Sébastien Schaefer

Asma Jeder

Giuseppe Sdanghi

Philippe Gadonneix

A Abdedayem

M T Izquierdo

G Maranzana

A Ouederni

Alain Celzard

V Fierro

Oxygen-promoted hydrogen adsorption on activated and hybrid carbon materials

Keywords: Activated carbons, Surface chemistry, Hydrogen adsorption, Acidic groups, polarized physisorption

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

Introduction

Olive stones (OS) are by-products of olive oil extraction that are mainly burnt for heating and cooking in domestic applications, or which are used for energy co-generation in the industry. OS represents 16-18 wt. % of the olive mass, and therefore huge amounts of OS are produced every year only in the Mediterranean basin. OS-derived activated carbons (ACs) have been produced and principally used for pollutant removal in the aqueous phase [START_REF] El-Sheikh | Characterization of activated carbon prepared from a single cultivar of Jordanian Olive stones by chemical and physicochemical techniques[END_REF][START_REF] Abdel-Ghani | Optimizing the preparation conditions of activated carbons from olive cake using KOH activation[END_REF][START_REF] Bacaoui | Optimization of conditions for the preparation of activated carbons from olivewaste cakes[END_REF][START_REF] Alslaibi | Comparative studies on the olive stone activated carbon adsorption of Zn2+, Ni2+, and Cd2+ from synthetic wastewater[END_REF][START_REF] Khalil | Decomposition of H2O2 on activated carbon obtained from olive stones[END_REF][START_REF] Yeddou | Removal of cyanide in aqueous solution by oxidation with hydrogen peroxide in presence of activated carbon prepared from olive stones[END_REF]. In these researches, the activation was carried out either physically, with CO 2 or steam [START_REF] El-Sheikh | Characterization of activated carbon prepared from a single cultivar of Jordanian Olive stones by chemical and physicochemical techniques[END_REF], or chemically, with KOH or H 3 PO 4 [START_REF] Abdel-Ghani | Optimizing the preparation conditions of activated carbons from olive cake using KOH activation[END_REF][START_REF] Bacaoui | Optimization of conditions for the preparation of activated carbons from olivewaste cakes[END_REF][START_REF] Alslaibi | Comparative studies on the olive stone activated carbon adsorption of Zn2+, Ni2+, and Cd2+ from synthetic wastewater[END_REF][START_REF] Khalil | Decomposition of H2O2 on activated carbon obtained from olive stones[END_REF][START_REF] Yeddou | Removal of cyanide in aqueous solution by oxidation with hydrogen peroxide in presence of activated carbon prepared from olive stones[END_REF]; KOH activation of OS produced ACs with surface areas as high as 1300 m 2 g -1 [START_REF] Bacaoui | Optimization of conditions for the preparation of activated carbons from olivewaste cakes[END_REF].

Hydrogen adsorption at cryogenic temperatures in Metal Organic Frameworks (MOFs) [START_REF] Farha | De novo synthesis of a metal-organic framework material featuring ultrahigh surface area and gas storage capacities[END_REF] exceeds the hydrogen storage capacities recommended by the US Department Of Energy (US DOE) for automotive applications (i.e., 7.5 wt. % of the whole mass of the storage system) [START_REF] Klebanoff | 5 years of hydrogen storage research in the U[END_REF].

However, MOFs are quite expensive and, most of the time, moisture-sensitive [START_REF] Huang | An in situ self-assembly template strategy for the preparation of hierarchical-pore metal-organic frameworks[END_REF]. ACs also exhibit high hydrogen storage capacities, around 6.4 wt. % in excess at 77 K and 4 MPa [START_REF] Fierro | Experimental evidence of an upper limit for hydrogen storage at 77 K on activated carbons[END_REF], but only 1 wt. % at 293K and 10 MPa [START_REF] Fierro | Experimental evidence of an upper limit for hydrogen storage at 77 K on activated carbons[END_REF][START_REF] Sircar | Design of high pressure differential volumetric adsorption measurements with increased accuracy[END_REF][START_REF] Xia | Enhanced room-temperature hydrogen storage in super-activated carbons: The role of porosity development by activation[END_REF][START_REF] Schaefer | Assessment of hydrogen storage in activated carbons produced from hydrothermally treated organic materials[END_REF][START_REF] Sdanghi | Hydrogen Adsorption on Nanotextured Carbon Materials[END_REF]. Moreover, ACs are cheaper, easier to synthesise than MOFs, and are often biosourced. In order to increase hydrogen-AC interactions, and hence hydrogen adsorption capacities, doping with metal nanoparticles or heteroatoms has been tested. On the one hand, doping with transition metal nanoparticles improves the adsorption of hydrogen by chemisorption and spill-over [START_REF] Stuckert | Characteristics of Hydrogen Storage by Spillover on Pt-Doped Carbon and Catalyst-Bridged Metal Organic Framework[END_REF][START_REF] Psofogiannakis | Fundamental studies and perceptions on the spillover mechanism for hydrogen storage[END_REF][START_REF] Zhao | Activated carbons doped with Pd nanoparticles for hydrogen storage[END_REF][START_REF] Schaefer | Physisorption, chemisorption and spill-over contributions to hydrogen storage[END_REF][START_REF] Pyle | Hydrogen storage in carbon nanostructures via spillover[END_REF][START_REF] Wei | Enhanced hydrogen storage performance of reduced graphene oxide hybrids with nickel or its metallic mixtures based on spillover mechanism[END_REF][START_REF] Guo | Pt-4, Pd-4, Ni-4, and Ti-4 catalyzed hydrogen spillover on penta-graphene for hydrogen storage: The first-principles and kinetic Monte Carlo study[END_REF] or polarised physisorption mechanisms whenever hydrogen is adsorbed on alkali earth or Si/SiC-doped carbon structures [START_REF] Duclaux | Deuterium adsorption in carbon single walled carbon nanotubes doped by lithium and potassium: Adsorption isotherms and in situ neutron diffraction[END_REF][START_REF] Pellenq | Strong physisorption site for H-2 in K-and Li-doped porous carbons[END_REF][START_REF] Wen | Tunable H-2 binding on alkaline and alkaline earth metals decorated graphene substrates from first-principles calculations[END_REF][START_REF] Schaefer | Rice straw-based activated carbons doped with SiC for enhanced hydrogen adsorption[END_REF][START_REF] Cho | Si-doping effect on the enhanced hydrogen storage of single walled carbon nanotubes and graphene[END_REF]. On the other hand, doping with heteroatoms such as boron [START_REF] Ariharan | Hydrogen storage on boron substituted carbon materials[END_REF][START_REF] Sankaran | Boron substituted carbon nanotubes -How appropriate are they for hydrogen storage?[END_REF][START_REF] Umegaki | Boron-and nitrogen-based chemical hydrogen storage materials[END_REF], nitrogen [START_REF] Ariharan | Nitrogen-incorporated carbon nanotube derived from polystyrene and polypyrrole as hydrogen storage material[END_REF][START_REF] Wang | Nitrogen-doped porous carbons with high performance for hydrogen storage[END_REF][START_REF] Xia | Hydrogen Storage in High Surface Area Carbons: Experimental Demonstration of the Effects of Nitrogen Doping[END_REF][START_REF] Zhao | Hydrogen adsorption on functionalized nanoporous activated carbons[END_REF][START_REF] Zhao | Hydrogen uptake of high surface area-activated carbons doped with nitrogen[END_REF] or even sulphur [START_REF] De Yuso | Synthesis of sulfurdoped porous carbons by soft and hard templating processes for CO2 and H-2 adsorption[END_REF] also increases the hydrogen adsorption potential of carbon-based materials [START_REF] Rangel | Theoretical study of hydrogen adsorption on nitrogen doped graphene decorated with palladium clusters[END_REF][START_REF] Petrushenko | Hydrogen physisorption on nitrogen-doped graphene and graphene-like boron nitride-carbon heterostructures: a DFT study[END_REF][START_REF] Zhao | Synthesis and characterization of Pt-N-doped activated biocarbon composites for hydrogen storage[END_REF]. This increase is due to the existence of permanent dipoleinduced dipole interaction or even quadrupolar interactions [START_REF] Lalonde | A zwitterionic metal-organic framework with free carboxylic acid sites that exhibits enhanced hydrogen adsorption energies[END_REF].

