Pedro F Dos Santos

Laurent Andre

Marion Ducousso

François Contamine

Pierre Cézac

Experimental Measurements of CO 2 Solubility in Aqueous MgCl 2 Solution at Temperature between 323.15 and 423.15 K and Pressure up to 20 MPa

Experimental CO2 solubility data in brine at high temperatures and pressures are important for CO2 capture and storage (CCS) applications. Many data has been acquired in single-salt aqueous solutions such as NaCl, CaCl2 and Na2SO4. CO2 solubility data in aqueous solutions containing MgCl2 are extremely scarce. In this work, new experimental data of CO2 solubility in MgCl2 solutions were acquired. Thirty-three experimental points of CO2 solubility have been reported in an original range of pressure (1.5 to 20 MPa), temperature (323.15, 373.15 and 423.15 K) and salinity (1, 3 and 5 mol/kg). Measurements were made using the potentiometric titration method. Results show that the CO2 solubility increases with the evolution of pressure and decreases with the evolution of MgCl2 concentrations. A discussion about the salting-out effect on MgCl2 solutions is made, and a comparison is proposed on the same effect in aqueous systems containing calcium chloride.

INTRODUCTION

CO2 capture and storage (CCS) in deep geological formations is an alternative for reducing the concentration of CO2 in the atmosphere and combating the effects of global warming [START_REF] Eiken | Lessons learned from 14 years of CCS operations: Sleipner, In Salah and Snøhvit[END_REF][START_REF] Torp | Demonstrating Storage of CO2 in Geological ReservoirsThe Sleipner and Sacs Projects[END_REF][START_REF] Farret | Captage et stockage géologique du CO2 : retour d'expérience et perspectives[END_REF] . Knowledge about the solubility of carbon dioxide in aqueous electrolyte solutions is extremely important for CCS [START_REF] Chabab | Thermodynamic study of the CO2 -H2O -NaCl system: Measurements of CO2 solubility and modeling of phase equilibria using Soreide and Whitson, electrolyte CPA and SIT models[END_REF][START_REF] Zhao | Carbon dioxide solubility in aqueous solutions of sodium chloride at geological conditions: Experimental results at 323.15, 373.15, and 423.15 K and 150 bar and modeling up to 573.15 K and 2000 bar[END_REF][START_REF] Shi | An improved model for CO2 solubility in aqueous electrolyte solution containing Na + , K + , Mg 2+ , Ca 2+ , Cland SO4 2under conditions of CO2 capture and sequestration[END_REF][START_REF] Tignor | CLIMATE CHANGE 2013 The physical science basis summary for policymakers, technical summary and frequently asked questions[END_REF] . The deep aquifers suitable for CO2 storage are in the temperature range between 313-423 K, with salinity ranging from 35,000 mg/L to more than 300,000 mg/L, containing mainly Cl -and Na + , with variable amounts of K + , Mg 2+ and Ca 2+8,9 .

Geochemical models could predict CO2 solubility according to the temperature, pressure and composition of the aquifers [START_REF] Shi | An improved model for CO2 solubility in aqueous electrolyte solution containing Na + , K + , Mg 2+ , Ca 2+ , Cland SO4 2under conditions of CO2 capture and sequestration[END_REF][START_REF] Duan | An improved model for the calculation of CO2 solubility in aqueous solutions containing Na + , K + , Ca 2+ , Mg 2+ , Cl -, and SO4 2[END_REF][START_REF] Duan | An improved model calculating CO2 solubility in pure water and aqueous NaCl solutions from 273 to 533 K and from 0 to 2000 bar[END_REF][START_REF] Springer | A thermodynamic model for predicting mineral reactivity in supercritical carbon dioxide: I. Phase behavior of carbon dioxide-water-chloride salt systems across the H2O-rich to the CO2-rich regions[END_REF] . However, experimental solubility points are required to reinforce their databases.

CO2 solubility in single-salt aqueous solutions such as NaCl, CaCl2 and Na2SO4 under CCS conditions is well documented [START_REF] Messabeb | Experimental Measurement of CO2 Solubility in Aqueous NaCl Solution at Temperature from 323[END_REF][START_REF] Messabeb | Experimental Measurement of CO2 Solubility in Aqueous CaCl2 Solution at Temperature from 323[END_REF][START_REF] Poulain | Experimental Measurements of Carbon Dioxide Solubility in Na-Ca-K-Cl Solutions at High Temperatures and Pressures up to 20 MPa[END_REF][START_REF] Poulain | Experimental Measurement of CO2 Solubility in a 1 mol/kgw CaCl2 Solution at Temperature from 323[END_REF][START_REF] Cruz | Experimental Study of Carbon Dioxide Solubility in Sodium Chloride and Calcium Chloride Brines at 333[END_REF][START_REF] Dos Santos | Experimental Measurement of CO2 Solubility in Aqueous Na2SO4 Solution at Temperatures between 303[END_REF] . The CO2-H2O-NaCl is the most investigated system, more than 1500 solubility points for pressure up 140 MPa, temperature comprises between 273.15-723.15 K and a molality up to 6 mol/kg have been published [START_REF] Chabab | Thermodynamic study of the CO2 -H2O -NaCl system: Measurements of CO2 solubility and modeling of phase equilibria using Soreide and Whitson, electrolyte CPA and SIT models[END_REF][START_REF] Messabeb | Experimental Measurement of CO2 Solubility in Aqueous NaCl Solution at Temperature from 323[END_REF][START_REF] Cruz | Experimental Study of Carbon Dioxide Solubility in Sodium Chloride and Calcium Chloride Brines at 333[END_REF][START_REF] Takenouchi | The binary system H2O-CO2 at high temperatures and pressures[END_REF][START_REF] Takenouchi | The solubility of carbon dioxide in nacl solutions at high temperatures and pressures[END_REF][START_REF] Rumpf | Solubility of carbon dioxide in aqueous solutions of sodium chloride: Experimental results and correlation[END_REF][START_REF] Yan | Measurement and modeling of CO2 solubility in NaCl brine and CO2-saturated NaCl brine density[END_REF][START_REF] Zhao | PHASE EQUILIBRIA IN CO2-BRINE SYSTEM FOR CO2STORAGE[END_REF][START_REF] Guo | Quantitative Raman Spectroscopic Measurements of CO2 Solubility in NaCl Solution from (273.15 to 473[END_REF][START_REF] Wang | Determination of CO2 Solubility in Water and NaCl Solutions under Geological Sequestration Conditions Using a Fused Silica Capillary Cell with in Situ Raman Spectroscopy[END_REF] . As far as the CO2-H2O-Na2SO4 system is concerned, to our knowledge 450 solubility points are available in the literature for pressure up to 20 MPa, a temperature range of 288.15-423.15 K and a molality 0.1-2.7 mol/kg [START_REF] Dos Santos | Experimental Measurement of CO2 Solubility in Aqueous Na2SO4 Solution at Temperatures between 303[END_REF][START_REF] Kobe | Confining liquids for gas analysis: Solubility of carbon dioxide in salt solutions[END_REF][START_REF] Gerecke | Ein Beitrag zur Gaslöslichkeit in Elektrolytlösungen: Untersucht am Beisp[END_REF][START_REF] Rumpf | An Experimental and Theoretical Investigation on the Solubility of Carbon Dioxide in Aqueous Solutions of Strong Electrolytes[END_REF][START_REF] Bermejo | The influence of Na2SO4 on the CO2 solubility in water at high pressure[END_REF][START_REF] Yasunishi | Solubility of Carbon Dioxide in Aqueous Mixed-Salt Solutions[END_REF][START_REF] Zhao | Measurement and Modeling of CO2 Solubility in Natural and Synthetic Formation Brines for CO2 Sequestration[END_REF][START_REF] Gilbert | CO2 solubility in aqueous solutions containing Na + , Ca 2+ , Cl -, SO4 2and HCO3 -: The effects of electrostricted water and ion hydration thermodynamics[END_REF] . The CO2-H2O-CaCl2 system has been investigated as well: more than 450 points were collected. On the other hand, the literature regarding the CO2-H2O-MgCl2 is practically scarce. Experimental studies in MgCl2 solutions at high temperature, pressure and salinity are reduced to Zhao et al [START_REF] Zhao | Experimental Studies and Modeling of CO2 Solubility in High Temperature Aqueous CaCl2, MgCl2, Na2SO4 and KCl Solutions[END_REF] and Tong et al [START_REF] Tong | Solubility of CO2 in Aqueous Solutions of CaCl2 or MgCl2 and in a Synthetic Formation Brine at Temperatures up to 423 K and Pressures up to 40 MPa[END_REF] . However some formation brines of Na-Ca-Cl type such as Weyburn (Li et al [START_REF] Li | Densities and Solubilities for Binary Systems of Carbon Dioxide + Water and Carbon Dioxide + Brine at 59 °C and Pressures to 29 MPa[END_REF] and Poulain et al [START_REF] Poulain | Experimental Measurements of Carbon Dioxide Solubility in Na-Ca-K-Cl Solutions at High Temperatures and Pressures up to 20 MPa[END_REF] or Mt Simon (Zhao et al [START_REF] Zhao | Measurement and Modeling of CO2 Solubility in Natural and Synthetic Formation Brines for CO2 Sequestration[END_REF] and Poulain et al [START_REF] Poulain | Experimental Measurements of Carbon Dioxide Solubility in Na-Ca-K-Cl Solutions at High Temperatures and Pressures up to 20 MPa[END_REF] ) contain magnesium (up to 0.049 mol/kg) as well. The acquisition of CO2 solubility data in magnesium chloride solutions close to saturation, despite the fact that natural formations reported in the literature have concentrations between 0.05 and 0.26 mol/kg [START_REF] Jacob | CO2 solubility in multi-component brines containing NaCl, KCl, CaCl2 and MgCl2 at 297 K and 1-14 MPa[END_REF][START_REF] Bachu | Interfacial Tension between CO2, Freshwater, and Brine in the Range of Pressure from (2 to 27) MPa, Temperature from (20 to 125) °C, and Water Salinity from (0 to 334 000) mg•L -1[END_REF][START_REF] Zhao | Measurement and Modeling of CO2 Solubility in Natural and Synthetic Formation Brines for CO2 Sequestration[END_REF][START_REF] Portier | Modelling CO2 solubility in pure water and NaCl-type waters from 0 to 300 °C and from 1 to 300 bar[END_REF] , are important in order to improve the database for geochemical models. Indeed, the optimisation of the numerical codes on a wide concentration range (up to salt saturation) increases their robustness and limits convergence issues when ionic strength of the solutions augments.

