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Mention a capacitor to a group of technically minded folks and, chances are, most will envision two parallel plates with plus and minus signs lining up on each. But the science of capacitance has evolved far beyond simple plate capacitors.
Researchers are squeezing orders of magnitude more power out of capacitors by tailoring electrolytic liquids, upping surface area, and boosting charge-transfer efficiency. The aim is a capacitor that can complement advanced rechargeable batteries by storing larger amounts of energy and capitalizing on capacitors’ traditional advantages: fast energy delivery, fast recharge, and reliability through hundreds of thousands of charge-discharge cycles.
Such capacitors could provide backup power, store energy from renewable sources, control blade position in wind turbines, and complement automotive batteries. The devices are taking to the air, too, in emergency-door-opening systems on Airbus 380s.
Capacitance 101
True capacitors use electrostatic methods, not the electrochemical reactions found in batteries, to store and release energy. Potential energy in capacitors comes from charges being displaced from equilibrium positions, not from consuming reactants. Charge in capacitors is stored on electrode surfaces, resulting in two major differences between capacitors and batteries: energy density and charge or discharge time.
Capacitors have less volumetric energy-storage capacity than batteries which store charge chemically throughout their volume. Advanced lithium-ion batteries have energy densities up to 180 W-hr/kg while state-of-the-art supercapacitors weigh in at 6 to 7 W-hr/kg. However, capacitors can charge and discharge up to 10,000 W/kg in a few seconds regardless of temperature. Compare this to the about 300 W/kg lithium-ion batteries discharge over times on the order of hours.
Although capacitors’ discharge time is often too short to be useful in direct competition against batteries, vehicle manufacturers are now working on pairing the two. Capacitors can quickly take up power from regenerative braking and deliver power boosts for acceleration, both of which can level the load on paired batteries and extend their lives.
The energy a capacitor can store is proportional to the square of the voltage for a given capacitance. That is:
E = ½CV2
where C = capacitance, Farads (F) and V = voltage. Devices with higher capacitance can store and supply more energy for a given voltage. Because of the square relationship, capacitors can store significant energy at 2.5 to 3 V, a factor which helps extend the life and safety of their components.
Capacitance, in turn, is determined by:
C = εrε0A ÷ d
where εr = the dielectric constant of the material separating positive and negative charges, ε0 = the dielectric constant of a vacuum, A = surface area, and d = charge-separation distance. So, greater active surface area and smaller charge-separation distance lead to more energy storage.
Electrostatic evolution
Dielectric capacitors store energy as charges on plates separated by a dielectric material. Dielectric thickness, which determines charge-separation distance, d, is on the millimeter scale. Capacitance for this type of capacitor is a few microFarads per gram.
In electrolytic capacitors (ECs), two metal layers are separated by an oxide film several microns thick and, in some designs, electrolyte-soaked paper. Charge stores within the dielectric oxide layer, so charge-separation distance is on the order of microns. Etching treatments that roughen the metal surface create more surface area than in a dielectric capacitor of comparable size. Combining greater surface area with smaller charge separation pushes the capacitance of electrolytic capacitors to a few hundred microFarads to several milliFarads per gram.
The most common supercapacitors, electrochemical double-layer capacitors (EDLCs), immerse symmetric positive and negative electrodes in an electrolyte. Charge stores electrostatically when electrolyte ions reversibly adsorb onto electrodes without reacting.
The term “double-layer” refers to the theory that while positively charged ions line up on the electrode surface, matching electrons line up inside the electrode, creating a double layer of charge carriers that can’t cross the liquid-solid boundary in a chemical reaction. The opposite — negative ions lining up against an absence of electrons — happens on the other electrode.
The interface between electrolyte ions and electrodes, and hence the charge-separation distance, is on the atomic scale. It is limited by the shell of solvent molecules, called the solvation shell or layer, that surrounds each ion. The tiny charge-separation distance combines with the high specific surface area (SSA) of carbon electrodes — around 2,000 m2/gm — to give supercapacitors capacitances on the order of 100 F/gm.
Powerful porosity
Carbon electrodes have such high SSA because they aren’t monolithic materials. They can be made from carbon fabrics, nanotubes, powders, and other material forms. The highest surface areas come from carbon powders formulated to have pores on the surfaces of their constituent particles.
Researchers have tried to understand how electrode pore size, surface area, and surface chemistry affect capacitor performance. Smaller pores mean higher surface area and, consequently, capacitance. But the surface area in pores that are smaller than electrolyte ions isn’t usable, so much research has focused on finding an optimal pore size and a way to narrow pore-size distributions.
