Madeleine Han

Isabel Gómez-Recio

Daniel Gutiérrez Martín

Nathaly Ortiz Peña

Maria Luisa Ruiz-González

Mohamed Selmane

José M González-Calbet

Ovidiu Ersen

Andrea Zitolo

Benedikt Lassalle-Kaiser

David Portehault
email: david.portehault@sorbonne-universite.fr

Christel Laberty-Robert

Benedikt Lassalle
email: kaiserbenedikt.lassalle@synchrotron-soleil.fr

Christel Laberty
email: robertchristel.laberty@sorbonne-universite.fr

Tuning of Oxygen Electrocatalysis in Perovskite Oxide Nanoparticles by the Cationic Composition

Keywords:

Manganese and cobalt perovskite oxides are among the most active precious metal-free electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), respectively. Herein, we question the role of the cationic composition and charge state in manganite, cobaltite and mixed Mn/Co perovskites on the mechanism of oxygen electrocatalysis for ORR and OER. We synthesize in molten salts a range of perovskite nanoparticles active in ORR (single B-site (LaMn)1-γO3 and (La0.7Sr0.3Mn)1-γO3), in OER (single B-site La0.67Sr0.33CoO3-δ) and in both ORR and OER (mixed B-site (LaMn0.6Co0.4)1-γO3).

By using operando X-ray Absorption Spectroscopy coupled to ex situ Electron Energy Loss Spectroscopy, we show that Mn and Co in single B-site perovskites undergo changes in oxidation states at the steady-state during electrocatalysis, while their oxidation states remain unchanged in the mixed Mn/Co perovskite during OER and ORR. We relate these distinct behaviors to modifications of the rate-determining steps of both the OER and ORR electrocatalytic cycles, triggered by an increased covalency of B-O bonds in the mixed perovskites. These results highlight how simple cationic substitutions, accompanied by a control of cationic vacancies, offer a pathway to tune oxygen electrocatalysis.

INTRODUCTION

Storing electrical energy is one of the scientific challenges in the transition from fossil fuels to clean and sustainable energy. Electrical energy can be stored into chemical bonds, by electrochemically splitting water to form dihydrogen and dioxygen gases. Most energy losses in low-temperature electrolyzers are due to slow kinetics of the oxygen evolution reaction (OER). Similar losses occur also in fuel cells that consume dihydrogen, due to the poor kinetics of the oxygen reduction reaction (ORR). The need to design active and cost-effective catalysts for OER and/or ORR is therefore providing a strong impetus to the understanding of the mechanisms of oxygen electrocatalysis. 1,2 ABO3 perovskite oxides, made of alkaline or rare-earth ions in the A-site and of first row transition metal (TM) cations in the B-site, are efficient OER and/or ORR electrocatalysts in alkaline solution. Some of them exhibit activities rivalling IrO2 benchmarks. 3 OER and ORR on perovskite oxides can occur through the adsorbate evolution mechanism (AEM), [4][START_REF] Suntivich | A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles[END_REF][START_REF] Song | A Review on Fundamentals for Designing Oxygen Evolution Electrocatalysts[END_REF] where the metal cations are the active sites, [START_REF] Song | A Review on Fundamentals for Designing Oxygen Evolution Electrocatalysts[END_REF][START_REF] Liu | Cobalt-Doped Perovskite-Type Oxide LaMnO3 as Bifunctional Oxygen Catalysts for Hybrid Lithium-Oxygen Batteries[END_REF][START_REF] Sun | Bifunctional LaMn0.3Co0.7O3 Perovskite Oxide Catalyst for Oxygen Reduction and Evolution Reactions: The Optimized Eg Electronic Structures by Manganese Dopant[END_REF][START_REF] Yoo | Role of Lattice Oxygen Participation in Understanding Trends in the Oxygen Evolution Reaction on Perovskites[END_REF] or through the lattice oxygen mechanism (LOM) involving anionic redox processes on two neighbouring sites. [START_REF] Song | A Review on Fundamentals for Designing Oxygen Evolution Electrocatalysts[END_REF][START_REF] Wang | Labile Oxygen Participant Adsorbate Evolving Mechanism to Enhance Oxygen Reduction in SmMn2O5 with Double-Coordinated Crystal Fields[END_REF] In the frame of AEM, the control of the OER/ORR electrocatalytic activity is usually correlated to the impact of the eg orbitals occupancy and of the covalency of B-O bonds on the kinetics of the rate-determining step. [4][START_REF] Suntivich | A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles[END_REF][START_REF] Song | A Review on Fundamentals for Designing Oxygen Evolution Electrocatalysts[END_REF][START_REF] Liu | Cobalt-Doped Perovskite-Type Oxide LaMnO3 as Bifunctional Oxygen Catalysts for Hybrid Lithium-Oxygen Batteries[END_REF][START_REF] Sun | Bifunctional LaMn0.3Co0.7O3 Perovskite Oxide Catalyst for Oxygen Reduction and Evolution Reactions: The Optimized Eg Electronic Structures by Manganese Dopant[END_REF][START_REF] Yoo | Role of Lattice Oxygen Participation in Understanding Trends in the Oxygen Evolution Reaction on Perovskites[END_REF][START_REF] Hwang | Perovskites in Catalysis and Electrocatalysis[END_REF] Likewise, the B-O covalency can also strongly influence the OER kinetics for perovskites operating through LOM. [START_REF] Suntivich | A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles[END_REF] A number of substituted perovskites have been designed according to these principles, in order to tune the eg occupancy and the B-O covalency, by substituting the A-site or B-site cations and by incorporating oxygen vacancies. [4][START_REF] Suntivich | A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles[END_REF][START_REF] Song | A Review on Fundamentals for Designing Oxygen Evolution Electrocatalysts[END_REF][START_REF] Hwang | Perovskites in Catalysis and Electrocatalysis[END_REF][START_REF] Grimaud | Double Perovskites as a Family of Highly Active Catalysts for Oxygen Evolution in Alkaline Solution[END_REF][START_REF] Grimaud | Activating Lattice Oxygen Redox Reactions in Metal Oxides to Catalyse Oxygen Evolution[END_REF][START_REF] Grimaud | Anionic Redox Processes for Electrochemical Devices[END_REF] Especially, mixed Mn/Co B-sites lead to OER/ORR bifunctional electrocatalysts. [START_REF] Sun | Bifunctional LaMn0.3Co0.7O3 Perovskite Oxide Catalyst for Oxygen Reduction and Evolution Reactions: The Optimized Eg Electronic Structures by Manganese Dopant[END_REF] More fundamentally, incorporating Mn on the B site of LaCoO3 reduces the average number of orbital-filling electrons and raises the O 2p band closer to the Fermi level, which favours OER kinetics. However, the role of each transition metal cation in such bimetallic B-site perovskites during OER and ORR electrocatalysis is still to be elucidated. Especially, to what extent the interplay between these cations can change the OER and ORR kinetics? This question is of utmost importance, as it would bring a more predictive approach for the choice of transition metals and of the overall composition of perovskite oxides towards efficient bi-functional electrocatalysts.

We report here the molten salts synthesis of manganese/cobalt perovskite nanoparticles with various compositions. Their high surface-to-volume ratio makes the use of X-ray absorption spectroscopy (XAS) relevant to extract information on the surface states. We then probe their electronic structure with operando XAS at the Mn and Co K-edges during OER and ORR electrocatalysis. The analysis of the resting state during electrocatalysis highlights reversible oxidation state changes on the B-site for perovskites built on solely Mn or Co cations.

On the other side, mixed manganese/cobalt materials do not show such changes despite similar electrocatalysis. These distinct behaviours point towards different kinetics of oxygen electrocatalysis mechanisms depending on the nature and composition of the B-sites in perovskite electrocatalysts.

RESULTS AND DISCUSSION

Synthesis and characterization of Mn and Co perovskite nanoparticles

The La(Mn,Co)O3± δ perovskite nanocrystals (LMCO) were synthesized by adjusting a procedure previously reported for manganite perovskites that relies on molten salts synthesis. [START_REF] Thi | Surface-Driven Magnetotransport in Perovskite Nanocrystals[END_REF][START_REF] Gonell | Versatile Molten Salt Synthesis of Manganite Perovskite Oxide Nanocrystals and Their Magnetic Properties[END_REF][START_REF] Gonell | Structure-Activity Relationship in Manganese Perovskite Oxide Nanocrystals from Molten Salts for Efficient Oxygen Reduction Reaction Electrocatalysis[END_REF] Briefly, the synthesis consists in reacting metal nitrate salts in adequate ratios into molten potassium nitrate for 1 h at 600 °C (see details in experimental section). Wavelength-dispersive X-ray fluorescence (WDXRF) indicates Co/Mn ratios of 0.34 and 0.49 at. for LMCO30 and LMCO40, respectively, and a La/(Mn+Co) ratio of 1.1 at. for both samples, which indicates an La excess of ~10% at. compared to the ideal perovskite composition (Table S- S-1). The nanowires are therefore attributed to La(OH)3 while the 30 nm nanoparticles correspond to the perovskite phase (Figure S-8). Note that La(OH)3 has little activity in ORR [START_REF] Gu | Lanthanum-Based Compounds: Electronic Band-Gap-Dependent Electrocatalytic Materials for Oxygen Reduction Reaction[END_REF] and OER, [START_REF] Weber | Atomistic Insights into Activation and Degradation of La0.6Sr0.4CoO3 Electrocatalysts under Oxygen Evolution Conditions[END_REF] and it is scarce enough to qualitatively assess the electrocatalytic properties of perovskite nanoparticles. The oxidation states of Mn and Co were evaluated on the pristine powders by recording the Mn and Co K-edge X-ray absorption near-edge structure (XANES) spectra (Figure 2a-b).

As A-site La 3+ is substituted by Sr 2+ from LMO to LSMO and SMO, a shift of the Mn Kedge toward higher energy is observed (Figure 2a Following the same analysis procedure, the Mn oxidation state is determined to be 3.6±0.1 and 3.5±0.2 in LMCO30 and LMCO40, respectively. The Co K-edge (Figure 2b) of LSCO is shifted to higher energy compared to LaCo 3+ O3δ (LCO), in agreement with charge balance, thus indicating an average Co oxidation between +3 and +4. A strong shift to lower energies, between Co 2+ and Co 3+ K-edges, is observed from LCO to LMCO40 and LMCO30. A linear interpolation from the absorption edge positions of Co(OH)2 (Co 2+ ) and LCO (Co 3+ ) yields a Co oxidation state in LMCO30 and LMCO40 at ca. 2.6. This trend is consistent with the concomitant increase in the average Mn oxidation state (Figure 2a) from LMO to LMCO30 and LMCO40 samples. Such electronic compensation between two coexisting transition metals has been observed in several manganite cobaltite oxides. [START_REF] Gholizadeh | Structural and Magnetic Features of La0.7Sr0.3Mn1-xCoxO3 Nano-Catalysts for Ethane Combustion and CO Oxidation[END_REF][START_REF] Yang | In Situ X-Ray Absorption Spectroscopy of a Synergistic Co-Mn Oxide Catalyst for the Oxygen Reduction Reaction[END_REF][START_REF] Kyômen | Correlation between Magnetic Properties and Mn/Co Atomic Order in LaMn0.5Co0.5O3+δ: I. Second-Order Nature in Mn/Co Atomic Ordering and Valence State[END_REF][START_REF] Joy | Spin States of Mn and Co in LaMn0.5Co0.5[END_REF][START_REF] Pecchi | Magnetic and Catalytic Properties of Perovskite-Type Mixed Oxides LaMn1-yCoyO3[END_REF] Because ORR and OER electrocatalytic properties of perovskites are strongly dependent on the stoichiometry, especially cationic [START_REF] Xu | New Undisputed Evidence and Strategy for Enhanced Lattice-Oxygen Participation of Perovskite Electrocatalyst through Cation Deficiency Manipulation[END_REF] and anionic vacancies, [START_REF] Tofield | Oxidative Nonstoichiometry in Perovskites, an Experimental Survey; the Defect Structure of an Oxidized Lanthanum Manganite by Powder Neutron Diffraction[END_REF] we performed Energy Electron Loss Spectroscopy (EELS) and Energy Dispersive X-ray spectroscopy (EDX) to evaluate the precise composition of single particles. By doing so, we could directly correlate transition metal oxidation states with the cationic composition and then address the question of non-stoichiometry in single particles. As for XANES, manganese and cobalt oxidation states were calculated by interpolating the M-L2,3 signal from calibration curves previously recorded (Figure 3 and Figure S-10) [START_REF] Varela | Atomic-Resolution Imaging of Oxidation States in Manganites[END_REF][START_REF] Wang | EELS Analysis of Cation Valence States and Oxygen Vacancies in Magnetic Oxides[END_REF] . The average oxidation states obtained from EELS for Mn and Co and for all assessed materials are consistent with those resulting from the XANES study (Table S-2). By combining XANES, EELS, EDX and WDXRF data, the compositions of LMCO30 and LMCO40 are estimated as (LaMn +3.6 0.75Co +2.6 0.25)0.94O3 and (LaMn +3.5 0.67Co +2.6 0.33)0.98O3 respectively.