Moreover, the extension of the concentration range allows the use of the codes for other application cases like industrial waters having high Mg contents. This highlights the need to collect CO2 solubility points for this system.

This study aims at acquire experimental data on CO2 solubility in aqueous MgCl2 solutions over a wide temperature range [323. 15-423.15 K], pressures up to 20 MPa and in three different salt concentrations [1, 3 and 5 mol/kg]. Solubility is measured using the potentiometric titration method as used by dos Santos et al [START_REF] Dos Santos | Experimental Measurement of CO2 Solubility in Aqueous Na2SO4 Solution at Temperatures between 303[END_REF] . A discussion of the salting-out effect is also proposed, comparing to the systems containing CaCl2 studied by Messabeb et al [START_REF] Messabeb | Experimental Measurement of CO2 Solubility in Aqueous CaCl2 Solution at Temperature from 323[END_REF] .

LITERATURE REVIEW

The solubility of carbon dioxide in single-salts systems has been widely studied over the years. However, with regard to the CO2-H2O-MgCl2 system, experimental data are scarce. Six studies are reported in Table 1. Regarding the studies at high pressure, this number drops by half. Only three studies reported CO2 solubility data at pressures other than atmospheric conditions; and at temperatures from 373.15 K, only two studies.

MATERIALS AND METHODS

Chemicals products.

The names of the compounds, their respective CAS registration numbers, as well as the source and purity of the chemicals used in our work are summarized in Table 2. The supplier states all purities values.

Apparatus Description and Operating Procedure

The acquisition of new CO2 solubility data is carried out on an experimental laboratory pilot, consisting mainly of an autoclave with a volume of 1 L thermostatically controlled in Hastelloy C276 (the experimental pilot scheme is shown in Figure 1). It can operate at temperatures between 293. 15 to ensure that all the degassed CO2 is captured and still dissolved in the liquid phase. Then, the amount of CO2 dissolved in the aqueous solution is determined by acid titration. Details on the CO2 solubility measuring technique and the reactor description can be found in dos Santos et al. [START_REF] Dos Santos | Experimental Measurement of CO2 Solubility in Aqueous Na2SO4 Solution at Temperatures between 303[END_REF] The experimental CO2 solubility results at a pressure range between 1.5-20 MPa and temperatures from 323.15 to 423.15 K in aqueous MgCl2 solutions at the ionic strength up to 15 mol/kg are listed in Table 3. (8) thermocouples; and (9) thermostatic baths (from dos Santos et al. [START_REF] Dos Santos | Experimental Measurement of CO2 Solubility in Aqueous Na2SO4 Solution at Temperatures between 303[END_REF] ).

Analysis

During the sampling process, a NaOH solution is used to retain the CO2 released from the sample, as well as the amount of gas still dissolved in the liquid phase, thus converting CO2 and HCO 3-into CO3 2-ions according to reactions (1) and (2). Then, the determination of the quantity of carbon dioxide contained into the syringe is performed by titration using a hydrochloric acid solution (HCl) thanks to an automatic titration unit from SI Analytics (TitroLine 7800).

CO 2(g) + 2 OH - (aq) ↔ CO 3 2- (aq) + H 2 O (l) (1)
HCO 3 - (aq) + OH - (aq) ↔ CO 3 2- (aq) + H 2 O (l) (2)
Because of the high pH, part of the dissolved magnesium in contact with the hydroxide ions reacts to form magnesium hydroxide, according to the equation (3). In aqueous systems containing MgCl2-NaOH, Mažuranić et al [START_REF] Mazuranic | Reaction Products in the System MgCl2-NaOH-H2O[END_REF] showed that Mg 2+ species is by far the predominant species in equilibrium with the solid phase Mg(OH)2, thus forming small amounts of solid in the solution. Despite the little amount of solid, the initial NaOH concentration is calibrated in order to the excess sodium hydroxide must be sufficient to ensure reactions (1), ( 2) and (3):

𝑀𝑔𝐶𝑙 2(𝑎𝑞) + 2 𝑁𝑎𝑂𝐻 (𝑎𝑞) → 𝑀𝑔(𝑂𝐻) 2(𝑠) + 2 𝑁𝑎𝐶𝑙 (𝑎𝑞) (3)
From the titration curve, three equivalence points can be detected, showed in the Figure 2 adapted from the previous paper (dos Santos et al. [START_REF] Dos Santos | Experimental Measurement of CO2 Solubility in Aqueous Na2SO4 Solution at Temperatures between 303[END_REF] ). The second equivalence point (V2) corresponds to the neutralization of CO3 2-into HCO3 -, according to reaction (5). Finally, the V3 equivalence point corresponds to the transformation of HCO3 -into H2CO3 (or CO2(aq)). (5)

OH - (aq

) + H + (aq) ↔ H 2 O (l) (4)
HCO 3 - (aq) + H + (aq) ↔ H 2 CO 3 (aq) (6)
Then, the amount of moles of CO2 can be determined and checked according to the following expression:

η CO 2 = η CO 3 2-= η HCO 3 -= C HCl (V 3 -V 2 ) 1000 (7)
The results reported in this work are the average of five successive titrations performed under each set of equilibrium conditions. Four validation criteria are used to validate the solubility points. For further information, one car refers to a previous publication (dos Santos et al. [START_REF] Dos Santos | Experimental Measurement of CO2 Solubility in Aqueous Na2SO4 Solution at Temperatures between 303[END_REF] ).

Experimental Uncertainty

The experimental uncertainty of the present work was evaluated by the analysis of variances (ANOVA) that consists of a statistical method based on repeatability and reproducibility experiments. The principle of the ANOVA methodology was applied and discussed in a previous paper (dos Santos et al [START_REF] Dos Santos | Experimental Measurement of CO2 Solubility in Aqueous Na2SO4 Solution at Temperatures between 303[END_REF] ). The repeatability and reproducibility experiments were performed at 373.15 K, 10 MPa and at 1 mol of MgCl2 per kilogram of water.

The uncertainty was obtained from the analysis of 3 different experiments (evaluation of reproducibility) performed under the same conditions with 5 titrations for each experiment (evaluation of repeatability). The average experimental uncertainty obtained from the ANOVA calculations is equal to 1.76%. The coverage factor is derived from the Student's distribution for two degrees of freedom and a 95% confidence level.

EXPERIMENTAL RESULTS

Validation of experimental protocol with the CO2-H2O-MgCl2 system

As shown previously, the amount of points in the literature of CO2 solubility in aqueous MgCl2 solutions is scarce, using only four analysis methods (Table 1). Since the potentiometric titration method is used for the first time for this chemical system, it must be validated. Thus, the CO2 solubility in aqueous MgCl2 solution at 1 mol/kg and at 373.15 K in the pressure range between 1.5 and 20 MPa was studied to validate the apparatus and experimental procedure.

