2024-03-11 10:30:07
A consortium of researchers from Bordeaux has filed a patent to transform waste from the paper industry into carbon polymers. These materials could be used for batteries or to store hydrogen.
From the paper industry’s kraft process, an unsavory residue escapes: black liquors, a sort of viscous sludge composed of lignin and hemicellulose. Rich in elements but difficult to recover, it is often used by paper makers in recovery boilers, in order to provide them with electricity and heat by cogeneration and to recover inorganic chemicals.
Researchers from several entities* based in Bordeaux are paving the way for another valorization, a priori interesting for the energy transition. By working for several years on obtaining new catalytic materials – notably thanks to the pioneering work of Hervé Deleuze – they understood that appropriate heat treatment of black liquor makes it possible to create carbon polymers. That is to say a potentially very useful material for electricity storage systems or hydrogen storage.
Heat treatment up to 900°C
The good results of the trials lead researchers to publish regularly on this subject. An article in Langmuir end of 2023 particularly evokes the transformation of black liquor from the manufacture of kraft paper into polymer, in the form of a porous monolithic material. Subsequently, the principles mentioned in this publication were the subject of a patent application by the researchers.
To obtain this new material, the treatment may seem quite trivial: “Black liquor is an alkaline solution that we emulsify, dry, then wash, and pass through an oven. A reducing atmosphere is created and the heat treatment is done in stages up to 900°C. The advantage is that the final product will almost always be the same, whatever the differences in quality of black liquors (oxygen and sulfur levels for example).”explains Rénal Backov, professor of integrative chemistry at the Paul Pascal Research Center.
The product obtained is a semi-graphitic material having a good balance between two types of carbon (SP3 and SP2), “which offers a good balance between microporosity and electronic conductivity”, specifies the specialist. These monoliths thus have a high specific accessible surface area of 600 to 1,500 m2/g, of the same order of magnitude as existing products on the market. It also has an open macroporosity releasing a pore volume of 1.13 cm3/g, allowing optimized mass transport. Enough to make them very good catalysts, while promoting a by-product of the paper industry.
Potential application in supercapacitors
The work was carried out as part of the major research program Post Petroleum Materials since they aim to find alternatives to carbon graphites made from resources of fossil origin. Other waste has also been tested, such as coconut or shrimp carcasses. But they are in limited quantity, while black liquors are produced at 65 million tonnes per year worldwide, which can make them an inexpensive product.
“We have already tested two conclusive applications for this material from black liquors. The first is to use it as an electrode in supercapacitors. These electrochemical storage systems are present in multiple aspects of daily life, such as the start&stop of cars, braking energy recovery systems of electric vehicles, etc. Our material reaches a capacitance of 145 F/g when subjected to 5 mV/s, a performance close to materials obtained with coconuts, explains Jacob Olchowka, CNRS research fellow at the ICMC in Bordeaux. The second application is the storage of hydrogen: we have demonstrated that the porous nature of our carbonaceous graphite, at room temperature and under a pressure of 40 bar, allows it to absorb 50% more hydrogen than a similar material commercially available ».
Bordeaux researchers are multiplying tests to refine the material and its applications. For example, it involves optimizing its role as an electrode for supercapacitors, by adding a tiny portion of carbon black. Or even use it as a negative electrode in sodium-ion batteries. A vast field to discover, and for which researchers are seeking support from industrialists.
* The Paul Pascal Research Center (CNRS / University of Bordeaux), the Institute of Condensed Matter Chemistry (CNRS / Bordeaux INP / University of Bordeaux) and the Institute of Molecular Sciences (CNRS / Bordeaux INP / University of Bordeaux )
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