Dopant-Induced Hexagonal to Orthorhombic Phase Transition in Fe_2–xMo_xP Nanorods and Its Influence on the Electrocatalytic Hydrogen Evolution Reaction

High Resolution Image Download MS PowerPoint Slide 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 Fe 2 P 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 G H ) on the Fe 2 P (210) surface. Accordingly, hexagonal and orthorhombic Fe 2– x Mo x P NRs ( x = 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 Fe 2 P phase transformed into orthorhombic Fe 2– x Mo x P when x ≥ 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– x Mo x P NRs ( x = 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 ( j ) of −10 mA/cm 2 compared to hexagonal Fe 2 P (η –10 = 378 mV) and orthorhombic Fe 2– x Mo x P (η –10 = 331–459 mV for x = 0.12–0.28) catalysts. The highest HER performance was achieved for Fe 1.91 Mo 0.09 P NRs with a dopant composition of 4.58%, consistent with composition-dependent Δ G 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 1.91 Mo 0.09 P 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.