Low-cost, layered transition-metal dichalcogenides (MX_2) based on molybdenum and tungsten have attracted substantial interest as alternative catalysts for the hydrogen evolution reaction (HER). These materials have high intrinsic per-site HER activity; however, a significant challenge is the limited density of active sites, which are concentrated at the layer edges. Here we unravel electronic factors underlying catalytic activity on MX_2 surfaces, and leverage the understanding to report group-5 MX_2 (H-TaS_2 and H-NbS_2) electrocatalysts whose performance instead mainly derives from highly active basal-plane sites, as suggested by our first-principles calculations and performance comparisons with edge-active counterparts. Beyond high catalytic activity, they are found to exhibit an unusual ability to optimize their morphology for enhanced charge transfer and accessibility of active sites as the HER proceeds, offering a practical advantage for scalable processing. The catalysts reach 10 mA cm^(−2) current density at an overpotential of ∼50–60 mV with a loading of 10–55 μg cm^(−2), surpassing other reported MX2 candidates without any performance-enhancing additives

Ajayan, Pulickel M.

Gu, Jing

Hackenberg, Ken P.

Keyshar, Kunttal

Liu, Yuanyue

Lou, Jun

Ogitsu, Tadashi

Vajtai, Robert

Wang, Y. Morris

Wood, Brandon C.

Wu, Jingjie

Yakobson, Boris I.

Yang, Yingchao

Zhang, Jing

Caltech Authors - Main

In the format provided by the authors and unedited.
Self-optimizing, highly surface-active layered metal dichalcogenide catalysts 
for hydrogen evolution 
Yuanyue Liu1+, Jingjie Wu1+, Ken P. Hackenberg1+,Jing Zhang1, Y. Morris Wang2, Yingchao 
Yang1, Kunttal Keyshar1, Jing Gu3, Tadashi Ogitsu2, Robert Vajtai1, Jun Lou1, Pulickel M. 
Ajayan1, Brandon C. Wood*2, Boris I. Yakobson*1 
1Department of Materials Science and Nano-Engineering, Rice University, Houston, TX 77005. 
2Lawrence Livermore National Laboratory, Livermore, CA 94550. 
3Department of Chemistry and Biochemistry, San Diego State University, San Diego, CA, 
92182. 
+These authors contribute equally 
*Correspondence to: brandonwood@llnl.gov, biy@rice.edu 
 
Supplementary Discussion: To evaluate the activity of the edges, we calculated the free 
energies of H adsorption on TaS2 and NbS2 edges. Although the structures of TaS2 and NbS2 
edges at working condition are unknown, and very likely they keep changing during the reaction 
as the materials are broken into small pieces, we take the most common structure of the MoS2 
edge at HER condition (as shown in the Supplementary figure 18 below)1,2, as a representative 
model for TaS2 and NbS2 edges.  
Our calculations show that the free energies of H adsorption on this specific type of edges are -
0.13 eV/H for TaS2 and -0.26 eV/H for NbS2. Compared with the free energies of H adsorption 
on the basal plane, 0.17 eV/H for TaS2 and 0.01 eV/H for NbS2, this specific type of edges likely 
does have some level of activity. However, we point out that these edges are less active than the 
MoS2 edge (0.05 eV/H; A closer-to-zero free energy suggests a higher activity, see the main text), 
which suggests that the better overall performance of NbS2 and TaS2 compared to MoS2 must 
have significant contributions from other factors. The most likely explanation is basal-plane 
activity, as our calculations indicate.   
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VOLUME: 2 | ARTICLE NUMBER: 17127
NATURE ENERGY | DOI: 10.1038/nenergy.2017.127 | www.nature.com/natureenergy 1
 
