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\title{}



\author[1]{Jong-Hwan  Park}%
\author[2]{Sun-Woo  Kim}%
\author[2]{So Young  Lee}%
\author[3]{Yuri Jung}%
\author[3]{Jae-Chul  Ro}%
\author[2]{Seong-Ju  Park}%
\author[2]{Hyoung-Juhn  Kim}%
\author[2]{Dong Han Seo}%
\author[3]{Su-Jeong  Suh}%
\affil[1]{Sungkyunkwan University College of Engineering}%
\affil[2]{Korea Institute of Energy Technology}%
\affil[3]{Sungkyunkwan University - Suwon Campus}%


\vspace{-1em}



  
  \date{March 15, 2025}


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\selectlanguage{english}
\begin{abstract}
Metal dichalcogenide based 2D materials, gained considerable attention
recently as a hydrogen evolution reaction (HER) electrocatalyst. In this
work, we synthesized MoSe2 based electrocatalyst via hydrothermal route
with varying phase contents (1T/2H) and respective HER performances were
evaluated under the acidic media (0.5M H2SO4), where best HER
performance was obtained from the sample consisting of mixed 1T/2H
phases, which was directly grown on a carbon paper (167mV at 10mA/cm2)
Furthermore, HER performance of electrocatalyst was further improved by
in-situ electrodeposition of Pt nanoparticles (0.15 wt\%) on the MoSe2
surface, which lead to significant enhancement in the HER performances
(133mV at 10mA/cm2). Finally, we conducted DFT calculations to reveal
the origin of such enhanced performances when the mixed 1T/2H phases
were present, where phase boundary region (1T/2H heterojunction) act as
a low energy pathway for H2 adsorption and desorption via electron
accumulation effect. Moreover, presence of the Pt nanoparticles tunes
the electronic states of the MoSe2 based catalyst, resulting in the
enhanced HER activity at heterointerface of 1T/2H MoSe2 while
facilitating the hydrogen adsorption and desorption process providing a
low energy pathway for HER. These results provide new insight on atomic
level understanding of the MoSe2 based catalyst for HER application.%
\end{abstract}%



\sloppy


\textbf{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.\textsuperscript{{[}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. \textsuperscript{{[}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.
\textsuperscript{{[}3{]}}Therefore, many researches have been directed
towards finding a new non-noble metal based electrocatalyst
\textsuperscript{{[}4{]}} with comparable HER activity of Pt/C catalyst.
\textsuperscript{{[}5{]}}Recently, various 2D Materials have been widely
studied as a promising candidate for HER. Amongst the 2D materials,
MoSe\textsubscript{2,}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
MoSe\textsubscript{2}(metallic-semiconducting transition).
\textsuperscript{{[}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.\textsuperscript{{[}6-b{]}} Therefore, careful optimization
of the phase and structure of the MoSe\textsubscript{2} is essential for
high HER performances. Moreover, MoSe\textsubscript{2} with the
optimized 1T and 2H mixed phases, have shown to improve the HER
activity,\textsuperscript{{[}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
MoSe\textsubscript{2}. \textsuperscript{{[}6{]}} Previous studies have
explored various strategies to enhance the catalytic performance of
MoSe\textsubscript{2} for the HER. For example, Jiang et
al.\textsuperscript{{[}7{]}} demonstrated the synthesis of
1T-MoSe\textsubscript{2} nanosheets with expanded interlayer spacing,
which showed superior electrochemical performance compared to
conventional MoSe\textsubscript{2}. 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. \textsuperscript{{[}8{]}} introduced
MoSe\textsubscript{2}/Mo core-shell 3D hierarchical nanostructures,
which significantly improved HER efficiency. Their study highlighted
that the 1T-MoSe\textsubscript{2}/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.\textsuperscript{{[}9{]}} investigated the effects of S-doping on
MoSe\textsubscript{2}, showing that doping induces a phase
transformation to 1T-MoSe\textsubscript{2}. 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.
\textsuperscript{{[}10{]}}developed V-doped MoSe\textsubscript{2}
nanosheets confined on carbon black (V-MoSe\textsubscript{2}/CB) using a
sol--gel process. This approach prevented agglomeration and yielded
ultra-thin nanosheets with short vertical lattice arrays. The
V-MoSe\textsubscript{2}/CB catalyst exhibited improved HER activity with
a small overpotential of 166 mV at 10 mA/cm\textsuperscript{2} 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 MoSe\textsubscript{2}-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. \textsuperscript{{[}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. \textsuperscript{{[}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, H\textsubscript{2}SO\textsubscript{4} 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
(\textasciitilde{}2 A/cm\textsuperscript{2}) 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.\textsuperscript{{[}13{]}}

Using the merits of above-mentioned materials/techniques, in this study,
we synthesized mixed phase 1T/2H MoSe\textsubscript{2} powder via
hydrothermal process where phase contents in MoSe\textsubscript{2} 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/cm\textsuperscript{2}under the acidic media (0.5 M
H\textsubscript{2}SO\textsubscript{4}) which was chosen as optimum
synthesis time. Then, we synthesized 1T/2H mixed phases of
MoSe\textsubscript{2} directly grown on a 3D porous carbon paper via
hydrothermal synthesis process for 36h, where enhanced HER performances
of 167 mV at 10 mA/cm\textsuperscript{2} were observed compared to the
conventional deposition process using a binder. Moreover, we conducted
an in-situ electrodeposition of Pt nanoparticle on
MoSe\textsubscript{2}, using the chronopotentiometry technique which
resulted in excellent HER performances with low overpotential of 133 mV
at 10 mA/cm\textsuperscript{2}, compared to the Pt-free, pristine
MoSe\textsubscript{2} 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 MoSe\textsubscript{2} based electro catalyst as well
as under the presence of Pt decoration on the mixed phases of
MoSe\textsubscript{2}, shedding a light on atomistic insight on HER
performances of 2D material based electrocatalyst such as
MoSe\textsubscript{2}.

