An electrolysis cell is a device that uses electrical energy to drive a chemical reaction that would not happen on its own. The most familiar example splits water into hydrogen and oxygen, but the same principle underpins chlorine production, metal refining, and even the conversion of carbon dioxide into useful fuels. The concept dates back to 1789, when two Dutch scientists first generated hydrogen using an electrostatic generator, and today more than a dozen competing cell designs are pushing toward commercial-scale green hydrogen production.
What Happens Inside an Electrolysis Cell
Every electrolysis cell has the same basic anatomy: two electrodes (an anode and a cathode) separated by some kind of ion-conducting medium, all connected to an external power supply. When voltage is applied, chemical reactions are forced to occur at each electrode. At the cathode, a substance gains electrons (reduction); at the anode, a substance loses them (oxidation). In water electrolysis, water molecules break apart: hydrogen gas bubbles off at the cathode and oxygen gas at the anode. The ion-conducting medium between them, whether a liquid electrolyte or a solid membrane, lets charged particles shuttle back and forth to complete the electrical circuit while keeping the product gases separated.
The minimum voltage needed to split water under standard conditions is about 1.23 volts, set by thermodynamics. In practice, every real cell needs more than that. The extra voltage, called overpotential, goes toward overcoming sluggish reaction kinetics at the electrode surfaces, resistance in the membrane or electrolyte, and the difficulty of moving reactants and products to and from the electrodes. Researchers partition these losses into activation overpotential (the energy penalty for kickstarting the reaction), ohmic overpotential (electrical resistance in the cell’s components), and mass-transport overpotential (the cost of shuttling molecules and ions where they need to go). How you define and measure those categories matters more than it might sound: different conventions for splitting them up can give substantially different numbers for the same cell, which has led to misinterpretations in the literature.1Journal of The Electrochemical Society. Interpreting Concentration and Activation Overpotentials in Electrochemical Systems: A Critical Discussion One recent study found that the true mass-transport overpotential in a PEM water electrolyzer at high current density was only about one-third of the value calculated by conventional methods.2International Journal of Hydrogen Energy. Reassessing mass transport overpotential in PEM water electrolysis: Overestimation and component decoupling
Temperature plays a useful role here. As temperature rises, the thermodynamic minimum voltage for water splitting drops because heat provides part of the energy the electricity would otherwise have to supply.3Journal of Power Sources. Thermodynamics of high-temperature, high-pressure water electrolysis That principle is what makes high-temperature electrolysis attractive, as we will see with solid oxide cells below.
The Four Main Water-Electrolysis Technologies
When people talk about electrolysis cells today, they usually mean one of four designs, each with its own trade-offs in cost, efficiency, maturity, and material requirements.
Alkaline Electrolysis
This is the oldest and most commercially mature approach. Industrial alkaline electrolyzers pump a concentrated potassium hydroxide solution (typically 20 to 30 percent by weight) through the cell, and a porous diaphragm sits between the electrodes to keep the hydrogen and oxygen apart while letting ions pass through. The diaphragm’s pore size is a balancing act: smaller pores block gas crossover more effectively but increase electrical resistance.4Chemical Reviews. Separators and Membranes for Advanced Alkaline Water Electrolysis Traditional alkaline cells run at relatively modest current densities, but a newer “zero-gap” design, where the electrodes press directly against the diaphragm, pushes performance much higher. Modeling work on a 20 Nm³/h electrolyzer showed that a zero-gap cell with a 0.1 mm diaphragm at 100 °C could reach a current density of 1.8 A/cm², though the thinner barrier and higher temperature also increase gas crossover, especially at low loads.5International Journal of Hydrogen Energy. Optimal operating parameters for advanced alkaline water electrolysis Lab-scale zero-gap cells using milder electrolyte concentrations and nanostructured electrodes have reached above 2.5 A/cm² at 2.3 V while maintaining adequate gas purity.6PubMed Central. Addressing Mass Transport Limitation and Gas Crossover Behavior in Zero-Gap Porous Separator Configurations for Alkaline Water Electrolysis
