High performance metal-free N/S-doped biochars towards oxygen reduction reaction
Renata Matos, A.J.S. Fernandes, Víctor Karim Abdelkader-Fernández, Andreia F. Peixoto, Diana M. Fernandes
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 (N 6 , SN 6 , S 3 N 3 and S 3 N 2 ) were used in order to optimize the ORR performance in alkaline medium. Doping with S 3 N 3 revealed to be more promising, as the resulting electrocatalyst, S 3 N 3 -CC, obtained the most positive onset potential ( E onset ), higher diffusion limiting current density ( j L ) and number of electrons transferred per oxygen molecule ( n O2 ) closer to 4. Moreover, this biochar showed excellent stability over 15 h of continuous use. The performance of S 3 N 3 -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. • Several heteroatom doped biochars were successfully prepared. • N-doping and N,S co-doping led to enhanced ORR activity. • The choice of dopant influenced significantly the ORR performance. • S 3 N 3 -CC achieved excellent ORR performance.