An electrolyzer is a device that uses electricity to split water molecules (\(\text{H}_2\text{O}\)) into hydrogen (\(\text{H}_2\)) and oxygen (\(\text{O}_2\)) through a process called electrolysis. This technology is gaining attention as a pathway for producing “green hydrogen,” which is created when the electricity powering the electrolyzer comes exclusively from renewable sources like solar or wind power. The resulting hydrogen gas is a clean energy carrier that can be stored, transported, and used across various sectors to help decarbonize heavy industry and transportation. Electrolyzers are a foundational technology for a future energy system that relies less on fossil fuels.
The Chemical Process of Electrolysis
The fundamental science behind an electrolyzer involves forcing a non-spontaneous chemical reaction to occur by introducing electrical energy. Water molecules are stable, and energy is required to break the strong covalent bonds holding the atoms together. The direct current (DC) applied provides the necessary energy input to drive the decomposition of water, which is the reverse of the reaction that occurs in a fuel cell.
This decomposition involves a reduction-oxidation (redox) reaction occurring across two distinct electrical terminals. When water is introduced, an electric current flows through the system’s conductive medium. At the positively charged terminal, the anode, water molecules lose electrons (oxidation), resulting in the formation of oxygen gas (\(\text{O}_2\)).
Simultaneously, at the negatively charged terminal, the cathode, hydrogen ions gain electrons (reduction) to form hydrogen gas (\(\text{H}_2\)). The overall chemical equation is \(\text{2H}_2\text{O} \rightarrow \text{2H}_2 + \text{O}_2\), meaning hydrogen is formed at twice the volume of the oxygen product.
Essential Internal Components
The chemical reactions are facilitated by three main physical components housed within the electrolyzer cell.
Electrodes
The two electrodes, the anode and the cathode, serve as the points where the electrical current enters and exits the water medium. They are coated with specialized catalyst materials that reduce the energy needed for the oxidation and reduction reactions to take place efficiently.
Electrolyte
The electrodes are in contact with the electrolyte, a substance that allows the flow of electrically charged ions between the terminals. Since pure water is a poor conductor, the electrolyte (a liquid solution or solid material) ensures the circuit is complete by enabling ion transport. The nature of the electrolyte varies significantly between different electrolyzer types.
Separator
Between the two electrodes sits a separator, often a porous diaphragm or a specialized membrane. This physical barrier prevents the mixing of the newly formed hydrogen and oxygen gases, which is crucial for safety as the mixture is highly explosive. The separator must also permit the movement of ions to maintain the flow of current and sustain the reaction.
Major Types of Electrolyzers
The three dominant commercial technologies for water electrolysis are differentiated primarily by the type of electrolyte and the operating temperature.
Alkaline Electrolyzer (AEL)
AEL is the most mature technology, used in industrial applications for over a century. AELs use a liquid solution, typically potassium hydroxide (\(\text{KOH}\)) or sodium hydroxide (\(\text{NaOH}\)), as the electrolyte, operating at moderate temperatures (60°C to 90°C). These systems have relatively low manufacturing costs and long operational lifespans, but they are often bulky and respond slowly to fluctuations in renewable power input.
Proton Exchange Membrane (PEM) Electrolyzer
PEM is a more advanced technology that uses a solid polymer membrane to conduct positively charged hydrogen ions (protons) from the anode to the cathode. PEM systems operate at lower temperatures (50°C to 80°C) and are valued for their compact design and ability to respond rapidly to variable renewable energy output. However, the acidic membrane environment requires expensive noble metals, such as platinum and iridium, as catalysts, leading to a higher initial system cost.
Solid Oxide Electrolyzer Cell (SOEC)
SOEC is a high-temperature approach utilizing a solid ceramic material as the electrolyte. SOECs operate at extremely high temperatures, typically ranging from 500°C to 1000°C. The elevated temperature allows the system to incorporate heat energy into the splitting process, potentially resulting in the highest overall efficiency and requiring less electrical energy. This design is well-suited for industrial settings where a source of waste heat is available, though high temperatures challenge material durability.