# Capturing Electrochemically Galvanic Exchange Events between Silver and Gold in Nonequilibrium and Equilibrium Realms

Bonito Aristide Karamoko, Erwan Oliviero, Yaovi Holade

► To cite this version:

Bonito Aristide Karamoko, Erwan Oliviero, Yaovi Holade. Capturing Electrochemically Galvanic Exchange Events between Silver and Gold in Nonequilibrium and Equilibrium Realms. Chemistry–Methods, 2025, pp.e202500030. 10.1002/cmtd.202500030. hal-05136675

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# Capturing Electrochemically Galvanic Exchange Events between Silver and Gold in Nonequilibrium and Equilibrium Realms

Bonito Aristide Karamoko, Erwan Oliviero, and Yaovi Holade*

The galvanic replacement reaction triggered by the electrochemical potential difference between two metals enables the design of multifunctional nanostructures for catalysis, sensing, and energy. However, there is still limited knowledge of the dynamics in the nonequilibrium and equilibrium realms to maximize material access or prevent structure damage. A methodology is reported to capture galvanic exchange events between atoms of electro-deposited bare Ag particles and aqueous $Au^{(III)}Cl_4^-$ species. As the population of $Au^{(III)}Cl_4^-$ species increases from the stoichiometric value to two orders of magnitude, the open-circuit potential

parameter that determines galvanic exchanges in the nonequilibrium and equilibrium domains follows a quasi-sigmoidal shape. An autocatalytic reaction network mechanism is postulated. The galvanic reaction can be interrupted whenever required to yield stable, separable intermediates that increase the rate of their own formation. The knowledge of the $Au^{(III)}Cl_4^-$ concentration-dependent potential-time curves permits to drastically reduce the reaction duration, from 85 to 13 min, while maintaining the material structure and properties, which will undoubtedly inspire new insights into other galvanic exchange processes.

## 1. Introduction

In recent decades, bimetallic materials have attracted interest in various fields such as catalysis, environmental science, medicine, and energy. Among the many synthetic routes for tuning the structure, galvanic replacement reaction (GRR) is a versatile method to obtain porous and hierarchical bimetallic structures

with adaptable properties.[1–8] GRR is thermodynamically, a spontaneous redox reaction driven by the difference of electro-chemical potential between two metals as opposed to the Kirkendall effect (KE) driven by the difference of the mutual diffusion coefficient of two matters.[9–11] In addition to beneficial effects for controlling particle structure at atomic scale, the knowledge of the thermodynamic and kinetic data of both GRR and KE are particularly useful for preventing or limiting severe material damages (void formation in alloys, dissolution of metals, sludge formation during metal plating, decrease of adhesion, and blistering of the coating).[3]

GRR entails the partial or complete replacement of atoms of the sacrificed metal, that is, the metal with the lowest redox potential. Consequently, the atoms of the metal with the highest redox potential take the place of the outgoing atoms, resulting in a porous bimetallic structure and/or lattice strains depending on the overall reaction stoichiometry.[1,12,13] This is the case for the GRR between Ag(0) and Au(III), as the standard redox potential of $AuCl_4^-$ / Au ($E^0 = 1.00 V_{SHE}$ (standard hydrogen electrode (SHE)) is more positive than that of $Ag^+/Ag$ ($E^0 = 0.80 V_{SHE}$) when considering bulk materials. Indeed, Pattadar et al.[4] showed that decreasing the size of particles results into reversing the thermodynamics of GRR between silver and gold or platinum, referred to as antigalvanic,[14] evoking the seminal Plieth's model[14,15] that links the metal oxidation potential to the particle size and the surface stress.[16] Despite a number of strategies in recent years to interrogate the GRR between silver and gold,[1,4,10,17–20] real-time monitoring of the dynamics of this redox reaction is limited. Various forms of silver crystals are typically the sacrificial template to synthesize porous (as at least two silver atoms are replaced by one atom of the incoming metal) bimetallic materials based on Au, Cu, Pt, Pd, etc.[3,8,10,17,18,20,21] However, most research focused on preparing unsupported metal particles often dispersed in

B. A. Karamoko, Y. Holade
Institut Européen des Membranes

IEM
UMR 5635
Université de Montpellier

ENSCM
CNRS
34090 Montpellier, France

E-mail: yaovi.holade@umontpellier.fr

E. Oliviero
Institut Charles Gerhardt Montpellier

ICGM
UMR 5253
Université de Montpellier

ENSCM
CNRS
34090 Montpellier, France

E. Oliviero
MEA Platform

Université de Montpellier
34090 Montpellier, France

Y. Holade
Institut Universitaire de France (IUF)

75005 Paris, France

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Supporting information for this article is available on the WWW under https://doi.org/10.1002/cmtd.202500030

