Self-assembled ultrathin NiCo2S4 nanoflakes grown on Ni foam as high-performance flexible electrodes for hydrogen evolution reaction in alkaline solution

Considerable efforts have been devoted on the design and fabrication of non-platinum electrocatalysts with high performance and low cost for hydrogen evolution reaction (HER). However, the catalytic activity of existing electrocatalysts usually subjects to the limited amount of exposed active sites. Herein, we propose that self-assembled ultrathin NiCo2S4 nanoflakes grown on nickel foam (NiCo2S4/Ni foam) can serve as excellent electrocatalyst for HER in alkaline solution with high activity and stability. The NiCo2S4/Ni foam electrodes were prepared by the complete sulfidation of networked ultrathin NiCo-layered double hydroxide nanoflakes grown on Ni foam (NiCo-LDH/Ni foam). The advantages of this unique architecture are that the ultrathin and porous NiCo2S4 nanoflakes can provide a huge number of exposed active sites, the highly-conductive Ni foam can promote the transfer of electrons, and the three-dimensional-networked structure can facilitate the diffusion and penetration of electrolyte. Electrochemical measurements reveal that NiCo2S4/Ni foam electrodes exhibit greatly improved performance than NiCo-LDH/Ni foam for HER in alkaline solution with low onset overpotential (17 mV), small Tafel slope (84.5 mV/dec) and excellent long-duration cycling stability (maintaining an onset overpotential of ~20 mV and an overpotential of 155 mV at 50 mA/cm2 after testing for 100,000 s). In addition, the highly-flexible NiCo2S4/Ni foam electrodes show no obvious catalytic degradation after bending for 200 times, confirming the high flexibility and robustness under severe conditions.