Electrocapillary Coupling during Electrosorption - Langmuir (ACS

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Electrocapillary Coupling during Electrosorption Qibo Deng*,† and Jörg Weissmüller†,‡ †

Institut für Werkstoffphysik und Werkstofftechnologie, Technische Universität Hamburg-Harburg, Hamburg, 21073 Germany Institut für Werkstoffforschung, Werkstoffmechanik, Helmholtz-Zentrum Geesthacht, Geesthacht, 21502 Germany

‡

ABSTRACT: The electrocapillary coupling coefficient, ς, measures the response of the electrode potential, E, to tangential elastic strain at the surface of an electrode. Using dynamic electro-chemo-mechanical analysis, we study ς(E) simultaneously with cyclic voltammetry. The study covers extended potential intervals on Au, Pt, and Pd, including the electrosorption of oxygen species and of hydrogen. The magnitude and sign of ς vary during the scans, and quite generally the graphs of ς(E) emphasize details which are less obvious or missing in the cyclic voltammograms (CVs). Capacitive processes on the clean electrode surfaces exhibit ς < 0, whereas capacitive processes on oxygen-covered surfaces are characterized by ς < 0 on Au but ς > 0 on Pt and Pd. The findings of ς < 0 during the initial stages of oxygen species adsorption and ς > 0 for hydrogen electrosorption agree with the trend that tensile strain makes surfaces more binding for adsorbates. However, the large hysteresis of oxygen electrosorption on all electrodes raises the question: is the exchange current associated with that process sufficient for its measurement by potential response during small cyclic strain?

I. INTRODUCTION The recent past has seen an increasing interest in the impact of mechanical deformation on electrode processes. The magnitude of the effect is measured by electrocapillary coupling parameters, ς, which take on different values depending on the electrode process. The parameters ς quantify the variation of a capillary force, the surface stress f, with superficial charge density q, and a Maxwell relation1−4 equates ς for any given electrode process to the respective variation of the electrode potential, E, with tangential strain, e. In other words, at any given value of E, one has ς = df /dq|e = dE /de|q

the potential scans of cyclic voltammetry, has been shown to provide data for ς during capacitive processes, in good agreement with ab initio computation. Dynamic stress analysis (DSA), which monitors the modulation of the surface stress prompted by electrochemical impedance-spectroscopy type oscillations of the electrode potential, provides an alternative approach to ς with consistent results.21,22 Here, we explore in how far DECMA data provide information on the electrocapillary coupling associated with electrosorption processes. We study noble metal electrodes in aqueous electrolyte and focus on the electrosorption of H and of oxygen species. Most studies of the electrocapillary coupling so far have emphasized capacitive processes. Here, experiment10,11,21,23−25 and ab initio computation9,26,27 testify to negative-valued ς for clean transition and noble metal surfaces, while ς > 0 is found for clean surfaces of Al28 and for capacitive processes on oxygen-covered surfaces of Pt24 and Pd.29 The findings thus imply that, for instance, clean Pt nanostructures will undergo net lateral expansion when positively charged, whereas the same process results in contraction when the surface is covered with oxygen species. The electrocapillary coupling during electrosorption is of relevance since ς here measures the variation of the electrosorption enthalpy with strain.14,19 Since the potential, Ead, of electrosorption scales with the adsorption enthalpy, Δhad, as zFEad = const − Δhad (where z and F denote valency and Faraday constant), the definition of ς as dE/de|q implies the variation of the adsorption potential with strain

(1)

Several studies point out the significance of the electrocapillary coupling at metal electrode surfaces for the surface electronic structure5−8 and relaxation9 and for the charge transfer between electrode and adsorbate.10,11 Furthermore, ς measures the efficiency of surfaces in enabling nanoporous metal actuation,12,13 and recent experiments demonstrate its central role in quantifying the coupling between mechanics and reactivity in strained-layer catalysts.14−18 A current review on electrocapillary coupling is given in ref 19. In spite of the significance of the electrocapillary coupling strength, there is still a paucity of data. Experiments have documented the variation of the surface stress with potential for electrosorption processes, yet few data sets contain data for q that would allow a precise estimate of ς. Recent work advertises Dynamic Electro-Chemo-Mechanical Analysis (DECMA) as a convenient experimental probe of electrocapillary coupling.20 DECMA uses a lock-in approach to detect the potential modulation when the electrode is subjected to cyclic elastic strain. The method, which can be applied simultaneously with © XXXX American Chemical Society

Received: April 9, 2014 Revised: August 1, 2014

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dx.doi.org/10.1021/la501353g | Langmuir XXXX, XXX, XXX−XXX

