Chitosan-derived nitrogen-doped carbon electrocatalyst for a sustainable upgrade of oxygen reduction to hydrogen peroxide in UV-assisted electro-Fenton water treatment

Abstract

The urgency to move from critical raw materials to highly available and renewable feedstock is currently driving the scientific and technical developments. Within this context, the abundance of natural resources like chitosan paves the way to synthesize biomass-derived nitrogen-doped carbons. This work describes the synthesis of chitosan-derived N-doped mesoporous carbon in the absence (MC-C) and presence (N-MC-C) of 1,10-phenanthroline, which acted as both a porogen agent and a second nitrogen source. The as-prepared MC-C and N-MC-C were thoroughly characterized and further employed as catalytic materials in gas-diffusion electrodes (GDEs), aiming to develop a sustainable alternative to conventional GDEs for H2O2 electrogeneration and photoelectro-Fenton (PEF) treatment of a drug pollutant. N-MC-C presented a higher content of key surface N-functionalities like the pyrrole group, as well as an increased graphitization degree and surface area (63 vs 6 m2/g), comparable to commercial carbon black. These properties entailed a superior activity of N-MC-C for the oxygen reduction reaction, as confirmed from its voltammetric behavior at a rotating ring-disk electrode. The GDE prepared with the N-MC-C catalyst showed greater H2O2 accumulation, attaining values close to those obtained with a commercial GDE. N-MC-C- and MC-C-derived GDEs were employed to treat drug solutions at pH 3.0 by the PEF process, which outperformed electro-oxidation. The fastest drug removal was achieved using N-MC-C, requiring only 16 min at 30 mA/cm2 instead of 20 min required with MC-C. The replacement of the dimensionally stable anode by a boron-doped diamond accelerated the degradation process, reaching an almost complete mineralization in 360 min. The main degradation products were identified, revealing the formation of six different aromatic intermediates, alongside five aliphatic compounds that comprised three nitrogenated structures. The initial N was preferentially converted into ammonium.