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hydrogène décarboné carbon-free hydrogen

Carbon-free hydrogen: how to go from gray to green?

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The industrial roll-out of hydrogen production only makes sense if it emits little or no carbon dioxide. Researchers at IMT schools are working on various alternatives to the use of fossil fuels, such as electrolysis and photocatalysis of water, plasma pyrolysis of methane, and pyrolysis and gasification of biomass.

Currently, the production of one ton of hydrogen results in 12 tons of CO2 emissions and 95% of the world’s hydrogen is produced from fossil resources. This is what we call gray hydrogen. A situation that is incompatible with the long-term roll-out of the hydrogen industry. Especially since, even if the CO2 emitted by current processes can be captured in a controlled environment, fossil resources will not be able to meet the government’s ambitions for this energy. It is therefore essential to develop other modes of “carbon-free hydrogen” production. Within the Carnot H2Mines network, researchers from the different IMT schools are working on processes that could turn the color palette of today’s hydrogen to green.

From blue to green

One process in line with the French government’s plan published last September is water electrolysis. This consists in separating an H2O molecule into hydrogen and oxygen using an electricity supply. This is a carbon-free solution, provided the electricity comes from a renewable source. But why turn an already clean energy into gas? “Hydrogen enables the storage of large amounts of energy over the long term, which batteries cannot do on a large scale to power an entire network,” explains Christian Beauger,  a researcher in materials science at Mines ParisTech. Gas therefore partly responds to the problem of intermittent renewable energies.

Researchers therefore want to improve the performance of electrolyzers in order to make them more competitive on the market. The goal is to find the best possible balance between yield, lifespan and reduced costs. Electrolyzers are made up of several electrochemical cells containing two electrodes and an electrolyte, as in the case of fuel cells. There are three main families: alkaline solutions with liquid electrolyte, polymer membrane technologies (PEM) and high-temperature systems based on ceramic solid oxide (SOC). Each presents its own problems.

At Mines ParisTech, Christian Beauger’s team is seeking to increase the lifespan of PEM electrolyzers by focusing on the materials used at the anode. “We are developing new catalyst supports in the form of metal oxide aerogels which must be electronically conductive and capable of resisting corrosion in a humid environment, at a temperature of 80°C and subjected to potentials often higher than 2 volts“, says the researcher. Another major problem also affects the materials: the cost of an electrolyzer. The catalyst present on PEM electrodes is iridium oxide, a compound that is too expensive to encourage widespread use of future high-power electrolyzers. For this reason, researchers are working on catalysts based on iridium oxide nanoparticles. This reduces the amount of material and thus the potential cost of the system.

Shedding light on photocatalysis

In the laboratory, an alternative using solar energy to break water molecules into hydrogen and oxygen is also being considered. This is photocatalysis. The semiconductors used can be immersed in water in powder form. Under the effect of the sun’s rays, the electron-hole pairs created provide the energy needed to dissociate the water molecules. However, the energy levels of these charge carriers must be controlled very precisely to be useful.

“We form defects in materials that introduce energy levels whose position must be compatible with the energy required for the process,” explains Christian Beauger. This ultra-precise work is delicate to carry out and determines the efficiency of photocatalysis. There is still a long way to go for photocatalysts, the most stable of which hardly exceed 1% in efficiency. But this method of hydrogen production should not be dismissed too quickly, as it is cheaper and easier to set up than a system combining a renewable energy source and an electrolyzer.

Turquoise hydrogen using methane pyrolysis

At Mines ParisTech, Laurent Fulcheri’s team, which specializes in plasma processes, is working on the production of hydrogen not from water, but from the pyrolysis of methane. This technique is still little known in France, but has been widely explored by our German and Russian neighbors. “This process requires electricity, as for the electrolysis of water, but its main advantage is that it requires about seven times less electricity than water electrolysis. It can therefore produce more hydrogen from the same amount of electricity,” he says.

