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.This research work was supported by the FICYT Regional Project (GRUPIN14-102) and Spanish MINECO/FEDER (CTQ2015-63552-R). G. A. F. thanks the MINECO for his predoctoral contract.Peer reviewe

Nitrogen-doped carbon microspheres with tunable porosity are investigated as electrocatalysts for the oxygen reduction reaction (ORR). The materials were synthesized by “nanocasting” involving the use of pyrrole as the carbon source and N-dopant, and porous silica microspheres as template. The engineered nitrogen-doped carbon particles combine several indispensable characteristics for a highly active metal-free carbon electrocatalyst: (i) a high content of nitrogen functionalities (∼8 wt%) mainly distributed in quaternary and pyridinic groups, which are highly active catalytic centers for the ORR reaction, and (ii) a high specific surface area (1200–1300 m² g⁻¹). Furthermore, the porosity of the N-doped microspheres can be modulated from a micro- to a mesoporous structure, i.e. from a micropore size distribution centered at ∼1 nm to a widely accessible mesoporosity with two mesopores systems (∼3 nm and ∼14 nm). The electrocatalytic activity of the N-doped carbon microspheres in the oxygen reduction reaction (ORR) was studied in both basic and acid media. Both types of materials catalyze the ORR via the efficient 4-electron process. However, the mesoporous carbon exhibits a more positive onset potential and a higher kinetic current density than the microporous microspheres and noteworthy the values are comparable to those of commercial Pt/C under basic conditions. Moreover, the mesoporous microspheres also show a better electrocatalytic activity than the microporous ones in acid medium, and a similar onset potential to that of Pt/C with a peroxide yield lower than 10%. A detailed comparison between the N-doped micro- and mesoporous microspheres reveals that the mesoporous material outperforms the microporous one not only in catalytic activity but also in durability in both electrolytes, which proves that the bimodal mesoporous structure acts as interconnected highways providing quick and full transport towards/from the catalytic sites for both reactant and products. This leads in turn to an effective metal-free carbon catalyst that can match the commercial Pt/C catalyst.

A procedure for the fabrication of N-doped hollow carbon spheres with a high rate capability for supercapacitors has been developed. The approach is based on a nanocasting method and the use of a nitrogen-rich compound (pyrrole) as a carbon precursor. The carbon particles thus produced combine a large BET surface area (∼1500 m² g⁻¹) with a porosity made up of mesopores of ∼4 nm, a high nitrogen content (∼6 wt%) and a capsule morphology which entails short ion diffusion paths derived from the shell morphology (thickness ∼60 nm). The porous properties of these hollow particles can be enhanced by means of an additional activation step with KOH. The activation process does not alter the hollow structure or spherical morphology, but strongly modifies the pore structure from a mesoporous network to a microporous one. The N-doped carbon capsules were tested in aqueous and organic electrolytes. In an aqueous medium (1 M H₂SO₄), the mesoporous carbon capsules offer the best performance due to the pseudocapacitive contribution of the N-groups, exhibiting a specific capacitance of ∼240 F g⁻¹ at 0.1 A g⁻¹ and a capacitance retention as high as 72% at 80 A g⁻¹. In contrast, in an organic electrolyte (1 M TEABF₄/AN), where the charge storage mechanism is based on the formation of the electric double-layer, the microporous capsules perform better due to the larger specific surface area. Thus, the microporous carbon capsules display a specific capacitance of up to 141 F g⁻¹ at 0.1 A g⁻¹ and an outstanding capacitance retention of 93% for an ultra-high discharge current density of 100 A g⁻¹.

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.This research work was supported by the Spanish Ministerio de Economía y Competitividad, MINECO (MAT2012-31651), Fondo Europeo de Desarrollo Regional (FEDER) and FICYT Regional Project (GRUPIN14-102). G. A. F. thanks the MINECO for his predoctoral contract and M. S. thanks the Spanish Ministerio de Ciencia e Innovación for her Ramón y Cajal contract.Peer reviewe

Supercapacitor technology is an extremely timely area of research with fierce international competition to develop cost-effective, environmentally friendlier EC electrode materials that have real world application. Herein, nitrogen-doped carbons with large specific surface area, optimized micropore structure and surface chemistry have been prepared by means of an environmentally sound hydrothermal carbonization process using defatted soybean (i.e., Soybean meal), a widely available and cost-effective protein-rich biomass, as precursor followed by a chemical activation step. When tested as supercapacitor electrodes in aqueous electrolytes (i.e. H₂SO₄ and Li₂SO₄), they demonstrate excellent capacitive performance and robustness, with high values of specific capacitance in both gravimetric (250–260 and 176 F g⁻¹ in H₂SO₄ and Li₂SO₄ respectively) and volumetric (150–210 and 102 F cm⁻³ in H₂SO₄ and Li₂SO₄ respectively) units and remarkable rate capability (>60% capacitance retention at 20 A g⁻¹ in both media). Interestingly, when Li₂SO₄ is used, the voltage window is extended up to 1.7 V (in contrast to 1.1 V in H₂SO₄). Thus, the amount of energy stored is increased by 50% compared to H₂SO₄ electrolyte, enabling this environmentally sound Li₂SO₄-based supercapacitor to deliver ~12 Wh kg⁻¹ at a high power density of ~2 kW kg⁻¹.