Review of liquid nano-absorbents for enhanced CO2 capture

Molecular Understanding of CO2 Absorption by Choline Chloride/Urea Confined within Nanoslits

Clarifying the potential relationship between the microstructure of nanoconfined choline chloride/urea (ChClU) and CO2 absorption performance is key to understanding the abnormal increase in CO2 under nanoconfinement. In this study, we used molecular dynamics simulations and grand canonical Monte Carlo (GCMC) to systematically study the mechanism underlying the absorption of CO2 by ChClU within nanoslits. According to the spatial distribution, ChClU can form two different laminar regions within nanoslits, namely, the interfacial region (region I) and beyond region I (region II). In region II, the interface induces rearrangement of ChClU, resulting in an increase in free volume and subsequent increase in CO2 solubility. In region I, changing the interface from hydrophobic to hydrophilic (e.g., S_I to S_IV) by setting the appropriate charge patterns, the urea molecules gradually change from "disordered" to "ordered standing" relative to the solid surface. The preferential orientation of the urea molecules causes competition between the ChClU's free volume and urea molecules, resulting in a non-monotonic change in CO2 solubility. Specifically, from S_I to S_III, the increase in urea molecules enhances the CO2 solubility. In S_IV, space for CO2 absorption is insufficient due to the accumulation of urea molecules, and thus CO2 solubility decreases.

Liquid CO2-Capture Technologies: A Review

Escalating global carbon dioxide (CO2) emissions have significantly exacerbated the climate impact, necessitating imperative advancements in CO2-capture technology. Liquid absorbents have received considerable attention in carbon capture for engineering applications, due to their high flexibility, reliability, and recyclability. Nonetheless, the existing technologies of liquid CO2 capture suffer from various issues that cannot be ignored, such as corrosion, elevated costs, and pronounced secondary pollution. More efforts are required to realize process optimization and novel absorbent innovation. This review presents nanofluids and other novel liquid absorbents such as ionic liquids, amino acids, and phase-change absorbents. The preparation, mechanisms of action, and influencing factors of nanofluid absorbents are discussed in detail to provide researchers with a comprehensive understanding of their potential applications. Further, the challenges (including energy loss, environmental and human health, barriers to application and capture performance, etc.) encountered by these innovative absorbents and techniques are also commented on. This facilitates side-by-side comparisons by researchers.

Electrochemical CO2 Conversion Commercialization Pathways: A Concise Review on Experimental Frontiers and Technoeconomic Analysis

Technoeconomic analysis (TEA) studies are vital for formulating guidelines that drive the commercialization of electrochemical CO2 reduction (eCO2R) technologies. In this review, we first discuss the progress in the field of eCO2R processes by providing current state-of-the-art metrices (e.g., faradic efficiency, current density) based on the recent heterogeneous catalysts' discovery, electrolytes, electrolyzers configuration, and electrolysis process designs. Next, we assessed the TEA studies for a wide range of eCO2R final products, different modes of eCO2R systems/processes, and discussed their relative competitiveness with relevant commercial products. Finally, we discuss challenges and future directions essential for eCO2R commercialization by linking suggestions from TEA studies. We believe that this review will catalyze innovation in formulating advanced eCO2R strategies to meet the TEA benchmarks for the conversion of CO2 into valuable chemicals at the industrial scale.

Quaternized Graphene for High-Performance Moisture Swing Direct Air Capture of CO2

Nowadays, moisture-swing adsorption technology still relies on quaternary ammonium resins with limited CO2 capacity under ambient air conditions. In this work, a groundbreaking moisture-driven sorbent is developed starting from commercial graphene flakes and using glycidyltrimethylammonium chloride for incorporation of CO2-sensitive quaternary ammonium functional groups. Boasting an outstanding CO2 capture performance under ultra-diluted conditions (namely, 3.24 mmol g-1 at CO2 400 ppm and 20% RH), the functionalized sorbent (fGO) features clear competitive advantages over current technologies for direct air capture. Notably, fGO demonstrated unprecedented moisture-swing capacity, ease of regenerability, versatility, selectivity, and longevity. These distinctive features position the fGO as an advanced and promising solution, showcasing its potential to outperform existing methods for moisture-swing direct air capture of CO2.

