New device stores electricity on silicon chips

New device stores electricity on silicon chips
Silicon chip with porous surface next to the special furnace where it was coated with graphene to create a supercapacitor electrode. Credit: Joe Howell / Vanderbilt)
(Phys.org) —Solar cells that produce electricity 24/7, not just when the sun is shining. Mobile phones with built-in power cells that recharge in seconds and work for weeks between charges.

These are just two of the possibilities raised by a novel supercapacitor design invented by material scientists at Vanderbilt University that is described in a paper published in the Oct. 22 issue of the journal Scientific Reports.

It is the first supercapacitor that is made out of silicon so it can be built into a silicon chip along with the microelectronic circuitry that it powers. In fact, it should be possible to construct these power cells out of the excess silicon that exists in the current generation of solar cells, sensors, mobile phones and a variety of other electromechanical devices, providing a considerable cost savings.

"If you ask experts about making a supercapacitor out of silicon, they will tell you it is a crazy idea," said Cary Pint, the assistant professor of mechanical engineering who headed the development. "But we've found an easy way to do it."

Instead of storing energy in chemical reactions the way batteries do, "supercaps" store electricity by assembling ions on the of a porous material. As a result, they tend to charge and discharge in minutes, instead of hours, and operate for a few million cycles, instead of a few thousand cycles like batteries.

These properties have allowed commercial , which are made out of activated carbon, to capture a few niche markets, such as storing energy captured by regenerative braking systems on buses and electric vehicles and to provide the bursts of power required to adjust of the blades of giant wind turbines to changing wind conditions. Supercapacitors still lag behind the electrical energy storage capability of lithium-ion batteries, so they are too bulky to power most consumer devices. However, they have been catching up rapidly.

New device stores electricity on silicon chips
Graph displays the power density (watts per kilogram) and energy density (watt-hours per kilogram) of capacitors made from porous silicon (P-Si), graphene-coated porous silicon and carbon-based commercial capacitors. Credit: Cary Pint / Vanderbilt

Research to improve the energy density of supercapacitors has focused on carbon-based nanomaterials like graphene and nanotubes. Because these devices store electrical charge on the surface of their electrodes, the way to increase their energy density is to increase the electrodes' surface area, which means making surfaces filled with nanoscale ridges and pores.

"The big challenge for this approach is assembling the materials," said Pint. "Constructing high-performance, functional devices out of nanoscale building blocks with any level of control has proven to be quite challenging, and when it is achieved it is difficult to repeat."

So Pint and his research team – graduate students Landon Oakes, Andrew Westover and post-doctoral fellow Shahana Chatterjee – decided to take a radically different approach: using porous silicon, a material with a controllable and well-defined nanostructure made by electrochemically etching the surface of a silicon wafer.

This allowed them to create surfaces with optimal nanostructures for supercapacitor electrodes, but it left them with a major problem. Silicon is generally considered unsuitable for use in supercapacitors because it reacts readily with some of chemicals in the electrolytes that provide the ions that store the electrical charge.

With experience in growing carbon nanostructures, Pint's group decided to try to coat the porous with carbon. "We had no idea what would happen," said Pint. "Typically, researchers grow graphene from silicon-carbide materials at temperatures in excess of 1400 degrees Celsius. But at lower temperatures – 600 to 700 degrees Celsius – we certainly didn't expect graphene-like material growth."

When the researchers pulled the porous silicon out of the furnace, they found that it had turned from orange to purple or black. When they inspected it under a powerful scanning electron microscope they found that it looked nearly identical to the original material but it was coated by a layer of graphene a few nanometers thick.

New device stores electricity on silicon chips
Transmission electron microscope image of the surface of porous silicon coated with graphene. The coating consists of a thin layer of 5-10 layers of graphene which filled pores with diameters less than 2-3 nanometers and so did not alter the nanoscale architecture of the underlying silicon. Credit: Cary Pint / Vanderbilt

When the researchers tested the coated material they found that it had chemically stabilized the silicon surface. When they used it to make supercapacitors, they found that the graphene coating improved energy densities by over two orders of magnitude compared to those made from uncoated and significantly better than commercial supercapacitors.

