# Unraveling Nickel Dissolution Kinetics in Sulfuric Acid via Element-Resolved Electrochemical Impedance Spectroscopy

Junsoo Han, Bernard Tribollet, Kevin Ogle

► To cite this version:

Junsoo Han, Bernard Tribollet, Kevin Ogle. Unraveling Nickel Dissolution Kinetics in Sulfuric Acid via Element-Resolved Electrochemical Impedance Spectroscopy. ACS Electrochemistry, 2025, 10.1021/acselectrochem.5c00428. [PDF] [ DOI ]

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# Unraveling Nickel Dissolution Kinetics in Sulfuric Acid *via* Element-Resolved Electrochemical Impedance Spectroscopy

Junsoo Han*<sup>[a]</sup>, Bernard Tribollet<sup>[a]</sup>, Kevin Ogle<sup>[b]</sup>

*corresponding author: Junsoo Han (junsoo.han@sorbonne-universite.fr)

**Abstract:** Dissolution of transition metals in acidic environments central to corrosion and electrocatalysis, yet decoupling the kinetics of individual electrochemical steps remains a longstanding challenge. We introduce a combined atomic emission spectroelectrochemistry - electrochemical impedance spectroscopy (AESEC-EIS) method enabling real-time, element-specific quantification of metal dissolution while simultaneously resolving interfacial impedance. Using nickel in deaerated sulfuric acid as a model system, we construct the first element-resolved Nyquist and Bode plots for an actively dissolving metal. The results reveal a Ni dissolution mechanism involving adsorbed intermediates acting as precursors to soluble Ni(II). A second anodic peak, induced by prior cathodic potential sweep, reflects surface modification. Kinetic modeling and EIS simulations confirm that surface-mediated pathway dominates under cathodic activation, whereas passivating route involving NiO formation prevails otherwise. AESEC-EIS offers a robust approach to decompose interfacial electrochemical kinetics and to elucidate metal dissolution mechanisms relevant to corrosion and electrocatalysis.

**Keywords:** element-resolved kinetics; nickel; corrosion; AESEC; EIS; interfacial electrochemistry

<footnote><sup>[a]</sup> Sorbonne Université, CNRS, Laboratoire Interfaces et Systèmes Electrochimiques, LISE, F-75005 Paris, France</footnote>

<footnote><sup>[b]</sup> Chimie ParisTech, PSL University, CNRS, Institut de Recherche Chimie Paris (IRCP), F-75005 Paris, France</footnote>
Introduction

Understanding the fundamental mechanisms of metal/alloy dissolution in acidic environments is essential for developing corrosion-resistant materials and improving the durability of electrocatalysts.[1-10] Among transition metals, nickel (Ni) plays a pivotal role in both contexts,[11-20] yet its electrochemical dissolution kinetics remain incompletely understood due to the complex interplay of anodic and cathodic processes at the metal - electrolyte interface.

The overall anodic dissolution of Ni in acidic media is commonly represented as:

$$Ni \rightarrow Ni^{2+} + 2e^{-} \qquad (1)$$

Mechanistic studies suggest that this process proceeds via two sequential one-electron steps involving an adsorbed hydroxide intermediate, NiOH(a).[1,2,8,21,22] This species may form either through dissociative chemisorption of water or through electrochemical adsorption of OH⁻.

$$Ni + H_{2}O \rightarrow Ni(OH)(a) + H(a) \quad (\text{chemisorption}) \qquad (2)$$

$$Ni + OH^{-} \rightarrow Ni(OH)(a) + e^{-} \quad (\text{electrochemical adsorption}) \qquad (3)$$

where H(a) denotes adsorbed hydrogen on the surface. On the cathodic side, the hydrogen evolution reaction (HER) occurs via the following mechanisms:[22-25]

$$H^{+} + e^{-} \rightleftharpoons H(a) \quad (\text{Volmer}) \qquad (4)$$

$$2H(a) \rightarrow H_{2} \quad (\text{Tafel}) \qquad (5)$$

$$H(a) + H^{+} + e^{-} \rightarrow H_{2} \quad (\text{Heyrovsky}) \qquad (6)$$

Under HER conditions, local generation of OH⁻ may also occur via water reduction:

$$H_{2}O + e^{-} \rightarrow OH^{-} + H(a) \qquad (7)$$

This OH⁻ may then react with Ni to form NiOH(a). **Reaction 2** represents a non-faradaic dissociative adsorption step in which water splits at the surface to produce NiOH(a) without contributing to the external current. **Reaction 3** corresponds to electrochemical adsorption, where OH⁻ formed at the interface reacts with Ni in a charge transfer step, directly contributing to the measured current. As the potential becomes less negative, **Reaction 3** becomes increasingly favored, enabling the oxidation of NiOH(a) to soluble Ni²⁺ or potentially contributing to passive film formation.

Although similar mechanisms have been proposed for other transition metals,[23-29] a universally accepted pathway for these metals' dissolution remains elusive. The formation and reactivity of surface intermediates are highly sensitive to sample preparation, surface structure, hydrodynamic conditions, and solution composition,[1,30,31] complicating direct mechanistic interpretation. Conventional electrochemical
techniques such as cyclic voltammetry,[1,32] and electrochemical impedance spectroscopy (EIS)[9,21] pro-
vide valuable kinetic insights; however, they measure only total current, hindering the separation of anodic
and cathodic processes as well as the identification of intermediate species. Electrochemical quartz crystal
microbalance (EQCM) has also been used to distinguish anodic and cathodic kinetics, typically under the
assumption of either pure dissolution or exclusive film formation, as in electroless plating.[2,22,33-36] How-
ever, to disentangle complex multi-element processes and pinpoint the behavior of individual species,
element-resolved analysis is indispensable.

To address these challenges, we present a combined atomic emission spectroelectrochemistry and electrochemical impedance spectroscopy (AESEC-EIS) method.[37-40] This approach enables real-time, element-specific quantification of metal dissolution rates while simultaneously capturing interfacial impedance response.[41-43] For the first time, we construct element-resolved Bode and Nyquist plots for a dissolving metal system, allowing the kinetic role of intermediate to be quantitatively assessed. Using pure Ni in deaerated 2 M H₂SO₄ as a model system, we (i) decouple anodic and cathodic reaction rates at open circuit; (ii) determine the impact of cathodic pretreatment on dissolution kinetics and interfacial impedance; (iii) identify the catalytic function of NiOH(a) in promoting Ni²⁺ formation; and (iv) validate a two-step anodic reaction mechanism through kinetic modeling and AESEC-EIS simulations. Beyond the Ni - H₂SO₄ system, the AESEC-EIS framework offers a broadly applicable platform for mechanistic analysis in corrosion science, electrocatalysis, and reactive metal dissolution.

**Experimental**

Materials

Nominally pure Ni (99.98%) was obtained from Sigma Aldrich. The sample was mechanically
ground with a series of SiC papers up to P4000 with deionized water, then dried with flowing N₂ gas. A
2 M H₂SO₄ solution was prepared from analytical grade materials (VWR) in deionized water. The elec-
trolyte was deaerated by bubbling Ar gas 15 min prior to the experiments and maintained during the
measurements. All the experiments presented here showed reproducible results from at least three repeated
measurements.