Electrolyte-Electrocatalyst Interfacial Effects of Polymeric Materials for Tandem CO2 Capture and Conversion Elucidated Using In Situ Electrochemical AFM

Integrating CO2 capture and electrochemical conversion has been proposed as a strategy to reduce the net energy required for CO2 regeneration in traditional CO2 capture and conversion schemes and can be coupled with carbon-free renewable electricity. Polyethylenimine (PEI)-based materials have been previously studied as CO2 capture materials and can be integrated in these reactive capture processes. PEI-based electrolytes have been found to significantly increase the CO2 loading, and impact selectivity and rate of product formation when compared to the conventional aqueous electrolytes. However, the influence of these materials at the catalyst-electrode interface is currently not well understood. In this study, PEI-based electrolytes were prepared and their impact on the morphology of a silver electrode performing electrochemical CO2 reduction (CO2R) was studied using in situ electrochemical atomic force microscopy (EC-AFM). The presence of PEI on the electrode surface could be distinguished based on nanomechanical properties (DMT modulus), and changes were observed as negative polarization was applied, revealing a reorganization of the PEI chains due to electrostatic interactions. These changes were impacted by the electrolyte composition, including the addition of supporting electrolyte KHCO3 salt, as well as CO2 captured by the PEI-based electrolyte, which minimized the change in surface mechanical properties and degree of PEI alignment on the electrode surface. The changes in surface mechanical properties were also dependent on the PEI polymer length, with higher molecular weight PEI showing different reconfiguration than the shorter polymer brushes. The study highlights that the choice of polymer material, the electrolyte composition, and CO2 captured impact the near-electrode environment, which has implications for CO2R, and presents EC-AFM as a new tool that can be used to probe the dynamic behavior of these interfaces during electrocatalysis.

Graphene Oxide Enhanced Monoethanolamine and Ethylenediamine Nanofluids for Efficient Carbon Dioxide Uptake from Flue Gas

The addition of nanoparticles in amine solutions to produce a stable amine-based nanofluid provides a high surface area for absorption and improves the absorption rate. In this work, nanofluids were prepared by dispersing graphene oxide (GO) in monoethanolamine (MEA) and ethylenediamine (EDA) solutions for adsorption of carbon dioxide (CO2) to further improve their absorption performance by providing more reaction sites on the GO framework. GO was synthesized using the modified Hummers method and characterized for physicochemical properties using SEM, EDS, FTIR, Raman analysis, and TGA. The FTIR spectra for the GO nanoparticles before absorption showed peaks attributed to C-C, H-C, and C-O bonding. After the absorption experiments, the FTIR spectra of GO showed peaks due to C-O-NH2, N-O-N, and N-H bonding. The BET analysis further confirmed the decrease in the surface area, pore volume, and pore diameter of the GO recovered from the nanofluids after the CO2 experiment, indicating an interaction between GO and amine molecules. The absorption process of CO2 by the nanofluid was performed in a custom-made pressure chamber whereby the CO2 gas was in direct contact with the absorption fluids. The obtained adsorption rate constant (k) for the reaction between CO2 and 30% MEA and EDA solutions was 0.113 and 0.131, respectively. Upon addition of 0.2 mg/mL GO in the base solution, k increased to 0.16854 and 0.17603 for the MEA and EDA nanofluids, respectively. The proposed mechanism involves GO nanoparticles interacting with the amine groups through the oxygen-rich groups of GO. This results in the formation of a zwitterion that readily reacts with CO2, resulting in a carbamate.

Molecular Recognition between Carbon Dioxide and Biodegradable Hydrogel Models: A Density Functional Theory (DFT) Investigation

In this research, we explore the potential of employing density functional theory (DFT) for the design of biodegradable hydrogels aimed at capturing carbon dioxide (CO2) and mitigating greenhouse gas emissions. We employed biodegradable hydrogel models, including polyethylene glycol, polyvinylpyrrolidone, chitosan, and poly-2-hydroxymethacrylate. The complexation process between the hydrogel and CO2 was thoroughly investigated at the ωB97X-D/6-311G(2d,p) theoretical level. Our findings reveal a strong affinity between the hydrogel models and CO2, with binding energies ranging from -4.5 to -6.5 kcal/mol, indicative of physisorption processes. The absorption order observed was as follows: chitosan > PVP > HEAC > PEG. Additionally, thermodynamic parameters substantiated this sequence and even suggested that these complexes remain stable up to 160 °C. Consequently, these polymers present a promising avenue for crafting novel materials for CO2 capture applications. Nonetheless, further research is warranted to optimize the design of these materials and assess their performance across various environmental conditions.

