Adnane Bouzina

Hubert Perrot

Catherine Debiemme-Chouvy
email: catherine.debiemme-chouvy@sorbonne-universite.fr

Ozlem Sel
email: ozlem.sel@college-de-france.fr

Interface Properties of 2D Graphene-Polydopamine Composite Electrodes in Protic Ionic Liquid-Based Electrolytes Explored by Advanced Electrogravimetry

Keywords: Reduced graphene oxide, polydopamine, ionic liquids, electrode/electrolyte interface, EQCM, ac-electrogravimetry

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Introduction

Continuous demand in performance enhancement of energy storage devices (e.g., batteries, supercapacitors) has been stimulating the quest of novel electrode materials, as well as new electrolytes. [1][2] For supercapacitor (SC) applications, various electrode materials (such as carbon materials, 3 transition metals and conducting polymers 4 ) and different class of electrolytes (ranging from aqueous, 5 organic, [START_REF] Kovalska | Organic electrolytes for graphene-based supercapacitor: Liquid, gel or solid[END_REF] and redox electrolytes [START_REF] Zhang | A Review of Redox Electrolytes for Supercapacitors[END_REF] ) have been widely studied. As an alternative to the relatively classical solvents, aprotic and protic ionic liquids (AILs and PILs) have also received a colossal interest as promising green electrolytes for SCs. 8- 9 Considering the diverse research efforts for finding novel electrode and electrolyte candidates, characterization of the processes occurring at the electrode/electrolyte interface (EEI) has become more important than ever. [START_REF] Hui | Advanced Electrochemical Analysis for Energy Storage Interfaces[END_REF][START_REF] Yu | Electrode-electrolyte interfaces in lithium-based batteries[END_REF][START_REF] Atkins | Understanding Battery Interfaces by Combined Characterization and Simulation Approaches: Challenges and Perspectives[END_REF][START_REF] Jeong | Mussel-Inspired Coating and Adhesion for Rechargeable Batteries: A Review[END_REF] Indeed, the ions and solvent molecules' behavior at the EEI plays an essential role and the electrolyte composition affect the overall energy storage performance. The challenge of monitoring the complex interface processes has stimulated the use of various analytical tools in combination with electrochemical methods, [START_REF] Hui | Advanced Electrochemical Analysis for Energy Storage Interfaces[END_REF][START_REF] Grey | Sustainability and in situ monitoring in battery development[END_REF][START_REF] Yang | In situ analyses for ion storage materials[END_REF] providing in situ structural, compositional and morphological information.

Among the techniques for interface characterization, electrochemical quartz crystal microbalance (EQCM) has started to attract a great deal of attention in the energy storage domain. The different modes (EQCM-R for motional resistance or multiharmonic EQCM-D for dissipation monitoring) permitted the method not only to characterize the gravimetric changes but also to study the morphological and viscoelastic/mechanical evolution of the electrode materials in response to an electrochemical solicitation. [START_REF] Levi | Application of a quartz-crystal microbalance to measure ionic fluxes in microporous carbons for energy storage[END_REF][START_REF] Ye | Charge Storage Mechanisms of Single-Layer Graphene in Ionic Liquid[END_REF][START_REF] Lemaire | Making Advanced Electrogravimetry as an Affordable Analytical Tool for Battery Interface Characterization[END_REF] The EQCM based methods have significantly contributed to the investigation of a variety of phenomena, inter alia electro-adsorption/desorption, [START_REF] Levi | Application of a quartz-crystal microbalance to measure ionic fluxes in microporous carbons for energy storage[END_REF][START_REF] Ye | Charge Storage Mechanisms of Single-Layer Graphene in Ionic Liquid[END_REF][START_REF] Goubaa | Dynamic Resolution of Ion Transfer in Electrochemically Reduced Graphene Oxides Revealed by Electrogravimetric Impedance[END_REF] insertion/disinsertion of ions within electrode materials, [START_REF] Lemaire | Making Advanced Electrogravimetry as an Affordable Analytical Tool for Battery Interface Characterization[END_REF] and solid-electrolyte interphase (SEI) formation/evolution [START_REF] Liu | In situ quantification of interphasial chemistry in Li-ion battery[END_REF][START_REF] Dargel | In situ real-time gravimetric and viscoelastic probing of surface films formation on lithium batteries electrodes[END_REF] can be listed. In the context of capacitive charge storage, EQCM has successfully been employed to study the electro-adsorption/desorption of ions on the carbon surface, which can be described as follows:

