Energy storage applications of activated carbons: supercapacitors and hydrogen storage

Porous carbons have several advantageous properties with respect to their use in energy applications that require constrained space such as in electrode materials for supercapacitors and as solid state hydrogen stores. The attractive properties of porous carbons include, ready abundance, chemical and thermal stability, ease of processability and low framework density. Activated carbons, which are perhaps the most explored class of porous carbons, have been traditionally employed as catalyst supports or adsorbents, but lately they are increasingly being used or find potential applications in the fabrication of supercapacitors and as hydrogen storage materials. This manuscript presents the state-of-the-art with respect to the preparation of activated carbons, with emphasis on the more interesting recent developments that allow better control or maximization of porosity, the use of cheap and readily available precursors and tailoring of morphology. This review will show that the renewed interest in the synthesis of activated carbons is matched by intensive investigations into their use in supercapacitors, where they remain the electrode materials of choice. We will also show that activated carbons have been extensively studied as hydrogen storage materials and remain a strong candidate in the search for porous materials that may enable the so-called Hydrogen Economy, wherein hydrogen is used as an energy carrier. The use of activated carbons as energy materials has in the recent past and is currently experiencing rapid growth, and this review aims to present the more significant advances.M.S. acknowledges the assistance of the Spanish MINECO for the award of a Ramón y Cajal contract.Peer reviewe

Graphitic carbon nanostructures from cellulose

Graphitic carbon nanostructures have been synthesized from cellulose via a simple methodology that essentially consists of the steps: (i) hydrothermal treatment of cellulose at 250 °C and (ii) impregnation of the carbonaceous product with a nickel salt followed by thermal treatment at 900 °C. The formation of graphitic carbon nanostructures seems to occur by a dissolution–precipitation mechanism in which amorphous carbon is dissolved in the catalyst nanoparticles and then precipitated as graphitic carbon around the catalyst particles. The subsequent removal of the nickel nanoparticles and amorphous carbon by oxidative treatment leads to graphitic nanostructures with a coil morphology. This material exhibits a high degree of crystallinity and large and accessible surface area.The financial support for this research work provided by the Spanish MCyT (MAT2008-00407) is gratefully acknowledged. M. S. acknowledges the assistance of the Spanish MCyT for the award of a Postdoctoral Mobility contract.Peer reviewe

Fabrication of porous carbon monoliths with a graphitic framework

[EN] Macro/mesoporous carbon monoliths with a graphitic framework were synthesized by carbonizing polymeric monoliths of poly(benzoxazine-co-resol). The overall synthesis process consists of the following steps: (a) the preparation of polymeric monoliths by co-polymerization of resorcinol and formaldehyde with a polyamine (tetraethylenepentamine), (b) doping the polymer with a metallic salt of Fe, Ni or Co, (c) carbonization and (d) the removal of inorganic nanoparticles. The metal nanoparticles (Fe, Ni or Co) formed during the carbonization step catalyse the conversion of a fraction of amorphous carbon into graphitic domains. The resulting carbon monoliths contain >50 wt.% of graphitic carbon, which considerably improves their electrical conductivity. The use of tetraethylenepentamine in the synthesis results in a nitrogen-containing framework. Textural characterization of these materials shows that they have a dual porosity made up of macropores and mesopores (∼2–10 nm), with a BET surface area in the 280–400 m2 g−1 range. We tested these materials as electrodes in organic electrolyte supercapacitors and found that no conductive additive is needed due to their high electrical conductivity. In addition, they show a specific capacitance of up to 35 F g−1, excellent rate and cycling performance, delivering up to 10 kW kg−1 at high current densities.The financial support for this research work provided by the Spanish MCyT (CQT2011-24776 and MAT2012-31651) is gratefully acknowledged. M. S. thanks the Spanish MCyT for the award of a Ramón y Cajal contract.Peer reviewe

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 m2 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).This research work was supported by the Spanish MINECO (MAT2012-31651). M.S. acknowledges the award of a Ramón y Cajal contract.Peer reviewe

Superior Capacitive Performance of Hydrochar-Based Porous Carbons in Aqueous Electrolytes

This is the accepted version of the following article: Fuertes, A. B. and Sevilla, M. (2015), Superior Capacitive Performance of Hydrochar-Based Porous Carbons in Aqueous Electrolytes. ChemSusChem, 8: 1049–1057. doi: 10.1002/cssc.201403267, which has been published in final form at http://dx.doi.org/10.1002/cssc.201403267. This article may be used for non-commercial purposes in accordance with the Wiley Self-Archiving PolicyBiomass-based highly porous carbons with excellent performances in aqueous electrolyte-based supercapacitors have been developed. The synthesis of these materials is based on the chemical activation of biomass-based hydrochar. The addition of melamine to the activation mixture leads to porous carbons with a porosity consisting of micropores/small mesopores. Furthermore, melamine promotes the introduction of nitrogen heteroatoms in the carbon framework, along with abundant oxygen functionalities, to improve the wettability. The materials produced in the presence or absence of melamine exhibit high specific capacitances in aqueous electrolytes (>270 F g−1 in H2SO4 and >190 F g−1 in Li2SO4). Additionally, the mesopores present in the melamine-based micro-/mesoporous carbons notably improve the ion-transport kinetics, especially in Li2SO4. Furthermore, in Li2SO4, they remain stable up to a cell voltage of 1.6 V; thus exhibiting superior energy and power characteristics than those in H2SO4.This research work was supported by the Spanish MINECO (MAT2012-31651). M. S. thanks the Spanish MINECO for her Ramón y Cajal contract.Peer reviewe

Carboxyl-functionalized mesoporous silica–carbon composites as highly efficient adsorbents in liquid phase