These conditions are selected because our results can be compared with data available in the literature [START_REF] Zhao | Experimental Studies and Modeling of CO2 Solubility in High Temperature Aqueous CaCl2, MgCl2, Na2SO4 and KCl Solutions[END_REF][START_REF] Tong | Solubility of CO2 in Aqueous Solutions of CaCl2 or MgCl2 and in a Synthetic Formation Brine at Temperatures up to 423 K and Pressures up to 40 MPa[END_REF] (Figure 3). Figure 3 shows a good agreement between our results and the data in the literature. We obtained consistent results with Tong et al 34 at 10 MPa (2 % of difference) and Zhao et al 33 at 15 MPa (3.03 % of difference). In particular, it means that the potential reaction of Mg 2+ with OH -(Equation 3) does not influence the acid titration.

In this way, the analytical method is therefore validated for the carbon dioxide solubility results for aqueous magnesium chloride systems.

at a pressure range between 1.5 MPa and 20 MPa. All experimental data are reported in Table 3. studied with a difference of less than 1 % from Zhao et al [START_REF] Zhao | Experimental Studies and Modeling of CO2 Solubility in High Temperature Aqueous CaCl2, MgCl2, Na2SO4 and KCl Solutions[END_REF] . Figure 4 shows that the solubility of CO2 increases with increasing pressure, and shows the opposite behavior when increasing the concentration, the solubility decreases. K up to 20 MPa: this study and previous studies [START_REF] Zhao | Experimental Studies and Modeling of CO2 Solubility in High Temperature Aqueous CaCl2, MgCl2, Na2SO4 and KCl Solutions[END_REF] .

Figure 5 shows the isotherms obtained at 373.15 K à 1 and 3 mol/kg. The results at 1 mol/kg were previously discussed for validation of the experimental protocol. At 3 mol/kg, the solubility points are consistent with those presented by Tong et al [START_REF] Tong | Solubility of CO2 in Aqueous Solutions of CaCl2 or MgCl2 and in a Synthetic Formation Brine at Temperatures up to 423 K and Pressures up to 40 MPa[END_REF] . Comparing CO2 solubility points of this study to those of Tong et al [START_REF] Tong | Solubility of CO2 in Aqueous Solutions of CaCl2 or MgCl2 and in a Synthetic Formation Brine at Temperatures up to 423 K and Pressures up to 40 MPa[END_REF] at 5 MPa and 15 MPa, the differences are of 2.2 % and 6.7 %, respectively. up to 20 MPa: this study and previous studies [START_REF] Zhao | Experimental Studies and Modeling of CO2 Solubility in High Temperature Aqueous CaCl2, MgCl2, Na2SO4 and KCl Solutions[END_REF][START_REF] Tong | Solubility of CO2 in Aqueous Solutions of CaCl2 or MgCl2 and in a Synthetic Formation Brine at Temperatures up to 423 K and Pressures up to 40 MPa[END_REF] .

At 423.15 K, as seen for 373.15 K, the set of data obtained in this study is in agreement with the experimental data available in the literature. At 1 mol/kg and 15 MPa, a slight deviation of 0.2 % is observed between the solubility point of this study and the one of Zhao et al [START_REF] Zhao | Experimental Studies and Modeling of CO2 Solubility in High Temperature Aqueous CaCl2, MgCl2, Na2SO4 and KCl Solutions[END_REF] . At 20 MPa and 1 mol/kg, this deviation is of 2.17 % between the solubility points of this study and the one of Tong et al [START_REF] Tong | Solubility of CO2 in Aqueous Solutions of CaCl2 or MgCl2 and in a Synthetic Formation Brine at Temperatures up to 423 K and Pressures up to 40 MPa[END_REF] . At 3 mol/kg, although there are no directly comparable data, there is a good agreement with the data published by Tong et al [START_REF] Tong | Solubility of CO2 in Aqueous Solutions of CaCl2 or MgCl2 and in a Synthetic Formation Brine at Temperatures up to 423 K and Pressures up to 40 MPa[END_REF] . up to 20 MPa: this study and previous studies [START_REF] Zhao | Experimental Studies and Modeling of CO2 Solubility in High Temperature Aqueous CaCl2, MgCl2, Na2SO4 and KCl Solutions[END_REF][START_REF] Tong | Solubility of CO2 in Aqueous Solutions of CaCl2 or MgCl2 and in a Synthetic Formation Brine at Temperatures up to 423 K and Pressures up to 40 MPa[END_REF] .

DISCUSSIONS

Salting-out effect in the CO2-H2O-MgCl2 system

Several studies on brines bringing discussions about the salting-out effect have been published. Koschel et al [START_REF] Koschel | Enthalpy and solubility data of CO2 in water and NaCl(aq) at conditions of interest for geological sequestration[END_REF] and Mohammadian et al [START_REF] Mohammadian | Measurement of CO2 Solubility in NaCl Brine Solutions at Different Temperatures and Pressures Using the Potentiometric Titration Method[END_REF] defined the salting-out effect as the calculation of the relative difference between the solubility of CO2 in saline solutions and the solubility of CO2 in pure water under the same conditions of temperature and pressure. The use of this definition, represented in the following equation, was used and discussed by Messabeb et al [START_REF] Messabeb | Experimental Measurement of CO2 Solubility in Aqueous CaCl2 Solution at Temperature from 323[END_REF] , Lara Cruz et al [START_REF] Cruz | Experimental Study of Carbon Dioxide Solubility in Sodium Chloride and Calcium Chloride Brines at 333[END_REF] and dos Santos et al [START_REF] Dos Santos | Experimental Measurement of CO2 Solubility in Aqueous Na2SO4 Solution at Temperatures between 303[END_REF] as well for NaCl, CaCl2 and Na2SO4 systems.

SO % = 100 × (m 0 -m) m 0 (8)
Where m 0 is the carbon dioxide solubility in water and m is the CO2 solubility in the brine at the same conditions of temperature and pressure. m 0 and m are expressed in terms of molality.

Table 4 shows the average values calculated for the salting-out effect and the standard uncertainties for the salting-out effect for the CO2-H2O-MgCl2 system. The standard uncertainties values are calculated by propagating CO2 solubility uncertainty in pure water and in the solution containing magnesium chloride. The calculation procedure is described in the paper of dos Santos et al. [START_REF] Dos Santos | Experimental Measurement of CO2 Solubility in Aqueous Na2SO4 Solution at Temperatures between 303[END_REF] and is discussed by Lara Cruz et al [START_REF] Cruz | Experimental Study of Carbon Dioxide Solubility in Sodium Chloride and Calcium Chloride Brines at 333[END_REF] . Figure 7 shows the salting-out effect calculated for the three salinities (1, 3 and 5mol/kg) in the CO2-H2O-MgCl2 system. Table 4 and Figure 7 highlight that temperature and pressure have very little influence on the salting-out effect. On the other hand, the molality of salt has a strong influence on the solubility of carbon dioxide, consequently on the SO% results. At 1 mol MgCl2/kg, the salting-out effect varies between 33 and 37 %, while at 3 mol/kg the values increase, reaching between 62 and 66 %. Near MgCl2 saturation, at 5 mol/kg, only one isotherm was generated (at 323.15 K), results show that the salting-out effect is 76.6 %.

