No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Abstract
The amount of ions stored within the electrochemical double layer is dominantly determined by the surface area of porous electrode. High surface area indicates high capacitance. However, this is not the case when the pore is extremely small, ∼less than 1 nm. To observe this phenomenon, we have carried out an experiment by comparatively investigating the electrochemical performance of activated carbon (AC) and sulfuric acid treated AC (SAC). The results show that the electrochemical performance of SAC involving the specific capacitance, rate capability and cycling stability is significantly increased as compared to pristine AC. These are attributed to the improved porosity by differentiating the respective contributions of electrochemical double layer capacity and pseudo-capacity from SAC and AC, respectively.
... KMnO 4 is an oxidizing chemical that can be utilized as a surface modification and ion transfer activator to improve the external functional groups of activated carbon materials [22]. The modification of AC by KMnO 4 significantly increases the number of micropores and reduces the pore size, effectively enhancing the material's specific surface area and providing more active adsorption sites for electron transfer [23]. According to a study, modifying the reaction time of AC and KMnO 4 can vary the shape of the composite material, allowing for the construction of the supercapacitor electrode [24]. ...
Baicalin is an index component used for the quality evaluation of Scutellaria baicalensisGeorgi with significant anticancer activity. It is very important to develop a highly selective and sensitive method for the detection of baicalin in complex samples, which is used for standardized planting, harvesting, and processing of Scutellaria baicalensis, quality control of Scutellaria baicalensis herbs and products. Based on this, a novel electrochemical sensor based on CS/ACK@CeO2-NPs composite material was constructed for the rapid and sensitive detection of baicalin in complex samples. It is noteworthy that cerium oxide nanoparticles (CeO2-NPs) were loaded on the KMnO4-modified activated carbon (ACK) and used in the construction of electrochemical sensors for the first time. The number of micropores of ACK was significantly increased, and the specific surface area of the material was effectively increased. The combination of ACK with cerium oxide nanoparticles (CeO2-NPs) avoided the agglomeration of CeO2-NPs, which produced a significant synergistic effect, increased the electron transfer rate, and provided more active adsorption sites for electron transfer. In addition, chitosan (CS) has good film-forming performance, so adding CS to the ACK@CeO2-NPs system not only improves the film-forming performance and mechanical properties but also makes the constructed sensor more stable and sensitive. After optimizing the conditions, the test displayed good linearity in the concentration range (2.24 × 10⁻⁴ ~ 4.48 × 10⁻⁷ mol/L) of baicalin with the lowest detection limit of 4.81 × 10⁻⁹ mol/L (S/N = 3) when it is used for the detection of baicalin in complex samples. This method provides a new approach for the rapid and sensitive detection of baicalin in complex samples. The application of electrochemical analysis techniques to the instant monitoring of active or indicator components in the materials of traditional Chinese medicine (TCM) and their preparations is expected to provide a sensitive, rapid, and reliable monitoring and analysis technique for the quality control of TCM and their products throughout their life cycle.
... Activated carbon materials play important roles as electrode materials and have been used to manufacture EDLCs, including commercially available commercial devices. This is because of the excellent electrochemical performance, resulting from their large surface area, tunable pore sizes, chemical resistance, non-corrosion properties, and good electrical conductivities [17][18][19][20][21][22][23][24]. However, the precursors of most carbon materials are usually derived from fossil fuels, which strongly detracts from the environmental benefits of their use. ...
For the preparation of activated carbon papers (APCs) as supercapacitor electrodes, impurity substances were removed from rice husks, before carbonization and various activation temperature treatments, to optimize electro chemical efficiency. The porosities and electrochemical performances of the ACPs depended strongly on activation temperature: The specific surface area increased from 202.92 (500 °C) to 2158.48 m² g⁻¹ (1100 °C). XRD and Raman analyses revealed that ACP graphitization also increased with the activation temperature. For activation at 1100 °C, the maximum specific capacitance was 255 F g⁻¹, and over 92% of its capacitance was retained after 2000 cycles.
