Alkaline electrolyzers are one of the most established and widely used technologies for producing hydrogen from water. They've been used in large-scale industrial applications for decades. The core of this process is a liquid electrolyte solution, typically potassium hydroxide (KOH) or sometimes sodium hydroxide (NaOH), which is excellent at conducting electricity.

Inside the electrolyzer, two electrodes—a cathode and an anode—are submerged in this alkaline solution. They are separated by a porous diaphragm that allows ions to pass through but keeps the produced hydrogen and oxygen gases separate. When an electric current is applied, water molecules are split.

At the negatively charged cathode, water molecules react with electrons from the power source to produce hydrogen gas and hydroxide ions ($OH^−$).

These hydroxide ions then travel through the diaphragm to the positively charged anode. There, they release their extra electrons, forming oxygen gas and water.

The biggest advantage of alkaline electrolyzers is their cost. They don't require expensive, precious metals like platinum or iridium for their catalysts. Instead, they can use more abundant and cheaper materials, such as nickel. This significantly lowers their initial capital cost.

They are also incredibly durable. Many alkaline electrolysis plants have operational lifespans exceeding 60,000 hours, making them a reliable workhorse for steady hydrogen production.

However, this technology has its drawbacks. The hydrogen gas produced is at a relatively low pressure, typically between 1 and 30 bar. For many applications, like fueling vehicles, the hydrogen must be compressed to much higher pressures, which requires additional equipment and energy.

Alkaline electrolyzers also have a slower response time to power fluctuations. This makes them less suitable for pairing directly with variable renewable energy sources like wind and solar, which can have rapid changes in output. Finally, they have a lower current density (200–400 mA/cm²) than some other electrolyzer types, meaning they need to be physically larger to produce the same amount of hydrogen.

Given their pros and cons, alkaline electrolyzers are best suited for large-scale, centralized hydrogen production where the power supply is constant. They are commonly used in industrial processes that need a steady stream of hydrogen, such as producing ammonia for fertilizers or in petroleum refining.

Time to check your understanding of alkaline electrolyzers.

Their cost-effectiveness and durability ensure that alkaline electrolyzers will remain a key technology for hydrogen production, especially in large industrial settings.