1

Supporting Information

Ultrahigh Hydrogen-Sorbing Nanofilms of Palladium Metallic-Glass

Baran Sarac, Yurii P. Ivanov, Tolga Karazehir, Marlene Mühlbacher, Baris Kaynak, A. Lindsay

Greer, A. Sezai Sarac and Jürgen Eckert

Fig S1. (a) GIXRD spectrum of the PdTF. (b) 3D profile analysis of a region of a Pd thin film 2 × 2 µ𝑚

performed by AFM.

Electronic Supplementary Material (ESI) for Materials Horizons.This journal is © The Royal Society of Chemistry 2019

2

Fig. S2 Overview of the MG nanofilm sputtered on a Si/SiO2 substrate. The seamless attachment

between the substrate and MG on the atomic-scale is observed.

3

Fig. S3 Structural evaluation of a hydrogenated Pd nanofilm investigated by HRTEM.

4

Fig. S4 Chronoamperometric saturation curves of the MG sample recorded by applying a potential step of

0.05 V between 0.4 V and 0.0 V. At each cycle, the samples are hydrogenated for 850 s. Hydrogen

evolution starts below 0.15 V (inset).

5

Fig. S5 Cyclic voltammograms of (a) MG and (b) Pd thin film samples. The samples are saturated at the

indicated potentials for 850 s before each CV is recorded. Despite the recording process is in steps of 0.05

V, for easier visualization and evaluation of the curves, potential intervals are plotted in steps of 0.1 V.

6

Fig S6. Dependence of charge transfer resistance, , of the MG and Pd thin films on the applied 𝑅𝐶𝑇

potential drawn against RHE.

Fig. S7 (a) HRSTEM investigation of the nanobubble formation (indicated by the gray zones within the

MG). (b) HRTEM investigation of the nanobubbles along the Si-MG interface.

7

Fig. S8 High-resolution chemical composition study of a Pd crystalline nanofilm. (a) HAADF-STEM

image, and (b) EELS analysis. The variation of the Pd contrast in (b) is due to the contributions of

scattering from differently oriented Pd grains, as clearly seen from the HAADF image.

8

Fig. S9 Thickness profile analysis of the cross-section of the thin films. HAADF and thickness analysis

(EELS) of the (a) MG nanofilm, (b) crystal Pd nanofilm.

9

Fig. S10 SAED patterns of the (a) superposition of diffraction from Si, Pt layer and hydrogenated Pd film,

(b) single Pt protection layer and (c) single Si substrate. The size of the SA aperture is 150 nm and the

thickness of Pd film is ~50 nm. HRTEM images of the (d,f) Pd film and (e) Pt layer. (g) The azimuthal profile

of the FFT patterns from Pt layer and Pd film. Both nanostructures have an fcc phase but clearly with a

different lattice parameter: for Pt and for hydrogenated Pd film .0.392 ± 0.002 𝑛𝑚 0.402 ± 0.002 𝑛𝑚

10

Fig. S11 (a),(c) HRTEM images of MG film after hydrogenation, (b),(d) corresponding FFT patterns. The

position of the (111) reflections of PdHx phase marked by circles. (e) the azimuthal profile of the FFT

pattern. The peak position is shifted towards in comparison with the deposited film (0.225 ± 0.002 𝑛𝑚

). The contribution from PdH crystals is marked by a dashed line, for (111) 0.220 𝑛𝑚 0.232 ± 0.002 𝑛𝑚

(lattice parameter is 0.402 ±0.002 nm)

11

Fig. S12 Hydrogen to metal (Pd) ratio of Pd and MG thin films in terms of the chronoamperometric

saturation potential.

12

Table 1 Hydrogen sorption of different materials in terms of maximum weight percent of stored hydrogen

(wt.% of ) and the atomic ratio of hydrogen to metal ratio (H/M). Commonly used pressurized & liquid 𝐻

have low amounts of storage per volume. Mg-rich powders have high storage but slow hydrogenation 𝐻2

kinetics. Complex light-metal hydrides (e.g. NaAlH4, LiBH4) have high storage but slow hydrogen release.

