Utilization of waste plastic for plasma-assisted deposition of carbon nanowalls on electrodes for sustainable high-performance energy-storage devices

Abstract

Current global policies, such as circular economy, electrification, and low carbon footprint dictate the replacement of fossil fuels for synthesizing electrodes of graphene supercapacitors with renewable sources. The traditional method for depositing graphene and similar structures is based on gaseous hydrocarbons. The possibility of replacing them with waste plastic is presented. We used propane and waste plastics (mostly polypropylene) for the deposition of a few µm-thick deposit of carbon nanowalls composed of vertically oriented multilayer graphene-like sheets on electrodes by plasma-enhanced chemical vapor deposition (PECVD). We studied their morphology and structure, as well as the performance of coin-shaped supercapacitors. The graphene-like coatings were deposited in plasma sustained at the pressure of 16 Pa by inductively coupled RF discharge in the H mode at the forward power of 500 W and power density approximately 8x106 W/m3. Such a powerful plasma caused thermal decomposition of the waste plastic. Optical emission spectroscopy showed significant differences only within the first few seconds of plasma treatment, which was enough to supply precursors for graphene growth. Both scanning and transmission electron microscopies showed similar structures for deposits prepared from propane or waste plastic, and the deposition rates of roughly 100 nm/s were observed. Waste plastic enabled the deposition of 2–3 times densely packed multilayer graphene-like sheets of a typical thickness of 5 nm at deposition times over 1 min. Raman spectroscopy indicated a somewhat lower concentration of defects for samples deposited from polypropylene, but the ID/IG versus the average distance between the neighboring multilayer graphene-like sheets followed the same line. The specific capacitance increased steeply with deposition time for deposits prepared from waste plastic and it was found to be linearly dependent on the ratio between the effective and projected areas of both precursors. The capacitance versus the ID/IG followed the same curve for both precursors. The results demonstrate the feasibility of replacing gaseous hydrocarbons with waste plastics for PECVD deposition of graphene electrodes.