In the race for the next-generation energy storage devices, lithium-ion batteries have become popular and made huge leaps in the last years. However, the power packs still show different drawbacks.
“What’s coming next are technologies that improve performance devices, hold more energy, last longer at a lower cost.” Bloomberg NEF Consultancy firm suggests. If we take a look to the Ragone plot, where different Energy Storage devices are chartered according to their Energy and Power density, there are some promising devices configuration already under development that can mean disruptive entrance in the Energy Storage landscape like the asymmetric and hybrid supercapacitors. These devices can be positioned in new niches within the Ragone plot with remarkable performances in different applications as aerospace, automotive, heavy transportation, power grids or electronics. There is a huge business opportunity regarding innovative devices.
Figure 1: Ragone plot for different energy storage systems.
EDCL supercapacitors are devices with a quite interesting penetration into the market (3M$ per yar) and with a quite interesting market growth potential (CAGR around 10%). Other devices, as pseudocapacitors, entered very recently in the market and others are still pushing to get into the industrial applications through the next years. In here we make a revision of some of the Energy Storage devices.
- Electrochemical double layer capacitors (EDLCs) or symmetric supercapacitor.
EDLCs are constructed using two carbon-based materials as electrodes, an electrolyte and a separator. EDLCs can either store charge electrostatically or via non faradic process, which involves no transfer of charge between electrode and the electrolyte. The principle of energy storage used by EDLCs is the electrochemical double layer. When voltage is applied, there is an accumulation of charge on electrode surfaces, due to the difference in potential there is an attraction of opposite charges, these results to ions in electrolyte diffusing over the separator and onto pores of the opposite charged electrode.
- Pseudocapacitors
Compared to EDLCs, that store charge electro-statically. Pseudocapacitors store charge via faradic process which involves the transfer of charge between electrode and electrolyte. When a potential is applied to a pseudocapacitor reduction and oxidation takes place on the electrode material, which involves the passage of charge across the double layer, resulting in faradic current passing through the supercapacitor cell.
The faradic process involved in pseudocapacitors allows them to achieve greater specific capacitance and energy densities compared to EDLCs. Examples are metal oxides, conducting polymers. Which leads to interest in these materials but due the faradic nature, it involves reduction-oxidation reaction just like in the case of batteries; hence they also suffer lack of stability during cycling and low power density.
- Hybrid
As we have seen EDLCs offer good cyclic stability, good power performance while in the case of pseudo capacitance it offers greater specific capacitance. In the case of hybrid system, it offers a combination of both, that is by combining the energy source of battery-like electrode, with a power source of capacitor-like electrode in the same cell. With a correct electrode combination, it is possible to increase the cell voltage, which in turn leads to an improvement in energy and power densities.
Currently, researchers have focused on the three different types of hybrid supercapacitors, which can be distinguished by their electrode configurations: Composite, Asymmetric and Battery-type.
- Composite configuration
Composite electrodes combine carbon-based materials with either metal oxides or conducting polymer in a single electrode, this means a single electrode will have both physical and chemical charge storage mechanisms. Carbon based materials offer capacitive double-layer of charge and high specific surface area which increases the contact between pseudocapacitive materials and electrolyte.
- Asymmetric configuration
Asymmetric hybrids combine Non faradic and Faradic processes by coupling and EDLC with a pseudocapacitor electrode. They are set up in a way that the carbon material is used as a negative electrode while either metal oxide or conducting polymer as positive electrode.
- Battery Type configuration
Battery type hybrid combines two different electrodes, like in the case of asymmetric hybrids but in this case, they are made up by combining a supercapacitor electrode with battery electrode. This configuration was set up so as to utilize both properties of supercapacitors and batteries in one cell.
These energy storage devices have advantages and limitations.
| Advantages | Virtually unlimited cycle life. Can be cycled millions of time High specific power. Low resistance enables high load currents Charges in seconds. No end of charge termination required Simple charging, draws only what it needs Safe. Forgiving if abused Excellent low-temperature charge and discharge performance |
| Limitations | Low specific energy. Holds a fraction of a regular battery Linear discharge voltage prevents using the full energy spectrum High-self discharge, higher than most batteries Low cell voltage. Requires series connections with voltage balancing High cost per watt |
New materials for new devices are required. Graphene, a one atom thick layer 2D structure has emerged as a unique carbon material that has potential for energy storage device applications because of its superb characteristics of high electrical conductivity, chemical stability, and large surface area.
Gnanomat is focused in the design, development and production of advanced nanomaterials with carbonaceous base to be used as electrode materials in Energy Storage Systems (ESS). Our aim is to improve electrochemical properties of commercial supercapacitors and batteries.