2.1. Materials and Equipment Used
The starches maize (M), Hylon V (HV), and Hylon VII (HVII) were obtained from Ingredion® (Manchester, UK). Pure amylose, pure amylopectin, chitosan (MMW), potassium iodide (KI), dimethyl sulfoxide (DMSO) 99.9% v/v, and para-toluenesulfonic acid monohydrate (p-TSA) were purchased from Sigma-Aldrich (Darmstadt, Germany). The ionic liquid (IL) 1-butyl-3-methylimidazolium chloride 99% (BMIMCl) was purchased from abcr (Karlsruhe, Germany). Ethanol 96% v/v, iodine (I2), and sodium hydroxide (pellets) were purchased from Scharlab (Barcelona, Spain). Tert-butanol (TBA) was obtained from Honeywell (Charlotte, NC, USA).
The amylose content in the commercially available starches was determined by a colorimetric method measuring the absorbance at 600 nm of a complex formed between amylose and iodine by UV-Vis spectrophotometry. The equipment used was Jasco V-630 UV-Vis spectrophotometer (Jasco Inc., Easton, MD, USA).
Fourier-Transform Infrared-Attenuated Total Reflection (FTIR-ATR) spectroscopy was performed using an Agilent Cary 630 FTIR (Agilent Technologies Inc., Santa Clara, CA, USA), after 32 scans with a spectral resolution of 4 cm−1 and a spectral range from 4000 to 500 cm−1. Proton nuclear magnetic resonance (1HNMR) spectroscopy was performed using a Bruker Avance DPX spectrometer (Bruker Corporation, Billerica, MA, USA) of 250 MHz (5.8 T) equipped with an automatic sample changer Bruker BACS-60 and a probe 1H/13C/31P.
Thermogravimetric analysis (TGA) was performed with a TA instrument Thermogravimetric Analyzer Q500 (Waters Corporation, Milford, MA, USA). The samples were loaded in flat platinum pans and the measurements were taken under a heating rate of 10 °C/min from room temperature to 900 °C.
Gas Permeation Chromatography (GPC) was performed using an Agilent 1200 Infinity GPC (Agilent Technologies Inc., Santa Clara, CA, USA) equipped with a 390-LC Multi Detector Suite.
High Resolution Scanning Electron Microscopy (HR-SEM) was performed using a Carl Zeiss MERLIN® Field Emission-Scanning Electron Microscopy (FE-SEM) microscope (Carl Zeiss AG, Oberkochen, Germany) equipped with a GEMINI® II column, and High-Resolution Transmission Electron Microscopy (HR-TEM), using a Jeol JEM-2011 scanning microscope (JEOL Ltd., Akishima, Tokyo, Japan).
Nitrogen adsorption and desorption isotherms were measured using a Nova 2200e Surface Area and Pore Analyzer from Quantachrome Instruments (Anton Paar, Graz, Austria). The surface area was determined by the application of multi-point Brunauer-Emmett-Teller (BET) equation and the pore size was calculated according to the Barrett, Joyner, and Halenda (BJH) average pore diameter equation, D = 4VBJH/SABJH.
Quantitative Nitrogen determination was measured by an Elemental Analyzer Eurovector EuroEA3000 (EuroVector S.p.A., Milan, Italy).
The micro-Raman spectra were recorded using a 532 nm laser excitation line with a dispersive spectrometer Jobin-Yvon LabRam HR 800 (Horiba Ltd., Kyoto, Japan), coupled to an optical microscope Olympus BXFM (Olympus Corporation, Tokyo, Japan).
2.3. Preparation of Mesoporous Carbons from a Modified Starbon® Based Process
2.3.1. Modified Starbon® Based Process
This process was carried out by dispersing a 10 wt% of the starch mixture with distilled water under strong magnetic stirring for 1 h. Then, the dispersion was ultrasonicated for 15 min (2200 J) using a tip (Ultrasonic processor Sonics VCX750 equipped with a solid 13-mm probe 630-0219, Sonics & Materials, Newtown, CT, USA) in a pulse mode of 1 s. After cooling at room temperature, the gel was maintained at 5 °C for 2 days to allow the retrogradation of the structure. After that time, 30 wt% of TBA with respect to the total water content was added to the gel and kept for 1 h in a rotatory mixer at lower speed rotation (5 rpm). Then, 1 wt% of p-TSA with respect to starch was added and kept for 1 h under rotation. The gel was then frozen with liquid nitrogen and dried for 2 days in a freeze-dryer (Scanvac Coolsafe 100-9 Pro Freeze-dryer, LaboGene A/S, Allerød, Denmark) at 0.02 mbar and room temperature. After freeze-drying, an expanded dried gel was obtained. The carbonization process was carried out under inert atmosphere (argon) in a multi-stage process from room temperature to 800 °C for 24 h in a laboratory chamber furnace (CWF 12/23, Carbolite Inc., Sheffield, UK).
