Copper Electrodeposition Kinetics Measured by Alternating Current Voltammetry and the Role of Ferrous Species

A double-walled glass container was used as the electrochemical cell hosting a three-electrode setup. The working electrode was a 6.58 mm diameter rotating copper disk (surface area 0.34 cm²), polished to a mirror with 1 micron alumina. The electrode was rotated at 800 revolutions per minute in experiments seeking controlled and forced mass transfer conditions. A coiled platinum wire and a saturated calomel electrode (0.241 V vs. SHE at 298 K) were used as counter and reference electrodes, respectively. All potentials are hereafter reported with respect to the standard hydrogen electrode. Solutions were prepared using reagent-grade sulfuric acid (95-98% purity, Sigma-Aldrich), copper (II) sulfate pentahydrate (99% purity, Alfa Aesar), and iron (II) sulfate pentahydrate (99+% purity, Alfa Aesar), and deionized water (100% purity, RICCA Chemical). Powders were weighed first, to obtain the necessary amount of cupric and ferrous ions, and then dissolved over a twenty-four hour period into a solution with the desired molarity of sulfuric acid.

Experiments were either performed at ambient temperature, 298 ± 5 K, or at water's freezing point, 273 K. For the latter, temperature was controlled by flowing ice water through the cell wall. Nitrogen gas was bubbled through all electrolytes for at least 10 minutes prior to electrochemical measurements. A summary of experimental solutions and conditions is presented in Table I.

Each run first consisted in recording the electrochemical impedance at the open-circuit potential to determine the uncompensated ohmic resistance. This value was obtained as the real component of the impedance measured when the imaginary component crosses the real axis on a Nyquist plot, typically for a frequency of around 100 kHz. The measured resistance was used to obtain the actual working electrode potential in post-experiment analysis, and all potential data are reported with such correction.

Measurements with a rotating working electrode were conducted for no more than 5 potential cycles with each freshly polished electrode. Non-rotating experiments were only run for one cycle. Except where noted, all data presented hereafter are with rotation of the cathode. The investigated potential range and the rotation rate were optimized to limit the hydrogen evolution reaction, to limit anodic dissolution of the copper electrode, and to ensure repeatability between cycles in both the DC component and higher AC harmonics. The scan-rate was limited to a fairly fast value in order to limit copper growth during each cycle. Only the third, fourth, and fifth cycles were analyzed. At the end of the last cycle, the cell was dismantled and the electrode was re-polished before the next run.

The direct current voltammograms were generated using a potentiostat (Reference 3000: Gamry Instruments, USA). For alternating current measurements, a sine wave created with a waveform generator (MOTU UltraLite-mk3 Hybrid: MOTU, USA) was superimposed onto the direct current ramp. The current response was collected using a data acquisition system (DT9837B: Data Translation, USA) at a 25 kHz acquisition frequency. A custom designed code (ver. 5.5.2; Scilab Enterprises, France) was used to analyze the results post-measurement. The complex current response was analyzed using a Fast-Fourier Transform (FFT), which presented a power spectrum with sharp peaks at multiples of the frequency of the AC sine wave (see Supplementary Figure S1). These peaks represented the transformed DC, fundamental, and higher harmonic components, and were isolated and converted back into the time domain using an inverse FFT. Thus, the DC and harmonic results could be individually plotted versus the DC potential.²¹

In absence of a full ACV model for the electrodeposition mechanism of copper, which presumably involves adsorption, the measured AC components were compared to a model drawn from experimental results for a simpler case: a reversible, single electron transfer system involving the ferri/ferrocyanide redox couple with 10 mM Fe(CN)₆³⁻ and 3 M KCl (see Supplementary Figure S2), with the overall reaction:

This reaction allows investigating a situation where the reaction-rate is quasi-reversible, highly dependent on mass-transfer, and the kinetics are relatively fast, features that are likely to be present in some steps of copper electrodeposition. Under these conditions, the fundamental harmonic presents one sharp, symmetrical peak at the half wave potential, E_1/2. The second harmonic is represented by the derivative of the fundamental harmonic, with two symmetrical peaks and a trough at E_1/2. The third harmonic is a derivative of this second harmonic, with three peaks, the middle peak being located at E_1/2. This pattern repeats for all higher harmonics.²⁰ In this well-studied system, peak heights scale proportionally with an increase in the electron transfer coefficient, α, and with a faster rate constant k₀. A large α favors more symmetrical peaks, in both height and shape.²² Four main features are thus of interest in analyzing experimental results: peak heights on both the anodic and cathodic sides of E_1/2, peak shape symmetry with respect to E_1/2, overall peak height, and the location of E_1/2.