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    2. Results and Discussion

\textbf{2.1 In-situ Pt decorated, (1T/2H) MoSe\textsubscript{2}synthesis
on a carbon paper}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1.} a) Schematic of the synthesis of
MoSe\textsubscript{2} bulk powder and MoSe\textsubscript{2} directly
grown on a carbon paper. b) Secondary electron microscope (SEM) image of
MoSe\textsubscript{2}-I-36h. c) High-resolution transmission electron
microscope (HR-TEM) image. d) Masked fast Fourier transform (FFT) of the
marked red box in c); green atom: Mo, yellow atom: Se, e) Cross-section
image of TEM of MoSe\textsubscript{2}-I-36h-Pt.
MoSe\textsubscript{2}-I-36h before and after Pt decoration.

\textbf{Figure 1} (a) presents the schematic of
MoSe\textsubscript{2}powder synthesis and in-situ grown
MoSe\textsubscript{2} on a carbon paper. In brief, to produce the
powder-type MoSe\textsubscript{2}, a solution containing 1.645 g
Na\textsubscript{2}MoO\textsubscript{4}[?]2H\textsubscript{2}O, 1.5492 g
Se powder, and 0.2595 g NaBH\textsubscript{4} dissolved for 0.5 h in a
mixture of 25 mL deionized water and 25 mL ethanol was prepared.
Subsequently, the dark-colored solution was transferred to a 100 mL
stainless steel autoclave and heated to 200\selectlanguage{ngerman}°C, maintained at varying
synthesis time. To obtain in-situ grown MoSe\textsubscript{2} on a
carbon paper, the carbon paper was added to the mixed solution. Pt
nanoparticles were decorated on the MoSe\textsubscript{2} surface using
chronopotentiometry at 10~mA~cm\selectlanguage{english}\textsuperscript{-2}~for 100~h. It was
confirmed using ICP-MS that the amount of Pt loading was about 0.15
wt\%. (\textbf{Table S1} ) Powder MoSe\textsubscript{2} and in-situ
grown MoSe\textsubscript{2} were denoted as MoSe\textsubscript{2}-P-X
and MoSe\textsubscript{2}-I-X, respectively, where X represents the
synthesis time.

Through SEM analysis, we observed variations in morphologies of
MoSe\textsubscript{2} powder dependent on the synthesis time. For
MoSe\textsubscript{2}-P-1 h, irregular particles shapes/sizes with
undesired composition of MoSe\textsubscript{2.86} were observed, as
shown in \textbf{Figure S1} (a) and \textbf{Figure S2} (a). The
compositions of MoSe\textsubscript{2}-P-6h, MoSe\textsubscript{2}-P-12h,
MoSe\textsubscript{2}-P-24h, MoSe\textsubscript{2}-P-36h, and
MoSe\textsubscript{2}-P-48h were analyzed by SEM-EDS shown
in\textbf{Fig. S2} (b)--(f). The obtained compositions were
MoSe\textsubscript{2.05}, MoSe\textsubscript{1.97},
MoSe\textsubscript{2.02}, MoSe\textsubscript{1.96},
MoSe\textsubscript{2.00}, respectively. MoSe\textsubscript{2} nanosheets
in \textbf{Fig. S1} (b)--(f) were generated with increasing synthesis
time to 6 h. The flower-like MoSe\textsubscript{2} nanosheet was
obtained from 12 h onwards. \textbf{Figure S3} shows XRD spectra of
MoSe\textsubscript{2} powder samples. For MoSe\textsubscript{2}-P-2H,
peaks at 13.1deg, 31.4deg, 37.8deg, and 55.94deg matched to (002), (100), (103),
and (110) planes of MoSe\textsubscript{2}-2H (JCPDS 00-029-0914). In the
case of MoSe-P-1 h, the peaks were corresponded to Se (JCPDS
01-086-2246) revealing the incomplete synthesis of
MoSe\textsubscript{2}. When the synthesis time was more than 6 h, we
observed the absence of the Se peak as well as the peak corresponding to
(103) plane of MoSe\textsubscript{2} originating from the 2H phase.
Moreover, the shift and broadening of peaks corresponding to (002) and
(100) planes of MoSe\textsubscript{2} were observed, indicating the
presence of mixed 1T and 2H phases of
MoSe\textsubscript{2}.\textsuperscript{{[}6-a{]}} \textbf{Figure S4}
exhibits Raman analysis of the MoSe\textsubscript{2}-P samples.
Intensity of a peak at 239 cm\textsuperscript{-1}, corresponding to
A\textsubscript{1g} of 2H-MoSe\textsubscript{2}, is diminished with
increasing synthesis time, whereas a peak at 281 cm\textsuperscript{-1},
matching to E\textsuperscript{1}\textsubscript{2g} of
1T-MoSe\textsubscript{2}, was increased with the synthesis time,
indicating the 1T phase of MoSe\textsubscript{2} increased as synthesis
time increased.\textsuperscript{{[}14{]}} Additionally, the peaks, which
are related to 1T phase, is clearly observed for the synthesis time of
36 h (J\textsubscript{1}=105 cm\textsuperscript{-1},
J\textsubscript{2}=149 cm\textsuperscript{-1}, J\textsubscript{3}=224
cm\textsuperscript{-1}) revealing the presence of mixed 1T and 2H phase
of MoSe\textsubscript{2}. \textsuperscript{{[}15{]}} In the case of
MoSe\textsubscript{2}-P-6h, the peaks at 121, 198, 277, and 352
cm\textsuperscript{-1} is matched to MoO\textsubscript{x} revealing the
incomplete formation of MoSe\textsubscript{2}.\textsuperscript{{[}16{]}}
\textbf{Figure S5 (a) and (b)} , shows the TEM images of the
MoSe\textsubscript{2}-P-36h and its selected area electron diffraction
(SAED) pattern, exhibiting that synthesized MoSe\textsubscript{2} was
polycrystalline in nature, reflected by the ring patterns in SAED with
respective lattice spacing values of 0.67 and 0.28 nm, which correspond
to (0 0 2) and (1 0 0) planes of 1T+2H phases of MoS\textsubscript{2}.
Furthermore, TEM-EDS elemental mapping analysis reveals the even
distribution of elements Mo and Se in samples with higher intensity of
Se being observed, as shown in \textbf{Figure S5} (c)-(e). We determined
that the optimized synthesis time was 36 h based on the detailed
material analysis revealing the presence of the 1T/2H mixed phases of
MoSe\textsubscript{2} and revealed the complete transformation of
precursors into MoSe\textsubscript{2} structures without the trace of
unreacted Se. The composition of MoSe\textsubscript{2}-P-36 h was
MoSe\textsubscript{2.24} which was confirmed using inductively coupled
plasma-mass spectrometry (ICP-MS). (\textbf{Table S1} ) Moreover, 36 h
sample exhibited highest HER performances under the 0.5 M
H\textsubscript{2}SO\textsubscript{4}electrolyte, compared to samples
synthesized at different time, where lowest overpotential of 177mV at
10mA/cm\textsuperscript{2} and lowest charge transfer resistance was
observed (R\textsubscript{ct}= 22\selectlanguage{greek}Ω\selectlanguage{english}). Furthermore, highest electrical
double layer capacitance (59.06 mF/cm\textsuperscript{2}) was observed
for the 36 h samples compared to the other samples with different
synthesis time (\textbf{Figure S6} and\textbf{S7} ).