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

© 2025 The Author(s). Chemistry - Methods published by Chemistry Europe and Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
solution with organic capping agents, which presents several challenges for accessing the true open-circuit potential (OCP) parameter that determines the evolution of GRR both in the nonequilibrium and equilibrium realms.[22–24] The commonly employed approach involves first preparing particles of the less noble metal in solution before introduction into a galvanic replacement mixture containing ionic species of the more noble metal, or vice versa.[25–27] This method poses the challenge of collecting the particles after GRR and transferring them to supports, as they may remain contaminated with organic surfactants, which is detrimental to optimizing their electrochemical response.[4] Therefore, preparing pristine metals directly on electrodes is essential to overcoming these challenges by capturing the GRR events and fully harnessing the potential of the derived materials.[4,28,29] Direct synthesis of particles on substrates can be achieved through several methods.[30–32] Among these, electrodeposition allows producing high-purity particles with precise control of the surface density by adjusting different parameters (current, potential, electrolyte, duration, etc.). Rasouli et al.[24] proposed Pd-Ni prepared by GRR between nickel electrodeposited on glassy carbon and [PdCl₄]²⁻ in aqueous solution, which highlighted a sigmoid shape of OCP-time profiles without any impact of the reactants on the duration of the GRR. While our earlier study of GRR between gold and silver electrodeposited on a gas diffusion electrode[18] showed a sigmoid shape, seminal study of Lo et al.[23] did not witness that when performing GRR of silver by palladium. We question herein whether GRR events between silver and gold with fine control over the gold salt concentration can be monitored electrochemically by linking the OCP-time profiles to an autocatalytic reaction network mechanism,[33,34] whereby the intermediates are stable, separable compounds. This end, we hypothesized that OCP monitoring as a function of time and the reactant concentration will provide accurate knowledge of GRR events, since OCP captures the collective and synchronous motions of the participating electrons, ions, and atoms.

## 2. Results and Discussion

### 2.1. Dynamics of Galvanic Replacement Reaction of Silver by Gold

To probe the above hypothesis, we electrodeposited bare Ag particles on a glassy carbon electrode (GCE) and then controlled the kinetics of GRR between silver and gold by regulating the population of Au(III) species in the solution while monitoring OCP to enhance the understanding of GRR dynamics. Operating the GRR in Figure 1a with a large excess of Au(III)Cl₄⁻ pushes the OCP (t > t₀) toward the Au(III)Cl₄⁻/Au(0) equilibrium potential as shown in Figure 1b since the mixed potential of the involved redox couples is expected to increase when GRR is running.[35] Since GRR is a redox reaction, the net equation can be described by three scenarios: 1) alloying reaction (Equation (1); 2) dealloying reaction (Equation (2)); and 3) full replacement (Equation (3)), where Kₐeq referred to as the equilibrium constant and a referred to as the activity. For micro or bulk materials, the redox potential data in Figure 1a indicate that Equation (3) is a quantitative reaction with an equilibrium constant Kₐeq,3 = 1.4 × 10¹⁰ at 25 °C. In fact, Figure 1b of the current-potential curves shows that during the GRR, if none of the reactants involved is in excess, the amplitude of the current corresponding to each half-cell decreases, causing OCP change. At the start of the reaction, the system is composed of silver atoms (capable of undergoing an oxidation reaction, the anode of the galvanic cell) and gold(III) ions (capable of undergoing a reduction reaction, the cathode of the galvanic cell). Therefore, both electron transfer and mass transport kinetics are also included in the GRR system.

$$x\text{AuCl}_4^- + 3\text{Ag} \rightleftharpoons \text{Ag}_{(3-x)}\text{Au}_x + 3x\text{Ag}^+ + 4x\text{Cl}^-; K_{\text{eq},1} = \frac{(a_{\text{Ag}^+})^{3x} \times (a_{\text{Cl}^-})^{4x} \times a_{\text{Ag}_{(3-x)}\text{Au}_x}}{(a_{\text{AuCl}_4^-})^x \times (a_{\text{Ag}^-})^3} \quad (1)$$

<figure><img src="image_5.png" /><figcaption>Figure 1. Developed methodology to access the dynamics of GRR. a) OCP measurement. b) Behavior of current-potential curves throughout GRR: Half-cell determination of OCP, C represents the concentration of dissolved species (ions), N represents the number of atoms for solid species (particles), t₀ represents the initial time, tₙ represents the final equilibrium time, and tᵢ represents the intermediate time (t₀ < tᵢ < tₙ).</figcaption></figure>
<figure><img src="image_6.png" /></figure>

$$
\begin{align}
& (1-x)\text{AuCl}_4^- + \text{Ag}_{(3-3x)}\text{Au}_x \rightleftharpoons \text{Au} + (3-3x)\text{Ag}^+ \nonumber \\
& \qquad + 4(1-x)\text{Cl}^-, K_{\text{eq},2} \nonumber \\
& = \frac{(a_{\text{Ag}^+})^{3-3x} \times (a_{\text{Cl}^-})^{4-4x} \times a_{\text{Au}}}{(a_{\text{AuCl}_4^-})^{1-x} \times a_{\text{Ag}_{(3-3x)}\text{Au}_x}} \tag{2}
\end{align}
$$

