Antonio B. Fuertes

Chemical and Structural Properties of Carbonaceous Products Obtained by Hydrothermal Carbonization of Saccharides

Carbon-rich-quick scheme: A carbon-rich solid product made up of uniform micrometer-sized spheres of tunable diameter has been synthesized by the hydrothermal carbonization of saccharides. These microspheres possess a core-shell chemical structure based on the different nature of the oxygen functionalities between the core and the outer layer (see figure).A carbon-rich solid product, here denoted as hydrochar, has been synthesized by the hydrothermal carbonization of three different saccharides (glucose, sucrose, and starch) at temperatures ranging from 170 to 240 degrees C. This material is made up of uniform spherical micrometer-sized particles that have a diameter in the 0.4-6 mum range, which can be modulated by modifying the synthesis conditions (i.e., the concentration of the aqueous saccharide solution, the temperature of the hydrothermal treatment, the reaction time, and type of saccharide). The formation of the carbon-rich solid through the hydrothermal carbonization of saccharides is the consequence of dehydration, condensation, or polymerization and aromatization reactions. The microspheres thus obtained possess, from a chemical point of view, a core-shell structure consisting of a highly aromatic nucleus (hydrophobic) and a hydrophilic shell containing a high concentration of reactive oxygen functional groups (i.e., hydroxyl/phenolic, carbonyl, or carboxylic).

Sustainable porous carbons with a superior performance for CO2 capture

Sustainable porous carbons have been prepared by chemical activation of hydrothermally carbonized polysaccharides (starch and cellulose) and biomass (sawdust). These materials were investigated as sorbents for CO2 capture. The activation process was carried out under severe (KOH/precursor = 4) or mild (KOH/precursor = 2) activation conditions at different temperatures in the 600–800 °C range. Textural characterization of the porous carbons showed that the samples obtained under mild activating conditions exhibit smaller surface areas and pore sizes than those prepared by employing a greater amount of KOH. However, the mildly activated carbons exhibit a good capacity to store CO2, which is mainly due to the presence of a large number of narrow micropores (<1 nm). A very high CO2 uptake of 4.8 mmol·g−1 (212 mg CO2·g−1) was registered at room temperature (25 °C) for a carbon activated at 600 °C using KOH/precursor = 2. To the best of our knowledge, this result constitutes the largest ever recorded CO2 uptake at room temperature for any activated carbon. Furthermore, we observed that these porous carbons have fast CO2 adsorption rates, a good selectivity for CO2–N2 separation and they can be easily regenerated.

Direct synthesis of highly porous interconnected carbon nanosheets and their application as high-performance supercapacitors.

An easy, one-step procedure is proposed for the synthesis of highly porous carbon nanosheets with an excellent performance as supercapacitor electrodes. The procedure is based on the carbonization of an organic salt, i.e., potassium citrate, at a temperature in the 750-900 °C range. In this way, carbon particles made up of interconnected carbon nanosheets with a thickness of <80 nm are obtained. The porosity of the carbon nanosheets consists essentially of micropores distributed in two pore systems of 0.7-0.85 nm and 0.95-1.6 nm. Importantly, the micropore sizes of both systems can be enlarged by simply increasing the carbonization temperature. Furthermore, the carbon nanosheets possess BET surface areas in the ∼1400-2200 m(2) g(-1) range and electronic conductivities in the range of 1.7-7.4 S cm(-1) (measured at 7.1 MPa). These materials behave as high-performance supercapacitor electrodes in organic electrolyte and exhibit an excellent power handling ability and a superb robustness over long-term cycling. Excellent results were obtained with the supercapacitor fabricated from the material synthesized at 850 °C in terms of both gravimetric and volumetric energy and power densities. This device was able to deliver ∼13 Wh kg(-1) (5.2 Wh L(-1)) at an extremely high power density of 78 kW kg(-1) (31 kW L(-1)) and ∼30 Wh kg(-1) (12 Wh L(-1)) at a power density of 13 kW kg(-1) (5.2 kW L(-1)) (voltage range of 2.7 V).

High density hydrogen storage in superactivated carbons from hydrothermally carbonized renewable organic materials

Hydrothermally carbonized organic materials (furfural, glucose, starch, cellulose and eucalyptus sawdust) have been used as precursors to produce high-surface area carbons. The synthesis methodology comprises two steps: (i) hydrothermal carbonization of organic materials and (ii) chemical activation with KOH as activating agent. In this way, activated carbon materials with a high surface area (up to 2700 m2 g−1) and narrow micropore size distribution in the supermicropore range (0.7–2 nm) are produced. The textural properties of the activated carbon products can be easily tuned by modifying the activating conditions (i.e., the activation temperature and the amount of KOH used). The activated carbon materials exhibit high hydrogen uptakes, up to 6.4 wt%, and large isosteric heats of adsorption, up to 8.5 kJ mol−1. In particular, the hydrogen storage density of the carbons is high and ranges between 12 and 16.4 µmol H2 m−2. The hydrogen storage density is closely related to the pore size of the carbons, with small micropores (ca. 1 nm) favouring a high density. Taking into account the high hydrogen storage capacities of these materials, as well as the simplicity of their synthesis procedure and the ready availability and low-cost of the raw precursors, it can be concluded that these activated carbons constitute a promising adsorbent for hydrogen storage.

Function search in a large transcription factor gene family in Arabidopsis: assessing the potential of reverse genetics to identify insertional mutations in R2R3 MYB genes.

