Synthesis of a New Class of Porous Carbon Materials from Polyacrylonitrile and Their Battery Applications

The development of mesoporous materials with well-defined pore size has been an ever-growing field over the last decade, finding versatile applications in academia and industry. Monoliths are functional macroporous materials with three-dimensional continuous interconnected pore structure in a single piece. They have received considerable attention due to their applications in chromatography, ion-exchange and catalysis which is attributed to their large permeability, fast mass transfer performance, high stability and ease of chemical modification. Polyacrylonitrile (PAN) is widely used as fibers in various industries. PAN fibers are also good precursor of high-quality carbon fibers. This study mainly deals with fabrication of mesoporous PAN monolith by thermally induced phase separation (TIPS) and its conversion to activated carbon monolith (ACM), which has large potential for battery applications. Furthermore, porous PAN particles were prepared by the dispersion polymerization, which were converted to porous carbon particles showing relatively large electrical capacity.

PAN is insoluble in water but soluble in mixture of water and DMSO. Monoliths of PAN with meso-scale pores were fabricated by dissolution of the polymer in the mixture of solvents by heating, followed by cooling: A newly developed facile and easy technique viz. TIPS.¹⁾ The shape of the monoliths could be modified by altering the shape of the vessel. The general procedure for the synthesis of mesoporous polymer monolith is illustrated in Figure 1. SEM images of the PAN monolith show that a continuous interconnected network of polyacrylonitrile was successfully prepared consisting of macropores. The skeleton size for the microstructures was ca. 0.6-1.0 µm whereas the pore sizes lie between 0.8-2.1 µm. A magnified view of the surface morphology of the PAN monolith shows that the skeleton of the macropores were uniformly porous all over with size of the pores lying in the mesoporous range. The BET surface area of the monoliths varied from 160-225 m²/g which depended on the preparation conditions e.g. solvent ratio, heating temperature and duration of homogenization. A highly porous ACM from the mesoporous PAN monolith via thermal treatment in two-steps: activation in air leading to cyclization and subsequent aromatization of the polyacrylonitrile moieties followed by carbonization in Ar or Ar/CO₂ mixture.²⁾ ACM thus produced had a nitrogen content of ca. 4.4 wt. % with high BET surface area (ca. 1400 m²/g) and narrow micropore size distribution. The average pore diameter of ACM was about 7.4 Å, which reveal the microporous structure. Cyclic voltammetry showed relatively large electrostatic capacitance of ACM.

Porous PAN materials were also obtained by dispersion polymerization of AN using poly(N-vinylpyrrolidone) as stabilizer in a mixture of DMF and methanol. The obtained monodisperse particles in the diameter of ca. 1 mm were converted to porous activated carbon particles with high BET surface area (ca.2600 m²/g), which also exhibited good capacitance properties even at high scan speed.

1) K. Okada, M. Nandi, J. Maruyama, T. Oka, T. Tsujimoto, K. Kondoh, H. Uyama, Chem. Commun., 47, 7422 (2011).

2) M. Nandi, K. Okada, A. Dutta, A. Bhaumik, J. Maruyama, D. Derks, H. Uyama, Chem. Commun., 48, 10283 (2012).