2.3.2. Modified Starbon® Based Process with a Previous Treatment of the Starches with the IL
In the first step of this process, the starches were dispersed in an IL. For this, BMIMCl was heated at 95 °C under mechanical stirring until completely melted. Then, 5 g of the starch or the mixture were added to the IL (20 wt% of starch) and stirred for 6 h at 95 °C by forming a viscous white dispersion. After that, the starch or the mixture was quantitatively separated from the IL using several washing steps. In the first step, 75 mL of distilled water was added dropwise, the dispersion was heated to 70 °C, and 150 mL of ethanol were added dropwise to precipitate the starch. The mechanical agitation was maintained overnight, and the next day the precipitate was separated by centrifugation at 4500 rpm for 10 min, which was followed by several washings with ethanol until complete elimination of IL, detected by FTIR at 1560 cm−1. Finally, the precipitate was separated by vacuum filtration and dried overnight at 80 °C and 60 mbar, obtaining a completely white powder.
In the following step, the Starbon®
based process was performed as explained before in Section 2.3.1
, and, finally, carbonized at 800 °C or 1000 °C by the same procedure.
2.7. Cell Assembly and Electrochemical Characterization
All electrochemical tests were carried out in the same reference battery cell system. An ECC-Air cell (EL-Cell, Hamburg, Germany) was used for testing all active materials. In the test system, a metal lithium chip as an anode (Reference EQ-Lib-LiC45, MTI Corp, Richmond, CA, USA) and a glass fibre filter paper (Whatman GF/A, Sigma Aldrich, Darmstadt, Germany) as a separator, which was soaked with 1M LiTFSI-TEGDME electrolyte for 5 min, were used. The electrolyte was prepared mixing 1 M bis(trifluoromethane)sulfonimide lithium salt (LiTFSI, Sigma-Aldrich-15224, Darmstadt, Germany) in tetraethylene glycol dimethyl ether (TEGDME, Sigma-Aldrich-172405, Darmstadt, Germany). O2-cathodes were prepared by bare coating of a slurry composed of the studied carbons, 5 wt% Polyvinylidene Fluoride (PVDF Kynar ADX 161, Arkema, Colombes, France) as a binder dispersed in 1-methyl-2-pyrrolidinone (NMP, Scharlau-ME05031000, Barcelona, Spain) and carbon black (Timcal Super C65, Timcal Ltd, Bodio, Switzerland) as a conductive additive with a 8:1:1 final dried weight ratio. They were mixed in a glass vial and stirred with an electromagnetic stirring method for at least 4 h and, finally, mixed with a homogenizer (Benchmark Scientific Inc. D1000, Edison, NJ, USA) for at least 10 min. Afterward, the prepared ink was spread onto a gas diffusion layer (GDL, SIGRACET 24 BA, SGL Group, Wiesbaden, Germany). The wet thickness was 100 μm and coating was made with the bar coating method. The coated GDL was dried in an oven at 50 °C overnight under vacuum 0.05 Pa. Subsequently, disk-shaped electrodes were punched with a diameter of 18 mm and an active carbon electrode loading ranged from 3.2 to 4.3 mg/cm2. The cell was assembled in an argon-filled glovebox and then connected to oxygen gas circulation (purity grade 4.5–99.995%) through the cathode with a flow rate of 5 mL/min during the experiments.
All electrochemical measurements were performed using a VMP3 and BCS810 multi-channel galvanostat-potentiostat (Bio-Logic SAS, Seyssinet-Pariset, France). A 2.15 and 4.35 V vs. Li/Li+ cut-off voltage was used for the galvanostatic cycling tests. The full discharge capacity was measured by discharging the cell at a constant current density of 100 mA/g of active carbon. Then another cell was assembled for the cyclability test and the performance of the material was studied by charging and discharging the battery cell at 50 mA/g with a limited capacity of 300 mAh/g.