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High-surface area carbons are produced from biomass-based products (wood sawdust and tannic acid) by means of an environmentally friendly process based on the use of sodium thiosulfate as activating agent and an inert salt (KCl) that serves as a confinement medium for the activation reaction. These porous carbons have high BET surface areas of up to 2650 m2 g-1, large pore volumes of up to 2.3 cm3 g-1 and a porosity that combines micro- and mesopores in different amounts depending on the quantity of activating agent employed. Such carbons have two additional remarkable properties: a) they are S-doped (2–6 wt% S) and b) they have good electrical conductivities in the 2.5–4.5 S cm-1 range. The above properties make these carbon materials highly attractive as supercapacitor electrodes. Indeed, when tested in a variety of electrolytes (H2SO4, TEABF4/AN and EMImTFSI) using commercial-level mass loadings, they show high specific capacitances (up to 200 F g-1, 140 F g-1 and 160 F g-1 in aqueous, organic and ionic liquid electrolytes, respectively) and high capacitance retention at high rates in all the electrolytes in combination with a good stability under cycling and floating modes.

A simple one-pot route for the synthesis of highly porous N-doped carbons with an excellent performance as supercapacitor electrodes is presented. The synthesis scheme is based on the carbonization of an organic salt, potassium citrate, at a temperature in the 700-900 ºC range, in the presence of urea. Such porous carbons combine ultra-high BET surface areas of up to 3350 m2·g-1, a large pore volume of up to 2.65 cm3·g-1, a porosity which can be regulated to have a micropore or micro-mesopore network and a notable nitrogen content in the 0.5-4.5 wt % range, two essential properties for their use in electrochemical devices. The presence of urea in the synthesis mixture is the key to pore development in these synthesized carbons. The applicability of this type of carbon as supercapacitor electrode material with an ionic liquid as electrolyte has also been demonstrated in this study.

A novel activating agent for the production of highly microporous carbons with textural properties that match those of superactivated carbons prepared by KOH activation, i.e. BET surface areas of 2600–3000 m2 g−1, pore volumes of ∼1.3–1.6 cm3 g−1 and pore size distributions in the supermicropore-small mesopore (<3 nm) region, is studied. It consists of a mixture of melamine and potassium oxalate, a substance which is less corrosive than KOH, imposing less technical restrictions. Additional advantages of this activating agent are that the morphology of the particles is not altered and, importantly, the product yield is almost double that of KOH activation. The advantageous textural characteristics of the produced materials are combined with a relatively good electronic conductivity of ∼2–3 S cm−1. When tested as supercapacitor electrodes using conventional electrolytes such as H2SO4 and TEABF4/AN, and less conventional ones such as EMImTFSI/AN, these carbons match the performance of benchmark KOH activated carbons and surpass that of commercial activated carbons specifically designed for supercapacitor applications.

Flexible supercapacitors have aroused a great deal of interest for their integration in portable, flexible and wearable electronic devices. In this context, graphene has emerged as an excellent building block for the fabrication of flexible electrodes. However, free-standing graphene films suffer from a certain lack of mechanical resistance, which limits their use. In this paper, we report on the fabrication of free-standing and flexible composites with enhanced robustness, consisting of a graphene layer deposited over a porous cellulose tissue. The coated graphene consists of two types of holey graphene units (i.e. wrinkled graphene sheets and graphene nanoscrolls) that produce closely interconnected and porous 3D graphene architectures. The graphene-tissue composites developed here have a thickness of around 60 µm and areal densities in the 0.6–2.4 mg cm−2 range. These composites have a very open structure that provides easy access to the electrolyte, thereby guaranteeing high ion-transport rates. In consequence, they show a remarkable capacitive performance in both liquid (1 M H2SO4) and solid (PVA-H2SO4) electrolytes. The supercapacitors assembled with these materials possess an areal capacitance of up to ~80 mF cm−2 at low rates in both kinds of electrolyte and around 60 mF cm−2 at 500 mA cm−2 in H2SO4 and 54 mF cm−2 at ca. 80 mA cm−2 in a gel electrolyte. What is more, these SCs are able to deliver ca. 9 µWh cm−2 of energy at a high power density of 100 mW cm−2.

A simple, cost-effective and scalable approach for the synthesis of N-doped mesoporous carbons that are effective as ORR catalysts is reported. The synthesis procedure involves two steps: a) production of mesoporous carbons using a template-free approach based on the carbonization of citrate salts of zinc and calcium, and b) N-doping by heat treatment in the presence of melamine. The resulting N-doped carbon possess a high specific surface area, a porosity made up exclusively of mesopores and a large amount of nitrogen functionalities (∼8–9 wt%). When used as an electrocatalyst for the oxygen reduction reaction (ORR), the metal-free carbon materials predominantly catalyze the 4 e− process, with an onset potential of 0.9 V (vs. RHE) and a superior kinetic current density (∼17 mA cm−2) to that of Pt/C at ∼0.6 V under basic conditions. In addition, the developed catalysts show a higher stability than commercial Pt/C and excellent electrocatalytic selectivity against methanol crossover.

A simple one-step synthesis methodology for the fabrication of mesoporous carbons with an excellent performance as supercapacitor electrodes is presented. The procedure is based on the carbonization of non-alkali organic salts such as citrate salts of iron, zinc or calcium. The carbonized products contain numerous inorganic nanoparticles (i.e. Fe, ZnO or CaO) embedded within a carbonaceous matrix. These nanoparticles act as endotemplate, which when removed, leaves a mesoporous network. The resulting carbon samples have a large specific surface area up to ∼1600 m2 g−1 and a porosity made up almost exclusively of mesopores. An appropriate heat-treatment of these materials with melamine allows the synthesis of N-doped carbons which have a high nitrogen content (∼8–9 wt.%), a large specific surface area and retain the mesoporous structure. The mesoporous carbon samples were employed as electrode materials in supercapacitors. They exhibit specific capacitances of 200–240 F g−1 in 1 M H2SO4 and 100–130 F g−1 in EMImTFSI/AN. More importantly, the carbon samples possess a good capacitance retention in both electrolytes (>50% in H2SO4 and >80% in EMImTFSI/AN at 100 A g−1) owing to their mesoporous structure which facilitates the penetration and transportation of ions.