In practice, researchers crack molecules of methane (formula CH4) at high temperature. “To do this, we use a gas in the plasma state to provide thermal energy to the system. It is the only alternative to provide energy at a temperature above 1,500°C without CO2 emissions and on an industrial scale,” says Laurent Fulcheri. The reaction thus generates two valuable products: hydrogen (25% by mass) and solid carbon black (75% by mass).  The latter is not to be confused with CO2 and is notably used in tire rubber, batteries, cables and pigments. The carbon is thus stored in the materials and can theoretically be recycled ad infinitum. “The production of one ton of carbon black by this method avoids the emission of 3 tons of CO2 compared to current methods”, adds the researcher.

This process has already proven itself across the Atlantic. Since 2012, researchers at Mines ParisTech have been collaborating with the American start-up Monolith Materials, which has developed a technology directly inspired by their work. Its location in Nebraska is not insignificant, as it gives it direct access to wind energy in the heart of the corn belt, a major agricultural area in the United States. The hydrogen produced is then transformed into ammonia to fertilize the surrounding corn farms.

Although the machine is working, the research of Laurent Fulcheri’s team, a major player in the start-up’s R&D, is far from over. “Hydrogen production is the simplest task, because the gas purification processes are fairly mature. On the other hand, the carbon black produced can have drastically different market values depending on its nano-structure. The objective is now to optimize our process in order to be able to generate the different qualities of carbon black that meet the demands of consumer industries,” says the researcher. Indeed, the future of this technology lies in the short-term valorization capacities of the two co-products.

Biomass processing: a local alternative

At IMT Mines Albi, Javier Escudero‘s team is working on thermochemical processes for the transformation of biomass by pyrolysis and gasification. Organic waste is heated to high temperatures in a reactor and converted into small molecules of synthesis gas. The hydrogen, carbon monoxide, methane and CO2 thus produced are captured and then recombined or separated. For example, the CO2 and hydrogen can be used to form synthetic methane for use in natural gas networks.

However, a scientific issue has yet to be solved: “The synthesis gas produced is always accompanied by inorganic molecules and large organic molecules called tars. Although their concentration is low, we still require an additional gas purification stage,” explains Javier Escudero. The result is an increase in processing costs that makes it more difficult to implement this solution on a small scale. The researcher is therefore working on several solutions. For example, the exploration of different catalyst materials that could accelerate certain reactions to separate molecules from waste, while eliminating tars.

This approach could be envisaged as a form of local energy recovery from waste. Indeed, these technologies would enable a small and medium-scale territorial network with reactor sizes adapted to those of the collection centers for green waste, non-recovered agricultural residues, etc. However, there is also a need to clarify the regulations governing this type of facility. “For the moment, the law is not clear on the environmental constraints imposed on such structures, which slows down their development and discourages some manufacturers from really investing in the method,” says the researcher.

There is no shortage of solutions for the production of carbon-free hydrogen. Nevertheless, the economic reality is that in order to be truly competitive, these processes will have to produce hydrogen cheaper than hydrogen from fossil fuels.

By Anaïs Culot

Power-to-gas, when hydrogen electrifies research

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Hydrogen is presented as an energy vector of the future. In a power-to-gas system, it serves as an intermediary for the transformation of electricity into synthetic methane. The success of this energy solution depends heavily on its production cost, so IMT researchers are working on optimizing the different processes for a more competitive power-to-gas solution.

Increasing production of renewable energy and reducing greenhouse gas emissions. What if the solution to these two ambitions were to come from a single technology: power-to-gas? In other words, the conversion of electricity into gas. But why? This method allows the storage of surplus electricity produced by intermittent renewable sources that cannot be injected into the grid. The energy is then used to produce hydrogen by electrolysis of water. The gas can then be consumed on site, stored, or used to power hydrogen vehicles. But these applications are still limited. This is why researchers are looking at transforming it into other useful products such as methane (CH4) to supply natural gas networks. What is the potential of this technology?

Costly hydrogen?

“The main issue with the development of power-to-gas today is its cost,” says Rodrigo Rivera Tinoco, a researcher in energy systems modeling at Mines ParisTech. If we take into account the cost of producing hydrogen using a low-temperature electrolyzer (PEM), the technology envisaged in power-to-gas installations, a 1 GW hydrogen reactor (almost the power equivalent of a nuclear reactor) would today cost €3 billion.” In September, the French government allocated a budget of €7 billion in aid for the development of the national hydrogen industry. A reduction in the production cost of this gas is therefore necessary. All the more so since power-to-gas technologies are destined to compete with other energy modes on the market.

