Carbon aerogels (CAGs) based on different types of starch (potato, maize, and rice) were obtained by the carbonization of organic aerogels. Precursors of carbon aerogels were prepared via the sol–gel polycondensation process according to the procedure described in our previous paper [25 (link)]. Briefly, starches of potato, maize, and rice origins (Sigma-Aldrich, Saint Louis, MO, USA) were dispersed in water with appropriate dilution ratio (the concentration of the solutions for potato and rice starches was 10 wt %, for maize starch it was 15 wt %). Suspensions of starches were stirred and heated up to the gelatinization temperature. Subsequently, the solvent exchange by immersing the aqueous gels for 12 days in the ethanol (96%, Avantor Performance Materials - formerly POCH S.A., Gliwice, Poland) was carried out. Afterwards, the alcogels were dried under ambient pressure in air at 50 °C for 1 day. As-obtained organic aerogels (OAGs) based on potato (OAGPS), maize (OAGMS) and rice starch (OAGRS) were analyzed in the context of thermal decomposition and morphology characteristics. Starch aerogels were then pyrolysed under argon flow (purity 99.999%, 50 mL∙min−1, Air Products, Allentown, PA, USA) at 700 °C, 800 °C, and 900 °C for 6 h, which allowed carbon aerogels (so called CAGPS, CAGMS, CAGRS, consequently) to be obtained. At this point, it should be mentioned that the OAGs for carbonization were prepared as coarse powders and CAGs after the heat treatment remained in this form. However, for electrochemical application, the CAGs need to be ground into uniform fine powders, and the samples in this form are presented in the following paper. That is why the grinding of CAGs samples after pyrolysis was performed in an agate mortar for about 30 min for each sample.
The thermal decomposition of organic aerogels was studied by means of thermogravimetric analysis coupled with evolved gas analysis with infrared spectroscopy detection (EGA(FTIR)-TGA/DTA/DTG method). The experiments were carried out using SDT Q600 thermobalance (TA Instruments, New Castle, DE, USA) coupled with a Fourier transform infrared (FTIR) spectrometer (Nicolet 6700 FTIR, Thermo Fisher Scientific, Waltham, MA, USA) by FTIR-TGA interface (Thermo Fisher Scientific, Waltham, MA, USA). The measurements were performed in an inert gas flow (N2, 20 mL∙min−1) for samples with the weight of 20 mg placed in a corundum crucible, in the temperature range of 20–1000 °C and at a heating rate equal to 5 °C∙min−1. The 2D and 3D FTIR spectral maps of evolved gaseous products were recorded with resolution of 4 cm−1 collecting eight scans for each spectrum. The morphology of the materials was characterized using an FEI Versa 3D (FEG—Field Emission Gun) scanning electron microscope (FEI Company, Hillsboro, OR, USA). The crystal structure of the carbon aerogels was characterized by powder X-ray diffraction (XRD) using BRUKER D2 PHASER (Billerica, MA, USA). The Cu Kα radiation ( in the range of 10–60° (2θ) with a step of 0.02° was used. To determine the amount (a weight percent) of carbon, hydrogen, and nitrogen elements in the obtained carbon compounds, the elemental analysis (CHN analysis) was conducted using micro analyzer vario MICRO cube coupled with microbalance (Elementar, Langenselbold, Germany). Before the CHN determination, the CAG samples were dried in vacuum oven under 80 mbar for 3 h at 80 °C. The evaluation of chemical composition was performed with an accuracy of 0.3%. The electrical conductivity (EC) studies were carried out using semi-4-probe method with 1 mA alternating current (at a fixed frequency of 33 Hz) within temperature range from −20 to +40 °C by means of state of the art Sigma1 apparatus. The powder samples (with a thickness of about 2.5 mm) were placed in a glass tube between the parallel flat and gold circular electrodes (with 5 mm in diameter) and pressed by an electrode piston until the measured resistance of the samples remained constant and appropriate electrical contact was assured. Porous features of the resulting samples were evaluated from N2 sorption at −196 °C measured with 3Flex v1.00 automated gas adsorption system (Micromeritics, Norcross, GA, USA). Before the analysis, the samples were degassed under vacuum at 350 °C for 24 h. The specific surface area (SBET) was determined by the single point surface area at .
The thermal decomposition of organic aerogels was studied by means of thermogravimetric analysis coupled with evolved gas analysis with infrared spectroscopy detection (EGA(FTIR)-TGA/DTA/DTG method). The experiments were carried out using SDT Q600 thermobalance (TA Instruments, New Castle, DE, USA) coupled with a Fourier transform infrared (FTIR) spectrometer (Nicolet 6700 FTIR, Thermo Fisher Scientific, Waltham, MA, USA) by FTIR-TGA interface (Thermo Fisher Scientific, Waltham, MA, USA). The measurements were performed in an inert gas flow (N2, 20 mL∙min−1) for samples with the weight of 20 mg placed in a corundum crucible, in the temperature range of 20–1000 °C and at a heating rate equal to 5 °C∙min−1. The 2D and 3D FTIR spectral maps of evolved gaseous products were recorded with resolution of 4 cm−1 collecting eight scans for each spectrum. The morphology of the materials was characterized using an FEI Versa 3D (FEG—Field Emission Gun) scanning electron microscope (FEI Company, Hillsboro, OR, USA). The crystal structure of the carbon aerogels was characterized by powder X-ray diffraction (XRD) using BRUKER D2 PHASER (Billerica, MA, USA). The Cu Kα radiation ( in the range of 10–60° (2θ) with a step of 0.02° was used. To determine the amount (a weight percent) of carbon, hydrogen, and nitrogen elements in the obtained carbon compounds, the elemental analysis (CHN analysis) was conducted using micro analyzer vario MICRO cube coupled with microbalance (Elementar, Langenselbold, Germany). Before the CHN determination, the CAG samples were dried in vacuum oven under 80 mbar for 3 h at 80 °C. The evaluation of chemical composition was performed with an accuracy of 0.3%. The electrical conductivity (EC) studies were carried out using semi-4-probe method with 1 mA alternating current (at a fixed frequency of 33 Hz) within temperature range from −20 to +40 °C by means of state of the art Sigma1 apparatus. The powder samples (with a thickness of about 2.5 mm) were placed in a glass tube between the parallel flat and gold circular electrodes (with 5 mm in diameter) and pressed by an electrode piston until the measured resistance of the samples remained constant and appropriate electrical contact was assured. Porous features of the resulting samples were evaluated from N2 sorption at −196 °C measured with 3Flex v1.00 automated gas adsorption system (Micromeritics, Norcross, GA, USA). Before the analysis, the samples were degassed under vacuum at 350 °C for 24 h. The specific surface area (SBET) was determined by the single point surface area at .
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