On negative charged surface of carbon (Q < 0) ,

Carbon

Cations Carbon Cations However, in the case of a multi-ions contribution to the interface processes, characterizing the role of respective species (cation, anion or solvent molecules present in the electrolyte) becomes rather difficult. [START_REF] Griffin | In situ NMR and electrochemical quartz crystal microbalance techniques reveal the structure of the electrical double layer in supercapacitors[END_REF][START_REF] Kwon | Characterization of the SEI on a Carbon Film Electrode by Combined EQCM and Spectroscopic Ellipsometry[END_REF][START_REF] Tsai | Electrochemical Quartz Crystal Microbalance (EQCM) Study of Ion Dynamics in Nanoporous Carbons[END_REF] To deconvolute the global electrogravimetric response into distinct contributions, the ac-electrogravimetry, a methodology which combines fast quartz crystal microbalance with electrochemical impedance spectroscopy (EIS) has been proposed as a complementary tool to interface characterization. [START_REF] Gabrielli | Ac-Electrogravimetry Study of Electroactive Thin Films. I. Application to Prussian Blue[END_REF][START_REF] Arias | New Insights into Pseudocapacitive Charge-Storage Mechanisms in Li-Birnessite Type MnO2 Monitored by Fast Quartz Crystal Microbalance Methods[END_REF] This advanced mode envelops the benefits of QCM and EIS in a single analytical tool, keeping in mind that it is not a basic coupling. Indeed, in addition to the classical impedance transfer function (TF), it allows a second transfer function to be obtained, the so-called electrogravimetric transfer function. [START_REF] Gabrielli | Ac-Electrogravimetry Study of Electroactive Thin Films. I. Application to Prussian Blue[END_REF][START_REF] Arias | New Insights into Pseudocapacitive Charge-Storage Mechanisms in Li-Birnessite Type MnO2 Monitored by Fast Quartz Crystal Microbalance Methods[END_REF] The main information that can be extracted from the ac-electrogavimetry includes: (i) the identification of involved species by their molar mass with clearly distinguishing between anions and cations, solvated ions and free solvent contributions, (ii) their kinetics of transfer at the EEI with their contribution in the charge balance, and (iii) the relative concentration changes of these species within the electrode material.

Due to the high relevance of the classical EQCM and ac-electrogravimetry for interface characterization, herein they were exploited to address the interface behavior of our previously optimized 2D graphene-polydopamine (ERGO-PDA) composite electrodes. [START_REF] Bouzina | Preventing Graphene from Restacking via Bioinspired Chemical Inserts: Toward a Superior 2D Micro-supercapacitor Electrode[END_REF] Regarding the electrolytes, we were particularly interested in protic ionic liquids (PILs) electrolytes (presence of labile hydrogens) over aprotic ionic liquids (AILs) (absence of labile hydrogens). This choice is motivated by the fact that PILs have the advantage of being inexpensive and easier to produce by a simple proton transfer between a Brønsted acid and a Brønsted base, compared to the AIL counterparts. [START_REF] Burrell | Preparation of protic ionic liquids with minimal water content and 15N NMR study of proton transfer[END_REF][START_REF] Brandt | An investigation about the cycling stability of supercapacitors containing protic ionic liquids as electrolyte components[END_REF] They are normally less viscous and can exhibit relatively higher conductivities than that of AILs, [START_REF] Brandt | An investigation about the cycling stability of supercapacitors containing protic ionic liquids as electrolyte components[END_REF][START_REF] Timperman | Phosphonium-based protic ionic liquid as electrolyte for carbon-based supercapacitors[END_REF] but present still lower conductivity compared to conventional organic electrolytes (such as acetonitrile [START_REF] Burrell | Preparation of protic ionic liquids with minimal water content and 15N NMR study of proton transfer[END_REF][START_REF] Brandt | An investigation about the cycling stability of supercapacitors containing protic ionic liquids as electrolyte components[END_REF] ). To further increase the ionic conductivity of the PILs, they have been used either as mixtures with organic solvents [START_REF] Brandt | An investigation about the cycling stability of supercapacitors containing protic ionic liquids as electrolyte components[END_REF] and more recently as PIL-water binary mixtures. [START_REF] Al-Zohbi | Impact of the aqueous pyrrolidinium hydrogen sulfate electrolyte formulation on transport properties and electrochemical performances for polyaniline-based supercapacitor[END_REF] However, understanding the interfacial behavior of PILs as an electrolyte component especially in binary mixtures still requires further investigation, which we address in the present study. As a promising PIL electrolyte candidate, pyrrolidinium hydrogen sulfate

([Pyr + ][HSO4 -]
) and its binary mixture with water has been chosen. [START_REF] Al-Zohbi | Impact of the aqueous pyrrolidinium hydrogen sulfate electrolyte formulation on transport properties and electrochemical performances for polyaniline-based supercapacitor[END_REF][START_REF] Anouti | Synthesis and Characterization of New Pyrrolidinium Based Protic Ionic Liquids. Good and Superionic Liquids[END_REF] The interfacial charge storage behavior of ERGO and ERGO-PDA is explored in the mentioned PIL based electrolyte, in the absence and presence of Na2SO4 and compared with that occur in pristine 0.5 M Na2SO4 (pH=2). Our advanced electrogravimetric study permitted us to highlight the peculiar behavior of chaotropic [Pyr + ] cations hindering the water molecules approach to the EEI and the possible implications of this behavior in charge storage performance of the 2D graphene-polydopamine composite electrodes.

Experimental

Preparation of the electrodes

The ERGO and ERGO-PDA electrodes were synthesized according to the procedure reported in our previous work. [START_REF] Bouzina | Preventing Graphene from Restacking via Bioinspired Chemical Inserts: Toward a Superior 2D Micro-supercapacitor Electrode[END_REF] Briefly, about 6 μL of the GO suspension was drop-casted on the gold electrode (0.2 cm 2 ) of a quartz resonator (9 MHz, AWSensors, Spain). The ERGO film was obtained after the electro-reduction of the GO under the chronoamperometry mode at -1 V vs saturated calomel electrode (SCE) for 10 min in a solution of 0.2 M PBS (pH 7.2) containing 5 mM of DA (1 mg•mL -1 ). In a consecutive step, the polydopamine (PDA) was electrodeposited by chronoamperometry at 0.8 V vs SCE for 5 min. Typical loadings (thicknesses) of the electrodes, ERGO and ERGO-PDA are 5.3 μg (180 nm), and 6.7 μg (200 nm), respectively.

