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.
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.
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!