Facile synthesis of mesoporous NixCo9−xS8 hollow spheres for high-performance supercapacitors and aqueous Ni/Co–Zn batteries

Porous micro/nanostructure electrode materials have always contributed to outstanding electrochemical energy storage performances. Co₉S₈ is an ideal model electrode material with high theoretical specific capacity due to its intrinsic two crystallographic sites of cobalt ions. In order to improve the conductivity and specific capacitance of Co₉S₈, nickel ions were introduced to tune the electronic structure of Co₉S₈. The morphology design of the mesoporous hollow sphere structure guarantees cycle stability and ion diffusion. In this work, Ni_xCo_9−xS₈ mesoporous hollow spheres were synthesized via a facile partial ion-exchange of Co₉S₈ mesoporous hollow spheres without using a template, boosting the capacitance to 1300 F g⁻¹ at the current density of 1 A g⁻¹. Compared with the pure Co₉S₈ and Ni-Co₉S₈-30%, Ni-Co₉S₈-60% exhibited the best supercapacitor performance, which was ascribed to the maximum Ni ion doping with morphology and structure retention, enhanced conductivity and stabilization of Co³⁺ in the structure. Therefore, Ni/Co–Zn batteries were fabricated by using a Zn plate as the anode and Ni-Co₉S₈-60% as the cathode, which deliver a high energy density of 256.5 W h kg⁻¹ at the power density of 1.69 kW kg⁻¹. Furthermore, the Ni/Co–Zn batteries exhibit a stable cycling after 3000 repeated cycles with capacitance retention of 69% at 4 A g⁻¹. This encouranging result might provide a new perspective to optimize Co₉S₈-based electrodes with superior supercapacitor and Ni/Co–Zn battery performances.

Mesoporous NiCo₉S₈-0.6 hollow spheres as a high-performance supercapacitor and aqueous Ni/Co–Zn battery.

A Co-MOF-derived Co9S8@NS-C electrocatalyst for efficient hydrogen evolution reaction

The exploitation of efficient hydrogen evolution reaction (HER) electrocatalysts has become increasingly urgent and imperative; however, it is also challenging for high-performance sustainable clean energy applications. Herein, novel Co9S8 nanoparticles embedded in a porous N,S-dual doped carbon composite (abbr. Co9S8@NS-C-900) were fabricated by the pyrolysis of a single crystal Co-MOF assisted with thiourea. Due to the synergistic benefit of combining Co9S8 nanoparticles with N,S-dual doped carbon, the composite showed efficient HER electrocatalytic activities and long-term durability in an alkaline solution. It shows a small overpotential of -86.4 mV at a current density of 10.0 mA cm-2, a small Tafel slope of 81.1 mV dec-1, and a large exchange current density (J 0) of 0.40 mA cm-2, which are comparable to those of Pt/C. More importantly, due to the protection of Co9S8 nanoparticles by the N,S-dual doped carbon shell, the Co9S8@NS-C-900 catalyst displays excellent long-term durability. There is almost no decay in HER activities after 1000 potential cycles or it retains 99.5% of the initial current after 48 h.