Angela Sanchez-Sanchez

Flavia L Braghiroli

M T T Izquierdo

Julien Parmentier

Alain Celzard

V Fierro

Synthesis and properties of carbon microspheres based on tannin-sucrose mixtures treated in hydrothermal conditions

Keywords: Tannin, Sucrose, Hydrothermal Carbonisation, Sustainable Carbons, Supercapacitor electrodes

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

Introduction

Carbon materials are applied to several fields of paramount importance such as catalysis, water and air depollution, and energy storage and conversion such as secondary batteries or supercapacitors. The production of " " from inexpensive carbon sources including lignocellulosic biomass or other by-products contained in food/agricultural waste, is of major importance nowadays. One way of achieving this target and producing functionalised carbon materials is applying hydrothermal carbonisation (HTC) as a low-cost, scalable, and industrially attractive first step of carbonisation prior to pyrolysis (Antonietti and Titirici, 2010;[START_REF] Celzard | Carbon gels derived from natural resources[END_REF][START_REF] Hu | Engineering carbon materials from the hydrothermal carbonization process of biomass[END_REF]. HTC maintains high oxygen content and allows the functionalisation of the resultant materials without the need of additional chemical reagents [START_REF] Unur | Functional nanoporous carbons from hydrothermally treated biomass for environmental purification[END_REF]. The use of moderate temperature and pressure conditions are fundamental for the onset of several reactions to which carbohydrates are submitted such as hydrolysis, dehydration, condensation, aromatisation and polymerisation [START_REF] Sevilla | Chemical and structural properties of carbonaceous products obtained by hydrothermal carbonization of saccharides[END_REF]). An insoluble black solid that contains higher carbon content than in the original material is obtained, and then a second-step of pyrolysis at 900°C can be applied for increasing the electrical conductivity and improving the textural properties of the final materials.

HTC has been applied to a broad range of carbohydrates and biomass: rice grains, oak leaves, pine needles [START_REF] Titirici | A direct synthesis of mesoporous carbons with bicontinuous pore morphology from crude plant material by hydrothermal carbonization[END_REF], microalgae [START_REF] Falco | Renewable nitrogendoped hydrothermal carbons derived from microalgae[END_REF] and prawn shells [START_REF] White | Naturally inspired nitrogen doped porous carbon[END_REF]. Mimosa tannin has also been submitted to HTC. Mimosa tannin is a phenolic material that has been used for the production of several materials such as resins/adhesives [START_REF] Pizzi | Advanced wood adhesives technology[END_REF], foams [START_REF] Tondi | Tannin-based rigid foams: A survey of chemical and physical properties[END_REF][START_REF] Letellier | Preparation and structural characterisation of model cellular vitreous carbon foams[END_REF], gels (Szczurek et al., 2011a(Szczurek et al., , 2011b) ) and recently N-doped carbon materials [START_REF] Braghiroli | Nitrogen-doped carbon materials produced from hydrothermally treated tannin[END_REF](Braghiroli et al., , 2015a)). This is explained by the fact that such condensed (flavonoid) tannins are quite reactive in the presence of aldehydes (e.g. formaldehyde), and another advantage is that they can auto-condensate, especially when heated or in presence of strong mineral acids [START_REF] Pizzi | Tannin-based wood adhesives[END_REF]. Fig. 1 shows the main flavonoid structure of the robinetinidol unit (resorcinol A-ring and pyrogallol B-ring) which constitutes approximately 70% of Mimosa tannin extract [START_REF] Bate-Smith | The phenolic constituents of plants and their taxonomic significance. I. Dicotyledons[END_REF][START_REF] Pizzi | Tannin-based wood adhesives[END_REF]Hemingway et al., 1989;[START_REF] Pizzi | Advanced wood adhesives technology[END_REF].

Figure 1 Various carbonaceous microspheres were obtained after tannin HTC, depending on temperature, reaction time and tannin concentration, and their production was found to obey a first-order kinetics with an activation energy of 91 kJ mol -1 [START_REF] Braghiroli | Kinetics of the hydrothermal treatment of tannin for producing carbonaceous microspheres[END_REF]. The mechanism of microspheres formation proceeds by nucleation, leading to a high number of small particles, followed by growth and coalescence, leading to bigger particles of lower surface area. The hydrochar yield investigated as a function of pH was found to increase from 64.4 % at unmodified pH (around 4.2 for the aqueous tannin solution) to 86.7 % when the pH was decreased to 1. The pH also influenced the morphology and the texture of the hydrochars, those prepared at non-modified pH being powders, whereas those obtained at low pH being stiff monoliths presenting a higher surface area, up to 800 m 2 g -1 after pyrolysis at 900°C. But in all cases, the micropore size distributions were the same (Braghiroli et al., 2015b).

Concerning sugars, [START_REF] Wang | Monodispersed hard carbon spherules with uniform nanopores[END_REF] submitted a sucrose solution (1.5 mol L -1 ) to HTC at 190°C during 5h to produce carbonaceous microspheres with an average diameter of 5 µm and a BET area as high as 400 m 2 g -1 after subsequent pyrolysis at 1000°C. Then, [START_REF] Sevilla | Chemical and structural properties of carbonaceous products obtained by hydrothermal carbonization of saccharides[END_REF] studied the hydrochars obtained after HTC of several saccharides in different conditions of temperature (170-240°C) and reaction time (0.5-15h) concluding that they presented a core-shell structure consisting of a hydrophobic core and a hydrophilic shell containing reactive oxygenated functional groups. Next, [START_REF] Falco | Renewable nitrogendoped hydrothermal carbons derived from microalgae[END_REF] observed the effect of adding glucose to microalgae organisms submitted to HTC, and found that such strategy allowed incorporating nitrogen in the final carbon matrix and also increasing the carbon yield from 18 to 47.5%.

In the present study, we used condensed tannin combined with commercial sugar (sucrose) to see the effects of such mixture on the preparation of carbon materials. Sucrose was selected because it is a commercial product, readily available and not expensive but also for comparison purposes with previous studies [START_REF] Wang | Monodispersed hard carbon spherules with uniform nanopores[END_REF][START_REF] Sevilla | Chemical and structural properties of carbonaceous products obtained by hydrothermal carbonization of saccharides[END_REF].

