Electrosorption of Hydrogen in Pd-Based Metallic Glass Nanofilms

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Electrosorption of hydrogen in Pd-based metallic glass nanofilms Baran Sarac, Tolga Karazehir, Marlene Mühlbacher, Baris Kaynak, Christoph Gammer, Thomas Schöberl, Abdulkadir Sezai Sarac, and Juergen Eckert ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00330 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 27, 2018

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ACS Applied Energy Materials

Electrosorption of hydrogen in Pd-based metallic glass nanofilms

Baran Sarac,†,*, Tolga Karazehir,‡,$ Marlene Mühlbacher,§ Baris Kaynak,# Christoph Gammer,† Thomas Schöberl,† A. Sezai Sarac,‡ Jürgen Eckert †,§ †

Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, 8700 Leoben, Austria

‡

Department of Chemistry, Polymer Science and Engineering, Istanbul Technical University, 80626 Istanbul, Turkey

$ Faculty

of Engineering, Department of Energy System Engineering, Adana Science and Technology University, 01250 Saricam,

Adana, Turkey §

Montanuniversität Leoben, Department Materials Physics, 8700 Leoben, Austria

#

Montanuniversität Leoben, Department Polymer Science, 8700 Leoben, Austria

ABSTRACT: As an efficient potential hydrogen storage and conversion system, hydrogen

electrosorption and evolution mechanisms in Pd-based metallic glass thin films (MGTF) are investigated. In this study, thin films of 55 nm thickness were deposited by dc magnetron sputtering. The amorphous structure of MGTFs and the atomically smooth interface between the MGTF and substrate were confirmed by transmission electron microscopy, whereas the composition dependent surface roughness was obtained via atomic force microscopy. The shifts in the broad diffraction maxima for the Si and Cu additions were evaluated by X-ray diffraction. The Pd thin film (PdTF) and MGTF working electrodes were chronoamperometrically saturated in 0.5 M H2SO4 solution. The formation of palladium hydride (PdHx) in the MGTFs was investigated by X-ray photoelectron spectroscopy. Cyclic voltammograms were subsequently recorded (between -0.2 V and 1.4 V) at sweep rates of 0.02 Vs-1. Electrochemical impedance spectroscopy of MGTFs and PdTF was performed in full spectrum including sorption, desorption and evolution of hydrogen in a conventional three-electrode configuration. Electrochemical circuit

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modeling provided the relationship between the composition-dependent hydrogen evolution and H-absorption/adsorption processes. The adsorption capacitance parameter Yad corresponding to α and β-hydrides formation in case of Pd0.79Si0.16Cu0.05 MGTF is ~5 times higher than that of the crystalline Pd thin film which is in line with the decrease in the charge transfer resistance Rct. Addition of Cu disturbs the symmetry of the glass formers, leading to remarkable changes in interfacial hydrogen bonding and diffusion of hydrogen into sub-layers. Compared to other Pdbased micron-sized materials, our findings show excellent volumetric hydrogen storage capacity of 4 times higher than that of the traditional counterparts of several microns, and of 50% higher than the Pd thin films of the same thickness, together with high tunable capacitance, charge transfer resistance and diffusivity depending on the glass-forming characteristics of the nano-sized MGTF. KEYWORDS: Thin films, metallic glass, palladium, dc magnetron sputtering, electrosorption,

hydrogen storage and conversion, electrochemical impedance spectroscopy, cyclic voltammetry 1. INTRODUCTION Current state-of-the-art hydrogen storage and conversion systems at hydrogen production sites, hydrogen refueling stations and onboard vehicles necessitates new materials with improved thermodynamics and high storage densities. Being the cleanest, lightweight and highly abundant material in the universe, hydrogen is counted to be one of the most efficient energy carrier that can meet nearly every end-use energy need 1-3. As compared to commonly adopted hydrogen and liquid hydrogen storage methods

1, 4-8

, metal-based storage is becoming more and more popular due to

the possibility of absorption of hydrogen in large quantities in smaller volumes at ambient conditions

9-11

. Among such systems, palladium can store about 900 times larger amounts of

hydrogen than its volume via the occupation of octahedral sites and formation of palladium hydride (PdHx)12-13. Unlike most metals and alloy systems, the dissociation of H2 molecules within Pd


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occurs with almost no activation barrier enabling these materials to serve as a catalyst for hydrogen sorption and desorption

14-15

. In addition, highly reversible sorption kinetics as well as metal-

hydride formation facilitate an extremely clean H2 gas storage 2. Furthermore, when the sample thickness drops below 100 nm, these thin films show larger plateau pressure for absorption and higher -phase solubility than bulk samples 16. Although Pd in element form suffices the critical properties for optimum hydrogen storage, i.e. rapid sorption kinetics, high volumetric gas storage, and high reversibility etc., its application field is fairly limited due to its relatively high price and moderate affinity to oxidizing or reducing environments under extreme conditions 17. Hence, to lower the costs and increase the durability, Pd alloys and composites have been widely investigated and employed in the last decade 1-2, 17-18. Among them, when compared to conventional Pd systems, a high passivation potential and lower passivation current density is observed for almost all Pd-based metallic glasses (MGs) containing Cu

