We reported a novel protocol to efficiently synthesize molybdenum carbonitride (MoCN) nanomaterials with dense active sites and high surface area. The key step in this protocol is the preparation of the catalyst precursor, which was obtained by polymerizing diaminopyridine in the presence of hydrogen carbonate. The abundant amino groups in the poly diaminopyridine bound numerous Mo species via coordination bonds, resulting in the formation of dense Mo active sites. The addition of hydrogen carbonate to the synthesis mixture resulted in CO2 gas evolution as the local pH decreased during polymerization. The in situ evolved CO2 bubbles mechanically broke down the precursor into MoCN nanomaterials with a high surface area. The synthesized MoCN materials were demonstrated as an electrocatalyst for hydrogen evolution reaction (HER). It exhibited an HER onset potential of -0.05 V (vs RHE) and a high hydrogen production rate (at -0.14 V vs RHE, -10 mA cm(-2)) and is therefore one of the most efficient, low-cost HER catalysts reported to date.