Physicochemical and electrochemical properties of microwave-assisted hydrochars and activated carbons from straws

It is of great significance to comprehensively study the physicochemical properties of microwave-assisted hydrochar Hydrochars (carbon-rich solids) from biomass can be converted to activated carbons via hydrothermal carbonization. This offers a promising way for the disposal of the lignocellulosic wastes in modern agriculture. In this study, a systematic investigation was made to explore the physicochemical and electrochemical properties of microwave-assisted hydrochar and alkali-activated carbon from straws. A microwave hydrothermal experiment was carried out using straws with different mass fractions of citric acid for the preparation of hydrochar and alkali-activated carbon. The physicochemical behaviors of the hydrochars and alkali-activated carbon were also investigated using various characterization techniques. The results showed that the yield, volatile matter and H content of the hydrochar from straws decreased, whereas, the ash, fixed carbon, C, S and higher heating value increased, as the mass fraction of citric acid increased. These parameters tended to be stable, when the acid mass fraction reached 10%. In the hydrochar, the conversion rate of oxygen, carbon, and calorific value first decreased, and then increased during the test. Hydrochar with the citric acid mass fraction of 10% exhibited the most abundant carbon microsphere structure, as well as the largest specific surface area and pore volume, with the mesopore volume of 0.06-0.10 cm3/g. After the 10% citric acid, the hydrochar was activated by KOH solution at 900℃. The yield of activated carbon was about 8%-11%, while, that of activated gas was about 32%-35%, mainly including CO and H2. The total output of combustible gases was 450-530 L/kg. The abundant pore structure was formed for the activated carbon at 900℃, where the specific surface area, total pore volume, and diameter of pores were 1 250-1 570 m2/g, 1.00-1.20 cm3/g, and 3.55-4.10 nm, respectively. The majority of pores were the mesopores of 0.69-0.81 cm3/g and micropores of 0.35-0.38 cm3/g. Compared with hydrochars, the activated carbon showed the lower strength of O-H, aliphatic C-H, C=O, C=C and C-C, while, the higher intensity of peak energy in C-O-C bond. Similar to the hydrochars, the graphitization degree of activated carbon decreased, due to the increasing disorder and defects in carbon, indicating suitable for the energy storage of electrode materials. At low scanning rates, the cyclic voltammetric curves of activated carbon behaved symmetrical rectangular shapes, indicating that the characteristic of a typical double electric layer capacitance occurred in the activated carbon. Nevertheless, the cyclic voltammetric curve was gradually deformed, as the scanning rate increased. When the current density was 1 A/g, the specific capacitances of activated carbon at 900°C based on hydrochars from rice straw, maize stover and rape stalk were 160.54, 150.12 and 155.17 F/g, respectively. The capacitance retention rates of activated carbon from rice straw, maize stover and rape stalk after 5000 cycles were 91.04%, 88.12% and 89.06%, respectively, showing a good cycle stability. Among different straws, the rice straw showed the highest yield, ash content, carbon conversion rate and energy conversion rate of hydrochar and activated carbon. The maize stover represented the highest fixed carbon, C and higher heating value of hydrochar and activated carbon, whereas, the rape stalk displayed the lowest carbon conversion rate of hydrochar and activated carbon. The hydrochar and activated carbon from rice straw indicated the largest specific surface area, total pore volume, mesopore volume and micropore volume, whereas, those from rape stalk showed the largest pore size. The activated carbon from rice straw demonstrated the strongest vibration absorption peak of oxygen-containing functional groups, the lowest graphitization degree, as well as the largest specific capacitance, and highest capacitance retention rate. The findings can be benefit to improving the quality of hydrochar, and the utilization of activated carbon as electrode materials in intelligent industry.