19-20

. The change in the passivation kinetics can be mainly linked with the glass-forming

characteristics of these compositions. In general, metallic glasses possess an exclusive collection of properties, i.e. excellent mechanical strength and hardness together with high wear and fatigue resistance

21-23

. Moreover, similar to thermoplastics, MGs can be easily formed into complex

geometries even under atmospheric pressure at relatively low temperatures 24-29. The first detailed study about the hydrogen absorption of Pd-Cu-Si metallic glass thin films (MGTFs) produced by magnetron sputtering shows that the compositional variation significantly influence the

response

30-31

. Nevertheless, the main challenge is the impossibility of

determining the phase transformations and kinetic parameters during hydrogen sorption and evolution using electric resistivity change due to

gas absorption. In this respect, the

3



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combination of cyclic voltammetry (CV) and electroimpedance spectroscopy (EIS) bring significant insight to all kinetic, diffusion and surface-related parameters 32-33. An alternative method for hydrogen storage is to utilize organic chemical hydrides, metal hydrides, and nanostructured carbon through physisorption 1-2. Despite its significant advantages (e.g. very low density, fast charging/decharging kinetics, long-term cycling stability etc.) , due to weak van der Waals forces between hydrogen and carrier material, temperature and pressure pronouncedly influence the amount and rate of hydrogen sorption

1, 4

. On the other hand, in

electrosorption, strong covalent bonding can be realized effectively at room temperature without the risk of corrosion and oxidation of the Pd-MGTF electrode in acid solutions 34-35. Metallic glasses absorb hydrogen by the distribution of interstitial sites in the amorphous phase of different partial free volume, where further diffusion into the sub-layers occurs by the dissociation of hydrogen molecules to hydrogen atoms 36-38. Figure 1 shows the schematics of the hydrogen electrosorption and evolution mechanisms. As explained by Gabrielli et al. in detail 39, the hydrogen reaction with Pd occurs in a two-step process. The first step is termed Volmer reaction, which is related to the hydrogen adsorption reaction: ⇌ The

(1)

atoms stem from the dissociation of

as an electrolyte, which is reduced by an

electron taken from the Pd atom. Depending on the applied potential, the adsorbed hydrogen can be desorbed into gaseous hydrogen either by (i) electrochemical desorption (Heyrovsky reaction): ⇌

(2)

or (ii) the Tafel reaction: 2

⇌2

(3)

The adsorption reaction is followed by hydrogen absorption into subsurface layers:

4


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⇌

(4)

The absorbed hydrogen in the subsurface layers can further diffuse into the bulk depending on the hydrogen concentration and the applied potential. The absorption of hydrogen atoms into Pd and the formation of PdHabs ( to  phase transition) results in a volumetric expansion of up to 10.4%

16, 40

. The critical aspect for the design of Pd-based nano-thin films is related to the

adjustment of the surface morphology and the glassy structure via composition optimization. For Pd-Si-Cu MG systems, Pd-centered trigonal prism cluster structures capped with 3 half octahedra (9 or 6 Pd atoms surrounding a Si atom) and Cu atoms acting as surrounding atoms of the Pd-Si tetragonal prism were postulated as the main non-translational structural motifs through analysis of the radial distribution function obtained by neutron diffraction and geometrical structure relaxation simulations

31, 41-43

. Another important study analyzing pair correlation functions has

revealed that Cu holes tend to fill the interstitial sites of the Si network 44.

Figure 1. Schematic illustration of the hydrogen sorption (adsorption and absorption) and evolution mechanisms in Pd-Cu-Si BMGs.

5



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Despite the fact that considerable progress has been made for crystalline metals and alloys in terms of hydrogen sorption and desorption kinetics, electrochemical studies of the relevant mechanisms in metallic glasses are still very limited

