Does anyone know the ideal voltage and current to do large scale electrolysis? I have seen wattage listed but never the voltage/amperage needed for hydrogen production.
Disclaimer: I’m an electrical engineer, not a chemist.
In brief, when you electrolyze water, you’re basically using a tub of water to complete an electrical circuit. At the cathode (negative pole), electrons are forced into the water. This provides the extra electrons needed to force H+ ions to become H2 molecules. Hence, hydrogen gas accumulates at the cathode. At the anode, the reverse happens. Electrons are pulled from the water molecules to complete the circuit, allowing O- ions to become O2 molecules, and oxygen gas accumulates at the anode. The voltage where the reaction begins to occur is about 1.5V, if I remember correctly. However, higher voltages don’t allow the reaction to occur faster, since the gas production is basically dependent on how many electrons (how much current) you can pump into and pull out of the water.
So, until someone more knowledgable comes along and tells me I’ve made a hash of my explanation, I’ll say that the ideal voltage and current for electrolysis is 1.5V and as much current as you can push through your apparatus.
The trouble is that at 1.5 V, you’re not going to get a whole lot of current. You always need an overpotential to get current to flow. The higher the overpotential, the greater the current (however, the higher the overpotential, the more energy converted to heat, rather than useful chemical energy). So as a zeroth order problem, you trade efficiency for speed.
The trick, of course, is in trying to minimize the overpotential, while still getting good current. Partly, this is done by adding by adding electrolytes to the water, to improve the conductivity. Actually, more frequent than splitting water into hydrogen and oxygen is splitting brine into hydrogen and chlorine. Electrode materials are also important. Currently, I think platinum is typical, but it would be desirable to have either lower cost or better working materials at the electrodes (this is also true of fuel cells, which are essentially electrolytic cells in reverse). An active area of research currently is to try to develop some sort of photocatalytic system to electrolyze water: See photocatalytic water splitting, as well as the more general artificial photosynthesis. In my opinion these being viable is very far in the future, but it is something receiving a lot of grant money at the moment.
Yes Consider automotive battery - 6 cells nominal 12 volts, produces hydrogen at 15 volts …(For THAT chemistry … difference voltage for different chemisty… but its the same idea) 2.5 volts each cell…
BUT note, the discussion so far suggested that they’d sit there and try to adjust the voltage to limit current… A tiny bit more voltage will map to a huge current…
But actually what they will be controlling is what engineers call a current source.
The simplest is to have a power supply with an internal resistance… Or just add a resistor to a low internal resistance power supply.
Modern , proper engineering… you use a transistor in a current amplifier configuration to limit current to the setting desired …
The voltage is a consideration, eg you need to provide cooling to the resistor or transistor, which is burning power at current * voltage drop … ( Watts when using Amps and Volts.)… So using like 100 volts and dropping it to 20 volts wastes 80% of the power… you don’t do that, you set the voltage to be just enough…
For reasons of efficiency, you do the same thing as an auto battery and run at increased voltage over multiple cells… eg 28 volts over 12 cells in series ?
I was looking at a large commercial unit to electrolyze water on the internet. They were saying it took 4.4 kiloWatts power for production of a cubic meter of hydrogen per hour, but did not specify any voltage. At 12 VDC that would be about 350 Amperes. At 100 VDC that would be about 44 amperes. I was curious if anyone had any idea what voltage they used in these. As you said if you get the voltage too high you don’t produce efficiently. If you get the amperage too high the unit is going to heat or even burn up. I’m not sure why they give a power but not a voltage or amperage. It would sure make a lot of difference in what you attempted to generate the power with.
At 1.5 to 12 VDC wouldn’t you have to add something to the water to start the current flow? Pure water would require that the electrodes be very close together which would be dangerous in a unit like this.
I appreciate the input.
First, I’ll thank you to acknowledge my PhD and call Doc, Doctor or best Herr Doktor.
Second, despite being descended from a family with a name ending in -ski, my heritage is Russian not Polish.
Finally, why do you insist I’m negative? You’re may be half empty. It may be half full. Let me refill it and get you some pretzels too!
On a serious note
prbeck I electrolyzed some water a while ago using directions from The Golden Book Of Chemistry Experiments. I should have used distilled water instead of tap water. I had to keep cleaning a build up of some kind of copper compound off the electrodes.
I’m curious as to just why you’re electrolyzing large quantities of water. I did it for fun (and it was), and to hear the distinctive barking noise that the Golden Book said pure hydrogen makes when burnt (It was cool).