N-, O- and P-doped hollow carbons: Metal-free bifunctional electrocatalysts for hydrogen evolution and oxygen reduction reactions

To realize clean and renewable energy conversion systems, such as fuel cells and water splitting devices, cost-effective and energy-efficient bifunctional metal-free electrocatalysts are highly needed. Herein, N-, O- and P-doped hollow carbons (NOPHCs) are successfully synthesized via pyrolysis of Co 2 P-containing polypyrrole (Co 2 P@PPY) precursors and removal of Co 2 P templates. The materials are proven to serve as efficient and stable metal-free bifunctional electrocatalysts toward hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) in basic media. Notably, NOPHC 10 -900, which is obtained using 0.10 g of Co 2 P as template and pyrolyzed at 900 °C, exhibits high electrocatalytic performance for HER with a small overpotential of 290 mV at −10 mA cm −2 , a low Tafel slope of 102 mV dec -1 , and a great long-term stability for 19 h. Moreover, it affords decent electrocatalytic performance for ORR, with positive onset potential, high electron-transfer number, good methanol crossover tolerance and outstanding durability. The excellent electrocatalytic performance is mainly due to the synergistic effect of the multi-heteroatoms dopants in it and the large density of active sites on the surface of the hollow carbons.