Recent progress in controlled carbonization of (waste) polymers

Abstract

The controlled carbonization of polymers is one of the most important polymer reactions. It represents the conversion process of polymeric precursors into carbonaceous materials in a “controlled” manner. In this review, we summarize the research progress on the controlled carbonization of (waste) polymers in preparing functional carbon materials with well-defined microstructures, fabricating advanced polymer composites with enhanced flame retardancy, and converting waste polymers into high value-added nanocarbon materials. The carbonization mechanisms of polymers with different compositions and chain structures are highlighted. How to match the degradation reaction of polymers with the carbonization reaction of the degradation products is addressed to improve the carbonization efficiency and optimize the growth of carbon materials. The applications of (waste) polymer-derived carbon materials in electrochemical energy storage and environmental remediation are reviewed. Current existing limitations and future perspectives are also given.

Introduction

The controlled carbonization of polymers entails the conversion process of polymeric precursors into carbonaceous materials with defined microstructures via a series of “controlled” chemical reactions. The controlled carbonization of polymers can be summed up into two aspects in our opinion. The first one is related to the “controlled” degradation and carbonization reactions of polymers. This is because polymers usually yield complex degradation products, which are challenging to be effectively converted into carbon materials. The other one is focused on the “controlled” microstructures of obtained carbon materials, which often play crucial roles in their properties and performance. Interest in the controlled carbonization of polymers in recent years has been attributed to such attractive applications as preparing advanced CNMs, fabricating polymer composites with enhanced flame retardancy, and converting waste polymers into valuable nanocarbon materials.

First, compared to traditional hydrocarbons such as gases (e.g., methane, ethylene, and propylene), liquids (e.g., methanol, ethanol, and acetonitrile), and solids (e.g., glucose and amino acid), the advantages of polymers as carbon precursors in preparing CNMs include the facile processability and controllable amount of carbon atom. For example, polymers as precursors are easily pre-formed according to the requirements of preparing fibers via the electrospinning technique of polymers and their composites with metal or metal oxide nanoparticles. Besides, the controllable amounts of carbon atom and heteroatoms such as nitrogen in polymer precursor are key to prepare graphene with controlled layers [1] and heteroatom-doped carbon materials [2]. Compared to biomass, synthetic polymers have well-defined structures and can yield carbons with relatively well controlled structures and content [3]. Second, polymer materials are widely used in the electronics and electrical industry, automobiles, and housing. However, the inherent flammability of polymers restricts many applications owing to safety concerns. Promoting carbonization of polymers during combustion plays a critical role in improving the flame retardancy of polymer materials because the number of flammable components diffusing into the flame area is reduced in carbonization reactions. Furthermore, carbon materials formed in-situ from polymers provide a good protective layer to enhance the flame retardancy of polymer materials. Third, the controlled carbonization of polymers provides a potential means to reutilize a large number of waste polymers for synthesizing CNMs. Waste polymers can be roughly categorized into waste synthetic polymers (i.e., plastics, rubber, and fibers) and waste biopolymers. Taking waste plastics as an example, it is estimated that the quantity of worldwide waste plastics has reached 6.4 billion tons since the 1950s, and ca. 250 million tons of plastic wastes will be generated every year by 2025. Most waste polymers are not biodegradable and cause many environmental problems such as “White Pollution” [4,5]. Sustainable development calls for low environmental impact technologies to recycle waste polymers in place of the current practices of landfilling and incineration [[6], [7], [8]]. Since polymers generally contain a rich carbon element, converting waste polymers into high value-added CNMs not only contributes to the reutilization of waste polymers, but also provides a low-cost way to prepare functional carbon materials.

According to the chemical reactions of a polymer backbone occurring at high temperatures, polymers can be classified into two categories: noncharring and charring. Noncharring polymers consist of many commercial plastics, e.g., PP and PE. During the carbonizing process, the backbone chains of noncharring polymers decompose into low-molecular compounds (such as propylene and benzene) through dehydrogenation, cyclization, aromatization, etc. The core questions in the conversion of noncharring polymers into CNMs are: (1) how to control the degradation behavior of noncharring polymers into low-molecular compounds that act as suitable carbon sources for the growth of CNMs, and (2) how to promote the carbonization of these degradation products. Charring polymers include many synthetic polymers, e.g., PF resin, PAN, and CPVC, as well as some biopolymers. Compared to noncharring polymers, the backbone chains of charring polymers in the carbonizing process generally do not degrade but undergo a series of chemical reactions such as cyclization, aromatization, and crosslinking, and gradually construct a carbon material frame [9]. Hence, adjusting these reactions is crucial to effectively convert charring polymers into carbon materials. Essentially, any substances that regulate these degradation and carbonization reactions are potential catalysts to control the carbonization of polymers.

The controlled carbonization of biomass was used to prepare ACs with varied pore sizes from the pyrolysis of bamboo by adjusting reaction temperature [10]. The concept of controlled carbonization of (waste) polymers in this review is different than that example. Moreover, the controlled carbonization of small molecules has been proposed. For instance, Zhang et al. prepared carbon nanoparticles by hydrothermal carbonization of sucrose using sulfuric acid [11]. Obviously, compared to traditional hydrocarbons, the controlled carbonization of (waste) polymers to prepare carbon materials with well-defined microstructures in this review is more challenging due to the complex degradation products of polymers. Taking polyolefins as an example, the degradation products are composed of gases (e.g., methane, ethylene, and propylene), liquids (e.g., 1,3-pentadiene and toluene), and semi-solids (e.g., long-chain alkenes).

Although there have been several reviews published recently on the reutilization of (waste) polymers to prepare CNMs [[12], [13], [14], [15]], little attention has been paid to the controlled carbonization of polymers. Which types of catalysts are proper for promoting the degradation and carbonization processes of polymers? How do the compositions and distributions of polymer degradation products influence the efficiency of carbonization and the morphology of CNMs? Only after clarifying these basic questions can we effectively implement controlled carbonization of polymers. We have studied the controlled carbonization of polymers since 2005 [16], and proposed the strategy of combined catalysts to promote the degradation of polymers and the subsequent carbonization of degradation products into nanocarbon products, e.g., CNTs. In this review, we summarize research progress in the controlled carbonization of polymers (Fig. 1). We first introduce several controlled carbonization strategies, e.g., combined catalysts, fast carbonization, active-template carbonization, and copolymer-template carbonization. Next, we discuss the applications of controlled carbonization of polymers in improving the flame retardancy of polymeric materials and preparing various CNMs with well-defined microstructures. Special focus is on the degradation and carbonization mechanisms of polymers with different compositions and chain structures. We discuss how to match the ratio of degradation reaction to carbonization reaction through the catalysis strategy to improve the carbonization efficiency and optimize the growth of carbon materials. Considering the numerous studies in the past two decades on preparing commercial carbon materials such as CB from polymeric precursors, these aspects are not covered in this review.

Moreover, the performance of carbon materials highly depends on their morphologies, porosities, and chemical compositions [17]. We accordingly address how to control the physical-co-chemical properties of (waste) polymer-derived carbons including morphology, porosity, and heteroatom doping. Porous carbons, because of their extraordinary and tunable physical-co-chemical properties, are promising candidates for many applications, e.g., electrochemical energy storage and environmental protection. The low-cost preparation of porous carbons with well-designed pore sizes is highly desirable. The controlled carbonization of waste polymers into porous carbons with high performance in electrochemical energy storage and environmental remediation can conceivably kill two birds with one stone. For this reason, we summarize the progress in the applications of polymer-derived porous carbons in the two fields above and finally point out the directions of polymer carbonization research.