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Letter
Effect of Intercalated Metals on the Electrocatalytic Activity of 1T-MoS for the Hydrogen Evolution Reaction 2
Nuwan H. Attanayake, Akila C. Thenuwara, Abhirup Patra, Yaroslav V. Aulin, Thi Tran, Himanshu Chakraborty, Eric Borguet, Michael L. Klein, John P Perdew, and Daniel R. Strongin ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00865 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017
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Effect of Intercalated Metals on the Electrocatalytic Activity of 1TMoS2 for the Hydrogen Evolution Reaction. Nuwan H. Attanayake1,3, Akila C. Thenuwara1,3, Abhirup Patra2,3, Yaroslav V. Aulin1,3, Thi M. Tran1 Himanshu Chakraborty1,3,4, Eric Borguet1,3, Michael L. Klein 1,3,4, John P. Perdew2,3 and Daniel R. Strongin1,3,* [1]
Department of Chemistry, Temple University, 1901 N. 13th Street, Philadelphia, PA 19122 (USA)
[2]
Department of Physics, Temple University 1925 N. 12th Street, Philadelphia, PA 19122 (USA)
[3]
Center for Computational Design of Functional Layered Materials (CCDM)
[4]
Institute for Computational Molecular Science, Temple University, 1925 N. 12th Street, Philadelphia,
Pennsylvania 19122 (USA)
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ABSTRACT: We show that intercalation of cations (Na+, Ca2+, Ni2+, and Co2+) into the interlayer region
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of 1T-MoS2 is an effective strategy to lower the overpotential for the hydrogen evolution reaction (HER).
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In acidic media the onset potential for 1T-MoS2 with intercalated ions is lowered by 60 mV relative to
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pristine 1T-MoS2 (180 mV). Density functional theory (DFT) calculations show a lowering in the Gibbs
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free energy for H-adsorption (GH) on these intercalated structures relative to intercalant-free 1T-MoS2.
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The DFT calculations suggest that Na+ intercalation results in a GH close to zero. Consistent with
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calculation, experiments show that the intercalation of Na+ ions into the interlayer region of 1T-MoS2
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results in the lowest overpotential for the HER.
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TOC GRAPHICS
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The release of CO2 into the atmosphere through anthropogenic activities, such as the combustion of
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fossil fuels has severe consequences for both flora and fauna. A proposed solution to this growing problem
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would be to use green energy from renewable energy sources such as wind and solar.1-3 Capture and
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storage of solar energy as a renewable energy source would be one of the most important steps toward
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turning away from the use of fossil fuels. Storing solar energy in batteries, however, is not currently a
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plausible solution due to their low energy densities. Hence, there is significant interest in developing
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methods to store solar energy in the form of chemical fuels. In this context, splitting water into hydrogen
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and oxygen using solar energy would be one such method to harness solar energy and store it in chemical
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bonds (i.e., hydrogen).4-6 Solar-induced water splitting has generally been investigated in two ways: (1)
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the direct photochemical splitting of water over suitable semiconductor catalyst and (2) the use of photo-
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voltaics to harness solar energy and to then split water with an electrolyzer.1 One crucial step in this latter
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effort is the hydrogen evolution reaction (HER) step:
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2H+ + 2e- → H2
(1)
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Precious metals such as Pt are still the most efficient HER catalysts, but they are not ideal for mass scale
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H2 production via water electrolysis due to their scarcity and high cost. Toward the goal of economic
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feasibility there is a significant research effort designed to find efficient and earth abundant electrocata-
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lytic materials that can catalyze the HER at the lowest possible overpotential. 2D transition metal dichal-
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cogenides (TMD) have shown significant promise for the HER in acidic media.7-9 Within this material
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class, WS2 and MoS2 are the most studied.10-12 This research effort has been inspired in part by the excel-
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lent catalytic activity of nano and micro sized MoS2 structures in petroleum refining industries.13 TMD
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are layered structures with van der Waals forces binding the sheets together. The activity of the TMD
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varies depending on structure and morphology. For example, bulk MoS2 was thought to be inactive to-
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wards HER until nano structures were realized and found to be excellent HER catalysts.8, 14-15 Three pol-
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ymorphs of MoS2 have been identified. The polymorphic semiconducting 2H type with a trigonal pris-
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matic coordination to six sulfur atoms was shown to have active sites on the edges on under coordinated
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Mo atoms. The basal planes of these materials were found in general to be inactive towards HER,14, 16
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while edge sites and sulfur anion vacancies provided sites for increased HER activity.17 Furthermore, it
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was shown that introduction of lattice strain in the layered sheets increased the HER activity. 18 Also, of
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note is that the interplanar conductivity of the TMD is 2200 times less than the conductivity along the
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basal plane.10 The paucity of active sites and low out of plane conductivity are key issues that limit the
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electrocatalytic activity of bulk 2H phase of MoS2 (i.e., 2H-MoS2) for the HER.