$$
\begin{align}
& \mathrm{AuCl}_{4}^{-} + 3\mathrm{Ag} \rightleftharpoons \mathrm{Au} + 3\mathrm{Ag}^{+} + 4\mathrm{Cl}^{-}, K_{\mathrm{eq},3} \nonumber \\
&= \frac{(a_{\mathrm{Ag}^{+}})^{3} \times (a_{\mathrm{Cl}^{-}})^{4} \times a_{\mathrm{Au}}}{(a_{\mathrm{Ag}})^{3} \times a_{\mathrm{AuCl}_{4}^{-}}} \tag{3} \\
&= \exp \left[ \frac{3F}{RT} \left( E^{0}_{\mathrm{AuCl}_{4}^{-}/\mathrm{Au}} - E^{0}_{\mathrm{Ag}^{+}/\mathrm{Ag}} \right) \right] \nonumber
\end{align}
$$

It is known that the standard redox potential decreases when the particle size diminishes, evening leading to antugal- vanic reaction.[4,14,15] So to circumvent such effects and also crystallographic orientation, we targeted micrometer-sized particles and preferentially Ag(111) surface (Figure 2a–f). As

indicated in Figure 1b, AgCl precipitation can interfere with GRR, so we performed GRR in brine solution which allows suppressing that according to silver-chloride speciation dia- grams.[36,37] One major goal for GRR of Ag–Au systems is target- ing Equation (1) wherein remaining 5–20 at% of Ag alloyed with 95–80 at% Au to form porous structure with cumulated three beneficial effects, namely, long-range geometric lattice deformation (strain), synergy (multiple metals), and short- range electronic charge transfer effects.[38]

2.2. Characterization of Electrodeposited Silver Electrodeaterial (GCE-Ag)

Aiming to locate the appropriate potentials for the ammonia- mediated electrodeposition of Ag microplate onto GCE, we first performed linear sweep voltammetry (LSV) measurements. The control experiments are shown in Figure 2a, the molar ratio

<figure><img src="image_7.png" /><figcaption>Figure 2. Ammonia-mediated electrodeposition of Ag particles onto a GCE. a) LSV in Ar-saturated solution [20 mV s⁻¹, 25 °C, 200 mM KNO₃, 0.06 mM AgNO₃, 8.2 mM NH₄OH (R = n(NH₄OH)/n(AgNO₃) = 137)]. b) CV (20 mV s⁻¹, 25 °C, 0.1 M KOH) before (GCE) and after electrodeposition (GCE-Ag). SEM of GCE-Ag synthesized: c) without and d) ammonia. e) TEM image, and f) SAED pattern of GCE-Ag.</figcaption></figure>
<figure><img src="image_8.png" /></figure>

$R=n(NH_4OH)/n(AgNO_3)$ was fixed to 137 (extended data in Figure S1a, Supporting Information, $R=0-1666$). The drastic shift in the commencement potential of Ag(I) to Ag(0) reduction, from $-0.16$ to $-0.35$ $V_{MSE}$ (mercury-mercurous sulfate electrode) confirms the complexation and the ability to regulate the electrodeposition thermodynamics and kinetics. These experimental values are close to the Nernstian potentials of $E_{eq}=-0.09$ and $-0.27$ $V_{MSE}$ for $Ag^+/Ag$ and $Ag^+(NH_3)_2/Ag$, respectively. The sharp current density increase at more negative potentials is credited to the hydrogen evolution reaction (HER) at the first electrodeposited silver clusters that catalyze HER, which is the foundation of the electroshock synthesis where the coalescence and the fusion of clusters into bigger particles is regulated.[31,39] Based on these results, we operated the chronoamperometry (Figure S1b, Supporting Information) at different potentials from $-0.38$ to $-0.70$ $V_{MSE}$ and reaction duration from 15 min to 4 h.

