Electrosynthesis of Carbon Nitride Quantum Dots and exploring its electrocatalytic activity for Oxygen Reduction Reaction

We here report a single-step electro-synthetic route for the transformation of melamine (in 1-butyl-3-methylimidazolium tetrafluoroborate) to graphitic carbon nitride quantum dots (g-C3N4 QDs) as confirmed by a variety of characterization tools including Fourier-transform infrared and x-ray diffraction. The formation of g-C3N4 QDs with size tuning occurs by varying the applied potential (-1.5 V, -3.0 V, -4.5 V) in a controllable manner in the same electrolyte as revealed from the photoluminescence at 420 (violet region), 502 (blue region), and 510 nm (green region) due to the modulation of the band gap. The dual emission in the violet region is excitation-independent, while the excitation-dependent features suggest contributions from the surface defects. The size-dependent morphology of these quantum dots is further investigated for electrocatalytic applications using oxygen reduction reaction (ORR) in alkaline media as a model reaction. The observed Tafel slope (~30 mV/dec) and the exchange current density reveal interesting kinetic enhancement compared to traditional Pt-based bench-mark electrocatalysts and superior stability upon cycling. With better performance parameters like current density values at a comparatively lower overpotential and sustained stability, these can replace the conventional precious metal electrocatalysts to facilitate cheaper fuel cells, metal-air batteries and water electrolyzers.