The transportation sector often seen as the most difficult to decarbonise, plays a significant role in the worldwide emissions of greenhouse gases (GHGs), making it imperative to implement effective strategies for GHG reduction. One promising approach involves the utilization of electro-fuels. Electro-fuels are generated by merging recycled CO2 gas with hydrogen obtained through the electrolysis of water, with the energy needed for production sourced from emissions-free electricity. Additionally, electro-fuels encompass recycled solid metal fuels, which are created through electrochemical reduction reactions of their respective oxides [1]. E fuels may help the sector achieve the 2050 decarbonisation goals.

The concept of e-fuels has, during the last 5-10 years, gained increased attention from different industries such as the automotive, fuel producers, and energy utility companies [2]. Electro-fuels are potentially of interest for all transport modes; they can be used in combustion engines and may not require significant investments in new infrastructure. However e-fuels are of special interest in sectors such as long-distance aviation and deep-sea shipping, where electrification opportunities are limited because liquid fuels with high energy density are difficult to substitute.

Sustainable fuels, combined with reductions in energy demand, can significantly reduce GHG emissions whilst not jeopardising other sustainability requirements regarding biodiversity, water resources, air quality, land use, and material sourcing. Their sustainability characteristics including feedstock availability, their cost-competitiveness, their technology readiness and technical specifications will make them Sustainable fuels for different transport modes and transport applications, given the global limitation of resources, the shift to sustainable fuels should be first driven by a significant increase in energy efficiency [3].

E-fuels are synthetic fuels, resulting from the combination of ‘green or e-hydrogen’ produced by electrolysis of water with renewable electricity and CO2 captured either from a concentrated source (e.g. flue gases from an industrial site) or from the air (via direct air capture, DAC). E-hydrogen has also been considered as part of this review. The table 1 below summarise the potential primary uses of e-fuels across different transport segments [4].

Table 1: E-fuel types and their potential uses [4].

This article aims to review the concept of e-fuels, their production and summarize the transport sector efforts to achieve the 2050 de-carbonization goals.

E-Fuel Production Pathways -Overview

Figure 1 gives an overview of the major production pathways for e-fuels. [5] E-fuels comprise gaseous and liquid fuels that are produced from electricity as main feedstock

Figure 1: E-Fuels production pathways [5].

The initial step in any production process involves the generation of electricity, typically sourced from power plants that utilize Renewable Energy Sources (RES). Subsequently, the production of e-fuels involves water electrolysis to produce hydrogen, which can be represented by the following equation:

2H2O → 2H2 + O2 (1)

 For water electrolysis, various technologies are employed, including alkaline electrolyzers, proton-exchange-membrane electrolyzers (PEM), and solid oxide electrolyzers. Alkaline electrolyzers are a well-established technology commonly used in industrial settings. However, they are limited in their ability to operate intermittently, typically functioning within a range of 20% to 40% of their nominal capacity [6,7]. PEM electrolyzers offer promise for more flexible operation and have already demonstrated the ability to adapt to real-world wind profiles in plants with a 4 MW nominal load [8]. On the other hand, solid oxide electrolyzers are a more recent technological development and have a lower level of technological maturity.

While it is feasible to directly utilize the hydrogen generated through water electrolysis in transportation or industrial applications, integrating hydrogen into existing gas infrastructure and storage systems is challenging due to its low volumetric energy density [6]. Consequently, further processing of hydrogen into high-energy-density fuels is required to ensure compatibility with existing infrastructure.

Various chemical and biological synthesis methods are known for transforming hydrogen into gaseous or liquid fuels like synthetic methane, methanol, diesel, gasoline, kerosene, OME, DME, and ammonia. Particularly, I will focus on processes that involve carbon dioxide as an additional feedstock, which can be obtained from the atmosphere, industrial processes, or power plant exhaust gases. Among these methods, the Sabatier process, Fischer-Tropsch synthesis, methanol synthesis, and DME synthesis are either commercially established or in advanced development stages.

