Quinoxaline-Functionalized Palladium(II)-NHC Electrocatalysts: Soaring Efficiency in a Low-Overpotential Hydrogen Evolution Reaction (HER)

In the pursuit of meeting the growing demand for a green hydrogen economy, the electrochemical hydrogen evolution reaction (HER) stands out as one of the most efficient and sustainable pathways. In this context, the rational design of effective electrocatalysts based on metal-N-heterocyclic carbene (NHC) complexes holds great promise, offering a highly versatile and robust approach. Proceeding in this direction, herein, we assessed the electrocatalytic performance of three Pd(II) complexes (Pd1, Pd2, and Pd3) based on quinoxaline-wingtip NHCs toward the hydrogen evolution reaction (HER) in acidic medium (0.5 M H2SO4). All of the complexes were synthesized via the transmetalation route and thoroughly characterized using various spectroscopic and analytical techniques, including single-crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD), and high-resolution mass spectra (HR-MS) analyses. Single-crystal X-ray crystallography reveals that variation of the imidazole N-substituent influences the coordination behavior of Pd(II), resulting in mononuclear and dinuclear complexes. Among the complex fabricated CC electrodes, Pd1 exhibited the best performance, affording a quite low overpotential of only −5 mV vs reversible hydrogen electrode (RHE) to deliver a current density of −10 mA/cm2 with a Tafel slope of 96 mV/dec, followed by Pd3 and Pd2. Pd1 also demonstrated excellent long-term operational stability over 24–25 h. The promising HER activity of Pd1 can be attributed to the presence of Cl···π interactions and rare-type Cl···Cl interactions, which facilitate the development of two-dimensional (2D) polymeric network-like structures, supported by density functional theory (DFT) studies and Hirshfeld surface analysis. Hence, the present study validated the successful application of rationally designed Pd(II)-NHC complexes as promising HER electrocatalysts.