Summary

Although hydrogen is the most abundant element in the universe, it rarely exists in pure form on Earth and must therefore be manufactured prior to use in industrial applications. This article provides a high-level overview of hydrogen production processes and cross-references them with the colour coding system commonly encountered in sustainability discourse.

Definitions


E	:	Specific Exergy $(kJ/kg)$
LHV	:	Lower Heating Value $(kJ/kg)$

Introduction

Hydrogen is the most abundant element in the universe, yet it rarely exists in pure form on Earth. Being highly reactive, it combines with other elements to form compounds such as water and methane. Therefore, in order to utilise Hydrogen for applications such as ammonia production or as an alternative fuel source, it must first be manufactured.

Due to hydrogen’s simple structure it can be produced from abundant resources (e.g. water) using sustainable energy, and is therefore considered an important energy carrier for economic decarbonisation. However, the degree to which hydrogen can enable decarbonisation is subject to its method of production and future technological development.

Hydrogen production processes differ along three primary vectors:

Feedstock - Molecules containing hydrogen such as water, natural gas, coal, biomass, organic waste or geological deposits.
Conversion mechanism - process technologies for liberating hydrogen from the feedstock such as reforming, gasification, pyrolysis, electrolysis, thermochemical cycling or biological conversion.
Energy source - The energy input to drive the conversion mechanism such as electricity, thermal heat or chemical energy

The permutation of these vectors can be distilled into the primary differentiator - the chemical process pathways:

Thermochemical
Electrochemical
Biological
Photocatalytic
Photoelectrochemical

Photocatalytic and Photoelectrochemical are emerging production processes, still in their infancy, and while listed above for completeness are not subsequently considered in this article.

Production Processes and Sustainability

Each hydrogen production technology involves trade-offs among economic, environmental, and availability objectives¹.

Given hydrogen’s potential in decarbonisation, and the broader societal interest in hydrogen, production processes are typically referenced using a colour-coding scheme. There are no universally agreed colour-coding definitions; however, they are intended to intuitively denote sustainability, with green hues indicating high sustainability and shades of grey as sustainability potential decreases.

The following sections consider each production method using an engineering taxonomy and cross-reference processes with the colour-coding system where available. However, colour-coding of hydrogen production processes is ambiguous in relation to a process’s true sustainability, and engineering measures such as exergy and carbon intensity should be the primary considerations.

Production Process Taxonomy

Thermochemical

Thermochemical processes use heat to promote chemical reactions that release hydrogen from feedstock that can be organic (typically fossil fuels) or inorganic (typically water). Thermochemical processes are currently used for majority of the world’s hydrogen production capacity.

Feedstocks

Fossil Fuel

Fossil fuel-based thermochemical processes use coal or natural gas to produce hydrogen². Here hydrogen is liberated from a fossil fuel, and carbon oxides are produced as by-products. These carbon-oxides contain greenhouse gases (e.g. $CO_2$ ), which if not captured and sequestered during production, are vented into the atmosphere and contribute to climate change.

Fossil fuel-based Hydrogen production methods account for approximately 99% of hydrogen production. In 2023, Hydrogen production had a cost of 920 Mt of CO2eq emissions per annum which was approximately 2.1% of total global CO2eq emissions.

Non-fossil Fuel

Non-fossil fuel-based thermochemical methods predominantly utilise water thermolysis; the thermal dissociation (splitting) of water. The downside of these methods is that extreme temperatures - typically above 2,500K - are required. While there are chemically assisted processes that operate at relatively lower temperatures such as the Sulphur-Iodine (S-I cycle), the absolutes temperatures required by this processes are still high (~1200K) and limit the practicality in providing sufficient heat to operate the process in a sustainable way.

Key Processes

Gasification - Converts water and carbon-rich feedstocks such as coal or biomass into synthesis gas (syngas). The syngas is a mixture of hydrogen and carbon monoxide which is than enriched using a water gas shift reaction from which the hydrogen is subsequently separated using a purification process such as pressure swing adsorption.

