The apparent cathodic current on the Ti surface in acidic solution is composed of three reactions: the reductions of the oxide film, H+, and O2. However, classical electrochemical tests usually provide coupled multi-reaction currents. Furthermore, dissolved oxygen makes in situ measurements significant due to the thermodynamic instability of Ti. In this study, the effects of pre-reduction and dissolved oxygen on hydrogen evolution reaction (HER) kinetics on the Ti surface were investigated by electrochemical and composition characterizations. The HER current was quantitatively separated from the apparent cathodic current from Ti, including the reductions of the oxide film, H+, and O2 by scanning electrochemical microscopy. The HER transfer coefficient and standard rate constant for Ti spontaneously passivated in air (P-Ti) are both 0.48/4.9 × 10–12 cm/s in aerated and O2-saturated solutions, while those for Ti activated by electro-reduction (EA-Ti) are 0.25/3.6 × 10–7 cm/s and 0.25/3.1 × 10–7 cm/s, respectively. The variable HER behavior is caused by the changes in the film electronic structure and composition except for the competition reduction between O2 and H+ as indicated by Mott–Schottky and surface-enhanced Raman spectroscopy. The deviation from 0.5 in the transfer coefficient implies that the Tafel slope and HER mechanism should be analyzed with caution.

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