Hydrogen, the New Green Gold: Are Materials Ready for Storage and Transport?

The current energy crisis is a stark reminder of the fragility of our societal model (economic, ecological, and social) and necessitates accelerating the energy transition.

To address this, attention is turning to hydrogen as a clean energy carrier, as its combustion does not emit CO2, which would be an excellent alternative to decarbonize our lifestyles.

Hydrogen is the most abundant element in the universe. On Earth, it is found in the form of dihydrogen (H2). This molecule raises as much interest (mobility, refining, chemistry, metallurgy, glassmaking, cement, electronics…) as questions (energy cost for its production, storage, transport, interaction with materials).

The industry today consumes 50 million tons of H2. The transport sector (for vehicle propulsion) is the most important field of application (trains, trucks, airplanes, cars, boats, buses). By 2050, it is estimated that H₂ will account for approximately 12% of global energy production, of which approximately 70% will be green H2 (i.e., low carbon).

For over 160 years, humans have exploited a prodigious energy source, oil, to motorize, mechanize, and facilitate all areas of human activity. Concretely, with one liter of gasoline (about 0.75 kg), one can produce 10 kWh.

This is the energy needed to take 5 showers or travel 50 km (on average) with an electric car… With one liter of liquid H2 (about 0.071 kg, therefore 10 times lighter than gasoline), one can produce about 3 kWh. In other words, with liquid H2, we obtain a specific energy (divided by mass) 3 times greater than gasoline.

The observation is clear: the same electrical energy is produced for a fuel mass 3 times lower. And in the field of transport, mass is a key factor: the heavier an object, the more energy is needed to move it.

Lightweighting is therefore fundamental to reducing energy consumption and consequently CO2 emissions. In 2035, Airbus plans the inaugural flight of the first liquid hydrogen plane, called Zero-e(1) (referring to zero CO2 emissions).

Half of the mass of this aircraft is composed of lightweight composite materials, combining carbon fibers and plastics, derived from fossil fuels (oil). Zero CO2 emissions: a dream in the form of an illusion or a revolution?

As a preliminary to this revolution, it should be noted that the production of hydrogen (liquid or gaseous) has a variable cost.

Indeed, H2 comes in all colors (black, brown, gray, blue, yellow, green, turquoise, and white – from most to least expensive) depending on the manufacturing process used, emitting more or less CO2. This difference is mainly due to the energy cost (more or less high) of these techniques.

For example, black hydrogen comes from the transformation of coal into gas. It is therefore energy-intensive and polluting. On the contrary, white hydrogen is present in its natural state on earth, so it is cleaner to exploit. But today, 95% of the hydrogen produced comes from fossil sources.

Based on current use (40 million private vehicles in France), if all French people opted for hydrogen, it would require a quantity of 5 million tons, or 300 TWh of electricity (which represents the energy cost to produce it).

This is equivalent to the production of 46 additional nuclear reactors(2), 30,000 wind turbines, or 6,000 km2 of photovoltaic panels. In short, producing H2 can be energy-intensive! To produce it, the efficiency (the ratio between the energy obtained and the energy required as input) must be competitive compared to other energy sources.

Let’s now look at the technological locks imposed by H2. And in particular the challenges imposed on materials in the H2 environment. Indeed, changing the energy production model (replacing, in particular, thermal engines using fossil fuels with gasoline) requires completely adapting the propulsion systems and fuel storage methods. This poses many scientific and technical problems in terms of materials.

To store hydrogen in liquid form, it must be brought to a temperature of -250°C. Technically difficult and expensive…

First, the storage problem: at atmospheric pressure, 1 kg of liquid H2 takes up about 800 times less volume than 1 kg of gaseous H2.

Practical, but complex to implement! To store hydrogen in liquid form, it must be brought to a temperature of -250°C (called cryogenic temperature). Technically difficult and expensive. Not to mention the embrittlement of materials at these temperatures, which we will discuss later.

Compressed H2 can also be used, but the pressure of the tanks can reach 700 bars (which corresponds to the technical limit of materials). Withstanding such high pressures requires the use of very strong and very light materials (so as not to weigh down the structure).

Hermetic materials are needed because H2 molecules are among the smallest and move very easily through most materials.

Typically, composite materials (like those used in aeronautics) combining carbon fibers and polymers (plastics) are relevant options to meet the strength requirements of tanks.

Scientific studies tend to show that gaseous hydrogen, used under certain temperature conditions, has little effect on plastics or elastomers(3) (for example, the rubber used in flexible hoses). However, rapid decompression of the gas(4) can be detrimental to these materials.

Hermetic materials are also needed because H2 molecules are among the smallest and move very easily through most materials. It is then necessary to use a liner (an envelope) – for example, metal hydrides – to ensure this tightness. Research has been conducted for over 20 years on this subject.

Today, we have reached a certain maturity to address these issues. The main challenge lies in cost control in order to be able to “democratize” these tanks and their application in different areas of everyday life. The field ranges from cars to rockets, including industry and power supply for high-altitude shelters to replace generators (currently powered by diesel).

Let’s go back to the embrittlement of metallic materials by liquid or gaseous hydrogen. Whether they are used for the storage or transport of hydrogen, embrittlement is a physical process during which the molecules will penetrate the material, more particularly in its microstructure consisting of grains (similar to crystals) “glued” to each other and promote its embrittlement.

The metal loses its ability to deform plastically (like a Carambar that is stretched) and becomes brittle (like a Carambar in the freezer).

This change in behavior generally results in an alteration of mechanical properties and premature failure over time (what is called material fatigue).

For example, in the field of aeronautics, the material problems are those encountered in gas turbines exposed to hydrogen and water vapor at high temperatures.

These turbines, generally made from superalloys (or metal alloys for prolonged use at high temperatures), undergo oxidation phenomena (formation in the presence of oxygen of an oxide layer that causes deterioration of the surface condition of the material), hot corrosion (a longer-term consequence of oxidation), and diffusion of H2 within the microstructure(5).

These are processes commonly encountered in petrochemical processes, engines, boilers, and nuclear power plant reactors.

As for composite materials in interaction with gaseous H2, the physical mechanisms involved are not at all the same due to the difference in microstructure (no grains as in metals) and chemical composition. They are therefore less sensitive to low temperatures and embrittlement compared to metallic materials.

Among other material locks concerning the storage (in tanks) or distribution of hydrogen (in pipes or conduits like city gas), fire resistance is particularly important.

This gas being highly flammable and explosive, the main challenge lies in the use of materials capable of maintaining their rigidity and resistance in critical service conditions (in case of fire).

Composite materials reinforced with carbon fibers are therefore a relevant solution, as they retain excellent mechanical properties under flame and high temperatures.

In particular, carbon fibers decompose at high temperatures but maintain a high level of mechanical performance under kerosene flame, but few studies have looked at the behavior under hydrogen flame to date.

In conclusion, material issues such as embrittlement, oxidation, corrosion, the requirement for high mechanical performance, and fire resistance illustrate the potential challenges related to materials in parts and infrastructure interacting with liquid or gaseous hydrogen.

Thus, the choice of materials for applications involving hydrogen is a question of compromise between energy cost, availability, recyclability, physical properties, and performance in service (under conditions of use).

Understanding the physical mechanisms involved is therefore fundamental to developing and making the use of H2 reliable in different industrial sectors.

The current state of knowledge and research in the field of materials (metallic and composite) interacting with hydrogen has been growing rapidly over the past ten years. They now make it possible to respond to many technological locks and to envisage a generalization of hydrogen applications in many industrial fields. Ultimately, the main challenge will be to produce it at a low energy cost.

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