Steam Methane Reforming (SMR) - Liberates hydrogen from methane by heating methane (from natural gas) with steam in the presence of a catalyst to produce a mixture of carbon dioxide and hydrogen. SMR can also utilise naphtha as a feedstock; however, using naphtha introduces process complexity and produces more carbon oxides.

Pyrolysis - Thermally decomposes methane in the absence of oxygen to produce hydrogen and solid carbon. Given the process is anaerobic, decomposition products don’t oxidise (i.e., carbon doesn’t react to form carbon oxides) and minimal greenhouse gas emissions are produced from the process.

Colour Coding

The following colour codes are often used as synonyms for thermochemical processes:

Black - Hydrogen produced by gasification with bituminous (black) coal feedstock.
Brown - Hydrogen produced by gasification with lignite (brown) coal feedstock.
Grey - Hydrogen produced by SMR with natural gas feedstock
Blue - Hydrogen produced using the SMR process (grey hydrogen) but with the addition of a Carbon Capture and Storage (CCS) or Carbon Capture and Utilisation (CCU) technology to reduce carbon emissions³.
Turquoise - Hydrogen produced via pyrolysis with methane feedstock. The heat source will dictate whether the Turquoise is more blue or green - providing heat via renewable or waste heat will make it greener, while gas-fired heaters will make it bluer.
Red - Hydrogen produced using heat from Nuclear reactors for high-temperature catalytic splitting of water molecules.
White - Hydrogen produced by naturally occurring geochemical processes within the Earth’s crust. Estimates claim approximately 23 million tonnes of white hydrogen is generated per annum. However, natural hydrogen does have challenges around geographic availability and costs of extraction and purification.

Additionally, there are several processes that are yet to have colour code consensus:

Biomass gasification - Produced from biomass using gasification. This is similar to black and brown hydrogen, with the exception that the biomass could be from a renewable resource, improving sustainability over these processes.
Solar thermochemical hydrogen - Similar to red hydrogen but generates heat through concentrated solar power rather than nuclear reactors. This method currently has efficiency and reliability issues due to the extreme temperature cycling of the reactor.

Electrochemical

Electrochemical processes use an electric current to dissociate hydrogen ions from water molecules, which are later separated and combined to form hydrogen gas. The key point of interest is that electrochemical processes can produce hydrogen from electricity and water, two inputs that can be viewed as abundant and sustainable⁴. Therefore, electrolysis can provide a pathway to achieving security and sustainability energy goals.

Key Processes

Electrolysis - Converts water into hydrogen and oxygen by applying a direct current. Electrolysis is primarily conducted in an electrolyser, of which for Hydrogen production there are four primary designs - Alkaline (AEL), Proton Exchange Membrane (PEM), Anion Exchange Membrane (AEM) and Solid Oxide (SOEC). The differences in design philosophy dictate the capital and operating costs as well as expandability and operating flexibility (e.g. ability to adapt to electricity supply variability).

Electrochemical Processes and their Colour Coding

Given their potential in providing renewable hydrogen, electrochemical production processes utilise a brighter palette then thermochemical processes:

Green - Hydrogen produced using electrolysis powered by renewable electricity sources such as wind, solar, hydro, tidal or geothermal.
Yellow - A sub-category of green hydrogen in which the renewable energy source is solar radiation (via photovoltaic or solar thermal power plants). However, note that the label ‘yellow’ is occasionally used to refer to hydrogen produced by electrolysis using electricity for the grid (which may or may not be supplied by renewable sources).
Pink - Hydrogen produced using electrolysis powered by electricity provided by nuclear power plants exclusively.
Purple - Hydrogen produced using high-temperature steam electrolysis where heat for vaporising water and electricity to drive electrolysis is provided by nuclear reactors. By using heat to vaporise the water to steam, the electricity requirements for electrolysis are lowered by approximately 35%.

Biological

Biological methods use microorganisms to produce hydrogen from organic matter. There are several biological processes for producing hydrogen, including photobiological, microbial biomass conversion and dark fermentation.