The loadings were estimated by QCM measurements of the quartz resonators before and after the electrode preparation. The thickness of ERGO or ERGO-PDA films were determined by field emission gun scanning electron microscopy (FEG-SEM) observation of the sample cross section (Figures S1a-b). The densities of the electrodes (surface area = 0.2 cm 2 ) were estimated to be 1.47 g•cm -3 and 1.67 g•cm -3 for ERGO and ERGO-PDA, respectively. The presence of N (characteristic of PDA) was detected for the ERGO-PDA by energy dispersive x-ray (EDX) analysis (Figure S1d).

Synthesis of the selected protic ionic liquid (PIL)

Pyrrolidinium hydrogen sulfate [Pyrr + ][HSO4 -] was synthesized as described by Anouti et al. [START_REF] Anouti | Transport properties in two pyrrolidinium-based protic ionic liquids as determined by conductivity, viscosity and NMR self-diffusion measurements[END_REF] The pyrrolidine (Sigma-Aldrich, 99%) (0.4 mol) was added into a two-necked round-bottomed flask which was placed in an ice bath and equipped with a reflux condenser and a dropping funnel, the latter serving for the addition of sulfuric acid (96% in water). An equimolar quantity of the sulfuric acid (0.4 mol) was added dropwise in the pyrrolidine containing flask under vigorous stirring for 2 h. The resulting product is a yellow liquid and viscous at room temperature, ascribed as [Pyrr + ][HSO4 -] (PIL).

Morphological and compositional characterizations

The cross-section of the ERGO and ERGO-PDA films was observed under a field emission gun scanning electron microscope (FEG-SEM) (Ultra55, Zeiss) operating at 5 kV. The energy dispersive spectroscopy (EDX) analysis was performed using a JEOL 2010 microscope (JEOL Ltd, Tokyo, Japan) to examine the composition of the ERGO and ERGO-PDA films (Figure S1c-d). The FEG-SEM micrographs of the cross-section and the EDX spectra of ERGO and ERGO-PDA are given in the Supporting Information file, Part I (Figures S1a-b).

Electrogravimetric characterizations

The electrogravimetric characterizations were performed in a standard three-electrode electrochemical configuration, by using ERGO or ERGO-PDA-loaded quartz resonator as the working electrodes, SCE as the reference electrode, and a platinum grid as the counter electrode. EQCM and ac-electrogravimetry measurements were carried out in three different electrolyte compositions: 0. ). [START_REF] Bizet | Immunodetection by quartz crystal microbalance. A new approach for direct detection of rabbit IgG and peroxidase[END_REF] To verify the gravimetric regime of the microbalance, the motional resistance (Rm) values and the resonant frequency of the quartz with and without loading in the air and in the electrolytes are measured with an Agilent network analyzer (Agilent 4294A impedance analyzer) (details are given in the Supporting Information file, Part II). The resonant frequency is obtained from the maximum of the real part of the electroacoustic admittance and the typical Butterworth-Van Dyke (BVD) equivalent circuit is used to fit this admittance to extract the values of the Rm. [START_REF] Alassi | Quartz Crystal Microbalance Electronic Interfacing Systems: A Review[END_REF] Then, the Rm values were converted into W (full width at half maximum of the resonance peak) to verify that the W ≪ f condition is fulfilled.

For ac-electrogravimetry measurements, [START_REF] Gabrielli | Ac-Electrogravimetry Study of Electroactive Thin Films. I. Application to Prussian Blue[END_REF] a four-channel frequency response analyzer (FRA, Solartron 1254) and a lab-built potentiostat were used. The modified working electrode, on the QCM surface, was polarized at selected potentials and a sinusoidal small amplitude potential perturbation (50 mV) was applied. The frequency range of the measurements was between 63 kHz and 10 mHz. The mass change, Δm, of the working electrode was measured simultaneously with the ac response, ΔI, of the electrochemical system, which allowed the mass/potential transfer function (TF) (∆m/∆E(ω)) to be measured along with the classical electrochemical impedance (ΔE/ΔI(ω)) at a given potential and frequency modulation, f (pulsation, ω = 2πf).

The electrochemical TF, (ΔE/ΔI(ω)), is presented in the form of charge/potential TF, ∆q/∆E(ω), which can decouple the charged species contributions as semi-circles, providing that their timeconstants are distinct from one another. Details of the ac-electrogravimetry theory and data fitting procedure are given in the previous references [START_REF] Goubaa | Dynamic Resolution of Ion Transfer in Electrochemically Reduced Graphene Oxides Revealed by Electrogravimetric Impedance[END_REF][START_REF] Gao | Tuning Charge Storage Properties of Supercapacitive Electrodes Evidenced by In Situ Gravimetric and Viscoelastic Explorations[END_REF] and in the Supporting Information File, Part III.

