On the Dominating Mechanism of the Hydrogen Evolution Reaction at

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On the dominating mechanism of the hydrogen evolution reaction at polycrystalline Pt-electrodes in acidic media Sebastian Watzele, Johannes Fichtner, Batyr Garlyyev, Jan Nicolas Schwämmlein, and Aliaksandr S. Bandarenka ACS Catal., Just Accepted Manuscript • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 29, 2018

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On the dominating mechanism of the hydrogen evolution reaction at polycrystalline Pt-electrodes in acidic media

Sebastian Watzele,a,b Johannes Fichtner,a Batyr Garlyyev,a Jan N. Schwämmlein,c Aliaksandr S. Bandarenkaa,b,*

a - Physik-Department ECS, Technische Universität München, James-Franck-Straße 1, 85748 Garching, Germany

b - Nanosystems Initiative Munich (NIM), Schellingstraße 4, 80799 Munich, Germany

c - Technical Electrochemistry, Technische Universität München, Department of Chemistry, Lichtenbergstraße 4, 85748 Garching, Germany

KEYWORDS: Electrocatalysis, Microelectrodes, Hydrogen evolution reaction mechanism, Polycrystalline platinum, electrochemical impedance spectroscopy

Corresponding Author: Tel. +49 (0) 89 289 12531, E-mail: [email protected] (A.S. Bandarenka) 1 ACS Paragon Plus Environment

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ABSTRACT

Hydrogen evolution reaction (HER) is of paramount importance for both fundamental and applied electrocatalysis. However, many issues related to understanding of this reaction remain unclear. These, for instance, include a surprising pH-dependence of the electrode activities and non-Tafel dependency of the HER-associated current for various electrocatalysts. Even the dominating mechanism for this reaction at different potentials is often difficult to reveal. In this manuscript, we use electrochemical impedance spectroscopy to estimate the relative contribution of the Volmer-Heyrovsky and Volmer-Tafel pathways to the overall hydrogen evolution process at polycrystalline Pt electrodes at pH=0, pH=1 and pH=2 as a function of the electrode potential. Pt-microelectrodes were used to facilitate impedance measurements at high current densities (up to ~1 A∙cm-2 in 1 M HClO4) and to overcome common complications due to the fast kinetics of this reaction. Our results show that it is possible to distinguish different reaction pathways experimentally at each electrode potential, using impedance measurements and demonstrate that the relative contributions of the Volmer-Heyrovsky and Volmer-Tafel pathways are in most cases comparable. Both mechanisms contribute differently to the total current at different electrode potentials and none of them can be considered as absolutely dominating at a given complex Pt surface. These findings can be particularly used for elaboration of theoretical models and interpretation of non-Tafel behavior of polarization HER-curves in acidic media.

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1. INTRODUCTION

The hydrogen evolution reaction (HER) attracts considerable attention due to its importance for the schemes of the provision of so-called renewable energy [1,2,3,4,5]. At the same time, this reaction is an important model electrocatalytic process to gain better understanding of electrocatalysis itself, e.g., to find clearer links between the electrode structure/composition and the resulting activity [6,7]. While it can also be recognized as the most studied electrocatalytic reaction, numerous important issues remain uncertain even for platinum-based electrodes, which demonstrate the highest activities towards the HER [8]. Among those issues, one can mention an unusual pH-dependence of the electrode activities [2,9], non-Tafel dependences of the polarization curves within wide potential ranges for various electrodes [10], uncertainties in the nature of the catalytic centers [11], or adsorbateadsorbate interactions, which complicate the overall reaction analysis [12]. The origin of the above-mentioned issues is largely due to experimental and methodological difficulties in investigating HER. While the mechanisms of this reaction were proposed at the times of the “beginning of electrochemistry”, their experimental distinguishability is difficult. Recently, there were significant efforts to overcome the situation and provide a more comprehensive picture of the reaction both from theoretical and experimental points of view, see e.g., refs [13,14,15]. Nevertheless, the choice of the methods remains largely limited. Electrochemical impedance spectroscopy is probably one of few choices nowadays to provide in-situ experimental data on HER under various conditions. Perhaps, initial successful attempts to apply this method in order to investigate mechanisms of the hydrogen evolution in more detail were undertaken by Conway et al. more than two decades ago [16,17]. However, 3 ACS Paragon Plus Environment