Various kinds of microspheres were seen by electron microscopy, depending on temperature, pH, and sucrose / tannin ratio. After a subsequent thermal treatment at 900°C, the final carbon materials presented developed textural properties especially at low pH. To understand better the influence of the various chemical and structural characteristics of the present carbons, some of them were tested as electrode materials and the reversibility of the oxygen groups reactions in acidic electrolyte was studied. Such electrochemical studies allowed separating f f w f if we were to start developing supercapacitor electrodes.

Materials and methods

Raw materials

Mimosa tannin, kindly supplied by SilvaChimica, was used in this study. The extraction process and the composition of the tannin extract was described elsewhere [START_REF] Braghiroli | Kinetics of the hydrothermal treatment of tannin for producing carbonaceous microspheres[END_REF].

Hydrothermal treatment

In a typical synthesis, 2.0 g of either tannin (T) alone, sucrose (S) alone, or tannin-sucrose mixture, were dissolved in 16.0 g of water, put inside a glass vial and then into a Teflon-lined autoclave for 24h installed in an oven preheated at 180°C. Different conditions of hydrothermal carbonisation (HTC) were studied, using various fractions of sucrose: 0, 25, 50, 75 and 100 wt. %. The addition of sucrose changed the pH from 4.2 (100% T) to 8 (100% S).

The resultant materials were called HsStT180_u, s and t being the percentages of sucrose and tannin, respectively, and u meaning unmodified pH. A second series of samples was prepared by fixing the pH at 2 by use of pTSA. Those materials were labelled HsStT180_2. The HTC temperatures were also varied, 160 or 200°C, and the corresponding samples were labelled HsStT160_u or HsStT200_u for unmodified pH experiments.

In all cases, the filtrates were recovered, washed with water, and then dried in a vacuum oven at 80°C. Pyrolysis was carried out under nitrogen flow in a tubular furnace heated at 1°C min -1 up to 900°C, and this temperature was hold for 3h. Pristine tannin (0S100T) was also carbonised in the same conditions and labelled C0S100T. In order to distinguish carbon materials from hydrochars w ' nning of the label; for instance, the sample H100T180_2 became CH100T180_2 after pyrolysis at 900°C.

Materials characterisation

Scanning electron microscopy (SEM) observations were carried out with a FEI-Quanta 400 apparatus for investigating the morphology of the samples, which were metallised beforehand.

A series of images was taken for each sample, and only one was provided here as an example.

The purpose was to evaluate the morphology of the samples and to observe general trends in the particle size, depending on the preparation conditions.

Pore texture parameters were obtained by krypton adsorption at -196°C for organic, or non-porous samples obtained after HTC, and by nitrogen adsorption at -196°C for pyrolysed materials, using a ASAP 2020 automatic apparatus (Micromeritics) in both cases. CO 2 adsorption at 0°C was also carried out on pyrolysed samples using a Micromeritics ASAP 2420 apparatus. The following parameters were determined: NLDFT surface area, S NLDFT (m 2 g -1 ); BET area, A BET (m 2 g -1 ); total pore volume, V 0.97 (cm 3 g -1 ); micropore volumes, V DR,N2

and V DR,CO2 (cm 3 g -1 ), from the N 2 or CO 2 adsorption isotherms; micropore volume, V NLDFT (cm 3 g -1 ), by applying the 2D-NLDFT-HS to both CO 2 and N 2 adsorption isotherms; and mesopore volume, V mes (cm 3 g -1 ), calculated as the difference V 0.97 -V DR,N2 . The pore size distributions (PSDs) were calculated by applying the 2D-NLDFT-HS method to both CO 2 and N 2 adsorption isotherms. More details are given elsewhere [START_REF] Schaefer | Assessment of hydrogen storage in activated carbons produced from hydrothermally treated organic materials[END_REF].

Ultimate analyses of samples were carried out in a CHONS elemental analyser (Vario El cube, Elementar) as reported elsewhere [START_REF] Celzard | Floating hollow carbon spheres for improved solar evaporation[END_REF]. Temperature-Programmed Desorption (TPD) experiments were performed with a custom-built set-up, consisting of a tubular quartz reactor placed inside an electric furnace. In a typical experiment, the sample was heated up to 1100°C in Ar atmosphere at a heating rate of 10°C min -1 . The amounts of CO and CO 2 were determined as reported elsewhere [START_REF] Tellez-Juárez | Hydrogen storage in activated carbons produced from coals of different ranks: Effect of oxygen content[END_REF].

X-ray photoelectron spectroscopy (XPS) results were recorded with an ESCAPlus OMICROM system equipped with a hemispherical electron energy analyser. Measurements were carried out as reported elsewhere [START_REF] Sanchez-Sanchez | Ordered mesoporous carbons obtained by soft-templating of tannin in mild conditions[END_REF].

Electrochemical characterisation was performed with a Biologic VMP3 multi-potentiostat.

The carbon electrodes were prepared by spreading onto a graphite foil a paste (~15 mg cm -2 ) made of the carbon material mixed with polytetrafluoroethylene (PTFE) binder and carbon black in the weight percent ratio of 85:10:5. The resulting electrodes were then pressed at 5

MPa and impregnated by 1 mol L -1 H 2 SO 4 electrolyte during 48 hours. Cyclic voltammetry (CV) tests were carried out at different scan rates between 0.5 and 100 mV s -1 and within the potential window of 0 -0.8 V in a three-electrode cell, using a platinum gauze as the counter electrode and a saturated calomel electrode (SCE) as the reference one. The specific capacitance (C, F g -1 ) was calculated form the CV curves according to Eq. ( 1):

(1)

where I (A) is the current, s (V s -1 ) is the scan rate, V (V) is the potential window and m (g) 167 is the mass of carbon in the electrode. 168

Results and discussion

Carbon yields and morphologies

To study the effects of the pH on the tannin / sucrose solution submitted to HTC, the same weight (1 g) of sucrose and tannin was used. Table 1 shows the main characteristics of these hydrochars (H50S50T) at different pH ranging from 1 to unmodified (4.2). Unlike H0S100T (100% tannin submitted to HTC -see Fig. 2), submitting 50S50T to HTC at different pH did not produce any change in the HTC yield. Being at the level of 61.5 ± 1.5 wt. %, the HTC yield of 50S50T was also always much lower than that of H0S100T. The A BET values (around 1 m 2 g -1 ) were almost the same for 0S100T, H0S100T and H50S50T, considering the intrinsic uncertainties of adsorption techniques as soon as very low surface areas are concerned.

