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. The overall research objective of this study was the synthesis and characterization of activated carbon fibers and nanofibers, as well as monoliths, in order to obtain suitable materials for hydrogen storage. The specific objectives of the research were: . Identify the optimal parameters of activation, in order to synthesize activated carbon fibers (ACFs) with suitable hydrogen storage characteristics. . Produce larger amounts of ACFs by up-scaling of the activation process. . To study the synthesis of activated carbon nanofibers (ACNFs). . To optimize the density of the ACFs and ACNFs by synthesizing monoliths from them. . Measure the adsorption of H2 on selected samples and evaluate their total storage capacities. . Estimate the capacities of H2 storage systems by taking into account the technical specifications of state-of-the-art H2 storage vessels. The main contribution of this work was the identification of the activation parameters for the synthesis of activated carbon fiber materials with tailored properties for hydrogen storage application. H2 adsorption measurements on ACFs confirm and consolidate results previously reported in the literature. Despite this, exceptionally high hydrogen adsorption amounts were measured for an ACNF at 298 K and 20 MPa. The total H2 storage capacity was established as a useful tool for material characterization. In addition, formulas have been developed for calculating the capacities of H2 storage systems. These formulas provide the opportunity to evaluate the performance of tanks and adsorbents for H2 storage via physisorption

Activated Carbon Fibre Monoliths for Hydrogen Storage

Porous adsorbents are currently investigated for hydrogen storage application. From a practical point of view, in addition to high porosity developments, high material densities are required, in order to confine as much material as possible in a tank device. In this study, we use different measured sample densities (tap, packing, compacted and monolith) for analyzing the hydrogen adsorption behavior of activated carbon fibres (ACFs) and activated carbon nanofibres (ACNFs) which were prepared by KOH and CO2 activations, respectively. Hydrogen adsorption isotherms are measured for all of the adsorbents at room temperature and under high pressures (up to 20 MPa). The obtained results confirm that (i) gravimetric H2 adsorption is directly related to the porosity of the adsorbent, (ii) volumetric H2 adsorption depends on the adsorbent porosity and importantly also on the material density, (iii) the density of the adsorbent can be improved by packing the original adsorbents under mechanical pressure or synthesizing monoliths from them, (iv) both ways (packing under pressure or preparing monoliths) considerably improve the storage capacity of the starting adsorbents, and (v) the preparation of monoliths, in addition to avoid engineering constrains of packing under mechanical pressure, has the advantage of providing high mechanical resistance and easy handling of the adsorbent

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

Gas Storage Scale-up at Room Temperature on High Density Carbon Materials

In relation to the current interest on gas storage demand for environmental applications (e.g., gas transportation, and carbon dioxide capture) and for energy purposes (e.g., methane and hydrogen), high pressure adsorption (physisorption) on highly porous sorbents has become an attractive option. Considering that for high pressure adsorption, the sorbent requires both, high porosity and high density, the present paper investigates gas storage enhancement on selected carbon adsorbents, both on a gravimetric and on a volumetric basis. Results on carbon dioxide, methane, and hydrogen adsorption at room temperature (i.e., supercritical and subcritical gases) are reported. From the obtained results, the importance of both parameters (porosity and density) of the adsorbents is confirmed. Hence, the densest of the different carbon materials used is selected to study a scale-up gas storage system, with a 2.5 l cylinder tank containing 2.64 kg of adsorbent. The scale-up results are in agreement with the laboratory scale ones and highlight the importance of the adsorbent density for volumetric storage performances, reaching, at 20 bar and at RT, 376 g l-1, 104 g l-1, and 2.4 g l-1 for CO2, CH4,and H2, respectively.Generalitat Valenciana and FEDER (project PROMETEO/2009/047)

Sorbent design for CO2 capture under different flue gas conditions

CO2 capture by solid sorbents is a physisorption process in which the gas molecules are adsorbed in a different porosity range, depending on the temperature and pressure of the capture conditions. Accordingly, CO2 capture capacities can be enhanced if the sorbent has a proper porosity development and a suitable pore size distribution. Thus, the main objective of this work is to maximize the CO2 capture capacity at ambient temperature, elucidating which is the most suitable porosity that the adsorbent has to have as a function of the emission source conditions. In order to do so, different activated carbons have been selected and their CO2 capture capacities have been measured. The obtained results show that for low CO2 pressures (e.g., conditions similar to post-combustion processes) the sorbent should have the maximum possible volume of micropores smaller than 0.7 nm. However, the sorbent requires the maximum possible total micropore volume when the capture is performed at high pressures (e.g., conditions similar to oxy-combustion or pre-combustion processes). Finally, this study also analyzes the important influence that the sorbent density has on the CO2 capture capacity, since the adsorbent will be confined in a bed with a restricted volume.The authors thank the Generalitat Valenciana and FEDER (project PROMETEO/2009/047) for financial support

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

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

Scale-up activation of carbon fibres for hydrogen storage

In a previous study, we investigated, at a laboratory scale, the chemical activation of two different carbon fibres (CF), their porosity characterization, and their optimization for hydrogen storage [1]. In the present work, this study is extended to: (i) a larger range of KOH activated carbon fibres, (ii) a larger range of hydrogen adsorption measurements at different temperatures and pressures (i.e. at room temperature, up to 20 MPa, and at 77 K, up to 4 MPa), and (iii) a scaling-up activation approach in which the obtained activated carbon fibres (ACF) are compared with those from laboratory-scale activation. The prepared samples cover a large range of porosities, which is found to govern their ability for hydrogen adsorption. The hydrogen uptake capacities of all the prepared samples have been analysed both in volumetric and in gravimetric bases. Thus, maximum adsorption capacities of around 5 wt% are obtained at 77 K, and 1.1 wt% at room temperature, respectively. The packing densities of the materials have been measured, turning out to play an important role in order to estimate the total storage capacity of a tank volume. Maximum values of 17.4 g/l at 298 K, and 38.6 g/l at 77 K were obtained.Osaka Gas Co., Ltd. supplied the two precursor materials. Financial help was received from the European Union (Marie Curie Research Training Network-HyTRAIN-Project reference: 512443), MEC (ENE2005-23824-E/CON), the Generalitat Valenciana (ACOMP06/089 and PROMETEO/2009/047), as well as MEC-CTQ2006-08958/PPQ

Material demands for storage technologies in a hydrogen economy

A hydrogen economy is needed, in order to resolve current environmental and energy-related problems. For the introduction of hydrogen as an important energy vector, sophisticated materials are required. This paper provides a brief overview of the subject, with a focus on hydrogen storage technologies for mobile applications. The unique properties of hydrogen are addressed, from which its advantages and challenges can be derived. Different hydrogen storage technologies are described and evaluated, including compression, liquefaction, and metal hydrides, as well as porous materials. This latter class of materials is outlined in more detail, explaining the physisorption interaction which leads to the adsorption of hydrogen molecules and discussing the material characteristics which are required for hydrogen storage application. Finally, a short survey of different porous materials is given which are currently investigated for hydrogen storage, including zeolites, metal organic frameworks (MOFs), covalent organic frameworks (COFs), porous polymers, aerogels, boron nitride materials, and activated carbon materials.Generalitat Valenciana and FEDER (Project PROMETEO/2009/047)