The electrocatalytic HER performance of as-fabricated VN@NC, WVN@NGC, bare GCE and 25% Pt/C samples was explored in acid and alkaline electrolytes employing conventional three-electrode configuration. To evaluate the influences of various related factors on electrochemical properties, the amounts of tungsten sources and the pyrolysis temperatures as key indicators were to analyze the catalytic activity in 0.5 mol/L H
2SO
4 and 1.0 mol/L KOH media respectively (Figs. S11 and S12 in Supporting information).
Figs. 4a and
e displayed the linear sweep voltammetry (LSV) curves of all the samples. Notably, WVN@NGC-900 required only the overpotentials of 143 mV and 158 mV to achieve a current density of 10 mA/cm
2 in 0.5 mol/L H
2SO
4 and 1.0 mol/L KOH media, respectively, outperforming the corresponding VN@NC-900 (361 mV) and VN@NC-900 (460 mV). The experimental results demonstrated that the incorporation of W into VN@NC could greatly enhance the electrocatalytic HER activity. In addition, the electrocatalytic HER performance over the resultant WVN@NGC-900 was better than that of the most of VN-based materials, as shown in Table S2 (Supporting information). Furthermore, the linear portion of Tafel slope derived from LSV curves was carried out to assess the behavior of reaction kinetics for as-prepared catalysts, as demonstrated in
Figs. 4b and
f. The WVN@NGC-900 displayed smaller numerical value (acid medium: 84 mV/dec, alkaline medium: 50 mV/dec) of Tafel slope compared with VN@NC-900 (acid medium: 410 mV/dec, alkaline medium: 188 mV/dec), revealing the Volmer-Heyrovsky mechanism was regarded as the predominant step of WVN@NGC-900 sample [
35]. To further analyze HER kinetics of synthetic samples during electrochemical process, the electrochemical impedance spectroscopy (EIS) test was performed in
Figs. 4c and
g. The charge transfer resistance (
Rct) for WVN@NGC-900 was lower than that of VN@NC-900 in both acid and alkaline solutions, which implied W dopant was quite advantageous to expedite electron transmission and favor HER kinetics. Furthermore, the equivalent circuit diagram and corresponding resistance were illustrated in the inset of
Figs. 4c and
g and Tables S3, S4 (Supporting information). By ascertaining the prominent HER electrocatalytic activity of WVN@NGC-900 ultrafine nanostructure, the double-layer capacitance (
Cdl) of as-fabricated catalysts was to calculated to estimate the electrochemically active area (ECSA). As we all know, the ECSA was proportionate to
Cdl value, namely the half of slope for as-synthesized catalysts, acquired from cyclic voltammetry (CV) at different scanning rates (Figs. S13 and S14 in Supporting information). As expected, the
Cdl values of WVN@NGC-900 (pH 0: 31 mF/cm
2, pH 14: 143 mF/cm
2) was larger than that of VN@NC-900 (pH 0: 16 mF/cm
2, pH 14: 63 mF/cm
2), which indicated the largest catalytically active sites of WVN@NGC-900 that boosting the electrocatalytic activity toward HER. To evaluate the durability of WVN@NGC-900, the time-dependent curves were measured, as shown in
Figs. 4d and
h. Evidently, the WVN@NGC-900 could sustain robust electrocatalytic hydrogen evolution with negligible decay for 50 h consecutive operation in both acid and alkaline electrolytes, confirming the superior durability in HER electrochemical process. After 50 h long-term
i-t measurement in both media, the microstructure and components of WVN@NGC-900 electrocatalyst maintained basically intact, certifying the excellent structural robustness throughout the electrochemical test in both 0.5 mol/L H
2SO
4 and 1.0 mol/L KOH conditions (Figs. S15-S18 in Supporting information).