Therefore, it can be concluded that, after HTC, the materials were not porous.

After a second thermal treatment at 900°C, A BET increased from 471 m 2 g -1 for tannin directly pyrolysed (C0S100T) to 639 m 2 g -1 for CH50S50T 180_u (i.e., at unmodified pH), and even up to 809 m 2 g -1 for CH50S50T 180_1 (i.e., at pH 1). The increase of A BET was also observed for the 100% tannin material (CH0S100T) having been submitted to HTC at low pH. Fig. 2 summarises the results of HTC yield and A BET of all these hydrochars H0S100T and CH0S100T (already demonstrated in (Braghiroli et al., 2015b)) and H50S50T and CH50S50T at pH values ranging from 4 to 1. The results on pure tannin are presented here for the sake of comparison to study the effect of sucrose addition.

Figure 2 It has already been mentioned that heating tannin in the presence of strong mineral acids favours its condensation/polymerisation reactions by different possible ways to form " " " ", which are high-molecular weight, insoluble molecules [START_REF] Pizzi | Tannin-based wood adhesives[END_REF]Braghiroli et al., 2015a). Tannin treated after HTC at 180°C was shown to have a structure similar to that of flavonoid tannin (see Fig. 1), but presents a much higher level of polymerisation, mainly through the opening of the heterocyclic C2-ring and subsequent partial condensation between C2 (open) to C6 or C8, and partly through some autocondensation C4 to C6 and C8 (Braghiroli et al., 2015a).

On the other hand, sucrose submitted to HTC undergoes hydrolysis that produces glucose and fructose, which decompose into hydroxymethylfurfural (HMF) as the main dehydration product, plus aldehydes (formaldehyde, pyruvaldehyde, etc.) and organic acids (acetic, formic, levulinic acids, etc.), hence producing a rapid pH change from 8 to 3 as shown in the literature [START_REF] Antal | Mechanism of formation of 5-(hydroxymethyl)-2-furaldehyde from d-fructose and sucrose[END_REF][START_REF] Ulbricht | A review of 5-hydroxymethylfurfural (HMF) in parenteral solutions[END_REF][START_REF] Asghari | Acid-catalyzed production of 5-hydroxymethyl furfural from d-fructose in subcritical water[END_REF]. Glucose and fructose can also undergo polymerisation and condensation reactions as reported elsewhere [START_REF] Antal | Mechanism of formation of 5-(hydroxymethyl)-2-furaldehyde from d-fructose and sucrose[END_REF][START_REF] Kabyemela | Kinetics of glucose epimerization and decomposition in subcritical and supercritical water[END_REF][START_REF] Luijkx | The role of deoxyhexonic acids in the hydrothermal decarboxylation of carbohydrates[END_REF][START_REF] Sevilla | Chemical and structural properties of carbonaceous products obtained by hydrothermal carbonization of saccharides[END_REF]. [START_REF] Titirici | Hydrothermal carbon from biomass: a comparison of the local structure from poly-to monosaccharides and pentoses/hexoses[END_REF] observed that the morphologies and chemical structures of carbons obtained from sucrose are directly related to those of carbons obtained from pure furan. It was also found that such intermediate molecules derived from saccharide dehydration as mentioned above coexist with furans and account for the observed changes of particle size, textural properties and particles aggregation.

When both materials (tannin and sucrose) coexist, we can assume that several reactive germs are obtained through hydrolysis of sucrose submitted to HTC. SEM photos show that the average microsphere diameter was reduced from 10 µm for H0S100T (see Fig. 3a) to 2-5 µm by simple addition of sucrose (see Fig. 3c) for H50S50T, whereas 100% sucrose produced spheres of average size 1-2 µm (see Fig. 3b). This finding suggests that sucrose reacts faster than tannin, producing more germs and, consequently, smaller spheres are obtained (see Fig. 3c). A BET (m 2 g -1 )

V 0.97

(cm 3 g -1 )
V DR,N2

(cm 3 g -1 )

V DR,CO2

(cm 3 g -1 )

V μ,NLDFT A similar effect was found by decreasing the pH down to 2. Small spherical particles of around 2 µm arranged in a necklace structure were indeed obtained in carbons based on either pure tannin (see Fig. 3d) or tannin-sucrose mixture (compare Fig. 3c,3f,3g and 3h) submitted to HTC at pH 2. No changes in particle diameter were observed, as their size was nearly constant around 2 µm, irrespective to the HTC temperature: 160, 180 or 200°C. This finding at lower pH is different from what was formerly observed for H0S100T at unchanged pH and 160°C, for which irregular spheres were found, indicating that HTC reaction was not finished.

(cm 3 g -1 ) C H N S O C/O No HTC 0S100T - - - - 0.27 - - - - 53
By increasing the HTC temperature from 180 to 200°C, bigger and rounded spheres were observed [START_REF] Braghiroli | Kinetics of the hydrothermal treatment of tannin for producing carbonaceous microspheres[END_REF].

Figs. 3b and 3e show SEM pictures of carbon materials derived from 100S0T submitted to HTC at unchanged pH (pH around 4) or at pH 2. HTC of 100S0T at unchanged pH produced y z f x y 1 μ agreement with other authors [START_REF] Titirici | Hydrothermal carbon from biomass: a comparison of the local structure from poly-to monosaccharides and pentoses/hexoses[END_REF][START_REF] Sevilla | Chemical and structural properties of carbonaceous products obtained by hydrothermal carbonization of saccharides[END_REF]. However, sucrose hydrolysis at pH 2 produced bigger spheres of around 5-10 µm. [START_REF] Zhao | Carbon spheres obtained via citric acid catalysed hydrothermal carbonisation of cellulose[END_REF] observed the effect of the acidic pH on the formation mechanism of carbon spheres when submitting cellulose to HTC. The compounds obtained from cellulose hydrolysis rapidly react with H + present in the system through dehydration and polymerisation reactions and lead to hydrochar nucleation. Besides that, they concluded that the acid concentration in the HTC system could also control the shape and the diameter of the carbon spheres.

