# Operando X-ray Absorption Spectroscopy of Co 3 Mn–CO 3 LDH: Formation and Structural Evolution under Electrochemical Conditions

Hani Farhat, Christine Mousty, Vanessa Prevot, Anthony Beauvois, Stéphanie

Belin, Valérie Briois, Claude Forano

► To cite this version:

Hani Farhat, Christine Mousty, Vanessa Prevot, Anthony Beauvois, Stéphanie Belin, et al.. Operando X-ray Absorption Spectroscopy of Co 3 Mn–CO 3 LDH: Formation and Structural Evolution under Electrochemical Conditions. Journal of Physical Chemistry C, 2024, 128 (49), pp.21023-21037. 10.1021/acs.jpcc.4c03883 . hal-04935470

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Operando X-ray Absorption Spectroscopy of

Co₃Mn-CO₃ LDH: Formation and Structural

Evolution under Electrochemical Conditions

Hani Farhatª, Christine Moustyª, Vanessa Prevotª, Anthony Beauvoisᵇ, Stéphanie Belinᵇ, Valérie Brioisᵇ*, Claude Foranoª*

ªUniversité Clermont Auvergne, CNRS, Clermont, Institut de Chimie de Clermont- Ferrand, F-63000 Clermont–Ferrand, France

ᵇSynchrotron SOLEIL, CNRS-UR1, BP 48, L'orme des Merisiers, 91192 Gif-sur-Yvette, France

KEYWORD

Layered Double Hydroxide (LDH); CoMn LDH Formation; Electrochemical Behaviour;

*Operando* Quick X-ray Absorption Spectroscopy (Q-XAS); Multivariate Curve Resolution with

Alternating Least Squares Analysis (MCR-ALS)
18 Abstract

The purpose of this paper is to elucidate the mechanism of formation of CoMn layered double
hydroxide (LDH) and to study the role of both metal cations, cobalt and manganese, in the
electrochemical behavior of this lamellar compound. Multivariate curve resolution with
alternating least square (MCR-ALS) analysis was applied to time-resolved synchrotron X-ray
Absorption Spectroscopy (QXAS) data, monitoring the speciation of Co and Mn valence state
under both Co₃Mn LDH formation and electrochemical processes. QXAS experiments at the Co
and Mn K-edges were recorded all over three steps of the synthesis process: i) fast
coprecipitation and two successive aging steps ii) under Ar and iii) in air, respectively. In the
first step, corresponding to the fast coprecipitation, a brucite-like Co/Mn(OH)₂ mixed phase
precipitates rapidly and no oxidation of metal cations occurs during the two first steps (i and ii).
While, a topotactic oxidation of Mn(II) into Mn(III) by oxygen occurs during the final drying
step under air atmosphere (iii), assisted by the intercalation of carbonate, leading then to the
formation of Co(II)₃Mn(III)-CO₃ LDH. Finally, the redox contribution of both metal cations,
Co(II) and Mn(III), in the electrochemical process is highlighted and a mechanism of electron
hopping between Co-Co sites is proposed.

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## Introduction

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Layered Double Hydroxides (LDH) containing transition metals (Mn, Fe, Co, Ni) have recently undergone renewed interests due to their potential applications in energy storage and conversion systems¹⁻² such as pseudo-capacitors³, oxygen reduction reaction⁴⁻⁵ and water splitting⁶ but also in catalysis⁷ and electrocatalysis⁸. The 2D structure of LDH is composed of alternative layers of [M²¹⁻ₓM³³ₓ(OH)₂]ₓ⁺ and [Xq⁻ₓ/q.nH₂O] interlayer domains. This structure allows for various combinations of divalent (M²¹) and trivalent (M³³) metals. Transition metals with mixed valence states (MVM), like CoFe, NiCo, NiMn, CoMn, CuCo, NiCoFe, are particularly interesting due to their potential to promote specific electrochemical properties⁹⁻¹⁰ such as redox or conducting properties. However, tailoring MVM composition in LDH, metal local structure and M-M vicinity remain a key strategy to control MVM synergistic behaviors for enhanced electrochemical performances⁷. While many synthetic routes have been used for the preparation of LDH-MVM, most of them were not effective in controlling accurately the metal valence state composition. LDH-MVM compositions containing Mn, Fe, Co, and Ni often exhibit a mix of +II, +III, and sometimes +IV oxidation states, depending on the association of MVM and the synthesis conditions. For LDH-MVM containing Mn, Fe, Co, Ni, the coexistence of +II (Mn, Fe, Co, Ni), +III (Mn, Fe, Co, Ni) and even +IV (Mn) oxidation states have been mentioned¹, ¹¹. However, the state of valence depends on both MVM association and conditions of LDH synthesis. The oxidation states and behaviors of transition metals like Ni, Co, Fe, and Mn within LDH are influenced by various synthesis methods and conditions. Recently, Tichit et al. ¹² reviewed comprehensive studies on synthesizing dicationic LDH, particularly Mg/Al LDH, revealing fast nucleation rates and multistep processes. Formation methods vary from
dissolution/reprecipitation for Mg/Al and Zn/Al LDH to surface reactions for M(II)/Fe(III) LDH, 58 with a one-step process for MII/Cr LDH. They emphasize the role of MVM on the LDH 59 formation processes. 60

Ni(II) typically remains in the +II state when combined with Co or Mn, except under specific 61 conditions like complexation with N-methylimidazole (MIM)¹³ or electrochemical reduction of 62 NO₃⁻ anions¹⁴. Fe can exhibit both +II and +III oxidation states when combined with Co, 63 maintaining a constant Fe(II)/Fe(III) molar ratio of 1 in some cases¹⁵⁻¹⁷. The oxidation state of 64 Co in LDH phases varies depending on synthesis conditions (e.g., presence of air, H₂O₂, or O₂ 65 during precipitation)¹⁸⁻¹⁹. The Co(II)/Co(III) molar ratio often self-adjusts to form the most 66 thermodynamically stable LDH phase, with partial oxidation of Co(II) into Co(III) not exceeding 67 50%²⁰. Mn can achieve various oxidation states from +II to +VII but obtaining a single oxidized 68 form in LDH is challenging. Monometallic Mn(II)Mn(III) LDH was synthesized recently²¹, and 69 Mn's oxidation degrees become more difficult to control when associated with d⁰ metal cation as 70 shown by X-ray photoelectron spectroscopy (XPS)²²⁻²³. Surprisingly, LDH based on Co/Mn 71 association has not been deeply investigated, yet Co and Mn hydroxide and oxohydroxide phases 72 display very active intrinsic electrochemical properties. Few papers report the interest combining 73 mixed valence states of cobalt and manganese in LDH structure for electrochemical applications. 74

CoMn LDH were evaluated as oxygen evolution catalysts²⁴⁻²⁷, pseudo-supercapacitors²⁸⁻³⁰ and 75 for H₂O₂ ³¹⁻³² and glucose electrode detection³³. Typically, these materials are synthesized using the 76 coprecipitation method (pH ~ 10) in the presence of an oxidizing agent (H₂O₂, O₂, air)²⁵, ³⁴⁻³⁵ 77 (Table 1). Pure CoMn LDH phases are obtained for Co/Mn molar ratio comprised between 2 and 78 4³⁶⁻³⁷. Best crystallinity is achieved at a Co/Mn ratio of 3:1³². Zhang et al.³⁸ utilized the 79 Impinging Stream-Rotating Packed-Bed reactor (IS-RPB) technique, a method that involves 80
high-speed mixing of co-precipitation reagents, to synthesize CoMn LDH with a Co/Mn molar ratio of 1.64. XPS results indicate that cobalt predominantly exists in the Co(II) form and manganese as Mn(III). Differently, when CoMn LDH are precipitated at the surface of multiple-walled carbon nanotubes (MWCNTs)³⁹, Co stabilizes at +II while Mn displays +III and +IV oxidation degrees.

