design of active and stable co–mo–sx chalcogels -s gels as ... · 2 ba2+- free electrolyte...

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Jakub SSubrahmaStamenko

1 Materia

2 Departm

3 Univers

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taszak-Jirkovanyam S. Kovic1, Mercou

als Science Di

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sity of Ljublja

Sulfur modif

entary Figureout (gray das(red). Pt(111)Absolute refle(blue curve) aperimentally m

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vský,1 ChrisKota,2 Kee-Curi G. Kanatzi

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S1a shows Csibly adsorbedad the Pt(111)ion of the pro

ble Co-Mohe hydrog

tos D. MaChul Chang,1

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nne National

western Unive

a, Slovenia

urfaces: Pt(1

c voltammetred sulfur in 0

Sad coverages of the (00L)Sad

- in 0.1M Ka. Inset shows

CVs and HERd sulfur. As ) electrode waocedure show

o-Sx chalcogen evoluti

alliakas,1,2 P1 Bostjan Gd M. Markovi

Laboratory, A

ersity, Evansto

11)-Sad

ry and b HER0.1M KOH ae (green) is a) crystal trunc

KOH containins the electron

R polarizationmentioned inas hold at the

wn in [S1]. Th

ogels as pHion reactio

Pietro P. LGenorio1,2, Dic1*

Argonne, IL,

on, IL, USA

R polarizationand in the pralso shown focation rod at ng Ba2+ (red density profi

n curves for thn the method e open circuithe surface cov

H-universon

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USA

n curve for Presence of B

for compariso-.035 V for Pcurve). Solidile of the fits.

he HER on Psection, to o

t potential in verage (ϴSad ~

sal catalys

manja Danilnik1, Vojisla

Pt(111) with (Ba2+ in 0.1M on. Scan rate Pt(111)- Sad

-δ

d lines are CT

Pt(111) and Ptobtain interme0.01 mM Na

~0.25ML) by

1

sts

lovic,1 av R.

(blue) KOH at 50 in 0.1

TR fits

t(111) ediate

a2S for Sad in

Design of active and stable Co–Mo–Sx chalcogelsas pH-universal catalysts for the hydrogen

evolution reaction

SUPPLEMENTARY INFORMATIONDOI: 10.1038/NMAT4481

NATURE MATERIALS | www.nature.com/naturematerials 1

© 2015 Macmillan Publishers Limited. All rights reserved

http://dx.doi.org/10.1038/nmat4481

2

Ba2+- free electrolyte (blue curve) and in the presence of 10-4 M Ba2+ is determined by integrating the charge under the Hupd peak (0.05 up to 0.35V vs. RHE). Inspection of Supplementary Figure S1a reveals that while in the Hupd potential region Ba2+ has no effect on adsorption of hydrogen, in the “butterfly region” (0.6 up to 0.85 V vs. RHE) the role of Ba2+ in formation of OHad adlayer is significant. This is in line with previously proposed role of non-covalent interactions in the formation of oxide on metal surfaces (see ref. [S2]). The inset of Supplementary Figure S1a summarizes the role of cations on the HER in alkaline solutions. Four points are noteworthy: (i) in KOH rather small differences in activity (within experimental errors) are observed between Pt(111) and Pt(111)-Sad; (ii) importantly, in the presence of Ba2+ small, yet clearly discernable, increase in activity is recorded on Sad-covered Pt(111); (iii) as for Au(111), on the Pt(111) surface covered with high ϴSad the HER is substantially deactivated relative to Pt(111); and (iv) in contrast to Au(111), due to higher intrinsic activity of Pt the promoting role of Ba2+ is much smaller. The latter observation is in line with the ORR on Au and Pt modified with Pbupd or Biupd [S3]; namely, while the 2e- reduction on Au is catalyzed to 4e- reduction on Au covered by Pbupd/Biupd for the same surface coverages by heavy metal adatoms the effect on Pt is unmeasurable. Based on results in Figure S1 and Supplementary Figure S1 we conclude that HER on Au-/Pt-Sad in alkaline solutions involves Volmer-Tafel pathway in which Sad

δ---Cn+--H2O network serve to promote water dissociation step followed by adsorption of Had intermediates on neighboring metal atoms that, finally, recombine to form H2.

Supplementary Figure S1b summarizes the SXS measurement for the Pt(111)-Sad interface. To explore the Pt-Sad interactions we performed the experiments on Pt(111) covered with rather high surface coverages by Sad, close to saturation. Notice that because Au(111) reconstruction is lifted in the presence of Sad, it was impossible to probe the Au-Sad interfaces, although many conclusions from the Pt-Sad systems can be adopted for the former system. Here, we use Crystal Truncation Rod measurements of sulfur adsorbed noble metal surfaces (e.g. Pt(111) in 0.1 M KOH solution. The data was taken at 11keV and acquired at bending magnet beamline 12-BM-B at the Advanced Photon Source (APS), Argonne National Laboratory. As shown in Supplementary Figure S1b and summarized in Supplementary Table S1, from analyses of crystal truncation rods [S2] we found that when Pt is covered by Sad

