Max voltage and current for electrolysis of water.

what is max value of voltage and current can be used for electrolysis of water to accelerate the hydrogen production rate within safety limit?

sort by: active | newest | oldest
jfryar302723 months ago

There are many factors that go into hydrolysis. Some of the main ones are surface area of cathode and anode, conductivity of the water, and resistance of conductors. I'm sure there are caculators out there somewhere.

iceng4 months ago

Pure water, de-ionized, no dissolved gases, is a non-conductor ! . . . . That means does not pass electric current and would sustain over 40 volts AC or DC..

But you will not be using pure water and you did not specify the kind, area and spacing of your electrodes or the ion in the water solution...

Simply put a DC Ammeter in series with a 0-12 or 0-20 volt adjustable DC power supply and initially adjust for 100ma and try to avoid more then an Ampere.. Current_flow, Not_Voltage, determines how much gas gets produced..

If the solution gets hot cut back on current. As you adjust for current the voltage will adjust for all other conditions...

DO NOT USE TABLE SALT = NaCl because one electrode will liberate Poison Chlorine Gas !!!

Chlorine gas just a tiny bit, not even visible, makes you feel like a Maga head cold with Scorched lungs, Runny weeping eyes and poor thinking...

electrolysis-of-water.gifGasCollection.GIFEE-H2O.jpg

It's not just a case of the more volts you supply the better the rate of production, there are a number of factors to consider.

Electrolysis begins at 1.229V and it's the current that largely determines the amounts of hydrogen and oxygen formed. Efficiency is key, I've read that electrolysis is most efficient between 1.24 and 1.48V, depending on who you ask. Other factors such as the choice of electrode material and the size and spacing of the electrodes will all influence production levels.

You also need to be extra careful when it comes to your electrode material, people often swear by the use of stainless steel, but this is highly dangerous. It can produce chlorine gas (which is poisonous) and the water will end up with chromates in it, which is illegal to dispose of down the drain.

When it comes to electrolysis I suggest you research Yull Brown and Michael Faraday for more information.

If I model the cell as a voltage source, Vrxn, in series with a resistor R, I can make realistic predictions about reaction rate, and efficiency.

Rather than saying, "it's the current that largely determines the amounts of hydrogen and oxygen formed", we can be more exact. Just realize that electrons are a reactant.

2 H2O +2 e-= H2 +2 0H- [cathode]
2 OH- = H2O + 0.5 O2 + 2 e- [anode]

H20 = H2 + 0.5 O2 [net reaction]

The rate at which electrons are being fed into the cell is the current. That is, assuming current is not, like flowing around the cell somehow, on some parallel circuit, or flowing into some other reaction.

For example, 0.2 mols of electrons per hour is:

0.2*(96485 coulombs)/(3600 seconds) = 5.36 C/s = 5.36 A

and that corresponds to a rate of,

0.1 mol/hour, for hydrogen production, and,
0.05 mol/hour, for oxygen production.

Regarding your statement, "Efficiency is key, I've read that electrolysis is most efficient between 1.24 and 1.48V, depending on who you ask."

Well, it doesn't have to be all about rumor and hearsay. If I have a believable number for Vrxn, the efficiency is simply:

Eff = (chemical power)/(total power input)

= (Vrxn*I)/(Vs*I)
= Vrxn/Vs

where Vs is the actual voltage applied across the cell.

Moreover, if I have a believable number for R, the power wasting series resistance, then the current is:

I = (Vs-Vrxn)/R

For example if Vrxn = 1.5 volts, and the actual voltage applied across the cell is 2.5 volts

Eff = Vrxn/Vs = (1.5 V)/(2.5 V) = 0.60 = 60%

Also, as part of this example, assume the current observed through the cell is 2.0 A, at Vs= 2.5 V.

Solve for R.
R = (Vs-Vrxn)/I = (2.5 V -1.5 V)/(2.0 A) = 0.5 ohm

Next consider what happens if I increase the applied voltage to the cell, to say, Vs=6.0 V.

Now the current in the cell is (Vs-Vrxn)/R = (6.0 V -1.5 V)/(0.5 ohm) is 9.0 A. So I am making hydrogen at 4.5 times the rate I was at 2.0 A.

However the efficiency is much worse:

Eff = Vrxn/Vs = (1.5 V)/(6.0 V) = 0.25 = 25%

i.e. 75% of the input electrical power is being wasted as heat.

So that's the trend, you know. This overvoltage, (Vs-Vrx), is wasting energy, making the cell less efficient. But at the same time, this rate at which the reaction happens is directly proportional to the overvoltage, since I=(Vs-Vrx)/R.

So I have to waste more power to make the reaction go faster, given a constant R.

Said another way: I have to waste some power to make the reaction happen at all, since 100% efficiency corresponds to Vs=Vrxn, and I=(Vs-Vrx)/R = 0.

Jack A Lopez4 months ago

If I were to model an electrolysis cell as an electrical circuit, one sort of simple, maybe too simple, way to do this is to just call it a constant voltage source in series with a resistor.

