Hydrogen as fuel
Hydrogen as a fuel for fuel cell electric vehicles
The transport sector is the most challenging with respect to transitioning to a 100% renewable society. It accounts for 14 % of global greenhouse gas emissions amongst economic sectors . Therefore, to achieve our goal of a fully renewable energy system, there must be a key focus focus on transportion.
Hydrogen has many applications, and many people see it as the clean fuel of the future when it is generated from water and returns to it oxidized. Hydrogen-powered fuel cells are increasingly seen as promising pollution-free sources of energy and are now being used in cars and buses. Furthermore, hydrogen is used in the chemical industry to produce ammonia for agricultural fertilizer (the Haber Bosch process) and cyclohexane and methanol, which are intermediates in the production of plastics and pharmaceuticals . Methanol is now also used as a fuel for transportation applications. This article will focus on hydrogen as a fuel for fuel cell electric vehicles (FCEVs).
Properties of hydrogen
Gaseous hydrogen has some outstanding specifications compared to other fuel types, as can be seen in table 1.
Lower explosion limit (%, air)
Upper explosion limit (%, air)
Flash point ͦC
Lowest ignition energy mJ
Density (20 ͦC, 1 bar)
Boiling point ͦC
Critical temperature ͦC
Critical pressure bar
Diffusion coefficient cm2/s
Table 1: Fuel specifications .
As can be seen in Table 1, hydrogen has a very wide flammability range (lower and upper explosion limit) compared to other fuels, at between 4% and 75%. The optimal combustion condition is a 29% hydrogen-to-air volume ratio. Detection sensors are almost always installed in hydrogen systems to quickly identify any leak and minimize the potential for undetected flames.
As mentioned above, hydrogen is the smallest known molecule. It has a low viscosity, which is why it is prone to leakage. In a confined space, leaking hydrogen can accumulate and reach flammable concentrations. Any gas other than oxygen is an asphyxiator in sufficient concentrations. In a closed environment, leaks of any size are a concern, as hydrogen is impossible for human senses to detect and can ignite in a wide range of concentrations in air. However, proper ventilation and the use of detection sensors can mitigate these hazards.
Hydrogen has the smallest ignition energy, much lower than that required for other common fuels. This means that small sparks can easily ignite it.
Hydrogen has high energy content by weight (density) but not by volume, which is a challenge for storage. In order to store sufficient quantities of hydrogen gas, it is compressed and stored at high pressures. As can be seen in Table 1, the critical pressure for gaseous hydrogen is 13 bar. For comparison, hydrogen is compressed to 350-700 bar in storage tanks in FCEVs. For safety, hydrogen tanks are equipped with pressure relief devices that prevent the pressure in the tanks from becoming too high .
The easiest way to decrease the volume of a gas, at constant temperatures, is to increase its pressure. Thus, at 700 bar, hydrogen has a density of 42 kg/m3, compared to 0.089 kg/m3 under normal pressure and temperature conditions. At this pressure, 5 kg of hydrogen can be stored in a 125 liter tank .
As can be seen in Figure 1, the density of hydrogen highly depends on the temperature and pressure.
Figure 1: Hydrogen density at different temperatures and pressures. 
Due to its weight, hydrogen has a high diffusion rate, which results in rapid dispersion. This means that if a hydrogen cloud comes into contact with an ignition source in an open space with no confinement, flames will propagate through a flammable hydrogen-air cloud at several meters per second, and even more rapidly if the cloud is above ambient temperature .
Hydrogen can, nevertheless, be used as safely as other common fuels when simple guidelines are followed. This will be dealt with in the sub-section: Standards and regulations.
The application of hydrogen in the transport sector
Hydrogen can be used in three different ways in relation to transportation:
- Fuel cell-electrical vehicles
- Hydrogen combustion engines
- Hydrogen-methane-mixtures for combustion engines
This paper will focus on FCEVs. There are two main reasons why FCEVs are superior to electric vehicles (EVs): 1) shorter refueling times; and 2) longer ranges . FCEV refueling times are only a few minutes, while an EV needs several hours fully recharge. FCEVs are more efficient than conventional internal combustion engines vehicles and produce no tailpipe emissions, only emitting water vapor and warm air. FCEVs use a propulsion system similar to that of electric vehicles, where energy stored as hydrogen is converted into electricity by the fuel cell. Furthermore, they are equipped with advanced technologies to increase efficiency, such as regenerative braking systems that capture the energy lost during braking and store it in a battery .
