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

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.