More than 92 genes encoding MYB transcription factors of the R2R3 class have been described in Arabidopsis. The functions of a few members of this large gene family have been described, indicating important roles for R2R3 MYB transcription factors in the regulation of secondary metabolism, cell shape, and disease resistance, and in responses to growth regulators and stresses. For the majority of the genes in this family, however, little functional information is available. As the first step to characterizing these genes functionally, the sequences of >90 family members, and the map positions and expression profiles of >60 members, have been determined previously. An important second step in the functional analysis of the MYB family, through a process of reverse genetics that entails the isolation of insertion mutants, is described here. For this purpose, a variety of gene disruption resources has been used, including T-DNA–insertion populations and three distinct populations that harbor transposon insertions. We report the isolation of 47 insertions into 36 distinct MYB genes by screening a total of 73 genes. These defined insertion lines will provide the foundation for subsequent detailed functional analyses for the assignment of specific functions to individual members of the R2R3 MYB gene family.

Chemical and structural properties of carbonaceous products obtained by pyrolysis and hydrothermal carbonisation of corn stover

The main properties of chars produced from corn stover, either by pyrolysis at 550°C (to produce biochar) or by hydrothermal carbonisation (to produce hydrochar), were studied. Carbonaceous materials were characterised by: SEM imaging, solid-state 13C NMR, FT-IR, Raman spectroscopy, and XPS. The following parameters were determined: elemental composition, cation exchange capacity, acid groups contents, BET, and yield. The hydrochar had a low ash content and low pH (4.7); recovery of C was high (57%), although only about half of the C was aromatic. Atomic O/C and H/C ratios in the hydrochar were higher than in the biochar. The same pattern was observed for the estimated concentration of carboxylic functional groups (0.07 compared with 0.04 mol/kg). The biochar had higher ash content than the hydrochar, and also higher pH (~10) (lime equivalence ~40 kg CaCO3/t). The C recovery (46%) was lower than in the hydrochar, although most of the C recovered was aromatic. Both chars could be used as soil amendments, for very different requirements. Soil responses and the residence times of the chars (especially the hydrochar) must be studied in detail to pursue long-term C sequestration.

Assessment of the Role of Micropore Size and N-Doping in CO2 Capture by Porous Carbons

The role of micropore size and N-doping in CO2 capture by microporous carbons has been investigated by analyzing the CO2 adsorption properties of two types of activated carbons with analogous textural properties: (a) N-free carbon microspheres and (b) N-doped carbon microspheres. Both materials exhibit a porosity made up exclusively of micropores ranging in size between <0.6 nm in the case of the pristine materials and up to 1.6 nm for the highly activated carbons (47% burnoff). The N-doped carbons possess ~3 wt % of N heteroatoms that are incorporated into several types of functional groups (i.e., pyrrole/pyridone, pyridine, quaternary, and pyridine-N-oxide). Under conventional operation conditions (i.e., T ~ 0-25 °C and P(CO2) ~ 0-1 bar), CO2 adsorption proceeds via a volume-filling mechanism, the size limit for volume-filling being ~0.7-0.8 nm. Under these circumstances, the adsorption of CO2 by nonfunctionalized porous carbons is mainly determined by the volume of the micropores with a size below 0.8 nm. It was also observed that the CO2 capture capacities of undoped and N-doped carbons are analogous which shows that the nitrogen functionalities present in these N-doped samples do not influence CO2 adsorption. Taking into account the temperature invariance of the characteristic curve postulated by the Dubinin theory, we show that CO2 uptakes can be accurately predicted by using the adsorption data measured at just one temperature.

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.

Sulfur-containing activated carbons with greatly reduced content of bottle neck pores for double-layer capacitors: a case study for pseudocapacitance detection

Synthesis of S-doped activated carbons (ACs) by carbonization and simultaneous activation of S-based polymers was found to be an efficient route to produce porous carbons for double layer capacitors (EDLCs) with high specific energy and power densities combined with low self-discharge. Here we investigate for the first time the processing-structure–property relationships related to the formation of polythiophene-derived ACs for EDLC applications. Sulfide bridges present in the polymer precursor were found to depress the shrinkage of the smallest micropores during the carbonization process and allow for the enhanced ion transport within the produced AC electrodes. The cyclic voltammetry (CV) measurements on S-doped ACs produced at 800 and 850 °C showed high specific capacitance (up to ∼200 F g−1) and no significant self-discharge in neutral aqueous electrolytes. More importantly, these capacitance values remained virtually identical for a sweep rate increasing from 1 to 50 mV s−1. The observed capacitance retention is quite remarkable for thick electrodes of ∼200 μm and a large AC particle size of 10–100 μm. It indicates great potential of the proposed synthesis technology for EDLCs operating at high frequencies and high currents. In the course of our systematic studies of AC performance in different electrolytes we found a strong correlation between the large pseudocapacitance and the significant self-discharge in ACs. We harness the difference between the characteristic times required to establish a double layer and that of the pseudocapacitive redox reactions and propose a simple method to estimate the fraction of pseudocapacitance. The proposed method is particularly valuable in cases when CV measurements do not show clear characteristic reduction–oxidation peaks.

Mesoporous carbons with graphitic structures fabricated by using porous silica materials as templates and iron-impregnated polypyrrole as precursor

Templated mesoporous carbons containing graphitic structures and a well-developed porosity were synthesised by using mesoporous silica materials (i.e. SBA-15 and silica xerogel) as templates and polypyrrole as carbon precursor. The polypyrrole was synthesised by in situ polymerization of pyrrole which was infiltrated into the silica pores containing FeCl3. The carbonization of polypyrrole in presence of iron allows carbon structures with some degree of graphitic order to be formed. The samples were characterised by XRD, nitrogen sorption, TEM, Raman and elemental analysis. The results show that the templated carbons have BET surface areas >1000 m2 g−1 and a porosity built up by two pore systems with sizes centred at ∼3–4 nm and at 10–14 nm. In addition, they have a good electrical conductivity due to the presence of graphitic structures in the carbon framework.