E.G. Calvo

Fast microwave-assisted synthesis of tailored mesoporous carbon xerogels

Resorcinol-formaldehyde carbon xerogels with several initial pH were synthesized using two different heating methods (conventional and microwave heating). The effect of the pH of the precursor solution and the method of synthesis employed on the textural and chemical properties of the final materials was evaluated. It was found that both methods produce tailored carbon xerogels depending on the initial pH and that the pores of the carbon xerogels become larger as the initial pH decreases. High pHs result in exclusively microporous carbon xerogels, while a decrease in the amount of NaOH added, i.e. lower pH, causes the materials to evolve firstly into micro-mesoporous samples and then into micro-macroporous carbon xerogels. The main difference between the two heating methods studied, apart from the duration of the synthesis (i.e. approximately 5 h for the microwave-assisted synthesis as opposed to several days by conventional methods) lies in the meso-macroporosity of the resulting materials, since microwave radiation produces mainly mesoporous carbon xerogels with a specific mesopore size over a wider range of pH than conventional synthesis. For example, the pH range for mesoporous MW samples is 4.5-6.5 while equivalent samples that are conventionally synthesized require an initial pH of between 5.8 and 6.5. This work also illustrates a simple and precise method for determining the gelation point (t(g)) of different pH resorcinol-formaldehyde mixtures, based on varying the energy consumed by the microwave device during the synthesis of organic gels, without the need for other more complicated techniques.

The effect of the carbon surface chemistry and electrolyte pH on the energy storage of supercapacitors

Energy storage in supercapacitors can be enhanced by optimising the electrostatic mechanism and the pseudo-faradaic reactions. In the latter case, it is essential to take into account the surface chemistry of the electrode material and the pH of the electrolyte. In this work, supercapacitors were assembled using three carbon xerogels containing different oxygen surface groups and various aqueous media (Na2SO4, H2SO4 and KOH), in order to evaluate the influence of the surface chemistry on the electrochemical behaviour of the cells. However, it is not only necessary to take into account the chemical nature of the electrodes, but also the combination of this surface chemistry and the pH of the electrolyte, since the performance of the electrode material depends on the type of electrolyte medium used. In the case of a carbon xerogel with quinone-type oxygen groups, it has been demonstrated that there are pseudo-capacitive effects when the pH of the electrolyte is lower than the point of zero charge (pHPZC) of the electrodes, i.e., when the electrode surface is positively charged. However, in the case of an alkaline electrolyte, the carbon samples studied presented similar capacitance values, suggesting that the negative charges make a lower contribution in these carbon-based energy storage systems.

Microwave-induced low temperature pyrolysis of macroalgae for unprecedented hydrogen-enriched syngas production

An efficient methodology based on low temperature microwave-induced pyrolysis has been developed for syngas production from macroalgae. The protocol provided unprecedented hydrogen production, with switchable H2/CO ratios depending on pyrolysis conditions which were found to remarkably improve conventional pyrolysis experiments even at significantly higher temperatures (400 vs. 800 °C). Arcing effects under microwave irradiation, which result in an interesting observed pseudo-catalytic effect promoted by the metal oxides contained in macroalgae, seem to account for the improved results.

Acid-based resorcinol-formaldehyde xerogels synthesized by microwave heating

The polymerization reaction that takes place between resorcinol and formaldehyde is spontaneous but slow. For this reason, compounds are often used to increase the reaction rate and reduce the synthesis time. These compounds can be basic or acidic and their nature and concentration can be used to modify the mechanisms of the reaction and the final properties of the materials. In this work, the differences in the final properties of the organic xerogels obtained with basic or acid boosters have been studied. It was found that, irrespective of the nature of the booster, none of the end-product materials showed any differences in their chemical properties. Moreover, the concentrations of the components of the precursor solution (i.e., monomers, water, and methanol) were observed to have the same effect on the porous properties of the materials regardless of whether an acidic or a basic booster was used. However, differences in the porous properties were observed. It was found that the methanol content was crucial to tailor the porosity over the entire nanoscale when an acidic booster is used. These results are of great importance as acidic boosters allows to decrease synthesis time and, hence, to produce more competitive materials.Graphical abstract