Langmuir ς ad = dEad /de|q = −

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1 dΔhad /de|Γ zF

(2)

with Γ the specific excess of adsorbate. The interest in this coupling coefficient arises as many modern catalyst materials exploit strained active layers, which exhibit modifications of the enthalpies of adsorption and of the energies of transition states in the interest of tayloring reactivity and selectivity of the catalyst.17,30−35 In the context of strained-layer catalysis, the coupling between strain and adsorption enthalpy has seen several studies by ab initio computation.36−40 It is well established that hydrogen adsorption on Pd and Pt makes the surface stress more negative, so that ς > 0 during electrosorption.24,29,41,42 For Pd this is consistent with H absorption in bulk inasmuch as tensile strain makes the crystal more binding for H, both in the bulk and at the surface. The difference in the respective mechanochemical coupling strengths can be understood as the result of the mechanics of misfitting solute in a solid continuum.29 The values of ς for H electrosorption derived from surface stress variation,24,29,41,42 cyclic voltammetry on strained layers,14,17 and DECMA15 are also consistent with the in situ observations on strainmodulated reactivity in ref 15, which show that the hydrogen evolution reaction on Pt and Au is accelerated by tensile strain while H forms a dilute adsorbate layer at low overpotential, whereas the trend is inverted and the reactivity diminished as the adsorbate layer becomes saturated at higher overpotential. The case of oxygen electrosorption is less obvious. Oxygen adatoms decorate regions of tensile strain on the Ru (0001) surface,43 and oxygen species adsorption makes the surface stress of Pt,24,41 Pd,29 and Au44 more negative while tensile strain makes Au surfaces more binding for O.45 These findings all consistently indicate ς < 0 for oxygen species adsorption. Yet, independently confirmed numerical values have not been established so far. Furthermore, OH electrosorption on Au is known to occur in several steps, with the initial hydroxide anion adsorption followed by proton desorption and finally a transition of the adsorbed oxygen into subsurface sites.46 In principle, each of these processes may be characterized by an independent value of ς. The issue has not been studied so far. Our study is designed to measure ς with emphasis on oxygen species and H electrosorption. By means of example, the noble metals Au, Pt, and Pd are investigated in dilute H2SO4.

Figure 1. Schematic display of the dynamic electro-chemo-mechanical analysis (DECMA) experiment, showing the general setup (left) and a blow-up of the electrode geometry (right). Labels: CE, WE, RE = counter, working, and reference electrode, respectively; RD = delay resistance.

capacitance, respectively. The value of ς emerges from the experiment independent of A. The reference electrode (RE) is Ag/AgCl in 3.5 M KCl (World Precision Instruments). All potentials in this work are quoted versus the standard hydrogen electrode (SHE) and are positive by 197 mV compared to potentials measured versus Ag/AgCl. The electrolytes are prepared from H2SO4 (Suprapur, Merck) and ultrapure water (18.1 MΩ cm, Sartorius) and deaerated with 99.9999% Ar gas. The glass electrochemical cell is cleaned in Piranha solution (5 volume parts of concentrated H2SO4 + 1 part of 30% H2O2) for 24 h and then rinsed with ultrapure water. The entire DECMA setup is housed in a stainless steel vessel with high-vacuum grade fittings. Prior to the start of an experiment, the vessel is repeatedly evacuated and backfilled with Ar to atmospheric pressure. The primary experimental data sets are obtained as follows. The real and imaginary components of the potential modulation amplitude, Ê , in response to the strain are detected by a lock-in amplifier (SR 7270, Signal Recovery). The grip displacement at any given time is read from the sensor in the piezo-actuator and is used as the reference signal in the lock-in amplifier. As described in ref 20, the displacement amplitude is converted into an area−strain amplitude, ê, accounting for the sample geometry and for the elastic transverse contraction. A delay resistance, RD, between the WE and the potentiostat (PG-Stat 302N AUTOLAB) acts as a low pass filter. This slows down the potential control by the potentiostat, ensuring that the strain cycles are approximately at constant charge. As a suitable value of RD, we here used 20 kΩ. As shown in ref 20, the lock-in amplifier here probes the strain-modulated potential difference between WE and RE. Real and imaginary parts of the response, ςre and ςim, are defined as follows. Let the excitation arise form the time- (t-) dependent strain e(t ) = e ̂ sin(ωt )

(3)

where the hatted quantity is amplitude and ω is the angular frequency. In the simplest case the electrode potential reacts immediately to the strain, in other words

II. EXPERIMENTAL PROCEDURES The procedures of this study are largely identical to those of our earlier report on dynamic electro-chemo-mechanical analysis (DECMA) on polarizable electrodes; see details in ref 20. In the interest of a selfcontained description, we present a brief display of the procedures. The working electrodes (WE) are 50 nm thin metal films (Au, Pt, or Pd), sputtered unto 125 μm thick polyimide (Upilex, UBE) substrates, ∼1 × 2 cm2 in size, with 1−2 nm titanium as an adhesion promoter. After deposition, the films were annealed for 1 h in vacuum of 10−7 mbar at 300 °C. For DECMA, a sample is mounted between clamps that serve to apply a cyclic strain (Figure 1). A piezo actuator (PI-840 Physik Instrumente) acts on one clamp to impose a sinusoidal cyclic elastic strain on the WE. The metal is facing down and is contacted from below by a standing meniscus. The area, A, of the electrode is determined a posteriori by analyzing the footprint of the meniscus on the sample, which is visible after the experiment in an optical microscope. A typical value is A = 0.8 cm2, with an ∼15% variation between experiments on different samples. The parameter A was used for reducing current and capacity values to current density and

E(t ) = ςe ̂ sin(ωt )

(4)

A more realistic scenario allows for the signal transport to be delayed, e.g. by ohmic resistance in series with a capacitance. The potential response is then

E(t ) = ςe ̂ sin(ωt + φ)

