The current energy crisis is a strong indicator of the fragility of our societal model (economic, ecological and social) and requires us to accelerate the energy transition. To answer this, attention is turning to hydrogen as a clean energy vector, because combustion does not emit CO2 (greenhouse gas responsible for global warming), which would provide an excellent alternative to decarbonizing our lifestyle.
Hydrogen is the most abundant element in the universe. On Earth it exists in the form of dihydrogen (H2). This molecule arouses as much interest (mobility, refining, chemistry, metallurgy, glassmaking, cement works, electronics, etc.) as it does questions (energy costs of its production, storage, transport, interaction with materials). The industry currently consumes 50 million tonnes of H2. The transport sector (for propulsion of vehicles) is the main area of application (trains, trucks, planes, cars, boats, buses). In 2050, the share of H₂ in global energy production is estimated to be around 12%, of which around 70% will be H2 green (ie low carbon).
For more than 160 years, humans have harnessed an amazing source of energy, oil, to motorize, mechanize and facilitate all areas of human activity. Concretely, we can produce 10 kWh with one liter of petrol (around 0.75 kg). This is the energy needed to take 5 showers or travel 50 km (on average) with an electric car… With one liter of H2 liquid (about 0.071 kg therefore 10x lighter), we can produce about 3 kWh. In other words, with H2 liquid, we therefore get a specific energy (divided by the mass) 3 times greater than petrol.
Lighter gas
The observation is obvious: the same electrical energy is produced for a fuel mass that is 3 times lower. And in the field of transport, mass turns out to be a key factor: the heavier an object is, the more energy it takes to move it. Lightweighting is therefore fundamental to reducing energy consumption and thus CO emissions.2. In 2035, Airbus plans the inaugural flight of the first plane with liquid hydrogen, called zero-e (which refers to zero CO emissions).2). Half of this aircraft’s mass consists of lightweight composite materials that combine carbon fibers and plastic, derived from fossil fuels (oil). Zero CO emissions2 : a dream in the form of illusion or revolution?
As a prelude to this revolution, it should be noted that the production of hydrogen (liquid or gas) has a variable price. In fact, H2 comes in all colors (black, brown, grey, blue, yellow, green, turquoise and white – from the most expensive to the cheapest) depending on the manufacturing process used, emitting more or less CO22. This difference is essentially due to the energy costs (more or less high) of these techniques. For example, black hydrogen comes from the conversion of coal to gas. It is therefore expensive in terms of energy and pollutes. Conversely, white is present in its natural state on earth, so it is easier to exploit. But today 95% of the hydrogen produced comes from fossil sources.
Based on current consumption (40 million private vehicles in France), if all French people chose hydrogen, it would require an amount of 5 million tons or 300 TWh of electricity (representing the energy cost to produce it). Or production of 46 additional nuclear reactors, 30,000 wind turbines or even 6,000 km2 of solar panels. In summary, H produces2 can be expensive in energy! To produce it, the efficiency (ratio between the energy obtained and the energy required as input) must be competitive with other energy sources.
What are the limitations of the materials?
Let us now look at the technological barriers imposed by H2. And especially the challenges imposed on materials in the environment in H2. Changing the energy production model (especially replacing thermal engines using fossil gasoline fuels) requires complete adaptation of propulsion systems and fuel storage methods. Which presents numerous scientific and technical problems in terms of materials.
First of all the storage problem: at atmospheric pressure, 1 kg of H2 liquid takes about 800 less volume than 1 kg of H2 gaseous. Practical but complex to implement! To store hydrogen in liquid form, it must be brought to a temperature of -250°C (so-called cryogenic temperature). Technically difficult and expensive. Not to mention the weakening of materials at these temperatures, which we will talk about again.
We can also use H2 compressed, but the pressure in the tanks can reach 700 bar (which corresponds to the technical limit of the materials). Supporting such high pressures requires the use of very strong and very light materials (so as not to weigh down the structure).
Typically, composite materials (such as those used in aviation) combining carbon fibers and polymers (plastics) are relevant options to meet tank resistance requirements. Scientific studies tend to show that hydrogen gas used under certain temperature conditions has little effect on plastics or elastomers (eg rubber used in flexible pipes). On the other hand, rapid gas decompression can be harmful to these materials.
Hermetic materials are also necessary because the H molecules2 is among the smallest and moves very easily through most materials. It is then necessary to use a liner (an envelope) – for example metal hydrides – to ensure this seal. For more than 20 years, research has been carried out on this topic. Today we have reached a certain maturity to respond to these questions. The biggest challenge lies in controlling costs to be able to “democratize” these thoughts and their application in different areas of everyday life. The field extends from cars to rockets, including industry and electrical supplies of high-altitude refuges to replace generators (currently powered by diesel).
Materials that can become fragile
Let us return to the weakening of metallic materials with liquid or gaseous hydrogen. Whether they are used to store or transport hydrogen, embrittlement is a physical process during which the molecules penetrate the material, specifically its microstructure, which consists of “glued” grains (like crystals against each other and encourage). its weakening. The metal loses its ability to deform plastically (like a Carambar being stretched) and becomes brittle (like a Carambar in the freezer).
This change in behavior generally results in a change in mechanical properties and premature failure over time (so-called material fatigue). For example, in the aviation field, the material problems are those encountered in gas turbines exposed to hydrogen and water vapor at high temperatures. These turbines, generally made of superalloys (or metal alloys for long-term use at high temperatures), are subject to oxidation phenomena (formation in the presence of oxygen of an oxide layer which causes deterioration of the surface condition of the material), hot corrosion (longer-term consequence of oxidation) and diffusion of H2 in the microstructure. These are processes that commonly occur in petrochemical processes, engines, boilers and reactors in nuclear power plants. As for composite materials that interact with H2 gas, 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 brittle compared to metallic materials.
Among the other material limitations regarding the storage (in tanks) or distribution of hydrogen (in pipes or lines such as town gas), fire resistance is particularly important. Since this gas is highly flammable and explosive, the main challenge lies in the use of materials capable of maintaining their stiffness and resistance under critical operating conditions (in case of fire). Composite materials reinforced with carbon fibers are therefore a relevant solution because they retain excellent mechanical properties under flames and high temperatures. In particular, carbon fibers degrade at high temperatures but maintain a high level of mechanical performance under a petroleum flame, but few studies to date have focused on the behavior under a hydrogen flame.
In conclusion, material issues such as brittleness, oxidation, corrosion, the requirement for high mechanical performance, fire resistance clearly illustrate the potential challenges associated with materials in parts and infrastructures that interact with liquid or gaseous hydrogen. Thus, the choice of materials for applications involving hydrogen is a matter of compromise between energy costs, availability, recyclability, physical properties and operational performance (under service conditions).
Understanding the physical mechanisms involved is therefore fundamental to developing and making use of H2 in various industrial sectors. Current knowledge and research in materials (metallic and composites) that interact with hydrogen has grown significantly over the last ten years. They now make it possible to respond to numerous technological obstacles and to envisage a generalization of hydrogen applications in several industrial fields. In the end, the biggest challenge will be to produce it at low energy costs.