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Toward the goal of making MoS2 a more efficient HER catalyst, it has been shown that the 2H-MoS2
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phase can be transformed into the polymorphic metallic 1T type MoS 2 (via the intercalation of Li ions19-
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stable nano-sheets (NS) of 1T-MoS2, with lattice distortions.24-26 These distorted metallic 1T-MoS2 NS
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show good electrochemical activity relative to the 2H phase on both the edge sites and the basal plane.26-
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27
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hough this charge has been proposed to reduce their electrocatalytic efficiency.7, 24
) with octahedral coordination of S around the Mo. Lithium intercalation followed by exfoliation yields
The exfoliated NS bear a surface negative charge that enables a stable colloidal solution in water, alt-
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A hypothesis tested in this research is that the electrocatalytic activity of the layered 1T-MoS2 NS for
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the HER can be enhanced by controlling its electronic and structural properties via the intercalation of
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metal cations. Prior studies have already shown examples where the electrocatalytic behavior for water
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splitting of particular layered materials are sensitive to the nature of metal/cations in the interlayer re-
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gion.28-30 To test this hypothesis for 1T-MoS2 we determined the electrocatalytic HER activity of this
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layered material individually intercalated with Co2+, Ni2+, Ca2+, and Na+. Density Functional Theory
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(DFT) calculations were employed to gain an insight into the Gibbs free energy of adsorption for hydrogen 4
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(ΔGH) on each of the intercalated samples. The value of ΔGH has been widely used as a descriptor for
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evaluating catalysts for HER.14, 31-33 In particular, an improved catalytic activity is observed for ΔGH val-
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ues close to zero.
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To prepare 1T-MoS2 samples we used a well-established method that yields lithium intercalated bulk
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2H-MoS2 by using strong reducing agents such as LiBH4 and n-butyl lithium.19 The intercalated material,
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LixMoS2, was then exfoliated in water to produce metallic monolayer NS of MoS2.34 Prior studies have
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shown that these NS can be restacked by adding cations to a NS containing solution.35
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To prepare intercalated samples for this study, solutions containing either Na+, Ca2+, Ni2+ or Co2+ ions
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were added separately to a solution containing 2 mg/mL 1T-MoS2 NS. This addition led to a precipitated
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phase. The solution was centrifuged and the precipitate was washed three times with deionized (DI) water
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(see SI). Samples individually prepared with Na2+, Ca2+, Ni2+, and Co2+ will be referred to hereafter as
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Na/1T-MoS2, Ca/1T-MoS2, Ni/1T-MoS2 and Co/1T-MoS2, respectively.
Figure 1. Comparison of (001) and (002) XRD reflections of bulk 2H-MoS2, 1T-MoS2 and metal cation-intercalated-1T-MoS2. 72
X-ray diffraction (XRD) was used to determine the interlayer spacing of the various samples. Figure 1
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exhibits diffractograms for bulk 2H-MoS2 and 1T-MoS2 NS that were collected for analysis upon drying, 5
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and 1T-MoS2 NS that immediately precipitated from solution after individual exposure to Co2+, Ni2+, Ca2+,
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and Na+. Inspection of the data shows that bulk 2H-MoS2 exhibits a (002) reflection associated with an
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interlayer spacing of 6.36 Ȧ.36-37 Exfoliated MoS2 sheets that stacked upon drying exhibit an interlayer
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spacing of 12.87 Ȧ, due to a water double layer that persists in the interlayer region.37 The comparatively
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weak Bragg reflections associated with 1T-MoS2 NS (Figure 1 and SI Figure S2 for complete diffracto-
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grams) after exfoliation and drying suggest that this material likely has poor long range order (relative to
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the other samples). An analysis of the XRD does show that the interlayer distance is a function of the
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intercalated metal cation. Based on an analysis of the (001) reflection, the presence of Na+ results in the
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smallest interlayer distance (12.33 Å), while Ni2+ results in the greatest interlayer distance (12.73 Å).