After the optimization of the MoSe\textsubscript{2} powder synthesis, we
conducted experiments to directly grow the 1T/2H mixed phase
MoSe\textsubscript{2} on a conductive support such as carbon paper.
Morphology and the microstructure of MoSe\textsubscript{2}-I-36h was
investigated using SEM and HR-TEM. As shown in \textbf{Figure 1} (b),
dense and flower-like MoSe\textsubscript{2} nanosheets were grown on the
carbon paper. In addition, through the HR-TEM analysis, the resultant
in-situ grown samples exhibited heterointerface of 1T/2H phases of
MoSe\textsubscript{2} as shown in \textbf{Figure 1} (c) and (d).
Elemental composition mapping was conducted using TEM-EDS elemental
mapping technique where Mo and Se was well distributed along the samples
with higher intensity being observed for the Se, like the 36h powder
case. (\textbf{Figure S8} ). After the Pt nanoparticle decoration, Pt
clusters were observed on the MoSe\textsubscript{2} surface
(\textbf{Figure 1} (e) and \textbf{Figure S9} ). Furthermore, decorated
Pt nanoparticle exhibited lattice spacing value of 0.20 nm, which
corresponds to (200) plane, which was confirmed via HR-TEM analysis
(\textbf{Figure S10)} . \textsuperscript{{[}17{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 2. Comparison of MoSe2-I-36h after Pt decoration. a} )
X-ray diffraction (XRD) spectra; green sphere and grey diamond indicate
MoSe\textsubscript{2} 1T/2H and carbon paper, b) Raman spectra of
MoSe\textsubscript{2}-I-36h; Magenta diamond, blue reverse triangle, and
red diamond denote MoO\textsubscript{x}, MoSe\textsubscript{2}-2H, and
MoSe\textsubscript{2}-1T.

\textbf{Figure 2} (a) displays XRD spectra of
MoSe\textsubscript{2}-I-36h before and after Pt cluster decoration.
Similar to MoSe\textsubscript{2}-P 36h case, MoSe\textsubscript{2}-I-36h
was composed of mixed phases of 1T/2H MoSe\textsubscript{2}. After Pt
decoration on the MoSe\textsubscript{2}, the peaks matching to 1T/2H
phases of MoSe\textsubscript{2} remained unchanged while Pt XRD peaks
were not detected due to low concentration. From the Raman spectra
in\textbf{Figure 2} (b), the intensity of the A\textsubscript{1g} peak
(2H phase) was decreased, whereas those of J\textsubscript{1},
J\textsubscript{2}, J\textsubscript{3}, and
E\textsuperscript{1}\textsubscript{2g} (1T phase peaks) increased
significantly after the Pt decoration, indicating the conversion of 2H
MoSe\textsubscript{2} to 1T MoSe\textsubscript{2} during the Pt
electrodeposition process. \textbf{Figure S11(a)} , shows the XPS
analysis of the MoSe\textsubscript{2}-I-36h-Pt sample where the presence
of Pt element was clearly confirmed compared to the pristine
MoSe\textsubscript{2}-I-36h. Furthermore, the intensities of the peaks
(229.3, 232.9 eV for Mo 3d and 54.7 and 55.7 eV for Se 3d, which were
related to 2H-MoSe\textsubscript{2}, were decreased while increase in
relative intensities of the 1T-MoSe\textsubscript{2} were observed after
the Pt electrodeposition process, as depicted in \textbf{Figures S11}
(b) and (c) which is in good agreement with the Raman
analysis.\textbf{Figures S11} (d) exhibit Pt 4f spectra, the peaks
located at 72.5 and 75.6 eV corresponding to Pt\textsuperscript{0} were
observed, while the peaks located at 73.6 and 76.8 eV corresponding to
Pt\textsuperscript{2+} might be attributed to the oxidation of Pt
nanoparticles during the electrochemical reaction
process.\textsuperscript{{[}18{]}}

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    2.2 Electrochemical properties\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 3.} a) LSV curves with 85\% iR correction at a scan rate
of 5 mV/s, b) Tafel slope, c) C\textsubscript{dl}, d) EIS, e) Comparison
of HER overpotential at 10 mA cm\textsuperscript{-1} for previously
reported HER electrocatalysts in 0.5 M
H\textsubscript{2}SO\textsubscript{4}, f) Chronopotentiometry curves at
10, 20, 30, 40, and 50 mA/cm\textsuperscript{-1} in 0.5 M
H\textsubscript{2}SO\textsubscript{4}.

