M. Luisa Ferrer

Deep eutectic solvents as both precursors and structure directing agents in the synthesis of nitrogen doped hierarchical carbons highly suitable for CO2 capture

Deep eutectic solvents (DESs) have been used in the synthesis of nitrogen-doped carbons exhibiting a hierarchical porous structure. The CO2 sorption capacity of these solid sorbents was extraordinary because of their relatively high nitrogen content and their bimodal porous structure where micropores provide high surface areas (ca. 700 m2 g−1) and macropores provide accessibility to such a surface. DESs were composed of resorcinol, 3-hydroxypyridine and choline chloride in 2 : 2 : 1 and 1 : 1 : 1 molar ratios. Polycondensation of resorcinol and 3-hydroxypyridine (with formaldehyde) promoted DES segregation in a spinodal-like decomposition process by the formation of a polymer rich phase and a depleted polymer phase. Thus, DESs played a multiple role in the synthetic process; the liquid medium that ensured reagents homogenization, the structure-directing agent that is responsible for the achievement of the hierarchical structure, and the source of carbon and nitrogen of the solid sorbent obtained after carbonization. Interestingly, the homogeneous incorporation of nitrogen at the solution stage of the synthetic process (rather than by post-treatment of the preformed carbon) allowed the achievement of significant nitrogen contents even in carbons obtained at relatively high temperatures (e.g. 8–12 at% for 600 °C and ca. 5 at% for 800 °C). It is worth noting that, despite thermal treatments at high temperatures tend to decrease the nitrogen content, the high surface area of the solid sorbents obtained at 800 °C contributed to a significant enhancement of CO2 capture while providing superior selectivity, recyclability and stability.

Deep Eutectic Solvents in Polymerizations: A Greener Alternative to Conventional Syntheses

The use of deep eutectic solvents (DESs) that act as all-in-one solvent-template-reactant systems offers an interesting green alternative to conventional syntheses in materials science. This Review aims to provide a comprehensive overview to emphasize the similarities and discrepancies between DES-assisted and conventional syntheses and rationalize certain green features that are common for the three DES-assisted syntheses described herein: one case of radical polymerization and two cases of polycondensations. For instance, DESs contain the precursor itself and some additional components that either provide certain functionality (e.g., drug delivery and controlled release, or electrical conductivity) to the resulting materials or direct their formation with a particular structure (e.g., hierarchical-type). Moreover, DESs provide a reaction medium, so polymerizations are ultimately carried out in a solventless fashion. This means that DES-assisted syntheses match green chemistry principles 2 and 5 because of the economy of reagents and solvents, whereas the functionality incorporated by the second component allows the need for any post-synthesis derivatization to be minimized or even fully avoided (principle 8). DESs also provide new precursors that favor more efficient polymerization (principle 6) by decreasing the energy input required for reaction progress. Finally, the use of mild reaction conditions in combination with the compositional versatility of DESs, which allows low-toxic components to be selected, is also of interest from the viewpoint of green chemistry because it opens up the way to design biocompatible and/or eco-friendly synthetic methods (principle 3).

Three-dimensional microchanelled electrodes in flow-through configuration for bioanode formation and current generation

Three-dimensional microchannelled nanocomposite electrodes fabricated by ice-segregation induced self-assembly of chitosan-dispersed multiwall carbon nanotubes are shown to provide a scaffold for growth of electroactive bacteria for use as acetate-oxidizing bioanodes in bioelectrochemical systems. The hierarchical structure provides a conductive surface area available for G. sulfurreducens colonization, with a flow through configuration along the electrode providing a substrate for bacterial colonization and bio-electrochemical processes. This configuration, whilst resulting in sub-monolayer biofilm coverage over the three-dimensional surface, is capable of providing acetate oxidation current densities of up to 24.5 A m−2, equating to a volumetric current density of 19 kA m−3, in the flow-through configuration. Such bioanodes, when operated in non-optimized flow-through microbial fuel cell configuration, provide a maximum power density of 2.87 W m−2, which is equivalent to 2.0 kW m−3 volumetric power density.

