Nano-engineered 2D Materials for CO₂ Capture

Abstract

Recently, nano-engineered two-dimensional (2D) materials have gained immense interest in various applications, including CO₂ capture. The precise atomic structure of 2D nanomaterials introduced various significant characteristics required for specific applications. Increasing levels of CO₂ in the environment is a concerning topic for surviving a sustainable life on Earth. Therefore, CO₂ capture and conversion into useful products have been recognized as the best approach to reduce the CO₂ level in the atmosphere. To capture CO₂, several materials have been studied and emphasised about their advantages and disadvantages. The recent progress in 2D materials, especially graphene-based materials, has shown their potential in CO₂ capture. Graphene-based materials, transition metal dichalcogenides (TMDCs), 2D transition metal oxides (TMOs), MXenes, boron nitrides, carbon nitrides, 2D metal–organic frameworks (MOFs) etc., are the various examples of 2D materials, which have been investigated for CO₂ capture. This chapter aims to provide a brief overview of the recent advantages in the nano-engineering of the various 2D materials for CO₂ capture. In particular, the recent development of emerging strategies such as doping, defects engineering, hetero-structural designing, and architectural functionalization of 2D nanomaterials for enhanced CO₂ capture are discussed thoroughly. The challenges and future outcomes have also been highlighted, which will open the directions for future research.

14.1 Introduction

Industrial modernisation and increased living standards have deteriorated environmental health. The ever-increasing demand and consumption of energy leading to the sharp rise of CO₂ concentration in the environment globally, which is directly or indirectly responsible for the deteriorating climate situation and global warming. CO₂ is one of the prime greenhouse gases (GHGs), and its high levels are not beneficial for the health of planet Earth. Several agencies noted that the CO₂ level in the atmosphere has surpassed the 415 ppm, which is the maximum record yet and alarming the current situation [1]. IPCC (International Panel on Climate Change) has also predicted that by 2100, the level of CO₂ will reach 590 ppm, which will raise the global temperature by 1.90 °C [2]. This exponential rise of CO₂ in the atmosphere can cause severe adverse effects on the planet, such as glaciers melting, and ice melting on Earth’s poles, which further raise the sea level [3]. The constant rise in global temperature can significantly affect the living survival on Earth. Therefore, there is a need to control the emission of CO₂ in the atmosphere [4] and to improve the research and development techniques for CO₂ capture and storage (CCS) [5] and convert the captured CO₂ into value-added products via different conversion routes [6].

Industrial CO₂ emission is the major source of CO₂ in the environment via pre-combustion or post-combustion treatments. To control the environmental CO₂ concentration, there is a foremost need to develop technologies to capture it from the atmosphere. Adsorption, absorption and membrane separation are the most applicable methods for CO₂ capture. However, except for adsorption, all other methods feature drawbacks such as corrosion, long processing period, high cost, and the generation of other harmful side products [7, 8]. Therefore, CO₂ capture via adsorption has attracted the attention of researchers globally. Several materials such as ionic liquids [9], polymers [10], carbon-based materials [11], zeolites [12], metal oxides [13], MXenes [14] and MOFs [15] have been proposed as favourable sorbent materials to capture the CO₂ from the atmosphere. Among all the class of 2D nanomaterials, for example, graphene oxide (GO), metal dichalcogenides, carbon nitrides, metal–organic frameworks, boron nitrides, MXenes etc., has gained immense interest in various applications, including CO₂ capture due to its outstanding characteristics such as high active surface area, high aspect ratio, excellent optical and mechanical properties and ease of functionalization [16]. Figure 14.1 represents the various examples of 2D nano-engineered materials used in CO₂ adsorption with their unique characteristics. In the last few decades, there is an exponential growth in the advancement of the nano-engineering of 2D materials using new strategies. 2D materials were fabricated with tunable characteristics, controlled orientations, and doped or combined with several other nanomaterials to synthesize 2D based nanocomposite materials with improved CO₂ capturing capacity.

Fig. 14.1

Examples of 2D materials, various ways to functionalize 2D materials and their specific characteristics to capture the CO₂

Several review articles have been published on CO₂ capture and conversion with a prime focus on the various technologies or using specific materials [5, 17,18,19,20]. For instance, the progress in using various 2D nano-engineered materials for CO₂ capture has been addressed. Recently, the significance of 2D nanomaterials such as graphene, boron nitrides, carbon nitrides, MXenes and metal chalcogenides have been proven to be successful candidates for the CO₂ capture. Further, in several reports, the analysis of the engineering aspects of 2D materials for improved CO₂ capture has also been reported [16]. Therefore, there is a need to put the application of various 2D nano-engineered materials in one frame for better understanding for the researchers. This chapter aims to methodically review the critical developments in applying different nano-engineered 2D materials in CO₂ capture. The functionalization of 2D nanomaterials or preparation of 2D nanomaterial-based nanocomposites can potentially improve the CO₂ capture aptitude of parent 2D nanomaterials. Additionally, the current scenario, opportunities, and challenges of applying nano-engineered 2D materials have also been discussed.

14.2 CO₂: Sources and Hazardous Effect

CO₂ gas is one of the major gases of GHGs emissions and causes global warming and climate change. CO₂ is one of the significant environmental emissions that can be through natural sources or anthropogenic activities. Figure 14.2 shows the possible sources of CO₂ emission in the environment [3]. Major natural sources of CO₂ emission include the CO₂ release from decomposition of biomass and vegetation, respiration, venting volcanos, ocean release and natural wildfires. Other than this, most of the CO₂ is released into the environment through human activities such as burning fossil fuels, deforestation, burning solid waste and biological matters, transportation, power generation, agricultural practices and industrial release. In one survey (2014), total global CO₂ emission was noticed to be 32.4 Gt CO₂, out of which the energy sector contributed majorly (82%), and the burning of fossil fuels was the prime cause. Amongst fossil fuels, oil consumption was the leading cause of CO₂ emission (34%), followed by coal (46%), gas fuels (1%), and the remaining was from the geothermal, nuclear, solar, hydro, biofuels, wind and waste [21]. Therefore, the ever-increasing demand for energy with the increase in population is one of the major causes of CO₂ release. Increasing CO₂ levels can be life-threatening to the planet, and there is a need to curb the CO₂ emissions or GHGs emissions to save life on Earth.

Fig. 14.2

Reproduced with permission from ref. [3]. Copyright 2022 CRC PRESS, Taylor, and Francis

Examples of several sources of CO₂ emission in the environment.