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To confirm that the intercalated samples maintained the 1T phase, Raman spectroscopy was carried out
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(Figure 2). The distinct phonon modes at 154 (J1), 219 (J2) and 327 cm-1 (J3), which are characteristic of
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metallic 1T-MoS2, are attributed to the formation of a super lattice structure.35 The presence of the 1T
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super lattice strucuture was also observed in selective area elecron diffraction (SAED) patterns of both
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1T-MoS2 and intercalated 1T-MoS2 samples (SI Figure S3).27, 38-39 The broadening of 404 (A1g) and 380
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cm-1 (E12g) modes, as well as suppressed intensity of E12g are also typical for 1T phase. These results
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suggest that the metallic nature of the 1T NS is retained upon stacking in 3D with intercalated metal
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cation.
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Figure 2. Raman spectra of 2H-MoS2 (black),1T-MoS2 (green), and metal ion intercalated 1TMoS2 samples. Additional evidence for the presence of the 1T phase and the super lattice structure was obtained from X-ray photoelectron spectroscopy (XPS). The Mo 3d region associated with MoS2 consists of two primary peaks at ~229 and 232 eV corresponding to the 3d3/2 and 3d5/2 components of Mo4+ (SI Figure S4). Similar to the analysis of XPS for 2H- and 1T-MoS2 in prior work, deconvolution of the peaks obtained in this study for MoS2 shows additional peaks with a shift to lower binding energy of ~1.0 eV, that corresponds to the 1T phase.25, 34 Analysis of the S 2p1/2 and 2p3/2 XPS also shows that each feature is composed of two contributions, due to the presence of the 2H and 1T phases of MoS2. The presence of two phases gives rise to the super lattice structure. Deconvolution of both the Mo 3d and S 2p regions shows that ~75% of the exfoliated MoS2 material is in the 1T phase. The intercalated samples also show a similar contribution of the 1T phase suggesting that metal ion intercalation does not alter the relative concentration of the 2H and 1T phase (see SI Figure S4-S6). Analysis of the samples with energy dispersive spectroscopy (EDS) suggests that the amount of intercalant varies in the range of 1-2% when 10 mM divalent cation solutions and 4-5 atomic % is observed with the 20 mM solution of monovalent Na+ cation (Figure S7 and S10). We did not observe an increase in the intercalated cation concentration when the 7
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MoS2 sheets were exposed to higher cation solution concentrations. We believe this result is because the exfoliated sheets flocculate only when the charge on the sheet is neutralized by the intercalated cations. XPS of the Ni and Co intercalated samples (Figure S6b and S6c) showed the presence of Ni2+ and Co2+, respectively, indicating that redox chemistry between the cations and MoS2 did not occur during the intercalation process. Figure 3 shows scanning electron microscopy (SEM) images of 1T-MoS2 and Co/1T-MoS2, and transmission electron microscopy (TEM) images of the exfoliated 1T-MoS2 and flocculated samples of Co/1TMoS2. The exfoliated 1T-MoS2 sheets are very thin and, upon flocculation, several sheets stack over one another as evident from the SEM images and TEM image contrast. SEM and TEM images of the other
Figure 3. SEM images of (a) 1T-MoS2 and (b) Co/1T-MoS2 and TEM images of exfoliated (c) 1T-MoS2 and (d) stacked Co/1T-MoS2 (refer to SI for images of other samples)
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intercalated samples are shown in the SI (Figure S8-S9). During the intercalation process we do not experimentally observe a difference in the sizes of the 1T-MoS2 NS that stack to form the intercalated structures. Electrochemical experiments were carried out to understand the effect of the intercalated metals on the HER activity of 1T-MoS2. Measurements were performed with a three electrode system (see SI for further information). Figure 4A exhibits polarization curves for the different samples. These data show the effect
Figure 4. A) Polarization plots of current density (j) vs V after iR correction showing enhanced catalytic activity of metal cation intercalated 1T-MoS2. The inset exhibits a magnified view emphasizing the differences in overpotential between the samples (at -10 mA cm-2); B) Tafel plots for 1T-MoS2 and metal cation intercalated 1T-MoS2 samples; C) Nyquist plot showing the decrease in charge transfer resistance after intercalation; D) Plots of scan rate vs current density used for ECSA calculations.
