Yu-Ping Ku

Konrad Ehelebe

Andreas Hutzler

Markus Bierling

Thomas Böhm

Andrea Zitolo

Mykhailo Vorokhta

Nicolas Bibent

Florian D Speck

Dominik Seeberger

Ivan Khalakhan

Karl J J Mayrhofer

Simon Thiele

Frédéric Jaouen

Serhiy Cherevko

Oxygen Reduction Reaction in Alkaline Media Causes Iron Leaching from Fe-N-C Electrocatalysts

Keywords: Fe-N-C, Oxygen Reduction Reaction, AEMFC, Fe dissolution, Fe redox transition

The electrochemical activity of modern Fe-N-C electrocatalysts in alkaline media is on par with that of platinum. For successful application in fuel cells, however, also high durability and longevity must be demonstrated. Currently, a limited understanding of degradation pathways, especially under operando conditions, hinders the design and synthesis of simultaneously active and stable Fe-N-C electrocatalysts. In this work, using a gas diffusion electrode half-cell coupled with inductively coupled plasma mass spectrometry setup, Fe dissolution is studied under conditions close to those in fuel cells, i.e. with porous catalyst layer and at current densities up to -125 mA•cm -2 . Varying the rate of oxygen reduction reaction, we show a remarkable linear correlation between the Faradaic charge passed through the electrode and the amount of Fe dissolved from the electrode. This finding is rationalized assuming that oxygen reduction and Fe dissolution reactions are interlinked, likely through a common intermediate formed during the Fe redox transitions in Fe species involved in oxygen reduction reaction, such as FeNxCy and Fe3C@N-C. Moreover, such a linear correlation allows the application of a simple metric -S-number -to report the material's stability. Hence, in the current work, a powerful tool for a more applied stability screening of different electrocatalysts is introduced, which allows on the one hand fast performance investigations under more realistic conditions, and on the other hand more advanced mechanistic understanding of Fe-N-C degradation in catalyst layers.

Introduction

On the way to a climate neutral hydrogen economy, [START_REF] Bockris | A Hydrogen Economy[END_REF] energy efficient, durable and affordable fuel cells (FCs) are needed. While proton exchange membrane fuel cells (PEMFCs) have been commercialized, further development of this technology is hindered by its dependency on Pt as the main electrocatalyst material for both hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR). [START_REF] Pivovar | Catalysts for fuel cell transportation and hydrogen related uses[END_REF] Therefore, research on highly active and stable platinum group metal (PGM)-free catalysts is booming, as summarized in recent reviews. [START_REF] Du | Achievements, challenges and perspectives on cathode catalysts in proton exchange membrane fuel cells for transportation[END_REF] Among all identified PGM-free candidates, iron-nitrogendoped carbon (Fe-N-C) is the most promising to catalyze the sluggish ORR. [START_REF] Martinez | Progress in the development of Fe-based PGM-free electrocatalysts for the oxygen reduction reaction[END_REF][START_REF] Speck | Single-Atom Catalysts: A Perspective toward Application in Electrochemical Energy Conversion[END_REF] Recent studies have theoretically and experimentally demonstrated that Fe-N-C catalysts are more active and stable in alkaline than in acidic media. [START_REF] Santori | Effect of Pyrolysis Atmosphere and Electrolyte pH on the Oxygen Reduction Activity, Stability and Spectroscopic Signature of FeNx Moieties in Fe-N-C Catalysts[END_REF][START_REF] Holby | Acid Stability and Demetalation of PGM-Free ORR Electrocatalyst Structures from Density Functional Theory: A Model for "Single-Atom Catalyst" Dissolution[END_REF][START_REF] Bae | Effect on the H2O2-Induced Deactivation of Fe-N-C Catalysts[END_REF] This indicates the unique possibility to use Fe-N-C as a relatively efficient, earth abundant and cheap cathode catalyst material in the alternative anion exchange membrane fuel cell (AEMFCs) technology. Due to the recent groundbreaking developments in membrane science, AEMFCs demonstrate great potential for successful commercialization. [START_REF] Serov | Catalytic Advantages, Challenges, and Priorities in Alkaline Membrane Fuel Cells[END_REF] With regards to durability and longevity of Fe-N-C catalysts in this environment, many questions remain however open. In particular, understanding of Fe x+ leaching and, consequently, an implementation of mitigation strategies to its suppression are considered main challenges. [START_REF] Martinez | Progress in the development of Fe-based PGM-free electrocatalysts for the oxygen reduction reaction[END_REF][START_REF] Santori | Effect of Pyrolysis Atmosphere and Electrolyte pH on the Oxygen Reduction Activity, Stability and Spectroscopic Signature of FeNx Moieties in Fe-N-C Catalysts[END_REF][START_REF] Choi | Stability of Fe-N-C Catalysts in Acidic Medium Studied by Operando Spectroscopy[END_REF][START_REF] Choi | Minimizing Operando Demetallation of Fe-N-C Electrocatalysts in Acidic Medium[END_REF][START_REF] Sgarbi | Electrochemical Transformation of Fe-N-C catalysts into Iron Oxides in Alkaline Medium and Its Impact on the Oxygen Reduction Reaction Activity[END_REF] So far, four key degradation mechanisms have been suggested to occur in parallel or series on Fe-N-C in acidic media, [START_REF] Speck | Single-Atom Catalysts: A Perspective toward Application in Electrochemical Energy Conversion[END_REF] providing also clues to possible degradation pathways in alkaline electrolytes:

(i)
Carbon corrosion. Corrosion of the carbon support leads to Fe demetallation and a drop in electron conductivity. [START_REF] Speck | Single-Atom Catalysts: A Perspective toward Application in Electrochemical Energy Conversion[END_REF][START_REF] Choi | Stability of Fe-N-C Catalysts in Acidic Medium Studied by Operando Spectroscopy[END_REF] It has been shown in various studies that the rate of this mechanism depends on temperature, [START_REF] Choi | Stability of Fe-N-C Catalysts in Acidic Medium Studied by Operando Spectroscopy[END_REF] electrochemical potential, [START_REF] Choi | Stability of Fe-N-C Catalysts in Acidic Medium Studied by Operando Spectroscopy[END_REF][START_REF] Chenitz | A specific demetalation of Fe-N4 catalytic sites in the micropores of NC_Ar + NH3 is at the origin of the initial activity loss of the highly active Fe/N/C catalyst used for the reduction of oxygen in PEM fuel cells[END_REF] and, as was demonstrated recently, on the presence or absence of O2 in aqueous electrolytes. [START_REF] Kumar | On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts[END_REF] (ii) Reactive oxygen species (ROS). Formed as intermediates or by-products of ORR, ROS were shown to cause Fe-N-C catalyst degradation in acidic conditions. [START_REF] Bae | Effect on the H2O2-Induced Deactivation of Fe-N-C Catalysts[END_REF][START_REF] Kumar | On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts[END_REF][START_REF] Lefèvre | Fe-based catalysts for the reduction of oxygen in polymer electrolyte membrane fuel cell conditions: determination of the amount of peroxide released during electroreduction and its influence on the stability of the catalysts[END_REF][START_REF] Choi | The Achilles' heel of ironbased catalysts during oxygen reduction in an acidic medium[END_REF] ROS can oxidize the carbon support surface, resulting in a decrease of the ORR turnover frequency (TOF) of the existing Fe active sites. The presence, amount and effect of ROS depend on the electrochemical potential, temperature, and presence of ORR. [START_REF] Bae | Effect on the H2O2-Induced Deactivation of Fe-N-C Catalysts[END_REF][START_REF] Choi | The Achilles' heel of ironbased catalysts during oxygen reduction in an acidic medium[END_REF] In an alkaline environment the effect of ROS attack on catalyst degradation was shown to be minor. [START_REF] Bae | Effect on the H2O2-Induced Deactivation of Fe-N-C Catalysts[END_REF][START_REF] Choi | The Achilles' heel of ironbased catalysts during oxygen reduction in an acidic medium[END_REF] (iii) Agglomeration. Agglomeration of active singleatom Fe sites to Fe-clusters was shown to be affected by temperature. [START_REF] Kumar | Durability: Effects of Single Atoms' Mobility and Clustering[END_REF] (iv) Fe dissolution. Direct Fe ion leaching from active sites (not indirectly triggered by carbon corrosion) is influenced by electrolyte pH, [START_REF] Santori | Effect of Pyrolysis Atmosphere and Electrolyte pH on the Oxygen Reduction Activity, Stability and Spectroscopic Signature of FeNx Moieties in Fe-N-C Catalysts[END_REF] potential, [START_REF] Choi | Stability of Fe-N-C Catalysts in Acidic Medium Studied by Operando Spectroscopy[END_REF] temperature, 10 chemical environment, [START_REF] Santori | Effect of Pyrolysis Atmosphere and Electrolyte pH on the Oxygen Reduction Activity, Stability and Spectroscopic Signature of FeNx Moieties in Fe-N-C Catalysts[END_REF] and water-flux across the active sites within micropores. [START_REF] Chenitz | A specific demetalation of Fe-N4 catalytic sites in the micropores of NC_Ar + NH3 is at the origin of the initial activity loss of the highly active Fe/N/C catalyst used for the reduction of oxygen in PEM fuel cells[END_REF] While it is likely that several degradation mechanisms are operative and the extent of each mechanism is different depending on the FC operating conditions, the Fe-N-C structure and morphology, and the active layer design, a fundamental understanding of the stability of Fe-N-C catalysts is still lacking. The dissolution of different Fecontaining species initially present in Fe-N-C catalysts could lead to various types and degrees of degradation. [START_REF] Sgarbi | Electrochemical Transformation of Fe-N-C catalysts into Iron Oxides in Alkaline Medium and Its Impact on the Oxygen Reduction Reaction Activity[END_REF] For instance, Fe leaching from atomically dispersed FeNxCy sites, which catalyze ORR in both acidic and alkaline media, will result in a decreasing active site density (SD). The turnover frequency (TOF) of active sites may also be modified. For example, Fe3C particles embedded within graphite (Fe3C@N-C) could act as an electron-donor to nearby FeNxCy sites [START_REF] Jiang | Understanding the High Activity of Fe-N-C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe-Nx[END_REF] and/or graphite layers. [START_REF] Deng | Iron Encapsulated within Podlike Carbon Nanotubes for Oxygen Reduction Reaction[END_REF] As a result, the adsorption of O2 can be optimized, and the TOF of active sites can be increased, especially in alkaline media. [START_REF] Sgarbi | Oxygen reduction reaction mechanism and kinetics on M-NxCy and M@N-C active sites present in model M-N-C catalysts under alkaline and acidic conditions[END_REF] Therefore, Fe leaching from Fe3C@N-C in alkaline media might cause a drop of TOF of originally boosted active sites. Furthermore, Fe leaching from different Fe species was observed during stability tests under various conditions. [START_REF] Choi | Stability of Fe-N-C Catalysts in Acidic Medium Studied by Operando Spectroscopy[END_REF][START_REF] Choi | Minimizing Operando Demetallation of Fe-N-C Electrocatalysts in Acidic Medium[END_REF][START_REF] Chenitz | A specific demetalation of Fe-N4 catalytic sites in the micropores of NC_Ar + NH3 is at the origin of the initial activity loss of the highly active Fe/N/C catalyst used for the reduction of oxygen in PEM fuel cells[END_REF][START_REF] Li | Identification of durable and nondurable FeNx sites in Fe-N-C materials for proton exchange membrane fuel cells[END_REF] As an example, in scanning flow cell (SFC) experiments coupled to inductively coupled plasma mass spectroscopy (ICP-MS), Fe dissolution in an Arpurged acidic electrolyte was assigned to leaching of inactive Fe sites. [START_REF] Choi | Stability of Fe-N-C Catalysts in Acidic Medium Studied by Operando Spectroscopy[END_REF][START_REF] Choi | Minimizing Operando Demetallation of Fe-N-C Electrocatalysts in Acidic Medium[END_REF] In contrast, for cathodes after H2/O2 PEMFC tests, the decrease of ORR activity was correlated to a loss of FeNxCy active sites using ex situ Mössbauer spectroscopy. [START_REF] Chenitz | A specific demetalation of Fe-N4 catalytic sites in the micropores of NC_Ar + NH3 is at the origin of the initial activity loss of the highly active Fe/N/C catalyst used for the reduction of oxygen in PEM fuel cells[END_REF][START_REF] Li | Identification of durable and nondurable FeNx sites in Fe-N-C materials for proton exchange membrane fuel cells[END_REF] This discrepancy illustrates that direct comparisons of the results from thin-film catalyst layers in aqueous model systems (AMS), e.g. rotating disk electrode (RDE) or SFC, and gas-diffusion cathodes in PEMFCs or AEMFCs are challenging, as highlighted in our recent review. [START_REF] Ehelebe | Limitations of aqueous model systems in the stability assessment of electrocatalysts for oxygen reactions in fuel cell and electrolysers[END_REF] Such differences may affect the intensity and the mechanisms of degradation significantly. A major discrepancy between AMS and real devices is the direct contact of the catalyst to gaseous O2, which cannot be realized in RDE or SFC. Moreover, even in AMS, the influence of O2 on Fe-N-C stability is rarely addressed as the stability has traditionally been tested in de-aerated acidic environment. However, degradation of PGM-free materials can be significantly affected by the presence of O2. [START_REF] Kumar | On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts[END_REF][START_REF] Speck | Mechanisms of Manganese Oxide Electrocatalysts Degradation during Oxygen Reduction and Oxygen Evolution Reactions[END_REF][START_REF] Osmieri | Durability evaluation of a Fe-N-C catalyst in polymer electrolyte fuel cell environment via accelerated stress tests[END_REF] Thus, using online ICP-MS, Speck et al. [START_REF] Speck | Mechanisms of Manganese Oxide Electrocatalysts Degradation during Oxygen Reduction and Oxygen Evolution Reactions[END_REF] identified different dissolution mechanisms for MnOx in Ar-and O2-purged alkaline media. On the one hand, Mn transient-dissolution occurs in both Ar and O2 environment, which was attributed to the redox transitions of Mn. On the other hand, Mn total dissolution is drastically increased in O2 environment. This was ascribed to the presence of ROS, such as HO2 -generated during the ORR. A similar trend has also been detected for Fe-N-C catalysts degradation in acidic media. Combining RDE measurements with Raman spectroscopy, Kumar et al. [START_REF] Kumar | On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts[END_REF] showed that both deterioration of ORR activity and the extent of carbon corrosion are significantly more severe in O2-compared to Ar-saturated electrolytes.