δ-, the expansion of Pt surface atoms in KOH is close to ~1.8%, which is significantly higher than ~1.4% observed for the Sad

δ- -free Pt(111) [S4, S5]. However, in the presence of Ba2+, the top Pt surface atoms relax to ~2.6%, signifying a strong effect of Sad

δ----Ba2+(H2O)y interactions on expansion of Pt surface atoms. As a consequence, the Pt-Ba2+ distance is shorter by ~5% and the concentration of Ba2+ in the double layer is almost 2-fold higher relative to Pt free of Sad

δ-, confirming our hypothesis that the Sadδ----Ba2+(H2O)y

interaction plays a very important role in the double layer structure and thus influences the reactivity of water.

Supplementary Table S1 | CTR fitting results for S adsorbed Pt(111) in 0.1M KOH Pt(111)/S Pt(111)/S-Ba2+ coverage Distance (Å) Coverage Distance (Å) Pt 0.76 ± 0.02 2.31 ± 0.01 1.01 ± 0.04 2.33 ± 0.01 S 1.35 ± 0.07 2.49 ± 0.03 0.5 ± 0.2 2.21 ± 0.08 Ba 0.14 ± 0.01 3.44 ± 0.04


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As can be seen PDF is based on a conventional X-ray or neutron powder diffraction experiment and it’s simply another representation of the diffraction data. The only difference is that PDF reflects both the long-range atomic structure, manifest in the Bragg reflections, and the local structure imperfections, manifest in the diffuse component of the diffraction pattern. The data analysis does not presume any periodicity therefore the PDF technique is very useful for examining if the distortions found in a specific single crystal are representative of the total bulk of the sample. The coordination number in the PDF data was estimated by converting the PDF plots (G(r)) to RDF (Radial Distribution Function, RDF = 4πρG(r)) by using an average density of ρ = 2.5 g/cm3 which was measured with a pycnometer for each chalcogel. Peaks in the RDF plots were fitted with Gaussian peaks and their area was used to estimate the coordination number by taking the relative ratios between the peak areas and normalizing against the M-M coordination number.


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9

the same chemical environment (see Supplementary Figures S8c and S8d). The width of the XPS spectra of CoSx and CoMoSx is wider than the one of MoSx (and Mo3S13) due to the polysulfides.

Supplementary Figure S8. XPS S 2p spectra for a CoSx, b CoMoSx, c MoSx, and d comparison of chalcogels’ S2p XPS spectra against crystalline (NH4)2Mo3S13. S 2p peaks are broader for CoSx and CoMoSx due to the presence of polysulfides in their structure.

In summary, all of the characterization data we obtained (Supplementary Table S2) enable us to elucidate several important morphological and structural features: (i) all chalcogels have amorphous structure with random porous networks; (ii) MoSx consists of robust Mo3S13 clusters interconnected with MoS4 tetrahedra; (iii) CoSx is more open structure with a random distribution of CoS8 octahedra with a fraction of Co2S6 blocks that contain Co-Co dimers; and (iv) CoMoSx consists of the Mo3S13 clusters interconnected by CoS8 octahedra that are capped by polysulfide oligomers in a compact and robust structure.


10

Supplementary Table S2 | Structural parameters for the chalcogel materials*.

Material Element Ex-situ Characterization In-situ Characterization

(E = -0.5 V vs RHE; pH 13)

M-M (Å)

M-S (Å)

S-S (Å)

NM-M NM-S Ratio NM-

M/M-S

M-M (Å)

M-S (Å)

S-S (Å)

NM-M NM-S Ratio NM-

M/M-S MoS2 3.16 2.41 3.16 - - -

- CoS2 3.90 2.32 2.12 - - - MoSx 2.81 2.35 2.14 2 7.4(6) 3.7(3) 2.81 2.38 - 0.5(2) 2.1(3) 4.2(4) CoSx 2.61 2.25 2.01 1 1.4(4) 1.4(4) 2.58 2.23 - 1.2(3) 2.1(3) 1.7(3)

CoMoSx Co-Co -

2.42 2.05 2

9.2(6) 3.1(2) 2.59 2.22

- 0.8(2) 2.0(3) 2.5(3)

Mo-Mo 2.76

- 1 2.73 2.39 0.9(2) 3.1(4) 3.4(3) Co-Mo - - - - 2.68 - - -

* Structural parameters obtained by Pair Distribution Function analysis (ex-situ PDF) and Extended X-ray absorption fine structure (In-situ EXAFS). Characteristic bond distances between the 3d metal center, the metal centers and the sulfur atoms and the sulfur-sulfur atoms, respectively. N = first shell coordination number. PDF is not a chemical element specific technique like EXAFS, therefore some peaks in PDF may result from overlapping vectors, e.g., Mo-S and Co-S will give one single broad peak. The coordination number in the PDF data was estimated by converting the PDF plots (G(r)) to RDF (Radial Distribution Function, RDF = 4πρG(r)) by using an average density of ρ = 2.5 g/cm3 which was measured with a pycnometer for each chalcogel. Peaks in the RDF plots were fitted with Gaussian peaks and their area was used to estimate the coordination number by taking the relative ratios between the peak areas and normalizing against the M-M coordination number. There is a close agreement between structural parameters for the as prepared TMSx chalcogels and TMSx chalcogels under the HER conditions given that the ratios of M-S to M-M are similar in both conditions.