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the kinetics of this process as well as hydrogen underpotential adsorption/desorption [18] enabled their detailed quantitative analysis only in some particular cases of Pt high-index single crystals of platinum group metals [19,20] and relatively high pH values [18]. A powerful approach to further overcome experimental challenges associated with the HER kinetics and mechanisms is to use microelectrodes: among others, they offer an opportunity to minimize the influence of the non-conductive phase formation or effectively resolve constituents of fast electrode processes. The benefit of performing fundamental electrocatalytic measurements including microelectrodes related to the hydrogen-involved reactions was recognized decades ago (see e.g., [21]); and impedance measurements, particularly using microelectrodes recently provided new insights into the electrochemical interface properties (see e.g., [22,23,24,25,26,27,28,29] and references therein). In this work, we aimed at answering the following fundamental and methodological electrocatalytic questions related to HER on Pt-electrodes: (i) is it possible to resolve commonly accepted mechanisms of hydrogen evolution at Pt microelectrodes using electrochemical impedance spectroscopy at different electrode potentials in acidic media; (ii) if the latter is possible, what are the relative contributions of the Volmer-Heyrovsky and Volmer-Tafel pathways to the overall reaction kinetics at different potentials and pH values; (iii) at higher current densities, which are important for practical applications, does one of the mechanisms become dominating?

2. EXPERIMENTAL

A Hg/HgSO4 electrode (Schott, Germany) and a Pt wire/mesh (99.9 %, Goodfellow, Germany) were used as the reference (RE) and counter electrodes (CE), respectively. 4 ACS Paragon Plus Environment

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However, all the potentials in this work are referenced to the reversible hydrogen electrode (RHE) scale. Polycrystalline Pt microelectrodes (Ch. Instruments, USA) with diameter of 25 µm were used as working electrodes in all the experiments. Ar-saturated (Ar 5.0, Air Liquide, Germany) and H2-saturated (H2 5.0, Air Liquide, Germany) 1 M, 0.1 M, 0.01 M HClO4 electrolytes were prepared using HClO4 (70% Suprapur®, Merck, Germany) in ultraclean water (Evoqua, Germany). The microelectrodes were electrochemically cleaned by cycling them in 0.1 M HClO4 solutions until stable characteristic voltammograms were obtained. A VSP-300 (Bio-Logic, France) potentiostat was used in all experiments. Electrochemical impedance spectroscopy measurements were performed within the potential range between 0.02 V and -0.1 V using AC-probing frequencies from 2 MHz to 1 Hz (10 mV amplitude). The impedance data were analyzed using the home-made software EIS Data Analysis 1.3 [30,31].

3. RESULTS AND DISCUSSION 3.1. Theoretical considerations Consider that the HER on Pt in acidic media proceeds according to the VolmerHeyrovsky mechanism (M refers to an adsorption site at the electrode surface) H+ + M + e- = MHads (1) MHads + H+ + e- = M + H2

(2)

or according to the Volmer-Tafel mechanism H+ +M + e- = MHads 2MHads = 2M + H2

(3)


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For simplicity, assume that the processes take place at different parts of the electrode simultaneously and quasi-independently. The sum of the partial currents formally originating from the Volmer-Heyrovsky, iVH, and Volmer-Tafel, iVT, reaction pathways gives the total Faradaic current, itot: itot = iVH + iVT

(4)

In order to obtain an equation for the impedance response due to the Volmer-Heyrovsky pathway, consider the equations for the corresponding reaction rates (in accordance with equations 1-2): 𝜈1 = 𝑁𝑉𝐻 (𝑘1 (𝐸)𝐹1 (𝜃) − 𝑘−1 (𝐸)𝐹−1 (𝜃))

(5)

𝜈2 = 𝑁𝑉𝐻 (𝑘2 (𝐸)𝐹2 (𝜃) − 𝑘−2 (𝐸)𝐹−2 (𝜃))

(6)

