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 )