N and B are the heteroatoms most often introduced, and their positive effect on hydrogen adsorption has been demonstrated [START_REF] Xia | Hydrogen Storage in High Surface Area Carbons: Experimental Demonstration of the Effects of Nitrogen Doping[END_REF][START_REF] Zhao | Hydrogen uptake of high surface area-activated carbons doped with nitrogen[END_REF][START_REF] Kang | Hydrogen adsorption on nitrogen-doped carbon xerogels[END_REF][START_REF] Wang | Hydrogen Storage Properties of B-and N-Doped Microporous Carbon[END_REF][START_REF] Yushin | Carbide-derived carbons: Effect of pore size on hydrogen uptake and heat of adsorption[END_REF]. The effect of oxygen content is not so clear. Some theoretical studies have shown that the oxygen-containing functionalities in wellordered materials, such as carbon nanotubes or graphene-like materials, should decrease hydrogen storage capacities both by steric hindrance in the pores and by increase of the mass of sorbent [START_REF] Georgakis | Molecular dynamics study of hydrogen adsorption in carbonaceous microporous materials and the effect of oxygen functional groups[END_REF]. On the contrary, other theoretical studies concluded that carbon oxidation could lead to significantly more attractive surfaces at low pressure and low coverage degree [START_REF] Gotzias | Effect of surface functionalities on gas adsorption in microporous carbons: a grand canonical Monte Carlo study[END_REF][START_REF] Gotzias | Theoretical study of hydrogen adsorption in oxygen functionalized carbon slit pores[END_REF]. As the amount of adsorbed hydrogen on ACs at room temperature is quite low (typically less than 1 wt. % in excess at 10 MPa [START_REF] Fierro | Experimental evidence of an upper limit for hydrogen storage at 77 K on activated carbons[END_REF][START_REF] Sircar | Design of high pressure differential volumetric adsorption measurements with increased accuracy[END_REF][START_REF] Xia | Enhanced room-temperature hydrogen storage in super-activated carbons: The role of porosity development by activation[END_REF][START_REF] Schaefer | Assessment of hydrogen storage in activated carbons produced from hydrothermally treated organic materials[END_REF]), the coverage remains very low and the effect of surface groups may be significant. Experimental studies showed that, after AC oxidation with nitric acid, hydrogen peroxide, ammonium persulfate, potassium permanganate or orthophosphoric acid [START_REF] Moreno-Castilla | Changes in surface chemistry of activated carbons by wet oxidation[END_REF][START_REF] Huang | Effect of surface oxides on hydrogen storage of activated carbon[END_REF][START_REF] Huang | Hydrogen adsorption on modified activated carbon[END_REF][START_REF] Lee | Effect of chemical treatments on hydrogen storage behaviors of multi-walled carbon nanotubes[END_REF], the hydrogen storage capacities decreased. This finding was explained by the corresponding combined effects of decreased microporous volume, increased steric hindrance, and more disordered nature of the adsorbed hydrogen layer after oxidation of the carbonaceous surface [START_REF] Huang | Effect of surface oxides on hydrogen storage of activated carbon[END_REF][START_REF] Huang | Hydrogen adsorption on modified activated carbon[END_REF]. Other authors showed that, despite the decrease of textural properties after oxidation, there is a positive effect of acidic and oxygen-containing functionalities on adsorption energy [START_REF] Huang | Hydrogen adsorption on modified activated carbon[END_REF], evidenced using the Dubinin-Astakhov model [START_REF] Agarwal | EFFECT OF SURFACE-ACIDITY OF ACTIVATED CARBON ON HYDROGEN STORAGE[END_REF] and explained by their strong electron-acceptor character [START_REF] Lee | Effect of chemical treatments on hydrogen storage behaviors of multi-walled carbon nanotubes[END_REF].

Finally, some authors did not conclude about the effect of oxygen content on hydrogen adsorption [START_REF] Tellez-Juarez | Hydrogen storage in activated carbons produced from coals of different ranks: Effect of oxygen content[END_REF], mainly due to the cryogenic adsorption temperature that was chosen (77 K), and for which the surface area has much more impact than small changes of surface chemistry [START_REF] Tellez-Juarez | Hydrogen storage in activated carbons produced from coals of different ranks: Effect of oxygen content[END_REF]. Besides, the corresponding materials also contained potassium residues in the form of salts (mainly K 2 CO 3 ) or metallic K, and such potassium-containing carbon structures may induce polarised physisorption effects. Therefore, their hydrogen adsorption properties could be improved [START_REF] Duclaux | Deuterium adsorption in carbon single walled carbon nanotubes doped by lithium and potassium: Adsorption isotherms and in situ neutron diffraction[END_REF][START_REF] Pellenq | Strong physisorption site for H-2 in K-and Li-doped porous carbons[END_REF].

In order to definitely state on the contribution of oxygen to hydrogen adsorption, the present study focuses on the hydrogen storage capacities obtained on activated carbons and hybrid carbon materials. The hybrid carbon materials were synthesised following a route similar to that used for the synthesis of activated carbons (i.e., KOH activation). The only difference was that the materials were washed only with water instead of HCl + water, leading to hybrid materials with a high carbonate content in the porous carbons. The latter were called so because of their very high content of minerals, instead of just ACs despite the fact that they followed the same classic activation process, using OS as precursor and KOH as activating agent. We unquestionably confirmed the positive effect of oxygen, and especially acidic functions, on the hydrogen adsorption properties at 273 K.

