Study on the Design of Electrocatalysts for Polymer Electrolysis in the Aqueous Solution
Authors: Kazuma Hisazaki , Maruyama Takahiro , Saida Takahiro 
Published: February 2024

Now, most plastic recycling is thermal recycling by incineration. As a matter of course, thermal recycling is not a sustainable recycling method for plastic material. Material recycling is being carried out in which some used plastics are molded and reused.[1] However, material recycling has the problem that cannot separate the pigments and dyes. This problem causes the limitation of color and quality degradation in the recycling of plastics. Therefore, we focused on the depolymerization and re-polymerization between polymers and monomers by electrolysis in order to develop a new recycling method without degrading the quality of the plastics.
Since most polymers are insulator materials, there has been almost no report about polymer electrolysis, and the electrochemical behavior is also unknown. [2, 3] We investigated to obtain basic information for the electrolysis of polymers, a monovalent alcohol molecule bonded to the terminal or side chain and a divalent alcohol molecule bonded to both ends were used as the model molecule, and their electrochemical behavior was investigated. In addition, electrolysis of PVA and PEG, which have similar structures to alcohol molecules, was conducted as the first step of depolymerization into monomers. In the case of PEG, the oxidation current, which may be attributed to the decomposition, was observed. In contrast, the decomposition behavior of PVA was not confirmed. Their results suggested that the oxidation of polymer may favor the presence of other atoms, such as oxygen, at the main chain rather than the functional group at the side chain. Nylon66, which had nitrogen atoms at the main chain, also showed oxidation currents. However, Nylon66 was only oxidized by the iron-based catalyst. Iron should form the surface oxide layer at the oxidation potential of Nylon66. However, it doesn’t understand whether metal or metal oxide is suitable for the electrolysis of polymer materials. This study electrolyzed Nylon66 using Ti foil and Ru and RuO2 powder to consider whether metal or oxide was suitable for polymer electrolysis.
The electrochemical cell was composed of a working electrode, a carbon counter electrode, a reversible hydrogen electrode (RHE) as a reference electrode, and 0.2 M NaOH. Cyclic voltammetry (CV) was conducted at room temperature in an inert atmosphere, and the scan rate was 50 mV s-1. Nylon66 was polymerized on the working electrode directly. Furthermore, before and after electrolysis, its molecular structure and electronic state on the electrode surface were confirmed by Raman and XPS.
All test catalysts in this study showed the oxidation current of Nylon66 around 1.2 V only in the first cycle. RuO2 demonstrated relatively high oxidation currents compared with other catalysts. In addition, Ti foil formed a thick oxide layer at the surface during CV measurement.
After electrolysis, the Raman spectrum of the Ti foil surface indicated that the C-O-C stretch vibration mode, which did not include in Nylon66, was observed. On the other hand, Ru and RuO2 did not generate new vibrational modes. However, the results of XPS suggested that the sp2 carbon species, C=C bonds, increased in all catalysts. Thus, Nylon66 was probably decomposed. Interestingly, Ru0 species were observed in RuO2 after electrolysis. This result meant RuO2 was partially reduced during electrolysis. It is thought that the lattice oxygen in the RuO2 crystal was possibly consumed by Nylon66 oxidation. The above results indicate that the lattice oxygen of oxide should play an important role in polymer electrolysis.
In this study, electrolysis of Nylon66 was performed using Ti and Ru, RuO2 electrocatalysts. Each catalyst, the oxidation current was observed only in the first cycle. The results of Raman and XPS suggested the partial decomposition of Nylonn66 in all catalysts. It was concluded that the presence of lattice oxygen in oxide is one of the keys to polymer electrolysis, and the metal oxide may be suitable as an electrocatalyst.
[1] T. Kamo, Science of the Field, 1, (2021) 28-44.
[2] O. R. Luca et al. Molecules, 25 (2020) 1-9.
[3] T. Jiang et al. Solar Energy Materials & Solar Cells, 204 (2020) 1-10.