This survey paper is pointed toward featuring past works of RH-filled polymer composites give data to applications and further examination in this space. Based on the information gathered, using RH-filled composites as an alternative material in aircraft and boats is extremely feasible, with their major thrusts being lightweight and low effort. However, more research into physical and synthetic treatments to strengthen the interfacial binding with the polymeric network is needed, as the fiber-polymer link is critical in determining the final composite qualities. To understand the pre-owned polymer mixes as the grid and optional fillers may influence the properties would give fascinating territories to be investigated. In this study, rice husk was mixed with glass fibers in (50 + 50) and (25 + 50) weight percentage. A glass fiber and polymer were used as binder in 25% of the mass of the fillers. A sufficient number of small specimens were produced to perform small-scale tests and assess properties such as compressive strength, thermal conductivity, dynamic stiffness, improvement in impact sound insulation, sound absorption and transmission loss. Here the data based on the rice husk and glass fiber oriented composite analysis is carried out to bring a optimized fiber reinforced polymer composites.
N, O-codoped hierarchical porous carbon (HDPC) is synthesized adopting pomelo peel as a biomass carbon source by ZnCl2 activation. The prepared HDPC possesses a high specific surface area of 1582 m² g⁻¹ and together with rich heteroatom of 5.2 wt% N and 5.5 wt% O. When used as electrode material of supercapacitor, it delivers a higher specific capacitance of 180 F g⁻¹ at 0.5 A g⁻¹, excellent rate capability with a capacitance retention of 75.6% at 10 A g⁻¹, and fantastic cycle stability with nearly 100% Coulombic efficiency at 5 A g⁻¹ for 5000 cycles in 6 M KOH.
Ketamine, a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, may prevent central sensitization and result in preemptive analgesia when administered before surgically induced trauma. The goal of this study was to determine whether intravenous low dose ketamine would reduce postoperative pain and cumulative analgesic requirements after total intravenous anesthesia (TIVA) with propofol and fentanyl.
Carbon supercapacitors, which are energy storage devices that use ion adsorption on the surface of highly porous materials to store charge, have numerous advantages over other power-source technologies, but could realize further gains if their electrodes were properly optimized. Studying the effect of the pore size on capacitance could potentially improve performance by maximizing the electrode surface area accessible to electrolyte ions, but until recently, no studies had addressed the lower size limit of accessible pores. Using carbide-derived carbon, we generated pores with average sizes from 0.6 to 2.25 nanometer and studied double-layer capacitance in an organic electrolyte. The results challenge the long-held axiom that pores smaller than the size of solvated electrolyte ions are incapable of contributing to charge storage.
Recently, the use of RFID(Radio Frequency Identification) for object identification is used more often, but the tag collision problem by the use of a radio frequency still exists. Therefore, in this paper, we analyzed tag identification time to minimize tag collisions and improve tag identification time by applying various tag anti-collision algorithms in the suggested method. As a result, we drew a conclusion about the number of optimum readers used in the RFID system environment. Significantly, we expect that the suggested method will be used more efficiently in the simultaneous multi-tags identification RFID system environment.
Large scale reduced graphene oxide (RGO) with sulfonic acid groups (SRGO) has been synthesized by a one-step facial method for high performance electrochemical capacitors (ECs). The as-prepared SRGO shows an enhanced specific surface from 254 to 434 m2 g−1 and bulk conductivity from 2 × 10−4 to 2.5 × 10−4 S m compared with that of pristine RGO. An asymmetric EC was assembled with SRGO as the working electrode and Pt as the counter electrode immersed in a 1 M neutral aqueous Na2SO4 solution. The results showed that a maximum specific capacitance of 130 F g−1 was achieved by the SRGO, which is much higher than that of pristine RGO. The SRGO also exhibits a high rate capability and superior cycling stability of more than 99% retention of the specific capacitance after 1000 cycles. Furthermore, it also exhibits excellent specific energy and power densities of 14.6 W h kg−1 and 50 W kg−1 at a current density of 100 mA g−1, respectively. These results suggest that our SRGO material could be a very promising and versatile building block for carbon-based materials as electrodes towards the next generation of high-performance ECs.
We have experimentally and theoretically clarified the effect of oxygen functional groups on capacitive performance of the photochemically treated activated carbon electrode. A high density of C=O group at the mouth of the micropores, where the chemically active edge sites are predominantly available, increase the energy barrier for ions to enter the pores, thereby resulting in a large decrease in the specific capacitance.
