How Much Hydrogen (or Oxygen) Will my Electrolyzer Make?

There are a lot of design considerations that go into an electrolyzer that will dictate what pressure they can operate at, their efficiency, safety, etc.  Today I will let you worry about all the mechanical design and talk a bit about the principles behind the electrolyzer and what this means to you (the designer).  Everything below applies primarily to PEM water electrolysis, but much of it may apply to other electrolyzer types as well.  If you want to skip the explanation, you can go straight to the handy spreadsheet.

 

Current

As you know, electrolyzers convert water and power (electricity) into Hydrogen and Oxygen. The interesting part to me was that the amount of H2 or O2 the electrolyzer generates is determined solely by the current.

This makes sense when you look at the physics of the electrolysis cell.  Since current is defined as the flow of electrons (or protons) and a Hydrogen molecule is just 2 protons and 2 electrons, it follows that when you put a certain number of electrons across the membrane (current), it will generate an equivalent number of Hydrogen molecules.

The exact amount is 0.007 Liters/minute @ STP (aka standard Liters per min, or SLPM) of H2 for every Amp that is put through each cell (0.007 SLPM/A/cell)

In practice, this gives you two variables to play with: Current and Number of Cells.  For example.  If you wanted 7 SLPM of H2 you could design a single cell eletrolyzer and pump 1000 A through it (0.007SLPM/A/cell * 1000A * 1 cell) or you could design one with 10 cells and only have to put 100 A through it (0.007 * 100A * 10 cells).  This allows you to get a rough estimate of how many cells you might need based on the current available.

Also, since the Hydrogen and Oxygen production are dictated completely by the current, this can sometimes be a convenient way to control production rates without actually having to measure the gas production or rely on other parameters that may change with time.

 

Voltage

The voltage that it takes to provide this current determines the overall efficiency, and thus the amount of power (P=V*I) required to generate your Hydrogen and Oxygen.

The voltage each cell will operate at is an experimentally determined value that can vary depending on the properties of the MEA (Catalyst types, Membrane thickness), temperature, current density, mechanical design, etc.  At any given set of conditions, an MEA (Membrane Electrode Assembly) will have a Voltage vs Current parameters (usually called an IV curve).

These curves will have lower voltages at lower current densities.  This means less power per unit of gas generated.  But since you are also providing less current you will have to have larger active areas and/or more cells to generate the same total amount of gas (but at a lower total power).

Basically, this means that you can achieve higher efficiencies, but it usually increases the stack costs since you have more cells and therefore more components.  Of course, some systems can call for higher stack costs because it results in lower overall system costs (or mass) by allowing you to use convenient, lower cost power supplies, fewer solar panels, etc.

 

There are of course many other factors that go into the proper selection for your Electrolyzer project.  We are here to help!  Let us know via e-mail or in the comments if you have any questions you would like us to tackle.

Have a great weekend!

 

Comments

  1. Brian Gauger says

    Great article, but I have one small nit to pick. Current flow is either defined as electron flow (in conductors), “hole” movement (in P-type semiconductors), or ion movement in solution (as here, in electrochemistry.)

    It follows that current can only defined as “proton flow” in the special case of H+ ion movement in solution. This may well be the case in one’s electrolyzer, but it is not GENERALLY true that “current is defined as the flow of electrons (or protons).”

    Very useful information, nonetheless. Thank you very much!

  2. Gerald Ogumerem says

    I need help with an electrolyzer design

    • Hello Gerald,

      I would highly recommend you contact our fuel cell design team by emailing them directly at MEA@fuelcellsetc.com.

      • Ok, fuel cells ETC, try this one on for size because you may have to do it for real.

        Supposing Diablo Canyon were reconfigures to use seawater and a single pass flow from the Pacific to the output of the final condenser. We do this because this produces at least 640,000 TAF of distilled water, and by design disposes of about 1/20th cubic mile of brine at 25%, the point at which solids begin saturating in the brine and precipitating out.