Figure 3 Table S1 shows the main characteristics of hydrochars and carbon materials prepared with 50% of sucrose (50S50T) at different HTC temperatures (160, 180 and 200°C) and different pH. Some of these results are summarised in Fig. 4 including the total carbon yield and the A BET as a function of HTC temperature at unchanged pH or at pH 2. The HTC yield was around 60% irrespective to the temperature or the pH. After pyrolysis, the total carbon yield appears to be constant, around 30%, whatever the HTC temperature or the pH used beforehand. In the same way, A BET did not change with HTC temperature but was higher for samples prepared at pH 2. Indeed, carbons prepared from hydrochars obtained at pH 2 presented an A BET of 735 ± 32 m 2 g -1 , while those prepared at unchanged pH had an A BET of 629 ± 10 m 2 g -1 .

Figure 4 In order to study the effect of the percentage of sucrose on the hydrochar characteristics, hydrochars based on various proportions of tannin and sucrose were prepared at unchanged pH and at pH 2, summarised in Table S2. We can observe by SEM photos in Fig. 5a to 5c that, by increasing the weight fraction of sucrose from 25 to 100% at unchanged pH, the f 10 2 μ nd they became more individualised and more monodispersed. Polydispersity and higher particle size thus appear to be a consequence of tannin presence in the system at unchanged pH. In the literature, changes of particles morphology produced by mixing two carbon precursors submitted to HTC were also evidenced for microalgae in the presence of glucose added in a mass ratio of 1:1. In that case, the final particles morphology was that of pure glucose but with a double higher average diameter, passing from 1 2 μ [START_REF] Falco | Renewable nitrogendoped hydrothermal carbons derived from microalgae[END_REF]. Coming back to the present situation, the effect of adding sucrose at pH 2 was opposite to that observed at unchanged pH. With the addition of sucrose at lower pH, the average sphere diameter increased from 1-2 μ to 2-3 μ and finally to around 5-10 μ when the proportion of sucrose increased from 25% (Fig. 5d), to 75% (Fig. 5d) until 100% (Fig. 5f), respectively.

Figure 5 Fig. 6a shows the HTC yield of all materials at different proportions of sucrose / tannin and at two pH, unchanged and 2. The dotted lines represent the theoretical yield calculated from the experimental yields of 0S100T and 100S0T (e.g. for 75S25T we have Y HTC, 75S25T = (75  Y HTC,100S0T / 100 + 25  Y HTC,0S100T / 100), in order to observe any synergetic effect between tannin and sucrose during HTC. At pH 2, the experimental HTC yield was very similar to the expected value, so we can conclude that there is no apparent interaction between sucrose and tannin regarding the yield. Acidic pH accelerates HTC reactions, both for sucrose and tannin precursors [START_REF] Lee | Dilute-Acid Hydrolysis of Lignocellulosic Biomass[END_REF][START_REF] Kim | Cellulose hydrolysis under extremely low sulfuric acid and high-temperature conditions[END_REF][START_REF] Trajano | Fundamentals of Biomass Pretreatment at Low pH[END_REF]) and no differences were observed. At unmodified pH, experimental HTC yield of tannin-sucrose mixtures were higher than those theoretically calculated, suggesting that HTC products of sucrose and tannin interact with each other. At pH around 4, sucrose yielded more solid materials. Its higher HTC kinetics compared to tannin led to more germs with a resulting lower particle size and an enhanced HTC yield. Since at pH 2 the experimental HTC yield was the same as calculated, it can be assumed that a low pH promotes fast reactions to liquid products [START_REF] Lee | Dilute-Acid Hydrolysis of Lignocellulosic Biomass[END_REF][START_REF] Kim | Cellulose hydrolysis under extremely low sulfuric acid and high-temperature conditions[END_REF][START_REF] Trajano | Fundamentals of Biomass Pretreatment at Low pH[END_REF] that do not interact with tannin. 6a but after pyrolysis at 900°C. As a general remark, the addition of sucrose to tannin slightly increased A BET . The highest A BET was found for the mixture 50S50T at both pH, but the main effect on the increase of surface area was due to the pH reduction. Fig. 7 shows that the product A BET  total carbon yield increased with the total carbon yield. This effect is opposite to what is obviously found for carbon activation, whether physical or chemical, which produces higher surface area when the carbon yield decreases, i.e., when the burn-off increases. Besides, by increasing the proportion of tannin mixed with sucrose in HTC conditions, an increase of the total carbon yield can be seen. However, the highest A BET was obtained at equal proportion (50:50) of T and S at pH 2. Such increase of surface area, from 476 to 791 m 2 g -1 , cannot be explained by the decrease of particle diameter from 10 µm (CH0S100T180_u) to approximately 2 μ (CH50S50T180_2), which accounts for less than 1% of the measured A BET . As previously discussed in Braghiroli et al. (2015b), the decrease of particle size might facilitate the evolution of volatile matter and then the development of textural properties.

Figure 7 All materials were found to be microporous, given their Type I nitrogen isotherms at -196°C, having a vertical part at low relative pressure P/P 0 and an almost horizontal part from P/P 0 = 0.02 to 1 as shown in Fig. 8a for CH50S50T180_2 and CH50S50T180_u. The pore size distribution calculated by NLDFT is presented in Fig. 8b for the same samples;

CH50S50T180_2 had some porosity in the mesopore range (between 2 and 10 nm), whereas

CH50S50T180_u presented an entirely microporous structure. The isotherm of sample prepared at pH 2 presented a slight slope showing the presence of pores broader than 2 nm.