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of the intercalated atoms on the overpotential () for HER associated with the different electrocatalysts (at a current density of 10 mA/cm2). The lowest overpotential in the MoS2 series studied was associated with Na/1T-MoS2 (=183 mV) and this material had an onset potential as low as 120 mV. This value was 50 mV lower than the value for pristine 1T-MoS2. The value of 230 mV vs RHE for 1T-MoS2 obtained in our study is consistent with prior HER studies for this same material.22-23, 26 An important conclusion that can be drawn from the data is that all the 1T-MoS2 materials with intercalated metal ions show a lower than does intercalant-free 1T-MoS2. While Na/1T-MoS2 exhibits the lowest value, all of the other intercalated metal ion-1T- MoS2 samples exhibit values that are within 12 mV of the Naintercalated samples and more than 40 mV lower than the associated with 1T-MoS2 (see Table 1). Figure 4B exhibits Tafel plots, derived from extrapolating the linear region of a plot of vs log j. The Tafel slope for Na/1T-MoS2 is 45 mV per decade, similar to the Tafel slope for 1T-MoS2 (also 45 mV decade-1). Tafel plots for all the samples are provided in the SI (Figure S11). Based on prior studies, the relatively low Tafel slope values obtained for our MoS2 samples imply that a two-electron-transfer process takes place in the HER, consistent with a Volmer-Heyrowsky mechanism where the rate limiting step in the process is the desorption of H2.40 The similar Tafel slopes for all the samples suggest that the intercalation process does not alter the HER mechanism that is associated with pristine 1T-MoS2. It is also important to mention that prior studies26 have suggested that removing some negative charge from exfoliated 1T-MoS2 NS (by exposing to iodine) also improves the catalytic HER activity of 1T-MoS2 (based on a lowering of ). This reduction in charge upon intercalation is emphasized in our research by zeta potential measurements (see Table S1). It was found, for example, that upon intercalation with Na+, the zeta potential decreased from a value of -43 mV to -29 mV. In view of this prior assertion, it is possible that the removal of some of the charge from the surface of 1T-MoS2 due to intercalation plays a role in facilitating the electron transfer between the NS and protons during the HER. To investigate the effect that intercalation had on the charge transfer resistance (Rct) of the samples, we carried out electrochemical impedance 10
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studies. Figure 4C exhibits data obtained at a value of 200 mV. 1T-MoS2 exhibits an Rct value of ~60 Ω, compared to a Rct = ~10 kΩ for the non-metallic 2H-MoS2 phase (figure SI 12). The lowest Rct values are observed for the intercalated 1T-MoS2 samples (Rct varies from 25 to 55 Ω), consistent with the superior HER activity of these samples. The electrochemically active surface area (ECSA) both gives an estimation of the active sites and is proportional to the double layer capacitance (Cdl). The slope of capacitive current for cyclic voltamograms (Figure SI13) plotted vs scan rate (Figure 4D) gives half the value of Cdl. The ECSA is proportional to the Cdl values (an increase in Cdl is associated with an increase in ECSA).41 The Cdl values are tabulated in Table 1. Based on these values, all the intercalated samples display a higher Cdl. Higher Cdl values imply that the capacitance on the catalyst has improved, suggesting that intercalated cations reside in the interlayer region. The improvement in the ECSA upon intercalation suggests that more sites are accessible for HER upon intercalation. We do not believe that the improved ECSA is due to changes in interlayer spacing, since all the intercalated samples have similar spacings. This improved ECSA is an interesting experimental observation considering that prior research of the 2H phase of MoS2 suggested that an increase in accessible reaction sites and lowering of the for this phase occurred with an expansion in interlayer distance.42 It may be the case that a reduced charge on the sheets upon cation intercalation may allow more reaction site accessibility for the proton reduction cycle, resulting in the decreased overpotential and increased ECSA. Chronopotentiometric measurements carried out at 10 mA/cm2 show that the intercalated samples are stable over a period of 24 h (see SI figure S14). Raman spectroscopy of Na/1T-MoS2 after the stability test shows that the 1T structure is preserved (SI figure S15). All the experimental data taken together show that the intercalated samples are stable and more active than intercalant-free 1T-MoS2.
