We report a comprehensive analysis of the catalytic oxygen reduction reaction (ORR) reactivity of four of today's most active benchmark platinum group metal-free (PGM-free) iron/nitrogen doped carbon electrocatalysts (Fe–N–Cs). Our analysis reaches far beyond previous such attempts in linking kinetic performance metrics, such as electrocatalytic mass-based and surface area-based catalytic activity with previously elusive kinetic metrics such as the active metal site density (SD) and the catalytic turnover frequency (TOF). Kinetic ORR activities, SD and TOF values were evaluated using in situ electrochemical NO₂ ⁻ reduction as well as an ex situ gaseous CO cryo chemisorption. Experimental ex situ and in situ Fe surface site densities displayed remarkable quantitative congruence. Plots of SD versus TOF (“reactivity maps”) are utilized as new analytical tools to deconvolute ORR reactivities and thus enabling rational catalyst developments. A microporous catalyst showed large SD values paired with low TOF, while mesoporous catalysts displayed the opposite. Trends in Fe surface site density were linked to molecular nitrogen and Fe moieties (D1 and D2 from ⁵⁷Fe Mössbauer spectroscopy), from which pore locations of catalytically active D1 and D2 sites were established. This cross-laboratory analysis, its employed experimental practices and analytical methodologies are expected to serve as a widely accepted reference for future, knowledge-based research into improved PGM-free fuel cell cathode catalysts.

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.