15, 45-50

. Here, we elucidate the hydrogen

electrosorption mechanism in MGTFs with nano-sized thicknesses. Sputtering of the films using physical vapor deposition (PVD), and their amorphous nature together with the quality of the SiMGTF interface will be discussed. The hydrogen sorption mechanism will be investigated via cyclic voltammetry and electrochemical impedance spectroscopy, and the change in the surface chemistry via H bonding will be clarified by X-ray photoelectron spectroscopy (XPS). The ultimate goal of this study is to assess the hydrogen sorption and evolution capability, as well as the resistive and capacitive properties in hydrogenated MGTFs of different composition, and highlight the key advantages compared to the conventional Pd films on multi-length scale. 2. EXPERIMENTAL SECTION Deposition of thin films by PVD. Pd-Si, Pd-Si-Cu, and reference Pd thin films were deposited in a custom-built laboratory-scale unbalanced dc magnetron sputtering system with a base pressure < 10-4 Pa. The deposition system was fitted with three 2″ diameter circular targets (Pd, 99.95% purity; Si, 99.999% purity; Cu, 99.99% purity) in a confocal arrangement. Different chemical compositions of the films were achieved by adjusting the dc power applied to the individual targets. Deposition rates were ~ 40 nm/min and final film thicknesses were ~ 55 nm for all films. Further details of the deposition of thin films are provided in the Supporting Information. Surface characterization using AFM. The AFM imaging was performed with a Dimension 3100 scanning probe microscope (Veeco, NY 11803, USA) in tapping mode using a standard silicon tapping mode cantilever with a nominal tip radius of 10 nm. The scan size was 20x20 µm2, and the scan rate was 1 Hz. The number of samples per line and number of lines were identically 512 6


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for all specimens. A filtering procedure at the cut-off frequency of 0.3 with a line thickness of 50 µm was used to obtain a smaller standard deviation. X-ray diffraction analysis. The surface sensitive X-ray diffraction measurement of thin films were conducted in the grazing incidence mode by Rigaku SmartLab 5-Axis X-ray diffractometer with Cu K radiation. The curve fitting for the first diffraction peak was performed by Pseudo Voigt function which is a convolution of Gaussian and Lorentzian peak fit. High resolution TEM analysis. Cross-sectional MGTF specimens were prepared by making a sandwich structure followed by slicing and polishing. To obtain high-quality TEM specimens Ar+ ion milling was carried out in a Gatan precision ion polishing system (PIPS II, model 695) with liquid N2 cooling. The TEM investigations were carried out using a JEOL 2100F operated at 200 kV. The TEM was equipped with an imaging spherical aberration corrector (CEOS) for HRTEM imaging. Surface chemistry study using XPS. Chemical analysis of the doped and undoped MG2 sample was performed by XPS (Kα Thermo Scientific Photoelectron Spectrometer) employing monochromatic Al Kα radiation (1486.6 eV). A survey scan was carried out with a pass energy of 200 eV and an energy resolution of 1.0 eV. The narrow resolution spectra were recorded with a pass energy of 10 eV and 0.1 eV steps. The spot size was 400µm with an analysis depth of 5 nm. The peaks were fitted using a Gaussian/Lorentzian mixed function employing Shirley background correction (Software Thermo Avantage v5.906). Details about carbon impurity and and thin oxide layer formation is provided in Supporting Information. Electrochemical measurements. Before the electrochemical measurements, the working electrode was held in N2-saturated 0.5 M H2SO4 solution by using the chronoamperometry experiment for 800 seconds. Cyclic voltammograms (CVs) were subsequently recorded in the 7



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potential range from -0.205 to 1.4 V at sweep rates of 0.02 Vs-1 in the same solution. Electrochemical impedance spectroscopy (EIS) was performed in a conventional three-electrode configuration with Pd or MGTF electrode, Ag/AgCl (KCl sat.), and a platinum wire as working, reference, and counter electrodes, respectively. All potentials in this study are given with respect to Ag/AgCl(KCl sat.). The redox potential of Ag/AgCl (KCl sat.) is +0.205 V vs. a standard hydrogen electrode at 25 °C. The measurements were carried out using a PARSTAT 2263 potentiostat (Princeton Applied Research, U.S.A.). The EIS diagrams were obtained in potentiostatic mode at various bias potentials, with an ac perturbation amplitude of 0.005 V in the frequency range from 10 kHz down to 10 mHz, with 5 points per decade. The applied potential was maintained with respect to the reference electrode. The ZSwin software was used to model the impedance diagrams. 3. RESULTS AND DISCUSSION Composition variation in MGTFs. Figure 2 shows the chosen MGTF compositions denoted as MG1, MG2, MG3, MG4, and MG5. Table S1, Supporting Information shows the magnetron sputter deposition parameters of investigated films and the corresponding compositions measured by XPS. Compositional region of interest was determined from the ternary phase diagram based on the heat of crystallization and the estimated supercooled liquid region

19

. The high mixing

enthalpy (-55 kJ/mol) and the size difference between Pd and Si (0.48 Å) favor glass-forming ability 51. The addition of Cu can disturb the symmetry of the glass (5 or 6 Pd atoms surrounding a Si atom at near-eutectic Pd100-xSix compositions), leading to an increase in the number density of trigonal prisms which accounts for the changes in the hydrogen sorption capacity

30, 38

. The

theoretical density decreases from 12 g/cm3 to 9.2 g/cm3 through the replacement of Pd by lighter

8


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elements like Si and Cu. Table 1 lists the compositional variations measured by XPS and the correlated theoretical density of each composition.

