PorousCarboninSupercapacitors

-A review

Submitted toProf. Yani Zhang

School of Materials Science, NPU, Xi’an, China

Prepared by Shameel FarhanPhD Student (Carbon foam core sandwich structures)Student ID 2013410005

HistoricalOverview1950s- General Electric started experimenting using porous carbon electrodes.1957-Becker developed a low voltage electrolytic capacitor with porous carbon electrodes.1966-Researchers at Standard Oil of Ohio developed electrical energy storage apparatus. 1970- The electrochemical capacitor patented by Donald L. Boos.1971-NFC marketed supercapacitor to provide backup power for computer memory. 1978-Panasonic marketed its "Gold caps” brand as a successful energy source for memory backup.

1982-PRI Ultracapacitor with low internal resistance.1991- Line was drawn between supercapacitor and battery electrochemical behavior.1994-Electrolytic-Hybrid Electrochemical Capacitor. 2007-An electrostatic carbon electrode with a pre-doped Li-ion electrochemical electrode.Current Research- To improve thecharacteristics, such as energy density, power density, cycle stability and reduce production costs by using new carbon materials like CNTs, graphene, aerogels, carbon foam etc.

A capacitor differs from a battery in that it can store a higher amount of energy, but for a

shorter period of time. This allows a supercapacitor to be used in applications that require larger amount of energy in repeated

bursts (for example, a camera flash). Batteries, however, supply the bulk of energy in most

devices since they can store and deliver energy over a slower period of time.

If you handle your super capacitors

carefully, you will die before they

do...Seriously!

What everyone wants is a device that can store a lot of energy

and charge or discharge quickly.

Fig.1 Standard lithium ion battery vs supercapacitor.

OverviewofSupercapacitorsElectrochemical capacitors, electric double-layer capacitor, supercondenser, pseudo capacitor, electrochemical double layer capacitor and ultra-capacitor, are electrochemical energy-storage devices characterized by high power densities and exceptional cycle lifetime required for handy electronics to heavy Industrial applications including hybrid vehicles. Performance of a supercapacitor depends on the surface area/cm of electrode, electrolyte and the separator used. Unlike conventional capacitors, supercapacitors do not have a dielectric, an electrical insulator that can be polarized with the application of an electric field. Instead, the plates of a supercapacitor are filled with two layers of the identical substance. This allows for separating the charge. Materials used in making supercapacitors are made from powdered, activated carbon. Various institutions have researched the possibility of using carbon nanotubes. Certain polymers as well

as graphene, a material made of tightly packed carbon atoms, are also used for production. By contrast, while supercapacitors have energy densities that are approximately 10% of conventional batteries, their power density is generally 10 to 100 times greater. This results in much shorter charge/discharge cycles than batteries. Additionally, they will tolerate many more charge and discharge cycles than batteries.

Many capacitors used in audio circuits have capacitances such as 470uf or 680uf (micro farads). Capacitors used in high frequency RF applications can be as small as 1pf (pico farad). The farad is a measure of capacitance (or storage capacity).

BasicDesignPrinciple construction of a supercapacitor is shown in figure 2. Ions are separated from the electrolyte by a charging current, and are propelled toward their respective electrodes. The membrane serves to separate the ions so that a net charge separation can be maintained. Notice that the ions on the electrode surface

are neutralized (save for a residual dipole field) by the opposite charge attracted to just below the surface of the electrode. This dual surface layer is called an electric double layer. All other parameters being equal, the number of ions stored is proportional to the surface area of the electrodes. Energy storage of a capacitor is proportional to the amount of charge stored, so a tenfold increase in capacitance will require new electrodes that are highly conductive (so large power levels can be generated) and provide more surface area than conventional supercapacitors.

Fig. 2 Basic supercapacitor; 1. Power source, 2. Collector, 3.polarized electrode, 4. Helmholtz

double layer, 5. Electrolyte having positive and negative ions, 6. Separator.

ApplicationsThey are often used in filtering applications, coupling or decoupling applications, or AC-DC smoothing applications (there are some large caps in standard AC-DC power supply that acts to smooth out the ripple on the line), hybrid vehicles and electric vehicles, self-powered equipment powered by human muscle, mechanically powered flashlight, enhancing performance for portable fuel cells, such as generators, and improving the handling of batteries.

