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Green vs Blue vs Grey Hydrogen

Updated 8 October 20267 min readNext-Gen & Off-Grid

The colours describe how the hydrogen was produced, not the gas itself: every H2 molecule is identical. Grey hydrogen comes from reforming natural gas and releases the resulting carbon dioxide. Blue hydrogen uses the same process but captures most of that CO2. Green hydrogen splits water using electricity, so its emissions are those of the electricity.

Key takeaways

  • The colour is a label for the production route. Chemically, the product is the same gas in every case.
  • Grey hydrogen's carbon dioxide is not a combustion by-product bolted on at the end — it comes out of the chemistry itself, because the carbon in methane has to go somewhere.
  • Blue is grey plus capture, so its footprint depends on two things the label does not state: how much CO2 is actually captured, and how much methane leaked upstream.
  • Green hydrogen's emissions are the emissions of the electricity that made it. Made with fossil electricity it can be worse than grey.
  • Hydrogen is an energy carrier, not a source. Every conversion step costs energy, which is why it earns its place where electrons cannot go rather than where they can.
On this page
  1. The colours describe the process, not the gas
  2. Grey: reforming natural gas
  3. Blue: the same plant, plus capture
  4. Green: splitting water with electricity
  5. The energy ledger
  6. Where hydrogen earns its place

The colours describe the process, not the gas

There is no green molecule and no grey molecule. Hydrogen is H2 — two protons, two electrons, the lightest and simplest chemistry there is. The colour code is industry shorthand for how it was made, and therefore for what was emitted, consumed and left behind along the way.

That distinction matters because it moves the argument to the right place. Comparing colours is not comparing fuels; it is comparing supply chains that happen to end in the same gas.

Three hydrogen production routes side by side. Grey takes natural gas and steam and releases carbon dioxide to the atmosphere. Blue uses the same process with a carbon capture stage that sends most of the carbon dioxide to storage. Green uses electricity and water in an electrolyser, producing hydrogen and oxygen with no carbon in the process at all.Three hydrogen production routes side by side. Grey takes natural gas and steam and releases carbon dioxide to the atmosphere. Blue uses the same process with a carbon capture stage that sends most of the carbon dioxide to storage. Green uses electricity and water in an electrolyser, producing hydrogen and oxygen with no carbon in the process at all.
Same product, three supply chains. The carbon in grey and blue comes from the feedstock itself; green has no carbon in the process to begin with.

Grey: reforming natural gas

Most hydrogen made today comes from steam methane reforming. Methane and steam are reacted over a catalyst at high temperature:

  • Reforming: CH4 + H2O → CO + 3H2
  • Water-gas shift: CO + H2O → CO2 + H2

Look at where the carbon goes. It entered as methane and leaves as carbon dioxide — not because something was burned carelessly, but because the process pulls hydrogen off the methane and the carbon must end up somewhere. A second source of CO2 comes from burning fuel to supply the heat the reaction needs, which is a separate, more dilute exhaust stream.

Process flow diagram of steam methane reforming. Natural gas and steam enter a reformer heated by burners, producing a syngas of carbon monoxide and hydrogen. A water-gas shift reactor converts the carbon monoxide with more steam into carbon dioxide and additional hydrogen, and a separation stage delivers pure hydrogen while carbon dioxide leaves as a concentrated process stream and the burners emit a second, dilute exhaust.Process flow diagram of steam methane reforming. Natural gas and steam enter a reformer heated by burners, producing a syngas of carbon monoxide and hydrogen. A water-gas shift reactor converts the carbon monoxide with more steam into carbon dioxide and additional hydrogen, and a separation stage delivers pure hydrogen while carbon dioxide leaves as a concentrated process stream and the burners emit a second, dilute exhaust.
Two streams of carbon dioxide leave the plant: a concentrated one from the process itself, and a dilute one from the furnaces that supply the heat.

Blue: the same plant, plus capture

Blue hydrogen is grey hydrogen with carbon capture attached. The chemistry is unchanged; what is added is the equipment to separate CO2 and send it to permanent storage.

