Home Brew Supercapacitor Whipped Up In The Kitchen

[Taavi] has a problem – a wonky alarm clock is causing him to repeatedly miss his chemistry class. His solution? Outfit his clock radio with a supercapacitor, of course! But not just any supercapacitor – a home-brew 400 Farad supercap in a Tic Tac container (YouTube video in Estonian with English subtitles.)

[Taavi] turns out to be quite a resourceful lad with his build. A bit of hardware cloth and some stainless steel from a scouring pad form a support for the porous carbon electrode, made by mixing crushed activated charcoal with epoxy and squeezing them in a field-expedient press. We’ll bet his roommates weren’t too keen with the way he harvested materials for the press from the kitchen table, nor were they likely thrilled with what he did to the coffee grinder, but science isn’t about the “why?”; it’s about the “why not?” Electrodes are sandwiched with a dielectric made from polypropylene shade cloth, squeezed into a Tic Tac container, and filled with drain cleaner for the electrolyte. A quick bit of charging circuitry, and [Taavi] doesn’t have to sweat that tardy slip anymore.

The video is part of a series of 111 chemistry lessons developed by the chemistry faculty of the University of Tartu in Estonia. The list of experiments is impressive, and a lot of the teaser stills show impressively exothermic reactions, like the reduction of lead oxide with aluminum to get metallic lead or what happens when rubidium and water get together. Some of this is serious “do not try this at home” stuff, but there’s no denying the appeal of watching stuff blow up.

As for [Taavi]’s supercap, we’ve seen a few applications for them before, like this hybrid scooter. [Taavi] may also want to earn points for Tic Tac hacks by pairing his supercapacitor with this Tic Tac clock.

[Thanks, Lloyd!]

41 thoughts on “Home Brew Supercapacitor Whipped Up In The Kitchen

    1. Are the bare, exposed wires intentional? Perhaps that’s the mode of operation: you short the super-capacitor with your hand to shut off the alarm, and that jolts you to a wakeful state?

  1. It seems too good to be true; even at only 1.2V, a 400F capacitor at that size would be remarkable, considering the rather crude construction.

    This capacitor would hold about 0.5 * 400 * 1.2^2 ~= 288J, while a D-cell sized supercapacitor from Maxwell (350F, 2.7V) would hold 0.5 * 350 * 2.7^2 ~= 1275J in what seems like roughly the same volume.

    I’m not quite convinced it actually functions as a capacitor, it could be a rechargeable battery instead. Batteries don’t necessarily need dissimilar anode and cathode materials, flooded lead-acid batteries are also build with just lead in both electrodes (although one is more porous than the other). Only when it is first charged, is the polarity determined.

    Also, I kind of doubt activated charcoal is actually that conductive, I’m fairly sure it’s amorphous carbon instead of graphite, and thus not very conductive. Also, I’d think the epoxy resin would insulate the pieces of charcoal from the iron mesh, and would seal the pores preventing the electrolyte from getting in. Even if they are not perfectly sealed, air needs to get out to let the electrolyte in.

    I think it’s yet another bogus youtube “instructable”, but I’m not sure, and it would be rather cool if it wasn’t.

    1. Looks like an electrolytic double-layer capacitor. Both the electrolyte and the plates store energy. It’s a like a hybrid between a capacitor and a battery, as it stores energy both electrostatically and electrochemically. The electrical conductivity of the activated carbon is sufficient to make a capacitor. However, I’m concerned about the epoxy sealing the pores in the charcoal. Activated charcoal is used because its porosity gives it an absurd effective surface area for charge storage. Using epoxy in this way likely drastically reduces the surface area. This device probably works primarily through electrochemical means. I’d have to make one and do some tests before making any definitive pronouncements. However, I find this particularly interesting because I worked in a materials science lab that was developing novel electrolytes and electrode materials for electrolytic double layer caps.

    1. “Science isn’t about WHY. It’s about WHY NOT. Why is so much of our science dangerous? Why not marry safe science if you love it so much. In fact, why not invent a special safety door that won’t hit you on the butt on the way out, because you are fired.” – Cave Johnson (1958)

      1. Yes, this is a battery, not a capacitor.
        Energy is being stored chemically not electrostatically.

        Neat to see, but knowing that it is a battery takes the project from a “THIS CHANGES EVERYTHING” to a “well, it’s good to see that people are still learning chemistry.”

          1. Ok, I’ll buy that. The video does suggest a linear discharge voltage which would mean capacitor. I was more confused by the text which implied that the first charge had to run overnight to allow something (electrodes/electrolytes?) to oxidize.

      2. There are different chemistries possible for super caps and a lot of them utilise storage of ions in the electrolyte in addition to the charge on plates the to increase capacity, which makes them partially batteries.

