An electrolyzer splits water into hydrogen and oxygen using electricity. At its simplest, it works like a battery in reverse: instead of producing electricity from a chemical reaction, it uses electricity to force a chemical reaction that wouldn’t happen on its own. Two electrodes sit in or against a water-based medium, electrical current flows between them, and water molecules break apart into hydrogen gas on one side and oxygen gas on the other.
The Basic Chemistry
Every electrolyzer has the same core components: two electrodes (an anode and a cathode), an electrolyte or membrane between them, and a power source. When voltage is applied, water molecules give up their hydrogen and oxygen atoms through two separate reactions happening simultaneously at each electrode.
At the anode (positive side), water loses electrons and releases oxygen gas. At the cathode (negative side), hydrogen ions pick up those electrons and form hydrogen gas. The electrons travel through an external electrical circuit from anode to cathode, which is the current that drives the whole process. Meanwhile, charged particles (ions) move through the electrolyte or membrane between the electrodes, completing the circuit internally. The type of ion that moves, and the direction it travels, is what distinguishes one electrolyzer technology from another.
PEM Electrolyzers
Proton exchange membrane (PEM) electrolyzers use a thin, solid polymer membrane as both the electrolyte and the barrier between the two gas streams. Water is fed to the anode side, where it splits into oxygen gas, positively charged hydrogen ions (protons), and electrons. The protons pass through the membrane to the cathode, while the electrons are forced through the external circuit. On the cathode side, protons and electrons reunite to form hydrogen gas.
PEM systems respond quickly to changes in power input, which makes them well suited for pairing with solar panels or wind turbines that produce variable electricity. They operate at moderate temperatures and can run at higher pressures, which reduces the energy needed to compress the hydrogen afterward. The main drawback is cost: PEM electrolyzers typically require precious metal catalysts at both electrodes, which drives up the price of the stack.
Alkaline Electrolyzers
Alkaline electrolyzers are the oldest and most commercially mature technology. Instead of a solid membrane, they use a liquid electrolyte, typically a potassium hydroxide solution at 25 to 30 percent concentration. The system runs at elevated temperatures between 70 and 100°C.
The chemistry works in the opposite direction from PEM. Hydroxide ions (negatively charged oxygen-hydrogen pairs) form at the cathode and travel through the liquid electrolyte to the anode. At the anode, those hydroxide ions release their electrons and produce oxygen gas and water. Hydrogen gas forms at the cathode. A porous separator between the electrodes keeps the hydrogen and oxygen streams from mixing while still allowing the hydroxide ions to pass through.
Because alkaline systems don’t need precious metal catalysts, they’re cheaper to build. They’re the dominant technology for industrial hydrogen production today. The trade-off is that they’re slower to ramp up and down, making them less ideal for fluctuating renewable power sources, and the liquid electrolyte requires more careful maintenance.
Solid Oxide Electrolyzers
Solid oxide electrolyzer cells (SOECs) take a fundamentally different approach by running at very high temperatures, typically 700 to 850°C. Instead of liquid water, they split steam. At the cathode, steam combines with electrons to produce hydrogen gas and negatively charged oxygen ions. Those oxygen ions migrate through a solid ceramic membrane to the anode, where they release electrons and form oxygen gas.
The extreme heat is actually an advantage. High temperatures make the electrochemical reactions more efficient, meaning less electricity is needed per unit of hydrogen produced. SOECs can also use non-precious metal catalysts, reducing material costs. If a source of waste heat is available (from an industrial process or a nuclear reactor, for example), the overall electricity demand drops significantly compared to any other electrolyzer type. The challenge is durability: ceramic components degrade faster at such high temperatures, and the systems take longer to start up and shut down.
AEM Electrolyzers
Anion exchange membrane (AEM) electrolyzers are a newer technology that tries to combine the best qualities of PEM and alkaline systems. Like PEM, they use a solid membrane, which allows a compact design with low electrical resistance. Like alkaline systems, they transport negatively charged ions (hydroxide) through the membrane rather than protons, which opens the door to using non-precious metal catalysts.
AEM systems can operate with pure water or very dilute alkaline solutions rather than the concentrated potassium hydroxide that conventional alkaline electrolyzers need. This simplifies the system and reduces corrosion concerns. The technology is still maturing, so large-scale commercial deployments are limited compared to PEM and alkaline, but it’s one of the most actively developed areas in hydrogen production.
Why Water Purity Matters
Electrolyzers are sensitive to what’s in the water you feed them. Impurities like dissolved minerals, chloride, or organic compounds can poison catalysts, corrode membranes, and degrade performance over time. PEM systems are particularly vulnerable because contaminants can block the membrane’s ability to transport protons.
Research-grade electrolyzers typically use ultra-pure water with a resistivity of 18.2 megaohm-centimeters, which is essentially as pure as water can get. Commercial systems don’t always need that level, but European testing guidelines call for high-purity water with conductivity below 1.0 microsiemen per centimeter at the inlet. PEM systems also require ion exchange resins in the water recirculation loop to continuously scrub out any impurities that accumulate during operation. For anyone planning an electrolyzer installation, water treatment is a real and ongoing operational cost, not an afterthought.
Efficiency and Energy Input
No electrolyzer converts 100 percent of the electricity it receives into hydrogen energy. Some energy is always lost as heat. The theoretical minimum voltage needed to split water is about 1.23 volts per cell, but real systems operate well above that. PEM electrolyzers typically run near 1.9 volts per cell, while solid oxide cells can operate closer to 1.1 volts because their high temperatures provide some of the energy thermally rather than electrically.
That voltage gap is a direct measure of efficiency. Lower operating voltage means less electricity wasted as heat per unit of hydrogen. This is why SOECs are often cited as the most electrically efficient option, particularly when paired with an external heat source. PEM and alkaline systems generally fall in a similar efficiency range to each other, with the specific numbers depending heavily on system design, operating conditions, and whether you’re measuring the stack alone or the full system including pumps, power electronics, and water treatment.
Falling Costs and Scale
The economics of electrolyzers are shifting quickly. The U.S. Department of Energy has set a 2026 target of $150 per kilowatt for uninstalled alkaline electrolyzer systems, assuming high-volume manufacturing. The stack itself, the core component, has a target of $50 per kilowatt. These are aggressive goals, but they signal the direction the industry is heading as manufacturing scales up and designs mature.
Cost reductions are coming from several directions: replacing precious metals with cheaper catalysts, manufacturing membranes and electrodes at higher volumes, and designing larger stacks that produce more hydrogen per unit. The combination of cheaper electrolyzers and cheaper renewable electricity is what makes “green hydrogen” (hydrogen produced with zero carbon emissions) increasingly competitive with hydrogen made from natural gas, which currently accounts for the vast majority of global production.