On the Path to nanocapacitors with 100 Times More Energy Storage

From MIT Technology Review, the University of Maryland has early stage research, the device will have to be scaled up to be practical, but initial results show that new nanoscale supercapacitors can store 100 times more energy than previous devices of its kind.

The arrays’ storage capacity is about 100 microfarads per square centimeter. The Maryland group describes making 125-micrometer-wide arrays, each containing one million nanocapacitors. The surface area of each array is 250 times greater than that of a conventional capacitor of comparable size.

New Scientist reports they currently hold 2500 joules in one kilogram. [only 0.7 W-h/kg now] A single kilogram can deliver a one megawatt peak burst of power. The next step is to tweak the design to improve its performance – for instance, the team will experiment with deeper pores that can each hold bigger capacitors and thus store more energy.

But surface area isn’t the only determinant of energy density. The Maryland group’s nanocapacitors also benefit from the very small spacing between their electrodes, and the work is unique in this respect, says Robert Hebner, director of the Center for Electromechanics at the University of Texas at Austin. Hebner was not involved in the Maryland research.

The total thickness of each nanocapacitor is just 25 nanometers, and the charges can pack very close together.

The nanocapacitor arrays can’t store much total energy because they’re so small. “Instead of making these little dots, we want to make a large area that contains billions of nanocapacitors to store large amounts of energy,” says Lee. Both he and Rubloff say that scaling up to a practical level is not trivial, but the pair is working together to make larger arrays. “There are many scale-up issues,” says Rubloff. “We’ll look at how large we can make these and still have all of them work.”

Even if this problem is solved, they’ll still have to make sure that they can effectively connect multiple arrays to one another. But Hebner says that this problem is not intractable, and he points to devices on the market, including sensitive magnetic detectors, that successfully overcome similar connectivity issues.

A 2006 “Introduction to Energy Sources” by the Indian Institute of Technology, had a chapter on supercapacitors.

In the double layer at plane electrodes, charge densities of about 16-50 μF/cm2 are commonly realized. Taking an average value of 30 μF/cm^2, the capacitance of a single Polarisable electrode with a typical surface area of 1000 m^2/g for porous materials leads to a specific capacitance 300 F/g. At 1 V in an aqueous electrolyte, the maximum storage energy, E, is E=CVi2/2= (300 X 12)/2=150 W-s/g, 150kJ/kg or 42 W-h/kg.

So to get to W-h/kg, we would need to know what the square meters per gram would be to get Farads/gram and what the voltage would be to get to W-h/kg.

The 25 nanometer thickness suggests 20-40 layers per micron. A gram with the density of water is one cubic centimeter. So 200,000-400,000 layers per centimeter. 20 to 40 m^2 of the material per CC. The material would then have more surface area in each layer to get to higher surface area than what has already been achieved.

The highest energy density supercapacitor in production is 30 Wh/kg. Presumably the 100 times better claim means the potentisl is 3,000 Wh/kg.

Experimental electric double-layer capacitors from the MIT LEES project have demonstrated densities of 30 W·h/kg and appear to be scalable to 60 W·h/kg in the short term, while EEStor claims their examples will offer capacities about 400 W·h/kg. For comparison, a conventional lead-acid battery is typically 30 to 40 W·h/kg and modern lithium-ion batteries are about 160 W·h/kg. In automobile applications gasoline has a net calorific value (NCV) of around 12,000 W·h/kg, which operates at 20% tank-to-wheel efficiency giving an effective energy density of 2,400 W·h/kg.

The other recent energy storage news is fast charging lithium ion batteries.

Extremely high rates can be achieved, at a 200C rate (corresponding to an 18 second total discharge) more than 100mAh g can be achieved, and a capacity of 60mAh g is obtained at a 400C rate (9 sec to full discharge). Such discharge rates are two orders of magnitude larger than those used in today’s lithium ion batteries. Typical power rates for lithium ion battery materials are in the range of 0.5 to 2 kW/kg. The specific power observed for the modified LiFePO4 (170kWkg at a 400C rate and 90kWkg at a 200C rate) is two orders of magnitude higher. At this point the researchers have only tested the cells to 50 cycles but have noted no degradation. The a small prototype cell can be fully charged in 10 to 20 seconds, compared with six minutes for cells made in the standard way.

This new ability to charge and discharge lithium-ion batteries within seconds blurs the distinction between batteries and ultracapacitors. Besides being able to charge one’s cellphone in seconds, this will have a major impact on electric cars. If electric grid power was available, an electric car with a 15kWh battery could be charged in five minutes. This would require the delivery of 180 kw of energy in that time frame.

Abstract for “Nanotubular metal–insulator–metal capacitor arrays for energy storage”.

Nanostructured devices have the potential to serve as the basis for next-generation energy systems that make use of densely packed interfaces and thin films. One approach to making such devices is to build multilayer structures of large area inside the open volume of a nanostructured template. Here, we report the use of atomic layer deposition to fabricate arrays of metal–insulator–metal nanocapacitors in anodic aluminium oxide nanopores. These highly regular arrays have a capacitance per unit planar area of 10 F cm-2 for 1-m-thick anodic aluminium oxide and 100 F cm-2 for 10-m-thick anodic aluminium oxide, significantly exceeding previously reported values for metal–insulator–metal capacitors in porous templates. It should be possible to scale devices fabricated with this approach to make viable energy storage systems that provide both high energy density and high power density.

Supplementary information for the nano supercapacitors.

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