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Hybrid Design

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: jimmy101 » Sun Jan 27, 2008 12:20 pm

1) Are certain acids/bases better for electrolysis than others?
2) If so, what makes one better than another (is it just which is stronger, or is there a desired strength)?


People generally avoid acids and salt (NaCl) since they tend to produce gases other than oxygen. (NaCl produces a fair amount of chlorine gas.) NaOH or KOH works about the best and are readily available. Bicarbonate (baking soda) is sometimes used and is probably the safest. In general you want the highest concentration of electrolyte you can get. With NaOH a 10%~20% solution is common. At this concentration the solution is corrosive to skin so be careful. Plastic, glass, iron/steel and copper will all handle concentrated solutions of NaOH, KOH or bicarbonate. (Don't use aluminum)

3) Do I need to keep on adding more into the water as it is split and I need to add more, or will it be reused?

Yes, you may need to add water. It is easy enough to figure out how fast you will consume the water. The molecular weight of water is 18g/mol, the density is 1g/cc. 18g water = 18cc (about 1.2 tablespoons) = 1 mol. 1 mol will create 11.2 liters of Oxygen and 22.4 liters of hydrogen (both at STP) for each ~1.2 tablespoons of water consumed. You'll create a fair amount of oxygen and hydrogen before you'll need to add water.

4) Does it matter how much I put in the water? ie: can I just pour a ton in just to be safe, or should I measure it out?

The more the better. The higher the ionic concentration the more current the cell will carry. The more current it carries the faster it will produce hydrogen and oxygen. For a science project you definitely need to know how much of the solid electrolyte you dissolved in your water. If the cell draws too much current it'll heat up. At high electrolyte concentrations it can get hot enough to boil, which is not good. Solutions of NaOH/KOH are kind of soapy, they'll foam and bubble which can be a problem. If the solution gets too hot with your power supply then drop the concentration of the electrolyte.

On the electrodes:

Just get a metal coffee can and cut the electrodes out of that. The honeycomb shape really isn't needed, though it does work better. You can make up for the less efficient shape by just using a larger piece of the can. If you want to buy stainless steel sheets I'm sure McMaster-Carr has something suitable. (http://www.mcmaster.com/ for example, part no. 3254K22, Stainless Steel Foil Soft Temper Type 304, .002" Thick, 12" W, 10'L, $23.97 Each)

You can make a perfectly useable cell using a squarish shaped Tupperware type plastic container with sealable lid. Cut a suitably sized hole in the lid and run a piece of Tygon tubing into the chamber. The other end of the tube is run up into a water displacement chamber as the gas receiver. It is easiest to not try to separate the hydrogen and oxygen gases. If you do want to separate the gases you'll need to increase the separation of the electrodes, add a small divider to the top of the cell and use two gas collection tubes and receivers. The electrolyte should only fill the container to perhaps 80% full (you want the air space at the top to keep the bubbling electrolyte from being forced out of the cell and into you receiver(s).)

The electrical connections to the electrodes can be tricky. You don't want a reactive metal like copper coming in contact with the solution, the copper will corrode pretty quickly. Also, you want to be sure the electrodes can't touch each other. One way to accomplish both is to cut the electrodes with a small tab on top. Then cut two slits in the top of the container and pass the tabs through the slits. Epoxy, glue, whatever, the electrodes to the top. Make your electrical connections to the parts of the tabs that protrude through the lid using wires with alligator clips.

What are you going to use as your power supply?

A note of warning, concentrated NaOH and KOH solutions are corrosive. Your cell will create heat (hence pressure), the cell is sealed and is normally operating at a pressure just slightly above atmospheric. If any thing happens that plugs the outlet gas hoses the cell will pressurize. If it is running hot enough to boil it will pressurize to dangerous levels almost instantly. A tupperware container won't take much pressure. If the cell ruptures you'll spray hot electrolyte all over the place. This is very bad if you are using NaOH or the like. Concentrated lye will damage skin, eyes, just about everything that is alive.

Your power supply is being used around conductive water solutions. This is any easy way to shock the yourself. A car battery (or charger) can put out hundreds of amps at 12V. This combination is very dangerous.

Think safety.
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Unread postAuthor: rp181 » Sat Feb 09, 2008 8:29 pm

two things:

If your using sodium hydroxide, dont use glass. It reacts with glass to form sodium silicate, slowely, but it still does.

I was luckey enough to find crystal drain cleaner (100% sodium hydroxide) at Lowes. Dont use galvanized steel, that has zinc, and NaOH reacts with it to form hydrogen gas, i tried with galvanized pipe.

Why not use welding oxygen tanks and get hydrogen from A reaction of sodium hydroxide and aluminum?

Dont use graphite, some oxygen will combine with the carbon to form Carbon dioxide.
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Unread postAuthor: Lentamentalisk » Sat Feb 09, 2008 8:56 pm

the idea was to do all the separating my self, for the science project, but you are right, the Al+NaOH reaction is much faster... I made a primitive hydrogen torch out of Aluminum foil and draincleaner in a soda bottle once, the flow was so high that the flame was self sustaining.
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Unread postAuthor: rp181 » Sun Feb 10, 2008 10:41 am

i did that too, except it melted all the bottles i put it in, except one sturdy water bottle
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Unread postAuthor: Lentamentalisk » Sun Feb 10, 2008 11:56 am

even better, i have some steel tea tins with easily removable lids that I drilled burner holes into, that then burns incredibly well. The plus side of using a steel container is that you can blowtorch the side to heat it up so that the reaction starts sooner ;)
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