Salting-out effect in the CO2-H2O-MgCl2 and CO2-H2O-CaCl2 systems

Salting-out effect for the systems CO2-H2O-MgCl2 and CO2-H2O-CaCl2 were compared to evaluate the influence of cations (Ca 2+ and Mg 2+ ). Messabeb et al [START_REF] Messabeb | Experimental Measurement of CO2 Solubility in Aqueous CaCl2 Solution at Temperature from 323[END_REF] published eighteen solubility points of CO2 in CaCl2-aqueous solutions distributed between three molalities 1, 3 and 6 mol/kg. Figure 8 presents the salting-out effect for this system. Comparing Figures 7 and8, one can observe that at 1 mol of salt/kg, the salting-out-effect is of 30-35 % for CaCl2 and of 33-37 % for MgCl2. At 3 mol/kg, it is of 60-65 % for CaCl2 and 62-67 % for MgCl2. Even though they are not directly comparable because they have different molalities -6 mol CaCl2/kg and 5 mol MgCl2/kg, it is possible to notice that close to the saturation of the salt, the saltingout effect is higher. Both results, on average, are greater than 75 %. The similarity regarding the salting-out effect of the two systems was highlighted previously by Tong et al [START_REF] Tong | Solubility of CO2 in Aqueous Solutions of CaCl2 or MgCl2 and in a Synthetic Formation Brine at Temperatures up to 423 K and Pressures up to 40 MPa[END_REF] concluding that in addition to its salinities, the charge of the ion is an important parameters in the thermodynamic equilibrium. Zhao et al [START_REF] Zhao | Experimental Studies and Modeling of CO2 Solubility in High Temperature Aqueous CaCl2, MgCl2, Na2SO4 and KCl Solutions[END_REF] describe that ion-water molecule interactions govern the behavior of CO2 solubility in aqueous saline solutions. In the case of Mg 2+ and Ca 2+ , they have very similar properties (Alkaline Earth metals, similar electronic structure and same charge). In solution with chloride ions, in the configuration 1 cation-2 anions, their properties do not differ, this probably explains the similarity between solubility values, and consequently in the results of the salting-out effect. In addition, Zhao et al. [START_REF] Zhao | Experimental Studies and Modeling of CO2 Solubility in High Temperature Aqueous CaCl2, MgCl2, Na2SO4 and KCl Solutions[END_REF] observed that the CO2 solubility in aqueous CaCl2 solutions is slightly higher than in aqueous solutions containing MgCl2. Consequently, the salting-out effect is slightly lower for CO2-H2O-CaCl2 systems. Zhao et al [START_REF] Zhao | Experimental Studies and Modeling of CO2 Solubility in High Temperature Aqueous CaCl2, MgCl2, Na2SO4 and KCl Solutions[END_REF] describes that the charge density of Mg 2+ is slightly higher than that of Ca 2+ because despite the same charge number, Mg 2+ has a smaller radius than Ca 2+ . This slight difference results in Mg 2+ being able to solvate more molecules of water, leaving fewer molecules available to dissolve CO2 than in an aqueous solution of CaCl2 with the same ionic strength (ion hydration and ion-water molecule interactions [START_REF] Collins | Charge density-dependent strength of hydration and biological structure[END_REF][START_REF] Samoilov | A new approach to the study of hydration of ions in aqueous solutions[END_REF] ). Figure 9 directly compares the salting-out effect values between the CO2-H2O-CaCl2 and CO2-H2O-MgCl2 systems at 1 and 3 mol/kg. Despite the proximity of the SO% values for both systems, see that the points are close to the dotted line, the SO% values tend slightly towards MgCl2, going in the same direction observed by Tong et al [START_REF] Tong | Solubility of CO2 in Aqueous Solutions of CaCl2 or MgCl2 and in a Synthetic Formation Brine at Temperatures up to 423 K and Pressures up to 40 MPa[END_REF] and Zhao et al [START_REF] Zhao | Experimental Studies and Modeling of CO2 Solubility in High Temperature Aqueous CaCl2, MgCl2, Na2SO4 and KCl Solutions[END_REF] .

CONCLUSIONS

Carbon dioxide solubility was evaluated in aqueous solutions containing MgCl2 at pressures up to 20 MPa, at temperatures of 323.15 K, 373.15 K and at 423.15 K and at salt concentrations of 1, 3 and 5 mol/kg. The analysis by pH-metric titration was used for the first time to study the CO2-H2O-MgCl2 system. Our experimental results showed to be in agreement with the literature data.

The ANOVA method allowed us to quantify the experimental uncertainty of the study.

Our experimental solubility data presented an uncertainty of 1.76 % obtained from repeatability and reproducibility experiment with ANOVA calculations. This study reports thirty-three new solubility points for CO2 magnesium chloride solutions.

The salting-out effect was also calculated for our studied conditions. It has been shown

that temperature and pressure have almost no influence on the results. On the contrary, the increase in salt concentration implies an increase in the salting-out effect -between 33-37 % at 1 mol/kg, 62-66 % at 3 mol/kg and 76 % at 5 mol/kg. A comparison of the salting-out effect with the CO2-H2O-CaCl2 system was also investigated which highlights that whatever the cation is (Ca 2+ or Mg 2+ ), the salting-out-effect is nearly the same for these two chlorinated systems.

  and 473.15 K. CO2 loading in the autoclave is performed using a thermostated volumetric pump (Top Industrie PMHP 200-200), which could operate, for a pressure up to 20 MPa and offers automatic pressure compensation after each sampling. Once pressure and temperature are reached, agitation is maintained for about an hour (minimum) to ensure thermodynamic equilibrium. The solubility of CO2 in the brine is determined by means of an acid-base titration: a sample of solution is taken from the reactor by means of a syringe, initially containing a solution of sodium hydroxide. The sodium hydroxide solution is in excess

Figure 1 .

 1 Figure 1. Scheme of the experimental cell. (1) CO2 bottle; (2) thermostatic volumetric pump;

Figure 2 .

 2 Figure 2. Curve of pH and pH derivative depending on the volume of added acid obtained from titration (adapted from dos Santos et al 18 ).

Figure 3 .

 3 Figure 3. Solubility of carbon dioxide in a 1 mol MgCl2•kg H2O -1 solution at 373.15 K up to 20 MPa: this study and previous studies 33,34 .

Figure 4 .

 4 Figure 4. Solubility of carbon dioxide in a 1, 3 and 5 mol MgCl2•kg H2O -1 solution at 323.15

Figure 5 .

 5 Figure 5. Solubility of carbon dioxide in a 1 and 3 mol MgCl2•kg H2O -1 solution at 373.15 K

Figure 6 .

 6 Figure 6. Solubility of carbon dioxide in a 1 and 3 mol MgCl2•kg H2O -1 solution at 423.15 K

Figure 7 .

 7 Figure 7. Evolution of the salting-out effect due to the presence of magnesium chloride as a

Figure 8 .

 8 Figure 8. Comparison of the salting-out effect in the CO2-H2O-CaCl2 system (Messabeb et al. 14 ).

Figure 9 .

 9 Figure 9. Comparison of salting-out effect between CO2-H2O-CaCl2 14 and CO2-H2O-MgCl2

Table 1 .

 1 Published CO2 Solubility Data for MgCl2 Solutions. causes the color of the indicator to fade. The titration return the solution to its original color. The sample CO2 is titrated and the mass recorded in micrograms. Their study was performed only at 297 K, investigating the solubility at pressures between 2.76-17.23 MPa and at two concentrations of salt, 0.4 and 2 mol/kg.

			Temperature	Pressure	𝐦 𝐌𝐠𝐂𝐥 𝟐	Analysis
	Authors	Year
			(K)	(MPa)	(mol/kg) a	method
	Kobe and				Analytical:
		1935	298.15	0.10446	3.151
	Williams 26				volumetry
	Yasunish and				Analytical:
		1979 288.15-308.15 0.101325 0.356-3.967
	Yoshida 37				volumetry

Table 2 .

 2 Characteristics of the Chemical Compounds Used in this Study.

	Chemical name	Formula	N° CAS	Source	Purity
	Carbon dioxide	CO2	124-38-9	Air Liquide	99.7 %wt
	Magnesium
	chloride	MgCl2.6H2O	7791-18-6	VWR	100% wt
	hexahydrate
	Sodium			Fisher
		NaOH	1310-73-2		46/51 %wt
	hydroxide			Scientific
	Hydrochloric
		HCl	7647-01-0	VWR	0.1 ± 2 % mol/L
	acid
	Pure water	H2O	7732-18-5	Barnstead	18.2 MΩ.cm

Table 3 .

 3 Experimental Data of CO2 Solubility in Magnesium Chloride Solutions at 323.15,

	373.15, and 423.15 K.
	𝐦 𝐌𝐠𝐂𝐥 𝟐	Temperature	Pressure	𝐦 𝐂𝐎 𝟐	𝐮(𝐦 𝐂𝐎 𝟐 )
	(mol/kg) a,b	(K) a	(MPa) a	(mol/kg) b	(mol/kg) c
			1.60	0.188	0.003
			3.03	0.338	0.006
			5.11	0.527	0.009
	1	323.15
			10.14	0.777	0.014
			15.04	0.840	0.015
			20.13	0.899	0.016
			1.60	0.109	0.002
			3.05	0.198	0.003
			5.04	0.310	0.005
	1	373.15
			10.17	0.550	0.010
			15.20	0.685	0.012
			20.18	0.769	0.014
			2.98	0.140	0.002
			4.97	0.237	0.004
	1	423.15	10.11	0.453	0.008
			15.08	0.619	0.011
			20.02	0.753	0.013
	3	323.15	5.03	0.303	0.005

a Standard uncertainties u are u(T) = 0.06 K, u(P) = 0.03 MPa and u(m MgCl 2 ) = 0.001 mol/kg. b Relative uncertainties u r are u r (m CO 2 ) = 0.0176. Molalities are expressed in moles per kilogram of water. c u(m CO 2 ) = 0.0176 • m CO 2 mol/kg.

Table 3

 3 At 323.15 K, fourteen solubility points were obtained, thirteen of which are completely new to the literature. Only one point at 15 MPa and 1 mol/kg MgCl2 solution was previously

	reports thirty-three points of solubility generated during the present work, where
	twenty-seven points are new to different conditions of T-P-msalt. Still analyzing Table 3, the
	CO2 solubility decreases with increasing temperature and salinity, a behavior opposite to the
	pressure. The increase in pressure allows the CO2 to dissolve in greater amounts in the liquid
	phase. Figures 4-6 compare all the experimental points for the CO2-H2O-MgCl2 system

available at 323.15 K, 373.15 K and 423.15 K, respectively. The T-P-msalt ranges presented here have not been studied so far, as shown in the isotherms present in the figures.