The Sabatier process stands out as a well-developed technology for converting hydrogen and carbon dioxide into gaseous synthetic methane [9,10] as illustrated by the following equation:

4H2 + CO2 → CH4 + 2H2O (2)

Synthetic methane production facilities are capable of operating at partial loads, with an efficiency of up to 40% of their nominal capacity.

On the other hand, Fischer-Tropsch synthesis is a widely used process for generating liquid e-fuels. This process converts a mixture of hydrogen and carbon monoxide (known as syngas) into hydrocarbons of varying chain lengths. Depending on specific conditions such as the hydrogen-to-carbon monoxide ratio in syngas, temperature, pressure, and catalyst used, one can produce different proportions of synthetic diesel, gasoline, or kerosene. However, operating the Fischer-Tropsch process with flexibility can be challenging due to its complex fluid dynamics and thermodynamic reactions [6].

Attempts at Pilot to Full Scale E-Fuel Production

Porsche and several partners [11] have started production of a climate neutral “e-fuel” aimed at replacing gasoline in vehicles with traditional internal combustion engines. The German automaker, owned by Volkswagen, said that a pilot plant in Chile started commercial production of the alternative fuel. In the pilot phase, Porsche expects to produce around 130,000 liters (34,342 U.S. gallons) of the e-fuel. Plans are to expand that to about 55 million liters (14.5 million U.S. gallons) by mid-decade, and around 550 million liters (145.3 million U.S. gallons) roughly two years later.

The most efficient power-to-liquid plant in Europe will be built by the technology group AVL in Graz (Austria) by 2023 [12]. It is intended to produce around 100,000 liters of so-called e Fuel per year. The completely synthetic fuel can be used instead of fossil fuels in combustion engines of all kinds and is produced in a CO2-neutral manner. The system should be a model for industrial use worldwide.

Norwegian has announced [13] a landmark partnership with Norsk e-Fuel to build the world’s first full scale e-fuel plant in Mosjøen, Norway. The plant will produce sustainable aviation fuels (SAF), marking an important milestone towards Norwegian's target of 45 percent emissions reduction by 2030.

E Fuels in Transport Sector

From a technical perspective, e-fuels are of significant interest for various modes of transportation, with a particular focus on medium to long-distance ocean transport, aviation, and heavy-duty road transport. These sectors face challenges in substituting high-energy-density liquid fuels with electrification through battery-electric propulsion [14]. Selma Brynolf and colleagues [15] have conducted a detailed review and assessment of the feasibility of e-fuels in different transport modes, partly from a techno-economic standpoint.

While road transport can potentially be decarbonized using alternatives to e-fuels, such as battery electric vehicles (BEVs) and hydrogen fuel cell electric vehicles (FCEVs), there are hurdles to overcome. BEVs have gained traction in passenger car applications, but transitioning to electric solutions for light trucks and heavy-duty vehicles is more complex due to factors like payload capacity, charging infrastructure, and range requirements. Such a transition would necessitate substantial investments in BEV charging points and a hydrogen distribution system for FCEVs [16]. In contrast, e-fuels are most cost-effective for light-duty vehicles, as the vehicle cost represents a larger portion of the overall cost.

The International Maritime Organization (IMO) aims to reduce the carbon intensity of all ships by at least 40% by 2030 compared to a 2008 baseline. Ships have traditionally relied on low-cost, high-sulfur fuels like heavy fuel oils and diesel, making the transition to zero-carbon fuels a costly endeavor. Different categories of ships with varying operational profiles, such as coastal, inland, and ocean-going vessels, present different decarbonization challenges. While liquefied natural gas (LNG) has been introduced [17] in shipping primarily due to stricter sulfur regulations, initiatives are underway to explore renewable marine fuels like hydrogen, methane, and methanol [18,19]. Battery-electric ships are already in operation for coastal and inland shipping, but for long-distance ocean-going vessels, batteries are not a viable option [20]. Making e-fuels cost-competitive with conventional marine fuels in the shipping sector will require substantial incentives.

The International Air Transport Association has committed to reducing aviation CO2 emissions by 50% by 2050. Achieving this goal requires the adoption of low or zero-emission alternative aviation fuels, such as hydrogen, e-fuels, or biofuels [21, 22]. E-fuels for aviation can be used in modified jet engines (e-jet fuels) or in fuel cells [23, 24]. Depending on the type of e-fuels considered, adjustments to conventional jet engines may be necessary. The most commonly discussed e-fuel for aviation is e-jet fuel produced from Fischer-Tropsch synthesis [21]. Other e-fuels under consideration for aircraft include methanol-to-jet (MTJ), liquefied methane, and liquefied hydrogen produced via water electrolysis. Ammonia is also mentioned as a potential e-fuel option for aviation [23].

E-Fuels and Environment

The Renewable & Low-Carbon Liquid Fuels Platform [25] asserts that renewable and low-carbon liquid fuels are a crucial, long-term technological solution for addressing carbon emissions. These fuels are expected to continue playing a significant role in decarbonizing various modes of transportation, including road, aviation, and maritime. While electrification and renewable gaseous fuels are strategic options for road transport, renewable and low-carbon liquid fuels are seen as enduring solutions in aviation and maritime sectors, with the potential for significant international adoption.

On October 27th of the previous year, EU negotiators, representing European governments and MEPs, [26]reached an agreement on revised CO? standards for new cars and vans, aiming to end the sale of new polluting combustion engine cars by 2035. This move is a crucial step towards Europe's goal of achieving climate neutrality by 2050. However, the German government raised last-minute objections to this landmark deal, advocating for the continued sale of new cars with internal combustion engines after 2035 and the inclusion of combustion engine cars powered by e-fuels in the future.

Despite major car manufacturers publicly committing to a complete transition to electric cars by 2035 or earlier, lobbies associated with oil, gas, and automotive suppliers argue that so-called "carbon-neutral" synthetic fuels, known as 'e-fuels,' offer a more affordable and accessible solution. They claim that e-fuels are compatible with Europe's climate neutrality and energy security objectives. E-fuels are produced through an expensive process that converts electricity into hydrogen, which is then combined with CO2 to create a liquid fuel resembling conventional gasoline, diesel, or aviation kerosene. Advocates argue that if renewable electricity is used, and CO2 is captured from the air, e-petrol and e-diesel can be considered climate-neutral fuels that also reduce pollution. Their argument centers on decarbonizing the fuel itself rather than focusing solely on decarbonizing engines.

To test the claims of these clean fuels, T&E (Transport & Environment) [27]decided to commission IFP Energies Nouvelles to conduct a series of laboratory-based tests simulating real-world driving conditions (WLTC and RDE). The aim was to measure the emissions of various e-petrol blends in an A-class (A180) Mercedes. The results of these lab and field simulations regarding pollutant emissions are as follows:

• NOx emissions did not show any significant difference for any of the tested e-fuels compared to today's petrol fuel. This indicates that e-fuels emit a similar amount of NOx pollution as fossil fuels, which means the use of e-petrol in cars is unlikely to have a substantial impact on NOx emissions, a key component of toxic NO2 pollution in European cities.
• There was a notable decrease in particle emissions in all tests. The number of particle emissions larger than 10 nm decreased by 97% in the lab test and by 81-86% in the RDE test cycle, compared to fossil fuels. This reduction was a significant improvement, well below legal limits, although there was no observed difference in particle mass emissions.
• Carbon monoxide emissions were significantly higher with the tested e-petrol blends, with emissions being nearly three times higher in the lab WLTC test and 1.2-1.5 times higher in the RDE test compared to fossil fuel. The largest increase in emissions occurred during engine startup, which is frequent in urban areas.
• Hydrocarbon emissions, which are harmful chemical compounds consisting of hydrogen and carbon, decreased by 23-40% in the WLTC test, but no significant difference was observed in the RDE test due to low emissions across all fuels.
• Ammonia emissions from two e-petrol blends roughly doubled in the RDE test, especially after engine startup (cold start), which is common in urban settings. These findings suggest that certain e-petrol blends may lead to an increase in ammonia emissions, a precursor to PM2.5 pollution.
• In summary, the testing results indicate that e-petrol is not a clean fuel and, except for particle emissions, is unlikely to substantially reduce the emissions of toxic pollutants, both regulated and unregulated, when compared to today's petrol fuel.

Hydrogen, Ammonia and Syn hydrocarbons combustion [28].

Hydrogen combustion in IC engines and ammonia combustion have the potential to increase NOx emissions, similar to fossil fuels. However, they reduce other pollutants like sulfur dioxide, carbon monoxide, heavy metals, and particulates. Selective Catalytic Reduction (SCR) can effectively reduce NOx emissions by 90%, but it requires ongoing maintenance for optimal operation.

When ammonia is used in Solid Oxide Fuel Cells (SOFCs), it can also produce NOx emissions, which can be mitigated using SCR technology. However, ammonia combustion also emits particulate matter and unburnt ammonia particles, necessitating the development of better engine calibration and combustion control technologies.

On the other hand, using synthetic hydrocarbons like e-diesel or e-kerosene instead of fossil fuels offers only marginal air quality benefits. These synthetic fuels still produce carbon monoxide (CO), NOx, and particulate matter emissions at levels similar to fossil-derived fuels. However, the concentration of particulate matter may be lower due to fewer impurities. NOx emissions from e-diesel are comparable to or lower than those from fossil-derived diesel, but additional deNOx exhaust treatments may be necessary to meet emission standards.

E fuels: Projected Production Costs

A glance at the recently published information on the projected production cost of e-fuels (e petrol & e diesel) show variations from source to source. For example situation published (in March 2023) by Germany [29] with respect e petrol  state; “With a more complex and energy intensive process of making e-fuels comes more prohibitive costs. Using a baseline with a retail e-petrol price in Germany in 2030 of €2.8/L ( Based on the ICCT’s estimate ) , this price means that in 2030 drivers would need to pay at least €210 to fill up their 75 L tank and about €2,300 minimum per year to cover 15,000km (Based on a fuel consumption of 5.4L/100km) . Based on current petrol prices in Germany (1,84/L , Euro-super 95(I) price on the 06/03/2023 ), filling up a tank costs around €140, meaning that, in 2030, e-petrol is expected to be 50% more expensive than fossil petrol today  as seen in figure 2 below. The likely huge price difference between fossil and synthetic petrol will create significant economic incentives for drivers to game the system.

Figure 2: Filling up with e petrol will be 50% more expensive.

Whereas e fuel alliance [30] on the  other hand state: With increased quantities of e-fuels being added gradually to conventional fuels, and production costs falling owing to economies of scale, e-fuels would be affordable for consumers in every phase of the market ramp-up. While the production costs for one litre of  e fuel in 2025 with a 4% blending rate with conventional fuels are estimated to be between EUR 1.61 and EUR 1.99, by 2050 they may decrease from anywhere between EUR 0.70 to EUR 1.33 per litre of e fuel with a 100% blending rate.

This means that in 2025, diesel will cost EUR 1.22 for customers at filling station. In 2050, e diesel will cost between EUR 1.38 and EUR 2.17 (according to current taxes and duties). In 2025, petrol with an e-fuels admixture will cost between EUR 1.34 and EUR 1.36; in 2050, prices for e petrol are expected to be between EUR 1.45 and EUR 2.24.