We probed the effectiveness of silver deposition on the working electrode (GCE-Ag) by three complementary methods, cyclic voltammetry (CV) in 0.1 M KOH electrolyte (Figure 2b), electron microscopy (Figure 2c-e), and electron diffraction (Figure 2f). The CV of Figure 2b confirms the successful electrodeposition of Ag as the main reduction peak for GCE-Ag at $\approx 0.9$ $V_{RHE}$ (reversible hydrogen electrode, calibration curve in Figure S2, Supporting Information) is characteristic of Ag₂O reduction (electrical charge of $Q=291$ µC) that was formed during the forward scan.[40,41] Using a metric charge of $Q_0 = 420$ µC cm⁻² for the Ag₂O monolayer[40] leads to an electrochemically active surface area of $Q/Q_0 = 0.69$ cm², which is relatively low for 73 µg of Ag (Figure S1b, Supporting Information) on GCE of 0.20 cm² thereby suggesting the formation of rather big particles. To probe such a hypothesis, we performed scanning electron microscopy (SEM, Figure 2c-e and S3, Supporting Information). While the microscale size is confirmed, different morphologies were obtained as a function of the molar ratio $R=n(NH_4OH)/n(AgNO_3)$. The morphology changed from a dendritic form for $R=0$ (Figure 2c) to a metallic-ball-shaped form for $R=1666$ (Figure S3, Supporting Information) via plate form (Figure 2d), which demonstrates that the morphology of the deposited silver can be controlled by adjusting the concentration of ammonia. Based on the additional investigated parameters of the applied potential (from $-0.38$ to $-0.70$ $V_{MSE}$, Figure S4, Supporting Information) and the duration (from 15 min to 4 h, Figure S5, Supporting Information) to account for the nucleation and the growth stages, the optimized conditions were $-0.45$ $V_{MSE}$, $R=137$, and 4 h for targeted hexagonal-shaped particles likely exposing Ag(111). To confirm this, we implemented transmission electron microscopy (TEM, Figure 2e). The intensity profiles extracted from high-resolution TEM (HRTEM) (Figure S6, Supporting Information) show a d-spacing value of 2.55 Å corresponding to Ag(111) since the final atomic composition is 92 at% Ag and 8 at% O from the scanning transmission electron microscopy coupled energy-dispersive X-ray spectroscopy (STEM-EDX) analysis reported in Figure S7, Supporting Information. The presence of oxygen can be explained by the natural oxidation of metals when exposed to air. To assess the crystallinity of the synthesized Ag particles, we performed selected area electron

diffraction (SAED). The SAED pattern of GCE-Ag in Figure 2f is made of small spots forming rings, thus highlighting the crystallinity of the Ag particles.

## 2.3. Monitoring Galvanic Replacement Reaction (GRR) of Silver at Glassy Carbon Electrode (GCE) by Gold (III) in Solution

Having characterized the template GCE-Ag, we further performed the real-time monitoring of the dynamics of the GRR through OCP based on the theory of Figure 1b and experimental set-up of Figure 1a wherein GCE-Ag was connected to a working electrode. The reference electrode was Ag/AgCl. It is worth noting that the results of Figure 3 for the control experiments on OCP measurements with pristine GCE (without silver particles), but with gold(III) species in the electrolyte solution, show only a Nernstian response of spontaneous increase of the onset potential, where the electrochemical Au(III) to Au(0) reduction reaction should experimentally begin (Equation (4)). After OCP stabilization, GCE analysis showed no trace of gold, which was to be expected since there was no reducing element to transform Au(III) to Au(0), referred to as “gold electrodeposition,” a well-known field.[39,42,43] According to Equation (4), the thermodynamic potential (theoretical) potential where $Au^{(III)}Cl_4^-$ to $Au^{(0)}$ reduction would start is $E_{eq}(Au^{(III)}Cl_4^-/Au^{(0)}) = 0.89$ V versus SHE = 0.69 V versus Ag/AgCl for 0.142 mM $KAuCl_4$. We can see that in the presence of silver, the profile of OCP(t) until the equilibrium potential is radically different. So, emphasis was next placed on varying the concentration of $KAuCl_4$ from 0.014 to 1.419 mM, which corresponds to molar ratio $R_{GRR} = n(KAuCl_4)/n(Ag) = 0.31-31.30$. Theoretically, the full exchange of silver by gold corresponds to $R_{GRR} = 0.33$, Equation (3). Figure 4a shows the OCP profiles, before ($t < 10$ min) and after ($t > 10$ min) injecting $KAuCl_4$ (0.014 mM ($R_{GRR} = 0.31$) is reported in Figure S8, Supporting Information, since GRR kinetics was too slow to reach equilibrium), which manifested a quasi-

<figure><img src="image_9.png" /><figcaption>**Figure 3.** Control experiments on the effect of gold(III) and silver(0) on the behavior of OCP versus time in the absence of Ag particles onto GCE and in the presence of Ag particles onto GCE (referred to as ‘GCE-Ag’): GCE in 3.5 M $NaCl$, GCE in 3.5 M $NaCl + 0.142$ mM $KAuCl_4$, and GCE-Ag in 3.5 M $NaCl + 0.142$ mM $KAuCl_4$.</figcaption></figure>
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>Figure 4. a) OCP versus time during GRR with different concentrations of KAuCl₄ [25 °C, magnetic stirring, 3.5 M NaCl + KAuCl₄]. b) Effect of Au(III) concentration on the time (denoted "t") required to reach equilibrium in panel (a). c,d) SEM images of silver-gold structures after GRR: c) 0.142, d) 0.709, and e) 1.419 mM KAuCl₄. f,g) TEM images, and h) SAED patterns.</figcaption></figure>

sigmoid shape that is often a signpost of an autocatalytic behavior.[33,34,44,45] As the GRR can be stopped at any time to yield stable and separable intermediate Ag₁⁻ₓAuₓ that enhances the rate of its own formation,[4] we postulated an autocatalytic reaction networks mechanism, which is accounted in the chemical reactions in solution. Mechanically, Ag first reacts with Au(III) to form Ag₁⁻ₓAuₓ, the latter reacts with the remaining Au(III) to give Ag₁⁻ₓ₂Auₓ₂ and so on in a cascade until the stable structure of Ag₁⁻ₓₙAuₓₙ (0 < xₙ < 1) at equilibrium with galvanic replacement medium is reached.