Interfacial-interaction induced modifications in segmental dynamics and free volume of the poly(ethyleneimine) canopy in nanoparticle-organic hybrid materials: an investigation by temperature dependent broadband dielectric and positron annihilation spectroscopy

Broadband dielectric spectroscopy, positron annihilation lifetime spectroscopy and differential scanning calorimetry are used to investigate the molecular dynamics, free volume and thermal behaviour of a poly(ethyleneimine), PEI, canopy in liquid-like nanoparticle-organic hybrid materials (NOHMs) consisting of alumina nanoparticles and nanorods as inorganic nanocores. It is confirmed that the highly branched PEI canopy in liquid-like NOHMs possesses an ordered structure having less entanglements of the side chains as compared to neat PEI. The size of the free volumes associated with the PEI canopy for nanoparticle- and nanorod-based NOHMs is larger and smaller, respectively, as compared to neat PEI. The time scales characterizing the segmental dynamics and side chain relaxations of the canopy in the nanorod-based NOHM are slowed down drastically as compared to the nanoparticle-based NOHM and neat PEI. These results confirm that the shape of the inorganic cores plays a deterministic role toward the structural arrangement (free volume) and segmental dynamics of the canopy polymer in liquid-like NOHMs.

Experimental study of CO2 capture by nanoparticle-enhanced 2-amino-2-methyl-1-propanol aqueous solution

2-Amino-2-methyl-1-propanol (AMP) is often used as a moderator to enhance the CO2 capture capacity of absorbents due to its unique spatial site resistance structure, and relatively few studies have been conducted on the enhancement of AMP aqueous solutions by nanoparticles for CO2 capture. In order to investigate the effect of nanoparticles on the CO2 capture performance of AMP aqueous solution, different nanofluids were formulated in this paper using a two-step method, and a bubbling reactor and an oil bath were used as the experimental setup for absorption/desorption, and through comparative experiments, it was found that the type of nanoparticles, the solid content, and the different parameters have great influences on the CO2 absorption load and desorption rate. The experimental results show that the addition of TiO2 nanoparticles to the AMP base solution can accelerate the absorption-desorption mass transfer rate of CO2, and there exists an optimal solid content of 1 g L-1 (±1.0%, ±2.5%); after multiple absorption-desorption experiments, good cycling performance can still be achieved. The experimental results of the nanofluid-promoted mass transfer mechanism are also illustrated and analyzed in this paper.

Dual Superlyophobic Materials for Under-Liquid Microfluid Manipulation, Immiscible Solvent Separation, and CO2 Blockage

Oily water purification, immiscible solvent separation, sensitive microreaction, and CO₂ blockage are of great interest because of their importance for the environment and demands of controllable microreactions. However, one specific material that can meet all the requirements has yet to be reported. Herein, we developed a simple environment-benign method to prepare specific dual superlyophobic materials to solve the problems mentioned earlier. The dual superlyophobic materials can maintain their dual superoleophobicity in various oil/water systems, and no additional surface modifications were required when the oil/water system was changed. Moreover, the materials can be used to separate oil/water mixtures with separation efficiencies greater than 99.50% even after 40 separation cycles and separate immiscible organic solvents with efficiencies over than 99.25% after 20 cycles. Separations of meal waste oily water at 60 °C and crude oil/water were also successfully performed. The materials can be further applied to manipulate and block CO₂ bubbles under liquid. The materials can also act as a platform for microdrop manipulation/microreaction under liquid.

Single atom solutions for carbon dioxide capture

New solvents are considered to be one of the effective methods to facilitate the reaction rate and lower the reaction energy barrier. However, the common method to develop a new solvent has come to a dead end. Thus, a single atom in solvent to produce a single atom solution is designed to create the breakthrough. Eight kinds of single atom solutions are prepared as new absorbents. Experiments prove the single atom in the solutions and their charge-producing effects. A density functional theory model is developed to analyze the microscale characteristics. Meanwhile, it has been applied in carbon dioxide capture. The CO₂ desorption rate is intensified in the single atom solution system due to the controlled reaction energy barrier. The results show that single atom solutions produce a maximum voltage of 2.12 V and, thus, contribute to near zero energy consumption by effectively harvesting the substantial waste heat below 373 K.

