Hydrogen Production: Electrolysis

Electrolysis is the process of using electricity to split water into hydrogen and oxygen, and the reaction takes place in an electrolyzer. Electrolyzers can range in size from small, appliance-size units that are well-suited for small-scale distributed hydrogen production, to large-scale, central production facilities that can be tied directly to nuclear power or other forms of electricity production.

How Do Electrolyzers Work?

Like fuel cells, electrolyzers consist of an anode and a cathode separated by an electrolyte. Different electrolyzers function in different ways, mainly due to the type of electrolyte material involved and the type of ion conducted—for example, positively charged hydrogen ions (H⁺) or negatively charged hydroxide ions (OH^-).

Proton Exchange Membrane Electrolyzers

In a proton exchange membrane (PEM) electrolyzer, the electrolyte is a specialized, solid plastic material that only allows hydrogen ions (protons) to pass through it. PEM electrolyzers typically operate at relatively low temperatures, about 70°C–90°C.

• At the anode: Direct current (DC) voltage applied to the electrolyzer provides the energy to split water molecules into oxygen molecules, positively charged hydrogen ions (protons), and electrons.

Anode reaction: 2H₂O → O₂ + 4H⁺ + 4e⁻ 

• Membrane transport and current flow: The specialized membrane allows hydrogen ions to pass through it to the cathode, while the electrons flow through an external circuit, driven by the potential difference of the applied voltage.
   
• At the cathode: Hydrogen ions combine with electrons from the external circuit to form hydrogen molecules (in a gaseous state). 

Cathode reaction: 4H⁺ + 4e⁻ → 2H₂

Alkaline and Anion Exchange Membrane Electrolyzers

Traditional alkaline electrolyzers and anion exchange membrane (AEM) electrolyzers function through the same core electrochemical process but use different electrolytes. Traditional alkaline electrolyzers use a liquid alkaline solution (potassium hydroxide or sodium hydroxide) with a porous separator, whereas AEM electrolyzers use a specialized solid polymer membrane that selectively conducts hydroxide ions. Alkaline electrolyzers typically operate at relatively low temperatures, about 60°C–90°C, and AEM electrolyzers can operate at even lower temperatures, about 50°C–80°C. 

• At the cathode: Applied DC voltage provides the electrical energy and electrons required to split water molecules into hydrogen gas and negatively charged hydroxide ions. 

Cathode reaction: 4H₂O + 4e⁻ → 2H₂ + 4OH⁻

• Separator transport and current flow: While the external circuit supplies a continuous flow of electrons to the cathode (driven by the potential difference of the applied DC voltage), the negatively charged hydroxide ions migrate through the electrolyte (liquid solution or solid membrane) toward the positively charged anode.
   
• At the anode: Hydroxide ions oxidize to form oxygen gas and water molecules, releasing their electrons back to the external circuit.

Anode reaction: 4OH⁻ → O₂ + 2H₂O + 4e⁻

Solid Oxide Electrolyzers

In a solid oxide electrolyzer—referred to as an SOEC, for solid oxide electrolyzer cell—the electrolyte is a solid ceramic material that selectively conducts negatively charged oxide ions (O^2-). SOECs operate at very high temperatures, about 650°C–850°C. SOECs can effectively use heat from various sources, including nuclear energy, to reduce the amount of electrical energy needed to produce hydrogen from water.

• At the cathode: Applied DC voltage supplies electrons that combine with high-temperature steam (H2O) to produce hydrogen gas and negatively charged oxide ions.

Cathode reaction: 2H₂O + 4e⁻ → 2H₂ + 2O²⁻

• Electrolyte transport and current flow: While the potential difference of the applied DC voltage drives a flow of electrons through the external circuit to the cathode, the negatively charged oxide ions (O²⁻) migrate through the solid ceramic electrolyte toward the positively charged anode. 
   
• At the anode: Oxide ions oxidize to form pure oxygen gas, releasing their electrons back into the external circuit.

Anode reaction: 2O²⁻ → O₂ + 4e⁻