Phosphate-Functionalized Carbon Monoliths from Deep Eutectic Solvents and their Use as Monolithic Electrodes in Supercapacitors

Porous carbon materials have received considerable attention in the last years because of their outstanding performance as substrates in a number of separation and catalytic processes as well as electrodes in fuel cells, supercapacitors, and batteries. 2] In sustainable terms, these latter electrochemical processes are especially attractive for the future of our society because of their capability to either provide an alternative to combustion of fossil fuels for energy production (e.g. , fuel cells) or to store energy (e.g. , supercapacitors and batteries). Particularly interesting are those nanostructured porous carbon materials or those carbon materials showing three-dimensional (3D) hierarchical porous textures (containing pores at different scales, from micropores to mesopores, up to macropores) that combine high specific surface areas with proper channels, allowing good diffusion of any substance (e.g. , analytes, adsorbates, electrolytes, etc.) to the entire surface of the material. A number of synthetic routes using different carbonaceous precursors in the presence of either hard or soft templates has been explored to date with the aim to control the textural properties of the resulting carbon materials. Among them, the use of ionic liquids (ILs) and deep-eutectic solvents (DESs) is especially interesting in sustainable terms due to their recent application to a number of green processes, both synthetic and catalytic. Sustainability is emphasized with those ILs and DESs capable to play multiple roles (e.g. , from reaction medium to carbonaceous precursors up to structure-directing agents) in the synthetic process and hence, promoting a significant economy of reagents. Thus, ILs and DESs have proved efficient in tailoring not only the textural properties, but also the composition of the resulting carbon materials. For instance, nitrogen-functionalized carbon materials have exhibited a remarkable capacity for CO2 adsorption [9] and they hold much potential as electrodes for supercapacitors. With regard to this application, the use of phosphate-functionalized carbon materials open interesting perspectives because of the widening of the operational voltage window with aqueous electrolytes and the subsequent increase of the energy density that can be attained. However, neither ILnor DES-assisted synthesis have yet been described for the preparation of phosphate-functionalized carbon monoliths (PfCMs). Herein, we describe the preparation of PfCMs by polycondensation of formaldehyde with a resorcinol-based DES—composed of resorcinol, choline chloride, and glycerol—and using phosphoric acid as catalyst. The ternary DES was prepared in a closed container by the complexation of resorcinol, choline chloride, and glycerol in a molar ratio 1:1:1 at 90 8C (see the Experimental Section). The inset in Figure 1 a shows that the resulting DES is a viscous liquid (the static viscosity is 314 cP at 22 8C, as determined by

Deep-Eutectic-Solvent-Assisted Synthesis of Hierarchical Carbon Electrodes Exhibiting Capacitance Retention at High Current Densities

