Mirko Kunowsky

Activated carbon monoliths for gas storage at room temperature

Porous materials are interesting candidates for gas storage in different applications. The present study analyses at room temperature the high pressure storage of H2, CH4 and CO2 in a number of porous carbons (eight monoliths and two powdered activated carbons). The samples cover a wide range of porosities and densities (monoliths having high porosity with moderate density or moderate porosity with high density) with the aim to discuss the relative importance that the sample surface area has on the volumetric storage capacity, in relation to the importance of the density of the material. Our results show that the gravimetric storage capacities of the three studied gases are controlled by the textural properties of the adsorbent, whereas the volumetric storage capacities are mainly controlled by the adsorbent density. High volumetric excess adsorption capacity values (for example, H2: 10 g l−1; CH4: 110 g l−1 and CO2: 440 g l−1) correspond to monoliths having high densities, despite their moderately developed porosities. This paper also compares these results with those obtained similarly (same gases and same experimental conditions) using the highest known surface area material (MOF-210). In summary, our volumetric results, obtained with commercially available ATMI monoliths and their CO2 activation, are, to the best of our knowledge, amongst the highest that have been reported; higher than the high surface area samples of the M3M monolith prepared from Maxsorb (SBET: 2610 m2 g−1) or MOF-210 (SBET: 6240 m2 g−1). Although a variety of MOFs have been reported to exceed our results, oftentimes these values are overestimated due to the fact that the volumetric capacity of MOFs was calculated using crystal density rather than experimentally measured density.

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.

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.