The regular CO₂ emission via natural and human activities is causing global warming and increasing the global temperature. IIPCC has stated that to avoid any devastating cost of CO₂ emission, the increase in global temperature should be restricted to 2 °C and it is most likely achieved 1.5 °C already [22]. Global warming and drastic climate changes can negatively impact the terrestrial and ocean ecosystem, water supply, food chain, health, weather condition and economic growth (Fig. 14.3). Acid rain is one of the disastrous consequences of increased CO₂ levels and can damage trees and the environment [23,24,25]. Global warming aids in melting the glaciers, particularly in Greenland and Antarctica, which adds water to oceans and seas. This exercise increases the sea level, which can cause floods, agricultural soil contamination with salt, destructive erosion and destruct habitat for living creatures such as fish, plants and birds. It is also apparent that the gaseous exchange between air and oceans will increase the ocean CO₂ concentration with the increase in atmospheric CO₂. This will increase the dissolved inorganic carbon and consequently decrease the ocean surface pH, resulting in ocean acidification [26]. Ocean acidification is threatening the habitat of marine creatures and will make a negative impact on the food chain. In one study, it has been noticed that the CO₂ concentration has risen from 378 to 410 ppm in the span of 14 years (i.e. 2005–2020), which is a clear indication that more drastic weather conditions are about to come [27, 28].

Fig. 14.3

An illustration of the impact of increasing CO₂ concentration in the environment

Therefore, there is an urgent need to curb CO₂ or GHGs emissions into the environment. Nowadays, several strategies are adopted to reduce atmospheric CO₂ levels, such as (a) absorption/adsorption of atmospheric CO₂, (b) progressive adaptation of renewable energy sources, (c) CCS and (c) CO₂ capture and utilization (CCU). Various porous materials, such as zeolites, can be considered as possible adsorbents for CO₂ capture, but the poor selectivity limits their industrial applications. In this regard, 2D nanomaterials have been fabricated to improve CO₂ adsorption/absorption and selectivity. The following sections will describe the fundamentals and mechanism of CO₂ capture using various 2D nanomaterials.

14.3 Mechanism of Carbon Dioxide Capture

Carbon dioxide capture is an important process that has been developed to reduce the amount of carbon dioxide emissions released into the atmosphere. Carbon dioxide capture involves trapping and storing carbon in various ways, including capturing it from industrial sources before it is emitted into the atmosphere or by directly removing it from ambient air. This technology can help mitigate climate change and reduce our dependence on fossil fuels for energy production. The most widely used form of carbon dioxide capture is direct air capture (DAC). DAC systems use filters to trap CO₂ molecules as they pass through them, which are then stored in tanks or other containers until they can be processed further for reuse or disposal. Other methods include pre-combustion techniques and oxyfuel combustion, which uses chemical absorption processes like amine scrubbing. These technologies have proven effective at reducing CO₂ emissions but require significant investment in infrastructure costs due to their complexity and costliness relative to other emission reduction strategies like renewable energy sources or efficiency improvements within existing facilities.

Overall, while there are many different mechanisms available for capturing carbon dioxide emissions before they enter Earth’s atmosphere, each one requires careful consideration when evaluating its potential effectiveness against environmental goals while also taking into account any associated economic implications related with implementation costs & benefits over time. Currently, adsorption is the most viable technology for CO₂ capture due to its straightforward operation and low cost. Adsorption occurs in two ways- strong chemical interactions (chemisorption) and weak physical interactions (physisorption) [29, 30]. Solid adsorbents benefit from the lack of substantial amounts of water/solvent compared to aqueous absorbents (use amine-based solvents), which results in less energy being used during CO₂ desorption. In addition, solid adsorbents have lower heat capacities than aqueous ones, reducing the energy needed to heat the adsorbent to desorb the CO₂. The solid adsorbents also offer fast kinetics of CO₂ adsorption/desorption and effortless operation [29]. Because of these advantages, development of novel adsorbents with high efficiency are utmost importance for remediation of CO₂ from the atmosphere. Figure 14.4 illustrates the various kinds of interactions taking place during CO₂ adsorption.

Fig. 14.4

Adsorption of CO₂ on the 2D materials-based adsorbents via physical and chemical adsorption

Solid adsorbents are further classified into three ways: low-temperature adsorbents (less than 200 °C), mid-temperature adsorbents (200–400 °C), and high-temperature adsorbents (greater than 400 °C) [31]. The low temperature adsorbents follow physisorption mechanism through electrostatic or van der Waals forces. Because of low interaction energy, desorption and active site regeneration can be done at low temperature conditions, resulting in lower energy and cost requirements. However, the major drawback of low temperature adsorbent is poor selectivity towards CO₂ [30]. Moreover, flue gas stream contaminants (such as NO_X, SO_X, particulate matter and water vapour) can deactivate the sorption site for CO₂ or out-compete CO₂. Furthermore, water vapours can selectively adsorb over CO₂ on low-temperature adsorbents, resulting in poor adsorbent stability due to deactivation [30]. Contrarily, high-temperature adsorbents offer better CO₂ selectivity and thus, they have been used in DAC conditions [32]. However, to replenish these adsorbents, a cyclic carbonation/decarbonation process is needed at high temperatures. In contrast, mid-temperature adsorbents provide reasonable selectivity as well as high performance for CO₂ capture. Additionally, these adsorbents are suitable for effective CO₂ capture and sequential catalytic conversion as they operate in the same temperature window [33].

14.4 Capture of CO₂ Using Nano-engineered 2D Materials

Regular emission of toxic and greenhouse gases into the environment has become a threat to the healthy planet. CO₂ is one of the major greenhouse gases released into the atmosphere via several natural and anthropogenic sources. The healthy management of these gases in the atmosphere is one of the essential requirements to balance survival. Several routes have been adopted to lower the CO₂ level, such as (a) control of the CO₂ emission, (b) CCS and (c) CCU conversion and utilization. Various 2D nano-engineered materials have been proven as suitable candidates for CO₂ capture and conversion. Lately, 2D materials such as graphene and graphene-based nanomaterials, 2D TMDCs, h-BN, 2D TMOs, MXenes, 2D MOFs, 2D covalent organic frameworks (COFs), g-C₃N₄, borophene, phosphorenes, nanoclays, etc. have been employed for CO₂ capture, (Fig. 14.1) [34]. The subsequent sections will discuss role of these 2D materials for CO₂ capture in details.

