Biotemplating of Al2O3-Doped, CaO-Based Material from Bamboo Fiber for Efficient Solar Energy Storage

The high-temperature sintering of CaO-based materials leads to the serious decay of energy storage performance during the calcination/carbonation cycle. To overcome the loss in porosity problem, an efficient CaO-based material for thermal energy storage was synthesized using bamboo fiber as the biotemplate. The synthesis parameters (bamboo fiber addition, pyrolysis, Al2O3 loading) and the energy storage reaction characteristics of CaO-based energy storage material were optimized on the basis of cyclic calcination/carbonation experiments. The results show that the sacrificed biotemplate enhances the porosity of the synthetic material, denoting improved energy storage density. The cumulative energy storage density of the templated material over 50 cycles is 24,131.44 kJ/kg higher than that of limestone. The carbonation conversion and energy storage density of the templated CaO-based material doped with 5 wt.% Al2O3 and 0.5 g bamboo fiber reach 0.75 mol/mol and 2368.82 kJ/kg after 10 cycles, respectively, which is 2.7 times as high as that of original limestone. The maximum apparent carbonation rate of the templated CaO-based materials in the 1st cycle corresponds to a 240% increment compared to limestone. The maximum calcination rate of the synthetic CaO-based material in the 12th cycle remains 93%, as compared with the initial cycle. The microstructure analysis reveals that the hierarchically-stable structure during the cycle is beneficial for a more effective exposure of surface reactive sites for CaO and inward/outward diffusion for CO2 molecules through CaO. The method using the sacrificed biological template provides an advanced approach to fabricate porous materials, and the composite CaO-based material provides high-return solar energy storage for a potential application in industrial scale.

CO2 Capture at Medium to High Temperature Using Solid Oxide-Based Sorbents: Fundamental Aspects, Mechanistic Insights, and Recent Advances

Carbon dioxide capture and mitigation form a key part of the technological response to combat climate change and reduce CO₂ emissions. Solid materials capable of reversibly absorbing CO₂ have been the focus of intense research for the past two decades, with promising stability and low energy costs to implement and operate compared to the more widely used liquid amines. In this review, we explore the fundamental aspects underpinning solid CO₂ sorbents based on alkali and alkaline earth metal oxides operating at medium to high temperature: how their structure, chemical composition, and morphology impact their performance and long-term use. Various optimization strategies are outlined to improve upon the most promising materials, and we combine recent advances across disparate scientific disciplines, including materials discovery, synthesis, and in situ characterization, to present a coherent understanding of the mechanisms of CO₂ absorption both at surfaces and within solid materials.

Mechanistic Understanding of CaO-Based Sorbents for High-Temperature CO2 Capture: Advanced Characterization and Prospects

Carbon dioxide capture and storage technologies are short to mid-term solutions to reduce anthropogenic CO₂ emissions. CaO-based sorbents have emerged as a viable class of cost-efficient CO₂ sorbents for high temperature applications. Yet, CaO-based sorbents are prone to deactivation over repeated CO₂ capture and regeneration cycles. Various strategies have been proposed to improve their cyclic stability and rate of CO₂ uptake including the addition of promoters and stabilizers (e. g., alkali metal salts and metal oxides), as well as nano-structuring approaches. However, our fundamental understanding of the underlying mechanisms through which promoters or stabilizers affect the performance of the sorbents is limited. With the recent application of advanced characterization techniques, new insight into the structural and morphological changes that occur during CO₂ uptake and regeneration has been obtained. This review summarizes recent advances that have improved our mechanistic understanding of CaO-based CO₂ sorbents with and without the addition of stabilizers and/or promoters, with a specific emphasis on the application of advanced characterization techniques.

Accurate Control of Cage-Like CaO Hollow Microspheres for Enhanced CO2 Capture in Calcium Looping via a Template-Assisted Synthesis Approach

Herein we report the development of synthetic CaO-based sorbents for enhanced CO₂ capture in calcium looping via a template-assisted synthesis approach, where carbonaceous spheres (CSs) derived from hydrothermal reaction of starch are used as the templates. Cage-like CaO hollow microspheres are successfully synthesized only using urea as the precipitant, and the formation mechanism of this unique hollow microsphere structure is discussed deeply. Moreover, cage-like CaO hollow microspheres possess an initial carbonation conversion of 98.2% and 82.5% under a mild and harsh conditions, respectively. After the 15 cycles, cage-like CaO hollow microspheres still possess a carbonation value of 49.2% and 39.7% under the corresponding conditions, exceeding the reference limestone by 85.7% and 148.1%, respectively. Two kinetic models are used to explore the mechanism of carbonation reaction for cage-like CaO hollow microspheres, which are subsequently proved to be feasible for analysis of chemical-controlled stage and diffusion-controlled stage in the carbonation process. It is found the unique hollow microsphere structure can significantly reduce the activation energy of carbonation reaction according to the kinetic calculation. Furthermore, the energy and raw material consumptions related to the synthesis of cage-like CaO hollow microspheres are analyzed by the life cycle assessment (LCA) method.

Optimization of the structural characteristics of CaO and its effective stabilization yield high-capacity CO2 sorbents

Calcium looping, a CO₂ capture technique, may offer a mid-term if not near-term solution to mitigate climate change, triggered by the yet increasing anthropogenic CO₂ emissions. A key requirement for the economic operation of calcium looping is the availability of highly effective CaO-based CO₂ sorbents. Here we report a facile synthesis route that yields hollow, MgO-stabilized, CaO microspheres featuring highly porous multishelled morphologies. As a thermal stabilizer, MgO minimized the sintering-induced decay of the sorbents’ CO₂ capacity and ensured a stable CO₂ uptake over multiple operation cycles. Detailed electron microscopy-based analyses confirm a compositional homogeneity which is identified, together with the characteristics of its porous structure, as an essential feature to yield a high-performance sorbent. After 30 cycles of repeated CO₂ capture and sorbent regeneration, the best performing material requires as little as 11 wt.% MgO for structural stabilization and exceeds the CO₂ uptake of the limestone-derived reference material by ~500%.

The economic operation of a carbon dioxide capture technique of calcium looping necessitates highly effective CaO-based CO₂ sorbents. Here, the authors report a facile one-pot synthesis approach to yield highly effective, MgO-stabilized, CaO-based CO₂ sorbents featuring highly porous multishelled morphologies.

Multishelled CaO Microspheres Stabilized by Atomic Layer Deposition of Al2 O3 for Enhanced CO2 Capture Performance

CO₂ capture and storage is a promising concept to reduce anthropogenic CO₂ emissions. The most established technology for capturing CO₂ relies on amine scrubbing that is, however, associated with high costs. Technoeconomic studies show that using CaO as a high-temperature CO₂ sorbent can significantly reduce the costs of CO₂ capture. A serious disadvantage of CaO derived from earth-abundant precursors, e.g., limestone, is the rapid, sintering-induced decay of its cyclic CO₂ uptake. Here, a template-assisted hydrothermal approach to develop CaO-based sorbents exhibiting a very high and cyclically stable CO₂ uptake is exploited. The morphological characteristics of these sorbents, i.e., a porous shell comprised of CaO nanoparticles coated by a thin layer of Al₂ O₃ (<3 nm) containing a central void, ensure (i) minimal diffusion limitations, (ii) space to accompany the substantial volumetric changes during CO₂ capture and release, and (iii) a minimal quantity of Al₂ O₃ for structural stabilization, thus maximizing the fraction of CO₂ -capture-active CaO.