Water molecules each contain one oxygen atom bonded to two hydrogen atoms, and breaking those bonds to free the oxygen requires a significant input of energy. Electrolysis, the process of running an electric current through water, is the most straightforward method and the one used aboard submarines and space stations to keep crews breathing. But electrolysis is only one approach. Nature figured out ways to pull dissolved oxygen from water hundreds of millions of years ago, and researchers today are exploring solar-driven and catalytic methods that could make the process far cheaper and more scalable.
Dissolved Oxygen Versus Bound Oxygen
Before getting into extraction methods, it helps to understand that “oxygen in water” can mean two very different things. The first is dissolved oxygen: free Oâ‚‚ molecules that have mixed into water from the atmosphere, much the way carbon dioxide dissolves into a soda. The amount that dissolves depends on temperature, pressure, and salinity. Cold, fresh water holds more dissolved oxygen than warm, salty water. The relationship between gas pressure and how much oxygen dissolves follows a well-characterized equilibrium described by Henry’s law, where the concentration of dissolved gas is proportional to its partial pressure in the atmosphere above the water.1Frontiers in Nuclear Engineering. The solubility of oxygen in water and saline solutions In seawater, various dissolved salts further reduce oxygen solubility, with each salt type having a measurable “salting out” effect.2Marine Chemistry. Solubility of oxygen in the major sea salts as a function of concentration and temperature
The second form is the oxygen chemically locked inside the water molecule itself. Liberating that oxygen means tearing apart the covalent bonds holding Hâ‚‚O together, which takes a lot more energy than simply coaxing dissolved gas out of solution. Every method discussed below falls into one of these two categories: harvesting dissolved Oâ‚‚ or splitting water molecules outright.
How Fish Extract Oxygen From Water
Fish are the most familiar example of organisms pulling dissolved oxygen out of water. Their gills are remarkably efficient at this, despite the fact that water contains far less oxygen per liter than air does. The key design feature is counter-current flow: blood moves through the gill filaments in the opposite direction to the water flowing over them.3Respiration Physiology. A dimensional analysis of oxygen transfer in the fish gill This arrangement ensures that blood encountering fresh, oxygen-rich water at the end of its path through the gill still has a lower oxygen concentration than that water, so diffusion continues across the entire surface rather than reaching equilibrium partway through.
Interestingly, a mathematical model of gill oxygen transfer found that the counter-current setup does not dramatically increase total oxygen uptake compared to a parallel-flow arrangement. Its real advantage is energy savings: the fish can pump less water across its gills to absorb the same amount of oxygen.4Journal of Theoretical Biology. Energy advantage of counter-current oxygen transfer in fish gills Gill anatomy is also finely tuned at the structural level. The spacing between gill lamellae, the thin plate-like structures where gas exchange happens, is surprisingly consistent across fish species of wildly different body sizes. This consistency reflects an optimization: spacing the lamellae too close together increases the surface area for diffusion but also raises the resistance to water flowing through, so there is a sweet spot that maximizes overall oxygen transfer.5PubMed Central. Optimal lamellar arrangement in fish gills
Some aquatic insects use a completely different trick. Rather than gills, certain marine insect pupae have structures called plastron-bearing spiracular gills, which trap a thin film of air against the body. Oxygen from the surrounding water diffuses into this air film, and the insect breathes from it much the way a terrestrial insect breathes from the atmosphere.6Nature. Plastron Respiration in Marine Insects The plastron acts like a permanent, self-replenishing air bubble, a surprisingly elegant solution to underwater breathing.
When Fish Breathe Air Instead
Not all fish rely exclusively on dissolved oxygen. Dozens of species have evolved some capacity for air breathing, gulping air at the surface and absorbing oxygen through modified swim bladders, intestinal tissue, or accessory respiratory organs. These fish are studied closely by evolutionary physiologists because they offer a window into the cardiovascular and respiratory changes that allowed vertebrates to transition from water to land hundreds of millions of years ago.7PubMed. Evolutionary and cardio-respiratory physiology of air-breathing and amphibious fishes The fact that air holds roughly 20 to 30 times more oxygen by volume than water helps explain why this adaptation has evolved independently in many unrelated fish lineages. The energetic payoff is enormous, especially in warm, stagnant water where dissolved oxygen runs low.
Splitting Water With Electricity
If you need pure oxygen rather than the small amounts dissolved in water, the standard approach is electrolysis: passing a direct electric current through water to break it into hydrogen gas and oxygen gas. In a typical setup, water contacts two electrodes. At the anode (positive electrode), water molecules lose electrons and release oxygen. At the cathode (negative electrode), the freed hydrogen ions pick up electrons and form hydrogen gas.
Modern systems often use a proton exchange membrane (PEM) to separate the two sides. Water is fed to the anode, where it is split into oxygen, protons, and electrons. The protons travel through the membrane to the cathode, while the electrons travel through an external circuit, and hydrogen forms on the other side.8Materials Science for Energy Technologies. Hydrogen production by PEM water electrolysis – A review PEM electrolysis can operate at high current densities and respond quickly to changes in power input, which makes it well suited for pairing with intermittent energy sources like wind or solar.