14.4.1 Graphene and Graphene-Based Nanomaterials

Since the discovery of graphene, it has been considered as an active and exciting material in wide range of applications such as lubrication, catalysis, supercapacitors, tissue engineering, 3D printing, sensors, desalination, water and air purification, drug delivery, solar cells and molecular imaging [35,36,37,38]. Graphene based materials such as graphene oxide (GO) and reduced graphene oxide (rGO) have also been widely used in the CO₂ capture as adsorbents and absorbents due to its favourable properties [39]. Graphene exhibit a 2D hexagonal morphology, which resembles more like a honeycomb network and provides extraordinary physicochemical, mechanical, thermal and electrical properties for its high efficiency in a wide range of applications [40, 41]. Theoretical graphene nanosheets demonstrate a very high active surface area (~2630 m².g⁻¹), which is one of the key characteristics for adsorption/absorption applications. Additionally, the high porosity of graphene-based materials, high thermal and chemical stability, and ease of functionalization of graphene nanosheets open the infinite possibilities to construct an adsorbent/absorbent with high CO₂ capture efficiency and selectivity [42]. The oxygen functionalities on the GO make its surface basic in nature, which attracts the CO₂, which is acidic in nature and helps in CO₂ adsorption. Moreover, the oxygen functionalities on GO surface open the prominent platform for easy modification of the adsorbent surface to enhance CO₂ adsorption.

Graphite and GO have been modified via several routes with different functional groups to enhance their capability for gas storage. Exfoliated rGO is one of the popular graphene derivatives in CO₂ capture and exists in the stacked few layers of defected graphene sheets with fewer oxygen functionalities. The route of GO reduction to prepare rGO also significantly affects the CO₂ adsorption capacity. As chemical/thermal reduction reduces the CO₂ adsorption capacity of rGO by 1.5 mmol.g⁻¹ than GO at ambient conditions [43], whereas on reducing GO hydrothermally, the CO₂ adsorption capacity enhances by 2.4 mmol.g⁻¹ at ambient pressure and 0 °C [44]. Additionally, on exfoliating the graphene or graphene derivatives sheets, the CO₂ adsorption efficiency increases due to exposure to a more active surface. Molecular dynamics simulation studies have also proven that under the assistance of H₂O molecules and the presence of active functional groups on the GO, surface enhances the CO₂ adsorption from the environment [45]. Further, the adsorption of CO₂ can be promoted by the functionalization and doping of the graphene sheets. CO₂ adsorption efficiency of rGO was compared with boron-doped rGO (B/rGO) under identical conditions. Doped boron significantly improves the adsorption characteristics of rGO from 1.3 mmol.g⁻¹ to 1.8 mmol.g⁻¹ [43]. Theoretical simulation studies reveal that the CO₂ molecule adsorbed on the graphene sheets with a parallel orientation and the distance between the graphene sheets and CO₂ was found to be 0.345–0.36 nm, which is attributed to the van der Waals interactions.

Doping or functionalizing graphene and its derivatives with nitrogen or nitrogen-based functional groups is another way to improve CO₂ adsorption efficiency. Polyaniline/hydrogen exfoliated graphene (PANI/HEG) based nanocomposite materials have excellent applications as CO₂ adsorbents [46]. PANI/HEG could adsorb 75 mmol.g⁻¹ CO₂ at room temperate and 11 bar pressure, which was almost 3.5 times higher than the CO₂ adsorbed by HEG (21.6 mmol.g⁻¹) only [47]. FTIR studies of CO₂-adsorbed PANI/HEG revealed that the interactions between the CO₂ and nitrogen atom present on adsorbent are majorly chemical interactions. Additionally, PANI/HEG also showed excellent recyclability, with 2–3% less adsorption efficiency. Amine functionalized graphene is one of the most popular graphene based adsorbent material, which has been investigated to capture the CO₂. Nucleophilic nature of amines strongly interacts with the electrophilic CO₂ and improve the CO₂ adsorption efficiency of GO. Shin et al. used polyethyleneimine (PEI) as an amine source to functionalize the GO and used it as an adsorbent material for CO₂ capture [48]. The amine-functionalized GO has shown an excellent adsorption capacity (84 mg.g⁻¹) for CO₂ adsorption, which was attributed to the appropriate PEI concentration and improved the electron donor and acceptor interaction between the adsorbate and adsorbent. Additionally, the loading of PEI on GO surface also enhances the selectivity of CO₂ adsorption over N₂ (Fig. 14.5). However, the excess loading of PEI further reduces the CO₂ capacity, which might be due to the unavailability of the active sites of the GO surface.

Fig. 14.5

Reproduced with permission from ref. [48]. Copyright 2016, Elsevier Publications

Adsorption curves of a GO and b PEI-GO for CO₂ and N₂ gases and c amount of CO₂ and N₂ gases adsorbed on GO and various PEI functionalized GO.

In another report, TEPA (Tetraethylenepentamine) was employed as an amine precursor to functionalize the GO to improve the CO₂ uptake [49]. Ultrasonic waves were used to activate the TEPA functionalized GO for CO₂ adsorption studies. Ultrasonic-activated TEPA-GO has shown higher CO₂ adsorption capacity (1.2 mmol.g⁻¹) than bare GO (0.3 mmol.g⁻¹) under identical conditions. This might be due to ultrasonic waves assisted in the exfoliation of graphene layers in the adsorbent material and provided a large number of active sites with functional groups for CO₂ adsorption. Consequently, the nucleophilic amine in the adsorbent material (TEPA-GO) interacts with electrophilic CO₂ efficiently and performs excellently. TEPA-GO also display excellent recycling stability, which is suitable for practical applications.