$$ \begin{aligned} \mathrm{AuCl}_{4}^{-}(aq) + 3\mathrm{e}^{-} & \rightarrow \mathrm{Au}(s) + 4\mathrm{Cl}^{-}(aq), & E_{\mathrm{eq}}(\mathrm{AuCl}_{4}^{-}/\mathrm{Au}) \\ & = E^{0}(\mathrm{AuCl}_{4}^{-}/\mathrm{Au}) + \frac{RT}{3F} \ln \left[ \frac{a_{\mathrm{AuCl}_{4}}}{(a_{\mathrm{Cl}})^{4}} \right] \end{aligned} \quad (4) $$

Herein, both the length of the induction period or the lag phase (I) and the exponential phase (II) are strongly influenced

by the population of AuCl₄⁻ species near the GCE-Ag interface while the value of OCP at the saturation phase (III) is quite similar, about 0.72 V<sub>Ag/AgCl</sub> (0.92 V<sub>SHE</sub>), which is closed to E<sub>eq</sub>(Au(III)Cl₄⁻/Au(0)) = 0.89–0.92 V versus SHE = 0.69–0.72 V versus Ag/AgCl for 0.142–1.419 mM KAuCl₄ (Equation (4)). For GCE-Ag material (Figure 3 and 4a), upon introducing AuCl₄⁻, the rapid shift in potential and stabilization is attributed to an increase in Ag(I) concentration due to the diffusion of AuCl₄⁻ ions near the electrode surface[18,23,24,46] since the GDE electrode does not stabilize around 0 V versus Ag/AgCl electrode (Figure 3). During the lag phase (I), the quasi-stable potential of 0–0.05 V<sub>Ag/AgCl</sub> is under both the electron transfer kinetics of cathodic reaction and the diffusion of AuCl₄⁻. The anodic reaction sites switch from silver atoms initially to Ag₁⁻ₓAuₓ wherein the diffusion of both Ag and Au atoms (possible contribution of hidden Kirkendall effect[9–11]) results in a substantial increase of the potential in the region (II).
<figure><img src="image_12.png" /></figure>

Figure 4b shows the log-log relationship between the concentration of AuCl₄⁻ and the cumulated duration of lag phase (I) and the exponential phase (II), which is referred to as τ and is 85 ± 5, 26 ± 3, 18 ± 2.7, and 13 ± 2 min for 0.142, 0.355, 0.709, and 1.419 mM AuCl₄⁻, respectively. The significant decrease of τ when augmenting the population of AuCl₄⁻ species supports our hypothesized autocatalytic reaction networks mechanism. Attempts to find a model (theoretical or numerical) to fit these experimental data have been inconclusive until now. Nevertheless, we would argue that the answer, which would be of great interest to the entire scientific community, lies in taking into account both the electron transfer and the mass transport kinetics (migration in an electric field, diffusion due to a chemical potential gradient, convection in a velocity field, steric effect, Frumkin correction).<sup>[47-51]</sup> As such, the implementation of

the recently developed Frumkin-Butler-Volmer size-modified Poisson-Nernst-Planck (FBV-SMPNP) for CO₂ electrolysis[⁵⁰] might work for GRR. Figure 4c-e and S9, Supporting Information, show the morphological evolution of the Ag₁-xAuₓ structure when the GRR was stopped upon reaching the saturation phase. When the population of AuCl₄⁻ species augments, the formation of holes becomes more pronounced, while the overall morphology of the microplates remains integral. At 0.142 mM, not all the silver atoms are attacked by Au(III) ions, which explains the gradual increase in the OCP curve. In contrast, at 1.419 mM, the introduction of Au(III) results in the formation of a highly porous structure, supporting the short duration of the lag phase, driven by the fast oxidation of Ag(0) by the large population of Au(III) species.[⁵²] In such a situation, it is also possible that subsequent gold deposits may form at the first gold sites rather than exactly replacing the

<figure><img src="image_13.png" /><figcaption>**Figure 5.** STEM-EDX elemental mappings of a) Ag, b) Au, and c) Ag + Au. d) EDX line-scan intensity profiles and STEM bright field image in the inset. e,f) Comparative performance, in 0.1 M KOH + 0.1 M D-glucose, of GCE-Ag_GRR-x, x = concentration of KAuCl₄ (mM): e) CV (50 mV s⁻¹, 25 °C) and f) Complex-plane Nyquist impedance at 0.25 V<sub>RHE</sub> (inset the equivalent electrical circuit R<sub>Ω</sub> + Q<sub>CPE</sub>/R<sub>ct</sub>) for 0.1 M KOH + 0.1 M D-glucose at 25 °C.</figcaption></figure>
<figure><img src="image_14.png" /></figure>

silver positions within the structure, due to the more favorable energy barrier and the possible contribution of the Kirkendall effect induced by the difference in the mutual diffusion coefficients of the two metals.