France wants to reach a cost of €50 per megawatt-hour in 2030. However, a low-cost but short-lasting technology would not be suitable. “To be cost-effective, systems must have a minimum 60,000 to 90,000 hour operating guarantee,” adds Rodrigo Rivera Tinoco. Currently, low-temperature electrolyzers (PEMs) have an operating life of between 30,000 and 40,000 hours. This is where research comes in. The objective is to optimize the energy efficiency of low-cost technology.

Which technology for which use?

Digital modeling enables the identification of the strengths and weaknesses of technologies prior to their installation. “We carry out technical and economic studies on the various water electrolysis processes in order to increase their efficiency and reduce their cost,” says Chakib Bouallou, an expert in digital modeling and energy storage at Mines ParisTech. Several technologies exist, but which one is the most suitable for storing renewable energy? On an industrial scale, low-temperature electrolyzers are mature and seem to respond to the intermittent nature of these energy sources.

However, in the evaluation phase, no technology is being ruled out. As part of the ANR MCEC project and in collaboration with Chimie ParisTech, Chakib Bouallou’s team is currently working on a solution based on molten carbonates that relies on the co-electrolysis of water and CO2. “Using performance curves of materials depending on the current, we estimate the efficiency of the systems under different usage scenarios. The overall analysis of this technology will then be compared to other existing techniques”, says the researcher. Indeed, the adaptability of a system will depend above all on the intended use. To complete these studies, however, experiments are essential.

Minerve: a demonstrator for research purposes

In order to gain the knowledge needed to make the transition to power-to-gas, the Minerve demonstrator was installed in 2018 on the Chantrerie campus north of Nantes. “The platform is first and foremost a research tool that meets the needs of experimentation and data collection. The results are intended to help develop simulation models for power-to-gas technologies,” explains Pascaline Pré, a process engineering researcher at IMT Atlantique. Equipped with solar panels and a wind turbine, Minerve also has an electrolyzer dedicated to the production of hydrogen converted, with CO2 in cylinders, into methane. This is then redistributed to a fuel distribution station for natural gas vehicles (CNG) and used for mobility.  The next step is to integrate CO2 capture technology from the combustion fumes of the site’s heating network boilers to replace the cylinders.

Carbon dioxide is very stable in the air. Turning it into useful products is therefore difficult. Pascaline Pré’s team is developing a new process to capture this gas by absorption using a solvent. The gas collected is purified, dried, compressed and sent to the methane plant. However, some hurdles need to be overcome in order to optimize this approach: “Solvent regeneration consumes a lot of heat. It would be possible to improve the energy efficiency of the device by developing an electrified microwave heating system,” explains the researcher. This concept would also reduce the size of the facilities needed for this process for a future industrial installation.

In the long term, Minerve should also serve as a basis for the study of many issues in the Carnot HyTrend project, which brings together part of the French scientific community to look at hydrogen. Within three years, initial recommendations on the different technologies (electrolysis, methanation, CO2 capture, etc.) will be published to improve the existing situation, as well as studies on the risks and environmental impacts of power-to-gas.

What about power-to-gas-to-power?

It is possible to go beyond current power-to-gas techniques by adding an oxycombustion step. As part of the ANR project FluidStory, Chakib Bouallou’s team focused on modeling a device based on three advanced technologies: low-temperature PEM electrolysis, methanation (allowing the storage of electricity in the form of gas) and oxycombustion power plants for the destocking stages. The first two steps are therefore the same as in a classical power-to-gas infrastructure as mentioned above. The difference here is that oxygen and CH4, obtained respectively by electrolysis of water and methanation, are stored in caves for an indefinite period of time. Thus, when the price of electricity rises, the oxy-fuel combustion process reuses these gases to produce electricity. The CO2 also emitted during this reaction will be reused by the methanation process in the next cycle.