Various graphene-based nanocomposite materials have also been investigated for CO₂ capture. In one study, Pokhrel et al. designed novel nanocomposite compromises of GO, Zeolite imidazolate framework (ZIF-8), and amine functional groups, which was present due to functionalization with 3-aminopropyl triethylsilane (APTES), PEI and ethylene diamine (ED) [50]. The amine groups inserted in the GO layers increased the interlayer spacing, which helped in improving CO₂ adsorption. Among all amine, functional groups precursors, APTES-modified GO-based nanocomposite showed the highest adsorption efficiency with a 36% increase in CO₂ adsorption capacity compared to unmodified GO. Additionally, the adsorption efficiency of APTES-modified GO nanocomposite increased in pre-adsorbed water conditions (10% RH), and successfully adsorbed 33% extra CO₂ than in dry adsorption conditions. This might be due to the generation of bicarbonates in the presence of moisture which provides more active sites of CO₂ adsorption. On comparing the ZIF-8/GO amine functionalized adsorbent with amine-modified ZIF-8,

Other than functionalized and doped graphene, 3D graphene, such as graphene-based hydrogels and aerogels, has also been studied for CO₂ adsorption. The 3D structure of the graphene-based adsorbent provides a more porous architecture for the adsorption studies. Hsan et al. prepared the chitosan (CS) embedded GO aerogels (CSGO) for the adsorption of CO₂ [51]. To construct crosslinking between the CS and GO, (1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride and N-hydroxy succinimide crosslinkers were used and followed by the freeze-drying process to obtain aerogel structure. CSGO was found to be excellent adsorbent material with 0.25 mmol.g⁻¹ adsorption capacity for CO₂ at 298 K and 1 bar pressure in comparison to raw CS (0.05 mmol.g⁻¹). The presence of nitrogen contents on the aerogel skeleton improves the selective CO₂ adsorption capacity. Therefore, introducing the nitrogen functionalities on the exfoliated graphene and its derivatives improved the CO₂ capture efficiency.

14.4.2 2D Transition Metal Oxide-Based Nanomaterials

2D transition metal oxide (TMOs)-based nanomaterials have been explored for various applications including electrochemistry, photochemistry, energy storage, tribology, catalysis, and environmental remediation [52,53,54,55,56]. Due to their remarkable properties such as high surface area, a large number of active sites, cost-effectiveness, ease of modification, abundance availability and microstructural features, metal oxide-based nanomaterials have been considered potent competitors in adsorption applications [53]. Additionally, oxide-based adsorbent material have been found highly selective towards CO₂ adsorption at high temperature conditions [57]. In dry atmospheric conditions, TMOs surface is usually terminated with O²⁻ anions, which are typically bigger than the cationic M⁺. This mismatching of size creates a lower coordination of cations on the surface and results in plenty of adsorption sites [58]. Therefore, several metal oxide-based nanomaterials are used in gas adsorption. TMOs can adsorb the CO₂ via physisorption or chemisorption or both (Fig. 14.6) [33]. Metal oxides usually exhibit a stronger affinity towards CO₂ and form a strong chemical interaction with CO₂ in the presence of other co-existing gases, which are adsorbed by weak physio-sorption only. This improves the selective adsorption application, which is indispensable for practical approaches.

Fig. 14.6

Reproduced with permission from ref [33]. Copyright 2022, Elsevier Publications

CO₂ adsorption on metal oxides via a chelation, b chemisorption, and c physisorption.

Magnesium oxide (MgO) and Calcium oxide (CaO) are the two most successful metal oxides for CO₂ capture. Other metal oxides such as CuO, MnO₂, and CeO₂ were also used in CO₂ capture but were comparatively less explored. For the adsorption of CO₂, MgO exhibits appropriate surface morphology with oxygen, which improves the adsorption capacity and lowers the regeneration energy consumption [59, 60]. Recently Hu et al. published a detailed review focused on the strategy development of MgO-based adsorbents for CO₂ capture [61]. CO₂ adsorption on MgO is majorly an acid–base type reaction followed by physical interactions. The acidic CO₂ reacts with the basic sites of MgO to form a complex (MgO–O-C = O) to get chemically adsorbed. Othman et al. have incorporated several metal oxide nanoparticles (MgO, MnO₂, ZnO, and CaO) on activated carbon nanofibers (ACNFs) via electrospinning and pyrolysis process and examined for the CO₂ capture [62]. MgO incorporated ACNFs were found to be with highest surface area (413 m².g⁻¹) and micropore volume (0.1777 cm³.g⁻¹) than other metal oxide ACNFs and pristine ACNFs. CO₂ adsorption capacity of MgO-ACNFs was also the highest (60 cm³.g⁻¹) at 298 K. Also, the incorporation of MgO nanoparticles on the activated carbon-based bamboo (BAC) enhanced the BAC CO₂ adsorption capacity by 112% [63]. The major driven force for the adsorption of CO₂ on MgO-BAC was physical adsorption forces than chemical interactions because the chemical interactions between MgO and CO₂ to form MgO–O-C = O (MgCO₃) was slow than physical attractions.

Recently, alkali metal nitrates have been mixed with MgO-based nanoadsorbents to improve their efficiency in CO₂ capture. Alkali metals in MgO melt during the CO₂ capture process and avoid the MgCO₃ formation of MgO surface [64]. Figure 14.7 demonstrates the application of molten alkali nitrates in MgO to enhance CO₂ capture via carbonation [64]. NaNO₃ and KNO₃ are the popular nitrate promoters used in MgO as they do not react with CO₂ directly [65]. However, alkali metal carbonates can also be used in place of alkali metal nitrates to improve the CO₂ capture efficiency of MgO-based adsorbents. Carbonate promotors in MgO enable easier melting during the CO₂ capture process, allowing the faster transport of CO₂ through the carbonate layer. Kwak et al. mixed the triple eutectic alkali carbonate (TEC) (Li₂CO₃, Na₂CO₃ and K₂CO₃) with MgO for enhanced CO₂ capture [66]. The TEC amount in MgO significantly affects the kinetics of CO₂ capture. The CO₂ capture process was classified into two key steps: (a) fast and large and (b) slow and small. Higher the amount of TEC promotes the step ‘a’ but does not affect step ‘b’.

Fig. 14.7

Reproduced with permission from ref. [64]. Copyright 2022, Royal Society of Chemistry

Diagrammatic illustration of MgO sorbent role with nitrate salts in enhancing the CO₂ capture capacity.

Other than MgO, CaO is also extensively examined metal oxide material for CO₂ capture due to its fast carbonation and easy regeneration. CaO is generally used for CO₂ storage and reacts with CO₂ reversibly, as shown in Eq. 14.1

$$CaO + C{O}_{2} \leftrightarrow CaC{O}_{3}$$

(14.1)

The adsorption of CO₂ takes place at 500 °C, and the exothermic reaction knows as carbonation. However the desorption of CO₂ takes place at higher temperature (800–950 °C) and the endothermic reaction is known as calcination or decarbonation [67]. The reversible adsorption and desorption of CO₂ using CaO are known as chemical looping. CaO-based adsorbents are commercially used to capture the CO₂ from cement plants. However, mechanical failure and excessive sintering are the significant drawbacks of CaO in CO₂ capture. Once CO₂ uses the initial layer of CaO to form the CaCO₃, the adsorption process slows down. Several works have been proposed to improve the adsorption efficiency of CaO. On using nanosized CaO, the number of active surface sites is increased for the adsorption studies. Florin et al. derived the CaO from nanosized CaCO₃ and used it for CO₂ capture [68]. The prepared CaO was exposed for 5 carbonation cycles (each cycle was run for 24 h) with no morphological impediment. Also, the material was used for 100 CO₂ carbonation and decarbonisation cycles (20 min for each cycle), representing the potential of nano CaO for CO₂ capture [68]. In summary, MgO and CaO are the most explored transition metal oxide-based nanomaterials for CO₂ capture, whereas there is a need to examine other metal oxides also.