Biological processes although promising are still in the experimental phases, and are typically difficult to achieve economic yield rates at scale.

Key Processes

Microbial Electrolysis - Bacteria breakdown (oxidise) organic matter, releasing electrons and protons in the processes, that when supplemented with a small external voltage to overcome thermodynamic barriers and drive the electrolysis. The energy consumption of microbial electrolysis is relatively low compared to standard electrolysis due to bacteria assisting in the breaking of chemical bonds. Additionally, this process has demonstrated high efficiencies in laboratory settings. However, costs of the electrode materials, maintaining an anaerobic environment and managing the microbial population add complexity to this process.

Photobiological Processes - Photobiological processes use microorganisms such as algae to convert solar energy into hydrogen. Depending on the organism, hydrogen molecules can be produced directly (biophotolysis) or indirectly through photofermentation of organic substrates. The cost of producing hydrogen using algae-based methods varies widely depending on several factors, such as the type of photobioreactor used, the cultivation method, and the downstream processing required. Generally, the cost of producing hydrogen using algae-based methods is higher than traditional methods, such as steam methane reforming.

Evaluating Production Methods

Evaluating hydrogen production methods is complex and requires consideration of multiple parameters across technical, economic, and environmental dimensions. Evaluation criteria can include hydrogen yield, energy and exergy efficiency, production cost, greenhouse gas emissions, technology readiness level, feedstock availability, scalability, and operational flexibility.

Energy and thermodynamic efficiency measures, such as LHV and exergy efficiency, respectively, are the best comparison methods for evaluating hydrogen production methods. However, a comprehensive techno-economic analysis needs to be undertaken for a specific production scenario, as such analyses are sensitive to system boundary conditions and do not account for input availability or scalability.

Lower Heating Value (LHV)

The Lower Heating Value (LHV) represents the amount of energy released during combustion when the water produced remains in the vapour phase (i.e. excludes the latent heat of condensation). Since hydrogen is typically consumed in systems where the combustion products are exhausted in the vapour phase, the LHV provides a useful practical basis for comparison of hydrogen production efficiencies.

The LHV efficiency is calculated as the ratio of the chemical energy contained in hydrogen to the total energy supplied to the production process. It is a straightforward measure of how input energy is converted into usable fuels and the most widely reported performance metric.

However, LHV cannot identify thermodynamic losses arising from irreversibilities within the process and obscures differences in how effectively each process utilises energy inputs.

Exergy Efficiency

The exergy efficiency is a measure of how a process converts the useful energy of its inputs, which may be electrical, thermal, or chemical energy, into the useful energy in its products, which, in the case of hydrogen, is primarily chemical energy. Exergy efficiency is regarded as one of the most comprehensive metrics for assessing thermodynamic performance and process improvement potential.

Exergy efficiency analysis is based on the first and second laws of thermodynamics and considers the degradation of energy quality as a result of a process. Therefore, exergy efficiency recognises that not all energy has the same ability to perform useful work, and identifies where work potential is destroyed. For example, 1 MJ of electrical energy contains approximately 1 MJ of exergy because electricity can be converted entirely to useful mechanical work. However, 1 MJ of low-temperature heat will contain less than 1 MJ of exergy because only a fraction of the thermal energy can be converted into work.

As it evaluates all forms of energy on the common basis of useful work potential, exergy analysis enables fair comparisons between processes with varying operating conditions and that utilise different energy sources. Furthermore, exergy destruction can be quantified for individual process units, allowing optimisation of components that contribute to inefficiency.

LHV vs Exergy

Key Difference	Exergy Efficiency	LHV Efficiency
What it measures	Maximum useful work potential	Total energy content (heat value)
Accounts for energy quality?	Yes	No - all energy counted equally
Penalizes irreversibilities by quality?	Yes - destruction of useful work potential	No - only quantity of energy loss
Identifies where losses occur?	Yes - exergy destruction in each scope	No - only overall energy balance
Best use	Process improvement, technology comparison	Simple benchmarking, energy balance, energy carrier comparison

Summary of Hydrogen Production Processes

A summary of the hydrogen production processes is reported below. Data such as GHG intensity and the LHV and exergy efficiencies should be considered as representative. These metrics vary significantly based on plant configuration, energy sources, and system-boundary scoping.