Figure 1 shows the DC signal measured during copper electrodeposition in a solution with 0.63 M Cu(II) and 1.84 M H₂SO₄, concentrations representative of industrial practices. Multiple analyses of the DC component do not allow for a clear distinction between the current responses for solutions with and without 0.054 M Fe(II).

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In ACV signals, as shown in Figure 2, the fundamental and higher harmonic components reveal harmonic peaks characteristic of a faradaic event, for all concentrations of H₂SO₄ (solutions 1a, 1b, 1c, Table I). The peak center for this faradaic event shall henceforth be denoted E₁. E₁^ω, as observed for the fundamental harmonic, does not align with E₁^2ω, E₁^3ω and E₁^4ω as observed in higher harmonics. The latter are positioned at 0.285 V/SHE across compositions and harmonics, while E₁^ω shifts toward a more cathodic potential with an increasing concentration of H⁺.

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In solutions with higher concentrations of H⁺, the peak heights become asymmetric with respect to E₁. In 1.84 M H₂SO₄, there is a considerable peak height difference between the two peaks of the second harmonic. In the third and fourth harmonic, the peaks that occur on the anodic side of E₁ are generally higher than their cathodic counterparts, with the exception of the fourth harmonic in the extreme case of 0.84 M H₂SO₄.

The peak shapes are also asymmetrical with respect to E₁. In solutions with higher concentrations of H⁺, the peaks at potentials cathodic to E₁ have a prominent shoulder around 0.150 V/SHE (see inset in Figure 2C), which is absent from their anodic counterpart.

Figures 3A and 3B shows the second and third harmonics for solutions that contained Fe(II) ions at 0.054 M (solutions 2a, 2b and 2c, Table I). The measurements of the fundamental harmonic for such solution (not shown) did not exhibit noticeable difference from the one measured for a solution free of iron presented in Figure 2A. In particular, they also exhibit a peak for E₁ at around 0.043 V/SHE. Other key features of the AC harmonics discussed above for solutions without Fe(II) (Figure 2) were more pronounced in solutions containing Fe(II). This is illustrated by the prominence of the shoulder at 0.150 V/SHE in such solutions (see inset Figure 3B). Likewise, the peak heights are more asymmetric than in solutions without Fe(II). Such solutions also show wider harmonics peaks, with the anodic and cathodic sides located further from each other (Figure 3C). In solutions with high concentrations of H⁺, peak heights were lower when Fe(II) ions were present (Figure 3D). As with peak distance, this effect is less pronounced at a higher pH.

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In order to investigate the possibility that the peak shoulder located at 0.150 V/SHE in Figure 2 was a second faradaic event, efforts were made to slow down electron transfer kinetics. The next set of experiments diverged from previous conditions in favor of a colder temperature (273 K) and a lower concentration of Cu(II). These solutions (solutions 3 and 4, Table I) were prepared with 0.01 M Cu(II) and 1.84 M H₂SO₄, both with and without 0.054 M Fe(II). Cyclic voltammetry experiments under these conditions (Figure 4) still do not show a significant difference between solutions with and without Fe(II).

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Figure 5 is an overlay of the fundamental and second harmonic from a solution at 273 K containing 0.01 M Cu(II), 1.84 M H₂SO₄ and 0.054 M Fe(II). As anticipated for such conditions, lower AC currents are measured, indicative that reaction rates were slower. Two separate faradaic events can be observed, at 0.33 V/SHE and 0.04 V/SHE, termed E₁ and E₂, respectively. These two distinct fundamental harmonic peaks line up with two distinct second harmonic troughs. The location of E₁^ω at low temperature (Figure 5) is shifted anodically compared to the room temperature data such that it is now aligned with E₁ observed for the higher harmonics.

