F. Suárez-García

Highly stable performance of supercapacitors from phosphorus-enriched carbons

Phosphorus-rich microporous carbons (P-carbons) prepared by a simple H(3)PO(4) activation of three different carbon precursors exhibit enhanced supercapacitive performance in 1 M H(2)SO(4) when highly stable performance can be achieved at potentials larger than the theoretical decomposition potential of water. This ability of P-carbons greatly enhances the energy density of supercapacitors that are capable of delivering 16 Wh/kg compared to 5 Wh/kg for the commercial carbon. An intercept-free multiple linear regression model confirms the strongest influence of phosphorus on capacitance together with micropores 0.65-0.83 nm in width that are the most effective in forming the electric double layer.

Activated carbons by pyrolysis of coffee bean husks in presence of phosphoric acid

Abstract Activated carbons (Acs) were prepared by pyrolysis of coffee bean husks in presence of phosphoric acid (chemical activities). Husks from Colombian coffee beans were impregnated with aqueous solutions of H3PO4 following a variant of the incipient wetness method. Diffenent concentrations were used to produce impregnation ratios of 30, 60, 100 and 150 wt.%. Activation was carried out under argon flow by heating to 723 K with 1 h soaking time. The porous texture of the obtained ACs was characterized by physical adsorptions of N2 at 77 K and CO2 at 273 K. The impregnation ration had a strong influence on the pore structure of these Acs, which could be easily controlled by simply varying the proportion of H3PO4 used in the activation. Thus, low impregnation ratio led to essentially microporous Acs. At intermediate impregnation ratios, ACs with wider pore size distribution (from micropores to mesopores) were obtained. Finally, high impregnation ratios yielded essentially mesoporous carbons with high surface area and pore volume.

Pyrolysis of apple pulp: chemical activation with phosphoric acid

Chemical activation of apple pulp with phosphoric acid has been carried out to prepare activated carbons. The materials were characterized by elemental analysis, N2 adsorption (77 K), CO2 adsorption (273 K) and X-ray diffraction. The effects of temperature, soaking time, impregnation ratio and degree of washing were studied. Maximum development of porosity was observed upon heating to 723 K plus 1 h soaking. The amount of phosphoric acid used in the impregnation step strongly influenced the porous texture, micropores being predominant at low impregnation ratios whereas larger amounts of phosphoric acid produced wide micropores and mesopores. The washing sep also had a crucial effect on the porosity accessible to different adsorbates. The results suggest the feasibility of the process from the point of view of both porous texture and carbon yield.

Tuning of texture and surface chemistry of carbon xerogels

The influence of different activation processes on the textural and surface chemical properties of carbon xerogels was studied. Carbon xerogels were prepared by the conventional sol-gel approach using resorcinol and formaldehyde; two different pHs of sol-gel processing led to carbon materials with distinct pore size distributions. The materials were subjected to controlled activation by three different methods: activation by oxygen plasma, activation by HNO(3), and activation by diluted air. Treatments with HNO(3) and diluted air created oxygen groups on the external surface as well as inside the pore channels, whereas plasma is more suitable for introducing oxygen groups selectively on the external surface. Nevertheless, it was shown that samples with wider pores can be oxidized to some extent on the pore interiors by plasma. Significant changes in total surface area by air activation were observed.

Pyrolysis of apple pulp: effect of operation conditions and chemical additives

Abstract The objective of this work was to evaluate the feasibility of using apple pulp (residue from apple pressing in cider manufacture) as a feedstock for activated carbon production. To this end, pyrolysis of apple pulp was studied using thermogravimetry. The effect of a series of operation variables (heating rate, temperature/time programs, gas flow and nature) was studied, attempting to increase the char yield at 400°C. Apple pulp exhibits two weight-loss steps, ascribable to decomposition of light fractions (hemicellulose) and cellulose. Changes in pyrolysis conditions had no effect on these two weight-loss steps and did not affect the yield of the process. Chemical additives (AlCl3, FeCl3, H3PO4, NH4Cl, KOH and ZnCl2) slightly affect the first step by inhibiting hemicellulose decomposition, while accelerating cellulose decomposition through dehydration reactions. Phosphoric acid exhibited the largest influence on the pyrolysis process. At concentrations higher than 30 wt.% H3PO4, the two weight loss steps ascribed to hemicellulose and cellulose decompositions overlapped, and above 100 wt.%, a single step was observed. It was concluded that impregnation with phosphoric acid can enhance the preparation of activated carbons by chemical activation of apple pulp.

Activated carbon fibers with a high content of surface functional groups by phosphoric acid activation of PPTA

Activated carbon fibers (ACFs) were prepared by chemical activation of poly(p-phenylene terephthalamide (PPTA) with phosphoric acid, with a particular focus on the effects of impregnation ratio and carbonization temperature on both surface chemistry and porous texture. Thermogravimetric studies of the pyrolysis of PPTA impregnated with different amounts of phosphoric acid indicated that this reagent has a strong influence on the thermal degradation of the polymer, lowering the decomposition temperature and increasing the carbon yield. As concerns surface chemistry, TPD and chemical analysis results indicated that the addition of phosphoric acid increases the concentration of oxygenated surface groups, with a maximum at an impregnation ratio of 100 wt.%. The resulting materials present uncommon properties, namely a large amount of oxygen- and phosphorus-containing surface groups and a high nitrogen content. Porosity development following H(3)PO(4) activation was very significant, with values close to 1700 m(2)/g and 0.80 cm(3)/g being reached for the BET surface area and total pore volume, respectively. The pore size distributions remained confined to the micropore and narrow mesopore (<10 nm) range.

