Organic porous tannin gels are versatile materials that can be used for several applications, mainly because of their final characteristics, which are normally tailored during synthesis steps. The tannin gel synthesis is based on a sol-gel process of soluble precursors. The colloidal suspension of solid particles (sol) is composed of tannin and a crosslinker dispersed in a solvent, commonly water. A schematic representation of a sol-gel process is given in Figure 6a.

Usually, the reactions start with an addition reaction between the main precursor (tannin) and the crosslinker (aldehyde), making these species more reactive. The partially hydrolyzed particles then crosslink by polycondensation reactions, releasing water molecules. Thus, the polymeric chains grow, forming a giant cluster of macroscopic size, becoming insoluble. This phenomenon is known as gelation [52]. The sol-gel transition is usually estimated by a significant change in viscosity through visual observation of the container tubes where the solutions are placed. Normally, reactions happen in a sealed flask at moderate temperature (50–85 °C), so that the whole solution can be converted into a gel [52], and when the viscous liquid is no longer flowing at an angle of 45°, the solution is considered gelled [53,54], establishing the gelling time (T_gel) [55]. The polymerization reaction in the aqueous medium generates a gel as a semi-solid system containing two phases: a solid based on a nanostructured network (spherical nodules) interconnected by narrow necks defined as "string-of-pearls", which is enclosed in a high porosity inter-penetrated solvent medium (second phase), as well as in some by-products formed during the polymerization reactions [52,56,57]. The size of these nodules depends directly on the synthesis conditions, especially the pH and the mass fraction of solids in the initial solution [58,59,60,61]. Usually, phenolic gels prepared under acid conditions lead to a tridimensional chain with large nodules, consequently developing larger pores and higher pore volumes [60,61,62,63]. By increasing the pH to an alkaline level, a less porous material is created, with smaller pores or non-porosity [60,61,64]. Since tannin gels are porous materials from a phenolic precursor, they also follow this trend, as presented in Figure 6b,c.

After gelation, the polymerization reactions still continue, since the network formed is highly flexible and its constituent chains can move in relation to each other [62]. At this point, small clusters still exist and covalent bonds cross-linking occurs within the main network [52,55,62]. Besides, a fraction of the water in the polymerized gel structure is present as methylol derivatives as −OH groups [62], and part of the porosity is still prone to evolve, since evaporation of water generates voids within the network. Therefore, gel drying should not be done right after gelling because an ageing step is required to ensure the maximum formation of crosslinks [52,62,65]. These additional reactions generate a porous material with better mechanical properties, i.e., are able to withstand capillary forces during the drying process [62].

The tannin gels formed are classified as physical or chemical, according to the type of crosslinks in their network structure [60]. The so-called physical gels are formed by weak bonds, usually based on Van der Waals and other secondary forces [66] such as hydrogen bonds [67]. Chemical gels instead have a structure based on strong covalent bonds crosslinks, establishing a reticulated network and an infused gel [67].

Hydrothermal carbonization (HTC) is a wet thermochemical conversion process to convert biomass (or wet biomass) into fuels in solid and liquid phases [100]. This technique has been known since 1913, but there has been an increasing interest in it only during the last decade. HTC uses moderate temperature (180−260 °C) and pressure (1−4.7 MPa) to transform biomass precursors into hydrochars with a hydrophilic shell and a hydrophobic core. Recently, the HTC technique has allowed the production of gels made from various precursors, e.g., phloroglucinol with some carbohydrates (glucose, fructose, or xylose) [101], borax and glucose [102], S-(2-thienyl)-l-cysteine and 2-thienyl carboxaldehyde [103], d-glucose and ovalbumin [104] and mimosa tannin in ammonia solution [105], without any crosslinker agent (formaldehyde) and at a much lower reaction time (e.g., 24 h).