Experimental

Materials synthesis

Olive stones (OS) were provided by a local factory in Gabès (Tunisia). They were already crushed with a particle size lower than 3 mm. OS were washed with hot distilled water for further production of hydrochars and activated carbons (ACs).

OS-derived hydrochars were prepared by hydrothermal carbonisation (HTC) at two different temperatures. For that purpose, 32 mL of water and 4 g of OS were poured in a Teflon-lined autoclave (inner volume 100 mL), which was introduced in a preheated oven at either 453 or 513 K. The autoclave remained inside for 6 hours. After cooling, the hydrochars were washed with bi-distilled water, dried at 378 K and labelled according to their HTC temperatures in degrees Celsius (i.e., HTC-180 and HTC-240).

Hydrochar activation was carried out by physical mixing with KOH using a KOH/hydrochar weight ratio equal to 2:1. The mixture was next submitted to thermal treatment under nitrogen flow. The temperature was increased at 10 K min -1 up to 1173 K. The latter temperature was held for 2 hours. After cooling under inert atmosphere, the resultant ACs were washed first with hot bi-distilled water, then with a 1 mol L -1 hydrochloric acid solution, and finally with hot bi-distilled water again until neutral pH. The ACs were labelled 180-KOH-W and 240-KOH-W. Part of them was also kept unwashed in order to investigate the effect on hydrogen adsorption of oxygenated functionalities in such carbonaterich ACs. These hybrid carbons were labelled 180-KOH-UW and 240-KOH-UW.

The materials 180-KOH-W and 240-KOH-W were also oxidised with ozone in a fixed bed reactor loaded with 0.85 g of sample under a constant ozone flow of 55 mg L -1 at 30°C for 2 hours. The as-modified materials were labelled 180-KOH-W-O3 and 240-KOH-W-O3, respectively.

In order to compare the aforementioned chemically activated carbons with a physically activated one, the remaining HTC-180 hydrochar was treated in a tubular furnace heated at 5 K min -1 up to 1173 K under nitrogen flow (100 mL min -1 ). Once the set point was reached, the N 2 flow was maintained 1 hour and then replaced by a CO 2 flow (50 mL min -1 ) for 2 hours. At the end, the oven was flushed with nitrogen again during cooling. The resultant AC was labelled CO2_2h.

Finally, six carbonaceous materials were characterised in depth: 180-KOH-W, 180-KOH-UW, 240-KOH-UW, 180-KOH-W-O3, 240-KOH-W-O3 and CO2_2h. The hydrogen adsorption and textural properties of these carbons were compared to those of ACs prepared by chemical activation of sucrose with KOH. Additional details about these materials, in terms of preparation method and main characteristics, can be found elsewhere [START_REF] Schaefer | Assessment of hydrogen storage in activated carbons produced from hydrothermally treated organic materials[END_REF].

Characterisation

Elemental analysis and ash content

Elemental analyses (EA) of samples were carried out with a CHONS elemental analyser (Vario El Cube, Elementar, Germany) to determine carbon, hydrogen, nitrogen and sulphur contents (wt. %) by combustion of the samples in a stream of pure O 2 . In a second step, oxygen was analysed with the same equipment using a different procedure.

The ash content was obtained by thermogravimetric analysis, using a protocol adapted from the ASTM D2866 -11. First, the sample was extensively dried at 383 K, and then the temperature was increased up to 923 K at 10 K min -1 in air and was maintained for 3 h at 923 K. Next, the temperature was increased a second time up to 1023 K and held for 1 h. The ash content was calculated as follows:

(1) where is the final mass after the two ramps, i.e., after treatment at 1023 K. The mass after the first ramp is expected to be similar to the one after the second ramp. Indeed, the oxidation/gasification of the carbonaceous part is supposed to be achieved at 923 K whereas the thermal decomposition of the carbonates occurs at much higher temperature (i.e., around 1173 K or slightly less [START_REF] Misra | WOOD ASH COMPOSITION AS A FUNCTION OF FURNACE TEMPERATURE[END_REF]).

X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was carried out with an ESCAPlus OMICROM system equipped with a hemispherical electron energy analyser. The spectrometer was operated at 10 kV and 15 mA, using a non-monochromatised MgK X-ray source (hν = 1253.6 eV) under vacuum (< 5  10 -9 Torr). Analyser pass energies of 50 eV and 20 eV were used for survey and detailed scans, respectively. The C1s peak at 284.5 eV was used for binding energy correction. A survey scan (1 sweep/200 ms dwell) was acquired between 1100 and 0 eV. Current region sweeps for N1s (410-390 eV), O1s (545-520 eV), C1s (300-280 eV)

and K2s (390-370 eV) regions were performed to quantify the atomic percentage of each element. The CASA data processing software allowed smoothing, Shirley-type background subtraction, peak fitting and quantification. Atomic percentages of each element were calculated from intensity ratios using Scofield sensitivity factors [START_REF] Scofield | HARTREE-SLATER SUBSHELL PHOTOIONIZATION CROSS-SECTIONS AT 1254 AND 1487EV[END_REF]. The K2s region was used instead of using the classical K2p region (310-290 eV) for potassium quantification, because the range of characteristic energies of K2p overlaps that of C1s [START_REF] Jansen | XPS OF NITROGEN-CONTAINING FUNCTIONAL-GROUPS ON ACTIVATED CARBON[END_REF].

Textural characterisation

The textural characterisation was performed by nitrogen and carbon dioxide adsorption at 77 and 273 K, respectively, using an automatic adsorption apparatus (ASAP 2020, Micromeritics). Adsorption data were treated using the Microactive® software from Micromeritics. Prior to gas adsorption, the samples were degassed under vacuum at 373 K until the pressure stabilised around 0.2 to 0.4 mPa for more than 48 h. Further degassing was carried out at the measuring port for at least 6 h. Cool and warm volumes were determined after nitrogen or carbon dioxide adsorption to avoid helium entrapment in the narrowest pores.

The pore size distributions (PSDs) were obtained using the non-local density functional theory (NLDFT) from the Solution of Adsorption Integral Equation Using Splines (SAIEUS®) routine and combining N 2 (77 K) and CO 2 (298 K) adsorption data [START_REF] Jagiello | 2D-NLDFT adsorption models for carbon slit-shaped pores with surface energetical heterogeneity and geometrical corrugation[END_REF]. The smoothing parameter λ was set constant and equal to 4.5 for all samples in order to allow comparison between the different PSDs. The average micropore width, L 0, NLDFT (nm) was calculated using this PSD. The mesopore volume was calculated by subtracting the micropore volume obtained from the NLDFT method, V mic NLDFT (cm 3 g -1 ), to the total pore volume, V tot (cm 3 g -1 ), directly measured by N 2 adsorption at relative pressure P/P 0 = 0.99. The NLDFT method was also used to determine the surface area, (m 2 g -1 ), by integrating the PSDs over the whole range of pore sizes [START_REF] Centeno | The assessment of surface areas in porous carbons by two modelindependent techniques, the DR equation and DFT[END_REF].