In order to bridge the gap between fundamental and applied research, gas diffusion electrode (GDE) half-cell setups have emerged as a tool to combine the advantages of AMS (three electrode setup, fast and comparable testing at standard operating conditions) and FCs (aqueous electrolyte-free catalyst layers and high current densities). [START_REF] Lin | Benchmarking high surface area electrocatalysts in a gas diffusion electrode: measurement of oxygen reduction activities under realistic conditions[END_REF][START_REF] Pinaud | Key considerations for high current fuel cell catalyst testing in an electrochemical half-cell[END_REF][START_REF] Ehelebe | Evaluating Electrocatalysts at Relevant Currents in a Half-Cell: The Impact of Pt Loading on Oxygen Reduction Reaction[END_REF][START_REF] Ehelebe | Fuel cell catalyst layer evaluation using a gas diffusion electrode half-cell: Oxygen reduction reaction on Fe-N-C in alkaline media[END_REF] In the present work, we used a GDE halfcell setup coupled with online ICP-MS (GDE-ICP-MS) [START_REF] Ehelebe | Platinum Dissolution in Realistic Fuel Cell Catalyst Layers[END_REF] to systematically investigate the impact of O2 atmosphere and a solid electrolyte interface on Fe demetallation in alkaline medium from Fe-N-C catalyst layers (CLs) operating at current densities up to -125 mA•cm -2 . Such a setup allows going beyond the previously reported SFC-ICP-MS setup, which was restricted to very low current densities and therefore of little interest to study degradation mechanisms that are intimately related to the ORR rate.

Experimental

Electrode manufacturing

There are two kinds of alkaline Fe-N-C gas diffusion electrodes (GDEs) in this work. One contains a commercial Fe-N-C catalyst (PMF-011904, [START_REF] Primbs | Establishing reactivity descriptors for platinum group metal (PGM)-free Fe-N-C catalysts for PEM fuel cells[END_REF] Lot 0601, Pajarito Powder) and the other contains a benchmark synthesized Fe-N-C catalyst (Fe0.5-NH3). A brief description of the manufacture of the electrode containing PMF-011904 can be found below, and a detailed version is in section 1.1.1 of the supporting information (SI). The synthesis of Fe0.5-NH3 and the manufacture of its electrode can be found in sections 1.1.2 and 1.1.3 of the SI, respectively. Note that in the Results and Discussion sections 3.1 and 3.2, the Fe-N-C catalyst refers to PMF-011904, while in the section 3.3 both Fe-N-C catalysts are considered and specified accordingly.

The alkaline Fe-N-C catalyst layer (CL) was fabricated on a gas diffusion medium including a microporous layer (H23C8, Freudenberg, 4x4 cm², 218 ± 5 μm). The CL with PMF-011904 was composed of 70 ± 0.3 wt% the commercial Fe-N-C catalyst and 30 ∓ 0.3 wt% commercial ionomer (Aemion TM HNN5-00-X, Ionomr). The anion exchange membrane (AEM) was prepared solely with this ionomer. The loading of the CL was determined to be 1.35 ± 0.05 mg•cm -2 . The thicknesses of the CL and AEM are 47.5 ± 5.2 μm and 0.5 ± 0.2 μm, respectively, measured from images (see Figure S2 and Table S2) taken via crosssectional imaging with a focused ion beam scanning electron microscope (Crossbeam 540 FIB-SEM, Zeiss). More detailed information can be found in section 1.2 of the SI.

GDE-ICP-MS measurements

Both on-line GDE-ICP-MS and ex situ ICP-MS measurements were performed. The GDE-ICP-MS setup and methodology have been introduced in our previous work. [START_REF] Ehelebe | Platinum Dissolution in Realistic Fuel Cell Catalyst Layers[END_REF] Also, Scheme S1 demonstrates the concept of the setup. To detect 56 Fe species, ICP-MS (Perkin Elmer, NexION 350) was operated in dynamic reaction cell (DRC) mode using CH4 (5.5, Air Liquide) to minimize the impact of the interference with [START_REF] Kumar | Physical and Chemical Considerations for Improving Catalytic Activity and Stability of Non-Precious-Metal Oxygen Reduction Reaction Catalysts[END_REF] Ar 16 O + . An internal standard of 100 μg•L -1 of Ge ((NH4)2GeF6, Merck Centripur) in 1 wt% HNO3 (Suprapur ® , Merck) was used. A four-point calibration was carried out with a blank and three standard Fe solutions (0, 1, 5, 25 μgFe•L -1 ) every day before measurements. For the standard solutions, the Merck Centripur ICP standard (Fe(NO3)3, 1000 mg•L -1 , in 2-3% HNO3) was first diluted 100 times with 1 wt% HNO3(aq), and then further diluted to the aimed concentrations with a 0.1 M NaOH electrolyte (Merck Suprapure). More details can be found in section 1.3 of the SI.

The electrochemical measurements were performed in 0.1 M NaOH in a GDE half-cell. [START_REF] Ehelebe | Evaluating Electrocatalysts at Relevant Currents in a Half-Cell: The Impact of Pt Loading on Oxygen Reduction Reaction[END_REF][START_REF] Ehelebe | Fuel cell catalyst layer evaluation using a gas diffusion electrode half-cell: Oxygen reduction reaction on Fe-N-C in alkaline media[END_REF] With GDE-ICP-MS, the impact of (i) gas supplied to the back of the GDE (Ar vs. O2, 50 ml•min -1 ) and (ii) the CL | electrolyte interface (AEM vs. no AEM) have been investigated. The measurements with O2 were conducted with galvanostatic techniques and consisted of three parts, (i) an activity test, (ii) an accelerated stress test of 200 cycles (AST-200), and finally (iii) a repeated activity test. (see Scheme 1). Then, with the potential responses obtained from the first activity test with O2, an electrochemical protocol with Ar-saturated environment was programed with potentiostatic techniques that reproduced the potentials experienced by the GDEs during the whole measurements with O2 (also see Scheme 1). Importantly, this protocol with Ar was applied to fresh samples, not to samples after the O2 protocol. After the whole Ar protocol, the activity test with O2 was performed. A detailed explanation of this experimental design can be found in section 1.4 of the SI. Each set of experiments was performed at least twice to show the result's reproducibility.