Today, most car manufacturers have opted for a solution that consists of storing hydrogen in gaseous form at high pressure. This enables the storage of enough hydrogen to allow a FCEVs to travel between 500 and 600 km between refuelings .
Hydrogen fuel technology has undergone a huge improvement in the last few years. In fact, there is significantly more energy and explosive potential in a gasoline fuel tank than in a hydrogen fuel cell tank. Furthermore, various sensors are emplaced on hydrogen-fuel cell vehicles to manage possible leaks, such as :
- Seal valves
- Seal fuel lines
An overview of the differences between gasoline/petrol cars, EVs and FCEVs can be seen in the table below.
Charging time (full charge)
≈ 5 min.
≈ 5 min. 
Efficiency (well-to-wheel) 
≈ 13 %
≈ 73 %
≈ 22 %
Up to ≈ 1200 
≈ 200-500 
Tailpipe emissions from tailpipe.
No tailpipe emissions and the environmental effects depend on power production in the energy system.
No tailpipe emissions.
Gravimetric energy density (MJ/kg) 
Production cost 
Depends highly on the location 
Consumption per 100 km
7.04 l/100 km 
17-20 kWh/100 km 
0.84 kg/100 km 
Cost per km (EUR/100 km)
Table 2: Comparison of different vehicle types.
The combining of short refueling times, high driving ranges and cost-effectivenessmake FCEVs the most optimal solution.
Hydrogen purity or quality is a term to describe the lack of impurities in hydrogen as a fuel gas. The purity requirement varies with the application. A hydrogen internal combustion engine can tolerate low hydrogen purity, whereas a hydrogen fuel cell requires high hydrogen purity to prevent catalyst poisoning. Impurities in hydrogen (even in the ppm and ppb range) have a severe effect on the performance of fuel cells. Thus, it is crucial to be able to detect any impurities before a fuel is used. An international standard (ISO 14687-2:2012) has been published to specify the impurities and levels that must be identified, as is shown in Table 3.
Total Sulphur compounds
Total halogenated compounds
Maximum particulates concentration
Table 3: Impurities and levels that must be identified for hydrogen to be a viable fuel in FCEVs .
Hydrogen can be stored physically as either a gas or liquid. It typically requires high-pressure tanks (350-700 bar tank pressure). Another possibility is the chemical storage of hydrogen, whereby it is stored on the surface of solids (by adsorption) or within solids (by absorption). The automotive application utilizes the physical storage of hydrogen .
- Compressed gas
The storage tanks in cars must withstand high pressures and be able to store hydrogen without any leakage. Several car manufacturers today use compressed hydrogen tanks in their cars, which are capable of 350 and 700 bars, depending on the automotive type (light duty/heavy duty).
As mentioned above, there are two main varieties of hydrogen fuel tanks. The most common hydrogen fuel tank for cars, trucks, buses and other vehicles is the compressed hydrogen gas tank. Most hydrogen fueling stations currently dispense compressed hydrogen and only a few carry cryogenic liquid hydrogen. This is because almost all car manufacturers have chosen to fuel their cars with compressed hydrogen gas. BMW is one exception, with their dual fuel “Hydrogen 7 automobile” that uses cryogenic hydrogen and gasoline.
One disadvantage of cryogenic hydrogen tanks in recent years has been that they can have boil-off problems. This means that the liquid hydrogen will, over time, find its way out of the tank and evaporate. This is only the case when the car is left alone in one place for a couple of weeks, e.g., an airport. Furthermore, it is necessary to store cryogenic liquid hydrogen at a temperature below negative 253 degrees Celsius to maintain its liquidity. This demands a technologically advanced freezer system.
Compressed hydrogen fuel tanks are now made of carbon fiber composites or carbon fiber and metal alloys and composites. The inner line of the tank is a high-molecular weight polymer that serves as a hydrogen gas permeation barrier. The outer shell is placed on the tank for impact and damage resistance. A pressure regulator and an in-tank gas temperature sensor are located in the tank’s interior in order to monitor the pressure and temperature during the gas-filling process.
Compressed hydrogen gas tanks can have different interiors.
- Open space
- Metal hydride technology
When the hydrogen is stored in the porous metal hydride material, the gas is released by adding a small amount of heat to the tank. The disadvantage of this is that metal hydrides are generally very heavy, which will cut down the range per liter of fuel in the vehicle.
The goal is to find a better way to store hydrogen that is not as costly as metal hydrides or related methods under development. Hydrogen tanks must be lighter, hold more volume and cost less than they presently do .