The formal rate constants k1(E) and k2(E) contain the proton concentration and depend on the electrode potential, E. The potential dependent functions F1(θ), F-1(θ), F2(θ), F-2(θ) also depend on the H-adsorption isotherms, accounting for possible adsorbate-adsorbate interactions. NVH is the fraction of adsorption sites, where the reaction takes place according to the Volmer-Heyrovsky mechanism compared to the Volmer-Tafel mechanism. The connection between the reaction rates ν1, ν2 and iVH is iVH = F(ν1 + ν2) = Fr0

(7)

where F is the Faraday constant. As equations (1) and (2) represent the two-stage mechanism with the interfacial charge transfer at each stage, it is also necessary to take changes in the fractional coverage of the adsorbed H-species into account [32]: 𝑞𝑎 𝑑𝜃 𝐹 𝑑𝑡

= 𝜈1 − 𝜈2 = 𝑟1

(8)

where qa is the charge necessary to form a complete monolayer of Hads and t is time. For a small AC-probing-amplitude, ∆E = Edc + Ẽexp(jωt), where Edc is an applied constant bias, Ẽ is the complex E-amplitude (phasor), j is the imaginary unit and ω is the


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angular frequency, the current, iVH, and the fractional coverage, θ, will oscillate around some quasi-constant values, idc,VH, θdc: ∆iVH = idc,VH + ῖVHexp(jωt)

(9)

∆θ = θdc + 𝜃̃exp(jωt) (10) where ῖVH and 𝜃̃ are the corresponding complex amplitudes. In the following, idc,VH and θdc are assumed to be constant and excluded from further analysis. Linearization of equations (7) and (8) gives: ∆𝑖𝑉𝐻 = (

𝜕𝑖𝑉𝐻 𝜕𝐸

) ∆𝐸 + ( 𝜃

𝑞𝑎 𝑑∆𝜃 𝐹 𝑑𝑡

𝜕𝑖𝑉𝐻 𝜕𝜃

𝜕𝑟

𝜕𝑟

) ∆𝜃 = 𝐹 ⌊( 𝜕𝐸0 ) ∆𝐸 + ( 𝜕𝜃0 ) ∆𝜃⌋ 𝐸

𝜃

𝜕𝑟

𝜕𝑟

= ( 𝜕𝐸1 ) ∆𝐸 + ( 𝜕𝜃1 ) ∆𝜃 𝜃

𝐸

𝐸

(11)

(12)

Taking into account Equations (9) and (10), Equations (11) and (12) can be rewritten as follows: 𝑖̃𝑉𝐻 𝐹

𝑞𝑎 𝐹

𝜕𝑟 𝜕𝑟 = ( 𝜕𝐸0 ) 𝐸̃ + ( 𝜕𝜃0 ) 𝜃̃ 𝜃

𝐸

𝜕𝑟 𝜕𝑟 𝑗𝜔𝜃̃ = ( 𝜕𝐸1 ) 𝐸̃ + ( 𝜕𝜃1 ) 𝜃̃ 𝜃

𝐸

(13)

(14)

From Equations (13) and (14), one can write the following equation for the complex admittance for the Volmer-Heyrovsky pathway:

𝑌̂𝐹,𝑉𝐻 =

𝑖̃ − 𝑉𝐸̃𝐻

𝜕𝑟0

= −𝐹 ( 𝜕𝐸 ) − 𝜃

𝐹2 𝜕𝑟0 𝜕𝑟 ( ) ( 1) 𝑞𝑎 𝜕𝜃 𝐸 𝜕𝐸 𝜃 𝐹 𝜕𝑟 𝑗𝜔− ( 1 ) 𝑞𝑎 𝜕𝜃 𝐸

𝐵

= 𝐴 + 𝑗𝜔+𝐶

(15)

−1 where A is equal to the inverse of the charge transfer resistance 𝑅𝑐𝑡,1 for the Volmer-

Heyrovsky pathway. From Equation (15), the complex impedance can be found as [32]:


