|Fig. 1: MCM-41 self-assembly. (1) Liquid-crystal phase and (2) Co- assembly template.|
Energy storage is a technique that employs a device or a media to store excess energy and to use the stored energy to perform useful operations at a later time. Storing energy allows human to balance the supply and demand of energy and is a possible solution to the energy crisis. Batteries and capacitors are the most common forms of energy storage devices, which are able to provide a maximum working voltage of hundreds of volts and capacitance of up to several kilo-farads. Capacitive energy storage is distinguished from other types of electrochemical energy storage by short charging times, long operational life, high power density and a low cost of power. These features are desirable for a range of applications, in electric vehicles as regenerative braking devices that capture the otherwise lost kinetic energy of a vehicle when the brakes are applied and storage for renewable energy supplies such as wind and wave power which come in short bursts. Conventional rechargeable batteries often charge too slowly to be useful in these contexts. In this report, the use of porous materials for the development of capacitor will be discussed.
Porous materials have been studied extensively, notably in applications regarding adsorption, catalysis, and capacitance. The International Union of Pure and Applied Chemistry (IUPAC) divides porous materials into three classes with respect to pore size: macroporous (> 50 nm), mesoporous (2-50 nm), and microporous (< 2 nm).  Ordered mesoporous materials have several unique structures that combine increased surface area and site activity with high adsorptivity and mass transport potential. Synthesis of these ordered porous materials often employs a self-assembly process of a surfactant molecules (block copolymer) and polymerizable inorganic species. The surfactant molecules self-assemble into a micelle due to the presence of a hydrophilic head and a hydrophobic tail. Micelles then pack into various arrays depending on the ratio of surfactants to inorganics, temperature, pH, packing parameters, humidity, and other factors affecting thermodynamic stability. After the inorganic precursor polymerize on the silica template and fix its structure, surfactant template is removed from the nanostructure, forming ordered pores in the materials. 
|Fig. 2: showing the difference between conventional capacitor and Electrical double layer capacitor.|
In a conventional capacitor, energy is stored by the removal of charge carriers from one plate to the other. Electrical double layer capacitors (EDLC) consist of two individual plates separated by an intervening insulator. Energy is harnessed by a physical separation of the charges on the Helmholtz double-layers formed between the surface charges and the diffused layer closed to the bulk electrolyte. The capacitance of the electrode/interface in an electrostatic is associated with an electrode-potential- dependent accumulation of electrostatic charge at the interface; therefore, the specific capacitance and energy storage characteristics of EDLC are limited by the capability of carbon electrodes to adsorb a large quantity of ions and attract the ions closer to the pore walls.  It is known that when a disordered material is used, for example activated carbon, ion transport is dramatically slow down as diffusion is heavily limited due to the irregularly curved pores with narrow bottle-necks.  Mesoporous materials offer high specific surface area and with the tunability of the self-assembly process, straight mesoporous channels allow fast ion transport and sub-nanometer pores enhances high specific capacitance.
Wang et al. has demonstrated using the ordered channel of mesoporous carbon, a shorter diffusion route (0.5-1 μm) and a lower ion-transport resistance has enhanced the capacitance.  They design a 3D architecture composed of macroporous, mesoporous and microporous materials. Macropores provides the least resistance to for an ion-buffering reservoirs; mesoporous region gives a low-resistant pathways for ions to diffuse through; and, micropores to strengthen the capacitance.
|Fig. 3: Schematic representation of the 3D hierarchical porous texture showing the self-supporting carbon network.|
The mesoporous wall consists of localized graphitic structure, which leads to enhanced electric conductivity. The energy and power densities can be calculated respectively as:
|E = 1/2 × CV2|
|P = iV/2|
Where C, V and i are the gravimetric capacitance, the cell voltage and the current density. The reported energy and power densities are 5.7 Wh/kg and 10 kW/kg, respectively at a drain time of 3.6 s; compared to those of activated carbon, 2.2 Wh/kg and 4 kW/kg, we can see significant improvements in this hierarchical porous materials. Their group further optimized the capacitance using an organic electrolyte solution and achieve an energy and power densities of 22.9 Wh/kg and 23 kW/kg under the same drain time. We can clearly see how this trimodal material with ordered porous structure has improved the traditional disordered microporous materials.
Another type of capacitor that is of high research interest is the pseudocapacitor. Differs from the EDLC, where charge storage is statically in the double layer, pseudocapacitor has a chemical reaction at the electrode. This charging/discharging reaction is reversible and since it is faradaic in origin, it is called "pseudocapacitance."  Pseudocapacitors combine features of both rechargeable battery and standard capacitor and have a substantially higher density then the normal EDLC, by at least an order of magnitude.  They have high potential to excel in areas like mobile digital technologies and flash photography.
Brezesinski et al. employed a novel nanocrystalline ordered mesoporous α-MoO3 (molybdite) and use it as redox pseudocapacitance.  Please refer to original paper for bright-field TEM micrograph of mesoporous α-MoO3 and computational modeling of Molybdite unit cell showing the layer-like arrangement of molybdenum and oxygen atom. They employed a sol-gel synthesis using the surfactant template with MoCl5 to produce a mesoporous film of α-MoO3 with crystalline structure. By comparing the redox pseudocapacitance of amorphous mesoporous and crystalline forms of MoO3, the team demonstrated that mesoporous films of iso-oriented α-MoO3 show impressive charge storage behavior because of a combination of the atomic scale structures and the nanoscale structure of the films.
Despite the high power density observed with this materials through the non-faradaic contribution from the double layer capacitance, lithium insertion were found to take place rapidly into the Van de Waals gap of the crystalline structure from LiClO4 electrolyte, which further increases the faradaic contribution from diffusion controlled Li+ insertion and charge transfer processes with surface atoms through pseudocapacitance. It is also found out that the facile ability of Li+ to be inserted to the interlayer gap is predominantly enhanced by the ordering structure of the mesopores and the short diffusion path lengths. Through the amenability of lithium ion insertion, optimization of both energy density and power density in a single material becomes possible.
The uses of porous materials, especially with ordered mesoporous materials, have shown high potentials in the making of capacitance for fast energy storage purpose due to their short cycle time, high power density and long operational life. Researchers have already demonstrated the incorporation of lithium ion into these types of capacitor. This phenomenon offers high potential for more rapid charging as well as meaning higher power densities might be achievable that are on a par with rechargeable lithium-type batteries rather than standard capacitors.
© John To. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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