Laure Cibien

Maxime Parot

Patrick Nkuigue Fotsing

Philippe Gaveau

Emmanuel Djoufac Woumfo

Julien Vieillard

Alfredo Napoli

Nicolas Brun
email: nicolas.brun@enscm.fr

Ionothermal Carbonization in [Bmim][FeCl 4 ]: an Opportunity for the Valorization of Raw Lignocellulosic Agrowastes into Advanced Porous Carbons for CO 2 Capture †

Ionothermal carbonization in [Bmim][FeCl 4 ]: an opportunity for the valorization of raw lignocellulosic

agrowastes into advanced porous carbons for CO 2 capture

Introduction

With the concept of biorefinery, the conversion of renewable feedstocks into energy and marketable products, e.g. platform chemicals and advanced materials, has constituted a major field of research. 1,2 This assessment is particularly true for the agro-industry sector, which generates a large amount of organic residues, namely agrowastes. These agrowastes are cheap, renewable and abundant, making them attractive raw materials for sustainable industrial processes. Amongst biorefinery's end products, porous carbonaceous materials 3,4 have attracted much interest as they offer a wide range of potential applications in energy storage, 5,6 catalysis 7 and environmental remediation [8][9][10] . Until relatively recently, the thermochemical conversion of renewable feedstocks into porous carbons was limited to dry pyrolysis and/or activation processes. In the past years, wet pyrolysis in water, also called hydrothermal carbonization (HTC), has been seen as a promising alternative to design porous carbonaceous materials on demand from renewable feedstocks. [11][12][13][14][15][16] Those carbonaceous materials, also called hydrochars, can be obtained from a large range of natural precursors, including untapped byproducts generated during degradation of lignocellulose in paper mills 17 and biorefineries 6 . The hydrothermal approach is based on the degradation/dehydration of (poly)saccharides into furan-based intermediates at relatively mild conditions; typically at a temperature between 130 and 250°C and at a pressure below 10 bars. HTC usually yields microscopic nonporous spheres with limited control over morphology and texture. When applied to soluble carbohydrates, e.g. monosacharides, HTC has been frequently combined with sacrificial templates [18][19][20][21][22][23] , catalysts and/or structuring agents [24][25][26][27] to yield porous hydrochars. Conversely, the hydrothermal carbonization of insoluble feedstocks, e.g. cellulose and raw lignocellulosic materials, remains limited in terms of textural and morphological control. 13 Since 2002 and the first article demonstrating their ability to dissolve cellulose, 28 ionic liquids (ILs) have been widely employed in biorefinery, both as derivatizing [29][30][31] and nonderivatizing 28,32 solvents. Particularly, short-chain imidazolium-based room temperature ILs with strong hydrogen bond acceptors as anions, such as 1-butyl-3-methylimidazolium chloride (i.e. [Bmim]Cl), appeared to be very efficient in solubilizing cellulose. R.D. Rogers et al. demonstrated that such solvation involves stoichiometric hydrogen-bonding between the hydroxyl protons of the polysaccharide and the anions of the IL. 28,33 From that moment on, few other hydrophilic ILs, with various anions (e.g. chloride, phosphate, phosphonate and acetate) and cations (e.g. imidazolium, ammonium and phosphonium), demonstrated similar abilities. 34 While the role of the anion seems to be clear (i.e. related to hydrogen bonding basicity), the way cationic structure affects the dissolution of cellulose is still open to discussion. 35 Besides, functional ILs with acidic properties have been employed both as solvent and catalyst for the hydrolysis and conversion of lignocellulosic biomass to platform chemicals. [36][37][38] Beyond dissolution and selective deconstruction, ILs were also employed for the thermochemical conversion of carbohydrates into porous carbonaceous materials. Usually, ionic liquid is involved either as a soft-template in water 39,40 or as a functional solvent playing the role of catalyst and/or porogenic agent [41][42][43][44] . While the first approach falls in the category of HTC, the second one can be regarded as ionothermal carbonization (ITC; by analogy with hydrothermal carbonization) yielding ionochars (by analogy with hydrochars). While full of promise, the ITC process is still a young topic with reaction mechanisms yet to elucidate and a limitless variety of carbon precursor/IL systems to investigate. Thus far, such ionothermal treatment has not been fully exploited and was mainly restricted to monosaccharides, [42][43][44] except for rare examples dealing with raw lignocellulosic agrowastes 45,46 . Using Bmim-type ILs with different anions (i.e. Cl -, BF 4 -and Tf 2 N -), Wang et al. 45 applied the ITC process to cellulose and sugar cane bagasse. The authors showed a correlation between the size of the anion and the textural properties of the ionochari.e. larger anions yield higher pore volume revealing a bit more the versatility of the ITC process. More recently, Xie et al. 46 applied the ITC approach to Jujun grass. After treatment at 180°C in a Lewis acid ionic liquid (i.e. 1butyl-3-methylimidazolium tetrachloroferrate, [Bmim][FeCl 4 ]) and subsequent chemical activation with KOH, the authors obtained hierarchical porous carbons with high surface area and significant mesoporous contribution. The beneficial input of the ITC pre-treatment in terms of porosity and capacitive performance was clearly evidenced, making this two-step approach really promising. However, in this last study, the authors considered ionochars as synthetic intermediates and mainly focused on the final activated carbons and their electrochemical properties. More recently, X. Li et al. 47,48 believe that this study further contributes to demonstrate that ITC can be a promising approach into the circular economy for the valorization of raw agrowastes into advanced carbonaceous materials.

Experimental section

Materials

Iron (III) chloride hexahydrate was purchased from Acros Organics. 1-Butyl-3methylimidazolium chloride (99 %) was purchased from Iolitec (Germany) and kept under vacuum to avoid hydration. Sigmacell cellulose (type 50, 50 µm) and kraft lignin were purchased from Sigma Aldrich. All reagents were used without further purification.

Preparation of the cocoa bean shells (CS)

The cocoa bean shells (CS) were collected from a local farm managed by the IRAD (Institute of Agricultural Research for Development) at Yaoundé, Cameroon. The shells were cut, washed with acidic water (1M HCl) to remove organic matter, sun-dried for 5 days, and heated at 70°C overnight to remove moisture. Finally, CS were ground and sifted at 160 µm.

Extraction of lignin by the Klason method

Lignin was extracted from CS and quantified using the Klason method as described in literature. 50 In brief, 3 g of CS were treated in 30 The as-obtained samples were named IC-240.

Preparation of [Bmim][FeCl 4 ]

[Bmim][FeCl 4 ] was prepared using the well-established procedure reported in literature. 42,51 Typically, equimolar amounts of FeCl

High temperature post-treatment

Pyrolysis

Pyrolysis was carried out in a horizontal tube furnace at 900 °C for 2 h with a heating rate of 5 °C min -1 under argon flow (50 mL min -1 ). Samples were then allowed to cool down to room temperature through a natural convection step. The as-obtained samples were named with the suffix P9.

CO 2 activation

CO 2 activation was carried out in a vertical tube furnace following two different procedures.

In a first procedure, the samples were pretreated at 950 °C for 10 min under nitrogen flow (100 mL min -1 ) with a heating rate of 10 °C min -1 and an intermediate drying plateau at 110°C for 60 min, and subsequently treated at 950°C for 120 min under a CO 2 enriched nitrogen flow (10 mL min -1 CO 2 , 90 mL min -1 N 2 ). The as-obtained samples were named with the suffix A1.

In a second procedure, the samples were treated at 950 °C for 130 min under a CO 2 enriched nitrogen flow (10 mL min -1 CO 2 , 90 mL min -1 N 2 ) with a heating rate of 10 °C min -1 and an intermediate drying plateau at 110°C for 60 min. The as-obtained samples were named with the suffix A2.

In all cases, samples were allowed to cool down to room temperature through a natural convection step.

Mass and carbon yields

The mass yield and the corrected mass yield were calculated as follows:

Residual atoms correspond to any atom present in the dried char and coming from the salt (FeCl 3 ) or the ionic liquid ([Bmim]Cl or [Bmim][FeCl 4 ]), i.e., iron, chloride and nitrogen excess. Iron and chloride contents were determined by SEM-EDX (section 2.7). Nitrogen content was determined by elemental analysis. The nitrogen excess coming from the ionic liquid was determined by subtracting the nitrogen coming form the initial biomass. HC-240

and HC-180 were used as references.

The carbon yield was calculated as follows:

Characterization

Scanning electron microscopy (SEM) analyses were performed on a Hitachi S-4800 electron microscope. SEM-EDX (Energy Dispersive X-ray spectroscopy) analyses were performed on a Zeiss EVO HD15 electron microscope. Transmission electron microscopy (TEM) analyses were performed on a JEOL 1200 microscope. High resolution TEM analysis was performed on a JEOL 2200FS microscope operated at 200 kV, from the MEA platform (Université de Montpellier). This microscope is equipped with a field emission gun (FEG) and an in-column Omega-type energy filter. STEM-EDX (Scanning Transmission Electron Microscopy) mapping were performed using a probe size of 1. °C min -1 in the 20-1000 °C range, under a dry air atmosphere (50 mL min -1 ).

Results and discussion

Ionothermal (ITC) versus hydrothermal (HTC) carbonization

In the manner of hydrothermal carbonization (HTC), ionothermal carbonization (ITC) is a thermochemical process which allows carbonization of carbohydrates and raw lignocellulosic materials under mild conditions (i.e. < 10 bars and < 240 °C). room-temperature IL which acts as a porogen and as a solvent with good moisture-stability and superior solvation properties compared to water. Thus, ITC of carbohydrates in BFe generated porous ionochars with a higher carbonization yield compared with analogous hydrochars obtained by the HTC process. These features were confirmed in our study with raw biomass, i.e., cocoa bean shell (CS). CS is an abundant and often underexploited lignocellulosic agrowaste from the cocoa industry that is mainly used as fuel for boilers or as fertilizer for cocoa trees. 49 The valorization of CS into valuable and marketable products will help improving waste disposal and developing local circular bioeconomy. 49,52,53 The cocoa shells employed in this study are constituted of ca. 25-30 wt.% lignin, ca. 55-60 wt.% holocellulose and ca. 6 wt.% ash (Figure S1, Tables S1 andS2). Figure 1a S3). Similarly, the textural properties of IC-Fe-240 were significantly improved. The nitrogen sorption isotherms obtained at 77 K are shown in Figure 2b. IC-Fe-240 displays a type I/II isotherm with a H3 hysteresis loop, related to micro-macroporous materials. This ionochar is mainly microporous with a BET equivalent specific surface area (SSA BET ) of 550 m 2 .g -1 (Figure 1c and Table S4). A similar shape was obtained for HC-Fe-240. This hydrochar presents a high SSA BET of 284 m 2 .g -1 with equal SSA contributions from micropores and macropores (Figure 1c and Table S4). On the contrary, HC-240 depicts a type II isotherm, typical of macroporous materials, with a low SSA BET of 20 m 2 .g -1 , while IC-240 is non-porous. These observations were confirmed by scanning (SEM, Figure S2) and transmission electron microscopy (TEM, Figure 2). As shown by TEM (Figure 2a), HC-240 is made of aggregated nanoparticles with a diameter of ca. 50-100 nm. These particles are mainly not dispersible and supported on the preserved natural macrostructure of the native CS (Figure S2). As for IC-240, SEM observations confirm the absence of porosity even at the macroscopic scale (Figure S2). As shown by SEM and TEM, the presence of iron chloride either in water or in BFe, significantly affects the nanoparticle size. HC-Fe-240 (Figure 2b) and IC-Fe-240 (Figure 2c) are made of aggregated nanoparticles with smaller diameters of ca. 10-40 nm, in good agreement with the higher external SSA obtained by nitrogen sorption analyses (Figure 1). These results suggest that iron not only has a role of Lewis acid catalyst in the carbonization process, it is also involved in the generation of porosity. Importantly, a synergistic effect was clearly evidenced when processing ITC in BFe. IC-Fe-240 shows improved mass yield, carbon yield and textural properties as compared with its two analogous obtained in the sole presence of [Bmim]Cl or FeCl 3 in water, i.e. IC-240 and HC-Fe-240 respectively. As suggested by Xie et al. for the ITC of carbohydrates, 42 BFe might closely interact with the biopolymers and the carbonaceous particles through hydrogen bonds, but also via the π-system of the imidazolium ring. As imidazolium is amphiphilic, complex liquid structures might form 54 and favor the nucleation of small particles and the generation of meso-and macropores. The chemical structure of the four samples was investigated by FTIR (Figure 3a) and solid-state 13 C CP NMR spectroscopy (Figure 3b). HC-Fe-240 and IC-Fe-240 present similar FTIR spectra (Figure 3a (aromatic υ C=C stretching vibrations in lignin). Moreover, the band centered at 1445 cm -1 is more intense. These bands can be attributed to the native ligneous components of CS. The band centered at 1040 cm -1 can be also attributed to υ C-O stretching vibrations in holocellulose.