Two details decide what that label is worth, and neither is contained in the word "blue":

  1. What fraction is actually captured. The concentrated process stream is comparatively straightforward to capture. The dilute furnace exhaust is much harder, and a plant that captures only the easy stream leaves a substantial share of its emissions uncaptured.
  2. What leaked upstream. Methane that escapes during extraction and transport never reaches the plant, so no capture equipment can touch it. Since methane is itself a potent greenhouse gas, upstream leakage can dominate the footprint of an otherwise well-captured plant.
Diagram of a blue hydrogen plant showing two carbon dioxide streams. The concentrated process stream is captured at high rates and sent to geological storage. The dilute furnace flue gas is harder to capture and is often partly released. A third arrow shows methane leaking upstream during extraction and transport, before the plant, where capture equipment cannot reach it.Diagram of a blue hydrogen plant showing two carbon dioxide streams. The concentrated process stream is captured at high rates and sent to geological storage. The dilute furnace flue gas is harder to capture and is often partly released. A third arrow shows methane leaking upstream during extraction and transport, before the plant, where capture equipment cannot reach it.
Capture rate and upstream leakage are the two numbers that decide what blue hydrogen actually delivers, and neither is implied by the colour.

Green: splitting water with electricity

Green hydrogen skips carbon entirely. An electrolyser passes a direct current through water: hydrogen forms at the cathode, oxygen at the anode, and the only material inputs are purified water and electricity.

2H2O → 2H2 + O2

The thermodynamics set a hard floor. Producing a kilogram of hydrogen requires at least about 39.4 kWh of energy if the water is supplied as liquid and the products brought back to ordinary conditions — that figure is the hydrogen's higher heating value, and no electrolyser beats it. Real systems use more, because of resistive losses, the overpotentials needed to drive the reactions at a useful rate, and the auxiliary equipment around the stack.

Cross-section of a proton exchange membrane electrolyser cell. Purified water enters at the anode where it splits into oxygen, protons and electrons. Protons cross the membrane to the cathode while electrons travel the external circuit from the power supply, recombining at the cathode to form hydrogen gas. Oxygen leaves at the anode side and hydrogen at the cathode side.Cross-section of a proton exchange membrane electrolyser cell. Purified water enters at the anode where it splits into oxygen, protons and electrons. Protons cross the membrane to the cathode while electrons travel the external circuit from the power supply, recombining at the cathode to form hydrogen gas. Oxygen leaves at the anode side and hydrogen at the cathode side.
The membrane passes protons and blocks gas, so hydrogen and oxygen leave as separate streams. Electrons take the external route, which is where the electricity does its work.

Three stack technologies dominate. Alkaline electrolysers are the long-established workhorse, using a liquid potassium hydroxide electrolyte. PEM cells use a solid polymer membrane, respond quickly to changing power and suit a variable renewable supply. Solid oxide cells run hot and can reach higher efficiency by taking part of the energy as heat rather than electricity, at the price of thermal cycling.

That responsiveness matters more than it sounds. An electrolyser paired with solar or wind must follow a supply that varies by the minute and the season — wind output, for one, rises with the cube of wind speed until the turbine deliberately caps it — which is the same intermittency problem faced by storage on the electricity side.

The energy ledger

Hydrogen is an energy carrier, not an energy source. Every step in the chain — electrolysis, compression or liquefaction, transport, and conversion back to useful work — takes its share.

Flow diagram tracing one hundred units of electricity through a hydrogen chain: electrolysis loses roughly a third, compression and storage take a further share, and converting hydrogen back to electricity in a fuel cell loses about half of what remains, leaving a fraction of the original input as delivered electricity. A parallel arrow shows the same electricity used directly, with only small transmission losses.Flow diagram tracing one hundred units of electricity through a hydrogen chain: electrolysis loses roughly a third, compression and storage take a further share, and converting hydrogen back to electricity in a fuel cell loses about half of what remains, leaving a fraction of the original input as delivered electricity. A parallel arrow shows the same electricity used directly, with only small transmission losses.
Illustrative proportions, not plant data. The lesson is structural: each conversion is a toll, and a round trip through hydrogen pays several of them.
The three routes compared by process
GreyBlueGreen
FeedstockNatural gas and steamNatural gas and steamPurified water
Energy inputHeat from burning fuelHeat from burning fuel, plus capture energyElectricity
Where the carbon comes fromThe methane molecule itselfThe same, mostly capturedNo carbon in the process
Main emission riskCO2 released directlyUncaptured share plus upstream methane leakageThe emissions of the electricity used
Plant responsivenessSteady, continuous operationSteady, continuous operationCan follow a variable supply, particularly PEM
Also producesWaste heatWaste heat, CO2 for storageOxygen, waste heat

Qualitative comparison. Emissions intensities vary widely by plant, by capture rate and by the electricity mix; the IEA publishes ranges, and any specific figure should come from the plant or the study being cited, not from the colour.