Carbon/boron nitride nanotubes and derivatives have high storage via physisorption due to very high

surface to volume ratio, however conflicting data on the hydrogen reversibility has been reported. Glass

microspheres have moderate storage at high temperatures, but duration of hydrogen intake takes several

hours to complete saturation. Zeolites and MOFs are porous, thermally robust and safe materials together

with low cost, nevertheless further structural optimization is necessary to enhance its hydrogen storage

capacity. Ti-based alloys (including HEAs) have moderate storage but hydrogenation is slow and generally

non-reversible. Pd and its crystalline compounds have low storage but excellent kinetic reversibility.

Material Temperature

(⁰C)

Pressure

(bar)

Time

(min)

Max

wt% of

H2

H/M Reference

MgH2- 5 mol% Fe2O3 300 2-15 20 1.37 - 1

La0.5Ni1.5Mg17 280 2.2 – 11.3 15 4.03 - 2

Mg – 10 wt% Al2O3 300 11 60 5.66 - 3

MgH2 – 1 at% Al 180 0.6 420 7.3 - 4

NaAlH4 211 ~160 1460 5.6 3.0 5

Na2LiAlH6 211 45 100 2.5 - 6

La0.90Ce0.05Nd0.04Pr0.01Ni4.63Sn0.32 100 5-10 6.6 0.95 - 7

Zr0.75Ti0.25Cr1.5Ni0.5 40 47 - 1.75 - 8

Ti45Zr38Ni17 260 40.5 7200 <3.00 1.6 9

13

FeTi 25 100 - 1.92 - 10

TiCr1.1V0.9 30 17 - 3.5 - 11

TiVZrNbHf (HEA) 299 53 600 2.7 2.5 12

Pd 25-30 0.02-1 - 0.56 0.6-

0.74

13-15,

Current

study

Pd – (Pt, Au, Rh) 25 1 - - 0.80 15, 16

Pd79Si16Cu5 25 1 0.166 2.49 2.63 Current

study

Amorphous Carbon Nanotube 17 ~24.5 6.6 0.8 - 1.1 - 17

SWNT bundles (Ø: 12 – 14 Å) -196 ~20 - ~5.2 - 18

Graphene sheet -196 0.4 19

Boron Nitride Nanotube 25 ~100 240 1.8 - 2.6 - 20

Nanostructured Graphite -196 10 - 7.4 0.95 21

Liquid CH4 - -161 - 25 - 21

BaRe <100 1 - 2.5-3.0 4.5 21

Glass microspheres 200 10 333 1.0-2.4 - 22

LiH slurry 25 1 40 25.2 - 23

LiBH4 600 70 ~20 8.0 - 24

KOH catalysed Carbon Aerogel -196 35 - 5.2 - 25

Zeolites -196 0.1 – 1.5 - 0.5 -

2.75

- 26

Metal-Organic Frameworks (-196) - 25 1 - 100 - 0.05 -

9.95

- 27

14

Fe10Ti10Al80 (µm-sized

amorphous powder)

25 4 - 2.5 ~2.5 28

MGTi5Fe6 (µm-sized partially

amorphous powder)

25 8.5 - - 1.8 29

Fe2Ti (µm-sized crystalline

powder)

25 8.5 - - 1.3 29

Liquid hydrogen -252 1 5.0 –

10.0

- 30

Compressed H2 25 2000 -

5000

- ~5.0 - 31

Materials and methods

Selection of composition: The selection criterion was based on the glass forming ability, where a near-eutectic

composition with a large ratio was chosen. Moreover, the difference between the onset and end of melting (𝑇𝑔/𝑇𝑙

viz. undercooled liquid region) is minimum for this composition, indicating a high homogeneity of the melt. 𝑇𝑙 ‒ 𝑇𝑚

Thin-film deposition. (composition in at%) metallic glass and polycrystalline Pd nanofilms were DC-𝑃𝑑79𝑆𝑖16𝐶𝑢5

magnetron sputtered onto a Si(001) substrate (20×7 mm2, B-doped, ρ = 1‒20 Ω cm). The deposition rate was ~40

nm/min in Ar atmosphere with the Ar flow set to 40 sccm to maintain a constant total deposition pressure of 0.4 Pa.