These features suggest that lignin, and to a lesser extent holocellulose, are less affected in the absence of iron. The chemical structure of these four samples was also investigated by 13 C solid-state NMR spectroscopy (Figure 3b). Two major domains can be clearly identified: the aliphatic region between 0 and 50 ppm and the aromatic region centered at 120-125 ppm. At first glance, the two samples treated in the absence of iron, i.e. HC-240 and IC-240, present more aliphatic carbons than HC-Fe-240 and IC-Fe-240. This observation is particularly pronounced for IC-240 with two narrow and intense peaks at 13 and 30 ppm and a shoulder at 19 ppm. This feature can be attributed to the partial degradation of [Bmim]Cl during the thermochemical treatment, as confirmed by GC-MS (Figure S3) and FTIR (Figure S4) of the recycled IL and by the presence of 6.7 wt.% of nitrogen in IC-240 (Table S3). This will be further discussed in section 3.4. In the aromatic region, while IC-240 displays a relatively narrow and intense peak centered at 122 ppm, HC-240 and, to a lesser extent, IC-Fe-240 and HC-Fe-240, present a shoulder centered at 145-150 ppm. This peak can be attributed to aromatic carbons bounded to lignin methoxy groups and suggests that lignin was mildly affected in these conditions. As for HC-240, this observation is in good agreement with FTIR spectroscopy. As for HC-Fe-240 and IC-Fe-240, however, this feature is contradictory with the conclusions drawn from FTIR. As reported by Baccile et al. 12 , the peak at 145-150 ppm can be also attributed to a nonsubstituted, protonated, α-carbon of the furan ring of furanic resins. At that stage, it is important to get back to basics of the HTC process applied to carbohydrates and lignocellulosic materials. The conversion of carbohydrates into hydrochars is assumed to proceed in three main steps: (i) hydrolysis into monosaccharide units if starting from oligo-or polysaccharides, (ii) dehydration of monosaccharides into furan derivatives, and (iii) polycondensation of furan derivatives into polyfuranic species. 3,12,13,15 At high temperature, typically at 240 °C, the carbonization process continues via intramolecular reactions (i.e., condensation, dehydration and decarbonylation reactions) which lead to the production of condensed polyaromatic domains. As for insoluble feedstocks, e.g. cellulose and raw lignocellulosic materials, fundamental differences in the HTC mechanism were observed. As reported by Baccile et al., 12,13 the polyfuranic structures usually reported from simple carbohydrates cannot be observed in cellulose and lignocellulosic-derived hydrochars, which yield higher amounts of arene groups coming either from condensed furanic units or from polyaromatic hydrocarbon clusters. In this article, the authors concluded that the hydrolysis of cellulose to glucose and subsequent dehydration and polycondensation into polyfuranic species is limited to the cellulose-water interface and accounts for a minor part of cellulose conversion. Meanwhile, the HTC of bulky cellulose follows chemical reaction pathways comparable to the classical dry pyrolysis process. The results obtained herein with HC-240 are in good agreement with the mechanism proposed by Baccile et al. based on hydrochars derived from cellulose and rye straw. Besides residual lignin, the presence of a shoulder at 145-150 ppm for IC-Fe-240 can suggest a higher furan-to-arene ratio in the carbonaceous structure. This feature could be explained by a better solubility of the cellulosic component in IL that would increase the cellulose-solvent interface. One may expect a similar behavior in [Bmim]Cl. However, no clear shoulder was observed at 145-150 ppm for IC-240. Even though the presence of this peak is still a matter of discussion, the presence of oxygenated functional groups in the carbonaceous framework of IC-Fe-240 was confirmed by elemental analyses (Table S3) and X-ray photoelectron spectroscopy (XPS, Figures 4 andS5, Tables S5 andS6). According to the simulated C1s XPS spectra, IC-Fe-240 depicts less sp 2 carbons and more oxygenated functional groups than HC-Fe-240 and HC-240 (Figure 4a, Table S5). This behavior might be explained by the coordination of [FeCl 4 ] -to the oxygen atoms of carbohydrates and lignin which may stabilize carbon oxygenated groups and generate micropores. This assumption was further supported experimentally. Firstly, according to SEM-EDX analyses (Table S3), IC-Fe-240 has more residual iron and chlorine than HC-Fe-240. Secondly, the simulated O1s XPS spectrum of IC-Fe-240 (Figure 4, Table S6) suggests the presence of Fe-O species (contribution at 531 eV), 55 together with oxygen atoms in a strongly electronegative environment (contribution at 534.5 eV). The latter might refer to oxygen atoms in the second coordination sphere of [FeCl 4 ] -anions. These contributions are less pronounced for HC-Fe-240 and HC-240 (Figure 4b-c, Table S6). Additionally, the O/C mass ratio of IC-Fe-240 extracted from XPS is significantly lower than the one obtained from elemental analysis (0.27 vs. 0.40 wt.% respectively, Table S6). This difference is less pronounced for HC-Fe-240 and HC-240. This feature suggests that the carbonization at the biomass-solvent interface is more advanced than the carbonization in the bulk, especially for the ITC process in BFe. It also confirms the presence of oxygenated functional groups in the aromatic carbonaceous framework of IC-Fe-240, mainly due to the recalcitrant ligneous fractions.

On the influence of hydration

At that stage, it is important to notice that ITC occurs under autogenous pressure induced by water produced during dehydration/condensation reactions and by low-boiling point organic by-products. The residual water present in IL and biomass might be also considered. The as-prepared ionochars were thoroughly characterized by elemental analyses (Table S7)

and nitrogen sorption at 77 K (Table S8). The results are compiled in S8) and pore volume (Figure 5c) reached a maximum for IC-Fe-240-10w of c.a. 616 m 2 .g -1 and 0.57 cm 3 .g -1 respectively. Particularly, a significant difference was observed for the contribution due to external pores (Figure 5c), i.e., meso-and macropores in which the nitrogen adsorption occurs in multimolecular layers. While the micropore volume kept stable from IC-Fe-240 to IC-Fe-240-10w with a value of c.a. 0.2 cm 3 .g -1 , the external pore volume increased from 0.26 to 0.36 cm 3 .g -1 while increasing the water content from 0 to 10 wt.%. Reversely, while increasing the water content from 10 to 25 wt.%, the external pore volume drastically decreased to 0.2 cm 3 .g -1 . As mentioned in section 3.1, BFe is amphiphilic and might form complex liquid structures governed by the assembly of hydrophobic and hydrophilic domains.

The addition of water may cause a change in the hydration state of BFe and modify the hydrophilic/hydrophobic balance that might affect these complex liquid structures. In our case, a small amount of water seems to be beneficial for the formation of ionic liquid nanodomains and, in fine, the generation of meso-and macropores in the ionochars. This result suggests that the use of wet starting products and agrowastes in the ITC process might be advantageous for the production of highly porous chars.

On the contribution of cellulosic and ligneous components

As mentioned in section 3.1, the ligneous fractions of CS seem to be mildly affected as compared to the cellulosic fractions. To get a better insight into the role of each component of the lignocellulosic material, commercial cellulose and kraft lignin were treated in BFe at 240°C for 20 hours, yielding IC-Fe-240-C and IC-Fe-240-L respectively. Those ionochars were compared with the ones obtained from raw CS (IC-Fe-240) and Klason lignin extracted from raw CS (IC-Fe-240-KL). The as-prepared ionochars were thoroughly characterized by elemental analyses (Table S9) and nitrogen sorption at 77 K (Table S10). The results are compiled in Figure 6. As shown in Figure 6a, the mass and carbon yields are significantly higher for ligneous materials than for cellulosic ones. This behavior can be explained by the fact that lignin contains more aromatic carbon and is more stable than hemicellulose and cellulose in these conditions. Regarding mass and carbon yields, CS and cellulose behaved similarly. This trend was confirmed by nitrogen sorption data. As shown in Figure 6b, IC-Fe-240-C and IC-Fe-240 display similar type I/II isotherms with H3 hysteresis loops. IC-Fe-240-C shows, however, a larger SSA BET of c.a. 736 m 2 .g -1 (550 m 2 .g -1 for IC-Fe-240) mainly due to micropores' contribution (Table S10 and Figure 6c). Reversely, IC-Fe-240-L depicts a type II isotherm and is mainly macroporous with a low SSA BET of c.a. 40 m 2 .g -1 . Evidently, cellulose and hemicellulose, which undergo more severe structural rearrangement than lignin during the ITC process, contribute more quantitatively to the generation of micropores in the final ionochars. IC-Fe-240-KL depicts intermediate textural properties with a SSA BET of c.a. 250 m 2 .g -1 . The presence of residual carbohydrates or a higher H unit content in the Klason lignin extracted from CS as compared with the commercial Kraft lignin (Figure S7) might explain such difference. To elucidate this behavior, the ionothermal carbonization of diff erent types of lignin and raw agrowastes is currently under investigation.

Another significant difference lies in the fact that IC-Fe-240-L contains twice more residual iron than the three other ionochars (i.e., 6 wt.% versus c.a. 3 wt.% for the others, Table S9). As mentioned in section 3.1, [FeCl 4 ] -moieties are willing to coordinate to the oxygen atoms of carbohydrates and lignin. As reported by Li et al., 48 the low absolute binding energy between the cation and the anion in BFe improves the efficient dissociation of [Bmim] + and [FeCl 4 ] -and favors the coordination interactions between Fe atom of [FeCl 4 ] - and O atom of ester and phenol groups from lignin. One may assume that the recalcitrant G and S units in lignin 48 are willing to form stable coordination complexes with [FeCl 4 ] -and to trap more iron atoms in the final carbonaceous framework.

Recyclability and reusability of [Bmim][FeCl 4 ]

To support the sustainability of the ITC process, the recyclability of BFe was investigated.

The iron-containing ionic liquid was thoroughlty characterized by FTIR spectroscopy and ICP atomic emission spectrometry (ICP-AES) before (BFe) and after ITC of raw CS at 180 and 240°C for 20 hours (BFe-r180 and BFe-r240 respectively, Figure 7a). While the chemical signature of [Bmim][FeCl 4 ] was preserved, the amount of iron decreased from 16.6 wt.% in fresh BFe to 16.3 wt.% in BFe-r180 and 16.0 wt.% in BFe-r240. This result confirms the loss of iron during ITC, partly due to the presence of remaining [FeCl 4 ] -complexes in ionochars.

One may also consider the fact that both BFe-r180 and BFe-r240 were collected only after the first filtration step, which allowed recovering c.a. 95 wt.% of the IL. [FeCl 4 ] -moieties and imidazolium cations were also present in the subsequent filtrates, making the recycling largely improvable. BFe-r240 was also characterized by GC-MS and no degradation products were detected, while imidazole by-products (e.g., imidazole, methylimidazole, 1-butylimidazole and N-Propionylimidazole) were identified for [Bmim]Cl recovered after ITC of CS at 240°C (Figure S3). These by-products were probably generated by the cleavage of the butyl and methyl side chains by hydroxyl radicals. 56 BFe-r240 was reused for the ITC of CS at 240°C for 20 hours, yielding IC-Fe-240-r1. This ionochar was compared with IC-Fe-240 (Tables S11 andS12). As shown in Figure 7b, IC-Fe-240 and IC-Fe-240-r1 display similar type I/II isotherms with H3 hysteresis loops. Meanwhile, the mass and carbon yields were not affected (Figure 7c). However, the SSA BET slightly decreased from 550 m 2 .g -1 for IC-Fe-240 to 455 m 2 .g -1 for IC-Fe-240-r1 (Table S12). This difference results from a reduction in the micropore contribution (Figure 7d), which is mainly related to the amount of [FeCl 4 ] - moieties in the reaction medium as mentioned above.