Nature provides a wide range of entities and/or systems with different functions that may serve as a source of bioinspiration for material chemists. Actually, materials exhibiting a 3D porous texture (combining pores at different scales, from macroto mesoup to micropores) mimic the hierarchical structure of different systems found in living organisms (e.g., the blood circulation or the respiratory system in mammalians). Structural organization at different scales is ultimately responsible for the outstanding properties offered by hierarchical materials (not only as stationary phases in separation and catalytic processes, but also as electrodes in fuel cells and capacitors) because they offer not only large surface areas, but also accessibility to such a surface. A number of synthetic routes have been explored by using different carbonaceous precursors and either exo or endo templates to modulate the porous texture of the resulting carbon structures. Recent efforts have also been focused on the preparation of porous carbon composites containing graphitic carbon entities (e.g., carbon nanotubes and nanohorns, or even graphene oxide) the challenge of which is double and resides in 1) the achievement of a homogenous dispersion of these entities throughout the monolith structure and 2) the preservation of high surface areas. Baumann and co-workers have recently performed a quite extensive and stimulating work on single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs) synthesized by resorcinol–formaldehyde polycondensation. Particularly interesting in terms of conductivity and surface area were those composites based on DWCNTs, although the authors expressed the convenience of substituting the surfactants used for carbon-nanotube (CNT) dispersion. Ionic liquids (ILs) and deep eutectic solvents (DESs, a new class of ILs obtained by complexion of quaternary ammonium salts with hydrogen-bond donors, such as acids, amines, and alcohols) have lately been the solvent of choice in a number of chemical processes because of special features: for example, they are nonreactive with water, nonvolatile, and biodegradable as well as excellent solvents for a wide variety of solutes, such as different substrates, enzymes, and even microorganisms of catalytic and biocatalytic interest. Of particular interest for the purpose of this work are those processes for which the capability both as solvents for CNT dispersion and structure-directing agents in the synthesis of different materials was demonstrated. Actually, ILs and DESs have been used as solvents for preparation of CNT-based carbon composites and even as carbonaceous precursors of both nontextured and textured carbons. In particular, we have recently described the preparation of DESs based on mixtures of resorcinol and choline chloride, the rupture of which (via resorcinol polycondensation and subsequent segregation of choline chloride) resulted in the formation of bimodal porous carbons. In this case, hierarchy was obtained through a synthetic mechanism that combined aspects from those original works used for synthesis of zeolites (i.e., based on DES rupture and controlled delivery of an organic template to the reaction mixture) and those used for synthesis of macroporous structures (i.e. , based on spinodal-like processes) . It is also worth noting the “green” character of the process as a result of the absence of residues and/or byproducts eventually released after the synthetic process, that is, one of the components forming the DES (e.g., resorcinol) becomes the material itself, whereas the second one (e.g., choline chloride) is fully recovered and can be reused in subsequent reactions. Based on these previous results, we considered that the use of DESs could open an interesting path for the preparation of hierarchical porous CNT composites. Herein, we describe the preparation of hierarchical porous multiwalled CNT (MWCNT) composites exhibiting high surface areas and outstanding conductivities through furfuryl alcohol (FA) condensation catalyzed by a protic DES based on complexes of para-toluene sulfonic acid [a] Dr. M. C. Guti rrez, Dr. D. Carriazo, Dr. R. Jim nez, Dr. J. M. Rojo, Dr. M. L. Ferrer, Dr. F. del Monte Instituto de Ciencia de Materiales de Madrid-ICMM Consejo Superior de Investigaciones Cient ficas-CSIC Campus de Cantoblanco, 28049-Madrid (Spain) E-mail : mcgutierrez@icmm.csic.es delmonte@icmm.csic.es [b] Dr. A. Tamayo Instituto de Ceramica y Vidrio-ICV Consejo Superior de Investigaciones Cient ficas-CSIC. Campus de Cantoblanco, 28049-Madrid (Spain) [c] Dr. F. Pic Centro Nacional de Investigaciones Metalurgicas-CENIM Consejo Superior de Investigaciones Cient ficas-CSIC Av. Gregorio del Amo s/n, 28040-Madrid (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201101679.

Microwave-assisted synthesis of NiCo2O4–graphene oxide nanocomposites suitable as electrodes for supercapacitors

Microwave irradiation combined with mild heating was demonstrated as a suitable synthetic method for the preparation of NiCo2O4 nanowires and NiCo2O4–graphene oxide nanocomposites. In both cases, the synthesis was carried out in just a few minutes, at relatively low temperatures and with a certain economy of reagents. In the particular case of the nanocomposites, NiCo2O4 was deposited in form of not only nanowires, but also ultrafine nanoparticles on the surface of graphene oxide sheets due to the preferred activation of these sheets under microwave irradiation. The nanocomposites exhibited a superior performance to NiCo2O4 nanowires as electrodes in pseudocapacitors with capacitance retentions of nearly 735 F g−1 at current densities of up to 33 A g−1.

Chitosan Gelation Induced by the in Situ Formation of Gold Nanoparticles and Its Processing into Macroporous Scaffolds

This work describes a simple synthetic route to induce chitosan (CHI) gelation by the in situ formation of gold nanoparticles (AuNPs). AuNPs were obtained by thermal treatment (e.g., 40 and 80 °C) of CHI aqueous solutions containing HAuCl(4) and in the absence of further reducing agents. The CHI hydrogels resulting after AuNP formation were submitted to unidirectional freezing and subsequent freeze-drying via ISISA (ice-segregation-induced self-assembly) process for the preparation of CHI scaffolds. The study of AuNP-CHI scaffolds by SEM and confocal fluorescence microscopy revealed a morphological structure characteristic of the hydrogel nature of the samples subjected to the ISISA process. Interestingly, not only the morphology but also the dissolution and swelling degree of the resulting CHI scaffolds were strongly influenced by the strength of the hydrogels obtained by the in situ formation of AuNP. We have also studied the catalytic activity AuNP-CHI scaffolds in the reduction of p-nitrophenol. The negligible dissolution and low swelling degree obtained in certain AuNP-CHI scaffolds allowed them to be used for more than four cycles with full preservation of the reaction kinetics.

