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Published on: June 7, 2018
Dopant-Induced Hexagonal to Orthorhombic Phase Transition in Fe2- Mo P Nanorods and Its Influence on the
Jordon Baker1, Danyang Wang1, Md Kawsar Alam1
1Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284-2006, United States.
Abstract:
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 (ΔG H) on the Fe2P (210) surface. Accordingly, hexagonal and orthorhombic Fe2- Mo 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 Fe2P phase transformed into orthorhombic Fe2- Mo P when x ≥ 0.11 (5.41%). The admixture of Mo caused variations in the surface chemistry, leading to a significant decrease in Feδ+ and Pδ- charges. The HER performance was observed to be both phase- and composition-dependent with mixed-phase Fe2- Mo 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/cm2 compared to hexagonal Fe2P (η-10 = 378 mV) and orthorhombic Fe2- Mo P (η-10 = 331-459 mV for x = 0.12-0.28) catalysts. The highest HER performance was achieved for Fe1.91Mo0.09P NRs with a dopant composition of 4.58%, consistent with composition-dependent ΔG 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 Fe1.91Mo0.09P 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.
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