Figure 2. (a) Compositional variation of the MGTFs shown in the Pd-Si-Cu triplot. The main investigations concerned the addition of Cu into a deep eutectic Pd-Si (16 - 18 at. % Si) MGTF. Note that the plot shows a maximum Si and Cu content of 40%. Crystalline Pd is depicted by a filled blue circle. (b) Enthalpy of formation between Pd, Si, and Cu atomic pairs, and their individual atomic radii. Table 1. Composition of each MGTF in atomic percent and the corresponding theoretical density.

Sample Name

Composition (at.%)

Pd

Theoretical Density (g/cm3) 12.020

MG1

Pd0.84Si0.16

10.025

MG2

Pd0.79Si0.16Cu0.05

9.889

MG3

Pd0.75Si0.16Cu0.09

9.778

MG4

Pd0.69Si0.18Cu0.13

9.428

MG5

Pd0.62Si0.18Cu0.20

9.226

X-ray diffraction analysis. The X-ray diffraction (XRD) studies in Figure 3 confirm the existence of changes between different Pd – Si – Cu MG compositions. The first diffractogram exhibits sharp diffraction peaks (Pd (111): 41.14°, Pd (200): 46.70°, Pd (220): 68.14°) corresponding to a pure

9



elemental Pd thin film. Addition of Si and Cu creates a fully amorphous structure with broad diffraction maxima. Table S2, Supporting Information displays the fitting details (red curves) of the first broad maximum for each composition. Addition of Cu to MG1 shifts the first broad maximum towards larger diffraction angles. Thus, the decrease in the average atomic distances is probably connected to the replacement of larger atoms (Pd) with smaller (Cu) ones.

Figure 3. Comparison of X-ray diffraction patterns (

∝

1.5418 Å) of the crystalline Pd sample

and various Pd-Si-(Cu) MG compositions used in this study. The diffraction patterns were fitted (red

10


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curves) using Pseudo Voigt function. The shift in the first broad maximum via Si and Cu addition is depicted by the dashed blue lines.

Interface and intrinsic properties evaluated by high resolution TEM. The interface adhesion between the MGTF and Si substrate is a crucial factor for the MGTF-Si assembly because hydrogen can easily penetrate through the sample and accumulate at the interface. This is an undesired way of hydrogen storage because, in this case, the hydrogen is not absorbed electrochemically but rather through physisorption, and cannot be easily desorbed 52. Moreover, hydrogen embrittlement can occur when unbonded hydrogen remains for a long time in the samples 17, 53. To study the structure of the MGTF-Si assembly, detailed TEM investigations were carried out on a cross section specimen. The bright-field TEM image in Figure 4a shows that the entire MGTF layer has a uniform thickness of 55

1 nm. The high-resolution TEM (HRTEM)

images of the interface (Figures 4b and 4c) confirm the seamless atomic adhesion with no visible gaps between substrate and the MGTF. A native oxide layer (amorphous SiO2) with a thickness of around 2 nm is present on the Si substrate. The HRTEM image from the MGTF (Figure 4d) corroborates the fully amorphous nature without any presence of lattice fringes. This is also confirmed by the diffraction pattern shown in Figure 4e.

11



Figure 4. (a) Overview image of a MGTF sputtered on Si. (b) HRTEM image of the Si/MGTF assembly, with a closer-up image (c) showing the seamless attachment between MGTF and Si. The native SiO2 layer is thinner than 2 nm. (d) HRTEM image of a MG1 sample and (e) the corresponding diffraction pattern indicating that the MGTF is fully amorphous. The same fully glassy structures were recorded for the other MGTF compositions.

Surface properties of MGTFs observed by AFM. Figure 5 illustrates the topographical investigations of 5 different MGTF compositions and of a reference Pd thin film (PdTF) on a Si substrate. The 3D AFM profiles were obtained from representative 2 μm X 2 μm sections. The 12


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topological features and surface evenness vary significantly as a function of composition. PdTF has a root mean square roughness

of 0.64 nm. Among the MGTFs, MG1 sample exhibits the

smoothest profile with an extremely low

3.37 nm is observed for the

of 0.27 nm.

MG2 glassy thin films with 5 at. % Cu addition. Increasing the Cu content up to 9 at.% results in the highest

of 8.32 nm. However, further increase of the Cu content decreases

to the

same value (3.37 nm) as for MG2. Although no clear trend can be observed, as compared to PdTF and Pd-Si MGTF, Cu addition up to a certain point increases the roughness of the MGTF. Because the surface features are very small compared to the total projected area of 4 μm2, only minor differences between the actual surface area (c.f.

4.005

approximated by the triangulation method is observed 54.

13


and

4.067

)


Figure 5. 3D surface topography of (a) Pd and (b-f) MGTFs for a representative 2 μm X 2 μm region measured by atomic force microscopy.

= root mean square of the surface roughness.

