Upcycling waste tires to affordable catalysts for the oxygen reduction reaction

Abstract

A significant yearly increase in the number of waste tires (WTs) has been observed globally. Nevertheless, methods for WT management are limited to the particulation of WTs for their use as a modifier in asphalt and concrete and burning WTs to obtain thermal energy in a power plant. Therefore, we designed a new approach to recycle WTs into valuable resources. We demonstrated the upcycling of WTs into affordable oxygen reduction reaction (ORR) catalysts by utilizing all components, including carbon black, rubber, and Fe particles. Under thermal oxidation, rubbers were transformed into cross-linked and stable structures and subsequently converted to carbonized rubbers on further heat treatment. In addition, heat treatment under ammonia gas flow enlarged the pore size of the WT-derived catalysts and provided nitrogen functional groups. Structural and textural analyses confirmed that the carbonized rubbers rendered an increase in the number of active sites for the ORR. Furthermore, it was revealed that the Fe particles survived thermal oxidation and also acted as active sites for the ORR, promoting catalytic activity. Our N-doped catalyst with Fe exhibited ORR activity comparable to that of a Pt/C catalyst. Therefore, we believe that our approach of envisioning upcycling pathways is an effective solution for the management of WTs.