HER performance of the MoSe\textsubscript{2} samples were evaluated
under the acidic media (0.5 M H\textsubscript{2}SO\textsubscript{4}).
LSV curves in \textbf{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 MoSe\textsubscript{2}-P and MoSe\textsubscript{2}-I samples was
approximately 5 mg. MoSe\textsubscript{2}-I-36h exhibited a lower
\selectlanguage{greek}η\selectlanguage{english}\textsubscript{10} (167 mV) than that of MoSe\textsubscript{2}-P-36h
(177 mV). Furthermore, HER activity of MoSe\textsubscript{2}-I-36h was
significantly enhanced by in-situ electrodeposition of Pt cluster,
resulting in \selectlanguage{greek}η\selectlanguage{english}\textsubscript{10} of 133 mV. The
\selectlanguage{greek}η\selectlanguage{english}\textsubscript{10}without iR correction of MoSe\textsubscript{2}-P-36h,
MoSe\textsubscript{2}-I-36h, MoSe\textsubscript{2}-I-36h-Pt, and Pt/C
were 196, 185, 150, and 23 mV, respectively (\textbf{Figure S12} ). The
lower Tafel slope, represents the higher intrinsic activity of the
electrocatalyst. The Tafel slope values of MoSe\textsubscript{2}-P-36h,
MoSe\textsubscript{2}-I-36h, MoSe\textsubscript{2}-I-36h-Pt, and Pt/C
were 120, 106, 102, and 30 mV/dec, respectively (Figure 3(b)). This
suggests that MoSe\textsubscript{2}-I-36h-Pt displayed higher intrinsic
activity compared to the other MoSe\textsubscript{2} samples which can
be confirmed by specific activity comparison (\textbf{Figure S13} ). The
double layer capacitance (C\textsubscript{dl}) can be extracted from CV
curves with different scan rate in non-Faradaic region (\textbf{Figure
S14} ). The C\textsubscript{dl} of MoSe\textsubscript{2}-I-36h-Pt (17.46
mF/cm\textsuperscript{2}) was improved compared to
MoSe\textsubscript{2}-I-36h (16.28 mF/cm\textsuperscript{2}), as
depicted in Figure 3(c). However, the C\textsubscript{dl} of
MoSe\textsubscript{2}-P-36h (59.06 mF/cm\textsuperscript{2}) was larger
than those of MoSe\textsubscript{2}-I samples. The
MoSe\textsubscript{2}-P-36h was bonded using polyvinylidene fluoride
(PVDF), however, the surface of carbon paper was exposed (\textbf{Figure
S15} (a)), which may improve C\textsubscript{dl,} whereas
MoSe\textsubscript{2}-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
(\textbf{Figure S15} (b) \textsuperscript{{[}19{]}} as revealed in our
previous study, the C\textsubscript{dl} of pure carbon paper was highest
compared to electrocatalyst-coated electrodes.\textsuperscript{{[}19{]}}
In addition, thicknesses of the MoSe\textsubscript{2}-P-36h and
MoSe\textsubscript{2}-I-36h were over 1000 nm, 605 nm, respectively
(\textbf{Figure S16} ). Its large thickness could contribute to the
increase in charge transfer resistance (R\textsubscript{ct}) for
MoSe\textsubscript{2}-P-36h due to an increased charge transfer distance
and presence of unwetted region leading to decreased active sties in the
electrocatalysts.\textsuperscript{{[}20{]}} Decoration of highly active
and conductive Pt nanoparticles on MoSe\textsubscript{2} surface
assisted in reducing the charge transfer resistance for
MoSe\textsubscript{2}-I-36h-Pt samples which lead to lowest
R\textsubscript{ct} values amongst the MoSe\textsubscript{2} based
samples. As displayed in Figure S6(b) and Figure 3(d),
R\textsubscript{ct} values of MoSe\textsubscript{2}-P-36h,
MoSe\textsubscript{2}-I-36h, MoSe\textsubscript{2}-I-36h-Pt, and Pt/C
were 22, 10.79, 6.69, and 0.08 \selectlanguage{greek}Ω, \selectlanguage{english}respectively, suggesting that the
lower interfacial resistance between the electrolyte and surface of
electrocatalysts, while charge transfer is facilitated in
MoSe\textsubscript{2}-I-36h-Pt sample. Finally,
MoSe\textsubscript{2}-I-36h-Pt showed a lower HER overpotential with
high specific mass activity in acidic media (0.5 M
H\textsubscript{2}SO\textsubscript{4}) compared to other previously
reported metal chalcogenide and MoSe\textsubscript{2} materials (Figure
3(e); \textbf{Table S3 \& Table S4} ). The stability of
MoSe\textsubscript{2}-I-36h-Pt was evaluated by chronopotentiometry
(Figure 3(f)). MoSe\textsubscript{2}-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/cm\textsuperscript{2}, 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 MoSe\textsubscript{2}-I-36h-Pt
under the acidic media (0.5 M H\textsubscript{2}SO\textsubscript{4}) at
a current density of 10mA/cm\textsuperscript{2}. After the 100 h of
stability test, MoSe\textsubscript{2}-I-36h-Pt electrocatalyst exhibited
slight increase in the overpotential (\selectlanguage{greek}Δ\selectlanguage{english}17mV), revealing the good
long-term stability of the resultant electrocatalyst under the acidic
media. (\textbf{Figure S17} )

\textbf{2.3 Characterizations of MoSe\textsubscript{2}-I-36h-Pt after
stability test}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 4.} Characterization of MoSe\textsubscript{2}-I-36h-Pt
after stability test. (a) Post-test SEM image of surface, (b) Post-test
XRD, (c) Post-test Mo 3d, (d) Post-test Se 3d, and (e) Post-test Pt 4f.

