With global energy demand rising, hydrogen, as a low-carbon energy carrier, is poised to become a cornerstone of the energy transition, offering a versatile, clean alternative with significant potential to reduce emissions toward net zero. Fig. 1 shows the global hydrogen production from renewables and fossil fuels between 2019–2024.
Unlike conventional direct electrification, which faces challenges in long-distance transport and storage, hydrogen’s high energy density and diverse production pathways make it a promising solution for large-scale, flexible energy use.
Hydrogen can be produced through thermochemical, biochemical, or electrochemical processes. To navigate this landscape, the industry has adopted a hydrogen color spectrum, to classify production pathways based on their feedstock and associated carbon footprint.
The main types of hydrogen production include gray, blue, green, and turquoise. Gray hydrogen relies on fossil fuels with no emissions control. Blue hydrogen, similar to gray, also uses fossil fuels but incorporates carbon capture. Green hydrogen is produced by splitting water using renewable electricity. Lastly, turquoise hydrogen splits natural gas into hydrogen and solid carbon.
Hydrogen-Generation Spectrum
Transitioning to a clean hydrogen economy requires navigating a wide range of production technologies (Fig. 2). Each method differs in its feedstock, energy requirements, emissions, and byproducts, creating distinct environmental and economic profiles. Grasping the core chemical reactions, operational conditions, and strategic benefits of each pathway is crucial to understanding the trade-offs that will define hydrogen’s role in the future global energy landscape. Below is a brief overview of the main hydrogen production technologies.
Gray Hydrogen via Steam Methane Reforming
Today, gray hydrogen makes up almost 75% of global hyrogen production. This type is generated through a mature and highly optimized process known as steam methane reforming (SMR). It is considered the most economically viable technique given the decades of process refinement. The main drawback of SMR is its substantial carbon footprint. The process is a multistage thermochemical conversion. It starts with the primary reforming reaction, where a methane source, typically natural gas, reacts with high-temperature steam (between 700°C to 1,100°C) in the presence of a metal-based catalyst. This is a strongly endothermic reaction, as it requires a significant external heat input. The output is a mixture of hydrogen and carbon monoxide (CO).
The core chemical transformation is:
CH4 + H2O (+heat) → CO + 3H2
To maximize the hydrogen yield, the produced CO undergoes a second step called the water-gas shift reaction. In this exothermic reaction, CO reacts with additional steam at lower temperatures (200°C to 400°C) to produce more hydrogen and carbon dioxide (CO2).
CO + H2O → CO2 + H2 (+heat)
Blue Hydrogen via Carbon Capture
Blue hydrogen represents an evolution of gray hydrogen, aiming to bridge the gap between today's fossil-fuel-based economy and a future low-carbon one. The concept of blue hydrogen is to continue using natural gas as a feedstock but to integrate carbon capture, utilization, and storage (CCUS) technologies to capture the CO2 generated during the process.
The most common approach involves retrofitting SMR plants, but this often results in modest capture rates as it is difficult to capture all emission streams. Consequently, new large-scale projects are favoring autothermal reforming, a process that uses pure oxygen and combines reactions into a single step. This creates a single, concentrated stream of CO2, making it easier and more cost-effective to achieve very high capture rates, often exceeding 90%.
Green Hydrogen: Electrolytic Production
Green hydrogen is produced via electrolysis, a process that uses electricity to split water into hydrogen and oxygen. The green designation is critically dependent on the electricity coming from renewable sources such as solar or wind, resulting in a process with zero direct greenhouse gas (GHG) emissions.
The fundamental reaction that occurs in an electrolyzer is:
2H2O (+electricity) → 2H2 + O2
There are three main electrolyzer technologies at different commercial maturity levels: alkaline electrolysis (AEL), proton exchange membrane (PEM) electrolysis, and solid oxide electrolysis (SOEC). AEL is the most mature and cost-effective technology, using liquid alkaline electrolyte. PEM offers faster response times and greater flexibility for integration. SOEC operates at high temperatures and offers higher electrical efficiency but remains less commercially developed.