This closed-cycle design therefore allows autonomous operation with regard to the required reagents, which is not possible in conventional power-to-gas setups. However, analyses aimed at better understanding its mechanics and the nature of the interactions between its components have yet to be conducted.

Looking towards power-to-X

The methanation at the heart of the processes mentioned so far is only one example of the transformation of hydrogen in contact with CO2. Indeed, these reactions, called hydrogenation reactions, are used to synthesize many chemicals usually obtained from fossil resources. At IMT Albi Mines, Doan Pham Minh’s team is working on the optimization of these processes. As well as methane production, researchers are targeting the synthesis of liquid biofuels, methanol, ethanol and other carbon-based chemicals. All these “X” compounds are therefore obtained from hydrogen and CO2. Two factors determine the nature of the result: the operating conditions (temperature, pressure, residence time, etc.) and the catalyst used. “This is what drives the reaction to a target product. Thus, by developing active, selective and stable catalytic materials, we will improve yields in synthesizing the desired product,” the researcher explains.

Methanol is of particular interest to industries. Indeed, this compound is everywhere around us and is used in particular for the surface materials of furniture, in paints, plastics for cars, etc. The same is true for ethanol, biofuels or chemical intermediates of renewable origin. Beyond the role of hydrogen in the national energy mix, the researcher therefore insists on its use by other high-consumption sectors: “It is widely used by the chemical industry and we must be ready to develop competitive and high-performance processes by anticipating future uses of hydrogen and power-to-X.”

By Anaïs Culot

Fuel cells in the hydrogen age

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Hydrogen-powered fuel cells are recognized as a clean technology because they do not emit carbon dioxide. As part of the energy transition aimed at transforming our modes of energy consumption and production, the fuel cell therefore has a role to play. This article will look at the technologies, applications and perspectives with Christian Beauger, a researcher specialized in material sciences at Mines ParisTech

How do fuel cells work?

Christian Beauger: Fuel cells produce electricity and heat from hydrogen and oxygen reacting at the heart of the system in electrochemical cells. At the anode, hydrogen is oxidized to protons and electrons —the source of the electric current— while at the cathode, oxygen reacts with the protons and electrons to form water. These two electrodes are separated by an electrolyte that is gas-tight and electronically insulated. As an ion conductor, it transfers the protons from the anode to the cathode. To build a fuel cell, several cells must be assembled (a stack). The nominal voltage depends on their number. Their size determines the maximum value of the current produced. The dimensions of the stack (size and number of cells) depend on what the device is to be used for.

Within the stack, the cells are separated by bipolar plates. Their role is to supply each cell with gas, to conduct electrons from one electrode to the other and to cool the system. A fuel cell produces as much electricity as heat, and the temperature of use is limited by the materials used, which vary according to the type of cell.

Finally, the system must be supplied with gas.  Since hydrogen does not exist naturally, it must be produced and stored in pressurized tanks. The oxygen used comes from the air, supplied to the fuel cell by means of a compressor.

What is the difference between a fuel cell and a battery?

CB: The main difference between the two is in their design. A battery is an all-in-one technology whose size depends both on the power and the desired autonomy (stored energy). On the contrary, in a fuel cell, the power and energy aspects are separated. The available energy depends on the amount of hydrogen on board, often stored in a tank. So a fuel cell has very varying levels of autonomy depending on the size of the tanks. The power is linked to the size of the stack. Recharging times are also very different. A hydrogen-powered vehicle can be refueled in minutes, compared to a battery that usually takes several hours to charge.

What are the different types of fuel cells?

CB: There are five major types that differ according to the nature of their electrolytes. Alkaline fuel cells use a liquid electrolyte and have an operating temperature of around 70°C. With my team, we are working on low-temperature fuel cells (80°C) whose electrolyte is a polymer membrane; these are called PEMFCs. PAFCs use phosphoric acid and operate between 150°C and 200°C. MCFCs have an electrolyte based on molten carbonates (600-700°C). Finally, those with the highest temperature, up to 1000 °C, use a solid oxide (SOFC), i.e. a ceramic electrolyte.