Pyrolysis and CO 2 activation: from ionochars to activated carbons

High temperature post-treatments (i.e., pyrolysis and CO 2 activation) were carried out on ionochars to further investigate their potential and extend their applicability. After pyrolysis at 900°C under argon (P9 series), porous carbon materials with increased carbon content (c.a. 90 wt.%) can be obtained (Tables S13 andS14). Interestingly, the remnant of iron in ionochars was converted to iron-carbon core-shell nanoparticles as shown by high-resolution TEM imaging (Figure 8a) and elemental EDX mapping (Figure 8b and Figure S8). According to X-ray diffraction performed on IC-Fe-240-P9 (Figure 8c), the core of these core-shell nanoparticles is composed of iron (α-Fe), iron carbide (Fe 3 C), and maghemite (γ-Fe 2 O 3 ).

TEM imaging also revealed that highly ordered graphitic layers surround the iron-based core (Figure S9). The carbothermal reduction of iron salts to α-Fe and Fe 3 C at high temperature and the formation of similar core-shell structures were already described for biomass derived hydrochars 57 and ionochars 42 . As a direct consequence, these ionochar-derived carbons are strongly attracted to magnets (insert in Figure 8a) and might be easily separable from solution by external magnetic fields. Besides, the textural properties of the ionochars were mainly preserved after pyrolysis, with type I/II isotherms with H3 hysteresis loops and a slight increase in micropore contributions (Table S13 and Figure 9a). Based on these advantageous properties, we are currently investigating their use in magnetic separation for wastewater treatment and depollution. To evaluate the potential of ITC as a pretreatment for the preparation of highly porous carbons (i.e., activated carbons), we also studied the physical activation of a few samples under a CO 2 -rich atmosphere at 950°C. HTC has been established as an efficient pretreatment in the transformation of carbohydrates and raw lignocellulosic biomass to activated carbons. 9,10 Recently, Xie et al. 46 also proposed to use ITC as an intermediate step prior to activation of raw biomass. All these studies used KOH activation at high temperature (between 600 and 900°C). The latter is a chemical activation commonly employed to prepare activated carbons with high SSA and narrow micropores. 58 In this study, we proposed to investigate another approach, i.e. the physical activation in a CO 2 -rich atmosphere. CO 2 activation has been less employed than KOH activation as it usually yields activated carbons with lower SSA. Nonetheless, CO 2 activation usually offers higher yields and carbon content compared to chemical activation. 58 Moreover, CO 2 activation is more advantageous in terms of time, cost and simplicity. 58 Two samples were submitted to CO 2 activation at 950°C: HC-240, the hydrochar serving as a reference; and IC-Fe-240-10w, the ionochar with the highest SSA BET . Two different procedures were employed: A1, when the samples were submitted to CO 2 activation for 2 hours at the plateau only; and A2, when the sample was submitted to CO 2 activation from the beginning of the heating ramp, starting from 110°C. As shown in Figure 9b, the activated carbons still display type I/II isotherms. Both pore volumes and SSA BET were significantly increased as compared with the carbons obtained by pyrolysis (P9 series, Table S15).

Considering textural properties, ITC seems to be favorable compared to HTC. After a similar activation procedure (A1), IC-Fe-240-10w-A1 presents a SSA BET of c.a. 1440 m 2 .g -1 , mainly due to micropores' contribution (SSA micropore of c.a. 1200 m 2 .g -1 ), while HC-240-A1 presents a SSA BET of c.a. 800 m 2 .g -1 (Table S15). Meanwhile, the activation procedure A2 allows reaching higher SSA. Thus, IC-Fe-240-10w-A2 presents a SSA BET of c.a. 2060 m 2 .g -1 , with a micropores' contribution of c.a. 1800 m 2 .g -1 . Interestingly, mass yields of 35-44 wt.% were obtained (Table S16).

Preliminary results for post combustion CO 2 capture

The hydrochars, ionochars and carbons prepared in this study were tested for post combustion CO 2 capture. The most representative isotherms recorded at 273 K are shown in Figure 10a. For a purpose of comparison, the CO 2 uptakes of each sample are represented in Figure 10b as a function of their corrected mass yield. We believe that these two parameters, i.e., mass yield and CO 2 uptake, are crucial to evaluate both the efficiency and the sustainability of the different materials prepared in this study. Interestingly, two regions can be clearly identified. The ionochars and ionochar-derived carbons gather in the upper right region, which corresponds to higher mass yields and/or higher CO 2 uptakes. Reversely, all the other materials gather in the bottom left region, which corresponds to lower mass yields and/or lower CO 2 uptakes. In another representation, we proposed to express the CO 2 uptakes of each sample per gram of CS treated, i.e. the CO 2 uptakes were weighted as a function of the total mass yields of each activation procedure. The same trend was observed (Figure S10). The ionochars and ionochar-derived carbons display weighted CO 2 uptakes superior to 0.8 mmol of CO 2 adsorbed per gram of CS treated, while the other materials were below 0.6 mmol.g -1 . These results further demonstrate that ITC in [Bmim][FeCl 4 ] is an advantageous pretreatment for the transformation of raw lignocellulosic agrowastes into advanced activated carbons for CO 2 capture.

The CO 2 uptake of the most efficient material, i.e., IC-Fe-240-10w-A2, was also tested at 293 and 298 K (Figure 10c). At 298 K and 1 bar, IC-Fe-240-10w-A2 was able to adsorb 4.4 mmol of CO 2 per gram. This value is amongst the highest values reported to date in literature for nitrogen-free biomass-derived carbons (Table 1). [8][9][10]58 To the best of our knowledge, it's also the highest CO 2 uptake reported to date for CO 2 activated biomass-derived carbons. 58,59 These results are highly promising and might encourage further studies in the field of ionochars-derived activated carbons. With this aim, we are currently investigating different chemical activation procedures (KOH and ZnCl 2 ) to produce ionochars-derived carbons with enhanced micropores' contribution. Thorough characterizations of the CO 2 adsorbents reported here are also under investigation such as cyclic stability, adsorption kinetics, and CO 2 /N 2 selectivity. Considering the advantageous textural properties of these materials, precombustion CO 2 capture at higher pressure should be also considered.

Conclusion

In this study, we investigated the ITC of a raw lignocellulosic agrowaste used as a model, i.e. of hydration over yields and textural properties was also investigated. We showed that the use of wet starting products and agrowastes in the ITC process might be advantageous for the production of highly porous chars. Importantly, the contributions of lignin and cellulose were highlighted and the recyclability of [Bmim][FeCl 4 ] was clearly evidenced, proving the sustainability of the ITC approach and making this process easily applicable to other raw agrowastes. The subsequent activation of the as-obtained ionochars with carbon dioxide allowed producing activated carbons with specific surface area superior to 2000 m 2 .g -1 and remarkable CO 2 uptake as high as 4.4 mmol.g -1 at 25°C and 1 bar. This value is the highest CO 2 uptake reported to date in literature for CO 2 activated biomass-derived carbons. To conclude, this work highlights the numerous advantages offered by the ionothermal carbonization for the valorization of raw lignocellulosic agrowastes into advanced porous carbons.

  represents the mass yield and the carbon yield for CS treated at 240°C for 20 hours in water (HC-240), in water with FeCl 3 (HC-Fe-240), in [Bmim]Cl (IC-240) and in BFe (IC-Fe-240). Both mass yield (43.4 wt.%) and carbon yield (73 wt.%) were significantly improved for IC-Fe-240 as compared with its analogous obtained in water or in [Bmim]Cl (Table

Figure 1 .

 1 Figure 1. (a) Mass and carbon yields, (b) Nitrogen sorption isotherms at 77 K and (c) specific surface areas of HC-240, HC-Fe-240, IC-240 and IC-Fe-240.

Figure 2 .

 2 Figure 2. TEM micrographs of (a) HC-240, (b) HC-Fe-240 and (c) IC-Fe-240.

  ) with two intense broad bands centered at 1600 (aromatic υ C=C stretching vibrations) and 1220-1280 cm -1 (υ C-O stretching vibrations in aromatic ethers and esters) and four shoulders centered at 1700 (υ C=O stretching vibrations of aromatic carbonyl groups), 1440 (aromatic υ C=C stretching vibrations), 1374 (δ CH3 bending vibrations of aromatic methyl groups) and 1074 cm -1 . These bands are mainly related to vibration modes in aromatic networks except for the one at 1074 cm -1 , which might correspond to alkoxy stretching vibrations (υ C-O ) of residual ester linkages between lignin and carbohydrates. HC-240 and IC-240 present similar FTIR spectra with three extra shoulders centered at 1040 (aromatic δ CH in plane deformations), 1270 (υ C-O stretching vibrations in lignin) and 1504 cm -1

Figure 3 .

 3 Figure 3. (a) FTIR and (b) 13 C CP-MAS NMR spectra of HC-240, HC-Fe-240, IC-240 and IC-Fe-240.

Figure 4 .

 4 Figure 4. (a) XPS data obtained from the deconvoluted C1s spectra of HC-240, HC-Fe-240 and IC-Fe-240. O1s XPS band-like spectra (black solid lines) and deconvoluted curves (colored filled curves) of (b) HC-240, (b) HC-Fe-240 and (c) IC-Fe-240.

  In our case, BFe was thoroughly dried prior to ITC (water content c.a. 0 wt.% according to TGA, Figure S6) and the water content in CS was systematically controlled prior to thermochemical treatment (c.a. 7.5 wt.% according to TGA). As both IL and lignocellulosic biomass are usually hard to keep dry, we proposed to study the influence of water over the carbonization process and to explore the possibility to extend ITC to wet starting products and agrowastes. In our study, the bound water in CS represented an amount of c.a. 0.6 wt.% in relation to BFe. To study the influence of hydration, we conducted three experiments at 240°C for 20 hours at increasing water content in relation to BFe (2, 10 and 25 wt.%, corresponding to the ionochars named IC-Fe-240-2w, IC-Fe-240-10w and IC-Fe-240-25w respectively) and we compared with ITC (0 wt.%, IC-Fe-240) and HTC (100 wt.%, HC-240).

Figure 5 .

 5 Figure 5a, IC-Fe-240 and IC-Fe-240-2w depict similar mass and carbon yields of c.a. 43 and 73-75 wt.% respectively. While increasing the water content from 2 to 25 wt.%, mass and carbon yields significantly decreased to reach 38 and 62 wt.% respectively for IC-Fe-240-25w. This feature can be attributed to the decrease in iron-based Lewis catalysts while diluting BFe. Meanwhile, the textural properties of the ionochars were significantly affected by the water content. As shown in Figure 5b, all the samples, except for the one treated in 100 wt.% water (i.e., HC-240), display a type I/II isotherm with a H3 hysteresis loop, related to micro-macroporous materials. Interestingly, both SSA BET (TableS8) and pore volume

Figure 5 .

 5 Figure 5. (a) Mass and carbon yields, (b) Nitrogen sorption isotherms at 77 K and (c) pore volumes of IC-Fe-240, IC-Fe-240-2w, IC-Fe-240-10w, IC-Fe-240-25w and HC-240.

Figure 6 .

 6 Figure 6. (a) Mass and carbon yields, (b) Nitrogen sorption isotherms at 77 K and (c) pore volumes of IC-Fe-240, IC-Fe-240-C, IC-Fe-240-L and IC-Fe-240-KL.

Figure 7 .

