A carbon material was pyrolyzed from waste tire granules and used to synthesize non-platinum group metal (NPGM) catalysts via dry ball-milling and wet synthesis methods using Fe or Co salts and 2,2’-bipyridine as precursors. These materials were studied with HR-SEM, SEM-EDX, and N2 sorption analysis methods as well as the rotating disk electrode method in alkaline solution (0.1 M KOH). All materials exhibited a heterogeneous morphology as well as low porosity and low specific surface area (92 < S BET < 150 m2 g-1). Although the carbon support itself did not show significant activity toward the oxygen reduction reaction (E onset = -0.24 V vs Hg/HgO), the modified NPGM materials had a considerably higher onset potential (∆E onset ~150 mV). It was shown that active NPGM catalysts can be synthesized from waste tire derived carbon materials.

A porous carbon material was synthesized using waste tire granules via pyrolysis. Six non-platinum-group metal catalyst materials were synthesized based on this carbon material, Fe(NO3)3 · 9H2O, and either dicyandiamide or guanidine carbonate as the nitrogen source. Synthesis parameters such as pyrolysis temperature, precursor mass ratios, and acid washing time were varied for each material. All catalysts were characterized physically using HR-SEM, SEM-EDX, and N2 sorption analyses as well as electrochemically using the rotating disk electrode method in 0.1 M KOH. The materials were rich in nanofibers, and exhibited moderate porosity and specific surface area (S BET = 77-194 m2 g-1). By modifying the synthesis parameters of the materials, the electrochemical properties were successfully improved, achieving E onset = 0.92 V vs RHE and E 1/2 = 0.82 V vs RHE (Fe-guan III 900). The number of transferred electrons in the ORR was n = 4 for the more active materials.

Carbon supported Pt catalyst materials were synthesized from waste tire derived carbon by two different methods (microwave and refluxing). Catalysts and carbons were physically (XRD, N2 sorption analysis, TGA) and electrochemically characterized (RDE, CV, PEMFC single cell measurements). The RDE measurements were performed in 0.1 M HClO4 solution. For PEMFC single cell measurements, membrane-electrode assembly systems (MEAs) were prepared from catalyst materials by using different methods of coating the cathode layer and additional treatments. Catalysts were sprayed onto the membrane or GDL. In addition, MEAs were prepared where the catalyst was pipetted into GDL in several drops. Some of the MEAs were additionally treated with extra layer of Nafion and hot-pressed. The highest power density (0.42 W cm−2) was achieved with the MEA where the Pt catalyst synthesized via refluxing method was coated on the GDL with multiple drops and additional treatments were applied.

Over the past decades, a considerable amount of work has been put into replacing expensive Pt in oxygen reduction reaction (ORR) catalysts for use in fuel cells, such as with Fe or Co and nitrogen co-doped carbon materials [1-3]. At the same time, a major environmental issue is the lack of adequate methods for repurposing or recycling waste tires. Herein, it was investigated whether waste tires could be used as a source of carbon for synthesizing ORR catalysts. To prepare the carbon support, waste tire granules were pyrolyzed at 1000 °C for three hours in Ar. This material was then used to synthesize four non-platinum-group metal (NPGM) catalysts via dry ball-milling [2] and wet synthesis [2, 3] methods. FeSO4∙7H2O or Co(NO3)2·6H2O salts and 2,2’-bipyridine were used as precursors. Wet synthesis required the precursors to be dissolved in water and mixed with the carbon material, after which the solution was dried. For the dry method, all precursors were mixed and ball-milled for one hour. The products of both synthesis methods were subsequently pyrolyzed at 800 °C for 1.5 h in Ar. All materials were characterized using HR-SEM, SEM-EDX, and nitrogen sorption methods. The materials were heterogeneous and contained additives commonly present in tires as constituents of tire filler materials or residual products of tire production (Si, S, Ca, and Zn). All materials exhibited low porosity and a rather low specific surface area (SBET = 92 to 150 m2 g-1). Electrochemical analyses were carried out in an O2 saturated 0.1M KOH solution using the rotating disk electrode method. Although the carbon support on its own did not show significant activity toward the ORR, the modified Co-N/C and Fe-N/C catalysts demonstrated substantially higher onset potentials (∆Eonset ~150 mV). The best electrochemical activity was achieved by the Co-N/C material synthesized using the wet method. These results show that waste tire-derived carbon materials can be used to successfully synthesize NPGM catalysts suitable for application in fuel cells. Acknowledgements This work was supported by PRG676 "Development of express analysis methods for micro-mesoporous materials for Estonian peat derived carbon supercapacitors" (1.01.2020- 31.12.2024), TK141 "Advanced materials and high-technology devices for sustainable energetics, sensorics and nanoelectronics" (1.01.2016−1.03.2023), and LLTKT20148 "Production of Polymer Electrolyte Membrane Fuel Cell" (10.02.2020−9.07.2021) References [1] F. Roncaroli, E. S. Dal Molin, F. A. Viva, M. M. Bruno, and E. B. Halac, Electrochimica Acta, 174, p. 66 (2015) [2] P. Teppor, R. Jäger, E. Härk, S. Sepp, M. Kook, O. Volobujeva, P. Paiste, Z. Kochovski, I. Tallo, and E. Lust, J. Electrochem. Soc., 167, 054513 (2020) [3] R. Jäger, P. E. Kasatkin, E. Härk, P. Teppor, T. Romann, R. Härmas, I. Tallo, U. Mäeorg, U. Joost, P. Paiste, K. Kirsimäe, and E. Lust, J. Electroanal. Chem., 823, p. 593 (2018)