Modulating Hydrogen Evolution Activity of Graphitic Carbon Nitride via In Situ Sulfur Doping: Role of Thiourea‐Melamine Ratio in Metal‐Free Electrocatalysis

The development of efficient metal‐free electrocatalysts is crucial for sustainable hydrogen production via the hydrogen evolution reaction (HER). In this work, graphitic carbon nitride (g‐C 3 N 4 ) was engineered via in situ sulfur doping by tuning the thiourea‐to‐melamine precursor ratio. Structural and surface analyses confirm successful sulfur incorporation into the g‐C 3 N 4 framework, generating defect‐rich architectures and modulating the electronic structure. Electrochemical investigations reveal that sulfur doping significantly reduces charge‐transfer resistance, indicating improved electrical conductivity compared to pristine g‐C 3 N 4 . Additionally, the use of 15 wt% acetylene black as a conductive additive further facilitates efficient electron transport during electrocatalysis. Among the series, the sample with a thiourea:melamine ratio of 2:1 exhibits the best HER performance, achieving an overpotential of −81 mV at 10 mA cm −2 and a Tafel slope of 69 mV dec −1 , with stable operation for 60 h. The enhanced catalytic activity is attributed to synergistic effects of optimized sulfur incorporation, increased electrochemically active surface area, improved charge transport, and favorable hydrogen adsorption–desorption kinetics. These findings demonstrate that precursor ratio‐controlled sulfur doping is an effective strategy to enhance the electrocatalytic performance of metal‐free g‐C 3 N 4 systems for HER applications.