## 3 Results and Discussion
The enhanced HER catalytic activity of the Co\({}_{3}\)Se\({}_{4}\) (20 s) catalysts can be attributed to better quality loading of the catalyst on the FTO substrate. However, the decrease in the activity of the Co\({}_{3}\)Se\({}_{4}\) (25 s) catalysts may be due to overloading of the catalysts on the FTO. This clumps the electroactive sites, resulting in a poor interface between electrode and electrolyte. Therefore, controlled loading of the catalysts over the substrates plays a vital role because it tends to generate more electroactive sites, which facilitates superior electrocatalytic activity. Overall,the Co\({}_{3}\)Se\({}_{4}\) catalysts deposited on the FTO substrate for 20 s delivered superior HER electrocatalytic activity by requiring only a very low overpotential of 215 mV to acquire an improved current density of 10 mA/cm\({}^{2}\). This overpotential (215 mV) required by the Co\({}_{3}\)Se\({}_{4}\) catalysts electrodeposited on the FTO substrate for 20 s seems very efficient compared to the previously reported results on metal selenides. To study the effect of Co\({}_{3}\)Se\({}_{4}\) catalyst on to the different electrode substrate, the Co\({}_{3}\)Se\({}_{4}\) film was deposited onto the Si substrate to compare electro-catalytic property under dark condition. In this study, highly P-doped n\({}^{+}\)-type Si was selected for the electrochemical proton reduction to generate hydrogen (Figure 2c) and it is the general method to investigate electro-catalytic performance [38]. As a result, electrochemical behaviors were very similar to those of performance from Co\({}_{3}\)Se\({}_{4}\) films on FTO substrate with increasing Co3 Se4 deposition time (Figure 4). A slightly higher anodic shift in FTO samples might be attributed to the difference between the intrinsic resistivity or the conductivity of Si and FTO substrates. To sum up, in terms of electro-catalytic behavior, the same tendency of Co\({}_{3}\)Se\({}_{4}\) co-catalyst film was identified from FTO and Si electrode substrate. To gain additional and more detailed information about the inherent properties of the electrodeposited Co\({}_{3}\)Se\({}_{4}\), the current density versus potential curves of the Co\({}_{3}\)Se\({}_{4}\) samples on GC and on FTO were converted to Tafel plots to study the rate limiting step of the HER reaction. First, the Tafel slopes shown in Figure 4d were calculated to be 66.2 and 42.5 mV/dec for Co\({}_{3}\)Se\({}_{4}\) on GC  
Figure 4: Electrochemical activity of cobalt selenide (Co\({}_{3}\)Se\({}_{4}\)) in 1 M HClO\({}_{4}\) with respect to deposition time on glassy carbon and FTO electrode from RDE system: (**a**) Polarization curves of bare GCE, bare FTO, Co\({}_{3}\)Se\({}_{4}\)(10) on GCE and Co\({}_{3}\)Se\({}_{4}\)(10) on FTO. (**b**) Polarization curves of bare FTO without any co-catalyst and FTO with cobalt selenide deposited for 5, 10, 15, 20 and 25 s. (**c**) the stability of electrodeposited Co\({}_{3}\)Se\({}_{4}\) during 7200 s at a steady potential of \(-\)0.2 V (vs. RHE) (**d**) HER Tafel slope of glassy carbon and FTO electrodeposited with cobalt selenide. and on FTO, respectively. The Co\({}_{3}\)Se\({}_{4}\)(20) electrocatalyst deposited on FTO yielded a consistently low Tafel value, substantiating the upgraded electrocatalytic activity of the material. This provided further evidence that hydrogen can be generated by a Heyrovsky reaction mechanism, by which one proton adsorbs to the electrode and one hydrated proton comes from the electrolyte (and they combine) to make hydrogen gas. The exchange current density of Co\({}_{3}\)Se\({}_{4}\) on GC was 1.62 \(\times\) 10\({}^{-6}\) A/cm\({}^{2}\) and that of Co\({}_{3}\)Se\({}_{4}\) on FTO was 4.47 \(\times\) 10\({}^{-8}\) A/cm\({}^{2}\), which values were derived from the results on variation in the substrate condition. In conclusion, electrodeposited Co\({}_{3}\)Se\({}_{4}\)(20) showed an effective hydrogen evolution mechanism with any of the conductive standard electrodes.