Mono- vs\nTri-nuclear Silver(I) and Gold(I) <i>N</i>‑Heterocyclic\nCarbene Complexes/Metallacycles as\nFree-Standing Carbon Cloth Electrodes for Hydrogen Evolution Reaction\nin Alkaline Medium

The synthesis of a Au(I) chlorido mono-carbene complex\n(<b>AuLCl</b>), a tri-nuclear Ag(I) tri-carbene metallacycle\n(<b>Ag</b><sub><b>3</b></sub><b>L</b><sub><b>3</b></sub><b>·3PF</b><sub><b>6</b></sub>), and\na tri-nuclear Au(I)\ntri-carbene metallacycle (<b>Au</b><sub><b>3</b></sub><b>L</b><sub><b>3</b></sub><b>·3PF</b><sub><b>6</b></sub>) via direct or transmetalation route has been\nreported. The new metal <i>N</i>-heterocyclic carbene (NHC)\ncomplexes are thoroughly characterized by <sup>1</sup>H, <sup>13</sup>C NMR, and single crystal X-ray diffraction (SCXRD) technique. The\nmolecular structure analysis of <b>AuLCl</b> and <b>Au</b><sub><b>3</b></sub><b>L</b><sub><b>3</b></sub><b>·3PF</b><sub><b>6</b></sub> reveals that both these\ncomplexes crystallize in a monoclinic crystal system and possess a\nlinear coordination geometry. The metal NHC complexes are fabricated\nas free-standing carbon cloth (CC) electrodes using a simple dip-coating\nand drying method. The deposition of the metal NHC complexes on the\nCC is investigated by field emission scanning electron microscopy\n(FE-SEM), and it is found that they are deposited as distinct, fine\nmicrogranules with varying sizes and shapes. The free-standing CC\nelectrodes of metal NHC complexes (<b>AuLCl</b>, <b>Ag</b><sub><b>3</b></sub><b>L</b><sub><b>3</b></sub><b>·3PF</b><sub><b>6</b></sub>, and <b>Au</b><sub><b>3</b></sub><b>L</b><sub><b>3</b></sub><b>·3PF</b><sub><b>6</b></sub>) are studied for their\npotential in the electrochemical hydrogen evolution reaction (HER)\nin an alkaline medium. The activity of the metal NHC complex CC electrodes\nis enhanced by incorporating conductive carbon ink in the electrode\nfabrication to develop complex-carbon electrodes (<b>AuLCl/C</b>, <b>Ag</b><sub><b>3</b></sub><b>L</b><sub><b>3</b></sub><b>·3PF</b><sub><b>6</b></sub><b>/C</b>, and <b>Au</b><sub><b>3</b></sub><b>L</b><sub><b>3</b></sub><b>·3PF</b><sub><b>6</b></sub><b>/C</b>). Among the fabricated free-standing CC electrodes, <b>Ag</b><sub><b>3</b></sub><b>L</b><sub><b>3</b></sub><b>·3PF</b><sub><b>6</b></sub><b>/C</b> exhibits the best HER activity with an overpotential of 187 mV vs\nRHE to reach a current density of 10 mA/cm<sup>2</sup> with a Tafel\nslope value of 169 mV/dec and charge transfer resistance (<i>R</i><sub>ct</sub>) value of 9.85 Ω. <b>Ag</b><sub><b>3</b></sub><b>L</b><sub><b>3</b></sub><b>·3PF</b><sub><b>6</b></sub><b>/C</b> is investigated\nfor its long-term operational stability for 24 h using the chronoamperometric\ntechnique and exhibited significant current retention. The integrity\nof the <b>Ag</b><sub><b>3</b></sub><b>L</b><sub><b>3</b></sub><b>·3PF</b><sub><b>6</b></sub><b>/C</b> CC electrode after the long-term operational stability\nanalysis is investigated by FE-SEM, exhibiting the robust and stable\nnature of the metallacycle-carbon composite electrode.