To obtain long-term dissolution stability data, the respective ASTs in Ar and O2 protocols were also extended from 200 to 2000 cycles, noted as AST-Ar-2k and AST-O2-2k, respectively. Every 500 cycles, two aliquots of solution were taken out from the well-mixed electrolyte for ex situ ICP-MS measurements. Following these extended ASTs, a final ORR activity test was also performed.

All values of potential are reported with respect to a reversible hydrogen electrode (RHE) and 100 % iRcorrected. The reference electrode (RE, Ag/AgCl Metrohm) was calibrated every day (𝐸 𝐴𝑔/𝐴𝑔𝐶𝑙 = 0.944 ± 0.0115 𝑉 𝑅𝐻𝐸 ). For potentiostatic measurements (Ar environment), the potential was 80 % iR-compensated during measurements and 20 % iR-corrected post measurements. The uncompensated resistance (Ru) was determined via electrochemical impedance spectroscopy (EIS) at open circuit potential (OCP). For galvanostatic measurements (O2), the data was 100 % post iR-corrected. The Ru was measured via EIS at each current step, as proposed previously. [START_REF] Pinaud | Key considerations for high current fuel cell catalyst testing in an electrochemical half-cell[END_REF][START_REF] Ehelebe | Evaluating Electrocatalysts at Relevant Currents in a Half-Cell: The Impact of Pt Loading on Oxygen Reduction Reaction[END_REF] In all experiments, the electrolyte was always purged with 50 ml•min -1 Ar.

Experimental details on characterization of catalysts and catalyst layers can be found in sections 1.5 -1.8 of the SI.

Results and Discussion

Online Fe Dissolution in Varied Conditions and the Respective Impacts on ORR performance

In order to gain insights into the Fe-N-C demetallation in realistic alkaline CLs during ORR, Fe dissolution in Arand O2-environments is first compared during the application of similar potential steps. Figure 1 shows the effect of O2 (or ORR) on the Fe dissolution at different potentials between 0.57 and 0.87 VRHE. The novel GDE-ICP-MS setup allows detection of Fe dissolution at current densities up to -125 mA•cm -2 (see Figure 1A). The highest current density that can be reached with this setup is defined by the electrolyte resistance, which is in turn mainly limited by the highest alkaline electrolyte concentration that the ICP-MS can tolerate, i.e. 0.1 M. For comparison: in AMS such as RDE, only a few mA•cm -2 can be reached due to the solubility limit of O2 in aqueous electrolytes. [START_REF] Choi | Stability of Fe-N-C Catalysts in Acidic Medium Studied by Operando Spectroscopy[END_REF][START_REF] Kumar | On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts[END_REF][START_REF] Hossen | Synthesis and characterization of high performing Fe-N-C catalyst for oxygen reduction reaction (ORR) in Alkaline Exchange Membrane Fuel Cells[END_REF] In Figure 1A and 1B, the electrochemical protocols in Ar and O2 environments before their respective AST-200 protocols are displayed. In the presence of O2, -125 mA•cm -2 can be reached due to ORR at around 0.6 VRHE. In an Ar environment the steady-state current density, which was not yet reached at the end of the 40-s step, is less negative than -0.2 mA•cm -2 at similar potentials. Under these electrochemical conditions, the Fe dissolution profiles (see Figure 1C, left hand-side axis, solid lines) show that significantly more Fe is dissolved in the presence of O2. In the potential range between 0.8 -0.6 VRHE, the amount of Fe dissolved during each 40-s step in O2 is about one order of magnitude higher than that in Ar (see Figure 1C, right hand-side axis, dotted lines). Also, at 0.87 VRHE, the Fe dissolution is already significantly higher in O2 than in Ar. Hence, it is clear that the amount of Fe dissolution would be underestimated if the electrochemical testing is executed in an environment without O2 as often done hitherto in AMS. [START_REF] Santori | Effect of Pyrolysis Atmosphere and Electrolyte pH on the Oxygen Reduction Activity, Stability and Spectroscopic Signature of FeNx Moieties in Fe-N-C Catalysts[END_REF][START_REF] Choi | Stability of Fe-N-C Catalysts in Acidic Medium Studied by Operando Spectroscopy[END_REF] In addition to the impact of the presence of O2 or ORR, the following two characteristic features of Fe-N-C demetallation can be observed for both Ar and O2 cases. Firstly, the Fe dissolution starts immediately when the cathodic currents or potential steps are applied. After moving back to open circuit potential (OCP), also the dissolution drops immediately. In general, the amount of dissolved Fe species rises with decreasing potential or/and increasing current density. Additionally, the dissolution rate at the regularly repeated steps performed at -5 mA•cm -2 (or at the corresponding potential, 0.87 V) is almost constant (see Figure S3C), indicating that the order of the applied steps at higher currents (or lower potentials) does not affect the dissolution behavior in the chosen protocol. Besides Fe dissolution, also the potential at these repeated steps merely drops 3 mV before the step to the lowest potential before the accelerated stress test (AST-200) (see Figure S3B).

To mimic fuel cell load cycles, we conducted the 200-cycle AST between -0.05 and -125 mA•cm -2 in O2 environment, which correspond to 0.88 ± 0.03 and 0.56 ± 0.04 VRHE, respectively. For comparison, the AST in Ar was conducted in a similar potential range, i.e. between 1.0 and 0.573 VRHE. Using these protocols, the total amount of dissolved Fe species during the O2-AST-200 is almost one order of magnitude higher than that during the Ar-AST-200 (see Figure 2A), in line with the trends seen already during the part of the protocol that is applied before the AST-200. Interestingly, despite the dramatically higher Fe dissolution during ORR compared to the one in Ar environment, the electric-charge-normalized dissolution rates during the ASTs in O2 and Ar are similar (see Figure 2B). This implies that Fe dissolution in Fe-N-C could be associated with a charge-transfer-related process that commonly occurs in both ASTs in Ar and O2. To further validate this linear correlation between total amount of Fe dissolved from and the total electric charge passed through the electrode, Fe dissolution in O2 for two different holding periods at -125 mA•cm -2 (40 s or 600 s) is compared in Figure S4A. The results confirm that Fe dissolution is fairly constant when being normalized to the applied electric charge. In contrast, when the same Fe dissolution data is normalized to the number of applied cycles, no correlation can be seen (see Figure S4B). All these results imply that the rate of Fe dissolution in alkaline media is directly proportional to the applied current density, at least within the corresponding investigated potential range. This finding could partly explain why Sgarbi et al. [START_REF] Sgarbi | Electrochemical Transformation of Fe-N-C catalysts into Iron Oxides in Alkaline Medium and Its Impact on the Oxygen Reduction Reaction Activity[END_REF] did not observe a notable deviation in Fe-N-C degradation between Ar-and O2-saturated alkaline environments with an RDE half-cell, where the current density of ORR is limited to around -5 mA/cm 2 or lower. While the fate of leached iron cations can depend on the Fe-N-C catalyst morphology (microporous Fe-N-C in the previous work [START_REF] Sgarbi | Electrochemical Transformation of Fe-N-C catalysts into Iron Oxides in Alkaline Medium and Its Impact on the Oxygen Reduction Reaction Activity[END_REF] vs. mesoporous Fe-N-C in the present work, the former possibly favoring reprecipitation in the catalytic layer and the latter favoring the transfer towards the bulk electrolyte), a 25 times higher current density achieved in a GDE half-cell here amplifies the difference and unveils the impacts of ORR on Fe-N-C degradation in alkaline media, of which a detailed evaluation can be found in the section 3.3.

In oxygen evolution reaction (OER) research, the reciprocal of the charge-normalized dissolution, the socalled S-number, has been well established as a metric for catalyst stability. [START_REF] Geiger | The stability number as a metric for electrocatalyst stability benchmarking[END_REF] Due to the correlation between electric charge and Fe dissolution, a similar concept can be applied here. The S-number of the presently studied commercial Fe-N-C would be around 10 [START_REF] Santori | Effect of Pyrolysis Atmosphere and Electrolyte pH on the Oxygen Reduction Activity, Stability and Spectroscopic Signature of FeNx Moieties in Fe-N-C Catalysts[END_REF] . This means that in the studied potential region between 0.57 and 0.87 VRHE, about one million electrons were exchanged in the ORR for every Fe atom that dissolved. Accordingly, the average probability of losing an Fe atom for each electron transfer event is approximately 10 -6 . Yet, as the degradation rate of Fe-N-C catalysts might depend on operation time, as shown for acidic ORR, [START_REF] Zhang | Is iron involved in the lack of stability of Fe/N/C electrocatalysts used to reduce oxygen at the cathode of PEM fuel cells?[END_REF] so would be their S-numbers. Hence, we additionally performed ex situ experiments with the respective ASTs extended from 200 to 2000 cycles, noted as Ar-AST-2k and O2-AST-2k. Especially, the S-numbers obtained during O2-AST-2k and evaluated for every 500 cycles (see Figure S5) suggest that the S-number around 10 6 is representative for the first 1500 cycles. Further discussion on how applicable this S-number is for AEMFCs can be found in the text accompanying Figure S5. Although ORR is a complicated multi-step 4-electron-transfer reaction with several intermediate states, [START_REF] Ramaswamy | Activity Descriptor Identification for Oxygen Reduction on Nonprecious Electrocatalysts: Linking Surface Science to Coordination Chemistry[END_REF][START_REF] Ramaswamy | Alkaline Anion-Exchange Membrane Fuel Cells: Challenges in Electrocatalysis and Interfacial Charge Transfer[END_REF] where the coordinated Fe cation could be variously unstable, the Snumber could be a suitable quantification metric to study the overall stability of Fe-N-C catalysts in various conditions and to compare different materials.

