On the use of diatomite as antishrinkage additive in the preparation of monolithic carbon aerogels

Abstract

We have synthesized flexible monolithic carbon aerogels by incorporating diatomite as antishrinkage agent during the sol-gel polycondensation of the precursors. The materials displayed elastic properties and preserved their pristine dimensions upon densification by carbonization even after etching of the diatomite, due to the open pore structure of the siliceous additive in the material's matrix. The aerogels displayed a well-developed porous structure, confirming that the polymerization of the precursors is not impeded; the effect of the additive was independent of the formulation of the aerogels, as evidenced by the behavior of aerogels with varied composition (molar ratio, pH, N-doping).After the early works of Pekala and coworkers on the synthesis of carbon aerogels by the solgel polycondensation of resorcinol (R) and formaldehyde (F) mixtures [1,2], the interest for these solids has raised due to their potential use in several fields. Despite their versatility, the technological applications of carbon aerogels have often been limited by their poor mechanical properties. The mechanical stiffness of carbon aerogels depends strongly on the 2 porosity and the degree of cross-linking of the precursors [2,3]. The usually fragile hydrogels are mechanically reinforced upon carbonization as they become more rigid with increased Young modules [2,3], but their deformation capacity decreases. In previous works we have reported the preparation of carbon aerogels using various precursors with enhanced conductivity and chemical resiliency to oxidation [4,5]. Despite the good electrochemical performance, the use of monolithic electrodes was limited due to the large shrinkage and deformation of the pieces after carbonization -leading to densification of the matrix-[5]; this is a challenge for electrochemical applications where the contact between the electrode material and the current collector is crucial to avoid efficiency losses due to resistance.Aiming at fabricating large monolithic electrodes with improved mechanical properties without adversely affecting their porosity and conductivity, we herein report the preparation of monolithic carbon aerogels with enhanced mechanical properties by using a low cost siliceous sacrificial additive (i.e. diatomite earth). The synthesis route is a simple modification of the conventional one reported elsewhere [4], and consists on allowing the sol-gel polymerization of the precursors in the presence of the additives; after the supercritical drying step, the hydrogels (series H-) were carbonized (series C-), and the siliceous additive was finally removed by HF etching (see further details in the ESI File). Selected formulations with different precursor's molar ratio (R, F and melamine, M) and additives (either diatomite (D), carbon black (B), or both) were prepared (Table 1), to show that the effect of the additive does not depend on the composition of the aerogels. From a macroscopic point of view ( Fig. 1), the materials ranged from translucent (pristine) to opaque when eith...

Synthesis of Porous and Mechanically Compliant Carbon Aerogels Using Conductive and Structural Additives

“…In a previous study we reported the two-step synthesis of N-doped carbon aerogels based on the prepolymerization of melamine-resorcinol-formaldehyde (MRF) mixtures [26,27]. Regardless of the solution pH and M/R molar ratio, the hydrogels prepared by the conventional one-step route displayed essentially a microporous character, as opposed to the meso-/macroporous network of those prepared upon the prepolymerization of the precursors.…”

“…The typical shrinkage of the monoliths after carbonization—due to the evolution of volatiles upon densification of the carbon matrix—is shown in Figure 6. Values up to a 30% reduction in volume are typically reached in RF and MRF aerogels (Table 3); this behavior is independent of the composition of the aerogels [27], although it is more pronounced for MRF formulations. For the samples CG-D and CG-DB prepared in the presence of diatomite, the carbonized disks preserved their dimensions, even after the removal of the siliceous skeleton, likely due to the open pore structure of the aerogels provided by the diatomite (Figure 5b), which would facilitate the evolution of the volatiles upon carbonization.…”

“…Organic hydrogels are mechanically reinforced upon carbonization at high temperature to render denser carbon gels, that become more rigid (although typical Young modules are still a factor of 100–1000 lower than those of silica glass) but their deformation capacity decreases [9,20,24]. In a previous study we reported the good electrochemical performance of binderless monolithic aerogel electrodes over powdered materials due to the combination of a nanoporous structure interconnected with a macroporous network [26,27,28]; their technological implementation at large scale was, however, limited due to the large shrinkage and deformation of the pieces after the carbonization; the contact between the bent monolithic electrodes and the current collector was compromised, leading to important efficiency losses due to resistance.…”

Synthesis of mechanically robust porous carbon monoliths for CO2 adsorption and separation

“…Gomis-Berenguer et al [9] synthesized carbon xerogels with ultrahigh micro-and mesopores from the activation of polymeric resins prepared by polycondensation of resorcinol/formaldehyde mixtures in basic medium and subcritical drying. Macías et al [10] prepared flexible monolithic carbon aerogels by incorporating diatomite as anti-shrinkage agent during the solgel polycondensation of resorcinol and formaldehyde. However, the mechanical strength of the carbon aerogel is far from being able to meet the practical application requirements.…”

Carbon Black as Conductive Additive and Structural Director of Porous Carbon Gels

“…In a previous study, we reported the successful polycondensation of resorcinol-formaldehyde mixtures in the presence of low amounts of additives (e.g., diatomite, carbon black), to render carbon gel/CB composites with enhanced electrical conductivity and mechanical features [25,26]. Aiming to evaluate the percolation threshold of carbon black in its role as conductive additive, we prepared a series of organic and carbon gels with a fixed molar ratio of reactants (R/F, R/C and R/W) and increasing amounts of CB additive.…”