Towards an efficient treatment and valorisation of plastic-wastes: Hydrogen production by electrolysis

Hydrogen is called to play an important role in the energy transition because of its ability to act as an energy carrier without emitting carbon dioxide. The production of carbon-free hydrogen is a major concern in order to make its use a sustainable solution. The electrolysis of water makes it possible to generate pure and carbon-free hydrogen from the various intermittent sources of renewable energy. Nonetheless, the energy demand for this process is elevated (>1.23 V at room temperature). The electro-oxidation of organic molecules was suggested as an answer to this problem because their thermodynamic oxidation potential is much lower than that of the oxygen evolution reaction, which takes place at the anode of the electrolysers. Organic waste could be a sustainable source of organic fuels if their electro-oxidation kinetics are sufficiently efficient. Nowadays, several problems are threatening the stability of life on the planet and the survival of humanity is in question. We have reached the point of having to reconsider our models of consumption and production of energy and goods. For this reason, not only energy sustainability must be evaluated, but also the fate of the waste that we produce daily. One of the most important and environmentally harmful wastes is plastic, which can also be considered an important source of hydrogen because of its organic nature. Taking advantage of this type of waste for the production of H₂ seems to be an attractive option, as long as the multiple limitations that this application represents can be overcome. The aim of this work is to explore in depth, for the first time, the electrolysis of polymers at low temperature for the production of hydrogen. This thesis work proposes a new approach to understand the electrolysis of plastic waste using two model polymers: polymethyl methacrylate (PMMA) and polyethylene glycol (PEG). The different composition of these polymers, as well as the use of different experimental strategies, allowed us to establish a series of significant discoveries on the feasibility of this type of process, as well as a series of improvements necessary to upturn their performance. The electro-oxidation of the lateral ester groups of PMMA has been approached according to three different strategies: (i) treatment of model molecules, (ii) solubilisation in a binary solvent, and (iii) direct attack by electrochemical methods. On the other hand, the electro-oxidation of aqueous solutions of PEG allowed us to deepen our understanding of the phenomena involved in the breaking of C-O bonds characteristic of a wide range of polymeric materials. For the electrochemical treatment of PMMA and PEG, the transport of macromolecules in the porosity of the electrodes strongly limits the kinetics of electrooxidation. A series of characterisation techniques were used to determine the impact of the porosity and the nanostructure of the different implemented electrodes. A more porous morphology of the Pt/Carbon catalyst associated with a better dispersion of the Pt nanoparticles makes possible to considerably increase the electrochemical conversion performance of the PEG. Finally, it is proposed that the technology to be implemented to recover plastic waste depends on the nature of the most easily electro-oxidisable chemical bonds in the main polymer chain. (author)