Rational design of an N,S-dual-doped metal-free catalyst from PPy for efficient electrochemical oxygen reduction

Abstract

Earth-abundant carbon-based materials have been explored as efficient, low-cost, metal-free alternatives to platinum and transition metal-based electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells and metal–air batteries. In contrast to conventional methods for the synthesis of metal-free catalysts relying on carbon nanotubes and graphene with inadequate porosity, herein, we report the synthesis of an N,S-dual-doped porous carbon (N,S-PC) catalyst derived from a polypyrrole (PPy) hydrogel impregnated with suitable N,S doping agents. The catalyst with the optimum structure and composition (N,S-PC(TS)) was obtained using thiosemicarbazide as the doping agent. This catalyst demonstrated excellent ORR activity in 0.1 M KOH with a positive onset (E_on) and half-wave (E_1/2) potential of 0.940 V and 0.825 V vs. RHE, respectively, which are close to those of Pt/C. In addition, the catalyst manifested excellent short-term stability, retaining 78% of its original current for 25 h and superior stability during potential cycling experiment. This catalyst displayed good selectivity towards the 4e⁻ ORR process with a low H₂O₂ yield (≤8.5%) in the potential range 0.4–0.1 V. Through this work, we attempt to popularize organic polymer hydrogel substrates having abundant nitrogen and unique three-dimensional network structures for the design of metal-free carbon catalysts for electrochemical energy applications.