Results and Discussion

The interface properties of the ERGO and ERGO-PDA composite electrodes were studied in [Pyr + ][HSO4 -]-water binary mixture in the absence/presence of Na2SO4 and compared with the behavior of electrodes in the baseline electrolyte, 0.5 M Na2SO4. In order to have a rational correlation between different media, the aqueous 0.5M Na2SO4 electrolyte pH is adjusted to 2 by H2SO4, since the pH of [Pyr + ][HSO4 -]-water binary mixture is 1.7 and 2, in the absence and presence of Na2SO4, respectively. The processes occurring at the interface during electrochemical cycling are studied by employing EQCM and ac-electrogravimetry tools. The EQCM responses of the ERGO and ERGO-PDA electrodes at a scan rate of 50 mV•s -1 in Na2SO4 at pH = 2 are shown in Figure 1. CV curves show a larger integral area for ERGO-PDA compared to ERGO (Figure 1a), indicating a higher electroactive surface area for the charge storage process of the composite electrode, keeping in mind that the electrodes have similar loadings (5.6 µg and 6.5 µg, respectively). Simultaneous microbalance frequency changes obtained during electrochemical cycling are converted into mass changes, justified by the electroacoustic impedance studies (Tables S1 andS2). In these equations, Ki represents the kinetics of interfacial transfer, Gi (inverse of the transfer resistance, Rti) describes the ease or difficulty of interfacial transfer for each species at the electrode/electrolyte interface, where ω is the pulsation, df is the average film thickness, F is the Faraday constant, and Mi is the molar mass of the involved species in charge compensation process. The charge/potential transfer functions, of both electrodes exhibit a relatively suppressed loop (Figure 2a), which can be related to the contribution of more than one charged species with a similar time constant. It should be noted that the loop diameter is larger for the ERGO-PDA composite film compared to the pristine ERGO, which indicates a better capacitive behavior of the composite electrode, in agreement with the increased integral area of the CV curves in Figure 1.

The data analysis involves testing of several configurations to fit the experimental curves. We consider several charged and non-charged species (also their hydrated counterparts) in Na2SO4 (pH=2) electrolyte, while keeping in mind that a good agreement between the experimental and the theoretical TFs in terms of shape and frequencies was required. Multi-species contribution is totally consistent with former results obtained for carbon based electrodes tested under similar conditions (in NaCl, KCl and LiCl). [START_REF] Goubaa | Dynamic Resolution of Ion Transfer in Electrochemically Reduced Graphene Oxides Revealed by Electrogravimetric Impedance[END_REF][START_REF] Gao | Tuning Charge Storage Properties of Supercapacitive Electrodes Evidenced by In Situ Gravimetric and Viscoelastic Explorations[END_REF] However, the repartition of each species is different depending on the electrodes' nature (Figure 2c and2d). It is noted that a significant free water contribution is observed in the potential range from -0.4 to 0.5 V vs SCE, with an enhancement in its quantity for the ERGO-PDA electrode (Figure 2d). The relative concentration increase of the hydrated sodium cation is more significant and its transfer kinetic is almost twice faster in ERGO-PDA than in ERGO (Figure 2e and2f). This is consistent with the higher d-spacing between the graphene sheets and the better rate capability of the ERGO-PDA electrode compared with the ERGO, that was reported in our previous work. [START_REF] Bouzina | Preventing Graphene from Restacking via Bioinspired Chemical Inserts: Toward a Superior 2D Micro-supercapacitor Electrode[END_REF] Multi-species presence in the charge-compensation and significant amount of water contribution prompted us to change the classical electrolyte composition to [Pyr + ][HSO4 -]water binary mixture in the absence and presence of Na2SO4. It is noted that this binary mixtures are of interest for enlarging the operating potential window when used as electrolytes in supercapacitor devices. decrease with increasing the water content, this phenomenon can be explained by the decrease of the charge carriers concentration due to dilution effect. [START_REF] Al-Zohbi | Impact of the aqueous pyrrolidinium hydrogen sulfate electrolyte formulation on transport properties and electrochemical performances for polyaniline-based supercapacitor[END_REF] It is noted that the obtained ionic conductivity values of the [Pyr + ][HSO4 -]-water binary mixture are in agreement with the reported values. [START_REF] Al-Zohbi | Impact of the aqueous pyrrolidinium hydrogen sulfate electrolyte formulation on transport properties and electrochemical performances for polyaniline-based supercapacitor[END_REF][START_REF] Anouti | Synthesis and Characterization of New Pyrrolidinium Based Protic Ionic Liquids. Good and Superionic Liquids[END_REF] In addition to ionic conductivity, we also measured the motional resistance In these two protic ionic liquid-based electrolytes, we can clearly see that both electrodes show a clear pseudo-capacitive contribution to the global capacitive response, as revealed by the broad reduction-oxidation peaks. Such a phenomenon was previously observed for functionalized activated carbon electrodes tested in similar protic ionic liquids, [START_REF] Mysyk | Pseudocapacitance of nanoporous carbons in pyrrolidinium-based protic ionic liquids[END_REF][START_REF] Hurt | Chemistry and Physics of Carbon[END_REF][START_REF] Montes-Morán | On the nature of basic sites on carbon surfaces: an overview[END_REF][START_REF] Jurewicz | Towards the mechanism of electrochemical hydrogen storage in nanostructured carbon materials[END_REF] and it was attributed to the (i) electrochemical reactions originated from the oxygenated groups of carbon surface (such as quinone/hydroquinone pairs or pyrone-like structures) which can accept two protons and two electrons and (ii) to the reversible hydrogen electrosorption. The ERGO-PDA composite shows more pronounced faradaic contributions (bumps at around 0.7 V vs SCE on CV, Figure 3b and S5b) than the pristine ERGO, which can be explained by the higher amount of catechol type groups for the ERGO-PDA due to the presence of the polydopamine.

Electrogravimetric behavior of ERGO and ERGO-PDA in [Pyr

Turning to the electrogravimetric behavior, based on the mass variation vs potential occuring during CV in PIL/water (Figure 3c and3d) and PIL/water + 0.5 M Na2SO4 (Figure S5c and S5d), we can say that the anions dominate the charge compensation process for both electrodes.