Where hydrogen earns its place

The energy ledger explains the pattern in how hydrogen is actually used. Wherever electricity can do the job directly, converting it into hydrogen first and back again later means paying two conversion tolls for nothing. Wherever the job needs a chemical reducing agent, a dense chemical feedstock, or very high process heat, electrons cannot substitute at all — and there hydrogen is not competing with electrification, it is competing with fossil feedstock. Home heating shows the gap most starkly: burning hydrogen returns at most its own energy content as heat, while a heat pump moves several units of heat for each unit of electricity, so the same electricity warms a building far more effectively through a wire than through an electrolyser and a boiler.

A ladder of hydrogen applications ordered by how well direct electrification can substitute. At the top, uses with no electric alternative such as ammonia production, oil refining and steel reduction. In the middle, high-temperature industrial heat and long-duration seasonal storage. At the bottom, uses where direct electricity is markedly more efficient, such as home heating and short-range passenger transport.A ladder of hydrogen applications ordered by how well direct electrification can substitute. At the top, uses with no electric alternative such as ammonia production, oil refining and steel reduction. In the middle, high-temperature industrial heat and long-duration seasonal storage. At the bottom, uses where direct electricity is markedly more efficient, such as home heating and short-range passenger transport.
Ordered by whether electrons can do the job instead. The top of the ladder is chemistry, where hydrogen is the feedstock; the bottom is where a wire and a heat pump win on physics.

Seasonal storage sits awkwardly in the middle: the round-trip efficiency is poor, but hydrogen can be stored in quantities and for durations that electrochemical batteries cannot approach, which is a different problem from the one solved by pumped hydro. And because the whole case for green hydrogen rests on the electricity behind it, the question of what a solar or wind system does with surplus output — store it, curtail it, or convert it — is the same question seen from the other end, as do solar panels work on cloudy days shows from the supply side.

Frequently asked questions

Is green hydrogen always the cleanest option?

Only if the electricity is clean. Electrolysis simply transfers the emissions of the electricity into the hydrogen, and because the process consumes far more energy than the hydrogen ends up carrying, grid electricity with a high fossil share can produce hydrogen with a larger footprint than reforming gas directly.

What is the difference between blue and grey hydrogen in practice?

The plant is largely the same. Blue adds equipment to capture carbon dioxide, most readily from the concentrated process stream and with more difficulty from the dilute flue gas of the furnaces. The share captured, and what happens to the CO2 afterwards, is what the colour is claiming.

Why not just burn hydrogen for home heating?

You can, but the energy ledger is unkind. Making hydrogen from electricity, moving it and burning it delivers a fraction of the heat that the same electricity would deliver through a heat pump, which moves several units of heat per unit of electricity rather than converting one to less than one.

Does hydrogen production need pure water?

Electrolysers need demineralised water, so seawater or river water requires treatment first. The quantity is modest compared with the energy involved, but the purity requirement is real and part of siting a plant.

What are the other hydrogen colours?

Black and brown refer to hydrogen from coal gasification. Turquoise refers to methane pyrolysis, which produces solid carbon rather than CO2. Pink or purple refers to electrolysis powered by nuclear electricity. All of them describe the input and the process, never the gas.

Sources

Named organisations whose published material underpins this article. Where no link is given, the source is named rather than linked.

Editorial Team

Research, drafting and review

Articles are drafted from primary engineering and physics references with AI-assisted tools, then reviewed and fact-checked line by line by a human editor before publication. We publish explanations, not recommendations: no products, no pricing, no country-specific rules, and no invented author personas.

Last reviewed 8 October 2026. How we research and review

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