Underpotential hydrogen deposition and hydrogen evolution reaction of MGTFs - Cyclic voltammetry. In this section, the cyclic voltammograms recorded in 0.5 M H2SO4 solution at a sweep rate of 0.02 Vs-1 for different thin film electrodes (Pd, MG1, MG2, MG3, MG4, MG5) are presented. The CV profiles are considered in four different potential regions, namely overpotential deposited hydrogen (OPD), underpotential deposited hydrogen (UPD), double-layer, and oxide region. Two voltammetric peaks observed represent hydrogen oxidation and reduction. The

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difference observed for the cathodic and anodic scans in terms of current density and potential is due to sorption of hydrogen into or desorption of hydrogen from the Pd-electrode at the UPD region. In this region, the reduction of the hydrogen ion by taking an electron from the PdTF or MGTF followed by the electrochemical sorption takes place. No hydrogen evolution is observed in the UPD region. The CVs recorded at the potential range of (-0.2) V – 0.4 V are shown in Figure 6. The cathodic local minimum mainly observed for the MGTFs corresponds to the H adsorption and absorption regions overlapping with each other. Shokhen and Zitoun reported for VAMoS2@Pd that the oxidation of adsorbed and absorbed hydrogen takes place at almost the same potential

55

. Therefore, the peaks related to the adsorbed and absorbed hydrogen cannot be

distinguished from each other

55

. The reason for this is possibly because there is not sufficient

amount of Pd in the sample which can ensure the separation of adsorption and absorption peaks 56

. The electrosorption behavior of hydrogen changes markedly with alloy composition, as it is

clearly seen in the sequence of voltammograms in Figure 6, and on a narrower potential scale in Figure 7. As seen in the cathodic scan, hydrogen sorption starts at around 0.14 V. For all the MGTFs, a small sorption cavity appears between 0.2 V – (-0.1) V, indicating that the ultimate sorption of H is reached in two energetically different steps. As the scan moves towards more negative values, hydrogen evolution dominates within the OPD region. In the anodic scan, hydrogen desorption starts at around (-0.1) V, which leads to an increase in cathodic current due to the already absorbed hydrogen 57. Since the second desorption peak cannot be clearly observed for the crystalline PdTF, the multi-step desorption phenomenon is unique to MGTFs. H desorption becomes more defined as shown in Figure 6. Palladium oxide (PdO) formation appears at about 1.0 V and reduction of PdO at about 0.5 V, which is separated from the hydrogen sorption-desorption peak. In the cathodic scan, at potentials larger than 1.3 V, the current

15



increase is due to the oxygen evolution reaction. The difference in peak current in the regions of PdO formation and reduction varies with the composition of the MGTFs; however, it is larger for the PdTF than for the MGTF electrodes. Additionally, the peak corresponding to the reduction of PdO shifts to more negative potentials probably due to the diffusion of dissolved oxygen from bulk palladium to the surface. A similar shape of the Pd electrode voltammogram (Figure 6a) was reported in literature for the VA-MoS2@Pd electrode, where Pd was electrodeposited on the edges of MoS2 flakes 55. For the cyclic voltammetry of samples with such small roughness values, the literature theoretically and experimentally proves the independence of the electrode roughness in relation to the shape of the CV curves and peak currents 58.

16


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Figure 6. Cyclic voltammograms of (a) Pd, (b) MG1, (c) MG2, (d) MG3, (e) MG4 and (f) MG5 recorded in N2-saturated 0.5 M H2SO4 electrolyte in the potential range of (-0.2) V to 1.4 V at sweep rates of 0.02 Vs-1. 17



Figure 7. CVs of (a) Pd, (b) MG1, (c) MG2, (d) MG3, (e) MG4 and (f) MG5 recorded in N2-saturated 0.5 M H2SO4 electrolyte in the potential range of -0.205 to 0.4 V at sweep rates of 0.01 Vs-1. 18


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Underpotential hydrogen deposition and hydrogen evolution reaction of MGTFs Electrochemical impedance spectroscopy. Electrochemical impedance spectroscopy (EIS) measurements of MGTF and PdTF electrodes were performed in the potential range from 0.4 V to (-0.3) V to investigate the double layer, UPD and OPD zones. 0.02 or 0.04 V potential intervals were selected depending on the scanned spectrum. Figure 8 shows impedance spectra of PdTF and MGTF electrodes measured in the potential range of 0.4 V – 0.08 V in steps of 0.04 V. Although sorption peaks of H in the UPD and OPD regions cannot be separated from the CV, the adsorption and absorption of hydrogen are distinguishable from the change in shape of the impedance spectrum by the evaluation of the high frequency (HF) semicircle, straight line, or low frequency arc on the Nyquist diagram. The data points in the potential range of 0.4 V – 0.2 V correspond to the double layer region, where the decrease in the radius of the semi-circle corroborates hydrogen adsorption on the TF surface. There are remarkable changes in hydrogen sorption at 0.08 V with a slight deviation from the semi-circle at low frequency (near dc) range, which is indicative of the electrochemically sorbed hydrogen within the sub-surface. Figure S1, Supporting Information shows the corresponding Bode phase plots measured in the potential range of 0.4 V – 0.08 V. The values shift to lower phase angles as the applied potential decreases. The nature of the dominant conductive behavior of the thin films, a resistor or a capacitor, affects the real and imaginary components of the impedance and the phase angle (φ), within the system at a given frequency range. The real part of the impedance is represented as resistive behavior, and its associated phase angle is φ = 0o. The imaginary part of the impedance is represented as capacitive behavior with a phase angle of φ = 90o. Nyquist diagrams recorded in the potential range of 0.04 V – (-0.04) V in steps of 0.02 V are shown in Figure 9. The EIS diagram obtained for the PdTF has a similar shape as the reported Pd films deposited on a gold substrate and Pd nanoparticles 33, 48. For the MGTF