Alkaline water electrolysis is the older and more widely deployed alternative. It uses a liquid alkaline solution (typically potassium hydroxide) as the electrolyte. While alkaline systems have historically been cheaper to build because they avoid expensive precious-metal catalysts, improving the stability and activity of low-cost anode materials remains an active area of research. Nickel-iron-based catalysts have emerged as particularly promising for the oxygen-producing side of alkaline electrolysis because they are abundant and relatively inexpensive.9PubMed. Advances in Stability of NiFe-Based Anodes toward Oxygen Evolution Reaction for Alkaline Water Electrolysis
Electrolysis Aboard Submarines and Space Stations
The most critical real-world application of water electrolysis is keeping people alive in sealed environments. U.S. nuclear submarines generate breathable oxygen by electrolyzing water using solid polymer electrolyte (SPE) cells. A low-pressure electrolyzer system developed for submarine use can produce up to 250 standard cubic feet per hour of oxygen at ambient pressure.10SAE International. Design and Operation of a Low Pressure Electrolyzer (LPE) for Submarine Applications More advanced submarine designs have pushed cell performance further, achieving current densities of up to 1,600 milliamperes per square centimeter of cell area. Some next-generation systems even combine oxygen generation with carbon dioxide reduction in a single electrochemical cell, simplifying the life-support hardware aboard.11International Conference On Environmental Systems. Oxygen Generator Cell Design for Future Submarines
The International Space Station relies on a similar oxygen generation system. In both submarines and spacecraft, the hydrogen byproduct can be vented overboard, reacted with carbon dioxide to recover water (via the Sabatier reaction), or simply dumped, depending on the mission profile. The core chemistry is identical to a lab electrolysis demo, just scaled up and made extremely reliable.
Chemical Oxygen Generators
When electrolysis hardware is too heavy, too complex, or unavailable, chemical oxygen generators offer a self-contained backup. The most common type is the oxygen candle, a solid block that releases oxygen when ignited. A typical formulation uses sodium chlorate as the main oxygen source (around 86% of the mixture), with manganese and cobalt oxide as catalysts and a small amount of clay as a binder. In testing, this type of candle produced an average oxygen flow of about 1.6 liters per minute, with a total output of roughly 28 liters and an effective oxygen production rate of about 54%.12Journal of Chemistry. Coupling Effect of Metals and Oxides on the Oxygen Supply Performance of Sodium Chlorate Oxygen Candle
Oxygen candles do not extract oxygen from water. They store it chemically and release it on demand. But they are widely used alongside water electrolysis systems as emergency backups on submarines and in mining rescue equipment, where losing the primary oxygen supply could be fatal within minutes.
Using Sunlight and Heat to Split Water
Electricity is not the only energy source that can crack water molecules apart. Thermochemical water splitting uses very high temperatures, typically from concentrated solar energy, to drive a series of chemical reactions that ultimately produce hydrogen and oxygen from water without direct electrical input. Certain metal oxide materials are heated until they release oxygen from their crystal structure, then cooled and exposed to steam, which re-oxidizes them and produces hydrogen. Perovskite-type materials have shown the ability to release oxygen at temperatures in the range of 1,000 to 1,400°C, which is lower than what older reference materials like ceria require.13The Journal of Physical Chemistry C. Investigation of Perovskite Structures as Oxygen-Exchange Redox Materials for Hydrogen Production from Thermochemical Two-Step Water-Splitting Cycles
Photocatalytic splitting takes a different route, using semiconductor materials that absorb sunlight and use the energy directly to break water molecules at their surface. Research on titanium dioxide (TiOâ‚‚), one of the most studied photocatalysts, has shown that the lifetime of photogenerated charge carriers in the material strongly determines how well it can split water. Producing one molecule of oxygen requires four photons, mirroring the four-electron process used in natural photosynthesis.14PubMed. Mechanism of photocatalytic water splitting in TiO2. Reaction of water with photoholes, importance of charge carrier dynamics, and evidence for four-hole chemistry The parallel to biology is striking, but current photocatalysts remain far less efficient than plant chloroplasts. Finding a stable semiconductor that can absorb a broad range of sunlight and split water at practical rates is one of the central challenges in the field.
The Problem With Splitting Seawater
Nearly all commercial and research electrolysis uses purified fresh water. Running the process with seawater introduces a nasty complication: chloride ions. When you apply voltage to seawater, the chloride ions oxidize at the anode alongside water, producing chlorine gas instead of (or in addition to) oxygen. Even though the oxygen evolution reaction is thermodynamically favored, the chlorine oxidation reaction is kinetically faster, meaning it tends to dominate in practice.15ACS Applied Energy Materials. Breaking the Scaling Relationship of Oxygen Evolution Reaction and Chlorine Oxidation Reaction via MnO2 Polymorphic Engineering for Selective Seawater Electrolysis Chlorine gas is toxic, corrosive to equipment, and an unwanted byproduct in any oxygen-generating application.
