Towards Sustainable Hydrogen Peroxide Electroproduction: Activated Carbon from Sewage Sludge As an Eco-Friendly Electrode Material

Abstract

Hydrogen peroxide (H2O2) has emerged as a chemical of paramount importance, finding diverse applications as a bleaching agent, medical disinfectant, and environmentally benign oxidant [1]. The industrial production of H2O2 currently relies on the anthraquinone oxidation process, primarily executed within large-scale industrial plants [1]. Herein lies a challenge— H2O2, typically concentrated to an 80% level, introduces inherent hazards concerning storage and transportation, prompting a quest for a safer alternative. One way to overcome such problems is by the in-situ electrogeneration of H2O2, which would allow an adjustable production of the chemical on demand directly for its application, avoiding storing costs and safety issues. The in-situ electrogeneration can be done by using a gaseous-diffusion electrode (GDE), which typically enables the application of elevated current densities or substantial overpotentials, consequently facilitating the generation of high concentrations of H2O2, since there is less limitation due to O2 mass transport to the electrode surface [2]. Commonly, these electrodes are constructed from amorphous carbon materials, due to their highly active surface area with oxygenated functional groups, excellent conductivity, and innate selectivity for H2O2 electroproduction. However, a notable challenge emerges – bare carbon's selectivity for H2O2 presents its highest response at high overpotentials, entailing heightened energy consumption. The studies in this field are currently focused on using these types of carbon as support for other catalytic materials that are more active and selective towards H2O2 electroproduction [3, 4]. Numerous studies in the literature have explored the use of carbon materials, with the most commonly employed carbon supports for such applications being Printex 6L (Orion) and Vulcan XC 72R (Cabot). These materials are obtained from the incomplete combustion of heavy petroleum products at high temperatures, being non-sustainable and environmentally unfriendly. A potential solution lies in the use of environmentally conscious carbon sources. While amorphous carbon can be obtained from various carbon-rich wastes through an activation process, their utility in H2O2 electrogeneration remains underexplored. This study sought to obtain activated carbon from sewage sludge, focusing on its efficacy in H2O2 electrogeneration. Acid (H3PO4) and alkaline (KOH/NaOH mixture) carbon activators were analyzed in concentrations ranging from 0 – 30 %m/v, temperatures of 400 – 900 ºC, and activation time from 0 – 2 h at the selected temperature. The obtained materials were analyzed through linear sweep voltammetry using an RRDE setup in N2-saturated and O2-saturated, in a 0.05 M K2SO4 (pH 3) electrolyte solution. Results have shown that the most successful materials for H2O2 electrogeneration were those produced under 900 ºC for 2h impregnated with 5 % m/v H3PO4 and 597 ºC, zero of plateau time impregnated with 20 % m/V mixture of KOH/NaOH in the ratio of 50.5 KOH: NaOH. These optimized materials exhibited remarkable selectivities of ≈ 90% and ≈ 81% at -0.8 V vs. Ag/AgCl, respectively. This level of electrocatalytic performance is on par with that of the commercial reference material, Printex 6L. These findings highlight the promise of our material as a viable carbon support for in-situ H2O2 electrogeneration, offering a more sustainable alternative compared to commercial black carbons. Acknowledgments The authors are sincerely grateful to the Brazilian research funding agencies, including the Brazilian National Council for Scientific and Technological Development - CNPq (grants no. 465571/2014-0, 302874/2017-8, and 427452/2018-0), São Paulo Research Foundation (FAPESP – grants no. #2023/04230-2, #2023/07750-7, 2022/04058-2, #2021/12053-8, #2019/04421-7, #2017/10118-0, and #2013/02762-5).