Oxygen Reduction Reaction Platinum Group Metal-free Electrocatalysts Derived from Spent Coffee Grounds

Fe-N-C materials are emerging as a class of cheap and highly-active platinum group metal-free (PGM-free) electrocatalysts for oxygen reduction reaction (ORR) in alkaline fuel cells. The material optimization together with the intelligent selection of the primary raw material can make Fe-N-Cs more competitive than PGMs in the current market. The addition of a Mn as a second metal can improve the ORR activity with a bifunctional catalysis, in which the presence of Mn in the vicinity of Fe permits a faster reduction of *OH intermediate after its transfer from Fe to Mn site [1], [2]. Moreover, the upcycling of spent coffee grounds (SCGs) as a costless carbon and nitrogen source can not only alleviate the environmental accumulation of the waste in the environment but also make the final electrocatalyst cheaper. Herein, bimetallic dual-site electrocatalysts were synthesized by 1) pyrolyzing SCGs (furnished by the illycaffè company) at 400, 600, 800 and 1000 °C, 2) activating the porosity of the as-obtained char with KOH and 3) functionalizing the activated carbon with iron(II) and manganese(II) phthalocyanine. The final materials showed a high degree of defectivity (increasing with the pyrolysis temperature) and specific surface area (up to 1820 m2 g −1) together with a large amount of pyridinic sites. The electrocatalysts were tested in a three-electrode system with rotating ring disk electrode using 0.1 M KOH electrolyte. The best material in terms of activity and selectivity was CFeMn_600, showing the same half-wave potential as Pt/C standard and a 4-electron production. These results could be due to high surface area, Fe-Mn synergy, and abundance of C-N defects. While the comparison between the only activated and functionalized electrocatalyst prove the fundamental role in the metal in pursuing the ORR, the poor performance of the non-activated CFeMn_600 (i.e. CFeMn_noA_600) compared to the activated CFeMn_600 justify the activation step in the material preparation and highlight the importance of surface area in the ORR electroactivity. The best performing CFeMn_600 retained its onset and half-wave potential after 2000 cycles of stability. Eventually, CFeMn_600 was tested as a cathodic electrocatalyst in alkaline exchange membrane fuel cells (AEMFC). The results show an open circuit voltage (OCV) of 0.890 V, close to the best state-of-the-art PGM-free materials, and a power density of 30 mW cm−2. Despite the low power density, this research shows the first application of a SCG-derived electrocatalyst in a real AEMFC and becomes a pioneer in the fuel cell study of waste-derived electrode materials.