To further approach more realistic AEMFC conditions, we additionally compare on-line Fe dissolution in GDEs without and with a thin AEM coated on top of the CL in an O2-saturated environment. For Pt/C catalysts in acidic media, a significant stabilizing impact of Nafion membranes on the net Pt dissolution was recently observed. [START_REF] Ehelebe | Platinum Dissolution in Realistic Fuel Cell Catalyst Layers[END_REF] This effect was attributed to the impeded masstransport of dissolved catalyst species through the membrane. However, for the present system, we cannot observe any significant impact of the AEM on Fe dissolution (see Figure 3C andS6). The diverging effects of the Nafion membranes and the AEM can be attributed to the following differences between the two systems. Firstly, the thickness ratio of CL to membrane in our AEM system is two to three orders of magnitude higher than that in the work mentioned above (see Table S2 and Figure S2). In our case, the doctor-blade coated AEM membrane is less than 1 μm thick, whereas in the literature Nafion membranes of 25 and 52 μm were used. [START_REF] Ehelebe | Platinum Dissolution in Realistic Fuel Cell Catalyst Layers[END_REF] Hence, the impacts of AEM on Fe-N-C demetallation in our study are diminished. Additionally, the varied physical properties of dissolved species and the membranes could contribute to the differing results. [START_REF] Speck | Single-Atom Catalysts: A Perspective toward Application in Electrochemical Energy Conversion[END_REF] For future work, commercial AEMs with increased thickness should be used to reveal the impact of the transport of dissolved Fe species through the membrane on the total Fe dissolution. After examining the effect of different relevant conditions on Fe dissolution, also the impact of catalyst dissolution on the catalyst activity should be compared (see Figure 3). First of all, when measuring ORR performance, no significant effect of the AEM on the initial activity (pristine) could be observed. Also, it is noteworthy that, if compared to the state-of-the-art AEMFCs literature values, [START_REF] Adabi | High-performing commercial Fe-N-C cathode electrocatalyst for anion-exchange membrane fuel cells[END_REF] the ORR performance of the used electrocatalyst is inferior. We attribute this discrepancy to the relatively low concentration of alkaline electrolyte, i.e. 0.1 M NaOH, which is the highest concentration that the on-line ICP-MS can tolerate. Nevertheless, the activity test here is not designed for obtaining maximum performance but for comparing the effect of ASTs performed in the absence and presence of O2 (ORR) on the same catalyst. Calculated from the polarization curves in Figure 3A, the relative loss of power density (PD) at 0.65 VRHE is compared to the relative loss of Fe content in Figure 3B & 3C, first-hand indicating if the dissolved Fe species originally contribute to the activity or not. During the Ar-AST-200, only negligible performance deterioration occurred, although Fe dissolution was measurable (See Figure 3, blue). This can be interpreted as a small Fe dissolution during potential cycling in absence of O2, but not specifically at the Fe active sites, or not at the most active sites. In contrast, regardless of whether there is an AEM coating or not, a similar and significant performance loss is detected after O2-AST-200 (see Figure 3B). The relative losses of PD are above 20%. However, the relative losses of Fe during the O2-AST-200 are only around 0.4 %, with or without an AEM coating (see Figure 3C). Similar trends of PD loss and Fe dissolution are also observed for Ar-AST-2k and O2-AST-2k (see Figure S7B & S7C). Especially, although similar Fe contents were lost during O2-AST-200 and Ar-AST-2k, the performance loss during O2-AST-200 is more than 50 times higher than that during Ar-AST-2k. This implies that ORR could trigger Fe dissolution from species that contribute highly to the activity, or trigger modifications of the bulk or surface of the carbon matrix, in turn leading to decreased turnover frequency of the active sites, or both.

Impacts of ORR on the Dissolved Fe Species and on the Carbon Matrix

To identify which Fe species were dissolved during Ar-AST and O2-AST, pristine and aged samples (no AST, Ar-AST-2k, and O2-AST-2k) were post mortem characterized using several ex situ techniques, including Xray absorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. First of all, the identified Fe species in this commercial Fe-N-C catalyst are FeNxCy, Fe3C@N-C, and exposed Fe3C nanoparticles. Fe3C is suggested as the major Fe species by the Fourier transform (FT) of the extended X-ray absorption fine structure (EXAFS) spectra, characterized by a peak at about 2.08 Å, assigned to Fe-Fe backscattering in Fe3C (see Figure S8). Also confirming the presence of Fe3C with HRSTEM, lattice planes of the Fe-containing particles were captured and are shown in Figure S9A. In their respective diffractograms in Figure S9B, lattice spacings of 2.5 Å and 2.4 Å were identified, which correspond well to the planes (200) and (121) in the crystalline structure of Fe3C. Furthermore, Figure S10 shows that many Fe3C particles are encapsulated in graphite shells of thicknesses varying from about 2 nm to 12 nm (or even thicker). Moreover, these figures exhibit the interlayer spacing of the graphite shell being around 3.5 Å, similar to reported value for Ndoped graphite shell around Fe3C nanoparticles, usually denoted as Fe3C@N-C. [START_REF] Kumar | Physical and Chemical Considerations for Improving Catalytic Activity and Stability of Non-Precious-Metal Oxygen Reduction Reaction Catalysts[END_REF] As introduced before, these embedded Fe3C particles are known to potentially boost the activity of the graphite shell or nearby FeNxCy active sites. Identifying FeNxCy, the XPS N-1s spectra (see

Ar O 2 O 2 -M Fe Content O 2 O 2 -M Figure 4A
) show a notable peak at around 399.5 eV, which can be assigned either to specific N species or to N-Fe bonds. [START_REF] Primbs | Establishing reactivity descriptors for platinum group metal (PGM)-free Fe-N-C catalysts for PEM fuel cells[END_REF][START_REF] Matanovic | Understanding PGM-free catalysts by linking density functional theory calculations and structural analysis: Perspectives and challenges[END_REF] Furthermore, spectrum images (see EDX maps in Figure S11) indicate that besides Fe-containing nanoparticles, there is an additional Fe signal that is homogeneously distributed, even below the nanometric scale; Additionally, HAADF-HRSTEM reveals comparably bright spots at the atomic scale (see Figure S12). As this method exploits the mass-thickness contrast, these bright spots are heavier elements and thus can be assigned to atomically dispersed Fe-sites within the remarkably lighter carbon support. TEM analyses reveal the presence of FeNxCy sites in this material although its relative amount is suggested to be minor (< 5 %rel.) according to the EXAFS spectra. Note that the presence of graphite shells on the Fe3C nanoparticles and the overall small amount of Fe in the catalyst explain the absence of Fe signal in XPS Fe-2p spectra (see Figure S13). In all cases in this study, it was not observed that the dissolved Fe species re-precipitate as Fe oxide, in contrast to a recent study performed in RDE conditions and for two other Fe-N-C catalysts, one rich in Fe3C@N-C and the other rich in atomically dispersed FeNxCy sites. [START_REF] Sgarbi | Electrochemical Transformation of Fe-N-C catalysts into Iron Oxides in Alkaline Medium and Its Impact on the Oxygen Reduction Reaction Activity[END_REF] The different tendencies of re-precipitation of leached Fe might be attributed to the different catalyst layer structures, ranges of current densities, and/or temperature. Although Fe dissolution is observed during Ar-AST-2k, the polarization curve, XPS N-1s and C-1s spectra barely change (see Figure S7, 4A & 4B). The unchanged XPS N-1s and C-1s spectra suggest the integrities of FeNxCy and Fe3C@N-C, respectively. Because if FeNxCy site density had significantly decreased, the N-Fe signal in the N-1s spectrum (399.5 eV) would drop; if Fe dissolved from Fe3C@N-C, the protecting graphite layer would already be compromised and thus the C-C signal in the C-1s spectrum would be lower. Therefore, Fe species which dissolve during Ar-AST-2k are likely the exposed Fe3C nanoparticles, which do not contribute to the activity, explaining why the performance remained almost intact. During O2-AST-2k, 3.4% of Fe and 34% of PD were lost (see Figure S7B & S7C), which is similar to the degradation of another Fe3C@N-C species-rich Fe-N-C catalyst in an RDE setup, after 10k cycles of similar potential limits and step duration to those used in this work. [START_REF] Sgarbi | Electrochemical Transformation of Fe-N-C catalysts into Iron Oxides in Alkaline Medium and Its Impact on the Oxygen Reduction Reaction Activity[END_REF] Herein, as the initial relative amount of FeNxCy sites is minor, the loss of FeNxCy sites could not be conclusively determined by either XANES or XPS. For example, in XPS N-1s spectra (see Figure 4A), the intensity at the N-Fe peak (399.5 eV) barely drops. Although there is a drop around 400.2 eV, this could not be exclusively assigned to a loss of FeNxCy or pyrrolic-N, of which the latter contributes to the peak at 401.1 eV and is noted as N3 in Figure 4A. [START_REF] Primbs | Establishing reactivity descriptors for platinum group metal (PGM)-free Fe-N-C catalysts for PEM fuel cells[END_REF] Nevertheless, even if the total Fe loss is comprised of some from FeNxCy sites, leading to decreasing SD, this specific loss would only partially explain the 34% loss of PD. On the contrary, Fe3C particles with and without graphite shell both largely disappear during O2-AST-2k, obviously observed with STEM (see Figure S14). Additionally, Figure 4B shows a drop of the C-C signal in the XPS C-1s spectrum and this confirms that the protecting graphite shell of Fe3C@N-C could be compromised and thus new Fe3C became exposed to the electrolyte, and started leaching. The loss of Fe3C@N-C could lead in turn to a drop of the average TOF of FeNxCy sites of this catalyst, which could explain the 34% decrease in PD.

Figure 4. XPS spectra of the Fe-N-C GDEs that underwent (i) no AST (grey, desh-dotted line), (ii) Ar-AST-2k (blue, solid line), and (iii) O2-AST-2k (red, solid line). A: XPS N-1s spectra, where N1, N2, N3, and N4 are assigned to pyridinic N, Fe-N, pyrrolic N, graphitic N, respectively. 30 B: XPS C-1s spectra, where the signal peaks are assigned to C species including those with C-C, C-O or -N, and C=O or =N bonds. [START_REF] Gupta | Highly Active and Stable Graphene Tubes Decorated with FeCoNi Alloy Nanoparticles via a Template-Free Graphitization for Bifunctional Oxygen Reduction and Evolution[END_REF] C: O-1s spectra, where the O species are assigned to O-C and O=C. [START_REF] Bae | Effect on the H2O2-Induced Deactivation of Fe-N-C Catalysts[END_REF] As the decreased C-C signal in XPS C-1s spectrum of O2-AST-2k (see Figure 4B) indicates a loss of graphite layers, the nature of the modification of the graphite structure during ORR in alkaline is further analyzed with Raman spectra and XPS O-1s spectra (see Figure S15 and 4C, respectively). Raman spectra show that the graphite crystallinity barely changed during O2-AST, or at least not as obviously as reported in acidic medium by Kumar et al. [START_REF] Kumar | On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts[END_REF] Moreover, compared to the XPS O-1s spectrum of the pristine sample, those of the aged samples show decreased signals of both C-O and C=O species (oxygen functional groups). The opposite trend was observed by Choi et al. [START_REF] Choi | The Achilles' heel of ironbased catalysts during oxygen reduction in an acidic medium[END_REF] and Bae et al. [START_REF] Bae | Effect on the H2O2-Induced Deactivation of Fe-N-C Catalysts[END_REF] upon the oxidation of graphite by ROS, suggesting a different nature of graphite modification in this study. Also, the decreased amounts of oxygen

C

functional groups suggest either the initially bonded O was reduced or that the O-bonded C was oxidized to gaseous CO or CO2, the latter of which is known as the classic carbon corrosion. However, the rate of classic carbon corrosion is expected to increase with potential, [START_REF] Choi | Stability of Fe-N-C Catalysts in Acidic Medium Studied by Operando Spectroscopy[END_REF][START_REF] Kumar | Physical and Chemical Considerations for Improving Catalytic Activity and Stability of Non-Precious-Metal Oxygen Reduction Reaction Catalysts[END_REF] while the rate of Fe dissolution increases with decreasing potential, in the studied potential range. Hence, the classic carbon corrosion is not considered dominating degradation mechanism under the studied conditions. To summarize the above analyses, while a negligible change in the graphite crystallinity and no further oxidation of the carbon matrix are suggested by the Raman and XPS O-1s spectra, respectively, the removal of a few graphite layers during ORR is indicated by XPS C-1s spectra.