Hydrothermal urea synthesis was employed to deposit CoMn LDH with varying Co/Mn compositions (R = 0.5, 1, 2, 3, 4) onto Ni foam (NiF)⁴⁰. XPS analysis of Co2p and Mn2p orbital energies reveals that cobalt exists predominantly in the Co(II) state and Mn in Mn(III) state with some Mn(IV) impurity. CoMn LDH with a unique composition of Co(II)₂Mn(III) was prepared by conversion of a cobalt-based zeolitic imidazolate framework structure with a Mn salt solution (MnSO₄(aq.), Mn(NO₃)₂(aq.), MnCl₂(aq.))⁴¹. As shown by XPS, cobalt is in the oxidation degrees +II, while the energy difference of 5.5 eV of the Mn3s confirms a single oxidation degree for manganese of +III. The different modes of synthesis of the CoMn LDH phases and their chemical compositions determined by XPS are summarized in Table 1. The M(II)/M(III) ratio of 2 appears to be the most commonly encountered.

Despite the numerous syntheses implemented for the preparation of CoMn LDH, it is evident that none of them allows for precise control of the Co and Mn valence states. Chemical characteristics, including metal valence state, metal distribution, structural defects, and platelet morphology - all of which are influenced by synthesis conditions - play a critical role in the electroactivity of CoMn LDH. The relationship between structure and property remains poorly understood, and the mechanisms of CoMn LDH formation have not been thoroughly investigated. This challenge stems from the rapid precipitation of LDH, which occurs too quickly for detailed mechanistic studies using standard characterization techniques.
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<table><caption>**Table 1:** Synthesis methods of CoMn LDHs described in the literature</caption><thead><tr><th>LDH</th><th>Synthesis</th><th>Oxidant</th><th>Composition</th><th>M<sup>II</sup>/M<sup>III</sup><br>(XPS)</th><th>Impurities</th><th>Ref</th></tr></thead><tbody><tr><td>Co<sub>2</sub>Mn</td><td>Coprecipitation (N<sub>2</sub>)</td><td>H<sub>2</sub>O<sub>2</sub></td><td>Co<sup>II</sup>Co<sup>III</sup><sub>2</sub>Mn<sup>III</sup></td><td>2</td><td>---</td><td>25</td></tr><tr><td>Co<sub>2</sub>Mn</td><td>Coprecipitation (N<sub>2</sub>)</td><td>H<sub>2</sub>O<sub>2</sub></td><td>Co<sup>II</sup><sub>2</sub>Mn<sup>III</sup></td><td>&lt; 2</td><td>Mn<sup>IV</sup></td><td>34</td></tr><tr><td>Co<sub>2</sub>Mn</td><td>Coprecipitation (N<sub>2</sub>)</td><td>Air</td><td>---</td><td>---</td><td>---</td><td>35</td></tr><tr><td>Co<sub>R</sub>Mn<br>(R = 1, 2, 3)</td><td>Coprecipitation (Air)</td><td>Air</td><td>---</td><td>---</td><td>R = 1<br>β-M(OH)<sub>2</sub><br>R = 1, 2</td><td>36</td></tr><tr><td>Co<sub>R</sub>Mn<br>(R = 1-5)</td><td>Coprecipitation (N<sub>2</sub>)</td><td>Air</td><td>---</td><td>---</td><td>β-Co(OH)<sub>2</sub><br>Mn(OH)<sub>2</sub><br>Mn(CO<sub>3</sub>)<br>R = 3, 4</td><td>32</td></tr><tr><td>Co<sub>1.64</sub>Mn/CF</td><td>IS-RPB</td><td>Air</td><td>Co<sup>II</sup><sub>1.64</sub>Mn<sup>III</sup><br>(Co<sup>II</sup>Co<sup>III</sup>)<sub>2</sub>(Mn<sup>II</sup>Mn<sup>I</sup>)</td><td>≤ 1.6</td><td>---</td><td>38</td></tr><tr><td>Co<sub>2</sub>Mn/PPy</td><td>IS-RPB</td><td>Air</td><td>---</td><td>---</td><td>---</td><td>42</td></tr><tr><td>Co<sub>2</sub>Mn/CNTs</td><td>Coprecipitation</td><td>H<sub>2</sub>O<sub>2</sub></td><td>Co<sup>II</sup>Mn<sup>III</sup></td><td>&lt; 2</td><td>Mn<sup>IV</sup></td><td>43</td></tr><tr><td>CoMn/MW<br>CNTs (R = 2-7)</td><td>Coprecipitation</td><td>O<sub>2</sub></td><td>Co<sup>II</sup><sub>5</sub>Mn<sup>III</sup></td><td>&gt; 5</td><td>Mn<sup>IV</sup></td><td>39</td></tr><tr><td>Co<sub>2</sub>Mn/CF</td><td>Coprecipitation</td><td>Air</td><td>Co<sup>II</sup><sub>2</sub>Mn<sup>III</sup></td><td>2</td><td>---</td><td>29</td></tr><tr><td>Co<sub>2</sub>Mn/NiF</td><td>Hydrothermal</td><td>Air</td><td>Co<sup>II</sup><sub>2</sub>Mn<sup>II</sup>Mn<sup>III</sup></td><td>&lt; 2</td><td>---</td><td>44</td></tr><tr><td>Co<sub>R</sub>Mn/NiF<br>(4 &lt; R &lt; 0.5)</td><td>Hydrothermal</td><td>Air</td><td>Co<sup>II</sup><sub>3</sub>Mn<sup>III</sup></td><td>&gt; 3</td><td>Pour R = 3 Mn<sup>IV</sup></td><td>40</td></tr><tr><td>Co<sub>2</sub>Mn/CC</td><td>Hydrothermal</td><td>Air</td><td>---</td><td>---</td><td>---</td><td>45</td></tr><tr><td>CoMn<sub>2</sub>/CC</td><td>Coprecipitation</td><td>Air</td><td>Co<sup>II</sup>Co<sup>III</sup>Mn<sup>III</sup><sub>2</sub></td><td>&lt; 2</td><td>---</td><td>41</td></tr><tr><td>Co:Mn/NiF<br>(1:4, 2:3,</td><td>Electrodeposition</td><td>Air</td><td>---</td><td>---</td><td>---</td><td>28</td></tr></tbody></table>
1:1, 3:2, 4:1,

9:1, 19:1)

$$
\begin{array}{l}
E_{\text{app}} = -1 \\
\text{V/AgAgCl} \\
\text{Electrodeposition}
\end{array}
$$

Co₉₉Mn/CF
P

$$
\begin{array}{l}
E_{\text{app}} = -0.9 \\
\text{V/ECS}
\end{array}
$$

Air

(CoIICoIII)₉₉MnIII

< 99

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107 CTNs Carbon nanotubes, MWCNTs: multiwall carbon nanotubes, CF: carbon fibers, CFP:

108 Carbon fiber paper, NiF: Nickel foam, PPY: polypyrrole,

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110 Time-resolved synchrotron X-ray Absorption Spectroscopy, also referred as Quick-XAS
111 (QXAS), is a unique tool appropriate to study fast kinetic processes with typical achievable time
112 resolution of several ms⁴⁷. QXAS experiments were used to study the mechanism of Co-Ru
113 alloys formation⁴⁸ or iron speciation in iron-organic matter aggregates⁴⁹. However, most of the
114 studies reported on the mechanism of LDH formation were supported by ex-situ experiments,
115 limiting then the identification of intermediates species. So far QXAS has not been implemented
116 to track the precipitation of metal hydroxides. Moreover, advanced technology in synchrotron
117 radiation equipment allows to monitor also fast kinetic processes in materials under *operando*
118 conditions in catalysis and energy sciences⁵⁰. QXAS has been typically implemented for tracking
119 reaction in heterogeneous catalysis⁴⁷, ⁵¹, electron transfer in metal complexes in homogeneous
120 catalysis⁵², heterogeneous surface reactivity⁵³. In this paper, we are the first to carry out QXAS
121 experiments at the Co and Mn K-edges in order to thoroughly characterize the valence state of
122 metals under the structural evolution toward Co₃Mn LDH phase and to elucidate its mechanism
123 of formation. LDH formation was also followed by *in situ* powder X-ray diffraction (XRD).
124 QXAS experiments were extended to the characterization of redox behavior of Co₃Mn under
125 cyclic voltammetry condition. Multivariate Curve Resolution with Alternating Least Square
126 (MCR-ALS)⁴⁷, ⁵⁴ minimization was used to analyze QXAS data, measured during the monitoring
127 of the speciation of Co and Mn valence state under the electrochemical activation. Its
128 combination with Modulation Excitation Spectroscopy X-ray Absorption Spectroscopy with
129 Phase Sensitive Detection (MEXAS-PSD) was applied to get insight on the chemical element
130 answering to the cyclic perturbation.

