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

Three ways to pull hydrogen from water or gas

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)⁸Steam 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 plant⁶SMR with CO2 capture
Natural gas plus steam → Hydrogen plus concentrated CO2 · 68.4% HHV, about 64% LHV (calculated), with 96.2% of CO2 captured⁶Autothermal reforming (ATR) with capture
Natural gas plus oxygen and steam → Hydrogen plus CO2 · 67.9% HHV, about 64% LHV (calculated), 94.5% captured⁶Methane 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

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 upstream⁴⁶	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%¹⁴	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 SMR⁴⁸	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	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

Global Hydrogen Review 2026: Production International Energy Agency, 2026
Global Hydrogen Review 2024: GHG emissions of hydrogen and its derivatives International Energy Agency, 2024
Comparison of Commercial, State-of-the-Art, Fossil-Based Hydrogen Production Technologies (costs in 2018 USD) US DOE National Energy Technology Laboratory, 2022
Recent Advances in Electrified Methane Pyrolysis Technologies for Turquoise Hydrogen Production (Rohani, Sudiiarova, Lyth, Badakhsh), Energies 18(9) 2393 MDPI, 2025
Technical Targets for Proton Exchange Membrane Electrolysis US Department of Energy, Hydrogen and Fuel Cell Technologies Office, 2023