Water adsorption

Water adsorption experiments were carried out using a 3-Flex (Micromeritics) automatic adsorption apparatus. 0.2 g of each material were first degassed as described in subsection 2.2.3, and dead volumes were determined after adsorption-desorption. Water adsorption isotherms were carried out at 293.15 K and analysed in two ranges of relative pressure. The water uptake in the lowest pressure range (P/P 0 < 0.15) was assumed to be related to the surface chemistry of the material, and essentially to oxygenated groups in agreement with previous studies [START_REF] Bandosz | Effect of surface chemistry on sorption of water and methanol on activated carbons[END_REF][START_REF] Mccallum | A molecular model for adsorption of water on activated carbon: Comparison of simulation and experiment[END_REF][START_REF] Salame | Study of water adsorption on activated carbons with different degrees of surface oxidation[END_REF][START_REF] Do | A model for water adsorption in activated carbon[END_REF][START_REF] Lodewyckx | Suggested improvements in the parameters used for describing the low relative pressure region of the water vapour isotherms of activated carbons[END_REF]. Water isotherms were linearized in the low-pressure range using Henry's law, Eq. ( 2), where Q (cm 3 g -1 ) is the STP amount of adsorbed water, and the asobtained Henry's constants, K, were then compared for the different materials.

(2)

Hydrogen adsorption

Hydrogen adsorption experiments were carried out at 288.15, 298.15 and 308.15 K using a HPVAII automatic adsorption apparatus (Micromeritics). The temperature control was set to 293.15 K for the cold volume measurement and then was changed to 298.15 K for the warm volume measurement and for the experiments. The pressure steps for adsorption were in the range 0.05 to 10 MPa, while those for desorption were in the range of 10 to 2 MPa. A 10 cm 3 cell was used for an average sample amount of 1 g. The contribution of the empty cell was systematically measured and subtracted to all data in order to improve the accuracy. The absolute adsorbed amount was calculated from the excess, using the following equation [START_REF] Schaefer | Physisorption, chemisorption and spill-over contributions to hydrogen storage[END_REF]:

(

where excess N ex and absolute N abs adsorbed amounts are both expressed in , (Pa) is the absolute pressure, (g mol -1 ) is the molecular weight of H 2 , its compressibility factor at the considered pressure and temperature T (K), is the density of liquid hydrogen (70.849 kg m -3 ) and R is the molar gas constant (8.314 J mol -1 K -1 ) [START_REF] Schaefer | Physisorption, chemisorption and spill-over contributions to hydrogen storage[END_REF]. The isosteric heat of adsorption, Q st (kJ mol -1 ), was calculated using the Microactive® software from Micromeritics, based on the Clausius-Clapeyron equation:

(4) applied to the three obtained isotherms. A (weighted) average Q st value was also calculated over the 0 -1 mmol.g -1 range of adsorbed hydrogen amounts.

Results and discussion

Composition and surface chemistry

C, N and K contents can be found in Table S1 of the supplementary information. Usually, the oxygen contained in quinoid carbonyls is named OI and modelled by a Lorentzian-Gaussian (70 % -30 % mix, respectively) peak centred in the 531-531.9 eV range [START_REF] Lakshminarayanan | Nitric acid oxidation of vapor grown carbon nanofibers[END_REF]. Using the same model of peaks, the carbonyl oxygens in esters, anhydrides and hydroxyls can be assigned to the 532.3-532.8 eV range and are usually named OII [START_REF] Lakshminarayanan | Nitric acid oxidation of vapor grown carbon nanofibers[END_REF][START_REF] Zielke | Surface-oxidized carbon fibers .1. Surface structure and chemistry[END_REF][START_REF] Zhou | Characterization of surface oxygen complexes on carbon nanofibers by TPD, XPS and FT-IR[END_REF]. The peak for noncarbonyl oxygen (ether type) in esters or anhydrides is located in the 533.1-533.8 eV range and is named OIII. The peak for oxygen contained in carboxylic acids is assigned to the 534.3-535.4 eV range and is named OIV. One last peak, which concerns adsorbed water or oxygen present on the surface of the material during the XPS analysis is assigned to the 536-536.5 eV range and is named OV [START_REF] Zielke | Surface-oxidized carbon fibers .1. Surface structure and chemistry[END_REF][START_REF] Zhou | Characterization of surface oxygen complexes on carbon nanofibers by TPD, XPS and FT-IR[END_REF]. In our case, the contributions of the OII and OIII peaks were merged due to their very important overlapping and to the similar chemical nature of these groups. Thus, here, OI still stands for the peak assigned to oxygen in the quinoid carbonyl groups, OII stands for the peak assigned to oxygen either contained in esters and anhydrides (in carbonyl groups and ether-like bridges) and in hydroxyl groups, OIII stands for oxygen in carboxyl groups, and O ads * corresponds to adsorbed oxygen (i.e., water or O 2 ), in agreement with other authors [START_REF] Terzyk | The influence of activated carbon surface chemical composition on the adsorption of acetaminophen (paracetamol) in vitro Part II. TG, FTIR, and XPS analysis of carbons and the temperature dependence of adsorption kinetics at the neutral pH[END_REF][START_REF] Park | Effect of KOH activation on the formation of oxygen structure in activated carbons synthesized from polymeric precursor[END_REF]. Due to the high content of inorganic matter, the assessment of the elemental composition using EA does not allow reaching 100 wt. %.