14.4.3 MXenes

Beyond graphene, transition metal carbon/nitride (MXene) is another class of 2D material that has drawn a lot of interest from research communities. By carefully etching the A layer from MAX phases, where M is a transition metal, A is a IIIA or IVA element, and X is C or N, MXenes have been synthesized [69]. Due to the high corrosivity of hydrofluoric acid, it has been employed to etch the covalently bonded IIIA or IVA element [70]. To avoid the toxicity of hydrofluoric acid, alternative environmentally friendly processes have also been implemented, including alkali treatment, Lewis acid etching, electrochemical etching and so on [71]. MXenes have received enormous attention for their functionality and characteristics in electrochemical charge storage, environmental remediation, electromagnetic interference shielding, catalysts and other applications [72]. Owing to high chemical and mechanical stability with outstanding thermal and electrical conductivities, it offers requisite features for CO₂ activation/conversion, capture and storage. Various termination functionalities (−O, OH, halogens, chalcogens or their mixtures) of MXenes make them attractive materials for adsorption processes. Additionally, these materials can be produced with high surface areas in range of 250–1000 m²/g that provide high reactive sites and porous functionalities for CO₂ capture and conversion [70]. Importantly, it was noticed that MXenes have higher adsorption energies compare to other 2D materials [73].

CO₂ adsorption on MXenes depends on many factors such as the thickness of sheets, defects, specific surface area, lone pair electrons and presence of CO₂-philic groups on the surface [74]. For example, Viñes et al. used 2D MXene, which has the general formula M_n+1X_nT_x (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, n = 1, 2, 3, 4), for effective CO₂ removal [75]. The maximum CO₂ abatement rate could be 8.25 mol CO₂ per kg MXene. Studied MXenes followed the given order for CO₂ capture: Ti₂CT_x > V₂CT_x > Zr₂CT_x > Nb₂CT_x > Mo₂CT_x > Hr₂CT_x > Ta₂CT_x > W₂CT_x. They exhibited high adsorption capacity despite of low CO₂ partial pressure and high temperature. The higher adsorption energy, large surface area, MXene/CO₂ charge transfer, and abundant adsorption sites could be the causes of the greater ability of MXenes to capture CO₂. The study summarized that the adsorption strength and conformation had the greatest influence on CO₂ desorption. In another study, dimethylsulfoxide intercalated Ti₃C₂T_x was prepared via wet chemistry using sodium fluoride and hydrochloric acid [76]. It demonstrated a high surface area of 66 m².g⁻¹ with a high volume capacity of 502 V.v⁻¹ and exhibited a high CO₂ adsorption uptake of 5.79 mmol.g⁻¹. However, this adsorption capacity was lower than the theoretical capacity (44.2 mmol.g⁻¹) of Ti₃C₂T_x with a specific surface area of 496 m².g⁻¹ [76]. Pristine Ti₃C₂T_x showed a lower surface area (21 m².g⁻¹) and lower CO₂ adsorption capacity. It is noteworthy that tailoring the surface area of MXenes is a potential approach for enhancing the CO₂ uptake. In another study, individual sheets of Ti₃C₂T_x carbides were employed to achieve efficient CO₂ adsorption (12 mol/kg) [77].

Numerous theoretical studies have been conducted to understand the insights of the CO₂ adsorption process on MXenes. Using density functional theory, Morales-Garcia et al. proposed that there was insignificant effect of a number of MXene sheets on CO₂ adsorption [78]. Using first-principles calculations, Sun et al. proposed the surface lone pair of MXenes account for high CO₂ absorption [79]. They selected nine MXene for the study such as Ti₂CT_x, Zr₂CT_x, Hf₂CT_x, V₂CT_x, Nb₂CT_x, Ta₂CT_x, Cr₂CT_x, Mo₂CT_x, and W₂CT_x. The CO₂ adsorption by MXenes followed the IVB > VB > VIB trend, which was linked to the lone pair electrons of MXene that shift to CO₂ during the absorption process. Thus, the occurrence of surface lone pairs on metals of MXenes could play a significant role in CO₂ adsorption. Another study used DFT calculations to examine the CO₂ adsorption and desorption on 2D M₂N materials (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) [80]. M₂N is better than Mo₂C for effective CO₂ uptake due to the presence of high electronegativity N atom layers and substantial adsorption energy (−3.13 eV).

Furthermore, MXene-based composites have been prepared to enhance the adsorption capacity for capturing CO₂ gas at the different sources. For example, MXene-based solid nanocomposite (Ti₃C₂/PEI/BO) adsorbent was fabricated using Ti₃C₂(OH)_x, polyethyleneimine and 1,2-butylene Oxide [81]. Ti₃C₂/PEI/BO adsorbent demonstrated high CO₂ adsorption capacity (2.60 mmol/g) during CO₂ (100%) as a purge gas at a high temperature (120 ℃). Furthermore, adsorbent also showed long-term cyclic stability and superior attrition resistance. In another study, thin-film matrix membranes were prepared by incorporating Ti₃C₂T_x nanosheets in Pebax-1657 (Pebax is the trade name of polyether block amide) for effective mixed gases (CO₂/N₂ and CO₂/H₂) separation [82]. The separation performance of the composite membranes exceeds Robeson’s upper bounds, while working within the target range for economical CO₂ remediation. The uniform galleries of MXene produced via H-bonding interaction between MXene and rubbery Pebax might be responsible for the high CO₂ capacity. Fast and specialized CO₂ transport was made possible by interfacial interactions and selective Ti₃C₂T_x composite nanochannels. The performance mechanism of MXene/Pebax membrane for selective capturing of CO₂ and N₂ can be depicted in Fig. 14.8. Because CO₂ has a higher quadrupole moment than N₂, it more easily affiliates with the MXene surface. The surface hydroxyl groups of MXene improved CO₂ capture efficiency and also resulted in highly selective adsorption of CO₂. MXene galleries were ~0.35 nm in diameter, bigger than CO₂ but smaller than N₂ [82]. It demonstrated that N₂ was constrained while CO₂ was qualified to infiltrate and diffuse across the membrane. Additionally, the adsorption capacity of various 2D materials-based adsorbents at different conditions is discussed in Table 14.1.