Table 4 .

 4 The Average Values of Salting-Out Effect and the Calculated Standard Uncertainties for salting-out effect in Magnesium Chloride Solutions at 323.15, 373.15, and 423.15 K.

	𝐦 𝐂𝐎 𝟐 (mol/kg)	Temperature (K)	𝐒𝐎 % ̅̅̅̅̅̅	𝐮 𝐒𝐎 % ̅̅̅̅̅̅
		323.15	33.56	1.99
				2.01
		373.15	33.02
	1
				1.90
		423.15	37.23
				1.14
		323.15	62.68
				1.14
	3	373.15	62.75
				1.02
		423.15	66.61
				0.72
	5	323.15	76.56

This work enriches the amount of experimental CO2 solubility data, aiming to help in the development of CO2 storage technology. We saw that, just like CaCl2, systems containing MgCl2 can strongly influence the amount of CO2 dissolved in aqueous solutions, consequently, in the gas storage capacity. And these experimental data can also be used to improve thermodynamic models that have parameters based on experimental data.

ACKNOWLEDGMENTS

The project leading to this publication has received funding from Excellence Initiative of Université de Pau et des Pays de l'Adour -I-Site E2S UPPA, a French "Investissements d'Avenir" programme. The Carnot ISIFoR and BRGM are gratefully acknowledged.

Data acquisition on the CO2-H2O-MgCl2 system at high salinity

The results of CO2 solubility for the CO2-H2O-MgCl2 system were obtained at three salinities, 1, 3 and 5 mol/kg and four different temperatures, 323.15 K, 373.15 K and 423.15 K,

AUTHOR INFORMATION

Corresponding Author

*Phone: +33559407820. E-mail: marion.ducousso@univ-pau.fr

Corresponding author address:

Université de Pau et des Pays de l'Adour -LaTEP-Bâtiment d'Alembert, Rue Jules Ferry, BP -64075 Pau, France

Notes

The authors declare no competing financial interest.


Laura Mallón

Christian Cerezo-Navarrete

Nuria Romero

Marta Puche

Jordi García-Antón

Roger Bofill

Karine Philippot

Luis M Martínez-Prieto

Xavier Sala

Ru nanoparticles supported on alginate-derived graphene as hybrid electrodes for the hydrogen evolution reaction

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

Introduction

The still massive consumption of fossil fuels triggers global warming through greenhouse gas (GHG) emissions and threatens both our lifestyle and the sustainability of our planet. 1 Thus, the development of renewable carbon-neutral fuels has become urgent and one of the pivotal societal challenges in the 21 st century. In this regard, the renewable production of H2 through water splitting (WS, H2O à H2 + ½O2) triggered by renewable energy sources (e.g. renewable electricity from sunlight or wind) is an appealing and clean alternative to steam reforming methods, which rely on fossil fuels and are intensive GHG emitters. 2 However, both WS half reactions, namely hydrogen and oxygen evolution reactions (HER and OER, respectively), occur through large activation energy barriers and, therefore, at high overpotentials. Thus, the development of highly active, sustainable and durable electrocatalysts, able to accelerate these reactions and lower the required overpotentials, is still a key challenge at the core of clean H2 production from water. Nanoparticles (NPs), with a high surface-to-volume ratio and tuneable surface properties, are interesting systems in (electro)catalysis and WS. In this respect, we have recently shown how the bottom-up synthesis of Ru NPs through the organometallic approach under mild conditions 3,4 allows obtaining ultra-small NPs with narrow size-distributions and controllable surface environments, which have a strong impact in their catalytic performance in both the HER 5,6,7 and the OER. 8,9 The immobilization of NPs onto electrodes such as highly conductive carbon-based materials or oxides/metal oxides is a common way to improve (a) their long-term stability, preventing their agglomeration and sintering under catalytic conditions, and (b) their electron-transfer rates in practical electrodes. In this context, carbon-based (nano)materials (i.e. graphene) have shown to be excellent choices due to their high electrical conductivity and long-term stability together with a versatile morphology and rich surface chemistry. 10 Heteroatomdoping (i.e. N, S, P, O, etc.) of these carbon-based (nano)materials proved to be a powerful strategy not only to tune the physicochemical properties of the conductive supports themselves (i.e. electrical conductivity of the C-based nanomaterials) but also to influence the properties and affinity of the nanoparticles deposited at their surface. 11,6 The presence of doping heteroatoms thus influences either the activity, selectivity and long-term stability of the resulting hybrid electrocatalysts, and represents an interesting strategy to finetune their overall performance. 12 With far higher surface area and charge carrier mobility than related graphitic materials, 13 graphene has emerged in recent years as a paradigmatic 2D carbon-support in electrocatalysis. However, practical application of graphene-based hybrid Please do not adjust margins electrocatalysts relies on the development of scalable and sustainable methods to produce this carbon-nanomaterial. In this regard, graphene production through biomass carbonization, a waste-treatment technology, is an attractive method to reduce electrode costs and increase the sustainability of electrode production. 14 Herein, we report the preparation of hybrid nanomaterials via the growth of ultra-small Ru NPs through organometallic synthesis on bare/P-doped graphene supports obtained from biomass (alginate from marine algae), and their evaluation as HER electrocatalysts under acidic conditions.

Results and Discussion

Synthesis and Characterization. Biomass-derived bare and P-doped graphene supports (G and P-G, respectively) where prepared by pyrolysis of alginate and phosphate-modified alginate at 900 o C, respectively, following reported methods. 15,16 Decomposition of the [Ru(COD)(COT)] (COD: cyclooctadiene and COT: cyclooctatriene) organometallic precursor under mild conditions (3 bar H2, r.t, 20 h) in the presence of G or P-G supports, previously ultrasonicated in THF, yielded the graphene-supported Ru NPs (Ru/G and Ru/P-G) (Figure 1, top). 17,18 After an optimized digestion process (see Experimental Section), inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis showed a metal content of 2.6 wt.% and 3.3 wt.% for Ru/G and Ru/P-G, respectively. The two samples where then analysed by transition electron microscopy (TEM). As shown in Figure 1a-c, Ru/P-G revealed spherical, well distributed and monodispersed NPs with a mean diameter of 1.5 ± 0.3 nm. In contrast, the use of non-doped graphene (G) as a support yielded slightly larger (1.9 ± 0.6 nm) and more aggregated NPs (Figure 1, d-f). The difference in size and dispersion suggests that phosphorous atoms present in the P-doped graphene facilitate the formation and stabilization of smaller Ru NPs, as previously observed in similar N-doped graphene systems. 17,18 Figures 1 andS1 (see ESI †) also allow to infer the different exfoliation degree of the two supports (G and P-G). The presence of phosphate and sodium ions during the preparation of P-G seems to facilitate the separation of the carbon sheets, yielding a more exfoliated graphene support. The lower roughness of the P-G material (vs. G) has been later corroborated by electrochemical measurements (ECSA and RF, see Table 1

below).

High-resolution TEM (HRTEM) images and electron diffraction patterns confirmed the crystallinity of the Ru NPs in both Ru/G and Ru/P-G. From HRTEM micrograph of Ru/P-G, measurement of interplanar distances (see ESI †, Figure S2a) provided values of 0.21 and 0.23 nm, in agreement with the (002), (100) and (100) atomic planes of the hexagonal compact crystalline (hcp) structure of bulk ruthenium.

