MakeShift 01: Sean Cahill’s “Alessandro Volta” Honorable Mention
by William Lidwell
May 13, 2005
As I stated in my analysis, I am skeptical that a battery could be made to work given the time and circumstances. This skepticism wouldn’t apply if Sean Cahill were in the SUV, however, as I am convinced that he is Volta reincarnated. An excellent submission and brilliant analysis! If anyone could make it work, I think he could. Congratulations Sean!
We gave Sean an “Alessandro Volta” Honorable Mention. Here’s his entry:
We will use the aluminum cola cans and the hydrogen peroxide as the basis for wet cells. A schematic of a cell is shown below:
The wet cells themselves will not deliver enough instantaneous current to start the car, so they must be used to recharge the car battery over several hours. The car battery can then be used to deliver the “cold cranking” amps to start the car (hopefully) for a few seconds. We must develop enough potential to “reduce” lead sulfate back into lead and lead peroxide, which is the reaction that powers the car battery. The six cells of the lead-acid cell need a total potential in excess of 6*2.12 V or 12.72 V in order to recharge.
The Electrochemistry
The reactions we will use are shown below:
The positive potential in this cell tells us (through the Gibbs equation) that this reaction will proceed spontaneously. Four of these cells in series will produce enough potential (13.75 V) to recharge the car battery. The Nernst equation tells us how much voltage we can expect to develop with these cells, and how to maximize that voltage, ΔE.
Where R is Boltzmann’s constant, T is absolute temperature, n is the number of electrons moved per reaction (6), and F is Faraday’s constant. The quantities in {} are the concentrations of the various important species in the cell reaction, raised to the power of their number in the balanced equation. Looking at the Nernst equation, one can see the quantities you would want to optimize in order to get peak performance.
The first quantity would be T, temperature. Unfortunately, hydrogen peroxide decomposes at about 80 C into oxygen and water, so getting it much hotter than “just uncomfortable to the touch,” or around 50 C, is probably a bad idea. Room temperature, 27 C, is 300 Kelvin. Raising the absolute temperature to 323 Kelvin gives minimal return for the risk to the cells. Probably best to leave the cells at ambient temperature, unless it gets quite cold.
Next, we want to minimize the quantities in the numerator and maximize the quantities in the denominator. As the aluminum [Al] is oxidized into [Al 3+], it will build up in the cell, slowing down the reaction and reducing the voltage delivered. Fortunately for us, one of the main ingredients in colas is phosphoric acid (H3PO4). Aluminum orthophosphate (AlPO4) is largely insoluble in aqueous (water-based) solutions, and thus aluminum ions will precipitate out of solution, keeping the aluminum ions that are in solution low, where we want it. A possible problem here is that aluminum orthophosphate will build up on the can walls, slowing down the reaction. We can deal with this using something called a “salt bridge.” More on this later.
We want the H2O2 concentration to be high as possible. Typical hydrogen peroxide found in a first aid kit is 3% by weight. It is impractical to concentrate the peroxide by distillation, as it will decompose at 80 C, so we will just have to live with diluting it as little as possible.
[H+] is the concentration of hydrogen ions (protons). The negative logarithm of proton concentration, otherwise known as pH, is a measure of the alkalinity (pH range 7-14) or acidity (pH range 0-7) of a solution. Phosphoric acid has a pH of about 2.7, while lime juice has a pH of about 2.8. Thus, as the pH of the cola solution gets higher (less acid) due to proton consumption, we can replenish the acid with lime juice as well.
Construction
We need to cut the lids off of four of the cola cans and reserve their liquid. Pour all the cola into whatever cooking container was used along with your sterno stove to prepare dinner last night. If you have no such container, pour the cola into the potato chip bag, and use two of the cola cans to start reducing the colas to cola syrup. Although heating the cola will cause the loss of CO2, and carbonic acid, the effect on pH is negligible. You can add the lime juice and reduce that as well. Remember, we want to dilute the peroxide as little as possible, so replenishing pH with an acidic syrup is preferable.
The cola cans for the cells must be free of any insulating coatings on the inside, so use whatever means might be in the tool box (sand paper, scotch brite, etc) or dirt with a little water. The more clean area, the better, but be careful not to gouge the metal. Also, clean a spot on the outside wall of the can where we will attach the anode wire.