Synthesis of macroporous poly(acrylic acid)–carbon nanotube composites by frontal polymerization in deep-eutectic solvents

Deep Eutectic Solvents (DESs) formed between Acrylic Acid (AA) and Choline Chloride (CCl) exhibit certain properties of ionic liquids (e.g. high viscosity) that make them suitable for frontal polymerization (FP). The use of DESs not only as a monomer but also as the solvent prevents the use of additional solvents (i.e. typically of organic nature) and offers a green tool for the synthesis of functional composites. We have recently explored this approach for the preparation of poly(acrylic acid) (PAA) and poly(methacrylic acid). In this work, we have taken advantage of the outstanding capability of DESs to solubilize and/or disperse a number of substances to incorporate – in a homogeneous fashion – carbon nanotubes (in this particular case, N-doped MWCNT – CNxMWCNTs) in the polymerizable DES. Interestingly, CNxMWCNTs also played the role of an inert filler in FP. The resulting PAA–CNxMWCNT composites exhibited some distinct features as compared to previous PAA also obtained via DES-assisted FP. For instance, PAA–CNxMWCNT composites can undergo swelling depending on the pH, as bare PAA. However, the presence of CNxMWCNTs allows the formation of a macroporous structure after submission to a freeze-drying process, the achievement of which was not possible in bare PAA. The combination of structural (e.g. macroporosity) and functional (e.g. stimuli responsive) properties exhibited by these materials besides an eventually high biocompatibility – coming from the green character of the DES-assisted synthesis – should make the resulting macroporous PAA–CNxMWCNT composites excellent candidates for their future application as biomaterials.

Phosphorus-doped carbon–carbon nanotube hierarchical monoliths as true three-dimensional electrodes in supercapacitor cells

Eutectic mixtures of the monohydrated form of p-toluenesulfonic acid (pTsOH·H2O) and triethyl phosphate (TEP) in a 1 : 1 molar ratio were used as the medium to disperse previously functionalized multiwalled carbon nanotubes (MWCNTs), and to catalyse the polycondensation of furfuryl alcohol. Hierarchically structured P-doped carbon–CNT composites were obtained after carbonization. The high surface area, the phosphate functionalization, the hierarchical structure and the good electrical conductivity exhibited by these composites provided remarkable metrics when they were used as electrodes in a supercapacitor cell, with energy densities of around 22.6 W h kg−1 and power densities of up to 10 kW kg−1 for operational voltages of up to 1.5 V. This performance surpasses any performance previously reported for electrodes weighing (at least) 10 mg per cm2 of current collector and using an aqueous electrolyte. Actually, the supercapacitor cell built up with these electrodes provided enough neat energy to turn an IR LED of 30 mW on and emit light over a certain period.

Deep eutectic assisted synthesis of carbon adsorbents highly suitable for low-pressure separation of CO2–CH4 gas mixtures

Ionic liquids and deep eutectic solvents (a new class of ionic liquids obtained by complexation of quaternary ammonium salts with hydrogen bond donors such as acids, amines, and alcohols among others) have been recently used as solvents and even as precursors in the synthesis of carbonaceous materials. The use of long-alkyl-chain derivatives of ionic liquids that, playing the role of structure directing agents, were capable of designing the mesoporous structure of the resulting carbons is of particular interest. Meanwhile, deep eutectic solvents proved efficient in tailoring the structure comprised between large mesopores and small macropores, but the control over the smaller ones (e.g. small mesopores and micropores) is still a challenge as compared to ILs. Herein, we have used deep eutectic solvents composed of resorcinol, 4-hexylresorcinol and tetraethylammonium bromide for the synthesis of carbon monoliths with a tailor-made narrow microporosity. Behaving as molecular sieves, these carbons exhibited not only good capacities for CO2 adsorption (up to 3 mmol g−1) but also an outstanding – especially at low pressures – CO2–CH4 selectivity.