131

132 **Method**

133 **Chemicals.** The reagents used for the preparation of the LDH phases, cobalt nitrate
134 hexahydrate (Co(NO₃)₂·6H₂O, 98%), manganese nitrate tetrahydrate (Mn(NO₃)₂·4H₂O, 97%)
135 and sodium carbonate (Na₂CO₃) were purchased from Honeywell and sodium hydroxide (NaOH,
136 98%) from Acros. All these chemicals were used as received without further purification.
137 Deionized water was employed for all experiments. NaOH (98%, Acros) was used as electrolyte.

138 **Preparation of materials.** The Co₃Mn-CO₃ LDH was synthesized using the coprecipitation
139 method. *Ex-situ* and *in-situ* syntheses were operating under similar condition, except for the
140 geometry of the reactor. Typically, 50 mL of a mixed Co(NO₃)₂·6H₂O and Mn(NO₃)₂·4H₂O
141 solution, with a total metal concentration equal to 0.16 M and a molar ratio R = Co(II)/Mn(II)
142 equal to 3, was added to 50 mL of a basic 0.2 M NaOH/0.1 M NaCO₃ solution, previously
143 purged for 1 h with Ar. Addition of metal salts was conducted over a period of 7 min in an Ar
144 atmosphere and under a vigorous stirring. Volumes and concentrations were previously adjusted
145 to reach a final pH of 10.0. For *ex-situ* preparation conditions, the precipitates were washed with
146 deionized water and centrifuged three times, and finally dried at room temperature under air
147 atmosphere for XRD and electrochemical characterization. Four coprecipitations were also
148 carried out using the same protocol but with different counter-anions (NO₃⁻, SO₄²⁻, Cl⁻, C₂O₄²⁻) at
149 equal concentration for these all anions.
**Quick XAS measurements and data analysis.** X-ray absorption near edge structure (XANES) and Fourier transforms of the Extended X-ray Absorption Fine Structure (EXAFS) measurements at the Co (7709 eV) and Mn (6539 eV) K-edges were performed at the ROCK beamline (SOLEIL synchrotron)⁵⁰, using the Si(111) Quick-EXAFS monochromator with the main goniometer aligned at 14.50 ° and 16.45° for an oscillation amplitude of 2.0° and 3.1°, respectively. Mirrors with a B₄C coating and a grazing incidence of 2.8 mrad with respect to the pink and monochromatic beams were used for harmonic rejection. Oscillation frequency of the crystal was chosen at 2 Hz giving rise per second of 2 spectra collected with ascending Bragg angles and 2 spectra collected with descending Bragg angles. The acquisition time was 250 ms for each spectrum. *Operando* QXAS spectra were collected either in transmission mode using 3 Oken ionization chambers filled with nitrogen or in fluorescence mode using a solid-state Passivated Implanted Planar Silicon (PIPS) detector from Canberra, Inc.

Energy alignment and normalization of the XAS spectra were performed using the Python 'normal Gui' graphical interface developed at the beamline for the fast handling of the Quick-XAS data⁵⁵. Multivariate Curve Resolution with Alternating Least Square⁴⁷ iterative fitting method provided an estimation of the chemical element molar fraction (C (%) as a function of time) of the species involved in the coprecipitation pathway or in the electrochemical oxidation process together with the spectra of the pure species. This method therefore allowed for disentangling the species in the reaction mixture. To this purpose, all the experimental spectra collected during the chemical processes have been gathered in an experimental matrix D and the bilinear decomposition of D into a matrix of concentration C and a matrix of pure spectra S<sup>T</sup>, according to the Beer Lambert relation governing the X-ray Absorption Spectroscopy (D = C.S<sup>T</sup> + E where E is a matrix containing the experimental noise), have been provided using the MCR-
ALS 2.0 Matlab ® toolbox developed by the Roma Tauler's group⁵⁶. Constraints such as non-negativity of the concentrations and of the QXAFS absorbances and closure relation of the concentrations were used to minimize the rotational ambiguity of the method⁵⁷.

EXAFS least square fitting was performed using IFEFFIT⁵⁸ available with Horae package. The amplitude reduction factor, S₀², and energy parameter, were derived using EXAFS analysis and fitting of Co₃O₄ and Mn₂O₃ for which crystallographic structures are known.

**In situ LDH precipitation monitoring.** The formation of the Co₃Mn-CO₃ LDH phase was investigated simultaneously by QXAS at the Co and Mn K-edges. The co-addition was performed using a cell developed at the Synchrotron SOLEIL⁵⁹ allowing reagent addition under vigorous magnetic stirring and under controlled atmosphere (Fig. S1). The optical path between the two Kapton windows used for letting the X-rays passed through was 3mm. Irrespective to the volume of metal salts added, fluorescence detection was used at the Mn K-edge for data recording whereas at the Co K-edge, this detection mode was only mandatory at the early stages of salt addition and replaced by transmission mode after addition completion. A total of 400 merged spectra were recorded to monitor the evolution of cobalt species over time (1 merged spectrum each 25 s). At the Mn K-edge, only the measurements in solution at the end of the salt addition was used due to an observed beam damage, as discussed in the results and discussion section and corresponds to an acquisition time of 50 seconds. After aging under Ar and air, the precipitate was washed with deionized water and centrifuged three times before to be measured as wet solid in transmission at both edges. The collected QXAS data were analyzed and discussed based on comparison with *ex situ* XANES spectra of Co and Mn reference products (CoO, Co₃O₄, MnO, Mn₂O₃, MnO₂) (Table S1).
**Electrochemical characterization.** QXAS measurements were also performed under electrochemical polarization. The experiments were made in 0.1 M NaOH using an electrochemical cell dedicated to *operando* QXAS<sup>60</sup>, including an Ag-AgCl electrode as the reference electrode and a platinum auxiliary electrode (Fig. S2). The electrochemical cell was tilted to 45° compared to the incident X-ray beam towards the PIPS in order to allow fluorescence collection. Carbon paper (CPE-AvCarb MGL190 from FuelCell Store) with a large surface area (1 cm<sup>2</sup>/side) was used to prepare the working electrode Co<sub>3</sub>Mn-CO<sub>3</sub>/CPE. Before use, the CPE was cleaned by sonication for 15 min in successively acetone, ethanol and water. Then a drop (2*150 µL) of a 2 mg/mL Co<sub>3</sub>Mn-CO<sub>3</sub> LDH aqueous suspension, stirred overnight, was deposited on the electrode surface, and dried in a desiccator for 3 h.