Some newer alkaline cells use ion-solvating membranes instead of traditional porous diaphragms. In these membranes, both positive and negative ions from the electrolyte carry charge. This introduces a wrinkle: as current flows, electro-osmosis drags potassium hydroxide toward the cathode, making the discharged electrolyte more concentrated than the feed. The membrane’s chemical stability allows operation up to 80 °C, and using high-surface-area catalysts like Raney nickel on the cathode further boosts performance.7PubMed Central. Experimental investigation into the functionality of alkaline water electrolysis with ion-solvating membrane in anode feed mode using diluted potassium hydroxide
Proton Exchange Membrane (PEM) Electrolysis
PEM cells replace the liquid electrolyte with a solid polymer membrane, most commonly a perfluorosulfonic acid material like Nafion. Protons generated at the anode migrate through this membrane to the cathode, where they combine with electrons to form hydrogen. The solid membrane allows compact design, high current densities, rapid start-up, and the ability to produce hydrogen at elevated pressure directly. PEM electrolysis is widely seen as a leading technology for coupling with intermittent renewables. The catch is cost: the acidic environment at the anode demands catalysts based on iridium, one of the rarest elements in Earth’s crust.8Chem Catalysis. Iridium management strategies for scalable proton exchange membrane water electrolysis A tubular PEM design tested with low iridium loading achieved an iridium mass activity up to three times higher than previously reported values, suggesting that clever engineering can stretch limited iridium supplies further.9International Journal of Hydrogen Energy. Tubular membrane electrode assembly for PEM electrolysis
Anion Exchange Membrane (AEM) Electrolysis
AEM electrolysis tries to combine the best of both worlds: the compact solid-membrane design of PEM with the alkaline environment that lets you use cheaper, non-noble-metal catalysts. The membrane conducts hydroxide ions instead of protons. This is a younger technology, but lab results are encouraging. A cell using earth-abundant catalysts delivered 1.0 A/cm² at just 1.57 V and 80 °C, reaching an energy conversion efficiency of about 75 percent.10Advanced Energy Materials. High‐Efficiency Anion Exchange Membrane Water Electrolysis Employing Non‐Noble Metal Catalysts Another study demonstrated a nickel-copper-phosphide catalyst operating in an AEM electrolyzer for over 300 hours with 95 to 97 percent current retention, showing that durability of non-precious catalysts is catching up.11PubMed. Tensile nanostructured hierarchically porous non-precious transition metal-based electrocatalyst for durable anion exchange membrane-based water electrolysis AEM membrane longevity remains an open question for industrial-scale deployment, but the ability to avoid iridium and platinum altogether makes this approach intensely interesting.
Solid Oxide Electrolysis Cells (SOECs)
Solid oxide cells operate at very high temperatures, typically 700 to 900 °C, using a ceramic electrolyte that conducts oxide ions. The extreme heat has a thermodynamic benefit: a larger share of the energy needed for water splitting comes from heat rather than electricity, so the electrical efficiency can be very high. SOECs also avoid noble-metal catalysts entirely, use no liquid electrolyte, and can run in reverse as fuel cells. They are particularly attractive where waste heat is available, such as next to nuclear reactors or industrial furnaces.12PubMed Central. High Temperature Solid Oxide Electrolysis for Green Hydrogen Production Their main drawbacks are slow start-up times and material degradation at high temperatures, which limit their suitability for following variable renewable power.
The Catalyst Bottleneck
Catalysts sit at the heart of every electrolysis cell, lowering the energy barrier for the reactions at each electrode. On the hydrogen-producing cathode side, the goal is a material that binds hydrogen atoms just strongly enough: too tight and the hydrogen product cannot leave the surface; too loose and the reaction barely starts.13Innovation Discovery. Non-precious Electrocatalysts for the Hydrogen Evolution Reaction Platinum is the benchmark, but many non-precious alternatives based on nickel, molybdenum, and phosphide compounds now approach platinum’s performance in alkaline environments.
The oxygen-producing anode is harder. In acidic PEM cells, the oxygen evolution reaction is inherently sluggish, involving multiple electron transfers, and most catalysts dissolve rapidly in the corrosive environment.14PubMed. Breaking the Activity and Stability Bottlenecks of Electrocatalysts for Oxygen Evolution Reactions in Acids Iridium oxide remains the only anode catalyst that is both active enough and stable enough for commercial PEM electrolyzers, and global iridium production is tiny compared to the amount that would be needed for a major scale-up of PEM capacity. Strategies to cope include reducing catalyst loading, improving iridium utilization through better electrode architectures, and recycling spent catalysts.15Chem Catalysis. Iridium management strategies for scalable proton exchange membrane water electrolysis AEM and alkaline technologies sidestep the problem entirely by running in an alkaline environment where nickel-iron and other earth-abundant materials do fine as oxygen catalysts.