AESEC measurements
The principle and detailed analytical method of the AESEC technique have been published elsewhere.[44] A Gamry Reference 600TM potentiostat connected to the AESEC system was used for the electrochemical measurement with a saturated calomel reference electrode (SCE) and a Pt foil counter electrode. For the AESEC linear sweep voltammetry (AESEC-LSV) experiments, open circuit dissolution was measured for an initial 1000 s until the system reached a reasonable steady state, then the potentiodynamic polarization experiment was carried out with a sweep rate of 0.5 mV s⁻¹. For the potentiostatic AESEC-EIS measurements, a constant potential was applied to the system for the first 1000 ~ 1500 s, then the AESEC-EIS was carried out at the same potential. For the open circuit AESEC-EIS measurement, the open circuit dissolution rate of Ni simultaneously with the open circuit potential (Eoc) were measured for the first approximately 1000 s then the AESEC-EIS was performed at the last potential value of the Eoc. The frequency range of the EIS measurement was from 10⁵ to 10⁻² Hz and the data were recorded with 7 points per decade using a 10 mVrms sinusoidal perturbation. The EIS results were recalculated by correcting the electrolyte resistance (Re),[45] and all EIS plots and simulations in this work are presented with the Re-corrected data. All measured EIS spectra were fitted using the measurement model approach,[46,47] which ensures consistency with the Kramers-Konig relations.

## AESEC Data Analysis

The atomic emission intensity of each element (M) at a characteristic wavelength (λ), IM, λ, was measured by the inductively coupled plasma atomic emission spectrometer (ICP-AES), Ultima 2C™, Horiba France. The Ni emission intensity at 237.6 nm wavelength was detected by a monochromator in the ICP-AES. The elemental concentration, CM, was calculated as:

$$C_M = (I_{M, \lambda} - I_{M, \lambda^0}) / \kappa_{\lambda} \qquad (8)$$

where IM, λ° is the background intensity, and κλ is the sensitivity factor of M obtained from an individual measurement using a standard ICP calibration method.[44] The elemental dissolution rates per unit surface area (A), vM, was calculated from CM using flow rate (f) of the electrolyte controlled by a peristaltic pump as:

$$v_M = f C_M / A \qquad (9)$$

It is often convenient to present the vM as an equivalent elemental current density, jM, using Faraday's law to facilitate comparison between the electrical current and the spectroscopic elemental dissolution rate signals as:

$$j_M = z_M F v_M \qquad (10)$$
where F is the Faraday constant (96485 C mol⁻¹), and z_M is the valence of the dissolving ions for oxidation (e.g., M → M²⁺ + z_Me⁻). The z_Ni = 2 was used in this work.

## Results and Discussions

*Open Circuit Dissolution of Ni in Deaerated 2 M H₂SO₄*

To decouple the individual contributions of anodic and cathodic reactions, AESEC-EIS measurements were performed at the open circuit potential (E<sub>oc</sub>) of nominally pure Ni in Ar deaerated 2 M H₂SO₄. After a stabilization period of 1000 s to ensure quasi-steady state conditions, potentiostatic AESEC-EIS was conducted. Time-resolved dissolution rate and current signals are presented in **Figure 1(a)**. The dissolution rate of Ni dissolution is expressed as an equivalent current density (j<sub>Ni</sub>) by assuming an oxidation state of z<sub>Ni</sub> = 2. To compare this with the measured electrical current (j<sub>e</sub>), j<sub>e</sub> is numerically convoluted with the residence time distribution (RTD) of the flow cell,<sup>[44,48]</sup> yielding the RTD-corrected current j<sub>e</sub>*. This correction enables direct, phase-resolved comparison between j<sub>Ni</sub> and j<sub>e</sub>*. The difference between the j<sub>e</sub>\* and j<sub>Ni</sub>, j<sub>Δ</sub> = j<sub>e</sub>\* - j<sub>Ni</sub>, accounts for both deposition processes, such as oxide formation, and reduction reactions. Assuming that oxide formation makes a minor contribution to the total oxidation current, the cathodic current density (j<sub>c</sub>) can be estimated from mass-charge balance as j<sub>c</sub> ≈ j<sub>Δ</sub>. This approach enables the independent evaluation of anodic and cathodic reaction rates at E<sub>oc</sub> using only AESEC data. In **Figure 1(a)**, j<sub>Ni</sub> and j<sub>c</sub> oscillated in-phase with the sinusoidal potential (see the inset), indicating a linear and quasi-reversible response. The consistently negative values of j<sub>Δ</sub> validate the assumption of no significant formation of insoluble products, confirming that j<sub>Δ</sub> = j<sub>c</sub> throughout the measurement.
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Element-resolved AESEC-EIS of Ni in deaerated 2 M H₂SO₄ at E<sub>oc</sub>. (a) Time-resolved dis-solution current and cathodic current. The inset shows the j<sub>e</sub>, j<sub>e</sub>*, j<sub>Ni</sub> and E. (b) Element-resolved Bode plot. Note that the Bode phase plot in (b) reflects the phase shift between E and j<sub>e</sub>. (c) Example of determining Δj<sub>Ni</sub> at f = 0.04 Hz for Eq. 11.</figcaption></figure>

From these time-resolved signals, the element-resolved Bode plot was constructed (Figure 1(b)) for the first time. At each frequency, the amplitude and phase of the current response for each species were extracted and used to compute the impedance:

$$Z_{jM} = |\Delta E/\Delta j_M|\{\cos(\varphi) + j \sin(\varphi)\} \quad (11)$$

where Δj<sub>M</sub> is the amplitude of the sinusoidal component for either j<sub>Ni</sub> or j<sub>c</sub> as illustrated in Figure 1(c), and φ is the phase shift between E and j<sub>e</sub>. Note that determining the EIS phase between E and j<sub>e</sub> (as well as j<sub>Ni</sub>) has no physical meaning, because j<sub>e</sub> is a modulated current signal used only to facilitate comparison with spectroscopic signals (e.g., j<sub>Ni</sub>) while accounting for the RTD. Figure 1(b) reveals the impedance of the anodic (Z<sub>jNi</sub>) and cathodic (Z<sub>jC</sub>) processes, alongside the total impedance modulus (|Z<sub>mod</sub>|). Notably, Z<sub>jC</sub> closely matches |Z<sub>mod</sub>| in the low-frequency regime (f < 10 Hz), indicating that the electrochemical impedance response was dominated by the cathodic reaction. Due to limitations in spectroscopic time
resolution and the RTD of the flow cell, element-resolved EIS data were reliably acquired only for $f \le 5$ Hz.

The corresponding Nyquist plot is presented in Figure 2(a). The experimental impedance data from Figure 1(a) were fitted using the equivalent circuit depicted in Figure 2(b). The fitting results are summarized in Table 1. In Figure 2(b), an inductance (L) is introduced in the cathodic branch of the circuit to account for the relaxation (or desorption) of adsorbed hydrogen intermediates.[49] This is supported by the low-frequency inductive loop in the Nyquist plot (Figure 2(a)), where the cathodic process dominates the impedance response (Figure 1(b)). The presence of such loops has been widely reported in HER studies on transition metals.[2,9,21-23,28,50,51] While the presence of a similar anodic reaction contributing to the inductive loop remains uncertain, the dominant contribution of the cathodic pathway justifies assigning the L exclusively to that branch.

Assuming a Langmuir-type adsorption behavior, both anodic and cathodic reactions were modeled as parallel processes, each coupled to the double layer capacitance ($C_{dl}$), represented as a constant-phase-element (CPE$_{dl}$). Graphical analysis of the impedance spectra[45] yielded CPE parameters of $Q_{dl} = 24.3$ µF cm$^{-2}$ s$^{(\alpha-1)}$ and $\alpha_{dl} = 0.99$. Using Brug's formula,[52] the calculated $C_{dl}$ was $22.1$ µF cm$^{-2}$, in excellent agreement with the circuit fit value of $19.6$ µF cm$^{-2}$, confirming the internal consistency of the model. The resistance associated with adsorbed anodic intermediates ($R_{ads, a}$) was $8.4 \times 10^5$ Ω cm$^2$, orders of magnitude higher than both the cathodic adsorption resistance ($R_{ads, c} = 1.0 \times 10^4$ Ω cm$^2$) and the anodic charge transfer resistance ($R_{ct, a} = 1.5$ Ω cm$^2$). This suggests that Ni was covered by adsorbed anodic species, or a passive film, with electron transfer remaining facile once the surface was accessible. Conversely, the cathodic branch exhibited a high charge transfer resistance ($R_{ct, c} = 794$ Ω cm$^2$), indicating that cathodic kinetics were limited by sluggish electron transfer. Together, these results point to an anodically passivated but kinetically competent Ni surface, where the overall electrochemical behavior was governed by the stability of the adsorbed anodic film and the activation controlled nature of the cathodic reaction.

