Surface functionalization to abate the irreversible capacity of hard carbons derived from grapefruit peels for sodium-ion batteries

Journal Pre-proof Surface functionalization to abate the irreversible capacity of hard carbons derived from grapefruit peels for sodium-ion batteries Luis A. Romero-Cano, Helena García-Rosero, Francisco Carrasco-Marín, Agustín F. Pérez-Cadenas, Linda V. González-Gutiérrez, Ana I. Zarate-Guzmán, Guadalupe Ramos-Sánchez PII:

S0013-4686(19)31844-4

DOI:

https://doi.org/10.1016/j.electacta.2019.134973

Reference:

EA 134973

To appear in:

Electrochimica Acta

Received Date: 27 April 2019 Revised Date:

24 September 2019

Accepted Date: 28 September 2019

Please cite this article as: L.A. Romero-Cano, H. García-Rosero, F. Carrasco-Marín, Agustí.F. PérezCadenas, L.V. González-Gutiérrez, A.I. Zarate-Guzmán, G. Ramos-Sánchez, Surface functionalization to abate the irreversible capacity of hard carbons derived from grapefruit peels for sodium-ion batteries, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.134973. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Surface functionalization to abate the irreversible capacity of hard carbons derived from grapefruit peels for Sodium-Ion batteries Luis A. Romero-Cano [a, b ,1*], Helena García-Rosero [b], Francisco Carrasco-Marín [b], Agustín F. Pérez-Cadenas [b], Linda V. González-Gutiérrez [c], Ana I. ZarateGuzmán [c,1], Guadalupe Ramos-Sánchez [a, d*] [a]

Departamento de Química. [d] Departamento de Ingeniería de Procesos e Hidráulica, CONACYT Fellow, Universidad Autónoma Metropolitana-Iztapalapa. 09340. Ciudad de México. MÉXICO [b] Grupo de Investigación en Materiales de Carbón, Facultad de Ciencias, Universidad de Granada. Av. Fuente Nueva, s/n. Granada, 18010. ESPAÑA [c] Centro de Investigación y Desarrollo Tecnológico en Electroquímica (CIDETEQ) S.C., Parque Tecnológico Sanfandila, Pedro Escobedo, Querétaro, 760703, MÉXICO * e-mail: [email protected] (GRS), [email protected] (LARC) [1]

Present address: Facultad de Ciencias Químicas. Universidad Autónoma de Guadalajara. Zapopan. Av. Patria 1201, C.P. 45129. Zapopan, Jalisco. MÉXICO

Abstract Carbon materials obtained from biomass are on the route to become the anode material for Na Ion Batteries; however, the large irreversible capacity limits their application, while the high synthesis temperature might have a great impact on the process sustainability. In this work, carbon materials derived from Grapefruit peels are proposed as sustainable anode materials for energy storage in Na-Ion batteries. The low temperature carbonization produces highly porous carbon presenting high irreversible capacity for Na intercalation. In order to increase the reversible capacity, a surface functionalization process with Melamine, Urea and Citric acid is proposed. The functionalization processes modify the disorder within the material and add functional groups on the surface leading to the attainment of higher reversible capacity; particularly the Urea functionalized sample

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possess a much higher reversible capacity and capacity retention at higher C-rates due to a less reactive surface and enhanced Na diffusivity. Thus, the functionalization has an important effect on the transport of sodium inside the pores but also on the increase of the disordered nature which permits stable sodium intercalation capacity, around 180 mAh g-1 after 60 charge/discharge cycles at several C-rates, which is a drastic improvement of the properties of unfunctionalized materials obtained at low carbonization temperature. This strategy fully exploited can lead to new avenues for the reutilization of biomass waste into energy storage applications. Keywords: Biomass derived anodes; Na+ intercalation; Hard Carbons; Carbonization at low temperatures; Functionalization effect; Stable operation anode.

1. Introduction Sun and Wind are vast primary resources of renewable energy, representing an important alternative to fossil fuels. However, these primary sources can only be competitive if they are coupled to energy storage systems as a way to balance the demand/consumption peaks as well as to provide strategies to implement a decentralized energy generation scheme. Energy storage systems based on electrochemical batteries are broadly utilized in several fields; however, in order to represent a viable alternative for renewable energy storage, the battery should possess several characteristics; among them, they should provide high energy density, possess high durability (similar to that of the generation system) and, very important, have low-price for all the system component`s. Lithium Ion batteries (LIB) have been long recognized as the electrochemical battery with highest specific energy and durability; however, the uneven distribution of Lithium on the earth crust and its high price

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make it a questionable alternative [1,2]. Due to these reasons, Na Ion batteries (NIB) are becoming a practical alternative due to the similar operation mechanism than LIB while the movable ion, sodium, presents a much higher availability and uniform worldwide distribution. Nowadays, several materials have been developed as cathodes for Na-ion batteries [3] leading to operation voltages and specific capacities closer to those obtained for LIB; however, the anodic material is still problematic since graphite, the prototypical LIB anode material is not stable for repeated Na intercalation/deintercalation processes. As an alternative, the search for new carbon structures is intended to increase the reversibility and durability towards sodium intercalation [4,5]. Among the different proposed materials, hard carbons obtained from the pyrolysis of cellulosic precursors at temperatures below 1500°C [6], exhibit specific capacities from 100 to 300 mAh g-1. The resulting structures are mainly highly disordered with a porosity defined by the pyrolysis temperature [7], exhibiting high specific capacities for Na intercalation, abruptly decaying in the following charge/discharge cycles. It has been demonstrated that the reactivity of the surface of graphitic materials is related to the surface chemistry and size which can be influenced by the synthesis method and performed posttreatments [8]. Hard carbons are usually synthesized at temperatures above 1000°C; thus, finding ways to lower the synthesis temperature might have a deep impact on the process sustainability. Therefore, the main objective of this work is to demonstrate that carbon materials synthesized with low energy conditions (600°C) can lead to adequate properties for Na+ intercalation. As an alternative to diminish the irreversible capacity and increase the performance of the materials the surface functionalization is proposed as an interesting alternative. This strategy is different to nitrogen doping, which has been extensively reported [9–11]. 3