Contrary to the ERGO-PDA composite, the pristine ERGO's mass response shows quite significant dependence on the scan rate in PIL/water binary mixture (Figure 3c). Its global mass varies from 0.33 µg.cm -2 at 10 mV•s -1 to 0.036 µg.cm -2 at 200 mV•s -1 (recorded at 0.9V) which suggests a slow transfer kinetics of anionic species in this electrode (Figure 3c). The acelectrogravimetric results in the PIL/water and PIL/water + 0.5 M Na2SO4 are presented in Figure 4 (at 0 V vs SCE). Charge/potential TF, , presents loops with a larger diameter in the case of the ERGO-PDA composite, evidencing the positive effect of the PDA on the global capacitive response of the electrode in this complex electrolyte (Figures 4a and4c). Through the fitting of experimental TFs using theoretical TFs mentioned in Eq.1 and Eq.2, only ionic species seem to participate in the charge balance. Figures 4b and4d show the electrogravimetric TF, , in the PIL/water in the absence and presence of Na2SO4, respectively. In both media, the ionic species are identified, demonstrating an interfacial kinetics of transfer from high to low as Pyr + , H + and SO4 2-, i.e., Pyr + species appear at high frequencies, H + at intermediate frequencies, and SO4 2-anions were detected at low frequencies.

The other possible configurations for the charge compensation process (e.g. HSO4 -or H3O + ) have been discarded due to the unsatisfactory fit of the charge/potential and mass/potential TFs (Figure S6 and S7). The absence of free water molecules in the two PIL-based electrolytes and/or hydrated ions can be explained by the chaotropic behavior of Pyr + cations, which permits them to remain effectively dehydrated, and acts as a barrier that prevents the intervention of water molecules, either in the hydration shell or as free H2O molecules, in the confined space between the graphene sheets. Even after adding 0.5 M Na2SO4 to the binary mixtrure PIL/water, Pyr + inhibits the highly kosmotropic Na + ions and water molecules approach to the interface, 45- 46 suppressing the contribution of water molecules (Figures 4d), substantially observed in the pristine aqueous electrolyte (Figure 2). Further analyses of the Ki and Gi parameters for the three ionic species in PIL/water and PIL/water + 0.5 M Na2SO4 (Tables 1 and2) provide insights about the dominant species in the charge compensation mechanism and the role of H + and SO4 2-. The ratio % Gi/Ki which relates the contribution of each species in the charge balance, was also calculated using Eq. 3 and Eq. S6-S9, at two different potentials (0 and 0.5V vs SCE). It is important to mention the major role of H + in the charge balance, compared to Pyr + and SO4 2-(Tables 1 and2), but its gravimetric contribution to global mass response is insignificant compared to that of the anions, SO4 2-, because of low molar mass of H + . It should be underlined that only the ac-electrogravimetric tool permits this kind of calculations.

The transfer kinetics of the ionic species detected in PIL/water electrolyte are higher in the ERGO-PDA composite electrode than that occurs in pristine ERGO (Table 1 and2). For instance, the anions, SO4 2-, are two times slower in ERGO, which can explain the observed scan rate dependency of the EQCM mass profiles of the pristine ERGO (Figure 3c). When 0.5 M Na2SO4 is present in PIL/water binary electrolyte, a significant improvement of transfer kinetics of the SO4 2-was observed (Table 2). This is probably due to the increase of the concentration of the anions in the solution by adding 0.5 M Na2SO4.

Correlation of the classical EQCM and ac-electrogravimetric analyses:

In order to validate our hypothesis implying a multi-species contribution in the charge balance, a methodology taking advantage of the complementarity between the EQCM and acelectrogravimetry is used. The applied approach uses the charge fractions and the flux direction of each species estimated from ac-electrogravimetry to obtain a total mac-QCM (Figure 5).

Equations S6-S9 permitted the charge fractions to be estimated using the -Gi/Ki values, which characterizes the concentration variation of each species due to a potential variation (∆C/∆E(ω)) (Equation 3) (Table 1, Through incorporation of Na2SO4 in PIL/water binary mixtures, we could clearly highlight the impact of the cations' nature on the interface behavior. Although Pyr + cations' contribution to charge compensation is minor (~5% in Table 1-2) compared to H + and SO4 2-, it has a significant impact in hindering the participation of Na + species and water molecules (either in the solvation shell or as free H2O molecules) into the charge compensation process.

In the next section, the electrochemical charge storage of ERGO-PDA composite is tested in PIL/water and PIL/water + 0.5M Na2SO4 solutions to investigate any possible consequence of such a hindrance on performance and will be compared to that in aqueous 0.5M Na2SO4 pH=2 electrolyte. .

Electrochemical charge storage performance of ERGO-PDA electrode:

Equation 6

In above equations, is the gravimetric capacitance, v is the potential scan rate, V1 and V2 are the low and high end potentials, respectively, I(V) is the response in current, m is the mass loading of the electrode, is the volumetric capacitance, is the density of the electrode (1.67 g•cm -3 ) and E is the energy density.

