Dopant-Induced Hexagonal to Orthorhombic Phase Transition in Fe Mo P Nanorods and Its Influence on the Electrocatalytic Hydrogen Evolution Reaction.

Electrochemical water splitting represents a sustainable method for producing molecular hydrogen, a promising clean energy alternative to fossil fuels. Iron phosphides have emerged as earth-abundant catalysts for the hydrogen evolution reaction (HER), where performance can be enhanced by admixing synergetic metals to produce bimetallic catalysts. Herein, we report a theoretical and experimental study that reveals the influence of dopant-induced hexagonal to orthorhombic phase transition on the catalytic activity and stability of FeP nanorods (NRs) for HER. Among eight metal dopants computationally studied, Mo has been identified as the most promising dopant owing to its optimum hydrogen binding free energy (Δ ) on the FeP (210) surface. Accordingly, hexagonal and orthorhombic Fe Mo P NRs ( = 0-14%) with average lengths and widths ranging from 50.9 ± 22.1 to 92.4 ± 43.8 nm and 3.8 ± 1.0 to 6.3 ± 1.8 nm, respectively, were colloidally synthesized to investigate the structure- and composition-dependent HER activity. Upon incorporation of Mo, the underlying hexagonal FeP phase transformed into orthorhombic Fe Mo P when ≥ 0.11 (5.41%). The admixture of Mo caused variations in the surface chemistry, leading to a significant decrease in Fe and P charges. The HER performance was observed to be both phase- and composition-dependent with mixed-phase Fe Mo P NRs ( = 0.03, 0.06, and 0.09) exhibiting superior catalytic activity and overpotentials (η) of 298, 267, and 222 mV, respectively at a current density () of -10 mA/cm compared to hexagonal FeP (η = 378 mV) and orthorhombic Fe Mo P (η = 331-459 mV for = 0.12-0.28) catalysts. The highest HER performance was achieved for FeMoP NRs with a dopant composition of 4.58%, consistent with composition-dependent Δ calculations. Although all compositions displayed a Volmer-Heyrovsky HER mechanism, the admixture of Mo improved the HER kinetics, producing the lowest Tafel slope (167.08 mV/dec) for FeMoP NRs. The incorporation of Mo improves the charge transfer resistance and preserves the stability of hexagonal and orthorhombic NRs in alkali environments with a negligible increase in η after 10 h of HER. This study advances the understanding of dopant-induced crystal structure transitions and paves the way for efficient and stable catalytic material design.