The actual composition of the sputtered films was Pd78.8Si16.0Cu5.2 determined by X-ray photoelectron spectroscopy

(XPS - Kα Thermo Scientific Photoelectron Spectrometer) analysis. The power values used at the magnetron

deposition targets were: Pd = 60 W, Si = 140 W, Cu = 5 W. For the deposition of polycrystalline Pd films a power of

60 W was used.

AFM imaging. A standard silicon tapping mode cantilever with a tip radius of 10 nm was inserted to a Dimension

3100 scanning probe microscope (Veeco, Plainview, NY) to scan a selected area of 2 x 2 μm2 at a scan rate of 1 Hz.

15

For both specimens, the number of samples per line and number of lines was chosen identical as 512. For the root

mean square, a filtering procedure at the cutoff frequency of 0.3 with a line thickness of 50 μm was used.

The real surface area was approximated by the triangulation method using the statistical quantities tool of Gwyddion

software 32. In this approximation, four triangles are formed by placing a point in the center of the one pixel-sized

rectangle. Thus, the area of one triangle is calculated by:

(S1)𝐴12 =

ℎ𝑥ℎ𝑦

41 + (𝑐1 + 𝑐2

ℎ𝑥)2 + (𝑐1 + 𝑐2 ‒ 2�̅�

ℎ𝑦 )2

where denotes the area of the first triangle, pixel dimensions along the axes, the (x,y) coordinates for 𝐴12 ℎ(𝑥,𝑦) 𝑐𝑖

the first and second points, and the (x,y) coordinates for the center point. The surface area of a pixel can be �̅�

approximated by summing the individual areas:

(S2)𝐴 = 𝐴12 + 𝐴23 + 𝐴34 + 𝐴41

X-ray diffraction. Structural characterization of the Pd and MG nanofilms was conducted by X-ray diffraction using

a Rigaku SmartLab 5-Axis X-ray diffractometer with Cu K radiation. A surface-sensitive grazing incidence mode was

implemented. The curve fitting for the first diffraction peak was performed by Pseudo Voigt function which is a

convolution of Gaussian and Lorentzian peak fit. For PdTF, Laplace peak function was utilized.

Electrochemical measurements. Prior to the electrochemical measurements, the working electrode was

chronoamperometrically saturated by immersing in an N2-saturated 0.5 M H2SO4 solution for 800 seconds at 0.2 V

(against RHE) using PARSTAT 2263 potentiostat (Princeton Applied Research, U.S.A.), which was sufficient to reach

the saturation limit of hydrogen. Cyclic voltammograms (CV) were subsequently recorded in the potential range

from 0 V to 0.6 V at sweep rates of 0.01 V s‒1 in the same solution.

Cyclic voltammograms of MG and Pd thin films saturated at the same potentials between 0.6 V – 0.0 V against the

RHE in steps of 0.05 V are provided in Fig. S5. Chronoamperometric hydrogen saturation (as shown in Fig. S4) was

performed before each CV cycle. As the applied potential decreases, a remarkable enlargement in the desorption

profile (anodic scan) is observed for the MG thin film sample. Furthermore, a small sorption cavity formation in Fig.

16

S5 can be noticed as the potential becomes 0.2 V. On the other hand, a relatively minor increase in the desorption

profile is observed for the Pd thin film sample without any indication of a sorption cavity.