ARTICLE

Please do not adjust margins Raman spectroscopy was used to analyse and study the properties of graphene materials, both starting supports, as well as Ru/G and Ru/P-G catalysts. In all samples, two bands at ca. 1360 cm -1 (band D) and ca. 1600 cm -1 (band G) were observed, together with a broad peak centred at ca. 3000 cm -1 (band 2D'), which is related to a high exfoliation degree (see ESI †, Figures S3 andS4). The ratio of intensities of D and G bands (ID/IG) is a key parameter to determine the percentage of defects on graphene supports, which are excellent anchoring points for metal NPs, improving their stabilization. 19 As shown on Figures S3a andS4a), P-G presents a higher ID/IG ratio than G (2.39 for G and 2.81 for P-G), thus indicating the presence of more defect sites on the P-doped support. Moreover, the incorporation of Ru NPs on graphene supports led to a slight decrease in the two ID/IG ratios, indicating a higher sp 2 domain due to the growth of Ru NPs over the defects (see ESI †, Figure S3b andS4b). [START_REF] Stankovich | 21 Graphene Oxide: Fundamentals and Applications[END_REF] X-ray photoelectron spectroscopy (XPS) analysis was performed on both graphene supports (G and P-G) and obtained hybrid nanomaterials (Ru/G and Ru/P-G) to determine the chemical composition and the nature of the C, P and Ru species present (Figures 2 andS5). The C 1s signals of G and P-G appear as broad bands at binding energy (BE) of 284.5 eV, which can be deconvoluted in three components (see ESI †, Figure S5a-b). The main peak at 284.5 eV (orange) is assigned to the carbon atoms of graphitic layers (sp 2 ). The peak at ca. 286.4-286.2 eV (green) is attributed to carbon atoms bonded to P and those present in epoxides or tertiary alcohols. The last peak centred at ca. 288.9-288.6 eV (blue) corresponds to carboxylic groups. 21 Similarly, the P 2p signal of P-G is the result of the convolution of two components (see ESI †, Figure S5c), a major peak at 132.9 eV corresponding to P atoms bonded to oxygen (blue curve), and a lower intensity one at 131.8 eV (green curve), for P-C bonds. These peaks were previously identified in similar P-doped graphenes 15 and fit well with the results obtained by FT-IR, where the characteristic P-O/P=O stretching band is observed at ca. 1160-1252 cm -1 for this support (see ESI †, Figure S6). In addition, the stretching vibration associated to aromatic C=C bonds from the graphene sheet appears at ca. 1600 cm -1 . Analysis of the P 2p signal allowed us to determine the P content in the doped support P-G, namely 1.15 at.%.

Since the overlapping between the Ru 3d and C 1s signals makes difficult their deconvolution and interpretation, the different oxidation states of Ru in Ru/P-G and Ru/G were identified upon analysing the Ru 3p region. Figure 2a displays the Ru 3p3/2 signal of the as-synthesized Ru/G, at a BE of 462.9 eV. The deconvolution of this signal reveals two contributions, a major one at 464.2 eV, attributed to Ru IV and characteristic of RuO2, and a secondary one at 462.7 eV corresponding to Ru 0 . The as-synthesized Ru/G surface approximately contains 64 % of Ru IV and 36 % of Ru 0 . Similarly, the Ru 3p3/2 signal of Ru/P-G (Figure 2b) presents a peak of BE at 463.5 eV, also containing two contributions, at 464.5 eV (RuO2) and 462.8 eV (Ru 0 ). The surface of the as-synthesized Ru/P-G contains ca. 65 % of Ru(IV) and 35% of Ru 0 . 31 P solid state NMR confirmed the presence of the dopant atoms in the P-doped graphene material and allowed to determine their chemical nature. 31 P MAS NMR spectra of both P-G and Ru/P-G display a broad peak between 10 and -40 ppm (see ESI †, Figure S7), as the result of the overlapped signals of phosphonate (7 to 10 ppm), [START_REF] Puziy | [END_REF] phosphate (ca. 0 ppm), [START_REF] Puziy | [END_REF] methaphosphate (-3 to -7 ppm), 23 elemental phosphorous (-14 to -17 ppm) 23 and/or polyphosphate (-22 ppm). 24 Elemental phosphorous comes from the reduction of the phosphate by carbon at high temperature during the pyrolysis of the P-doped graphene. 23 The presence of metaphosphate and polyphosphate groups could derive from the condensation of Na2HPO4 at 900 o C. 23Error! Bookmark not defined. Thus, most P-O bonds observed by XPS are due to phosphate-like structures.

Electrocatalytic performance in the HER.

The HER performance of Ru/G and Ru/P-G was evaluated in 1 M H2SO4 aqueous solution after their dispersion in THF (2 mg/mL) and drop-casting onto a glassy carbon rotating disk electrode (GC-RDE). A three-electrode configuration was used being the drop-casted GC-RDE the working electrode, SCE (saturated calomel electrode, KCl sat.) and a Pt wire the reference (RE) and counter (CE) electrodes, respectively. The polarization curves of Ru/G and Ru/P-G at t=0 s are shown in Figure 3 (bold lines). A change in the current density is observed when scanning towards reductive potentials, which is attributed to their catalytic activity to reduce protons to H2.

The catalytic performance was significantly improved after a currentcontrolled bulk electrolysis at j = -10 mA/cm 2 for ca. 12-20 h (Figure 3a, dashed lines). As presented in Figure 3 and Table 1, whereas Ru/G and Ru/P-G show a η10 of 233 mV and 243 mV, respectively, a shift on the polarization curves is observed after the reductive process, Please do not adjust margins improving the η10 to 29 mV and 15 mV, respectively. This behaviour is attributed to a change in the oxidation state of surface Ru atoms of the NPs: the partially oxidized surface of the as-synthesized NPs (see XPS analysis, Figure 2) is reduced to metallic Ru when submitted to a reductive treatment. The reduced species Ru/G-r and Ru/P-G-r are more active in HER than their as-synthesised counterparts Ru/G and Ru/P-G, displaying higher current densities and lower overpotentials. We previously reported a deep study of the proposed RuO2/Ru interconversion with 4-phenylpyridne stabilized RuNPs, 5 confirming by XPS a total disappearance of the RuO2 peak under reductive catalytic conditions. The electrocatalytic performance of all the systems has been then compared by following the benchmarking methodology reported by Jaramillo et al. 25 First, the double-layer capacitance (CDL) was estimated from the capacitive current in a non-Faradaic region using Eq. 1 shown below. Then, the electrochemically active surface area (ECSA) and roughness factor (RF) of all electrodes and supports were calculated from the obtained CDL (see ESI †, Figures S8 andS9) according to equations 2 and 3.

i = νCDL

Eq. 1

ECSA [cm 2 ] = 𝑪 𝑫𝑳 𝑪 𝒔 Eq. 2 RF = 𝑬𝑪𝑺𝑨 𝑺

Eq. 3

The ECSA value allows calculating the specific current density (js) of the electrodes, which is the current density per "real" electroactive area at a given overpotential. Thus, the current density, js, normalized per ECSA at η= -100 mV, has been calculated for all the Ru materials before and after the reductive treatment/activation, as well as for the bare supports (i.e. G and P-G). Long-term stability is, together with kinetics, a key parameter for a catalyst to be potentially relevant in the HER. Thus, in a current controlled experiment (i.e. chronopotentiometry), Ru/G-r and Ru/P-G-r electrodes were hold at a constant current density of j = -10 mA/cm 2 for 12 h while monitoring the change in the required overpotential. Both systems showed almost no change for η10 and almost identical LSV polarization curves before and after 12h under catalytic turnover (Figure 4). Finally, a Faradaic efficiency of 97-98% was determined by quantifying the H2 evolved by the systems during a 20 min chronoamperometry using a H2-Clark electrode and comparing with the maximum theoretical amount of H2 calculated from the total charge passed through each respective electrode (Figure 5). This result confirms the production of H2 as the sole reaction taking place.

ARTICLE

Please do not adjust margins The fate of the hybrid electrodes under turnover conditions has been studied by TEM after performing a 2-h CP to each system (j = -10 mA/cm 2 ). Each material was recovered from the electrode by sonication in THF and drop-casted onto a TEM grid. Interestingly, the presence of small NPs onto the graphene supports is still visible (see ESI †, Figures S10 andS11). Moreover, the reductive potential applied for 2 h did not change significantly the size and morphology of the Ru NPs. In order to verify the nature of the NPs on-top of the graphene supports, EDX analysis of Ru/P-G has been performed after 2 h under catalytic conditions (see ESI †, Figure S12). The results confirm the presence of Ru in the sample, therefore corroborating the TEM data, and highlight the stability of the supported Ru NPs after short-term catalytic turnover.

Conclusions

Bare and P-doped graphenes arising from the pyrolysis of biomass (alginate from marine algae) were used as a support for the growth and stabilization of ultra-small Ru/RuO2 NPs through organometallic synthesis. Compared to bare counterpart, Pdoped graphene allowed obtaining smaller and better dispersed NPs in hybrid electrodes of lower roughness and electroactive surface area. Electrochemical activation of the supported nanocatalysts by reduction of the passivating RuO2 layer, offers excellent HER electrocatalysts under acidic conditions (η10 of 29 mV and 15 mV for the bare and P-doped electrodes, respectively). P doping, identified as surface phosphates by 31 P solid state NMR, induces a general improvement of all HER benchmarking parameters determined, including overpotential (η10) and exchange (j0) and specific (js) current densities. All studied systems show excellent long-term stability and selectivity for hydrogen generation with no sign of deactivation after 12 h under turnover conditions and quasi quantitative Faradaic efficiencies. Altogether, this work evidences how the combination of an efficient nanocatalyst synthesis method with the doping of graphene supports provides performant hybrid electrodes for the production of hydrogen.