## 2.4. Compositional Analysis and Electrochemical Properties Postgalvanic Replacement

Having almost the same potential in region (III) suggests that, irrespective of the initial $AuCl_4^-$ concentration, the $Ag_{1-x}Au_x$ atomic composition would be the same, calling for elemental analysis. We have therefore performed the compositional analysis by STEM-EDX. Indeed, both TEM (Figure 4f,g) and SAED (Figure 4h) analysis confirm the porous structure and the crystallinity of the particles after GRR; however, TEM intensity profiles after GRR are inconclusive since silver and gold have similar lattice parameters (Figure 1a), the reason why X-ray diffraction was not done. The elemental maps (Figure 5a–c) and the STEM-EDX line profiles (Figure 5d) reveal a coexistence of Ag and Au and their homogeneous distribution at the atomic level. The hexagonal shape of the Ag plates is preserved during GRR, even at high molar ratio $R_{GRR} = 31.30$, which is 100 times the stoichiometric one (Equation (3)). The determined atomic composition (Figure S10, Supporting Information) of 8 at% Ag and 92 at% Au is consistent with the bulk measurement (Table S1, Supporting Information), wherein gold represents $90 \pm 2$ at%. Consequently, even though SEM of Figure 4c–e showed a morphological difference, the compositional analysis highlights that the final equilibrium composition is independent of the concentration of the $KAuCl_4$ under these working conditions. These new results, obtained by coupling direct monitoring and ex situ analysis, clearly demonstrate a significant time reduction in the GRR of Ag by Au, from over 85 min to around 13 min.

We then investigated whether or not this gain in reaction time could be accompanied by at least a maintenance of the catalytic activity of the materials. We therefore considered the electrooxidation of glucose as a model reaction for the electrocatalytic upgrading of cellulosic biomass to platform molecules. The CV results of Figure S11a, Supporting Information, confirm that silver alone does have a noteworthy electrocatalytic activity for glucose oxidation, the positive peak at 1.30 $V_{RHE}$ corresponds to Ag-to-Ag₂O conversion, and the reverse process being the peak at 0.85 $V_{RHE}$. Gold-based catalysts have been extensively studied for glucose oxidation due to their enhanced catalytic activity and greater resistance to surface poisoning.[42,53,54] Hence, combining gold and silver by GRR to create porous alloy catalysts could potentially improve the performance. The CV of $Ag_{1-x}Au_x$ electrodes in the absence of D-glucose in Figure S11b, Supporting Information, highlights the change in CV of pristine Ag, confirming the presence of gold. Although the atomic composition is similar, the electrochemical behavior is slightly altered in the oxide-forming region, impacting glucose oxidation processes (Figure 5e). Glucose oxidation starts at $\approx 0.1 V_{RHE}$; however, the catalyst prepared by the GRR condition of 0.7–1.4 mM $KAuCl_4$ shows higher electrocatalytic kinetics in the potential ranges of 0.2–0.5 $V_{RHE}$, which could be explained by the increased

porosity (SEM images) that augments the number of active sites. Indeed, in Figure 5e, the oxide peak area is lower for GCE-Ag_GRR-1.419 than for GCE-Ag_GRR-0.355, meaning that normalization of Figure 4g by the electrochemically active surface area would lead to a much higher specific activity for GCE-Ag_GRR-1.419 than for GCE-Ag_GRR-0.355. This contributes to the reduction in charge transfer resistance ($R_{ct}$) as shown by potentiostatic electrochemical impedance spectroscopy (EIS) at 0.25 $V_{RHE}$ (Figure 5f and Table S2, Supporting Information), $R_{ct} = 127, 71$, and $87 \Omega \ cm^2$ for GCE-Ag_GRR-0.355, GCE-Ag_GRR-0.709, and GCE-Ag_GRR-1.419, respectively. Looking ahead, it would certainly be of fundamental interest to seek theoretical models capable of predicting these experimental results or, in a more practical context, to integrate the catalyst synthesis methodology into practical electrolyzer supports, such as gas diffusion electrodes.