Indeed, the inorganic fraction, which mainly contains potassium carbonate, remains partly undetected when using the CHONS elemental analyser. This undetected inorganic fraction can be composed of K 2 CO 3 (i.e., having survived the combustion of the sample), K 2 O (potassium oxide), KHCO 3 (potassium bicarbonate) or KOOCH (potassium formate). The latter chemical species can be formed from K 2 CO 3 in the presence of CO 2 [START_REF] Tounsadi | Experimental design for the optimization of preparation conditions of highly efficient activated carbon from Glebionis coronaria L. and heavy metals removal ability[END_REF], CO 2 /H 2 O mix and H 2 [START_REF] Ramirez | Tandem Conversion of CO2 to Valuable Hydrocarbons in Highly Concentrated Potassium Iron Catalysts[END_REF]. Potassium can also be volatilised in equivalent processes [START_REF] Tounsadi | Experimental design for the optimization of preparation conditions of highly efficient activated carbon from Glebionis coronaria L. and heavy metals removal ability[END_REF]. Consequently, the missing mass fraction from the EA (up to 15.7 wt. % for the material 180-KOH_UW) was modelled as a fifth compound, in addition to the four aforementioned chemical species. For each model compound and materials, the elemental composition from EA was corrected and presented in Table S2. The comparison of the K contents using EA and XPS is also given in Oxygen contents determined by XPS were always higher than obtained by EA, which indicates a higher amount of O on the surface. Oxygenated functional groups are indeed more likely to be found at the surface due the higher reactivity of the carbon atoms located at the edges of the carbon layers [START_REF] Boehm | Surface oxides on carbon and their analysis: a critical assessment[END_REF] and to the easier contact with the activation agent [START_REF] Figueiredo | Modification of the surface chemistry of activated carbons[END_REF]. Similar reasoning can be applied to potassium. Due to the activation process of olive stones, potassium carbonate is most likely located on the surface of the activated carbon. and in Table S3 (in percentage of oxygen content). The washed materials (180_KOH_W_O3 and 240_KOH_W_O3) have similar ash content to the CO 2 -activated one (CO2_2h). The origin of the ashes in sample CO2_2h resides in the precursor itself. Indeed, it is well known that lignocellulosic biomass contains metal salts, oxides and hydroxides. In the case of olive stones, the metals are mostly K + and Ca 2+ [START_REF] Eliche-Quesada | Olive Stone Ash as Secondary Raw Material for Fired Clay Bricks[END_REF][START_REF] Miranda | Combustion analysis of different olive residues[END_REF].

As no signal in the K2p region was detected for these three materials (180_KOH_W_O3, 240_KOH_W_O3 and CO2_2h), K must be absent from the outer surface. The presence of K (1.9 at. %) in 180_KOH_W might be due to the intercalation of potassium in the graphite-like domains of the AC [START_REF] Enoki | Hydrogen alkali-metal graphite ternary intercalation compound[END_REF]. and EA (O-EA, corrected values) and the ash content; the best (linear) correlation was found between O-EA and ash content. Therefore, O-EA should be more representative than O-XPS, the latter being always higher due to the more oxidised character of the surface. Thus, in the following, O-EA results will be considered as representative of the total oxygen content, while O-XPS results will be considered only for discussing phenomena taking place on the surface. For the sake of comparison with previous studies [START_REF] Schaefer | Assessment of hydrogen storage in activated carbons produced from hydrothermally treated organic materials[END_REF], the O-EA that will be used in the following figures is the uncorrected one (i.e., taken from Table S1). 240_KOH_UW and 180_KOH_W_O3, respectively. All the materials had very similar pore sizes (0.76 nm ± 0.02 nm) while CO2_2h had an average pore width of 0.65 nm and a much lower micropore volume (Figure S5). The washed materials (180_KOH_W, 180_KOH_W_O3 and 240_KOH_W_O3) exhibited much higher surface areas, from 1217 to 1478 m 2 g -1 , than the unwashed ones (180_KOH_UW, 240_KOH_UW), from 536 to 655 m 2 g -1 , likely due to the opening of pores formerly blocked by potassium salts. The surface areas of the washed materials are in agreement with those reported in the literature for OS-derived ACs [START_REF] Bacaoui | Optimization of conditions for the preparation of activated carbons from olivewaste cakes[END_REF][START_REF] Roman | Control of pore development during CO(2) and steam activation of olive stones[END_REF][START_REF] Ubago-Perez | Granular and monolithic activated carbons from KOH-activation of olive stones[END_REF]. Figure S6 shows the exceptionally high fractions of pores narrower than 0.5 nm for the OS-derived materials, compared to those of sucrose-derived ACs [START_REF] Schaefer | Assessment of hydrogen storage in activated carbons produced from hydrothermally treated organic materials[END_REF]. This is due to the high lignin content in OS.

Porous texture and affinity for water

Indeed, differently from hemicellulose and cellulose that are also present in biomass, lignin essentially produces micropores when submitted to carbonisation [START_REF] Montane | Activated carbons from lignin: kinetic modeling of the pyrolysis of Kraft lignin activated with phosphoric acid[END_REF][START_REF] Fierro | Influence of the demineralisation on the chemical activation of Kraft lignin with orthophosphoric acid[END_REF][START_REF] Fierro | Kraft lignin as a precursor for microporous activated carbons prepared by impregnation with ortho-phosphoric acid: Synthesis and textural characterisation[END_REF][START_REF] Borrero-Lopez | High added-value products from the hydrothermal carbonisation of olive stones[END_REF][START_REF] Jeder | The severity factor as a useful tool for producing hydrochars and derived carbon materials[END_REF][START_REF] Rowlandson | Toward Process-Resilient Lignin-Derived Activated Carbons for Hydrogen Storage Applications[END_REF]. and 180_KOH_W materials were type II according to the IUPAC classification [START_REF] Ng | Nanoporous materials with enhanced hydrophilicity and high water sorption capacity[END_REF]; they had an S-shape and presented a strong increase of adsorbed amount at high relative pressure,