The above unveils the impacts of ORR on the origins of dissolved Fe from Fe-N-C and on its graphite structure in alkaline media. With the presented data, some derivations on the degradation mechanism of Fe-N-C catalysts in alkaline media in realistic catalyst layers are made in the further course.

Fe Leaching Mechanism in Fe-N-C Catalysts in Alkaline Media

The degradation of Fe-N-C proceeds through diverse mechanisms in different electrochemical potential ranges or pH environments. [START_REF] Santori | Effect of Pyrolysis Atmosphere and Electrolyte pH on the Oxygen Reduction Activity, Stability and Spectroscopic Signature of FeNx Moieties in Fe-N-C Catalysts[END_REF][START_REF] Choi | Stability of Fe-N-C Catalysts in Acidic Medium Studied by Operando Spectroscopy[END_REF][START_REF] Choi | The Achilles' heel of ironbased catalysts during oxygen reduction in an acidic medium[END_REF] It has to be noted that the current work only allows derivations for the potential range between 1.0 and approximately 0.6 VRHE in alkaline media. This, however, corresponds well to a usual potential range of AEMFCs under load. [START_REF] Mustain | Durability challenges of anion exchange membrane fuel cells[END_REF] Under the investigated conditions, the Fe dissolution is directly proportional to the applied charge for the Fe3C@N-C species-rich Fe-N-C but also another benchmark Fe-N-C with dominant Fe species as FeNxCy (see Figure S16), the latter of which has been noted as Fe0.5-NH3 and well characterized previously. [START_REF] Zitolo | Identification of catalytic sites for oxygen reduction in iron-and nitrogen-doped graphene materials[END_REF] In other words, for the two mainstream Fe-N-C catalysts for ORR in alkaline media, Fe dissolution can be largely correlated to charge transfer events. To start with the Fe3C@N-C species-rich Fe-N-C, this correlation ties Fe dissolution from exposed Fe3C particles to charge transfer events responding to electrochemical potential cycling during Ar-AST. Also, the same correlation associates Fe dissolution from Fe3C@N-C with charge transfer events in ORR catalytic cycle. This association confirms that the dissolved Fe from Fe3C@N-C were originally involved in ORR, at least indirectly. For example, Jiang et al. [START_REF] Jiang | Understanding the High Activity of Fe-N-C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe-Nx[END_REF] reported that the step of adsorption of O2 is more favored owing to additional electron provided to N atoms in FeNxCy sites by neighboring metallic Fe. In contrary, in Fe0.5-NH3 the dominant species FeNxCy is barely assisted by other Fe species to catalyze ORR. Meanwhile, the initial involvement of the dissolved Fe species in ORR is confirmed by the direct correlation between Fe dissolution and charge transfer events during ORR. Therefore, the dissolved Fe species during ORR in Fe0.5-NH3 should be mainly assigned to FeNxCy and thus Fe dissolution from FeNxCy sites can also be associated with the charge transfer events in ORR catalytic cycle. Consequently, both N-coordinated Fe at FeNxCy sites and the core-shell interface Fe of Fe3C@N-C could be destabilized by one or more step(s) in the ORR catalytic cycle. To rationalize which step(s) dominates the Fe dissolution from Fe3C@N-C and FeNxCy in alkaline media, the potential destabilizing steps, namely, the adsorption of O2, the reduction of reaction intermediates, and the Fe redox transitions, are discussed below.

In the ORR where Fe3C@N-C is involved, O2 molecules could adsorb on activated graphite [START_REF] Deng | Iron Encapsulated within Podlike Carbon Nanotubes for Oxygen Reduction Reaction[END_REF] and/or TOF-boosted FeNxCy site. [START_REF] Jiang | Understanding the High Activity of Fe-N-C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe-Nx[END_REF] If O2-adsorbing graphite is oxidized, then the initially protected Fe3C@N-C could be exposed and lost. However, as discussed previously, XPS C-1s and O-1s spectra shown in Figure 4B & 4C suggest that the graphite was not further oxidized but removed. Thus, although the observed removal of graphite layers could be ORRtriggered, this may hardly be attributed to oxidation of graphite following the adsorption of O2. On the other hand, according to density functional theory (DFT) calculations conducted by Aoyama et al. [START_REF] Aoyama | Oxygen reduction reaction of FeN4 center embedded in graphene and carbon nanotube: Density functional calculations[END_REF] , the adsorption of O2 molecules on Fe at FeNxCy sites can shift the Fe position from in-plane to out-of-plane. This could lead to a destabilization of the N-coordinated Fe cation and therefore to its dissolution. Although the DFT study was conducted for an acidic condition, we could not rule out the possibility that a similar destabilizing effect also occurs in alkaline media.

Secondly, the Fe active sites could also be destabilized by ORR intermediates, mainly HO2 -(the dominating form of H2O2 in alkaline) or ROS. [START_REF] Ramaswamy | Activity Descriptor Identification for Oxygen Reduction on Nonprecious Electrocatalysts: Linking Surface Science to Coordination Chemistry[END_REF] The impact of H2O2 on the stability of Fe-N-C in acidic media has been systematically studied. [START_REF] Speck | Single-Atom Catalysts: A Perspective toward Application in Electrochemical Energy Conversion[END_REF][START_REF] Kumar | On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts[END_REF][START_REF] Lefèvre | Fe-based catalysts for the reduction of oxygen in polymer electrolyte membrane fuel cell conditions: determination of the amount of peroxide released during electroreduction and its influence on the stability of the catalysts[END_REF][START_REF] Choi | The Achilles' heel of ironbased catalysts during oxygen reduction in an acidic medium[END_REF][START_REF] Choi | Unraveling the Nature of Sites Active toward Hydrogen Peroxide Reduction in Fe-N-C Catalysts[END_REF] Choi et al. [START_REF] Choi | The Achilles' heel of ironbased catalysts during oxygen reduction in an acidic medium[END_REF] and Bae et al. [START_REF] Bae | Effect on the H2O2-Induced Deactivation of Fe-N-C Catalysts[END_REF] performed H2O2-treatment on Fe-N-C in acidic or alkaline media, and found notable declines of the ORR activity after H2O2-treatments in acidic yet a negligible decrease after an H2O2-treatment in alkaline. This suggests that compared to H2O2 in acidic, HO2 -is less harmful to Fe-N-C in alkaline environment, and thus its impact on the degradation process is potentially less pronounced. Actually in this work, as previously mentioned, the amounts of oxygen functional groups decrease during ORR in alkaline (see Figure 4C) as opposed to the effect of H2O2-treatment in acid. [START_REF] Bae | Effect on the H2O2-Induced Deactivation of Fe-N-C Catalysts[END_REF] Moreover, a great discrepancy is observed between the potential dependencies of the electric chargenormalized Fe dissolution detected in this work and the HO2 -Faradaic efficiency reported for a similar catalyst in a previous work [START_REF] Hossen | Synthesis and characterization of high performing Fe-N-C catalyst for oxygen reduction reaction (ORR) in Alkaline Exchange Membrane Fuel Cells[END_REF] (see Figure 5). Yet, it has to be considered that the Faradaic efficiency was determined in a rotating ring-disk electrode (RRDE) setup, where ORR is severely limited by the restricted mass transport of dissolved O2 when the potential is below 0.8 VRHE. This could lead to deviations when the RRDE result is compared to the dissolution data from this work, gathered from GDE halfcell measurements, where those mass transport limitations do not play a significant role. [START_REF] Lin | Benchmarking high surface area electrocatalysts in a gas diffusion electrode: measurement of oxygen reduction activities under realistic conditions[END_REF][START_REF] Pinaud | Key considerations for high current fuel cell catalyst testing in an electrochemical half-cell[END_REF][START_REF] Ehelebe | Evaluating Electrocatalysts at Relevant Currents in a Half-Cell: The Impact of Pt Loading on Oxygen Reduction Reaction[END_REF] Overall, the results point against a dominant impact of HO2 -or ROS on the stability of Fe-N-C in alkaline media and O2 environment; however, with currently available data, it cannot be conclusively excluded either. [START_REF] Hossen | Synthesis and characterization of high performing Fe-N-C catalyst for oxygen reduction reaction (ORR) in Alkaline Exchange Membrane Fuel Cells[END_REF] Thirdly, in a previous online dissolution study, the onsets of transition-metal dissolution in PGM-free catalysts (manganese oxide MnOx) could be correlated to the corresponding redox transition potentials. [START_REF] Speck | Mechanisms of Manganese Oxide Electrocatalysts Degradation during Oxygen Reduction and Oxygen Evolution Reactions[END_REF] It was proposed that the reconstruction of the surface MnOx coordination during redox transitions could lead to the so-called transient-dissolution. Although the structures of MnOx and the Fe species in Fe-N-C are different and this demetallation mechanism has not been identified responsible for degradations of Fe-N-C catalysts yet, a similar concept could still be applied. Namely, the ORRtriggered redox transition of the Fe on the core-shell interface of Fe3C@N-C leads to the coordination reconstruction of the core-shell interface of Fe3C@N-C. Consequently, the continuous reconstruction of the coreshell interface may create defects on the interface, potentially detaching the graphite shell from the Fe3C core, and thereby exposing the Fe3C core to the electrolyte. Additionally, during ORR the Fe 3+ /Fe 2+