 
Supplementary figure 1| MX2 surface activity and its electronic origin. (A) Schematic of 
MX2 surface catalyzed HER. M: blue; X: yellow. (B) and (C) Electronic densities of states (DOS) 
of representative pristine and H-adsorbed MX2 systems. DOS plots represent a coverage of H:M 
= 1:16, with the filled states shown in gray (energies are referenced to vacuum for the pristine 
case). Left/right panels represent metallic (here, TiS2) and semiconducting (here, MoS2) variants. 
Insets show charge density isosurfaces for states within the energy range of the Fermi level to -
0.025 eV below. H: black; charge density isosurface: red. For the metallic system (left panel), H 
adsorption does not change the overall DOS profile, instead shifting the Fermi level slightly to 
reflect complete charge transfer from the H adsorbate. The charge density distribution shows that 
the transferred electrons are delocalized throughout the M layer. For the semiconducting system 
(right panel), the DOS profile also remains largely intact, and the H occupies a very shallow n-
type dopant level. The charge density distribution shows that this state is quasi-localized in space. 
In the dilute adsorption limit, both cases are well described by εLUS. (D) Correlation between the 
εLUS descriptor and the surface adsorption energy Ea (Eq. 1, Methods; negative indicates stronger 
binding) of H for a test series of MX2 candidates. H-NbS2, H-TaS2 and T-TaS2 are shown as 
filled circles. 
 
 
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 Supplementary figure 2 | Structures of various phases of MX2 and testing of other common 
descriptors. Top: the red arrow indicates the primitive cell. Bottom: adsorption energy Ea (Eq. 1, 
Methods) as a function of the charge on the X atom (left); the d-band center of bulk M (right). 
The d-band center values are taken from Ref. 3. 
 
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 Supplementary figure 3 | Computed Gdiff and Gtot (Eqs. 2 and 3, Methods) for H adsorption 
on the group-5 MX2 candidates. Top: Coverage-dependent Gtot. Bottom: Gdiff for the system in 
vacuum (left), in solvent based on an implicit solvation model (center), and in solvent with a 
large electric field pointing towards the electrode (right). See Methods for calculation details. For 
calculation of Gdiff,, results are based on adsorption on a 4x4 supercell; Pt and Ni results assume 
adsorption on the (111) surface; and the MoS2 edge is modeled using a nanoribbon with edge 
structure taken from Ref. 2.  
 
  
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 Supplementary figure 4 | Physical characterization of as-grown H-TaS2 flake. (a) SEM 
image. (b) High-resolution TEM image showing lattice. (c) Raman spectroscopy. (d) XRD 
pattern of H-TaS2 showing preferred (00l) orientation on SiO2/Si substrate.  
 
0 10 20 30 40 50 60 70
SiO2 substrate
 
(004)(003)(002)
In
te
ns
ity
 (a
.u
.)
2
(001)
c d
150 200 250 300 350 400 450
A1gE
1
2g
 
 
In
te
ns
ity
 (a
. u
.)
Raman shift (cm-1)
E1g
b
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 Supplementary figure 5 | Physical characterization of as-grown H-NbS2 flake. (a) SEM 
image. (b) High-resolution TEM image showing lattice. (h) EELS confirming presence of Nb 
and S in H-NbS2. (c) Raman spectroscopy.  
  
a b
c d
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 Supplementary figure 6 | Electrical Conductivity of H-TaS2 and H-MoS2 flakes. (a) 
Measurement of the in-plane and (b) out-of-plane resistivity of pre-cycled H-TaS2, and (c) in-
plane resistivity of H-MoS2. 
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 Supplementary figure 7. Physical characterization of T-MoS2 and T-TaS2. SEM images for 
(a) T-MoS2 and (b) T-TaS2. Raman spectroscopy for (c) T-MoS2 and (d) T-TaS2 . The T-MoS2 
has a typical thickness around 100 nm which is comparable to the TaS2 after 5000 cycles. The 
emergence of new Raman shifts at 198, 225, and 284 cm−1 associated with the phonon modes of 
T-MoS2, clearly confirm the formation of T-MoS2 from exfoliation treatment of commercial H-
MoS2 plates. 
 