Zinc oxide (ZnO)-based porous carbon composite nanofibers (ZnO-CCNFs) are prepared by one-step electrospinning and subsequent thermal treatment using zinc acetate, as the pore generator and ZnO precrursor. In particular, the PAN-based nanofiber paper contains in-frame incorporated nitrogen surface functionalities, due to its large residual nitrogen content in the char. The N functionalities doped at the graphite edges enhances their capacitance by the pseudocapacitive effect.
Therefore, the ZnO-CCNFs showed higher capacitance (163 Fg-1 at 1 mAcm-2) and energy density (20.80-14.80 Whkg-1 in the power density range of 400-20,000 Wkg-1) than the control sample of carbon nanofibers (CNFs) in aqueous electrolyte. The combination of the high surface area of CNFs with the large capacity of surface functional groups such as N, O, and ZnO as the faradic electrode material affords the advantages of both the double layer capacitance and the pseudocapacitance, thereby offering potential applications for supercapacitors.
Carbon nanomaterials are used as an electrode of electric double layer capacitors (EDLCs). In this research, we used arc black (AcB) and carbon nanoballoon (CNB) as the electrode material. AcB was produced by an arc discharge of graphite in N2 atmosphere, and CNB was formed by a heat treatment of AcB. CNB is graphitic, and the particle shape is hollow. CNB has a higher specific capacitance than AcB at a high scan rate. In order to increase the specific capacitance of EDLC, CNB was oxidized at 625 °C in the air. By oxidization, the outer shell of CNB forms wrinkle. We call this material oxidized CNB (Ox-CNB). AcB, CNB, and Ox-CNB were used for the EDLC electrodes and were compared with commercially available activated carbon (AC). Cyclic voltammetry and electrochemical impedance spectroscopy of the EDLC electrodes were measured by an electrochemical measurement system. The specific capacitance of Ox-CNB (29 F/g) was larger than that of AC (16 F/g) at a scan rate of 500 mV/s. Furthermore, Ox-CNB had a high conductivity as a result of impedance measurement. Ox-CNB is an excellent electrode material of EDLC when using at a high charge/discharge rate.
Corn starch based solid biopolymer electrolytes doped with lithium acetate (LiOAc) and plasticized with glycerol are prepared by solution cast technique. In unplasticized system, the maximum room temperature conductivity of (2.07 +/- 0.53) x 10(-5) S cm(-1) is obtained by the electrolyte consists of 75 wt.% starch and 25 wt.% LiOAc. In plasticized system, addition of 30 wt.% glycerol to the highest conducting unplasticized electrolyte has further increase the conductivity up to (1.04 +/- 0.10) x 10(-3) S cm(-1). Results from X-ray diffraction (XRD) explain that the enhancement of conductivity is contributed by the degree of crystallinity of electrolytes. Linear sweep voltammetry (LSV) shows that the highest conducting plasticized electrolyte is stable up to 2.1 V. The highest conducting plasticized electrolyte is used in the fabrication of an electrochemical double layer capacitor (EDLC). The EDLC is characterized using cyclic voltammetry (CV) and galvanostatic charge-discharge measurements. From CV, the specific capacitance (C-s) of fresh EDLC is calculated to be 33.00 F g(-1) at scan rate of 0.5 mV s(-1). The value of C-s increases as the scan rate decreases. From the charge-discharge measurement, the value of C-s is almost constant at similar to 33.31 F g(-1) for 1000 cycles.
Ordered mesoporous carbon films with a 3D pore structure (COU-3) were synthesized by a softtemplating method using resorcinol (R)/formaldehyde (F) as a carbon source and Pluronic F127 as a template agent. The carbon films were prepared by a spin-coating technique. From the results of X-ray diffraction pattern, field emission scanning electron microscope and transmission electron microscope observation, the structure of the COU-3 film was determined to be an Im3m structure whose c-axis is oriented perpendicular to the film surface. The pores run in the directions both perpendicular and parallel to the film surface. The porous structure of the film is different from the ordered mesoporous carbon powders and thick films (COU-1 and COU-2) which were synthesized using the same precursor solution. In the preparation of the COU-3 films, the self-assembly and condensation of a RF-F127 composite immediately occurred during the spin-coating. In this process, the RF-F127 composites are arranged on a substrate surface and an ordered structure spreads from the substrate surface to an upper region. The electrochemical performance of electrochemical double layer capacitor was examined. The CV curves are flat and rectangular in shape, and did not change even at a high scan rate, indicating that the internal resistance in grain boundary and the contact resistance with the substrate can be ignored in the COU-3 film. Additionally, the COU-3 film shows good cycle stability after 250 cycles.