(5)

or equivalently E(t ) = ςree ̂ sin(ωt ) + ςime ̂ cos(ωt )

(6)

where φ is a phase shift, tan φ = ςim/ςre

(7)

With these conventions, delayed response is represented by a negative phase-shift φ as well as oppositely-signed values of the real and imaginary parts of the experimental potential−strain coupling coefficient, ςre and ςim. B


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The discussion of phase shifts is complicated by the fact that the electrocapillary coupling parameter ς may take on either sign. Thus, even in the case of immediate response (as in eq 4), a sign inversion in ς lets the signal appear phase-shifted by 180°. In order to remove this ambiguity and to connect φ to a possible delayed response, we take φ to represent the delay of the actual signal relative to the signal which would be expected in the case of immediate response. This notion is consistent with eqs 3−7. We used a strain frequency, ν = ω/2π, of 20 Hz with amplitude ê ≈ 2 × 10−4. The amplitude of strain-induced cyclic potential variation was on the order of 0.05−0.5 mV. Resolving the potential variation of an electrode process in constant charge mode requires that the impedance of the lock-in amplifierhere governed by the SR 7270’s input resistance of 10 MΩ and capacitance of 25 pFis much higher than that of the electrode process. We used electrochemical impedance spectroscopy (EIS) at the same frequency and in the same potential range as the DECMA experiments to verify this requirement. The EIS used a root-mean-square voltage perturbation of 50 mV and potential step of 10 mV. At ∼500 Ω, the impedance magnitude was largest during oxygen electrosorption on Au electrodes. As this value is more than four orders of magnitude less than the amplifier impedance, we conclude that the condition for resolving, in DECMA, each electrode process contributing to the cyclic voltammogram (CV) and EIS is satisfied in our experiments.

Figure 2. Results of dynamic electro-chemo-mechanical analysis (DECMA) at cyclic strain frequency 20 Hz for Au (left column) and Pt (right column) thin film electrodes. Top row: cyclic voltammograms for current density, j, versus electrode potential, E. Middle row: differential capacitance, cAC = −1/(ωZIm), measured by electrochemical impedance spectroscopy at 20 Hz. Bottom row: DECMA signal measured simultaneously with the CVs. Real and imaginary components of the potential−strain response, ς. The potential range is divided into three parts: hydrogen electrosorption (magenta shade), pseudocapacitive process (yellow), and oxygen electrosorption (cyan). Arrows indicate sweep direction. Electrolyte: 10 mM H2SO4, potential sweep rate: 2 mV/s for the simultaneous CVcum-DECMA scan, 0.5 mV/s for the EIS scan.

III. RESULTS A. General Remarks. All DECMA experiments used 10 mM H2SO4 as the working electrolyte. Typical measurements used linear potential scans with a scan rate of 2 mV/s, a potential interval of 1.6 V, and a total cycle time of around 30 min, corresponding to a frequency of 0.6 mHz. As detailed above, strain cycles at a frequency of 20 Hz were applied simultaneously. Since the strain cycles were more than 4 orders of magnitude faster than the potential cycles of the CV, the impact of strain on the potential could be readily separated by the lock-in technique. The lock-in time constant was set to around 200 ms, which is 4 cycles of strain variation (20 Hz) in measurement. We note that the potential scan rates, Ė , of the CV and of the strain cycles were much closer to each other. The root-mean-square (RMS) potential scan rate in response to the cyclic strain is Ė = √2πêvς, which evaluates to 18 mV/s if ς = 1 V. This is only about one order of magnitude faster than the potential variation in the CV. The similarity of the potential scan rates suggests that similar processes were probed by cyclic voltammetry and DECMA. We also carried out EIS at the same frequency as the strain cycles. The AC-capacitance was computed from the imaginary part, ZIm, of the impedance according to cAC = −1/ωZIm. In its two columns, Figure 2 shows the DECMA data for ς(E) for (111)-textured polycrystalline gold and platinum, respectively, along with the CVs and with the AC-capacitance data. DECMA data for Pd are shown separately in Figure 3. The CVs of Figures 2 and 3 were measured simultaneously with the potential−strain response. In this way, the features of ς(E) may be directly correlated with the established signatures of the various electrode processes. The EIS data were obtained in situ but in separate scans at lesser effective scan rate. In a series of successive potential scans, the graphs of ς(E) are typically found reproducible from the third cycle onward, and all DECMA data shown refer to samples after at least two prior cycles. The general features of ς(E) were highly reproducible between samples, and the sample-to-sample reproducibility in the values of ς for individual electrode processes was within ∼8%.

Figure 3. Results of dynamic electro-chemo-mechanical analysis of Pd. Top row: in situ cyclic voltammogram. Bottom row: potential−strain response, ς, at cyclic strain frequency 20 Hz. The sweep directions are indicated by arrows. Electrolyte: 10 mM H2SO4, potential sweep rate: 2 mV/s.

