Hydrogen possesses a high gravimetric energy capacity and it is used as a green energy source in automobiles since it eliminates CO2 emissions. A variety of hydrogen storage materials such as metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), highly porous carbonaceous materials are currently emerging. Researchers at the University of Nottingham, U. K., have developed a process to prepare porous carbon using fresh and smoked cigarette butts (filters) by a sequence of treatments – hydrothermal carbonisation followed by activation.
Unused (F group) and smoked (S group) filters containing cellulose acetate as the main ingredient were used as the starting materials. The F and S group cigarette filters were ground to form a fluffy white or yellow-brown mass, mixed with water (at a ratio of 1 g filter to 10 mL water), hydrothermally carbonized in a stainless steel autoclave to 250 °C for 2 h and the resultant carbonaceous matter (hydrochar) was dried at 112 °C. The hydrochars derived from F and S group cigarette filters were denoted as FF-hydrochar and SF-hydrochar, respectively.
The hydrochars were ground with KOH (KOH/hydrochar ratio = 4), activated at 600, 700 and 800 °C for 1 h and allowed to cool under N2. The resultant activated carbons were washed initially using 2M HCl followed by deionized water to remove the residual acidity and dried at 112 °C. The activated carbons were designated as FF-4T (from FF-hydrochar) and SF-4T (from SF-hydrochar, respectively in which 4 represents the KOH/hydrochar ratio and T refers to the activation temperature.
Fig. 1 Schematic of the conversion of cigarette butts (filters) to activated carbon
The porosity and pore size distribution of FF and SF series activated carbons is found to increase with an increase in activation temperature from 600 to 800 °C. For FF-4T activated carbons, both the apparent surface area and pore volume are increased with an increase in activation temperature with a maximum apparent surface area of 4113 m2/g and pore volume of 1.87 cm3/g are obtained for FF-4800. In contrast, the trend is reversed for SF-4T activated carbons in which a maximum apparent surface area of 4310 m2/g and pore volume of 2.09 cm3/g are obtained for SF-4600. Among all the samples evaluated, SF-4600 has the highest apparent surface area of 4310 m2/g with a micropore surface area of 3867 m2/g, which is 90% of the total surface area ever reported for activated carbons. This is due to the presence of metal additives such as K, Ca, Na, Mg, etc., in the smoked filters which could have acted as activating agent besides KOH. The high surface area and high microporosity with a significant proportion of pores are < 1 nm in size of SF-4600 are the important attributes needed for hydrogen storage materials.
Assessment of hydrogen uptake properties of FF-4T and SF-4T series activated carbons at -196 ºC and 0 – 40 bar (cryo-storage conditions required for low pressure vehicular hydrogen storage) indicates that SF-4600 contributes to the highest hydrogen uptake. A combination of high apparent surface area, high microporosity and high oxygen content (16 – 31 wt% with oxygen functional groups such as COOH, C-OH and O-C=O) enables SF-4600 to achieve a high hydrogen uptake.
Fig. 2 Excess and total hydrogen uptake at -196 °C of activated carbons derived from (a) fresh cigarette filters and (b) smoked cigarette filters/butts; (c) Bench marking of hydrogen uptake of SF-4600 with high surface area MOFs
T.S.N. Sankara Narayanan
For more details, the reader may kindly refer: T.S. Blankenship and R. Mokaya, Cigarette butt-derived carbons have ultra-high surface area and unprecedented hydrogen storage capacity, Energy Environ. Sci., 2017, DOI: 10.1039/C7EE02616A
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