Dopant-Induced Hexagonal to Orthorhombic Phase Transition in Fe_2–<i>x</i>Mo_<i>x</i>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 Fe₂P 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<i>G</i> _H) on the Fe₂P (210) surface. Accordingly, hexagonal and orthorhombic Fe_2-<i>x</i> Mo _<i>x</i> P NRs (<i>x</i> = 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 Fe₂P phase transformed into orthorhombic Fe_2-<i>x</i> Mo _<i>x</i> P when <i>x</i> ≥ 0.11 (5.41%). The admixture of Mo caused variations in the surface chemistry, leading to a significant decrease in Fe^δ+ and P^δ- charges. The HER performance was observed to be both phase- and composition-dependent with mixed-phase Fe_2-<i>x</i> Mo _<i>x</i> P NRs (<i>x</i> = 0.03, 0.06, and 0.09) exhibiting superior catalytic activity and overpotentials (η_-10) of 298, 267, and 222 mV, respectively at a current density (<i>j</i>) of -10 mA/cm² compared to hexagonal Fe₂P (η_-10 = 378 mV) and orthorhombic Fe_2-<i>x</i> Mo _<i>x</i> P (η_-10 = 331-459 mV for <i>x</i> = 0.12-0.28) catalysts. The highest HER performance was achieved for Fe_1.91Mo_0.09P NRs with a dopant composition of 4.58%, consistent with composition-dependent Δ<i>G</i> _H 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 Fe_1.91Mo_0.09P 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_-10 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.