B. Real and Imaginary Parts of the Potential-Strain Response. The results for the electrocapillary coupling in Figure 2 show the real and imaginary components of the DECMA signal for ς(E). In all instances the real component is C


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electrosorption in the CV, until essentially reversible behavior, free of hysteresis, is reached near and beyond the capacitive minimum of the coupling strength. D. Platinum Electrode. The results for Pt are shown in the right-hand column of Figure 2. The CVs of this and of the Pd electrode (Figure 3) differ from that of Au inasmuch as the regime of H underpotential deposition (UPD) is included. We note in particular the very high AC-capacitance in this regime, which indicates that H is cyclically adorbed/desorbed during the EIS potential cycles. By contrast, the capacitance remains low in the oxygen adsorption regime, suggesting that the oxygen coverage does not respond to the EIS potential cycles. This observation applies to all three electrodes of our study. Similar to Au, ς of Pt is negative for dominantly capacitive processes near zero charge, with the value of −1.3 V. The initial stages of oxygen adsorption also reveal similar phenomena as for Au, yet the features are less expressed, with unspecific humps rather than the extrema observed in Au. By contrast, the increase in ς(E) at potentials positive of the first oxygen electrosorption peak in the CV is much stronger in Pt, and it leads to a change in sign with ς > 0 at the largest potentials. This positive value is retained during the backward (negativegoing) scan, with a nearly constant value of ς = 0.6 ± 0.1 V in the entire 700 mV of potential interval before the onset of oxygen desorption. As in Au, the reduction of the surface is accompanied by a steep decrease in ς, which here reverts to negative, while not entirely recovering the value of the positivegoing scan. The hydrogen UPD regime is distinguished by almost reversible behavior, with practically identical graphs of potential−strain response during positive- and negative-going scans. The most obvious trend is a more positive response as the potential goes to negative. The strongest positive coupling is reached before the onset of strong Faraday currents, with ς ≈ +1.2 V. More detailed inspection reveals that the graph of ς(E) exhibits upward and downward peaks and points of inflection in this potential range. Yet, even though ς(E) was measured simultaneously with the CV, the potentials of the features in ς(E) are not obviously correlated to the potentials of the UPD peaks of the CV. E. Palladium Electrode. As the electrocapillary coupling in Pd has been explored in several previous studies, we here add a short discussion of DECMA of a Pd electrode. The results (Figure 3) are quite similar to those of Pt, both in the qualitative features of the graphs of ς(E) and in the numerical values. The largest negative coupling strength in the nominally capacitive regime is again −1.3 V; the positive values peak at +0.5 V for capacitive processes on oxygen-covered Pd and at +1.1 V for H electrosorption. While the CV of Pd does exhibit a UPD peak, the graph of ς(E) is here featureless and simply shows the strong increase as the potential goes negative in the UPD regime. F. Time-Dependent ς during Oxygen Electrosorption. During potential scans through the regime of oxygen species electrosorption, the ς values of Pt and Pd essentially saturate at a positive value. For Au, ς retains its negative sign in this regime. However, the coefficient continuously increases throughout the oxygen-covered part of the CV. We therefore found it of interest to verify if ς of Au will eventually reach positive values after sufficiently long dwell time at very positive potential. The two bottom graphs of Figure 4 show the coupling coefficient for Au during holds at E = 1.2 and 1.65 V.

the dominant one. Our discussion focuses on this component rather than on the magnitude, since this connects most naturally to the inversions in sign that are an inherent part of the variation of electrocapillary coupling with potential. As the most important feature of the imaginary component of ς(E), we note that, except for small potential intervals in the Pt data set, the real and imaginary components are systematically of opposite sign. According to section II, this represents a delayed response. Since the imaginary component of ς(E) vanishes in certain potential regions, the delay cannot simply be an artifact of the electronics. Instead, the observation relates to relaxation in the double-layer which is more or less pronounced depending on the electrode process. This issue will be picked up in the Discussion. C. Gold Electrode. The CV of the Au electrode (Figure 2a) exhibits the well-known, extended potential range of nominally capacitive behavior, bounded by regimes of oxygen electrosorption and of hydrogen evolution. The AC-capacitance (Figure 2b) has a pronounced maximum in the vicinity of the capacitive regime. The capacitance is considerably smaller in the regime of oxygen electrosorption at the positive end of the potential interval of our data, while an upward trend in cAC announces itself near the negative potential vertex of the scan. The magnified part of the CV in Figure 2a reveals weak features within the capacitive regime, with the feature marked by label S commonly attributed to an order−disorder transition in the sulfate adlayer.47,48 This emphasizes the significant adsorption even in the nominally capacitive part of the CV. The H electrosorption is at −0.3 to −0.4 V vs SHE (see ref 15 and references therein), at or beyond the negative end of our potential window. The electrocapillary coupling at more negative potentials is not accessible due to the Faraday current of the hydrogen evolution reaction.15 On the potential axis, the peaks for oxygen species adsorption and desorption (labels B and D, respectively, in the CV) are substantially displaced from each other. Au shares this pronounced hysteresis of oxygen electrosorption with the other electrodes of this study. We now turn to the results for electrocapillary coupling. Starting out at negative E, Figure2c) shows ς strongly decreasing with increasing potential. The slightly enhanced current during the negative-going branch of the CV beyond E ∼ +0.15 V (label E) has a correspondence in a break (label e) in the graph of ς(E). An analogous feature is apparent in the DSA data of ref 21 yet receives no explanation there. Since H electrosorption on Au requires considerably more negative potential (see above), the desorption of an anion would be a possible explanation. The electrocapillary coupling strength reaches an extremum at E = 0.55 V, with ς = -1.7 V. Positive-going scans then bring ς through a maximum (label s in the figure). As this maximum coincides with the peak S in the CV of Figure 2a), it can be associated with the ordering of the sulfate adlayer. The next features in ς(E) are a sharp increase (at label a) that coincides with the onset of OH-adsorption, label A, in the CV, and a weak maximum (label b) coinciding with the electrosorption peak of the CV (label B there). Upon further increase of E, the coupling coefficient reaches a plateau. When the scan direction is inverted, the small current in the CV indicates an essentially capacitive polarization of the oxygen-covered electrode surface. In this regime, ς exhibits a weak linear increase with decreasing E, until the coupling drops abruptly at the onset of oxygen desorption (label C in the CV). The graph of ς(E) then reflects the hysteresis of the oxygen species D


