In this paper, we focus on the recent advances on the physical chemistry of lignin. Emerging trends of incorporating lignin in promising future applications such as controlled release, saccharification of lignocelluloses, bioplastics, composites, nanoparticles, adsorbents and dispersants, in electro-chemical applications and carbon fibers, are also reviewed. We briefly describe the complexity of the lignin structure that influences the solution behavior, both as a macromolecule and a colloid, as well as the potential of being a renewable precursor in the development of high-value applications. Special attention is paid on summarizing the present knowledge on lignin colloidal stability and surface chemistry.

Lignin: Recent advances and emerging applications

Heteroatom-doped porous carbon materials (HPCMs) have found extensive applications in adsorption/separation, organic catalysis, sensing, and energy conversion/storage. The judicious choice of carbon precursors is crucial for the manufacture of HPCMs with specific usages and maximization of their functions. In this regard, polymers as precursors have demonstrated great promise because of their versatile molecular and nanoscale structures, modulatable chemical composition, and rich processing techniques to generate textures that, in combination with proper solid-state chemistry, can be maintained throughout carbonization. This Review comprehensively surveys the progress in polymer-derived functional HPCMs in terms of how to produce and control their porosities, heteroatom doping effects, and morphologies and their related use. First, we summarize and discuss synthetic approaches, including hard and soft templating methods as well as direct synthesis strategies employing polymers to control the pores and/or heteroatoms in HPCMs. Second, we summarize the heteroatom doping effects on the thermal stability, electronic and optical properties, and surface chemistry of HPCMs. Specifically, the heteroatom doping effect, which involves both single-type heteroatom doping and codoping of two or more types of heteroatoms into the carbon network, is discussed. Considering the significance of the morphologies of HPCMs in their application spectrum, potential choices of suitable polymeric precursors and strategies to precisely regulate the morphologies of HPCMs are presented. Finally, we provide our perspective on how to predefine the structures of HPCMs by using polymers to realize their potential applications in the current fields of energy generation/conversion and environmental remediation. We believe that these analyses and deductions are valuable for a systematic understanding of polymer-derived carbon materials and will serve as a source of inspiration for the design of future HPCMs.

Polymer-Derived Heteroatom-Doped Porous Carbon Materials.

This paper reviews the research and development works conducted over the past few decades on carbon fiber reinforced metal matrix composites (CFR-MMC). The structure and composition of carbon fiber and its bonding to metal matrix have an impact on the properties of the resulting CFR-MMC remarkably. The research efforts on process optimization and utilizing of carbon fibers are discussed in this review. The effect of carbon fiber on structural, physical and mechanical properties of metal matrix composite are studied as well. This review also provide an overview of the research to date on various fabrication methods that is used for production of CFR-MMC.

Carbon fiber reinforced metal matrix composites: Fabrication processes and properties

Graphene fiber: a new trend in carbon fibers

Advanced electrochemical energy storage devices (EESDs) that can store electrical energy efficiently while being miniature/flexible/wearable/load-bearing are much needed for various applications ranging from flexible/wearable/portable electronics to lightweight electric vehicles/aerospace equipment. Carbon-based fibers hold great promise in the development of these advanced EESDs (e.g., supercapacitors and batteries) due to their being lightweight, high electrical conductivity, excellent mechanical strength, flexibility, and tunable electrochemical performance. This review summarizes the fabrication techniques of carbon-based fibers, especially carbon nanofibers, carbon-nanotube-based fibers, and graphene-based fibers, and various strategies for improving their mechanical, electrical, and electrochemical performance. The design, assembly, and potential applications of advanced EESDs from these carbon-based fibers are highlighted. Finally, the challenges and future opportunities of carbon-based fibers for advanced EESDs are discussed.

Carbon-Based Fibers for Advanced Electrochemical Energy Storage Devices

This Review gives an overview of precursor systems, their processing, and the final precursor-dependent structure of carbon fibers (CFs) including new developments in precursor systems for low-cost CFs. The following CF precursor systems are discussed: poly(acrylonitrile)-based copolymers, pitch, cellulose, lignin, poly(ethylene), and new synthetic polymeric precursors for high-end CFs. In addition, structure-property relationships and the different models for describing both the structure and morphology of CFs will be presented.