19



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electrodes, a high-frequency depressed semicircle with a low frequency straight line showing a downward curvature on the Nyquist diagrams between 0.04 V and 0 V indicates the presence of two time constants. This region corresponds to the range of Pd-H formation and no hydrogen evolution, in other words, to a charge-transfer resistance related capacitance loop. In the low frequency range, a capacitive behavior is demonstrated with a spike corresponding to the hydrogen penetration up to the electrode blocking substrate. The diameter of the depressed semicircle decreases with decreasing potential. Moreover, an increase in slope towards the imaginary part of the Nyquist diagram is observed, which indicates the increase of the capacitance due to increase in hydrogen sorption. Figure S2, Supporting Information shows the corresponding Bode phase plots measured in the potential range of 0.04 V – (-0.04) V. The shapes of the Bode plots do not show ideal capacitive response since the phase angle

for each sample is lower than 90°. The

Bode plots also display low and medium frequency (LF and MF, respectively) relaxation time constants. In the LF region,

increases with decrease in potential. In the MF region, a maximum

corresponding to the characteristic frequency ( with decrease in potential.

) is observed in the EIS patterns and

decreases

shifts to higher frequencies with decrease in potential 48. Figure 10

shows the impedance spectra of the PdTF and MGTF electrodes obtained in the potential range of (-0.06) V – (-0.1) V. For the MGTFs, in contrast to PdTF, LF straight lines parallel to each other are shifted to higher real part of impedance

with decrease in potential, and the HF semicircle

is more pronounced compared to that of the 0.04 V – (-0.04) V range. The diameter of the semicircle for the MGTF electrodes compared to the potential range of 0.04 V – (-0.04) V increases with decrease in potential. Conversely, for the Pd sample, the LF straight lines shift to low resistances with decrease in potential, and the diameter of the semicircles decreases with decrease in potential. Figure S3, Supporting Information shows the Bode phase diagram of the PdTF and

20


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MGTF electrodes in the potential range of (-0.06) V – (-0.1) V. The Bode phase angles in the MF region rise and intersect with each other upon the decrease in potential except that for the Pd electrode. A different trend for V.

remains constant and

is observed compared to the potential range of 0.04 V – (-0.04) slightly increases with decrease in potential (except for Pd where

shifts to high frequency with decrease in ). Figure 11 and Figure S4, Supporting Information display the Nyquist and corresponding Bode phase plots in the potential range of (-0.11) V – (0.2) V, respectively. The diameter of the semi-circle keeps decreasing as the potential decreases from -0.11 V to -0.2 V. For this region, the LF straight lines bend more towards the real part of the diagram (

) with the decrease in potential. This, in turn, indicates the influence of the hydrogen

evolution reaction (HER), which is more visible for the potential of -0.18 V and -0.2 V on the HF semicircle 32. The phase angles slightly decrease with decrease in potential. Figure 12 presents the Nyquist diagram of Pd and MGTF electrodes in the potential range of (-0.21) V – (-0.3) V. The hydrogen evolution is visible with a HF semi-circle and the LF arc. This suggests a HER leading to an electron transfer, and therefore, the LF arc appearing in this potential range is attributed to the Heyrovsky reaction. It was reported that since HER has very fast kinetics, the impedance measurements only performed at lower potential intervals (0.005 V) might lead to an apparent LF semi-circle 48. The corresponding Bode phase plots of PdTFs and MGTFs in the potential range of (-0.21) V – (-0.3) V are shown in Figure S5, Supporting Information. The

values for the MGTF

electrodes are shifted to higher frequencies with decrease in potential. In addition, related to the HER reaction decreases with decrease in potential, but

in the MF

in the MF for the Pd

electrode slightly increases. It is evident that the content of palladium in the electrodes strongly influences the shape and features of the impedance spectra. Figures S6 and S7, Supporting Information. show the Bode Magnitude and Bode Phase plots of MGTF and PdTF samples for the 21



Page 22 of 49

UPD and HER regions, respectively. MG3 followed by PdTF show the highest Bode Magnitude at very low frequencies near DC (cf. | | | |

227 Ω and | |

1129 Ω and | |

3067 Ω for -0.04 V,

1367 Ω for -0.28 V). Compared to MGTFs, PdTF is more sensitive

to changes in frequency, which is possibly because the absorption for PdTF is more pronounced at relatively lower potentials (see Figure 10 and 11). Similarly, compared to other MGTFs of interest, the Bode Phase angle is much lower for the MG3 at constant potentials of -0.04 V and 0.28 V at every frequency, confirming the resistive behavior of this alloy type. More detailed information pertaining to the explanation of the large Rs difference observed for the MG3 sample is given in the Supporting Information.