Solving this problem is a major focus of current materials science research. Strategies include engineering the anode catalyst to be selective for oxygen over chlorine, applying protective coatings, and designing membranes that keep chloride ions away from the electrode surface. Progress has been steady but the challenge is steep: seawater contains a complex mixture of salts and impurities that cause corrosion and degrade catalysts over time.16PubMed Central. Comprehensive Chlorine Suppression: Advances in Materials and System Technologies for Direct Seawater Electrolysis If direct seawater electrolysis can be made practical, it would eliminate the need for expensive desalination as a pre-treatment step, a significant cost saving for coastal hydrogen and oxygen production.
Artificial Photosynthesis
Plants split water as part of photosynthesis, using sunlight to strip electrons from water molecules and releasing oxygen as a byproduct. Researchers have been trying to build artificial systems that mimic this process for decades. The approach typically involves linking a light-absorbing molecule (a chromophore) to a water oxidation catalyst. Early work paired ruthenium-based chromophores with manganese-based catalysts, directly inspired by the manganese cluster at the heart of the natural photosynthetic machinery.17PubMed. Artificial photosynthesis: from nanosecond electron transfer to catalytic water oxidation
More recent catalyst design has focused on improving turnover numbers (how many oxygen molecules a single catalyst molecule can produce before it degrades) and reducing overpotential (the extra voltage needed beyond the theoretical minimum). Ruthenium catalysts with negatively charged ligands, such as carboxylate groups, have shown dramatic improvements in both stability and oxygen production rates. The negatively charged groups stabilize the catalyst in its high-energy active states, allowing it to cycle more times before breaking down.18PubMed. Highly efficient bioinspired molecular Ru water oxidation catalysts with negatively charged backbone ligands Artificial photosynthesis is still far from commercial deployment, but the field has moved from proof-of-concept demonstrations to catalysts that actually work at meaningful rates.
Artificial Gills
One of the more imaginative approaches to getting oxygen from water is the artificial gill: a device that extracts dissolved Oâ‚‚ from water the way a fish gill does, rather than splitting water molecules. The concept involves pumping water past a gas-permeable membrane, allowing dissolved oxygen to diffuse across, and collecting it on the other side for breathing.
A prototype developed using concentrated hemoglobin solution as the oxygen carrier showed that the idea is physically feasible. The hemoglobin picks up oxygen from seawater at lower temperatures and releases it at higher temperatures, mimicking how blood loads and unloads oxygen in a living body. The enhancement factors, a measure of how much better the carrier performs than a plain membrane, were about 3 for oxygen uptake and 16 for oxygen release. Scaling the system up for a resting human, the researchers estimated a required membrane surface area of about 64 square meters and an inspiratory oxygen pressure adequate for normal breathing.19Journal of Membrane Science. Development of a compact artificial gill using concentrated hemoglobin solution as the oxygen carrier
A different design used a molybdenum porphyrin compound dissolved in a solvent as the oxygen carrier, with visible light driving the uptake and release cycle. When light hits the carrier, it changes the molecule’s oxygen affinity, allowing it to grab oxygen from water under one condition and dump it into air under another.20Journal of Membrane Science. The photoresponse of a molybdenum porphyrin makes an artificial gill feasible Both designs demonstrate that the physics works, but the engineering challenges are formidable. A 64-square-meter membrane is not something you strap to a diver’s back. Still, the concept hints at future devices that could supply oxygen in underwater habitats or emergency scenarios without the energy cost of full electrolysis, pulling free dissolved gas from the ocean rather than cracking molecular bonds.
Why Most Research Focuses on Hydrogen
If you read the scientific literature on water splitting, you will notice that almost all of it frames the goal as hydrogen production rather than oxygen production. That is because hydrogen is the economically valuable product: it serves as a clean fuel, an industrial feedstock, and a potential energy storage medium. Oxygen is the byproduct. For every two molecules of water you split, you get two molecules of hydrogen and one molecule of oxygen. In most large-scale electrolysis facilities, the oxygen is either vented to the atmosphere or sold as a low-value industrial gas.
The exception is in closed environments, where the oxygen is the whole point. Submarines, spacecraft, and potentially future lunar or Martian habitats all need reliable oxygen far more than they need hydrogen. In those contexts, the economics flip entirely. The hydrogen becomes the waste product (or gets repurposed for carbon dioxide scrubbing), and the oxygen is what justifies the energy expenditure. This disconnect between the research community’s focus and the life-support community’s priorities means that some of the most practical oxygen-from-water technology sits in engineering reports from defense contractors and space agencies rather than in the high-profile chemistry journals.
The Discovery That Started It All
The understanding that water contains oxygen, and that oxygen can be liberated from it, traces back to the late 18th century. Joseph Priestley isolated oxygen in 1774, though he called it “dephlogisticated air” and continued using that term for the rest of his life. It was Antoine Lavoisier who demonstrated that air contained a distinct element, named it oxygen, and showed it combined with hydrogen to form water, effectively disproving the prevailing phlogiston theory of combustion.21PubMed. Fire-air and dephlogistication. Revisionisms of oxygen’s discovery The realization that water was not an element but a compound of two gases was one of the pivotal moments in the history of chemistry, and every method of extracting oxygen from water discussed here flows directly from it.