It is important to investigate the electrocatalyst's properties after
the stability test. The post-test morphology of
MoSe\textsubscript{2}-I-36h was well maintained, as displayed
in\textbf{Figure 4a} . XRD analysis reveals an increase in intensity of
a peak at 13.1\selectlanguage{ngerman}°, indicating the possible phase transformation from 1T to
2H-MoSe\textsubscript{2} during the stability test (Figure 4(b)).
Moreover, such phase transition was confirmed by post-test Raman
analysis where reduction in 1T phase MoSe\textsubscript{2} peaks were
observed compared to the MoSe\textsubscript{2}-I-36h-Pt sample before
the test. (\textbf{Figure S18} ) This could be attributed to differences
in electrochemical stability between the 1T and 2H phase where, unstable
1T phase may undergo transformation to a more energetically favorable 2H
phase of MoSe\textsubscript{2} during the stability test which may have
resulted in slight increase in overpotential shown in Figure 3f. The
post stability test, Mo 3d and Se 3d peak analysis also reveals the
relative increase in the 2H peaks (Figure 4 (c) and (d) compared to the
sample before the stability test (Figure S11). These changes might have
originated from the transformation of MoSe\textsubscript{2} from 1T to
2H-MoSe\textsubscript{2} phase during the stability test. However, the
surface state of MoSe\textsubscript{2}-I-36h-Pt still maintained 1T/2H
mixed phases of MoSe\textsubscript{2} despite the phase conversion. With
respect to Pt 4f, the binding energies were nearly unchanged (Figure
4(e)), suggesting the properties of Pt were rarely affected by the
stability test.

\textbf{2.4 DFT study of HER mechanism}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure 5. DFT calculations of MoSe\textsubscript{2} based
electrocatalyst;} Fully relaxed geometry. (a) 1T/2H
MoSe\textsubscript{2}, (b) 1T MoSe\textsubscript{2}, (c) 2H
MoSe\textsubscript{2}. 1T/2H MoSe\textsubscript{2} with Pt cluster and
the optimized geometry when H atom adsorbs different Pt positions; (d)
Pt1, (e) Pt2, (f) Pt3, (g) Pt4. Grey, light purple, light green, and
white ball indicate Pt, Mo, Se, and H, respectively, (h) Charge density
difference of 1T/2H MoSe\textsubscript{2}; yellow and blue contour are
charge accumulation and depletion regions, respectively, (i) Gibbs free
energy of HER with different active sites, (j) Partial density of states
for Se 4p and H 1s; E\textsubscript{p} indicates p-band center, (h)
Partial density of states for Pt 5d; E\textsubscript{d} indicates d-band
center.

We conducted density functional theory (DFT) to reveal the role of
heterostructure of 1T/2H mixed phases of MoSe\textsubscript{2} and the
1T/2H mixed phase MoSe\textsubscript{2} surface with Pt cluster. The
fully relaxed MoSe\textsubscript{2} structures, which are 1T/2H
MoSe\textsubscript{2}, 1T MoSe\textsubscript{2}, 2H
MoSe\textsubscript{2}, and 1T/2H MoSe\textsubscript{2} with Pt,
respectively, were prepared for calculating the Gibbs free energy of
hydrogen adsorption (\selectlanguage{greek}Δ\selectlanguage{english}G\textsubscript{H}), as illustrated
in\textbf{Figure 5} (a)--(g). As depicted Figure 5(h), the charge
density accumulation was found for the interface between 1T and 2H
MoSe\textsubscript{2}, resulting in the enhanced HER
activity.\textsuperscript{{[}21{]}} The HER activity can be estimated by
\selectlanguage{greek}Δ\selectlanguage{english}G\textsubscript{H} value. When the \selectlanguage{greek}Δ\selectlanguage{english}G\textsubscript{H} value approaches
0 eV, the HER activity increases owing to the optimal balance between
the adsorption and desorption reaction of H atoms on the active
sites.\textsuperscript{{[}22{]}} The \selectlanguage{greek}Δ\selectlanguage{english}G\textsubscript{H} values of 1T/2H
MoSe\textsubscript{2} of point 1, point 2, and point 3 are 1.76, 0.93,
and 1.81 eV, respectively (\textbf{Figure S20} (a) and (b) in Supporting
Information). Their corresponding structures are shown in Figure 5(a)
and Figure S20(c) and (d) in the Supporting Information. Figure 5(i)
shows \selectlanguage{greek}Δ\selectlanguage{english}G\textsubscript{H} values of 1T MoSe\textsubscript{2} (1.62 eV),
2H MoSe\textsubscript{2} (2.27 eV), 1T/2H MoSe\textsubscript{2} (0.93
eV), 1T/2H MoSe\textsubscript{2} with Pt1 (--0.28 eV), 1T/2H
MoSe\textsubscript{2} with Pt2 (--0.24 eV), 1T/2H
MoSe\textsubscript{2}with Pt3 (--0.18 eV), 1T/2H MoSe\textsubscript{2}
with Pt4 (--0.61 eV). The 1T/2H MoSe\textsubscript{2} exhibits the lower
\selectlanguage{greek}Δ\selectlanguage{english}G\textsubscript{H}values for the than those of individual 2H
MoSe\textsubscript{2} and 1T MoSe\textsubscript{2} phases, indicating
that the presence of heterointerfaces between 1T/2H phases in
MoSe\textsubscript{2} enhances the HER activity. The improved HER
activity may be attributed to the electron accumulation at the
heterointerface, where optimized \selectlanguage{greek}Δ\selectlanguage{english}G\textsubscript{H} was observed at the
interface between 1T MoS\textsubscript{2} and 2H
MoS\textsubscript{2}.\textsuperscript{{[}21{]}} Furthermore, the
presence of the Pt cluster improves HER activity of Se site at the
heterointerface, showing lower Gibbs free energy of 0.88 eV
(\textbf{Figure S21(a)} ).