From the Fe dissolution data during ORR, any destabilizing step during the ORR catalytic cycle could be suspected to be responsible for the Fe demetallation in Fe-N-C. However, more insights may be gained from the surprisingly similar values of the electric chargenormalized Fe dissolution during the O2-AST and Ar-AST (see Figure 2B &S16). This implies that the key destabilizing factor to the Fe dissolution in Fe-N-C in alkaline media could be one that is related to charge transfer events and that also occurs in both O2-AST and Ar-AST. Among the three above mentioned destabilizing factors during ORR, only Fe redox transitions also take place during the Ar-AST. Indeed, charge-discharge processes may result in the Fe dissolution from exposed Fe3C particles. [START_REF] Choi | Minimizing Operando Demetallation of Fe-N-C Electrocatalysts in Acidic Medium[END_REF][START_REF] Ujimine | Electrochemical characteristics of iron carbide as an active material in alkaline batteries[END_REF] Moreover, the correlation between Fe 3+ /Fe 2+ redox transition and the stability of Fe at FeNxCy sites in the absence of O2 could be supported by the findings revealed by Li et al. [START_REF] Li | Identification of durable and nondurable FeNx sites in Fe-N-C materials for proton exchange membrane fuel cells[END_REF] They reported that the Fe active sites that undergo reversible Fe 3+ /Fe 2+ redox transitions due to potential switches between 0.2 and 0.8 VRHE (FeN4C12) are less stable than those where the charge of Fe ions (2+) is constant and independent of the potential switches (FeN4C10). This supports the idea that the N-coordinated Fe could be less stable during its redox transitions even in the absence of O2.

Hence, we propose that the Fe dissolution from FeNxCy sites in alkaline media can be attributed to the instability of the coordinated Fe during the Fe 3+ /Fe 2+ redox transition. As for Fe3C@N-C in alkaline media, the ORR-triggered Fe redox transition destabilizes the core-shell interface and potentially detaches the graphite shell from the Fe3C core, at least locally. If the exposed Fe on the partially covered Fe3C core is still involved in the ORR occurring at its neighboring active sites, the ORR-triggered Fe redox transition would continue destabilizing this exposed Fe and thus cause its dissolution. Or if the exposed Fe is not involved in any ORR, it would be destabilized by its redox transition triggered by potential cycling as in the Ar case. This hypothesis can also explain the different amounts and the varied ORR-activity contributions of the dissolved Fe species in the absence and presence of ORR: On the one hand, the Fe redox transition in an Ar-saturated environment would only be triggered by the change of electrochemical potential, occurring only twice in each AST cycle. Also, the transitioned Fe species do not necessarily have high activity. On the other hand, in the presence of O2, the Fe involved in ORR continuously undergo redox transitions in the first half of each AST cycle. Moreover, the more active the FeNxCy sites are, the more frequent the involved Fe species undergo the redox transitions. Therefore, during the O2-AST, the Fe dissolution from the FeNxCy or Fe3C@N-C that contribute highly to the ORR activity is more significant than that during the Ar-AST.

Conclusions and Outlook

In the present work, Fe demetallation of Fe-N-C catalyst layers in alkaline media is studied at realistic conditions, such as O2 environment, elevated current densities, and a thin AEM on the catalyst layer. We show that Fe dissolution is significantly enhanced in O2 environment compared to Ar. Additionally, Fe dissolution is shown to be directly proportional to the applied charge. We identified that in the Fe3C@N-C species-rich Fe-N-C, Fe is dissolved from exposed Fe3C nanoparticles responding to potential cycling, while the Fe dissolution from Fe3C@N-C that are involved in ORR follows ORR-triggered removal of the protective graphite shell. Moreover, we discovered for both Fe3C@N-C species-rich and FeNxCy species-rich Fe-N-C catalysts, a strong correlation between Fe redox transitions and the Fe dissolution in the presence and absence of ORR in the studied potential range, 1.0 -0.57 VRHE. This leads to our hypothesis that the instability of Fe during the redox transitions could be predominantly responsible for the Fe demetallation in Fe-N-C catalysts in alkaline media. This hypothesis could help rationalizing the different scales and origins of the Fe dissolution in the presence and absence of ORR. For future work, Fe-N-C catalysts with solely Fe3C@N-C species, solely FeNxCy species, and with both, should be evaluated with GDE-ICP-MS in different potential regions. In addition to the applied ex situ techniques in this work, the Fe dissolution data should be accompanied with post mortem Mössbauer spectroscopy (necessitating 57 Fe enriched samples), [START_REF] Chenitz | A specific demetalation of Fe-N4 catalytic sites in the micropores of NC_Ar + NH3 is at the origin of the initial activity loss of the highly active Fe/N/C catalyst used for the reduction of oxygen in PEM fuel cells[END_REF][START_REF] Li | Identification of durable and nondurable FeNx sites in Fe-N-C materials for proton exchange membrane fuel cells[END_REF] in order to rationalize the structurestability relationship of Fe-N-C catalysts in more detail. Based on that, strategies for improving the stability of Fe-N-C catalysts dedicated for AEMFCs need to be developed, since the dominant degradation mechanism of Fe-N-C catalysts might be different in alkaline compared to acidic environment.

Scheme 1 .

 1 Scheme 1. Electrochemical protocols of the GDE-ICP-MS measurements in O2 and Ar environments, respectively. The protocol with O2 consists of an initial activity test, an AST of 200 cycles and a final activity test. The applied current densities are shown on the left hand-side of the upper scheme and the measured steady-state potential responses are shown on the right hand-side. This set of potentials is then applied in the protocol with Ar, shown in the lower scheme.

Figure 1 .

 1 Figure 1. Comparison of Fe-N-C demetallation between Arand O2-saturated alkaline (0.1 M NaOH) environments before the accelerated stress test (AST-200), via gas diffusion electrode setup coupled to inductively coupled plasma mass spectrometry (GDE-ICP-MS). A: Current density profile. B: Potential profile. C-left axis (solid lines): Fe dissolution rate normalized to catalyst loading. C-right axis (dotted lines): Amount of dissolved Fe species at different potentials (VRHE) normalized to catalyst loading.

Figure 2 .

 2 Figure 2. Comparison of the quantitative results for the Fe dissolution in alkaline (0.1 M NaOH) during the AST-200 between Ar-and O2-saturated environments. A: Amount of dissolved Fe species normalized to catalyst loading. B: Amount of dissolved Fe species normalized to the catalyst loading and normalized also to the applied electric charge (O2 environment) or measured electric charge (Ar environment).

Figure 3 .

 3 Figure 3. A: The polarization curves of the Fe-N-C GDEs in the following five conditions. (i) Before any AST, and without the thin AEM (Pristine, black), (ii) before any AST, and with the thin AEM (Pristine-M, grey), (iii) after the Ar-AST-200, and without the thin AEM (Ar-AST-200, blue), (iv) after the O2-AST-200, and without the thin AEM (O2-AST-200, red), and (v) after the O2-AST-200, and with the thin AEM (O2-M-AST-200, yellow). B & C: The relative losses of the power density at 0.65 VRHE (B) and the Fe content (C) over the Ar-AST-200, O2-AST-200, or O2-M-AST-200 procedures. Note that the calculation of the loss of the Fe content is explained in section 1.3 of the SI.

Figure 5 .

 5 Figure 5. A: Correlation of the electric charge-normalized Fe dissolution with the applied potential during ORR in a GDE half-cell. The electric charge-normalized Fe dissolution is the integrated amount of Fe dissolution normalized to the applied charge and catalyst loading. B: Correlation of the Faradaic efficiency of HO2 -with the applied potential during ORR in a rotating ring-disk electrode (RRDE) half-cell, 31 reprinted from Journal of Power Sources, 375, Hossen, M. M., et al., Synthesis and characterization of high performing Fe-NC catalyst for oxygen reduction reaction (ORR) in Alkaline Exchange Membrane Fuel Cells, 214-221, Copyright (2018), with permission from Elsevier. Fe-NMG and Fe-NMP stand for Fe-N-C catalysts synthesized using different N-C precursors, Nicarbazin, Methylimidazole and Glucoril for Fe-NMG, and Nicarbazin, Methylimidazole and Pipemedic Acid for Fe-NMP.[START_REF] Hossen | Synthesis and characterization of high performing Fe-N-C catalyst for oxygen reduction reaction (ORR) in Alkaline Exchange Membrane Fuel Cells[END_REF]

  𝑁 4 𝐹𝑒 2+ ⋯ 𝑂 𝑎 𝐻 𝑏 ⟶ 𝑁 4 𝐹𝑒 3+ ⋯ 𝑂 𝑎 𝐻 𝑏 𝑁 4 𝐹𝑒 3+ ⋯ 𝑂 𝑐 𝐻 𝑑 + 𝑒 -⟶ 𝑁 4 𝐹𝑒 2+ ⋯ 𝑂 𝑐 𝐻 𝑑

	redox transition
	(see Equation (1) and (2) below) at FeNxCy sites and its
	corresponding changes of coordination could result in
	transient instability of the coordinated Fe, and thus
	potentially drive its dissolution.

-

(1)

ACKNOWLEDGMENT YPK acknowledges Jonas Möller for software development for data analysis, Matej Zlatar, Christian Göllner, Dr. Attila Kormanyos, and Dr. Daniel Escalera for technical support during ICP-MS measurements, Yi-Jia Tsai for visualizing the graphic abstract, Funda Arslan and Guangxin Liu for sharing crucial electrochemical equipment. KE acknowledges Heinrich Böll Foundation for financial support. The project CREATE leading to this application has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 721065.

ASSOCIATED CONTENT

The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

A scheme of GDE-ICP-MS setup; experimental details; details of data analysis; SEM images for thickness analysis; supporting dissolution and electrochemical data; FT-EXAFS spectra; STEM micrographs; and Raman spectra.

