An electrolyzer is a device that uses electricity to split water into hydrogen and oxygen. It’s the core technology behind “green hydrogen,” a fuel source that produces no carbon emissions when the electricity comes from renewable sources like wind or solar. Think of it as the reverse of a fuel cell: where a fuel cell combines hydrogen and oxygen to generate electricity, an electrolyzer consumes electricity to pull water apart into its two component gases.
Global installed electrolyzer capacity hit 2 gigawatts in 2024, with another gigawatt added in the first half of 2025 alone. China accounts for 65% of that total. The technology is scaling fast, but it’s still early compared to the hundreds of gigawatts that energy agencies say will be needed to decarbonize heavy industry, shipping, and chemical production.
How Water Splitting Works
Every electrolyzer has the same basic architecture: two electrodes (an anode and a cathode) separated by a material called an electrolyte. When you run electricity through the system, water molecules break apart at the anode, releasing oxygen gas and positively charged hydrogen ions. Those hydrogen ions travel through the electrolyte to the cathode, where they pick up electrons and form hydrogen gas. Oxygen exits one side, hydrogen exits the other.
The electricity provides the energy needed to break the strong bonds holding water molecules together. The electrolyte’s job is to let ions pass between the two electrodes while keeping the hydrogen and oxygen gas streams separate. That separation matters because mixing the two gases creates an explosive hazard. The type of electrolyte used is the main difference between the three major electrolyzer technologies on the market today.
Alkaline Electrolyzers
Alkaline electrolyzers are the oldest and most mature type, with industrial use dating back to the 1920s. They use a concentrated potassium hydroxide solution (typically 20% or more by weight) as the electrolyte, with a porous separator called a diaphragm sitting between the two electrodes. The diaphragm’s pores fill with the liquid electrolyte, allowing hydroxide ions to migrate through while keeping the gas streams apart. The electrodes are usually made from inexpensive nickel-based materials, which is a big reason these systems cost less upfront than the alternatives.
The trade-off is speed. Alkaline electrolyzers respond to power changes in seconds, not milliseconds, and their ramp-up rate tops out around 0.3% to 17% of full power per second. That’s fine for steady industrial operation but limits their usefulness for fast grid-balancing tasks. They ramp down faster, at roughly 25% per second, because shutting off a chemical reaction is simpler than starting one.
PEM Electrolyzers
PEM (proton exchange membrane) electrolyzers replace the liquid electrolyte with a thin solid polymer membrane. Hydrogen ions pass directly through this membrane from the anode to the cathode. Because the membrane is solid, PEM systems are more compact, can operate at higher pressures, and produce very pure hydrogen.
Their standout feature is response time. PEM electrolyzers can adjust output in milliseconds, with ramp-up rates between 10% and 80% of full power per second. That makes them well suited for pairing with intermittent renewable energy sources, where power output can swing rapidly as clouds pass over a solar farm or wind speeds shift. The downside is cost. A 1-megawatt PEM system currently runs about $2,000 per kilowatt of capacity installed, based on Department of Energy modeling at today’s low manufacturing volumes. At that price point, and using renewable electricity at roughly $0.03 per kilowatt-hour, the hydrogen produced costs between $5 and $7 per kilogram.
Solid Oxide Electrolyzers
Solid oxide electrolyzers take a fundamentally different approach by running at extreme temperatures, typically between 500°C and 1,000°C. At those temperatures, some of the energy needed to split water comes from heat rather than electricity, which can push overall efficiency significantly higher. The electrolyte is a dense ceramic material that conducts oxygen ions at high temperatures.
These systems make the most sense in industrial settings that already produce waste heat, such as steel mills or nuclear plants, where that thermal energy would otherwise be lost. They can also co-produce syngas (a mix of hydrogen and carbon monoxide) when fed carbon dioxide alongside steam, making them versatile tools for chemical manufacturing. The challenge is durability: running ceramics at furnace-like temperatures causes materials to degrade over time. Current optimization targets suggest lifespans above 50,000 hours are achievable, which translates to roughly six years of continuous operation. Response time is in the seconds range, similar to alkaline systems.
Comparing the Three Technologies
- Alkaline: Lowest cost, longest track record, nickel-based electrodes, liquid electrolyte. Best for steady, large-scale hydrogen production where rapid power adjustments aren’t needed.
- PEM: Fastest response (milliseconds), compact design, solid membrane, higher capital cost. Best for pairing with variable renewable energy and applications requiring high-purity hydrogen.
- Solid oxide: Highest potential efficiency, operates at 500–1,000°C, ceramic electrolyte. Best where industrial waste heat is available and continuous operation is planned.
Why Electrolyzers Matter for the Energy Grid
Electrolyzers do more than just make hydrogen. They can act as flexible loads on the electrical grid, absorbing excess renewable energy that would otherwise be curtailed. When a wind farm generates more power than the grid can use at 2 a.m., an electrolyzer can soak up that surplus and store it as hydrogen for later use in fuel cells, industrial processes, or transportation.
This grid-balancing role is why response time matters so much. PEM systems, with their millisecond reaction speeds, can provide what grid operators call “ancillary services,” fine-tuned adjustments that keep the grid’s supply and demand in balance moment to moment. Alkaline and solid oxide systems respond in seconds, which is still fast enough for many balancing tasks but not for the most demanding frequency regulation services. All three types ramp down faster than they ramp up, a useful characteristic since reducing demand quickly is often what the grid needs most during sudden supply drops.
What’s Next: AEM Technology
A fourth type called the anion exchange membrane (AEM) electrolyzer is in active development. AEM systems aim to combine the best of both worlds: the low-cost electrode materials of alkaline systems with the compact, membrane-based design of PEM systems. That would mean lower capital costs than PEM and lower operating costs than alkaline.
The technology isn’t there yet. Industry prototypes show stable performance but lag behind PEM and alkaline systems in output, while lab results from universities show impressive performance but poor long-term stability. Bridging that gap between academic breakthroughs and commercial reliability remains the central challenge. Most advancements from 2016 to 2024 have not yet transitioned to commercial products, but the pace of research is accelerating as manufacturers look for ways to bring hydrogen production costs down.