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Figure 4. Time-dependence of ς with oxygen species adsorption on Au, Pt, and Pd surfaces, respectively. The applied potentials (vs SHE) are indicated in the figure.

Figure 5. Details of the DECMA signal for Au in the oxygen electrosorption regime. (a) Potential scan conditioning the surface. CV (top graph), starting with a clean surface at 0.85 V, scans through oxygen species adsorption and stops during the backward scan right in front of the reduction peak (red, solid graphs) of the CV or at its maximum (blue, dotted graphs). Note the reproducibility of the results for ς (bottom graphs). (b) Hold at open circuit after stopping the CV. Time- (t-) dependent open circuit potential Eoc (top) is monitored along with ς (bottom).

Prior to the hold, the surface was reduced by a CV at 100 mV/ s, going to −0.3 V negative vertex and then to the hold potential. While a minor upward drift is seen at the lesser hold potential, ς at the higher potential is constant and remains negative even after a 2000 s hold. This suggests that the negative ς is an equilibrium property of the oxygen-covered gold surface and not the result of sluggish oxidation kinetics. For comparison, Figure 4 also shows the results of analogous experiments with Pt and Pd. The holds here start when the CVs are interrupted at potentials prior to the sign inversion of ς. A continuous upward drift is apparent. This is consistent with the slow increase in oxygen coverage at a potential that is known from previous studies, e.g. ref 24. G. Impact of Desorption Rate. We discuss the ς data of present experiments as the signature of a cyclic potential variation at constant charge. This refers to an essentially reversible reaction of the surface to the imposed cyclic strain. Yet, the strain cycles are superimposed to a CV in which processes such as oxygen adsorption and surface reduction involve large hysteresis and are therefore irreversible in the time frame of the strain cycles. This raises the question: is the irreversible electrosorption current strain-dependent and could this be responsible for the downward peaks that are a prominent feature of the Au DECMA signal near oxygen electrosorption? The ease with which the strain-dependence of Faraday currents can be monitored by DECMA15 emphasizes the issue. As a check we have monitored the potential−strain coupling during the very slow desorption of oxygen under open circuit conditions. Slow desorption means small current, so that a conceivable modulation of the desorption current with strain will make a much reduced contribution to the experimental potential−strain coupling. Figure 5 refers to potential scans that start out from a clean surface. A CV under conditions as in Figure 2 goes through the oxygen electrosorption and then stops either at E = 1.14 V, at the very onset of the desorption peak, or at E = 1.05 V, at the peak position. The cell is then brought to open circuit, the potentiostat is switched to chrono-potentiometry mode at open circuit, and ς along with the open circuit potential, Eoc, is recorded as a function of time. The graphs of ς(E) in Figure 5a emphasize the reproducibility of the DECMA signal during the CV. At open circuit, Eoc drifts slightly to negative, providing an unspecific signature of the surface reduction. By contrast, ς varies strongly, decreasing smoothly from its initial, oxygencovered surface value of −0.7 V to a final value of −1.8 V. This latter value agrees with that of the clean surface before the onset of oxygen species electrosorption.

The above results have two implications: First, the DECMA signatures measured during the CV of Figure 2a at finite scan rate are perfectly consistent with the findings during the much slower reduction of the open circuit experiment in Figure 5. This implies that the DECMA data are not affected by the rate of the reduction or oxidation processes during the CV. Second, as compared to the open circuit potential, the DECMA data provide a much more sensitive probe of the surface state, clean or oxygen covered. H. Impact of Cyclic Strain Frequency. With an eye to discussing the impact of the oxygen electrosorption kinetics on Au for the DECMA signal, we have examined the frequencydependence of ς in the respective part of the CV. Figure 6a

Figure 6. Frequency-dependence of DECMA signals for Au in the oxygen electrosorption regime. (a) electrocapillary coupling coefficient ς at strain frequencies 2 Hz (red, solid graphs) and 20 Hz (blue, dotted graphs), measured simultaneously with the cyclic voltammograms (top graphs) of current I versus electrode potential E at the potential scan rate 2 mV/s. (b) ς at potential E = +1.3 V (peak labeled b in graph a) versus strain frequency ν.

compares the results of DECMA measured at strain frequency 2 Hz to the 20 Hz signal that underlies the remaining data in this work. Throughout most of the potential interval, the electrocapillary coupling coefficients are seen to depend only weakly on the frequency. However, the feature (label b) which coincides with the electrosorption peak of the CV (label B) reacts strongly with the frequency change: The respective ς value increases by more than 1 V when the frequency is reduced. A study of intermediate frequencies (Figure 6b) E


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different electrode processes which are schematically illustrated in Figure 8. Their discussion starts out with capacitive processes

reveals this trend to be systematic. We also found the trend reproducible when the frequency was scanned up and down. Furthermore, a similar frequency-dependence is apparent in the oxygen desorption regime during the negative-going part of the scan in Figure 6a. The frequency dependence in a narrow potential range points toward a slow process that affects the potential−strain coupling exclusively when the surface is partly covered with oxygen. This process is suppressed on clean or completely oxygen-covered surfaces.