One-step fabricated Zr-supported, CaO-based pellets via graphite-moulding method for regenerable CO2 capture

Calcium looping (CaL) belongs to a promising high-temperature CO₂ capture technology because of adopting cheap and extensive source CaO-based sorbents. However, CaO-based sorbents are prone to occur the issues of sintering and elutriation in the fluidized-bed reactors. To further enhance the practicability, the Zr-supported, CaO-based sorbent pellets were produced using the graphite-moulding method. Different synthesis modes (i.e., sol-gel, hydration-mixing, and wet-mixing) were compared for preparing CaO-based composite slurry before pelletization. Sol-gel is the promising synthesis mode to prepare Zr-supported, CaO-based pellets with outstanding CO₂ capture performance due to achieving a more uniform distribution of inert CaZrO₃ spacer. Moreover, the Zr-based stabilizer content within sorbent pellets produced by the combined method of sol-gel and graphite-moulding was further studied. The higher content of Zr-based stabilizer promotes the enhancement of cyclic stability and mechanical strength of Zr-supported, CaO-based pellets. After 17 cycles, the sorbent pellets containing 20 wt% of Zr-based stabilizer display a high CaO carbonation conversion of 74.1 %. Moreover, CaO-based pellets with 20 wt% of Zr-based stabilizer possess a higher compression strength of 4.84 ± 1.08 MPa, which is as high as 1.8 times that of the pellets with 5 wt% of Zr-based stabilizer.

Nanofluid preparation, stability and performance for CO2 absorption and desorption enhancement: A review

In recent years, the importance of capturing CO₂ has increased due to the necessity of minimizing climate change and the detrimental effects of CO₂ emissions from industrial processes. CO₂ absorption, as one of the most mature carbon capture technologies, has been improved by introducing nanosized particles into liquid absorbents. Nanofluids have been the subject of interest in many studies recently due to their tremendous impact on absorption. This review comprehensively examines the CO₂ absorption behavior for nanofluids through the investigation of different absorption systems. Potential mechanisms for improving the absorption/regeneration performance of nanoabsorbents as well as the synergistic effects of physicochemical properties of nanofluids, such as viscosity and density on CO₂ capture behavior, are reviewed. Nanofluid enhancement factors in terms of absorption rate and capacity towards CO₂ are also compiled. Mathematical models, which have been proposed for calculating mass transfer coefficient and mass diffusivity, are comprehensively outlined. The paper discusses conventional methods for nanofluid preparation affecting the physicochemical properties of nanofluids. Strategies for enhancing nanofluid stability, as well as approaches to examine their stability are discussed. Finally, nanoparticle concentration, types and size of them, and selection of the base liquid absorbent as the key factors influencing the CO₂ removal process by nanofluids, are considered in this paper, as well.

Effect of Nanoparticles on the Thermal Stability and Reaction Kinetics in Ionic Nanofluids

Nowadays, the incorporation of nanoparticles into thermal fluids has become one of the most suitable strategies for developing high-performance fluids. An unconventional improvement of thermo–physical properties was observed with the addition of 1% wt. of nanoparticles in different types of fluids, such as molten salts, allowing for the design of more thermally efficient systems using nanofluids. Despite this, there is a lack of knowledge about the effect that nanoparticles produce on the thermal stability and the decomposition kinetics of the base fluid. The present study performs IR- and UV-vis spectroscopy along with thermogravimetric analysis (TGA) of pure nitrate and nitrate based nanofluids with the presence of SiO₂ and Al₂O₃ nanoparticles (1% wt.). The results obtained support that nanoparticles accelerate the nitrate to nitrite decomposition at temperatures below 500 °C (up to 4%), thus confirming the catalytic role of nanoparticles in nanofluids.