Here are some common electrode materials:
Activated carbons are made by heat treating carbon-rich precursors like polymers, wood, coconut shells, or pitch in inert atmospheres then “activating” them in carbon dioxide, water vapor, or potassium hydroxide to increase their SSA and pore volume. Activated-carbon electrodes produce up to 120 F/gm capacitance and are the most widely used electrode material. However, they usually have a broad distribution of pore sizes ranging from less than 2 nm to over 50 nm in diameter.
Templated carbons have pore-size distributions that center more narrowly around 2 nm or larger mesopores. To make them, manufacturers fill samples of nanoporous silicon oxide or zeolite (aluminosilicate) with carbon precursors, then dissolve the host material, leaving carbon that mimics the host-material structure. These electrodes produce capacitance around 100 to 200 F/gm depending on the electrolyte used.
Carbide-derived carbons are formed when titanium carbide and other metal-carbide precursors are etched with chlorine to produce metal chlorides and carbon powders or films. The metal leaches from the carbide leaving carbon atoms to organize themselves into an amorphous structure. Researchers found that by controlling process time and temperature they could dial in carbon pore size to nanometer accuracy and maintain a narrow pore-size distribution. Carbon powders produced this way have average pore sizes between 0.6 and 10 nm.
Researchers feared subnanometer pores would be too small to accommodate electrolyte ions and their solvation shells. But capacitance normalized for surface area
(μF/cm2) sharply increased for pores that matched the diameter of the ion, indicating that electrolyte ions leave solvent molecules behind to enter smaller pores.
Carbide-derived carbons are cheaper to make than other nanostructured carbon electrode materials, and by-products like titanium chloride are sought-after commodities in themselves, for producing nanoscale titanium dioxide, for example. This gives capacitor manufacturers more-affordable carbon electrode materials that perform better than those previously available.
Amped-up electrolytes
Electrolytes — aqueous, organic, or ionic — also play a major role in supercapacitor performance. Manufacturers choose electrolytes in part based on the cell voltage they can withstand without decomposing because higher voltages mean higher energy density.
Aqueous electrolytes suspend charge-carrying ions in water. This makes them inexpensive and easy to produce. However, they can only operate up to about 1 V without breaking down.
Most EDLCs, therefore, use organic electrolytes, which can operate at up to 2.7 V. An organic electrolyte suspends charge-carrying ions in a sea of carbon-based solvent molecules. Acetonitrile is the current state-of-the-art solvent for organic electrolytes. However, acetonitrile produces flammable vapor at room temperature and is a developmental toxin, so manufacturers are also turning to propylene carbonate which has a 275°F flash point and little toxicity.
Researchers are working on solventless ionic liquid electrolytes formed from salts that are liquid at room temperature. Because there is no solvent to break down, capacitors with ionic liquid electrolytes can operate with voltage windows up to 3 V or larger and promise energy densities closer to those of batteries. For example, EDLCs using carbide-derived carbon electrodes and an ionic liquid electrolyte of 1-ethyl-3-methylimidazolium (EMI) and bis(trifluoromethanesulphonyl)imide (TFSI) have capacitance of 160 F/gm at 60°C and 3 V.
Such ionic electrolytes are commercially available. However, the energy density of EDLCs with these electrolytes is limited because their room-temperature conductivity is a few milliSiemens per centimeter (mS/cm). Conductivities perk up as temperatures rise, so these electrolytes are mostly used when application temperatures are too high for conventional electrolytes. Researchers are currently looking for ionic liquid electrolytes with room-temperature conductivities of 40 mS/cm that don’t break down or react with common electrode materials at up to 4 V. They also have to be stable for the more than 100,000 cycles end users expect from supercapacitors.
A final challenge for EDLCs using ionic electrolytes is charge transfer. As device voltages approach 4 V, interfaces between active materials and current collectors have to be carefully designed so they don’t impede efficient electron flow. In a similar way, the porous separators that divide positive and negative carbon electrodes need to be as thin as possible to boost capacitance, but they can’t let the electrodes touch and short circuit the device.
Carrying charge
Reducing the resistance or impedance between electrodes and external circuitry boosts capacitor performance regardless of the electrode and electrolytes being used. EDLCs with organic electrolytes usually have aluminum foils or grids that collect charge from electrodes. Surface treatments like carbon-based sol gels can cut down on resistance losses at the interface between the current collector and electrode.
Researchers have also looked at tweaking the design of aluminum current collectors on the nanoscale. For instance, growing carbon nanotubes perpendicular to the surface of aluminum current collectors can halve resistance at the interface.
Current collectors can also be made entirely of carbon, specifically nanotube papers with conductivities around 100 S/cm. The material doesn’t corrode in aqueous electrolytes and is both tough and flexible. Such papers are expensive compared to aluminum, but cost is expected to fall as techniques improve and more producers emerge.