To investigate relationship between electronic structures and HER
performance, we calculate partial density of states (PDOS) of 1T/2H
MoSe\textsubscript{2} with Pt, 1T/2H MoSe\textsubscript{2}, 1T
MoSe\textsubscript{2}, and 2H MoSe\textsubscript{2} for Se atom and H
atom, as shown in \textbf{Figure 5} (j). The bonding strength of H atom
at active site can be confirmed by p-band center (E\textsubscript{p}).
When E\textsubscript{p} is upshifted to Fermi energy
(E\textsubscript{F}), the bonding strength is
increased.\textsuperscript{{[}23{]}} The E\textsubscript{p} of 1T/2H
MoSe\textsubscript{2} with Pt, 1T/2H MoSe\textsubscript{2}, 1T
MoSe\textsubscript{2}, and 2H MoSe\textsubscript{2} for Se 4p were
--2.67, --2.75, --3.89, and --4.13 eV, respectively. Upshift of PDOS
of H 1s to E\textsubscript{F} indicates the increased H atom activation,
as observed in 1T/2H MoSe\textsubscript{2}. Therefore, the electron
accumulation at the heterointerface, improved the bonding strength
between Se and H atom, leading to more favorable H activation which
contributes to the increased HER activity. In addition, the Pt cluster
promotes the HER activity at the heterointerface, which attributed to
the modulation of electronic structure, as shown in Figure 5(j).

When d-band center (E\textsubscript{d}) is close to E\textsubscript{F},
the bonding strength of the adsorbate is enhanced. When H atom is
adsorbed on different Pt sites, \selectlanguage{greek}Δ\selectlanguage{english}G\textsubscript{H} of Pt1, Pt2, Pt3,
and Pt4 are --0.28, --0.24, --0.18, and --0.61 eV, respectively.
E\textsubscript{d} values of above sites are --2.71, --2.81, --2.94, and
--1.84 eV, as depicted in \textbf{Figure 5} (i) and (k), indicating that
the Pt1, Pt2, and Pt3 sites near the Se atoms are favorable for hydrogen
adsorption and desorption. Although the Pt4 site shows the low HER
activity, its site has higher HER activity than 1T
MoSe\textsubscript{2}, 2H MoSe\textsubscript{2}, and 1T+2H
MoSe\textsubscript{2}. Additionally, as shown in \textbf{Figure S22} ,
Pt3 sites of 1T MoSe\textsubscript{2}+Pt and 2H MoSe\textsubscript{2}+Pt
exhibit reduced Gibbs free energies (--0.24 and --0.23 eV), indicating
the interaction between Pt and Se moderately control bonding strength of
Pt and H regardless of the MoSe\textsubscript{2} slab types. 1T+2H
MoSe\textsubscript{2} shows the lowest \selectlanguage{greek}Δ\selectlanguage{english}G\textsubscript{H}, indicating
that heterostructure is more favorable for HER compared to 1T
MoSe\textsubscript{2} and 2H MoSe\textsubscript{2} when the Pt cluster
is decorated onto the substrate. Based on DFT results, the high HER
activity of the 1T/2H MoSe\textsubscript{2}+Pt cluster is attributed to
the synergistic effect of the heterostructure of 1T/2H
MoSe\textsubscript{2} and the Pt cluster.

In summary, we synthesize 1T/2H mixed phase
MoSe\textsubscript{2}heterostructure directly grown on a carbon paper
using a hydrothermal method. Pt cluster is decorated on the
heterostructure using an in-situ electrochemical deposition approach.
This two-step approach enhances the interfacial adhesion and reduces the
interfacial resistance between the carbon paper and the electrocatalyst
compared to that of using MoSe\textsubscript{2} powder with binder. As a
result, directly grown sample (MoSe\textsubscript{2}-I-36h) exhibits
higher HER activity than the 1T/2H MoSe\textsubscript{2} powder
(MoSe\textsubscript{2}-P-36h) due to direct bonding between the
substrate and electrocatalyst which improves charge transfer. In
addition, the HER activity is significantly improved when the small
amount of 0.15 wt\% Pt was decorated on the heterostructure of 1T/2H
MoSe\textsubscript{2}. From DFT results, we found that the
heterostructure interface (phase boundary between 1T and 2H
MoSe\textsubscript{2}) act as a site with low \selectlanguage{greek}Δ\selectlanguage{english}G\textsubscript{H}which
would act as a primary active region in the electrocatalyst. Such
reduction in \selectlanguage{greek}Δ\selectlanguage{english}G\textsubscript{H} at the phase boundary is attributed to
the electron accumulation, improved H activation, and enhanced bonding
strength between Se and H atom. In addition, the decorated Pt cluster
act as not only an independent active site but also tuning the
electronic structure both Pt and Se active site which enhanced the HER
activity.

