Glucosamine derived hydrothermal carbon electrodes for aqueous electrolyte energy storage systems

Nitrogen-doped porous hard carbons are synthesized by hydrothermal carbonization method (HTC) using glucosamine as biosource and treated at different carbonization temperatures in nitrogen environment (500, 750, 1000 °C). The electrochemical performances of hard carbons electrode materials for aqueous electrolyte sodium ion batteries are examined to observe the effect of two different voltage ranges (−0.8–0.0) V and (0.0–0.8) V in 1.0 M Na2SO4 aqueous electrolyte. The best electrochemical performances are acquired for the 1000 °C treated glucosamine (GA-1000) porous carbon sample that provides ~96 F/g capacitance value in the negative voltage range (between −0.8 and 0.0) V. The sodium diffusion coefficient of the GA-1000 carbon calculated by electrochemical impedance measurements is found to be 1.5 × 10−14 cm2/s.

Moving from organic electrolyte to the use of aqueous electrolyte, it is a straightforward method to avoid the high cost and safety problems associated with organic liquid electrolytes.The abundance of water, utilizing inexpensive sodium salts such as Na 2 SO 4 , NaNO 3 , NaCl whose ionic conductivity is about 10 times higher than that of organic electrolytes make these electrolytes even more attractive.High ionic conductivity enables to obtain much more cycles capacity.However, the major disadvantage of aqueous electrolyte batteries is the low thermodynamic stability of water (1.23 V) that results in low energy density [2,[25][26][27].Gogotsi and Dyatkin reported the specific capacitance of the porous carbon spheres in which 138 F/g at 2 mV/s and 91 F/g at 100 mV/s in 1.0 M Na 2 SO 4 aqueous electrolyte were obtained [28].Whitacre et al. demonstrated that specific capacitance values of 200 F/g in an aqueous Na 2 SO 4 electrolyte can be achieved with hard carbons synthesized from low cost food-grade carbohydrates [2].Sevilla et al. found that the supercapacitor performances of N-doped carbon from glucosamine/carbon nanotube composites were 50~60 F/g in 1.0 M H 2 SO 4 electrolyte [29].Lu et al. prepared coin type supercapacitor electrodes from high surface area activated carbon produced from corn by hydrothermal method.The capacitance values in aqueous, organic and inorganic electrolytes reached to 222 F/g, 202 F/g and 188 F/g, respectively [30].Altinci and Demir synthesized a sponge-like porous carbon with a high surface area by using the hydrothermal carbonization (HTC) method of pistachio shells with the activation step.They reported 166 F/g in capacitance value in 1 M KOH electrolyte at 0.5 A/g current density [31].
The aim of this study is to accomplish high surface area, porous, amorphous N-doped carbons from glucosamine precursor with HTC method that allow the adsorption-desorption of sodium ions and to investigate their electrochemical performance.For this purpose, carbonaceous material derived from glucosamine were carbonized in inert nitrogen environment at different temperatures (500, 750, 1000 °C).By utilizing scanning electron microscopy (SEM), X-Ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) and Brunauer-Emmett-Teller (B.E.T), morphological and structural examinations were investigated.Electrochemical measurements were performed by cyclic voltammetry (CV), galvanostatic charge/discharge test and electrochemical impedance spectroscopy (EIS).

Synthesis of glucosamine derived n-doped carbon
Commercially available 2.0 g of D(+)-glucosamine.HCl was mixed in 18 g distilled water in a magnetic stirrer for 45 min.Then the aqueous solution of D(+)-glucosamine.HCl was placed in a Teflon inlet autoclave and kept for 20 hours in a furnace at 180 °C to proceed hydrothermal carbonization (HTC) process.According to the HTC synthesis route, the applied temperature (180 °C) is sufficient enough to first dehydration of glucosamine and then complete dehydration to form carbonaceous materials [17,19,29] After HTC step, the carbonaceous material was washed with water and ethanol by soxhlet extraction then placed in a vacuum furnace overnight to dry.The resulting sample was quoted as GA-HTC.Afterwards, different carbonization temperatures 500, 750, 1000 °C were applied in a tubular furnace with an inert N 2 gas for 6 hours in order to improve the conductivity.Further carbonized N-doped carbons derived from glucosamine were named as GA-500, GA-750 and GA-1000, respectively.

