Cubic core–shell structure of NiCoSx/CoS2 as a high-efficiency tri-functional catalyst for Zn–air battery and overall water splitting

Abstract

Cubic core–shell NiCoS_x/CoS₂ composite catalyst was successfully prepared on the basis of K₃[Co(CN)₆]. First, Ni²⁺ is substituted for K⁺ in K₃[Co(CN)₆]₂ to prepare the binary metal ion precursor of Ni₃[Co(CN)₆]₂. Afterward, coating Co(OH)₂ on the surface of Ni₃[Co(CN)₆]₂ could not only maintain the cubic structure of Prussian blue analogue but also build a composite interface between two phases. After the Ni₃[Co(CN)₆]₂/Co(OH)₂ precursor is sulfided, a porous NiCoS_x/CoS₂ composite catalyst material with a cubic structure is obtained. NiCoS_x/CoS₂ catalysts showed excellent oxygen evolution reactions and oxygen reduction reactions. The overpotential was only 310 mV to achieve a current density of 20 mA cm⁻² and the half-wave potential can reach 0.80 V. During the exploration of the hydrogen evolution reaction, the catalyst exhibited 253 mV overpotential at 10 mA cm⁻². When this catalyst was used in Zn–air batteries as the cathode, it showed excellent cycle stability (210 h) and small voltage gap (0.80 V) at 5 mA cm⁻². Meanwhile, soft-pack flexible batteries assembled with NiCoS_x/CoS₂ can be charged and discharged stably at any bending angle.