Activated Carbon Fiber Monoliths as Supercapacitor Electrodes

Activated carbon fibers (ACF) are interesting candidates for electrodes in electrochemical energy storage devices; however, one major drawback for practical application is their low density. In the present work, monoliths were synthesized from two different ACFs, reaching 3 times higher densities than the original ACFs’ apparent densities. The porosity of the monoliths was only slightly decreased with respect to the pristine ACFs, the employed PVDC binder developing additional porosity upon carbonization. The ACF monoliths are essentially microporous and reach BET surface areas of up to 1838 m2 g−1. SEM analysis reveals that the ACFs are well embedded into the monolith structure and that their length was significantly reduced due to the monolith preparation process. The carbonized monoliths were studied as supercapacitor electrodes in two- and three-electrode cells having 2 M H2SO4 as electrolyte. Maximum capacitances of around 200 F g−1 were reached. The results confirm that the capacitance of the bisulfate anions essentially originates from the double layer, while hydronium cations contribute with a mixture of both, double layer capacitance and pseudocapacitance.Financial support through the projects of reference MAT2014-57687-R, GV/FEDER (PROMETEOII/2014/010), and University of Alicante (VIGROB-136) is gratefully acknowledged

The contribution of sulfate ions and protons to the specific capacitance of microporous carbon monoliths

The monoliths studied in this work show large specific surface areas (up to 1600 m2 g-1), high densities (up to 1.17 g cm -3) and high electrical conductivities (up to 9.5 S cm-1). They are microporous carbons with pore sizes up to 1.3 nm but most of them below 0.75 nm. They also show oxygen functionalities. The electrochemical behavior of the monoliths is studied in three-electrode cells with aqueous H2SO4 solution as electrolyte. This work deals with the contribution of the sulfate ions and protons to the specific capacitance of carbon monoliths having different surface areas and different contents of oxygen groups. Protons contribute with a pseudocapacitance (up to 152 F g -1) in addition to the double layer capacitance. Sulfate ions contribute with a double layer capacitance only. At the double layer, the capacitance of the sulfate ions (up to 291 F g-1) is slightly higher than that of protons (up to 251 F g-1); both capacitances increase as the surface area increases. The preference of protons to be electroadsorbed at the double layer and the broader voltage window of these ions account for their higher contribution (70%) to the double layer capacitance. © 2014 Elsevier B.V. All rights reserved.Financial support through the projects MAT2011-25198, MP 1004 and PROMETEO/2009/047 is gratefully acknowledged. V.B. thanks MINECO for R&C contract.Peer Reviewe

The contribution of sulfate ions and protons to the specific capacitance of microporous carbon monoliths

The monoliths studied in this work show large specific surface areas (up to 1600 m2 g-1), high densities (up to 1.17 g cm-3) and high electrical conductivities (up to 9.5 S cm-1). They are microporous carbons with pore sizes up to 1.3 nm but most of them below 0.75 nm. They also show oxygen functionalities. The electrochemical behavior of the monoliths is studied in three-electrode cells with aqueous H2SO4 solution as electrolyte. This work deals with the contribution of the sulfate ions and protons to the specific capacitance of carbon monoliths having different surface areas and different contents of oxygen groups. Protons contribute with a pseudocapacitance (up to 152 F g-1) in addition to the double layer capacitance. Sulfate ions contribute with a double layer capacitance only. At the double layer, the capacitance of the sulfate ions (up to 291 F g-1) is slightly higher than that of protons (up to 251 F g-1); both capacitances increase as the surface area increases. The preference of protons to be electroadsorbed at the double layer and the broader voltage window of these ions account for their higher contribution (70 %) to the double layer capacitance.Financial support through the projects MAT2011-25198, MP 1004 and PROMETEO/2009/047 is gratefully acknowledged. V.B. thanks MINECO for R&C contract

Contribution of Cations and Anions of Aqueous Electrolytes to the Charge Stored at the Electric Electrolyte/Electrode Interface of Carbon-Based Supercapacitors

For their use in supercapacitors, aqueous electrolytes of acidic (H2SO4), neutral (Na2SO4, K2SO4), and basic (NaOH, KOH) nature are studied, using two microporous binder-free and self-standing carbon cloths as electrodes. The carbon cloths show similar porosities and specific surface areas but different contents in surface oxygen groups. The working potential window and the specific capacitance associated with the cations and anions are measured. From these parameters, the charges stored by the cations and anions at the electric electrolyte/electrode interface are deduced. The charge stored by the cations is higher than that stored by the anions for the three types of electrolytes. The differences between cations and anions are higher for the acidic and basic electrolyte than for the neutral electrolyte and also higher for the carbon cloth with the highest content in surface oxygen groups. The charge stored by the cations follows the sequence H3O+ > Na+ or K+ from the basic electrolytes > Na+ or K+ from the neutral electrolytes. The charge stored by the anions follows the sequence SO42– > HSO4– > OH–. The results here reported provide a better understanding on the electric double layer of carbon-based supercapacitors. Those results are also of interest for asymmetric and hybrid supercapacitors.Financial support from the projects of reference MAT2014-57687-R and FCT-M-ERA-NET/0004/2014, PCIN-2015-024 are gratefully acknowledged

Dense carbon monoliths for supercapacitors with outstanding volumetric capacitances