Their operating principle is the same, but they do not present the same problems. Temperature influences the choice of materials for each technology, but also the context in which they are used. For example, SOFCs take a long time to reach their operating temperature and therefore do not perform optimally at start-up. If an application requires a fast response, low-temperature technologies should be preferred. Overall, PEMFCs are the most developed.

What are the technical challenges facing fuel cell research?

CB: The objective is always to improve performance, i.e. conversion efficiency and lifespan, while reducing costs.

For the PEMFCs we are working on, the amount of platinum required for redox reactions should be reduced. Limiting the degradation of the catalyst support is another challenge. For this purpose, we are developing new supports based on carbon aerogels or doped metal oxides, with better corrosion resistance under operating conditions. They also provide a better supply of gas (hydrogen and especially air) to the electrodes. We have also recently initiated research on platinum-free catalysts in order to completely move away from this expensive material.

Another challenge is cooling. One option to make more efficient use of the heat produced or to reduce cooling constraints in the mobility sector is to be able to increase the operating temperature of PEMFCs. The Achilles heel here is the membrane. With this in mind, we are working on the development of new composite membranes.

The path for SOFCs is reversed. With a much higher operating temperature, there are fewer kinetic losses at the electrodes and therefore no need for expensive catalysts. On the other hand, the heavy constraints of thermomechanical compatibility limit the choice of SOFC constituent materials. The objective of the research is therefore to lower the operating temperature of SOFCs.

Where do we find these fuel cells today?

CB: PEMFCs are the most widespread and marketed, primarily in the mobility sector. The fuel cell vehicles offered by Hyundai or Toyota, for example, carry a fuel cell of about 120 kW. Electricity is generated on board by the fuel cell hybrid to a battery. The battery preserves the fuel cell during strong accelerations. Indeed, although it is capable of rapidly supplying the required energy, this driving phase accelerates the degradation of the core materials. Fuel cells can also be used as range extenders as originally developed by SYMBIO for Renault electric vehicles. In this case, hydrogen takes over when the battery weakens. The fuel cell can then recharge the battery or power the electric motor.

Another example of commercialization is micro-cogeneration, which makes it possible to use the electricity and heat produced by the fuel cell. In Japan, the Ene Farm program, launched in 2009, has enabled tens of thousands of residential cogeneration systems to be marketed, built using PEMFC or SOFC stacks with a power output of around 700 W.

You mentioned the deterioration of materials and the preservation of fuel cells in use: what about their lifespan?

CB: Lifespan is mainly impacted by the stability of the materials, especially those found in the electrodes or that make up the membrane. The highly oxidizing environment of the cathode can lead to the degradation of the electrodes and indirectly of the membranes. The carbon base of PEMFC electrodes has a particular tendency to oxidize at the cathode. The platinum on the surface can then come away, agglomerate, or migrate towards the membrane to the point of degrading it. Ultimately, the target for vehicles is 5,000 hours of operation and 50,000 hours for stationary applications. We must be at two-thirds of that goal now.

Read more on I’MTech Hydrogen: transport and storage difficulties

What are the prospects for fuel cells now that hydrogen is receiving investment support?

CB: Applications for mobility are still at the heart of the issue. Interest is shifting towards heavy vehicles (buses, trains, light aircraft, ships) for which batteries are insufficient. Alstom’s iLint hydrogen train is being tested in Germany. The aeronautics sector is also conducting tests on small aircraft, but hydrogen-powered wide-body aircraft are not for the immediate future. PEMFCs have the advantage of offering a wide range of power to meet the needs of the various uses from mobile applications (computer, telephone, etc.) to industry usage.

Finally, it is difficult to talk about fuel cells without talking about hydrogen production. It is also often talked about as a means of storing renewable energy. To do this, the reverse process to that used in the fuel cell is required: electrolysis. The water is dissociated into hydrogen and oxygen by applying a voltage between the two electrodes.

Overall, it should be remembered that fuel cell deployment only makes sense if the method of hydrogen production has a low carbon footprint. This is one of the major challenges facing the industry today.

Interview by Anaïs Culot.

For more information about fuel cells:

Turning exhaust gases into electricity, an innovative prototype