Abstract:

In recent years, electrochemical water splitting that involves the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has attracted widespread attention because the process is clean, environmentally friendly, and the generated oxygen/hydrogen gas can be converted into electricity in a fuel cell. However, the HER and OER kinetics are sluggish and require highly efficient electrocatalysts for enhancing the reaction rate. Currently, precious metals such as Pt and RuO₂ have shown excellent HER and OER performance, respectively, but their practical applications are limited by their scarcity and high price. Therefore, the use of 3d transition metals, such as iron (Fe) and nickel (Ni), as electrocatalysts has attracted significant attention. To obtain a catalytic performance similar to that of precious metals, several methods have been explored, such as alloying 3d transition metals with precious metals, compositing a variety of transition metals, or requiring good carbon-based materials as a supporter. Metal-organic framework (MOF)-derived nanomaterials have emerged as some of the most promising non-precious metal bifunctional electrocatalysts. The MOF structure consists of metal-based units and special organic ligands, and after annealing, metal atoms can be converted into unsaturated metal-based active sites, and the organic ligands can be converted into carbon support. This could accelerate the charge transfer efficiency and can be beneficial for achieving excellent HER and OER performance. However, bifunctional electrocatalysts derived from nickel (Ni)-based MOFs have not been studied intensively, and their catalytic activity and stability remain to be improved. Herein, a Ni-MOF precursor was synthesized via a liquid-phase coordination reaction using Ni²⁺ and benzene-1, 3, 5-tricarboxylic acid. The obtained Ni-MOF samples were annealed under H₂/Ar atmosphere at 600, 700, and 800 ℃, named Ni/C-H₂-600, Ni/C-H₂-700, Ni/C-Ar-700, and Ni/C-H₂-800, respectively. During high-temperature annealing treatment, Ni nanoparticles were grown in situ on a rod-shaped carbon substrate to form novel hybrid architecture Ni/C nanoparticles used as a high-performance bifunctional electrocatalyst for overall water splitting. The HER overpotential of Ni/C-H₂-700 was 120 mV at a current density of 10 mA∙cm⁻², which is much lower than that of Ni/C-H₂-600 (250 mV), Ni/C-H₂-800 (348 mV), and Ni/C-Ar-700 (275 mV). Ni/C-H₂-700 required an OER overpotential of 350 mV to achieve a current density of 10 mA∙cm⁻², which is lower than that of Ni/C-H₂-600 (370 mV), Ni/C-H₂-800 (430 mV), and Ni/C-Ar-700 (380 mV). Furthermore, Ni/C-H₂-700 showed the idea of HER and OER durability. Presumably, good structural properties and the abundant surface area of the carbon substrate elevated HER/OER activity owing to the synergistic advantages of accessible active sites and enhanced electronic conductivity.

Key words: Ni-based MOFs, Water splitting, Hydrogen evolution reaction, Oxygen evolution reaction, Electrocatalyst