Biochar from residues of anaerobic digestion and its application as electrocatalyst in Zn–air batteries

Background

Biochar, the product obtained by pyrolysis of biomass, is a new eco-friendly material with excellent properties and many promising applications. Among them it can be used as cathode in Zn-air batteries with very satisfactory results

Methods

The residue was obtained by anaerobic digestion of a mixture of corn silage (10%), malt (10%), and cattle manure (80%), aiming to biogas production. It was first freeze–dried and then it was subjected to pyrolysis up to 800^oC.

Significant findings

The biochar was physicochemically characterized. It has moderate specific surface area, sufficient sp²/sp³ ratio and metal and non–metal surface chemical species. The biochar demonstrated satisfactory electrocatalytic performance, both as oxygen reduction and oxygen evolution catalyst. When applied as electrocatalyst in Zn–air batteries it reached an open–circuit potential of 1.45 V, a short–circuit current density of 200 mA cm^–2 and maximum power density of around 62 mW cm^–2. Its energy density was 927 Wh kg^–1, (at 20 mA), and 518 Wh kg^–1 (at 100 mA). In a charge–discharge mode at 10 mA, the potential varied between 1.35 and 1.90 V. These data, show that the waste biomass can be used as inexpensive material for Zn–air batteries and offers a useful approach to combine waste management with energy storage.