gregxy
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Cell for high current electrolysis of water
Hi,
I would like to build a compact cell to produce H2 & O2 from water in short bursts at high current density (at least 2A/cm^2). What would be the
best choice for electrodes and electrolyte?
I'd like to make the cell by stacking the electrodes between layers of absorbant material saturated with electrolyte to give about 8 cells in series.
This would then be run at 12V.
I could use mesh electrodes to make it easier for the gas to
escape.
People seem to use nickel electrodes with KOH but I don't understand why the anode is not destroyed.
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hodges
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There are a lot of plans for homemade electrolysis equipment on http://www.oupower.com/.
Some metals form conducting oxides that prevent anodes from being consumed. An obvious case of this is lead anodes forming PbO2. This is also true
for the chromium in stainless steel. I'm not sure about nickel.
Hodges
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hodges
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I have done a bit of experimenting with electrolyis over the past week or so. I used a 1-liter plastic bottle, into which I inserted two long
stainless steel screws through the bottom. The problem was to prevent leaks around the screws. I finally accomplished this using a glue gun. I
temporarily poked holes near the screws so I could get the glue in, then sealed these. This worked for a while, although after a few explosions
inside the bottle it started leaking a bit.
I made a hole in the bottle top, and glued in a sports needle of the type used to inflate basketballs. I filled the bottle nearly to the top with
sodium hydroxide solution.
With two amps of current flowing, there was not much gas coming out. When I tried to light it, flame would immediately go into the bottle and
explode, blowing out the flame. So I tried squishing the needle with a pliers to restrict the size of the opening. By doing this, and increasing the
current to 4 amps, I was able to get a tiny pinhead-sized flame to burn at the end of the needle. It would burn about 10 seconds before exploding
into the bottle and thus extinguishing itself. I'm guessing if I could use a higher current of say 10 amps I could sustain the flame. 10 amps is
still only going to produce a few mililiters of gas per second. My variable power supply is only good for 5 amps, and at 4 amps the voltage was over
10 volts meaning that I had a lot of resistance loss and the water would heat up pretty fast.
I could probably get this to work by putting a few of these in series (such as two stainless steel plates connected to the voltage with several
unconnected plates of the same size between the two plates to form cells). Anyone have any better ideas for the gas output tube?
If anyone decides to experiment with this setup be sure that the bottle is almost completely filled with water so that there is not much space for the
gas mix to accumulate in the bottle. The flame <i>will</i> eventually go into the bottle and explode. Even with just a few ml of space
at the top of the bottle the explosions sounded like a muffled cannon.
Hodges
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12AX7
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Use a whole lot more electrode. (Just how much voltage are you applying for 4 amps between two screws??) You may consider using a reclosable vessel
of some sort, so you can mount interleaved plates for an electrode structure.
Tim
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gregxy
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My goal is to make a weapon actuator for a fighting robot (battlebot). I want to place the H2/O2 generator in a metal cylinder with a piston, generate
several hundred ml of gas and then ignite it to drive the piston.
To be effective I need to generate the gas in a short period, say 10 seconds. The device also needs to be shock proof sot
it can't have free liquid.
I currently thinking of absorbing NaOH sloution into fabric sandwitched between Stainless steel plates.
This would then be run at 24V with 20 to 100 amps.
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tumadre
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I would take several old nickel cadmium batteries and remove the plates and rebuild the cell, as to alow the *fast* evolution of gas.
This would be the most effective as the voltage can be as low as 1.6 volts per cell, if you can keep the KOH concentrated, you may have to add more if
you increase the water content.
you must rebuild the cell compleatly, not just adding water through a hole drilled in the battery, the fiber is packed far too tight in the battery to
allow gas to escape,
the reason for packing the fiber so tight is to allow the H2 and O2 to be reduced and oxidized by the plate exposed to the gas.
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simple
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From what i know you need 10+ VDC to deplite water efecently.
The good way is to use salty water( say 0.5%) and IrO dased anode and Titanium for cathode. Keep the voltage above 10VDC as it causes more water
deplition than chlorine production at this level.
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not_important
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Salt water will give you chlorine, no matter what voltage, even if you are getting O2 as well. It also takes much more expensive electrodes.
Hydroxide, K or Na, with stainless steel or nickel (better) electrodes. The closer you can get the electrodes, the lower the ohmic drop and the less
voltage needed to run the cell (down to the minimum needed to split H2O and overcome electrode overvoltage).
The hydroxide will absorb CO2 and needs to be replaced ever so often.
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simple
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| Quote: | Originally posted by not_important
Salt water will give you chlorine, no matter what voltage, even if you are getting O2 as well. It also takes much more expensive electrodes.
Hydroxide, K or Na, with stainless steel or nickel (better) electrodes. The closer you can get the electrodes, the lower the ohmic drop and the less
voltage needed to run the cell (down to the minimum needed to split H2O and overcome electrode overvoltage).
The hydroxide will absorb CO2 and needs to be replaced ever so often. |
Must agree. With salty water you always get some chlorine output. It is only matter of the balance, how much chlorine to O2. However using catalitic
electrode will make a reaction a lot faster. If you need fast production catalist is the way to go
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len
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It turns out that getting H2 and O2 out of water is not as easy as the anecdotal drawings in textbooks would suggest. If you use a C electrode or the
iron nails they show, the carbon soon disintegrates while the solution turns black, the iron dissolves, and no O2 forms. Salts such as MgSO4 conduct
poorly, and a coating of Mg(OH)2 soon stops all current. If you use a non-volatile acid such as H2SO4, it starts getting reduced and SO2 is
liberated.
After a lot of experimenting I finally found a method that works. NaOH 30% solution gives maximum possible conductivity, while a SS anode hardly
corrodes. (Just as not_important says. He somehow always knows).
The cell operates at 20A at about 5V. The NaOH heats up, moreover a salt solution has negative resistance, and so you have all ingredients for
runaway! The power supply should be of the current control variety. 10L of H2 gas are produced in about 1/2hr, collect under water, and finally the
water forced upwards so H2 fills a balloon.
For the construction I used a 100mm plumbers PVC pipe, about 150mm long. A PVC end cap was glued to the bottom end. SS sheet about 0.5mm thick and
100mm wide was then made to line the inside of the PVC pipe. A hole drilled at the top and afixed with a nut/bolt to the outside formed the anode
terminal. A copper pipe 50mm diameter, terminating in a 10mm copper pipe formed the cathode. The 10mm pipe was the H2 outlet and also acted as a
terminal.
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tumadre
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quote:
"For the construction I used a 100mm plumbers PVC pipe, about 150mm long. A PVC end cap was glued to the bottom end. SS sheet about 0.5mm thick and
100mm wide was then made to line the inside of the PVC pipe. A hole drilled at the top and afixed with a nut/bolt to the outside formed the anode
terminal. A copper pipe 50mm diameter, terminating in a 10mm copper pipe formed the cathode. The 10mm pipe was the H2 outlet and also acted as a
terminal."
any one tried using the NIOOH and NI +KOH technology?
what if you robot flips over and that H2 pushes NaOH into your steel piston?
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not_important
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Alkalies are not that hard on iron and steel. On the other hand, acids tend to chew up iron and chloride ion is a great promoter of corrosion of
ferrous metals.
I thought that NiOOH was for batteries, while this is just electrolysis.
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