Turquoise Hydrogen via Methane Pyrolysis
Turquoise hydrogen is an emerging pathway that decarbonizes natural gas without CCUS technologies integration. The process, known as methane pyrolysis, involves heating methane gas (CH4) to very high temperatures (>1,000°C), and "cracking" the molecule into hydrogen gas and solid carbon.
The chemical reaction is:
CH4→ C(s)+2H2
The primary advantage is that it produces no gas CO2. Alternatively, carbon is captured as a solid product (i.e., graphene), avoiding complex infrastructure required for CO2 transportation or sequestration. The economic viability of turquoise hydrogen is directly linked to the market value of this solid carbon, which can have wide utilization including electronics, batteries, coatings, filtration, and biomedical applications.
Key Challenges
Global adoption of hydrogen faces several technical, economic, and logistical challenges. These include:
- High Production Costs. This is considered the primary barrier for low-carbon hydrogen compared to gray hydrogen, which is the most mature technology.
- Infrastructure and Storage. Hydrogen’s low density means it requires large or high-pressure storage, making its transportation and storage more complex compared to conventional fuels. Pipelines, compressors, and facilities often need upgrades to handle hydrogen safely and efficiently.
- Scaling Up Production. Meeting global low-emissions targets means expanding the hydrogen-production supply chain in a short period of time, which is a major challenge at this stage.
- Energy System Integration. Adapting current energy systems to accommodate hydrogen requires ensuring efficient integration between conventional and novel technologies.
Conclusions
Hydrogen represents a promising energy resource in the industry to replace conventional fuels and reduce GHG emissions. The global hydrogen economy is currently progressing through a critical transition phase, where the path forward involves a strategic combination of various technologies to be deployed. Key challenges such as cost, infrastructure, storage, transport, and system integration, remain to be addressed through targeted research and innovation, as well as policy support.
The hydrogen landscape will not be uniform, but it is moving steadily toward a more sustainable and diversified energy future.
For Further Reading
Hydrogen as an Alternative Fuel: A Comprehensive Review of Challenges and Opportunities in Production, Storage, and Transportation by M. Bhuiyan and Z. Siddique
Review on Techno-Economics of Hydrogen Production Using Current and Emerging Processes: Status and Perspectives by M. Nemittallah, A. Alnazha, and U. Ahmed, et al.
Hydrogen Energy Systems: A Critical Review of Technologies, Applications, Trends, and Challenges by M. Yue, H. Lambert, and E. Pahon, et al.
Ali Alshuwaikhat is a petroleum engineer at Saudi Aramco, a position he’s held since 2018. He is currently with the reservoir engineering technology division under EXPEC Advanced Research Center. His specialty is improved oil recovery applications in reservoir management, with a focus on implementing best-in-class strategies and practices aiming to maximize reservoir recovery and optimize sweep efficiency. His previous experience includes reservoir management, production engineering, reservoir description, and petrophysics. He has several publications as a main author and coauthor on real-time production optimization, assessing novel techniques, and machine-learning applications for formation evaluation. He holds a BS in petroleum engineering from the University of Houston.
Zuhair AlYousef is a petroleum engineer at Saudi Aramco, a position he's held since 2008. The emphasis of his work has been on CO2 storage and EOR. He is currently leading the gravity override mitigation, the gas mobility control, and CO2 sequestration research, and contributing to high-impact projects within the area of CO2-EOR and sequestration. He has authored and coauthored numerous technical papers and filed several patents applications. He has been appointed to serve as an associate editor on the editorial board of the Journal of Geoenergy Science and Engineering (2021-2024), the SPE Journal (2023-current), and the Journal of Petroleum Exploration and Production Technology (2019-current). He holds a BS from King Fahd University of Petroleum and Minerals (KFUPM) and MS and PhD degrees from Texas A&M University, all in petroleum engineering.