ClassificationSupercapacitors are divided into three families, based on electrode design:

Double-layer capacitors –with carbon electrodes or derivates with much higher electrostatic double-layer capacitance than electrochemical pseudo capacitance

Pseudo capacitors – with metal oxide or conducting polymer electrodes with a high amount of electrochemical pseudo capacitance

Hybrid capacitors – capacitors with asymmetric electrodes, one of which exhibits mostly electrostatic and the other mostly electrochemical capacitance, such as lithium-ion capacitors

New Research

Now researchers at UCLA have used a standard DVD writer to make such electrodes. The electrodes are composed of an expanded network of graphene that shows excellent mechanical and electrical properties as well as exceptionally high surface area.

The process is based on coating a DVD disc with a film of graphite oxide that is then laser treated inside a DVD writer to produce high-quality graphene electrodes. Graphite oxide is a compound of carbon, oxygen, and hydrogen made by treating graphite with sulfuric and phosphoric acids combined with potassium permanganate, an extremely strong oxidizer. When graphite oxide is placed in a basic solution, it exfoliates into monomolecular layers with a graphene-like structure. These layers were then collected on an ordinary

DVD disc. The disk was then written on, a number of passes being made.

Fig. 3 UCLA researchers develop new technique to scale up production of graphene micro-

supercapacitors.

The action of the 5 milliwatt IR laser on the graphite oxide was to reduce the material, thereby producing isolated but intertangled graphene monolayers. The surface area of the resulting electrodes was 1,520 square meters per gram - about a third of an acre, and 3-5 times the surface area of activated carbon electrodes. Graphene's intrinsic surface area is 2,630 square meters (about two thirds of an acre) per gram.

The UCLA research team investigated several different types of supercapacitor chemistry using the laser scribed graphene electrodes. They found the supercapacitors were surprisingly robust against flexure, surviving thousands of folds with no significant change in capacitance. Their highest energy storage supercapacitor was based on using the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate as the electrolyte. The supercapacitor exhibited a capacitance of 276 Farads per gram, and an operating voltage of 4 volts. This corresponds to an energy density of over 600 watt-hours per kilogram, or about four times that of lithium-ion batteries. In practice, the energy density will be smaller,

owing to support structures, but such supercapacitors should be able to give lithium-ion batteries a run for their money.

Fig. 4 Schematic showing the structure of laser scribed graphene supercapacitors created by

UCLA researchers.

The researchers didn't stop there, though. They began to play around with electrodes. Kaner said, "We placed them side by side using an interdigitated pattern, akin to interwoven fingers. This helped to maximize the accessible surface area available for each of the two electrodes while also reducing the path over which ions in the electrolyte would need to diffuse. As a result, the new supercapacitors have more charge capacity and rate capability than their stacked counterparts."

A few years ago, a group of researchers at the Lawrence Berkeley National Laboratory began working on creating micro-supercapacitors. Using micro fabricationmethods similar to those which are already being used to create microchips for electronic devices, these researchers etched electrodes of monolithic carbon film into a substrate of conductive titanium carbide. The result was micro-supercapacitors that had an energy storage density at least twice as much as existing supercapacitors.

Fig. 5 Berkeley Lab chemist John Chmiola is developing a new breed of micro-supercapacitors.

OvercomingtheLimitationsofSupercapacitorsLike a battery, a standard capacitor stores electrical energy. Whereas a battery can both produce and store electrons, a capacitor can only store them. And although a battery can dump its charge slowly through the course of hours, a capacitor dumps its charge in mere seconds.

Supercapacitors also have a low energy density and can only hold 1/5th to 1/10th the energy of a standard battery. Because of the organic electrolyte used in supercapacitors, the fast energy discharge of a supercapacitor is much higher than that of a battery. Supercapacitors are low voltage devices: in order to achieve a practical working voltage, several need to be strung together. And at present, mass production of supercapacitors has not been something that is cost effective. For example, if you wanted to use a supercapacitor to charge your laptop now, you might have to spend hundreds of dollars on dozens of supercapacitors. When connected together, these series of

supercapacitors would create a laptop that would no longer be very mobile.

Because of these limitations, using supercapacitors in our home electronics and mobile devices is not yet feasible. The wonders of graphene seem to know no bounds. Not only is it one of the strongest materials known, is both highly conductive and piezoelectric, it can generate electricity from flowing water and now it is being used to make better supercapacitors.

