Introduction to Electrolyzers

Electrolyzers use electricity to break water into hydrogen and oxygen. The electrolysis of water occurs through an electrochemical reaction that does not require external components or moving parts. It is very reliable and can produce ultra-pure hydrogen (> 99.999%) in a non-polluting manner when the electrical source is renewable energy.

The hydrogen produced from an electrolyzer is perfect for use with hydrogen fuel cells. The reactions that take place in an electrolyzer are very similar to the reaction in fuel cells, except the reactions that occur in the anode and cathode are reversed. In a fuel cell, the anode is where hydrogen gas is consumed, and in an electrolyzer, the hydrogen gas is produced at the cathode. The disadvantage of electrolyzers is the requirement of electrical energy to complete the reaction. Ideally, the electrical energy needed for the electrolysis reaction should come from renewable energy sources such as wind, solar or hydroelectric sources. Electrolyzers are useful and ideal when incorporated into certain stationary, portable, and transportation power systems. Some examples of applications in which electrolyzers would be particularly advantageous are long-term field use, fuel cell–powered vehicles, and portable electronics. A sufficient amount of hydrogen can be generated before it is used, and therefore, could be a beneficial addition to a system that uses solar and wind power.

Some of the advantages of using electrolyzers are:

1. The hydrogen produced is very pure.
2. It can be produced directly at the location, and at the time at which it will be used and does not necessarily have to be stored.
3. It is a much cheaper method than gas supplied in high-pressure cylinders.

There are more than enough solar and wind natural resources globally to produce all the hydrogen needed for stationary, transportation, and portable applications. Electrolysis has the potential to meet the cost requirements specified by many governments around the world.

Types of Electrolyzer Designs

There are many ways to build and configure an electrolyzer, and different electrolytes can be used just like in fuel cells. However, one difference from fuel cells is that high-temperature systems cannot be used because the water would have to be supplied as steam. Electrolyzers can be divided into two main designs: unipolar and bipolar. The unipolar design typically uses liquid electrolyte (alkaline liquids), and the bipolar design uses a solid polymer electrolyte (proton exchange membranes). Potassium hydroxide was a commonly used electrolyte in the past, but recently PEM membranes are more typical.  The construction of an electrolyzer is very similar to a battery or fuel cell; it consists of an anode, a cathode, and an electrolyte.


Alkaline Electrolyzer

Alkaline electrolyzers usually use an aqueous potassium hydroxide (KOH) solution as the electrolyte. Other frequently used electrolytes include sulfuric acid (H2SO4), potassium hydroxide (KOH), sodium chloride (NaCl) and sodium hydroxide (NaOH). The typical concentration of an electrolyzing solution is 20 – 30 weight % to provide a balance between ionic conductivity and corrosion resistance.

Alkaline electrolyzers work well at operating temperatures between 25 – 100 °C and pressures of 1 – 30 bar respectively. Commercial alkaline electrolyzers have current densities in the range of 100 - 400 mA/cm2. The chemical reactions for the alkaline electrolyzer are:

• Anode: 4H2O + 4e–  2H2 + 4OH
• Cathode: 4OH- + O2 + 4e+ 2 H2O
• Overall: 2 H2O → 2H2 + O2

The overall construction of an alkaline electrolyzer is straightforward. It has a unipolar design consisting of two metal electrodes suspended in an aqueous electrolyte solution. When electricity is supplied to the electrodes, hydrogen and oxygen gas is generated on each electrode. The electrolyzer must be designed so that each gas is collected and removed from the electrolyzer efficiently. The engineer must ensure that the gases do not mix because, in the presence of a spark, a hydrogen and oxygen mixture is flammable.

PEM - Based Electrolyzer

The polymer electrolyte membrane (PEM)-based electrolyzer is very popular, and many modern electrolyzers are built with PEM technology. The PEM electrolyzer uses the same type of electrolyte as a PEM fuel cell. The electrolyte is a thin, solid ion-conducting membrane, which is used instead of the aqueous solution. These electrolyzers use a bipolar design and can be made to operate at high differential pressures across the membrane. The reactions are as follows:

• Anode: 4H+ + 4e → 2H2
• Cathode: 2H2O → O2 + 4H+ + 4e
• Overall: 2H2O (l) + 4H+ + 4e→ 2H2 + O2 + 4H+ + 4e

PEM electrolyzers are popular because many of the typical problems of PEM fuel cells are not applicable. Water supplied to the cathode can also be easily used to cool the cell, and water management is much simpler since the positive electrode must be flooded with water. Hydrogen produced by this type of electrolyzer is of high purity. The only issue is the presence of water vapor in the system. Water diffuses through the electrolyte as in fuel cells; therefore, electrolyte designers use various techniques to avoid this occurrence. A common method is to use thicker electrolytes than those used in fuel cells.