22


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Figure 8. Nyquist plots of (a) Pd, (b) MG1, (c) MG2, (d) MG3, (e) MG4 and (f) MG5 recorded in N2saturated 0.5 M H2SO4 electrolyte in the potential range of 0.4 V– 0.08 V.

23



Figure 9. Nyquist plots of (a) Pd, (b) MG1, (c) MG2, (d) MG3, (e) MG4 and (f) MG5 recorded in N2saturated 0.5 M H2SO4 electrolyte in the potential range of 0.04 V – (-0.04) V. The insets in the figures are a magnified view of the high frequency region of the Nyquist diagram.

24


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Figure 10. Nyquist plots of (a) Pd, (b) MG1, (c) MG2, (d) MG3, (e) MG4 and (f) MG5 recorded in N2saturated 0.5 M H2SO4 electrolyte in the potential range of (-0.06) V – (-0.1) V.

25



Figure 11. Nyquist plots of (a) Pd, (b) MG1, (c) MG2, (d) MG3, (e) MG4 and (f) MG5 recorded in N2saturated 0.5 M H2SO4 electrolyte in the potential range of -0.11 V – (-0.2) V.

26


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Figure 12. Nyquist plots of (a) Pd, (b) MG1, (c) MG2, (d) MG5, (e) MG3 and (f) MG4 recorded in N2saturated 0.5 M H2SO4 electrolyte in the potential range of (-0.21) V – (-0.3) V.

27



Kinetic parameters. In order to model the impedance spectra of the UPD region obtained from the PdTF and MGTFs, an equivalent electrical circuit (EEC1, R(QR)Q) was constructed (inset to Figure 10a). The equivalent circuit consists of a solution resistance, Rs, in series with a parallel combination of a constant phase element (CPE1) corresponding to the double layer capacitance and charge transfer resistance (Rct), and a series combination of a constant phase element (CPE2) corresponding to the adsorption capacitance (Cad). The hydrogen evolution resistance, Rev, was not considered for the EEC1 in the UPD region due to its very high value, which significantly lowers the quality of fitting 48. Thus, a second equivalent electrical circuit (EEC2, R(QR)(QR)) (inset to Figure 10a) was proposed to model the impedance diagram, which consists of Rs, CPE1, Rct, CPE2, and Rev for the OPD region. Since the hydrogen evolution reaction takes place in the OPD region, using Rev improves the quality of the fit. In this case, two capacitive semicircles are observed in the Nyquist diagram, where the LF one is the HER. Rev decreases with decreasing potential. However, due to the fast kinetics of HER it is difficult to estimate the curve at very low potential range. All the impedance spectra can be well approximated by the proposed EEC1 and EEC2 with a low value of the least squares, χ2, between 10-4 and 10-5. The study of MGTF electrodes reveals that the electrode composition strongly influences Rct, double layer capacitance, and H UPD kinetics. Figure 13a shows Rct as a function of applied potential. A similar shape of the charge transfer resistance vs. potential graph was reported in the literature 48, 57. Rct exhibits a strong effect with decrease in the Pd content of the electrode. Among the MGTFs in the UPD region (Figure 10a, highlighted region), the highest charge-transfer resistance is observed for MG1 (Rct_max_MG1 = 885 .cm2), followed by MG4, MG3, MG5, and MG2 (Rct_max_MG2 = 485 .cm2), respectively. At lower potentials (below the range of (-0.1) V – (-0.15) V, depending on the MGTF electrode), Rct 28