\textbf{3. Conclusions}

In this work, we successfully prepared self-standing 1T/2H mixed phases
of MoSe\textsubscript{2} on a carbon paper using a direct growth
approach via hydrothermal synthesis method, where post in-situ Pt
decoration was carried out by electrodeposition method. The
electrocatalyst (Pt decorated1T/2H mixed phase of MoSe\textsubscript{2})
prepared by this simple two step approach exhibit high HER activity (133
mV at 10 mA/cm\textsuperscript{2}) under the acidic medium (0.5 M
H\textsubscript{2}SO\textsubscript{4}). The direct growth of 1T/2H mixed
phase MoSe\textsubscript{2} on a carbon paper contributes to the lower
R\textsubscript{ct} with reduction in the overpotential by 10 mV at a
current density of 10 mA/cm\textsuperscript{2} compared to the
electrocatalyst prepared by conventional (electrocatalyst + binder)
approach. Moreover, low concentration (0.15 wt\%) decoration of Pt
contributes to significant enhancement in the HER activity with further
reduction in R\textsubscript{ct} compared to the pristine 1T/2H
MoSe\textsubscript{2} electrocatalyst without the Pt deposition. Such
results are well supported by the DFT calculation and provides insight
on enhanced HER activity at the phase boundary region in
MoSe\textsubscript{2} compared to the 1T or 2H alone regions due to the
electron accumulation at the heterointerface, while Pt clusters
interacts with Se site in MoSe\textsubscript{2} while perturbing the
electronic states of MoSe\textsubscript{2}, which further improves the
HER activity and confirmed through \selectlanguage{greek}Δ\selectlanguage{english}G\textsubscript{H} and
E\textsubscript{d} analysis, highlighting a origin of phase dependent
HER performances in MoSe\textsubscript{2} based electrocatalyst in the
presence of a dopant, which will pave a way for developing advanced
metal dichalcogenide 2D material based electrocatalyst in future.

\textbf{4. Experimental sections}

Sodium molybdate dihydrate
(Na\textsubscript{2}MoO\textsubscript{4}[?]2H\textsubscript{2}O, 99.5\%
purity; Sigma aldrich), selenium powder (Se, 99.5\% purity, Sigma
aldrich), sodium borohydride (NaBH\textsubscript{4}, 97\% purtiy;
Daejung), and ethanol (C\textsubscript{2}H\textsubscript{5}OH, 99.5\%
purity; Daejung) were used without purification. Deionized (DI) water of
18.2 M\selectlanguage{greek}Ω \selectlanguage{english}cm\textsuperscript{-1} was obtained using a water purification
system (V-RO up 15-S, A.F ENG, Korea).



\textbf{4.2 Preparation of MoSe\textsubscript{2} powder with 2H.}

To obtain pristine 2H MoSe\textsubscript{2}, the synthesis procedure was
the same in section 2.1, and the synthesized solution was slow cooled to
25 \selectlanguage{ngerman}°C in a furnace.

\textbf{4.3 Preparation of 1T/2H mixed phase
MoSe\textsubscript{2}directly grown on a carbon paper.}

To synthesize MoSe\textsubscript{2} directly onto a carbon paper
(HCP020N; Shanghai Hesen Electric Co. Ltd), a mixed solution containing
precursors and a 2×4 cm\textsuperscript{2} carbon paper was transferred
to a 100 mL stainless-steel autoclave. The synthesis duration was 36 h,
and the solution was rapidly cooled to 25 °C within 15 min. In-situ
MoSe\textsubscript{2} with varying synthesis times denoted
MoSe\textsubscript{2}-I-Y (Y = 36), where I represent `in-situ' and Y
represent synthesis time. The loading amount onto the carbon paper was
about \textasciitilde{}5 mg. For the Pt loading on the
MoSe\textsubscript{2}-I-36h-Pt sample was 0.15 wt\%
(\textasciitilde{}7.5ug/cm\textsuperscript{2}).

\textbf{4.4 Preparation of Pt/C electrode}

Pt/C (nominally 20\% on carbon black, Thermo Fisher Scientific) 30 mg
and 5 mg polyvinylidene fluoride (PVDF) were mixed in 500 \selectlanguage{greek}μ\selectlanguage{english}L
N-methyl-2-pyrrolidone (NMP). The 50 \selectlanguage{greek}μ\selectlanguage{english}L ink solution was deposited
evenly onto a 1\selectlanguage{ngerman}×1 cm\textsuperscript{2} carbon paper. The electrode was
dried in an oven at 80 °C for 24 h. The loading amount onto the carbon
paper was about \textasciitilde{}5 mg.

\textbf{4.5 Material properties}

The morphologies of the MoSe\textsubscript{2} samples were observed
through scanning electron microscopy (SEM, Philips, XL30 ESEM-FEG) at
the MEMS Sensor Platform Center of SungKyunKwan University (SKKU). The
microstructures of the samples were monitored by field-emission TEM
(FE-TEM, JEOL LTD, JEM-2100F) at an accelerating voltage of 200 kV. To
exactly observe Pt cluster on the MoSe\textsubscript{2}-I-36h-Pt, a
focused ion beam (FIB, FEI, Quanta 3D FEG) was selected. Elemental
analysis of Mo, Se, and Pt was conducted using inductively coupled
plasma mass spectrometry (ICP-MS, PerkinElmer, NexION2000). Phase
identification of as-synthesized MoSe\textsubscript{2} samples was
performed using multiple-purpose X-ray diffraction (XRD, Rigaku,
SmartLab 9kW) with Cu K\selectlanguage{greek}\textsubscript{α} \selectlanguage{english}source (\selectlanguage{greek}λ \selectlanguage{english}= 0.15406 nm, 45 kV,
200 mA) in the range of 10--90\selectlanguage{ngerman}°. In the case of MoSe\textsubscript{2}-I
samples, grazing-incidence XRD method at an 0.5° was chosen to avoid the
strong intensity of a substrate. The surface states were characterized
using a Raman spectrometer (HORIBA, LabRam HR Evolution) with 532 nm
laser in the range of 100--400 cm\textsuperscript{-1} and X-ray
photoelectron spectroscopy (XPS, Vg Scienta, EASC 2000) with an Al
anode.



