Nature-Inspired Design of Nano-Architecture-Aligned Ni5P4-Ni2P/NiS Arrays for Enhanced Electrocatalytic Activity of Hydrogen Evolution Reaction (HER)

The projection of developing sustainable and cost-efficient electrocatalysts for hydrogen production is booming. However, the full potential of electrocatalysts fabricated from earth-abundant metals has yet to be exploited to replace Pt-group metals due to inadequate efficiency and insufficient design strategies to meet the ever-increasing demands for renewable energies. To improve the electrocatalytic performance, the primary challenge is to optimize the structure and electronic properties by enhancing the intrinsic catalytic activity and expanding the active catalytic surface area. Herein, we report synthesizing a 3D nanoarchitecture of aligned Ni5P4-Ni2P/NiS (plate/nanosheets) using a phospho-sulfidation process. The durability and unique design of prickly pear cactus in desert environments by adsorbing moisture through its extensive surface and ability to bear fruits at the edges of leaves inspire this study to adopt a similar 3D architecture and utilize it to design an efficient heterostructure catalyst for HER activity. The catalyst comprises two compartments of the vertically aligned Ni5P4-Ni2P plates and the NiS nanosheets, resembling the role of leaves and fruits in the prickly pear cactus. The Ni5P4-Ni2P plates deliver charges to the interface areas, and the NiS nanosheets significantly influence Had and transfer electrons for the HER activity. Indeed, the synergistic presence of heterointerfaces and the epitaxial NiS nanosheets can substantially improve the catalytic activity compared to nickel phosphide catalysts. Notably, the onset overpotential of the best-modified ternary catalysts exhibits (35 mV) half the potential required for nickel phosphide catalysts. This promising catalyst demonstrates 70 and 115 mV overpotentials to attain current densities of 10 and 100 mA cm-2, respectively. The obtained Tafel slope is 50 mV dec-1, and the measured double-layer capacitance from cyclic voltammetry (CV) for the best ternary electrocatalyst is 13.12 mF cm-2, 3 times more than the nickel phosphide electrocatalyst. Further, electrochemical impedance spectroscopy (EIS) at the cathodic potentials reveals that the lowest charge transfer resistance is linked to the best ternary electrocatalyst, ranging from 430 to 1.75 Ω cm-2. This improvement can be attributed to the acceleration of the electron exchangeability at the interfaces. Our findings demonstrate that the epitaxial NiS nanosheets expand the active catalytic surface area and simultaneously elevate the intrinsic catalytic activity by introducing heterointerfaces, which leads to accommodating more Had at the interfaces.