Hydrogen can be produced by splitting water using an electrolyzer, a process called electrolysis. Electrolyzers can vary in size from compact, appliance-sized equipment that is well suited for small-scale distributed hydrogen production to large-scale, central production facilities that may be directly connected to sustainable or other non-greenhouse-gas-emitting modes of electricity generation. Electrolyzers consist of an anode and a cathode separated by an electrolyte. Different electrolyzers work in slightly different ways, mostly due to the different forms of electrolyte content used. In a polymer electrolyte membrane (PEM) electrolyzer, the electrolyte is a solid specialty plastic material.
- Water reacts at the anode to form oxygen and positively charged hydrogen ions (protons).
- The electrons flow through an external circuit and the hydrogen ions selectively move across the PEM to the cathode.
- At the cathode, hydrogen ions combine with electrons from the external circuit to form hydrogen gas.
- Alkaline Electrolyzers: Alkaline electrolyzers operate with hydrogen being generated on the cathode side via the transport of hydroxide ions (OH-) through the electrolyte from the cathode to the anode. Electrolyzers using a sodium or potassium hydroxide liquid alkaline solution as the electrolyte have been commercially available for many years. Recent approaches are showing promises on the lab scale with solid alkaline exchange membranes.
- Solid Oxide Electrolyzers: Hydrogen is generated in a slightly different way by solid oxide electrolyzers that use a solid ceramic material as the electrolyte that selectively conducts negatively charged oxygen ions (O2-) at elevated temperatures. i) Water at the cathode combines with external circuit electrons to form hydrogen gas and negatively charged oxygen ions. ii)The oxygen ions pass through the solid ceramic membrane and react at the anode to form oxygen gas and generate electrons for the external circuit.