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𝑍̂𝐹,𝑉𝐻 = 𝑅𝑐𝑡,1 +

𝑅 2 |𝐵|

where 𝑅𝑎 = 𝐶−𝑅𝑐𝑡,1

𝑐𝑡,1 |𝐵|

and 𝐶𝑎 = 𝑅2

1

𝑐𝑡,1 |𝐵|

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1

(16)

1 +𝑗𝜔𝐶𝑎 𝑅𝑎

can be called “adsorption resistance” and “adsorption

capacitance”, respectively. Equation (16) can be formally represented by an equivalent combination of the corresponding passive elements, the resistances and the capacitance, as shown in Figure 1A. However, it should be emphasized that there are no real capacitances or resistances in the system. These elements just reflect the “AC-behavior” of the kinetic equations in simpler terms. The impedance response resulting from a Volmer-Tafel pathway can formally be taken into account comparatively easy. Equations (3) largely determine the model. In contrast to the Volmer-Heyrovsky pathway, only one electron transfer takes place at each adsorption site for a Volmer-Tafel mechanism. Additionally, the reaction H+ + M + e- = MHads is not complicated by the fast subsequent charge transfer [15]. Therefore, certain diffusion limitations are expected at high current densities (see Supporting Information for details). The corresponding linear part of the AC-response due to the Volmer-Tafel pathway can be written as: ∆𝑖𝑉𝑇 = (

𝜕𝑖𝑉𝑇 𝜕𝐸

𝜕𝑖

)

𝐶∗

∆𝐸 + ( 𝜕𝐶𝑉𝑇∗ ) ∆𝐶 ∗ 𝐸

(17)

where C* is the surface concentration of the electroactive species. After similar transformations, the following equation for the complex Faradaic impedance resulting from the Volmer-Tafel pathway alone can be obtained: 𝑍̂𝐹,𝑉𝑇 = 𝑅𝑐𝑡,2 + 𝑍̂𝐷𝑖𝑓𝑓

(18)

Where 𝑍̂𝐷𝑖𝑓𝑓 is the semi-infinite Warburg diffusion impedance originating from the term 𝜕𝑖

( 𝜕𝐶𝑉𝑇∗ ). Equation (18) can be formally represented by an equivalent combination of the 8 ACS Paragon Plus Environment

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corresponding passive elements connected in series as shown in Figure 1B. According to the Dolin-Erschler-Randles approximation [33,34], the impedance of the electric double layer, ZDL, is connected in parallel to the Faradaic impedances. In general, the response of the double layer can be approximated by the constant phase element, with the impedance, given as: ZDL = ZCPE = 𝐶 ′

1

𝑛 𝐷𝐿 (𝑗𝜔)

(19)

where the exponent n accounts for the frequency dispersion of the double layer and CDL’ is proportional but not equal to the true double layer capacitance if n is significantly lower than 1. However, as the focus of this work is set to the kinetic analysis, we assume here that CDL’ ≡ CDL, irrespective of the values of n. Together with the uncompensated resistance of the electrolyte, RU, the complete equivalent electric circuit (EEC) can be constructed (Figure 1C). An explanation for the choice of each single circuit element is given in Supporting Information. (A)

(B)

(C)

Figure 1. Models (equivalent electric circuits) for the analysis of impedance data expected for the hydrogen evolution reaction at Pt microelectrodes. (A,B) Equivalent circuits for the Faradaic part of impedance associated with (A) the Volmer-Heyrovsky and (B) Volmer-Tafel pathways. (C) A complete equivalent electric circuit. See Section 3.1. for the element explanation. 9 ACS Paragon Plus Environment

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Taking into account the physical meaning of the charge transfer resistances: 𝑅𝑐𝑡,1 = −

𝑅𝑐𝑡,2 = −

1 𝜕𝑖 ( 𝑉𝐻 ) 𝜕𝐸

1 𝜕𝑖 ( 𝑉𝑇 )

(20)

(21)

𝜕𝐸

and the properties of the physico-chemical equations (exponential functions) describing the corresponding kinetics, the ratio Rct,1/Rct,2 in the absence of significant diffusion limitations gives an estimate for the relative contribution of the Volmer-Tafel (VT) and the VolmerHeyrovsky (VH) mechanisms to the total current. It is now a key question if the analysis of experimental impedance data can confirm the hypothesis resulting from the theoretical considerations described here, and if the use of microelectrodes facilitates the resolution of the constituents of the fast electrocatalytic process.