Stability of chalcogel materials

To establish not only the electrochemical performance changes during cycling, monitoring materials dissolution is relevant not only to account for the performance loss but, to a first approximation, correlate to the number of active sites. We use ICP-MS analysis to determine the amount of Co and Mo lost during potential cycling. The results are shown only for pH 1 as in alkaline media precipitation of TMn+ hinders their detection in solution, even by ICP-MS.

The ICP-MS results shown in Supplementary Table S1 indicate three main trends for the chalcogels materials. First, the chalcogels are most stable as crystalline form than as amorphous. Second, chalcogels containing cobalt shows grater dissolved metal contents in solution than the one containing only Mo. These results are close related to the activity trend observed, where amorphous materials are much more active than crystalline and within amorphous, CoSx are more active than MoSx. For last, the combination of Co and Mo centers in CoMoSx suggests a remarkable stability synergy, observing a reduced cobalt dissolution rate when compared to pure CoSx at the same time that the Mo becomes slightly less stable in CoMoSx than in MoSx. Additional to elucidating the stability of the TM centers by following their dissolution, their correlation with the activity observed for the HER indicates the active role the TM centers play in the HER mechanism.


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42

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12

Effect of pH on the HER for Chalcogel materials

To understand the reaction mechanism for the hydrogen evolution reaction, pH can be used as powerful probe as the change in the reactant nature for hydrogen formation can tell us what the limiting step in the reaction pathway is. Supplementary Figure S9 show the polarization curves in both acid and alkaline for all materials used here. The general trend observed for all materials is an enhancement of the reaction rate at lower pH values, as the reactant nature is the hydronium ion, much easier to brake to form H atoms than water, as in the case of pH 13. CoMoSx on the other hand is the only material not showing this pronounced tendency, which leads us to point in a rate determining step change from the other materials. While braking hydronium or water is the usual limiting step for the chalcogel, the combination of CoSx and MoSx units in CoMoSx enables the reaction to break the initial reactants at much faster rate, turning the HER limited only by the recombination step of Hads. This indicates that this material can provide even further activity enhancement if the appropriate modification to enhance the Tafel step is selected.

References

[S1] Park, I., Xu, B., Atienza, D.O., Hofstead-Duffy, A.M., Allison, T.C., Tong, Y.J., Chemical state of adsorbed sulfur on Pt nanoparticles. ChemPhysChem 12, 747, 2011.

[S2] Strmcnik, D. et al. The role of non-covalent interactions in electrocatalytic fuel-cell reactions on platinum. Nat. Chem. 1, 466–472 (2009).

[S3] Adžić, R. R.; Tripković, A. V; Marković, N. M. J. Electroanal. Chem. Interfacial Electrochem. 114 (1), 37–51 (1980).

[S4] Tidswell, I.M., Lucas, C.A., Markovic, N.M, Ross, P.N., Surface-structure determination using anomalous x-ray scattering: underpotential deposition of copper on Pt(111). Phys. Rev. B 51, 10205-10208 (1995).

[S5] Markovic, N. M. & Ross, P. N. Jr Surface science studies of model fuel cell electrocatalysts. Surf. Sci. Rep. 45, 117-229 (2002).

[S6] Chupas, P. J. et al. Rapid-acquisition pair distribution function (RA-PDF) analysis. J. Appl. Crystallogr. 36, 1342-1347 (2003).

[S7] Hammersley, A. P., Svensson, S. O., Hanfland, M., Fitch, A. N. & Hausermann, D. Two-dimensional detector software: From real detector to idealised image or two-theta scan. High Pressure Res 14, 235-248 (1996).

[S8] Egami, T. & Billinge, S. J. L. Underneath the Bragg peaks: structural analysis of complex materials (Pergamon Press, Elsevier, 2003).

[S9] Juhas, P., Davis, T., Farrow, C. L. & Billinge, S. J. L. PDFgetX3: a rapid and highly automatable program for processing powder diffraction data into total scattering pair distribution functions. J. Appl. Crystallogr. 46, 560-566, doi:10.1107/s0021889813005190 (2013).


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[S10] Farrow, C. L. et al. PDFfit2 and PDFgui: computer programs for studying nanostructure in crystals. J. Phys.: Condens. Matter 19, 335219 (2007).

[S11] Laajalehto, K.; Kartio, I.; Nowak, P. XPS study of clean metal sulfide surfaces. Appl. Surf. Sci. 81, 11, (1994)


design of active and stable co–mo–sx chalcogels -s gels as ... · 2 ba2+- free electrolyte...

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