Residual metallic impurities within carbon nanotubes play a dominant role in supposedly “metal-free” oxygen reduction reactions

Oxygen reduction reactions (ORRs) are of key importance in the area of electrochemical energy production. The replacement of the Pt industrial standard with heteroatom doped metal-free carbon nanotubes was previously suggested for ORRs. Here we will show that the ORR electrocatalysis on these supposedly metal-free materials is likely in fact due to the presence of residual metallic impurities within the carbon nanotubes and that these impurities play a dominant role.