 7 Figure 7. (a) FTIR spectra of BFe, BFe-r180 and BFe-r240. (b) Nitrogen sorption isotherms at 77 K of IC-Fe-240 and IC-Fe-240-r1. (c) Yields and (d) specific surface areas of IC-Fe-240 and IC-Fe-240-r1.

Figure 8 .

 8 Figure 8. (a) HR-TEM micrograph and (b) STEM-EDX mapping of a core-shell structure observed in IC-Fe-240-P9. (c) XRD patterns of IC-Fe-240 and IC-Fe-240-P9. The upper right insert is a picture of IC-Fe-240-P9 attracted to a magnet.

Figure 9 .

 9 Figure 9. Nitrogen sorption isotherms at 77 K of some ionochars after (a) pyrolysis at 900°C under argon and (b) CO 2 activation at 950°C.

Figure 10 .

 10 Figure 10. (a) CO 2 sorption isotherms at 273 K of some ionochars after pyrolysis or CO 2 activation. (b) CO 2 uptake at 273 K and 1 bar expressed in mmol.g -1 as a function of the corrected mass yield of the hydrochars, ionochars and carbons prepared in this study. (c) CO 2 sorption isotherms at 273, 293 and 298 K of IC-Fe-240-10w-A2.

  cocoa bean shells from Cameroon, in [Bmim][FeCl 4 ]. The beneficial inputs of the ITC approach in [Bmim][FeCl 4 ] towards mass yield, carbon yield and specific surface area were clearly evidenced. The coordination of [FeCl 4 ] -to the oxygen atoms of lignocellulosic materials and ionochars seems to stabilize carbon oxygenated groupsaccounting for enhanced mass and carbon yieldsand to favor the generation of micropores. The influence

  In this study, we propose to provide further insights into the ITC process applied to lignocellulosic biomass via thorough textural and structural characterizations and comparison with hydrochars analogous. With this aim, we applied the ITC approach to a raw lignocellulosic agrowaste model, i.e. cocoa bean shells (CS) from Cameroon. The cocoa industry is a pertinent model as it generates a large amount of agrowastes. Through on-farm processing, ca. 80 wt.% of the cocoa fruit is discarded as residues, including cocoa pod husk, cocoa pulp and CS.49 Herein, both [Bmim]Cl, a well-known commercial ionic liquid, and[Bmim][FeCl 4 ], a Lewis acid ionic liquid analogous, were employed as reaction media for the treatment of CS. The beneficial inputs of the ITC approach towards mass yield, carbon yield and specific surface area were clearly evidenced. Importantly, the contributions of lignin and cellulose were highlighted and the recyclability of [Bmim][FeCl 4 ] was clearly evidenced,

demonstrated synergistic effects that make [Bmim][FeCl 4 ] an efficient catalyst for the depolymerization of lignin through γ-O ester bond cleavage. Even though the authors focused on the production of methyl phydroxycinnamate, a platform molecule for a variety of fine chemicals, rather than on the production of ionochars, this study further supported the relevance of using [Bmim][FeCl 4 ]

for the conversion of lignocellulosic materials.

proving the sustainability of the ITC approach and making this process easily applicable to other raw agrowastes. The subsequent activation of the as-obtained ionochars with carbon dioxide allowed producing activated carbons with specific surface area superior to 2000 m 2 .g - 1 and remarkable CO 2 uptake as high as 4.4 mmol.g -1 at 25°C and 1 bar. This value is amongst the highest values reported to date in literature for nitrogen-free biomass-derived carbons. In this contribution, we wish to provide useful insight and understanding of the transformation mechanism involved in the ITC of lignocellulosic biomass in [Bmim][FeCl 4 ]. Besides, we

2.4. Hydrothermal carbonization (HTC)

	mL of concentrated sulfuric acid (72 wt.%
	H 2 SO 4 ) at 25 °C for 1 hour under vigorous stirring. The slurry was diluted with deionized
	water to decrease the H 2 SO 4 concentration to 4 wt.%, autoclaved at 121 °C for 60 min and
	filtrated through a 0.22 µm PVDF membrane. The insoluble lignin fraction, namely Klason
	lignin (KL), was dried at 100°C overnight in a vacuum oven and weighed. The ash content
	was determined by thermogravimetric analysis under a dry air atmosphere. The soluble lignin
	fraction was quantified by UV-vis spectroscopy.
	Typically, CS powder (1 g) was dispersed in deionized water (10 mL), sealed in a Teflon
	lined autoclave (37 mL) and heated at 240 °C for 20 h. After cooling down to room
	temperature (through quenching in cold water), samples were filtered, successively washed
	two times with deionized water (50 mL) and two times with ethanol (50 mL), then dried at 80
	°C overnight. The as-obtained samples were named HC-240. Similar experiments were
	carried out in the presence of FeCl 3 . FeCl 3 .6H 2 O (10.86 g, corresponding to 40.2 mmol of
	FeCl 3 ) was mixed with deionized water (c.a. 4 mL) until the gauge of a 10 mL volumetric
	flask was reached. The 10 mL solution was mixed with CS powder (1 g) and sealed in a
	Teflon lined autoclave (37 mL) and heated at 240 °C for 20 h. After filtration, washing and
	drying, the as-

obtained samples were named HC-Fe-240. 2.5. Ionothermal carbonization (ITC) 2.5.1. ITC in [Bmim]Cl

	Typically, CS powder (1 g) was mixed with [Bmim]Cl (10.8 g), sealed in a Teflon lined
	autoclave (37 mL) and heated at 240 °C for 20 h. After cooling down to room temperature
	(through quenching in cold water), samples were filtered, successively washed two times with
	deionized water (50 mL) and two times with ethanol (50 mL), then dried at 80 °C overnight.

2.5.3. ITC in [Bmim][FeCl 4 ]

	0.24 mol) were mixed in a round-bottom flask for 30 min at room temperature under vigorous
	stirring. [Bmim][FeCl 4 ] was then dried under vacuum at 100 °C overnight.
	Typically, CS (1 g) was dispersed in [Bmim][FeCl 4 ] (13.56 g, c.a. 10 mL and 40.2 mmol),
	sealed in a Teflon lined autoclave (37 mL) and heated at 240 °C for 20 h. After cooling down
	to room temperature (through quenching in cold water), samples were filtered, successively
	washed two times with deionized water (50 mL) and two times with ethanol (50 mL), then
	dried at 80 °C overnight. The as-obtained samples were named IC-Fe-240. Some experiments
	were conducted in 10 mL of [Bmim][FeCl 4 ]/water mixtures. The as-obtained samples were
	named IC-Fe-240-yw, where y is the water content in wt.%.
	All experiments were performed at least in triplicate. All the data reported in this study
	correspond to average values. Standard deviation was used to measure the statistical
	dispersion and error bars were included on all graphs accordingly.
	3 .6H 2 O (64.86 g, 0.24 mol) and [Bmim]Cl (41.92 g,

  Ionochars and hydrochars were degassed at 150 °C and carbons (i.e., materials post-treated at high temperature) were degassed at 250 °C for 6 h under high vacuum (ca. 0.1 Pa) before physisorption measurements.13 C solid-state NMR was carried out on a VARIAN VNMRS 300 MHz spectrometer at 75.44 MHz. A VARIAN T3 MAS probe was used with 3.2 mm ZrO 2 rotors. Experiments were performed using Cross Polarization Magic Angle Spinning (CP-MAS) with a contact time of 1 ms and a recycle delay of 5 s.Rotors were spun at 15 kHz. Plastic inserts were removed from the rotor and short caps were used to minimize the 13 C background signal. Spectra were normalized and the background signal was removed by subtracting the spectrum obtained with the empty rotor. FTIR spectroscopy was performed in attenuated total reflectance (ATR) mode on a PerkinElmer Spectrum Two spectrometer. XRD patterns were recorded on a Bruker D8 Advance

5 nm and X-rays measured with a silicon drift detector (30 mm 2 , JEOL) with a collection solid angle of 0.13sr. N 2 (at 77 K) and CO 2 (at 273, 293 and 298 K) physisorption experiments were carried out on a Micromeritics 3Flex apparatus.

diffractometer with a Bragg-Brentano geometry and equipped with a Bruker Lynx Eye detector, with the Kα radiation of Cu (λ = 1.5418 Å) and an angular step size of 0.02° into the 10°-80° interval. X-ray photoelectron spectroscopy (XPS) analyses were performed on a ESCALAB 250 ThermoElectron spectrometer using a monochromatized Al Kα source (h = 1486.6 eV) and a 500 µm spot size. GC-MS was performed on a Shimadzu QP2010SE gas chromatograph-mass spectrometer with a Phenomenex Zebron ZB-5ms column (30m x 0.18mm x 0.18µm). Injection: Split 1:30 @ 250 °C. Oven program: 50 °C (hold 2 min) 50-280 °C @ 22 °C/min (Hold 2 min). Thermogravimetric analyses (TGA) were carried out using a Netzsch Simultaneous Thermal Analyzer STA 409 PC Luxx, with a heating rate of 10

  12,13 ITC of carbohydrates in [Bmim][FeCl 4 ] (BFe) was previously described by Xie et al., and the multiple role played by this iron-containing IL was clearly evidenced.42 FeCl 3 acts as a Lewis acid catalyst involved in the different stages of the carbonization process. Its combination with [Bmim]Cl yields a

Table 1 .

 1 A selection of nitrogen-free biomass-derived carbons that have amongst the highest CO 2 uptake values reported to date in literature for post combustion CO 2 capture at 298 K. CO 2 activated carbons are highlighted in grey. ‡ Values related to IC

-Fe-240-10w-A2.

	Biomass	Pretreatment Activation	SSA BET (m 2 . g -1 )	CO 2 uptake at 298 K (mmol.g -1 ) 0.15 bar 1 bar	Reference
	Cocoa bean shells	ITC	CO 2 950°C	2060	1.3	4.4	Our study ‡
	Sawdust	n/a	KOH 700°C	1575	1.3	4.6	8
	Camellia japonica	HTC	KOH 700°C	1353	1.5	5.0	9
	Sawdust	HTC	KOH 600°C	1260	n/a	4.8	10
	Date stones	n/a	KOH 800°C	2367	1.1	4.4	60
	Grass cuttings	HTC	CO 2 800°C	841	1.1	2.8	61
	Coconut shells	n/a	CO 2 800°C	1327	n/a	3.9	62
	Olive stones	n/a	CO 2 800°C	1215	n/a	3.1	63

Acknowledgements

This project was supported by Chimie Balard Cirimat Carnot institute through the ANR program N°16 CARN 0008-01. This work was partially supported by INSA Rouen, Rouen University, CNRS, Labex SynOrg (ANR-11-LABX-0029). Several characterizations were performed with the support of the "Balard Plateforme d'Analyses et de Caractérisation (PAC Balard)". The authors would like to thank Didier Cot for SEM, Bertrand Rebière for SEM-EDX, Franck Godiard for TEM, Erwan Oliviero for STEM-EDX, Christine Biolley for ICP-AES, Pascale Guiffrey for GC-MS, Thomas Cacciaguerra for XRD and Emmanuel Fernandez for solid-state NMR.

Conflicts of Interest

There are no conflicts to declare.


Mohamed Baccour

Nicolas Louvain

Johan G Alauzun

Lorenzo Stievano

P Hubert Mutin

Bruno Boury

Laure Monconduit
email: laure.monconduit@umontpellier.fr

Nicolas Brun
email: nicolas.brun@enscm.fr

Carbonization of polysaccharides in FeCl3/BmimCl ionic liquids: Breaking the capacity barrier of carbon negative electrodes in lithium ion batteries

Introduction

Li-ion batteries (LIBs) occupy in our days a prominent place in a large variety of applications, ranging from portable electronic devices to electric vehicles, to stationary applications. With the advent of next generation cathode materials, expected to yield outstanding high specific capacities (i.e. 1675 mAh g -1 for sulfur cathodes), [START_REF] Yang | Nanostructured sulfur cathodes[END_REF] the development of negative electrode materials with higher performance is mandatory to replace Li metal electrodes which still present a lot of issues. To date, graphite remains the most used negative electrode material in commercially available LIBs. However, its specific capacity remains relatively low (372 mAh g -1 ), which might cause an important mismatch with the next generation of positive electrodes. To overcome these downsides, porosity appears as a key parameter.