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Table 1: Summary of electrochemical data Sample
η (mV)
Tafel Rct (Ω) Slope (mV/dec)
Cdl (mF cm-2)
1TMoS2
230
45
60
3.75
Na/1TMoS2
183
45
25
9.20
Ca/1TMoS2
190
46
30
7.35
Ni/1TMoS2
191
47
30
6.30
Co/1TMoS2
195
46
40
6.15
Finally, it is useful to revisit the electrochemical polarization results in view of DFT calculations. Specifically, we calculated the hydrogen atom adsorption free energy change (ΔGH) for 1T-MoS2 in the absence and presence of the different intercalated cations. Prior studies have generally shown that ΔGH is an excellent descriptor to assess the electrochemical HER activity of catalytic surfaces.14, 31-33 In particular, optimal catalytic activity is associated with a ΔGH 0.14, 32 DFT calculations were performed in the Vienna Ab Initio Simulation Package (VASP)43 within the projector augmented wave method (PAW).44 The newly developed meta-GGA SCAN45 and its non-local van der Waals corrected form, SCAN+rVV1046 were used as the exchange-correlation approximations (Refer to the SI for detailed information including geometries). Figure 5 summarizes the computational results. The largest ΔGH value is obtained for 1T-MoS2 in the absence of intercalant, while the Na+ intercalated sample is associated with the ΔGH value closest to zero.
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Figure 5. Free energies of hydrogen atom adsorption on the basal plane of pristine 1TMoS2 as well as on the intercalated 1T-MoS2. These calculations are consistent with the experimental finding that the highest and lowest overpotential are associated with 1T- MoS2 and Na/1T- MoS2. Figure S14 exhibits density of states (DOS) plots for the different 1T- MoS2 systems. The presence of the localized d-states of Co and Ni at the Fermi level (EF) are prominent features for Ni/1T- MoS2 and Co/1T- MoS2 while delocalized sp states characterize the Na/1TMoS2 and Ca/1T- MoS2 DOS plots. Interestingly, while the contributions of the different intercalants to the DOS of 1T- MoS2 are varied, all these intercalated systems are more effective HER catalysts than 1TMoS2 without intercalant. These calculations suggest that the introduction of DOS at the EF, whether from a transition metal or sp metal, favorably affects HER by a lowering of ΔGH from the value associated with intercalant-free 1T- MoS2. To summarize, we have used a solution-based technique to intercalate metal cations into the interlayer region of 1T- MoS2 to improve the efficiency of the material for the HER. Intercalation likely lowers the for the HER at least in part by increasing the ECSA of the intercalated samples. Also, DFT calculations suggest that the intercalation of either a sp or d-band metal cation into the 1T phase of MoS2 results
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in a change of ΔGH, relative to the ΔGH associated with the intercalant-free material, that can be related to enhanced activity for the HER. ASSOCIATED CONTENT Supporting Information. Experimental Conditions, DFT calculations and structure. Figures S1-S17 as cited in the text. Zeta potentials. AUTHOR INFORMATION Corresponding Author * Daniel R. Strongin. Email:
[email protected] Author Contributions All authors have given approval to the final version of the manuscript.
ACKNOWLWEDGEMENTS This work was supported by the Center for the Computational Design of Functional Layered Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0012575. A.P. was supported by the National Science Foundation under Grant No. DMR-1607868 (J.P.P.). ABBREVIATIONS TMD, Transition metal dichalcogenide; HER, Hydrogen Evolution Reaction; DFT, Density Functional Theory; DOS, Density of States.
REFERENCES 14
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