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Green vs Blue vs Grey Hydrogen

Updated 21 September 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.

Grey, blue and green hydrogen production comparedThree production routes drawn side by side. Grey takes natural gas and steam into a reformer and releases carbon dioxide to the atmosphere along with the hydrogen. Blue uses the same reformer but adds a capture stage that sends most of the carbon dioxide to geological storage. Green passes electricity and purified water through an electrolyser, producing hydrogen and oxygen with no carbon in the process. All three produce the same hydrogen molecule.Greyreform natural gasnatural gas+ steamreformerCO2 to airH2Bluereform, then capturenatural gas+ steamreformercapturemost CO2to geological storagethe share that escapes captureH2Greensplit water with electricityelectricitypurified waterelectrolyserno carbon hereH2O2footprint = footprint of the electricitySame molecule, every timeTwo protons and two electrons. Nothing about the gasrecords how it was made, which is exactly why thecolours had to be invented.Grey and blue start from a molecule that already contains carbon, so carbon dioxide is a product of the chemistryitself, not a by-product of heating. Green starts from water, so its emissions are whatever the electricity broughtwith it — which can be very little, or a great deal.
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.

Steam methane reforming, step by stepProcess flow for steam methane reforming. Natural gas and steam enter a reformer heated by burners, where methane and water react to form carbon monoxide and hydrogen. The gas passes to a water-gas shift reactor where carbon monoxide and more steam form carbon dioxide and further hydrogen. A separation stage delivers pure hydrogen and a concentrated carbon dioxide stream, while the burners that heat the reformer emit a second, dilute exhaust.natural gas (CH4)steam (H2O)Reformercatalyst, hightemperatureburners supply the heatCO + H2Shift reactormore steam in,more hydrogen outCO2 + H2Separationpurify the hydrogenH2concentrated CO2 streamfrom the process itself: easier to capturedilute exhaustfrom the burners: harder to captureThe two reactionsCH4 + H2O → CO + 3H2reforming: strips hydrogen off methaneCO + H2O → CO2 + H2shift: converts the carbon monoxide, gaining one more H2The carbon entered as methane, so it has to leave as carbon dioxide: grey hydrogen's emissions come fromthe chemistry itself, not from a badly run plant.
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.
What blue hydrogen captures, and what escapes itA blue hydrogen plant with three carbon paths. The concentrated process stream of carbon dioxide is captured at a high rate and sent to geological storage. The dilute flue gas from the furnaces is harder to capture and is often partly released. Methane leaking upstream during extraction and transport never reaches the plant at all, so no capture equipment can act on it.Gas fieldand pipeline1. Methane leakagebefore the plant: no capture equipment can reach itReformerplus shift andseparation2. Dilute flue gaslow concentration, hard to captureH23. concentrated CO2Capturegeological storagepermanent only if it stays put, which is aquestion about the site, not the plantthe uncaptured shareno plant captures everythingTwo numbers decide what "blue" means:the capture rate, and the upstream leak rate.
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.

Inside a proton exchange membrane electrolyser cellCross-section of a PEM electrolyser cell. Purified water enters on the anode side, where it splits into oxygen gas, protons and electrons. The protons cross the solid polymer membrane to the cathode. The electrons cannot cross the membrane, so they travel through the external circuit from the power supply and recombine with the protons at the cathode to form hydrogen gas. Oxygen leaves on the anode side and hydrogen on the cathode side.anodecathode+−membranepasses protons, blocks gas and electronsH+H+H+H+protons cross the membranepower supplye−e−purified water inO2 outH2 outAt each electrodeanode: 2H2O → O2 + 4H+ + 4e−cathode: 4H+ + 4e− → 2H2Overall: water in, hydrogen and oxygen out. Thermodynamics sets a floor of about 39.4 kWh per kilogram of hydrogen.
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, 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.

What each conversion step costs in a hydrogen chainIllustrative flow diagram following one hundred units of electricity through a hydrogen round trip. 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, so only a small fraction of the original input is delivered. A second bar shows the same electricity used directly, where only transmission losses apply.Electricity → hydrogen → electricity100 units of electricity inafter electrolysislost as heat in the stackafter compression and storagedelivered againlost converting backThe same electricity used directlydelivered, less transmission lossesIllustrative proportions, not plant data: real efficiencies depend on the electrolyser, the storage pressure and theconversion device. The structural point holds regardless — each step is a toll, and a round trip pays several.This is why hydrogen is chosen where electrons cannot do the job, not where they can.
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.

Where hydrogen competes well, and where electricity winsA ladder of hydrogen applications ordered by how readily direct electrification can substitute. At the top sit uses with no electric alternative: ammonia and fertiliser, oil refining, and reducing iron ore for steel. In the middle sit high-temperature industrial heat, shipping and aviation fuels, and long-duration seasonal storage. At the bottom sit uses where a wire and a heat pump are markedly more efficient, such as home heating and short-range passenger transport.electrons can substitute more easilyHydrogen as a chemical feedstockAmmonia and fertiliser · oil refining · reducing iron ore for steelThese processes need hydrogen atoms, not energy in general. Electricity cannot substitute, so thecomparison here is with today's fossil-derived hydrogen, which is where the colours matter most.Hard to electrify, still contestedVery high process heat · shipping and aviation fuels · long-duration seasonal storageElectrification is difficult rather than impossible. Hydrogen's poor round-trip efficiency is weighed againstits ability to be stored in quantities and for durations that batteries cannot reach.Where a wire wins on physicsHome heating · short-range passenger transport · most stationary powerA heat pump moves several units of heat per unit of electricity; a hydrogen boiler converts one unit intoless than one, after paying for electrolysis and delivery first.The ladder is about substitutability, not enthusiasm. Hydrogen is strongest exactly where it is not competingwith electricity at all, and weakest where a cable would have done the job with fewer conversions.
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 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 21 September 2026. How we research and review

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