PorouscarbonaselectrodematerialinsupercapacitorsIn principle, any pore size can be adjusted and inserted into carbon, ranging from ultra-micropores to μm-sized macropores. It has been shown many times that the different approaches are largely compatible with each other, with the consequence that many hierarchical materials could be designed over the last few years. The biggest challenge for carbon materials with well-defined pore sizes in supercapacitor applications is the reduction of price. No material could compete with the very low costs and the scalability of activated carbons so far. Graphenes and carbon onions show better conductivities. The pore sizes of templated carbons are more narrowly distributed. Aerogels and fibers can directly be fabricated into the desired electrode shape. VA-CNTs are orientated and perfectly connected to a current collector.

Electro sorption of electrolyte molecules on the surface of a porous carbon material is the key process in supercapacitors. High power densities can be accomplished if the electrolyte has fast access to the surface of the electrode material. This can be ensured either by lowering the particle size of the carbon material down to the nanometer scale or by introducing internal porosity. The size, geometry, and distribution of these pores

Fig. 6 Energy density of carbon materials with different pore size depends on the potential window (A). Optimal pore size increases with increasing operating voltage window (B) and passes a maximum (C).

significantly influence the final performance of the supercapacitor device. The pore size distribution should be span over micropore scale (dpore < 2 nm) and the mesopore scale (2 nm < dpore < 50 nm). There is an optimal micropore size, which is different for each electrolyte system and at different voltage windows. Pore size distribution and uniformity are important for increasing the capacitance. Various techniques like activation, templating and etching etc are used to tailor the required pore size dictribution.

Tailoring microporisity in electrodes

The capacitance (C) of a supercapacitor electrode depends on the electrode specific surface area (A), and the distance (d) between the adsorbed ions and the electrode surface as shown in equation 1.

C = (εA)/d (1)

The dielectric constant (ε) is set by the used electrolyte. The values of A and d are significantly influenced by the size of the micropores. An “optimal” micropore size is

not easily available because the actual energy density also depends on the ion size andoperating voltage window

At a given pore size, the maximal energy density increases with increasing cell voltage and saturates at high voltages when no additional charge can be accommodated within the pore (Fig. 6A). This saturation energy density increases (high voltages) when the pores get larger since more charge can be stored. Therefore, at high voltages large pores are preferable (Fig. 6B). At low voltages however, the energy density is smaller for large pores because an electro-neutral zone is formed in the centre of the large pore, which does not contribute to stored charge (Fig. 6C).

Synthesis of materials with very narrow pore size distributions (PSD) is encouraged if the capacitance performance should be optimized. Experiments such as small-angle neutron scattering and nuclear magnetic resonance investigations (NMR) are used to directly probe the ion adsorption and electrosorption in microporous carbons. All of these studies impressively show the importance of uniformly adjusting the micorpore size in supercapacitor electrodes with low dispersity.

Templating is the method of choice if materials with uniform and ordered pore systems should be synthesized. This strategy is faced with the accusation of being expensive and badly up-scalable, but its academic benefit is non-questionable. Again, these materials are currently of outstanding importance since questions on electrolyte–carbon interaction or the principle influence of pore size/geometry/connectivity on supercapacitor performance need to be answered. The variety of templates is versatile and carbons with uniform micropore sizes are derived from different zeolite templates (e.g. Y, X13, beta, L, ZSM-5). If the zeolite channels are 3D-connected the

replica carbon is even ordered. The specific surface areas of these carbons even exceed 4000 m2 g−1. Activation of carbonaceous sources is a comparably low-price technique to introduce porosity. Activated carbons are obtained from the carbonization and activation of resources such as coals, peat, special woods, coconut shells and synthetic organic polymers etc. They are amorphous and contain nitrogen or oxygen. The main activating agents are carbon dioxide, steam, potassium hydroxide, zinc chloride and phosphoric acid. All activation processes differ in the fraction of pore sizes they create.This is generally observed, since all these agents act as dehydrating agents, stabilizing the carbon structure, giving higher carbon yields. The best of them exceed capacitances of 300 F g−1 in aqueous electrolytes. But activation is not only suitable for natural carbon resources. Recent attempts use natural resources such as fungi as carbon precursors. Hierarchical carbons derived from KOH activation of these sources show surface areas exceeding 2000 m2 g−1 and very narrow micropores.