IV. DISCUSSION A. Double-Layer Relaxation. We first discuss the possible origin of the imaginary component of ς. This component is remarkable inasmuch as the measurement of a potential variation under (approximately) open circuit conditions with a high impedance lock-in amplifier will per se require negligible response time. This, along with the variation of the imaginary component with the potential, prompts our conclusion that this signal is the signature of a relaxation process in the double layer. Figure 7 schematically illustrates such a process, here

Figure 8. Schematic representation of electrode processes on a strained surface: (a) capacitive process on nominally clean surfaces; (b) oxygen-species electrosorption; and (c) capacitive process on oxygen-covered surfaces.

on nominally clean surfaces (Figure 8a). Here, our experiments in 10 mM H2SO4 find that Au, Pt, and Pd electrodes exhibit an extremum with maximum magnitude of ς within the capacitive regime on the clean surfaces. For Au this is consistent with previous findings from DECMA20 and dynamic stress analysis (DSA).21 For Pt in 100 mM HClO4, DSA reveals no extremum of ς(E) in the capacitive regime.22 The deviation between the results of the two studies may indicate a dependency of the electrocapillary coupling on the nature of the electrolyte. Yet, DECMA data for Au exhibit little variation when different electrolytes (HClO4 and H2SO4) and different concentrations are studied. This argues against a significant impact of the electrolyte on ς in the capacitive regime. In this context it is remarkable that, compared to DSA, the DECMA data for Pt reveal more detail at all potentials. This suggests that the weak extremum may simply not be resolved in the DSA data of ref 22. The potential of the extremum in ς(E) has been linked, empirically, to the potential of zero charge.20 Yet, the extremum appears here correlated with weak peaks in the CV that may be linked to sulfate electrosorption or adlayer ordering. This suggests that the decrease in |ς| at potentials positive of the minimum may be due to electron transfer to adsorbed sulfate. A decrease in the magnitude of the electrocapillary coupling due to partial charge transfer to adsorbed anions has been proposed by Haiss10,11 and agrees well with later findings in refs 23 and 24. Experiment and theory consistently document negativevalued ς for clean and charge-neutral transition metal surfaces in electrolyte;11,21,23−25 the finding is also consistent with the work function−strain response of the corresponding surfaces in vacuum.26 Our data for ς for Au, Pt, and Pd in the nominally capacitive regime agrees with these findings. Taking the value of ς at the capacitive extremum as an estimate for the electrocapillary coupling strength, ςzc, at zero charge, we find the magnitude values −1.98, −1.34, and −1.31 V for Au, Pt, and Pd, respectively. Magnitude and trend compare favorably to ab initio density functional theory (DFT) results, which suggest ς = −1.86, −1.00, and -0.98 V, respectively, for (111) surfaces of the three metals in vacuum.26 C. Capacitive Processes on Oxygen-Covered Surfaces. Concerning the electrosorption of oxygen species, our data show two obvious observations: First, ς varies relatively strongly with the oxygen coverage. Starting out with negative values during the early stages of adsorption, ς for all electrodes subsequently increases. For Au, ς remains negative in the entire oxygen electrosorption regime, while Pt and Pd show a

Figure 7. Response of the electrode potential to a fast strain jump, δe. Schematic cross sections of the double-layer illustrate the immediate strain of an otherwise unrelaxed double-layer, followed by slower relaxation.

prompted by a jump in the strain. The response of the potential may then be separated into (1) an immediate response at constant double-layer structure and (2) a subsequent slower relaxation in which the double layer structure may adjust by rearrangement of ions and/or by change of the adsorbate coverage. If this latter process enhances the potential response, then the experiment will observe a delayed response (negative phase angle φ). By contrast, if the relaxation diminishes the potential response, then the experiment will offer a potential variation that runs ahead (positive φ). The experimental data almost invariably shows real and imaginary signals of opposite sign, implying (in view of eq 7) negative phase angle and, therefore, delayed response. In other words, the relaxation enhances the potential variation. This is surprising in view of arguments (to be discussed below) which suggest that, as compared to strain without ionic relaxation, adsorption will generally diminish the magnitude of the potential−strain response. The detailed nature of this relaxation awaits further studies. B. Capacitive Processes on Nominally Clean Surfaces. As anticipated in section III, the results of our study may be discussed in relation to electrocapillary coupling during the F


