Comprehensive Enhancement in Electrocatalytic Oxygen Evolution Performance of Nickel and Cobalt Complexes Derived from Π-Conjugated N-heterocyclic Carbene Ligands Through Carbon Composite Strategy

Electrocatalysis for oxygen evolution reaction (OER) amidst various advanced technologies for sustainable, clean and highly efficient energy generation plays a key role. In this study, we report nickel (6 and 7) and cobalt (8 and 9) complexes of N-heterocyclic carbene (NHC) ligands to obtain molecular electrocatalysts for OER application. The corresponding triazolium bromide salts (4 and 5) and complexes (6-9) were characterized using various spectral and analytical techniques. Further, NHC complexes were employed as molecular electrocatalysts for OER in alkaline media, which resulted in the excellent overpotentials of 373-424 mV vs. RHE to achieve the current density of 50 mA cm-2. The observed overpotential values have been further improved to 323-373 mV vs. RHE for complex-graphitic carbon composites (6-CP to 9-CP). Interestingly, complexes and carbon composites displayed low overpotential in the range of 375-433 mV vs. RHE, illustrating a higher current density of 100 mA cm-2. A lower Tafel slope value was observed for complex carbon composites, indicating improved OER kinetics. The composite 6-CP displayed stability for up to 210 min, authenticated by the amperometric measurement. The charge transfer resistance was calculated using the electrochemical impedance technique. The electron transfer mechanism was studied by the RRDE experiment by reducing the evolved oxygen to peroxide. Excellent overpotential and lower Tafel slope values along with low charge transfer resistance make these NHC complex-carbon composites an appropriate design for the optimal OER electrocatalysts. Tailored NHC ligands featuring pi-conjugated appendants along with the establishment of durable pi-pi stacking interactions between the NHC complexes (Ni/Co) and graphitic carbon substrates to boost the oxygen evolution performance are studied.