Material characterization
Morphological characterizations of the synthesized carbons were investigated by scanning electron microscope (SEM Philips XL30).Thermogravimetric analyses, TGA, were performed using Perkin Elmer 4000 instrument at the temperature between 30-700 °C with a 10 min/°C heating range in an inert nitrogen gas environment.X-ray diffraction (Bruker D8 diffractometer 2θ mode, Cu Kα radiation, λ = 1.5406 nm) patterns of the sample were recorded in the range of 2θ = 0-90º.Surface area and pore size distribution evaluation of hydrothermal carbonization carbon sample and carbonized N-doped carbon samples were determined by using the multi point Brunauer-Emmett-Teller (B.E.T) analyzer (Quantachrome Autosorb Instruments) via nitrogen adsorption isotherms at 77 K under vacuum.FTIR spectroscopy (Perkin Elmer Spectrum 100) was used to determine the bonds of N-doped carbon samples (GA-500, GA-750, GA-1000) and bare hydrothermal carbon, GA-HTC.

Electrochemical measurements
For the electrochemical tests, initially electrodes were prepared by a slurry formation in which 80 wt.% active mass of the N-doped carbon derived from glucosamine (GA-500, GA-750, GA-1000), 10 wt.% of Ketjen black (KB) conductive additive and 10 wt.% of polyvinylidene fluoride (PVDF) binder were mixed in N-methyl-2-pyrrolidone (NMP) solution for 20 h.Later, electrode slurry was coated on the graphite plate in the form of a thin film to have an area of 2 cm × 1 cm.The working electrode has ~2.0 mg active material loading and ~30 μm coating thickness.For the counter electrode, a cleaned graphite plate with a blank surface was used.Half-cell test measurements were carried out in two operating voltage range (0.0 -0.8) V and (between -0.8 and 0.0) V according to the 3-electrode cell configuration.Ag/AgCl electrode was used as the reference electrode and 1.0 M Na 2 SO 4 dissolved in bi-distilled water was used as aqueous electrolyte.Electrochemical Impedance Spectroscopy (EIS) was carried out between 0.1 mHz and 0.2 MHz with a magnitude of 5 mV voltage.

Synthesis and characterization
Morphological investigations of carbonaceous materials which was synthesized from glucosamine sources via hydrothermal carbonization (HTC) method was firstly determined by SEM in Figure 1.HTC is a simple, safe and inexpensive method to obtain carbonaceous structures at low temperature, using only water and pure glucosamine precursors as an input.On the other hand, the resulting particles have full of functional groups and low electronic conductivity in nature.Thus, at varying temperatures (500, 750, 1000 °C) the samples were further heat treated in an inert nitrogen gas environment.The intercalation of the nitrogen heteroatom into the graphitic structure with high temperature has been contributed to the electron transport in the conduction band [32].From the SEM images, agglomerated and porous texture can be clearly observed independently of the type of the samples.Even though the changes in particle size are not visible upon heat treatment, the structure turned out to be more porous that is beneficial for Na-ion adsorption/desorption during the electrochemical cell performances.
The surface area of all the synthesized carbon materials was determined by nitrogen adsorption-desorption isotherm at 77K by Brunauer-Emmett-Teller (BET) analysis.Isotherm and pore width graphs shown in Figures 2a-2d).There is a clear change at the isotherm curves and pore size distribution of the materials depending on the temperature treatment.While micro and mesopores were distributed in the hydrothermal carbonized glucosamine sample GA-HTC, the presence of upward micropores with a sharp distribution was observed in the GA-750 sample, which was carbonized at 750 °C.Therefore, a high surface area has been obtained at high temperature carbonization as expected.In other words, it can be said that the material has been carbonized at high temperatures and a more porous structure was provided.Adsorption isotherms type (IV-V) hysteresis are shown for a mesoporous and microporous substance according to the IUPAC isotherm classification.Figure 2 desorption hysteresis can be often connected with narrow pores [33].B.E.T method to derive the surface area from adsorption-desorption isotherm data, thus, Equation (1) was used for the B.E.T linear isotherm equation below: Where the ( P/P 0 ) term is relative pressure, C is the B.E.T constant that is the intercept at the linearization fit, V m is the monolayer adsorbed gas quantity (cc/g) [34].
BET surface areas of the GA-HTC and GA-750 carbon samples are 14.047 and 610.368 cm 2 /g and the total pore volumes are 0.018 cc/g and 0.278 cc/g, respectively.It is clearly seen in Figure SI-1a and Figure SI-1b, the surface area of the GA-750 sample increased considerably after the carbonization process in the nitrogen environment after HTC.It can be said that the carbonization treatment effectively increases the surface area and total pore volume of the carbon sample.When looking at the pore size distribution graphs that determined by the density functional theory (DFT), the GA-HTC sample has micro and mesoporous structure although the GA-750 sample mostly has micropores of approximately 0.3 nm in size that stated in the subgraph (Figure 2d).The carbon structures having micropores and mesopores facilitate the adsorption and desorption of sodium ion species into the structure and increases the diffusion rate by shortening the diffusion pathway.Adsorption-desorption isotherms were summarized at Table 1.
As seen in Figures 3a-3b), bare glucosamine diffraction pattern showed a crystalline structure and had a sharp characteristic peaks.Conversely, an amorphous carbon typical wide peak was observed at 23º (002) peak plane position for hydrothermal carbonization glucosamine (GA-HTC) and other samples carbonized at different temperatures (GA-500, GA-750 and GA-1000).As the carbonization temperature increases, the weak (101) graphitic carbon layer peak appears around 43º that is ascribed to regular turbostractic carbon structure as well as increase of the amount of nitrogen [20,22,35].Scherrer and Bragg equation were used for the calculation of the crystalline plane size and graphitic carbon interlayer space.d (002) and crystallite sizes of carbon samples were determined and summarized in Table 2.The interlayer distance of the carbonaceous glucosamine via hydrothermal carbonization at 180 °C was found to be approximately 0.7 nm, and, as the temperature increased, distance narrowed to 0.58 -0.56 nm.Since this distance is wider than the distance between graphene layers, it created a favorable distance for the insertion of sodium ion.In the FTIR analysis of N-doped carbons inherited from glucosamine, the peak at 700 cm -1 indicated by a pointed star was attributed to the bending mode in graphite-like areas with nitrogen atoms [36].The presence of N atoms in the carbon network was evident by the C -N and N -CH 3 bonds at 1250 -1372 cm -1 and 1200 -1600 cm -1 .C = N and C -O in amides showing repetitive units in glucosamine appearing at nearly 1650 cm -1 -1590 cm -1 were attributed to stretch vibration.The peaks observed between 1450 and 1250 cm -1 correspond to the bond groups of C = C, C = N and C = CO, respectively.The band at 1621 cm -1 can be associated with different groups particularly the C = N stretch vibration or the C -O stretch vibration in amides [37].The band at 1252 cm -1 can be related to C -O stretching vibration, C -C skeleton, C -N and N -H stretch and bending in amides [38].All these peaks referred as dash line in FTIR spectra (Figure 4a).The sharp peak appearing between 2100 -2300 cm -1 for the GA-750 and GA-1000 carbons can be associated with the C ≡ N band [37].
When looking at the thermogravimetric analysis curves with an inert nitrogen gas in the range of 10-700 °C temperature in Figure 4b, the pure glucosamine has lost approximately 69% of its weight.On the other hand, GA-1000, GA-750, and GA-500 have only small weight loss resulting from the unbound water at approximately 100 °C in which they kept their remaining mass around 93 wt.%, 89 wt.%, and 86 wt.%, respectively.After 100 °C, high amount of carbons was obtained by GA-1000, GA-750 and GA-500.However, GA-HTC sample lost its unbound water similarly then degraded and lost about 35 wt% indicating that GA-HTC has full of functional groups as depicted at the FTIR in Figure 4a.

