Biochar from olive tree twigs and spent malt rootlets as electrodes in Zn-air batteries.

Biochars, i.e. porous carbons obtained by pyrolysis of biomass, can act as electrocatalysts for oxygen evolution and oxygen reduction reaction. In the present work, two biochars have been prepared by using materials of completely different biomass origin: olive-tree twigs and spent malt rootlets (brewery wastes). Both biomass species were subjected to pyrolysis under limited oxygen supply and then they were activated by mixing with KOH and pyrolysis again. The obtained biochars were characterized by several techniques in order to determine their structural characteristics and the composition of their active components. Despite their different origin, the two biochars demonstrated similar structural and compositional characteristics thus highlighting the importance of the pyrolysis and activation procedure. Both biochars were used as electrocatalysts in the operation of rechargeable Zn-air batteries, where they also demonstrated similar electrocatalytic capacities with only a small advantage gained by olive-tree-twigs biochar. Compared to bare nanoparticulate carbon (carbon black), both biochars demonstrated a marked advantage towards oxygen evolution reaction.