According to the cationic compositions measured, some samples present an excess of Mn 3+ or of Mn 4+ compared to the ideal perovskite stoichiometry. The former must be compensated by oxygen vacancies. The latter must account for cationic vacancies, since manganese and cobalt perovskites cannot accommodate oxygen over-stoichiometry through interstitial sites. [START_REF] Tofield | Oxidative Nonstoichiometry in Perovskites, an Experimental Survey; the Defect Structure of an Oxidized Lanthanum Manganite by Powder Neutron Diffraction[END_REF][START_REF] Cortés-Gil | Novel Insights into the Magnetic Behavior of Non-Stoichiometric LaMnO3+δ Nanoparticles[END_REF][START_REF] Reller | Superstructures Formed by the Ordering of Vacancies in a Selective Oxidation Catalyst: Grossly Defective CaMnO3[END_REF][START_REF] Alonso | Influence of the Synthetic Pathway on the Properties of Oxygen-Deficient Manganese-Related Perovskites[END_REF][START_REF] Cortés-Gil | Ferromagnetism in a New Manganese-Related Brownmillerite: La0.5Sr0.5MnO2.5[END_REF][START_REF] Hagenmuller | Nonstoichiometry in the Perovskite-Type Oxides: An Evolution from the Classical Schottky-Wagner Model to the Recent High Tc Superconductors[END_REF][START_REF] Mastin | Structural and Thermal Properties of La1-xSrxCoO3-δ[END_REF][START_REF] Hansteen | Divalent Manganese in Reduced LaMnO3-δ -Effect of Oxygen Nonstoichiometry on Structural and Magnetic Properties[END_REF][START_REF] Pishahang | Oxygen Non-Stoichiometry and Redox Thermodynamics of LaMn1-xCoxO3-δ[END_REF] With these considerations in mind, and using the oxidation state values obtained from XANES and EELS, we could evaluate the most likely compositions of the pristine samples (Table S-3 andTable 1). Accordingly, LSCO exhibits some anionic vacancies, LSMO and LMCO40 hold cationic vacancies, but the mixed B-site perovskite LMCO40 stands out with the highest amount of cationic vacancies.

Sun et al. have suggested that the Co-rich perovskite LaMn0.3Co0.7O3δ is prone to oxygen vacancies, 8 like cobalt perovskites. In the Mn-rich sample reported herein, the expected behaviour may be closer to manganese perovskites, which can accommodate oxygen vacancies and cationic vacancies. The charge compensation mechanism for cationic vacancies observed herein for the mixed Mn/Co B-site perovskite relies only on changes of oxidation states for the B-site cations. This strongly differs from the Co/Fe B-site perovskites, which were suggested to compensate cationic vacancies by oxygen vacancies. 26

OER and ORR electrocatalytic properties of perovskite nanoparticles

The electrocatalytic properties of the synthesized perovskites have been evaluated for the OER and the ORR in an O2-saturated 0.1 M KOH electrolyte with a rotating disk 3-electrode setup (Figure 4 and S-11). The currents were normalized versus the specific surface area of each electrode material measured by N2 sorption analysed by the BET method (Table S no noticeable surface restructuration, whether amorphization or transformation into metal oxyhydroxides, in agreement with our previous observations for LSMO. [START_REF] Gonell | Structure-Activity Relationship in Manganese Perovskite Oxide Nanocrystals from Molten Salts for Efficient Oxygen Reduction Reaction Electrocatalysis[END_REF] The apparent absence of amorphization is consistent with previous reports [START_REF] Grimaud | Double Perovskites as a Family of Highly Active Catalysts for Oxygen Evolution in Alkaline Solution[END_REF][START_REF] Li | Surface Reconstruction of Perovskites for Water Oxidation: The Role of Initial Oxides[END_REF][START_REF] Mefford | Water Electrolysis on La1-xSrxCoO3-δ Perovskite Electrocatalysts[END_REF] for perovskites of compositions similar to those we studied (LMO, LSMO, LSCO for Sr/(La+Sr) < 0.4 at.).

Indeed, surface restructuration is linked to the ability of lattice oxygen anions to undergo exchange with the electrolyte [START_REF] Grimaud | Double Perovskites as a Family of Highly Active Catalysts for Oxygen Evolution in Alkaline Solution[END_REF][START_REF] Li | Surface Reconstruction of Perovskites for Water Oxidation: The Role of Initial Oxides[END_REF] and anionic redox reactivity. [START_REF] Grimaud | Double Perovskites as a Family of Highly Active Catalysts for Oxygen Evolution in Alkaline Solution[END_REF][START_REF] Moysiadou | Mechanism of Oxygen Evolution Catalyzed by Cobalt Oxyhydroxide: Cobalt Superoxide Species as a Key Intermediate and Dioxygen Release as a Rate-Determining Step[END_REF] This reactivity is favored when the O p-band is close to the Fermi level. In the perovskites considered herein, the O pband is sufficiently far from the Fermi level [START_REF] Grimaud | Double Perovskites as a Family of Highly Active Catalysts for Oxygen Evolution in Alkaline Solution[END_REF][START_REF] Mefford | Water Electrolysis on La1-xSrxCoO3-δ Perovskite Electrocatalysts[END_REF] to hinder surface reconstruction during electrocatalysis. We also perform our operando investigations at moderate current densities, which again hinders such evolution. S-6). For LMO (Figure 5a), the starting Mn oxidation state is below 3, thus below the value of the pristine powder. This suggests reduction during the ink processing and dispersion in ethanol. Then, the oxidation state further decreases overtime in the dry state, down to below 2.8. This phenomenon is attributed to reduction under the beam in the presence of carbon when the material is not cooled down in the electrolyte. [START_REF] Weatherup | Environment-Dependent Radiation Damage in Atmospheric Pressure X-Ray Spectroscopy[END_REF] Under increasingly reducing conditions (0.6, 0.4 and 0.2 V/RHE), the differential spectra exhibit a rising peak near the K-edge at 6550 eV compared to the reference open circuit voltage (OCV, 0.9 V/RHE) state. This peak indicates a decrease of the edge energy by 1 eV, thus a reduction of manganese under ORR conditions, down to a Mn oxidation state of ca. 2.5 at 0.4 V/RHE, under ORR conditions. The differential peak remains identical after the potential is set back to the OCV. Then, when the electrode is maintained at the OCV, the Mn oxidation state rises up to its original value of 2.9. Therefore, the Mn reduction under ORR conditions is reversible. When increasing the potential from the OCV to 1. and S-8) is similar to the one observed for LMO, but with higher Mn oxidation state due to Sr 2+ doping. When the potential is reduced to 0.2 V/RHE, the Mn average oxidation state decreases from 3.3 to 2.7, indicating the formation of Mn 2+ under ORR reduction in a perovskite that is initially composed of mixed valence Mn 3+/4+ . A similar observation was made for an electrodeposited δ-KxMnO2-yꞏzH2O electrode. [START_REF] Risch | Redox Processes of Manganese Oxide in Catalyzing Oxygen Evolution and Reduction: An in Situ Soft X-Ray Absorption Spectroscopy Study[END_REF] We then studied the Co electronic state in LSCO during ORR and OER by monitoring the cobalt K-edge XANES as a function of time (Figure 5b and Table S decreasing the potential down to ORR conditions (0.4 V/RHE), the differential spectrum displays an intense peak at the Co K-edge at 7720 eV, a small peak at 7730 eV and a large bump at 7770 eV, with a decrease of the Co oxidation state from 3.2 to 2.9. [START_REF] Grimaud | Double Perovskites as a Family of Highly Active Catalysts for Oxygen Evolution in Alkaline Solution[END_REF] Co is also bound to less oxygen anions during ORR than during OER according to the FT-EXAFS ( We evaluated the evolution of the anionic compositions of the samples probed operando from the average Mn and Co oxidation states, with the assumptions made above for the pristine samples (Table 1). The compositions are steady when the electrodes are immersed in the basic electrolyte (OCV conditions). During electrocatalysis, some samples exhibit significant composition deviations from the pristine states, but they are maintained in the stable stoichiometry range of manganese and cobalt perovskites, [START_REF] Tofield | Oxidative Nonstoichiometry in Perovskites, an Experimental Survey; the Defect Structure of an Oxidized Lanthanum Manganite by Powder Neutron Diffraction[END_REF][START_REF] Cortés-Gil | Novel Insights into the Magnetic Behavior of Non-Stoichiometric LaMnO3+δ Nanoparticles[END_REF][START_REF] Reller | Superstructures Formed by the Ordering of Vacancies in a Selective Oxidation Catalyst: Grossly Defective CaMnO3[END_REF][START_REF] Alonso | Influence of the Synthetic Pathway on the Properties of Oxygen-Deficient Manganese-Related Perovskites[END_REF][START_REF] Cortés-Gil | Ferromagnetism in a New Manganese-Related Brownmillerite: La0.5Sr0.5MnO2.5[END_REF][START_REF] Hagenmuller | Nonstoichiometry in the Perovskite-Type Oxides: An Evolution from the Classical Schottky-Wagner Model to the Recent High Tc Superconductors[END_REF][START_REF] Mastin | Structural and Thermal Properties of La1-xSrxCoO3-δ[END_REF][START_REF] Hansteen | Divalent Manganese in Reduced LaMnO3-δ -Effect of Oxygen Nonstoichiometry on Structural and Magnetic Properties[END_REF][START_REF] Pishahang | Oxygen Non-Stoichiometry and Redox Thermodynamics of LaMn1-xCoxO3-δ[END_REF] thus supporting the stability of these materials under ORR and OER conditions. LMO, LSMO and LSCO are reduced under ORR conditions, which is compensated by the creation of oxygen vacancies. These transformations are reversible and the materials recover compositions close to the initial ones under oxidative OER conditions (LSMO is inactive in OER and has not been evaluated in these conditions). The mixed B-site perovskite LMCO40 stands out from the other samples, as we do not observe any significant change in the composition during electrocatalysis, as mentioned above for the Mn and Co oxidation states.

The mixed B-site perovskite exhibits a deviation of Mn and Co oxidation states compared to single B-site perovskites. Mn 2+ forms operando under ORR conditions in LMO and LSMO (average oxidation states below +3, Figure 7 and Table S-5). Likewise, Co 4+ is detected operando under OER conditions (average oxidation states above +3, Figure 7). On the opposite, Mn 2+ and Co 4+ are not observed in LMCO40, which contains under all conditions mixed valences Mn 3+/4+ and Co 2+/3+ . Hence, an electron transfer occurs from Mn 2+ to Co 4+ in the mixed perovskite. Although Mn and Co species are usually considered as active sites of ORR and OER electrocatalysts, 4,[START_REF] Suntivich | A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles[END_REF] respectively, one may notice that Mn 3+ and Mn 4+ encountered in LMCO40 are proposed to be intermediates in the water oxidation reaction for several OERactive manganese oxides [START_REF] Gorlin | Mn3O4 Supported on Glassy Carbon: An Active Non-Precious Metal Catalyst for the Oxygen Reduction Reaction[END_REF][START_REF] Gorlin | In Situ X-Ray Absorption Spectroscopy Investigation of a Bifunctional Manganese Oxide Catalyst with High Activity for Electrochemical Water Oxidation and Oxygen Reduction[END_REF] as well as in the Oxygen Evolving Complex of Photosystem II. [START_REF] Kern | Structures of the Intermediates of Kok's Photosynthetic Water Oxidation Clock[END_REF] Conversely, Co 2+ present in LMCO40 has been proposed as an ORR active species. [START_REF] He | A Co/Metal-Organic-Framework Bifunctional Electrocatalyst: The Effect of the Surface Cobalt Oxidation State on Oxygen Evolution/Reduction Reactions in an Alkaline Electrolyte[END_REF] Therefore, Mn and Co cations may act together as active sites in LMCO40 for both ORR and OER.