Figure 12 presents the different organic gels prepared under HTC conditions with: (a) tannin aqueous solution at low pH [106]; (b) tannin aqueous solution in presence of a metal salt; and (c) evaporated aminated tannin. The first gels (Figure 12 a) were elaborated under HTC conditions (180 °C for 24 h) simply with tannin solution in a very acidic environment (pH lower than 3). Three main reactions occurred with acidic tannin solution under heat: (i) the formation of soluble anthocyanidin and catechin [70,107]; (ii) the rearrangement to phlobaphenes, which are insoluble in water and have high molecular weight (Figure 2) [18,108] and; (iii) possibly condensation between the free radical coupling of B-ring catechol units in the presence of atmospheric oxygen [70]. In the same HTC conditions, tannin dissolved with metal salts, especially chromium, lead to a nano-structure powder material, but its microscope structure showed small particles aggregated together in a nodular form, which is typical of a gel structure [94]. This is due to the tannin characteristic of being good metal chelators (Figure 4). Thus, metal ions were chelated with most of hydroxyl groups in tannin-based polymer networks under HTC conditions (Figure 12b).

Another gel produced under the same hydrothermal conditions as previously mentioned was prepared from aminated tannin. In a typical synthesis, tannin was dissolved in ammonia solution at 28 wt.% and left in a fume hood to evaporate overnight. The solid dried residue recovered was then mixed with water. The final mixture after HTC conditions generated a monolithic material, having the same typical structure of a gel made from tannin and formaldehyde, but in absence of any crosslinker or condensation catalyst [105,109]. The material is composed of very long chains polymerized to an “infinite” three-dimensional network that were not even detected by MALDI-ToF analysis due to their high molecular weight. The mechanism of gel formation is based on selfcondensation and partly dehydrated tannin, mainly through the heterocycle opening [105,109]. The same reactions were also reported during tannin amination under ambient conditions [110]. However, under HTC conditions, the formation of this gel is probably due to the creation of −NH− bridges with the amination of C3’ and C4’ (Figure 2) according to ¹³C NMR analysis, as shown in Figure 12c.

The evaporated aminated tannin gel was also dried accordingly to produce xerogel, cryogel and aerogel at the same conditions as mentioned before with tannin-formaldehyde gels [111]. After HTC, these gels were homogeneous and had monolithic shapes, but they were quite fragile, with some cracks here and there. The mass fraction of evaporated aminated tannin was varied at 11, 18, and 27 wt.% to check any possible physicochemical differences with more concentrated gels. Interestingly, xerogels had great porosity, and the drying method had an important impact on the most diluted gel (11 wt.%). At such concentration, the porosity was improved in this sequence: xerogel, cryogel and aerogel (102, 219, and 295 m²/g, respectively). The same trend was also observed for tannin-formaldehyde gels under normal conditions (Figure 10).

A hydrothermal cryogel made from graphene oxide sheets and tannin was developed by Deng et al. [112]. The mechanism of gel formation was based on the self-assembly of graphene oxide with tannin acting as both reductant and surface functionalization agent. The final monolith with a three-dimensional structure had a mesoporous structure and a good performance for strontium removal due to its important amount of oxygen functional groups (see more details in Section 2.6). A summarized description of organic tannin gels made through HTC synthesis conditions and their final physicochemical properties can be seen in Table 2.

Thermal treatments are very well-known techniques used to transform organic-derived materials into carbonaceous materials from different precursors. Typically, carbonization and activation are the main methods used for the synthesis of carbon gels. Carbonization is based on heating up a dried organic material in an inert atmosphere (nitrogen, helium or argon) at temperatures between 600 and 1000 °C. At such conditions and at slow heating rates, the volatiles are released in a certain form to avoid the shrinkage of the final material structure [115]. Although it may vary from one precursor to another, the influence of temperature, residence time and heating rate have been well studied. For example, it has been demonstrated that temperatures above 900 °C are suitable for removing most volatile compounds [77], developing the porosity of the tannin gel, improving its micropore volume and surface area, and widening the narrow pores as well. This was indeed confirmed by Szczurek et al. [77] through FTIR analysis of the tannin-formaldehyde gel structure after drying and carbonizing at different temperatures (300–900 °C). At lower temperatures (300–600 °C), the functional groups (mostly oxygenated) were still present in their chemical structure, whereas at higher temperatures (700–900 °C), the same groups were drastically reduced due to their gasification during carbonization. Thus, temperature has a positive influence on the formation of microporosity, surface area and fixed C parameters, but it has a negative influence on carbon yield, average porosity, and functional groups that will be volatilized (e.g., N, H, and O content).

