter-deposited on a flexible base material and patterned into an inter-digital structure using very narrow laser beams. The same laser technique is used in treating the graphene-oxide film to control the pore size and conductivity. This one-step process both reduces the oxygen containing groups between the graphene layers and generates the ultra-fine pores necessary for high ion storage capacity. The inter-digital layout, fine pore size and layered structure provide the most direct within-layer ion path and give the supercapacitor its exceptionally high energy and power densities. The metal current collectors are subsequently deposited on the film and the layers assembled along with the electrolyte into the required final shape.

An important distinction between supercapacitors and batteries, such as the lithium-ion battery, is that supercapacitors are based on physics and not chemistry.  A chemical battery deteriorates over time due to repeated chemical reactions degrading the materials in the electrodes.  The acceptable industry standard is about 1,000 cycles, after which the ability to store energy falls away rapidly.  As supercapacitors do not involve the use of chemical reactions they do not suffer chemical degradation.  In theory they can last for more than 10,000 cycles, giving a battery life ten times longer than existing chemical battery lives. 

The slowly reducing supercapacitor discharge curve is different from what most battery powered devices currently expect - a reasonably constant voltage from brand new until nearly flat. To match this during discharge a supercapacitor includes an electronic circuit that takes the slowly decreasing supercapacitor voltage and regulates it to a constant voltage across the output terminals – in the AA battery case at a constant 1.5 volts.  In addition to its regulatory role, the electronics includes circuitry for protecting itself and the supercapacitor from the destructive currents that may flow due to an unduly heavy load or short circuit across the output terminals. The actual mechanism can be either switching off entirely for a while or limiting the available current to some nominal value. 

 


 

Market analysis for supercapacitors

 Graph showing the increasing Supercapacitor Market Size

Data from: “Supercapacitor Market -­‐ Global Industry Analysis, Trend, Size, Share and Forecast 2015 – 2023”, Transparency Market Research, April 2016