3.2. Experimental results

Figure 2A shows a characteristic cyclic voltammogram of the Pt microelectrode in Arsaturated 1 M HClO4. The voltammogram demonstrates features typical for polycrystalline Pt in acidic media, which are related to the adsorption and desorption of hydrogen (between ~0.03 V and ~0.4 V) and hydroxyl (between ca 0.7 V and 0.9 V) species. The figure shows an approximate onset of ca 20-30 mV of the HER in Ar-saturated electrolyte, which is slightly more positive (ca 20-30 mV) compared to the thermodynamic equilibrium potential of the 10 ACS Paragon Plus Environment

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HOR/HER at standard conditions (0 V vs RHE). This is due to the absence of hydrogen in the electrolyte, causing a potential shift according to the Nernst equation. Therefore, impedance measurements in Ar-saturated electrolyte were performed at potentials ≤0.02 V vs RHE.

(A)

(B)

(C)

Figure 2. (A) Typical cyclic voltammogram of a Pt microelectrode in Ar-saturated 1 M HClO4 at a scan rate of 50 mV/s. An approximate onset of for the hydrogen evolution is ~0.02 V vs RHE. (B) Voltammograms of a Pt microelectrode in H2-saturated 1 M HClO4, 0.1 M HClO4 and 0.01 M HClO4 to indicate the HER activity. Microelectrodes enable practically iR-free measurements revealing remarkably high intrinsic current densities. (C) Tafel plots for the dependences shown in (B).


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Figure 2B shows typical voltammograms characterizing the hydrogen evolution reaction in the H2-saturated HClO4 electrolytes with three different pH values, i.e., 0, 1 and 2. As it is known, the activity (pH corrected cases) of Pt electrodes towards this reaction depends on proton concentration in the solution, being the highest for the most acidic conditions. The origin of such a behavior is, however, still under debate [2,9,35]. It is also clear from Figure 2C that at a given electrode potential the quasi-Tafel slopes are dissimilar for the currents measured at different pH values; and, in general, a non-linear behavior of the current-potential dependency in the logarithmic scale is evident from Figure 2C. Figure 3 shows examples of impedance spectra together with fitting results, which suggest that it is problematic to achieve good fitting assuming only one of the well-accepted mechanisms. Typical electrochemical admittance spectra of the Pt microelectrodes recorded in 1 M, 0.1 M, and 0.01 M HClO4 at different potentials are presented in Figure 4. As it can also be seen from the Figure, the model shown in Figure 1C is able to accurately describe the impedance response at all the investigated potentials: the normalized root-mean-squared deviations were less than 2.5% in all the cases. Moreover, the relative errors of the key individual EEC elements of this work were in the overwhelming majority of cases less than 10%, confirming the expectations outlined in Section 3.1. In other words, these low values indicate significance of the contributions of these elements to the overall fitting.


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(A)

(B)

(C)

Figure 3. Typical impedance Nyquist plots (symbols) of the Pt microelectrode spectra taken at E=-0.05 V vs RHE in H2-saturated (A) 1 M HClO4, (B) 0.1 M HClO4 and (C) 0.01 M HClO4. The dashed/dotted lines represent attempts to fit the spectra assuming only VolmerHeyrowsky and only Volmer-Tafel mechanisms, respectively. Only considering both mechanisms, satisfactory fitting can be achieved.

Figure 5 shows impedance spectra obtained in the H2-saturated electrolytes. As it can be seen from the Figure, it is also possible to achieve a good fitting with low root-mean-square deviations, further supporting the idea about a possibility to distinguish two HER mechanisms using impedance spectroscopy and Pt microelectrodes. Indeed, the spectra can’t be fitted using solely the model for the Volmer-Heyrowsky or the Volmer-Tafel mechanism as


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demonstrated in Figures 3 (A)-(C). Fitting under the assumption of just one mechanism fails, and the combination of both mechanisms allows satisfactory fitting. (A)

(B)

(C)

(D)

Figure 4. Typical admittance Nyquist plots (symbols) of the Pt micro-electrode spectra taken at different potentials in Ar-saturated (A) 1 M HClO4, (B) 0.1 M HClO4 and (C) 0.01 M HClO4. (D) Examples of the phase shifts plotted as a function of the AC probing frequency for Pt micro-electrodes in Ar-saturated 1 M HClO4, 0.1 M HClO4 and 0.01 M HClO4 at -0.09 V. Solid lines represent fitting to the equivalent circuit shown in Figure 1C.

