Plastic-derived substrate-grown carbon nanotubes as freestanding electrode for hydrogen evolution in alkaline media

Switching from a linear approach of waste management to a circular approach by transforming plastic waste into a higher value product is a promising direction towards plastic waste treatment. In this study, plastic pyrolysis gas was used to fabricate carbon nanotubes (CNTs) based freestanding, binder-free electrode for hydrogen evolution reaction (HER). This study investigated the quality of CNTs synthesized on conductive carbon paper, semiconductive silicon and dielectric glass substrates, and their HER performance in 1 M KOH. CNTs synthesized on silicon were observed to be hollow-core, whereas nanotubes grown on glass and carbon paper displayed bamboo-like morphology, possessing better graphitization than CNTs grown on silicon. This is in agreement with HER performance, with the as-prepared electrodes requiring an overpotential of 267 mV, 241 mV and 216 mV to achieve 10 mA/cm2 for silicon, glass and carbon paper respectively. Despite being poorly conductive, the glass substrate electrode achieved a lower overpotential than the silicon electrode. The as-prepared silicon electrode also faced a delamination issue likely attributed to the lower surface energy of the hydrophobic silicon surface. The proposed approach thus showed that the in-situ fabricated electrodes performed better than separately synthesized CNTs prepared into electrodes by 27.4% and 14.2% for carbon paper and glass substrates respectively. The improved performance of the freestanding, binder-free electrodes can be linked to the lower charge-transfer resistance and reduced contact resistance between the CNTs and substrate.