• Energy Research

The current work relates to the development of synthetic calcium oxide (CaO) based compositions as candidate materials for energy storage under a cyclic carbonation/decarbonation reaction scheme. Although under such a cyclic scheme the energy density of natural lime based CaO is high (∼ 3MJ/kg), the particular materials suffer from notable cycle-to-cycle deactivation. To this direction, pure CaO and CaO/Al2O3 composites have been prepared and preliminarily evaluated under the suggested cyclic carbonation/decarbonation scheme in the temperature range of 600-800°C. For the composite materials, Ca/Al molar ratios were in the range between 95/5 and 52/48 and upon calcination the formation of mixed Ca/Al phases was verified. The preliminary evaluation of materials studied was conducted under 3 carbonation/decarbonation cycles and the loss of activity for the case of natural CaO was obvious. Synthetic materials with superior stability/capture c.f. natural CaO were further subjected to multi-cyclic carbonation/decarbonation, via which the positive effect of alumina addition was made evident. Selected compositions exhibited adequately high CO2 capture capacity and stable performance during multi-cyclic operation. Moreover, this study contains preliminary experiments referring to proof-of-principle validation of a concept based on the utilization of a CaO-based honeycomb reactor/heat exchanger preliminary design. In particular, cordierite monolithic structures were coated with natural CaO and in total 11 cycles were conducted. Upon operation, clear signs of heat dissipation by the imposed flow in the duration of the exothermic reaction step were identified.

Abstract Physisorption of hydrogen in nanoporous materials offers an efficient and competitive alternative for hydrogen storage. At low temperatures (e.g. 77 K) and moderate pressures (below 100 bar) molecular H2 adsorbs reversibly, with very fast kinetics, at high density on the inner surfaces of materials such as zeolites, activated carbons and metal–organic frameworks (MOFs). This review, by experts of Task 40 ‘Energy Storage and Conversion based on Hydrogen’ of the Hydrogen Technology Collaboration Programme of the International Energy Agency, covers the fundamentals of H2 adsorption in nanoporous materials and assessment of their storage performance. The discussion includes recent work on H2 adsorption at both low temperature and high pressure, new findings on the assessment of the hydrogen storage performance of materials, the correlation of volumetric and gravimetric H2 storage capacities, usable capacity, and optimum operating temperature. The application of neutron scattering as an ideal tool for characterising H2 adsorption is summarised and state-of-the-art computational methods, such as machine learning, are considered for the discovery of new MOFs for H2 storage applications, as well as the modelling of flexible porous networks for optimised H2 delivery. The discussion focuses moreover on additional important issues, such as sustainable materials synthesis and improved reproducibility of experimental H2 adsorption isotherm data by interlaboratory exercises and reference materials.

Abstract The efficient storage of energy combined with a minimum carbon footprint is still considered one of the major challenges towards the transition to a progressive, sustainable and environmental friendly society on a global scale. The energy storage in pure chemical form using gas carriers with high heating values, including H 2 and CH 4 , as well as via electrochemical means using state-of-the-art devices, such as batteries or supercapacitors, are two of the most attractive alternatives for the combustion of finite, carbon-rich and environmentally harmful fossil fuels, such as diesel and gasoline. A few-step, reproducible and scalable method is presented in this study for the preparation of an ultra-microporous (average pore size around 0.6 nm) activated carbon cloth (ACC) with large specific area (> 1200 m 2 /g) and pore volume (~ 0.5 cm 3 /g) upon combining chemical impregnation, carbonization and CO 2 activation of a low-cost cellulose-based polymeric fabric. The ACC material shows a versatile character towards three different applications, including H 2 storage via cryo-adsorption, separation of energy-dense CO 2 /CH 4 mixtures via selective adsorption and electrochemical energy storage using supercapacitor technology. Fully reversible H 2 uptake capacities in excess of 3.1 wt% at 77 K and ~ 72 bar along with a significant heat of adsorption value of up to 8.4 kJ/mol for low surface coverage have been found. Upon incorporation of low-pressure sorption data in the ideal adsorbed solution theory model, the ACC is predicted to selectively adsorb about 4.5 times more CO 2 than CH 4 in ambient conditions and thus represents an appealing adsorbent for the purification of such gaseous mixtures. Finally, an electric double-layer capacitor device was assembled and tested for its electrochemical performance, constructed of binder-free and flexible ACC electrodes and aqueous CsCl electrolyte. The full-cell exhibits a gravimetric capacitance of ~ 121 F/g for a specific current of 0.02 A/g, which relative to the ACC's specific area, is superior to commercially available activated carbons. A capacitance retention of more than 97% was observed after 10,000 charging/discharging cycles, thus indicating the ACC's suitability for demanding and high-performance energy storage on a commercial scale. The enhanced performance in all tested applications seems to be attributed to the mean ultra-micropore size of the ACC material instead of the available specific area and/or pore volume.

The solid-state pyrolysis of acetylenedicarboxylic acid, monopotassium salt at 300 °C in air results in bulk quantities of a micron-sized yet macroporous oxidized carbon, which inherently possesses high content of metal-binding sites, such as carboxylate groups, free radicals, and ether/hydroxyl units. Besides its high oxygen content, the solid is stable in water and does not leach or disorient, while it also exhibits an appreciable thermal stability, at temperature exceeding 200 °C in air. Several techniques including TEM/SEM, TGA, Raman/FT-IR, XPS, EPR, and potentiometric titrations were employed for the characterization of the solid. Furthermore, liquid phase adsorption experiments revealed that the material is an efficient heavy metal adsorbent due to the presence of diverse surface-accessible binding sites, showing unusually high metal uptake capacities for Pb2+ and Cu2+ ions (ca. 4.5 mmol g−1).

The cyclic carbonation/calcination reaction of CaO is discussed as a thermochemical energy storage system. Especially the high reaction temperature enables high theoretical energetic efficiencies. A severe issue is the strong cycle-to-cycle degradation of the material due to sintering. In order to overcome this, two different approaches are studied in this work: (1) Intermediate hydration of natural CaO to regenerate the sorbent. (2) Preparation of pure CaO and CaO/Al2O3 composites with different Ca/Al molar ratios. All materials prepared are structurally and morphologically characterized and for the evaluation of the sorbents, the CO2 uptake capacity during carbonation reaction is measured over multiple cycles. Besides the successful proof of an optimized cyclic stability, the energetic efficiency and storage density of the synthesized samples is calculated and compared to the benchmark material, natural CaO. In case of storage density, values of up to 3.5 times and in case of energetic efficiency, a factor of 1.2 referred to natural CaO are obtained within the 20th cycle.