A Review on Enhancing Solvent Regeneration in CO2 Absorption Process Using Nanoparticles

The employment of nanoparticles in solvents is a promising method to reduce the energy consumption during solvent regeneration. Numerous experimental and theoretical studies have been conducted to investigate the remarkable enhancement of nanoparticles. Yet, there are limited reviews on the mechanistic role of nanoparticles in enhancing the solvent regeneration performance. This review addresses the recent development on the employment of various nanoparticles, which include metals oxides, zeolites and mesoporous silicas, to enhance the mass and heat transfer, which subsequently minimize the solvent regeneration energy. The enhancement mechanisms of the nanoparticles are elaborated based on their physical and chemical effects, with a comprehensive comparison on each nanoparticle along with its enhancement ratio. This review also provides the criteria for selecting or synthesizing nanoparticles that can provide a high regeneration enhancement ratio. Furthermore, the future research prospects for the employment of nanoparticles in solvent regeneration are also recommended.

Nanoscale Hybrid Electrolytes with Viscosity Controlled Using Ionic Stimulus for Electrochemical Energy Conversion and Storage

As renewable energy is rapidly integrated into the grid, the challenge has become storing intermittent renewable electricity. Technologies including flow batteries and CO₂ conversion to dense energy carriers are promising storage options for renewable electricity. To achieve this technological advancement, the development of next generation electrolyte materials that can increase the energy density of flow batteries and combine CO₂ capture and conversion is desired. Liquid-like nanoparticle organic hybrid materials (NOHMs) composed of an inorganic core with a tethered polymeric canopy (e.g., polyetheramine (HPE)) have a capability to bind chemical species of interest including CO₂ and redox-active species. In this study, the unique response of NOHM-I-HPE-based electrolytes to salt addition was investigated, including the effects on solution viscosity and structural configurations of the polymeric canopy, impacting transport behaviors. The addition of 0.1 M NaCl drastically lowered the viscosity of NOHM-based electrolytes by up to 90%, reduced the hydrodynamic diameter of NOHM-I-HPE, and increased its self-diffusion coefficient, while the ionic strength did not alter the behaviors of untethered HPE. This study is the first to fundamentally discern the changes in polymer configurations of NOHMs induced by salt addition and provides a comprehensive understanding of the effect of ionic stimulus on their bulk transport properties and local dynamics. These insights could be ultimately employed to tailor transport properties for a range of electrochemical applications.

Application of Nanofluids in CO2 Absorption: A Review

The continuous release of CO2 into the atmosphere as a major cause of increasing global warming has become a growing concern for the environment. Accordingly, CO2 absorption through an approach with maximum absorption efficiency and minimum energy consumption is of paramount importance. Thanks to the emergence of nanotechnology and its unique advantages in various fields, a new approach was introduced using suspended particles in a base liquid (suspension) to increase CO2 absorption. This review article addresses the performance of nanofluids, preparation methods, and their stability, which is one of the essential factors preventing sedimentation of nanofluids. This article aims to comprehensibly study the factors contributing to CO2 absorption through nanofluids, which mainly addresses the role of the base liquids and the reason behind their selection.

Engineered assembly of water-dispersible nanocatalysts enables low-cost and green CO2 capture

Catalytic solvent regeneration has attracted broad interest owing to its potential to reduce energy consumption in CO₂ separation, enabling industry to achieve emission reduction targets of the Paris Climate Accord. Despite recent advances, the development of engineered acidic nanocatalysts with unique characteristics remains a challenge. Herein, we establish a strategy to tailor the physicochemical properties of metal-organic frameworks (MOFs) for the synthesis of water-dispersible core-shell nanocatalysts with ease of use. We demonstrate that functionalized nanoclusters (Fe₃O₄-COOH) effectively induce missing-linker deficiencies and fabricate mesoporosity during the self-assembly of MOFs. Superacid sites are created by introducing chelating sulfates on the uncoordinated metal clusters, providing high proton donation capability. The obtained nanomaterials drastically reduce the energy consumption of CO₂ capture by 44.7% using only 0.1 wt.% nanocatalyst, which is a ∽10-fold improvement in efficiency compared to heterogeneous catalysts. This research represents a new avenue for the next generation of advanced nanomaterials in catalytic solvent regeneration.