Process	Principal inputs	LHV efficiency	Exergy efficiency	Indicative production-cost⁵	Representative GHG intensity ( $kgCO_2e/kgH_2$ )	Commercial status and key qualification
Coal gasification	Coal, oxygen and steam	60%⁶	50.9% (reactor level)⁷	Low	22–26 without carbon capture⁸	Commercial and emissions-intensive; Costs dependent on coal price, plant location and carbon policy
Steam methane reforming (SMR)	Natural gas and steam	76%⁶	78.9% (reactor level)⁷	Low	10–12 without carbon capture⁸	Mature and widely deployed; emissions include both the reforming process and energy supply
SMR with carbon capture and storage (CCS)	Natural gas, steam and energy for capture	69% with a 95% capture assumption⁶	~65%⁹	Moderate	3-5 assuming > 90% capture $CO_2$ rates	Total lifecycle abatement must be assessed rather than inferred from the capture rate alone
Methane pyrolysis	Methane and high-temperature heat or electricity	35-38% when considering only hydrogen as a product	46.9% when hydrogen is the useful product; potentially about 80% when solid carbon is recovered as a useful coproduct⁷	Moderate	2-3 for plasma process¹⁰	Emerging; durable handling or use of the solid-carbon coproduct is essential
Biomass gasification	Dry biomass, oxygen and/or steam	46%¹¹	45-50%¹²	Moderate	Feedstock- and boundary-specific; may be low or net-negative only when sustainable biomass and durable $CO_2$ storage are demonstrated	Pilot/development pathway for dedicated hydrogen production; feedstock logistics are important constraints
Thermochemical water splitting	Water and concentrated solar or nuclear heat	~32.5%¹³	31.8% ¹³	High to Very High	Potentially low, but dependent on heat source, materials and plant construction ¹⁴	Research and demonstration; high-temperature materials, reactor durability and heat cycling remain challenges
Low-temperature water electrolysis (PEM example)	Purified water and electricity	61% LHV efficiency¹⁵	87.9% (reactor level)⁷	High¹⁵	0.4–2.7 with renewable electricity⁸	Commercial; actual cost and emissions are dominated by electricity price, utilisation and generation mix
High-temperature solid-oxide electrolysis (SOEC)	Steam, electricity and high-temperature heat	80-85%¹⁶	73-78%¹⁶	High	0.3-3.3 depending on the electricity and heat sources	Emerging commercial technology; offers high efficiency where suitable high-temperature heat and steady operation are available
Microbial electrolysis	Organic matter or wastewater, microorganisms and a small electrical input	Bench-scale results are not yet representative of commercial systems	No representative commercial value	Very High	Feedstock-, electricity- and treatment-boundary-specific; $CO_2$ is produced when organics are oxidised	Early research and development; scale-up, electrode cost and microbial stability remain barriers¹⁷
Photobiological production	Water and sunlight; some pathways also consume organic substrates	No robust commercial benchmark	No representative commercial value	Very High	Potentially low, but dependent on cultivation, reactor energy and downstream purification	Early research; low production rates, low solar-to-hydrogen efficiency and oxygen inhibition are key challenges¹⁸