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Figure 6 shows the harmonics for the isolated first and second faradaic events. With the exception of the E₁ offset by around 25 mV, no significant difference in peak height or shape are observed for this event in solutions with or without Fe(II). In presence of Fe (II), E₂ (Figure 6, insets) is more cathodic and separated from E1, with consistently lower harmonic peaks and more distinctive features than in absence of Fe(II). Compared to the harmonics at E₁, and the results of the ACV model (see Appendix 2), the E₂ peaks are atypical. E₂^3ω and E₂^4ω are located at a more cathodic potential than E₂^ω or E₂^2ω. This shift is similar to the one observed in Figure 2A, where the location of E₁ shifts between the fundamentals and the higher harmonics. The observed faradaic event at E₂ is distorted towards more anodic potentials due to overlap with the faradaic event at E₁, with the strongest effect on the fundamental harmonic. Higher harmonics appear less susceptible to such overlap, pertaining more strictly to the charge transfer of interest, with peak positions stabilizing in the third and fourth harmonic.

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Although the peaks at E2 (Figures 6c and 6d) are strongly asymmetrical, they split at a consistent potential with alternating peaks (for odd harmonics) and troughs (for even harmonics). This is a definitive characteristic of a faradaic event in AC voltammetry, with the observed asymmetry being typical of an irreversible reaction.^20,25 Such a current response shape has also been predicted for reactions involving an adsorption step.²⁵

In order to further investigate the electron transfers occurring at E₁ and E₂, measurements conducted in absence of rotation of the working electrode are reported (Figure 7). The absence of rotation did not affect the E₁ signals, while peak currents at E₂ increased and the peak split in higher harmonics is more pronounced. Furthermore, the harmonics at E₂ in absence of rotation exhibit a similar shape to E₁, and are in more qualitative agreement with the ACV model.

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Depending on the experimental conditions highlighted earlier, one or two faradaic events may be distinguished during copper electrodeposition. In industry-level concentrations and at room temperature, DC measurements indicate an inflection point at 0.25 V/SHE (Figure 1), a distinct event E₁ also observed in AC measurements in second and higher harmonics. At lower temperatures and lower copper concentration, the location of E₁ shifts anodically, as would be expected from a system with Nernstian behavior. The location of E₁ at a potential close to the standard electrode potential for Cu(II) reduction to Cu metal (Table I), and the lack of any other thermodynamically possible faradaic reactions occurring at that potential lead us to assign E₁ to the following reaction:

This first step, according to the previous literature, is the slowest faradaic step in the overall copper electrodeposition reaction. DC measurements in industrial concentrations also indicate a second inflection point at 0.05 V/SHE, E₂ (Figure 1), an event not noticed as clearly as E₁ in AC measurements under the same conditions. Instead, the second and third harmonic currents from E₁ exhibit shoulders overlapping with the current response at E₂. The shoulders are most prominent in low pH solutions, which have been shown to be associated with a smaller α.^11,26 The loss of peak symmetry could be attributed to a decrease in α, or to the increasing prominence of the event at E₂. Slowing overall reaction kinetics, either through decreasing pH, decreasing Cu(II) concentration, or decreasing temperature, point to slower events at E₁ and E₂, increasing the prominence of the shoulder. Furthermore, at low pH, the harmonics for E₁ are more asymmetrical, again indicating overlap with the currents from E₂. This overlap is consistent with ACV models for multiple electron transfer steps.²⁰

In the extreme case shown in Figures 5 and 6, the two reduction steps at E₁ and E₂ are completely separated. In these cold solutions with low concentrations of Cu(II), no shoulders on the cathodic side of the reaction are observed. This confirms that the second event corresponds to a distinct electron transfer reaction, and that the shoulders observed under industrial conditions are due to the overlap of E₁ and E₂ currents. It is likely, therefore, that the event E₂ is very fast compared to E₁, such that it is often masked by E₁ current responses. The event E₂ exhibits atypical second and higher harmonic currents exhibiting strongly asymmetrical shapes, pointing to an irreversible reaction or a reaction involving an adsorbed species.^20,25

It is possible that the hydrogen evolution reaction (HER) occurs in parallel with the reaction at E₂, although studies of hydrogen evolution on copper electrodes report large overpotentials for HER.^4,5,27 Furthermore, Fe(II) ions have been shown to be a catalyst for HER,²⁷ which contradicts the decrease in currents observed for E₂ in systems in the presence of Fe(II) (Figure 6).