Fig. 14.8

Reproduced with permission from ref. [82]. Copyright 2020, American Chemical Society

Proposed mechanism for transporting and separating CO₂ and N₂ by Ti₃C₂T_x/Pebax membranes.

Table 14.1 Comparison of CO₂ adsorption capacities of various 2D materials

14.4.4 Hexagonal Boron Nitride (h-BN)

Hexagonal boron nitride (h-BN) nanosheet is made of sp²-conjugated boron and nitrogen atoms that create a 2D structure. It has high thermal stability (up to 800 °C), excellent thermal conductivity (300–2000 Wm.K⁻¹), high Young’s modulus (0.8 TPa) and superior fracture strength (165 GPa) [99]. h-BN nanosheet has been prepared using both top-down and bottom-up approaches. Tunning and functionalization of pores and structures of BN via selecting appropriate precursors during synthesis is a crucial strategy to achieve excellent CO₂ adsorbent. For instance, a few layered h-BN (1–4 layers) with a high surface area (927 m².g⁻¹) were produced chemically using boric acid and urea and found to be advantageous for CO₂ adsorption [100]. Yang et al. synthesized three-dimensional (3D) flower-like BN nanosheets (FBNNSs) by template-free cylinder compressing route using boron oxide and guanidine hydrochloride as starting precursors [101]. FBNNSs demonstrated efficient CO₂ adsorption up to 74.4 mg.g⁻¹ at 1 bar, which was ascribed to the large surface area (1114 m².g⁻¹), pore volume (0.7 cm³.g⁻¹), hierarchical pore availability, and plentiful edge functionalities (−OH/−NH₂). The CO₂ adsorption mechanism on nanosheets was governed by hydrogen bonding, and weak van der Waals interaction. FBNNSs showed high stability and performance even after 10 cycles of recyclability. Additionally, in another study, a new type of micropore-rich BN fibres was produced by introducing a hexamethylenetetramine surfactant in the conventional porous BN precursor (melamine-diborate) [93]. Because of excessive micropores, micropore-rich BN fibres showed better CO₂ capture capacity (2.85 mmol.g⁻¹) than original porous BN fibres (3.19). These thermally stable BN fibres can be reused in several cycles without loss their adsorption property. Moreover, large surface area (1900 m².g⁻¹) BN can be produced using multiple N precursors (mixing biuret or melamine with urea), exhibiting high adsorption capacity of 1.6 mmol.g⁻¹ at 1 bar, 25 °C and 8.3 mmol.g⁻¹ at 20 bar, 25 °C. More notably, CO₂ adsorption capacity decreased to 1.1 mmol.g⁻¹ (1 bar and 25 °C) after palletization [90]. C-doped BN fibre was also prepared using melamine and boric acid with final treatment at 1100 °C [102]. Excellent sorption property (3.71 mmol.g⁻¹ at 273 K) of doped fibre was attributed to micropores (formed by doping of C) and more negative charges on BN (induced by structural defects), resulting in high CO₂ affinity via chemisorption interaction. Thus, it is noteworthy that the adsorption performance of BN sheets/fibres can be improved by slight modification in their structure and composition.

To improve CO₂ capture performance, BN can be engineered with metallic nanoparticles. For instance, in-situ Cu-incorporated BN nanofibers (BNNF) were prepared using BN precursors (melamine and boric acid) and Cu source (Copper dichloride dihydrate) for efficient uptake of CO₂ (Fig. 14.9a) [94]. It was observed that Cu²⁺ ions speed up the nucleation process during the reaction between melamine and boric acid, which is discovered to be a crucial component for producing Cu@BNNF with uniform and tiny diameters (Fig. 14.9b–d). Interestingly, the presence of Cu on BNNF improves the CO₂ affinity, the adsorption capacity of the BN nanofibers enhanced from 1.34 to 2.77 mmol.g⁻¹ (1 bar and 273 K) (Fig. 14.9e). The Cu@BNNF also exhibited high recycling stability, unlike several other BN-based absorbents (Fig. 14.9f). Moreover, CO₂ adsorption mechanism on Cu-loaded BNNS was investigated with density-functional theory calculations. It was discovered that adding Cu on BNNF can lead to enhanced electron interaction between BNNF and the CO₂ molecule, resulting in better CO₂ affinity and adsorption capacity. In another study, Yang et al. synthesized 3D hierarchically cubic/spherical morphologies of functionalized BN nanosheets (FBNNSs)/ZnO superstructures using the evaporation-mediated solvothermal approach [95]. These materials offer several benefits, including scalability, inexpensive technology and high throughput. As-produced FBNNSs/ZnO superstructures showed a high CO₂ adsorption performance of 124.5 mg.g⁻¹ from 0 to 1 bar at 273 K and presented reasonable reusability of 10 cycles with an average adsorption capacity up to 115.7 mg.g⁻¹. The superior adsorption performance of superstructure for CO₂ removal was described by van der Waals interaction, chemisorption and hydrogen bonds from the ZnO and amino groups of FBNNs.

Fig. 14.9

Reproduced with permission from ref. [94]. Copyright 2020, American Chemical Society

a Synthesis of Cu@BNNF; SEM b and TEM images c–d exhibit uniform and smaller diameter of Cu@BNNF; e adsorption quantity of Cu@BNNF and BNNF; f CO₂ adsorption and desorption cycling for the Cu@BNNF.

14.4.5 Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDCs) have recently gained attention for their potential application in carbon dioxide and other gases capture [103]. TMDCs possess unique physical and chemical properties, such as high surface area and strong adsorption abilities, which make them ideal candidates for capturing CO₂ from air or exhaust streams. In addition to their excellent adsorptive properties, they also exhibit a low cost of production compared to other materials used in carbon capture technologies [104, 105].

