Developing hydrogen storage materials from Cigarette butts – Stepping towards the reality of achieving hydrogen economy

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

Upcycling waste polyethylene to high performance graphitic carbon – A new avenue for utilization of plastic waste

Linear low density polyethylene (LLDPE) is one of the most widely used plastics, mainly as a packaging film, contributes to the generation of a huge volume of plastic waste. Only 5.8 % of waste LLDPE was recycled in 2014, while the remaining wastes were buried in landfills, posing a huge challenge for solid waste management.

Researchers from Korea Institute of Science and Technology, Korea University of Science and Technology, Konkuk University, Republic of Korea and Georgia Institute of Technology, USA have developed a process to LLDPE to graphitic carbon and demonstrated that it would be possible to convert typical household LLDPE waste products such as cling wrap and poly-gloves into high quality carbon materials. (Choi et al., High performance graphitic carbon from waste polyethylene: thermal oxidation as a stabilization pathway revisited,
Chem. Mater., DOI: 10.1021/acs.chemmater.7b03737).

Thermal oxidation was used as a pre-treatment to modulate the chemical structure of LLDPE. During the thermal oxidative pre-treatment (~330 °C) the ‘non carbonizable’ LLDPE was successfully transformed into an ordered carbon (50% yield). The aliphatic LLPDE chain is reorganized into thermally stable cross-linked polyaromatic moieties with a cyclized ladder structure (Scheme 1), which makes them suitable for carbonization.

Scheme 1 Proposed chemical structural transformation of aliphatic LLDPE chains into cyclized polyaromatic moieties through thermal oxidation

The thermally stable cross-linked polyaromatic moieties, in turn, guides a spontaneous transition into well-stacked polyaromatic carbon structures with a further increase in temperature from 330 to 2400 °C (Scheme 2). The resultant product possesses a high quality graphitic structure that exhibits superior degree of ordering and electrical performance.

Scheme 2 Growth of basic structural unit (BSU) of thermally oxidized LLDPE samples during carbonization and graphitization processes

It is a low-cost production route for graphitic carbon materials, which would find application in energy storage and flexible, printed electronics.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Choi et al., High performance graphitic carbon from waste polyethylene: thermal oxidation as a stabilization pathway revisited, Chem. Mater., DOI: 10.1021/acs.chemmater.7b03737