The volumetric capacitance values are quite significant in all three different electrolytes studied, highlighting the suitable 2D structure of ERGO-PDA electrodes for charge storage. Only a slight enhancement in capacitance values was observed in the PIL based electrolytes compared to the pristine electrolyte (325 F•cm -3 in PIL-water (40/60 wt%) versus 311 F•cm -3 in 0.5 M Na2SO4 (pH=2)) and the rate performance of the composite electrode is quasi-similar in the three electrolytes (Figure 6a). The dependence of the capacitance values on the scan rate can be explained by the gradual decrease of pseudo-capacitive contribution from redox reactions with the increase of the scan rate. Furthermore, the repeated CV cycles at a high scan rate (1000 mV•s -1 ) in 0.5 M Na2SO4 (pH=2) and in PIL/water (60/40 wt%) reveal the excellent cycling stability of the ERGO-PDA electrode for 10000 cycles in two electrolytes (Figure 6b). The cycling stability of the ERGO-PDA electrode was also evaluated by the continuous galvanostatic charge-discharge (GCD) (Figure 6c) and the voltage floating (aging) (Figure S11) methods in PIL/water (60/40) + 0.5 M Na2SO4. As illustrated in Figure 6c, after 10000 continuous cycles at current density of 1 mA•cm -2 , 97% of capacitance retention is achieved.

Figure S11 shows the potential-holding curve for 30 h, which shows no significant changes in the GCD curves throughout the aging time, signifying that the ERGO-PDA electrode exhibits excellent stability and no degradation.

We could notice a significant difference when the volumetric energy density of the ERGO-PDA electrode is calculated (Equation 6). Figure 6d shows that ERGO-PDA has a significantly high volumetric energy density in PIL based electrolytes. In the PIL-water binary mixture, the composite electrode can deliver an energy density of 16.3 at a scan rate of 10 mV.s -1 , which is almost twice higher than what the same electrode can achieve in the pristine aqueous electrolyte (8.7

.

) at the same scan rate. Even at a high cycling rate

(1 V•s -1 ), ERGO-PDA electrode can still deliver an energy density of 6.1

.

, which is comparable or higher than most of the recent reports on EDLC electrodes tested in aqueous electrolyte evaluated under similar discharge rates. [START_REF] Bouzina | Preventing Graphene from Restacking via Bioinspired Chemical Inserts: Toward a Superior 2D Micro-supercapacitor Electrode[END_REF][START_REF] Li | Achieving Ultrahigh Volumetric Energy Storage by Compressing Nitrogen and Sulfur Dual-Doped Carbon Nanocages via Capillarity[END_REF][START_REF] Tao | Towards ultrahigh volumetric capacitance: graphene derived highly dense but porous carbons for supercapacitors[END_REF] According to the equation used for energy density calculation (Equation 6), this significant improvement of the energy density can be explained by the larger potential window in the [Pyr + ][HSO4 -]-water binary mixture (60/40 wt%): 1.2 V versus only 0.9 V in 0.5 M Na2SO4 (pH=2), which is more evidently seen in the CV curves presented in Figure 1, 3 and S5. This behavior is fully in line with the electrogravimetric results which reveal that the water molecules are not implied in the interfacial electrochemical processes when PIL is present in the electrolyte.

Conclusions

The processes occurring at the vicinity of the electrode/electrolyte interface are intimately correlated to the electrochemical performance and highly impacted by the choice of the electrolyte composition. Herein, a thorough analysis of interfacial behavior of 2D graphenepolydopamine (ERGO-PDA) composite electrodes was performed in protic ionic liquid In the order to verify the gravimetric regime of our films in air and the electrolyte, the registered motional resistance was converted to total resonance width, (where /2 = is the halfwidth at half-height of the resonance peak). Contrary to the motional resistance, the has the

Eq. S1

where A is the piezoelectrically active area (0. The ac-electrogravimetry methodology and theoretical background were previously discussed by Gabrielli et al. [3][4] Briefly, ac-electrogravimetry consists of coupling electrochemical impedance spectroscopy (EIS) with a fast quartz crystal microbalance (QCM) used in ac-mode.

It allows the mass change, Δm, of the working electrode to be measured simultaneously with the AC response, ΔI, of the electrochemical system. This leads to Table S3. Gi /Ki values obtained from the fitting of the ac-electrogravimetric data measured in 0.5 M Na2SO4 (pH=2) at different potentials for ERGO electrode.

  Potential of zero charge)On positive charged surface of carbon (Q > 0) Potential of zero charge)

3. 1 .Figure 1 .

 11 Figure 1. EQCM results in (a) CV response and (b) mass change vs applied potential of ERGO and ERGO-PDA at a scan rate of 50 mVs -1 in 0.5 M Na2SO4 (pH=2). The current responses are normalized by the electrode loadings (panel a) whereas the m is normalized by the gravimetric surface area of the electrode (panel b).

19 Figure 2 . 1 ∑

 1921 Figure 2. Experimental and theoretical ac-electrogravimetric data of ERGO and ERGO-PDA at 0 V vs SCE, (a) ∆q/∆E(ω) and (b) ∆m/∆E(ω) TFs. Evolution of the relative concentration change, Ci-C0, in (c) ERGO and in (d) ERGO-PDA of the charged and non-charged species over the applied potential range, estimated from the ac-electrogravimetric data fitting. Kinetic constants of interfacial transfer of the hydrated cations (Na + • 4H2O) (Kc1) (e) and its relative concentration change, Cc1-C0 in ERGO and ERGO-PDA thin films (f). All the measurements are performed in 0.5 M Na2SO4 (pH=2).

