Figure 3. a) LSV curves with 85% iR correction at a scan rate of 5 mV/s, b) Tafel slope, c) Cdl, d) EIS, e) Comparison of HER overpotential at 10 mA cm-1 for previously reported HER electrocatalysts in 0.5 M H­2SO4, f) Chronopotentiometry curves at 10, 20, 30, 40, and 50 mA/cm-1 in 0.5 M H2SO4.
HER performance of the MoSe2 samples were evaluated under the acidic media (0.5 M H2SO4). LSV curves in Figure 3 (a) shows that HER can be enhanced by direct growth approach on the current collector compared to conventional electrode preparation method. The loading amount onto the carbon paper of MoSe2-P and MoSe2-I samples was approximately 5 mg. MoSe2-I-36h exhibited a lower η10 (167 mV) than that of MoSe2-P-36h (177 mV). Furthermore, HER activity of MoSe2-I-36h was significantly enhanced by in-situ electrodeposition of Pt cluster, resulting in η10 of 133 mV. The η10without iR correction of MoSe2-P-36h, MoSe2-I-36h, MoSe2-I-36h-Pt, and Pt/C were 196, 185, 150, and 23 mV, respectively (Figure S12 ). The lower Tafel slope, represents the higher intrinsic activity of the electrocatalyst. The Tafel slope values of MoSe2-P-36h, MoSe2-I-36h, MoSe2-I-36h-Pt, and Pt/C were 120, 106, 102, and 30 mV/dec, respectively (Figure 3(b)). This suggests that MoSe2-I-36h-Pt displayed higher intrinsic activity compared to the other MoSe2 samples which can be confirmed by specific activity comparison (Figure S13 ). The double layer capacitance (Cdl) can be extracted from CV curves with different scan rate in non-Faradaic region (Figure S14 ). The Cdl of MoSe2-I-36h-Pt (17.46 mF/cm2) was improved compared to MoSe2-I-36h (16.28 mF/cm2), as depicted in Figure 3(c). However, the Cdl of MoSe2-P-36h (59.06 mF/cm2) was larger than those of MoSe2-I samples. The MoSe2-P-36h was bonded using polyvinylidene fluoride (PVDF), however, the surface of carbon paper was exposed (Figure S15 (a)), which may improve Cdl, whereas MoSe2-I-36h can be attributed to the even deposition of electrocatalyst on a carbon paper surface which minimized exposure of bare carbon paper which would contribute to additional capacitance (Figure S15 (b) [19] as revealed in our previous study, the Cdl of pure carbon paper was highest compared to electrocatalyst-coated electrodes.[19] In addition, thicknesses of the MoSe2-P-36h and MoSe2-I-36h were over 1000 nm, 605 nm, respectively (Figure S16 ). Its large thickness could contribute to the increase in charge transfer resistance (Rct) for MoSe2-P-36h due to an increased charge transfer distance and presence of unwetted region leading to decreased active sties in the electrocatalysts.[20] Decoration of highly active and conductive Pt nanoparticles on MoSe2 surface assisted in reducing the charge transfer resistance for MoSe2-I-36h-Pt samples which lead to lowest Rct values amongst the MoSe2 based samples. As displayed in Figure S6(b) and Figure 3(d), Rct values of MoSe2-P-36h, MoSe2-I-36h, MoSe2-I-36h-Pt, and Pt/C were 22, 10.79, 6.69, and 0.08 Ω, respectively, suggesting that the lower interfacial resistance between the electrolyte and surface of electrocatalysts, while charge transfer is facilitated in MoSe2-I-36h-Pt sample. Finally, MoSe2-I-36h-Pt showed a lower HER overpotential with high specific mass activity in acidic media (0.5 M H2SO4) compared to other previously reported metal chalcogenide and MoSe2 materials (Figure 3(e); Table S3 & Table S4 ). The stability of MoSe2-I-36h-Pt was evaluated by chronopotentiometry (Figure 3(f)). MoSe2-I-36h-Pt showed slight increase in overpotential by 6, 5, 5, 2, and 2 mV at a current density of 10, 20, 30, 40, and 50 mA/cm2, respectively, where good HER stability was demonstrated under the varying current densities. Besides the HER stability evaluation at different current densities, we also performed long term stability test of the MoSe2-I-36h-Pt under the acidic media (0.5 M H2SO4) at a current density of 10mA/cm2. After the 100 h of stability test, MoSe2-I-36h-Pt electrocatalyst exhibited slight increase in the overpotential (Δ17mV), revealing the good long-term stability of the resultant electrocatalyst under the acidic media. (Figure S17 )