The determination of , and phases and the hydrogen evolution region (HER) was through the electron 𝛼 𝛽 𝛼 + 𝛽

transfer of hydrogen obtained from the equivalent circuit model (ECM). In Fig S6, a sharp increase in slope 𝑅𝐶𝑇

around 0.28 V indicates a faster drop in the resistivity, and thus, a change in the hydrogenation mechanism and

higher amounts of hydrogen sorption ( ). The subsequent increase in with a maximum around 0.06 V is 𝛼→𝛼 + 𝛽 𝑅𝐶𝑇

the result of the saturation of the phase and the formation of the -hydride phase via the absorption of hydrogen 𝛼 𝛽

on the active sites of the metallic glass in bulk form. On the contrary, only a very small increase in the PdTF (around

0.16 V) is observed. This finding clearly shows that hydrogenation behavior of MG is more pronounced. The final

decrease in MG below 0.04 V is due to the initiation of the hydrogen evolution reaction (HER) and its dominancy

over the phase formation at negative potentials. The regions indicated on this curve matches well with the anodic 𝛽

scan of the CV curve in Fig. 2a. The change in slope during HER transition, a big desorption hump in the + 𝛽→𝛽 𝛽

region followed by a transition into a second small desorption hump in the region, and finally a smooth part 𝛼 + 𝛽

of the anodic curve on higher potentials ( region) are the characteristics of the MG sample.𝛼

To find the charge due to hydrogen interaction, the area under the desorption curves at each cycle was divided by

the scan rates of . This calculation method reported by Kumsa et al.33 only takes the hydrogen atom 0.02 𝑉𝑠 ‒ 1

desorption area lying between the double layer and hydrogen evolution regions into account. Hydrogen in moles

were then calculated from , where is charge at each cycle, Faraday constant ( ), and 𝑛 =

𝑄𝐹 ∗ 𝑛 𝑄 𝐹 ~96485 𝐶𝑚𝑜𝑙 ‒ 1

number of electrons transferred in the cell reaction (1 in this case). Hydrogen in weight percent (wt.%) is then 𝑛

calculated by dividing into its standard atomic weight (1.00794 u). The mole of Pd is calculated from the mass of the

investigated region (from the surface area of 0.16 cm2, thickness of 52.5 nm, and density of 9.889 g cm-3) divided by

its standard atomic weight (106.42 u). The maximum wt. % of hydrogen sorbed is subsequently calculated from

. The mass of the Pd within MG is found from its atomic ratio (0.79) within the 𝑤𝑡.% (𝐻) =

𝑤𝑡.% (𝐻)𝑤𝑡.% (𝐻) + 𝑤𝑡.% (𝑃𝑑)

metallic glass.

17

Electrochemical impedance spectroscopy (EIS) was performed in a conventional three-electrode configuration with

a Pd or a metallic-glass nanofilm electrode, Ag/AgCl (KCl 3.5 M), and platinum wires as working, reference, and

counter electrodes, respectively. The Potentiodynamic EIS measurements were carried out using a PARSTAT 2263 at

potential steps of 10 mV or 20 mV in the frequency range from 10 kHz down to 100 mHz with 5 points per decade.

The amplitude of the sinusoidal voltage signal was 5 mV. The redox potential of Ag/AgCl (KCl 3.5 M) is +0.205 V vs.

a standard hydrogen electrode at 25°C. The EIS data were simulated with an electrical circuit model (ECM) using the

ZSimpWin V.3.10 (AMETEK SI) software.

In order to separate the influence of the adsorption capacitance, , from the double layer capacitance, , a 𝐶𝑎𝑑 𝐶𝑑𝑙

constant phase element (Q2 in ECM1 & ECM2) is included. The adsorption capacitance parameter and exponent 𝑌𝑎𝑑

are the components of . While the parameter is related with the surface roughness, describes the 𝑛𝑎𝑑 𝐶𝑎𝑑 𝑛𝑎𝑑 𝑌𝑎𝑑

ability of a material to store electrical charge during (de)hydrogenation. Our previous study has shown that of 𝑛𝑎𝑑