The total calculated impedance ($Z_{total, calculated}$) was reconstructed from the individual anodic ($Z_a = Z_{jNi}$) and cathodic ($Z_c = Z_{jC}$) impedance components according to:

$$ \frac{1}{Z_{total, calculated}} = j\omega C_{dl} + \frac{1}{Z_a} + \frac{1}{Z_c} \qquad (12) $$

This calculated result (filled circles, Figure 2(a)) shows strong agreement with the measured electrochemical impedance (open circles, Figure 2(a)), validating the separation and recombination of element-resolved Nyquist plot contributions. These findings justify the measurement of $Z_a$ and $Z_c$ via AESEC-EIS.
and confirm that, at open circuit, Ni dissolution and HER occur in parallel (Figure 2(b)), with the low-frequency impedance dominated by the cathodic pathway. AESEC-EIS provides both chemical specificity (via $j_{Ni}$) and electrochemical kinetics (via $Z_{jM}$), offering mechanistic insights beyond the capabilities of conventional techniques.

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Nyquist plot and equivalent circuit modeling of AESEC-EIS data obtained from Figure 1. (a) Experimental impedance (open circles) calculated response using $Z_{jNi}$ and $Z_{jC}$ from Eq. 12 (filled circles), and equivalent circuit fit (line). (b) Equivalent circuit - CPE<sub>dl</sub>: CPE of double layer; $R_{ct, a}$: charge transfer resistance of anodic reaction; CPE<sub>a</sub>: CPE of anodic reaction; $R_{ads, a}$: resistance of adsorbed species from anodic reaction; $R_{ct, c}$: charge transfer resistance of cathodic reaction; $R_{ads, c}$: resistance of adsorbed species from cathodic reaction, and L: inductance for adsorbed species relaxation (desorption).</figcaption></figure>

<table><caption>**Table 1.** Fitting result for pure Ni in deaerated 2 M $H_2SO_4$ at $E_{oc}$, obtained from the AESEC-EIS equivalent circuit shown in Figure 2(b). The equivalent circuit fitting was performed using the in-house software SIMAD™ (https://www.lise.upmc.fr/en/simad). The standard deviation of the fit is reported as a percentage (%).</caption><thead><tr><th>R<sub>e</sub><br>Ω cm<sup>2</sup></th><th>Q<sub>dl</sub><br>μF cm<sup>-2</sup> s<sup>α-1</sup></th><th>α<sub>dl</sub></th><th>R<sub>ct, a</sub><br>Ω cm<sup>2</sup></th><th>Q<sub>a</sub><br>μF cm<sup>-2</sup> s<sup>α-1</sup></th><th>α<sub>a</sub></th><th>R<sub>ads, a</sub><br>Ω cm<sup>2</sup></th><th>R<sub>ct, c</sub><br>Ω cm<sup>2</sup></th><th>R<sub>ads, c</sub><br>Ω cm<sup>2</sup></th><th>L<br>H cm<sup>-2</sup></th></tr></thead><tbody><tr><td>4.4<br>± 0.01%</td><td>21.5<br>± 1.1%</td><td>0.99<br>± 0.11%</td><td>1.5<br>± 3.8%</td><td>88.8<br>± 0.96%</td><td>0.86<br>± 0.17%</td><td>8.4 x 10<sup>5</sup><br>± 5.7%</td><td>794<br>± 0.72%</td><td>1.0 x 10<sup>4</sup><br>± 8.6%</td><td>3.4 x 10<sup>4</sup><br>± 0.13%</td></tr></tbody></table>

## Potential Dependent Dissolution of Ni: Effect of the Cathodic Potential Step

AESEC-linear sweep voltammetry (AESEC-LSV) experiments were conducted to investigate the active-passive transition of Ni under different pretreatment conditions. Based on the $j_{Ni}$ profiles, five domains were identified in Figure 3 - I: cathodic; II: active (Tafel); III: active-passive; IV: first passive; and V: second passive. Figure 3(a) shows the AESEC-LSV initiated at $E_{oc}$ - 10 mV following 1000 s of
open circuit exposure without cathodic pretreatment. **Figure 3(b)** shows AESEC-LSV began from -0.40 V vs. SCE after the same open circuit duration. In domain **I**, the decay in $j_{Ni}$ reflects washout of preformed soluble Ni species, rather than active electrochemical dissolution. **Figures 3(c) and 3(d)** show similar experiments with only 10 s of open circuit exposure, isolating the role of cathodic pretreatment. These results suggest that the second oxidation peak ($a_2$) arises from oxidation of Ni species formed during the cathodic step or revealed by the removal of blocking species (e.g., at grain boundaries). When scanning from -0.40 V vs. SCE, $a_2$ was consistently observed, as shown in **Figures 3(b) and 3(d)**.

Across all four conditions, a plateau labeled $a_3$ was observed in domain **IV** at similar potentials, likely associated with passive film formation and dissolution.[6,7,52-55] In this regime, $j_{Ni} \approx j_e^*$, suggesting a quasi-steady state process. However, after cathodic pretreatment (**Figures 3(b) and 3(d)**), $j_{Ni} > j_e^*$ in domain **IV**, indicating continued dissolution of a Ni species formed or revealed during the cathodic step. Mass-charge balance yielded an unaccounted charge of 118 mC cm$^{-2}$ in domain **III** (from $j_e^* - j_{Ni}$), and an excess dissolution of -125 mC cm$^{-2}$ in domain **IV**. These comparable values suggest that Ni species generated in domain **III** dissolve in domain **IV**. At more anodic potentials (domains **IV** and **V**), $j_{Ni} > j_e^*$ under all conditions, consistent with sluggish dissolution kinetics of oxidized Ni species.[56-58]

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 3.** AESEC-LSV of pure Ni in deaerated 2 M H₂SO₄ under different pretreatment conditions: (a) E₀c (1000 s) → LSV from E₀c - 10 mV; (b) E₀c (1000 s) → LSV from -0.40 V vs. SCE; (c) E₀c (10 s) → LSV from E₀c - 10 mV; (d) E₀c (10 s) → LSV from -0.40 V vs. SCE.</figcaption></figure>

To investigate the influence of cathodic pretreatment on dissolution kinetics near the active-passive transition domain, potentiostatic step AESEC-EIS experiments were performed within the domain III in Figure 3. A potential of -0.05 V vs. SCE, corresponding to the onset of the a₂, was applied either directly or following cathodic polarization to -0.40 V vs. SCE. The chosen cathodic potential lay below the equilibrium potential for Ni²⁺/Ni (E°Ni²⁺/Ni = -0.292 V vs. SHE = -0.533 V vs. SCE in 2 M H₂SO₄ at T = 25 °C), minimizing reduction of soluble Ni²⁺ while still altering the surface.