Interconnected and hierarchical pore structures were shown to improve the diffusion of lithium ions, while high specific surface areas promote efficient charge-transfer processes at the electrode-electrolyte interface. [START_REF] Roberts | Porous carbon spheres and monoliths: morphology control, pore size tuning and their applications as Li-ion battery anode materials[END_REF][START_REF] Kim | Alginic acid-derived mesoporous carbonaceous materials (Starbon (R)) as negative electrodes for lithium ion batteries: Importance of porosity and electronic conductivity[END_REF] Note that such high surface area can also severely impact the efficiency of the first cycle. Different approaches and precursors were developed for the synthesis of advanced porous carbons with high specific surface areas and pore volumes. [START_REF] Kim | Alginic acid-derived mesoporous carbonaceous materials (Starbon (R)) as negative electrodes for lithium ion batteries: Importance of porosity and electronic conductivity[END_REF][START_REF] Titirici | Sustainable carbon materials[END_REF][START_REF] White | Always Look on the "Light" Side of Life: Sustainable Carbon Aerogels[END_REF][START_REF] Yao | Carbon-Based Materials for Lithium-Ion Batteries, Electrochemical Capacitors, and Their Hybrid Devices[END_REF][START_REF] Dutta | Hierarchically porous carbon derived from polymers and biomass: effect of interconnected pores on energy applications[END_REF][START_REF] Lu | Biomass-derived carbon electrode materials for supercapacitors[END_REF] In particular, biosourced porous carbons have attracted increasing interest due to the abundance and the large variety of readily available and often undervalued bioresources. [START_REF] Titirici | Sustainable carbon materials[END_REF] Various routes were developed to transform (carbonize) biomass into porous carbon, such as physical or chemical activation, [START_REF] Heo | H2O2/steam activation as an eco-friendly and efficient top-down approach to enhancing porosity on carbonaceous materials: the effect of inevitable oxygen functionalities on CO2 capture[END_REF] dry pyrolysis, but also wet pyrolysis in water [START_REF] Hu | Engineering Carbon Materials from the Hydrothermal Carbonization Process of Biomass[END_REF][START_REF] Titirici | Chemistry and materials options of sustainable carbon materials made by hydrothermal carbonization[END_REF][START_REF] Titirici | Back in the black: hydrothermal carbonization of plant material as an efficient chemical process to treat the CO2 problem?[END_REF] or in less conventional media, for example ionic liquids. [START_REF] Liu | Biomass-derived hierarchical porous carbons: boosting the energy density of supercapacitors via an ionothermal approach[END_REF][START_REF] Xie | Hierarchical porous carbonaceous materials via ionothermal carbonization of carbohydrates[END_REF][START_REF] Lee | Ionothermal carbonization of sugars in a protic ionic liquid under ambient conditions[END_REF] Ionic liquids (ILs) have been widely employed for the synthesis of advanced carbonaceous materials, as precursors, [START_REF] Fellinger | 25th Anniversary Article: "Cooking Carbon with Salt": Carbon Materials and Carbonaceous Frameworks from Ionic Liquids and Poly(ionic liquid)s[END_REF][START_REF] Zhang | Carbon materialization of ionic liquids: from solvents to materials[END_REF] porogens, [START_REF] Wu | Structure and morphology controllable synthesis of Ag/carbon hybrid with ionic liquid as soft-template and their catalytic properties[END_REF] catalysts and/or solvents. [START_REF] Xie | Hierarchical porous carbonaceous materials via ionothermal carbonization of carbohydrates[END_REF] Thanks to their easily tunable properties (e.g., viscosity, polarity, and thermal stability), ILs can be seen as designer solvents, especially for the deconstruction and conversion of raw lignocellulosic biomass and biopolymers. [START_REF] Wang | Ionic liquid processing of cellulose[END_REF][START_REF] Swatloski | Dissolution of cellose with ionic liquids[END_REF] This feature has made ILs prime candidate solvents for the preparation of biosourced carbonaceous materials. For instance, 1-butyl-3methylimidazolium chloride (BmimCl) proved to be highly efficient when used in the solvation of cellulose. [START_REF] Swatloski | Dissolution of cellose with ionic liquids[END_REF][START_REF] Remsing | Mechanism of cellulose dissolution in the ionic liquid 1-n-butyl-3-methylimidazolium chloride: a C-13 and Cl-35/37 NMR relaxation study on model systems[END_REF][START_REF] Zhang | 1-Allyl-3-methylimidazolium chloride room temperature ionic liquid: A new and powerful nonderivatizing solvent for cellulose[END_REF] When mixed with iron(III) chloride (FeCl 3 ), BmimCl yields BmimCl/FeCl 3 compounds that display all characteristics of conventional ionic liquids (i.e., high temperature stability, high solvation ability and negligible vapor pressure) together with singular properties such as paramagnetism, moisture-stability and intermediate Lewis acidity. [START_REF] Sitze | Ionic liquids based on FeCl3 and FeCl2. Raman scattering and ab initio calculations[END_REF] Benefiting from these unique features, Xie et al. proposed the use of BmimFeCl 4 (obtained from an equimolar mixture of BmimCl/FeCl 3 ) for the thermochemical treatment of simple saccharides [START_REF] Xie | Hierarchical porous carbonaceous materials via ionothermal carbonization of carbohydrates[END_REF][START_REF] Xie | Mesoporous graphite nanoflakes via ionothermal carbonization of fructose and their use in dye removal[END_REF][START_REF] Lin | Ionothermal synthesis of microporous and mesoporous carbon aerogels from fructose as electrode materials for supercapacitors[END_REF] and even raw lignocellulosic biomass. [START_REF] Liu | Biomass-derived hierarchical porous carbons: boosting the energy density of supercapacitors via an ionothermal approach[END_REF] This thermochemical approach, which allows carbonization under mild conditions (i.e., < 10 bars and < 240 °C), has been called ionothermal carbonization (ITC), by analogy with the hydrothermal carbonization (HTC) process. ITC in BmimFeCl 4 allowed obtaining carbonaceous materialscalled ionocharswith high specific surface areas and high pore volumes. According to the authors, BmimFeCl 4 had a multiple role: (i) a porogen; (ii) a Lewis acid catalyst for the carbonization process, and (iii) a recyclable solvent. [START_REF] Xie | Hierarchical porous carbonaceous materials via ionothermal carbonization of carbohydrates[END_REF] Besides BmimFeCl 4 , organic ILs were also employed for the ionothermal treatment of saccharides and raw lignocellulosic compounds. [START_REF] Zhang | Updating Biomass into Functional Carbon Material in Ionothermal Manner[END_REF] Using Bmim-type ILs with different anions (i.e. Cl -, BF 4 and Tf 2 N -), Wang et al. [START_REF] Zhang | Updating Biomass into Functional Carbon Material in Ionothermal Manner[END_REF] showed a correlation between the size of the anion and the textural properties of the ionocharlarger anions yielding higher pore volumefurther confirming the versatility of the ITC process. Ionochars prepared from various ILs were successfully employed as adsorbents, [START_REF] Xie | Mesoporous graphite nanoflakes via ionothermal carbonization of fructose and their use in dye removal[END_REF] electrocatalysts [START_REF] Zhang | Updating Biomass into Functional Carbon Material in Ionothermal Manner[END_REF][START_REF] Zdolsek | Electrocatalytic Activity of Ionic-Liquid-Derived Porous Carbon Materials for the Oxygen Reduction Reaction[END_REF] and electrode materials for supercapacitors [START_REF] Liu | Biomass-derived hierarchical porous carbons: boosting the energy density of supercapacitors via an ionothermal approach[END_REF][START_REF] Lin | Ionothermal synthesis of microporous and mesoporous carbon aerogels from fructose as electrode materials for supercapacitors[END_REF].

While full of promise, the ITC process is still a young topic with reaction mechanisms yet to elucidate and a limitless variety of carbon precursor/IL systems to investigate. In this article, we proposed to further explore the ITC process using a set of BmimCl/FeCl 3 compounds and polysaccharides for the preparation of carbon negative electrodes in Li-ion batteries (LIBs). The use of four polysaccharides (starch, chitosan, pectin and alginic acid) and three different FeCl 3 /BmimCl molar ratios (0.5, 1 and 1.5) allowed tuning both textural properties and structural functionalities. We demonstrated a correlation between the ionic structures in FeCl 3 /BmimCl ILs at different molar ratios and the nanoporosity of the asprepared ionochars. The amount of residual iron species entrapped within the carbonaceous network was adjusted and provided an additional control over the degree of graphitization.

Importantly, the external pore volume and the incorporation of nitrogen functional sitesthrough the use of chitosanwere identified as important parameters to improve the electrochemical performance of the as-obtained carbons. In this contribution, we wish to show the versatility offered by the ITC process for the synthesis of advanced carbon materials and to produce new insight regarding the use of such materials as negative electrodes in LIBs.

Experimental section

Materials

Alginic acid from brown algae (from Macrocystis pyrifera), pectin from citrus peel (Galacturonic acid ≥74.0 %), chitosan (low molecular weight, 50-190 kDa), starch from potato and N-Methyl-2pyrrolidone (NMP) (anhydrous, 99.5 %) were purchased from Sigma-Aldrich (France). Carboxymethyl cellulose sodium salt (CMC; DS = 0.9, Mw = 250,000) was purchased from Aldrich. Iron (III) chloride hexahydrate was purchased from Acros Organics.

1-Butyl-3-methylimidazolium chloride (99 %) was purchased from Iolitec (Germany). Super P (>99 %) was purchased from Alfa Aesar (France). All reagents were used without further purification.

Preparation of hydrochars, ionochars and carbon materials

Carbon materials were synthesized by ionothermal carbonization (ITC) of polysaccharides followed by dry pyrolysis under argon. The first step consisted in the thermochemical treatment of polysaccharides in an ionic liquid (IL) in an autoclave, under autogenous pressure. To do so, three FeCl 3 /BmimCl ILs were prepared by mixing and slightly heating (up to 60 °C) the two components (i.e. FeCl 3 and BmimCl) at three different FeCl 3 /BmimCl molar ratios (0.5; 1; and 1.5). As obtained iron-based ionic liquids were then dried at 120 °C under vacuum for 6 hours. Four polysaccharides (i.e. chitosan, starch, pectin and alginic acid) were used as carbon precursors. The preparation of the porous carbon materials (Fig. 1) consisted in treating 1 g of polysaccharides in 10 mL IL in a Teflon lined stainless steel autoclave (23 mL volume) at 240 °C during 20 h. The as-obtained aromatic carbon networks, called ionochars, were washed twice in demineralised water (50 mL) and once in ethanol (50 mL), filtered with a 0.22 µm PVDF membrane in order to eliminate and recover the IL, and dried in an oven at 80 °C. Ionochars derived from chitosan, starch, pectin and alginic acid were named respectively CI0.5, SI0.5, PI0.5 and AI0.5 when the used IL was prepared with a FeCl 3 /BmimCl molar ratio of 0.5. In the case of FeCl 3 /BmimCl molar ratios of 1 and 1.5, ionochars were named respectively XI1 and XI1. 5 (X = C,S,P,A). As a comparison, an ionochar was prepared by treating 1 g of starch in 10 mL BmimCl in a Teflon lined stainless steel autoclave (23 mL volume) at 240 °C during 20 h. The resulting material was named SI0. The dried ionochars were pyrolyzed at 900 °C for 2 h with a heating rate of 10 °C min -1 under argon flow (45 mL min -1 ). As-obtained carbon materials were named XPY (X = C, S, P, A; corresponding to the polysaccharide) and (Y = 0.5, 1, 1.5; corresponding to the FeCl 3 /BmimCl molar ratio). An alternative procedure was also applied to ionochars obtained from chitosan with the lowest FeCl 3 /BmimCl molar ratio (i.e., 0.5). In order to increase the amount of residual iron within the carbonaceous framework, the washing step before pyrolysis was replaced by a simple filtration through a 0.22 µm PVDF membrane without any addition of water or ethanol. After oven-drying and subsequent pyrolysis, the as-obtained carbon was named CP0.5nw. CP0.5nw was then washed twice in 1 M hydrochloric acid. The as-prepared carbon was named CP0.5nw-HCl.

