Oxygen Reduction Reaction Driven by PGM-free Electrocatalysts in a Circular Economy Framework

Global carbon emissions demand a shift toward sustainable energy systems, yet the green transition relies heavily on costly and scarce raw materials. Circular economy promotes resource recovery and waste utilization. Fuel cells (FCs) produce clean energy but are limited by the slow oxygen reduction reaction (ORR), typically catalyzed by expensive platinum-group metals (PGMs). Similarly, the synthesis of hydrogen peroxide (H₂O₂) depends on PGM-based, energy-intensive methods. Developing PGM-free electrocatalysts from waste-derived carbons offers a green alternative. These materials exhibit tunable selectivity between the 4-electron ORR for FCs and the 2-electron ORR for green H₂O₂ production. Among them, transition metal–nitrogen–carbons (TM–Nx–Cs) with earth-abundant metals (Fe, Co, Ni, Mn), combine high efficiency, cost-effectiveness, and structural versatility. Carbons can be prepared from C-rich non-valorized wastes through pyrolysis in anoxic atmosphere. Upcycling valorizes waste and reduces the final price of the material. To ensure high efficiency, chemical activation with KOH, creating interconnected pores, and functionalization with metal phthalocyanine, aiming to install the active sites, are used. The electrochemical analysis via rotating ring disk electrode (RRDE) technique enables the comparison of ORR electroactivity, kinetics and selectivity in different conditions, e.g. pH. However, RRDE is not always enough to evaluate the real application of these materials. Hence, fuel cell tests, though complex and expensive, can be used to assess their applicability in real devices, while gas diffusion electrode (GDE) can verify the potential scalability of H2O2 electrosynthesis. This thesis investigates green strategies to develop PGM-free electrocatalysts for ORR via waste upcycling. First, cigarette butts, one of the most littered wastes globally, were converted into Fe–Nx–C electrocatalysts via a synthesis process comprising pyrolysis, activation, and functionalization. Using different cigarette components (paper, tobacco, filter) as feedstocks, all derived catalysts exhibited high activity, comparable to Pt/C in alkaline media, with strong selectivity toward the 4-electron pathway due to pyridinic nitrogen sites—becoming suitable for FCs. Next, spent coffee grounds, another abundant biomass waste rich in C and N, were used to produce dual-site Fe–Mn–Nx–C electrocatalysts. They demonstrated superior ORR performance compared to Pt/C in 0.1 M KOH, and in alkaline membrane FCs achieved an open-circuit potential of 0.89 V and a peak power density of 30 mW cm−2, highlighting the beneficial synergistic effect of dual-metal sites. The study also explored how pH influences ORR activity, selectivity, and kinetics in Fe–N–C materials. By systematically correlating physicochemical properties—such as active site distribution, surface chemistry, and morphology—with electrochemical behavior, this work provided the first comprehensive evaluation of pH-dependent ORR performance in such electrocatalysts. Finally, the thesis introduced a green electrochemical route for H₂O₂ electrosynthesis using waste lignin–derived carbon materials doped with atomically dispersed metals (Fe, Co, Ni). They achieved over 90% peroxide selectivity in RRDE tests, while galvanostatic electrolysis at near-neutral pH identified the sample obtained at 400 °C (L_400) as the most efficient, with energy consumption lower than current industrial methods. Moreover, Fe-functionalized L_400 acted as an effective electro-Fenton catalyst, degrading 97% of the pollutant lisinopril, proving its environmental remediation potential. In summary, this work demonstrates how waste upcycling can drive circular, cost-effective, and sustainable pathways for designing multifunctional PGM-free electrocatalysts for both clean energy conversion and green chemical production.