## 3. Conclusion

In summary, this work contributes new elements to our understanding of galvanic replacement reactions, whereby the partial or complete replacement of the metal with the lowest redox potential by atoms of the metal with the highest redox potential occupying the place of the outgoing atoms generally results in a porous multimetal structure and/or lattice deformations depending on the overall stoichiometry of the reaction. Directly capturing the dynamics of this redox process in the nonequilibrium and equilibrium realms via OCP transients enables the proposing of an autocatalytic reaction network mechanism. The duration of the induction period or lag phase and the exponential phase are strongly influenced by the population of $Au^{(III)}Cl_4^-$ species near the $Ag^{(0)}$ interface, while the OCP value at the saturation phase (III) is around 0.92 $V_{SHE}$, which is closed to $E_{eq}(Au^{(III)}Cl_4^- / Au^{(0)}) = 0.89–0.92 V_{SHE}$ for 0.142–1.419 mM $KAuCl_4$. While offering a versatile synthesis route that drastically reduces the duration of the galvanic reaction, from 85 to 13 min, without altering the structure and properties of the material, attempts to model this quasi-sigmoidal pattern of experimental data remain inconclusive to date. We are therefore releasing our research findings to enable others to move forward where we, for the moment, cannot. Indeed, the answer, which would be of great interest to the entire scientific community, would lie in taking into account both the kinetics of electron transfer and that of mass transport (migration, diffusion, convection, steric effect, Frumkin correction). The present methodology, which contributes to the knowledge of how electrons, atoms, and ions move synchronously to form stable porous structures from the exchange ratio of 3Ag per 1Au for the silver-gold system, could inspire new advances in nanostructure synthesis by exploring more complex reaction systems through different galvanic exchange ratios. Indeed, quantifying the driving force of the galvanic exchange by obtaining the missing thermodynamic and kinetic data (rate constant and redox potential as a function of time and nanoparticle size) is fundamental to proposing new catalytic architectures such as high-entropy porous materials for various practical applications in catalysis, sensing, and energy.
<figure><img src="image_15.png" /></figure>

# 4. Experimental Section

## Chemicals and Materials

The nonrotating stainless-steel rod connected to a Change Disk RDE PTFE tip (Glassy Carbon Disk Insert, 5.0 mm OD × 4.0 mm thick) was purchased from Pine Research Instrumentation (USA) and used as the working electrode, hereafter referred to as GCE. Prior to each electrochemical measurement, the glassy carbon disk insert was polished mechanically and sequentially with an alumina slurry of decreasing particle sizes of 3, 1, and 0.05 µm. After washing with ultrapure water, it was cleaned by sonication for 5 min. Potassium hydroxide (KOH, 99.98% (trace metal basis), Acros Organics), potassium nitrate (KNO₃, Sigma-Aldrich, 99.0% min), silver nitrate (AgNO₃, Premium, 99.9995% (metals basis), Alfer aesar), potassium tetrachloroauric (III) (KAuCl₄·xH₂O, 99.9%, Alfer aesar), ammonia solution (NH₄OH, 30%, for analysis-ACS, Carlo Erba Reagents), sodium chloride (NaCl, 100%, VWR), and D-(+)-glucose 99.5%), Sigma-Aldrich) were used as received. Nitrogen (N₂, 4.5) gas was ultrapure (Air Liquide, France) and all named ultrapure water (MQ) is provided by Milli-Q Millipore source (18 MΩ cm at 20 °C). For electrochemical experiments, all glassware used was first immersed for at least 12 h in an acidified potassium permanganate solution to oxidize all impurities, especially organic ones (KMnO₄ at 3 g L⁻¹ and H₂SO₄ at 2.25 mol L⁻¹), which is followed by washing with a solution of water + H₂SO₄ + H₂O₂ (1 vol. each).<sup>[55-57]</sup> For the used reactor for the synthesis, a washing by aqua regia (2 vol. HCl + 1 vol. HNO₃) was implemented for 15–30 min. We end by rinsing with cold and hot water in the following order: cold, hot, cold.

## Electrodeposition of Silver onto Glassy Carbon Electrode

The electrodeposition experiments were performed at controlled temperature of 25 ± 2 °C in a conventional three-electrode glass cell (single compartment). A glassy carbon plate electrode (Alfer Aesar) and a mercury-mercurous sulfate electrode (Hg|Hg₂SO₄|K₂SO₄ saturated, MSE, OrigaLys ElectroChem SAS) were used as the counter and reference electrodes, respectively. The silver particles were deposited onto the GCE, which served as the working electrode in the electrochemical setup. Typically, a mixture composed of 94 mL of 200 mM KNO₃, 6 mL of 1 mM AgNO₃ stock solution (prepared in 200 mM KNO₃), and varying volumes of ammonia for R = n(NH₄OH)/n(AgNO₃) (R = 0, 62, 137, and 1666) are gently stirred and deaerated with N₂ for about 15 min before the experiment. The electrodeposition was carried out using chronoamperometry. The working electrode was first subjected to an open-circuit potential measurement for 30 s (excluding the previous degassing step), followed by the silver deposition measurement; the used potentiostat was VIONIC (Metrohm). The deposition potentials, ranging from -0.38 to -0.70 V versus MSE, were selected based on the control LSV data gathered in Figure S1, Supporting Information. For the optimization, the electrodeposition was performed for different durations: 15 min and 1, 2, and 4 h.