E Fuels: Advantages and Barriers

Advantages

• E-fuels offer a substantial reduction in CO2 emissions compared to their fossil-based counterparts, making them an attractive option for promoting low-CO2 mobility in Europe.
• E-fuels possess a greater energy density than electricity, making them suitable for use in sectors like aviation and shipping where electric alternatives are limited.
• Existing infrastructure such as gas transport networks, liquid fuel distribution systems (pipelines), filling stations, and energy storage facilities can still be used for the transportation and storage of E-fuels.
• Liquid E-fuels are relatively inexpensive and easier to store and transport compared to electricity. They can be stored in large-scale stationary facilities and mobile vehicle tanks, helping to offset seasonal supply variations and enhance energy security.
• Some E-fuels can be deployed without significant modifications to existing vehicle engines. Liquid E-fuels provide an alternative technology for reducing greenhouse gas emissions in both new and existing vehicles due to favorable combustion characteristics.
• E-fuels, such as methane and liquid E-fuels, can potentially be blended with natural gas and conventional fossil fuels, respectively, as long as they meet the required specifications.

Barriers

• The production of E-fuels involves inherent thermodynamic conversion losses, necessitating a substantial number of new renewable generation plants.[31]
• E-fuels exhibit lower energy efficiency compared to battery electric vehicles (BEVs). BEVs have an energy use efficiency that is 4-6 times higher than E-fuels in combustion engines. Fuel cell vehicles also outperform liquid E-fuel cars in terms of efficiency, with a fuel cell vehicle achieving an efficiency of around 26-35%, while a liquid E-fuel car has an efficiency of about 13-15% [32].

• E-fuels are synthetic gaseous and liquids, resulting from the combination of ‘green or e-hydrogen’ produced by electrolysis of water with renewable electricity and CO2 captured either from a concentrated source or from the air. Potentially Electro-fuels are of interest for all transport modes; but of special interest in sectors such as long-distance aviation and deep-sea shipping. Porsche and partners have started production of a climate neutral “e-fuel” aimed at replacing gasoline in vehicles with traditional IC engines. The most efficient power-to-liquid plant in Europe will be built by the technology group AVL in Graz, by 2023.
• While road transport can potentially be decarbonized using alternatives to e-fuels, such BEVs FCEVs, but transitioning to electric solutions for light trucks and heavy-duty vehicles is more complex due to factors like payload capacity, charging infrastructure, and range requirements
• Different categories of ships with varying operational profiles, such as coastal, inland, and ocean-going vessels, present different decarbonization challenges. Initiatives are underway to explore renewable marine fuels like hydrogen, methane, and methanol. For aviation e-jet fuel produced from Fischer-Tropsch synthesis, methanol-to-jet (MTJ), liquefied methane, and liquefied hydrogen are being researched.
• Lab and field simulations testing regarding pollutant emissions abatement yielded results such as NOx emissions did not show any significant difference for any of the tested e-fuels compared to today's petrol, notable decrease in particle emissions in all tests, Carbon monoxide emissions were significantly higher, hydrocarbon emissions, decreased by 23-40% in the WLTC test and Ammonia emissions from two e-petrol blends roughly doubled in the RDE test. Hydrogen and ammonia combustion in IC engines have the potential to increase NOx but they reduce other pollutants.
• Projected production cost of e-fuels (e petrol & e diesel) show variations from source to source. As per Germany Using a baseline retail e-petrol price in 2030 of €2.8/L against a current petrol prices 1,84/L, e-petrol is expected to be 50% more expensive than fossil petrol today.
• Among the major advantages Liquid e-fuels are relatively inexpensive and easier to store and transport compared to electricity. They can be stored in large-scale stationary facilities and mobile vehicle tanks, helping to offset seasonal supply variations and enhance energy security.
• While the potential for e-fuels to decarbonize the transport sector has been discussed above, many uncertainties in terms of production costs, vehicle costs and environmental performance remain. For e-fuels to play a significant role in transportation, their attractiveness relative to other transport options needs to be improved. Incentives will be needed for e-fuels to be cost-effective.