The amperometric response and the QXAS signal of Co₃Mn-CO₃/CPE were recorded simultaneously upon cycling electrochemical polarization between 0.0 to 0.6 and then back to 0.0 V vs. Ag-AgCl using a potentiostat Biologic SP-150 controlled by EC-Lab software. In total, twenty-two potential cycles were performed over a period of time of 6000 s. For the 1st and 22nd potential cycle, corresponding the time intervals 0 - 1800 s and 4200 - 6000 s, the potential was modulated at low scan rate (v) using the pulse voltammetry technic with a potential amplitude of 10 mV and a pulse width of 15 s, leading to a scan rate of v = 0.66 mV s⁻¹. Twenty intermediate potential cycles, recorded between 1800 and 4200 s, were performed using classic cyclic voltammetry technic at v = 10 mVs⁻¹. Acquisition of QXAS signal was triggered with the potential cycling and recorded during the whole electrochemical polarization (22 cycles), in total 6000 s. To improve the signal to noise ratio 25 spectra recorded successively were merged, leading to a time resolution of 12.5 s. Phase Sensitive Detection (PSD) analysis was finally applied on the MEXAS experiments to unravel the active species in the electrodes and let silent
218 the spectator ones⁶¹. The reproducibility of the *operando* experiments was verified with four
219 different electrodes.

220 The stability of the Co₃Mn-CO₃/CPE soaked in 0.1 M NaOH was studied as a function of time
221 (100 min, 5 h and 13 h). The alkaline medium caused a slow alteration of the LDH, with a small
222 loss of material (15 %) during the first 100 min, corresponding to the period of time required to
223 perform the electrochemical experiment. Meanwhile X Rays irradiation has no significant effect.
224 Hence, one can consider that the electrode is stable during the *operando* electrochemical
225 measurements. However, a longer stay (t > 5 h) in 0.1 M NaOH caused an irreversible oxidation
226 of the LDH, labelled hereafter COx-Co₃Mn-CO₃/CPE. The collected data were compared to *ex-
227 situ* XAS spectra of the pristine and the oxidized Cox-Co₃Mn-CO₃/CPE films, and other
228 reference samples (Table S1).

229 **Ex situ physical characterization.** XRD analyses were performed using theta-theta
230 PANalytical X'Pert Pro diffractometer equipped with a Cu anticathode (λKα₁ = 1.540598 Å,
231 λKα₂ = 1.544426 Å) and an X'Celeretor detector. XRD pattern was recorded in the range 3 -
232 90°/2θ with a step size of 0.033422°. Scanning electron microscopy (SEM) images of the
233 Co₃Mn-CO₃ films were recorded directly on a carbon paper electrode using a JSM-7500F Field
234 Emission Scanning Electron Microscope operating at an acceleration voltage of 3 KV and
235 magnifications of x500, x5,000, x50,000. UV-Vis spectra were recorded with Shimadzu v-2400
236 spectrophotometer.

237
238
239
240
**Results & Discussion**
241 *Ex situ monitoring of Co₃Mn LDH phase*

242 As noticed in the introduction, LDH can be formed with a wide variety of M<sup>II</sup>/M<sup>III</sup> compositions which make them unique materials with diverse physical and chemical properties.
243 Indeed, as it was already discussed in highly key reference review<sup>62-64</sup> the formation of pure
244 LDH phases depends on many parameters, i.e. the nature of M<sup>II</sup> and M<sup>III</sup> cations and the
245 interlayered anions, the M<sup>II</sup>/M<sup>III</sup> molar ratio, the structural organization (cations and/or anion
246 ordering, stacking sequence) and the layer and interlayer domains interactions. Standard molar
247 Gibbs free energies of formation and dissolution of LDH have been determined to estimate and
248 compare the feasibility of LDH formation and their stability depending on their composition<sup>65</sup>.
249 The method of synthesis is also a key factor that impact the formation of LDH<sup>66-67</sup>. Recently,
250 Farhat et al.<sup>32</sup> have demonstrated the importance of the Co(II)/Mn(II) ratio (R) for the
251 coprecipitation of pure Co(II)<sub>R</sub>Mn(III) LDH phases. Minor impurities were present for 1.0 < R <
252 5.0, except for the compound with R = 3 which corresponds to the purest and the best
253 crystallized LDH structure. The valence states of Co and Mn and a completed chemical
254 composition of Co<sub>3</sub>Mn-LDH (Co(II)<sub>0.9</sub>Co(III)<sub>0.1</sub>)<sub>3.1</sub>Mn(III)(OH)<sub>8.2</sub>(CO<sub>3</sub>)<sub>0.59</sub>(OH)<sub>0.14</sub>·2.14H<sub>2</sub>O
255 were proposed based on a combination of complementary analytical techniques, i.e. energy
256 dispersive X-ray (EDX) microanalysis, XPS, thermogravimetric analysis (TGA), and XRD
257 refined using the Rietveld method.
258

259 To better understand the formation mechanism of this pure Co₃Mn LDH phase, the role of
260 metal salt counter-anions (X = NO₃⁻, CO₃²⁻, SO₄²⁻, Cl⁻, C₂O₄²⁻) in the coprecipitation process was
261 first investigated. The resulting solids were characterized by XRD (Fig. S3). Results were
262 unexpected. While the coprecipitation of LDH phases intercalated with NO₃⁻, SO₄²⁻, Cl⁻, C₂O₄²⁻
263 anions are usually obtained for many other M(II)M(III) LDH systems<sup>68</sup> the intercalation of these
anions in Co₃Mn LDH failed. Co₃Mn-CO₃ was the only LDH phase obtained. For the other anions (X = NO₃⁻, SO₄²⁻, Cl⁻, C₂O₄²⁻), β-Co(OH)₂ (COD 1548810) was the major phase. The presence of an amorphous component was probably due to the precipitation of unidentified Mn-containing phases. These results highlight the key role of the carbonate anions in the formation process and in the stabilization of Co₃Mn LDH structure, questioning the mechanism of formation and why the NO₃⁻, SO₄²⁻, Cl⁻, C₂O₄²⁻ anions are not intercalated in CoMn LDH under such synthesis conditions. At this stage, it is pointed out that intercalation of CO₃²⁻, confirmed by FTIR (Fig. S4), may arise from two sources: i) an initial input and/or ii) a contamination by atmospheric CO₂ that may occur over the all steps of the synthesis. Moreover, intercalation of anions assumes that Mn(II) must be oxidized into Mn(III) in the presence of anions, a process that raises questions given the chosen experimental conditions.

In order to determine the role of the atmosphere on the oxidation process during the last step of formation, namely the drying step, the fresh prepared precipitate was dried either under air atmosphere or under vacuum followed by in-situ analysis using XRD (Fig. S5). Surprisingly, the Co₃Mn-CO₃ LDH phase was formed only under air. Conversely, drying under vacuum resulted in a mixture comprising β-Co(OH)₂ phase, residual Na₂CO₃, and an amorphous manganese-containing product. This highlights the critical role of atmospheric oxygen in partially oxidizing divalent metals to trivalent states, which is essential for forming the LDH structure. Drying under air atmosphere, favors not only the oxidation of M(II) but also the adsorption of CO₂ and its conversion into carbonate anions, simultaneous reactions that allow the achievement of the LDH phase. This mechanistic process was monitored by in-situ XRD, using a cell maintained under continuous flow of controlled atmosphere, either under air or under N₂/O₂ gas mixture (CO₂-free reconstituted air). X-ray diffractograms were acquired hourly from 1 hour to 15 hours. It is
noted that under continuous air flow, the (003), (006), (012), (015), (110) and (113) diffraction lines, that characterized the LDH structure, appear and are continuously growing in intensity (Fig.1A), while under CO₂-free atmosphere (reconstituted air), the expected LDH pattern is totally absent (Fig.1B). This is a direct experimental evidence that not only O₂ is necessary to achieve metal oxidation but also that carbonate anions (CO₃²⁻), generated from the dissolution of CO₂ from air, are involved in the formation of the LDH phase. The basic condition of the reacting medium (pH 10.0) ensures the dissolution of sufficient CO₂ content into the aqueous phase and its transformation into the carbonate form (CO₃²⁻) following the well-known pH-dependent carbonate speciation CO₂/HCO₃⁻/CO₃²⁻. Moreover, this result explains the reason why NO₃⁻, SO₄²⁻, Cl⁻, C₂O₄²⁻ anions were not intercalated in the Co₃Mn under such synthesis conditions.