MGTFs and PdTF has little influence on the adsorption kinetics, whereas is the sole parameter to describe the 𝑌𝑎𝑑

adsorption behavior in the considered scan range.34 The UPD (above 0.04 V) and HER (below 0.04 V) regions were

described by the following circuits (S3) and (S4), respectively:

(S3)

(S4)

XPS Analysis. Chemical analysis of the as-sputtered/hydrogenated Pd-MG nanofilms were performed by XPS

employing a monochromatic Al Kα radiation of 1486.6 eV. A pass energy of 200 eV and an energy resolution of 1.0

eV was used to conduct the survey scan. Narrow resolution spectra were recorded with a pass energy of 10 eV and

18

0.1 eV steps. The spot size was 400 µm with an analysis depth of 5 nm. The peaks were fitted using a

Gaussian/Lorentzian mixed function and Shirley background correction (Software Thermo Avantage v5.906).

Transmission electron microscopy (TEM). For TEM investigations, cross-sectional specimens were prepared by a

focused ion beam (FIB) protocol on a Helios NanoLab DualBeam microscope equipped a field emission gun (FEG)

electron source and a high-resolution ion column. For the FIB sample preparation, a protective Pt layer was coated

on the samples after hydrogenation. This layer has an fcc structure with a lattice parameter of

confirming the literature finding.35 In order to obtain high-quality TEM specimens with 𝑎𝑓𝑐𝑐_𝑃𝑡 = 0.392 ± 0.002 𝑛𝑚

no post-induced crystallization, ion milling was carried out under low-voltage conditions (final cleaning at 2 kV for

tens of minutes). HRTEM studies were performed on a Cs-corrected Titan G2 60-300 TEM (FEI, Netherlands)

equipped with a high brightness gun, monochromator and Gatan Quantum GIF (Gatan, USA). Imaging and electron

energy loss spectroscopy (EELS) were performed at 300 kV. The optical conditions of the microscope for EELS

imaging and spectroscopy were defined to obtain a probe-size of 0.2 nm, with a convergence semi-angle of 10 mrad,

and a collection semi-angle of 12 mrad. The thickness of the areas of interest was obtained from zero-loss spectrum

imaging.

References

1. K. S. Jung, E. Y. Lee and K. S. Lee, J. Alloys Compd., 2006, 421, 179-184.2. Q. Li, K. C. Chou, K. D. Xu, L. J. Jiang, Q. Lin, G. W. Lin, X. G. Lu and J. Y. Zhang, Int. J. Hydrogen

Energ., 2006, 31, 497-503.3. M. Y. Song, J. L. Bobet and B. Darriet, J. Alloys Compd., 2002, 340, 256-262.4. H. Imamura, K. Masanari, M. Kusuhara, H. Katsumoto, T. Sumi and Y. Sakata, J. Alloys Compd.,

2005, 386, 211-216.5. B. Bogdanovic and M. Schwickardi, J. Alloys Compd., 1997, 253, 1-9.6. L. Zaluski, A. Zaluska and J. O. Strom-Olsen, J. Alloys Compd., 1999, 290, 71-78.7. V. Iosub, A. Latroche, J. M. Joubert and A. Percheron-Guegan, Int. J. Hydrogen Energ., 2006, 31,

101-108.8. M. Bououdina, H. Enoki and E. Akiba, J. Alloys Compd., 1998, 281, 290-300.9. A. M. Viano, R. M. Stroud, P. C. Gibbons, A. F. Mcdowell, M. S. Conradi and K. F. Kelton, Phys.

Rev. B, 1995, 51, 12026-12029.10. L. Zaluski, A. Zaluska, P. Tessier, J. O. Stromolsen and R. Schulz, J. Alloys Compd., 1995, 227, 53-

57.11. D. S. dos Santos, M. Bououdina and D. Fruchart, Int. J. Hydrogen Energ., 2003, 28, 1237-1241.12. M. Sahlberg, D. Karlsson, C. Zlotea and U. Jansson, Sci. Rep., 2016, 6, 36770.13. F. A. Lewis, Platinum Met. Rev., 1960, 4, 132-137.

