<p>A novel bifunctional electrocatalyst for water splitting was constructed with the CoSe/MoSe₂ heterojunction encapsulated within a nitrogen-doped carbon matrix (Co₁Mo₂Se/Co-N-C). This catalyst was synthesized via a facile one-step high-temperature calcination process. By optimizing the molar ratio of <i>n</i>(Co)/<i>n</i>(Mo) and the calcination temperature, a unique architecture was achieved featuring uniformly dispersed nanoparticles, well-defined heterointerfaces, and isolated Co atoms embedded in the carbon layer. Such structural features facilitated efficient transfer of electrons and maximized exposure of active sites. Electrochemical evaluations in 1.0 mol·L⁻¹ KOH demonstrated that Co₁Mo₂Se/Co-N-C exhibited excellent hydrogen evolution reaction performance, requiring an overpotential of only 63 mV to reach 10 mA·cm⁻² with a Tafel slope of 60 mV·dec⁻¹, comparable to that of commercial Pt/C. For oxygen evolution reaction, the catalyst achieved an overpotential of 328 mV at 10 mA·cm⁻² and a Tafel slope of 97 mV·dec⁻¹. Furthermore, a full water splitting cell based on this catalyst reached 10 mA·cm⁻² at an applied voltage of 1.623 V. These results highlight synergistic effects of the heterojunction and the nitrogen-doped carbon matrix, offering a promising strategy for the sustainable hydrogen production.</p>