Mono- vs Tri-nuclear Silver(I) and Gold(I) N-Heterocyclic Carbene Complexes/Metallacycles as Free-Standing Carbon Cloth Electrodes for Hydrogen Evolution Reaction in Alkaline Medium

Abstract

The synthesis of a Au(I) chlorido mono-carbene complex (AuLCl), a tri-nuclear Ag(I) tri-carbene metallacycle (Ag₃L₃·3PF₆), and a tri-nuclear Au(I) tri-carbene metallacycle (Au₃L₃·3PF₆) via direct or transmetalation route has been reported. The new metal N-heterocyclic carbene (NHC) complexes are thoroughly characterized by ¹H, ¹³C NMR, and single crystal X-ray diffraction (SCXRD) technique. The molecular structure analysis of AuLCl and Au₃L₃·3PF₆ reveals that both these complexes crystallize in a monoclinic crystal system and possess a linear coordination geometry. The metal NHC complexes are fabricated as free-standing carbon cloth (CC) electrodes using a simple dip-coating and drying method. The deposition of the metal NHC complexes on the CC is investigated by field emission scanning electron microscopy (FE-SEM), and it is found that they are deposited as distinct, fine microgranules with varying sizes and shapes. The free-standing CC electrodes of metal NHC complexes (AuLCl, Ag₃L₃·3PF₆, and Au₃L₃·3PF₆) are studied for their potential in the electrochemical hydrogen evolution reaction (HER) in an alkaline medium. The activity of the metal NHC complex CC electrodes is enhanced by incorporating conductive carbon ink in the electrode fabrication to develop complex-carbon electrodes (AuLCl/C, Ag₃L₃·3PF₆/C, and Au₃L₃·3PF₆/C). Among the fabricated free-standing CC electrodes, Ag₃L₃·3PF₆/C exhibits the best HER activity with an overpotential of 187 mV vs RHE to reach a current density of 10 mA/cm² with a Tafel slope value of 169 mV/dec and charge transfer resistance (R_ct) value of 9.85 Ω. Ag₃L₃·3PF₆/C is investigated for its long-term operational stability for 24 h using the chronoamperometric technique and exhibited significant current retention. The integrity of the Ag₃L₃·3PF₆/C CC electrode after the long-term operational stability analysis is investigated by FE-SEM, exhibiting the robust and stable nature of the metallacycle-carbon composite electrode.