Multifunctional N,S-doped biochar decorated with transition metals as electrocatalysts for energy conversion applications

Electrocatalytic oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) play key roles in the development of green and sustainable energy systems. However, these require high performance electrocatalysts. Non-precious transition metals supported on heteroatom doped carbon materials, including biochars, are potential substitutes to the benchmark electrocatalysts containing Pt-group metals. Herein, we prepared four composites containing transition metal species (Co, Fe, Mn and Ni) immobilized in a N,S co-doped biochar through a simple wet impregnation followed by pyrolysis approach. Among the prepared composites (Co/N,S-CC, Fe/N,S-CC, Mn/N,S-CC and Ni/N,S-CC), Co/N,S-CC presented trifunctionality, achieving good HER and OER performance and excellent ORR results. Its performance was explained by a combination of factors: ( i ) the promising co-doped biochar support, containing high content of active sites for ORR and HER, ( ii ) the formation of Co–N bonds, which promoted the interaction between support and Co particles and ( iii ) the synergistic effect between Co species and the conductive biochar support. The other showed electrocatalytic activity towards at least one of the evaluated reactions. Therefore, this work provides a new pathway to fabricate efficient trifunctional electrocatalysts, derived from biowaste/biomass, which are crucial to devices that switch between energy generation and storage modes. • Biochars were successfully prepared by simple wet impregnation/pyrolysis approach. • N,S co-doping improved hydrophobicity fundamental for HER and OER. • Fe/N,S-CC and Ni/N,S-CC presented bifunctionality. • Co/N,S-CC presented trifunctionality for HER, OER and ORR.