Several studies have been conducted on material-based hydrogen storage to further improve storage potential. These studies have investigated metal hydride, chemical hydrogen storage and sorbent materials . Scientists and researchers are currently working on this issue and, as with many other technology-driven challenges, the future will most likely hold a variety of viable solutions.
Nevertheless, however, today’s hydrogen fuel tanks are safe due to several safety measures and requirements related to ATEX (European directives for controlling explosive atmospheres) approval requirements.
Standards and regulations
Hydrogen has a few unique properties that require special consideration, as described in the first section, Properties of Hydrogen. The same considerations apply as with any fuel; safe handling depends on knowledge of its particular physical, chemical and thermal properties and consideration of safe ways to accommodate these. Hydrogen, when handled with knowledge, is a safe fuel.
To ensure that hydrogen is handled properly, the International Organization for Standardization (ISO) is developing international safety standards; the Canadian Hydrogen Installation Code (CHIC), for instance, defines the requirements applicable to the installation of hydrogen equipment while the Society of Automotive Engineers (SAE) defines standards whereby the principal emphasis is placed on the transportation industry. Several standards for hydrogen applications have also been published during the last few years :
Hydrogen fuel quality – Product specification
Hydrogen generators using water electrolysis process – Industrial, commercial, and residential applications
Hydrogen fuel – Product specification – Part 1: All applications except proton exchange membrane (PEM) fuel cell for road vehicles
Hydrogen fuel – Product specification – Part 2: Proton exchange membrane (PEM) fuel cell applications in road vehicles
Hydrogen fuel – Product specification – Part 3: Proton exchange membrane (PEM) fuel cell application in stationary appliances
Hydrogen generators using fuel processing technologies – Part 1: Safety
Hydrogen generators using fuel processing technologies – Part 2: Test methods for performance
Hydrogen generators using water electrolysis process – Part 1: Industrial and commercial applications
Hydrogen generators using water electrolysis process – Part 2: Residential applications
Storage and transport:
There are international standards being developed specifically for both the stationary and portable storage of hydrogen, which is critical for ensuring safety in the hydrogen industry.
Gaseous hydrogen – Cylinders and tubes for stationary storage
Transportable gas storage devices – Hydrogen absorbed in reversible metal hybride
Safety is a critical factor to be considered and is vital for satisfying community expectations and furthermore, to ensuring workforce and environmental health and safety.
Basic considerations for the safety of hydrogen systems
Hydrogen detection apparatus – Stationary applications
Many standards exist for transportation utilization.
Gaseous hydrogen land vehicle refueling connection devices
Gaseous hydrogen – Land vehicle fuel containers
Gaseous hydrogen – Thermally activated pressure relief devices for compressed hydrogen vehicle fuel containers
Gaseous hydrogen – Fueling stations – Part 1: General requirements
Gaseous hydrogen – Fueling stations – Part 2: Dispenser
Gaseous hydrogen – Fueling stations – Part 3: Valves
Gaseous hydrogen – Fueling stations – Part 5: Fueling station hoses
Gaseous hydrogen – Fueling stations – Part 6: Fittings
Gaseous hydrogen – Fueling stations – Part 8: Fuel quality control
Liquid hydrogen – Land vehicle fueling system interface
Liquid hydrogen – Land vehicle fuel tanks
Gaseous hydrogen and hydrogen blends – Land vehicle fuel tanks
Gaseous hydrogen land vehicle refueling connection devices
Gaseous hydrogen – Fueling stations – Part 1: General requirements
SAE J2600: 2015-08
Compressed Hydrogen Surface Vehicle Fuelling Connection Devices
SAE J2601: 2014-07
Standard, Fueling Protocols for Light Duty Gaseous Hydrogen Surface Vehicles
SAE J2799: 2014-04
Standard, Hydrogen Surface Vehicle to Station Communications Hardware and Software
The ISO is currently developing new standards relating to hydrogen applications. Furthermore, companies that manufacture hydrogen and fuel cell products and build hydrogen fueling stations use many features that continue to be validated through safety tests. Hydrogen has been safely produced, stored, transported and used in large amounts in industrial applications  .
Benefits of hydrogen as a fuel for fuel cell electric vehicles:
+ No tailpipe pollution
+ Non-toxic, generated from water and returning to water when oxidized
+ Range and refueling time is comparable to ICEs
+ 14 times lighter than air, rise and disperses rapidly
+ High energy content by mass
Drawbacks of hydrogen as a fuel for fuel cell electric vehicles:
- Low viscosity – difficulties in storing hydrogen
- Highly explosive and dangerous in closed environments
- Pressurized gaseous fuel requires special engines and infrastructure
- Low energy content by volume
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