Or, in other words, I've divided this system into two parts, one that is turning electricity into useful chemical work (the voltage source, like an ideal battery cell), and the other that is turning electricity into heat (the resistor), essentially wasted electrical energy.

The constant voltage source is small, only about 1.23 volts,

https://en.wikipedia.org/wiki/Electrolysis_of_wate...

and the current through the cell multiplied by that voltage represents the rate at which actual useful chemical work being done. I mean,this represents the place where energy is going is into delta-H. You know, enthalpy of the products (e.g. 1 H2(g) + 0.5 O2(g)) minus enthalpy of the reactants (e.g. 1 H2O(l)).

Curiously, when I do that math, taking the enthalpy of formation for liquid water, and diving it by the amount of charge in 2 mols of electrons transfered, I get:

(285830 J)/(192970 C) = 1.48 V

Which, strangely, is not exactly the same answer (1.23 V, as mentioned above) as you get from looking up the, numbers in a standard reduction potential table,

https://en.wikipedia.org/wiki/Standard_electrode_p...

and finding the sum potential of two half-cell reactions.

Anyway, the other place electrical energy is going, is waste heat. That is, current through the cell multiplied by the overvoltage (The total cell voltage minus 1.23 V, or maybe it's 1.48 V?) , or if we can pretend that part is a resistor, call it I^2*R, is the rate at which energy is being wasted, as heat.

Moreover, if I imagine that this resistance is due to a kind of bulk resistivity,

R =rho*l/A

then there are consequences in terms of the geometry of the cell. To make R small, I want to make l, the distance between the plates, small, and I want to make A, the area of the plates, large.

The goal is to make R small, to make I^2*R small, and the geometry that does this is big wide plates, spaced very close together.

You can see this geometry in other electrochemical cells, particularly batteries. Flat cells, or two flat plates rolled into a spiral, informally called a, "jelly roll", are common. I recall the Wikipedia article for, "Nickel–metal_hydride_battery", had a nice picture of this "jelly roll" geometry, here:

https://en.wikipedia.org/wiki/File:Nimh_disassembl...

Also the geometry in PEM (proton exchange membrane) electrolysis cells,

https://en.wikipedia.org/wiki/Proton_exchange_memb...

is essentially wide flat electrodes, spaced closely together.


However, for an electrolysis cell (without a separator membrane) making gases at both anode and cathode, particularly if you want to keep those two gases separate, a design with small distance between the plates will be troublesome, because the gases are trying to diffuse into each other.

So here is an important design choice for your homemade water electrolysis cell:

Are you going to try to separate the gases, and collect them in separate containers?

Or are you going use close electrode spacing, and just let the gases from each electrode mix together, thus producing a mostly stoichiometric mixture {2 H2 + 1 O2} ?

This design choice is important, especially if you want to store more than a few liters, or around 0.1 mol, or around 20 KJ by enthalpy value, or around 5 ampere*hours in terms of the amount of electric you pushed through the cell to make this gas.

The reason why, is because stoichiometric {2 H2 + 1 O2} is explosive. A small spark can set it off, and it goes BOOM! I recall a previous discussion about this topic here,

https://www.instructables.com/answers/Why-have-neu...

The reason this was memorable to me is, the venerable Kiteman said,

"In UK schools, it is illegal to detonate more than one litre of the mixture indoors, because of the damage to hearing caused by the explosion."


Really, for me, the part that was interesting is that someone actually decided to make a rule, and uh, draw the line (which in this case is a volume) at 1.0 liters of STP volume of produced gas mixture.

Many (most?) homemade electrolysis cell designs that you'll find populating your search results, like KOR's design linked previously, will be the kind that simply mix the anode and cathode gases together, and a consequence of this, the gas produced cannot be stored safely.

So you kind of have to search harder to find the designs that are keeping the gases separate.

I recall, Cody of the Cody'sLab Youtube channel, built a design of this type. Also he was doing other things too, like drying (removing water vapor), compressing, and storing the separated hydrogen and oxygen, and later burning it via a homemade welding torch.

The only problem is Cody's videos are not polished and pretty (kind of the exact opposite style from Grant Thompson, the KOR), and Cody kind of rambles from one topic to the next. Don't get me wrong. This guy is completely brilliant! It is just that you'll probably have to spend like an hour of video watching time, across several of his videos, to figure out the details of what he is actually doing for any one particular project, of which is working on several, apparently simultaneously.

Cody's Water Spltter
(Published on Mar 23, 2015)


Burning Water
(Published on May 27, 2015)


Cody's Hydrogen Generator Finished for 2500 Subscribers!
(Published on Jun 10, 2015)


Testing My Oxyhydrogen Torch
(Published on Jun 12, 2015)

rickharris4 months ago

Define safety?

I imagine you can ramp up the voltage until you get electrical breakdown of the fluid.

A lot depends on exactly what is in the water.

Distilled water has a very high resistance.

Suggest you look at this instructable.

https://www.instructables.com/id/How-to-Convert-Wa...

There is more productivity in HOW you design the generator then in increasing the voltage and current.

I won't go into how dangerous it is to generate more than a small amount of Hydrogen.