Gas Crossover and Safety
Hydrogen and oxygen are produced on opposite sides of the cell, but small amounts inevitably migrate across the separator. If hydrogen accumulates in the oxygen stream (or vice versa) above about four percent, the mixture enters the flammable range. This gas crossover is the central safety challenge for electrolysis cells, and the mechanisms differ between technologies. In PEM electrolyzers, crossover is dominated by diffusion of dissolved hydrogen through the membrane, driven by supersaturation in the catalyst layer: at 1 A/cm² and atmospheric pressure, diffusion accounts for roughly 98 percent of the total hydrogen permeation.16Journal of The Electrochemical Society. Hydrogen Crossover in PEM and Alkaline Water Electrolysis: Mechanisms, Direct Comparison and Mitigation Strategies In alkaline electrolyzers, the dominant pathway is not permeation through the diaphragm itself but mixing of the electrolyte loops, where dissolved gas carries over into the opposite compartment through the circulating liquid.17Journal of The Electrochemical Society. Hydrogen Crossover in PEM and Alkaline Water Electrolysis: Mechanisms, Direct Comparison and Mitigation Strategies
Overall, hydrogen permeation across the separator is more than ten times lower in alkaline cells than in PEM cells under comparable conditions, mainly because hydrogen is far less soluble in concentrated potassium hydroxide than in water.18Journal of The Electrochemical Society. Hydrogen Crossover in PEM and Alkaline Water Electrolysis: Mechanisms, Direct Comparison and Mitigation Strategies Crossover gets worse at higher pressures, since more gas dissolves in the liquid or membrane. One mitigation approach embeds a third electrode inside the PEM membrane that electrochemically pumps permeating hydrogen back to the cathode side. In tests at a cathode pressure fifty times higher than the anode, this technique considerably improved cell efficiency at low current densities.19Electrochimica Acta. High-pressure water electrolysis: Electrochemical mitigation of product gas crossover Catalytic recombiners, which safely react any stray hydrogen and oxygen back into water, are another standard safety measure.20International Journal of Hydrogen Energy. Hydrogen safety aspects related to high-pressure polymer electrolyte membrane water electrolysis
How Dynamic Operation Affects Durability
If green hydrogen is the goal, electrolysis cells need to run on power from wind turbines and solar panels, which means their output swings constantly. This dynamic operation poses durability challenges that do not arise when a cell runs at steady state. In PEM electrolyzers, one of the key degradation mechanisms is dissolution of iridium from the anode catalyst layer. Over time, this thins the catalyst and increases the voltage penalty for driving the reaction.21Journal of The Electrochemical Society. Electrolyzer Durability at Low Catalyst Loading and with Dynamic Operation How fast you ramp the voltage matters: tests using triangular voltage cycling found that a tenfold increase in ramping rate roughly doubled the degradation rate, likely because fast voltage swings intensify bubble formation at the catalyst-membrane interface.22Journal of Electrochemical Science and Technology. Effect of Ramping Rate on the Durability of Proton Exchange Membrane Water Electrolysis During Dynamic Operation Using Triangular Voltage Cycling
The way you shut down an electrolyzer between active periods also makes a big difference. A comparative study testing three operating modes over 365 cycles found a clear degradation hierarchy. On/standby mode, where the cell is kept at a low voltage between active bursts, caused the most damage: resistance ballooned, sulfur-enriched zones appeared in the membrane, and the anode’s porous structure partially collapsed. Continuous operation showed intermediate wear. Surprisingly, a clean on/off mode, where the cell was fully shut down between cycles, preserved both electrochemical performance and physical structure the best, recording the lowest resistance increase and no significant morphological damage.23Energy Conversion and Management. Influence of dynamic operating cycles on the performance and degradation of PEM electrolyzers For operators coupling electrolyzers with variable renewables, this suggests that a full shutdown may be gentler on the cell than idling at standby.
Membrane thinning from chemical attack is another concern under fluctuating conditions. Simulations modeling PEM electrolyzers powered by real wind profiles showed that extended off periods and frequent part-load operation significantly accelerated fluoride release, a marker of membrane degradation.24Journal of Power Sources. Chemical degradation of PEM electrolyzers powered by unsteady wind energy The upshot: dynamic operation is technically feasible, but the operating strategy needs careful design to avoid shortening the stack’s life.
Porous Transport Layers and Cell Engineering
Between the catalyst layer and the flow channels that distribute water and collect gas, there sits a porous transport layer (PTL). This component has to do several jobs at once: let water reach the catalyst, let gas bubbles escape, conduct electricity, and withstand mechanical compression without crushing. At high current densities above about 2 A/cm², the PTL often becomes the bottleneck, because bubbles trapped in its pores block water access. Conventional PTLs are random fibrous or sintered structures, but researchers are shifting toward deterministic architectures, such as laser-perforated sheets and integrated bipolar plate designs, to give bubbles cleaner exit paths.25International Journal of Hydrogen Energy. Current status and future trends in anode porous transport layers for high-efficiency PEM water electrolysis Getting this layer right is one of the less glamorous but most practical engineering challenges in scaling up electrolyzers.
