A basic alkaline electrolyzer splits water into hydrogen and oxygen using electricity, and you can build one with stainless steel plates, a potassium hydroxide solution, and a sealed container. The minimum voltage needed to split water is 1.23 volts, but real-world cells need more, typically 1.8 to 2.5 volts per cell, because of resistance losses and the extra energy required to release gas bubbles from the electrodes.
This guide covers the core design of an alkaline electrolyzer: choosing materials, mixing the electrolyte, assembling the cell, and handling the hydrogen gas safely.
How an Alkaline Electrolyzer Works
An alkaline electrolyzer is a pair of metal electrodes submerged in a water-based alkaline solution, separated by a porous barrier. When you apply DC voltage across the electrodes, water molecules break apart. Hydrogen gas forms at the cathode (negative plate) and oxygen gas forms at the anode (positive plate). The alkaline solution doesn’t get consumed in the reaction. It provides hydroxide ions that carry electrical charge between the plates, completing the circuit through the liquid.
Industrial alkaline electrolyzers run at 62% to 82% electrical efficiency, meaning that proportion of the input energy ends up stored as hydrogen. A DIY cell will land toward the lower end of that range, but the basic chemistry is identical.
Materials You Need
The electrode plates are the heart of the system. Use 316L stainless steel rather than 304. Both will work initially, but 316L contains molybdenum, which makes it significantly more resistant to corrosion in high-pH environments. Testing shows 316L develops a more stable passive film, with higher resistance to the intergranular corrosion that eventually eats through 304 plates. You can source 316L as flat plate stock, typically 1 to 2 mm thick, cut to fit your cell housing.
For the cell housing, you need a container that won’t react with a strong alkaline solution. HDPE (high-density polyethylene), polypropylene, or acrylic all work. Many builders use thick-walled HDPE containers or fabricate a sealed box from acrylic sheet with chemical-resistant gaskets. The housing must be sealed well enough to capture gas without leaking, but not so airtight that pressure builds dangerously. A fitting at the top connects to your gas outlet tubing.
Other materials include:
- Separator/diaphragm: A porous sheet between the plates that lets ions pass but keeps hydrogen and oxygen gas streams from mixing. Common DIY options are fiberglass mesh or purpose-made alkaline-resistant membrane material.
- Tubing and fittings: Chemical-resistant tubing (silicone or polyethylene) to route gas out of the cell.
- DC power supply: A bench power supply or battery capable of delivering 2 to 3 volts per cell at your desired amperage.
- Potassium hydroxide (KOH): The electrolyte. Available as flakes or pellets from chemical suppliers.
Mixing the Electrolyte
Potassium hydroxide outperforms sodium hydroxide as an electrolyte because it has higher ionic conductivity, which means lower electrical resistance in your cell and more hydrogen per watt. The standard concentration for alkaline electrolysis is 25% to 30% KOH by weight. For a liter of solution, that means dissolving roughly 250 to 300 grams of KOH in distilled water.
Experimental data confirms that concentrations below 15% produce noticeably less hydrogen and can cause the cell to heat up faster, a thermal sensitivity issue where low-concentration systems lose more energy to waste heat. Concentrations in the 15% to 20% range work adequately, but 25% to 30% is the sweet spot used in commercial systems.
KOH is highly caustic. It generates heat when dissolving in water. Always add the flakes to the water slowly, not the reverse, and use chemical-resistant gloves and eye protection. Use only distilled or deionized water. Tap water contains minerals that will contaminate your electrodes and reduce performance over time.
Assembling the Cell
The simplest configuration is two plates facing each other inside the housing, separated by 3 to 5 mm of space with the diaphragm between them. More advanced designs use a “dry cell” stack, where multiple plates are sandwiched together with gaskets between them and electrolyte flows through channels. The stacked design is more compact and efficient, but harder to build and seal properly. Start with a simple two-plate wet cell to understand the basics.