indicating the very hydrophilic nature of the materials surface. The water isotherm of 180_KOH_W presented a smooth step at moderate relative pressure that might be associated to a combination of type VI isotherm with an unusual stepwise water adsorption [START_REF] Ng | Nanoporous materials with enhanced hydrophilicity and high water sorption capacity[END_REF]. Those of 180_KOH_W_O3 and 240_KOH_W_O3 were type IV according to the IUPAC classification [START_REF] Ng | Nanoporous materials with enhanced hydrophilicity and high water sorption capacity[END_REF]; the pronounced knee at low relative pressure is indicative of their hydrophilic nature and the multiplicity of adsorption sites for water. The water adsorption isotherm on the CO2_2h material was type V, suggesting the lower hydrophilic character of this material compared to the others [START_REF] Ng | Nanoporous materials with enhanced hydrophilicity and high water sorption capacity[END_REF]. The insert in Figure 4.a) shows the linear fit of the water isotherms in the low-pressure range (P/P 0 < 0.15) in order to obtain the apparent Henry's constant, corresponding to the slope. O content obtained by EA and XPS measurements. The best linear fit was obtained when using the O content from EA, giving relevant information on the bulk. There are indeed important differences between the surface chemistry of the outer surface of carbon particles and that of their inner pores [START_REF] Boehm | Surface oxides on carbon and their analysis: a critical assessment[END_REF][START_REF] Figueiredo | Modification of the surface chemistry of activated carbons[END_REF]. Water adsorption was measured at equilibrium, giving time to water molecules to diffuse towards the narrowest pores and, therefore, experiments lasted more than 10 days. For this reason, EA is more representative than XPS for describing the impact of surface chemistry on adsorption phenomena. Figure S8 shows the very poor correlation between the aforementioned Henry's constant and the textural parameters (i.e., average micropore size and microporous volume). Thus, within the low pressure range (P/P 0 between 0 and 0.2), the water adsorption process is mostly driven by specific interactions between H 2 O molecules and oxygenated adsorption sites and, possibly, carbonates. The surface groups influencing most the adsorption of H 2 O within the low-pressure range are the carboxyls (OIII from Figure 4.c.)). Indeed, acidic groups are well known H-bond donors that make them very attractive for water adsorption and that can explain this trend [START_REF] Fletcher | Role of surface functional groups in the adsorption kinetics of water vapor on microporous activated carbons[END_REF][START_REF] Hamad | Theoretical study of the adsorption of water on a model soot surface: I. Quantum chemical calculations[END_REF]. The poor quality of the linear regression is due to the low amount of such surface groups (OIII < 5.6 at. % on the total atomic content). in agreement with previous results [START_REF] Schaefer | Assessment of hydrogen storage in activated carbons produced from hydrothermally treated organic materials[END_REF][START_REF] Cabria | The optimum average nanopore size for hydrogen storage in carbon nanoporous materials[END_REF]. Cabria et al. indeed suggested that the optimal pore size for hydrogen adsorption on activated carbon at room temperature is around 0.6 nm [START_REF] Cabria | The optimum average nanopore size for hydrogen storage in carbon nanoporous materials[END_REF]. Above 0.4-0.5 cm 3 g -1 , the pore volume in the 0.5-0.7 nm range was almost constant, and thus the increase of pore volume and hydrogen adsorption capacity was due to pores within the 0.7-2 nm range. This increase was less pronounced than the one induced by the increase of pore volume in the 0.5-0.7 nm range. We had already shown [START_REF] Zhao | Optimization of activated carbons for hydrogen storage[END_REF] that, although the optimal pore diameter for hydrogen storage ranges from 0.5 to 0.7 nm [START_REF] Rzepka | Physisorption of hydrogen on microporous carbon and carbon nanotubes[END_REF][START_REF] Gogotsi | Importance of pore size in high-pressure hydrogen storage by porous carbons[END_REF], pores wider than 0.7 nm are essential to reach high hydrogen storage capacities at 77 K. This is due to the necessity of high surface area, which cannot be developed without broadening the PSD. The same conclusion applies to hydrogen storage at room temperature. Some authors suggested to take into account both the average micropore size and the specific surface area to better understand the influence of surface properties on hydrogen adsorption [START_REF] Sevilla | Energy storage applications of activated carbons: supercapacitors and hydrogen storage[END_REF]. According to those authors, dividing the absolute hydrogen adsorbed by the surface area and representing this amount (H 2, abs /S NLDFT ) versus the average micropore size, L 0 , should reveal the influence of surface chemistry on hydrogen adsorption under specific pressure and temperature conditions. Figure 7.a) shows the evolution of H 2, abs /S NLDFT with L 0 , at 2 and 10 MPa: the increase of L 0 produced a decrease of H 2, abs /S NLDFT , whatever the pressure. Indeed, the increase of the average micropore size induced a decrease of adsorption potential, which can be modelled by the Steele's potential [START_REF] Steele | Physical interaction of gases with crystalline solids. 1-Gas-solid interactions and properties of isolated adsorbed atoms[END_REF][START_REF] Steele | Physical interaction of gases with crystalline solids. 2-Dimensional second and third virial coefficients[END_REF].

Hydrogen adsorption

The differences between carbons derived from OS (OS-ACs) and from sucrose (S-ACs) can be attributed either to the surface chemistry or to the PSD. If the average pore size is a good indicator of textural properties, it does not account for the shape of the PSDs. Figure

7.b)

shows that OS-ACs and S-ACs follow the same trends when it comes to compare the microporous volume and L 0 . This shows the necessity of using other tools to compare the PSDs. Thus, the use of the principal component analysis with the R software (FactoMineR® package) allowed performing hierarchical clustering for all the materials. Herein, we considered the pore volumes in the different ranges of pore sizes (i.e., <0.5, 0.5-0.7, 0.7-2 nm and >2nm) as principal components. Here, it is possible to estimate that the PSDs of OS-ACs is less adapted than those of S-ACs for hydrogen adsorption, due to the importance of the narrowest pores (i.e., <0.5 nm) (see again Figures S5 andS6). Indeed, too narrow pores (i.e., below 0.5 nm) are less desirable for hydrogen adsorption in activated carbons than wider ones (i.e., between 0.5 and 0.7 nm). This finding is in agreement with the conclusions of Cabria et al. [START_REF] Cabria | The optimum average nanopore size for hydrogen storage in carbon nanoporous materials[END_REF], and Xia et al. [START_REF] Xia | Enhanced room-temperature hydrogen storage in super-activated carbons: The role of porosity development by activation[END_REF],

according to which the optimum pore size for hydrogen adsorption is around 0.6 nm. In contrast, the changes of L 0 with the O content were not significant for OS-ACs, and therefore the effect of both parameters can be separated. The increase of O content (around 15 wt. %) for this set of samples led to an increase of 26 % of the hydrogen storage capacity.

Although it has been evidenced that intercalated K can improve the hydrogen adsorption properties of the materials through polarised physisorption or chemisorption mechanism [START_REF] Duclaux | Deuterium adsorption in carbon single walled carbon nanotubes doped by lithium and potassium: Adsorption isotherms and in situ neutron diffraction[END_REF][START_REF] Pellenq | Strong physisorption site for H-2 in K-and Li-doped porous carbons[END_REF][START_REF] Enoki | Hydrogen alkali-metal graphite ternary intercalation compound[END_REF], the correlation between H 2, abs /S NLDFT and O content was better than that based on K content (Figure S10), indicating that oxygen must play a major role in the improvement of the surface adsorption properties. In addition, the materials 180_KOH_W, 180_KOH_W_O3 and 240_KOH_W_O3 have been previously washed. Thus, as evidenced above, they do not contain a large amount of potassium, so the observed effect is mostly due to the oxygenated surface groups. Concerning the samples 180_KOH_UW, and 240_KOH_UW, the additional positive effect can be attributed to both the oxygen groups and to the remaining K 2 CO 3 . Since K 2 CO 3 also contains oxygen, the oxygen content is one of the main factors explaining the increase in adsorbed hydrogen amount per unit of surface area. Thus, in the case of these hybrid materials, even if the potassium (either intercalated or present in K 2 CO 3 ) might play a role in such adsorption enhancement, the oxygen content is a better descriptor than the potassium content. This positive effect of oxygen on hydrogen adsorption was also confirmed by Figure 9.

Here, the three parameters H 2, abs /S NLDFT , L 0 , and the oxygen content, are taken into account in the form of a 3D-plot (Figure 9 and their strong electron-acceptor character. When fitting a linear equation to the hydrogen capacities plotted as a function of the amounts of different types of oxygen, we observed that the determination coefficients and the slopes were both maximum when using OIII and O ads *.

