Nanostructured mesoporous carbons are widely used in catalysis, adsorption and energy storage. Unlike conventional carbons, the textural properties of nanostructured carbons can be easily adjusted ...during synthesis. This design of the mesostructure allows to process large molecules by addressing the diffusional limitations/pore blockage which might occur in purely microporous carbon materials. Many synthetic approaches have been reported for the synthesis of mesoporous materials, either with a disordered mesoporosity (carbon gels), or with ordered porous systems (templated carbons). Carbon gels are synthesized by sol-gel processing through the polymerization of hydroxybenzenes and aldehydes. On the other hand, carbons with a periodically arranged mesoporous network can be obtained from the same precursors in the presence of templates, which may be rigid inorganic solids (hard-templating) or supramolecular assemblies (soft-templating). Controlling the surface chemistry is also a critical step in the development of high-performance materials; this can be achieved by doping with heteroatoms and functionalization with surface groups. The design of both texture and surface chemistry allows tuning the properties, thus leading to materials that meet the requirements of the targeted applications. The fundamentals, recent advances and the main challenges related to the preparation of nanostructured mesoporous carbon materials are presented in this review.
Porous carbons derived from metal-organic frameworks (MOFs) are promising materials for a number of energy- and environment-related applications, but their almost exclusively microporous texture can ...be an obstacle to their performance in practical uses. Here, we introduce a novel strategy for the generation of very uniform mesoporosity in a prototypical MOF, namely, zeolitic imidazolate framework-8 (ZIF-8). The process, referred to as “nanopore lithography”, makes use of graphene oxide (GO) nanosheets enclosing ZIF-8 particles as masks or templates for the transfer of mesoporous texture to the latter. Upon controlled carbonization and activation, nanopores created in the GO envelope serve as selective entry points for localized etching of carbonized ZIF-8, so that such nanopores are replicated in the MOF-derived carbonaceous structure. The resulting porous carbons are dominated by uniform mesopores ∼3–4 nm in width and possess specific surface areas of ∼1300–1400 m2 g–1. Furthermore, we investigate and discuss the specific experimental conditions that afford the mesopore-templating action of the GO nanosheets. Electrochemical characterization revealed an improved capacitance as well as a faster, more reversible charge/discharge kinetics for the ZIF-8-derived porous carbons obtained through nanopore lithography, relative to those for their counterparts with standard activation (no GO templating), thus indicating the potential practical advantage of the present approach in capacitive energy storage applications.
Graphene and graphene-based materials have shown great promise in many technological applications, but their large-scale production and processing by simple and cost-effective means still constitute ...significant issues in the path of their widespread implementation. Here, we investigate a straightforward method for the preparation of a ready-to-use and low oxygen content graphene material that is based on electrochemical (anodic) delamination of graphite in aqueous medium with sodium halides as the electrolyte. Contrary to previous conflicting reports on the ability of halide anions to act as efficient exfoliating electrolytes in electrochemical graphene exfoliation, we show that proper choice of both graphite electrode (e.g., graphite foil) and sodium halide concentration readily leads to the generation of large quantities of single-/few-layer graphene nanosheets possessing a degree of oxidation (O/C ratio down to ∼0.06) lower than that typical of anodically exfoliated graphenes obtained with commonly used electrolytes. The halide anions are thought to play a role in mitigating the oxidation of the graphene lattice during exfoliation, which is also discussed and rationalized. The as-exfoliated graphene materials exhibited a three-dimensional morphology that was suitable for their practical use without the need to resort to any kind of postproduction processing. When tested as dye adsorbents, they outperformed many previously reported graphene-based materials (e.g., they adsorbed ∼920 mg g–1 for methyl orange) and were useful sorbents for oils and nonpolar organic solvents. Supercapacitor cells assembled directly from the as-exfoliated products delivered energy and power density values (up to 15.3 Wh kg–1 and 3220 W kg–1, respectively) competitive with those of many other graphene-based devices but with the additional advantage of extreme simplicity of preparation.