The behaviour we observe during electrocatalysis can be discussed in the mechanistic frame of OER/ORR on perovskites. [START_REF] Grimaud | Activating Lattice Oxygen Redox Reactions in Metal Oxides to Catalyse Oxygen Evolution[END_REF][START_REF] Grimaud | Anionic Redox Processes for Electrochemical Devices[END_REF] LSCO (Sr/(La+Sr) < 0.4 at.) has been shown to perform alkaline OER through the AEM. [START_REF] Song | A Review on Fundamentals for Designing Oxygen Evolution Electrocatalysts[END_REF][START_REF] Mefford | Water Electrolysis on La1-xSrxCoO3-δ Perovskite Electrocatalysts[END_REF][START_REF] Rong | A Fundamental Relationship between Reaction Mechanism and Stability in Metal Oxide Catalysts for Oxygen Evolution[END_REF] For perovskites that do not undergo surface amorphization, OER (and ORR) following the AEM are usually considered to occur through four coupled proton-electron transfers. [START_REF] Grimaud | Activating Lattice Oxygen Redox Reactions in Metal Oxides to Catalyse Oxygen Evolution[END_REF][START_REF] Grimaud | Anionic Redox Processes for Electrochemical Devices[END_REF] In this mechanism, the rate-determining step (RDS) is often the formation of a B m+ (OOH) intermediate from B (m+1)+ (O 2-) [START_REF] Grimaud | Anionic Redox Processes for Electrochemical Devices[END_REF] (step 2 on Figure 8a)

or its deprotonation to form a B (m+1)+ (O2 2-) species (step 3 on Figure 8a). Operando XAS shows that the OER resting state for LSCO corresponds to an accumulation of B species with increased oxidation state. This might indicate either that the formation of the O-O bond (step 2) is the RDS, or that it is a pre-equilibrium exists between B (m+1)+ (O 2-) and B m+ (OOH), displaced towards B m+1 (O 2-). In this case, step 3 would then be the RDS. Although the latter case has been proposed for cobalt oxyhydroxide operating through the LOM, [START_REF] Moysiadou | Mechanism of Oxygen Evolution Catalyzed by Cobalt Oxyhydroxide: Cobalt Superoxide Species as a Key Intermediate and Dioxygen Release as a Rate-Determining Step[END_REF] to our knowledge there has been no evidence of a pre-equilibrium for perovskites operating through the AEM mechanism. [START_REF] Song | A Review on Fundamentals for Designing Oxygen Evolution Electrocatalysts[END_REF][START_REF] Mefford | Water Electrolysis on La1-xSrxCoO3-δ Perovskite Electrocatalysts[END_REF][START_REF] Rong | A Fundamental Relationship between Reaction Mechanism and Stability in Metal Oxide Catalysts for Oxygen Evolution[END_REF] Considering the O-O bond formation (step 2) as the RDS is also consistent with most mechanistic proposals. [START_REF] Grimaud | Activating Lattice Oxygen Redox Reactions in Metal Oxides to Catalyse Oxygen Evolution[END_REF][START_REF] Grimaud | Anionic Redox Processes for Electrochemical Devices[END_REF] A similar reasoning can be made for LMO during OER.

Under ORR conditions, the RDS is expected to be the protonation and reduction of B (m+1)+ (O 2-) species (step IV Figure 8b), or the binding of O2 and release of OH -concomitant with an increase of the B oxidation state (step I Figure 8b). [START_REF] Grimaud | Anionic Redox Processes for Electrochemical Devices[END_REF] Operando XAS data collected during ORR on all single B-site perovskites studied herein (LMO, LSMO and LSCO) show reduced B-site cations in the resting state as compared to their initial states. Although we cannot exclude the occurrence of a pre-equilibrium, we note that this possibility has not been proposed in previous mechanistic ORR proposals, 4,[START_REF] Hwang | Perovskites in Catalysis and Electrocatalysis[END_REF][START_REF] Grimaud | Anionic Redox Processes for Electrochemical Devices[END_REF] and that the displacement of a surface hydroxide group by O2 (step I Figure 8b) is the most common RDS for ORR on perovskite oxides, 4 in agreement with our data. For both OER and ORR, our data for single B-site perovskites are then consistent with RDSs corresponding to the reaction of B(O 2-) or B(OH -) moieties to form peroxo species. The mixed B-site perovskite LMCO40 exhibits a behaviour opposite to single B-site compounds, as the oxidation state of the B-site cations is unchanged in the resting state. To understand this drastic difference, we further examine the impact of the ex situ and operando chemical compositions of the perovskites on the nature of the B-O bonds. The covalency of the B-O bond in manganese perovskites is increased by substituting the B-site cation with a more electronegative metal, such as cobalt, and by increasing the manganese oxidation state. [START_REF] Song | A Review on Fundamentals for Designing Oxygen Evolution Electrocatalysts[END_REF][START_REF] Hwang | Perovskites in Catalysis and Electrocatalysis[END_REF][START_REF] Grimaud | Anionic Redox Processes for Electrochemical Devices[END_REF] The B-O covalency is also enhanced by incorporating cationic vacancies. [START_REF] Song | A Review on Fundamentals for Designing Oxygen Evolution Electrocatalysts[END_REF] We note that under OER operating conditions, LMCO40 exhibits the highest Mn oxidation state and the highest amount of cationic vacancies (Table 1) among the studied compounds. Hence, the electronic state and composition of the mixed Mn/Co perovskite enhance the B-O covalency. This increase of covalency has been shown [4][START_REF] Suntivich | A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles[END_REF][START_REF] Song | A Review on Fundamentals for Designing Oxygen Evolution Electrocatalysts[END_REF] to increase the rate of both OER and ORR, especially the steps involved in the formation of peroxo species, which we identified above as plausible OER/ORR RDSs for the single B-site perovskites. We can then assume that due to increased covalency, these steps could be fast enough on the mixed B-site perovskite to trigger a change in the RDSs.

Increasing the covalency of the B-O bond facilitates the deprotonation of the hydroxo or peroxo ligands, which makes the existence of pre-equilibra for steps 1 and 3 (Figure 8a) of the OER AEM cycle even less likely than for the single B-site perovskites.

Again with the assumption that there is no pre-equilibrium for OER on perovskite surfaces operating through the AEM mechanism, [START_REF] Song | A Review on Fundamentals for Designing Oxygen Evolution Electrocatalysts[END_REF][START_REF] Mefford | Water Electrolysis on La1-xSrxCoO3-δ Perovskite Electrocatalysts[END_REF][START_REF] Rong | A Fundamental Relationship between Reaction Mechanism and Stability in Metal Oxide Catalysts for Oxygen Evolution[END_REF] the identification by XAS of unchanged oxidation state suggests that steps 1 or 3 (Figure 8a In this energetic configuration, some O 2p electrons are transferred to the metal d band, so that the redox processes occur on the anionic sublattice. [START_REF] Song | A Review on Fundamentals for Designing Oxygen Evolution Electrocatalysts[END_REF][START_REF] Hwang | Perovskites in Catalysis and Electrocatalysis[END_REF][START_REF] Grimaud | Anionic Redox Processes for Electrochemical Devices[END_REF] According to the absence of change of the oxidation states of B cations at the OER/ORR steady-states, the LOM RDSs for the mixed Bsite perovskite would be hydroxyl deprotonation (step 1, Figure 8c) or hydroxyl uptake (step 4, Figure 8c) for OER and O-O splitting (step III, Figure 8d) or hydroxyl regeneration (step IV, opposite to step 1 in OER, Figure 8d) for ORR. As the same step would not be RDS for the two opposite reactions OER and ORR, then the LOM RDSs would be probably hydroxyl uptake for OER (step 4, Figure 8c) and O-O splitting for ORR (step III, Figure 8d). This discussion relates to the catalytic cycle commonly proposed for LOM on perovskites where no pre-equilibrium is considered, [START_REF] Song | A Review on Fundamentals for Designing Oxygen Evolution Electrocatalysts[END_REF][START_REF] Grimaud | Anionic Redox Processes for Electrochemical Devices[END_REF] but mechanisms involving a pre-equilibrium have been unambiguously identified for OER involving lattice oxygen on cobalt oxyhydroxides. [START_REF] Moysiadou | Mechanism of Oxygen Evolution Catalyzed by Cobalt Oxyhydroxide: Cobalt Superoxide Species as a Key Intermediate and Dioxygen Release as a Rate-Determining Step[END_REF][START_REF] Surendranath | Mechanistic Studies of the Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH[END_REF] Accordingly, that more complex catalytic cycles should probably be considered as well for perovskites, which is out of the scope of this study. While the present data do not allow distinguishing between AEM and LOM, we note that LOM is usually observed for materials exhibiting significant amounts of oxygen vacancies, which are not observed for the mixed Bsite perovskite examined here. However, LOM usually occurs together with AEM at a varying extent. [START_REF] Grimaud | Anionic Redox Processes for Electrochemical Devices[END_REF] Hence, we do not discard the occurrence of both mechanisms in the mixed B-site perovskite, although our data indicate changes in the rate-determining steps between single and mixed B-site perovskites during OER and ORR.

CONCLUSION

By synthesizing single and mixed B-site perovskite nanoparticles, we have designed bifunctional electrocatalysts for alkaline water oxidation and oxygen reduction. We have combined advanced electron energy loss spectroscopy coupled to transmission electron microscopy with ex situ and operando X-ray absorption spectroscopy (XAS) in order to provide detailed insights into the perovskite composition and the evolution of the electronic states of manganese and cobalt cations in the perovskite B-site. Probing the resting state during electrocatalysis by operando XAS enabled to propose rate-determining steps for water oxidation and oxygen reduction reactions. For single B-site manganite and cobaltite perovskites, these steps agree with the classical coupled proton-electron transfer mechanism where the B-site cations are the active sites. On the opposite, the absence of change of oxidation state in the resting state for a mixed Mn/Co B-site perovskite indicates a modification of the rate-determining step. We relate this behaviour to an increase of the B-O covalency through the formation of the cationic vacancies upon partial cationic substitution. Therefore, our work

shows how the interplay between cationic substitutions, electron transfer between transition metals and the formation of cationic vacancies, can be adjusted to modify the kinetics of oxygen electrocatalysis.

MATERIALS AND METHODS

The synthesis of strontium doped lanthanum manganite (LSMO) was performed according to a procedure reported previously. [START_REF] Thi | Surface-Driven Magnetotransport in Perovskite Nanocrystals[END_REF]

Synthesis of manganite cobaltite LMCO40

The manganite perovskites were synthesized from the nitrate precursors La(NO3)3•6H2O (Sigma-Aldrich, > 99 %, 8.1 mmol), Mn(NO3)2•4H2O (Aldrich, 98 %, 5.6 mmol), Co(NO3)2•6H2O (Alfa Aesar, > 98 %, 2.7 mmol) and KNO3 (Sigma-Aldrich, > 99%, 80 mmol).

The powder mixtures were coarsely grinded with a few drops of ethanol in a ball miller (20 Hz, 2 min) and dried in a vacuum oven (60 °C) for at least 24 hours. For each sample, 2.5 g of the corresponding powder was put into an alumina crucible and placed for one hour in a muffle furnace under air and preheated at 600 °C. The crucible was then removed from the hot oven and cooled down at room temperature, under air. The recovered black solid was dispersed in deionized water (c.a. 60 mL). The dispersion was centrifuged (20 000 rpm for 20 min) then the transparent supernatant was removed. The remaining black solid was redispersed in deionized water and washed by ten centrifugation-redispersion cycles until the supernatant conductance reached a value of 20 µS cm -1 . The powders were finally dried under vacuum at 60 °C overnight.

