Tannins are eco-friendly, bio-sourced, natural, and highly reactive polyphenols. In the past decades, the understanding of their versatile properties has grown substantially alongside a continuously broadening of the tannins’ application scope. In particular, recently, tannins have been increasingly investigated for their interaction with other species in order to obtain tannin-based hybrid systems that feature advanced and/or novel properties. Furthermore, in virtue of the tannins’ chemistry and their high reactivity, they either physicochemically or physically interact with a wide variety of different compounds, including metals and ceramics, as well as a number of organic species. Such hybrid or hybrid-like systems allow the preparation of various advanced nanomaterials, featuring improved performances compared to the current ones. Consequently, these diverse-shaped materials have potential use in wastewater treatment or catalysis, as well as in some novel fields such as UV-shielding, functional food packaging, and biomedicine. Since these kinds of tannin-based hybrids represent an emerging field, thus far no comprehensive overview concerning their potential as functional chemical building blocks is available. Hence, this review aims to provide a structured summary of the current state of research regarding tannin-based hybrids, detailed findings on the chemical mechanisms as well as their fields of application.

A promising route to monolithic, hollow sphere carbon assemblies based on sustainable precursors with a tailored nanostructure is presented. These carbon assemblies, recently termed carbon spherogels, are generated via a polystyrene sphere template-based sol-gel process of mimosa tannin and biomass-derived 5-(hydroxymethyl)furfural. By completely replacing petroleum-based precursors (especially toxic formaldehyde) highly porous, nanoscale carbon monoliths are obtained, which are investigated as state-of-the-art, sustainable electrode materials for energy storage. This study defines the required synthesis parameters, in particular the highly acidic initial pH and a tannin/water ratio of at least 0.05 or lower, for a successful and homogeneous generation of these biobased carbon spherogels.

As a result of the global demand for sustainable products, a suitable alternative to the resorcinol-formaldehyde aerogels, which are frequently used as precursors for carbon aerogels, is searched for. In this study, the replacement of petroleum-derived formaldehyde with a natural, biobased crosslinker, namely 5-(hydroxymethyl)furfural (5-HMF) is shown, and the synthesis of renewable, monolithic tannin aerogels is demonstrated. Compared to well-known tannin-formaldehyde aerogels, this green alternative shows lower reactivity of the crosslinker associated with lower gelation times as well as lower specific surface areas at the organic stage. Nonetheless, the morphologies and synthesis-structure relationships follow similar trends for both tannin-based aerogels, e.g., the pore size is influenced by the initial pH in the same manner. The turnover to carbon aerogels by a carbothermal treatment results in enhanced high-specific surface areas of the tannin-5-HMF-based carbon aerogels, which are similar and even slightly outperform those obtained from tannin-formaldehyde aerogels. This suggests that they are a convenient alternative for carbon aerogel applications. Graphical Abstract

In this work, polyvinylidene fluoride (PVDF) aerogels with a tailorable phase composition were prepared by following the crystallization-induced gelation principle. A series of PVDF wet gels (5 to 12 wt.%) were prepared from either PVDF–DMF solutions or a mixture of DMF and ethanol as non-solvent. The effects of the non-solvent concentration on the crystalline composition of the PVDF aerogels were thoroughly investigated. It was found that the nucleating role of ethanol can be adjusted to produce low-density PVDF aerogels, whereas the changes in composition by the addition of small amounts of water to the solution promote the stabilization of the valuable β and γ phases. These phases of the aerogels were monitored by FTIR and Raman spectroscopies. Furthermore, the crystallization process was followed by in-time and in situ ATR–FTIR spectroscopy. The obtained aerogels displayed specific surface areas > 150 m2 g−1, with variable particle morphologies that are dependent on the non-solvent composition, as observed by using SEM and Synchrotron Radiation Computed micro-Tomography (SR-μCT).

Tannin-silica hybrid materials are expected to feature excellent mechanic-chemical stability, large surface areas, high porosity and possess, after carbothermal reduction, high thermal stability as well as high thermal conductivity. Typically, a commercially available tetraethoxysilane is used, but in this study, a more sustainable route was developed by using a glycol-based silica precursor, tetrakis(2-hydroxyethyl)orthosilicate (EGMS), which is highly water-soluble. In order to produce highly porous, homogeneous hybrid tannin-silica aerogels in a one-pot approach, a suitable crosslinker has to be used. It was found that an aldehyde-functionalized silane (triethoxysilylbutyraldehyde) enables the covalent bonding of tannin and silica. Solely by altering the processing parameters, distinctly different tannin-silica hybrid material properties could be achieved. In particular, the amount of crosslinker is a significant factor with respect to altering the materials’ properties, e.g., the specific surface area. Notably, 5 wt% of crosslinker presents an optimal percentage to obtain a sustainable tannin-silica hybrid system with high specific surface areas of roughly 800–900 m2 g−1 as well as a high mesopore volume. The synthesized tannin-silica hybrid aerogels permit the usage as green precursor for silicon carbide materials.