As a comparison, a hydrochar was prepared by treating 1 g of starch in 10 mL water in a Teflon lined stainless steel autoclave (23 mL volume) at 240 °C during 20 h. The dried hydrochar underwent pyrolysis as described previously for ionochars. The resulting carbon material was named SHP.

Characterization

Scanning electron microscopy (SEM) images were obtained using a Hitachi S-4800 electron microscope. Transmission electron microscopy (TEM) images were acquired with a JOEL 1200 microscope. N 2 physisorption experiments were carried out at -196 °C on a Micromeritics 3Flex; ionochars were degassed at 150 °C and carbons (pyrolyzed ionochars) were degassed at 250 °C for 6 h under high vacuum (ca. 0.1 Pa) before physisorption measurements. Raman spectra were acquired on a Horiba Jobin-Yvon LabRAM ARAMIS microspectrometer with an excitation wavelength of 473 nm. XRD patterns were recorded using a PANalytical X'Pert Pro MPD diffractometer, with the Kα radiation of Cu (λ = 1.5418 Å) and a step size of 0.033° into the 10°-70° interval. X-ray photoelectron spectroscopy (XPS) analyses were performed on a ESCALAB 250 ThermoElectron spectrometer using a monochromatized Al Kα source (h = 1486.6 eV) and a 500 µm spot size. Galvanostatic electrochemical characterizations were carried out at room temperature on a BTS 3000 instrument (Neware Battery).

Electrode preparation

Electrode slurries with different formulations were prepared using carbonaceous materials derived from polysaccharides as active materials AM, Super P as conductive carbon additive CA and polyvinylidene fluoride (PVDF) or carboxymethylcellulose sodium salt (CMC; DS = 0.9, Mw = 250,000) as the binder. Three AM:CA:binder weight ratios were used (i.e., 80:10:10, 90:0:10 and 70:18:12). In the case of PVDF, N-methyl-2-pyrrolidone (NMP) was used as solvent, whereas deionised water was used in the case of CMC. Slurries were mixed using an agate grinding jar containing four 4 mm diameter agate balls. A Fritish Pulverisette 7 was used to mill the mixture at 500 rpm for 1 h and then the slurry was tape casted uniformly at 150 µm onto 18 µm thick copper current collector. Electrodes with a diameter 12.7 mm were cut using a disk cutter and dried under vacuum for 6 h at 80 °C. Coin cells were assembled under Ar atmosphere (O 2 < 0.5 ppm, H 2 O < 0.5 ppm) in glove box (MBraun).

Lithium metal was used as reference and counter electrode. The electrolyte used was LP30 (1M LiPF 6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC:DMC = 1:1). Whatman glass fiber disk were used as separators. The electrochemical galvanostatic measurements were carried out in the voltage of 3.00-0.01 V versus Li + /Li at different current densities.

Results and discussion

Ionochars and ionochar-derived carbons

Carbon materials were synthesized in a three-step procedure as described in Fig. 1: (i) ionothermal carbonization (ITC) of polysaccharides in FeCl 3 /BmimCl ILs, (ii) separation of the as-obtained ionochars, and (iii) pyrolysis under argon flow at 900 °C to obtain porous carbons. The first step, so-called ITC, is a thermochemical process which allows carbonization of (poly)saccharides under mild conditions (i.e. < 10 bars and < 240 °C) and proceeds in a similar way as hydrothermal carbonization (HTC). [START_REF] Falco | Hydrothermal Carbon from Biomass: Structural Differences between Hydrothermal and Pyrolyzed Carbons via C-13 Solid State NMR[END_REF][START_REF] Falco | Morphological and structural differences between glucose, cellulose and lignocellulosic biomass derived hydrothermal carbons[END_REF] Therefore, the conversion of polysaccharides into ionochars is assumed to proceed in three main steps: (i) hydrolysis into monosaccharide units, (ii) dehydration of monosaccharides into furan derivatives, and (iii) polycondensation of furan derivatives into polyfuranic species. [START_REF] Titirici | Sustainable carbon materials[END_REF][START_REF] Falco | Hydrothermal Carbon from Biomass: Structural Differences between Hydrothermal and Pyrolyzed Carbons via C-13 Solid State NMR[END_REF][START_REF] Falco | Morphological and structural differences between glucose, cellulose and lignocellulosic biomass derived hydrothermal carbons[END_REF][START_REF] Titirici | Hydrothermal carbon from biomass: a comparison of the local structure from poly-to monosaccharides and pentoses/hexoses[END_REF] At high temperature, typically at 240 °C as described here, the carbonization process continues via intramolecular reactions (i.e., condensation, dehydration and decarbonylation reactions) which lead to the production of condensed polyaromatic domains. At that stage, it is important to notice that ITC occurs under autogenous pressure induced by water produced during dehydration/condensation reactions and by low-boiling point organic by-products. ITC of carbohydrates in BmimFeCl 4 was previously described by Xie et al., and the key role of this IL was clearly evidenced. [START_REF] Xie | Hierarchical porous carbonaceous materials via ionothermal carbonization of carbohydrates[END_REF] FeCl 3 acts as a Lewis acid catalyst involved in the different stages of the carbonization process. Its combination with BmimCl yields a room-temperature IL which acts as a porogen and as a solvent with superior solvation properties compared with water. Thus, ITC of carbohydrates in BmimFeCl 4 generates porous ionochars with a higher carbonization yield compared with analogous hydrochars obtained by the HTC process.

Herein, ionochars were synthesized from four polysaccharideschitosan (C), starch (S), pectin (P) and alginic acid (A) in BmimFeCl 4 . The nitrogen sorption isotherms and textural properties of the four corresponding ionochars are presented in Figure S1. Significant differences were observed depending on the polysaccharide. While ionochars derived from pectin (PI1) and alginic acid (AI1) display type-I isotherms related to microporous materials, starch-derived ionochar (SI1) displays a type I/II isotherm with a H3 hysteresis loop, related to micro-macroporous materials. [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF] A slight H4 hysteresis loop was observed for AI1, suggesting the presence of slit-like mesopores. Unlike the three other polysaccharides, chitosan yielded a nonporous ionochar (CI1; Fig. S1). As mentioned previously, BmimFeCl 4 is supposed to act as a porogen and as a catalyst during the ITC process. This feature was demonstrated by comparing a hydrochar obtained from starch through HTC in water (SH), an ionochar obtained from starch through ITC in BmimCl (SI0) and the analogous obtained through ITC in BmimFeCl 4 (SI1). While SI1 displays a BET equivalent specific surface area (SSA) of 200 m 2 g -1 and a mass yield of 50 wt.% (Table S1), both SH and SI0 are non-porous and present lower mass yields (36 and 38 wt.%, respectively, Table S1).

After pyrolysis at 900 °C, the shapes of isotherms and the hysteresis loops were preserved for PP1, AP1 and SP1 (Fig. 2a). A significant increase in SSA, inherent to the selfactivation process directed by the gases produced during pyrolysis, was noticed for each sample (Table 1). This feature was also observed for the carbon derived from chitosan (CP1), which displays a type-I isotherm (Fig. 2a). While SHP and SP1 display similar SSAs and micropore volumes (Table 1), SP1 displays a higher external pore volume due to the contribution of large mesopores and/or narrow macropores. The textures observed by SEM (Fig. 3) are in good agreement with the nitrogen physisorption data. Smooth surfaces were observed for the microporous carbons (PP1, AP1 and CP1), while a meso-macroporous structure made of aggregated nanoparticles was obtained for the starch-derived carbon (SP1).

Larger particles, typical of HTC-derived carbons, were observed for SHP (Fig. 3a).

Micrometric hollow spheres were observed for the pectin derived materials (Fig. 3c), suggesting supramolecular self-assemblies between pectin and imidazolium cations. Besides the equimolar FeCl 3 /BmimCl mixture, two other FeCl 3 /BmimCl molar ratios, 0.5 and 1.5, were used for the ITC of the four polysaccharides. The nitrogen sorption isotherms of the as-prepared carbons after pyrolysis are shown in Fig. 2b (ratio 0.5) and Fig. 2c (ratio 1.5), and the corresponding textural properties are reported in Table 1. For each polysaccharide, both SSA and pore volume (especially external pore volume) reached a maximum for the lowest FeCl 3 /BmimCl molar ratio (i.e., 0.5). In a recent study, Li et al. [START_REF] Li | Ionic Structures of Nanobased FeCl[END_REF] observed the formation of nanostructures in FeCl 3 /BmimCl ILs at different molar ratios. The sizes of the local domains were found to be tens of nanometers. Interestingly, the authors showed that the compactness and the thermal stability of the nanostructures in FeCl 3 /BmimCl ILs were significantly higher when decreasing the FeCl 3 /BmimCl molar ratio from 1 to 0.6. These features were explained based on the stronger inter-ionic interactions in ILs with lower FeCl 3 /BmimCl molar ratios, i.e., when Cl -and FeCl 4 -coexist. Li et al. [START_REF] Li | Ionic Structures of Nanobased FeCl[END_REF] further supported this statement by comparing conductivities and glass transition temperatures. In our study,

(a) (b) (c)
carbons obtained in ILs displaying nanostructures with higher compactness and higher thermal stability (i.e., molar ratio of 0.5) exhibit higher SSA and higher external pore volume (Table 1). Thus, ionic structures in FeCl 3 /BmimCl ILs appear as a critical factor that might impact supramolecular assemblies during the ITC process and tune, in fine, the nanoporosity of carbons.

Table. 1:

Textural properties of the carbon materials prepared in this study. † BET Specific Surface Area (SSA); ‡ Total pore volume determined at P/P 0 0.95 (adsorption);  Micropore and external pore volumes determined by the t-plot method.

Samples

SSA † (m 2 g -1 )

Pore volume ‡ (cm 3 g -1 )

Micropore volume  (cm 3 g -1 )

External pore volume  (cm 3 Besides nanoporosity, the influence of ITC and FeCl 3 /BmimCl molar ratio over the carbon microstructure was investigated by Raman spectroscopy. Fig. 4a shows the Raman spectra for the four carbon materials obtained in IL with a FeCl 3 /BmimCl molar ratio of 1. D and G bands can be observed at 1345 and 1583 cm -1 , respectively. Also, two broad peaks were observed in the 2650 to 3150 cm -1 region. They are related to the 2D (2690 cm -1 ) and D + G bands (2960 cm -1 ) of carbonaceous materials, and are typical of disordered pseudographitic structures with a low level of graphitization. Carbon materials derived from starch and prepared by ITC in FeCl 3 /BmimCl ILs with 0.5, 1 and 1.5 molar ratios were compared to an analogous starch-derived carbon prepared by HTC in pure water. The obtained Raman spectra of all the carbons were similar (Fig. 4b), showing that neither ITC (as compared with HTC, see sample SHP), nor the FeCl 3 /BmimCl molar ratio affected the degree of graphitization after pyrolysis. The same feature was observed for the other polysaccharidederived carbons. This behavior can be explained by the low concentration and/or heterogeneous dispersion of the residual iron species embedded in the ionochars (Table 2). In fact, after ITC, the ionochars were thoroughly washed with water and ethanol, to remove FeCl 3 /BmimCl before pyrolysis. In order to increase the amount of residual IL (and the amount of residual iron) within the carbonaceous framework, the washing step before pyrolysis was replaced by a short filtration without any addition of water or ethanol. This experiment was performed with chitosan in IL with a FeCl 3 /BmimCl molar ratio of 0.5 (sample named CP0.5nw). The amount of residual iron in CP0.5nw was significantly increased as compared with CP0.5; the proportion of iron estimated by SEM-EDX was 4.5 at.% in CP0.5nw against 0.17 at.% in CP0.5 (Table 2). The presence of residual iron during pyrolysis affected both the nitrogen content (Table 2) and the textural properties (Fig. 5a and Table 1). While CP0.5 displays a type I/II isotherm with a H3 hysteresis loop, CP0.5nw displays a type I/II isotherm with a H2(a) hysteresis loop. This H2(a) hysteresis loop is ascribed to pore-blocking effects and cavitation-induced evaporation of nitrogen trapped in pores connected by narrow necks. Moreover, CP0.5nw displays a lower pore volume and a lower SSA as compared with CP0.5. As shown in Fig. 5b, significant differences were also observed by Raman spectroscopy. As for CP0.5nw, the G band is thinner and is significantly more intense than the D band. The I D /I G ratio decreases from 0.8 for CP0.5 to 0.34 for CP0.5nw, suggesting a higher degree of graphitization in CP0.5nw.