The most common type of TMDC used for CO₂ capture is molybdenum disulfide (MoS₂). This material has been found to be highly effective at trapping CO₂ molecules due to its large surface area and strong affinity towards the gas molecule [106]. MoS₂ can be produced synthetically via chemical vapour deposition methods and direct chemical synthesis using S and Mo precursors or extracted from natural sources like shale deposits [107,108,109,110]. The synthesis approaches are relatively inexpensive, reducing the cost of CO₂ adsorption compared with traditional methods used in CO₂ removal systems such as cryogenic distillation or amine scrubbing processes [111]. Additionally, this material does not require any post-treatment steps after use, making it an attractive option for industrial applications where cost savings is essential. Recently, various studies have been reported for CO₂ capture using pristine or modified MoS₂. According to Sun et al., the strength of the applied electric field affects how strongly CO₂ adsorb on the MoS₂ [112]. When an electric field is applied, CO₂ is adsorbed on the material and desorbed when the electric force is relaxed. It becomes a possible carbon capture material because of this distinctive quality. The Cu-modified MoS₂ nanosheets were prepared for the CO₂ reduction process by Shi et al. [113]. The Cu/MoS₂ hybrid (0.44 cm³/g) showed better adsorption capacity compared to pristine MoS₂ (0.22 cm³.g⁻¹). Thus, Cu nanoparticles have improved the adsorption capacity since CO₂ can now be adsorbed on both Cu and host MoS₂ nanosheets.

In addition, the surface of MoS₂ can be tuned to obtain high reactive edge sites via doping or creating defects for the adsorption of a specific gas. Wu et al. studied the role of nitrogen doping on MoS₂ (defect-rich and defect-free structure) reactivity towards CO₂ adsorption [114]. N-doped MoS₂ with Mo defects demonstrated the highest CO₂ adsorption capacity because of more electrostatic and covalent interactions with the CO₂. On the contrary, the defect-free MoS₂ followed weak van der Waals interactions, which resulted in weak CO₂ adsorption. Also, the defective and N-doped MoS₂ showed high selectivity of CO₂ over N₂ [114]. In another study, MoS₂ nanocomposites were prepared using rGO and polypyrrole (PPy) to enhance the reactive sites for CO₂ adsorption [96]. Nanocomposite with the highest PPy concentration exhibited the highest CO₂ adsorption capacity (13 cm³.g⁻¹) compared to MoS₂ (6 cm³.g⁻¹) due to the presence of excessive nitrogen binding sites (Fig. 14.10). Therefore, the adsorption capability of MoS₂ nanosheets can be improved by structural modifications using doping/defect or making composite with other active species.

Fig. 14.10

Reproduced with permission from ref. [96]. Copyright 2015, American Chemical Society

CO₂ adsorption of the MoS₂ nanosheets, rGO–MoS₂, rGO–MoS₂/PPy-600, rGO–MoS₂/PPy-300, and rGO–MoS₂/PPy-150 nanocomposites.

14.4.6 Carbon Nitride

Like graphene, graphitic carbon nitride (g-C₃N₄) is made of hexagonally ordered heptazine (tri-s-triazine) units bonded across tertiary amines. The carbon nitride term is basically used for CN material with elemental compositions C/N (x) of less than 1.0, in which the main constituting elements are carbon and nitrogen atoms are either substituted for carbon atoms or bonded as nitrogen functionalities. In the literature, the materials with CNx with x > 1.0 are frequently referred to as N-rich carbon nitrides [115]. Because of its excellent physicochemical characteristics, g-C₃N₄ has gained significant attention for energy and environmental applications [116, 117]. Unlike other 2D materials, g-C₃N₄ can be commonly synthesized by direct thermal decomposition of N-rich precursors such as urea or melamine. Conventional top-down exfoliation of g-C₃N₄ is not popular because of densely packed heptazine units. The affinity of pristine g-C₃N₄ and CO₂ is low due to the poor alkalinity of N species in g-C₃N₄, which result in limited utilization in CO₂ removal. Nevertheless, amine functionalization of carbon nitride is the way to enhance CO₂ uptake through improving adsorbent-adsorbate interactions. For instance, polyethylenimine (PEI)-functionalized g-C₃N₄ was synthesized using the physical impregnation method to enhance the adsorption capacity and selectivity of g-C₃N₄ towards CO₂ [118]. Due to the presence of excessive amine groups and stacked pores, the highest CO₂ adsorption capacity of PEI-g-C₃N₄ was found to be 3.77 mmol.g⁻¹ at 100 °C and ambient pressure, which was better than pristine g-C₃N₄. In another study, N-rich ionic liquid functionalization of g-C₃N₄ was performed with ionic liquid 1-Butyl-3-Methylimidazolium bis(trifluoromethyl sulfonyl)imide ([BMIM][TFSI])  for enhanced uptake of CO₂ [83]. The NS-g-C₃N₄/[BMIM][TFSI] demonstrated excellent adoption capacity to 42.93 mmol.g⁻¹ under nearly ideal conditions, better than bulk g-C₃N₄ (8.54 mmol.g⁻¹). The high sorption uptake was ascribed to high N content (56.6 atomic%) that acts as reactive sites for CO₂ interactions. At the same time, ionic liquid molecules on the g-C₃N₄ surface offer extra physicochemical adsorption mechanism thus increasing the CO₂ uptake.

Additionally, more active sites on g-C₃N₄ can be generated by making a composite of it with other nanomaterials. Carbon nitride-ZnO hybrid adsorbent was prepared for CO₂ adsorption. The hybrid material showed a remarkable adsorption capacity of 6.11 mmol.g⁻¹ (1 atm and 25 °C), which was three times better than g-C₃N₄ [85]. It was summarized that the high CO₂ affinity of hybrid material is governed by basicity, amount of amine precursors, framework stability, surface area and surface porosity. However, Kim et al. used RGO aerogel as a template for the evolution of carbon nitride with a large surface area for CO₂ uptake [84]. The schematic of the preparation of carbon nitride aerogels for CO₂ capture is described in Fig. 14.11. The as-prepared structure exhibited high adsorption properties (0.43 mmol.g⁻¹) at ambient conditions with high selectivity against the N₂ and excellent regeneration capacity. The increased CO₂ capture capacity of carbon nitride aerogels was attributed to the microporous edges of carbon nitride that provide the optimal CO₂ affinity via dipole-induced dipole interactions.

Fig. 14.11

Reproduced with permission from ref. [84]. Copyright 2015, American Chemical Society

Synthesis of carbon nitride aerogels for the selective adsorption of CO₂.