298

<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

299
**Figure 1.** Evolution of X-ray diffractograms of Co₃Mn-CO₃ dried (A) under air atmosphere and (B) under CO₂-free reconstituted air (N₂/O₂). Insert (C) XRD of samples dried after 15h00 for both conditions.

**In-situ monitoring of Co₃Mn LDH phase formation by QXAS**

To track the evolution of the oxidation state of Co and Mn elements during the synthesis, in situ monitoring of Co₃Mn LDH coprecipitation was conducted using QXAS experiments at the Co and Mn K-edges (see experimental section). The experiment involved three distinct steps carried out under different atmospheres:

First Step: Addition of metal salts under Ar for a short duration (7 min) to facilitate rapid coprecipitation.

Second Step: Aging of the coprecipitated solid under Ar (73 min).

Third Step: Further aging under air by opening the synthesis reactor (76 min).

The evolution of Co K-edge and Mn K-edge XANES spectra is illustrated in figures 2 and 3, respectively.

Initially, the Co K-edge absorption signal stabilizes rapidly after the completion of salt addition (step 1), and it remains constant throughout the subsequent synthesis steps (Ar aging, air aging) (Fig. 2). The energy position at 7718 eV (corresponding to 50% of the normalized absorbance) indicates the presence of Co(II). The strong similarity with β-Co(OH)₂ XANES spectra and EXAFS data (Fig. 4(a, b, c) & Table S1) confirms the formation of Co(II) layered hydroxides with an environment similar to that of Co(OH)₂ in the early stage of coprecipitation and its stability over the aging steps (Fig. 2 A & B).

321
<figure><img src="image_4.png" /><figcaption>**Figure 2.** *In situ* Co K-edge XANES monitoring of CoMn LDH coprecipitation (A) under Ar comprising 7 min of salts co-addition (step 1, blue lines) followed by 73 min of aging (step 2, red lines) and (B) aging under air (step 3). *In-situ* XANES monitoring of the first 50 minutes of drying under air of the wet solid: (C) at the Mn K-edge and (D) at the Co K-edge.</figcaption></figure>

In a preliminary study, a continuous increase of the energy position of the Mn K-edge X-ray absorption rising edge from 6546 eV to 6548.5 eV (Fig. S6) was observed during the salt addition and Ar-aging steps (step 1+2) and up to 6550 eV during the aging step under air (step
3). This shift reflects a progressive oxidation of Mn (from Mn(II) to Mn(III)) throughout the entire coprecipitation process, even under O₂-free conditions. This oxidation was attributes to irradiation damage caused by high-energy X-rays. This damage was restricted to the permanent floc covering the reactor Kapton window. Unfortunately, due to the reflection measurement configuration required for fluorescence detection at the Mn K edge and associated low-penetration depth fluorescence sensitivity, this beam radiation damage on floc was critical for data recording. Therefore, to mitigate the irradiation damage, filters were used to reduce the photon density and the cell was regularly moved during the experiment with a short exposure time at each position. In such conditions, no evolution of the Mn absorption signal was observed. Utilizing the high time-resolution of QXAS experiments (few ms), data were recorded at the end of each ageing step and evidences the formation of Mn(II) hydroxides species⁶⁹ which are stable until the air aging stage (Fig. 3). It is noteworthy that measurement in transmission being possible due to larger penetrating strength of X-rays at the Co K edge (7709 eV) compared to Mn (6539 eV), irradiation damage was not effective for Co K-edge XANES experiment presented (Fig. 2 A & B).
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Mn K-edge XANES spectra for Co₃Mn LDH coprecipitation recorded in solution at the end of the 7 min lasting co-addition of cobalt and manganese salts (fluorescence signal – blue dots) and on the wet solid obtained after ageing first in argon (73 min) and second in air (80 min) (red line – transmission signal).</figcaption></figure>

According to the positions of rising K-edge of Co (7717.9 eV) and Mn (6547.0 eV), it can be concluded that the suspended solid after precipitation, Ar ageing and air ageing, was a mixture between Co(OH)₂ and Mn(OH)₂, and probably more realistically a brucite-like solid solution Co₁-xMnₓ(OH)₂. No oxidation occurred at these steps, no intercalation of anions and the LDH was not formed yet.

The structural evolution to Co₃Mn LDH occurs during the drying step under air as evidenced by the initial 50 minutes of drying monitored by QXAS at the Mn and Co K-edges shown in Fig. 2 C & D and S6. The data in Figure 2 C & D demonstrate that drying under air leads to the
selective transformation of the wet solid into the Co₃Mn LDH with the Co atoms retaining a local environment similar to that in β-Co(OH)₂ with a +II oxidation state. Approximately 50% of the wet solid is transformed into Co₃Mn LDH within the first 50 minutes of drying, as illustrated in Figure S7. Comparative analysis of the Mn K-edge spectra (Table S1) reveals a shift to higher energy from 6547.0 eV for the wet solid to 6550.3 eV for the dried Co₃Mn LDH (Fig. 4), providing clear evidence of the total oxidation of Mn(II) to Mn(III).

Following drying under air, the Co₃Mn-CO₃ LDH phase is formed, with Co²⁺ and Mn³⁺ cations located in the octahedral sites of the M(OH)₂ layers. The typical M-O bond lengths are measured at 2.08 Å for Co and 1.94 Å for Mn, with M-M distances ranging from 3.12 to 3.13 Å (Table 2), consistent with the unit cell parameter 'a' (3.11378(3) Å) determined by Rietveld refinement³². QXAS experiments confirm data from the *in situ* XRD study (Fig. 1) and the anion impact on LDH formation. Indeed, the presence of CO₂ is a necessary condition for the formation of the Co₃Mn LDH phase which is only stabilized by CO₃²⁻ anions.

Considering that both Co and Mn cations are initially in a brucite-like M(OH)₂ environment during the early stage of coprecipitation, and conversion into LDH occurs at the final drying step where dissolution/reprecipitation is likely not favored, we conclude that the conversion mechanism from the brucite-like phase to LDH is driven by Mn(II) oxidation. This oxidation is a result of the lower oxidation potential for Mn(OH)₃/Mn(OH)₂ (E° = +0.05 V) compared to Co(OH)₃/Co(OH)₂ (E° = 0.4 V)⁷⁰.

The neutralization of the emerging positive charge of the brucite-like Co(II)₃Mn(III) layer is simultaneously counter-balanced by the insertion of carbonate anions in the 2D structure. The adsorption of CO₂ on the basic surface of metal hydroxides initiates the formation of adsorbed CO₃²⁻ anions which are subsequently intercalated to stabilize the Co₃Mn-CO₃ LDH phase. The
conversion of CO₂ into CO₃²⁻ anions arises in the basic environment of the LDH structure containing H₂O and /OH⁻ anions. It is important to note that the oxidation of Mn²⁺ to Mn³⁺ by O₂ in air under basic conditions is associated with the formation of OH⁻, which could facilitate the basic conversion of CO₂ from the air into CO₃²⁻, while preserving the integrity of the metal hydroxide layers. This process culminates in the final chemical composition (Co₂.27Mn₀.73(OH)₆(CO₃)₀.43(OH)₀.1.56.H₂O), as previously reported³².

Scheme 1 outlines the three-steps process for the formation of Co₃Mn LDH:

1. initial coprecipitation of Co₀.75Mn₀.25(OH)₂ solid solution,

2. oxidation of Mn²⁺ by O₂ from the air,

3. basic conversion of CO₂ into CO₃²⁻ followed by CO₃²⁻ intercalation into LDH.

① Coprecipitation

<figure><img src="image_6.png" /></figure>

② Mn²⁺/Mn³⁺ Oxidation

③ CO₂/CO₃²⁻ conversion & CO₃²⁻/OH⁻ intercalation

<figure><img src="image_7.png" /></figure>

Scheme 1. Proposed mechanism of Co₃Mn LDH formation.