As the carboxylic acids are very hydrophilic, we can estimate that there is a strong correlation between the oxygen content due to the OIII type functions and the oxygen content related to O ads *. Thus, the strong electron-acceptor character of acidic functions, and especially carboxylic acids, together with the difference of electronegativity between carbon and oxygen inducing the formation of partial charges and polarised physisorption thus [START_REF] Duclaux | Deuterium adsorption in carbon single walled carbon nanotubes doped by lithium and potassium: Adsorption isotherms and in situ neutron diffraction[END_REF][START_REF] Pellenq | Strong physisorption site for H-2 in K-and Li-doped porous carbons[END_REF], should play a major role on hydrogen adsorption. In addition, it has been evidenced that carboxylic functions can induce surface charge-induced dipoles and surface charge-quadrupole interactions [START_REF] Lalonde | A zwitterionic metal-organic framework with free carboxylic acid sites that exhibits enhanced hydrogen adsorption energies[END_REF]. Moreover, some authors suggest that oxygen contained in the carbon structure promotes the accessibility of hydrogen to the surface [START_REF] Lee | Influence of oxygen-functional groups on carbon replicas for hydrogen adsorption[END_REF]. Thus, if ACs having high oxygen content exhibited lower hydrogen adsorption capacities it was due to o the lack of appropriate textural properties, such as pore size. The isosteric heats of adsorption Q st were calculated using the isosteric method and a set of three isotherms for each material (283.15, 298.15 and 313.15 K). The Q st of two of the OSderived materials are presented in Figure 11.a). The uncertainty in the calculation of Q st calculated by the isosteric method is generally estimated to be about 30 % of the value obtained. For the sake of clarity, the uncertainties have not been presented in the graphs. Q st exhibits a decrease in the very low coverage range, 0-0.2 mmol g -1 , which is due to the saturation of the most attractive adsorption sites when adsorption proceeds. The oscillations of Q st with the surface coverage are due to measurement uncertainties. Although the error may seem quite large, it has been proved that the isosteric method gives similar Q st as those given by the application of the modified Dubinin-Astakhov equation [START_REF] Sdanghi | Application of the modified Dubinin-Astakhov equation for a better understanding of high-pressure hydrogen adsorption on activated carbons[END_REF]. Adsorption calorimetry is also a relevant technique for the determination of Q st . Kloutse et al. [START_REF] Kloutse | Isosteric heat of hydrogen adsorption on MOFs: comparison between adsorption calorimetry, sorption isosteric method, and analytical models[END_REF] compared the Q st values for H 2 adsorption on MOFs and found a good correlation between the values obtained by the isosteric and the calorimetric-volumetric methods. Figure 11.b) shows, for the sake of comparison, the average Q st of all other materials in the 0-1 mmol g -1 range, as a function of L 0 . The uncertainty is, in most cases, between 0.1 and 2.5 kJ.mol -1 . Most Q st values are higher than the theoretical ones and, at given pore sizes, higher than those of S-ACs. Indeed, in the 0.6-1.0 nm pore size range, the theoretical isosteric heats of adsorption are in the 6-7 kJ mol -1 range [START_REF] Schindler | The theoretical maximum isosteric heat of adsorption in the Henry's law region for slit-shaped carbon nanopores[END_REF] whereas those of S-ACs and OS-ACs are in the 7-9 kJ mol -1 and 6-11 kJ mol -1 ranges, respectively. These high values are due to polarised physisorption phenomena. Indeed, the presence of oxygen or alkali induces the formation of partial charges enhancing the adsorption on the materials surface [START_REF] Duclaux | Deuterium adsorption in carbon single walled carbon nanotubes doped by lithium and potassium: Adsorption isotherms and in situ neutron diffraction[END_REF][START_REF] Pellenq | Strong physisorption site for H-2 in K-and Li-doped porous carbons[END_REF].

Finally, the surface groups were modelled using the Avogadro® software (Figure 12). The calculation of the partial charges and the mapping of the electrostatic potential was carried out after a geometric optimization of the local environment of the surface groups, using the Gasteiger-Marsili methodology and a Van-der-Waals surface model [START_REF] Gasteiger | New model for calculating atomic charges in molecules[END_REF]. It can be noticed that the maximum partial charges are obtained for the ester bridge (+0.706 e), anhydride (+0.706 e) and carboxyl (+0.634 e) structures. Thus, if the ester bridges and anhydride structures are excluded, the mapping of electrostatic potential (MEP) and local partial charges explains the good correlation between the hydrogen adsorption properties and the amount of oxygen of type OIII, corresponding to carboxyls that indeed have the strongest partial charges. It also indicates that, for this set of OS-derived samples, the type OII might be mainly composed of hydroxyl and carbonyl groups. As explained above (Figure 10) and according to some authors, the electron-acceptor (H-donor) character of the carboxyl groups could explain such behaviour [START_REF] Duclaux | Deuterium adsorption in carbon single walled carbon nanotubes doped by lithium and potassium: Adsorption isotherms and in situ neutron diffraction[END_REF][START_REF] Pellenq | Strong physisorption site for H-2 in K-and Li-doped porous carbons[END_REF]. Carboxyl groups might promote surface chargequadrupole and surface charge-induced dipole interactions and, therefore, increase the hydrogen adsorption performances [START_REF] Lalonde | A zwitterionic metal-organic framework with free carboxylic acid sites that exhibits enhanced hydrogen adsorption energies[END_REF]. In addition, structures containing carboxyls can form surface alkali metal complexes (e.g. with lithium and sodium) [START_REF] Srinivasu | Quantum chemical studies on hydrogen adsorption in carbon-based model systems: role of charged surface and the electronic induction effect[END_REF]. This suggests that such alkali metal complexes could possibly promote specific interactions between hydrogen and the substrate (i.e., multiple Coulomb and Kubas interactions) [START_REF] Srinivasu | Quantum chemical studies on hydrogen adsorption in carbon-based model systems: role of charged surface and the electronic induction effect[END_REF][START_REF] Kim | Crossover between multipole Coulomb and Kubas interactions in hydrogen adsorption on metal-graphene complexes[END_REF].

Conclusion

Activated and hybrid carbon materials with high oxygen contents, up to 26.6 wt. % according to elemental analysis, and high surface areas, up to 1478 m 2 g -1 , were prepared from olive stones (OS). XPS analysis of oxygen content allowed concluding that, despite the basic nature of the activating agent (KOH), the resultant ACs had an amphoteric nature. We established that acidic functions at the surface of the carbons had a strong influence on water adsorption in the low-pressure range (i.e., P/P 0 < 0.15) but also on hydrogen adsorption.