Nyquist plots in **Figure 4** reveal clear differences between conditions. Both spectra display a distorted capacitive loop bending toward negative real impedance at low frequencies ($f < 0.3$ Hz), indicative of negative differential resistance (NDR),[27,28,50,59-65] a signature of competing charge transfer and pseudo passivating processes. This is consistent with the negative slope region in AESEC-LSV (**Figure 3**) ($\partial j/\partial E < 0$, i.e., polarization resistance, $R_p < 0$). The presence of NDR suggests that the system resided near the transition between active dissolution and passivation. Notably, after cathodic pretreatment, (-0.40 V → -0.05 V vs. SCE) an additional capacitive loop appeared in the intermediate-frequency range ($10 > f > 1$ Hz), as highlighted in the inset of **Figure 4**. This feature was not observed, or potentially masked, in the untreated case (-0.05 V vs. SCE). This new time-constant likely arose from surface-bound intermediates formed or reactivated during cathodic polarization. The additional time-constant suggests that cathodic polarization modified the surface chemistry in a way that persisted even after the potential was returned to the anodic domain.
Time-resolved AESEC-EIS data support this distinction. In **Figures 5(a) and 5(b)**, $j_{Ni}$ and $j_e^*$ oscillated in-phase with E, consistent with charge transfer controlled dissolution. In both cases, the faradaic yield was high ($j_{Ni}/j_e^* = 0.92 \pm 0.04$), suggesting incomplete passivation, consistent with the impedance behavior observed in the Nyquist plots (**Figure 4**). The cathodically pretreated surface exhibited a higher $j_{Ni}$ ($6.3 \pm 0.3 \text{ mA cm}^{-2}$, **Figure 5(b)**) than the untreated surface ($3.5 \pm 0.3 \text{ mA cm}^{-2}$, **Figure 5(a)**), indicating enhanced reactivity. This trend was reproducible through repeated measurements. The inset of **Figure 5(b)** shows distinct transient shoulder peaks in $j_e$, indicated by arrows, which were absent in the untreated case (**Figure 5(a)**). This transient current may be attributed to the oxidation of kinetically distinct surface species formed during the cathodic step. These features cannot be explained solely by harmonic distortion or multi-frequency artifacts, as discussed in **Supplementary Information A1**.

Lissajous plots at $f = 0.01 \text{ Hz}$ further support this interpretation.[66,67] For the untreated sample, the $j_e$ vs. E loop shows a monotonically negative slope throughout the AC cycle (**Figure 6(a)**). In contrast, the pretreated sample (**Figure 6(b)**) displays both negative and positive slopes, reflecting a surface undergoing time-dependent chemical transformation even under constant applied potential. The origin of these Lissajous features can be understood by inspecting the AC potential excursion (peak-to-peak potential, $V_{pp} = 2\sqrt{2}V_{rms}$) relative to the AESEC-LSV response. In **Figure 3(c)**, where no cathodic step was applied, the $V_{pp}$ remains entirely within the negative slope region of the $j_e$ vs. E, explaining the consistently negative slope of the Lissajous plot in **Figure 6(a)**. In contrast, **Figure 3(d)** (with cathodic pretreatment) shows that $V_{pp}$ spans both negative and positive slope regions of $j_e$ vs. E, corresponding to the inflection observed in the Lissajous loop of **Figure 6(b)**. Thus, the difference in Lissajous morphology directly reflects the underlying modification of interfacial kinetics by cathodic activation.

Taken together, the emergence of an intermediate-frequency time-constant, the elevated $j_{Ni}$ values, and the shoulder feature in $j_e$ collectively confirm that cathodic pretreatment induces a kinetically distinct surface state. These findings underscore the sensitivity of AESEC-EIS in capturing transient interfacial dynamics with both chemical and electrochemical resolution.
<figure><img src="image_7.png" /><figcaption>**Figure 4.** AESEC-EIS results for Ni in 2 M H₂SO₄ at -0.05 V: Nyquist plots obtained after direct anodic polarization (-0.05 V vs. SCE) and following cathodic pretreatment (-0.40 V → -0.05 V vs. SCE). All potentials are referenced relative to the SCE.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_9.png" /></figure>

**Fig. 5.** Time-resolved AESEC-EIS for Ni in 2 M H₂SO₄ at (a) -0.05 V, and (b) at -0.40 V → -0.05 V. In (b) j_e convolution (j_e*) was performed only for the anodic current. All potentials are referenced relative to the SCE.
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Corresponding Lissajous plots ($j_M$ vs. E) from the AESEC-EIS measurements shown in Fig. 5: (a) -0.05 V, and (b) -0.40 V → -0.05 V at $f=0.01$ Hz. All potentials are referenced relative to the SCE.</figcaption></figure>

<figure><img src="image_11.png" /></figure>

To investigate the onset and evolution of passivation, AESEC-EIS experiments were conducted at 0.06 V vs. SCE, a potential situated near the active-passive (domain **III**) and passive (domain **IV**) regimes as defined in **Figure 3**. Two conditions were compared: (i) direct polarization to 0.06 V vs. SCE and (ii) the same polarization following a cathodic treatment at -0.40 V vs. SCE, to probe how the surface state established by cathodic activation influences the subsequent formation of the passive film.

Nyquist plots under these conditions are shown in **Figure 7**. In the untreated case (0.06 V vs. SCE), the low-frequency impedance response ($f<1$ Hz) was dominated by a large capacitive loop, consistent with the progressive formation of a passive film that impedes interfacial charge transfer. The cathodically pretreated surface (-0.40 V → 0.06 V vs. SCE) exhibited a significantly reduced second capacitive loop along with the appearance of a low-frequency inductive loop ($f<0.01$ Hz, inset). The presence of the inductive feature suggests desorption or transformation of adsorbed intermediates.[8,29,68] These results indicate that dynamic surface reactivity persisted even under nominally passivating conditions. In summary, cathodic activation altered the characteristics of the second capacitive loop, consistent with the results observed in **Figure 4**, and introduced additional time-constants.

The corresponding time-resolved AESEC-EIS results further highlight the contrast between the two surface conditions. For the untreated surface (Figure 8(a)), both $j_{Ni}$ and $j_e^*$ gradually decreased over the first ~1000 s. Throughout this period, $j_{Ni}$ remained consistently lower than $j_e^*$, indicating the progressive formation of a passive film that inhibits metal dissolution.[69] The faradaic yield remained below unity
(j<sub>Ni</sub>/j<sub>e</sub>* = 0.88 ± 0.02), indicating that a portion of the anodic current was consumed in forming non-dis-
solving surface phases, such as oxides or hydroxides. Moreover, the suppression of oscillatory features in
j<sub>Ni</sub> further supports the conclusion that the interface had transitioned into a passive state.[38-40] In contrast,
the cathodically pretreated surface (Figure 8(b)) exhibited a markedly different response: both j<sub>Ni</sub> and j<sub>e</sub>*
increased over time, and notably, j<sub>Ni</sub> consistently exceeded j<sub>e</sub>*, resulting in a faradaic yield of 1.07 ± 0.06.
This divergence indicates that Ni species formed or exposed during the cathodic activation step continued
to dissolve under anodic polarization. The steadily increasing trend in j<sub>Ni</sub> (and j<sub>e</sub>*) , along with its higher
value (j<sub>Ni</sub> = 5.8 ± 0.4 mA cm<sup>-2</sup> vs. 3.1 ± 0.1 mA cm<sup>-2</sup> in the untreated case), suggests that the surface
remained chemically and electrochemically active over the course of the measurement.

Further confirmation of the distinct interfacial states is provided by the post-polarization dissolu-
tion response. Upon returning to Eoc, the untreated sample exhibited a sharp Ni dissolution peak (Figure
8(a), bottom inset), consistent with the chemical dissolution of a passive film formed during polarization
at 0.06 V vs. SCE. This transient feature was absent in the cathodically pretreated case (Figure 8(b)),
indicating that no substantial passivating layer had formed. Instead, the surface remained dissolution-
active throughout the experiment, as indicated by both the elevated jNi and the presence of an inductive
loop in Figure 7. The jNi trend in this post-polarization (i.e., Eoc, Figure 8(b)) domain closely followed
je*, indicating that the decaying jNi signal was due to the RTD of the flow cell, rather than chemical disso-
lution.[44] The dynamic response of je* further supports this interpretation. In the untreated case (Figure
8(a); upper inset), je* reached a steady state plateau and exhibited minimal modulation by the applied
sinusoidal potential, suggesting charge transfer is limited by a passive film.[38] By contrast, the cathodi-
cally pretreated surface (Figure 8(b)) displayed a je* signal that remained strongly modulated by the ap-
plied potential, indicating sustained interfacial reactivity and dynamic electron transfer.