The functionalization process is not only intended to diminish the amount of dangling bonds but also to serve as a way to homogenize the surface chemistry of the carbon material; moreover, the functional groups might provide extra capacitive storage mechanisms leading to a higher capacity at higher c-rates [12–15]; it is important to mention that surface functionalization has been poorly investigated and its effect is not fully understood [16]. Different sources have been proposed to obtain carbon materials [17,18], in this work, grapefruit peels were used as cellulosic precursor for the synthesis of carbon materials trough the carbonization at low temperatures to form meso and micropores in the carbonized material [19]. Grapefruit peels are not only very cheap precursors but also represent an alternative for their final disposition since their huge volume represent a problem in traditional removal processes. Then, the resulting carbonaceous materials are functionalized with nitrogen functional groups, using relatively cheap and simple chemical treatments in order to form a more reversible system. [20,21]. The obtained materials were used as anodes in Sodium Ion batteries and their properties were correlated with the morphological and physicochemical properties.

2. Experimental (Materials and Methods)

2.1 Preparation of functionalized carbons from grapefruit peels. The carbon materials were prepared from grapefruit peels (Citrus x Paradisi); in a first step, peels were washed with abundant distilled water, cut into small pieces and dried at 110 ºC for 12 hours; then a pyrolysis process was carried out in a tubular stainless steel

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furnace up to 600 ºC for 1 hour with a heating ramp of 5 ºC min-1 under nitrogen atmosphere (flow of 100 cm3 min-1). The obtained materials were named GPC. Carbon materials functionalization was carried out using a NaOH and citric acid treatment followed by, a further modification process with urea or melamine. Firstly, the carbon materials were washed and stirred for 1 h in a 0.1 M NaOH solution in a 10% w/v ratio at room temperature. The materials were washed with distilled water and then dried overnight at 110 ºC. Afterwards, citric acid treatment was used to introduce carboxyl groups onto the carbon surface; those functionalities serve as binding sites for the amino groups; for this purpose, 15 g of the GPC were immersed in 300 mL of a 0.6 M citric acid solution and kept under stirring for 2 hours at 80 ºC; finally the carbon material was washed until constant pH values were reached. Dry samples were labeled as GPC-AC. Urea and melamine modification were performed by the incipient wetness impregnation method in a carbon to nitrogen precursor weight ratio of 0.5 for urea and 14.3 for melamine (GPC-AC/ urea or melamine), the obtained samples were then designed as GPC-AC-U for samples modified with urea and GPC-AC-M for samples modified with melamine. For this purpose, the nitrogen containing precursor was dissolved in a minimum quantity of solvent, then the solution was slowly added into a container with the solid, then the solvent in excess was eliminated by evaporation. Once dried, the samples were thermally treated at the same conditions as the initial pyrolysis process (600 ºC for 1 hour with a heating ramp of 5 ºC min-1).

2.3 Characterization of prepared materials.

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The specific surface area (SBET) of the prepared carbons was studied by nitrogen adsorption-desorption isotherms at -196 °C in a QUADRASORB-SI equipment (Quantachrome Ins). In addition, mercury intrusion porosimetry studies were carried out in a Quantachrome Poremaster, to obtain pore volume, total pore area accessible to mercury and pore size distribution curves for the larger macropores and mesopores. The morphology of the materials was studied by means of scanning electron microscopy (SEM-EDX) in a JEOL model JSM-6510LVEDS microscope and a high-resolution transmission electron microscopy (HRTEM) HADF FERI TITAN. Elemental analysis (Carbon, nitrogen, hydrogen, oxygen and Sulphur) for all samples was obtained in a Thermo Finningan Flas EA1112 CHNS-O elemental analyzer. The functional groups introduced on the materials were studied by FTIR spectroscopy in the region of 400 to 4000 cm-1 using a Bruker Tensor 27 spectrophotometer equipped with the ATR module. To evaluate the degree of graphitization, X-ray diffraction (XRD) experiments were carried out. These experiments were performed at room temperature with a Bruker D8 advance diffractometer at 0.17° min-1, from 10 to 70° (2θ); additionally, a Thermo Scientific DRX RAMAN micro spectrophotometer was used at a laser wavelength of 780 nm and a power of 4 mW in a range of 200 to 2400 cm-1. Finally, X-ray photoemission spectroscopy (XPS) was used to characterize the surface chemistry of the carbons on its outer surface using a K-Alpha+ - Thermo Scientific spectrometer. XPS spectra were obtained with a monochromatic source of Al Kα X-ray (1486.71 eV) working at 150 W, 15 kV and 10 mA and a pressure base of 3 x 10-8 Torr in the analytic chamber; for wide-scan spectra, an energy range of 0 – 1350 eV with an energy step of 80 eV and a step size of 1 eV was utilized. Once the peaks were determined, high

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resolution spectra were obtained at an energy step of 40 eV and a step size of 0.1 eV. Each spectral region of interest (C1s, O1s and N1s) was scanned several times in order to obtain acceptable signal to noise ratios. For obtaining the number of components, the position of each peak and the peak area, after correcting the background signal, the resulting spectra were adjusted to Lorentz and Gauss plots (Voigt function). Assignment of peaks was conducted following the most recent literature.