Experimental Section

General procedures and starting materials Most of chemical operations were carried out using standard Schlenk tubes, Fischer-Porter bottle techniques under nitrogen atmosphere or in a glovebox under nitrogen atmosphere. Tetrahydrofuran (THF) was purified before use by distillation under argon atmosphere through filtration in the column of a solvent purification system (SPS). The organometallic precursor, [Ru(COD)(COT)], was purchased from Nanomeps (Toulouse), and alginic acid sodium salt and sodium phosphate dibasic from Sigma Aldrich. All reagents were used without purification.

Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). TEM and HRTEM images were performed at the "Servicio de Microscopía Electrónica" of Universitat Politècnica de València (UPV) by using a JEOL JEM 1400 Flash electron microscope operating at 120 kV with a point resolution of 3.8 Å and a JEOL JEM 2010 electron microscope working at 200 kV with a resolution point of 2.35 Å, respectively. The average particle size for both supported-Ru NPs was obtained by measuring more than 100 particles of each material by using the Elemental Analysis (EA). EA analyses of nitrogen, carbon and hydrogen were determined with a Euro EA3000 Elemental Analyzer (EuroVector) using sulfanilamide as reference.

Entry Electrode Ø (nm) Ru (wt.%) η10 (mV) Tafel Slope (b) (mV/dec) j0 (mA/cm 2 ) ECSA (cm 2 ) RF |js| (η =100 mV) (mA/cm 2 ) 1 G 19
Inductively coupled plasma atomic emission spectroscopy (ICP-AES). ICP-AES analyses of Ru/G and Ru/P-G were carried out at the ITQ by using a Varian 715-ES ICP-Optical Emission Spectrometer, in order to determine the Ru content. The samples for ICP were prepared following a modified digestion method previously reported. 27 A small portion of the material (30 mg) was suspended in 21 mL HCl-HNO3 (6:1) and then, the solution was sonicated for 90 minutes. Then, the samples were digested at 180 °C for 15. Finally, they were cooled down until room temperature, diluted with 100 mL of water and afterwards analyzed by ICP-AES.

Solid-state MAS-NMR spectroscopy. 31 P analyses were performed at the ITQ on a Bruker Avance 400WB instrument equipped with a 4 mm probe with a sample rotation frequency of 10 kHz. Measurements were carried out in a 4 mm ZrO2 rotor.

Raman Spectroscopy. Raman spectra were recorded with a 514 nm laser excitation in a Renishaw in via Raman spectrometer equipped with a Lyca microscope. The samples (powder) were deposited on an Al support and measured in the region between 0 and 3000 cm -1 with a resolution of < 4 cm -1 .

X-ray photoelectron spectroscopy (XPS). XPS analyses were recorded using a SPECS spectrometer equipped with a Phoibos 150MCD-9 multichannel detector using Mg-Kα (1235.6 eV) and Al-Kα (1483.6 eV) irradiation from a dual source. The pressure during the measurements was kept under 10 -9 Torr. The quantification and titration of the spectra were performed with the help of the CASA software, referencing them to the C1s peak (284.5 eV).

Infrared spectroscopy (IR). FT-IR spectra were recorded on a Nicolet 8700 Thermo spectrometer in the range 4000-600 cm -1 from samples prepared as KBr pellets.

X-ray Powder Diffraction (XRD). Powder samples were analysed using a CUBIX PANalytical diffractometer equipped with a PANalytical X´Celerator detector. X-ray monocromatic radiation of CuKα (λ1=1.5406 Å, λ2=1.5444 Å, I2/I=0.5) was employed.

Synthesis of graphenes

G. G was synthesized by pyrolysis of alginate according to previous literature. 15,16 In particular, the alginic acid sodium salt was pyrolized at 900 °C for 2 h (rate of 10 °C/min) under Ar atmosphere. The resulting graphitic powder was sonicated during 1 hour in water to obtain the G support.

P-G. P-G was synthesized by pyrolysis of phosphate-modified alginate according to previous literature, 15 dissolving 0.5 g of alginic acid sodium salt in a sodium phosphate dibasic aqueous solution (1.6 g in 50 mL of water). After that, the resulting black powder was pyrolized and sonicated in the same way that G support. EA: C=73.54%, H: 0.96%, S: 0.331% XPS: P=1.15 % IR: (KBr pellet, cm -1 ), 1160 cm -1 (ν P=O).

Synthesis of graphene

Table of Contents

Ultra-small Ru NPs grown on biomass-derived bare/P-doped graphene supports yield efficient and durable electrocatalytic H2 production from water.

Figure 1 .

 1 Figure 1. Top: Synthesis of Ru/G and Ru/P-G following the organometallic approach. Bottom: TEM images and size distribution histograms of Ru/P-G (a-c) and Ru/G (d-f).

Figure 2 .

 2 Figure 2. XPS signal of the Ru 3p band of as-synthesized Ru/G (a) and Ru/P-G (b).

Figure 3 .

 3 Figure 3. a) Polarization curves of Ru/G (grey line), Ru/P-G (red line) before (bold) and after (dashed) a reductive process at j = -10 mA/cm 2 in 1 M H2SO4. G (wine line), P-G (black line) blanks are also shown. b) Tafel plots of Ru/G, Ru/G-r, Ru/P-G and Ru/P-G-r in 1 M H2SO4. Same colour code as in (a).The difference in nature and in catalytic performance between assynthesised samples and their reduced analogous was also evidenced through the Tafel plots (Fig.3b), obtaining improved overall kinetics with the reduced systems. The Tafel slope (b) allows defining the rate determining step (rds) of the catalytic reaction. Both Ru/G and Ru/P-G show a Tafel slope close to 120 mV, a consistent value with the Volmer step as rds (adsorption of H + to form the M-H species on the NPs, typically b ≈ 120 mV/dec). In contrast, Ru/G-r and Ru/P-G-r show a Tafel slope of 48 mV/dec and 49 mV/dec, respectively, suggesting that the HER follows the Volmer-Heyrovsky mechanism with the Heyrovsky step (H2 electrodesorption with a proton from the solution, b ≈ 40 mV/dec) as the slowest path in the HER process.

Figure 4 .

 4 Figure 4. LSV of Ru/G-r (a) and Ru/P-G-r (b) before (dashed lines) and after (solid lines) a 12 h chronopotentiometry experiment at a j = -10 mA/cm 2 . 26

Figure 5 .

 5 Figure 5. H2-monitored current-controlled bulk electrolysis of Ru/G (a) and Ru/P-G (b) in 1 M H2SO4. The produced H2 was quantified in the gas phase using a Clark-type electrode.

Figure S1 .

 S1 Figure S1. TEM images of G (a-c) and P-G (d-f) supports.

Figure S2 .

 S2 Figure S2. HRTEM images, chosen expanded zone and corresponding Fourier Transform Analysis with planar reflections for Ru/P-G (a) and Ru/G (b).

Figure S3 .

 S3 Figure S3. Raman spectrum and ID/IG ratio of G (a) and Ru/G (b).

Figure S4 .

 S4 Figure S4. Raman spectrum and ID/IG ratio of P-G (a) and Ru/P-G (b).

Figure S5 .

 S5 Figure S5. XPS signal of the C 1s bands of G (a) and P-G (b), and the P 2p band of P-G (c).Deconvoluted peak component analysis is shown for each case (baseline as red curve.

Figure

  Figure S6 FT-IR spectra of P-G (top; black) and Ru/P-G (bottom; red).

Figure S7 .

 S7 Figure S7. 31 P MAS NMR spectra of a) P-G and b) Ru/P-G. Asterisks (*) mark the spinning side bands. The positions of phosphonate (7 ppm), phosphates (0 ppm), metaphosphonates (-7 ppm), phosphorous (-14 ppm) and polyphosphate (-22 ppm) are indicated on the zoomed part.

Figure S8 .

 S8 Figure S8. Representative multi CV experiment at different scan rates for CDL determination of Ru/G (a) and Ru/P-G (c). Plot of current values at 0.25 V (vs. SCE) for the different scan rates in 1 M H2SO4 for Ru/G (b) and Ru/P-G (d).

Figure S9 .

 S9 Figure S9. Representative multi CV experiment at different scan rates for CDL determination of Ru/G-r (a) and Ru/P-G-r (c). Plot of current values at 0.30 V and 0.25 V (vs. SCE) for the different scan rates in 1 M H2SO4 for Ru/G-r (b) and Ru/P-G-r (d), respectively.

Figure S10 .

 S10 Figure S10. TEM images and corresponding histogram of Ru/G after 2 h under catalytic conditions (j = -10 mA/cm 2 ).

Figure S11 .

 S11 Figure S11. TEM images and corresponding histogram of Ru/P-G after 2 h under catalytic conditions (j = -10 mA/cm 2 ).

Figure S12 .

 S12 Figure S12. TEM/EDX analysis of Ru/P-G after 2 h under catalytic conditions (j = -10 mA/cm 2 ).