Tailoring small mesoporosity in electrodes

Templating can be subdivided into soft- and hard-templating. The hard templates, mostly nanoscaled silica materials, are infiltrated with carbon precursors (e.g. sucrose, furfuryl alcohol, phenolic resin, pitches, acetonitrile), which are then polymerized and finally carbonized. In most instances, subsequent removal of the template is necessary. The soft-templating approach uses surfactants as structure-directing agents and carbon precursors (primarily based on resin) that interact with the surfactant and assemble around the formed micelles. Varying the surfactant/precursor ratio gives access to different mesostructures. The size of the

carbon precursor and its pre-polymerization degree determine the final pore size. In addition, the pore symmetry (cubic, hexagonal, wormlike) as well as the pore connectivity can be controlled leading to advanced ion transport properties.The final pore size can further be changed if additional activation steps are conducted. Templated carbons are primarily amorphous and often badly conductive. In consequence, conducting additives such as nanotubes or carbon black are added if these materials are applied in supercapacitor applications.

Tailoring large mesopore and macroporisity

Carbons with large mesopores or even macropores are accessible using colloidal templates such as silica nanoparticles. These particles are either commercially available or can be synthesized using emulsion, spray pyrolysis or Stöber approaches. Carbon precursors, such as resins, acrylonitrile, pitches, or carbon hydrates first polymerize and then carbonize around the sol template structure followed by dissolution of the template. The final pore size is mainly determined by the silica particle size. Larger mesopores can be introduced if mesocellular silica foams (MCFs) are utilized as templates. Important structural properties such as pore sizes and the degree of graphitization are precisely controllable by the elevated synthesis temperature. These materials show specific capacitances as high as 240 F g−1 in aqueous electrolytes. This technique is useable for template particles of different sizes providing a versatile access to carbons of different mesopore sizes with different amounts of additional micropores. Macroporous carbons are also synthesized using silica or polymer opal templates giving especially three-dimensional ordered carbon replicas. These materials show capacitances

as high as 120 F g−1 in organic electrolytes.Carbon aerogels are a class of macroporousopen cell foams with very low mass densities and large pore volumes that are derived by sol–gel chemistry. A molecular precursor, often based on resorcinol, is cross-linked into a gel via polymerization. The resulting hydrogel displays a three-dimensionalnetwork of interconnected nanometer-sized particles. This material has to be dried under particular conditions such as supercritical drying or freeze drying. The arrangement and connectivity of the primary particles are influenced by catalyst parameters and reaction conditions and therefore dictate the final properties of the aerogel such as electric conductivity, pore size, surface area, or pore volume. Aerogels often exhibit large, broadly distributed and flexible macropores, accompanied by large pore volumes. The latter can be a drawback in supercapacitor applications because the low density limits the volumetric capacitance/energy density which is a general disadvantage of electrode materials containing large pores. An advantage of this sol gel approach however is the simple access to shaped materials such as monoliths or thin films avoiding the additional necessity for current collectors in the final supercapacitor device. Hierarchical aerogels are of certain relevance as well and are most often synthesized by combining this approach with templating or activation processes.

In the light of cost reduction it is straight-forward to synthesize porous carbons from abundantly available natural bio-resources or waste products that accumulate during agricultural production. The carbonization and subsequent chemical activation of wastes such as cow manure and pulp-mill sludge yielded, depending on activation steps and temperature, broadly distributed meso-/macroporous carbons with surface areas up

to 700 m2 g−1 and pores around 25–100 nm. In recent years, the hydrothermal carbonization of natural carbon precursors was established as a very useful route for the production of carbon materials. This route can be applied to different types of organic materials such as algae, wood sawdust, starch, or cellulose. Nitrogen-doping of the resulting carbons can be achieved by using amino-containing biopolymers such as chitosan or D-glucosamine increasing the conductivity and therefore the performance in supercapacitor applications. Monolithic, flexible, sponge-like, carbonaceous aerogels have been synthesized applying a hydrothermal process to water melons. These materials have pores of approximately 45 nm and can be loaded with metal oxides resulting in high capacitive materials. The presence of heteroatoms within these materials has several advantages. Oxygen-rich carbons made from seaweeds, only possess low surface area around 200 m2 g−1 but comparatively high capacitance due to the oxygen functionalities that act in pseudo faradic charge/transfer reactions. Carbon nanotubes with high aspect ratios that are arranged perpendicular to a substrate are known as vertically aligned carbon nanotubes. Their porosity matches with the distance between the single (but multi-walled) nanotubes and is therefore well-defined and controllable with the synthesis parameters. They can be synthesized by catalytic thermal chemical vapor deposition (CVD) either under atmospheric or vacuum conditions using a variety of carbon precursors such as acetylene, ethylene, or alkanes.

