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

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 O-2 and H-2 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 H-2 and O-2.