\textbf{4.7 Density functional theory (DFT)}

Quantum ESPRESSO (QE) code version 7.2 was used to theoretically
determine the effect of the heterostructure and Pt
decoration.\textsuperscript{{[}27{]}} The projected augmented wave (PAW)
pseudopotentials were employed to show core--valence interactions. The
Perdew--Burke--Ernzerhof (PBE) functional of the generalized gradient
approximation (GGA) was used. The Van der Waals force was also corrected
using the ``Grimme-D3'' code, built in QE. The cutoff values, including
the plane-wave wavefunction of 55 Ry and charge density of 310 Ry, were
considered for convergence. A smearing code (marzari-vanderbilt) was
introduced to enhance the convergence.

The crystallographic information file (CIF) of
MoSe\textsubscript{2}(mp-1634) on Materials Project website was used to
design the supercell.\textsuperscript{{[}28{]}} The supercell of
MoSe\textsubscript{2} was optimized using the
Broyden--Fletcher--Goldfarb--Shanno (BFGS) algorithm with a force
threshold of 3.8×10\selectlanguage{english}\textsuperscript{-4} Ry/Bohr (\selectlanguage{english}[?]0.01 eV/\selectlanguage{english}) and a total
energy convergence threshold of 1.0x10\selectlanguage{english}\textsuperscript{-5} Ry (\selectlanguage{english}[?]0.0001
eV). The Monkhorst-pack k-point grid of 4 x 4 x 1 was selected to obtain
the optimized structure without spin polarization calculation owing to
non-magnetic property.\textsuperscript{{[}29{]}} Additionally, to
prevent interactions between the periodic boundaries along the z-axis, a
vacuum of 20 \selectlanguage{english} was applied in the supercell. Subsequently, the Gibbs
free energies (\selectlanguage{greek}Δ\selectlanguage{english}G) of the reactions in the HER mechanism for
MoSe\textsubscript{2} and MoSe\textsubscript{2} with Pt cluster were
calculated and are presented below.\textsuperscript{{[}23{]}}

\(\Delta G_{H}=\Delta E_{H}+\Delta E_{\text{ZPE}}-T\Delta S_{H}\)(4)

\(\Delta E_{H}=E_{\text{MoS}e_{2}+H}-E_{\text{MoS}e_{2}}-\frac{1}{2}E_{H_{2}}\)(5)

where \selectlanguage{greek}Δ\selectlanguage{english}G\textsubscript{H} is the Gibbs free energy of hydrogen
adsorption. \selectlanguage{greek}Δ\selectlanguage{english}E\textsubscript{H}, E\textsubscript{MoSe2+H},
E\textsubscript{MoSe2}, and E\textsubscript{H2} is the adsorption energy
of hydrogen, the total energies of the MoSe\textsubscript{2} slab with
hydrogen and the same structure without hydrogen, respectively.
E\textsubscript{H2} indicates the total energy of a gas-phase hydrogen
molecule. \selectlanguage{greek}Δ\selectlanguage{english}E\textsubscript{ZPE} and \selectlanguage{greek}Δ\selectlanguage{english}S\textsubscript{H} represent the
changes in zero-point energy, entropy between the adsorbed atomic
hydrogen and hydrogen in gas phase, respectively.

The DOS of 2H MoSe\textsubscript{2}, 1T+2H MoSe\textsubscript{2}, and
1T+2H MoSe\textsubscript{2} with Pt cluster were calculated for a
k-point grid of 6 \selectlanguage{ngerman}× 6 × 1 to obtain the exact electronic structure. To
reveal heterostructure effect, p-band center of Se (E\textsubscript{p})
was calculated using following equation:\textsuperscript{{[}23{]}}

\(E_{p}=\selectlanguage{greek}\frac{\int_{-\infty}^{+\infty}{E\times\rho dE}}{\int_{-\infty}^{+\infty}\text{ρdE}}\selectlanguage{english},\)(6)

where E\textsubscript{p}, \selectlanguage{greek}ρ, \selectlanguage{english}and E represent the p-band center, density
of p states projected onto the Se atom, and energy level, respectively.
When Pt cluster was decorated on the heterostructure slab, d-band center
(E\textsubscript{d}) was estimated as follows:\textsuperscript{{[}24{]}}

\(E_{d}=\selectlanguage{greek}\frac{\int_{-\infty}^{+\infty}{E\times\rho dE}}{\int_{-\infty}^{+\infty}\text{ρdE}}\selectlanguage{english},\)(7)

\textbf{Author contribution}

J.H.P and S. W. K contributed equally to this work. J.H.P and S. W. K
conducted experiments data curation \& analysis and manuscript
preparation. J.C.R, S. J. P, and S.Y.L contributed to data analysis.
Conceptualization, Supervision and writing---review and editing was
conducted by D.H.S, H.J.K and S.J.S.

\textbf{Acknowledgements}

This work was supported by the GRRC program of Gyeonggi Province (GRRC
Sungkyunkwan 2023-B01) and the Ministry of Trade, Industry and Energy
(No. 20017190). D. H. S acknowledges the support of Korea Institute of
Energy Technology Evaluation and Planning (KETEP) and the Ministry of
Trade, Industry \& Energy (MOTIE) of the Republic of Korea (No.
20224000000100). D.H.S and S.J.P acknowledges the research support of
KENTECH foundation research grant supported by MOTIE.

\textbf{Conflicts of Interest}

The authors declare no conflict of interest.

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