A team of researchers in Italy has transformed chars—the least valuable by-product of waste-tire pyrolysis—into highly efficient Oxygen Reduction Reaction (ORR) catalysts for use in alkaline fuel cells and metal air batteries. A paper on their work is published in the Journal of Power Sources.

The dramatic growth in the number of combustion engine powered vehicles is responsible in large part for our environmental problems, both due to the emission of greenhouse gases and in terms of solid waste disposal. With regard to the latter, scrap tires are a major long-standing and unresolved issue. Currently, about 1.5 billion waste tires are produced yearly, including 2.6 million tons for just Europe alone. More than 80% of the worldwide production of scrap tires is concentrated in a few macro-areas (USA, EU, China, India and Japan), where strict protocols and regulations have been applied (e.g. EU Directive 31/1991).

It should also be mentioned that tires are particularly problematic considering that in their worn-out state their mass is essentially the same as when they were produced. In particular, the chemical composition of scrap tires poses serious problems. Landfill storage is prohibited in most countries, due to flammability and leakage of toxic chemicals. For this reason, whole tires are recycled to produce mats for playgrounds, athletic surfaces, and also to make cement; since 1994 the fraction of waste tires going to landfills has decreased from 60% to 5% in 2015.

Thermal treatments, such as pyrolysis, have been developed as an efficient strategy to reclaim useful chemicals, often in the form of fuels, or from scrap tires. … The solid residuals, often referred to as chars, have been tentatively used to produce carbon black and/or activated carbon with some success in the production of activated carbon electrodes for supercapacitors. Chars contain significant amounts (from a few to several thousand ppm) of different metals embedded in a heterogeneous matrix in the form of isolated chemical entities or particulate matter (e.g. oxides, sulfides, etc.). Such compounds are introduced during vulcanization and other stages of rubber manufacture. The heterogeneous nature of the mixture makes an affordable separation and recovery of metals from scrap tires very difficult.

In this paper, we propose an effective way to exploit the presence of nano-sized metallic or metal oxides/sulfide particles dispersed in the carbonaceous matrix obtained after char formation. We demonstrate that a simple post pyrolysis heat treatment can be used to produce materials with high activity for the electrocatalytic oxygen reduction reaction (ORR).

—Passaponti et al.

The team used chars produced by Microwave Assisted Pyrolysis (MAP). They found that two main factors contributed to the enhanced ORR activity: (1) a large specific surface area and porosity; and (2) the presence of a high concentration of ZnO particles embedded within the carbon matrix.

Resources

• Maurizio Passaponti, Luca Rosi, Matteo Savastano, Walter Giurlani, Hamish A. Miller, Alessandro Lavacchi, Jonathan Filippi, Giovanni Zangari, Francesco Vizza, Massimo Innocenti (2019) “Recycling of waste automobile tires: Transforming char in oxygen reduction reaction catalysts for alkaline fuel cells,” Journal of Power Sources, Volume 427, Pages 85-90 doi: 10.1016/j.jpowsour.2019.04.067