Non-covalent Immobilization of Metal N-Heterocyclic Carbene Complexes onto Carbon Cloth as Bifunctional Electrodes for Overall Water Splitting in Alkaline Medium

Abstract

The use of molecular complex-modified electrodes presents avenues for their rational and simplistic design to serve as efficient catalysts in emerging electrocatalytic applications. Herein, three molecular electrocatalysts, CoLBr₂, NiLBr₂, and PdLBr2, were synthesized from the pyridine-functionalized N-heterocyclic carbene (NHC) ligand (HLBr) and physisorbed onto CC to obtain complex-modified free-standing electrodes. These complex-modified electrodes were investigated for their hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activity in 1 M KOH. Among the complex-modified electrodes, the CoLBr₂ electrode exhibited the best HER activity with an overpotential of −255 mV vs RHE at 10 mA/cm² and a Tafel slope of 155 mV/dec. For the OER activity, the NiLBr₂ electrode exhibited the best performance with an overpotential of 376 mV vs RHE at 10 mA/cm² and a Tafel slope of 86 mV/dec. The bifunctional nature of the complex-modified free-standing CC electrodes enabled the assembly of a symmetric alkaline electrolyzer, i.e., CoLBr₂//CoLBr2, with a cell voltage of 1.81 V at 10 mA/cm². The post-stability analysis of the complex-modified electrodes revealed that the complexes possessed chemical stability despite undergoing long-term stability tests at high overpotentials. These findings authenticate the versatility of metal NHC complexes for fabricating molecularly modified free-standing electrodes for the HER, OER, and overall water splitting, paving the way for the development of sustainable energy conversion technologies.