For specific applications (e.g., energy storage systems), gel materials could have very high porosity and surface area, reaching up to 3000 m²/g [116]. Thus, another largely known method for improving the textural properties is the activation method. Different organic precursors can be submitted to an activation process (physical or chemical) before or after carbonization. In physical activation, organic gels are introduced in an oven in an inert atmosphere, but also in the presence of CO₂ or steam at temperatures between 700 and 900 °C [117]. In chemical activation, the organic gel is in contact with a chemical agent instead such as H₃PO₄, KOH, NaOH, ZnCl₂, etc. at temperatures between 400 and 800 °C, which are considerably below the physical activation temperatures [118]. However, the disadvantage of this process is that the final material must be washed to remove residual chemical agents and possibly inorganic materials formed during this kind of activation [119]. The carbonized or activated carbons have higher specific surface area, higher pore volume, resistance against being attacked in acid and basic solutions, hardness, ignition at low temperature (200–500 °C), hydrophobicity and higher electrical conductivity which can be applied for catalysis, gas/air purification, and in the mining, chemicals, water, food, and pharmaceutical industries [120,121].

As mentioned before, carbon gels prepared under normal or hydrothermal conditions present textural properties tunable by pH, mass fraction, drying conditions, and others. In the case of gels prepared under normal conditions in presence of formaldehyde, the influence of the pH follows the trend: larger nodules for gels prepared under acid pHs and smaller nodules at alkali pHs (Figure 13a,b). In relation to their textural properties, Table 1 summarizes the main findings. In brief, tannin-formaldehyde gels after carbonization or activation reached the highest values of surface areas (S_BET) of 1420 m²/g and 1810 m²/g, respectively. In contrast, the highest S_BET values for organic gels were attained at 880 m²/g. Thus, the thermal treatment is crucial on the development of porosity, and consequently on the surface area. In addition, these values are comparable to phenol-formaldehyde [122] and resorcinol-formaldehyde [63] aerogels (up to 520 m²/g and 695 m²/g, respectively) and their carbon derivatives (up to 720 m²/g and 580 m²/g, respectively).

Table 2 also summarizes the description of tannin carbon gels preparation and their physicochemical and textural properties under HTC conditions. Tannin carbon gels made with different pHs under such conditions had a significant influence on the reaction rate, carbon yield and on the physicochemical characteristics of the final products. Firstly, by modifying the reaction medium, the hydrochar yield increased from 65 wt.% (HTC of tannin solution at non modified pH of 4.2) to 87 wt.% (HTC of tannin solution at pH 1) [106]. The low pH had a positive impact on the final carbon yield and on the lower nodule diameter (Figure 13c,d). The pH of tannin gels made under hydrothermal conditions somehow did not follow the same tendency as for tannin-formaldehyde gels. The gel structure with a lower nodule diameter (at low pH) might have promoted the evolution of volatiles, improving the development of its surface area with almost the same microporous size distribution (800 m²/g and 91% micropores) when compared to that with no pH modification (600 m²/g and 96% of micropores) [106].

Carbon aero-, cryo- and xerogels made from HTC conditions had also similar physicochemical characteristics of gels made from phenol/resorcinol-formaldehyde carbon aerogels [122,123] previously discussed. For example, surface areas of an aerogel made from HTC of evaporated aminated tannin at 27 wt.% and posterior carbonization at 900 °C reached values of up to 900 m²/g, having a mixture of well-developed microporosity (54%) as well as wider microporosity and mesoporosity (46%). However, these gels made under HTC were not considered to be monolithic compared to tannin-formaldehyde gel materials. Thus, possible applications for such gels would be in electrochemistry, as the material must be crushed and pressed during the preparation of carbon gel electrodes (see Table 3).