We have found that hydrogen adsorption is controlled by the following parameters, listed by decreasing order of importance: specific surface area, average micropore size, surface chemistry and shape of the pore size distribution. We have evidenced the influence of the average micropore size, surface chemistry and shape of the pore size distribution by dividing the adsorbed amount by the specific surface area. Comparing OS-ACs with sucrose-based ones (S-ACs), it was observed that OS-derived materials have similar average micropore sizes to S-ACs, but have generally lower hydrogen capacities. This finding was explained by the shape of the pore size distribution that is less adapted than that of S-ACs because of its high fraction of very narrow pores (below 0.5 nm). This trend was evidenced by a statistical (Principal Component) analysis that was run on the different pore volumes (<0.5 nm, 0.5-0.7 nm and 0.7-2 nm).

Even though the alkali metal can play a role in improving the hydrogen adsorption properties, surface chemistry -particularly through the oxygen content -has a strong influence on hydrogen adsorption near room temperature. At 10 MPa, the variation in oxygen content explains most of the 26% increase of adsorbed hydrogen amount per unit of surface area. The acidic functions seem to play an important role in such improvement.

Figure S1 .

 S1 Figure S1. Due to the similarity of results between EA and XPS when only considering

Figure 1 .

 1 Figure 1.a) shows the XPS spectra in the O1S region and Figure 1.b) shows a comparison

Figure 1 :

 1 Figure 1: a) XPS spectra for the O1S region of the materials, b) oxygen content from elemental analysis (EA) versus the one from XPS, c) deconvolution of the O1S spectrum of the 180_KOH_UW material, d) oxygen content in different oxygen-containing functions (XPS) versus total oxygen content obtained by XPS (all the results are expressed with respect to the total mass/quantity of matter).

Figure 1 .

 1 Figure 1.c) shows an example of O1s peak deconvolution for the sample 180_KOH_UW,

Figure 2 .

 2 Figure 2.a) shows the K content as a function of ash content. Without surprise, the

Figure 2 :

 2 Figure 2: a) Potassium content from XPS versus ash content, b) oxygen content from EA (corrected values) or XPS versus ash content.

Figure

  Figure S2 shows the differential thermogravimetric curves of all carbonaceous materials

Figure 2 .

 2 Figure 2.b) shows clear trends between the oxygen contents determined by XPS (O-XPS)

Figure

  Figure S4.a) and b) shows N 2 and CO 2 adsorption isotherms, respectively. All N 2

Figure 3 .

 3 Figure 3.a) shows the PSDs of the OS-derived materials. All showed two peaks in the

Figure 3 :

 3 Figure3: a) Pore size distributions obtained from the NLDFT method for all OS-derived materials and b) Total (V tot ), microporous (V mic ) and mesoporous (V meso ) pore volumes, and micropore and mesopore fractions.

Figures 4 and

 4 Figures 4 and S7 show water adsorption isotherms at 295.13 K. Those of 180_KOH_UW

Figure 4 :

 4 Figure 4: a) H 2 O adsorption isotherms (303.15 K) and corresponding linear regressions in the low-pressure range (insert), b) Apparent Henry's constants versus O content obtained from EA (empty symbols) or XPS (full symbols) and c) apparent Henry's constants versus O content for different types of surface groups (OI, OII, OIII and O ads *).

Figures 4 .

 4 Figures 4.b) and S8 show the as-obtained apparent Henry's constants as a function of the

Figure 5 .

 5 Figure 5.a) shows the hydrogen adsorption isotherms at 298.15 K on all the materials

Figure 5 :

 5 Figure 5: a) Excess hydrogen adsorption (full symbols)desorption (empty symbols) isotherms (298.15 K), and b) comparison between adsorbed excess and absolute adsorbed amounts at 298.15 K and 10 MPa.

Figure 6 .

 6 Figure 6.a) shows the changes of the excess H 2 adsorbed amount with the micropore

Figure 6 :

 6 Figure 6: a) Excess adsorbed hydrogen amount at 298.15 K and at two different pressures (2 MPa and 10 MPa) as a function of micropore volume, and b) dependence between the volume of different ranges of pore sizes (i.e., 0-0.5, 0.5-0.7 and 0.7-2 nm) and the micropore volume.

Figure 7 .

 7 c) shows the results of this clustering and it can be noticed that, regarding the PSDs, OS-ACs and S-ACs belong to two different families.

Figure 7 :

 7 Figure 7: a) Absolute adsorbed amount of hydrogen (298.15 K) per unit of surface area as a function of average micropore size at two different pressures (2 MPa and 10 MPa), b) Microporous volumes as a function of average micropore size, and c) Clustering realised by principal component analysis on the pore volumes corresponding to different ranges of pore sizes (<0.5, 0.5-0.7 and 0.7-2 nm) for all the materials.

Figure 8 .

 8 Figure 8.a) shows the quantity H 2, ads /S NLDFT at 10 MPa for S-ACs and OS-derived

Figure 8 :

 8 Figure 8: a) Absolute adsorbed hydrogen amount per unit of surface area (H 2, abs /S NLDFT ) at 10 MPa and 298.15 K versus oxygen content from elemental analysis, and b) Dependence between average micropore size and oxygen content from elemental analysis (uncorrected data).

  .a)). The correlation circle from the Principal Component Analysis (Figure 9.b)) shows a positive effect of O content on H 2, abs /S NLDFT , while the increase of the average micropore size has a negative effect.

Figure 9 :

 9 Figure 9: a) 3D plot of the absolute adsorbed amount of hydrogen per unit of surface area at 298.15 K and 10 MPa versus the average micropore size and the oxygen content (obtained from EA), and b) Correlation circle obtained from a Principal Component Analysis between the two aforementioned quantities.

Figure 10 :

 10 Figure 10: Correlation between the amounts of the different types of oxygen from XPS and the adsorbed hydrogen amounts per unit of surface area (10 MPa, 298 K)

Figure 11 :

 11 Figure 11: a) Isosteric heats of adsorption of samples 180_KOH_W and 240_KOH_W_O3 (obtained using the 288.15, 298.15 and 308.15 K adsorption isotherms),and b) average isosteric heats of adsorption versus average micropore size compared to the theoretical ones for carbon slit-shaped pores[START_REF] Schaefer | Assessment of hydrogen storage in activated carbons produced from hydrothermally treated organic materials[END_REF] (with red dashed line from Schindler and LeVan[START_REF] Schindler | The theoretical maximum isosteric heat of adsorption in the Henry's law region for slit-shaped carbon nanopores[END_REF]).

Figure 12 :

 12 Figure 12: Mapping of electrostatic potential (MEP) and local charges on oxygen-containing model structures of activated carbons.

Acknowledgement

This study was partly supported by the French PIA project "Lorraine Université d'Excellence," reference ANR-15-IDEX-04-LUE. We gratefully acknowledge the financial support of CHEERS and TALiSMAN projects, funded by FEDER.