In this work an activated carbon modified by nitric acid has been used as the electrodes in capacitive deionization (CDI) process for the desalination of an aqueous electrolytic solution. The experimental results have shown that the modification could greatly increase the salt removal from the solution. The desalination efficiency was increased about 15%, and the desalination kinetics was improved in the form of rate constant from 0.09208 to 0.09922. It has been found that the modification greatly increased the oxygen-containing functional groups on the surfaces of activated carbon, leading to the increases of the capacitance and the reduction of the charging resistance, which might be attributed to the improvement of the desalination.
Novel electrolyte additive for electrochemical capacitors has been reported. It has been demonstrated for the first time that addition of humic acids (HA) to KOH-based electrolyte significantly increases capacitance of symmetrical capacitors with electrodes made of activated carbon. Specific capacitances determined by means of galvanostatic charge/discharge, cyclic voltammetry and electrochemical impedance spectroscopy consistently showed increases for HA concentrations ranging from 2% w/w up to saturated solution with maximum positive effect observed for 5% w/w of the additive. The capacitance increase has been attributed to complex faradaic processes involving oxygen-containing groups of HA molecules. Due to abundant resources, low cost and easy processability the reported solution can find application in electrochemical capacitor technologies.
Multi-walled carbon nanotubes (MWCNTs) were simultaneously fluidized and oxidized with gaseous ozone in a vertical reactor. Two different varieties of MWCNTs were compared to determine the versatility of the treatment and to elucidate the effect of defects on the oxidation behavior of MWCNTs. The extent of oxidation and nature of functional groups introduced on the nanotube surfaces were determined using Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and Boehm titration, and structural changes were monitored with Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). After only a few minutes of treatment, non-graphitic impurities were removed from the MWCNTs and significant levels of oxidation (~8 at% O) were achieved with very little damage to the nanotube sidewalls. Short O3 exposure resulted in primarily hydroxyl functionalities while longer exposure led to the formation of mainly carboxylic acid groups. Aliphatic defects present in the commercially-produced MWCNTs were found to play an important role in the oxidation mechanism. Because of its ability to remove impurities and evenly oxidize the sidewalls of nanotubes without the use of any solvents, the fluidized O3 reaction developed in this study was found to be an attractive option for industrial-scale MWCNT functionalization.
Nitrogen-doped reduced graphene oxide (N-RGO) and reduced graphene oxide (RGO) have been synthesized by microwave-assisted hydrothermal method to discern the actual contribution of nitrogen-containing functional groups on the specific capacitance (CS,T) in acidic and alkaline electrolytes. Material characterization reveals similar porosity, electrolyte-accessible surface area, element composition, and graphitic crystallinity between N-RGO and RGO except the difference in the nitrogen content. In 1 M H2SO4, additional pseudocapacitance provided by pyridinic-N and pyrrolic-N/pyridone-N is clearly observed at the potential negative to 0.6 V (vs. RHE) while this contribution in pseudocapacitance diminishes in 1 M KOH due to the lack of proton in the electrolyte for these basic functional groups to undergo redox reactions. The double-layer capacitance of N-RGO in 1 M H2SO4 in higher than that in 1 M KOH owing to the presence of N-containing functional groups which increase the electronic charge density of graphene and favor proton adsorption in the acidic electrolyte. The contribution of nitrogen-containing functional groups on CS,T in acidic media is more pronounced than that in the alkaline electrolyte. This finding is crucial for the future application of N-doped carbons in supercapacitors to achieve full utilization.
The novel biacidic carbon has been synthesized via one-step hydrothermal carbonization of glucose, citric acid, and hydroxyethylsulfonic acid at 180 °C for only 4 h. The novel carbon had an acidity of 1.7 mmol/g with the carbonyl to sulfonic acid groups molar ratio of 1:3, which was confirmed by IR, XPS, TPD, SEM, and BET analyses. The catalytic activities of the carbon were investigated through esterification and oxathioketalization. The results showed that the carbon owned the comparable activities to sulfuric acid, which indicated that the carbon holds great potential for the green processes.