14.4.7 2D Metal–Organic Frameworks

Metal–organic frameworks (MOFs) are periodic network-structured porous coordination polymers, created by carefully coordinating the congregation of metal ions or clusters with organic ligands. Due to the diversities of metal ions/clusters and organic ligands, MOFs have shown a large surface area, high porosity, unique framework structure and tailorable pore, and open metal sites, which make them suitable for married applications, including CO₂ capture [119]. When the MOFs dimension is reduced to two-dimension, ultrathin 2D MOFs layers have more outward remarkable characteristics, offering various scientific applications [120]. Like graphene, 2D MOFs can be prepared using either top-down or bottom-up methods [41, 120]. For example, Zhang et al. applied a bottom-up room temperature approach for preparing 2D MOFs using Cu(II) propeller and different ligands in a dimethylformamide/water solvent system [97]. The size and surface functionalization can be adjusted by changing the solvent ratio and ligands, respectively (Fig. 14.12). The findings of the gas sorption experiments indicated that nitro and amine-functionalized 2D MOFs exhibited better CO₂ sorption selectivity over CH₄ and N₂, indicating that these materials may also be applied for carbon capture from flue gas (CO₂/N₂). The highest CO₂ adsorption capacity was found to be 1.78 mmol.g⁻¹ (1 atm, 273 K) for [Cu₂(nbdc)₂(dabco)]_n. Sun et al. synthesized Co based 2D Co-MOF-UPC-32 with persistent porosity for high CO₂ and H₂ adsorption [121].

Fig. 14.12

Reproduced with permission from ref. [97]. Copyright 2018, American Chemical Society

a Schematic shows the structure of 2D MOFs; b TEM images (with electron diffraction patterns in insets) and AFM topographic images of synthesized 2D MOFs; c CO₂ sorption capacity for CO₂, N₂ and CH₄.

Particularly in hot and humid environments, structural instability of MOFs is a major issue because of brittle coordination bonds. This restricts their capability to capture CO₂, especially in pre- or post-combustion processes. On the other hand, covalent organic frameworks (COFs) are joined by covalent bonds and are structurally stable to be used in harsh CO₂ capture settings. In fabricating membranes for the capture/separation of CO₂ gas, 2D COFs are primarily used as active constituents [122,123,124,125]. Lately, 3D MOFs and 2D COFs have been applied to fabricate dual-layered membranes for CO₂/H₂ separation [122]. 2D COFs were accommodated on vertical binding sites of MOFs, producing composite membranes with exceptional H₂/CO₂ selectivity (32.9) and superior permeability.

14.4.8 Other 2D Materials

Nanoclays consist of layered mineral silicate with excellent mechanical properties. Due to its high abundance and low cost, it has been used as a solid adsorbent for environmental remediation. Clay-based materials have also been studied for their potential to adsorb CO₂, but they have a major drawback [98, 126, 127]. The efficiency of CO₂ uptake under wet conditions is low due to the presence of diffused H₂O, which avert the trap of gas molecules. In dry conditions, however, high uptake can be expected because of nano-channels present in clay that allow easy intercalation of CO₂. The moisture issue has led researchers to investigate ways in which this could be overcome, such as by modification or impregnation with other compounds like polymers and surfactants or by making composites with other nanomaterials such as carbon nanotubes and graphene oxide sheets. For example, montmorillonite (MMT) clay was first modified with polyphosphoric acid and used to make a composite with rGO for CO₂ capture [98]. The MMT/rGO hybrid exhibited a reasonable CO₂ adsorption capacity of 0.5 mmol.g⁻¹ at 1 bar and 25 °C. In another method, porous MMT was modified with octadecylamine to adsorb CO₂, which showed excellent adsorption of CO₂ (7.16 mmol.g⁻¹) compared to pristine MMT (only 3.47 mmol.g⁻¹) at room temperature and 50 bar [127].

Other 2D materials, such as borophene [128, 129], phosphorene [130] and graphyne (comprised of both sp and sp² carbons) [131], have mainly been studied theoretically for CO₂ capture and separation. The adsorption of gas molecules (CO, CO₂, NH₃, NOx, and CH₄) on these 2D materials was investigated by DFT and grand canonical Monte Carlo (GCMC) calculations. The findings suggest they are potential candidates for excellent adsorption of CO₂ at ambient conditions.

14.5 Conclusion and Future Directions

The distinct and extraordinary properties of 2D materials (e.g., high surface area and porosity, tailorable structure, surface functionalities, chemical and thermal stability) have been fundamental driving forces for researchers to explore their potential in CO₂ capture and utilisation. Lately, 2D materials such as graphene and graphene-based nanomaterials, TMDCs (e.g., MoS₂), h-BN, 2D TMOs, MXenes, 2D MOFs, 2D COFs, g-C₃N₄, borophene, phosphorene, and nanoclays have been employed for CO₂ capture. This chapter summarised an overview of various sources of CO₂ and commonly employed mechanisms of CO₂ removal technologies, as well as recent developments related to the use of 2D materials for capturing CO₂. Usually, the CO₂ adsorption efficiency is controlled by external factors, including pressure, temperature, and contaminations. Various strategies, such as doping, defect engineering, and the formation of composites and hybrids, have been employed to improve the CO₂ adsorption capability of 2D materials. The importance of controlling and functionalising pores of adsorbent for CO₂ uptake is discussed thoroughly. Adsorption performance could also be improved by functionalizing 2D materials and their composites with CO₂-philic components. Moreover, new 2D materials such as phosphenes, borophene, 2D MOFs/COFs, and graphyne are mostly theoretically studied for CO₂ capture. Therefore, more research is required to establish their CO₂ capture and conversion potential.

There are still many limitations of commercializing 2D materials-based adsorbents technology for CO₂ capture. For CO₂-containing mixed gases, more focus should be placed on the high selectivity and high-performance CO₂ adsorption at moderate temperatures and low CO₂ concentration. The design and development of multi-functional 2D materials are critical to future technologies, where materials can concurrently capture CO₂ from gas streams and convert it into value-added chemicals using in-situ heterogeneous catalysis. The selectivity over other gases and low performance are the major troublesome for industrial adoption of 2D materials for CO₂ capture and conversion. Much research efforts are needed to match the industrial scale efficiencies. Still, there is a high preparation cost and non-uniform quality of the materials at large-scale production. Therefore, dedicated research is needed for the sustainable, affordable synthesis and processing of 2D materials-based technologies. Additionally, machine learning tools can be employed to fast-track the discovery of advanced 2D materials, experimental designs and to achieve the best possible performance.