From starch to carbonaceous foam: The effects of structure and conformation of the feedstock

Highlights

• Effects of structure/conformation on the thermal properties of starch were studied.

• Structure and conformation of feedstocks determine the preparation of CF.

• Hydrogen bonding is not a primary factor affecting the thermal properties.

• Influence mechanism on the preparation of CF has been proposed.

Abstract

Three-dimensional carbon/carbonaceous foams (CFs) have attracted great research interest in recent years and have been widely used in many military and civilian fields. Replacing traditional raw materials (e.g., pitches and petrochemical raw material derived resins) with renewable biomass resources is becoming one of the trends in the research of CFs. In the present work, CF was prepared from starch. The effects of the structure and conformation of polysaccharides on the preparation of CF were studied from multiple perspectives and multiple scales using a variety of starches as raw materials. It is proved that the conformation of the raw material, a factor that is almost neglected in the research of pitches derived CFs, has a significant influence on the preparation of starch derived CF. However, hydrogen bonding, which is ubiquitous in biomass feedstocks, is not a primary factor affecting the thermophysical and thermochemical properties of the feedstocks in this study. It is not the key to the foaming state of starch derived CF. The results of this work will be instructive for the preparation and structural regulation of biomass derived CFs.

Introduction

As three-dimensional porous carbonaceous products, carbon/carbonaceous foams (CFs) have received increasing attention worldwide since they were developed in the late 1960s. Especially in the past two decades, the number of related studies has increased exponentially (Fig. S1). They have been widely studied and applied in the military and civilian fields, including energy storage, oil/water separation, electromagnetic shielding, sensors, catalysis, water purification and so on [[1], [2], [3], [4], [5], [6], [7], [8]]. A variety of feedstocks may be used to prepare CFs. Among them, pitches are conventional and most commonly used. Some non-renewable synthetic resins have also been reported as precursors. In recent years, there emerges another attractive trend worthy of attention, namely the use of biomass to make CFs [[9], [10], [11], [12], [13], [14], [15]]. As an important member of the biomass family, environmentally friendly and renewable polysaccharides (such as starch, cellulose, chitosan, etc.) have huge reserves (150 billion tons/year) [16]. Previous studies have shown that starch can be used to make CFs [[17], [18], [19]]. Interestingly, however, the same or similar preparation strategies are not applicable to cellulose, despite the fact that both cellulose and starch are composed of glucose units and their only difference is the glycosidic linkages. Then we found that, even if only starch was used, different starches may lead to carbonaceous products with very different appearance and microstructure [20]. Unfortunately, there has been little research on the mechanism of converting polysaccharides into CFs. Any deep understanding of the above phenomena is barely reported. Therefore, taking starch as a prototype to understand the conversion mechanism will be beneficial to broaden the range of raw materials for the preparation of CFs. This also makes sense for the structural regulation of biomass-derived CFs.

The formation mechanism of CFs depends on their preparation strategy. According to the nature of the raw materials, there are two main strategies to obtain CFs: blowing method and template method. In the case where pitches are used as raw materials, the blowing method is usually employed [21]. In addition to pitches, this method is also applicable to some other thermoplastic materials, such as phenolic resins, tannin, sucrose, and so on. For some polymer foams (or polymer foams impregnated with resins) which already have a foam structure (e.g., melamine foam, melamine-formaldehyde foam, polyurethane foam, etc.), it is also feasible to obtain CFs by directly carbonizing them. This method is usually classified as a template method [6]. Besides, some inorganic or organic sacrificial particles can also be used as templates. In view of the characteristics of the pitches and their extensive use as raw materials in the preparation of CFs, and also considering the diversity of CFs obtained by the blowing method, this method is still a widely used strategy for the production of CFs. Also because of this, the researches on the mechanism of the blowing method are mainly focused on the systems using pitches. Researchers generally believe that the procedure of foaming in the preparation of CFs involves dissolution and nucleation of gases, formation and growth of bubbles, further coalesce of bubbles and the formation of cells [[21], [22], [23], [24]]. In addition to the rheological characteristics of the precursors, different parameters such as temperature, initial pressure, pressure drop rate, and solvent and/or gas proportions during the foaming process are key factors affecting the cell structure of the CFs [[25], [26], [27], [28], [29]].

Most of the reported methods for preparing CFs from biomass can be classified as blowing method. The studies on the mechanism of preparing CFs from pitches are of reference value for the preparation of CFs from biomass. However, due to the diversity of the structure and physicochemical properties of biomass, the preparation of CFs from biomass is inevitably different from those using pitches. The present work intends to explore the effects of the structure and conformation of polysaccharides on the preparation of CFs. By using starch as a prototype, we expect the results to provide useful guidance for the preparation and structural regulation of biomass-derived CFs.