Original draft preparation, conceptualization, writing—review and editing of the manuscript.

 Funding

All funding managed by the author himself.

Acknowledgments

Author deeply Acknowledge the support my daughter Jasdeep in the preparation and formatting of this manuscript. 

Conflicts of Interest

Authors declare no conflict of interest.

1. Ababneh H, BH Hameed. Electrofuels as emerging new green alternative fuel: A review of recent literature. Energy Conversion and Management. 2022; 254.
2. Maria Grahn et.al. Review of electrofuel feasibility-cost and environmental impact. Prog. Energy 4 2022; 032010.
3. Study Report by TRAN Committee: Assessment of the potential of sustainable fuels in transport. PE 733. 2023.
4. Marta Yugo and Alba Soler; A look into the role of e-fuels in the transport system in Europe (2030–2050) Concawe Review. 2019; 28: 1.
5. Maximilian Borning et al; Opportunities and Challenges of Flexible Electricity-Based Fuel Production for the European Power System, Sustainability. 2020, 12, 9844;
6. Tremel A. Electricity-Based Fuels; Springer International Publishing: Cham, Switzerland, 2018.
7. Schiebahn S, et al; Power to gas: Technological overview, systems analysis and economic assessment for a case study in Germany. Int. J. Hydrogen Energy 2015, 40, 4285-4294.
8. Kopp M et al. Energiepark Mainz: Technical and economic analysis of the worldwide largest Power-to-Gas plant with PEM electrolysis. Int. J. Hydrogen Energy 2017; 42: 13311-13320.
9. Sterner M, Eckert F, Thema M. In book, Energiespeicher: Bedarf, Technologien, Integration, 2nd ed; Sterner M, Stadler I, Eds; Springer: Berlin/Heidelberg, Germany, 2014; 327-493.
10. Toro C, Sciubba E. Sabatier based power-to-gas system: Heat exchange network design and thermoeconomic analysis. Appl. Energy 2018; 229: 1181-1190.
11. Gray N, et al; Decarbonising ships, planes and trucks: an analysis of suitable low-carbon fuels for the maritime, aviation and haulage sectors. Adv. Appl. Energy. 2021; 1:100008.
12. Selma Brynolf et al;Review of electrofuel feasibility—prospects for road, ocean, and air transport 2022 Prog. Energy. 2022; 4: 042007.
13. Kramer U et al. Defossilizing the transportation sector: options and requirements for Germany Research Association for Combustion Engines, FVV Prime Movers Technologies, Report 2018
14. Faber J et al. Fourth IMO GHG Study (Delft: CE Delft). 2020.
15. Balcombe P, Brierley J, Lewis C, Skatvedt L, Speirs J, Hawkes A, Staffell I. How to decarbonise international shipping: options for fuels, technologies and policies Energy Convers. Manage. 2019; 182 72–88
16. Malmgren E. Towards sustainable shipping: evaluating the environmental impact of electrofuels Mechanics and Maritime Sciences (Gothenburg: Chalmers University of Technology). 2021.
17. Andersson K, Brynolf S, Hansson J and Grahn M. Criteria and decision support for a sustainable choice of alternative marine fuels Sustainability. 2020; 12: 3623
18. Drunert S, Neuling U, Zitscher T, Kaltschmitt M. Power-to-Liquid fuels for aviation—processes, resources and supply potential under German conditions. Appl. Energy. 2020; 277: 115578
19. Malins C. What role is there for electrofuel technologies in European transport's low carbon future? Cerulogy Report. 2017.
20. Goldmann A et al. A study on electrofuels in aviation Energies. 2018; 11: 392
21. Bruce S et al. Opportunities for Hydrogen in Commercial Aviation (CSIRO). 2020.
22. The Renewable & Low-Carbon Liquid Fuels Platform, towards Transport Fuels’ Transformation.
23. The Future Cost of Electricity-Based Synthetic Fuels; Frontier Economics, 2018