Achieving highly exposed active sites of molybdenum carbide with the assistance of NH4Cl for efficient hydrogen evolution

Highlight • NH4Cl-assisted synthesis of Mo2C with highly exposed active sites is demonstrated. • The adequate exposure of active sites is crucial to excellent HER performance. • NH4Cl can enlarge the interlayer spacing and weaken the thermostability of C3N4. • The in-situ introduction of NH4Cl can purify the superfluous inactive carbon. • Different hydrogen halides, different structural and catalytic features of materials. Molybdenum carbide (Mo2C) shows tremendous potentials in electrocatalytic hydrogen evolution reaction (HER). Nonetheless, the rational design of Mo2C and establishment of structure-performance correlation are still challenging. Herein, a facile NH4Cl-assisted approach has been realized to fabricate Mo2C by employing ammonium heptamolybdate and melamine as molybdenum and carbon sources, respectively. By virtue of the intercalation into the interlayers of intermediate graphitic carbon nitride, HCl molecules released from NH4Cl play a pivotal role in purifying the superfluous carbon matrix, which is a common byproduct during the preparation process of carbide. Furthermore, the participation of NH4Cl can sharply decrease in carbon content and significantly enhance the exposure of active carbide sites, which is proved to be the most essential factor affecting the catalytic activity of materials. As a result, the as-prepared catalyst presents remarkable HER performance in alkaline media, with a low overpotential of 126 mV at 10 mA cm−2 as well as small Tafel slope of 60.97 mV dec−1. Impressively, it even surpasses commercial Pt/C in the pragmatically meaningful high-current region (>80 mA cm−2), demonstrating its great potential to substitute the noble metal catalysts in practical alkaline HER process.