The graphene layer acts as an atomically thin protective coating. Pint and his group argue that this approach isn't limited to graphene. "The ability to engineer surfaces with atomically thin layers of materials combined with the control achieved in designing porous materials opens opportunities for a number of different applications beyond energy storage," he said.

"Despite the excellent device performance we achieved, our goal wasn't to create devices with record performance," said Pint. "It was to develop a road map for integrated energy storage. Silicon is an ideal material to focus on because it is the basis of so much of our modern technology and applications. In addition, most of the silicon in existing devices remains unused since it is very expensive and wasteful to produce thin wafers."

Pint's group is currently using this approach to develop that can be formed in the excess materials or on the unused back sides of and sensors. The supercapacitors would store excess the electricity that the generate at midday and release it when the demand peaks in the afternoon.

"All the things that define us in a modern environment require electricity," said Pint. "The more that we can integrate power storage into existing and devices, the more compact and efficient they will become."


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Exploring supercapacitors to improve their structure

More information: www.nature.com/srep/2013/13102 … /full/srep03020.html
Journal information: Scientific Reports

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Oct 23, 2013
Okay, so you have a material with 1/10th the energy density of lead acid batteries - how do you propose to integrate it to the photovoltaic cell?

The voltage of a PV cell comes from the recombination current of electrons displaced by the incoming radiation through a PN (diode) junction. The forward voltage drop over this junction is what is visible to the external circuit as the source voltage. The load connects in parallel with the junction, diverting some of the recombination current out of the cell, creating the characteristic V-I curve of the PV cell.

Now, if you simply connect a capacitor to the output of a PV cell, the capacitor leaks backwards through the cell's PN junction the moment the sun stops shining. You'd need a blocking diode in series with the capacitor, but such a diode would also have a forward voltage drop and would consume a large part of the output power.

Oct 23, 2013
Now, if you simply connect a capacitor to the output of a PV cell, the capacitor leaks backwards through the cell's PN junction the moment the sun stops shining. You'd need a blocking diode in series with the capacitor, but such a diode would also have a forward voltage drop and would consume a large part of the output power.

I didn't get the impression the capacitors would be directly connected, one per cell. Rather, I'd expect cells in series through a single blocking diode, then to some sort of regulator that would convert the available power to something optimal for charging the capacitor(s) at whatever their current charge level is.

Oct 23, 2013
Ratings bot user blotto has vandalized Eikka's interesting comment already as part of Al Gore's campaign to smear Climatology skepticism. This account now seems to follow Eikka's posts here, seen in the Activity tab:
http://phys.org/p...activity

We have nuclear.
We already *have* nuclear.

"Can renewable energies provide all of society's energy needs in the foreseeable future? It is conceivable in a few places, such as New Zealand and Norway. But suggesting that renewables will let us phase rapidly off fossil fuels in the United States, China, India, or the world as a whole is almost the equivalent of believing in the Easter Bunny and Tooth Fairy." / "The problem is that, by drinking the kool-aid, you are also pouring it down the throats of my dear grandchildren and yours. The tragedy in doing so is much greater than that of Jim Jones' gullible followers, who forced their children to drink his kool-aid. All life will bear the consequences." - James Hansen, 2011

Oct 23, 2013
NikFromNYC, I appreciate that you've strong views on climate change, but I don't think that obsessing about who's upvoting and downvoting posts represents a significant addition to the debate. In addition, that quote you're using actually serves to make those of us (such as myself) who would like to see action on climate change look a bit rabid and unreasonable - the evidence speaks for itself, drama just serves to make your side's argument appear weaker.

The people who've already made up their minds on this issue won't be persuaded by your arguments, and those who are still undecided will be turned off by radicalism, which is to the climate change denialists' benefit.

Oct 23, 2013
artificially boost their own egos with rating bots


The bots serve a threefold purpose.

One is to make opposing ideas seem less credible by flooding the ratings so that it appears that many people are disagreeing with the other guy. They rarely seem to be used for upvoting anyone, because that would quickly give away who's controlling them.

The second purpose is to defeat the filtering system in order to spam and preach misinformation and political agenda, since people can't use the filter when relevant comments are downrated to the same level as the cranks.