Electrical double layer capacitors (EDLCs) with activated sucrose-derived carbons (ASCs) as electrodes are reported. The carbons were prepared by the pyrolysis of sucrose followed by the activation with CO2 gas for 1–5 h at 900 °C to tune the pore size distribution and the specific surface area (SSA). The porosity of the ASCs has been characterized using N2 and CO2 adsorption measurements. The activation increased the SSA from ∼200 to 3000 m2 g−1 and produced pores mostly in the 0.4–2 nm range. The pyrolysis of sucrose without CO2 activation produces a carbon with specific capacitance as low as 4 F g−1, whereas selected ASCs exhibit specific capacitance in excess of 160 F g−1 and excellent frequency response in a two-electrode EDLC cell with 1 M H2SO4 electrolyte. The activation time of 4 h resulted in the most promising electrochemical performance. Excellent ASC stability was confirmed by extensive electrochemical characterization after 10,000 charge–discharge cycles.
The dependence of the voltammetric surface charge q* on solution pH and potential scan rate has been investigated using a set of RuO2 electrodes prepared by thermal decomposition of RuCl3 at temperatures in the range 300–500°C. The systematically higher charge in KOH than in HClO4 in the same potential range (vs rhe) is attributed to the stabilization of higher oxidation states of surface Ru atoms in the alkaline environment. The variation of q* with v, the potential scan rate, is shown to be linearizable as a function of v. It is thus possible to extrapolate the values of q* to v=0 and v=∞, respectively. The extrapolation enables an “inner” surface to be discriminated from an “outer” surface. The former is pointed out to be composed by the regions of difficult accessibility for the proton-donating species assisting the surface redox reactions. Reasons why the “screened” surface appears to be higher in alkaline than in acid solutions, are discussed. It is stressed that only working with a set of RuO2 electrodes prepared at different temperatures it is possible to discover meaningful correlations.
The surface of pyrolyzed sugar carbons (PSCs) was treated with H2O2, HNO3, or H2SO4 reagents. Surface modification of these materials improved their electrode performance. FTIR, Raman, NMR, and X-ray diffraction (XRD) spectroscopic techniques were used to characterize structures of the treated and untreated carbons. The charge/discharge capacity, potential profile, and irreversible capacity loss have been investigated. The correlations between cell capacities, H/C ratios, and Raman vibrational modes are discussed.
To examine whether acid treatment of a non-graphitizing hard carbon influences positively or negatively its electrochemical ano-dic performance, this study reports the effects of sulfuric acid treatment on the microstructural changes and electrochemical per-formance of PAN-based hard carbons prepared at various temperatures. It was found that PAN-based hard carbons heat treated at 900 °C (TAN9) exhibit an increased reversible capacity by up to 8% following the sulfuric acid treatment (TAN9S series). Since small changes in microstructure, except for slight reductions in surface area and crystallite size (L a) value, were observed after sulfuric acid treatment, it was speculated that the capacity responses of samples in series TAN9S were due to the introduction of new functional groups such as ASO 3 H and ASO 4 H. As the newly intro-duced functional groups are strong acids but their conjugates (ASO À 3 and ASO À 4) are weak bases, those conjugates were thus con-sidered to be able to react with Li + ion relatively weakly and reversibly. For PAN-based hard carbons heat treated at 1100 °C (TAN11), there were much smaller changes, as compared with series TAN9S samples, in surface chemistry and microstructure by sulfuric acid treatment. Consequently, we observed an analogous but smaller influence of sulfuric acid oxidation on electrochem-ical performance. The samples from series TAN11S exhibited very stable cycling behavior. It was suggested that the acidity/basicity of surface functional groups may be an influential factor for improving the electrochemical performance of hard carbon-based ano-dic materials for lithium ion batteries.