Splitting Seawater Instead of Fresh Water
About 97 percent of Earth’s water is salty, so the idea of splitting seawater directly is appealing, especially for coastal hydrogen hubs. The reality is harder than it sounds. Seawater brings chloride ions, which can oxidize to chlorine gas at the anode instead of (or alongside) oxygen, poisoning catalysts and corroding cell components. Dissolved calcium and magnesium precipitate as scale on electrodes and membranes. Marine organisms cause biofouling. All of these degrade performance and shorten cell life.26Journal of Power Sources. Direct and hybrid seawater electrolysis: Materials, cell designs, and techno-economic realities Most practical large-scale projects today desalinate seawater first and then feed purified water into a conventional electrolyzer, since desalination adds only a small fraction of the total energy and cost. Direct seawater electrolysis remains an active area of catalyst and membrane research, with selective catalysts being developed to favor oxygen evolution over chlorine evolution, but it is not yet commercially competitive.
What Green Hydrogen Actually Costs
The economics of electrolytic hydrogen depend heavily on the price of electricity, the utilization rate of the electrolyzer, and the capital cost of the stack. A techno-economic comparison of the four main electrolyzer types, modeled over a 25-year horizon with photovoltaic and wind power, found that at a refueling capacity of 500 kg of hydrogen per day, the levelized cost ranged from about $7.2 to $8.7 per kilogram. Alkaline systems came in cheapest at around $7.2/kg, followed by AEM at $7.6/kg, SOEC at $7.9/kg, and PEM at $8.7/kg. The alkaline system also had the shortest payback period at roughly 8.4 years.27International Journal of Hydrogen Energy. Modelling, design and techno-economic comparison of electrolyzer technologies for photovoltaic-wind grid-tied hydrogen refueling infrastructure Those numbers are for a specific grid-tied refueling scenario and will vary with electricity prices, subsidies, and scale, but the broad ordering gives a sense of where each technology sits today. For context, hydrogen produced from natural gas with steam methane reforming typically costs around $1 to $3 per kilogram depending on the region and whether carbon capture is included, so green hydrogen still carries a significant premium.
Electrolysis Cells Beyond Hydrogen
Water splitting dominates the headlines, but electrolysis cells have been workhorses of the chemical industry for well over a century. The chlor-alkali process uses brine electrolysis to produce chlorine and sodium hydroxide, two of the most widely used industrial chemicals. Electrowinning cells extract metals like copper, zinc, and aluminum from solution by plating them onto a cathode. A recent advance in manganese electrowinning illustrates how cell design innovation carries across applications: a three-chamber cell with a proton-exchange membrane achieved a current efficiency of about 85 percent and cut energy consumption by over 1,400 kWh per ton compared to conventional industrial practice, while also eliminating the problematic buildup of manganese dioxide sludge on the anode.28Separation and Purification Technology. Energy-efficient and advanced electrowinning of metallic manganese within a novel H+ exchange membrane cell using a Ti/IrO2−RuO2−SiO2 anode
High-temperature solid oxide cells can also run co-electrolysis, feeding in both steam and carbon dioxide simultaneously. The steam provides the main electrochemical reaction at the cathode, while CO₂ is converted largely through thermochemical side reactions inside the porous electrode. The result is a mixture of hydrogen and carbon monoxide called syngas, which is the starting material for making synthetic fuels, methanol, and a range of other chemicals. Managing syngas composition requires careful control of operating conditions, electrode materials, and microstructure, because the kinetics of CO₂ reduction and the diffusion of the gas mixture through the electrode are both slow.29Journal of Power Sources. Reactions and mass transport in high temperature co-electrolysis of steam/CO2 mixtures for syngas production If this technology matures, it could turn surplus renewable electricity and captured CO₂ into drop-in liquid fuels, closing a carbon loop rather than simply producing hydrogen.
A Surprisingly Long Track Record
Electrolysis cells are often discussed as if they are emerging technology, but their roots run deep. The very first electrochemical generation of hydrogen happened in 1789, using an electrostatic generator. Within a year of Volta’s invention of the battery in 1800, multiple experimenters were decomposing water with voltaic piles. By around 1900, an estimated 400 industrial alkaline electrolyzers were already operating worldwide, and hydroelectric-powered electrolysis plants were producing hydrogen for ammonia fertilizer well into the twentieth century.30Electrochemical Power Sources: Fundamentals, Systems, and Applications. The history of water electrolysis from its beginnings to the present What changed is not the principle but the context: the urgency of decarbonization, the falling cost of renewable electricity, and decades of materials science progress have turned an old technology into a centerpiece of the energy transition. The challenge now is less about whether electrolysis cells work and more about making them cheap, durable, and large enough to matter at the scale the climate demands.