Electrooxidation of Poly(methyl-methacrylate) and Poly(ethylene glycol): Hydrogen Production from Plastic Wastes Electrolysis

Abstract

Introduction. Green H2 production by water electrolysis at low temperature has some bottlenecks mostly dealing with the anodic reaction, the oxygen evolution reaction (OER), given its thermodynamic and kinetic limitations [1]. One strategy to reduce the energy requirements for this H2 production route is the replacement of the conventional OER (with a thermodynamic potential of 1.23 V vs. Reversible Hydrogen Electrode (RHE)) by a less-energy intensive reaction like the electrooxidation of organic molecules (in some cases presenting a thermodynamic potential as low as c.a. 0 V vs RHE). This approach also addresses the possibility of converting organic wastes into value-added chemicals avoiding the CO2 emissions derived from conventional treatments (e.g., incineration) [2, 3]. Our research group is focused on the electrochemical valorization of several organic wastes like polyethylene glycol (PEG) or poly(methyl-methacrylate) (PMMA). Both are commonly found in urban and industrial water effluents and concern has been raised about the true fate of these polymers in the environment. Herein, the possibility of generating H2 and value-added organic molecules from these plastic wastes has been explored. Experimental. A binary solvent strategy was followed to dissolve the PMMA polymer, by using 1 M H2SO4 in 80% v/v 2-propanol/water electrolyte. Then the electrooxidation of PMMA was studied in a 2-electrode batch cell at 70ºC with different PMMA concentrations (0.1-2 wt.%). On the other hand, the electrooxidation of ethylene glycol (EG) and PEG with various molecular weights (from 200 to 4000 g mol-1), which are soluble in water, was carried out in aqueous solutions (PEG concentrations from 1 to 20 g/L) between 30 and 80 °C by using a Polymer Electrolyte Membrane (PEM) reactor. Pt/C electrodes were used in all cases. A commercial Pt cathode was employed and the anode material was prepared by the spray-deposition of a Pt(20 wt.%)/C catalyst powder onto a carbon paper gas-diffusion layer. The liquid products at the anodic compartment were analysed by size-exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) spectroscopy while the production of hydrogen in the cathodic compartment was online monitored with a mass spectroscopy. Results and discussion. In the case of PMMA electrooxidation, despite the dissolution challenge, polymer macromolecules showed to partially block the accessibility of the Pt active sites on the electrode and strongly degraded its electrochemical performance [4]. In the case of PEG, it was much more easily electrooxidised, at low cell voltages, from around 0.4 V at 80°C (Figure 1a), near to values recorded with EG, suggesting a first step linked with the electrooxidation of the terminal groups of the macromolecules chain. Current densities larger than 100 mA.cm-2 at 0.8 V and 80°C were achieved for low molecular mass molecules (200 and 400 g/mol). A decrease of the PEG molecular weight in the anodic solution was demonstrated by SEC, confirming that the electro-oxidation process likely follows two main mechanisms, involving the consumption of terminal –OH or intermediate -C-O-C- bonds cleavage (Figure 1b), depending on the polymer molecular weight. This study demonstrates the proof-of-concept of the low-temperature electro-oxidation of polymers containing C-O bonds in their main chain. Conclusions. With a further macroporous electrode enegineering and catalyst development, the electrolysis of plastic wastes stands as a promising technology not only for the production of hydrogen from macromolecules by electrochemical means, but also as a potential pathway to depolymarize plastic wastes, thus increasing their degradability. References. [1]. Y. Cheng, S.P. Jiang, Advances in electrocatalysts for oxygen evolution reaction of water electrolysis-from metal oxides to carbon nanotubes, Prog. Nat. Sci.: Mater. 25 (2015) 545-553.[2]. M. Simões, S. Baranton, C. Coutanceau, Electrochemical valorisation of glycerol, ChemSusChem. 5 (2012) 2106–2124[3]. Y.X. Chen, A. Lavacchi, H.A. Miller, M. Bevilacqua, J. Filippi, M. Innocenti, et al. Nanotechnology makes biomass electrolysis more energy efficient than water electrolysis. Nat. Commun. 5 (2014) 1-6.[4]. N. Grimaldos-Osorio, F. Sordello, M. Passananti, J. González-Cobos, A. Bonhommé, P. Vernoux, A. Caravaca. From plastic-waste to H2: A first approach to the electrochemical reforming of dissolved Poly(methyl methacrylate) particles. Int. J. Hydrogen Energy. 48 (2023) 11899-11913. Acknowledgments The authors gratefully acknowledge the French institution “Ecole Urbaine de Lyon” (EUL - Institut de Convergences) for funding a PhD grant of Dr. N. Grimaldos-Osorio. Figure 1