Activated carbons (ACs) were prepared by microwave-assisted heat treatment of petroleum coke with KOH as activation agent, and characterized by infrared spectroscopy and nitrogen adsorption technique with the aim of studying the effect of activation time on the properties of ACs for electrodes in electric double layer capacitors (EDLCs). The electrochemical properties of AC electrodes in EDLCs were studied by cyclic voltammetry, constant current charge–discharge and electrochemical impedance spectroscopy. The results show that the specific surface area (SBET) and total pore volume of ACs goes through a maximum as the activation time increases. At 35 min of the activation time, the as-made AC (denoted as AC-35) has a SBET of 2312 m2/g. With AC-35 as the electrode, its specific capacitance in EDLC at a current density of 50 mA/g can reach 342.8 F/g, and remains at 245.6 F/g even after 800 cycles while the energy density of the capacitor remains at 8.0 Wh/kg. The results have demonstrated that the microwave-assisted heat treatment is an efficient approach to the preparation of ACs with high performance for EDLCs.
Carbon functionalized with sulfonic acid groups has been synthesized using the one-step hydrothermal carbonization of furaldehyde and hydroxyethylsulfonic acid aqueous solution at 180 °C for 4 h. The carbon exhibited high acidity and comparable activities to sulfuric acid for the traditional acid-catalyzed reactions, which indicated that it has great potential for environment-friendly processes. The copolymerization method provides an efficient procedure for the synthesis of various functionalized carbons by one-step hydrothermal carbonization.
Reduction of a colloidal suspension of exfoliated graphene oxide sheets in water with hydrazine hydrate results in their aggregation and subsequent formation of a high-surface-area carbon material which consists of thin graphene-based sheets. The reduced material was characterized by elemental analysis, thermo-gravimetric analysis, scanning electron microscopy, X-ray photoelectron spectroscopy, NMR spectroscopy, Raman spectroscopy, and by electrical conductivity measurements. (c) 2007 Elsevier Ltd. All rights reserved.
Extending the Capability of Supercapacitors
Supercapacitors have porous electrodes that can store more charge per volume in electrical double layers than conventional parallel plate capacitors. However, the porous electrodes cause poor performance in filter circuits that eliminate residual alternating current ripple from rectified direct current. Miller et al. (p. 1637 ) fabricated electrodes with a high surface area for ionic adsorption by growing graphene sheets in the vertical direction off a metal surface. Such capacitors may be able to perform the same filtering tasks as conventional capacitors but take up less space.
In capacitive deionization (CDI), an electrical potential difference is applied across oppositely placed electrodes, resulting in the adsorption of ions from aqueous solution and a partially ion-depleted product stream. CDI is a dynamic process which operates in a sequential mode; i.e., after a certain ion adsorption capacity has been reached, the applied voltage is reduced, and ions are released back into solution, resulting in a solution concentrated in ions. The energetic input of CDI is very small, while there are no ion-exchange materials involved that need to be replaced regularly. Here we present a dynamic process model for CDI which includes the storage and release of ions in/from the polarization layers of the electrodes. The charge and ion adsorption capacity of the polarization layers is described using the equilibrium Gouy¿Chapman¿Stern (GCS) model, while the charge transfer rate from bulk solution into the polarization layer is modeled according to Ohm¿s law, i.e., depends solely on an electric field term across a mass-transfer layer. An important element in the model is the differential charge efficiency: the effective salt removal rate relative to the electronic current, for which an analytical expression is derived based on the GCS model. We present results for the effluent salt concentration and electron current, both as function of time, based on a process model that assumes ideal mixing in the CDI unit cell. The theoretical results are in very good agreement with an example data set.
Two series of activated carbon monoliths (discs) have been prepared by chemical activation of olive stones with phosphoric acid or zinc chloride, without the use of any binder. A conforming step was introduced between impregnation with the chemical and heat treatment. Two equivalent series of granular activated carbons were also prepared in order to analyse the effect of conforming pressure on the porosity of the final activated carbon. The evolution of microporosity and the micropore size distribution has been followed by gas adsorption (N2 at −196 °C and CO2 at 0 °C) and immersion calorimetry into three liquids with different molecular dimensions (dichloromethane, benzene and 2,2-dimethylbutane). The experimental results indicate that activation by both chemicals produces a large development of microporosity but the differences between the granular and monolithic forms are more noticeable when using phosphoric acid. Thus, there is mainly a reduction in the interparticle space and macroporosity during the formation of the discs prepared using zinc chloride whereas there is an additional reduction in the volume and dimension of the meso- and microporosity when using phosphoric acid. The different behaviour of the two chemicals has been related to their effect on the precursor along the impregnation step. Authors acknowledge the financial support from MEC (The project MAT2004-03480-C02-02).