Electrochemical measurements
Cyclic voltammetry (CV) and galvanostatic charge/discharge electrochemical measurements were performed in 1.0 M Na 2 SO 4 (pH ≈ 5.8-6.0)solution that offers safe, cheap and effective technology in comparison with organic electrolytes.Half-cell tests were run in a 3-electrode configuration lab-scale system using two types of voltage ranges i) (between -0.8 and 0.0) V negative voltage and ii) (0.0-0.8) V positive voltage at 0.37 A/g current density against to Ag/AgCl reference electrode.
The absence of any reduction-oxidation peaks on the CVs in the aqueous electrolyte solution demonstrate that N-doped hard carbon acts as a non-Faradic capacitive electrode, and ions diffuse in the structure with the principle of adsorption and desorption on the amorphous surface [39].Since the oxidation and reduction reaction did not occur for the carbon anode in aqueous electrolyte half-cell experiments, charge-discharge capacitance were expressed as farad per gram active substance in capacity calculations.
GA-500, GA-750 and GA-1000 cyclic voltammetry, galvanostatic measurements at (between -0.8 and 0.0) V voltage window were given in Figure 5 and Figure 6, respectively.The peak seen in the first cycle around (-0.3) V in Figure 5 may result from the presence of functional groups that have not been completely reduced during the carbonization step.Since same CV peaks existed in the first cycles appeared at the other carbon samples, it could be said that the nonreduced functional structures remain in the carbonization step.In later cycles, typical rectangular shaped curves have been obtained in CV curves of carbon electrodes meaning that no reduction/oxidation peak in the voltammogram.
Considering the galvanostatic cycling shown in Figure 6, all three electrodes performed a characteristic capacitive behavior with a triangle voltage-time curves.GA-500 reached a very low capacitance values of 8.06 F/g at 0.37 A/g in (between -0.8 and 0.0) V negative voltage range, which cannot be compared with other GA-750 and GA-1000 carbons.
, d) pore size distribution of GA-750.The GA-750 sample, has a stable discharge capacity while its capacity has reached an acceptable result in 1.0 M aqueous electrolyte recorded as 86 F/g at the (between -0.8 and 0.0) V. Lastly, galvanostatic capacity measurement of the GA-1000 sample lead the value of 95.5 F/g.Sum of the negative voltage performances of GA-500, GA-750 and GA-1000 in 1.0 M Na 2 SO 4 at Figure 6, the samples showed stable cycle performances over near 200 cycles, and the best discharge capacitance attained for the GA-1000.As a second set of experiments, the measurements performed in parallel with the positive voltage (0.0-0.8) V operating range however almost no capacity was obtained from the GA-500 electrode.Capacitance values for GA-750 and GA-1000 realized in 1.0 M electrolyte concentrations are shown in Figures 7a-7d).N-doped carbon samples resulted sufficient performances at the negative voltage, whereas they could not reach to high performances while working at positive voltage.The reason could be explained by the surface charges of those carbon that have positive zeta-potential after the first hydrothermal carbonization step.As depicted in the literature, the zeta potentials shift to negative region when the carbonization temperatures increase from 750 -1000 °C, thus the surface of the electrodes are negatively charged at the working pH value [40].Therefore, as proven in Figure 7c, the best performance was found at the negative operating voltage rather than at the positive voltages.
C-rate capacities of GA-1000 as an anode electrode were tested at (between -0.8 and 0.0) V in the negative voltage range for 10 cycles from high current density (7.4 A/g) to low current density (0.37 A/g) (Figure 7c).When discharged at a current density of 7.4 A/g, the capacity value was approximately 35 F/g.When the current density was reduced to 0.37 A/g, the highest discharge capacity was obtained as shown in Figure 7d.The electrochemical impedance spectrum of GA-1000 was inquired in order to reveal the sodium ion diffusion property of glucosamine derived N-doped porous carbon electrode.The Warburg impedance on the Nyquist plot gives a straight linear part with a 45 ° phase in the EIS.45 ° line at the low frequency region on the Nyquist chart can be associated with sodium-ion diffusion.In energy storage systems, the porosity of the electrodes causes a similar characteristic 45° line on the Nyquist chart [41].The charge transfer resistance at the solid electrolyte interface is attributed to a semicircle at high frequency.From the semicircular endpoint data from the high frequency region to the low frequency region, the total resistance of the electrochemical test cell can be estimated [42,43].Sodium ion diffusion coefficient was estimated from Equation (2) via the Warburg low frequency diffusion estimation.Where R is the absolute gas constant, T is the room temperature (298 K), A is the electrode surface area (2 × 1 cm 2 ), n is the number of electron transferred, F is the Faraday constant (96,500 C/mol), C is concentration of Na + (1.0 M) and σ is the Warburg factor, which was calculated from the slope of real impedance ( Z Re ) versus the angular frequency (ω −0.5 ) shown in Figure 8-8b) with using the Equation (3) below.

