We report on the results of a comparative study of the structural and physicochemical characteristics of cotton fiber samples carbonized by the microwave and conventional thermal methods. The temperature evolution of the samples during the microwave carbonization process has been established. The morphological heterogeneity of the fiber surface over the microwave-carbonized sample cross section has been found. It is shown that fibers of the surface layers are subjected to the destruction of two kinds: (i) numerous transverse brittle fractures and fiber coloring at the places of swelling (a sharp increase in the fiber diameter) and (ii) fluffing of the surface into twisted fibrils with a transverse size of 50–300 nm due to destruction of the outer layers of a fiber secondary wall. In the central part of a sample, fibers are destructed via the formation of longitudinal interfibrillary slits and stratification of the fiber secondary wall, which leads to the formation of pores 50–200 nm in size. It has established that, during the microwave carbonization process, the central part of a sample is almost completely freed up of impurities, which are deposited onto fibers of the surface layers. It is demonstrated that the integral adsorption capacity of the microwave-carbonized sample is higher than the adsorption capacity of the sample carbonized by the thermal method (126 and 47 mg/g, respectively). It has been found that, under microwave exposure for more than 10 min, areas with an adsorption capacity of about 350–450 mg/g arise in the carbonized material, which is comparable with the value for the samples activated by the conventional method.

In this work, hierarchically porous composites were prepared in the form of activated carbon cloth (CC) Busofit T–1–055 filled with an electrically conductive polymer, polyaniline (PANI), for use as pseudocapacitive electrodes of electrochemical supercapacitors (SCs). CC fibers have high nanoporosity and specific surface area, so it was possible to deposit (via the chemical oxidative polymerization of aniline) a significant amount of PANI on them in the form of a thin layer mainly located on the inner surface of the pores. Such morphology of the composite made allowed the combining of the high capacitive characteristics of PANI with the reversibility of electrochemical processes, high columbic efficiency and cyclic stability rather typical for carbon materials of double-layer SCs. The highest capacitance of composite electrodes of about 4.54 F/cm² with high cyclic stability (no more than 8% of capacity loss after 2000 charge–discharge cycles with a current density of 10 A/cm²) and columbic efficiency (up to 98%) was achieved in 3 M H₂SO₄ electrolyte solution when PANI was synthesized from an aniline hydrochloride solution with a concentration of 0.25 M. Trasatti analysis revealed that 27% of specific capacitance corresponded to pseudocapacitance, and 73% to the double-layer capacitance.

The structure, elemental and phase compositions, and adsorption activity of lint cotton samples subjected to microwave and thermal carbonization have been investigated. It is found that impurities are nonuniformly deposited on surface layer fibers during microwave carbonization, practically not contaminating fibrils. Microwave heating causes more active development of the porous structure of fiber. The amorphous phase content is higher in the thermally carbonized sample. The most likely crystalline phases formed in the samples during carbonization are identified to be the graphite phase with a hexagonal lattice and the carbon phase with a tetragonal lattice. The adsorption activity of the microwave-carbonized sample is higher by a factor of almost 3 than that of the thermally carbonized one; in addition, the heating time is much shorter, and the output of ready material is larger. The results obtained demonstrated good prospects of the microwave carbonization method.

The morphology and surface composition of microwave carbonized cotton lint fibers and secondary micro- and nanostructures formed on their surfaces during the carbonization process were studied by the methods of scanning electron microscopy and X-ray microanalysis. During microwave carbonization, the impurity elements existing in the original cotton fibers were shown to move from the central area of the sample to the edge zones and deposit there as continuous phosphorus-containing coatings and micro- and nanostructures of various morphologies and compositions. Phosphorus and calcium were found to be the most difficult impurities to remove. The detected black areas characterize activated carbon comprising no impurity elements. The identified fiber surface structures were characterized by four types: monolithic carbon deposits with a bumpy and lamellar structure, thin transparent films (presumably graphene-like structures), carbon nanofibers and nanotubes (straight and curved, with a diameter of 70 to 150 nm and a length of several microns). These results may be applicable to optimize the production of carbon materials by simultaneous carbonization of cotton fibers with the secondary nanostructures formed on their surface, which may be useful for new applications of carbon materials in such fields as energy storage, electronics, and biomedical engineering.