Organic and Carbon Gels

Since the first report on organic gels based on the polycondensation of resorcinol with formaldehyde presented by Pekala in 1989, the number of publications, on both organic gels and carbon gels has experimented an enormous increase to the point where nowadays are published every year more than a hundred papers covering topics ranging from variations in the synthesis to the potential applications of this vast family of porous materials. This is due to the fact that, by controlling the synthesis conditions, it is possible to obtain materials with a suitable porosity for a specific application and even also with predetermined chemical properties, something that is practically impossible to achieve with any other porous materials. However, even after almost 30 years of continuous researching at laboratory scale, their industrial production and commercialization are still marginal compared with that of competitive materials. This chapter summarizes how the physicochemical properties of organic and carbon gels can be designed by controlling all the variables involved in the synthesis process. The chapter also addresses the most challenging problem of their mass production, i.e., scaling-up of production methods currently used in the labs.

This chapter presents the most recent updates about sol-gel chemistry of phenolic molecules and the corresponding materials: xerogels, cryogels, and aerogels. The structure and properties of the latter, whether in the organic or carbon forms, are detailed and actual and potential applications are reported.

After an introduction about plant polyphenols in general, the focus is mainly given to condensed (flavonoid) tannins, shown to be the most relevant raw material for preparing resins, and hence gels. Lignin is considered as well, despite its lower reactivity and its less reproducible character, because of its industrial importance. Details about the nature and the properties of the carbon that can be obtained by pyrolysis of crosslinked polyphenols are also given.

Tannin-formaldehyde resins and mixed formulations associating resorcinol, soy protein, lignin, phenol, or surfactant are then discussed in terms of reactivity and ability to produce highly porous gels, depending on the experimental conditions of synthesis (dilution, pH, amount of crosslinker, etc.) and drying (subcritical, supercritical, or lyophilization). The porous structure of those materials is also explained in relation to gelation time and mechanical properties of the corresponding hydrogels. Derived carbons gels, including N-doped, formaldehyde-free materials, and activated carbon gels, are also considered.

Mechanical and thermal properties of organic gels, as well as electrochemical properties of carbon gels, are next introduced. Finally, recent developments including one-step microwave synthesis of xerogels, carbon xerogel microspheres having the characteristics of carbon molecular sieves, and elastic gels behaving as rubber springs with tunable elastic properties, all biosourced and tannin-based, are presented.

Aerogels are sol-gel derived porous solids with structural properties, such as porosity, pore size, pore and solid phase connectivity that can be tailored over a wide range to provide unique material properties for different fields of applications, such as filters and adsorbers, catalyst supports, electrodes for electrical energy storage, and materials for lightweight construction or thermal insulation.

In this context, carbon aerogels and their organic precursor represent an important class of aerogels with very different physical properties at similar structural characteristics. This is due to the different intrinsic properties of the respective backbone components: At given meso- and macrostructure carbon aerogels are characterized by high thermal and electrical conductivity, significant mechanical brittleness, high porosity of the backbone phase related to micropores (<2 nm), and thus specific surface areas up to about 2000 m²/g. In contrast, the respective organic precursors exhibit very small electrical conductivities and a significantly reduced heat transfer via the aerogel backbone phase; they furthermore may be mechanically more flexible and are limited to specific surface areas below 1000 m²/g. The chapter provides an overview over typical structural and physical properties of carbon aerogels and their precursors.

Understanding structure–property relationships and optimizing aerogels for different applications requires reliable characterization techniques. The review article addresses different characterization techniques as well as the problem of artifacts upon structural characterization; the latter is due to the unique combination of small pore sizes and large porosities characteristic for aerogels. With that in mind, the article alternative, in part even more powerful approaches.

The air and water pollution is a main challenge in the preservation of our planet for the future generations. Pollution is a changing problem, in such a manner that the technology and materials used for decontamination processes should be continuously improved. The two main options to address this goal are based on the optimization of adsorption or catalytic processes. Both approaches are surface processes and thus materials with fitted physicochemical properties to be applied in these processes are needed.

The main advantage of the sol-gel synthesis is its flexibility and the multitude of possible combinations of the different synthesis conditions, thus allowing the preparation of materials with a wide range of different characteristics, which can be fitted at the nanometric scale. Carbon gels are nanomaterials prepared by specifically designed synthetic processes and obtained from pure reactants. Their properties can be fitted during or after the gelation/carbonization. In this chapter, the peculiarities of the carbon aerogels are presented together with the statement of the pollution problem. The analysis of the adsorbents and/or catalysts requirements for its use in environmental processes is described, as well as the possibilities to fit the synthesis processes to the needed physicochemical properties. Finally, the performance of carbon gels in both adsorption and catalytic processes is presented, taking into account both the water and air treatments. Carbon gels present excellent qualities and excellent performances in many different approaches used for decontamination. The use of alternative raw material and safe, fast, and conventional techniques of production of advanced carbon gels with fitted properties can favor definitively the industrial production and application of this kind of materials.

Due to their conductivity and adjustable pore texture, carbon gels have been considered as alternative materials to activated carbons, graphite, and carbon blacks in electrochemical applications such as electrochemical double-layer supercapacitors, lithium (or sodium) ion batteries, and proton exchange membrane fuel cells. This chapter reviews the use of carbon gels with tailored pore texture as electrode materials for the three applications. In each case, the advantages and drawbacks are identified and discussed, especially regarding the processing of carbon gel materials into composite electrodes, i.e., layers made from a powdery carbon and a binder. Like in catalysis processes, carbon gels may give quite interesting properties to the final electrochemical device, provided their features are properly tuned and the electrode processing is mastered. Indeed, pore texture adjustment often leads to decreasing mass-transport issues, and the possibility to modify the surface chemistry is an asset towards electrode/electrolyte interface optimization.