Conclusion
In this study, N-doped amorphous carbons were synthesized derived from D(+)-glucosamine.HCl as a source via the inexpensive, safe, one-step hydrothermal carbonization method.Subsequently, hydrothermal carbon (GA-HTC) was carbonized at 500, 750 and 1000 °C under N 2 gases atmosphere in order to increase the surface area and electrical conductivity of the resulting electrodes.The characterization of the synthetized amorphous carbon materials was made using SEM, XRD, FTIR, TGA, and B.E.T adsorption-desorption isotherm.These characterizations supported the successful synthesis of the amorphous N-doped carbons with high surface facilitating easy adsorption-desorption of the Na-ion species into the carbon structure.These N-doped carbons performed properly when used as electrodes in energy storage systems owing to microporosity and the presence of nitrogen heteroatoms functionalities in the structure.Electrochemical data have collected using cyclic voltammetry and galvanostatic charge/discharge method in 1.0 M Na 2 SO 4 aqueous electrolyte at two different voltage ranges (between -0.8 and 0.0) V and (0.0-0.8) V.These parameters indicated that non-Faradic capacities depend on the operating voltage range and carbonization temperatures.The capacitance values were found to be 8.06 F/g, 86.9 F/g and 95.5 F/g for GA-500, GA-750 and GA-1000, respectively, at negative voltage.On the other hand, much lower values were obtained at the positive voltage (0.0-0.8 V) due to the negatively charged surfaces of the electrodes.EIS measurements were applied to the highest performed electrode (GA-1000) and its Na ion diffusion coefficient was calculated to be 1.5 × 10 -14 cm 2 /s that is comparable with the literature values.

Table 1 .
Adsorption-desorption properties of N-doped carbons driven from glucosamine.

Table 2 .
XRD data of the pure glucosamine and carbons derived from glucosamine that carbonized at different temperatures.