New Future for Paper? Think Energy Storage Devices

The coming years will see more and more renewable sources, plug in electric vehicles, and even more electronic devices requiring energy storage. All these require an inexpensive , light, scaleable, and efficient energy storage. Energy storage is especially critical to further development of some intermittent renewables like wind and solar. From the policy standpoint, having reliable and inexpensive energy storage can make it easier to achieve renewable portfolio standards, help with electricity peak demands by offsetting them, or make electric vehicles more accessible. Well, paper just may be the answer. 

Recently, scientists at Stanford coated office paper with nanotubes and nanowires to make it conducting. Moreover, they produced lightweight, flexible battery and supercapacitor (high-energy density capacitor). The discovery was unexpected as the researchers substituted paper for plastics in their previous research as a base-layer for the coating [1]. More porous paper holds this special “ink” made with carbon nanotubes better, making for a more durable battery and capacitors. 

The device is made from an off-the-shelf office paper. The paper is coated with carbon nanotube “ink”. Because of its porosity and good absorbance, the ink rheology is not very important and nanotubes bind well to it. The Proceedings of National Academy of Sciences paper [2] reports a resistance of 1 Ω (Ohm)/sq (per square). This is comparable to a resistance of a 100W light bulb (its metal filament) and not very different from an internal resistance of a regular AA alkaline battery. This paper can be folded, crumpled or even dipped in acid and it still retains its unique properties. Professor Yi Cui, said that one day, he could use a brush to paint his walls with an energy storage device (energy-storing wallpaper, anyone?) [2,4]. 

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Conducting paper rolled into a tube, Ref. 3

Conducting paper rolled into a tube, Ref. [3] 

Paper for Supercapacitors 

For comparison, a 50nm layer of gold (very good conductor) deposited on a sheet of standard Xerox paper had resistance of 7 Ω (Ohms)/sq (it was 1Ω for the coated paper). By coating both sides of paper with the carbon nanotube ink, the researchers created a supercapacitor with capacity of 200F/g (Farad per gram) in sulfuric acid electrolyte. This capacity was not severely diminished even under high current loadings of 40A/g. The gold-based supercapacitors made in a similar fashion had about 4-5 times lower capacity. Ultimately, the supercapacitor performed well even at 40,000 cycles with capacitance losses less than 3%. In MIT Technology Review article [5], Nicholas Kotov, professor of chemical engineering at the University of Michigan, says that the dipping method to make the supercapacitors is “simple and nice”. 

Paper for Batteries 

Because a large weight of batteries, up to 20% is due to heavy metallic current collectors, the paper can also be used in batteries to reduce weight while still maintaining reasonable resistance. Thus a higher gravimetric energy density can be accomplished [1].  Such battery proof of concepts made by the Stanford group achieved over 500 cycle times with high capacity retention. Paper was stable in the electrolyte over the period of 3.5 months and the battery showed only small self-discharge. 

Below is a video from Dr. Cui’s group at Stanford showing how simple it is to make this paper (of course, the “secret sauce” of the nanotube ink is not revealed) 

Nanotubes + ink + paper = instant battery

What I personally like about this technology is that is seems “ready to go”, or pretty close to commercialization. Who knows, in a strange twist of fate, maybe in a year or two, our electronic readers such as kindles and iPads will rely on paper they replaced for their energy storage. 

References: 

[1] Paper Battery Shows Promise for Grid, Vehicle Energy Storage
http://www.nytimes.com/gwire/2009/12/08/08greenwire-paper-battery-shows-promise-for-grid-vehicle-e-59107.html

[2] Hu, L., Choi, J.W., Yang, Y., Jeong, S., La Mantia, F., Cui, L., Cui, Y., 2009, “Highly Conductive Paper for Energy-Storage Devices,” Proceedings of National Academy of Sciences
http://www.pnas.org/content/106/51/21490.full.pdf+html?sid=502de90b-f1a2-49d3-8c4f-12c052917c89

[3] “Battery Made of Paper Charges Up,” BBC News, Tuesday, December 8, 2009, 16:38 GMT
http://news.bbc.co.uk/2/hi/technology/8401566.stm

[4] “Making Powerful, Lightweight Batteries From Nothing But Nanotube Ink and Paper” Popular Science, Clay Dillow, Posted 12.08.2009 at 11:15 am
http://www.popsci.com/science/article/2009-12/making-powerful-lightweight-batteries-nothing-nanotube-ink-and-paper

[5] “Batteries Made from Regular Paper”, MIT Technology Review, Katherine Bourzac, December 8, 2009
http://www.technologyreview.com/energy/24097/?a=f

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