Enhancing Electrochemical Hydrogen Evolution Performance of N-Heterocyclic Carbene-Coordinated Palladium(II) Complexes with Conductive Carbon: Insights from Hydrogen Oxidation Reactions

The systematic design of molecular electrocatalysts for electrochemical hydrogen evolution reaction (HER) is an avenue that is still relatively unexplored. Palladium (Pd)-based molecular electrocatalysts were constructed from N-heterocyclic carbene functionalized with 1-allyl-1,2,4-triazole and coumarins. The role of coumarin substitutions on the activity of the electrocatalysts for HER in an acidic medium was assessed. The Pd complex bearing 7,8-benzocoumarin substitutions was found to exhibit the best activity with overpotential of −570 mV to attain a current density of 10 mA cm –2 with a Tafel slope value of 187 mV dec –1 . The HER activity of the Pd complexes were improved by incorporating with conductive carbon with the best electrode exhibiting an improved overpotential of −468 mV and Tafel slope value of 161 mV dec –1 . The developed electrocatalyst demonstrated exceptional stability for up to 12 h, with minimal efficiency decline, as evidenced by post-stability linear sweep analysis. Additionally, the post-HER analysis indicates the structural and molecular integrity of the electrocatalysts, reaffirming their prospects as models for designing electrocatalysts. Hydrogen oxidation reaction (HOR) studies were conducted by using a rotating ring disk electrode setup. The concurrent current responses of the disk and ring electrodes confirmed the HER activity of the prepared electrocatalysts.