Electrolyzer Efficiency

There are many factors that influence the performance of electrolyzers. Some of these include the overall design, the materials used, and the operating temperature and pressure. Operating at higher temperatures will increase the efficiency, but will also increase the corrosion rate of the electrolyzer materials. The electrolyzer efficiency is calculated in the same way as a fuel cell. The fuel cell efficiency is given by the formula:

And the inverse of this formula is the efficiency of the electrolyzer:

The losses in electrolyzers are the same as fuel cells, and typical values for Vcell and Vel_cell are 1.6 – 2.0 V depending upon the current density. The stack efficiency should also include the power losses due to the electricity needed for the pumps, valves, sensors, and controller, and the amount of energy put into the stack. Typical operating efficiencies of commercial electrolyzer units are about 60 to 70 percent.

Opportunities for Electrolysis

Integrating electrolyzers with a renewable energy system creates unique opportunities for providing power in the future. Renewable energy systems can connect to the utility grid through power electronics. The power electronics convert the alternating current (AC) from the grid to direct current (DC) power required by the electrolysis cell stack. Both PV and wind energy systems can be used as an electricity source. In many of the wind/electrolyzer systems used today for producing hydrogen, the electrolyzer uses the AC from the wind turbine directly.

There are many research and development projects that are being conducted globally that analyze and compare hydrogen production from solar and wind power and the electric grid. In these studies, the hydrogen is produced through electrolysis, and then compressed, and stored, to power an engine during periods with higher energy requirements. These projects will explore the coproduction of electricity and hydrogen to address the intermittent nature of solar and wind power, to create electricity when the energy demand is high. These studies also include the potential use of hydrogen for vehicle use. These research projects are studying multiple electrolyzer technologies; their abilities to be brought on- and off-line quickly; and the development of AC−DC and DC−DC converters to use the solar wind turbine to the electrolyzer to achieve efficiency gains.

Electrolysis can help to reduce the intermittent electricity production from renewable resources. Hydrogen systems can make hydrogen, and store it for later use – which can improve the capacity factor of renewable energy systems. This would help to make renewable energy constant or used for peak periods. By allowing the coproduction of hydrogen and electricity, the utility could optimize its production and storage system. Both solar and wind systems can benefit from producing electricity along with hydrogen. Some studies have shown that systems that are optimized for hydrogen and electricity generation have lower hydrogen prices – even when electricity is sold at a very low price.


Electrolysis uses electricity to break water into hydrogen and oxygen. This process can produce ultra-pure hydrogen (> 99.999%) in a non-polluting manner when the electrical source is renewable energy. The hydrogen can also be produced directly at any location, at the time that it is needed; therefore, it does not necessarily have to be stored. This is the ideal method of producing hydrogen for hydrogen fuel cells. If this system is designed properly, it can be a much cheaper method than gas supplied in high-pressure cylinders. Electrolyzers would be very useful if they are integrated into a stationary, portable or transportation power systems to generate hydrogen. It would also be a useful addition to a system that uses solar and wind power because hydrogen can be used to power fuel cells when the solar and wind power is intermittent. In the future, electrolysis can be used in conjunction with the hydrogen needed from wind and solar resources.

Dr. Colleen Spiegel Posted by Dr. Colleen Spiegel

Dr. Colleen Spiegel is a mathematical modeling and technical writing consultant (President of SEMSCIO) and Professor holding a Ph.D. and an MSc degree in Engineering. She has seventeen years of experience in engineering, statistics, data science, research & technical writing work for many companies as a consultant, employee, and independent business owner. She is the author of ‘Designing and Building Fuel Cells’ (McGraw-Hill, 2007) and ‘PEM Fuel Cell Modeling and Simulation Using MATLAB’ (Elsevier Science, 2008). She previously owned Clean Fuel Cell Energy, LLC, which was a fuel cell organization that served scientists, engineers, and professors world-wide.