Most of the carbohydrates submitted to HTC needed a second thermal treatment, at higher temperature, to increase the porosity of the resultant carbon materials. However, glucose after HTC and subsequent pyrolysis at 900°C was poorly porous and had low surface areas, around 30-50 m 2 g -1 [START_REF] Zhao | Sustainable nitrogen-doped carbonaceous materials from biomass derivatives[END_REF]. Thanks to the reactivity of tannins in hydrothermal conditions and posterior pyrolysis at 900°C, highly porous material were obtained (A BET > 600 m 2 g -1 ) confirming its potential as precursor for the production of porous carbons.

Figure 8 As for the chemical composition, Fig. 9a shows the van Krevelen diagram for all hydrochars and carbon materials prepared from tannin and sucrose at different pH and HTC temperatures (160, 180 and 200°C). The main arrow (from 100S0T to HTC_900°C, i.e., from pure sucrose to all carbons obtained by HTC followed by pyrolysis at 900°C) indicates the loss of oxygen and hydrogen, and therefore the increase of carbon content after both thermal treatments (HTC at 160-200°C and pyrolysis at 900°C) [START_REF] Van Krevelen | Graphical-statistical method for the study of structure and reaction processes of coal[END_REF]. The percentage of carbon increased after HTC and obviously even more after pyrolysis. After HTC, samples prepared at unmodified pH retained more oxygen, 30-36 wt. %, than those prepared at pH 2, 28-33 wt. %. After pyrolysis at 900°C, carbon derived from hydrochars prepared at unmodified pH retained more oxygen, 6-10 wt. %, than those prepared at pH 2, 3-6 wt. %.

Fig. 9b shows the total amounts of CO and CO 2 gases in mmol g -1 that have been lost

during a thermal treatment (TPD analysis) up to 1100°C for materials at different T/S proportions. CO 2 derives from precursors comprising carboxylic acids, lactones and anhydride groups, and CO derives from quinonic and phenolic groups [START_REF] Figueiredo | Modification of the surface chemistry of activated carbons[END_REF]. Boudouard equilibrium also takes place, modifying the CO/CO 2 ratio. The total amount of released gases (CO + CO 2 ) was higher for materials HsStT_u prepared without pH modification (0.065-0.075 mmol g -1 ) than for HsStT_2 prepared at pH 2 (0.036-0.052 mmol g -1 ). This is in good agreement with their higher amount of oxygen, 32.4-36.3 wt. %, compared to 30.6-32.7 wt. % in HsStT_2 materials, measured by EA (Table 1 and Table S1).

Indeed, tannin in the presence of strong acids undergoes dehydration and condensation reactions [START_REF] Pizzi | Tannin-based wood adhesives[END_REF]Braghiroli et al., 2015a), which might have reduced the amount of oxygen in hydrochars. C* appeared at an unusually low BE, 283.6 eV. This peak may be associated to pre-graphitelike C in highly aromatic materials (Cagniant et al., 1998;[START_REF] Weckhuysen | Characterization of surface carbon formed during the conversion of methane to benzene over Mo/H-ZSM-5 catalysts[END_REF].

The O1s high-resolution spectra of the hydrochars presented, in general, two oxygen contributions (Table 2): O1, assigned to C=O double bonds in quinone-type groups, carbonyls and carboxylic acids; and O2, related to -OH bonds in phenols, C-O-C in ether groups and C=O bonds in ester and anhydride groups. The O1s high-resolution spectra exhibited generally O1, O2 and O3 contributions, O3 being assigned to chemisorbed oxygen, single C-O bonds in esters and anhydrides, and C-O bonds in carboxylic groups and/or water.

Finally, the deconvolution of the N1s high-resolution spectra was only possible for samples with surface nitrogen concentrations higher than 1 at. %. Curve fitting was not possible for the N1s high-resolution spectrum of H50S50T180_3 hydrochar, due to the low area of the peak. N1s high-resolution spectra of both hydrochars and carbons exhibited three contributions assigned to three types of functional groups: N6, related to pyridinic nitrogen; N5, related to pyrrolic nitrogen; and NQ, assigned to quaternary nitrogen [START_REF] Briggs | Structural characterization of nitrogen-enriched coals[END_REF][START_REF] Dongil | Surface chemical modifications induced on high surface area graphite and carbon nanofibers using different oxidation and functionalization treatments[END_REF][START_REF] Weidenthaler | X-ray photoelectron spectroscopic studies of PAN-based ordered mesoporous carbons (OMC)[END_REF].

In general, pyrolysis decreased the relative concentrations of the groups related to CII and OII, while increasing the relative amounts of the groups related to the peaks CIII, CIV and OI.

This finding means that the concentration of ethers, phenols, esters and anhydrides decreased whereas the concentration of quinone-type groups, carbonyls and carboxylic acids increased as a consequence of pyrolysis.

Concerning the structure of the carbons obtained by pyrolysis at 900°C of the aforementioned hydrochars, it can be added that none of these materials are graphitic, as evidenced by their XRD patterns showing very broad and poorly defined ( 002) and ( 10l) bands (see for instance [START_REF] Tondi | Tannin-based rigid foams: A survey of chemical and physical properties[END_REF] and the supplementary information of (Celzard et al., 2019) for typical patterns of carbons based on tannin and tannin-sugar produced at 900°C). The corresponding Raman spectra demonstrated the same, but also that these materials are not graphitisable either, unless transition metals are added as graphitisation catalyst, which was not the case here. Typical Raman spectra of carbons based on tannin and tannin-sugar produced at 900°C can be found elsewhere [START_REF] Celzard | Floating hollow carbon spheres for improved solar evaporation[END_REF][START_REF] Braghiroli | Hydrothermal Treatment of Tannin: A Route to Porous Metal Oxides and Metal/Carbon Hybrid Materials[END_REF], and prove that the present materials are definitely highly disordered, i.e. very poorly crystallised, carbons.