2.4 Electrochemical characterization as Na ion battery electrodes The electrodes were prepared following the methodology reported by Guzmán and col., [22] briefly, slurries were prepared containing 80 wt % of the as synthesized material, 10 wt % Super-P carbon battery grade (particle size 40 nm from Denka Singapore Private Ltd) and 10 wt % PDVF as binder (Aldrich Chemical Company). The slurry was prepared in a three step process consisting on dissolving PVDF in N-metilpirrolidone, (Aldrich Chemical Company, NMP) with magnetic stirring in a ratio of 0.2 mL of NMP per 6 mg PVDF. Then, the conductive additive was added to the obtained solution and it was stirred for another 20 minutes. Finally, the obtained carbon was added to the stirring chamber and the mixture was heated at 50°C and stirred during 20 hours in enclosed vessels in order to avoid NMP evaporation and solid phases sedimentation. The obtained slurry was coated onto a carbon coated aluminum foil by the Dr. Blade technique using a 100 µm coating thickness. After coating, the electrode was dried first at 50°C in air atmosphere, and then at 120°C during 10 hours under vacuum. After drying the electrodes were roll pressed using rotating rollers at 90°C using a 30 µm roll to roll

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separation in a MSK-HRP-01 (MTI). Finally, the electrodes were circle shaped cut (18 mm diameter). All electrochemical experiments were performed in an ECC-Combi cell (EL-CELL) using cleansed sodium foil as counter and reference electrode. The electrolyte was prepared with equivalent volumetric amounts of Ethylene Carbonate (EC), Propylene Carbonate (PC) and Dimethyl Carbonate (DMC) containing 1M NaPF6. The electrolyte was impregnated in a glass fiber separator (Whatman) and sandwiched between the carbon containing electrode and sodium foil. All measurements were performed in a multi potentiostat/galvanostat VMP3 (Biologic Science Instruments). For charge/discharge experiments, the upper cutoff voltage limit was set at 3 V and the limit of the discharge voltage was set at 0.003 V. The rate performance test consisted of subsequent full discharges at C/10, 1C, 5C, 10C, and C/10, taking into account the theoretical capacity of graphite. The electrochemical characterization was performed after the cells recently assembled and subjected to a period of stabilization of 2 hours. EIS spectra were collected after this period to detect assembly irregularities, and under stable OCP conditions before the charge/discharge cycles. A wave amplitude of 10 mV was used because it was small enough to allow linearization of the response to the input signal, but also high enough to yield a response that was detectable from the measurement noise. The frequency was varied from 1 MHz to 0.01 Hz at 8 points per decade.

3. Results and Discussion 3.1 Physical-chemical characterization of prepared carbons

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The textural properties of the hard carbon materials, studied by nitrogen physisorption and mercury intrusion porosimetry are presented in Table 1. The prepared materials possess low surface area (SBET) between 3.2 – 10.9 m2 g-1, which is typical of hard carbons prepared from cellulosic residues at low pyrolysis temperature [23] (supplementary information Fig. S1a shows the N2 adsorption isotherms). The surface area determined by mercury intrusion porosimetry is very similar to that obtained by nitrogen physisorption; thus, it can be concluded that the larger channels do not conduce to micropores inside the materials, in such a way that the total area is completely exposed and is totally attributed to macro and mesoporous connections. It is noted that all materials are essentially meso and macroporous with an average pore diameter (area) of 0.03 µm. The average pore diameter as a function of the mercury intrusion volume corresponds to the 50% percentile of the pore volume in the cumulative curve, in such a way that the higher value, the higher amount of connections present in the porous network of the material. On the other hand, due to the functionalization the pore volume of all the samples is modified. The most representative effect can be observed in the total pore volume, which decreases when the functionalization of the hard carbons with nitrogen functional groups is carried out, indicating constrictions at the entrance of the mesopores making them less accessible to the mercury intrusion. This effect can be analyzed in detail with the pore size distribution (Supplementary material Fig. S1b), in which it is observed that the treatment with urea and melamine collapses the porous texture of the material, making it narrower. An important effect in the pore size distribution can be seen for the GPC-AC-M sample: in the 10 - 100 µm range the porosity is completely obstructed, which may be associated with the fact that the melamine molecules are more voluminous than the urea molecules, due to this, the channels in which Urea was anchored are narrowed after the carbonization process. 9

The morphology of the hard carbons was studied by SEM (Fig. 1). It is observed that the hard carbons possess, as their main characteristic, the presence of well-structured walls and channels without a definite direction, being able to clearly identify the formation of disordered channels. For the GPC-AC-M sample the channels are obstructed due to agglomerates of melamine decomposition products that have formed during the synthesis of the material. SEM images at lower magnification (supplementary material Fig. S2) shows that the porosity mainly arises from pores within the particles and not due to voids formed between particles. For all obtained samples, the HRTEM images show similar characteristics; it is observed the absence of defined crystalline domains while the presence of highly disordered structures is clearly observed in all regions. Specifically, the sample GPC-AC-U, shows the formation of structures made up of highly disordered carbon sheets interconnected by macropores. The interconnected regions between macropores are uniformly perceptible in all the samples (Highlighted region in Fig. 2a and 2b). The macropore walls show highly disordered structures with no graphitic domains (Fig. 2c, close up); however, in the connected parts, the presence of more ordered structures than in the walls of macropores, can be observed (Fig. 2d). The presence of diffuse "graphitic" domains, formed by layers of stacked graphene, are very similar to those previously reported by Yuan & Zhu [24]. The average spacing between the diffuse graphitic layers is approximately 0.47 nm such that open spacing could facilitate the intercalation of Na ions between the graphene layers [25– 27]. The diffraction patterns of all the studied samples displayed similar behavior (Supplementary Material, Fig. S3), a very wide peak (002) around 2θ = 23°, and a small 10