Connect the positive terminal of your power supply to the anode plate and the negative terminal to the cathode. The gas outlet fittings should be positioned at the highest point of each chamber so bubbles rise naturally into the tubing. If you’re using a single-chamber design without a diaphragm, the output will be a mixed hydrogen-oxygen gas (sometimes called HHO), which is more dangerous to handle than separated gases because it’s already in an explosive ratio.
Roughening the electrode surfaces with coarse sandpaper (80 to 120 grit) before assembly helps gas bubbles detach faster. Bubbles clinging to the plate surface act as insulators, increasing the voltage needed to maintain current flow. This added voltage requirement, called bubble overpotential, is one of the biggest efficiency losses in small electrolyzers. Adding a tiny amount of surfactant to the electrolyte can also reduce bubble coalescence and help them release from the plates more quickly.
How Much Hydrogen to Expect
The theoretical yield follows Faraday’s law of electrolysis: every 96,485 coulombs of charge (one ampere flowing for 96,485 seconds, or about 26.8 amp-hours) liberates one mole of hydrogen gas. At standard temperature and pressure, one mole of hydrogen occupies 22.4 liters. So each amp-hour of current produces roughly 0.42 liters of hydrogen in a perfectly efficient cell.
In practice, expect 60% to 75% of that theoretical yield from a well-built DIY cell, giving you around 0.25 to 0.31 liters of hydrogen per amp-hour. If you’re running 10 amps, that’s about 2.5 to 3.1 liters per hour. Increasing current increases production, but also increases heat. If your electrolyte temperature climbs above 60 to 70°C, you risk damaging seals and gaskets. Moderate current and good ventilation around the cell keep temperatures manageable.
Higher electrolyte temperatures do improve conductivity, since ions move faster and encounter less resistance as the solution warms. This is why commercial electrolyzers often run at 60 to 80°C intentionally. For a DIY setup without temperature-rated seals, letting the cell warm to 40 to 50°C is a reasonable compromise that improves efficiency without stressing your materials.
Gas Handling and Safety
Hydrogen is flammable in air at concentrations between 4% and 75%, and it ignites with very little energy. The primary risk with any electrolyzer is a flame traveling back through the gas line into the cell, where it could ignite the accumulated gas and rupture the housing. This is called a flashback.
The standard protection is a bubbler, sometimes called a flashback arrestor. This is a sealed jar partially filled with water, positioned between the electrolyzer and whatever you’re feeding hydrogen to. The gas line from the electrolyzer enters at the bottom of the jar, forcing gas to bubble up through the water before exiting through a second line at the top. If a flame tries to travel back through the line, the water column extinguishes it. Use at least 3 to 4 inches of water depth in the bubbler.
Commercial flash arrestors add extra layers of protection: a stainless steel flame barrier that physically stops combustion, a check valve that prevents reverse gas flow, and a thermal cutoff that shuts the system down if temperatures reach 165°C. For any setup beyond basic experimentation, adding an inline check valve between the bubbler and the electrolyzer is a worthwhile precaution.
Never store hydrogen in a sealed, rigid container without a pressure relief valve. Never operate the electrolyzer in an enclosed space without ventilation. Even small leaks accumulate quickly in a closed room.
Improving Performance Over Time
After your first build is running, there are several ways to push efficiency higher. Adding more electrode plates in a series or series-parallel configuration increases the active surface area, which lets you produce more gas at the same voltage. Each cell in a stack needs about 2 to 2.5 volts, so a 12-volt power supply can drive 5 to 6 cells in series.
Electrode conditioning also matters. New stainless steel plates often perform poorly for the first few hours because they haven’t developed their catalytic surface layer. Running the cell at moderate current for 12 to 24 hours before expecting peak output lets the plates develop a dark oxide coating that actually improves gas production. Don’t scrub this layer off during maintenance.
Monitor your electrolyte level and concentration over time. Water gets consumed by the reaction (it’s being split into gas, after all), so the KOH concentration slowly rises as the water level drops. Top off with distilled water periodically to keep the concentration in the 25% to 30% range. If the solution turns brown or develops sediment, the electrodes are corroding and you may need to replace them or switch to a higher-grade steel.