Synthesis of cobaltite LSCO

The cobaltite perovskites were synthesized from the nitrate precursors La(NO3)3ꞏ6H2O

(Sigma-Aldrich, > 99 %, 4.7 mmol), Sr(NO3)2 (Aldrich, 99 %, 2.4 mmol), Co(NO 3)2ꞏ6H2O

(Alfa Aesar, > 98 %, 6.9 mmol) and NaNO2 (Sigma-Aldrich, > 99%, 471 mmol). The solid mixture was coarsely grinded with a ball miller (20 Hz, 2 min) and dried in a vacuum oven (60 °C) for 3 hours and grinded again with a mortar. It was then dried again in the vacuum oven for more than 24 hours to avoid extra-hydration of precursors. 2.5 g of the powder was put into an alumina crucible and placed in a muffle furnace under air and preheated at 700 °C for one hour. The crucible was then removed from the oven and cooled down at room temperature. The recovered black solid was dissolved in deionized water (c.a. 100 mL).

The dispersion was centrifuged (20 000 rpm) then the transparent supernatant was removed.

The remaining black powder was redispersed in deionized water, washed and dried as the manganese perovskites described above.

Synthesis of cobaltite LCO

The cobaltite perovskite LCO was obtained by solid state synthesis. La2O3 (6 mmol) and Co3O4 (4.1 mmol) previously dried were grounded and heated at 1100 C for 24 h.

X-ray diffraction

Powder X-ray diffraction (XRD) was performed on a Bruker D8 advance diffractometer equipped with a Cu Kα source (Kα1= 1.5406 Å, Kα2= 1.5443 Å) in reflection mode, with an accelerating voltage of 40 kV and a current of 40 mA. The PDF reference cards for La0.67Sr0.33MnO3, La0.67Sr0.33CoO3 and La(OH)3 were 04-014-4312, 04-014-1579 and 04-005-8587 respectively. The ICSD reference card for LaMnO3 is 1542145.

Overall compositions

The atomic composition was measured by wavelength-dispersive X-ray fluorescence (WDXRF) on pelletized powders with a Bruker S8 Tiger spectrometer equipped with a Rh tube.

Transmission electron microscopy

Transmission electron microscopy (TEM) was performed on a FEI Spirit 2 apparatus operating at 120 kV. Qualitative STEM-EDX elemental mapping was performed on a JEOL JEM 2100

Plus LaB6 working at an accelerating voltage of 200 kV. The powders were deposited on carbon-coated copper grids by drop casting an ethanolic suspension of the particles.

Scanning transmission electron microscopy-Energy Electron Loss Spectroscopy (EELS) was performed in a probe spherical aberration corrected microscope JEOL JSM-ARM200F (Cold Emission Gun) operating at 120 kV. High angle-annular dark field (HAADF)

images were acquired at 68-280 mrad inner and outer collection semi-angles. EELS experiments were acquired using a GIF-QuantumER with a collection and covergence semiangle of 18 mrad and 20.3 mrad, respectively. Dual EELS function of the GIF-QuantumER spectrometer was used to record simultaneously zero-loss peak and core region in order to minimize the uncertainty on the energy shift. EEL spectrums were acquired by using spectrum line mode, with 0.25 and 0.1 eV energy dispersion per channel, an acquisition time of 0.5 s over an average total number of 100 to 150 points per particle, in a total of 20 particles per composition, and pixel size of 2 Å. Principal component analysis was performed on EELS data set to de-noise the spectra by using the MSA (multivariate statistical analysis) plugins for the GatanDMS analysis toolbox [START_REF] Watanabe | Analysis of Spectrum-Imaging Datsets in Atomic-Resolution Electron Microscopy[END_REF] .

Transition metal oxidation states have been analyzed from electron loss near energy edge structure (ELNES) of Mn L2,3 and Co-L2,3 edges. Numerical values were estimated by M-L2,3 interpolation in the fitting curves reported by M. Varela et al. [START_REF] Varela | Atomic-Resolution Imaging of Oxidation States in Manganites[END_REF] and Z. L. Wang et al. [START_REF] Wang | EELS Analysis of Cation Valence States and Oxygen Vacancies in Magnetic Oxides[END_REF] , respectively and error bars by repeating L2,3 calculation, modifying integrate window position ± 1 pixel.

Quantitative EDX was performed in a JEOL-JEM GRAND ARM 300cF microscope (Cold Emission Gun) equipped with SDD CENTURIO spectrometer, operated at 120 kV. The experimental error of this technique is in the order of 1 at.%.

Ex situ electrochemical measurements

A composite ink was prepared from the electrocatalyst (10 mg), activated acetylene black (Alfa Aesar, 99.9 %, 10 mg) and absolute ethanol (VWR, 10 mL). It was sonicated for 2 hours and Nafion dispersed in ethanol (Sigma Aldrich, 5 % dispersion, 435 µL) was added. The suspension was sonicated again for 30 min. The electrochemical experiments were carried out less than 12 hours after starting the preparation of the ink. elements Si drift fluorescence detector). The Co K-edge spectra were collected between 7510 and 8560 eV in a step mode with a scan rate of 3.5 eV.s -1 and an integration time of 0.07 s per point for a recording time between 6 and 7 min. The Mn K-edge spectra were collected between 6340 and 7390 eV in a step mode with a scan rate of 3.5 eVꞏs -1 with an integration time of 0.07 s per point for a recording time of 5 min per scan. The XAS spectra were recorded in fluorescence mode and normalized by a I0 measured before the sample. The operando electrochemical cell was similar to the one described by Lassalle et al. [START_REF] Lassalle-Kaiser | In Situ/Operando Studies of Electrocatalysts Using Hard X-Ray Spectroscopy[END_REF] The reference was a saturated calomel electrode (0.244 V/NHE). The counter electrode was a platinum wire in a frit.

The working electrode was a glassy carbon plate covered with an electrocatalyst ink (ca. 20 µLꞏcm -2 and 20 µg of catalyst per cm -2 , geometric active surface of ca. 1 cm -2 ). The plate was glued on a hole on the side of the cell delimiting the geometric active surface so that one side was in contact with the electrolyte, and the other side was directly exposed to the incident Xrays. In this configuration, X-rays are only absorbed by the components of the working electrode, namely the glassy carbon plate whose thickness is 60 µm and the electrocatalyst ink.

The vertical position of the working electrode allows oxygen bubbles that are formed during OER to escape to the surface of the aqueous electrolyte, and thus refresh the species near the electrocatalysts. The cell is large enough to add an oxygen bubbling inlet for ORR/OER experiments.

Operando XAS measurements

All Table S-1). These values are slightly higher than those expected from elemental WDXRF analysis (Table S-1). This discrepancy is attributed to the uncertainties in the evaluation of compositions from XRD and WDXRF, which amount to few percents for each technique. We note that this do not rule out the possible presence of amorphous Mn and Co-rich phases, although such components could not be detected by TEM and the in situ behaviour we observed by XAS, with constant Mn/Co oxidation state during electrocatalysis, cannot be ascribed to such phases, which on the opposite undergo changes of redox states during electrocatalysis. [2] Likewise, Co3O4 was implemented in the Rietveld analysis, but no corresponding peak could be detected and the fit was not improved by adding this third phase, so that we discard the presence of the spinel at amounts above the detection threshold of powder XRD.

  1). Reference samples LaMn +3 O3±  (LMO, Figures S-1 and S-2), La0.7Sr0.3MnO3±  (LSMO, Figures S-2 and S-3), La0.75Sr0.25CoO3- (LSCO, Figures S-4 and S-5) and SrMn +4 O3- (SMO) with similar particle size were synthesized by adjusting the procedure described above (see SI). LaCo +3 O3- (LCO) was obtained through a solid-state reaction (SI). Transmission electron microscopy (TEM) (Figure 1a-b) reveals two populations of objects: nanoparticles with an average diameter of ca. 30 nm (Figure S-6) and nanowires of diameter ca. 40 nm and lengths ranging from 100 to 300 nm (Figure 1c). Scanning transmission electron microscopy (STEM) elemental mapping coupled to X-ray energy dispersive (EDX) analysis (Figure 1c-f) evidences the homogeneous distribution of La, Mn and Co within the nanoparticles, while only La is detected in the nanowires. X-ray powder diffraction (XRD) reveals that the powder consists in the perovskite phase as major component, with lanthanum trihydroxide as a side-product (Figure 1g). The content of La(OH)3 is evaluated to ~10 wt.% according to Rietveld refinement of the powder XRD patterns (Figure S-7), in acceptable agreement with WDXRF data (Table S-1). Rietveld refinement also indicated no noticeable amounts of crystalline cobalt spinel (Figure S-7, Table

Figure 1 .

 1 Figure 1. TEM images of (a) LMCO30 and (b) LMCO40, corresponding to LaMn0.7Co0.3O3 ±δand LaMn0.6Co0.4O3± δ , respectively. (c) STEM and corresponding (d) La, (e) Mn, and (f) Co STEM-EDX maps of the LMCO30 sample. (g) Powder XRD patterns of the LMCO30 and LMCO40 samples. The patterns are indexed versus the LaMnO3± δ structure (black) and La(OH)3 (red) references (see Materials and Methods section).

  ), indicating an increase in the average Mn oxidation state, as expected from charge balance. We observe the same trend when substituting Mn by Co from LMO to LMCO30 and LMCO40. Linear combination fitting analysis (LCF) of LSMO, LMCO30 and LMCO40 with LMO and SMO as references (Figure S-9) enables estimating the Mn oxidation state to 3.4±0.1 in LSMO, close to the expected value of 3.3 according to the composition evaluated by WDXRF.

Figure 2 .

 2 Figure 2. (a) Mn K-edge XANES spectra of powders: LMO, LSMO, LMCO30, LMCO40. The references for SMO is reported from Celorrio et al (CC-BY 4.0). 25 (b) Co-K-edge XANES spectra of powders: LCO, LSCO, LMCO30, LMCO40 and Co(OH) 2 . The inserts magnify the half-edge jump regions.

Figure 3 .

 3 Figure 3. (a) Low and (b) high magnification HAADF-STEM images of LMCO40. (c, d) Corresponding EELS sum spectrum, (e) Mn and (f) Co oxidation state measured along the dashed line in (b).

  electrochemical surface area (ECSA) show a similar trend for the mixed B-site perovskite (Figure S-11b). In order to probe the role of B-site cationic substitution in manganites, we have monitored chemical and structural changes of the nanomaterials during electrocatalysis by using operando X-ray absorption spectroscopy (XAS) at the Mn and Co K-edges.

Figure 4 .

 4 Figure 4. Cyclic voltammograms of LMO (LaMnO3), LMCO30 (LaMn0.7Co0.3O3δ ) and LMCO40 (LaMn0.67Co0.33O3δ ) samples in an O2-saturated 0.1 M KOH electrolyte using a rotating disk electrode (1600 rpm) at 10 mVꞏs -1 . Electrocatalyst loading: 30 µgꞏcm -2 disk.

19

 19

Operando X-ray absorption spectroscopy of single metal B-site perovskiteelectrocatalysts

  Figures S-14 and S-15). Oxidation states retrieved from these measurements are also summed up in Table S-5. Each figure exhibits two panels: the first panel (Figures5a and c) shows the electrochemical potential at which the electrode is set, together with the oxidation state of the studied transition metal determined by linear interpolation from the absorption edge energy at half-edge jump of

Figure 5 .

 5 Figure 5. Operando XAS study of (a and b) LMO and (c and d) LSCO during OER and ORR electrocatalysis. (a and c) Time evolution of the oxidation state (red) at different potentials (blue). The period when the potential is not defined (a) corresponds to the dry electrode. The oxidation state is calculated by linear interpolation from the absorption edge positions of pristine LMO and SMO for Mn, and Co(OH)2 and LCO for Co. (b and d): XANES spectra at these potentials, from which the first OCV spectrum is subtracted. The dashed vertical line is the half-edge jump energy of the first OCV spectrum.