3 Then,

 3 Figure 2b depicts anexample of a mass/potential TF at 0 V vs SCE where three charged species participate to the electroadsorption process, i.e., Na + • 4H2O, H3O + , SO4 2-, also revealing an indirect contribution of free H2O to interfacial process. The interfacial transfer kinetics (Ki) of these four species obtained from the fitting process are presented as a function potential in FigureS3. Hydrated sodium species are exchanged at high frequencies, the protons, H3O + , participate at intermediate frequencies and SO4 2-at low frequencies, corresponding to the loop located in the fourth quadrant, as seen in Figure2b. The kinetic constants of the free solvent molecules are found to be close to that of Na + • 4H2O which suggests that they are electrodragged together with hydrated Na + .Another important outcome of the ac-electrogravimetry is the relative concentration changes of the active species of, Ci -C0 as a function of the potential. This quantity is estimated from the concentration potential transfer function at low frequencies as follows:→ Equation Figure2cand 2d present Ci-C0 of individual species identified from acelectrogravimetry in the charge compensation process of ERGO and ERGO-PDA, respectively.

  + ][HSO 4 -]-water binary mixture: PIL, pyrrolidinium hydrogen sulfate [Pyr + ][HSO4 -] is a viscous liquid. For this reason, the optimum composition of [Pyr + ][HSO4 -]-water binary mixture is determined based on the ionic conductivity and the viscosity. Figure S4 shows the variation of the ionic conductivity of [Pyr + ][HSO4 -]-water binary mixture as a function of water mass ratio (wt%) at room temperature. From this figure, it is clear that the ionic conductivity of the mixture increases progressively as the water content becomes higher. After reaching a maximum value of 184.5 mScm -1 for a [Pyr + ][HSO4 -]-water weight ratio of 40/60, the ionic conductivity starts to

(Figure 3 .

 3 Figure 3. (a-b) CV and (c-d) mass change vs applied potential of ERGO and ERGO-PDA at different potential scan rates in PIL/water (40/60 wt%).

Figure 3

 3 Figure3shows the EQCM responses of the ERGO and ERGO-PDA in the PIL/water (40/60 wt%) electrolyte. FigureS5shows the equivalent measurements in PIL/water + 0.5 M Na2SO4.

Figure 4 .

 4 Figure 4. Experimental and theoretical ac-electrogravimetric data of ERGO and ERGO-PDA at 0 V vs SCE, (a) ∆q/∆E(ω) and (b) ∆m/∆E(ω) in [Pyr + ][HSO4 -]-water binary mixture (60/40 wt%) and (c) ∆q/∆E(ω) and (d) ∆m/∆E(ω) in [Pyr + ][HSO4 -]-water binary mixture (60/40 wt%) in presence of 0.5 M Na2SO4.

2 ,Figure 5 .

 25 Figure 5. Comparisons of total mass variation estimated from ac-electrogravimetry (mac-QCM) and from classical EQCM (mEQCM) for ERGO and ERGO-PDA electrodes in (a, b) 0.5 M Na2SO4 (pH=2), (c,d) PIL/water (60/40 wt%) and (e, f) PIL/water (60/40 wt%) + 0.5 M Na2SO4.

4 Figure 6 . 6

 466 Figure 6. (a) Volumetric capacitance vs. potential scan rate in 0.5 M Na2SO4 (pH=2), PIL/water (60/40) and PIL/water (60/40 wt%) + 0.5 M Na2SO4, (b) capacitance retention of ERGO-PDA in 0.5 M Na2SO4 (pH=2) and PIL/water (60/40 wt%) + 0.5 M Na2SO4 at a scan rate of 1 V•s -1 , (c) capacitance retention of ERGO-PDA in PIL/water (60/40 wt%) + 0.5 M Na2SO4 at a current density of 1 mA•cm -2 and (d) the energy density of ERGO-PDA as a function of the scan rate, compared in three different electrolytes.

[ 3 . 8 . 7 .Figure S1 .

 387S1 Figure S1. FEG-SEM micrographs of the cross-section of (a) ERGO and (b) ERGO-PDA. EDX analysis of (c) ERGO and (d) ERGO-PDA.

  same dimension as the frequency (Hz) and can therefore be linked to the variation of the resonant frequency. The condition for gravimetric regime is described as| | ≪ | / |, withis the overtone which equals to 1 in the present case. The following equation can be used to determine the coefficient joining R1 and : 1-2 32 μ

∆∆Figure S2 .Figure S3 .

 S2S3 Figure S2. (a-b) CV and (c-d) mass change vs applied potential of ERGO and ERGO-PDA at different potential scan rates in 0.5 M Na2SO4 (pH=2).

Figure S6 .Figure S7 .Figure S8 .

 S6S7S8 Figure S6. Experimental and theoretical curves for ac-electrogravimetry analyses of ERGO-PDA at 0 V vs SCE. (a) ∆q/∆E(ω), (b) ∆m/∆E(ω) in [Pyr + ][HSO4 -]-water binary mixture (60/40 wt%) in presence of 0.5 M Na2SO4.

Figure S9 .Figure S10 .

 S9S10 Figure S9. The theoretical mass variation of the (a) ERGO and (b) ERGO-PDA electrodes, separating the global mass variation into several species contribution (calculated using Eq.S11) in PIL/water (40/60 wt%).

Table 1 .

