Enhancing short-term electron exchange in pyrogenic carbonaceous materials through post-pyrolysis oxidative treatments

Abstract

Pyrogenic carbonaceous materials (PCMs) can mediate environmentally beneficial redox reactions through electron exchange with contaminants and microorganisms. Electron exchange kinetics and capacities of PCMs are generally too low for applications that operate on short time scales, such as contaminant pump and treat reactors and biological activated carbon systems. Here, we aimed to increase electron exchange kinetics and capacities over the short term (hours) using a high-surface area activated carbon cloth (ACC) derived from a highly pure novoloid phenol-aldehyde feedstock subjected to oxidative treatments [hydrogen peroxide (H₂O₂) and nitric acid (HNO₃)]. Mediated electrochemical reduction experiments revealed substantial increases in both the magnitude and rate of electron accepting capacity (EAC) resulting from oxidative treatment. HNO₃-treated ACC reached a maximum EAC of 9.16 ± 0.36 mmols e⁻ per g-AC (30 min HNO₃ treatment) within 5–6 hours, a 5.1× increase relative to the non-treated ACC. The initial electron transfer rate (within the first 30 min) of the HNO₃-treated ACC reached a maximum of 0.11 ± 0.02 mmols e⁻ per g-AC per min, which was ∼3.5× faster than the non-treated ACC. We observed both an increase in oxygen content throughout the ACC and decrease of micropore volume after HNO₃ treatment, suggesting that electron exchange kinetics were improved by the appearance of additional redox-active oxygen groups and improved transport of the electron mediator in and out of the ACC pores. H₂O₂ treatment increased only the exterior surface oxygen content and was associated with loss of some redox functionality over time. These findings further our understanding of the physicochemical properties that contribute to PCM electron transfer and help transition reactive PCMs toward applications that would benefit from rapid redox transformations.