one at approximately 2θ = 43° (100), revealing the highly disordered nature of the samples. All samples, after functionalization present both wide peaks, thus although the surface has changed the bulk remained unchanged for all samples. The interlayer distance for all samples is around 0.38 nm after functionalization which is higher than the value reported for graphitic domains. The presence of small well-defined peaks at 2θ = 29.38° and 40° are most probably related to inorganic impurities like quartz and not to graphitic domains, since the combined citric acid/NaOH treatment eliminates the presence of these signals; then, after the second treatment the signals re-appear indicating the transformation of biomass forming extra inorganic impurities. Additionally, it is not possible to form graphitic domains at such low pyrolysis temperature [28]. The RAMAN spectra obtained for all the materials (Fig. 3a) indicate the presence of topological defects in all samples, in such a way that the structure of the carbons is mostly disordered. A parameter to determine the degree of graphitization and therefore, evaluate the degree of disorder on the structure, is by determining a relationship between the intensities of the bands D and G (ID/IG) [3,7]. The experimental evidence shows that this parameter is dependent on the amount of the nitrogen present in the material, since ID/IG intensity ratios are obtained in the order: GPC-AC (1.22) < GPC (1.24) < GPC-AC-M (1.26) < GPC-AC-U (1.39) These results indicate that a certain amount of defects is being introduced into the carbons structure (bulk) by means of the functionalization process, especially through the urea treatment. The elemental analysis indicates the total amount of nitrogen, hydrogen, sulfur and carbon in the obtained carbons (Table 2). All samples, independently of the functionalization present already important amounts of nitrogen and traces of sulfur; on the other hand, 11

carbon and hydrogen content is almost invariant and the rest of the components is mainly oxygen. The carbon, hydrogen, oxygen and nitrogen species observed on the GPC sample (un-functionalized), are observed due to the nature of the waste used as a precursor in the preparation of the hard carbons, since being a natural material it has predominantly cellulose units; on the other hand, the sulfur presence occurs due to the presence of proteins containing thiol groups. Sulfur species remained constant independently on the functionalization process, thus, they can be regarded as bulk sulfur species as they are not modified by surface treatments. For nitrogen species, the total amount of nitrogen in the original sample is high, but the 30% of original Nitrogen content is removed during the acid treatment; moreover, as the functionalization with urea and melamine is performed, the nitrogen content is again increased which indicates effective functionalization process. On the other hand, the carbon content increases after the acid treatment which indicates effective formation of C=O species. This preliminary analysis indicates that the functionalization is performed effectively, having an important effect on the surface chemistry of the samples. In order to get insights on the specific functional groups formed during the carbonization and functionalization, FTIR was utilized to analyze the samples surface (Fig. 3b). The FTIR spectra of GPC sample shows signals related to the presence of oxygen functional groups – C=O, –COO- [[29],[30,31]] but also signals corresponding to –NH3+ groups characteristic of materials from cellulosic origin. After the functionalization process with NaOH and citric acid (GPC-AC) the sample spectra shows the formation of signals corresponding to carboxylic groups (-COOH), indicating that the functionalization of the sample was effectively induced. During the NaOH treatment the ester methyl groups on the surface are

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removed [32] increasing the amount of oxygen functional groups, then during the citric acid treatment, as the condensation product (citric acid anhydride) is combined with hydroxyl groups on the carbon surface, ester bonds are forming, increasing the amount of carboxylic groups on the surface [33]. The presence of the carboxylic groups serves as anchoring sites for the introduction of nitrogen functional groups. The process occurs in a similar way to the acid-based neutralization reaction, where the R-COOH groups provide an H+ to the amino groups with free electron pair in the nitrogen, so that these groups act as a Lewis base accepting H+ leading to amino-functionalization of the carbon material. The functionalization with urea causes the disappearance of the carboxylic signals and the formation of absorption bands characteristic of nitrogen functional groups at 1380, 1230 y 875 cm-1, corresponding to groups –NH, -CN and –NH2, respectively [34]. Finally the melamine functionalized carbon material (GPC-AC-M) present predominantly the presence of characteristic signals of –NH y –NH2 [34], however the functionalization process is not totally effective, due to the remnant presence of the band corresponding to carbonyl groups. Thus, GPC-AC-M contains both, nitrogen and carboxylic surface functional groups; moreover, the partial substitution is also in agreement with elemental analysis showing lower amount of nitrogen. In this work, the low temperature treatment and nature of the precursors allows the formation of Nitrogen functional groups on the surface, rather than nitrogen doped carbon [35]. Finally, with the aim to complement the functionalization information made to the carbons, XP spectroscopy studies were carried out. The survey spectra are presented in Supplementary material Fig. S4, in them, signals associated to C1s, O1s and N1s regions are predominant, without traces of other elements. The results of the quantitative analysis is