19

14. B. D. Adams and A. C. Chen, Mater. Today, 2011, 14, 282-289.15. M. Łukaszewski and A. Czerwiński, J. Solid State Electrochem., 2011, 15, 2489-2522.16. M. Łukaszewski, K. Hubkowska, U. Koss and A. Czerwiński, J. Solid State Electrochem., 2012, 16,

2533-2539.17. T. K. Zhao, G. M. Li, L. H. Liu, L. Du, Y. N. Liu and T. H. Li, Fullerenes, Nanotubes, Carbon

Nanostruct., 2011, 19, 677-683.18. P. Lachance and P. Benard, Int. J. Green Energy, 2007, 4, 377-384.19. A. Zuttel, P. Sudan, P. Mauron, T. Kiyobayashi, C. Emmenegger and L. Schlapbach, Int. J.

Hydrogen Energ., 2002, 27, 203-212.20. R. Z. Ma, Y. Bando, H. W. Zhu, T. Sato, C. L. Xu and D. H. Wu, J. Am. Chem. Soc., 2002, 124, 7672-

7673.21. L. Schlapbach and A. Zuttel, Nature, 2001, 414, 353-358.22. S. Dalai, S. Vijayalakshmi, P. Shrivastava, S. P. Sivam and P. Sharma, Microelectron. Eng., 2014,

126, 65-70.23. A. W. McClaine, S. Tullmann and K. Brown, Progress Report for Fuel Cell Power Systems, III.B.3

Chemical Hydride Slurry for Hydrogen Production and Storage U.S. Department of Energy - Energy Efficiency and Renewable Energy Office for Transportation Techniques, Lexington, MA 10/2000.

24. M. Au, W. Spencer, A. Jurgensen and C. Zeigler, J. Alloys Compd., 2008, 462, 303-309.25. H. Y. Tian, C. E. Buckley, S. B. Wang and M. F. Zhou, Carbon, 2009, 47, 2128-2130.26. J. X. Dong, X. Y. Wang, H. Xu, Q. Zhao and J. P. Li, Int. J. Hydrogen Energ., 2007, 32, 4998-5004.27. M. P. Suh, H. J. Park, T. K. Prasad and D. W. Lim, Chem. Rev., 2012, 112, 782-835.28. M. D. K. Dewa, S. Wiryolukito and H. Suwarno, Energy Procedia, 2015, 68, 318-325.29. H. Suwarno, Adv. Mater. Res. (Durnten-Zurich, Switz.), 2011, 277, 129-136.30. A. Bourane, M. Elanany, T. V. Pham and S. P. Katikaneni, Int. J. Hydrogen Energ., 2016, 41,

23075-23091.31. R. Krishna, E. Titus, M. Salimian, O. Okhay, S. Rajendran, Ananth Rajkumar, J. M. G. Sousa, A. L.

C. Ferreira, J. C. Gil and G. J., in Hydrogen Storage, ed. J. Liu, IntechOpen, September 5th 2012, DOI: 10.5772/51238.

32. P. Klapetek, D. Nečas and C. Anderson, Statistical Analysis, http://gwyddion.net/documentation/user-guide-en/statistical-analysis.html, (accessed 01/02/2018).

33. D. W. Kumsa, N. Bhadra, E. M. Hudak, S. C. Kelley, D. F. Untereker and J. T. Mortimer, J. Neural Eng., 2016, 13.

34. B. Sarac, T. Karazehir, M. Mühlbacher, B. Kaynak, C. Gammer, T. Schöberl, A. S. Sarac and J. Eckert, ACS Appl. Energy Mater., 2018, 1, 2630-2646.

35. Y. Waseda, K. Hirata and M. Ohtani, High Temp. - High Pressures, 1975, 7, 221-226.