394
From XRD Data, we must notice that insertion of CO₃²⁻ occurs without any phase segregation or staging as reported by Ma et al.⁷¹ for topochemical conversion of Co₁-xFeₓ(OH)₂ and Co₁-xNiₓ(OH)₂ into LDH under oxidizing halogen agents. Electron transfer between Mn(II)(OH)₆ and O₂ species must be fast, occurring probably through an outer sphere electron transfer process according (eq.1)⁷²⁻⁷³.

$$2\text{Co(II)}_3\text{Mn(II)(OH)}_8 + 1/2\text{O}_2 + 2\text{H}^+ \rightarrow 2\text{Co(II)}_3\text{Mn(III)(OH)}_8^+ + \text{H}_2\text{O} \quad (\text{eq.1})$$

The rapid charge redistribution observed may be facilitated by the narrow band gap (bg) width of the cobalt hydroxide matrix, which exhibits semiconductive properties. Specifically, β-Co(OH)₂ or Co₂Mn-CO₃ LDH displays band gap widths of 2.85 eV and 2.66 eV, respectively. This characteristic suggests that the material can efficiently support charge transfer processes. Hartree-Fock calculations conducted on clusters of brucite-like Fe(OH)₂ white rust undergoing topochemical oxidative conversion into Fe(II)₂Fe(III)(OH)₆(CO₃)₀.₅.nH₂O green rust⁷⁴, have predicted self-diffusion of Fe³⁺ hole polaron along the layer and hopping rates between 10³ s⁻¹ (70 K) to 10¹⁰ s⁻¹ (300 K), the fastest valence interchange reaction occurring between two adjacent Fe²⁺-Fe³⁺ cations at a distance of 5.6 Å. Due to strong structural similarities between FeFe and CoMn systems, similar phenomenon could be involved in the topochemical conversion of the Co(II)₃.₁Mn(II)(OH)₈.₂ solid solution into (Co(II)₀.₉Co(III)₀.₁)₃.₁Mn(III)(OH)₈.₂ (CO₃)₀.₅₉(OH)₀.₁₄.2.14H₂O) LDH phase.
<figure><img src="image_8.png" /><figcaption>**Figure 4.** XANES spectra, EXAFS spectra and corresponding Fourier Transform of the wet (blue) and dried (red) Co<sub>3</sub>Mn LDH at (A, B, C) Co K-edge and (D, E, F) Mn K-edge. For the Co K-edge, spectrum obtained for Co(OH)<sub>2</sub> is reported in green for comparison purpose.</figcaption></figure>

<table><caption>**Table 2.** Structural data of Co<sub>3</sub>Mn LDH from EXAFS analysis (Co & Mn K-edges). Fitting range: Δk: 3.5 -10.7 Å<sup>-1</sup> and ΔR: 1.0-3.4 Å.</caption><thead><tr><th rowspan="2"></th><th colspan="4">Co K-edge (7717.9±0.7 eV) (s<sub>0</sub><sup>2</sup> = 0.85)</th><th colspan="4">Mn K-edge (6549.5±1.3 eV) (S<sub>0</sub><sup>2</sup> = 0.74)</th></tr><tr><th>N</th><th>R(Å)</th><th>σ<sup>2</sup> (Å<sup>2</sup>)</th><th>Rf/χ<sup>2</sup>_red</th><th>N</th><th>R(Å)</th><th>σ<sup>2</sup> (Å<sup>2</sup>)</th><th>Rf/χ<sup>2</sup>_red</th></tr></thead><tbody><tr><td rowspan="2">β-Co(OH)<sub>2</sub></td><td>6 O</td><td>2.10</td><td>0.0075</td><td>0.53%/526</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>6 Co</td><td>3.19</td><td>0.0068</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
<table><tbody><tr><td rowspan="2">Sample before drying</td><td>6 O</td><td>2.08</td><td>0.0102</td><td>0.83%/250</td><td>6 O</td><td>2.17</td><td>0.0143</td><td>1.00%/209</td></tr><tr><td>4.4 M</td><td>3.17</td><td>0.0101</td><td></td><td>4.4 M</td><td>3.24</td><td>0.0079</td><td></td></tr><tr><td rowspan="2">Sample after drying (Co<sub>3</sub>Mn-LDH)</td><td>6 O</td><td>2.08</td><td>0.0116</td><td>1.24%/7081</td><td>6 O</td><td>1.94</td><td>0.0083</td><td>2.62%/5387</td></tr><tr><td>6 M</td><td>3.12</td><td>0.00076</td><td></td><td>6 M</td><td>3.13</td><td>0.0058</td><td></td></tr></tbody></table>

421

422

423

## Operando QXAS monitoring electrochemical behavior of Co<sub>3</sub>Mn-CO<sub>3</sub>

The electrochemical behavior of CoMn LDH ($1 \le R \le 5$) has been investigated in 0.1 M NaOH by Farhat et al.<sup>32</sup> It was shown that the presence of Mn in the cobalt-based LDH structures amplifies their electrochemical responses, in comparison with the lower electrochemical responses of β-Co(OH)<sub>2</sub> and CoAl LDH. An optimum electrochemical response was obtained with the Co/Mn ratio equal to 3. Cyclic voltammograms of Co<sub>3</sub>Mn-CO<sub>3</sub> coated on a carbon electrode, recorded in 0.1 M NaOH, displayed a first irreversible peak, then a stable quasi-reversible signal was observed for the following cycles (Fig. S8A). Electrochemical impedance measurements highlighted a decrease of the charge transfer resistance for the oxidized film compared to the pristine Co<sub>3</sub>Mn-CO<sub>3</sub> modified electrode under open circuit potential (Fig. S8B)<sup>32</sup>. Finally, the presence of Na<sup>+</sup> in the oxidized phase was confirmed by XPS and EDX, with a (Co + Mn)/Na molar ratio equal to 2.94. As suggested by Su et al.<sup>75</sup> for CoAl LDH, synthesized by a “memory effect” route, sodium cations may be intercalated simultaneously with hydroxyl groups during the electrochemical process.

Ex situ XPS, XANES and EXAFS characterizations<sup>32</sup> have demonstrated that the electrochemical oxidation of (Co<sup>II</sup><sub>0.9</sub>Co<sup>III</sup><sub>0.1</sub>)<sub>3.1</sub>Mn<sup>III</sup> LDH leads to the full oxidation of Co(II) into Co(III) and Mn(III) into Mn(IV). It was reported in the literature, that X-ray absorption
spectroscopy can be combined with electrochemistry to elucidate the relationship between the electrochemical behavior and the structure of electroactive materials, such as LDH$^{76}$. To identify the contribution of both Co and Mn cations in the electrochemical response of Co₃Mn-CO₃, *operando* QXAS experiments recorded simultaneously at the Co and Mn K-edges were performed. The working electrode was a Co₃Mn-CO₃ thin film deposited on a carbon paper electrode (CPE). The SEM images show that the modified electrode Co₃Mn-CO₃/CPE consists of monodispersed hexagonal platelets (100-300 nm large) fully covering the surface of the CPE fibers (Fig. 5).

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

<caption>**Figure 5.** Scanning electron microscopy (SEM) images of Co₃Mn-CO₃ particles deposited on carbon paper electrode (CPE).</caption>

QXAS spectra were acquired, at sub-second time-scale, to follow the variation of the Co and Mn oxidation states during the electrochemical polarization between 0 to 0.6 V vs. Ag-AgCl (forward scan) and 0.6 to 0 V vs. Ag-AgCl (reverse scan) using 0.1 M NaOH as electrolyte solution. Twenty-two potential cycles were recorded over a period of time of 6000 s. The first
and the last cycles were recorded at very low scan rate (ν = 0.66 mV s⁻¹), 20 intermediate CVs were recorded at 10 mV s⁻¹ to study the signal repeatability (see experimental part). After this electrochemical treatment, the color of the film changed from pink to black. The shifts of the X-ray absorption energies, pointed at the rising K-edge of Co and Mn, were observed simultaneously during the potential scans, as shown in Fig. 6 A and B. It appears that the energies of the rising K-edge of Co and Mn increase respectively from 7718.2 to 7722.0 eV (Fig. 6A) and from 6550.3 to 6552.7 eV (Fig. 6B). Comparison of these energy values with those reported for *ex situ* XANES data for reference minerals (Table S1) suggests that the electrochemical activation forces the oxidation of Co(II) to Co(III) and Mn(III) to Mn(IV) species.