These results confirm that the cathodic pretreatment fundamentally altered the surface chemistry
of Ni, delaying or suppressing passivation even at potentials where oxide growth typically dominates.
This transformation may occur *via* removal of blocking surface oxides or generation of metastable, reac-
tive Ni-based intermediates at high energy sites. The AESEC-EIS methodology captures this shift with
simultaneous chemical and electrochemical resolution, enabling a mechanistic understanding of how in-
terfacial conditioning modulates early stage passivation behavior.
<figure><img src="image_12.png" /><figcaption>**Figure 7.** AESEC-EIS results for Ni in 2 M H₂SO₄ at 0.06 V vs. SCE: Nyquist plots obtained after direct anodic polarization (0.06 V vs. SCE) and following cathodic pretreatment (-0.40 V → -0.05 V vs. SCE). All potentials are referenced relative to the SCE.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 8.** Time-resolved AESEC-EIS for Ni in 2 M H₂SO₄ at (a) 0.06 V vs. SCE, and (b) at -0.40 V → 0.06 V vs. SCE. In (b), the j_e convolution (j_e*) is performed only for the anodic current. All potentials are referenced relative to the SCE.</figcaption></figure>

To evaluate the electrochemical behavior of Ni within the fully passive regime, AESEC-EIS measurements were conducted at 0.15 V and 0.50 V vs. SCE, both corresponding to domain **V** in the AESEC-LSV profile (**Figure 3**; 0.50 V vs. SCE is not shown but exhibited the same tendency). In this domain, the influence of cathodic pretreatment was deemed negligible, as the AESEC-LSV data (**Figures 3(b)** and
**3(d)** showed nearly identical dissolution responses regardless of whether a cathodic step had been applied. Accordingly, all measurements presented in this section were performed without prior cathodic polarization.

**Figure 9** presents the Nyquist plots at 0.15 V and 0.50 V vs. SCE. At the lower potential (0.15 V vs. SCE), the impedance spectrum features a capacitive loop accompanied by a low-frequency excursion into the negative real axis (i.e., NDR, $R_p < 0$). This behavior mirrors the response previously observed at 0.05 V vs. SCE (**Figure 4**) and indicates a persistent competition between metal dissolution and surface passivation. At the higher potential of 0.50 V vs. SCE, the impedance magnitude was greater than at 0.15 V vs. SCE, and the Nyquist plot no longer exhibited the NDR, indicating the formation of a stable, resistive passive film.[70] The disappearance of the NDR and the marked increase in total impedance confirm that the surface has transitioned into a strongly passivated regime, with minimal ongoing interfacial reactivity.

Time-resolved AESEC-EIS data further support this interpretation. At 0.15 V vs. SCE (**Figure 10(a)**), both $j_{Ni}$ and $j_e^*$ gradually decayed over the course of the measurement, consistent with passivation kinetics. The signal also displayed well-defined low frequency oscillations with a near -90° phase shift relative to the E (see inset), i.e., $R_p < 0$. This phase relationship suggests that the passive film was still evolving and had not yet fully developed under this applied potential. At 0.50 V vs. SCE (**Figure 10(b)**), the system exhibited markedly different behavior. Both $j_{Ni}$ and $j_e^*$ decayed monotonically over time, with no discernible low-frequency oscillations in $j_{Ni}$, and at significantly lower rates than those observed at 0.15 V vs. SCE (**Figure 10(a)**). The absence of phase-shifted oscillatory response in $j_{Ni}$ suggests the presence of a mature, stable passive film that effectively suppresses surface reactivity. These observations are consistent with **Figure 9**, where the large diameter of the semicircle indicates substantial charge transfer resistance and a strongly passivated surface state.

These results delineate the transition from an evolving, partially passivated interface at intermediate potentials (0.15 V vs. SCE) to a fully developed passive film at more anodic potentials (0.50 V vs. SCE).[71,72] The AESEC-EIS technique captures not only the impedance signatures associated with this transition but also the corresponding changes in real-time elemental dissolution kinetics (i.e., $j_{Ni}$), enabling direct correlation between passive film formation and the suppression of electrochemical activity.
<figure><img src="image_15.png" /><figcaption>**Figure 9.** AESEC-EIS for Ni in 2 M H₂SO₄ at 0.15 V and 0.50 V vs. SCE: Nyquist plots obtained at corresponding potentials. All potentials are referenced relative to the SCE.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Figure 10.** Time-resolved AESEC-EIS at (a) 0.15 V, and (b) at 0.50 V vs. SCE. All potentials are referenced to SCE. Ni dissolution, marked with an arrow at E<sub>oc</sub> in both cases, indicates dissolution of the passive film formed during AESEC-EIS.</figcaption></figure>

*Kinetic Model of Ni Dissolution in H₂SO₄*

To rationalize the experimental observations across all anodic potential domains and polarization conditions, a mechanistic kinetic model was developed based on literature-reported pathways for Ni dissolution in acidic media.[9,29,73] The model incorporates key surface and solution phase reactions described
in previous sections.[74] The proposed mechanism, illustrated schematically in **Figure 11**, consists of the following key steps:

a. Formation of the surface intermediate NiOH(a):

$$ \underset{k_1}{\mathrm{Ni} + \mathrm{H}_2\mathrm{O} \rightleftharpoons} \underset{k_1}{\mathrm{NiOH}(\mathrm{a})} + \mathrm{H}^+ + \mathrm{e}^- \quad (i_1) \qquad (13) $$

This step is treated as a quasi-equilibrated electrochemical adsorption process governed by potential dependent forward and backward rate constants, $k_1$ and $k_1$.

b. Electrochemical oxidation of NiOH(a) to a soluble species:

$$ \underset{k_2}{\mathrm{NiOH}(\mathrm{a}) \rightarrow} \mathrm{NiOH}^+ + \mathrm{e}^- \quad (i_2) \qquad (14) $$

c. Homogeneous transformation of $NiOH^+$ into $Ni^{2+}$ in bulk solution:

$$ \mathrm{Ni(OH)}^+ + \mathrm{H}^+ \rightarrow \mathrm{Ni}^{2+} + \mathrm{H}_2\mathrm{O} \quad (\text{bulk}) \qquad (15) $$

This step does not contribute to the interfacial impedance response.

d. Electrochemical oxidation of NiOH(a) to a passivating NiO film:

$$ \underset{k_4}{\mathrm{NiOH}(\mathrm{a}) \rightarrow} \mathrm{NiO} + \mathrm{H}^+ + \mathrm{e}^- \quad (i_3) \qquad (16) $$

e. Chemical dissolution of NiO by $HSO_4^-$:

$$ \underset{k_5}{\mathrm{NiO} + 2\mathrm{HSO}_4^- \rightarrow} \mathrm{Ni}^{2+} + 2\mathrm{SO}_4^{2-} + \mathrm{H}_2\mathrm{O} \qquad (17) $$

<figure><img src="image_17.png" /><figcaption>**Figure 11.** Schematic image of Ni dissolution kinetics in deaerated 2 M $H_2SO_4$.</figcaption></figure>

In this mechanistic framework, NiOH(a) occupies a central role. It may either (i) be electrochemically oxidized to $NiOH^+$ and eventually yield $Ni^{2+}$ in solution, or (ii) undergo further oxidation to form a passivating NiO layer. The balance between these two fates determines whether the system favors active
dissolution or passivation. The supporting electrolyte, 2 M H₂SO₄, fully dissociates in its first step (H₂SO₄ → H⁺ + HSO₄⁻) but only partially in the second (HSO₄⁻ ⇌ H⁺ + SO₄²⁻, pKa = 1.99 at 25°C), yielding an estimated [HSO₄⁻]:[SO₄²⁻] ratio of 195:1, assuming an activity coefficient of 1. This composition justifies the stoichiometry in **Reaction 17** and implies that HSO₄⁻ is the dominant complexing anion under all experimental conditions (i.e., Le Châtelier's principle).

