Construction of double-shell Ni3Se4@Co3Se4 microsphere for hybrid Zn-based supercapacitor with superior rate and energy density

In this work, a series of double-shell structure with Ni3Se4 as the outer shell and Co3Se4 as the inner shell (Ni3Se4@Co3Se4) are constructed by a hydroxide coating and then Se2−-etching strategy via metal-organic framework as precursor, which provides rich ions diffusion channels for excellent rate performance and cycle stability. The optimized double-shell structure (Ni3Se4@Co3Se4–1:3) presents a specific capacitance of 1120.4 F g−1 at the current density of 1 A g−1, and an outstanding rate performance of maintaining 92.1 % (the current density increased 10 times). Furthermore, an hybrid Zn-based supercapacitor device (Ni3Se4@Co3Se4–1:3//rGO-Zn) is fabricated with reduced graphene oxide coated Zn plate as negative electrode, which delivers a discharging platform (1.5–1.8 V) and high energy density of 271.2 Wh kg−1, is about 6 times higher than the hybrid supercapacitor device assembled with AC as negative (Ni3Se4@Co3Se4–1:3//AC) at similar power density. Meanwhile, a mixed energy storage hypothesis of OH−-anions involved faradic redox reaction, Zn2+-cations involved adsorption/desorption process has been proposed, and the Se-sites may be the initial adsorption center for the surface Zn2+-cations. Interestingly, the synthesis strategy of Ni3Se4@Co3Se4 double-shell structure can be extended to other elements system by changing the core with Ni- or Mn-based Metal Organic Frameworks.