[EN] Mesoporous silica–carbon composites functionalized with acidic oxygen groups were prepared by oxidation of a layer of carbon deposited inside the silica pores. The synthesis procedure involved the following steps: (a) removal of the silica surfactant, (b) impregnation of the silica pores with a carbon precursor, (c) carbonization and (d) oxidation with ammonium persulfate. The resulting silica–carbon composites contained around 25–30 wt.% of carbonaceous matter with a high density of acid oxygen groups attached to the deposited carbon layer (i.e. –COOH, –Cdouble bond; length as m-dashO and –OH). The structural characteristics of the parent silica were retained by the oxidized composite materials, which exhibit a high surface area, a large pore volume and a well-ordered porosity made up of uniform mesopores. The oxygen-functionalized silica–carbon composites were found to be excellent adsorbents of basic dyes (e.g. methylene blue) and heavy metals (i.e. Cu2+, Zn2+ and Pb2+) in aqueous media.This work was supported by the Spanish MICINN (Project CQT2011-24776). M.S. and P.V-V. acknowledge Ramon y Cajal and JAE-Predoc contracts, respectively.Peer reviewe

Free-standing hybrid films based on graphene and porous carbon particles for flexible supercapacitors

Free-standing flexible solid-state supercapacitors are attracting attention as a power supply for electronic equipment. Here we report a novel strategy to fabricate free-standing flexible hybrid papers made up of porous carbon particles combined with graphene sheets. The synergetic effect between the carbon particles and the graphene sheets entails two important advantages: (a) binder-free electrodes formed by carbon particles can be built with the assistance of the graphene sheets and (b) the restacking of the graphene sheets is avoided to a great extent due to the fact that the carbon particles act as spacers. These hybrid papers combine important properties for their use in solid-state supercapacitors: (a) large specific surface area, (b) good electrical conductivity, (c) high packing density and (d) excellent flexibility. They exhibit a volumetric electrochemical performance which is clearly superior to electrodes fabricated with carbon particles agglomerated with a binder. In addition, they achieve an excellent areal capacitance (103 mF cm−2) at current densities as high as 1400 mA cm−2 and are able to deliver a large amount of energy (∼12 μW h cm−2) at high power densities (316 mW cm−2). In this work, a robust, flexible and high-performance solid-state supercapacitor has been assembled using such hybrid papers.This research work was supported by the FICYT Regional Project (GRUPIN14- 102), and the Spanish MINECO-FEDER (CTQ2015-63552-R).Peer reviewe

Synthesis of Graphitic Carbon Nanostructures from Sawdust and Their Application as Electrocatalyst Supports

We present a novel and facile synthetic method for fabricating graphitic carbon nanostructures (GCNs) from sawdust. This method is based on the use of catalysts (Fe or Ni) that allows the direct conversion of sawdust into highly graphitized carbon material. The following procedure was used to obtain these graphitic nanoparticles:  (a) impregnation of the sawdust particles with iron or nickel salts, (b) carbonization of the impregnated material at a temperature of 900 or 1000 °C, and (c) selective removal of the non-graphitized carbon (amorphous carbon) by an oxidant (KMnO4). The resulting carbon is made up of nanosized graphitic structures (i.e., nanocapsules, nanocoils, nanoribbons), which have a high crystallinity, as evidenced by TEM/SAED, XRD and Raman analysis. These GCNs were used as supports for platinum nanoparticles. Such prepared electrocatalysts show an electrocatalytical surface area close to 90 m2.g-1 Pt, and they present a similar or higher electrocatalytic activity toward methanol electrooxidation than the Pt/Vulcan electrocatalyst prepared in the same conditions.The financial support for this research work provided by the Spanish MCyT (MAT2005-00262, MAT2004-01479 and FEDER) is gratefully acknowledged.Peer reviewe

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 PCO2 ∼ 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.The financial support for this research work provided by the Spanish MINECO (MAT2012-31651) is gratefully acknowledgedPeer reviewe

Supercapacitive Behavior of Two Glucose-Derived Microporous Carbons: Direct Pyrolysis versus Hydrothermal Carbonization

This is the accepted version of the following article: Sevilla, M., Yu, L., Ania, C. O. and Titirici, M.-M. (2014), Supercapacitive Behavior of Two Glucose-Derived Microporous Carbons: Direct Pyrolysis versus Hydrothermal Carbonization. CHEMELECTROCHEM, 1: 2138–2145. doi: 10.1002/celc.201402233, which has been published in final form at http://dx.doi.org/10.1002/celc.201402233. This article may be used for non-commercial purposes in accordance with the Wiley Self-Archiving PolicyThe physical and chemical characteristics of activated carbons produced from glucose and hydrothermally carbonized glucose are compared for the first time, as well as their performance as electrodes in supercapacitors with aqueous electrolyte (H2SO4). Both KOH-activated carbons exhibit similar textural properties, with Brunauer–Emmett–Teller surface areas of ≈1400–1500 m2 g−1 and a pore volume of ≈0.70 cm3 g−1, with the pore size distribution centered in the micropore range. When tested as supercapacitor electrodes, the activated carbon produced from hydrothermally carbonized glucose exhibits a superior rate capability, owing to lower equivalent distributed resistance (being able to work at an ultrahigh discharge current of 90 A g−1), as well as higher specific capacitance (≈240 F g−1 vs. ≈220 F g−1 for the glucose-derived activated carbon at 0.1 A g−1). Both supercapacitors have excellent robustness, even for a large cell voltage of 1.2 V in 1 M H2SO4.This research work was supported by Spanish MINECO (MAT2012-31651 and CTM2011-23378). M.S. acknowledges the award of the Ramón y Cajal contract. M.M. Titirici and Linghui Yu are grateful to the Max-Planck Society for financial support for this project.Peer reviewe