 1 Ki (kinetics of transfer) and Gi (inverse of the transfer resistance), values obtained from the fitting of the ac-electrogravimetric data measured in "[Pyr + ][HSO4

	% G i /K i

-]-water binary mixture (60/40 wt%)" at 0 and 0.5 V vs. SCE for ERGO and ERGO-PDA electrodes. Electrode 0V vs SCE ([Pyr + ][HSO 4 -]-water binary mixture (60/40 wt%)) K i (10 -6 cm•s -1 )

G i (10 -8 mol•s -1 •cm -2 •V -1 )

vs SCE ([Pyr + ][HSO 4 -]-water binary mixture (60/40 wt%)) K i (10 -6 cm•s -1 ) G i (10 -8 mol•s -1 •cm -2 •V -1 ) % G i /K i

		Pyr +	H +	SO 4 2-	Pyr +	H +	SO 4 2-	Pyr +	H +	SO 4 2-
	ERGO	226	72	4	0.7	6	-0.2	2	64	33
	ERGO-PDA	377	134	7	3	16	-0.4	4	63	33

Table 2 .

 2 Ki (kinetics of transfer) and Gi (inverse of the transfer resistance), values obtained

	from the fitting of the ac-electrogravimetric data measured in "[Pyr + ][HSO4 -]-water binary
	mixture (60/40 wt%) in presence of 0.5 M Na2SO4" at 0 and 0.5 V vs. SCE for ERGO and
	ERGO-PDA electrodes.

Electrode 0 V vs SCE ([Pyr + ][HSO 4 -]-water binary mixture (60/40 wt%) in presence of 0.5 M

Na 2 SO 4 ) K i (10 -6 cm•s -1 )

					G i (10 -8 mol•s -1 •cm -2 •V -1 )		% G i /K i
		Pyr +	H +	SO 4 2-	Pyr +	H +	SO 4 2-	Pyr +	H +	SO 4 2-
	ERGO	594	205	31	2	12	-0.6	4	70	26
	ERGO-PDA	952	368	38	4	25	-1	4	64	32
			0.5V vs SCE ([Pyr + ][HSO 4 -]-water binary mixture (60/40 wt%)
				in presence of 0.5 M Na 2 SO 4 )
		K i (10 -6 cm•s -1 )	G i (10 -8 mol•s -1 •cm -2 •V -1 )		% G i /K i
		Pyr +	H +	SO 4 2-	Pyr +	H +	SO 4 2-	Pyr +	H +	SO 4 2-
	ERGO	197	112	20	1	9	-0.5	5	70	25
	ERGO-PDA	408	196	31	4	19	-1	6	68	26

Table S1 .

 S1 The resonant frequency and the motional resistance of the blank quartz. ERGO and ERGO-PDA electrodes at different states.

	Resonant

frequency (Hz) Motional resistance (R 1 ) (Ω) Blank quartz in air 9 000 535 6 Blank quartz in Na 2 SO 4 (pH=2) 8 997 578 395 ERGO in air 8 994 020 7 ERGO-PDA in air 8 994 943 8 ERGO in Na 2 SO 4 (pH=2) 8 989 272 449 ERGO-PDA in Na 2 SO 4 (pH=2) 8 988 830 486

calculated during the film generation Electrode coating Air Na 2 SO 4 (pH=2)

				2 10 -4 •m²), 26 is the piezoelectric stress
	coefficient (9.65 10 -2 A•s •m -2 for AT-cut quartz),	is the density of the quartz (2.648 10 3
	kg•m -3 ), 0 is the resonant frequency (9 000 520 Hz), and is the shear modulus (2.947 10 10
	kg•m -1 •s -2 ).
	The value of 13.82 Hz•Ω -1 was found for the W/R1 ratio and used to obtain Table S2, which
	compares the bare and loaded quartz resonators in air and in electrolyte. The ratios |	|/| |
	of the ERGO and ERGO-PDA electrodes measured in air and in 0.5 M Na2SO4 (pH=2) are
	presented in Table S2. A ratio |	| /| |	1 ensures the gravimetric regime of the coating.
	Table S2: |	|/| | of the ERGO and ERGO-PDA electrodes measured in air and in 0.5 M
	Na2SO4 (pH=2).
		| |/| | ERGO 0.0001	0.09
		ERGO-PDA	0.0036	0.12

Part III. Ac-electrogravimetry theoretical background

Table S4 .

 S4 Gi/Ki values obtained from the fitting of the ac-electrogravimetric data measured in 0.5 M Na2SO4 (pH=2) at different potentials for ERGO-PDA electrode.

	(V/SCE)								2-
	0.5	2	3	3	-10	11	17	17	55
	0.3	2	6	7	-8	9	26	30	35
	0	1	6	3	-9	5	32	16	47
	-0.2	1	6	12	-7	4	23	46	27
	-0.4	1	5	11	-9	4	19	42	35
	Potential G C1 /K C1	G C2 /K C2	G H2O /K H2O	G SO4 2-/K SO4 2-	%	%	%	%
	(V/SCE)	(*10 4 )	(*10 4 )	(*10 4 )	(*10 4 )	G C1 /K C1	G C2 /K C2	G H2O /K H2O	G SO4 2-/K SO4 2-
	0.5	7	11	11	-11	18	27	28	27
	0.4	6	11	6	-7	20	37	20	23
	0.3	4	6	9	-6	16	25	36	24
	0.2	3	10	17	-7	8	27	46	19
	0	2	8	5	-6	9	38	24	29
	-0.1	2	8	18	-8	6	22	50	22
	-0.3	2	8	23	-8	5	20	56	18
	-0.4	2	7	28	-9	4	16	61	19

Acknowledgements

The authors thank "Doctoral school, "Chimie Physique et Chimie Analytique de Paris-Centre" ED388, French Ministry of Education Scholarship".

Notes

The authors declare no competing financial interest.

Corresponding Authors *E-mail: ozlem.sel@college-de-france.fr *E-mail: catherine.debiemme-chouvy@sorbonne-universite.fr

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