Guillermo A. Ferrero

Fe–N-Doped Carbon Capsules with Outstanding Electrochemical Performance and Stability for the Oxygen Reduction Reaction in Both Acid and Alkaline Conditions

High surface area N-doped mesoporous carbon capsules with iron traces exhibit outstanding electrocatalytic activity for the oxygen reduction reaction in both alkaline and acidic media. In alkaline conditions, they exhibit more positive onset (0.94 V vs RHE) and half-wave potentials (0.83 V vs RHE) than commercial Pt/C, while in acidic media the onset potential is comparable to that of commercial Pt/C with a peroxide yield lower than 10%. The Fe-N-doped carbon catalyst combines high catalytic activity with remarkable performance stability (3500 cycles between 0.6 and 1.0 V vs RHE), which stems from the fact that iron is coordinated to nitrogen. Additionally, the newly developed electrocatalyst is unaffected by the methanol crossover effect in both acid and basic media, contrary to commercial Pt/C. The excellent catalytic behavior of the Fe-N-doped carbon, even in the more relevant acid medium, is attributable to the combination of chemical functions (N-pyridinic, N-quaternary, and Fe-N coordination sites) and structural properties (large surface area, open mesoporous structure, and short diffusion paths), which guarantees a large number of highly active and fully accessible catalytic sites and rapid mass-transfer kinetics. Thus, this catalyst represents an important step forward toward replacing Pt catalysts with cheaper alternatives. In this regard, an alkaline anion exchange membrane fuel cell was assembled with Fe-N-doped mesoporous carbon capsules as the cathode catalyst to provide current and power densities matching those of a commercial Pt/C, which indicates the practical applicability of the Fe-N-carbon catalyst.

N-doped porous carbon capsules with tunable porosity for high-performance supercapacitors

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.

The influence of pore size distribution on the oxygen reduction reaction performance in nitrogen doped carbon microspheres

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.

From Soybean residue to advanced supercapacitors

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.

One-pot synthesis of microporous carbons highly enriched in nitrogen and their electrochemical performance

Microporous carbons with large nitrogen contents (6–23 wt%) have been successfully synthesized by the co-carbonization of an alkali organic salt and melamine. In this way, the processes of carbonization, activation and incorporation of nitrogen heteroatoms into the carbon backbone are integrated in only one step. The general applicability of this simple procedure has been proved by using it with a variety of organic salts, including potassium gluconate, and sodium salts of gluconate, citrate and alginate. In particular, the materials produced from potassium gluconate have a narrow micropore size distribution centered at around 0.7–0.8 nm, BET surface areas up to 1040 m2 g−1, and pore volumes of ∼0.3–0.4 cm3 g−1. It was found that the presence of abundant N-groups enhances the electrochemical performance of these materials in 1 M H2SO4. They exhibit high specific capacitances (surface basis) in the 16.6–23 μF cm−2 range and a good electro-oxidation stability as evidenced by the fact that carbon oxidation is shifted to more positive potentials by as much as 500 mV with respect to undoped carbon. In particular, a supercapacitor built with the carbon material synthesized at 850 °C using a melamine/potassium gluconate weight ratio of 2 showed an excellent robustness over a voltage window of 1.2 V in 1 M H2SO4, providing a maximum energy density of 10.2 W h kg−1 (7.9 W h L−1) and a maximum power density of 5.7 kW kg−1 (4.4 kW L−1).

Free-standing hybrid films based on graphene and porous carbon particles for flexible supercapacitors

Free-standing flexible solid-state supercapacitors are attracting attention as a power supply for electronic equipment. Here we report a novel strategy to fabricate free-standing flexible hybrid papers made up of porous carbon particles combined with graphene sheets. The synergetic effect between the carbon particles and the graphene sheets entails two important advantages: (a) binder-free electrodes formed by carbon particles can be built with the assistance of the graphene sheets and (b) the restacking of the graphene sheets is avoided to a great extent due to the fact that the carbon particles act as spacers. These hybrid papers combine important properties for their use in solid-state supercapacitors: (a) large specific surface area, (b) good electrical conductivity, (c) high packing density and (d) excellent flexibility. They exhibit a volumetric electrochemical performance which is clearly superior to electrodes fabricated with carbon particles agglomerated with a binder. In addition, they achieve an excellent areal capacitance (103 mF cm−2) at current densities as high as 1400 mA cm−2 and are able to deliver a large amount of energy (∼12 μW h cm−2) at high power densities (316 mW cm−2). In this work, a robust, flexible and high-performance solid-state supercapacitor has been assembled using such hybrid papers.

Aqueous Dispersions of Graphene from Electrochemically Exfoliated Graphite

A facile and environmentally friendly synthetic strategy for the production of stable and easily processable dispersions of graphene in water is presented. This strategy represents an alternative to classical chemical exfoliation methods (for example the Hummers method) that are more complex, harmful, and dangerous. The process is based on the electrochemical exfoliation of graphite and includes three simple steps: 1) the anodic exfoliation of graphite in (NH4 )2 SO4 , 2) sonication to separate the oxidized graphene sheets, and 3) reduction of oxidized graphene to graphene. The procedure makes it possible to convert around 30 wt % of the initial graphite into graphene with short processing times and high yields. The graphene sheets are well dispersed in water, have a carbon/oxygen atomic ratio of 11.7, a lateral size of about 0.5-1 μm, and contain only a few graphene layers, most of which are bilayer sheets. The processability of this type of aqueous dispersion has been demonstrated in the fabrication of macroscopic graphene structures, such as graphene aerogels and graphene films, which have been successfully employed as absorbents or as electrodes in supercapacitors, respectively.