# Low-carbon hydrogen and e-fuels: Science and limits

Three ways to pull hydrogen from water or gas

Source page: https://www.materialinsights.org/energy/green_hydrogen_carriers/science/

What this tab covers What physically happens in the conversion, step by step, the science behind it, and the hard limits that physics sets on how well it can ever work.

Low-carbon hydrogen is made three main ways: electricity splits water; natural gas is reformed and the CO2 captured and stored; or methane is cracked into hydrogen and solid carbon. The world uses almost 100 Mt of hydrogen a year, but low-emissions output was only almost 1 Mt in 2025.

#### The conversion, step by step
**Water electrolysis**
Electricity plus water → Hydrogen plus oxygen · 61% LHV for a 2022 PEM system (55 kWh/kg)^[8](https://www.energy.gov/eere/fuelcells/technical-targets-proton-exchange-membrane-electrolysis)**Steam methane reforming (SMR), no capture**
Natural gas plus steam, heat at about 900 C → Hydrogen (via syngas and shift) · 75.4% HHV, about 71% LHV (calculated) in the NETL reference plant^[6](https://www.netl.doe.gov/projects/files/ComparisonofCommercialStateofArtFossilBasedHydrogenProductionTechnologies_041222.pdf)**SMR with CO2 capture**
Natural gas plus steam → Hydrogen plus concentrated CO2 · 68.4% HHV, about 64% LHV (calculated), with 96.2% of CO2 captured^[6](https://www.netl.doe.gov/projects/files/ComparisonofCommercialStateofArtFossilBasedHydrogenProductionTechnologies_041222.pdf)**Autothermal reforming (ATR) with capture**
Natural gas plus oxygen and steam → Hydrogen plus CO2 · 67.9% HHV, about 64% LHV (calculated), 94.5% captured^[6](https://www.netl.doe.gov/projects/files/ComparisonofCommercialStateofArtFossilBasedHydrogenProductionTechnologies_041222.pdf)**Methane pyrolysis**
Methane plus heat (about 1,500 C without catalyst; plasma 1,000-3,500 C) → Hydrogen plus solid carbon · At most about 60% of the methane's energy ends up in hydrogen (calculated: 4 kg methane per kg H2)^[7](https://www.mdpi.com/1996-1073/18/9/2393)

#### The science

Hydrogen is rarely found in bulk: it must be pulled out of water or hydrocarbons, which takes energy. In reforming, steam and methane react over a nickel catalyst (CH4 + H2O → CO + 3H2), absorbing heat; a shift reactor then turns CO into CO2 and more hydrogen. Overall CH4 + 2H2O → CO2 + 4H2, so half the hydrogen comes from the water, and the feed alone gives a fixed 5.5 kg of CO2 per kg of hydrogen (calculated), plus CO2 from the gas burned to heat the reformer. In an SMR that flue gas is dilute and costly to capture. An ATR burns part of the gas with pure oxygen inside the reactor, so nearly all the CO2 leaves in one concentrated stream. Pyrolysis breaks the C-H bonds without oxygen (CH4 → C + 2H2), leaving 3 kg of solid carbon per kg of hydrogen (calculated) instead of CO2, but only about 60% of the methane's energy goes into hydrogen. Electrolysis uses no fossil feed: its emissions depend on the electricity.