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>
468
469 **Figure 6.** Evolution of (A) Co and (B) Mn K-edges XAS spectra of Co₃Mn-CO₃/CPE during
470 *operando* electrochemical experiment in 0.1 M NaOH, corresponding to 22 potential scans
471 between 0 to 0.6 V vs. Ag-AgCl (forward scan) and 0.6 to 0 V vs. Ag-AgCl (reverse scan), over
472 a period of 6000 s.

473
474 Figure 7A displays the variation of the applied potential as a function of time and the
475 corresponding current response of Co₃Mn-CO₃/CPE in 0.1 M NaOH. To elucidate the different
476 chemical species involved in the electrochemical behavior of Co₃Mn-CO₃ LDH, MCR-ALS
477 analysis was employed, detailed in Supplementary Information (Fig. S9 and S10), on the Co and
478 Mn XANES spectra recorded over time (6000 s) that are directly related to the instantaneous
479 applied potential and therefore to the current. Two components were identified at the Mn K-
480 edge, Mn-LDH corresponding to the Mn site in the pristine LDH material and a new species,
481 called Mn-Cp2, appearing during the electrochemical treatment, for which the concentration
482 profiles are reported in Fig.7 B. At the Co K-edge, three species were identified: the original Co-
483 LDH species and two new species, called Co-Cp2 and Co-Cp3, which concentration profiles are
484 given in Fig.7 C. The energy values associated with these species are provided in Table S1.

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<figure><img src="image_12.png" /><figcaption>**Figure 7.** (A) Co₃Mn-CO₃/CPE current/time/voltage curves in 0.1 M NaOH during QXAS spectra recording, concentration profiles of (B) Mn and (C) Co components determined by MCR-ALS as function of time during the potential scanning.</figcaption></figure>
The Mn-LDH component undergoes nearly complete disappearance (about 90%) during the first period of time (0 – 900 s, corresponding to the first anodic scan). Concurrently, a new oxidized Mn(IV) component, termed Mn-Cp2, emerges and remains stable throughout the experiment duration (900 to 6000 s) (Fig. 7B). Similarly, the Co(II) species from the Co₃Mn-CO₃ LDH layer disappears during the same period of time (0 – 900 s) (Fig. 7C), while two new components (Co-Cp2 and Co-Cp3) emerge. Their relative contribution changes with the potential over the whole electrochemical experimentation period. In the following CV cycles (between 1800 and 600 s), the contribution of Mn atoms to electron transfer seems to be negligible, with only Co undergoing the redox process. The evolution of these species appears to be reversible, during the 20 potential cycles, as highlighted by the measured current intensities (Fig. 7A in blue) and the fraction of Co-Cp2 and Co-Cp3 which perfectly follows the applied potential (Fig. 7C).

<figure><img src="image_13.png" /></figure>

**Figure 8.** Evolution of current (i) and Mn-Cp2, Co-Cp2 and Co-Cp3 profiles as a function of applied potential for the (A) 1st scan and (B) 22th scan of Co₃Mn-CO₃/CPE in 0.1 M NaOH.
Evolution of amperometric current and concentration profiles of the newly formed species at
Mn and Co K-edges as a function of applied potential is shown in Fig. 8 for the first (A) and the
last cycle (22th, B). During the first potential scan (Fig. 8A), the current-potential curve shows a
complex shape with several oxidation peaks (a₁, a₂ and a₃) that may result from an oxidation
process involving one Mn and two Co species. As depicted in Fig. 8A, the contribution of Mn-
Cp2 begins to increase concomitantly with the rise in anodic current (Ipa) at E = 0.100 V vs. Ag-
AgCl. Co-Cp2 component starts to appear as soon the Mn-Cp2 is appearing and its rate of
formation is very similar to the Mn component to reach finally a maximum proportion of 30 % at
0.330 V. Then the contribution Co-Cp2 starts to decrease to 0 %, whereas the Co-Cp3 increases
to reach a maximum value of 100 % at 0.600 V. During the reverse scan, the current
progressively decreases to reach a small cathodic peak c₁ at 0.180V. At this potential, the
contribution of Co-Cp2 and Co-Cp3 are almost equal to 50 %.

The rate of formation of these species during the first forward scan, between 0 and 0.6 V,
calculated as a first deviation (dM-Cp/dt), is compared to the current evolution as a function of
potential in Fig. S11. Even if the curves dCo-Cp2/dt and dCo-Cp3/dt vs. E are noisy, this
comparison provides some information about the contribution of the metal cations on the
amperometric response. The rate of formation dMn-Cp2/dt follows the same evolution than the
current to reach a maximum value at 0.389 V, closed to the peak a₂ (Fig. S11A). Figure S11B
shows that Co-Cp2 contributes mainly to the small current hump a₁ at ≈0.200 V, whereas the rate
of formation dCo-Cp3/dt starts to increase when the potential is higher than 0.300 V. In this
potential region (0.350-0.600 V), the current tends to be constant.
As said before, the evolution of these species Co-Cp2 and Co-Cp3 appears to be reversible during the following potential cycles where the two components Co-Cp2 and Co-Cp3 are always out of phase. This is confirmed during the last cycle (Fig. 8B), with a contribution of 50 % of Co-Cp2 and 50 % Co-Cp3 around 300 V (peak a') and 200 V (peak c₁), whereas it is 0 % for Co-Cp2 and 100 % of Co-Cp3 at 0.600 V (a₃) (Fig. 8B). Since it was reported that OER may occur at CoMn LDH surface²⁵, the peak a₃ at 0.600 V may be due to the concomitant oxidation of Co(II) to form Co-Cp3 with a small contribution OER.

To confirm the results obtained with MCR-ALS analysis, the 20 potential cycles were thus first merged into one single cycle on which phase sensitive detection (PSD) was applied (Fig. S12). Spectra obtained at both the Co and Mn K-edges from the merged cycles did not show significant differences. This observation can question the existence of Co-Cp2 and Co-Cp3 determined by MCR-ALS analysis, which are very close to each other. However, applying the PSD to the period at the Co K-edge, we obtained phase-resolved spectra with a typical shape that can be interpreted as a difference of spectra⁷⁷. As shown in figure S12E, the scaled difference between Co-Cp2 and Co-Cp3 exhibits exactly the same shape as the phase-resolved spectra. Unlike for the Co K-edge, the PSD applied to the single period at the Mn K-edge leads to noise only (Fig. S12F). These results enforce those obtained from MCR-ALC analysis, i.e. that Co is active during the electrochemical treatment of the activated Co₃Mn LDH while Mn is clearly inactive. Furthermore, the same shape of the difference between Co-Cp2 and Co-Cp3; and the phase-resolved spectra at Co K-edge clearly ensures that these two species are the only ones imply in the electrochemical response of the activated Co₃Mn LDH.

The reversible conversion of Co-Cp2 into Co-Cp3 at a rate up to 70% suggests that nearly all cobalt sites are accessible to the redox process thanks to an intralayer electron hopping occurring
between the two Co sites, such as oxidized Co(III) and reduced Co(II) species present in the LDH layer $[Co(III)_{3-x}Co(II)_xMn(IV)(OH)_8]^{+(5-x)}$ (with x = 0 – 2.1).