AUTHOR INFORMATION


Mohammed-Ridha Mahi

Nikolay Nicolov

Louis Coma

Joseph Grondin

Christian Aupetit

Raphaël Méreau
email: raphael.mereau@u-bordeaux.fr

Thierry Tassaing
email: thierry.tassaing@u-bordeaux.fr

What the anion tells us about the structure of tetrabutylammonium salt/phenol-based deep eutectic solvents? A combined infrared spectroscopic and DFT study

Keywords: Deep eutectic solvents, hydrogen bonds, Tetrabutyl ammonium salts, Phenol, Infrared, DFT

In this study, a series of new Deep eutectic solvents (DESs) composed of phenol as the HBD (Hydrogen Bond Donor) and tetrabutyl ammonium (TBA) salts with different molecular anions as HBA (Hydrogen Bond Acceptor) were prepared and investigated using ATR-IR spectroscopy and DFT calculations in order to understand the solvation mechanism at work in these DESs. In particular, the OH stretching mode of phenol is significantly shifted to lower frequencies in the DESs in comparison with that reported for pure phenol. Similarly, characteristic vibrational modes of the anions in the investigated DESs display significant shifts in frequency in comparison with that measure for the pure TBA salt. As a representative example, the frequency shifts observed for the azide anion as a function of the HBA:HBD molar ratio were fully interpreted by the DFT calculations putting in evidence strong hydrogen bonding interactions that occurs between the anion and the phenol molecules. Interestingly, a minimum of the calculated free energy interaction of -25.9 kcal/mol was found at a 1:3 HBA:HBD molar ratio that is consistent with thermodynamic results reported for the Cholinechloride/Phenol DES for which the lowest freezing temperature was reported at this ratio.

1-Introduction

Deep eutectic solvents or DESs are a new class of solvents with promising properties for a large number of applications such as gas separation and sequestration, electrochemistry, catalysis and the extraction of natural compounds for cosmetic or pharmaceutical applications. [START_REF]Deep Eutectic Solvents for Medicine, Gas Solubilization and Extraction of Natural Substances[END_REF][START_REF] Hansen | [END_REF][3][4] These solvents are usually prepared by mixing two cheap and safe components, a hydrogen bond donor (HBD) with a hydrogen bond acceptor (HBA) and characterized by a significant depression in melting points compared to those of the pure constituents. The first example was provided by Abbott et al. [5] as they discovered that mixing choline chloride (ChCl) and urea could produce a mixture with a significantly lower melting point.

Since then, a large number of DESs have been proposed combining different HBD and HBA at various molar ratios where HBD typically involve molecules with OH or NH group such as amide, amine, alcohols, organic acids, whereas HBA usually involves inorganic salts such as choline chloride and quaternary ammonium salts. As reported in many recent publications on DESs, hydrogen bonding is believed to be the key intermolecular force leading to the melting point depression observed during the DESs formation. [6] Nevertheless, anion-cation interactions as well as steric effects could play also a significant role. Therefore, efforts are still needed to identify the solvation mechanism at work at a molecular level that govern the structure-property relationships in this class of solvents.

In this context, Infrared spectroscopy is one of the most powerful tool to investigate hydrogen bond interactions and, although it might be also useful to detect impurities that may be present in a sample, a number of recent research works have primarily used it to identify the key molecular interactions that occurs in DESs in comparison with the pure constituents trying to elucidate the DESs structure. [7][8][9][START_REF] Ali | The journal of physical chemistry[END_REF][11][12] It is interesting to notice that most of the infrared studies reported so far are devoted to the study of the hydrogen bonded structure of the HBD in the DESs and little to no information is obtained on the HBA structure from IR spectroscopy. Indeed, we remember that the majority of DESs that have been prepared and studied so far are traditionally constituted of an HBA consisting of a quaternary ammonium salt comprising Cl - as the anion (although Br -and I -are proposed to a lesser extent). Therefore, it is not possible to get information about the solvation of the anion through the infrared spectroscopy. Thus, we propose to use quaternary ammonium salts comprising molecular anions in order to detect by infrared spectroscopy characteristic vibrational modes of the anion. Thus, we will get information on the structure of DESs from the view point of the anion and how the anion is solvated in the DESs in comparison with the pure ammonium salt. To the best of our knowledge, there are no studies on DESs that propose quaternary ammonium salts with molecular anions.

We are only aware about Choline and Geranate (CAGE) Deep Eutectic Solvent that can be used as an antiseptic agent for preventive and therapeutic applications. [13] It might be due to the fact that atomic anions are necessary to form a DES while ammonium salts including molecular anion do not form DESs with typical HBD. Nevertheless, using molecular anions would allow to detect them by IR spectroscopy and therefore obtain crucial information on the solvation of the anion in the DESs.

On the other hand, Choline Chloride/phenol-based deep eutectic solvents that have been reported about ten years ago by Guo et al. [14] are known to show low viscosity that is of prime importance for industrial applications. For example, it is possible to separate phenols from model oils with quaternary ammonium salts via forming deep eutectic solvents. [15] In particular, it was found that a deep eutectic was formed at about 75 % of phenolic compounds (DES 1:3). Compared with an ideal mixture of the two components, the freezing temperature of the phenol-ChCl (1:3) system depresses greatly from 127 °C to reach a freezing point of -20.1°C [15] and the phenol-TBACl (1:2) displays a freezing point of -1.1°C. [16] In a recent paper by Fan et al. [17], it was found that ammonium salt/phenol based deep eutectic solvents were less viscous than ammonium salt/glycolic acid-based DES as the interaction network stability of the latter is more robust than that of the phenolics-based one due to the strong covalency of hydrogen bond. This is the main reason that ammonium salt/phenol-based DESs show low viscosity. Therefore, phenol appears to be an interesting HBD model molecule in order to investigate the possibility to form new ammonium salt/phenol-based DESs considering various molecular anions. Thus, the aim of this work is to investigate a series of phenol/TBA salts mixture with various anions (Cl -(Chloride), N3 -(Azide), NCO -(Cyanate), SCN -(Thiocyanate), MDA - (Malondialdehyde), OAc -, (Acetate) H2PO4 -(Phosphate monobasic) that could form deep eutectic solvents at room temperature. Through ATR-IR experimental studies and DFT calculations, we demonstrate a peculiar solvation effect of phenol on the anion in these DESs that could be responsible for the eutectic behavior with a significant depression in the melting point of the DES compared to those of the pure components.

2-Experimental

Preparation of deep eutectic solvents

All chemicals purchased from the "Aldrich" company with analytical grade (97-99%) were used as received without any purification (see table S1 in ESI1). As the eutectic mole fraction was found to be 1:3 for the ChCl/Phenol and AmmoniumCl/Phenol DESs as previously reported [14,17], we have prepared all the DESs at this molar ratio. Few DES have been also prepared at other molar ratios (1:1, 1:2 and 1:4). The solid mixture was heated up to 80°C and constantly stirred for 2 hours until the formation of homogeneous transparent liquid. Upon cooling, we have checked that the mixture was still liquid at room temperature (see ESI 2).

ATR-IR spectroscopy

All DESs and their components were investigated by ATR-FTIR spectroscopy using a homemade Si ATR accessory suitable for high temperature (up to 100°C) coupled with a ThermoOptek interferometer (type 6700) equipped with a globar source, a KBr/Ge beam splitter and a DTGS (Deuterated TriGlycine Sulphate) detector. The spectra were recorded in the spectral range 4000-400 cm -1 using a resolution of 4 cm -1 and averaging 50 scans to improve the signal to noise ratio. The stainless steel cell with a volume of about 3 mL screwed above the Si crystal provides one port for the inlet of mixture. A magnetic stirrer was placed into the cell to ensure good homogenization of the mixture. The ATR cell was heated using cartridge heaters disposed in the periphery of its body. A thermocouple was used and located close to a cartridge heater for the temperature regulation with an accuracy of about ±1 °C. The mixture composed of the DESs was disposed in the cell at ambient temperature. Then, the cell was heated up to the desired temperature.

Computational details

The approach used in this study involved first simulated annealing for each of the different scenarios (TBAN3/phenol DESs at various molar ratios: 1:1, 1:2, 1:3, and 1:4) using the AMPAC program [20] at the AM1-D3H4 level of calculation. [18,19] Then, DFT calculations of geometries, energies, and vibrational frequencies reported in this paper were carried out with the M06-2X functional [START_REF] Zhao | [END_REF] using the 6-311G(d,p) basis set implemented in the Gaussian 16 package. [START_REF] Frisch | Gaussian 16 Rev.B[END_REF]. In order to determine the Boltzmann-weighted average IR spectra, the most stable conformations (within 15 kcal.mol -1 ) were examined using DFT (see ESI 4 and 5). To compare the experimental vibrational frequencies with the calculated ones, a scaling factor of 0.946 [23] has been used.

3-Results and discussion

3-1 Preparation and ATR-IR spectra of the phenol/TBA salts

The melting properties of the pure constituents used in this study that are all solids at room temperature are displayed in Table S2 (See ESI 2). Upon preparing the TBA /phenol salts mixture at different molar ratios with the various molecular anions (N3 -(Azide), NCO - (Cyanate), SCN -(Thiocyanate), MDA -(Malondialdehyde), OAc -, (Acetate) H2PO4 -(Phosphate monobasic), we observe that all the investigated mixtures are liquids at room temperature (see table S2). The ATR-IR spectra of the new TBA salt/phenol DESs obtained at a molar ratio 1:3 are reported in figure 1. The spectrum of pure phenol and the TBACl/phenol DES at a molar ratio 1:3 is also reported for comparison. As a general rule, all the spectra of the new DESs investigated in this study display a similar shape than that reported for the TBACl/phenol DES with characteristic peaks that belong to the phenol and the TBA cation (see figure 1). Nevertheless, depending on the DES, we observe some variation of the broad band at about 3300 cm -1 in pure phenol that is associated with its OH stretching mode and characteristic of the hydrogen bond network of phenol. Thus, this profile is always detected at lower wavenumber in the DESs comparing to that of pure phenol.

Therefore, the hydrogen bonding network that phenol molecules adopts in the DESs seems to be stronger than in pure phenol whatever the nature of the TBA salt. In addition, we can observe for the new DESs the characteristic peaks that are associated with the anion. Although the characteristic peaks of the anions MDA -, OAc -and H2PO4 -are difficult to analyze as they are superimposed over the phenol spectrum below 1700 cm -1 , the characteristic bands of the asymmetric stretching mode of the anions of TBAN3, TBASCN, and TBANCO in the DESs spectra can be clearly observed at about 2030, 2050 and 2150 cm -1 , respectively. Thus, the observation by infrared spectroscopy of the asymmetric stretching mode of these anions should allow to obtain information on how the anion is solvated in the DESs in comparison with the pure ammonium salt.