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transition to positive ς. Second, the backward scan toward more negative E, for which the CV suggests essentially capacitive processes on the oxygen-covered surface (Figure 8C), is accompanied by a slowly varying (Au) or even nearly constant (Pt, Pd) value of ς. We first focus on the second observation, which is well compatible with previous reports, specifically the sign-inversion for ς during capacitive processes on oxygen-covered Pt22,24 and Pd.29 The lack of features in ς(E) during this part of the potential scan is consistent with a stable oxygen adsorbate structure and with the potential variation affecting only the space charge in the outer Helmholtz layer. One study also found a positive-valued ς in what is discussed as oxygen-covered Au.49 Our present experiments, and specifically Figure 4, failed to reproduce that finding. In fact, the surface of ref 49 was created during dealloying of Ag−Au, and the presence of residual Ag on the surface is thus conceivable. Ag would more readily bind oxygen species and might then behave similarly to oxygen-covered Pt and Pd. In view of eq 2, the trend for stronger binding of adsorbates on laterally expanded surfaces translates into a negative-valued ςad for oxygen species electrosorption (process of Figure 8b). This is not compatible with the DECMA result of a positivevalued ς during negative-going scans on oxygen-covered Pt and Pd. The discrepancy supports our explanation of the DECMA signal in terms of a dominant contribution of capacitive processes rather than strain-induced cyclic adjustments of the oxygen coverage. D. Oxygen-Species Electrosorption. Preliminary to the discussion of our DECMA results during oxygen species electrosorption, the equivalence (or not) between the experimental potential−strain response and the electrocapillary coupling coefficient for oxygen electrosorption deserves a closer examination. Even though the experiment explores processes at open circuit, the lock-in amplifier has a finite internal capacity and its potential measurement therefore requires a finite charge transfer. When several electrode processes are simultaneously active, the experimental signal will then be dominated by the process with the smallest impedance. In our study, the CVs of all electrodes testify to significant hysteresis for oxygen electrosorption, with the desorption peak shifted on the potential axis by ∼300 mV relative to the adsorption peak. This suggests an activation barrier and thermally activated exchange between the adsorbate and solution with a small exchange current density or, in other words, a high impedance. The AC-capacitance data support this notion, with the capacitance much smaller in the electrosorption region than on the nominally clean surface. Furthermore, we had argued above that even this smaller capacitance originates from processes in the outer Helmholtz layer (“capacitive charging”) rather than reversible oxygen species electrosorption. The above picture would suggest that, where peaks in the CV indicate transitions between clean and adsorbate-covered states of the surface, ς simply jumps from its clean-surface value to that of capacitive processes on the adsorbate-covered surface. The signal would then not supply information on the straindependent energetics of adsorption. This type of jump appears indeed supported by the Au data of Figure 2a and c, where the adsorption and desorption peaks in the CV at points B and D, respectively, are accompanied by jumps in the 20 Hz DECMA signal starting at points a and c, respectively. For Pt and Pd the desorption features also agree with the present picture, as do the roughly constant ς during the

negative-going potential scans on the oxygen-covered surface and (for Pt) the absence of adsorption or desorption peaks in the AC-capacitance data. In view of significant structure in the graphs of ς(E) in the initial stages of oxygen species adsorption, the situation for adsorption on these two electrodes is less obvious. Yet, the data is at least compatible with an extended potential region of adsorption and a gradual change of the DECMA signal, starting out with ς for capacitive processes on the nominally clean surface and evolving toward the value characteristic of capacitive processes on the oxygen-covered electrode. As an exception from the dominantly capacitive origin of the DECMA signal of the oxygen covered surfaces, the inspection of the frequency dependence of the potential−strain response on Au (Figure 6) reveals small potential intervals in the anodic and cathodic scans where a slowand therefore not capacitiveprocess dominates at low frequency. The potentials coincide with the deposition or stripping of adsorbed oxygen species, as inferred from the CV. As we have ruled out that ς might reflect reversible oxygen species adsorption/desorption processes in response to strain, it appears likely that the slow process is one where adsorbed oxygen species reorder on the surface. This conjecture is indeed consistent with the notion that the peak labeled B in the CV represents the so-called ”replacement-turnover” process, combining the replacement of adsorbed anions by OH with a reconstruction process.50 Our data is consistent with the notion that the reconstruction couples to strain, that the contribution of this coupling to the net DECMA signal is enhanced at lesser strain frequency, and that the associated electrocapillary coupling parameter is near zero or even positive. We also found it of interest to compare the DECMA results to independent approaches to ςad. For dissociative adsorption of O2 on Au(111), DFT suggests that the O2 chemical potential, μO2, varies with the lattice parameter, a, as dμO2/d lna = 13 eV.45 Accounting for the definition of the area strain, δe = 2 δ ln a, and for an equivalent charge transfer of 4 electrons per O2, eq 2 yields ςad = −1.63 V for gaseous oxygen adsorption on Au. This value is quite close to the −1.9 V at the minimum of the experimental ς(E) near the onset of oxygen electrosorption on Au. Yet, in view of our discussion above, the agreement is likely fortuitous. For Pt, DFT gives a surface stress change of −3.2 N/m when 1/4 monolayer of atomic O adsorbs.41 By using the definition of ς as df/dq (eq 1) and again 2 electrons per O, one here finds ςad = −2.7 V. At −1.0 V, our experiments for Pt and Pd (Figures 2 and 3) at the onset of oxygen electrosorption give a much weaker coupling. It is therefore remarkable that experimental surface stress measurements on porous electrodes suggest ςad = −0.8 ± 0.1 V and −0.86 ± 0.15 V for oxygen electrosorption on Pt and Pd, respectively.24,29 This is more readily compatible with the present observations. Yet, here again, we tend to qualify the agreement as fortuitous. E. Hydrogen Electrosorption. The electrosorption of hydrogen is distinguished from that of oxygen species by its much faster kinetics. This is evident by the lack of hysteresis in the CVs and by the massively increased AC-capacitance during H UPD. The fast electrosorption kinetics has also been directly confirmed by potential relaxation experiments which, for Au in acidic solution, find the H coverage following potential jumps with a characteristic transition time constant, τ, of ∼1 ms.46 This is much faster than the strain cycles of our experiments at G