A commercially available dense carbon monolith (CM) and four carbon monoliths obtained from it have been studied as electrochemical capacitor electrodes in a two-electrode cell. CM has: (i) very high density (1.17 g cm−3), (ii) high electrical conductivity (9.3 S cm−1), (iii) well-compacted and interconnected carbon spheres, (iv) homogeneous microporous structure and (v) apparent BET surface area of 957 m2g−1. It presents interesting electrochemical behaviors (e.g., excellent gravimetric capacitance and outstanding volumetric capacitance). The textural characteristics of CM (porosity and surface chemistry) have been modified by means of different treatments. The electrochemical performances of the starting and treated monoliths have been analyzed as a function of their porous textures and surface chemistry, both on gravimetric and volumetric basis. The monoliths present high specific and volumetric capacitances (292 F g−1 and 342 F cm−3), high energy densities (38 Wh kg−1 and 44 Wh L−1), and high power densities (176 W kg−1 and 183 W L−1). The specific and volumetric capacitances, especially the volumetric capacitance, are the highest ever reported for carbon monoliths. The high values are achieved due to a suitable combination of density, electrical conductivity, porosity and oxygen surface content.Financial support from projects MAT2011-25198, MP1004 and PROMETEO/2009/047 is gratefully acknowledged. V.B. thanks MINECO for R&C contract

Synthesis of fibrous activated carbons and monoliths for hydrogen storage

For a future energy sector, hydrogen is considered a clean alternative to other fuels. As an ideal secondary energy carrier it can be produced from renewable energy sources (e.g., solar, wind, biomass, etc.), and converted very efficiently to electricity in fuel cells, emitting only water. However, one of the main obstacles which impedes the introduction of this technology is the absence of efficient storage solutions. This is strongly related to the low density of hydrogen, which exists as a supercritical fluid under normal conditions. In order to use hydrogen as a fuel, a number of different technologies are considered today. Among them, the high pressure storage in adsorbent materials is a promising technology. The adsorbents for such kind of application require very specific features, depending on the thermophysical storage conditions

Activated Carbon Fiber Monoliths as Supercapacitor Electrodes

Activated carbon fibers (ACF) are interesting candidates for electrodes in electrochemical energy storage devices; however, one major drawback for practical application is their low density. In the present work, monoliths were synthesized from two different ACFs, reaching 3 times higher densities than the original ACFs' apparent densities. The porosity of the monoliths was only slightly decreased with respect to the pristine ACFs, the employed PVDC binder developing additional porosity upon carbonization. The ACF monoliths are essentially microporous and reach BET surface areas of up to 1838 m g. SEM analysis reveals that the ACFs are well embedded into the monolith structure and that their length was significantly reduced due to the monolith preparation process. The carbonized monoliths were studied as supercapacitor electrodes in two-and three-electrode cells having 2 M HSO as electrolyte. Maximum capacitances of around 200 F g were reached. The results confirm that the capacitance of the bisulfate anions essentially originates from the double layer, while hydronium cations contribute with a mixture of both, double layer capacitance and pseudocapacitance.Professor A. Linares-Solano, now retired, is deeply acknowledged for helpful discussions about preparation of carbon monoliths, activation, and porosity. Financial support through the projects of reference MAT2014-57687-R, GV/FEDER (PROMETEOII/2014/010), and University of Alicante (VIGROB-136) is gratefully acknowledged. Edurne Laurín is acknowledged for the preparation of SEM samples.Peer Reviewe

Hydrogen storage in CO2-activated amorphous nanofibers and their monoliths

Amorphous carbon nanofibers (CNFs), produced by the polymer blend technique, are activated by CO2 (ACNFs). Monoliths are synthesized from the precursor and from some ACNFs. Morphology and textural properties of these materials are studied. When compared with other activating agents (steam and alkaline hydroxides), CO2 activation renders suitable yields and, contrarily to most other precursors, turns out to be advantageous for developing and controlling their narrow microporosity (< 0.7 nm), VDR(CO2). The obtained ACNFs have a high compressibility and, consequently, a high packing density under mechanical pressure which can also be maintained upon monolith synthesis. H2 adsorption is measured at two different conditions (77 K / 0.11 MPa, and 298 K / 20 MPa) and compared with other activated carbons. Under both conditions, H2 uptake depends on the narrow microporosity of the prepared ACNFs. Interestingly, at room temperature these ACNFs perform better than other activated carbons, despite their lower porosity developments. At 298 K they reach a H2 adsorption capacity as high as 1.3 wt.%, and a remarkable value of 1 wt.% in its mechanically resistant monolith form.Spanish MICINN (Project MAT2009-07150) and Generalitat Valenciana and FEDER (Project PROMETEO/2009/047)

Material design using molecular modeling for hydrogen storage

International audienceUsing grand canonical Monte-Carlo simulations, the adsorption capacities and isos-teric heats of hydrogen on activated carbons, graphite nanofibers, and bundles of carbon nanotubes are estimated for identical thermodynamic states. These computations allow a systematic, meaningful, and unbiased comparison of the adsorption properties of hydrogen in such porous materials. The comparison shows that the hydrogen storage capacity can he optimized, but only to a limited extent, in adjusting the material pore sizes and functionalizing a part of the adsorption sites. Therefore, at room, temperature and up to 70.0 MPa, for the three models of carbonaceous adsorbents, the hydrogen maximal excess adsorption is of the order of 2% of the adsorbent mass