Hydrogen is produced primarily by splitting it from the molecules it’s bound to, whether that’s natural gas, coal, water, or biomass. Nearly all of today’s hydrogen comes from fossil fuels: about 47% from natural gas, 27% from coal, and 22% from oil as a byproduct. Only around 4% is made through electrolysis (splitting water with electricity), and just 1% of total global output qualifies as truly renewable.
Steam Methane Reforming: The Dominant Method
The workhorse of hydrogen production is steam methane reforming, or SMR. Natural gas (mostly methane) is mixed with high-temperature steam inside a reactor packed with a catalyst. The methane and steam molecules break apart and recombine into carbon monoxide and hydrogen gas. This reaction happens at extreme conditions: temperatures between 800 and 880°C and pressures of 20 to 30 atmospheres.
A second step, called the water-gas shift reaction, then converts that carbon monoxide into carbon dioxide while releasing even more hydrogen. The overall result is that one molecule of methane plus two molecules of water yields one molecule of CO₂ and four molecules of hydrogen. SMR is efficient and relatively cheap, which is why natural gas dominates global hydrogen supply. The trade-off is obvious: for every kilogram of hydrogen, a significant amount of carbon dioxide goes into the atmosphere. Hydrogen made this way is commonly called “grey hydrogen.”
Blue Hydrogen: Adding Carbon Capture
Blue hydrogen uses the same SMR process but bolts on carbon capture and storage technology to trap the CO₂ before it reaches the atmosphere. The appeal is straightforward: you keep the low cost of natural gas while dramatically cutting emissions. In practice, the climate benefit depends on how well the capture system performs and how much methane leaks during natural gas extraction and transport.
Research published in Applied Energy found that blue hydrogen’s sustainability becomes questionable when carbon capture rates fall to 85% or below, especially if methane leakage from the supply chain exceeds about 1%. Even at a 90% capture rate, higher methane leakage rates (1% to 2.5%) can undermine the climate advantage enough that blue hydrogen doesn’t scale significantly in decarbonization models through 2050. So while the technology works in principle, its real-world climate value hinges on tight control over the entire natural gas supply chain.
Coal Gasification
In countries with abundant coal reserves, particularly China, hydrogen is produced through gasification. The process works in three stages. First, coal is heated with oxygen or steam in a high-temperature reactor, triggering exothermic (heat-releasing) reactions that bring the mixture up to operating temperature. Second, in the pyrolysis stage, the intense heat breaks the coal into lighter hydrocarbons, tar, and char. Third, a reduction phase converts CO₂ and steam into carbon monoxide, hydrogen, and methane.
Coal gasification produces a synthesis gas (a mix of hydrogen and carbon monoxide) that can be further processed to isolate pure hydrogen. Higher temperatures, elevated pressures, and more steam relative to the coal all increase hydrogen yields. The process can also be paired with carbon capture, and proponents point out that it can reduce sulfur and nitrogen oxide emissions compared to simply burning coal. Still, coal-based hydrogen carries the heaviest carbon footprint of any production method unless capture technology is added.
Electrolysis: Splitting Water With Electricity
Electrolysis passes an electric current through water to break it into hydrogen and oxygen. When that electricity comes from renewable sources like wind or solar, the result is “green hydrogen,” with zero direct carbon emissions. The chemistry is simple, but the engineering comes in several forms.
Alkaline electrolyzers are the oldest and most mature technology. They operate at relatively low temperatures and use a liquid alkaline solution to conduct ions between electrodes. They’re reliable and inexpensive but respond more slowly to fluctuating power inputs, which matters when paired with variable renewables.
Proton exchange membrane (PEM) electrolyzers use a solid polymer membrane instead of a liquid electrolyte. They can ramp up and down quickly, making them a better match for solar and wind power that varies throughout the day. The downside is higher cost, partly because they rely on precious metal catalysts.
Solid oxide electrolysis cells operate at much higher temperatures (typically 700 to 850°C). The heat dramatically boosts efficiency by reducing the electrical energy needed to split water. These systems can achieve far higher current densities than alkaline or PEM units, meaning more hydrogen per unit of electrode area. They’re best suited for applications where waste heat is available, such as industrial facilities or nuclear plants.
Regardless of the type, electrolysis requires about 9 kilograms of water for every kilogram of hydrogen produced, based purely on the chemistry. In reality, water consumption is higher because the input water needs purification and some is lost to cooling. This water demand is manageable in most regions but could become a limiting factor in arid areas if green hydrogen scales massively.
Solar-Driven Water Splitting
A newer approach skips the separate solar panel and electrolyzer entirely, using specialized materials that absorb sunlight and split water directly on their surface. The most advanced version, called a photovoltaic-electrochemical (PV-EC) system, has reached 32% solar-to-hydrogen efficiency in laboratory settings. Photoelectrochemical cells, which integrate the light-absorbing and water-splitting functions into a single device, have hit 19% efficiency. Only PV-EC systems have so far demonstrated the kind of long-term durability (over a decade) needed for practical deployment. These technologies are still largely in the research phase, but they represent a path toward simpler, potentially cheaper green hydrogen.
The Color-Coded Shorthand
The hydrogen industry uses a color system to quickly signal how hydrogen was made and what its carbon footprint looks like:
- Grey: Natural gas via SMR, no carbon capture. The cheapest and most common method today.
- Blue: Natural gas via SMR with carbon capture and storage. Lower emissions, but dependent on capture efficiency and methane leak rates.
- Brown/Black: Coal gasification without carbon capture. The most carbon-intensive route.
- Green: Water electrolysis powered by renewable electricity. Near-zero emissions but currently the most expensive.
- Pink/Purple: Electrolysis powered by nuclear energy. Low-carbon, with the steady power output that suits electrolyzers well.
Cost Differences Across Methods
Grey hydrogen from natural gas is the cheapest to produce, typically falling in the range of $1 to $2 per kilogram depending on regional gas prices. Blue hydrogen costs more because of the added carbon capture equipment, generally landing between $1.50 and $3 per kilogram. Green hydrogen remains the most expensive, with costs that have historically ranged from roughly $3 to $8 per kilogram, driven largely by the price of renewable electricity and electrolyzer hardware.
The gap is narrowing. Falling costs for solar and wind power, combined with larger and cheaper electrolyzers, are pulling green hydrogen costs down. Government subsidies and carbon pricing in several countries are simultaneously pushing fossil-based hydrogen costs up. Most energy analysts expect green hydrogen to become cost-competitive with grey hydrogen in favorable regions within the next decade, particularly in places with abundant, cheap solar or wind resources.
Why It Matters Which Method Wins
Hydrogen is used today mainly in oil refining and ammonia production for fertilizers. Its potential role is much larger: storing renewable energy, fueling heavy trucks and ships, replacing coal in steelmaking, and heating buildings. Whether hydrogen actually delivers climate benefits depends almost entirely on how it’s produced. Grey hydrogen from natural gas generates substantial CO₂. Green hydrogen from renewables generates virtually none. The production method is the whole ballgame, which is why the shift from fossil-based to electrolysis-based hydrogen is one of the most closely watched transitions in the energy sector.