From ashes to porous hierarchical nanocarbon electrode: Upcycling secondary waste materials through self-catalytic chemical vapour deposition

Metal and metal oxide particles are abundant in various ash-based wastes. Utilising these as catalyst sources for the fabrication of carbon nanomaterials could present a valuable approach to reduce our reliance on non-renewable and costly catalyst sources, thereby facilitating large-scale nanomaterial production. In this context, secondary waste materials (SWMs) are by-products resulting from the (complete or partial) combustion of carbon-rich sources or other industrial processes and their disposal poses a serious environmental problem. In this study, we demonstrate a novel strategy to upcycle SWMs as catalysts, as received, for the growth of carbon nanoarchitectured electrodes through microwave plasma-enhanced chemical vapour deposition (MPECVD), without the need for functionalisation. Firstly, 10 SWMs were selected to fabricate porous hierarchical nanocarbon (PHN) electrodes by phase-inversion and subsequent catalytic MPECVD growth. Secondly, distinct growth conditions, both in the presence and absence of CH4 as an external carbon source were applied, resulting in conductive electrodes, on which acetaminophen oxidation was performed. Results show that not all SWMs, despite originating from similar processes, work as a catalyst. In particular, principal component analysis suggests the presence of calcium oxosilicate and calcium‑magnesium‑iron carbonate as potential catalysts, which are present in two SWMs. Contrary to what might be expected, the occurrence of metals, such as Fe, Ni, is not a sufficient factor for the catalytic growth of carbon nanostructures. Interestingly, water vapour adsorption isotherms suggest the formation of different porous networks according to the specific SWMs. Finally, the development of waste-derived catalysts fosters the concept of upcycling, converting waste into higher-value products, thus closing the loop on resource utilisation and minimising waste generation.