Two CeO2/carbon nanotube composites were prepared in both basic and neutral medium thus using different deposition procedures of the cerium oxide. The modification of the synthesis conditions varied ...the hydrolysis rate of the cerium oxide precursor, leading to materials with different morphology, texture and surface chemistry, significantly changing their electrochemical performance. The different mechanisms of charge storage occurring on the surface of the materials were thoroughly studied to investigate the origin of the observed changes in the electrochemical properties. To that end, a combination of sweep wave voltammetry and deconvolution analysis in acid (1M H2SO4), basic (1M NaOH) and neutral (1M Na2SO4) electrolytes was conducted. The faradaic activity of the composite prepared by slow hydrolysis (containing well-dispersed and small ceria particles) is enhanced as compared with the composite prepared by fast hydrolysis, which originates bulky particles.
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•Fine identification of charge storage mechanisms in EDCL electrodes.•Unravelling the pseudofaradaic activity of CeO2/carbon nanotube composite.•Different active phases were identified as function of the used electrolyte.•Potential application for the characterization of a wide variety of electrodes.
Hierarchical carbons with different boron contents (from 0.42 to 2.37 wt.%) were prepared by combination of soft-templating and hydrothermal approaches. This newly developed strategy enables adequate ...control of the chemical state of the boron functionalities in carbon materials since the self-generated pressures favour B–C bond formation. The total content of oxygen increases simultaneously with the boron content, and its speciation is also influenced by the boron chemical state distribution: the presence of C–B–C (BC3 and BC2O) species induces the oxidation of phenols to carbonyls. The electrochemical performance of the prepared carbons was tested in a three-electrode cell configuration (1 M H2SO4) showing good specific capacitances per BET specific surface area (20–36 μF/cm2). The presence of boron on the carbon backbone improves both the charge propagation and the pseudocapacitive properties. Nevertheless, the electrochemical properties are only enhanced up to some level of boron doping (around 1.8 wt.%). A two-electrode supercapacitor built with a carbon prepared by a mild hydrothermal synthesis (50 °C during 28 h) delivered energy densities similar to those of commercial activated carbons. This could lead to high volumetric capacitance, since the boron doped carbons present much higher density.
To overcome the challenges of Na-O2 batteries with respect to efficiency, capacity, and cycle life as well as to develop cheap, metal-free, and abundant electrocatalysts, we explored boron and ...nitrogen-functionalized graphene aerogels prepared by the hydrothermal self-assembly of graphene oxide with subsequent thermal reduction. The results showed an improve of both the cycling overpotential and the coulombic efficiency for the functionalized graphene aerogels. However, the nitrogen-containing cathode presented a shortened cycle life and decreased charging stability. The postmortem analysis of the full discharge, and the full discharge and charge cathodes demonstrated that nitrogen functionalization triggered the formation of solid parasitic products that passivate the cathode surface, thus resulting in a poorer electrochemical performance. By contrast, functionalization with boron-containing groups demonstrated to be a more promising strategy due to minimized parasitic products, leading to lower oxygen reduction and evolution overpotentials with a concomitantly enhanced cell efficiency vs. the undoped cathodes. This resulted in a cycle life of 70 cycles at a relatively high current density of 0.1 mA cm−2 with a capacity cut-off of 0.5 mAh cm−2. Our study underscores that functionalization with heteroatoms simultaneously alters multiple characteristics of graphene-based materials, including their chemistry, texture and morphology, which in turn presents a critical impact on the electrochemical response of the resultant Na-O2 cells.
Metal-free electrocatalysts for the electrochemical conversion of gases constitute an important asset for a sustainable energy transition. Nucleotides act as redox mediators in the electron transport ...chain to reduce oxygen in cellular respiration. The biomimicry of such an efficient natural mechanism could be utilized to address the challenges associated with electrochemical gas conversion technologies, such as sluggish kinetics and high overpotentials. Multiple descriptors are generally reported to benchmark the activity of electrocatalysts where the turnover frequency (TOF) is claimed to be the most accurate criterion. Here, a library of graphene nanosheets-nucleotide hybrid materials was prepared, and the electrocatalytic performance towards ORR/OER reactions of a graphene-flavin mononucleotide hybrid was evaluated by rotating disc electrode experiments and TOF estimation. The determination of catalyst loading and dispersion is especially relevant when assessing the intrinsic activity of a catalyst and, therefore, the amount of nucleotide electrocatalyst loaded into the graphene support was thoroughly quantified by a combination of characterization techniques. Density functional theory calculations supported the observed experimental trends, both on the adsorption rate of a given nucleotide on graphene and the catalytic activity of a specific hybrid material. This work constitutes an avenue to predict nature-mimicking electrocatalysts for efficient energy storage.
