Robert Mokaya

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.

High density hydrogen storage in superactivated carbons from hydrothermally carbonized renewable organic materials

Hydrothermally carbonized organic materials (furfural, glucose, starch, cellulose and eucalyptus sawdust) have been used as precursors to produce high-surface area carbons. The synthesis methodology comprises two steps: (i) hydrothermal carbonization of organic materials and (ii) chemical activation with KOH as activating agent. In this way, activated carbon materials with a high surface area (up to 2700 m2 g−1) and narrow micropore size distribution in the supermicropore range (0.7–2 nm) are produced. The textural properties of the activated carbon products can be easily tuned by modifying the activating conditions (i.e., the activation temperature and the amount of KOH used). The activated carbon materials exhibit high hydrogen uptakes, up to 6.4 wt%, and large isosteric heats of adsorption, up to 8.5 kJ mol−1. In particular, the hydrogen storage density of the carbons is high and ranges between 12 and 16.4 µmol H2 m−2. The hydrogen storage density is closely related to the pore size of the carbons, with small micropores (ca. 1 nm) favouring a high density. Taking into account the high hydrogen storage capacities of these materials, as well as the simplicity of their synthesis procedure and the ready availability and low-cost of the raw precursors, it can be concluded that these activated carbons constitute a promising adsorbent for hydrogen storage.

Acidity and catalytic activity of the mesoporous aluminosilicate molecular sieve MCM-41

The acidity and catalytic properties of aluminosilicate mesoporous molecular sieves with the MCM-41 structure and bulk Si/Al ratios in the 10–60 range have been investigated. The incorporation of 4-coordinate aluminium into the structure of MCM-41 generates both BrØnsted and Lewis acid sites in amounts increasing with the degree of incorporation. However, the BrØnsted/Lewis acid population ratio is independent of the content of aluminium. The number and strength of acid sites generated are comparable to those of a pillared acid-activated clay and lower than in zeolite H-Y with Si/Al=3.65. Aluminosilicate MCM-41 is a moderate catalyst for the conversion of cumene which proceeds predominantly via catalytic cracking to propene and benzene. The sample of MCM-41 with the highest content of framework aluminium (Si/Al=10) has the largest number of BrØnsted acid sites and exhibits highest catalytic activity.

Hydrogen Storage in High Surface Area Carbons: Experimental Demonstration of the Effects of Nitrogen Doping

The influence of nitrogen doping on the hydrogen uptake and storage capacity of high surface area carbon materials is presented in this report. To generate suitable study materials, we have exploited the relationship between synthesis conditions and textural properties of zeolite-templated carbons to generate a range of high surface area carbons with similar pore size distribution but which are either N-doped or N-free. For N-doped carbons, the nitrogen content was kept within a narrow range of between 4.7 and 7.7 wt %. The carbon materials, irrespective of whether they were doped or not, exhibited high surface area (1900-3700 m(2)/g) and pore volume (0.99 and 1.88 cm(3)/g), a micropore surface area of 1500-2800 m(2)/g, and a micropore volume of 0.65-1.24 cm(3)/g. The hydrogen uptake varied between 4.1 and 6.9 wt %. We present experimental data that indicates that the effect of N-doping on hydrogen uptake is only apparent when related to the surface area and pore volume associated with micropores rather than total porosity. Furthermore, by considering the isosteric heat of hydrogen adsorption and excess hydrogen uptake on N-free or N-doped carbons, it is shown that N-doping can be beneficial at lower coverage (low hydrogen uptake) but is detrimental at higher coverage (higher hydrogen uptake). The findings are consistent with previous theoretical predictions on the effect of N-doping of carbon on hydrogen uptake. The findings, therefore, add new insights that are useful for the development of carbon materials with enhanced hydrogen storage capacity.

Ultrastable Mesoporous Aluminosilicates by Grafting Routes.

A combination of postsynthesis grafting and hydrothermal treatment offers an excellent route for the synthesis of ultrastable mesoporous aluminosilicates with enhanced acidity and catalytic activity. The stability observed (>150 h in boiling water; 4 h at 1000 degrees C) is, for mesoporous silicates, remarkable. Unusually the hydrothermal treatment is beneficial with respect to the use of the stable aluminosilicates as solid acid catalysts.