## Galvanic Replacement Reaction between Silver (0) and Gold (III)

GRR between silver (0) and gold (III) was performed with the electro-deposited silver obtained under the optimized conditions of R = n(NH₄OH)/n(AgNO₃) = 137, a potential of -0.45 V versus MSE and a duration of 4 h (about 0.68 ± 0.03 µmolAg, that is a loading of 3.46 µmolAg cm⁻² or 374 µgAg cm⁻²). GRR was carried out in a temperature-controlled glass cell at 25 ± 2 °C. The replacement reaction was carried out in a solution containing 12.872 mL of 3.5 M NaCl and 2.128 M stock solution of different concentrations of KAuCl₄ to obtain final concentration in the reactor of x mM KAuCl₄, wherein x = 0.014,

0.142, 0.355, 0.709, and 1.419, which correspond to 0.21, 2.13, 5.32, 10.63, and 21.28 µmolAur, respectively. First, the electrode was immersed in an outgassed 12.872 mL solution of 3.5 M NaCl, and the OCP was recorded using a small reference electrode (Ag/AgCl, CH Instruments, Inc.) while gently stirring. After stabilization, 2.128 mL of KAuCl₄ was introduced at a rate of 0.75 mL min⁻¹. The duration of the replacement reaction depended on the concentration of KAuCl₄. Once the equilibrium potential was reached, the reaction was quenched by rapidly rinsing the electrode several times with MQ water.

## Physicochemical Characterization

The morphology of the materials was analyzed using a Hitachi S4800 SEM. Elemental analysis was conducted locally via EDX. SEM-EDX analyses were also performed with a Zeiss EVO HD15 electron microscope. HRTEM and STEM were carried out using a JEOL 2200FS microscope equipped with a field emission gun operating at 200 kV. STEM-EDX was performed with an Oxford Instrument X-Max N 100 TLE 100 mm² windowless SDD detector.

## Electrochemical Measurements

The electrochemical measurements were performed using the above potentiostat (VIONIC, Metrohm, Violntello1.5 software). The reference electrode was Hg|HgO|KOH 0.1 M (MOE, RE-61AP Reference Electrode for alkaline solution, BAS Inc.). First, we performed the calibration using a platinum bulk as the working electrode, a MOE as the reference electrode, and a glassy carbon plate as the counter electrode. For all measurements conducted in KOH, the potentials were adjusted to the reversible hydrogen electrode (RHE). To this end, CV was recorded over a potential range of -0.9 to -0.8 V versus MOE under a continuous flow of high-purity H₂ from an onsite electrolyzer (HYGEN 200, CLAIND, Italy). The calibration relationship is given by E(V vs RHE) = E(V vs MOE) + ΔE, ΔE = 0.85 V at 25 °C in 0.1 M KOH, as indicated by the calibration curve shown in Figure S2, Supporting Information. The glucose oxidation reaction was conducted in a 0.1 M KOH containing 0.1 M D-glucose. Potentiostatic electrochemical impedance spectroscopy (EIS, 100 kHz to 100 mHz, 10 mV amplitude, 10 points per decade, unstirred solution) was performed at 0.25 V versus RHE. The voltammograms reported and potentials applied during EIS were not corrected by the iR-drop (ohmic resistance of 50–53 Ω cm² in 0.1 M KOH + 0.1 M D-glucose). All electrochemical measurements were performed using a conventional three-electrode temperature-controlled glass cell at (25 ± 2) °C.

## Acknowledgements

This work was funded by I-SITE Excellence Program of the University of Montpellier under the Investissements France 2030 (project NANOGATE), French National Research Agency (project MASTERS, grant no. ANR-22-CE43-0004), and Institut Universitaire de France (IUF).

## Conflict of Interest

The authors declare no conflict of interest.

## Author Contributions

**Bonito Aristide Karamoko:** data curation (equal); formal analysis (equal); investigation (equal); methodology (equal); validation
<figure><img src="image_16.png" /></figure>

(equal); writing—original draft (equal); writing—review and editing (equal). **Erwan Oliviero:** formal analysis (equal); investigation (equal); methodology (equal); writing—review and editing (supporting). **Yaovi Holade:** conceptualization (lead); data cura-tion (lead); formal analysis (lead); funding acquisition (lead); investigation (lead); methodology (lead); project administration (lead); resources (lead); supervision (lead); validation (lead); writing—original draft (lead); writing—review and editing (lead).

Data Availability Statement

The data that support the findings of this study are available from
the corresponding author upon reasonable request.

**Keywords:** autocatalytic reactions • electrochemical kinetics • electron transfer • galvanic replacement reactions • glucose electrooxidation

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Manuscript received: February 26, 2025

Revised manuscript received: April 18, 2025

Version of record online: