Co9S8@N,P-doped porous carbon electrocatalyst using biomass-derived carbon nanodots as a precursor for overall water splitting in alkaline media

Abstract

In this study, we first synthesized Co₉S₈@N-doped porous carbon (Co₉S₈@NC) using shrimp-shell derived carbon nanodots as a carbon/nitrogen source in the presence of CoSO₄ by a one-step molten-salt calcination method. This was followed by low-temperature phosphorization in the presence of NaH₂PO₂, whereby Co₉S₈@N,P-doped porous carbon (Co₉S₈@NPC) was finally obtained using the Co₉S₈@NC as a precursor. The results demonstrated that the molten-salt calcination approach can effectively create a pyrolytic product with a porous structure and improve the material’s surface area, which is favourable for electrocatalysis-related mass transport and the exposure of catalytic active sites during electrocatalysis. As an electrocatalyst, Co₉S₈@NPC exhibits higher catalytic activity for the hydrogen evolution reaction (HER) than Co₉S₈@NC in an alkaline medium. Among all the investigated Co₉S₈@NPC catalysts, Co₉S₈@NPC-10 (mass ratio of NaH₂PO₂ to Co₉S₈@NC = 10 : 1) displays the best HER activity with an overpotential of 261 mV at 10 mA cm⁻² in the alkaline medium. Interestingly, Co₉S₈@NPC-10 also displays good catalytic activity for the oxygen evolution reaction (OER) in this study. Owing to its bifunctional catalytic activity towards the HER and OER, the fabricated Co₉S₈@NPC-10 was simultaneously used as an anode and cathode material to generate O₂ and H₂ from overall water splitting in the alkaline medium, exhibiting a nearly 100% faradaic yield. This study would be helpful to the design and development of high performance non-precious metal electrocatalysts to be applied in overall water splitting to produce H₂ and O₂.