Block Copolymer Based Porous Carbon Fiber-Synthesis, Processing, and Applications.

Carbon is an abundant material with remarkable thermal, mechanical, physical, and chemical properties. Each allotrope has unique structures, properties, functionalities, and corresponding applications. Over the past few decades, various types of carbon materials such as graphene, carbon nanotubes, carbon quantum dots, and carbon fibers have been produced, finding applications in energy conversion and storage, water treatment, sensing, polymer composites, and biomedical fields. Among these carbon materials, porous carbons are highly interesting owing to their large surface areas and massive active sites to interact with molecules, ions, and other chemical species. The pore size and pore size distributions can be tunable (micro-, meso-, and macro-pores), providing chemical species with hierarchical structures to transport with low resistances. In this context, designing carbon precursors and preparing porous carbon with desired structures, properties, and functionalities are highly significant. Polymers are versatile carbon precursors. Designing the polymer precursors that facilitate the formation of well-controlled pores is an effective strategy to prepare porous carbons. In particular, porous carbon fibers (PCFs) in a fibrous format offer additional features of hierarchical porosity control, increased surface area, and fast ion transport. The most common approach to synthesizing PCFs is to use sacrificial agents (e.g., homopolymers of polystyrene (PS) and poly(methyl methacrylate) (PMMA), inorganic nanoparticles, and other additives) in a matrix of polyacrylonitrile (PAN) as the carbon fiber precursor. However, the nonuniform mixing of sacrificial agents in the PAN matrix results in PCFs with nonuniform pores and wide pore size distributions. Moreover, complete removal of the inorganic additives is challenging and sometimes requires the use of hazardous chemicals. Therefore, developing innovative methods for synthesizing PCFs is imperative to advance these engineering materials for emerging applications. In this Account, we summarize our efforts on the use of block copolymer precursors to prepare PCFs with tunable pore sizes and pore size distributions for a series of applications. First, we will introduce the synthesis methodologies for preparing PCFs. We have used reversible addition-fragmentation chain transfer (RAFT) polymerization to synthesize block copolymer precursors. Second, we will discuss the effects of preparation conditions on the properties of PCFs. The mechanical and electrical properties highly depend on the composition of the block copolymer, pyrolysis conditions, and humidity level during the fiber spinning process. Lastly, we will discuss the effects of controlled porosity on the surface area, electrical/ionic conductivity, and polymer-matrix interactions, which are crucial for applications including energy storage (e.g., batteries and supercapacitors), fiber-reinforced polymer composites, separation, and filtration.