The microstructure of carbons derived from chitosan was further investigated by XRD. XRD patterns (Fig. 5c) showed a broad and non-intense signal centered at about 26° for CP0.5, CP1 and CP1.5, which corresponds to the reflection of carbon atoms in the (002) plane. This feature is in good agreement with Raman spectroscopy and confirms that these three samples are amorphous. On the contrary, the peak centered at 26° is more intense and narrower for CP0.5nw, suggesting a better orientation of the aromatic carbon layer slices and a higher degree of graphitization, as already supported by Raman spectroscopy. Besides the peak associated with carbon, CP0.5nw shows several reflection peaks between 42° and 47° typical of iron carbide, Fe 3 C (Fig. S2). The intense reflection peak centered at 44.5° and the weak reflection peak centered at 65° also suggest the presence of Fe(0) in this sample. The carbothermal reduction of iron salts to iron carbide at high temperature is well known and was already described in ITC processes by Taubert et al. starting from BmimFeCl 4 . [START_REF] Xie | Hierarchical porous carbonaceous materials via ionothermal carbonization of carbohydrates[END_REF][START_REF] Gobel | Synthesis of mesoporous carbon/iron carbide hybrids with unusually high surface areas from the ionic liquid precursor [Bmim][FeCl4[END_REF] The presence of Fe(0) in Fe 3 C was also reported. [START_REF] Gobel | Synthesis of mesoporous carbon/iron carbide hybrids with unusually high surface areas from the ionic liquid precursor [Bmim][FeCl4[END_REF] The treatment of CP0.5nw in HCl yielded CP0.5nw-HCl and allowed the partial removal of the iron species. In the XRD pattern of CP0.5nw-HCl, the reflection peaks related to Fe 3 C were less intense (Fig. 5c) and the reflection peaks related to Fe(0) totally vanished (Fig. S2). These observations were confirmed by TEM. As shown in Fig. 6a-b, CP0.5 is mainly amorphous and no iron-containing particles could be identified, while CP0.5nw presents a snail-like graphitic carbon structure surrounding iron-based particles. These snail-like graphitic carbon structures were also observed by high resolution TEM (Fig. 6c), and graphitic plans were clearly identified. The profile of intercross distance (Fig. 6d) gave a plan-to-plan distance of about 0.34 nm, which corresponds to the distance between carbon layers in graphite.

Electrochemical performance as negative electrodes in lithium ion batteries

The different carbon materials synthesized in this study were tested as active materials (AM)

for the preparation of negative electrodes in Li-ion batteries. The four polysaccharide-derived carbons based electrodes (see the electrode preparation in the experimental part) treated in IL with a FeCl 3 /BmimCl molar ratio of 1 were compared by galvanostatic charge-discharge measurements (Fig. S3 and Table 3). As shown in Table 3, the first discharge capacities are 741, 634, 602 and 498 mAh g -1 for SP1, CP1, AP1 and PP1, respectively. All samples show a low coulombic efficiency (CE) for the first cycle: 38, 51, 44 and 50 % for SP1, CP1, AP1 and PP1, respectively. This poor CE is often correlated with the high surface area of the AM which promotes the irreversible degradation of the electrolyte and the formation of a solid electrolyte interphase (SEI). After about ten cycles, all samples show a stabilization of both capacity (270, 289, 245 and 231 mAh g -1 ) and coulombic efficiency (97, 98, 98 and 99 % for SP1, CP1, AP1 and PP1, respectively), with a good stability over 50 cycles (See Table 3).

The electrode derived from chitosan (CP1) presents the highest capacity. This result can be explained by the presence of nitrogen coming from chitosan (4.4 wt.% as shown in Table 2).

In fact, pyridinic and quaternary nitrogen atoms are expected to improve electronic conductivity and to introduce more defects and active sites in carbon frameworks, enhancing both lithium storage capacity and rate capability in carbon-based negative electrodes. [START_REF] Wu | The recent progress of nitrogen-doped carbon nanomaterials for electrochemical batteries[END_REF] At that stage, it is important to note that the carbons obtained from starch (SP1), pectin (PP1)

and alginic acid (AP1) display low nitrogen contents (≤ 1.0 wt.% according to elemental analyses), coming most probably from the presence of residual imidazolium moieties. In a previous study, Kim et al. [START_REF] Kim | Alginic acid-derived mesoporous carbonaceous materials (Starbon (R)) as negative electrodes for lithium ion batteries: Importance of porosity and electronic conductivity[END_REF] showed that the mesoporosity and the intrinsic electronic conductivity are two very important parameters when dealing with carbon-based negative electrodes for LIBs. Based on our results, nitrogen content seems to prevail over mesoporosity, as the specific capacity of SP1the sole mesoporous carbon of the seriesis significantly lower as the one of CP1 (Table 1). The influence of the IL (i.e. , the FeCl 3 /BmimCl molar ratio) used during ITC over the electrochemical performance was assessed for the materials obtained from starch (Fig. 7a), chitosan (Fig. 7b), pectin (Fig. 7c) and alginic acid (Fig. 7d). For all polysaccharides, a clear trend can be emphasized. SP0.5, CP0.5, PP0.5 and AP0.5, which are the samples with the largest external pore volume of their respective series (Table 1), present the highest reversible specific capacity (between 400 and 450 mAh g -1 at first cycle). As for the carbons derived from starch, pectin and alginic acid (i.e., the nitrogen-free polysaccharides), a clear correlation was observed between the external pore volume and the specific capacity (Table 1 and Fig. 7). Unlike these three polysaccharides, the external pore volume cannot be seen as the sole key parameter to impact the specific capacity of the chitosan-derived carbons. While CP1.5 displayed a higher external pore volume (0.14 cm 3 g -1 ) than CP1 (0.02 cm 3 g -1 ), CP1

clearly showed a higher specific capacity (Fig. 7b). As mentioned above, the nitrogen content seems to prevail over mesoporosity. This statement was confirmed also in this case, since CP1.5 has a lower nitrogen content than CP1 (Table 2), probably due to the larger amount of residual iron which appears to be responsible for nitrogen removal during pyrolysis. [START_REF] Ohtsuka | Nitrogen Removal during Atmospheric-Pressure Pyrolysis of Brown-Coal with Iron[END_REF] This behavior was also confirmed with CP0.5nw (Table 2). Electrodes formulation: active material, Super P (conductive carbon additive) and PVDF (binder) in the mass ratio of 80:10:10. Current density: 0.1 A g -1 . The specific capacities were calculated based on the mass of active material.

To further optimize the properties of these materials as negative electrodes in LIBs, the influence of the electrode formulation over electrochemical performance was studied. Thus, CP1 electrodes were prepared using different binders (CMC vs. PVDF) and different slurry compositions (Fig. S4). On the one hand, CMC allowed reaching higher specific capacities than PVDF. This behaviour was already observed by Kim et al. [START_REF] Kim | Alginic acid-derived mesoporous carbonaceous materials (Starbon (R)) as negative electrodes for lithium ion batteries: Importance of porosity and electronic conductivity[END_REF] for polysaccharide-derived carbon electrodes. On the other hand, for the CMC series, increasing the mass ratio CA:CP1 from 0:90 to 18:70 allowed increasing the reversible specific capacity of CP1 by c.a. 80 %, i.e. from 200 to 360 mAh g -1 (after subtracting the capacity contribution from Super P, ca. 40 mAh g -1 , as its intrinsic specific capacity is of ca. 150 -200 mAh g -1 ). [START_REF] See | Reversible Capacity of Conductive Carbon Additives at Low Potentials: Caveats for Testing Alternative Anode Materials for Li-Ion Batteries[END_REF] A similar trend was observed with CP0.5 (Fig. 8).

Considering the relatively high electronic conductivity of Super P (c.a. 280 S m -1 ) [START_REF] Kim | Sustainable polysaccharide-derived mesoporous carbons (Starbon (R)) as additives in lithium-ion batteries negative electrodes[END_REF], this feature suggests that both CP0.5 and CP1 are probably still limited by their low intrinsic electronic conductivity.

The electrochemical performance of CP0.5, CP0.5nw and CP0.5nw-HCl were compared also using an electrode formulation AM:CA:CMC of 70:18:12,. Interestingly, the reversible specific capacity of CP0.5 and CP0.5nw were very close (500 and 550 mAh g -1 respectively after 50 cycles; Fig. 8), while that obtained for CP0.5nw-HCl was more than 50 % lower (c.a. 200 mAh g -1 after 50 cycles). Thus, the presence of residual iron species, mainly Fe 3 C

and Fe(0), seems to prevail over the degree of graphitization in boosting the electrochemical performance.

Conclusions

In this study, we have demonstrated that the ionothermal carbonization (ITC) of various polysaccharides in FeCl 3 /BmimCl ionic liquids (ILs) can offer new opportunities for the preparation of carbon negative electrodes in Li-ion batteries (LIBs). We identified and varied several experimental parameters that allowed adjusting the chemical (nitrogen and iron content), the textural (specific surface area and pore volume) and the structural (degree of graphitization) properties of the as-obtained ionochars and carbon materials. We have demonstrated that the FeCl 3 /BmimCl molar ratio, which is related to the compactness and the thermal stability of ionic nanostructures in these ILs, is an important parameter allowing the tuning of the nanoporosity of the ionochars. We provided new insight into the influence of several important properties of the as-obtained carbon materials (i.e., external pore volume, nitrogen functional sites, residual iron species and degree of graphitization) over their electrochemical performance. The best electrochemical performance was obtained for the carbons prepared from chitosan (i.e., the carbon displaying the highest nitrogen content) and pre-treated in the ionic liquid with the lowest FeCl 3 /BmimCl ratio (i.e., 0.5) which yielded carbons with higher pore volumes. The presence of residual iron species, mainly Fe 3 C and Fe(0), seems to prevail over the degree of graphitization in boosting the electrochemical performance. This work further highlights the versatility offered by the ionothermal approach for tayloring the textural, structural and electrochemical properties of carbon materials, with potential applications in the field of energy storage and conversion..

Conflicts of interest

There are no conflicts to declare.

Fig. 1 .

 1 Fig. 1. Schematic representation of the procedure employed to prepare porous carbonaceous materials from the ionothermal carbonization of polysaccharides.

Fig. 2 .

 2 Fig. 2. N 2 physisorption isotherms of carbon materials prepared in this study.

Fig. 3 . 1 FeCl 3 / 1 FeCl 3 /

 31313 Fig. 3. SEM images of carbon materials obtained via HTC of starch (SHP), and via ITC in a 1:1 FeCl 3 /BmimCl IL of starch (SP1), alginic acid (AP1), pectin (PP1), chitosan (CP1) and in a 0.5:1 FeCl 3 /BmimCl IL of chitosan (CP0.5).

Fig. 4 .

 4 Fig. 4. Raman spectra of carbon materials (a) derived from four polysaccharides by ionothermal carbonization in IL with molar ratio FeCl 3 /BmimCl = 1 and (b) derived from starch and carbonized in different ILs and water.

Fig. 5 .