  4 and then 1.8 V/RHE, the aforementioned differential peak disappears and a small dip appears at 6555 eV, indicating an oxidation of manganese, up to an oxidation state of 3.2 from 1.4 V/RHE and beyond. Finally, the electrode was brought back to the OCV. The flat differential spectrum indicates that the electrode material recovered its initial state and that its oxidation under OER is reversible. The Fourier Transform (FT) of the Extended X-ray Absorption Fine Structure (EXAFS) at the Mn K-edge (Figure S-18) shows a first peak at 1.42 Å ascribed to the first Mn-O distance. The magnitude of this peak is correlated to the number of oxygen anions in the Mn coordination sphere. It increases during OER and decreases during ORR, which indicates filling of oxygen vacancies in OER conditions and the creation of oxygen vacancies in ORR conditions. The evolution of LSMO under electrocatalytic conditions (Figures S-19 to S-23, Tables S-7

Figure S- 24 )

 24 , in agreement with the evolution of the Mn coordination described above for LMO and LSMO. When the electrode is brought back to the OCV, the Co average oxidation state recovers its initial value of 3.1 at the OCV. The differential spectrum also shows a decrease in the intensity of the peak at 7720 eV, but a remaining peak at 7730 eV. Therefore, although changes in the oxidation state of Co during OER and ORR appear reversible, some structural changes under ORR conditions remain after operation. Overall, cobalt in LSCO follows the same trend as manganese in LMO and LSMO: the transition metal oxidizes during OER and reduces during ORR. These changes are quasi-reversible, since bringing back the electrode materials from ORR conditions to the OCV is not sufficient to fully recover the initial state within 1 h.Operando X-ray absorption spectroscopy of mixed metal B-site perovskite electrocatalystsWe then assessed the evolution during electrocatalysis of the redox states for the mixed Mn and Co perovskite with the largest Co content, LMCO40 (Figures S-25 to S-30 and Table S-10), as it exhibits higher activity than LMCO30. Manganese and cobalt K-edge spectra were recorded successively at each potential step. XANES spectra were collected and averaged for 20 and 40 min at the Mn (Figure6a-b) and Co (Figure6c-d) K-edges, respectively, in order to reach a sufficient signal-to-noise ratio for each metal despite their different contents. The Mn and Co oxidation states measured operando are ca. +3.4 and +2.6, respectively. They are higher and lower, respectively, than those of their single metal counterparts (see Figure7and TableS-5). For each element, no modification in the overall shape of the spectra nor in the edge energies is observed after 1 hour at a constant potential, whether under ORR or OER conditions. Contrary to single B-site perovskite detailed above, no significant change of the Mn and Co coordination spheres is observed by EXAFS from OER to ORR conditions (Figures S-29 and S-30). This indicates that no change of oxidation state or of local structure occur for both the manganese and cobalt cations. This behaviour is in sharp contrast with the clear changes of oxidation states observed for the single B-site perovskites LMO, LSMO and LSCO.

Figure 6 .

 6 Figure 6. Evolution of XANES spectra and oxidation state of (a and b) Mn and (c and d) Co of LMCO40 from operando XAS during OER/ORR electrocatalysis. (a and c) Time

Figure 7 .

 7 Figure 7. Operando evolution of the manganese (red) and cobalt (blue) oxidation states in perovskites under ORR and OER conditions.

Figure 8 .

 8 Figure 8. (a, b) Classical four-steps AEM mechanism of proton-coupled electron transfer for (a) the OER and (b) the ORR. (c, d) Possible LOM mechanism based on an anionic redox process for (c) the OER and (d) the ORR. Green circles in (a) and (b) highlight the ratedetermining steps (RDS) for single B-site perovskites. Orange circles show the plausible RDS for the mixed B-site perovskite LCMO40. Dashed boxes highlight the major species at the steady state detected by operando XAS.

Figure 8b )

 8b Figure 8b) are the OER and ORR RDSs, respectively. The second hypothesis corresponds to a more frequent case for perovskites (Figure8a-b).[START_REF] Suntivich | A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles[END_REF] It is also consistent with the lower impact of the increased B-O covalency on the deprotonation of OOH ligands as compared to OH ligands. Hence, for the mixed perovskite, we identify the OOH deprotonation (step 3) and the hydroxyl regeneration (step IV) as the most plausible RDSs for OER and ORR, respectively.Noteworthy, an increase of the B-O covalency can also trigger a switch of the ORR/OER mechanism, from cation-based AEM (Figure8a-b) to anion-based LOM (Figure8c-d).[START_REF] Song | A Review on Fundamentals for Designing Oxygen Evolution Electrocatalysts[END_REF][START_REF] Hwang | Perovskites in Catalysis and Electrocatalysis[END_REF] Indeed, while AEM occurs when the oxygen p band is lower in energy than the metal d band, favoring the metal cation as adsorption center, LOM can occur when the oxygen p band comes closer to or overlaps with the metal d band.

  The rotating disk electrode (RDE, Bio-logic) glassy carbon substrate (ø = 3 mm) was previously polished consecutively on a 1 µm paper (Buehler TexMet C, 5 min) and a 0.05 µm paper (Buehler ChemoMet, 5 min). 2 µL of the ink was then dropped on the substrate. The deposit was dried for 30 min in air. A few microliter of electrolyte was then dropped on the substrate to wet the electrode. After 30 min, the substrate was connected to the RDE. The 0.1 M KOH (Alfa Aesar, 85 %) electrolyte was prepared the day of the electrochemical experiments.The electrochemical measurements were carried out in a plastic beaker to prevent any glass dissolution from a glass beaker in the alkaline electrolyte. The electrolyte was saturated with dioxygen by bubbling for 30 min before the beginning of the experiment and all along the measurement during both OER and ORR experiments. The reference electrode was a saturated Ag/AgCl electrode (E 0 = 0.197 V/SHE). The counter electrode was a platinum wire separated from the electrolyte with a frit. The working electrode was on RDE (Bio-logic) equipped with nanoparticle ink-coated glassy carbon substrate tip. The cyclic voltammograms were recorded at a sweep rate of 10 mVꞏs -1 over 5 cycles with the RDE rotating at 1600 rpm. For the OER/ORR experiments (LMO, LSCO and LMCO samples), the cyclic voltammograms were recorded from the OCV to 1.8 V/RHE, then 0.4 V/RHE, then back to OCV.XAS setupXAS spectra were recorded on the SAMBA beamline at the SOLEIL synchrotron (Si(220) monochromator, ∆E/E = 6 x 10 -5 , Canberra Ge 35-pixel fluorescence detector) and on the B18 beamline at the DIAMOND synchrotron (Si(111) monochromator, ∆E/E = 1.4 x 10 -4 , 4

  operando XAS experiments were preceded by a control cyclic voltammogram. The control CVs and the potential steps are summarized in the supplementary information. For the study of LMCO40, more scans were recorded at a given potential to account for the lower amount of Co with respect to Mn. SUPPORTING INFORMATION. Compositional, structural and morphological characterization of the different samples (Tables S-1 to S-4, Figures S-1 to S-10), electrochemical characterization performed ex situ (Figure S-10) and in situ coupled to post-OER analysis (Tables S-5 to S-10, Figures S-12 to S-30). SAMBA beamline at Synchrotron SOLEIL have been funded by SOLEIL under the User Proposal 20181146. The experiments at the B18 beamline a Diamond Light Source have been funded by Diamond under the proposal SP21747-1. J.M.G.C. and M.L.R.G. acknowledge financial support by the Spanish Ministry of Science and Innovation through Research Project PID 2020-113753RB-100. The National Centre for Electron Microscopy (ELECMI National Singular Scientific Facility) is also acknowledged for provision of access to corrected aberration microscopy facilities.

Figure S- 2 .

 2 Figure S-2. TEM images of (a) LMO and (b) LSMO and their (c and d) respective diameter distribution (numbers between brackets are the number of particles counted).

Figure S- 3 .

 3 Figure S-3. Powder XRD pattern of the LSMO sample.

Figure S- 4 .

 4 Figure S-4. Powder XRD pattern of the LSCO sample.

Figure S- 5 .

 5 Figure S-5. TEM image of the LSCO sample.

Figure S- 6 .

 6 Figure S-6. Diameter distribution of (a) LMCO30 and (b) LMCO40.

Figure S- 7 .

 7 Figure S-7. Rietveld refinement of the powder XRD patterns of LMCO30 and LMCO40 samples. The patterns were refined using the Rietveld method as implemented in the DDM, Derivative Difference Minimization program (RDDM = 8.05% and 8.36% for LMCO30 and LMCO40, respectively),[1] by using as initial parameters the reference PDF+4 cards 04-023-5052 (SG R-3C) and 00-036-1481 for the perovskite and La(OH)3 phases, respectively. The refined lattice parameters of the perovskites are a = b = 5.5180(6) Å, c = 13.396(3) Å and a = b = 5.5167(7) Å, c = 13.409(3) Å, for LMCO30 and LMCO40, respectively, in full agreement with the expected perovskite structure. The weight ratio La(OH)3/perovskite evaluated from the Rietveld analysis is 17 and 20 wt. ratio (Table S-1). These values are slightly higher than those expected from elemental WDXRF analysis (TableS-1). This discrepancy is attributed to the uncertainties in the evaluation of compositions from XRD and WDXRF, which amount to few percents for each technique. We note that this do not rule out the possible presence of amorphous Mn and Co-rich phases, although such components could not be detected by TEM and the in situ behaviour we observed by XAS, with constant Mn/Co oxidation state during electrocatalysis, cannot be ascribed to such phases, which on the opposite undergo changes of redox states during electrocatalysis.[2] Likewise, Co3O4 was implemented in the Rietveld analysis, but no corresponding peak could be detected and the fit was not improved by adding this third phase, so that we discard the presence of the spinel at amounts above the detection threshold of powder XRD.

Figure S- 8 .

 8 Figure S-8. HRTEM image of LMCO40, showing the single crystal character of a nanoparticle.

Figure S- 9 .

 9 Figure S-9. XANES linear combination fitting (LCF) of (a) LSMO, (b) LMCO30 and (c) LMCO40. The reference XAS spectrum of SMO is taken from Celorrio et al.4

Figure S- 10 .

 10 Figure S-10. STEM study of (a-d) LMCO30 and (e-g) LMO. HAADF-STEM images of (a) LMCO30 and (e) LMO. The dashed lines are those used for recorded the corresponding EELS sum spectra (b, f). (c,g) Mn and (d) Co oxidation states evaluated along the analysis lines shown in (b,f).

Figure S- 11 .

 11 Figure S-11. (a) Ex situ cyclic voltammogram of LSCO in an O2-saturated 0.1 M KOH electrolyte using a rotating disk electrode (1600 rpm) at 10 mVꞏs -1 . Electrocatalyst loading: 30 µgꞏcm -2 disk. The current has been normalized versus the BET surface area. (b) Ex situ cyclic voltammograms of mixed B-site perovskites LMCO30 and LMCO40 normalized versus the measured electrochemical surface area (ECSA = 106 and 59 cm 2 oxide/cm 2 RDE for LMCO30and LMCO40, respectively, by assuming a specific capacitance of 40 μF cm 2 oxide 2 ) recorded at the same loading, rotation speed and with the same electrolyte. For the measurements in this last graph, an additional step has been added to prepare the ink before deposition: after mixing the components, the ink has been roll milled overnight at 60 rpm in a vial filled with zirconium beads (diameter 2 mm, mass ratio catalyst/zirconium = 0.02 %).

Figure S- 12 .

 12 Figure S-12. Non-normalized operando XANES spectra of (a) LMO (Mn-K), (b) LSMO (Mn-K), (c) LSCO (Co-K) and (d) LMCO40 (Mn-K in red and (Co-K in green) during ORR and OER experiments. The dissolution of Mn and Co are investigated by measuring the intensity of the XAS edge before normalization, as it is directly correlated to the concentration of the excited element. The monitored edge height is calculated as the difference between the intensity before the edge (ca. 6500 and 7700 eV for Mn and Co respectively) and after the edge (ca. 6800 or 7800 eV for Mn and Co respectively). In figure S-11, the edge intensity is given as percentage of the intensity of the first XAS measured in the electrolyte, ie. the first OCV.

Figure S- 17 .

 17 Figure S-17. Control cyclic voltammogram of LMO in an O2-saturated KOH 0.1 M electrolyte during the operando experiment. Scan rate: 100 mVꞏs -1 .

Figure S- 18 .

 18 Figure S-18. FT-EXAFS at the Mn K-edge of LMO during OER (1.8 V/RHE) and ORR (0.4 V/RHE).

Figure S- 28 .

 28 Figure S-28. Operando XANES spectra of LMCO40 during OER and ORR experiment on Co K-edge. Dry electrode is in black. Successive potentials are applied: 1.0, 1.8, 0.4 and 1.0 V/RHE. In the frame is a zoom at the half-jump area.

Figure S- 29 .

 29 Figure S-29. FT-EXAFS at the Mn K-edge of LMCO40 during OER (1.8 V/RHE) and ORR (0.4 V/RHE).

Figure S- 30 .

 30 Figure S-30. FT-EXAFS at the Co K-edge of LMCO40 during OER (1.8 V/RHE) and ORR (0.4 V/RHE).

Table 1 .

