Hydrogen electrolysis is a process that uses electrical energy to split water molecules (\(\text{H}_2\text{O}\)) into hydrogen (\(\text{H}_2\)) and oxygen (\(\text{O}_2\)). This electrochemical technique is the foundation for producing “green hydrogen,” a clean energy carrier whose only byproduct is water vapor when it is consumed in a fuel cell or burned. Since splitting water is non-spontaneous, an external energy source—electricity—must be supplied to drive the reaction forward. This process takes place within a specialized apparatus known as an electrolyzer, which manages the flow of current and the separation of the resulting gases.
Essential Components of an Electrolyzer
The core of the hydrogen production system is the electrolytic cell, which contains several components working in concert to facilitate the water-splitting reaction. At the heart of this cell are two conductive plates called electrodes: the anode, which is the positive terminal, and the cathode, which is the negative terminal. These electrodes provide the surfaces where the chemical reactions of oxidation and reduction occur.
The electrodes are submerged in or coated with an electrolyte, which is a substance necessary for conducting electrically charged ions between the two terminals. Without the electrolyte, pure water does not conduct electricity effectively enough for the process to occur efficiently. Depending on the type of electrolyzer, this electrolyte can be a liquid solution, such as potassium hydroxide, or a solid polymer membrane.
A separator, often referred to as a membrane, is placed between the anode and cathode to keep the generated hydrogen and oxygen gases strictly separated. This physical barrier is porous or selectively permeable, allowing specific ions to pass through while preventing the mixing of the two product gases. Finally, a direct current (DC) power source is connected to the electrodes, providing the necessary electrical potential to drive the entire electrochemical process.
The Step-by-Step Electrochemical Reaction
The process of electrolysis begins when the DC electrical current is applied to the electrolyzer cell, initiating the chemical breakdown of the water molecule. At the positively charged anode, water molecules surrender their electrons in a process called oxidation. This reaction produces oxygen gas (\(\text{O}_2\)) and positively charged hydrogen ions, known as protons (\(\text{H}^+\)), according to the half-reaction \(2\text{H}_2\text{O} \rightarrow \text{O}_2 + 4\text{H}^+ + 4\text{e}^-\).
The electrons released during this oxidation flow out of the anode and travel through the external electrical circuit toward the cathode. Meanwhile, the positively charged hydrogen ions are attracted across the electrolyte or membrane toward the negatively charged cathode.
Once at the cathode, the hydrogen ions complete the circuit by combining with the electrons that traveled through the external wire in a process called reduction. This final step creates pure hydrogen gas (\(\text{H}_2\)), as described by the half-reaction \(4\text{H}^+ + 4\text{e}^- \rightarrow 2\text{H}_2\). The overall chemical equation for the entire water-splitting process is \(2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2\), showing that twice as much hydrogen gas is produced as oxygen gas.
Major Types of Electrolysis Systems
The materials used for the electrolyte and electrodes define the major types of electrolysis systems.
Alkaline Electrolyzers (AEL)
Alkaline Electrolyzers (AEL) are the most mature technology, utilizing a liquid electrolyte, typically a solution of potassium hydroxide. AEL systems operate at relatively low temperatures and pressures and require less expensive materials, such as nickel, for their electrodes.
Proton Exchange Membrane (PEM) Electrolyzers
Proton Exchange Membrane (PEM) electrolyzers employ a solid polymer membrane as the electrolyte, which conducts protons (\(\text{H}^+\)) across the cell. This technology allows for a compact design and offers a quicker response time to fluctuations in electrical power input, making it suitable for integration with intermittent renewable energy sources like wind and solar. However, PEM systems usually require expensive catalysts, such as platinum and iridium, for high performance.
Solid Oxide Electrolysis Cells (SOEC)
Solid Oxide Electrolysis Cells (SOEC) use a solid ceramic material as the electrolyte. SOEC operates at very high temperatures, often between \(700^\circ\text{C}\) and \(850^\circ\text{C}\). This high operating temperature increases the overall efficiency because much of the energy required for the reaction is supplied as heat rather than electricity.
Measuring Energy Requirements and Efficiency
The amount of energy required to split water has a theoretical minimum, which is measured by the voltage needed to overcome the chemical bonds. Thermodynamically, this minimum is \(1.23\text{ volts}\) at standard conditions. In practice, a higher voltage is necessary to maintain the reaction rate, a phenomenon known as overpotential. Commercial electrolyzers operate at a minimum of \(1.5\text{ volts}\) per cell to overcome internal resistance and energy losses.
The efficiency of an electrolyzer is measured by comparing the energy content of the output hydrogen to the electrical energy input. An ideal system would require \(39.4\text{ kilowatt-hours}\) (\(\text{kWh}\)) of electricity to produce one kilogram of hydrogen gas. Due to inherent inefficiencies, commercial systems consume between \(50\text{ and }55\text{ kWh}\) of electricity per kilogram of hydrogen produced.
Another measure is Faradaic efficiency, which quantifies the proportion of electrical current that actually contributes to splitting the water, rather than being lost to side reactions or parasitic effects. Improving both the voltage and Faradaic efficiency of electrolyzers is a primary focus for researchers aiming to lower the overall cost of green hydrogen production.