In order to evaluate the sensitivity of the asymmetric stretching mode of the anion to the structure of the DES, TBA salt/phenol mixtures with the NCO -and N3 -anion have been prepared at a molar ratio 1:1, 1:2 and 1:4 that do also display a stable liquid phase at room temperature. The ATR-IR spectra of the TBAN3/Phenol DESs at a molar ratio 1:1, 1:2, 1:3 and 1:4 are reported in figure 2. For comparison, the ATR-IR spectra of pure phenol (liquid state at 50°C) and the pure TBAN3 salt (solid at room temperature) have been also reported. The corresponding TBANCO-Phenol DESs at the same molar ratios are reported in the supplementary information (ESI 3, Figure S1). Thus, upon an increase of the molar ratio from 1:1 to 1:4, the peak associated with the 3 mode of the N3 -anion is progressively shifted to higher wavenumbers as found at about 2040 cm -1 in the DES 1:4 and only at about 1995 cm -1 in the pure TBA salt. The OH band of phenol is more difficult to analyze as it displays a broad profile that is characteristic of the hydrogen bond network of phenol. However, we qualitatively observe that this profile that is barely detected in the 1:1 mixture shifts to higher wavenumbers towards that of pure phenol upon an increase of the molar ratio from 1:2 to 1:4. These observations seem to indicate that the hydrogen bond network of phenol molecules in the presence of the anion in the DESs is stronger than that observed in pure phenol. All these observations could be related to the formation of specific hydrogen bonding interactions between the anion and phenol molecules in these DESs.

In order to evaluate the sensitivity of such hydrogen bonding interactions to temperature effects, we have investigated the DES TBAN3/Phenol at a molar ratio 1:3 at different temperatures from 25°C up to 90°C (see figure 3). As a general trend, we do not observe any Figure 3: ATR-IR spectra of the TBAN3/Phenol DES at a molar ratio 1:3 at different temperature from 25°C up to 90°C. The spectrum of the pure TBAN3 salt (liquid at 90°C) is also reported and has been scaled in intensity for a qualitative comparison with the spectra of the DES at various temperature. significant variation of the spectrum of the DES upon an increase of the temperature from 25 up to 90°C. In particular, the broad band at about 3200 cm -1 associated with the OH stretching mode of phenol do not displays significant evolution. However, we observe a weak shift towards lower wavenumbers of the 3 mode of the N3 -anion in the DES from 2032 cm -1 at 25°C up to 2028 cm -1 at 95°C (see insert of figure 3). This trend put in evidence that only a weak lowering of the anion-phenol hydrogen bond interactions is observed in this DES upon a temperature increase that is even barely detected in the OH stretching region. The shift to lower wavenumbers is consistent with the fact that the 3 mode of the N3 -anion in pure TBAN3 characteristic of the N3 -anion "free" from hydrogen bonds is found at much lower wavenumber at 1992 cm -1 .

3-2 Analysis of the anion solvation by DFT calculations

In order to correlate the observed evolution of the asymmetric stretching mode of the N3 - anion with the hydrogen bonding structure of the TBAN3/Phenol DESs for the different molar ratios 1:1, 1:2, 1:3 and 1:4, we have performed DFT calculations on TBAN3-phenol complexes for different molar ratios 1:1, 1:2, 1:3 and 1:4. For each ratio, several geometries (i.e. different orientations of phenol molecule(s) with respect to TBAN3) were considered and have been used to evaluate their stabilization energies and to generate a Boltzmann-weighted average IR spectra for a given TBAN3:Ph ratio (see ESI 4 for details).

Structural and energetic considerations

The lowest energy structure of the fully optimized geometries of the TBAN3: Phenol species are presented in figure 4, namely, the TBAN3 salt (a), the TBAN3:Ph 1:1 complex (b); the TBAN3:Ph 1:2 complex (c); the TBAN3:Ph 1:3 complex (d) and the TBAN3:Ph 1:4 complex (e). For all the complexes investigated, the donating hydroxyl group of the phenol molecule(s) points towards a nitrogen atom of the N3 -anion that plays the role of the HBA (see figure 4).

The calculated binding enthalpies/Gibbs free energies of the complexes, obtained by difference between the enthalpies/Gibbs free energies of the interacting system (TBAN3:Ph) and the isolated TBAN3 and phenol molecules, are reported in table 1.

Table 1: Interaction enthalpies (ΔH), Gibbs free energies (ΔG), calculated M06-2X/6-311G(d,p) (3 (N3 -) DFT) and experimental (3 (N3 -) Exp) frequencies associated with the asymmetric stretching normal mode for the TBAN3:Ph complexes. It appears that the calculated binding enthalpies vary from -26.7 kcal mol -1 for the TBAN3:Ph 1:1 complex, up to -74.5 kcal mol -1 for TBAN3:Ph 1:4. Such values put in evidence that strong interactions occurs in theses complexes such as H-bonds between phenol and the azide anion as well as CH/ hydrogen bonds between the aliphatic chain of TBA + and the aromatic core of phenol molecule(s). Moreover, increasing the number of phenol molecules around the anion lead to an increase of the stabilization energy of the complex. Such strong hydrogen bonding structures are consistent with the experimental observation of the OH stretching profile of phenol molecules at lower frequencies in the DESs in comparison with pure phenol and is also put in evidence by calculated Gibbs free energies that vary from -13.3 kcal mol -1 for the TBAN3:Ph 1:1 complex, up to -25.1 kcal mol -1 for TBAN3:Ph 1:4. However, we observe that a minimum Gibbs free energy is obtained for the 1:3 complex hence putting in evidence entropic effects that compete with enthalpic effects when the number of phenol molecules increases in the TBAN3/Phenol complex. Therefore, it appears that the most stable structure for these TBAN3/Phenol DESs is expected to be reached for a 1:3 molar ratio. Such result is fully consistent with thermodynamic results reported for the Choline-chloride/Phenol DES for which a 1:3 HBA:HBD molar ratio has been found to display the lowest freezing temperature. [14] Vibrational spectra

In order to correlate the DFT calculations with the experimental infrared spectra, the vibrational analysis of the TBAN3/Phenol complexes has been performed starting from the optimized structures (see ESI 4). In figure 5, the scaled calculated frequencies associated with the asymmetric stretching normal mode of the azide anion in TBAN3 and its complexes with Phenol are compared with the experimental values (Table 1). It appears that whatever the number of phenol molecules, we observe a systematic difference between the calculated and experimental absolute values of about 40 cm -1 . As far as we know that numerical values of scaling factors can vary slightly as a function of the database used to establish it, the value used in this study might not be perfectly appropriate to scale the asymmetric stretching mode of the azide anion.

In addition, as DFT methods use harmonic approximations, discrepancies can arise because of anharmonic and finite temperature effects. Therefore, such uncertainties in the DFT calculations of vibrational frequencies could explain the systematic difference of about 40 cm - 1 observed between the calculated and experimental frequencies. However, the observed shift with the number of phenol molecules to higher frequencies of the asymmetric stretching mode 3 (N3 -) in the complexes is well reproduced by the calculations. Interestingly, the experimentally observed non-linear evolution of the shift as a function of the number of phenol molecules is particularly well reproduced by the calculations. Therefore, we can postulate that the calculated equilibrium geometry reported for the TBAN3/Phenol complexes should correspond to the most probable hydrogen bonding structure that TBAN3 and phenol molecules adopts in these DESs.

4-Conclusion

A series of new DESs were obtained by mixing Phenol with TBA salts with various anions (Cl - , N3 -, NCO -, SCN -, MDA -(Malondialdehyde), OAc -, H2PO4 -). The analysis of the ATR-IR spectra of these DESs reveals that the OH stretching mode of phenol is significantly shifted to lower frequencies in the DESs in comparison with that reported for pure phenol. The characteristic peaks that are associated with anions were also detected. In particular, the characteristic band of the asymmetric stretching mode of the N3 -anion in the TBAN3/Phenol DES was found to display significant shifts upon a variation of the temperature and the TBAN3/Phenol molar ratio from 1:1 to 1:4. The frequency shifts observed for the azide anion as a function of the HBA:HBD molar ratio were well reproduced by the DFT calculations putting in evidence strong hydrogen bonding interactions that occurs between the anion and the phenol molecules. A minimum of the calculated free energy interaction of -25.9 kcal/mol was obtained at a 1:3 molar ratio that is consistent with the lowest melting point depression observed experimentally at this ratio for the Choline chloride-Phenol DES [14]. This peculiar hydrogen bonded structure where phenol molecules are strongly hydrogen bonded to the azide anion of the TBAN3 salt is expected to occur in the common TBA salts with atomic anion (Cl -, Br -, I -).

Such structure rearrangement of phenol molecules with TBA salts is believed to be the main reason for the formation of a liquid phase at room temperature resulting from a significant depression in the freezing temperature the DES compared to those of the pure components.

Figure 1 :

 1 Figure 1: ATR-IR spectra of the TBA salt/phenol DESs at a HBA-HBD ratio of 1:3 investigated in this study. The spectrum of pure phenol (liquid at 50°C) is reported for comparison.

Figure 2 :

 2 Figure 2: ATR-IR spectra of the TBAN3/Phenol DES with various HBA-HBD ratios 1:1, 1:2, 1:3 and 1:4. The spectrum of pure phenol (liquid at 50°C) and the pure TBAN3 salt (solid at room temperature)

Figure 4 :

 4 Figure 4: Optimized structures of the TBAN3/Phenol complexes at M06-2X/6-311G(d,p) level of calculation. TBAN3 (a), TBAN3:Ph 1:1 complex (b); TBAN3:Ph 1:2 complex (c); TBAN3:Ph 1:3 complex (d); TBAN3:Ph 1:4 complex (e). Hydrogen bonds are displayed as dashed lines with the intermolecular distances in angstrom.

  with a factor of 0.946 (from[23])

Figure 5 :

 5 Figure 5: Comparison of the experimental and scaled calculated frequencies at the M06-2X/6-311G(d,p) computational level of the C-I stretching normal mode of "free" C4F9I and [C4F9I-LB] complexes. Ascaling factor of 0.946 was used to scale the calculated frequencies.[23]

ACKNOWLEDGMENTS

The authors acknowledge the Region Nouvelle Aquitaine and the program "Défi in situ en conditions extrêmes" of the MITI of CNRS for the financial support to the infrared and Raman equipment and to the GDR 2035 SolvATE for fruitful discussions. Computer time was provided by the Pôle Modélisation HPC facilities of the Institut des Sciences Moléculaires UMR 5255 CNRS -Université de Bordeaux, co-funded by the Nouvelle Aquitaine region.