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presented in Table 3, which was performed using the following sensitivity factors (provided by the manufacturer): 0.205 for C1s, 0.66 for O1s and 0.42 for N1s. The nitrogen content of 0.30% and 0.05% is observed for the functionalized carbons GPC-AC-U and GPC-AC-M; therefore, from these studies and those obtained by elemental analysis it can be concluded that most of the Nitrogenous functionalities added to the materials are found within the porous network, since XP spectroscopy is an analysis performed on the outermost layer of the material (3 - 5 nm). The high resolution spectra in the region of C1s, (Fig. 4a and Fig. S5a), can be deconvoluted into four different species in a similar way to that reported for similar carbons prepared from grapefruit peels [36]. The first signal at 284.7 eV, is attributed to C=C-C bonds, which is the most intense signal in the spectra, due to the semi-graphitized plates existing in the materials. The second signal at 285.9 eV corresponds to C-O bonds, while the signals at 287.5 eV and 289.1 eV, are related to bonds C=O and O-C=O, respectively. These signals are complemented with those observed in the region of the spectrum corresponding to O1s present in the materials, Fig. 4b and Fig. S5b, which can be deconvoluted into three peaks centered at 531.1, 533.2 and 534.2 eV, corresponding to group bonds, C=O, C-O and COOH, respectively [37,38]. It is observed that these signals increase in intensity after modification with citric acid indicating that the functionalization with carboxyl groups has been carried out correctly. Likewise, it is observed that the percentage of oxygen contained in the samples decreases from 18% to 9% after the treatment with urea and melamine, in such a way that the introduced oxygenated groups have served as anchoring sites for the nitrogenous functionalities (-COO-NH2). Finally, it is observed that the spectra in high resolution corresponding to the N1s of the samples GPC-AC-M and GPC-AC-U differ from

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each other (Fig. 4c and Fig. S5c), which suggests that the different nitrogen precursors lead to different Nitrogenous functionalities in the material [37–40]. For GPC-AC-M the XP spectrum can be deconvoluted into 3 species attributed to: 398.7 eV -NH2 (~37%), 400.2 eV pyridine-N (~32%) and 400.8 eV quaternary-N (~ 31%). While for GPC-AC-U the XP spectrum has been deconvoluted into 4 species attributed to bonds: 398.7 eV -NH2 (~61%), 400.2 eV pyridine-N (~19%), 401.3 eV quaternary-N (~14%) and 403.7 eV pyridine-Noxide (~ 6%). In such a way that the functionalization with urea produces mainly amino groups functionalities and conserves in a smaller proportion oxygenated functionalities linked with nitrogen, which may be due to the C=O group of urea. Therefore, the functionalization process induces changes on the porous characteristics, mainly the constriction of pores; on the other hand, the bulk is not affected as detected by DRX, but definitely a higher degree of the amorphous nature is increased (Raman). The existence of functional groups is detected within the pore walls whose nature depends on the treatment carried out, but most of them remain within the pores as XPS is not able to detect drastic changes 3.2 Electrochemical Characterization The discharge vs potential curves obtained for the GPC-AC-U as anode material (Fig. 5a) indicates a very high initial capacity. All samples show a similar behavior typical of nongraphitic materials [3], providing a high specific capacity in the first cycle, the capacity on the first discharge cycle follow the trend: GPC-AC-U (422 mAh g-1) > GPC-AC-M (354 mAh g-1) > GPC (337 mAh g-1) > GPC-AC (67 mAh g-1).

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It is important to note that, differently from mesoporous carbons [41,42] where a high amount of non-reversible capacity is observed as a result of SEI formation (plateau in the range of 1 to 0.4V), the functionalized samples show a smooth decay of voltage in the first cycle. dQ/dV curves were calculated for the first and subsequent cycles (included as inset in Fig. 5 and Fig S6a), as they can confirm the presence of extra processes related to electrolyte decomposition and SEI formation. The dQ/dV curves for GPC show a welldefined peak at 0.8 and others at potentials lower than 0.4 V, which is directly associated to electrolyte decomposition of electrolyte due to the presence of a very high amount of defects and oxygen functional groups leading to irreversible capacity. After citric acid treatment, the GPC-AC sample only shows a small shoulder at 0.4 V, indicating the homogenization of the chemical surface. The GPC-AC sample, with a higher number of carboxylic groups on the surface shows the lowest specific capacity on the first cycle. This behavior is due to the high concentration of carboxylic groups on the surface, which probably lead to a very strong interaction with Na Ions, leading to a passivating effect. The functionalized samples, on the other hand, display a smoother discharge curve during the first cycle and subsequent ones. After the first cycle, the specific capacity is diminished; however, it is mainly stabilized during the first cycles leading to stable charge/discharge curves. The capacity diminishment is very low after the first cycle, especially for the GPCAC-U, obtaining a specific capacity of 187 mAh g-1 in the 10th cycle, which afterwards diminishes only to 174 mAh g-1 in the 60th cycle (Supplementary information, Fig. Sup 6b). It is also important to recall that the reversible capacity in the first cycles is improved from a 25 % (of the initial capacity) in GPC to almost 60% in the GPC-AC-U sample. Moreover, the irreversible capacity is only observed from the first to the second cycle,