Results with a stationary working electrode show that the ACV signal at E₂ has a stronger dependence on rotation than at E₁, with the event E₂ appearing more distinct and better defined in absence of rotation (Figure 7). For a mass-transfer controlled quasi-reversible reaction, ACV signals have been shown to be insensitive to the mass-transfer conditions for low-enough frequencies of excitation.²⁸ This result applies to our AC measurements for reaction 3 at E₁, which involves soluble species and did not exhibit variation with or without rotation. Nevertheless, it can be postulated that in absence of rotation, the soluble product (Cu(I)) for reaction 3 stays in the vicinity of the electrode and is less dispersed toward the bulk of the electrolyte. Such Cu(I), confined to the electrode surface is then more available for further reaction, explaining the enhanced AC signal at E₂.

The faradaic event E₂ therefore appears as a second electron transfer step involving surface confined species, most likely following reaction 5:

Previous authors have reported such a reaction occurring at similar potentials, suggesting that the reaction involves Cu(I) as an adsorbed species and is a very fast electron transfer reaction.¹⁸ Such a scheme is in agreement with the mechanism presented in introduction, with reaction 5 involving surface species, e.g. adsorbed species. That reaction 5 can be affected by the amount of Cu(I) supplied from reaction 3 and possibly the mass-transfer conditions is a feature that can be evidenced thanks to the combination of ACV and RDE techniques.

The features of the fundamental harmonics for copper electrodeposition recorded in proton concentrated solutions remain anomalous, in particular with regards to the shift of E₁^ω from the Nernst reduction potential for reaction 1. Such a shift is not observed in conditions where reaction 5 can be isolated (see Figure 2 vs Figure 5 for example), suggesting that the measured reduction potential for reaction 1 in industrial-simulating concentrations represents a value between the Nernst potentials for reactions 1 and 2. This is also consistent with ACV models for a mechanism involving two successive electron transfer steps.²⁰

In very acidic solutions, the influence of Fe(II) on the electron transfer kinetics is more predominant at low pH. With Fe(II) present in the electrolyte, the harmonic peaks for reaction 3 remain strongly asymmetrical, even at a higher pH, as opposed to measurements without Fe(II). This suggests that reaction 5, which has harmonics better resolved in solutions with high concentration of proton, is influenced by Fe(II), the presence of which enhances the separation between reaction 3 and 5 (see Figures 3A and 3B).

Under conditions where the two reduction steps are distinguishable, no difference in harmonic shape for reaction 3 in solutions with and without iron can be observed (Figure 6). Conversely, reaction 5 is highly influenced by the presence of Fe(II) (Figure 6, insets). Measurements in solutions containing Fe(II) reveal consistently decreased current values; and peak shapes that more qualitatively fit the ACV model than measurements without Fe(II).

When Fe(II) is present in the system, reaction 3 appears at more anodic potentials, while reaction 5 occurs at more cathodic potentials. Fe(II) ions therefore appear to be hindering reaction 5, thus enhancing separation between the two electron transfers, while not influencing reaction 3. Those results are a key step in isolating and studying the two reactions independently, and in ultimately extracting kinetic parameters for Cu(II) reduction.

Our results allow us to support the hypothesis that the electron transfers in copper reduction obey the following mechanism:

The inconsistent trends with pH for step 3 (reaction 5), however, may also indicate that Fe(II) affects the intermediate chemical reaction 4:

in which case water solvation shell interactions between Fe(II) and Cu(I) would have to be studied.

Lack of a fully developed model for ACV experiments on such mechanism involving adsorption limits the extent to which kinetics for Cu(I) reduction and the influence of Fe(II) on this reduction can be quantitatively characterized. It is suggested that future work be focused on developing such a model.