Concerning the morphology, carbon spheres have a wide range of applications, depending on their diameter, pore texture and surface chemistry. For example, they can be used in batteries [START_REF] Park | Carbon microspheres with well-developed micro-and mesopores as excellent selenium host materials for lithium-selenium batteries with superior performances[END_REF] or supercapacitors [START_REF] Zhou | Ultrahigh volumetric capacitance and cyclic stability of fluorine and nitrogen co-doped carbon microspheres[END_REF], as catalysts [START_REF] Frusteri | Carbon microspheres preparation, graphitization and surface functionalization for glycerol etherification[END_REF][START_REF] Maciá-Agulló | Synthesis of Carbon-based Solid Acid Microspheres and Their Application to the Production of Biodiesel[END_REF] or even as boosters to produce pure water as we reported recently [START_REF] Celzard | Floating hollow carbon spheres for improved solar evaporation[END_REF]. 1.

With respect to surface chemistry, the most remarkable difference in their surface chemistry is that CH50S50T180_u exhibited higher surface oxygen content than the other two carbons, and that CH50S50T180_1 exhibited higher relative concentrations of quinone and carbonyl groups (OI peak). The latter are the most electroactive oxygen functionalities in acidic electrolytes [START_REF] Inagaki | Carbon materials for electrochemical capacitors[END_REF]. Choosing such materials for electrochemical tests was therefore supposed to clarify the understanding of these new carbon materials.

At the lowest scan rate of 0.5 mV s -1 , cyclic voltammetry (CV) curves of C0S100T and CH50S50T180_1 exhibited more defined peaks of both anodic and cathodic currents than CH50S50T180_u despite possessing a lower concentration of oxygen on their surfaces (Fig. 10a). The presence of redox peaks is due to a contribution of the surface oxygen groups to the total capacitance of the electrodes by a pseudocapacitance mechanism. Even at this low scan rate of 0.5 mV s -1 , CV curves of CH50S50T180_u exhibited a highly distorted profile with respect to the rectangular shape of an ideal electric-double layer capacitor (ELDC). This fact may indicate the existence of kinetic limitations for ion transport, and agrees with the highly microporous texture of the carbon, whose percentage of narrow porosity (width < 0.7 nm) was the highest of the three materials, 84.0%, and whose percentage of mesoporosity was the lowest one, 0.0%. At the relatively low scan rate of 10 mV s -1 , the shape of the CV curves of all the materials clearly deviated from the rectangular one, and no clear peak could be seen (Fig. 10b). This suggests that the behaviour of the supercapacitor electrodes becomes less capacitive and more resistive, due to kinetic limitations, and that pseudocapacitance contribution is negligible, respectively [START_REF] Mcdonough | Influence of the structure of carbon onions on their electrochemical performance in supercapacitor electrodes[END_REF].

The studied electrode materials yielded similar values of specific capacitance in the entire range of investigated scan rates. At the lowest scan rate of 0.5 mV s -1 , specific capacitances of 51, 57 and 58 F g -1 were exhibited by C0S100T, CH50S50T180_1 and CH50S50T180_u, respectively (Fig. 10c). These capacitance values progressively decreased when the scan rate increased, due to the increasing resistance to diffusion of electrolyte ions into the pores. At 100 mV s -1 scan rate, the specific capacitance was 15 F g -1 for all the materials (Fig. 10c), which corresponds to capacitance retentions of 30.4, 26.6 and 26.3% for C0S100T, CH50S50T180_1 and CH50S50T180_u, respectively (Fig. 10d). The lower capacitance retention of CH50S50T180_1 and CH50S50T180_u compared to C0S100T indicates the existence of more important kinetic limitations at increasing scan rates due to their high fractions of narrow porosity, 71.4 and 84.0%, respectively. The CV results agree with a previous study on molasses-derived carbons, in which the materials prepared by HTC exhibited, in the studied potential window, lower capacitances than those that were directly pyrolysed, mainly due to their narrow porosities and low electrical conductivities [START_REF] Sanchez-Sanchez | Sugarcane molasses as a pseudocapacitive material for supercapacitors[END_REF].

Figure 10

In order to determine the pseudocapacitance contribution to the overall capacitance of the materials, interfacial capacitances at different scan rates, C / S NLDFT , were plotted versus oxygen surface concentrations (Fig. S7a of the Supporting Information section). The interfacial capacitance was calculated by considering S NLDFT instead of A BET , since it has been previously shown that this approach is more appropriate for materials mainly comprising narrow pores, such as in the present case [START_REF] Sanchez-Sanchez | Sugarcane molasses as a pseudocapacitive material for supercapacitors[END_REF]. In general, higher surface oxygen concentrations were not related to higher interfacial capacitances in the entire range of studied scan rates. This should mean that the pseudocapacitance contribution of the materials is lower than the capacitive contribution, for which the porous texture is the key parameter.

Oxygen functional groups have a great influence on the electrochemical properties of carbon materials [START_REF] Anjos | Pseudocapacitance and performance stability of quinone-coated carbon onions[END_REF][START_REF] Tanaka | Surface modification of soft-templated ordered mesoporous carbon for electrochemical supercapacitors[END_REF]. Hydroxyl, carbonyl and, especially, quinone groups, have been proved to yield more capacitance than carboxyl, anhydride or lactone groups in acidic aqueous electrolytes thanks to their Faradaic reactions [START_REF] Inagaki | Carbon materials for electrochemical capacitors[END_REF]. Redox reactions involving these groups are the following [START_REF] Oh | Oxygen functional groups and electrochemical capacitive behavior of incompletely reduced graphene oxides as a thin-film electrode of supercapacitor[END_REF]:

Hydroxyls:

Carbonyls:

Quinones:

Carboxylic acids:

where the symbol > represents the surface of the carbon material. Protons, H + , are not only simply adsorbed on the oxygen functional groups, but are involved in the electrochemical reactions.

As a consequence of these Faradaic reactions, redox peaks can appear on the anodic and cathodic branches of the CV curves of the carbon materials. CV curves obtained in a threeelectrode cell in 1 mol L -1 H 2 SO 4 electrolyte, measured at 0.5 mV s -1 scan rate and considering the first cycle of cycling, are displayed in Fig. 11. These curves exhibited two redox processes associated to peaks 1 and 2 appearing in the anodic and cathodic branches, which are represented y " x" " " are defined by two parameters:

(i) peak current, I p , which is the current at the maximum of the peak; and (ii) peak potential, E p , which is the potential at the maximum of the peak (Fig. 11a, 11c and 11e).