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while after the second cycle, the capacity remains practically constant, being a drastic improvement in comparison to previous reports [35]. Surprisingly, for all samples, the voltage plateau below 100 mV, classic for hard carbon used as Na storage materials, is not directly observed (dQ/dV indicates that the process slightly appears at the end of the discharge curve); this behavior is related to the fact that the low pyrolysis temperature is not able to form micropores capable to store Na ions on the micropore surface [43]. However, the extra capacity below 100 mV classically observed in Hard Carbons is challenging, since its relative closeness to 0 V vs Na+/Na0 increases the probability for Na plating and dendrite formation on the long-term operation; thus, the behavior observed in the samples here reported, indicates a safer operation during cycling. Rate capability tests were further performed at several C-rates (Fig. 5b). The initial capacity of GPC samples is high; however, it presents the fastest capacity decay during the first cycles, this behavior indicates that the functionalization process allows the stabilization of the surface leading to the attainment of higher specific capacities. The sample stabilized with melamine although presents a higher stability than GPC samples still presents capacity decay during all the C-rates, indicating that melamine is not as effective as Urea; moreover, the sample shows a drastic diminishment of BET area, which results in the diminishment of specific capacity from the very first cycle. It has been proposed that the diminishment of capacity is probably related to two factors: SEI formations and the irreversible insertion of Na+ getting trapped inside the structure of non-graphitized carbons; SEI formation (as demonstrated by dQ/dV curves) is observed mainly on GPC and GPC-AC samples, but also at a lower degree on GPC-U and GPC-M samples, probably due to the formation of a less reactive surface. However, irreversible 17

capacity is observed in all samples; thus, Na trapping should play a significant role, due to the presence of disordered graphitic planes, only those with a low plane to plane separation get trapped while those with a higher separation allows reversible sodium intercalation [44,45]. Once the irreversible capacity is utilized, the remaining sites allow the attainment of a stable capacity. In order to get insights on the effect of the functionalization process, electrochemical impedance spectroscopy was used to determine sodium intercalation properties on the synthesized hard carbon samples. The Nyquist plots and their corresponding equivalent circuits are shown in Figure 6a. The ohmic resistance (Re) includes the intrinsic resistance of the active material and the electrolyte and the contact resistance at the interface between the material and the current collector. The first semicircle was assigned to a CPE (Constant Phase Element) to simulate the capacitance related to the surface of the material. For the non-functionalized sample (GPC) the process of formation of an RSEI is observed, which is attributable to the resistance in the solid-electrolyte interface [46]. Once oxygenated and nitrogenous groups have been introduced to the material, RSEI decreases dramatically, so that the formation is no longer perceptible in the graph, indicating a decrease in the formation of the solid-electrolyte interface layer. This effect can be attributed to the increase in the functional groups which improve the hydrophobicity of the material thus making the surface more wettable with electrolyte [47,48]. The second semicircle corresponds to charge transfer resistance (RCT) while the inclined line is related to mass transfer process within the porous structure. In order to obtain kinetic parameters, impedance data were fitting to the equivalent circuit proposed by Zhang et al. [46] using ZVIEW software, version 2.7, the results are shown in Table 4.

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The RSEI of sample GPC was 10.97 Ω being higher with respect to GPC-AC (1E-5 Ω), GPC-M (0.0017 Ω) and GPC-U (0.0064 Ω), this is due to the presence of oxygenated and nitrogen groups present in samples GPC-AC, GPC-M, GPC-U which promote the diminishment of hydrophobic nature of material allowing the electrolyte to diffuse towards the surface of material [49]. On the other hand, the RCT of the original sample, GPC, was 296 Ω, and began to increase for each of the functionalized samples GPC-AC (703 Ω), GPC-U (1065 Ω), and GPC-M (1721 Ω). This increase in the charge transfer resistance can be explained due to the textural modifications that occur in the material when the chemical functionalization was carried out. The introduction of the functional groups results in the constriction of channels and pores of the material, as can be seen in the pore size distribution, Fig. 8b. The increase in RCT is proportional to the decrease in the pore volume, in such a way that the ions that migrate between anode and cathode are trapped inside the new porous network, so they require more energy to leave the material, this behavior would also explain the fact of observing high non-reversible energy storage capacities in the first cycle of charge/discharge. Furthermore, the sodium-ion diffusion coefficients of these four samples were calculated (Fig. 6b). The straight lines at the low frequency region are ascribed to the sodium ion diffusion in the bulk of the electrode materials (Warburg diffusion) and the slopes of these straight lines could be used to determine the Warburg impedance coefficient (σw). Then the sodium-ion diffusion coefficient is calculated using equation 1 [50,51] D = (R2T2) * (dE/dx) / 2A2n4F4c2σw2

Eq. 1

where, σw is the Warburg impedance coefficient, D is sodium-ion diffusion coefficient, R is the gas constant, T is the absolute temperature, A is the surface area of the material (the 19

BET area is used as an approximation), n is the number of electron transferred per molecule during electrochemical reaction, F is Faraday constant, and c is molar concentration of Na+, (dE/dx) is approximated as (∆E/∆x) at the beginning of the discharge curve. The sodiumion diffusion coefficients are calculated to be 1.95 x 10-13 cm2 s-1, 2.56 x 10-11 cm2 s-1, 9.96 x 10-12 cm2 s-1 and 1.56 x 10-11 cm2 s-1, respectively, for GPC, GPC-AC, GPC-M and GPC-U. Therefore, the functionalization process leads to several interesting characteristics of the materials as anodes for sodium ion batteries; first, the surface is stabilized at the expenses of increased charge transfer resistance; second, the carboxylation, however, leads to a pseudo-capacitive behaviour which avoids the intercalation into the graphitic domains; and third, the functionalization with urea allows the stabilization of the surface but also providing faster paths for the transport of the sodium ions and enhanced capacity retention. As a way to explain the effect of the functionalization process with the maximum specific capacity obtained in these materials, figure 7 and 8 provide the relation between the physicochemical characteristics with the capacity on the 60th cycle. Figure 8 relates the three main effects of the functionalization, a) modification of the porosity, b) inclusion of groups that modify the disorder of the graphitic planes and c) changes on the surface chemistry. One of the main factors affecting the capacity is the porosity of the samples, as predicted by Fick´s transport laws, first the access of the electrolyte into the porosity induces a faster transport of Na, in this regard as the pore becomes narrower the distance that the ion has to travel to intercalate into the anode surface is lower, but also very narrow pore volume could lead to a very limited amount of electrolyte which on its turn, depletes the amount of sodium very quickly, thus the maximum capacity is obtained for medium pore volume (GPC-AC-U) while the samples modified with melanine (GPC-AC-M) 20

