Conversely, the higher HER activity of the VO-TiO2 electrode is ascribed to the presence of oxygen vacancies and polarons. To further explore the role of vacancies and polarons on HER activity at pH=1, we conducted a Tafel analysis for both electrodes. Figure shows that the Tafel slopes of a VO-TiO2 nanoarray depend slightly on the electrode potential, ranging between 59 and 86 mV/dec. On pristine TiO2, the spread of Tafel slopes exhibit broader range, spanning from 79 to 154 mV/dec depending on the electrode potential. While a single Tafel slope alone cannot fully quantify the intrinsic electrocatalytic activity of either material, comparison of the Tafel slopes reveals that intrinsic HER activity of VO-TiO2 nanoarray is significantly higher than that of pristine TiO2. We also note that the Tafel slope of VO-TiO2 at lower overpotentials is close to the cardinal value of 60 mV/dec, commonly considered as an indicator of HER following the Volmer-Heyrovsky mechanism, with the Heyrovsky step identified as the rate-determining step. Finally, we utilized in situ Raman spectroscopy to investigate the structural stability of both pristine TiO2 and VO-TiO2 electrodes. The results depicted in Figure show that the Raman bands within the potential range from 0 VSHE to -2.0 VSHE overlap with the bands at OCP. These measurements provide compelling evidence that both electrodes maintain their structural integrity under operation conditions.