To simulate the system response under different experimental conditions, the reaction network was numerically solved using a custom in-house implementation of the SIMADTM software. The interfacial parameters obtained from fitting are summarized in **Table 2**, including the rate constants (kᵢ), transfer coefficients (bᵢ), site densities (β₁ for NiOH(a), β₂ for NiO), and the diffusion properties of H⁺ (D_H⁺: diffusion coefficient; δ_H⁺: diffusion length). The Nyquist plots comparing experimental data with simulations from the kinetic model are shown in **Figure 12**. The corresponding steady state reaction rates obtained from the model are summarized in **Table 3**. A detailed description of the parameter determination procedure is provided in **Supplementary Information A2**.

A key outcome of the kinetic model is the marked difference in steady state NiOH(a) coverage (θ̄₁) between cathodically treated and untreated samples. Following cathodic activation (-0.40 V → -0.05 V vs. SCE), θ̄₁ ≈ 10⁻⁴ mol cm⁻² (**Table 2**), which is two to three orders of magnitude higher than under untreated conditions (θ̄₂ ≈ 10⁻⁷ mol cm⁻²). In contrast, the steady state NiO coverage (θ̄₂) remains low on cathodically pretreated surfaces (θ̄₂ ≈ 0.995 ~ 0.998) but dominates on untreated ones (θ̄₂ ≈ 0.9999). The fraction of the surface remaining uncovered by either intermediate (1 - θ̄₁ - θ̄₂) is roughly two orders of magnitude higher in cathodically treated samples, indicating greater availability of reactive sites. These results are consistent with the enhanced reactivity and suppressed passivation observed in AESEC-EIS following cathodic conditioning.

The kinetic balance between **Reactions 13** and **14**, as revealed by the steady state rate comparison in **Table 3**, is particularly insightful. For cathodically activated surfaces, the rate of NiOH(a) formation (**Reaction 13**, k₁(1 - θ̄₁ - θ̄₂)), equals the rate of its electrochemical oxidation to NiOH⁺ (**Reaction 14**, k₂β₁θ̄₁). This near equivalence indicates a dynamic steady state that enables continuous Ni dissolution through sustained intermediate turnover. In contrast, untreated surfaces exhibit a significant rate imbalance: the rate of **Reaction 13** exceeds that of **Reaction 14** by three orders of magnitude, leading to transient accumulation of NiOH(a) at the interface. This excess intermediate is subsequently diverted toward
passivating NiO formation *via* **Reaction 16** ($k_4\beta_1\bar{\theta}_1$). This mechanistic shift is consistent with the experimentally observed suppression of $j_{Ni}$ and the onset of passive behavior. In cathodically treated systems, the rate of **Reaction 14** is one to two orders of magnitude higher than in the untreated case, while the rate of forward **Reaction 13** is reduced by two orders of magnitude. This shift promotes enhanced formation of $NiOH^+$ and reduces the accumulation of $NiOH(a)$, thereby suppressing its conversion to $NiO$.

A comparison between the two cathodically treated cases reveals that **Reaction 14** proceeds more rapidly at -0.40 V → 0.06 V vs. SCE, while **Reaction 16** is comparatively slower. This trend aligns with the AESEC results: $j_{Ni} < j_e^*$ for -0.40 V → -0.05 V (**Figure 5(b)**) whereas $j_{Ni} > j_e^*$ for -0.40 V → 0.06 V vs. SCE (**Figure 8(b)**), indicating a more active dissolution regime under the more anodic condition.

In summary, the kinetic modeling suggests that $NiOH(a)$ serves as a dynamic intermediate whose fate, toward either continued dissolution or passivation, is governed by both the applied potential and surface preconditioning. Cathodic activation enhances the formation and turnover of this intermediate, promoting the dissolution pathway *via Reactions 14*, while suppressing passivation *via Reactions 16*. This mechanistic shift accounts for the higher faradaic efficiency, the emergence of the $a_2$ peak, and the additional time-constants observed in AESEC-EIS following cathodic pretreatment.

<table><caption>**Table 2.** Kinetic and interfacial parameters for Ni oxidation in 2 M $H_2SO_4$ at the indicated applied potentials, obtained from EIS model fitting. Parameters include electrochemical rate constants ($k_i$), transfer coefficients ($b_i$), site densities ($\beta_i$), diffusion coefficient of $H^+$ ($D_{H^+}$), diffusion length of $H^+$ ($\delta_{H^+}$), and surface coverages ($\bar{\theta}_i$). 1 - $\bar{\theta}_1$ - $\bar{\theta}_2$ represents the fraction of electrochemically active bare Ni surface.</caption><thead><tr><th></th><th>-0.4 V → -0.05 V</th><th>-0.05 V</th><th>-0.4 V → 0.06 V</th><th>0.06 V</th></tr></thead><tbody><tr><td>k<sub>1</sub> / mol cm<sup>-2</sup> s<sup>-1</sup></td><td>2.0 x 10<sup>-5</sup></td><td>0.11</td><td>2.4 x 10<sup>-5</sup></td><td>0.42</td></tr><tr><td>b<sub>1</sub> / V<sup>-1</sup></td><td>16</td><td>2</td><td>3</td><td>1</td></tr><tr><td>k<sub>-1</sub> / cm s<sup>-1</sup></td><td>1.0 x 10<sup>-8</sup></td><td>2.7 x 10<sup>-2</sup></td><td>1.1 x 10<sup>-8</sup></td><td>0.29</td></tr><tr><td>b<sub>-1</sub> / V<sup>-1</sup></td><td>27</td><td>88</td><td>26</td><td>95</td></tr><tr><td>k<sub>2</sub> / s<sup>-1</sup></td><td>10</td><td>2.2 x 10<sup>-1</sup></td><td>10</td><td>4.6 x 10<sup>-1</sup></td></tr><tr><td>b<sub>2</sub> / V<sup>-1</sup></td><td>5</td><td>36</td><td>16</td><td>14</td></tr><tr><td>k<sub>4</sub> / s<sup>-1</sup></td><td>1.7 x 10<sup>-4</sup></td><td>4.8</td><td>2.3 x 10<sup>-4</sup></td><td>6.9</td></tr><tr><td>b<sub>4</sub> / V<sup>-1</sup></td><td>6</td><td>84</td><td>11</td><td>9</td></tr><tr><td>k<sub>5</sub> / s<sup>-1</sup></td><td>1.0 x 10<sup>-4</sup></td><td>5.0 x 10<sup>-7</sup></td><td>2.8 x 10<sup>-4</sup></td><td>4.8 x 10<sup>-7</sup></td></tr><tr><td>β<sub>1</sub> / mol cm<sup>-2</sup></td><td>9.0 x 10<sup>-4</sup></td><td>2.4 x 10<sup>-2</sup></td><td>1.0 x 10<sup>-4</sup></td><td>7.7 x 10<sup>-2</sup></td></tr></tbody></table>
<table><caption>**Table 3.** Steady state reaction rates (mol cm<sup>-2</sup> s<sup>-1</sup>) calculated from the EIS kinetic model for the indicated applied potentials. The chemical dissolution of NiO (Reaction 17) is, by definition, equal to the rate of Reaction 16 under steady state conditions.</caption><tr><td>β<sub>2</sub> / mol cm<sup>-2</sup></td><td>5.1 x 10<sup>-9</sup></td><td>9.5 x 10<sup>-2</sup></td><td>8.6 x 10<sup>-9</sup></td><td>1.2 x 10<sup>-1</sup></td></tr><tr><td>D<sub>H<sup>+</sup></sub> / cm<sup>2</sup> s<sup>-1</sup></td><td>9.3 x 10<sup>-5</sup></td><td>9.3 x 10<sup>-5</sup></td><td>9.3 x 10<sup>-5</sup></td><td>9.3 x 10<sup>-5</sup></td></tr><tr><td>δ<sub>H<sup>+</sup></sub> / µm</td><td>17</td><td>61</td><td>34</td><td>60</td></tr><tr><td>Q<sub>dl</sub> / Ω<sup>-1</sup> cm<sup>-2</sup> s<sup>α</sup></td><td>2.4 x 10<sup>-5</sup></td><td>3.2 x 10<sup>-5</sup></td><td>2.6 x 10<sup>-5</sup></td><td>2.8 x 10<sup>-5</sup></td></tr><tr><td>α<sub>dl</sub></td><td>0.737</td><td>0.681</td><td>0.751</td><td>0.764</td></tr><tr><td>θ&#773;<sub>1</sub></td><td>0.000332718</td><td>0.000000419</td><td>0.000104236</td><td>0.000000108</td></tr><tr><td>θ&#773;<sub>2</sub></td><td>0.998154863</td><td>0.999937544</td><td>0.995612651</td><td>0.999976273</td></tr><tr><td>1 - θ&#773;<sub>1</sub> - θ&#773;<sub>2</sub></td><td>1.5 x 10<sup>-3</sup></td><td>6.2 x 10<sup>-5</sup></td><td>4.3 x 10<sup>-3</sup></td><td>2.4 x 10<sup>-5</sup></td></tr></table>