Generally, gels have been used for a great deal of applications such as diapers, contact lenses, medical electrodes, breast implants, paints, coatings, adhesives, drug delivery, controlled release of different molecules, adsorbents, and columns for materials separation (chromatography), separation technology (ion exchange), biosensors, catalysis, actuation, and sensing through artificial muscles [129]. However, only a few applications of organic and carbon gels made from tannin are presented in Table 3, according to the available literature. The removal of metals (e.g., Cr²⁺, Cu²⁺, Pb²⁺, and Zn²⁺) was investigated by the usage of tannin-gel type already explained in Section 1.3 (Tannin complexation of metals). They are less porous than gels described in this review, as they did not follow the same procedure of ageing, solvent exchange, and drying methods. Despite their lower porosity, tannin-gels have been reported to be mostly applied as adsorbents for the removal of metal contaminants from water at a concentration above the threshold fixed by environmental regulators. This is because tannin presents phenolic hydroxyl molecules that have a specific affinity to metal ions, and the ability to chelate them. An example of the tannin chelation process is presented in Figure 4. The advantage of using tannin-gel as adsorbent over tannin is that a gel type material is insoluble in water whereas tannin powder is very soluble, and it must be insolubilized or immobilized after the water treatment process.

It is also noticed that the applications of tannin gels are proposed depending on the gel type, namely aero-, cryo- and xero-gel. For example, two types of organic aerogels made from tannin-soy-formaldehyde and tannin-formaldehyde were tested as thermal insulators. This is because of their very low density (0.05−0.84 g/cm³) and high mesopore volumes (up to 2.3 cm³/g) [54,60]. The first aerogel presented a thermal conductivity of 0.033 W/mK, which was higher than that of air (0.026 W/mK). The authors reported that even though high porosity was attained, its volume of mesopores was only half of the total pores volume thus, narrower pores and fewer macropores would be necessary to reduce its thermal conductivity [54]. The second aerogel (tannin-formaldehyde), however, presented a thermal conductivity close to the air (0.027 W/mK) due to its low density and a significant fraction of very narrow pores. Additionally, the first aerogels showed a filamentous structure different from the typical “string-of-pearls” found for tannin-formaldehyde gel and thus, the thermal conductivity might have presented a distinct mechanism for both microstructures. This finding demonstrated that depending on the gel type and its physicochemical and textural characteristics (density, pores distribution, surface area, etc.), they can be selected for very specific applications.

For example, mesoporous materials are very interesting for water treatment because an appropriate pore size distribution is crucial in order to adsorb molecules of different sizes. Indeed, organic molecules (high-size molecule) were found to be successfully adsorbed by materials with larger micropores and mesopores [130]. Bimodal porosity was also found to be interesting for various applications because of the accessibility of ions or compounds to be transported from wider pores to micropores. On the other hand, micropores were found to play an important role in enhancing the specific capacitance. However, micropores with complex pore structure are not very useful for their capacity to store electrical energy. For example, pores lower than 0.7 nm may contribute to improving the specific capacitance, but they are not accessible at a high discharge rate [131]. Thus, the pore structure and pore size distribution must be optimized in order to have materials with high capacitance performances.

Researchers were motivated to explore the electrochemical properties of hydrothermal tannin carbon gels not only because of their great developed porosity, but also due to the enhancement of their functional groups, especially nitrogen and oxygen connected to their surface. This is because supercapacitor performances depend on the surface area accessible to the electrolyte ions, the presence of mesopores, and some heteroatoms, such as oxygen and nitrogen for the improvement of wettability and electronic conductivity [132,133,134]. Indeed, the final evaporated aminated tannin under HTC conditions presented nitrogen compounds in their structure, which had an important influence on the capacity of such materials to store electrical energy. According to Table 3, these materials presented the highest specific capacitance (up to 390 F/g) at scan rate of 2 mV/s [111]. Such values were higher than those reported for biosourced carbon gels in the available literature [126,135,136,137]. This finding is due to the combination of functional groups of oxygen and nitrogen that contributed to the enhancement of pseudo-capacitance through faradaic effects [60,111]. However, the authors highlighted that to achieve even better electrochemical performances at higher scan rates, the mesostructuration within 3 and 13 nm must be created in the structure of the carbon gel for the accessibility of micro- or ultramicro-pores. So, further studies are still needed for an effective application of tannin carbon gels in energy storage.