 
 
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Supplementary figure 8 | Electrochemical evolution recorded periodically during potential 
cycling for H-TaS2 electrode. (a) Polarization curves recorded after different cycles. The initial 
polarization curve was recorded after 4 cycles. (b-c) EIS after correspondent cycles. Solid lines 
are fit to the equivalent circuit model described in Methods. (d) Relative change of current 
density j (at -0.1 V vs. RHE) for H-TaS2 as a function of the change in the effective double-layer 
capacitance C for H-TaS2. The capacitance is derived from the constant phase element (CPE) in 
the EIS equivalent circuit. The power of n used in the CPE fits are 0.78, 0.72, 0.76, 0.68, 0.70, 
0.72 for 2, 1000, 2000, 3000, 4000 and 5000 cycles, respectively.  
  
-0.3 -0.2 -0.1 0.0 0.1
-200
-150
-100
-50
0
 
 
j (
m
A
 c
m
-2
)
E (V vs. RHE)
 Initial
 Post-300 cycles
 Post-1000 cycles
 Post-2000 cycles
 Post-2500 cycles
 Post-3500 cycles
 Post-4500 cycles
 Post-5000 cycles
H-TaS2
a b
0 20 40 60
0
5
10
15
 
 
-Z
'' 
(O
hm
)
Z' (Ohm)
0 200 400 600
0
100
200
 
 
-Z
'' 
(O
hm
)
Z' (Ohm)
Initial
1000 cycles
2000
2000
3000
4000
5000
c d
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 Supplementary figure 9 | Electrochemical evolution recorded periodically during potential 
cycling for H-NbS2 electrode. (a) Polarization curves recorded after different cycles. The initial 
polarization curve was recorded after 2 cycles. (b) The corresponding Tafel slope evolution. (c-d) 
EIS after correspondent cycles. Solid lines are fits to the equivalent circuit model described in 
Methods. (e) Cycle-dependent evolution of current (recorded at -0.1 V vs. RHE), charge transfer 
resistivity, and capacitance for H-NbS2. 
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 Supplementary figure 10 | Polarization curves recorded periodically during potential 
cycling for H-MoS2 electrode.  The initial polarization curve was recorded after 2 cycles. 
 
 
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 Supplementary figure 11 | HER of H-TaS2 under potentiostatic operation. (a) 
Chronoamperometric response at constant potential of -0.54 V vs. RHE (not iR corrected). (b) 
Linear sweep voltammetry of initial and after 12 and 24 hours. (c) Corresponding EIS data. 
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Supplementary figure 12 | Morphological, chemical and structural characterization of H-
TaS2 on a glassy carbon plate before and after cycling. (a) SEM images showing 
morphological evolution of H-TaS2 at different magnifications before (left) and after (right) 5000 
cycles. The bottom left shows a blank image, confirming no nano-scaled H-TaS2 before cycling. 
(b) EDS characterization of H-TaS2 before (left) and after (right) cycling for densely covered 
regions shown in the SEM images below. (c) TEM images for H-TaS2 before (left) and after 
(right). Insets are HETEM showing no crystalline structure change. 
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 Supplementary figure 13 | Morphological, chemical and structural characterization of H-
NbS2 on a glassy carbon plate before and after cycling. (a) SEM and TEM images showing 
morphological evolution of H-NbS2 at different cycles. The inset at the bottom right image 
shows the HRTEM of H-NbS2 after 12000 cycles (b) XPS data for fine scan of S 2p and Nb 3d 
of H-NbS2 cycling. (c) Thickness profiles for H-NbS2 before (left) and after (right) cycling, 
based on the AFM images below. Four thickness profiles are shown for the cycled sample to 
confirm thickness uniformity. (d) Raman spectroscopy of H-NbS2 before and after cycling.  
 