<table><thead><tr><th>/ mol cm<sup>-2</sup> s<sup>-1</sup></th><th>-0.4 V → -0.05 V</th><th>-0.05 V</th><th>-0.4 V → 0.06 V</th><th>0.06 V</th></tr></thead><tbody><tr><td>Forward Reaction 13</td><td>3.0 x 10<sup>-8</sup></td><td>7.0 x 10<sup>-6</sup></td><td>1.0 x 10<sup>-7</sup></td><td>9.9 x 10<sup>-6</sup></td></tr><tr><td>Backward Reaction 13</td><td>1.8 x 10<sup>-18</sup></td><td>5.1 x 10<sup>-12</sup></td><td>4.3 x 10<sup>-18</sup></td><td>2.0 x 10<sup>-11</sup></td></tr><tr><td>Reaction 14</td><td>3.0 x 10<sup>-8</sup></td><td>2.0 x 10<sup>-9</sup></td><td>1.0 x 10<sup>-7</sup></td><td>3.8 x 10<sup>-9</sup></td></tr><tr><td>Reaction 16</td><td>5.1 x 10<sup>-13</sup></td><td>4.8 x 10<sup>-8</sup></td><td>2.4 x 10<sup>-14</sup></td><td>5.8 x 10<sup>-8</sup></td></tr><tr><td>Reaction 17</td><td>5.1 x 10<sup>-13</sup></td><td>4.8 x 10<sup>-8</sup></td><td>2.4 x 10<sup>-14</sup></td><td>5.8 x 10<sup>-8</sup></td></tr></tbody></table>
<figure><img src="image_18.png" /><figcaption>**Figure 12.** Comparison of the experimental results (circles) with kinetic model simulations (squares) for Ni dissolution in 2 M H₂SO₄ under various applied potential conditions.</figcaption></figure>

*Mechanistic Insights - Open Circuit Dissolution*

The combination of AESEC-EIS and AESEC-LSV enabled, for the first time, simultaneous resolution of electrochemical impedance and element dissolution kinetics at open circuit. From the elemental Lissajous plot shown in **Figure 13**, obtained at *f* = 0.005 Hz from the AESEC-EIS at *E*<sub>oc</sub> in **Figure 1**, the cathodic Tafel slope obtained via AESEC-EIS (b<sub>c, j<sub>c</sub></sub> = 28 mV decade<sup>-1</sup>) is characteristic of a Volmer-Tafel mechanism,<sup>[75,76]</sup> in which the recombination of H(a) (**Reaction 5**) is rate limiting. In contrast, AESEC-LSV (**Figures 3(b) and 3(d)**) yields a higher slope (b<sub>c, j<sub>c</sub></sub> = 112 mV decade<sup>-1</sup>), consistent with a Volmer-Heyrovsky mechanism dominated by electrochemical desorption (**Reaction 6**).<sup>[67,76]</sup> This difference highlights the sensitivity of HER kinetics to measurement timescale and H(a) surface coverage: under quasi-steady state EIS conditions, H(a) accumulates and facilitates recombination, while dynamic LSV suppresses H(a) buildup, favoring desorption.
<figure><img src="image_19.png" /><figcaption>**Figure 13.** Elemental Lissajous plot of nominally pure Ni in 2 M H₂SO₄ at E₀c and f = 0.005 Hz, obtained from Figure 1(a).</figcaption></figure>

A similar trend was observed for anodic dissolution of Ni. The Tafel slope determined from jNi (bA, jNi) under AESEC-EIS at E₀c (bA, jNi = 110 mV decade⁻¹) was higher than that observed from AESEC-LSV (66 - 74 mV decade⁻¹), indicating a slower anodic reaction under higher H(a) coverage. These results confirm that interfacial hydrogen not only governs HER kinetics but also inhibits Ni oxidation,[23,77,78] demonstrating the strength of AESEC-EIS in isolating half-reaction contributions with chemical and temporal specificity.

*Mechanistic Insights - Effect of Cathodic Potential Step*

Cathodic pretreatment significantly altered the anodic dissolution kinetics of Ni. **Figure 3** showed that the cathodic activation step gave rise to a reproducible second oxidation peak (a₂)[2,3,56,79] that was absent in untreated samples. AESEC-EIS revealed an additional intermediate-frequency time-constant (**Figures 4 and 7**) and elevated Ni dissolution rate (jNi) following cathodically preconditioning, indicating enhanced interfacial reactivity.

In alkaline media, the a₂ peak is commonly associated with Ni(II) → Ni(III) transitions (e.g., Ni(OH)₂ → NiOOH),[80,81] but such transformations are thermodynamically inaccessible under strong acidic conditions. Here, the a₂ peak coincided with a rise in jNi and je* (and je), ruling out purely solid-state oxidation (e.g., NiO → Ni₃O₄)[82,83] and supporting the oxidation of an interfacial, catalytically active Ni(II) species, namely NiOH(a). Kinetic modeling indicated that, following cathodic pretreatment, NiOH(a) formation (Reaction 13) and oxidation to NiOH⁺ (Reaction 14) occurred at nearly identical
rates, implying steady state turnover of the intermediate and active promotion of the dissolution pathway. Thus, the a₂ peak serves as a mechanistic signature of NiOH(a)-mediated dissolution catalysis. In contrast, untreated surface exhibited a mismatch between **Reaction 13** and **Reaction 14**, favoring transient NiOH(a) accumulation and diversion into passivating NiO (**Reaction 16**).

The effect of cathodic pretreatment likely arises from interfacial competition between H⁺ and OH⁻ adsorption. Using equilibrium surface coverage models, the fractional coverages of NiH(a) and NiOH(a) were estimated from standard Gibbs free energies of adsorption.[⁶⁹,⁸⁴⁻⁸⁸] Under bulk acidic conditions ([H⁺] ≈ 4 M) and cathodic potentials, NiH(a) dominates (θH⁺ ≈ 1). However, local OH⁻ generation during HER (**Reaction 7**) may transiently raise the interfacial pH, enabling NiOH(a) formation via electrochemical adsorption (**Reaction 3**), particularly at grain boundaries or defect sites on polycrystalline Ni.[²²,⁶⁸]

While H chemisorption on Ni (NiH(a))[⁸⁴,⁸⁹,⁹⁰] is well established through temperature-programmed desorption (TPD) studies,[⁹¹,⁹²] Ni does not exhibit distinct underpotential deposition (UPD) features in acidic media, unlike Pt-group metals.[⁹³,⁹⁴] Thus, although NiH(a) may influence surface chemistry, it is not electrochemically resolvable under the present conditions. The absence of soluble Ni species in the NiH(a) oxidation pathway (NiH(a) → Ni + H⁺ + e⁻) and the clear jₙᵢ peak observed simultaneously with jₑ in AESEC-LSV at the a₂ peak indicate that NiOH(a), rather than NiH(a), is the dominant electroactive intermediate responsible for the a₂ peak and for the enhanced jₙᵢ following cathodic pretreatment.