Figure 6 represents an example of the dependencies of the key EEC-parameters of the equivalent circuit shown in Figure 1C. While the above-mentioned EEC parameters depend 14 ACS Paragon Plus Environment

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on complex factors, which are often difficult to predict (surface concentrations of H+ in the Ar-saturated solutions etc), one can e.g., observe a well-expected behavior of the double layer “capacitance” being much higher in more concentrated electrolytes in both H2-free and H2saturated electrolytes (Figure 6A,G). The differences in the “capacitance” values between the Ar-saturated and H2-saturated electrolytes are caused by different exponents n (Equation 19); and this difference is likely caused by different double layer properties due to specific adsorption of reaction intermediates [36,37]. However, further discussion regarding this is outside the scope of this manuscript.


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(A)

(B)

(C)

(D)

Page 16 of 26

Figure 5. Typical admittance Nyquist plots (symbols) of the Pt microelectrode spectra taken at different potentials in H2-saturated (A) 1 M HClO4, (B) 0.1 M HClO4 and (C) 0.01 M HClO4. (D) Examples of the phase shifts plotted as a function of the AC probing frequency for Pt microelectrodes in H2-saturated 1 M HClO4, 0.1 M HClO4 and 0.01 M HClO4 at -0.09 V. Solid lines represent fitting to the equivalent circuit shown in Figure 1C.


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Figure 6. The dependences of the main parameters of the equivalent circuit shown in Figure 1C on the electrode potential for the Pt microelectrodes in (A-F) Ar-saturated and (G-L) H2saturated 1 M HClO4, 0.1 M HClO4 and 0.01 M HClO4 electrolytes. Dotted lines are the guides to the eyes.


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The charge transfer resistances related to the Volmer-Heyrovsky and Volmer-Tafel steps (Figure 6B,C,H,I) demonstrate complex dependencies associated with the fact that according to Equations 20-21 they are derivatives of the DC-currents related to the corresponding mechanisms. However, even in this case one can notice that the resistances are in general lower in more acidic media in accordance to the trends depicted in Figure 2B,C. The dependences of other constituents of the Faradaic impedance are Ra and Ca for the Volmer-Heyrovsky mechanism (Figures 6D, E, J, K, which according to Equation (16) should in general correlate with each other, as observed) and the Warburg parameter, AW, for the Volmer-Tafel mechanism. For the latter, it is clear from Figures 6F and L that the diffusion limitations exist, but they are not process determining within the investigated potential range. As stated earlier, further insight into the dominating mechanism of the hydrogen evolution in the investigated systems can be obtained from a simple ratio: Rct,1/Rct,2. Due to the physical meaning of these parameters (Equations 20-21) and the properties of the functions governing the current, this ratio illustrates a relative contribution of the VolmerHeyrovsky and Volmer-Tafel pathways into the overall current due to the hydrogen evolution reaction at different pH values: 𝑅𝑐𝑡,1 𝑅𝑐𝑡,2

𝑁

𝐹(𝜃)

𝐶∗

∝ 𝑁 𝑉𝑇 ∙ 𝐹(𝜃) 𝑉𝑇 ∙ 𝐶𝑉𝑇 ∗ 𝑉𝐻

𝑉𝐻

(22)

𝑉𝐻

where C*VH and C*VT are the effective surface concentrations of the electroactive species, which can be found at the sites where the Volmer-Heyrovsky and Volmer-Tafel pathways take place. For Equation (22), we assume that the transfer coefficients for two mechanisms are equal to each other and the functions, which describe the adsorbate-adsorbate contributions, are dissimilar (new parameter, NVT, is the fraction of the adsorption sites where the reaction takes place according to the Volmer-Tafel mechanism).


