Manufacture and application of lignin-based carbon fibers (LCFs) and lignin-based carbon nanofibers (LCNFs)

Environmental issues and constantly diminishing petroleum resources are considerable barriers inhibiting modernization, and vast efforts have been exerted to address these problems. Carbon fibers (CFs) are carbon materials with high mechanical strength and functionality for applications in construction, electronics, transportation, and aviation. Currently, most CFs are produced from polyacrylonitrile, a petroleum-based, unsustainable, and non-renewable chemical of relatively high price. Interestingly, lignin is an inexpensive, highly accessible, and renewable resource. It has been utilized to fabricate lignin-based carbon fibers (LCFs), which have met rapid development during the past two decades. In this review, LCFs are generalized by focusing on their steps of manufacture. Resource types and corresponding pretreatments ensure the processability of spinning and thermal treatments. Fibers are formed via spinning methods, including melt-spinning, wet-spinning, dry-spinning, and electrospinning. The next step is the most significant process of stabilization, in which fibers are oxidized, crosslinked, and thermally stabilized for pyrolysis. Subsequent to carbonization and/or additional processes (activation and graphitization), LCFs are obtained. Each step can influence the terminal performance of LCFs, which is discussed in detail. Recently produced LCFs of sub-micron size, also known as lignin-based carbon nanofibers (LCNFs), are detailed. Furthermore, attributed to the excellent performance and low cost of LCFs and LCNFs, they have been applied in various fields, predominantly for electronic devices such as batteries and supercapacitors. Our review is concluded with opinions on the potential for further advancement of this promising material. This review details recent progress in the conversion of technical lignins to multi-functional, high-value, and promising carbon fiber materials, and discusses their applications.