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sufficient for its measurement by potential response during fast strain cycles? Our results suggest that capacitive processes in the outer Helmholtz layer, which occur simultaneously with the electrosorption and have a much lesser impedance, dominate the experimental ς value throughout most of the potential interval of oxygen electrosorption. The surface of gold at intermediate oxygen coverage represents an exception. Here, the potential−strain response at frequency ≲1 Hz is dominated by a process that can be suppressed at ≳30 Hz. We propose that this observation reflects the coupling of a reconstruction during the replacement−turnover processan established part of the sequence of events during oxygen electrosorption on goldto strain. Contrary to oxygen, the kinetics of H electrosorption is indeed sufficient for the adsorbate to equilibrate with the strained surface on a time-scale much faster than the strain cycles, suggesting that the process can be probed by DECMA. Overall, the present findings support the notion, motivating our study, that DECMA affords an experimental characterization of the electrocapillary response during electrosorption.

a frequency of 20 Hz, thereby justifying the assumption of quasi instantaneous equilibration. The faster kinetics of hydrogen electrosorption, as compared to the case of oxygen discussed above, suggests that the DECMA signal indeed represents ςad for H. This notion is at least consistent with the sign of the coefficient, which becomes positive in the hydrogen electrosorption regime, as would here be expected from the trends of stronger binding in response to tensile strain. In fact, the experimental ς of Pt and Pd are numerically compatible with previous reports. DFT puts the surface stress change during adsorption of one monolayer of hydrogen on Pt(111) at -4.6 N/m.41 As above, this can be converted to an electrocapillary coupling parameter. The result is ςad = +1.9 V, not too dissimilar from the maximum ς near the negative potential vertex of our scan, +1.42 V. At ς = +1.5 V, the results of experimental surface stress measurements for H on Pt24 are even closer. The agreement is similar for palladium. Our experimental maximum value of ς = +1.36 V here compares to +1.4 V from the shift of the potential of H UPD with strain14 and to +1.2 V from surface stress measurement.29 This latter value is also quantitatively consistent with a model analyzing the impact of strain on the chemical potential of a misfitting solute atom near a solid surface.29 It is remarkable that the DECMA signal reaches the theoretical value for hydrogen electrosorption only at potentials negative of H UPD. This may suggest that H displaces another adsorbate with a strongly negative coupling coefficient and that this latter coupling controls the experimental net value of the coupling coefficient unless the surface is nearly fully covered with hydrogen. This picture could also explain why the UPD peaks in the CV of Pt have no direct correspondence in features of the ς(E) graph. Hydrogen adsorption on Au is weak in the potential range of our study. Yet, the numerical value ς = +0.55 V at the lower potential vertex agrees with results on the mechanical modulation of the hydrogen evolution rate in electrocatalysis, which indicate a lower bound at ςad ≥ +0.5 V.15

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Q.D. acknowledges support by the China Scholarship Council. This study is part of preliminary work for the pending proposal WE 1424/16-1 with the Deutsche Forschungsgemeinschaft. A critical reading by Anja Michl is gratefully acknowledged.

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REFERENCES

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V. SUMMARY AND CONCLUSIONS Our study used dynamic electro-chemo-mechanical analysis (DECMA) for investigating the electrocapillary coupling coefficients, ς, of the noble metals Au, Pt, and Pd in potential intervals including the electrosorption of oxygen species and of hydrogen. The magnitude and sign of ς vary during the scans, and quite generally the graphs of ς(E) emphasize details which are less obvious or missing in the CVs. The strong signature of the sulfate adlayer ordering in DECMA provides an example. Capacitive processes on the clean electrode surfaces exhibit ς < 0, and the magnitudes of the coefficients reflect the trends suggested by density functional theory for the work function−strain response of clean transition metal surfaces in vacuum. Capacitive processes on oxygen-covered surfaces are characterized by ς < 0 on Au but ς > 0 on Pt and Pd. This is compatible with previous findings based on the surface stress variation with charge on porous metals, and with recent findings from dynamic stress analysis. The findings of ς < 0 during the initial stages of oxygen species adsorption and of ς > 0 for hydrogen electrosorption agree with the established trend that tensile strain makes surfaces more binding for many adsorbates. Yet, the large hysteresis of oxygen electrosorption on all electrodes raises the question: is the exchange current associated with that process H


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Electrocapillary Coupling during Electrosorption - Langmuir (ACS

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