collapses the porosity (Figure 8b). Once the ion is closer to the graphitic layers, the amorphous characteristic of the samples have a higher importance, in figure 8c it can be observed that the enhanced capacity can be related to the higher amorphicity as obtained from Raman characterization, i.e. the higher the amorphicity the higher the possibility to have higher layer to layer separation and thus higher reversible sodium intercalation. And finally, the functionalization process also allows the modification of the electronic structure facilitating ion some cases the diffusion of sodium but also leading to capacitive behaviour (GPC-AC), but in any case the nitrogen content can be directly related to the Id-Ig ratio which correlates with the amorphicity of the samples. Table 5 presents the comparison of the specific capacity obtained for several non-graphitic carbons utilized as anodes for sodium ion batteries and NaPF6 as electrolyte. The specific capacity obtained on these types of materials is similar in comparison to commercial carbons and very high and stable in comparison to graphite, it is worth noting that even if other materials present higher capacity, the great advantage of the samples here reported arise from the utilization of the very low-cost precursors, the low temperature synthesis and the elimination of the plateau at potentials lower than 0.1 V. Moreover, these results indicate that the functionalization has a profound effect, affecting the porosity and disorder on the graphitic planes. The functionalization with other functional groups and partial functionalization open new avenues for the development of high capacity anodes for sodium ion batteries with enhanced capacity retention at higher c-rates and durability.

4. Conclusions

21

In this work, a surface functionalization strategy is applied to increase the reversible capacity as anode materials for Sodium ion batteries of biomass derived carbon materials synthesized at low temperature. It was demonstrated that the functionalization process has a deep effect on the accessibility of the electrolyte to the porosity of the materials and the sodium diffusion coefficient; also, it affects the reactivity of the surface, allowing a stable and more reversible sodium intercalation. The improved reversible capacity is mostly related to the structural surface disorder caused by the introduction of nitrogen functional groups between the non-graphitized layers; the functional groups, on the other hand, improve the surface-electrolyte contact improving the Surface hydrophobic character due the presence of the functional group. These properties can be fully exploited by modifying the functional group length and connectivity leading to even better improvements of hard carbons synthesized at low temperatures.

5. Acknowledgments The authors thanks to the “Red de Almacanamiento de Energía - CONACYT” for their support in conducting this research. Also, the CONACyT project PN-2016/2551 and the Ministry of Economy and Competitiveness FEDER funds (Spain), through the “Proyecto Retos” NO. CTQ2013-44789-R. Also, the authors thanks to Julio Aguirre for their support during the studies of X-ray diffraction. Luis A. Romero-Cano and Ana I. Zarate-Guzman thanks the CONACyT (Mexico) for the support received with the scholarship numbers 378307 and 256943. Helena García-Rosero gratefully acknowledges to COLCIENCIAS (Colombia) for supoorting her PhD studies.

22

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30

TABLES Table 1. Textural properties of carbon materials derived from grapefruit peels: a) After carbonization (GPC), b) modified with NaOH and citric acid (GPC-AC), c) functionalized with urea (GPC-AC-U) and d) functionalized with melamine (GPC-AC-M)

N2

Mercury intrusion porosimetry

physisorption

Average Sample

SBET

Vm, N2

SHg

pore diameter

Mesopores Macropores vol.

vol.

-1

-1

2

-1

-1

pore volume

(Volume) 2

Total

-1

-1

m g

mL g

m g

µm

mL g

mL g

mL g

GPC

10.9

0.004

11.0

23.25

0.048

0.677

0.725

GPC-AC

6.5

0.002

6.7

32.62

0.069

0.682

0.751

GPC-AC-U

9.7

0.003

9.8

37.23

0.121

0.533

0.654

GPC-AC-M

3.2

0.001

3.8

27.86

0.091

0.299

0.390

31

Table 2. Organic Elemental Analysis (bulk analysis) of carbon materials derived from grapefruit peels: a) After carbonization (GPC), b) modified with NaOH and citric acid (GPC-AC), c) functionalized with urea (GPC-AC-U) and d) functionalized with melamine (GPC-AC-M).