 1 Chemical compositions of pristine samples

LMO, LSMO, LSCO and LMCO40 and

  their evolution during ORR and OER estimated from operando XANES.

		Pristine	OCV	ORR	OER
	LMO	LaMnO3	LaMnO2.9	LaMnO2.7	LaMnO2.95
	LSMO (La0.7Sr0.3Mn)0.98O3 (La0.7Sr0.3Mn)0.98O3 La0.7Sr0.3MnO2.75	-
	LSCO	La0.75Sr0.25CoO2.98 La0.75Sr0.25CoO2.93 La0.75Sr0.25CoO2.83 La0.75Sr0.25CoO2.98
	LMCO40		(LaMn +3.5 0.67Co +2.8 0.33)0.98O3

Table S - 1 .

 S1 Oxidation state of transition metals in perovskites calculated by XANES and STEM-EELS and chemical composition determined by STEM-EDX. STEM-EELS and STEM-EDX measurements were performed on single particles to ensure the homogeneity of the compositions.

		XANES	EELS		EDX
	Sample	Mn	Co	Mn	Co	La	Sr	Mn	Co
	LMO	3.0	N/A	N/A	N/A
	LSMO 3.4±0.1	N/A	3.6±0.1	N/A	37.0	16.0	46.9
	LSCO	N/A	3.2	N/A	N/A
	LMCO30 3.6±0.2	2.6	3.7±0.2 2.6±0.3 50.2		36.6 12.85
	LMCO40 3.5±0.2	2.6	3.7±0.2 2.8±0.1 48.9		34.8 16.21

Table S - 3 .

 S3 Anionic (x) and cationic (γ) stoichiometries for LMO, LSMO, LSCO and LMCO40 in the pristine state and in ORR and OER conditions, considering cationic (x=3-δ, ABO3-δ) or anionic ((AB)γO3) lattices fully occupied, respectively. The highlighted cells correspond to the most likely compositions, reported in Table1of the main text.

	Pristine	OCV	ORR	OER
	ABO x (

AB) γ O 3 ABO x (AB) γ O 3 ABO x (AB) γ O 3 ABO x (AB) γ O 3

	Sample	x	γ	x	γ	x	γ	x	γ
	LMO	3.00	1.00	2.90	1.00	2.70	1.11	2.95	1.02
	LSMO	3.05	0.98	3.00	0.98	2.75	1.09
	LSCO	2.98	1.01	2.93	1.03	2.83	1.49	2.98	1.01
	LMCO40	3.16	0.95	3.12	0.95	3.12	0.96	3.12	0.96

Table S -

 S 4. BET specific surface area of the pristine samples.

		LMO	LMCO30 LMCO40	LSMO	LSCO
	BET surface area (m 2 g -1 )	35	35	35	35	16
			S-10

Table S - 5 .

 S5 Oxidation states interpolated from the edge energies measured operando at halfedge jump of the Mn and Co K-edges in LMO, LSMO, LSCO and LMCO40 in ORR and OER conditions.

		Pristine	OCV		ORR (0.4	OER (1.8
				pH=13	V/RHE)	V/RHE)
	Sample	Mn	Co	Mn	Co	Mn	Co	Mn	Co
	LMO	3.0	N/A	2.8	N/A	2.4	N/A	2.9	N/A
	LSMO	3.4	N/A	3.3	N/A	2.8	N/A	-	N/A
	LSCO	N/A	3.2	N/A	3.1	N/A	2.9	N/A	3.2
	LMCO40	3.5	2.6	3.4	2.6	3.4	2.6	3.4	2.6

Table S - 6 .

 S6 Operando study of LMO. Potential of each steps and the number of scans acquired. No potential was applied during the steps in blue.

	Potential (V/RHE)	Dry	OCV	0.8	0.6	0.4	0.2	0.9	1.4 1.8 0.9
	Scans	16	16	9	16	16	16	16	8	8	8

ACKOWLEDGEMENTS

The authors acknowledge Patricia Beaunier from the Institut des Materiaux de Paris Centre for STEM. This work was supported by the French national agency for research (ANR) under the project InSiChem ANR-16-CE05-0011. This project has received funding from the European Research Council (ERC) Consolidator Grant GENESIS under the European Union's Horizon 2020 research and innovation programme (grant agreement n° 864850). The experiments at the

Supporting information

Tuning of Oxygen Electrocatalysis in Perovskite Oxide Nanoparticles by the Cationic Composition

Madeleine Han 1,2 , Isabel Gómez-Recio, 2 Daniel Gutiérrez Martín, 3 Nathaly Ortiz Peña, 4 Maria Luisa Ruiz-González, 3 Mohamed Selmane, [START_REF] Suntivich | A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles[END_REF] José M. González-Calbet, 3 Ovidiu Ersen, 4 Andrea Zitolo 1 , Benedikt Lassalle-Kaiser*


Entre Bataille

Maria Marques

Christel Mercanti-Guerin

Christophe De Lassus

Bezes

Maria Mercanti-Guerin

Christel De Lassus

Christophe Bezes
email: c.bezes@istec.fr

Quand L'experientiel

Directeurs Marques

Artistiques

Synergie Ou Bataille

Entre Marques

Directeurs Artistiques

QUAND L'EXPERIENTIEL EN LUXE SE CONSTRUIT SUR INSTAGRAM : SYNERGIE OU

Keywords: luxe, Instagram, expérientiel, directeurs artistiques, IA luxury, Instagram, experiential, artistic directors, AI

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Sur Instagram, les marques de luxe initient des conversations avec leurs clients via trois canaux : leurs comptes officiels, les influenceurs (blogueurs, vlogeurs, YouTubers…) et les comptes de leurs managers. Ainsi, le directeur artistique ou DA prend-il la parole de façon très scénarisée pour présenter les nouvelles collections, produits, partenariats 1 ... Souvent très médiatisé, il contribue à l'expérience de la marque en étant, parfois, plus ou autant connu que cette dernière. Néanmoins, assez peu de recherches académiques en gestion sont consacrées au DA, personnage pourtant central dans cet univers [START_REF] Hyunjoo | Recognizing Design: The Artistic Director as Silent Designer[END_REF]. Ceci est dû à son statut ambigu et aux multiples significations que le terme "Directeur Artistique" peut recouvrir dans d'autres 1 Ipsos, 2019 World Luxury Tracking (WLT), Kolsquare, plateforme pour l'optimisation de campagnes d'Influence Marketing, décembre 2019 domaines que celui de la mode : de planner designer en passant par créateur ou artiste entrepreneur [START_REF] Pillet-Anderlini V | Création, industrie et communication : la figure du créateur à l'épreuve du storytelling de la création dans la mode[END_REF]. Alors que les communications Instagram développées par les DA sont en forte progression 2 , il est donc intéressant d'en comprendre l'effet. C'est pourquoi cette recherche propose de mixer un courant de recherche fort en marketing, le secteur du luxe et une personne clé liée à cet univers et source d'un nouvel expérientiel : le DA. L'apport du DA à la dimension expérientielle du luxe permet de dresser une typologie de la collaboration qu'il entretient avec la marque. Plusieurs questions se posent : est-il un relai efficace des publications de la marque et plus généralement de son univers ? Travaille-t-il plus à promouvoir son image que celle de la marque ? Peut-il parfois être perçu davantage comme un concurrent que comme un allié ? Son positionnement parfois très éloigné de la marque peut il brouiller le positionnement de cette dernière ou au contraire le porter et le moderniser ? L'originalité de notre recherche réside dans la visualisation des relais (ou pas) que le DA peut faire de l'expérience créée en ligne par la marque. La création d'expérience est analysée dans un espace social très concurrentiel pour le luxe (celui d'Instagram) au sein duquel il est possible de quantifier sa capacité à générer de l'engagement ou, au contraire, à publier des contenus à contrecourant de l'univers de la marque. Dans une première partie, nous analysons les liens entre luxe, social et expérientiel. Nous interrogeons les dynamiques d'influence qui traverse ce secteur et les modélisations les plus pertinentes de la "luxemosphère" qui impliquent des approches narratives différentes (Brun et al., 2013) plus adaptées au contexte digital et qui changent la façon de "vivre le luxe". Dans une deuxième partie, une étude empirique couple l'utilisation de l'intelligence artificielle et l'analyse de contenu pour aboutir à une vision globale (images et commentaires) des stratégies sociales des DA et des comptes corporate des marques. Les implications théoriques de cette recherche portent sur une nouvelle méthodologie d'études adaptée à un réseau social visuel et une revisite du concept d'influence expérientielle à l'heure des réseaux sociaux. Les implications managériales vont de la difficile gestion des influenceurs salariés à l'élaboration de stratégies expérientielles congruentes avec l'image de la marque.

Luxe, réseaux sociaux et influenceurs expérientiels

Les réseaux sociaux permettent au luxe de moderniser son image, de toucher de nouvelles cibles et de faire émerger de jeunes marques. Le contenu publié par les marques a augmenté la perception favorable du secteur, l'image de marque et l'intention d'achat des consommateurs [START_REF] Kwon | Consumers' Social Media Advocacy Behaviors Regarding Luxury Brands: An Explanatory Framework[END_REF]. Les effets des stratégies SNSs (social media marketing and brand publishing on social networking sites) portent sur les réactions des consommateurs, la notoriété de la marque et l'attitude, la mise en scène de soi sur les réseaux (Kretz et de Valck, 2010 ;[START_REF] Okonkwo | The luxury brand strategy challenge[END_REF]. Ces expériences sociales renforcent la qualité des relations et l'identification aux marques chez les Millennials [START_REF] Ladwein | L'ambivalence dans la décision d'acheter des produits de luxe : le cas des millennials[END_REF]. Plusieurs thématiques émergent : 1) la représentation de la marque : publier le contenu de la marque met en avant les innovations et les nouveaux produits, suscite l'appréciation des clients, permet d'intégrer des applications ou d'afficher des contenus associés à la valeur de sa marque plutôt qu'à son domaine d'expertise [START_REF] Phan | Social media and luxury brand management: The case of Burberry[END_REF]. 2) Le shopping social : promotion, catalogue électronique et stories dédiées à la conversion, avant-première exclusive comme promotion des nouvelles collections, campagnes publicitaires en avant-première, diffusion en direct des défilés avec focus sur les produits portés, récompenses exclusives pour les fans, jeux-concours, annonces des soldes [START_REF] Saravanakumar | Social media marketing[END_REF].

3) La création de communautés: animation de communautés de marque, accroissement du nombre de fans, détection des insights consommateurs via ces communautés, cocréation et travail sur la viralité, développement de l'engagement (Pentina et Koh, 2012). 4) La captation de la data : mise en place d'outils de social listening adaptés, définition et suivi des KPI, évaluation des campagnes sociales, enrichissement grâce aux réseaux sociaux des CRM on-boarding [START_REF] Liu | Examining the impact of luxury brand's social media marketing on customer engagement: Using big data analytics and natural language processing[END_REF]. Parmi ces thématiques, l'expérientiel et l'influence concernent un nombre important de recherches [START_REF] Tafesse | An experiential model of consumer engagement in social media[END_REF]. L'influenceur peut être source d'expérientiel organisée autour de sa personne ou être médiatisée par cette dernière. Les mécaniques liant évènement fortement expérientiel et influenceur sont couramment pratiquées dans le luxe. Ce dernier a une fonction particulière dans l'entreprise qui lui donne autorité pour être le "chef d'orchestre" de l'expérientiel de marque. C'est cette fonction particulière qui est dévolue au directeur artistique. Dans ce cadre, les liens expérientiel-influenceur sont à étudier.

