Temperature-Driven Structural Evolution in N-Doped Carbon Dots for Tailored Electrocatalytic CO ₂ ER–HER Pathways

We report tunable electrocatalytic activity for CO₂ reduction (CO₂ER) and the hydrogen evolution reaction (HER), exhibited by nitrogen-doped carbon dots (NCDs) synthesized via thermolysis of citric acid and urea (1:3 molar ratio) at 140, 240, 300, and 400 °C. Microscopic and spectroscopic analyses revealed a structural transition from hydrogen-bonded molecular clusters to disordered graphitic domains with an increase in the synthesis temperature. Linear Sweep Voltammetry (LSV) and Chronoamperometry studies revealed the highest current density for NCDs synthesized at 300 °C in a CO₂-saturated medium (4.3 mA cm^-2) and a maximum Faradaic efficiency (FE) of 69.96% for the formation of CO₂-reduced products (methanol, acetate, and <i>n</i>-propanol in 26.13:27.55:21.87% ratio) with minimum HER activity (FE of 25.03%). In contrast, NCDs synthesized at 400 °C exhibited a strong selectivity toward HER with an FE of 64.82% while demonstrating poor catalytic activity toward CO₂ER (FE of 30.18%). The electrocatalytic activity of the synthesized NCDs and their selectivity toward either CO₂ER or HER has been quantitatively correlated with the evolution of their domain behavior as well as the bonding environment of nitrogen.