Natalia Rey-Raap

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.

Towards a feasible and scalable production of bio-xerogels.

HYPOTHESIS The synthesis process of carbon xerogels is limited, mainly due to two drawbacks that prevent their introduction onto the market: (i) the long time required for producing the material and (ii) the reagents used for the synthesis, which are costly and harmful to the environment. Microwave radiation is expected to produce a reduction in time of more than 90%, while the use of tannin instead of resorcinol will probably result in a cost-effective carbonaceous material. EXPERIMENTS Resorcinol-tannin-formaldehyde xerogels containing different amounts of tannin, either with or without a surfactant (sodium dodecyl sulphate), were synthesized by means of two different heating methods: conventional and microwave heating. The effects of the surfactant, the heating method and the addition of tannin upon the porous structure and the chemical composition of the final materials were evaluated. FINDINGS It was found that the addition of surfactant is essential for obtaining highly porous xerogels when using tannins. The heating method also plays an important role, as conventionally synthesized samples display a greater volume of large pores. However, tannins are less sensitive to microwave radiation and their use results in tannin-formaldehyde xerogels that have a porous structure and chemical composition similar to those of resorcinol-formaldehyde xerogels.

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