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10 Comments To "Introduction to Electrolyzers"

Farmer in CA On 07.16.2021
We use an alkaline electrolyzer to produce HOCL for use as a sanitizing agent at our fresh produce packing facility. Our cells and membranes are cylindrical in shape. We inject a brine solution into the source water and apply 19v DC and between 90-120 amps to produce between 4-6 liters per minute of Anolyte. Our ideal end product is 6.5-7 pH, ORP 800-1000+ mV and FAC 300-500+ ppm. Which material would be best suited as a replacement membrane? Reply to this comment
Fuel Cell Store On 07.16.2021
For HOCl generation from a brine solution, the industry would usually use a cation exchange membrane. Most commercial anion exchange membranes would have oxidation and blister formation issue when they are exposed to electrolysis conditions encountered in the brine electrolyzer hardwares. You can use any one of the following membranes for your HOCl generation application: Nafion N324: Nafion N424: Nafion N438: Nafion N2050TX:
Vijay Venugopal On 06.15.2021
Can we use filtered seawater, that is alkaline nature, directly into the Electrolyzers? Reply to this comment
Fuel Cell Store On 06.15.2021
Hello Vijay, Proton exchange membrane (or PEM) electrolyzers require the use of ultrapure water such as distilled water or deionized water for their operation. Use of other water sources usually creates a lot of lifetime, side reaction, and unwanted by-product generation issues. Alkaline electrolyzers require the use of ultrapure alkaline electrolytes such as NaOH or KOH dissolved in distilled water or deionized water for their operation. Use of other water sources would drastically shorten the membrane lifetime again. None of the electrolyzer hardware on our website is suitable for filtered seawater since most filtration would roughly clean up the ions and the remaining ppm level anions or cations would still create problems.
Shabnam Daneshmand On 05.06.2021
Hi, I have a question. We want to produce 2000 H2 per day using PEM electrolyzers. I want to know that how much kw power is needed by each FEM and how many hours do they work to produce 2000 kg H2 each day? Reply to this comment
Fuel Cell Store On 05.06.2021
It is often difficult to get an exact number for power consumption due to component power efficiencies. An estimate using the power needs of the C30 that is the largest unit we represent and can only generate about 65 kg/day, would put the power needs around 65 kWh/kg H2 produced. Scaling this out to 2000 kg/day would need 31 units which would have the combined power need of around 2000 kWh/kg H2.
David Graham Atkinson On 09.22.2020
I would like your advice. We are a hydrogen technology, based in Alberta Canada. we are wanting to test to see if we can pre treat ionised water prior to electrolysis. which machine would show if we could reduce the energy use to the electrolysis process in our own experiment? Reply to this comment
Fuel Cell Store On 05.06.2021
Hello David, Most commercial electrolyzers are either based on PEM or alkaline chemistry. In PEM electrolyzers usually a cation exchange membrane is used, and such membranes would require high purity deionized water (16-18 Megaohms) to have long lifetime. Alkaline electrolyzers, on the other hand, would use caustic solutions for their operation. One way to see how the quality of the water supply is affecting the electrolysis performance is to test the water directly in an electrolyzer module that is suitable to the water quality you have and obtain the IV curves. You can then determine the cell voltage at a particular current or current density. The higher the quality of the water (meaning lower contaminants in the water source), the lower the cell voltage would be. You can simply calculate the power consumption at the specific current or current density and determine the efficiency.
Vedansh On 07.06.2020
is it safe to build it at home?? can we use any metal for electrodes or if no then what is best for electrodes?? Reply to this comment
Fuel Cell Store On 05.06.2021
Hello Vedansh, If someone has sufficient electrochemical and engineering background to build such systems, then it is safe to build it at home or office or in a research lab. Since electrolyzers require the usage of highly specialized components (CCMs with precious metal catalysts, anode and cathode diffusion media, endplates, a DC power supply, and deionized water as the water source, etc.), we usually advise researchers or hobbyists to explore some of our educational kits listed on the Fuel Cell Store and try to learn as much as possible from those kits before they are ready to assemble theirs. You can contact us for your other questions at directly and our highly experienced science and engineering team would be happy to direct you to the correct parts.

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