Raman spectroscopy provides additional information about the structures of FJH-synthesized samples. For TC-1, a characteristic peak is observed at around 686 cm -1 , corresponding to the W-C stretching mode. However, this peak becomes broader and weaker for TC-2, TC-3, and TC-4, as the phase-pure WC emerges . The peak located at approximately 250 cm -1 is ascribed to oxygen bonding (i.e., WC-O), which is due to the oxidation of the sample surface in air. The W-C bonding is further confirmed by XPS data (Fig. ), with no detectable oxygen bonding to tungsten or carbon, ruling out the formation of tungsten oxides. Furthermore, three main characteristic peaks are observed in all the samples at approximately 1340, 1594, and 2700 cm -1 , attributed to the D-band (diamond-like carbon), G-band (graphitic carbon), and 2D-band (layered structure of graphite or graphene), respectively (Fig. ) . The relative intensity ratio of the Gband to D-band, indicative of the graphitic degree of the carbon content, exhibits a direct relationship with the FJH synthesis duration of tungsten carbides, suggesting a more welldefined crystalline structure with increasing FJH durations . The microstructural morphology of the synthesized tungsten carbide samples by FJH was investigated by SEM. Specifically, TC-1 exhibits the uniform formation of densely connected nanoparticles with sizes ranging from 30 to 50 nm (Fig. and). For TC-2, connected nanoparticles ranging from 50 to 260 nm in size cover the entire carbon fiber surface forming a porous-like structure (Fig. and). Moreover, a porous structure comprised of uniform nanoparticles with an average size of approximately 70 nm forms on the surface of the TC-3 catalyst (Fig. and), resulting in a higher surface area and more active sites for the HER compared to the other catalysts. In addition, the element distribution of tungsten is uniform throughout the catalysts, suggesting well-dispersed active sites for effective HER (Fig. ). As shown in Figs. and, a dense layer is formed on the surface of carbon fiber, indicating the tungsten carbide particles are sintered to be interconnected, which can hinder effective HER. The HER performance of the synthesized tungsten carbide samples with different FJH durations was evaluated in an acidic environment using LSV. Fig. depicts the polarization curves obtained in 0.5 M H2SO4 for all fabricated tungsten carbide samples and a commercial Pt/C catalyst. The overpotential of TC-4 at a current density of 10 mA/cm 2 was determined to be 180.97 mV, which is the lowest among the tungsten carbide samples but higher than that (44.19 mV) of Pt/C. The overpotentials of TC-1, TC-2, and TC-3 catalysts were 239.02 mV, 194.98 mV, and 187.57 mV, respectively, indicating a decrease in overpotential with increasing FJH synthesis durations. This trend may be attributed to the co-existence of three active components (W, WC, and W2C) and their potential synergistic effects, resulting in modulated hydrogen binding energy and enhanced charge transfer kinetics . At a high current density of 4 A/cm 2 , TC-3 exhibited the best performance (η = 387 mV) among the fabricated tungsten carbide samples, significantly outperforming TC-1 (469.73 mV) and TC-4 (418.39 mV). Remarkably, TC-3 also outperformed Pt/C at current densities exceeding 630 mA/cm 2 , demonstrating its exceptional kinetics at high current densities. The superior hydrogen evolution performance of TC-3 suggests the importance of the synergetic effect between the two phases (i.e., W2C and WC) and especially their improved interface charge transfer.