Cost-effective preparation of metal-free electrocatalysts by phosphoric acid activation of lignocellulosic materials for oxygen reduction reaction

Activated carbons have been prepared by phosphoric acid activation of lignocellulosic precursors, an industrial method, followed by heat treatment in ammonia. Thus, a cost-effective, scalable and metal-free electrocatalyst was developed for use in oxygen reduction reaction (ORR) at fuel cell cathodes. The physicochemical properties of the activated carbons have been analyzed by elemental analysis, X-ray photoelectron spectroscopy, X-ray diffraction, and nitrogen adsorption. The ORR electrocatalytic performances of the activated carbons have been investigated by cyclic voltammetry and linear sweep voltammetry in an alkaline electrolyte. The results showed that phosphorus-containing groups are key to endowing phosphoric-acid-activated carbons with comparable electrocatalytic activity to that of commercial Pt/C. This was because these phosphorus-containing groups facilitated the formation of both nitrogen-containing groups and defects in the microstructure. Besides, heat treatment of phosphoric-acid-activated carbons in ammonia produced a highly developed mesopore structure and thus kinetically facilitated the ORR.