Notes

This is a restatement of the energy trilemma in which an energy source makes tradeoffs between energy security, affordability and sustainability objectives. ↩︎
Methods utilising methane as a feedstock can substitute fossil fuels for a ‘more’ sustainable sources of methane, such as biogas from anaerobic digestion of organic waste. However, this introduces additional expenses and impacts unit economics. ↩︎
One study suggests that total CO2 emissions blue hydrogen were only 9-12% lower than grey hydrogen due to increased fugative methane emissions from increased natural gas usage to power the carbon capture. ↩︎
While we often talk about renewables having negligible emissions this is only in the context of operation. Significant emissions can be produced during the manufacture of the components i.e. steel, membranes, cathodes etc ↩︎
Levelised production costs are specific to many factors. Indicative costs are based on ranges of reported studies, with low being < USD $2.5/\mathrm{kg\,H_2}$ ; Moderate being < USD $4/\mathrm{kg\,H_2}$ ; high < USD $6/\mathrm{kg\,H_2}$ and Very High > USD $6/\mathrm{kg\,H_2}$ ↩︎
IEA 2020 cost model assumptions are reported on an LHV basis and use 60% efficiency for coal gasification, 76% for SMR without CCS, and 69% for SMR with CCS; the CCS case assumes a 95% capture rate: Global average levelised cost of hydrogen production by energy source and technology, 2019 and 2050. ↩︎ ↩︎ ↩︎
Exergy efficiency is defined here as $\eta_{ex}=Ex_{out}/Ex_{in}$ , where $Ex_{out}$ is the exergy recovered in the selected useful products and $Ex_{in}$ is the chemical exergy of the reactants plus the exergy supplied as heat. Reactor-level values and operating assumptions are from Megía et al., Comparative Analysis of Energy and Exergy Performance of Hydrogen Production Methods, Entropy 22 (2020), 1286. The model excludes most pumps and compressors and does not include upstream production of electricity or feedstocks. Its generic water-electrolysis result is not specific to PEM technology. For methane pyrolysis, treating solid carbon as a recoverable coproduct changes the useful-product boundary and substantially increases the result. ↩︎ ↩︎ ↩︎ ↩︎
International Energy Agency, GHG emissions of hydrogen and its derivatives, Global Hydrogen Review 2024. Values cover lifecycle emissions to the production gate; the system boundary should be stated whenever values from other sources are compared. ↩︎ ↩︎ ↩︎
65% exergy efficiency modelled by Fan et al. ↩︎
GHG intensity varies significantly for 0.5-1.5 $kgCO_2e/kgH_2$ for low-carbon electricity and carefully sourced gas to 3-5 $kgCO_2e/kgH_2$ for carbon intensive electricity with fugitive methane emissions. Ng et al reported 2.78 $kgCO_2e/kgH_2$ for plasma-arc methane pyrolysis with natural gas feedstock. ↩︎
US Department of Energy, Technical Targets for Hydrogen Production from Biomass Gasification. The DOE explicitly identifies these as H2A model results rather than operating-plant observations. ↩︎
Iribarren et al state exergy efficiency of the plant as 48%. Toonssen et al62-66% with heat recovery. ↩︎
32.5% LHV energy efficiency and 31.8% exergy efficiency for an Aspen Plus-modelled Cu–Cl cycle driven by recovered cement-slag heat, integrated with a reheat Rankine cycle and hydrogen compression to 750 bar presented by shaq, Dincer and Naterer. ↩︎ ↩︎
US Department of Energy, Hydrogen Production: Thermochemical Water Splitting. ↩︎
US Department of Energy, Technical Targets for Proton Exchange Membrane Electrolysis. ↩︎ ↩︎
Representative optimised standalone case: 85.15% LHV and 83.41% exergy; approximately 79% and 78%, respectively, when high-pressure hydrogen compression is included. See AlZahrani and Dincer. ↩︎ ↩︎
US Department of Energy, Hydrogen Production: Microbial Biomass Conversion. ↩︎
US Department of Energy, Hydrogen Production: Photobiological. ↩︎

Hydrogen Production Methods

Summary

Definitions

Introduction

Production Processes and Sustainability

Production Process Taxonomy

Thermochemical

Feedstocks

Fossil Fuel

Non-fossil Fuel

Key Processes

Colour Coding

Electrochemical

Key Processes

Electrochemical Processes and their Colour Coding

Biological

Key Processes

Evaluating Production Methods

Lower Heating Value (LHV)

Exergy Efficiency

LHV vs Exergy

Summary of Hydrogen Production Processes