 
 
  
 
 
 
 
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 Supplementary figure 14 | Absence of contaminants in H-TaS2 as evidenced by CO 
stripping voltammetry results.  (a) CO stripping voltammetry for a long-term cycled H-TaS2 
and (b) a reference Pt/C (Pt loading 0.3 μg cm-2) electrode in CO-free solution of 0.5 M H2SO4. 
Solid line is CO stripping, dashed line is background cyclic voltammogram collected 
immediately after CO stripping. The CO is adsorbed at a constant potential of 0.20 V vs. RHE 
for 5 min. Scan rate is 50 mV/s.  
 
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 Supplementary figure 15 | Absence of contaminants in H-TaS2 and H-NbS2as evidenced by 
XPS survey scan of electrodes after cycling. (a) H-TaS2 and (b) H-NbS2. Bottom panels show 
zoomed-in regions of 50-100 eV. The survey scan shows no other metal contaminations on the 
electrodes or below the detecting limit. 
  
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 Supplementary figure 16 | Faradaic efficiency of H2 evolution reaction on H-TaS2 electrode. 
(a) Current versus time at -0.4 V vs. RHE, with (b) periodically measured production moles of 
H2. The red line represents theoretical production moles calculated from the charges passed 
during electrolysis. (c) Gas chromatography trace of H2 collected from the HER compartment.  
 
 
Supplementary figure 17 | Polarization curves of Pt shown in different scales. The literature 
results are included for comparison. 
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 Supplementary figure 18| The most common structure of MoS2 edge at HER condition (see 
JACS 2015 137 6692; JACS 2005 127 5308 etc). 
 
 
Supplementary table 1. Comparison of HER activity 
Sample Catalyst 
loading 
(μg/cm2) 
j0 
(A/cm2) 
Tafel slope 
(mV/decade) 
|j| @ -0.15 
V vs. 
RHE 
(mA/cm2) 
|V| vs. 
RHE @ 
j=-10 
mA/cm2 
(V) Ref 
Nano particulate 
MoS2 N/A 1.3-3.7×10-7 55-60 0.1 
0.17 
4 
Particulate MoS2 4 4.6×10-6 120 0.5 0.3 5 
Double gyroid 
MoS2 60 6.9×10-7 50 1 
0.28 
6 
Edge exposed 
MoS2 film 8.5 2.2× 10-6 105-120 0.06 
>0.4 
7 
Edge exposed 
MoS2 film 22 1.71-3.40× 10-6 115-123 0.1 
0.21 
8 
30 nm MoS2 3400-3900 5.0× 10-5 66 10.3 0.15 9 
T- MoS2 50 N/A 40 1 0.2 10 
T- MoS2 N/A N/A 43 2 0.2 11 
MoS2/RGO 285 5.1×10-6 41 8 0.16 12 
Defect-Rich 
MoS2 285 8.9×10-6 50 
3 0.2 13 
Electrodeposited 
MoS2  N/A N/A 106 
2 >0.4 14 
MoS2/CNT- 650 2.91×10-5 100 2 0.28 15 
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graphene 
NanoflakesWS2 350 N/A 48 1 >0.4 16 
WS2/RGO 400 N/A 58 2 0.26 17 
T-WS2 6.5 1×10-6 60 1 0.23 18 
T-WS2 1000±200 <10-4 70 14 0.14 19 
MoSX/CNT 102 33.11× 10-6 40 25 0.11 20 
H-MoS2 100 3.4×10-6 120 
 
0.08 
 
>0.4 
This 
work 
T-MoS2 50 2.5× 10-6 78 0.2 
 
0.29 
This 
work 
T-TaS2 80 6.4× 10-8 92 0.02 
 
>0.4 
This 
work 
H-TaS2 55 7.8× 10-5 37 
 
160 
 
0.06 
This 
work 
H-NbS2 10 1.3×10-3 28 
 
128 
 
0.05 
This 
work 
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Self-optimizing, highly surface-active layered metal dichalcogenide catalysts for hydrogen evolution

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Self-optimizing, highly surface-active layered metal dichalcogenide catalysts for hydrogen evolution

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