        There are Lithium-ion super caps the are quite expensive but hold more charge than more conventional super caps.

      3. In fact, I’m pretty sure it’s a battery.

        The iron in the electrodes and the NaOH in the electrolyte probably form an metal-oxide battery similiar to the Ni-Fe battery invented a hundred years ago, which consisted of iron plates with NiOH electrolyte in between.

        1. “The action which takes place in an Edison cell, both in charging and discharging, is a transfer of oxygen from one electrode to the other, or from one group of plates to the other, hence this type of cell is sometimes called an oxygenlift cell. In a charged cell the active material of the positive plates is superoxidized, and that of the negative plates is in a spongy or deoxidized state.[15]”

          The voltage of an Edison cell is approximately 1.2 Volts.

  2. What is limiting the voltage to max 1.2 volts in this homebrew contraption? And how can epoxy-drenched pieces of carbon have an electrical connection between themselves and the grid?

    1. I think the V limits are below where the electrolyte/dialectric/insulator material break down and/or arc. In a capacitor you do not want the electrolyte/dielectric to conduct the plates or you would just have a fancy bulky piece of wire. The epoxy serves as a dialectric

      1. The oxidation-reduction potential of iron is 1.21 Volts. Beyond that, other chemical reactions in the cell take place.

        This device is not a capacitor, but a simple weak metal-oxide battery where oxygen is shuttled from one electrode to another via the electrolyte. The actual reaction happens between the iron scouring pads acting as electrodes, oxidizing one while reducing the other.

        If you substituted one of the electrodes with nickel coated wire and applied 1.6 Volts to charge it, the capacity would increase tremendously.

        1. So, in this case, he wasted his time coating the steel with the carbon / epoxy? He could have fit more plates in the same package for higher capacity if that is the case.

  3. To everyone saying it is a battery, it is not. it’s a type of hybrid/pseudo-capacitor called an Electrical double-layer capacitor (EDLC).

    While have elements of electrochemistry, specifically ion migration they are fundamentally capacitors. I built some capacitor cells using a similar method using Mn02 nanorods as the active electrode material a few semesters ago. The specific capacitance was measured at 38F per gram. I don’t know how much mass the cell in the video has but it seems to be around the same order.

    Some research groups have achieved up 168F/g using Mn02. This is 5-10% of the energy density of lithium cells!

    1. Supercaps are a pain to use as their voltage decreases linearly with remaining charge following Q = C*V. So it basically means that you’ll need a very efficient boost converter that squeeze the last bit of charge out of it at very low voltages or you’ll only use like 10% of its capacity. These days, there are converters from “energy harvesting” chips that could make it more usable. A battery on the other hand has a relative flat discharge curve so voltage remains almost constant until it runs out of juice.

      Chemical bonds stores more energy per weight and because of the flat discharge curve are a lot less complex to use. There are applications where supercap/ultracaps are great at, replacing ordinary AA cells isn’t one of them.

      1. Although the energy of the capacitor is relative to V^2 so the top half of the voltage range contains 75% of the energy.

        If your device can operate on, say 5 volts down to 2.5 volts, you don’t really need any sort of boost converter.

    2. You can. Who knows if he has that kind of access where he is in Estonia. I don’t picture the having the greatest reatil access. ..and for those who say internet, really? Ever try shipping something to Estonia? You’d think you were buying your package first class tickets on the Concord. …plus those eastern block countries really have a hacker spirit to them. There are a lot of people there that build first and order later only if they have to.

  4. Is the energy stored by an electric field set up by charged particles on the surfaces of adjacent plates, then it’s a capacitor.
    Or is the energy stored by reversible chemical changes due to the flow of electricity. Then it’s a battery.
    I don’t know any chemistry so I can’t tell.

    1. Like Mark said, It’s both. Electrolytic capacitors can have much higher energy density because of this effect. Super capacitors take it to a new level by drastically increasing the surface area of the electrodes, which increases the capacity.

  5. Lots of interesting ideas here:)
    Well, Estonia is kind of similar to Elbonia-swampy and huge postal costs from US. Except it is heavily internetized – for example, you can elect or pay taxes by internet.

  6. This is not a supercap. It’s an Aluminum Air battery. The lower voltage is probably because of the higher resistance. The metal scouring pad he’s wrapping is an aluminum one. The entire build is directly lifted from the Al-air battery in the chemicum link, right down to using a tic tac container. There’s no circuitry added to handle charging or discharging the supercap, because this is a drop in replacement for his battery. He called it a supercap, probably because it sounds cooler than “check out my aluminum foil battery!” This goes under the heading homebrew suprecrap.

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