Elemental Analysis Material

N

C

H

S

(%wt)

(%wt)

(%wt)

(%wt)

GPC

1.71

76.26

1.74

0.04

GPC-AC

1.38

80.20

1.99

0.06

GPC-AC-U

4.74

81.75

1.82

0.06

GPC-AC-M

2.33

79.17

1.81

0.06

32

Table 3. Binding energies (eV) of the C1s, O1s, and N1s regions of carbon materials derived from grapefruit peels: a) After carbonization

(GPC), b) modified with NaOH and citric acid (GPC-AC), c) functionalized with urea (GPC-AC-U) and d) functionalized with melamine (GPC-AC-M)

Sample

C1s

specie

GPC

284.7

C=C-C

285.9

GPC-AC

GPC-U

GPC-M

FWHM

%

(eV)

peak

1.39

%

O1s

specie

%O

N1s

specie

% peak

%N

75

531.4

C=O

57

14.28

398.7

NH2

53

0.01

C-O

17

532.8

C-O

23

400.3

Pyridine-N

25

287.5

C=O

5

533.8

C-OOH

20

401.2

Quaternary-N

22

289.1

O-C=O

3

284.6

C=C-C

72

531.6

C=O

43

398.5

NH2

39

285.9

C-O

15

532.7

C-O

32

400.2

Pyridine-N

33

287.6

C=O

7

533.7

C-OOH

25

401.0

Quaternary-N

29

289.2

O-C=O

6

284.5

C=C-C

66

531.3

C=O

50

398.7

NH2

61

286.2

C-O

19

532.9

C-O

30

400.2

Pyridine-N

19

287.9

C=O

12

534.4

C-OOH

20

401.3

Quaternary-N

14

289.4

O-C=O

3

403.7

Pyridine-N-oxide

6

284.7

C=C-C

398.7

NH2

37

286.2

1.33

1.77

1.36

peak

18.60

9.82

82

531.6

C=O

47

C-O

10

532.9

C-O

26

400.2

Pyridine-N

32

287.7

C=O

5

534.0

C-OOH

27

400.8

Quaternary-N

31

289.5

O-C=O

2

33

9.90

0.01

0.30

0.05

Table 4. Parameters derived from Nyquist data fitting to equivalent circuit.

Sample

Re (Ω Ω)

RSEI (Ω Ω)

CPESEI (F)

RCT (Ω Ω)

CPEDL (F)

Χi2

GPC

4.924

10.97

4.50E-06

296.6

0.3141

0.0023

GPC-AC

2.074

1.00E-05

1.30E-06

703.9

0.1962

0.0065

GPC-AC-M

2.938

0.0017

1.18E-06

1721

0.3062

0.0055

GPC-AC-U

3.289

0.0064

8.23E-06

1065

0.4215

0.0010

34

Table 5. Survey of the specific capacity for Na intercalation obtained for different types of non-graphitized carbons

Tpyrolisis

SBET

C1

Cn

n

Precursor

Reference 2

-1

ºC

m g

mAh g

1100

137.0

Sugar

1100

Grapefruit peels

-1

-1

mAh g

cycles

280

270

200

[52]

70.0

265

265

40

[53]

600

9.7

421

174

60

Present study

Lychee seeds

500

6.61

312

146

100

[54]

Wood

1100

---

361

292

---

[55]

Corn cob

1300

3.7

300

275

100

[56]

cotton

1300

38

315

305

100

[57]

Argan shell

1300

2.6

360

312

100

[58]

sucrose

1100

8.5

353

282

200

[56]

Sucrose

1100

7

359

346

60

[59]

Carbon dots

1200

549.8

328

149

5000

[60]

Sucrose powder with graphene oxides

35

FIGURE CAPTIONS

Fig. 1. SEM images of carbon materials derived from grapefruit peels: a) After carbonization (GPC), b) modified with NaOH and citric acid (GPC-AC), c) functionalized with urea (GPC-AC-U) and d) functionalized with melamine (GPC-AC-M) Fig. 2. HRTEM images of the sample functionalized with urea (GPC-AC-U) at different magnifications. Fig. 3. (a) RAMAN spectra and (b) FTIR spectra (The main functional groups are marked in each IR spectrum) of carbon materials derived from grapefruit peels: a) GPC, b) GPCAC, c) GPC-AC-U and d) GPC-AC-M. Fig. 4. Deconvolution of the high-resolution XP spectra into the main species: a) C1s, b) O1S y c) N1s region, for carbon materials functionalized with urea (GPC-AC-U) and functionalized with melamine (GPC-AC-M). Fig. 5. (a) Galvanostatic charge/discharge experiments for carbon material functionalized with urea (GPC-AC-U), (b) Rate capability of carbon materials: ○ After carbonization (GPC), □ modified with NaOH and citric acid (GPC-AC),

functionalized with urea

(GPC-AC-U) and △ functionalized with melamine (GPC-AC-M). Experimental conditions: Current density 37 mA g-1, 1 M NaPF6/EC:PC:DMC where EC: Ethilene Carbonate, PC: Propilen Carbonate and DMC: Dimethyl Carbonate. Inset shows dQ/dV as function of V. Fig. 6. a) Nyquist plot and equivalent circuit corresponding to samples: o GPC,
GPCAC, ◊ GPC-M and ∆ GPC-U. b) Relationship between real resistance and frequency, the slope (Warburg impedance coefficient) of which was used to calculate the sodium-ion diffusion coefficient. Fig. 7. Proposed sodium storage mechanism in non-graphitized mesoporous carbons functionalized with nitrogen functional groups. Fig. 8. Effect of the characteristics of hard carbons materials:
After carbonization (GPC),  modified with NaOH and citric acid (GPC-AC),




functionalized with urea

(GPC-AC-U) and
functionalized with melamine (GPC-AC-M). a) Relation of pore 36

volume with capacity, b) Pore size distribution, c) Relation of ID/IG with capacity and d) relation of nitrogen content in hard carbons with ID/IG.

37

FIGURES

Figure 1

38

Figure 2

39

Figure 3

40

Figure 4

41

Figure 5

42

Figure 6

43

Figure 7

44

Figure 8

45