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

Abstract

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 1allyl-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 7,8benzocoumarin 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.

Nickel(II)–N-Heterocyclic Carbene Complex and its Carbon Nanotube Composites as Efficient Bifunctional Electrocatalysts for Hydrogen and Oxygen Evolution Reactions and Mercury-Sensing Applications

“…In this study, we report a benzimidazole-based donor-functionalized NHC precursor with a coumarin substituent that was synthesized and further complexed with nickelocene to access the title bis- NHC-coordinated nickel­(II) complex with diamagnetic behavior and possess electrocatalytic applications. This nickel NHC-based electrocatalyst possesses a π-conjugated ligand system, which provides effective coordination to the metal center and leads to efficient electron transfer during the evaluation of hydrogen and oxygen evolution studies and mercury-sensing applications …”