Carbon Fibers: Precursor Systems, Processing, Structure, and Properties

All-cellulose composites (ACCs) are prepared from high-strength rayon fibers and cellulose pulp. The procedure comprises the use of a pulp cellulose solution in the ionic liquid (IL) 1-ethyl-3-methyl imidazolium acetate ([EMIM][OAc]) as a precursor for the matrix component. High-strength rayon fibers/fabrics are embedded in this solution of cellulose in the IL followed by removal of the IL. Different concentrations of cellulose in the IL are investigated and the mechanical properties of the final ACCs are determined via tensile, bending, and impact testing. ACCs prepared in this study show mechanical properties comparable to thermoplastic glass fiber-reinforced plastics. Apart from being bio-based, they possess several advantages such as biodegradability and full recyclability. The recycling of ACCs is successfully demonstrated in several cycles by using the recycled cellulose for subsequent matrix preparation.

Ionic Liquid Approach Toward Manufacture and Full Recycling of All‐Cellulose Composites

Reversible-addition fragmentation-transfer (RAFT) polymerization of acrylonitrile (AN) was performed with 2-(2- cyano-2-propyl-dodecyl)trithiocarbonate as RAFT agent and azobis(isobutyronitrile) as initiator. Linear polyacrylonitrile (Mn 5 133,000 g/mol, PDI 5 1.34) was prepared within 7 h in 86% isolated yield. High-yield copolymerization with methyl methacrylate (MMA) was performed and copolymerization parameters were determined according to Kelen and T€ ud€ os at 90 � C in ethylene carbonate yielding rAN 5 0.2 and rMMA 5 0.42. The molecular weights, polydispersity indices (PDIs), and MMA content of the copolymer were adjusted in a way that precur- sor fibers could be prepared via wet spinning. These precursor fibers had round cross-sections and a dense morphology, showing tenacities of 40-50 cN/tex and elastic moduli of 900- 1000 cN/tex at a fineness of 1 dtex and an elongation of 13- 17%. Precursor fibers were oxidatively stabilized and then car- bonized at different temperatures. A maximum tensile strength of 2.5 GPa was reached at 1350 � C. Thermal analysis, infrared and Raman spectroscopy, wide-angle X-ray scatter- ing, scanning electron microscopy, and tensile testing were used to characterize the resulting carbon fibers. V C 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1322-1333

Carbon Fibers Prepared from Tailored Reversible-Addition-Fragmentation Transfer Copolymerization-Derived Poly(acrylonitrile)-co- Poly(methylmethacrylate)

The manufacture of high mechanical strength cellulose-based carbon fibers (CFs) is accomplished in a continuous process at comparably low temperatures and with high carbon yields. Applying a sulfur-based carbonization agent, i.e., ammonium tosylate (ATS), carbon yields of 37% (83% of theory), and maximum tensile strengths and Young's moduli up to 2.0 and 84 GPa are obtained already at 1400 °C. For comparison, the use of the well-known carbonization aid ammonium dihydrogenphosphate ((NH4)H2PO4), ADHP, is also investigated. Both the precursor and the CFs are characterized via elemental analysis, wide-angle X-ray scattering, Raman spectroscopy, scanning electron microscopy, and tensile testing. Thermogravimetric analysis coupled with mass spectrometry/infrared spectroscopy discloses differences in structure formation between ATS and ADHP-derived CFs during pyrolysis.

Cellulose-Derived Carbon Fibers with Improved Carbon Yield and Mechanical Properties

Cellulose derivative fibers were prepared via phosphorylation of cellulose with the ionic liquid (IL) 1,3-dimethylimidazolium methyl-H-phosphonate [MMIM] + [MMP] − in the spinning dope and subsequent fiber formation in a dry–wet-spinning process. The thus obtained precursor fibers were carbonized at different temperatures. In order to receive carbon fibers in high carbonization yields, the degree of substitution (DS) was adjusted. The rheological behavior of the spinning dope was studied and the spinning and carbonization parameters were optimized. Moreover, the precursor fiber tensile and structural properties were compared to pure cellulose fibers. According to thermal analysis coupled with evolved gas analysis (TGA-EGA) of the derivative and pure cellulose fibers, the carbonization yields could be almost doubled via the applied functionalization of cellulose and differences in the relative amounts of released gases during carbonization were studied. Both, precursor and carbon fibers were analyzed by, wide-angle X-ray scattering (WAXS), Raman spectroscopy, scanning electron microscopy (SEM), and tensile testing.

Johanna M. Spörl

Papers

Carbon Fibers: Precursor Systems, Processing, Structure, and Properties

Ionic Liquid Approach Toward Manufacture and Full Recycling of All‐Cellulose Composites

Carbon Fibers Prepared from Tailored Reversible-Addition-Fragmentation Transfer Copolymerization-Derived Poly(acrylonitrile)-co- Poly(methylmethacrylate)

Cellulose-Derived Carbon Fibers with Improved Carbon Yield and Mechanical Properties

Carbon fibers prepared from ionic liquid-derived cellulose precursors