 5 Fig. 5. (a) N 2 physisorption isotherms and (b) Raman spectra of CP0.5 (0.17 at.% Fe) ‡ and CP0.5nw (4.5 at.% Fe) ‡ . (c) X-ray powder diffraction patterns of the five carbons derived from chitosan: CP0.5 (0.17 at.% Fe) ‡ , CP0.5nw (4.5 at.% Fe) ‡ , CP0.5nw-HCl (1.0 at.% Fe) ‡ , CP1 (0.45 at.% Fe) ‡ and CP1.5 (2.6 at.% Fe) ‡ . ‡ Iron contents according to SEM-EDX. X-ray diffraction profiles assigned to graphite [26-1079] (*); and Fe 3 C [01-085-1317] (¤). The intense reflection centered at 44.5°, mainlypresent in CP0.5nw, can be assigned to Fe 3 C (left shoulder or section of the reflection) and Fe(0). The reflection at 65°, mainly present in CP0.5nw, can be assigned to Fe(0).[START_REF] Gobel | Synthesis of mesoporous carbon/iron carbide hybrids with unusually high surface areas from the ionic liquid precursor [Bmim][FeCl4[END_REF]

Fig. 6 .

 6 Fig. 6. TEM images for (a) CP0.5 (0.17 at.% Fe) ‡ and (b) CP0.5nw (4.5 at.% Fe) ‡ . (c) Highresolution TEM images for CP0.5nw and (d) profile of intercross distance. ‡ According to SEM-EDX.

Fig. 7 .

 7 Fig. 7. Discharge/charge capacity as function of cycles for carbons derived from: (a) starch treated hydrothermally and ionothermally in three ILs (FeCl 3 /BmimCl = 0.5, 1, 1.5); (b) chitosan; (c) pectin; and (d) alginic acid treated in three ILs (FeCl 3 /BmimCl = 0.5, 1, 1.5).

Fig. 8 .

 8 Fig. 8. (a) Cycling test of CP0.5 (0.17 at.% Fe) ‡ , CP0.5nw (4.5 at.% Fe) ‡ and CP0.5nw-HCl (1.0 at.% Fe) ‡ , (b) rate capability test and Coulombic efficiency of CP0.5 (0.17 at.% Fe) ‡ and CP0.5nw (4.5 at.% Fe) ‡ . The specific capacities were calculated based on the mass of active material. ‡ According to SEM-EDX.

Table 2 .

 2 Weight proportion of nitrogen, N/C atomic ratio and atomic proportion of iron of the carbon materials used in this study.

		 XPS; † Elemental analysis; ‡ SEM-EDX.
	Samples	N/C (at.) 	N (wt.%) †	Fe (at.%)
	CP1.5	0.017	2.0	2.6 ‡ (0.09) 
	CP1	0.032	4.7	0.45 ‡ (0.09) 
	CP0.5	0.029	4.4	0.17 ‡ (n/a) 
	CP0.5nw	0.018	0.2	4.5 ‡ (0.58) 
	CP0.5nw-HCl	0.016	-	1.0 ‡ (0.57) 

Table 3 .

 3 Capacities and coulombic efficiency at the 1 st , 10 th , and 50 th cycles for carbons derived from

	polysaccharides and treated in IL (FeCl 3 /BmimCl = 1)
		Discharge	Reversible capacity (mAh g -1 )	Coulombic efficiency (%)
	Sample	capacity at 1 st	at 1 st	at 10 th	at 50 th	at 1 st	at 10 th	at 50 th
		cycle (mAh g -1 )	cycle	cycle	cycle	cycle	cycle	cycle
	SP1	741	294	270	263	38	97	98
	CP1	634	323	289	280	51	98	100

Acknowledgements

Financial support was received by the European Commission in the framework of POROUS4APP project (H2020 GA no. 666157). The authors would like to thank Didier Cot (IEM, France) for SEM analyses, Erwan Oliviero (ICGM) for High resolution TEM analyses, Valérie Flaud (ICGM) for XPS analyses and Lea Daenens (ICGM) for technical help in the collection of the Raman spectra.

Notes and references


Corto Bastien

Antoine Deleforge
email: antoine.deleforge@inria.fr

Cédric Foy
email: cedric.foy@cerema.fr

MEAN ABSORPTION COEFFICIENT ESTIMATION FROM IMPULSE RESPONSES: DEEP LEARNING VS. SABINE

We consider the problem of estimating the mean absorption coefficients of a room from an impulse response using supervised learning on simulated training sets. Two neural network architectures and two training dataset designs are considered. The proposed approach is shown to yield smaller estimation errors than the classical Sabine and Eyring formulas, despite not relying on any geometrical information on the room. Simulated results demonstrate the robustness of the approach under different challenging acoustic conditions.

INTRODUCTION

To improve the listening quality of a room, the main parameters an acoustician can act on are the absorption coefficients of its surfaces. These are generally frequencydependent and are typically expressed within a given octave band, here, b ∈ {.125, .25, .5, 1, 2, 4} kHz. To obtain the acoustic diagnosis of a room and deduce a renovation plan, one would ideally need to know the absorption profile α i (b) of each surface i in the room. No method is currently known to reliably estimate these quantities in situ, so in practice, the mean absorption coefficients ᾱ(b) weighted by the area S i of each surface are estimated instead. The conventional method to estimate them is to calculate the room's reverberation times RT 60 (b) by Schroeder integration on a measured room impulse response (RIR) [START_REF] Schroeder | New method of measuring reverberation time[END_REF] and to use these in the celebrated Sabine's law or its slightly more precise variant from Eyring:

ᾱEyring (b) = -ln (1 -0.163 • V /(S • RT 60 (b))) (1)
where V denotes the room's volume and S = i S i its total surface. These simple formulas stemming from reverberation theory [START_REF] Kuttruff | Room Acoustics -Fifth edition[END_REF] hold well under diffuse homogeneous sound fields such as those encountered in approximately cubic rooms with uniform absorption profiles. However, they are no longer accurate in environments such as corridors or acoustically treated rooms. In this study, we investigate the possibility of overcoming these limitations using supervised learning on large simulated training datasets. We show that neural networks trained to map RIRs to their corresponding ᾱ yield more accurate estimates than Sabine and Eyring, especially in non-homogeneous conditions, despite not needing to know V or S.

TRAINING SETS AND NETWORKS

As a good compromise between realism and computational time, we simulated RIRs using the hybrid shoe-box room acoustic simulator Roomsim [START_REF] Schimmel | A fast and accurate "shoebox" room acoustics simulator[END_REF]. One of its attractive features is to combine the image-source method [START_REF] Allen | Image method for efficiently simulating small-room acoustics[END_REF] to accurately model early specular reflections and an efficient raytracing method called diffuse rain to efficiently capture late scattering effects. The training and development datasets are respectively composed of 15k and 5k RIRs corresponding to shoe-box rooms with lengths and widths uniformly drawn at random in [1.5, 10] and heights in [2.5, 4]. These ranges incorporate homogeneous, elongated and flat geometries that are typical of housing and office buildings. A (source, receiver) pair spaced by at least 1 m is placed uniformly at random in each simulated room with a minimum distance of 0.5 m to each surface.

Two different approaches are considered to sample the acoustic properties of rooms. In the Unif. dataset, the 36 absorption coefficients (6 surfaces, 6 octave bands) are drawn uniformly and independently at random between 0 and 1. While this approach seems natural, we observed that over 90% of resulting RT 60 (b) values were lower than 0.4 s, which is not representative of commonly encountered room acoustics. To correct this, we designed a reflectivity-biased (RB) dataset as follows: (i) for each surface type (walls, floor, ceiling), toss a coin, (ii) on heads, draw a reflective, frequency-independent absorption profile uniformly at random in [0.1, 12], (iii) on tails, draw a non-reflective, frequency-dependent absorption profile, uniformly at random within ranges depending on the surface type, as defined in Table 1. These ranges were chosen to incorporate about 100 commonly encountered absorption profiles manually selected from available lab measurements 1 2 3 . The resulting dataset features better spread RT 60 s from 0 to 2 s. Finally, since scattering coefficients are harder quantities to define and measure in practice, a unique coefficient for all surfaces in each octave band was drawn uniformly at random, in [0, 1] for the Unif. dataset, and in [0, 0.3] for octave bands < 1 kHz and [0.2, 1] otherwise for the RB dataset, following measures in [START_REF] Vorländer | Definition and measurement of random-incidence scattering[END_REF].

Simulated RIRs are downsampled to 16 kHz, cropped to 0.5 s, corrupted by additive white Gaussien noise with a 30 dB signal-to-noise ratio (SNR) and normalized to have maximum value of 1, yielding input vectors in R 8000 . Two neural network architectures are considered. The first one is a multilayer perceptron (MLP) with 3 fully connected hidden layers of successive dimensions 128, 64 and 32, each followed by exponential linear units (ELUs). The second one is a convolutional neural network (CNN) starting with 3 1D-convolutional hidden layers with respective filter sizes 33, 17, 9 and number of filters 64, 32 and 16, each followed by a maxpool-4 layer and ELUs. The resulting output of dimension 2000 is then passed through a fully connected hidden layer of size 32 with ELUs. For each network, a fully-connected output layer followed by sigmoids is used to yield the desired output ᾱ∈ [0, 1] 6 , with a mean squared error loss. Networks are optimized over training sets using the default Pytorch implementation of ADAM and batches of size 1000. Parameters yielding the lowest loss on development sets over 200 epochs are kept.

RESULTS AND CONCLUSION

Absolute errors on absorption coefficients over different test sets containing 500 simulated RIRs each are reported in Fig. 1. In (a), only absorption profiles corresponding to real materials 1 2 3 are used. We see that networks trained on the naive Unif. set do not succeed in outperforming classical approaches. However, estimation errors twice smaller as Eyring and with much less variance are obtained using the RB training set. In (b), we see that as expected, the advantage of supervised learning having access to full RIRs is clearest under room geometries departing from the homogenous assumption of reverberation theory. In (c), we see that performances of CNN-RB de-grade under low RT 60 's, which is expected as less information is then available in RIRs, and in (d), that it degrades under higher noise. Nevertheless, the supervised learning approach preserves a significant advantage in all explored conditions. Finally, we constructed a test set with similar RT 60 and geometry as RB, but using the pyroomacoustic simulator [START_REF] Scheibler | Pyroomacoustics: A python package for audio room simulation and array processing algorithms[END_REF], which significantly differs from Roomsim by handling only frequency-independent absorption profiles, not incorporating diffusion, and using sinc kernels instead of finite impulse response filters. The mean errors of Eyring and CNN-RB were respectively 0.093 and 0.085 on this test set. This encouragingly suggests that the supervised learning approach did not severely overfit the specific training simulator. Future work will include testing the approach on measured RIRs and extending it to the much more challenging estimation of individual surface absorption profiles, by leveraging multiple RIRs per room.

Figure 1 .

 1 Figure 1. Errors on mean absorption coefficients using the proposed neural networks (MLP, CNN) trained on two datasets (Unif., RB) vs. reverberation theory (Sabine, Eyring). (a) Realistic absorption profiles, (b) Homogenous vs. flat vs. elongated room geometries, (c) RT 60 (b) < 0.5 s vs. RT 60 (b) ∈ [0.5, 1.5] s vs. RT 60 (b) > 1.5 s, (d) Varying SNR.

Table 1 .

 1 Absorption coefficients ranges in the RB dataset.

	1 https://www.acoustic-supplies.com/absorption-coefficient-chart/
	2 https://www.acoustic.ua/st/web absorption data eng.pdf
	3 http://www.acouphile.fr/materiaux.html
	freq. (Hz)		125	250	500	1k	2k	4k
	Walls Floor Ceiling	α min i α max i α min i α max i α min i α max i	0.01 0.50 0.01 0.20 0.01 0.70	0.01 0.50 0.01 0.30 0.15 1.00	0.01 0.30 0.05 0.50 0.40 1.00	0.01 0.12 0.15 0.60 0.40 1.00	0.01 0.12 0.25 0.75 0.40 1.00	0.01 0.12 0.30 0.80 0.30 1.00