Efficient Electrochemical Reforming of Water-Insoluble C‑Only Plastic Wastes

Abstract

We report here the efficient electrochemical reforming of hydrocarbon polymer wastes, i.e. composed of C-C and C-H bonds only, in aqueous solution at 3 V. Anodic degradation of these chemically resilient wastes is achieved with Faradaic efficiencies of up to 32% on a Ni/Sb-doped SnO₂ electrode. The hydrophobic plastic particles, initially present as large aggregates, are solubilized during the early stages of the reaction, which is essential to achieve high reforming efficiencies. Cathodic H₂ generation is demonstrated with Faradaic and energy efficiencies of up to 57% and 30%, respectively. Under optimized conditions, electroreforming requires ca. 0.10 kWh/g of plastic degraded, which is >120 times more efficient than that previously reported on boron-doped diamond anodes. If scaled up, energy costs as low as ca. 2000$/ton could be achieved, while the H₂ generated could cover up to ca. 70% of these costs. CO₂ emissions, expected to be ranging from 1.65 to 13.02 kg_CO2eq/kg_H2, are competitive with conventional plastic-to-H₂ high-temperature processes. Our results support the industrial potential of plastic electroreforming to efficiently treat chemically resilient plastic wastes.

Keywords: electrocatalysis; hydrogen; plastic waste; polypropylene; polystyrene; reforming.

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Electroreforming of C-only plastic wastes. (a) Schematic showing coupled anodic mineralization of plastic wastes via direct oxidation and mediated by HO^• radicals, and cathodic H₂ generation. Bottom left inset: Photograph of an aqueous plastic waste stock solution showing significant aggregation of the plastic particles in solution and at the air–water interface. (b-d) Typical photographs of the electrochemical cell filled with an aqueous 0.75 M LiClO₄ electrolyte containing PS nanoparticles at a concentration of 0.46 g/L: (b) Before electroreforming; (c) After 30 min of electroreforming at 6 V and moderate stirring: Significant foaming occurs, which prevents reliable and efficient electroreforming of the plastic wastes; (d) After 10 h of electroreforming at 4.5 V under vigorous stirring: The plastic electrolyte solution is clear due to efficient mineralization of the plastic particles.

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Plastic waste particles. (a–c) Photographs of the plastic wastes used: polystyrene (PS) waste obtained from tape casing (a), referred to as PS₁, and Petri-dish (b), referred to as PS₂, and polypropylene (PP) waste obtained from centrifuge tube (c). The plastic wastes were used to prepare: (d) monodisperse nanoparticle dispersions with a particle diameter d ∼ 100 nm via dissolution–precipitation and (f) plastic powders composed of highly polydisperse particles with sizes up to ca. 20 μm via mechanical grinding. (e, g) Typical SEM images of the resulting nanoparticles (e) and particle powders (g).

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Plastic waste mineralization. (a) Electroreforming at 4.5 V of PS₁ nanoparticles prepared via dissolution/precipitation. FE_{PS1‑to‑CO2} (left axis, blue circles) and plastic degradation efficiency (right axis, green circles), as a function of electroreforming duration. Two data points are provided for 10 h of electroreforming. (b) FE_{plastic‑to‑CO2} (blue bars, left axis) after 30 h electroreforming at 3 V of PS₁, PS₂ and PP particles. np: nanoparticle dispersions prepared via dissolution/precipitation. Powders: particle powders prepared via mechanical grinding. The corresponding residual TOC is shown for each experiment (red bars, right axis). The expected TOC originating from the stock solution containing 0.46g/L of plastic particle is shown as a comparison, labeled Plastic solution.

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H₂ generation during plastic electroreforming. (a) FE_H2 (in blue) and FE_O2 (in green) measured via GC for different plastic particles. The control electrolysis experiment performed in pure 0.75 M LiClO₄ with a Pt cathode and NATO anode is labeled “No plastic”. (b) Detected H₂ (black square) and O₂ (red circles) concentrations in the chamber as a function of time during the electroreforming of PS₂ nanoparticles. The expected gas concentration corresponding to 100% Faradaic efficiency is provided as a guide for both H₂ (blue triangles) and O₂ (green inverted triangles). (c) Measured H₂/O₂ ratio. (d) Energy efficiency of H₂ generation (light orange bars, left axis) and estimated percentage of the energy costs that could be recovered by the sale of the H₂ produced during the electroreforming experiment (red bars, right axis).