Preparation and hydrogen storage capacity of templated and activated carbons nanocast from commercially available zeolitic imidazolate framework

A commercially available zeolitic imidazolate framework (ZIF), namely Basolite Z1200 (BASF), has been used as template for nanocasting of highly microporous ZIF-templated carbon. The “hard template carbonization technique” consists of liquid impregnation of furfuryl alcohol into the pores of the ZIF followed by polymerization and then carbonization during which the ZIF template is removed to generate the microporous carbon (90–95% microporosity) with a surface area of 900–1100 m2 g−1 and a pore volume of ca. 0.7 cm3 g−1. Chemical activation (with KOH at a relatively low temperature of 700 °C for 1 h and a carbon/KOH weight ratio of 1 : 4) of the ZIF-templated carbons increases their porosity by between 30 and 240% depending on their carbonization temperature, to achieve a surface area of up to 3200 m2 g−1 and a pore volume of 1.94 cm3 g−1. Despite the drastic increase in porosity, the activated ZIF-templated carbons retain significant microporosity with micropores contributing 80–90% of surface area and 60–70% of pore volume. This occurs because the activation process mainly enhances existing porosity rather than creating new larger pores. The activation enhances the hydrogen uptake capacity of the ZIF-templated carbons by between 25 and 140% from 2.6–3.1 wt% to the range 3.9–6.2 wt% (at −196 °C and 20 bar). The increase in hydrogen uptake after activation is closely related to rises in the micropore surface area and micropore volume rather than overall increase in porosity. Due to their microporous nature, the carbons exhibit high hydrogen storage density in the range 13.0–15.5 μmol H2 m−2, which is much higher than that of most high surface area activated carbons.

Hollow spheres of crystalline porous metal oxides: A generalized synthesis route via nanocasting with mesoporous carbon hollow shells

Hollow spheres and shells of crystalline porous metal oxides have been nanocast using hollow spheres of mesoporous carbon as hard template. The metal oxides are fabricated from alkoxide precursors within the pore channels of the carbon templates. Remarkably, only one infiltration cycle was required to introduce (metal alkoxide) molecular precursors into the pores of the carbon templates. Removal of the carbon by calcination (at 500–600 °C) results in porous metal oxides with predominantly hollow sphere morphology, thus demonstrating the replication of the hollow sphere morphology from carbon to metal oxide. The metal oxides (titania, zirconia, alumina and magnesia) exhibit highly crystalline frameworks and relatively high surface area. The surface area is particularly high for alumina (γ-Al2O3, 212 m2 g−1) and titania (anatase, 100 m2 g−1). Mixed (MgO–Al2O3) or binary (MgTiO3) metal oxides with relatively well formed hollow sphere morphology and high surface area (154 m2 g−1 for MgTiO3 and 322 m2 g−1 for MgO–Al2O3) may also be nanocast.

On the synthesis and characterization of ZSM-5/MCM-48 aluminosilicate composite materials

Composite micro/mesoporous ZSM-5/MCM-48 materials have been prepared using a simple two step crystallisation process. The synthesis process involved the assembly of precursor zeolite species, containing ZSM-5 units at various stages of crystallisation, into a mesostructured material. By varying the time allowed for the crystallisation of the precursor zeolite species (between 2 and 8 hours) it was possible to modify the composition of the composite materials (i.e., zeolite/MCM-48 ratio). Furthermore, it was possible to obtain relatively pure forms of both end members (ZSM-5 and MCM-48) from the precursor zeolite species after appropriate aging. The resulting composite materials were characterized by powder XRD, nitrogen sorption, IR spectroscopy, TGA, SEM and TEM and found to exhibit varying levels of zeolitisation. The ZSM-5 present in the composite materials was made up of nanosized zeolite crystallites. The textural properties of the composite materials were, in some cases, comparable to conventional mesoporous materials. The acidity and hydrothermal stability of the composite materials was found to be dependent on the extent of zeolitisation, i.e., ZSM-5/MCM-48 ratio.

Electrical Double-Layer Capacitance of Zeolite-Templated Carbon in Organic Electrolyte

Downlo Electrical Double-Layer Capacitance of Zeolite-Templated Carbon in Organic Electrolyte C. Portet,* Z. Yang, Y. Korenblit, Y. Gogotsi,** R. Mokaya, and G. Yushin A. J. Drexel Nanotechnology Institute and Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA School of Chemistry, University of Nottingham, Nottingham NG7 2RD, United Kingdom School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA

Ultrahigh surface area polypyrrole-based carbons with superior performance for hydrogen storage

Ultrahigh surface area carbons (3000–3500 m2 g−1) have been obtained via chemical activation of polypyrrole with KOH. The carbon materials exhibit large pore volumes (up to ∼2.6 cm3 g−1) and possess two pore systems: one of pores in the micropore range (∼1.2 nm) and the other in the small mesopore range (2.2–3.4 nm). Some carbons also possess a significant proportion of 0.6 nm pores. Tuning of the carbon textural properties was achieved through the control of the activation parameters (temperature and amount of KOH). The carbons achieve an excellent hydrogen storage capacity of up to 7.03 wt% at −196 °C and 20 bar, which is the highest ever reported for one-step activated carbons and amongst the best for any porous material. The gravimetric hydrogen uptake of the carbons translates to a very attractive volumetric density of up to 37 g H2 L−1 at 20 bar. These carbons exhibit excellent gravimetric and volumetric capacity due to the fact that their high porosity is not at the detriment of packing density.