Figure 11 For the samples C0S100T and CH50S50T180_1, no second anodic peak, I p,2 ox , was seen, indicating the irreversibility of the associated redox process. In contrast, the first peak could be clearly observed both in the anodic and cathodic branches and appeared in the five cycles studied at scan rates from 0.5 to 5 mV s -1 for C0S100T, and from 0.5 to 2 mV s -1 for CH50S50T180_1 and CH50S50T180_u. This suggests that the associated redox system may be quasi-reversible in the aforementioned scan rate intervals. The highly distorted CV curves with respect to the expected rectangular shape of an ideal electric-double layer capacitor at scan rates above 2 or 5 mV s -1 did not allow differentiating clear redox peaks. Therefore, the reversibility of the redox system 1 was studied up to a maximum scan rate of 5 mV s -1 . The formal potentials of peak 1 vs SCE reference, calculated as E 0 = 1/2 (E p,1 ox + E p,1 red ), were 0.425, 0.424 and 0.444 V for C0S100T, CH50S50T180_1 and CH50S50T180_u, respectively. The reversible character of the redox process was evaluated through the following criteria:

1) Peak separation, E = E p,1 ox -E p,1 red  0.059 / n, where E p,1
ox and E p,1 red (V) are the maxima of the anodic and cathodic peaks, respectively, and n is the number of transferred electrons: Considering the transfer of 1 electron, the peak separation was lower than 0.059 V at the lowest scan rate of 0.5 mV s -1 for the three materials, suggesting the reversibility of the redox process. E generally increased above 0.059 V as the cycle number increased, which indicates that the redox process was progressively less reversible due to the gradual transformation of surface groups into non-electroactive species. However, E was progressively lower than 0.059 V at the scan rate of 0.5 mV s -1 for the sample C0S100T, suggesting a higher reversibility of the redox process (Fig. 11b,11d and 11f). This can be explained by the improvement of the surface wettability during the cycling process, which facilitates the contact between the surface functionalities and the electrolyte ions, so that redox reactions are easier to occur, and also by the higher fraction of pores involved in the materials as the cycling process goes on [START_REF] Jiang | Highly ordered macroporous woody biochar with ultra-high carbon content as supercapacitor electrodes[END_REF]. Varying the scan rate in cyclic voltammetry allows varying the diffusion layer thickness, thus enabling the control of the electrochemical process either by electron transfer or by mass transport. The diffusion layer is thicker at low scan rates, and thinner as the scan rate increases. Electrochemical processes reflect the competition between reactions at the electrodes and diffusion, so that faster scan rates favour electrochemical irreversibility, which is controlled by the electron transfer rate, and the peak potentials are thus shifted. This trend is clearly observed in Fig. S7b of the Supporting Information section, which shows the progressive separation of maxima of the peaks associated to the redox process 1 at increasing scan rates from 0.5 to 2 mV s -1 for a representative sample, C0S100T (McDonough et al., 2012). When the peak potentials remain constant with increasing scan rates, the involved redox process is reversible. Fig. 12b, 12d and 12f show the progressive increase of the peak potentials of the studied samples, E p ox,red , at increasing scan rates, ln s, indicating the quasi-reversibility of the involved redox system [START_REF] Gómez | Graphene-conducting polymer nanocomposite as novel electrode for supercapacitors[END_REF].

2) Ratio of anodic to cathodic currents, I p,1 ox / I p,1 red  1, where I p,1 ox and I p,1 red (A) are the currents at the maximum of the anodic and cathodic peaks, respectively: When the peak currents, I p , for the oxidation and reduction reactions are the same, i.e., I p,1 ox / I p,1 red = 1, the redox process is reversible [START_REF] Bard | Electrochemical methods: fundamentals and applications[END_REF]. The current ratios of the studied samples at scan rates ranging from 0.5 to 2 mV s -1 were 0.98  0.04, which indicates the quasi-reversibility of the redox process.

3) Scan rate dependence of peak current, I p ox,red  s 1/2 , where I p ox,red (A) is the current at the maximum of the anodic or the cathodic peaks and s 1/2 (V 1/2 s -1/2 ) is the square root of the scan rate: A linear relationship between I p ox,red and s 1/2 indicates that the redox process is quasi-reversible [START_REF] Gómez | Graphene-conducting polymer nanocomposite as novel electrode for supercapacitors[END_REF] As shown in Fig. 12a, 12b and 12c, I p ox,red versus s 1/2 leads to linear relationships for the studied samples, with determination coefficients close to 1, thus indicating that the concerned redox processes are quasi-reversible. From these linear relationships and applying the Randles-Sevcik equation (Eq. 2), the diffusion coefficients, D 0 , of the processes were calculated [START_REF] Mcdonough | Influence of the structure of carbon onions on their electrochemical performance in supercapacitor electrodes[END_REF]. The Randles-Sevcik equation reads:

(2)

where I p (A) is the peak current, n is the number of electrons transferred in the reaction, F (C mol -1 ) is the Faraday constant, A (cm 2 ) is the electrode area, C (mol L -1 ) is the concentration of the diffusing species in the bulk of the electrolyte, D 0 (cm 2 s -1 ) is the corresponding diffusion coefficient, s (V s -1 ) is the scan rate, R (J mol -1 K -1 ) is the universal gas constant and T (K) is the temperature. Considering that n = 1 and C = 1 mol L -1 (H 2 SO 4 ), the diffusion coefficients were found to be 1.602  10 -13 , 1.156  10 -13 and 6.973  10 -14 cm 2 s -1 for C0S100T, C50S50T180_1 and C50S50T180_u, respectively. These values are considerably lower than those obtained for well-established redox processes in other doped carbons used as electrode materials, such as graphene-polyaniline (PANI) nanocomposites or PANI systems alone, suggesting a hindered ion transport [START_REF] Gómez | Graphene-conducting polymer nanocomposite as novel electrode for supercapacitors[END_REF]. Therefore, activation for opening the porosity and developing it further appears necessary for improving the electrochemical performance of these materials. For instance, we have recently shown that CO 2 activation allows increasing the surface area of tannin-derived mesoporous carbons up to nearly 2000 m 2 /g and maintaining high-rate capability when increasing the current density up to 80 A/g [START_REF] Castro-Gutiérrez | High-Rate Capability of Supercapacitors Based on Tannin-Derived Ordered Mesoporous Carbons[END_REF]. The same kind of behaviour can therefore be expected with the present samples.