And the third purpose is to drive down other user's average ranks to make it difficult to distinguish reasonable commenters from the cranks and pundits. Level the playing field for yourself.

The best remedy is to simply not give a damn about the ratings, because they're completely meaningless at this point anyhow.

Oct 25, 2013
I didn't get the impression the capacitors would be directly connected, one per cell. Rather, I'd expect cells in series through a single blocking diode, then to some sort of regulator that would convert the available power to something optimal for charging the capacitor(s) at whatever their current charge level is.


They would be manufactured one per cell, because the cells are made one by one, so that when the device is assembled the cells in a panel would then have to be wired up on both sides separately, and that's more complicated.

And the regulator you're talking about is an MPPT that tries to moderate the current draw from the cell to get maximum energy out. If you start charging an empty capacitor at 0 volts, the panel experiences a short circuit and the efficiency of the charging process is poor until the voltage rises up to where the bend of the V-I curve is. That's because the power is amps times volts.

http://www.chuck-...age1.gif

Oct 25, 2013
It may seem like a trick question to ask what the voltage of a 12 Volt battery is when you short it out, but the answer is zero.

Practically no energy is spent in the external circuit because almost no voltage is acting over the piece of low-resistance wire connecting the battery terminals. Yet, current is flowing, and since the external circuit (the wire) doesn't have any appreciable voltage between its ends, all the voltage must act inside the battery, and the energy dissapates there as heat.

And that's what happens to PV cells when you put a load on them. Unless the load pulls just the right amount of current to keep the voltage at the right level, it's going to waste a portion of the energy.


Oct 25, 2013
which will be enough current to create enough of the heat you are talking about to melt your conductor within about 1 second.


That depends on your battery, and your conductor.

The reason why the voltage across the short-circuit drops to zero, or close to it, is because the battery too is current limited. Car batteries for example can sustain a coupe hundred amps, which for a bar of metal with less than milliohms of resistance shouldn't do anything in a hurry.

The reason why a monkey wrench etc. flashes red in a second when you short the battery with it is because it has a positive temperature coefficient - that is, the resistance increases with temperature, so that the hotter it gets the more voltage - therefore power - is dissapated in the resistance until the resistance starts to limit the current. It starts heating up like that at the contact point at the battery terminals where the point contact offers the initial resistance and the initial hot spot.

Oct 25, 2013
Which is btw. why you should keep your car's battery terminals clean and check the cable shoes occasionally to see that they're not rusted or loose. Once you get a contact problem, the contact point starts to heat up and your cable starts to heat up, which may result in a fire.


Oct 28, 2013
Ratings bot user blotto has vandalized Eikka's interesting comment already as part of Al Gore's campaign to smear Climatology skepticism. This account now seems to follow Eikka's posts here, seen in the Activity tab:
http://phys.org/p...activity

We have nuclear.
We already *have* nuclear.

nic the hick from nyc, i look at what people smarter than you, technologists, elon musk, google, etc are betting on, where they put their money. (yeah i know: it's big libtard conspiracy. fukushima was a conspiracy. it's funny though, because the the kind of shit that you can't make up is actually in plain sight: dick cheney who was on the payroll of the company that invented fracking and helped push through the epa loopholes currently exempting that industry from regulation, now singing the praises of fracking without the slightest regard the cognitive dissonance. the irony.)

so anyway, i thnk i'll take a pass on putting my money where your mouth is.

Oct 28, 2013
When talking about short circuit, comparing a PV cell to a battery is a horrible completely unrelated nonsensical comparison.


Not at all. Haven't you ever heard of Norton-Thevenin circuit equivalents? This is basic circuit theory.

http://www.allabo.../10.html

A PV cell acts like a Norton's circuit, and the battery acts like the Thevenin's circuit, and their behaviour for an external load is broadly speaking the same, aside for a few minor details that result in different characteristic V-I curves.

A solar cell can be modeled as a battery just the same.

Also there is zero chance a dirty battery post will start a fire from your wires heating up...Where do you get this crap?


From the fire department.

One of the most common causes for vehicle fires is electrical faults resulting from faulty connections, because the contact point to a dirty oxidized battery terminal or other connection point has high resistance and starts to heat up.


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