High performance metal-free N/S-doped biochars towards oxygen reduction reaction

Abstract

The lack of cost-effective electrocatalysts towards oxygen reduction reaction is hindering the large-scale application of promising energy conversion technologies, such as fuel cells or metal-air batteries. In this context, metal free N and S doped carbon materials have been successfully applied to oxygen reduction reaction (ORR) electrocatalysis. Heteroatom doped biochars, in particular, represent a cost-effective and sustainable replacement to Pt-based electrocatalysts. Herein, we developed several N-doped or N and S co-doped biochars, using shrimp shells as biomass feedstock. Four dopants containing different N/S ratios (N6, SN6, S3N3 and S3N2) were used in order to optimize the ORR performance in alkaline medium. Doping with S3N3 revealed to be more promising, as the resulting electrocatalyst, S3N3-CC, obtained the most positive onset potential (Eonset), higher diffusion limiting current density (jL) and number of electrons transferred per oxygen molecule (nO2) closer to 4. Moreover, this biochar showed excellent stability over 15 h of continuous use. The performance of S3N3-CC was mainly explained by a high content of pyridinic and graphitic N, a high ratio between pyridinic and pyrrolic N, the introduction of thiophenic S and an optimized N/S ratio.