Influence of Porous Texture and Surface Chemistry on the CO2 Adsorption Capacity of Porous Carbons: Acidic and Basic Site Interactions

Doped porous carbons exhibiting highly developed porosity and rich surface chemistry have been prepared and subsequently applied to clarify the influence of both factors on carbon dioxide capture. Nanocasting was selected as synthetic route, in which a polyaramide precursor (3-aminobenzoic acid) was thermally polymerized inside the porosity of an SBA-15 template in the presence of different H3PO4 concentrations. The surface chemistry and the porous texture of the carbons could be easily modulated by varying the H3PO4 concentration and carbonization temperature. Porous texture was found to be the determinant factor on carbon dioxide adsorption at 0 °C, while surface chemistry played an important role at higher adsorption temperatures. We proved that nitrogen functionalities acted as basic sites and oxygen and phosphorus groups as acidic ones toward adsorption of CO2 molecules. Among the nitrogen functional groups, pyrrolic groups exhibited the highest influence, while the positive effect of pyridinic and quaternary functionalities was smaller. Finally, some of these N-doped carbons exhibit CO2 heats of adsorption higher than 42 kJ/mol, which make them excellent candidates for CO2 capture.

Synthesis, characterization and dye removal capacities of N-doped mesoporous carbons.

Nitrogen-doped ordered mesoporous carbons were synthesized by chemical vapor deposition, using acetonitrile as carbon and nitrogen source and SBA-15 as mesoporous silica template. Their porous texture, structural order and surface chemistry were studied as a function of the experimental conditions (acetonitrile stream concentration and deposition time). A non-doped ordered mesoporous carbon was also prepared by the same procedure using propylene as carbon source. Methylene blue, methyl orange and fuchsin acid were selected as probe molecules to investigate the dye adsorption behavior on the ordered mesoporous carbons. Both N-doped and non-doped ordered mesoporous carbons adsorbed large amounts of these three dyes demonstrating the importance of mesoporosity, especially for the adsorption of larger dyes (e.g. fuchsin acid). The presence of nitrogen functional groups was detrimental for the adsorption of the basic dye (methylene blue). On the other hand, the nitrogen functionalities improved the adsorption kinetics for both acid and basic dyes, and the N-doped samples achieved 100% of their maximum adsorption capacities in less than 15 min.

Preparation and porous texture characteristics of fibrous ultrahigh surface area carbons

Microporous carbon adsorbents with fibrous morphology and very high surface area and pore volume development were prepared from poly (m-phenylene isophthalamide) fibres. The precursor polymer was impregnated with aqueous solutions of phosphoric acid, carbonised and physically activated with CO2 to different burn-off degrees. The char yield following pyrolysis increased with the impregnation ratio. Additionally, pre-impregnation with phosphoric acid strongly enhanced the reactivity of chars towards CO2, there being a positive correlation between reactivity, impregnation ratio and oxygen content in the chars. The porous texture of the obtained materials was characterized by physical adsorption of N2 at 77 K and CO2 at 273 K. The pores enlarged slightly with increasing burn-off in CO2, though remaining restricted to the micropore region. No pores larger than 3–4 nm were ever found in any of the activated materials, even at high burn-offs. Ultrahigh surface area carbons with BET surface areas over 2000 m2 g−1 and pore volumes in excess of 1.10 cm3 g−1 were obtained at high burn-offs, displaying an extensive network of supermicropores considerably wider than those obtained under equivalent conditions in the absence of the H3PO4 additive. To complement these observations, scanning tunneling microscopy was performed on the ultrahigh surface area carbons. Visual evidence was provided of an extensive network of ∼2 nm pores, consistent with the physical adsorption measurements.

Effects of the mesostructural order on the electrochemical performance of hierarchical micro–mesoporous carbons

Hierarchical ordered porous carbons show a better behavior as electric double layer capacitors (EDLCs) than disordered carbons especially at high rates thanks to their unique architectural design. In the present work, we demonstrate that mesostructural order is a key factor for the performance of carbons at high rates. For that, two series of hierarchical micro–mesoporous carbons were prepared by chemical activation with KOH following two different procedures. SBA-15 silica was infiltrated by chemical vapor deposition of propylene and the resulting carbon–silica composite was split into two halves. One part was directly activated and, subsequent to the activation process, the template was eliminated. The second part was first subjected to the template removal and the resulting CMK-3 type carbon was further activated. Thus, carbons obtained by both these methods present bimodal pore size distributions and different mesostructural order as a function of the activation conditions. Carbons prepared from the direct activation of the composite present a high conservation of the ordered mesoporous structure. The electrochemical performance of both carbon series was tested in an acid aqueous electrolyte (1 M H2SO4) by cyclic voltammetry and impedance spectroscopy. A highly ordered mesoporous structure significantly improves the specific capacitance at high scan rates due to enhancement in the diffusion of molecules.