1. Introduction
Green hydrogen has emerged as an important clean energy source for
replacing the conventional energy source, such as fossil fuel which
could contribute in decreasing the greenhouse effect. Electrochemical
water electrolysis (EWE), the process used to generate green hydrogen,
consisting of hydrogen evolution reaction (HER) and oxygen evolution
reaction (OER), has been widely studied over the decades due to its
advantageous merits including high hydrogen production rate and ability
to generate high purity hydrogen without other
byproducts.[1] However, large amount of energy is
needed to split the water, although the theoretical potential for the
water electrolysis is 1.23 V, additional energy is required due to the
presence of overpotential arising from the electrocatalyst in each HER
& OER processes which hinder energy efficient production of hydrogen.
Therefore, developing advanced electrocatalyst with lower overpotential
with high activity, low charge transfer resistance and excellent
electrical conductivity is highly desired for energy efficient, high
performing water electrolysis. [2]
Specifically, in HER process, the use of noble catalyst such as Pt/C is
widely used as an electrocatalyst, owing to its low overpotential with
high activity along with its great conductivity. Yet, the use of
expensive, noble metal catalyst is one of the roadblocks in cost
effective production of green hydrogen. [3]Therefore, many researches have been directed towards finding a new
non-noble metal based electrocatalyst [4] with
comparable HER activity of Pt/C catalyst. [5]Recently, various 2D Materials have been widely studied as a promising
candidate for HER. Amongst the 2D materials, MoSe2,metal dichalcogenides based 2D material gained considerable interests as
a promising candidate for HER due to its tunable d-spacing with lamellar
structure facilitating the ion transport and tunable conductivity
provided by controlling the phase of the MoSe2(metallic-semiconducting transition). [6] Amongst
the phases in metal dichalcogenide based 2D materials, 2H phase
(semiconducting phase) shows low HER performance compared to the 1T
phase (metallic phase) due to its lower conductivity.[6-b] Therefore, careful optimization of the phase
and structure of the MoSe2 is essential for high HER
performances. Moreover, MoSe2 with the optimized 1T and
2H mixed phases, have shown to improve the HER activity,[6-a] Although further study is needed to
investigate the origin of the high HER performance induced by mixed
phase of metal dichalcogenide based electrocatalyst such as
MoSe2. [6] Previous studies have
explored various strategies to enhance the catalytic performance of
MoSe2 for the HER. For example, Jiang et al.[7] demonstrated the synthesis of
1T-MoSe2 nanosheets with expanded interlayer spacing,
which showed superior electrochemical performance compared to
conventional MoSe2. They reported a low overpotential of
179 mV and a small Tafel slope of 78 mV/dec, attributing these
improvements to the high conductivity and increased number of active
sites in the 1T phase. Qu et al. [8] introduced
MoSe2/Mo core-shell 3D hierarchical nanostructures,
which significantly improved HER efficiency. Their study highlighted
that the 1T-MoSe2/Mo core-shell structures exhibited a
low Tafel slope of 34.7 mV/dec, suggesting enhanced electron transfer
efficiency between the catalyst and electrode. Li et al.[9] investigated the effects of S-doping on
MoSe2, showing that doping induces a phase
transformation to 1T-MoSe2. This transformation has led
to a high purity of the 1T phase, resulting in a low overpotential of
167 mV and a Tafel slope of 54 mV/dec, thereby enhancing HER
performance. Additionally, Ren et al. [10]developed V-doped MoSe2 nanosheets confined on carbon
black (V-MoSe2/CB) using a sol–gel process. This
approach prevented agglomeration and yielded ultra-thin nanosheets with
short vertical lattice arrays. The V-MoSe2/CB catalyst
exhibited improved HER activity with a small overpotential of 166 mV at
10 mA/cm2 and a Tafel slope of 65 mV/dec, attributed
to optimized electronic structure and enhanced electrocatalytic
performance. These studies have collectively illustrated various methods
to maximize the catalytic efficiency of MoSe2-based
materials for HER. Furthermore, often metal-based dopants could be
introduced to improve the HER performances of the metal dichalcogenide
based electrocatalyst, where Pt nanoparticles/dopant has shown to be
effective in enhancing the HER performances. [11]
Besides the development of the high performing electrocatalyst, HER
performances can be affected by the method in which electrocatalyst
deposition takes place on the current collector. Conventionally,
electrocatalyst deposition is often carried out via drop-casting
process, where electrocatalyst is often mixed with non-conductive
binders such as Nafion or PVDF, which are then pasted onto a conductive
current collector. The used of non-conductive binders often, obstructs
the active sites of the electrocatalyst while increasing the resistance
in the electrodes. In contrast, direct growth of electrocatalyst on the
current collector is often favored due to its fast electron transfer
between the electrocatalyst and the substrate, resulting in enhanced HER
activity with long-term stability without the masking effect which could
be more favorable method of electrocatalyst deposition compared to the
conventional process. [12]
Finally, media in which HER takes place, is an important factor
determining the HER performances. Generally, use of the pure water
induces high overpotential of water electrolysis due to its high
electrolyte resistance. Often, H2SO4 or
KOH based electrolytes are used, which reduce electrolyte resistance, to
increase the efficiency of water electrolysis and to decrease the
overpotential. Especially, water electrolysis under acidic operation
conditions has advantages such as high current density (~2
A/cm2) and low gas crossover compared to alkaline
water electrolysis (AWE). Therefore, water electrolysis under acidic
media is favorable for the designing of compact water electrolysis cell.[13]
Using the merits of above-mentioned materials/techniques, in this study,
we synthesized mixed phase 1T/2H MoSe2 powder via
hydrothermal process where phase contents in MoSe2 were
controlled by the synthesis time, then the HER performances of the
resultant electrocatalysts were first evaluated. The best HER
performances were demonstrated by the samples with synthesis time of 36
h which exhibited overpotential of 177 mV at 10 mA/cm2under the acidic media (0.5 M H2SO4)
which was chosen as optimum synthesis time. Then, we synthesized 1T/2H
mixed phases of MoSe2 directly grown on a 3D porous
carbon paper via hydrothermal synthesis process for 36h, where enhanced
HER performances of 167 mV at 10 mA/cm2 were observed
compared to the conventional deposition process using a binder.
Moreover, we conducted an in-situ electrodeposition of Pt nanoparticle
on MoSe2, using the chronopotentiometry technique which
resulted in excellent HER performances with low overpotential of 133 mV
at 10 mA/cm2, compared to the Pt-free, pristine
MoSe2 powder. Finally, we conducted a thorough
theoretical study (DFT calculation) on varying HER performances under
various 1T/2H phase conditions to explore the origin of phase dependent
HER performance in MoSe2 based electro catalyst as well
as under the presence of Pt decoration on the mixed phases of
MoSe2, shedding a light on atomistic insight on HER
performances of 2D material based electrocatalyst such as
MoSe2.