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.
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 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.
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.
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.
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.