Once the cans are prepared, mix up some potato chips with a small quantity of water, just enough to make a thick paste. Form this paste, in a thin layer, to the inside walls and bottom of the can, trying not to leave any exposed metal. This potato chip paste will serve as a barrier to contain the cola solution, while being an extremely conductive electrolyte because of the NaCl (table salt). Aluminum chloride is highly soluble in aqueous solution (69 g/100 ml), so the aluminum ions will diffuse through the potato layer and precipitate in the presence of the phosphoric solution. The aluminum ion concentration will thus remain quite low, without passivating the aluminum.
Next, we need to create our graphite cathodes. We need as much graphite as possible to be exposed to solution, so we will go through the following procedure. Pull the metal band holding the eraser off the pencils. Scrape paint off pencils. Cut pencils in half and put into boiling water for several minutes using the sterno stove. The pencil wood will swell slightly, and the glue holding the wooden halves will soften, allowing the wood sheathing around leads to be carefully removed. The wood pieces can be used to hold the graphite electrodes in position in the cells, as shown in the figure, although any non conductive material that bridges the can opening may be used.
An aluminum can is 65 mm in diameter, 130 mm tall, and the wall is about 100 microns thick. Assuming we can consume about 80 microns, we can estimate the amount of current that may be delivered. The total exposed aluminum has an area of 0.030 m2. The volume of aluminum we might expect to consume per cell is therefore 2.4 x 10-6 m3. This is about 6.5 g. The atomic mass of aluminum is 26.98 g/mol. This means each cell can consume about 0.24 moles of aluminum, or 1.45e23 atoms, given enough of the other constituents. A 1 liter 3% solution of hydrogen peroxide contains about 1.27 moles of H2O2, and divided up amongst four cells gives 0.32 moles. Given that Al to H2O2 consumption ratio is 2:3, peroxide would seem to be the limiting constituent. 0.32 moles H2O2 will yield 0.64 moles of electrons, or 62,000 coulombs. Lead-acid batteries have 85-95% charging efficiency. This means we can deliver a substantial fraction of 15 ampere-hours, given a long enough time and enough acid. Pretty respectable, and well more than necessary to turn the starter/solenoid for a few tens of seconds.
Charging
The conductors to wire up to the electrodes and interconnect the cells can be gotten from the jumper cables. Jumper cables are usually made from stranded copper cable. We can strip back some insulation and cut several strands (at least 5). Be careful not to cut through the entire cable. Unwind at least a foot from the cable and cut off. Replace the insulation and tape back into place as well as possible. Wire up four assembled cells like those shown in the figure, and then connect them in series (anode to cathode), without electrolyte. You should have one free anode at one end, and one free cathode at the other. Prop up the cans as well as possible to keep them from spilling, but don’t let them touch each other or the ground (to prevent “parasitic” leakage current). You might use the tent for this purpose.
Wiring up the cells.Before proceeding, put the key in the ignition and turn to accessory position to see the state of the battery voltage. This will help you gauge the progress of recharging later. Remove the key from the ignition. Disconnect the car battery cables from the car battery to prevent any leakage paths for charging current.
Pour peroxide into each of the fully assembled cells to no more than 70% of capacity of the cell, as peroxide quantity allows. Add acid syrup to about 90% of capacity. The graphite electrode should be as much immersed into the liquid as is possible without getting the copper wire into the solution. Stir gently if desired with an insulating stick, careful not to disturb the potato layer or to get liquid on the copper. Connect the unconnected anode (aluminum can wire) to the positive terminal of the car battery, and the unconnected cathode (graphite electrode wire) to the negative terminal of the battery using the jumper cables.
Next Steps
Be patient. Take care of yourselves: hydrate with the remaining water, energize yourselves with the apples and bananas, and stay warm with the sleeping bags. After an hour or so, you can check to see if the process is working. Make sure no power-consuming functions are running: doors closed, radio off, lights off, etc. Disconnect the jumper cables, reconnect the car battery cables, and open the car door. If the dome light comes on, even dimly, the process is working. Disconnect the car battery cables, and reconnect the jumper cables. Wait a few more hours. Reconnect the car battery cables, and put ignition key in accessory to see how battery voltage is doing. Dome light should be getting brighter. Battery voltage should be above 12.5 V if charging is going well.
If more peroxide and acid solutions remain outside of the cells, you may mix up what remains (about 3 parts peroxide to 1 part acid syrup). Remove some of the existing cell solution and add new solution into each of the cells as space allows.
Check the progress periodically. When car voltage exceeds 13 V, or time has run out, it’s time to try and start the car. Leave the engine running so the generator/alternator will continue charging the battery. Clean up your mess and leave no trace of your little adventure in electrochemistry.