To further characterize the oxidized phase formed after electrochemical treatment (EOx-CoMn-CO$_3$/CPE), its structural characteristics were investigated using EXAFS data (Fig. 9) and X-ray diffraction pattern (Fig. 10). The EXAFS signals at both Co (Fig. 9 (A)) and Mn K-edges (Fig. 9 (B)) for EOx-Co$_3$Mn-CO$_3$/CPE were compared to those obtained with the chemically oxidized Co$_3$Mn-CO$_3$ LDH in NaOH (COx-Co$_3$Mn-CO$_3$/CPE). Fitted metal-oxygen and metal-metal distances are reported in Table S2. Consistent with previous findings by Farhat et al.³², the oxidation of Co$_3$Mn-CO$_3$ LDH results in shorter metal-oxygen and metal-metal distances compared to pristine Co$_3$Mn-CO$_3$ LDH (Table 2). These distances are similar to those found for Co(III) and Mn(IV) in the octahedral sites of the ordered spinel Co$_3$O$_4$ and MnO$_2$ structures respectively. Interestingly, comparison of EXAFS signals between EOx-Co$_3$Mn-CO$_3$/CPE and COx-Co$_3$Mn-CO$_3$/CPE reveals minimal differences.

However, despite these structural similarities, the cyclic voltammetry curves of EOx-Co$_3$Mn-CO$_3$/CPE and COx-Co$_3$Mn-CO$_3$/CPE exhibit distinct shapes (Fig. S13), with the oxidation peak of EOx-Co$_3$Mn-CO$_3$/CPE appearing at a lower anodic potential. This suggests that electron transfer is more facile in EOx-Co$_3$Mn-CO$_3$/CPE compared to COx-Co$_3$Mn-CO$_3$/CPE, probably due to a change of mass-transfer properties of EOx-Co$_3$Mn-CO$_3$/CPE, linked to the flux of OH$^-$ and insertion/deinsertion of Na$^+$ within the LDH structure occurring during the reversible electrochemical oxidation process.
<figure><img src="image_14.png" /><figcaption>**Figure 9.** Fourier Transform of the EXAFS signal of COx-Co₃Mn-CO₃/CPE (red) and EOx-Co₃Mn-CO₃/CPE (blue) at (A) Co and (B) Mn K-edges.</figcaption></figure>

From XRD data, the limited number of diffractions lines and the very low intensity of the (110) line (60° 2θ) result from a preferred orientation of the LDH platelets on the CPE substrate. Additionally, the broad diffraction pattern of the CPE substrate (Fig. 10) renders the identification of the oxidized phase difficult. However, the diffractions lines LDH at 11.7901° (7.505 Å) and 23.5853° (3.771 Å) can be easily assigned to the (003) and (006) diffraction lines
of the pristine Co₃Mn-CO₃ material. A new doublet of (00l) harmonic diffractions lines pointed at 9.6193° (9.193 Å); 19.1383° (4.637 Å) is observed which corresponds to the oxidized phase.

This harmonicity is clearly in favor of a layered structure with an expanded basal spacing (9.446 Å) compared to the the pristine phase (7.523 Å).

<figure><img src="image_15.png" /></figure>

<caption>**Figure 10.** X-ray diffractograms of EOx-Co₃Mn-CO₃/CPE.</caption>

Infrared analysis of EOx-Co₃Mn-CO₃ (data not shown) shows intense vOH and v₃(CO₃) vibration bands which confirms that the converted structure retains some structural characteristic of the pristine LDH, including hydroxylated layer domains and intercalated carbonate anions³².

Finally, the presence of Na⁺ in the oxidized phase suggests its participation in the electrochemical process.

Generally, electrochemical oxidation of electroactive cations, namely Ni(II) or Co(II), leads a decrease in basal spacing in the LDH structure⁷⁸⁻⁷⁹ due to an increase in coulombic interactions between intercalated OH⁻ anions from the electrolyte and the more positively charged hydroxide
layers. For instance, using *operando* XRD measurements, Taviot et al. found an interlayer distance of 7.44 Å for Co₂Al LDH oxidized in KOH.

These data shows that Co₃Mn-CO₃ displayed a high affinity with carbonate and its oxidation leads to a highly charged hydroxide layers where an exchange of carbonate by OH⁻ is not so favorable than with CoAl LDH. It is thus not surprising that a complex charge compensation process takes place implying a reversible flux of OH⁻ anions and a counter flux of dehydrated Na⁺, as suggested in earlier papers<sup>75, 79</sup> The expansion of interlayer distance may be attributed to the co-intercalation of Na⁺/OH⁻ electrolyte ions alongside the intercalated CO₃²⁻ anions. The similar expanded layered oxidized-Co₃Mn phase was obtained under ex-situ electrochemical activation (Fig. S8C) as demonstrated in a recent paper<sup>32</sup>.

## Conclusion

*Operando* Quick-XAS experiments were used for the first time to go insight the formation mechanism and the electrochemical behavior of Co₃Mn LDH. Fast coprecipitation of a mixture of Co(II) and Mn(II) salts under argon allows the formation of Co(II)₁-xMn(II)x (OH)₂ solid solution that is stable even under Ar or Air aging as shown by XANES spectra at Co and Mn K-edges of the aged precipitate before drying. Under air drying the mixed CoMn brucite-like phase undergoes a topochemical oxidation of Mn(II) into Mn(III) in a pseudo solid state leading to the formation of Co(II)₃Mn(III)-CO₃ LDH.

On the other side, *operando* QXAS coupled with potential scanning experiments demonstrate that electrochemical activation of Co(II)₃Mn(III)-CO₃ LDH ends to the irreversible oxidation of Mn(III) in Mn(IV), while only cobalt cations Co(II) undergo a reversible oxidation process. Two different cobalt species Co-Cp2 and Co-Cp3 are involved in the redox process with an opposite
phase. Moreover, it appears that the electrolyte cation, namely sodium, is involved in the electrochemical process causing an expansion of the interlayer domain. Oxidation of Co and Mn is likely accompanied by partial deprotonation of the layers, simultaneous with the co-intercalation of Na⁺ and OH⁻ ions. Further electrochemical experiments, such as electrochemical impedance and electrochemical quartz microbalance, would provide additional information about the electron transfer mechanism in this LDH material, highlighting the role of electrolyte cations and hydroxyl anions.

## Acknowledgements

The authors acknowledge the SOLEIL scientific committee that elected this project for accessing to the ROCK beamline experiments (proposal 20190617). The work at ROCK was supported by a public grant overseen by the French National Research Agency (ANR) as a part of the "Investissements d'Avenir" program ref: ANR-10-EQPX-45.

The authors acknowledge Dr. Sophie Pelon from Université Paris Saclay (Institut de Chimie Physique) for scientific advises on in-situ QXAS electrochemical experiments and the reviewers for their constructive comments.

## Supporting Information

Electrochemical cell - XRD data – Q-XAS data – XAS analysis-Electrochemical data

## Corresponding Authors

Dr. Claude Forano<sup>a</sup>; claudie.forano@uca.fr

Dr. Valérie Briois<sup>b</sup>; valerie.briois@synchrotron-soleil.fr
639 **Present Address**

640 <sup>a</sup>Université Clermont Auvergne, CNRS, Clermont, Institut de Chimie de Clermont- Ferrand, F-

641 63000 Clermont-Ferrand, France

642 <sup>b</sup>Synchrotron SOLEIL, CNRS-UR1, BP 48, L'orme des Merisiers, 91192 Gif-sur-Yvette, France

643 **Author Contributions**

644 H. Farhat: Ex-situ synthesis and characterizations of CoMn LDH materials

645 C. Mousty: Implementing and monitoring ex-situ electrochemistry experiments

646 V. Prevot: Expertise in LDH synthesis and realization of LDH modified electrodes

647 A. Beauvois: QXAS data processing

648 S. Belin: Implementing electrochemistry experiments at SOLEIL

649 V. Briois: QXAS analysis and data processing

650 C. Forano: Implementing in situ LDH synthesis at SOLEIL and ex-situ structural analysis

651 **Funding Sources**

652

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<figure><img src="image_16.png" /><figcaption>QXAS</figcaption></figure>

E (V/Ag-AgCl)

<figure><img src="image_17.png" /></figure>

927

928

929