Descriptor-Based Insights into Hydrazine Selectivity in Electrochemical Ammonia Oxidation on Single-Atom Centers of MXenes.

Selective electrochemical synthesis of hydrazine (N2H4) from ammonia oxidation offers a promising route toward value-added nitrogen chemicals but remains challenging due to competing oxidation pathways. Here, density functional theory calculations are employed to investigate hydrazine formation at electrochemically formed single-atom centers on 14 carbide- and nitride-based MXenes (MXene-SACs). Hydrazine selectivity is assessed using three descriptors, namely the thermodynamic free-energy span G max(U), the kinetic descriptor G ‡(U) associated with the N-N coupling activation barrier, and the hydrazine desorption energy (ΔG des) governing product release from the catalytic surface. Evaluation of G max(U) identifies Nb2C-, V2C-, Cr2C-, and V2N-SAC motifs as promising materials for efficient hydrazine formation. However, transition state calculations for N-N coupling barriers modify the thermodynamic activity trends, as reflected by G ‡(U), and indicate that selectivity remains generally biased toward N2 formation. A combined descriptor assessment shows that carbide-based MXene-SAC motifs, particularly V2C and Cr2C, remain the most promising hydrazine-competitive candidates due to their balanced intermediate stabilization and favorable desorption energetics, while V2N-SAC lies closest to the ideal hydrazine-selective regime in descriptor space. Overall, the descriptor-guided screening strategy provides a transferable basis for analyzing catalyst selectivity toward hydrazine formation in ammonia electro-oxidation and related catalytic systems.