Polyaromatic hydrocarbon-based 3D coordination polymeric materials: synergistic effect of nickel(ii) nitrate and tetraalkynyl anthracene towards electrocatalytic hydrogen production

Abstract

The exigency in the development of clean energy sources has led to the exploration of a wide variety of non-toxic, inexpensive, and highly efficient electrocatalysts for the production of hydrogen as a potential clean energy carrier. Drawing inspiration from the exceptional properties of Ni-based electrocatalysts for the hydrogen evolution reaction, herein, a simple methodology was adopted using ultrasonic treatment of Ni(NO₃)₂·6H₂O with tetra- and dialkynylated anthracenes at room temperature to synthesize a series of five 3D coordination polymeric materials. Amongst them, the material 3D-CPM-a generated from a Ni(II) coordination polymer of tetraalkynylated anthracene containing the phenyl methanone moiety gave an exceptional overpotential of 94 ± 14 mV at −100 mA cm⁻² for the HER at 90 °C. In contrast, the individual activities of Ni(NO₃)₂·6H₂O and tetraalkynylated anthracene were poor, exhibiting high overpotentials of 202 mV and 412 mV, respectively. The Ni(II)-alkynylated anthracene complex apparently contained 3D-dimeric structures with hexacoordinated Ni, as evidenced through FT-IR spectroscopy, NMR, EPR, XPS, MALDI and DFT calculations. The Ni–O bond in 3D-CPM-a facilitated ligand-to-metal charge transfer, and the π-conjugated framework of the ligand helped in increasing the stability while amplifying the catalytic activity. The DFT studies indicated the involvement of a Volmer–Tafel mechanism with favourable hydrogen adsorption energies. The high electrochemical active surface area (ca. 2300 cm²), high faradaic efficiency (ca. 88%) and good electrochemical stability highlight the significance of these Ni(II)-alkynylated anthracene 3D coordination polymers towards hydrogen production, thus paving the way for futuristic, inexpensive and easy-to-synthesise non-noble metal-based catalysts for effective water electrolysis.