#### Limits set by science

| Limit | Value | Why it exists | Best today |
|---|---|---|---|
| **Feed carbon in reforming** | 5.5 kg CO2 per kg H2 from chemistry alone (calculated); 10-12 kg CO2-eq per kg for unabated gas including fuel and upstream^[4](https://www.iea.org/reports/global-hydrogen-review-2024/ghg-emissions-of-hydrogen-and-its-derivatives)^[6](https://www.netl.doe.gov/projects/files/ComparisonofCommercialStateofArtFossilBasedHydrogenProductionTechnologies_041222.pdf) | Stoichiometry fixes the feed CO2; only capture can remove it. | Modelled capture of 94.5-96.2% |
| **Rising cost of deep capture** | Abatement about USD 60-85 per t CO2 at 55-70% capture; USD 85-110 above 90%^[1](https://www.iea.org/reports/global-hydrogen-review-2026/production)^[4](https://www.iea.org/reports/global-hydrogen-review-2024/ghg-emissions-of-hydrogen-and-its-derivatives) | The concentrated process CO2 is captured first; the last tonnes come from dilute flue gas. | One large gas-with-capture project started in 2025 |
| **Electricity carbon threshold** | Grid below about 200-240 g CO2 per kWh to beat unabated SMR^[4](https://www.iea.org/reports/global-hydrogen-review-2024/ghg-emissions-of-hydrogen-and-its-derivatives)^[8](https://www.energy.gov/eere/fuelcells/technical-targets-proton-exchange-membrane-electrolysis) | At 55 kWh/kg, a 200 g/kWh grid gives about 11 kg CO2 per kg H2 (calculated), the same as gas reforming. | Building the renewable plants embeds 0.4-2.7 kg CO2-eq per kg H2 |
| **Pyrolysis carbon by-product** | 3 kg solid carbon per kg H2 (calculated)^[7](https://www.mdpi.com/1996-1073/18/9/2393) | Carbon black sales help, but at scale the carbon would swamp those markets. | Plasma, microwave, resistive and molten-metal reactors at pilot to early commercial scale |

#### Sources

1. [Global Hydrogen Review 2026: Production](https://www.iea.org/reports/global-hydrogen-review-2026/production) International Energy Agency, 2026
2. [Global Hydrogen Review 2024: GHG emissions of hydrogen and its derivatives](https://www.iea.org/reports/global-hydrogen-review-2024/ghg-emissions-of-hydrogen-and-its-derivatives) International Energy Agency, 2024
3. [Comparison of Commercial, State-of-the-Art, Fossil-Based Hydrogen Production Technologies (costs in 2018 USD)](https://www.netl.doe.gov/projects/files/ComparisonofCommercialStateofArtFossilBasedHydrogenProductionTechnologies_041222.pdf) US DOE National Energy Technology Laboratory, 2022
4. [Recent Advances in Electrified Methane Pyrolysis Technologies for Turquoise Hydrogen Production (Rohani, Sudiiarova, Lyth, Badakhsh), Energies 18(9) 2393](https://www.mdpi.com/1996-1073/18/9/2393) MDPI, 2025
5. [Technical Targets for Proton Exchange Membrane Electrolysis](https://www.energy.gov/eere/fuelcells/technical-targets-proton-exchange-membrane-electrolysis) US Department of Energy, Hydrogen and Fuel Cell Technologies Office, 2023

## Other sections

- [Summary](https://www.materialinsights.org/energy/green_hydrogen_carriers/index.md)
- [Energy overview](https://www.materialinsights.org/energy/green_hydrogen_carriers/energy/index.md)
- [Correlation](https://www.materialinsights.org/energy/green_hydrogen_carriers/correlation/index.md)
- [Value chain](https://www.materialinsights.org/energy/green_hydrogen_carriers/valuechain/index.md)
- [Unit economics](https://www.materialinsights.org/energy/green_hydrogen_carriers/economics/index.md)
- [Engineering methods](https://www.materialinsights.org/energy/green_hydrogen_carriers/methods/index.md)
- [Equipment and suppliers](https://www.materialinsights.org/energy/green_hydrogen_carriers/equipment/index.md)
- [Overview](https://www.materialinsights.org/energy/green_hydrogen_carriers/overview/index.md)
- [Production route](https://www.materialinsights.org/energy/green_hydrogen_carriers/routes/index.md)
- [Production](https://www.materialinsights.org/energy/green_hydrogen_carriers/production/index.md)
- [Feedstocks](https://www.materialinsights.org/energy/green_hydrogen_carriers/feedstocks/index.md)
- [Cycles](https://www.materialinsights.org/energy/green_hydrogen_carriers/cycles/index.md)
- [Trading](https://www.materialinsights.org/energy/green_hydrogen_carriers/trading/index.md)
- [Movement](https://www.materialinsights.org/energy/green_hydrogen_carriers/movement/index.md)
- [Regulation](https://www.materialinsights.org/energy/green_hydrogen_carriers/regulation/index.md)
- [Characteristics](https://www.materialinsights.org/energy/green_hydrogen_carriers/characteristics/index.md)
- [Emissions](https://www.materialinsights.org/energy/green_hydrogen_carriers/emissions/index.md)
- [Relationships](https://www.materialinsights.org/energy/green_hydrogen_carriers/links/index.md)