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subsequently starts to decrease due to the hydrogen evolution reaction. In the OPD region, minimum Rct is obtained for the MG5 (197 Ω.cm2) electrode followed by MG2. At more positive potentials Rct increases sharply to 1.41 kΩ.cm2 until 0.4 V. For the PdTF electrode, the trend is different than for the MGTF electrodes, where the observed maximum between -0.3 V and 0.0 V for the MGTF electrodes shifts to a more positive potential presenting a shoulder at -0.05 V. This distinction can be explained by the difference of the hydrogen adsorption properties of the Pd electrode compared to the MGTF electrodes as revealed also by CV (see Figure 6), where the barely visible peak around 0.0 V is different than the pronounced peaks for the MGTF electrodes. The decrease in Rct with decrease in potential is ascribed to the formation of the α-phase of hydrogen in Pd which saturates with the Rct minima around 0 V. At even lower potentials the charge-transfer resistance increases due to the saturation of the α-phase. This can be attributed to free Pd sites at the surface occupied by H forming Pd-H. Hence, there is a higher resistance for the formation of the β-phase of hydrogen in Pd leading to an increase in Rct. Thus, the increase in Rct revealing a maximum at -0.15 V is attributed to the saturation of the α-phase and the formation of the β-phase. However, the decrease in Rct towards negative potentials is due to the onset of hydrogen evolution. This is because the hydrogen concentration at the electrode surface is initially very low and increases with negative overpotential; thus, when hydrogen evolution starts, Rct becomes smaller 33. For more positive potentials, above 0.0 V, a broad shoulder is observed around 0.2 V corresponding to the double layer region. In this region, for MG1, Rct rises to approximately 30 times of its original value. On the other hand, the rise of Rct (7 times) and the steepness of the shoulder is not as pronounced for Pd. The increase of Rct is in good agreement with the addition of Cu into the MGTFs. Q1 and Q2 indicating the CPE1 and CPE2, respectively, are given as equation 5 and 6, respectively 59, and the impedance values Z are calculated as:

29



1/ 1/

∗ ω

Page 30 of 49

(5)

∗ ω

(6)

where Ydl is the double layer capacitance parameter, Yad is the hydrogen adsorption capacitance parameter,

√ 1 (imaginary number),

2

(angular frequency, f being frequency), and

ndl and nad are the capacitance exponents related to the double layer and hydrogen adsorption. The double layer capacitance (Cdl) can be estimated from the value of CPE1, using the following equation 48: (7)

1 1

Here, Ydl is a CPE constant, and n is a CPE exponent. The double layer capacitance as a function of applied potential is shown in Figure 13b. With increase of the potential between -0.15 V and 0.15 V, the double-layer capacitance tends to increase giving a maximum for Pd and the MGTF electrodes. In the literature, a similar maximum in the dependence of the double-layer capacitance vs. potential was reported for the hydrogen UPD region. This can be linked to the metal/solution interface changes with the potential of UPD hydrogen adsorption or the influence of ionic adsorption 33, 60. At -0.1 V, the onset of HER revealed from the CV measurements, as well as a minor second maximum are observed for MGTF electrodes. The second maximum appears at more positive potential and lower capacitance density compared to that for the Pd electrode. Moreover, the Cdl values tend to increase and give another maximum between 0.2 and 0.4 V indicating the double layer region, in agreement with the CV measurements. Cdl in this range is larger for the MG5 electrode. By using a quartz crystal microbalance a mass increase was found at UPD for polycrystalline Pt, and is was corroborated 30


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that the double layer is the reason for the ionic adsorption starting well before metal oxidation 61. This is in agreement with our capacitance measurements where the maximum is reached before the oxide region 62. For the MG1 electrode, low capacitance in the double layer region (maximum of 0.06 mFcm-2) and low surface roughness (

0.27 nm, see Figure 5) were observed. This

decrease can be related to the decrease in the surface roughness with decreasing Cu content in the MGTF electrodes. In the same region, Cdl reaches to maximum of 0.51 mFcm-2 at 0.3 V for the MG5 electrode with

3.48 nm, see Figure 5).

Figure 13. (a) Dependence of Rct (logarithmic scale) on applied potential for Pd, MG1, MG2, MG3, MG4, and MG5 recorded in N2-saturated 0.5 M H2SO4 electrolyte in the potential range of 0.4 V – (-0.3) V. The inset shows the EEC1 for UPD and EEC2 for OPD regions. (b) Dependence of the double layer capacitance Cdl on the applied potential for Pd, MG1, MG2, MG3, MG4, and MG5 recorded in N2-saturated 0.5 M H2SO4 electrolyte in the potential range of 0.4 V – (-0.3) V.

The separation of Cad from Cdl is possible by impedance measurements by introducing the Q2 parameter into the EEC to consider the adsorption capacitance. The variation of the Q2 capacitance parameter, Yad, and the Q2 exponent nad is given in Figures 14a and 14b, respectively. Yad increases 31



up to -0.2 V corresponding to the increase in the amount of absorbed and adsorbed hydrogen. However, the findings show that Yad decreases to 6.8x10-5 Ssncm-2 below -0.2 V due to the beginning of HER, which changes the equation from the purely capacitive vertical line to two semicircles on the complex plane plots. Yad increases to 5.9x10-3 Ssncm-2 for the MG2 electrode and reaches a maximum in the potential between 0.0 V and 0.4 V (Figure 10a). Hoshi et. al. 63 reported that the detected adsorbed sulfuric acid anion at 0.3 and 0.4 V via infrared reflection absorption spectra verifies the sulfuric acid anion adsorption of Pd(111) below 0.4 V. The specific adsorption of bisulfate anions takes place within the potential range of 0.3

Electrosorption of Hydrogen in Pd-Based Metallic Glass Nanofilms

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