Activation of polymer blend carbon nanofibres by alkaline hydroxides and their hydrogen storage performances

In the present work we study the hydroxide activation (NaOH and KOH) of phenol-formaldehyde resin derived CNFs prepared by a polymer blend technique to prepare highly porous activated carbon nanofibres (ACNFs). Morphology and textural characteristics of these ACNFs were studied and their hydrogen storage capacities at 77 K (at 0.1 MPa and at high pressures up to 4 MPa) were assessed, and compared, with reported capacities of other porous carbon materials. Phenol-formaldehyde resin derived carbon fibres were successfully activated with these two alkaline hydroxides rendering highly microporous ACNFs with reasonable good activation process yields up to 47 wt.% compared to 7 wt.% yields from steam activation for similar surface areas of 1500 m2/g or higher. These nano-sized activated carbons present interesting H2 storage capacities at 77 K which are comparable, or even higher, to other high quality microporous carbon materials. This observation is due, in part, to their nano-sized diameters allowing to enhance their packing densities to 0.71 g/cm3 and hence their resulting hydrogen storage capacities.

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.

High-Pressure Methane Storage in Porous Materials: Are Carbon Materials in the Pole Position?

Natural gas storage on porous materials (ANG) is a promising alternative to conventional on-board compressed (CNG) or liquefied natural gas (LNG). To date, Metal–organic framework (MOF) materials have apparently been the only system published in the literature that is able to reach the new Department of Energy (DOE) value of 263 cm3 (STP: 273.15 K, 1 atm)/cm3; however, this value was obtained by using the ideal single-crystal density to calculate the volumetric capacity. Here, we prove experimentally, and for the first time, that properly designed activated carbon materials can really achieve the new DOE value while avoiding the additional drawback usually associated with MOF materials (i.e., the low mechanical stability under pressure (conforming), which is required for any practical application).

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.

Changes in the storage and sink of carbon dioxide in subsurface atmospheres controlled by climate-driven processes: the case of the Ojo Guareña karst system

A comprehensive environmental monitoring program was conducted in the Ojo Guareña cave system (Spain), one of the longest cave systems in Europe, to assess the magnitude of the spatiotemporal changes in carbon dioxide gas (CO2) in the cave–soil–atmosphere profile. The key climate-driven processes involved in gas exchange, primarily gas diffusion and cave ventilation due to advective forces, were characterized. The spatial distributions of both processes were described through measurements of CO2 and its carbon isotopic signal (δ13C[CO2]) from exterior, soil and cave air samples analyzed by cavity ring-down spectroscopy (CRDS). The trigger mechanisms of air advection (temperature or air density differences or barometric imbalances) were controlled by continuous logging systems. Radon monitoring was also used to characterize the changing airflow that results in a predictable seasonal or daily pattern of CO2 concentrations and its carbon isotopic signal. Large daily oscillations of CO2 levels, ranging from 680 to 1900 ppm day−1 on average, were registered during the daily oscillations of the exterior air temperature around the cave air temperature. These daily variations in CO2 concentration were unobservable once the outside air temperature was continuously below the cave temperature and a prevailing advective-renewal of cave air was established, such that the daily-averaged concentrations of CO2 reached minimum values close to atmospheric background. The daily pulses of CO2 and other tracer gases such as radon (222Rn) were smoothed in the inner cave locations, where fluctuation of both gases was primarily correlated with medium-term changes in air pressure. A pooled analysis of these data provided evidence that atmospheric air that is inhaled into dynamically ventilated caves can then return to the lower troposphere as CO2-rich cave air.

Applications for CO2-Activated Carbon Monoliths: I. Gas Storage

Carbon monoliths with high densities are studied as adsorbents for the storage of H2, CH4, and CO2 at ambient temperature and high pressures. The starting monolith A3 (produced by ATMI Co.) was activated under a CO2 flow at 1073 K, applying different activation times up to 48 h. Micropore volumes and apparent surface areas were deduced from N2 and CO2 adsorption isotherms at 77 K and 273 K, respectively. CO2 and CH4 isotherms were measured up to 3 MPa and H2 up to 20 MPa. The BET surface area of the starting monolith (941 m2/g) could be significantly increased up to 1586 m2/g, and the developed porosity is almost exclusively comprised of micropores <1 nm. Total storage amounts take into account the compressed gas in the void space of the material, in addition to the adsorbed gas. Remarkably, high total storage amounts are reached for CO2 (482 g/L), CH4 (123 g/L), and H2 (18 g/L). These values are much higher than for other sorbents with similar surface areas, due to the high density of the starting monolith and of the activated ones, for which the density decreases only slightly (from 1.0 g/cm3 to 0.8 g /cm3 upon CO2 activation). The findings reveal the suitability of high density activated carbon monoliths for gas storage application. Thus, the amounts of stored gas can be increased by more than a 70 % in the case of H2 at 20 MPa, almost 5.5 times in the case of CH4 at 3 MPa, and more than 7.5 times in the case of CO2 at 3 MPa when adsorbents are used for gas storage under the investigated conditions rather than simple compression. Furthermore, the obtained results have been recently confirmed by a scale-up study in which 2.64 kg of high density monolith adsorbent was filled a tank cylinder of 2.5 L (Carbon, 76, 2014, 123).

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.

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.