Un luxe de plus en plus expérientiel et social

Longtemps apanage des élites, les marques de luxe font face à une demande croissante qui les ont conduites à revoir leurs stratégies de conquête. Elles visent une clientèle plus large et, dans les sociétés occidentales, plus tournée vers l'émotion que vers la consommation ostentatoire [START_REF] De | Access to the luxury brand myth in pop-up stores. A netnographic and semiotic analysis[END_REF]Batat, 2019a et b). Dès lors, la compréhension des dimensions de l'expérience est devenue clé sur Internet [START_REF] Geerts | Le luxe et Internet : évolutions d'un paradoxe[END_REF]. Le marketing expérientiel engage sensoriellement les consommateurs et ne concerne pas uniquement la propriété (Loussaief et Damay, 2018). L'expérientiel devient de plus en plus social et multidimensionnel. Si les recherches sur l'expérientiel sont très nombreuses [START_REF] Filser | Le marketing de la production d'expérience : statut théorique et implications managériales[END_REF][START_REF] Roederer | Marketing and experiential consumption[END_REF][START_REF] Pulh | Muséo-parcs et réenchantement de l'expérience muséale : le cas de la Cité des Arts et des Sciences de Valencia[END_REF], le digital et a fortiori Instagram ont fait évoluer le concept [START_REF] Kwon | Consumers' Social Media Advocacy Behaviors Regarding Luxury Brands: An Explanatory Framework[END_REF]. Le luxe est l'illustration de cet expérientiel social en plein essor [START_REF] Batat | Digital luxury: transforming brands and customer experiences[END_REF]. Le processus de consommation expérientielle ne se limite pas à l'acte d'achat [START_REF] Holbrook | The millennial consumer in the texts of our times: Experience and entertainment[END_REF] mais se déploie avant et après l'interaction avec l'espace marchand [START_REF] Arnould | Consumer Culture Theory (CCT): twenty years of research[END_REF]. La Fashion Week est, désormais, vécue comme une expérience à 360°avec applications dédiées, casques virtuels et réalité augmentée qui enrichissent les perceptions des spectateurs sur les réseaux. Les plateformes de réalité virtuelle comme Livit sont utilisées par des marques comme Moschino pour créer des shows sociaux, expérientiels et technologiques. Des drones chez Dolce Gabbana ont ouvert le bal du défilé de la Milan Fashion Week en 2018 pour projeter des images sur grand écran complètement « instagramables ». L'expérientiel social du luxe prend aussi en compte l'interactivité client-marque. Les expériences considérées comme des divertissements (défilés de mode dans les boutiques de créateurs) impliquent souvent un faible degré d'implication de la part du client. Mais l'adjonction d'une ressource permettant des rapports humains interactifs accroît l'implication et l'interactivité. Améliorer l'expérience émotionnelle devient central pour les marques pour générer de l'engagement. Les émotions passent par la fierté des DA qui défilent après le spectacle, la visualisation des couturières au travail, la reconnaissance des mannequins pour le créateur. La modélisation de l'expérientiel permet l'émergence de mécanismes reposant sur la célébrité (Mathys et al., 2016). Particulièrement, le modèle SEM (strategic experientiel modules) couple le social à la création. Ce modèle [START_REF] Schmitt | Experiential marketing: A new framework for design and communications[END_REF] inclut les expériences sensorielles (SENSE), les affectives (FEEL), cognitives créatives (THINK), physiques et comportementales, (ACT), et les expériences d'identité sociale générées par la relation avec un groupe ou une culture de référence (RELATE). Toutes ces dimensions se retrouvent dans les modes de communication utilisés dans le luxe.

La mise en scène du luxe sur Instagram, vers une perte de contrôle ?

Pourquoi les clients du luxe se connectent-ils autant sur Instagram qui semble représenter le réseau du luxe et de l'expérience ? Pour [START_REF] Quach | From connoisseur luxury to mass luxury: Value co creation and co-destruction in the online environment[END_REF], Instagram apporte des innovations majeures en narration visuelle et de dimensions émotionnelles. Les stories, les carrousels, les publicités dynamiques et les hashtags dédiés aux évènements (fashion week, expositions, inaugurations de boutique ou lancements de produit) ont installé Instagram comme le réseau social de référence des marques de luxe [START_REF] Tobin | The role of social connection in satisfaction with Instagram photographs[END_REF]. Par le biais des influenceurs, les marques cherchent à communiquer auprès de leurs clients, notamment sur Instagram, réseau privilégié de l'influence [START_REF] Abidin | Visibility labour: Engaging with Influencers' fashion brands and OOTD advertorial campaigns on Instagram[END_REF]. Les influenceurs sont représentatifs de l'obsession de la mise en scène ciblée et digitale (selfies), de la culture de l'ostentation et de la recherche de l'influence sous toutes ses formes. Les évènements ou les expositions marque (branded exhibitions) facilitent la cocréation de contenus visuels et leur dissémination sur le réseau [START_REF] Koivisto | Pixelize me!': digital storytelling and the creation of archetypal myths through explicit and implicit self-brand association in fashion and luxury blogs[END_REF]. Les utilisateurs d'Instagram ont l'engagement et la fidélité à la marque les plus élevés par rapport à Facebook, Twitter, et Snapchat [START_REF] Phua | Gratifications of using Facebook, Twitter, Instagram, or Snapchat to follow brands: The moderating effect of social comparison, trust, tie strength, and network homophily on brand identification, brand engagement, brand commitment, and membership intention[END_REF]. La gestion des influenceurs sur Instagram est jugée très souvent problématique. Leur contenu peut échapper aux marques et les instagrammeurs chercher leur intérêt propre [START_REF] Quach | From connoisseur luxury to mass luxury: Value co creation and co-destruction in the online environment[END_REF]. La tendance est donc de pratiquer l'employee advocacy (incitations pour les employés de partager les contenus sociaux de la marque) et de disposer d'influenceurs du type employés ambassadeurs [START_REF] Tsarenko | When and why do social resources influence employee advocacy? The role of personal investment and perceived recognition[END_REF]. Le tableau 1 montre l'absence du DA comme objet d'étude. En effet, cet endossement particulier d'une marque par son directeur artistique ne correspond ni à un endossement traditionnel par une célébrité totalement extérieure à la marque, ni à un endossement nouveau par des micro-célébrités [START_REF] Ambroise | L'endossement par les célébrités : clarifications conceptuelles, synthèse critique et perspectives de recherche[END_REF].

Tableau 1 -Principales recherches traitant des influenceurs du luxe sur Instagram

Typologie des influenceurs Effets sur les marques sponsorisées

Micro-célébrités Etat d'esprit et ensemble de pratiques d'auto présentation dans lesquels les utilisateurs de manière stratégique vont se mettre en scène afin d'accroître l'attention [START_REF] Marwick | Instafame: Luxury Selfies in the Attention Economy[END_REF]. Individus ordinaires qui atteignent la célébrité sur Instagram grâce à leur esprit d'entreprise (Abidin, 2015 ;[START_REF] Duffy | Having it all" on social media: entrepreneurial femininity and selfbranding among fashion bloggers[END_REF].

Authenticité, proximité, sympathie. Confiance via l'identification à l'égard de l'émetteur. Dégradation possible de l'image de la marque si les contenus apparaissent trop artisanaux (effets bricolage ou assemblage). [START_REF] De | Marketing through instagram influencers: The impact of number of followers and product divergence on brand attitude[END_REF]. Blogueuses de mode, acteurs, personnalités issues du show-business ou de la beauté pratiquant l'endossement implicite ou explicite. [START_REF] Indvik | Congruence between spokesperson and product type: a matchup hypothesis perspective[END_REF][START_REF] Loureiro | A passion for fashion: The impact of social influence, vanity and exhibitionism on consumer behavior[END_REF] Les facteurs de succès de ces influenceurs résident dans la désirabilité sociale, la bienveillance perçue ressentie par leurs fans, Accroissement de la couverture et de la notoriété. Bouche à oreille positif. Endossement et commerce social. Dégradation possible du caractère exclusif de la marque (influenceurs multimarques). Perte du sentiment de proximité (influenceur vu comme un média promotionnel) leur compétence et leur intégrité [START_REF] Cheung | Consumer Purchase Decision in Instagram Stores : The Role of Consumer Trust[END_REF].

Figures médiatiques, célébrités

Pairs

Influence entre pairs et émergence d'influenceurs au sein de communautés de pratique ou de cohortes (Sherman et al., 2016) Imitation ou mimétisme de consommation

Influenceurs virtuels, avatars virtuels

Egéries virtuelles, Brand Avatars -Avatars imitant les modèles réels ou figurant des représentations idéales de l'homme. Versions hybrides et transformées du corps humain [START_REF] Andersson | Virtual Avatars, Virtual Influencers & Authenticity[END_REF] Modernisation des marques, Retombées Presse, curiosité. Identification floue par certains consommateurs du caractère non-humain de l'influenceur.

Le directeur artistique, un influenceur sous influence ?

Les directeurs artistiques sont aujourd'hui, profondément influencés par les réseaux sociaux dans leurs créations autant que dans la façon de promouvoir leurs collections. Leurs lignes artistiques sont soumises aux règles sociales de réseaux comme Instagram [START_REF] Rocamora | Mediatization and digital media in the field of fashion[END_REF]. Le défilé largement retransmis sur Facebook ou Instagram devient un moment essentiel de la médiatisation de la marque mais aussi du DA. Ce super-influenceur qu'est le DA est chargé de rajeunir des marques traditionnelles, d'intégrer de nouveaux codes visuels, de faire émerger de nouvelles marques. Il devient chef d'orchestre du Live de la marque et de sa présence sur le réseau. Néanmoins, sa marque personnelle forte et son importance au sein de l'entreprise le placent au-dessus d'un certain nombre de sujétions. Son statut le positionne, à la fois, dans et hors de l'entreprise ce qui n'est, pas le cas, des simples influenceurs ou ambassadeurs. Cet ambassadeur employé est un champion de marque dont la gestion s'avère plus délicate qu'un simple ambassadeur influenceur. Certains directeurs artistiques sont-ils trop dissonants ou insuffisamment congruents avec la marque qui les emploie ? L'expérience consommateur qu'ils proposent n'est-elle pas trop éloignée de l'expérientiel habituel de la marque ?

Méthodologie et résultats

Notre recherche s'appuie sur l'analyse des comptes Instagram des directeurs artistiques (DA) et des marques de luxe pour lesquelles ils travaillent. Le corpus étant particulièrement délicat à définir, les marques de luxe étudiées ont été choisi avec soin : 19 des principales marques de luxe sélectionnées sur Instagram et leurs 20 directeurs artistique (une des maisons ayant deux créateurs). Ont été sélectionnées, de façon exhaustive, toutes les marques de luxe disposant de comptes Instagram de plus de 10.000 membres en 2020 et travaillant avec des directeurs artistiques possédant aussi un compte actif personnel. Deux types d'indicateurs ont été étudiés : 1) les images postées via l'API Cloud Vision de Google3 . Cet outil d'intelligence artificielle catégorise les éléments qu'il reconnaît dans les images et leur associe des mots les qualifiant (Figure 1). De plus en plus utilisé en recherche [START_REF] Andrade | Computational Vision and Business Intelligence in the Beauty Segment-An Analysis through Instagram[END_REF], il transforme les images en mots et permet de réaliser des analyses de contenus sur des réseaux sociaux très visuels comme Instagram. Les objets détectés dans les images publiées par les DA et leurs marques ont été analysés ; 2) Les commentaires liés aux publications. Les publications les plus commentées ont été consignées via NCapture du logiciel NVivo, puis leurs textes analysés (captions). 5 Le diagramme de surface permet de comparer des noeuds en fonction du nombre de références qu'ils contiennent. Ainsi, la figure 2 montre une place importante du noeud "Show, collection, event" au sein du corpus.

DA/ Marques : des synergies vertueuses et des associations contre-productives

Des diagrammes de comparaison des thématiques partagées sur les Instagrams des DA et de leurs marques permettent d'établir une classification des collaborations 6 . Elles vont de la synergie vertueuse entre une marque et son DA, à des associations qui peuvent gêner voire brouiller les stratégies sociales des marques. Certains DA mettent en ligne des publications en accord avec la stratégie de contenu de la marque pour laquelle ils travaillent ; d'autres exposent des univers très différents, voire disruptifs, par rapport au positionnement de leur marque. Ils brouillent parfois de façon involontaire le positionnement voulu de la marque. Enfin, une petite catégorie de DA s'inscrit en concurrence avec leur marque (détention d'une marque propre, faible proximité avec leur marque employeur…).