Sustainable metal-free nucleotide/graphene hybrids are prepared as promising electrocatalysts for gas conversion reactions. Non-covalent π-π interactions are facilitated through liquid adsorption and the amount of adsorbed nucleotide in graphene is assessed using various techniques. The trend observed experimentally aligns consistently with the adsorption energies computed through DFT calculations. The riboflavin-containing hybrid shows a better ability to catalyze OER than graphene. Display omitted
Either chemical activation (KOH) or physical activation (CO
2) were performed on ordered mesoporous carbons (OMCs) obtaining simultaneously remarkable microporosity development and mesoporous system ...preservation. Bimodal micro–mesoporous carbons with narrow and tuneable pore sizes distributions are obtained.
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► Microporosity is developed in OMC preserving the ordered structure. ► Carbons with bimodal, narrow and tuneable pore size distribution are prepared. ► Both physical and chemical activations increase significantly the micropore volume. ► Mesoporous order is lost under hard chemical activation conditions.
Ordered mesoporous carbon (OMC) with hexagonal arrays of tubes (CMK-3) was synthesized by direct replication, using SBA-15 as template and propylene as carbon source. Microporosity was developed in these OMCs with either carbon dioxide or potassium hydroxide as activating agents; effects of these activation treatments on their pore structure were discussed. Porous texture was characterized by gas adsorption, morphology was examined by SEM and the degree of order of porous structure was evaluated by X-ray diffraction and TEM micrography. The specific surface area, microporous volume and total pore volume were markedly enhanced upon activation of CMK-3, but the long-range ordered structure gradually became more disordered. Potassium hydroxide activation develops the materials microporosity in a greater extent than with CO
2, nevertheless moderate temperatures and activating agent proportions must be used in order to preserve the ordered mesoporous structure.
Activated carbon xerogels with a cellular morphology were obtained from hydrothermally carbonized glucose-graphene oxide (GO) hybrids and tested as supercapacitor electrodes. The effect of the ...chemical activation (using KOH) on the nanometer-scale morphology, local structure, porous texture and surface chemistry of the resulting carbon materials was investigated and correlated with their electrochemical behaviour. The electrochemical performance of the activated xerogels was studied in a three-electrode cell using 1 M H2SO4 as the electrolyte. The results underlined the relevant role played by the xerogel nanomorphology; more specifically, xerogels with cellular structures exhibiting well-connected, continuous and very thin (∼5–15 nm) carbon walls (prepared with lower amounts of activating agent) favored ionic diffusion and electronic conduction compared to materials with broken, thicker walls (obtained from higher amounts of activating agent). The effect of nanomorphology and local structure was also made apparent when the xerogels were used as actual supercapacitor electrodes. Particularly, a symmetric capacitor assembled from a carbon xerogel with very thin walls and relatively high graphitic character delivered a much higher specific capacitance than that of a commercial activated carbon (223 vs 153 F g−1 at 100 mA g−1) as well as a significantly improved retention of capacitance at high current densities.
Sodium-oxygen batteries hold great promise for the transition to a non-fossil fuel economy due to their high theoretical energy density. One of the most important components of these devices is the ...air-cathode, where the electrons available at the solid electrode, the Na+ ions present in the liquid electrolyte and oxygen gas react to form sodium oxides as discharge products. The kinetics of the discharge/charge reactions depend significantly on the boundary points between the solid-liquid-gas reaction phases, known as triple phase boundary (TPB). The density of TPB points (and therefore the battery efficiency) can be maximized by incorporating perfluorinated polymers on the cathode formulation. Thus, this type of polymers enhance oxygen transport properties which favour the diffusion of gaseous components in detriment to liquid electrolytes on solid electrodes. In this work, polytetrafluoroethylene (PTFE) and fluorinated ethylene propylene (FEP) polymers were added in different weight ratio to commercial graphene nanoplatelets (GNPs) cathodes. The critical physical properties affecting the formation of the TPB have been identified and correlated to sodium-oxygen battery performance. These key properties, which are crucial to modulate the oxygen diffusion within the cathode structure, have been identified for the first time in this work for aprotic metal air devices. This approach is of outmost importance for the development or efficient electrochemical storage devices where oxygen gas is involved.
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