The integrated AESEC-EIS analysis, supported by kinetic modeling and interfacial simulations, reveals a surface-mediated mechanism for Ni dissolution in acidic sulfate media, governed by dynamic formation and reactivity of the adsorbed intermediate. This species plays a pivotal mechanistic role, functioning either as a catalytic precursor for Ni²⁺ release or as a branching point toward passivation.

Following cathodic polarization, the Ni surface exhibited enhanced reactivity characterized by high NiOH(a) coverage, suppressed NiO formation, and increased jₙᵢ. Kinetic simulations demonstrated that the rate of NiOH(a) formation (**Reaction 13**) was approximately balanced by its electrochemical oxidation to soluble NiOH⁺ (**Reaction 14**), establishing a near steady state intermediate pool. The resulting pathway, Ni + H₂O → NiOH⁺ + H⁺ + 2e⁻, effectively served as a catalytic loop,[²¹,⁷⁷,⁹⁵] promoting continuous Ni²⁺ release. The congruence between jₙᵢ and jₑ* under these conditions (assuming zₙᵢ = 2) further supports this two-electron oxidation mechanism. By contrast, untreated Ni surfaces exhibited low NiOH(a) coverage and dominant NiO formation, driven by an imbalance in reaction rates: **Reaction 13** proceeded rapidly, while **Reaction 14** remained kinetically sluggish. The accumulated NiOH(a) was diverted toward passivating oxide (**Reaction 16**), as confirmed by both EIS results and dissolution kinetic
modeling coupled with AESEC. Under these conditions, $j_{Ni}$ consistently fell below $j_e^*$, and the $a_2$ peak was absent, hallmarks of a passivating interfacial regime.

A numerical simulation of potential-dependent surface coverages and interfacial pH (Figure 14), though phenomenological, elucidated the role of surface site density of NiOH(a) ($\beta_1$) in governing Ni dissolution, with $\beta_2$ (NiO) fixed at $10^{-6}$ mol cm$^{-2}$. In the high initial $\beta_1$ case ($10^{-4}$ mol cm$^{-2}$), both formation and turnover of NiOH(a) were accelerated, yielding low $\theta_1$ and enhanced interfacial acidification, which facilitated rapid, sustained dissolution consistent with a catalytic mechanism in which NiOH(a) acts as a transient, non-accumulating intermediate. In contrast, low initial $\beta_1$ case ($10^{-6}$ mol cm$^{-2}$) led to NiOH(a) accumulation and limited conversion to soluble products, reflecting a non-catalytic regime favoring passivation.

These findings underscore the critical role of interfacial site availability and intermediate dynamic in controlling the dissolution-passivation balance. Integrated AESEC-EIS with kinetic modeling and interfacial simulation provides a quantitative platform for dissecting complex electrochemical pathways.

<figure><img src="image_20.png" /><figcaption>**Figure 14.** Simulated potential-dependent evolution of NiOH(a) coverage ($\theta_1$), NiO coverage ($\theta_2$), and interfacial pH. Simulations were performed using the kinetic parameters in Table 2 under two representative surface site density conditions: high NiOH(a) site density ($\beta_1 = 10^{-4}$ mol cm$^{-2}$; dashed curves) and low site density ($\beta_1 = 10^{-6}$ mol cm$^{-2}$; solid lines). The site density of NiO, $\beta_2$, is fixed at $10^{-6}$ mol cm$^{-2}$. The differential equations presented in the **Supplementary Information, Eqs. A2.2 - A2.4** were solved using the backward differentiation formula method. The following parameters were used: $D_{H^+} = 9.3 \times 10^{-5}$ cm$^2$ s$^{-1}$ and $\delta_{H^+} = 20$ µm. Pre-exponential factors for Eq. A2.1 are $k_i$ ($i = 1, -1, 2, \text{and } 4$) = 0.001, with units as follows: $k_1$ [mol cm$^{-2}$ s$^{-1}$], $k_{-1}$ [cm s$^{-1}$], and $k_2$ and $k_4$ [s$^{-1}$]. Additionally, $k_5 = 10^{-7}$ s$^{-1}$.</figcaption></figure>
¹ and $b_i = 20 \text{ V}^{-1}$ are used. The activity coefficient of $H^+$ in 2 M $H_2SO_4$ is set to 0.1194,[⁹⁶] yielding a bulk pH of 0.32, which is used to estimate the interfacial pH profile from the steady state $H^+$ concentration at the interface ($\overline{C}_{H^+}$, Eq. A2.8).

## Conclusions

The combined AESEC-EIS approach enabled element-resolved impedance analysis of Ni dissolution in $H_2SO_4$, revealing distinct anodic and cathodic contributions to the overall response. At open circuit, dissolution kinetics were strongly influenced by $H(a)$: steady state conditions under AESEC-EIS led to higher surface $H(a)$ coverage and inhibited Ni oxidation, while dynamic potential sweeps in AESEC-LSV promoted desorption and enhanced dissolution. Cathodic pretreatment further modified surface reactivity by generating active intermediates or exposing bare Ni at high-energy sites, resulting in a new anodic peak and a catalytic $NiOH(a)/NiOH^+$ cycle. In contrast, untreated surfaces accumulate $NiOH(a)$, favoring $NiO$ formation and passivation. Modeling and simulations linked these behaviors to the balance between surface coverage, local acidification, and reactive site density, establishing AESEC-EIS as a powerful tool for dissecting dissolution mechanisms at reactive metal interface.

## Key findings:

* AESEC-EIS enables simultaneous element-resolved impedance and dissolution kinetics, distinguishing anodic and cathodic contributions.

* Surface hydrogen coverage critically governs the balance between dissolution and inhibition.

* Cathodic pretreatment induces a reactive surface state that activates Ni oxidation through a catalytic $NiOH(a)/NiOH^+$ cycle.

* Interfacial modeling links local acidification and reactive site density to the transition between dissolution and passivation regimes.

## Supplementary Information:

* **A1:** Discussions on possible effect of harmonics on the shoulder peaks in **Figure 5(b)** (PDF).

* **A2:** EIS kinetic modeling calculations (PDF).

## Author contributions

J.H. conceived the main conceptual ideas, planned/conducted the AESEC-EIS measurements and following analysis, and wrote the manuscript. B.T. developed and analyzed the EIS kinetic model and edited the
manuscript. K.O. conceived the AESEC-EIS coupling, analyzed AESEC data, developed the kinetic model, and edited the manuscript.

**Acknowledgement**

This work was supported by the French National Research Agency, *Agence Nationale de Recherche - Jeune Chercheur et Jeune Chercheuse (ANR-JCJC)*, Grant No. ANR-22CE08-0015-01 (*QUEENE*), and by the French government's "France 2030" initiative through the *PEPR-DIADEM* (Priority Research Programs and Equipment - Integrated Devices for Accelerating the Deployment of Emerging Materials) program, managed by the *ANR*, Grant No. ANR-23-PEXD-0006. K. Ogle was supported by the *ANR*, Grant No. ANR-20-CE08-0031 (*Tapas 2020*). This work was also supported by the European Research Council Starting Grant (*ERC-STG*) under the European Union's Horizon 2020 research and innovation program (Grant Agreement No. 101220159). The authors express their gratitude to Prof. François Huet (Sorbonne Université, LISE, CNRS) for useful discussions.

**Competing interests**

The authors declare no competing financial interests.
## Graphical abstract

### Element-resolved EIS

<figure><img src="image_21.png" /></figure>

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