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The ratios Rct,1/Rct,2 are plotted in Figure 7A as the function of the electrode potential for the Ar-saturated and in Figure 7B for the H2-saturated electrolytes. While there are noticeable differences between the H2-saturated and Ar-saturated solutions, one can see that the trends at each pH are similar, and the relative contributions of two mechanisms depend on the electrode potential. However, it is also clear that for the majority of the potentials in the investigated electrode potential range, one cannot consider any of the mechanisms as absolutely dominating. The dependences shown in Figure 7 pose several fundamental and methodological questions particularly related to nanostructured Pt-electrocatalysts, where the surface structure is complex. At least three of them are listed below. (i) Would common extrapolation of any part of “linear” DC-curves shown in Figure 2C indeed give the true HER/HOR exchange current density at 0.00 V vs RHE? (ii) If so, to which mechanism kinetics would it belong to? (iii) Do often reported arbitrary “Tafel slopes”, which change with the electrode potential, have some fundamental physical meaning in the light of methodical challenges in distinguishing the two mechanisms and quantification of the kinetics of them? Other complications arise when considering Figure 7C. Apart from the fact the Tafel slopes change with the electrode potential, it is clear that it is also different for the same ratio Rct,1/Rct,2, at higher and lower current densities. The most probable reason for this is different adsorbate-adsorbate interactions, which, unfortunately, cannot be described purely by e.g., Langmuir, Temkin or Frumkin isotherms. Therefore, one can note that consideration of HER as a "simple" reaction is only true from a historical point of view.


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(A)

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(B)

(C)

Figure 7. The ratio Rct,1/Rct,2, illustrating a relative contribution of the Volmer-Heyrovsky and Volmer-Tafel pathways into the overall current due to the hydrogen evolution reaction at different pH values as indicated in the Figure. The bold symbols designate extrema where the relative contribution of the Volmer-Tafel pathway is maximal for the corresponding pH values. The data obtained in (A) Ar-saturated, and (B) H2-saturated electrolyte solutions. The error bars account only independent measurements. Dotted lines are the guides to the eyes. (C) Determination of the exchange current densities using “classical” approach accounting the situation when Rct,1/Rct,2 ≈ 1 (bold symbols) at the higher current densities for each pH. The error bars in (A) and (B) refer to independent measurements.


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Taking into account that the higher current densities are more important for practical applications, one can consider quasi-linear parts in Figure 7C, which correspond to the ratio Rct,1/Rct,2 ≈ 1 at higher current densities. The values of ~160 mA/cm2, ~85 mA/cm2 and ~10 mA/cm2 were obtained for pH=0, 1 and 2, respectively. The former is in agreement with the values presented in [38]. One should again notice that these values were obtained without the influence of the iR-factor due to the use of microelectrodes.

4. SUMMARY AND CONCLUSIONS

In this work, we used electrochemical impedance spectroscopy in an attempt to distinguish and estimate relative contributions of the Volmer-Heyrovsky and Volmer-Tafel pathways to the HER at polycrystalline Pt electrodes at pH=0, pH=1 and pH=2 as a function of the electrode potential. Pt-microelectrodes were used to facilitate EIS measurements at higher current densities and overcome complications due to the fast kinetics of this process. Our results show that it is possible to distinguish different reaction pathways experimentally up to 0.1 V vs RHE. However, they contribute differently to the total Faradaic current at different potentials and, at the same time, none of them can be considered as absolutely dominating. These findings can be used for elaboration of theoretical models and interpretation of non-Tafel behavior of polarization HER-curves in acidic media.


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ACKNOWLEDGMENTS

We are thankful to Prof. Dr. Hubert A. Gasteiger (Technical University Munich) for fruitful discussions regarding this manuscript. Financial support from the cluster of excellence Nanosystems Initiative Munich (NIM), the Deutsche Forschungsgemeinschaft (DFG; project BA 5795/3-1) and IGSSE (Technische Universität München; project 11.01) is gratefully acknowledged.

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On the Dominating Mechanism of the Hydrogen Evolution Reaction at

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