Catalytic solvent regeneration is of interest to reduce energy consumption in CO2 separation, however, the development of engineered nanocatalysts remains a challenge. Here, a new avenue is presented for the next generation of advanced metal-organic frameworks (MOFs) in energy-efficient CO2 capture.

Water-Dispersible Nanocatalysts with Engineered Structures: The New Generation of Nanomaterials for Energy-Efficient CO2 Capture

The high energy demand of CO₂ absorption-desorption technologies has significantly inhibited their industrial utilization and implementation of the Paris Climate Accord. Catalytic solvent regeneration is of considerable interest due to its low operating temperature and high energy efficiency. Of the catalysts available, heterogeneous catalysts have exhibited relatively poor performances and are hindered by other challenges, which have slowed their large-scale deployment. Herein, we report a facile and eco-friendly approach for synthesizing water-dispersible Fe₃O₄ nanocatalysts coated with a wide range of amino acids (12 representative molecules) in aqueous media. The acidic properties of water-dispersible nanocatalysts can be easily tuned by introducing different functional groups during the hydrothermal synthesis procedure. We demonstrate that the prepared nanocatalysts can be used in energy-efficient CO₂ capture plants with ease-of-use, at very low concentrations (0.1 wt %) and with extra-high efficiencies (up to ∼75% energy reductions). They can be applied in a range of solutions, including amino acids (i.e., short-chain, long-chain, and cyclic) and amines (i.e., primary, tertiary, and primary-tertiary mixture). Considering the superiority of the presented water-dispersible nanocatalysts, this technology is expected to provide a new pathway for the development of energy-efficient CO₂ capture technologies.

Dynamic Mixing Behaviors of Ionically Tethered Polymer Canopy of Nanoscale Hybrid Materials in Fluids of Varying Physical and Chemical Properties

An emerging area of sustainable energy and environmental research is focused on the development of novel electrolytes that can increase the solubility of target species and improve subsequent reaction performance. Electrolytes with chemical and structural tunability have allowed for significant advancements in flow batteries and CO₂ conversion integrated with CO₂ capture. Liquid-like nanoparticle organic hybrid materials (NOHMs) are nanoscale fluids that are composed of inorganic nanocores and an ionically tethered polymeric canopy. NOHMs have been shown to exhibit enhanced conductivity making them promising for electrolyte applications, though they are often challenged by high viscosity in the neat state. In this study, a series of binary mixtures of NOHM-I-HPE with five different secondary fluids, water, chloroform, toluene, acetonitrile, and ethyl acetate, were prepared to reduce the fluid viscosity and investigate the effects of secondary fluid properties (e.g., hydrogen bonding ability, polarity, and molar volume) on their transport behaviors, including viscosity and diffusivity. Our results revealed that the molecular ratio of secondary fluid to the ether groups of Jeffamine M2070 (λ_SF) was able to describe the effect that secondary fluid has on transport properties. Our findings also suggest that in solution, the Jeffamine M2070 molecules exist in different nanoscale environments, where some are more strongly associated with the nanoparticle surface than others, and the conformation of the polymer canopy was dependent on the secondary fluid. This understanding of the polymer conformation in NOHMs can allow for the better design of an electrolyte capable of capturing and releasing small gaseous or ionic species.

Spherical amine grafted silica aerogels for CO2 capture

The objective of this research was to develop a novel spherical amine grafted silica aerogel for CO₂ capture. A spherical silica gel was synthesized by dropping a sodium silicate based silica sol into an oil bath. Amine grafting was achieved by bonding 3-aminopropyltriethoxysilane onto the framework of the silica gel. The spherical amine grafted silica gels were dried using vacuum drying to prepare the spherical amine grafted silica aerogels (SASAs). The synthetic mechanism of the SASAs was proposed. The structures and the CO₂ adsorption performances of SASAs were researched. The amine loading of the SASAs increased with the grafting time, however, the specific surface area and pore volume sharply decreased owing to the blockage of the pore space. Excess amine loading led to the decrease of the CO₂ adsorption capacity. The optimal CO₂ adsorption capacity was 1.56 mmol g⁻¹ with dry 1% CO₂ and at 35 °C. This work provides a low-cost and environmentally friendly way to design a capable and regenerable adsorbent material.

The objective of this research was to develop a novel spherical amine grafted silica aerogel for CO₂ capture.