External surface area and inter-particular porosity

As per definition a surface curvature is called a pore if its cavity is deeper than wide. One straightforward approach is to lower the

particle size of the carbon material down to the nanometer scale because the external specific surface area of a sphere increases with its decreasing diameter. These materials can be synthesized at temperatures as low as 200 °C and various functional groups (C–H, C O, C–O, and C N) can be introduced. Besides conventional fiber production technologies like melt-spinning or melt-blowing, electrospinning of polymer solutions – primarily polyacrylonitrile (PAN) is a promising technology for synthesizing carbon fibers with diameters of few tens of nanometers to a few micrometers. For the preparation, the polymer solution is charged to a potential in the 10 to 30 kV range and ejected from the tip of a needle. The resulting polymer jet is accelerated and elongated while flying through the space and finally collected as an ultrathin fiber web on a counter electrode. With regard to supercap applications this technology is highly beneficial due to small fiber diameters and therefore short diffusion pathways, additional inter-fiber macroporosity, flexibility, self-standing, and the expandability of additional coating steps when processed to the final device. Surface areas of these webs are usually in the range of 500–2000 m2 g−1. The fiber pore structure can be adjusted by the pyrolysis regime only to a limited degree. Moreover, the interfiber macroporosity is not fixed because of the high flexibility of the fiber web. Micro- and mesopores are formed due to the evolution of volatile substances during pyrolysis (e.g., CO, CO2) and therefore largely depend on the used precursor. However, it is mainly the subsequent activation step that adds the majority of meso- and micropores. The most widely applied activation technique is based on steam. Pore insertion without activation is based on sacrificial templates such as Nafion or carbon precursor mixtures with different carbonization. Most of the fibers

show good capacitances in the range of 130–180 F g−1 in aqueous electrolytes. A capable strategy to enlarge the specific surface area and to better control the porosity in the nanofibers is to combine electrospinning with a carbide-derived carbon approach. Carbon nanofibers are hard to graphitize and suffer from low conductivity. Therefore, conductive agents such as CNTs or carbon black can be added to the electrospinning approach.

Likewise even though not strictly porous, graphene is a noteworthy electrode material as well. Graphene is a single layer of sp2 hybridized carbon atoms in a two-dimensional honeycomb lattice. It is the basic building block for other carbon structures such as nanotubes, fullerenes and graphite. It stands out due to chemical stability and excellent electrical conductivity. It is not a typical porous material, but rather an ideal flat surface with a high theoretical surface area of 2630 m2 g−1, which makes it a perfect model system for studying electrolyte double layer behavior in supercapacitors. Indeed, in theory graphene does not contain any wormlike, poorly accessible pores, but its layered structure can hinder electrolyte diffusion and mass transport. Therefore, graphenes with curved morphologies were

synthesized by exfoliation. The reduction of graphene oxide introduces mesopore-like structures which allows for a better accessibility of especially large electrolyte molecules such as ionic liquids. But chemical activation with KOH also leads to significant increase in the surface area, additional mesopores of 2–5 nm and a better surface accessibility. Large micrometer-sized pores can be introduced if graphene layers are cross-linked in a sol–gel process resulting in a hydroge. Composites of graphenes with other materials such as carbon spheres, vertically aligned nanotubes, carbon black or metal nanoparticles are also beneficial since these additives act as spacers and separate the sheets from each other. The resulting hierarchical materials show higher capacitances than the single components. Recent attempts also use templates such as silica nanoparticles to introduce mesopores during CVD graphene synthesis. Finally it should be mentioned that currently many efforts are being made to dope graphenes with nitrogen to further increase the specific capacitance. Graphenes also enjoy the advantage of being utilized in ultrathin supercapacitors, printable electronics or nano-devices.

Table 1 Comparison of different carbon materials and their properties in EDLC electrodes

Material Activated carbon

Templated carbon

Carbide-derived carbon

Carbon aerogel

Carbon fiber

Graphene VA-CNT

Graphene oxide

a Theoretical values.Price Low High Medium Medium Medium Medium High HighScalability High Low Medium Medium High Medium Low LowSurface area [m2 g−1]

2000 <4500 <3200 <700 <200 2630a 1315a 500

Conductivity Low Low Medium Low Medium High High VariableGravimetric capacitance

Medium High High Medium Low Medium Low Low

Volumetric capacitance

High Low High Low Low Medium Low Low

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