Table 1

 1 The obtained results are summarized in Table1. Regarding the bare supports, ECSA and RF data support the higher roughness of the non-doped support G (RF of 279.43 for G vs. 51.76 for P-G) already observed by TEM (see ESI †, FigureS1). Introduction of Ru NPs onto the carbon supports slightly increases the RF values in both cases (

	, entries 2 and

Ru/G-r. For comparison purposes, the most relevant HER benchmarking parameters for the hybrid electrocatalysts developed in this work are collected in Table

S1

(see ESI †) together with those of reported graphene-supported Ru-based systems and metal-free graphene-based materials. As can be inferred from the collected data, Ru/G-r and Ru/P-G-r show way higher HER performance than bare graphene-based electrodes and lay among the three state-ofthe-art Ru-based systems.

Table 1 .

 1 Summary of physico-chemical and HER electrocatalytic data (1 M H2SO4) for the hybrid electrodes studied in this work.

  Please do not adjust margins stirring bar. Prior to each measurement, both compartments were purged with Ar. Unisense H2-NP Clark electrode was used to measure the hydrogen evolved in the gas phase during a chronopotentiometry. The Clark electrode was calibrated by adding different volumes of 99 % pure hydrogen at the end of the experiment. Plotting ic as a function of ν yields a straight line with slope equal to CDL. In this way, 8 different scan rates were used (5, 10, 25, 50, 75, 100, 250 and 500 mV/s), holding the working electrode at each potential vertex for 10 seconds prior to the next step. The ECSA was obtained by dividing the calculated capacitance by a tabulated value (CS = specific capacitance) that depends on the material used and solution (for C, in 1 M H2SO4 CS=13-17 μF/cm 2 ). The roughness factor (RF) was calculated by dividing the ECSA by the geometrical surface area (S) of the RDE.Please do not adjust margins accumulation of H2 bubbles onto the surface of the employed carbon electrodes. 27 T. Suoranta, M. Niemelä, P. Perämäki, Talanta 2014, 119, 425-429.

	Double-layer capacitance (CDL) and electrochemically active
	surface area (ECSA) determination: CDL was estimated by	-supported Ru NPs
	performing CV measurements with different scan rates. A non-	Ru/G and Ru/P-G: In a Schlenk tube, 10 mg (0.032 mmol) of
	faradaic region was chosen from the LSV (typically a 0.1 V	[Ru(COD)(COT)] were dissolved in 5 mL THF (previously
	window about OCP), where no redox process takes place and all	deoxygenated). At the same time, a Fischer-Porter bottle was
	the measured current is due to double-layer charging. Based on	charged with G or P-G (100 mg) and dispersed in 50 mL of
	this assumption, the charging current (ic) can be calculated as	deoxygenated and anhydrous THF by ultrasonication (90 min).
	the product of the electrochemical double-layer capacitance	Then, the [Ru(COD)(COT)] solution was transferred to the
	(CDL) by the scan rate (ν).	Fischer-Porter bottle and it was pressurized with 3 bar of H2.
		After 20 h under vigorous stirring at room temperature, the
		pressure was released and Ru/G or Ru/P-G were separated
		from the suspension by filtration under air through a polyamide
		membrane (Whatman® membrane filters, 47mm×0.45 μm) and
		washed with THF (100 mL). The resulting black precipitates were
		dried overnight in an oven at 60 °C. The size of the NPs was
		measured by TEM on a population of at least 100 NPs, affording
		a Ru NP mean diameter of 1.9 (0.6) nm and 1.5 (0.3) nm for
		Ru/G or Ru/P-G, respectively. Ru contents (ICP-AES): 2.6 % and
		3.3 % for Ru/G or Ru/P-G, respectively.
		Electrochemical measurements
		All the electrochemical experiments were performed with a
		BioLogic SP-150 potentiostat. The solutions were degassed
		previous to the electrochemical analysis with an Ar flow. Ohmic
		potential (IR) drop was automatically corrected at 85 % using
		the Biologic EC-Lab software for linear sweep voltammetry. 1 M
		H2SO4 solution was prepared by mixing 56.1 mL of 95-97 %
		H2SO4 with Milli-Q water up to 1 L. A glassy carbon rotating disk
		electrode (RDE, ф = 0.3 cm, S = 0.07 cm 2 ) was used as working
		electrode. The electrode preparation started from a 2 mg/mL
		dispersion of each material by adding 1 mg of Ru material in 0.5
		mL of THF. Long-time sonication was applied to prevent NP
		aggregation over the C support. Then, an aliquot of 5 μL was
		drop-casted on the surface of the GC/RDE (S = 0.07 cm 2 ), and
		dried. A 5 μL-drop of Nafion (0.02% w/w in water and 1-
		metanol) was finally added and dried prior to the
		electrochemical measurements. The RDE was rotated at 3000
		rpm in order to ensure complete removal of in situ formed H2
		bubbles during catalytic turnover. A Pt wire was used as counter
		electrode (CE) and a standard calomel electrode (SCE,
		Hg/Hg2Cl2, KCl sat.) was used as a reference electrode (RE). The
		potentials reported versus normal hydrogen electrode were
		transformed as follows (ENHE = ESCE + E 0 SCE), where E 0 SCE = 0.244
		V.
		A 10 mL two-compartment cell with a proton exchange
		membrane between the two compartments was used for

faradaic efficiencies calculation. The CE was placed in one compartment and the WE and RE were placed in the other one together with the Clark electrode. Both compartments were filled with c.a. 7 mL of 1 M H2SO4 solution and equipped with a ARTICLE J. Name., 2013, 00, 1-3 | 7

Acknowledgements

The authors thank Instituto de Tecnología Química (ITQ), Consejo Superior de Investigaciones Científicas (CSIC), Universitat Politècnica de València (UPV), Universitat Autonoma de Barcelona for the facilities. Electron Microscopy Service of the UPV for TEM facilities and J. A. Vidal-Moya (ITQ, CSIC-UPV) for NMR measurements are also thanked. We also gratefully acknowledge Prof. A. Corma for his contribution to this research. C. C.-N. acknowledges Generalitat Valenciana predoctoral fellowship (GVA: ACIF/2019/076) and L. M. the UAB for a PhD grant. This work was financially supported by the MINECO/FEDER project PID2019-104171RB-I00. J. G.-A. acknowledges the Serra Húnter Program. CNRS and Université de Toulouse-Paul Sabatier are also acknowledged.

Author Contributions

L.M. performed the electrochemical experiments and characterization of the electrodes after catalysis and participated in writing -review and editing. C. C-N. and M.P. performed the synthesis and structural/spectroscopic characterization of the hybrid electrodes. N.R. coordinated the tasks between researchers and participated in writing -review and editing. R.B., K.P. and J.G-A supervised the project and participated in writing -review and editing. X.S. and LM.M-P. conceived the idea of the project and wrote the original draft with input from other authors. All authors contributed to the design of experiments, analysis of the results and preparation of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Supporting Information

Ru nanoparticles supported on alginate-derived graphene for the hydrogen evolution reaction

Laura Mallón, a, c Christian Cerezo-Navarrete, b Nuria Romero, a Marta Puche, b Jordi García-Antón, a Roger Bofill,* a Karine Philippot,* c Luis M. Martínez-Prieto* b and Xavier Sala* a Table S1. Comparison of the most relevant graphene-derived and Ru/graphene-based HER nanoelectrocatalysts under acidic conditions. Parameters: mean diameter (Ø), onset overpotential (η0, mV), overpotential at |j| =10 mA•cm -2 (η10, mV), Tafel slope (b, mV•dec -1 ), exchange current density (|j0|, mA•cm -2 ), specific current density (|jS|, mA•cm -2 ) and turnover frequency (TOF, s -1 ). Unless otherwise stated, electrolyte is 0.5 M H2SO4.

Entry

Catalyst Ø (nm)

Ref. Legend: GCE-S-GNs-1000=glassy carbon modified sulfur-doped graphene nanosheets heat treated at 1000 °C, GLC=graphene-layered carbon, RGO=reduced graphene oxide, GnP=graphene nanoplatelets. N-G=N-doped graphene; P-G=P-doped graphene; N,P-G=N,Pdoped graphene; N,B-CN=N,B-doped carbon nanosheets, N-CN=N-doped carbon nanosheets, N,P-CN=N,P-doped carbon nanosheets, N,s-CN=N,S-doped carbon nanosheets, P,N-G=P,Ndoped graphene, VG=vertical graphene, N-VG=N-doped vertical graphene, Ru/NG-750= Ru nanoclusters on N-doped graphene prepared at 750 °C, Ru@CN=Ru NPs over N-doped carbon, Ru-NGA= Ru-modified N-doped graphene aerogel, Ru@NC= Ru NPs embedded in N-doped carbon.