Figure 6 3. 2

 62 Figure 6

Figure 9

 9 Figure 9

Fig. 1 :Fig. 2 :

 12 Figure12

Fig. 3 :Fig. 4 :

 34 Fig.3: Typical SEM images of carbon materials (i.e., pyrolysed at 900°C) based on: tannin

Fig. 6

 6 Fig. 6: a) HTC yield of materials made at 180°C at different T/S proportions and of

Fig. 7 :

 7 Fig. 7: BET areas A BET (open symbols) and product Total carbon yield  A BET (full symbols)

Figure 10 .

 10 Figure 10. Cyclic voltammetry (CV) results obtained in a three-electrode cell in 1 mol L -1 H 2 SO 4 electrolyte: (a) CV curves recorded at 0.5 mV s -1 scan rate; (b) CV curves recorded at 10 mV s -1 scan rate; (c) Specific capacitance of the samples at scan rates between 0.5 and 100 mV s -1 ; (d) Capacitance retention at scan rates between 0.5 and 100 mV s -1 .

Figure 12 .

 12 Figure 12. (a, c, e) Dependence of the anodic and cathodic peaks currents, I p,ox and I p,red , on the square root of the scan rate, s 1/2 ; (b, d, f) Evolution of the peak potentials, E p,ox and E p,red ,

  Figure 1

Figure 3

 3 Figure 3

Figure 4 37

 4 Figure 4

  Figure 5

  Figure 7

  Figure 8

  Figure 9

  Figure 10

  Figure 11

Figure 12 -

 12 Figure12

Table 1 :

 1 Textural properties and elemental analysis obtained for materials 50S50T after HTC and subsequent pyrolysis made at different pH. 217

		Yield (wt. %)	Textural Properties	Elemental Analysis (wt. %)
	Sample	HTC Carbon Total	S NLDFT (m 2 g -1 )

Table 2 .

 2 Atomic surface concentrations of C, N and O, and contributions to the C1s, O1s and N1s high-resolution XPS spectra of the materials in terms of peak area fraction (%) at given binding energies (BE, eV). PSDs are clearly not the same, as suggested by the data of Table

	Sample name	Concentration (at. %)			C1s peak BE (eV) Area (%)				O1s peak BE (eV) Area (%)			N1s peak BE (eV) Area (%)
		C	O	N	C*	CI	CII	CIII	CIV	CV	O1	O2	O3	N6	N5	NQ	NX
	0S100T	80.4	19.6	-	-	284.4 66.1	285.8 29.6	288.2 2.5	289.2 0.9	291.0 0.9	531.7 24.8	532.8 75.2	-	-	-	-	-
	H50S50T180_1	81.9	16.8	1.3	-	284.4 65.9	285.7 27.3	287.6 5.6	289.0 0.7	290.3 0.4	531.4 36.1	532.7 63.9	-	398.5 12.2	399.7 59.9	401.1 27.9	-
	H50S50T180_2	83.0	15.7	1.3	-	284.4 73.7	285.8 19.4	287.6 4.6	288.9 1.9	290.6 0.4	531.4 41.7	532.7 58.3	-	398.5 28.1	399.7 58.5	401.1 13.4	-
	H50S50T180_3	87.1	12.6	0.3	283.6 9.0	284.5 31.4	286.0 44.3	287.6 12.2	289.0 1.0	290.1 2.1	531.8 7.2	533.3 60.8	534.5 32.0	-	-	-	-
	H50S50T180_u	84.2	15.8	-	-	284.4 73.7	285.7 20.7	287.6 3.7	289.0 1.6	290.3 0.3	531.7 39.5	532.8 60.5	-	-	-	-	-
	H100S0T180_u	81.9	17.9	0.3	-	284.4 65.4	285.6 25.8	287.6 8.1	289.2 0.3	290.4 0.4	531.5 40.1	532.8 59.9	-	-	-	-	-
	C0S100T	97.1	2.9	-	-	284.4 66.4	285.5 25.5	287.6 5.7	289.2 1.8	290.5 1.5	531.8 40.4	533.0 54.0	534.5 5.6	-	-	-	-
	CH50S50T180_1	97.5	2.5	-	-	284.5 68.7	285.5 23.8	287.6 5.2	289.2 1.4	290.4 1.5	531.6 48.1	532.9 49.4	534.5 2.5	-	-	-	-
	CH50S50T180_2	97.2	2.8	-	-	284.5 65.8	285.5 25.8	287.6 5.6	289.0 0.6	290.2 2.2	531.6 41.4	532.9 47.0	534.5 11.9	-	-	-	-
	CH50S50T180_3	94.8	3.4	1.7	-	284.4 66.2	285.5 25.3	287.6 5.9	289.2 1.1	290.4 1.5	531.3 46.5	532.8 51.0	534.5 2.5	398.2 28.8	400.3 32.7	401.1 38.5	-
	CH50S50T180_u	87.9	11.7	0.40	-	284.4 75.9	286.0 19.5	287.6 3.2	289.2 0.7	290.1 0.4	531.9 53.3	533.2 46.7	-	-	-	-	-
	CH100S0T180_u	97.7	2.3	-	-	284.5 67.1	285.6 25.4	287.7 4.8	289.2 0.6	290.4 2.1	532.0 49.2	533.0 47.8	534.5 3.0	-	-	-	-

Acknowledgements

The present research was partly made possible by the support of the Region Lorraine and University of Lorraine 'J . F gratefully acknowledge the financial support of the CPER 2007-2013 "S ion du Pôle F G E " F Général des Vosges), regional (Région Lorraine), national (DRRT and FNADT) and European (FEDER) funds.

a)