        We will be selling the distilled water, but we must dispose of the brine.

        So we run the brine into a collection of electrolysis cells and strip off the Hydrogen at the anode, That hydrogen will be sold on the hydrogen economy we hear so much about that really doesn’t exist yet.

        So, using the brine dump from Diablo Canyon for feed stock, How much hydrogen (in moles, for convenient calculations) will those cells produce?

        I am trying to get somebody to consider the use of the heat used to boil water for steam to, as a “waste byproduct” , thus creating free distilled water in cubic mile lots absolutely free at the output of the final condenser.

        Having shown this is quite a doable thing, and quite effective and efficient way to meet California’s water and power needs, I need to find out how much power you need to do it (so I can calculate how much more distilled water you make). and how much hydrogen you produce so I can estimate how much profit yould make doing that.

        • Hi Earl,

          Thanks for your comments. Unfortunately, using salt water will not work with a PEM Electrolyzer. The salts will plug the membrane and cease operations of the electrolyzer.

  3. how do you determine the cell area of the electrolyser or do all electrolysers have cell area of 100cm2

  4. why H2 Gas unit is taken as SLPM , why not in cubic meter/min?

  5. my electrolyser is running at 2000A with 54cells
    how much cubic meter hydrogen gas will it produce per hour

  6. william ndifuna says

    Hi,

    I want to design a pem electrolyser to produce hydrogen for cooking as a fuel to replace a daily household grid electricity consumption of 25.6 kwh (this is the only parameter I have/ know). source of power will be solar.
    I want to know;
    – the quantity of h2 I need to produce to burn to replace 25.6 kwh of grid energy.
    – the size of electrolyzer needed to produce this quantity of hydrogen
    – the amount of input power from solar that is required
    – anything else that is required. more importantly I wan to know how to arrive at the values.

    thank you

  7. Well, my actual question is, if you electrolytically decompose a cubic mile of sea water, how much gaseous hydrogen do you collect?

    This is because I propose desalinating large quantities of sea water in sequestering enough sea water to lower sea level (9k cubic miles lowers the sea by a foot) and sequestering the sea water by using it to irrigate farmland. in the process you end up with a brine of a chosen concentration, which being salt water does carry electric current, and more importantly proton (hydrogen nuclei) drift. Having produced the brine, we squeeze the remaining hydrogen out by6 decomposing it.

    Since even at moderate concentrations the essential volume of just water in a brine remains the same, knowing how much hydrogen makes up a cubic mile of sea water is all I need to know.

    • Imjustamoniker says

      So brine is just water with HUGE concentrations of sodium (SALT) in them, there is probably some more pollutants in the solution as well. You could electrolyze the brine but please keep in mind for every unit of oxygen and hydrogen you remove, there will be salt left. You will eventually have to do the nasty part of removing the salt from your electrolyzer, but this will probably just destroy the electrolyzer. I recommend to anyone wanting to go the electrolyzer route, try to get the most pure water you can use, your electrolyzer will last longer and you’ll have far less pollutants to deal with in the end.

  8. Hi,

    I want to design a pem electrolyser to produce hydrogen for my clinic as a fuel to replace a daily household grid electricity consumption of 25.6 kwh (this is the only parameter I have/ know). source of power will be solar. I want to connect 5 stacks of PEM Fuel cell, each 6.8 KW in design.
    I want to know;
    – the quantity of h2 I need to produce to burn to replace 25.6 kwh of grid energy.
    – the size of electrolyzer needed to produce this quantity of hydrogen
    – the amount of input power from solar that is required
    – anything else that is required. more importantly I wan to know how to arrive at the values.

    thank you

  9. Earl Moreo says

    In order to make sure my decimal places get it right, I need to know the number of moles of hydrogen there are in a cubic mile of water.

    The reason I need to know that is to calculate how much energy it would take to completely strip a cubic mile of hydrogen, so that I can calculate the cost if doing that.

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