Clay-supported graphene materials: application to hydrogen storage

The present work refers to clay–graphene nanomaterials prepared by a green way using caramel from sucrose and two types of natural clays (montmorillonite and sepiolite) as precursors, with the aim of evaluating their potential use in hydrogen storage. The impregnation of the clay substrates by caramel in aqueous media, followed by a thermal treatment in the absence of oxygen of these clay–caramel intermediates gives rise to graphene-like materials, which remain strongly bound to the silicate support. The nature of the resulting materials was characterized by different techniques such as XRD, Raman spectroscopy and TEM, as well as by adsorption isotherms of N2, CO2 and H2O. These carbon–clay nanocomposites can act as adsorbents for hydrogen storage, achieving, at 298 K and 20 MPa, over 0.1 wt% of hydrogen adsorption excess related to the total mass of the system, and a maximum value close to 0.4 wt% of hydrogen specifically related to the carbon mass. The very high isosteric heat for hydrogen sorption determined from adsorption isotherms at different temperatures (14.5 kJ mol−1) fits well with the theoretical values available for hydrogen storage on materials that show a strong stabilization of the H2 molecule upon adsorption.

New insights on electrochemical hydrogen storage in nanoporous carbons by in situ Raman spectroscopy

In situ Raman spectroscopy was exploited to analyze the interaction between carbon and hydrogen during electrochemical hydrogen storage at cathodic conditions. Two different activated carbons were used and characterized by different electrochemical techniques in two electrolytes (6 M KOH and 0.5 M Na2SO4). The in situ Raman spectra collected showed that, in addition to the D and G bands associated to the graphitic carbons, two bands appear simultaneously at about 1110 and 1500 cm−1 under cathodic conditions, and then they disappear when the potential increases to more positive values. This indicates that carbon–hydrogen bonds are formed reversibly in both electrolytes during cathodic conditions. Comparing the two activated carbons, it was confirmed that, in both electrolytes, the hydrogenation of carbon atoms is produced more easily for the sample with lower amount of surface oxygen groups. In KOH medium, for the two samples, the formation of carbon–hydrogen bonds proceeds at more positive potential with respect to the thermodynamic potential value for hydrogen evolution. Furthermore, changes in the shape of the D band (due to an intensity increase of the D1 band) during the formation of carbon–hydrogen bonds suggest that hydrogenation of the carbon atoms increases the number of edge planes.

Pseudocapacitance of zeolite-templated carbon in organic electrolytes

Carbon and graphene-based materials often show some amount of pseudocapacitance due to their oxygen-functional groups. However, such pseudocapacitance is generally negligible in organic electrolytes and has not attracted much attention. In this work, we report a large pseudocapacitance of zeolite-templated carbon (ZTC) based on the oxygen-functional groups in 1 M tetraethylammonium tetrafluoroborate dissolved in propylene carbonate (Et4NBF4/PC). Due to its significant amount of active edge sites, a large amount of redox-active oxygen functional groups are introduced into ZTC, and ZTC shows a high specific capacitance (330 F g−1). Experimental results suggest that the pseudocapacitance could be based on the formation of anion and cation radicals of quinones and ethers, respectively. Moreover, ZTC shows pseudocapacitance also in 1 M lithium hexafluorophosphate dissolved with a mixture of ethylene carbonate and diethyl carbonate (LiPF6/EC+DEC) which is used for lithium-ion batteries and lithium-ion capacitors.

Design of Activated Carbon/Activated Carbon Asymmetric Capacitors

Supercapacitors are energy storage devices that offer a high power density and a low energy density in comparison with batteries. Their limited energy density can be overcome by using asymmetric configuration in mass electrodes, where each electrode works within their maximum available potential window, rendering the maximum voltage output of the system. Such asymmetric capacitors are optimized using the capacitance and the potential stability limits of the electrodes, with the reliability of the design largely depending on the accuracy and the approach taken for the electrochemical characterization. Therefore, the performance could be lower than expected and even the system could break down, if a well thought out procedure is not followed. In this work, a procedure for the development of asymmetric supercapacitors based on activated carbons is detailed. Three activated carbon materials with different textural properties and surface chemistry have been systematically characterized in neutral aqueous electrolyte. The asymmetric configuration of the masses of both electrodes in the supercapacitor has allowed to cover a higher potential window, resulting in an increase of the energy density of the three devices studied when compared with the symmetric systems, and an improved cycle life.

Novel synthesis of a micro-mesoporous nitrogen-doped nanostructured carbon from polyaniline

Nanostructured carbons with relatively high nitrogen content (3–8%) and different micro and mesoporosity ratio were prepared by activation of polyaniline (PANI) with a ZnCl2–NaCl mixture in the proportion of the eutectic (melting point 270 °C). It was found that the activated carbons consisted of agglomerated nanoparticles. ZnCl2 plays a key role in the development of microporosity and promotes the binding between PANI nanoparticles during heat treatment, whereas NaCl acts as a template for the development of mesoporosity of larger size. Carbons with high micropore and mesopore volumes, above 0.6 and 0.8 cm3/g, respectively, have been obtained. Furthermore, these materials have been tested for CO2 capture and storage at pressures up to 4 MPa. The results indicate that the nitrogen groups present in the surface do not seem to affect to the amount of CO2 adsorbed, not detecting strong interactions between CO2 molecules and nitrogen functional groups of the carbon, which are mainly pyridinic and pyrrolic groups.