Les DA relais

Les DA relais (figure 3) partagent de nombreuses thématiques avec leurs marques. Certains peuvent être assimilés à de simples relais des marques ; d'autres relaient leur communication en apportant leur univers personnel, ce qui enrichit leur positionnement. Dans les deux cas, ils entretiennent une relation forte avec la marque pour laquelle ils travaillent et sont soumis à un vrai challenge. Cette attitude peut s'expliquer en ce qui concerne Sophie de La Fontaine, descendante du créateur de Longchamp. Un DA comme Pierpaolo Piccioli travaille depuis 1999 pour Valentino. Il a dû prouver sa valeur après le départ chez Dior de Maria Grazia Chiuri. Riccardo Tisci a travaillé pour de nombreuses marques de luxe avant de connaître la consécration chez Valentino. Recruté par Burberry en 2018, il a pour challenge de sauver cette marque. Olivier Rousteing est un DA relais extrêmement actif sur son compte à la fois personnel et professionnel. Réputé comme un maître des réseaux sociaux et de la communication, il procure à Balmain un positionnement avant-gardiste. Le risque de s'appuyer sur les DA Relais pour une marque est qu'une forte congruence peut aboutir à une « valeur positive froide » en termes de construction d'image, c'est-à-dire sans émotion supplémentaire (Mandler 1982, p.22). 4.2 L'expérientiel : indicateur des jeux de pouvoir et des modes de coordination L'organisation particulière des maisons de luxe, à la fois faiseurs de roi et soumis à la volonté d'indépendance de DA qui les surpassent parfois, en capital social pose la question de la coordination des actions entreprises sur les réseaux sociaux. Une lecture par la littérature sur les organisations offre une vision intéressante de ce que beaucoup d'auteurs nomment l'enchevêtrement des modes de coordination. Ces modes sont fondés sur des organisations en réseau et révolutionnés par les réseaux sociaux. Comme le souligne Froehlicher (2001, p. 11), « l'existence de zones d'incertitude dans les champs d'action fournit un degré d'autonomie irréductible à l'acteur par rapport à la structure. Dans cette optique, la frontière de l'organisation est une ressource pour un acteur évoluant à son avantage simultanément dans un milieu interne et externe ». Les acteurs jouent sur une représentation conflictuelle de l'organisation où l'efficacité d'une marque est la résultante d'un équilibre précaire dans les jeux de pouvoir. Dans le cas des marques de luxe, le recrutement de DA prestigieux est souvent salué par les analystes financiers. Une nouvelle génération de DA émerge qualifiée de « producteurs de contenus créatifs ». Comme Virgil Abloh ou Kanye West 7 , ces DA peuvent venir d'univers très éloignés de la mode (musique, travail de l'image). Ils arrivent avec une dimension créative, des collaborations et des équipes qui se mettent temporairement au service de la marque. La force de leur réseau est un miroir de leur pouvoir d'influence et non l'inverse. Ainsi, les trois modes de coordination (la hiérarchie, le marché et la coopération) sont remis en question. Les DA n'obéissent à aucune hiérarchie tant que le marché les adoube et valide les bons résultats financiers qu'ils peuvent générer. Dans ce cas, ils sont libres d'imposer leur mode de collaboration et leur type d'expérientiel.

Conclusion

A l'heure où les influenceurs sont remis en question, cette recherche pointe du doigt les contraintes liées à la gestion de contenu sur Instagram. La multiplication des comptes de marque et associés (fans, salariés, influenceurs) rend problématique la recherche d'une cohérence de positionnement. Le statut particulier d'influenceurs comme les DA rend d'autant plus délicat le rapport de forces qui peut s'instaurer, entre ces influenceurs pratiquant un self branding assumé [START_REF] Gandini | Digital work: Self-branding and social capital in the freelance knowledge economy[END_REF] et des marques qui fondent leur différence sur leur caractère exclusif. Les implications managériales de cette recherche mettent en évidence la nécessité de pratiquer une stratégie spécifique en fonction des profils de ces salariés. De nouvelles voies de recherche pourraient être initiées sur le concept d'attachement affectif entre un créateur et sa marque. La prise en compte des récits de vie des DA sur Instagram et de leur histoire personnelle émergent comme des variables essentielles permettant d'expliquer pourquoi et comment une collaboration peut être réellement fructueuse. La typologie proposée de l'influence (relais, brouillage, concurrence) peut être appliquée à d'autres secteurs à l'heure où se multiplient les interrogations sur l'efficacité des stratégies d'influence et la gestion des influenceurs. D'aucuns prédisent même la fin des influenceurs et pourraient être confortés par les résultats de notre étude8 . Plus généralement, cette recherche permet d'aller plus loin dans la clarification du concept de champion de marque et, notamment, sa capacité à aider les marques à gérer le changement. Cet effet d'entraînement est illustré par la capacité de certains DA à mettre en avant les équipes (couturiers, casteurs…), à s'inscrire dans l'histoire des marques (clins d'oeil à des créations prestigieuses passées) mais aussi à imposer un style plus personnel qui s'intègre ou pas à l'ADN de leur marque employeur. Enfin, elle offre une illustration de l'utilisation de l'intelligence artificielle appliquée à la recherche sur les réseaux sociaux. Si l'analyse des réseaux sociaux (ARS) s'est, jusque-là, centrée sur les interactions sociales en termes de liens et de noeuds, l'intelligence artificielle permet d'ajouter une dimension plus qualitative de catégorisation des images et de transformation de ces dernières en items. Très utilisée dans la gestion des commentaires, avis, sélection des posts les plus viraux, elle trouve là un nouveau champ d'application et ouvre des voies futures de recherche face à un Internet de plus en plus soumis à la force des images. En dernier lieu, elle illustre l'indispensable transformation des marques de luxe qui passe par un expérientiel renouvelé. Les DA disruptifs et ré-inspirants pour leurs marques peuvent réussir cette transformation comme ils peuvent échouer. Si le lien entre expérientiel renouvelé et nouveau positionnement de marque est largement observé dans la littérature [START_REF] Hackley | Entertainment marketing and experiential consumption[END_REF], l'intégration d'émetteurs à la fois salariés et indépendants dans la production d'expérientiels dissonants voire concurrents de la marque n'est pas encore traitée. Il préfigure les problématiques des marques de luxe en termes de gestion de leurs hauts potentiels qui disposent de communautés de fans qui leur sont propres. Il questionne également les recherches portant sur les champions de marque et les mécaniques d'employee advocacy [START_REF] Morhart | Rand-specific leadership: Turning employees into brand champions[END_REF]. Fondées sur les seules performances en termes de Reach (couverture sur cible), elles devraient prendre également en compte des indicateurs plus qualitatifs liés aux positionnements des marques et à leurs impératifs de transformation.

  Figures médiatiques, célébritésFigures populaires ayant construit leur notoriété en dehors du réseau Instagram[START_REF] De | Marketing through instagram influencers: The impact of number of followers and product divergence on brand attitude[END_REF]. Blogueuses de mode, acteurs, personnalités issues du show-business ou de la beauté pratiquant l'endossement implicite ou explicite.[START_REF] Indvik | Congruence between spokesperson and product type: a matchup hypothesis perspective[END_REF][START_REF] Loureiro | A passion for fashion: The impact of social influence, vanity and exhibitionism on consumer behavior[END_REF] Les facteurs de succès de ces influenceurs résident dans la désirabilité sociale, la bienveillance perçue ressentie par leurs fans,

Figure 1 -

 1 Figure 1 -Design de la recherche via l'API Cloud Vision de Google

Figure 2 -

 2 Figure 2 -Typologie des noeuds identifiés, diagramme de surface 5

6

  Deux matériaux ont nourri notre analyse. Google Cloud Vision en traduisant les images partagées sur Instagram en mots a fourni les codes qui sont à la base de l'analyse sémantique. Ces codes pouvaient être la dénomination d'objets, des couleurs ou des sentiments. Ncapture disponible sur le logiciel NVivo a capté et enregistré les différents comptes Instagram étudiés et enrichi notre matériel d'étude. Lors d'une première étape, nous avons effectué un encodage manuel à partir des occurrences les plus fréquentes. Nous l'avons complété par un encodage automatique proposé par NVivo qui utilise des algorithmes d'apprentissage automatique pour effectuer un encodage global des passages de texte contenant des mots similaires à ceux des contenus encodés précédemment. Pour parfaire notre codage, nous avons utilisé les requêtes NVivo pour rechercher et analyser des mots ou des locutions dans nos sources et noeuds. NVivo propose via la recherche textuelle d'afficher toutes les correspondances dans un noeud d'aperçu ce qui nous a permis d'établir les diagrammes de comparaison.

Figure 3 -

 3 Figure 3 -Le DA relais Allessandro Michele et Gucci (politique de contenu centrée sur le produit, les valeurs de la marque et sa communauté)

Figure 4 -

 4 Figure 4 -Le DA Brouillage Silvia Venturini (faible similarité de contenus avec sa marque et un univers personnel riche et éloigné de son quotidien professionnel)

Figure 5 -

 5 Le DA Concurrence Karl Lagerfeld (grande proximité avec Chanel même si de nombreuses publications poussent sa marque propre)4. Discussion

4.1 Différents types de DA pour différents types d'expérientiels.

  Le tableau 2 permet de cartographier les contenus publiés par les DA en fonction du type d'expérientiel qu'ils proposent à leurs abonnés sur Instagram. Il est issu d'un croisement entre l'analyse de contenu pratiquée et le modèle expérientiel SEM[START_REF] Schmitt | Experiential marketing: A new framework for design and communications[END_REF]. Il montre chez les DA un fort expérientiel fondé sur les cinq dimensions du modèle. Casey Cadwallader, DA de Mugler développe avec succès un registre qui met en scène des silhouettes épurées et joue sur des matières fluides enveloppant les corps (latex, laçage, laine). Piccioli (Valentino) travaille sur l'esprit du temps en offrant des expériences nostalgiques et sentimentales (photographies de rue dans les grandes métropoles du monde et sur tous les continents). Vacarello (Yves Saint Laurent) propose des concerts Rock et des spectacles de Paris en plein air où conceptualisation et créativité se rejoignent. Surridge (Cavalli) théâtralise ses espaces d'exposition (Arche de Noé, bestiaire et monde animal). Rousteing (Balmain) développe des rencontres expérientielles en compagnie d'influenceurs et symbolise le DA engagé sur des sujets liés à la responsabilité sociétale de la mode.

				Tableau 2 : Typologie des DA en fonction des types d'expérientiel proposés
		s	l	l		SENSE	FEEL	THINK	ACT	RELATE
					s
				e
		n
			i		e t	Expérience	Expérien	Expérience	Expérience	Expérience tirée
	i	o	t			sensorielle	ce	cognitive et	tirée d'un	d'une relation
		s		n	n		affective	créative	mode de vie	avec un groupe,
					a
		n	i	e					une
		e		r	n				culture de référence
					i
			é
					m
		m
			p
	i
					o
				s
				x
	D
				e
				e
					d
						Mode,	Amour de	Backstage,	Voyage,	Communicati
						beauté,	la marque,	séances de	photograph	on, média,
						tissus,	valeurs,	créativité,	ie,	collaborations,
						couleurs,	nostalgie	mise en	peinture,	communautés,
						corps,	et	scène des	recherche	prises de paroles
						matières	sentimen	équipes,	d'inspiration	publiques, prises
							ts	visualisati	dans des	de position sur
								on des	univers	des
								ateliers	éloignés de	sujets de société
									la mode
	Principa		Waightkel	Garcia	Vevers	Surridge	Gvasalia
	ux DA		ler	Piccioli	Vacarello	Jenden	Rousteing
						Andrew	Tisci	Lagerfeld	Ramsey-Levi	Ghesquière
						Michele	Kim
						Cadwallader	Venturini
							De La
							Fontaine
	Concernant l'expérience sensorielle,

https://pro.iconosquare.com/resources/instagram-marketing-trends-and-benchmarks-2018

L'API Vision dispose de modèles de machine learning pré-entraînés. Cet outil permet d'attribuer des étiquettes à des images et les classe rapidement dans des millions de catégories prédéfinies. Il peut détecter des objets et des visages, lire du texte imprimé ou manuscrit, et intégrer des métadonnées au catalogue d'images.

Nous avons effectué des diagrammes de grappes qui permettent de regrouper les sources, les noeuds de thème ou les cas qui partagent des mots, des valeurs d'attribut ou un encodage similaires. Ceci nous a permis de regrouper des thématiques et des individus donc, par exemple un contenu "voyages" avec certains DA ou certaines marques.

https://fr.fashionnetwork.com/news/La-nouvelle-generation-des-directeurs-artistiques-ces-createurs-d influence,1081871.html

https://nypost.com/2020/06/06/kylie-jenner-and-the-end-of-the-instagram-influencer-business-model/